Surface Temperature Dependence of Methane Activation on Ni(111)

Vibrational state resolved measurements of methane's dissociation on Ni(111) show a strong surface temperature dependence near the translational energy threshold for reaction. The reactivity of molecules excited to nu = 1 of the nu(3) C-H stretching vibration and incident on the surface with a translational energy of 40 kJ mol(-1) increased 8-fold as the surface temperature increased from 90 to 475 K. This enhancement is much larger than that reported for earlier studies at higher incident energies. These results support recent calculations that predict an important role for lattice deformation in transition state access. At higher surface temperatures, surface phonon excitation allows substrate atoms to sample lattice geometries with more favorable transition state energetics. We have also measured the coverage-dependent reactivity of these molecules at both surface temperatures and report a simple model that quantitatively predicts the observed coverage-dependent reactivity.