NEMCA

Although the term non-Faradaic process has been used for many decades to describe transient electrochemical processes where part of the current is lost in charging-discharging of metal-electrolyte interfaces, in all these cases the Faradaic efficiency, A, is less than 1 (100%). Furthermore such non-Faradaic processes disappear at steady state. Electrochemical promotion (NEMCA) must be very clearly distinguished from such transient non-Faradaic processes for two reasons ... 

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... 

Figure 1.2. Experimental setup used in NEMCA experiments.

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. 

In a typical NEMCA experiment the reactants (e.g. C2H4+02) are co-fed over a conductive catalyst which also serves, at the same time, as the working electrode in a solid electrolyte cell ... 

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. 

It is clear that in case (a) the rate, r, of the catalytic reaction (e.g. CO oxidation) will not be affected while in case (b) the rate increase, Ar, will at most equal I/nF (e.g. direct reaction of O2 with CO). In case (c), however, the new species introduced electrochemically onto the catalyst surface will interact with coadsorbed reactants and will change the catalytic properties of the catalyst surface in an a priori unpredictable manner, which is nevertheless not subject to Faraday s law. Thus in cases (a) and (b) there will be no NEMCA but in case (c) it is entirely logical to anticipate it. Even in case (b) one may anticipate NEMCA, if the product remains on the surface and has some catalytic or promotional properties. 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

Electrochemical promotion (NEMCA) bears several similarities with electrolysis in the sense that potential application controls the rate of a process. This is shown in Fig. 1.6, prepared by N. Anastasijevic, a member of the team which made the first NEMCA observations with aqueous... 

The importance of NEMCA in electrochemistry, surface science and heterogeneous catalysis has been discussed by Bockris,7 Wieckowski,8 Pritchard9 and Haber10 respectively. Electrochemical promotion, or NEMCA, has found its position in recent years as a separate section in practically all new general or graduate level textbooks on electrochemistry13,14 and catalysis.15... 

Electrochemical promotion or NEMCA is the main concept discussed in this book whereby application of a small current (1-104 pA/cm2) or potential ( 2 V) to a catalyst, also serving as an electrode (electrocatalyst) in a solid electrolyte cell, enhances its catalytic performance. The phenomenology, origin and potential practical applications of electrochemical promotion, as well as its similarities and differences with classical promotion and metal-support interactions, is the main subject of this book. 

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. 

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). 

Figure 3.3. Electrode configuration for SEP (a) and for electrochemical promotion (or NEMCA) studies (b). The latter can be carried out using the fuel-cell type configuration (c) or the single chamber type configuration (d).

Also the similarity between the remote control spillover mechanism of Fig. 3.5 and the mechanism of electrochemical promotion (NEMCA) already outlined in Figure 1.4c and thoroughly proven in Chapter 5, should be noted. In electrochemical promotion the solid electrolyte is the donor phase and the conductive catalyst is the acceptor phase, using Delmon s terminology. 

A difference between the two systems is that in NEMCA experiments the spillover-backspillover rate can be accurately measured and controlled by simply measuring the imposed current or potential. Another difference is that in electrochemical promotion experiments backspillover provides a promoting species, not an active site, to the catalyst surface. This latter difference can however be accommodated by a broader definition of the active site . 

Figure 3.6. Spatial variation of the electrochemical potential, jl02-, of O2 in YSZ and on a metal electrode surface under conditions of spillover (broken lines A and B) and when equilibrium has been established. In case (A) surface diffusion on the metal surface is rate limiting while in case (B) the backspillover process is controlled by the rate, I/nF, of generation of the backspillover species at the three-phase-boundaries. This is the case most frequently encountered in electrochemical promotion (NEMCA) experiments as shown in Chapter 4.

A typical experimental setup for NEMCA studies is shown in Figs. 4.1 and 4.2. Two types of catalytic-electrocatalytic reactors can be used ... 

Figure 4.1. Electrode configuration for NEMCA studies using (a) the fuel cell type reactor and (b) the single-chamber type reactor.

Figure 4.4. Scanning tunneling micrograph of the surface of a Pt catalyst used in NEMCA studies Scan size 62 A Vbias=0.5 V, Itunnd=15 nA.1 Reprinted with permission from Elsevier Science.

Figure 4.6.Scanning electron micrographs of a Ag catalyst-electrode deposited on YSZ and used for NEMCA studies10 (a) Top view (b) Cross section of the Ag/YSZ interface. Reprinted with permission from Academic Press.

The deposition of thin conductive oxide films on flat zirconia components has also received considerable attention both for fuel cell applications20 and also for SEP21 and NEMCA studies.22,23 The interested reader is referred to the original references for experimental details. 

Both the counter and the reference electrodes are essential for fundamental NEMCA studies. They need not be of the same material with the catalyst. The counter electrode-solid electrolyte interface does not have to be polarizable. In fact, it is advantageous when it is not, because then most of the applied potential difference ends up as overpotential at the catalyst and not at the counter electrode. 

The reference electrode-solid electrolyte interface must also be non-polarizable, so that rapid equilibration is established for the electrocatalytic charge-transfer reaction. Thus it is generally advisable to sinter the counter and reference electrodes at a temperature which is lower than that used for the catalyst film. Porous Pt and Ag films exposed to ambient air have been employed in most previous NEMCA studies.1,19... 

Since electrochemical promotion (NEMCA) studies involve the use of porous metal films which act simultaneously both as a normal catalyst and as a working electrode, it is important to characterize these catalyst-electrodes both from a catalytic and from an electrocatalytic viewpoint. In the former case one would like to know the gas-exposed catalyst surface area A0 (in m2 or in metal mols, for which we use the symbol NG throughout this book) and the value, r0, of the catalytic rate, r, under open-circuit conditions. 

When one starts NEMCA experiments only r0 is important to measure and this is very easy. However the subsequent measurement of NG and Io is quite important for a better understanding the system and this we will discuss here. The measurement of tPb and Ntpb is discussed in Chapter 5. 

Although NEMCA is a catalytic effect taking place over the entire catalyst gas-exposed surface, it is important for its description to also discuss the electrocatalytic reactions taking place at the catalyst-solid electrolyte-gas three phase boundaries (tpb). This means that the catalyst-electrode must also be characterized from an electrochemical viewpoint. When using YSZ as the solid electrolyte the electrochemical reaction taking place at the tpb is ... 

Again the extent to which such parallel reactions contribute to the measured current is not very easy to quantify. However, fortunately, such a quantification is not necessary for the description of NEMCA. What is needed is only a measure of the overall electrocatalytic activity of the metal-solid electrolyte interface or, equivalently, of the tpb, and this can be obtained by determining the value of a single electrochemical parameter, the exchange current I0, which is related to the exchange current density i0 via ... 

The exchange current I0 is an important parameter for the quantitative description of NEMCA. As subsequently analyzed in this chapter it has been found both theoretically and experimentally1,4 19 that the order of magnitude of the absolute value A of the NEMCA enhancement factor A defined from ... 

A typical electrochemical promotion (EP), or in situ controlled promotion (ICP) or NEMCA experiment utilizing YSZ, an O2 conductor, as the promoter donor is shown in Fig. 4.13. The reaction under study is the oxidation of C2H4 on Pt ... 

Figure 4.13. NEMCA Rate and catalyst potential response to step changes in applied current during C2H4 oxidation on Pt T=370°C, p02=4.6 kPa, Pc2h4=0.36 kPa. The experimental (t) and computed (2FNG/I) rate relaxation time constants are indicated on the figure. See text for discussion. ro=1.5-10 8 mol O/s, Ar=38.5-10 8 mol O/s, I/2F=5.2-10 12 mol O/s, pmax=26, Amax=74000, Ng=4.240 9 mol Pt.4 Reprinted with permission from Academic Press.

Then let us examine the rate relaxation time constant x, defined as the time required for the rate increase Ar to reach 63% of its steady state value. It is comparable, and this is a general observation, with the parameter 2FNq/I, (Fig. 4.13). This is the time required to form a monolayer of oxygen on a surface with Nq sites when oxygen is supplied in the form of 02 This observation provided the first evidence that NEMCA is due to an electrochemically controlled migration of ionic species from the solid electrolyte onto the catalyst surface,1,4,49 as proven in detail in Chapter 5 (section 5.2), where the same transient is viewed through the use of surface sensitive techniques. 

So the explanation for Figure 4.13 and for all NEMCA studies utilizing O2 conductors, such as YSZ, is simply the following Promoting anionic species O8 (accompanied by their compensating charge 5+ in the metal) are generated in an electrochemical step at the tpb ... 

How can we confirm this sacrificial promoter model By simply looking at the r vs t transient behaviour of Figure 4.13 or of any galvanostatic NEMCA experiment upon current interruption (1=0). 

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