Electrocatalysis

4 Electrocatalysis. There is a particular type of ECE electrode reaction mechanism which is designated as liCE (the arrows indicate that the second electron transfer consists of a inverse reaction with respect to the first). This process is called electrocatalysis and is of importance in inorganic chemistry.13 

Before proceeding it is necessary to clarify the terminology of two fairly similar processes, at least as far as their nomenclature is concerned redox catalysis and electron-transfer chain catalysis. 

Redox catalysis, which will not be discussed in detail herein, consists of lowering the kinetic barrier of the reduction (or the oxidation) process of a species, which thermodynamically is little inclined to be reduced (or oxidized), by use of a redox mediator. The latter has the role of carrying electrons towards (or away from) the low redox-active original species. 

If we consider a particular species Ox, which is difficult to reduce, the above process can be described by the following scheme  

By applying a potential to the electrode equal to the reduction potential of the catalyst (the redox mediator) the catalyst is reduced, but, upon contact with the oxidized form Ox, a redox reaction takes place in which Ox is reduced to Red and the mediator reoxidized. At this point the continuous cathodic reduction of the catalyst reactivates the whole process and the catalytic cycle is repeated. 

The science of electrocatalysis provides the connection between the rates of electrochemical reactions and the bulk and surface properties of the electrodes on which these reactions proceed. 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

In redox reactions, the electrode is not inert in the full meaning of the term. It serves not only to feed current through the electrolyte but also acts as a catalyst (as a catalytic electrode ) determining the rates and special features of electrochemical reactions occurring at its surface. 

The degree to which an electrode will influence the reaction rates is different for different electrochemical reactions, hi complex electrochemical reactions having parallel pathways, such as a reaction involving organic substances, the electrode material might selectively influence the rates of certain individual steps and thus influence the selectivity of the reaction (i.e., the overall direction of the reaction and the relative yields of primary and secondary reaction products). 

Historically, electrocatalytic science developed from investigations into cathodic hydrogen evolution, a reaction that can be reahzed at many metals. It was found in a number of studies toward the end of the nineteenth century that at a given potential, the rate of this reaction differs by severaf orders of magnitude between metals. In one of the first theories of hydrogen evofution, the recombination theory of hydrogen overvoltage, the rate of this reaction was finked directfy to the rate of the catalytic 

The whole discipline of electrocatalysis emerged in this context mainly in relation to a vivid interest in electrochemical energy conversion and it is at present one of the most important areas of electrochemical research. 

This resulted in a need for appropriate characterization of the structural and electronic properties of electrode surfaces and detection of adsorbed intermediates in electrode reactions. 

Several factors have contributed to this goal in the recent past development of electrochemical techniques for the study of complex reactions at solid electrodes, use of physical methods such as ESCA, Auger, LEED, etc. for the study of surfaces in the ultrahigh vacuum (UHV) environment and in situ techniques under the same conditions as the electrode reaction. Ellipsometry, electroreflectance, Mossbauer, enhanced Raman, infrared, electron spin resonance (ESR) spectroscopies and measurement of surface resistance and local changes of pH at surfaces were incorporated to the study of electrode kinetics. 

Electrode reactions offer the possibility of fine control of the kinetics through the electrode potential, but unfortunately they lack selectivity. For instance, if the potential of a mercury electrode is poised at a sufficiently negative value in a solution containing several metal ions such as Pb2+, Tl+, Cu2+, etc., all of them will be deposited. 

An electrocatalytic reaction is an electrode reaction sensitive to the properties of the electrode surface. An electrocatalyst participates in promoting or suppressing an electrode reaction or reaction path without itself being transformed. For example, oxygen reduction electrode kinetics are enhanced by some five orders of magnitude from iron to platinum in alkaline solutions or from bare carbon to carbon electrodes modified with Fe phthalocyanines or phenylporphyrins. For a comprehensive discussion of the subject, the reader is referred to refs. (76, 95, and 132-136). 

The use of nanoparticles for electrocatalysis has been vell documented [40a, 199a-d]. Improving the performance of electrochemical energy devices has driven the research on the use of nanoparticles for catalyzing various redox reactions vith the aim of using them in fuel cells. Since the literature on this topic is vast, we have attempted to summarize only the recent literature on the electrocatalysis of certain reactions. Excellent revie vs on various aspects of electrocatalysis are already available [40a, 199a]. 

The high dispersity inside the nano-honeycomb matrix and the high surface area of the nanopartides leads to very good electrocatalytic activity. The electrocatalytic activities of nanosized platinum particles for methanol, formic add and formaldehyde electrooxidation have been recently reported [215]. The sensitivity of the catalyst particles has been interpreted in terms of a catalyst ensemble effect but the detailed microscopic behaviour is incomplete. Martin and co-workers [216] have demonstrated the incorporation of catalytic metal nanopartides such as Pt, Ru and Pt/Ru into carbon nanotubes and further used them in the electrocatalysis of oxygen reduction, methanol electrooxidation and gas phase catalysis of hydrocarbons. A related work on the incorporation of platinum nanopartides in carbon nanotubes has recently been reported to show promising electrocatalytic activity for oxygen reduction [217]. 

Just as in chemical catalysis, electrocatalysis provides a reaction path which lowers the energy of activation (see Fig. 3.1) and hence increases the rate of reaction, i.e., the current density for a given overvoltage. We can distinguish between the situation when a species in solution acts as a catalyst and a process where the reactive intermediate has to be adsorbed on the electrode. We have already met an example of the former in Section 3.1.3, namely the catalysis of the epoxidation of propylene by means of bromide ions. 

Fletcher, in his interesting book. Industrial Electrochemistry, demonstrates reasoning that can sometimes lead to a decision on the most probable reaction path when an adsorption step is involved. Electrocatalysis is of major importance industrially. Use of dimensionally stable anodes in the chlorine industry is a good example. 

In the context of electrode reactions, a reaction model is an expression for the dependence of current density on reactant concentration, electrode potential, and mass transfer coefficient. One or two other variables may also be important, such as temperature or pH, A reaction model is different from a reaction mechanism in that all that is needed for the model is an expression for the current density as a function of several parameters. Often it is not necessary to consider some very short-lived reaction intermediates which are an essential part of a reaction mechanism. As we shall see in the two examples given at the end of this chapter, it is often necessary to group reaction steps and resulting products together otherwise the complexity of the reaction model will detract from its usefulness. 

We will derive a typical expression for a reaction model and then consider ways of obtaining the necessary numerical values for the constants and variables in the expression. The chapter concludes with two examples of reaction models, one for an inorganic and the other for an organic. 

Many electrode reaettoi only occur at a measurable rate at very high overpotentials, i.e. the exchange current density is low. The art of electrocatalysis is to provide alternative reaction pathways which avoid the slow step and permit the reaction to be carried out with a high current density close to the reversible potential i.e. to increase the exchange current density. 

In general the catalyst may be an adsorbed or a solution-free species. Although there are many examples of the latter (e.g, the epoxidation of propylene catalysed by 6r ) 

Such reactions arc probably better classilied as indirect electrode reactions (Chapter 6). Only reactions catalysed by adsorbed species will be conaidmd here. Certainly, many such reactions (c.g. CI21 O2 and H2 evolution) are very important in electroch ical technology. 

The hydrogen evolution reaction is historically very important since its study has contributed much towards our iind ttanidlQg of electrode reactions. It is 

This discussion will assume an acidic medium, although the modifications to include higher pH are obvious. The adsorbed hydrogen atom is formed by the reaction  

The application of modified electrodes can be exploited in such technologies as energy storage, microelectrochemical devices, supramolecular chemistry, elec-trochromic displays, electrocatalysis, solar energy conversion and electroana-lysis.  

Attachment of polymeric films to electrode surfaces for electroanalysis is usually undertaken to achieve at least one or a combination of the following goals. To achieve these objectives modifiers with electrocatalytic, molecular recognition, preconcentration, or permselective properties are applied to electrode surfaces. 

Enhanced signals over unmodified electrodes or generation of a signal where none was previously detected 

Increased stability of the electrode and reproducibility toward detection of the analyte of interest 

Initial research into the application of PMEs focused on their potential use in electrocatalysis. Much of this work centered on preformed redox polymers containing coordinated electroactive metal complexes because of the synthetic flexibility and the ability to control loading of the electrocatalytic center in the modifying layer. Electrostatic binding of electroactive ions into ionomeric polymer films is a convenient procedure for preparing electrocatalytic layers, although care must be taken to minimize leaching of the electroactive center.  

This chapter provides a critical review of transition metal macrocycles, both in intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. Fundamental aspects of electrocatalysis, oxygen reduction and transition metal macrocycles will be highlighted in this brief introduction, which should serve as background material for the subsequent more specialized sections. 

The kinetics of electrochemical reactions are often modified by the nature of the electrode material, and by the presence of atomic and molecular species either adsorbed on the surface or in the bulk solution [14]. Electrocatalysis is primarily concerned with the study of this phenomenon and, particularly, with the factors that govern enhancements in the rates of redox processes. Implicit in this general statement is the ability of the species responsible for these effects, or electrocatalyst, or the electrode itself, to carry out the reaction numerous times before undergoing possible deactivation. Electrocatalytic processes in which the electrode simply serves as a source or sink of electrons to generate solution phase species that 

The potency of an electrocatalyst is usually defined in terms of the potential required to carry out a specific process at a prescribed rate, or current, per unit area of electrode. Platinum, for example, promotes hydrogen evolution and hydrogen oxidation in aqueous electrolytes, at very high rates at potentials very close to the thermodynamic redox potential for the reaction H+(aq) I e - - ViH2(g), that is, small overpotentials, T. Hence, it is a far more potent electrocatalyst than, for example, Hg or carbon, for which the onset for either reaction occurs at potentials far removed from that predicted value, that is, large T.  

The overall reaction is unlikely to be reversible and coverages by each species will be determined by kinetic rather than thermodynamic factors. Even so such dissociative adsorption processes are very important and are at the heart of the electrocatalysis necessary for fuel cells because the direct loss of an electron from potential fuels always requires a substantial overpotential. 

The coverage by organic fragments cannot be described in terms of isotherms (since the adsorption is not an equihbrium process) and, indeed, it is usually difficult to identify with certainty the structure of the adsorbed species. The information available is generally deduced from measurement of the charges for 

According to the Buder-Volmer equation, the dependence of q on 7 is linear in a range of few millivolts around the reversible electrode potential, whereas it becomes logarithmic at q 50-100 mV away from equilibrium conditions, depending on the degree of reversibility of the specific electrode reaction  

Equation (7.17) is the Tafel equation and expresses the way in which the applied potential difference operates to enhance the reaction rate [22]. Since the unit of q is volts, the units of a and b are also volts a is called the Tafel intercept, that is, the overpotential at 7 = 1 (which depends on the units of 7, A or mA or pA) b is known as the Tafel slope, that is, the variation of q per decade of current. 

The current 7 is an extensive quantity, in that it depends on the size of the electrode. For this reason, the reaction rate is conveniently referred to the unit surface area (7/S=j, current density). Even so, the current density continues to be an extensive quantity if referred to the geometric (projected) surface area since electrodes are as a rule rough and the real surface does not coincide with the geometric surface [23]. Conversely, b is an intensive quantity, in that it depends only on the reaction mechanism and not on the size of the electtode. The term b is the most important kinetic parameter in electrochemistry also because of the easy and straightforward procedure for its experimental determination. Most electrode mechanisms can be resolved on the basis of Tafel lines only. 

The targets of electrocatalysis are at the basis of recent developments in the field of water electrolysis. First, it is necessary to distinguish between materials evaluation and materials selection. The former is the search for materials with better and better properties for the wanted electrode process. The latter implies global considerations of applicability. This is probably what makes academic research differ from R D. The former is favored by scientifically exciting performance, in the latter it is necessary to find a compromise between, for instance, activity and stability or between efficiency and economic convenience. 

Point (i) above implies the search for new procedures to prepare a given electrode material or new composite materials to look for possible synergetic effects. 

Ni-NiOH2 composite electrode is fully determined by the NiOH2 particles. These two examples show that composites can be suitable electrocatalytic electrodes. Though it is clear that more research has to be done to find out the full potential of electrochemical composite deposition in the preparation of electroactive electrodes. 

Corresponding to this is the idea of biosensors that could be implanted in the body for the electroanalysis of conceivably any chemical in the body. Thus, it may become possible to adsorb enzymes on the surface of electrodes and then tune these enzymes to react with appropriate biomolecules, as represented in Fig. 4.114. How would conducting polymers figure in such devices They might be useful as the biosensor itself, since, being organic, they are more likely to interact positively with enzymes and biochemicals than metal electrodes would. 

Electrocatalytic processes have received considerable attention in the last decades because of their application in synthesis and sensing. The term catalysis is used for describing the modification in the reaction rate of a given chemical reaction by effect of the addition of a catalyst species. Two essential conditions have to be accomplished by catalytic processes the thermodynamics of the reaction becomes unaltered, and the catalyst stays unchanged. Additionally, a common demand for catalysis is that the catalytic process involves small concentrations of the catalyst. In the most general view, the rate of the reaction can be either increased (positive catalysis) or decreased (negative catalysis), although, obviously, positive catalysis is preferentially desired. 

The term electrocatalysis will be used in the following for designing electrochemical processes involving the oxidation or reduction of a substrate species, S, whose reaction rate is varied in the presence of a given catalytic species. Cat. The effect of electrocatalysis is an increase of the standard rate constant of the electrode reaction resulting in a shift of the electrode reaction to a lower overpotential at a given current density and a current increase. The faradaic current resulting from the occurrence of a catalytic electrode mechanism is called catalytic current. For a positive electrocatalysis, the current obtained in the presence of the catalyst must exceed the sum of the currents obtained for the catalyst and the substrate, separately, under selected experimental conditions (Bard et al., 2008). Three possible situations can be discerned  

The catalyst and the substrate are in the same phase, usually dissolved in the bulk solution (homogeneous catalysis). 

The catalyst or the substrate is immobilized (via fixation, functionalization, adsorption, etc.) at the electrode surface (heterogeneous catalysis). 

The catalyst is electrochemically generated at the electrode/electrolyte interface. 

During the last few decades Pt-based nanocatalysts have been introduced to Fuel Cell Technology to improve the exploitation of the expensive noble metal component via a substantial enhancement of the active catalyst surface using particles in the 2-5 nm range. [143, 144]. In essence, a fuel cell is an electrochemical reactor 

Size- and shape-controlled cubo-octahedral platinum(o) nanoparticles of 4nm average size stabilized by sodium polyacrylate showing 111 and 100 surfaces were used to prepare Vulcan supported electrocatalysts. Cyclic voltammetric CO oxidation studies carried out by the thin film rotating disc method show two different sites of CO oxidation (Fig. 2.19). This can be assigned to differences in the activity of the crystal surfaces and is in agreement with single crystal studies. TEM results after cyclic voltammetric characterization show a complete absence of agglomerations. 

Theoretical calculations have proven to be very usefijl in designing efficient FC catalysts based on high-throughput experiments generating trimetaUic nanosystems having PtRu alloyed with Co, Ni, or W [168]. For ejample, the activity trend observed with real Pt, PtRu, PtRuNi, and PtRuCo catalysts generated by sputtering techniques corresponds exactly to the trends predicted by theory (see Fig. 2.20(a) and (b)). 

In the following sections three typical examples of our nanochemical methodology for the preparation of advantageous fuel cell catalysts are discussed in the form 

The two bottom layers represent Ru atoms while Pt atoms form the top surface atoms. The predicted surface activities of various ternary alloys are shown in the plot (b). [Reproduced with permission from the American Chemical Society (168)]. 

The Chlorine Evolution Reaction. The anodic reaction of most interest to the chlor-alkali industry is the chlorine evolution reaction. The thermodynamics of the Cl2-Cl reaction at equilibrium 

Several reviews addressing the polarization behavior. CP adsorption, competition between CP adsorption and OH codeposition, oxide film formation and CP ion discharge, as well as the kinetic aspects on oxide-covered and oxide-free surfaces have been published. Section 4.5 reviews the mechanistic aspects of chlorine evolution, focusing on the nature and characterization of the adsorbed intermediates [26,27]. 

The chlorine evolution reaction proceeds on an oxidic surface covering the base metal substrate. Denoting the substrate covered with an oxide film as S-O, the various 

The observation f the negative reaction oirder for the chlorine evtddtidft ifeadtiOh With respect to the H ions is a que ttefiti iilssue [33], 

Irrespective (rf hilndh stAibme (SMSS) fs tte irite ooiiiw oiltting step idaiia  

Carbon shows reasonable electrocatalytic activity for oxygen reduction in alkaline electrolytes, but it is a relatively poor oxygen electrocatalyst in acid electrolytes. A detailed discussion on the mechanism of 

The rate and mechanism are different on the basal plane and edge sites of carbon. The reactions involving oxygen are two to three orders of magnitude slower on the basal plane than on the edge sites, because of the weak adsorption of oxygen molecules on the basal plane surface [34J. 

The overpotentials for oxygen reduction and evolution on carbon-based bi-functional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize 

In redox flow batteries such as Zn/Cl2 and Zn/Bij, carbon plays a major role in the positive electrode where reactions involving Clj and Bfj occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br, in graphite electrodes. The reversible redox potentials for the CI2 and Br2 reactions [Eq. (8) and (9)] 

In the Zn/Cl2 battery, carbon is utilized in both electrodes, serving as a flowthrough positive electrode and a substrate 

The catalysis of electrode reactions is possible both by species attached to the electrode surface and by species dissolved in the electrolyte. Examples of 

The oxidation of propylene to propylene oxide is probably unknown as a direct electrode reaction, but in the presence of bromide ion in slightly alkaline solution, say pH 9, the conversion is possible in high yield by the route 

The term electrocatalysis is, however, more commonly applied to systems where the oxidation or reduction requires bond formation, or at least a strong interaction of the reactant, intermediates, or the product with the electrode surface. The catalyst is the electrode material itself or a species adsorbed from solution. This chapter will discuss this more limited definition of electrocatalysis (note also that simple electron transfer reactions which are pictured as occurring by an outer sphere mechanism and may have very high exchange current densities, are not normally considered within electrocatalysis — in this book they are dealt with in Chapter 3). 

How may electrode reactions which involve surface chemistry be recognised in the laboratory Most obviously, the I-E characteristics will depend very strongly on the choice of electrode material, and with some electrodes, at least, the reaction will occur up to several volts away from the reversible potential. More detailed analysis of the EE data will reveal that  

however, possible to identify several types of electrocatalysts, for example 

A detailed discussion on the mechanism of oxygen reduction and evolution on carbon was presented by Kinoshita [1]. The experimental studies suggest that oxygen reduction in alkaline electrolytes is first order in O2 concentration. There is evidence that the reaction mechanism is not the same on different carbon electrodes, as illustrated by Equations 10.2-10.7 for graphite and carbon black. Graphite  

In the Zn/Ch battery, carbon is utihzed in both electrodes, serving as a flow-through positive electrode and a substrate for the zinc negative electrode. The requirements are listed below. Chlorine, flow-through electrode  

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One factor contributing to the inefficiency of a fuel ceU is poor performance of the positive electrode. This accounts for overpotentials of 300—400 mV in low temperature fuel ceUs. An electrocatalyst that is capable of oxygen reduction at lower overpotentials would benefit the overall efficiency of the fuel ceU. Despite extensive efforts expended on electrocatalysis studies of oxygen reduction in fuel ceU electrolytes, platinum-based metals are stiU the best electrocatalysts for low temperature fuel ceUs. 

M. A. Gutjahr, in G. Sandstede, ed.. From Electrocatalysis to Fuel Cells, Univ. of Washington Press, Seattie, 1972, pp. 143—156. 

Oxidation can also occur at the central metal atom of the phthalocyanine system (2). Mn phthalocyanine, for example, can be produced ia these different oxidation states, depending on the solvent (2,31,32). The carbon atom of the ring system and the central metal atom can be reduced (33), some reversibly, eg, ia vattiag (34—41). Phthalocyanine compounds exhibit favorable catalytic properties which makes them interesting for appHcations ia dehydrogenation, oxidation, electrocatalysis, gas-phase reactions, and fuel cells (qv) (1,2,42—49). 

Electrodes. At least three factors need to be considered ia electrode selection as the technical development of an electroorganic reaction moves from the laboratory cell to the commercial system. First is the selection of the lowest cost form of the conductive material that both produces the desired electrode reactions and possesses stmctural iategrity. Second is the preservation of the active life of the electrodes. The final factor is the conductivity of the electrode material within the context of cell design. An ia-depth discussion of electrode materials for electroorganic synthesis as well as a detailed discussion of the influence of electrode materials on reaction path (electrocatalysis) are available (25,26). A general account of electrodes for iadustrial processes is also available (27). 

EC mechanism, 34, 42, 113 E. Coli, 186 Edge effect, 129 Edge orientation, 114 Electrical communication, 178 Electrical double layer, 18, 19 Electrical wiring, 178 Electrocapillary, 22 Electrocatalysis, 121 Electrochemical quartz crystal, microbalance, 52 Electrochemihuiiinescence, 44 Electrodes, 1, 107... 

G. Jarzabek and Z. Borkowska, in Electrocatalysis, P. Novae, A. Pomianowski, and J. 

Conducting polymers have found applications in a wide variety of areas,44 45 and many more have been proposed. From an electrochemical perspective, the most important applications46 appear to be in batteries and supercapacitors 47,48 electroanalysis and sensors49-51 electrocatalysis,12,1, 52 display and electrochromic devices,46 and electromechanical actuators.53... 

Theoretical aspects of mediation and electrocatalysis by polymer-coated electrodes have most recently been reviewed by Lyons.12 In order for electrochemistry of the solution species (substrate) to occur, it must either diffuse through the polymer film to the underlying electrode, or there must be some mechanism for electron transport across the film (Fig. 20). Depending on the relative rates of these processes, the mediated reaction can occur at the polymer/electrode interface (a), at the poly-mer/solution interface (b), or in a zone within the polymer film (c). The equations governing the reaction depend on its location,12 which is therefore an important issue. Studies of mediation also provide information on the rate and mechanism of electron transport in the film, and on its permeability. 

Like other ion-exchange polymers, conducting polymers have been used to immobilize electroactive ions at electrode surfaces. Often the goal is electrocatalysis, and conducting polymers have the potential advantage of providing a fast mechanism for electron transport to and from the electrocatalytic ions. 

Small-Particle Effects and Structural Considerations for Electrocatalysis Kinoshita, K. 14... 

Detailed and shorter39 45 reviews of the electrochemical promotion literature prior to 1996 have been published, mainly addressed either to the catalytic or to the electrochemical community. Earlier applications of solid electrolytes in catalysis, including solid electrolyte potentiometry and electrocatalysis have been reviewed previously. The present book is the first on the electrochemical activation of catalytic reactions and is addressed both to the electrochemical and catalytic communities. We stress both the electrochemical and catalytic aspects of electrochemical promotion and hope that the text will be found useful and easy to follow by all readers, including those not frequently using electrochemical, catalytic and surface science methodology and terminology. 

The reader must have already identified some of the basic concepts which play a key role in understanding the electrochemical activation of heterogeneous catalysis catalysis, electrocatalysis, promotion, electrochemical promotion, spillover, backspillover. It is therefore quite important to define these terms unambiguously so that their meaning is clearly determined throughout this book. 

Electrocatalysis Again by definition, an electrocatalyst is a solid, in fact an electrode, which can accelerate a process involving a net charge transfer, such as e.g. the anodic oxidation of H2 or the cathodic reduction of 02 in solid electrolyte cells utilizing YSZ ... 

G.-Q. Lu, and A. Wieckowski, Heterogeneous Electrocatalysis A Core field of Interfacial Science, Current opinion in Colloid and Interface Science 5, 95 (2000). 

I.V. Yentekakis, Y. Jiang, S. Neophytides, S. Bebelis, and C.G. Vayenas, Catalysis, Electrocatalysis and Electrochemical Promotion of the Steam Reforming of Methane over Ni Film and Ni-YSZ cermet Anodes, Ionics 1, 491-498 (1995). 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and [Pg.306]

The rates of C2H4, C2H6 and C02 formation depend exponentially on UWr and O according to equation (4.49) with a values of 1.0, 0.75 and 0.4, respectively, for I>0, and of 0.15, 0.08 and 0.3, respectively for I<0. Linear decreases in activation energy with increasing have been found for all three reactions.54 It should be emphasized, however, that, due to the high operating temperatures, A is near unity and electrocatalysis, rather than NEMCA, plays the dominant role. 

As shown in Figure 9.31, butane is formed electrocatalytically (Ab t < 1) since no gaseous H2 is supplied, thus Abutis restricted to its electrocatalysis limits (tA negative potential region of electrocatalysis, electrochemically promoted formation of isomerization products continues with large A and p values (Fig. 9.31). 

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