A Galvanostatic NEMCA Transient Revisited

Both the TPD spectra (Fig. 5.2b) and the cyclic voltammograms (Fig. 5.2c) show clearly the creation of two distrinct oxygen adsorption states on the Pt surface (vs. only one state formed upon gas phase 02 adsorption, Fig. 5.2b, t=0). 

The weakly bonded O adsorption state is populated almost immediately (Figs. 5.2b and 5.2c). The strongly bonded O adsorption state is populated over a time period of the order 2FNG/I. This is exactly the time period which the catalytic rate needs to reach its electrochemically promoted value (Fig. 5.2a). 

The latter acts as a sacrificial promoter. It is a promoter, as it forces oxygen to populate the weakly bonded (and highly reactive) oxygen adsorption state. It is also sacrificed as it is consumed by C2H4 at a rate I/2F, equal to its rate of supply. 

In view of the above physical meaning of A it is clear why A can approach infinite values when Na+ is used as the sacrificial promoter (e.g. when using j -Al203 as the solid electrolyte) to promote reactions such as CO oxidation (Fig. 4.15) or NO reduction by H2 (Fig. 4.17). In this case Na on the catalyst surface is not consumed by a catalytic reaction and the only way it can be lost from the surface is via evaporation. Evaporation is very slow below 400°C (see Chapter 9) so A can approach infinite values. 

Since A expresses the ratio of the rates of consumption of the two oxygen states by C2H4 one has  

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