Redox properties of doped and supported copper–ceria catalysts

Abstract

Copper-doped ceria catalysts feature in a variety of catalytic reactions. One important application is selective hydrogen combustionviaoxygen exchange, which forms the basis of cyclic oxidative dehydrogenation. This paper describes the synthesis of monophasic (doped) and biphasic (supported) Cu–ceria catalysts, that are then characterized using a combination of temperature programmed reduction (TPR) and X-ray diffraction (XRD) methods. The catalysts are analyzed both as fresh samples and after redox cycling at 550–800 °C. TPR and XRD characterization clarify the role of the active sites on the catalyst surface and the copper–ceria interactions. Depending on the catalyst type, reduction occurs at ∼110 °C, ∼150 °C, or ∼190 °C. The reduction at 110 °C is ascribed to highly dispersed copper species doped in the ceria lattice, and that at 190 °C to CuO crystallites supported on ceria. Remarkably, both types converge to the 150 °C feature after redox cycling. The reduction temperature of the doped catalyst increases after redox cycling, indicating that stable Cu clusters form at the surface. Conversely, the reduction temperature of the “supported” catalyst decreases after redox cycling, and the CuO crystallites disappear. With this knowledge, a copper-doped ceria catalyst is analyzed after application in selective hydrogen combustion (16 consecutive redox cycles at 550 °C). No CuO crystallites are observed, and the sample reduces at ∼110 °C. This suggests that copper-doped ceria is the active oxygen exchange phase in selective hydrogen combustion.