The Role of Copper Valence States in CuZnAl Catalysts for CO2-to-Methanol Conversion
CuZnAl (CZA) is a classic industrial catalyst for methanol synthesis from syngas, but its catalytic performance for CO2 hydrogenation to methanol is suboptimal. The catalytic mechanism of Cu species in CZA remains challenging. This study systematically investigates the valence state changes of active Cu species in CZA catalysts and their influence on catalytic performance by modifying catalysts with varying amounts of electron donor K, thereby identifying the catalytic function of Cu species with different valence states. H2-TPR, XPS, and HR-TEM characterizations reveal that highly dispersed K species supported on CZA catalysts inhibit the reduction of CuO, resulting in a small amount of Cu2O active species being produced under reaction conditions, thus causing a decrease in catalytic activity. Furthermore, XRD and Cu LMM spectra show that the proportion of Cu0 in K-modified CZA catalysts increases with K loading, but a higher proportion of Cu0 species on the surface obviously promotes the reverse water gas shift (RWGS) reaction. According to the results of in situ infrared spectroscopy, CZA catalyst follows the reaction pathway mediated by HCOO* in the hydrogenation of CO2 to methanol.