Paper SS1-WeA5
Abstraction of ¹⁸O Atoms Chemisorbed on Pd(111) by ¹⁶O Atoms Incident from the Gas-Phase

Wednesday, October 17, 2007, 3:00 pm, Room 608

We investigated the reactions of gas-phase ¹⁶O atoms with ¹⁸O atoms initially chemisorbed on the Pd(111) surface using direct product monitoring and temperature programmed desorption. We find that ¹⁶O¹⁸O molecules desorb promptly at surface temperatures as low as 100 K during the atom-surface collisions, which suggests that ¹⁸O abstraction occurs by a direct reaction with incident ¹⁶O atoms. Only about 20% of the initially adsorbed ¹⁸O evolves as ¹⁶O¹⁸O during the beam exposures, independent of the surface temperature from 100 K to 500 K. At all temperatures, the ¹⁶O¹⁸O desorption rate initially rises to a maximum and decays thereafter with increasing beam exposure. Above 200 K, a second maximum in the desorption rate also appears after longer exposures, with the intensity of this maximum increasing with increasing surface temperature. We find that the abstraction kinetics is primarily determined by the distribution of oxygen phases that develop on the surface during oxygen uptake from the beam. Specifically, the data suggests that the observed ¹⁶O¹⁸O desorption arises from direct reactions between gaseous ¹⁶O atoms and ¹⁸O atoms chemisorbed either on the bare metal or on top of a surface oxide, and that the abstraction of oxygen atoms incorporated within both surface and bulk oxides occurs with low probability. As a result, only a fraction of the ¹⁸O atoms are abstracted before incorporating into oxide phases during the beam exposure. The second maximum in the ¹⁶O¹⁸O desorption rate is attributed to abstraction of ¹⁸O atoms chemisorbed on top of a developing surface oxide. The increase in this rate maximum with increasing surface temperature will be discussed within the context of an exchange process between oxygen atoms within the surface oxide and oxygen atoms chemisorbed on top of the surface oxide, and a kinetic analysis of the exchange will be presented.