Isolated Zr Surface Sites on Silica Promote Hydrogenation of CO₂ to CH₃OH in Supported Cu Catalysts

Abstract

Copper nanoparticles supported on zirconia (Cu/ZrO₂) or related supported oxides (Cu/ZrO₂/SiO₂) show promising activity and selectivity for the hydrogenation of CO₂ to CH₃OH. However, the role of the support remains controversial because most spectroscopic techniques provide information dominated by the bulk, making interpretation and formulation of structure-activity relationships challenging. In order to understand the role of the support and in particular of the Zr surface species at a molecular level, a surface organometallic chemistry approach has been used to tailor a silica support containing isolated Zr(IV) surface sites, on which copper nanoparticles (∼3 nm) are generated. These supported Cu nanoparticles exhibit increased CH₃OH activity and selectivity compared to those supported on SiO₂, reaching catalytic performances comparable to those of the corresponding Cu/ZrO₂. Ex situ and in situ X-ray absorption spectroscopy reveals that the Zr sites on silica remain isolated and in their +4 oxidation state, while ex situ solid-state nuclear magnetic resonance spectroscopy and catalytic performances show that similar mechanisms are involved with the single-site support and ZrO₂. These observations imply that Zr(IV) surface sites at the periphery of Cu particles are responsible for promoting CH₃OH formation on Cu-Zr-based catalysts and provide a guideline to develop selective CH₃OH synthesis catalysts.