DFT Studies on Catalytic Properties of Isolated and Carbon Nanotube Supported Pd9 Cluster – I: Adsorption, Fragmentation and Diffusion of Hydrogen

The processes of adsorption, fragmentation and diffusion of hydrogen on a small palladium clusterhave been investigated by means of DFT and DFT/MM approaches. These studies have beenperformed by considering a D3h symmetry Pd9 in the isolated state as well as when supported on aportion of single-walled armchair(6,6) carbon nanotube. The hydrogen fragmentation process easilyoccurs on the bare Pd9 cluster, involving energy barriers of 25–35 kJ mol 1 and the drop in spinmultiplicity on passing from the reactant to the product. The atomic hydrogen diffuses through thecluster atoms with energy barriers, which do not exceed 20 kJ mol 1, with some positions clearlyidentifiable 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 intoaccount.