DFT studies on catalytic properties of isolated and carbon nanotube supported Pd9 cluster–I: adsorption, fragmentation and diffusion of hydrogen

Abstract

The processes of adsorption, fragmentation and diffusion of hydrogen on a small palladium cluster have been investigated by means of DFT and DFT/MM approaches. These studies have been performed by considering a D_3h symmetry Pd₉ in the isolated state as well as when supported on a portion of single-walled armchair(6,6) carbon nanotube. The hydrogen fragmentation process easily occurs on the bare Pd₉ cluster, involving energy barriers of 25–35 kJ mol⁻¹ and the drop in spin multiplicity on passing from the reactant to the product. The atomic hydrogen diffuses through the cluster atoms with energy barriers, which do not exceed 20 kJ mol⁻¹, with some positions clearly identifiable as the most stable. In the case of the palladium supported system, which is a better model to simulate experimental conditions, calculations predict that the hydrogen fragmentation barrier is reduced by ca. 15 kJ mol⁻¹, with respect to that of the unsupported system, while the energetics of the diffusive process is not significantly affected by the support, if the reduction of the number of sites available in the same palladium cluster, as well as their geometry, are taken into account.