Paper SS+AS+NC-FrM11
The Adsorption of Water on a PdO(101) Thin Film on Pd(111)

Friday, October 24, 2008, 11:40 am, Room 207

Palladium oxide (PdO) is an excellent catalyst for the oxidation of hydrocarbons and CO under oxygen-rich conditions. Unfortunately, however, many fundamental questions about the surface chemistry of PdO have remained unanswered since it has been challenging to prepare well-defined PdO surfaces for detailed experimental investigations. In this talk, I will discuss our recent experimental study of the adsorption of water on a PdO(101) thin film that was grown in ultrahigh vacuum by oxidizing Pd(111) using an oxygen atom beam. From temperature programmed desorption (TPD) measurements, we find that water can populate several adsorbed states on PdO(101). Physisorbed water molecules populate both monolayer and multilayer states on PdO(101), with desorption from these states yielding TPD peaks at 150 and 197 K, respectively. The desorption temperature from the physisorbed monolayer is about 35 K higher for PdO(101) compared with Pd(111), which suggests that hydrogen-bonding enhances the physisorption bond strength on the oxide. We also observe a relatively broad desorption feature of water from PdO(101) that appears to consist of two components centered at 320 K and 350 K. We present evidence that these components arise from distinct states of molecularly and dissociatively chemisorbed water, respectively, and that both forms of chemisorbed water bind to coordinatively unsaturated Pd cations that are prevalent on the PdO(101) surface. Finally, although the dissociative chemisorption of water presumably involves hydrogen transfer to an oxygen anion of the oxide, we observe only small amounts of oxygen exchange between water and the oxide in experiments with oxygen-labeled water. This finding suggests that water dissociation produces inequivalent hydroxyl groups on PdO(101).