Palladium chemisorption complexes

Freund s group at the Fritz Flaber Institute have put much emphasis on linking surface science studies with applied catalysts through replicating the latter with model systems without having to resort to the complexity of the real system. A system they have studied in detail is that of nitric oxide chemisorption at a palladium-alumina model catalyst, where they isolated different... 

Second, we shall present our approach as to how to probe the palladium surfaces in more complex Pd/support catalysts, especially when the so-called metal-support interactions are expected. We shall develop our idea of how to use such chemical probes as a catalytic reaction (alkane catalytic conversion) or chemisorption in order to see important changes in the catalytic behavior. When possible, an adequate reference to available data from more sophisticated physical techniques is made. 

The reactivity of palladium and copper cluster models toward diazirine has been compared using the LCGTO-MCP-LSD method <1996SUS11> such calculations were performed to give an insight into the differential bond scission experimentally observed in the thermal decomposition of diazirine on palladium and copper surfaces. Stronger chemisorption was evident with palladium and furthermore, partial diazirine lowest-unoccupied molecular orbital (LUMO) occupation only occurred for the copper cluster model systems. The calculated N-N bond order was significantly decreased in the copper complexes of excited state diazirines, whereas palladium complexes remained unperturbed. 

In our case, CO chemisorption and XPS techniques have allowed us to prove how such gaseous pretreatment can increase the dispersion of palladium by decreasing the size of its crystallites. Chen and Ruckenstein [19] already observed the formation of a [PdO] surface complex which spreads on alumina... 

The importance of relativistic phenomena both in coordination complexes and in chemisorption has been reviewed. For complexes containing coordinated ethene or other unsaturated hydrocarbons, comparable quantitative information on all the Group 10 metals is extremely hard to come by, but calculations on various ethene and ethyne complexes (Table 4.13) performed by the non-local quasi-relativistic DF method are instructive. For each complex the bond energy is in the sequence Ni > Pt > Pd marked differences in the stabilities and reactivities of complexes of the type M"P2(CH3) (M = Pd, Pt P = PPhs) were also noted. In this context, it is never remarked that nearly all reactions homogeneously catalysed by metal salts or complexes, and metal-mediated reactions, involve elements from the first and second rows, and very rarely a third row element. Ruthenium, rhodium and palladium feature often osmium, iridium and platinum hardly at all. This is because very generally the complexes of the third row elements are too stable to be reactive. 

Pti-x ZXjc supported on carbon or alumina, Kt/Kb is proportional to x, suggesting electron transfer from platinum to zirconium, as predicted by the Engel-Brewer theory, and (2) chemisorption of sulfur on platinum has been shown to decrease electron density of the surface, while carbon has the opposite effect. The ratio Kt/Kb was very large for ruthenium, about 10 for rhodium and about unity for palladium, which may help to explain their different activities in these and other reactions. An extensive kinetic study of the hydrogenation of mixtures of benzene and toluene on NiA zeolite has however revealed a situation of some complexity, and it is not certain that the original simple concept is totally valid. 

Arenes can also bridge tw o metals, as shown in Figure 2.35. In one example shown, the arene bridges and sandwiches two palladium atoms. The arene in the dinuclear nickel complex also bridges two metals. The arene in the osmium complex binds symmetrically to each of the three metal atoms on the triangular face of a cluster. This compound provides a molecular view of the chemisorption of benzene on a metal surface. ... 

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