Ce_1-xRh_xO_2-δ Solid Solution Formation in Combustion-Synthesized Rh/CeO₂ Catalyst Studied by XRD, TEM, XPS, and EXAFS

Ionically dispersed Rh over CeO 2 in Rh/CeO 2 catalysts prepared by a single step solution combustion method is shown to improve the redox property and catalytic activity. The H 2 /Rh ratio obtained from hydrogen uptake measurement was 5.4, 2.4, and 2.1, respectively in 0.5, 1, and 2% Rh/CeO 2, indicating a significant contribution from the reduction of CeO 2 in the presence of Rh. In 1% Rh/CeO 2, the light-off temperature for CO oxidation is about 80 °C lower compared to Rh metal and 190 °C lower than that of Rh 2 O 3 . 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 CeO 2 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 CeO 2 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/CeO 2 is shown to be due to the formation of a Ce 1 - x Rh x O 2 - δ type of solid solution with −□−Rh 3+ −O−Ce 4+ − kind of linkages on the surface.