5334
J . Phys. Chem. 1984,88, 5334-5342
Reaction of Hexane Soot with N02/N20, M. S. Akhter, A. R. Chughtai, and D. M. Smith* Department of Chemistry, University of Denver, Denver, Colorado 80208 (Received: April 17, 1984)
Qualitative and quantitative aspects of the reaction between NOz/N20, and hexane soot have been studied by FT-IR. The rapid reaction near room temperature yields several surface species, including C-N02, C-ONO, and C-N-N02. The formation of these functionalities has been confirmed through reaction with "NO2 and N1*OZ.The dependence of the initial rate on the pressure of NO2 and N204,obtained from treatment of the data using the Elovich equation, has been derived from the change in infrared band absorbance as a function of time. Separate manometric experiments have confirmed these dependencies and yielded the overall activation energy for the reactions. Plausible reaction mechanisms are discussed.
Introduction The oxides of nitrogen are of considerable current interest to atmospheric and environmental chemists. They are formed predominantly in the high-temperature or primary combustion zones of both stationary and mobile power sources and are common atmospheric pollutants. Like hydrocarbons, oxides of nitrogen contribute to the formation of photochemical smog. Fossil fuel combustion is the principal source of man-made nitrogen oxides although some chemical processes, such as nitric acid plants and fertilizers, are responsible for local emissions. Oxides of nitrogen and nitrate aerosols have been studied extensively1S2because of their importance in atmospheric chemistry. Much of the work on NO, ( N O NO2) and nitrate aerosols has concentrated on gas-phase photochemical reactions, and on gaseous and aerosol products formed through such reaction^.^,^ Direct measurements of interactions of NO, with aerosols or environmental surfaces are more limited. Some investigators have examined adsorbed products of reactions of NO, with particulate Those studies have demonstrated the formation of nitrate and other nitrogen-containing species on exposure of particulate matter to NO,. Because of the apparent atmospheric burden of elemental carbon, its possible role in heterogeneous reactions has been considered by a number of investigators. For example, several types of carbon have been the recent subjects of laboratory studies to examine the effect of particulate carbon on reactions of probable importance in atmospheric chemistry. Novakov and co-workers have used ESCA to examine the role of soot in the catalytic oxidation of sulfur dioxide to ~ u l f a t eand ~ ! ~gas-carbon surface reactions of nitric oxide and ammonia.' Cofer et al.,8.9Chang et al.,1° Britton and Clarke," and Kasaoka et al.lz have demonstrated the catalytic role of carbon in the oxidation of sulfur dioxide by nitrogen dioxide. In this work, the reaction of NOz with soot, the formation of new surface species on carbon, and the kinetics of the reaction have been examined.
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(1) E. A. Shuck and E. R. Stephens, "Oxides of Nitrogen", J. N. Pitts, Jr., and R. L.Metcalf. Eds., Wiley-Interscience, New York, 1969, Ado. Environ. Sci., Vol. 1, p 73. (2) G. M. Hidy and C. S. Burton, Int. J . Chem. Kine?.,Symp., No. 1,509 ( 1975). (3) A. E. Ore1 and J. H. Seinfeld, Enuiron. Sci. Technol., 11, 1000 (1977). (4) A. B. Harker, L. W. Richards, and W. E. Clark, Atmos. Enuiron., 11, 87 (1977). (5) T.Novakov, S. G. Chang, and A. B. Harker, Science, 186,259 (1974). (6) R. Brodzinsky, S. G. Chang, S. S. Markowitz, and T. Novakov, J . Phys. Chem., 84, 3354 (1980). (7) S. G. Chang and T. Novakov, Atmos. Enuiron., 9, 495 (1975). (8) W. R. Cofer, D. R. Schryer, and R. S. Rogowski, Atmos. Enuiron., 14, 571 (1980). (9) W. R. Cofer, D. R. Schryer, and R. S. Rogowski, Atmos. Enuiron., 15, 1281 (1981). (10) S . G. Chang, R. Toosi, and T. Novakov, Atmos. Enuiron., 15, 1287 (1981). (11) L. G. Britton and A. G. Clarke, Atmos. Enuiron., 14, 829 (1980). (12) S. Kasaoka, T. Kosaka, Y. Haya, and E. Sasaoka, Nippon Kuguku Kuishi, 11, 1737 (1977).
0022-3654/84/2088-5334$01.50/0
Experimental Section Soot samples with an average weight of 9.3 mg were collected on CaF2 disks (24.8 mm X 4 mm) from the flame of burning n-hexane and each freshly prepared sample introduced into an especially designed cell. The description of the cell, preparation of the soot, and the physical and chemical properties of this material have been reported e1~ewhere.l~The cell was sealed and evacuated to torr for at least 2 h, after which the infrared spectrum was taken and stored in computer memory. Nitrogen dioxide (99.5%) was admitted at various pressures (5, 15, 30, 60, 90, and 120 torr) into the cell at 22 "C in separate replicate experiments. At various time intervals, the NOz gas was quickly evacuated from the system, followed by pumping for 15 min, after which the spectrum was taken and stored in memory. The cumulative times of contact between N O 2 and soot in these experiments were from 1 to 45 min. At each time interval, subtraction of the original soot absorbance spectrum from that of soot after contact with NO2 was carried out. Integrated absorbance values, ~ A ( Dde,) for all bands were determined by integrating the area of each new band appearing in the subtracted spectra. The rate of formation of these bands, their assignments, and their dependence on the pressure of NO2 were studied. During the spectroscopic rate studies it was difficult to determine the loss of N O z after reaction with the soot because of the small fraction of the total which reacted. For this, a manometric apparatus was designed which is shown in Figure 1. This apparatus made it possible to study the rate of depletion of NO2 as a function of the pressure of NO2. Each experiment was performed with a 1200-mg sample of hexane soot which reacted with NO2 at pressures ranging from 64 to 240 torr at various temperatures. The pressure of NO2 was recorded every 30 s. From these experiments, the activation energy and initial rate dependence on the pressure of NO2 were calculated for the reaction. UV-grade n-hexane was from either J. T. Baker Chemical Co. or Burdick and Jackson Laboratories, Inc. l4NOZ(99.5%) was provided by Matheson and was used without further purification. l5NOZ(15N,99%) was from the Isotope Labelling Corp. Infrared spectra of both l4NOZand 15NOzwere taken to check the purity of these gases and also to check for traces of NO, a possible impurity in NOz. CaF, disks were supplied by International Crystal Corp. No reaction was observed between the NOz and CaF2 disk under the conditions of this study. Results and Discussion Figure 2 is a typical FT-IR spectrum of hexane soot (evacuated at lo4 torr for 4 h) taken at 22 O C with 1000 scans at a resolution of 4 cm-I. Characteristic and complex absorption bands occur at 1260, 1440, 1590, 1700-1800, and 3040 cm-'. The high-noise region above 3200 crn-', showing a small broad band due to (13) M. S.Akhter, A. R. Chughtai, and D. M. Smith, Appl. Spectrosc., in press.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5335
Reaction of Hexane Soot with N 0 2 / N 2 0 4
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Figure 2. Infrared spectrum of hexane soot in the 2000-1200- and 3200-270O-cm-' regions.
hydrogen-bonded OH, is not included here. The relative intensities of these bands vary somewhat with the conditions of soot collection,
as reported e1~ewhere.l~It is evident that the 1700-1800- and 1260-cm-' regions each consist of more than a single band, having a shoulder on the higher frequency side of a more intense band; a shoulder at about 1630 cm-' on the 1590-cm-l band is also noticeable. The assignments of these infrared bands have been discussed in detail and reported elsewhere.13J4 Figure 3 is the spectrum of soot following its reaction with 120 torr of NO, for 1 min at 22 OC. The bands at 1660, 1540, 1340, 1305, and 1280 cm-' indicate the formation of new species. (14) J. R. Keifer, M. Novicky, M. S. Akhter, A. R. Chughtai, and D. M. Smith, SPIE, 289, 184 (1981).
5336 The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 30.64991
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Subtraction spectra (Figure 4) show one additional band which lies beneath the envelope in the region of 1500-1660 cm-I. This band is observed at 1565 cm-' as a shoulder on the 154O-cm-' band after subtraction. The subtracted spectra clearly indicate that the intensities of these new bands formed on carbon increase with contact time. Over a period of 64 h, the intensities increase to a maximum intensity; such a fully developed spectrum is shown in Figure 5. After contact with 15N02under similar conditions, as shown in Figure 6, isotopic shifts of all new bands except those at 1775, 1730, and 1400 cm-' were observed. The isotopic shifts demonstrate that the absorptions at those frequencies result from
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Figure 7. Soot spectrum subtracted from that of soot contacted with NOz (60 torr, 22 "C), P = torr: (A) 1, (B) 2, (C) 3, (D) 4, (E) 5, (F) 10, (G) 20, (H) 45 min.
nitrogen-containing surface species. The infrared frequencies and their assignments will be discussed in terms of related pairs. The unshifted bands a t 1775, 1730, and 1400 cm-l will be discussed separately. Separate but less definitive experiments with N1802have indicated the origin of the oxygens associated with the new surface species at 1660, 1540, 1340, and 1280 cm-' to be in the NOz reagent. In addition to these bands a broad band in the 17001800-cm-' region, shifted about -10 cm-' with N1802,is formed. Band broadening in these experiments, the result of the use of a 50% N'802/N'602 mixture which yields two overlapping absorptions, is due to functionalities containing both l6O and l80. The broad band in the 1700-1800-cm-' region presumably consists of the 1775- and 1730-cm-' bands previously observed in the 15N02 experiments (see Figure 6). The results suggest that these two bands, at 1775 and 1730 cm-I, are due to newly formed carbon-oxygen functionalities, probably lactone and alkylcarbonyl groups. 1660- and 1280-crn-' Bands. Tarte's analysis of 15 nitrites revealed that the trans and cis forms of the nitrite structure absorb as a double band in the ranges 1681-1653 and 1613 cm-I. The frequencies show a steady stepwise decrease as the size of the attached group is increased, the highest being methyl nitrite at 1681 and 1625 cm-' and the lowest amyl nitrite at 1653 and 1613 cm-I. These ranges have been confirmed by similar studies of Haszeldine et al.16J7 With these data and other spectral measurements reported in the literature,18-20 it appears that the (chelated) 1660-cm-' band can be assigned to the R-0-N=O group. The nonchelated nitrite absorption occurring at a somewhat lower frequency than 1660 cm-' could be assumed to lie beneath the broad 1600-cm-' band. The band at 1280 cm-' can be attributed to the R-O single-bond stretching frequency,21,22 although (15) P. Tarte, J. Chem. Phys., 20, 1570 (1952). (16) R. N. Haszeldine and J. Jander, J . Chem. SOC.,691 (1954). (17) R. N. Haszeldine and J. H. Mattinson, J . Chem. SOC.,4172 (1955). (18) N. B. Colthup, L. H. Daly, and S. E. Wiberley, "Introduction to Infrared and Raman Spectroscopy", Academic Press, New York, 1975, p 328. (19) L. J. Bellamy, "The Infrared Spectra of Complex Molecules", Vol. 1, Chapman and Hall, London, 1975, p 331. (20) L. J. Bellamy, "The Infrared Spectra of Complex Molecules (Advances in Infrared Group Frequencies)", Chapman and Hall, London, 1980, p 221. (21) M. St. C. Flett, J . Chem. SOC.,962 (1951).
Reaction of Hexane Soot with NO2/N204 the frequency reported for R-0 varies widely from compound to compound. The isotopic studies with I5NO2,which demonstrated a shift of the 1660- and 1280-cm-' bands of -30 cm-' in each case, are consistent with this assignment. These compare with calculated (harmonic oscillator) values -37 and -29 cm-' for the 1660- and 1280-cm-l bands, respectively. Qualitatively, the intensities of the 1660- and 1280-cm-' bands increase simultaneously with time as shown in Figure 7. It is differicult to measure quantitatively the integrated absorbance for the 1280-cm-' band. 1540- and 1340-cm-' Bands. Barnes and c o - ~ o r k e r sreported ~~ nitro group frequencies at 1550 and 1340 cm-I, assignments which were confirmed by Raman data on compounds containing this Smith et aLz5 also confirmed this assignment of the C-NO2 group by studying the simpler nitroparaffins. Colthup26 reported that C-NO2 compounds absorbed at different frequencies from those containing the 0-NO, group. Haszeldine?' Brown,,* and K ~ r n b l u malso ~ ~have reported frequency ranges for alkylnitro compounds. They are 1567-1550 (asymm) and 1379-1368 (symm) cm-I. Lunn30has studied the effect of substitution at the a-carbon atom and reported the frequencies of C-NO2 bands shifted by steric and inductive effects. Aromatic nitro groups absorb strongly at 1540-1500 cm-' and somewhat more weakly at 1370-1330 cm-3.31-34 Kross and F a ~ s e lwho , ~ ~studied 34 compounds, reported that the influence of molecular environment, the effect of conjugation, and substitution effects influence the frequency of C-NO,. In this work, we found two bands, one at 1540 cm-l and another at 1340 cm-l, which apparently are due to C-NO,. This assignment is consistent with the isotopic studies with ISNO,,which demonstrated a shift of the 1540- and 1340-cm-' bands of -35 and -30 cm-I, respectively. These compare with calculated (harmonic oscillator) values of -39 and -34 cm-I for the 1540- and 1340-cm-' bands. The intensities of these two bands also increase simultaneously with time as shown in Figure 7. 1565- and 1305-cm-' Bands. Two assignments for the 1565and 1305-cm-' bands are possible in principle. One possibility is -N-NO,, a nitramine, and the other is C-NO, in an environment different from the one absorbing at 1540 and 1340 cm-l. The -N-NO, band should show an asymmetric NO, frequency at much the same position as C-NO2 compounds, whereas the symmetric stretch absorption is toward lower frequencies. Leiber et al.35studied 17 N-nitro compounds of various types. In 14 of these, the symmetric NO2vibration is within the range 1315-1 260 cm-I, while the exceptions (which absorb at higher frequencies) are either acid or salt forms in which ionic structures would be expected to have an influence on the absorption frequency. The asymmetric frequency falls in a wider range and appears to be more influenced by the nature of the substituents. BellamyIg reported that in -N-NO,, the NO2 absorption is in the ranges 1587-1530 (asymm) and 1292-1260 (symm) cm-'. The other possible assignment is the C-NO, group. The 1565-cm-' band is consistent with the range of asymmetric frequencies observed for this group, but the 1305-cm-' band does (22) 0. E. Shreve, M. R. Heether, H. B. Knight, and D. Swern, Anal. Chem., 22, 1498 (1950). (23) R. B. Barness, R. C. Gore, U. Liddel, and V. Z. Williams, "Infrared Spectroscopy", Reinhold, 1944, p 44. (24) J. H. Hibbew and E. Teller, "The Raman Effect and Its Chemical Applications", Reinhold, 1939, p 231. (25) D. C. Smith, C. Y. Pan, and J. R. Nielson, J . Chem. Phys., 18, 706 (1950). (26) N. B. Colthup, J . Opt. SOC.Am., 40, 397 (1950). (27) R. N . Haszeldine, J . Chem. SOC.,2525 (1953). (28) J. F. Brown, Jr., J. Am. Chem. SOC.,77, 6341 (1955). (29) N . Kornblum, H. E. Ungnade, and R. A. Smiley, J. Org. Chem., 21, 377 (1956). (30) W. H. Lunn, Spectrochim. Acta., 16, 1088 (1960). (31) R. J. Francel, J . Am. Chem. SOC.,74, 1265 (1952). (32) W. C. Lothrop, G . R. Handrick, and R. M. Hainer, J . Am. Chem. SOC.,73, 3581 (1951). (33) R. R. Randle and D. H. Whiffen, J . Chem. SOC.,4153 (1952). (34) R. D. Kross and V. A. Fassel, J . Am. Chem. Soc., 78,4225 (1956). (35) E. Lieber, D. R. Levering, and L. J. Patterson, Anal. Chem., 23, 1594 (1951).
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
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