STRUCTURE AND REACTIVITY OF SUPPORTED PALLADIUM CATALYSTS .1. PD SIO2PREPARED FROM PDCL2/

The effect of precalcination conditions on the final metal dispersion of 0.6 wt% Pd/SiO2 catalyst prepared from PdCl2 was studied. Upon redu ction at 500 degrees C, differently precalcined (in Ar or O-2/He, at v arious temperatures) samples exhibited substantial variations in metal dispersion measured by H-2 chemisorption (H/Pd between 0.04 and 0.66) . A variety of methods have been employed in order to get an insight i nto the chemistry of the calcination/reduction steps. Calcination in O -2/He at temperatures > 500 degrees C leads to the essential removal o f chlorine and the formation of large crystallites of PdO, which, upon reduction in H-2, form a poorly dispersed metal phase. On the other h and, samples precalcined at 300 degrees C contain highly dispersed pal ladium chloride species. However, their reduction in H-2 only results in moderate metal dispersions (H/Pd=0.25). Higher metal dispersions ar e achieved by precalcination at 450-500 degrees C (H/Pd=0.66). The tem perature-programmed experiments, X-ray diffraction, electron paramagne tic resonance, and UV-visible diffuse reflectance spectroscopy demonst rate incorporation of oxygen into the palladium chloride precursor dur ing the precalcination step. Because the O-2/He-pretreated samples sho w higher metal dispersions than those precalcined in Ar, the incorpora tion of oxygen into Pd-Cl complexes deposited on silica seems to play a beneficial role in preserving a highly dispersed Pd-containing phase . Conversion of neopentane catalyzed by differently pretreated 0.6 wt% Pd/SiO2 samples showed only very minor variations (TOFs, selectivitie s) with metal dispersion, similarly to previous results obtained on a series of Pd/SiO2 catalysts prepared from chlorine-free precursors.