Unveiling the Origin of Alkali Metal (Na, K, Rb, and Cs) Promotion in CO(2) Dissociation over Mo(2)C Catalysts

Molybdenum carbide (Mo(2)C) is a promising and low-cost catalyst for the reverse water−gas shift (RWGS) reaction. Doping the Mo(2)C surface with alkali metals can improve the activity of CO(2) conversion, but the effect of these metals on CO(2) conversion to CO remains poorly understood. In this study, the energies of CO(2) dissociation and CO desorption on the Mo(2)C surface in the presence of different alkali metals (Na, K, Rb, and Cs) are calculated using density functional theory (DFT). Alkali metal doping results in increasing electron density on the Mo atoms and promotes the adsorption and activation of CO(2) on Mo(2)C; the dissociation barrier of CO(2) is decreased from 12.51 on Mo(2)C surfaces to 9.51–11.21 Kcal/mol on alkali metal-modified Mo(2)C surfaces. Energetic and electronic analyses reveal that although the alkali metals directly bond with oxygen atoms of the oxides, the reduction in the energy of CO(2) dissociation can be attributed to the increased interaction between CO/O fragments and Mo in the transition states. The abilities of four alkali metals (Na, K, Rb, and Cs) to promote CO(2) dissociation increase in the order Na (11.21 Kcal/mol) < Rb (10.54 Kcal/mol) < Cs (10.41 Kcal/mol) < K (9.51 Kcal/mol). Through electronic analysis, it is found that the increased electron density on the Mo atoms is a result of the alkali metal, and a greater negative charge on Mo results in a lower energy barrier for CO(2) dissociation.