Activation Energy and Preexponential Factor Dependence on Work Function

7 Activation Energy and Preexponential Factor Dependence on Work Function 

It has been known since the early days of electrochemical promotion that upon varying Uwr and thus E , not only the catalytic rates, r, are changing in a frequently dramatic manner, but also the activation energy of the catalytic reaction is also significantly affected. An example was already presented in Fig. 4.28 which shows that both C2H4 and CH4 oxidation on Pt/YSZ conform to equation (4.50) with an values of -1 and -3, respectively. 

This linear variation in catalytic activation energy with potential and work function is quite noteworthy and, as we will see in the next sections and in Chapters 5 and 6, is intimately linked to the corresponding linear variation of heats of chemisorption with potential and work function. More specifically we will see that the linear decrease in the activation energies of ethylene and methane oxidation is due to the concomitant linear decrease in the heat of chemisorption of oxygen with increasing catalyst potential and work function. 

It is quite interesting that ln(r°/r0°), where r0° is the open-circuit preexponential factor, also varies linearly with Uwr and O (Figs. 4.35 to 4.37) and in fact, when plotted as kbT0ln(r°/ro°) where T is the isokinetic temperature discussed below, with the same slope as EA. This decrease in apparent preexponential factor with increasing 3 can be attributed to the reduced binding strength and thus enhanced mobility of chemisorbed oxygen on the catalyst surface. 

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