Paper SS-TuM4
Vibrationally Hot Precursors – Key Reactants in Catalytic Hydrocarbon Activation?

Tuesday, October 21, 2008, 9:00 am, Room 208

Gas-surface reactions are often classified as direct or precursor-mediated based on whether their reactivity scales with surface or gas temperature. This interpretation is rooted in the assumption that as gas temperature and kinetic energy increase, trapping probability and hence reactivity for a precursor-mediated process falls. Application of this rubric has led to the conclusion that methane dissociation generally follows a direct mechanism for reaction on a wide range of transition metals. We suggest that this classification scheme might not properly account for the role of a precursor-mediated mechanism that involves vibrationally hot molecules. In this new mechanism, the vibrational energy content (and hence reactivity) of reagents that trap on the surface would also scale with increasing gas temperature, thus clouding the distinction between direct and precursor-mediated mechanisms. If a vibrationally hot precursor mechanism exists, it could well dominate reactivity of important industrial reactions under processing conditions. Relative to the molecules typically studied in beam-surface scattering studies, the methane molecules in an industrial steam-reforming reactor have low translational energy but high vibrational energy. This is because the many vibrational degrees of freedom in a polyatomic molecule can result in a chemically significant vibrational energy content at the elevated temperatures of a steam-reforming reactor. Prior beam-surface scattering studies have not found compelling evidence for this channel in part because the low kinetic energy molecular beams used to ensure adequate trapping probabilities are generally expanded from a room-temperature nozzle source where essentially all molecules are in the vibrational ground state. Studies that quantify vibrational effects in trapping and vibrational quenching on metals, in contrast, do point to a potentially important role for vibrationally excited precursors in catalytic reactions. The presentation will detail this new mechanism for gas-surface reactivity and describe recent results from beam-surface scattering measurements that use both thermal and laser excitation of methane vibrations to assess the importance of this trapping-mediated channel for vibrationally hot molecules.