Palladium clusters, size-selective

Figure 9.8 Relative reactivities (a) and selectivities (b) of palladium cluster size in the formation of C6H6, C4H8 and C4H6 in the polymerisation of acetylene. (Reproduced from Ref. 29).

A size-selective synthesis of nanostructured transition metal clusters (Pd, Ni) has been reported166, as has the preparation of colloidal palladium in organic solvents167, the latter of which is an active and stable catalyst for selective hydrogenation. The use of microwaves in the preparation of palladium catalysts on alumina and silica resulted in hydrogenation catalysts with improved crystallite size and activity168. 

Scheme 1 describes the synthesis of some palladium full-shell clusters with different ligands. The ligand molecules influence not only the cluster size but, as will be shown, especially the catalytic selectivity [6]. 

The TPR spectra of the different products of the polymerization of acetylene on small, monodispersed palladium clusters supported on defect-rich MgO thin films are shown in Figure 6. Striking atom-by-atom size-dependent reactivities and selectivities are observed. Only the three reaction products C5H5, C4H8, and C4H6 are detected. Remarkably, no C3H , C5H , and are formed, indicating the absence... 

Fig. 23. Catalytic formation of benzene molecules on different palladiiun cluster sizes by the temperature-programmed-reaction (TPR) experiment. The benzene was produced through the cyclo trimerization of acetylene catalyzed by size-selected palladium clusters, Pdni supported on a MgO film. (Adapted from Ref. 35.)...

Metal clusters may also find applications in catalysis. Here the smaller the particle size, the higher the number of their surface atoms [144,145]. This makes the particles interesting candidates for catalytic applications. Crooks used palladium clusters for hydrogenation and showed an elevated reactivity with the particles, depending on the particle size [146]. Nanoparticles are also used as the sensitive layer in selective gas detectors [147]. 

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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