Dr Valérie Keller

Prof Bowker Michael

Prof Merci

Didier Leonard

Dr Mireille Richard

Laetitia Oliviero

Dr Christophe Geantet

Dr Eric Puzenat

Dr Pavel Afanasiev

Luis Cardenas Merci

de m'avoir sélectionné parmi tous ces candidats et de m'avoir fait confiance pour le concours de l'Ecole Doctorale. Merci à tous les quatre de votre aide et de votre accompagnement.

Quatre encadrants c'était quatre domaines de compétences assez variés et donc une tonne de connaissances que j'ai pu emmagasiner. J'ai appris la photocatalyse, la chimie des sulfures et la spectroscopie des photoélectrons auprès des meilleurs. Mais j'ai aussi développé un esprit critique et une rigueur scientifique -dont le domaine de la photocatalyse aurait bien besoin. Pour cela, je vous suis extrêmement reconnaissant.

Quatre encadrants c'est aussi beaucoup, beaucoup de travail mais vous m'avez fait confiance pour trier dans nos diverses idées et pour mener mon projet. J'ai énormément gagné en autonomie lors de ces 3 ans alors, merci infiniment. Nos réunions à 5 fortes têtes (4 au début) vont me manquer. Petit regret, je ne suis parti en congrès qu'avec Christophe. J'espère qu'on y remédiera dans le futur…. (A noter qu'une marche pour le climat Eric ne compte pas). Merci d'ailleurs pour ces nombreux congrès que j'ai pu effectuer. J'y ai rencontré beaucoup de personnes très inspirantes.

Merci au Dr. Laurent Lombez, ex-mentor, dont la rigueur en physique des semiconducteurs et dont la bible Physics of Solar Cells -P. Würfel ont joué un rôle clé dans ma thèse.

Merci aux services scientifique qui ont grandement contribué à ce projet. Merci à Yoann, Mimoun, Pascale, Laurence et Nicolas. Un merci particulier pour Laurence qui a été suffisamment patiente pour me former sur notre TEM (« le papy ») et à Pascale qui le matin même de ma soutenance m'aidait encore à extraire mes résultats BET.

Merci aux services administratifs et informatiques qui ne font pas le travail le plus visible dans un projet de recherche mais qui reste pourtant crucial.

Le doctorat c'est une formation par la recherche mais elle est d'autant plus enrichissante lorsque c'est une formation par l'enseignement. Alors merci aux étudiants que j'ai eu la chance d'encadrer en stage : Pierre, Mathis, Alexandre et en particulier Alison Leang. Ton autonomie, ta rigueur et ta force de proposition m'ont été d'une grande aide pour compléter mes travaux de thèse. Merci également à Paola, Pauline, Sonia, Florence et la Prof. Anne Giroir Fendler pour ces trois années d'enseignement à l'Université Claude Bernard Lyon 1.

Merci à tous les membres de l'IRCELYON que j'ai côtoyés au cours de ces trois années, permanents et non-permanents ! Merci à tous les membres de mon équipe de recherche. Merci aux permanents, Dorothée, Chantal, Stéphane, Laurent, Ruben, Jean-Marc, Gilles. J'ai eu la chance d'interagir et de collaborer avec beaucoup d'entre vous. Merci pour notre discussion scientifique, ou autres qui m'ont permis de voir les coulisses de la science et du CNRS. Merci pour vos remarques pertinentes lors de mes (nombreuses) interventions en réunion d'équipe.

Merci à tous les étudiants ECI2D pour les moments qu'on a pu partager au cours de ces 3

ans. Merci à Zoé, Elodie, Nelly, Camella, Camila, Junjie, Santiago, Alissa, Jeannot, Eugenie, Thomas, Fefe, Maxime D., Clément Mo., Davide, Mathieu, Nha, Christophe Jr., Maxime (miaou) et Caroline Dessal (non je ne t'appellerai pas Caro). Merci pour les fous rires, les soirées, les viennoiseries, les parties de cartes dans cette salle polyvalente, bref merci pour votre soutient quotidien. Un merci particulier au bureau du FUN, le calme et le sérieux qui règne dans cette pièce ont joué un rôle évident dans mon processus de rédaction. Merci à Santi avec qui on a passé des moments très sympas lors du MACSVIII et du FCCAT2. Merci à la future Dr. Blaser et à M. Len pour leur soutien et nos soirées Kaamelott (merci M. Alexandre Astier au passage… Et puisque j'en suis aux personnes inspirantes, merci Cal Newport, Hal Elrod et David Allen pour vos ouvrages).

Merci à ma famille et à mes amis pour avoir participé à la personne que je suis, une thèse c'est l'association entre un sujet et une personne. Merci également pour votre soutien indéfectible, merci de m'avoir accompagné par vos pensées ou par votre présence lors de ma soutenance de thèse. Merci de me stimuler lors des repas de familles pour expliquer ce que je fais et de me donner l'occasion de défendre ce beau métier qu'être chercheur.

Et enfin, un immense merci à Pauline ! Tu as démarré cette thèse à mes côtés comme P. Reumaux, tu l'as fini comme P. Maheu. Cela n'a pas toujours été facile alors merci pour ton soutien vital au cours de cette thèse. Merci pour ton écoute, ta patience et ta bienveillance. Cette thèse n'aurait surement pas vu le jour sans toi. Merci d'être ce que tu es et de m'accepter comme je suis. Merci de t'être engager à vivre aux côtés d'un chercheur pour le reste de nos vies. Je te dédie cette thèse.

Parce qu'elle m'appelait « notre savant Cosinus » et qu'elle espérait me voir « grand chercheur devant l'Eternel ». Reproduced from [80] with Na 2 S/Na 2 SO 3 . Reproduced from [61] and [62] RuS 2 /TiO 2 (-SiO 2 ). Reproduced from [102] and [104], respectively, with permissions. ....................... Reproduced from [1] with permission. (right) Diagram of the electron energy during its excitation from valence band to conduction band; two processes can occur with absorption or emission of a phonon (Γ). Adapted from [10]

Index of symbols

Figure VII-18:

The three energy levels diagrams for the relative positions of rutile and anatase. Type I is called included, Type II is called staggered. For the latter, energy levels of anatase can be more negative than rutile (Type II-anatase) or they can be less negative (Type II-rutile). Adapted from [1] with permission.

List of tables

General introduction

Photocatalysis is a field of heterogeneous catalysis. It can play a key role in the context of greener chemistry and renewable energy. Indeed, a photocatalytic process addresses two aims at the same time. On the one hand, it can convert solar energy in molecules, thus, storing a promising but intermittent source of energy into chemical energy. On the other hand, it produces valuables molecules, at room temperature, from solar energy that is clean, abundant and free.

Photocatalysis would be a process to store solar energy in renewable hydrogen (H 2 ). This work aims to produce H 2 from the photocatalytic dehydrogenation of alcohols. A forecast would be that H 2 is obtained from bio-based alcohols.

The objective is therefore to produce efficiently H 2 from alcohol. To tackle this issue, this manuscript provides a fundamental approach. To do so, it provides structure-activity relationships between the intrinsic properties of the photocatalysts and their performances.

Transition metal sulfides have highly tunable properties. They activate hydrogen, they have promising electrochemical behaviors and their optical properties vary from one transition metal sulfide to another. Therefore, different supported transition metal sulfides provide a whole range of properties.

Those materials are ideal to establish a structure-activity relationship.

Chapter I defines concepts about photocatalysis, solar fuels and the reaction of interest, the photocatalytic dehydrogenation of alcohols.

Chapter II provides a state of the art on the key properties of transition metal sulfides and their use in catalytic reactions. It also reports photocatalytic results already obtained with supported transition metal sulfides.

Chapter III reminds the steps of a photocatalytic mechanism. It evidences the difficulties of comparing one photocatalyst to another, from the literature, and it exposes the experimental methodologies used to obtain accurate photocatalytic results. Finally, it also reports the methods of characterization of the photocatalysts.

22

Chapter IV suggests that the photocatalytic dehydrogenation of propan-2-ol is an ideal reaction to get an insight in the photocatalytic mechanism. Mechanism, macro-and micro-kinetic of the propan-2ol dehydrogenation is discussed.

Chapter V screens seven transition metal sulfides supported on titania for this reaction.

Photocatalytic activities are interpreted in light of the intrinsic properties of the sulfides (i.e. H 2 activation, electrochemical behavior, morphology). The latter were determined with hydrodesulfurization, electrocatalytic hydrogen production or structure characterizations.

Chapter VI focuses on a RuS 2 /TiO 2 photocatalyst. It reports the influence of six parameters on the photocatalytic activity: the flux and the energy of the incident photons, the mass of photocatalyst, the co-catalyst loading, the size of the co-catalyst and the composition of the titania support.

Chapter VII reports a methodology combining UV-Vis diffuse reflectance and photoelectron spectroscopies to establish the electronic structure of powders TiO 2 and RuS 2 /TiO 2 photocatalysts and to have an insight on the valence band states. It is applied to the sulfide-based photocatalyst to interpret their photocatalytic results.

Solar Energy: A promising solution for clean and abundant energy

120 000 Terawatt (TW) / year of solar energy reach the surface of the Earth. In comparison, humans consumed 18-19 TW during 2018 [1,2] and it is estimated that in 2030 they will consume 23.4 TW [3]. Consequently, technologies that would convert at least 0.02 % of total solar energy input would overcome the growing energy demand. In other words, Earth receives enough solar energy in 1 h to supply the world energy consumption of a year [4].

The two main drawbacks of solar energy are its intermittency and its space dilution. The amount of solar energy received on Earth varies as a function of the hours of the days, as a function of the seasons and as a function of the latitude or the longitude. To replace fossil fuels or nuclear, solar energy must be supplied continuously. That is why it requires storage technologies.

Batteries are one solution; photovoltaic device converts a fraction of the sunlight in electricity and then batteries store it. Photovoltaic is a well established technology that provides 1.5 % of the worldwide electricity production [5,6]. Depending on the application, various types of batteries exist [7] but they are costly and suffer from long term stability issues (unsuitable for seasonal storage).

Thermal energy is another solution. Concentrating solar power converts the thermal part of solar energy in heat which is then stored trough: sensible heat storage, latent heat storage and thermochemical heat storage. The latter is the most promising because of its energy storage density.

The different technologies are reviewed in [4] by Carillo et al.

Finally, the solution studied in this work is to store solar energy in chemical bonds, synthesizing energetic molecules thanks to solar energy. Those molecules are called solar fuels.

Solar Fuels 1.2.1 Photocatalysis and production of solar fuels

Converting solar energy in chemical energy can be done by photocatalysis. According to the IUPAC, photocatalysis is a "change in the rate of chemical reaction or its initiation under the action of ultraviolet, visible, or infrared radiation in the presence of a substancethe photocatalystthat absorbs light and is involved in the chemical transformation of the reaction partners." As any catalyst, the photocatalyst must "regenerate its chemical composition after each cycle of such interactions." [8].

Light is absorbed by the photocatalyst; such interaction photogenerates electrons in the conduction band and holes in the valence band. The first reduce molecules and the second oxidize molecules.

Paternity of photocatalysis is ambiguous [9,10]. In 1910, in Russia, Plotnikov, introduced for the very first time the words "photokatalyse" and "photokatalytisch" (respectively photocatalysis and photocatalyst) [11]. Thanks to M. Landau the word "photocatalyse" appeared in France in 1913. He reported that thorium and uranium based catalyst, in presence of light, enhance the decomposition of the oxalic acid [12]. The earliest work about titanium dioxide (TiO 2 ) is from Carl Renz. Without mentioning the word "photokatalyse", he reported darkening of TiO 2 under sunlight illumination in presence of glycerol [13].

At the Institute of Research on Catalysis, in the 60s', Pr. Teichner conducted researches on TiO 2 powder. He faced incoherence during measurements of electrical conductivity. He concluded that those variations depend on the amount of sunshine in the laboratory. He also reported that TiO 2 turns from white to blue color under vacuum and UV irradiation due to oxygen vacancies [14]. He initiated a research field on photocatalysis in Lyon starting with the partial photooxidation of alkanes and olefinic hydrocarbons [15].

For sure, the paper published in Nature by Honda and Fujishima [16] is not the first work to report photocatalysis with TiO 2 . It is highly cited (more than 24,300 citations at the beginning of 2019)

as the first report of TiO 2 for photocatalysis, but it was published later (in 1972) and it dealt with "photoelectrocatalysis" and not "photocatalysis".

Since its discovery, photocatalysis was used for many applications: air or water treatment through the oxidation of organic molecules (pesticide, dyes, volatile organic compound, etc.) [17,18] and/or synthesis, to catalyze for instance C-C or C-N bonds formation [19][20][21]. More recently, scientists were interested in photocatalysis for energy purpose, which is the topic of this Ph.D. thesis. Among those photocatalytic applications, Nozik [22] and Bard [23] distinguished two photochemical processes: . Thermodynamic forbids such a reaction; the light source brings enough energy to overcome the energy barrier. This distinction is not unanimous in the litterature [9,24]. It is based on the thermodynamic of a reaction. While the IUPAC definition is based only on the kinetic of a reaction. As a reminde, if the rate of a chemical reaction is increased thanks to light, it is photocatalytic. In addition, IUPAC and the textbook reference of MD. Archer and J. Barber defined the photosynthesis differently [25,26]. The word photosynthesis only referred to the natural process converting CO 2 and H 2 O in organic compounds (Section 1.2.3 describes this phenomenon).

We choose to follow these recommendations. Photosynthesis stands for the natural process and photocatalysis stands for any process accelerated in presence of light and a photocatalyst. However, a photocatalytic reaction can be endergonic or exergonic, as schemed on Going from ( ) to ( ) releases energy to power a system. The input energy can come from the chemical potential of the photogenerated electron-hole pairs [27,28]. It is free energy carried by photons and due to the conservation of energy (photons do not react with each other), carried by the electron-hole pairs.

Photocatalysis performs two tasks at the same time. On one hand, photocatalytic endergonic reactions store solar energy in energetic molecules, so-called solar fuels. On the other hand, photocatalysis is a way to produce those molecules through cleaner ways. Solar energy is free and abundant and the reaction occurs at room temperature.

Solar fuels contain hydrogen, methane or methanol. The first is produced from alcohol dehydrogenation/reforming or water splitting. The others are produced from CO 2 reduction. This work exclusively deals with hydrogen production from alcohols.

Hydrogen: the most promising solar fuels

Various industrial processes use hydrogen as a reagent but hydrogen also emerges as a strategic energy vector for this century [29,30] [31]. The hydrogen field is more and more developed. Thanks to the appropriate technologies, hydrogen would contribute to reach the 7 th of the 17 th Sustainable Development Goals: "Ensure access to affordable, reliable, sustainable and modern energy for all" for the United Nation Program [32].

For instance, four Japanese companies (Chiyoda Corporation, Mitsubishi Corporation, Mistui & Co, Ltd and Nippon Yusen Kabushiki Kaisha) planned to deliver the first hydrogen supply chain for 2020 [33]. It means that the industry knows how to produce H 2 , to transport it and how to use it to provide energy for all. Research is undergoing to make H 2 competitive with other energy vectors.

Hydrogen is also a way to tackle greenhouse gas effect. Figure I-2 plots the proportion of greenhouse gases released in 2016, in France by sector. It shows that the main source of emissions is the transport (29 %); heating and supplying buildings emits 19 % of the greenhouse gases. Hydrogen may help to limit those emissions. As a way of storage it participates to the rise of renewable energy.

It can power various means of transports, releasing only water vapor. The boat Energy Observer is making a world tour thanks to H 2 produced from sea water [34]; in Germany, Alstom launched a regional train powered by H 2 [35]; in Paris, H 2 is supplying a hundred of taxis [36]. In the north of France (Dunkerque), the project GRHYD run by ENGIE expects to power an entire neighborhood with Hythane, a mix of H 2 and natural gases [37]. Addition of hydrogen to the natural gas enhances the energetic yield of the process and reduces the amount of greenhouse gases released during the combustion.

Figure I-2:

Carbon dioxide emissions by sector, in France, in 2016. Proportions were made based on a total emission of 463 MtCO 2 eq. Adapted from [38].

The main challenge for the hydrogen economy is that less than 1.0 % of its production is from renewable resources [31]. The main processes are: the steam methane reforming (equ. 1.1), the partial oxidation of hydrocarbons (equ. 1.2) or the Auto Thermal Reforming which combines both.

1.1

1.2

To pretend to be a clean and a renewable vector of energy, production of hydrogen must be decarbonated (produced from water) or at least earth surface carbon neutral (produced from biomass [39]). Several pathways exist including solar energy [40]. The most developed technology is to use photovoltaic devices to provide electricity and to perform water electrolysis. Water electrolysis (equ. 1.3) already produced 4 % of the total hydrogen but all the electricity does not come from renewable sources. The reaction is endothermic ( ):

1.3

This process is a two-step production, first photovoltaic panels convert photons in electricity and then the electricity converts water in hydrogen. Up to now, the record from the literature are at 24.4 % and 18 % with and without light concentrator [41,42]. Photocatalysis and photoelectrocatalysis are onestep production processes; they may pretend to higher efficiency.

Green and sustainable production of H 2 is not competitive yet. But Jaramillo and co-workers predicted (in 2013) that a single bed particle suspension, with solar to hydrogen efficiency of 10 %, would produce 1 kg of H 2 for $1.60 [43]. As a comparison they reported a cost of $1.00-$5.00 per kg of H 2 with steam methane reforming.

Farming 19%

Wastes 4%

Transport 29%

Buildings 19%

Industry 18%

Energy production 11%

Artificial photosynthesis -Water splitting

Photosynthesis is a light driven process in which the nature converts carbon dioxide and water in dioxygen and sugar [26]:

1.4

The antennas chlorophyll in plants or algae absorb light and funnel the energy to two photosystems (PS): PSI and PSII. The latter use water as a renewable source of electrons and protons:

1.5

Those are then involved in the Benson-Calvin cycle with the ATP and the NADPH to convert CO 2 in sugars. Usually, the PSI provides additional photogenerated electrons (than those from the PSII) and transfers them to a ferredoxine that reduces NAP + in NADPH. This pathway leads to the overall balanced reaction 1.6. But under certain conditions the PSI transfers the electrons to a [Fe-Fe]hydrogenase to produce H 2 gas. In that case, the balanced reaction is:

1.6
It is an endergonic reaction:

. In analogy to such a system, an artificial photosynthetic system aims to convert carbon dioxide and water, much faster, in high-energy content chemical species. It must be composed of a light-harvesting antenna, some charge separation units and of multi-electron transfer catalysts.

The CO 2 reduction part is more difficult to address: it involves more species (e -, H + , CO 2 ) and the catalysts suffer from a lack of selectivity [44]. However, this field is gaining more interest [45]. Rongé et al. proposed in [46] a chart reporting the different records of solar-to-fuel-efficiencies. It reports both the record for water-splitting and CO 2 reduction. The work reported here does not address the CO 2 reduction, it is focused on the hydrogen production.

Most of scientists divide the water splitting in two parts, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Such a terminology comes from the electrocatalytic hydrogen production; through misuse of language, those terms are also employed in photocatalysis. They focused on one of the two reactions using a "sacrificial agent" for the second half reaction. This molecule must not limit the overall activity of the system. Thanks to this strategy, they can design a photocatalyst composed of two materials. One oxidizes the water and the other reduces it.

This architecture is called a Z-scheme [47]. They may separate spatially the two half reactions; it is the case for a photoelectrochemical cell for which the oxidation and the reduction occur on two different electrodes [48].

For simplicity, this project is only dedicated to suspended particles and to one semiconductor with a co-catalyst. The water-splitting with such a system was already reported in the literature [49][50][51][52][53]. However, the photocatalytic water splitting has a larger thermodynamic barrier ( ). This is mostly due to the oxidation of water in O 2 . It requires the coordination of two molecules of water and four proton-coupled electorn transfer [54]. Generating an overpotential higher than 1.23, because of overpotential losses, in the same particle induces undoubtedly low efficiency, because of charge carriers that can recombine. Pinaud et al. theorized a maximum solar to hydrogen efficiency of 11 % with a single semiconductor [43]. With a Z-scheme system, it would be 25 %.

An alternative is to convert organic compounds that are thermodynamically favored, into hydrogen. In comparison, the of alcohols is much lower than water, it induces for glucose [55] or for glycerol [56] or methanol [57]. This project is therefore dedicated to the photocatalytic conversion of alcohols, easier to perform. From a practical point of view, notice also that oxidation of alcohol instead of water does not lead to an explosive O 2 /H 2 mixture. It could eliminate the need for a gas separation membrane [54].

Photocatalytic conversion of alcohols i. Photocatalytic H 2 production from biomass

The photocatalytic conversion of carbohydrates is promising for the field of solar fuels. Biomass would be a way to produce clean and renewable H 2 . Figure I-3 shows that biomass is composed of lignocelluloses, starch, sugar-based crops or vegetable oils crops [58]. Those elements are converted in a significant amount of carbohydrates with a wide variety of functionalities such as hydroxyl groups or carboxylic acid. Some have a great interest for the photocatalytic production of H 2 [59]. The latter can also be called photocatalytic hydrogen evolution reaction (PHER). Lignocellulose has the asset to be an agriculture waste product. Its use to produce H 2 would not impact food production [60]. Almost all the biomass derivatives, showed on Figure I-3, were already reported to produce H 2 . For simplicity, various mono-alcohol systems were extensively studied and few works were reported on more applied systems. with permission.

ii. Photocatalytic dehydrogenation of model alcohols

Bowker and co-workers [61][62][63] as well as Idriss and co-workers [56,64,65] screened various alcohols. They showed that the PHER activity of TiO 2 -based materials vary as a function of the alcohol. They correlated the PHER rate with intrinsic properties of the alcohols: number of α-H, C:O ratio, polarity, polarizability and the oxidation potential [56,64,65]. Correlation with the oxidation potential is nuanced in Chapter IV and the corresponding Annexes.

In addition, the literature is confusing on the distinction between the three processes that can occur during the photocatalytic conversion of an alcohol-water mixtures [29,66]. Both, water and alcohol can supply protons according to the following reaction:

1.7

1.8

Therefore, hydrogen production may originate from three processes. The first one is the photocatalytic reforming of an alcohol with water that produces H 2 , both molecules react. The second one is the photocatalytic dehydrogenation of an alcohol producing H 2 ; water does not react, it only dilutes the alcohol. The third one is the photocatalytic water-splitting; the alcohol is a hole scavenger it consumes holes while water is reduced in H 2 . To claim water-splitting, labeling experiments is mandatory to attest that H 2 is formed with the protons from the water and not from the alcohol. In addition, dehydrogenation and reforming are often confused because only the production of H 2 is monitored and the products of the oxidation are not discussed.

The most studied alcohols are the ethanol, the propan-2-ol and the methanol. Table I-1 reports several papers on their photoconversion.

Methanol-water mixture is the most studied system. Kollmansberger et al. evidenced with a quadrupole mass spectrometer, the formation of methyl formate and formaldehyde [67]. Selli and coworkers identified eight products for the photocatalytic conversion of a methanol-water mix (See Table I-1) [68]. They proposed a mechanism to explain their formation as well as a microkinetic model [69]. They evidenced three parallel oxidation paths: an indirect one for which the radical mediated the oxidation step, a direct one for which the holes in the valence band of the photocatalyst

35

react with the species adsorbed at the M-TiO 2 interface and a water-assisted one for which water helps the holes and the surface species reaching each other. In addition, in situ AT-IR analysis showed that some reaction are only catalytic, it is the case for the formation of diethyl ether [70].

Those studies evidenced how complex is the photocatalytic conversion of methanol-water mixture. It supports the fact that the amount of H 2 produced often differs from the expected stoichiometry. Kandiel et al. [66] obtained n H2 = 2.6 n(formaldehyde, formic acid, CO 2 ) and Kennedy et al. [71] n H2 = 2.2 n CO2 , without detecting any other products.

The acetaldehyde results from the dehydrogenation of ethanol. It is often the main product of the photocatalytic conversion of an ethanol-water mixture [72]. Pt/Nb-TiO 2 acetone, propanone disopropyl acetal [75] The photoconversion of a propan-2-ol-water mixture seems to be simplier. Teichner et al.

reported the formation of only acetone for Pt/TiO 2 [78]. Fontelles-Carceller et al. also detected propanone and diisopropyl acetal with Pt/Nb-TiO 2 photocatalyst (See Table I-1) [75]. The intermediate between propan-2-ol and acetone, the 2-propoxide was evidenced by FT-IR analysis [79] and solid state NMR [80]. Gu et al. combined

iii. Glycerol, a biodiesel waste to valorize

Kennedy et al. reported photocatalytic conversion of cyclic alcohols and polyols [71]. The most studied polyol is the glycerol. Transesterification of vegetable oil or animal fat gives biodiesel and glycerol is the main by-product of this process (10 wt%) [29,81]. Reforming of glycerol gives renewable hydrogen and has a positive impact on the biodiesel economy:

(1.9) and non-glycerol organic matter). They may compete with glycerol for their adsorption on active sites.

They also analyzed the composition of gas and liquid phase and concluded that the mechanism varied between the two systems.

iv. From ideal systems to more applied ones

Impurities are an issue for the PHER activity; Lopez-Tenllado et al. described another:

adsorption competition between the different alcohols of a mixture. They studied binary mixture of C-3 alcohols and they showed that the number of hydroxyl function or its position influence the adsorption strength at the surface of TiO 2 . For instance, the presence of propan-1-ol completely inhibits the conversion of propan-2-ol [86].

Despite that, studies progressively tend from mono-alcohol systems to more complex and more applied one. In the 90s' Kawai and Sakata pioneered the PHER from carbohydrates such as sugars, starches and/or cellulose [87,88]. Later, the group of Kondarides reported the PHER with polysaccharides such as lactose, starch and cellulose [83]. They also produced H 2 with acetic acid, formic acid or acetaldehyde [89]. Fu et al. reported the photoreforming of glucose and sucrose with various metals supported on TiO 2 [90]. Zhang et al. produced H 2 with natural sunlight up to 2.5 μmol/h (830 μmol/h/g bio ) from cellulose directly immobilized on the photocatalyst (Pt/P25) [91].

Kuehenel and Reisner reviewed works on the photocatalytic reforming of unprocessed biomass: various sort of grass, rice or wood derivatives and seaweeds [55]. They emphasized the importance of alkaline solution to enhance biomass solubility and also show that efficiencies of the photocatalysts are still low. With Pt/TiO 2 (5 wt% Pt), Kawai and Sakata reported r H2 = 27 μmol/h for the chlorella algae (270 μmol/h/g bio. ) and r H2 = 33 μmol/h for the laver (330 μmol/h/g bio. ) [88]. ) with CdS/CdO x quantum dots (using also a Co(BF) 4 molecular co-catalyst) [94]. To compare they obtained lower r H2 for the conversion of α-cellulose (0.99 μmol/h; 10 μmol/h/g bio. ) and for the conversion of hemicellulose (0.89 μmol/h; 18 μmol/h/g bio ).

One key aspect of the photoconversion of biomass is to improve stability of the photocatalyst to face complex and aggressive mixtures. A second one is that biomass contains many molecules and additional phenomena can occur. Mechanistic insight are required to convert selectively biomass in high valuable molecules. For such a purpose, the Ph.D. project focused on the photoconversion of mono-alcohol systems.

Photocatalytic materials

Two parts compose an efficient photocatalyst: a semiconductor absorber and a co-catalyst.

The first behaves like the antenna in a natural photosynthetic system (See Section 1.2.3); the second behaves like a reaction center and their junction enhances the charge carrier separation. Notice that, the absorber may also act as a reaction center; a semiconductor alone is also a photocatalyst. Notice also that the co-catalyst may also act as an absorber if it is a semiconductor. Those terms are only proposed to distinguish the two parts of a photocatalyst.

Absorber

The photocatalytic activity of an inorganic semiconductor depends on various parameters. It includes: different phases, presence of multiple phases, surface area, crystallinity, presence of dopants and crystallite size. Influence of some parameters are reviewed in [95].

One key criterion is the size of the band gap (E g ). It governs the kind of photons that the semiconductor absorbs (Section 3.1.1 provides more details). It goes from 1.1 eV (Si, λ=1130 nm) to 5.0 eV (ZrO 2, λ=250 nm) depending on the phase [96][97][98]. Ultraviolet rays, visible light and infrared rays account, respectively, for 4 %, 53 % and 43 % of the total incident energy of the sun. It means that one of the challenges is to absorb and use visible light to produce hydrogen [57].

A second key criterion is the position of the valence band and of the conduction band. They set the oxidation and the reduction potential of the photogenerated holes and electrons. Their positions must allow an electronic transfer from the catalyst to the targeted molecule (See Chapter III for more details on the mechanism). For instance bare WO 3 , Fe 2 O 3 or BiVO 4 cannot be used for HER reaction [97].

Because of those requirements, the most studied photocatalysts are the TiO 2 -based ones [45] or the CdS-based ones [99]. Alternatives are investigated, for instance: CuGa x In 5-x S 8 [100], g-C 3 N 4 [101], VO 2 [102], SrTiO 3 [103] or tantalates [104]. SrTiO 3 has a dielectric constant 2-3 times higher than TiO 2 . This parameter permits a better charge carrier separation (See Chapter III for more details on the key properties of a photocatalyst) [105]. ZnIn 2 S 4 is also extensively reported in the literature as a support for sulfides co-catalysts [99,106].

Among those candidates, CdS has a small band gap (2.5 eV) and thus, it absorbs visible light.

The main drawback with CdS is its photocorrosion which releases toxic cadmium ions. To overcome this issue, it is often combined with other semiconductor [94,107] or it can be involved in a solid solution. Cd 1-x Zn x S solid solutions are more and more studied for higher stability and higher PHER activity [108,109]. The Cd/Zn ratio is tuned to obtain the appropriate band structure.

On the contrary, TiO 2 has a large band gap (3.2 eV for anatase and 3.0 eV for rutile, the two most known polymorphs) [110]. An whole range of works from the literature are dedicated to modify TiO 2 , allowing visible light absorption. Ma et al. reviewed various strategies: doping with cations to lower the conduction band position, doping with anions to shift the valence band upward or, surface sensitization [45]. Among them, doping with nitrogen is the most studied strategy [111]. Materials can be also co-doped [82]. A last pathway use plasmonic effect shifting the photoabsorption profile of TiO 2 [112].

The most efficient and commercial absorber is the TiO 2 Degussa P25. It is a mix of anatase and rutile. It is the most active support reported to date. The exact nature of such a high PHER activity is not completely understood. One hypothesis is a particular heterojunction between the two TiO 2 phases.

Electrons migrate to one polymorph and holes migrate to the other. Works from the literature argue on the relative energy levels ( and of anatase and rutile). To answer, several works determine the electronic structure of anatase and rutile, separately [113][114][115][116][117].

The Ph.D. project is focused on the co-catalyst. To avoid the contribution of a synergetic effect occurring in multiphase absorber, the co-catalyst were mainly supported on monophasic and commercials TiO 2 .

Co-catalyst

Reasons to add a co-catalyst is extensively described in Section 3.1. The band alignment at the interface between the absorber and the co-catalyst enhance the charge carrier separation. The co-catalyst may also become an active site for one of the two half-reaction. This section lists some analytical techniques to evidence a co-catalytic effect. Then it briefly describes the use of metallic cocatalyst in the literature. Chapter II establishes a state of the art for their transition metal sulfides counterpart. Photoluminescence is observed only if the photogenerated charge carriers recombine. TRPL measurements allow calculating the lifetime of free charge carriers [118]. Some recent studies measured the photoluminescence intensity with and without a co-catalyst. They interpreted the difference as a lower recombination rate and concluded that the co-catalyst improves the charge carrier separation [101,[119][120][121]. For instance, Wang et al. reported a lifetime of 5.10 ns for Mn 0.5 Cd 0.5 S and of 5.43 ns when CoS x is added [122]. which affect the adsorption/desorption dynamic [70].

i. Ways to characterize the co-catalyst role

A
TAS consists in exciting a material with a light pulsed and measuring its absorption along the time. It detects shorts-lived intermediates and it allows studying the kinetic of holes and electrons separately [123].

Finally, TRMC measures variations of the microwave power reflected by a sample [124]. For instance, it provides the mobility or the lifetime of the charge carriers. Warman et al. showed with TRMC that the presence of propan-2-ol delayed charge carrier recombination. Molecule prevent the accumulation of holes on the TiO 2 surface; they act as scavengers [START_REF] Warman | Electronic processes in semiconductor materials studied by nanosecond timeresolved microwave conductivity-III. Al 2 O 3 , MgO and TiO 2 powders[END_REF].

A second way to evidence a co-catalytic effect is to determine the electronic structure of a given material. The band positions of the absorber and of the co-catalyst indicate if the electrons and the holes can transfer between the materials.

Mott-Schottky analysis gives the energy of the conduction band minimum ( ). The maximum of the valence band ( ) is deduced with the band gap. Mott-Schottky analysis consists in measuring the capacitance of the layer formed at the electrolyte/semiconductor interface. Plotting the capacitance as a function of the electrode potential gives and the donor density [113,119]. Careful correlation must be done with the PHER activity; it was shown that the electrolyte may induce surface charges giving an inappropriate [START_REF] Nosaka | Reconsideration of Intrinsic Band Alignments within Anatase and Rutile TiO 2[END_REF][START_REF] Kullgren | Resolving the Controversy about the Band Alignment between Rutile and Anatase: The Role of OH -/H + Adsorption[END_REF].

Photoemission spectroscopies (UPS and XPS) give the energy of the . The is deduced with the band gap. UPS has a better resolution than XPS but it is also more sensitive to surface states.

For this reason, UPS studies are mainly focused on model materials [START_REF] Liu | XPS and UPS Characterization of the TiO 2 /ZnPcGly Heterointerface: Alignment of Energy Levels[END_REF][START_REF] Borodin | Characterizing TiO 2 (110) surface states by their work function[END_REF]. Jaegermann and coworkers made an extensive work on this topic [START_REF] Md | Preparation of RuO 2 /TiO 2 Mesoporous Heterostructures and Rationalization of Their Enhanced Photocatalytic Properties by Band Alignment Investigations[END_REF][START_REF] Md | Band alignment investigations of heterostructure NiO/TiO 2 nanomaterials used as efficient heterojunction earth-abundant metal oxide photocatalysts for hydrogen production[END_REF][START_REF] Kashiwaya | Fermi Level Positions and Induced Band Bending at Single Crystalline Anatase (101) and (001) Surfaces: Origin of the Enhanced Photocatalytic Activity of Facet Engineered Crystals[END_REF]. They prepared materials in a high vacuum chamber and measured their valence band position with XPS and UPS. They used the resulting electronic structures to interpret the PHER activity of powders but it is not taken into account that morphology and chemical composition may differ between model and powder materials.

For this reason, a methodology to study powders by UPS was developed during this Ph.D. [START_REF] Maheu | UPS and UV spectroscopies combined to position the energy levels of TiO 2 anatase and rutile nanopowders[END_REF], it is detailed in Chapter VII.

A last technique is reported to probe a co-catalytic effect. It is the electrochemical impedance spectroscopy (EIS). A lower radius indicates a lower resistance for charge transport in the materials.

EIS experiments reported lower radius in presence of a co-catalyst [119,[START_REF] Guo | Enhanced hydrogen evolution via interlaced Ni 3 S 2 /MoS 2 heterojunction photocatalysts with efficient interfacial contact and broadband absorption[END_REF]. They also studied the influence of the co-catalyst loading on the radius [101,120].

ii. Metallic co-catalyst

Metals were extensively studied as co-catalysts. First, it should be noticed that those photocatalytic materials are often referred as metal co-catalyst but often it is a mix of metal and oxides counterpart. Heiner et al. performed XPS analyses showing that among their six M/TiO 2 (M=Au, Cu, Pt, Co, Ru, Pd), at least four contained oxides (RuO x , PtO x , PdO x , CuO 2 , CuO) [START_REF] Hainer | Photocatalytic Hydrogen Generation Using Metal-Decorated TiO 2 : Sacrificial Donors vs True Water Splitting[END_REF].

Yasuda et al reviewed their activity (supported on TiO 2 ) in various alcohol-water systems [START_REF] Yasuda | Sacrificial hydrogen production over TiO 2 -based photocatalysts: Polyols, carboxylic acids, and saccharides[END_REF].

Trasatti found a relationship between the work function of the metals and their activity for the electrocatalytic production of hydrogen [START_REF] Trasatti | Work function, electronegativity, and electrochemical behaviour of metals[END_REF]. He evidenced that the higher the work function was, the higher the PHER rate was. He also evidenced a volcano relationship between the H 2 exchange current and the metal-hydrogen atom bond strength. This behavior was extensively modeled by Norskov et al. [START_REF] Nørskov | Trends in the Exchange Current for Hydrogen Evolution[END_REF][START_REF] Montoya | Materials for solar fuels and chemicals[END_REF]. For those reasons, it is often the same metal reported as efficient co-catalysts such as Pd or Pt (See Table I-2). They are close to the optimum of the volcano curve discussed above and they have the lowest work function.

Attempts to study less costly metals were made. For instance, Cu-based co-catalyst showed significant performance but particles oxidized during synthesis or photocatalytic test [START_REF] Hainer | Photocatalytic Hydrogen Generation Using Metal-Decorated TiO 2 : Sacrificial Donors vs True Water Splitting[END_REF][START_REF] Choi | Hydrogen production from methanol/water decomposition in a liquid photosystem using the anatase structure of Cu loaded TiO 2[END_REF][START_REF] Bandara | Highly stable CuO incorporated TiO 2 catalyst for photocatalytic hydrogen production from H 2 O[END_REF].

Table I-2:

Selected publications reporting M/TiO 2 for the conversion of a methanol-water mixture.

Metals are ranked following their activity for the photocatalytic hydrogen production. Scarcity of some metals is also an issue for the photocatalytic production of renewable hydrogen. Figure I-5 depicts the availability of some elements of the periodic table; it confirms issues with Pt and Pd. For Au, its acquisition is the problem; gold is extracted from countries in conflicts.

Pt

To substitute costly and scarce elements, transition metals such as V, Mn, Ni, Cu or Mo would be suitable in the short term. The only element that is plentiful and can be used as a co-catalyst is the iron (See Figure I-5) but it tends to be oxidized [START_REF] Jellinek | Inorganic Sulfur Chemistry[END_REF].

Note that Ti and O are widely available making TiO 2 suitable for high scale applications, which is not the case for ZnO or CdS. Note also that elemental sulfur is plentiful. As a result, the scarcity of MS x /TiO 2 photocatalysts depends only on the scarcity of the metal. A promising candidate would be FeS x /TiO 2 . In addition, FeS x has already shown a catalytic activity for H 2 production. Indeed, Fe-S bonds are involved in the three types of hydrogenase enzymes. For instance, the hydrogenase from Clostridium pasteurianum has a catalytic [6Fe-6S] cluster. With a system based on this enzyme, a TOF around 21 000 s -1 (-0.7 V vs Ag/AgCl) was reported for the electrocatalytic hydrogen production [START_REF] Madden | Catalytic Turnover of [FeFe]-Hydrogenase Based on Single-Molecule Imaging[END_REF].

Figure I-5: Element Scarcity Periodic

Cost and availability

Résumé:

Production photocatalytique d'hydrogène avec des sulfures de métaux de transitions supportés sur TiO2.

La photocatalyse est une voie de synthèse prometteuse de l'hydrogène comme carburant solaire. La production photocatalytique est un moyen, à la fois de stocker l'énergie solaire sous forme d'énergie chimique et de produire des carburants de manière renouvelables en utilisant l'eau ou des alcools biosourcés comme matière première.

L'objectif de cette thèse est d'étudier la déshydrogénation photocatalytique d'alcools à l'aide de sulfures de métaux de transitions, supportés sur TiO 2 (MS x /TiO 2 ). Ces sulfures de métaux de transitions ont des propriétés d'activation de l'hydrogène, des propriétés électrochimiques et des propriétés optiques intéressantes.

Une série de sept MS x /TiO Photocatalysis is a promising way to synthesize H 2 as a solar fuel. On one hand, the photocatalytic H 2 production stores solar energy under chemical energy. On the other hand, it produces H 2 with a renewable process using water and bio-based alcohols as a feedstock. This Ph.D thesis aims to study the photocatalytic dehydrogenation of alcohols with transition metal sulfides supported on TiO 2 (MS x /TiO 2 ). Those transition metal sulfides have versatile and highly tunable properties. They can activate H 2 , they have promising electrochemical behavior and optical properties.

Seven MS x /TiO 2 (M = Co, Ni, Cu, Mo, Ru, Ag, Hg) are therefore studied. The photocatalytic dehydrogenation of propan-2-ol is used as a model reaction. Structure-activity relationships are found between the intrinsic properties of the MS x /TiO 2 and their photocatalytic activity. Measuring an apparent activation energy provides additional mechanistic insights.

It shows that the photocatalytic production of hydrogen is mostly limited by the charge carrier separation and by the electronic transfer. Therefore a method combining the UPS and the UV-Visbile absorption spectroscopies has been develop to establish the electronic structure of photocatalytic powders.

This work concludes that the electronic structure plays a crucial role in photocatalysis. With RuS 2 /TiO 2 photocatalyst, the electron transfer is evidenced as the rate-determining step of the photocatalytic dehydrogenation of propan-2-ol.

Key

Hydrogenation reactions on sulfide catalysts: hydrotreatments

Transition metal sulfides were widely studied for hydrotreatment [1,2]. In particular, they catalyze reactions of hydrodesulfurization (HDS). It consists in removing sulfur from natural gas or refined petroleum products. The more sulfur is removed from petroleum, the less automotive vehicles or industrial furnaces emit SO 2 when burning petroleum. The latter is a greenhouse gas.

For this process, catalyst must be stable at high temperatures and sulfur must not poisoned such a catalyst. That is why supported transition metal sulfides are suitable. The chemical equation for the HDS process is the following:

2.1

For study purposes, organic molecules containing sulfur are used to measure the rate of hydrodesulfurization (r HDS ) of a given catalyst. Dibenzothiophene and thiophene are among those molecules.

60

The Sabatier principle is responsible of the volcano plot on Figure II-2. MS x must not interact too weakly or too strongly with H 2 or reactive intermediates. For low E M-S , sulfurs are highly labile and leave the particles surface without reacting. For high E M-S , sulfurs are highly bonded and cover the particles; no active sites remain to adsorb H 2 . Therefore, the curve reaches an optimum with at its top RuS 2 , Rh 2 S 3 or mixed sulfides. open circles are predictions for mixed sulfide. Lines are the theoretical rate of: direct desulfurization (solid grey line), hydrogenation (dashed grey line), sum of both (solid black line). Reproduced from [4] with permission.

The microreversibility principle states that at equilibrium, each individual step of a complex reaction occurs in such a way that the forward and reverse rates are equal. So, a catalyst which breaks the H-H bond in HDS reaction may catalyze the formation of the H-H bond. This is a reason to study MS x for photocatalytic or electrocatalytic hydrogen evolution reaction (HER). In both cases, two H atoms must be coupled to form a hydrogen molecule.

Hydrogen evolution reaction with sulfide electrocatalysts

The most studied MS x for electrocatalytic HER is MoS 2 . In 2005, it was first revealed to be active for the electrocatalytic HER, one of the two half-reactions of water splitting:

2.2

The best catalyst for such a reaction is Pt. Because of scarcity and cost, MS x and MoS 2 in particular, are studied.

Three strategies exist to optimize the HER activity of a catalyst: increasing the number of active sites, increasing the intrinsic activity of each active site or enhancing the electrical contact between the electrode and the catalyst [5,6]. Those strategies can be addressed simultaneously to obtain higher activity. The third one is indirectly related to sulfide materials. It is therefore not discussed in this chapter. Few systems are reported in [7]. Seh et al. emphasized that the gain of activity is often higher when increasing intrinsic activity rather than the number of active sites [6].

A necessary but insufficient condition for having a high intrinsic activity is to have a hydrogen adsorption free energy (

) close to 0 [8]. This is due to the two HER mechanisms: the Volmer-Heyrowky mechanism and the Volmer-Tafel mechanism [9,10]:

1: Volmer step: 2.3 2a: Heyrovksi step:

2.4 2b: Tafel step: 2.5

Volmer step corresponds to the adsorption of the proton at the electrode surface and the Heyrovksi / Tafel steps correspond to their desorption. If the hydrogen atom binds too weakly to the surface, step 1 is the rate-determining step; if the hydrogen atom binds too strongly to the surface, step 2a or 2b is the rate-determining step. Such a mechanism gives rise to a volcano curve between the logarithm plot of the exchange current density and . Gerischer and Parson first theorized this plot, independently [11,12]. Later, Trasatti collected experimental data and constructed the first volcano curve [13].

62

To compare the activity of electrocatalysts prepared one with thiomolybdate decomposition followed by sulfidation under H 2 S. To increase the amount of edges, various strategies were reviewed in [5,6,9] for instance. It goes from nanostructuring

MoS 2 [16][17][18][19] to amorphous MoS x [20,21]. It gives lower Tafel slopes and less negative η 10 .

Another way to increase the amount of active sites is activating the basal plane for HER. Li et al.

succeeded introducing sulfur vacancies and strain [22]. Density functional theory (DFT) calculations evidenced that such a strategy shift the close to the optimal value. Linear sweep voltammetry (LSV) curves evidenced improved performances (See Table II

Variability of the electronic structure of transition metal sulfides

The electronic structure of a semiconductor (i.e. position of the valence and the conduction bands) sets the oxidation potential of photogenerated holes and the reduction potential of photogenerated electrons (more details are provided in Chapter III).

It is therefore crucial to know the exact position of and for a semiconductor (or for a metal) involved in a photocatalytic process. Size of indicates the photon energy that can be absorbed by the semiconductor.

Xu and Schoonen compiled the and the of 57 oxides and 56 metal sulfides [32]. It is mainly a calculation work but they also reviewed experimental data to compare their results. Notice that energy levels vary depending on the calculation method. This work has the merit to use the same methodology for calculating energy levels of 113 compounds so, relative comparison can be made. Figure II-3 also illustrates that , and vary depending on the sulfide phases. Only binary sulfides are depicted on this figure; Boldish and White evidenced that can be tuned between 0.4 and 2.4 eV preparing binary, ternary or quaternary sulfides [33]. In addition, a phenomenon called quantum size confinement varies the size of depending on the particle size [34]. The smaller the particles are, the larger the is. It emphases that various MS x supported on TiO 2 provide a whole range of electronic structures. It allows studying its influence on the PHER activity.

Those paragraphs reports the of the main sulfides phases used in this manuscript. They report experimental and calculation works that complete data on Figure II-3 [32]. They also give an idea in which range, the band gap of those sulfides can vary depending on particle size.

The band gap of bulk α-HgS is reported close to 2.0 eV [35]. Preparation of HgS films at various temperatures evidenced that the increases from 1.9 to 2.4 eV when the particle size decreases from 8 to 4 nm [36]. Wang et al. reported a band of 2.5 eV for 8 nm β-HgS particles [37]. It is unclear if for particles of 8 nm the difference is due to different phases (α-HgS and β-HgS) or experimental errors.

In average, the of Ag 2 S particles is close to 1.0 eV. Boldish et al. reported a gap of 0.9 eV (without indicating particle size) [33]; it agrees with Figure II-3. Several studies conclude that between 4.7 and 10.7 nm, the varies between 1.2 and 1.0 eV [38][39][40][41] [50]. They reported CoS 2 as a ferromagnetic metal; it agrees with previous calculations [51]. CoS also has a metallic behavior [46] while intermediate structures such as Co 9 S 8 were reported as semiconductors. Few works reported a of 0.9-1.0 eV for Co 9 S 8 nanoparticles [52][53][54]. It must be noticed that those determinations were made by diffuse reflectance spectroscopy. They extrapolated the band gap because the apparatus cannot acquire spectra below 1.5-1.0 eV. It seems to lack exactness.

RuS 2 is reported as a compound with a direct band gap. Fiecheter and co-workers reported RuS 2 films prepared with reactive magnetron sputtering. Depending on the excitation sources (direct-current or radio-frequencey) and on the preparation temperatures they reported from 2.1 eV to less than 1.0 eV (experimental limit) [55,56]. For (100)-and (111)-oriented films they reported a size of 1.9 eV [57]. Huang and Chen also reported an indirect band gap of 1.4 eV for RuS 2 single crystal (111) [58] and Vaterlaus et al. reported an indirect band gap of 1.2 eV [59].

Regarding, the amount of sulfide phase available and using quantum size effect, it gives access to a whole range of band gaps. Except the influence of the quantum size effect, the theoretical work of

Sulfides as co-catalysts for photocatalytic production of H 2

Transition metal sulfides are good candidates for photocatalytic applications. They activate H 2 , they are active for the electrocatalytic H 2 production and they have tunable electronic structure. This section summarizes the use of sulfides as co-catalyst for the PHER.

It must be noticed that comparison between photocatalysts or co-catalysts studied in different laboratories is difficult. Conditions of the experiments varied: preparation methods, reactor geometry, light source, oxidized molecule, support and so on. Chapter III provides more details. That is why screenings of photocatalysts in similar conditions are crucial. However, in addition to our recent publication [60], only two studies screened sulfides as co-catalysts: six for Shen et al. [61] and seven for Tabata et al. [62]. [25]. Two MoS 2 /TiO 2 were prepared, one with the thioacetamide reflux method detailed in Section 5.1 and the other with a common impregnation followed by sulfidation with H 2 S. The first catalyst is more efficient than the second one and an enhancement factor of 20 is reported (See entry 1).

Screening of various MS x as co-catalysts for the PHER

Improving strategies include preparation of amorphous MoS x sulfides for increasing the amount of unsaturated sulfur. They can be obtained by heating (NH 4 ) 2 MoS 4 with the support [68,69] or photoreducing this precursor on the support [70]. In Table II-2, entries 2 to 4 show that independently of the support or of the reactive mixture, amorphous MoS x are always more active than the crystalline one. Notice also that the preparation decomposing (NH 4 ) 2 MoS 4 is more efficient than the photoreducing one. The enhancement factor is higher as well as the AQY (See entries 2 and 4).

Graphene can be used to enhance the contact, and therefore the charge transfers, between the cocatalyst and the support [71,72] However, MoS 2 also acts as a co-catalyst of ZnIn 2 S 4 [81], g-C 3 N 4 [82], black phosphorous [83] CdS [84,85] and Mn x Cd 1-x S [86] Among the various strategies reported in Table II-2, the one with the highest enhancement factor is, in average, the preparation of amorphous MoS x .

Alternative transition metal sulfides for the PHER

Other transition metal sulfides were reported for the PHER. Among them, seven (including MoS x ) are studied in this work. Some selected publications are discussed below and catalytic activities are reported in Table II

-2.
Because WS 2 has a similar structure than MoS 2 , it is also highly studied; [87] reviewed some systems.

CoS x was reported as a co-catalyst with an enhancement factor, 39 for TiO 2 (See entry 12) [88].

With sulfide-based support, this factor is much lower: 3.6 for CdS and 2.4 for Mn 0.5 Cd O.5 S (See entry 11) [89]. Despite what they claimed, their catalyst suffered from deactivation. It lost 20 % of its activity after 5 cycles. On the contrary, Wang et al. constructed stable (after 5 cycles) Co 9 S 8 @ZnIn 2 S 4 cages [53]. They evidenced a favorable electronic transfer, using diffuse reflectance spectroscopy and Mott-Schotky analysis to determine the electronic structure. They reported a synergetic factor of 3 (i.e. r H2 (Co 9 S 8 @ZnIn 2 S 4 ) = 3 [r H2 (Co 9 S 8 ) + r H2 (ZnIn 2 S 4 )]).

NiS/TiO 2 catalyst, reported in entries 13, 14 and 15, were all reported for the photocatalytic conversion of methanol. Among those three works, Huang et al. reported the highest co-catalytic effect [90]. The activity of the TiO 2 nanotubes is enhanced by a multiplying factor of 79. It was prepared using sodium hydroxide, nickel nitrate, thiourea and glycol. Except the MoS x -based photocatalyst, NiS/TiO 2 has the highest enhancement factor.

NiS x co-catalysts were also reported on other supports such as g-C 3 N 4 [91,92], HNb 3 O 8 [93],

CuGa 3 S 5 [62] or even, metal-organic framework [94]. Xu and Xu reviewed other catalysts [95]. One of the highest multiplying factor ( 114) is reported by Kadi et al. for a g-C 3 N 4 support [92]. They even reported a synergetic factor of 34.

NiS co-catalyst is the most reported nickel sulfide phases [95]. However Zhang et al. reported a better activity with NiS 2 (See entry 17). NiS 2 was even reported as an unsupported photocatalyst [96].

Tuning its morphology (nanospheres, nanocubes, nanoparticles), they reported various activity with a AQY at 420 nm of 6.8 % for the nanopsheres (See entry 16). Ni 3 S 2 shows also an activity [97].

HgS-based photocatalyst was already reported for depollution applications [98]. In this work, it is the first time that such a material is studied for the PHER. Therefore, [60] is the first report from the literature in which HgS/TiO 2 produces hydrogen.

RuS 2 -based materials were often reported for photoelectrocatalytic applications [57,99,100] but rarely for the PHER. Hara et al. reported PHER activity for RuS 2 /TiO 2 and surprisingly for RuS 2 /SiO 2 [101,102]. SiO 2 only dispersed the RuS 2 photocatalyst producing H 2 . For similar purposes, RuS 2 particles were immobilized on thiol-modified polystyrene particles [103]. They produced H 2 from a mix propan-2-ol and Na 2 S/Na 2 SO 3 .

Hara et al. [102] but also Nguyen et al. [104] reported the photocatalytic "water decomposition"

with RuS 2 /TiO 2 (-SiO 2 ) heterostructures. The first performed the experiment using Na 2 S/Na 2 SO 3 as a sacrificial agent; the second did not provide enough details on the experimental conditions. They both reported a co-catalytic effect of RuS 2 (See entries 18 and 19) but they disagreed on its origin. With RuS 2 /TiO 2 (-SiO 2 ). Reproduced from [102] and [104], respectively, with permissions. Ag 2 S can be also used in more complex structures. Li et al. reported a ZnS-In 2 S 3 -Ag 2 S solid solution. The Ag 2 S content tunes the band gap between from 2.88 to 2.58 eV. They reported a AQY of 9.9 % for the photoconversion of Na 2 S-Na 2 SO 3 /water. It is also reported as an interfacial layer between silver nanowires and multimetallic sulfide nanoparticles such as MnZnGaSnS [109]. Its true role is unclear but it is reported as an adhesive between the two active sites.

Sulfides are involved as catalyst but also as sensitizer, this is the case for CuS. It was reported as a sensitizer for TiO 2 by Manjunath et al. CuS has a 2.0 eV band gap, absorbing visible light [110].

It was prepared with hydrothermal synthesis leading to CuS-TiO 2 rather than co-catalytic particles dispersed on TiO 2 . A synergetic factor of 1.7 is calculated. More surprisingly, at least for TiO 2 -based material, they reported production of H 2 with pure water. According to the order of magnitude (r H2 = 2.0 μmol/h), it can be due to remaining sacrificial agent. In two years, Wang et al. studied CuS and NiS-CuS supported on P25 (See entries 25 and 26). They reported promising co-catalytic effects with activities 32 and 28 times higher than bare P25, respectively. The interest of adding NiS is arguable because it increases the catalyst cost without increasing its activity.

Less common sulfides were also reported for photocatalytic or photoelectrocatalytic purpose, such as VS 2 [111], FeS 2 [112,113] or Bi 2 S 3 [114,115].

Among all the systems reported in

PY (thioacet. reflux) = 7 % (λ < 400 nm) [25] a-MoS x TiO 2 -P25 Methanol r H2 (a-MoS x /TiO 2 ) = 8 r H2 (c- MoS 2 /TiO 2 ) = 864 r H2 (TiO 2 ) AQY (amorph.) = 7.45 % (λ = 367 ± 20 nm) b [68] a-MoS x g-C 3 N 4 Lactic acid r H2 (a-MoS x /g-C 3 N 4 ) = 5.5 r H2 (c-MoS x /g-C 3 N 4 ) = 90 r H2 (g- C 3 N 4 )
Unk.

[69] a-MoS x

AQY = 3.2 % (λ = 360 nm) b [116] CoMoS 2 rGO/g- C 3 N 4 TEOA r H2 (CoMoS2/rGO/g-C3N4) = 38 r H2 (rGO/g-C3N4) Unk. [75] Cu 2 MoS 4 CdS Lactic acid r H2 (CuMoS 4 /CdS) = 4.2 r H2 (CdS) Unk. [76] MoS 2 (1T- & 2H-) TiO 2 Na 2 SO 3 / Na 2 S r H2 (MoS 2 /TiO 2 ) = 20 r H2 (TiO 2 )
Unk.

[79]

1910 μmol/h AQY = 0.55 % b [110] CuS TiO 2 (P25) methanol r H2 (CuS/P25) = 32 r H2 (P25) Unk. [64] NiS & CuS TiO 2 (P25) methanol r H2 (CuS-NiS/P25) = 28 r H2 (P25) Unk. [120] CuS CdS lactic acid r H2 (CuS/CdS) = 18.7 r H2 (CdS) AQY = 9.85 % (λ = 475 nm) b [121] VS 2 g-C 3 N 4 TEOA r H2 (VS 2 /g-C 3 N 4 ) = 26.5 r H2 (CdS) AQY = 2.25 % (λ = 420 nm) b [111]

Additional comments

Regarding the study of sulfides based photocatalysts, few additional comments can be made. For their study, the Na 2 S/Na 2 SO 3 redox-couple diluted in water is highly used. It serves for stability but also to report "true" water splitting. Indeed, while with alcohol the origin of the H 2 is ambiguous (alcohols can be oxidized and provide the protons becoming hydrogen), at first glance, nor Na 2 S or Na 2 SO 3 can generate protons. But the following process can occur [122]:

2.6
To claim "true" water-splitting, labeling experiments with D 2 O must be performed [123,124].

In addition, studies often report comparison between sulfides and noble metal co-catalyst. They mostly used supported Pt as a benchmark catalyst. However, it is well established that Pt is poisoned by sulfur. Comparison between sulfides-based catalyst with Pt-based catalyst in Na 2 S or Na 2 SO 3 might be unfair. To attest the superiority of a catalyst, it should also be compared for the photoconversion of another reactive mixture.

Finally, almost all the studied study the influence of the co-catalyst loading. However, noble metal-based systems are also sensitive to such phenomenon. Noble metal and sulfides should be compared at the same atomic ratio but also at their co-catalyst optimum.

A rigorous comparison is the work of Yuan et al. [116]. They compared the activity of MoS 2 /TiO 2 and M/TiO 2 (M=Pt, Pd, Rh, Au and Ru) for the photoconversion of methanol-water mixture. They reported MoS 2 /TiO 2 1.6 times more active than Pt/TiO 2 (with both 0.5 wt% of cocatalyst). They also optimized their co-catalyst loading; Pt/TiO 2 has an optimum at 1.5 wt%. It shows a similar activity than MoS 2 /TiO 2 but with much more co-catalyst.

Conclusions and objectives

Sulfides are materials with highly tunable properties. They were highly studied for hydrotreatment and reactions requiring H 2 activation. For such properties, studies were made in the past decades on electrocatalytic and photocatalytic HER.

Many sulfide phases have already shown a co-catalytic effect for the PHER. However, comparison of PHER efficiency measured in different laboratories is always difficult because of the experimental conditions that vary. The few systematic comparison of PHER performance for a series of sulfides are phenomenological. They did not provide clear structure-activity relationships. To this day, reason of the co-catalytic effect of sulfides remains unclear. In the next chapter we proposed such correlations.

We choose to establish such a correlation using the photocatalytic dehydrogenation of alcohols.

Main Scheme 1 depicts its mechanism. This work aims to study different facets of the mechanism by limiting the amount of variable parameters.

The oxidation half-reaction ( -Main Scheme 1) is studied in Chapter IV. Among the several alcohols that can be photocatalytically converted, we choose to focus this work on the photocatalytic dehydrogenation of propan-2-ol. Its mechanism is fully established and a micro-kinetic model is proposed. Such a reaction was used as a model reaction to screen seven MS x supported on the same TiO 2 (MS x /TiO 2 ). Chapter V reports the results. TiO 2 surface contains the main active sites for the oxidation half-reaction. Therefore, if TiO 2 is unchanged and if the same molecule is oxidized for the seven MS x /TiO 2 , variation of activities must be correlated to the intrinsic properties of the MS x .

Their ability to activate H 2 (governing -Main Scheme 1) was estimated using thiophene hydrodesulfurization and electrocatalytic hydrogen production. The latter also estimates the activity of each MS x to reducte protons (governing -Main Scheme 1). Those intrinsic properties are correlated with the PHER activity of the seven MS x /TiO 2 .

81

Chapter VI provides additional mechanistic insights It reports the influence of five parameters on the PHER activity: the incident flux of photons, the mass of photocatalyst, the MS x loading, and preliminary results on the influence of the size of the MS x particles and on the TiO 2 support.

Finally, Chapter VII is mainly dedicated to the electronic structure of the photocatalysts. The latter govern ( -Main Scheme 1). Correlations between the electronic structure of the MS x /TiO 2 and their PHER activity are formulated.

Main Scheme 1: Mechanism for the photocatalytic dehydrogenation of alcohols with MS x /TiO 2 .

Chapter III:

From theoretical to experimental photocatalysis

This chapter reports the main concepts of the photocatalytic mechanism. It explains the methodology used in this work to perform photocatalytic experiments with accuracy and to limit the amount of variable in the mechanism. It also reviews the different methods used to synthesize and characterize the photocatalysts.

Mechanism of the photocatalytic conversion of alcohols

Photocatalysis is a field of heterogeneous catalysis. It therefore involves molecules adsorption / desorption, chemical reactions but also interaction of the catalyst with light and charge carriers. It results in a complex mechanism. Section 3.1 reviews the different steps of a photocatalytic mechanism; it involves many parameters. They are not all involved at the same timescale or on the same site. Therefore, understanding which parameters are crucial and how to optimize them is the key to do photocatalysis by design.

Main Scheme 2 represents a non-exhaustive list of those parameters. Takanabe reviewed those parameters in [1] [2]. In addition to those reviews, the following textbooks gives an overview on photocatalysis and on its mechanism [3][4][5].

There are several phenomena to distinguish. On one side, a semiconductor absorbs photons and converts them in charge carriers. On the other side, chemical reactions occur at the surface of the photocatalysts.

Main Scheme 2: Scheme of the photocatalytic dehydrogenation of alcohols with a co-catalyst (A) supported on a semiconductor (SC).

Mechanism: Excitons generation

The wavelength and the incident flux of photons sent into the reactor vary from one lamp to another. also depends on the oldness of the lamp. Those photons reach the optical window; its size depends on the geometry of the reactor. Glassware and the solution inside the reactor filter a part of the light. The incident flux of photons reaching the photocatalyst is written as follow:

3.1

Then, the photocatalyst receives those photons and convert them in excitons (electron-hole pairs).

3.2

Note that, the exponent " V " corresponds to the total volume whereas the exponents " S " and " B " are for surface and bulk. As an example: and .

The size of the semiconductor band gap commands which wavelength can be absorbed (equ. 3.3) [3] and the Beer-Lambert law imposes the amount of photons absorbed (equ. 3.4):

3.3 3.4
is the mass of photocatalyst, is the height of the irradiated reactive mixture and is the surface of the optical window.

(cm -1 ) and (cm 2 g -1 ) are the absorption coefficients of the semiconductor. They depend on the semiconductor composition, on the amount of doping (extrinsic/intrinsic) and on the amount of defects. Section 6.2 demonstrates the second part of the equ. 3.4.

Some of the transition metal sulfides studied in the project are also semiconductors that can absorb photons. As a result, we assume:

3.5

95

SC subscript stands for the semiconducting support and A subscript stands for the co-catalyst. It means that the mass of photocatalyst ( ), the surface area of the support ( ) and the co-catalyst loading ( ) influence the absorption step.

As a result, the on Main scheme 2 is defined as follow:

3.6 3.7

Section 5.3.1 studies the influence of and on the overall PHER activity.

Mechanism: Charge separation, transport and/or recombination

After photons absorption, the amount of excitons is available. The charge carriers may recombine in a phonon ( ); light energy is therefore lost as thermal energy. Or, charge carrieres may be separated:

3.8

3.9

Charge separation occurs if thermal energy (25 meV at room temperature) is available to overcome the exciton binding energy. Bohr theory expressed the binding energy of a Mott-Wannier exciton ( ) as follow [1]:

3.10
is the dielectric constant of the semiconductor, is the mass of the electron and is the reduced effective mass deduced from the one of the electrons ( ) and of the holes ( ): .

The electronic structure imposes those parameters; shape of the valence band and of the conduction band commands the and the . The higher the the lower the is and therefore the easier is the charge separation. For instance, equals 100-200 for TiO 2 whereas it equals 5-10 for CdS and ZnO [6,7]. It results in a close to 1 meV for TiO 2 .

Only charge carriers at the surface of the particles are involved, so they must travel up to the surface of the particles:

3.11

Depending on the A/SC electronic structure, charge carriers are located on the surface of the support or on the co-catalyst. Their transport has two driving forces: it can be a diffusion phenomenon due to a concentration gradient and/or it can be a drift phenomenon due to a potential gradient.

The diffusion coefficient ( ) and the carrier mobility ( ) command the diffusion phenomena. The effective mass and so the shape of the electronic structure influences :

3.12

The mobility is considered to be a better intrinsic property of a material than its conductivity. Latter depends on the temperature, the purity and the synthesis conditions.

Drift phenomenon is influenced by the potential gradients at the various interfaces: SC bulk -SC surf. , SC surf. -A, SC surf. -reactive mixture, A surf. -reactive mixture During transport process, free charge carriers can again recombine with their counterpart. Those recombination phenomena impose a carrier lifetime for the holes and the electrons. This lifetime gives the carrier diffusion length. It is the average distance above which a charge carrier recombines with a probability of 100 %. Charge carrier must find an active site before reaching this limit. Dopant concentration (wanted or defects) influence the carrier lifetime, as well as the cristallinity or the nanostructured aspect of the surface.

The electron mobility in a TiO 2 equals 1-10 cm2.V-1.s-1, the one of the holes is ten-hundread times higher [6,7]. It results in a lifetime of 10 -11 s and 10 -13 s for electrons and holes, respectively.

The presence of a co-catalyst limits the charge carrier recombination and improves their transport. It bends the band diagram of the photons absorber and modifies and . The band alignment and the potential gradient at the SC/A interface drives the charge carrier away from their generation site. This spatial separation limits the electron-hole recombination [1].

To sum up, on Main Scheme 2, , and depend on the electronic structure and the morphology of the photocatalysts. They depend on the nature of the absorber and of the co-catalyst and on their interface. In Chapter V and Section 6.4.2, either the sulfides or the support are screened. It modifies the electronic structure of the all architecture which is discussed in Chpater 7.

Mechanism: Chemical reactions

Photocatalytic mechanism ends with the successful consumption of the surface electrons and holes for the reduction and the oxidation reactions. Photocatalysis is a field of heterogeneous catalysis. So, the amount of active sites plays a key role on the PHER activity. Many studies reported the same trends for the PHER rate as a function of the co-catalyst loading [8][9][10][11]. Such phenomenon will be addressed in Section 6.3.

To perform photocatalysis, the charge carriers must transfer from the photocatalyst to the molecule. So the quasi-Fermi level of the electrons ( ) must be more negative (versus normal hydrogen electrode (NHE)) than the reduction potential ( ) of the molecule and the quasi-Fermi level of the holes ( ) must be more positive (vs NHE) than the oxidation potential ( ) of the molecule [12]. The difference between the two quasi-Fermi levels defines the chemical potential of electron-hole pairs: . A photon, promote an electron from the VB to the CB so it transfers its quantities (energy, momentum, angular momentum) to the electron-hole pairs. Those are then transferred either to phonons through recombination or to free charge carriers [3,13].

98

The quasi-Fermi levels are linked to the valence band minimum ( ) and to the conduction band maximum ( ):

3.13 3.14 is the Boltzman constant, and are the effective density of states of the conduction and of the valence band, respectively, and are the concentration of electron and hole, respectively.

The order of magnitude of and induce a ratio between 0.1 and 100 and so and [2].

In this work, the reduction potential is always the proton reduction one: . varies with the nature of the alcohol.

(vs NHE) can be calculated as follow:

3.15
is the standard Gibbs free energy of the reaction, is the number of holes involved and is the Faraday constant (96,485 C.mol -1 ).

values are summarized in Table III-1. Overall, it is easier to oxidize an alcohol than water. This is the reason why studies shift from water-splitting to photoconversion of alcohol (See Section 1.2.4). Among the alcohol, it is important to notice that the depends on the type of reaction. For the reforming of alcohols, the are ranked: methanol < ethanol < isopropanol. For the dehydrogenation of alcohols, the are ranked: isopropanol < ethanol < methanol. It shows how important it is to establish a clear mechanism. As a result, and on Main Scheme 2 depend on and on . Therefore (equ. 3.13 and 3.14) they depend on and .

It is also important to keep in mind that both oxidation and reduction should be considered. If

, electrons accumulate on the co-catalyst and if , holes accumulate on the support. In both cases, they might modify the photocatalyst. Propan-2-ol 64 0.33 [14] 183 0.11 [14,16] It must be kept in mind that photocatalysis cannot be simplified as a two steps process with on one side charge carrier generation and their use in electrocatalysis on the other side. However, an insight on the electrocatalytic activity of a co-catalyst may help to understand its PHER activity.

The Tafel equation estimates the electrocatalytic activity of a given material. It requires some hypotheses, for instance, it neglects the reverse reaction keeping only the reduction current of the Butler-Volmer equation [4]. The equation reads:

3.16
is the exchange current, is the transfer coefficient, is the number of electrons involved in the reaction and is the overpotenial. .

with the density of current is called a Tafel plot. The lower the slope of this graph is, the more the catalyst is efficient [17]. A second criterion is the voltage required to reach a current density of -10 mA/cm2 (η 10 ). Those criteria allow discussing the electrocatalytic activity of our sulfides in Section 5.5.2. 100

Photocatalytic test and appropriate methodology

As shown in Chapter II, photocatalysts are mostly compared based on their PHER rate despite the fact that this rate depends on plenty of parameters. It is influence for instance by the incident flux of photons, the mass of photocatalyst or the molecule to oxidize. That is why, it is so difficult to compare two photocatalysts from two different works. Few papers reported the good practice in photocatalysis [18][19][20][21][22]. The next sections describe the methodologies used to provide PHER results that are reproducible. To report the efficiency of a given material, the amount of photocatalyst must not alter its performance. Some papers choose to normalize their PHER rate with the photocatalyst mass, so-called specific PHER rate. Herrmann showed that the PHER rate linearly increases with the mass until to reach a plateau [19]. This behavior is modeled with equ. 3.4 (latte in the manuscript re-written as 6.5), depends on the semiconductor and on the size of the optical window of the reactor.

Photocatalytic Set-Up

Mass normalization in the linear region do not induce an error but it does in the plateau region.

The specific PHER rate is underestimated [20,21]. For being accurate, a curve must be reported in works on photocatalysis. It must be done each time the support change;

according to (1. 4), depends on its absorption coefficient and its specific surface.

curve allows choosing the optimal mass of photocatalyst in between the linear and the plateau region. At this mass, no normalization is needed and the PHER rate is optimal.

101

Size of the optical window also influence the . So to reproduce the results from the literature, schemes of the reactor and its dimension must be provided. In this work, the geometry of the reactor is always the same and its circular optical window is of 20 cm 2 .

The reactive mixture must be also chosen wisely. Section 1. Before irradiation, the system is purged with an Ar flow (100 mL/min) for 30 min to conduct the test under anaerobic conditions. The Ar flow is then fixed to 20mL/min during the photocatalytic test.

Gases are analyzed by micro-gas chromatography (μ-GC) with thermal conductivity detectors (Agilent Technologies, 300 A). Four columns, described in Table III- The μ-GC quantifies gases thanks to prior calibration of the apparatus. For most of the gases analyzed by the columns A, B and C, it was made with standard gas mixtures. For acetone and isopropanol a particular methodology was used to obtain results for Chapter IV (See also Annexes, Figure S8 for additional details). It gives a response factor (i.e. ratio between surface of the peak and molar ratio) of 9.4 10 6 for the acetone, and 9.9 10 5 for the isopropanol. Factor responses of the other gases vary among the thesis because of the oldness of the columns.

The μ-GC provides the hydrogen concentration ( ) which leads to the PHER rate: .

The photoreactor is a semi batch reactor, the mass balance for the hydrogen can be written as in equ.

3.17 with the output molar flow rate and the accumulation term.

With a proper stirring, no hydrogen accumulates. In addtion, between 10 °C and 45 °C, the mole fraction solubility is between 1.576 10 -5 and 1.317 10 -5 [23]; the amount of hydrogen dissolved in water is negligible. The accumulation term is supposed close to zero. Therefore, the PHER rate can be calculated using equ. 3.18.

It takes around 30 min to the system to reach a steady state. An average value of among 1 h is used to calculate from equ. 3.18. Except in Chapter 4, all the are calculated at the steady state. Commonly, is between 10 and 20 mL/min and is calculated assuming the gas perfect law verified.

3.17

3.18

Irradiance measurement & Photon Yield calculation

Herrmann reported in [19] a linear dependence between the PHER rate and the flux of photons ( ). Above a threshold, he reported another relationship:

. It means that the (see equ. 1.8) should be known with accuracy and must be provided in the experimental method. This information is occasional in the literature. Providing the power of the lamp is not enough; its intensity varies depending on its oldness, on the light bulb and on the brand. In addition the varies depending on the size of the optical window. was measured each month and the flux of photons will be given along the manuscript.

104

LED and HPK lamps were mainly used to measure the overall efficiency of a given catalyst.

With the LED lamp, the intensity can be tuned. This feature is used in Section 6. More details on the lamps are provided in the Annexes (See Figure S2 andS3). The PHER rate divided by the flux of photons gives the efficiency or the yield of the reaction.

The IUPAC Recommandations of 2011 [24] defined six measures: Quantum Efficiency (QE), Quantum Yield (QY), Apparent Quantum Efficiency (AQE), Apparent Quantum Yield (AQY), Photonic Efficiency (PE) and Photonic Yield (PY).

Others are discussed in [21]; for instance, the solar to hydrogen efficiency (STH) is useful for practical application. It is defined as the chemical energy of hydrogen produced divided by the solar energy input from incident sunlight.

Regarding the six common measures, efficiency is for polychromatic light source whereas yield is for monochromatic light source. Then, a yield is calculated following three ways: advised to calculate the AQY (equ. 3.20), taking the total amount of photons send on the photoreactor [26].

However, the AQY can be increased, only removing, with light filters, photons that are not used in the photocatalytic process. For instance, it has no sense to report AQY with a HPK or a Xe lamp. It has the merit of being transposable from one study to another but it is not representative of the efficiency of a photocatalyst.

For those reasons the PY is mostly used for this work. It is calculated as the PHER rate over a photon flux for a given wavelength range. So wavelength can be chosen depending on the band gap of the absorber to take into account that they are not excited by the same photons. It allows comparing the efficiency of co-catalyst deposited on different semiconductor. PY is in this case the amount of H 2 produced over the amount of charge carriers received by the co-catalyst.

The experiments are made in a quartz reactor, the glassware and the water cut the light below 290 nm. To take this phenomenon into account, the reactor was put between the lamp and the fiber optic probe during irradiance measurement. Most of the studies were made with pure anatase; red line on Figure III-2 depicts its absorption spectra (approximated to its Kubelka-Munk function). The 106 anatase absorbs photons with a wavelength below 390 nm. According to equ. 3.3 (Section 3.1.1), the flux of absorbable photons (

) is: The red filled part on Figure III-2 evidences the photons that can be absorbed. Notice that for the LED, which is monochromatic there is not much difference between the PY and the AQY. To follow IUPAC nomenclature [24], PY and AQY should be called photonic efficiency and apparent quantum efficiency when using the HPK or Xe lamp. Efficiency is for polychromatic lamp and Yield is for monochromatic lamp. For sake of simplicity, no distinction will be made in the manuscript and only Yield terms are used herein.

In addition, curve must be also reported in the literature. This correlation will be discussed in details in Section 6.1.1. The linear dependence between and is valid only below a threshold. So above, it might be irrelevant to normalize by .

Finally, a careful measurement of the photon flux must be done. An infamous publication is the one of Yan et al. [27] in which they claim a QE for hydrogen production of 93 %. This methodology provides two additional parameters to have an insight in the photocatalytic mechanism.

Experimental error

The PY is the ratio between the PHER rate and the photon flux. The experimental error on PY is 15 %, it is the sum of the error of the two components. The main error on is the 5 % error for the μ-GC analysis; the flow rate is well-regulated. Several irradiance measurements were made at different positions in the photoreactor and along 2 h. It estimates the error on the flux of photon to 10 %.

From the hundred of photocatalysts studied along the three years of PhD, the absolute error on was estimated to 1.0 kJ/mol. The relative error about was calculated thanks to the following equ.

Synthesis and characterization of the photocatalysts

The catalysts were prepared with a thioacetamide reflux method reported in [28]. More details will be given in Section 5.1. The different analytic techniques, to characterize the catalysts, are detailed below.

XRD measurement

X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance A25 diffractometer equipped with a Ni filter (with Cu-Kα, radiation 0.154184 nm) and a 1-dimensional multistrip detector (Lynxeye, 192 channels on 2.95°). Scherrer equation calculates the size of the crystallites:

3.25

3.26

Å for the Cu Kα1 radiation and is the Bragg angle; and are the line broadening at half maximum intensity, measured and the one intrinsic to the apparatus. Latter is taken equal to 0.06 for the apparatus of the institute. Diffractogram were mainly analyzed with Diffrac.Eva software using standard JCPDS files for phase identification.

Transmission electron microscopy

Transmission electron microscopy (TEM) analysis was carried out on a JEOL 2010 LaB6 device with an accelerating voltage 200 keV. Its point to point resolution is 0.19 nm. The apparatus contains an Energy-dispersive X-ray spectroscopy (EDX) analyser (Link ISIS -Oxford Instrument). The samples were dispersed in pure ethanol by ultrasound, and then put onto a holey carbon on a copper grid. To study copper-based sample, titania grids (on a holey carbon) were used. To protect them from air oxidation, samples were immediately introduced into the TEM vacuum chamber. The analysis of images was carried out using Digital Micrograph Gatan™ software. 110

Elemental analysis: CHNS and ICP-OES

The metal content in the synthesized solids was determined, after dissolution in an acid mixture, by inductively coupled plasma optical emission spectrometry (ICP-OES Activa Jobin Yvon). The acids employed varied depending on the metal to quantify.

For Co, Mo, Ag, Ni and Cubased materials the following procedure was used: dissolution in H 2 SO 4 , HNO 3 and HF followed by an evaporation and a dilution in HNO 3 . For Ru-based materials a dissolution in H 2 SO 4 , HNO 3 and HF under 150 °C during one night was employed.

To quantify Hg three procedures were compared. The one providing repeatable results is the dissolution of Hgbased materials in H 2 SO 4 , HNO 3 , HF and KMnO 4 under 150 °C during one night.

Elemental analysis of light elements (CHNS) was performed on an analyzer Thermo Fisher Flash 2000. Around 1 mg of sample mix with vanadium oxide was placed in a tin basket. Vanadium oxide shift the equilibrium to the formation of SO 3 -and improve the sulfur quantification. The basket is then burnt and a gas chromatograph column separates the elements. A thermal conductivity detector quantifies them.

Temperature Programmed Reduction

Temperature-programmed reduction (TPR) consists in detecting gazes with mass spectrometer upon linear heating of the sample. [29,30] It was carried out in a quartz reactor from room temperature to 1050 °C at a rate of 5 °C.min -1 . The samples (50-100 mg) were heated under H 2 flow (50 ml min -1 ). During the process, reduction of the MS x occurs, following the reaction: At high temperature, H 2 reduces the core sulfurs.

Because of that it is possible to have an estimation of the amount (i.e. area under the peak) of surface sulfurs (S S ), core sulfurs (S C ) and total sulfurs (S T = S S + S C ). The ratio S S /S T permits to calculate the size of transition metal sulfides particles: thanks to a geometrical model based on the crystallographic phase it is possible to link the ratio S S /S T with the number of unit cell. Geantet and coworkers [31,32] used this method to estimate the size of bulk and supported RuS 2 particles. Results corresponded to size determined with high resolution TEM.

Afanasiev argued in [33] that low temperature reduction is kinetically controlled and high temperature reduction is thermodynamically controlled. For the first case, intensity of the peak is proportional to the H 2 flows and to the amount of surface exposed (consequently, to the mass of catalyst); for the second case, the intensity is constant. For the first case, position of peaks maxima is unchanged but for the second, higher masses or higher H 2 flows increase the length of the rising domain of the curve. As a consequence, position of TPR peaks must be interpreted carefully; the exact position of TPR "core reduction" peak is not relevant.

Photoelectron Spectroscopy: XPS and UPS

Photoelectron spectroscopy analyses were conducted in an integrated ultrahigh vacuum system:

an Axis Ultra DLD system (Kratos Analytical). The analysis chamber is equipped with a hemispherical electron energy analyzer and a micro-channel plate detector with high sensitivity for all electrons -irrespective of their kinetic energy. Spin-orbit splitting from the Handbook of XPS were used to fit the doublet. [34] A relativistic ratio of 0.5, 0.67 and 0.75 were applied for the 2p, 3d and 4f orbitals, respectively. The relative sensitivity factors (RSF) of the kratos library were used.

UPS spectra were obtained with the following method. To acquire wide spectrum (from 20 to -5 eV) the parameters are: a pass energy between 5 or 20 eV (to maximize the intensity), an aperture of 110 μm, a dwell time of 200 ms and a step of 25 meV. To obtain a zoom at lower energy (for states identification and determination) the step can be 15 or 5 meV. Number of scans varies to limit the signal-to-noise ratio. Sample preparation, photoelectron spectroscopy as well as data interpretation will be discussed in Chapter VII.

UV-Vis Absorption Spectroscopy

Two experimental set-ups were used to measure the UV-Vis diffuse reflectance. The first one provides many absorption spectra in a short period of time. The second one has a better resolution but it requires more time to collect a spectrum.

For the first set-up, six optical fibers illuminate the powder sample with a Deuterium-Halogen light source (AvaLight-DHS, Avantes). A seventh fiber collects the diffused light that is then analyzed by a CCD detector (AvaSpec-2018, Avantes).

The second set-up is a Perkin Elmer Lambda 45 spectrometer equipped with an integrating sphere (RSA-PE-20). The apparatus illuminates the sample from 315 nm to 1100 nm with a Tungsten halogen lamp and from 200 nm to 315 nm with a Deuterium lamp.

Thiophene hydrodesulfurization

Principle of thiophene HDS was already introduced in Section 2.1. Prior to catalytic tests, all samples are re-activated under standard conditions: treatment under a flow of 10 % H 2 S/H 2 at 350 °C for 2 h. The reaction is carried out in a fixed-bed flow microreactor filled with 100-200 mg of catalyst.

The following conditions are applied: the temperature varies from 280 °C to 340 °C, the gas flow is 50 mL/min and the partial pressure of thiophene is 2.7 kPa.

The plug-flow reactor model is used to calculate the specific rate ( ) as follow [35]:

3.29
is the thiophene molar flow (mol/s), is the mass of catalyst (g) and is the thiophene conversion.

It is measured at a steady state, after at least 16 h on-stream. is intentionally calculate at a conversion lower than 10 %.

Electrocatalytic hydrogen evolution reaction

First, an electrochemical ink is prepared. It contains 5 mg of powder to sudy, 800 μL of ethanol and 400 μL of a 0.5 wt%. Nafion solution. 1 mg of acetylene black carbon is added to increase the conductivity of the ink. Then, 10 μL of this ink is dropped on a glassy carbon electrode in rotation. It has a surface of 0.0706 cm 2 . For a perfect dropping, the electrode is loaded with 0.71 mg/cm 2 of catalyst.

Measurements were performed at ambient temperature on a Voltalab three-electrode potentiostat (PGZ100 all-in-one). Pt is the counter electrode and the reference is a saturated calomel electrode (SCE). The electrolyte is 0. S5). Photocatalytic activity depends on the reactive mixture and on the oxidation potential of the alcohol (See Annexes, Figure S6). This conclusion stands for RuS 2 /TiO 2 but also for other MS x /TiO 2 (See Annexes, Figure S7 and [2]).

Photonconversion of methanol gives the highest initial PHER rate but after 2 h some catalysts, such as RuS 2 /TiO 2 , deactivate. In addition, the photocatalytic conversion of methanol can be a onestep dehydrogenation but also a reforming pathway with several parallel or sequential reactions. On the contrary, this chapter shows that the photocatalytic conversion of propan-2-ol occurs via a dehydrogenation pathway. In red, it indicates the chemicals that are detected by the μ-GC.

Methane, propane and acetaldehyde were detected in the gas phase with Pt/TiO 2 . Acetaldehyde and propene were detected in the gas phase with RuS 2 /TiO 2 . Section 4.1.4 discusses their formation.

These by-products are produced in negligible quantities. For instance, with Pt/TiO 2 and the amount of acetaldehyde produced with RuS 2 /TiO 2 is close to the detection limit. In contrast to the works from the literature (See Section 1.2.4), no CO 2 was formed. In our conditions, no propan-2-ol reforming occurs.

The literature works mostly report the hydrogen evolution and rarely the consumption of alcohols and the formation of by-products. For slurry experiments, this is in part related to the analytical difficulties. In contrast to H 2 , the alcohols and their oxidation products are distributed between gas and liquid phases. To take into account such distribution and to quantify the products in both liquid and gas phases, we tried to calibrate acetone and propan-2-ol (See Annexes, Figure S8). To do so, known amounts of acetone / propan-2-ol were injected in the photoreactor filled with pure water. Then, a calibration curve was established thanks to Henry's law [3]. It correlates the molar fraction of acetone / propan-2-ol in the liquid phase and its partial pressure detected with the μ-GC. For a given flux of gas vector, the balance can be calculated between the total amount of acetone / propan-2-ol remaining in the liquid phase and the one in the gas flow. Such calibration assumes that gas/liquid equilibrium is reached at any instant due to a small Ar-flow.

Results

125

After that, the PHER rate becomes higher than the one of the acetone (See Table IV-1). It could be due to the accumulation of acetone in the liquid phase. As, PY (Pt/P25) = 13 PY (RuS 2 /PC500) it explains why the extended initial period lasts longer for RuS 2 /PC500.

It can also be due to the formation of side products, not observed with the μ-GC. One candidate could be the propanone diisopropyl acetal in equilibrium with the acetone [4] (See Annexes for extended discussion). Notice also that GC x GC chromatography was performed on the liquid phase.

After 120 h of test with RuS 2 /TiO 2 it evidences no additional molecules than remaining propan-2-ol and produced acetone. In summary, the photocatalytic conversion of a propan-2-ol-water mixture can be considered as a single-pathway reaction:

4.4

Dehydrogenation gives hydrogen and acetone in a 1:1 proportion at least during the first stages, when the modification of the liquid reactants composition could be neglected. Then, after 30 h (10 h with Pt/TiO 2 ) on-stream, the analysis became more complex. However it seems to happen due to chemical reaction engineering issues (mass balance in the presence of gas-liquid equilibrium), rather than to purely chemical reasons. In addition, it is confirmed that with the MS x /TiO 2 photocatalysts, the propan-2-ol is always in excess and diffusion phenomenon should not be limiting. 127

Influence of the concentration on the kinetic

Based on the results discussed above, the kinetic of the reaction is studied within the first 10 h.

During this period, the reaction is undoubtedly a dehydrogenation pathway. Figure IV-2 reports the amount of hydrogen and of acetone produced along the time for various initial concentrations of propan-2-ol with Pt/TiO 2 photocatalyst. No additional gas was observed than those discussed above.

At higher concentration the reaction seems to follow the same pathway: no additional products are detected.

The amount of hydrogen produced during the first 10 h evolved linearly. This is also the case for acetone (except for high concentration test, for which acetal formation or acetone accumulation in the liquid phase could be considered).

The initial rates of H 2 and acetone production were calculated as a function of the propan-2-ol concentration and they are reported on Figure IV-3.

128
photocatalysts but the first catalyst is almost 30 times more efficient than the second one. However, the experimental conditions differ; test was made at 40 °C for Pt/TiO 2 and at 20 °C for RuS 2 /TiO 2 , the two co-catalysts are supported on two commercial TiO 2 (P25 and PC500) and the incident flux of photons is almost two times higher for tests with Pt/TiO 2 .

RuS 2 and Pt co-catalysts were compared in similar experimental conditions, both supported on PC500. Pt/TiO 2 is 7.7 times more efficient than RuS 2 /TiO 2 . depends on the molecules that are adsorbed on the same sites [5]. So to express , the adsorption competition between propan-2-ol, acetone and hydrogen is first studied. For both photocatalysts, a Langmuir-Hinshelwood mechanism is evidenced. Table S3 summarized the fitting parameters. The results obtained with RuS 2 co-catalyst do not show a clear difference between the non-dissociative and the dissociative mechanisms. The fitting data obtained with the Pt co-catalyst argues for a dissociative mechanism. This was already evidenced in previous works [6,7]. It concludes that the initial rate of hydrogen production is written:

4.8
This relationship is valid for " " corresponding to Pt or RuS 2 co-catalyst and it can be extrapolate to any co-catalyst with similar behavior. For instance, the other MS x /TiO 2 studied in this work.

This section concludes that the photocatalytic dehydrogenation of propan-2-ol follows a

Langmuir-Hinshelwood kinetics. It also evidences that the mechanism is dissociative. During the adsorption step, the amphoteric OH surfaces dissociate the propan-2-ol as follow:

4.9

Then, the photogenerated holes oxidize the alkoxide and the photogenerated electrons reduce the protons leading to the targeted H 2 .

Influence of the temperature on the kinetic

To complete this kinetic study it is also crucial to understand how the temperature influences the However, the rate-determining step may also be another phenomenon with a low-energy barrier.

It can be the charge carrier separation or the electronic transfer from TiO 2 to the co-catalyst ( on Main Scheme 3). It can also be one of the chemical steps, the proton reduction ( on See Main

Scheme 3) or the hydrogens coupling ( on Main Scheme 3) but in those cases, the must be higher. The order of magnitude is however not sufficient as an argument to conclude.

To discriminate the rate-determining step and the key properties of a photocatalyst, a whole range of properties influencing the rate constants (See Main Scheme 3) are studied in the next chapters. It is made thanks to a screening of various MS x or various experimental conditions. Photocatalytic reforming of ethanol Pt x /TiO 2 17.5 [12] Photocatalytic reforming of tert-butanol Pt x /TiO 2 5 [12] Photocatalytic reforming of glycerol Ti@TiO 2 27 [10] Various values were measured for the photocatalytic conversion of three alcohols with RuS 2 /TiO 2 (See Figure S5). To identify the key properties of the MS x /TiO 2 photocatalysts, their activity should not depend on the reactive mixture. Therefore, the next chapters study photocatalysts for the same reaction: the dehydrogenation of propan-2-ol.

Prior to that, and were determined at different concentrations of propan-2-ol (See shown that those electrons and holes do not contribute significantly to the H 2 production.

Formation of by-products

Acetone and H 2 are the two main products during the photocatalytic dehydrogenation of propan-2-ol but few by-products were also observed: methane, propane and acetaldehyde for Pt/TiO enhance PHER rates, reported in Chapter V, this phenomenon is also neglected.

Theoretical kinetic study

The evolution of , and are written as follow :

Preliminary conclusions

The main conclusion is that does not depend on ( on Main Scheme 3). The H-H coupling is not the rate-determining step of the photocatalytic dehydrogenation of propan-2-ol.

As discussed in Section 2.2, the electrocatalytic HER depends mostly on the ability of the catalyst to activate H 2 and to catalyze the H-H coupling (Heyrovksi or Tafel step). The closer the is to 0, the higher electrocatalytic HER rate is [21,22]. On the contrary, it seems that such a property is not a key parameter for the PHER. The various ability of the MS x to activate H 2 is used in Chapter V to probe such a hypothesis.

According to equ. 4.20, the key steps of the overall photocatalytic mechanism are, the transfer of the electrons from the conduction band of TiO 2 to the one of the co-catalyst :

or the proton reduction at the surface of the co-catalyst :

In the first case, the key parameter of the photocatalyst is its electronic structure. The overall PHER activity is driven by the band alignment at the interface between TiO 2 and the cocatalyst. To investigate such a hypothesis, the PHER activity of MoS x /TiO 2 , HgS/TiO 2 , NiS 2 /TiO 2 and RuS 2 /TiO 2 are reported (Chapter V) as well as their electronic structure (Chapter VII).

In the second case, the key parameter of the photocatalyst could be the proton affinity of the co-catalyst or its . The latter imposes the reduction potential of the photogenerated electrons.

To investigate such a hypothesis, the PHER activity of seven MS x /TiO 2 and the electrocatalytic activity of the bulk sulfides are reported (Chapter V). In addition, Chapter VI studies RuS 2 , with a given activity for H + reduction, supported on five TiO 2 supports.

Conclusions and discussion

Various alcohol-water mixtures were compared to determine which one is the best to measure the PHER activity of the MS x /TiO 2 photocatalysts. We have chosen to focus this work on a propan-2-olwater mixture (50 %vol.). Sulfide-based catalyst are more stable in this reactive mixture and the photocatalytic reaction has a single pathway: propan-2-ol is dehydrogenated in acetone and hydrogen.

The mechanism for the photocatalytic conversion of alcohols is rarely provided. In this work, we succeed to report the evolution curve of almost all the compounds involved in the reaction. Additional experiments are needed to report accurately the propan-2-ol consumption.

A mechanism and a micro-kinetic model is proposed thanks to a study of the PHER rates as a function of time and concentration with Pt/TiO 2 and RuS 2 /TiO 2 catalysts. A dissociative Langmuir-Hinshelwood mechanism is evidenced. The rate of hydrogen and acetone production are expressed as:

With the rate constant and the equilibrium constant.

The influence of the temperature is also studied. It shows that PHER rates for all photocatalysts follow an Arrhenius law. It provides the Arrhenius parameters: an apparent activation energy ( ) and a pre-exponential factor ( ). Combining the two expressions, must depend on the number of active sites and on the coverage: « ».

In the following chapters, photocatalytic tests are performed in the same reactive mixture

Preparation of the seven MS x /TiO 2 photocatalysts

Seven MS x /TiO 2 (with M = Ag, Co, Cu, Hg, Mo, Ni, Ru) solids were prepared with the thioacetamide reflux method already reported by Girel et al. [2]. It is based on the hydrolysis of the thioacetamide [3]:

CH 3 CSNH 2 + 2H 2 O CH 3 CO 2 H + H 2 S + NH 3 5.1
The hydrogen disulfide produced in-situ is continuously refluxed to cover the TiO 2 surface. When the precursor is injected, the sulfides are formed on the surface. To obtain various co-catalyst loading we changed the amount of TiO 2 powder, metallic precursor and of thioacetamide Because transition metal sulfide is less stable than the oxide when you go from the left to the right side of the periodic table, strongly oxophilic transitions metals such as Zr, V or Nb were not tried. The thioacetamide reaction was carried out with twelve of the first, second and third row metals.

For most of the metals tried, the color of the powder was grey. It combines white TiO 2 powder and the dark color of sulfides. An inspection of the color change, after the thioacetamide reflux, suggested that for Mn, Fe, W and Pt other products were formed. Platinum product remained yellow that is the color of the precursor solution. Mn, Fe and W seemed to be oxides. Synthesis with zinc nitrate does not give a precipitate. Consequently, seven MS x /TiO 2 were retained for further characterizations and catalytic tests. For this chapter, our primary goal was to study those materials prepared with the same method.

Another material was prepared named TiO 2 *. It is the bare PC500 which has followed the same preparation method than MS x /TiO 2 : PC500 was put with thioacetamide in water, and reflux during 1.5 hour. Powder was washed two times with ethanol and dried under N 2 . It is used as a reference for the MS x /TiO 2 . Chemical analysis evidenced a S / Ru ratio of 1.9 for bulk RuS x and The surface area of the bulk is much lower than PC500 support, therefore, it limits the adsorption of impurities and it evidences an under stoichiometric RuS 2-x . Additional characterizations were performed to determine the chemical composition of the cocatalysts. To complete characterizations of supported sulfides, bulk counterparts were also prepared in the same conditions. It is assumed that with similar experimental conditions, the sulfide phases are the same for supported and bulk counterparts.

Characterization of the seven MS x /TiO 2 photocatalysts

Elementary composition of the photocatalysts: ICP-OES & CHNS

Crystallographic phase of the co-catalysts: XRD analysis

XRD analysis determines the crystalline phase of some of the catalysts. Patterns show the presence of elemental sulfur, rhombohedral (00-024-1206) or monoclinic (04-007-3019), potential residues of the synthesis method. For Ag sulfide, the main TPR peak lies in the same temperature range for bulk and supported AgS x (a). It evidences deposition of Ag 2 S phase on TiO 2 . Peaks below 300 °C on the black curve are assigned to sulfide impurities. Their reduction is not observed on bulk due to lower surface area.

TMS

TPR curve for bulk RuS x (b) agree with those already reported in the litterature [5]. TPR curves for bulk and supported RuS x are similar, except a peak at 300 °C for the supported sulfide. It is due to the thioacetamide reduction (discussed below). A catalyst extensively washed after the synthesis does not exhibit this peak on its TPR curve (See Annexes, Figure S13). The thioacetamide peak represents For copper, cobalt, mercury and ruthenium supported sulfides -for which particles were not observed with TEM-the core reduction is observed above 300 °C. It evidences the success of the deposition of MS x on TiO 2 . TPR curve obtained for CoS x (e) and HgS x (f) are too complex to clearly assign reduction processes. Further investigations would be required.

For bulk copper sulfide, the TPR curve (c) has similarities with the one reported by Ziolek et al. [6]. The core reduction starts close to 500 °C for bulk CuS and close to 700 °C for supported CuS. It confirms that supported particles are smaller than bulk counterpart (See Table V-3).

TPR curve of NiS 2 /TiO 2 (d) is rather complex, with several apparent maxima, as reported in [7].

The NiS 2 phase is reduced until the metallic Ni 0 going through several intermediates: NiS , Ni 3 S 2 , or Ni 9 S 8 as examples.

TPR curve of MoS x /TiO 2 (g) exhibits two reduction peaks above 600 °C. Those phenomena were already reported in the literature [8,9]. First peak is the reduction of sulfurs atoms bonded to the edges of the slabs. Second peak is the progressive reduction of sulfur atoms from edges to basal plane.

TPR curve of PtS x /TiO 2 (h) exhibits small peaks close to 600 °C. This temperature range matches with the PtS reduction reported in [10]. However, the low intensity of the peak disproves the complete sulfidation of Pt (140 μmol of PtS estimated, 0.09 wt%). The low-temperature peaks may be due to impurities or at least S weakly bonded to Pt. It evidences a Pt poisoned by sulfur rather than a true PtS x cluster deposited on TiO 2 . PtS x /TiO 2 will be no longer discussed for photocatalytic purpose because of its unclear chemical nature. It has a reasonable activity, a PY of 0.32 %.

Signal of other masses is recorded over time (See Annexes, Figure S14). Those assignments discriminate three types of sulfur: the one from thioacetamide impurities (S thio. ), the one from the sulfides at the surface (S surf. ) and in the bulk (S bulk ) (See Figure V-6).

A geometrical model, developed by Geantet et al., correlates the S surf. /(S surf. + S bulk ) ratio with the size of particles that have a pyrite structure [11]. Therefore, TPR measurement gives a size for the RuS 2 particles of 1.3 ± 0.2 nm. It agrees with the size estimated with STEM images on Figure V-4.

Notice that the major drawback of microscopy is its lack of statistical interpretation. Less than ten images are often taken from a fraction of a given sample. Size estimated with TPR curve and geometrical model give an average size of a 100 mg sample. Notice that a size of 1.3 ± 0.2 nm was also determined with this method for the extensively washed RuS 2 /TiO 2 (See Figure S8). TPR experiments confirm that seven MS x phases were successfully supported on commercial TiO 2 (PC500). It also estimates the size of the RuS 2 particles, a catalyst of interest in this work. No particular cautions were used to perform the PHER tests so no particular cautions were used to insert the samples in the XPS analysis chamber.

165

As a reference, the TiO 2 * (i.e. a thioacetamide treated TiO 2 ) and the thioacetamide powder were analyzed with XPS (See Annexes, Figure S15). Sulfur states from the thioacetamide have a similar binding energy (BE) than the sulfur from the supported transition metal sulfides. However, XPS analysis also evidenced that the surface concentration of those states can be neglected (See Annexes, Figure S16).

The surface Hg/S ratio is close to one (See Table V-4), it confirms the deposition of HgS on V -4). Those binding energies correspond to β-HgS; α-HgS states have BE, 1.0 eV higher [12]. It shows the limitations of the phase assignment with the XRD performed on bulk sulfides.

S 2p -SO x

before fitting the tricky range between 280 and 288 eV. Two chemical states were found at 460.5 eV and 464.1 eV. They correspond to ruthenium sulfides / metallic ruthenium (87 %) and to a more oxidized compound (13 %). Ru 0 (metal) and Ru II (sulfide) have the same BE [13,14]. Consequently, the Ru 3d 5/2 were fitted with the same states and the same proportions. Applying the spin-orbit splitting of 4.2 eV fits the Ru 3d 3/2 orbitals (See Figure V-9, right). The missing contribution to match the experimental spectrum with the fit (dots) is the C sp 2 state with a BE of 284.5 eV.

Figure V-9:

XPS spectra of the unsupported ruthenium sulfide. In the binding energy range of Ru 3p

(left) and the C 1s & Ru 3d one (right). Dotted line show the result of the fit. Reproduced from [1] with permission.

The parameters of the asymmetric line shape were optimized with the bulk RuS x sample (F=0.001, E=400 and p=60; See equ. 3.28, Section 3.3.5). The same methodology was kept for the supported RuS x . First, the contributions of the Ru 3d 5/2 orbitals are fitted. Inspired from the bulk RuS x two sates provide a good fit. Then the spin orbit splitting is applied and the missing contributions are due to the carbon states. The small contribution at 166-171 eV corresponds to sulfate groups [15]. Except a small global shift between the BE of supported and bulk samples, spectra are similar. The supported one has more carbon impurities and has a higher SO x / S tot ratio likely due to the higher surface area of TiO 2 for sulfur and carbon adsorption.

Atomic ratio % / (% Ru 0 -RuS x ) decreases from 3.5 for bulk to 1.4 for supported RuS x (See Table V-4). It demonstrates that the thioacetamide reflux method gives a RuS 2-x /TiO 2 .

Figure V-10:

XPS spectra for bulk (down) and supported (up) RuS x : Ru 3d -C 1s (left), S 2p (right).

On all spectra, black line is the experimental spectrum, color lines are the fitting contributions and black dots constitute the resulting fitted spectrum. Reproduced from [1] with permission.

Ag 2 SO 4 , Ag 2 O and Ag 2 S have similar BEs, the bonded elements do not affect the BE of the Ag I state [16]. Ag 3d spectra evidence two states, one at 367.2 eV and another at 366.1 eV (See Annexes, Figure S17 andTable V-4). According to the literature, the first is assigned to Ag I and the second to AgO [17,18]. A particularity of silver is that oxide state has a lower BE than metal and sulfides counterparts [16].

Three states are observed in the energy range of the S 2p orbitals (See Table V S15). properties such as high or low , the exciton binding is therefore too higher and charge carrier are not well-separated. For instance, MoS 2 has a low [29].

Because the morphology can change between bulk and supported RuS 2 , its activity was also measured directly on the TiO 2 using a particular range of photon energy to only excite RuS 2 . An action spectrum reports the photon yield (or the PHER rate) measured at different wavelength. Figure V-13

shows the action spectra of NiS 2 /TiO 2 and RuS 2 /TiO 2 .

TiO 2 -anatase (CristalACTIV TM PC500) has a band gap of 3.15 eV (See section 7.1.1 for its determination) so it can only absorb photons with . The diffuse reflectance spectrometer has a lower limit at 1050 nm so measurements only conclude that NiS 2 and RuS 2 have a band gap lower than 1.2 eV. For NiS 2 , the literature is unclear if it has a metallic or a semiconducting behavior.

In the second case, the size of the band gap is close to 0.3 eV [30,31]. For RuS 2 , the literature reports an indirect band gap between 1.2 and 1.4 eV [31][32][33]. It suggests that NiS 2 and RuS 2 can absorb all the photons from the solar simulator (Xe lamp). Figure V-13 depicts the range of photons that can be absorbed by each semiconductor.

In the domain for which the PC500 does not absorb (i.e. 490 nm, 445 nm and 425 nm), no H 2 is detected (See red squares on Figure V-13). In the domain for which the PC500 absorbs the PY measured is significant. It means that only the photons that are absorbed by TiO 2 participate to the H 2 production. The MS x absorbs photons but they may rapidly recombine. This conclusion is also valid for MoS x /TiO 2 (See Annexes, Figure S18) and can be assumed for all the MS x /TiO but its peak is below the detection limit and cannot be quantified. also depends on the surface available to absorb the photons. So it depends on the amount of TiO 2 and its surface area. Section 6.2 and Section 6.3 discuss, respectively, the influence of the mass of photocatalyst and of the co-catalyst loading. Influence of the incident photons is extensively discussed in Section 6.1.

Photon Yield comparison

The PHER activity was compared for the seven MS x /TiO 2 under similar experimental conditions. Tests were performed with: the same geometry of reactor (the optical window influence the amount of photons received), the same reactive mixture (the propan-2-ol-water mix discussed in chapter IV), same TiO 2 support (CristalACTiV TM PC500 -anatase), the same mass of photocatalyst (50 mg for not being limited by photon absorption, See Section 6.2) and with the same amount of cocatalyst (See Table V-1). Similar experimental conditions allows correlating variations of the PHER activity with intrinsic properties of the MS x .

Figure V-14 shows the PY of the seven MS x /TiO 2 and the corresponding references: the bare TiO 2 and a thioacetamide treated TiO 2 (TiO 2 *). The second one is PC500 which has followed the same preparation conditions than the MS x /TiO 2 (See Section 5.1).

Presence of thioacetamide shows a harmful effect of those impurities: (TiO 2 ) = 13.8 PY(TiO 2 *).

Carbon impurities must be adsorbed at the surface of TiO 2 , limiting the amount of active sites for photon absorption. It can also limit the H 2 production consuming charge carriers through, for instance, photocatalytic hydroxylation. Despite the presence of remaining thioacetamide (or acetic acid) that was revealed by C analysis (See

Arrhenius parameters for the first MS x /TiO 2 series

To go further in our investigation, the and the provide useful information. They are calculated from an Arrhenius law: PY = f (1/T); Section 3.2.3 explains the procedure. Notice that the Arrhenius law is applied to a dimensionless PY, it results in a dimensionless . Table V-5 reports PY, and .

The low order of magnitude for was already noticed in chapter IV. Those results confirm that the adsorption of propan-2-ol on the TiO 2 surface is not the rate-determining step:

varies depending on the sulfide co-catalysts, whereas the oxidation half-reaction mainly occurs on the TiO 2 surface. The rate-determining step must be correlated to the presence of the co-catalyst. The rate-determining step might be: the H 2 desorption, the catalytic H + reduction, the catalytic H-H coupling or another step such as the electronic transfer from the TiO 2 to the MS x .

From the parameters reported in Table V show the reproducibility of the preparation methods.

The second screening was performed in similar conditions, unless the incident light was more intense (LED source: 11 200 μE/h instead of HPK lamp: 1 400 μE/h). The photon yield is much lower with the LED lamp (See Figure V-15). Section 6.1 discusses the influence of the lamp intensity, for such a light intensity, the PHER rate is not proportional to the flux of photons. Ratios PY (MS x /TiO 2 ) / PY (TiO 2 ) (so-called enhancement factor), on the contrary, were similar during the tests with the two lamps (See Table V -6). It confirms that even if some limitations occurred, the enhancement factor is still relevant to discuss reports from the litterature (See Section 2.4). This ratio is independent on the incident flux of photons.

The photocatalytic conversion of propan-2-ol was never reported for MoS x /TiO 2 or any MS x /TiO 2 . However, in [1], we reported an enhancement factor of 25 for the photocatalytic conversion of methanol. This enhancement factors is lower than those reported in Section 2.4, for the photocatalytic conversion of methanol, between 20 -864 (See Table II A correlation between and the number of active sites also explains the photocatalytic behaviors of RuS 2 /TiO 2 and MoS x /TiO 2 . Both have the same E app but the of RuS 2 is 10-25 times higher than MoS x (See Table V -6). It is due to the morphology of the two catalyst. RuS 2 particles are small and well-dispersed whereas MoS x /TiO 2 is made of larger MoS x particles (See Section 5.2.3). In addition, it was evidenced that only the edges are active to produce hydrogen [36]. Number of RuS 2 active sites is therefore higher than the MoS x active sites. ). For both, T=20 °C.

Preliminary conclusions

Seven MS x were supported on the same titania support, with the same preparation technique.

Their PHER activity was measured for the photocatalytic dehydrogenation of propan-2-ol.

Among the screening, RuS S19). Pt and RuO x were supported on PC500 support with the photodeposition method described at the beginning of Chapter IV. With this technique, the ruthenium salt might not be completely reduced and reffered as RuO x [37].

Stability of the seven MS x /TiO 2

The aim of such a screening is to establish a structure-activity type relationship correlating the PHER activity with intrinsic properties of the MS x . Before doing that, it must be confirmed that the asprepared MS x phases and so their intrinsic properties are kept along the PHER tests.

At a given temperature, the PY does not significantly change with time but to measure and the temperature increases up to 45 °C and it lasts more than 20 h per cycle. One cycle consists in measuring the PY at 10, 20, 35 and 45 °C; each measurement lasts 3 to 4 h. Few examples of evolution curves are given in the Annexes (See Figure S20). It is therefore crucial to check the stability of the photocatalysts regarding temperature and time. ). This phenomenon is stronger for the most efficient catalysts. The higher the r H2 is, the higher the amount of H + and/or e -in contact with the MS x is. They may progressively reduce NiS 2 and RuS 2 in metals. It seems to have a positive effect on PY and on .

Evolution of the PHER activity along time

To explain the photocatalyst activation two hypotheses were made. Both were made because H 2 S was produced at the beginning of all the PHER test. Either H 2 S is produced to a conversion of the MS x in a more active species, an oxide or a metal, or H 2 S is produced due to the progressive removing of impurities blocking the active sites. Thioacetamide content is however sufficiently reduced for not being observed by attenuated total reflection infrared spectroscopy (ATR-IR). Comparison between TiO 2 and NiS 2 /TiO 2 shows three vibrations corresponding to thioacetamide (and carbon) impurities. Vibrations at 1096 and 1051 cm -1 are due to the C-N stretching of thioacetamide [38]. Vibrations between 1490 and 1350 cm -1 are due to the bending of -CH group and vibration at 2972 cm -1 is due to the symmetric stretching of the C-H bond. Those vibrations are no longer observed after the NiS 2 /TiO 2 photocatalyst was used in a PHER test. However, characterization before and after PHER tests are scarce.

Depending on the MS x and on the lamp used for the test, a small leaching occurs (See chemical analysis for some selected catalysts in the Annexes, Table S4). For a PHER test, with HgS/TiO 2 , under LED illumination (11 200μE), all the HgS is leached into the solution. This phenomenon is explained below with XPS.

Ag 2 S/TiO 2 is the only catalyst for which the PY decreases during the test. The other suffers from a leaching but it has not a negative impact on their PHER activity.

One hypothesis would be that the sulfides also act as a co-catalyst in solution. This phenomenon was already reported in [39]. To elude this hypothesis for RuS 2 /TiO 2 , a cycling experiment was performed and discussed in Section 5. XPS analyses on HgS/TiO 2 evidenced that the HgS phase is kept after a test under a lamp with an intensity of 1400 μE/h but not for the test under a lamp with an intensity of 11 200 μE/h.

For the low intensity test, Hg II and S 2-states were observed with a BE close to 100 and 161 eV, respectively. The total Hg / S ratio is of 0.35 (three times lower than before test). The S 2-state represents now 35 % of the total sulfur (93 % before test). A major part of the sulfide was converted in sulfate.

For the high intensity test, neither the sulfur nor the mercury were observed by XPS. We assume the high photon flux induces a high density of electrons and holes. The latter reacted faster than electrons so, they accumulated either at the surface of TiO 2 or on the HgS particles and damaged them.

Presence of H 2 may also help to reduce Hg II in Hg 0 ( ). This new state leach easily in the solution. Depending on the lamp intensity, and followed by compete:

5.7

It explains why the PHER behavior of HgS/TiO 2 differs under HPK and LED lamps. In the first case, a . of 33 kJ/mol was measured and in the second case it was 25 kJ/mol (See Table V Notice that the value obtained with RuO x /TiO 2 is 15 kJ/mol (See Annexes, Figure S19). It disproves the hypothesis of a complete oxidation of the co-catalyst. After that, the catalyst was recovered and suspended in a fresh reactive mixture (50 %vol. of propan-2ol in water) for another test. Seven runs were made following such a procedure. For the first run, 120 mg of photocatalyst was used. It is far above the threshold were PY = f(mass) curve reaches a plateau (See Section 6.2 for more details). Even if few mg of catalyst are lost during the recovering process, the activity would be independent of the mass variation.

The PY increases between the first and the second run; a lower is responsible of such an enhancement. After that, between the second and the last run, no difference of activity is observed: the three parameters PY, and did not vary (except that is lowered during the third run).

Two hypotheses are considered to explain the difference between the first and the second run.

Either the thioacetamide / H 2 S impurities that were released at the beginning of the test poisoned the reactive mixture. Renewing the reactive mixture, thus, increases the activity. Or, the catalyst is modified during the first run and it is stable during the other runs. 161.9 ± 0.1 eV and at 279.9 ± 0.1 eV are still observed [13].

However, / "RuS x -Ru 0 " ratio decreases showing that progressively, the state at 279.9 eV is converted from RuS 2 to Ru 0 . The activity increases during the 160 h-experiement. It indicates that Ru 0 is more efficient than RuS 2 . In addition, the activity is unchanged between run 2 and run 7 for the cycling experiment. After one test and after seven tests, the / "RuS x -Ru 0 " equals 1.0 and 0.1, respectively (See Table V-7). This decreasing has no impact on the activity. Ru 0 , poisoned with sulfur may be the only responsible of the PHER activity of the material. In-situ experiment would be interesting to understand the nature of the active site and the conversion of RuS 2 in Ru 0 . In fact, it was by X-ray absorption spectroscopy that metallic domain can be formed at the surface of RuS 2 particles.

It stands for RuS 2 supported in a zeolite and tested for tetralin hydrogenation [40].

To establish reliable correlation RuS 2 /TiO 2 must be studied as-prepared and tests must not exceed few days. To summarize, the presence of adsorbed thioacetamide at the surface of the photocatalyst was evidenced but it does not influence the PHER activity. Regarding the co-catalyst, XPS analyses have shown a mix of sulfides and oxides in the as-prepared catalyst. After their use in a photocatalytic test, the ratio oxide / sulfide increases depending on the sulfide phase. It is however unclear how much this oxide phase contributes to the PHER activity, in-situ experiment would provide additional mechanistic insights.

Intrinsic activity of the seven MS x /TiO 2

Photocatalytic alcohol dehydrogenation follows a complex mechanism. The second one is the electrocatalytic HER. It is an electrochemical process; sulfides catalyze the conversion of protons in H 2 (See Section 2.2). Electrocatalytic HER models the PHER step for which protons are reduced in hydrogen radicals.

It should however be kept in mind that the three reactions do not involve the same molecular entities: HDS converts H 2 forming H 2 S, the electrocatalytic HER converts H + in H° forming H 2 and the photocatalytic dehydrogenation converts hydroxyl functions in H + , that are reduced in H°, forming H 2 . It might limit the approximations proposed above. but also with the hydrogenation properties reported in [41]. HgS was never studied for HDS and here, it shows no activity for thiophene HDS. These experiments establish a rank of the MS x /TiO 2 for their capacity to activate H 2 :

Thiophene HDS: probe for H-H coupling

RuS 2 > MoS x >> CoS x > NiS 2 CuS Ag 2 S > HgS Figure V-24:
Reaction rate of the MS x /TiO 2 for the thiophene hydrodesulfurization. Adapted from [1] with permission.

Electrocatalytic HER: probe for H + reduction

Electrocatalytic HER activities were measured with bulk sulfides for two reasons. Bare titania is inactive and with a low amount of sulfides a non-negligible amount of carbon black would be required to measure a current. Its presence may give a false intrinsic activity of the sulfides. Besides, for supported sulfides, the MS x -TiO 2 interface plays a key role, as well as the electronic transfer between the two particles. The aim was to study only the intrinsic electrocatalytic activity of sulfides, so only bulk sulfides were studied. Notice that between supported and bulk sulfides, size may differ and slightly change their electrochemical behavior.

Benck et al recommended two parameters to estimate the electrocatalytic activity of a material [42]. Both are obtained with linear sweep voltammetry (LSV) curves depict on Figure V-25. The first one is the voltage required to reach a current of -10 mA/cm 2 (η 10 ) and the second one is the Tafel slope. It is obtained, plotting the overpotential as a function of the current logarithm (See Sections 3.3.8 and 2.2 for methodology). The lower the Tafel slope is, the lower the voltage required to obtain a significant current is and so, the higher the electrocatalytic performances are. (mol.g -1 .s -1

Cu Mo Ag Hg Co Ni Ru

)

Table V-8 reports the two parameters. High HER performance corresponds to a low Tafel slope and a low value of η 10 [42]. So those criteria establish the following rank for H + reduction:

MoS x > RuS 2 Ag 2 S > CuS > CoS x > NiS 2 HgS
HgS has a much lower Tafel slope than NiS 2 but it has a higher η 10 (See Table V-8). It is not relevant to establish a proper rank for them. Notice that it is the first time that HgS is reported as an electrocatalyst and it has a poor activity.

Figure V-25:

Electrocatalytic HER activity of the seven bulk sulfides with on the left, the Linear Sweep Voltammetry curves and on the right, the Tafel plots. Reproduced from [1] with permission.

HER activity for MoS x is similar than the one reported by our group (the Tafel slope is lower but the η 10 is higher) [2]. Many studies reported innovative MoS x -based electrocatalyst; they have an increased density of edges sites or an increased intrinsic activity. Section 2.2 reports selected publications (-η 10 = 200 -300 mV, Tafel slope of 40-70 mV/decade) but in average, the MoS x prepared with the thioacetamide reflux method has a lower activity. It is not so surprising, this preparation method is not claimed to enhance the amount of edge sites or their intrinsic activity.

Similar conclusion is made for the CoS x electrocatalyst.

Stability of the NiS 2 catalyst is discussed below. But even during the second LSV (η 10 = -500 mV, Tafel slope of 140 mV/decade), the catalyst is less efficient than those reported in the literature (-η 10 = 200 -300 mV, Tafel slope of 40-50 mV/decade) [43,44].

- information on the stability of the sulfides. It must however be kept in mind that in electrocatalysis, the current is much higher than in photocatalysis. Therefore, a catalyst stable during electrocatalysis may be stable during photocatalysis but the reverse is not true.

RuS 2 follows an activation between the first and the second LSV (See Figure V-26) but no reduction peak is observed. It seems to be impurities that are removed from the surface of the catalyst.

Then the HER activity of RuS 2 during LSV 2 to 4 is stable.

For the NiS 2 , HER activity decreased from the 1 st to the 3 rd LSV before to stabilize (See Figure

V-26)

. A reduction peak is observed during the first LSV. The LSV trend and the Tafel slope for the second LSV are similar to the activity of NiS 2 reported in [46]. So the reduction peak might be due only to the reduction of surface impurities. Between the 2 nd and the 3 rd LSV, it seems that the catalyst is removed from the electrode surface. Such a phenomenon was already reported for NiS 2 [43].

The reduction peak observed during the 1 st LSV of NiS 2 was also observed for HgS, CuS, Ag 2 S and CoS x . While the HgS and the CoS x activities stabilize after their 1 st LSV, the CuS deactivates and the Ag 2 S activates. The CuS and the Ag 2 S are progressively reduced in Cu 0 and Ag 0 . The first is a suitable electrocatalyst for oxidation half reaction, not reduction [47]. The second is a suitable electrocatalyst for HER [48].

The MoS x is stable according to its LSV curves and cyclic voltammetry experiments on

Conclusions and discussion

First of all, action spectra (See Section 5.3.1) evidence that the sulfides do not provide additional charge carriers. They absorb photons but the photogenerated carriers recombine. It implies that the on Main Scheme 4 depends only on TiO 2 and not on the absorption coefficient of MS x .

Consequently, the MS x /TiO 2 are compared for the same amount of incident electro-hole pairs. The various PY, and measured are therefore due to the various intrinsic properties of the MS x .

Main Scheme 4: Mechanism for the photocatalytic dehydrogenation of propan-2-ol with MS x /TiO 2 .

Because Ag 2 S is not stable (as well as HgS that leached during the high lamp intensity screening) and because stability of CoS x and CuS were not studied, conclusions are mainly based on HgS (first screening), NiS 2 , RuS 2 and MoS x co-catalysts.

The PY was first compared with the intrinsic properties of the MS x /TiO 2 photocatalysts: morphology (See Section 5.2.3), electrochemical HER (See Section 5.5.2) and thiophene HDS (See Section 5.5.1).

We conclude that the photocatalyst with the highest PY is the one with small and well-dispersed RuS 2 particles. The latter have a good behavior for H + reduction and for H 2 activation. Small particles increase the amount of active sites for H 2 production but also the interface between TiO

This step is either governed by the electrocatalytic activity of the MS x ( ) and/or by the limiting amount of electrons (

). In the first case, the electrochemical behavior of the MS x is the key parameter to maximize the PY. In the second case, the electronic structure of the MS x /TiO 2 is the key parameter to maximize the PY. Up to now, nothing permits to discriminate which step is the key one.

The second case is however more likely. In the case of HgS/TiO 2 , the PHER activity is only driven by the enhance charge carrier separation, HgS has no activity for HDS and has a poor activity for electrocatalytic HER. It is therefore assume that the photogenerated electrons do not even reach the

209

Chapter IV and V conclude that the H-H coupling is not the rate-determining step of the photocatalytic process. The rate-determining step could be either the transfer of the electrons from TiO 2 to the MS x or the proton reduction at the MS x surface. To distinguish one step from the other, an extensive mechanistic study is reported in this chapter.

It was conducted with the RuS 2 /TiO 2 photocatalyst. The previous chapter has shown that it is the photocatalyst with the highest PHER activity. In addition, it has the highest rate of HDS and so a good ability to activate H 2 ( , Main Scheme 5), RuS 2 is the second best electrocatalyst the HER ( & , Main Scheme 5) and RuS 2 particles are small and well-dispersed. The standard parameters for a heterogeneous catalytic process are also studied. Sections 6.2, 6.3 and 6.4.1 discuss, respectively, the influence of the mass of photocatalyst, of the co-catalyst loading and of the size of the co-catalytic partciles on the PHER efficiency.

The influence of those parameters was already reported in several works (See Section 2.4).

However, it is the first time that the Arrhenius parameters were measured: the and .

Finally, while the other chapters are focused on various MS x supported on the same titania support (PC500), section 6.4.2 discusses the PHER activity of a RuS 2 co-catalyst on various titania support.

In contrast to PY, the does not depend on the incident flux of photons (See Figure VI-1, right). For a given photocatalyst (RuS 2 /TiO 2 ), it concludes that photon absorption is not the ratedetermining step. The incertitude on the , is not negligible but in average, it seems to be constant (See Annexes, Figure S21). The threshold between the two trends (r H2 = cte and r H2 = cte

) is estimated at 2800 ± 200 μE/h. HPK and Xe lamps have an intensity low enough for calculating accurately PY. With LED lamp at high intensity, it should be kept in mind that the PY is underestimated.

Thresholds between the trends r H2 = cte and r H2 = cte were reported in the literature: 251 μE/h for TiO 2 -P25 [2] and 36 110 μE/h for CdS [3]. It shows that this threshold depends on various parameters such as the photocatalytic material but also the geometry of the reactor.

It means that work from the literature must report the r H2 = f ( ) curve for each photocatalyst.

It would avoid underestimating the efficiency normalizing the r H2 by whereas their relation is not linear (See Photon Yield evolution on Figure VI-1, left).

Influence of the photon energy on the PHER activity

Action spectra, described in Section 5.3.1, evidence that PY increases when increases. It can be due to the absorbance properties of TiO 2 . The latter absorbs more photons at higher photon energy (See evolution of on Figure V-13). Another hypothesis is that the PY increasing is not due to the increasing amount of photons absorbed but it is due to itself.

To go further, and were measured at three wavelengths: 395 nm, 365 nm and To perform an action spectrum, filters are used and the resulting incident flux of photons is low (close to 100 μE/h). Photon absorption might be the rate-determining step. However, does not linearly evolve with : 130 μE/h at 3.4 eV and 100 μE/h at 3.65 eV.

In addition, an of 20 kJ/mol is measured at 3.14 eV, the photon energy for which the photon absorption could be limiting ( ).

This value is the same than the ones measured at much higher flux of photons (See energy and so, depending on the temperature of the resulting charge carriers. Such phenomenon needs to be studied in detail, with, for instance, luminance measurements, [5].

It seems that some electrons are not energetic enough to transfer from TiO 2 to RuS 2 surface.

Increasing the photon energy increase the amount of electrons transferred to RuS 2 . As a result, the concentration of active sites increases so does the . (See Figure VI-3, right).

The incident photons strongly influence the PHER activity. These information are however rarely provided in the experimental section of literature reports. Notice also that above a lamp intensity threshold, the PHER activity is not linear with and calculating PY, QY or AQY might be reconsidered. already reported in the literature for a Pt/TiO 2 system [1,6]. The shape of the curve is similar for RuS 2 /TiO 2 and Pt/TiO 2 . The nature of the co-catalyst does not influence this behavior; it must be due mostly to the light absorption by TiO 2 . Increasing the mass of photocatalyst induces a linear increase of TiO 2 surface, responsible of photons absorption. In the limiting case, TiO 2 absorbs all the light, addition of new photocatalyst to the slurry does not increase the overall H 2 production.

This trend follows a derivative of a Beer Lambert law [7]. It has been already introduced in Section 3.1.1 (equ. 3.4); demonstration is provided below:

6. 1
is the mass of photocatalyst, is the height of the irradiated reactive mixture and is the surface of the optical window;

(cm -1 ) and (cm 2 g -1 ) are the absorption coefficients of the semiconductor.

Irradiated surface of catalyst (

) can be expressed as follow:

6.2

6.3
is the surface area of the photocatalyst and is a constant (h/E):

Equ. 6.3 is integrated thanks to equ. 6. 1 and 6.2. According to the previous section, it is assumed that the PHER rate linearly increases with (or with the incident flux of photons for a given ) (experiments were performed with ):

6.4

6.5

And so:

6.6

Fitting of the experimental data (on Figure VI-4, (a)) gives equals 0.62 cm 2 g -1 and equals 2.5

x 10 -6 m -2 . It has a reliability factor of 0.034.

At low mass (before 40 mg), the TiO 2 surface limits photon absorption. So the PY linearly increases with the mass. At high mass (above 70 mg), amount of incident photons limits the PHER rate; TiO 2 absorbs all the photons available. The curve reaches a plateau and equ. 6.7 defines the maximum P which depends on the experimental set-up ( ) and on intrinsic parameters of the photocatalyst ( ). For this reason should be reported in the littérature for each experimental set-up or support, as adviced in [8].

6.7

For each mass, the PY was measured at various temperatures to calculate and . At low mass of photocatalyst, the amount of photons absorbed and therefore of free charge carriers is not optimal. varies at low mass because the amount of free charge carriers limit the overall hydrogen production.

To explain the influence of on one hypothesis would be that depends on the density of photons available per particle ( ):

6.8

The lower the mass of photocatalyst is, the higher the density of photons per TiO 2 particles is and therefore, the higher the probability for a given TiO 2 surface to absorb photons is. This probability decreases until reaching a threshold for which all the photons are absorbed: and . Adding more mass only increases the amount of TiO 2 sites that do not receive a photon. After normalization, depends on . According to equ. 6.7, also depends on . Pre-exponential factor

Influence of the co-catalyst loading on the PHER activity

Most of publications discussed in Section 2.4, report that the PHER rate has a maximum as a function of the MS x loading [9][10][11][12][13][14]. It is not a phenomenon occurring only with sulfide-base catalysts [1,15,16].

Catalytic activity increases with an increase co-catalyst loading, up to a threshold; after this threshold, it decreases. Two main hypotheses are proposed to explain this phenomenon: (i) the excessive co-catalyst may shield the TiO 2 surface, limiting its photon absorption; (ii) the excessive cocatalyst may act as a recombination center [17].

Since few years a habit is to attest a co-catalytic effect reporting photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) measurements [10,13,[18][19][20]. The first informs on the rate of recombination: the lower the PL intensity is, the lower this rate is (See Section 1.3.2 for more details). The second informs on the electron transfer resistivity between the support and the cocatalyst: the lower the radius of the semi-circle is, the lower this resistivity is (See Section 1.3.2 for more details). If a MS x is deposited on a support and if it reduces the PL intensity and the radius of the semi-circle in EIS, it is concluded that the MS x has a co-catalytic effect.

Recently Shao et al. went further performing PL and EIS experiments for VS 2 supported on g-C 3 N 4 , with different VS 2 loading [18] (See Annexes, Figure S22). They reported a lower PL intensity and a lower EIS radius with a higher VS 2 content. Thank to the arguments above, they simply concluded to a co-catalytic effect of VS 2 . An alternative interpretation of their results is proposed here.

They evidenced an optimal PHER activity for a VS 2 loading of 2.8 wt%. They also reported a constant decreasing of the PL intensity with the VS 2 loading and a minimum at 2.8 wt% of the radius of the EIS semi-circle (See Annexes, Figure S22).

A conclusion would be that the recombination rate decreases with an increasing VS 2 loading. It eludes the hypothesis that decreasing activity above the loading optimum is due to additional recombination centers. On the contrary, a loading higher than 2.8 wt% seems to increase the resistivity for the electron transfer. Above this optimum, VS 2 may aggregate. An electron that migrates from g-C 3 N 4 to a molecule has to cross several VS 2 particles; it decreases the PHER rate.

The increased resistivity for the electron transfer is an alternative explanation of the cocatalyst optimum than a shielding effect. Notice that this phenomenon occurs even for semiconductor with a large surface area and that are not easily shielded. Measuring and for the RuS 2 /TiO 2 catalyst would provide additional information. Influence of the co-catalyst loading may also inform on what could limit the H 2 production.

Figure shows the PY of RuS 2 /TiO 2 as a function of the Ru loading. From 0 to 0.84 wt%, PY increases. The higher the amount of RuS 2 active sites is, the higher the amount of H 2 produced is.

Chapter V concludes that the rate-determining step is either the H + reduction or the electron transfer from TiO 2 to RuS 2 . In both cases, increasing the amount of RuS 2 increases the amount of active sites for H + reduction and/or the TiO 2 -RuS 2 interface for electron transfer.

Above 0.84 wt% (around 60,000 particles per μm 2 ), PY decreases while increasing the cocatalyst loading.

Figure VI-5 (b) evidences a continuous decrease of . However, two trends can be distinguished, one for which the drastically decreases until 0.33 wt% and a second for which the decrease is lower. Below 0.33 wt% the first add of RuS 2 particles enhance charge carriers separation significantly. Above 0.33 wt%, it is less beneficial, RuS 2 particles may aggregate and the electron transfer from TiO 2 to H + is more and more difficult to enhance, as discussed for VS 2 /g-C 3 N 4 above. To give an idea, 0.33 wt% Ru supported on PC500 corresponds to 24,000 particles per μm 2 . Three hypotheses are discussed to explain the decreasing.

A first hypothesis is that decreases because of a shielding effect. The higher the RuS 2 content is, the lower the TiO 2 surface is unoccupied, the lower the amount of photogenerated electrons is and so the lower the amount of active species is. The optimum would be due to an increasing amount of but a decreasing amount of . However at a 0.12 wt% of RuS 2 loading (9,000 particles per μm 2 ), the light should not be shielded but already decreases.

(

222

The second hypothesis is that decreases because of its dependence to (See equ. 6.8). For a given flux of absorbable photons, the higher the co-catalyst loading is, the lower the density of electrons per RuS 2 active sites is. However, should reach a minimum as on Figure VI-4 (c): all the photogenerated electrons are located on a RuS 2 particle, giving a maximal .

Finally, we proposed a third hypothesis to explain the decreasing.

is not the only active species, TiO 2 can also reduce H + . So can be influenced by the amount of both, and active sites. The more RuS 2 is added, the more species replace to produce H 2 . Even if the active sites are less active than RuS 2 they are more numerous than .

To take into account the contribution of both RuS 2 and TiO 2 for H + reduction, the photon yield can be written as a function of two terms: active sites (which influences the pre-exponential factor) is higher. When the RuS 2 loading is increased, the overall tends to that is low. They reported an enhanced PHER activity with smaller Pd particles (2.3 ± 0.6 nm). Similarly, we tailored the size of supported Pt particles using various titania form, various Pt loading and various temperature treatments. We evidenced an optimal PHER activity with 1.2 ± 0.3 nm Pt particles for the photocatalytic conversion of ethanol in gas and liquid phase [22].

A preliminary study is made to study the influence of the size of the RuS 2 particles on the PHER activity and on the Arrhenius parameters ( and ). Sections 5.2.2 and 5.2.3 report microscopy analyses and a geometric model, used to interpret TPR curve. It estimates the size of the RuS 2 particles as 1.3 ± 0.2 nm, for the sample prepared with the thioacetamide reflux method. In the literature, larger particles were obtained with the impregnation and sulfidation method [23]. For RuS 2 supported on Al 2 O 3 , the size of the RuS 2 particles were tuned between 3.0 and 4.0 nm, performing the sulfidation between 300 and 600 °C. To compare between soft thioacetamide reflux method and standard procedure [23][24][25], but also to obtain different size of RuS 2 particles, RuS 2 /TiO 2 catalyst was prepared with the impregnation and sulfidation method. TEM images shows that RuS 2 particles have a size of 2.1 ± 0.3 nm when they are impregnated and sulfurized at 350 °C. Therefore, the support has an effect on the particle growth,

The PHER activity were compared for the samples obtained with the two preparation methods.

Figure VI-8 reports the PY, the and the . It evidences that the thioacetamide reflux method sample has a better activity than its counterpart. This conclusion was already reported for MoS x /TiO 2 [25]. However, and were never compared for the two preparation methods.

The decreases while the size of the particles increases. also decreases for larger particles. The smaller the particles are, at a given loading, the higher the number of active sites is. It confirms that depends on the number of active sites.

Because of the quantum size confinement [26], the smaller the particles are, the larger their band gap should be. A hypothesis would be that the impregnated sample has a lower E g ; its position can be lower than for the thioacetamide sample. The electron transfer from the conduction band of TiO 2 to the one of RuS 2 is therefore favored in the case of the impregnated sample. However, the lower is not enough to counterbalance the lower and as a result, its PY is almost twice higher than for the thioacetamide sample. Notice also that CristalACTiV TM PC500 is not well-crystalized. Its particles have an irregular shape and a mean size of 7 nm (See Annexes, Figure S32 for XRD measurment). PC500 is not appropriate for TEM study as obtaining images of RuS 2 was difficult. To go further in such a study, RuS 2 particles should be supported on a more appropriate support.

Influence of the TiO 2 support on the PHER activity

Until now, we used the same TiO 2 : a commercial CristalACTIV TM PC500. This section reports preliminary results for which the co-catalyst is always the same (bulk RuS 2 ) and several TiO 2 supports were screened in order to confirm that the rate-determining step is the electron transfer and not the proton reduction which mostly depends on the co-catalyst activity.

RuS 2 was first prepared with the thioacetamide reflux method (See Section 5.1) and then, deposited onto five supports. RuS 2 and TiO 2 powders were mechanically mixed and sonicated for depostion. It avoids the presence of impurities, depending on the surface area of TiO 2 . The powders were prepared with a similar stoichiometric ratio but additional characterizations are needed to confirm it.

Among the supports, one is pure anatase (CristalACTIV TM PC500), one is pure rutile (nanorods, prepared with the method reported in [27], one is pure brookite [28] and the others are mixed anataserutile phases, the well-known P25 (Aeroxide ® Evonik; 80 / 20 wt%, anatase / rutile) and Aeroxide ® P90 Evonik (92 / 8 wt%, anatase / rutile). Variations of PY are not surprising; TiO 2 supports have different surface area and they do not absorb the same amount of photons (their and their absorption coefficient depend on powder composition). The following rank for the TiO 2 polymorphs is observed: anatase > rutile > brookite.

Surprisingly, RuS 2 /P90 is more active than RuS 2 /P25 whereas P25 is often reported as the best commercial titania photocatalyst [29].

However, variations indicate that the rate-determining step does not depend on RuS 2 but on the couple TiO 2 -RuS 2 . It concludes that the rate-determining step cannot be proton reduction, which is mainly performed by RuS 2 but it may be the electron transfer from TiO 2 to RuS 2 . Notice also that mixed titania phases (P25 and 90) have the same than PC500. This result will be explained in Section 7.2.4 based on anatase and rutile electronic structures.

Additional experiments are required to determine the electronic structure of the resulting RuS 2 /TiO 2 . However, it is well-known that from one TiO 2 support to another, the E F , the and the vary [30,31]. Therefore, band alignment at the TiO 2 -RuS 2 interface also varies as well as the amount of electrons reaching RuS 2 particles. It concludes that the rate-determining step is the electron transfer or the charge carrier separation.

In both cases, the key property of a photocatalyst is its electronic structure.

That is why the last chapter of this work is focused on how to determine the electronic structure of photocatalytic powders. Some correlations are proposed between the PHER activity of MS x /TiO 2 and their electronic structure.

Conclusions and discussion

Section 6.2 and 6.3 reports the influence of the mass of RuS 2 /TiO 2 and the Ru weight loading on the PHER activity. In summary, at low mass of catalyst (<40 mg), the photon limits the PHER activity. That is why photocatalytic tests were performed with at least 40 mg of photocatalyst.

Increasing the RuS 2 content favors the rate-determining step. Its addition decreases the until 0.33 wt%, after, its effect is less significant. RuS 2 particles might aggregates, the TiO 2 -RuS 2 interface or the RuS 2 active sites no longer evolve linearly with the RuS 2 loading.

Those studies also show that depends on , on and on the amount of active sites. It should be understand that the active sites can be both the TiO 2 and/or the RuS 2 surface.

That is why when the RuS 2 loading is increased the H 2 production occurs exclusively on the RuS 2 particles and . As , the overall therefore decreases when the RuS 2 loading is increased. The decrease of and the one of leads to an optimal loading for the PHER activity. It is evidenced at 0.84 wt% Ru for the RuS 2 /TiO 2 photocatalyst.

Chapter V concludes that the H-H coupling is not the rate-determining step. Ability of the MS x to activate H 2 does not influence the measured for the photocatalytic dehydrogenation of propan-2-ol. Section 6.4.2 also concludes that the H + reduction is not rate-determining step for RuS 2 /TiO 2 .

were measured with the same RuS 2 , the one reducing H + , on various titania supports.

values vary showing that it depends on the RuS 2 -TiO 2 combination. In addition, we have evidenced (See Section 6.1.2) that photons with a higher energy are more efficient for the photocatalytic dehydrogenation of propan-2-ol, they are more easily transferred. Those experiments concludes that the transfer of the electrons is the rate-determining step with the RuS 2 /TiO 2 photocatalyst.

To investigate this phenomenon, the following chapter is dedicated to the use of spectroscopic techniques for establishing band diagrams.

Chapter VII:

Electronic structure

of MS x /TiO 2 photocatalysts

This chapter aims to present a methodology to determine the electronic structure of the MS x /TiO 2 powders. It combines the photoelectron and the diffuse reflectance spectroscopies. The electron structure then used to establish correlations with their PHER activity. The work dedicated to the electronic structure of TiO 2 -PC500 is published in [1].

238

-Dilution mixes the grains of the sample with the one of the diluent. The more the sample is diluted, the more the average grain size tends toward the one of the diluent. The resulting sample has grain size larger than wavelength and its scattering coefficient is constant.

-Dilution removes specular reflection -Dilution increases the reflectance above 0.6. Below this threshold, significant deviations from the Kubelka-Munk theory occur. Any work reporting should not be interpreted with the Kubelka-Munk theory.

All the absorption spectra below are reported using BaSO 4 as a diluent. The standard procedure was to mix 30 mg of TiO 2 -based powder with 900 or 1200 mg of BaSO 4 (mass ratio of 1:30 or 1:40).

Then an iterative procedure was used to dilute again the resulting power and get concentration from Adapted from [1] with permission. anatase gives an insight in the one of the MS x /TiO 2 . Work on TiO 2 is reported in [1] for commercial anatase (CristalACTiV TM PC500) and rutile (US Reasearch Nanomaterials 99.9 % rutile). It is assumed that the absorbed and the emitted phonons have the same energy. Therefore, the average between the two intercepts " " and " " gives . For anatase-PC500, the band gap measures 3.15 ± 0.05 eV. It is close to the expected value for anatase phase [11].

iii. Optical properties of TiO 2 -anatase

It is unclear why phonons contribution was observed only at low TiO 2 concentration. Adapted from [10] with permission.

Figure VII-1 shows that thin transitions can be resolved for samples that were diluted enough. To investigate those transitions, we proposed in [1] to calculate a density of transitions, t(hν), defined as followed: Each transition corresponds to a population of electrons excited from the valence band states to the conduction band states. To assign those transitions it is mandatory to identify those states. Section 7.2, UPS is used to identify the states from which the electrons are photogenerated.

iv. Optical properties of the seven sulfides

The 3.2-1.3 eV energy range provides pieces of information about sulfides absorption. The spectrometer has a lower limit of 1.3 eV. For sulfides that have a lower band gap, it can be extrapolated but such a methodology may lack of reliability.

CoS x and NiS 2 are metallic, at least they absorb photons with higher energy than 1.3 eV (See Figure left). Such a behavior agrees with the literature (See Section 2.3 or [12]). CuS shows a transition at 1.6 eV but below, F KM starts increasing again. This phenomenon was also reported in [13] but unexplained. No band gap was measured for CuS and it disagrees the commonly reported 2.0 eV band gap. [17]. One of the fitting parameter is . However, such a procedure requires preliminary pieces of information on band structure and it does not take into account contribution of interface or defects. As a result, this method is not suitable for a non-ideal material such as photocatalytic powders studied here.

Two additional methods are proposed in the literature, to determine [19,20].

The first one consists in approximating the valence band edge to a linear curve. The is then taken as the intersection between such a curve and the zero intensity level. The valence band edge follows a Fermi Dirac distribution [21] and at room temperature, the Maxwell Boltzman approximation can be used. It results in a linear extrapolation of the valence band edge. Such method may induce a lack of information on the first photoemitted electrons, for which the signal increases of few percent. In addition, the plot of linear curve appears subjective.

For photochemical processes, the electrons at the edges of the valence band are crucial. The method determining must consider them. That is why we proposed another method in [1]. is defined as the inflexion point of the valence band spectrum i.e. the energy for which the first electrons can be photoexcited. To determine it, the spectrum is twice derived. corresponds to the energy for which the 2 nd derivative of the valence band spectrum has an isolated zero and changes sign.

The fit of the valence band edge proposed by Kraut et that the spectrum for bulk RuS 2 is similar than the one reported in [22].

The main asset of this technique is to provide the level of any powder with a commercial XPS apparatus; it can be done when core levels are analyzed with XPS. The main drawback of such method is its resolution. The XPS apparatus has an intrinsic resolution of 0.5 eV. The error is increased by the way to determine (See below); we therefore estimated an experimental error of 0.8 eV. Regarding NiS 2 /TiO 2 , MoS x /TiO 2 and RuS 2 /TiO 2 , it seems that the more negative the (vs NHE) is, the higher the PY is. The higher the energy barrier between the energy of the and the E° (H + /H 2 ) is, the higher the PY is. This energy barrier does not follow the evolution, it may confirm that H + reduction is not the rate-determining step. However, few meV distinguish the catalysts energy levels, it does not allow to conclude. To confirm those hypotheses, UPS, which has a better resolution for the valence band, is used. It should determine the with a better accuracy. and UPS (21.2 eV) do not provide the same information [24,25].

In a photoelectron spectroscopy process, photons are sent with an energy, [24]. This energy overcomes the binding energy, Eb, between the electron and its corresponding state. The electron must overcome the work function of the sample. As a result, the kinetic energy of the photoemitted electron is written:

7.6
The first difference is that, XPS excites deeper energetic states. It is possible to study a broad range of : core level, valence band and Auger transitions at the same time whereas UPS is limited only to the VB region (BE < 40 eV). The second one is that, X-ray are more energetic so photogenerated electrons have a higher (See equ. 7.6)) and thus, a longer path length. The information depth for XPS and UPS are around 10 nm and 2-3 nm, respectively. The third one is that, UPS source of radiation has a narrow line width and a high photon flux intensity. Resolution for XPS varies from 0.3 eV to 0.7 eV whereas for UPS it is lower [24]. The UPS set-up used in this work has a resolution of 0.2 eV; notice that literature reports apparatus with a resolution up to few meV [25,26].

Finally, photoemission cross section (i.e the probability of an electron of being ejected) is much higher in UPS than in XPS for the valence band states.

252

That is why XPS was used to get preliminary information on the valence band states during core levels analysis while UPS (He I discharge) was used to get more resolved pieces of information on the valence band electron density.

UPS on TiO 2 powder to measure

To interpret PHER efficiency based on its electronic structure, the latter must be determined for the photocatalytic nanopowders. As discussed in Section 1. The sputtered Ag foil was also used to estimate the resolution of the apparatus (Figure VII-12, right, inset). For such a purpose, two methods are reported in the literature, one by Schalf et al. [30] and another by Reinert et al. [24] The first one approximates the edge as three lines. The resolution is the space between the two intersections. The second one estimates the resolution as the full width at half maximum of a Gaussian fitting the Fermi edge.

With a pass energy of 10 eV and a spatial aperture of 110 μm, the resolution for UPS is estimated to be 0.2 ± 0.03 eV. As a comparison, the one measured by XPS is higher than 0.5 eV (See Annexes, Figure S26). On those substrates, powder was deposited with two methods.

For the first one, powders are ultrasonically dispersed in heptane or in pentane (4 g/L). Then, several drops are deposited on a conducting ITO substrate. This preparation method was proposed for zeolites [31] and further applied to other oxides such as V 2 O 5 or bismuth molybdate [32].

For the second one, a thin film of TiO 2 powder is prepared by electrophoretic deposition (EPD).

This technique was first used to prepare samples of dye-sensitized solar cells [33]. Particles of a stable suspension were deposited applying a DC electric field [34]. EPD duration, electrodes distance and composition of the solution can influence the thickness and the morphology of the film [35]. One of the most promising charging system is a mixture of iodine, acetone and water [36]. 50 mg of iodine was dispersed in 100 ml of acetone and stirred until the iodine particles were completely dissolved.

Then, 2-3 g/L of photocatalytic powder was sonicated in the charging solution for 10 min and leaved for approximately 1 h. The acetone follows a keton-enol reaction; it releases protons that adsorb at the surface of the TiO 2 particles. It acts as the driving force to reach the electrode. Charging solution was poured in a glass vessel containing ITO/glass substrates as cathode and anode with a separation distance of around 1 cm. A stabilized power supply (type elc AL 936N) established a constant voltage Based on the results obtained with the 2 nd derivative method, the first approach provides similar results, independently of the type of alkane, = 3.0 ± 0.2 eV and E cut-off = 17.5 ± 0.2 eV (See Figure . The second approach provides the same but an E cut-off 0.3 eV lower. Three calibration methods could be used: spectra are all shifted to the same E cut-off and difference of are discussed; spectra are all shifted to the same and difference of E cut-off are discussed; as the experimental setup was already calibrated, spectra are discussed as-obtained.

The main drawback of preparing powder samples is that it requires solvent. Residues may induce an interfacial dipole between the substrate and the powder. It may globally shift the spectrum [30].

This is why the third way is irrelevant; spectrum must be calibrated post-acquisition.

The first method can be argue as follow. The analyzer detects photons only if the vacuum chamber, the analyzer and the sample are in contact. So the Fermi levels and therefore their E cut-off (See equ. 7.7 are equalized. E cut-off of the samples, analyzed on the same substrate, can be aligned and relative position of the valence band states can be discussed. This method was applied on molecular system in [37] and on oxysulfide powders [38].

The more reliable way to get the absolute energy of the maximum of the valence band is to have an internal calibration. With an appropriate dropping of the alkane-suspension on Ag foil it is In addition, to drop the alkane suspension, the clean Ag foil must be taken out of the XPS chamber and so, being exposed to the atmosphere. The UPS spectrum of such oxidized Ag was performed (See Annexes, Figure S27). Valence band states are modified but the position of the Fermi level is unchanged.

A similar attempt was made with an Au foil but the TiO 2 states was not well resolved on the spectrum combining the TiO 2 and the Au states. Valence band states of titania powders UPS is a three-step process [39]. First, an incoming photon is absorbed by the material and excites the latter. Second, the photogenerated electron travels to the surface of the sample and it is ejected into vacuum. Finally, an analyzer collects electrons by a finite acceptance angle measuring their E kin. .

During the trajectory to the surface, electrons may suffer from inelastic collisions reducing their E kin . Electrons collected without inelastic collision are called primary electrons and the other, secondary electrons. These two types of electrons provide different pieces of information. Primary electrons, being not altered by other states allow determining the binding energy of the electronic state while secondary electrons form the background. The resulting spectrum is a superposition of the two electron populations.

In UPS, the energy of the incident photons is 21.2 eV. As a result, the E kin. of the photoemitted electrons is low (See equ. 7.6)), they are more willing to suffer from inelastic collisions. Proportion of secondary over primary electrons is therefore much higher than for XPS (hν = 1464.4 eV). So secondary electrons will significantly contribute ( to the valence band spectrum ( .

Their contribution must be removed to identify the true density of states ( .

8

In the literature, different types of background functions are reported for valence band subtraction. Fleming et al advised a "a + cxd" form [40], Laidani and co-workers advocated a Shirley type background [41][42][43] and Payne et al. used a Tougaard type background [44]. Those backgrounds suit for higher photon energy, so-called core-level spectra, for which the secondary electrons contribute less. That is why Henrich and co-workers developed a background dedicated to UPS analysis [45]. It takes into account the energy dependence of the cross-section for the creation of secondary electrons by primary electrons. It also considers the secondary-electron cascade process, i.e.

secondary electrons which generate other secondary electrons. This phenomenon induces an exponential shape which is not embodied by standard core-level background.

This background depends on two parameters: , the energy above which the signal is close to zero and , the energy below which the spectrum is only composed of the background:

7. 9

Note that the energy position of does not strongly influence the background shape.

However, is chosen quite arbitrarily, few try and error procedures are needed. may induce few variations for quantification of the states but it does not influence their binding energy.

After background subtraction, spectra can be decomposed using Lorentzian Assymetric Finit Contribution can be assigned as follow. The state IV at 11 eV is due to surface OH groups [46].

Localized molecular orbital (MO) models [47], energy band diagram [48] and DOS calculations [48][49][50][51] were made on TiO 2 -anatase. Those works help to assign the three remaining contributions (I, II and III). Electrons from the non-bonding (π) O 2p orbitals induce I. The PDOS of Ti 3d increases only at higher energy [48,51]; the second and the third states are hybridized Ti 3d -O 2p orbitals.

Electrons from the π-hybridized orbitals induce II state and electrons from the bonding (σ) hybridized orbitals induce III state. with permission.

Density of each states can be determined for the three samples, prepared with the dropping methods and the EPD.

Figure VII-18:

The three energy levels diagrams for the relative positions of rutile and anatase. Type I is called included, Type II is called staggered. For the latter, energy levels of anatase can be more negative than rutile (Type II-anatase) or they can be less negative (Type II-rutile). Adapted from [1] with permission.

As the of anatase is more negative than the one of rutile, photogenerated holes have a higher oxidative potential in anatase than in rutile. As the of rutile is less negative than the one of anatase, photogenerated electrons have a higher reduction potential in rutile than in anatase. Anatase should be a better oxidant and rutile should be a better reductant.

All the PHER tests discussed in this manuscript were performed in a photoreactor made of pyrex glassware; it is often used in laboratories. Such glassware filters the incoming photons below 290 nm.

So, photons reaching the photocatalysts have an energy below 4.3 eV. This allows defining an energy threshold (E thr. ) corresponding to the deepest energy that can be photoexcited in the photoreactor (See Anatase and rutile can be mixed in photocatalytic powder. This is the case in P25, the famous commercial photocatalyst [53]. If the relative position of anatase and rutile are not modified when the heterostructure is formed, the electrons should migrate from rutile to anatase and the holes from anatase to rutile ( S33). Current applied to deposit charging particles damaged the RuS 2 phase.

On the contrary, RuS 2 -based samples were successfully prepared with the dropping method. It is therefore used for electronic structure analysis.

UPS analyses on RuS 2 -based powders

For UPS interpretation, the same methodology than the one reported in Section 7.2.2 is applied.

A Henrich-type background is subtracted from the recorded spectra (I tot ). Such inconstancy reinforce the hypothesis of an interfacial dipole that prohibits discussing E cut-off value, with this preparation method, and that absolute energy levels calibration is mandatory.

Conclusions and discussion

This chapter aims to determine and to understand the electronic structure of transition metal sulfides supported on TiO 2 .

Thanks to photoelectron spectroscopy, the valence band maximum is measured. XPS gives the with an accuracy of 0.5-0.8 eV. It can be performed simultaneously with core levels analyzes.

UPS gives the with a better accuracy, 0.2 eV than XPS. Photocatalytic materials are powder to analyze their valence band with UPS, it requires preparation of powder layer, two methods were compared. This is not needed for XPS analysis.

A way to get an accurate absolute position with UPS, is to prepare an inhomogeneous layer of powder on an Ag foil. UPS spectrum thus contains both powder and Ag states. Those of the latter are well-defined and well-positioned, they are used as an internal calibration.

UV-Vis diffuse reflectance spectroscopy is combined with the Kubella-Munk theory to obtain UV-Vis absorption spectra. Thanks to them, the band bap of a semiconductor is estimated. The band gap of TiO 2 powders (3.15 eV for anatase and 3.0 eV for rutiles) as well as the seven supported sulfides were determined. For instance, RuS 2 and MoS x have an indirect band gap of 1.0 and 1.2 eV, respectively.

Combining XPS and UV-Vis diffuse reflectance spectroscopy gives the and the of RuS 2 /TiO 2 , NiS 2 /TiO 2 , MoS x /TiO 2 and HgS/TiO 2 relatively to TiO 2 .

It demonstrates that the electronic transfer is favored from TiO 2 to NiS 2 , MoS x or HgS. For the latter, electrons can transfer to HgS particles but their energy is not enough to reduce H + .

For RuS 2 , its conduction band is higher than TiO 2 ; the electron transfer is not favored. The photon yield is not the only parameter used to estimate the PHER activity of the photocatalysts. In this work we proposed to measure two kinetic parameters an apparent activation energy ( ) and a pre-exponential factor ( ). Both are obtained from an Arrhenius law that correlates the PY to the temperature.

Thos three parameters are therefore used to investigate the mechanism for the photocatalytic dehydrogenation of propan-2-ol with MS x /TiO 2 photocatalysts. The mechanism is depicted below.

Main Scheme 6: Mechanism for the photocatalytic dehydrogenation of propan-2-ol with MS x /TiO 2 :

the electronic transfer between the TiO 2 and the MS x are the key steps.

Chapter 4 discusses about the appropriate reaction for evaluating the efficiency of a given catalyst. For the photoconversion of an alcohol-water mixture, it is not always clear if it is a dehydrogenation or a reforming process that occurs. In the first case, the efficiency of photocatalyst is measure for one single step while in the second case, several reactions occur. The photocatalytic conversion of a propan-2-ol-water mixture is proposed as a model reaction to study the PHER activity of the MS x /TiO 2 . Only the dehydrogenation occurs: propan-2-ol is converted in acetone and hydrogen.

The mechanism for this reaction is established and macro-/micro-kinetic models are proposed. It evidences the conversion of propan-2-ol follows a dissociative Langmuir-Hinshelwood kinetic. It results that depends on the propan-2-ol concentration. A 50%vol. of propanol dilute in water is therefore used as the standard reactive mixture.

It is believed that for the same reaction, the variation of the PHER activity of the MS x /TiO 2 should not be due to the oxidation half-reaction ( on Main Scheme 6). Studying the PHER activity of MS x /TiO 2 , for the photocatalytic dehydrogenation of propan-2-ol is therefore a way to understand the activity of the MS x for the production of H 2 .

281

This work aims to compare the influence of the intrinsic activity of the transition metal sulfides on the overall PHER activity. To be reliable, the MS x must receive the same amount of reagent (i.e. photogenerated electrons) to produce H 2 . Section 6.1.1 shows that at low lamp intensity the PHER rate linearly evolves with the incident flux of photons. Above a threshold, , it is therefore advised to calculate an enhancement factor (i. e. the ratio between PY (MS x /TiO 2 ) and PY (TiO 2 )). The incident flux of photons is always measured to compare the PHER activities of the MS x co-catalyst.

Section 5.3.1 evidences that TiO 2 is the only semiconductor responsible of photon absorption ( on Main Scheme 6). Therefore, in Chapter V, seven MS x are supported on the same commercial TiO 2 (anatase), with the same thioacetamide reflux preparation method. The latter generated the amount of electron-hole pairs to produce H 2 .

The PHER activity of a photocatalyst is influenced by the mass of photocatalyst in the photoreactor. In Section 6.2 it is shown that the relationship between the two parameters depends only the TiO 2 properties. Again, supporting the MS x on the same commercial TiO 2 (anatase) and measure PHER activity at a similar mass is a reliable way to compare co-catalysts.

In summary, measuring the PY, the and the of MS x /TiO 2 photocatalysts, with the same TiO 2 support, with the same mass of catalyst and under a known flux of photons is an appropriate way to compare the intrinsic activity of MS x .

The kinetic model, reported in Section 4.2, mainly concludes that the PY depends on two rate constants, the one for the transfer of the electrons from the TiO 2 to the MS x ( on Main Scheme 6) and the one for the H + reduction ( on Main Scheme 6). The rate-determining step cannot be the H-H coupling ( on Main Scheme 6).

It is confirmed experimentally in Chapter 5. The latter reports a screening of seven MS x /TiO 2 .

Thiophene HDS estimates the ability of the MS x to activate H 2 . values are also measured for the photocatalytic dehydrogenation of propan-2-ol. It is shown that the ability to activate H 2 has no influence on and therefore the H-H coupling is not limiting the PHER activity.

282

Structural properties of the catalyst as well as their electrochemical behavior were estimated.

First structure properties were obtained thanks to TPR, microscopy and XPS analyses, for instance.

The second was determined with the electrocatalytic HER. No direct correlation was found between the PHER activity of MS x /TiO 2 and the electrochemical properties of MS x .

Regarding the morphology, it is evidence that depends on the amount of active sites. It therefore depends on the co-catalyst loading, on the mass of photocatalyst, on the size of the cocatalytic particles but also on the amount of electrons reaching the co-catalytic particles. Indeed the true active sites are the therefore, their concentration influence . All the parameters influencing

are not yet identified and understood. For instance the work made on the influence of the size of the RuS 2 particles on the PHER activity should be completed.

It is shown that the electronic structure is crucial in a photocatalytic process.

The study of HgS/TiO The highest initial activity is obtained with pure methanol but the catalyst rapidly deactivates.

The PY decreased four times between the beginning of the test and after 20 h (See Figure S4). It is assumed that methanol or intermediates adsorb at the surface of the active sites, blocking the hydrogen production. Water may help to remove such intermediates. To confirm such a hypothesis, water could be added after the deactivation of the catalyst. Characterizations of the catalyst recovered after test are also needed, such as XPS. Another alcohol screening was performed with a RuS 2 /TiO 2 (1.12 wt% Ru) catalyst (See Figure S5). Photoconversion of methanol, ethanol and propan-2-ol (dilute in water) are compared from a thermodynamic and a kinetic point of view. The Gibbs free energy ( °) for the dehydrogenation of methanol, propan-2-ol and ethanol equal 65.3, 42.2 and 25.2 kJ/mol at 20 °C, respectively [1].

From a thermodynamic point of view, a hypothesis would be that the higher ° is, the more difficult it is to cross the thermodynamic energy barrier. Consequently, the rate of hydrogen production should be higher for the dehydrogenation of the propan-2-ol. Figure S5 (left) disproves it with PY (methanol-water) ≈ 2 PY (propan-2-ol-water). Concerning the Gibbs free energy, an interesting parameter to calculate is the power of chemical energy ( ) stored during a photocatalytic process; is the rate of production:

° °
And also the electrical to chemical energy efficiency:

is the electrical power of the lamp.

With RuS 2 /TiO 2 , at 20 °C and , (methanol), (ethanol) and (propan-2-ol) equal 1.00 μW, 0.5 μW and 0.2 μW respectively. For such a reason, the dehydrogenation of methanol would be more suitable for industrial storage of solar energy in hydrogen. Notice that depends on the flux of photons. The latter depends on the electrical efficiency of the light, converting a current in a flux of photons. To increase , the photocatalyst must be optimized as well as the optoelectronic set-up.

Full cycle analysis should take into account the electrical power used to regulate the temperature.

An optimum should found between increasing the temperature to increase the rate of hydrogen production and limiting the electrical power used for the cryostat.

The manuscript brings mostly a fundamental point of view on the photocatalytic mechanism.

This study is out of the scope of the manuscript. Figure S11 reports of the distribution of the diameter ( ). A mean diameter of 2.7 ± 0.6 nm is estimated but, but the Sauter mean diameter [7] can be also calculated:

It equals 3.06 nm. In addition, for a fcc metal such as Pt, the dispersion ( ) is then deduced with [7]: Figure S16 shows that the atomic surface concentration of the sulfur from thioacetamide impurities is much lower than the sulfur states from the supported RuS 2 . It is therefore consistent to neglect the thioacetamide impurities when discussing the surface concentration of the sulfide states eV, the μ-GC detects hydrogen but its peak is below the detection limit and cannot be quantified. Pt/TiO 2 and RuO x /TiO 2 were prepared with the photodeposition method. Precursor salts of H 2 PtCl 6 .xH 2 O and RuCl 3 .xH 2 O were used for the two respective photocatalyst. A complete reduction of the Ru cation is unlikely so it might be in an oxide state. Their PHER activity is compared with the best photocatalyst of the seven MS x /TiO 2 screening: a RuS 2 /TiO 2 catalyst (See Figure S19). Samples were prepared by the EPD method. Reproduced with permission from [12].

It

Figure I- 1 :

 1 Figure I-1: Energetic diagram for an exergonic reaction (left) and an endergonic reaction (right). .....

Figure I- 2 :

 2 Figure I-2: Carbon dioxide emissions by sector, in France, in 2016. Adapted from [39]. ....................

Figure I- 3 :

 3 Figure I-3: Biomass-derived oxygenated molecules. Reproduced from [59] with permission. ............

Figure I- 4 :

 4 Figure I-4: Schematic mechanism of the photocatalytic reforming of propan-2-ol with Pt/TiO 2 .

  with permission. ................................................................................................

Figure I- 5 :

 5 Figure I-5: Element Scarcity Periodic Table from the EuChemS: "The 90 natural elements that make up everything. How much is there? Is that enough?" [144]. .................................................................

Figure II- 1 :

 1 Figure II-1: Mechanism for the thiophene hydrodesulfurization process: from thiophene to butene or butane. Reproduced from [3] with permission. .....................................................................................

Figure II- 2 :

 2 Figure II-2: Rate for the hydrodesulfurization of dibenzothiophene plotted as a function of the binding energy between metal and sulfur. Reproduced from [4] with permission. ............................................

Figure II- 3 :

 3 Figure II-3: Electronic structure of TiO 2 -anatase and seven transition metal sulfides. Data from[32].

Figure II- 4 :

 4 Figure II-4: PHER activities of MS x /ZnIn 2 S 4 /Pt (left) and MS x /CuGa 3 S 5 (right) from aqueous solution

  with permission. ..................................................

Figure II- 5 :

 5 Figure II-5: Band diagrams and the proposed electron transfer for (left) RuS 2 /TiO 2 and for (right)

Figure III- 1 :

 1 Figure III-1: Photocatalytic set-up.......................................................................................................

Figure III- 2 :

 2 Figure III-2: Irradiance spectrum for the HPK lamp , the LED and the Xe lamp. ..............................

Figure III- 3 :

 3 Figure III-3: Methodology to measure and for a given photocatalyst. ........................

Figure IV- 1 :

 1 Figure IV-1: Amount of substances detected during the photocatalytic conversion of a propan-2-olwater mixture.. .....................................................................................................................................

Figure IV- 2 :

 2 Figure IV-2: Amount of hydrogen produced (left) and of acetone produced (right) along 10 h.. .......

Figure IV- 3 :

 3 Figure IV-3: Initial rate of hydrogen (red circles) and of acetone (green triangles) production as a function of the initial concentration of propan-2-ol.. ..........................................................................

Figure IV- 4 :

 4 Figure IV-4: Amount of propan-2-ol (IPA), acetone (Ac.) and H 2 along the time. ...........................

Figure IV- 5 :

 5 Figure IV-5: Arrhenius plot with the photon yield measured at four temperatures for Pt/P25, RuS 2 /PC500, bare P25 and bare PC500. .............................................................................................

Figure IV- 6 :

 6 Figure IV-6: Apparent Arrhenius parameters ( and ) as a function of propan-2-ol concentration for its photocatalytic conversion with RuS 2 /TiO 2 (1.15 wt% Ru). ...............................

Figure IV- 7 :

 7 Figure IV-7: Amount of methane (left) and propane (right) produced along 16 h of test.. .................

Figure IV- 8 :

 8 Figure IV-8: The PHER activity of RuS 2 /TiO 2 , NiS 2 /TiO 2 and CoS x /TiO 2 for the conversion of propan-2-ol-water mixture.. .............................................................................................................................

Figure V- 1 :

 1 Figure V-1: Diffractogram of commercial CristalACTIV TM PC500, NiS x /TiO 2 (with 1.0 wt% Ni) and RuS x /TiO 2 (with 3.7 wt% Ru) ..............................................................................................................

Figure V- 2 :

 2 Figure V-2: XRD patterns of CoS x , RuS x and MoS x amorphous phases. Reproduced from [1] with permission. ..........................................................................................................................................

Figure V- 3 :

 3 Figure V-3: Diffractogram of bulk TMS (a: AgS x ; b: CuS x ; c: NiS x ; d: HgS x ). Reproduced from [1] with permission. ..................................................................................................................................

Figure V- 4 :

 4 Figure V-4: STEM (HgS x & RuS x ) & TEM (AgS x , NiS x , MoS x and CoS s ) images of selected MS x /TiO 2 . Reproduced from [1] with permission. ..............................................................................

Figure V- 5 :Figure V- 6 :

 56 Figure V-5: (black) TPR curve of MS x /TiO 2 with M=Ag (a), Ru (b), Cu (c), Ni (d), Co (e), Hg (f), Mo (g), Pt (h). (red) TPR curves of bulk MS x with M=Ag (a), Ru (b), and Cu (c). Adapted from [1] with permission. ................................................................................................................................... 159-160 Figure V-6: Fit of the TPR curve of RuS 2 /TiO 2 .. ................................................................................

Figure V- 7 :

 7 Figure V-7: Metal content determined with chemical analysis (ICP-OES) versus surface metal content determined with XPS...........................................................................................................................

Figure V- 8 :

 8 Figure V-8: XPS spectra for HgS/TiO 2 . ..............................................................................................

Figure V- 9 :

 9 Figure V-9: XPS spectra of the unsupported ruthenium sulfide. ........................................................

Figure V- 10 :

 10 Figure V-10: XPS spectra for bulk (down) and supported (up) RuS x . Reproduced from [1] with permission. ..........................................................................................................................................

Figure V- 11 :

 11 Figure V-11: XPS Ni 2p spectra for bulk (down) and supported (up) NiS x .. ......................................

Figure V- 12 :

 12 Figure V-12: XPS spectra for supported MoS x : Mo 3d -S 2s (left), S 2p (right).. ............................

Figure V- 13 :

 13 Figure V-13: Action spectra of NiS 2 /TiO 2 (left) and RuS 2 /TiO 2 (right). .............................................

Figure V- 14 :

 14 Figure V-14: Photon Yield of the seven MS x /TiO 2 (2.6-3.0 metal atoms per 10 nm 2 ) and the supports (TiO 2 is the as-purchased PC500 and TiO 2 * is the thioacetamide treated TiO 2 ).................................

Figure V- 15 :

 15 Figure V-15: The PHER activity of four MS x /TiO 2 (MoS x , HgS, NiS 2 and RuS 2 ) and the reference TiO 2 *.. .................................................................................................................................................

Figure V- 16 :

 16 Figure V-16: (left) The PHER activity of the seven MS x /TiO 2 and the reference TiO 2 * along five temperature cycles. Reproduced from [1] with permission. (right) is determined measuring the PY at 10, 20, 35 and 45 °C. ...........................................................................................................

Figure V- 17 :

 17 Figure V-17: Amount of hydrogen disulfide produced as a function of: (a) the amount of hydrogen produced in 2 h by each MS x /TiO 2 , (b) the co-catalyst weight loading, (c) the mass of photocatalyst, (d) the photon yield along the time for two NiS 2 /TiO 2 and two TiO 2 catalysts...................................

Figure V- 18 :

 18 Figure V-18: TPR curve of NiS 2 /TiO 2 before and after a PHER test. .................................................

Figure V- 19 :

 19 Figure V-19: ATR -IR spectra of TiO 2 and NiS 2 /TiO 2 before and after a PHER test. .......................

Figure V- 20 :

 20 Figure V-20: XPS spectra of MoS x /TiO 2 , RuS 2 /TiO 2 and NiS 2 /TiO 2 before (down) and after (up) the photocatalytic dehydrogenation of propan-2-ol. Reproduced from [1] with permission. ...................

Figure V- 21 :

 21 Figure V-21: The PHER rate as a function of time for 50 mg of RuS 2 /TiO 2 ......................................

Figure V- 22 :

 22 Figure V-22: The PHER activity (PY and on the left; PY and on the right) of RuS 2 /TiO 2 during seven cycles.. .........................................................................................................

Figure V- 23 :

 23 Figure V-23: XPS spectra of RuS 2 /TiO 2 before test (down), after the long time experiment (middle) and after seven tests (up). ....................................................................................................................

Figure V- 24 :

 24 Figure V-24: Reaction rate of the MS x /TiO 2 for the thiophene hydrodesulfurization. Adapted from [1] with permission. ..................................................................................................................................

Figure V- 25 :

 25 Figure V-25: Electrocatalytic HER activity of the seven bulk sulfides with on the left, the Linear Sweep Voltammetry curves and on the right, the Tafel plots. Reproduced from [1] with permission. .............................................................................................................................................................

Figure V- 26 :

 26 Figure V-26: (left) Linear sweep voltammetry curves (right) cyclic voltammetry. From top to bottom curves are obtained for: RuS 2 , NiS 2 , MoS x ..........................................................................................

Figure V- 27 :

 27 Figure V-27: Schematic representation of the photogenerated electrons reacting on the TiO 2 particles while they travel to the HgS particles, attracted by the band alignment. Adapted from [1] with permission. ..........................................................................................................................................

Figure VI- 1 :

 1 Figure VI-1: The PHER activity of RuS 2 /TiO 2 (1.15 wt% Ru) as a function of the incident flux of photons. . .............................................................................................................................................

Figure VI- 2 :

 2 Figure VI-2: Irradiance measurements performed at different spot of the photoreactor. ....................

Figure VI- 3 :

 3 Figure VI-3: Photon yield and Arrehnius paramaters ( on the left; on the right) measured at three photon energies for the photocatalytic dehydrogenation of propan-2-ol with RuS 2 /TiO 2 . .....

Figure VI- 4 :

 4 Figure VI-4: The PHER activity of RuS 2 /TiO 2 (0.33 wt% Ru) as a function of the mass ..................

Figure VI- 5 :

 5 Figure VI-5: The PHER activity of RuS 2 /TiO 2 (40 mg) as a function of the co-catalyst loading. ......

Figure VI- 6 :

 6 Figure VI-6: (left) Contribution of the two "" terms (See equ. 6.9) to the overall PY .....

Figure VI- 7 :

 7 Figure VI-7: Microscopy images for RuS 2 /TiO 2 : STEM for thioacetamide reflux method and TEM for impregnation & sulfidation method.....................................................................................................

Figure VI- 8 :

 8 Figure VI-8: The PHER activity of RuS 2 /TiO 2 prepared with the impregnation and sulfidation method (2.1 nm) or with the thioacetamide reflux method (1.3 nm).. .............................................................

Figure VI- 9 :

 9 Figure VI-9: The PHER activity of bulk RuS 2 deposited on TiO 2 support (anatase, rutile, brookite, and mixed phases).. ....................................................................................................................................

Figure VII- 1 :

 1 Figure VII-1: UV-Vis absorption spectra for TiO 2 -anatase diluted in BaSO 4 . Adapted from [1] with permission. ..........................................................................................................................................

Figure VII- 2 :

 2 Figure VII-2: (left) Normalized Kubelka-Munk functions for bare TiO 2 and the seven MS x /TiO 2 (0.1 wt% of powder diluted in BaSO 4 ) ................................................................................................

Figure VII- 3 :

 3 Figure VII-3: (left) Plot of (α) 1/2 versus photon energy for TiO 2 -anatase diluted with BaSO 4 (390 ppb).

  with permission. ...............................................................................

Figure VII- 4 :

 4 Figure VII-4: Transitions density t versus the incident photon energy for TiO 2 -anatase diluted in BaSO 4 (390 ppb). Reproduced from [1] with permission. ..................................................................

Figure VII- 5 :

 5 Figure VII-5: Tauc plot with an exponent 0.5 for supported NiS 2 , CuS and CoS x on the left and supported Ag 2 S, HgS, RuS 2 and MoS x on the right.. ..........................................................................

Figure VII- 6 :

 6 Figure VII-6: Tauc plot for the RuS 2 /TiO 2 photocatalyst. ..................................................................

Figure VII- 7 :

 7 Figure VII-7: Kraut method [17] applied to RuS 2 /TiO 2 , bulk RuS 2 and TiO 2 . ...................................

Figure VII- 8 :

 8 Figure VII-8: Band structure diagrams for commercial TiO 2 -anatase, RuS 2 /TiO 2 heterostructure and bulk RuS 2 . ............................................................................................................................................

Figure VII- 9 :

 9 Figure VII-9: XPS spectra for commercial TiO 2 -anatase, NiS 2 /TiO 2 , RuS 2 /TiO 2 , HgS/TiO 2 and MoS x /TiO 2 heterostructures. ................................................................................................................

Figure VII- 10 :

 10 Figure VII-10: Band structure diagrams for commercial TiO 2 -anatase, NiS 2 /TiO 2 , RuS 2 /TiO 2 , HgS/TiO 2 and MoS x /TiO 2 heterostructures. ........................................................................................

Figure VII- 11 :

 11 Figure VII-11: Photoionization cross section of the O 2p orbitals, as a function of the incident photon energy. Data extracted from [27].........................................................................................................

Figure VII- 12 :

 12 Figure VII-12: Full He I UP spectrum (black line) and its first derivative (red square) of ITO substrate (left) and Ag foil (right). Inset shows the Fermi edge of Ag and the determination of the apparatus resolution. Adapted from [1] with permission. ...................................................................................

Figure VII- 13 :

 13 Figure VII-13: He I UP spectrum of the TiO 2 -anatase on ITO substrate. Adapted from [1] with permission. ..........................................................................................................................................

Figure VII- 14 :

 14 Figure VII-14: (left) He I UP spectrum of the TiO 2 -anatase on Ag foil. Sample was prepared dropping deliberately small amount of TiO 2 -heptane suspension. Reproduced from [1] with permission. (right) energy diagram determining the absolute position. ....................................................................

Figure VII- 15

 15 Figure VII-15: Resulting (black line) for TiO 2 -anatase samples, prepared by the drop of a TiO 2 -heptane suspension. Reproduced from [1] with permission .......................................................

Figure VII- 16 :

 16 Figure VII-16: (left) obtained after subtraction of I back with the Henrich or the Tougaard background (right) Area under the four contributions fitting the (red) and (blue). Reproduced from [1] with permission. ..................................................

Figure VII- 17 :

 17 Figure VII-17: Absolute electronic structure of the two commercial TiO 2 powders (anatase and rutile). Adapted from [1] with permission. .....................................................................................................

  ..................................................................................................................................

Figure VII- 19

 19 Figure VII-19: Resulting (grey line) for bulk RuS 2 (left) and RuS 2 /TiO 2 (right), prepared by the drop of a powders-pentane suspension. .........................................................................................

Figure VII- 20

 20 Figure VII-20: Resulting (grey line) for bare TiO 2 , RuS 2 /TiO 2 (1.5 wt% Ru) and RuS 2 /TiO 2 (6.2 wt%). ............................................................................................................................................

Figure VII- 21 :

 21 Figure VII-21: Absolute band structure diagrams and the valence band states for commercial TiO 2anatase (left), bulk RuS 2 (right) and the resulting RuS 2 /TiO 2 heterostructure (middle). .....................

  photocatalysis and photosynthesis. The first concerns exergonic reactions (downhill reactions:

Figure I- 1 .

 1 photocatalytic reaction can be endergonic or exergonic, as schemed on Figure I-1. Solar fuels are

Figure I- 1 :

 1 Figure I-1: Energetic diagram for an exergonic reaction (left) and an endergonic reaction (right).

Figure I- 3 :

 3 Figure I-3: Biomass-derived oxygenated molecules. They are classed into generic families (in italics,

  gas phase chromatography and FT-IR analysis to propose the mechanism sketched on Figure I-4. The propan-2-ol chemisorbed at the surface of TiO 2 giving the 2-propoxide. The latter reacts with a hole to generate a proton and the acetone. An breaks the C-C bond of the acetone and yields to . CH 3 radical and acetic acid. The two products form CH 4 , CO 2 and ethane. It should be noticed that the presence of is unclear. It could be due to the oxidation of water generating another proton but this is not discussed.

Figure I- 4 :

 4 Figure I-4: Schematic mechanism of the photocatalytic reforming of propan-2-ol with Pt/TiO 2 . (a) is

  first way to evidence a co-catalytic effect is to study the kinetic of the charge carriers with and without a co-catalyst. Time-resolved spectroscopies are powerful techniques to study the carrier dynamics in photocatalytic processes. Time-resolved photoluminescence (TRPL), time-resolved infrared, transient absorption spectroscopy (TAS) and time-resolved microwave conductivity (TRMC) are some examples.

Time-resolved infrared

  spectroscopy distinguishes free electrons in the conduction band and shallow trapped electrons. It also evaluates their respective life time. With this technique Chiarello et al. showed another role of the Pt co-catalyst. They evidenced new active sites at the Pt-TiO 2 interface

Mot- clés :

 clés Carburants solaires, Production photocatalytique d'H2, Déshydrogénation d'alcools, Sulfures de métaux de transitions, Dioxyde de titane, Structure électronique, UPS, RuS2/TiO2 Summary: Study of Titania supported transition metal sulfides for the photocatalytic production of hydrogen.

  photocatalytic hydrogen production with transition metal sulfidesbased catalysts.

59

 59

Figure II- 1

 1 Figure II-1 evidences the mechanism for thiophene HDS. After HDS, the MS x may catalyze the

Figure II- 1 :

 1 Figure II-1: Mechanism for the thiophene hydrodesulfurization process: from thiophene to butene or

Figure II- 2 :

 2 Figure II-2: Rate for the hydrodesulfurization of dibenzothiophene plotted as a function of the

Figure II- 3 :

 3 Figure II-3: Electronic structure of TiO 2 -anatase and seven transition metal sulfides. A is the electron

Figure II- 5 (

 5 Figure II-5 (left), Hara et al. argued that electrons transfer from RuS 2 to TiO 2 and holes from TiO 2 to

Figure II- 5 :

 5 Figure II-5: Band diagrams and the proposed electron transfer for (left) RuS 2 /TiO 2 and for (right)

5

 5 Cd 0.5 S Na 2 SO 3 / Na 2 S r H2 (CoS x /Mn 0.5 Cd 0.5 S) = 2.4 r H2 (Mn 0.5 Cd 0.5 S) r H2 (CoS x /CdS) = 3.6 r H2 (CdS) AQY = 2.35 % (λ = 420 nm) b [89]

Figure III- 1

 1 Figure III-1 depicts the photocatalytic set-up. It is composed of a semi-batch slurry reactor filled

2 . 4

 24 has already shown that the oxidizing molecule (and its oxidation potential, equ. 3.15, influence the PHER rate. Concentration of such a molecule has also an influence. It may follow a Langmuir-Hinshelwwod behavior [19]. In addition, some molecules can influence the pH of the solution. The designation of "sacrificial agent" hides how much it influences the PHER rate. It means that from one reactive mixture to another the efficiency of a given material vary. Few alcohols-water mixture are screened in Chapter IV.Otherwise, the same one is used all along this work. It is assume that if the alcohol to oxidize is the same and if activity variations are observed, it must be due to a change in the reduction half-reaction, the one of interest.

Figure III- 1 :

 1 Figure III-1: Photocatalytic set-up.

2 ,

 2 compose the chromatograph. From the beginning of the Ph.D. project to March 2019, an OV1 was used to analyze the alcohols and the ketones. Since, it was replaced by a Wax to have a better separation between the molecules. It gives a careful quantification of the molecules for the mechanistic study details in Chapter IV.

  Figure III-2 show their respective irradiance spectra. It is obtained with a radiometer device: a

Figure III- 2 :

 2 Figure III-2: Irradiance spectrum for the HPK lamp , the LED and the Xe lamp (from left to right).

  photoreactor and because of shadowing effect of the co-catalytic particles, it is impossible to measure

3 . 22 corresponds

 322 to 3.2 eV (390 nm) for the anatase and 4.3 eV (equivalent to 290 nm) is the threshold for glassware filtration.

Figure III- 3 :

 3 Figure III-3: Methodology to measure and for a given photocatalyst, case study of

3 . 27 A

 327 Thermo Prolab quadrupole mass-spectrometer detects the H 2 S produced. No calibration of the quadruple detector was made. The amount of gases released is arbitrary. Interpretation of sulfides TPR curves is rather phenomenological; TPR is used as a fingerprint technique and informs on the chemical nature of supported sulfide. It is composed of multiple well-separated reduction maxima. At low temperature, H 2 reduces the weakly bonded sulfurs.

28 F

 28 Photoelectron Spectroscopy (UPS). XPS is performed with a monochromatized Al source, having

123 4. 1

 1231 Therefore, for stability and simplicity reasons, this work compares the PHER activity of MS x /TiO 2 catalysts for the dehydrogenation of propan-2-ol. Photocatalytic conversion of a propan-2-ol-water mixture 4.1.1 Photocatalytic dehydrogenation of propan-2-ol The μ-GC detects six components during the photocatalytic conversion of a propan-2-olwater mixture with Pt/TiO 2 and five components with RuS 2 /TiO 2 . In both cases, the propan-2-ol is mainly dehydrogenated to acetone and hydrogen, following this overall reaction (with A the cocatalyst: Pt or RuS 2 ):

  for propan-2-ol consumption are not satisfying. Almost five molecules are consumed for one acetone and one H 2 produced. The calibration methodology might be inappropriate because of the high boiling point of the alcohol (T eb, propan-2-ol = 82.5 °C). Even at low Ar-flow, the equilibrium is not completely established for the propan-2-ol giving an approximate calibration. Therefore, Figure IV-1 depicts the relative consumption of the propan-2-ol and the absolute production of hydrogen and acetone along several days on-stream.

Figure IV- 1 :

 1 Figure IV-1: Amount of substances detected during the photocatalytic conversion of a propan-2-ol-

  acetaldehyde (A. U.) Amount of propan-2-ol (A. U.) propan-2-ol (A. U.)

Figure IV- 2 :

 2 Figure IV-2: Amount of hydrogen produced (left) and of acetone produced (right) along 10 h. The

Figure IV- 3

 3 Figure IV-3 shows that the rate of hydrogen and acetone production increases as a function

Figure IV- 3 :

 3 Figure IV-3: Initial rate of hydrogen (red circles) and of acetone (green triangles) production as a

Figure

  Figure on the left was obtained with Pt/TiO 2 (1.0 wt% Pt, P25, , T=40 °C)

129 Figure

 129 Figure IV-4 shows the consumption of propan-2-ol along the time for three different

Figure IV- 4 :

 4 Figure IV-4: Amount of propan-2-ol (IPA), acetone (Ac.) and H 2 along the time. Three photocatalytic

10 Figure

 10 photoreactor's temperature; PHER rates (or photon yields) were measured at four temperatures

Figure IV- 5 : 133 should

 5133 Figure IV-5: Arrhenius plot with the photon yield measured at four temperatures for Pt/P25,

Figure IV- 6 )

 6 Figure. It aims eliminating the possibility that an insufficient concentration limits the overall

Figure IV- 6

 6 Figure IV-6 evidences that the concentration has no influence on the and on the .

Figure IV- 6 :

 6 Figure IV-6: Apparent Arrhenius parameters ( , red squares and , blue squares) as a

135 In

 135 Photon absorption

2 and propene and acetaldehyde for RuS 2 /

 2 TiO 2 . They are all produced in negligible quantities but their formation can still be explained. It should also be noticed that none of those products are observed in dark conditions. Acetaldehyde is due to the oxidation of the C 2 carbon instead of the C 1 carbon bearing the hydroxyl group. At the same time, the CH 3 radical reacts with H 2 giving methane. CH 4 is therefore a secondary product that requires prior production of H 2 . It agrees with the convex function observed on the CH 4 evolution curves on Figure IV-7 (left): are the reaction centers and and are the adsorption sites. Regarding the Pt/TiO 2 photocatalytic system, methane and propane are produced. Figure IV-7 reports their evolution at four temperatures. It gives apparent activation energies of 8 kJ/mol for the propane formation and of 11 kJ/mol for the methane formation. Those values are close to the one of the H 2 production Figure IV-5).

Figure IV- 7 :

 7 Figure IV-7: Amount of methane (left) and propane (right) produced along 16 h of test. The

  0

Figure IV- 8 : 140 4. 2

 81402 Figure IV-8: The PHER activity of RuS 2 /TiO 2 , NiS 2 /TiO 2 and CoS x /TiO 2 for the conversion of

(

  50 %vol. of propan-2-ol in water) to keep the coverage constant and to study other parameters influencing the .The micro-kinetic study concludes on the following expression for the PHER rate:It does not depend on the H-H coupling step ( or on the Main Scheme 3) but either on the electron transfer step or on the proton reduction step. To confirm such hypotheses and to identify the rate-determining step, seven MS x /TiO 2 with different electrochemical behavior and electronic structures are studied in the following chapters.Chapter IV proposes the photocatalytic dehydrogenation of propan-2-ol as a good descriptor of the PHER activity. For the same reaction and the same support, several transition metal sulfides were screened in Chapter V to establish a correlation between the PHER activity of MS x /TiO 2 and their intrinsic properties. Chapter VI reports the influence of five parameters: the incident flux of photons, the mass of photocatalyst, the co-catalyst loading, the size of the co-catalytic particles and the type of TiO 2 support for the same photocatalyst: RuS 2 /TiO 2 . It provides additional pieces of information on how those parameters influence the Arrhenius parameters: and . x /TiO 2 were synthesized with the same preparation method providing a whole range of properties. They were tested for the photocatalytic dehydrogenation of propan-2-ol (Section 5.3), the thiophene hydrodesulfurization (HDS) (Section 5.5.1) and the electrocatalytic hydrogen evolution reaction (HER) (Section 5.5.2) in order to study various facets of the photocatalytic mechanism and to propose structure-activity relationships. A main part of this work is published in[1].

Figure V- 1

 1 shows diffraction patterns for NiS x /TiO 2 , RuS x /TiO 2 and TiO 2 only. For the three, diffraction peaks are indexed as anatase phase (PDF 00-021-1272). When the NiS x or the RuS x co-catalysts are deposited on TiO 2 with the thioacetamide reflux method, it does not modify the XRD pattern of TiO 2 . This preparation method drop-off the co-catalyst at the TiO 2 surface and does not dope TiO 2 . Scherrer equation (Section 3.3.1) estimates the crystallite size for the anatase phase around 7 nm.

Figure V- 1 :

 1 Figure V-1: (black) Diffractogram of commercial CristalACTIV TM PC500 ; (blue) Diffractogram of

Figure V- 3 :

 3 Figure V-3: (black) Diffractogram of bulk TMS (a: AgS x ; b: CuS x ; c: NiS x ; d: HgS x ) (blue & red) Miller indexes based on matching patterns (a: Ag 2 S; b: CuS; c: NiS 2 & S; d: HgS). Reproduced from [1] with permission.

Figure V- 4

 4 Figure V-4 shows microscopy images of the supported sulfide catalysts. It reveals that dispersion

Ag 2 S

 2 and NiS 2 are poorly dispersed with a size of particles around 100 nm. HgS x and MoS x particles give one order of magnitude with smaller particles (10 nm range). MoS x particles are amorphous and the electron beam rapidly transformed it, into MoS 2 fringes. Finally, in the case of copper (See Annexes, FigureS12), cobalt and ruthenium sulfides, microscopy analysis evidenced a high dispersion. HRTEM and STEM did not evidence CuS x and CoS x particles. Species are ultradispersed of subnanometer size and evidenced by EDX. On the contrary, the relatively high mass of Ru allows detecting particles with HREM. Size of clusters were measured to be 1-1.5 nm.Those results can be compared with the size of bulk particles estimated with Scherrer equation (See Section 5.2.1). It shows that supported NiS 2 particles are larger and supported CuS particles are smaller than their bulk counterparts.

Figure V- 4 :Figure V- 5 :

 45 Figure V-4: STEM (HgS x & RuS x ) & TEM (AgS x , NiS x , MoS x and CoS s ) images of selected MS x /TiO 2 . Reproduced from [1] with permission.

  For instance, m/z = 27 and m/z = 13 correspond to HCN and CH fragment, respectively ( and ratios vary as a function of the sample, respectively, between 10-80 and 0.2 -2). It distinguishes H 2 S formed through thioacetamide reduction and the one formed through MS x reduction. m/z=34 signal (H 2 S fragment) without the m/z = 27 (HCN fragment) and m/z = 13 (CH 3 /NH fragment) signals indicate MS x reduction. On average, m/z = 27 and m/z = 13 signals increase between 150 °C and 400 °C. Therefore the thioacetamide reduction can be identified on the TPR curves of CoS x /TiO 2 , NiS 2 /TiO 2 and RuS 2 /TiO 2 (See Figure V-5).

Figure V- 6 :

 6 Figure V-6: Fit of the TPR curve of RuS 2 /TiO 2 . It distinguished sulfur from thioacetamide (S thio.) and the one from RuS 2 (S surf. and S bulk ). Spectrum was calibrated thanks to the total amount of sulfur released during the TPR and the total amount measured in CHNS.

  analysis: XPS measurements XPS measurements provide information on the dispersed and/or amorphous sulfide phases, that were unsuccessfully characterized by microscopy or XRD. It also evidences an influence of the TiO 2 support on the HgS phase.

Figure V- 7

 7 Figure V-7 reports the content of metal atoms determined with ICP-OES and XPS. The first

Figure V- 7 :

 7 Figure V-7: Metal content determined with chemical analysis (ICP-OES) versus surface metal content

  0

TiO 2 .

 2 Cinnabar (α-HgS) and metacinnabar (β-HgS) are the two phases that can be synthesized with soft method. For bulk HgS, XRD diffractogram (See Figure V-3 (d)) evidences the formation of cinnabar. For supported HgS, XPS spectra (See Figure V-8) evidences Hg II and S 2-states at 100.0 eV and 161.1 eV, respectively (See Table

Figure V- 8 :

 8 Figure V-8: XPS spectra for HgS/TiO 2 : Hg 4f (left), S 2p (right). On the S 2p spectrum, black line is

Figure V- 10

 10 Figure V-10 depicts two states for the supported ruthenium: one at 279.9 eV and another at

Figure V- 11 :

 11 Figure V-11: XPS Ni 2p spectra for bulk (down) and supported (up) NiS x . On all spectra, black line is the experimental spectrum, color lines are the fitting contributions and black dots constitute the

Figure V- 11

 11 Figure V-11 shows a small global shift between the BE of bulk and supported samples. Notice

Figure V- 12

 12 Figure V-12 reports two states for the molybdenum sample: one at 227.9 eV and another at 230 eV (See Table V-4). The S 2s contributes at 225.6 eV. The first one corresponds to Mo IV and the

  2 . It means that during the screening of the seven MS x the flux of photons absorbed ( depends only on TiO 2 . In this chapter, MS x are supported on the same commercial TiO 2 (CristalACTIV TM PC500). Therefore, the MS x were compared with the same incident flux of photons and therefore of electron-hole pairs ( . The equ. (3.6) and (3.7) can be re-written as follow: flux of photons that is available, is the absorption coefficient of TiO 2 , is the mass of photocatalyst, is the surface area of TiO 2 and is the co-catalyst loading. So is independent of the absorption coefficient of the MS x .

Figure V- 13 :

 13 Figure V-13: Action spectra of NiS 2 /TiO 2 (left) and RuS 2 /TiO 2 (right): absorption spectrum is given as the Kubelka-Munk function (black line) and the Photon Yield is measured for different energy of

  Photon energy (eV)

Figure V- 14 :

 14 Figure V-14: Photon Yield of the seven MS x /TiO 2 (2.6-3.0 metal atoms per 10 nm 2 ) and the supports

  Photon Yield (%)

Figure V- 16

 16 Figure V-16 shows that the activity (PY and ) of the photocatalysts varies along different

Figure V- 16 :

 16 Figure V-16: The PHER activity of the seven MS x /TiO 2 and the reference TiO 2 * along five

Figure V- 17 :

 17 Figure V-17: Amount of hydrogen disulfide produced as a function of: (a) the amount of hydrogen

  Photon Yield (%)

  Figure V-19 shows ATR-IR spectra of TiO 2 and NiS 2 /TiO 2 (before and after the PHER test).

Figure V- 18 :

 18 Figure V-18: TPR curve of NiS 2 /TiO 2 before and after a PHER test.

Figure V- 19 :

 19 Figure V-19: ATR -IR spectra of TiO 2 and NiS 2 /TiO 2 before and after a PHER test.

4 . 4 .

 44 XPS analyses were performed on the catalysts before and after the PHER test. It provides pieces of information about their modification. CuS/TiO 2 and CoS x /TiO 2 were not characterized.XPS analyses onAg 2 S evidenced that after 30 h of test (under a lamp intensity of 1400 μE/h) no more S x states are observed. The Ag-based co-catalyst is converted from Ag 2 S to AgO x or Ag 0 . It explains why the catalyst deactivates along the test (See Figure V-16).

Figure V- 21 :

 21 Figure V-21: The PHER rate as a function of time. 50 mg of RuS 2 /TiO 2 were dispersed in 50 %vol. of propan-2-ol in water.

192

  The reusability of the catalyst was also tested. A standard PHER test was performed with RuS 2 /TiO 2 : the PY was measured at 10, 20, 35 and 45 °C during at least 3 h for each temperature.

Figure

Figure V- 22 :

 22 Figure V-22: The PHER activity (PY and on the left; PY and on the right) of RuS 2 /TiO 2

193

  Photon Yield (%)

Figure V- 23 :

 23 Figure V-23: XPS spectra of RuS 2 /TiO 2 before test (down), after the long time experiment (middle) and after seven tests (up): Ru 3d -C 1s (left) and S 2p (right). On all spectra, black line is the

Figure V- 24

 24 Figure V-24 reports the reaction rates of the different MS x /TiO 2 for thiophene HDS. RuS 2 /TiO 2 is the most active catalyst and MoS x the second. It agrees with the volcano curve discussed in Section 2.1

Table V- 8 :

 8 Figure V-26 reports the multiple LSV curves and cyclic voltammetry experiments. They provide

Figure V- 26 .

 26 Figure V-26.

Figure V- 26 :

 26 Figure V-26: (left) Linear sweep voltammetry curves (right) cyclic voltammetry. From top to bottom

  Screening of seven transition metal sulfides supported on titania 200

2 and RuS 2 201

 2 Figure V-14). It shows either that HDS and electrocatalytic HER are not good descriptors for the steps

202

  HgS particles as proposed onFigure V-27. Electrons are attracted in the direction of the HgS particles, due to the band alignment but they react on the TiO 2 active sites.

Figure V- 27 :

 27 Figure V-27: Schematic representation of the photogenerated electrons reacting on the TiO 2 particles while they travel to the HgS particles, attracted by the band alignment. Adapted from [1] with

Figure V- 15 (

 15 Figure V-15 (See Section 5.3.1) reports the PY of TiO 2 , HgS/TiO 2 , NiS 2 /TiO 2 and RuS 2 /TiO 2 for

  at different light intensities. It is not surprising because results from an Arrhenius law applied to PY, therefore is already normalized by . It results that except the value of PY, all the conclusions made about and in the previous chapters and in the following sections are independent on the flux of photons.

Figure VI- 1 :

 1 Figure VI-1: The PHER activity of RuS 2 /TiO 2 (1.15 wt% Ru) as a function of the incident flux of

Figure VI- 2 :

 2 Figure VI-2: Irradiance measurements performed at different spot of the photoreactor: the very bottom of the reactor (below the 1 st cm of height), the middle (between 1 and 4 cm above the bottom

  340 nm (See Figure VI-3). Both parameters vary depending on . decreases while increases (See Figure VI-3, left).

Figure VI- 1 ,

 1 Figure VI-1, right). Having more energetic photons seem to decrease the values that were

Figure VI- 3 :

 3 Figure VI-3: Photon yield and Arrehnius paramaters ( on the left; on the right) measured

6 . 2

 62 Photon Yield (%)

Figure VI- 4

 4 Figure VI-4 (b) and (c), respectively, show their evolution as a function of the mass.Figure VI-4 (d)

Figure VI- 4 :

 4 Figure VI-4: The PHER activity of RuS 2 /TiO 2 (0.33 wt% Ru) as a function of the mass. (a) Photon

Figure VI- 5

 5 Figure shows that continuously decreases until 9 ± 5 (6.2 wt%). This decrease is

Figure VI- 5 :

 5 Figure VI-5: The PHER activity of RuS 2 /TiO 2 (40 mg) as a function of the co-catalyst loading. (a)

6 . 9 equals

 69 29 kJ/mol (See Figure VI-5, (c) at 0.0 wt% Ru) and is taken as the lowest value on Figure VI-5, (b) (16 kJ/mol). Then, and can be optimized to fit properly the data (See Annexes for comparison between experimental and theoretical data, Figure S23). Figure VI-6 reports the resulting pre-exponential factor and the contribution of each " " term. It indicates that the more RuS 2 co-catalyst is added, the more the RuS 2 term contributes (See Figure VI-6 (left)) and therefore, . is much lower than because it depends on the RuS 2 loading. At 0.84 wt% Ru, the is low enough to compensate the low due to the co-catalytic effect of RuS 2 . Above this threshold, so PY reaches a plateau (See Figure VI-5 (a)) because and are almost constant.

Figure VI- 6 :

 6 Figure VI-6: (left) Contribution of the two "" terms (See equ. 6.9) to the overall PY; (right) pre-

  size of the RuS 2 particles on the PHER activityThe influence of the size of the co-catalytic particles on the PHER activity has gaining more interest. Very recently, Abdullah et al. tailored the size of the Pd particles, supported on TiO 2[21].

First, RuCl 3 .

 3 xH 2 O was deposited by an incipient wetness impregnation on TiO 2 . The amount of TiO 2 and metallic precursor was adjusted to get a 0.3 wt% Ru catalyst. Sample was dried at 110 °C under an ambient atmosphere, for 1 h. The powder was then sulfurized under 10 %vol. of H 2 S in N 2 , 5 °C/min temperature ramp was applied until reaching 350 °C then, temperature was kept constant during 3 h.

Figure VI- 8 :

 8 Figure VI-8:The PHER activity of RuS 2 /TiO 2 prepared with the impregnation and sulfidation method

Figure VI- 9

 9 Figure VI-9 reports the PY measured at 20 °C of the resulting photocatalysts and deduced

Figure VI- 9 :

 9 Figure VI-9:The PHER activity of bulk RuS 2 deposited on TiO 2 support (anatase, rutile, brookite,

  Photon Yield (%)

3. 3

 3 wt% to 9.8 ppb (five dilution of 1:40, TiO 2 :BaSO 4 ).ii. Additional informations provided diluting solid samplesA meticulous dilution of the powder inBaSO 4 provides additional pieces of information. For bare TiO 2 , it should be noticed that a sample with a concentration of 3.4 wt% has a minimum reflectance of 25 % (spectra not shown). Even the sample with a concentration of 0.35 wt% should not be interpreted with the Kubelka-Munk theory because it has a minimum reflectance of 55 % (See green line on Figure VII-1). A concentration lower than 16 ppm is needed for obeying the Kubelka-Munk requirements. Diluting further the sample gives better resolved transitions; it evidences two of them at 4.6 and 5.0 eV. The artefact at 3.9 eV is due to the spectrometer switching from tungsten lamp to deuterium one. At 3.25 eV, an additional peak is observed (See Figure VII-1, left inset). It might be due to the contribution of the lowest energetic excitons. The electron photoexcited at this energy was near the valence band maximum and reaches an energy near the conduction band minimum. Such additional features advocate the use of dilution before to perform UV-Vis absorption spectroscopy on powders.

Figure VII- 1 :

 1 Figure VII-1: UV-Vis absorption spectra for TiO 2 -anatase diluted in BaSO 4 . The TiO 2 weight

Figure VII- 2 (

 2 Figure VII-2 (left) shows the absorption spectra of TiO 2 and the seven MS x /TiO 2 catalysts. At

Figure VII- 2 ( 2 h

 22 Figure VII-2 (right) determines the band gap energy (E g ) of TiO 2 in pristine TiO 2 and in

TiO 2 -

 2 anatase is known for having an indirect band gap[8]. Therefore, the Figure VII-2 is the Tauc plot for bare TiO 2 and the MS x /TiO 2 photocatalysts, with an exponent 0.5. The band gap of TiO 2 is estimated between 3.15 and 3.3 eV. This variation is within the error bar. It concludes that the presence of the MS x does not modify the TiO 2 band gap. Sulfides do not dope TiO 2 , they are only deposited on its surface. For such a reason, understanding the optical properties of commercial TiO 2 -

Figure VII- 2 :

 2 Figure VII-2: (left) Normalized Kubelka-Munk functions for bare TiO 2 and the seven MS x /TiO 2

Figure VII- 3 :

 3 Figure VII-3: (left) Plot of (α) 1/2 versus photon energy for TiO 2 -anatase diluted with BaSO 4 (390

7 . 4

 74 It normalizes the amount of transitions occurring in a range of energy by the total amount of transitions. Figure VII-4 plots the transition densities of TiO2-anatase. It is possible to decompose this curve in four contributions named α, β, γ and δ (from low to high photon Normal distributions are used: . μ corresponds to peak position and σ is the standard deviation. A least square method provides those parameters that are reported in inset on Figure VII-4.

Figure VII- 4 :

 4 Figure VII-4: Transitions density t versus the incident photon energy for TiO 2 -anatase diluted in

  MoSx has a band gap of 1.2 eV (See Figure VII-5, right). It is close enough to the value measured or calculated for bulk MoS 2 [12,14]. According to that Tauc plot, Ag 2 S and HgS have an indirect band gap of 1.0 and 1.4 eV, respectively.Table VII-1 summarizes results.

Figure VII- 5 :

 5 Figure VII-5: Tauc plot with an exponent 0.5 for supported NiS 2 , CuS and CoS x on the left and

Figure VII- 7 :

 7 Figure VII-7: Kraut method [17] applied to RuS 2 /TiO 2 , bulk RuS 2 and TiO 2 . From left to right the

Figure VII- 8 :

 8 Figure VII-8: Band structure diagrams for commercial TiO 2 -anatase, RuS 2 /TiO 2 heterostructure and

Figure VII- 9

 9 Figure VII-9 shows the spectra. It allows using Kraut method on supported sulfides. The main

Figure VII- 9 :

 9 Figure VII-9: XPS spectra for commercial TiO 2 -anatase, NiS 2 /TiO 2 , RuS 2 /TiO 2 , HgS/TiO 2 and

Figure VII- 10

 10 Figure VII-10 depicts the resulting band diagrams of NiS 2 , RuS 2 , HgS and MoS x supported on

Figure VII- 10 :

 10 Figure VII-10: Band structure diagrams for commercial TiO 2 -anatase, RuS 2 /TiO 2 , NiS 2 /TiO 2 ,

7. 2 UPS to study the valence band of commercial TiO 2

 22 for valence band investigation UPS and XPS are both photoemission processes. UPS uses ultraviolet photons for excitations while XPS uses X-ray photons. Because the energy of the incident photons differs, XPS (1486.6 eV)

  Figure VII-11 illustrates this behavior for the O 2p orbital. The latter has a main interest for the VB study of TiO 2 . Photoemission cross section is directly correlated to the intensity of the source of photons. Thus intensity of the resulting signal with a He I discharge lamp (21.2 eV) should be 4000 times higher than with Al Kα radiation.

Figure VII- 11 :

 11 Figure VII-11: Photoionization cross section of the O 2p orbitals, as a function of the incident photon

Figure VII- 12 :

 12 Figure VII-12: Full He I UP spectrum (black line) and its first derivative (red square) of ITO

Figure VII- 13 :

 13 Figure VII-13: He I UP spectrum of the TiO 2 -anatase on ITO substrate. Samples were prepared by the

  possible to prepare an imperfect layer[1]. UPS spectrum can then show both, the Ag states and the powder states (See Figure VII-14, green curve). Fermi level of the Ag foil observed on blue and green spectra allows working on an absolute energy scale. One of the d band of Ag is therefore measured 6.2 eV below its Fermi level corresponding to -10.4 eV (See Figure VII-14, right). Green spectrum evidences a difference of 2.3 eV between this d state and the one of TiO 2 (III; state assignment is discussed in the following section). Finally, on Figure VII-13 (red spectrum on Figure VII-14), the valence band maximum is 5.4 eV below III state. It gives an absolute value of -7.1 eV for TiO 2anatase. Notice that to obtain such spectrum a low amount of powder must be deposited on the Ag foil. Several tries are required to get an optimal sample.

Figure VII- 14 :

 14 Figure VII-14: (left) He I UP spectrum of the TiO 2 -anatase on Ag foil. Sample was prepared dropping

(

  LF) functions, as depicted on Figure VII-15. It exhibits the difference states of the valence band. To fit the spectrum, at least four contributions are needed . It is mainly inspired from the work of Fleming et al. who performed UPS on TiO 2 /Si (100) grown by molecular beam deposition [40].

Figure VII- 15 :

 15 Figure VII-15: Resulting (black line) for TiO 2 -anatase samples, prepared by the drop of a

Figure VII- 16 :

 16 Figure VII-16: (left) obtained after subtraction of I back with the Henrich or the Tougaard

Figure VII- 17 :

 17 Figure VII-17: Absolute electronic structure of the two commercial TiO 2 powders (anatase and

Figure VII- 17

 17 Figure VII-17). E thr. equals -8.25 eV for anatase and -8.0 eV for rutile. Between and E thr. , π -O 2p

Figure VII- 21 :

 21 Figure VII-21: Absolute band structure diagrams and the valence band states for commercial TiO 2 -

Figure S1 :

 S1 Figure S1: (a) Dimensions of the semi-batch photoreactor. (b) Photoreactor filled with a photocatalyst

Figure S4 :

 S4 Figure S4: Photocatalytic activity of RuS 2 /TiO 2 (0.3 wt% Ru) for the dehydrogenation of propan-2-ol

Figure S5 :

 S5 Figure S5: Photocatalytic activity of RuS 2 /TiO 2 (1.15 wt% Ru) for the dehydrogenation of propan-2-

Figure S6 :

 S6 Figure S6: Activity of Pd/TiO 2 (left) and RuS 2 /TiO 2 (1.15 wt% Ru) (right) for the photoconversion of

Figure S7 :

 S7 Figure S7: PHER parameters (rate of hydrogen production at 20 °C and E a,app ) of RuS/TiO 2 (1.6 wt%

Figure S9 :

 S9 Photon Yield (%)

  equals 0.36. 100 mg of Pt/TiO 2 (1.0 wt% Pt) was employed for the test. It means that 3.08 10 18 atoms of Pt are deposited on P25 and because of the dispersion atoms.

Figure S10 :

 S10 Figure S10: TEM images of Pt/P25 (1.0 wt% Pt) (left) and of P25 (right).

Figure S11 :

 S11 Figure S11: Pt particle size histograms of Pt/P25, determines with 189 particles on TEM images.

Figure S13 :Figure S15 :

 S13S15 Figure S13: TPR curve of bulk RuS 2 (red), RuS 2 /TiO 2 washed 2 times (black) and 5 times (blue). H 2 S

Figure S16 :

 S16 Figure S16: XPS spectra of TiO 2 * (left) and RuS 2 /TiO 2 (0.8 wt% Ru) (right). On all spectra, black

Figure S17 :

 S17 Figure S17: XPS spectra of AgS x /TiO 2 before (bottom spectrum) and after (top spectrum) the

  Figure S18: Action spectra of MoS x /TiO 2 : absorption spectrum is given as the Kubelka-Munk

Figure S19 :

 S19 Figure S19: PHER parameters for the dehydrogenation of propan-2-ol with RuS 2 /TiO 2 (0.8 wt% Ru),

Figure S20 :

 S20 Photon energy (eV)

Figure S21 :Figure S23 :Figure S26 :

 S21S23S26 Figure S21:The PHER rate obtained for the dehydrogenation of propan-2-ol with RuS 2 /TiO 2 . It is

Figure S27 :

 S27 Figure S27: Full He I UP spectrum of Ag foil substrates. Black line is the spectrum of the Ag foil

  Figure S28: He I UP spectrum of the TiO 2 -anatase (red) and TiO 2 -rutile (blue) on ITO substrates.

Figure S29 :

 S29 Figure S29: Resulting I signal (black line) for TiO 2 -anatase and TiO 2 -rutile samples. Four contributions

Figure S30 :

 S30 Figure S30: (left) UV-Vis absorption spectra for TiO 2 -rutile diluted in BaSO 4 . The TiO 2 weight

Figure S31 :

 S31 Figure S31: Tauc plot for TiO 2 -rutile (187 ppb) diluted in BaSO 4 . Adapted from [12] with permission.

Figure S32 :

 S32 Figure S32: XRD diffraction patterns for the anatase phase (CristalACTiV TM PC500) (left) and for the

Figure S33 :

 S33 Figure S33: XPS analysis of the S 2p states on the RuS 2 /TiO 2 powder as-prepared (black line) and the

Figure S34 :

 S34 Figure S34: He I UP spectrum of the inhomogeneous dropping of RuS 2 /TiO 2 powder on an Ag foil

Figure S35 :Figure S36 :

 S35S36 Figure S35: Full He I UP spectrum of the bulk RuS 2 with three different aperture of the analyzer.

Table I -

 I 1: Selected publications reporting the products of the photoconversion of alchol-water mixture. ................................................................................................................................................. Table I-2: Selected publications reporting M/TiO 2 for the conversion of a methanol-water mixture.. . Table II-1: Selected works from the literature about the electrocatalytic hydrogen evolution reaction with transition metal sulfides. ............................................................................................................... Table II-2: Selected publications of photocatalytic materials using sulfides as co-catalyst. ................. Table III-1: Gibbs free energy and standard oxidation potential for selected reactions. ....................... Table III-2: Analytical parameters of the micro gas chromatograph .................................................. Table VII-1: Summarized of the optical properties of the seven MS x /TiO 2 determined with UV-Vis absorption.. .......................................................................................................................................... Summarized of the density of π -O 2p and Ti-O hybridized states, obtained with the three preparation methods. Reproduced from [1] with permission. ....................................................

	Table VII-2:

Table IV-1: Amounts of H 2 and acetone produced at different on-stream times with Pt/TiO 2 and one with RuS 2 /TiO 2 catalysts. .................................................................................................................... Table IV-2: Selected publications reporting an measured under photocatalytic conditions. Table V-1: Metal loading of the as-synthesized photocatalysts. . ....................................................... Table V-2: Elemental amount (Metal, N, C, H, S) of the as-synthesized photocatalysts. ................... Table V-3: Phases identified by XRD for bulk TMS and their crystallite size estimated with Scherrer equation. .............................................................................................................................................. Table V-4: Binding energy and concentration of states evidenced for MS x /TiO 2 . .............................. Table V-5: Summary of PY, and the pre-exponential factor for the seven MS x /TiO 2 and TiO 2 *. .................................................................................................................................................. Table V-6: The PHER activity of four MS x /TiO 2 (photon yield, apparent activation energy and preexponential factor) at two lamps intensity (HPK lamp: 1400 μE/h, LED lamp: 11 200 μE/h). ......... Table V-7: Relative concentration of sulfur and ruthenium at the surface of the RuS 2 /TiO 2 catalyst before and after tests. . ......................................................................................................................... Table V-8: Electrocatalytic HER performance of the seven bulk sulfides. .........................................

Table I - 1 :

 I1 Selected publications reporting the products of the photoconversion of alchol-water mixture.

	Reactive mixture	Photocatalyst	Products (Hydrogen each time)	Reference
		Pt/TiO 2 (110)	formaldehyde, methyl formate	[67]
		Pt/Nb-TiO 2	formic acid, methyl formate, (formaldehyde)	[75]
	Methanol-water
		M/TiO 2	formaldehyde, acetaldehyde, methyl formate, CO,	[68-70]
		(M=Pt, Ag, Au)	CO 2 , CH 4 , dimethyl ether
		Pt/TiO 2	acetaldehyde, acetic acid, 2,3-butanediol, CH 4 , CO 2 [73]
		Pt/TiO 2	acetaldehyde, acetic acid, 2,3-butanediol, acetal, … [74]
	Ethanol-water	Pt/TiO 2	acetaldehyde, 1,1-diethoxyethane, CH 4 , CO 2	[76]
		Pt/Nb-TiO 2	acetaldehyde, ethyl acetate	[75]
		Cu x O/TiO 2	acetaldehyde, 1,1-dietoxyethane, CH 4 , CO 2	[77]
		Pt/TiO 2	Acetone	[78]
		Pd/TiO 2	CO 2 , CH 4 (gaseous phase)	[61]
	Propan-2-ol-water	Pt/TiO 2	Acetone, 2-propoxide, CH 4 , CO 2 , acetic acid	[79]
		TiO 2	acetone, CO 2 , 2-propoxide	[80]

  They used a standard Pt/TiO 2 (0.5 wt% Pt, Degussa P25)[83] [81]. Bowker et al.compared their results with the one of Kondarides et al.[84]. They reported Pd/TiO 2 and Au/TiO 2 (0.5 wt% Pd, 2.0 wt% Au, Degussa P25) and they concluded the following rank for the co-catalysts: Pd > Pt > Au. Montini et al proposed a mechanism for the photocatalytic conversion of glycerol-water mixture with a Cu x O/TiO 2 photocatalyst[77]. It explains the formation of no less than eleven products.Ribao et al. evidenced the issue of chemicals purity in glycerol. They reported the PHER from crude and synthetic glycerol with TiO 2 /graphene oxide/Pt[85]. The PHER rate is 5.6 lower in crude than in synthetic glycerol. They attributed it to the presence of impurities (4 % of methane, salts, ashes

Luo et al. showed an enhanced PHER activity with the presence of glycerol: r H2 (water + glycerol) = 16.4 r H2 (water only). It was obtained with a Pt supported on a B, N, co-doped TiO 2 [82]. Kondarides et al. reported the appropriate stoichiometry 1:7 and 3:7 for glycerol:H 2 and CO 2 :H 2 , respectively.

  Speltini et al., with a

	similar material (Pt/P25, 5 wt% Pt), produced H 2 from rice husk under natural sunlight [92]. They
	reported a PHER rate of 4 μmol/h (7.1 μmol/h/g bio. ) Kasap et al. reported the PHER with a range of
	unprocessed biomass samples: paper, cardboard, bagasse, wooden branch and sawdust. The latter
	showed the highest r H2 , equals to 0.6 μmol/h (1.5 μmol/h/g bio. ) with a carbon nitride-based
	photocatalyst [60].

Lignin itself received little attention. It absorbs UV light limiting its conversion with wide band gap semiconductors. Kadam et al. succeeded to produce H 2 in the visible with C, N, S-doped ZnO/ZnS [93]. Wakerley et al. reported r H2 = 0.2 μmol/h (410 μmol/h/g bio.

  Table from the EuChemS: "The 90 natural elements that make up everything. How much is there? Is that enough?" [144].

are not the only reasons to use MS x as co-catalyst, they provide a whole range of properties. Section

	2.1 and 2.2 describe promising ability to activate H 2 and
	promising electrochemical behavior. Last but not least, MS x are mainly semiconductors. Varying the
	MS x phases or the size of the particles (due to quantum size effect) tunes the size of the band gap. So
	MS x /MO x materials give access to a large scope of electronic structures. It is useful to understand their
	influence on the photocatalytic activity.

  2 (M = Co, Ni, Cu, Mo, Ru, Ag, Hg) ont été étudiés. La réaction de déshydrogénation photocatalytique du propan-2-ol est utilisée comme réaction modèle. Des corrélations sont établies entre les propriétés intrinsèques de ces MS x /TiO 2 et leur activité photocatalytique. De plus, la mesure d'énergie d'activation d'apparente apporte une compréhension supplémentaire sur les mécanismes photocatalytiques.Cette dernière montre que la production photocatalytique d'hydrogène est principalement limitée par les phénomènes de séparation et de transfert de charges dans les photocatalyseurs. Ainsi, une méthodologie combinant la spectroscopie de photoélectrons UV et la spectroscopie d'absorption UV-Visbile a été mis en place pour déterminer la structure électronique des poudre photocatalytiques.Ce travail conclue sur le caractère central de la structure électronique en photocatalyse. Dans le cas du photocatalyseur RuS 2 /TiO 2 , le transfert électronique est l'étape cinétiquement déterminante pour la déshydrogénation photocatalytique du propan-2-ol.

, Benck et al. recommended to provide the potential to reach a current density of interest such as 10 mA/cm 2 or 1 mA/cm 2 . Table

	II-1 reports
	those parameters for selected materials. Tafel slopes are also reported in this table. It corresponds to
	the slope of log(i) = f( ); It gives the additional voltage required to increase the current by an order of
	magnitude. The

lower the Tafel slope is, the better the catalyst is. MoS 2 has gaining more interest during the past decades. DFT

			calculations revealed that MoS 2
	edges possess a	of 0.08 eV whereas basal plane has a	of 1.92 eV [14]. Jaramillo et al. then
	showed, experimentally, that MoS 2 edges are the catalytic sites for the HER. Since, studies on MoS 2
	were mainly dedicated to increase the amount of MoS 2 edges [15].
	A MoS 2 prepared with standard procedure has little activity (See Table II-1, entry 1). Girel et al.

Table II -1: Selected

 II works from the literature about the electrocatalytic hydrogen evolution reaction with transition metal sulfides. a determined graphically. b corrected with the internal resistances from electrode substrate or solution.

	Entry	Tafel slope (mV/decade)	η 10 (-mV vs RHE)	η 1 (-mV vs RHE)	Ref.
	MoS 2 (thiomolybdate)	105-125	232-246		[25]
	MoO 3 -MoS 2 nanowires	50-60	254 b	205 a, b	[16]
	Double gyroid MoS x	50	206 b	166 a, b	[17]
	Petaled-MoS 2 /Mo	68	240	Undet.	[19]
	Petaled-MoS 2 /Au	69	280	Undet.	[19]
	MoS 2 (chemical vapor deposition)	98	Undet.	290 a	[22]
	Strained vacancy MoS 2 (chemical vapor deposition)	60	170	90 a	[22]
	1T -MoS 2	40	207	160 a	[26]
	2H -MoS 2	75-85	300 a, b	215 a, b	[26]
	Exfoliated 1T-WS 2	70	151 b	Undet.	[24]
	FeS 2	56	Undet.	217	[27]
	FeS 2	62-77	Undet.	210 a	[28]
	CoS 2	49	Undet.	230	[27]
	CoS 2	44-47	Undet.	190 a	[28]
	CoS x (electrodeposition)	Undet.	Undet	350 a	[29]
	CoWS x (electrodeposition)	78	280	200	[29]
	CoMoS x (electrodeposition)	85	250	180	[29]
	CuS	113	600	Undet.	[30]
	Ni 3 S 2	97	335	Undet.	[31]
	NiS	124	474	Undet.	[31]
	NiS 2 (electron beam evaporation and sulfidation)	128	454	Undet.	[31]
	NiS 2 (electron beam evaporation and sulfidation)	52	192	128	[27]
	NiS 2	41-46	260 a	200 a	[28]
	65

we proposed to compare the ratio between the r H2 of the supported co-catalyst with the r H2 of the bare support, called enhancement ratio. Both were measured in similar conditions and are comparable.

	good co-catalyst for reduction [62,63]. Therefore, under illumination, Pt particles are photodeposited
	on the NiS located on the ZnIn 2 S 4 . It multiplies the number of junction that photogenerated electrons
	must cross. So r H2 (Pt/NiS/Zn 2 S 4 ) < r H2 (Pt/Zn 2 S 4 ) (See Figure II-4).
	On the contrary, CuGa 3 S 5 has no Pt on it, the MS x act as a co-catalyst for H 2 production and the
	rate-determining step concerns the reduction half-reaction. CuS already showed good behavior for
	oxidation [61] so it is not surprising that it is the best co-catalyst of ZnIn 2 S 4 . But its behavior is
	unclear: it is also described as a co-catalyst for reduction half-reaction [64]. It enhanced the activity of
	Cd 0.4 Zn 0.6 S almost as much as NiS or CoS (r H2 (NiS/Cd 0.4 Zn 0.6 S) = 1.3 r H2 (CuS/Cd 0.4 Zn 0.6 S) = 1.2 r H2
	(CoS/Cd 0.4 Zn 0.6 S) = 6.8 r H2 (Cd 0.4 Zn 0.6 S) [63].
	CuS/ZnIn 2 S 4	NiS/CuGa 3 S 5
	Ag 2 S/ZnIn 2 S 4	FeS/CuGa 3 S 5
	SnS/ZnIn 2 S 4	Ru 2 S 3 /CuGa 3 S 5
	ZnIn 2 S 4	Ag 2 S/CuGa 3 S 5 CoS/CuGa 3 S 5
	MnS/ZnIn 2 S 4	PdS/CuGa 3 S 5
	CoS/ZnIn 2 S 4	CuGa 3 S 5
	NiS/ZnIn 2 S 4	CuS/CuGa 3 S 5
	Shen et al. reported the PHER activity of MS x /Pt/ZnIn 2 S 4 [61]. They prepared them at room temperature, mixing ZnIn 2 S 4 , metallic precursor and Na 2 S.9H 2 O. They photodeposited Pt at the beginning of the test. CuS, Ag 2 S and SnS enhanced the PHER activity of Pt/ZnIn 2 S 4 whereas MnS, NiS and CoS lowered its activity (See Figure II-4). They argued that CuS acts as the oxidation co-catalyst and that Pt acts as the reduction co-catalyst (they did not provide evidences). Tabata et al. reported the PHER activity of MS x /CuGa 3 S 5 [62]. They prepared them in situ. They mixed the support, the metallic precursors and the Na 2 S/Na 2 SO 3 -aqueous solution in the photoreactor. A color change indicated formation of sulfides. They claimed, without evidences, that the MS x and the Figure II-4: PHER activities of MS x /ZnIn 2 S 4 /Pt (left) and MS x /CuGa 3 S 5 (right) from aqueous solution with Na 2 S/Na 2 SO 3 . Reproduced from [61] and [62]with permission. Except those studies, they are many more sulfide co-catalysts reported for the PHER; Table II-2 reviews some of those works. Sulfides were deposited on various support and studied in various reactive mixture. In addition, the incident flux of photons is rarely provided or it must be recalculated (See Table II-2); the reactor's temperature is even less reported. As discussed previously, it 0 1 0 2 0 3 0 4 0 5 0 complicates comparison between the efficiency of co-catalysts. To overcome this issue, 0 20 40 60 80 100 120 140 160 180 200 220 r H2 (μmol/h) r H2 (μmol/h)
	CuGa 3 S 5 have no physical junction. They reported a higher PHER rate with the MS x , except for CuS
	(See Figure II-4).
	With Pt as co-catalyst of ZnIn 2 S 4 , electrons are easily transferred to the Pt and used for H 2
	production. The rate-determining step concerns the oxidation half-reaction. NiS is often reported as a

71 2.4.2 MoS x -based photocatalysts for the PHER

	MoS 2 is a low band gap semiconductor (See Figure II-3 and Section 2.3) so it can act as a co-
	catalyst but also as a sensitizer. In the second case, the MoS 2 provides additional photons which results
	in an increasing activity. For instance, Guo et al. reported a MoS 2 /TiO 2 heterostructure that absorbs
	from ultraviolet-visible to near-infrared wavelength [65]. The MoS 2 with S-vacancies has a localized
	surface plasmon resonance so, they absorbed visible light; the MoS 2 has low band gap close to 1.1 eV
	and absorbs infrared photons while TiO 2 absorbs UV photons. It results in an 8 times higher activity
	than bare TiO 2 . It is low if the heterostructure also provides photons from visible and infrared. In
	addition to those optical properties, MoS 2 was reported with interesting hygroscopic properties [66].
	Daeneke et al. showed a proof of concept using sulfur-rich MoS x device to produce H 2 from water
	vapor.

MoS 2 is, as well as the electrocatalytic HER, the most studied co-catalyst for the PHER.

	Strategies for improving its activity are similar to those reported for the HER in Section 2.2. Edge sites
	are more active than basal plane for hydrogen production. The aim is therefore to increase the density
	of edge sites. Some are reviewed by Chen et al. for MoS 2 /TiO 2 photocatalysts [67]. The work of Girel
	et al. is suitable for comparison because no particular strategy was applied

To improve the amount of MoS 2 edges, the

  . Chang et al. almost doubled their activity adding reduced graphene oxide to MoS 2 /CdS [72]. catalyst can also be doped. Xu et al. doped with Co

	MoS 2 can be nanostructured to improve the ratio edge / bulk; the latter are inactive for H 2
	production. Liu et al. proposed to support on TiO 2 , vertical single (or few-) layers MoS 2 nanosheets
	[73]. Lin et al. created sulfur vacancies removing the K + intercalated between MoS 2 layers [74].
	[75] and Hong et al. with Cu [76]. CoMoS x catalysts were already promising for HDS (See Figure II-2
	and [77,78]. First, it increases the efficiency of a heterojunction graphene oxide (rGO) with g-C 3 N 4
	(See entry 7). Second, it improves the catalyst stability; while the activity of MoS 2 /rGO/g-C 3 N 4
	decreases of 18 % after 4 cycling test in TEAO, with CoMoS 2 /rGO/g-C 3 N 4 it decreases of only 3 %.
	The gain of activity for Cu 2 MoS 4 (See entry 8) is much lower (See entries 7 and 8) than CoMoS 2 and
	it lacks stability [76].
	Finally, 1T

-phase is reported as more efficient than the

2H-phase. Another strategy consists, therefore, in stabilizing the 1T-phase. Liu et al. claimed to support on TiO 2 nanorods a mix phase 2H-/1T-MoS 2 (evidenced by XPS and Raman) [79]. They reported an enhancement factor of 20. Mao et al. applied the same synthetic and characterization methodology to support 1T-MoS 2 on a ZnCoS solid solution [80]. They reported a much lower enhancement factor (See entries 9 and 10) but with a significant AQY of 15.15 %. MoS 2 supported on TiO 2 were extensively analyzed because this work deals with TiO 2 supports.

  Gaikwad et al. proposed a photocatalyst to oxidize with RuS 2 and reduced with Pt [105]. RuS2 only, shows a co-catalytic effect on the Cd 0.5 Zn 0.5 S support (See entry 19). They even reported H 2 production with a Pt-RuS 2 /Cd 0.5 Zn 0.5 S under solar irradiation. However, sulfides can also catalyze the oxidation half-reaction. Ag 2 S is sometimes reported as a co-catalyst for reduction or oxidation. Zhang et al. reported it as a co-catalyst for bare oxisulfides (Sm 2 Ti 2 S 2 O 5 and Gd 2 Ti 2 S 2 O 5 ) (See entry 21) but also for Rh/Sm 2 Ti 2 S 2 O 5 and Pt/Sm 2 Ti 2 S 2 O 5 ; Ag 2 S increases 8-9 times their PHER activity [106]. It confirms that Ag 2 S catalyst and the noble metals do not catalyze the same reaction. For similar reason, 2.0 wt% of Ag 2 S increased 10.5 times the PHER

activity of Pt/CdS (See entry 22). To eludes the possibility that Ag 2 S acts as a sensitizer -it absorbs photons up to 800 nm -action spectrum was performed. In the wavelength range for which Ag 2 S only absorbs, no significant amount of H 2 was produced

[107]

. On the contrary, Di et al. reported Ag 2 S as a catalyst for reduction, it enhances the CdS activity (See entry 23). They measured the E F of Ag 2 S, CdS and Ag 2 S/CdS with Kelvin probe measurement and advocated an electron transfer from CdS to Ag 2 S [108].

Table II -2, the best ones contain MoS x . They have one of the highest enhancement factor and AQY. It should however be noticed that NiS shows a promising co-catalytic effect with an enhancement factor up to 79 (See entry 13) and that CuS/CdS is the second catalyst with the highest AQY (See entry 27).Table II - 2 :

 IIII2 Selected publications of photocatalytic materials using sulfides as co-catalyst. a If the opposite is not clearly indicate, molecules are diluted in water. TEAO and TEA stand for triethanolamine and trimethylamine, respectively. EY and ErY stand for eosyne Y and Erythrosin Yellowish. b No multiplying factor "2" is used, that is why it can differ from the publication,and; measuring efficiency is discussed in Chapter III c Flux of photon is not directly provided in the publication, it was recalculated.

	Entry MS x phase	Support	Reactive mixture a	Activity	Flux of photons & Efficiency b	Refs.
				r H2 (thioacet. reflux-
	MoS 2	TiO 2	Methanol	MoS 2 /TiO 2 ) ≈ 3 r H2 (impre. & sulfide. -MoS 2 /TiO 2 ) ≈ 20 r H2
				(TiO 2 )

Table III - 1 :

 III1 Gibbs free energy and standard oxidation potential for selected reactions.

	Oxdidant	Balanced equation		Reference
	Water	474	1.23	[14]
		103	0.53	[14]
	Methanol
		16 -9	0.02 -0.03	[14-16]
		74	0.38	[14]
	Ethanol
		97	0.08	[14,16]

Table III - 2 :

 III2 Analytical parameters of the micro gas chromatograph

	Parameters	Column A	Column B	Column C	Column D
	Column type	Molecular Sieve	PPU	Alumina	OV1 / Wax
		(5 Å)
	Gases analyzed	He, H 2 , O 2 , N 2 ,	CO 2 , H 2 O, H 2 S,	Various alkanes	Isopropanol,
		CH 4 , CO	ethane	and alkenes	Methanol, acetone
	Back flush (s)	5	/	/	/
	Column temperature (°C) 90	85	90	50 / 55
	Column pressure (psi)	28	25	28	28 / 30
	Gaz vector	Ar	He	He	He / He

  1.1 Applying different electrical power from 140 mW to 2450 mW gives a flux of photons between 900 μE/h and 16 300 μE/h. The Xe lamp is equipped with filters providing six different monochromatic sources of light (centered at 490 nm, 445 nm, 425 nm, 395 nm, 365 nm and 340 nm). It allows measuring the PHER rate at those wavelengths, so-called action spectrum. Results are reported in Section 5.3.1.

measured for all the photocatalysts of the Ph.D. project followed an Arrhenius law between 5 °C and 45 °C. It allows calculating an apparent activation energy ( ) and a pre-exponential factor ( ) from the Arrhenius law

  See Figure III-1). It permits to measure the PY at different temperatures, commonly it is made at 10, 20, 35 and 45 °C. It requires at least 3 h to reach the steady state and to have at least 10 points to calculate an average.

	3.2.3	Arrhenius parameters measurement:	&
	To go further in the investigation, a new methodology (in the photocatalysis field) is used. It
	consists in measuring apparent activation energy (	) for photocatalysts.
	A cryostat (Julabo Cryostat F32-ME) supplies water recirculation at a given temperature to
	regulate the one of the photoreactor (The PHER rates according to equ. 3.23 Note that the mathematical determination of	is independent of the
	flux of photons, only	is influenced.
			107

To this date it is the highest value in the literature for sulfides-based photocatalyst. After recalculation of their QE, a value of 410 % was found. It can be due to a mismeasurement of either the PHER rate or to the photon intensity. :

3.23

Notice that is deduced from an Arrhenius law applied to PY, it is therefore normalized by the incident flux of photons. Assuming a standard pre-exponential factor of an Arrhenius law, . Figure III-3 shows a standard procedure with RuS 2 /TiO 2 . The %H 2 is measured at least at four temperatures, here: 10 °C, 20 °C, 35 °C and 45 °C. Natural logarithm of PY is plotted as a function of 1/T. Slope of the linear regression provides the and the intersection with the x-axis provides the

The results obtained with the RuS 2 /TiO 2 (PC500) catalyst enables extending the mechanism established for metal-based photocatalyst to transition metal sulfides -based photocatalyst

  After a purge of the photoreactor with Ar, light was switched on. Then, a H 2 PtCl 6 xH 2 O salt was added to reduce the Pt cation at the surface of the TiO 2 ; the holes being consumed to oxidize the

	alcohols:
	4.1
	Chapter IV:
	Kinetic of the
	photocatalytic
	dehydrogenation of
	propan-2-ol
	Chapter IV focuses on the entire mechanism of the photocatalytic conversion of alcohol-water mixtures
	1 M H 2 SO 4 aqueous solution, it is purged with bubbling Ar prior any
	experiment. Linear Sweep Voltammetry (LSV) scans were carried out from 0.1 to -0.8 V vs SCE at 5 and especially of the propan-2-ol. It studies not only the reduction half-reaction but also the oxidation

mV/s rate. Cyclic Voltammetry (CV) scans were carried out from 0.5 to -1.0 V vs SCE at 200 mV/s rate. half-reaction. A mechanism and a micro-kinetic model are proposed for the photocatalytic dehydrogenation of propan-2-ol. 122 mixture. . This catalyst was prepared with the thioacetamide reflux method described in Section 5.1 Before working on the photocatalytic dehydrogenation of propan-2-ol, various reactive mixtures were tried. Results are reported in the Annexes. RuS 2 /TiO 2 was studied for the photocatalytic conversion of five reactive mixtures: pure methanol, pure propan-2-ol, diluted methanol (50 %vol. in water), diluted propan-2-ol (50 %vol. in water) and diluted ethanol (50 %vol. in water) (See Annexes, Figures S4 and

Table IV -

 IV 1 reports the results of the catalytic tests performed with Pt/TiO 2 and RuS 2 /TiO 2 .

	During the extended initial period, the H 2 :acetone stoichiometry is 1:1, within the accuracy range
	of the analytic technique. It lasts 10 h for Pt/TiO 2 and 30 h for RuS 2 /TiO 2 .

Table IV - 1 :

 IV1 Amounts of H 2 and acetone produced at different on-stream times with Pt/TiO 2 and one with RuS 2 /TiO 2 catalysts.For RuS 2 /TiO 2 (20 %vol. of acetone), it is 25 times higher. It

		Pt/TiO 2		RuS 2 /TiO 2
		t = 10 h	t =40 h	t = 20 h	t = 110 h
	n (H 2 produced) (mmol)	7	18	0.4	3
	n (acetone produced) (mmol)	7	6	0.5	1
	For Pt/TiO 4.2
					4.3
	Dehydrogenation of the acetone can also occurs in parallel to the dehydrogenation of propan-2-
	ol, all along the test. Therefore, it would contribute to the overall PHER rate. This reaction was studied
	in details to see how it contributes to the overall PHER rate.

2 , after 30 h, the propan-2-ol is almost consumed (See Figure IV-1). The liquid phase is saturated in acetone so the latter can be dehydrogenated. The absence of CO 2 excludes the photocatalytic reforming of acetone (equ. 4.2). It might convert the acetone in cyclopropanone (equ. 4.3) but additional characterizations are needed.

An acetone-water mixture was photoconverted in hydrogen with the two catalytic systems. The PHER rate from propan-2-ol is 5 times higher than with acetone, for Pt/TiO 2 (10 %vol. of acetone).

126

concludes that the photocatalytic dehydrogenation of acetone is negligible comparing to the dehydrogenation of propan-2-ol.

  Regarding the MS x /TiO 2 catalytic systems, the highest amount of hydrogen produced overall, during the Ph.D. project is 7.8 mmol of H 2 . It was obtained during a 250 h test with one of the best catalyst (RuS 2 /TiO 2 -1.6 wt% Ru). The reactive mixture initially contains around 330 mmol of propan-2-ol. It concludes that no tests were performed during this work at a conversion higher than 2.3 %. Therefore,

during all the tests, the propan-2-ol is always in excess and with a sufficient stirring, the diffusion of the propan-2-ol to the catalyst surface should not limit the PHER.

  The trends are similar for Pt/TiO 2 and RuS 2 /TiO 2

	Amount of H 2 (mol)	0 0.04 Isopropanol/Water V/V % 2 4 6 8 0.00 0.01 0.02 0.03 100% 50% 20% 10% 5% 2%	10	Amount of acetone (mol)	0 0.04 Isopropanol/Water V/V % 2 4 6 8 0.00 0.01 0.02 0.03 100% 50% 20% 10% 5% 2%	10
		time (hours)			time (hours)

Table IV - 2 :

 IV2 Selected publications reporting an measured under photocatalytic conditions.

	Experimental conditions	Photocatalyst	(kJ/mol)	Reference
		Pt/P25	9.2
	Dehydrogenation of propan-2-ol, liquid phase	Pd/P25	16	[8]
		Ni/P25	15
		Pd/TiO 2	10.5
	Photocatalytic conversion of ethanol, liquid phase	Pt/TiO 2	15.5	[11]
		Rh/TiO 2	26.5
	Photodegradation of phenol, liquid phase	TiO 2 supports	9 -20.5	[13]

  Mo 7 O 24 (NH 4 ) 6 .4H 2 O (Alfa Aesar), Ni(NO 3 ) 2 (H 2 O) 6 (Sigma Aldrich), RuCl 3 .xH 2 O (Sigma Aldrich).

	The following precursors are used: AgNO 3 (Sigma Aldrich), Co(NO 3 ) 2 (H 2 O) 6 (Sigma Aldrich),
	Cu(NO 3 ) 2 (H 2 O) 2.5 (Acros Organics), HgCl 2 ("A la licorne" droguerie produits chimiques),
	Commercial TiO 2 has an anatase crystallographic structure and contain 85 wt% of TiO 2 crystallized. It
	benefits of a high surface area, 350 m 2 /g. [4] XRD analysis measures a crystalline domain of 7 nm
	(See Section 3.3.1).

Commonly, 2.0 g of commercial TiO 2 (CristalACTiV TM PC500) (referred later as PC500) and 900 mg of thioacetamide (Alfa Aesar) are suspended in 100 mL of deionized water. Suspension is refluxed under stirring for 20 min (90 °C, 350 rpm). Then, a metallic precursor is added with a 350 μmol equivalent of metal. Sulfur precursor is thus, in large excess (n S = 35 x n metal ). Mixture is kept under thioacetamide reflux during 1 h. In the case of the MoS x deposition, ammonium heptamolybdate is used as a precursor and it requires adding 2.0 ml of 2.5 M HCl. Indeed, Peeters et al. reported a better conversion of thioacetamide in H 2 S under acidic conditions.

[3]

Precipitate is isolated, washed at least two times with ethanol and dried under N 2 .

Bulk transition metal sulfides were prepared in the same conditions, without adding TiO 2 . A 3500 μmol equivalent of metal is added in the solution containing 2.0 g of thioacetamide (n S = 7.5 x n metal ).

Table V -

 V 1 shows the metal loading measured by ICP-OES for each photocatalyst. It shows that loadings are similar to the expected ones, 300 000 atoms of metal per μm 2 was successfully deposited on TiO 2 .

The structure activity relationship that is discussed later will be based on materials prepared in the same conditions and with the same co-catalyst loading.Table V - 1 :

 V1 Metal loading of the as-synthesized photocatalysts. a Experimental loadings were measured by ICP-OES. b S BET PC500 equals 350 m 2 /g. TableV-2 reports the elemental amount of C, H, N, S that were quantified for TiO 2 * and RuS x -NiS x -AgS x -CoS x /TiO 2 . It evidences the presence of organic impurities: 0.1-0.2 wt% of N, up to 2.0 wt% of C and around 1.0 wt% of H. The thioacetamide reflux method is based on the hydrolysis of thioacetamide in H 2 S and acetic acid (See equ. 5.1). Even after washing, it

	MS x	Precursor	wt% metal	N atomes / 10 nm 2
			expected	measured a measured a
	AgS x AgNO 3	1.9	1.9	≈ 3.0
	HgS x HgCl 2	3.5	3.1	≈ 2.7
	NiS x	Ni(NO 3 ) 2 (H2O) 6	1.0	0.95	≈ 2.8
	CuS x Cu(NO 3 ) 2 (H2O) 2.5	1.1	1.1	≈ 3.0
	CoS x Co(NO 3 ) 2 (H2O) 6	1.0	0.90	≈ 2.6
	RuS x RuCl 3 .xH 2 O	1.7	1.6	≈ 2.7
	MoS x Mo 7 O 24 (NH 4 ) 6 .4H 2 O	1.7	1.7	≈ 3.0

seems that thioacetamide or acetic acid which did not react are adsorbed at the TiO 2 surface.

  Influence of those impurities will be extensively studied in Section 5.4.2. other organic impurities. Because of that, amount of S includes both, sulfur bonded to the metal and sulfur from impurities. The S/Metal is therefore the upper limit of the MS x stoichiometry.Stoichiometries are higher than the expected ones: RuS 2 , NiS 2 , NiS, Ni 3 S 2 , Ag 2 S, Co 9 S 8 , CoS 2 , etc.

Concentrations of N, C, and H do not correspond to their respective amount in the molecule of thioacetamide (weight ratio C/N should equal 1.7 for thioacetamide) (See Table V-2). It indicates the 154 presence of

Table V - 2 :

 V2 Elemental amount (Metal, N, C, H, S) of the as-synthesized photocatalysts. Metal content was measured by ICP-OES ant the other elements by CHNS. a Ratio includes S bonded to the metal and S from impurities. b After removing the sulfur from the thioacetamide impurities with Figure V-6.

	Sample	wt% metal wt% N wt% C wt% H wt% S	C/N	S/Metal a
	RuS x /TiO 2	1.58	0.17	0.61	0.88	1.72	3.6	3.4 (2.5) b
	NiS x /TiO 2	0.95-0.96	0.13	1.12	0.96	1.60	8.6	3.1
	AgS x /TiO 2	1.89	0.08	1.69	0.97	0.52	21	0.9
	CoS x /TiO 2	0.89-0.90	0.08	1.10	1.02	1.14	14	2.35
	TiO 2 *		0.09	1.96	1.20	0.16	22

Table V -

 V

							(111)	(120)		(102) (013) (-122) (-113) (022)	AgS x				(102)	(103) (006)	(110)
	Intensity (a. u.)	(100)	(-102) & (110) (-111) (012)	(-112)		(-121)	(031)	(-202)	(210)	(-104) (-210) (-104) (211) & (220) (-114) & (122) Ag 2 S: PDF 01-080-5476 (133)	Intensity (a. u.)	(002)	(100)	(101)	(105)	(108)	CuS x CuS : PDF 00-006-0464 (213) (208) (1011) (116)
	10	20				30		40			50	60	70	80		10	20	30	40	50	60	70	80
									angle (2 )					angle (2 )
																	(101)
	Intensity (a. u.)	(113)	(222)	(026)	(313) (111)	(200)	(210)	(211)	(220)	NiS x NiS 2 : PDF 04-003-1992 S: PDF 04-012-7311 (311) (331) (222) (023) (321)		Intensity (a. u.)	(100)	(003)	(012)	(103)	HgS x (111) HgS: PDF 04-002-6787 (113) (202) & (015) (021) (104) (110) (114) & (203) (116) (006) (211) (024) (122) & (205)
	10	20				30		40			50	60	70	80		10	20	30	40	50	60	70	80
									angle (2 )						angle (2 )
												Phase identified	t crystallite (nm)	PDF file
							AgS x				Ag 2 S				50	01-080-5476
							HgS x				HgS				15	04-002-6787
							NiS x				NiS 2				10	04-012-7311
							CuS x				CuS				7	00-006-0464

3: Phases identified by XRD for bulk TMS and their crystallite size estimated with Scherrer equation.

resulting S sulfide /Ru molar ratio equals 2.5 and concludes to an over- stoichiometric RuS 2+x .

  the area under the curve. The true amount of sulfur detected by elemental analysis (See TableV-2) can be corrected. The

	0.4 29 % of						1.6
					CoS x /TiO 2				HgS/TiO 2
	Amount of H 2 S (μmol)	0.1 0.2 0.3	thioacetamide reduction		Amount of H 2 S (μmol)	0.4 0.8 1.2	Amount of H 2 S (μmol)	0.00 0.05 0.10 0.15 0.20	200	400 Temperature (°C) 600 800	1000
			0.0						0.0
			200	400	600	800	1000		200	400	600	800	1000
				Temperature (°C)				Temperature (°C)
		1.0						0.8
	Amount of H 2 S (μmol)	0.2 0.4 0.6 0.8		MoS x /TiO 2	Amount of H 2 S (μmol)	0.2 0.4 0.6	Amount of H 2 S (μmol)	0.00 0.02 0.04 0.06 0.08 0.10	Temperature (°C) 400 600 800 PtS x /TiO 2 200	1000
		0.0						0.0
			200	400	600	800	1000		200	400	600	800	1000
				Temperature (°C)					Temperature (°C)
							161

Table V - 4 :

 V4 Binding energy and concentration of states evidenced for MS x /TiO 2 . Spectra are calibrated with the intense C 1s state at 284.5 eV.

			Binding Energy (eV)		Binding Energy (eV)	S / M
		[relative concentration] -M states	[relative concentration] -S states
	TiO 2 *					163.7 [37%]	169.5 [63%]
	HgS/TiO 2	100.0 -				161.1			168.9	1.0
						[93%]			[7%]	(	/
									= 0.95)
	Bulk RuS x	279.6 -	280.1 -					161.5	166.8	3.5
								[96%]	[4%]	(	/Ru II
		[95%]	[5%]						= 1.5)
	RuS x /TiO 2	279.9 -	281.0 -					161.9	166.6 [15%]	1.9
								[70%]	& 168.5	(	/Ru II
		[93%]	[7%]						[15%]	= 1.5)
	Bulk Ag 2 S	367.7 -	365.9 -			160.8 [70%]		0.4
		[0.2%]	[99.8%]			& 161.8
						[14%] &
						163.0 [16%]
	Ag 2 S/TiO 2	367.2 -	366.1 -			160.4 [23%]		168.7	0.8
		[90%]	[10%]			& 161.2			[37%]
			10			[40%]
	Bulk NiS 2	852.9 -	854.3 -		856.7 -	undet.		undet.	undet.	undet.
	NiS 2 /TiO 2	[42%] 852.3 -[49%]	4 6 8 [32%] 854.7 -[34%] M / Ti (%) (XPS)		HgS NiS 2 MoS x [26%] RuS 2 857.0 -[17%]		RuS 2 161.1 [41%] & 163.7	167.9 [46%]	(	1.3 / = 1.1)
			2	RuS 2	Ag 2 S		[13%]
	MoS x /TiO 2 227.9 -[39%]	0 [61%] 230.0 -	1	2 N metal / 10 nm 2 of TiO 2 3 4 5 6 161.7 [83%]	7	8	168.2 [17%]	0.7 (S x /Mo IV = 1.5)
						164

  Figure V-10 shows the pyrite state (S-S) 2-at 161.9 eV (See Table V-4)[13]. It attest the presence of RuS 2 on TiO 2 .

		18000
		16000	Ru 3p		Ru / RuS x	35000 30000		C 1s & Ru 3d		Ru 3d 5/2 -RuS x
	Intensity (cps)	12000 14000					Intensity (cps)	20000 25000 15000	Ru 3d 3/2 -RuS x O y Ru 3d 3/2 -RuS x C 1s -C-H				Ru 3d 5/2 -RuS x O y
		10000					RuS x O y	10000
		8000						5000
		500	490	480	470	460	450	290	288	286	284	282	280	278	276	274
				Binding Energy (eV)						Binding Energy (eV)
							167

. The first state is metallic Ru or its sulfide counterpart

[13]

. The second state is a more oxidized one, a sulfate or an oxide.

Table V

 V

	-4 reports an atomic ratio %	/%	a bit lower than 0.5 (for both the bulk and the
	supported sample).

It suggests the presence of Ag 2 O in addition to Ag 2 S and Ag 2 SO 4 .

  Ni 2p orbitals are weakly sensitive to Ni oxidation or the nature of ligand attached to Ni[20]. So NiS, NiS 2 or Ni 3 S 2 states have similar BE[21]. In addition, satellite contributions for Ni 2p orbitals make the fit of the spectrum more difficult. Figure V-11 compares the Ni 2p XPS spectra obtained for bulk and supported NiS x . XRD evidenced only a NiS 2 phase for the bulk (See Figure V-3, c) so its XPS spectra must only show NiS 2 and amorphous states. Three states and the corresponding satellite peaks allow fitting the experimental spectrum (See Table V-4).

The first narrow state corresponds to NiS 2 , the second state is a nickel oxide and the third state is NiSO 4

. BE of those states agree with the ones reported in the literature

[20,[22][23][24]

. Sulfate also contributes in the S 2p range of energy close to 168 eV and pyrite state, has the same BE than RuS 2 , close to 161.2 eV. The additional state at 163.7 eV correspond to the sulfur from thioacetamide impurities (See Table V-4 and Figure

170 To summarize, the seven MS x were supported on the same titania support, with the same thioacetamide reflux preparation method and with the same content, as confirmed by elementary analyses. The previous characterizations identify the following photocatalytic material: Ag 2 S/TiO 2 , RuS 2+x /TiO 2 , CuS/TiO 2 , CoS x /TiO 2 , NiS 2 /TiO 2 , HgS/TiO 2 , MoS x /TiO 2 .

  . The S 2s contributes at 225.6 eV. The first one corresponds to Mo IV and the second one to Mo V[25,26]; Mo VI contributes at higher energy 232-233 eV[27,28].Ag 2 S, HgS, NiS 2 and CuS bulk are crystallized so XRD analysis identifies the sulfide phase while the other MS x are amorphous. TPR and XPS analyses confirm the Ag 2 S and NiS 2 phases supported on TiO 2 . It also provides information on the amorphous and/or well-dispersed MS x , for instance it confirms an over stoichiometric RuS 2+x phase.XPS analyses also evidences that co-catalysts are composed of a mix of sulfide and oxide.

		600								500
	Intensity (counts)	500	Mo V : 61 %		S 2s Mo IV : 39 %	Intensity (counts)	400 450	S 2p -SO x	S 2p -S 2-
		400
		240	236	232	228	224	220		350	172	168	164	160	156
				Binding Energy (eV)					Binding Energy (eV)
	Figure V-12: XPS spectra for supported MoS x : Mo 3d -S 2s (left), S 2p (right). On all spectra, black
	line is the experimental spectrum, color lines are the fitting contributions and black dots constitute the
	resulting fitted spectrum.

The first state can be assigned to MoS 2 . The state at 161.7 eV corresponds to S 2-and it evidences the presence of MoS 2 . For the second state, such a BE could correspond to: an oxysulfide

[26]

or an MoS 3 . The ratio % S 2-/ % (Mo IV + Mo V ) evidence an under stoichiometric MoS 0.7 (See Table

V

-3), it denies the hypothesis of a MoS 3 but it is rather surprising. The stoichiometry would show the presence of non sulfurized Mo but the characteristic states are not observed.

Additional experiments are required to determine if it is due to the thioacetamide reflux method that does not completely sulfurized the catalyst or if it is due to the deliberate absence of precautions to deal with the MS x /TiO 2 for PHER tests or characterization.

Finally, microscopy evidences the photocatalyst morphology. NiS 2 and Ag 2 S particles measure 100 nm. The size of the HgS and MoS x particles are close to 10 nm. CuS, CoS x and RuS 2+x particles are small (below 5 nm) and well-dispersed. The RuS 2+x particles measure 1.3 ± 0.2 nm; it was evidenced with STEM but also by combining TPR measurement and a geometrical model.

5.

3 Photocatalytic activity of the seven MS x /TiO 2

	5.3.1	Photon absorption
	Section 3.3.1 describes the absorption step of a photocatalytic process. Some transition metal
	sulfides are semiconductors, in the visible range they could act as a sensitizer. If they only enhance
	charge carrier separation and catalyze the chemical reactions, they act as a co-catalyst.
	30 mg of bulk RuS 2 was tested for the photocatalytic dehydrogenation of propan-2-ol (50 %vol.

of propan-2-ol in water). It showed no activity. It may be due to a lower surface area than the TiO 2 support. It reduces the number of active sites absorbing photons. It can also be due to intrinsic

seven MS x have a co-catalytic effect on titania supports.

  Table V-2), all the MS x enhance the activity of TiO 2 *.Indeed, all the MS x /TiO 2 have a higher PY than TiO 2 * and except CuS/TiO 2 , all the MS x /TiO 2 have a higher PY than TiO 2 (PY (CuS/TiO 2 ) = 0.9 PY (TiO 2 )), (See FigureV-14). It concludes that the

	Surprisingly, molybdenum sulfide which is by far the most studied co-catalyst (See Section 2.4.2)
	is not the most efficient one. The thioacetamide reflux method does not provide an efficient MoS x -
	based photocatalyst. Comparison with the literature is made at the end of Section 5.3.4. The most
	active photocatalyst is RuS 2 /TiO 2 : PY (RuS 2 /TiO 2 ) = 272 PY (TiO 2 *) = 1.9 %. HgS/TiO 2 also shows a
	reasonable activity, fifty times higher than TiO 2 *. It is the first time that such a material is reported for
	photocatalysis, as well as RuS 2 /TiO 2 .

Table V - 5 :

 V5 Summary of PY, and the pre-exponential factor for the seven MS x /TiO 2 and TiO 2 *. Photocatalysts are separated in three families, corresponding to red, blue and black colors.

	MS x	Photon Yield (%)	(kJ/mol)
	RuS 2	2.3 ± 0.3	19.0 ± 1.0	50 ± 10
	NiS 2	0.97 ± 0.1	26.0 ± 1.0	400 ± 100
	CoS x	0.5 ± 0.1	27.0 ± 1.0	240 ± 60
	HgS	0.42 ± 0.05	33.0 ± 1.0	2700 ± 750
	Ag 2 S	0.25 ± 0.05	26.0 ± 1.0	110 ± 30
	MoS x	0.14 ± 0.05	19.0 ± 1.0	3 ± 1
	CuS	0.09 ± 0.05	29.0 ± 1.0	100 ± 35
	TiO 2	0.09 ± 0.01	28.0 ± 1.0	90 ± 30
	TiO 2 *	0.007 ± 0.001	41.0 -46.0	1400 -11600
	5.3.

-5, the seven photocatalyst can be divided into three families. The first one contains supported RuS 2 and MoS x . Their PY is mainly due to a low (19-20 kJ/mol) ( contributes faintly). The second one contains HgS/TiO 2 and TiO 2 * for which mainly contributes to the PY ( contributes faintly). Notice that the PY value is too low for TiO 2 * to obtain reliable and value. For the others, the two terms contribute significantly.

4 Arrhenius parameters for the second MS x /TiO 2 series

  NiS 2 /TiO 2 , RuS 2 /TiO 2 , HgS/TiO 2 and MoS x /TiO 2 photocatalysts were prepared a second time to

for NiS 2 , RuS 2 and MoS x the lamp intensity has no influence on the rate- determining

  -2). It is not surprising because no particular strategy was applied to enhance the amount of edges. RuS 2 /TiO 2 is the best photocatalyst of this screening, it shows an enhancement factor of 28 in methanol[1] and 30 in propan-2-ol. It is similar to the PHER activities obtained in other reactive mixture with RuS 2 /TiO 2 , RuS 2 /SiO 2 -TiO 2[34,35] (See TableII-2).

	Notice on Figure V-15 that PY have similar trends at high and at low intensity. However, it is
	important to pay attention to the Arrhenius parameters (	and	) reported in Table V-6: for
	HgS/TiO 2 and TiO 2 *, the contribution of	and	differ. For the other, the	values are
	the same. It shows that, surface shielded by thioacetamide impurities. Therefore,	(TiO 2 *, LED lamp)	(TiO 2 *,
	HPK lamp)	(TiO 2 , HPK lamp). However, even if TiO 2 * is no longer limited by the amount of
	photon absorbed, the carbon impurities limit the number of active sites and it results that	(TiO 2 ,
	LED lamp)	(TiO 2 , HPK lamp) = 3	(TiO 2 *) (See Table V-6).

step. It reinforces the conclusion that will be detailed in Section 6.1: values do not depend on the incident flux of photons.

At low lamp intensity, with TiO 2 *, the rate-determining step is the absorption of photons ( > 41 kJ/mol, See Table

V

-6)

. At high lamp intensity, the excess of photons compensate the TiO 2

Table V - 6 :

 V6 The PHER activity of four MS x /TiO 2 (photon yield, apparent activation energy and preexponential factor) at two lamps intensity (HPK lamp: 1400 μE/h, LED lamp: 11 200 μE/h). a Metal loading measured by ICP-OES. b Results differ between Table V-5 and Table V-6 because another photocatalyst was used to compare them at the same loading. The PHER activity of four MS x /TiO 2 (MoS x , HgS, NiS 2 and RuS 2 ) and the reference TiO 2 *. Bar charts depict the photon yield. Red squares depict the apparent activation energy. On the

	Photocatalyst	Lamp intensity (μE/h)	Metal (N atomes /10 nm 2 ) a	PY (MS x /TiO 2 )/ PY (TiO 2 )	(kJ/mol)
	HgS/TiO 2	1400 11200	2.7 3.0	4.7 3.0		33 25	2680 15
	NiS 2 /TiO 2 RuS 2 /TiO 2 Figure V-15: left, tests were performed with a HPK lamp ( 1400 b 2.8 b 11200 2.8 1400 2.7 11200 2.0	10.8 12.9 25.0 30.0	23 b 25 19 20 ) and on the right with a LED 60 b 90 40 25
	MoS x /TiO 2 lamp (	11200 1400	2.6 3.0	1.4 1.6		18.5 19	1 3
		1400				28	80
	TiO 2	11200				30	60
		1400				41 -46	1400 -11 600
	TiO 2 *	11200				32	20

It is the best efficiency for a RuS 2 /TiO 2 photocatalyst to this date and it is among the best MS x -based photocatalyst

  2 /TiO 2 is the best photocatalyst. It significantly enhances the activity of bare TiO 2 : PY (RuS 2 /TiO 2 ) = 30 PY (TiO 2 ).

(See Section 2.4).

However, the

activity of RuS 2 /TiO 2 is 7-8 times lower than Pt/TiO 2 (a standard in photocatalysis) and 2.5 times lower than RuO x /TiO 2

(See Annexes, Figure

  Two hypotheses are considered. The first hypothesis is that the rate-determining step varies depending on the MS x . The three families described in Section 5.3.3 would have various intrinsic properties and they would not be limited by the same step. This is at least the case for TiO 2 * studied under HPK lamp. The second hypothesis is that the rate-determining step is the same for bare TiO 2 and the seven MS x /TiO 2 . Then the value depends on the intrinsic activity of the MS x . To distinguish those two hypotheses, the intrinsic activity of MS x are studied in Section 5.5 but also in chapters VI and VII and structure-activity relationships are proposed. TiO 2 and TiO 2 are more efficient than MoS x /TiO 2 and TiO 2 *, respectively (See TableV-6). As the of NiS 2 /TiO 2 is 8 times higher than RuS 2 /TiO 2 it may confirm that NiS 2 /TiO 2 is not only

	50 Those MS x /TiO 2 photocatalysts are of great interest because they provide a whole range of
	intrinsic properties. Measuring PY at different temperature also provides	and	. They
	provide a mechanistic insight and	informs on the rate-determining step.	30 40	E a,app (kJ/mol)
					20
		/	MoS x	H g S N i S 2 RuS 2
				MS x /TiO 2
	This work evidences that the number of active sites influences	. It explains why
	RuS 2 /
		180

composed of the large 100 nm particles evidenced by microscopy images (Figure V-4) but also of small and well-dispersed particles evidenced combining XPS and ICPO-OES (Figure V-7). Chapter VI discusses the influence of other parameters influencing .

TiO 2 with M= 5.4.2 Drawback of the thioacetamide reflux method: impurities i. Thioacetamide impurities: evidence from the photocatalytic test

  During the first 2 h of a PHER test, H 2 S is produced in parallel to H 2 . The μ-GC quantifies the To understand the origin of H 2 S, Figure V-17 shows the amount of H 2 S produced as a function of three parameters, the PHER rate (a), the co-catalyst loading (b) and the mass of photocatalyst. If H 2 S is due to the co-catalyst reduction, thanks to the H 2 produced, it follows this overall mechanism:

		cycle 1	cycle 2
	TiO 2 * therefore, it disproves a potential hydrolysis of remaining thioacetamide (See equ. 5.1): cycle 3 cycle 5 E a,app (kJ/mol) MS x /amount produced. Notice that H 2 S is released under illumination, its production is photocatalytic and cycle 4 5.4
			5.5
			5.6
	Figure V-17 (d) evidences a lower activity for TiO 2 * (thioacetamide treated), as it was already
	discussed in section 5.3.2. But the difference between a NiS 2 /TiO 2 washed two times and a NiS 2 /TiO 2
	washed seven times is not significant. Those washing steps decrease the C content from 1.1 wt% to
	0.2 wt%. n H2S is, as expected, decreased four times (7.5 μmol to 1.9 μmol) but the r H2 only increases of
	10 % (from 7.8 μmol/h to 8.6 μmol). Interestingly, it is the	which is responsible of this

Figure V-17 (a & b) show that n H2S does not evolve linearly with the amount of H 2 or MS x involved during the reaction. According to the equ. 5.6, H 2 S is not due to the MS x reduction. On the contrary, Figure V-17 (c) shows n H2S increasing linearly as a function of the mass of photocatalyst. It argues that surface impurities are responsible of the H 2 S releasing. It can be H 2 S

that desorbs under illumination or it can be thioacetamide that is photocatalytically decomposed in H 2 S. Notice also that the amount of H 2 S released (n H2S ) is lower than the amount of sulfur contained in the material (See Table

V

-2). For instance, 50 mg of CoS x /TiO 2 contains 18 μmol and it releases less than 1 μmol. increasing (from 25 to 80) and not the (from 21 kJ/mol to 22 kJ/mol). It confirms that depends on the number of active sites, and the latter are occupied by thioacetamide impurities.

We conclude that thioacetamide (

or derivatives) have a negligible influence on the PHER activity of MS x /TiO 2 .

  The thioacetamide reflux method consists in first, covering TiO 2 surface with thioacetamide and then, adding a metal precursor. Where the sulfide is formed, it can be assumed that most of the thioacetamide is consumed. Thioacetamide will only block active sites at the surface of TiO 2 , less photons might be absorbed but sulfides are still available to produce H 2 .

For the NiS 2 /TiO 2 , MoS x /TiO 2 and RuS 2 /TiO 2 catalysts, the proportion of S x decreased and the SO x one increased after a PHER test. However, the and are still observed. The MS x /TiO 2 contain MoS 2 , RuS 2 and NiS 2 states after test

  Figure V-20 compares the XPS spectra of NiS 2 /TiO 2 , MoS x /TiO 2 and RuS 2 /TiO 2 , before and after a PHER test of 24 h. It evidences that sulfide phases are kept after tests. Samples contain carbon impurities. Before test, it is remaining thioacetamide or acetic acid from the synthesis. After test, it is propan-2-ol or acetone adsorbed on TiO 2 . This carbon may prevent the detection of all the metal or sulfur on the surface. It leads, for instance, to an increase of Ru and Mo (See Figure V-20, right).For the MoS x /TiO 2 catalyst, the Mo/S ratio decreases. It agrees with what was observed with elemental analysis (See TableS4). The content of Mo V has increased; MoS x might be converted in more oxidized species but not in MoO 3 (no Mo VI observed).For the RuS 2 /TiO 2 , states close to 162 and 280 eV attest the presence of RuS 2 after the PHER test. It is confirmed with the % / (% Ru -RuS x ) ratio close to 2. A non-negligible amount of RuS 2 is however converted in RuO x or in sulfate. For the NiS 2 /TiO 2 , NiS x state (See Figure V-20, red lines) is still observed. The % / % NiS x ratio equals 2.8; it confirms that no Ni 0 was formed (NiS x and Ni 0 has similar BE[20]). NiO

			Mo 3d -S 2s / Ru 3d -C 1s / Ni 2p			S 2p
		78 % Mo V (0.69 % at )		22 % Mo IV (0.19 % at )			34 % SO x (0.13 % at )		66 % S 2-(0.25 % at )
						S 2s
	MoS 2 /TiO 2	61 % Mo V (0.32 % at )		39 % Mo IV (0.21 % at )			17 % SO x (0.07 % at )		83 % S 2-(0.33 % at )
						S 2s
		240	236	232	228	224	220	172	168	164	160	156
			Binding Energy (eV)				Binding Energy (eV)
		7 % at CH/CC 5 % at COC/COH 4 % at COOH		66 % RuO a S b (1.0 % at ) 34 % RuS x /Ru 0	(0.4 % at ) 20 % SO x2	23 % SO x1 (0.5 % at )		57 % S 2-2 (1.1 % at )
						(0.5 % at )
	RuS 2 /TiO 2		1 % at COOH 2 % at COC/COH 5 % at CH/CC	(0.5 % at ) 7 % RuO a S b (0.04 % at ) 93 % RuS x /Ru 0	15 % SO x2 (0.15 % at )	15 % SO x1 (0.15 % at )		70 % S 2-2 (0.7 % at )
		295	290	285	280	275	172	168	164	160	156
			Binding Energy (eV)				Binding Energy (eV)
				38 % NiO z
				(0.13 % at )
	NiS 2 /TiO 2
		868	864	860	856	852	848
			Binding Energy (eV)
	Figure V-20: XPS spectra of MoS x /TiO 2 , RuS 2 /TiO 2 and NiS 2 /TiO 2 before (down) and after (up) the
	content after test (See Figure V-20, left). So discussion on the absolute surface concentration must be photocatalytic dehydrogenation of propan-2-ol: Mo 3d -S 2s, Ru 3d -C 1s, Ni 2p (left) and S 2p
	nuanced. (right) On all spectra, black line is the experimental spectrum, color lines are the fitting contributions
							189

-6)

.

Surprisingly, the is smaller with the Hg I /Hg 0 leached in solution and. The value would be expected close to the one measured with bare TiO 2 (44 kJ/mol with the HPK and 32 kJ/mol with the LED). It means that the mercury has a co-catalytic effect without being deposited on the TiO 2 . Such a phenomenon was already reported by Tabata et al [39]. y and NiSO 4 states are still evidenced by XPS and an additional state is observed (See Figure V-20, green lines), with a BE of 855.7 eV. Its chemical state is unknown, it might be another oxide formed during the PHER test.

For the three photocatalysts, the MS x states are still observed after the PHER test but they are progressively oxidized.

and black dots constitute the resulting fitted spectrum. Relative concentration and semi-absolute atomic concentration (in parentheses) are provided on the spectra. Reproduced from

[1]

with permission.

49 % NiS x (0.49 % at )

	5.4.4	Long time experiment and cycling tests
	Long time experiments were performed on two RuS 2 /TiO 2 photocatalysts. One with a Ru loading
	of 0.8 wt% (See Figure V-21) and another with a loading of 1.6 wt% (not shown). Each time, the
	temperature was regulated at 20 °C during 160 h and 50 mg of photocatalysts were used. In both cases,
	the photocatalyst did not deactivate but, the PHER rate constantly increased until 48 h. After that, it
	stabilized and PY is close to 2.4-2.5 %.
	After 160 h, the PY was measured at four temperatures for the 1.6 wt% Ru sample. It gives an
	value of 20 kJ/mol and a	of 80. It shows no clear difference with the	(19 kJ/mol)
	measured after one day of test and after 160 h (Table V-5). Only the	increased from 50 to 80. If
				10 % NiO y		52 % S 2-2
	17 % NiSO 4 (0.06 % at )		35 % NiS x (0.12 % at ) (0.03 % at )	48 % SO x (0.2 % at )	(0.2 % at )
		17 % NiSO 4	35 % NiO y (0.35 % at )	46 % SO x (0.6 % at )	41 % S 2-2 (0.5 % at )
		(0.17 % at )				13 % S thio.
						(0.2 % at )
				172	168	164	160	156
					Binding Energy (eV)

the RuS 2 is converted to a more active state, it has no influence on the rate-determining step but it may influence the number of active

sites. Either the amount of exposed increases and/or the amount of electrons that are transferred to the co-catalyst increases.

Ru 3d -RuS x /Ru 0 Ru 3d -RuO a S b

	after cycle test	C 1s -CH/CC			after cycle test
	C 1s -COC/COH C 1s -COOH				S 2p -SO x2	S 2p -SO x1	2 S 2p -S 2-
	after long test					after long test
	before test						before test
	295	290	285	280	275	172	168	164	160	156
			Binding Energy (eV)			Binding Energy (eV)

Table V - 7 :

 V7 Relative concentration of sulfur and ruthenium at the surface of the RuS 2 /TiO 2 catalyst before and after tests. A table with the semi-absolute atomic concentrations is provided in the Annexes (TableS5).

		(%)	SO x,1 (%)	SO x,2 (%) Ru-RuS x (%) Ru-RuO a S b (%) S x / RuS x
	before test	78	9	13	75	25	1.41
	after standard test	55	29	17	61	40	1.01
	after long time exp.	78	9	13	64	36	0.29
	after cycling test	48	24	28	68	32	0.08

HDS models the PHER step for which two hydrogen radicals or and entities are coupled, forming H 2.

  To get an insight in it, we propose to investigate other reactions that inform on the catalyst's properties.The first one is the thiophene HDS. It is a refining process; sulfides catalyze the reaction between chemisorbed hydrogen and thiophene, breaking the C-S bond (See Section 2.1). It estimates the ability of a catalyst to dissociate the H-H bond and, because of the microreversibility principle, a catalyst breaking such a bond can catalyze its formation. Thiophene

Table VII - 1 :

 VII1 Summarized of the optical properties of the seven MS x /TiO 2 determined with UV-Vis absorption. a size of the ban gap is extrapolated. Tauc plot for the RuS 2 /TiO 2 photocatalyst. (left) Sample is diluted in BaSO 4 for a final content of 625 ppm, the scan rate of acquisition is 60 nm/min; (right) Sample is diluted in BaSO 4 for a final content of 16 ppm, the scan rate of acquisition is 60 nm/min.

	Compound		Type of band gap	Other transitions
	Ag 2 S/TiO 2	1.0 ± 0.1 eV a	indirect	2.6 and 2.0 ev
	HgS/TiO 2	1.4 ± 0.1 eV	indirect
	CoS x /TiO 2		metallic behavior
	NiS 2 /TiO 2		metallic behavior
	CuS/TiO 2	undet.	undet.	1.6 ± 0.1 eV
	MoS x /TiO 2	1.2 ± 0.1 eV a	indirect
	RuS 2 /TiO 2	1.0	indirect	1.9	eV	(direct
				transition)
	Figure VII-6:

  [START_REF] Kraut | Precise Determination of the Valence-Band Edge in X-Ray Photoemission Spectra: Application to Measurement of Semiconductor Interface Potentials[END_REF], proposed a method to measure the valence band maximum ( ) of a semiconductor, using XPS[17]. The method is illustrated with the example of RuS 2 /TiO 2 on Figure VII-7. It has been recently used to study MoS 2 /TiO 2 [18]. To determine the , Kraut et al. fitted the valence band edge of Ge and GaAs(110) samples with a Chelikowsky-Cohen non local pseudo-potential density of states (DOS)

		7.1.2	Kraut method to position	(MS x ) relatively to	(TiO 2 )
	The	is measured relatively to a core-level. In this work, Ti 2p 3/2 and Ru 3d 5/2 states were
	chosen with, respectively, an energy level of:	and	.	is the energy difference
	between those core levels (	). Therefore, the difference,	, between
	the valence band maxima of RuS 2 and of TiO 2 (	and	) equals:
						7.5
				1.00E-011
				8.00E-012	direct transition: 1.9 eV
			h ) 2 KM (F	4.00E-012 6.00E-012		excitonic peak
				2.00E-012
				0.00E+000
				1.2	1.4	1.6	1.8	2.0
						h (eV)

  al. cannot be applied without a previous knowledge of the valence band states. It was not applied to our materials but linear extrapolation and the 2 nd derivative methods are compared in Section 7.2.2. For TiO 2 and RuS 2 /TiO 2 powders, spectra are adjusted with the TiO 2 state at 458.5 eV. For bulk and supported RuS 2 powders, spectra are adjusted with the RuS x state at 279.9 eV. It gives the spectra on Figure VII-7 and data lead to = 2.3 eV. Valence band spectrum of RuS 2 /TiO 2 evidences the same states than TiO 2 with an additional state around 0.5 eV. It correspond to the intense state on the RuS 2 spectrum. Those states will be extensively discussed in Section 7.2 and 7.3 but notice already

Performing UPS on nanopowders was the aim of this study.

  The methodology discussed below is reported in[1].Two types of substrates were studied for preparing samples. The first substrate is a commercial conducting ITO layer on glass substrate (R S =30-60 Ω/sq). The second one is an Ag foil. The two substrates must be thin enough to maximize the electrical contact between the powder and the detector. Both substrates were chemically cleaned by sonication in ethanol, water and finally in acetone. In addition, the Ag foil was cleaned, in the XPS chamber, with Ar ion etching. Figure VII-12 is the UPS spectra of both substrates. Equ. 7.6 explains the UPS spectrum feature on Figure VII-12. Electrons with high kinetic energy are the one emitted from the Fermi edge ( ) (See the sharp Fermi edge of Ag on Figure VII-12, right). Electrons with just enough kinetic energy to leave the sample form a sharp cut-off ( ) This is valid only in the case for which the work function of the sample is equal to the one of the analyzer; if the sample is in contact with the sample holder and the analyzer their Fermi levels are aligned. The measured for ITO is 17.0 eV; it agrees with the one reported by Beerbom et al. [28].The one of the Ag foil allows calculating a work function of 4.2 eV, close to the values reportedfor Ag[29]. Measurements on known ITO and Ag foil substrate confirm the accurate calibration of the

	(	). And so:
			100	h =21.2 eV 10.7 Mbarn
		Cross Section (Mbarn)	0.001 0.01 0.1 1 10	h =1486.4 eV 2.6 10 -4 Mbarn h =40.8 eV 6.8 Mbarn x 4115	7.7
			1E-4
				0	200 400 600 800 1000 1200 1400
					Energy (eV)

3.2, UPS is mainly conducted on model materials such as single crystals.

apparatus. ITO is a suitable substrate for UPS measurement. Below 10 eV, the energy range of interest, ITO does not evidence significant states. On the contrary, Ag does but it limits the charging effects.

Table VII

 VII

					1 4 1 2 1 0	8	6	4	2	0	-2
						Binding energy (eV)
	12	10	8	6	4	2		0
						π -O 2p orbitals	π -hybridized orbitals	σ -hybridized orbitals
	Preparation method
								(%)	(%)	(%)
	Drop of a TiO2-heptane suspension			18	35	47
	Drop of a TiO2-pentane suspension			19	40	41
	Electrophoretic deposition					23	49	29

-2 summarized the results. It shows that density of π -O 2p states are independent of the preparation method but the density of the hybridized states varies. No explanation was found but it must be kept in mind when comparing this experimental DOS.

Table VII-2:

Summarized of the density of π -O 2p and Ti-O hybridized states, obtained with the three preparation methods. Reproduced from

[1]

with permission.

It indicates that the step prior proton reduction is the transfer from anatase to RuS 2 and not from rutile to RuS 2 . It confirms the type II-anatase alignment on Figure VII-17. Electrons photogenerated in rutile first transfer to anatase and then to RuS 2 ; electrons photogenerated in anatase directly transfer to RuS 2 .

  See Figure VII-17). It agrees with the PHER activity of RuS 2 / -rutile -antase -P90 -Vis absorption spectrum (Figure VII-4). Indeed, four transitions were observed at 4.1 (α), 4.5 (β), 5.0 (γ) and 5.4 eV (δ). Crossing the 3.15 band gap in energy means that for each transition, 0.95 (α), 1.35 (β), 1.85 (γ) and 2.25 eV (δ) are additional energies to excite deeper valence band states and send them in more energetic conduction band states. The only states that can give rise to transitions α and β are the π -O 2p orbitals. Transitions γ and δ may be due to the excitation of πhybridized orbital or to π -O 2p orbitals. The energy of the incident photons is not enough to excite the σ -hybridized state.The methodology described in the previous Sections determined the absolute electronic structure of nanopowders. It was applied to oxysulfide powders prepared at Sorbonne university. Tuning the Gd/Ce ratio in (Gd,Ce) 2 O 2 S gives a band gap between 2.1 and 4.7 eV. Position of the valence band maximum were determined with UPS on those powders[38].

	P25 reported in Section 6.4.2. The	measured for RuS 2 /P25 and RuS 2 /P90 were the same than
	for RuS 2 /PC500 (pure anatase) but their	differ from the one of RuS 2 /rutile (nanorods). In
	addition, Chapters IV, V and VI argue that the rate-determining step is the electron transfer from the
	support to RuS 2 .

The four valence band states are centered for anatase at -9.05 ± 1.3, -10.2 ± 1.9, -12.3 ± 2.8 and -on its UV-7.

3 Absolute electronic structure of RuS 2 /TiO 2 7.3.1 Sample preparation

  Section 7.2.2 reports two preparation methods to obtain powder layer for UPS analysis. EPD and dropping method were used to obtain RuS 2 /TiO 2 layer on ITO and Ag foil. Unfortunately, with the EPD method, XPS analyses reveal that RuS 2 is totally converted into sulfate (See Annexes,

	Figure

intense contribution of the Ru 4d t 2g states between 0 and 2 eV below the valence band maximum. Below 2 eV it is the hybridized Ru 4d e g and S 3p orbitals that contribute.

  -19 (left) shows the resulting for bulk RuS 2 . First it can be concluded that the valence band spectrum differs with the one acquired with XPS (See Figure VII-7, red line). It is due to the difference of the cross-sections for the Ru & S states at 1486.6 and 21.2 eV. In addition, PDOS of RuS 2 are reported in few works[22,[54][55][56]. They all evidenced an The orbitals O 2p π from the O 2 contamination can also contribute. Density of the first states are reported two-three times higher than the hybridized ones. For the experimental DOS obtained by UPS, the relative intensity of those states is reversed, density of the Ru 4d t 2g states are four times lower than hyb. I or hyb. II. (See Figure.

Figure VII-19 (right) shows the UPS spectrum of RuS 2 /TiO 2 . The four TiO 2 states, already discussed in Section 7.2.3 are observed between 3 and 12 eV. The relative density of π -O 2p states, π 268 thanks to that,

the absolute position of RuS 2 was measured, 0.6 eV below the Fermi edge of the Ag foil. It therefore locates the valence band maximum at -4.8 eV. Figure VII-20:

  Resulting (grey line) for bare TiO 2 , RuS 2 /TiO 2 (1.5 wt% Ru) and RuS 2 /TiO 2 (6.2 wt%). Inset zooms the low binding energy range of the spectrum. Ru 4d t 2g states between in RuS 2 /TiO 2 and bulk RuS 2 is negligible.Proximity of their Fermi level, may favor electrons transfer from TiO 2 to RuS 2 but those levels cannot be determined accurately with UPS. For instance, the difference of E cut-off measured for bulk RuS 2 /ITO and TiO 2 /ITO gives, thanks to equ. 7.7, the difference of Fermi levels. The one of TiO 2 is measured 1.6 eV above the one of RuS 2 , a band alignment would create a significant energy barrier between TiO 2 and RuS 2 which refutes the co-catalytic effect of RuS 2 for alcohol dehydrogenation.

		RuS 2 /TiO 2 -6.2 wt% Ru
		RuS 2 /TiO 2 -1.5 wt% Ru
	I signal (a. u.)	TiO 2	Intensity (a. u.)
			4	3 Binding energy (eV) 2 1	0
	7.3.3 Figure VII-21 depicts the band diagram of bare TiO 2 -PC500, bulk RuS 2 and RuS 2 /TiO 2 . Error bar Absolute electronic structure for RuS 2 -based powders is estimated to be 0.2 eV due to: intrinsic resolution of the apparatus (See Figure VII-12, right) and error that can be made to determine . Band diagrams show that and of RuS 2 are both less negative than for anatase. It is a type II-anatase according to Figure VII-18 For the heterostructre, . It means that the RuS 2 co-catalyst imposes the valence band maximum after Fermi levels equalized. TiO 2 valence band states (O 2p -π and Ti-O hybridized) can be compared before and after junction with RuS 2 . Their energies are shifted, closer to the vacuum, of less than 0.1 eV. It indicates that and are close to each other and that is closer to the vacuum level. The shift of the 7 6 5 4 3 2 1 0 -1 Binding energy (eV)
		269

  RuS 2 -based powders were also investigated combining those techniques. For bulk RuS 2 , valence and conduction band positions are at -4.8 eV and -3.8 eV. RuS 2 exhibits four states, one due to Ru 4d -t 2g orbitals, it imposes the valence band maximum and three, deeper in energy, that are due to hybridized (Ru 4d e g -S 3p) orbitals.We conclude that electron transfers are not favored from TiO 2 to RuS 2 . This transfer is the ratedetermining step for the photocatalytic dehydrogenation of propan-2-ol with RuS 2 /TiO 2 catalyst. A low portion of photogenerated electrons, that are energetic enough, can transfer from TiO 2 to RuS 2 , the TiO 2 , HgS/TiO 2 and MoS x /TiO 2 it is unclear if the electronic transfer is also the ratedetermining step or if it is the proton reduction. Additional characterizations are needed. This work evidences that MS x /TiO 2 are promising photoactive materials. Transition metal sulfides were already evidenced as suitable co-catalysts for the PHER. However, literature lacks comparative studies and mechanistic insights. It is impossible to compare PHER activity, from the literature, due to various experimental conditions, for instance: light intensity, mass of photocatalyst, reactive mixture.Addition of an MS x enhances the PHER activity of bare TiO 2 . It is often explained by a band alignment at the MS x -TiO 2 interface, enhancing charge carrier separation but proofs are scarce.Regarding their PHER activity, no MS x /TiO 2 reported in this manuscript compete with Pt/TiO 2 for the photocatalytic dehydrogenation of propan-2-ol. The best photocatalyst reported in this work is RuS 2 /TiO 2 ; Pt/TiO 2 and RuO x /TiO 2 are 7.7 times and 2.5 times more efficient than RuS 2

	Combining UPS and UV-Vis diffuse reflectance spectroscopy gives the energy levels of PC500,
	used along this work as a TiO 2 support. It has	= -7.1 eV and	= -3.95 eV. UPS, in addition,
	gives the experimental density of valence band states. TiO 2 powders (anatase or rutile) have four main
	states: π -O 2p orbitals, π -hybridized (Ti-O) orbitals, σ -hybridized (Ti-O) orbitals and OH
	adsorbed on its surface. The first state is the only one that can be photoexcited during a PHER test
	(λ > 290 nm).
	limiting parameter is the amount of and the decreases with photons higher in energy (See Section 6.4.2). Increasing the RuS 2 active sites. It explains why the increases loading (See Section 6.3) also increases the surface of the TiO 2 -RuS 2 interface and thus, the amount of . and For NiS 2 /Conclusions Perspectives
		279

To confirm those results, RuS 2 -based powders were extensively studied with UPS, a more resolved technique.

However, this study has a great interest from a fundamental point of view. Transition metal sulfides have various properties, from their ability to activate H 2 to their optical properties, including their promising electrochemical behavior. The study of various supported transition metal sulfides provides a whole range of intrinsic properties. Those were correlated with the PHER activity of the MS x /TiO 2 catalysts.

  2 provides mechanistic insights. Indeed, HgS co-catalyst significantly increases the PHER activity whereas it has no activity for H 2 activation nor electrocatalytic H 2 production. The only reason of this enhancement is that the HgS conduction band is lower than TiO 2 one (See Section 7.1.3). Photogenerated electrons are driven from the TiO 2 to the HgS particles enhancing charge carrier separation. However those electrons must react on the TiO 2 active sites to produce H 2 . The band structure of HgS determined combining XPS and DRS evidence that it cannot reduce protons. RuS 2 catalyst, supported on five TiO 2 supports (anatase, rutile, brookite or mixed phases), gives various activity. varies as a function of the TiO 2 -polymorph. It confirms that the proton reduction which occurs mostly on RuS 2 ( on Main Scheme 6) is not the rate-determining step. The latter can be either the electron transfer or the charge carrier separation but in both cases the electronic structure of MS x /TiO 2 plays a key role. UPS and UV-Vis absorption spectroscopies investigate the band alignment of RuS 2 /TiO 2 heterostructure. UPS measures the valence band maximum of RuS 2 , TiO 2 and RuS 2 /TiO 2 . UV-Vis absorption spectroscopy gives the optical band gap of each semiconductor. It shows that the electron Conclusions and Perspectives 283 transfer is not favored in RuS 2 /TiO 2 , the conduction band of RuS 2 is higher in energy (i. e. closer to the vacuum level) than TiO 2 . The amount of active sites limits the overall PHER activity However, as RuS 2 has a co-catalytic effect on TiO 2 , some electrons must migrate from the TiO 2 to RuS 2 . It is confirmed with PHER tests, RuS 2 /TiO 2 is photoexcited at different wavelength and a lower is measured for higher photon energy. Hot electrons can therefore transfer before thermalization and the amount of . The amount of electrons reaching the RuS 2 particles therefore decreases and increases . This conclusion stands for RuS 2 /TiO 2 ; it is unproved if the electron transfer is the ratedetermining step for all the MS x /TiO 2 . Electronic structure should be determined for other materials (it has been done combining XPS and UV-Vis absorption spectroscopies but the XPS resolution is not enough). UPS also provides the valence band states of those materials. It evidences the states that are mainly involved in photocatalytic processes. For bare TiO 2 it is the non bonding O 2p orbital; for RuS 2 it is the Ru 4d t 2g . It is now important to understand how the sulfide influences this Ru state. Is it detrimental? In that case, sulfide has less interest as a co-catalyst than metallic or oxide counterpart. It is benefic? In that case, the PHER activity can be tuned with different M/S stoichiometry. A material of interest would be NiS x because various Ni/S stoichiometry can be obtained.

Table S1 :

 S1 Sum up of and for the photocatalytic dehydrogenation of methanol, ethanol and propan-2-ol with RuS 2 /TiO 2 (1.15 wt% Ru). A LED lamp, working at a power of 950 mW provides

	μ	. Temperature is regulated at T=20 °C.
		rH2 (μmol/h)	° (kJ/mol)	(mW)	η (%)
	methanol	56	65.3	1.0	0.11
	ethanol	43	42.2	0.5	0.05
	propan-2-ol	28	25.9	0.2	0.02

  The highest PHER rate reached with Pt/TiO 2 is 3190 μmol/h. Assuming that two are required to form one H 2 , the amount of must be at least of 1.07 10 18 /s. The experiment is performed with a lamp intensity of 17 500 μE/h. The quantum yield must be at least 18%, so the must receive more than 5.27 10 17 electrons/s. Lifetime of and are required to remove the time component and to conclude. Assuming that they are similar, the 1.11 10 18 atoms of Pt contain at least 1.07 10 18 and 5.27 10 17 per

				40
					Mean 2.7 nm
					Standard Deviation 0,6 nm
				30
			Amount	20
				10
				0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 0
					Size (nm)
	second. Without free	,		. So amount of	represent less
	that 67 % of the total -species. In addition	can be superior to zero and the quantum yield can
	be higher than 18%. A preliminary conclusion is that ,	but lifetime of
	and	would be required to conclude.
	Microkinetic model: additional comments
	Assuming that	is not neglected in comparison to . Notice that	is still mandatory
	to produce H 2 . The approximation of the stationary state is applied to	which is extremly
	reactive:

Table S4 :

 S4 Elemental amount (metal and S) of selected MS x /TiO 2 before and after their used for the photocatalytic dehydrogenation of propan-2-ol under a HPK / LED lamp. a measured by ICP-OES. S content obtained by CHNS. c Those samples were studied by XPS.

			before test		after test
			wt% metal a	S / M b	wt% metal a	S / M b
		RuS 2 /TiO 2 c	1.6	3.4	1.0	3.0
	HPK lamp	NiS 2 /TiO 2 c Ag 2 S/TiO 2	0.95 1.9	3.1 0.9	0.5 1.6	0.8 0.2
		RuS 2 /TiO 2	1.15	6.0	1.15	2.8
	LED lamp	NiS 2 /TiO 2 HgS/TiO 2	0.72 3.6	3.1 1.6	0.1	2.2
		MoS x /TiO 2 c	1.45	1.7	1.41	0.6

b

Table S5 :

 S5 Semi-absolute atomic concentration of sulfur and ruthenium at the surface of the RuS 2 /TiO 2 catalyst before and after tests.

(%

at ) SO x,1 (% at ) SO x,2 (% at ) Ru-RuS x (% at ) Ru-RuO a S b (% at ) S x / RuS x

	before test	2.13	0.24	0.36	1.51	0.51	1.41
	after standard test	1.03	0.55	0.31	1.00	0.66	1.01
	after long time exp.	1.01	0.11	0.17	3.46	1.93	0.29
	after cycling test	0.28	0.14	0.16	3.55	1.65	0.08

Main Scheme 5: Mechanism for the photocatalytic dehydrogenation of propan-2-ol with MS x /TiO 2 .

Chapter I:

Production of solar fuels

Chapter I introduces the concept of solar fuels and the photocatalytic production of H 2 . This Ph.D.

project is focused on the production of H 2 with the photocatalytic dehydrogenation of alcohols. This chapter provides an overview of this reaction and of the photocatalysts used.

68

Xu and Schoonen [32] agrees the experimental works discussed above. As any powder is unique, of the seven MS x /TiO 2 studied in this manuscript will be measured (See Section 7.1.1). Those experimental data will be compared with those works.

121

The PhD project focuses on the PHER with titania supported transition metal sulfides. It aims to identify the rate-determining step and the key parameters influencing the PHER rate. To do so, it is important to study the two half-reactions, namely the reduction and the oxidation part of the overall mechanism ( and on Main Scheme 3). The reduction part is the half-reaction of interest, as it leads to the targeted H 2 . It is always reported in the literature, measuring a PHER rate, whereas the oxidation of alcohols, is seldom discussed in detail [1]. An entire mechanism is rarely provided.

Section 1.2.4 explains how the literature is ambiguous on the photocatalytic conversion of alcohol-water mixtures. Depending on the study and on the alcohol molecule, the photocatalytic conversion is described as a dehydrogenation or a reforming process. So a well-defined system should be selected prior to study the intrinsic properties of the photocatalyst.

Main Scheme 3: Mechanism for the photocatalytic dehydrogenation of alcohols with MS x /TiO 2 .

We believe that the photocatalytic dehydrogenation of propan-2-ol would be the simplest reaction. Ambiguity about the oxidation half-reaction is avoided and therefore, it may be a good descriptor of the performance of a given photocatalyst. This reaction is extensively studied in this chapter with two types of photocatalysts: a reference Pt/TiO 2 (1.0 wt% Pt, P25) and RuS 2 /TiO 2 (1.15 wt% Ru, PC500), representative of the sulfide-based photocatalysts.

The results obtained with the benchmark Pt/TiO 2 (P25) catalyst leads to a mechanism and a micro-kinetic model for the photocatalytic conversion of a propan-2-ol-water mixture. It was prepared with a photodeposition method. TiO 2 powder (Degusa P25) was suspended in the reactive

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Propene was observed with only three co-catalysts among the seven supported sulfides studied in Chapter V: NiS 2 , CoS x and RuS 2 .

Propene is a well-known product of the thermal catalytic dehydration of propan-2-ol [17,18]. The latter occurs easily on many oxides, including titania. However, propene production does not occur in dark condition. The reaction is photocatalytically driven. The following mechanism is therefore proposed:

°

In presence of H 2 , propene could be hydrogenated in propane but this is not observed for any of the MS x /TiO 2 materials studied.

Figure IV-8 reports the PY and the apparent activation energies for the propene formation with three photocatalysts. The best one is NiS 2 /TiO 2 : r propene = 2.5 μmol/h. In this case, the propene formation rate is 21 lesser than the hydrogen formation one. This ratio is similar for CoS x (x 14) or higher for RuS 2 (x 97). It seems therefore reasonable to neglect, in the micro-kinetic study, the dehydration in comparison to the dehydrogenation pathway.

The is higher for the dehydration of the propan-2-ol than for its dehydrogenation. The measured values are 3 times lower to the one usually obtained in thermal dehydration of propan-2-ol [19,20]. It is in agreement with the fact that propene formation is photocatalytic. While the sulfide phase and the presence of light does not influence the rate-determining step for the propan-2-ol dehydration, it seems to influence the one of the propan-2-ol dehydrogenation (See squares on Figure . This is extensively studied in Chapter V. The previous macro kinetic model evidences a dissociative mechanism (See Section 4.1.2). Therefore, the propan-2-ol dissociate in and , on adjacent pair sites: and (See equ. 4.9). Therefore, we assumed:

. Consequently, the PHER rate (equ.

Influence of the incident photons on the PHER activity

This section discusses the influence of the incident photons on the PHER efficiency of RuS 2 /TiO 2 (1.15 wt% Ru on PC500). During light absorption (See on Main Scheme 5), TiO 2 converts photons in electron hole pairs. The influence of the amount and of the energy of the incident photons, reaching the bottom of the photoreactor, are studied.

Influence of the photon flux on the PHER activity

To study the influence of the incident amount photons on the PHER rate and on and , the incident amount of photons was tuned from 0 to 16 500 μE/h (See extrapolates the initial slope. Therefore, calculating a PY, defined as the ration between r H2 and is inappropriate at intensity higher than 1800 -2500 μE/H, it underestimates the efficiency of a catalyst.

It explains why the orders of magnitude for PY, reported on Figure V-15 (See Section 5.3.1), differ between the HPK and the LED lamp.

Herrmann theorized that at high intensity, the PHER rate does not evolve linearly with but with , a constant [1] (or See Annexes). It was also reported experimentally for TiO 2 [2] and CdS [3]. Herrmann explained this relationship by a predominant charge carrier recombination. Too much photons are absorbed by the TiO 2 surface, charge carriers are not transferred fast enough to the co-catalyst and most of them recombine.

TiO 2 -anatase provides smaller particles than Al 2 O 3 [23]. TPR experiment was performed with the impregnated sample (See Annexes, Figure S24). It shows a similar shape than for the sample obtained with the thioacetamide reflux method. The ratio of surface sulfur over total amount of sulfur is estimated at 0.65. The geometrical model discussed in Section 5.2.3 gives a particle size of 1.3 nm; it disagrees with TEM images. It shows the limitation of the model. Indeed, such preparation method involves H 2 S that can be adsorbed on the large surface area PC500; all surface sulfur used in the model may not be related to RuS 2 .

XPS spectra (See Annexes, Figure S25) confirm a similar composition for the impregnated sample and for the one obtained with the thioacetamide reflux method. They show the RuS x /Ru 0 state at 279.9 eV and a more oxidized one with a higher binding energy for fresh catalyst. Again, as no particular precaution was taken to perform the PHER test, none was taken to insert the sample in the XPS chamber.

Thioacetamide reflux method Impregnation and sulfidation method

Preliminary study with UV-Vis & XPS

To determine the band diagram of the MS x /TiO 2 photocatalysts, two parameters are experimentally measured: the valence band maximum ( ) and the optical band gap (E g ). The first is obtained with photoelectron spectroscopy and the second is obtained with diffuse reflectance spectroscopy (DRS). XPS analysis has a lower resolution than UPS but it is easier to perform.

Therefore, a first study of four MS x /TiO 2 were made combining DRS and XPS. The electronic structure of RuS 2 /TiO 2 was then better resolved combining DRS and UPS

UV-Vis Absorption spectroscopy to determine i. Kubelka-Munk theory and its use

In 1931, Kubelka and Munk developed the most used theory for the diffusion reflection of layers [2]. It concerns plane-parallel layer with an infinite thickness. The latter is needed to neglect the transmittance in the theory [3]. In practice, powders are prepared in a sample holder with an appropriate thickness, at least 4 and 5 mm to prevent transmission phenomenon.

Kubelka-Munk approximation relates UV-Vis absorption spectra and DRS.

The full demonstration of the so-called Kubelka-Munk equation can be found in [4] or in [5]. The equation reads:

is the Kubelka-Munk function, is absorption coefficient defined by Lambert-Bouguer's law (also named ), is the scattering coefficient and is the reflectance. It assumes that the two coefficients are constant through the all sample volume and that the scattering coefficient is independent of the wavelength. However, it occurs only when the average grain size is large as compared to the wavelength.

Pure BaSO 4 is often used as a reference; it has a low absorbance in the wavelength range of interest and its scattering coefficient is almost independent of the wavelength [6]. BaSO 4 must also be used to dilute samples [4,7] active sites or they require an additional energy from the lattice to transfer to RuS 2 .

However, the main drawback of such determination is the XPS resolution (close to 0.5 eV). With the methodology used for determining this error bar is estimated to 0.8 eV. Within such an error, an electron transfer between anatase and RuS 2 can be favored or not (as plotted).

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of 14.9 V during 1 min. With their low boiling point, charging additives are easily removed, which is crucial for UPS, a surface sensitive technique.

Therefore, EPD technique gives a homogeneous and well-controlled TiO 2 layers on a conducting sample without impurities while the so-called "dropping method" is easier to process and the resulting inhomogeneity is crucial to perform internal calibration, as discussed below. Either the and the of rutile are in-between those of anatase (type I) or the and the of one polymorph are more negative than those of the other.

A type II-anatase alignment was already reported in the literature, combining XPS and UV-Vis absorption spectroscopy [49,50] or Kelvin probe force microscopy [52] Ko et al. used DFT calculations (FHI-AIMS code) to predict the type of alignment as a function of rutile and anatase particle size. XRD analyses and Scherrer equation give a diameter of 7-8 nm and 18-24 nm for the anatase and the rutile, respectively (See Annexes, Figure S32). According to their model, a type I (i.e and of rutile between those of anatase) should be obtained.

- S6). The lower the energy barrier between the valence band maximum and the oxidation potential of the alcohol is, the faster the holes are consumed and the less they can recombine with electrons.

The difference between the two trends is due to the choice of the . Al-Azri et al took the corresponding to the reforming of the six alcohols studied. They assumed completer reforming without providing experimental evidences. differs if it is a reforming or a dehydrogenation process. For instance, (methanol) > (propan-2-ol) whereas (methanol) < (propan-2-ol) (See Table III-1). That is why, it is crucial to discriminate between dehydrogenation and reforming reactions by analyzing the other products of the reaction.

Calibration of the experimental set-up

Figure S8 depicts the calibration curve for propan-2-ol on left and acetone on right. Known amounts of liquid acetone were injected in pure water (See insets). A calibration curve was established between the molar fraction of acetone after each injection and peak area on the chromatograph. For a given flux of gas vector, it correlates the amount of acetone in the liquid phase and the one detected by μ-GC. Calibration assumes that gas/liquid equilibrium is reached with a small Ar-flow. The same procedure is used to calibrate propan-2-ol. Curves were corrected to take into account the stripping occurring in the semi-batch reactor.

However, for propan-2-ol, the relationship is no longer linear. Slopes below and above a molar fraction of 0.09 are different. Notice that tests are performed starting at 0.19 (commonly, 25 mL of propan-2-ol in 50 mL of water). Among the side reactions that could occur, Fontelles et al. reported the formation of propanonediisopropyl-acetal. It results from the reaction between one acetone and two propan-2-ol molecules [3]. Zhu and co-workers reported the photocatalytic coupling of one acetone and one propan-2-ol forming the 2,3-dimetyl-2,3-butanediol [4,5]. Hunger and Horvath reported the catalytic coupling of two molecules of propan-2-ol forming a diisopropyl ether [6].

Adsorption of propan-2-ol and acetone on TiO 2

Table S2: Sum up of the three photocatalytic tests performed with Pt/TiO 2 and with an initial propan-2-ol concentration of: 20, 10 or 5 %vol. IPA and Ac. stand respectively for propan-2-ol and acetone.

[IPA] (t=0) (%vol.)

[IPA] (t=0) And therefore:

In absence of a co-catalyst, the charge carrier recombination governs the overall activity of bare TiO 2 . It is assumed:

. In that case the PHER rate is simplified in:

Assuming that each is written as , an analogy with the Arrhenius law gives and .

When Pt or RuS 2 are deposited on the TiO 2 surface, photogenerated electrons and holes are spatially separated. The recombination rate is low and so the term can be neglected:

Annexes

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Two special cases can then be detailed:

-If :

Therefore, the and the corresponds, respectively, to and . The ratedetermining step is the proton reduction on the co-catalytic particles.

-If :

Therefore, the and the corresponds, respectively, to and . The rate-determining step is the electron transfer from the TiO 2 conduction band to the co-catalyst. Notice that in that case, is independent of ; it disagrees with the macro kinetic model. At low lamp intensity, the charge carriers are consumed fast enough by the molecules, the ratedetermining step is either the oxidation or the reduction half-reaction.

If electrons are less numerous than holes:

The approximation of the stationary state is used on :

And so the rate of production is proportional to .

If holes are less numerous than electrons, the oxidation must be the rate-determining step and it is assumed that radical is easier to oxidize than propan-2-ol.

The approximation of the stationary state is used on :

And so the rate of production is also proportional to :

At low lamp intensity, when the charge carriers are consumed fast enough by the molecules, the rate of production is proportional to .

On the contrary, at high intensity, charge carriers accumulate and the recombination is becoming