PROTECTION OF A RH/AL2O3 CATALYST UNDER EXTREME ENVIRONMENTAL-CONDITIONS

The removal of isolated Al-OH groups by the silation of Rh/Al2O3 catalysts is effective in preventing catalyst degradation under conditions where CO adsorption occurs. This is because isolated Al-OH groups are essential as oxidizing agents for the formation of isolated Rh(I)(CO)2 species, causing Rh-Rh bonds to be broken in metallic Rh crystallites in the presence of CO(g). We have found that the functionalization of isolated Al-OH species by (CH3)3SiCl leads to the formation of (CH3)3Si-O-Al = (alumasiloxane) surface species which are very stable. Both oxidation-reduction cycles and hydrolysis at elevated temperatures are shown to be ineffective in breaking metallic Rh-Rh bonds on the silated catalyst in the presence of CO. This suggests that silation by alkyl halosilanes may provide a practical route for the protection of Rh/Al2O3 catalysts from oxidative degradation under extreme conditions encountered in automotive catalysts.