Carbon dioxide activation and hydrogenation over copper-based catalysts studied by near-ambient pressure X-ray photoelectron spectroscopy

Carbon neutrality has been proposed as a commitment by the international communities for the well-being of human. To achieve carbon neutrality, one practical approach is to synthesize fuels and chemicals through CO2 hydrogenation to replace fossil fuels. CO2 hydrogenation in industrial scale relies on the development of highly efficient catalysts, which, however, is still a challenging task due to the lack of fundamental knowledge on the catalytic mechanism. The aim of this thesis is promoting the understanding of the gas-surface interaction and the structure-performance relationships for Cu-based CO2 hydrogenation catalysts. In this thesis, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and conventional catalyst characterization and evaluation methods were applied. Initially, size-selected Cu nanoparticles (Cu NPs) were deposited on carbon and silica, and their thermal stability was investigated by XPS and TEM. Under ultra-high vacuum (UHV) condition, the agglomeration of Cu NPs was found to be size-dependent. Cu NPs were better stabilized on SiO2 support than carbon support. Moreover, the CO2 hydrogenation environment affects the sintering of Cu NPs, as it causes the redispersion and valorization of Cu species. Then, In/Cu model catalysts were prepared and investigated by NAP-XPS. It was found that the deposition of In on the surface of a Cu foil led to the formation of Cu-In alloy, whereas upon CO2 exposure, In was partially oxidized to In2O3-x and Cu remains metallic. CO2 was activated on the defective In2O3-x sites mainly in the form of carbonate. An easier CO2 activation was observed on the In2O3-x sites adjacent to Cu-In alloys by quantifying the surface density of oxygen vacancies and carbonates.Afterwards, practical and model Cu/CeO2-x catalysts were synthesized and investigated for CO2 hydrogenation. NAP-XPS revealed that partially reduced ceria with oxygen vacancies is responsible for the CO2 activation into carbonate. H2 can dissociate on metallic Cu and spillover to the ceria sites, facilitating the conversion of carbonate to CO via formate intermediate and the regeneration of the oxygen vacancies. Therefore, at the Cu/CeO2-x surface, the coupled Cu+/Cu0 and Ce4+/Ce3+ redox pairs on the metal-oxide interfaces are active sites for the RWGS reaction via the formate route.Based on the findings above, a series of practical catalysts consist of Cu and Cu-In sites supported by ZrO2 and CeO2 supports were systematically studied. Their structural properties and the catalytic performance were found to be support dependent. The CO2 conversion of the Cu-In/ZrO2 catalyst was higher than that of the Cu/ZrO2 catalyst, but it is opposite for the Cu/CeO2 and Cu-In/CeO2 catalysts. The support dependent RWGS activity was found to correlate with different active sites: Cu-In alloys on ZrO2 for promoted RWGS activity, and separated Cu and In2O3 on CeO2 for a suppressed activity. These results demonstrate that the active center and catalytic performance of Cu-based catalysts can be tuned by the oxide support.In short, the understanding in the surface science of CO2 hydrogenation on Cu-based catalysts was promoted in this thesis. The resulting knowledge can be applied in both the interpretation of the behaviors of existing catalysts, and the rational design of novel catalysts with better performance. This thesis contributes fundamental insights to producing fuels and chemicals from CO2 in industrial scale.