Balandin Volcano Plot

The Balandin volcano plot illustrates a relation between a certain type of catalytic activity index, A, versus some enthalpic function, He, related with the heat of chemisorption [25], In Figure 2.7... 

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

Clearly, the log r vs. AGads slopes will be twice as great for the second step as for the first, as shown by the corresponding plots A and B for each step, respectively. Such plots are called Volcano plots, and their existence was first discussed in the case of purely catalytic processes by Balandin they were theoretically discussed by Parsons and Gerischer for the... 

Thns, a plot of reaction rate against adsorption energy of the intermediate species is bell or volcano-shaped and has a distinct maximum. This bell-shaped relation between the rate of a catalytic chemical reaction and the adsorption energy of an intermediate was described first by Aleksei Balandin in the 1930s and is usually associated with his name. This basic relation is preserved in more complex situations when the simplifying assnmptions made above no longer hold. 

Sabatier and Balandin had predicted a relationship between catal)dic activity and heat of adsorption. If a solid adsorbs the reactants only weakly, it will be a poor catalyst, but if it holds reactants, intermediates or products too strongly, it wiU again perform poorly. The ideal catalyst for a given reaction was predicted to be a compromise between too weak and too strong chemisorption. Balandin transformed this concept to a semiquantitative theory by predicting that a plot of the reaction rate of a catal)Tic reaction as a function of the heat of adsorption of the reactant should have a sharp maximum. He called these plots volcano-shaped curvesl This prediction was confirmed by Fahrenfort et al." An example of their volcano-shaped curve is reproduced in Fig. 9.1. They chose the catalytic decomposition of formic acid... 

---