Ce1-xRhxO2-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 Ce1-xRhxO2-δ type of solid solution with −□−Rh3+−O−Ce4+− kind of linkages on the surface.