IN-SITU ELECTRON-MICROSCOPY STUDIES OF PALLADIUM SUPPORTED ON AL2O3 SIO2, AND ZRO2 IN OXYGEN

Controlled atmosphere electron microscopy (CAEM) has been used to obse rve and compare directly the behavior of palladium particles supported on gamma-alumina, silica and zirconia, when heated in the presence of oxygen, Palladium is converted to PdO on all three supports at temper atures in excess of 325 degrees C and undergoes a wetting and spreadin g action to form thin flat crystallites. At temperatures above 750 deg rees C a significant fraction of the PdO disproportionates to form met allic Pd, which subsequently undergoes a rapid interaction with both a lumina and silica at 800 degrees C to form compounds that probably con sist of mixtures of the respective oxides; however, the precise compos ition of these species cannot be determined, It is suggested that this phenomenon is responsible for deactivation of these catalyst systems at high temperatures. This type of interaction was not observed when z irconia-supported palladium specimens were treated under the same cond itions, In this case, the support appeared to stabilize the formation of PdO up to 900 degrees C. It is significant that on all supports a f raction of the palladium remains in the form of PdO up to the highest temperatures even though bulk thermodynamics predict that it should al l be in the metallic state. (C) 1995 Academic Press, Inc.