Catalysts ozone destruction

Chlorine atoms are also very efficient ozone destruction catalysts, as noted originally by Stolarski and Cicerone (4) ... 

This simple oxygen-only mechanism consistently overestimates the O3 concentration in the stratosphere as compared to measured values. This implies that there must be a mechanism for ozone destruction that the Chapman model does not account for. A series of catalytic ozone-destroying reactions causes the discrepancy. Shown below is an ozone-destroying mechanism with NO/NO2 serving as a catalyst ... 

In 1970 I then proposed that ozone destruction by NO and NO2 as catalysts (10) could occur via the pair of reactions ... 

Chlorine atoms can also act as a catalyst for the destruction of ozone. The first step of a proposed mechanism for chlorine-catalyzed ozone destruction is... 

The production of chlorofluorocarbons, or CFCs, has been banned by international treaty because of their deleterious effect on the ozone layer. The ozone layer absorbs much of the Sun s dangerous UV radiation before it reaches the Earth s surface. CFCs are extremely stable in the lower atmosphere (one reason why they are so useful), but when they reach the stratosphere they decompose, producing potent catalysts of ozone destruction. Ozone destruction is most evident above Antarctica during the spring, when this region is exposed to the Sun for the first time in months. Dichlorodifluoromethane (CF2CI2) is a typical CFC. 

The first of these reactions will result in the generation of a single ozone molecule. The second reaction produces the NO that leads to catalytic ozone destruction. The relative rates of these two reactions is in an approximate ratio of 9 1, favoring the first. Since NO is a catalyst for O2 destruction (a single NO will destroy many ozone molecules before being removed), N2O is believed to exert a significant control on stratospheric O3 concentrations. 

For the last two decades, attention has been focused on redressing the ozone depletion in the earth s protective layer. It is believed that chlorine radicals dissociated from chlorofluorocarbons (CFCs), upon irradiation of sun s UV in the stratosphere, promotes the ozone depletion. Hence, in addition to development of CFC alternatives there is an urgent need for the safe disposal of CFCs. Several processes such as pyrolysis, incineration, photocatalysis, oxidative destruction over metal oxide or zeolite catalysts and destruction at very high temperatures ( by plasma technique ) are reported in the literature for the disposal of CFCs[ 1-5]. But all these processes yield harmful products like CO, HF/F2 etc. Catalytic conversion of chlorinated organics in presence of hydrogen seems to be a better technique as it yields either hydrofluorocarbons(HFCs) or hydrochlorofluorocarbons(HCFCs) whose ozone depletion potential is either zero or very low and yet most of these products act as CFC alternatives. 

In these reactions NO is not consumed while it destroys ozone. Rather, NO acts as a catalyst to ozone destruction in a pure oxygen atmosphere. Because it is faster, the catalytic cycle proceeds several times during the same time interval in which the 03 loss reaction of the Chapman mechanism occurs once. 

Thus these additional reactions in the stratosphere decrease significantly the rate of ozone destrnction processes, especially nnder contamination of the atmosphere by chlorofluorocarbons and nitrous oxides as the catalysts of ozone destruction reactions. 

It was not until recently that N2O was recognized as an atmospheric pollutant which contributes to stratospheric ozone destruction and greenhouse effects. This gave more emphasis to the search of active catalysts to decompose N2O. 

Catalysts which are in the same phase as the reactants (e.g. they are all in solution) are termed homogeneous catalysts. Examples of homogeneous catalysis include the hydrolysis of sugar (page 254), and the catalysis of ozone destruction by CT (see page 394). Another example is the reaction of persulfate (S20 (aq)) and iodide (I (aq)) ions which obeys the overall reaction... 

Since the rate of ozone destruction depends on [Cl], can Cl be considered a catalyst for the reaction of Equation 18.10 ... 

The UV photometric ozone monitor is relatively maintenance free except for periodic calibration by iodimetry and changing of the inlet filter and catalyst. Hence, UV photometry is suitable for continuous monitoring of atmospheric ozone and has been adopted as a standard method along with the ethene-CL method in major counters. UV ozone monitors with higher concentration ranges (up to 10 vol%) are also available. In high-concentration ozone measurements, ozone-free reference should be prepared separately for calibration, because of insufficient activity of the catalyst for ozone destruction. For a humid air sample, a water trap (or humidifier) is necessary. 

The ozone destruction catalyst should be checked regularly to be sure that it has not been damaged or exhausted. The temperature of the destruction beds will auto-accelerate at excessive ozone concentrations, leading to an explosion. 

The depletion of ozone in the stratosphere by Cl atoms provides an example of the lowering of activation energy by a catalyst. Ozone is normally present in the stratosphere and provides protection against biologically destructive, short-wavelength ultraviolet radiation from the sun. Some recent ozone depletion in the stratosphere is believed to result from the Cl-catalyzed decomposition of O3. Cl atoms in the stratosphere originate from the decomposition of chlorofiuorocarbons (CFCs), which are compounds manufactured as refrigerants, aerosol propellants, and so forth. These Cl atoms react with ozone to form CIO and O2, and the CIO reacts with O atoms (normally in the stratosphere) to produce Cl and O2. 

Several reaction mechanisms are suggested on the role of halogens for the ozone destruction, e.g. chlorine acting as a catalyst... 

Fig. 2.1 displays a catalytic cycle containing Cl and CIO thus, formally ozone destruction has a chlorine catalytic cycle. For gas-phase catalytic reactions, often the catalyst should have a radical nature with a relatively low value of activation energy for formation of a catalytic complex. 

Many reagents act as contact catalysts for the destruction of ozone a study of some of them has been made. ... 

Catalysts are immensely beneficial in industry, but accidental catalysis in the atmosphere can be disastrous. Recall from Box that the chemishy of ozone in the stratosphere involves a delicate balance of reactions that maintain a stable concentration of ozone. Chlorofiuorocarbons (CFCs) shift that balance by acting as catalysts for the destruction of O3 molecules. 

Fig. 3.9. Photochemical formation and non-catalytic destruction of ozone. UV-C radiation (200-280 nm wavelength) UV-B radiation (280-320 nm wavelength). Note how high-quality energy (UV radiation) is converted into lower quality energy (heat). Catalysts such as freons or nitrogen oxides can destroy ozone (e.g. Cl + 03 —> CIO + 02).

You probably know that compounds called chlorofluorocarbons (CFCs) are responsible for depleting the ozone layer in Earth s stratosphere. Did you know, however, that CFCs do their destructive work by acting as homogeneous catalysts Use the Internet to find out how CFCs catalyze the decomposition of ozone in the stratosphere. To start your research, go to the web site above and click on Web Links. Communicate your findings as a two-page press release. 

CFCs are nearly ideal substances for attacking ozone molecules and damaging the ozone layer. On the one hand, they tend to be very stable, even in the stratosphere. Many CFCs have half-lives of 100 years or more that means that once they have escaped into the upper atmosphere, they are likely to remain there for very long periods. On the other hand, some small number of CFC molecules do dissociate to form chlorine free radicals, with the ability to destroy ozone molecules. Although the number of CFC molecules that do dissociate is relatively small, the actual number is not important since chlorine free radicals that are generated in the process are used over and over again. That is, they are catalysts in the destruction of ozone and are not, themselves, used up in their reactions with ozone molecules. 

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