The thermal decomposition of ozone

As was discussed earlier, ozone plays an important part in the chemistry of the troposphere, where its excess is harmful, and in stratospheric chemistry, where its shortage is also detrimental. Ozone can decompose by several mechanisms thermal, photochemical, homogeneous catalysis reactions and under the action of solid surfaces. In the laboratory, the latter effect can be controlled by a suitable treatment of the reactor walls, as well as by a study of the rate of reaction as a function of the surface/volume ratio. In order to eliminate photochemical and homogeneous catalysis reactions, the chemical reaction must be carried out in the absence of radiations and catalytic additives, such as halogenated substances. The mechanism put forward to interpret the thermal reaction, can be written as follows  

This is an open sequence reaction, as the O atom produced by reaction (39) disappears in reaction (40) without being regenerated later on. 

In order to establish the kinetic laws of the reaction, the QSSA is applied first of all to the very reactive intermediate O  

The following expression for the quasi-stationary concentration of the O atom is deduced  

Equation (44) means that reaction (39) is at quasi-equilibrium. The rate of reaction can be written  

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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. 

Another simple reaction with a complicated reaction rate law is Reaction 1-5, 203(gas) 302(gas), which may be accomplished thermally or by photochemical means. The reaction rate law for the thermal decomposition of ozone is d /df= c5[03] /[02] when [O2] is very high, and is d /dt=ks [O3] when [O2] is low. 

Benson, S.W., and A.E. Axworthy (1965) Reconsiderations of the rate constants from the thermal decomposition of ozone, J. Chem. Phys., 42,2614. 

Pure Ozone. Luminosity may be obtained simply by the thermal decomposition of ozone. Schuller ( 4) noted that ozonized water emitted a weak light, and Meyer (19) expressed the opinion that this phenomenon really was responsible for the light emission by phosphorus. Beger (5) by working at 350° C. was able to show that the luminescence could be attributed to the transformation of ozone into oxygen. This was the conclusion which Dewar (18) also drew from his vacuum experiments. 

Most of the known data on the thermal decomposition of ozone can be explained quantitatively in terms of the simple atomic mechanism... 

Discussion. The simple atomic mechanism (Reactions 1 to 3) appears to be compatible with the qualitative features of the thermal decomposition of ozone. An attempt was made, therefore, to test quantitatively. 

It thus appears that a possible and fast mechanism for the production of ozone is by way of oxygen atoms which act as catalysts for the conversion of 02 O3. Because oxygen atoms are essentially slow in destruction of ozone, the limiting stationary process must be the destruction of ozone via the same type of process which is responsible for oxygen destruction—e.g., electron bombardment—or else the increase in temperature of the discharge which would finally provoke the thermal decomposition of ozone and make Reaction 3 a limiting process. 

Reaction of Carbon Monoxide with Oxygen Atoms from the Thermal Decomposition of Ozone... 

The reaction between carbon monoxide and oxygen atoms produced by the thermal decomposition of ozone was studied in the range 80-160 C The chemiluminescence from 002( 82) was used to follow the course of the reaction. The effect of added carbon dioxide, tetrafluoromethane and oxygen on the kinetics and chemiluminescence was investigated. It is concluded that there are simultaneous bimolecular and third body channels for the reaction of CO with 0-atoms to produce electronically excited C02 ... 

The reaction of oxygen atoms with carbon monoxide is an important reaction in many combustion systems. Although there is an extensive literature on this reaction Q) there is disagreement and uncertainty on the molecularity of the reaction, on the kinetic parameters and on the mechanism of the chemiluminescence. We have investigated this reaction using 0-atoms from the thermal decomposition of ozone. This has advantages compared to systems where... 

The rate of the photodissociation of ozone in the example shown in Figure 11.9 may seem slow. But it is actually tremendously faster than what we would see in the absence of ultraviolet light. The rate constant, k, for the thermal decomposition of ozone in the dark at 2S°C is just 3 x 10 ozone under these conditions ... 

It is of interest to ascertain the time at which the steady concentration is attained. Consider in this connection the thermal decomposition of ozone, the mechanism of which involves the following elementary steps [38]... 

Benson, S.W. Axworthy, A.E., 1965 Reconsiderations of the Rate Constants from the Thermal Decomposition of Ozone , in The Journal of Chemical Physics, 42 2614. 

Phosphoms oxyfluoride is a colorless gas which is susceptible to hydrolysis. It can be formed by the reaction of PF with water, and it can undergo further hydrolysis to form a mixture of fluorophosphoric acids. It reacts with HF to form PF. It can be prepared by fluorination of phosphoms oxytrichloride using HF, AsF, or SbF. It can also be prepared by the reaction of calcium phosphate and ammonium fluoride (40), by the oxidization of PF with NO2CI (41) and NOCl (42) in the presence of ozone (43) by the thermal decomposition of strontium fluorophosphate hydrate (44) by thermal decomposition of CaPO F 2H20 (45) and reaction of SiF and P2O5 (46). 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Ozone is formed in certain chemical reactions, including the action of fluorine on water (p. 323) and the thermal decomposition of iodic(VII) (periodic) acid. It is also formed when dilute (about 1 M) sulphuric acid is electrolysed at high current density at low temperatures the oxygen evolved at the anode can contain as much as 30 % ozone. 

Rice, C.P., Sikka, H.C.,and Lynch, R.S. Persistence ofdichlobenil in a farm pond, / Agric. FoodChem., 22(3) 533-535,1974. Rice, F.O. and Murphy, M.T. The thermal decomposition of five-membered rings, / Am. Chem. Soc., 64(4) 896-899,1942. Richard. Y. and Brener. L. Removal of pesticides from drinking water by ozone, in Handbook of Ozone Technology and Applications, Volume II. Ozone for Drinking Water Treatment, Rice, A.G. and Netzer, A.. Eds. (Montvale, M A Butterworth Publishers, 1984), pp. 77-97. 

Ozonization of the polypropylene powder creates the peroxidic species in the polymer, as well. The activation energy [41] of the thermal decomposition of these peroxides is 100 kJ/mol. In the decomposition of peroxides more than one type of radicals was trapped. Moreover, the three exotherms (peak at 40,90, and 130 °C) were observed on DSC thermograms of ozonized sample which also indicates the presence of several types of peroxides. Besides the peroxidic bonds in polymer, selective thermal decomposition may occur also with such bonds in the polymer as, e.g., with end groups containing the initiator moieties [42], This, however, takes place at higher temperatures than it corresponds to usual temperatures at which the thermo-oxidation starts. 

Thus, decomposition of PAN in the presence of NO can lead to the formation of OH radicals and conversion of NO to NOj merely from thermal reactions. This could be very important because PAN can be an indicator of the urban ozone-forming potential (see Section 8.4.1) and of the organic-oxidizing capacity of the urban air mass. The thermal decomposition of PAN also leads to the formation of NOj radicals. 

According to eqn. (109a), only CO may be replaced by inert gas without lowering the overall second limit pressure, and this is contrary to experiment. The situation can be modified by inclusion of further reactions, so that some semblance of agreement is obtained. However, there is a stronger objection. There is no evidence that CO reacts directly with O3 at 350 °C, the formation of CO2 in these conditions being due to reaction with oxygen atoms which arise from the thermal decomposition of the ozone[379, 380]. 

The photochemical production of ozone is of limited industrial interest because the practical yield is much lower than that produced by silent electric discharge. However, there are a number of scientific problems connected with photochemical ozone production which have not yet been solved, and the number continually increases, because of the important role which ozone plays in the atmosphere. As a constituent of the atmosphere (about 100 millionth parts thereof at the earth s surface), ozone forms a protective screen because it absorbs radiations of wave lengths below 3000 A. which are deleterious to life. Furthermore, the heat liberated by such absorption and by the exothermic decomposition of ozone creates in the higher atmosphere (at approximately 40 km.) a warm layer which helps to establish thermal equilibrium on our planet. 

Because all heat dissipated in the gas must eventually be conducted to the walls, the maintenance of a low temperature in the discharge requires a narrow gap (walls close together), low currents, and low space velocities (low specific rates of O3 production). These requirements are opposite to the conditions which would be suggested by the known sensitivity of ozone to wall catalysis and the enhanced rate of destruction of such carriers as 0, 0 , and at walls. All of these contribute to low efficiency O3 production. To the extent that heat is not successfully dissipated, the mean temperature rises and the thermal decomposition of O3 becomes increasingly important. 

Thus, if a 10% ozone mixture is produced, complete absorption by turpentine will reduce 100 cm (cubic centimeters) of gas to 90 cm. On the other hand, if this 10% mixture is heated in order to decompose ozone to oxygen, the 10 cm of ozone present yields 15 cm of oxygen for a total gas volume of 105 cm. In other words, if reactions (2) and (3) hold (i.e., the formula of ozone is O3), then the diminution of volume upon complete reaction with ozone must be twice the expansion of volume upon thermal decomposition of ozone. The reproducibility of this experiment was verified and allowed the assignment of the formula despite its relatively low abundance in the mixture. 

Even small admixtnres of halogens, like bromine and chlorine, also have a significant negative effect on prodnction and stability of ozone (Benson Axwortly, 1957). Such admixtures sometimes lead to ozone explosions. The halogens stimulate catalytic thermal decomposition of ozone via the fast chain mechanism, which in the case of chlorine, for example, starts with the formation of atomic chlorine and CIO radicals in the chain initiation reactions (Schumacher, 1957) ... 

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