Decomposition heterogeneous reactions

The catalyzed decomposition of ozone is known to be responsible for the ozone hole (Figure A) that develops in Antarctica each year in September and October, at the end of winter in the Southern Hemisphere. No ozone is generated during the long, dark Antarctic winter. Meanwhile, a heterogeneous reaction occurring on clouds of ice... 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

The stoichiometry of decomposition of [Ni(NH3)4](NCS)2 was dependent on the method of salt preparation [1126]. Ammonia was lost in three successive steps (—NH3, —NH3, —2 NH3) from the solution-prepared salt, but the first intermediate could not be isolated from the similar reaction of material prepared by heterogenous reaction. The difference in behaviour was ascribed to differences in perfection of the crystallites resulting from the alternative preparative methods. 

Consideration thus far has been on only balanced reactions which occur in one phase, that is, homogeneous reactions. There are, of course, a great many reactions which occur between substances in different phases, and these are known as heterogeneous reactions. Numerous reversible, heterogeneous reactions are known, and it is pertinent now to bestow consideration on how far the law of mass action can be applied to such cases. The familiar reaction of the decomposition of calcium carbonate thermally - a well-known example of a reversible reaction represented by the equation... 

The transport of heat a fine example for this factor may be cited thermal decomposition of limestone endothermically to lime. The heterogeneous reaction is chemically shown as ... 

Hence, the copper surface catalyzes the following reactions (a) decomposition of hydroperoxide to free radicals, (b) generation of free radicals by dioxygen, (c) reaction of hydroperoxide with amine, and (d) heterogeneous reaction of dioxygen with amine with free radical formation. All these reactions occur homolytically [13]. The products of amines oxidation additionally retard the oxidation of hydrocarbons after induction period. The kinetic characteristics of these reactions (T-6, T = 398 K, [13]) are presented below. 

Cuf1101-HC00 The decomposition of formic acid on metal and oxide surfaces is a model heterogeneous reaction. Many studies have since shown that it proceeds via a surface formate species. Thus on Cu 110) adsorbed formic acid is found at low temperature. On heating to 270 K deprotonation occurs, giving rise to the surface formate, which in turn decomposes at 450 K with evolution of H2 and C02- In previous studies, particularly with vibrational spectroscopy, it had been demonstrated that the two C-0 bonds are equivalent and that the symmetry is probably C2v [19]. A NEXAFS study by Puschmann et al. [20] has subsequently shown that the molecular plane is oriented perpendicular to the surface and aligned in the <110> azimuth. 

Decomposition of gaseous COS proceeds measurably at temperatures in excess of 350 °C. Early investigators90 showed that in static systems between 350 and 600 °C a heterogeneous reaction... 

These are analogous to the reactions postulated for hydrocarbon systems by Laidler et al.115,176. With certain hydrocarbons there is almost no inhibition region, and the main effect of nitric oxide is to accelerate the rate of decomposition as in the present instance. Further work on this interesting aspect of the decomposition of S2F10 is obviously desirable. In addition, attention should be given to the effects of inert gases on the orders of the individual reactions in the above mechanism, and on the rate of the heterogeneous reaction. 

Section 2 deals with reactions involving only one molecular reactant, i.e. decompositions, isomerisations and associated physical processes. Where appropriate, results from studies of such reactions in the gas phase and condensed phases and induced photochemically and by high energy radiation, as well as thermally, are considered. The effects of additives, e.g. inert gases, free radical scavengers, and of surfaces are, of course, included for many systems, but fully heterogeneous reactions, decompositions of solids such as salts or decomposition flames are discussed in later sections. Rate parameters of elementary processes involved, as well as of overall reactions, are given if available. 

The transformations described in this chapter include mostly heterogeneous reactions at the surface of metallic lithium. Processes of this type can become too slow on a preparative scale at low temperatures. This is why either they have to be carried out at elevated temperarnres (possible decomposition reactions) or an activation (e.g. ultrasound) of the lithium metal is necessary. In spite of the relatively high reaction temperatures, very selective reactions are observed, when this method is applied to the presented systems. Thus, in addition to Section II. A (Deprotonation), this section contains many visualisations of X-ray structural analyses. 

IR spectrometric data reveal that the thermolysis of octa(methylsilsesquioxane) is a heterogeneous reaction involving the formation of a non-volatile solid phase similar to SiOj In a helium atmosphere at 150-300 °C (CHjSiO j)g almost completely evaporates without marked decomposition. When heat i in the air, however, this compound begins to decompose slowly at 270 °C, and at 450 °C an exo-effect attributed to the oxidation by air oxygen is observed When the temperature is raised to 500 °C, 66% of (CH3SiOi,5)g are oxidized to SiOj. 

Spinodal decomposition of a phase into two phases (heterogeneous reactions)... 

In addition, such small vessels have high surface-to-volume (S/V) ratios, which may increase the relative contributions of reactions that occur on the surface. One such heterogeneous reaction is the decomposition on surfaces of 03 to form 02 obviously, the faster this decomposition, the lower the concentrations of O, that will be observed during the chamber run. 

The top panel of Fig. 17.2 (Ts = 800 K) reveals that there is very little decomposition of the silane in the gas phase, which is a result of the relatively low temperature. As a result the net growth rates should be expected to be quite low, since the silane sticking coefficient is so low. At a surface temperature of Ts = 1300 K, however, the decomposition of silane to silylene in the gas-phase boundary layer is nearly complete. The relatively high silylene concentrations should lead to high growth-rates. The peak in the silylene profile at about 1.5 mm above the surface results from the competition between production by the homogeneous decomposition reaction and consumption at the surface by heterogeneous reaction. 

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