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

Abstract

The effect of pretreatment conditions on the final metal dispersion and chemistry of precalcination of Pd/SiO₂ catalysts prepared from Pd(NH₃)₄(NO₃)₂ and palladium acetylacetonate [Pd(acac)₂] have been studied. The fate of the Pd(NH₃)₄²⁺/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 N₂, N₂O and NO are also liberated at various stages of precalcination in O₂–He. The latter finding, not observed by others, is explicable by the known propensity of palladium to catalyse NH₃ oxidation. Pretreatment of the Pd(NH₃)₄²⁺/SiO₂ in He leads to well dispersed reduced Pd species, but an analogous pretreatment in O₂–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 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/SiO₂, depends on the temperature of the pretreatment. The low-temperature treatment (at 250 °C), followed by reduction in H₂, 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 H₂.