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.

Graphical abstractSurface phase diagrams for MoS2 M- and S-edges are revisited on the basis of DFT calculations including for the first time entropic and enthalpic vibrational corrections to electronic energies. Free-energy barriers along edge reduction pathways are reported.Figure optionsDownload full-size imageDownload high-quality image (81 K)Download as PowerPoint slideHighlights► MoS2 edges phase diagrams are revisited with DFT calculations of free energies. ► Free-energy barriers along M- and S-edges reduction pathways are reported. ► S vacancies and SH groups may coexist on M-edges in conditions relevant to HDS. ► Calculated surface groups vibration frequencies agree with available experiments. ► BEP relationships are found for the associative desorption of H2 and H2S.