Trajectory surface hopping study of photocatalyzed H₂ dissociation on a gold cluster

Modelling of nonadiabatic reactions for heterogeneous photocatalysis involving absorbates on metal nanoparticles provides insight for the interpretation of experiments. In this paper, photoinduced H₂ dissociation in a Au₆H₂ model complex has been investigated using Time-Dependent Density Functional Theory (TDDFT), and with a decoherence corrected fewest switches surface hopping (DC-FSSH) approach that includes all degrees of freedom of the Au₆H₂ cluster in the photodynamics. The excited states of this cluster near the equilibrium geometry mostly involve weakly entangled combinations of transitions between occupied orbitals with 60% gold d-orbital character and unoccupied orbitals that are 95% sp, with little variation between different excited states for energies close to what is the Au plasmon energy for larger clusters. Both adiabatic and nonadiabatic process play significant roles in H-H bond dissociation, with adiabatic dissociation always being fast and nonadiabatic dissociation involving slow or fast mechanisms and little variation in the dissociation dynamics when different excited states are considered. In all cases both hydrogen atoms end up chemisorbed on the Au cluster, in contrast to earlier work which suggested that dissociation was dominated by one or both H atoms going to the gas phase. Most H-H bond dissociation reactions take place via the nonadiabatic pathway and leading to both hydrogens chemisorbed on the nearest Au atom, but others lead to H's on different Au atoms. H₂ desorption from the Au₆ cluster competes with hydrogen dissociation, and is always nonadiabatic for this model. Charge transfer between the adsorbed H₂ molecule and the Au₆ cluster is found to make a minor contribution to H-H dissociation. Instead, the calculations show that nonadiabatic transitions between metal localized states are dominant, and that the lowest excited metal-localized state adiabatically evolves into an H₂ dissociative state. These calculations provide new insights to an important model system for plasmon mediated photocatalysis.