J. Phys. Chem. 1987, 91, 1565-1568
1565
A Fourier Transform Infrared Study of the Rate Constant of the Homogeneous Gas-Phase Reaction N205 H20 and Determination of Absolute Infrared Band Intensities of N,O, and HNOB
+
J. Hjorth,* G. Ottobrini, F. Cappellani, and G. Restelli Commission of the European Communities, Joint Research Centre, Ispra Establishment, 21 020 Ispra (Va), Italy (Received: April 18. 1986; In Final Form: September 2, 1986)
N205was reacted with water vapor in large FEP-Teflon bags and its decay followed by sampling in a White gas cell using infrared Fourier transform spectroscopy. A rate constant for the homogeneous N2O5 + HzO gas-phase reaction at 296 2 K and 740 Torr total pressure equal to (1.48 f 0.42) X lo-,' cm3 molecule-' s-' was observed. Critical analysis of the results suggests that the true homogeneous rate constant might be lower. The measured absolute intensities of the N205 710-780- and 1225-1270-cm-I bands were 4.57 X and 3.81 X cm molecule-', respectively. The measured HNO, absolute intensities of the overlapping band systems us, 2u9, and v5 + u9 - v9 (840-930 cm-I) and u,, u4 (1270-1350 cm-I) were 2.21 X lo-'' and 4.29 X lo-'' cm molecule-I, respectively.
*
Introduction The importance of the homogeneous gas-phase reaction
NzOS
+ HzO
-
2HN03
(1)
has been under discussion for some years now in the context of the transformation of NO, to HNO, at night in the troposphere (see, eg., ref 1 and 2) where the heterogeneous reaction of N,05 with liquid water (droplets or wet aerosols) also occurs. The first determination of the rate constant of the homogeneous reaction was performed by Morris and Niki in 1973 using a 67-L Pyrex glass cell as reaction chamber and infrared absorption spectroscopy to measure the NzOs and H 2 0 concentrations. An upper limit equal to 1.3 X cm3 molecule-' s-' at 298 K was determined., Computer modeling studies of smog chamber data (see, e.g., ref 4) and experimental time concentration profiles of the NO, radical in ambient air,5 however, give support to a much lower rate constant for reaction 1. This has been apparently substantiated by the result of a determination performed by Tuazon et aL6 indicating a value, after allowing for the contribution of the cm3molecule-' s-l from heterogenous reaction, equal to 1.3 X an originally estimated upper limit equal to 2.4 X lo-" cm3 molecule-I s-l a t 298 f 1 K. The experiments were performed in 3800- and 5800-L reaction chambers, mixing purified N2O5 with purified air and using long-path Fourier transform spectrometry to measure N2O5and HNO,, while a dry bulb/wet bulb technique was used to evaluate water vapor concentrations. In the present study large FEP-Teflon bags were used as reaction chambers, taking advantage of their low surface reactivity and low surface-to-volume ratio (6 m-l). Air in the bag was sampled and analyzed by transferring it to a Pyrex glass White cell coupled to a Fourier transform spectrometer for quantitative measurement of reactants and reaction products. Unlike the experiment of ref 6, N 2 0 5was prepared in this case by mixing N O z with excess 0, in purified air. Concentrations of N20s, HNO,, and H,O were calculated from infrared absorption data by using in the case of N,05 and HNO, integrated band intensities calibrated for the range of optical depths (up to 23 Torr cm) and the instrumental resolution (1 cm-I) used. An upper limit for the rate constant equal to (1.48 0.42) X
*
(1) Richards, L. W. Atmos. Environ. 1983, 17, 391. ( 2 ) Jones, C. L.; Seinfeld, J. H. Atmos. Enuiron. 1983, 17, 2370. (3) Morris, Jr., E. D.; Niki, H. J . Phys. Chem. 1973, 77, 1929. (4) Atkinson, R.; Lloyd, A. C.; Winges, L. Atmos. Emiron. 1982, 16, 1341. (5) Platt, U.; Perner, D.; Winer, A . M.; Morris, G. W.; Pitts, Jr., J. Geophys. Res. Lett. 1980, 7 , 89. ( 6 ) Tuazon, E. C.; Atkinson, R.; Plum, C. N.; Winer, A. M.; Pitts, Jr., J. N. Geophys. Res. Lett. 1983, 10, 953.
0022-3654/87/2091-1565$01,50/0
cm3 molecule-' s-l was measured. This value is in agreement with the value of ref 6 in spite of the differences in the reaction chambers and in the chemical reactants applied in the two studies. Using curve of growth analysis we measured the absolute infrared intensities of the NzOs bands around 743 and 1246 cm-' and 3.81 X cm and found them equal to 4.57 X molecule-', respectively; those of the HNO, overlapping band systems us, 2u9, and us + u9 - v9 (840-930 cm-I) and u3, u4 (1270-1350 cm-I) were measured and found to be 2.21 X and 4.29 X cm molecule-', respectively, with an estimated accuracy for all the data between 10 and 15%. Experimental Section NO, (500 ppmv in air) was mixed in 1.5-m3 volume, 6-m-I surface-to-volume ratio FEP-Teflon (Dupont, type A) bags along with purified air and an excess of 0, to convert NO2 to NO, and N205;N20s was obtained at concentrations in the range 0.5-3 ppmv with a half-life in the bag in the range 3-7 h depending on water content. 0, was prepared by passing purified air from an AADCO generator through a silent discharge. Water vapor concentrations were kept in the range 300-2000 ppmv. The bags filled to about 740 Torr total pressure were kept in the dark at 296 f 2 K during the experiment. 0, concentrations were measured by UV absorption using a Dasibi instrument; N205, HNO,, and H 2 0 were measured by long-path Fourier transform infrared spectroscopy. Infrared spectra were obtained by transferring gas samples from the bag through Teflon tubing to an evacuated 25-L 70-m optical beam path Pyrex glass cell mounted in the sample compartment of a Bruker IFS 113 V FT spectrometer. Repeated samplings of bags with different concentrations of reactants and the use of different transfer systems t o the gas measuring cell did not show any evidence that the transfer procedure used had any influence on the measured spectroscopic data. Spectra were recorded at 2-, 1-, and 0.06-cm-I instrumental resolution with Happ-Genzel apodization. For each sample three spectra were subsequently collected by coadding 100 scans per spectrum. The sequencing of spectra at regular time intervals allowed for the correction of N20s absorbance values for the heterogeneous loss of N2Os at the cell wall following injection of the sample. Half-life values from 10 to 20 min were measured for the experimental conditions used. Since water vapor slowly permeated through the Telfon film, its concentration inside the bag was frequently measured during each experiment. N 2 0 5and HNO, concentrations were measured from the integrated absorbances of their bands: 1225-1 270 cm-' for N205 and 1270-1350 cm-' for HNO,. While the N205 band was practically free from interferences by other absorptions, spectral subtraction was used previous to calculation of the HNO, band 0 1987 American Chemical Society
lOOxam coadded
-0.1 1200
1220
1240
1260
1280
1300
1320
1340
1360
1380
1400
0.5
HNO3
N2 0 5
H2 0
-0.1
lh0
1220
1240
1260
1280 1300 1320 WAVENUMBERS CM-1
1340
1360
1380
4
1400
Figure 1. Absorbance vs. wavenumber plot in the 1200-1400-cm-’ spectral region showing correction for H20absorption previous to calculation of spectral absorbances; 740 Torr; 296 f 2 K; I-cm-’ instrumental resolution (full-width at half-maximum).
intensity to correct for H 2 0 absorption in the region (Figure 1). In order to convert spectral absorbances to gas concentrations, integrated band intensities, IBI = l/cL.fbandlog(Zo/ZJ dv (c and L are the concentration and the optical beam path, Io and I , the incident and transmitted intensity at wavenumber v), derived as discussed in the following and equal to 1.76 X lo-’’ and 2.02 X lo-’’ cm molecule-’ for the N205 and HN03 band, respectively, were used. The water vapor concentration was evaluated from the absorption due to two coincident H 2 0 transitions at 21 36.144 cm-I. For the experimental conditions, gas-phase composition and instrumental resolution, used, this spectral feature was free from interferences due to absorptions of species different from H20, and even at the highest water vapor concentrations employed (52000 ppmv) it was not affected by spectral saturation phenomena. cm2 molecule-’, A peak absorption coefficient of 0.3 X calculated from the data of ref 7, was used to derive the H 2 0 concentrations from spectra recorded at 0.06-cm-I resolution. These data have been estimated to be affected by an uncertainty of f 2 5 % which accounts for the uncertainties in the tabulated line intensity and pressure broadening coefficient and for the effect of the convoluting instrumental function. Absolute Calibration of N 2 0 5and HNO, Absorptions. N2O5 was generated in a 400-L evacuable Pyrex glass-Teflon gas cell with a 76-m multiple reflection system on line with a Bruker FT spectrometer. Spectra were recorded at 1-cm-’ resolution following different concentrations of N205(up to a few ppmv) in the gas cell. Between recordings of the spectra a known volume of the gas phase (2-3% of the total) was taken from the middle of the cell by using a Teflon tube and passed through a 0.1% N a O H solution contained in two impingers in series. The pressure in the cell was kept constant by supplying pure air from a Teflon bag at constant pressure; in the calculations an (7) AFGL, Atmospheric Absorption Line Parameters Compilation, 1982 edition. Rothman, L. S . , et al. Appl. Opr. 1983, 22, 2247.
allowance was made for the dilution of the gas. The solution of the impingers was then analyzed for nitrate and nitrite ion content by ion chromatography. N205was found to form only nitrate ions in the NaOH solution where it was absorbed with close to 100% efficiency; no nitrate ions were found above the sensitivity of the apparatus in the second impinger. N205was always accompanied in the gas phase by variable amounts of HN03 due to the reaction with water vapor occurring in the NO2, Oj, and zero air mixing phase. Due to the excess of 03,the concentration of NOz in the gas mixture could be considered negligible. Since the concentration of NO3 was also calculated to be small in comparison to that of N2O5 and HN03, the integrated intensities of the N205and HNO, bands could be derived from the relationship (NO,) = 2 X aabsNiO5 ,BabsHNo,,where (NO,) is the amount of nitrate ions in the impinger solution normalized to the volume of the gas sampled, absNZOs and abSHN0, are the measured integrated absorbances of infrared bands, and a and @ are the coefficients optimized by a least-squares fitting procedure, used to calculate the IBI values. The standard error on the determination of the regression coefficients from the results of nine calibrations was 5% and 8% respectively for NzOSand HNO,, which, however, does not account for systematic errors in the measurements. In a subsequent set of experiments, HN03 in zero air was introduced in the gas cell and IBI values for the HNO, bands were measured as previously discussed with calibration by ion chromatography. These values were found to be in agreement within &5% with the value previously derived from the N205-HN03 system. IBI values of 1.76 X lo-’’ and 2.02 X lo-’’ cm molecule-’ for the 1225-1270- band and 1270-1350-cm-’ band of N2O5 and HNO,, respectively, were obtained. These values when compared to other data given in ref 8 and 9 show a good agreement for N205
+
(8) Cantrell, R. A.; Stockwell, W. R.; Anderson, L. G.; Bukarow, K. L.; Perner, D.; Schmeltekopf, A,; Calvert, J. G.; Johnston, H. S. J . Phys. Chem. 1985, 89, 139.
FT-IR Study of the N205
+ H 2 0 Reaction
1567
The Journal of Physical Chemistry, Vol. 91, No. 6,1987
TABLE I: Absolute Band Strength S = .fbudKvdu = l/cLJ"ln ( I o / I J du of N205 and HNO, Bands Measured in This Work Compared to Existing Literature Data' absolute band strength S, cm molecule-' NZOS HN03 ref 710-780 cm-l 1225-1 270 cm-I 840-930 cm-' 1270-1350 cm-l this work (4.57 f 0.5) x 10-17 (3.81 0.4)X (2.21 0.2) x 10-17 (4.29 0.6)X 10 4.51 x 10-17 12 (2.78 0.13) X (2.36f 0.09) X lo-'' 13 (2.60 0.13)X (5.06 0.25) X 14 (2.39A 0.12) x 10-17 15 (2.57i 0.11) x 1 0 4 7 (5.14i 0.26) X
*
*
*
*
*
*
'The value of the band strength derived from the curve "optical cross section vs. wavelength" measured in ref 10 is quoted in ref 12. In ref 13 and ref 14 the integration limits for the HN03 bands at 850-920 and 1275-1350 cm-I; in ref 15 respectively 820-950 and 1250-1375 cm-I. N2Os
DECAY WITH
Nz05 DECAY WITH H 2 0
H 2 0 xlo-" 41
0
o
1
z
3
4
5
6
7
a
9
io
,
11
I
12x10'17
[H201 rnoIccuics-1.cm'
0.0 0.00
[,,,
0.06
0.12
, , ) ,
0.18
, , ,
0.24
,
,,,,,
0.30
, , ) , ,
0.36
, , , , ;
0.42
TIME x H20 CONCENTRATION moiecules.cm-3,s .IO'*'
Figure 2. N205decay (In ([Nz05]o/[N205],))plotted vs. the product of time and H20concentration; linear least-squares fit to the data with 90% confidence limits.
while a significant difference appears for H N 0 3 .
Results and Discussion
+
Rate Constant for the Homogeneous Reaction N2O5 H 2 0 . The results have been evaluated by using a procedure of linear regression on the data In ([N205]o/[N205],) vs. the product [ H 2 0 ] t . The experimental data are shown in Figure 2. From the analysis a rate constant at 296 f 2 K equal to (1.48 f 0.42) X 10-*' cm3molecule-' s-' at the 90% significance level was derived with the error limits estimated by t statistics. The y intercept of the regression line was not significantly different from zero. Correction of the N2O5 concentration for decay in the measuring cell was made from three spectra recorded in sequence by taking into account deviations from first-order decay. This results in a spread associated with the [N,O5] values of *5%. This figure, which in turn introduces an uncertainty associated with the quantity In ([N205]o/[N20S],)ranging from 100% for the smaller values to 10% for the larger ones, accounts for most of the scatter of the experimental data which can be seen in Figure 2. In the gas mixtures used in the present study the predominant nitrogen compounds were N2O5 and HNO,; NO3 and NO, were at much lower concentrations. Using the steady-state expression given in ref 10 for the NO3 concentration in an 03-N02 mixture and the rate constants derived in the same article, for the conditions of this experiment, we found that [NO3] was typically at the level of 100 ppbv, and [NO*] was somewhat lower. These values can be compared to [N205] which was at the level of a few ppmv. Accordingly, the influence of the interconversion of NO, species on the concentration of N2O5 during the experiment is so relatively (9) Massie, S.T.; Goldman, A.; Murcray, D. G.; Gille,J. C. Appl. Opt. 1985. 24, 3426. (10) Graham, R. A.; Johnston, H. S. J . Phys. Chem. 1978,82,254.
Figure 3. Observed second-order rate constant for N2O5 decay vs. [H20]-'; linear least-squares fit to the data with 90% confidence limits.
small that it can be considered negligible. Another possible source of error might arise from the loss of NO3 to the bag walls. If this process is a significant sink of NO, for the system, the rate constant is overestimated; however, the good agreement of the rate constant measured in this experiment with that obtained in ref 6 supports the hypothesis that the wall loss of NO3 is not an important factor. In fact the NO3 concentration in the two systems is significantly different, being relatively much higher in the present experiment because of the presence of 03. The loss of NzO5 could then be due to three possible mechanisms, namely: a second-order homogeneous gas-phase reaction between N2Os and H20, a second-order heterogeneous (wall) reaction between N205 and H 2 0 , and a first-order N2O5 loss to the reaction chamber walls. From the data of this experiment it is impossible to distinguish between the effects of the two second-order reactions. Consequently, the two rate constants will be included in one term. As shown in Figure 3 the rate constant found by second-order kinetics decreases with increasing water vapor concentration. This observation suggests the addition of a first-order term to obtain an expression in the form: In ([N20s]o/[N,05],) = k"[H,O]t
+ k't
where k" is the rate constant for the two second-order reactions and k' is the first-order wall loss rate constant. Optimization of these parameters with respects to the experimental data leads to values for k" of (0.90 f 0.21) X cm3 molecule-' s-' and for k' of (1.8 f 0.7) X lo-' s-' at a 90% confidence level. The value for k1corresponds to a half-life of N2O5 with respect to first-order wall loss of about 11 h. This analysis could indicate that the true value of the rate constant of the homogeneous gas-phase reaction N 2 0 5 H20 is lower than the previously indicated upper limit. Generally quite large amounts of nitric acid were observed to be formed in the bags upon the initial mixing of NO, and O3 (up to about half of the N205 was converted to H N 0 3 ) . The subsequent behavior of H N 0 3 appeared complex however, and no conclusions could be derived from the analysis of these data. Absolute Band Strengths of N205and HiV03. Absolute band du = l/cLJ"In (&,/Iu) du were derived from strengths S = jbandKv
+
J. Phys. Chem. 1987, 91, 1568-1573
1568 a
N205 (1225-1270 cm-'1 H NO3 (840
- 930 cm-')
I
I
i
I'.
04-
0
5
IO
15
20
1
15
C I (torr.cm)
Figure 4. Curves of growth relative to the N 2 0 51225-1270-cm-' band and to the HNO, 840-930-cm-l band system ( u s , 2v9, u5 ug - v9); 296 f 2 K.
+
curve of growth analyses]' for the N2O5 710-780- and 12251270-cm-I bands and for the H N 0 3 840-930- and 1270-1350cm-I bands. The calibration technique described in the Experimental Section for evaluation of the IBI values was also used in this case. Figure 4 shows two curves of growth for the N205 1225-1 270-cm-' band and the HN03 840-930-cm-l band system. (1 1) Goody, R. M. Atmospheric Radiation; Oxford University: London, 1964; Vol. 1.
The S values were derived from a regression to the points in the linear region of the curve of growth. The results of this work are given in Table I along with existing literature data. The uncertainties associated with the values measured in this work were derived from the spread in the measured data and the estimated accuracy of the calibration procedure. A significant disparity is apparent between the N z 0 5band strengths obtained in the present experiment and the values, measured by curve of growth analysis, given in ref 12. The comparison with the value measured for the 1225-1270-cm-' band in ref 10 shows a much better agreement. In the case of H N 0 3 the comparison with the band strengths given in ref 13 and ref 15 shows that the values measured in the present experiment are lower by 17-19% for both bands. On the other hand, satisfactory agreement is apparent for the strengths of the 840-930-cm-l band, measured in this work and in ref 14, the difference lying within the uncertainties of the determinations. Acknowledgment. The authors gratefully acknowledge the contributions of Mr. G. Melandrone for the setup of the multipass cell-FTS system and of Mrs. H. Geiss for the ion chromatographic analysis. Registry No. N205,10102-03-1;HNO,, 10102-43-9;H20,7732-18-5. (12) Lovejoy, R. W.; Chackerian, Jr., C.; Boese, R. W. Appl. Opt. 1980, 19, 744. (13) Goldman. A.:, Kvle. , . G. T.: Bonomo. F. S.ADDI.ODt. 1971. 10. 65. (14j Goldman, A.; Bonomo, F.'S.;Vaiero, F. P. J:;'Goo;vitch, D.; Boese, R. W. Appl. Opt. 1981, 20, 172. (15) Giver, L. P.; Valero, F. P. J.; Goorvitch, D.; Bonomo, F. S. J . Opt. SOC.Am. 1984, BI, 715 I
A High-Temperature Photochemlstry Kinetics Study of the Reaction of O(3P) Atoms with Ethylene from 290 to 1510 K Khaled Mahmud, Paul Marshall, and Arthur Fontijn* Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 (Received: June 13, 1986; In Final Form: September 24, 1986)
The 0 + C2H, reaction has been investigated in a high-temperature photochemistry (HTP) reactor at temperatures from 290 to 1510 K and pressures from 60 to 500 Torr. Ground-state 0 atoms were generated by flash photolysis of C 0 2 and monitored by timeresolved atomic resonance fluorescence with pulse counting. Measurementswere made under pseudo-first-order conditions [O]
