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

• PdO on alumina decomposes in a two-step process during thermal ramping.
• Decomposition initiates at the PdO surface near the bulk PdO decomposition temperature forming a Pd shell.
• The remaining PdO core decomposes at temperatures above the bulk PdO decomposition temperature.

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.

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