Insights into water–gas shift reaction mechanisms over MoS2 and Co-MoS2 catalysts: a density functional study

Abstract

In order to investigate WGS reaction mechanisms on pure MoS2 and Co-MoS2 catalysts and to understand the role of Co, detailed WGS reaction processes including the HCOO-mediated path, the COOH-mediated path, and the redox path on both catalysts were studied by density functional theory (DFT+D) calculations. Comprehensive research found that the favorable pathways of WGS mechanisms over molybdenum sulfide catalysts were far more complicated than the pure redox mechanism. From the results, the redox mechanism was just predominant in the WGS reaction on the Mo termination of Co-MoS2 catalysts and on the S termination of MoS2, in which H2O decomposed into oxygen atom and two hydrogen atoms in a stepwise manner, and afterwards CO2 was generated via CO oxidation by O atom. For the S termination of Co-MoS2 and Mo termination of MoS2, however, the associative mechanism (COOH-mediated) was prevailing, wherein the COOH intermediate was formed by OH reacting with CO. Moreover, it was proposed that the dissociation of COOH into CO2 and H was the rate-determining step for the Mo termination of pure MoS2, whereas for the S termination of MoS2, the cleavage of the second O–H bond in H2O determines the whole reaction process. Besides, the second O–H bond breaking was also the rate determining step for the Mo termination of Co-MoS2. For the S termination of Co-MoS2, the first O–H bond breaking became the rate determining step. Moreover, further comparisons revealed that the S edge was prior to the Mo edge for the WGS reaction after doping Co, and in general, Co-promoter facilitated the reaction, which decreased the overall barriers of WGS reaction by about 0.5 eV.