Volcano-type reactions

Inspection of Table 6.1 shows that reactions exhibiting volcano-type (maximum type) behaviour with respect to D are those where the kinetics also exhibit a maximum with respect to A and D so that the rate is always positive order in A or D and at the same time negative (not zero) order in D or A respectively. 

Rule G3 A reaction exhibits volcano-type behaviour when both the electron donor D and electron acceptor A are strongly adsorbed on the catalyst surface. 

In the context of Langmuir-Hinshelwood type kinetics, Rule G3 can be expressed as  

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The mles of electrochemical promotion follow directly from Table 6.1 For example, as shown in Table 6.1 all purely electrophobic reactions are positive order in D and zero or negative order in A. All purely electrophilic reactions are positive order in A and zero or negative order in D. Volcano-type reactions are always positive order in one reactant and purely negative order in the other. Inverted volcano-type reactions are positive order in both reactants. 

Inspection of Table 6.1 shows the following rule for inverted volcano type reactions ... 

Figure 6.20. Model predicted electrochemical promotion kinetic behaviour (a) and (b) volcano-type reaction, (c) and (d) inverted volcano-type reaction.

Many reactions exhibit both electrophobic and electrophilic behaviour over different UWr and O ranges leading to volcano-type51 (Fig. 4.16) or inverted-volcano-type (Fig. 4.25) behaviour.70... 

Figure 6.2. (Top) Definitions of local electrophobic and local electrophilic behaviour for two reactions exhibiting global volcano-type behaviour (a) and global inverted-volcano-type behaviour (b). (Bottom) Corresponding variations in surface coverages of adsorbed electron donor (D) and electron acceptor (A) reactants. As shown in this chapter volcano-type behaviour corresponds in general to high reactant coverages, inverted-volcano-type behaviour corresponds in general to low reactant coverages.

Figure 9.9. Rate and catalyst potential response to application of negative currents (a,b), for the case of volcano-type" behaviour (a) and S-type behaviour (b) of the reaction rate, and to application of positive currents (c,d) see text for discussion. Conditions (a) pco 2 kPa, Po2=2 kPa, T=350°C, catalyst Cl (b) pCo=2 kPa, p02=4 kPa, T=350°C, catalyst Cl. (c,d) pCo =0.73 kPa, po2=0.86 kPa, T=402°C, catalyst C2. Reprinted with permission from Academic Press.1 ...

Interestingly as shown in Fig. 9.17 the reaction exhibits pronounced volcano-type behaviour with the rate of N2 production maximized for Uwr=-0.3 V. 

The parameter a in Equation (11.6) is positive for electrophobic reactions (5r/5O>0, A>1) and negative for electrophilic ones (3r/0Oelectrochemical promotion behaviour is frequently encountered, leading to volcano-type or inverted volcano-type behaviour. However, even then equation (11.6) is satisfied over relatively wide (0.2-0.3 eV) AO regions, so we limit the present analysis to this type of promotional kinetics. It should be remembered thatEq. (11.6), originally found as an experimental observation, can be rationalized by rigorous mathematical models which account explicitly for the electrostatic dipole interactions between the adsorbates and the backspillover-formed effective double layer, as discussed in Chapter 6. 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

Inspection of Table 3 shows that reactions exhibiting volcano-type (maximum type) behavior with respect to are those where the kinetics also exhibit a maximum with... 

From Eq. (86), one can study the dependence of catalytic reactions on catalyst work function. Four types of rate-work function dependence have been identified experimentally, that is, electrophobic (3r/3 > 0), electrophilic (3r/3 < 0), volcano (where the rate exhibits a maximum with varying ) and inverted volcano type, where the rate exhibits a minimum with varying <1> (Fig. 52). 

The mechanism often invoked on zeolitic-type catalysts with a partially oxidised hydrocarbon as a possible intermediate of the reaction could account for these results. Indeed, there are similarities such as the conversion of NO which follows a volcano-type curve and there is an increase of activity parallel to an increase of the oxygen coneentration in the reacting mixture. 

One recognizes here Sabatier behavior in the volcano-type dependence of the reaction rate as a function of the adsorption equilibrium constant for CO. One can also note the positive order of the rate constant in CO pressure at the left of the Sabatier maximum and the negative order in CO pressure to the right of the Sabatier maximum. 

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. 

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