Cédric Bara

Anne-Félicie Lamic-Humblot

Emiliano Fonda

Anne-Sophie Gay

Anne-Lise Taleb

Mathieu Elodie D Devers

D Digne

Gerhard D Pirngruber

Mathieu Digne

Xavier Carrier
email: xavier.carrier@upmc.fr

Surface-dependent sulfidation and orientation of MoS 2 slabs on alumina-supported model HDS catalysts

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Heterogeneous catalysts based on Transition Metal Sulfides (TMS) have long been used to eliminate heteroatoms (S, O, N) and metals from crude oil or petroleum fraction for environmental reasons (to decrease polluting gas emission during oil combustion) but also for industrial concerns in order to protect catalysts used downstream in the refinery [START_REF] Toulhoat | Catalysis by transition metal sulphides from molecular theory to industrial application[END_REF]. Among all impurities, sulfur represents a major problem since it leads to sulfur dioxide emission, responsible of acid rain and catalytic converter poisoning [START_REF] Song | An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel[END_REF]. Therefore, legislation on the maximum tolerated sulfur content in fuels is more and more stringent which implies a constant improvement of hydrotreating catalysts. More recently, TMS-based catalysts have also attracted a great interest for biomass processing as hydrogenation/decarboxylation catalysts for renewable lipids and hydrodeoxygenation of pyrolysis oils from 2 nd generation lignocellulosic biomass [START_REF] Furimsky | Hydroprocessing challenges in biofuels production[END_REF].

TMS-based heterogeneous catalysts are classically described as made of molybdenum or tungsten sulfide slabs (Mo(W)S 2 ) promoted at the edge by Co or Ni atoms,dispersed over a porous polycrystalline oxide support of high surface area, mostly -alumina [START_REF] Toulhoat | Catalysis by transition metal sulphides from molecular theory to industrial application[END_REF][START_REF] Prins | Handbook of Heterogeneous Catalysis[END_REF][START_REF] Topsøe | Hydrotreating Catalysis Science and Technology[END_REF][START_REF] Topsøe | In situ Mössbauer emission spectroscopy studies of unsupported and supported sulfided cobalt-molybdenum hydrodesulfurization catalysts: evidence for and nature of a cobalt-molybdenum-sulfur phase[END_REF]. One important and still largely debated question concentrates on the role of the oxide support on the sulfide active phase in terms of size, morphology, orientation or sulfidation degree. Even subtle differences, such as alumina polymorphism, have been shown to be a decisive factor.

As a matter of fact, it has been demonstrated that promoted CoMoS catalysts supported on -Al 2 O 3 show a higher intrinsic activity (per Mo atoms) than the same system supported on -Al 2 O 3 [START_REF] Laurenti | Intrinsic potential of aluminasupported oMo catalysts in HDS: omparison between γ c , γ T , and δ-alumina[END_REF]. These differences were explained in terms of better dispersion for the MoS 2 active phase and weaker metal-support interactions on -Al 2 O 3 . However, a rational understanding of these differences is excluded in industrial catalysts due to the use of high surface area, polycrystalline oxide particles that expose a variety of crystallographic planes with numerous defects and a multiplicity of surface hydroxyl groups. Yet, a molecular-scale understanding of the support influence is obviously a pre-requisite for a scientific design of industrial hydrotreating catalysts.

Surface-science type studies can help resolving this issue by using planar surfaces with wellcontrolled surface structure [START_REF] Bara | A review of surface science approaches for the preparation of alumina-supported hydrotreating catalysts[END_REF]. Specific reactivity of various surface orientations can thus be studied and compared to polycrystalline industrial materials. Specifically, Sakashita and Yoneda [START_REF] Sakashita | Orientation of MoS 2 clusters supported on two inds of γ-Al 2 O 3 single crystal surfaces with different indices[END_REF][START_REF] Sakashita | Effects of surface orientation and crystallinity of alumina supports on the microstructures of molybdenum oxides and sulfides[END_REF] studied non-promoted sulfided Mo catalysts on oriented Al 2 O 3 thin films prepared by electron beam evaporation of -Al 2 O 3 onto MgAl 2 O 4 wafers. These authors claimed that MoS 2 slabs are oriented parallel to the support on the most predominant -Al 2 O 3 (110) surface and show the highest sulfidation degree with respect to the other (100) and (111) orientations. However, the use of thin films epitaxially grown on an oriented substrate as models for -Al 2 O 3 has been questioned [START_REF] Bara | A review of surface science approaches for the preparation of alumina-supported hydrotreating catalysts[END_REF]. More recently, we showed that -alumina single crystals can be used as a suitable model for -Al 2 O 3 since there are many analogies between the surface structure of -Al 2 O 3 , the metastable polycrystalline industrial support, and -Al 2 O 3 , the thermodynamically stable alumina polymorph from which single crystal wafers can be grown [START_REF] Bara | A review of surface science approaches for the preparation of alumina-supported hydrotreating catalysts[END_REF][START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF].

We have recently followed this surface-science approach using well defined -alumina single crystal wafers of different orientations (i.e A , C (0001), M and R planes) combined with an aqueous-phase deposition method of Mo denoted equilibrium adsorption (or selective adsorption). The deposition from aqueous phase is closer to industrial procedures of catalysts preparation than more classical UHV surface science deposition methods [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF]. It was shown that the surface speciation of alumina hydroxyls controls the Mo adsorption at the oxide/water interface and in turn the strength of metal-support interactions leading to various sulfidation degrees depending on the surface orientation. Structural analogies between  and  alumina allow the following extrapolation to polycrystalline supports: we can expect a heterogeneous Mo distribution over industrial -Al 2 O 3 supported catalysts, with a strong metal-support interaction and low sulfidation degree on the predominant (110) surface, and lower metal-support interactions, but higher sulfidation degrees on the (111) and (100) planes.

To step forward in understanding industrial catalysts, the present contribution broadens the previous approach by using an alternative aqueous-phase deposition method, i.e. a simple impregnation (without filtration and washing steps), which mimics the classical incipient wetness impregnation (also known as dry impregnation) used with shaped oxide supports.

This method has two advantages with respect to equilibrium/selective adsorption: first weakly reactive surfaces (such as the C (0001) plane in the present case) [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF] can be used since weakly interacting Mo species will not be washed out with a rinsing step and second, the amount of deposited Mo can be easily controlled so that various orientations with identical Mo surface density can be prepared. Hence, model catalysts have been prepared on various -alumina single crystal wafers (A , C (0001), M and R planes) and sulfided at different temperatures to study the role of each -alumina surface on the nature of the active phase: sulfidation degree, MoS 2 nanoparticle size and metal-support interaction with XPS, AFM and TEM. The orientation of the active phase has also been investigated by surface EXAFS spectroscopy and high resolution scanning transmission electron microscopy in high angle annular dark field mode (HR HAADF-STEM). The role of the synthesis route toward Mo adsorption and sulfidation is finally discussed with respect to the surface hydroxyl speciation. following a procedure already reported in ref [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF]. A final calcination step was performed in air in a muffle furnace overnight at 700 °C. Mo impregnation was carried out through deposition of a 100 µL droplet of an aqueous solution of ammonium heptamolybdate tetrahydrate, (NH 4 ) 6 Mo 7 O 24 .4H 2 O (Merck, > 99% purity) at the natural pH (5.2) of the solution. Two different Mo surface loadings were considered: 3.5 at.nm -2 ([Mo] = 5.8.10 - 6 mol.L -1 ) and 0.9 at.nm -2 ([Mo] = 1,5.10 -6 mol.L -1 ) in order to be close to the surface saturation of the A (11 0) and M planes (about 4 at.nm -2 ) or the R plane (1 at.nm -2 ) as shown in ref [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF]. Most of the results presented in the paper focus on the highest surface concentration (3.5 at.nm -2 ). After Mo deposition, the model catalysts were dried 1 h at room temperature under primary vacuum to remove the solvent. It was also noticed that this step favored a homogeneous spreading of the 100 µL droplet over the wafer. After drying, samples were calcined 2 h at 450 °C in air in a muffle furnace.

Materials and Methods

Sulfidation of model catalysts (MoS 2 /-Al 2 O 3 )

Conventional gas phase sulfidation was performed in a glass reactor under a flow of 2 L.h -1 of 15 mol% H 2 S/H 2 at atmospheric pressure and constant temperature (100, 200, 300, 400 or 450 °C) for 2 h. The reactor was then cooled down to 80 or 150 °C (depending on sulfidation temperature) for 1 h under argon (2 L.h -1 ) to remove any excess of sulfur. Catalysts were then kept under argon at room-temperature before analysis.

Characterization

XPS: XPS spectra were recorded with an Omicron (ESCA+) instrument using a monochromatic Al X-ray source (h = 1486.6 eV) with an accelerating voltage of 14 kV and a current intensity of 20 mA (overall energy resolution was about 0.8 eV). Spectra were collected at a takeoff angle of 90° under a pressure lower than10 -9 mbar. More information can be found in ref [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF]. XPS was used to quantify the sulfidation degree through the methodology developed by Gandubert et al. [START_REF] Gandubert | X-ray photoelectron spectroscopy surface quantification of sulfided CoMoP catalysts. Relation between activity and promoted sites. Part II: Influence of the sulfidation temperature[END_REF] by integrating the contributions of MoS equipped with a 150 x 150 x 5 µm 3 scanner (J-scanner). AFM analyses were performed using the peak force tapping mode (PFT), recently developed [START_REF] Adamcik | Single-step direct measurement of amyloid fibrils stiffness by peak force quantitative nanomechanical atomic force microscopy[END_REF] with silicon tip on Si 3 N 4 cantilevers. It was checked that no significant evolution of morphology occured during AFM imaging. The root mean square surface roughness (R rms ) is expressed by the square root of the sum of the squares of the individual heights and depths from the mean line.

X-ray absorption:

Surface EXAFS in Grazing-Incidence geometry (Grazing-Incidence X-ray Absorption Spectroscopy or GI-XAS) was conducted on the SAMBA beamline at the SOLEIL synchrotron facility (Saint-Aubin, France) [START_REF] Baudelet | SOLEIL, un nouvel outil puissant pour les sciences des matériaux[END_REF]. Spectra were collected in the fluorescence mode with a Canberra 36 pixels solid-state Ge detector at the Mo K edge (20 keV). The orientation of the wafers was varied with respect to the polarization of the synchrotron beam. These orientations will be denoted parallel and perpendicular throughout the text when the electric-field vector of the synchrotron beam is respectively parallel and perpendicular to the surface of the wafer.

For sulfide samples, planar catalysts were transferred from the sulfidation reactor to the EXAFS sample holder in a glove box and then fixed with carbon tape on a goniometer head in a sealed plastic box filled with argon in order to prevent any oxidation. Reference compounds were collected in transmission mode by diluting the samples in cellulose. XAS data analysis was carried out with the Demeter package [START_REF] Ravel | HEPHAESTUS: data analysis for x-ray absorption spectroscopy using IFEFFIT[END_REF].

It has to be noted that coordination numbers reported in this paper are effective coordination numbers as defined by Equ. 1 [START_REF] Shirai | X-ray absorption fine stucture for catalysts and surfaces[END_REF] that depend on the polarization of the synchrotron beam.

For example, the effective coordination numbers (N effective ) will be three times higher than the actual number of neighbors (N real ) when the chemical bond (directed along r ) is parallel to the electric field vector e cos θ = .

Equ. 1

Results and Discussion

Molybdenum oxide dispersion: AFM

AFM images were recorded after aqueous-phase impregnation of Mo (high loading 3.5 at.nm - 2 ) on A , C (0001), M and R planes and subsequent calcination at 450°C in order to investigate the Mo dispersion at the oxide state. Surface roughness analysis, before

Mo deposition, shows a relatively smooth surface for all orientations (i.e. R rms of 0.10-0.15 ± 0.01 nm). However, succeeding impregnation and calcination lead to three different behaviors depending on the surface orientation (Figure 1). On the C (0001) plane, a large part of the Mo oxide particles are aggregated forming large clusters with a width of 600 to 900 nm. These clusters are composed of a collection of individual particles about 7 to 22 nm wide and 1 to 4 nm high. The remaining alumina surface is mostly free of Mo even if few individual nanoparticles are still discernable. Only a slight increase of the surface roughness is observed up to 0.40 ± 0.01 nm since large parts of the wafer are uncovered leading to a heterogeneous distribution of Mo on the C (0001) surface.

On A and M planes, a drastic increase of the surface roughness is observed up to 4.0 ± 0.01 nm due to the presence of two sets of nanoparticles: aggregates (average height of 20-70 nm and width of 100-500 nm) and individual particles (average height of 1-9 nm and width of 5-30 nm). The M plane slightly differs from the A plane by the presence of steps (0.2 to 0.3 nm high). This faceting behavior has already been observed by others after high temperature treatment [START_REF] Wiehl | High-resolution transmission electron microscopic investigations of molybdenum thin films on faceted α-Al 2 O 3[END_REF][START_REF] Heffelfinger | Mechanisms of surface faceting and coarsening[END_REF][START_REF] Curiotto | Surface morphology and composition of c-, a-and m-sapphire surfaces in O 2 and H 2 environments[END_REF] but Mo oxide nanoparticles are not specifically located along these steps. On the R plane, no aggregation is observed and small Mo oxide nanoparticles (average height of 1-4 nm and width of 2-11 nm) are homogeneously distributed on the surface. Hence, the surface roughness increases only slightly (0.30 ± 0.01 nm) with respect to the bare surface (0.15± 0.01 nm).

The aggregation of molybdenum oxide nanoparticles observed with AFM can be taken as a good indication of the strength of metal-support interactions at the oxide state [START_REF] Parker | Kinetic model for sintering of supported metal particles with improved size-dependent energetics and applications to Au on TiO 2 (110)[END_REF][START_REF] Campbell | Insights into catalysis by gold nanoparticles and their support effects through surface science studies of model catalysts[END_REF][START_REF] Bartholomew | Studies in Surface Science and Catalysis[END_REF]. The homogeneous distribution of small nanoparticles on the R plane reveals strong metalsupport interactions responsible of nanoparticle stabilization. Conversely, low metal-support interactions on the C (0001) plane leads to a large aggregation of particles [START_REF] Campbell | The Energetics of Supported Metal Nanoparticles: Relationships to Sintering Rates and Catalytic Activity[END_REF]. A and but it completes the conclusions to the C (0001) plane which was not adsorbing Mo through equilibrium adsorption [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF].

Molecular structure of the oxide phase: GI-XAS

The structure of Mo oxide nanoparticles obtained after calcination (high loading 3.5 at.nm -2 ) was investigated using grazing-incidence X-ray absorption specroscopy (GI-XAS) by varying the orientation of the wafers with respect to the polarization of the synchrotron beam (parallel vs perpendicular). First, Mo coordination was determined by comparing the XANES region of the model catalysts with Mo reference compounds where Mo is 6-fold coordinated (ammonium heptamolybdate, noted AHM and molybdenum trioxide, noted MoO 3 ) or 4-fold coordinated (sodium molybdate, Na 2 MoO 4 ). Octahedral or tetrahedral coordinations show distinct features in the pre-edge region (1s to 4d transition at 20 000-20 010 eV, Figure 2)

with enhanced intensity for four-fold coordination [START_REF] Tougerti | Synergy between XANES Spectroscopy and DFT to Elucidate the Amorphous Structure of Heterogeneous Catalysts: TiO 2 -Supported Molybdenum Oxide Catalysts[END_REF]. whose positions and relative intensities are in good agreement with AHM features. Thus, it can be concluded that the speciation of molybdenum in the oxide state is relatively similar on A , C (0001), M and R planes which can be explained by the presence of polymolybdate-like species with Mo located in edge-shared octahedral [START_REF] Tougerti | Synergy between XANES Spectroscopy and DFT to Elucidate the Amorphous Structure of Heterogeneous Catalysts: TiO 2 -Supported Molybdenum Oxide Catalysts[END_REF][START_REF] De Boer | Remarkable spreading behavior of molybdena on silica catalysts: an in situ EXAFS-Raman study[END_REF]. More specific information on a potential surface-dependent orientation of supported polymolybdate species can be obtained by comparing the spectra when the electric field vector of the synchrotron beam is set parallel or perpendicular to the surface of the single crystal wafer. In the case of an oriented adsorption of polymolybdate on the surface, the XANES region should exhibit dissimilarities between both polarizations [START_REF] Tougerti | Surface Science Approach to the Solid-Liquid Interface: Surface-Dependent Precipitation of Ni(OH) 2 on α-Al 2 O 3 Surfaces[END_REF][START_REF] Shirai | Polarized Total-Reflection Fluorescence EXAFS Study of Anisotropic Structure Analysis for o Oxides on α-Al 2 O 3 (0001) as Model Surfaces for Active Oxidation Catalysts[END_REF][START_REF] Chun | Polarization-dependent total-reflection fluorescence XAFS study of Mo oxides on a rutile TiO 2 (110) single crystal surface[END_REF] since Mo atoms are non equivalent in polyoxomolybdate species and each of them contributes directly to the relative intensity of the different maxima [START_REF] Tougerti | Synergy between XANES Spectroscopy and DFT to Elucidate the Amorphous Structure of Heterogeneous Catalysts: TiO 2 -Supported Molybdenum Oxide Catalysts[END_REF].

For the C (0001), A and M planes, no or very minor changes are discernible in the global XANES shape when the polarization is changed (Figure 2) suggesting that molybdates species are not specifically oriented on these three facets. One weak maximum is observed at 20 023 eV for the M plane in parallel polarization along with a slight shift of the pre-edge maximum. However, these small differences should not be overinterpreted since this spectrum is contaminated with numerous diffraction peaks from the alumina substrate throughout the energy range that modify the overall shape of the spectrum.

Parallel Perpendicular

Conversely, on the R plane, modest but discernable changes can be observed throughout the XANES region for the parallel and perpendicular polarizations: width of the pre-edge region and relative intensity between 20 020 and 20 040 eV. These differences suggest that molybdates species are specifically oriented on this surface in agreement with a previous work from Tougerti et al. [START_REF] Tougerti | Surface Science Approach to the Solid-Liquid Interface: Surface-Dependent Precipitation of Ni(OH) 2 on α-Al 2 O 3 Surfaces[END_REF] for Ni adsorption on the same surface.

This polarization dependence is further demonstrated by examination of both the EXAFS part (Figure 3a) and the Fourier-Transform of the EXAFS region for the R surface for both polarizations (Figure 3b). In Figure 3b Qantitative analysis of the EXAFS results was performed by fitting the spectra obtained on the R surface in the range of R = 1.0-3.6 Å (Table 1). The Mo-O shell can been fitted

with one O contribution at 1.77 Å with coordination numbers of 2.0 (parallel) and 3.6

(perpendicular) depending on the polarization. In fact, the variety of Mo-O distances in polymolybdates is responsible of constructive and destructive interferences in the EXAFS signal leading to a challenging determination of the number of oxygens neighbors [START_REF] Koningsberger | X-ray absorption: principles, applications, techniques of EXAFS, SEXAFS, and XANES[END_REF][START_REF] Clausen | X-ray absorption spectroscopy studies of calcined Mo/Al 2 O 3 and CoMo/Al 2 O 3 hydrodesulfurization catalysts[END_REF][START_REF] Leliveld | A Mo-K Edge XAFS Study of the Metal Sulfide-Support Interaction in (Co) Mo Supported Alumina and Titania Catalysts[END_REF].

Nevertheless, the clear difference shown for the two polarizations strongly suggests that the polymolybdate-like species are more extended in the perpendicular direction.

Scheme 1: "Edge-on" orientation of an heptamolybdate-like species on the R plane of -Al 2 O 3 .

is the wave vector and is the electric field vector of the synchrotron beam. Parallel and perpendicular polarizations refer to the orientation of with respect to the surface of the wafer.

The second shell can be fitted with two Mo contributions at 3.21 and 3.41 Å with similar coordination numbers of 0.6 (Figure 3c, Table 1). These results are in good agreement with those obtained for Mo catalysts supported on alumina powders [START_REF] Carrier | Physical techniques and catalyst preparation: Determining the interactions of transition-metal complexes with oxide surfaces[END_REF].

3,2Å

XANES analysis showed that polymolybdate-like species are predominant and heptamolybdate can be taken as a simplified model of these surface polymolybdates. If one considers an orientation "edge-on" of the polymolybdate on the surface i.e. adsorption via the smaller edge of the polyoxo compound), Mo-Mo bonds at 3.2 and 3.4 Å will be parallel to the electric field vector in perpendicular polarization (right on scheme 1) which will explain a strong enhancement of their contribution in the EXAFS signal [START_REF] Shirai | Polarized Total-Reflection Fluorescence EXAFS Study of Anisotropic Structure Analysis for o Oxides on α-Al 2 O 3 (0001) as Model Surfaces for Active Oxidation Catalysts[END_REF]. Conversely, extinction of these Mo-Mo contributions will occur in parallel polarization since Mo-Mo bonds at 3.2 and 3.4 Å will be perpendicular to the electric field vector (left on scheme 1).

The EXAFS analysis points to a structuring effect of the R plane on Mo surface species at the oxide state. On this surface, enhanced contributions of the Mo-O and Mo-Mo shells in perpendicular polarization suggest that molybdenum polyoxo clusters are more extended in the perpendicular direction as schematically represented on Scheme 1. This effect can be related to the conclusions drawn from AFM measurements (vide supra) that demonstrated the strongest metal-support interactions on the R plane.

In the following sections, the influence of the oxide support on the genesis and orientation of the active phase (i.e molybdenum sulfide) will be discussed.

Genesis of the molybdenum sulfide active phase

Sulfidation is a crucial step in the catalyst synthesis route since it transforms the catalytically inactive molybdenum oxide phase into the active sulfide phase (i. Following this general procedure, the sulfidation degree (i.e. relative contribution of MoS 2 to the total contribution of Mo) has been determined for each model catalyst at different sulfidation temperatures (Figure 5). For all catalysts, sulfidation starts at low temperature (100 °C) and reaches about 90 % at 450 °C in good agreement with conventional supported catalysts [START_REF] Gandubert | X-ray photoelectron spectroscopy surface quantification of sulfided CoMoP catalysts. Relation between activity and promoted sites. Part II: Influence of the sulfidation temperature[END_REF]. One main difference among all model catalysts comes from a much higher sulfidation degree (53 %) at 100 °C on the C (0001) surface as compared to the other orientations (15-30 %) and to conventional catalysts [START_REF] Gandubert | X-ray photoelectron spectroscopy surface quantification of sulfided CoMoP catalysts. Relation between activity and promoted sites. Part II: Influence of the sulfidation temperature[END_REF]. Similarly, a maximum sulfidation degree of 90 % is reached as soon as 300°C on the C (0001) plane whereas the same ratio is only achieved at 450 °C for the other facets.

These results can be connected to different strengths of metal-support interactions as explained above. Namely, weak metal-support interactions at the oxide state on the C (0001) plane can explain extensive sulfidation at low temperature. These results are in line with the AFM results and also with the negligible reactivity of the C (0001) plane toward Mo or Ni adsorption previously reported [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF][START_REF] Tougerti | Surface Science Approach to the Solid-Liquid Interface: Surface-Dependent Precipitation of Ni(OH) 2 on α-Al 2 O 3 Surfaces[END_REF]. Al 6c -µ 2 -OH surface hydroxyls wich are the only adsorption sites present on the C (0001) plane [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF] form weak bonds with the polyoxo clusters which explains a full sulfidation at much lower temperature than the other alumina orientations. A and M planes display similar sulfidation degree (except at 100 °C where a difference is observed). Moreover, as a general trend, these two surfaces show a slightly higher sulfidation degree as compared to the R plane while these differences level off at 450°C. This conclusion is also in agreement with AFM results that showed stronger metalsupport interactions at the oxide state for the R plane. This is also in line with the structuring effect of this latter surface for Mo oxide species observed with EXAFS. Moreover, these results derived from catalysts prepared by impregnation are also in full agreement with our previous results obtained for catalysts prepared by selective adsorption [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF].

Stronger metal-support interactions on the R plane were explained by the speciation of surface hydroxyls, Al 4c -µ 1 -OH, that will favor stronger bonds with the oxide phase leading to smaller oxide particles well dispersed and anchored to the surface and hardly sulfidable. AFM images (Figure 6) obtained on sulfided samples at 450°C show a homogeneous dispersion of MoS 2 nanoparticles for A , M and R planes whereas large aggregates are observed on the C (0001) plane in agreement with the large clusters previously detected on this surface at the oxide state (Figure 1). Examination of cross section profiles confirms a similar and homogeneous particle density on A , M and R orientations with an average particle height of 1-1.5 nm while a highly heterogeneous distribution is observed on the C (0001) plane with larger particles in height (2-8 nm).

It has also to be noted that comparison of Figure 1 (oxide state) and Figure 6 (sulfide state)

shows a spreading of Mo particles during sulfidation for A and M planes.

Aggregates observed at the oxide state, are no longer present after sulfidation. This behavior may be related to intermediate metal-support interactions as suggested by the different results reported above. Following recent in-situ TEM results [START_REF] Hansen | Growth Mechanism for Single-and Multi-Layer MoS 2 Nanocrystals[END_REF], we may infer that small particles are probably sulfided first and then grow by adding Mo from larger particles (layer-by-layer growth).

TEM analysis was performed to confirm the influence of the support orientation on the size (length of the fringes) and stacking of MoS 2 slabs. Model catalysts were sulfided at 300 °C only, since XPS results (Figure 5) showed that differences among the various orientations in The stacking distribution (Figure 8) does not change drastically between the four crystallographic orientations. However, it can be noticed that the C (0001) plane displays a slightly higher stacking degree which is also visible in Figure 7. This result is in agreement with Sakashita [START_REF] Sakashita | Effects of surface orientation and crystallinity of alumina supports on the microstructures of molybdenum oxides and sulfides[END_REF] who showed that aggregates at the oxide state lead to highly stacked slabs In order to obtain information on the orientation of MoS 2 slabs, model catalysts have been further investigated at the sulfide state by GI-XAS and HAADF-STEM.

Support effect on the orientation of MoS 2 slabs

As for TEM analysis, model catalysts were sulfided at 300 °C under H 2 S(15%)/H 2 before GI-EXAFS experiments and analyzed using two polarizations (i.e parallel and perpendicular) as discussed previously for the oxide phase.

Mo K-edge XANES spectra of sulfided model catalysts are compared to bulk MoS 2 in Figure 9. The spectrum of the latter reference compound (blue curve, Figure 9) shows a shoulder at 20 014 eV and a single broad band at 20 032 eV in agreement with literature data [START_REF] Cesano | Model oxide supported MoS 2 HDS catalysts: structure and surface properties[END_REF]. The global shape of the XANES spectra of Mo/-Al 2 O 3 model catalysts exhibit similar features confirming sulfidation as shown above with XPS (Figure 5) [START_REF] Koningsberger | X-ray absorption: principles, applications, techniques of EXAFS, SEXAFS, and XANES[END_REF][START_REF] Boudart | Study by synchrotron radiation of the structure of a working catalyst at high temperatures and pressures[END_REF][START_REF] Clausen | Extended x-ray absorption fine structure study of the cobalt-molybdenum hydrodesulfurization catalysts[END_REF]. Partial erosion of the shoulder at 20 014 eV also confirms that a full sulfidation has not been reached for all model catalysts as expected from the sulfidation temperature (i.e 300 °C) [START_REF] Leliveld | The sulfidation of γalumina and titania supported (cobalt)molybdenum oxide catalysts monitored by EXAFS[END_REF].

Very small polarization effects can be distinguished for the A , M and R orientations (Figure 9) which implies that there is no or little preferential orientation of the agreement with reported data for supported MoS 2 due to the small size and low crystallinity of these particles [START_REF] Cesano | Model oxide supported MoS 2 HDS catalysts: structure and surface properties[END_REF][START_REF] Calais | Crystallite size determination of highly dispersed unsupported MoS 2 catalysts[END_REF]. More quantitative information was obtained by fitting the experimental spectra for the model catalysts (see supporting information). All model catalysts can be fitted with a first shell at 2.36 Å with a coordination number between 2.4 and 5.3 and a second shell at 3.16 Å with a coordination number between 0.8 and 2.1. These data are in line with results reported for conventional catalysts supported on -alumina [START_REF] Shimada | EXAFS study on the dispersion of molybdenum sulfide catalysts on γalumina[END_REF][START_REF] Kulkarni | EXAFS study of the molybdenum/titania hydrodesulfurization catalysis[END_REF][START_REF] Yoshimura | Molybdate catalysts prepared by a novel impregnation method: effect of citric acid has a ligand on the catalytic activities[END_REF][START_REF] Matsubayashi | Structural change of supported Ni-Mo sulfide catalysts during the hydrogenation of coalderived liquids[END_REF][START_REF] Yokoyama | Deactivation of Co-Mo/Al 2 O 3 hydrodesulfurization catalysts during a one-year commercial run[END_REF]. The size of MoS 2 slabs can be potentially determined based on the coordination number of Mo second neighbors [START_REF] Calais | Crystallite size determination of highly dispersed unsupported MoS 2 catalysts[END_REF][START_REF] Shido | Why EXAFS underestimated the size of small supported MoS 2 particles[END_REF]. In fact, a coordination number of 1-2 as found above for model catalysts should correspond to a slab size of about 1-2 nm following the model of Shido and

Prins [START_REF] Shido | Why EXAFS underestimated the size of small supported MoS 2 particles[END_REF]. This size is clearly underestimated with respect to TEM analysis (Fig. 8) and this discrepancy has been largely debated in the literature and explained by several effects including a high degree of disorder and/or existence of TEM-invisible small MoS 2 particles [START_REF] Calais | Crystallite size determination of highly dispersed unsupported MoS 2 catalysts[END_REF][START_REF] Shido | Why EXAFS underestimated the size of small supported MoS 2 particles[END_REF].

The EXAFS signal was very weakly affected by changing the polarization for the A , M and R planes as already observed for the XANES region, meaning that MoS 2 slabs are not specifically oriented on these surfaces. Conversely, a more noticeable polarization effect is observed for the C (0001) plane in line with XANES results. The FT (Figure 10) shows that the intensity of the second shell (Mo neighbors) is higher for the discussed above since it has been shown by DFT calculations that the absence of specific interactions between the active phase and the support will favor a basal orientation of MoS 2 [START_REF] Arrouvel | Edge wetting effects of γ-Al 2 O 3 and anatase-TiO 2 supports by MoS 2 and CoMoS active phases: A DFT study[END_REF][START_REF] Arrouvel | A density functional theory comparison of anatase (TiO 2 )-and γ-Al 2 O 3 -supported MoS 2 catalysts[END_REF]. Sulfide model catalysts have also been analyzed by HR HAADF-STEM in order to confirm the orientation of MoS 2 slabs determined by GI-EXAFS. In fact, with STEM in the HAADF detection mode, intensity is proportional to the squared atomic number (Z²) of the observed atoms [START_REF] Jesson | Incoherent imaging of crystals using thermally scattered electrons[END_REF][START_REF] Nellist | Direct Imaging of the Atomic Configuration of Ultradispersed Catalysts[END_REF], making feasible the detection of MoS 2 slabs oriented perpendicular to the incident electron beam and thus parallel to the surface and possibly the determination of the associated 2D morphology [START_REF] Carlsson | Supported metal sulphide nanoclusters studied by HAADF-STEM[END_REF][START_REF] Alphazan | Highly Active Nonpromoted Hydrotreating Catalysts through the Controlled Growth of a Supported Hexagonal WS 2 Phase[END_REF][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][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[END_REF]. The analysis was also performed on samples sulfided at 300 °C to compare HAADF-STEM images to GI-EXAFS results. On the C (0001) plane, a preferential basal orientation of MoS 2 slabs was observed (Figure 11) in full agreement with XAS results. The average slab size is also larger (with a distribution in the 2-6 nm range) with respect to the other orientations as shown previously by TEM (Figure 8). On the M plane, STEM images show a random orientation with a similar ratio of edge and basal bonded MoS 2 slabs (Figure 11) wich confirms the absence of polarization effect in XAS. One-layer thick MoS 2 slabs are predominant on this surface as reported by TEM. Finally, on the R surface, STEM results are less straightforward than with TEM and AFM. One-layer thick MoS 2 slabs are observed which are mostly basal bonded with a size distribution in the range of 1-4 nm in good agreement with TEM analysis.

C plane M plane R plane

Basal-bonding

Basalbonding

Edgebonding

Edgebonding

Basalbonding

However, there is also another group of edge-bonded MoS 2 clusters with a higher stacking degree (up to 6) and larger average size around 8 nm suggesting that this sample is probably more heterogeneous than the one observed with conventional TEM.

Nevertheless, HAADF-STEM observations are qualitatively in line with GI-EXAFS and confirm the support effect on the orientation of the active phase: predominant basal orientation on the C (0001) plane and random (basal and edge bonding) on the M and R planes.

Influence of synthesis route: equilibrium adsorption vs impregnation

The results reported in the present contribution are related to model catalysts prepared by a simple impregnation of Mo in order to mimic the classical incipient wetness impregnation used for industrial catalysts. These results, especially sulfidation degree, can be compared to our previous study on the same type of model systems but prepared by equilibrium adsorption (selective adorption) [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF]. The latter technique is more selective since only specifically adsorbed species remain on the surface thanks to a washing step which removes weakly bound Mo species. This comparison will be focused on the A and R orientations since i) the C (0001) plane is not adsorbing Mo through equilibrium adsorption [START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF] and ii) the A and M planes show a similar behavior due to the same type of surface hydroxyls [START_REF] Bara | A review of surface science approaches for the preparation of alumina-supported hydrotreating catalysts[END_REF].

The Mo loading investigated is different for the two surfaces considered since the nature and number of each surface adsorption sites are different and hence, the maximum Mo surface density that can be reached by equilibrium adsorption is also different (i.e about 3.5 at.nm -2 for the A plane and 1.0 at.nm -2 for the R plane).

Figure 12 compares the sulfidation degree for both orientations and for both deposition methods. On both planes, the sulfidation degree is always lower for equilibrium adsorption for all sulfidation temperatures: about 10 to 20 % lower for the A plane and 20 to 30 % for the R plane. These differences are the highest for the surface showing the strongest metal-support interactions.

These differences may be explained by a more homogeneous spreading of polyoxo Mo species on the surface through equilibrium adsorption due to the deposition procedure: large volume of solution, long equilibration time (5 h). Moreover, the washing step removes nonchemically adsorbed Mo species that would be easier to sulfide. Hence, all other things being equal, equilibrium adsorption will lead to more evenly spread molybdates in stronger interaction with the surface, probably via Al-O-Mo chemical bonds and consequently to a lower sulfidation degree as compared to impregnation.

From model systems to industrial catalysts

We have shown previously [START_REF] Bara | A review of surface science approaches for the preparation of alumina-supported hydrotreating catalysts[END_REF][START_REF] Bara | Aqueous-phase preparation of model HDS catalysts on planar alumina substrates: support effect on Mo adsorption and sulfidation[END_REF] slabs could be associated to type II on the C (0001) plane, i.e. weaker metal-support interactions and higher activity than type I. However, this analogy has to be restrained since i) type II structures do not necessarily involve multi-stacked slabs [START_REF] Candia | Effect of sulfiding temperature on activity and structure of cobalt-molybdenum/γ-alumina catalysts. II[END_REF][START_REF] Hinnemann | A density functional study of the chemical differences between Type I and Type II MoS 2 -based structures in hydrotreating catalysts[END_REF] and ii) the size of the slabs are another key parameter since large particles (as on the C (0001) plane) will have a reduced number of edge sites leading to lower catalytic activity. Furthermore, it has to be noted that our results are at variance with the work of Sakashita [START_REF] Sakashita | Effects of surface orientation and crystallinity of alumina supports on the microstructures of molybdenum oxides and sulfides[END_REF] who suggested a perpendicular orientation of MoS

Conclusions

A model approach has been followed in order to gain a molecular-scale insight into the Mo speciation at the oxide and sulfide states in alumina-supported hydrotreating catalysts. Several

Figure 1 :

 1 Figure 1: Representative AFM images (5 x 5 µm 2 , Peak Force Tapping mode, z-scale 4 nm) of planar Mo/α-Al 2 O 3 model catalysts with a Mo surface loading of 3.5 at.nm -2 after impregnation and calcination at 450 °C for 2 h for various alumina orientations: C (0001), A , M and R

  intermediate behavior with stabilization of one group of oxide particles and sintering of the other group. These results are in full agreement with our previous work on A , M and R planes after selective adsorption of Mo

Figure 2

 2 Figure 2 clearly shows that for all model catalysts, the pre-edge XANES feature is similar to the reference compounds where Mo is in octaedral coordination (i.e AHM and MoO 3 ) which rules out a tetrahedral symmetry for Mo. A closer look at the edge region also shows that model catalysts show three successive maxima at 20 023, 20 037 and 20 053 eV (Figure 2)

Figure 2 :

 2 Figure 2: Mo K-edge XANES spectra of reference compounds and model Mo catalysts in parallel (black curve) and perpendicular (red curve) orientations with respect to the electric-field vector of the synchrotron beam. Model catalysts are supported on the C (0001), A , M and R planes of -Al 2 O 3 . Samples were prepared by Mo impregnation (high loading 3.5 at.nm -2 ) and calcination at 450°C for 2h. (Norm µ(E): normalized absorption).

  (Fourier Transforms), the relative intensity of the first peak at 1.2 Å corresponding to the Mo-O shell is more intense for the perpendicular polarization. Furthermore, the second shell (Mo-Mo) is only visible in this perpendicular polarization. For the sake of comparison, the FT of AHM has been reported and the second coordination shell (Mo-Mo) displays also two maxima at slightly lower Mo-Mo distances (i.e 2.7 and 3.2 Å) with a shoulder on the first peak. This pattern has been explained in the literature by the presence of three different types of Mo atoms in AHM with four different Mo-Mo distances from 3.19 to 3.43 Å[START_REF] Evans | Crystal structure of the heptamolybdate(VI) (paramolybdate) ion, [Mo 7 O 24 ] 6in the ammonium and potassium tetrahydrate salts[END_REF][START_REF] Carrier | Physical techniques and catalyst preparation: Determining the interactions of transition-metal complexes with oxide surfaces[END_REF].

Figure 3 :

 3 Figure 3: a) k 3 -weighted Mo K-edge EXAFS spectra for Mo/-Al 2 O 3 on the R plane calcined at 450°C for 2 h in parallel (black curve) and perpendicular (red curve) polarizations as well as for Ammonium Heptamolybdate (AHM, green curve), b) Corresponding Fourier transforms (Δk = 4-10 Å - 1 ) uncorrected for phase shift and c) the real part of the Fourier-filtered FT (ΔR = 1.0-3.6 Å) of the model catalyst in perpendicular polarization (red curve) and best fit (blue curve).

Table 1 :

 1 EXAFS fitting results of the Fourier filtered (Δk = 4-10 Å -1 , ΔR = 1.0-3.6 Å) EXAFS signal for the parallel and perpendicular polarizations of the model catalyst Mo/-Al 2 O 3 on the R plane. N are effective coordination numbers. Only one σ 2 are fitted for the two Mo-Mo shells in perpendicular polarization and a single ΔE 0 value for all shells. Standard errors in EXAFS are N ± 20%, R ± 0.04 Å,  2 ± 20% and  ± 10%.

  The presence of Mo-Mo contributions only on the FT in perpendicular polarization is also in favor of a specific orientation of the polymolybdates. If one takes into account the structure of heptamolybdate, Mo-Mo bonds at 3.2 and 3.4 Å are indeed present (scheme 1). Of course, pure heptamolybdate species do no exist anymore as such on the surface after calcination but

  MoS 2 ). Metal-support interactions play a key role at this step which requires sulfidation of Mo-O-Al linkages (if anay) to substitute oxygen atoms by sulfur atoms [10,35-37]. Hence, different metal-support interactions should eventually lead to different sulfidation degrees. Calcined model catalysts supported on A , C (0001), M and R planes with a Mo loading of 3.5 at.nm -2 were sulfided in gas phase (15 % H 2 S/H 2 ) at several temperatures from 100 to 450 °C. Quantification of the sulfidation degree was performed by XPS spectroscopy based on the decomposition of Mo3d, S2s and S2p peaks taking into account the occurrence of three different Mo species: molybdenum oxide (MoO 3 ), molybdenum oxysulfide (MoO x S y ) and molybdenum sulfide (MoS 2 ) [38,39]. An example of the decomposition of XPS spectra is shown in Figure 4 for a model catalyst sulfided at 450 °C and supported on the M plane. The S2p peak can be decomposed in two contributions at 161.8 and 163.3 ± 0.1 eV assigned to S 2-species involved in the MoS 2 phase and S 2 2species related to oxysulfide compounds respectively [39]. The 220-230 eV region is composed of five contributions. Three of them at 228.9, 230.3 and 232.8 ± 0.1 eV are assigned to Mo3d for MoS 2 , MoO x S y and MoO 3 respectively in good agreement with literature data [38,39]. The two other contributions at lower binding energy (i.e 226.0 and 227.5 ± 0.1 eV) are ascribed to S2s contributions (S 2-and S 2 2-respectively) in agreement with the S2p photopeak.

Figure 4 :

 4 Figure 4: Decomposition of S2p, Mo3d and S2s XPS peaks for a model Mo/-Al 2 O 3 catalyst supported on the M plane (3.5 atMo/nm²) and sulfided at 45 °C for 2 h.

Figure 5 :

 5 Figure 5: Sulfidation degree for model Mo/-Al 2 O 3 catalysts supported on C (0001), A , M and R planes as a function of sulfidation temperature with a Mo surface loading of 3.5 at.nm -2 .

Figure 6 :

 6 Figure 6: Representative AFM images (2.0 × 2.0 µm 2 , peak force tapping mode in air, z-scale 8.0 nm) obtained on model Mo/-Al 2 O 3 catalysts supported on C (0001), R , A and M planes and sulfided at 450 °C (Mo surface loading is 3.5 at.nm -2 ). Cross sections were taken at the position indicated by the white dashed lines on the images.

  degree are smoothed at higher temperatures. Size distributions determined from TEM images (Figure7) are presented on Figure8. The trends are following those observed for the sulfidation degree: the highest average size (5,1 ± 0.1 nm) is obtained for the C (0001) plane with an important proportion of MoS 2 slabs (34 %) having a length above 5 nm, the lowest size (3,7 ± 0.1 nm) for the R plane and intermediate but similar sizes (4.1-4.2 ± 0.1 nm) are obtained for the A and M surfaces. Hence, both AFM and TEM reveal larger MoS 2 particles on the C (0001) plane.

Figure 7 :

 7 Figure 7: Typical TEM images of model Mo/-Al 2 O 3 catalysts supported on A and C (0001) planes sulfided at 300°C in H 2 S/H 2 (Mo surface loading is 3.5 at.nm -2 ).

Figure 8 :

 8 Figure 8: MoS 2 size and stacking distribution for model Mo/-Al 2 O 3 catalysts supported on C (0001), R , A and M planes sulfided at 300°C in H 2 S/H 2 (Mo surface loading is 3.5 at.nm - 2 ).

Frequency

MoS 2

 2 phase on these surfaces. Conversely, a more pronounced polarization effect appears for the C (0001) plane with an erosion of the shoulder at 20 014 eV and a more intense broad band at 20 032 eV for the perpendicular polarization. It is interesting to note at this point that there is no direct and simple relation for the orientation of Mo clusters between the oxide and sulfide state. In fact, GI-XAS experiments at the oxide state showed a preferential orientation of polyoxomolybdate on the R plane and random orientation on the C (0001) plane while the situation is exactly reversed after sulfidation.

Figure 9 :

 9 Figure 9: Mo-K edge XANES spectra of bulk MoS 2 (blue curve) and sulfided model Mo/-Al 2 O 3 catalysts in parallel (black curve) and perpendicular (red curve) polarizations supported on the C (0001), A , M and R planes. Samples were prepared by Mo impregnation with a Mo surface loading of 3.5 at.nm -2 , calcination at 450°C for 2h and sulfidation at 300 °C under H 2 S/H 2 (Norm µ(E): normalized absorption).

Figure 10 :

 10 Figure 10: a) k 3 -weighted Mo K-edge EXAFS spectra of bulk MoS 2 (blue curve) and sulfided model Mo/-Al 2 O 3 catalysts (Mo surface loading is 3.5 at.nm -2 ) supported on the C (0001) and R planes in parallel (black curve) and perpendicular (red curve) polarizations, b) Corresponding Fourier Transforms (Δk = 4-9.5 Å -1 ) uncorrected for phase shift.

  polarization. Quantitative fitting yields a coordination number of 1.8 Mo in parallel polarization and 0.8 Mo in perpendicular polarization. This difference can be explained by a preferential orientation of MoS 2 slabs parallel to the surface. One hypothesis could involve a basal bonding of MoS 2 slabs on the support (Scheme 2). In this case, Mo-Mo bonds are parallel to the electric field vector in parallel polarization leading to an enhancement of the Mo-Mo contribution. In perpendicular polarization, Mo-Mo linkages are perpendicular to the electric field vector which explains a decrease of the Mo-Mo contribution. However, if all MoS 2 slabs were basal bonded, the Mo-Mo contribution should completely vanish in perpendicular polarization. Since the second Mo shell is present for both polarizations, it is more likely that only part of MoS 2 slabs are basal bonded while another part is randomly oriented. It can be inferred that basal bonded slabs are the largest ones while the smallest ones are randomly oriented. Another alternative hypothesis would involve a tilted geometry for 100% of MoS 2 slabs with respect to the surface. However, this hypothesis appears much less plausible since AFM (Figure6) showed a rather heterogeneous distribution of MoS 2 on the surface which is not in agreement with a unique orientation of MoS 2 clusters.

Scheme 2 :

 2 Scheme 2: Basal orientation of MoS 2 slabs on the C (0001) plane of -Al 2 O 3 and change of polarization with respect to the synchrotron beam: parallel on the left and perpendicular on the right.is the wave vector and is the electric field vector. (

Figure 11 :

 11 Figure 11: Typical HR HAADF-STEM images of sulfided model Mo/-Al 2 O 3 catalysts (Mo surface loading is 3.5 at.nm -2 ) supported on the C (0001), M and R orientations of -Al 2 O 3 . Samples were prepared by Mo impregnation, calcination at 450°C for 2h and sulfidation at 300°C under H 2 S/H 2 .

Figure 12 :

 12 Figure 12: Comparison of sulfidation degree as a function of sulfidation temperature for model Mo/-Al 2 O 3 catalysts prepared by impregnation (red triangles) and by equilibrium adsorption (blue circles) on the A plane (Mo loading of 3.5 at.nm -2 ) and on the R plane (Mo loading of 1.0 at.nm - 2 ).

3 (Scheme 3 :

 33 Scheme 3: Surface-dependent speciation of Mo species at the oxide and sulfide state on α-Al 2 O 3 surfaces based on the results described in this work and proposed relationship with γ-Al 2 O 3 by taking into account the speciation of surface OH 14 and the morphology of γ-Al 2 O 3 particles as shown in Digne et al.[60] M/S stands for Metal/Support.

• 3 Sulfidation• 3 C

 33 Dispersed and oriented oxide particles • Strong M/S interactions -Al 2 O 3 A and M planes: Al 6c -µ 1,2,3 -OH • Intermediate sulfidation degree • No specific orientation Type I/II ? -Al 2 O MoOx aggregates and dispersed particles • Intermediate M/S interactions -Al 2 O metal-support interactions are postulated leading to shorter and less stacked MoS 2 slabs and to lower sulfidation degree with respect to the C (0001) facet. As a consequence the nature of the active phase is different on the A /M planes and on the C (0001) plane. Hence, this dissimilarity is in disagreement with our previous assumption [11] that the sulfide phase was of type II on A /M planes and therefore on the (100) plane of -Al 2 O 3 . The situation is probably more complex with an intermediate case between type I and type II structures.

  2 slabs on the (100) -Al 2 O 3 surface while our GI-EXAFS and HAADF-STEM results reveal a random orientation of MoS 2 slabs (i.e basal and edge bonding). Finally, the strongest metal/support interactions are observed on the R plane with a homogeneous dispersion (AFM) and specific orientation (GI-EXAFS) of Mo oxide nanoparticles. Conversely, no specific orientation of MoS 2 slabs has been found from GI-EXAFS analysis suggesting that restructuring of the supported Mo phase occurs during sulfidation. Low sulfidation degree along with small and weakly stacked MoS 2 slabs could be associated to type I structures on this surface and the associated (110) -Al 2 O 3 surface.

-Al 2 O

 2 3 surfaces with different orientations (A , C (0001), M and R ) were used as surrogates for -Al 2 O 3 . The surface relationship between  and -Al 2 O 3 polymorphs is summarized in Scheme 3. Mo deposition was performed by impregnation to mimic industrial catalyst synthesis procedure and to get a proper control of the molybdenum surface density which facilitates comparison between model surfaces. AFM characterization of the calcined catalysts reveals different dispersions for Mo oxide nanoparticles which can be related to the strength of metal-support interactions. Three differents behaviors have been documented: i) weak metal-support interactions on the C (0001) plane, ii) intermediate on A and M planes and iii) strong on the R plane that can be related to speciation of surface hydroxyls. A specific orientation of Mo oxide species has also been pointed out by GI-XAS spectroscopy on the R plane revealing a structuring effect of the facet in agreement with the high strength of metal-support interactions. The Mo sulfidation capacity on each model surface is a direct consequence of the previous findings. A maximum sulfidation degree (90 %) at low temperature (300°C) was observed for the C (0001) plane with a larger MoS 2 slab size and stacking as compared to A , M and R facets. A predominant basal orientation of MoS 2 slabs on the C (0001) plane was also pointed out by GI-EXAFS in agreement with the absence of specific metalsupport interactions while a random orientation was observed for the other surfaces. A , and M surfaces present an intermediate sulfidation degree while the R plane shows the weakest sulfidation degree and MoS 2 slab size. Comparison among preparation routes (i.e equilibrium adsorption vs. impregnation) indicates that equilibrium adsorption leads to the weakest sulfidation degree which can be explained by Mo species in stronger interaction with the surface due to a more efficient spreading during deposition. Extension of these results to  alumina suggests a surface-dependent Mo sulfidation and orientation on industrial hydrotreating catalysts. They confirm that the precise control of the alumina morphology might lead to more active catalyst. contribution to HAADF-STEM analyses at the Université de Strasbourg (IPCMS-UMR 7504 CNRS).

Preparation of model catalysts (MoOx/-Al 2 O 3

) 1 cm 2 single crystal -alumina wafers were purchased from Mateck for the A (11 0), C (0001) and R planes and from SurfaceNet for the M orientation. The wafers were first chemically cleaned with successive washing steps in water, HNO 3 and NH 3

  2 ,MoO x S y and MoO 3 . The sulfidation degree (% MoS 2 ) was calculated as the ratio of the contribution of MoS 2 over the total contribution of Mo (MoS 2 + MoO x S y + MoO 3 ). Sulfided samples were rapidly transferred (less than 1 min.) from the sulfidation reactor to the XPS analysis chamber and it was checked that this fast transfer prevents any reoxidation of MoS 2 .

	TEM: Conventional TEM images were collected with a JEOL 2010 microscope operating at
	200 kV for determining the average size (length of a fringe) and stacking (number of fringes)
	of MoS 2 particles using ImageJ 1.45 software [13]. TEM samples were obtained by scratching
	the surface with a razor blade in order to concentrate the clusters of active phase into an
	ethanol drop before spreading this drop on a carbon-coated copper grid.
	High-resolution high angle annular dark field scanning transmission electron microscopy (HR
	HAADF-STEM) analyses were performed using a JEOL 2100 F FEG TEM/STEM instrument
	operating at 200 kV, equipped with a Cs aberration probe corrector. For HAADF acquisition,
	the camera length was 10 cm corresponding to inner and outer diameters of annular detector
	of about 60 and 160 mrad. Model catalysts (MoS 2 /-Al 2 O 3 wafers) were scratched with a
	diamond tip scriber. The as obtained alumina fragments were suspended in ethanol and
	sonicated. A drop of the suspension was deposited on a Cu grid covered with a holey carbon
	membrane.
	AFM: Topographic AFM images on MoS 2 /-Al 2 O 3 samples were obtained on a Nanoscope
	VIII Multimode AFM from Bruker Nano Inc. (Nano Surfaces Division, Santa Barbara, CA)

Acknowledgements

e gratefully ac nowledge . M thivier and . Calers for assistance in XPS analysis and S. Casale for TEM experiments. GI-XAS Experiments were performed on the Samba beamline at SOLEIL Synchrotron (France) under proposal number 20131167. We are grateful to the SAMBA beamline staff for valuable help. We express gratitude to O. Ersen for his

Supporting Information

Supplementary data associated with this article can be found: quantitative EXAFS analysis of sulfided model Mo/-Al 2 O 3 catalysts.


G Hubert

Ground Albedo Neutron impacts to Seasonal Variations of Cosmic-Ray-induced Neutron in medium Geomagnetic Latitude and Antarctica: Impacts on Soft Error Rate

Keywords: Cosmic-ray induced neutrons, ground albedo neutron, seasonal variations, GEANT4, high-altitude stations

This paper investigates the impact of ground albedo neutrons in the terrestrial radiation field and their seasonal variations. A modeling methodology is proposed, it is based on ATMORAD (atmospheric shower modeling) and GEANT4 simulations taking into account the soil characteristics and hydrogen content. This method was validated thanks to neutron measurements performed in two high altitude stations located in medium geomagnetic latitude and Antarctica (Pic-du-Midi and Concordia, respectively). Thus, the chemical rock composition, the density, the water content and the snow pack impacts to neutron spectrum were investigated. Simulated and measured spectra were compared. Analyses show that variations are dominant in the thermal domain (i.e. En < 0.5 eV) and lesser degree in epithermal and evaporation domains (i.e. 0.5 eV < En < 0.1 MeV and 0.1 MeV < En < 20 MeV, respectively). The last part investigates the impact induced by the evaporation neutron contribution to Single Event Rate (more impacted by the ground albedo neutron). Results show that evaporation neutrons contribution to the total SER should be assessed for highsensitive technologies.

I. INTRODUCTION

In terrestrial fields, the Single Event Upset (SEU) occurence is attributed to thermal neutron, fast neutron and alpha emissions [START_REF] Peronnard | Real-life SEU experiments on 90nm SRAMs in Atmospheric Environment: measures vs. predictions done by means of MUSCA SEP3 platform[END_REF]- [START_REF] Heidel | Alpha-particle-induced upsets in advanced CMOS circuits and technology[END_REF]. More recently, impacts of protons [START_REF] Rodbell | Low-energy Proton-Induced Single-event-Upsets in 65 nm node, Silicon-on-Insulator, Latches and Memory Cells[END_REF]- [START_REF] Hubert | Impact of the Solar Flares on the SER dynamics on Micro and Nanometric Technologies in the Geostationary Orbit[END_REF] and muons [START_REF] Sierawski | Muon-Induced Single Event Upsets in Deep-Submicron Technology[END_REF] [START_REF] Hubert | Impact of scaling on the soft error sensitivity of bulk, FDSOI and FinFET technologies due to atmospheric radiation[END_REF] were demonstrated. The evaluation of the failure rate requires one to associate the radiation field and the cross section deduced directly by experiments or simulations. As mentioned in the JEDEC JESD89A standard [START_REF] Jesd89a | Measurement and reporting of alpha particle and terrestrial cosmic ray-induced soft errors in semiconductor devices[END_REF], for actual generations of devices, the neutron energy threshold has been characterized to be 10 MeV. Nevertheless, recent works describe the importance to take into account the low-energy threshold in determining an accurate estimate of Soft Error Rate (SER), particularly for nanoscale devices. Indeed, Yahagi et al. show in [START_REF] Yahagi | Threshold energy of neutron-induced single event upset as a critical factor[END_REF] that the energy neutron threshold can induce a significant difference in SER estimations for SRAM as small as the 130 nm technological node.

The precise knowledge of the fluence rate and spectrum of Manuscript received July 11, 2016. This work is supported by the IPEV (French Polar Institute, Institut Paul-Emile Victor) in the framework of the program no. 1112 (CHINSTRAP).

Guillaume Hubert is with the French Aerospace Laboratory (ONERA), Toulouse, France (phone: 33-562252885; fax: 33-562252569; e-mail: Guillaume.hubert@onera.fr). atmospheric cascade particles induced by galactic and solar cosmic rays is important. Secondary cosmic radiations are produced by the interaction of primary cosmic rays with the nuclei of the constituents of the atmosphere [START_REF] Sierawski | Impact of lowenergy proton induced upsets on test methods and rate predictions[END_REF]. They can interact further with atoms and nuclei of the atmosphere to produce extensive showers of neutrons, protons, photons, electrons, positrons, pions, and muons. In the terrestrial environment, neutron fluence rates and spectra can be impacted by some mechanisms as the environmental water content (hydrometry, snow and vegetation) [START_REF] Schron | Monitoring Environmental water with ground albedo neutrons and correction for incoming cosmic rays with neutron monitor data[END_REF], the interaction of alpha particles emitted by radon [START_REF] Grieder | Cosmic Rays at Earth: Researcher's Reference Manual and Data Book[END_REF] (underground environment), by the weather condition [START_REF] Kuzhevskij | Neutron field of the Earth: Origin and dynamics[END_REF] or by seismic activities [START_REF] Lockwood | Correlation of meteorological parameters with cosmic ray intensities[END_REF]. Besides, cosmic and terrestrial sources, atmospheric neutrons may be also be generated by lightning discharges [START_REF] Volodichev | The flows of neutrons of space radiation and from terrestrial crust[END_REF].

Neutron detectors (commonly named neutron monitors NM, [START_REF] Oh | Record Neutron Monitor Counting Rates from Galactic Cosmic Rays[END_REF] [START_REF] Mcdonald | Integration of neutron monitors data with spacecraft observation: a historical perspective[END_REF]) and neutron spectrometers are usually used to monitor the dynamics of incoming cosmic-ray neutrons of high energy (cascade neutron i.e. E > 20 MeV) because local environmental conditions have no influences. However, neutrons below energy of 10-20 MeV are most affected by the local environment. This fact is the result of the interaction of atmospheric neutrons with the ground matter, inducing secondary neutrons referred to as ground albedo neutrons. Hydrogen in soil, air and snow determines the amount of ground albedo neutron in the sensitive energy range from 1 eV to 20 MeV. The investigation of ground albedo neutrons is not simple because neutron spectrum measurements are required over a long time allowing one to analyze the impact of seasonal changes.

A network composed by neutron spectrometers operated in high-altitude stations in medium geomagnetic latitude and Antarctica Environment has been active since 2011 [START_REF] Hubert | Analyses of Cosmic Ray induced-Neutron based on spectrometers operated simultaneously at mid-latitude and Antarctica high-altitude stations during quiet solar activity[END_REF]- [START_REF] Cheminet | Characterization of the IRSN neutron multisphere spectrometer (HERMEIS) at European standard calibration fields[END_REF]. More precisely, data recorded since 2011 were available in the Pic-du-Midi station [START_REF] Hubert | Analyses of Cosmic Ray induced-Neutron based on spectrometers operated simultaneously at mid-latitude and Antarctica high-altitude stations during quiet solar activity[END_REF], and since December 2015 in Concordia (Antarctica). To improve the data analyses, Hubert et al. presents in [START_REF] Hubert | Radiation effects investigations based on atmospheric radiation model (ATMORAD) considering GEANT4 simulations of extensive air showers and solar modulation potential[END_REF] an atmospheric radiation model named ATMORAD and based on simulations of Extensive Air Showers, the Force-Field Approximation to model the galactic cosmic rays (GCR) and the solar modulation potential. This approach is used to estimate the atmospheric radiation environment for terrestrial/atmospheric applications and to monitor the solar potential. Thus, the knowledge of variations of thermal (i.e. En < 0.5 eV), epithermal (0.5 eV < En < 0.1 MeV) and evaporation (0.1 MeV < En < 20 MeV) domains can improve cosmic radiation monitoring [START_REF] Clem | Atmospheric yield functions and response to secondary particles of neutron monitors[END_REF], and can be used to specify spectrum in SER calculations.

In this work, the main objective is to investigate the impact of ground albedo neutron in spectrum modifications, particularly in the thermal, epithermal and evaporation domains. In a first part, the impact of ground albedo neutrons was investigated using ATMORAD and GEANT4 simulations with the aim to improve the atmospheric model taking into account the impact of local environment. The approach was validated considering measured spectra from the Concordia and Pic-du-Midi stations. The second part is devoted to evaluate the impact of radiation field improvements on the SER calculations.

II. EXPERIMENTS, METHODS AND SIMULATIONS

A. Neutron spectrometers network in High-Altitude stations

In May 2011, a neutron spectrometer [START_REF] Cheminet | Characterization of the IRSN neutron multisphere spectrometer (HERMEIS) at European standard calibration fields[END_REF] extended to the high energy was firstly operated at the summit of the Pic-du-Midi in the French Pyrenees (see table I and [22][23]). A second spectrometer was operated simultaneously since February 2015 at the summit of the Pico dos Dias (Observatory of Pico dos Dias) depending from the LNA (National Astrophysics Laboratory) in Brazil (table I). In the framework of the Continuous High-altitude Investigation of Neutron Spectra for Terrestrial Radiation Antarctic Project (CHINSTRAP) supported by IPEV (French Polar Institute), a third neutron spectrometer composed of six spheres was installed at Concordia research station (Antarctica) [START_REF] Hubert | Analyses of Cosmic Ray induced-Neutron based on spectrometers operated simultaneously at mid-latitude and Antarctica high-altitude stations during quiet solar activity[END_REF]. The station is located at Dome C on the Antarctic Plateau (75° 06' S, 123° 23' E) at height of 3,233 meters above the sea level. The site has an almost zero rigidity cutoff (R < 0.01 GV), i.e., no geomagnetic shielding even for low-energy particles. [START_REF] Hubert | Radiation effects investigations based on atmospheric radiation model (ATMORAD) considering GEANT4 simulations of extensive air showers and solar modulation potential[END_REF][23] [START_REF] Hubert | Continuous high-altitude measurements of cosmic ray neutron and SEU/MCU at various locations: correlation and analyses based-on MUSCA-SEP3[END_REF] CONCORDIA [START_REF] Hubert | Analyses of Cosmic Ray induced-Neutron based on spectrometers operated simultaneously at mid-latitude and Antarctica high-altitude stations during quiet solar activity[END_REF] Pico dos Dias [START_REF] Hubert | Long and short-term Atmospheric Radiation analyses based on coupled Measurements at High Altitude remote Stations and extensive Air Shower modeling[END_REF] Fig. 1 : Location and view of the neutron spectrometer operated in Concordia since December 2015.

Pic-du-Midi

These three high-altitude stations constitutes a neutron spectrometer network which records simultaneously the neutron fields and their short/long time dynamics (i.e. seasonal variations and solar flare events). This work is exclusively based on data recorded in the Pic-du-Midi and Concordia stations. Measurements were used to validate the proposed modeling approach and to investigate the ground albedo neutron impact on radiation field variations. The ACROPOL platform has been presented in previous work [START_REF] Hubert | Continuous high-altitude measurements of cosmic ray neutron and SEU/MCU at various locations: correlation and analyses based-on MUSCA-SEP3[END_REF], thus the Fig. 1 focus on the new installation in Concordia (location in Antarctica, the shelter, and the neutron spectrometer) while the table I summarizes station characteristics such as altitude, latitude and longitude. Data record by the neutron spectrometers consist in a series of counts M d recorded at the end of every cycles of tacq (five minutes). The fluence responses R d (E) (calculated by Monte Carlo simulations with the MCNPX2.6 and GEANT4 codes [START_REF] Cheminet | Characterization of the IRSN neutron multisphere spectrometer (HERMEIS) at European standard calibration fields[END_REF]) of the set of Bonner spheres are mandatory to derive the neutron differential fluence rate

) (E E φ &
from a series of measurements M d during the acquisition time tacq (and an absolute uncertainty ε d ) with an unfolding procedure. Thus, equation 1 can be iteratively solved by using a code named GRAVEL starting with a default spectrum [START_REF] Matzke | Unfolding procedures[END_REF][28]

∫ = ± max min ) ( ) ( ) / ( d d acq d E E E dE E E R t M φ ε & (Equ. 1)
The relative uncertainty was investigated in [START_REF] Cheminet | Characterization of the IRSN neutron multisphere spectrometer (HERMEIS) at European standard calibration fields[END_REF] and it depends to number N int of acquisition cycles and M d , the objective consist in obtaining a compromise

B. Atmospheric showers and ground albedo neutron modeling

The using of physical Monte-Carlo simulations (GEANT4 in this work) allows one to investigate the neutron transport in the material environment. Thus, it is possible to integrate intrinsic characteristics (soil composition, porosity) and to take into account seasonal changes (snow, humidity in the rock). Usually the terrestrial radiation field is based on atmospheric showers physical modeling, and consideration of soil characteristics is basic or neglected.

Indeed, high-energy neutrons propagate through the atmosphere and generate low-energy neutrons by evaporation processes. The soil further slows neutrons down especially when water is present and they quickly moderate into the thermal regime. Fast neutrons which are reflected from the soil travel large distances before they get detected. Most of the detected neutrons originate from the first tens of meters around the sensor and are able to penetrate the soil down to 90 cm. The total neutron spectrum at ground level will have two components: the spectrum of neutrons from atmospheric showers and the neutron spectrum albedo.

Ground albedo neutron

Neutron directed towards the ground, atmospheric shower Snow thickness e snow … Fig. 2 : Overview of the approach, the neutron spectrum result from the sum of atmospheric spectrum deduced from ATMORAD and ground albedo neutron calculated thanks to GEANT4 simulations.

As presented in Fig. 2, the general approach consists of considering a relatively simple scene geometry (ground plane), but to refine the level of detail in the modeling of soil with a description of the rock implying composition, density and porosity, humidity level then the incorporation of a layer of varying thickness snow. The neutron spectrum is the sum of the atmospheric spectrum deduced from ATMORAD and the ground albedo neutron spectrum calculated thanks to GEANT4 simulations.

1) Radiation field input data

The first step before to quantify the impact of the local environment, is to define the incident neutron spectrum unmodified by the local environment (expect the atmosphere). The reference spectrum is provided by ATMORAD considering only downward-propagating neutrons. Thus, the backscattered particles are not considered and they will be replaced by the ground albedo neutron calculated from GEANT4 and considering the soil characteristics and dynamics (Pyrenees in the case of the Pic-du-Midi). Thus, a high number of particles (neutron and proton, according to the reference spectrum) are considered in GEANT4 simulations.

2) Geometric modeling and physics

The geometry of the simulated scene in GEANT4 consists of a series of rectangular shapes nested in one another, including the neutron/proton sources, the soil definition, the hydrogen content characteristics, the snow thickness and the atmosphere (defined as the air). As shown in Fig. 3, the volume "World" consists of vacuum is a cube whose edge is 200 m. A smaller cube of 180×180×180 m 3 is the volume of the atmosphere filled with air. The bottom half of this cube is occupied by a parallelepiped with a square face of 170×170 m 2 and 80 m deep. This volume represents the ground. This volume may reside another parallelepiped with same section but of variable thickness. Pic-du-Midi and Concordia characteristics are very different. The local environment in Concordia is well known and stable over time (i.e. only composed by ice and few precipitation levels). The Pic-du-Midi description is more complex because it is not simple to know precisely the geological compositions and hydrologic properties. Then, the strategy consists to determine the configuration "bare rock" (without snow) closest to the experimental results obtained in summer. Once the reference configuration defined, calculations by varying the snow thickness will allow deducing the spectra obtained during the winter.

The modeling is based on the GEANT4 code Version 10.1 developed by a World-wide collaboration and distributed by CERN [START_REF][END_REF]. The "emlowenergy" physical model is used; this physical model has been designed on top of Livermore physics for validation of new low-energy models. Intra-nuclear cascades are treated with the Bertini model (BERT), the cross sections of neutrons are high precision (HP) and finally the inclusion of thermal treatment of elastic scattering (TS: thermal scattering) in molecules of water is ensured.

3) Modified spectrum calculation

Neutron spectra for a given environment (i.e. ω, ρ and snow thickness) is equal to the contribution of the spectrum of the incident neutrons directed towards the ground and albedo backscattered or reissued by the ground neutrons. Four sensitive volumes were considerer, using volumetric fluence scorers (indicated s) located 1, 2, 5 and 100 meters above the scene, respectively. Each scorer allows one to calculate the spectral distribution gs k φ considering the atmospheric and the backscattered contributions, the index g, k and s denote the neutron energy group, the primary particle type (neutron or proton) and the selected sensitive volume. The last volume (i.e. 100 m) is indented to discriminate the atmospheric shower contribution. The total fluence of incident particles n φ is derived from the number of particles and the area of the source disk using Equation 2, when N k is the total neutron or proton number considered in the simulation. The source was considered as isotropic over the upper hemisphere.

)

ATMORAD tot ATMORAD cas n Exp cas g snow g gs n snow g s n t E e E t e E φ φ φ φ ε ρ ω φ ε ρ ω φ / , , , , , , , , , ⋅ ⋅ ∆

A. Seasonal variation analyses

Variations in the neutron daily fluence rate recorded in the Pic-du-Midi were presented in previous works [START_REF] Hubert | Long and short-term Atmospheric Radiation analyses based on coupled Measurements at High Altitude remote Stations and extensive Air Shower modeling[END_REF] from June 2011 to April 2015. Updated measurements are presented in Fig. 4 until June 2016. Results show that total neutron flux variations are sinusoidal with a period of one year. This periodicity can be attributed to the local water/snow environmental dynamics. Snow cover and water content in the soil may alter the ground albedo neutrons and their transportation (absorption and backscattering). Then, it is possible to show [START_REF] Hubert | Long and short-term Atmospheric Radiation analyses based on coupled Measurements at High Altitude remote Stations and extensive Air Shower modeling[END_REF] that seasonal oscillations are predominant in the thermal part and to a lesser extends on the epithermal and evaporation regions.

Indeed, the soil is wetter during the summer as a result of the higher precipitation level and melting winter snow. In winter, precipitation induces a snowfall of up to several meters thick. The soil is relatively dry because water does not infiltrate. Thus, at mountain altitude, the most abundant amount of hydrogen is provided by snowfall in winter.

The amount of snow deposited on the ground was measured in the Pyrenees at different altitudes by automatic stations Nivôse managed by Météo France. There are four stations in the mountains of the Pyrenees, with altitudes ranging from 2140 m to 2445 m. This is therefore a range of altitude close to the summit of the Pic du Midi although slightly lower (2885 m at the Pic-du-Midi). The data from these stations allowed estimation of the average variation over three decades Thus, Fig. 5 presents the neutron flux recorded during the period 2011-2016 as function of the averaged thickness of accumulated snow. Neutron flux decreases exponentially with the snow thickness which is consistent with the neutron-matter interaction mechanisms. Moreover, it is interesting to note a low decrease in neutron flux from a snow thickness of approximately 100 cm.

B. Ground albedo neutron modeling

Impacts of the the chemical rock composition, its density and the water content were simulated to compare the calculated spectra and the measured spectra (summer). Then, a snowpack was added in the reference configuration to take 0018-9499 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNS.2016.2614540, IEEE Transactions on Nuclear Science into account the effect induced by the snow during the winter.

1) Validation of the methodology based on Concordia data

Concordia measurements are available since December 2015. The soil composition and the hydrogen content are well known and stable over time because there is little precipitation and the scene description can be reduced to an ice water thickness. The first measurements (December to June) can be used to assess the relevance of the modeling approach.

Thus, Table II presents the measured flux (average data from December to June 2016) and the calculated flux issued from GEANT4 simulations and for the four energy domains. In simulations, the "ground" volume was composed by H 2 0 ice using 0.35 g/cm 3 density. The incident spectrum is issued from ATMORAD, considering the altitude, period, geomagnetic conditions and atmospheric description of Concordia. A good agreement were obtained between simulation and measured fluxes. Analyses can be applied to the data obtained from the Picdu-Midi station, implying to define the rock composition, the porosity and the mass fraction of water and the snow thickness variations.

2) Impact of the rock configuration

To evaluate the effect of the chemical composition of the rock, four rocks referenced in table III were considered (without snow and moisture: e neigne = 0 and ω = 0 cm). The porosity ε of the rock was set at 0.15 in the four cases. Thus, Fig. 6 shows fluxes obtained considering the four compositions and considering each energy domain. The influence of the nature of the modelled rock is relatively low in the output spectra (albedo + incident). This can be explained by the fact that the alumina and the calcite have similar molecular weights (102 g/mol and 100 g/mol, respectively). The main variation is observed for the epithermal neutron, but less than 5 %. A first conclusion is that composition rock does not seem to be a major factor. However, the rock #3 which is very close to the traditional granite, an inherent component to the Pyrenees, will be considered as a reference in the next analyses. Fig. 6 : Neutron flux in /s/cm² for each energy domain, GEANT4 results obtained for 4 rocks chosen for their chemical composition.

3) Impacts of the porosity and the mass fraction of water

The impact of the porosity and the mass fraction of water were investigated. The porosity was considered equal to 0.3, and the mass fractions of water ω were considered equal to 0, 5, 10, 15 and 20%, respectively.

Neutron fluxes obtained for each energy domain (thermal, epithermal, evaporation and cascade) were presented in Fig. 7. A water content of 15% gives the closest spectrum of experimental configuration with evaporation peaks and cascading substantially identical, slightly higher epithermal region (21%) and slightly lower thermal peak (-22%). Fig. 8 shows neutron spectrum issued from measurements (summer) and optimal simulation (ATMORAD + GEANT4 ground albedo neutron) i.e. considering a porosity of 0.30, and wet rocks of 15 %.

Pic-du-Midi, summer ATMORAD + GEANT4 (ε = 0,3 -ε = 0.15) ATMORAD (atmospheric shower) Fig. 8 : Neutron spectrum issued from measurements (summer) and simulation (ATMORAD + GEANT4 ground albedo neutron) considering a porosity of 0.30 and wet rocks of 15 %.

4) Impacts of the snow thickness

In this part, the impact of the addition of a snowpack (occurring during the winter) in the neutron spectrum is investigated.

Thus, a liquid water layer was considered in the simulation, the thickness of this layer varies from 5 to 300 mm to model the snow depth estimated at the Pic-du-Midi. An a priori equivalence of 1 cm of snow to 1 mm of rain was considered. The simulation results are shown in Fig. 9. Fig. 9 : Neutron flux in /s/cm² for each energy domain. Comparison between measurements and simulations considering a porosity of 0.30, a wet rocks of 15 % and for various liquid water layers with a thickness from 5 to 300 mm. The experimental spectrum given by the spectrometer during wither is plotted.

Fluxes are compared with fluxes obtained experimentally with the neutron spectrometer in February 2015 (winter). The comparison is relevant for cascade and evaporation domains, while significant shifts were observed for thermal and epithermal domains. Thus, simulations overestimate the magnitude of the thermal peak (up to 55% for cases and 30 mm of water) and epithermal contribution. Fig. 10 shows neutron spectrum issued from measurements (winter) and optimal simulation (ATMORAD + GEANT4 ground albedo neutron) i.e. considering a porosity of 0.30, a wet rocks of 15 % and a liquid water layer characterized by a thickness of 300 mm.

Energy (MeV)

Pic-du-Midi, winter Simulations (ε = 0,3 -ω = 0,15) 300 mm H 2 0 (ATMORAD + GEANT4) ATMORAD (atmospheric shower) Fig. 10 : Neutron spectrum issued from measurements (winter) and simulation (ATMORAD + GEANT4 ground albedo neutron) considering a porosity of 0.30 a wet rocks of 15 % and a liquid water layer with a thickness of 300 mm.

IV. CONTRIBUTION OF THE GROUND ALBEDO NEUTRON MODELING TO THE SOFT ERROR RATE As feature sizes in nano-scale devices continue to shrink, the elementary charge (critical charge Q crit ) required to induce an SEU continues to decrease. Indeed, the critical collected charge required to cause an error in a circuit reduces due to the reduction of operation voltage and capacitance of transistors. Previous works [START_REF] Sierawski | Effect of scaling on Muon-Induced Soft Errors[END_REF][32] provide critical charge estimation for current or future devices. Thus, nanoscale technologies are characterized by a critical charge which may be on the order of 0.1 fC. As shown in [START_REF] Yahagi | Threshold energy of neutron-induced single event upset as a critical factor[END_REF], neutron cross sections are significant for neutron energies below 10 MeV. This part of the paper is devoted to investigating the impact of the evaporation neutrons on SER rates, then to evaluating the improvement due to the presence of water.

Thus, a SRAM array composed by 100 × 100 cells in a 28 nm planar bulk CMOS has been considered using MUSCA SEP3 [START_REF] Hubert | Operational SER calculations on the SAC-C orbit using the Multi SCAles Single Event Phenomena Predictive Platform (MUSCA SEP3)[END_REF] [START_REF] Hubert | Single-Event Transient Modeling in a 65-nm Bulk CMOS Technology based-on Multi-Physical Approach and Electrical Simulations[END_REF]. The modeling of radiation effects in nanoscale devices implied to take into account high level physical description. Thus, realistic secondary ion track 3D structures issued from GEANT4 simulations were considered [START_REF] Raine | Impact of the Radial Ionization Profile on SEE Prediction for SOI Transistors and SRAMs Beyond the 32-nm Technological Node[END_REF]- [START_REF] Hubert | Impact of the Radial Ionization Profile of proton on SEU Sensitivity of nanoscale SRAMs[END_REF]. Carriers and charges evolve according to mechanisms as well as drift (electric field), diffusion (carrier concentration gradient), collection and recombination processes. Fig. 11 presents the FIT rate for the SRAM cells considering neutron spectra for the ground reference (NYC environment deduced from ATMORAD without soil description) and allowing to distinguish contributions induced by evaporation neutron (< 20 MeV) and by neutron energy less than 1 MeV. At low neutron energy, typically less than few MeV, the elastic scattering is the predominant mechanism. For high-energy neutrons, SEUs are induced more by inelastic nuclear (spallation) collisions where the ionization arises from the combined effects of the recoiling in and secondary particles emitted from the nucleus.

The results clearly show that evaporation neutron contribute significantly to SER, around 10 to 25 % considering the typical critical charge. Moreover, it is interesting to note that neutrons characterized by energies less than 1 MeV induce a contribution of a few percent (not negligible but insufficient to present a significant additional problem). For the very low critical charge, this contribution reaches 5 to 10 %, but this is unrealistic from a technological point of view. The event multiplicity can be easily extracted from simulations. Multiple events represent about 20 % of events when the spectrum was considered, while evaporation neutrons contribute to less than 2 % of the SER. Thus, results presented in Fig. 11 indicate the significant impact of evaporation neutron; it is interesting to evaluate the interest to improve the neutron spectrum (i.e. the consideration of ground albedo neutron) in SER calculations. Fig. 12 presents the predicted FIT rates obtained with a critical charge equal to 0.1 fC and distinguishing the total, cascade and evaporation contributions. In accordance with Fig. 7, the soil porosity was considered equal to 0.3 and results were presented considering the mass fractions of water ω max from 0 to 20%. Results show that the evaporation neutron contribution, although significant, induces relatively low impact on SER. This depends strongly on the critical charge, which in this case was considered realistic. Fig. 13 presents the SER contribution induced by evaporation and cascade neutrons as function of the liquid water thickness from 0 to 300 mm and considering a critical charge equals to 0.1 fC. This result confirms the previous analyses, i.e. evaporation neutrons weakly affect the rate, on the order of 20 to 35% . Whatever the hypothesis of hydrogen content in the soil (wet or liquid water thickness), the cascade neutrons are not impacted. This characteristic of quasi invariance with respect to the hydrogen environment was already used to monitor the cosmic ray from cascade neutron recording. Similar analyses were conducted with low critical charges (i.e. 0.05 fC), which foreshadows future critical charge levels. Thus, the evaporation neutron contributions becomes more significant (up to 40 %), however the main contributor to SER are cascade neutrons. Then, evaporation neutrons constitute a large part of the terrestrial spectrum and their contribution to the total SER should be assessed for high-sensitive technologies. Moreover, to provide an exhaustive overview, it would be necessary to consider the alpha-SER, the proton and the muon contributions to SER.

V. CONCLUSION

This work investigates the ground albedo neutron influences on seasonal variations of the atmospheric neutron spectrum. The role of hydrogen content on ground albedo neutron generation was demonstrated with GEANT4 simulations considering soil description and hydrogen characteristics (porosity, wet, snow thickness). To validate the modeling approach and investigate the seasonal variations, this work is based on two neutron spectrometers operating in remote altitude stations, at the Pic-du-Midi and Concordia station, respectively.

Data analyses and GEANT4 simulations validate the role of hydrogen content in the ground albedo neutron generations. Thus, the modeled scene representative to the Pic-du-Midi was simulated for various hydrogen properties (composition, density, porosity and wet level) and snow thickness. The orders of magnitude of the calculated thermal fluence rate are consistent with measurements obtained during summers and winters. These investigations focused to the mountain environment and demonstrate the complex processes that need to be considered to quantify the ground neutron environment. A database which contains details of albedo neutrons (fluence, energy, and angle) was developed considering discrete neutron energies, the typical rock (granite, argils, snow/ice, etc.), the porosity and the humidity and the snow thickness. The coupling of ATMORAD and this database has improved the ground radiation field specifications for the terrestrial environment and the analyses of the ground neutron measurements.

The knowledge of variations of thermal, epithermal and evaporation domains can improve cosmic radiation monitoring. Indeed, variations implied corrections in the yield functions [START_REF] Clem | Atmospheric yield functions and response to secondary particles of neutron monitors[END_REF] of equipments (spectrometers or monitors). As shown in results, cascade neutrons (En > 20 MeV) are weakly impacted, this property is usually used in cosmic ray analyses.

The last part investigates the contribution of the ground albedo neutron modeling to the SER. The results show clearly that evaporation neutron contribute significantly to SER, around 20 to 35 % considering the typical critical charge. Moreover, neutrons characterized by energies less than 1 MeV induce a contribution of a few percents (not negligible but insufficient to present a significant additional problem for SEU analysis). Similar analyses were conducted with low critical charges (i.e. 0.05 fC), which foreshadows future critical charge levels. Thus, the evaporation neutron contributions becomes more significant (up to 40 %), however the main contributor to SER are cascade neutrons.

Snow: 170 ×Fig. 3 :

 1703 Fig.3: Description of the modeled scene from GEANT4. The elementary volumes representing the atmosphere, soil and snow are represented. The particle generation disc is also visible at the top of the atmosphere. The cylindrical volumes sensitive samplings at different altitudes are also presented (dimensions in meter).

  of the flux in each sensitive volume s n φ & is normalized to the conditions of the Pic-du-Midi or Concordia neutron spectrometer measured by equalizing the proportion of experimental cascade neutrons Exp cas φ & and determined from the unit spectrum given by ATMORAD ATMORAD cas φ &. Through simulations, it is possible to calculate the modified neutron fluence rate considering the scorer s and the neutron energy E (equation 3). This is based on the local environment description with only a few input parameters i.e. the nature of the soil, the mass fraction of water in the rock, the porosity and the thickness of snow. g E and g E ∆ denote the neutron energy and the neutron energy range, respectively.(

=(Equ. 3 )

 3 III. GROUND ALBEDO NEUTRON IMPACT TO SPECTRUM VARIATIONS

Fig. 5 :

 5 Fig. 5 : Neutron flux recorded from June 2011 to June 2016 in the Pic-du-Midi, versus the averaged thickness of accumulated snow in the Pyrenees (measured closed to the Pic-du-Midi station).

20 Fig. 7 :

 207 Fig. 7 : Neutron flux in /s/cm² for each energy domain. Comparison between measurements and simulations considering a porosity of 0.30 and various wet rocks from 0 to 20%.

Fig. 11 :

 11 Fig.11: Predicted FIT rate for the 28 nm bulk CMOS cell considering neutron spectrum in ground and allowing to distinguish contributions induced by evaporation neutron (< 20 MeV) and by neutron energy less than 1 MeV.

Fig. 12 :Fig. 13 :

 1213 Fig.12: Predicted failure rate as function of the wet rock fraction, considering the neutron spectrum in ground (ATMORAD), and allowing to distinguish the evaporation contribution and the cascade contribution.

TABLE I .

 I CHARACTERISTICS OF PIC-DU-MIDI AND CONCORDIA STATIONS

Pic-du-Midi, France Concordia station, Antarctica Altitude

		2885 m	3233 m
	Latitude	42°55'N	75°06 ' S
	Longitude	0°08'E	123°19 ' E
	Cut-off rigidity	5.6 GV	< 0.001 GV
	Atmospheric depth	700.7 g/cm²	635 g/cm²
	Start Operating	May 2011	December 2015

  This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNS.2016.2614540, IEEE Transactions on Nuclear Science

0018-9499 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

  of accumulated snow depth.

		0.12	Y1	Y2	Y3	Y4	Y5
		0.11
	Neutron flux in n/cm²/s	0.06 0.07 0.08 0.09 0.10	Pic-du-Midi measurments,
		0.05	from June 2011 to June 2016
			total neutron flux
		0.04
		01/06/2011	01/06/2013	01/06/2015

Date

Fig. 4 : Neutron flux variations from June 2011 to June 2016 in the Pic-du-Midi.

TABLE II .

 II MEASURED AND CALCULATED FLUXES IN CONCORDIA

	Domain	Measurements from Dec. to June 2016	Simulation H20 ice, density = 0.35 g/cm 3
	Thermal	3.96E-02	3.54E-02
	Epithermal	3.27E-02	5.07E-02
	Evaporation	6.10E-02	6.26E-02
	Cascade	5.19E-02	5.61E-02

TABLE III .

 III MASS FRACTIONS OF SELECTED ROCKS

	Rock	SiO 2	Al 2 O 3	CaCO 3
	#1	75 %	25 %	0 %
	#2	70 %	14 %	17 %
	#3	49 %	8 %	43 %
	#4	46 %	0 %	54 %

& & & &

VI. ACKNOWLEDGMENTS

This work is supported by the IPEV (French Polar Institute, Institut Paul-Emile Victor) in the framework of the program n. 1112 (CHINSTRAP, Continuous High-altitude Investigation of Neutron Spectra for Terrestrial Radiation Antarctic Project).

The author thanks Dr. R. Cabanac, Mr. F. Lacassagne and the technical staff for their supports at the Pic-du-Midi station (OMP, Midi-Pyrenees Observatory). I'm thankful to the personnel of the Concordia station for their great help and hard work in harsh conditions, the scientific coordinator Doris Tuillier from IPEV and Luciano Milano.