A density functional theory study of the formation of COS from CO and H2S on cesium sulfide

Alkali metals can promote the performance of MoS2 in methanethiol (CH3SH) synthesis from CO/H2/H2S mixtures. Recently, it has been found that alkali sulfides and most prominently Cs2S can also catalyze the reaction between CO and H2S to COS and H2, COS acting as an intermediate in CH3SH formation (M. Yu et al. J. Catal. 2022, 405, 116-128). Here, we study the nature of the active sites and the mechanism of the CO + H2S → COS + H2 reaction for the 6 low-index Miller planes of Cs2S. While CO adsorbs weakly, strong dissociative adsorption of H2S results in HS* and H* intermediates, which further stabilize the (001) facet as the dominant surface termination. The main reaction pathway towards COS involves the association of CO* and SH* to COSH* followed by its dehydrogenation in a Langmuir-Hinshelwood mechanism. Reactions of CO* with lattice S atoms have prohibitively high barriers due to the strong Cs-S bonds in Cs2S. Overall, the reaction rate is dominated by the (001) facet with small contributions of the (010), (011) and (101) surfaces. COSH* formation, its dehydrogenation to COS, and COS desorption compete as rate-controlling steps on these surfaces.