The activation of alkanes on Ni(100)

Abstract

The dissociative chemisorption of saturated C 1C 4 hydrocarbons was examined on Ni(100) using supersonic molecular beam techniques. For all the alkanes investigated no measurable adsorption was observed for incident translational energies less than 10 kJ mol at a surface temperature of 500 K. The initial sticking probability, s 0, for each alkane increased as the translational kinetic energy of the beam increased. However, the translational energy required to achieve comparable dissociation probabilities for each alkane increased as the carbon chain length increased, suggesting an apparent increase in the activation energy for adsorption. The scattering distribution for each alkane was measured at incident kinetic energies below the dissociation threshold at a surface temperature of 500 K. For scattered methane the angular distribution was almost specular in nature, indicating direct inelastic scattering. For butane the angular distribution was cosine, indicating the incident molecules exchanged energy with the surface before returning to the vacuum. Ethane and propane exhibited patterns intermediate to the methane and butane cases. The apparent increase in activation energy for alkanes with increasing carbon chain length may be explained by transfer of translational energy in the larger molecules out of the reaction coordinate into dissipative channels involving phonon excitation in the surface and/or to internal degrees of freedom of the molecule. The initial sticking probability for propane at 120 kJ mol incident beam energy was independent of surface temperature from 300 to 700 K, suggesting a direct mechanism for dissociation. For all the alkanes investigated the effect of carbon build-up on the adsorption could be described by a Langmuirian model second-order in carbon coverage. Large ensembles were not required for alkane activation. Each carbon adatom in the alkane blocked a small (3 or less) number of “sites” on the surface, and the decrease in the number of sites blocked per carbon adatom suggested that the carbon chain remained at least partially intact on adsorption.