Kinetic isotope effect in direct ethane dissociation on Pt(111)

Abstract

Supersonic molecular beam techniques were used to study the dynamics of direct C2H6 and C2D6 dissociation on Pt(111). The initial dissociation probabilities for both isotopes, S0(C2H6 and S0(C2D6), increased with normal translational energy, En, over the entire range of En studied. A significant normal kinetic isotope effect was observed; the ratio S0(C2H6/S0(C2D6) decreased from 2.7 to 1.6 as En was increased from 80 to 118 kJmol. A one-dimensional quantum mechanical tunneling model based on an Eckart potential barrier quantitatively accounts for these observations. After correcting for translational energy dissipation to the lattice, the extracted barrier heights, V0, and widths, L, are 123 kJmol, 1.1 A and 130 kJmol, 1.1 A for C2H6, and C2D6, respectively. The larger barrier height for the direct dissociation of C2D6 relative to C2H6 can be attributed to differences in zero point energy for C-H(D) stretching motion. Neither. S0(C2H6) nor S0(C2D6) exhibited a dependence on nozzle temperature in these experiments suggesting that excitation of the normal vibrational motions of methyl rocking and deformation, C-C stretching and torsion do not promote direct ethane dissociation on Pt(111) under these experimental conditions.