The NEMCA Effect

Following Vayenas et al., the effect of current or potential on the catalysis activity is usually described by two parameters (we consider here the use of an O conductor)  

In the case of a Faradaic effect, all oxygen which is transported electrochemically through the electrolyte reacts at the anode (A = 1). A reaction exhibits the NEMCA effect when A 1. When A 1, the reaction is termed electrophobic (the catalytic reaction is promoted by a positive current or overpotential), while a reaction accelerated by a negative current or potential is termed electrophilic. 

The NEMCA effect was explained by taking into account the increase in the 

An important characteristic of NEMCA is that, for any reaction, the magnitude of I A can be estimated by the equation  

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By 19884 it became obvious that the NEMCA effect, this large apparent violation of Faraday s law, is a general phenomenon not limited to a few oxidation reactions on Ag. Of key importance in understanding NEMCA came the observation that NEMCA is accompanied by potential-controlled variation in the catalyst work function.6 Its importance was soon recognized by leading electrochemists, surface scientists and catalysis researchers. Today the NEMCA effect has been studied already for more than 60 catalytic systems and does not seem to be limited to any specific type of catalytic reaction, metal catalyst or solid electrolyte, particularly in view of... 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

Despite the surprise caused by the first literature reports of such large non-Faradaic rate enhancements, often accompanied by large variations in product selectivity, in retrospect the existence of the NEMCA effect can be easily rationalized by combination of simple electrochemical and catalytic principles. 

C.G. Vayenas, I.V. Yentekakis, S.I. Bebelis, and S.G. Neophytides, In situ Controlled Promotion of Catalyst Surfaces via Solid Electrolytes The NEMCA effect, Ber. Buns. Phys. Chem. 99(11), 1393-1401 (1995). 

In the experiments of Figs. 4.13 and 4.14, the A value at steady-state are 74,000 and 770 respectively. A reaction is said to exhibit the NEMCA effect when A >1. 

Figure 4.23. Comparison of predicted and measured enhancement factor A values for some of the early studies of catalytic reactions found to exhibit the NEMCA effect.1,19 Reprinted with permission from Elsevier Science.1...

As already noted the strength of chemisorptive bonds can be varied in situ via electrochemical promotion. This is the essence of the NEMCA effect. Following initial studies of oxygen chemisorption on Ag at atmospheric pressure, using isothermal titration, which showed that negative potentials causes up to a six-fold decrease in the rate of 02 desorption,11 temperature programmed desorption (TPD) was first used to investigate NEMCA.29... 

I.V. Yentekakis, and S. Bebelis, Study of the NEMCA effect in a single-pellet catalytic reactor, J. Catal. 137, 278-283 (1992). 

Equation (5.18) plays a key role in understanding and interpreting the NEMCA effect and it is therefore important to discuss it in some detail. Equation (5.19) is discussed in detail in Chapter 7 in connection with the absolute potential scale of solid state electrochemistry. 

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 rate of ammonia production was enhanced by more than 1100% in the nitrogen rich regime (Figs 9.33 and 9.34), upon potential application of -IV between the working electrode and the Ag reference electrode. The extent of the NEMCA effect depends strongly on the kinetic regime of the reaction. Very pronounced non-faradaic behavior is observed in the regime 0.33

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The NEMCA effect in aqueous electrochemistry may be of considerable technological value, for example in the electrochemical treatment of toxic organics or the production of useful industrial chemicals. 

The possibility of application of the NEMCA effect in conventional flow reactors and of its extension to oxide catalysts may be of great importance in the future, though both the nature of the migrating, spillover species and their effect on the molecular-scale mechanism require further studies B. Grzybowska-SwierkoszandJ. Haber, Annual Reports on Chemistry, 1994)... 

The observed increase, Ar, in catalytic rate, expressed in mol O/s, exceeds by far the rate, I/2F, of O2- supply to the catalyst electrode. Consequently this is an example of electrochemical promotion, or non-Faradaic electrochemical modification of catalytic activity (the NEMCA effect), which results from the migration of promoting O2- species on the catalyst surface upon positive current application. 

NEMCA effect — The term NEMCA is the acronym of Non-faradaic Electrochemical Modification of Catalytic Activity. The NEMCA effect is also known as electrochemical promotion (EP) or electropromotion. It is the effect observed on the rates and selectivities of catalytic reactions taking place on electronically conductive catalysts deposited on ionic (or mixed ionic-electronic) supports upon application of electric current or potential (typically 2 V) between the catalyst and a second (counter or auxiliary) electrode also deposited on the same support. The catalytic reactants are usually in the gas phase. 

The product selectivity is also significantly affected by the NEMCA effect, which is usually quite reversible, although at lower temperatures a permanent NEMCA effect is often observed [iv, v]. 

Electrochemical and surface spectroscopic techniques [iii, v] have shown that the NEMCA effect is due to electro chemically controlled (via the applied current or potential) migration of ionic species (e.g., Os, NalS+) from the support to the catalyst surface (catalyst-gas interface). These ionic species serve as promoters or poisons for the catalytic reaction by changing the catalyst work function O [ii, v] and directly or indirectly interacting with coadsorbed catalytic reactants and intermediates [iii—v]. 

The magnitude of the NEMCA effect for a given catalytic system is commonly described by two parameters, the rate enhancement ratio, p, (= r/r0, where r and rQ are the electropromoted and unpromoted reaction rate values) and the faradaic efficiency, A, (= (r - r0)/(I/nF)), where I is the current, F is the Faraday constant, and n is the charge of the promoting ion. The magnitude of A can be predicted from the parameter 2Fro/I0, where I0 is the exchange current of the catalyst-support interface [v]. 

During investigation of the NEMCA effect the rates of catalytic reactions were found to depend on catalyst work function, , via the equation In(r/r0) = aty/k T where a is a reaction-specific constant and kB is the... 

The NEMCA effect is closely related to classical promotion and to the phenomenon of metal-support interactions (MSI) with oxide supports and that MSI can be viewed as a self-driven NEMCA microsystem where the promoting O2- ions are thermally migrating from the support to the dispersed catalyst nanoparticles and replenished in the support by gaseous 02 [v]. 

An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers transported from the carrier to the metal particles are oxygen ions, which diffuse to the surface of the metal particles, thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters. Work is currently in progress to elucidate the mechanism of induction of metal-support interactions by carrier doping. 

The phenomenon of EPOC or NEMCA effect was first reported in solid electrolyte systems [23, 195-205], but several NEMCA studies already exist using aqueous electrolyte systems [23, 30, 31,145] or Nafion membranes [23]. The EPOC phenomenon leads to apparent Faradaic efficiencies, A, well in excess of 100% (values up to 105 have been measured in solid-state electrochemistry and up to 102 in aqueous electrochemistry). This is due to the fact that, as shown by a variety of surface science and electrochemical techniques [23, 40, 195-198, 206-209], the NEMCA effect is due to electrocatalytic (Faradaic) introduction of promoting species onto catalyst-electrode surfaces [23, 196], each of these promoting species being able to catalyze numerous (A) catalytic turnovers. 

C. H. Hamann, A. Hamnett, W. Vielstich, Electrochemical modification of catalytic activity in heterogeneous chemical reactions-the NEMCA effect, in Electrochemistry,... 

In the electrochemical conversion of hydrocarbons the NEMCA (non-faradaic electrochemical modification of catalytic activity) effect has been reported frequently over metal anodes [13] and rarely over metal oxide anodes [14]. The NEMCA effect is known to promote the rate of oxidation and, to the knowledges of the authors, such enhancement in catalytic activity is generally observed over the metal anodes which have original catalytic activity, e.g. Pt, Pd, Rh and Ag, and is also observed as a non-linear function of the electric current. In the present study, we observed an almost linear increase of activity with increase in the electric current. Lacking a reference electrode, it is beyond the scope of this work to elaborate on the work function of the anode material. However, it is likely that the contribution of the NEMCA effect is neglisible and the electrochemically generated ooxygen species operates in the partial oxidation of alkanes. 

In the experiment of Fig. 2 the maximum A value is 74,000. A reaction exhibits the NEMCA effect when lAI >1. Depending on the observed sign of A, catalytic reactions are termed electrophobic (A>1) or electrophilic (A<-1) A values ranging from 3x10 [6,9,14,15] and down to -SxlO [9,14,15] have been measured. Relatively safe predictions about the order of magnitude of A can be made as discussed below. 

In Situ Controlled Promotion of Catalysis the NEMCA Effect... 

The measurement of A is important for determining if a reaction exhibits the NEMCA effect, but its magnitude is not a fundamental characteristic of a catalytic reaction, since... 

The NEMCA effect does not appear to be limited to any specific type of catalytic reaction, metal catalyst or electrolyte, particularly in view of the recent demonstration of NEMCA using aqueous electrolytes. The catalyst, however, must be electronically conductive and the only report of NEMCA on an oxide catalyst is for the case of Ir02 which is a metallic oxide. It remains to be seen if NEMCA can be induced on semiconductor catalysts. 

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