KINETIC ISOTOPE EFFECT IN DIRECT ETHANE DISSOCIATION ON PT(111)

Supersonic molecular beam techniques were used to study the dynamics o f direct C2H6 and C2D6 dissociation on Pt(111). The initial dissociati on probabilities for both isotopes, SO(C2H6) and SO(C2D6), increased w ith normal translational energy, E(n), over the entire range of E(n) s tudied. A significant normal kinetic isotope effect was observed; the ratio S0(C2H6)/S0(C2D6) decreased from 2.7 to 1.6 as E(n) was increase d from 80 to 118 kJ/mol. A one-dimensional quantum mechanical tunnelin g model based on an Eckart potential barrier quantitatively accounts f or these observations. After correcting for translational energy dissi pation to the lattice, the extracted barrier heights, V0, and widths, L, are 123 kJ/mol, 1.1 angstrom and 130 kJ/mol, 1.1 angstrom for C2H6 and C2D6, respectively. The larger barrier height for the direct disso ciation of C2D6 relative to C2H6 Can be attributed to differences in z ero point energy for C-H(D) stretching motion. Neither S0(C2H6) nor SO (C2D6) exhibited a dependence on nozzle temperature in these experimen ts suggesting that excitation of the normal vibrational motions of met hyl rocking and deformation, C-C stretching and torsion do not promote direct ethane dissociation on Pt(111) under these experimental condit ions.