Heterogeneous solid electrolytes

Meier J (1987) Defect chemistry emd conductivity effects in heterogeneous solid electrolytes. J Electrochem Soc 134 1524-1535... 

J. Maier, Heterogeneous solid electrolytes, in Superionic solids and solid electrolytes,... 

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. 

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... 

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... 

M. Stoukides, Applications of Solid Electrolytes in Heterogeneous Catalysis, Industrial Engineering Chemistry Research 27, 1745-1750 (1988). 

Consequently the absolute potential is a material property which can be used to characterize solid electrolyte materials, several of which, as discussed in Chapter 11, are used increasingly in recent years as high surface area catalyst supports. This in turn implies that the Fermi level of dispersed metal catalysts supported on such carriers will be pinned to the Fermi level (or absolute potential) of the carrier (support). As discussed in Chapter 11 this is intimately related to the effect of metal-support interactions, which is of central importance in heterogeneous catalysis. 

Similarly to charge-transfer processes at solid-electrolyte interfaces, the ET rate for heterogeneous reactions at ITIES is determined by the flux of reactants to the interface as well... 

A great many electrolytes have only limited solubility, which can be very low. If a solid electrolyte is added to a pure solvent in an amount greater than corresponds to its solubility, a heterogeneous system is formed in which equilibrium is established between the electrolyte ions in solution and in the solid phase. At constant temperature, this equilibrium can be described by the thermodynamic condition for equality of the chemical potentials of ions in the liquid and solid phases (under these conditions, cations and anions enter and leave the solid phase simultaneously, fulfilling the electroneutrality condition). In the liquid phase, the chemical potential of the ion is a function of its activity, while it is constant in the solid phase. If the formula unit of the electrolyte considered consists of v+ cations and v anions, then... 

The aim of this review is to present and discuss recent work on solid electrolyte electrochemical cells relevant to in-situ catalyst sensing. Consequently, the area of SEP will be concentrated upon, however, appropriate closed-circuit or ampero-metric studies will also be discussed. This review is intended to also introduce the reader familiar with heterogeneous catalysis to the electrochemical concepts and techniques required to fully appreciate the research work in this field. 

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... 

Figure 5.41 Selective-ion electrodes (a) glass membrane (b) liquid ion exchange (c) homogeneous solid membrane (d) heterogeneous solid membrane (e) solid membrane without reference electrode (/) gas-permeable membrane 1, sensing electrode 2, electrolyte, 2(e) ohmic contact, 2(f) gas-permeable membrane 3, membrane sur-port 4, reference electrode, 4(f) outer electrode body, 5(b) liquid ion exchanger 5(f) electrode body 6(b) reference electrode body, 6(f) electrolyte 7, liquid junction.

Hydrocarbons Oxidation by heterogeneous catalysis C02 (ideally) Used in solid-electrolyte cells... 

Now, we are going to consider another extreme model of surface heterogeneity, when Ay s are not correlated at all. That model has already been elaborated by Rudzihski and co-workers [133,134], for the case of adsorption of liquid mixtures of non-electrolytes on heterogeneous solid surfaces. The relation between the one-dimensional adsorption energy distributions Xi( i) are, for instance such, as shown in Fig. 17. 

Since in the last decade significant results have been achieved in the theory of adsorption from non-electrolytic liquid mixtures on heterogeneous solids (see reviews [187-189] and references therein), these results can be utilized in the study of heterogeneity of proton binding sites on inorganic oxides. 

Additional difficulties in formulating an adsorption theory for the liquid - solid interface result from a variety of interactions between components of a liquid mixture and from a complex structure of the adsorbent, which may possess different types of pores and strong surface heterogeneity. Our considerations will be limited to physical adsorption on heterogeneous solid surfaces of components of comparable molecular sizes from non-electrolytic (non ideal or ideal) miscible binary liquid mixtures. 

The reversibility of the Pt(02)IZr02(Ca0) membrane oxygen electrode in molten KCl-NaCl has been studied in a number of works. Thus, the authors of Refs. [65, 236, 237] reported the application of the electrode with a Zr02(CaO) solid electrolyte membrane for the potentiometric investigation of some heterogeneous acid-base equilibria and the construction of the row of cation acidity in the KCl-NaCl melt. 

In addition to the potential technological applications of electrochemical modification of catalytic activity, the ability of solid electrolytes to dose reversibly, precisely, and in situ catalyst surfaces with promoters, by "knob-turn" variation of the catalyst potential and work function, provides a unique opportunity for the systematic study of the role of promoters and poisons in Heterogeneous Catalysis. 

Aside from potential technological applications, the use of p"-Al203 as the solid electrolyte allows for a detailed, in situ and systematic investigation of the role of alkali promoters in heterogeneous catalysis. 

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. 

We will focus on the investigation of the phenomenon of electrochemical promotion by using YSZ as the solid electrolyte. Two types of heterogeneous catalytic gas reactions will be discussed. One of them is the catalytic combustion of ethylene over RUO2 or I1O2 catalysts and the other is the reduction of NO by propylene in presence of oxygen over Rh catalysts. 

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