Naslov
Heterogeneous Photocatalytic Degradation of Fluorochlomethanes with Simulated Tropospheric Light

Autori
Filby, Gordon ; Mintas, Mladen ; Güsten, Hans

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Skup
German-Yugoslav Symposium on Enviromental Chemistry in Air and Water

Mjesto i datum
Rovinj, Hrvatska, 1980

Vrsta sudjelovanja
Poster

Vrsta recenzije
Nije recenziran

Ključne riječi
Heterogena fotokatalitička degradacija
(heterogeneous photocatalytic degradation of fluorochlomethanes)

Sažetak
The release of man-made chlorofluoromethanes (CFM’ s) into the atmosphere has received worldwide attention because of potential damage to the stratospheric ozone layer (Rowland and Molina, 1975 ; Güsten, 1976). Current observations and theory indicate that the commenly used refrigeranta and aerosol propellants CFCl3 and CFCl2 are not degradaded ib the troposphere. Because of the very slow vertical diffusion of the CFM’ s in the order of 30 years only 1% per year will be transported into the stratosphere. Thus, even a very inefficient tropospheric removal process might constitute a tropospheric sink. Computer model calculations show that, for example, xith tropospheric removal at a rate of 1% per year only half of the anthropogenic CFM’ s would diffuse into the stratosphere. Estimates of the rates for suggested tropospheric sinks of CFM’ s such as dissolution in the ocean, trapping in Antertic snow or reaction with atmospheric ions have proven to be quantitatively of minor importance. There are, however, indications that CFM’ s can be photolysed catalytically on sand with simulated tropaspheric solar light (> 300 nm) (Ausloos et al., 1977). Large siliceous land surfaces such as deserts might act in this respect as sinks (Alyea et al., 1978). We suggest an additional photocatalytic process as a possible tropospheric sink for CFM’ s, namly the heterogeneous photocatalytic degradation on the surface of atmospheric aerosols and dusts. The lower troposphere contains a lagre number of adsorptive and potentially catalytic aerosols which provide a large surface area. For example, one gram of particles of 0.1 μ m radius has an surface area of 10 m2. Since their chemical composition are minly oxides these aerosols are potentially good photocatalysts. Laboratory experiments with simulated tropospherical sun light in the presence of ZnO indicate a heterogeneous photolytic decomposition of the non-light absorbing CFM’ s. The photolytic degradation was followed with ESCA spectroscopy (Table 1). * Visiting scientist from the University of Zagreb, Faculty of Technology Department of Organic Chemistry. Table 1 Photocatalytic degradation of CF2Cl2 on ZnO. Dependence of binding energies and halogen/Zn ratio of the irradiation time. Irradiation time (hrs) Binding energy (eV)* F/Zn+ Cl/Zn+ Zn O F Cl 0 1021.7 532.1 685.3 199.4 0.03 0.007 54 1021.9 532.2 685.3 199.7 0.03 0.014 175 1022.0 532.2 685.3 199.9 0.031 0.019 364 1023.2 532.2 685.3 199.98 0.027 0.024 652 1023.7 532.3 685.3 200.2 0.029 0.03 * C(1s) = 285.0 eV as standard + initial halogen/Zn ratios are due to non-removal impurities present in the zinc oxide The increasing Cl/Zn ratio indicates a photocatalytically induced chlorine cleavage in CFM’ s on the ZnO surface. Interesting is the constancy of the F/Zn ratio during the course of the irradiation. This we interpret as indicating a desorptin of the fluorine containing rasidue from the surface at one or more stages in the reaction sequence. A tentative reaction mechanism for the heterogeneous photocatalytic degradation of CFM’ s on oxide surface: (XO2+, e¯ ; ) + CFCl3 → XO2+ + CFCl3¯ ; CFCl3¯ ; → Cl¯ ; + • CFCl2 CFM’ s exibit an extremely large electron cross section for electron capture. Contrary to many other small molecules in the gas phase dissociative electron capture occurs here without activation energy (Illenberger et al., 1978).

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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