Aktivace vazby C-H lehkých alkanů na Pt(111): Disociativní koeficient ulpění, Evans-Polanyiova rovnice a přenos energie mezi plynem a povrchem
Abstrakt
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.