C−H Bond Activation of Light Alkanes on Pt(111): Dissociative Sticking Coefficients, Evans−Polanyi Relation, and Gas−Surface Energy Transfer

Abstract

Effusive molecular beam experiments were used to measure alkane dissociative sticking coefficients, S(Tg,Ts), for which the impinging gas temperature, Tg, and surface temperature, Ts, could be independently varied. The 400−1000 K temperature range examined should be relevant to heterogeneously catalyzed industrial processes such as the steam reforming of alkanes. Methane, ethane, and propane all showed increasing dissociative sticking as either Tg or Ts were increased—indicative of an activated reaction mechanism. Effusive beam experiments with gas impinging along the surface normal and Tg = Ts = T determined Sn(T), a close approximation and formal upper bound to the thermal dissociative sticking coefficient, S(T), appropriate to reaction with a thermal ambient gas. Activation energies determined from Sn(T) for methane, ethane, and propane are Ea = 58, 43, and 34 kJ mol−1, respectively. An Evans−Polanyi plot of Ea for alkane dissociative chemisorption versus the alkane thermal desorption energy, ED, is linear with a slope of −0.94. Assuming that the alkane ED serves as a good approximation to the van der Waals stabilization of the chemisorbed alkyl radical product of dissociative chemisorption, the slope of the Evans−Polanyi plot indicates a late transition state for alkane dissociative chemisorption on Pt(111). A microcanonical unimolecular rate theory (MURT) model of dissociative chemisorption was used to analyze the effusive molecular beam 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 with a mean energy down parameter of α = 350 cm−1 for ethane, and α = 1400 cm−1 for propane, sufficed to provide a good description of the Sn(Tg,Ts) data. The methane Sn(Tg,Ts) values reported here for effusive molecular beams are roughly 2.5 times smaller than expectations based on MURT analysis of earlier, higher energy, supersonic molecular beam experiments.