Structural and Chemical Evolution of an Inverse CeO_<i>x</i>/Cu Catalyst under CO₂ Hydrogenation: Tunning Oxide Morphology to Improve Activity and Selectivity

Small nanoparticles of ceria deposited on a powder of CuO display a very high selectivity for the production of methanol via CO2 hydrogenation. CeO2/CuO catalysts with ceria loadings of 5%, 20%, and 50% were investigated. Among these, the system with 5% CeOx showed the best catalytic performance at temperatures between 200 and 350 °C. The evolution of this system under reaction conditions was studied using a combination of environmental transmission electron microscopy (E-TEM), in situ X-ray absorption spectroscopy (XAS), and time-resolved X-ray diffraction (TR-XRD). For 5% CeOx/Cu, the in situ studies pointed to a full conversion of CuO into metallic copper, with a complete transformation of Ce4+ into Ce3+. Images from E-TEM showed drastic changes in the morphology of the catalyst when it was exposed to H2, CO2, and CO2/H2 mixtures. Under a CO2/H2 feed, there was a redispersion of the ceria particles that was detected by E-TEM and in situ TR-XRD. These morphological changes were made possible by the inverse oxide/metal configuration and facilitate the binding and selective conversion of CO2 to methanol

Atomic Structural Origin of the High Methanol Selectivity over In₂O₃–Metal Interfaces: Metal–Support Interactions and the Formation of a InO_<i>x</i> Overlayer in Ru/In₂O₃ Catalysts during CO₂ Hydrogenation

CO2 hydrogenation to methanol is of great environmental and economic interest due to its potential to reduce carbon emissions and produce valuable chemicals in one single reaction. Compared with the unmodified traditional Cu/ZnO/Al2O3 catalyst, an indium oxide (In2O3)-based catalyst can double the methanol selectivity from 30–50 to 60–100%. It is worth noting that over catalysts involving various active metals dispersed on indium oxide (M/In2O3, M = Pd, Ni, Au, etc.), although the methanol yield is boosted, the selectivity remains similar to that of plain In2O3 despite the distinct chemical properties of the added metals. To investigate the phenomena behind this behavior, here we used RuO2/In2O3 as a test catalyst. The results of ambient pressure photoelectron spectroscopy, in situ X-ray absorption fine structure, and time-resolved X-ray diffraction indicate that the structure of the RuO2/In2O3 catalyst is highly dynamic in the presence of a reactive environment. Specifically, under CO2 hydrogenation conditions, Ru clusters facilitate the reduction of In2O3 to generate In2O3–x aggregates, which encapsulate the Ru systems in a migration driven by thermodynamics. In this way, the Ru0 sites for CH4 production are blocked while creating RuOx–In2O3–x interfacial sites with tunable metal–oxide interactions for selective methanol production. In an inverse oxide/metal configuration, indium oxide has properties not seen in its bulk phase that are useful for the binding and conversion of CO2. This work reveals the dynamic nature of In2O3-based catalysts, providing insights for a rational design of materials for the selective synthesis of methanol