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International audience; MoS2 is a promising low-cost catalyst for the hydrogen evolution reaction (HER). In order to enhance the HER activity of the inert basal plane, we perform an extensive density functional theory screening (DFT) of substitutional doping. Substituted basal planes may have many more active sites per MoS2 formula unit compared to the active sites located on the edges. We assess the electrocatalytic activity by explicitly considering the electrochemical potential via grand canonical DFT in combination with the linearized Poisson-Boltzmann equation. We screen 17 abundant elements for molybdenum doping and 5 elements for sulfur substitution. For the first time in such a screening study, we investigate not only the adsorption of H, but also of OH and H2O to explore the solvent effect, since the reaction takes place in an aqueous medium. Two other phenomena that could hinder hydrogen production at these sites are investigated, namely H2S release and the (local) segregation/dispersion tendency of the dopants in the basal surface. Compared to the pristine MoS2 nanosheets, our results show that most of the dopants significantly enhance the electrocatalytic activity. Considering all the evaluated factors, we identify the most promising systems: Dimers of Ti, Zr, and Hf, and the substitution of S by P are predicted to lead to stable active sites at the basal level with overpotentials of about 0.2 V.

International audience; MoS2 is a promising low-cost catalyst for the hydrogen evolution reaction (HER). However, the nature of the active sites remains controversial. By explicitly considering the electrochemical potential using grand canonical density functional theory (DFT) in combination with the linearized Poisson-Boltzmann equation, we revisit the active sites of 2H-MoS2. We start with the edges and we assess the influence of the presence of water and the competition between the release of H2S and H2. In our calculations, we consider the 50% S-edge and the 50%S Mo-edge, as they have been shown to be the most stable edge structure. Our calculations reveal that the Mo-edge is very likely to be reconstructed in the presence of water. In particular, H2S formation is only weakly endothermic (0.4 eV). Hence, under HER conditions where there is no sulfur reservoir, sulfur is gradually replaced by oxygen. Fortunately, the 0%S Mo-edge covered by OH is active for HER, as shown in our previous study. For the 50% S-edge, the termination is more stable with respect to H2S release (2.5 eV) and OH is stably adsorbed on the Mo-Mo sites. Our grand canonical DFT computations suggest that this edge is active for HER. The release of H2S at this edge is less likely to take place compared to H2 generation but is still possible as it is endothermic by only 0.21 eV after adsorption of OH. Once this OH substitutions happen, it leads to a 0%S S-edge that is predicted to be inactive for HER. For the basal plane, the thermodynamics of proton reduction as a function of electrochemical potential shows that three basal plane defects exhibit thermodynamic overpotentials below 0.2 V3 and the single sulfur vacancy (Vs) is the most abundant defect. Therefore, we present a detailed mechanistic study of HER on this defect and evaluate the Vomer, Tafel and Heyrovsky transition states for thedifferent possible reaction steps by considering the activation energy as a function of the electrochemical potential.

International audience

International audience

International audience; Lindqvist isopolyanions [NbxW6–xO19](2+x)– (x = 0–4 and 6) were prepared and their spectroscopic and thermal properties were determined by Raman and IR spectroscopy as well as TGA/DSC. The structure of the [NbW5O19]3– anion obtained as single crystal was determined. Ni-promoted alumina-supported hydrodesulfurization (HDS) catalysts were prepared from the best soluble NbW polyoxometalates. In the calcined catalysts, better dispersion of the metallic species is observed when using NbW isopolyanions instead of the conventional ammonium metatungstate. The presence of niobium was expected to introduce acidity leading to isomerization property in classical NiW HDS catalysts. In HDS reaction conditions (under hydrogen pressure and sulfided environment) the cyclohexane isomerization into methylcyclopentane activity of niobium-based catalysts was found up to 5 times superior to that of conventional NiW catalyst, showing the beneficial effect of niobium for this reaction.

National audience; La microscopie électronique en transmission est une technique de choix pour caractériser la phase active au sein de catalyseurs hétérogènes. Elle permet de déterminer des tailles de particules, de vérifier des homogénéités de distribution au sein du support. Les catalyseurs à base de sulfures de métaux de transition, en particulier le sulfure de molybdène, sont largement utilisés dans le domaine de l'hydrotraitement. L'activité et la sélectivité de ces catalyseurs sont liées à la morphologie, la taille et le degré d'empilement des feuillets de MoS2, phase active déposée dans un support poreux (alumine, silice...). En MET champ clair, les feuillets ne sont visibles que si leur plan basal est orienté parallèlement à l'axe optique du microscope. Par ailleurs, par manque de contraste avec le support, les feuillets de taille inférieure à 1 nm sont peu visibles. La microscopie électronique en transmission en mode balayage couplée à un détecteur champ sombre annulaire à grand angle (STEM-HAADF), en raison de la dépendance de l'intensité du signal avec Z, permet d'augmenter le contraste entre des phases de numéros atomiques très différents et est donc une technique intéressante pour la caractérisation de phases actives au sein de catalyseurs. Par ailleurs, l'utilisation d'un MET doté d'un correcteur d'aberration sphérique donne accès à des résolutions atomiques en mode STEM-HAADF. L'objectif de l'étude est d'appliquer le STEM-HAADF haute résolution à la caractérisation fine des morphologies et répartitions des feuillets de MoS2. Deux matériaux ont été caractérisés : l'un constitué de MoS2 dont le précurseur a été déposé dans une silice mésostructurée SBA15 par une méthode d'imprégnation à sec ; l'autre constitué de MoS2 dont le précurseur a été encapsulé lors de la synthèse de la silice.

The HAADF-STEM mode is an excellent characterization tool for the investigation of the crystalline structure, morphology and spatial distribution of the active phases deposited or inserted in a support, due principally to the important difference between the atomic numbers of active phase and material support. In this general framework, the characterization of the MoS2 nanostructured phase deposited on a carbon support or on silica microspheres was the subject of several pioneering works. The aim of this work is to illustrate the benefits of using high resolution HAADF imaging for characterizing the morphology and the precise localization of MoS2 sulfide phase in various industrial supports. Using the HR mode, different types of localization of the MoS2 phase (in the walls or filling the pores of a SBA15 matrix) have been evidenced according to the preparation method, as well as the presence of small metallic clusters incorporated in the walls of the mesoporous network. In addition, combining the tomographic approach with the HAADF imaging allows a precisely description of the 3D distribution of the nanostructured phase on the support material. In this context, the distributions of sulfide nanoparticles within the disordered mesoporous networks of silica microspheres have been determined.