Methane dissociation on Ni(111): the effects of lattice motion and relaxation on reactivity

The effects of lattice motion and relaxation on the dissociative adsorption of methane on a Ni(111) surface are explored. Electronic structure methods based on the density functional theory are used to compute the potential energy surface for this reaction. It is found that, in the transition state and product regions, there are forces causing the Ni atom over which the molecule dissociates to move out of the surface. In order to examine the extent to which the lattice might pucker during this reaction, high dimensional fully quantum scattering calculations are carried out. It is found that a significant amount of lattice puckering can occur, even at large collision energies, lowering the barrier to reaction and increasing the dissociative sticking probability. This is shown to be in contrast to the predictions of the surface oscillator model. While we observe similar puckering forces for this reaction on Pt(111), our calculations suggest that the puckering on this surface will be considerably less due to the larger metal atom mass. The "laser off" reactivities of CD(3)H on Ni(111) are computed, and it is demonstrated that there can be significant contributions to the reactivity from vibrationally excited molecules, particularly at lower collision energies, or when a large nozzle temperature is required to attain the necessary collision energy for reaction. Comparisons are made with recent experiments with regard to the variation of reactivity with collision energy, vibrational state, and surface temperature.