Catalytic activity electron work function

The connection between electronic work function and catalytic activity was shown some time ago, especially by Suhrmann (1-4) and Czesch (1), who found a simple relation between the changes of the work functions of different metal surfaces upon adsorption of H atoms, and the catalytic activities of such surfaces for the recombination of H atoms. 

The electrical conductivity of the solid would also vary as a function, of the impurity nature and the chemical effect of these must be different. However, experimental results show that the relationship between conductivity and catalytic activity is much more complex. This is probably due to the fact that the conductivity of polycrystalline semiconductors often is not affected by changes in the Fermi level of the surface. Thus there must be another connection between changes in electron work functions of modified catalysts and their adsorptive and catalytic activities. 

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). 

The scientific literature abounds in attempted correlations between the catalytic activities of metals and some set of their bulk properties (electron work function, bond energy of intermediates, etc.). Such correlations would help in understanding the essence of catalytic action and enable a conscious selection of the most efficient catalysts for given electrochemical reactions. 

The enhancement in the catalytic activity is due to the electrochemical supply of H+to the catalyst which decreases the catalyst work function and thus strengthens the chemisorptive bond of electron acceptor N while at the same time weakening the bonds of electron donor H and NH3. 

However, when the reductions were carried out with lithium and a catalytic amount of naphthalene as an electron carrier, far different results were obtained(36-39, 43-48). Using this approach a highly reactive form of finely divided nickel resulted. It should be pointed out that with the electron carrier approach the reductions can be conveniently monitored, for when the reductions are complete the solutions turn green from the buildup of lithium naphthalide. It was determined that 2.2 to 2.3 equivalents of lithium were required to reach complete reduction of Ni(+2) salts. It is also significant to point out that ESCA studies on the nickel powders produced from reductions using 2.0 equivalents of potassium showed considerable amounts of Ni(+2) on the metal surface. In contrast, little Ni(+2) was observed on the surface of the nickel powders generated by reductions using 2.3 equivalents of lithium. While it is only speculation, our interpretation of these results is that the absorption of the Ni(+2) ions on the nickel surface in effect raised the work function of the nickel and rendered it ineffective towards oxidative addition reactions. An alternative explanation is that the Ni(+2) ions were simply adsorbed on the active sites of the nickel surface. 

The explanation for the apparent correlation between catalytic activity and electron affinity of metals cannot be as simple as that which has been advanced for the homogeneous catalysts. This is because chemisorption on metals (unlike the splitting of hydrogen by metal ions in solution ) is an exothermic process and, hence, as shown earlier, catalytic activity depends not only on a low activation energy of adsorption but also on a low heat of adsorption. The interpretation applied earlier to homogeneous catalysts can account for an inverse dependence of Ea on the work function, but does not suggest any obvious reason why Q should show a similar dependence. 

As early as 1928, Roginskil and Schul tz (4) stressed the importance of electronic considerations, and Rideal and Wansbrough-Jones (5) related the work function of metals to the activation energy for their oxidation. Brewer, 1928 (6), Schmidt, 1933 (7), and Nyrop, 1935 (8) proposed that the surface must be capable of effecting ionization of the adsorbed species in some catalytic processes. Lennard-Jones (9) in his... 

The decisive factor for the catalytic activity is the metal-carbon monomer molecule has to be inserted. Essential content of the present work is the study of the stability of this metal-carbon bond as a function of the electron donor-acceptor properties of the ligands. Thus the activity of the catalytic center can be tailored. Furthermore, it could be shown experimentally, that the coordination of the monomer molecule to the free site of the active species provokes an additional destabilization of the metal-carbon bond. 

As will be discussed later, we can conclude from other evidence that the surface of a catalyst for ammonia synthesis is actually heterogeneous in respect to catalytic activity and that, in all probability differences in the amount and nature of the promoters cause variations of electronic properties, observable by changes of the work function and of the catalytic activity. This conclusion may be expressed by stating that the activity change caused by promoters cannot be considered solely as a local action a point which has been especially well clarified by recent work on catalysts which are semiconductors. 

The simple relation of catalytic activity with respect to the exchange to the work function of electrons of oxides is missing. 

Copper and silver complexes bearing trispyrazolylborate ligands have shown catalytic activity towards several organic transformations involving the functionalization of unsaturated carbon-carbon bonds or several saturated E—H bonds, particularly of carbon-hydrogen bonds. The tunability of this class of ligands from both steric and electronic perspectives allows the control of those catalytic capabilities. On the basis of the work already described, the potential for their use on other, yet unreported transformations seems feasible. 

The influence of adsorbed Si, P, S, and Cl on the medium-pressure cyclotrimerization of acetylene to benzene over Pd(l 11), (100), and (110) has been studied by Logan et al. (113). Whereas both sulfur and chlorine decrease the activity, silicon increases the activity. The effect of phosphorous depends on the crystal face. According to their work function measurements, sulfur withdraws electron density from the Pd surface (as is also expected for chlorine), whereas Si donates electron density, and P has the least effect on the work function. Thus, the qualitative influences on catalytic activities correlate with the influences of the additives on the electronic character of the surface. In addition, Si decreases the carbon coverage seen in postreaction AES from —82 to —70% of a monolayer, whereas sulfur and chlorine increase the amount of carbonaceous residue. The authors interpreted these results by suggesting that the electron-donating ligands keep the Pd surface cleaner for the desirable reaction by... 

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