THE EFFECT OF ALUMINA PHASE-STRUCTURE ON THE DISPERSION OF RHODIUM ALUMINA CATALYSTS/

Aluminas from various sources were thermally treated in air to obtain a range of phases (delta, theta, and alpha) with BET surface areas bet ween 5 and 90 m(2)/g. Rhodium was impregnated onto the aluminas at a c onstant loading of 0.6 mu mol Rh/m(2) (BET). The resultant Rh-Al2O3 ca talysts were hydrothermally aged by treating in 10% H2O/air at 1223 K for 24 h. Temperature-programmed reduction and a noble metal surface a rea measurement based on methanation of adsorbed CO were used to chara cterize the state of the Rh after aging. Three broad forms of Rh were identified: occluded, strongly interacted, and noninteracted. Apparent dispersions ranged from 3 to 11%, with the highest values obtained fo r alpha-alumina. A pronounced loading effect was observed for the Rh/a lpha-alumina catalysts; dispersions ca. 10% were obtained only on samp les with loadings less than about 1 mu mol Rh/m(2) (BET). Higher loadi ngs (>1.5 mu mol Rh/m(2) (BET)) resulted in apparent dispersions of 3% or less. These results suggest that the most effective way to maximiz e the Rh dispersion in high-temperature aged alumina-based automotive catalysts is to isolate the Rh on alpha-phase alumina at loadings less than 1 mu mol/m(2) (BET). Presteaming of selected aluminas at 1223 K for 24 h prior to Rh impregnation decreased the BET surface area and r esulted in less occlusion of Rh during subsequent aging. (C) 1995 Acad emic Press, Inc.