Evidence for kinetic and oxidative stabilization of Rh on Mo-promoted Rh 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 active Rh centers. This promotion leads to enhanced oxygenate selectivity. Explanations of the promotional influence of the early transition-metal oxides fall into three categories: (i) kinetic stabilization of small Rh aggregates, (ii) wetting and spreading of the transition-metal oxide, leading to decoration of the surface of Rh crystallites, and (iii) oxidative stabilization of Rh aggregates, altering Rh interactions with sub-carbonyls during reaction. Characterization of the RhMo γ-Al₂O₃ system provides supporting evidence for both kinetic stabilization and oxidative stabilization of Rh aggregates and suggests that these 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 aggregates. Even after extended on-stream testing (> 10 h), where metal aggregation is unavoidable, high-resolution transmission electron microscopy, associated energy dispersive spectroscopy, and the use of fast-Fourier transform post-processing to produce optical diffractograms highlight differences in the metal-aggregate morphology.