Microstructural Changes and Volatilization of Rh and Rh/Ce on SiO2 and Al2O3 in NO + CO

Dramatic changes in the microstructures of Rh and Rh/Ce supported on SiO2 and Al2O3 occur after treatments in NO + CO at temperatures as low as 300°C. Silica-supported Rh was nearly completely volatilized by treatment in NO + CO at 300°C, but the addition of Ce was found to retard this effect. Examination of the effects of the individual gases showed that NO alone disperses Rh over the SiO2 and CO alone volatilizes the dispersed Rh formed by treatment in NO. However, heating in CO alone had no effect on undispersed Rh. The activity of Rh on porous SiO2 for NO + CO at 200°C decreased by a factor of five after heating overnight at 300°C due to loss of Rh. On alumina, heating in NO + CO had much less effect and only a few smaller particles disappeared even after extensive treatment at 400°C. Heating in NO alone, however, did disperse the Rh into small (∼20 Å) particles on the Al2O3. Similar changes were observed for Rh/Ce. The NO + CO activity on alumina actually increased slightly by heating at 300°C for extended time. XPS indicates the presence of small amounts of oxidized Rh on both supports after heating in NO + CO, but cannot identify whether this is the dispersed Rh or the oxidized surface of Rh particles. We speculate that dispersion max be through the formation of a mobile nitrosyl complex and volatilization by formation of volatile carbonyls.