We use density functional theory to examine 24 transition states for methane dissociation on five different metal surfaces. In our calculations, the nonlocal exchange-correlation effects are treated within the generalized gradient approximation using the Perdew-Burke-Ernzerhof functional. In all cases, the minimum energy path for dissociation is over a top site. The barriers are large, 0.66-1.12 eV, and relatively insensitive to the rotational orientation of the (nonreacting) methyl group and the azimuthal orientation of the reactive C-H bond. There is a strong preference on the Pt surfaces for the methyl fragment to bond on the top site, while on the Ni surfaces there is a preference for the hollow or bridge sites. Thus, during the dissociation on Pt, only the low mass H atom needs to significantly move or tunnel, while on Ni, both the dissociating H and the methyl fragment move away from the top site. For all 24 configurations there is a strong force at the transition state to pucker the metal atom over which the reaction occurs. The resulting magnitude of the variation in the barrier height with the motion of this atom varies a bit from surface-to-surface, but is of the order of 1 eV/A. We derive a model for the effective reaction barrier height that includes the effects of lattice motion and substrate temperature and compare with recent experiments and other theoretical studies.

The barrier to the dissociative adsorption of methane on metal surfaces is generally large, and its height can vary with the motion of the lattice atoms. One fully quantum and three different mixed quantum-classical approaches are used to examine this reaction on Ni(111) and Pt(111) surfaces, using potential energy surfaces derived from density functional theory. The three approximate methods are benchmarked against the exact quantum studies, and two of them are shown to work reasonably well. The mixed models, which treat the lattice motion classically, are used to examine the lattice response during the reaction. It is found that the thermal motion of the lattice atoms strongly modifies the reactivity, but that their motion is not significantly perturbed. Based on these results, new models for methane reactions are proposed based on a sudden treatment of the lattice motion and shown to agree well with the exact results. In these new models, the reaction probability at different surface temperatures is computed from static surface reaction probabilities, allowing for a quantum calculation of the reaction probability without having to explicitly treat the motion of the heavy lattice atoms.

The nature of the lattice motion during the dissociation of methane on Ni(111) is analyzed in great detail, and various models for including lattice effects are explored. It is shown that the thermal vibrations of the lattice strongly modify the reactivity, but that the lattice motion is relatively unperturbed by the incident molecule during the collision, in contrast with several earlier predictions. Based on these studies we propose a new model to describe the effects of lattice motion, which agrees well with exact quantum calculations.

The dissociative chemisorption of methane on metal surfaces is of great practical and fundamental interest. Not only is it the rate-limiting step in the steam re-forming of natural gas, but also the reaction exhibits interesting mode-specific behavior and a strong dependence on the temperature of the metal. Electronic structure methods are used to explore this reaction on various Ni and Pt surfaces, with a focus on how the transition state is modified by motion of the metal lattice atoms. These results are used to construct models that explain the strong variation in reactivity with substrate temperature, shown to result primarily from changes in the dissociation barrier height with lattice motion. The dynamics of the dissociative chemisorption of CH4 on Ni and Pt is explored, using a fully quantum approach based on the reaction path Hamiltonian that includes all 15 molecular degrees of freedom and the effects of lattice motion. Agreement with experiment is good, and vibrational excitation of the molecule is shown to significantly enhance reactivity. The efficacy for this is examined in terms of the vibrationally nonadiabatic couplings, mode softening, mode symmetry, and energy localization in the reactive bond.

Water dissociation is the rate-determining step (RDS) in the industrially important water gas shift (WGS) reaction. Low temperature Cu catalysts are limited by a higher barrier to dissociation whereas Ni surfaces with lower barriers for this reaction are deactivated by carbon deposition due to CO dissociation. Density functional theory (DFT) calculations are performed on a series of overlayer and subsurface bimetallics starting with Ni(111) and Cu(111) to understand the synergistic catalytic activity of Cu/Ni bimetallics toward H 2 O dissociation which is the RDS. Surface parameters like surface energy, work function and density of states were calculated and were correlated with the change in reactivity. Transition state (TS) calculations showed that addition of Ni to Cu(111) surfaces decreased dissociation barriers while the scenario is reversed when Cu atoms replace Ni in Ni(111) surface with no linear relation with any calculated surface properties in both cases. Linear relations were found to correlate well the reaction energies with the activation energy barriers. Effects of surface temperature were included by determining the change in the barrier heights and barrier locations with lattice atom motion calculated from TS calculations. Dissociation probabilities calculated at different surface temperatures using semiclassical methods showed that increase in surface temperature increases dissociation probabilities where the extent of increase is strongly dependent on the change in barrier heights. Overall, Ni addition to Cu(111) surface proved beneficial while the Cu addition to Ni(111) surface proved detrimental to H 2 O dissociation.