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 CeO2 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 (H2-TPR). It has been confirmed that the highly dispersed copper oxide (CuOx) clusters, as well as the strong interaction of Cu-[Ox]-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 O2- or H2-atmosphere and then tested for the CO-PROX reaction at a space velocity (SV) of 60 000 mL·h–1·gcat–1. The prereduced 5 and 10 wt % Cu samples exhibited excellent catalytic performance with high CO conversions (>50%, up to 100%) and O2 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 O2- and H2-pretreated samples after CO-PROX at 80 °C. Furthermore, the extended X-ray absorption fine structure (EXAFS) fitting results, together with the corresponding H2-TPR data distinctly determined that the highly dispersed CuOx (x = 0.2−0.5) cluster, other than the Cu–[Ox]–Ce (x = 0.7−3.2) structure, were the crucial active species for the studied CO-PROX reaction.