(356e) Sintering Studies On Model Catalytic Systems

Sintering is one of the most important deactivation mechanisms of heterogeneous catalysts. Direct electron microscopy observations of commercial catalysts are inherently difficult due to the tortuous pore structure of such materials. Model catalytic systems consisting of Pt and Pd particles on flat oxide substrates provide a means for improving the visibility of metal particles and to improve counting statistics. We use high resolution scanning electron microscopy (HRSEM) to observe the metal particles in the nano-size regime relevant to heterogeneous catalysis. Our initial experiments were performed in vacuum to gain insight into the relative role of vapor phase versus surface transport of adatoms. We observe a dramatic loss of Pd due to evaporation caused by the high vapor pressure of Pd at this temperature. The rate of metal evaporation predicted by the Langmuir model was compared to the observed loss of Pd and the results suggest that the surface transport pathway is dominant even in vacuum. Hence, measurements of metal particle loss from these model catalysts provide a simple approach to investigate the role of the support on catalyst sintering. In this presentation, we will report on the role of the support modification via CeO2, ZrO2, and BaO on the transport of Pd adatoms during sintering at elevated temperatures. We compare the behavior of the model catalysts with those of conventional supported catalysts to relate these fundamental studies to rates of catalyst sintering.