Transient In situ DRIFTS Investigation of CO₂ Hydrogenation to Methanol over Unsupported CuGa Catalysts

Gallium oxide-based catalysts are promising candidates for CO₂ hydrogenation to methanol, whilst mechanistic understanding of the catalytic systems is not yet fully understood. Here, transient in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies of CO₂ hydrogenation over the unsupported CuGa and CuGaN (with N doping) catalysts were conducted. The findings show that N doping could possibly promote the formation of surface Cu⁺ sites on the interface between Cu and Ga₂O₃, and hence increasing the selectivity to methanol. However, N doping prohibited the reduction of Ga₂O₃ to oxygen-deficient Ga₂O_3-x, which is unfavorable for CO₂ conversion. Additionally, during DRIFTS the steady-state isotope transient kinetic analysis (SSITKA) was performed (under H₂/D₂ isotopic switching conditions). The SSITKA results confirm that the Cu⁺-bound formates were likely the key intermediates for methanol synthesis over the catalyst, and the N doping could also weak the interaction between CO* species and the catalyst surface, and thus facilitating the selectivity towards methanol rather than CO. Findings of the work show that the partial nitridation of the metal oxides could be the strategy to promote methanol synthesis selectivity.