AVS 51st International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuA

Paper SS1-TuA7
Adsorption Energies of Small Alkane Molecules on MgO(100), Pt(111), and C(0001)/Pt(111) by Temperature Programmed Desorption

Tuesday, November 16, 2004, 3:20 pm, Room 210B

Session: Hydrocarbon Reactions on Metal Surfaces
Presenter: S.L. Tait, Jr., University of Washington
Authors: S.L. Tait, Jr., University of Washington
Z. Dohnálek, Pacific Northwest National Laboratory
C.T. Campbell, University of Washington
B.D. Kay, Pacific Northwest National Laboratory
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The adsorption of small hydrocarbon molecules at oxide and metal surfaces represents an important scientific problem with applications in catalysis and sensors. We have used molecular beams and temperature programmed desorption (TPD) to study the adsorption of small alkane molecules, C@sub n@H@sub 2n+2@ (n=1-10), on MgO(100), Pt(111), and a two-dimensional graphite film on Pt(111). Hydrocarbon molecules are deposited on the surface by a highly collimated molecular beam with a well-defined incident angle and kinetic energy. The adsorption energies and pre-exponential factors are extracted from TPD data. Simulations based on the results of this analysis show excellent agreement with experiment. The desorption prefactors increase with alkane chain length by more than five orders of magnitude over the range of alkane molecules studied. Adsorption energy on each surface increases linearly with alkane chain length with a y-intercept value near zero. Prior studies have found a non-zero intercept by treating the pre-exponential factor as constant with alkane chain length. This increase in pre-exponential factor can be attributed entirely to the increase in the rotational entropy in the transition state theory, assuming that the adsorbed initial state is not a free rotor but that the transition state is. Pacific Northwest National Laboratory is a multiprogram National Laboratory operated for the U. S. Department of Energy by Battelle Memorial Institute under contract DE-AC06-76RLO 1830. SLT supported by a UW/PNNL Joint Institute for Nanotechnology fellowship.