CO₂ hydrogenation on Cu-catalysts generated from Zn^II single-sites: Enhanced CH₃OH selectivity compared to Cu/ZnO/Al₂O₃

The hydrogenation of CO₂ to CH₃OH is mostly performed by a catalyst consisting mainly of copper and zinc (Cu/ZnO/Al₂O₃). Here, Cu-Zn based catalysts are generated using surface organometallic chemistry (SOMC) starting from a material consisting of isolated Zn^II surface sites dispersed on SiO₂ – Zn^II@SiO₂. Grafting of [Cu(OtBu)]₄ on the surface silanols available on Zn^II@SiO₂ followed by reduction at 500 °C under H₂ generates CuZn_x alloy nanoparticles with remaining Zn^II sites according to X-ray absorption spectroscopy (XAS). This Cu-Zn/SiO₂ material displays high catalytic activity and methanol selectivity, in particular at higher conversion compared to benchmark Cu/ZnO/Al₂O₃ and most other catalysts. In situ XAS shows that CuZn_x alloy is partially converted into Cu(0) and Zn(II) under reaction conditions, while ex situ solid state nuclear magnetic resonance and infrared spectroscopic studies only indicate the presence of methoxy species and no formate intermediates are detected, in contrast to most Cu-based catalysts. The absence of formate species is consistent with the higher methanol selectivity as recently found for the related Cu-Ga/SiO₂.