PREPARATION AND PRETREATMENT EFFECTS ON METAL DECORATION IN RH/LA2O3

Ethane hydrogenolysis and cyclopropane hydrogenation were used to probe changes in the decorative overlayer in Rh/La2O3 resulting from variations in reduction temperature, weight loading, treatment in oxygen followed by low temperature reduction, and method of precursor decomposition. Increasing reduction temperature resulted in an increase in the extent of decoration, especially between 200-degrees-C and 400-degrees-C, as evidenced by a suppression of cyclopropane hydrogenation and ethane hydrogenolysis rate. For a series of catalysts with increasing weight loading, reduction at 500-degrees-C resulted in significant decoration in each case. However, only a slight effect of weight loading may be suggested. An oxidation/low temperature reduction cycle partially reversed the decorative effect. No difference in decoration was observed due to calcination vs. reduction for precursor decomposition when the catalysts were exposed to the same final reduction temperature. Due to the smaller ensemble size required for its sites, cyclopropane hydrogenation was found to be more sensitive than ethane hydrogenolysis to changes in surface structure for these heavily decorated catalysts. Because of limitations in determining free rhodium surface area, the extent of coverage of the rhodium by the decorative overlayer was only able to be determined qualitatively.