Using Effusive Molecular Beams and Microcanonical Unimolecular Rate Theory to Characterize CH_4 Dissociation on Pt(III)

摘要

The dissociative sticking coefficient for CH_4 on Pt(111) has been measured as a function of both gas temperature (T_g) and surface temperature (T_s) using effusive molecular beam and angle-integrated ambient gas dosing methods.The experimental results are used to optimize the three parameters of a microcanonical unimolecular rate theory (MURT) model of the reactive system.The MURT calculations allow us to extract transition state properties from the data as well as to compare our data directly to other molecular beam and thermal equilibrium sticking measurements.We find a threshold energy for dissociation of E_0=52.5+-3.5 kJ mol~(-1).Furthermore,the MURT with an optimized parameter set provides for a predictive understanding of the kinetics of this C-H bond activation reaction,that is,it allows us to predict the dissociative sticking coefficient of CH_4 on Pt(111) for any combination of T_s and T_g even if the two are not equal to one another,indeed,the distribution of molecular energy need not even be thermal.Comparison of our results to those from recent thermal equilibrium catalysis studies on CH_4 reforming over Pt nanoclusters (approx 2 nm diam) dispersed on oxide substrates indicates that the reactivity of Pt(111) exceeds that of the Pt nanocatalysts by several orders of magnitude.