THE PREPARATION AND CHARACTERIZATION OF ULTRADISPERSED RHODIUM CATALYSTS

This study was directed at finding routes to the preparation of heterogeneous catalysts in which the active sites were comprised of isolated rhodium atoms on a support material. The approach taken was to prepare very highly dispersed catalysts via the low temperature reduction of supported organometallic compounds. The resulting catalysts were then tested for catalytic activity for the hydrogenation and isomerization of 1-butene. The observed rates and selectivities (ratio of isomerized product to hydrogenated product) were used as a probe of the nature of the active site. This probe was rendered more meaningful by establishing a well-characterized set of catalysts on which comparisons could be based.; Chemisorption and catalytic studies indicated that catalysts with relatively high rhodium content, in the vicinity of 1.0%, invariably consisted of metal crystallites. For these catalysts, both the isomerization and hydrogenation of 1-butene were demonstrated to be structure insensitive over a particle size range of 10-100 (ANGSTROM). The effects of changing the impregnation solvent, organometallic complex, reduction temperature, or support were related to observable quantities such as the dispersion or the catalytic activity.; Catalysts approaching atomic dispersions were observed when the catalysts had a relatively low rhodium content. On oxide supports such as silica and alumina, catalytic activity for the test reaction is lost as the atomically dispersed state is approached. The loss in activity appears to result from a strong interaction with surface hydroxyls on the support. In contrast, this behavior is not observed with the low loading catalysts prepared on a hydroxyl-free carbon black. As atomic dispersion is approached on carbon, one observes a shift in selectivity toward isomerization. The shift is caused by a decrease in the activity for catalytic hydrogenation while isomerization remains structure insensitive. This observation was rationalized in terms of electronic and geometric effects as the atomically dispersed state is approached.