Promotional Rules

The reader already familiar with some aspects of electrochemical promotion may want to jump directly to Chapters 4 and 5 which are the heart of this book. Chapter 4 epitomizes the phenomenology of NEMCA, Chapter 5 discusses its origin on the basis of a plethora of surface science and electrochemical techniques including ab initio quantum mechanical calculations. In Chapter 6 rigorous rules and a rigorous model are introduced for the first time both for electrochemical and for classical promotion. The kinetic model, which provides an excellent qualitative fit to the promotional rules and to the electrochemical and classical promotion data, is based on a simple concept Electrochemical and classical promotion is catalysis in presence of a controllable double layer. 

On the basis of the above simple rules one can formulate the following two promotional rules ... 

Table 4.2 lists the same catalytic systems but now grouped in terms of different reaction types (oxidations, hydrogenations, reductions and others). In this table and in subsequent chapters the subscript D denotes and electron donor reactant while the subscript A denotes an electron acceptor reactant. The table also lists the temperature and gas composition range of each investigation in terms of the parameter Pa/Pd which as subsequently shown plays an important role on the observed r vs O global behaviour. Table 4.3 is the same as Table 4.2 but also provides additional information regarding the open-circuit catalytic kinetics, whenever available. Table 4.3 is useful for extracting the promotional rules discussed Chapter 6. 

C.G. Vayenas, S. Brosda, and C. Pliangos, Rules and Mathematical Modeling of Electrochemical and Chemical Promotion 1. Reaction Classification and Promotional Rules,/. Catal., in press (2001). 

Consequently the generalized promotional rules (6.11) and (6.12) can also be written as ... 

The above global and local promotional rules suggest, in a straightforward manner, the following three practical rules for promoter selection with respect to rate maximization911 ... 

Needless to remind that the above practical promotional rules are applicable for modest (e.g. <0.2) coverages of the promoting species so that site-blocking by the promoter does not become the dominant factor limiting the catalytic rate. 

Tables 6.2 to 6.9 summarize all local (Table 6.2), global (Tables 6.3 to 6.7), fundamental (Table 6.8) and practical (Table 6.9) promotional rules. Tables 6.6 and 6.7 provide some obvious extensions to monomolecular reactions, also in good agreement with experiment. All the rules can be summarized by the inequalities 6.11, 6.12 and 6.13 ...

Table 6.2. Local Electrochemical Promotion Rules (Langmuir-Hinshelwood mechanisms)...

Table 6.3. Global Electrochemical Promotion Rules - Rules G1 G2...

In the previous section it was shown that the global electrochemical promotion rules G1 to G7 cover not only all cases of electrochemical promotion studied up to date but also the general trends observed with classical chemical promotion. 

The crucial task remains of examining to what extent it can also describe the effect of promotion, electrochemical or classical, on catalytic reaction kinetics. More specifically we will examine to what extent it can predict the four main types of global r vs O dependence and all the associated local and global electrochemical and chemical promotional rules. 

Metcalfe107,108 has recently modeled electrochemical promotion using 02 conductors and derived108 equation (6.66) using transition state theory and the concept of a partially charged transition state.108 Despite this interesting theoretical study,108 which is consistent with the basic experimental electrochemical promotion observations Eqs. (4.49) and (4.50) little is still known, experimentally or theoretically about the parameter AR and its possible relationship to A and Aa. Consequently, and in order not to introduce adjustable parameters, we will set XR equal to zero in the subsequent analysis and will show" that it is possible to derive all local and global promotional rules in terms of only four parameters... 

The mathematical model of equations (6.63) to (6.65) is in excellent qualitative agreement with experiment as shown in Figures 6.18 to 6.25. It describes in a semiquantitative manner all electrochemical promotion studies up to date and predicts all the local and global electrochemical and classical promotion rules LI, L2 and G1 to G7. 

The excellent prediction by the model of all global promotion rules is not only qualitative. The predicted p values ( 102 for IV variation in UWR, Fig. 6.18a) is in excellent agreement with experiment (e.g. Fig. 4.24). Also the pmax values ( 10-20) predicted for volcano and inverted volcano behaviour are in very good agreement with experiment. Finally the Xd, Xa which are used ( 0.15) are physically very reasonable. For example for Uwr-1 V at 673 K it is EM7, thus the XD and X.A values used in the simulations, which are physically very reasonable, give exp(Xjn), and thus p, values between 10 2 and 102 in good qualitative agreement with experiment. 

This is a typical example of purely electrophilic behaviour and of global promotional rule G2. As shown in Figure 8.20 the rate is positive order in P02 for po2<4 kPa and negative order in pC3H6 f°r Pc3H6>0-2 kPa. As expected from rule G2 (dr/dPA>0, dr/d D<0) the reaction, within this gaseous composition, exhibits purely electrophilic behaviour (Fig. 8.21) with p values up to 6 and -A values up to 3000. 

As shown on this Figure and also in Fig. 8.29 increasing Uwr and O above their open-circuit potential values leads to a local "volcano , i.e. the rate goes through a maximum. This is consistent with the global promotional rule G3 and the observed rate dependence on pco/po2 (Fig- 8.30) where it is interesting to observe that the rate maximum is only moderately affected by the applied potential. 

The observed inverted volcano behaviour, as well as the kinetic behaviour are qualitatively similar to the case of CO oxidation on Ag/YSZ (Figs. 8.34 and 8.35)2 38 and thus present a nice example of promotional rule G4. Similar is the electrochemical promotion behaviour of CO oxidation on Au which has also been studied by Sobyanin and coworkers.40 41... 

The reaction was investigated under atmospheric pressure and at temperatures 500°C to 600°C, where the only product was CO, as Pd, contrary to Rh, does not adsorb C02 dissociatively.59 This difference in reaction pathway is also reflected in the NEMCA behaviour of the system, since in the present case CO formation is enhanced (by up to 600%) not only with decreasing catalyst potential and work function, but also enhanced, although to a minor extent, via catalyst potential increase (Fig. 8.56). Enhancement factor A values up to 150 were measured. The reaction exhibits typical inverted volcano behaviour, which is characteristic of the weak adsorption of the reactants at the elevated temperature of this investigation, and thus of promotional rule G4. 

This is a truly exciting electrochemical promotion system which can serve as an excellent example for illustrating the two local and three of the four global promotional rules described in Chapter 6. The reason is that under open-circuit conditions the reaction is positive order in both reactants, as can be seen in subsequent figures. 

It is also worth noting the transition from volcano-type kinetics to s-shape type kinetics with respect to pC2H4 with decreasing potential (Fig. 8.73). Equally good agreement with the generalized promotional rule (Eq. 6.12) is shown by the kinetics with respect to po2 (Fig. 8.74)... 

In a more recent study19 Harkness and coworkers found that at lower temperatures where the r vs PC2H4 dependence exhibits a maximum1,19 the r vs Uwr (thus O) dependence also exhibits a maximum (volcano behaviour) in excellent agreement with the global promotional rule G3 discussed in Chapter 6. 

The kinetics depicted in Figures 9.4 in conjunction with Figure 9.5 and 4.30 provide an excellent example of promotional rules L2, and G2 (electrophilic behaviour), as well as rule G3 (volcano type behaviour). As long as the rate is negative order in C2H4 and positive order in po2 (Fig. 9.4)... 

As already discussed in Chapter 6 (Figure 6.25) the observed complex rate dependence of CO oxidation on pco, P02 and UWR (O) (Figs. 4.16, 4.31, 9.6 and 9.7) can be described in a semiquantitalive fashion by the effective double layer model presented in Chapter 6. The system provides an excellent paradigm of the promotional rules Gl, G2 and G3 which are summarized by the general inequalities (6.11) and (6.12) written specifically here for the CO oxidation system ... 

The kinetics depicted in Figures 9.13 and 9.14 are extremely instructive and provide a classical example of global promotional rule G2 (electrophilic behaviour). 

As already discussed in Chapter 6, Figure 9.19 provides a nice example of global promotional rule Gl The rate is clearly positive order in the electron donor C6H6 (Fig. 9.19) and at the same time is enhanced with increasing O (electrophobic behaviour). 

Then analyze Table A. 1 on the basis of the local and global promotional rules of Chapter 6. 

This is a direct consequence of global promotional rule Gl An electronegative promoter (e.g. O2) will enhance the catalytic rate only when the catalyst surface is predominantly covered by an electron acceptor reactant (e.g. O). If there is little or no O on the surface then 02 will act as a reactant (Faradaic behaviour) and not as a promoter. 

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