A combination of time-resolved X-ray diffraction (TR-XRD), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy was used to carry out in situ characterization of Cu/CeO2 nanocatalysts during the hydrogenation of CO2. Morphological effects of the ceria supports on the catalytic performances were investigated by examining the behavior of copper/ceria nanorods (NR) and nanospheres. At atmospheric pressures, the hydrogenation of CO2 on the copper/ceria catalysts produced mainly CO through the reverse water–gas shift (RWGS) reaction and a negligible amount of methanol. The Cu/CeO2-NR catalyst displayed the higher activity, which demonstrates that the RWGS is a structure-sensitive reaction. In situ TR-XRD and AP-XPS characterization showed significant changes in the chemical state of the catalysts under reaction conditions, with the copper being fully reduced and a partial Ce4+ → Ce3+ transformation occurring. A more effective CO2 dissociative activation at high temperature and a preferential formation of active bidentate carbonate and formate intermediates over CeO2(110) terminations are probably the main reasons for the better performance of the Cu/CeO2-NR catalyst in the RWGS reaction.

The metal−oxide interaction changes the surface electronic states of catalysts deployed for chemical conversion, yet details of its influence on the catalytic performance under reaction conditions remain obscure. In this work, we report the high activity/stability of a ceria-supported Ru−nanocluster (<1 nm) catalyst during the dry reforming of methane. To elucidate the structure−reactivity relationship underlying the remarkable catalytic performance, the active structure and chemical speciation of the catalyst was characterized using in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS), while the surface chemistry and active intermediates were monitored by in situ ambient-pressure Xray photoelectron spectroscopy (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Methane activates on the catalyst surface at temperatures as low as 150 °C. Under reaction conditions, the existence of metal−support interactions tunes the electronic properties of the Ru nanoclusters, giving rise to a partially oxidized state of ruthenium stabilized by reduced ceria (Ru δ+ − CeO 2−x ) to sustain active chemistry, which is found to be very different from that of large Ru nanoparticles supported on ceria. The oxidation of surface carbon is also a crucial step for the completion of the catalytic cycle, and this is strongly correlated with the oxygen transfer governed by the Ru δ+ −CeO 2−x interactions at higher temperatures (>300 °C). The possible reaction pathways and stable surface intermediates were identified using DRIFTS including ruthenium carbonyls, carboxylate species, and surface −OH groups, while polydentate carbonates may be plain spectators at the measured reaction conditions.

Strong bonding interactions between a transition metal and a substrate or support is one of the most effective strategies to immobilize subnanometer scale clusters or atoms in heterogeneous catalysis. We show that such a type of phenomenon can take place on a Mo2N surface. Combined experimental and theoretical studies show that strong metal–support interactions between face-centered cubic-structured γ-Mo2N and cobalt have been confirmed to effectively anchor subnanometer Co clusters and prevent their aggregation. The results of X-ray absorption near edge structure, ambient pressure X-ray photoelectron spectroscopy, and density functional theory revealed electronic perturbations in the nitride-bonded cobalt not seen on a strongly active oxide such as CeO2. A charge transfer from Co to Mo2N was observed with a significant stabilization of the Co 3d levels, which prevents the full decomposition of CO2. The subnanometer Co loaded on γ-Mo2N catalysts exhibited very high selectivity to the product CO, whereas the undesirable methanation activity, typically inevitable on traditional Co/oxide catalysts, was successfully suppressed. As a consequence of the electronic perturbations induced by the nitride, the cobalt was not able to fully dissociate the CO2 molecule to generate C or CH x fragments necessary for methane production. Under reaction conditions, the strong bonding between Co and γ-Mo2N maintained the subnanometer geometry of Co, leading to a remarkable selectivity and stability.