Luz Zavala Sanchez

Xavier Portier

Françoise Maugé

Laetitia Oliviero

Luz A Zavala-Sanchez

Francoise Maugé

Promoter location on NiW/Al 2 O 3 sulfide catalysts: parallel study by IR/CO spectroscopy and High Resolution STEM-HAADF microscopy

Keywords: HR STEM-HAADF, NiWS, Citric acid, CO adsorption, IR spectroscopy, contrast variations, HDS, promoter location, sulfide nanoparticle morphology

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Graphical Abstract

INTRODUCTION

Nowadays, it is fundamental to direct the efforts in the field of hydrodesulfurization (HDS) science towards highly active and sustainable catalysts. The demand for more ecological petroleum products that have lower sulfur content is growing due to the environmental and health problems caused by SO x emissions. The industrial HDS process uses Co (or Ni)promoted Transition Metal Sulfides (TMS) supported catalysts. The bimetallic sulfide phases, commonly called CoMoS, NiMoS or NiWS phases, corresponds to a nanolayer of Mo(W)S 2 decorated by the promoter at its edges. [START_REF] Topsøe | In Situ Mössbauer Emission Spectroscopy Studies of Unsupported and Supported Sulfided Co-Mo Hydrodesulfurization Catalysts: Evidence for and Nature of a Co-Mo-S Phase[END_REF] For years, the outstanding catalytic properties of NiW-based catalysts have attracted great attention due to its good performance in hydrogenation (HYD) and hydrodenitrogenation (HDN) reactions, especially in severe working conditions. [START_REF] Minaev | NiWS/Al 2 O 3 Hydrotreating Catalysts Prepared with 12-Tungstophosphoric Heteropolyacid and Nickel Citrate: Effect of Ni/W Ratio[END_REF][START_REF] Stanislaus | Recent Advances in the Science and Technology of Ultra Low Sulfur Diesel (ULSD) Production[END_REF] The strong hydrogenation function of WS 2 is also beneficial in hydrocracking (HCK). [START_REF] Füchtbauer | Morphology and Atomic-Scale Structure of Single-Layer WS 2 Nanoclusters[END_REF] In addition, NiW/Al 2 O 3 sulfide catalysts have a high potential for the conversion of heavy fractions. [START_REF] Cooper | Aromatic Saturation of Distillates: An Overview[END_REF] With the incoming emphasis on the production of ultra-low sulfur transport fuels in particular from heavy feedstocks, there is renovated interest in going deep insight into the structure of such NiW/Al 2 O 3 sulfide catalysts. [START_REF] Carlsson | Morphology of WS 2 Nanoclusters in WS 2 /C Hydrodesulfurization Catalysts Revealed by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) Imaging[END_REF] The MoS 2 based catalysts including those promoted by Co and Ni, have been extensively investigated at atomic level through surface science study of model catalysts as well as through theoretical and experimental studies of industrial-like catalysts. By contrast, the WS 2 phase and its promoted counterparts are poorly understood at the atomic level. [START_REF] Füchtbauer | Morphology and Atomic-Scale Structure of Single-Layer WS 2 Nanoclusters[END_REF] Even though NiWS catalysts provide an improvement in the catalytic activity in aromatic hydrogenation, their use is not as widespread as CoMoS system. Often several difficulties arise that limit the use of NiW/Al 2 O 3 catalysts such as (i) the difficulty to reach a high level of sulfidation of the NiW phase due to strong metals-support interaction and (ii) the unfavorable formation of the mixed active phase of NiWS catalyst compared to the Mobased catalysts. [START_REF] Suárez-Toriello | Influence of the Solution PH in Impregnation with Citric Acid and Activity of Ni/W/Al 2 O 3 Catalysts[END_REF][START_REF] Breysse | Influence of Sulfidation Conditions on the Properties of NiW/Al2O3 Hydrotreating Catalysts[END_REF] The lower concentration of NiWS in the catalysts is associated with the differences of sulfidation temperature of the metal supported species, which cause a premature sulfidation of the Ni and a subsequent segregation of the phases of Ni and Wsulfides. [START_REF] Breysse | Influence of Sulfidation Conditions on the Properties of NiW/Al2O3 Hydrotreating Catalysts[END_REF] One strategy to overcome this challenge is the use of chelating agents during the preparation stage of the NiW phase. Indeed, organic chelating agents have been successfully used to prepare highly active Mo-based HDS catalysts with increased promotion degree. [START_REF] Li | Essential Role of Citric Acid in Preparation of Efficient NiW/Al 2 O 3 HDS Catalysts[END_REF][START_REF] Yoshimura | Preparation of Nickel-Tungstate Catalysts by a Novel Impregnation Method[END_REF][START_REF] Lélias | Effect of EDTA Addition on the Structure and Activity of the Active Phase of Cobalt-molybdenum Sulfide Hydrotreatment Catalysts[END_REF][START_REF] Lélias | Effect of NTA Addition on the Formation, Structure and Activity of the Active Phase of Cobalt-molybdenum Sulfide Hydrotreating Catalysts[END_REF] Chelating agents simplify the procedure of catalyst preparation since they are usually applied in a one-step co-impregnation procedure. Chelating agents favor the formation of strong complexes during the preparation and lead to a weakening of the metal-support interaction, all these features limit the separated sulfidation of the metal and promoter. [START_REF] Valencia | Kinetic Study of NiMo/SBA-15 Catalysts Prepared with Citric Acid in Hydrodesulfurization of Dibenzothiophene[END_REF] Among the various organic additives used, addition of citric acid (CA) has been well documented as an effective method to prepare a highly active bimetallic catalysts for HDS reactions. [START_REF] Suárez-Toriello | Influence of the Solution PH in Impregnation with Citric Acid and Activity of Ni/W/Al 2 O 3 Catalysts[END_REF][START_REF] Fujikawa | Highly Active HDS Catalyst for Producing Ultra-Low Sulfur Diesel Fuels[END_REF][START_REF] Santolalla-Vargas | Effects of PH and Chelating Agent on the NiWS Phase Formation in NiW/γ-Al 2 O 3 HDS Catalysts[END_REF] The impact of CA addition on the structure of the active phase has been investigated for both Mo and W based catalysts. [START_REF] Chen | IR Spectroscopy Evidence of MoS2 Morphology Change by Citric Acid Addition on MoS2/Al 2 O 3 Catalysts -A Step Forward to Differentiate the Reactivity of M-Edge and S-Edge[END_REF][START_REF] Chen | Role of Citric Acid in Preparing Highly Active CoMo/Al 2 O 3 Catalyst: From Aqueous Impregnation Solution to Active Site Formation[END_REF][START_REF] Chen | How the CO Molar Extinction Coefficient Influences the Quantification of Active Sites from CO Adsorption Followed by IR Spectroscopy? A Case Study on MoS 2 /Al 2 O 3 Catalysts Prepared with Citric Acid[END_REF][START_REF] Garcia | Effet Du Support Sur La Morphologie et l'activité Des Catalyseurs d'hydrodésulfuration[END_REF][START_REF] Zavala-Sanchez | High Resolution STEM-HAADF Microscopy on γ -Al 2 O 3 Supported MoS 2 Catalyst -Proof by Image of the Changes in Dispersion and Morphology of the Slabs with Addition of Citric Acid[END_REF][START_REF] Chen | Effect of High Pressure Sulfidation on the Morphology and Reactivity of MoS 2 Slabs on MoS 2 /Al 2 O 3 Catalyst Prepared with Citric Acid[END_REF][START_REF] Van Haandel | Influence of Sulfiding Agent and Pressure on Structure and Performance of CoMo/Al 2 O 3 Hydrodesulfurization Catalysts[END_REF] However, for W-based catalysts, no clear evidence of the effect of the additive in the sulfide phase structure has been presented so far.

As a proven technique, low-temperature CO adsorption followed by infrared spectroscopy (IR/CO) has been used to characterize sulfide catalysts since several decades.

It is now well established that IR/CO spectra of unpromoted Mo-based catalysts present two different CO adsorption bands, [START_REF] Bachelier | SULFIDED CoMo/Al2O3 CATALYSTS : CARBON MONOXIDE CHEMISORPTION AND SURFACE STRUCTURES[END_REF] which are located at ~2110 and ~2070 cm -1 that accounts for Mo sites on M-edge and S-edge sites of MoS 2 slabs. Quantification of the sites of these two edges allowed to account for the morphology of the MoS 2 slabs (S-edge/Medge ratio). These results are in agreement with morphologies observed by STEM-HAADF for the same catalysts. [START_REF] Zavala-Sanchez | High-Resolution STEM-HAADF Microscopy on a γ-Al2O3 Supported MoS2 Catalyst-Proof of the Changes in Dispersion and Morphology of the Slabs with the Addition of Citric Acid[END_REF] In addition, very recently through a parallel between IR/CO experiments and DFT calculations, it was possible to point out that the two IR/CO bands at 2126 and 2066 cm -1 observed on sulfide W/Al 2 O 3 catalysts also reflect the M-edge and Sedge sites of WS 2 slabs. [START_REF] Zavala-Sanchez | Structural Sites Investigation and Quantification of WS2/Al2O3 Catalysts : Coupling between IR/CO and DFT Calculations for a Deep Insight into Edge Sites[END_REF] In general, IR/CO is particularly effective for the characterization of surface sites of the sulfide catalysts, since it enables to characterize both the acidity of the support, the amount and the nature of the edge sites of the sulfide sites as well as the sulfide slab geometry. [START_REF] Hadjiivanov | Characterization of Oxide Surfaces and Zeolites by Carbon Monoxide as an IR Probe Molecule[END_REF] The observations by high resolution transmission electron microscopy (HRTEM) provide morphological information like length and stacking of the supported TMS slabs. 27- 30 When it comes to the sulfide slab shape observation, one of the obstacles is that HRTEM images suffer from contrast disturbances due to the support, particularly when this is an insulator as strong as alumina. Only slabs parallel to the electron beam can be observed. In contrast, with high angle annular dark-field scanning transmission electron microscopy (STEM-HAADF), images are formed by collecting electrons scattered at high angles by a dark field annular detector, whereby the intensities of the images depend on the atomic number of the elements. HR STEM-HAADF or Z-contrast image mode offers a significant advantage. Indeed, it facilitates the distinction of the elements according to their atomic number (Z), since the contrast is directly proportional to Z 1.7 , even for objects of a single atomic plane. [START_REF] Nellist | Direct Imaging of the Atomic Configuration of Ultradispersed Catalysts[END_REF] This mode is suitable for the study of small nanoparticles formed by heavy elements, especially if they are deposited on a light-weighted support such as industrial alumina. Despite experimental evidence of differences in activity, NiWS catalysts are mostly assumed to behave similar, in some way, to the much more studied HDS CoMoS catalyst. Lately, STEM-HAADF has been successfully applied to investigate the shapes of WS 2 . Recently, Alphazan et al. [START_REF] Alphazan | Highly Active Nonpromoted Hydrotreating Catalysts through the Controlled Growth of a Supported Hexagonal WS2 Phase[END_REF][START_REF] Alphazan | Improved Promoter Effect in NiWS Catalysts through a Molecular Approach and an Optimized Ni Edge Decoration[END_REF] reported on highly active W-based catalysts that their enhanced activities are explained by the 2D-hexagonal morphology of WS 2 particles.

Nevertheless, some questions remain: what about the morphology of promoted-W catalysts prepared with chelating agents such as citric acid? And more challenging, what about the location of the promoter? And finally, how the promoter decoration pattern is specifically in industrial-type catalysts?

In this work we report the effect of citric acid (CA) as a chelating agent on activity and structure of NiW supported catalysts. This work aims to explain the beneficial role of the citric acid at a molecular level. The formation of NiWS sites during catalyst sulfidation was in situ followed by CO adsorption. The improved thiophene HDS activity on NiW catalysts prepared with CA can be related to the enhanced amount of NiWS sites as evidenced by CO adsorption followed by IR spectroscopy. The consequent modifications of the use of CA on the morphology parameters such as average slab length, stacking and shape of the nanoparticles are analyzed by HR TEM and HR STEM-HAADF.

EXPERIMENTAL SECTION

Catalysts preparation

The

Composition and textural properties

Chemical analysis by Inductively Coupled Plasma

The elemental contents of Ni, W, and S in sulfide catalysts were determined by ICP method. Oxide catalysts were sulfided ex-situ in a glass reactor under 10% H 2 S/H 2 (30 mL min -1 ) flow. Catalysts were sulfided at 673K with a rate of 3 K min -1 during 2h. Then, the temperature was decreased to room temperature under ultra-pure N 2 flow (30 mL min -1 ).

The catalyst inside the reactor was introduced in a dry glove box under Ar flow, to avoid air contact. Afterwards, inside of the dry glove box, 50 mg of sample was dissolved in hydrofluoric acid, aqua regia, boric acid, and water with a total volume of 100 ml. Tungsten and sulfur amounts present in the catalysts were quantified by an inductively coupled plasma atomic emission spectrometry (ICP-AES) using a PerkinElmer Optima 330DV ICP instrument.

Nitrogen physisorption analysis

Nitrogen adsorption/desorption isotherms of oxide and sulfide catalysts were measured at -196 °C using Micrometrics Model ASAP 2020 volumetric adsorption analyzer. Oxide samples were sulfided ex-situ in a glass reactor at 673 K (heating rate of 3 K min -1 ) under 30 ml min -1 of 10% H 2 S/H 2 for 2 h. After that, the reactor was flushed by Ar at 623 K and cooled down to room temperature. Then, the catalysts were degassed at 523 K under vacuum for 12 h prior to analysis. Specific surface areas were determined from the BET equation. The total volume was calculated from the volume adsorbed at P/P 0 = 0.95. Pore size distributions of solids were estimated by Barrett-Joyner-Halenda (BJH) method.

Thiophene hydrodesulfurization test

The thiophene hydrodesulfurization (HDS) test was carried out with a differential glass reactor. Since the catalytic bed is very thin, it can be considered that the specific velocity corresponds to the initial velocity. Sulfidation procedure before thiophene HDS test, was performed at different temperatures (573-773 K) with a heating rate of 3 K min -1 at 0. In order to maintain a differential reactor, the thiophene conversion was controlled below 10%. The reaction rate (r s ) (mol h -1 kg -1 ) was calculated using the following formula:

(

Where x is the thiophene conversion (%), F is the molar flow rate of thiophene (mol h -1 ), m cat is the mass of the catalyst after sulfidation, respectively. The initial catalyst mass was corrected by precisely weighting the catalyst after catalytic test.

Low temperature CO adsorption followed by IR spectroscopy

The series of catalysts were characterized in situ by low-temperature (~100 K) CO adsorption followed by IR spectroscopy (IR/CO). CO adsorption was performed at low temperature to avoid any reaction with the catalyst surface. The oxidic catalyst was grounded and pressed into a self-supported wafer (~10 mgprecisely weighted, for a disc of 2.01 cm 2 ) and positioned into a quartz IR cell with CaF 2 windows. Before to the adsorption experiments, the sample was sulfided in situ in the IR cell. For this, the catalysts was sulfided with a gas mixture containing 10% H 2 S/H 2 (30 ml min -1 ) at 673 K (sulfiding flow starting from room temperature) for two hours with a heating rate of 3 K min -1 . After sulfidation, the cell was flushed with Ar and evacuated down to P<1˟10 -4 Pa until the catalyst reached room temperature. CO adsorption at low temperature (100 K) was carried out introducing controlled CO pressure measured in a standard volume called small volume. These small calibrated doses of CO were introduced into the IR cell until equilibrium pressure of 133 Pa was reached. After each CO introduction, FTIR spectra were recorded with 256 scans and a resolution of 4 cm -1 using a Nicolet Nexus FTIR spectrometer equipped with a MCT detector (FTIR=Fourier Transform Infrared;

MCT=Mercury-Cadmium-Tellurium). All spectra were normalized to a disc of 5 mg cm -2 .

The bands of the adsorbed CO species were obtained by subtracting the spectra recorded after and before CO introduction. The spectra were decomposed with Peakfit V4.12 using ''Autofit peak III-deconvolution'' and considering pseudo Voigt functions. For all the spectra, the second derivative function was used to ensure the good position of the bands in the spectral decomposition. For the decomposition of all spectra, the center and full width at half height (FWHH) of each peak are allowed to vary in a small range (± 3 cm -1 ).

Microscopy observation by HRTEM and HR STEM-HAADF

For microscopy observations, the oxide catalysts were sulfided at 673 K (heating rate of 3 K min -1 ) at a pressure value of 0.1 MPa for 2 h under a 30 mL min -1 flow of 10% H 2 S/H 2 .

Then, to limit detrimental exposure to air, the sulfided catalysts were unloaded from the sulfidation reactor under argon flow and poured into absolute ethanol. A few drops of the suspension of catalyst were put on a 300 mesh copper grid with holey carbon film. The images of sulfided catalysts presented in this work have been taken using a double corrected JEOL ARM 200F cold FEG microscope operated at 200 kV. First, length and stacking degree distributions of sulfide slabs were determined manually by measuring at least 300 slabs per sample from HRTEM images. The image treatment was performed using the commercial software GMS3 from GATAN (DIGITALMICROGRAPH) and free Mesurim software. The average length ( ) and the average stacking degree ( ) were calculated according to the following distribution modes given by the equations ( 2) and ( 3):

(2)

Where ( ) is the average length, n i the number (frequency) of slabs having the length within the discrete interval i (nm), and n, the total number of particles or slabs. The number of slabs per stacked particle was determined to obtain the average stacking degree ( ):

Where n i is the number of slabs in N i layers and n, the total number of particles or slabs. The nanostructure and morphology of the NiWS slabs were analyzed by high resolution scanning transmission electron microscopy (HRSTEM) using a high angle annular dark field (HAADF) detector. All the STEM images presented in this manuscript were acquired in HAADF mode. The acquisition of one image lasted about 30 s with a resolution of 1024 pixels x 1024 pixels, (30 μs of exposure time for each spot in the scanning mode). During this acquisition and despite exposure to the electron beam, the shapes of the particles remained unchanged, meaning that the slabs were quite stable under the electron beam.

RESULTS AND DISCUSSION

Elemental analysis

Elemental analysis was performed by ICP after the sulfidation stage (T s =673K) of the catalysts in order to verify the sulfidation degree of the NiW catalysts (Table I). All the catalysts presented sufficient sulfur to form WS 2 , that is, the molar concentration of sulfur is about twice that of tungsten. It is also observable that the S/W ratio increases with citric acid amount. Thus, this is an indication that the presence of citric acid favors the sulfidation of W.

Textural properties

The nitrogen adsorption-desorption isotherms for both dried (oxidic) and sulfided NiW/Al 2 O 3 catalysts correspond to a type IV isotherm according to IUPAC classification with a H1 hysteresis loop, characteristic for mesoporous materials. [START_REF] Sing | Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity[END_REF] Table II presents the effect of sulfidation on the specific surface area and pore diameter of the catalysts prepared with and without citric acid. It is interesting to note that as the concentration of the citric acid in the impregnation solution increases, the oxide catalyst suffers from a decrease in its surface area, possibly due to the blockage of the pores of the alumina by citric acid molecules. However, when carrying out the sulfidation process, it is observed how the catalysts progressively recover their surface area. This is a consequence of the low decomposition temperature (~448 K) of the citric acid relative to the sulfidation temperature (Ts=673 K). If we consider the surface area (252 m 2 g -1 ) and pore size diameter of the pure γ-alumina carrier (12.5 nm), it is observed that the textural properties are almost maintained at the final stage of the catalyst with just a small decrease in surface area and pore size that is more marked when the amount of CA increases. This is likely related to the rising of the amount of carbonaceous deposits formed by CA on the catalyst. the optimal sulfidation parameters to use for W-based catalysts. In order to guarantee an optimal sulfidation procedure for the series of samples studied in this work, we carried out catalysts activation by varying the sulfidation temperature in the range of 573 K up to 723 K with 50 K intervals. To follow the sulfidation procedure impact, thiophene HDS performances were tested for all these catalysts sulfided in different conditions (Figure 1).

For NiW(CA/Mo=0)/Al 2 O 3 , its HDS reaction rate increases with the sulfidation temperature, it exhibits the lowest HDS rate compared to catalyst prepared with CA.

Moreover, in addition to being the catalyst with the lowest catalytic activity, it is also the only catalyst that shows an increase in reaction rate after sulfidation at 673 K. This indicates that the catalyst is not completely activated even at this high temperature.

This is in agreement with the expected difficulty of sulfidation reported for W based catalysts.

As a general trend, the addition of citric acid increased the catalytic activity as observed for the NiW(CA/Mo= x)/Al 2 O 3 with x=0.5,1.0 and 2.0. It is noticeable that even a small concentration of CA leads to a clear beneficial effect. Moreover, the catalytic tests showed that an increase in the CA content accelerates the rate of reaction at any of the sulfidation temperatures. For the three samples prepared with CA, it was found that the sulfidation temperature at which the highest reaction rate is obtained was 673 K. If the catalysts are sulfided at higher temperature (T s =723 K), a decrease in activity is observed.

This is linked to an agglomeration of the sulfided particles. This detrimental effect of high T s is not observed with the catalyst prepared without CA. This can be explained by an improvement of the sulfidation degree at higher sulfidation temperature that counterbalanced the active phase sintering effect. Also, this can be related to a stronger anchorage of the sulfided phase on the support in absence of CA that limits the sintering.

The beneficial effect of the addition of CA in the catalysts can be explained by the fact that, by adding citric acid, an increase in the amount of Ni-AC complexes that will interact with tungsten is promoted, which makes it possible to form more NiWS active sites. This assumption will be explored by low temperature CO adsorption followed by IR spectroscopy.

Effect of the citric acid amount

The influence of citric acid amount on the thiophene HDS activity of the sulfided NiW(CA/M=x)/Al 2 O 3 is presented in Figure 2. NiW(CA/M = 0)/Al 2 O 3 catalyst exhibits the lowest HDS rate, and a general trend of increasing activity with the addition of citric acid is observed. The thiophene HDS rate gradually increases from 29 mol h -1 kg -1 on NiW(CA/M=0)/Al 2 O 3 to 56 mol h -1 kg -1 on NiW(CA/M=2)/Al 2 O 3 . As already reported, the addition of citric acid in NiWS catalysts increases the catalytic activity. [START_REF] Suárez-Toriello | Influence of the Solution PH in Impregnation with Citric Acid and Activity of Ni/W/Al 2 O 3 Catalysts[END_REF] The catalytic activity was also compared to the counter partner CoMo/Al 2 O 3 . It is observed that the NiW series present enhanced activity and that the increase extent of HDS rate with CA is similar for both systems. characterize the sulfide phase. After evacuation at 298 K, no more CO adsorbed on sulfide sites is detected. As shown in a recent paper, these bands are respectively associated to CO adsorbed on M-edge and S-edge of the WS 2 slabs. [START_REF] Zavala-Sanchez | Structural Sites Investigation and Quantification of WS2/Al2O3 Catalysts : Coupling between IR/CO and DFT Calculations for a Deep Insight into Edge Sites[END_REF] This confirms that the CO adsorption on the unpromoted W sites is weak. Conversely, on NiW(CA/M=0)/Al 2 O 3 catalyst, the main bands observed at 2133 and 2094 cm -1 remain after evacuation at 298 K with decreased intensity. The evacuation also puts in evidence the presence of a new component at 2078 cm -1 . As observed in Figure 3, the presence of low wavenumber bands at ~1930 cm -1 is also noted in Figure 4.

A more accurate view can be obtained by analyzing the spectra for the first doses of CO introduced on NiW catalyst without CA (Figure 5). Figure 5 also presents the corresponding second derivatives calculated in order to specify the position of the various bands. The presence of the bands ascribed to non-promoted sites is also highlighted by this way. In conclusion, it can be considered that CO adsorbed on the sulfide phase of NiW Next, we focus on the assignation of these five contributions. As mentioned before, comparison between spectra obtained on NiW and W catalysts allows assigning the two bands at 2121 and 2066 cm -1 to the CO adsorbed on unpromoted W sites, located on Medge and S-edge respectively. The question now concerns the assignment of the bands at 2133, 2094 and 2075 cm -1 . These wavenumbers are very close to those reported on NiW catalysts by Zuo et al. [START_REF] Zuo | The Formation of the Active Phases in Sulfided NiW/Al2O3catalysts and Their Evolution during Post-Reduction Treatment[END_REF] and Duchet et al. [START_REF] Duchet | Carbon Monoxide and Oxygen Chemisorption and Functionalities of Sulphided Ni-W/Al2O3 Hydrotreating Catalysts[END_REF] The poor spectral quality of the spectra presented in the early work of Duchet et al. [START_REF] Duchet | Carbon Monoxide and Oxygen Chemisorption and Functionalities of Sulphided Ni-W/Al2O3 Hydrotreating Catalysts[END_REF] makes difficult specifying if these bands are characteristics of promoted or unpromoted W sites. Nevertheless, this work shows that increasing the temperature of gas phase sulfidation in the range 673-873 K, preferentially enhances the intensity of the band at ~2090 cm -1 . [START_REF] Duchet | Carbon Monoxide and Oxygen Chemisorption and Functionalities of Sulphided Ni-W/Al2O3 Hydrotreating Catalysts[END_REF] Later, Zuo et al. [START_REF] Zuo | The Formation of the Active Phases in Sulfided NiW/Al2O3catalysts and Their Evolution during Post-Reduction Treatment[END_REF] studied the influence of the catalysts composition: increasing the Ni/W ratio (0.17 to 0.41) increases the CO uptake with development of the three bands. Thus, these bands were ascribed to NiWS sites. It was also reported that the wavenumber of CO band at ~2096 cm -1 is very close to that observed for Ni sulfide catalyst (2098 cm -1 ). Moreover, this band presents a specific behavior upon H 2 treatment of sulfided NiW catalysts. Indeed, it markedly increases whereas the two other bands at 2127 and 2079 cm -1 tend to disappear. Zuo et al. [START_REF] Zuo | The Formation of the Active Phases in Sulfided NiW/Al2O3catalysts and Their Evolution during Post-Reduction Treatment[END_REF] proposed that the three bands characterized interaction between Ni and W but in a different way: the band at ~2096 cm -1 is ascribed to Ni particles perturbed by WS x phase whereas the bands at 2127 and 2079 cm -1 would characterize NiWS sites with different environments.

This interpretation can be refined considering previous assignments based on DFT calculation of CO adsorbed on Ni-promoted system. [START_REF] Travert | Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study[END_REF] Thus, these bands could correspond to NiWS sites but with different degrees of promotion (partially and totally). Hence, the bands at 2094 and 2078 cm -1 would correspond to CO adsorbed on partially promoted sites whereas the band at 2131 cm -1 would correspond to CO adsorbed on totally promoted NiW sites. This is also in agreement with the electron transfer on NiWS that should be from Ni to W. Hence, compared to unpromoted WS 2 sites (2121 cm -1 ), v(CO) wavenumbers on partially promoted sites are detected at lower frequencies (2094 and 2078 cm -1 ). On totally promoted sites, CO only interacts with Ni atoms. Compared to pure NiS x sites (2098 cm -1 ), the CO frequency is indeed at a greater value (2131 cm -1 ). Such a rather high CO wavenumber was also reported for promoted sites of NiMo catalysts (2120 cm -1 ). 37 Hence, CO spectra can account for partially and totally promoted NiWS sites (Table III).

Going further in the assignment of promoted sites to M-and S-edge would require complementary experiments. As for the low wavenumber and small contribution at ~1927 cm -1 , it is ascribed to metallic Ni in accordance to reference. 26

Figure 7 shows the influence of CA amount on the total area of the spectra ascribed to CO in interaction with the sulfide sites. To go further, these spectra were decomposed as shown in Figure 6 and using the parameters mentioned in Table III. An increase of the three ν(CO) bands (2133, 2094 and 2078 cm -1 ) specific of NiWS sites is reported in Figure 7. These three NiWS bands increase in a very similar proportion with CA addition. In parallel and as expected, the intensity of the specific bands of non-promoted W sites (2121 and 2066 cm -1 ) decreases, however slightly, with CA addition. They are detected whatever the CA amount is, indicating that, even for the highest CA amount i.e., for the catalyst that presents the greatest amount of NiWS, some edges of WS 2 slabs stay unpromoted. Figure 7 also points out a parallel between the increase of ν(CO) bands characteristic of NiWS sites and the rise of HDS activity. This parallel, between IR characterization and activity tests,

shows that the increase of HDS activity on catalyst prepared with CA can be related to the enhanced formation of NiWS sites. These results can be compared with the ones obtained by Suárez-Toriello et al. [START_REF] Suárez-Toriello | Influence of the Solution PH in Impregnation with Citric Acid and Activity of Ni/W/Al 2 O 3 Catalysts[END_REF] where it was shown that citric acid coordinates the Ni atoms, The majority species in an acidic pH medium would be under the forms [Ni(HCit)(Cit) (H 2 O) 4 ] 3-and [Ni(Cit) 2 (H 2 O) 4 ] 4-. These ionic species will interact with the support and form AlO-Ni-AC complexes. [START_REF] Suárez-Toriello | Influence of the Solution PH in Impregnation with Citric Acid and Activity of Ni/W/Al 2 O 3 Catalysts[END_REF][START_REF] Santolalla-Vargas | Effects of PH and Chelating Agent on the NiWS Phase Formation in NiW/γ-Al 2 O 3 HDS Catalysts[END_REF] All these features are beneficial for NiWS phase formation. In addition, as shown on Mo-based catalysts, [START_REF] Dominguez Garcia | New Insight into the Support Effect on HDS Catalysts: Evidence for the Role of Mo-Support Interaction on the MoS 2 Slab Morphology[END_REF] CA likely decreases the sulfide phase-support interaction. This will make tungsten more available and allows WS 2 sulfidation at lower temperature avoiding early nickel sulfidation. Thus, this feature should also favored NiWS formation. Moreover, as shown previously, changing the W-support interaction could also affect the morphology of the sulfide phase. But, the Figure 8C graph also shows that the addition of CA significantly increases the number of very small slabs of about 1 nm. Similarly, the average stacking degree (Figure 8D) is not significantly different between the samples (from 2.0±0.1 to 2.3±0.2 when citric acid is added). To get information about the shape of the sulfided particles and their local composition, STEM-HAADF analysis was performed on NiW catalysts prepared without or with CA (Figure 9). It has to be mentioned that, compared to HRTEM, only a limited number of zones of the image are exploitable by STEM-HAADF. Indeed, due to the high metal loading, it is difficult to get the isolated sulfided slabs that are required for obtaining atomic scale observations. Nevertheless, some differences appear between the catalysts prepared without (Figure 9A andB) and with CA (Figure 9C andD). First, regarding the particles greater than 1 nm, no clear difference in their morphology could be evidenced, they all appear as hexagonal shaped more or less rounded. However, the most remarkable features is the detection of tiny particles of less than one nanometer as well as agglomerates of few atoms on NiW(CA/M = 2)/Al 2 O 3 sample (Figure 9C andD). Such small particles could not be observed on catalyst prepared without CA (Figure 9A andB). This is in accordance with the previous results obtained by HRTEM.

The next question was how to get the nature of the atoms observed on the HR-STEM HAADF images, and particularly the nature of the edge atoms. EELS or EDX analyses were carried out but due to the very low signal (CPS), the results were not conclusive regarding the promoter localization. Among the difficulties, one has to quote the very limited amount of matter under the beam for each analyzed spot during the scanning process. The high insulating properties of the support is also a disadvantage for TEM observation. Nevertheless, thanks to the contrast sensitivity of the HAADF technique, it is possible to quantify the variations in contrast between Ni and W atoms. Indeed, they present strong differences in their Z number, contrast in HAADF mode being sensitive to the Z 1.7 , a good contrast between these two atoms is expected (Z 1.7 (W) / Z 1.7 (N) = 5.

22).

This technique can thus be used to study metal atom distribution within monolayer catalysts. [START_REF] Nikulshina | MoW Synergetic Effect Supported by HAADF for Alumina Based Catalysts Prepared from Mixed SiMo n W 12-n Heteropolyacids[END_REF][START_REF] Nikulshina | Enhancing the Hydrodesulfurization of 4,6-Dimethyldibenzothiophene through the Use of Mixed MoWS2 Phase Evidenced by HAADF[END_REF] By this way, it should be possible to obtain intensity profiles from lines of atoms located in different regions of the projected slab.

To perform such analysis, a special attention was taken to analyze non-overlapping particles with monolayer stacking. In addition, images of these isolated slabs present an excellent resolution and show nicely the atomic arrangement in the slab plane. As a matter of fact, the images of the particles answering to these criteria are scarce. A detailed inspection of the particle presented in Figure 10 reveals variations in the contrast of the atoms located at the edges and in the bulk. shows distances between the atoms of 0.311±0.010 nm. This is in good agreement with the distances obtained by EXAFS for W-W atoms (0.316 nm) in WS 2 /Al 2 O 3 . [START_REF] Thomazeau | EXAFS Characterization of New Active Phases for Catalytic Hydrotreatment: Two Cations Disulfide Layers in the MoxW(1-x)S2 Lamellar Solid Solution[END_REF] In the region (A), majority of the atoms of the edges are less bright compared to the bulk (W) atoms of the region B. This suggests the presence of Ni atoms at this sulfided edge. Interactomic distances measured at this edge reach 0.317±0.010 nm. From the atomic model reported by Girleanu et al., [START_REF] Girleanu | Magnifying the Morphology Change Induced by a Nickel Promoter in Tungsten(IV) Sulfide Industrial Hydrocracking Catalyst: A HAADF-STEM and DFT Study[END_REF] the Ni-Ni distances were found to have a value of 0.32 nm, i.e., slightly greater than the one measured in the core of the particle. This is in agreement with the experimental value measured between the atoms of the region A. Thus, the interatomic distance as well as the low contrast of the atoms located at region A, allow to assert the presence of only Ni atoms at this edge.

In the region C ( Analysis of contrast and distance at core and edge of a sulfide particle in another zone was also performed (Figure 11). As for the atoms located in the core of the particle, their contrast stays constant and the interatomic distances oscillate around 0.311 nm. At the periphery of this particle, one can see one atom next to brighter one whose interatomic distance is 0.281 nm. By their side, there are three less bright atoms (with the lowest contrast at the edge), that exhibit an interatomic distance of around 0.317 nm. This

Figure 1 .

 1 Figure 1. Effect of sulfidation temperature of NiW(CA/M=x)/Al 2 O 3 catalysts on the thiophene HDS activity.

Figure 2 .

 2 Figure 2. Effect CA addition on thiophene HDS activity of sulfided NiW(CA/M=x)/Al 2 O 3 catalysts sulfided at 673 K and CoMo(CA/M=x)/Al 2 O 3 catalysts sulfided at 623 K.

3. 4 .Figure 3

 43 Figure3shows the IR spectra of adsorption of 133 Pa of CO (Tads ~100 K) on sulfided

Figure 3 .Figure 4 .

 34 Figure 3. IR spectra of CO adsorption (133 Pa CO at equilibrium, T ads =100K) on NiW(CA/M=x)/Al 2 O 3 catalysts sulfided at 673K. The inset zooms on the low wavenumber bands.

Figure 4

 4 Figure 4 compares spectra taken on NiW(CA/M=0)/Al 2 O 3 and W(CA/M=0)/Al 2 O 3

20 Figure 5 .

 205 Figure 5. IR spectra of first CO doses (0.002 to 0.012 μmol CO·mg -1 ) adsorbed at 100 K on NiW(CA/M=0)/Al 2 O 3 sulfided at 673 K. Each spectrum is presented with its second derivative function (lighter curves).

  and without CA gives rise to at least five bands at 2133, 2121, 2094, 2078 and 2066 cm -1 .

Figure 6 .

 6 Figure 6. Decomposition of the IR spectrum of CO adsorption (PCO=133 Pa at equilibrium, T=100 K) on NiW(CA)/Al 2 O 3 catalyst sulfided at 673 K with Peakfit V4.12.

  hence, it prevents the formation of insoluble Ni species of the [Ni(OH) 2 (H 2 O) 4 ] type in the impregnating solution and promotes the formation of soluble Ni-AC complexes.

Figure 7 .

 7 Figure 7. Effect of citric acid addition on the areas of different bands of the sulfided NiW (CA/M=x)/Al 2 O 3 catalysts obtained by CO/IR. In this graphic the HDS thiophene rate divided by a factor of ten is presented to evidence the parallel growth of certain bands along with the catalytic activity.

3. 5 .

 5 Electron microscopy by HRTEM and STEM HAADF Electron microscopy observations of NiW(CA/M=0)/Al 2 O 3 and NiW(CA/M=2)/Al 2 O 3 was performed by HRTEM and HR-STEM HAADF techniques. The objective was to account for change of sulfide phase morphology i.e. slab size and stacking, shape of the sufide slabs, as well as to try to get information on the nature of the sulfide slab edges.Figure 8 shows typical HRTEM images of NiW(CA/M = 0)/Al 2 O 3 and NiW(CA/M = 2)/Al 2 O 3catalysts. The distributions of length and stacking of the sulfide slabs were obtained after a careful analysis of more than 300 slabs Figure8(C and D). It should be mentioned that in this work, we were able to reliably account for slabs of about 1 nm. These could be obtained by adjusting the local contrast of each zone analyzed of the high resolution pictures and also by taking the images with underfocus to increase the Fresnel fringes of the slabs when parallel to the beam. It was observed that the addition of citric acid does not lead to a significant decrease of the average slab length (3.11 ± 0.14 nm to 2.95 ± 0.14 nm).

Figure 8 .

 8 Figure 8. HRTEM images of NiW(CA/M=0)/Al 2 O 3 (A) and NiW(CA/M=2)/Al 2 O 3 (B), with their slab length distribution (C) and stacking number distribution (D).

Figure 9 .

 9 Figure 9. HR STEM-HAADF micrographs of NiWS particles of NiW(CA/M=0)/Al 2 O 3 (A and B) and NiW(CA/M=2)/Al 2 O 3 (C and D).

Figure 10 .Figure 10 ,

 1010 Figure 10. Variations of contrast and interatomic distances at different zones of a sulfided slab of NiW (CA/Mo=2)/Al 2 O 3 .

Figure 10 -

 10 C), atoms present different contrasts one from each other, this feature suggests different natures of the atoms at this edge. Interactomic distances measured at this edge reach a lower value than that measured in the core of the particles and on edge A. The DFT calculations of Girleanu et al. determined Ni-W distances in the range 0.274-0.285 nm. 43 Thus, the accordance of measured distances in region C as well as the change of contrasts of the atoms located allow to conclude to the presence of both Ni and W atoms at the edge C.

Figure 11 .

 11 Figure 11. HR HAADF-STEM image of sulfided NiW(CA/M=2)/Al 2 O 3 catalyst. Inset of the figure shows the contrast variations and characteristic distances observed in the atoms located in the periphery and in the bulk of the slab.

  indicates the coexistence of Ni and W atoms at the sulfide slab edge. Similar analysis of the several sulfide particles by STEM-HAADF shows that, as expected, only W atoms are present in the core of the particles whereas the edges can expose either exclusively Ni atoms or Ni and W atoms with various configurations. Presence of totally promoted edges and partially promoted edges are highlighted by STEM-HAADF in accordance with IR assignments.The aim of this study was to obtain an atomic level analysis of the structure of the active sites of Ni promoted WS 2 particles supported on γ-Al 2 O 3 prepared with and without citric acid.Thiophene hydrodesulfurization test shows that citric acid addition during the preparation stage markedly increases the catalytic activity. Parallel with IR/CO indicates that this is a consequence of the increased formation of NiWS sites in presence of citric acid. A careful analysis of the IR/CO spectra at various CO coverages was performed. First, it shows that unpromoted sites characterized by bands at 2121 cm -1 (W on M-edge) and 2066 cm -1 (W on S-edge) are still detected whatever the amount of citric acid. Second, it points out that the NiWS sites give rise to three bands. Parallel with previous works allowed to assign the 2094 and 2077 cm -1 bands to partially promoted sites and the band at 2131 cm -1 to totally promoted sites.HRTEM shows that citric acid addition favors the formation of very small particles of about 1 nm. HR STEM HAADF images confirm this and, even provides evidence for the formation of clusters of few atoms. Moreover, by analyzing contrast and interatomic distances on atomic scale images of sulfide particles obtained by HR STEM HAADF, it was possible to identify the nature of the core and edge atoms of the particles. The metal atoms of particle core are only W ones whereas the edges of the particles could be either fully promoted with detection of only Ni atoms, or partially promoted with detection of both Ni and W atoms. Hence, local and global analysis of NiW slabs are in good agreement to account for more fully promoted and partially promoted NiW sites by CA addition

Table I .

 I Elemental analysis of NiW catalysts before and after sulfidation at 673K.

	Sulfide catalyst	% wt.W	% wt. Ni	Ni/(W+Ni) *	S/(W+Ni) *
	NiW(CA/M=0)/Al 2 O 3	21	12	0.4	1.7
	NiW(CA/M=0.5)/Al 2 O 3	22	11	0.3	1.9
	NiW(CA/M=1)/Al 2 O 3	21	10	0.3	2.0
	NiW(CA/M=2)/Al 2 O 3	20	10	0.3	2.2
	* Atomic ratio

Table II .

 II Textural properties of NiW catalysts before and after sulfidation at 673K.

	Catalyst	SBET (m 2 g -1 )	Pore diameter (nm)
		dried	sulfided	dried	sulfided
	NiW(CA/M=0)/Al 2 O 3	193	224	10.6	11.9
	NiW(CA/M=0.5)/Al 2 O 3	160	221	10.9	11.7
	NiW(CA/M=1)/Al 2 O 3	107	212	10.5	11.4
	NiW(CA/M=2)/Al 2 O 3	89	207	9.7	11.1
	3.3.

Thiophene HDS activity 3.3.1. Effect of sulfidation temperature

It is generally reported that W-based catalysts present a greater difficulty for the sulfidation process than the Mo-based ones. This translates into that, compared to Mo-based catalysts, higher activation temperatures are required, as well as longer sulfidation times. It should be noted that nowadays there is no established protocol or widely accepted process regarding

Acknowledgements

L. Zavala-Sanchez acknowledges the Mexican National Council for Science and Technology (CONACYT) for the Ph.D. grant. The authors acknowledge LABEX EMC3 for the financial support to MIMOSA project. X. Portier from CIMAP laboratory would like to thank the ANR (Agence Nationale pour la Recherche) for its funding through an EQUIPEX project (grant # AWR-11-EQPX-0020) and also the Normandie Region for its financial support that allowed the use of the JEOL ARM 200F microscope.

Author information:

Corresponding author: laetitia.oliviero@ensicaen.fr ORCID Laetitia Oliviero: https://orcid.org/0000-0002-7931-439X Luz Zavala-Sanchez: https://orcid.org/0000-0002-9643-3825

Notes:

The authors declare no competing financial interest.


Luz Zavala Sanchez

Xavier Portier

Françoise Mauge

Laetitia Oliviero
email: laetitia.oliviero@ensicaen.fr

Luz Zavala-Sanchez

Françoise Maugé

High Resolution STEM-HAADF microscopy on γ-Al 2 O 3 supported MoS 2 catalyst -Proof by image of the changes in dispersion and morphology of the slabs with addition of citric acid

Keywords: HR STEM-HAADF, MoS 2, Citric acid, CO adsorption, IR spectroscopy, Hydrotreatment catalyst, Sulfide nanoparticle morphology

published or not. The documents may come

High Resolution STEM-HAADF microscopy on γ-Al2O3

supported MoS2 catalyst -Proof by image of the changes in dispersion and morphology of the slabs with addition of citric acid.

Introduction

In order to face with the severe environmental legislations as well as the decrease of light oil resources, heavy crude oils have to be refined into ultra-low sulfur content transportation fuel. Co (or Ni)-promoted Transition Metal Sulfides (TMS) supported catalysts have been the most important hydrodesulfurization (HDS) catalysts in petroleum refineries for many years. To meet the very restrictive regulation demands, a challenge is to develop catalysts with improved activity and controlled selectivity. Hence, there is a strong concern in the implementation of characterization methods able to account for the local structures of the active sites of industrial catalysts as well as their role in the HDS reaction. Indeed, atomic-scale structural information of catalysts prepared under the same conditions as those used industrially can lead to a more rational design of catalysts for industrial applications. Conventional HDS catalysts consist of a bimetallic (CoMo, NiMo) sulfide phase supported on alumina. The sulfide phase is presented as nanoparticles with lamellar structure. High Resolution Transmission Electron Microscopy (HRTEM) observations account for dimensional parameters of these nanoslabs and report values of average stacking (~2 to 4) and average length (~3 to 6 nm) that depend on the sulfidation conditions [START_REF] Bremmer | The effect of oxidation and resulfidation on (Ni/Co)MoS 2 hydrodesulfurisation catalysts[END_REF][START_REF] Toledo-Antonio | Upgrading HDS activity of MoS 2 catalysts by chelating thioglycolic acid to MoO x supported on alumina[END_REF]. Since several decades, the most accepted structure model of the catalytic active sites on sulfided CoMo/Al 2 O 3 catalysts is the one proposed by Topsøe [START_REF] Topsøe | Importance of Co-Mo-S Type Structures in Hydrodesulfurization[END_REF][START_REF] Brorson | The morphology of MoS 2 , WS 2 , Co-Mo-S, Ni-Mo-S and Ni-W-S nanoclusters in hydrodesulfurization catalysts revealed by[END_REF]. The very well-known MoS 2 structure establishes that the TMS slab present, in its crystallographic (0001) plane (2D perspective), two types of edges: the Mo-terminated edge (M-edge) and the sulfurterminated edge (S-edge). Theoretical calculations [START_REF] Byskov | Edge termination of MoS 2 and CoMoS catalyst particles[END_REF] as well as Scanning Tunneling Microscopy (STM) of TMS [START_REF] Lauritsen | Atomicscale structure of Co-Mo-S nanoclusters in hydrotreating catalysts[END_REF] confirmed the presence of these two edges on model MoS 2 phase and reported that metal promoters (Co or Ni) have different affinities towards M-edge and S-edge. On silica supported MoS 2 catalysts, Okamoto et al. showed by X-ray absorption spectroscopy techniques that the local edge structures of the MoS 2 slabs affect the Co-anchoring. This study also indicates that M-edge and S-edge sites have different intrinsic activities in HDS reactions, S-edge being more active and more favorable for the promotion and the formation of CoMoS phase [START_REF] Okamoto | Effect of sulfidation atmosphere on the hydrodesulfurization activity of SiO 2 -supported Co-Mo sulfide catalysts: Local structure and intrinsic activity of the active sites[END_REF]. Thus, these previous studies performed on model catalysts report that morphology of MoS 2 slabs, in particular the type and amount of the edges exposed, has a significant influence on both the CoMoS site concentration and their intrinsic activity. Chen et al. reported on an advance towards industrial catalysts by showing that CO adsorption followed by infrared spectroscopy characterization (IR/CO) can discriminate between M-edge and S-edge sites of alumina-supported MoS 2 phase. These results were based on a parallel study between Density Functional Theory (DFT) calculations and variations in position and intensity of IR bands of CO adsorbed on MoS 2 /Al 2 O 3 sulfided at various temperatures. Two ν(CO) contributions were distinguished, one at 2115 cm -1 ascribed to CO in interaction with M-edge sites and one at 2065 cm -1 ascribed to CO in interaction with S-edge sites [START_REF] Vradman | High loading of short WS 2 slabs inside SBA-15: promotion with nickel and performance in hydrodesulfurization and hydrogenation[END_REF][START_REF] Huang | SBA-15 as support for Ni-MoS 2 HDS catalysts derived from sulfur-containing molybdenum and nickel complexes in the reaction of HDS of DBT: An all sulfide route Catal[END_REF][START_REF] Nava | CoMo/Ti-SBA-15 catalysts for dibenzothiophene desulfurization[END_REF]. Since the molar extinction coefficient for each contribution was determined, the IR/CO technique allowed to inform on the amount on Mo sites on M-and S-edges of sulfided Mo/Al 2 O 3 , and thus, accounted for the shape of the MoS 2 nanoslabs. This way, IR demonstrated to be a reliable technique to recognize the morphology of the particles as well as the quantification of active sites exposed by the slab. It has been also shown that MoS 2 morphology is strongly influenced by the sulfide phase -support interactions [START_REF] Dimitrov | Synthesis and characterization of W-SBA-15 and W-HMS as supports for HDS[END_REF]. On MoS 2 /Al 2 O 3 , the addition of an organic additive or a chelating agent during the impregnation stage can alter these interactions [START_REF] Vradman | High loading of short WS 2 slabs inside SBA-15: promotion with nickel and performance in hydrodesulfurization and hydrogenation[END_REF][START_REF] Kisielowski | Imaging MoS 2 nanocatalysts with single-atom sensitivity[END_REF]. Thereby, chelating agent addition can change MoS 2 slab morphology of Mo catalyst deposited on industrially used support such as γ-Al 2 O 3 . However, direct information regarding the morphology of the MoS 2 prepared in presence of citric acid remains scarce. Since this information is obtained by IR/CO characterization i. e., by an indirect method, the question that arises now is: how to obtain a direct observation of these morphological changes? The morphology of TMS slabs deposited on an industrial support such as γ-alumina is usually characterized by HRTEM [START_REF] Vradman | High loading of short WS 2 slabs inside SBA-15: promotion with nickel and performance in hydrodesulfurization and hydrogenation[END_REF][START_REF] Huang | SBA-15 as support for Ni-MoS 2 HDS catalysts derived from sulfur-containing molybdenum and nickel complexes in the reaction of HDS of DBT: An all sulfide route Catal[END_REF][START_REF] Nava | CoMo/Ti-SBA-15 catalysts for dibenzothiophene desulfurization[END_REF][START_REF] Dimitrov | Synthesis and characterization of W-SBA-15 and W-HMS as supports for HDS[END_REF]. HRTEM observations provide an insight on general parameters such as the length, stacking of the slabs and defects such as bending or dislocations. When it comes to the morphological study, one of the obstacles is that TEM images undergo contrast perturbation due to the support, particularly when this one is such a strong insulator like alumina. To achieve the observation, the orientation of the MoS 2 slabs in relation to the direction of the incident electron beam is a determining factor. Only MoS 2 slabs that are oriented perpendicular to the [0001] axis are observable by means of HRTEM. This is explained because along these directions, parallel to the slabs, the atomic density is quite high, therefore the resulting contrast in the image is easily visible. Whereas for the top view images of the MoS 2 slabs (along the [0001] crystallographic direction) the contrast is very low because the electron beam is crossing only one atomic layer (in the case of one slab). Moreover, although the HRTEM technique allows the observation of metallic particles of subnanometric dimensions, when these particles are immersed in an irregular matrix such as γ-Al 2 O 3 , their location is complex. Easily, particles of such minimal dimensions can be unnoticed and be confused as part of the support (γ-Al 2 O 3 ). For this reason, it is necessary to process the images in order to modify the intensities for a possible interpretation and detection of the slabs. Some advances in HRTEM technique have allowed to obtain images of MoS 2 nanoparticles along the [0001] direction of unsupported films and TMS with conductive supports with single-atom sensitivity. For instance, Kisielowski et al. reported imaging of greater than 10 nm length MoS 2 particles along the [0001] direction with resolved information about the shape and structure of the nanocatalysts [START_REF] Kisielowski | Imaging MoS 2 nanocatalysts with single-atom sensitivity[END_REF]. In spite of the fact that catalysts were prepared in an industrial style, they were synthesized on a non-industrial support (graphitic powder). The selection of the support for preparing catalysts plays a major role, since each support has its own characteristics and presents different interactions with the active phase. These different interactions between the active sites and the support translate into changes in morphology, size and activity among others. Furthermore, carbon supports are conductive and this allows a better observation of the slabs by means of the HRTEM mode. Nevertheless, this is unattainable with the catalysts supported on alumina. Some researchers have explored chemical attack treatments in an attempt to separate the TMS particles from the support. A method used to overcome the contrast disturbance from the alumina support is to dissolve the support by HF etching [START_REF] Li | Application of HF etching in a HRTEM study of supported MoS 2 catalysts[END_REF]. Nevertheless, for CoMoS catalysts, this leads to strong alterations. Indeed, it was found that the dissolution of the support by HF solution led to a partial destruction of the active phase with loss of cobalt and a rearrangement of the MoS 2 particles [START_REF] Berhault | The single-layered morphology of supported MoS 2 -based catalysts-The role of the cobalt promoter and its effects in the hydrodesulfurization of dibenzothiophene[END_REF].

Experiments of scanning tunneling microscopy (STM) at atomic resolution, provided an overview of the effect of sulfo-reductive atmospheres, sulfidation agents, promoter atoms, and type of support in the equilibrium morphology of single-layer MoS 2 [START_REF] Lauritsen | Atomicscale structure of Co-Mo-S nanoclusters in hydrotreating catalysts[END_REF][START_REF] Walton | MoS 2 nanoparticle morphologies in hydrodesulfurization catalysis studied by scanning tunneling microscopy[END_REF][START_REF] Besenbacher | Recent STM, DFT and HAADF-STEM studies of sulfide-based hydrotreating catalysts: Insight into mechanistic, structural and particle size effects Catal[END_REF]. However, majority of these results are not quite scalable to the commercial catalysts because the preparation and characterization conditions are significantly different from the standard ones used in operation conditions. For instance, in the reported STM studies MoS 2 nanoparticles were supported on Au. Indeed, the sample preparation requires conditions of ultra-high vacuum (UHV) and an oriented substrate like Au (111) or carbon. However, this tailored particle growth on a conductive substrate presents strong differences compared with a commercial catalyst, where the active sites are dispersed in an insulator and multi-faceted surface support like amorphous silica-alumina (ASA) or γ-Al 2 O 3 .

Advances in electron microscopy have opened new routes to address the atomic-scale structure of catalytic materials [START_REF] Nellist | Direct Imaging of the Atomic Configuration of Ultradispersed Catalysts[END_REF][START_REF] Hillyard | Detector geometry, thermal diffuse scattering and strain effects in ADF STEM imaging[END_REF]. Compared to classical image modes, bright-field (BF) and dark-field (DF), High Resolution Scanning Transmission Electron Microscopy in High-Angle Annular Dark Field mode (HR STEM-HAADF) or Z-contrast image mode offers a significant advantage. Indeed, it facilitates the distinction of the elements according to their atomic number (Z), since the contrast is directly proportional to Z 1.7 , even for objects of a single atomic plane [START_REF] Nellist | Direct Imaging of the Atomic Configuration of Ultradispersed Catalysts[END_REF]. HR STEM-HAADF mode is suitable for the study of small nanoparticles made of heavy elements, especially if they are deposited on a support composed of lightweight elements, such as industrial alumina. Recently, Alphazan et al. [START_REF] Alphazan | Highly active nonpromoted hydrotreating catalysts through the controlled growth of a supported hexagonal WS 2 phase[END_REF][START_REF] Alphazan | Improved promoter effect in NiWS catalysts through a molecular approach and an optimized Ni edge decoration[END_REF] reported on highly active W-based catalysts prepared through controlled surface chemistry that their enhanced activities are explained by the 2D hexagonal morphology of WS 2 particles. W catalysts have been chosen because this metal provides better contrast with Al 2 O 3 compared to Mo due to the difference in atomic number. Nevertheless, to obtain better crystallized and dispersed slabs, the sulfidation was performed at higher temperature (T s = 600°C) than the usual operating one, and the selected metal coverage was considerably lower than the commonly used. When the metal load decreases, the overlapping of many particles is limited, which improves the quality of the observation. In addition, a higher sulfidation temperature results in more crystalline structures with higher stability under the electron beam. Thus, these modifications can lead to morphological changes, i.e. S-edge/M-edge ratio of the sulfided slabs can be modified compared to that of catalysts prepared in a standard manner [START_REF] Chen | IR Spectroscopic Evidence for MoS 2 Morphology Change with Sulfidation Temperature on MoS 2 /Al 2 O 3 Catalyst[END_REF][START_REF] Chen | On the morphology of MoS 2 slabs on MoS 2 /Al 2 O 3 catalysts: the influence of Mo loading[END_REF]. Therefore, a gap arises between catalytic activity and morphology studies.

Hence, the aim of the present work is to obtain a direct characterization of the slab morphology of Al 2 O 3 supported MoS 2 catalysts prepared in exactly the same conditions of synthesis and sulfidation than the ones for the catalytic application. With this objective, a parallel study by means of HR STEM-HAADF (direct approach) and IR/CO spectroscopy method (indirect approach) is carried out on Mo catalysts prepared without or with citric acid. Modification of the slab parameters (length, stacking, and shape) according to the preparation conditions is analyzed via HR STEM-HAADF observations and compared with HRTEM and IR/CO results. The coherence between the results from these different approaches validates the indirect characterization of the MoS 2 slab by IR/CO method. Thus, the study is a proof by image of the IR spectroscopic results that gives new insight into the characterization of active sites of TMS catalyst. O PROLABO) to obtain a CA/Mo = 2. Afterwards, the solution was added to the pretreated γ-Al 2 O 3 support (SASOL, specific surface area of 252 m 2 .g -1 and pore volume of 0.84 mL.g -1 , precalcined in air at 723 K for 2 h), strongly shaken and left for maturation during 2 h. Finally, the catalysts were dried at 383 K for 16 h. Note that these catalysts were not calcined in order to keep the chelating agent if present in its initial form. Hereinafter, the Mo/Al 2 O 3 catalysts are denoted Mo(CA/Mo = x)/Al 2 O 3 . X refers to the molar ratio of CA to Mo. x = 0 or 2.0.

Experimental procedures

Catalyst preparation

HRTEM and High-resolution Scanning Transmission Electron Microscopy -High Angular Annular Dark Field (HR STEM-HAADF).

All TEM images presented hereinafter have been taken using a double corrected JEOL ARM 200F cold FEG microscope operated at 200 kV. First, slab length and stacking degree distributions of sulfide slabs were determined manually by measuring around 200 slabs per sample from HRTEM images. The image treatment was performed using the commercial software from GATAN (DIGITALMICROGRAPH).

The nanostructure and morphology of the MoS 2 slabs were analyzed by high resolution scanning transmission electron microscopy (HR STEM) using a high angle annular dark field (HAADF) detector. The catalyst precursors were firstly sulfided at 623 K (heating rate of 3 K min -1 ) and 0.1 MPa for 2 h under a 30 mL min -1 flow of 10%H 2 S/H 2 . Then, to limit detrimental exposure to air, the sulfided catalysts were unloaded from the sulfidation reactor under argon flow and poured into absolute ethanol. A few drops of the suspension of catalyst were put on a 300 mesh copper grid with holey carbon film. All the images presented in this manuscript were acquired in HAADF mode and the acquisition of an image lasted about 30 s. During this acquisition and despite exposure to the electron beam, the shapes of the particles remained unchanged, meaning that the slabs were quite stable under the electron beam. The possible effect of damage associated with the heating effects caused by the incident beam in the MoS 2 nanolayers is always a factor to consider. The advisable acceleration voltage is a low beam energy such as 80 kV. In our study, we did not observe significant changes during the acquisition of the images at 200 kV. Observations at 80 kV were also performed, but no significant difference in terms of slab stability was noticed. Therefore, it was concluded that the operation with an acceleration of 200 kV is compatible with our purpose of morphological analysis.

Two-Dimensional (2D) quantitative morphology analysis from HAADF-TEM images.

In a previous work, Baubet et al. reported a reliable methodology for quantification of the morphology from HR HAADF-STEM images [START_REF] Baubet | Quantitative Two-Dimensional (2D) Morphology-Selectivity Relationship of CoMoS Nanolayers: A Combined High-Resolution High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HR HAADF-STEM) and Density Functional Theory (DFT) Study ACS Catal[END_REF]. Two methods were compared. The first one consists of the determination of the S-edge/Medge ratio through the measurement of the lengths of the regular shaped particle. For the second one, a 2D shape morphology index was determined by analysis of the slabs whatever its shape taking into account ten parameters. The results obtained for catalysts sulfided under different conditions were comparable with both methods and were consistent with what was predicted by DFT calculations. In this work, following a similar approach, a sample of a dozen observable isolated particles for both Mo(CA/Mo = 0)/Al 2 O 3 and Mo(CA/Mo = 2)/Al 2 O 3 samples was taken through contrast optimization in our images, to obtain statistical information about the morphology of the MoS 2 slabs. In these regular slabs, a contour was drawn by a simple shape model with straight edges and the corresponding edge lengths were measured, as well as the area of the particle with the help of the Mesurim Pro software. The limited number of particles is explained by the difficulty to observe isolated specimens with faceted edges in these realistic samples.

For assigning M-edge and S-edge, we rely on DFT calculations that predicted that M-edge is energetically more stable than S-edge in a wide range of sulfur chemical potential and, therefore, that the morphology of MoS 2 is truncated triangle terminated by mainly M-edge under typical sulfidation and HDS conditions [START_REF] Schweiger | Shape and Edge Sites Modifications of MoS 2 Catalytic Nanoparticles Induced by Working Conditions: A Theoretical Study[END_REF].

Particle dispersion analysis

Considering that for each slab, the dispersion is the ratio of edge atoms per total atoms, the calculated dispersion values were obtained through the measurement of slab area and perimeter with the following equation:

S-edge/M-edge ratio calculation

Given that the morphology of these slabs is well in accordance with the prediction of DFT, we consider them to be nanoparticles in "equilibrium". Therefore, we rely on the previous DFT calculations [START_REF] Schweiger | Shape and Edge Sites Modifications of MoS 2 Catalytic Nanoparticles Induced by Working Conditions: A Theoretical Study[END_REF] to assign the chemical nature of the edge. Indeed, for the sulfidation process that we used, the particles must exhibit a majority of M-edge. Hence, for all the slabs observed, the largest edge is assigned to the Medge and then we consider alternation of S-and M-edges.

For each particle, all edges were measured carefully. Considering the largest edge length being an M-edge (L M ) and taking into account the intercalation between M-edge and S-edge, the S-edge/M-edge ratio is calculated by the following equation ( 2):

With L Si and L Mi , the lengths of each S-edge and M-edge respectively.

IR spectroscopy of CO adsorption (IR/CO)

The MoS 2 edge sites of these catalysts were in-situ characterized by low-temperature CO adsorption followed by IR spectroscopy. The oxidic catalyst was grounded and pressed into a self-supported wafer (10 mg for a disc of 2.01 cm 2 ) and positioned into a quartz IR cell with CaF 2 windows. Prior to the adsorption experiments, the sample was sulfided in situ in the IR cell. For this purpose, the catalysts was sulfided in a gas mixture containing 10% H 2 S/H 2 (30 ml.min - 1 ) at 623 K for two hours with a heating rate of 3 K.min -1 . After sulfidation, the cell was flushed with Ar and evacuated down to P<1 10 -4 Pa until the catalyst reached room temperature. Then, CO adsorption was performed at low temperature (~100 K) to avoid any reaction with the catalysts surface. Small calibrated doses of CO were introduced into the IR cell until equilibrium pressure of 133 Pa. After each CO introduction, FTIR spectra were recorded with 256 scans and a resolution of 4 cm -1 using Nicolet Nexus FT-IR spectrometer equipped with a MCT Detector. The quantification of each edge sites present in the series of catalysts was obtained after the spectral decomposition performed in Peakfit V4.12 using '' Autofit peak IIIdeconvolution ''. In the spectral decomposition, the values of the center and the full width at half maximum (FWHM) correspond to those previously reported on [START_REF] Lauritsen | Atomicscale structure of Co-Mo-S nanoclusters in hydrotreating catalysts[END_REF]. The used CO molar extinction coefficients are 20 ± 3 (M-edge) and 35 ± 9 (S-edge) cm•μmol -1 as determined in SI of [START_REF] Lauritsen | Atomicscale structure of Co-Mo-S nanoclusters in hydrotreating catalysts[END_REF].

Results and discussion

Length and stacking degree determined by HRTEM analysis

In figure 1 representative images (A-B) and distributions of length and stacking (C-D) of the sulfide slabs obtained for Mo(CA/Mo = 0)/Al 2 O 3 and Mo(CA/Mo = 2)/Al 2 O 3 catalysts are presented. A careful process of measuring the length of the slabs present in each sample was carried out. The addition of citric acid leads to a decrease of the average slab length from 2.9 ± 0.9 nm to 1.7 ± 0.9 nm. Also to an increase of the average stacking number (1.9 to 2.2) is noticed. While the presence of particles stacked in two or more slabs is significant in both samples, the amount of the rather small monolayer slabs observed in the Mo(CA/Mo = 2)/Al 2 O 3 sample was much higher than in the Mo(CA/Mo = 0)/Al 2 O 3 sample. In both samples, interestingly, those particles that were not found stacked are characterized for a minimum slab length. This could suggest that only longer particles will stack into two or more. Larger slabs will stack while the small ones will be better dispersed in the surface. Such results are in accordance with previously reported results and ascribed to the decrease interaction between the MoS 2 slabs and the support due to the addition of citric acid [START_REF] Chen | IR spectroscopy evidence of MoS 2 morphology change by citric acid addition on MoS 2 /Al 2 O 3 catalysts -A step forward to differentiate the reactivity of M-edge and Sedge[END_REF].

Dispersion calculated from HR HAADF-STEM images.

In figure 2 (A andC) the HR STEM-HAADF micrographs representative of Mo(CA/Mo = 0)/Al 2 O 3 and Mo(CA/Mo = 2)/Al 2 O 3 are shown respectively. Observed particles are characterized by a good contrast and thus the acquisition mode is adequate to reveal the 2D morphology of slabs in industrial-grade alumina support. Indeed, planar MoS 2 slabs are displayed with atomic resolution of the Mo atoms (bright dots) present in the slabs. These HR HAADF-STEM observations reveal differences in slab growth between the two samples analyzed. Indeed, the general topology of Mo(CA/Mo=0)/Al 2 O 3 is characterized by MoS 2 slabs grown in the form of plates and by a contrast variation among the slabs (figure 2 (A)). It is also noticeable that some of the slabs in Mo(CA/Mo = 2)/Al 2 O 3 are much smaller than the ones observed in Mo(CA/Mo = 0)/Al 2 O 3 sample . In both samples, brighter areas can be attributed to an increase in the density of Mo that is related to the agglomeration of stacked slabs present. However, according to figure 2 (C), the particles whose length is less than 1 nm, are appreciated with an intensity of homogeneous brightness, which suggests that they are particles of thickness of a single atomic layer (not stacked). In figure 2 (B andD), the contour of the more isolated slabs in both samples is presented. By optimization and variation of contrast by region, a dozen slabs per sample were drawn in order to carry out a statistical analysis of their length and dispersion (eq. ( 1)). Note that the longest length observed in the 2D particles images was taken into account for determining the diameter of the MoS 2 slabs. Results are gathered in Table I and figure 3. In accordance with the results obtained from HRTEM, HR STEM HAADF study reveals that the addition of citric acid promotes the reduction of MoS 2 particle size. However, for both samples the value obtained by the HR STEM HAADF study is lower than the one obtained by the HRTEM study, underlying that the smallest particles can be hard to distinguish in HRTEM. Moreover, the latter approach is less accurate since the slab length measurements are done from the projection of the slabs which is not necessarily the longest distance of the slabs. Additionally, the distribution in length obtained by both studies is also different: more homogeneous lengths are observed in HR STEM HAADF than in HRTEM. Finally, the fact that the 2D morphology of the slabs is revealed by the HR STEM HAADF study allows calculating realistic dispersion values for each particle. The average value of dispersion is then significantly increased with the addition of citric acid: from 0.48 ± 0.08 for the Mo(CA/Mo=0)/Al 2 O 3 sample to 0.60 ± 0.12 for the Mo(CA/Mo=2)/Al 2 O 3 sample.

MoS 2 slab morphology (shape) and S-edge/M-edge ratio

As shown in images at greater magnification in figures 4(A-C), the MoS 2 particles in Mo(CA/Mo=0)/Al 2 O 3 have a geometry, whose shape resembles a triangle with slightly truncated edges. Accurate observation of picture of figure 4(B) shows a particle with this shape superimposed on a larger one, leading to a different projected pattern. Inset of figure 4(B) shows a Fast Fourier Transform (FFT) of a single region of the slab in the image, and the as expected six-fold symmetry of the Mo atoms arrangement is clearly evidenced. In order to rationalize these morphology observations, calculation of the S-edge/M-edge ratio for a dozen observed particles has been performed. Since HR HAADF-STEM analysis does not allow one to identify the type of edge exposed by the nanoparticles, as done in ref. [START_REF] Baubet | Quantitative Two-Dimensional (2D) Morphology-Selectivity Relationship of CoMoS Nanolayers: A Combined High-Resolution High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HR HAADF-STEM) and Density Functional Theory (DFT) Study ACS Catal[END_REF], the longest length was ascribed to M-edge since this is the most stable edge at the sulfidation conditions we used [START_REF] Chen | IR Spectroscopic Evidence for MoS 2 Morphology Change with Sulfidation Temperature on MoS 2 /Al 2 O 3 Catalyst[END_REF]. The Sedge/M-edge ratio from HR HAADF-STEM were then calculated following eq. ( 2): the distribution in S-edge/Medge ratio is reported in figure 5 and the average value is 0.37 ± 0.12 for Mo(CA/Mo=0)/Al 2 O 3 and 0.78 ± 0.10 for Mo(CA/Mo=2)/Al 2 O 3 (Table I).

Table I.

Parameters calculated from the HR STEM-HAADF images for both catalyst.

The significant increase in average S-edge/M-edge ratio value and the clear difference in distribution pattern agree with the more isotropic character of the slabs for the catalyst prepared with citric acid as previously observed.

Morphology based on IR/CO of MoS 2 /Al 2 O 3 catalysts prepared with citric acid

IR/CO spectroscopy has revealed that several parameters during the preparation and sulfidation stage can modify the morphology of MoS 2 slabs of supported catalysts [START_REF] Chen | IR Spectroscopic Evidence for MoS 2 Morphology Change with Sulfidation Temperature on MoS 2 /Al 2 O 3 Catalyst[END_REF][START_REF] Chen | IR spectroscopy evidence of MoS 2 morphology change by citric acid addition on MoS 2 /Al 2 O 3 catalysts -A step forward to differentiate the reactivity of M-edge and Sedge[END_REF][START_REF] Chen | Effect of high pressure sulfidation on the morphology and reactivity of MoS 2 slabs on MoS 2 /Al 2 O 3 catalyst prepared with citric acid[END_REF][START_REF] Chen | On the morphology of MoS 2 slabs on MoS 2 /Al 2 O 3 catalysts: the influence of Mo loading[END_REF]. For example, and for the present study, the addition of citric acid promotes the growth of S-edge (IR/CO band at 2065 cm - 1 ), while it inhibits that of M-edge (IR/CO band at 2115 cm -1 ) (figure 6). From these variations, it is expected that the morphology of MoS 2 slabs is progressively modified from a slightly truncated triangle with predominant M-edge to a more hexagonal shape with closer amount of M-edge and Sedge, as the quantity of citric acid present in the preparation stage is increased. Our HR STEM-HAADF images and deduced results reveal indeed a change in morphology from one sample to another with the addition of citric acid. However, HR STEM-HAADF images also show that not only model forms (triangle and hexagon) are present. Indeed, the obtained images mainly evidenced particles changing from slightly truncated to more-faceted shapes, but the same trend is observed, i. e. the S-edge/M-edge ratio increases with citric acid addition. Another important point is that the increase in dispersion (i. e. decrease in slab length) deduced from the HR STEM-HAADF study is also in accordance with the global increase in edge sites concentration obtained by the IR/CO method.

Conclusion

One guideline of this work was to characterize catalysts prepared in conditions close to the ones used for applications and not specifically chosen to meet the requirements of these advanced techniques of characterization. This has made very delicate the observation of the shape of sulfide slabs by electron microscopy. However, atomic-scale images of γ-Al 2 O 3 supported Mo nanoparticles were obtained by HR STEM-HAADF for sulfide catalysts prepared without and with citric acid. HR STEM-HAADF observations and subsequent exploitation highlighted that the addition of citric acid during the preparation step of Mo catalysts changes the shape of the MoS 2 nanoslabs. Indeed, the S-edge/M-edge ratio obtained by HR STEM-HAADF study is significantly increased by addition of citric acid as it was previously obtained by the IR/CO method. Moreover, the study shows that HR STEM-HAADF allows more easily to distinguish the smallest slabs (< 2 nm) from the oxide support. So, this HR STEM-HAADF study is a proof by image of the results obtained by the IR/CO method. The direct observations validate the variation of morphology of MoS 2 nanoslabs detected indirectly by the IR/CO method thus consolidating also the ν(CO/Mo) band attribution to CO interacting on the two different edges.

As a consequence, this study strengthens the importance of coupling techniques like microscopy that allows a direct observation of the shape of a group of sulfided slabs and IR spectroscopy that allows an indirect detection of the shape but of a large number of sulfided slabs. The present study paves the way for further investigations on promoted catalysts comparable to the ones used in the HDS industry.

  perimeter (nm) d = atomic diameter Mo (nm) N c = Number of corner atoms in the corner A = Particle area (nm 2 )

Figure 1 .

 1 Figure 1. TEM analysis: images of Mo(CA/Mo=0)/Al 2 O 3 (A) and Mo(CA/Mo=2)/Al 2 O 3 (B), slab length distribution (C) and stacking number distribution (D).

Figure 2 .

 2 Figure 2. Processing of HR STEM-HAADF micrographs of MoS 2 particles of Mo(CA/Mo = 0)/Al 2 O 3 (A and B) and Mo(CA/Mo = 2)/Al 2 O 3 (C and D). In figures B and D the contour of selected contrasted particles is shown.

Figure 3 .

 3 Figure 3. Histogram of the measured diameter of the particles observed in HR HAADF-STEM

Figure 4 .

 4 Figure 4. HR STEM-HAADF micrographs of MoS 2 particles of Mo(CA/Mo = 0)/Al 2 O 3 and Mo(CA/Mo = 2)/Al 2 O 3 of different zones. Inset B and B' FFT of a single slab and inset C) zoom on the filtered image.Furthermore, when analyzing filtered images (inset in figure4(C) as an example), it was determined that the interatomic distance between the atoms of Mo is ~0.31 nm corresponding to that of the Mo atoms in a MoS 2 crystal (0.317 nm). In general, the particles observed for Mo(CA/Mo = 2)/Al 2 O 3 displayed different morphologies than the ones for Mo(CA/Mo = 0)/Al 2 O 3 . Addition of citric acid leads to (1) a significant amount of faceted-shape slabs as shown in figures 4(A'-C') and (2) some rather symmetrical shaped slabs.

Figure 5 .

 5 Figure 5. Histogram of the S-edge/M-edge ratio based on the measured edge lengths for Mo(CA/Mo=0)/Al 2 O 3 and Mo(CA/Mo=2)/Al 2 O 3

Figure 6 .

 6 Figure 6. IR spectra of CO adsorption on sulfided Mo(CA/Mo = 0 or 2)/Al 2 O 3 catalysts.

  Two Mo/Al 2 O 3 catalysts, without and with citric acid (CA), were prepared by a one-step pore volume impregnation method. The aqueous impregnation solutions were prepared by dissolving a fixed amount of ammonium heptamolybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 •4H 2 O MERCK) in order to reach 3 Mo atoms.nm -2 of Al 2 O 3 support. CA was incorporated at this stage by dissolving the required amount of citric acid (C 6 H 8 O 7 •H 2

Acknowledgements L. Zavala-Sanchez acknowledges the Mexican National Council for Science and Technology (CONACYT) for the Ph. D. grant. The authors acknowledge LABEX EMC3 for the financial support to MIMOSA project.