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.

No abstract

The methane activation and methane dry reforming reactions were studied and compared over 4 wt % Ni/CeO2 and 4 wt % Ni/CeZrO2 (containing 20 wt % Zr) catalysts. Upon the incorporation of Zr into the ceria support, the catalyst exhibited a significantly improved activity and H2 selectivity. To understand the effects of the Zr dopant on Ni and CeO2 during the dry reforming of methane (DRM) reaction and to probe the structure–reactivity relationship underlying the enhanced catalytic performance of the mixed-oxide system, in situ time-resolved X-ray diffraction (TR-XRD), X-ray absorption fine structure (XAFS), and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) were employed to characterize the catalysts under reaction conditions. TR-XRD and AP-XPS indicate that ceria–zirconia supported Ni (Ni/CeZrO2) is of higher reducibility than the pure ceria supported Ni (Ni/CeO2) upon the reaction with pure CH4 or for the methane dry reforming reaction. The active state of Ni/CeZrO2 under optimum DRM conditions (700 °C) was identified as Ni0, Ce3+/Ce4+, and Zr4+. The particle size of both nickel and the ceria support under reaction conditions was analyzed by Rietveld refinement and extended XAFS fitting. Zr in the ceria support prevents particle sintering and maintains small particle sizes for both metallic nickel and the partially reduced ceria support under reaction conditions through a stronger metal–support interaction. Additionally, Zr prevents Ni migration from the surface into ceria forming a Ce1–x Ni x O2–y solid solution, which is seen in Ni/CeO2, thus helping to preserve the active Ni0 on the Ni/CeZrO2 surface.

This paper describes the effect of halide anions (X = Cl, Br, I) immobilized on the surface of nanostructured silver electrocatalysts on the efficiency and the mechanism of CO 2 reduction to CO in aqueous carbonate solutions. A simple oxidation−reduction cycle on Ag foil in the presence of halide anions produces high-surface-area nanostructured catalysts mainly composed of metallic Ag with a small amount of halide anions attached to the electrode surface (X−Ag) as demonstrated by XPS, XRD, and SEM studies. The activity of X−Ag electrocatalysts in 0.1 M NaHCO 3 at pH 6.8 is significantly higher than that of Ag foil or Ag nanoparticles with comparable surface area and morphology. The activity enhancement is attributed to the formation of active catalytic sites, presumably Cl − − Ag n + clusters on the surface of metallic Ag, as evidenced by XPS analysis. The activity of X−Ag catalysts is in the order Cl > Br > I, which is consistent with the proposed model of an active site. The Tafel analysis of electrochemical CO 2 reduction points to the sensitivity of the mechanism of electrocatalysis on the nature of X.

No abstract