Molecular Orbital Study of H₂ and CH₄ Activation on Small Metal Clusters. 2. Pd₃ and Pt₃

The electronic structure of Pd 3 and Pt 3 clusters and the detailed reaction mechanism of activation of H 2 and CH 4 on these clusters have been studied with a density functional method. Full geometry optimization has been carried out and led to the reaction mechanisms that are dramatically different from those of a previous work where only limited potential energy scans were carried out. In the Pd 3 + H 2 system, Pd 3, like Pd 2, activates H 2 without barrier. For the activation of the C−H bond in CH 4 with Pd 3, although the final products are found to be similar in energy compared to the case of Pd 2, the activation barriers on Pd 3 are much higher than those on Pd 2 . This difference has been explained in terms of the large repulsion from the s 1 d 9 configurations of Pd atoms in Pd 3, whereas Pd atoms in Pd 2 adopt mainly the less repulsive d 10 configuration. In the case of Pt 3 + H 2 /CH 4, the reactions basically follow the same pattern as in the Pt 2 systems. Namely H−H and C−H are broken at first on a single Pt atom, and then one H atom migrates to other Pt atom(s). No activation barrier has been found on either the singlet or the triplet state for H−H activation, and a smaller activation barrier height compared to the Pt 2 case has been obtained for the C−H activation. Results from the current series of studies are consistent with the recent experimental observations on the reactivities of unsupported Pd n and Pt n .