HIGH-TEMPERATURE BEHAVIOR OF RH/AL2O3 CATALYSTS

The behavior of highly dispersed Rh/Al2O3 catalysts (0.15% Rh) has been investigated at extreme temperatures up to 1400 K under vacuum and under CO(g) pressure. Infrared spectroscopic studies have shown that the removal by heating of isolated Al-OH groups on the support, beginning near 400-600 K, decreases the tendency of the Rh(x)0 species to form Rh(I) species (as Rh(I)(CO)2) following CO adsorption. This is in agreement with the work of others where isolated Al-OH groups have been shown to be necessary for oxidation of supported Rh(x)0 species in the presence of CO. Heating under vacuum or under CO(g) yields similar behavior, although additional degradation of Rh(x)0 sites for CO chemisorption occurs under CO(g), possibly due to carbon deposition. Heating to 1400 K causes the conversion of gamma-Al2O3 to alpha-Al2O3 and results in the complete loss of CO chemisorption capability for the Rh(x)0 species as judged by IR spectroscopy.