Platinum catalysts electronic structure

Determination of the Atomic and Electronic Structure of Platinum Catalysts by X-ray Absorption Spectroscopy... 

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]. 

Two stable compounds of formula C6H8 react with bromine and with KMn04. On hydrogenation with a platinum catalyst at 25°, both absorb two moles of hydrogen and form cyclohexane. Write possible structures for these substances and explain how electronic spectra may be used to tell which compound is which. 

Such electronic transfer induced by sulfur adsorption was also pointed out by using cinnamic acid as a probe molecule (48). The UV-visible reflexion spectra of adsorbed cinnamic acid on nonpoisoned and partly poisoned platinum catalysts shows that adsorption on pure platinum induces a shift of the peaks toward the higher wavelengths and an appearance of fine structure. Sulfurization of platinum induces a further enhancement of higher wavelength peaks. Binding energy of cinnamic acid is thus increased by sulfur adsorption on Pt catalysts. 

One way to clarify the role of the adsorbate is the in situ investigation of platinum catalysts with electron spectroscopy, which is capable of quantitatively determining the abundance and chemical structure of the carbon species present. This has been done for a platinum black [66] material used after various activation procedures in n-hexane conversion reactions. It was... 

The differences in selectivity between catalysts cannot be explained only in terms of the strength of reactant adsorption. A tentative explanation lies in the preference of platinum for concerted addition of protons to adsorbed alkenes with simultaneous electron transfer (25). The electronic structure of the surface intermediate of the concerted step appears to lead to halide cleavage. Palladium, on the other hand, can participate in insertion reactions (305) and promotes surface reaction between hydrogen atoms and adsorbed alkenes 4Sa. It is possible that palladium adsorbs vinyl halides on two different sites or at two different states, dependent on potential, one of which... 

The alloy catalysts used in these early studies were low surface area materials, commonly metal powders or films. The surface areas, for example, were two orders of magnitude lower than that of platinum in a commercial reforming catalyst. Hence these alloys were not of interest as practical catalysts. The systems emphasized in these studies were combinations of metallic elements that formed continuous series of solid solutions, such as nickel-copper and palladium-gold. The use of such systems presumably made it possible to vary the electronic structure of a metal crystal in a known and convenient manner, and thereby to determine its influence on catalytic activity. Bimetallic combinations of elements exhibiting limited miscibility in the bulk were not of interest. Aspects of bimetallic catalysts other than questions related to the influence of bulk electronic structure received little attention in these studies. 

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