CO₂ Hydrogenation to CH₃OH on Supported Cu Nanoparticles: Nature and Role of Ti in Bulk Oxides vs Isolated Surface Sites

The selective hydrogenation of CO₂ to CH₃OH is a crucial part of efforts to mitigate climate change via the methanol economy. Understanding the nature and role of active sites is essential for designing highly active and selective catalysts. Here, we examine Cu nanoparticles dispersed on TiO₂ and Ti-containing SiO₂ supports, where the Ti moieties of these materials are reducible to different extents, using a surface organometallic chemistry approach, together with state-of-the-art characterization methods (UV-vis, infrared (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies and in situ transmission electron microscopy (TEM)). Cu nanoparticles are small and well-dispersed on isolated, dimeric, and oligomeric Ti moieities on SiO₂ when reduced or when the material has been oxidized via exposure to air, but they are small only in the latter case for Cu dispersed on the bulk oxide TiO₂. Large Cu nanoparticles, present on TiO₂ when reduced, redisperse upon exposure to air, likely associated with the facile oxidation of the reduced TiO₂ surface, and agglomerate again when reduced in situ within the electron microscope. CH₃OH formation rates and selectivities on Cu/TiO₂ are low as a result of these large Cu nanoparticles and the ability of TiO₂ to catalyze the hydrogenation of CO₂ to CO. After accounting for the CO formation rates of the support itself, the CH₃OH selectivities are similar for all catalysts (and greater than that for Cu/SiO₂, where SiO₂ is considered as an innocent support), suggesting that Ti sites in all materials have a similar nature and role. These Ti sites act as Lewis acid sites, whose presence is evidenced by pyridine adsorption studies using IR and NMR spectroscopies, that stabilize the same surface intermediates at the interface of Cu nanoparticles and the support, despite differences in the reducibility of these Ti species.