A theoretical study of the catalytic properties of Pt/Fe nanoclusters

Extended Huckel, density functional (DF) and Hartree-Fock (HF) calculations are performed to investigate the catalytic properties of pure Pt and mixed Pt/Fe nanoclusters. The metallic aggregates are modelled in terms of a single atom, a cuboctahedral 13-atom or a truncated octahedral 201-atom structure, made of pure platinum or alloyed with iron. Their interaction is considered with typical unsaturated organic functional groups, such as a phenyl ring, a carbon-carbon double bond and a carbon-oxygen double bond, taken as representative of the three reaction centres of cinnamaldehyde. At the DF level, it is found that iron has a strong tendency to mix with platinum, with the latter tending to segregate to the surface. Moreover, the strength of the interaction with the organic functional groups follows the order: C=C much greater than C=O > Phi. A single Pt atom is already a good model for small aggregates. The effect of alloying with iron is not to change this order, but (a) to substantially depress the activity of these systems; (b) to diminish the preference toward the carbon-carbon bond, possibly implying a more subtle catalytic behaviour. The results of the extended Huckel or HF approaches in some cases compare reasonably well with the DF ones, but in others they differ, in which cases we believe the DF ones to be more reliable. (C) 2002 Elsevier Science B.V. All rights reserved.