AVS 50th International Symposium
    Surface Science Thursday Sessions
       Session SS2-ThM

Paper SS2-ThM11
The Water Formation Reaction on a Palladium and Platinum Catalysts, Studied with Laser-induced Fluorescence and Kinetic Modelling

Thursday, November 6, 2003, 11:40 am, Room 327

Session: Catalysis III: High vs. Low Pressures
Presenter: A. Johansson, Goteborg University and Chalmers University of Technology, Sweden
Authors: A. Johansson, Goteborg University and Chalmers University of Technology, Sweden
M. Forsth, Goteborg University and Chalmers University of Technology, Sweden
A. Rosen, Goteborg University and Chalmers University of Technology, Sweden
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The noble metals palladium and platinum are widely used as catalysts where the most important application is said to be the cleaning of exhaust gases. In addition, high-temperature data, especially for palladium are very scarce. In this study we have studied water formation on the palladium metal at high temperatures (1300 K) and moderate pressures (13-26 Pa). The OH desorption was studied with laser-induced fluorescence and the water production with microcalorimetry as a function of the hydrogen mixing ratio, @alpha@@sub H2@. With basis on the experimental data a theoretical model was also obtained using Chemkin. The result was compared with similar experiments for platinum. The following differences were observed. The maximum in OH desorption occurs at the same @alpha@@sub H2@ for both platinum and palladium, however the maximum in water production is measured to be at different @alpha@@sub H2,H2Omax@; at 20% for platinum and 40% for palladium. From the model the initial sticking coefficients for hydrogen and oxygen are found to be about a factor of ten larger on Pd than for Pt. The OH desorption was also seen to have a first order coverage dependence on palladium, no such behaviour was found for platinum. The main route for water formation on a platinum surface is the hydrogen addition reaction however, the main water forming reaction on palladium is not straightforward to determine because of the coverage dependent OH desorption. The reverse of the hydrogen addition reaction was found to be more important for the OH formation on Pd than on Pt. The apparent desorption energy as a function of @alpha@@sub H2@ was also measured for palladium. It was seen that it is increasing with @alpha@@sub H2@ to a maximum and then stays constant. It is interesting to note that the maximum in apparent desorption energy occurs at the same @alpha@@sub H2@ as the maximum in water production. The same phenomenon is also observed on platinum, but at a different @alpha@@sub H2@.