NEMCA enhancement factor

The exchange current I0 is an important parameter for the quantitative description of NEMCA. As subsequently analyzed in this chapter it has been found both theoretically and experimentally1,4 19 that the order of magnitude of the absolute value A of the NEMCA enhancement factor A defined from ... 

Figure 4.23. Comparison of predicted and measured enhancement factor A values for some of the early studies of catalytic reactions found to exhibit the NEMCA effect.1,19 Reprinted with permission from Elsevier Science.1...

The hydrogenation of CO and C02 on transition metal surfaces is a promising area for using NEMCA to affect rates and selectivities. In a recent study of C02 hydrogenation on Rh,59 where the products were mainly CH and CO, under atmospheric pressure and at temperatures 300 to 500°C it was found that CH4 formation is electrophobic (Fig. 8.54a) while CO formation is electrophilic (Fig. 8.54b). Enhancement factor A values up to 220 were... 

The reaction was investigated under atmospheric pressure and at temperatures 500°C to 600°C, where the only product was CO, as Pd, contrary to Rh, does not adsorb C02 dissociatively.59 This difference in reaction pathway is also reflected in the NEMCA behaviour of the system, since in the present case CO formation is enhanced (by up to 600%) not only with decreasing catalyst potential and work function, but also enhanced, although to a minor extent, via catalyst potential increase (Fig. 8.56). Enhancement factor A values up to 150 were measured. The reaction exhibits typical inverted volcano behaviour, which is characteristic of the weak adsorption of the reactants at the elevated temperature of this investigation, and thus of promotional rule G4. 

It was found that both the catalytic rates and the selectivity to the various products can be altered significantly (rate changes up to 250% were observed) and reversibly under NEMCA conditions. Depending on the product, electrophobic or electrophilic behaviour is observed as shown in Fig. 8.57. In addition to the selectivity modification due to the different effect on the rate of formation of each product, acetaldehyde, which is not produced under open circuit conditions is formed at negative overpotentials (Fig. 8.58). Enhancement factor A values up to 10 were observed in this complex system.59... 

The NEMCA effect was first demonstrated on Pt and Ag electrodes using 8 mol% YjOj-stabilized ZrOj (YSZ), an O " conductor, as the solid electrolyte. Electrochemical 0 pumping to the catalyst electrode was found to cause an up to 60-fold (6000%) steady-state reversible enhancement in the rate of oxidation. Furthermore, this steady-state rate increase was found to be up to 3 x 10 times higher than the steady-state rate of supply of O to the catalyst, i.e., the enhancement factor A or apparent faradaic efficiency for the process is 3 x 10 . More recently, the rate of C2H4 oxidation on Rh electrodes supported on YSZ was found to reversibly increase by a factor of 90, again with faradaic efficiency values of the order of 10 . ... 

Table 13.4 provides a list of the reactions which have been studied already and shown to exhibit NEMCA. In order to compare different catalytic reactions, it is useful to define two dimensionless parameters, i.e., the enhancement factor or faradaic efficiency A and the rate enhancement ratio p. The former is defined as ... 

FIGURE 13.14. Comparison of predicted and measured enhancement factor A values for the first 12 catalytic reactions found to exhibit the NEMCA effect. (From Vayenas, C.G, Bebelis, S., Yentekakis, I.V., Tsiakaras, R, Karasali, H., and Karavasilis, Ch., in Proceedings of the 10th Int. Congress on Catalysis, Guczi, L., Solymosi, E, and Tetenyi, R, Eds., Elsevier Science Publishers, B.V., Amsterdam, 1993, 2139. With permission.)... 

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