Reactivity of a Zirconia-Copper Inverse Catalyst for CO2 Hydrogenation

Copper-zirconia catalysts have been shown to be effective for methanol synthesis via CO2 hydrogenation, yet the active phases and reaction mechanism remain uncertain. In this work, an inverse model catalyst ZrO2/CuO2/Cu(111) was prepared by mass-selected ion deposition and tested for CO2 hydrogenation under near-ambient pressure (AP) reaction conditions by using X-ray photoelectron spectroscopy (NAP-XPS) and infrared reflection-absorption spectroscopy (NAP-IRAS). The spatial resolution afforded by the small entrance cone of the AP-XPS spectrometer was used to resolve regions of the surface with and without Zr deposition. Carbon 1s core level spectra of the ZrO2/Cu2O/Cu(111) regions of the surface under 500 mTorr of CO2 + H-2 (1:3 ratio) show evidence for reaction intermediates including carbonate (CO3*), formate (HCOO*), and HxCO* species, with methoxy having the highest surface concentration at 500-600 K. These intermediates are confirmed by IRAS vibrational spectra. In regions of the surface without Zr, the Cu2O/Cu(111) is reduced to metallic Cu, and the surface intermediates are different and are present at much lower concentrations. The observed surface intermediates and their temperature dependence suggest a mechanism in which CO2 is adsorbed on zirconia as carbonate (CO3*) and then converted to HCOO* and HxCO* hydrogenated intermediates that ultimately lead to methoxy (CH3O*), the final surface-bound precursor for methanol. Overall, the results clearly demonstrate the promotional effects of small ZrO2 particles for enhancing the reactivity of Cu surfaces for CO2 hydrogenation.