Heterogeneous decomposition of trichlorofluoromethane on carbonaceous surfaces

Abstract

The interaction of trichlorofluoromethane with activated charcoal has been investigated by dynamic mass spectrometry up to 750 K. Prior physical adsorption, revealed in programmed desorption experiments, is followed by irreversible first-order decay with formation of nearly equimolar amounts of HCl above 550 K. This unexpectedly fast process has an apparent activation energy of only 59.4 kJ mol-' and is demonstrably catalytic. The mechanism of C1,CF decomposition on carbon surfaces and its possible impact on atmospheric chemistry are discussed.We have recently presented direct evidence showing that carbon surfaces efficiently catalyse the atomization of molecular chlorine and the breakdown of polychlorinated methanes.' Such results are particularly relevant to the free-radical mechanisms underlying the synthesis and decomposition of chlorocarbons and, moreover, provide a rationalisation for the erratic kinetic behaviour often observed in such systems.2 Early reports had already identified charcoal as a catalyst in thermal chlorination^.^ Since carbon is the most abundant element associated with pollution particulates,* and considering that the collision frequency between gas molecules and particles in highly contaminated urban environments can reach up to ca. 1 s -' ,~ and that current estimates of the tropospheric lifetime of chlorofluorocarbons span the wide range 30-150 years, we decided to undertake studies on the decay of C1,CF with the specific goal of assessing the likelihood of its decomposition on atmospheric carbonaceous surfaces. Clearly we face one of those rare kinetic problems which require one to explore chemical change over long, rather than short, time period^.^ The results reported in this paper confirm the remarkable activity of carbonaceous surfaces towards chlorocarbon decomposition and, in conjunction with simple considerations, suggest the possibility of an active role for soot as a tropospheric sink for such species. ExperimentalKinetic studies were performed in a 300 cm3 heated Pyrex reactor coupled to an analytical mass spectrometer (EMBA 11, Extranuclear Laboratories) via a molecular leak for continuous monitoring of reactants and pr0ducts.l It was verified that C1,CF (Freon 11, Matheson) is perfectly stable in the gas phase below 750 K, as expected from kinetic and thermochemical considerations." On the other hand the addition of small amounts (0.5-1.0 g) of activated carbon (Mallinckrodt, 0.5 O h ash, specific area 684 m2 g-l, as determined by nitrogen adsorption B.E.T. measurements) dispersed on glass wool to minimize mass-transfer artifacts, led to the rapid first-order decay of C1,CF at pressures between 0.02 and 0.1 1 Torr in the range 555-750 K. The most intense ion signal in the mass spectrum of the reactant at m / z = 101 (C1,CF') was used for this purpose. Moreover, rates were found to be proportional to the amount of carbon added, confirming the catalytic nature of a process which leads to the formation of nearly equimolar amounts of HCl as the sole gaseous product. The last...

Heterogeneously catalysed hydrolytic decomposition of CFCs

“…An alternative approach to incineration is the catalytic oxygenation of C 1 and C 2 CFCs by means of O 2 and/or H 2 O to give CO 2 over acidic oxide catalysts. [11][12][13][14][15][16][17][18][19][20] A variety of catalysts, zeolites, TiO 2 , ZrO 2 and V 2 O 5 , have been reported. ZrO 2 (PO 4 ) and V 2 O 5 , for example, are particularly active.…”

Decomposition of dichlorodifluoromethane of titania/silica

“…CFCs can be destroyed by some techniques such as reduction with sodium naphthalenide (Oku et al, 1989) or hydrogen (Witt et al, 1981), decomposition by solar energy (Blake, 1988), and decomposition on activated charcoal (Colussi and Amorebieta, 1987). However, incineration seems to be the most practical technique when a large amount of CFCs must be destroyed.…”

Liquid-Phase Adsorption of 1,1-Dichloro-1-fluoroethane by Various Adsorbents

“…Because CFCs are highly stable chemical compounds, safe disposal of all CFCs is not easy. Incineration or chemical destruction has been popular methods, and investigations have been conducted by many researchers to examine the various facets of incineration and chemical destruction (Colussi and Amorebieta, 1987;Blake, 1988;Okazaki and Kurosaki, 1989;Oku et al, 1989;Imamura et al, 1990Imamura et al, , 1991Imamura et al, , 1992Aida et al, 1990;Miyatani et al, 1992;Cheung and Kurup, 1994;Greene, 1992, 1995;Nagata et al, 1994;Kiefer et al, 1994;Bell et al, 1995). Because of high operating temperatures and because metal catalysts were required in most of the chemical destruction of CFCs, it is a relatively difficult and expensive process.…”