Solid Catalytic Reactions

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. 

In many catalytic reactions, solid, liquid, and gas phases are involved, and the phase behavior often has a strong influence on mixing and mass transfer and consequently on the catalytic performance. Supercritical fluids, especially supercritical CO2, have gained considerable attention as environmentally benign solvents (e.g., (94y). The combined use of in situ transmission and ATR-IR spectroscopy together with video monitoring is a promising approach for elucidation of the behavior of a... 

However, before extrapolating the arguments from the gross patterns through the reactor for homogeneous reactions to solid-catalyzed reactions, it must be recognized that in catalytic reactions the fluid in the interior of catalyst pellets may diSer from the main body of fluid. The local inhomogeneities caused by lowered reactant concentration within the catalyst pellets result in a product distribution different from that which would otherwise be observed. 

Fluidized bed noncatalytic reactors. Fluidized heds are also suited to gas-solid noncatalytic reactions. All the advantages described earlier for gas-solid catalytic reactions apply. As an example. 

Catalytic Reactions This use has provided the greatest impetus for use, development, and research in the field of flmdized solids. Some of the details pertaining to this use are to be found in the preceding pages of this sec tion. Reference should also be made to... 

The most common heterogeneous catalytic reaction is hydrogenation. Most laboratory hydrogenations are done on liquid or solid substrates and usually in solution with a slurried catalyst. Therefore the most common batch reactor is a stirred vessel, usually a stirred autoclave (see Figure 2.1.1 for a typical example). In this system a gaseous compound, like hydrogen, must react at elevated pressure to accelerate the process. 

Reactions that are catalyzed by solids occur on the surfaces of the solids at points of high chemical activity. Therefore, the activity of a catalytic surface is proportional to the number of active centers per unit area. In many cases, the concentration of active centers is relatively low. This is evident by the small quantities of poisons present (material that retards the rate of a catalytic reaction) that are sufficient to destroy the activity of a catalyst. Active centers depend on the interatomic spacing of the solid structure, chemical constitution, and lattice structure. 

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

From a theoretical point of view the study of the kinetics of coupled catalytic reactions makes it possible to investigate mutual influencing of single reactions and the occurrence of some phenomena unknown in the kinetics of complex reactions in the homogeneous phase. This approach can yield additional information about interactions between the reactants and the surface of the solid catalyst. 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

Both heated stages [224] and ambient temperature gas environments [225,226] have been developed for use in electron microscopy and both are combined [227,228] in the controlled atmosphere instrument. Pressures of up to 30 kPa and temperatures up to 1500 K have been used in studies of a wide variety of solid—gas and catalytic reactions [ 229]. 

Section 8 deals with reactions which occur at gas—solid and solid—solid interfaces, other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reaction at the liquid—solid interface (and corrosion), involving electrochemical processes outside the coverage of this series, are not considered. With respect to chemical processes at gas-solid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in to the consideration of the kinetics and mechanism of catalytic reactions. 

By 19884 it became obvious that the NEMCA effect, this large apparent violation of Faraday s law, is a general phenomenon not limited to a few oxidation reactions on Ag. Of key importance in understanding NEMCA came the observation that NEMCA is accompanied by potential-controlled variation in the catalyst work function.6 Its importance was soon recognized by leading electrochemists, surface scientists and catalysis researchers. Today the NEMCA effect has been studied already for more than 60 catalytic systems and does not seem to be limited to any specific type of catalytic reaction, metal catalyst or solid electrolyte, particularly in view of... 

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. 

Promotion We use the term promotion, or classical promotion, to denote the action of one or more substances, the promoter or promoters, which when added in relatively small quantities to a catalyst, improves the activity, selectivity or useful lifetime of the catalyst. In general a promoter may either augment a desired reaction or suppress an undesired one. For example, K or K2O is a promoter of Fe for the synthesis of ammonia. A promoter is not, in general, consumed during a catalytic reaction. If it does get consumed, however, as is often the case in electrochemical promotion utilizing O2 conducting solid electrolytes, then we will refer to this substance as a sacrificial promoter. 

There are, however, numerous cases where electronegative additives can act as promoters for catalytic reactions. Typical examples are the use of Cl to enhance the selectivity of Ag epoxidation catalysts and the plethora of electrochemical promotion studies utilizing O2 as the promoting ion, surveyed in Chapters 4 and 8 of this book. The use of O, O8 or O2 as a promoter on metal catalyst surfaces is a new development which surfaced after the discovery of electrochemical promotion where a solid O2 conductor interfaced with the metal catalyst acts as a constant source of promoting O8 ions under the influence of an applied voltage. Without such a constant supply of O2 onto the catalyst surface, the promoting O8 species would soon be consumed via desorption or side reactions. This is why promotion with O2 was not possible in classical promotion, i.e. before the discovery of electrochemical promotion. 

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... 

Nevertheless there are some reactions which never change. Thus NO reduction on noble metals, a very important catalytic reaction, is in the vast majority of cases electrophilic, regardless of the type of solid electrolyte used (YSZ or P"-A1203). And practically all oxidations are electrophobic under fuel lean conditions, regardless of the type of solid electrolyte used (YSZ, p"-Al203, proton conductors, even alkaline aqueous solutions). 

S. Bebelis, M. Makri, A. Buekenhoudt, J. Luyten, S. Brosda, P. Petrolekas, C. Pliangos, and C.G. Vayenas, Electrochemical activation of catalytic reactions using anionic, cationic and mixed conductors, Solid State Ionics 129, 33-46 (2000). 

In view of the above physical meaning of A it is clear why A can approach infinite values when Na+ is used as the sacrificial promoter (e.g. when using j "-Al203 as the solid electrolyte) to promote reactions such as CO oxidation (Fig. 4.15) or NO reduction by H2 (Fig. 4.17). In this case Na on the catalyst surface is not consumed by a catalytic reaction and the only way it can be lost from the surface is via evaporation. Evaporation is very slow below 400°C (see Chapter 9) so A can approach infinite values. 

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

As already analysed in Chapter 5, once the backspillover species originating from the solid electrolyte have migrated at the metal/gas interface, then they act as normal (chemical) promoters for catalytic reactions. For example, Lambert and coworkers via elegant use of XPS18 have shown that the state of sodium introduced via evaporation on a Pt surface interfaced with P"-A1203 is indistinguishable from Na5+ introduced on the same Pt surface via negative (cathodic) potential application. 

For a given catalytic reaction does the rate enhancement ratio p depend only on UWr or does it also depend on the nature of solid electrolyte (YSZ, P"-A1203, Ti02, Ce02) ... 

Do all catalytic reactions on conducting catalysts deposited on solid electrolytes exhibit electrochemical promotion (NEMCA) behaviour ... 

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