Paper SS+HC+NS-WeA11
Surface Temperature Effects in CH₄ Dissociation on Flat and Stepped Nickel Single Crystals

Wednesday, November 1, 2017, 5:40 pm, Room 25

A modified King and Wells molecular beam reflectivity method was used to obtain thermal and quantum state resolved initial sticking probabilities (S₀) for CH₄ on nickel single crystals at elevated surface temperatures (500 to 1000K). We recorded the reactivity of a supersonic molecular beam of methane with and without laser excitation to v=1 of the v₃ antisymmetric C-H stretching vibration. Square wave modulation of the laser source during CH₄ deposition provided simultaneous real-time measurement of S₀^LaserOff and S₀^v3, resulting in a dramatic reduction in data acquisition time, a significant reduction in experimental error, and the opportunity to measure S as a function of accumulating surface coverage. On a Ni(111) surface, both laser-off and vibrational state-resolved measurements of S₀ were essentially independent of surface temperature, within error, from 1000 to 800K and then decreased linearly as T_surf dropped from 800 to 500K. These findings are consistent with the predictions of Reaction Path Hamiltonian calculations by Jackson and Guo over a wide range of incident kinetic energies (96 to 167 kJ/mol). The calculations suggest that at low incident energies, reactivity occurs predominantly at on-top sites, but as energy increases, reactions begin to occur at bridge sites as well. We will also present more recent measurements on the low step density Ni(997) surface that provide insight into the role of step-edge nickel atoms at these industrially relevant surface temperatures.