Laetitia Oliviero

Arnaud Travert

Elizabeth Dominguez

Jianjun Chen

Françoise Maugé
email: francoise.mauge@ensicaen.fr

Catalysis by sulfides: Advanced IR/CO spectroscopy for the identification of the most active sites in HDS reactions

Keywords: Hydrodesulfurization (HDS), Molybdenum disulfide (MoS 2 ), Support effect, Co carbonyl nitrosyl, Slab morphology, Active site structure, Infrared (IR) spectroscopy, CO adsorption

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Introduction

The production of clean fuels is more than ever a major industrial challenge to meet both the high demand for fuel and the urgency of environmental issues. Hydrotreatment is a key refinery process to remove impurities such as sulfur, nitrogen and metals even from highly contaminated feedstocks.

The performance and stability of new generations of hydrotreating catalysts have been significantly improved. Even if the latest generations of hydrotreating catalysts are still essentially based on the classical composition i.e. sulfide Mo(W) promoted by Co(Ni) supported on alumina, a more thoughtful choice of support characteristics, the addition of dopants and, above all, a better control of the sulfidation stage allowed their performances to be greatly improved [START_REF] Tanimu | Advanced Hydrodesulfurization Catalysts: A Review of Design and Synthesis[END_REF][START_REF] Kang | A review on the Mo-precursors for catalytic hydroconversion of heavy oil[END_REF][START_REF] Oliviero | Organic additives for hydrotreating catalysts: A review of main families and action mechanisms[END_REF].

These optimizations were also obtained thanks to the development of well-adapted characterization methods. It should be mentioned that the characterization of these catalytic systems is challenging due to various specificities such as the sulfidation step (which leads to long-term contamination of the equipment), the high sensitivity of the sulfide catalysts to air-contact (which makes in situ characterizations much more relevant), the small size of the sulfide slabs (≤ 2 nm) and, most of all, the different forms of the elements present in the catalyst of which only a minority fraction is active for hydrotreatment. One can quote the alumina support, the Co spinel, the Co sulfide, the unpromoted MoS 2 that are mostly inactive phases whereas, as early demonstrated by Topsøe's group [START_REF] Topsøe | On the state of the Co-Mo-S model[END_REF][START_REF] Topsøe | In situ Mossbauer emission spectroscopy studies of unsupported and supported sulfided CoMo hydrodesulfurization catalysts: Evidence for and nature of a Co-Mo-S phase[END_REF][START_REF] Wivel | On the catalytic significance of a CoMoS phase in CoMo Al2O3 hydrodesulfurization catalysts: Combined in situ Mössbauer emission spectroscopy and activity studies[END_REF][START_REF] Wivel | Mössbauer emission studies of calcined CoMo Al2O3 catalysts: Catalytic significance of Co precursors[END_REF], the so-called CoMoS phase actively participate to the reaction. This so-called CoMoS phase was related to the presence of Co atoms deposited on the edge of the MoS 2 phase [START_REF] Topsøe | On the state of the Co-Mo-S model[END_REF][START_REF] Topsøe | In situ Mossbauer emission spectroscopy studies of unsupported and supported sulfided CoMo hydrodesulfurization catalysts: Evidence for and nature of a Co-Mo-S phase[END_REF][START_REF] Wivel | On the catalytic significance of a CoMoS phase in CoMo Al2O3 hydrodesulfurization catalysts: Combined in situ Mössbauer emission spectroscopy and activity studies[END_REF][START_REF] Wivel | Mössbauer emission studies of calcined CoMo Al2O3 catalysts: Catalytic significance of Co precursors[END_REF].

Among the various characterization techniques that account for atomic level structures of surface sites of in situ sulfided supported catalysts, infrared (IR) spectroscopy of probe molecule is one of the most relevant. In the early 80's, it was showed that the IR bands of adsorbed NO make possible to distinguish Mo sites from Co (or Ni) sites and that nice relationships could be obtained between IR/NO and HDS activity [START_REF] Topsøe | Characterization of the structures and active sites in sulfided CoMo Al2O3 and NiMo Al2O3 catalysts by NO chemisorption[END_REF][START_REF] Topsøe | IR and AEM evidence for the edge location of the promotor atoms in Co-Mo-S type structures[END_REF][START_REF] Topsøe | Active sites and support effects in hydrodesulfurization catalysts[END_REF][START_REF] Topsøe | IR Spectroscopic Studies of the Nature of Surface Sites in Hydrotreating Catalysts[END_REF]. In agreement with CoMoS model, IR/NO shows that optimizing the MoS 2 slab dispersion and their edge coverage by promotors leads to improved HDS activity.

NO adsorption has been extensively used for sulfided catalysts characterization by several groups [START_REF] Portela | The adsoprtion of Nitric Oxide on supported Co-Mo Hydrodesulfurization Catalysts: a Review[END_REF][START_REF] Parker | IR spectroscopy of hydrotreating catalysts[END_REF][START_REF] Ramírez | Effect of boron addition on the activity and selectivity of hydrotreating CoMo/Al2O3 catalysts[END_REF][START_REF] Steinfeldt | VOx species on alumina at high vanadia loadings and calcination temperature and their role in the ODP reaction[END_REF][START_REF] Reinhoudt | Characterization of the active phase in NiW/γ-Al2O3 catalysts in various stages of sulfidation with FTIR(NO) and XPS[END_REF]. However, NO has some drawbacks, the main one been its reactivity that may lead to corrosive adsorption on sulfide catalysts [START_REF] Shuxian | X-ray photoemission study of oxygen and nitric oxide adsorption on MoS2[END_REF][START_REF] Redey | The surface chemistry of molybdena-alumina catalysts reduced in H2 at elevated temperatures[END_REF][START_REF] Goldwasser | On the surface chemistry of molybdenaalumina catalysts prepared from Mo(CO)6[END_REF]. At the same period, IR spectroscopy of adsorbed CO was also developed and prove to be a powerful alternative for sulfide catalysts characterization. CO adsorption can be implemented at low temperature (~100 K), which suppress the potential reactive interactions with some surface species, as well as of enhancing the interaction of CO with the weak adsorption sites of the sulfide phase and of the support. IR/CO gives rise to two main bands specific of unpromoted Mo sites (at ~2110 cm -1 ) and CoMoS sites (at ~2070 cm -1 ) (or NiMoS at ~2122 cm -1 ) [START_REF] Bachelier | Carbon-Monoxide and oxygen as probe molecules for HDS active-sites[END_REF][START_REF] Bachelier | Sulfided cobalt-molybdenum/alumina catalysts: carbon monoxide chemisorption and surface structures[END_REF][START_REF] Delgado | Site characterizationon reduced and on sulfided Mo/Al2O3 catalysis by adsorption of probe molecules[END_REF][START_REF] Zaki | Low-temperature CO adsorption and state of molybdena supported on alumina, titania, ceria and zirconia -An Infrared Spectroscopic investigation[END_REF][START_REF] Angulo | Formation of promoted sites from Ni(CO)4 decomposition on sulfided Mo/Al2O3 catalysts. A comparative IR study with conventional Ni-Mo/Al2O3 catalysts, using carbon monoxide as a probe molecule[END_REF][START_REF] Qin | Characterization of sulfided Co-Mo/Al2O3 by CO NO co-adsorption with IR spectroscopy[END_REF][START_REF] Maugé | FTIR Study of CO Adsorption on sulfided Mo/Al2O3 unpromoted or promoted by metal Carbonyls: titration of sites[END_REF]. The determination of the molar extinction coefficients of these IR/CO band makes possible to account for the amount of sites and thus dispersion and degree of promotion of the sulfided slabs. Parallel between qualitative and quantitative IR/CO data and performances of the sulfided catalyst allows a strong insight into the active sites, as calculation of their TOF [START_REF] Dujardin | Towards the characterization of active phase of (Co)Mo sulfide catalysts under reaction conditions-Parallel between IR spectroscopy, HDS and HDN tests[END_REF][START_REF] Chen | IR spectroscopy evidence of MoS2 morphology change by citric acid addition on MoS2/Al2O3 catalysts -A step forward to differentiate the reactivity of M-edge and S-edge[END_REF][START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF] However, accurate spectral analysis points out that more than the two main bands initially quoted are presented in the IR/CO signal on sulfide phase. Indeed on unpromoted catalyst, according to the preparation methods (organic additives, temperature and pressure of sulfidation, nature of the support), IR spectra of Mo catalyst can show CO bands at ~2110 but also at 2070 cm -1 [START_REF] Travert | Parallel between infrared characterisation and ab initio calculations of CO adsorption on sulphided Mo catalysts[END_REF][START_REF] Dujardin | Towards the characterization of active phase of (Co)Mo sulfide catalysts under reaction conditions--Parallel between IR spectroscopy, HDS and HDN tests[END_REF]. To go deeper insight into the structures of the edge sites of sulfide slabs parallels between IR/CO experiments and DFT calculations were done [START_REF] Travert | Parallel between infrared characterisation and ab initio calculations of CO adsorption on sulphided Mo catalysts[END_REF][START_REF] Travert | CO Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study[END_REF]. Theoretical calculations allowed to assign them to CO adsorbed on the two types of MoS 2 edges. Further parallel between IR/CO experiments, and high resolution electron microscopy (HREM) and high angle annular dark field (HAADF) observations confirmed their assignments to M-and S-edge sites of MoS 2 slabs. Determination of the molar extinction of these bands make IR/CO a way to account for the average shape of MoS 2 slabs (S-/M-ratio).

In the present studies, we focus on the local structures of promoted edge sites of supported CoMo catalysts. Indeed, at least three components can be discriminated in the CoMoS signal. DFT calculations showed that many parameters can slightly modify the CO frequency such as the location of the CoMoS sites (S-or M-edges), the degree of promotion of the edges, the center for CO adsorption (Mo or Co) [START_REF] Travert | CO Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study[END_REF]. Hence, to elucidate which site structure the experimentally observed CO bands correspond to, we studied sulfided catalysts with (i) different sulfided slabs morphologies by varying the nature of the support (silica, alumina, titania) or adding organic additive (citric acid (CA)), and thus varying the interaction sulfided phase-support, and (ii) step by step Co/Mo ratio changes by in situ introduction of controlled amounts of the Co promotor on MoS 2 slabs presenting different morphologies. This was done using chemical vapor deposition (CVD) using Co(CO) 3 NO), an in situ promotion method developed by our group [START_REF] Maugé | FTIR Study of CO Adsorption on sulfided Mo/Al2O3 unpromoted or promoted by metal Carbonyls: titration of sites[END_REF]. In addition, in order to go deeper and discriminate between the different sites giving rise to bands with close wavenumbers, 2D Inversion of InfraRed Spectra (2D-IRIS) was used. This chemometric method allows recovering contributions of species differing by their affinity with the surface on overlapped spectra [START_REF] Stelmachowski | Speciation of adsorbed CO2 on metal oxides by a new 2-dimensional approach: 2D infrared inversion spectroscopy (2D IRIS)[END_REF]. In the present work, 2D-IRIS was applied to the study of CO adsorption isotherms (at 100 K) of Mo and CoMo sulfide catalysts. The insight provided by IR/CO characterization coupled with 2D-IRIS showed a step forward for characterization of Mo and CoMoS sites of supported sulfide catalysts. Hence, this makes IR/CO able to distinguish for the presence of non-promoted Mand S-edges sites, from partially promoted as well as totally promoted on M-and S-edges sites.

Interestingly, this work allowed establishing relationships between atomic scale site structure and intrinsic HDS activity, and evidenced that the partially promoted M-edge sites present the highest TOF value.

Materials and methods

Catalysts preparation

Oxide form preparation

A series of promoted CoMo catalysts were prepared by wetness impregnation method on different supports: Al 2 O 3 (gamma alumina, 248 m 2 /g, Sasol), SiO 2 (silica gel, 506 m 2 /g, Merk) and TiO 2 (mixture of anatase and rutile phases, 59 m 2 /g, Degussa). Before impregnation, the supports were calcined and sieved between 0.2-0.5 mm. The same molybdenum density (3 atoms Mo per nm 2 ) and Co/(Co + Mo) molar ratio equal to 0.3 were targeted. Heptamolybdate tetrahydrated salt (Alfa Aesar, 99%) and cobalt nitrate (Alfa Aesar, 98%) were used as molybdenum and cobalt precursors respectively. Both precursors were dissolved in an adjusted water volume and dispersed inside the pores of the support by a maturation step of 2 hours. After impregnation, the catalysts were dried at 383 K (3 K/min) during 16 hours and then calcined at 673 K (3 K/min) during 4 hours. So called oxide catalysts are then obtained.

Sulfidation procedures

Ex situ procedure: for ICP and TEM analysis, oxide catalysts were sulfided ex-situ in a glass reactor connected with 10% H 2 S/H 2 (30ml/min) flow at atmospheric pressure. Catalysts were heat from room temperature up to 623 K with a rate of 3 K/min and then maintained at 623K during 2 h. After, the temperature was cooled down to 298 K under N 2 flow (30ml/min). Then the catalyst inside the reactor was introduced in a dry box under Ar flow, to avoid totally air contact.

For IR experiment and activity test, in situ sulfidation in the IR cell and reactor respectively was performed with the same parameters of gas flow, pressure and temperature.

CVD preparation in the IR cell

Two types of Mo/Al 2 O 3 catalysts were prepared by an one-step pore volume impregnation method in the presence or absence of citric acid (CA). The impregnation solutions were firstly prepared with or without citric acid ( was firstly evacuated at 623 K for 1 h in the IR cell with the final pressure reaches 10 -3 Pa. Subsequently, small calibrated doses of Co(CO) 3 NO vapor (0.8852 cm 3 ) at different pressures (precisely measured) was introduced onto the catalyst surface at room temperature. These doses correspond to 12, 21, 32, 50, 85, 115, 185, 290, 394 umol/g catalyst (from 0.07 to 2.32% Co). After that, the sample was re-sulfided at 623 K for 2 hours in 10% H 2 S/H 2 gas flow.

Catalyst characterization

Textural properties

Nitrogen adsorption/desorption isotherms were measured on calcined supports and oxide catalysts at 77 K using Micrometrics Model ASAP 2020 volumetric adsorption analyzer. Samples were degassed at 523 K under vacuum for 12 h prior to analysis. Specific surface areas were determined from the BET equation.

Elemental analysis by Inductively Coupled Plasma (ICP)

Inside the dry box, 50 mg of sample was dissolved in 100 mL of a mixture of hydrofluoric acid, aqua regia, boric acid and water. Quantification of molybdenum, cobalt and sulfur amount present in the catalysts was then determined by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) using a PerkinElmer Optima 330DV ICP instrument.

Transmission Electron Microscopy (TEM)

Transmission electron microscopy (TEM) was performed on a JEOL 2200 FS FEG operated at 200 kV. After ex-situ sulfidation and transfer into the glove box, suspensions in ethanol absolute were prepared. TEM analyses were carried out using a few drops of the suspension on a copper grid. All TEM images were recorded at the same magnification and digitized using a 4092x4092 px 2 GATAN Ultrascan camera. Slab length and stacking degree distributions of sulfide slabs were determined manually from at least 300 slabs. The measure was performed using Mesurim program.

The TEM pictures and image results of analysis are presented in SI.

Infrared spectroscopy

Infrared characterizations were performed in a low temperature cell adapted to adsorb carbon monoxide at very low temperature (100 K). Firstly, oxide catalyst in wafer form was activated in situ at 623 K under 10% H 2 S/H 2 flow during 2 hours. Then, the catalyst was flushed with Ar during 10 minutes and left 1 h under vacuum at 623 K. Finally, it was cooled down to 100 K under high vacuum (1.10 -4 Pa).

CO adsorption was performed in two different ways. The conventional experiments were performed introducing small CO doses by a calibrated volume and then a equilibrium pressure of 133 Pa to obtain sites saturation. Isotherm experiments were also carried out increasing equilibrium CO pressure up to sites saturation. All IR spectra were recorded with a Nicolet Magna 550 FT-IR spectrometer equipped with a MCT detector. Every spectrum was normalized to a disc of 5 mg /cm 2 and all experiments were duplicated (maximum area variation of 5%). Equilibrium/saturated spectra were decomposed using OMNIC program and Voigt function. The wavenumbers of band maxima were determined by polynomial interpolation of the negative peaks of the second derivative of the spectra. The precision on these frequencies was in the order of +/-0.6 cm-1 in the worst case (i.e.

for spectra with S/N ratio of 20) and much higher in all other cases [START_REF] Cameron | Precision in Condensed Phase Vibrational Spectroscopy[END_REF].

Inversion of infrared spectra (2D-IRIS, [33])

2D-IRIS applied to spectra of adsorbed species recorded at equilibrium pressures is a data processing methodology consisting in assessing the energy distribution of the adsorption sites. Such a distribution can be defined as:

where is the reduced Gibbs free energy of adsorption, and is the number of sites characterized by a reduced adsorption Gibbs free energy . Assuming that the local adsorption isotherm has the same functional form for all sites the adsorption isotherm is given by: where is the total uptake at pressure . Inversion of this integral equation allows recovering the distribution function from the experimental isotherm [START_REF] House | Adsorption on heterogeneous surfaces[END_REF][START_REF] Heuchel | Evaluation of the energy distribution function from liquid/solid adsorption measurements[END_REF][START_REF] Dąbrowski | Comparison of energy-distribution functions calculated for gas-solid and liquid-solid adsorption data[END_REF] The above expression can be extended to the evolution of the absorbance of the IR spectra of the adsorbate recorded at equilibrium pressure:

Where is the absorbance measured at wavenumber and pressre and the 2D distribution function is related to the distribution function of adsorption sites by: Where is the absorption coefficient at wavenumber of the species of reduced Gibbs free adsorption energy , and is the surface area of the self-supported disc of catalyst. Inversion of the above integral equation allows recovering the distribution function from the set of experimental spectra recorded at equilibrium pressures of the probe molecule or optical isotherms. The numerical procedure to recover the distributions functions has been detailed

elsewhere [START_REF] Stelmachowski | Speciation of adsorbed CO2 on metal oxides by a new 2-dimensional approach: 2D infrared inversion spectroscopy (2D IRIS)[END_REF]. The analysis of the spectra with 2D-IRIS was carried out with SpectroChemPy [38] using Langmuir local isotherms and a reference pressure p° = 133 Pa (1 Torr).

Catalytic activity: Thiophene test

Thiophene reaction was carried out in a differential reactor at 623 K and atmospheric pressure with a thiophene partial pressure of 8 kPa in a mixture of hydrogen (91.2 kPa) and H 2 S (2.1 kPa).

The outlet gas was analyzed by Varian 3900 gas chromatograph equipped with flame ionization (FID) detector. Thiophene reaction was running during 18 h to obtain a stable reaction rate. The reaction rate was calculated as r HDS = (F/m)•X, where F/m is the molar flow rate of thiophene per gram of sulfided catalyst and X is thiophene conversion which is below 5%. The initial mass of oxide catalyst (around 25 mg precisely weighted) was corrected to take into account the sulfidation step using a correction factor calculated from the infrared experiments. The distribution function obtained on CoMo/Al 2 O 3 catalyst differs from that obtained on the non-promoted catalyst by a shift of the signal related to adsorption on weak Lewis acid sites and OH groups toward higher values of . This reflects a weakening of the acidity of these sites which could originate from the insertion of Co ions in the alumina lattice and/or long-range interaction with the sulfide phase. Regarding the distribution function related to CO adsorption on the sulfide phase, it is striking that while the position of the spot corresponding to the non-promoted sites does not change much ( = 2110 cm -1 ,  =-5), the contributions associated to the broad and intense bands characteristic of the promoted phase appear at significantly more negative values (~ -5.5 --6.5).

Results

General characterizations of (Co)Mo catalysts

This indicates that such bands correspond to CO species more strongly bound to the sulfide phase.

Moreover, the position of the maxima becomes also more negative as the wavenumber decreases, which is consistent with a strengthening of the adsorption with back-donation from the sulfide phase d electrons to the  CO orbitals. This is very different from the case of the non-promoted catalyst for which the distribution function associated to the band characteristic of CO adsorbed on the S-edge is located at significantly higher  values that the one associated to the non-promoted Medge. Moreover, the distribution function obtained for the promoted catalyst does not show any contribution at the position observed for CO adsorbed on the S-edge of the non-promoted catalyst .

This is a strong indication that the correspond sites (Mo centers on non-promoted S-edge) are absent on CoMo/Al 2 O 3 .

A B

The same procedure was applied to silica supported catalysts. Figure 3 shows the spectroscopic isotherms and the distribution functions of the absorbances obtained for nonpromoted and promoted catalysts supported on SiO 2 . The bands corresponding to CO adsorption on silanol groups (2155 cm -1 ), physisorbed and oxysulfide [START_REF] Labruyere | Structure des sites sulfures des catalyseurs d'hydrotraitement: approche combinee par spcetroscopie IR et modelisation moleculaire[END_REF] species (2135 cm -1 ) led for both catalysts to distribution of absorbances located at high  values, indicating a weak adsorption.

Adsorption on the sulfide phase of the non-promoted Mo/SiO 2 led to two resolved bands at 2110 and 2070 cm -1 assigned to CO adsorption on non-promoted M-edge and S-edges, respectively. The and CoMo/SiO 2 were considered as a basis for partitioning the area of the bands assigned to CO adsorption on non-promoted and promoted sites on the spectra obtained at CO saturation. Two peaks at about 2098 and 2110 cm -1 were used for non-promoted sites [START_REF] Dujardin | Towards the characterization of active phase of (Co)Mo sulfide catalysts under reaction conditions-Parallel between IR spectroscopy, HDS and HDN tests[END_REF][START_REF] Le Guludec | From Gas to Liquid Phase Sulfidation: An IR Spectroscopy Study[END_REF]; three peaks at about 2083 cm -1 , 2073 cm -1 and 2057 cm -1 were used to account for the three components characteristic of promoted sites. Low frequency peaks (at 2038, 2020 and 2000 cm -1 ) have also been used to account for the broadening of the CoMoS band in the low wavenumber range. The decompositions were performed using OMNIC program and using Voight functions and are presented in SI.

Quantification of sites was performed distinguishing non-and promoted sites and using extinction coefficients of 15 and 43 cm•µmol -1 respectively [START_REF] Chen | IR spectroscopy evidence of MoS2 morphology change by citric acid addition on MoS2/Al2O3 catalysts -A step forward to differentiate the reactivity of M-edge and S-edge[END_REF][START_REF] Maugé | FT-IR study of carbon monoxide adsorption on sulfided molybdenum/alumina unpromoted or promoted by metal carbonyls: titration of sites[END_REF]. The quantification of the Mo sites and main components of CoMoS sites is summarized in Table 3.

Effect of Co deposition amount on the promoted site bands

The overlapping (CO/CoMoS) bands detected between 2090 and 2000 cm -1 , has to be attributed to specific Co environment as totally or partially promoted edges. For this purpose, CoMoS site formation upon cobalt addition using CVD method was performed in-situ in the IR cell.

Increasing amounts of Co(CO) 3 NO were contacted with sulfided Mo/Al 2 O 3 and Mo(CA/Mo=1)/Al 2 O 3 catalysts and the resulting edge sites were probed by CO adsorption (Figures SI7 andSI8). These two catalysts were chosen since the addition of citric acid leads to a change in the slab morphology of Mo/Al 2 O 3 catalyst, i.e. the catalyst prepared with citric acid has higher S-/M-edge ratio than the one prepared without citric acid [START_REF] Chen | IR spectroscopy evidence of MoS2 morphology change by citric acid addition on MoS2/Al2O3 catalysts -A step forward to differentiate the reactivity of M-edge and S-edge[END_REF]. In order to highlight the position of the different band components, Figure 5 shows the second derivative of the CO adsorption spectra obtained after increasing CVD doses. In both experiments, two CO stretching bands appear at 2055 and 2072 cm -1 from the first CVD dose up to the last one. Both bands increase with Co addition at the same time than the band ascribed to CO in interaction with non-promoted M-edge sites decreases. The only difference between the two samples is the disappearance of the band around 2070 cm -1 ascribed to (CO/S-edge) of unpromoted catalyst, from the first CVD dose indicating the promotion of this edge as well. Thus, these two bands at 2055 and 2072 cm -1 can be ascribed to partially promoted sites, but without any evidence of edge site location. Moreover, a third band appears at 2082 cm -1 with the last Co(CO) 3 NO doses. Accordingly, this 2082 cm -1 band can be attributed to fully promoted edge. This attribution is further confirmed by the fact that the 2082 cm

Catalytic activity

Thiophene HDS activity rate of CoMo catalysts are presented in Figure 6a and Figure 6b on a mass and surface area basis respectively, knowing that the surface area of the support varies but the loading was kept constant at 3 Mo at.nm support -2 . In both cases, CoMo/Al 2 O 3 catalyst presents the highest catalytic activity. However, CoMo/SiO 2 catalyst presents higher HDS rate than CoMo/TiO 2 if the HDS rate is represented per gram of catalyst, while CoMo/TiO 2 catalyst presents higher HDS rate than CoMo/SiO 2 catalyst when HDS rate is expressed per surface area. It is well known that thiophene is not an optimal molecule to study the selectivity of HDS catalysts. However, some differences between the product distributions were found as shows Figure 7. In particular, it appears that CoMo/TiO 2 favors the HYD pathway as was shown for Mo/TiO 2 catalyst since the percentage of THT formed is significantly higher for this catalyst as compared to

CoMo/Al 2 O 3 .

Discussion

Understanding CoMoS site formation is a requirement for the catalyst improvement. For that, series of catalysts using different supports have been studied. The goal of using different supports was to obtain MoS 2 slabs with different morphologies, varying the metal-support interaction, and then to determine the Co affinity for M-and/or S-edge sites. For similar purpose, CVD Co Thus, the edge decoration by Co has been studied by the IR/CO coupled with 2D-IRIS method.

From the stretching frequencies and the Gibbs free energies of CO on the corresponding adsorption site (), it is possible distinguishing M-from S-edge sites for non-promoted catalysts as well as three different promoted sites. The challenge now is to specify the structure of these different

CoMoS sites and to determine if they present different intrinsic activities.

Location of the CoMoS site

In previous studies, metal-support interaction was reported as the main factor that influences MoS 2 slab characteristics [START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF][START_REF] Chen | IR Spectroscopic Evidence for MoS2 Morphology Change with Sulfidation Temperature on MoS2/Al2O3 Catalyst[END_REF][START_REF] Arrouvel | A density functional theory comparison of anatase (TiO2)-and gamma-Al2O3-supported MoS2 catalysts[END_REF]. Indeed, IR/CO spectra clearly shows that S-/M-edge ratio of Mo catalysts change with the support nature. Thus, the strongest metal-support interaction reported for Mo/TiO 2 catalyst leads to a really low S-/M-edge ratio that corresponds to a triangle-like slab shape. Besides, the weakest metal-support interaction of Mo/SiO 2 catalyst leads to a high S-/Medge ratio corresponding to a truncated triangular shape near to a perfect hexagon. And, catalyst with medium metal-support interaction as Mo/Al 2 O 3 catalyst presents slabs of truncated triangular shape in between the two previous catalysts.

In order to gain information on the edge location of the promotor and knowing that the nature of the support drastically changes the S-/M-edge ratio of the supported MoS 2 slabs, the promoted sites/non-promoted sites ratio (promotion degree for CoMo catalysts, calculated from

Table 3) was plotted with the S-/M-edge ratio (morphology for the Mo catalysts) for the three supports as shows Figure 8a. For higher S-/M-edge ratio, higher promotion degree is obtained but without a clear relationship. It can also be noticed that the promotion degree is higher than the S-/M-edge ratio. This result can be interpreted as Co decoration occurs on both S-and M-edge sites of MoS 2 nanoparticles. Additionally, as shows Figure 2

Structure of the CoMoS sites

On the spectra of CVD experiments of the two Mo catalysts, the second derivative method points out the presence of three components whose detection depends on the amount of Co deposited (Figure 5). From these intensity changes with Co/Mo ratio, one can ascribe the bands at 2055 and 2072 cm -1 to partially promoted sites, while that at 2082 cm -1 can be attributed to fully promoted edge. But without any specification about their edge location.

On the spectra of classically prepared catalysts supported on the different oxides, three CoMoS band are also detected at very close wavenumbers. Parallel between the morphology of MoS 2 phase on the different supports (Figure 8a) and the proportion of the three different promoted sites (Figure 8b) was performed to get information about the edge location. *data from [START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF] To go further, the turn over frequency (TOF) of promoted sites on the different supports was calculated. It was calculated taking into account the TOF values obtained previously for nonpromoted sites on Mo catalysts on the three supports [START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF] and the concentration of non-and promoted sites such as :

r HDS = n M-edge • TOF M-edge + n CoMoS • TOF CoMoS ,
where n is site concentration, r HDS is thiophene HDS rate and TOF (turnover frequency). promoted sites allowed to go further in the identification of the most active partially promoted sites.

CoMo/Al 2 O 3 that presents the highest proportion of partially promoted M-edge sites (2072-74 cm -1 ) compared to partially promoted S-edge sites (2054-56 cm -1 ) has the highest TOF CoMoS whereas CoMo/SiO 2 that has a lower proportion of partially promoted M-edge sites compared to partially promoted S-edge sites presents a lower TOF CoMoS .

This highlights that partially promoted M-edge sites present greater intrinsic activity than partially promoted S-edge sites and that totally promoted M-edge sites. Previous works already reported changes in the promoted site properties according to the degree of promotion [START_REF] Arrouvel | A density functional theory comparison of anatase (TiO2)-and gamma-Al2O3-supported MoS2 catalysts[END_REF][START_REF] Walton | MoS2 nanoparticle morphologies in hydrodesulfurization catalysis studied by scanning tunneling microscopy[END_REF]. DFT calculations shows that the sulfur-metal bond energy at the M-edge decreases with the increase of the Co content on this edge. From XPS quantification, DFT models and toluene-hydrogenation measurements, Gandubert et al showed that the mixed Co-Mo sites at the M-edge of CoMoS particles present greater intrinsic catalytic activity than sites with higher Co amount that is in nice agreement with our results. In conclusion, the structure of the promoted sites strongly impacts their intrinsic activities. This study highlights that IR/CO is able to report for the environment of the different promoted sites formed on supported CoMo catalysts and in particular to account for the most active promoted sites i.e. the partially promoted M-edge sites through specific IR/CO band (at 2073 cm -1 ).

Conclusions

The IR spectroscopy of CO adsorbed on supported sulfide CoMo catalysts that present different sulfide slab morphologies (S-/M-edge ratios) , allowed to provide evidence for the presence of promoted sites with different structures. The 2D-IRIS method that is based on the inversion of CO adsorption isotherms obtained by IR spectroscopy was applied for the first time to sulfide catalysts to account for the Gibbs free adsorption energies for each adsorption sites of the support and the sulfide phase. 2D-IRIS method allowed to discard the presence of non-promoted Sedge sites in the contribution characterizing promoted sites (2090-2000 cm -1 ). By applying second derivative functions to the IR/CO spectra of CoMo catalysts, three main components of promoted sites were evidenced at about 2057, 2073 and 2083 cm -1 .

Step-by-step in situ CVD Co deposition on Mo catalysts with different morphologies shows that bands at ~2073 and ~2057 cm -1 characterize partially promoted sites whereas ~2083 cm -1 is specific of completely promoted sites. Further, analysis of the promoted sites bands for catalysts supported on alumina, silica and titania that present different sulfide slab morphologies were performed. Firstly, all the results are in favor of Co promoting M-edge as well as S-edge. Secondly, variations with S-/M-edge ratio of Mo catalysts, with Co/Mo ratio deposited by CVD and parallel with DFT calculated CO frequencies allowed to assign 2072-74 and 2082-85 cm -1 bands to M-edge sites, respectively partially and completely promoted, and 2054-57 cm -1 to S-edge sites, partially promoted. Finally, parallel with HDS activity shows that catalytic activity cannot be directly related to the amount of CoMoS sites for CoMo catalysts supported on different oxides. This points out that the intrinsic activity of the three types of promoted sites is different. Indeed, CoMo/Al 2 O 3 that presents the highest TOF(CoMoS), has the highest proportion of partially promoted M-edge sites. Whereas CoMo/TiO 2 catalyst that presents the lowest TOF(CoMoS), has the lowest proportion of partially promoted M-edge sites as well as the highest proportion of totally promoted M-edge sites. This highlights that partially promoted Medge sites (CO=2072-74 cm -1 ) present greater intrinsic activity than totally promoted M-edge sites (CO=2082-85 cm -1 ). This makes IR/CO not only an important tool for measuring the amount of non-promoted on M-and S-edge sites, and promoted sites but also a way to account for the different types of promoted sites and more specifically the most active types of promoted sites of CoMo supported catalysts.

  wavenumber is recorded for (CO/S-edge) band detected at 2065 cm -1 and (CO/CoMoS) band with a maximum at 2075 cm -1 . For Mo/TiO 2 showing mostly (97%) M-edge sites, the eventual contribution of (CO/ S-edge) in the (CO/CoMoS) massif can be neglected (Figure1C). Thus, one question arises for catalysts that present MoS 2 nanoparticles with S-edge sites: does the massif ascribed to (CO/CoMoS) contain also contribution of the (CO/ S-edge) band? To answer this question, the 2D-IRIS method will be applied to (Co)Mo/Al 2 O 3 and (Co)Mo/SiO 2 catalysts.

Figure 1 . 2 2D

 12 Figure 1. IR spectra of CO at 133 Pa equilibrium pressure for Mo (green line) and CoMo (thick blue line) sulfide catalysts supported on Al 2 O 3 (A) , SiO 2 (B) and TiO 2 (C).

Figure 2 .

 2 Figure 2. Spectroscopic isotherms (top) and 2D-distribution functions of the absorbances (bottom) of CO adsorbed on (A) Mo/Al 2 O 3 and (B) CoMo/ Al 2 O 3 catalysts.

  corresponding distribution functions are close from those obtained for the CO bands assigned to the sulfide phase of Mo/Al 2 O 3 (Figure 2A) with maxima located at close positions. Again, the  value obtained for CO adsorption on the S edge ( = -4,8) was larger than that corresponding to CO adsorption on the M-edge ( = -5,5). By contrast, on CoMo/SiO 2 catalyst,  value obtained for CO adsorption on the M-edge increases ( = -4,8) and the distribution functions associated to the CO bands located at ~ 2080 -2060 cm -1 has maxima located at slightly lower  values. This indicates that these latter sites lead to stronger CO adsorption, in line with the results obtained on Al 2 O 3 .

Figure 3 . 3 . 2 . 3

 3323 Figure 3. Spectroscopic isotherms (top) and 2D-distribution functions of the absorbances (bottom) of CO adsorbed on (A) Mo/SiO 2 and (B) and CoMo/SiO 2 catalysts.

Figure 4 .

 4 Figure 4. Bottom: IR spectra of CO adsorption from small CO doses up to saturation at equilibrium pressure (133Pa). Top: second derivative of IR spectra. A: CoMo/Al 2 O 3 , B: CoMo/SiO 2 ; C: CoMo/TiO 2

- 1 band

 1 starts to be visible earlier for Mo/Al 2 O 3 catalyst than for Mo(CA)/Al 2 O 3 catalyst. Indeed, MoS 2 slabs are smaller on Mo(CA/Mo=1)/Al 2 O 3 and thus present higher amount of total edge sites than Mo/Al 2 O 3 catalyst does [42].

Figure 5 .

 5 Figure 5. Second derivatives of IR spectra of CO on (Co)Mo/Al 2 O 3 catalysts prepared by CVD. (A) Co deposition on Mo/Al 2 O 3 , (B) Co deposition on Mo(CA)/Al 2 O 3 . From violet to yellow: nonpromoted to Co-promoted catalysts at increasing Co content (from 0.07 to 2.32 %wt Co deposited).

Figure 6 .

 6 Figure 6. Thiophene reaction rates per gram (a) and per surface area (b) on promoted catalysts

Figure 7 .

 7 Figure 7. Percentage of different products converted in thiophene HDS reaction for CoMo catalysts supported on Al 2 O 3 , SiO 2 and TiO 2

  , the decrease of non-promoted M-edge sites pointed out on IR spectra of CoMo catalysts and the absence of non-promoted S-edge site evidenced by 2D-IRIS on CoMo/Al 2 O 3 catalysts are also in favored of promoted sites formed on both sulfide slab edges. Nevertheless, one has to recall that changes in morphology between unpromoted and promoted catalysts can occur. STM images had showed on Au support[START_REF] Walton | MoS2 nanoparticle morphologies in hydrodesulfurization catalysis studied by scanning tunneling microscopy[END_REF] the variation of triangular shaped slabs toward more truncated ones by promotion. However, in case where changes of sulfided morphology are unlikely, as for in situ Co deposition on MoS 2 /Al 2 O 3 , the S-edge band is not more detected from the first CVD Co doses on Mo(CA) catalyst. This result is also in favor of promoted sites formed on S-edges. Hence, there are strong indications that Co decoration occurs on both S-and M-edge sites of supported MoS 2 nanoparticles.

Figure 8 .

 8 Figure 8. a) Comparison between MoS 2 morphology detected for non-promoted catalysts and promotion degree obtained for promoted catalysts. b) Contribution of the three main ν(CO/CoMoS) components for CoMo catalysts supported on Al 2 O 3 , SiO 2 and TiO 2 .

  Figure8bpoints out that their proportions strongly depend on the support nature. It appears that the component at 2054-57 cm -1 follows the same trend than the S-/M-edge ratio detected for non-promoted catalyst. It could be then proposed that this component corresponds to partially promoted S-edge sites. On the other hand, the component detected at 2082-85 cm -1 follows the reverse trend. It could be ascribed to totally promoted M-edge sites. As for the band at 2072-74 cm -1 that corresponds to partially

Table 1

 1 summarizes the physicochemical properties of the sulfide catalysts on different supports. It can be noted that CoMo/SiO 2 catalyst loss 2/5 of the surface area presented by the support after the impregnation. This was previously observed on non-promoted catalysts prepared in similar way and ascribed to the small pore size of the support (6 nm)[START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF].ICP analysis of the sulfided samples confirm the target loading in Co and Mo. The sulfur content is high enough for CoMo/Al 2 O 3 and CoMo/TiO 2 to have complete Mo sulfidation. For CoMo/TiO 2 , the S/(Mo+Co) ratio is greater than 2, which accounts for a partial sulfidation of the support itself.

Conversely, the lower S/(Co+Mo) value for CoMo/SiO 2 catalyst suggest uncomplete Mo sulfidation.

Table 1 .

 1 Textural properties and elemental analysis of catalysts Figure SI2 for promoted catalysts supported on Al 2 O 3 , SiO 2 and TiO 2 . The average of slab length and stacking number are summarized in Table 2. CoMo/Al 2 O 3 catalyst shows the smallest slab length and stacking number. CoMo/TiO 2 and CoMo/SiO 2 catalysts have close slab length and CoMo/SiO 2 catalyst have around double stacking number.

	Catalysts	Surface area (m 2 /g) Support Oxide catalyst	% wt Mo	% wt Co	at. ratio S/(Mo+Co)
	CoMo/SiO 2	506	307	16	3	1.7
	CoMo/Al 2 O 3	248	235	8	2	2.1
	CoMo/TiO 2	59	58	2	0.4	2.8*

* TiO 2 support is partially sulfided Transmission electron microscopy was performed to determine the slab length and stacking number for promoted catalysts. A minimum of 60 images (Figure

SI1

) were taken for each sample to be sure about the validity of the measure. Distribution of slabs length and stacking are shown in

Table 2 .

 2 Average slabs length and stacking number for CoMo catalysts supported on Al 2 O 3 , SiO 2 and TiO 2. Mo catalysts supported on Al 2 O 3 , SiO 2 and TiO 2 . The spectra obtained at 133 Pa equilibrium pressure of CO are compared with those obtained on Mo catalysts in Figure 1. The spectra of the three Mo catalysts presents bands at about 2114 and 2070 cm -1 that account for CO adsorption on M-and S-edges Mo sites, respectively. As already shown, the marked changes in band ratios point out that different MoS 2 morphologies were obtained for Mo catalysts supported on Al 2 O 3 , SiO 2 and TiO 2 [29]. Figure 1A shows the comparison of IR spectra of CO adsorption at equilibrium pressure for Mo/Al 2 O 3 and CoMo/Al 2 O

	Catalyst	Average length (nm)	Average stacking number
	CoMo/SiO 2	4.5	3.3
	CoMo/Al 2 O 3	2.9	1.4
	CoMo/TiO 2	4.7	1.7

3 catalysts. (CO/S-edge) band shows a maximum at 2070 cm -1 , while the overlapping stretching bands of (CO/CoMoS) show a maximum at 2074 cm -1 . Similar case is presented for (Co)Mo/SiO 2 catalyst as shows Figure

1B

. Close

Table 3 :

 3 Sites quantification for CoMo catalysts supported on Al 2 O 3 , SiO 2 and TiO 2

		Non promoted sites Promoted sites
	Wavenumber (cm -	2116-2099	Total	Main components
	1 )		2090-2000	2082-2085 2070-2074	2054-2057
	Catalyst	µmol edge sites /g catalyst
	CoMo/SiO 2	25	29	6	9	10
	CoMo/Al 2 O 3	61	72	19	26	17
	CoMo/TiO 2	28	21	9	5	4

  deposition were also performed on alumina-supported MoS 2 slabs with different morphologies.

	HDS rate (mol/(h.kg)) a) HDS rate (mol/(h kg))		1.4E-04	HDS rate (mol/(h.m 2 b) HDS rate (mol/(h m 2 )) ))
					1.2E-04
					1.0E-04
					8.0E-05
					6.0E-05
					4.0E-05
					2.0E-05
					0.0E+00
	CoMo/SiO 2	CoMo/Al 2 O	3	CoMo/TiO 2		CoMo /SiO2 CoMo/SiO 2	CoMo /Al2O3 CoMo/Al 2 O 3	CoMo /TiO2 CoMo/TiO 2
		0.7
		0.6
		0.5
	Selectivity	0.3 0.4					Butane 1 Butene 2 Butene
							THT
		0.2
		0.1
		0.0
			CoMo /SiO2 CoMo/SiO 2	CoMo /Al2O3 CoMo/Al 2 O 3	CoMo /TiO2 CoMo/TiO 2

Table 4 .

 4 Figure 9. Comparison of a) unpromoted and promoted sites concentration, b) concentration of the 3 types of CoMoS sites with thiophene HDS rate for CoMo catalysts supported on Al 2 O 3 , SiO 2 and TiO 2 TOF values (in h -1 ) of non-and promoted sites for CoMo catalysts supported on Al 2 O 3 , SiO 2 and TiO 2 .

	1.40	CoMoS/Mo S-/M-edge
		CoMo /SiO2 CoMo/SiO 2	CoMo /Al2O3 CoMo/Al 2 O 3 CoMo/TiO 2 CoMo /TiO2	CoMo/SiO 2	CoMo/Al 2 O 3 CoMo/TiO 2

Table 4

 4

	shows that the TOF(CoMoS) values are different for the three catalysts. CoMo/Al 2 O 3
	presents the highest TOF(CoMoS) value and the highest promotion factor (TOF (CoMoS)/TOF
	(unpromoted Mo)), whereas CoMo/TiO 2 has the lowest TOF(CoMoS) value as well as the lowest
	promotion factor, and CoMo/SiO 2 presents in-between values. These differences are related to

different proportions of the various types of CoMoS site structures that constitute the promoted sites of these catalysts. Parallel between TOF CoMoS values and proportion of S-and M-edge partially

Acknowledgement

E. Dominguez Garcia thanks the financial support of the French Ministry of Research through her PhD grant. J. Chen thanks The Natural Science Foundation of China (No. 21606047), and Normand IEPE Federation for the financial supports. Erwan Oliviero is thanked for the electron microscopy analysis.

promoted sites, the determination of the location is less direct. One can note the decrease of this component when the component specific to fully-promoted M-edge sites (2082-85 cm -1 ) increases suggesting the assignment of the band at 2072-74 cm -1 to partially promoted M-edge sites.

Previous paper by Travert et al reported the changes of CO stretching vibrations calculated by DFT versus the different promotion degree (0, 25%, 100%) of M-and S-edges of Co-promoted MoS 2 slabs [START_REF] Travert | CO Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study[END_REF]. The DFT calculations show that promoted sites can present many different structures giving rise to different (CO) frequencies some being separated by only few cm -1 (Table SI4). This points out that the calculated wavenumbers cannot be directly used to assign the bands experimentally observed. However, analysis of the calculated values on CO frequencies of the different sulfide sites, shows interesting trends: i) the (CO) frequency of the fully-promoted sulfide sites is greater than that of partially promoted sulfide sites , which well agrees with the attribution we previously drawn from CVD experiments, ii) for similar site structure, the (CO) wavenumber of CO on M-edge sites is higher than the wavenumber of (CO) on S-edge sites. This last point is an additional support to propose that band at 2072-74 cm -1 , that is located at greater wavenumber than 2054-57 cm -1 band, correspond to partially promoted sites on M-edge.

Finally, from the advanced IR/CO results obtained by 2D-IRIS, Co CVD and comparison between (Co)Mo supported on the three different supports and from comparison with the trends resulting from the DFT calculation, the structure of the sites corresponding to the three CO/CoMoS bands could be determined.

Influence of the CoMoS site structure on the HDS activity

The metal-support interaction has been revealed as a factor that influences the CoMoS site formation and affects the promotor localization and promotion degree of the edge. Thus, the impact of the CoMoS site structures on the HDS activity has been studied on the CoMo catalysts supported on the different supports. Firstly, Figure 9a that compares the concentration of non-promoted and promoted sites with thiophene HDS rate, does not show any clear relationship. In particular, CoMo/TiO 2 that presents amount of promoted sites higher to that of CoMo/Al 2 O 3 , clearly exhibits a lower activity. This highlights that the concentration of CoMoS sites is an important factor but not the only factor that determines HDS activity. Indeed, Figure 8b points out a nice parallel between the HDS activity and the amount of partially promoted sites (bands at 2072-74 and 2054-57 cm -1 ).

CoMo/Al 2 O 3 that presents the highest concentration of partially promoted sites, has the highest activity. Compared to CoMo/Al 2 O 3 , CoMo/TiO 2 catalyst presents a lower amount of partially promoted sites but a higher amount of totally promoted sites, and a lower activity.


Fengqian Chen

Dr Xinyu Guan

Hui Li

Dr Jiehua Ding

Prof Liangkui S Zhu

Dr Bin Tang
email: bin.tang@deakin.edu.au

Valentin Valtchev

Yushan Yan

Shilun Qiu

Qianrong Fang
email: qrfang@jlu.edu.cn

Dr ] F Chen

Prof Q Qiu

Fang

Prof V Valtchev

Prof Y Yan

Three-Dimensional Radical Covalent Organic Frameworks as Highly Efficient and Stable Catalysts for Selective Oxidation of Alcohols

Keywords: covalent organic frameworks, radical, functionalization, selective, oxidation of alcohol

With excellent designability, large accessible inner surface, and high chemical stability, covalent organic frameworks (COFs) are promising candidates as metal-free heterogeneous catalysts. In this work, we, for the first time, report two three-dimensional (3D) radicalbased COFs, termed JUC-565 and JUC-566, in which radical moieties (2,2,6,6-tetramethylpiperidine-1-oxy, TEMPO) are uniformly decorated on the channel walls via a bottom-up approach. Based on grafted functional groups and suitable regular channels, these materials open up the application of COFs as highly efficient and selective metal-free redox catalysts in aerobic oxidation of alcohols to relevant aldehydes or ketones with outstanding turn over frequency (TOF) up to 132 h -1 , which has exceeded other TEMPO-modified catalytic materials tested under similar conditions. Meanwhile, these stable COF-based catalysts could be easily recovered and reused for multiple runs. This study thus promotes 3D functional COFs anchored with stable radicals for potential applications in organic synthesis and material science.

Scheme 1. Strategy for constructing 3D radical-based COFs. Molecular structures of TEMPO-TPDA (a) as a linear 2-connected building unit as well as TAPM (b) or TAPA (c) as a tetrahedral 4-connected building unit. Two novel 3D radical-based COFs, JUC-565 (d) and JUC-566 (e), prepared by the condensation reaction of TEMPO-TPDA and TAPM or TAPA. A single dia framework (f) and 10-fold interpenetrated dia net (g) in 3D radical-based COFs.

The stable organic nitroxyl radicals, such as 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO), have been widely studied as highly efficient metal-free redox catalysts. [4] Individual TEMPO molecules and their derivatives have been employed in a broad range of chemical transformations, but the recovery and reuse of catalysts are highly desirable since TEMPO is considerably expensive. As a result, various supporting materials have been studied for the immobilization of TEMPO, such as silica, amorphous polymers, and metal-organic frameworks (MOFs). [5] However, it was still difficult to prepare catalysts combining excellent recyclability and high catalytic activity due to irregular pores or poor stability in traditional supporting materials. In our considerations, COFs might be promising candidates for TEMPO immobilization and corresponding redox catalysis due to their large exposed inner surface, uniform nano-channels, highly designable structures, and excellent chemical stability. Nevertheless, only a few reports of TEMPO-decorated COFs were available to date, [6] and they were limited to post-modified 2D frameworks, which has extremely restricted their performance. Herein, we report two 3D TEMPO-based COFs via bottom-up strategy, termed JUC-565 and JUC-566. Different from previously reported 2D TEMPO-based COFs, TEMPO groups as radical moieties were uniformly anchors on the channel walls with high density. Since the whole framework was connected via strong covalent bonds, these materials showed high chemical stability in strong basic or acidic aqueous solutions. Based on rich radical units, interconnected channels and outstanding stability, these functional COFs have been demonstrated to be highly selective and reusable metal-free redox catalysts for the application in aerobic oxidation of alcohols to relevant aldehydes or ketones with remarkable turnover frequency (TOF) as high as 132 h -1 , which is the highest ever reported. To the best of our knowledge, this study represents the first case of 3D functional COFs decorated by TEMPO radicals and their application as metal-free redox catalysts for aerobic oxidation of alcohols.

The synthesis of 3D radical-based COFs was carried out by solvothermal condensation of TEMPO-TPDA (TPDA = [1,1′:4′,1″terphenyl]-4,4″-dicarbaldehyde), and 1,3,5,7-tetrakis(4-aminophenyl) adamantane (TAPA) or tetra(4-aminophenyl)methane (TAPM) in a mixture of dioxane, mesitylene, and acetic acid at 120 °C for 3 days (Scheme 1). A variety of methods have been utilized for structural confirmation. Scanning electron microscopy (SEM, Figures S1 andS2) images demonstrated isometric nanocrystals building random aggregates for both COFs. Fourier transform infrared (FT-IR) spectra showed new peaks at 1658 cm -1 for both COFs, which is a typical characteristic of the imine bond. At the same time, almost complete disappearance of C=O (1696 cm -1 for TEMPO-TPDA) and N-H stretching (~3332 cm -1 for TAPA and ~3394 cm -1 for TAPM) proved well transformation of aldehyde and amine groups (Figures S3 andS4). Furthermore, thermogravimetric analysis (TGA) illustrated good thermal stability (~300 °C under N 2 ) of 3D radical-based COFs (Figures S5 andS6). Outstanding chemical stability was also proved by the preserved PXRD patterns after immersing in different organic solvents and strong basic (1 M NaOH) or acidic (1 M HCl) aqueous solutions for one week (Figures S7-10). Moreover, the radical character was probed by electron paramagnetic resonance (EPR) spectroscopy at room temperature showing peaks at g = 2.0096 for JUC-565 and 2.0097 for JUC-566, respectively, which is well consistent with that of TEMPO radical (g = 2.0097, Figures S12-14). The crystalline structures were verified by the powder X-ray diffraction (PXRD) and structural simulations (Figure 1). After a geometrical energy minimization by the Materials Studio software package [START_REF]Materials Studio ver. 7.0[END_REF] based on 10-fold interpenetrated dia nets and disordered pendant functional groups, [START_REF] Bonneau | [END_REF] S2 andS3). The simulated PXRD patterns were in good agreement with the experimental ones. Moreover, the experimental PXRD patterns were carried out by full profile pattern matching (Pawley) refinements. Peaks at 4.31, 6. 14, 6.76, 8.68, 9.77 and 13.15° for JUC-565 belong to the (110), ( 200), ( 210), ( 220), (310), and (330) Bragg peaks of space group P-4 (No. 81); peaks at 3.97, 5.70, 6.39, 8.05, 9.01 and 12.54° for JUC-566 correspond to the (110), ( 200), ( 210), ( 220), (310), and (330) Bragg peaks of the space group P-4 (No. 81). The calculated results were well consistent with the measured ones with good agreement factors (Rp = 1.15% and ωRp = 1.65% for JUC-565; Rp = 0.73% and ωRp = 1.09% for JUC-566). In addition, we also tried alternative structures with 9-or 11-fold interpenetrated dia topology; however, there was an apparent difference between the simulated and experimental PXRDs (Figures S15-22). Also, the PXRD pattern of JUC-565 was in good agreement with that of LZU-79 with an approximate length of linkers, and 10-fold interpenetrated dia net, which has been proved by its single-crystal X-ray diffraction (Figure S23). [9] In terms of these results, the obtained COFs are proposed to be the expected architectures possessing microporous channels (~0.8 nm for JUC-565 and ~0.9 nm for JUC-566), which are uniformly decorated by TEMPO radicals (Figure 2 and Figure S24). The porosity and specific surface areas of 3D radical-based COFs were determined by N 2 adsorption measurements at 77 K. As shown in Figure 3, both COFs exhibited sharp gas uptake at low pressure (below 0.1 P/P 0 ), which reveals their microporous nature. An inclination of the isotherm in the 0.8-1.0 P/P 0 range and slight desorption hysteresis can be observed due to the presence of textural mesopores. [10] The Brunauer-Emmett-Teller (BET) surface areas were calculated as 313 m 2 /g for JUC-565 and 482 m 2 /g for JUC-566 (Figures S25 andS26), which are much higher than those from 2D radical COFs, i.e., only 5.2 m 2 /g for [TEMPO] 100% -NiP-COF [6a] and 34 m 2 /g for T-COF-100. [6b] The nonlocal density functional theory (NLDFT) was utilized to estimate their pore size distributions. Both COFs demonstrated microporous cavities with ~0.9 nm for JUC-565 and ~1.0 nm for JUC-566 (Figure 3, inset), which are in good agreement with pore sizes predicted from their crystal structures.

Encouraged by the abundant TEMPO radicals, good chemical stability, high surface areas and suitable regular pores of 3D radical-based COFs, JUC-566 was chosen to evaluate the catalytic performance of a metal-free heterogeneous catalyst for aerobic oxidation of hydroxyl to carbonyl. Typically, the catalytic reaction was carried out in a glass vial with COF catalyst (5.0 mol% based on nitroxide radical), NaNO 2 (30.0 mol%) as dioxygen activator, 1,3-dibromo-5,5-dimethylhydantoin (DBDMH, 7.5 mol%) as co-catalyst, and anhydrous acetic acid as solvent (AcOH, 0.5 mL) under oxygen atmosphere (O 2 balloon) at room temperature. As shown in Table 1, successful oxidation of a broad spectrum of alcohols to corresponding carbonyl compounds with high yields was achieved. For example, the reaction of benzyl alcohol could be completed in 10 min with a 99% yield (enter 1). Notably, its turn over frequency (TOF) was calculated as high as 132 h -1 , which has surpassed previous reported TEMPO-anchored materials under similar reaction conditions (Table S1), such as MOFs (19.2 h -1 for UiO-67-TEMPO), [11] amorphous polymers (40 h -1 for TEMPO-CMP-4), [12] ordered mesoporous silica (111 h -1 for SBA-15-ABNO), [13] oxide nanoparticles (125 h -1 for MNST). [14] The para-substituted benzyl alcohols could also be transformed to the corresponding aldehydes in almost quantitative yields (Table 1, entries 2-6). Meanwhile, increasing reaction time was required with larger substrates, possibly triggered by the slow diffusion through COF channels. Subsequently, the reaction was extended to some other alcohols, including secondary alcohols, which will produce ketones (entries 7 and 8). Noteworthy, all selected substrates demonstrated high yields, illustrating the generality of this catalysis system. To further confirm the function of JUC-566 in the catalytic transformation, we attempted to remove it by filtration after 15 mins of reaction, and an immediate termination of reaction was observed (Figure 4a). This result prove the heterogeneous nature of catalytic reaction. More importantly, thanks to the uniform channels of the frameworks, JUC-566 demonstrated an amazing size slectivity. For instance, only 5% of the target product was detected even after 60 min under similar conditions for 3,4,5-trimethoxybenzyl alcohol, which possessed the dimension of 1.0 × 1.1 nm (larger than the aperture of JUC-566 of about 0.9 nm, entry 9, Figure 4b). Meanwhile, a controlled experiment was also carried out with the catalysis of TEMPO molecules under similar conditions, and 99% transformation of 3,4,5-trimethoxybenzyl alcohol was achieved in 20 minutes (Table 1, entry 10). These results illustrated that the catalytic reaction took place in the COF's channels and that JUC-566 possessed excellent size selectivity over substrate dimensions. After that, the cycling performance of this catalyst was also explored with 4-nitrobenzyl alcohol as the substrate. After the reaction was completed, the catalysts could be recovered by simple filtration and used for the next run. After five cycles, a high yield of 95% could still be observed (Figure 4c). Furthermore, the preserved PXRD pattern of recovered JUC-566 confirm its structural integrity after the recycle tests (Figure S11). In summary, through using a pre-designed TEMPO-based monomer, we develop two robust 3D radical COFs, JUC-565 and JUC-566. These materials are provided with high stability, large surface areas, suitable regular pores, abundant and accessible TEMPO moieties on the channel walls. Combining high efficiency and good reusability, 3D radical-based COFs exhibit extraordinary selective catalytic performance for aerobic oxidation of various alcohols to the corresponding aldehydes or ketones with excellent TOF up to 132 h -1 , which is much higher than other reported TEMPO-decorated catalytic materials under similar conditions, including MOFs, silica, amorphous polymers, and oxide nanoparticles. These results thus open an avenue towards designing 3D functionalized COFs and promote their applications in organic synthesis and material science.

Figure 1 .

 1 Figure 1. PXRD patterns of JUC-565 (a) and JUC-566 (b).

Figure 2 .

 2 Figure 2. Extended structure of JUC-565: (a) single dia network and (b) 3D porous framework viewed along the c axis. The pink balls represent TEMPO radicals. C, green; H, yellow, N, blue; O, pink.

  their unit cell parameters were acquired (a = b = 28.838 Å, c = 7.1132 Å and α = β = γ = 90° for JUC-565; a = b = 31.438 Å, c = 7.6704 Å and α = β = γ = 90° for JUC-566; Tables

Figure 3 .

 3 Figure 3. N 2 adsorption-desorption isotherms at 77 K for JUC-565 (a) and JUC-566 (b). Inset: pore-size distribution calculated by fitting on the NLDFT model to the adsorption data.

a

  Reaction conditions: 0.2 mmol of alcohol, 8.0 mg of JUC-566 (5.0 mol% of nitroxide radical), 4.2 mg of NaNO 2 (30.0 mol%), and 4.3 mg of DBDMH (7.5 mol%) in 0.5 mL of CH 3 COOH with an O 2 balloon at room temperature. b Determined by 1 H NMR spectroscopy. c Reaction conditions: the catalyst was replaced with TEMPO molecule and all other conditions remained the same.

Figure 4 .

 4 Figure 4. (a) Yield vs time curve for aerobic oxidation controlled by the addition or removal of COF crystals. (b) Yield vs time curves for aerobic oxidation with substrates of different sizes. (c) Recyclability study of aerobic oxidation with 4-nitrobenzyl alcohol as the substrate.

Table 1 .

 1 Aerobic oxidation of alcohols using JUC-566 a .

	entry	alcohol	product	time	yield	b
	1			10 min	99%
	2			10 min	99%
	3			20 min	99%
	4			30 min	99%
	5			40 min	98%
	6			40 min	99%
	7			40 min	98%

Acknowledgments

This work was supported by National Natural Science Foundation of China (22025504, 21621001, and 21390394), 111 project (BP0719036 and B17020), China Postdoctoral Science Foundation (2020TQ0118 and 2020M681034), and the program for JLU Science and Technology Innovative Research Team. V.V., Q.F. and S.Q. acknowledge the collaboration in the framework of China-French joint laboratory "Zeolites".

Conflict of interest

The authors declare no conflict of interest. Received: ((will be filled in by the editorial staff)) Published online on ((will be filled in by the editorial staff))


Stephan N Steinmann
email: stephan.steinmann@ens-lyon.fr

Wei Zhi

Seh

Understanding electrified interfaces

Understanding electrified interfaces is crucial to enabling a multitude of applications, including photo(electrocatalysis), super-and pseudocapacitors, batteries, etc. However, reaching an atomistic understanding of electrified interfaces remains challenging and will require the combination and development of refined computations and experiments.

Electrified interfaces are a key component of a variety of technologies, including (photo)electrocatalysis for the production of chemicals (for example, H2, NH3 and CO2 reduction products) [START_REF] Seh | Combining theory and experiment in electrocatalysis: Insights into materials design[END_REF] , super-and pseudocapacitors, large-scale redox flow batteries and portable lithium-ion batteries.

Electrified interfaces consist of an electrode and a liquid or solid electrolyte. This contact leads to the formation of an electric double layer. Practical electrodes expose a high surface area to maximize the contact with the electrolyte. Since molecules and ions coming from the bulk need to reach the electrode, these interfaces are generally non-crystalline and are, thus, complex at the atomic level. Moreover, if an (electro-)chemical reaction occurs at the electrified interface, the system is in a non-equilibrium state. Therefore, single-crystal surface models do not adequately describe electrified interfaces, and atomistic modelling is pushed to its limits. 2

[H1] The challenge of atomistic description

For a realistic description of the physical and chemical processes in large, disordered systems, such as electrified interfaces, atomistic simulations need to be performed over sizeable timescales (for nanoseconds if coupled to biased exploration, otherwise even longer). Furthermore, electronic structure methods or interatomic potentials that capture the polarization in the presence of an electrochemical potential need to be considered to capture (electrochemical) reactivity. The experimental characterization of electrified interfaces is also challenging. Compared to the bulk electrode and electrolyte, the number of atoms forming the interface is generally small, so they tend to be difficult to detect experimentally. Furthermore, the non-crystalline nature of practical electrified interfaces prohibits the use of diffraction techniques, which are the most suitable methods to achieve atomic resolution. Nevertheless, advanced surface-sensitive techniques, such as operando variants of spectroscopies and microscopies, promise (near) atomic resolution and chemical characterization of electrified interfaces in the next decade under operating conditions.

To gain a deeper understanding of electrified interfaces, and thus, finer handles to design improved interfaces, the community needs to embrace the challenge of reaching an atomistic description. [START_REF] Seh | Combining theory and experiment in electrocatalysis: Insights into materials design[END_REF] Here, we suggest three complementary avenues to tackle this challenge: (a) autonomous workflows, (b) machine learning and (c) integrated computations and experiments. Our discussion is geared towards electrocatalysis; however, most aspects also apply to other electrified interfaces.

[H1] Computational screening and autonomous workflows High-throughput and high-performance computing enables fast computational screening. Single crystal surfaces without a detailed description of the electrolyte are highly simplified models for electrified interfaces. Their simplicity enables efficient screening of electrode materials for various systems, such as catalysis and batteries. The systematic use of computations can guide experiments, but should also incorporate a feasibility component, that is, addressing the stability of the proposed active sites and materials. The screening of surfaces for assessing their activity and stability does not, in principle, require significant user knowledge and, thus, lends itself for high-throughput computing. [START_REF] Back | Discovery of Acid-Stable Oxygen Evolution Catalysts: High-Throughput Computational Screening of Equimolar Bimetallic Oxides[END_REF] However, the corresponding infrastructure for system setup and analysis largely relies on inefficient, inhouse scripting and programming. Therefore, these approaches require human intervention and remain time-consuming in practice. One of the potential ways to address these issues is to develop autonomous workflows.

Autonomous workflows are highly beneficial to perform detailed mechanistic studies. In particular, they can be used in evaluation of relevant surface states as a function of the operating conditions (electrochemical potential, solvent and supporting electrolyte), evaluation of energies of various reaction intermediates by automatic 'docking' of intermediates on the surface, and determination of their energies as a function of the electrochemical potential by grand-canonical density functional theory (DFT). The identified intermediates should also be allowed to 'back-couple' to the surface state, i.e., the intermediates, rather than species derived from the solvent and electrolyte, can become the dominating species on the surface. In addition, to make experimentally relevant predictions, micro-kinetic models are required to describe the overall activity and selectivity. However, such models rely on the determination of the relevant activation energies, for which no generally accepted method exists for reactions at electrified interfaces.

[H1] Limitations of machine learning

Machine learning has the potential to change our workflows and (atomic) vision of electrified interfaces. Here, we focus on atomistic modelling, which excludes more global system-based machine learning techniques, such as surrogate models and data-driven rationalizations and predictions. [START_REF] Toyao | Machine Learning for Catalysis Informatics: Recent Applications and Prospects[END_REF] At the atomic scale, machine learning can play several roles, including the generation of relevant geometries for high-throughput computing and autonomous workflows, automation of decision making during autonomous workflows (for example, which intermediates to follow, which transition states are relevant), and powering of atomistic simulations, that is, replacing costly DFT computations by faster energy evaluations. The possibility to reach (near) DFT accuracy for orders of magnitude larger systems with orders of magnitude lower computational cost is enticing: it would allow computations of larger, and thus more realistic, models for electrified interfaces as compared to what is attainable with DFT. However, powering atomistic simulations with machine learning potentials is not trivial, in particular, for electrified interfaces.

The fundamental problem is that the interatomic potentials of many low-cost machine learning potentials do not consider atomic charges and polarizability. These physically motivated quantities may not be necessary for the computation of organic molecules and certain materials, [START_REF] Bartók | Machine learning unifies the modeling of materials and molecules[END_REF] but are key to the understanding of electrified interfaces. The electronic structure (and thus, the chemical properties) of electrodes, adsorbates and molecules depend on the electrochemical potential. If electrons are properly described in the simulations, the electrochemical potential can be accounted for by surface charge, electric fields, or explicit cation-electron pairs, since all three methods allow tuning of the Fermi level and thus the energy of electrons. [START_REF] Abidi | Atomistic modeling of electrocatalysis: Are we there yet?[END_REF] By contrast, low-cost machine learning potentials do not define a Fermi level. System-specific potentials can still mimic similar effects via geometrical parameters, which may explain the attractiveness of such approaches. [START_REF] Batchelor | High-Entropy Alloys as a Discovery Platform for Electrocatalysis[END_REF][START_REF] Chen | Identifying Active Sites for CO2 Reduction on Dealloyed Gold Surfaces by Combining Machine Learning with Multiscale Simulations[END_REF] However, this pragmatic solution leads to 'disposable' interatomic potentials (force fields), that is, these potentials are useful only for the specific application for which they have been designed.

To overcome the limitations of machine learning potentials and to enable computation of electrified interfaces, more physically motivated functional forms need to be developed and adopted. [START_REF] Xie | Incorporating Electronic Information into Machine Learning Potential Energy Surfaces via Approaching the Ground-State Electronic Energy as a Function of Atom-Based Electronic Populations[END_REF] In addition, training sets should be made available in the form of curated datasets, similar as for molecular chemistry (quantum-machine.org). For example, the NOMAD Repository and Archive (nomad-lab.eu) could be used to centralize the available datasets and to compare results from various computational setups for identical systems. Combining datasets of various origins via machine learning (typically transfer learning) would allow the 'homogenization' of inhomogeneous data, further increasing the reusability of the original data.

[H1] Integrating computations and experiments

A general challenge for electrified interfaces is the lack of a gold standard for electronic energy evaluations for surfaces and solids. By contrast, in molecular chemistry, complete basis set coupled cluster singles, doubles and perturbative triples, or CCSD(T), computations are widely accepted as gold standard for all but the most problematic electronic structures. Carefully converged quantum Monte Carlo simulations could provide the required accuracy for interfaces; however, the computational cost is currently only compatible with small model systems. [START_REF] Doblhoff-Dier | Quantum Monte Carlo Calculations on a Benchmark Molecule-Metal Surface Reaction: H 2 + Cu(111)[END_REF] In other words, the accuracy of commonly available DFT data compared to the reality (i.e., experiment or exact quantum mechanics) might be insufficient to produce experimentally relevant data. As a consequence, reproducing DFT data at a lower computational cost (i.e., via machine learning potentials) might not be sufficient to achieve physically relevant results. Therefore, experimental validation of computational insights remains indispensable for the foreseeable future.

To validate computational models, surface-sensitive operando techniques need to be developed to characterize electrified interfaces in detail. Such techniques can be broadly classified into spectroscopic and microscopic methods. Spectroscopic techniques, such as Xray photoelectron spectroscopy, extended X-ray absorption fine structure, Fourier-transform infrared spectroscopy, Raman spectroscopy and ultraviolet-visible spectroscopy, can provide valuable information about the chemical species present at interfaces. For example, ambient pressure X-ray photoelectron spectroscopy performed at a synchrotron can be used to characterize solid/liquid interfaces, provided that the electrolyte layer is thin enough (< 20 nm) to allow measurement. [START_REF] Handoko | Understanding heterogeneous electrocatalytic carbon dioxide reduction through operando techniques[END_REF] Scanning probe microscopy, such as scanning tunnelling microscopy, atomic force microscopy and scanning electrochemical microscopy, enable highresolution topographic imaging of interfaces with atomic resolution. For example, scanning electrochemical microscopy, which uses a scanning tip as a miniaturized electrode, can be applied to study the local electrochemical behaviour and surface reactivity of liquid/solid/gas interfaces. [START_REF] Handoko | Understanding heterogeneous electrocatalytic carbon dioxide reduction through operando techniques[END_REF] Liquid phase-transmission electron microscopy has also emerged as a promising technique to study real-time morphological and compositional changes in the presence of flowing electrolytes under controlled electrochemical conditions. However, several challenges remain in the operation of this technique. The electron beam can induce radiolysis and sample damage, in addition to creating artificial hot spots with high electrochemical activity near the electrode tip. [START_REF] Handoko | Understanding heterogeneous electrocatalytic carbon dioxide reduction through operando techniques[END_REF] Thus, operando characterization techniques need to be explored that reflect real electrochemical conditions, ideally, operating high-throughput using smart automation and robotics.

Finally, high-quality data collected by experimental techniques can be fed into computational models to validate and refine them in a tight feedback loop. Machine learning using deep generative models also enables the inverse design for a directed, automated search of the entire chemical space. Given a specified materials property, the ultimate goal of inverse design is to predict a feasible, optimal structure and composition within the chemical space.

In summary, integrating computations and experiments will transform the way we understand and discover new electrified interfaces and materials in the future.

Acknowledgements

This work was supported by the Singapore National Research Foundation (NRF-NRF2017-04) and by Région Auvergne Rhône-Alpes through the project Pack Ambition Recherche 2018 MoSHi.

Author contributions

Both authors contributed to researching data, discussion of content, writing and reviewing/editing the manuscript.

Competing interests

The authors declare no competing interests.