Surface volcano plot

Lambert and coworkers,7 18 25 who were first to study this interesting system, have shown that the nature of the anion (nitrate or carbonate) formed on the catalyst surface in presence ofNa+ plays an important role in the sharpness of the volcano plot obtained upon varying Uwr.-... 

A very useful analysis of catalytic reactions is provided for by the construction of so-caUed volcano plots (Figure 1.2). In a volcano plot, the catalytic rate of a reaction normahzed per unit reactive surface area is plotted as a function of the adsorption energy of the reactant, product molecule, or reaction intermediates. 

Figure 8.25. Predicted volcano plots for ammonia synthesis, showing the turnover frequency versus the relative bonding strength of N atoms to the surface for ammonia concentrations of 5%, 20%, and 90%. The left-hand panel corresponds to conditions of...

The critical role of the M/M—OH redox system in determining the population of the surface active metal sites is, with high probability, the actual reason for the strong predictive power of the M—Ox bond strength with regard to the relative rates of ORR at different metal surfaces. In fact, a better presentation of the volcano plot would be obtained by using, for the ordinate of the plot the value (1 /Z + 1) exp(— /RT),... 

For the ascending branch of the volcano plot, the term (1/Z + 1) could serve by itself as an effective ORR activity predictor, whereas, for the descending branch, (1/Z + 1) becomes close to unity at 0.85 V, and the exponential factor exp(—A//, /R70, then determines the ORR rate based on the residual interaction of dioxygen with the (excessively) noble metal catalyst surface. 

The expression (1/Z+ 1)] exp[— AHl /RT] at 0.85 V, better reflects the reality of a partially oxidized Pt surface and the critical effect of active site availability on the rate of the ORR. Effects of site availability were not considered in the calculations in Nprskov et al. [2004] of ORR activity for various metals. The expression used to calculate activity defined the ordinate parameter in the ORR volcano plots presented. This parameter was defined in Nprskov et al. [2004] as kT min,- log(k,/ko). 

Figure 3.16 Volcano plot for the hydrogen evolution reaction (HER) for various pure metals and metal overlayers. Values are calculated at 1 barof H2 (298K) and at a surface hydrogen coverage of either 0.25 or 0.33 ML. The two curved lines correspond to the model (3.24), (3.25) transfer coefficients (not included in the indicated equations) of 0.5 and 1.0, respectively, have also been added to the model predictions in the figure. The current values for specific metals are taken from experimental data on polycrystalline pure metals, single-crystal pure metals, and single-crystal Pd overlayers on various substrates. Adapted from [Greeley et al., 2006a] see this reference for more details.

Fig. 20. The volcano plot for HCOOH decomposition on high surface area catalysts ( ) and single crystals ( ) (102a). Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1979.

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). 

Figure 1.9 Volcano plot showing dependence of rate on strength of adsorption the upper part shows the corresponding variation of surface coverage 6.

The Sabatier principle of catalysis also finds extensive application in the area of electrocatalysis reactants should be moderately adsorbed on the catalyst/electro-catalyst surface. Very weak or very strong adsorption leads to low electrocatalytic activity. This has been demonstrated repeatedly in the literature by the use of volcano plots (Figs 23-25). In these plots, the electrocatalytic activity is plotted as a function of the adsorption energy of the key reactant or some other parameter related to it in a linear or near-linear fashion, such as the work function of the metal [5], or the metal—H bond strength when discussing the H2 evolution reaction (Fig. 24) [54] or the enthalpy of the lower-to-higher oxide transition when examining the O2 evolution reaction (Fig. 25) [55]. 

In the above we have deliberately stayed close to Vannice s model, and changed only the nature of the rate-determining step from H-assisted C-O bond breaking (15) into (irreversible) hydrogenation of surface carbon (28). Consequently, the overall rate equation (30) differs from Eq. (17) only in that it contains the term 0c instead of Ochoh- When assuming further that Cads instead of CHOHajs is the most abundant surface intermediate, the model can be made formally identical to that of Vannice, including the explanations it offers for the volcano plot and the compensation effect. 

On the left of the transition series the product 6a b is very high, but the vacancy term, which is essential for the reaction to proceed, is very small, near zero, due to site blockage. On the right hand side of the volcano plot the adsorbates are much more weakly bound and the opposite is the case, namely the 9a b term is very small, and there are many vacancies on the surface, In between these two extremes there is a good distribution of adsorbate on the surface with vacancies for adsorption. 

High activity catalysts are generally metal oxides in which the metal can assume more than one valence state, are p-type semiconductors, and produce highly mobile chemisorbed surface oxygen (a consequence of this last characteristic is an intermediate heat of adsorption of O, that produces the familiar volcano plot). 

Thus, electrocatalysts for the ORR have to present electronic structures that result in adsorption forces that strike these two competing steps while strong adsorption leads to facilitated 0-0 bond breaking, weak adsorption tend to facilitate the O-H bond formation (hydrogen addition). This produces the so-called volcano plot of the activity as a function of the adsorption strength on the catalyst surface [7-9]. 

---