Evolution of Pd/SiO2 catalysts prepared from chlorine-free precursors

The effect of pretreatment conditions on the final metal dispersion and chemistry of precalcination of Pd/SiO 2 catalysts prepared from Pd(NH 3 ) 4 (NO 3 ) 2 and palladium acetylacetonate [Pd(acac) 2 ] have been studied. The fate of the Pd(NH 3 ) 4 2+ /SiO 2 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 , N 2 O and NO are also liberated at various stages of precalcination in O 2 –He. The latter finding, not observed by others, is explicable by the known propensity of palladium to catalyse NH 3 oxidation. Pretreatment of the Pd(NH 3 ) 4 2+ /SiO 2 in He leads to well dispersed reduced 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 /SiO 2 precursor, an oxidizing atmosphere and temperatures 250 °C are essential for complete removal of carbonaceous species. After decomposition in He, the catalytic role of a carbonaceous material, retained by Pd/SiO 2 , depends on the temperature of the pretreatment. The low-temperature treatment (at 250 °C), followed by reduction in H 2 , supposedly leads to a loosely packed carbonaceous residue, leaving a considerable part of the palladium surface still capable of 2,2-dimethylpropane conversion. On the other hand, a high-temperature decomposition of the Pd(acac) 2 precursor (at 500 °C) in He greatly suppresses the activity. It is speculated that the decomposition in He at 500 °C leads to a carbide-like surface material, which is not eliminated by reduction in H 2 .