Solid-state catalysis

Second, in many cases the solid state catalysis was incorporated, as contaminants, within the growing polymer making an additional purification step necessary in the polymer processing to rid the polymer of this undesired material. 

The electronic structure of a solid metal or semiconductor is described by the band theory that considers the possible energy states of delocalized electrons in the crystal lattice. An apparent difficulty for the application of band theory to solid state catalysis is that the theory describes the situation in an infinitely extended lattice whereas the catalytic process is located on an external crystal surface where the lattice ends. In attempting to develop a correlation between catalytic surface processes and the bulk electronic properties of catalysts as described by the band theory, the approach taken in the following pages will be to assume a correlation between bulk and surface electronic properties. For example, it is assumed that lack of electrons in the bulk results in empty orbitals in the surface conversely, excess electrons in the bulk should result in occupied orbitals in the surface (7). This principle gains strong support from the consistency of the description thus achieved. In the following, the principle will be applied to supported catalysts. 

Yet another application of solid-state catalysis occurs in the desulfurization of petroleum. Natural petroleum includes various molecules that contain sulfur atoms. Combustion of this petroleum produces S02, which must be removed from the exhaust to prevent air pollution. One way to prevent pollution by S02 is to remove the sulfur from the petroleum before it is used for fuel—the desulfurization of petroleum. One type of sulfur containing molecules found in petroleum are thiols, which can be written R—SH, where R represents a molecular fragment containing a long chain of carbon atoms. In desulfurization the goal is to remove the sulfur from this molecule to produce a hydrocarbon (R—H) ... 

Enzymes exhibit a multi-point contact with the substrate, that means a structural flexibility to facilitate catalysis of a reaction. These distinctive features differ from those of the active sites employed by soluble transition metal complexes and solid state catalysis. Furdier important factors in enz5miatic catalysis are simultaneous acid and base catalysis and hydrophobic/hydrophihc interactions at the same time. 

These expressions have direct analogues for equilibria in solution which may in part explain their common usage in the analysis of solid state catalysis. 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

On the basis of these redox potentials it seems likely that direct electron release to the benzenediazonium ion takes place only with iodide. This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light, or other special procedures (Sec. 10.6). For bromo- and chloro-de-di-azoniations, catalysis by cuprous salts (Sandmeyer reaction, Sec. 10.5) is necessary. For fluorination the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen (see Sec. 10.4). With astatide (211At-), the heaviest of the halide ions, Meyer et al. (1979) found higher yields for astato-de-diazoniation than for iodo-de-diazoniation, a result consistent with the position of At in the Periodic System. It has to be emphasized, however, that in investigations based on measuring yields of final products (Ar-Hal), the possibility that part of the yield may be due to heterolytic dediazoniation is very difficult to quantify. 

Figure 1.6. Common features of Heterogeneous Catalysis, Fuel Cell operation, Electrolysis and Electrochemical Promotion 1. Solid state catalyst, 2. Adsorption, 3. AG < 0, 4. Yield control via DC current or voltage application (Adapted from N. A. Anastasijevic).

Due to the interdisciplinary nature of electrochemical promotion, which involves elementary but important concepts from at least five different fields (catalysis, surface science, electrochemistry, solid state ionics, chemical reaction engineering) we have structured the book in such a way to make it possible for readers from all the above fields to follow the entire book. 

Thus Chapter 2 discusses the phenomenology and basic concepts of classical promotion, a subject quite familiar to catalysis and surface science researchers and graduate students, at a level which should be comfortable to electrochemists, solid state ionics and chemical reaction engineering researchers. 

Chapter 3 discusses solid electrolytes and some of their early applications in fuel cells and catalysis. This material is quite familiar to the solid state ionics community but may be helpful to surface scientists, aqueous electrochemists and chemical reaction engineers. 

Chapter 7 introduces the concept of absolute electrode potential in solid state electrochemistry. This concept has some important implications not only in solid state electrochemistry but also, potentially, in heterogeneous catalysis of supported catalysts. 

Work function, a quantity of great importance in surface science and catalysis, plays a key role in solid state electrochemistry and in electrochemical promotion. As will be shown in Chapter 7 the work function of the gas-exposed surface of an electrode in a solid electrolyte cell can be used to define an absolute potential scale in solid state electrochemistry. 

In this Appendix we summarize some of the most common questions asked by physical chemists when they first encounter NEMCA. There are also questions asked after years of exposure in this area. They have been sampled by the authors from more than 100 presentations in International Conferences on Catalysis, Electrochemistry, Solid State Ionics and Surface Science. Some of the questions are easy to answer, some are difficult and there are even some for which there is still no definitive answer. For the sake of the reader who may want to test his understanding up to this point we are first listing the questions separately, then proceed with their answer. 

This equation gives (0) = 0, a maximum at =. /Km/K2, and (oo) = 0. The assumed mechanism involves a first-order surface reaction with inhibition of the reaction if a second substrate molecule is adsorbed. A similar functional form for (s) can be obtained by assuming a second-order, dual-site model. As in the case of gas-solid heterogeneous catalysis, it is not possible to verify reaction mechanisms simply by steady-state rate measurements. 

Laboratory, where he worked with John Longo and Allan Jacobson on the synthesis and characterization of mixed metal oxides and their application in heterogeneous catalysis. He joined the chemistry faculty of Northwestern University in 1984 where he is now Professor of Chemistry and an active member of the Center for Catalysis and Surface Science and the Materials Research Science and Engineering Center. Kenneth Poeppelmeier has published over 250 research papers and supervised approximately 40 Ph.D. students in the area of inorganic and solid state chemistry. He is a Fellow of the American Association for the Advancement of Science (AAAS) and the Japan Society for the Promotion of Science (JSPS) and has been a Lecturer for the National Science Council of Taiwan (1991), Natural Science Foundation of China (1999) and Chemistry Week in China (2004), and more recently an Institut Universitaire de France Professor (2003). 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

The structure of the active catalyst and the mechanism of catalysis have not been completely defined. Several solid state complexes of BINOL and Ti(0-/-Pr)4 have been characterized by X-ray crystallography.158 Figure 2.4 shows the structures of complexes having the composition (BIN0Late)Ti2(0-/-Pr)6 and (BINOLate)Ti3(O-/-Pr)10. 

Many catalysts, both immobilized (on solid state supports) and heterogeneous, contain phosphines and other phosphorus compounds, so that solid state NMR has become an invaluable tool in the study of catalysis. 

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