Synergy between Defects, Photoexcited Electrons, and Supported Single Atom Catalysts for CO₂ Reduction

Dispersed atomic catalysts can achieve high catalytic efficiency and have the potential to enable chemical transformation of inert molecules like CO2. The effect of surface defects on photocatalytic reduction of CO2 using supported single atom catalysts however requires clarification. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (Ov) and photoexcited electrons on supported single atom Cu catalysts and CO2 reduction. Adsorption of Cu was strong to the TiO2 surface, and charges of the Cu atoms were highly dependent on whether surface defects were present. Cu atoms with Ov aided in the adsorption of activated bent CO2, which is key to CO2 reduction. Our results also show that CO2 dissociation (CO2* → CO* + O*), which is a proposed initial step of CO2 reduction to hydrocarbon products, occurs very readily for a single Cu atom in an Ov, with barriers of ∼0.19 eV. Such low barriers do not occur with Cu over a stoichiometric surface. Furthermore, the presence of a photoexcited electron leads to a substantial increase in reaction rate for Cu over a stoichiometric surface; the Cu/TiO2 surface is largely inert in the absence of photoexcited electrons. Experimental results corroborate these theoretical calculations and show that activation of CO2 occurs most readily for TiO2 catalysts with dispersed Cu and Ov. CO2 photoreduction also occurs most readily for TiO2 catalysts with dispersed Cu and Ov, compared to TiO2 or Cu over stoichiometric TiO2 catalysts. We also modeled atomic Pt to understand how metals besides Cu may behave. We found that Pt over TiO2 also activates CO2 but that dissociation of CO2 over Pt with Ov does not occur as readily as for Cu with Ov. Our results show that tailoring TiO2 surfaces with defects in conjunction with specific atomic catalysts like Cu may lead to fast desirable photoreduction of CO2