Solid electrolytes potentiometry

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

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. 

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... 

H.-H. Hildenbrand, and H.-G. Lintz, Solid electrolyte potentiometry aided study of the influence of promotors on the phase transitions in copper-oxide catalysts under working conditions, Catalysis Today 9, 153-160 (1991). 

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. 

Propene to acrolein. Hildenbrand and Lintz87,88 have used solid electrolyte potentiometry to study the effect of the phase composition of a copper oxide catalyst on the selectivity and yield of acrolein during the partial oxidation of propene in the temperature range of 420-510°C. Potentiometric techniques were used to determine the catalyst oxygen activity, and hence the stable copper phase, under working conditions. Hildenbrand and Lintz used kinetic measurements to confirm that the thermodynamically stable phase had been formed (it is known that propene is totally oxidised over CuO but partially oxidised over ). 

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). 

The use of IR spectroscopy and solid electrolyte potentiometry has provided additional experimental evidence for these types of models. Lind-strom and Tsotsis (93,120) report an absorption band around 2120 cm for the CO/O2 reaction on Pt in addition to the typically observed Pt-CO band around 2070 cm". This new band was assigned to CO molecules adsorbed on an oxidized Pt surface. The intensity of this band oscillated counterphase to the reaction rate, but with a much lower amplitude than the 2070-cm" ... 

To our knowledge it is the first time that the oxygen activity profile along a bed of an oxidic catalyst during partial oxidation of an aldehyde could be determined by use of the Solid Electrolyte Potentiometry. 

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)... 

The fundamentals of gas titration with solid electrolyte cells are described in detail within the entries titration and coulometry, and special aspects are also treated within solid electrolyte, potentiometry, and amperometry. Therefore, the focus is set here to the most important errors of gas titration with SE cells. These errors are related mainly to the peripheral parameters as... 

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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