Paper SS1-TuM12
Mechanistic Insights Into Methane Activation from State-Resolved Beam-Surface Scattering Measurements

Tuesday, October 16, 2007, 11:40 am, Room 608

State-resolved surface scattering experiments provide detailed insight into the mechanistic basis for methane activation. We use infrared laser excitation of molecules in a supersonic molecular beam to prepare reagents with a well-defined internal and translational energy. The state-resolved reaction probabilities that we measure reveal which molecular motions (methane stretching and bending, translation, and surface atom motion) best promote reaction. Earlier work in our lab has shown that on Ni(111), C-H stretching excitation is significantly more effective than translational energy or bending excitation in promoting methane dissociation. Recent measurements demonstrate our ability to exert bond-selective control over methane dissociation. We excite the C-H stretch in CHD₃ and detect exclusively reaction products from the C-H bond cleavage channel. In contrast, thermal excitation of CHD₃ vibrations leads to a preference for C-D bond cleavage. This observation contradicts statistical pictures of gas-surface activation and indicates that energy exchange during direct dissociation is not complete on the timescale of reaction. The presentation will highlight recent experimental results from our lab, discuss how these results teach us about the reaction coordinate for methane activation, and explore how limited energy flow during reaction impacts our understanding of the energetics of hydrocarbon activation.