Electronic structure of catalysts

VII. Cathodoluminescence Imaging for Elucidation of Electronic Structures of Catalysts... 

Further developments in the fundamental approach to the electronic structure of catalysts were made possible by the development of the quantum mechanical treatments of solids which followed the work of Sommerfeld, Bloch and others. Similarly, Pauling s resonating valence bond treatment has lent further impetus to consideration of metallic catalysts. 

The electronic structure of catalysts has been shown to be important in adsorption, and since adsorption is a necessary step in heterogeneous catalytic reactions, it would be expected that changes in the electronic structure would influence the rate of reaction. 

In considering the effect of the electronic structure of catalysts on activity, Dowden (33) suggested that carbides, and similarly nitrides and carbonitrides, should be less active for synthesis than the corresponding metal since the interstitial atoms may contribute electrons to the unfilled d-shells of the metal, which are believed to be essential for the catalytic activity of transition metals in hydrogenation reactions. This hypothesis is supported by the low activity of cobalt carbide compared with that of reduced cobalt (28,29). For iron catalysts the hypothesis... 

In view of their potential importance as a means of monitoring changes in the electronic structure of catalysts, well-founded procedures for quantitative analysis of X-ray absorption-edge data are clearly highly desirable. In the past the absence of such techniques has led to clearly contradictory results for... 

It is evident from the above discussion that catalyst characterization is an activity important for scientific understanding, design, and troubleshooting of catalyzed processes. There is no universal recipe as to which characterization methods are more expedient than others. In the opinion of the writer, we will see continued good use of diffraction methods and electron microscopy, surface analysis, IR spectroscopy, and chemisorption methods, increased use of combined EM and ESCA analyses for determining the dopant dispersion, increased use of MAS-NMR and Raman spectroscopies for understanding of solid state chemistry of catalysts, and perhaps an increased use of methods that probe into the electronic structure of catalysts, including theory. 

Nprskov and coworkers have successfully developed a d-band theory in relating the adsorption properties of rate-limiting intermediates in catalytic process to electronic structure of catalyst surfaces [44, 45]. According to this theory, the... 

The change in the electronic structure of a bulk metal catalyst, in consequence of its transformation into the hydride, influences respectively the metal surface atoms (ions) or, strictly speaking, their d orbitals. Recent achievements and the present knowledge of the subject only permit us so far to formulate such general conclusions. 

Bis(imino)pyridine iron complex 5 as a highly efficient catalyst for a hydrogenation reaction was synthesized by Chirik and coworkers in 2004 [27]. Complex 5 looks like a Fe(0) complex, but detailed investigations into the electronic structure of 5 by metrical data, Mossbauer parameters, infrared and NMR spectroscopy, and DFT calculations established the Fe(ll) complex described as 5 in Fig. 2 to be the higher populated species [28]. 

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

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]. 

In spite of the importance of having an accurate description of the real electrochemical environment for obtaining absolute values, it seems that for these systems many trends and relative features can be obtained within a somewhat simpler framework. To make use of the wide range of theoretical tools and models developed within the fields of surface science and heterogeneous catalysis, we will concentrate on the effect of the surface and the electronic structure of the catalyst material. Importantly, we will extend the analysis by introducing a simple technique to account for the electrode potential. Hence, the aim of this chapter is to link the successful theoretical surface science framework with the complicated electrochemical environment in a model simple enough to allow for the development of both trends and general conclusions. 

Wakisaka M, Mitsui S, Hirose H, Kawashima K, Uchida H, Watanabe M. 2006. Electronic structures of Pt-Co and Pt-Ru alloys for CO-tolerant anode catalysts in polymer electrol3de fuel cells studied by EC-XPS. J Phys Chem B 110 23489-23496. 

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