Effect of Potassium on Methanol Steam Reforming on the Cu(111) and Cu(110) Surfaces: A DFT Study

The potassium (K) additive is of great importance in improving the efficiency of catalytic reactions in numerous industrial reaction processes. In this work, density functional theory calculations are carried out to investigate the reaction mechanism as well as selectivity and activity of the methanol steam reforming (MSR) reaction on pure and K-added Cu(111) and Cu(110) surfaces. Our results suggest that the K adatom can improve the selectivity of MSR toward CO2 by stabilizing the weakly adsorbed H2CO species and enabling it to react with OH to yield H2COOH followed by continuous dehydrogenation into CO2 and activity of MSR by favoring the rate-limiting steps for H3CO, H2O, and H3COH dehydrogenation. It is found that there are two key factors contributing to K promotion effects: one is the direct K–O bonding interaction that would stabilize nearly all oxygenates including H2CO species and the stabilized oxygenates of transition states (TSs) result in the lower dissociation barriers of key steps on both Cu surfaces; the other is the indirect interaction between K and adsorbates mediated by added surface electrons donated by K, which would affect the magnitude of interaction energies between two fragments of TSs and thus energy barriers of key steps. The fact that more electrons are donated from K to Cu(111) than to the Cu(110) surface and further to the TS complexes would result in the stronger promotion effect of K on the MSR reaction over Cu(111) compared with Cu(110). Our results emphasize the essence of the promotion effect of alkali additives on complex reaction systems.