Electrochemical Promotion Using Aqueous Electrolytes

The recent discovery of NEMCA in aqueous systems is of considerable theoretical and practical importance. In this case, no ion migration (back-spillover) is necessary to account for the observed behavior, which appears again to be due to the effect of changing potential and work function on the binding strength of adsorbates. Here, there appears to be only one 

Hydrogen and oxygen are consumed on the Pt surface at a rate rc by the catalytic reaction 

Under open-circuit conditions, the catalyst potential VwR E (r.h.e.) takes values of the order 0.4-0.85 V, that is, -0.35 to +0.1V on the normal hydrogen electrode scale (n.h.e.), depending on the hydrogen-to-oxygen ratio. 

When a positive current I is applied between the catalyst electrode and the Pt counter electrode, then the catalyst potential E changes to more positive values (Fig. 90), and the following electrochemical (net charge-transfer) reactions take place at the surface of the Pt catalyst electrode  

Consequently, if were to remain constant, application of a positive current would increase by less than I/2F (Arn I/2F) and would decrease or increase kq again by less than 7/2F(-//2F A/ o I/2F), 

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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. 

Electrochemical promotion (EP) denotes electrically controlled modification of heterogeneous catalytic activity and/or selectivity. This recently discovered phenomenon has made a strong impact on modem electrochemistry/ catalysis/ and surface science. Although it manifests itself also using aqueous electrolytes/ the phenomenon has mainly been investigated in gas-phase reactions over metal and metal oxide catalysts. In the latter case, the catalyst, which is an electron conductor, is deposited in the form of a porous thin film on a solid electrolyte support, which is an ion conductor at the temperature of the catalytic reaction. Application of an electric potential on the catalyst/support interface or, which is equivalent, passing an electric current between catalyst and support, causes a concomitant change also in the properties of the adjacent catalyst/gas interface, where the catalytic reaction takes place. This results in an alteration of the catalytic behaviour, controllable with the applied potential or current. 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

During the last few years it has become apparent that the active use of sohd electrolyte cells offers some very interesting possibilities in heterogeneous catalysis it was found that solid electrolyte cells can be used not only to study, but also to influence catalytic phenomena on metal surfaces in a very pronounced and reversible manner. Work in this area prior to 1988 had been reviewed. Then in 1988 the first reports on NEMCA appeared in the literature. Since then the NEMCA effect has been described for more than 40 catalytic reactions,and work prior to 1992 has been reviewed in a monograph. In addition to the group which first reported this novel effect, the groups of Sobyanin, Lambert et al., Stoukides et al., and Haller et al. have also contributed recently to the NEMCA literature. Very recently, the NEMCA effect was also demonstrated in an aqueous electrolyte system by Anastasijevic et al. and by Neophytides et al. The term Electrochemical Promotion in Catalysis has also been proposed by Pritchard to describe the NEMCA effect. 

A second major event in the saga of polymer conductors was the discovery that the doping processes of polyacetylene could be promoted and driven electrochemically in a reversible fashion by polarising the polymer film electrode in a suitable electrochemical cell (MacDiarmid and Maxfield, 1987). Typically, a three-electrode cell, containing the (CH) film as the working electrode, a suitable electrolyte (e.g. a non-aqueous solution of lithium perchlorate in propylene carbonate, here abbreviated to LiC104-PC) and suitable counter (e.g. lithium metal) and reference (e.g. again Li) electrodes, can be used. 

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