Angle-Resolved Thermal Dissociative Sticking of Light Alkanes on Pt(111): Transitioning from Dynamical to Statistical Behavior

Gas-surface reactivity involving small molecules can exhibit significant dynamical deviations away from statistical behavior that complicate quantitative modeling of catalytic processes. This study examines dissociative chemisorption of light alkanes to determine if a transition toward statistical behavior can be identified at some threshold level of molecular complexity. Angle-resolved thermal dissociative sticking coefficients (ar-SCs) were measured for methane, ethane, and propane on Pt(111) using effusive molecular beams at a temperature of 700 K. The ar-DSCs were peaked around the direction of the surface normal, and the angular variation flattened with increasing alkane size from roughly cos(12)theta to only a 20% variance with angle. Precursor-mediated microcanonical trapping models of the gas-surface reactivity were used to simulate the ar-DSCs while maintaining consistency with other nonequilibrium supersonic and effusive molecular beam experiments. This work indicates that dissociative chemisorption of ethane is dynamically biased similar to methane, wherein the mean vibrational efficacy for promoting reaction relative to normal translational energy is 0.40 and molecular translations parallel to the surface and rotations are spectator degrees of freedom. In contrast, propane's reactivity shows no discernible evidence for dynamical deviations away from statistical behavior.