Palladium energy levels

The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its Is electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. 

Figure 16. Energy level diagram indicating modification of energy levels of palladium and hydrogen upon formation of palladium hydride. Same remarks as those in Figure 3.

Field-effect transistors (Appendix C) are miniature cousins of the Kelvin probe. The most common is the insulated gate field-effect transistor. The heart of the insulated gate field-effect transistor is the Metal-Insulator-Semiconductor (MIS) capacitor. Let us form this capacitor from palladium (to be modulated by hydrogen), silicon dioxide (insulator), and p-type silicon (semiconductor), and examine the energy levels in this structure (Fig. 6.32). 

In a Thought Experiment, the junction is disassembled (Fig. 6.32) by division through the insulator and the two halves are first treated as electrically isolated objects. In the ensuing equations, we use the common symbol for the work function of a material. There are three electron work functions to be considered that of palladium 0pd, that of an arbitrary metal which does not interact with hydrogen 0m, and that of silicon 0su The insulator is considered to be ideal which means that it does not contain mobile charges. Therefore, it does not have a defined Fermi level. Because the two halves are not connected, their energy levels are in an arbitrary undefined position with respect to each other. On the other hand, metal M and palladium (as well as the M and silicon) form ohmic junctions, meaning that the... 

Fig. 6.32 Energy-level diagrams of disassembled palladium gate insulator silicon junction (please see text for discussion)...

Metal complexes of heterocyclic compounds display reactivities changed greatly from those of the uncomplexed parent systems. All of the -electron system(s) of the parent heterocycle can be tied up in the complex formation, or part can be left to take part in alkenic reactions. The system may be greatly stabilized in the complex, so that reactions, on a heteroatom, for example, can be performed which the parent compound itself would not survive. Orbital energy levels may be split and symmetries changed, allowing hitherto forbidden reactions to occur. In short, a multitude of new reaction modes can be made possible by using complexes dimerization of azirines with a palladium catalyst serves as a typical example (Scheme 81). A variety of other insertion reactions, dimerizations, intramolecular cyclizations, and intermolecular addition reactions of azirines are promoted by transition metals. 

To explain the different behavior for nanoparticles, it is important to realize that for palladium not only absorption in the bulk of the material is important, but also the sorption of hydrogen near the surface should be considered. It is known that palladium forms surface hydrides, with about 1 H per Pd surface atom. However, sorption in subsurface sites is also important, as evidenced by studies on (flat (110) surfaces [45]. Figure 10.9 shows a schematical representation of the energy levels for the hydrogen atom at the surface, in subsurface sites, and absorbed in the bulk. 

Coordination complexes of palladium are catalysts in many of the reactions we will discuss in this chapter. Palladium metal, Pd(0), has a [Kr]4d electron configuration. Thus, the 4d energy level is full and the 5s and 5p orbitals of Pd(0) are vacant. Coordination complexes of Pd(0) that have four ligands have tetrahedral geometry in which the hgands are linked by sp hybrid orbitals. 

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

In the electron transfer theories discussed so far, the metal has been treated as a structureless donor or acceptor of electrons—its electronic structure has not been considered. Mathematically, this view is expressed in the wide band approximation, in which A is considered as independent of the electronic energy e. For the. sp-metals, which near the Fermi level have just a wide, stmctureless band composed of. s- and p-states, this approximation is justified. However, these metals are generally bad catalysts for example, the hydrogen oxidation reaction proceeds very slowly on all. sp-metals, but rapidly on transition metals such as platinum and palladium [Trasatti, 1977]. Therefore, a theory of electrocatalysis must abandon the wide band approximation, and take account of the details of the electronic structure of the metal near the Fermi level [Santos and Schmickler, 2007a, b, c Santos and Schmickler, 2006]. 

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