Non-Faradaic Electrochemical Modification of Catalytic Activity NEMCA

D. Tsiplakides, S. Neophytides, O. Enea, M.M. Jaksic, and C.G. Vayenas, Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) of Pt Black Electrodes Deposited on Nafion 117 Solid Polymer Electrolyte, /. Electrochem. Soc. 144(6), 2072-2088 (1997). 

V.A. Sobyanin, V.I. Sobolev, V.D. Belyaev, O.A. Mar ina, A.K. Demin, and A.S. Lipilin, On the origin of the Non-Faradaic electrochemical modification of catalytic activity (NEMCA) phenomena. Oxygen isotope exchange on Pt electrode in cell with solid oxide electrolyte, Catal. Lett. 18, 153-164 (1993). 

Electrochemical promotion, or non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) came as a rather unexpected discovery in 1980 when with my student Mike Stoukides at MIT we were trying to influence in situ the rate and selectivity of ethylene epoxidation by fixing the oxygen activity on a Ag catalyst film deposited on a ceramic O2 conductor via electrical potential application between the catalyst and a counter electrode. 

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

A controlled modification of the rate and selectivity of surface reactions on heterogeneous metal or metal oxide catalysts is a well-studied topic. Dopants and metal-support interactions have frequently been applied to improve catalytic performance. Studies on the electric control of catalytic activity, in which reactants were fed over a catalyst interfaced with O2--, Na+-, or H+-conducting solid electrolytes like yttrium-stabilized zirconia (or electronic-ionic conducting supports like Ti02 and Ce02), have led to the discovery of non-Faradaic electrochemical modification of catalytic activity (NEMCA, Stoukides and Vayenas, 1981), in which catalytic activity and selectivity were both found to depend strongly on the electric potential of the catalyst potential, with an increase in catalytic rate exceeding the rate expected on the basis of Faradaic ion flux by up to five orders of... 

During the last 15 years the closely related phenomenon of electrochemical promotion [9-12], or non-Faradaic electrochemical modification of catalytic activity, NEMCA effect [9-12], has been discovered and studied for more than 60 catalytic reactions [13,14]. 

Electrochemical promotion of catalysis (EPOC) or non-Faradaic electrochemical modification of catalytic activity (NEMCA) is the phenomenon whereby application of small current density (1-lO pA/cm ) or potential ( + 2V) between a conductive catalyst, deposited on a solid electrolyte, and a second (catalytically inert) electrode, also deposited on the solid electrolyte, enhances the catalytic performance of the catalyst [9-14,16-25] (Figure 1). 

This novel effect has been termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect [5-15]) or electrochemical promotion [16] or in situ controlled promotion [20]. Its importance in catalysis and electrochemistry has been discussed by Haber [18], Pritchard [16] and Bockris [17], respectively. In addition to the group which first reported this new phenomenon [5-7], the groups of Lambert [12], Haller [10], Sobyanin [8], Comninellis [13], Pacchioni [21] and Stoukides [11] have also made important contributions in this area, which has been reviewed recently [14,15]. In this review the main phenomenological features of NEMCA for oxidation reactions are briefly surveyed and the origin of the effect is discussed in the light of recent kinetic, surface spectroscopic and quantum mechanical investigations. 

The pronounced reversible promotional phenomena observed upon varying the electrical potential of metal catalysts interfaced with solid electrolytes are known as Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect ). Electrochemical Promotion (EP), or In situ Controlled Promotion (ICP). The three terms, i.e., NEMCA effect, EP, or ICP are used interchangeably in this chapter as they refer to the same phenomena. 

The use of solid electrolytes as active catalyst supports to induce the effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) [16] or in situ controlled promotion [17] and alter the catalytic properties of metal catalysts has been described in detail previously [16-18]. A goal of the present work was to explore the possible relationship between the NEMCA effect [16-18] and the effect of dopant-induced-metal-support interactions [6,7,9,10]. 

In this Chapter, the progress recently made in the field of electrochemical promotion (EP) of catalytic gas reactions is reviewed. The phenomenon consists of electrochemical polarization of metal or metal oxide electrodes interfaced with solid electrolytes which result in a pronounced increase in the catalytic reaction rate. The effect is also termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect), since the rate increase may exceed the ionic current by several orders of magnitude. The promotion is not limited to the electrochemically polarized interface between catalyst and solid electrolyte, but extends to the entire catalyst surface exposed to the reactive gas. In fact, one of the major challenges in the field of electrochemical promotion is to elucidate the exact mechanism by which the promoting effect propagates from one interface to the other. 

In single-chamber reactor, the A value would be possible to exceed the unity. This means that there is additional hydrogen from the gas phase that accompanies the electrochemical hydrogen in the same reaction. This phenomenon is called non-Faradaic electrochemical modification of catalytic activity (NEMCA). The concept of the NEMCA effect is different from Faradaic effect. In NEMCA, an electrode will serve as a catalyst for two simultaneous processes, chemical processes and electrochemical processes. 

Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA)... 

Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA), Fig. 4 Typical example of electrochemical modification of oxygen rai Pt/YSZ catalyst (a) gaseous adsorption, (b) electrochemical adsrnption, and (c) mixed adsorption [5]... 

During the last few years a new application of sohd electrolytes has emerged. It was found that the catalytic activity and selectivity of the gas-exposed electrode surface of metal electrodes in solid electrolyte cells is altered dramatically and reversibly upon polarizing the metal/solid electrolyte interface. The induced steady-state change in catalytic rate can be up to 9000% higher than the normal (open-circuit) catalytic rate and up to 3 x 10 higher than the steady-state rate of ion supply. " This new effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) has been already demonstrated for more than... 

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