Elementary steps in the formation of highly dispersed palladium in NaY: I. Pd ion coordination and migration

Abstract

TPR, TPD, TPO, and TPMS have been used to determine the location and chemical state of Pd supported on NaY. Oxidation of the ammine ligands of Pd(NH 3) 2+ 4 is found to be a stepwise process, producing Pd(NH 3) 2+ 2 ions in supercages and Pd(NH 3) 2+ and Pd 2+ ions in sodalite cages. The relative abundance and location of these ions can be controlled by the calcination program. The size of Pd particles after reduction depends on the location and coordination of Pd ions after calcination. Calcination below 250 °C leaves Pd ions in the supercages, where their coordination to ammine ligands decreases with increasing calcination temperature. The H/Pd ratio after reduction shows a strong positive correlation with the calcination temperature. It is concluded that the Pd particle size in the kinetic regime is controlled by the relative rates of nucleation and crystal growth, which in this case are determined by the relative abundances of the Pd tetraammine and diammine ions. At a calcination temperature of 300 °C Pd ions lose the third ammine ligand allowing the monoammine Pd ions to migrate into sodalite cages, where the remaining ligand is destroyed at 400 °C.