Identification of Transient Intermediates and Active Species in Atomic CZA Catalysts for CO2 Hydrogenation to Methanol

Direct hydrogenation of carbon dioxide to methanol is a promising strategy for carbon capture and utilization (CCU). Copper–zinc–alumina (CZA) catalysts are widely used for this transformation, yet the nature of the active Cu and Zn species and the reaction intermediates remains debated due to their sensitivity to feed composition and temperature. This challenge is compounded by the high metal loading in conventional CZA catalysts, which obscures active species signals with background contributions from spectator species. To address this, we synthesized model CuZn/Al2O3 catalysts using bimetallic coordination complexes, achieving low metal loadings that yield small Cu clusters and Cu+ single atoms adjacent to isolated Zn2+ sites. In situ XANES and UV–vis data were analyzed using multivariate curve resolution–alternating least-squares (MCR–ALS), revealing that Cu dispersion and reagglomeration─phenomena suspected in industrial systems─also occur at low loadings. Operando and modulation excitation with phase sensitive detection DRIFTS (ME-PSD-DRIFTS) showed: (a) Cu clusters dissociate H2 and activate CO2 via monodentate formate; (b) Al2O3 stabilizes Cu+ under reducing conditions, with Cu content correlating with methanol productivity via CO hydrogenation; and (c) Zn in ZnAl2O4 promotes CO2 activation through reactive carbonate formation and enhances oxygenate conversion kinetics. ZnAl2O4 also acts as a structural promoter, facilitating CO2 conversion via reverse water gas shift (RWGS) and CO hydrogenation. These findings reveal new structure–activity relationships, highlighting the role of the mixed-metal interface in stabilizing transient intermediates and providing some guidance in the rational design of improved catalysts for CO2 valorization.