Temperature-programed reduction of unpromoted MoS_2-based hydrodesulfurization catalysts: Experiments and kinetic modeling from first principles

摘要

In this report, we have addressed the mechanism of reduction of sulfided γ-Al_2O_3 supported MoS_2 catalytic nanoparticles by dihydrogen gas, starting from the state 100% sulfur covered and ending in 50% sulfur covered Mo-edges, with the production of gaseous hydrogen sulfide. We have prepared and characterized a consistent set of oxidic catalysts precursors with variable loadings in Mo, presulfided these catalysts under high chemical potential of sulfur, ensuring prevalence of 100%S covered Mo-edges (triangular nanoparticles), and finally acquired temperature-programed reduction (TPR) spectra. We have performed DFT calculations of the free-energy barriers along the reduction pathway, elaborated analytical models of TPR spectra with free-energy activation barriers and some measurable catalysts characteristics as inputs, and discriminated among various sticking coefficient models with respect to the experimental data. We find that the rate-determining step for the onset of reduction is the dissociative chemisorption of H_2 on two adjacent capping S_2 dimers yielding mono-sulfhydrilated S_2 pairs in mutual hydrogen bonding situations, with a free-energy barrier of 1.5 eV. The observed decrease of TPR spectra peak temperatures with increasing Mo content results from an increase in the ratio of the area available for reactive adsorption over the total surface area as the loss of dispersion is overcompensated by the increasing active Mo-edge area.