Impurity effects on small Pd clusters: A relativistic density functional study of Pd₄X, X = H, C, O

We carried out relativistic density functional calculations to investigate systematically the effect of main group element impurities H, C, and O on a Pd₄ cluster. We determined a bridging coordination for Pd ₄H as most stable, whereas several other local minima are energetically close. The interaction of C with Pd₄ is strong enough to restructure the cluster, resulting in two Pd₂ units bridged by 4-fold coordinated C, but other isomers are again almost degenerate. Nearly degenerate isomers of Pd₄O exhibit 2- and 3-fold coordination of O. In the most stable structures, the binding energies of the impurities, 295 kJ/mol for Pd₄H, 655 kJ/mol for Pd₄C, and 367 kJ/mol for Pd₄O, are large enough to allow bond breaking of common small molecules when they interact with an ensemble of Pd₄ clusters. Interestingly, the noteworthy relativistic effect on the properties of Pd ₄ also affects the interaction with impurity atoms. Comparison with other metals reveals similarities with Ni₄X and differences from Ir₄H, Ir₄C, and Pt₄H.