Methane dissociation on stepped Ni surfaces resolved by impact site, collision energy, vibrational state, and lattice...

We explore the dynamics and kinetics of methane dissociation on the steps of Ni(211) and the terraces of Ni(111), as models for step and terrace sites, respectively, on a real Ni catalyst. A quantum approach is used to compute state resolved sticking probabilities, S₀, and the thermally averaged sticking is computed from both S₀ and more standard transition state methods. While the barriers can be much lower on the step edges, the terrace atoms can make important contributions to the overall reactivity if the step density is not too high and/or at higher temperatures. At 500 K, we find that for reaction on the step edge, sticking is dominated by molecules with either one or two quanta of bending vibration excited, with translational energies of about 0.10-0.35 eV or 0-0.2 eV, respectively. These energies are well below the rigid lattice activation energies, and reaction requires both a significant conversion of vibrational energy into motion along the reaction path and puckering of the lattice atom over which the molecule dissociates. We show that the average amount of puckering, which lowers the barrier to reaction, is about 0.28 Å at 500 K. Reactions are dominated by collisions at impact sites within a few tenths of an Å of the minimum barrier pathway at the step edge. Our computed sticking probabilities for reaction on the step at 500 K are in good agreement with available experimental data.