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J. Phys. Chem. B 2000, 104, 978-982
Chemical Reactions of Organic Molecules Adsorbed at Ice 1. Chlorine Addition to Propene James D. Graham and Jeffrey T. Roberts* Department of Chemistry, UniVersity of Minnesota Minneapolis, Minnesota 55455-0431 ReceiVed: April 28, 1999; In Final Form: NoVember 17, 1999
A new ice-catalyzed chemical reaction is reported, namely the addition of Cl2 to the C-C double bond of propene (C3H6) to form 1,2-dichloropropane. The reaction, which was investigated with temperatureprogrammed desorption mass spectrometry (TPD), was carried out on ultrathin (10-100 monolayer thick) films of ice deposited on single-crystal metal substrates under ultrahigh vacuum. The Cl2-addition product was identified as 1,2-dichloropropane on the basis of its fragmentation pattern in the mass spectrometer. 1,2-Dichloropropane formation occurs below 150 K, and no chlorohydrin (e.g., 2-chloro-1-propanol) evolution is ever observed. The reaction of coadsorbed propene and chlorine is very different from what occurs in aqueous solutions, where chlorohydrin formation occurs readily. Possible mechanisms of the ice-surfacecatalyzed reaction are discussed.
Introduction Chemical reactions that are promoted or catalyzed by the surface or near surface regions of ice particles in type II polar stratospheric clouds (PSCs) have been shown to play a role in the annual sequence of events that ultimately opens the Antarctic ozone “hole.”1 At present, the most important processes occurring over ice are believed to be2-5 ice
HCl + ClONO2 98 Cl2 + HNO3 ice
HCl + HOCl 98 Cl2 + H2O ice
N2O5 + H2O 98 2HNO3
(1) (2) (3)
Heterogeneous processing of atmospheric chlorine also occurs over type I PSCs, which consist of H2O/HNO3 mixtures,5,6 and in the presence of sulfate aerosol particles.7-9 Laboratory work on the subject of heterogeneous chemistry in the stratosphere has generally been restricted to model cloud and aerosol particles at temperatures and pressures that mimic those encountered in the stratosphere.2-5,8,10,11 The approach has been extraordinarily successful with regard to the determination of reaction kinetics and product distributions. However, an understanding of the mechanisms of these reactions is still lacking. There is disagreement, for instance, over the nature of reactive HCl in eqs 1 and 2, specifically whether HCl is bound to the ice surface as part of a molecularly12,13 or dissociatively adsorbed phase,4,14 or whether it is confined to the near surface region, perhaps in a liquidlike layer.14 The principal difficulty with extracting mechanistic information under stratospherically mimetic conditions is that the relevant reactions are rapid and multistep. One approach for circumventing this obstacle is to work at lower temperatures, where the overall reaction rates are lower, and the elementary steps involved in a surface-mediated transformation, e.g. adsorption, reaction, and desorption, can be isolated and studied * To whom correspondence should be addressed. Tel.: (612) 625-2363 Fax: (612) 626-7541. E-mail:
[email protected].
individually.15 A more subtle issue is that very few heterogeneous reactions have actually been identified as important in the stratosphere. Because detailed information is available on only a small number of reactions, the relationships between reactant structure and reactivity are not well established. Were such relationships established, a deeper understanding of mechanism would likely emerge. Here we report the observation of a new heterogeneous transformations over ice, specifically the addition Cl2 to propene (C3H6) to form 1,2-dichloropropane. The reaction was identified and studied in ultrahigh vacuum at 175 K or less, conditions that are far removed from those of the stratosphere. The reaction is not likely to be important in the stratosphere, where propene is essentially nonexistent. Nevertheless, the finding is relevant to the subject of heterogeneous atmospheric processing because it suggests possible mechanisms for reactions on the surfaces of ice and related materials. Experimental Section Experiments were conducted in two stainless steel vacuum chambers described elsewhere.15-17 Base pressures were typically ∼10-8 Pa. One of the chambers was used for temperatureprogrammed desorption (TPD) measurements, and the other was used for Fourier transform reflection-absorption spectroscopy (FTIRAS). Two single-crystal metal surfaces, W(100) and Pt(111), were employed as substrates for growth of the ice films. Temperatures were measured using thermocouple junctions spotwelded to the edges of the crystals (W-5% Re/W-26% Re on tungsten, chromel/alumel on platinum). Electronic ice points substituted for reference junctions. Temperature accuracy and precision are estimated as (1 K. Gases were introduced into the reaction vessels via stainless steel leak valves and directed dosers. During directed dosing, a sample was positioned approximately 5 mm in front of the tube used to transport gas from behind the leak valve into the reaction vessel. The geometry of the directed doser is such that the effective pressure at an ice was greater than the pressure rise in the reaction vessel as a whole. The enhancement factor of a doser may be readily determined through suitable calibration experiments, for instance by studying the adsorption of CO on
10.1021/jp991407x CCC: $19.00 © 2000 American Chemical Society Published on Web 01/19/2000
Organic Molecules Adsorbed at Ice 1
Figure 1. Single reflection infrared spectra of (a) amorphous and (b) crystalline ice, both deposited on Pt(111). The films were 12 monolayers thick.
the clean Pt(111) substrate18 or the condensation of H2O onto cold (
