EVIDENCE FOR KINETIC AND OXIDATIVE STABILIZATION OF RH ON MO-PROMOTEDRH AL2O3/

Carbon monoxide hydrogenation studies on silica- and alumina-supported Rh catalysts conducted in our laboratories as well as those reported in the literature suggest that the addition of early transition-metal oxides to the catalyst surface exerts a promotional influence on activ e Rh centers, This promotion leads to enhanced oxygenate selectivity. Explanations of the promotional influence of the early transition-meta l oxides fall into three categories: (i) kinetic stabilization of smal l Rh aggregates, (ii) wetting and spreading of the transition-metal ox ide, leading to decoration of the surface of Rh crystallites, and (iii ) oxidative stabilization of Rh aggregates, altering Rh interactions w ith sub-carbonyls during reaction. Characterization of the Rh-Mo/gamma -Al2O3 system provides supporting evidence for both kinetic stabilizat ion and oxidative stabilization of Rh aggregates and suggests that the se processes contribute to the enhanced oxygenate selectivity of these materials. Hydrogen and carbon monoxide chemisorption results provide evidence for highly dispersed and oxidatively stabilized Rh aggregate s. Even after extended on-stream testing (> 10 h), where metal aggrega tion is unavoidable, high-resolution transmission electron microscopy, associated energy dispersive spectroscopy, and the use of fast-Fourie r transform post-processing to produce optical diffractograms highligh t differences in the metal-aggregate morphology.