Free-energy profiles along reduction pathways of MoS2 M-edge and S-edge by dihydrogen: a first-principles study

We present the results of DFT calculations of free energy profiles along the reaction pathways starting from 50% coverage of MoS2 M-edge and 100% coverage of S-edge by sulfur, and leading to 37% coverage, i.e. creation of anionic vacancies, upon reduction by dihydrogen and production of H2S. Significant entropic and enthalpic corrections to electronic energies are deduced from the sets of normal modes vibration frequencies computed for all stationary and transition states. On that basis, we revisit and discuss the surface phase diagrams for M- and S- edges as a function of temperature, H2 partial pressure and H2S/H2 molar ratio, with respect to ranges of conditions relevant to industrial hydrotreating operations. We show that in such conditions, anionic vacancies on the M-edge, and surface SH groups on the M- and S- edges, may coexist at equilibrium. Moderate activation barriers connect stationary states along all paths explored

Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage

Advanced DFT models of complex catalysts, such as amorphous silica–alumina and supported subnanometric platinum particles, bridge the gap between the ideal surface model and the industrial catalyst.</p

Evidence for H2-Induced Ductility in a Pt/Al2O3 Catalyst

International audienceFocusing on a highly dispersed 5 wt % Pt/Al2O3 catalyst for industrial hydrogenation and dehydrogenation reactions, we employ inelastic neutron scattering (INS) spectroscopy to obtain the vibrational fingerprint of the hydrogenous species formed under different H2 equilibrium pressure and temperature. The INS profiles are interpreted on the basis of systematic DFT simulations on 26 different PtxHy/γ-Al2O3 models, indicating that the INS spectra are a unique fingerprint of the PtxHy/γ-Al2O3 model morphology, size (x), H-coverage (y), and typology of Pt–H species. We fit the experimental INS spectrum of Pt/Al2O3 measured under higher H-coverage conditions with a linear combination of the simulated spectra. We find that 47% of the spectrum can be ascribed to Pt55Hy/γ-Al2O3 clusters completely solvated by H atoms (y = 81 and 91) and in weak interaction with the support, followed by the disordered Pt34H54/γ-Al2O3 model (36%) and by the smaller Pt13H32/γ-Al2O3 one (18%). These results are in good agreement with the particle size distribution previously determined by TEM and confirmed by XAS. A second INS spectrum collected under lower H-coverage conditions exhibits the typical features of less hydrogenated PtxHy models in stronger interaction with the γ-Al2O3 support, as well as bands associated with the formation of −OH species at the support by H-spillover. Overall, our study reveals the relevance of combined INS and DFT analysis to quantify the versatile atomic scale’s properties of Pt/Al2O3 catalyst in terms of cluster morphology, size, typology of Pt–H species, and cluster/support interaction depending on the H-coverage, providing important insights about their behavior under hydrogenation conditions

Prediction of optimal catalysts for a given chemical reaction

We show that the optimal catalyst for a given reaction equalizes the free energies of intermediates in an adsorbed phase, and in consequence is described by a surface energy proportional to the enthalpy of this reaction.</p

Revisiting γ-Alumina Surface Models through the Topotactic Transformation of Boehmite Surfaces

International audienceThe rational understanding of γ-alumina (γ-Al2O3) supported catalysts requires an ever more improved atomic scale determination of the support’s surface properties. By using density functional theory (DFT) calculations, we show how the structural and energetic surface properties of alumina crystallites intrinsically depend on its synthesis pathway. Considering the case study of the topotactic transformation of boehmite (γ-AlOOH) into γ-Al2O3 taking place during calcination, we propose a methodology to mimic this pathway by reconstructing relevant slabs of boehmite into γ-alumina slabs following 3 steps: dehydration, contraction/translation and Al migration into spinel or non-spinel sites. On the one hand, we confirm the reliability of some earlier , and surface structures determined by standard bulk cleavage approach. Moreover, we find new γ-alumina surfaces harboring Brønsted acid sites (BAS) and Lewis acid sites (LAS) with specific local structures. More strikingly, we find that the basal surface of alumina inherited from the (0 1 0) basal surface of boehmite, exhibits a larger number of isolated µ2-OH groups than the lateral surface. For the lateral (respectively ) orientation, four (respectively three) thermodynamically competing surfaces are identified, including models earlier proposed. These results are induced by finite size and morphology effects during the topotactic transformation of boehmite crystallites. Thanks to a thorough comparative analysis of morphology and nature of BAS and LAS as a function of thermal treatment and water pressure for each surface, we identify coherent chemical families of surfaces across the main crystallographic orientations. These features open the door to a better differentiation of the reactivity of the basal alumina surfaces from the lateral ones

Size-Dependent Structural, Energetic, and Spectroscopic Properties of MoS 3 Polymorphs

International audienceIn spite of the interest of a-MoS3 based nanomaterials, the amorphous nature of the MoS3 phase makes it challenging to control and understand its chemical reactivity. In particular, the type of the structural building unit such as Mo3 triangular vs Mo chain is still debated, while the ambivalent interpretation of the nature of sulfur species (S2–, S22–) and Mo–Mo bonds leads to ambiguous interpretations of spectroscopic data and reactivity. By density functional theory (DFT), we simulate the energetic, structural, and spectroscopic features of relevant zero-dimensional 0D-, 1D-, and 2D-MoS3 triangular, chain-like polymorphs, including unprecedented ones (ring, wave, and helix) and revisit the interpretation of EXAFS, IR-RAMAN, and XPS experimental data. We analyze how MokS3k clusters of a few k atoms may grow up to infinite (periodic replica) MoS3 polymorphs. The evolution of the growth energy and the computed IR spectra within the density functional perturbation theory level suggest the coexistence of various polymorphs in the MoS3 phase as a function of sizes. Molecular dynamics simulations reveal how the small triangular MokS3k oligomers may transform into condensed MoS3 patches resembling embryos of the 2D 1T′-MoS2 phase. Finally, we discuss some plausible transformation pathways from one polymorph to another