The effect of cobalt promoter on the CO methanation reaction over MoS2 catalyst: a density functional study

Abstract

The potential mechanism of sulfur-resistant CO methanation was theoretically investigated via density functional theory (DFT + D) calculations. Comparisons were made between modified Co–MoS₂ and pure MoS₂ catalysts and we highlighted the distinguished CO methanation pathway in the presence of Co-promoter. Multiple intermediates were formed at different catalytic sites during the reaction, which further increased the mechanism complexity. The results obtained from Co–MoS₂ imply that the CH₃OH species could be formed along the most feasible reaction pathway on Mo catalyst termination; the subsequent dissociation of CH₃OH into CH₃ and OH was found to be the rate determining step with a reaction barrier of 29.35 kcal mol⁻¹ at 750 K. On the S edge of Co–MoS₂, the CH₂OH intermediate could be formed as a result of CH₂O reacting with adsorbed hydrogen, and subsequent CH₂OH dissociation was noted to release CH₂. Afterwards, consecutive hydrogenation of CH₂ led to the final CH₄ yield. On S catalyst termination, it was suggested that the CHO intermediate formation played a key role as the rate-determining step with the reaction barrier of 19.56 kcal mol⁻¹ at 750 K. By comparing the CO methanation energy profiles over different samples, it was discovered that the Co-promoter did possess promoting effects at both the Mo edge and the S edge of the catalyst; note that this enhancement at the Mo edge was superior to that at the S edge, especially for larger scale applications. Moreover, after doping with Co, the OH species was easier to remove in terms of H₂O molecules, which created enough vacant active sites for a continuous reaction.