Effusive molecular beam was used to measure alkane dissociative sticking coefficients, S(Tg,Ts), with independently varied gas and surface temperature, Tg and Ts. Methane, ethane, and propane all showed increasing S as either Tg or Ts were increased—indicative of an activated reaction mechanism.
Activation energies were determined for all 3 gases, yielding a linear Evans−Polanyi plot of Ea for alkane diss. chemisorption vs. the alkane thermal desorption energy, ED, with a slope of −0.94, indicating a late transition state, assuming that ED serves as a good approximation to the van der Waals stabilization of the chemisorbed alkyl radical. A MURT model of diss. chemisorption was used to analyze the experiments.
Explicit accounting of the gas−surface energy transfer for the nonequilibrium experiments became increasingly important as the alkane size was increased. A simple exponential down model of the molecule/phonon collision step size distribution provided a good description of the Sn(Tg,Ts) data.