Volcano relation

The microkinetic models in this section are built upon BEP-relations of the type described above. It will be shown that an underlying BEP-relation in general leads to the existence of a volcano relation. We shall also use the microkinetic models in combination with the universal BEP-relation to explain why good catalysts for a long range of reactions lie in a surprisingly narrow interval of dissociative chemisorption energies. 

Figure 4.44. Contracted energy diagrams for CO methanation over Ni, Ru, and Re (Left). BEP-relation for CO dissociation over transition metal surfaces (right-top) and the corresponding volcano-relation for the turnover frequency (right-bottom). Adapted from [55,140].

J. O M. Bockris and S. U. M. Kahn, Surface Electrochemistry, pp. 292-294, Plenum, New York, 1993. Theory of volcano relations in electrochemical reactions. 

Modern DFT has become a powerful tool to understand, predict, and discover electrochemical catalysts with improved ORR activity and stability. Computational free energy reaction diagrams provide insight into the potential-determining elementary reaction step of the ORR as a function of atomistic descriptors (surface-related properties) of the catalyst material. DFT-based volcano relations have been established pointing to improved catalyst systems. 

In a first approximation under non-FC conditions, the Tafel slope may be attributed to the action of the change in the free energy of electrons (i.e., the change in Fermi level of the electrode with respect to the electrostatic energy of the ions in the solution) on the fractional electronic change on the transition state. However, a persuasive case can be made that the experimental Volcano relations and Tafel slopes represent the same phenomenon. 

Parsons (23) derived a theoretical relation for the dependence of log Iq on the standard Gibbs energy (AG °) of chemisorption of H at the metal, and its form is a volcano relation as shown in Fig. 3. The physical basis of this relation is discussed in more detail in Section XII, and its relation to modern data on log I o and is shown in Fig. 16 later. 

It was noted early (16) that there was a periodic relation of I o or Ini o to properties of metals across the periodic table (Fig. 15), such as lattice energy and electron work function, O. This is illustrated in one of such plots, versus O, shown in Fig. 2 for the kinetics of cathodic Hj evolution on various electrode metals. Early plots of In I o versus gave two or three distinguishable regions with an apparent maximum in 4> for increasing In Iq values (75), that is, a volcano relation lying on its side (see further comments below). 

Let us look back at Figure 1.17. That is a so-called VOLCANO relation happens that if you plot on the ordinate axis the velocity of a reaction and on the ventricle abscissa axis bonding of the named substrates to an important radical bonded to the surface (as an intermediate). Bonding (including the heat of sublimation) of the catalyst substrate, one often finds a volcano shape. Now, in Table 1.3, I have listed the melting points of a number of refractory (i.e., stable) metals. What do you see Bonds to platinum will be in the middle of this group. It will bond well enough, but not too much so that reactants just adhere to the surface and do not desorb. 

The six possible combinations of three simple reactions [discharge, electrochemical desorption, and combination steps, reactions (47)-(49)] will all show volcano relations. An analysis shows that their rates are as given below. For simplicity, all reactions are considered to occur for AGads 5 0 at the V value considered, i.e., Langmuir conditions. 

The question is whether the volcano relations of Figure 16 type for oxygen result from the same cause as for the hydrogen electrode. Fortunately, activation energy data (at 460 mV overpotential) are availableThese are... 

Hence, volcano relations should also arise as a function of redox potential. 

The most studied reaction has been methanol oxidation (reviewed in Ref. 132), though volcano relations have been noted for ethyleneand formic acid oxidation. Useful reviews on methanol oxidation and on specific electrocatalysis have been given by McNichol. Rates of organic oxidations are low. In general, fuels such as methanol are, therefore, reformed to give hydrogen for more efficient oxidation in a fuel cell. Reactions normally involve processes such... 

Finally, why are oxgyen reactions, such as dioxygen reduction or hydrocarbon oxidation, slow compared with hydrogen reactions for materials at the top of the volcano relation. This is clear from Figure 4. Oxygen intermediates... 

Fig. 2.4 A volcano relation between catalytic activity and adsorption strength...

Unsubstituted BaAli20i9 sample showed very poor activity. For the samples containing transition metal ions, the following order of reactivity was found Mn Cu>Fe Co=Cr Ni. The same reactivity order was found over LaMAlnOi9 samples, which indicated that combustion activity is primarily determined by the nature of the transition metal ion. As shown in Figure 8, a volcano correlation was found between catalytic activity (expressed as Tio /o) and the difference of standard formation enthalpies of trivalent and divalent M-oxides. This suggests that CH4 oxidation occurs according to a redox mechanism that involves a reversible cycle. The volcano relation... 

Michaud JP, Krupitsky D, Grove JS, Anderson BS (2005) Volcano related atmospheric toxicants in Hilo and Hawaii volcanoes national park implications for human health. Neurotoxicology 26(4) 555-563... 

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