We present the results of DFT calculations of free energy profiles along the reaction pathways starting from 50% coverage of MoS 2 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 H 2 S. 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, H 2 partial pressure and H 2 S/H 2 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 Sedges, may coexist at equilibrium. Moderate activation barriers connect stationary states along all paths explored. Keywords : Density Functional Theory, HDS catalysts activation, Free energy surface phase diagrams, Dihydrogen activation, Brönsted-Evans-Polanyi relationships, sulfur vacancies, SH and MoH surface groups vibrational frequencies determing the hydrogenation state of the catalyst in HDS working conditions will help for a better control of the surface chemical species: coordinatively unsaturated metallic sites (CUS), hydride, sulfhydryl group, sulfur anions. Most of these species are suspected to be involved in the elementary steps of HDS reactions [45]: reactant adsorption, monohydrogenation, hydrogenolysis, and sulfur vacancy regeneration. Since most industrial hydroprocessing catalysts are based on MoS 2 nanoparticles, and it is now well established that catalytic activity is localized at their ³0-edge´DQG ³6-HGJH´ VXUIDFHV > @ ZH HVWDEOLVK KHUH WKURXJK ILUVW-principles (DFT) calculations including for the first time thermal and entropic effects on the catalyst, the free energy profiles of dihydrogen and hydrogen sulfide exchanges with such surfaces, including elementary steps of dissociative adsorption, associative desorption, and surface diffusion 2 / 31 · © §1 · © § h T k m P T k T k B i i B B trans i S P P i , m i , being the pressure and the mass of the particle i respectively

We present the results of first-principles plane-wave pseudopotential calculations of piezoelectric coefficients of first and second order for a total of nine III-V binary phases (AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb) of zinc-blende semiconductors. These coefficients are used to calculate the piezoelectric fields for [111]-oriented quantum wells (QWs) with different well-barrier combinations and various dimensions. We derive an approximate analytic expression for the strain tensor in the case of pseudomorphic growth along an arbitrary growth direction. Together with the piezoelectric coefficients, this allows a simple calculation of the piezoelectric field up to second order in strain for an arbitrary growth direction and any material combination within the nine III-Vs presented here. Nonlinear contributions to the polarization are shown to be of significant magnitude for all the materials presented. In some cases the field is increased by the second-order terms; in some cases it is decreased. We analyze the chemical trends of the obtained coefficients. We compare our results to available experiments and find good agreement in one-third of the cases, while for the remaining cases the calculated field is larger to significantly larger than in the measurements. We discuss the popular experimental techniques and highlight possible reasons for the discrepancies.

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