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

TL;DR

This study uses first-principles DFT calculations to analyze free energy profiles and surface phase diagrams of MoS2 edges during reduction by dihydrogen, revealing conditions for vacancy and surface group coexistence.

Contribution

It provides detailed free energy and phase diagrams for MoS2 edges, incorporating entropic and enthalpic corrections, which enhances understanding of their behavior in industrial conditions.

Findings

Anionic vacancies on M-edge can coexist with surface SH groups.

Moderate activation barriers are found along reduction pathways.

Surface phase diagrams depend on temperature, pressure, and molar ratios.

Abstract

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.