Homomolecular reaction

Two general approaches to the study of the exchange reaction have been made namely the homomolecular reaction (e.g., [10]) and the heterol dic reaction (e.g., [11]). In the former, a mixture of 02/ 02 is scrambled over the oxide surface and in the latter, gas-phase 2 is reacted directly with the oxide itself. The former reaction leads solely to the production of 3 02 ... 

If high concentrations of oxygen-18 are used, it should be noted that isotopic scrambling of oxygen molecules, known as homomolecular reactions, can occur at low temperatures under the influence of light, electric discharges, ozone, chlorine, or oxide surfaces (Ogg, 1953 Ogg and Sutphen, 1953 Johnston and O Shea, 1953). 

Almost all the crystalline materials discussed earlier involve only one molecular species. The ramifications for chemical reactions are thereby limited to intramolecular and homomolecular intermolecular reactions. Clearly the scope of solid-state chemistry would be vastly increased if it were possible to incorporate any desired foreign molecule into the crystal of a given substance. Unfortunately, the mutual solubilities of most pairs of molecules in the solid are severely limited (6), and few well-defined solid solutions or mixed crystals have been studied. Such one-phase systems are characterized by a variable composition and by a more or less random occupation of the crystallographic sites by the two components, and are generally based on the crystal structure of one component (or of both, if they are isomorphous). 

As will be discussed in Sect. 5, linear free energy relations allow comparisons between the kinetics of heterogeneous (electrode) and homogeneous homomolecular charge transfer reactions. 

It is generally concluded that 0 species are the most reactive in CO oxidation and in homomolecular O2 exchange under illumination. The latter reaction proceeds via... 

Homomolecular O2 exchange is very slow on ZnO at 298 K in the dark. However, cooling to 77 K produces a sudden increase in rate. There is debate over the active intermediate for this strange, non-photocatalytic reaction. Tanaka et al. and Hirota et al. favour O4 and 4 species, respectively, whereas Russian workers conclude that these cannot be involved. Tanaka et al. published e.s.r. studies using " 02 in support of neutral O4 species, but Gundrizer et al. state that such species in liquid O2 are inactive in exchange and, therefore, that it is more likely for the intermediate on ZnO to be a non-radical , molecularly adsorbed species. It is not clear in their paper what exactly is meant by this nor how such species can function as intermediates. 

In Section II the perovskite and related structures are briefly introduced. In Section III the methods most frequently used for perovskites preparation are described comparatively. Sections IV, V, and VI refer to the bulk and surface properties of perovskites. Some of these properties will facilitate understanding of the catalytic action of these compounds. Section VII includes a review of the reactions where perovskite oxides were used as catalysts. Some of them are described separately (Sections VII,A-H). These include reactions that were more extensively studied or reactions that may have an increasing interest in the near future. Section VII,I includes less studied reactions such as oxygen homomolecular exchange, hydrogen and NH3 oxidations, N20 decomposition and dehydro-... 

The dissociation of molecular nitrogen may be investigated separately by measuring the rate of the homomolecular exchange reaction (80-87)... 

Similarly, measurements of the rate of the homomolecular exchange reaction Ha + Dg = 2HD may be evaluated provided that the isotope effect resulting from a mass ratio 1 2 is taken in account. Since these reactions have been reviewed by other authors (107,108), no discussion is included in this article. 

Studies of the homomolecular and heterolytic exchange processes are generally in the form of the measurement of rates under isothermal conditions. However, studies have also been made of temperature programmed isotopic exchange, in which the oxide is subjected to a temperature ramp under the reaction atmosphere, and the partial pressures of various isotopic oxygen species is determined as a function of temperature (e.g., Refs. 20-21). The photoactivation of oxygen exchange has also been reported in a number of studies which have been performed under UV irradiation (e.g.. Refs. 18, 22, 23). 

The dependence of equilibrium constant for reaction [9.116] on e is similar in a concentration range from 0.5 to 1.0 molar fraction of aldehyde, if the degrees of homomolecular association for aldehydes and acetals are smaller than for alcohol and water. 

By integrating eq. (8-23) and using eq. (8-22), we can calculate the concentration of electronic charge carriers at the surface,, and thus we can calculate the rate of dissociation, if we assume a certain type of disorder in the thin film layer, and if the proper boundary conditions are used. As an example [22, 23], let us consider the oxidation of Ni to NiO. For thin oxide layers, the oxidation process approximately obeys a cubic rate law between 250 °C and 400 °C. Simultaneous studies of oxidation and homomolecular isotope exchange according to the reaction = 2 at the NiO surface have been carried out. These... 

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