Solid Electrolyte Potentiometry SEP

When a solid electrolyte component is interfaced with two electronically conducting (e.g., metal) films (electrodes), a solid electrolyte galvanic cell is formed (Fig. 3). Cells of this type with YSZ solid electrolyte are used as oxygen sensors. The potential difference that develops spontaneously between the two electrodes (W and R designate the working and the reference electrode, respectively) is given by 

The measured potential difference is related to the oxygen activity, a , on the catalyst surface 

In addition to solid electrolyte potentiometry, the techniques of cyclic voltammetry and linear potential sweep have also been used recently in solid electrolyte cells to investigate catalytic phenomena occurring on the gas-exposed electrode surfaces. The latter technique, in particular, is known in catalysis under the term potential-programmed reduction (PPR). With appropriate choice of the sweep rate and other operating parameters, both techniques can provide valuable kinetic and thermodynamic information about catalytically active chemisorbed species and also about the NEMCA effect, as analyzed in detail in Section III. 

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Wagner first proposed the use of such galvanic cells in heterogeneous catalysis, to measure in situ the thermodynamic activity of oxygen O(a) adsorbed on metal electrodes during catalytic reactions.21 This led to the technique of solid electrolyte potentiometry (SEP).22 26... 

Solid electrolyte electrochemical cells can be operated in a variety of ways (the three modes of operation are illustrated schematically in Figure 2). Such a cell may be operated potentiometrically in order to investigate the behaviour of a catalyst of interest. This technique has become known as solid electrolyte potentiometry (SEP). The catalyst itself is deposited in the form of an electrode... 

The use of equation (3.2) to study the behaviour of catalysts is known as solid electrolyte potentiometry (SEP). Wagner38 was the first to put forward the idea of using SEP to study catalysts under working conditions. Vayenas and Saltsburg were the first to apply the technique to the fundamental study of a catalytic reaction for the case of the oxidation of sulfur dioxide.39 Since then the technique has been widely used, with particular success in the study of periodic and oscillatory phenomena for such reactions as the oxidation of carbon monoxide on platinum, hydrogen on nickel, ethylene on platinum and propylene oxide on silver. 

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

In a recent study (4) kinetic measurements in a CSTR were combined with simultaneous in situ measurement of the thermodynamic activity of oxygen adsorbed on the catalyst by using the technique of solid electrolyte potentiometry (SEP). The technique originally proposed by C. Wagner (1) utilizes a solid electrolyte oxygen concentration cell with one electrode also serving as the catalyst for the reaction under study. It has already been used to study the oxidation of ethylene on Ag (5) and on Pt (6). 

When the solid electrolytes are used as membranes, there are three different operation modes, as shown in Fig. 3. Id. Mode 1 is under open circuit operation, in which no net current passes through the membrane. The reactor in this mode often serves as a sensor or an in situ characterization technique for catalytic gas-solid reactions under work conditions, named solid electrolyte potentiometry (SEP)... 

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