Nature of the two-step temperature-programmed decomposition of PdO supported on alumina

Abstract

Temperature-programmed decomposition of PdO supported on high-surface-area alumina occurs in a two-step process starting near the temperature of bulk PdO decomposition, but slowing to near-zero rates as the bulk oxide decomposition temperature is traversed. Most of the oxide decomposes in a second step 40–60°C above the bulk PdO decomposition temperature. We present evidence suggesting that a thin Pd shell forms initially on the surface of PdO particles and propose that the resulting core–shell structure is connected with the observed meta-stability. Both the experimental results and order-of-magnitude estimates indicate that neither small particle effects (metal-support interaction or surface energy) nor low oxygen permeability through the Pd shell explain the two-step process. We tentatively ascribe the higher-temperature PdO decomposition step to the contribution of strain energy to the Pd shell imparted by epitaxy with the underlying PdO lattice.