Elementary steps in the formation of highly dispersed palladium in NaY: II. Particle formation and growth

Abstract

Calcination conditions which leave Pd ions in Pd NaY in two reproducible states have previously been defined: Low-temperature calcination ( T c = 250 ° C) leaves Pd(NH 3) 2+ x in supercages, but high-temperature calcination ( T c = 500 ° C) places Pd 2+ quantitatively in sodalite cages. It has now been found that the processes which lead to Pd particle formation during reduction with H 2 are entirely different for these two cases. Reduction of Pd ions in supercages results in the formation of primary particles ( d < 7 A ̊ ) which rapidly migrate and coalesce to larger particles, whose migration is impeded by the size of the supercage windows. Reduction of Pd ions in sodalite cages, however, leads to the formation of isolated atoms or dimers which are trapped in these cages. Their subsequent release into the supercage network is an activated process. The ratio of adsorbed hydrogen to reduced palladium, in this case, initially increases with temperature, then passes through a maximum, indicating that isolated Pd atoms are incapable of dissociatively chemisorbing H 2.