Atomic-Scale Insights into CO2 and H2O Co-Adsorption on Sr2Fe1.5Mo0.5O6 Surfaces: Role of Electronic Structure and Dual-Site Interactions

Co-electrolysis of CO2 and H2O offers a promising route for efficient and controllable syngas production from greenhouse gases and water. However, the atomic-scale reaction mechanism remains elusive, especially on complex oxide surfaces. In this study, we employ density functional theory (DFT) to investigate the adsorption and activation of CO2 and H2O on the FeMoO-terminated (001) surface of Sr2Fe1.5Mo0.5O6 (SFM), a double perovskite of growing interest for solid oxide electrolysis. Our results show that CO2 strongly interacts with surface lattice oxygen, adopting a bent configuration with substantial charge transfer. In contrast, H2O binds more weakly at Mo sites through predominantly electrostatic interactions. Co-adsorption analyses reveal a bidirectional interplay: pre-adsorbed H2O enhances CO2 binding by altering its adsorption geometry, whereas pre-adsorbed CO2 weakens H2O adsorption due to competitive site occupation. This balance suggests that moderate co-adsorption may facilitate proton–electron coupling, while excessive coverage of either species suppresses activation of the other. Bader charge analysis, charge density differences, and projected density of states highlight the key role of Fe/Mo–O hybridized states near the Fermi level in mediating surface reactivity. These results, obtained for a perfect defect-free surface, provide a theoretical benchmark for disentangling intrinsic molecule–surface and molecule–molecule interactions, and offer guidance for designing high-performance perovskite electrocatalysts for CO2 + H2O co-electrolysis.