Variable (\textit{T}$_{g}$, \textit{T}$_{s}$) Measurements of Alkane Dissociative Sticking Coefficients

Dissociative sticking coefficients $S(T_{g}, \quad T_{s})$ for CH$_{4}$ and C$_{2}$H$_{6}$ on Pt(111) have been measured as a function of gas temperature ($T_{g})$ and surface temperature ($T_{s})$ using an effusive molecular beam. Microcanonical unimolecular rate theory (MURT) was employed to extract transition state characteristics [e.g., $E_{0}$(CH$_{4})$ = 52.5$\pm $3.5 kJ/mol$^{-1}$ and $E_{0}$(C$_{2}$H$_{6})$ = 26.5$\pm $3 kJ/mol$^{-1}$]. MURT allows our $S(T_{g}, \quad T_{s})$ values to be directly compared to other supersonic molecular beam and thermal equilibrium sticking measurements. The $S(T_{g}, \quad T_{s})$ depend strongly on $T_{s}$, however, only for CH$_{4}$ is a strong $T_{g}$ dependence observed. The fairly weak $T_{g}$ dependence for C$_{2}$H$_{6}$ suggests that vibrational mode specific behavior and/or molecular rotations play stronger roles in the dissociative chemisorption of C$_{2}$H$_{6}$ than they do for CH$_{4}$. Interestingly, thermal $S(T_{g}=T_{s})$ predictions based on MURT modeling of our CH$_{4}$/Pt(111) data are three orders of magnitude \textit{higher} than recent thermal equilibrium measurements on supported Pt nanocrystallite catalysts [J. M. Wei, E. Iglesia, J. Phys. Chem. B \textbf{108}, 4094 (2004)].