Permanent NEMCA

One of the most interesting and potentially important from a practical viewpoint aspect of electrochemical promotion is the permanent NEMCA effect first discovered and studied by Comninellis and coworkers at Lausanne. 

Upon positive current application the rate of C2H4 oxidation increases by a factor of 13 (p=14) with a A value of the order of 100. The important aspect of the figure is that upon current interruption neither the rate nor O return to their initial open- circuit values (Fig. 4.49). There is a permanent 

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Figure 4.50. Transient effect of constant applied current (I=+300 pA) on the rate of C2H4 oxidation on Ir02 and on 75mol% Ir02 - 25%Ti02 and 25% Ir02 - 75%Ti02 composite catalysts deposited on YSZ. Note the decrease in p upon increasing the Ti02 content and the appearance of permanent NEMCA in all cases.124...

As already discussed in Chapter 4 (Figs. 4.49 and 4.50) the Ir02-Ti02/YSZ system also exhibits interesting permanent NEMCA behaviour. 

It is worth noting in Figure 4.49 that the permanent NEMCA behaviour is much more pronounced for the 25%Ir02-75%Ti02 catalyst, which is again consistent with the O2 donor character of Ti02. 

It is worth noting that at lower temperatures (Figs. 4.51, 4.52) positive current application leads to pronounced permanent NEMCA behaviour while more reversible NEMCA behaviour is obtained above 400°C (Fig. 8.61). 

Positive potentials lead to p values up to 20. (Figure 4.52). Negative currents also enhance the rate and selectivity but to a lesser extent (Fig. 8.64). Permanent NEMCA behaviour is also observed with positive currents at lower temperatures (Fig. 4.52). Overall, however, electrochemical promotion is not as pronounced as in the case where propene is used. This can be attributed to the much stronger electron donor character of C3H6 relative to CO which, as already noted in this chapter, behaves predominantly as an electron acceptor. Thus positive potentials weaken CO bonding to the surface while they enhance CjH6 chemisorption. 

They first reported and studied permanent NEMCA and via cyclic voltammetry established the dependence of metal/solid electrolyte capacitance on porous metal film mass, which confirms the O2" backspillover promoting mechanism. 

Particulate matter combustion of, electrochemically promoted, 525 Dinex process, 525 European and US standards, 526 Permanent NEMCA characteristics, 176 potential for catalyst preparation, 177 Perovskites... 

The product selectivity is also significantly affected by the NEMCA effect, which is usually quite reversible, although at lower temperatures a permanent NEMCA effect is often observed [iv, v]. 

When the factor y is unity, the electrochemical promotion is termed reversible. Several cases of irreversible modification have been reported from our laboratory. This effect has also been termed permanent electrochemical promotion or permanent NEMCA effect. 

The variation of the reaction rate between the two extremes, i.e., the open-circuit reaction rate, o, and the steady-state promoted reaction rate, A st, was fully reversible. A possible irreversible contribution to the promotion effect (permanent NEMCA) was avoided by limiting the polarizing current to 10 (tA. The current dependence of the steady-state promoted catalytic reaction rate was also investigated. The total increase in reaction rate, was a continuously increasing function... 

The proposed model of two-stage process is well supported by the cyclic voltammetric experiments presented in Section III.4. The fast reversible stage, attributed to formation of an electric double layer at the catalyst/gas interface via backspillover of promoters, has been discussed in detail in Section III.5. To explain the slow irreversible pretreatment. This phenomenon is called permanent electrochemical promotion or permanent NEMCA effect. The similarity between the regions of rate increase and decrease indicates that similar mechanisms are involved during current application and interruption, but the enhancement of the open-circuit rate indicates that the electrochemical promotion of the Ir02 catalyst is not reversible. This behavior of an oxide catalyst is different from that of a metal catalyst for which the electrochemical promotion is usually reversible. ... 

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