MOLECULAR-ORBITAL STUDY OF H-2 AND CH4 ACTIVATION ON SMALL METAL-CLUSTERS - I - PT, PD, PT-2, AND PD-2

The electronic structure of Pd/Pt dimer and the detailed reaction mech anism of H-2 and CH4 activation on these clusters have been studied wi th density functional (B3LYP) and complete active space second-order p erturbation (CASPT2) theories. It was found that B3LYP calculations ga ve reliable results on the electronic structures of the Pd/Pt dimers, in comparison with our CASPT2 calculations and data from previous theo retical investigations. Full geometry optimization has been carried ou t in the current study in contrast to previous work where only limited potential energy scans have been carried out, which led to dramatical ly different reaction mechanisms. In the case of Pt-2+H-2/CH4, H-WC-H activation preferentially takes place at first on one metal atom via s tructures far from planar, then one of the H atoms migrates to the eth er Pt atom with negligible barrier. On both the singlet and the triple t state, H-H activation is barrierless, while C-H activation has a dis tinct barrier on the singlet state for reaction starting from the grou nd triplet state Pt-2. In contrast, Pd-2 is found to activate the H-H bond without barrier on the singlet state, while the triplet states ar e very high in energy. In the CH4 activation, two paths, referred as s ymmetric and asymmetric paths, respectively, have been found. The char acters of the metal dimers and the differences between Pd-2 and Pt-2 s ystems have been analyzed based on MO diagrams. Results from the curre nt study are consistent with the recent experimental observations of C ox et al. on the reactivities of unsupported Pd-n and Pt-n. (C) 1998 A merican Institute of Physics. [S0021-9606(98)00820-4]