A NEMCA Experiment Galvanostatic and Potentiostatic Transients

A typical electrochemical promotion (EP), or in situ controlled promotion (ICP) or NEMCA experiment utilizing YSZ, an O2 conductor, as the promoter donor is shown in Fig. 4.13. The reaction under study is the oxidation of C2H4 on Pt  

The Pt film, with a surface area corresponding to NG=4.2-10 9 mol Pt, measured via surface titration of oxygen with C2H4,1,4 is exposed to po2 — 4.6 kPa, PC2H4 = 0.36 kPa at 370°C in a continuous flow gradientless (CSTR) reactor of volume 30 cm3. The rate of C02 formation is monitored via an infrared analyzer.1,4 

Then let us examine the rate relaxation time constant x, defined as the time required for the rate increase Ar to reach 63% of its steady state value. It is comparable, and this is a general observation, with the parameter 2FNq/I, (Fig. 4.13). This is the time required to form a monolayer of oxygen on a surface with Nq sites when oxygen is supplied in the form of 02 This observation provided the first evidence that NEMCA is due to an electrochemically controlled migration of ionic species from the solid electrolyte onto the catalyst surface,1,4,49 as proven in detail in Chapter 5 (section 5.2), where the same transient is viewed through the use of surface sensitive techniques. 

Thus the average lifetime, x, of O on the catalyst surface (x=TOF ) equals 770 s or 13 min. This then should be the time needed for the rate, r, to decay to its unpromoted value upon current interruption. This is in excellent agreement with experiment (Fig. 4.13) and nicely confirms the sacrificial promoter concept of NEMCA The promoter (O5 ) is sacrificed by eventually reacting with the oxidizable species (C2H4). But before being sacrificed , i.e. consumed, it has caused on the average the reaction of A extra oxygen atoms with the oxidizable species. 

This is the essence of NEMCA. The reader can check the validity of the sacrificial promoter concept in all NEMCA galvanostatic transients of this book. 

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