PNNL

Abstract

Coverage-dependent desorption kinetics parameters are obtained from high quality temperature programmed desorption (TPD) data for seven small n-alkane molecules on MgO(100). The molecules, CNH2N+2 (N = 1-4, 6, 8, 10), were each studied for a set of 29 initial coverages at a heating ramp rate of 0.6 K/s as well as at a set of nine ramp rates in the range 0.3 to 10.0 K/s. The inversion analysis method with its least-squares prefactor optimization discussed in the accompanying article is applied to these data. This method allows for accurate determination of prefactors and coverage-dependent desorption energies. The pre-exponential factor for desorption increases dramatically with chain length from 1013.1 to 1019.1 s-1 over the range N = 1-10. We show that this increase can be physically justified by considering the increase in rotational entropy available to the molecules in the gas-like transition state for desorption. The desorption energy increases with chain length as Ed(N) = 6.5 + 7.1 N, which implies an incremental increase of 7.1±0.2 kJ/mol per CH2.