Mechanisms of ozone decomposition

Mechanism of ozone decomposition in water depends on the presence of chemical species that can initiate, promote and/or inhibit its decomposition. The most accepted ozone decomposition mechanism is given in Figrtre 3. 

Tomiyasu, H Fukutomi, H Gordon, G. Kinetics and mechanism of ozone decomposition in basic aqueous soiution. inorganic Chemistry, 1985 24 (19), 2962-2966. 

Tomiyasu H, Fukutomi H, Gordon G (1985) Kinetics and Mechanisms of Ozone Decomposition in Basic Aqueous Solutions, Inorganic Chemistry 24 2962-2985. 

On the other hand, the indirect type of ozonation is due to the reactions of free radical species, especially the hydroxyl radical, with the organic matter present in water. These free radicals come from reaction mechanisms of ozone decomposition in water that can be initiated by the hydroxyl ion or, to be more precise, by the hydroperoxide ion as shown in reactions (4) and (5). Ozone reacts very selectively through direct reactions with compounds with specific functional groups in their molecules. Examples are unsaturated and aromatic hydrocarbons with substituents such as hydroxyl, methyl, amine groups, etc. [45,46],... 

Li, W., Gibbs, G.V., Oyama, S.T. Mechanism of ozone decomposition on a manganese oxide catalyst. 1. in situ Raman spectroscopy and ab initio molecular orbital calculations. J. Am. Chem. Soc. 120, 9041-9046 (1998)... 

Figure 3. Scheme of ozone decomposition mechanism in water. P = promoter (e.g. ozone, methanol), S = scavenger or inhibitor (e.g. /-butanol, carbonate ion), I = initiator (e.g. hydroxyl ion, perhydroxyl ion) (adapted by Beltran [35]). 

Keto groups are introduced during an ozone (Z) stage [11,12], Besides the action of hydroxyl radicals, which can be generated in an ozone stage either by slow decomposition of ozone in water or be triggered in the presence of metal ions, also an ionic pathway by the common 1,3-insertion mechanism of ozone has been proposed [13,14], leading to the eventual formation of lactones. 

The role of oxygen derived short-lived radical species in AOPs is still a field of active research. However, extensive knowledge has been accumulated on the mechanism of ozone aqueous chemistry (Rice, 2001, Gottschalk, 2000, Hoigne, 1998). The decomposition of molecular ozone in water by a radical-type chain reaction involves species such as hydrogen trioxide (HO ) with a lifetime of about 10 ps and HO radicals. The latter equilibrate with their dissociation products in about 30 ps, i.e. O3 and OH (Hoigne, 1998) or they decompose into O2 and HO promoting the chain reaction. 

The fractional value for n—i.e., the apparent order of reaction—is probably due to the complex nature of the competing reactions and the instability of the intermediates. In other words, the mechanism and the kinetics of ozone decomposition in aqueous solution and the possible intermediate unstable ozonide-ring leading from cyanide to cyanate will affect the apparent order of reaction. 

Ozone is formed by the photolytic decomposition of NO2 yielding oxygen radicals and by the reaction sequence NO2 — HNO3 — NO3 O3. In particular, the reaction of NO2 with hydroxyl radicals to form HNO3 increases ozone concentration because two radicals, NO2- and -OH, which catalyze ozone decomposition, are removed. Other radicals are also important for ozone destruction in the stratosphere, especially chlorine oxides see Chlorine, Bromine, Iodine, Astatine Inorganic Chemistif). The mechanism of ozone destruction is complicated as there many compounds involved. Chlorine nitrate and dinitrogen pentoxide can act as reservoir species for CIO-, NO2-, and NO3- radicals. 

Aqueous Phase. In pure water, the decomposition of ozone at 20°C iavolves a complex radical chain mechanism, initiated by OH and propagated by O2 radical ions and HO radicals (25). O3 is a radical ion. 

The decomposition of ozone, 03, to diatomic oxygen, 02, is believed to occur by a two-step mechanism ... 

As a final example of numerical simulations, consider the base-catalyzed decomposition of ozone in aqueous solution. This multistep reaction is controversial in that contradictory mechanisms have been suggested.33 34 The set of reactions that appears to be the most consistent with the experimental data is shown in Table 5-1, with a set of rate constants. Most of these values were reported in the literature, but several were refined to give agreement with experiments that measured the decline in concentration O3. 

Chain reaction. A possible mechanism for the chlorine-catalyzed decomposition of ozone is... 

FIGURE 13.16 A representation of a proposed one-step mechanism for the decomposition of ozone in the atmosphere. This reaction takes place in a single bimolecular collision. 

FIGURE 13.17 An alternative, two-step mechanism for the decomposition of ozone. In the first step, an energized ozone molecule shakes off an oxygen atom. In the second step, that oxygen atom attacks another ozone molecule. 

Mechanism II begins with fast reversible ozone decomposition followed by a rate-determining bimolecular collision of an oxygen atom with a molecule of NO. The rate of the slow step is as follows Rate = 2[N0][0 This rate expression contains the concentration of an intermediate, atomic oxygen. To convert the rate expression into a form that can be compared with the experimental rate law, assume that the rate of the first step is equal to the rate of its reverse process. Then solve the equality for the concentration of the intermediate ... 

The net reaction for this two-step mechanism is the conversion of an O3 molecule and an oxygen atom into two O2 molecules. In this mechanism, chlorine atoms catalyze ozone decomposition. They participate in the mechanism, but they do not appear in the overall stoichiometry. Although chlorine atoms are consumed in the first step, they are regenerated in the second. The cyclical nature of this process means that each chlorine atom can catalyze the destruction of many O3 molecules. It has been estimated that each chlorine atom produced by a CFC molecule in the upper stratosphere destroys about 100,000 molecules of ozone before it is removed by other reactions such as recombination CF2 Cl -b Cl CF2 CI2... 

C15-0051. Do the following for the ozone decomposition mechanism (a) Express the rate in terms of O2 formation, (b) Relate the rate of O3 consumption to the rate of O2 production, (c) If O2 forms at a rate of 2.7 X 10" M, state how fast ozone disappears. 

C15-0078. A student proposes the following mechanism for the atmospheric decomposition of ozone to... 

The gas phase decomposition of ozone is catalyzed by nitrogen pent oxide. Two mechanisms that purport to explain this process are given below. Can relatively simple kinetic measurements be used to eliminate one of the proposed mechanisms from further consideration Explain. 

As the reaction temperature is increased, chemiluminescence is observed in the reactions of ozone with aromatic hydrocarbons and even alkanes. Variation of temperature has been used to control the selectivity in a gas chromatography (GC) detector [35], At room temperature, only olefins are detected at a temperature of 150°C, aromatic compounds begin to exhibit a chemiluminescent response and at 250°C alkanes respond, giving the detector a nearly universal response similar to a flame ionization detector (FID). The mechanisms of these reactions are complex and unknown. However, it seems likely that oxygen atoms produced in the thermal decomposition of ozone may play a significant role, as may surface reactions with 03 and O atoms. 

The second term characterizes the chain decomposition of ozone. The mechanism of chain reaction was found to include the following reactions ... 

A knowledge of the kinetics of the decomposition of ozone is essential for the understanding of the chemistry of some important processes which occur in earth s atmosphere. Yet, in spite of numerous studies and the structural simplicity of ozone, the mechanism of its ultraviolet photolysis is still uncertain. Electronically and vibrationally excited species are involved in ozone decomposition and the current knowledge of the chemical behavior of such intermediates is still in its infancy. 

Alder, M. G., and G. R. Hill. The kinetics and mechanism of hydroxide ion catalyzed ozone decomposition in aqueous solution. J. Amer. Chem. Soc. 72 1884-1886, 1950. 

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