n-alkanes on Pt(111) and on C(0001)/Pt(111):Chain length dependence of kinetic desorption parameters

摘要

We have measured the desorption of seven small n-alkanes (C_NH_(2N+2),N=1-4,6,8,10) from the Pt(111) and C(0001) surfaces by temperature programed desorption.We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J.Chem.Phys.122,164708 (2005)].There we showed an increase in the desorption preexponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value.We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface,but is a general effect for desorption of the small n-aikanes.This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption.In this work,we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface.We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here.On each surface,the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value.Prior results of other groups have yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values.We demonstrate that by allowing the prefactor to increase according to our model,a reanalysis of their data resolves this y-intercept problem to some degree.