The Use of Alkali Ion Conductors

It has been known since 19911 that p -Al203, a Na+ conductor,2,3 can also induce pronounced electrochemical promotion (NEMCA) behaviour on metal surfaces. Here the dominant electrocatalytic reaction is  

Ethylene oxidation on Pt/p -Al203 was studied1 at temperatures 150° to 300°C, but most of the NEMCA experiments were carried out at 290°C. Experimental details about the cell can be found in ref. 1. The open-circuit kinetic behaviour was found to be similar to the case of C2H4 oxidation on Pt/YSZ (Chapter 8), i.e. the rate expression  

Also the rate plateau at low O values is strongly reminiscent of the observed behaviour with Pt/YSZ. Thus the behaviour is qualitatively very similar. The inserted 0Na abscissa in Fig. 9.2 is constructed on the basis of Eq. (4.25) or of the following equivalent form of the Helmholz equation  

It is worth emphasizing, however, that in both cases C2H4 oxidation exhibits electrophobic behaviour, that the relaxation time constants x can be estimated from similar formulae (equations (4.32) and (9.3)) and that the enhancement factors A can again be estimated from the same formula1 (equation 4.20). 

The fact that very small Na coverages (0Na=O.O15) suffice to induce pronounced (up to 70%) decreases in catalytic rate (i.e. the Na toxicity 10,11 is 0.70/0.015 = 47) rules out the possibility of any geometric interpretation 

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