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. 

High selectivity of 4,4 -DIBP was observed in the catalysis of HM. The selectivity of 4,4 -DIBP was constant during the reaction with the accumulation of 2- and 3-IPBP and decrease of the selectivity of 4-IPBP. These results show that the alkylation proceeds by a consecutive reaction mechanism. The alkylation of 4-IPBP occurred regloselectlvely to give 4,4 -DIBP. Other Isomers, 2- and 3-IPBP, do not participate in the reaction because these Isomers are too sterically bulky to enter the pore of HM. On the other hand, catalyses of HY and HL were nonselective for the formation of 4,4 -DIBP. Three isomers of IPBP take part in the alkylation, which is controlled by the electronic factor of reactant molecules at low temperatures and by the stability of product molecules at higher temperatures. 

One of the aims of research in catalysis is to identify the factors that contribute to the catalytic activity of solids. Among the various factors, a distinction is usually made between the geometric factor and the electronic factor. According to Balandin (1969), who emphasized the role of the geometric factor, activity depends on the presence of a... 

Despite various attempts, no single universal correlation between bulk properties and catalytic activity of solids has been found. It is now recognized that the geometric factor and the electronic factor cannot be separated from one another and that catalytic activity should be considered along with catalyst selectivity to arrive at an understanding of heterogeneous catalysis (Sachtler, 1981). 

The considerations of this reaction of Oz evolution on an oxide catalyst again show the importance of electronic factors and bonding. However, the discussion covers only the essentials the reality of the catalysis of perovskites in oxygen evolution involves several other factors that can be referred to here only briefly. 

In the next section experimental and theoretical approaches will be discussed to settle the dispute about the relevance of the electronic factor (117) to catalysis. 

---