Density functional calculations within the generalized gradient approximation have been used to investigate the lowest energy electronic and geometric structures of neutral, cationic, and anionic Pd(n) (n=1-7) clusters in the gas phase. In this study, we have examined three different spin multiplicities (M=1, 3, and 5) for different possible structural isomers of each neutral cluster. The calculated lowest energy structures of the neutral clusters are found to have multiplicities, M=1 for Pd(1), Pd(3), Pd(5), Pd(6), and Pd(7), while M=3 for Pd(2) and Pd(4). We have also determined the lowest energy states of cationic and anionic Pd(n) (n=1-7) clusters, formed from the most stable neutral clusters, in three spin multiplicities (M=2, 4, and 6). Bond length, coordination number, binding energy, fragmentation energy, bond dissociation energy, ionization potential, electron affinity, chemical hardness, and electric dipole moment of the optimized clusters are compared with experimental and other theoretical results available in the literature. Based on these criteria, we predict the four-atom palladium cluster to be a magic-number cluster.

Density functional theory (DFT) studies have revealed the energetically favorable reaction paths for oxidation of CO on Pd(4) cluster. Adsorption of various species such as O(2), 2O, O, CO, CO(2), and coadsorbate combinations, including O(2)+CO, 2O+CO, O+CO, and O+CO(2) on neutral, cationic, and anionic Pd(4) clusters were investigated. The results indicate that Pd(4)(+) and Pd(4) are more effective for catalyzing CO in comparison with Pd(4)(-). It is further observed that dissociated oxygen is a superior oxidant for CO oxidation on Pd(4)(q) (q = 0, 1, -1) than molecular and atomic oxygen.

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The binding of CO and NO to Pd atom supported on H-ZSM-5 zeolite have been studied with density functional theory using both simple cluster and embedded cluster models. The adsorption energies of CO and NO are found to be 2-3 kcal/mol stronger in embedded cluster approach than those that are in cluster model. We discuss trends in bond lengths, adsorption energies and vibrational frequencies of the adsorbed species with relation to the magnitude of charges using NPA scheme of NBO method.

The present study explores favorable chemical state of zeolite-supported Pd4 clusters using the ONIOM (our-own-N-layered integrated molecular orbital + molecular mechanics) approach, as implemented in the Gaussian03 code. Lowest-energy structures of Pd4 in the gas phase as well as on a zeolite support are found to be in the triplet state. The calculations reveal that reverse hydrogen spillover from bridging OH groups of zeolite support onto Pd4 results in a hydrogenated Pd4H/Zeo((m − 1)H), m = 1−3 species, which are energetically preferable over the bare zeolite-supported form of Pd4/Zeo(mH). The process of single hydrogen transfer from the zeolite support to the Pd4 cluster is found to be exothermic and more favorable than two- and three-proton transfer processes.