Spectroscopic properties of 41 electronic states of Pd2

The structure and properties of small Pd clusters

The zero-temperature minimal energy structure of small free-standing Pd clusters (14≤N≤21, where N is the number of atoms in the cluster), their characteristics and their magnetic configurations are investigated. Results obtained using five different phenomenological many-body potentials (implemented in combination with a genetic algorithm search) are refined by means of various density functional theory (DFT) techniques. The agreement and differences between the results obtained with our procedure, using these five potentials, are displayed in detail. While phenomenological potentials yield values that approach the minimal energies of larger clusters, as compared with DFT results, they fail to predict the right symmetry group for some of the clusters with N>14. We find that the minimal energy configurations are not necessarily associated with high symmetry of the atomic arrangement. Actually, several cases of previously overlooked low symmetry structures turn out to have lower energies than more symmetric ones.

PdnCO (n = 1,2): Accurate Ab Initio Bond Energies, Geometries, and Dipole Moments and the Applicability of Density Functional Theory for Fuel Cell Modeling

Electrode poisoning by CO is a major concern in fuel cells. As interest in applying computational methods to electrochemistry is increasing, it is important to understand the levels of theory required for reliable treatments of metal-CO interactions. In this paper we justify the use of relativistic effective core potentials for the treatment of PdCO and hence, by inference, for metal-CO interactions where the predominant bonding mechanism is charge transfer. We also sort out key issues involving basis sets and we recommend that bond energies of 17.2, 43.3, and 69.4 kcal/mol be used as the benchmark bond energy for dissociation of Pd2 into Pd atoms, PdCO into Pd and CO, and Pd2CO into Pd2 and CO, respectively. We calculated the dipole moments of PdCO and Pd2CO, and we recommend benchmark values of 2.49 and 2.81 D, respectively. Furthermore, we tested 27 density functionals for this system and found that only hybrid density functionals can qualitatively and quantitatively predict the nature of the sigma-donation/pi-back-donation mechanism that is associated with the Pd-CO and Pd2-CO bonds. The most accurate density functionals for the systems tested in this paper are O3LYP, OLYP, PW6B95, and PBEh.

Nucleation and growth on defect sites: experiment-theory comparison for Pd/MgO(001)

It is well established that nucleation of metal clusters on oxide and halide surfaces is typically dominated by defect sites. Rate equation models of defect nucleation have been developed and applied to these systems. By comparing the models with nucleation density experiments, energies for defect trapping, adsorption, surface diffusion and pair binding have been deduced in favourable cases, notably for Pd deposited on Ar-cleaved MgO(001). However, the defects responsible remain largely unknown. More recently, several types of ab initio calculation have been presented of these energies for Pd and related metals on MgO(001) containing several types of surface defect; these calculated values are surveyed, and some are widely divergent. New rate equation nucleation density predictions are presented using the calculated values. Some calculations, for some defect types, are much closer to experiment than others; the singly charged F(s)(+) centre and the neutral divacancy emerge as candidate defects. In these two cases, the Pd/MgO(001) nucleation density predictions agree well with experiment, and the corresponding surface defects deserve to be taken seriously. Energy and entropy values are discussed in the light of differences in calculated charge redistribution between the metal atoms, clusters and (charged) surface defects, and (assumed or calculated) cluster geometries.