Crystal Plane Effect of Ceria on Supported Copper Oxide Cluster Catalyst for CO Oxidation: Importance of Metal–Support Interaction

Copper–ceria as one of the very active catalysts for oxidation reactions has been widely investigated in heterogeneous catalysis. In this work, copper oxide (1 wt % Cu loading) deposited on both ceria nanospheres with a {111}/{100}-terminated surface (1CuCe-NS) and with nanorod exposed {110}/{100} faces (1CuCe-NR) have been prepared for the investigation of crystal plane effects on CO oxidation. Various structural characterizations, especially including aberration-corrected scanning transmission electron microscopy (Cs-STEM), X-ray absorption fine structure (XAFS) technique, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), were used to precisely determine the structure and status of the catalysts. It is found that the copper oxides were formed as subnanometer clusters and were uniformly dispersed on the surface of the ceria support. The results from XAFS combined with the temperature-programmed reduction technique (H₂-TPR) reveal that more reducible CuO_x clusters with only Cu–O coordination structure exclusively dominated in the surface of 1CuCe-NS, while the Cu species in 1CuCe-NR existed in both CuO_x clusters and strongly interacting Cu-[O_x]-Ce. In situ DRIFTS results demonstrate that the CeO₂-{110} face induced a strongly bound Cu-[O_x]-Ce structure in 1CuCe-NR which was adverse to the formation of reduced Cu­(I) active sites, resulting in low reactivity in CO oxidation (r_CO = 1.8 × 10^–6 mol_CO g_cat^–1 s^–1 at 118 °C); in contrast, CuO_x clusters on the CeO₂-{111} face were easily reduced to Cu­(I) species when they were subjected to interaction with CO, which greatly enhanced the catalytic reactivity (r_CO = 5.7 × 10^–6 mol_CO g_cat^–1 s^–1 at 104 °C). Thus, for copper–ceria catalyst, in comparison with the well-known reactive {110}_CeO2 plane, {111}_CeO2, the most inert plane, exhibits great superiority to induce more catalytically active sites of CuO_x clusters. The difference in strength of the interaction between copper oxides and different exposed faces of ceria is intrinsically relevant to the different redox and catalytic properties.