Electrophobic Reactions

Both C2H4 oxidation on Pt/YSZ (Fig. 4.13) and C2H4 oxidation on Rh/YSZ (Fig. 4.14) are electrophobic reactions, i.e. the rate r is an increasing function of catalyst potential UWr. They are therefore enhanced with positive currents (I>0) which leads to an increase in UWr- As we will see soon this is one of the four main types of experimentally observed r vs UWr behavior. 

Depending on the rate behaviour upon variation of the catalyst potential UWr and, equivalently work function , a catalytic reaction can exhibit two types of behaviour, electrophobic or electrophilic. These terms, introduced since the early days of electrochemical promotion, are synonymous to the terms electron donor and electron acceptor reaction introduced by Wolkenstein113 in the fifties. Electrochemical promotion permits direct determination of the electrophobicity or electrophilicity of a catalytic reaction by just varying UWr and thus 0. 

A typical example of an electrophobic reaction is the oxidation of C2H4 on Pt4,59 (Fig. 4.13), Rh50 and Ag11,12,49,77 under fuel-lean conditions.59... 

Many reactions exhibit both electrophobic and electrophilic behaviour over different UWr and O ranges leading to volcano-type51 (Fig. 4.16) or inverted-volcano-type (Fig. 4.25) behaviour.70... 

Typical examples of electrophobic reactions are shown on Fig. 4.28 for the catalytic oxidation ofC2H4 and ofCH4 on Pt/YSZ. As also shown in this figure, increasing also causes a linear variation in activation energy Ea ... 

Thus in Table 4.3 we add to Table 4.2 the last, but quite important, available piece of information, i.e. the observed kinetic order (positive order, negative order or zero order) of the catalytic reaction with respect to the electron donor (D) and the electron acceptor (A) reactant. We then invite the reader to share with us the joy of discovering the rules of electrochemical promotion (and as we will see in Chapter 6 the rules of promotion in general), i.e. the rules which enable one to predict the global r vs O dependence (purely electrophobic, purely electrophilic, volcano, inverted volcano) or the basis of the r vs pA and r vs pD dependencies. 

These features are common regardless of the type of solid electrolyte and promoting ion used. It is also in general noteworthy that the electrophobicity or electrophilicity of a reaction studied under the same experimental conditions sometimes changes upon changing the solid electrolyte. 

Nevertheless there are some reactions which never change. Thus NO reduction on noble metals, a very important catalytic reaction, is in the vast majority of cases electrophilic, regardless of the type of solid electrolyte used (YSZ or P"-A1203). And practically all oxidations are electrophobic under fuel lean conditions, regardless of the type of solid electrolyte used (YSZ, p"-Al203, proton conductors, even alkaline aqueous solutions). 

AEact aactAO OLac O for electrophobic reactions aact>0 for electrophilic reactions (5.73)... 

ELECTROPHOBIC, ELECTROPHILIC, VOLCANO AND INVERTED VOLCANO REACTIONS RATIONALIZATION, RULES, AND PREDICTIONS... 

Figure 6.2. (Top) Definitions of local electrophobic and local electrophilic behaviour for two reactions exhibiting global volcano-type behaviour (a) and global inverted-volcano-type behaviour (b). (Bottom) Corresponding variations in surface coverages of adsorbed electron donor (D) and electron acceptor (A) reactants. As shown in this chapter volcano-type behaviour corresponds in general to high reactant coverages, inverted-volcano-type behaviour corresponds in general to low reactant coverages.

The mles of electrochemical promotion follow directly from Table 6.1 For example, as shown in Table 6.1 all purely electrophobic reactions are positive order in D and zero or negative order in A. All purely electrophilic reactions are positive order in A and zero or negative order in D. Volcano-type reactions are always positive order in one reactant and purely negative order in the other. Inverted volcano-type reactions are positive order in both reactants. 

Inspection of Table 6.1 shows the following rule for electrophobic reactions Rule Gl A reaction exhibits purely electrophobic behaviour ((dr/dO)PA 0) when the kinetics are positive order in the electron donor (D) reactant and negative or zero order in the electron acceptor (A) reactant. 

Rule Gl A reaction exhibits purely electrophobic behaviour ((drld >)PA0) when the electron acceptor reactant (A) is strongly adsorbed and much more strongly adsorbed on the catalyst surface than the electron donor reactant (D). 

Rule G6 A monomolecular reaction is electrophobic for an electron donor adsorbate and electrophilic for an electron acceptor adsorbate. 

Another, and simpler, manifestation of rule Gl coming from the classical promotion literature is shown in Fig. 6.13. The rate of the oxidative dehydrogenation of C3H8 to C3H6 is first order in propane and near zero order in 02.84 As expected from rule Gl the reaction exhibits electrophobic behaviour. 

Rule LIWhen the rate is negative or zero order in the electron acceptor A and positive order in the electron donor D then the reaction exhibits electrophobic behaviour. 

Figure 6.19. Model predicted electrochemical promotion kinetic behaviour (a) and (b) electrophobic reaction, (c) and (d) electrophilic reaction.

In the former case (Xo>0) the r vs behaviour is electrophobic and the reaction order with respect to pD decreases with increasing [Pg.322]

Figure 6.21. Model predicted electrochemical promotion behaviour for a monomolecular reaction (a) electrophobic (b) electrophilic.

In summary the oxidation of C2H4 on Pt is one of the most thoroughly studied reactions from the point of view of NEMCA and, in view of its rather simple mechanistic scheme, one of the most thoroughly understood systems. Under fuel-lean conditions the reaction is a classical example of global promotional mle Gl, i.e. electrophobic behaviour. 

The oxidation of C2H4 to CO2 on Rh has been investigated13 at temperatures 300° to 400°C. The reaction exhibits very strong electrophobic behaviour and the rate can be reversibly enhanced by up to 10,000% by... 

It is worth noting that below the isokinetic point (T < T ) the reaction exhibits electrophobic behaviour, i.e. dr/dUwR > 0, while for T > T the reaction becomes electrophilic. At T = T the NEMCA effect disappears (see also the curve for T=370°C in Fig. 8.6). 

The oxidation of C2U4 on Ru02 deposited on YSZ was studied by Wodiunig20,21 at temperatures 250°C to 450°C. The reaction exhibits specta-cular electrophobic behaviour with p values up to 115 and A values up to 4400. 

Thus for low PC2H4/P02 ratios the rate is first order in C2H4 and zeroth order in 02. In excellent agreement with global rule Gl, the reaction exhibits pronounced electrophobic behaviour (Fig. 8.15). The p value of 115 is the highest reported so far for an oxidation reaction. 

The effect of catalyst overpotential and potential on the rates of these two reactions is shown in Figs. 8.45 and 8.46. They both exhibit electrophobic behaviour for Uwr>U r and electrophilic behaviour for UWR< U, i.e. the reaction exhibits pronounced inverted volcano behaviour. 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

Figure 8.68 shows a typical galvanostatic transient under oxidizing gaseous conditions. The reaction rate is enhanced by a factor of 20 (p=21) and the faradaic efficiency A (=Ar/(I/2F)) is 1880. The behaviour is clearly electrophobic (dr/dV Xi) and strongly reminiscent of the case of C2H4 oxidation on Pt/YSZ (Fig. 4.13) with some small but important differences ... 

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