Structural investigation of combustion synthesized Cu/CeO₂ catalysts by EXAFS and other physical techniques: Formation of a Ce_1-xCu_xO_2-δ solid solution

The structure and chemical environment of Cu in Cu/CeO₂ catalysts synthesized by the solution combustion method have been investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) spectroscopy, x-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and extended x-ray fine structure (EXAFS) spectroscopy. High-resolution XRD studies of 3 and 5 atom % Cu/CeO₂ do not show CuO lines in their respective patterns. The structure could be refined for the composition Ce_1-xCu_xO_2-δ (x = 0.03 and 0.05; δ ∼ 0.13 and 0.16) in the fluorite structure with 5-8% oxide ion vacancy. High-resolution TEM did not show CuO particles in 5 atom % CU/CeO₂. EPR as well as XPS studies confirm the presence of Cu²⁺ species in the CeO₂ matrix. Redox potentials of Cu species in the CeO₂ matrix are lower than those in CuO. EXAFS investigations of these catalysts show an average coordination number of 3 around the Cu²⁺ ion in the first shell at a distance of 1.96 Å, indicating the O^2- ion vacancy around the Cu²⁺ ion. The Cu-O bond length also decreases compared to that in CuO. The second and third shell around the Cu²⁺ ion in the catalysts are attributed to -Cu²⁺-O^2--Cu²⁺- at 2.92 Å and -Cu²⁺-O^2--Ce⁴⁺- at the distance of 3.15 Å, respectively. The present results provide direct evidence for the formation of a Ce_1-xCu_xO_2-δ type of solid solution phase having -□- Cu²⁺-O-Ce⁴⁺- kind of linkages.