Highly Dispersed Copper Oxide Clusters as Active Species in Copper-Ceria Catalyst for Preferential Oxidation of Carbon Monoxide

Abstract

Copper-ceria is one of the very active catalysts for the preferential oxidation of carbon monoxide (CO-PROX) reaction, which is also a typical system in which the complexity of copper chemistry is clearly exhibited. In the present manuscript, copper–ceria catalysts with different Cu contents up to 20 wt % supported on CeO 2 nanorods were synthesized by a deposition–precipitation (DP) method. The as-prepared samples were characterized by various structural and textural detections including X-ray diffraction (XRD), Vis-Raman, transmission electron microscopy (TEM), ex situ / in situ X-ray absorption fine structure (XAFS), and temperature-programmed reduction by hydrogen (H 2 -TPR). It has been confirmed that the highly dispersed copper oxide (CuO x ) clusters, as well as the strong interaction of Cu-[O x ]-Ce structure, were the main copper species deposited onto the ceria surface. No separated copper phase was detected for both preoxidized and prereduced samples with the Cu contents up to 10 wt %. The fresh copper–ceria catalysts were pretreated in either O 2 - or H 2 -atmosphere and then tested for the CO-PROX reaction at a space velocity (SV) of 60 000 mL·h –1 ·g cat –1 . The prereduced 5 and 10 wt % Cu samples exhibited excellent catalytic performance with high CO conversions (>50%, up to 100%) and O 2 selectivities (>60%, up to 100%) within a wide temperature window of 80–140 °C. The in situ XAFS technique was carried out to monitor the structural evolution on the copper–ceria catalysts during the PROX experiments. The X-ray absorption near edge spectra (XANES) profiles, by the aid of linear combination analysis, identified the oxidized Cu(II) were the dominant copper species in both O 2 - and H 2 -pretreated samples after CO-PROX at 80 °C. Furthermore, the extended X-ray absorption fine structure (EXAFS) fitting results, together with the corresponding H 2 -TPR data distinctly determined that the highly dispersed CuO x ( x = 0.2−0.5) cluster, other than the Cu–[O x ]–Ce ( x = 0.7−3.2) structure, were the crucial active species for the studied CO-PROX reaction.