EVOLUTION OF PD SIO2 CATALYSTS PREPARED FROM CHLORINE-FREE PRECURSORS/

The effect of pretreatment conditions on the final metal dispersion an d chemistry of precalcination of Pd/SiO2 catalysts prepared from Pd(NH 3)(4)(NO3)(2) and palladium acetylacetonate [Pd(acac)(2)] have been st udied. The fate of the Pd(NH3)(4)(2+)/SiO2 precursor strongly depends on precalcination conditions. In helium, a vast majority of the ammine ligands desorb in a stepwise fashion, whereas considerable amounts of N-2, N2O and NO are also liberated at various stages of precalcinatio n in O-2-He. The latter finding, not observed by others, is explicable by the known propensity of palladium to catalyse NH3 oxidation. Pretr eatment of the Pd(NH3)(4)(2+)/SiO2 in He leads to well dispersed reduc ed Pd species, but an analogous pretreatment in O-2-He results in the formation of an even more dispersed PdO species. In precalcination of the Pd(acac)(2)/SiO2 precursor, an oxidizing atmosphere and temperatur es greater than or equal to 250 degrees C are essential for complete r emoval of carbonaceous species. After decomposition in He, the catalyt ic role of a carbonaceous material, retained by Pd/SiO2, depends on th e temperature of the pretreatment. The low-temperature treatment (at 2 50 degrees C), followed by reduction in H-2, supposedly leads to a loo sely packed carbonaceous residue, leaving a considerable part of the p alladium surface still capable of 2,2-dimethylpropane conversion. On t he other hand, a high-temperature decomposition of the Pd(acac)(2) pre cursor (at 500 degrees C) in He greatly suppresses the activity. It is speculated that the decomposition in He at 500 degrees C leads to a c arbide-like surface material, which is not eliminated by reduction in H-2.