Electronic factors in catalysis

In the early 1950 s, the geometric approach to the interpretation of catalytic activity was largely abandoned in favour of the so-called elec- 

Relative values for log ksp for ethylene hydrogenation and for 5 for various metals 

In sharp contrast to the results obtained in, for example, the palladium—gold-catalysed para-hydrogenation conversion [317], formic acid 

It should be noted that, in the interpretation of activity patterns of alloy catalysts, extreme care is needed to ensure that the surface composition is known. It has been shown [321,322] with copper—nickel alloys, which show two phases in the composition range 2—80% copper, that, within this miscibility gap, the surface composition remains constant at 80% Cu—20% Ni, independent of the nominal bulk composition. Furthermore, the surface composition may vary depending upon the catalyst pretreatment [322], No miscibility gap occurs with palladium—gold or palladium-silver alloys [323]. 

In the apparent absence of a correlation between hydrogenation activity and the bulk electronic properties of alloys, Sachtler et al. [321,322, 

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Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

Dahl S, et al. Electronic factors in catalysis the volcano curve and the effect of promotion in catalytic ammonia synthesis. Appl Catal A Gen. 2001 222(1—2) 19—29. 

Although many studies of this type were conducted, the approach was not very fruitful in elucidating the so-called electronic factor in catalysis by... 

Experimental data on chemisorption and catalysis indicate that the different types of atoms in the surface of an alloy, such as nickel and copper, largely retain their chemical identities, although their bonding properties may be modified (6,7). At present the electronic factor in catalysis by metals generally is viewed in terms of localized chemical bonding effects similar to the "ligand effects of organometallic chemistry (6). 

The recent interest in electronic factors in catalysis has produced two significant theories. The first is that with the metals the electronic configuration, in particular of the d-band, is an index of catalyst activity. The second is that with the oxides, activity may be controlled by the semiconducting property. Hitherto, these theories have been regarded as unrelated to one another. 

The next advance came from the application of Fermi-Dirac statistics to the electrons in metals, which led to the band theory of a quasi-continu-ous series of energy levels, and to the concept of Brillouin zones, which is of special value for alloys. This sets the stage for a detailed study of the electronic factor in catalysis on metals. 

The concept of electronic factors in catalysis deals with the relationship between the electronic structure of solids, which depends on their physical properties, and the reactivity of adsorbed intermediates. 

For a meaningful discussion of electronic factors in catalysis it is necessary to briefly review the nature of chemisorption bonds. Two theories of the metallic state have been accepted, the electron band theory and the valence bond theory. Both theories recognize the existence of two separate functions for valence electrons in metals one function is to bind the atoms together and the other is to account for magnetic and conductive properties. In the electron band theory, as particularly applied to the transition metals, the s-electron energy band is broad with a low maximum... 

Dahl S, Logadottir A, Jacobsen CJH, Nprskov JK. Electronic factors in catalysis the volcano curve and the effect of promotion in catalytic am-monia synthesis. Appl Catal A 2001 222 19-29. Ertl G, Lee SB, Weiss M. Adsorption of nitrogen on potassium promoted Fe(lll) and (100) surfaces. Surf Sci 1982 114 527-545. 

Interest in studying formic acid adsorption on metals by XPS and UPS was stimulated largely by its use as a probe molecule for investigating the role of the electronic factor in heterogeneous catalysis as in the work of Schwab (70), Dowden and Reynolds (71), Eley and Leutic (72), and Fahren-fort et al. (73). The advantages of XPS and UPS are fourfold. 

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