Environ. Sci. Technol. 1999, 33, 4232-4238
Atmospheric Chemistry of Methylcyclopentadienyl Manganese Tricarbonyl: Photolysis, Reaction with Hydroxyl Radicals and Ozone T. J. WALLINGTON,* O. SOKOLOV, AND M. D. HURLEY Ford Motor Company, 20000 Rotunda Drive, Mail Drop SRL-3083, Dearborn, Michigan 48121-2053 G. S. TYNDALL AND J. J. ORLANDO Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado 80303 I. BARNES AND K. H. BECKER Bergische Universita¨t Wuppertal, Physikalische Chemie-FB9, Gauss-Strasse 20, D-42097 Wuppertal, Germany
Likely atmospheric loss processes for MMT include photolysis, reaction with ozone, and reaction with OH radicals. It is known that MMT undergoes photolysis when exposed to sunlight in the gas (3) and aqueous phase (4). However, a quantitative UV-visible spectrum of MMT is not available, and calculations of its rate of photolysis are not possible. There have been no studies of the rates of reaction of MMT with either OH radicals or ozone, and the potential atmospheric importance of these reactions is not known. To improve our understanding of the atmospheric chemistry of MMT, its UV-visible absorption spectrum was measured at the National Center for Atmospheric Research (NCAR) and the Bergische Universita¨t Wuppertal, and the kinetics of reactions 1 and 2 at 296 K were studied at the Ford Motor Company.
R. VOGT
O3 + MMT f products
(1)
Ford Forschungszentrum Aachen GmbH, Su ¨ sterfeldstrasse 200, D-52072, Aachen, Germany
OH + MMT f products
(2)
2. Experimental Section The atmospheric fate of MMT has been studied using laboratory smog chamber systems. MMT absorbs strongly in the UV-visible region from 210 to 400 nm with σ (333 nm) ) (1.9 ( 0.2) × 10-18 cm2 molecule-1 and undergoes photolysis at a rate estimated to be (1.3 ( 0.1) × 10-2 s-1 for a typical summer day at a latitude of 40° N. Photolysis gives CO in a molar yield that is indistinguishable from 100% and an unidentified species believed to be methylcyclopentadienyl manganese dicarbonyl (MMD), which also undergoes rapid photolysis liberating additional CO. Reaction of MMT with OH and O3 proceeds rapidly with rate constants of kOH ) (1.1 ( 0.3) × 10-10 and kO3 ) (7.7 ( 1.9) × 10-18 cm3 molecule-1 s-1. During the day, the atmospheric loss of MMT proceeds essentially entirely via photolysis. At night reaction with O3 can be significant.
1. Introduction MMT (methylcyclopentadienyl manganese tricarbonyl) is an additive that can be used to increase the octane number of gasoline. The addition of 0.033 g of MMT (0.0083 g of Mn) increases the octane number, (R + M)/2, of 1 L of gasoline by approximately 0.6 unit (1). The use of MMT in gasoline is controversial with concerns being raised over its possible adverse impacts on human health and exhaust treatment catalysts (2). MMT is a liquid with a vapor pressure of approximately 0.02 Torr at room temperature. Quantification of the environmental impact of MMT emissions requires a detailed understanding of the atmospheric chemistry of MMT. * Corresponding author phone: (313)390-5574; fax: (313)594-2923; e-mail:
[email protected]. 4232
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2.1. UV-Visible Absorption Spectrum Measurements at NCAR. The UV absorption spectrum of MMT was measured over the range of 210-450 nm, using a diode array spectrometer system described in detail elsewhere (5). The system consists of a 90-cm-long Pyrex absorption cell equipped with quartz windows. The output from a deuterium lamp is collimated through the absorption cell, focused onto the entrance slit of a 0.3-m Czerny-Turner spectrograph, and dispersed onto a 1024-element diode array detector. The spectrograph was equipped with 300 grooves mm-1 grating and provides spectra with a resolution of 0.6 nm. The wavelength scale of the spectrometer system was calibrated using emission lines from a low-pressure Hg Penray lamp. Spectra were obtained from the summation of 100 exposures of the diode array, each of 0.2-s duration. Spectra were measured both with the cell evacuated (Io) and with the cell filled with a gaseous sample (I) and converted to absorbance (A): A ) ln (Io/I). MMT liquid was obtained from Aldrich Chemical Co. at a stated purity of >99%. Gas-phase MMT samples were obtained by expanding the vapor above a liquid sample into the absorption cell. Before use, the liquid MMT sample was purified by multiple freeze-pump-thaw cycles. The sample was further purified by continual pumping to remove volatile impurities (presumably CO, formed from decomposition of the MMT). Purified samples were transferred to the absorption cell, and absorbance spectra were recorded at room temperature (296 ( 2 K). 2.2. UV and IR Absorption Spectrum Measurements at the Bergische Universita1 t Wuppertal. The UV absorption spectrum of MMT was measured in a 480-L volume reactor equipped with multiple reflection mirror optics for simultaneous in situ measurement in the UV and IR. The UV spectrometer consists of a modified 22-cm monochromator (SPEX) and a diode array detector (EG & G PAR 1412). A spectral resolution of 0.6 nm was achieved with a grating of 1200 lines/mm covering a spectral range of 70 nm on the 10.1021/es990350p CCC: $18.00
1999 American Chemical Society Published on Web 10/19/1999
diode array detector. The UV and IR light sources were a deuterium lamp and a globar, respectively. The optical path lengths in the UV and IR were preadjusted to 28.03 and 51.6 m, respectively. The wavelength scale was calibrated using atomic emission lines from a low-pressure mercury lamp. MMT liquid was obtained from Aldrich Chemical Co. at a stated purity of >99% and used without further purification. MMT was introduced into the chamber by injecting small amounts (1-5 µL) of the liquid into a stream of air used to fill the chamber. The absolute concentration of gaseous MMT in the chamber was calculated from the volume injected, density of the liquid, molecular weight of MMT, and chamber volume. After introduction of MMT, the reactor was filled with air to 1000 mbar, and the UV and IR spectra were recorded at room temperature (296 ( 2 K). The UV and IR absorbance increased linearly with MMT concentration in accordance with Lambert-Beer’s law, ln (Io/I) ) σcl. 2.3. Measurement of k1 and k2. The rate constants for reactions 1 and 2 were measured by relative rate techniques using a FTIR smog chamber system described previously (6). All experiments were performed in 750 Torr of air at room temperature (296 ( 2 K). The kinetics of reaction 1 were measured relative to reactions 3 and 4 by monitoring the decay of MMT, propene, and 2,3-dimethyl-1,3-butadiene when mixtures containing these compounds were exposed to ozone in 750 Torr of air at 296 K. Propene and 2,3-dimethyl1,3-butadiene were selected as reference compounds because of their convenient IR features and commercial availability and because their reactivities toward ozone are known and are expected to be comparable to that of MMT.
O3 + C3H6 f products
(3)
O3 + CH2dC(CH3)C(CH3)dCH2 f products
(4)
Experiments to measure k1 were performed in a darkened laboratory to prevent the photolysis of MMT. The experimental procedure was as follows. First, mixtures of 0.0-0.6 mTorr of MMT, 7 mTorr of propene, 7-8 mTorr of 2,3dimethyl-1,3-butadiene, and 470-600 mTorr of n-nonane were prepared in 200-400 Torr of pure dry air. To scavenge OH radicals that are formed in the reaction of unsaturated hydrocarbons with ozone (7), n-nonane was present at a concentration that was at least 100 times greater than that of MMT. Nonane reacts with OH radicals with a rate constant that is 1.0 × 10-11 cm3 molecule-1 s-1 (7). A 100-fold excess of nonane ensures that reaction of OH radicals with MMT is of negligible importance in the ozone experiments. A stream of oxygen was then passed through a silent discharge to generate ozone and was flowed into the chamber resulting in a typical increase in pressure in the chamber of 100-200 Torr. Rapid introduction of 100-200 Torr of air to bring the total pressure to 750 Torr was used to mix the gases in the chamber after which the decay of the reactants was monitored using FTIR spectroscopy. To study the reactivity of MMT toward OH radicals mixtures containing 0.17 mTorr of MMT, 8 mTorr of CH3ONO, 8 mTorr of NO, and 1 mTorr of either propene or cyclohexene in 750 Torr total pressure of air diluent at 296 K were subjected to UV irradiation. Photolysis of CH3ONO is a well-known source of OH radicals:
CH3ONO + hν f CH3O + NO
(5)
CH3O + O2 f HCHO + HO2
(6)
HO2 + NO f NO2 + OH
(7)
The relative decay rates of MMT and the reference (propene or cyclohexene) were monitored using FTIR spectroscopy and used to derive the rate constant ratios k2/k8 and k2/k9:
FIGURE 1. UV absorption spectra for MMT measured at NCAR (thin line, 210-410 nm) and Wuppertal (thick line, 305-375 nm). The spectrum measured at NCAR has been scaled to σ (333 nm) ) 1.9 × 10-18 cm2 molecule-1.
OH + C3H6 f products
(8)
OH + cyclohexene f products
(9)
Control experiments showed that photolysis was a significant loss process for MMT (but not for the other reactants) in the chamber. As discussed in section 3.3, appropriate corrections were computed to account for MMT loss via photolysis. All reagents except CH3ONO and O3 were obtained from commercial sources at purities >99.9%. Ultrahigh-purity synthetic air was used as the diluent gas in all experiments. CH3ONO was prepared by the dropwise addition of concentrated H2SO4 to a saturated solution of NaNO2 in methanol and was devoid of any detectable impurities using FTIR analysis. Ozone was prepared by passing ultrahigh-purity oxygen through a silent electrical discharge. Unless otherwise stated, all uncertainties quoted in the present manuscript are 2 standard deviations from regression analyses.
3. Results 3.1. UV-Visible Absorption Spectrum Measured at NCAR and Bergische Universita1 t Wuppertal. The UV-visible spectrum of MMT was measured at both NCAR and Bergische Universita¨t Wuppertal. At NCAR, the spectrum was recorded over the wavelength range of 210-410 nm while at Wuppertal the spectrum was recorded over the region of 305-375 nm. As seen from Figure 1, where the spectra can be compared, the shape of the spectra recorded in both laboratories was indistinguishable. The spectrum has a maximum at 330335 nm, with a shoulder peaking near 240 nm and a stronger absorption band extending into the vacuum UV. Measurable absorption (σ > 1 × 10-20) extends to wavelengths above 400 nm. The spectrum measured at NCAR was acquired by allowing saturated MMT vapor (approximately 20 mTorr; 4) into the absorption cell. It is difficult to measure pressures of ≈20 mTorr with great accuracy, and the experiments at NCAR were not used to derive absolute absorption cross sections. In the experiments at Wuppertal, known volumes of liquid samples were injected into the chamber. Three separate injections of 1.3, 2.15, and 3.0 µL of MMT were made into the chamber, and the resulting UV spectra scaled linearly (to within (5%) with the MMT concentration. Values of σ (333 nm) ) (1.9 ( 0.2) × 10-18 cm2 molecule-1 and σ (1960 cm-1) ) 1.33 × 10-17 cm2 molecule-1 (both base e) were obtained. The quoted errors reflect uncertainties associated with the reproducibility of the measurements and our estimate of possible systematic errors associated with VOL. 33, NO. 23, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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uncertainties in the UV and IR path length, calibration of the microsyringes, and sample purity. The spectra given in Figure 1 have been scaled to σ(333 nm) ) 1.9 × 10-18 cm2 molecule-1. The gas-phase MMT spectra in Figure 1 have a shape similar to the liquid-phase spectrum reported by Garrison et al. (4), although insufficient experimental details are available in that paper to obtain absolute cross section data. To determine a first-order photolysis rate ( j) for MMT in the atmosphere, wavelength-dependent cross section, σ(λ), quantum yield, φ(λ), and actinic flux, I(λ), data are required:
j)
∫σ(λ)φ(λ)I(λ) dλ λ
Since no gas-phase photolysis quantum yield data are available for MMT (to the best of our knowledge), a quantum yield of unity was initially assumed for all wavelengths to determine an upper limit to its photolysis rate. Under this assumption and using actinic flux data from Finlayson-Pitts and Pitts (8), a photolysis rate of (1.3 ( 0.1) × 10-2 s-1 is obtained for a solar zenith angle of 25° (representative of a typical summer day at 40° N), corresponding to a photolysis lifetime of only 80 s. This, of course, would make photolysis the dominant atmospheric loss process for MMT. Previous estimates of the photolysis lifetime for MMT of a minute or two in both the gas phase (3) and in aqueous solution (4) have been reported, consistent with our findings. Note however that, in the aqueous-phase studies of Garrison et al. (4), a quantum yield of 0.13 is reported, which implies much higher absorption cross sections than measured in our study. It is of course possible that the actual photolysis quantum yield for MMT is less than unity, leading to a longer atmospheric lifetime. However, this does not appear to be the case. The photolysis quantum yield for MMT can be estimated by comparing the photolysis rates of Cl2 (7.3 × 10-4 s-1) and MMT (7.1 × 10-3 s-1) in the chamber at Wuppertal. Given that the MMT absorption spectrum is uniformly 7.1 ( 1.0 times stronger than Cl2 in the 310-370nm region and that the MMT photolysis rate in the Wuppertal chamber (see description below) is 9.7 ( 1.5 times that of Cl2, it follows that the MMT quantum yield is similar to that of Cl2si.e., near unity. The photolability of metal-carbonyl compounds in the near-UV (where the photon energy greatly exceeds the typical metal-CO bond strength) via elimination of one or more CO ligands is well documented (9). 3.2. Study of MMT Photolysis at Bergische Universita1 t Wuppertal. The products formed following the photolysis of MMT in 760 Torr of air were investigated in experiments at the Bergische Universita¨t Wuppertal. Mixtures containing (1.80-3.06) × 1013 molecule cm-3 of MMT in 760 Torr of air were irradiated using the output from Philips TL05 40W fluorescent lamps (320 nm < λ < 360 nm, λmax ) 340 nm). Control experiments were performed to test for loss of MMT in the chamber in the dark; there was no observable change (
