Revealing the Mechanism of Converting CO₂ into Methanol by the Cu₂O and Oxygen Vacancy on MgO: Experiments and Density Functional Theory

Given the great significance of defect and Cu compounds for the reduction of CO₂ as well as the few reaction mechanisms of converting CO₂ into different hydrocarbons, the effects of oxygen vacancies and Cu₂O on the reduction of CO₂ were thoroughly investigated, and possible mechanisms were also proposed. A series of Cu₂O/O_v-MgO catalysts were synthesized for photothermal catalytic reduction of CO₂ to methanol under visible-light irradiation, among which the 7%Cu₂O/O_v-MgO composite exhibited the best reduction activity and the yield of methanol was 19.78 μmol·g^-1·h^-1. The successful composite of Cu₂O and O_v-MgO can yield a loose and porous nanosheet, uniform distribution, favorable absorbance and photoelectric performance, and increased specific surface area and adsorption ability of CO₂, which are all vital to the adsorption and conversion of CO₂. The introduction of oxygen vacancy and Cu₂O not only promotes the adsorption of CO₂ but also provides more electron-triggered CO₂ activation. Density functional theory (DFT) calculation was also performed to reveal the reaction mechanism for effective enhanced CO₂ reduction to ethanol or methanol by the comparison of CuO/MgO and Cu₂O/O_v-MgO composites, illustrating the reaction pathways of different products. By comparing the key steps in determining the selectivity of C₁ or C₂, the kinetic barriers of obtaining CH₃OH for the Cu₂O/O_v-MgO composite with CH₃OH as the main product were found to be lower than those of generating CH₂*, while the opposite is true for CuO/MgO composites, whereby it may be easier to obtain more C₂ products. These insights into the reaction mechanism of converting CO₂ into different hydrocarbons are expected to provide guidance for the further design of high-performance photothermal catalytic CO₂ reduction catalysts.