ZnO decorated with Au-Cu nanoclusters: a model system for investigating the CO2 conversion to methanol

Conventional modern catalysts for the thermal catalytic conversion of CO2 to MeOH involve the usage of Cu based materials, mainly in the form of CuO/ZnO/Al2O3 ternary catalysts with not well-defined Cu/(Cu+Zn) ratios. It has been suggested, by DFT calculations, that the active sites might be Cu defects substituted with Zn atoms [1], however, it has not yet been definitely established the role of each component of the catalyst. Nevetheless, the literature studies seem to agree that the main active region of these materials could be the Cu/Zn interfaces [2]. The understanding of these interfaces and of the defects generated at the contact point of the two metals, is therefore crucial for the mechanism understanding of these materials. In this context, thiolate protected metal nanoclusters (NCs) with general formula (Mn(SR)m) might come in handy in understanding the role of surface defectivity by creating atomic precise catalytic active sites and a truly monodisperse catalytic surface that provides ideal conditions for structure reactivity correlation studies. Due to their excellent catalytic properties and atomic scale tunability, these materials have captivated the interest of scientists in the last years, and today we can produce metal doped Au NCs or non-noble metal NCs easily in the liquid phase [3]. In this context, the synthesis, characterization and testing of Cu/Au NCs supported on ZnO, might give some interesting insights into the catalytic activity of CO2 methanolation. Moreover, by exploiting Electron Spin Resonance (ESR), to detect and monitor paramagnetic defects centers in the ZnO lattice (e.g. Vo•, Zni•) some interesting information about the defects generated at the surface or bulk of these catalyst could be obtained. UV-vis and MALDI analysis proved the successful generation of Cu mono and bi-doped Au NCs. The preliminary catalytic tests in CO2 methanolation evidenced satisfactory conversion and selectivity even at low cluster loadings (0.5% wt/wt total metal content). ESR monitoring shed interesting light on the native defects modification of the ZnO support both upon NCs deposition onto the oxide surface and after subjecting the material to catalytic conditions. These data seem to support the generation of surface or sub-surface defects (at lower magnetic field) and bulk defects (at higher field). Surface defects, mainly oxygenated species (e.g VO• and VO related species), appear to be sensitive to external conditions (e.g atmosphere and temperature), while bulk defects (e.g. Zni•) appear more sensitive to the deposition of clusters on the surface. In conclusion, this work may lay the groundwork for the development of a suitable protocol for studying in depth the ZnO defects modification during the thermo-catalytic reduction of CO2 to MeOH.