In Situ Studies of Copper-Zirconia-Zinc Oxide Model Catalysts for CO₂Hydrogenation

A combination of in situ X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRAS) was used to investigate the formation of surface intermediates from CO₂ hydrogenation on copper-zirconia-zinc oxide model catalysts under reaction conditions. Copper clusters with different numbers of atoms (n = 1, 4, 13) were deposited onto bare and ZrO₂-modified ZnO powder supports to systematically examine the effects of Cu cluster size and the synergy between metal and metal-oxide components. Under low pressure CO₂ hydrogenation conditions (CO₂:H₂ = 1:9, 0.4 mbar, 300–600 K), XPS and IRAS identify the most prominent intermediates as carbonate (CO₃*), formate (HCOO*) and methoxy (CH₃O*), which is the final surface-bound intermediate leading to methanol. The temperature profiles are consistent with a mechanism in which CO₂ is adsorbed as carbonate species (HCO₃*, CO₃*) followed by hydrogenation reactions to formate (HCOO*) and methoxy (CH₃O*), but the relative yields strongly depend on surface composition. Specifically, the presence of ZrO₂ promotes CO₂ adsorption and activation and improves the thermal stability of the Cu clusters against loss of surface area. Moreover, the ternary Cu₄/ZrO₂/ZnO surface is significantly more active than Cu₄/ZnO and ZrO₂/ZnO surfaces for the formation of methoxy (CH₃O*), indicating that Cu–ZrO₂ interfaces promote the formation of key intermediates leading to methanol. Finally, the yields of intermediates are similar for all Cu cluster sizes (Cu₁, Cu₄ and Cu₁₃), indicating that the primary role of Cu is to provide H atoms via H₂ dissociation and spillover. These molecular-level insights provide a fundamental understanding of the enhanced efficiency of ternary Cu–ZrO₂–ZnO catalysts and establish design principles for developing improved catalysts for CO₂ conversion.