Molecular-orbital models for the catalytic activity and selectivity of coordinatively unsaturated platinum surfaces and complexes

Electronic structures have been calculated for 5-, 6-, and 10-atom Pt clusters, as well as for a Pt(PH₃)₄ coordination complex, using the self-consistent-field X-alpha scattered-wave (SCF-Xα-SW) molecular-orbital technique. The 10-atom cluster models the local geometry of a flat, unreconstructed Pt(100) surface, while the 5- and 6-atom clusters show features of a stepped Pt surface. Pt(PH₃)₄ resembles the chemically similar homogeneous catalyst Pt(PPh₃)₄. Common to all these coordinatively unsaturated complexes are orbitals lying near or coinciding with the highest occupied molecular orbital (“Fermi level”) which show pronounced d lobes pointing directly into the vacuum. Under the hypothesis that these molecular orbitals are mainly responsible for the chemical activities of the above species, one can account for the relative similarities and differences in catalytic activity and selectivity displayed by unreconstructed Pt(100) surfaces, stepped Pt surfaces or particles, and isolated Pt(PPh₃)₄ coordination complexes. The relevance of these findings to catalyst-support interactions is also discussed. Finally, relativistic corrections to the electronic structures are calculated and their implications on catalytic properties discussed.