Tunable catalytic reactivity of small palladium clusters supported on graphene: a first-principles study

The catalytic reactivity is controlled by the binding strength between the catalyst surface and reaction intermediates. So our capability to tune the binding strength to an ideal value, i.e. the value corresponding to the top of the volcano relation, is crucially important. We have performed systematic density-functional theory calculations to investigate hydrogen adsorption on Pd-n (n = 1-5) clusters which are supported on graphene. The calculated results show that the hydrogen adsorption energies depend not only on the cluster size, but also on its shape, and that with n >= 3 hydrogen adsorption on the Pdn clusters is stronger than on Pd(111). The reactivity of the supported clusters is mainly determined by a combined effect of atomic configuration, orbital hybridization of Pd-d and H-s states and graphene support. We further characterize the hydrogen adsorption energy by the chemical and geometric contributions. The former is almost the only contribution for the planar clusters. However, when the shape of the cluster transforms from the planar configuration to the three-dimensional one, the role of the latter cannot be neglected. Our calculations also show that it is possible to survive/eliminate the 2D or 3D Pd clusters by controlling the gaseous environment during the sintering or annealing. The present work provides a theoretical base for exploring the applications of Pd-based clusters as a highly reactive catalyst in hydrogen-involved reactions.