Abstract
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 X-ray photoelectron spectroscopy (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Methane activates on the catalyst surface at temperatures as low as 150 degrees 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 delta+-CeO2-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 delta+-CeO2-x interactions at higher temperatures (>300 degrees 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.