Volcano-type plot

In Fig. 4.52 the activity of H2 oxidation for different metals is plotted as a function of heat of oxygen adsorption. A volcano-type plot is formed. A maximum in rate is found for the metal that can dissociate 02, but does not bind CO or oxygen too strongly. Gold cannot dissociate 02, tungsten is inactive because it... 

When one tries to correlate the electrocatalytic activity of metals with some fundamental property of the system, the result is often a "volcano- type" plot, as shown in Fig. 7F. 

Volcano type plots for hydrogen metal intermediates show a onestep process277 involving Pt as a hydrogenation catalyst. 02 evolution - no matter what mechanism is operative - has been found to proceed via two catalytic steps and volcano type plots are not possible in this case. 

Here, as in the case of the HOR, a volcano -type plot emerges that indicates that of the elementary metals, Pt is best. However, it is evident from this figure that there is room for improvement if a nanostructured material can be found with the optimum binding energies for the reaction intermediates. Several approaches that attempt to achieve this balance in binding energy are possible. 

Figure 6.3. Examples for the four types of global electrochemical promotion behaviour (a) electrophobic, (b) electrophilic, (c) volcano-type, (d) inverted volcano-type, (a) Effect of catalyst potential and work function change (vs I = 0) for high (20 1) and (40 1) CH4 to 02 feed ratios, Pt/YSZH (b) Effect of catalyst potential on the rate enhancement ratio for the rate of NO reduction by C2H4 consumption on Pt/YSZ15 (c) NEMCA generated volcano plots during CO oxidation on Pt/YSZ16 (d) Effect of dimensionless catalyst potential on the rate constant of H2CO formation, Pt/YSZ.17 n=FUWR/RT (=A(D/kbT).

Zi et al. [91] studied Y zeolites dealuminated with H4EDTA and (NH4)2SiF6. The surface acidity of the zeolites was determined by n-butyl-amine titration, and the amount of strong (Ho < -3.0) acid sites was correlated with the cumene conversion. Both values showed a volcano-type curve if plotted against the amount of Al in the framework. The maximum in these plots was found to be around 30 alumimuns per unit cell, which agrees with the results pubhshed by DeCanio et al. [82]. 

Sabatier s Principle is illustrated in Fig. 6.40 where the ammonia rate is plotted for similar conditions versus the type of transition metals supported on graphite. The theory outlined so far readily explains the observed trends metals to the left of the periodic table are perfectly capable of dissociating N2 but the resulting N atoms will be bound very strongly and are therefore less reactive. The metals to the right are unable to dissociate the N2 molecule. This leads to an optimum for metals such as Fe, Ru, and Os. This type of plot is common in catalysis and is usually referred to as a volcano plot. 

No catalyst has an infinite lifetime. The accepted view of a catalytic cycle is that it proceeds via a series of reactive species, be they transient transition state type structures or relatively more stable intermediates. Reaction of such intermediates with either excess ligand or substrate can give rise to very stable complexes that are kinetically incompetent of sustaining catalysis. The textbook example of this is triphenylphosphine modified rhodium hydroformylation, where a plot of activity versus ligand metal ratio shows the classical volcano plot whereby activity reaches a peak at a certain ratio but then falls off rapidly in the presence of excess phosphine, see Figure... 

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... 

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). 

In long-term studies, however, if a drug is effective, people may stay on it longer than they remain on the control arm. In such cases, comparing annualized rates relative to time of exposure may be more relevant. In case where the adverse events occur most frequently at the beginning of exposure, piecewise rates (e.g., with the first 30 days and after 30 days) might be more informative. Depending on the type of metric chosen, the volcano plots may present not risk difference on the x-axis, but another measure such as hazard ratio, risk ratio, or difference between annualized event rates. 

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