Gas-solid transformations

The corresponding preparation methods may be grouped into two main streams based upon the gas-solid and liquid-solid nature of the transformations. Gas-solid transformation methodsare broadly used in the context of ultrafme oxide powder synthesis. Liquid-solid transformation methods follow a bottom-up approach. A number of methods have been developed, among which some generally used are discussed below ... 

Gas-Solid Transformation Methods Gas-solid transformation methods are restricted to Chemical Vapor Deposition (CVD) and Pulsed Laser Deposition (PLD). 

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. 

Most microscopic theories of adsorption and desorption are based on the lattice gas model. One assumes that the surface of a sohd can be divided into two-dimensional cells, labelled i, for which one introduces microscopic variables Hi = 1 or 0, depending on whether cell i is occupied by an adsorbed gas particle or not. (The connection with magnetic systems is made by a transformation to spin variables cr, = 2n, — 1.) In its simplest form a lattice gas model is restricted to the submonolayer regime and to gas-solid systems in which the surface structure and the adsorption sites do not change as a function of coverage. To introduce the dynamics of the system one writes down a model Hamiltonian which, for the simplest system of a one-component adsorbate with one adsorption site per unit cell, is... 

Semiconductor photocatalysts in a form of colloids, powders, porous granules, thin films or bulk solids including single crystals (used in model studies) provide both liquid phase and gas phase transformations. Comprehensive reviews in this field can be found in monographs [4] (Chapters by N.S.Lewis and M.L.Rosenbluth M.Gratzel M.Schiavello and A.Sclafani P.Pichat and J.-M.Herrmann G.A.Somorjai T.Sakata H.Tributsch M.A.Fox H.Al-Ekabi and N.Serpone D.F.Ollis, E.Pelizzetti and N.Serpone) [8] (Chapter by Yu.A.Gruzdkov, E.N.Savinov and V.N.Parmon) and [3]. 

Specific research subjects have emerged with respect to improved descriptions of specific phenomena. Some time ago, it was speculated that gas-solid interactions and turbulence effects on reaction kinetics would be important areas of advance in the modeling art. Gas-solid interactions include both chemical formation of aerosols and reactions on surfaces of pre-existing suspended particulate matter. Because of differing effects of a material in the gas phase and in some condensed phase, it will be important to characterize transformation processes. The achex (Aerosol Characterization hYperiment) program recently carried out under the direction of Hidy will provide an extensive data base with which to test new ways of treating the gas-solid interaction problem. 

As with solid-to-gas transformations, solid-to-solid transformations are... 

There is a large class of industrially important heterogeneous reactions in which a gas or a liquid is brought into contact with a solid and reacts with the solid transforming it into a product. Among the most important are the reduction of iron oxide to metallic iron in a blast furnace the combustion of coal particles in a pulverised fuel boiler and the incineration of solid wastes. These examples also happen to be some of the most complex chemically. Further simple examples are the roasting of sulphide ores such as zinc blende ... 

Phase transformations in heterogeneous catalysis have been described recently by topochemical kinetic models [111-115]. These models were taken from solid chemistry, where they had been developed for "gas-solid reactions. The products of such reactions are solids. When gas is in contact with the initial solid, the reaction rate is negligible. But as nucleates of the phase... 

Braga, Dario, Polymorphism, Crystal Transformations and Gas-Solid Reactions, 7, 325. 

Tanaka, K., Fujimoto, D., Oeser, T., Imgartinger, H., and Toda, F. (2000) Chiral Inclusion Crystallization of Tetra(p-bromophenyl)ethylene by Exposure to the Vapor of Achiral Guest Molecules A Novel Racemic-to-Chiral Transformation Through Gas-Solid Reaction, Chem. Commun., 413-414. 

Penzien and Schmidt reported the first absolute asymmetric transformation in a chiral crystal. [10] They showed that enone 4,4 -dimethylchalcone 1, although being achiral itself, crystallizes spontaneously in the chiral space group P2 2 2 (Scheme 1). When single crystals of this material are treated with bromine vapor in a gas-solid reaction, the chiral dibromide 2 is produced in 6-25% ee. In this elegant experiment, it is the reaction medium, the chiral crystal lattice, that provides the asymmetric influence favoring the formation of one product enantiomer over the other, and the chemist has merely provided a non-chiral solvent (ethyl acetate) for the crystallization and a nonchiral reagent (bromine) for the reaction. 

It is well known that heterogeneous photocatalysis can be successfully used to photodegrade or to transform a wide range of molecules in liquid-solid and in gas-solid systems. Nevertheless, the knowledge of fundamentals of photocatalysis is essential to understand the mechanistic aspects and to find the parameters that influence the process under investigation. Moreover, the development of new photocatalysts and their application in the various research fields is a mandatory task. 

The martensitic transformation temperature, equal to ss 200 K for stoichiometric Ni2MnGa, linearly increases with increasing x in substituted Ni2+, Mri , Ga solid solutions, reaching approximately 320-330 K in the alloys with x = 0.18 — 0.19 [4] (alloys with a higher Ni excess have not been studied so far). Note that there exists a scatter in the data as for the martensitic transformation temperature, as well as for other relevant properties of the alloys, which is probably caused by a different degree of ordering [13, 14] and/or deviation from the nominal composition of the alloys. 

There are two main steps in catalyst preparation The hrst consists of depositing the active component precursor, as a divided form, on the support and the seeond of transforming this precursor into the required active component which depending on the reaction to be catalyzed can be found in the oxide sullided or metallic state A large majority o( deposition methods involve aqueous solutions and the liquid solid intei face In some cases, deposition can be also performed trom the gas phase and involves the gas solid interlace... 

The MC and MD methods permitting the variation of the shape of the cell are best suited for the study of phase transitions in solids. These methods have been used to study phase transitions of a few solids in the last few years. Among these are monatomic solids such as rare gas solids, ionic solids, and molecular solids. There are, however, some inherent limitations in these methods. While certain transitions are readily investigated by these methods, others are more difficult. The b.c.c. to f.c.c. transformation of monatomic solids is an example of a transition that is readily observed (5, 7) (see Figs. 2 and 3). This transition has been observed as a function... 

Column 8 of Table 3 is a list of the reactions. In addition to the wide range of catalytic transformations with gas-phase reactants, the list includes a representative selection of noncatalytic gas-solid reactions characterized by phase transformations combined with redox reactions of the solid. Iron catalysts were investigated in the presence of a liquid environment, and the results show that there is no fundamental problem with the use of X-rays penetrating a thin film of a liquid electrolyte or a solvent. 

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