CO₂ Activation and Hydrogenation on Cu-ZnO/Al₂O₃ Nanorod Catalysts: An In Situ FTIR Study

CuZnO/Al₂O₃ is the industrial catalyst used for methanol synthesis from syngas (CO + H₂) and is also promising for the hydrogenation of CO₂ to methanol. In this work, we synthesized Al₂O₃ nanorods (n-Al₂O₃) and impregnated them with the CuZnO component. The catalysts were evaluated for the hydrogenation of CO₂ to methanol in a fixed-bed reactor. The support and the catalysts were characterized, including via in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The study of the CO₂ adsorption, activation, and hydrogenation using in situ DRIFT spectroscopy revealed the different roles of the catalyst components. CO₂ mainly adsorbed on the n-Al₂O₃ support, forming carbonate species. Cu was found to facilitate H₂ dissociation and further reacted with the adsorbed carbonates on the n-Al₂O₃ support, transforming them to formate or additional intermediates. Like the n-Al₂O₃ support, the ZnO component contributed to improving the CO₂ adsorption, facilitating the formation of more carbonate species on the catalyst surface and enhancing the efficiency of the CO₂ activation and hydrogenation into methanol. The synergistic interaction between Cu and ZnO was found to be essential to increase the space-time yield (STY) of methanol but not to improve the selectivity. The 3% CuZnO/n-Al₂O₃ displayed improved catalytic performance compared to 3% Cu/n-Al₂O₃, reaching a CO₂ conversion rate of 19.8% and methanol STY rate of 1.31 mmolg_cat^-1h^-1 at 300 °C. This study provides fundamental and new insights into the distinctive roles of the different components of commercial methanol synthesis catalysts.