Influence of Step Defects on the H2S Splitting on Copper Surfaces from First-Principles Microkinetic Modeling

An atomic level insight into the chemistry of hydrogen sulfide (H2S) splitting by metallic copper (Cu) is a necessary prerequisite for understanding sulfur poisoning mechanism of Cu-based water-gas shift (WGS) catalysts. In the present work, we have combined periodic density functional theory predictions and a detailed microkinetic modeling of the H2S dissociation on the stepped-defect (211), (311), and regular (111) faces of Cu to define the effect of step defects upon the reaction. The results indicate that on each surface examined, the dissociative adsorption facilely leads to the formation of element sulfur (S) via a stepwise H-S bond cleavage mechanism, with the initial molecular adsorption of H2S preferred as the rate-limiting step. It has also been pointed out that the SH disproportionation reaction does not open an alternative path for surface atomic sulfur production under the studied reaction condition. These surfaces are all predicted to be significantly covered by the S species after sufficient exposure to a realistic environment containing only several ppm of H2S. Furthermore, it is confirmed that (i) the full decomposition process is structure sensitive, and (ii) the driving force behind the step-enhanced activity of Cu toward this reaction arises not from kinetic but from thermodynamic factors. More importantly, our calculations have demonstrated that the H2S tolerance of Cu steps (and other defects) for the WGS reaction is worsened by a factor of approximately 10(3) as compared to a perfectly regular surface. Because these deficient sites are known as the most active sites of Cu-based shift catalysts in the absence of sulfur-containing species, it appears to be impossible to improve their activity without a dramatic loss of sulfur resistance through simply tuning catalyst surface morphology.