Electrophobic and Electrophilic Reactions

Depending on the rate behaviour upon variation of the catalyst potential UWr and, equivalently work function t , 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 catalytic reaction is termed electrophobic1,19,54 with increasing catalyst work function O when its rate increases 

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 

Typical examples of electrophilic reactions are the reduction of NO by ethylene on Pt32 and the CO oxidation on Pt under fuel-rich conditions.51,62 

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Electrophobic and Electrophilic Reactions. - Depending on the rate behavior upon varying the catalyst potential and equivalently work function eO, catalytic reactions are divided in two large groups (Tables 1, 2, and 3). 

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... 

Electrophobic and electrophilic catalytic reactions are similar to electrocatalytic oxidations and reduction reactions, respectively, since in the cases of electrophobic catalytic reaction and electrocatalytic oxidations the rate increases with increasing potential, whereas in the case of electrophilic catalytic reactions or electrocatalytic reductions the rate increases with decreasing potential. 

Some reactions exhibit both electrophobic and electrophilic behavior over different Vwr and e0 ranges, leading to volcano-type or inverted-volcano-type behavior." ... 

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. 

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. 

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

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

This is the first and obvious application of Electrochemical Promotion, which was already proposed in 1992.2 Electrochemical promotion allows one to quickly and efficiently identify the electrophobic or electrophilic nature of a catalytic reaction and thus (Rules G1 to G4, Chapter 6) to immediately decide if an electronegative or electropositive, respectively, promoter is needed on a conventional catalyst. It also allows one to identify the optimal coverage, Op, of the promoting electronegative or electropositive species. 

A reaction exhibits electrochemical promotion when A > 1, whereas electrocatalysis is limited when A < 1. A reaction is termed electrophobic when A > 1 and electrophilic when A < —1. In the former case, the rate increases with catalyst potential, JJ, whereas in the latter case the rate decreases with catalyst potential. A values up to 3 x 105 [23, 210] and p values up to 150 [23] have been found for several systems. More recently, p values between 300 and 1200 [211, 212] have been measured for C2H4 oxidation on Pt. 

These rules have recently been shown to apply to supported heterogeneous catalysts as well, owing to the thermally induced spillover of ions from the support to the metal/gas interface [23] and concomitant establishment of an EDL at the metal-gas interface [23, 27]. According to these rules, a reaction is electrophobic (dr/d< > > 0) when dr/dpD > 0 and dr/dpA < 0 and electrophilic (dr/d< > < 0) when dr/dpu < 0 and dr/dpA > 0. In the former case the rate, is enhanced with electronegative promoters, whereas in the latter case it is enhanced with electropositive promoters. 

A reaction exhibits electrochemical promotion when lAI > 1, while electrocatalysis is limited to I Al < 1. A reaction is termed electrophobic when A > 1 and electrophilic when A < 1. In... 

C2H6 oxidation on Pt has been investigated" in a single-pellet reactor at temperatures between 400 and 500°C. The reaction exhibits electrophobic behavior for positive currents (Vwr > V r ) and electrophilic behavior for negative currents (Vwr < V r ) (Figs. 37 and 38). In the former case, p is up to 20 and A up to 3(X). In the latter case, p is up to 7 and A up to -100. The open-circuit kinetic behavior indicates that the catalyst surface is predominantly covered with oxygen and that the cov-... 

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). 

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

The parameter a in Equation (11.6) is positive for electrophobic reactions (5r/5O>0, A>1) and negative for electrophilic ones (3r/0Oelectrochemical promotion behaviour is frequently encountered, leading to volcano-type or inverted volcano-type behaviour. However, even then equation (11.6) is satisfied over relatively wide (0.2-0.3 eV) AO regions, so we limit the present analysis to this type of promotional kinetics. It should be remembered thatEq. (11.6), originally found as an experimental observation, can be rationalized by rigorous mathematical models which account explicitly for the electrostatic dipole interactions between the adsorbates and the backspillover-formed effective double layer, as discussed in Chapter 6. 

Figure 12.5. Ethylene oxidation on Pt finely dispersed on Au supported on YSZ.7 Effect of the current 1 on x 1, where x is the time constant measured during a galvanostatic transient experiment with I as the applied current x is obtained by fitting either r/r0=exp(-t/x) or l-exp(-t/x) to the experimental data depending on the sign of the current and whether the reaction is electrophilic or electrophobic, (a) Positive values of I for electrophilic (squares, T=371°C, pO2=18.0 kPa, Pc2H4=0-6 kPa) and electrophobic behavior (circle, T=421°C, p02=l 4.8 kPa, Pc2H4 CU kPa) (b) negative currents, electrophilic behavior (T=421°C, p02=14.8 kPa, pC2H4=0.1 kPa. Reprints with permission from Academic Press.

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