Palladium catalyzed oxidation of amorphous carbon: A study by in situ transmission electron microscopy

Abstract

Palladium clusters, 10-30 nm in size, were grown in UHV on amorphous carbon in a transmission electron microscope (TEM). The Pd catalyzed oxidation of carbon was then followed in situ in the TEM between 720 and 800 K at a pressure p of O2 with 0.25 < p (10-4 mbar) < 1.0). The behavior of hundreds of individual clusters was recorded on videotape in real time. Clusters moved as they dug irregular channels throughout the carbon film. They exhibited liquid-like behavior and frequent change of contrast (flashing). These active clusters started to move after an induction period. Some clusters remained inactive. Active clusters coalesced with inactive clusters to form active clusters. After some time, active clusters became and remained inactive. The site time yield of catalyzed oxidation, defined as the number of C atoms reacting per exposed Pd atom per second was almost independent of temperature and proportional to pressure of O2. Under the conditions of this work, it appears that the rate determining step of the oxidation of carbons is the dissociative chemisorption of O2 on Pd, with every O2 molecule sticking to the Pd surface reacting to C with formation of CO2. © 1993 Academic Press, Inc.