Metal catalysis, electronic factors

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. 

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. 

It is intriguing that analysis of the volcano curve predicts that the apex of the curve occurs at AH(H2)ads = 0 (formally, AG = 0) [26]. This value corresponds to the condition D(M-H) = 1/2D(H-H), that is, forming an M-H bond has the same energetic probability as forming an H2 molecule. This condition is that expressed qualitatively by the Sabatier principle of catalysis and corresponds to the situation of maximum electrocatalytic activity. Interestingly, the experimental picture shows that the group of precious transition metals lies dose to the apex of the curve, with Pt in a dominant position. It is a fact that Pt is the best catalyst for electrochemical H2 evolution however, its use is made impractical by its cost. On the other hand, Pt is the best electrocatalyst on the basis of electronic factors only, other conditions being the same. 

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). 

Some metal hydride values have been determined (Table the values are markedly ligand dependent, as seen for example, by replacement of one CO of HCo(CO)4, a strong acid, by PPhj, which decreases the acidity of 7 pK units However, the values, which must be a measure of the polarity of the M—H bond under a certain set of conditions (those of the titration procedures), have proved of little value in predicting the chemistry of the metal hydrides, e.g., whether behavior is more typical of Co "( H), Co ( H), or Co (H). This is critical in catalysis, particularly in the direction of addition of the M—H bond across olefinic groups (i.e., in olefin insertion), which is important in hydrogenation, hydroformylation, and isomerization . This is a complex question and, as well as electronic factors, steric factors, solvent polarity, the presence of radical initiators, and even temperature changes, can be important. 

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. 

In this Section we consider the enhancement of plasma-chemical conversion and product selectivity due to a foreign substance in the solid phase. The substance may be placed in the plasma, in the spatial afterglow, in a cold trap for collecting the products or in all of these regions. Typical solids which have been used include several transition metals and some of their oxides It appears that there is some connection between the degree of catalysis and the electron work functions of the metal catalysts. So far as conventional catalysis is concerned attempts to correlate the electronic factor with catalytic activity and chemisorption — the precursor of catalysis — has largely been unsuccessful (see Sect. 6). 

For the acid catalysed conversion of hydrocarbons, the reaction mechanisms in absence of sterical hinderance are rather well understood, so that molecular shape-selective effects exerted by constrained environments can be isolated [8,9]. Shape-selective catalysis is also possible when other than acid functions are confined to the intracrystalline void volumes of zeolite crystals, e.g. metal [10,11], bifunctional [12] and basic functions [13]. Nowadays, catalysis on zeolites with organic substrates containing heteroatoms receives much attention. Molecular shape-selectivity seems to be superimposed on electronic factors determining the selectivities [14,15]. 

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... 

Transition metal catalysis is a major driving force for development of new approaches in organic synthesis, medicinal chemistry, preparation of biologically active and pharmaceutical molecules, as well as in numerous applications related to material science and molecular electronics. Recent advances in green and sustainable chemistry emphasized the key role of waste-free chemicals production. Especially critical in fine chemicals synthesis is that high values of E-factor are not uncommon. Increasing demand in very complex molecular structures enforces implementation of sophisticated multistep synthetic procedures and further complicates the waste/product balance. On the other hand, so far most of the commodity chemicals remain to be produced by classical procedures, which are not green. 

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