$Ce_{1-x}Rh_xO_{2-\delta}$ Solid Solution Formation in Combustion-Synthesized $Rh/CeO_2$ 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 $Rh_2O_3.$ 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 $Ce_{1-x}Rh_xO_{2-\delta}$ type of solid solution with $-0-Rh^{3+}- O-Ce^{4+}-$ kind of linkages on the surface