ALKANE DISSOCIATION DYNAMICS ON PT(110)-(1X2)

Supersonic molecular beam techniques were used to study the reactive adsorption dynamics of methane and ethane on Pt(110)-(1x2). The initial dissociative sticking probability, S0, was measured as a function of surface temperature, incident translational energy, incident total vibrational energy, and incident polar angle at two azimuthal orientations. Under all experimental conditions, both alkanes dissociated via direct collisional activation. Over the range of translational energies studied here neither S0(CH4) nor SO(C2H6) exhibited a dependence on nozzle temperature in these experiments suggesting that excitation of the normal vibrational motions of methyl deformation, methyl rocking, C-C stretching, and torsional vibrational modes do not play a significant role in the direct dissociation of either alkane on Pt(110)-(1x2) under these experimental conditions. The C-H stretching modes were not sufficiently populated to determine the extent of their participation. Methane and ethane displayed almost identical initial reaction probabilities at a fixed incident translational energy and polar angle, similar to our findings for methane and ethane dissociation on Pt(111). However, the reactivity of both species was about a factor of 2 lower on Pt(110)-(1x2) than observed on Pt(111) at a fixed incident translational energy and polar angle. When the crystal was positioned such that the tangential velocity component of the beam was incident along the atomic rows (the [110BAR] direction) the dissociation of both alkanes exhibited normal energy scaling. When the azimuthal orientation was rotated 90-degrees such that the tangential velocity component of the beam was directed perpendicular to the close-packed rows (the [001] direction), the initial dissociation probabilities of both alkanes appeared to scale with E(i) cos0.5 theta(i). This is the first reported observation of non-normal energy scaling for direct alkane activation and is attributed to the corrugation of the surface microstructure.