Structural investigation of combustion synthesized Cu/CeO(2) catalysts by EXAFS and other physical techniques: Formation of a Ce(1-x)Cu(x)O(2-delta) solid solution

Abstract

The structure and chemical environment of Cu in Cu/CeO(2) 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 percent Cu/CeO(2) do not show CuO lines in their respective patterns. The structure could be refined for the composition Ce(1-x)Cu(x)O(2-delta) (x = 0.03 and 0.05; delta similar to 0.13 and 0.16) in the fluorite structure with 5-8 percent oxide ion vacancy. High-resolution TEM did not show CuO particles in 5 atom percent Cu/CeO(2). EPR as well as XPS studies confirm the presence of Cu2+ species in the CeO(2) matrix. Redox potentials of Cu species in the CeO(2) 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 Angstrom, indicating the O2- 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+-O2--Cu2+ - at 2.92 Angstrom and -Cu2+-O2--Ce4+- at the distance of 3.15 Angstrom, respectively.The present results provide direct evidence for the formation of a Ce(1-x)Cu(x)O(2-delta) type of solid solution phase having -square-Cu2+-O-Ce4+- kind of linkages.