Regensburg 2007 – scientific programme
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O: Fachverband Oberflächenphysik
O 57: Surface Chemical Reactions and Heterogeneous Catalysis II
O 57.8: Talk
Thursday, March 29, 2007, 17:30–17:45, H38
Dehydrogenation kinetics of methyl on various Pt surfaces as studied by temperature-programmed XPS. — •Christian Papp, Barbara Tränkenschuh, Regine Streber, Reinhard Denecke, and Hans-Peter Steinrück — Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen
We studied the reaction of methyl (CH3) on different platinum surfaces by temperature-programmed XPS at the synchrotron radiation facility BESSY II in Berlin [1]. The quantitative analysis yielded the transition temperatures for the subsequent dehydrogenation steps from CH3 to CH to atomic C, which depend on the crystallographic orientation of the substrate. Corresponding experiments with deuterated methyl (CD3) show the same dehydrogenation sequence; however, the transition temperatures are approx. 20 K higher, due to the kinetic isotope effect. By assuming first order reaction kinetics and by applying a simple Redhead analysis [2] to our data, we were able to estimate the activation energies. The difference in the activation barriers for dehydrogenation of deuterated and non-deuterated methyl to methylidyne agrees well with the difference in zero-point energies of the C-H and C-D vibrations. This result is in agreement with expectations from DFT calculations [3], which show that the rate limiting step in methyl dissociation should be the breaking of the first C-H bond.
[1] Papp et al., J. Phys. Chem. C accepted.
[2] Redhead, Vacuum 62 (1962) 203.
[3] Petersen et al., J. Phys. Chem. B 108 (2004) 5909.