Palladium clusters electronic properties

Baetzold used extended Hiickel and complete neglect of differential overlap (CNDO) procedures for computing electronic properties of Pd clusters (102, 103). It appeared that Pd aggregates up to 10 atoms have electronic properties that are different than those of bulk palladium. d-Holes are present in small-size clusters such as Pd2 (atomic configuration 4dw) because the diffuse s atomic orbitals overlap strongly and form a low-energy symmetric orbital. In consequence, electrons occupy this molecular orbital, leaving a vacant d orbital. For a catalytic chemist the most important aspect of these theoretical studies is that the electron affinity calculated for a 10-atom Pd cluster is 8.1 eV. This value, compared to the experimental work function of bulk Pd (4.5 eV), means that small Pd clusters would be better than bulk metal as electron acceptors. 

Figueras et al. (105) found some direct evidence for electron-deficient palladium clusters on various cation-exchanged forms of zeolite Y from CO adsorption experiments. In particular, a correlation was observed between the turnover number for benzene hydrogenation and the CO stretching frequency. The shift toward higher frequency with increasing support acidity was considered as evidence for increased electron acceptor properties of the support. Further studies will, however, be required to provide a more detailed understanding of this phenomenon. 

In the past, d-band theory has been successfully used to explain the reactivity of a wide range of catalysts. It was taken for granted that the participation of the structureless and wide sp band was not relevant. However, we want to emphasize that this approach is an oversimplification that could lead to wrong results. We have included two different pictures of the same system in order to show the behavior of the electron bands during the adsorption process. The electronic properties of a hydrogen atom approaching to the surface of a three palladium atoms cluster on Au(lll) are shown in Figure 1.16. 

In this section we shall try to review available data on catalytic properties of electron-deficient palladium on supported catalysts. The term electron deficient we shall consider to mean very small clusters of Pd on various supports, and/or palladium ions stabilized by virtue of their presence in an appropriate chemical environment. 

Palladium Ensembles in Zeolites. Small metal particles or clusters have attracted great interest during the last decade. The optical, electronic and catalytic characteristics of clusters are expected to change from bulk properties to molecular properties within a certain size-range. " This change is represented by the transition of the electronic band structure of a crystal to the molecular orbital levels of species few atoms in size. Since the cluster size determines the relative population of coordination sites and possibly its molecular symmetry, it is thought to be responsible for modified selectivities in a number of catalytic reactions. Controlled synthesis of stable clusters with defined size is of particular interest, because this would potentially allow to fine-tune the properties of electronic materials and catalyst systems. 

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