Photodecomposition of chloromethanes adsorbed on silica surfaces

Abstract

Irradi a tion of CC I4 , CFCI3 , a nd CF2C1 2 in the prese nce of C2 H6 in vesse ls co ntaining s ilica sa nd or fused qua rt z tubing results in th e formati on of chl orine-cont aining produ cts. The form ation of these compounds occurs at wavelength s ex tending up to a pproximately 400 nm, that is, at wavele ngths wel l beyond the a bsorpti on threshold of the ch lo romethanes in the gas p hase . It is s uggested that CCI, adsorbed on silica surfaces ph otod issociates to yield CCI" and CCl2 s pec ies. The poo r mate ri a l ba la nce obtained in these experim ents ind ica tes that several of the chl orine -co ntaining fragme nts are strongly adsorbed o n the su rface. At a CCI, pressure of 13 Pa (0.1 torr), ph otolys is with 366 nm light in th e prese nce of sa nd resu lt s in the decomposit ion of one mo lec ule for every 10' photo ns stri king th e sudace. Under oth erw ise id e nt ical co ndit ions, the photon-induced breakdown of CFC I3 and CF2CI2 is res pectively o nl y JO pe rce nt ur 3 pe rc e nt as effi c ie nt.Key words: Chl oron1etha nes, photoche mist ry; q ua ntum yields; q ua rtz; sa nd; surface reacti ons; tropospheri c s ink. . IntroductionIn a recent study [1 r from this laboratory, it was shown th a t in th e gas phase, the photodecompositi on c ross secti on of CC14 at 313 nm was ~ 3 .7 ± 0 .4 X 10-26 cm 2 mo lec ul e-I at 300 K. Because sun l ight reach ing sea level consists mainly of wa velengths a bove 320 nm , thi s laboratory result suggests tha t in th e troposphe re gaseous CC14 will not be di ssociated by light. Th e p hotodi ssoc iation cross section of f1u orines ubst ituted meth a nes a t th e wavele ngth s whi ch reach the ear th's surface s hou ld be eve n smalle r th a n th at [or CC14 , s ince th e abso rpti on s pec tra of th e f1u orin e-substituted compounds are shifted to shorter wavele ngth s [2]. The refore, c hloromethanes and fluorochloromethanes released to the a tmospher e would be expected to diffuse to the stratosph ere, unless s ome mec hanism for their re moval other th an p hotodecomposition exists. As Molina and Rowland [2] suggested , th ese compounds in the stratosphere would absorb h igh ene rgy ph otons and undergo photodissociation to produ ce chlorin e atoms , which in turn would be expected to unde rgo a c ha in re ac tion resulting in the re moval of ozone molec ules .Howe ve r, the re were available two pi eces of evide nce whi ch ind icated that possibly CCl4 wh en adsorbed on cel·tain sltl-faces d oes undergo photodecomposition at wavelength s as long as th ose wh ich reach the earth's surface . Th e first such observa ti o n was that the cross section for ph otodeco mpos iti on of CC I4 showed a n appare nt increase by a facto r of fi ve wh e n the measure ments were made in quartz ra th er th an Pyrex 2 vessels. Second ly, measurements of the co ncentra ti on of CCl4 in th e tro pos phere in the e astern he mis ph ere as a functi on of la titude fro m 50 N to 3 0 S have shown th at there is a pro nou nced minimum in the CC14 concentrati on in the...

Mineralization of CCl4 and CCl2F2 on Solid Surfaces

“…The first evidence for this was provided by Junge, Schütz, and Lovelock, who found that the concentrations of N20 and CCI4 in air masses originating over the Sahara were lower than in regions to the north or south [1]. In support of this idea we were able to show in the laboratory that organic compounds adsorbed on solids can undergo photochemical degradation to C02 (photomineralization) [2], and Ausloos et al carried out experiments in which CCI4, CCI3F, and CC12F2 adsorbed on silica were dissociated by long-wave UV light (?, > 300 nm), which cannot be absorbed by these compounds in the gas phase [3]. Further confirmation of these findings came from experiments with sunlight, rather than artifical light, where both the disappearance of the halocarbons [4] and the formation of C02 [5] were observed.…”

Dissociation of Molecules by Slow Electrons

“…(244) Photodecomposition is possible at lower photon energies when the chlorofluoromethanes are adsorbed on surfaces such as grains of sand. (245,246) It has been suggested that weakly bound electrons on these surfaces are responsible for this effect. (244) Electron attachment studies of these compounds are also important because they are most easily detected in the environment using the electron capture detector.…”

Atmospheric chemistry: response to human influence

“…Data from this laboratory (Kaplan et al 1978) indicate that fresh water systems can provide both sources and sinks for N 2 0 . It would seem possible that aqueous systems could play an important role and that biological uptake might represent a major sink (see, for example, Brice, Eggleton & Penkett 1977), though heterogeneous chemistry could also contribute (Ausloos, Rebbert & Glasgow 1977). The issue is clearly unresolved.…”

“…The results shown here were obtained with a model which assumed that stratospheric photolysis was the only removal process for CFCl 3 and CF 2 Cl 2 • The perturbation to ozone would be smaller if there were additional important sinks (cf. Ausloos et al 1977). Inclusion of CO and H 2 emissions diminishes the perturbations to total ozone, as discussed above.…”