Ms Mr Johann Pascale Jezequel

Ms Maya Dewalle

Mr Ahmed Marinova

Ms Camila Addad

Palombo

Ms Mr Xuemei Tong Li

Ms Anouchka Liu

Mr Bang Kimene

Mr Xiang Gu

Mr Feng Yu

Mr Zhiping Niu

Ms Ye

Ms Xiaofeng Haiqin Quan

Mr Yue Yi

Mr Jian Wang

Mr Wei Zhang

Mr Shaohua Jiang

Mr Chengnan Xie

Mr Alberto Li

Ms Karen Mazzi

Mr F Silva

Ms Ana Alex

Ms Sofia

Alexandre

Keywords: Bimetallic catalyst, gold alloy, glucose oxidation, synergistic effect

Ré sumé

Depuis qu'Haruta et Hutchings ont ré vé lé dans les anné es 80 l'activité de l'or dans l'oxydation du CO et l'hydrochloration de l'é thylè ne, les catalyseurs basé s sur ce mé tal ont pris un rôle de plus en plus important dans le domaine de la catalyse et leur utilisation s'est é normé ment dé veloppé e, notamment pour la synthè se de monomè res d'acé tate de vinyle, la synthè se directe du peroxyde d'hydrogè ne à partir de H 2 et O 2 , et l'oxydation des hydrocarbures et des alcools. Cependant, depuis le dé but de leur histoire, les systè mes catalytiques à base d'or ont toujours é té affecté s par une grande variation des ré sultats catalytiques obtenus en fonction de la mé thode de pré paration et du type de support utilisé . En effet, de nombreuses é tudes ont souligné l'importance de la bonne combinaison de ces paramè tres pour bien dé finir la morphologie des particules d'or et les interactions mé tal-support, toutes deux capables de modifier profondé ment l'activité et/ou la sé lectivité du catalyseur. Par consé quent, beaucoup d'attention a é té accordé e à la pré paration des catalyseurs à base d'or afin d'é tablir autant que possible la relation entre la mé thode de pré paration, le support choisi et les caracté ristiques des maté riaux catalytiques produits.

Contrairement au Pt ou au Pd, les catalyseurs basés sur l'or ont montré une trè s bonne sé lectivité en produits dé siré s et une ré sistance à l'empoisonnement lorsque de l'oxygè ne molé culaire est utilisé comme oxydant. Cependant, les catalyseurs à l'or ont souvent montré une faible activité par rapport aux catalyseurs basé s sur le Pt ou le Pd et ne conviennent pas à l'oxydation des alcools en milieu basique. Les systè mes bimé talliques pourraient cependant aider à dé passer ces limites, en combinant les proprié té s des deux mé taux constitutifs. Une grande amé lioration des proprié té s catalytiques a dé jà é té observé e pour de nombreuses ré actions lorsque l'on combine l'or avec un autre mé tal pour former de nouveaux sites actifs en induisant des effets de synergie. Tandis que dans le cas des catalyseurs monomé talliques à l'or, l'activité est surtout influencé e par la taille des particules et la nature du support, dans le cas des catalyseurs bimé talliques ils sont gé né ralement des facteurs secondaires. En effet, la structure associant l'or et le second mé tal joue alors un rôle pré pondé rant dans la dé termination de la performance catalytique. L'optimisation et la maitrise de l'architecture des nanoparticules bimé talliques constitue donc un objectif majeur afin de pouvoir é tablir des relations structure-activité permettant la conception rationnelle de nouveaux catalyseurs performants. Le type de morphologie pré paré dé pend du protocole de synthè se, de la miscibilité des deux mé taux et du traitement post-synthè se.

La composition (ratio entre les deux mé taux) est bien entendu é galement un paramè tre important influant sur les performances catalytiques. Cependant, la question de savoir quel est le meilleur ratio des deux mé taux dans le catalyseur optimal pour une ré action donné e et pourquoi, reste toujours sans ré ponse. Dans ce travail, l'oxydation du glucose en absence de base a é té choisie comme une ré action modè le repré sentative pour é tudier ces facteurs. catalyseurs Au-Ag supporté s sur SiO 2 pré paré s par une mé thode d'adsorption de sol avec diffé rents ratios de mé taux à pH 9,5. L'effet de synergie entre Au et Ag et sa dé pendance au rapport molaire Ag/Au ont é té é tudié s dans l'oxydation du glucose. Un effet de synergie a é té observé par rapport à l'é chantillon de ré fé rence Au/SiO 2 dans le cas des é chantillons bimé talliques avec un rapport molaire Ag/Au = 50/50. Une activité synergique a é té rapporté e dans les trois systè mes à pH é levé . Cependant, l'effet synergique dans l'oxydation du glucose en absence de base a peu ou pas é té é tudié . Ce cas de figure est pourtant trè s inté ressant car sans ajout de base le mé canisme de la ré action pourrait s'en trouver changé avec des effets sur la sélectivité.

En consé quence, l'objectif principal de cette thè se est de ré aliser une é tude systé matique approfondie de la corré lation entre les proprié té s physico-chimiques et catalytiques de catalyseurs hé té rogè nes bimé talliques pour l'oxydation du glucose en absence de base dans le milieu ré actionnel. En particulier, des é ventuels effets synergiques positifs entre les mé taux constituant les nanoparticules des systè mes catalytiques bimé talliques seront recherché s. Dans ce contexte, un ensemble de catalyseurs mono-et bimé talliques supporté s et à base d'or ont é té synthé tisé s, caractérisés et testés pour l'oxydation du glucose en absence de base en utilisant les é quipements de criblage à haut dé bit de la plateforme REALCAT.

Le pré sent manuscrit est organisé comme suit : Le chapitre I fait le point sur la litté rature sur les catalyseurs bimé talliques, en particulier les catalyseurs bimé talliques à base d'or et sur l'oxydation du glucose dans des conditions exemptes de base. Le chapitre II concerne la description des mé thodes et dispositifs expé rimentaux. Il dé crit en dé tail la pré paration des catalyseurs, leur caracté risation, la mé thode d'é valuation de leurs performances catalytiques, ainsi que les mé thodes analytiques. Le chapitre III met l'accent sur l'oxydation du glucose sur des catalyseurs bimé talliques à base d'or et de palladium avec des ratios de mé taux, des supports et des mé thodes de pré paration diffé rents. Le chapitre IV concerne l'étude de l'oxydation du glucose en absence de base sur des catalyseurs à base d'or combiné avec d'autres mé taux (Cu, Pt et Bi). Une comparaison avec les catalyseurs Au-Pd y est effectué . Enfin, le manuscrit se termine par la conclusion gé né rale et la proposition de quelques perspectives à cette é tude.

Comme dit pré cé demment, la litté rature nous apprend que les proprié té s des systè mes catalytiques à base d'or sont fortement affecté es par la mé thode de pré paration utilisé e en relation avec le type de support utilisé et, bien sûr, par le rapport des mé taux le composant. Ces paramè tres dé terminent la morphologie des nanoparticules mé talliques, les interactions mé tal-mé tal et mé tal-support, ce qui influe sur l'activité et la sé lectivité des catalyseurs. Dans ce travail de thè se, des sé ries de catalyseurs Au-X (où X = Pd, Cu, Pt, Bi) avec diffé rents ratios molaires et supporté s sur ZrO 2 ou TiO 2 ont é té pré paré s par diffé rentes mé thodes puis caractérisés et testés pour l'oxydation du glucose en absence de base. Il a systématiquement été tenter d'établir une relation entre les conditions de synthè se (et donc les proprié té s physico-chimiques des solides) et leurs performances catalytiques. La recyclabilité des catalyseurs les plus prometteurs a é galement é té é tudié e.

Les catalyseurs bimé talliques de la sé rie Au-Pd ont é té supporté s, soit sur TiO 2 , soit sur ZrO 2 , et tous deux ont é té pré paré s, soit par une mé thode de sol-immobilisation, soit par une mé thode de ré duction-pré cipitation. La quantité totale de mé taux dé posé s sur le support é tait de 1% en poids thé orique et le rapport entre Au et Pd a é té varié . Les analyses é lé mentaires ré alisé es par ICP-OES et XRF ont ré vé lé que les charges ré elles en mé taux é taient proches des valeurs thé oriques. Les tests catalytiques ont é té ré alisé s dans un ré acteur discontinu à diffé rentes tempé ratures, sous 5 bar d'air et durant 5 h.

Certains catalyseurs ont é té choisis pour tester diffé rentes conditions de ré action. Une tempé rature plus é levé e é tait favorable à l'activité mais pas à la sélectivité. De 60 ˚C à 100 ˚C, l'activité catalytique augmente énormément tandis que la sélectivité en acide gluconique diminue lé gè rement. Les essais de recyclabilité ont é té effectué s sur le catalyseur le plus prometteur, c'est-à -dire celui à 0,5% en poids de Au et 0,5% en poids de Pd sur TiO 2 pré paré par la mé thode d'immobilisation de sol. Aprè s trois essais, le catalyseur conservait l'inté gralité de ses performances catalytiques sans aucun traitement ré gé né ratif entre les tests. Au contraire, le catalyseur à 0,7% en poids de Au et 0,3% en poids de Pd sur ZrO 2 pré paré par sol-immobilisation a montré une diminution constante de son activité catalytique au cours des 3 cycles ré alisé s. Le support joue donc un des rôles les plus importants dans la stabilité des catalyseurs. Des effets de synergie et d'anti-synergie entre le palladium et l'or ont é té observé s lors des essais catalytiques. Un effet synergique é vident a notamment é té montré sur les catalyseurs de la sé rie Au-Pd/TiO 2 pré paré s par la mé thode de sol-immobilisation, alors qu'il y avait une anti-synergie sur les catalyseurs de la sé rie Au-Pd/ZrO 2 pré paré s par la mé thode de pré cipitation-ré duction. L'analyse XPS a permis de dé montrer que la présence d'espèces Au +δ et Pd +2 jouait un rôle important dans la ré action d'oxydation du glucose. L'analyse TEM a ré vé lé que la taille des particules mé talliques des catalyseurs pré paré s par la mé thode d'immobilisation du sol é tait plus petite que celle des catalyseurs pré paré s par la mé thode de ré duction-pré cipitation. Les images TEM et le diagramme XRD ont montré qu'il n'y avait pas de changement de morphologie des supports au cours de la pré paration des catalyseurs et des ré actions. Le PVA utilisé lors de la pré paration pour disperser les mé taux joue un rôle important lors de la pré paration des catalyseurs mais é galement pendant la ré action s'il n'est pas totalement éliminé lors des pré traitements. Il a non seulement un rôle de stabilisant pour obtenir des nanoparticules de taille contrôlé e et bien distribué es à la surface du support, mais il contribue é galement à obtenir une meilleure sé lectivité en acide gluconique et à la stabilité des catalyseurs. Une é tude de calcination du PVA ré siduel a é té ré alisé e. Elle montre que la taille des particules de mé tal augmente et que le Pd est oxydé pendant ce traitement. Il a é té é tabli que la taille des particules de mé tal n'est pas le facteur le plus dé terminant pour obtenir une activité é levé e. En effet, la nature du support est la plus importante dans l'oxydation du glucose. Les catalyseurs supporté s sur ZrO 2 ont montré une meilleure activité mais moins de stabilité en ré action. En revanche, les catalyseurs supporté s sur TiO 2 pré sentent une meilleure stabilité mais une moins bonne activité pour cette ré action. Les deux supports ont donc des avantages et des inconvé nients.

Les catalyseurs bimé talliques de la sé rie Au-Cu ont é té supporté s, soit sur TiO 2 , soit sur ZrO 2 , et tous deux ont é té pré paré s par un procé dé de ré duction-pré cipitation.

Aucun changement significatif de morphologie des nanoparticules et des supports a é té constaté lors de la pré paration des catalyseurs et aprè s ré action. Un effet synergique é vident a é té dé montré sur les catalyseurs de la sé rie Au-Cu/TiO 2 pour une ré action à 60 °C. En revanche, une anti-synergie a é té observé e dans les mê mes conditions de tests pour les catalyseurs de la sé rie Au-Cu/ZrO 2 . Comme pour la sé rie Au-Pd, la sé rie Au-Cu supporté e sur ZrO 2 est plus active que les catalyseurs supporté s sur TiO 2 . Le rapport Au/Cu n'affecte pas la sé lectivité lorsque la teneur en or est supé rieure à 0,3% en poids. La sé lectivité en acide gluconique sur les catalyseurs supporté s sur ZrO 2 est bien meilleure que sur les catalyseurs supporté s sur TiO 2 . Ceci est diffé rent des catalyseurs de la sé rie Au-Pd où la sé lectivité en acide gluconique n'é tait pas trè s diffé rente sur TiO 2 ou ZrO 2 . Le rapport pondé ral optimal est Au/Cu = 0,5 : 0,5. Ainsi, le catalyseur à 0,5% en poids de Au et 0,5% en poids de Cu sur TiO 2 a é té choisi pour les essais de recyclabilité . Contrairement à ce qui avait é té observé pour le catalyseur à 0,5% en poids de Au et 0,5% en poids de Pd sur TiO 2 , une diminution progressive de la conversion du glucose a é té observé lorsque le catalyseur a é té utilisé plusieurs fois. En effet, la conversion du glucose é tait de 45,7% pour le premier test, puis a diminué à 35,2% dans le second, et à 28,5% pour le troisiè me. La sé lectivité en acide gluconique est resté e pratiquement toujours la mê me autour de 65%. Une lixiviation importante du cuivre au cours de la ré action a é té confirmé e par les analyses ICP-OES et XPS ce qui explique cette dé sactivation progressive.

Finalement, il a donc é té conclu que les systè mes Au-Pd seront plus performants si supporté s sur TiO 2 alors que les systè mes Au-Cu seront meilleurs si supporté es sur ZrO 2 lorsqu'ils sont pré paré s par une mé thode de pré cipitation-ré duction.

Des sé ries Au-Bi et Au-Pt supporté es sur ZrO 2 ont é galement é té pré paré es par la mé thode de pré cipitation-ré duction pour comparaison. Le bismuth en tant que second mé tal n'a pas joué de rôle positif par rapport à Pd ou Cu. En effet, un effet anti-synergique est observé . La sé lectivité en acide gluconique sur les catalyseurs de la sé rie Au-Bi é tait trè s mauvaise et les catalyseurs non stables. En ce qui concerne les catalyseurs de la sé rie Au-Pt, le catalyseur à 0,75% en poids de Pt et 0,25% en poids de Au sur ZrO 2 pré sente la meilleure conversion de glucose (67,8%) et la meilleure sé lectivité en acide gluconique (80,5%). Contrairement à d'autres catalyseurs à base d'or, les catalyseurs Au-Pt/ZrO 2 é taient toujours actifs lorsque la teneur en or é tait infé rieure à 0,3% en poids. Aucun effet synergique é vident n'a é té observé pour cette sé rie. Ainsi, il ne semble donc pas intéressant d'associer ce métal avec l'or pour obtenir de meilleures performances dans l'oxydation du glucose en absence de base. Au final, seuls Pd et Cu apportent un effet de synergie positif avec Au.

Pour la suite du projet, il serait inté ressant d'effectuer des tests consé cutifs supplé mentaires du catalyseur à 0,5% en poids de Au et 0,5% en poids de Pd sur TiO 2 pré paré par la mé thode sol-immobilisation afin de valider sa stabilité et confirmer sa recyclabilité . Il serait inté ressant de dé terminer la duré e de vie de ce catalyseur. Si le catalyseur se dé sactive finalement aprè s un certain nombre d'essais consé cutifs, il serait important de trouver une mé thode pour le ré gé né rer, comme un traitement sous hydrogè ne ou par un autre agent ré ducteur. Au cas où le catalyseur pourrait ê tre trè s bien ré gé né ré , il deviendrait sans doute trè s inté ressant d'un point de vue industriel. D'un autre côté , une é tude ciné tique de la ré action mené e sur ce catalyseur utilisé sans base serait inté ressante.

List of Abbreviations, Acronyms and Symbols

General introduction

Since Haruta and Hutchings first disclosed the peculiarity of gold-based catalysts in CO oxidation and ethylene hydrochlorination in the 80s this metal has reached a very special status in the field of catalysis [START_REF] Haruta | Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0[END_REF][START_REF] Hutchings | Vapor phase hydrochlorination of acetylene: Correlation of catalytic activity of supported metal chloride catalysts[END_REF]. Actually, the use of gold-based catalysts has enormously expanded. It is now widely used in vinyl acetate monomer synthesis, in the direct synthesis of hydrogen peroxide from H 2 and O 2 , and in the oxidation of a variety of hydrocarbons and alcohols [START_REF] Silva | Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols[END_REF]. However, gold-based catalytic systems were always affected by a high variation of their catalytic properties depending on the preparation method employed, which is related to the kind of support used. In fact, many studies stated the importance of the preparation conditions in determining the morphology of the gold particles and the type of metal-support interactions, both being able to profoundly modify the activity and/or the selectivity of the catalyst. For this reason, a lot of attention has been paid to the preparation of gold catalysts in order to assess as much as possible the relation between support/preparation method and the characteristics of the produced catalytic materials [4,[START_REF] Comotti | Aerobic oxidation of glucose with gold catalyst: Hydrogen peroxide as intermediate and reagent[END_REF].

Unlike Pt or Pd, Au-based catalysts showed very good selectivity into desired products and high resistance to O 2 -poisoning when molecular oxygen is used as the oxidant. However, gold-based catalysts often showed low activity compared to Pt or Pd-based catalysts and are not suitable for alcohol oxidation in alkaline conditions.

However, bimetallic systems could overpass such limitations, combining the properties associated with the two constituent metals. There was a great enhancement of catalytic properties in many reactions when combining gold with other metals to form new active sites induced by synergistic effects [START_REF] Haruta | Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0[END_REF][START_REF] Hutchings | Vapor phase hydrochlorination of acetylene: Correlation of catalytic activity of supported metal chloride catalysts[END_REF][6][START_REF] Hutchings | Catalysis by gold[END_REF][START_REF] Bianchi | Selective oxidation of glycerol with oxygen using mono and bimetallic catalysts based on Au, Pd and Pt metals[END_REF][START_REF] Dimitratos | Synergetic effect of platinum or palladium on gold catalyst in the selective oxidation of D-sorbitol[END_REF]. While in case of monometallic gold catalysts the activity is mainly influenced by the particle size and the nature of the support, in the case of bimetallic catalysts they are secondary factors of importance [START_REF] Benkó | Bimetallic Ag-Au/SiO 2 catalysts: Formation, structure and synergistic activity in glucose oxidation[END_REF][START_REF] Haynes | Particle size effect in glucose oxidation with Pd/CB catalysts[END_REF]. Actually, the arrangement between gold and the other metal atoms plays the most important role in determining the catalytic performance [START_REF] Ferrer | Three-layer core/shell structure in Au-Pd bimetallic nanoparticles[END_REF][START_REF] Mu | Synergetic effect of surface and subsurface Ni species at Pt-Ni bimetallic catalysts for CO oxidation[END_REF]. Therefore, controlling and optimizing the architecture of the bimetallic nanoparticles (noted NPs in the following) constitutes the major goal to get high performance and to merge synthesis with structure-activity relationships (the so-called -catalysis by design‖), that ultimately allow the rational design of performing catalysts. The type of morphology obtained strongly depends on the synthetic protocol, the miscibility of both metals, but also on post-synthesis treatment [START_REF] Silva | Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols[END_REF]. The composition is obviously strongly influencing the catalytic performance as shown in many previous studies [START_REF] Silva | Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols[END_REF][START_REF] Benkó | Bimetallic Ag-Au/SiO 2 catalysts: Formation, structure and synergistic activity in glucose oxidation[END_REF][START_REF] Heidkamp | Ceria supported gold-platinum catalysts for the selective oxidation of alkyl ethoxylates[END_REF][START_REF] Comotti | Mono-and bimetallic catalysts for glucose oxidation[END_REF]. Thus, the question of which is the best metal ratio in the optimal catalyst for a given reaction and why, still remains unanswered.

In this work, we have chosen the oxidation of glucose as a representative reaction to study these factors. Comotti et al. [START_REF] Comotti | Mono-and bimetallic catalysts for glucose oxidation[END_REF] have studied the activity of Au-Pt and Au-Pd NPs supported on activated carbon compared to monometallic (Au, Pt, Pd, Rh) catalysts in glucose oxidation at controlled and uncontrolled pH. Hermans et al. [START_REF] Hermans | Preparation of Au-Pd/C catalysts by adsorption of metallic species in aqueous phase for selective oxidation[END_REF] have also reported synergistic activity in glucose oxidation at high pH over Au-Pd/C catalysts prepared by impregnation in aqueous solution. The synergistic effect was related to high Pd surface content. Timea et al. [START_REF] Benkó | Bimetallic Ag-Au/SiO 2 catalysts: Formation, structure and synergistic activity in glucose oxidation[END_REF] have reported SiO 2 -supported Au-Ag catalysts prepared by sol adsorption method with different metal ratios at pH 9.5.

The Au-Ag bimetallic effect and its dependence on the Ag/Au molar ratio was studied in glucose oxidation where a synergistic activity increase was observed in the case of the bimetallic samples with less than Ag/Au = 50/50 molar ratio when compared to the Au/SiO 2 monometallic reference sample. Synergistic effect has been reported for the three bimetallic systems at high pH. However, this synergistic effect in base free oxidation of glucose was seldom studied. However, it would be very interesting because there is no base added. The oxidation products of glucose are various but gluconic acid was chosen as the desired product in this work because the selectivity to this acid is strongly depending on the catalyst properties [START_REF] Zhang | Heterogeneously catalyzed hydrothermal processing of C5-C6 sugars[END_REF]. In the absence of a base the isomerization of glucose to fructose is slowed down. Thus the selectivity to gluconic acid should be enhanced. In addition, this route would be a one-step synthesis of gluconic acid from glucose instead of the acidification of a salt obtained at high pH.

Moreover, it will be easier to study the occurrence of any synergistic effect with a base-free system.

In consequence, the main objective of this thesis is to achieve a thorough systematic study of the correlation between the physicochemical and catalytic properties of bimetallic heterogeneous gold-based catalysts for the base-free oxidation of glucose. Especially, the occurrence of any synergetic effect in these catalytic systems will be studied. In this context, a set of different mono-and bimetallic supported gold-based catalysts has been synthesized, characterized and tested in the base free oxidation of glucose. The high-throughput equipment of the REALCAT platform i was used to accelerate the collection of experimental data.

The present manuscript is organized as follows: Chapter I provides an overview of the state-of-the-art on gold-based bimetallic catalysts and on oxidation of glucose under base-free conditions. Chapter II describes the experimental set-ups and procedures. It details the catalysts preparation, characterization, the method of evaluation of their catalytic performances, as well as the analytical methods used. Chapter III focuses on the results obtained in the oxidation of glucose over gold-palladium catalysts with different metal ratios, supports and prepared by different methods. Chapter IV deals with the oxidation of glucose over gold catalysts combined with different metals, namely Cu, Pt, Bi, in a view to compare them with the gold-palladium system. The manuscript is then ended by the general conclusions and some perspectives to this study.

i www.realcat.fr

Chapter I

Literature Review

Supported Au nanoparticles catalysts

As said above, Haruta and Hutchings were the first to report in the 80s gold as being of interest for catalysis [START_REF] Haruta | Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0[END_REF][START_REF] Hutchings | Vapor phase hydrochlorination of acetylene: Correlation of catalytic activity of supported metal chloride catalysts[END_REF][START_REF] Silva | Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols[END_REF]. Nowadays, Au nanoparticles-based catalysts are widely used for the vinyl acetate monomer synthesis, for the direct synthesis of hydrogen peroxide from H 2 and O 2 , and in the oxidation of hydrocarbons and alcohols.

In order to better understand the behavior of supported Au catalysts, firstly the terms nanoparticle (NP) and metallic nanoparticle should be defined.

Nanoparticles

The prefix nano-was adopted as an official SI (International System) prefix, meaning 10 -9 of an SI base unit at the 11 th General Conference on Weights and Measures (CGPM) in 1960. In 2000, the National Nanotechnology Initiative (NNI)

established the definition of nanotechnology, which is the ability to work at a molecular level, atom by atom, to create large structures with fundamentally new organization [START_REF] Roco | The long view of nanotechnology development: the National Nanotechnology Initiative at 10 years[END_REF][START_REF] Wojcieszak | Study of supported nickel catalysts prepared by aqueous hydrazine method. Hydrogenating properties and hydrogen storage. Support effect. Silver additive effect[END_REF].

Nanoscience is the study of phenomena occurring in the 1-100 nm range.

Nanomaterials are those which have structured components with at least one dimension that is smaller than 100 nm.

NPs are nanosized structures in which at least one of its phases has one or more dimensions (length, width or thickness) in the nanometer scale (from 1 to 100 nm). The properties of material at this level can change dramatically because of the so-called -quantum effects‖ [START_REF] Aharonov | Topological quantum effects for neutral particles[END_REF][START_REF] Hooft | Computation of the quantum effects due to a four-dimensional pseudoparticle[END_REF]. Without changing the chemical composition, but only reducing the particle size, materials can exhibit new properties such as different color, elasticity, conductivity, better and even new chemical reactivity [START_REF] Philip | Nanostructured materials[END_REF][START_REF] Federico | Nanostructured surfaces: challenges and frontiers in nanotechnology[END_REF][START_REF] Khan | Nanoparticles: Properties, applications and toxicities[END_REF]. The size and surface of NPs control the properties of the nanomaterials. They are interrelated because the surface to volume (S/V) ratio increase as the size decreases. Because surface atoms tend to be coordinatively unsaturated, there is a large energy associated with this surface. The smaller the nanocrystal, the larger the contribution made by the surface energy to the overall energy of the system [START_REF] Burda | Chemistry and properties of nanocrystals of different shapes[END_REF].

NPs can be classified into different classes based on their properties, shapes or sizes. The different groups include fullerenes, metallic NPs, ceramic NPs, and polymeric NPs [START_REF] Khan | Nanoparticles: Properties, applications and toxicities[END_REF]. In this work, only metallic NPs were studied in more details as they can be used as efficient catalysts for a number of reactions of interest.

Metallic nanoparticles

The term metallic nanoparticle is used to describe nanosized metals with dimensions (length, width or thickness) within the size range 1-100 nm. The existence of metallic NPs in solution was first recognized by Faraday in 1857 [START_REF] Faraday | The bakerian lecture: Experimental relations of gold (and other metals) to light[END_REF] and a quantitative explanation of their color was given by Mie in 1908 [START_REF] Gustav | Beiträ ge zur Optik trüber Medien, speziell kolloidaler Metallösungen[END_REF].

Metallic NPs exhibit unusual optical, thermal, chemical and physical properties that are due to the combination of a large proportion of high energy surface atoms compared to the bulk solid with the nanometer scale mean free path of an electron in a metal [START_REF] Panigrahi | General method of synthesis for metal nanoparticles[END_REF]. Synthesis of metal NPs has received considerable attention in the past two decades in regard of their numerous potential applications. There are generally two routes for the preparation of nanoscale particles: top-down and bottom-up. Top-down methods consist in reducing macroscopic particles to the nanosize scale, e.g. by mechanical grinding of bulk materials. On the contrary, bottom-up methods start with atoms that aggregate in solution or even in the gas phase to form particles of definite size under appropriate experimental conditions. Top-down processes are hardly suited for preparing particles with uniform shape and particle size smaller than 100 nm. On the contrary, bottom-up routes are much better for generating uniform particles with distinct size, shape and structure.

Monometallic Au supported catalysts

Among the various kinds of metal NPs, gold NPs catalyst is one of the most important. In order to reduce the amount of gold consumed and to be able to recycle it easily, supports are usually used. The primary role of the support is to avoid coalescence and agglomeration of the gold NPs. This is an important issue since the catalytic activity of gold NPs diminishes considerably as the particle size grows beyond 10 nm [START_REF] Comotti | The catalytic activity of -naked‖ gold particles[END_REF]. Characteristics of the support such as surface area, presence of surface hydroxyl groups, density of defects and crystal phase influence the adsorption ability of the support. In addition, supports can go further than simple carriers and can play a direct or indirect role in gold-catalyzed reactions as will be discussed later [START_REF] Corma | Supported gold nanoparticles as catalysts for organic reactions[END_REF].

All the general methods used for preparing metal-supported catalysts have been tested in the case of gold. However, gold presents peculiar characteristics, namely a low melting point and a low affinity for oxides, which permit the production of gold particles with a catalytic relevant activity, i.e., with nanometric (1-9 nm) diameter.

There are several ways to prepare gold particles. The most commonly used preparation methods are impregnation, deposition-precipitation and co-precipitation but there are also other special techniques such as vapor-phase deposition, sol immobilization and grinding [START_REF] Corma | Supported gold nanoparticles as catalysts for organic reactions[END_REF]. All these techniques are presented in the following sections.

Methods used for monometallic Au-based catalysts preparation

Impregnation

In this commonly used method the support is contacted with a solution of the metal precursor, and then it is aged, dried and calcined. Depending on the volume of solution with respect to the pore volume of the support, two types of impregnation can be distinguished: the so-called -incipient wetness‖ impregnation if the solution volume does not exceed the pore volume of the support and the -wet‖ impregnation when an excess of solution is used. The characteristics of the catalyst obtained strictly depend on the post-treatment conditions (rate of heating, final temperature, duration of the heat treatment, atmosphere under which it is done) and, obviously, on the type of supporting material. In fact, during the calcination step sintering of the precursor and reaction between the metal precursor and the support might occur. Moreover, the precursor can also react and evolve from the chemical point of view (thermal decomposition of nitrates for instance). The use of different conditions can lead to different metal-support interactions, which are of fundamental importance for the catalytic applications because they modify the electronic properties of the active phase. In the case of gold, the precursors generally used (such as HAuCl 4 for instance) spontaneously decompose to yield the metal at T > 200 °C. However, as the effect of temperature is detrimental to the metal dispersion, the reduction step is generally enhanced in presence of H 2 or, even better, the reduction in the liquid phase can be done by NaBH 4 or hydrazine without heating, i.e. without calcination [START_REF] Su | XPS and DRS of Au/TiO 2 catalysts: effect of pretreatment[END_REF][START_REF] Pestryakov | Physicochemical study of active sites of metal catalysts for alcohol partial oxidation[END_REF][START_REF] Pestryakova | Influence of modifying additives on electronic state of supported gold[END_REF][START_REF] Margitfalvi | Au/MgO catalysts modified with ascorbic acid for low temperature CO oxidation[END_REF].

Conventional impregnation with chloroauric acid (HAuCl 4 ) has led to much less active catalysts than deposition-precipitation (DP) or co-precipitation (CP) methods [START_REF] Lee | Supported Au catalysts for low-temperature CO oxidation prepared by impregnation[END_REF]. However, the simplicity of this methodology and the convenience of using chloroauric acid as the gold source, make impregnation very attractive for industrial scale-up purposes, and because of that, much research has been dedicated to try to improve this preparation method. The effect of the aging step during the preparation protocol was studied [START_REF] Lee | Supported Au catalysts for low-temperature CO oxidation prepared by impregnation[END_REF]. In the case of Au/γ-Al 2 O 3 catalyst for CO oxidation the HAuCl 4 solutions were aged in pH ranging from 5 to 11 for 15 to 720 min. UV/vis spectroscopy showed that during aging, hydrolysis of the AuCl 4 -complex gave rise to several species. It was found that Au particle size and loading were influenced by the precursor speciation. A significant improvement in the impregnation method has been reported using a two-step procedure in which impregnation of alumina with chloroauric acid was followed by washing of the excess of Au precursor and treatment of the solid with a strong base to convert the chloride to an absorbed hydroxide [START_REF] Xu | The preparation of highly dispersed Au/Al 2 O 3 by aqueous impregnation[END_REF]. Drying and calcination at 400 °C further yielded a catalyst with Au particles having an average diameter of 2.4 nm. The activity of the catalyst was comparable to catalysts prepared by deposition precipitation and it was found resistant to hydrothermal sintering. Quite recently [START_REF] Baatz | New innovative gold catalysts prepared by an improved incipient wetness method[END_REF] it was also disclosed that in addition to the pH effect, a relevant role is played also by the chloride ions concentration in the solution. In fact, HAuCl 4 dissolved in 2 M HCl, after reduction by H 2 at 250 °C, produced metallic particles of 1-2 nm mean diameter deposited on Al 2 O 3 . It should be noted that, in contrast to gas phase oxidation, the catalytic behavior in liquid phase oxidation appeared to be not so much affected by the presence of chloride [START_REF] Baatz | New innovative gold catalysts prepared by an improved incipient wetness method[END_REF]. A detailed study on the ion exchange between Al 2 O 3 and HAuCl 4 has also been reported as a function of pH, which in turn can modify both the species in the solution and the charge on the support surface. It should be noted that the ion adsorption on the surface of the support strictly depends on the isoelectric point (IEP) of the supporting material. The interaction between the charge that the surface assumes in solution and the ionic gold species, can be fruitfully used for well-established method that allows a certain control in the average particle size of the gold NPs is the variation of the pH of the gold salt-solid support suspension during the first stage of the adsorption on the solid surface. pH value of the adsorption is a crucial parameter in the preparation procedure of gold NPs supported on metal oxides. As a general rule, the particle size decreases in the range 25 to 5 nm when the pH increases from 4 to 9 [START_REF] Uphade | Synthesis and characterization of Ti-MCM-41 and vapor-phase epoxidation of propylene using H 2 and O 2 over Au/Ti-MCM-41[END_REF][START_REF] Bond | Gold-catalysed oxidation of carbon monoxide[END_REF][START_REF] Bond | Catalysis by gold[END_REF][START_REF] Sun | A review of the different techniques for solid surface acid-base characterization[END_REF].

The impregnation method using as-precipitated wet metal hydroxides as supports and Au phosphine complexes as gold source has also been studied [START_REF] Liu | Active oxygen species and mechanism for low-temperature CO oxidation reaction on a TiO 2 -supported Au catalyst prepared from Au(PPh 3 )(NO 3 ) and as-precipitated titanium hydroxide[END_REF][START_REF] Kozlov | Supported gold catalysts prepared from a gold phosphine precursor and as-precipitated metal-hydroxide precursors: Effect of preparation conditions on the catalytic performance[END_REF].

As-precipitated hydroxides were impregnated with an acetone solution of Au(PPh 3 )(NO 3 ), probably because a high concentration of OH at the surface is needed for a suitable interaction between the support and the organo-gold complex. This method has been named as liquid-grafting (LG). Vacuum drying at room temperature was followed by air calcination. Changes in the activities of both Au/Fe oxide and Au/Ti oxide catalysts have been attributed mainly to changes of the Au particle size distribution occurring during calcination.

Cationic complex such as Au(ethylenediamine) 2 Cl 3 can interact with negatively charged carbon surface in aqueous solution providing Au NPs well dispersed on the support surface. In contrast, using HAuCl 4 , largely aggregated Au metallic particles were obtained principally due to the redox properties of activated carbon (Figure I-1).

In fact, the higher variety of activated carbon functionalities than those present at the surface of oxides can promote other electrostatic interactions. Using this method Au on α-Fe 2 O 3 , MnOx and ZnO can be prepared with good dispersion. Addition of magnesium citrate during or after the precipitation to play the roles of anticoagulating agent was shown to be beneficial in some cases, such as for Au/TiO 2 . The effect of aging, calcination temperature and ratio of the precursors were studied for CO oxidation in the presence of O 2 or H 2 (PROX), but no general trends have been depicted [START_REF] Kahlich | Kinetics of the selective low-temperature oxidation of CO in H 2 -rich gas over Au/α-Fe 2 O 3[END_REF][START_REF] Lee | Effect of drying conditions of Au-MN CO-precipitates for low-temperature CO oxidation[END_REF][START_REF] Bailie | Hydrogenation of but-2-enal over supported Au/ZnO catalysts[END_REF][START_REF] Tsubota | Preparation of highly dispersed gold on titanium and magnesium oxide[END_REF].

Deposition-Precipitation

This technique consists in the precipitation of a metal hydroxide or carbonate on the surface of a support via the reaction of a base with the precursor of the metal. It was the first efficient method reported to produce highly active gold supported NPs. Rapid nucleation and growth in the solution leads to large crystallites unable to enter into the support pores, leading to a heterogeneous distribution of the metal. To produce good precipitation distributions, an effective mixing and a very slow addition of the base solution must be accomplished. Urea was found to be the best base and is now widely used in many deposition-precipitation preparations. It is usually added at room temperature and, by rising the temperature to 90 °C, it slowly hydrolyses generating ammonium hydroxide homogeneously through the solution. The rate of precipitation is generally higher than that of hydrolysis and, in this way the pH of the solution remains practically constant. After the deposition-precipitation step, the product is filtered, washed, dried and calcined as in the co-precipitation procedure. The optimum pH range for precipitation that also assures an efficient metal utilization (> 90%) is primarily dictated by the isoelectric point (IEP) of the supporting material. This leads to the main constraint of this method, that is, the inapplicability of the so-called acidic oxide (IEP < 5) such as SiO 2 (IEP = 2). This method offers the advantage of locating all the active metal onto the support surface, so that no precious metal is wasted in the bulk of the support as in the co-precipitation method. It is also capable of producing very narrow particle size distributions. However, the result is very sensitive to the nature of the support. For example, although this method is well established for the preparation of Au on non-acidic oxides, the use of this method applied to active carbon failed in producing highly dispersed Au NPs (Figure I-2) [4,[START_REF] Mitsutaka | Chemical vapor deposition of gold nanoparticles on MCM-41 and their catalytic activities for the low-temperature oxidation of CO and of H 2[END_REF].

A

Other methods of preparation

These methods could generally be referred as particular cases of impregnation and can be subdivided as follows.

a. Metal vapor deposition

Dimethyl-Au(III)-acetyl acetonate can be vaporized in a vacuum system by heating and deposited on a support. The method is highly efficient and does not suffer from the limitations of DP preparation, i.e., it is applicable to any kind of support.

Ligand exchange between Me 2 Au(acac) and the surface hydroxyl groups and adsorbed

b. Solid grinding

Dimethyl-Au(III)-acetyl acetonate was used as solid material thus avoiding any solvent in a procedure called solid grinding that was fruitfully applied for preparing gold on porous coordinated polymers and then extended to other supports such as active carbons. The gold precursor is ground together with the support using different apparatus, such as mortar or ball milling, and then calcined in air at 300 °C. Similar interactions between Me 2 Au(acac) and the support surface as in the case of vapor deposited can be supposed. The advantage of this technique is the easy applicability to any kind of support [START_REF] Okatsu | Deposition of gold nanoparticles on carbons for aerobic glucose oxidation[END_REF][START_REF] Ishida | Influence of the support and the size of gold clusters on catalytic activity for glucose oxidation[END_REF].

The mean diameters of the Au particles on nanoporous carbon (NPC) were calculated by HAADF (high-angle annular dark-field scanning transmission electron microscopy) STEM and TEM to be 1.9 and 2.6 nm, respectively. The mean diameter of Au particles on Al 2 O 3 was calculated to be 2.6 nm by TEM and HAADF STEM.

c. Metallic sol immobilization

The immobilization of pre-formed metallic sols is also widely applied. The advantage of using this technique principally lies in its applicability regardless of the type of support employed and the possible control on particle size/distribution, obtaining normally highly dispersed metal catalyst. The method is based on the preparation of Au NPs and their subsequent immobilization on a support. Therefore, generally no subsequent catalyst reduction is needed. Thus modifications on morphology and properties of the material which can occur during calcination are avoided. For catalytic applications particle size has to be ranged from 2 to 25 nm and sols have to show good stability. Common procedures include: reduction of metal salts, photochemical or thermal decomposition, and reduction of organometallic complexes.

Conclusions

To sum up, it can be said that the preparation method should be carefully chosen in regards of the purpose of the Au-based catalyst which is desired. Actually, the different preparation methods lead to different final catalysts even if the composition is the same.

For example, sol-immobilization method will lead to small and narrow dispersed supported NPs while solid grinding is more suitable to get bigger particles. For catalytic applications sol-immobilization is generally very suitable. This method of preparation has often been used in the literature to prepare catalysts for liquid phase reaction, like the oxidation of glucose or of benzyl alcohol. The precipitation-reduction method is also of interest for the preparation of heterogeneous catalysts and it will be used in this work for comparison [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF][START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF].

Characterization

A detailed characterization study of supported gold NPs requires the combination of different experimental techniques, each of them providing specific information that complements the data obtained from the others. TEM allows determining the shape and particle size distribution of the gold NPs and their location. However, two words of caution are needed: (1) There is always a risk that particles are not uniformly distributed over the support, and it is desirable to inspect a number of different areas of the sample in order to obtain a representative impression; (2) Reduction of oxidized species to metal can occur in ultrahigh vacuum (UHV) under the influence of the electron beam, and this can change the composition of the material entering the instrument. X-ray diffraction affords a mean size estimate from line broadening, but it only senses particles large enough to give coherent diffraction. Wide-angle x-ray scattering (WAXS) [START_REF] Galvagno | Chemical reactivity of supported gold: IV. Reduction of NO by H 2[END_REF], small-angle x-ray scattering (SAXS) [START_REF] Galvagno | Chemical reactivity of supported gold: IV. Reduction of NO by H 2[END_REF], and its -anomalous‖ relative (ASAXS) [START_REF] Benedetti | ASAXS Investigation of a Au/C Catalyst[END_REF] have also been used. Extended x-ray absorption fine structure (EXAFS) [START_REF] Bassi | Chemical reactivity of supported gold: III. Atomic binding and coordination of gold from X-ray absorption fine structure spectroscopy[END_REF][START_REF] Cocco | Chemical reactivity of supported gold. A structural study by small-angle x-ray scattering and x-ray absorption spectroscopy[END_REF] yields a mean gold-gold distance and a coordination number from which particle size can be obtained; other information such as gold-oxygen distances may also be found by this technique.

X-ray photoelectron spectroscopy (XPS) serves to establish the elemental composition, Au oxidation states on the surface. Fourier-transform infrared spectroscopy (FT-IR) also helps to analyze the elemental composition on the surface of the catalyst. Both methods serve to establish the strength of interaction with probe molecules.

Concerning the interaction of gold NPs with the support, it has been proposed that they can lead to the stabilization of charged gold atoms at the interphase between gold and the support [START_REF] Guzman | Co oxidation catalyzed by supported gold: Cooperation between gold and nanocrystalline rare-earth supports forms reactive surface superoxide and peroxide species[END_REF][START_REF] Abad | Garcí a, A collaborative effect between gold and a support Induces the selective oxidation of alcohols[END_REF][START_REF] Carrettin | Stabilization of Au(III) on heterogeneous catalysts and their catalytic similarities with homogeneous Au(III) metal organic complexes[END_REF][START_REF] Carrettin | Nanocrystalline CeO 2 increases the activity of Au for CO oxidation by two orders of magnitude[END_REF]. Evidence for the presence of gold atoms having positive or There are numerous reports showing that the preparation procedure determines the nature of the gold dispersion on the support. TEM imaging is the most important experimental technique to establish the particle size distribution and gold dispersion on the surface. However, it is very common that the poor contrast between the support and the gold NPs makes difficult the accurate measurements of particle size distribution. In addition, there is a natural bias in this technique towards larger particles that are more easily detectable than the smaller ones. Also in many cases, the detection limit of TEM is a few nanometers and it could be that catalytically more active smaller particles are barely detected or not detected at all. Also XPS, determining the atomic ratio of Au 4f 7/2 on the surface and comparing this value with the elemental analysis can be used to address gold dispersion on the external surface with respect to the bulk in the whole support particle [START_REF] Su | XPS and DRS of Au/TiO 2 catalysts: effect of pretreatment[END_REF]. XPS can also serve to detect and quantify the presence of other elements and particularly the presence of Cl that is considered as a poison for many gold-catalyzed reactions. Thus, Gaigneaux and co-workers by measuring XPS on samples after adsorbing AuCl 4 -following either the anionic adsorption or the deposition-precipitation protocol, have detected a small signal corresponding to Cl 2p that could not be accurately quantified due to its weakness [START_REF] Cellier | Investigation of the preparation and activity of gold catalysts in the total oxidation of n-hexane[END_REF]. This result implies that most of the chloride ions are replaced by surface hydroxyl groups in the adsorption by interaction with the surface hydroxyl groups, but also cast doubts about what could be the catalytic activity of the supported gold catalyst in the total absence of chloride. By using XPS, it has been established that thermal treatment produces the migration of ions and NPs over the support surface, leading to an increase in the average particle size [START_REF] Lee | Supported Au catalysts for low-temperature CO oxidation prepared by impregnation[END_REF][START_REF] Akita | Analytical TEM study on the dispersion of Au nanoparticles in Au/TiO 2 catalyst prepared under various temperatures[END_REF][START_REF] Haruta | Size-and support-dependency in the catalysis of gold[END_REF][START_REF] Haruta | Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts[END_REF][START_REF] Minicò | Catalytic combustion of volatile organic compounds on gold/iron oxide catalysts[END_REF][START_REF] Park | Effects of pretreatment conditions on CO oxidation over supported Au catalysts[END_REF][START_REF] Cunningham | Preparation and catalytic behaviour of sub-nanometer gold deposited on TiO 2 by vacuum calcination[END_REF][START_REF] Boccuzzi | Au/TiO 2 nanosized samples: A Catalytic, TEM, and FTIR study of the effect of calcination temperature on the CO oxidation[END_REF].

IR spectroscopy using CO as a probe and monitoring the C=O stretching vibration appearing in the region about 2100 cm -1 has also been employed to assess the presence of Au(III), Au(I) and Au(0) (Figure I-5) [START_REF] Cellier | Investigation of the preparation and activity of gold catalysts in the total oxidation of n-hexane[END_REF][START_REF] Boccuzzi | Au/TiO 2 nanosized samples: A Catalytic, TEM, and FTIR study of the effect of calcination temperature on the CO oxidation[END_REF][START_REF] Henao | In situ transient FTIR and XANES studies of the evolution of surface species in CO oxidation on Au/TiO 2[END_REF][START_REF] Kung | Understanding Au-catalyzed low-temperature CO oxidation[END_REF][START_REF] Boccuzzi | FTIR study of the electronic effects of CO adsorbed on gold nanoparticles supported on titania[END_REF][START_REF] Boccuzzi | Au/TiO2 nanostructured catalyst: pressure and temperature effects on the FTIR spectra of CO adsorbed at 90 K[END_REF][START_REF] Concepción | Stabilization of cationic gold species on Au/CeO 2 catalysts under working conditions[END_REF]. This technique is based on the different Lewis acid strength of the gold atoms depending on their oxidation state. the potential of CO as a probe to characterize gold NPs interacting with the solid support (taken from ref. [START_REF] Concepción | Stabilization of cationic gold species on Au/CeO 2 catalysts under working conditions[END_REF]).

However, it has to be noticed that both XPS and FT-IR using CO as probe may disguise the oxidation state of gold on the support they are characterizing. Thus, CO is a strong reducing agent that can decrease the population of Au(I) and Au(III) while recording the IR. This can certainly be diminished by performing the adsorption at low temperatures. Also, the high vacuum required for XPS and the energy of the soft X rays can lead to the reduction of a significant fraction of Au(I) and Au(III) ions. In any case, it is believed that the percentages of positive gold atoms estimated by XPS or CO titration are lower limit values of the real positive gold atoms that can be present before analysis.

XPS, XRD and BET surface area characterization of the samples were respectively used to determine the surface Au NPs oxidation state, the mean particle size and the textural properties of the support oxide phase [START_REF] Minicò | Influence of catalyst pretreatments on volatile organic compounds oxidation over gold/iron oxide[END_REF]. ICP and XRF could be used to determine the metal loading on the support by carrying out an elemental analysis [START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF][START_REF] Megí As-Sayago | Gold catalysts screening in base-free aerobic oxidation of glucose to gluconic acid[END_REF][START_REF] Miedziak | Base-free glucose oxidation using air with supported gold catalysts[END_REF].

Au-based bimetallic nanoparticle catalysts

Though there are a lot of advantages of using gold-based monometallic catalysts, they often showed low activity compared to Pt or Pd-based catalysts and were found not suitable for alcohol oxidation in alkaline conditions. However, bimetallic Au-based systems could overpass such limitations, combining the properties of Au and another metal. Hence, a great enhancement of catalytic properties was found in many reactions when combining gold with another metal through forming new active sites and inducing synergistic effects [START_REF] Haruta | Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0[END_REF][START_REF] Hutchings | Vapor phase hydrochlorination of acetylene: Correlation of catalytic activity of supported metal chloride catalysts[END_REF][6][START_REF] Hutchings | Catalysis by gold[END_REF][START_REF] Bianchi | Selective oxidation of glycerol with oxygen using mono and bimetallic catalysts based on Au, Pd and Pt metals[END_REF][START_REF] Dimitratos | Synergetic effect of platinum or palladium on gold catalyst in the selective oxidation of D-sorbitol[END_REF].

Bimetallic nanoparticles (NPs) considered here are nanoparticles comprising two different metals inside their structures, such as alloy or core-shell. These NPs show novel optical, electronic and catalytic properties in comparison with monometallic NPs and have hence attracted huge attention. Indeed, NPs composed of two different metal elements possesses not only the combined properties of the two individual metals, but also present sometimes synergetic properties [START_REF] Zaleska-Medynska | Noble metal-based bimetallic nanoparticles: the effect of the structure on the optical, catalytic and photocatalytic properties[END_REF][START_REF] Sharma | Novel development of nanoparticles to bimetallic nanoparticles and their composites: A review[END_REF]. Hence, the structure of the metallic NPs plays a very important role in the catalytic performance. The two most commonly encountered structures are introduced in the following sections.

Structure of the NPs

In bulk metals, atoms are arranged in various geometries, each metal having its own mode of atom placement. The resulting crystal structure is usually simple and depends on the nature of the metal and on other conditions such as temperature. In the case of metals NPs, the 3D arrangement of atoms may be similar to the crystal structure of the corresponding bulk metal, but in some cases, they may have a rather amorphous structure, depending on preparative conditions.

Bimetallic NPs, which are composed of two kinds of metal elements, can have a crystal structure similar to the bulk alloy as well. But, in addition, they can adopt other types of structures. Such structures, defined by the distribution modes of the two elements, include the random alloys, the alloys with an intermetallic compound, and core-shell structures.

Alloy structure

In a bulk, two kinds of metal elements often provide an alloy structure. If the atomic sizes of the two elements are similar, then it will generally be a random alloy.

When the atom sizes are quite different and the atomic ratio of the two elements is big enough, then they form an intermetallic compound. In the case of bimetallic NPs, these kinds of alloy structures seem to be more easily produced than in the case of bulk metals. In fact, bimetallic NPs between precious metals and light transition metals have such alloy structures.

Mallin and Murphy [START_REF] Mallin | Solution-phase synthesis of sub-10 nm Au-Ag alloy nanoparticles[END_REF] reported that gold-silver alloy NPs were synthesized via reduction by sodium borohydride of mixtures of HAuCl 4 and AgNO 3 with different molar ratio in the presence of sodium citrate as a capping agent which was used to inhibit the overgrowth and aggregation of NPs in water. Solution concentrations were adjusted to avoid the precipitation of AgCl during the course of the reaction (Table I-2). reduction by water. The photocatalytic behavior in this system is completely different to that measured for Au/P25 (hydrogen evolution) and Cu/P25 (lower activity, but similar methane selectivity).

There are many reports talking about alloy structure [START_REF] Miner | Synthesis, characteristics and catalytic activity of monodisperse platinum alloys with gold and palladium[END_REF][START_REF] Li | Formation and characterization of Au/Pd surface alloys on Pd(111)[END_REF][START_REF] Wilson | From core-shell to alloys: The preparation and characterization of solution-synthesized AuPd nanoparticle catalysts[END_REF][START_REF] Ding | Atomic structure of Au-Pd bimetallic alloyed nanoparticles[END_REF][START_REF] Al-Rifai | Deactivation behavior of supported gold palladium nanoalloy catalysts during the selective oxidation of benzyl alcohol in a micropacked bed reactor[END_REF][START_REF] Wan | Base-free aerobic oxidation of 5-Hydroxymethyl-furfural to 2,5-Furandicarboxylic acid in water catalyzed by functionalized carbon nanotube-supported Au-Pd alloy nanoparticles[END_REF][START_REF] Dimitratos | Oxidation of glycerol using gold-palladium alloy-supported nanocrystals[END_REF]. The metals in this structure exhibit very special properties than monometallic catalysts. Layered Ag-Au (core-shell) colloids have been described in fewer studies than Ag-Au alloys. In most of these systems, segregation occurs during reduction so that the more noble metal constitutes the core and the less noble metal forms the shell of a cluster [START_REF] Treguer | Dose rate effects on radiolytic synthesis of gold-silver bimetallic clusters in solution[END_REF]. with a Ag colloid and then added a reducing agent (p-phenylenediamine) [START_REF] Srnová -Šloufová | Core-shell (Ag)Au bimetallic nanoparticles: Analysis of transmission electron microscopy images[END_REF].

Šloufová et al. used for the preparation of Ag-Au colloids (silver core, gold shell) the modified seed-growth method of Turkevich and Kim [START_REF] Turkevich | Palladium: Preparation and catalytic properties of particles of uniform size[END_REF], who used this method for preparation of gold-plated palladium colloids. A sample of the Ag colloid (mean particle size 9 ± 2 nm) prepared as mentioned above was employed as a source of particle cores for preparation of a series of (Ag)Au bimetallic colloids with the expected core-shell structure and varying thicknesses of Au shells (Table I-3). A variation of the seed-growth procedure described in Kim's research and based on the reduction of HAuCl 4 with NH 2 OH' HCl was used to grow the Au shell [START_REF] Turkevich | Palladium: Preparation and catalytic properties of particles of uniform size[END_REF].

Preparation methods

As said above, optimizing the architecture of bimetallic NPs constitutes a major goal to merge synthesis with structure-activity relationships and ultimately allow a rational design of the bimetallic catalysts. The type of morphology prepared depends on the synthetic protocol, on the miscibility of both metals, and, of course, on post-synthesis treatments [START_REF] Silva | Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols[END_REF]. Among them the preparation method is the most determining factor.

As said above for monometallic catalysts, metal NPs can be prepared by subdivision of bulk metals or by growth of particles, which are obtained from molecular or ionic precursors. The former is a physical method, whereas the latter is a chemical preparative method which is more suitable to obtain small and uniform NPs.

The precise control of the atom aggregation is the most important step to get the desired size and to lead to the uniformity of the metal NPs.

Since the days of Faraday [START_REF] Faraday | The bakerian lecture: Experimental relations of gold (and other metals) to light[END_REF][START_REF] Toshima | Bimetallic nanoparticles novel materials for chemical and physical applications[END_REF] the presence of a non-ionic surfactant (Carbowax) by γ radiolysis. Sometimes when one of the metal is more easily reduced than the other, core/shell like NPs could also be produced by this method [START_REF] Zhang | Synthesis of Au/Pt bimetallic nanoparticles with a Pt-rich shell and their high catalytic activities for aerobic glucose oxidation[END_REF].

Successive reduction of metal ions

Successive reduction of two metal salts can be considered as one of the most suitable methods to prepare core-shell structured bimetallic particles [START_REF] Srnová -Šloufová | Core-shell (Ag)Au bimetallic nanoparticles: Analysis of transmission electron microscopy images[END_REF]. In 1970, Turkevich and Kim tried to grow Au on Pd NPs to obtain gold-layered Pd NPs [START_REF] Turkevich | Palladium: Preparation and catalytic properties of particles of uniform size[END_REF].

The deposition of one metal element on pre-formed monometallic NPs of another metal seems to be the most effective method. For this purpose, however, the second element must be deposited on the surface of pre-formed particles.

Ligand-stabilized Au-Pd [START_REF] Schmid | Catalytic properties of layered goldpalladium colloids[END_REF] and Au-Pt [START_REF] Schmid | Ligand-stabilized bimetallic colloids identified by HRTEM and EDX[END_REF] In the case of Au-Pt bimetallic NPs, uniform heterogeneous agglomerates with a large spherical Au core surrounded by small (≈ 5 nm diameter) spherical Pt particles were obtained. By EDX measurements, only Pt was found on the surface of the bimetallic agglomerates. Probably, a broad miscibility gap of gold and platinum between 2 and 85 wt.% Au causes this phenomenon [START_REF] Schmid | Ligand-stabilized bimetallic colloids identified by HRTEM and EDX[END_REF].

In the case of bimetallic Au-Pd system, on the contrary, a rather continuous structure was formed. TEM investigations of Au-Pd bimetallic NPs show somewhat irregular structures. No agglomeration of small particles was observed. The EDX spectrum of such a single particle showed only Pd on the surface. The reverse successive reduction also worked and Pd core-Au shell structured bimetallic NPs were produced.

Base-free glucose oxidation over Au-based bimetallic catalysts

In this work, the influence of factors such as structure of the bimetallic Au-based NPs, support, composition, preparation method and synergistic effect between both metals on the catalytic performance in base-free oxidation of glucose was studied.

Biomass is the most abundant and sustainable carbon source nowadays.

Carbohydrates constitute 75% of the annual renewable biomass with glucose as monomer of cellulose being the most abundant monosaccharide [START_REF] Corma | Supported gold nanoparticles as catalysts for organic reactions[END_REF][START_REF] Corma | Chemical Routes for the Transformation of Biomass into Chemicals[END_REF]. The oxidation of carbohydrates can provide new compounds and materials with interesting physicochemical properties, and this process is the source of a variety of high-added-value chemicals used in foods, cosmetics, detergents, and pharmaceuticals (for example, vitamin C). Biocatalytic and stoichiometric, as well as homogeneous and heterogeneous catalytic methods have been applied for the oxidation of glucose.

Efficient conversion of glucose to valuable compounds, so-called platform chemicals, is of great importance and it is a current hot topic of interest in chemistry [START_REF] Climent | Converting carbohydrates to bulk chemicals and fine chemicals over heterogeneous catalysts[END_REF][START_REF] Song | Conversion of glucose and cellulose into value-added products in water and ionic liquids[END_REF]. Within the glucose platform chemicals, d-gluconic acid obtained by aerobic oxidation of glucose is an important compound (Scheme I-1). It is widely used as environmentally friendly chelating agent and water-soluble cleansing agent in food and pharmaceutical industries with an annual market of about 100,000 tons per year [START_REF] Bellardita | Photocatalytic conversion of glucose in aqueous suspensions of heteropolyacid-TiO 2 composites[END_REF].

It is usually produced by fermentation of glucose with microbial species such as

Aspergillus niger, Penicillium sp., Zymomonas mobilis, G. oxydans and Gluconobacter sp. [START_REF] Liu | A mathematical model for gluconic acid fermentation by aspergillus niger[END_REF][START_REF] Znad | Production of gluconic acid from glucose by Aspergillus niger: growth and non-growth conditions[END_REF][START_REF] Sankpal | Optimization of fermentation conditions for gluconic acid production using Aspergillus niger immobilized on cellulose microfibrils[END_REF]. However, in the fermentation medium, the accumulation of gluconic acid inhibits the microbes function leading to low yields and slow overall reaction rate [START_REF] Bellardita | Photocatalytic conversion of glucose in aqueous suspensions of heteropolyacid-TiO 2 composites[END_REF][START_REF] Liu | A mathematical model for gluconic acid fermentation by aspergillus niger[END_REF]. In recent years, due to the complexity of fermentation process, increased research efforts have been devoted to find environmentally friendly alternative, such as technology based on heterogeneous catalysis.

Glucose is the monosaccharide most extensively studied in oxidation reactions.

The target of our reaction is to produce gluconic acid but getting a high selectivity is challenging because many compounds can be obtained from glucose oxidation. A summary of these compounds is presented in Scheme I-1, and the oxidation methods used are given in Table I-5. Different functions can be oxidized in a monosaccharide such as glucose, giving many valuable oxidation compounds: oxidation of the anomeric center (C1), the primary alcohol at C6, and the secondary alcohols at C2-C4. Cleavage of the vicinal diol and over-oxidation, giving degradation products, can also occur.

Because of the multi-functionality of carbohydrate molecules, important features of the catalytic oxidation systems are the regio-and chemoselectivity. Heyns and co-workers because at this pH the gluconic anions are easily desorbed from the catalyst surface, while at acidic pH the carboxylic acid remains strongly adsorbed, giving over-oxidation reactions, which lead to degradation products. Continuous glucose oxidation processes have also been reported [START_REF] Kunz | Katalytische oxidation von isomaltulose[END_REF].

The heterogeneous catalytic oxidations of sugars are performed mainly with air or oxygen in aqueous medium, under mild conditions, and in the presence of a supported noble metal catalyst. These reaction conditions, along with the facts that the reactants are renewable resources and the products are environmentally benign because of their biodegradability, make the catalytic oxidation of carbohydrates a paradigm of green chemistry. Carbohydrate oxidation on metal catalyst has been reviewed by Besson et al. [START_REF] Besson | Selective oxidation of alcohols and aldehydes on metal catalysts[END_REF].

The oxidation of alcohols and polyols to chemical intermediates represents a demanding target. Supported platinum and palladium NPs are generally acknowledged as effective catalysts for the oxidation of polyols, for example, in carbohydrate chemistry for the oxidation of glucose to gluconic acid [START_REF] Hutchings | Catalysis by gold: Recent advances in oxidation reactions[END_REF][START_REF] Hashmi | Gold catalysis[END_REF]. However, these catalysts, although well established, have disadvantages, namely they exhibit poor selectivity with complex substrates. Rossi [START_REF] Biella | Application of gold catalysts to selective liquid phase oxidation[END_REF][START_REF] Biella | Selective oxidation of D-glucose on gold catalyst[END_REF] and co-workers have shown in their seminal studies that supported gold-palladium NPs can be very effective catalysts for the oxidation of alcohols, including diols. However, the presence of a base was found to be essential for activity -it is considered to be essential for the first hydrogen These catalysts were also shown to be effective with gas-phase reactants and in this case no base addition was required, of course. These studies have been extended to the oxidation of sugars, and similar high catalytic efficiency for the oxidation of glucose has been observed [START_REF] Comotti | Oxidation of alcohols and sugars using Au/C catalysts: Part 2. Sugars[END_REF][START_REF] Beltrame | Aerobic oxidation of glucose: II. Catalysis by colloidal gold[END_REF]. Subsequent studies by Porta and Prati have shown further aspects of this complex oxidation [START_REF] Porta | Selective oxidation of glycerol to sodium glycerate with gold-on-carbon catalyst: an insight into reaction selectivity[END_REF]. Basheer et al. [START_REF] Basheer | Development and application of a simple capillary-microreactor for oxidation of glucose with a porous gold catalyst[END_REF] have shown that a simple capillary reactor can be used for the selective oxidation of glucose, thus

showing that oxidations in a flow reactor are feasible using a supported gold catalyst [START_REF] Hashmi | Gold catalysis[END_REF].

In many catalytic studies the support-catalyst interaction is a crucial factor controlling reactivity. Interestingly, Rossi and co-workers have shown that -naked‖ gold colloidal particles can be very effective catalysts for the oxidation of glucose to gluconic acid [START_REF] Comotti | The catalytic activity of -naked‖ gold particles[END_REF]. The initial rates for these non-supported particles were identical to the rates observed with Au/C catalysts operated under the same conditions, hence confirming that, in this case, the carbon support is of limited importance in the origin of the catalyst activity. The support catalyst interaction is, however, essential for the observation of a stable catalyst system. Subsequently Mertens et al. [START_REF] Mertens | Gold nanoclusters as colloidal catalysts for oxidation of long chain aliphatic 1,2-diols in alcohol solvents[END_REF] have also shown that colloidal gold can catalyze the oxidation of 1,2-diols. Recently, Tsunoyama et al. [START_REF] Tsunoyama | Size-specific catalytic activity of polymer-stabilized gold nanoclusters for aerobic alcohol oxidation in water[END_REF][START_REF] Tsunoyama | Colloidal gold nanoparticles as catalyst for carbon-carbon bond formation: Application to aerobic homocoupling of phenylboronic acid in water[END_REF] have shown that benzyl alcohol can be oxidized using oxygen in aqueous media with gold nanoclusters stabilized on polymers.

Over supported Au-based catalysts

Gold-based catalysts have shown better performance than palladium or platinum-based ones for the oxidation of carbohydrates (Figure I-8 adapted from [START_REF] Biella | Selective oxidation of D-glucose on gold catalyst[END_REF]). One of the most interesting properties of Au is its ability to convert almost all types of aldoses (e.g., glucose, maltose, xylose) to corresponding aldonic acids (e.g., gluconic acid). Comotti et al. [START_REF] Comotti | The Catalytic Activity of -Naked‖ Gold Particles[END_REF][START_REF] Besson | Conversion of Biomass into Chemicals over Metal Catalysts[END_REF] reported high activity and selectivity to gluconic acid using non-supported Au NPs. However, these systems showed relatively low stability [START_REF] Baatz | Influence of the preparation conditions on the properties of gold catalysts for the oxidation of glucose[END_REF], which was improved by immobilization on carbon [START_REF] Biella | Application of gold catalysts to selective liquid phase oxidation[END_REF][START_REF] Biella | Selective oxidation of D-glucose on gold catalyst[END_REF]. Taking into account these observations, the gold-support interaction was declared to play a crucial role in the formation of a stable catalytic system [START_REF] Okatsu | Deposition of gold nanoparticles on carbons for aerobic glucose oxidation[END_REF][START_REF] Comotti | Oxidation of alcohols and sugars using Au/C catalysts: Part 2. Sugars[END_REF][START_REF] Hashmi | Gold-catalyzed organic reactions[END_REF].

Various supports were studied using Au NPs of the same size [START_REF] Da Silva | Combining active phase and support optimization in MnO 2 -Au nanoflowers: Enabling high activities towards green oxidations[END_REF]. Different catalytic activities were observed indicating that a specific metal-support interaction between Au and support is governing the activity [START_REF] Önal | Structure sensitivity and kinetics of d-glucose oxidation to d-gluconic acid over carbon-supported gold catalysts[END_REF]. On the contrary, Ishida et al. observed that the size of gold NPs plays a more essential role than the nature of the support [START_REF] Okatsu | Deposition of gold nanoparticles on carbons for aerobic glucose oxidation[END_REF][START_REF] Ishida | Influence of the Support and the Size of Gold Clusters on Catalytic Activity for Glucose Oxidation[END_REF]. Excellent performance in terms of activity and recycling was observed for Au/TiO 2 system. Cao et al. [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF] prepared 1wt.%Au/TiO 2 catalysts using different methods of preparation and the catalysts were tested in base-free oxidation of glucose.

The catalyst prepared by the sol-immobilization method, using polyvinylalcohol (PVA) as stabilizing ligand and NaBH 4 as reductant showed the highest catalytic activity and gave a good yield to gluconic acid (64%). The glucose reaction conditions were 0.3

MPa O 2 , 160 °C for 1 h. PVA is well known that type of stabilizer that is used to cap the nanoparticles affects their stability and in turn affects its catalytic activity.

Nanoparticles synthesized using PVA are often small and stable [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF]. The effects of post-synthesis treatments by heating in air (at 250-550 °C for 3 h) or treatment with water to remove the residual PVA were studied. The effect of the quantity of stabilizing ligand PVA added during the preparation was also studied. The lower activity of Au/TiO 2 catalysts in the presence of the PVA thorough the formation of core-shell structure was reported by Villa et al. [START_REF] Villa | Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts[END_REF]. Indeed, as described in their work, the maximum catalytic activity was reached for the catalysts calcined at 250-350 °C.

Higher temperature permits to remove more PVA from the catalyst surface. Moreover, the best PVA to Au ratio was 0.1, as observed by the authors. The group also reported results obtained with magnesium oxide modified with Au using the sol-immobilization method. It showed an excellent selectivity towards gluconic acid [START_REF] Miedziak | Base-free glucose oxidation using air with supported gold catalysts[END_REF].

Puyu Qi et al. [START_REF] Qi | Catalysis and reactivation of ordered mesoporous carbon-supported gold nanoparticles for the base-free oxidation of glucose to gluconic acid[END_REF] used an ordered mesoporous carbon (CMK-3)-supported Au catalyst in the aerobic oxidation of glucose with O 2 under base-free conditions.

Catalytic tests showed that conversion remarkably increased but selectivity decreased when oxygen pressure and reaction temperature were increased. Glucose conversion to gluconic acid reached over 92% with 85% selectivity at 110 °C, 0.3 MPa oxygen pressure, and 2 h of the reaction time. Hydrogen peroxide was generated during reaction, and the relationship between hydrogen peroxide produced in-situ and the formation of the fructose by-product was discussed. Low glucose/Au molar ratio minimized fructose formation. A 92% gluconic acid yield was obtained after 15 min of reaction when the molar ratio of glucose/Au was set to 100. The spent catalyst treated with an aqueous solution of NaOH at 90 °C could convert glucose up to 87%, which was close to the result obtained with the as-prepared catalyst and excluded the effect of any alkaline residue.

Wang et al. [START_REF] Wang | Insights into the stability of gold nanoparticles supported on metal oxides for the base-free oxidation of glucose to gluconic acid[END_REF] studied Au catalysts supported on nano-or micro-sized metal oxides in the base-free oxidation of glucose to gluconic acid. The pH of the reaction solution was kept uncontrolled or lowered, in certain cases, by the addition of a mineral acid. They studied also the stability of these catalysts. They observed that the irreversible deactivation of the Au catalysts was due to the leaching and sintering of gold NPs. To improve the stability due to the hydrothermal sintering, they studied lowest Au loadings. They observed that, during the oxidation reaction, the Au surface density affected significantly their agglomeration tendency. The better stability was observed for 0.02 wt.% Au/μCeO 2 sample. Adsorption of different reactive species on the catalyst surface could be also responsible for the reversible deactivation of the catalyst. This adsorption could be however removed by the calcination of the used catalyst (preferentially at 325°C under static air) [START_REF] Wang | Insights into the stability of gold nanoparticles supported on metal oxides for the base-free oxidation of glucose to gluconic acid[END_REF].

Base-free oxidation of glucose using microwave heating was studied by Rautiainen et al. [START_REF] Rautiainen | Microwave-assisted base-free oxidation of glucose on gold nanoparticle catalysts[END_REF]. High conversion and selectivity to gluconic acid were obtained on Au supported carbon catalysts. Moreover, the use of microwave heating permitted to a remarkably shortened reaction time. Indeed, after only 10 min of reaction, high yield of gluconic acid was obtained (76%) using H 2 O 2 as oxidant and Au/Al 2 O 3 (0.09 mol.% Au) catalyst. In addition, high turn-over frequencies (TOFs = 10,000 h -1 ) were observed due to the microwave assisted oxidation. In terms of stability, the catalyst activity after 4 runs remained constant even though some increase in Au particle size was observed.

Miedziak et al. [START_REF] Miedziak | Base-free glucose oxidation using air with supported gold catalysts[END_REF] reported the selective synthesis of gluconic acid from glucose under mild conditions. They showed that this reaction could give good results even without any sacrificial base addition or pH control if a basic support is used.

To conclude, the oxidation in a base free medium is quite challenging and there is still a need to enhance the efficiency of the catalysts used at low pH. Gold NPs-based catalysts permit to obtain high efficiency in neutral or low pH conditions [START_REF] Zhang | Heterogeneously catalyzed hydrothermal processing of C5-C6 sugars[END_REF][START_REF] Wang | Insights into the stability of gold nanoparticles supported on metal oxides for the base-free oxidation of glucose to gluconic acid[END_REF][START_REF] Rautiainen | Microwave-assisted base-free oxidation of glucose on gold nanoparticle catalysts[END_REF].

Table I-6 summarizes the previous reports on base-free glucose oxidation on supported Au NPs catalysts. Entry 1 demonstrates 100% yield of gluconic acid over 1%Au/C catalyst, which is already a perfect yield. However, the reaction temperature was 100 ˚C which was a little too high. Thus new catalysts at milder reaction need to be developed. And by the same time, more energy could be saved.

Over bimetallic Au-based Catalysts

As said before, bimetallic gold-based catalysts have many advantages compared with monometallic gold catalysts. The combination of gold with one or more other metal(s) has been reported to enhance the activity and the selectivity to the desired acid in the oxidation of glucose under base-free conditions [START_REF] Derrien | Aerobic oxidation of glucose to glucaric acid under alkaline-free conditions: Au-based bimetallic catalysts and the effect of residues in a hemicellulose hydrolysate[END_REF]. Though there was an example of 1%Au/C, over which the conversion and selectivity to gluconic acid reached 100%. However, the reaction temperature used was high (Table I-6, entry 1).

Another example shows that the oxidation of glucose using Au-Pd catalyst at 60°C

gave better results than monometallic gold [START_REF] Zhang | Heterogeneously catalyzed hydrothermal processing of C5-C6 sugars[END_REF]. The advantage of the addition of a second metal was clearly shown when working in acidic medium. Pt showed a better effect if compared to Pd, which means that Pt had a boosting effect where only gluconic acid is produced [START_REF] Comotti | Mono-and bimetallic catalysts for glucose oxidation[END_REF]. Furthermore, high catalytic activity was also attributed to the presence of an Ag-Au core-shell structure as charges could be easily transferred between the core and the shell and oxygen is activated leading to the attack of the desired functional group. The catalytic activity changed according to the composition of the system and the best catalytic activity was obtained for 20% Ag and 80% Au [START_REF] Zhang | Preparation of novel Au/Pt/Ag trimetallic nanoparticles and their high catalytic activity for aerobic glucose oxidation[END_REF].

Recently Hutchings' group [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF][START_REF] Miedziak | Base-free glucose oxidation using air with supported gold catalysts[END_REF] studied several Au bimetallic catalysts for the oxidation of glucose and cellobiose. They found that the preparation method plays a crucial role in the catalytic activity of supported Au catalysts especially in the case of base-free oxidation processes. As can be seen from Table I-7, the calcined catalysts exhibited higher glucose conversion than dried ones. They authors concluded that the interaction between the substrate and the PVP used for the synthesis influence the catalytic activity. The results obtained for bi-and tri-metallic systems confirmed that there is a positive effect of the presence of multiple metals in the catalyst in comparison with a monometallic system. A simple change in the ratio of the metal-to-metal changes the catalytic results, which offers the advantage of fine-tuning the catalytic system.

Moreover, it is possible to obtain a sufficient contact between the active sites and reactants, even using a protective agent (such as polymers) that can provide high stability [START_REF] Ishida | Influence of the support and the size of gold clusters on catalytic activity for glucose oxidation[END_REF].

Mechanism

The oxidation of an alcohol to an aldehyde over metal catalysts occurs in three main steps, as reported by Davis et al. [START_REF] Davis | Selective oxidation of alcohols and aldehydes over supported metal nanoparticles[END_REF]. The first step is the alcohol adsorption on the metal surface, which produces an adsorbed metal alkoxide and a metal hydride (or a water molecule, if metal hydroxide species are involved). After this adsorption, a carbonyl species and a metal hydride are produced via the β-hydride elimination. The final step is the oxidation of metal hydride by O 2 to regenerate the metal surface (or metal hydroxide species). As reported, this last step may lead also to the production of water (on metals that dissociatively adsorb oxygen) or hydrogen peroxide (on metals that do not dissociatively adsorb oxygen such as gold) (Scheme I-2) [START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF]. Both molecules would desorb to regenerate the active metal surface. In this way the carboxylic acid is produced by a subsequent oxidation of the aldehyde through a geminal diol intermediate. Indeed, aldehydes may undergo reversible hydration to geminal diols. This hydrated diol can then also adsorb to a metal surface and undergo a β-hydride elimination to form a carboxylic acid. It was observed that the presence of surface adsorbed hydroxides, similar to an alkaline solution, facilitates both the formation of an adsorbed alkoxide (or geminal diol) and the elimination step of a β-hydride towards the production of aldehyde and carboxylic acid.

A similar mechanism corresponding to a modified oxidative dehydrogenation was also suggested as the most probable for glucose oxidation to gluconic acid using gold catalysts [START_REF] Comotti | Aerobic oxidation of glucose with gold catalyst: Hydrogen peroxide as intermediate and reagent[END_REF][START_REF] Prüße | Catalytic conversion of renewables: Kinetic and mechanistic aspects of the gold-catalyzed liquid-phase glucose oxidation[END_REF]. However, a deeper understanding was reported by Rossi et al. [START_REF] Beltrame | Aerobic oxidation of glucose: II. Catalysis by colloidal gold[END_REF],

which developed one of the best molecular models for carbohydrates oxidation in the presence of gold catalysts. Taking into account that this reaction was considered to be first order (for oxygen), the authors suggested a fast adsorption of the glucose on gold.

In this way, the rate-determining step would be the glucose oxidation by oxygen dissolved in the liquid (aqueous) phase. Hydrogen peroxide could be detected, but it decomposed quickly under the reaction conditions. In a subsequent investigation Rossi et al. [START_REF] Comotti | Aerobic oxidation of glucose with gold catalyst: Hydrogen peroxide as intermediate and reagent[END_REF][START_REF] Beltrame | Aerobic oxidation of glucose: II. Catalysis by colloidal gold[END_REF] showed that these oxidations do not proceed by a radical pathway but delivers gluconate and hydrogen peroxide by a two-electron mechanism. The hydrogen peroxide decomposes because of the basic medium before it reaches a concentration at which it would efficiently compete with oxygen as the oxidant.

Scheme I-2 Mechanistic model for glucose oxidation. BS: basic sites.

As could be noted, various transformations are involved in the process. In the case of gold catalysts these steps were shown to be highly dependent on the presence of the hydroxyl species. As the formation of Au-alkoxide is, with no doubt, crucial for further oxidation of glucose [START_REF] Larew | Concentration dependence of the mechanism of glucose oxidation at gold electrodes in alkaline media[END_REF], in a base-free oxidation the presence of local basicity form the support or/and from the second metal is decisive. In some studies [START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF], traces of gluconic acid were observed during the base-free oxidation reaction and uronic acid was the main product. Thus, it could be expected that the oxidation of glucose may proceed through the formation of gluconic acid (oxidation of the aldehyde at carbon atom C1) (Scheme I-2). The further step of this reaction is the oxidation of the primary hydroxyl at carbon atom C6 to yield glucuronic acid, an intermediate, which then undergoes oxidation to glucaric acid as descripted in Scheme I-2. Although the oxidation of glucose directly to dicarboxylic acids was already demonstrated to occur on a Pt/C catalyst using oxygen as an oxidant [START_REF] Larew | Concentration dependence of the mechanism of glucose oxidation at gold electrodes in alkaline media[END_REF][START_REF] Dirkn | The preparation of D-glucaric acid by the oxidation of D-gluconic acid catalysed by platinum on carbon[END_REF][START_REF] Dirkx | The oxidation of gluconic acid with platinum on carbon as catalyst[END_REF], the oxidation of free gluconic acid into glucaric acid was also demonstrated on Au/C catalysts [START_REF] Derrien | Aerobic oxidation of glucose to glucaric acid under alkaline-free conditions: Au-based bimetallic catalysts and the effect of residues in a hemicellulose hydrolysate[END_REF][START_REF] Yang | Morphological transition of gold nanostructures induced by continuous ultraviolet irradiation[END_REF]. It has been reported that the rate of the reaction between chemisorbed oxygen and adsorbed gluconic acid is much lower than the reaction rate between chemisorbed oxygen and adsorbed glucose [START_REF] Dirkx | The oxidation of gluconic acid with platinum on carbon as catalyst[END_REF]. However, it was also demonstrated that gluconic acid, formed from glucose oxidation, is oxidized much faster than if it is used as a starting molecule, under the same reaction conditions. This could be explained by the low pH of the initial solution in the case of gluconic acid oxidation and its adsorption on the catalysts surface that could deactivate the active sites [START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF].

Factors influencing activity

Effect of the size of gold particles

Many studies have shown that the catalytic activity of gold is dependent on the crystal planes step density, because energy barrier of oxygen on these steps is lower, active oxygen is easier to release, and the oxidation is accelerated [START_REF] Yang | Morphological transition of gold nanostructures induced by continuous ultraviolet irradiation[END_REF][START_REF] Daniel | Gold nanoparticles: Assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology[END_REF][START_REF] Sun | One-step synthesis and size control of dendrimer-protected gold nanoparticles: A heat-treatment-based strategy[END_REF][START_REF] Sharma | Solvent-assisted one-pot synthesis and self-assembly of 4-aminothiophenol-capped gold nanoparticles[END_REF][START_REF] Roy | A new organic compound for the synthesis of gold nanoparticles[END_REF][START_REF] Kim | Effects of PVP and KCl concentrations on the synthesis of gold nanoparticles using a solution plasma processing[END_REF]. Therefore, the catalytic activity of the Au NPs will be directly governed by their particle size and this parameter should be effectively controlled. Different preparative methods for gold colloids are known, most of them, based on reduction of tetrachloroauric acid HAuCl 4 as discussed above [START_REF] Daniel | Gold nanoparticles: Assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology[END_REF].

Luo et al. [START_REF] Luo | Self-catalyzed, self-limiting growth of glucose oxidase-mimicking gold nanoparticles[END_REF] found that the size and shape of NPs are generally controlled by external factors such as time, precursor, reaction temperature and/or surfactant concentration. It was stated that lack of an external influence eventually leads to the uncontrolled growth of the NPs. This could be responsible for the loss of the nanoscale properties of the materials. In the presence of gold precursor (HAuCl 4 ) it provokes the particles growth, which is responsible for the decrease of the activity and the surface passivation with gluconic acid (product of the reaction) leading to the deactivation of the system [START_REF] Luo | Self-catalyzed, self-limiting growth of glucose oxidase-mimicking gold nanoparticles[END_REF].

Megí as et al. [START_REF] Megí As-Sayago | Influence of gold particle size in Au/C catalysts for base-free oxidation of glucose[END_REF] prepared and immobilized a series of gold colloids on commercial activated carbon. The influence of the colloid preparation and stability were studied and related to the gold particle size in the final catalyst. The ratio between PVA (PolyVinyl Alcohol) and Au, and NaBH 4 and Au were studied. Two limiting values were found in order to achieve stable colloid and homogeneous particles size distribution, the PVA/Au ratio should be superior to 0.85 and NaBH 4 /Au superior to 3. The catalysts showed an important activity in the glucose to gluconic acid oxidation reaction, leading to gluconic acid yield close to 90% in base-free mild conditions. The size-activity correlation and probable mechanism were also discussed.

The structure/size sensitivity shows optimal size for maximum conversion for particles of around 9 nm, although the normalization to exposed surface gold atoms (TOF) moves this optimum to gold particles in the 15-20 nm range, pointing out a possible influence of the glucose adsorption step on the reaction mechanism. Besides, the catalysts showed a very good recyclability with the loss of activity of only 10%

after the fourth cycle [START_REF] Megí As-Sayago | Influence of gold particle size in Au/C catalysts for base-free oxidation of glucose[END_REF].

Ishida et al. [START_REF] Okatsu | Deposition of gold nanoparticles on carbons for aerobic glucose oxidation[END_REF][START_REF] Ishida | Influence of the support and the size of gold clusters on catalytic activity for glucose oxidation[END_REF][START_REF] Ishida | Gold catalysts: Towards sustainable chemistry[END_REF] have also demonstrated that the size of Au NPs supported on metal oxides during glucose oxidation strongly influences the overall TOFs. They concluded that the reaction occurs at the surface of the Au particles. The same trends were observed for the base-free catalytic oxidation reactions. supplies with reactive oxygen. While for Au supported on inert materials, where the oxygen supply most likely proceeds via direct dissociative adsorption on the Au particles, the size of the latter plays a crucial role. Moreover, the impact of the support on the catalytic activity also depends on the type of catalyzed reaction. Ishida et al. [START_REF] Okatsu | Deposition of gold nanoparticles on carbons for aerobic glucose oxidation[END_REF][START_REF] Ishida | Influence of the support and the size of gold clusters on catalytic activity for glucose oxidation[END_REF] reviewed the impact of supported gold catalyst on the catalytic oxidation of glucose.

Contrary to the gas phase CO oxidation, in the case of glucose oxidation, the catalytic activity of gold NPs was influenced more significantly by the size than by the nature of the support. Turnover frequency (TOF) given per surface Au atom (Table I-8) confirmed these results. The selectivity to gluconic acid remained constant at high level (above 98 %). Therefore, after choosing a suitable support, more attention should be paid to the influence of the preparation method on the size of the Au NPs. Although the size dependencies on the TOF values was not the same as the observed by Comotti et al. [START_REF] Comotti | Mono-and bimetallic catalysts for glucose oxidation[END_REF], the size of the Au particles plays a leading role. In the case of oxidation carried out in the liquid phase, the oxidation occurs at the Au surfaces and not at the perimeter interfaces between Au particles and the supports, as suggested for the CO oxidation. The activation energy is not the same on different supports. It could be concluded that metal-oxide supports affect electronically the gold clusters.

Megias-Sayago et al. [START_REF] Megí As-Sayago | Gold catalysts screening in base-free aerobic oxidation of glucose to gluconic acid[END_REF] have reported the base-free oxidation of glucose using molecular oxygen over various Au-supported catalysts.

Effect of reaction temperature

The conversion of glucose was found to be highly dependent on the temperature and 100% conversion was reached at 150 °C in case of Au/CeO 2 [START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF]. However, as shown in Table I-9 the yield to acid decreased drastically. Indeed, the formation of different by-products was observed (several peaks in the HPLC spectra). The highest yield to corresponding sugar acid was achieved at the temperature of 70 °C. Above this temperature, several other over-oxidation products appeared, which was due to the thermal decomposition and polymerization. A similar trend of decrease of selectivity with increase of temperature was reported Au/CMK-3 catalysts [START_REF] Qi | Catalysis and reactivation of ordered mesoporous carbon-supported gold nanoparticles for the base-free oxidation of glucose to gluconic acid[END_REF] for base-free oxidation of glucose. At relatively low temperature (namely 70 °C), no byproduct was observed (Table I-9). When the temperature was increased, some byproducts appeared.

However, when over 1%Au/C, it reached 100% yield of gluconic acid at 100 ˚C [START_REF] Biella | Selective oxidation of D-glucose on gold catalyst[END_REF].

It suggests that the reaction temperature depends a lot on the type of catalyst.

Effect of reaction time

According to the research of Qi et al. [START_REF] Qi | Catalysis and reactivation of ordered mesoporous carbon-supported gold nanoparticles for the base-free oxidation of glucose to gluconic acid[END_REF], as the reaction time lapsed with the gradually increasing conversion, the selectivity toward gluconic acid tapered off regularly. At the same time, the concentration of hydrogen peroxide also dropped drastically, in agreement with the tendency observed for the gluconic acid selectivity. In contrast, there were more fructose and other byproducts appeared. According to a previous report [START_REF] Comotti | Aerobic oxidation of glucose with gold catalyst: Hydrogen peroxide as intermediate and reagent[END_REF], when glucose molecules react with oxygen and water, aside from gluconic acid, hydrogen peroxide is also produced. Thus, the more rapidly the main reaction occurs, the more hydrogen peroxide is produced. The author proved that the generated hydrogen peroxide would act as an oxidant to oxidize glucose and decompose to oxygen. Biella et al. [START_REF] Biella | Selective oxidation of D-glucose on gold catalyst[END_REF] also studied the oxidation of glucose without controlling the pH. Gold on carbon showed a lower reaction rate than under pH-controlled conditions

In summary, increasing the temperature and oxygen pressure could promote the reaction rate and conversion, and the improving effect was more obvious when the oxygen pressure was increased at a relatively high temperature. For a catalyst when other reaction conditions were fixed, the selectivity reduced gradually along with time, but for the different catalysts that reached the same conversion, the reaction that last less time would have a better selectivity toward gluconic acid.

Stability

Wang et al. [START_REF] Wang | Insights into the stability of gold nanoparticles supported on metal oxides for the base-free oxidation of glucose to gluconic acid[END_REF] NPs and the presence of more anchoring sites that could prevent the growth of Au NPs.

Generally, in the case of base-free oxidation of glucose, the deactivation of the catalysts is observed due to the adsorption of acids on the catalysts surface (Table I-10). Some authors claimed that the reactivation of the catalysts could be done by calcination.

However, the calcination step would lead to a severe loss of activity because the diameter of the Au NPs was obviously larger than that of the as-prepared catalyst.

Miedziak et al. [START_REF] Miedziak | Base-free glucose oxidation using air with supported gold catalysts[END_REF] also reported a drastic decrease in activity for a bimetallic Au-Pd catalyst, giving a conversion of less than 20% in the second use. The conversion could be improved by only 5% through a calcination treatment. Qi et al. [START_REF] Qi | Catalysis and reactivation of ordered mesoporous carbon-supported gold nanoparticles for the base-free oxidation of glucose to gluconic acid[END_REF] discussed two different regeneration methods, including NaOH treatment and calcination, for an Au/CMK-3 catalyst that had lost gradually activity upon recycling during glucose oxidation. On the basis of the results, the used catalyst treated with 50 mL of 0.1 mol/L NaOH for 30 min at 90°C showed the best performance with 87% conversion. The characterization study indicated that after the recycling experiments, abundant carboxylic acids were adsorbed on the surface of the catalyst, which was the predominant factor that caused deactivation. Both NaOH treatment and calcination could effectively remove the adsorbates, and atmospheric calcination was more efficient in this frame. However, given the appearance of large particles during the calcination procedure, the NaOH treatment method seems to be more appropriate.

Therefore, one can conclude that the NaOH treatment is a very simple and useful way to regenerate used catalysts that suffered from adsorbate deactivation in glucose oxidation.

Megias-Sayago et al. [START_REF] Megí As-Sayago | Gold catalyst recycling study in base-free glucose oxidation reaction[END_REF] studied the viability of immobilized gold colloids on carbon as catalysts for the base-free glucose oxidation reaction with a special emphasis made on catalysts' recycling, operational life and possible routes for deactivation/reactivation under batch conditions. The observed catalytic behavior is related to all possible manners of deactivation, like gold metal state changes (particle size agglomeration or leaching), support modifications or active sites blocking by intermediates.

Conclusion

The bibliographic study has shown that gold-based catalysts are effective in catalyzing the oxidation of glucose, particularly in the presence of a base in the reaction media. Moreover, combined with a second metal, the activity and stability of gold catalysts can be enhanced. But, why this kind of phenomenon happens, and what is the relationship between physicochemical properties and catalytic performance remains to be answered. A few studies [START_REF] Benkó | Bimetallic Ag-Au/SiO 2 catalysts: Formation, structure and synergistic activity in glucose oxidation[END_REF][START_REF] Comotti | Mono-and bimetallic catalysts for glucose oxidation[END_REF][START_REF] Hermans | Preparation of Au-Pd/C catalysts by adsorption of metallic species in aqueous phase for selective oxidation[END_REF] have revealed some of the reasons under high pH value. However, there is less study of the synergistic effect over bimetallic catalyst in glucose oxidation under base-free condition where the reaction mechanism can be different. Especially, the question of which is the best metal ratio in the optimal catalyst for the oxidation of glucose and why, still remains unanswered.

In this literature review, the method used for the catalyst preparation was shown to strongly influence its physicochemical properties and hence the catalytic performances.

Among the different heterogeneous catalysts preparation methods as it was discussed above, the sol-immobilization seems to be the most suitable method for the preparation of catalysts with small and narrow dispersion of NPs. Precipitation-reduction method will also be used as a comparison because it can yield very small NPs as well and is easy to operate. In that purpose, we have prepared a series of monometallic or bimetallic Au-X (where X = Pd, Cu, Pt, Bi) supported catalysts. As TiO 2 and ZrO 2 are widely used stable supports, they were chosen for getting a good dispersion of the metallic NPs. The ratio of Au/X will be mainly discussed in order to reveal any synergistic effect between the two metals. The weight ratio between Au and X (where X = Pd, Cu, Bi) will be 0.9/0.1, 0.7/0.3, 0.5/0.5, 0.3/0.7 and 0.1/0.9. As for Au-Pt catalysts, the weight ratio between Au and Pt will be 0.75/0.25, 0.5/0.5, 0.25/0.75.

According to the literature, synergistic effect may occur in comparison with the purely monometallic catalysts (Au, Pd, Cu, Pt and Bi).

Finally, based on the literature, the reaction conditions applied in this study will be as follows: temperature range [60-150 ˚C], pressure 5 bar, reaction time 5 h, under air atmosphere, initial glucose concentration 0.01 g/g cata , 21.9 mg catalysts (initially defined for 1%Au/ZrO 2 , glucose/Au = 100 molar ratio and kept for all the other catalytic tests).

Chapter II Experimental Introduction

Based on the literature survey which was detailed in the previous chapter a strategy of research has been defined for this work. A library of supported Au-X (X = Pd, Cu, Pt, Bi) bimetallic catalysts has been prepared, characterized and tested for glucose oxidation. Of course, the characterization study of these series of catalysts had required the use of different techniques and instruments to determine their physical and chemical properties, their morphology and structure, and to identify the species which have a direct impact on their catalytic performance.

This chapter described in detail the experimental procedures and equipment used in this thesis and is divided in 3 parts. The first part describes the methods of preparation and characterization of the library of catalysts used in this work. The second part focuses on the description of the set-ups and protocols used for the catalytic tests. Finally, the third part is dedicated to the analysis of products of the glucose oxidation reaction.

Synthesis and characterization of the catalysts

Catalyst preparation

On the basis of the literature survey, sol-immobilization method and precipitation-reduction methods were chosen for the synthesis of the series of catalysts used in this work. By these two methods, the particle size of the metal nanoparticles could be kept below 10 nm [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF], which is highly important to get a good activity of the gold-based catalyst as we have seen previously. Especially, in the sol-immobilization method, by adjusting the time of stirring, reduction agent concentration, concentration of precursors, the particle size could be very well controlled [START_REF] Villa | Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts[END_REF]. The support provides the surface of the catalysts, where the metals are loaded and dispersed. The stability of the support is obviously also very important for the final catalyst. ZrO 2 and TiO 2 were chosen to play this role as they are very known to be stable in the selected operating conditions and they provide reasonable surface areas [START_REF] Wang | Influence of a ZrO 2 support and its surface structures on the stability and nucleation of Ptn (n = 1-5) clusters: A density functional theory study[END_REF]. The catalysts preparation methods are described in details in the following sections.

Sol-immobilization method

The sol-immobilization method was adapted from the literature with a view to get small and narrow dispersed nanoparticles on both supports [START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF]. It consisted in the preparation of the Au and/or X (X = Pd, Cu, Pt, Bi) nanoparticles stabilized with polyvinyl alcohol (PVA) and their subsequent immobilization on an appropriate amount of support. As said above, titanium dioxide (TiO 2 ) and zirconium dioxide (ZrO 2 )

were chosen as supports of the bimetallic catalysts with a view to reduce the A calcination oven from ThermoFisher (up to 800°C) was used for the thermal treatment of catalysts. The heat treatment procedures were as follows.

(i) The catalysts were placed under static atmosphere, increased from room temperature to 250 ˚C at a speed of 3 ˚C /min. Maintained for 3 h. Then decreased the temperature to room temperature by 0.3 ˚C/min.

(ii) The catalysts were placed under static atmosphere, increased from room temperature to 400 ˚C at a speed of 3 ˚C /min. Maintained for 2 h. Then decreased the temperature to room temperature by 0.3 ˚C/min.

Catalyst characterization

The catalysts prepared were characterized in order to determine their physical properties, such as the specific surface area, the particles size, composition and structure, etc. The techniques used in the present work to characterize the catalysts are described in the following.

X-ray fluorescence (XRF)

X-ray fluorescence (XRF) is one of the most common non-destructive analytical techniques used to determine the elemental composition of a material. Its principle is simple: The sample is irradiated with X-rays, which knock out electrons from atoms, leaving them in an excited state. During the relaxation of these electrons, the excess energy is released in the form of X-ray radiations, which intensity depends directly on the composition of the material. Therefore, it is possible to determine a material composition by detecting the X-rays that come out of the sample.

In the present work, metal loadings were determined by a Brüker M4 Tornado X-Ray spectrometer with Rh tube of radiation. To obtain accurate quantification of the metals present in the catalysts, each sample was irradiated 30 times in order to largely cover the catalysts surface. The mean value of the metal content was then calculated.

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

The ICP-OES is another very common analytical technique used for quantitative, semi-quantitative or qualitative determination of the elements constituting the catalysts.

This technique uses an inductively coupled plasma to produce exited atoms and ions that emit electromagnetic radiations at wavelengths characteristic of a particular element [START_REF] Stefá Nsson | New methods for the direct determination of dissolved inorganic, organic and total carbon in natural waters by Reagent-Free™ Ion Chromatography and inductively coupled plasma atomic emission spectrometry[END_REF].

In this present work, an Agilent 720-ES ICP-OES was used for determining the metal loading combined with a Vulcan 42S robot. The Vulcan 42S robot is an automated digestion system combining the two essential steps in sample preparation prior to analysis by ICP: sample digestion and sample preparation. The digestion procedure was as follows: firstly, 10 mg of the solid samples were weighted and then 2.4 mL of aqua regia were distributed by the robot on each sample which was then heated for 2 h up to 110 °C, add 2 mL of aqua regia and heated for 2 h at 110 °C again, and then add 2 mL more of aqua regia and heated for 3 h at 110 °C. After that make volume up to 20 mL with ultra-pure water, add the internal standard La (50 µL), and then analyze by ICP. The TEM electron microscopy images were recorded placing a drop of the particles in ethanol onto a carbon film supported on a cooper grid. The samples were studied using a FEI Tecnai G2 20 microscope with a LaB6 cathode operating at 200 kV and a FEI Titan Themis 300 with X-FEG emitter operating from 60 to 300 kV.

Experimental setup

The catalytic tests were carried out first on a classical autoclave reactor from Top Industry and then on a High-Throughput Screening Pressure Reactor (SPR) from Unchainedlabs.

Autoclave reactor

Six preliminary reactions were carried out using an autoclave reactor from Top Industrie (Figure II-4) in order to find a suitable set of reaction conditions before using the high-throughput equipment to compare the whole series of catalysts prepared. After several tests with different parameters (temperature, time, pressure, etc.), a suitable set of reaction conditions was found for comparing the catalysts. Indeed, at 60 °C, using a glucose/Au molar ratio of 100, for 5 h, and at 700 rpm of stirring the yield of gluconic acid was around 30% over 1 wt.% Au/ZrO 2 . These conditions were kept as standard ones for the following tests in the SPR equipment.

Screening Pressure Reactor (SPR)

Most of the catalytic results presented in this thesis were obtained on a Screening Pressure Reactor (SPR) multi reactor system from Unchainedlabs. It is equipped with In order to study and rank the catalysts, mild conditions were chosen, as said above. In a standard reaction test, the required amounts of catalyst were placed in each reactor (Figure II-5) and 2 mL of the aqueous glucose solution (1 wt.%) were then added. The reactors were then flushed with air at room temperature for 3 times before setting the air pressure at 5 bars. Then, the reactors were sealed and the heating was started (the temperature ramp rate was 5 ˚C/min). When the desired temperature was reached, stirring was set at 700 rpm and the reaction started and maintained for 5 hours.

At 60°C the final pressure was around 6 bars. At the end of the reaction, the reactors were cooled down. When the temperature reached 40 ˚C, the valves of the reactor vessel were opened to release the pressure. Finally, the reaction media were filtered to eliminate the catalysts and the liquid recovered were used to prepare the solution for HPLC analysis.

HPLC analysis of products and calculation

In The main problem encountered was the very close retention times of glucose and gluconic acid using the Rezex organic acid H + 300 x 7.80 mm column. Would the analyzed sample contain both molecules, no separation of the peaks would be observed on the spectrum. However, glucose could not be detected by the UV detector at certain UV wavelength while gluconic acid could be. Thus, combining the UV and the RI detectors was the solution we chose.

In a view to find the best UV wavelength for glucose and gluconic acid detection a Shimadzu HPLC-MS 2020 equipped with a Photodiode Array (PDA) detector SPD-20A which has a whole UV range from 190 to 700 nm was used. Below 213 nm UV detector could detect glucose, but above this value there was no more signal for glucose. Gluconic acid and glucuronic acid could be detected from wavelength from 190 nm to 250 nm. As a result, the wavelength of 225 nm was chosen as a suitable common wavelength to detect gluconic acid and glucuronic acid using the Dionex ultimate 3000 HPLC.

Finally, the retention time of glucose was 13.18 min (RI), gluconic acid was 13.21 min (RI) and 13.02 min (UV), glucuronic acid was 11.78 min (RI) and fructose was 14.42 min (RI).

Because of the low stability of the gluconic acid, sodium gluconate was used instead to get a better precision of analysis. Commercial gluconic acid was only 49~53% in water. Some crystallization appeared in the solution. After being injected into HPLC, sodium gluconate would keep the same form as gluconic acid theoretically:

Sodium gluconate + H + = Gluconic acid + Na + Gluconic acid and sodium gluconate with the same concentration were injected into HPLC 6 times separately. The concentrations detected by HPLC were almost the same (Table II-1) validating the fact that sodium gluconate can be used for the calibration instead of gluconic acid. Considering the UV peaks area, the concentration of gluconic acid could then be calculated. Then, the corresponding RI area peak of the gluconic acid could be calculated. Hence, the surface of the peak originating from glucose can be obtained by subtraction of the gluconic acid RI peak area from the total RI peak area. Finally, the concentration of glucose could be calculated using the calibration curve. The However, non-supported nanoparticles are easy to agglomerate and grow during the preparation and reaction. In consequence, titanium dioxide (TiO 2 ) and zirconium dioxide (ZrO 2 ) were chosen as supports for the bimetallic Au-Pd NPs to prevent the agglomeration phenomenon and to help dispersing them homogeneously. Both oxides are stable and very good support, especially TiO 2 that is widely used in the photocatalytic reactions [START_REF] Bellardita | Photocatalytic conversion of glucose in aqueous suspensions of heteropolyacid-TiO 2 composites[END_REF][START_REF] Cojocaru | Influence of gold particle size on the photocatalytic activity for acetone oxidation of Au/TiO2 catalysts prepared by dc-magnetron sputtering[END_REF][START_REF] Vià | Visible-light-controlled oxidation of glucose using titania-supported silver photocatalysts[END_REF][START_REF] Zhang | Dramatic visible photocatalytic degradation performances due to synergetic effect of TiO 2 with PANI[END_REF].

Sol-immobilization and precipitation-reduction methods described in Chapter II were used to prepare the catalysts. By these two methods, the average particle size of the metal nanoparticles should be kept in the 1-10 nm range [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF]. Sol-immobilization method can also help to obtain a good balance between activity, selectivity and stability of the metallic nanoparticles as described previously in the literature [START_REF] Villa | Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts[END_REF]. The techniques of characterization described in Chapter II (ICP, XRF, XRD, TEM, XPS, etc.) were used to study the catalysts.

This chapter is divided into three parts. The first and second parts are dedicated to the studies of the Au-Pd bimetallic catalysts supported on TiO 2 and ZrO 2 , respectively.

Two different methods of preparation, namely the sol-immobilization (noted SIM) and the precipitation-reduction (noted PR), were used and compared. Finally, in the last part of this chapter, the important role of the support is discussed.

Au-Pd NPs supported on titanium dioxide

As said in Chapter II, the TiO 2 used in this work was purchased from Sigma-Aldrich (P25). The main distinct polymorphs of TiO 2 are anatase and rutile

(Figure III-1)
. Anatase is thermodynamically less stable than rutile [START_REF] Ma | Removal of arsenic from aqueous solution by two types of nano TiO 2 crystals[END_REF]. In P25 the ratio between anatase and rutile phases is about 3:1. As mentioned in Chapter II, the bimetallic catalysts which will be denoted as x%Auy%Pd (where x% and y% correspond to the gold and palladium wt.% respectively) in the following were prepared by two different preparation methods:

Anatase Rutile

sol-immobilization (SIM) and precipitation-reduction (PR).

Sol-immobilization method

A series of seven Au-Pd catalysts containing 1 wt.% of total metal loading (theoretical value) and covering the whole range of composition from pure Au to pure Pd was prepared by the sol-immobilization method (SIM) and then characterized using the different methods presented in Chapter II.

ICP-OES and XRF analyses

In order to determine quantitatively the amount of metal really present in the synthesized catalysts, the ICP-OES technique was used. The results obtained are presented in Table III-1. X-ray fluorescence was also used to measure qualitatively the elemental composition of the catalytic materials. It has to be noted that the quantity of oxygen detected by this technique is not very accurate. For this reason, the absolute elemental composition cannot be precisely determined. However, the ratio between each metal can be estimated. The most important advantage of the XRF if compared to ICP is that it can be performed quickly without complex sample preparation (mineralization is not needed). The result of the analysis also is quickly obtained, taking typically only 10 to 30 minutes [START_REF] Steve | Advantages & Disadvantages of XRD and XRF[END_REF]. The results of the XRF analyses obtained are also shown in Table III-1. The theoretical and real loadings are generally very close for gold whatever the technique used to estimate them but for Pd some differences are observed with some inconsistencies between XRF and ICP analyses. The differences observed between the two techniques of analyses could be due to an imperfect sample preparation for ICP whereas the differences between the theoretical and actual Pd loadings are more likely related to the method that was used for the preparation of the catalysts. Indeed, it is possible that not all the Pd precursor was adsorbed on the support during the reduction by sodium borohydride. Therefore, some Pd could have stayed in the solution and then been eliminated during centrifugation and washing.

As a result, the Au/Pd ratios determined by ICP and XRF shows various differences when compared with the theoretical ratios. This may also be due to the very low metal loadings, which were less than one percent in weight.

The difference in the Au/Pd ratio observed in ICP and XRF could also be explained by the presence of a high concentration of oxides in the sample, which cannot be measured quantitatively by XRF [START_REF] Vrebos | Trends in quantification in XRF[END_REF]. As listed in Table III-1 the gold content determined by XRF agreed well with the results from ICP. In contrast, there are sometimes differences between the Pd compositions obtained by XRF and ICP. This suggests that the palladium atoms in the samples were probably in oxidized state. Given that XRF cannot detect all the elements precisely (as ICP-OES does [START_REF] Mccomb | Rapid screening of heavy metals and trace elements in environmental samples using portable X-ray fluorescence spectrometer, a comparative study[END_REF]), the Au/Pd ratio of the 0.9%Au0.1%Pd and 0.7%Au0.3%Pd catalysts were considered in the following as being 16.60 and 3.58, respectively.

Given that XRF cannot detect all the elements so precisely as ICP-OES does, the exact metal loadings were considered to be the ones obtained from ICP-OES for the following discussion. As observed, there is a narrow distribution of the particles sizes with the majority being in the 2-3 nm range for each catalyst. Unfortunately, it was difficult to detect the metal particles in 1%Pd/TiO 2 catalyst prepared by sol-immobilization method.

However, palladium was deposited on the support, and confirmed by ICP-OES and XRF techniques. This will also be confirmed by XPS as it will be discussed below. The Pd particles were just probably too small to be distinguished from TEM images. It can be concluded from these images that Au-Pd nanoparticles prepared by SIM were dispersed homogeneously on the TiO 2 support. It is of importance that the mean sizes of Au-Pd particles in the bimetallic Au-Pd/TiO 2 samples with different metal ratios were quite similar in a 2-3 nm range (Table III-2). The mean size of Au-Pd particles in such a range is believed to be beneficial for glucose oxidation [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF][START_REF] Cao | An investigation into bimetallic catalysts for base free oxidation of cellobiose and glucose[END_REF].

This also is interesting for the study of the effect of gold and palladium structure and ratio on catalytic performance. The surface chemical composition of the x%Auy%Pd/TiO 2 catalysts, as determined by XPS, are reported in Table III components, in a fixed intensity ratio (1.5), also keeping the same full width at FWHM values. Pd 3d peaks have asymmetric peak shape for metal while Pd oxides have symmetric peak shapes [START_REF] Moulder | Handbook of X-ray photoelectron spectroscopy[END_REF]. .00 eV and can also be assigned to Pd 0 and Pd +2 , respectively [START_REF] Moulder | Handbook of X-ray photoelectron spectroscopy[END_REF]. The Pd 3d spectrum of 0.9%Au0.1%Pd/TiO 2 and 0.7%Au0.3%Pd/TiO 2 catalysts consist of one component located at BE = 334.69 and 334.68 eV, respectively, which can be assigned to metallic palladium species, namely Pd 0 [START_REF] Moulder | Handbook of X-ray photoelectron spectroscopy[END_REF]. Surprisingly, in 1%Pd/TiO 2 catalyst no Pd 3d photoelectron peak was observed as shown on Figure III-5. This suggests that there is no palladium on the surface, or that its concentration is very low.

Catalytic test

Prior to the evaluation of the catalytic performance of the series of catalysts, a blank test was done in the standard reaction conditions. As shown in Table III-5, 6% of glucose conversion was obtained, while the selectivity to gluconic acid was only 9.0%.

In contrast, over TiO 2 the conversion of glucose was only 3.3% and selectivity to gluconic acid raised up to 39.6%. Blank test was also done at 80 ˚C by adding only the TiO 2 support. 3.6% of glucose conversion was observed, and no gluconic acid was detected. The carbon balance of all the blank tests was in the 94-98% range. Therefore, a volcano-type plot was observed across the range of bimetallic catalyst compositions tested. This confirmed the presence of a synergistic effect between Pd and Au. The selectivity to gluconic acid was always around 75% except for the 1%Au/TiO 2 catalyst where the selectivity to gluconic acid was only 53%. The oxidation reaction at 80 ˚C had the same activity trend, across the range of Au-Pd catalyst composition tested. This is remarkable and suggests an underlying similarity between the reaction pathways. The selectivity to gluconic acid was maintained at 75% more or less without any influence of the Pd content in the catalyst. In contrast, at 100 ˚C, a linear relation between catalyst activity and gold content was observed. This could be explained by the fact that, at this temperature, glucose conversion reached 100% over all catalysts.

The selectivity to gluconic acid was kept around 65%, which is less than at lower temperature, while around 10% selectivity to fructose was observed. It suggests that increasing the temperature will increase the conversion of glucose but the selectivity to desired gluconic acid will be decreased because of the isomerization reaction of glucose to fructose occurred. The activity results for 0.1%Au0.9%Pd/TiO 2 catalyst

were not shown in Figure III-6 because the conversion was nearly null and very close to the blank test. For example, at 60 ˚C the conversion of glucose was 4.3% and the selectivity to gluconic acid was 38.8%. Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 5 h of reaction.

Recyclability test

The recyclability of the 0.5%Au0.5%Pd/TiO 2 catalyst was studied in the following reaction conditions. Temperature, initial pressure, gas atmosphere, duration of the reaction, catalyst amount and initial reactant concentration were set as follow: 80 ˚C, 5 bar, air, 5 h, 21.9 mg and 1 wt.% glucose in water.

A series of three tests was performed consecutively. After the first run (denoted as 1 st ), the product solution was taken out from the reactor by a syringe. The catalyst was recovered by centrifuge, and then dried at 100 ˚C overnight. The next day the 2 nd run was done. The same amount of catalyst was weighted and added to the reactor. 2 mL 1wt.% glucose was then introduced. After that, the reactor was closed and the test was performed again in the same conditions as previously. After the second run, the same operation was repeated for the third time. To ensure the repeatability of the results, each test was repeated at least 2 times. The standard deviation is represented by the error bars in Figure III-7. The results obtained presented a progressive increase of the catalytic performance of the catalyst. The selectivity remained around 75%, and the carbon balance kept around 85%. Comparing the results after three run, the conversion of glucose increased progressively from 53.3% to 80.5%. Especially, it seems that the catalyst was activated during the 1 st run. Catalyst became more and more stable. The conversion was enhanced to 74.8% in the 2 nd run and even higher in the 3 rd run. Combining the XPS data of the fresh catalyst with the ones after use (Table III-9 and III-10), it can be said that more palladium appeared on the surface, and that the percentage of Pd +2 decreased. The main reason of activity enhancement could be attributed to the role of PVA. During the first run, the reaction environment was like a post-synthesis treatment of the catalyst. Part of the PVA was washed out from the catalyst. The presence of PVA has been demonstrated to lower the activity of Au catalysts but increase the stability of the catalyst upon recycling, prolonging considerably the catalyst life. These effects strictly depend on the amount of the protective agent [START_REF] Villa | Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts[END_REF]. The effect of PVA will be discussed further in this thesis (Section 1.3 and Section 3).

Precipitation-reduction method

Another preparation method used in this work was precipitation-reduction (PR).

As in the previous section, the techniques of characterization described in Chapter II were used to study the prepared catalysts.

ICP-OES and XRF analyses

The results of the ICP-OES and XRF analyses of the Au-Pd/TiO 2 PR series are showed in Table III-11. The theoretical and real loadings are generally very close but some differences are still observed for a few samples. Especially, the loadings of gold were quite close to the theoretical ones. But the real loadings of palladium were once again generally lower than the expected ones. The reason is the same as said previously (Section 1.1.1). prepared by PR method. These two catalysts were chosen typically to compare with Au-Pd/TiO 2 synthesized by SIM method. The other samples were not analyzed by TEM. At least 300 particles were counted and an average value and distributions were calculated from them. The average particle size of 1%Au/TiO 2 is 4.3 ± 1.9 nm, and the average particle size of 1%Pd/TiO 2 is 2.6 ± 1.6 nm. an almost linear trend. The selectivity to gluconic acid was maintained at around 70% when the Au content was in the range 0.3-1wt.%. It has to be noted that the selectivity to gluconic acid was only 31.4% when the gold content of the catalyst was 0.1wt.%.

Though the gold contributes a lot to the selectivity to gluconic acid in the glucose oxidation reaction, Au content in 0.1wt.%Au0.9wt.%Pd/TiO 2 was too low and the metal in this catalyst was mainly palladium [START_REF] Ishida | Influence of the support and the size of gold clusters on catalytic activity for glucose oxidation[END_REF]. The conversion of glucose was 11.3% and selectivity to gluconic acid was 53.6% over 1%Pd/TiO 2 , which was very different from the performance over 1%Pd/TiO 2 catalyst prepared by SIM method. Thus, it is

Comparison of both preparation method

As mentioned before, the bimetallic Au-Pd series on TiO 2 catalysts were prepared by two different methods: sol-immobilization using polyvinyl alcohol as a stabilizer and sodium borohydride as reducing agent and precipitation-reduction also using sodium borohydride as reducing agent. In this section, the effect of the preparation method on the physico-chemical properties of the obtained catalysts and, hence, on their catalytic performance will be discussed. However, at the same glucose conversion, the selectivity to gluconic acid over catalysts prepared by SIM is better compared to the catalysts prepared by PR. Au content (%) [START_REF] Minicò | Influence of catalyst pretreatments on volatile organic compounds oxidation over gold/iron oxide[END_REF]

˚C

It seems that there was no activity of palladium for glucose oxidation. Moreover, the PVA played a role of protective agent, and lead to a higher selectivity as already noticed in the literature [START_REF] Villa | Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts[END_REF][START_REF] Campisi | Selectivity control in palladium-catalyzed alcohol oxidation through selective blocking of active sites[END_REF]. However, it is not easy to remove all the PVA from the catalysts by washing during the preparation procedure, so a few researchers chose to remove some of the PVA to get a higher catalytic performance by post-synthesis treatment like calcination or reflux method [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF][START_REF] Villa | Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts[END_REF][START_REF] Cao | An investigation into bimetallic catalysts for base free oxidation of cellobiose and glucose[END_REF].

Au-Pd supported on zirconium dioxide

Zirconium dioxide (ZrO 2 ) is a very stable support and also very commonly used. It was chosen for a comparison with TiO 2 . Au-Pd supported on ZrO 2 catalysts were also produced by both SIM and PR methods as previously for TiO As previously a series of Au-Pd bimetallic catalysts were prepared by the two preparation methods: sol-immobilization and precipitation-reduction.

Sol-immobilization

The preparation method and the techniques of characterization used to study the series of catalysts synthesized by SIM were presented in Chapter II.

ICP-OES and XRF analyses

Table III-12 shows the results of the elemental ICP-OES and XRF analyses of the Au-Pd/ZrO 2 series prepared by sol-immobilization method. Following the same trend as seen previously for the Au-Pd/TiO 2 series the theoretical and actual loadings are generally close but some differences are still observed for a few samples. The real loadings of gold were quite close to the theoretical ones. But the real loadings of palladium were generally lower than the theoretical data. of the theoretical value. There might have been some problems during the preparation.

The difference of Au/Pd ratio between ICP and XRF could be explained by the same reasons already discussed in Section 1.1.1.

XRD

The diffractograms of 0.5%Au 0.5%Pd/ZrO 2 , 0.7%Au0.3%Pd/ZrO 2 prepared by sol-immobilization method and of the fresh support ZrO 2 for comparison are shown in III-13 gather the results. The Au-Pd particles in 0.7%Au0.3%Pd/ZrO 2 catalyst tend to agglomerate rather than to be well dispersed on the support. In addition, fewer particles were counted. In the fresh catalyst, only 114 particles were counted and the average metal particle size was estimated to 4.9 nm. In the case of the 1wt.%Au/ ZrO 2 catalyst more than 300 particles could be counted, and the average particle size of gold was 6.5 nm with a more narrow distribution.

Recyclability test

The recyclability test for the 0.7wt.%Au0.3wt.%Pd/ZrO 2 catalyst was carried out in the following conditions: Temperature, initial pressure, gas atmosphere, duration of the reaction, catalyst amount and initial reactant concentration were set as follows: 80 ˚C, 5 bar, air, 5 h, 21.9 mg and 1 wt.% glucose in water. In total, three tests were performed consecutively. Firstly, the fresh catalyst was tested and after the reaction the product solution was taken out with a syringe. The catalyst was recycled by centrifuge, and then dried at 100 ˚C overnight. The catalyst recovered was noted as 1 st run. The reaction was then carried out again (2 nd run). The same amount of catalyst was weighted and added to the reactor. 2 mL of 1wt.% glucose aqueous solution was then introduced. After that, the reactor was closed and the test was performed again in the same conditions as previously. After the second run, the same operation was applied for the third time. To ensure the repeatability of the results, each test was repeated at least 2 Table III-18 presents the ICP results of fresh 0.7%Au0.3%Pd/ZrO 2 catalysts and after the first and the second runs. The concentration of gold reduced slightly after each run while the concentration of palladium remains the same. As a result, the Au/Pd ratio became slightly smaller after each run. The change in the ICP results is not so significant, so the loss of gold should not be the main reason of the decrease of the catalytic performance. III-19). Thus, the particle sizes of this catalyst after calculation were not reliable. It suggests that leaching of metals occurred during the tests. However, the ICP-OES data before and after reaction confirmed that no leaching took place. So the only explanation is that there was a serious agglomeration and metallic particles were very difficult to be found in TEM images. As a result, the decrease of activity was attributed to NPs agglomeration.

Precipitation-reduction method

Precipitation-reduction (PR) method was also used to prepare the Au-Pd/ZrO 2 series. This method of preparation and the techniques of characterization used to study the series of catalysts were described in Chapter II.

ICP-OES and XRF analyses

Table III-22 presents the results of the ICP and XRF elemental analyses of the Au-Pd/ZrO 2 series prepared by precipitation-reduction method. The real loadings of gold were always around 80% of the theoretical value. The real loadings of palladium were also around 80% of the theoretical value when the Pd content was less than 0.3wt.% while the ratio decreased to 70% when the Pd content was more than 0.5wt.%.

When comparing the Au/Pd ratio determined by ICP-OES and XRF, the latter are smaller. It could be explained by the same reasons discussed previously in Section 1.1.1.

Catalytic test

The catalytic tests were carried out at 60 ˚C. In a typical reaction, 21.9 mg catalysts and 2 mL of the aqueous glucose solution (1 wt.%) were loaded in the reactor, which was then purged with air three times before closing, and the pressure was maintained at 5 bar. The reaction duration time was 5 h. The procedure was the same as previously. Figure III-23 presents the activity data obtained across the range of different gold content (wt.%). A strong anti-synergy between Au and Pd was observed, whereby the introduction of Pd resulted in a significant decrease in catalytic activity. Especially when the percentage of Au were 0.46wt.% and 0.61 wt.%. Notably, this time the selectivity to gluconic acid was not as stable when the Pd loading increased. When gold content was 0.24wt.% and 0.46wt.%, the selectivity to gluconic acid was 32.3% and 44.3%, respectively. Detailed results are gathered in Annex -Table 7.

Comparison of both preparation method

As we have seen before, the bimetallic Au-Pd series of catalysts supported on ZrO 2 were prepared by sol-immobilization and precipitation-reduction methods using sodium borohydride as reducing agent. In this part, the influence of the preparation method on the catalytic performance is discussed. The results are shown in Figure As observed, when the Au content was higher than 0.7wt.%, the conversion of glucose over the catalysts prepared by PR method were much higher than over the catalysts prepared by SIM method. In contrast, when the Au content was less than 0.7wt.%, the catalysts prepared by SIM method showed better or similar activity as for PR. At isoconversion the catalysts prepared by SIM method showed better selectivity to gluconic acid than the catalysts prepared by PR method, which is in the agreement with the PVA theory. Actually, PVA played a role of protective agent, and could formed special tunnels for reactant and product, hence enhancing the selectivity [START_REF] Villa | Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts[END_REF][START_REF] Campisi | Selectivity control in palladium-catalyzed alcohol oxidation through selective blocking of active sites[END_REF]. The 0.7%Au0.3%Pd/ZrO 2 catalyst prepared by SIM method seems to be optimal concerning the PVA content as it showed much better activity than the catalyst with the same gold and palladium loading obtained by PR method and also a higher selectivity to the desired product. TEM images (Figure III-19 and III-22) revealed that the metal particle size was 4.9 nm and 5.3 nm, respectively. As the particle sizes are almost the same other factors influence the catalytic activity. This could be the action of PVA. In the case of the palladium monometallic catalyst prepared by PR method relatively high activity was obtained (14%) but the selectivity to gluconic acid was very low (14.5%).

Figure III-24 Effect of the preparation method on the catalytic performances of Au-Pd series on ZrO 2 for the glucose oxidation. Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

Discussion

As the Au-Pd catalysts prepared by SIM method and supported on both TiO 2 and ZrO 2 exhibited a synergistic effect it is of interest to discuss the role of the support. At 60 ˚C, the ZrO 2 -supported catalysts showed a much higher activity than the TiO 2 -supported ones, more particularly when the gold loading was in the 0.3 to 1 wt.% range. As far as the selectivity is concerned the role of the support seems to be less important except for the lowest gold loading or when the gold loading is 1 wt.%. In the range 0.3 to 0.9 wt.% the selectivity to gluconic acid seems to be rather constant whatever the conversion and the type of support.

At 80 ˚C, a similar trend was observed for the activity which is always better for ZrO 2 -supported catalysts when the gold loading exceeded 0.3 wt.%. The selectivity to gluconic acid was almost the same over all the catalysts supported on both oxides when Glucose Conversion% PR Glucose Conversion% SIM Gluconic acid Selectivity% PR Gluconic acid Selectivity% SIM gold content was superior to 0.1wt.%. It suggests that the selectivity to gluconic acid did not depend on the support properties, or on the glucose conversion but only on the metal itself. The selectivity to gluconic acid enhanced significantly for 1.0wt.% gold content when the temperature increased. This might be due to the PVA removing and thus the tunnels for reactant creation during the reaction at 80 ˚C which also contributed to the increase in the conversion of glucose [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF].

As shown previously in Table III ). This might be due to the particles size and surface area of the bare oxides.

Indeed, the ZrO 2 particle size (around 5 μm) was much bigger than that of TiO 2 (around 21 nm). And the TiO 2 surface area (53 m² /g) was much higher than those of ZrO 2 (6 m² /g). A smaller particle size and a higher surface area of the support led to a better distribution of the metal particles and also to smaller metal particles hence impacting the catalytic properties and mainly the activity because of metal-support interactions.

As the most active catalyst did not have the smallest particle size it suggests that the support played also a significant role in the catalytic activity. ZrO 2 is p-type semiconductor while TiO 2 is n-type semiconductor. ZrO 2 provides oxygen vacancies and O 2 is easily adsorbed on the surface of a p-type semiconductor, and it is easier to get electrons from this type of oxide. This seems to contribute to get a better activity for base-free glucose oxidation reaction.

Nevertheless, the recyclability tests revealed that ZrO 2 is not able to prevent the metal particles from agglomeration which occurred during the reaction. Contrary to that, TiO 2 showed very good resistance to agglomeration. The particle size and distributions did not change during the reaction. The catalysts after reaction even showed better activity than the fresh ones.

˚C

Glucose Conversion % TiO₂ Glucose Conversion % ZrO₂ Gluconic acid Selectivity% TiO₂ Gluconic acid Selectivity% ZrO₂ increase selectivity and stability [START_REF] Villa | Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts[END_REF]. However, these effects are strictly depending on the amount of PVA still present in the catalysts after pre-treatment. Normally, further treatment would be required to remove the extra PVA. Hence, thermal treatments (calcination at 250 ˚C and 400 ˚C) were used in an attempt to remove most of the remaining PVA [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF] so we applied the same treatments to our series of catalysts. The 8and 9). The reaction was carried at 60 ˚C. In comparison with the results obtained over the fresh catalyst (Figure III-17), a significant increase of activity for 1 wt.% Au/ZrO 2 could be observed, especially when the catalyst was calcined at 400 ˚C. This could be explained by the removal of residual PVA. For the other catalysts with different Au contents varying degrees of activity decline were observed whenever calcined at 250 ˚C or 400 ˚C. Both showed a volcano-like curve with the top at 0.4wt.% gold content. The selectivity to gluconic acid showed no difference before and after calcination. Through the excess of PVA was also removed, the decrease of activity could be attributed to the oxidation of palladium. During the calcination, most of the metallic palladium was transformed to an oxide under static air. More oxidized palladium covered the surface of gold [START_REF] Silva | Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols[END_REF], which prevented the glucose to be in contact with the active centers of the catalyst.

It is also possible that the thermal treatment induced structural changes within the catalysts. Indeed, calcination has been previously reported to lead to changes in particle size [START_REF] Cao | Base-free oxidation of glucose to gluconic acid using supported gold catalysts[END_REF][START_REF] Lopez-Sanchez | Facile removal of stabilizer-ligands from supported gold nanoparticles[END_REF]. This also explained why catalysts calcinated at 400 ˚C exhibited lower activity than when calcination was done at 250 ˚C except for the 1 wt.% Au where PVA effect was the determining factor. Table III-23 shows the weight loss of catalysts during the calcination at 400 ˚C. The weight loss was found very small and might be related to some water release and the removal of PVA.

Introduction

A positive synergistic effect between the metals has been revealed in the previous chapter in the case of Au-Pd for the base-free oxidation of glucose. In this last chapter, more bimetallic gold-based catalysts were studied. Copper, bismuth and platinum were chosen as a second metal since they are well known to exhibit very good activity in oxidation reactions, especially Pt which is already used as a mono-metallic catalyst for oxidation reaction [START_REF] Haruta | Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0[END_REF][START_REF] Hutchings | Vapor phase hydrochlorination of acetylene: Correlation of catalytic activity of supported metal chloride catalysts[END_REF][6][START_REF] Hutchings | Catalysis by gold[END_REF][START_REF] Bianchi | Selective oxidation of glycerol with oxygen using mono and bimetallic catalysts based on Au, Pd and Pt metals[END_REF][START_REF] Dimitratos | Synergetic effect of platinum or palladium on gold catalyst in the selective oxidation of D-sorbitol[END_REF]. Among these metals copper was mainly studied. As it is easily dissolved in acid solution, SIM method is not suitable for the preparation of this kind of catalyst. Precipitation-reduction method was therefore the only method used to prepare the catalysts studied in this chapter.

The same TiO 2 and ZrO 2 as in the previous chapter were used as the supports of the metals, aiming to promote the distribution of nanoparticles and the stability of the catalyst and compare with the results obtained from Au-Pd series.

As described in Chapter II, the catalysts samples denoted as x%Auy%Cu (where x% corresponds to the gold wt.% and y to the copper wt.%) were prepared by precipitation-reduction (PR) method.

Study of the Au-Cu catalysts series 1.1 Supported on titanium dioxide

The same titanium dioxide (TiO 2 ) used in Chapter III for Au-Pd catalysts was used here as the support for Au-Cu nanoparticles. The structures of anatase and rutile phases were presented in previous chapter (Figure . The techniques of characterization described in Chapter II were used to study the Au-Cu/TiO 2 catalysts prepared by precipitation-reduction (PR) method.

1.1.1 ICP-OES and XRF analyses ICP-OES technique was used to determine the amounts of metal present in the synthesized catalysts quantitatively. Table IV-1 presents the results obtained. The actual loadings are generally close but a little bit less than the theoretical ones. It seems normal as some metals were lost during the preparation. Comparing to what was obtained for Pd, Cu seems to be more easily reduced and hence the real content is close to the theoretical values. As the metal loading increased, the copper was not loaded as well as gold. For example, for 1 wt. % Au in theory the actual loading was 0.92 wt.%, while for the copper loading of 1%Cu in theory the actual loading was only 0.81 wt.%. This could be due to the easy oxidation of copper.

Table IV-1 also shows the results obtained from X-ray fluorescence (XRF). As said in Chapter III, since the oxygen detected by XRF is not very accurate, only the ratios between metals are reliable. The Au/Cu ratios were in good agreement with the theoretical ones and with the data obtained from ICP-OES technique. The XRD analysis of the samples was carried out to determine the crystalline structure of the gold and copper species. 0.5%Au0.5%Cu/TiO 2 , prepared by precipitation-reduction method and fresh support TiO 2 as comparison were chosen (Figure IV-1). However, due to the sensitivity of the technique, only the presence of pure TiO 2 could be observed. were counted. An average size of the particles and distributions of the size were calculated. As observed, the metal nanoparticles were well distributed on the support in most of the cases. However, in (f) and (g), especially in (g), it was very difficult to distinguish the nanoparticles. This might be because the molar masses of Ti and Cu are very close, so the contrast between each element is not good. Another possibility is that the metal nanoparticles were too small to be observed. ICP-OES results showed that there was 0.09wt.% of gold and 0.76wt.% of copper in 0.1%Au0.9%Cu/TiO 2 catalysts and 0.81wt.% of Cu in 1%Au/TiO 2 catalyst. It suggests that the size of the nanoparticles had reached the resolution limitation of the TEM technique. found very close, in the range 2.7 to 3.4 nm. However, the mean size of gold nanoparticles in 1%Au/TiO 2 catalyst was 4.3 nm. The mean sizes of metal nanoparticles in 0.1%Au0.9%Cu/TiO 2 and 1%Cu/TiO 2 were not considered as being reliable because of the limited number of particles counted (Table IV-2). a function of the different ratio between Au and Cu. Some shifts were also detected.

That is because these two metals can easily formed alloy metals, which affects the binding energies [START_REF] Neaţu | Gold-copper nanoalloys supported on TiO 2 as photocatalysts for CO 2 reduction by water[END_REF][START_REF] Wilson | From core-shell to alloys: The preparation and characterization of solution-synthesized AuPd nanoparticle catalysts[END_REF]. For example, in the 1%Au/TiO 2 sample, the Au 4f curve fitting shows Au 4f 7/2 components at BE = 83.08 and 85.74 eV. These two values were in good agreement with those reported in the literature for Au 0 and Au +δ , respectively [START_REF] Park | Effects of pretreatment conditions on CO oxidation over supported Au catalysts[END_REF][START_REF] Casaletto | XPS study of supported gold catalysts: the role of Au 0 and Au +δ species as active sites[END_REF][START_REF] Moulder | Handbook of X-ray photoelectron spectroscopy[END_REF]. In the 0.7%Au0.3%Cu/TiO 2 sample, the curve fitting of Au 4f 7/2 spectrum indicated two Au 4f 7/2 components at BE = 83.15 and 84.97 eV that also could be attributed to Au 0 and Au +δ species, respectively. However, there was a small shift of BE values in comparison with the gold monometallic catalyst. As more Cu and less gold were introduced in the catalysts, the shifts of BE values became bigger.

The Cu 2p XPS spectrum of 0.1%Au0.9%Cu/ TiO 2 showed a main signal and a satellite peak. The main signal could be deconvoluted into four contributions, indicating different oxidation states of copper. The contribution at 932.41 eV could be assigned to the presence of CuOH, the peak at 931.54 eV could be assigned to Cu +1 and the metallic copper, and the last one at 934.22 eV could be attributed to the CuO species [188,[START_REF] Onal | Propylene epoxidation: High-throughput screening of supported metal catalysts combinatorially prepared by rapid sol-gel method[END_REF]. The satellite peak confirmed the presence of CuO species on the surface of this sample. The satellite peak arised when the emitted photoelectron lost part of its kinetic energy to excite a valence electron to an unoccupied d orbital of Cu.

The presence of satellite peak was, therefore, an indication of the existence of d 9 partially filled orbitals in Cu fundamental state, and thus it appeared only when Cu +2 species was present [START_REF] Neaţu | Gold-copper nanoalloys supported on TiO 2 as photocatalysts for CO 2 reduction by water[END_REF][START_REF] Onal | Propylene epoxidation: High-throughput screening of supported metal catalysts combinatorially prepared by rapid sol-gel method[END_REF][START_REF] Amorós-Pé Rez | Romá n-Martí nez, Cu/TiO 2 photocatalysts for the conversion of acetic acid into biogas and hydrogen[END_REF][START_REF] Hai | Modification of TiO 2 by bimetallic Au-Cu nanoparticles for wastewater treatment[END_REF][START_REF] Jia | Synergistic catalysis of Au-Cu/TiO 2 -NB nanopaper in aerobic oxidation of benzyl alcohol[END_REF]. When Cu loading was less than 0.5wt.%, the satellite peak was almost not observed because the signal intensity was very low, but CuO existed in most samples. The Au/Cu atomic ratio on the surface of 0.9%Au0.1%Cu/TiO 2 was 1.44 (Table IV-3), which indicated that the bulk was rich in gold. On the contrary, other catalysts were rich in copper. This is in good agreement with results obtained by ICP-OES and At 60 ˚C, as the introduced quantity of Au is increased, the catalytic activity is also increased to reach a maximum at 0.3-0.45wt.% of Au and then it decreased. The least active catalyst tested was the monometallic Cu catalyst, which reached only 1.5% conversion of glucose, forming a volcano plot across the range of bimetallic catalyst compositions tested. This confirmed the presence of a synergistic effect. The selectivity to gluconic acid was always around 50% except over 0.1% Au0.9%Cu/TiO 2 catalyst for which a sudden decrease of the selectivity to gluconic acid was observed.

The same observation was done at 80 ˚C but as the temperature increased, the activity and selectivity increased as well. The trend of activity was very close to that observed at 60 ˚C except for 0.1% Au0.9%Cu/TiO 2 , for which there was a remarkable increase of activity when the temperature was increased from 60 to 80 ˚C. As reaction temperature was increased to 100 ˚C, an almost linear inverse relation between catalyst activity and gold content was observed. This was because at this temperature, the glucose conversion was close to or reached 100% over all the catalysts. The selectivity to gluconic acid was maintained at roughly 60%. There was a little decrease in comparison to lower temperatures because of the generation of fructose as by-product.

When the gold content was more than 0.1wt.%, the selectivity to gluconic acid was maintained more or less at the same level whatever the Au content in the catalyst at a given temperature. This suggests that gold played a decisive role in the selectivity to gluconic acid in the oxidation of glucose. This effect was not obvious over 0.1% Au0.9%Cu/TiO 2 because the content of gold was too low to be the only key factor. Some other factors may affect the final result of the reaction. The morphology of the support did not change visually. The particle size of the metal reduced slightly after the reaction (Table IV-6). This can be explained by the fact that copper on the surface could be easily oxidized under air. During the reaction, acid was generated and then some of the copper and copper oxide was dissolved.

Supported on zirconium dioxide

The same zirconium dioxide (ZrO 2 ) as the one used in Chapter III was chosen for comparison. The techniques of characterization described in Chapter II were used to study the Au-Cu/ZrO 2 catalysts prepared by precipitation-reduction method.

ICP-OES and XRF analyses

Table IV-9 presents the results obtained by ICP-OES technique for the Au-Cu/ZrO 2 series prepared by PR method. Following the same trend as for the other catalysts series, the theoretical and actual loadings were generally found quite close but some differences were still observed for a few samples. As for gold, the actual loading of 0.9%Au0.1%Cu, 0.3%Au0.7%Cu, and 0.1%Au0.9%Cu were close to theoretical value while the actual loading of the other catalysts were a little bit lower than the expected values. As for copper, there was a similar observation for 0.5%Au0.5%Cu and 1%Cu, especially 0.9%Au0.1%Cu, for which the theoretical value was 0.1 while the actual value was 0.03. For the other catalysts the measured loadings were close to the theoretical ones. Table IV-9 also shows the data determined by XRF technique. For the same reasons as before, only metals ratio could be used in a reliable manner.

Comparing the Au/Cu ratios determined by XRF and ICP-OES with the expected theoretical ones, it can be said that the ratios determined by ICP-OES were very close to the theoretical values except for the 0.9%Au0.1%Cu catalyst where there was a significant leaching of copper. In contrast, except for 0.9%Au0.1%Cu, the Au/Cu ratios determined by XRF were much lower than the theory. That indicates that there was a lot of copper in higher oxidation state

Catalytic test

The reaction was carried out at 60 ˚C and the reaction conditions were the same as the ones used previously for the Au-Cu/TiO 2 catalysts (Section 1.1.5). Figure IV-12 presents the catalytic performances of the whole series. A significant anti-synergistic effect could be observed in the 0.3-0.4wt.%Au range. Actually, as more and more copper was introduced in the catalyst, its activity decreased progressively until 0.29 wt.% gold content, and then increased, forming a valley-like curve. The selectivity to gluconic acid was always maintained around 80%, except over 0.1%Au0.9%Cu where the selectivity to gluconic acid was only 6%. Annex 2 -Table 13 shows all the detailed results.

Au-Bi supported on zirconium dioxide

Bismuth was chosen as another metal to be combined with Au to form bimetallic catalysts. The ratios between gold and bismuth were the same as for the other catalysts series. The support used was the same ZrO 2 as previously and the preparation method was precipitation-reduction. The characterization study of this series is presented below.

ICP-OES and XRF analyses

ICP-OES and XRF analyses were also carried out for the Au-Bi series of catalysts to determine the actual metal loadings (Table IV-10). Gold loading followed the same trend as before, and the actual loadings were very close, but a little bit lower than the theoretical values. As for bismuth, it obeyed the same rule as gold except for 0.7wt.% Bi and 0.9wt.%Bi, especially the latter. In these cases, there was an obvious loss of bismuth during the preparation. This might be due to an incomplete reduction of the bismuth nitrate or by a significant loss during the washing of the catalyst during the preparation. Most of the results obtained by XRF were smaller than those obtained by 14). The trend of catalytic activity was similar to the one observed for the Au-Cu/ZrO 2 series, forming a valley-like curve. As the quantities of gold decrease, the catalyst activity decreased rapidly and maintained at the bottom until the gold content was less than 0.45wt.%. This time, the selectivity to gluconic acid was surprisingly various. For example, the selectivity to gluconic acid was around 73% over 1% Au/ZrO 2 catalyst while it was around 32% over 0.7%Au0.3%Bi/ZrO 2 catalyst and 43% over 0.5%Au0.5%Bi/ZrO 2 catalyst. The existence of Bi on the surface might reduce the active center of the surface Au.

Au-Pt supported on zirconium dioxide

Platinum was also chosen as another metal to be combined with Au to yield bimetallic catalysts. The ratios between gold and platinum were a little bit different as for the previous series as shown in Table IV-11. The same ZrO 2 as previously was used as support and the preparation method was precipitation-reduction. Derrien et al. also reported similar Au-Pt/ZrO 2 catalysts for the base-free glucose oxidation at high pressure. The metal loading was around 3.5wt.% and gluconic acid was the intermediate product [START_REF] Derrien | Aerobic oxidation of glucose to glucaric acid under alkaline-free conditions: Au-based bimetallic catalysts and the effect of residues in a hemicellulose hydrolysate[END_REF]. The characterization study of this series is presented below. Au content (%)

ICP-OES and XRF analyses

Table IV-11 presents the results of Au-Pt/ZrO 2 series obtained from ICP-OES and XRF. Following the same trend as before, the actual and theoretical loadings of gold are generally close but the former was a little bit lower than the latter. The same observation was made for platinum. For the same reasons as before, the absolute metal contents determined by XRF were not as reliable as the metal loadings determined by ICP-OES. Hence, only these latter were considered for the following discussion. 15). As the quantities of gold increased, the catalytic activity increased a little bit and then decreased.

The top conversion of glucose was observed over 0.75%Au0.25%Pt/ZrO 2 catalyst.

Similar observation was on selectivity to gluconic acid. The highest selectivity was 80.5% also over the catalyst 0.75%Au0.25%Pt/ZrO 2 . The lowest selectivity was 62.9% over 1%Pt/ZrO 2 catalyst. As more Pt introduced, more glucuronic acid was produced.

The maximum selectivity to glucuronic acid was observed over 1%Pt/ZrO 2 , 8.7%.

There was some fructose generated over 0.25%Au0.75%Pt/ZrO 2 catalyst. The carbon balance also increased from 83.3% to 93.3% as more and more Pt introduced. Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

Discussion

Effect of the second metal:

The combination of gold with other metal has already been reported to enhance the activity and the selectivity to the desired acid in the oxidation of sugars under base-free conditions. For example, the oxidation of glucose using Au-Pd catalyst at 60°C gave better results than monometallic gold [START_REF] Miedziak | Base-free glucose oxidation using air with supported gold catalysts[END_REF]. The advantage of the addition of a second metal was clearly shown when working in acidic medium. Pt showed a better effect if compared to Pd, which means that Pt had a boosting effect where only gluconic acid is produced [START_REF] Comotti | Mono-and bimetallic catalysts for glucose oxidation[END_REF]. Furthermore, high catalytic activity was also attributed to the presence of a core-shell structure as charges could be easily transferred between the core and the shell and oxygen is activated leading to the attack of the desired functional group. The catalytic activity changed according to the composition of the system. In addition to bimetallic nanoparticles, the use of trimetallic systems was also reported. In agreement with these previous papers, as could be seen from Figures IV-18 and IV-19, the highest activity was also observed in our work for Pt-based bimetallic catalyst in both Au and Pt rich samples. The activity of the Pt catalysts was two times higher than that observed for Pd samples. In the same time the Bi based samples showed anti-synergetic effect. In case of the Cu samples the activity was much higher for Au-rich catalysts. Au-Cu alloy systems supported on different materials were already explored as catalysts for oxidation reactions [START_REF] Liu | Au-Cu Alloy nanoparticles confined in SBA-15 as a highly efficient catalyst for CO oxidation[END_REF][START_REF] Pina | Highly selective oxidation of benzyl alcohol to benzaldehyde catalyzed by bimetallic gold-copper catalyst[END_REF]. Generally, Au 0 is believed to be the active site while, Cu 0 plays an important role in activation of oxygen molecules [START_REF] Thota | Colloidal Au-Cu alloy nanoparticles: synthesis, optical properties and applications[END_REF]. However, it was observed that Au 0 -Cu 0 nanoparticles alone are not catalytically active in their original form. The formation of an Au-CuO x structure has been found to be essential to obtain high activity. As suggested in the literature [START_REF] Liu | Structural changes of Au-Cu bimetallic catalysts in CO oxidation: In situ XRD, EPR, XANES, and FT-IR characterizations[END_REF] the Cu 0 component enriched the surface of the alloy nanoparticles with CuOx can provide active oxygen species. Moreover, Zhan et al. [START_REF] Zhan | Crystal structural effect of AuCu alloy nanoparticles on catalytic CO oxidation[END_REF] have also pointed out that the thickness of CuO x on the surface is also crucial because a thick CuO x layer will prevent O 2 from approaching Au 0 active sites and slowing down activation. In our case four different Cu species were detected (Table IV Moreover, as generally accepted, the catalytic rate is exponentially dependent on the adsorption energy between the active sites and reactants. Adsorption energy is determined by the composition, size and facet of the nanocrystals. In addition, ligands and the metal alloy may undergo a charge transfer process, tuning the surface electronic structure of the nanoparticles, and therefore affect their catalytic activity [START_REF] Gilroy | Bimetallic nanocrystals: syntheses, properties, and applications[END_REF]. Also, the influence of crystal structure and atomic ordering (intermetallic vs. random alloy) on the catalytic activity is very important. It has been demonstrated that ordered Au-Cu alloys exhibited enhanced catalytic properties [START_REF] Kim | Electrochemical activation of CO 2 through atomic ordering transformations of AuCu nanoparticles[END_REF]. However, the mechanism of how atomic ordering affects catalytic performance still needs to be studied.

One of the examples is the

In case of the Au-Pd bimetallic catalysts the remarkable increase in the catalytic activity in oxidation reactions is already well established [START_REF] Silva | Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols[END_REF][START_REF] Enache | Solvent-Free oxidation of primary alcohols to aldehydes using Au-Pd/TiO 2 Catalysts[END_REF][START_REF] Dimitratos | Pd and Pt catalysts modified by alloying with Au in the selective oxidation of alcohols[END_REF][START_REF] Ketchie | Selective oxidation of glycerol over carbon-supported AuPd catalysts[END_REF][START_REF] Kesavan | Solvent-free oxidation of primary carbon-hydrogen bonds in toluene using Au-Pd alloy nanoparticles[END_REF][START_REF] Silva | Magnetically recoverable AuPd nanoparticles prepared by a coordination capture method as a reusable catalyst for green oxidation of benzyl alcohol[END_REF].

For alcohols oxidation, Au-Pd nanoparticles were found to be twice as active as Pd alone for oxidation reaction. It was proposed that Au is an electronic promoter of Pd [START_REF] Enache | Solvent-Free oxidation of primary alcohols to aldehydes using Au-Pd/TiO 2 Catalysts[END_REF]. Composition effect was also reported, where a progressive increase in catalytic activity was observed on increasing the Pd content, with the optimum ratio of Au:Pd in ca. 1:1.85, and a progressive decrease of activity with further increase in the Pd content [START_REF] Pritchard | Direct synthesis of hydrogen peroxide and benzyl alcohol oxidation using Au-Pd catalysts prepared by sol immobilization[END_REF]. In our case the Au-Pd catalysts were dried at 100 o C for 16 h and the surface Pd/Au molar ratios were determined by XPS.

From the XPS data it could be deduced that gold was mostly reduced and the metallic character increased with the increase of Pd in the samples. In the same time the oxidation state of Pd progressively increases from Pd 0 to Pd 2+ with the increase of gold content in the sample. It could be concluded that there should be an optimum ratio between Pd 0 and Pd 2+ species that will maintain a high activity as well as high selectivity.

Taking this into account it can be concluded that the understanding of the structure and composition of the Au-Pd bimetallic catalysts is key for the design of more active catalysts and to find robust structure-activity relationships.

However, the exact characterization of these samples is often difficult due to interference of the support (metal support interactions) or low metal loadings.

For carbohydrates oxidation, the activity of the Au-Pd nanoparticles depends on the composition: gold-rich nanoparticles presented a lower activity when compared with the palladium-rich nanoparticles, but a maximum of activity (at 80 °C) was reached with Au 1 Pd 4 nanoparticles (Figure III -17). Moreover, the activity for all samples decreased after calcination, but the effect was more promunced in palladium-rich samples. Besides the positive effect expected of cleaning the surfaces from the capping agents (PVA), the segregation of palladium and its oxidation seems to play a major role in the catalytic activity.

After calcination at 400°C under air, Pd 0 was converted to oxide PdO, and the activity dramatically droped to values even lower than those obtained with the Pd monometallic sample. It could be firstly concluded that the gold-rich samples continue in their alloyed forms with a small increase in surface Pd, as observed by the XPS data compared to the non-calcined samples. Secondly, the Pd-rich samples present metal segregation with an increasing presence of palladium oxide. These results suggest a metallic segregation for the calcined AuPd nanoparticles as already reported by Meenakshissundaram et al. [START_REF] Meenakshisundaram | Oxidation of alcohols using supported gold and goldpalladium nanoparticles[END_REF].

Support effect

For a more refined analysis of the catalytic activity results, one has to keep in mind that the overall activity of a given catalyst is determined not only by structural properties of metal particles (particle size, exposed surface, crystallographic facets) but also by surface chemical processes (adsorption, O 2 spillover, oxygen bond formation).

In selective oxidation of carbohydrates over metal supported catalysts, it is believed that the reaction occurs both on the metal and on the support through the oxygen spillover. In this way, the acidity of the support (strength and concentration of acidic sites) plays an important role [START_REF] Zaki | In situ FTIR spectra of pyridine adsorbed on SiO 2 -Al 2 O 3 , TiO 2 , ZrO 2 and CeO 2 : general considerations for the identification of acid sites on surfaces of finely divided metal oxides[END_REF][START_REF] Emeis | Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts[END_REF][START_REF] Boudjahem | Study of nickel catalysts supported on silica of low surface area and prepared by reduction of nickel acetate in aqueous hydrazine[END_REF]. The contribution of the support may be important because it can furnish adsorption sites for the organic molecule, and in the vicinity of the metal particles, the adsorbed molecule can react with the spilled-over oxygen from the metal, thus contributing to the overall activity. Remarkably, the highest reaction rates at 80 °C were obtained for TiO Here, the trend is totally inversed in comparison with the series supported on TiO 2 . When gold content was over 0.5wt.%, Cu-based catalysts always showed a better activity than the Pd-based ones. The activity trend is inversed for lower gold content.

For higher Au contents the selectivity to gluconic acid was systematically better over Cu-based catalysts in comparison with Pd-based ones.

The role of the support is therefore very strong for this reaction in base-free conditions. To sum up Au-Pd catalysts are more efficient if supported on TiO 2 while Au-Cu catalysts are better when supported on ZrO 2 when a precipitation-reduction method of preparation is used. In addition Pd seems to be a better choice than Cu as a second metal even if some agglomeration in the 0.5%Au0.5%Pd/TiO 2 was observed by TEM (Figure IV-24).

Actually, serious leaching of Cu was observed after the catalyst reuse (Table IV via direct dissociative adsorption on the Au particles, the size of the latter plays a crucial role. Moreover, the impact of the support on the catalytic activity also depends on the type of catalyzed reaction. Ishida et al. [START_REF] Okatsu | Deposition of gold nanoparticles on carbons for aerobic glucose oxidation[END_REF][START_REF] Ishida | Influence of the support and the size of gold clusters on catalytic activity for glucose oxidation[END_REF] reviewed the impact of supported gold catalyst on the catalytic oxidation of glucose. Contrary to the gas phase CO oxidation, in the case of glucose oxidation, the catalytic activity of gold nanoparticles was influenced more significantly by the size than by the nature of the support.

Megias-Sayago et al. [START_REF] Megí As-Sayago | Gold catalysts screening in base-free aerobic oxidation of glucose to gluconic acid[END_REF] have reported the base-free oxidation of glucose using molecular oxygen over various Au-supported catalysts. The TOF calculated on the basis of Au dispersion for the different catalyst showed that the activity is in the following order: Au/Al > Au/Ce > Au/Zr. When comparing the selectivity to gluconic acid, the results showed that alumina-supported sample presents the highest (95%) selectivity. In the same time, with zirconia containing samples, showed the lowest selectivity to gluconic acid, favoring some by-products formation. As reported in the study of alcohols oxidation mechanisms over heterogeneous catalysts reviewed by Davis et al. [START_REF] Davis | Selective oxidation of alcohols and aldehydes over supported metal nanoparticles[END_REF] the oxidation of an alcohol to an aldehyde on metal catalyst occurs in three main steps. The first step is the alcohol adsorption on the metal surface, which produces an adsorbed metal alkoxide and a metal hydride (or a water molecule, if metal hydroxide species are involved). After this adsorption a carbonyl species and a metal hydride are produced via the β-hydride elimination. The final step is the oxidation of metal hydride by O 2 to regenerate the metal surface (or metal hydroxide species). As reported this last step may leads also to the production of water (on the metals that dissociatively adsorbed oxygen) or hydrogen peroxide (on the metals that do not dissociatively adsorb oxygen such as gold). Both molecules would desorb to regenerate the active metal surface. In this way the carboxylic acid is produced by a subsequent oxidation of the aldehyde through a geminal diol intermediate. Indeed, as reported in the literature, aldehydes may undergo reversible hydration to geminal diols. This hydrated diol can then also adsorb to a metal surface and undergo a β-hydride elimination to form a carboxylic acid. It was observed that the presence of the surface adsorbed hydroxides, similar to an alkaline solution facilitate both the formation of an adsorbed alkoxide (or geminal diol), and the elimination step of a β-hydride towards the production of aldehyde and carboxylic acid. A similar mechanism corresponding to a modified oxidative dehydrogenation mechanism was also suggested as the most probable for glucose oxidation to gluconic acid using gold catalysts [START_REF] Comotti | Aerobic oxidation of glucose with gold catalyst: Hydrogen peroxide as intermediate and reagent[END_REF][START_REF] Prüße | Catalytic conversion of renewables: Kinetic and mechanistic aspects of the gold-catalyzed liquid-phase glucose oxidation[END_REF]. Concerning the reaction of glucose oxidation, a deprotonated geminal diol formed in alkaline solution is adsorbed at the catalyst surface. It may then undergo a β-hydride elimination to form gluconic acid. It was showed, that after gluconic acid desorption, both the non-dissociatively adsorb oxygen species and a metal hydride reacted to form an adsorbed peroxide species. This peroxide species desorbs subsequently from the catalyst surface. In the same way the adsorption of sugar molecules on a metal surface is facilitated by surface hydroxyl species. It is possible due to the hydrogen bonds with the aldehyde (or geminal diol intermediate). As could be expected various transformations are involved in the process. The formation of enediol intermediate is with no doubt, crucial for more further oxidations of glucose [START_REF] Larew | Concentration dependence of the mechanism of glucose oxidation at gold electrodes in alkaline media[END_REF]. A lot of attention has been also paid to the mechanism of the oxidation of alcohols during homogeneous catalytic reaction using gold catalysts. Rossi et al. [START_REF] Beltrame | Aerobic oxidation of glucose: II. Catalysis by colloidal gold[END_REF] developed one of the best molecular models for carbohydrates oxidation in the presence of gold catalysts. Since, as showed above the oxidation of an aldehyde to a carboxylic acid proceeds via an aldehyde hydrate, it should be the same during the oxidation of an alcohol. On the basis of various kinetic results obtained for oxidation of glucose to gluconic acid activation energy of 47 ± 1.7 kJ.mol -1 was obtained which is quite similar to the value of 49.6 ± 4.4 kJ.mol -1 obtained in the case of the enzymatic oxidation.

Moreover taking into account that this reaction is considered a first order reaction (in oxygen), the authors suggested a fast adsorption of the glucose on gold. In this way the rate-determining step would be glucose oxidation by oxygen dissolved in the liquid (aqueous) phase. Hydrogen peroxide could be detected, but it decomposed quickly under the reaction conditions. These glucose oxidations can reach an initial TOF of 50120 h -1 . In a subsequent investigation Rossi et al. [START_REF] Comotti | Aerobic oxidation of glucose with gold catalyst: Hydrogen peroxide as intermediate and reagent[END_REF][START_REF] Beltrame | Aerobic oxidation of glucose: II. Catalysis by colloidal gold[END_REF] showed that these oxidations do not proceed by a radical pathway but delivers gluconate and hydrogen peroxide by a two-electron mechanism. The hydrogen peroxide decomposes because of the basic medium before it reaches a concentration at which it would efficiently compete with oxygen as the oxidant. Scheme IV-1 shows the mechanism for that oxidation reaction. In aqueous medium the two cyclic diastereomers of glucose and the acyclic aldehyde form are in equilibrium. Nucleophilic addition of hydroxide will form (a), which is adsorbed onto the catalyst (b). Oxygen then reacts with (a) to form the peroxy complex (c), and the hydrogen atom of the coordinated aldehyde hydrate is eliminated as a proton. The produced gluconic acid (e) then leaves the catalyst and hydrogen peroxide is also released. While gold colloids with a mean particle diameter of 3.6 nm lie on the border between heterogeneous and homogeneous catalysis, Shi and co-workers recently described the selective oxidation of alcohols to aldehydes or ketones using complexes of gold soluble in toluene. This reaction also works in air; for example, with benzyl alcohol, a 100% conversion and a 99% selectivity towards benzaldehyde was detected by gas chromatography [START_REF] Guan | Highly selective aerobic oxidation of alcohol catalyzed by a gold(I) complex with an anionic ligand[END_REF]. All of these routes require O 2 dissociation on the catalyst surface. This process is actually not favored on Au catalysts [START_REF] Schubert | Co oxidation over supported gold catalysts--inert‖ and -active‖ support materials and their role for the oxygen supply during reaction[END_REF][START_REF] Schubert | Influence of H 2 O and CO 2 on the selective CO oxidation in H 2 -rich gases over Au/α-Fe 2 O 3[END_REF]. Moreover, it could be inhibited by water or hydroxides that adsorb on Pt-group metals. The activation barriers for O 2 dissociation on Au [START_REF] Schmid | Catalytic properties of layered goldpalladium colloids[END_REF] and Pt( 111) confirm the noble nature of Au as compared to Pt-group metals (105 and 64 kJ.mol -1 , respectively). The production of peroxide during oxidation reactions over Au [START_REF] Kung | Understanding Au-catalyzed low-temperature CO oxidation[END_REF][START_REF] Zope | Inhibition of gold and platinum catalysts by reactive intermediates produced in the selective oxidation of alcohols in liquid water[END_REF] indicates that O 2 is first reduced during these reactions before dissociation. Then the activation of O 2 should occur through the dissociation of peroxide (OOH*) and hydrogen peroxide (HOOH*) intermediates [START_REF] Ide | The important role of hydroxyl on oxidation catalysis by gold nanoparticles[END_REF].

The steady-state coverage of hydroxide on the surface is probably limited by the ability of the metal to accommodate the excess negative charge. As was demonstrated by Davis et al. [START_REF] Ide | The important role of hydroxyl on oxidation catalysis by gold nanoparticles[END_REF] the peroxide that forms can subsequently dissociate on Au to form To develop a highly efficient process for the oxidation of carbohydrates which would not require the use of any hazardous or dangerous chemicals, the engineering of advanced heterogeneous catalysts is necessary. Progress in this field was already made in the past starting from simple monometallic Au supported catalyst with the incorporation of other metals into the system.

It is now well established that supported gold-based nanoparticles exhibited the best catalytic performances in oxidation processes. However, another important progress is needed in this field. Actually, even though it was possible to obtain high performances and high selectivity, these catalysts are not able to yield enough high rates of reaction or to maintain their activity in acidic or neutral media. Even if the gold catalysts can oxidize C5/C6 sugars without the need of base addition; the issues with catalysts deactivation must be solved. This phenomenon is directly related to the leaching of the metal in acidic media or to the saturation of the catalyst surface with adsorbed products or by-products. However, the oxidation in neutral or acidic media gives the advantage of the direct formation of the desired acid in one-step reaction.

The use of unwanted and environmentally dangerous chemicals such as NaOH could be eliminated, which is positive from an economical point of view (increase of the equipment lifetime due to the elimination of strong corrosive media, elimination of the final gluconate-acid transformation which the concomitant production of salt which must be treated). Thus, it is important to focus on the development of catalysts that will have better activity and stability in base-free conditions.

In this thesis, the electronic structure of gold-based nanoparticles and hence their catalytic properties were modified by exploring factors such as the method of preparation, the nature of the support and the second metal added to gold. The properties of the metallic nanoparticles of the prepared catalysts were studied in details using TEM and XPS techniques. Series of Au-X catalysts (where X = Pd, Cu, Pt or Bi) with different metals molar ratio supported on ZrO 2 or TiO 2 were prepared by sol-immobilization or precipitation-reduction methods to study the relationship between the above-mentioned parameters and the catalytic performances. The objective was to find the best second metal in the Au-based system, as well as the optimal Au/Me ratio, that will modify the electronic and catalytic properties of Au for glucose base-free oxidation. The reaction temperature, which plays an important role on the catalytic performance, recyclability of the best catalysts and the role of PVA were also studied.

Two series of Au-Pd bimetallic catalysts supported on TiO 2 and ZrO 2 prepared by sol-immobilization and precipitation-reduction methods were synthesised. The catalysts were characterized and tested in different reaction conditions. It was observed that higher temperature increased the activity but in the same time negatively impacted the selectivity to desired gluconic acid. The recyclability tests were carried out over the most promising catalyst (0.5%Au0.5%Pd/TiO 2 prepared by sol-immobilization method) and it was found that after three runs, the catalyst kept the integrality of its catalytic performances without any regenerative treatments. On the contrary, the 0.7%Au0.3%Pd/ZrO 2 catalyst prepared by sol-immobilization showed a constant decrease of catalytic activity after 3 runs due to progressive deactivation because of the agglomeration of metal particles. Therefore, the support played a crucial role in the stability of the catalysts. Synergetic and anti-synergetic effects between palladium and gold in nanoparticles were observed during the catalytic tests. Obvious synergetic effect was shown over the Au-Pd/TiO 2 series catalysts prepared by sol-immobilization method, while there was an anti-synergetic effect over the Au-Pd/ZrO 2 series catalysts made by PR method. XPS analysis demonstrated that Au +δ and Pd +2 played important role in the reaction of base-free glucose oxidation. TEM analysis revealed that the metal particle size of the catalysts prepared by sol-immobilization method was smaller than the catalysts prepared by precipitation-reduction method.

Au-Cu series bimetallic catalysts were supported on TiO 2 and ZrO 2 and prepared by precipitation-reduction method. A synergetic effect was observed over the Au-Cu/TiO 2 series catalysts at 60 ˚C. In contrast, there was an anti-synergetic effect over the Au-Cu/ZrO 2 series catalysts. The Au/Cu molar ratio did not affected the activity of the catalysts. However, it was found that the activity of the Au-Cu catalysts strongly depended on the presence of Au + and CuOH species.

Au-Bi and Au-Pt series catalysts supported on ZrO 2 were prepared by precipitation method for comparison with the Au-Pd and Au-Cu series. Bismuth as a second metal did not show any beneficial effect as compare to Pd and Cu. The evident anti-synergistic effect observed as the selectivity to gluconic acid over Au-Bi series catalysts was very low. In case of the Au-Pt series, the 0.75%Au0.25%Pt/ZrO 2 catalyst exhibited the best conversion of glucose 67.8% and best selectivity to gluconic acid 80.5%. Unlike other gold-based catalysts, Au-Pt/ZrO 2 catalysts were still active when gold content was less than 0.3wt.%. Among all the second metal selected (Pd, Cu, Pt, Bi), 1%Pt/ZrO 2 monometallic sample showed 32.8% of glucose conversion and 62.9% selectivity to gluconic acid while others reached only 14.0% conversion of glucose and very low selectivity to gluconic acid.

From a practical point of view, it is important to stress that, under certain operating conditions (60°C, air as oxidant) and for specific chemical compositions (Au-Pd and Au-Cu) or support (TiO 2 and ZrO 2 ), we could prepare gold-based bimetallic catalysts with superior catalytic performances than monometallic gold catalysts. From a fundamental point of view, a good correlation was established between the performances observed for these catalysts and the oxidation state of both metals. The nature of the support, which, mainly, governs the catalytic performances of the catalysts, seems to strongly impact the mechanisms of the oxidation in base free conditions.

This work raised some important questions and opened interesting prospects for later research. We shall evoke three of the main ones: iii) We concluded that the mechanism of the glucose oxidation in base-free conditions implies the in-situ formation of H 2 O 2 . The role of the basic sites in its in-situ formation must be studied in more details.

Comotti

  et al. ont é tudié l'activité des nanoparticules AuPt et AuPd supporté es sur charbon actif en les comparant aux catalyseurs monomé talliques (Au, Pt, Pd, Rh) dans l'oxydation du glucose à pH contrôlé et non contrôlé . Hermans et al. ont é galement rapporté une activité synergique dans l'oxydation du glucose à pH é levé sur des catalyseurs Au-Pd/C pré paré s par impré gnation en solution aqueuse. L'effet synergique é tait alors lié à une teneur é levé e en surface de Pd. Timea et al. ont rapporté des

Figure I- 1

 1 Figure I-1 TEM image of Au/AC prepared by impregnation method [4].

  reduction step is normally needed after DP deposition. Generally, this step is carried out by calcination at T > 227 °C and usually enlargement of Au NPs is observed by increasing calcination temperature. Quite recently, chemical reductions have been proposed with the advantage of obtaining smaller NPs. It should also be noted that post-treatment at high temperature is expected to increase the support-metal interaction (SMSI) that can play a significant role in catalytic performances.

Figure I- 2

 2 Figure I-2 TEM image of Au/AC prepared by DP method [4].

H 2 O

 2 molecules (Al 2 O 3 ) and hydrogen bonding between oxygen atoms of Me 2 Au(acac) and hydroxyl groups on the support surface (SiO 2 ) assured the immobilization of the gold precursor on the support surface. After deposition, the samples were calcined at 300 °C to burn out the organic ligands in the gold precursor. The activity observed for prepared catalysts generally increased with their gold dispersion characterized by transmission electron microscopy. Finely dispersed Au NPs on Al 2 O 3 (d(Au) < 5 nm) catalysts may be easily prepared by the CVD method. Au/Al 2 O 3 samples prepared by the traditional DP method generally contain more large gold crystallites (d(Au) > 7 nm).

For improving their resistance

  against coagulation, aqueous colloidal solution of metal particles have been stabilized by three methods: (a) surface potential and/or charge density are increased by the adsorption of surface active long-chain ions (i.e., surfactants); (b) Van der Walls forces are reduced by adsorption of relatively rigid hydrophilic macromolecules (i.e., dextrin, starch); (c) besides these stabilizing effects depending on Coulomb or van der Waals forces, a third type of stabilization, -steric stabilization‖, has been considered. A crucial point in this technique is represented by the immobilization step (Table I-1 and Figure I-3).

Figure I- 3

 3 Figure I-3 Influence of the support functionalities on immobilization of Au sol. (a) carbon nanotubes (CNTs); (b) functionalized CNTs [4].

  negative charge density can be obtained by various techniques. XP spectroscopy of the Au 4f 7/2 level gives an experimental band that can be fitted as corresponding to the contribution of Au(0), Au(I) and Au(III) (FigureI-4). In general, the population of Au(I) is significantly smaller than that of Au(III).

Figure I- 4

 4 Figure I-4 XP spectra of the Au 4f 7/2 level recorded for gold NPs supported on nanoparticulate ceria (reprinted from ref. [64])

Figure I- 5

 5 Figure I-5 CO stretching bands recorded at liquid-N 2 temperature upon admitting CO on three different Au/CeO 2 catalysts: (A) as prepared, (B) reduced at 100 ˚C in H 2 , and (C) reduced at 200 ˚C. The samples have been exposed -in situ‖ for the CO oxidation reaction at 100 ˚C for 2 h prior to CO adsorption experiments. This figure illustrates

2. 1 . 2

 12 Core-shell nanoparticles In a core-shell structure, one metal element forms an inner core and the other element surrounds the core to form a shell (Figure I-6). This bimetallic structure is characteristic of poly(N-vinyl-2-pyrrolidone) (PVP) stabilized bimetallic NPs of precious metals. PVP-stabilized Au-Pd and Au-Pt bimetallic NPs were prepared by alcohol co-reduction. The colloidal dispersions of Au-Pd and Au-Pt bimetallic NPs obtained by simultaneous reduction can be examined by UV/VIS spectra and transmission electron microscopy [95-99].

Figure I- 6 A

 6 Figure I-6 A so-called core-shell structure

[ 120 -

 120 Scheme I-1 Summary of Chemicals Obtained via Glucose Oxidation

  abstraction -and this is a significant difference from Pd and Pt catalysts, which are effective in acidic as well as basic conditions. The catalysts were based on Au/C systems and they were observed to be effective for a wide range of substrates (FigureI-7).

Figure I- 7

 7 Figure I-7 Oxidation of alcohols. Reaction conditions: [alcohol] = 0.35 M; alcohol/M = 1000:1 (mol/mol); NaOH = 1 equiv; p(O 2 ) = 3 atm; T = 70 °C [126]

Figure I- 8

 8 Figure I-8 Comparison of the catalytic activity of Au and different bi-and tri-metallic catalysts for glucose oxidation at pH=7 (Adapted from [128])

Figure I- 9

 9 Figure I-9 Activity in glucose oxidation at different pH and temperature (Reprinted with permission from [139].

Figure I- 10

 10 Figure I-10 Selectivity to gluconic acid at different pH (Reprinted with permission from [139].

  agglomeration and to help the dispersion of the nanoparticles. For instance, in a typical procedure for preparing Au-Pd/ZrO 2 catalyst 0.1 g of a 1wt.% aqueous solution of PVA was first added to an aqueous solution of Tetracholoroauric acid and to Pd(NO 3 ) 2 diluted in distilled water under vigorous stirring to form a 60 mL solution. A freshly prepared solution of NaBH 4 (0.021 g was dissolved in 5 mL H 2 O) was then added to form a light-red sol. Within 30 min of sol generation, the colloid (acidified at pH 1 by a 1 mol/L solution of sulfuric acid) was immobilized on 0.5 g of ZrO 2 under vigorous stirring conditions. The amount of support was calculated to give a total final metal loading of 1 wt.%. After 2 h, the slurry was separated by centrifugation (30 min, 4700 rpm), then the solid was washed with hot water (2 × 70 mL) and ethanol (1 × 70 mL) and dried at 80 °C overnight[START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF]. FigureII-1shows schematically the procedure of sol-immobilization method which was used.

Figure II- 1

 1 Figure II-1 Procedure of sol-immobilization method

Figure II- 2

 2 Figure II-2 Procedure of precipitation-reduction

1. 2 . 3 N

 23 2 adsorption/desorption (BET method) BET (Brunauer-Emmett-Teller) theory is well-known and widely used for the exploitation of physical adsorption-desorption isotherms of gas molecules on a solid surface. The concept of this theory is an extension of the Langmuir monolayer molecular adsorption to multilayer adsorption with the following hypotheses: (a) gas molecules physically adsorb on a solid in layers infinitely; (b) there is no interaction between each adsorption layer; and (c) the Langmuir theory can be applied to each layer.The BET method is widely used in surface science for the calculation of surface areas of solids by physical adsorption of gas molecules. Knowing the monolayer capacity of an adsorbent (i.e., the quantity of adsorbate which can be accommodated as a monolayer on the surface of a solid, the adsorbent) and the area of the adsorbate molecule, the surface area can be calculated.Regarding the adsorption gases, N 2 physisorption (at -196 °C) is generally considered to be the most suitable method for determining the surface area of non-porous and porous materials. In this study, N 2 physisorption measurements were carried out on a TriStar II Plus apparatus from Micromeritics. Before measurements, the samples were degassed on a VacPrep061 equipment (Micromeritics) at 75 °C in vacuum for 80 min and at 250 °C for 3 h.1.2.4 X-ray powder diffraction (XRD)XRD is the most commonly used analytical technique for elucidation of phase composition in heterogeneous catalysts. In principle, as X-rays interact with atomic planes of a crystalline sample, each plane creates a specific reflection with the incident rays. Usually, the sample is in a powdery form, consisting of fine grains in the form of single crystallites. The positions and the intensities of the peaks can be used for identifying the underlying structure (or phase) of a solid material. This technique permits the analysis of the chemical structure and the orientation of a crystal, to identify (by comparing the diffractograms obtained with reference data issued from ICDD files), quantify the crystalline phases and determine the crystallite size. The analysis duration time varies between 1 min to a few minutes according to the precision required for the analysis.XRD measurements in this study were carried out on Brüker X-Ray Diffractometer D8 Discover. The samples were scanned in the 2θ range of 20° to 70°.1.2.5 X-ray photoelectron spectroscopy (XPS)XPS also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist in the near surface region (in the top 1-10 nm layer) of a solid material. Electrons of each element have characteristic binding energies. X-ray photons generated by an X-ray source excite a sample, causing the emission of core electrons. These electrons are detected and counted for the given specific binding energies, which then form the spectrum. The numbers of detected electrons are thus related to the number of the specific element and can be used for quantification of composition (FigureII-3). As the energy of the X-rays with a particular wavelength is known (E photon ), the electron binding energy (E binding ) of each of the emitted electrons can be determined by using the following equation. (Eq. II-1) E 𝑏𝑖𝑛𝑑𝑖𝑛𝑔 = E 𝑝ℎ𝑜𝑡𝑜𝑛 -𝐸 𝑘𝑖𝑛𝑒𝑡𝑖𝑐 -𝜑 (Eq. II-1)Where E kinetic is the kinetic energy of the emitted electron measured by the instrument and φ is the work function of the spectrometer.

Figure II- 3

 3 Figure II-3 Photoelectron effect

Figure II- 4

 4 Figure II-4 Top Industrie autoclave reactor used for the first catalytic tests

  24 parallel stainless-steel batch reactors of 6 mL each (FigureII-5). SPR is an automated high pressure, high temperature reactor that operates automatically at temperatures up to 400 °C and pressures up to 50 bar. The SPR mixes the contents of the reactor vessels via orbital shaking under controlled pressure and temperature conditions.

Figure II- 5

 5 Figure II-5 SPR block and reactors

  order to analyze the products of the reaction, a Dionex ultimate 3000 HPLC (Figure II-7) was used. It is equipped with a fixed wavelength UV detector and aRefractive Index (RI) detector. As far as the stationary phase is concerned, a Rezex organic acid H + 300 x 7.80 mm was used. The mobile phase use was a 0.0025 mol/L sulfuric acid solution used at room temperature with a 0.45 mL/min flow rate.

Figure II- 7

 7 Figure II-7 Dionex ultimate 3000 HPLC

  calculating equations are as follows: C gluconic acid = k 1 * UVA gluconic acid RIA gluconic acid = C gluconic acid / k 2 RIA glucose = RIA gluconic acid+glucose -RIA gluconic acid C glucose = k 3 * RIA glucose (C represents concentration, UVA represents UV area, RIA represents RI area, k 1 , k 2 , k 3 are the response factors.) Calibrations were carried out and the calibration curves were obtained as shown in Annex 1.IntroductionGold-palladium (Au-Pd) bimetallic nanoparticles have shown significantly enhanced catalytic activity in comparison with monometallic Au or Pd in many reactions, such as CO oxidation, vinyl acetate monomer synthesis, the direct synthesis of hydrogen peroxide from H 2 and O 2 , and the oxidation of hydrocarbons, carbohydrates and alcohols[START_REF] Silva | Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols[END_REF][START_REF] Ryabenkova | The selective oxidation of 1,2-propanediol to lactic acid using mild conditions and gold-based nanoparticulate catalysts[END_REF][START_REF] Torrente-Murciano | Selective oxidation of salicylic alcohol to aldehyde with O 2 /H 2 using Au-Pd on titanate nanotubes catalysts[END_REF][START_REF] Khawaji | Au-Pd bimetallic nanoparticles immobilised on titanate nanotubes: A highly active catalyst for selective oxidation[END_REF][START_REF] Khawaji | Au-Pd NPs immobilised on nanostructured ceria and titania: impact of support morphology on the catalytic activity for selective oxidation[END_REF][START_REF] Landon | Direct synthesis of hydrogen peroxide from H 2 and O 2 using Pd and Au catalysts[END_REF].

Figure III- 1

 1 Figure III-1 The unit cells of anatase and rutile. Ti atoms are gray, O atoms are red.

1. 1

 1 .2 XRD X-ray powder diffraction (XRD) was used to bring information on the structural make-up, content and size of the crystalline structures in presence in the catalysts samples. More particularly, XRD analysis was carried out here to try to determine the crystalline structure of the gold and palladium species. However, the X-ray diffraction patterns of the whole Au-Pd/TiO 2 series were the same. For this reason, only 0.5%Au0.5Pd%/TiO 2 and 1% Au/TiO 2 XRD patterns are here presented. The diffractogram the fresh TiO 2 support was also recorded for comparison. Due to the limitations of the technique which does not allow detecting low metal contents (<5 wt.%) or small metal particles (<5 nm)[START_REF] Zieliński | Hydrogen storage on nickel catalysts supported on amorphous activated carbon[END_REF] and due to the probable good and homogeneous dispersion of the metals on the titania surface[START_REF] Cao | An investigation into bimetallic catalysts for base free oxidation of cellobiose and glucose[END_REF], X-ray diffraction patterns of the catalysts generally did not show peaks originated from metals. The results obtained are shown in FigureIII-2. The patterns only show the presence of pure TiO 2 with anatase and rutile-crystal structures, which is in good agreement with the standard data from the JCPDS PDF card 00-021-1272 and 04-003-0648, respectively. This indicates that the crystalline structure of the support did not change during the preparation.

Figure III- 2

 2 Figure III-2 Diffractograms of different samples

Figure III- 3

 3 Figure III-3 TEM micrographs of Au-Pd nanoparticles loaded on TiO 2 prepared by Sol-immobilization and the corresponding Au-Pd particle size distribution. (a) 1%Au/TiO 2 , (b) 0.9%Au0.1%Pd/TiO 2 , (c) 0.7%Au0.3%Pd/TiO 2 , (d) 0.5%Au0.5%Pd/TiO 2 , (e) 0.3%Au0.7%Pd/TiO 2

Figure III- 4

 4 Figure III-4 shows the EDX spectrum and mapping of elements of one particle on the 0.5%Au0.5%Pd/TiO 2 catalyst. It demonstrates that the Au-Pd particle is in an alloy structure.

Figure III- 4

 4 Figure III-4 (a) HAADF image and (b) the corresponding EDX spectrum from a single Au-Pd nanoparticle supported on TiO 2 prepared by the sol immobilization method. The HAADF image and montage of Au, Pd EDX elemental maps in (c) shows the co-existence of both elements within the nanoparticle

Figure III- 6

 6 Figure III-6 Oxidation of glucose over a series of mono-and bi-metallic catalysts containing Au and Pd supported on TiO 2 , Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 5 h of reaction.

Figure III- 7

 7 Figure III-7 Recyclability of the 0.5%Au0.5%Pd/TiO 2 catalyst for the oxidation of glucose Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 80˚C, 5 h of reaction.

Figure

  Figure III-8 TEM micrographs of 0.5%Au0.5%Pd/TiO 2 fresh and used. (a) Fresh catalyst (b) Catalyst after 1 st run, (c) Catalyst after 2 nd run

  Figure III-10 Diffractograms of different samples

Figure III- 11

 11 Figure III-11 TEM micrographs of Au-Pd nanoparticles loaded on TiO 2 prepared by PR method and the corresponding Au-Pd particle size distribution. (a) 1%Au/TiO 2 , (b) 1%Pd/TiO 2 ,

  the influence of both preparation methods in determining the catalytic properties on glucose oxidation.

Figure III- 12

 12 Figure III-12 Oxidation of glucose over a series of mono-and bi-metallic catalysts containing Au and Pd supported on TiO 2 , Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 80 ˚C, 5 h of reaction.

  Comparing the TEM images (Figure III-3 and Figure III-11 for SIM and PR methods, respectively), it is clear that the mean size of the NPs of the catalysts prepared by SIM was smaller than for PR. This fits well with the role of PVA, which was used as a stabilizer to get small and narrow distributed nanoparticles. The results of ICP-OES and XRF revealed that the amount of metal were almost the same whatever the method of preparation (TableIII-1 and TableIII-6). Therefore, the results of the catalytic tests over catalysts prepared by both methods are presented in Figure III-13 in function of the theoretical Au loading. As observed, the catalytic performances of the catalysts prepared by PR showed much better activity compared to the catalysts prepared by SIM.

Figure III- 13

 13 Figure III-13 Effect of the preparation method on the catalytic performances of Au-Pd series on TiO 2 for the glucose oxidation. Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 80 ˚C, 5 h of reaction.

  2 . The physicochemical polymorph of ZrO 2 is baddeleyite as presented in Figure III-14.

Figure III- 14

 14 Figure III-14 Crystal structure of baddeleyite[START_REF] Drabińska | Growth kinetics of epitaxial graphene on SiC substrates[END_REF]. Zr atoms are cyan, O atoms are red.

Figure III- 15 .

 15 Figure III-15. As before, no peaks originating from Au or Pd were detected. The patterns only showed the presence of pure ZrO 2 with baddeleyite-crystal structure, which is in good agreement with the stand data from the JCPDS PDF card 04-004-4339. No crystalline structure of the support change during the preparation.

2 Au( 111 )

 2111 Figure III-15 Diffractograms of ZrO 2 (Aldrich) supported samples

Figure III- 16

 16 Figure III-16 TEM micrographs of Au-Pd nanoparticles supported on ZrO 2 prepared by Sol-immobilization and the corresponding Au-Pd particle size distribution. (a) 0.7%Au0.3%Pd/ZrO2, (b) 1%AuZrO 2

  times. The results obtained are presented in Figure III-18. The standard deviation is represented by the error bars. After 3 consecutive tests, the results obtained showed a progressive decrease of the catalytic performances over the 0.7wt.%Au0.3wt.%Pd/ZrO 2 catalyst. The selectivity to gluconic acid remained constant at 77%. The carbon balance increased a little bit. The conversion of glucose decreased from 59% to 11.3%. It suggests that the catalyst was progressively deactivated. The TEM image (Figure III-19) showed the reason. It is because of the agglomeration of the metal nanoparticles. More and more particles agglomerated during the reaction.

Figure III- 18

 18 Figure III-18 Recyclability of the 0.7%Au0.3%Pd/ZrO 2 catalyst for the oxidation of glucose. Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 80˚C, 5 h of reaction.

Figure III- 19

 19 Figure III-19 TEM micrographs of Au-Pd nanoparticles loaded on ZrO 2 prepared by Sol-immobilization and the corresponding Au-Pd particle size distribution. (a) 0.7%Au0.3%Pd/ZrO 2 , (b) 0.7%Au0.3%Pd/ZrO 2 1 st run, (c) 0.7%Au0.3%Pd/ZrO 2 2 nd run After the first run agglomeration of the particles became more serious. See Figure III-19 (b). After the second run, there were no more than 5 particles visible on the TEM grid (TableIII-19). Thus, the particle sizes of this catalyst after calculation were not

ZrO 2

 2 for comparison are shown in Figure III-21. As before, the diffractograms showed only the presence of pure ZrO 2 with baddeleyite-crystal structure, which is in good agreement with the data obtained from the JCPDS PDF card 04-004-4339.

2 Au( 111 )

 2111 Figure III-21 Diffractograms of different samples

Figure III- 22

 22 Figure III-22 shows the TEM images of 1%Au/ZrO 2 (a) and 0.7%Au0.3% Pd/ZrO 2 (b), and at least 300 particles were counted. Average diameter of metal nanoparticles on 1wt.% Au/ZrO 2 was 4.2 ± 1.4 nm and on 0.7%Au0.3% Pd/ZrO 2 was 5.3 ± 2.1 nm.

Figure III- 22

 22 Figure III-22 TEM micrographs of Au-Pd nanoparticles loaded on ZrO 2 prepared by PR method and the corresponding metal particle size distribution. (a) 1%Au/ZrO 2 , (b) 0.7%Au0.3%Pd/ZrO 2 .

Figure III- 23

 23 Figure III-23 Oxidation of glucose over a series of mono-and bi-metallic catalysts containing Au and Pd supported on ZrO 2 , Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

Figure III- 25

 25 Figure III-25 and Figure III-26 present the influence of the nature of the support on the catalytic performances obtained at 60 ˚C and 80 ˚C, respectively.

Figure III- 25

 25 Figure III-25 Influence of the nature of the support on the catalytic performances of Au-Pd series for the glucose oxidation. Reaction conditions: 2 mL of 1 wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

Figure III- 26

 26 Figure III-26 Influence of the nature of the support on the catalytic performances of Au-Pd series for the glucose oxidation. Reaction conditions: 2 mL of 1 wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 80 ˚C, 5 h of reaction.

  performances obtained with the post-treated catalysts in the base-free oxidation of glucose are shown in Figure III-27 and Figure III-28 (detailed results are presented in Annex -Table

Figure III- 27

 27 Figure III-27 Catalytic performance of Au-Pd/ZrO 2 catalysts (SIM) in the oxidation of glucose after calcination at 250 ˚C. Reaction conditions: 2 mL of 1 wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

Figure III- 28

 28 Figure III-28 Catalytic performance of Au-Pd/ZrO 2 catalysts (SIM) in the oxidation of glucose after calcination at 400 ˚C. Reaction conditions: 2 mL of 1 wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

Figure IV- 1

 1 Figure IV-1 Diffractograms of TiO 2 and 0.5%Au0.5%Cu supported on TiO 2

Figure IV- 2

 2 Figure IV-2 presents TEM images for different Au/Cu ratios supported on TiO 2 . At least 300 particles, except for (f) where 59 particles and for (g) where 18 particles

Figure IV- 2

 2 Figure IV-2 TEM micrographs of Au-Cu nanoparticles loaded on TiO 2 prepared by precipitation-reduction method and the corresponding Au-Cu particle size distribution. (a) 1%Au/TiO 2 , (b) 0.9%Au0.1%Cu/TiO 2 , (c) 0.7%Au0.3%Cu/TiO 2 , (d) 0.5%Au0.5%Cu/TiO 2 , (e) 0.3%Au0.7%Cu/TiO 2 , (f) 0.1%Au0.9%Cu/TiO 2 , (g) 1%Cu/TiO 2

  Figure IV-3 (a) HAADF image and (b) the corresponding EDX spectrum from a single Au-Cu nanoparticle supported on TiO 2 prepared by the sol immobilization method. The HAADF image and montage of Au, Cu EDX elemental maps in (c) shows the co-existence of both elements within the nanoparticle.

Figure IV- 4

 4 Figure IV-4 show the XPS spectra of Au 4f and Cu 2p of the bimetallic Au-Cu/TiO 2 series. Table IV-3 and IV-4 show the results of chemical composition of all the investigated catalysts in details. As an example, Figure IV-5 and IV-6 show the peak fitting of 0.3%Au0.7%Cu/TiO 2.In all the Au-Cu/TiO 2 catalysts prepared by precipitation-reduction method, the Au 4f 7/2 photoelectron peak is located at a binding energy (BE) value between 83.08 and 85.74 eV. These values can be assigned to metallic Au 0 and oxidized Au +δ species[START_REF] Casaletto | XPS study of supported gold catalysts: the role of Au 0 and Au +δ species as active sites[END_REF][START_REF] Moulder | Handbook of X-ray photoelectron spectroscopy[END_REF]. The curve fitting of the Au 4f core-level spectrum was performed as described in Chapter III. In FigureIV-4, different Au 4f and Cu 2p photoelectron peaks of surface gold and copper was found as

Figure IV- 5

 5 Figure IV-5 XPS curves of the Au 4f photoelectron peak and its fitting in the 0.3%Au0.7%Cu/TiO 2 catalyst

Figure IV- 7

 7 Figure IV-7 Base-free oxidation of glucose over a series of mono-and bi-metallic catalysts containing Au and Cu supported on TiO 2 , Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% catalyst (21.9 mg), air pressure = 5 bar, 5 h of reaction.

Figure IV- 8

 8 Figure IV-8 Recyclability of the 0.5wt.%Au0.5wt.%Cu/TiO 2 catalyst for the oxidation of glucose. Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 80˚C, 5 h of reaction.

Figure IV- 9

 9 Figure IV-9 TEM micrographs of 0.5%Au0.5%Cu/TiO 2 prepared by precipitation-reduction method (used). (a) 0.5%Au0.5%Cu/TiO 2 1st run, (b) 0.5%Au0.5%Cu/TiO 2 2nd run

Figure

  Figure IV-11 Diffractograms of 0.5%Au0.5%Cu/ ZrO 2 and ZrO 2

Figure IV- 12

 12 Figure IV-12 Base-free oxidation of glucose over a series of mono-and bi-metallic catalysts containing Au and Cu supported on ZrO 2 Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

1 . 3

 13 Comparison of both supportsA volcano-like curve was observed for the activity vs. Au content plot for the Au-Cu/TiO 2 catalysts series indicating that there was a positive synergistic effect between Au and Cu on TiO 2 (Figure IV-7 60 ˚C). In contrast, an obvious valley-like curve could be seen for the Au-Cu/ZrO 2 catalysts demonstrating there was an anti-synergistic effect in this case (Figure.

Figure IV- 13

 13 Figure IV-13 gathered the data obtained for both series for an easier comparison. It shows the effect of TiO 2 and ZrO 2 as supports on the catalytic performance over the Au-Cu series catalysts for base-free glucose oxidation. It has to be noted that the gold and copper loadings for a given catalyst composition were almost the same over TiO 2 and ZrO 2 , which permits a direct comparison of the performances. As already observed for other catalysts series, when the gold content was above 0.3wt.%, the selectivity to gluconic acid was maintained almost constant whatever the catalyst composition, indicating that the Au/Cu ratio had no effect on the selectivity. The selectivities to gluconic acid over ZrO 2 -supported catalysts were much better than the ones obtained over TiO 2 -supported catalysts. This is different from what was observed for the Au-Pd series (Chapter III -Figure where TiO 2 and ZrO 2 played a similar role as for the selectivity to gluconic acid when the gold content was between 0.3 and 0.9wt.%. A

  TiO₂ Gluconic acid Selectivity% ZrO₂ ICP-OES, including the Au/Bi ratios but most of the time the XRF and ICP-OES results are consistent.

Au( 111 )

 111 Figure IV-14 Diffractograms of ZrO 2 supported samples

Figure IV- 15

 15 Figure IV-15 Base-free oxidation of glucose over a series of mono-and bi-metallic catalysts containing Au and Bi supported on ZrO 2 Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

Figure

  Figure IV-16 Diffractograms of ZrO 2 supported samples

Figure IV- 17

 17 Figure IV-17 Base-free oxidation of glucose over a series of mono-and bi-metallic catalysts containing Au and Bi.

  Au-Pt-Ag system prepared by reduction with NaBH 4 . Studies confirmed the best catalytic activity and highest TOF were observed with a mass ratio of 70/20/10 [149].

Figure IV- 18 .Figure IV- 19 .

 1819 Figure IV-18. Glucose conversion at 60°C for Au rich (0.75% Au -0.25% Me) bimetallic catalysts (with Me = Bi, Pt, Pd or Cu). Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

  impossible to register and discuss the Auger Cu LVV region. The discussion was made using the CuOH and Cu 2+ species that are consider active sites in the oxidation reaction (as discussed above). As could be seen from Figure IV-20 the conversion of glucose increases when the total % of oxidized Cu increases (i.e. considering CuOH and CuO).

Figure IV- 20 .

 20 Figure IV-20. Conversion of glucose at 60°C versus total % of oxidized copper (CuOH+CuO) as obtained from XPS.

Figure IV- 21 .

 21 Figure IV-21. Glucose conversion at 100°C versus the BE of Au 4f 7/2 region

  2 supported catalysts (Figure IV-22) as compared to ZrO 2 (Figures IV-23).

Figure IV- 22 .

 22 Figure Effect of the second metal on the catalytic performances of Au-X/TiO 2 (X = Pd or Cu) series of catalysts prepared by PR method for the base-free glucose oxidation. Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 80 ˚C, 5 h of reaction.

Figure IV- 23

 23 Figure IV-23 presents the performances of the series of catalysts supported on ZrO 2 . Here, the trend is totally inversed in comparison with the series supported on

  Figure Effect of the second metal on the catalytic performances of the Au-X/ZrO 2 (X = Pd or Cu) series of catalysts prepared by PR method for the base-free glucose oxidation. Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 60 ˚C, 5 h of reaction.

- 5 )

 5 leading to a strong instability of this kind of bimetallic catalysts.

Figure IV- 24

 24 Figure IV-24 ADF image of Pd agglomeration of 0.5%Au0.5%Pd/TiO 2

  10nm

4. 3 .

 3 Insight into the mechanism of carbohydrates oxidation in base-free conditionsThe mechanism of the carbohydrates oxidation in basic medium is already well known. Several roles for O 2 during the oxidation reactions in water at high pH have been already proposed. The most important of them include (i) the formation and direct participation of atomic O during reaction, (ii) the reduction of atomic O to water through H atoms adsorbed on the surface, (iii) the direct oxidation of the intermediate with atomic O to form the O-insertion product acids, (iv) and the removal of strongly bound organic adsorbates from the surface[START_REF] Ide | The important role of hydroxyl on oxidation catalysis by gold nanoparticles[END_REF].

Scheme IV- 1 :

 1 Scheme IV-1: Mechanistic model by Rossi et al. for glucose oxidation. Adapted from[START_REF] Comotti | Aerobic oxidation of glucose with gold catalyst: Hydrogen peroxide as intermediate and reagent[END_REF]

  atomic O and hydroxide, with a barrier of 83 kJ.mol -1 . However, as the authors concluded, the more probable step is the further reduction of OOH* to H 2 O 2 *, with a barrier of only 48 kJ.mol -1 . The decomposition of hydrogen peroxide over Pt and Pd has a barrier of only 29 and 5 kJ.mol -1 , respectively. The low calculated barrier for peroxide decomposition on Pd as compared to Au could explain the synergetic effect observed in our work for Au-Pd catalysts as compared to Au-Pt used tested at identical reaction conditions.Although the role of O 2 during selective oxidation is an indirect one that does not

i)

  We allotted the crucial role of the support in the mechanism of the base-free oxidation reaction. The existence of weak or/and metal-support interactions should be taking into account. However, the presence of surface defects or/and impurities that may play the role of local basic site and facilitate the formation of H 2 O 2 should be studied. A FTIR and UV-visible study of the adsorption of carbohydrates on these supports, as well as a fine chemical analysis of the support structure, could bring answers to these questions.ii) High activity was observed for the supported Au-Pd and Au-Cu catalysts obtained by precipitation-reduction method. That was ascribed to the existence of bimetallic structures in which gold would wet the second metal. A more complete physicochemical study (EXAFS, HRTEM, XPS-LEIS, Raman) should give more information on the structure of these bimetallic particles. Further, a FTIR and UV-visible study of the competitive adsorption and reduction of Au + and Me + ions on different supports would give useful information on the mechanism of formation of the bimetallic aggregates (core shell, perfect alloy, separated phases).

  preparing well-controlled Au-supported catalyst. Any surface is characterized by a pH called PZC (Point of Zero Charge) or IEP (IsoElectric Point) at which the surface is neutral. At pH below the IEP the surface is negatively charged while at pH over the IEP, the surface is positively charged. The isoelectric points of metal oxides have been determined and serve to classify the supports as acidic or basic depending if the surface

of the support is able to release a proton or to react with them. As an example, let's consider TiO 2 which is one of the supports most widely used in heterogeneous catalysis for dispersing gold NPs

[START_REF] Moreau | Gold on titania catalysts for the oxidation of carbon monoxide: control of pH during preparation with various gold contents[END_REF][START_REF] Moreau | CO oxidation activity of gold catalysts supported on various oxides and their improvement by inclusion of an iron component[END_REF]

. Below pH 6, the surface of titania is positively charged and can strongly adsorb through coulombic interactions anions such as AuCl 4 or AuCl 3 (OH) -. Based on this adsorption, one possibility to form gold NPs on a solid surface starts with spontaneous Au anion adsorption at acid pH values

[START_REF] Murphy | Raman spectroscopy of gold chloro-hydroxy speciation in fluids at ambient temperature and pressure: a re-evaluation of the effects of pH and chloride concentration[END_REF][START_REF] Murphy | The effects of temperature and pressure on gold-chloride speciation in hydrothermal fluids: a Raman spectroscopic study[END_REF]

. Oxides with IEP around 7 (TiO 2 , CeO 2 , ZrO 2 , Fe 2 O 3 ) produced very active species, whereas acidic support (SiO 2 ) or basic (MgO) usually appeared less active. In this regard, the amphoteric character of Al 2 O 3 (IEP 8-9) is more sensitive to pH variation. A

Table I -

 I 1 Effect of the support on Au particle size during the immobilizing step[START_REF] Dimitratos | Gold on titania: Effect of preparation method in the liquid phase oxidation[END_REF][START_REF] Villa | Au on nanosized NiO: A cooperative effect between Au and nanosized NiO in the base-free alcohol oxidation[END_REF].

	Support	TEM (nm)
	NiO commercial	3.8
	Nano NiO	3.6
	SiO 2	4.0
	MgO	3.8
	TiO 2	4.0
	H-mordenite	3.8
	AC X40S	3.0
	AC Norit	3.6

Table I

 I

	-2 Metal molar fraction and diameter [85]

Metal molar fraction % Average diameter (nm) a Ag Au

	Neatu et al. [87] reported gold-copper nanoalloys supported on TiO 2 for the CO 2
	100	0	17.2 ± 2.4
	75	25	4.9 ± 0.5
	50	50	5.6 ± 1.0
	25	75	6.2 ± 0.9
	0	100	7.2 ± 1.2
		a. The particle size is averaged over 100 nanoparticles.
	Han's group [86] reported a palladium-gold alloy catalyst for low-temperature CO
	oxidation. Performance was optimal for a catalyst of bulk composition Pd 4 Au 1 , a
	mixture of Pd 90 Au 10 (72.5 at. %) and Pd 31 Au 69 (27.5 at. %), that was remarkably active
	at 300 K and more stable than a pure Au catalyst.

Table I

 I

			-3 Metal ratio and diameter
		(Ag)Au-0.8	(Ag)Au-0.6	(Ag)Au-0.4	(Ag)Au-0.3	(Ag)Au-0.2
	Au/Ag (atomic) a	3.82	1.62	0.85	0.51	0.29
	Au/Ag (atomic) b	3.93	1.57	0.79	0.55	0.31
	Diameter (nm) a	16	12	11	10	10
	Diameter (nm) b	15.3	12.3	10.9	10.4	9.8
			a. measured b. calculated
	Ferrer et al. [12] described the internal structure of Au-Pd NPs exhibiting newly
	discovered three-layer core/shell morphology, which was composed of an evenly
	alloyed inner core, an Au-rich intermediate layer, and a Pd-rich outer shell. Sanchez et
	al. [80] also made similar structure Au-Pd catalyst.
	The following Table I-4 is a summary of bimetallic catalysts [107].

Table I -4 Summary of the bimetallic DENs as catalysts Metal system Synthetic route a Composition of DENs Structure of DENs Average diameter/nm Catalyzed reaction Property

 I

	PdAu	C	G6-Q 116 (Pd n Au 150-n )	Alloy	ca. 1.8	Allyl alcohol	70-200% (m)
				S	G6-Q 116 [Au 55 ](Pd n )	Core/shell	1.8-2.3	Allyl alcohol	30-38% (Pd)
				S	G6-OH[Pd 55 ](Au n )	Core/shell	2.4-2.9	Allyl alcohol	Precipitation
								Oxidation c
	PdAu	C	G4-NH 2 (Pd 27.5 Au 27.5 )	Alloy	ca. 1.8	CO	35˚C (Pd)
	PtAu d	TD	G5-OH(Pt 16 Au 16 )	Alloy	<3.0	CO	2 (Pt), 8.5 (c)
								Reduction
	PtAu	E	G4-NH 2 (Pt x Au y )	Alloy	-	O 2	Good
								Application
	AuAg	C	G6-OH(Au 27.5 Ag 27.5 )	Alloy	ca. 1.5	No	-
				S	G6-OH[Au 55 ](Ag n )	Core/shell	ca. 1.9	No	-
				C, S	G6-OH[AuAg ] 55 (Au n )	Core/shell	1.5-2.7	No	-
	AgM t	e	PD	Gn-OH(Ag x M ty )	Alloy	-	No	-
				PD	Gn-OH[Ag x ](M ty )	Core/shell	-	No	-
	CuM t	e	PD	Gn-OH(Cu x M ty )	Alloy	-	No	-
				PD	Gn-OH[Cu x ](M ty )	Core/shell	-	No	-
				a. C = Co-complexation, S = Sequential loading, PD = Partial displacement reaction, TD = Total displacement
			reaction, E = Electrodeposition. b. Enhanced percentage in turnover frequency compared to the monometallic DENs
				and their physical mixture (denoted m) in brackets. c. PdAu catalyst prepared using G4-NH 2 (Pd 27.5 Au 27.5 ) lowered
				temperature of complete (100%) CO conversion compared to that of Pd catalyst prepared via G4-NH 2 (Pd 55 ) (in
			PdAu system), relative rate times of Pt 16 Au 16 to those of Pt 32 and co-metallic Pt 32 + Au 32 (denoted c) catalysts at 100
			˚C (in PtAu system), and relative rate times of Pt 23 Cu 23 to that of Pt 45 catalyst at 60 ˚C (in PtCu system). d. PtAu and
				PtCu alloy catalysts were obtained by removing G5-OH using thermolysis treatment. e. M t = Au, Pt, and Pd.

  bimetallic NPs were prepared by Schmid et al. by successive reduction. Au colloids, with a diameter of about 18 nm, could be covered by Pt or Pd shells when the chloride precursors, H 2 PtCl 6 or PdCl 2

	were reduced. The original red color due to the Au NPs changed to brown-black when
	covered with Pt and Pd atoms. Addition of the water-soluble ligand p-H 2 NC 6 H 4 SO 3 Na
	(sodium sulfanilate) stabilized the obtained bimetallic NPs.

Table I -

 I 6 Catalytic performances observed in base-free glucose oxidation with supported Au NPs. Al 2 O 3 , Ce/Al: CeO 2 (20%wt)/Al 2 O 3 , Ce: CeO 2 , Ce25/Zr CeO 2 (25%wt)/ZrO 2 , Ce50/Zr: CeO 2 (50%wt)/ZrO 2

	Entry	Catalyst			Particle size (nm)	Preparation method	Glucose/Au T a (°C) Time (h)	Oxidant	Conv. (%)	Gluconic acid Selectivity (%)	Ref.
	1 2	1% Au/C 0.5%Au/nCeO 2	f	4.4 1.8	SIM e DP e	1000 140	100 65	6 2	0.3 MPa O 2 0.23 MPa O 2	100	100 95	[128]
	3	1.4%Au/nCeO 2		2.5	DP	140	65	2	0.23 MPa O 2		99
	4	1.6%Au/nZrO 2		1.7	DP	140	65	2	0.23 MPa O 2		99
	5 6	1.3%Au/nTiO 2 0.6% Au/μCeO 2	f	3.0 2.0	DP DP	140 140	65 65	2 2	0.23 MPa O 2 0.23 MPa O 2		99 96	[145]
	7	1.1% Au/μCeO 2	2.9	DP	140	65	2	0.23 MPa O 2		98
	8	0.8%Au/μZrO 2		2.1	DP	140	65	2	0.23 MPa O 2		98
	9 10	1.2%Au/μTiO 2 0.5% Au/MgO			2.0	DP SIM	140 1600	65 60	2 24	0.23 MPa O 2 air b		98 100	[82]
	11	1% Au/CMK-3 c	3.0	See Ref.	1000	110	2	0.3 MPa O 2		88
	12	1% Au/SBA-15	3.6	See Ref.	100	110	2	0.3 MPa O 2		92
	13	1% Au/CNTs			2.9	See Ref.	100	110	2	0.3 MPa O 2		83
	14	1% Au/graphene	3.2	See Ref.	100	110	2	0.3 MPa O 2		74	[144]
	15 16	1% Au/graphite 1% Au/AC g	3.7 3.5	See Ref. See Ref.	100 100	110 110	2 2	0.3 MPa O 2 0.3 MPa O 2		84 91
	17	1% Au/ZrO 2			2.5	See Ref.	100	110	2	0.3 MPa O 2		92
	18 d 19 d 20 21 22 23 24	2.3%Au/MgAl 2 O 4 1.8%Au/Al 2 O 3 2%Au/Al h 2%Au/Ce/Al h 2%Au/Ce h 2%Au/Ce25/Zr h 2%Au/Ce50/Zr h	3.8 2.4 5.1 ± 1.5 3.4 ± 0.8 3.9 ± 1.5 3.4 ± 0.8 3.4 ± 0.8	DP DP DAE e DAE DAE DAE DAE	870 1110 100 100 100 100 100	120 120 120 120 120 120 120	1/6 1/6 18 18 18 18 18	2.2 eq H 2 O 2 2.2 eq H 2 O 2 0.1MPa O 2 0.1 MPa O 2 0.1 MPa O 2 0.1 MPa O 2 0.1 MPa O 2		93 87 95 96 90 86 87	[146] [81]
	25	1%Au/TiO 2			2.1	SIM	438	160	1	0.3 MPa O 2		95	[147]

a. Conventional heating b. Atmospheric air c. Au supported on mesoporous carbon d. microwave heating e. SIM: sol-immobilization DP: deposition-precipitation DAE: direct anionic exchange f. n: nanosized μ: microsized g. AC: activated carbon h. Al:

Table I -

 I [START_REF] Hutchings | Catalysis by gold[END_REF] Oxidation of glucose over bimetallic Au catalysts (Adapted from[START_REF] Cao | An investigation into bimetallic catalysts for base free oxidation of cellobiose and glucose[END_REF])

	Catalyst	Post-synthesis	Glucose conversion	Gluconic acid
		treatment	(%)	Yield (%)
	0.5%Au-0.5%Pd/TiO 2	No	65	60
	0.5%Au-0.5%Pd/TiO 2	Air	70	63
	0.5%Au-0.5%Pd/TiO 2	reflux a	48	40
	0.5%Au-0.5%Pt/TiO 2	No	80	70
	0.5%Au-0.5%Pt/TiO 2	Air	68	62
	0.5%Au-0.5%Pt/TiO 2	Reflux	71	65
	Reaction conditions: glucose 0.20 g, water 20.0 g, catalyst 0.05 g, reaction temperature 160°C, reaction time 1 h, O 2

0.3 MPa. a. reflux: the dried catalyst was refluxed with hot water

Table I -

 I [START_REF] Bianchi | Selective oxidation of glycerol with oxygen using mono and bimetallic catalysts based on Au, Pd and Pt metals[END_REF] Turnover frequencies for glucose oxidation (glucose reacted (mol) per number of surface Au atoms (mol) per second) versus the diameter of the Au particles.

	Support	Al 2 O 3 a	Al 2 O 3 b	ZrO 2 a	ZrO 2 b	TiO 2 b	CeO 2 b	CeO 2 a	NPC a AC a
	Au particles size/nm	2.6	4.4	3.3	3.6	2.9	4	4.1	2.4	5.4
	TOF/mol glucose mol surfaceAu -1 s -1	41	7	45	28.5	24	19	22	3	10
	a,b indicate samples prepared by SG and DP, respectively. Reaction conditions: 5 wt.% glucose solution (175 mL),
	Au catalyst (30 mg), O 2 (120 mL/min), 50°C, pH 9.0. Adapted from [54].

Table I -

 I [START_REF] Dimitratos | Synergetic effect of platinum or palladium on gold catalyst in the selective oxidation of D-sorbitol[END_REF] Oxidation of glucose by Au/CeO 2 catalyst as adapted from[START_REF] Wojcieszak | Selective oxidation of glucose to glucuronic acid by cesium-promoted gold nanoparticle catalyst[END_REF]

	Temperature (°C) Time (min)	Conversion (%)	Acid yield (%)
	70	240	12	12
	110	240	65	53
	130	240	89	48
	150	240	100	23

Table I -

 I 10 Recyclability of Au based catalysts in base-free oxidation of glucose.

	Catalyst	First run	Second run Third run Fourth run	Ref.
	Au-Pd/MgO	62,0	24,4	18,7	-	[82]
	Au/CMK-3	92,0	-	-	70,0	[165]
	Au/nCeO 2	88,0	69,0	62,0	56,0	[145]
	Au/CeO 2	88,0	53,0	40,1	31,1	[145]

Table II -

 II 1 Comparison of gluconic acid and sodium gluconate

	Name	Con.1

a Con.2 a Con.3 a Con.4 a Con.5 a Con.6 a Average

	Gluconic acid	9.96	10.11	10.08	10.04	10.14	10.21	10.09
	Sodium Gluconate 10.35	10.34	10.37	10.36	10.33	10.30	10.34

a. Con. is the abbreviation of concentration. Unit is mg/g

Table III -

 III 1 ICP and XRF analyses of the Au-Pd/TiO 2 catalysts prepared by sol-immobilization method

	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP	Measured metal loading (wt.%) -XRF
		Au	Pd	Au/Pd	Au	Pd	Au/Pd	Au	Pd	Au/Pd
	1%Au	1	-	-	0.97	-	-	0.96	-	-
	0.9%Au0.1%Pd	0.9	0.1	9	0.83	0.05	16.60	0.84	0.07	12.00
	0.7%Au0.3%Pd	0.7	0.3	2.33	0.68	0.19	3.58	0.68	0.13	5.23
	0.5%Au0.5%Pd	0.5	0.5	1	0.45	0.31	1.45	0.48	0.16	3.00
	0.3%Au0.7%Pd	0.3	0.7	0.43	0.27	0.24	1.13	0.31	0.15	2.07
	0.1%Au0.9%Pd	0.1	0.9	0.11	0.09	0.58	0.16	0.10	0.42	0.24
	1%Pd	-	1	-	-	0.67	-	-	0.32	-

Table III -

 III 2 Mean value of the metal particle size in x%Auy%Pd /TiO 2 made by sol-immobilization method

	Catalysts	Particles size (nm)
	1%Au	2.5 ± 0.6
	0.9%Au0.1%Pd	2.5 ± 0.7
	0.7%Au0.3%Pd	2.7 ± 0.7
	0.5%Au0.5%Pd	2.6 ± 0.7
	0.3%Au0.7%Pd	2.4 ± 0.8

Table III

 III

	-3 Relative chemical composition of all the investigated catalysts. Elemental concentrations are expressed
			as atomic percentage (at. %)
	Catalysts	Au 4f 7/2	Pd 3d 5/2	Au/Pd	Au/Pd a	Ti 2p	O 1s	C 1s
	1%Au	0.254	ND b	-	-	15.055	42.722	41.969
	0.9%Au0.1%Pd	0.173	0.082	2.11	3.91	10.802	32.704	56.239
	0.7%Au0.3%Pd	0.131	0.073	1.79	3.31	10.92	32.037	56.84
	0.5%Au0.5%Pd	0.101	0.044	2.30	4.23	11.772	33.57	54.512
	0.3%Au0.7%Pd	0.054	0.063	0.86	1.59	11.931	35.341	52.611
	1%Pd	ND	ND	-	-	7.283	26.568	66.149

a. Weight ratio between gold and palladium b. Not detected

  -3. In all catalysts prepared by SIM method, the Au 4f 7/2 photoelectron peak is located at a BE value between 83.22 and 85.99 eV, as reported in Table III-4. These values are typical of metallic Au 0 and oxidized Au +δ

	species, respectively [182, 183]. The curve fitting of the Au 4f core-level spectrum is
	performed by using two spin-orbit split Au 4f 7/2 and Au 4f 5/2 components, separated by
	3.7 eV, in a fixed intensity ratio (1.33), keeping the same full width at half-maximum
	(FWHM) values. Peaks in the Au 4f region have an asymmetric peak shape. In the
	meanwhile, Pd 3d 5/2 photoelectron peak is located at a BE value between 334.55 and
	336.93 eV, and these values are typical of metallic Pd 0 and Pd +2 species, respectively
	[184]. Pd 3d was well separated by 5.26 eV by using spin-orbit Pd 3d 5/2 and Pd 3d 3/2

Table III -

 III 4 Relative surface distribution of gold and palladium species the pure TiO 2 support. The same oxidation states of gold were detected on the surface of the 0.5%Au0.5%Pd/TiO 2 catalyst. The curve fitting of Au 4f 7/2 spectrum indicates two Au 4f 7/2 components at BE = 83.38 and 85.92 eV that also can be attributed to Au 0 and Au +δ species, respectively[START_REF] Park | Effects of pretreatment conditions on CO oxidation over supported Au catalysts[END_REF][START_REF] Casaletto | XPS study of supported gold catalysts: the role of Au 0 and Au +δ species as active sites[END_REF][START_REF] Moulder | Handbook of X-ray photoelectron spectroscopy[END_REF]. There is a little shift of BE value comparing to previous sample. The oxidized Au +δ species are also a small fraction of 9.4% in total Au. The other samples are quite similar to the above two catalysts, including the catalyst after reaction. The Au 4f 7/2 photoelectron peak is located at a BE value between 83.22 and 85.99 eV. Au 0 and Au +δ are found among them.

				4f 7/2	4f 5/2		Au 4f
							1%Au
	Intensity (a.u.)						0.7%Au0.3%Pd 0.9%Au0.1%Pd
							0.5%Au0.5%Pd
							0.3%Au0.7%Pd
		78	81	84	87	90	93
				Binding Energy (eV)
							Pd 3d
	The Pd 3d curve fitting of the 0.5%Au0.5%Pd/TiO 2 sample shows Pd 3d 5/2
	3d 5/2 components at BE = 334.80 and 336.16 eV, which can be assigned to Pd 0 and Pd +2 , 1%Pd
	(a.u.)				3d 3/2	0.9%Au0.1%Pd
	Intensity					0.7%Au0.3%Pd
						0.5%Au0.5%Pd
						0.3%Au0.7%Pd
	Catalysts	328	Au 4f 7/2 BE (eV) 332	Surface Au species 336 340 Binding Energy (eV)		Pd 3d 5/2 BE (eV) 344	Surface Pd species
	1%Au Figure III-5 XPS curve of the Au 4f and Pd 3d photoelectron peak in the catalysts 83.51 Au 0 (95.0%) ND b	ND
	85.99 Figure III-5 presents the XPS spectra of the Au 4f and Pd 3d XPS regions in the Au +δ (5.0%)
	0.9%Au0.1%Pd catalysts. Different Au 4f and Pd 3d photoelectron peaks of surface gold and palladium 83.49 Au 0 (95.9%) 334.69 Pd 0 (100%)
	85.99 was found as a function of the different compositions. Because these two metals Au +δ (4.1%)
	0.7%Au0.3%Pd together could easily form alloys, they could affect the binding energy of each other 83.31 Au 0 (94.0%) 334.68 Pd 0 (100%)
	[90].		85.76	Au +δ (6.0%)
	0.5%Au0.5%Pd For instance, the Au 4f curve fitting of the 0.7%Au0.3%Pd/TiO 2 sample shows Au 83.38 Au 0 (90.6%) 334.80 Pd 0 (71.8%)
	85.92 4f 7/2 components at BE = 83.31 and 85.76 eV, which can be assigned to Au 0 and Au +δ , Au +δ (9.4%) 336.16 Pd +2 (28.2%)
	0.3%Au0.7%Pd respectively [72, 182, 183]. A small fraction of oxidized Au +δ species are found among 83.22 Au 0 (87.9%) 335.05 Pd 0 (87.8%)
	85.65 metallic gold and gold-palladium particles as indicated by the relative distribution of Au +δ (12.1%) 337.00 Pd +2 (12.2%)
	1%Pd		ND		ND		ND	ND
				b. Not detected

Au species in Table III-4. These metallic gold species are present in the sample as very small nanoparticles undetectable by XRD, since the diffraction pattern shows only the reflections of respectively

[START_REF] Moulder | Handbook of X-ray photoelectron spectroscopy[END_REF]

. Their relative distribution reveals a significant fraction of oxidized Pd +2 on the sample surface, as reported in Table

III

-4. A little shift of BE value of Pd 3d 5/2 components is observed in the Pd 3d curve fitting of 0.3%Au0.7%Pd/TiO 2 sample, which are 335.05 and 337

Table III -

 III 5 Blank test for the oxidation of glucose

	Temperature	Glucose	Gluconic acid	Glucuronic acid	Fructose	Carbon
	/˚C	Conversion %	Select%	Select%	Select%	Balance%
	60 a	6.0	9.0	0.0	0.0	94.5
	60 b	3.3	39.6	0.0	0.0	97.6
	80 b	3.6	0.0	0.0	0.0	95.9
	a. Blank test without adding catalyst. b. Blank test adding only support TiO 2 . Reaction conditions: 2 mL 1wt.%
	glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 5 h of reaction.
	Glucose oxidation was carried out at 60 ˚C, 80 ˚C, 100 ˚C and 150 ˚C, respectively.
	Because degradation of glucose happened at high temperature, lots of by-products were

generated during the oxidation of glucose at 150°C. The products solution became brown after 2 h of reaction. The carbon balance was very bad. Thus, the tests at 150 ˚C were not considered in the following discussion. The main product of the oxidation reaction was gluconic acid. Besides, sometimes glucuronic acid or fructose could also be observed. The carbon balance of the reactions at 60 ˚C was nearly 100%, while at 80 ˚C it was around 90% and at 100 ˚C it was around 75%. All the detailed results of each test are given in Annex 2 -Table

1

to Table

3

.

Figure III-6 presents the catalytic performances vs. gold content at 60 ˚C, 80 ˚C, 100 ˚C, respectively. As a reminder, the gold content considered here was the one determined by ICP analysis. To normalize and be able to compare the reactivity results, the catalytic activity was expressed as moles of glucose converted per mole of gold in the catalyst. It is clear that it lead to two different trends. At 60 ˚C, as the introduced quantities of palladium increased, the catalytic activity increased to a maximum at 0.45 wt.% Au, and then decreased. The less active catalyst tested was the monometallic Pd catalyst, for which only 1.5% conversion of glucose was reached (Table

III-6

).

Table III -

 III 6 Effect of temperature on the catalytic performance of 1wt.% Pd/TiO 2 for the oxidation of glucose

	Temperature	Glucose	Gluconic acid	Glucuronic acid	Fructose	Carbon
	/˚C	Conversion %	Select%	Select%	Select%	Balance
	60	1.2	35.3	0.0	0.0	98.9
	80	3.8	53.6	0.0	0.0	98.2
	100	13.5	53.6	0.0	13.2	95.5

Table III -

 III 7 ICP analysis of the 0.5%Au0.5%Pd/TiO 2 catalysts prepared by sol-immobilization method This could be explained that the analyses were made in the ICP error range. The catalyst kept the same loading after the first run. What is more important is that the Au/Pd ratio remained almost the same over the different runs.

	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP
		Au Pd	Au/Pd	Au	Pd	Au/Pd
	0.5%Au0.5%Pd	0.5 0.5	1	0.45	0.31	1.45
	0.5%Au0.5%Pd 1 st run a	0.5 0.5	1	0.36	0.25	1.44
	0.5%Au0.5%Pd 2 nd run a	0.5 0.5	1	0.40	0.26	1.54
	a. Recycled 0.5%Au0.5%Pd/TiO 2 catalyst
	ICP results for this 0.5%Au0.5%Pd/TiO

2 catalyst after first and second runs are shown in Table

III-7

. The quantity of gold and palladium decreased slightly after the first reaction. This could suggest some leaching issues during the first run. However, after the second run, there was more gold in the catalyst than the catalyst after the first run.

Table III -

 III [START_REF] Bianchi | Selective oxidation of glycerol with oxygen using mono and bimetallic catalysts based on Au, Pd and Pt metals[END_REF] Mean value of the metal particle size in 0.5%Au0.5%Pd /TiO 2 made by sol-immobilization method TiO 2 catalyst. The Au 4f curve fitting of the 1 st run sample shows Au 4f 7/2 components at BE = 83.06 and 85.91 eV, assigned to Au 0 and Au +δ species, respectively, which shifted slightly comparing to the fresh 0.5%Au0.5%Pd/TiO 2 catalyst. Au +δ species take 6.5% of the total surface gold, and the rest was metallic gold, which is less than 9.4% of oxidized gold on the fresh 0.5%Au0.5%Pd/TiO 2 catalyst. In the 2 nd run sample, Au 0 and Au +δ species are located at BE = 83.05 and 85.23 eV, respectively. The latter shifted from 0.7 eV, and occupied 8.4% of the total surface gold, which is more or less the same as in the fresh catalyst. The Pd 3d curve fitting of the 1 st and 2 nd run sample shows Pd 3d 5/2 components at BE = 334.56 and 336.78 eV, which can be assigned to Pd 0 and Pd +2 species, respectively. The binding energy did not change as compared to the fresh catalyst. However, the percentage of Pd +2 species in total surface palladium is becoming less and less important (see TableIII-9 and Table III-10)

		Catalysts		Particles size (nm)
	0.5%Au0.5%Pd		2.6 ± 0.7
	0.5%Au0.5%Pd 1 st run a	2.9 ± 0.7
	0.5%Au0.5%Pd 2 nd run a	2.7 ± 0.7
		a. Recycled 0.5%Au0.5%Pd/TiO 2 catalyst
		4f 7/2	4f 5/2			Au 4f
				0.5%Au0.5%Pd_2	nd run
	(a.u.)
	Intensity			0.5%Au0.5%Pd_1	st run
					0.5%Au0.5%Pd
	78	81	84	87	90	93
		Binding Energy (eV)
		3d 5/2			Pd 3d
				3d 3/2
				0.5%Au0.5%Pd_2	nd
	(a.u.)
	Intensity			0.5%Au0.5%Pd_1	st
					0.5%Au0.5%Pd
	328	332	336	340		344
		Binding Energy (eV)
	Figure III-9 XPS curve of the Au 4f and Pd 3d photoelectron peak in 0.5%Au0.5%Pd/TiO 2 catalyst
	Figure III-9 shows XPS spectra of the Au 4f and Pd 3d regions in
	0.5%Au0.5%Pd/

Table III -

 III 9 Relative chemical composition of 0.5%Au0.5%Pd/TiO 2 catalyst. Elemental concentrations are expressed as atomic percentage (at. %)

	Catalysts	Au 4f 7/2	Pd 3d 5/2	Au/Pd	Au/Pd a	Ti 2p	O 1s	C 1s
	0.5%Au0.5%Pd	0.101	0.044	2.30	4.23	11.772	33.57	54.512
	0.5%Au0.5%Pd 1 st run b	0.11	0.077	1.43	2.63	14.495	36.535	48.783
	0.5%Au0.5%Pd 2 nd run b	0.092	0.063	1.46	2.68	11.719	32.857	55.269

a. Weight ratio between gold and palladium b. Recycled 0.5%Au0.5%Pd/TiO 2 catalyst Table III-10 Relative surface distribution of gold and palladium

species Catalysts Au 4f 7/2 BE (eV) Surface Au species Pd 3d 5/2 BE (eV) Surface Pd species

	0.5%Au0.5%Pd	83.38	Au 0 (90.6%)	334.80	Pd 0 (71.8%)
		85.92	Au +δ (9.4%)	336.16	Pd +2 (28.2%)
	0.5%Au0.5%Pd 1 st run a	83.06	Au 0 (93.5%)	334.55	Pd 0 (83.6%)
		85.91	Au +δ (6.5%)	336.79	Pd +2 (16.4%)
	0.5%Au0.5%Pd 2 nd run a	83.05	Au 0 (91.6%)	334.56	Pd 0 (89.6%)
		85.23	Au +δ (8.4%)	336.78	Pd +2 (10.4%)
		a. Recycled 0.5%Au0.5%Pd/TiO 2 catalyst

Table III -

 III [START_REF] Haynes | Particle size effect in glucose oxidation with Pd/CB catalysts[END_REF] ICP and XRF analyses of the metal contained in Au-Pd/TiO 2 series made by precipitation-reduction

					method
	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP	Measured metal loading (wt.%) -XRF
		Au	Pd	Au/Pd	Au	Pd	Au/Pd	Au	Pd	Au/Pd
	1%Au	1	-	-	0.93	-	-	0.92	-	-
	0.9%Au0.1%Pd	0.9 0.1	9	0.82 0.08	8.63	0.83 0.07	11.86
	0.7%Au0.3%Pd	0.7 0.3	2.33	0.66 0.14	2.79	0.66 0.14	4.71
	0.5%Au0.5%Pd	0.5 0.5	1	0.45 0.26	1.62	0.49 0.21	2.33
	0.3%Au0.7%Pd	0.3 0.7	0.43	0.31 0.43	0.60	0.32 0.28	1.14
	0.1%Au0.9%Pd	0.1 0.9	0.11	0.11 0.64	0.14	0.14 0.35	0.40
	1%Pd	-	1	-	-	0.76	-	-	0.38	-

Table III -

 III 11 also shows the Au/Pd ratios determined by ICP and XRF. The XRF data are quite similar to that obtained for the SIM preparation method (Table III-1), they both are a little bigger than the excepted ones. However, the ICP analysis shows that the Au/Pd ratios of Au-Pd/TiO 2 series prepared by PR method are very close to theoretical ratios which are much better than the Au-Pd/TiO 2 series prepared by SIM method. TiO 2 , 1%Au/TiO 2 prepared by PR method and fresh support TiO 2 were chosen representatively for the XRD analysis. The results obtained are shown in Figure III-10. As previously said, X-ray diffraction patterns for the catalysts did not show peaks originating from the metals. The patterns only showed the presence of pure TiO 2 with anatase and rutile-crystal structures, which are in good agreement with the stand data from the JCPDS PDF card 00-021-1272 and 04-003-0648, respectively.

	1.2.2 XRD
	0.5%Au0.5%Pd/

Table III -

 III [START_REF] Ferrer | Three-layer core/shell structure in Au-Pd bimetallic nanoparticles[END_REF] ICP and XRF analyses of the metal contained in Au-Pd/ZrO 2 series made by sol-immobilization methodThe comparison of the Au/Pd ratio determined by ICP and XRF for each sample with the theoretical Au/Pd metal ratio showed quite a lot of difference. Gold loadings detected by ICP-OES are around 80-90% of the theoretical ones. The palladium loadings detected by ICP-OES are much lower, especially in the case of palladium monometallic sample, the loading of which was only 0.32wt.% as compared to 1wt.%

	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP	Measured metal loading (wt.%) -XRF
		Au	Pd	Au/Pd	Au	Pd	Au/Pd	Au	Pd	Au/Pd
	1%Au	1	-	-	0.86	-	-	0.73	-	-
	0.9%Au0.1%Pd	0.9 0.1	9	0.84 0.07 12.00 0.71 0.12	5.92
	0.7%Au0.3%Pd	0.7 0.3	2.33	0.57 0.19	3.00	0.55 0.29	1.90
	0.5%Au0.5%Pd	0.5 0.5	1	0.40 0.34	1.18	0.39 0.49	0.80
	0.3%Au0.7%Pd	0.3 0.7	0.43	0.27 0.50	0.54	0.22 0.65	0.34
	0.1%Au0.9%Pd	0.1 0.9	0.11	0.08 0.55	0.15	0.05 0.73	0.07
	1%Pd	-	1	-	-	0.32	-	-	0.43	-

Table III -

 III [START_REF] Mu | Synergetic effect of surface and subsurface Ni species at Pt-Ni bimetallic catalysts for CO oxidation[END_REF] Mean value of the metal particle size in Au-Pd /ZrO 2 made by sol-immobilization method

	Catalysts	Particles size (nm)	Particles counted
	0.7%Au0.3%Pd	4.9 ± 2.2	114
	1%Au	6.5 ± 1.6	> 300
	2.1.4 XPS
	The bimetallic 0.7%Au0.3%Pd catalyst supported on ZrO 2 was characterized by
	X-ray photoelectron spectroscopy to get information concerning the nature and
	chemical state of the surface species.

Table III -

 III 14 Relative chemical composition of 0.7%Au0.3%Pd/ZrO 2 catalyst. Elemental concentrations are expressed as atomic percentage (at. %)

	Catalysts	Au 4f	Zr 3d	O 1s	C 1s
	0.7%Au0.3%Pd	0.170	17.947	42.634	39.248
	Table III-15 Relative surface distribution of gold and palladium species
	Catalysts	Au 4f 7/2 BE (eV)	Surface Au species
	0.7%Au0.3%Pd	83.62		Au 0

Table III -

 III [START_REF] Heidkamp | Ceria supported gold-platinum catalysts for the selective oxidation of alkyl ethoxylates[END_REF] and III-15 show the XPS results for 0.7wt.%Au0.3wt.%Pd/ZrO 2 sample. The Au 4f 7/2 photoelectron peak is located at 83.62 eV which indicates a metallic Au 0[START_REF] Casaletto | XPS study of supported gold catalysts: the role of Au 0 and Au +δ species as active sites[END_REF]. No other chemical state of gold could be found. Because Pd 3d and Zr 3p share the same photoelectron peak around BE = 332 eV, the concentration of palladium on the surface of the catalyst could not be calculated.

2.1.5 Catalytic test

Glucose oxidation was carried out at 60 ˚C and 80 ˚C on the Au-Pd/ZrO 2 series of catalysts. Other reaction conditions were the same as in the case of the catalysts supported on TiO 2 oxide. Firstly, blank tests were done. The results are presented in Table III-16. ZrO 2 showed almost no activity at both temperatures.

Table III -

 III [START_REF] Hermans | Preparation of Au-Pd/C catalysts by adsorption of metallic species in aqueous phase for selective oxidation[END_REF] Effect of temperature on the catalytic performance of fresh support ZrO 2 for the oxidation of glucoseThe results of the catalytic tests of the Au-Pd/ZrO 2 series catalysts are shown in Figure III-17. The catalytic trends at 60 ˚C and 80 ˚C are totally different. At 60 ˚C, asthe quantity of gold decreases, the catalytic activity increased and reach the maximum at 0.57 wt.% of gold. Then a sudden decrease of the catalytic activity occurred as the gold content decreased to 0.4 wt.%. As the quantity of gold continuously decreases, the catalytic performance reached a new maximum, forming a two peaks plot across the whole range of the bimetallic catalyst compositions. The reaction test for these catalysts were performed a second time, but the same phenomenon happened. The selectivity to gluconic acid was always around 80% except over 0.3%Au0.7%Pd/ZrO 2 catalyst where it was around 65%. However, at 80 ˚C, as the quantity of gold decreases, the catalytic activity maintained the same level until 0.57 wt.%, and the increased to the maximum at 0.4 wt.% of gold. This behavior was very different with that observed at 60 ˚C. This suggests that there was a synergistic effect. The selectivity to gluconic acid was kept at around 75%, which was a bit lower as compared to the results obtained at 60 ˚C. It could be explained by the isomerization of glucose to fructose. Detailed results are given in Annex 2 -Table5 and 6.

	Temperature	Glucose	Gluconic acid	Glucuronic	Fructose	Carbon
	/˚C	Conversion %	Select%	acid Select%	Select%	Balance
	60	2.9	3.4	0.0	0.0	97.2
	80	1.7	40.3	0.0	0.0	99.2
	Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar,
			5 h of reaction.

Figure III-17 Oxidation of glucose over a series of mono-and bi-metallic catalysts containing Au and Pd supported on ZrO 2 , Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 5 h of reaction.

Table III -

 III 17 presents the catalytic test results over 1% Pd/ZrO 2 catalyst. It showed very low activity whenever at 60 ˚C or 80 ˚C.

Table III -

 III [START_REF] Zhang | Heterogeneously catalyzed hydrothermal processing of C5-C6 sugars[END_REF] Effect of temperature on the catalytic performance of 1% Pd/ZrO 2 for the oxidation of glucose

	Temperature	Glucose	Gluconic acid	Glucuronic acid	Fructose	Carbon
	/˚C	Conversion %	Select%	Select%	Select%	Balance
	60	0.2	100.0	0.0	0.0	100
	80	4.8	65.3	0.0	0.0	98.1

Reaction conditions: 2 mL 1wt.% glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5 bar, 5 h of reaction.

Table III -

 III [START_REF] Roco | The long view of nanotechnology development: the National Nanotechnology Initiative at 10 years[END_REF] ICP analysis of the metal contained in Au-Pd/ZrO 2 series made by sol-immobilization method

	100
	80
	60				Conversion%
	40				Gluconic acid Selectivity%
					Carbon Balance
	20
	0
	1st run	2ed run	3rd run
	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP
		Au Pd	Au/Pd	Au	Pd	Au/Pd
	0.7%Au0.3%Pd	0.7 0.3	2.33	0.57	0.19	3.00
	0.7%Au0.3%Pd 1 st run a	0.7 0.3	2.33	0.55	0.20	2.75
	0.7%Au0.3%Pd 2 nd run a 0.7 0.3	2.33	0.54	0.20	2.70
	a. Recycled 0.7%Au0.3%Pd/ZrO 2 catalyst

Table III -

 III [START_REF] Wojcieszak | Study of supported nickel catalysts prepared by aqueous hydrazine method. Hydrogenating properties and hydrogen storage. Support effect. Silver additive effect[END_REF] Mean value of the metal particle size in x%Auy%Pd /ZrO 2 made by sol-immobilization method ZrO 2 catalyst before and after catalytic test was also done. TableIII-20, Table III-21 and Figure III-20 present the results. It shows that there was an increase of surface gold concentration after the first run and then decrease after the second run. But the change was small.

	Catalysts	Particles size (nm)	Particles counted
	0.7%Au0.3%Pd	4.9 ± 2.2	114
	0.7%Au0.3%Pd 1 st run a	5.9 ± 0.9	46
	0.7%Au0.3%Pd 2 nd run a	5.0 ± 3.2	5
	1%Au	6.5 ± 1.6	> 300
	a. Recycled 0.7%Au0.3%Pd/ZrO 2 catalyst
	XPS analysis of 0.7wt.%Au0.3wt.%Pd/

Table III -

 III 20 Relative chemical composition of 0.7%Au0.3%Pd/ZrO 2 and catalyst used once and twice. Elemental concentrations are expressed as atomic percentage (at. %) Figure III-20 XPS curve of the Au 4f photoelectron peak in 0.7%Au0.3%Pd/ZrO 2 catalyst before and after catalytic tests

							Au 4f
					0.7%Au0.3%Pd_2 n d run
	(a.u.)
	Intensity				0.7%Au0.3%Pd_1 s t run
						0.7%Au0.3%Pd
	78	81	84	87	90	93
		Binding Energy (eV)
	Catalysts		Au 4f	Zr 3d	O 1s	C 1s
	0.7%Au0.3%Pd	0.170	17.947	42.634	39.248
	0.7%Au0.3%Pd 1 st run a	0.198	15.782	40.042	43.979
	0.7%Au0.3%Pd 2 nd run a	0.148	16.740	41.593	41.519
		a. Recycled 0.5%Au0.5%Pd/TiO 2 catalyst
	Table III-21 Relative surface distribution of gold and palladium species
	Catalysts	Au 4f 7/2 BE (eV)	Surface Au species
	0.7%Au0.3%Pd		83.62		Au 0
	0.7%Au0.3%Pd 1 st run a		83.51		Au 0
	0.7%Au0.3%Pd 2 nd run a	83.58		Au 0
		a. Recycled 0.7%Au0.3%Pd/TiO 2 catalyst

Table III -

 III [START_REF] Philip | Nanostructured materials[END_REF] ICP and XRF analyses of the metal contained in Au-Pd/ZrO 2 series made by precipitation-reduction

					method
	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP	Measured metal loading (wt.%) -XRF
		Au	Pd	Au/Pd	Au	Pd	Au/Pd	Au	Pd	Au/Pd
	1%Au	1	-	-	0.88	-	-	0.81	-	-
	0.9%Au0.1%Pd	0.9	0.1	9	0.84	0.08	10.5	0.73	0.11	6.64
	0.7%Au0.3%Pd	0.7	0.3	2.33	0.61	0.22	2.77	0.57	0.31	1.84
	0.5%Au0.5%Pd	0.5	0.5	1	0.46	0.32	1.44	0.38	0.38	1
	0.3%Au0.7%Pd	0.3	0.7	0.43	0.24	0.47	0.51	0.21	0.60	0.35
	0.1%Au0.9%Pd	0.1	0.9	0.11	0.09	0.61	0.15	0.05	0.76	0.07
	1%Pd	-	1	-	-	0.70	-	-	0.82	-
	2.2.2 XRD
	0.5%Au0.5%Pd/ZrO 2 , 1%Au/ZrO 2 prepared by PR method and fresh support

Table III - 19

 III19 -1 and Table III-12 the metal loadings were basically the same on TiO 2 or ZrO 2 when the SIM method of preparation was used. However, the TEM images (Figure III-3 and Figure III-19) revealed that the metal particles size was almost the double on ZrO 2 compared to TiO 2 (Table III-2 and

Table III -

 III [START_REF] Federico | Nanostructured surfaces: challenges and frontiers in nanotechnology[END_REF] Au-Pd on ZrO 2 series catalysts calcined at 400 ˚C

Catalyst Before calcination /mg Weight loss after calcination (in %)

	1%Au	124.6	0.6
	0.9%Au0.1%Pd	108.6	0.9
	0.7%Au0.3%Pd	110.5	0.9
	0.5%Au0.5%Pd	112.2	0.6
	0.3%Au0.7%Pd	110.6	0.5
	0.1%Au0.9%Pd	105.0	0.8
	1%Pd	104.4	0.9

Table IV -

 IV 1 ICP and XRF analyses of the metal contained in Au-Cu/TiO 2 series made by precipitation-reduction

					method
	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP	Measured metal loading (wt.%) -XRF
		Au Cu Au/Cu Au	Cu Au/Cu Au	Cu Au/Cu
	1%Au	1	-	-	0.92	-	-	0.92	-	-
	0.9%Au0.1%Cu	0.9 0.1	9	0.86 0.09	9.56	0.84 0.10	8.40
	0.7%Au0.3%Cu	0.7 0.3	2.33	0.72 0.29	2.48	0.67 0.30	2.23
	0.5%Au0.5%Cu	0.5 0.5	1	0.45 0.41	1.10	0.48 0.47	1.02
	0.3%Au0.7%Cu	0.3 0.7	0.43	0.30 0.64	0.47	0.31 0.66	0.47
	0.1%Au0.9%Cu	0.1 0.9	0.11	0.09 0.76	0.12	0.13 0.85	0.15
	1%Cu	-	1	-	-	0.81	-	-	0.93	-
	1.1.2 XRD

Table IV -

 IV 2 Mean value of the metal particle size in x%Auy%Cu /TiO 2 prepared by precipitation-reduction method

	Catalysts	Particles size (nm)
	1%Au/TiO 2	4.3 ± 1.9
	0.9%Au0.1%Cu/TiO 2	3.3 ± 1.0
	0.7%Au0.3%Cu/TiO 2	3.1 ± 1.0
	0.5%Au0.5%Cu/TiO 2	3.4 ± 1.0
	0.3%Au0.7%Cu/TiO 2	2.7 ± 1.0
	0.1%Au0.9%Cu/TiO 2

  Table IV-4 also reveals that CuOH was present whenCu loading was less than 0.9wt.%. In the case of 1%Cu/TiO 2 catalyst, only Cu +1 /Cu 0 and CuO could be observed.

	Au 4f Table IV-4 Relative surface distribution of gold and copper species
	Catalysts		Au 4f 7/2 BE (eV)	Surface Au species	Cu 2p 3/2 BE (eV)	Surface Cu species
								1%Au
	1%Au		83.08		Au 0 (93.5%)			-	-
	(a.u.)	85.74		Au +δ (6.5%)	0.9%Au0.1%Cu
	Intensity 0.9%Au0.1%Cu	83.07		Au 0 (100%)	0.5%Au0.5%Cu 0.7%Au0.3%Cu 931.45	Cu 0 /Cu +1 (56.0%)
								932.34	CuOH(44.0%)
						0.3%Au0.7%Cu
	0.7%Au0.3%Cu		83.15		Au 0 (92.1%)			931.44	Cu 0 /Cu +1 (65.0%)
						0.1%Au0.9%Cu
		75	78 84.97	81	84 Au +δ (7.9%) 87	90	93 932.51	CuOH (30.7%)
					Binding Energy (eV)			934.13	CuO (4.3%)
	0.5%Au0.5%Cu		83.34		Au 0 (92.0%)			Cu 2p 931.46	Cu 0 /Cu +1 (52.4%)
			84.91		Au +δ (8.0%)			1%Cu 932.48	CuOH (34.0%)
	(a.u.)				0.9%Au0.1%Cu 934.21	CuO (13.6%)
	0.3%Au0.7%Cu Intensity	83.45 85.06		Au 0 (93.6%) Au +δ (6.4%)	0.7%Au0.3%Cu 0.5%Au0.5%Cu 0.3%Au0.7%Cu 931.42 932.40	Cu 0 /Cu +1 (49.9%) CuOH (36.0%)
						0.1%Au0.9%Cu 934.04	CuO (14.1%)
	0.1%Au0.9%Cu		83.64		Au 0 (87.2%)			931.54	Cu 0 /Cu +1 (36.5%)
		924	931 85.56		938 Au +δ (12.8%) 945	952		959 932.41	CuOH (40.3%)
				Binding Energy (eV)
	Figure IV-4 XPS curves of the Au 4f and Cu 2p photoelectron peak in the catalysts series 934.22 CuO (23.2%)
	1%Cu		-		-			932.05	Cu 0 /Cu +1 (85.0%)
	Table IV-3 Relative chemical composition of all the investigated catalysts. Elemental concentrations are expressed 934.23 CuO (15.0%)
			as atomic percentage (at. %) δ. Represent 1 and/or 3.
	Catalysts	Au 4f 7/2	Cu 2p 3/2	Au/Cu	Au/Cu a	Ti 2p	O 1s	C 1s
	1%Au	0.26	-		-	-			24.75	57.344	17.647
	0.9%Au0.1%Cu	0.246	0.171	1.44	4.45	23.319	57.036	19.227
	0.7%Au0.3%Cu	0.179	0.582	0.31	0.96	22.371	56.908	19.96
	0.5%Au0.5%Cu	0.171	0.829	0.21	0.64	22.936	56.909	19.155
	0.3%Au0.7%Cu	0.118	1.27		0.09	0.29	23.594	57.654	17.364
	0.1%Au0.9%Cu	0.051	1.216	0.04	0.13	18.411	51.351	28.972
	1%Cu	-	1.157	-	-			17.204	49.964	31.675
			a. Weight ratio between gold and copper

  Table IV-1). Comparing the value of Au/Cu bulk weight ratio, the Au/Cu ratio on the surface is smaller. It reveals that more copper stayed on the surface of the catalysts rather than in the bulk of the catalyst. The other reaction conditions were the same as the ones used for the Au-Pd series in Chapter III. The test results are presented in Figure IV-7. Detailed results are given in Annex 2 -Table 10 to 12.

	XRF (1.1.5 Catalytic test	Au 0
	Similarly to what was done with the Au-Pd series catalysts, catalytic tests for the
	Au-Cu/TiO 2 series catalysts were carried out at 60, 80 and 100 ˚C, respectively
	(Figure IV-7).	Au +δ
			Cu 0
		CuOH
	Satellite
		CuO	Cu +1

Table IV -

 IV 5 ICP and XRF analyses of the metal contained in 0.5%Au0.5%Cu/TiO 2 catalyst made by precipitation-reduction method

	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP
		Au Cu Au/Cu Au	Cu Au/Cu
	0.5%Au0.5%Cu	0.5 0.5	1	0.45 0.41	1.10
	0.5%Au0.5%Cu 1 st run a	0.5 0.5	1	0.41 0.02	20.5
	0.5%Au0.5%Cu 2 nd run a	0.5 0.5	1	0.41 0.00	-
	a. Recycled 0.5%Au0.5%Cu/TiO 2 catalyst
	Figure IV-9 presents the metal particle size distribution of 0.5%Au0.5%Cu/TiO 2
	catalyst after first run and the second run. In comparison to the micrograph of the
	0.5%Au0.5%Cu/TiO 2 catalyst (d) in Figure IV-2, there was no significant difference.

Table IV -

 IV 6 Mean value of the metal particle size in 0.5%Au0.5%Cu/TiO 2 catalyst made by PR method TiO 2 catalyst before and after reaction. The Au 4f curve fitting of the 1 st run sample shows Au 4f 7/2 components at BE = 83.76 eV, assigned to Au 0 species, which was shifted of 0.42 eV in comparison to the fresh 0.5%Au0.5%Cu/TiO 2 catalyst.There was only metallic gold but no Au +δ existed on the surface of the catalyst. The same is observed in the 2 nd run sample, Au 0 species is located at BE = 83.26 eV, which shifts only of 0.08 eV and was very close to the signal observed on the fresh catalyst (Table IV-7 and TableIV-8). The intensity of the Au 4f signal for the catalysts after reaction was weak in comparison to that of the fresh catalyst. The gold on the surface of the catalyst was lost. As for Cu, no peak can be viewed on Cu 2p spectra of 0.5%Au0.5%Cu/TiO 2 catalyst after reaction. This is in agreement with the ICP-OES results (TableIV-5). Very little amount of the gold and most of the Cu was lost in the solution during the glucose oxidation reaction.

		Au 4f Table IV-8 Relative surface distribution of gold and copper species
			Catalysts Au 4f 7/2		Particles size (nm) Cu 2p 3/2
	Catalysts			Surface Au species		Surface Cu species
		0.5%Au0.5%Cu/TiO 2 BE (eV)	3.4 ± 1.0 0.5%Au0.5%Cu_2 BE (eV) nd run
	0.5%Au0.5%Cu 1 st run a (a.u.) 0.5%Au0.5%Cu 83.34 Au 0 (92.0%)	3.0 ± 0.9 931.46	Cu 0 /Cu +1 (52.4%)
		0.5%Au0.5%Cu 2 nd run a a. Recycled 0.5%Au0.5%Cu/TiO 2 catalyst 3.3 ± 0.9 0.5%Au0.5%Cu_1 84.91 Au +δ (8.0%) 932.48 st run Intensity 934.21	CuOH (34.0%) CuO (13.6%)
	Figure IV-10 presents XPS spectra of the Au 4f and Cu 2p in 0.5%Au0.5%Cu 1 st run a 83.76 Au 0 (100%) --
	0.5%Au0.5%Cu -90 93 a. Recycled 0.5%Au0.5%Cu/TiO 2 catalyst δ. Represent 1 and/or 3. 0.5%Au0.5%Cu 2 nd run a 83.26 Au 0 (100%) 0.5%Au0.5%Cu/78 81 84 87 Binding Energy (eV)	-
							Cu 2p
		(a.u.)				0.5%Au0.5%Cu
		Intensity
					0.5%Au0.5%Cu_1 st	run
					0.5%Au0.5%Cu_2 nd	run
		924	931	938	945	952	959
				Binding Energy (eV)
	Figure IV-10 XPS curves of the Au 4f and Cu 2p photoelectron peak in 0.5%Au0.5%Cu/TiO 2 catalyst before and
				after reaction
	Table IV-7 Relative chemical composition of 0.5%Au0.5%Cu/TiO 2 catalyst. Elemental concentrations are
			expressed as atomic percentage (at. %)
	Catalysts	Au 4f 7/2	Cu 2p 3/2	Au/Cu	Au/Cu a		Ti 2p	O 1s	C 1s
	0.5%Au0.5%Cu	0.171	0.829		0.21	0.64	22.936	56.909	19.155
	0.5%Au0.5%Cu 1 st run b	0.1	-		-	-	13.156	33.874	52.87
	0.5%Au0.5%Cu 2 nd run b	0.046	-		-	-		6.627	24.128	69.199
	a. Weight ratio between gold and copper b. Recycled 0.5%Au0.5%Cu/TiO 2 catalyst
									123

Table IV -

 IV [START_REF] Dimitratos | Synergetic effect of platinum or palladium on gold catalyst in the selective oxidation of D-sorbitol[END_REF] ICP and XRF analyses of the metal contained in the Au-Cu/ZrO 2 series prepared by the

				precipitation-reduction method
	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP	Measured metal loading (wt.%) -XRF
		Au Cu Au/Cu Au	Cu Au/Cu Au	Cu Au/Cu
	1%Au	1	-	-	0.88	-	-	0.81	-	-
	0.9%Au0.1%Cu	0.9 0.1	9	0.86 0.03 28.67 0.73 0.02	36.50
	0.7%Au0.3%Cu	0.7 0.3	2.33	0.54 0.26	2.08	0.48 0.27	1.78
	0.5%Au0.5%Cu	0.5 0.5	1	0.39 0.38	1.03	0.37 0.45	0.82
	0.3%Au0.7%Cu	0.3 0.7	0.43	0.29 0.65	0.45	0.21 0.66	0.32
	0.1%Au0.9%Cu	0.1 0.9	0.11	0.10 0.87	0.11	0.04 0.84	0.05
	1%Cu	-	1	-	-	0.83	-	-	0.99	-
	1.2.2 XRD
	As previously done for the other catalysts series, the 0.5%Au0.5%Cu/ZrO 2

prepared by PR method and the fresh support ZrO 2 for comparison were analyzed by XRD. The patterns only showed the presence of pure ZrO 2 with baddeleyite-crystal structure. This indicates that the crystalline structure of all the supports in each catalyst did not change during the preparation (Figure IV-11).

  3 and 0.9wt.%. A sudden decrease of selectivity was observed when the gold content was less than 0.3wt.%. Over most of the Au-Cu catalysts the conversions of glucose were higher on ZrO 2 -supported catalysts than on TiO 2 . Similar observations were made on Au-Pd/ZrO 2 and Au-Pd/TiO 2 catalysts. It could be explained by the fact that ZrO 2 is a p-type semiconductor while TiO 2 is n-type semiconductor. O 2 is easier to be adsorbed on the surface of a p-type semiconductor, and it is easier to get electrons from this type of oxide. This contributed to the higher activity for glucose oxidation reaction. Thus, catalysts supported over ZrO 2 were more active for a given Au/Cu ratio. Figure IV-13 Effect of the support on the catalytic performances of the Au-Cu series in glucose base-free oxidation.

	Reaction conditions: 2 mL 1wt.% of glucose aqueous solution, 1wt.% bimetallic catalyst (21.9 mg), air pressure = 5
	bar, 60 ˚C, 5 h of reaction.

Table IV -

 IV 10 ICP and XRF analyses of the metal contained in Au-Bi/ZrO 2 series made by PR method Figure IV-14 presents the diffractogram patterns of the 0.5%Au0.5%Bi/ZrO 2 catalyst and of the fresh support ZrO 2 for comparison. The former was chosen to represent the whole Au-Bi/ZrO 2 series of catalysts. As observed before, gold and bismuth were not detected by this technique because of the too low metal loadings. The patterns only showed the presence of pure ZrO 2 with baddeleyite-crystal structure.

	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP	Measured metal loading (wt.%) -XRF
		Au Bi Au/Bi	Au	Bi	Au/Bi	Au	Bi	Au/Bi
	1%Au	1	-	-	0.88	-	-	0.81	-	-
	0.9%Au0.1%Bi	0.9 0.1	9	0.85 0.10	8.50	0.71 0.09	7.89
	0.7%Au0.3%Bi	0.7 0.3	2.33	0.67 0.31	2.16	0.51 0.25	2.04
	0.5%Au0.5%Bi	0.5 0.5	1	0.45 0.47	0.96	0.35 0.39	0.90
	0.3%Au0.7%Bi	0.3 0.7	0.43	0.20 0.53	0.38	0.15 0.45	0.33
	0.1%Au0.9%Bi	0.1 0.9	0.11	0.06 0.28	0.21	0.02 0.28	0.07
	1%Bi	-	1	-	-	0.90	-	-	0.80	-
	2. 2 XRD

Table IV -

 IV [START_REF] Haynes | Particle size effect in glucose oxidation with Pd/CB catalysts[END_REF] ICP and XRF analyses of the metal contained in the Au-Pt/ZrO 2 series prepared by the precipitation-reduction method

	Catalysts	Theoretical metal loading (wt.%)	Measured metal loading (wt.%) -ICP	Measured metal loading (wt.%) -XRF
		Au Pt Au/Pt	Au	Pt	Au/Pt	Au	Pt	Au/Pt
	1%Au	1	-	-	0.88	-	-	0.81	-	-
	0.75%Au0.25%Pt	0.7 0.3	2.33	0.55 0.17	3.24	0.50 0.23	2.17
	0.5%Au0.5%Pt	0.5 0.5	1.00	0.46 0.42	1.10	0.31 0.41	0.76
	0.25%Au0.75%Pt	0.3 0.7	0.43	0.25 0.68	0.37	0.13 0.63	0.21
	1%Pt	-	1	-	-	0.78	-	-	0.91	-
	3.2 XRD
	0.5%Au 0.5%Pt/ZrO 2 and fresh support ZrO 2 were chosen for XRD analysis

. As before, only pure ZrO 2 was observed.

  -4 and FigureIV-4): Cu 0 , Cu + , CuOH and CuO. For all catalysts the Cu is mostly in Cu 0 /Cu + state. Indeed, the BE energy corresponds well to the metallic and/or Cu + states. However, due to the very low Cu

Acknowledgements I would like to sincerely acknowledge my grant from China Scholarship Council to financially support my doctor studies in France. This work was performed in UCCS (Unité de Catalyse et Chimie du Solide -CNRS UMR 8181) and Ecole Centrale de Lille. I would like to express my sincere gratitude to my supervisors, Prof. Sé bastien

Analytically pure chemicals (D-Glucose, D-Fructose, D-Gluconic acid sodium salt, D-Glucuronic) were used for calibration as said above. Firstly, 0.0555 mol/L solutions of each compound were prepared. Then these solutions were diluted in water at 75%, 50%, 25%, 5%, 1% in volume. Thus, 6 points were used to plot the calibration curve of each product.

Chapter III

Base-free oxidation of glucose over Au-Pd catalysts

Chapter IV Base-free glucose oxidation over Au-X (with X= Cu, Bi or Pt) catalysts and comparison with Au-Pd catalysts involve incorporation into the acid products but instead regenerates hydroxide ions the molecular oxygen is required for the reaction to proceed. Zope et al. [START_REF] Zope | Inhibition of gold and platinum catalysts by reactive intermediates produced in the selective oxidation of alcohols in liquid water[END_REF][START_REF] Zope | Influence of reaction conditions on diacid formation during Au-catalyzed oxidation of glycerol and hydroxymethylfurfural[END_REF] proposed that O 2 played an indirect role in the mechanism by removing electrons from the metal surface generated by substrate oxidation [START_REF] Zope | Reactivity of the gold/water interface during selective oxidation catalysis[END_REF]. Thus, O 2 closes the catalytic cycle by serving as an electron scavenger and regenerating hydroxide ions that are consumed in the reaction to produce acid products. Evidently, the role of the basic site on the catalyst surface is important. It activates O-H and C-H bonds. Thus the activity of the catalysts will be related to the optimum number of metal-support interfacial sites, which may contain hydroxyl groups (surface basic sites). If hydroxyl groups are supplied through the solution medium (basic solution), the metal-support interface is much less important so that even bulk gold becomes an active catalyst. The mechanism of the oxidation in base-free conditions is still controversed and discussed. Adapted deep characterization methods and operando studies will be needed to elucidate it.

Chapter V

General conclusion and perspectives

Mots clé s :

Catalyseurs bimé talliques, or alliages, oxydation du glucose, effet de synergie

Bimetallic catalysts for oxidation of carbohydrates: looking for synergetic effects

Abstract :

The supported bimetallic nanoparticles are particularly attractive catalysts due to the increased activity and stability over their monometallic counterparts. In this thesis, gold-based solids have been studied as catalysts for the selective base-free oxidation of glucose. It has been demonstrated that the variation of the molar ratio between gold and the second metal (Pd, Pt, or Cu) has a different impact on the catalytic performances depending on the nature of the second metal, the support and the method of preparation. TiO 2 supported Au-Pd series prepared by the sol-immobilization method and Au-Cu series prepared by the precipitation-reduction method showed a significant synergistic effect, particularly when the ratio of the two metals was 1. XPS analysis showed that the Au +δ , Pd +2 and CuOH species played an important role in the base-free reaction. Bismuth as the second metal showed no beneficial effect, unlike palladium and copper. The Au-Pt catalysts supported on ZrO 2 proved to be still active when the gold content was less than 0.3 wt.%. Ultimately, the nature of the support has a very important impact on the mechanism of the base-free reaction conducted on gold-based catalysts. The reason lies in the interactions of the support with the bimetallic phase thus favoring the formation of the active species.