THERMAL EVOLUTION OF THE SUPPORT PROMOTER EFFECT IN RH/LA2O3

Suppression of CO chemisorption and the rates of ethane hydrogenolysis and cyclopropane hydrogenation along with a - 1-eV shift in the Rh 3d 5 2 binding energy were observed for a 200†C-reduced Rh/La2O3 catalyst compared to Rh/SiO2. These results are consistent with the decoration model proposed for the metal-support interaction between La2O3 and noble metals, where the decorating species play the role of a promoter. The presence of these effects after such a mild pretreatment suggests that dissolution of the support by acidic precursor solutions during impregnation/drying and/or migration of support species during the early stages of reduction are probably the primary causes of the decoration. The extent of decoration appeared to be increased upon reduction of the La2O3-supported catalyst at 400°C as evidenced by a further suppression of CO chemisorption and cyclopropane hydrogenation rate. H2 uptake, which greatly exceeded CO uptake after the 200°C reduction suggesting spillover of H2 to decorating species and/or the support, increased further after a 400°C reduction indicating an increased capacity of the decorating species and support for spillover H2. This thermal modification of the decorative overlayer occurs concurrent with the dehydration of La(OH)3 to LaOOH and La2O3 and the Rh-catalyzed decomposition of La2(CO)3)3, those compounds which constitute the support after impregnation and/or exposure to air. © 1991.