Ce_1-xRh_xO_2-δ solid solution formation in combustion-synthesized Rh/CeO₂ catalyst studied by XRD, TEM, XPS, and EXAFS

Abstract

Ionically dispersed Rh over CeO₂ in Rh/CeO₂ catalysts prepared by a single step solution combustion method is shown to improve the redox property and catalytic activity. The H₂/Rh ratio obtained from hydrogen uptake measurement was 5.4, 2.4, and 2.1, respectively in 0.5, 1, and 2% Rh/CeO₂, indicating a significant contribution from the reduction of CeO₂ in the presence of Rh. In 1% Rh/CeO₂, the light-off temperature for CO oxidation is about 80 °C lower compared to Rh metal and 190 °C lower than that of Rh₂O₃. The enhanced redox property and CO oxidation activity of the catalyst has been correlated with the structure. The X-ray diffraction (XRD) pattern could be refined to the fluorite structure with Rh substituting in the Ce site. Transmission electron microscopy (TEM) images show only CeO₂ crystallites of about 50 nm and no evidence of any metal particles up to 1 atom % Rh. X-ray photoelectron spectroscopy (XPS) studies demonstrate that Rh is dispersed in the +3 oxidation state on CeO₂ with enhanced Rh ion concentration in the surface layers. An average coordination number of 2.5 at a distance of 2.05 Å in the first shell is obtained around Rh ions from extended X-ray absorption fine structure (EXAFS) spectroscopy, indicating an oxide ion vacancy around the Rh ion. The correlations at 2.72 and 3.16 Å correspond to Rh-Rh and Rh-Ce interactions, respectively. Thus, the enhanced catalytic activity of Rh/CeO₂ is shown to be due to the formation of a Ce_1-xRh_xO_2-δ type of solid solution with -O-Rh³⁺-O-Ce⁴⁺- kind of linkages on the surface.