Products of the H
+ NOp Reaction
The Journal of Physical Chemistry, Vol. 82,
No. 19, 1978 2139
Vibrationally Excited OH in the Products of the H 4- NOz Reaction Geoffrey K. Smitht and Edward R. Fisher Deparfment of Chemical Engineering and Research Institute for Engineering Sciences, Wayne State Universlty, Detroit, Michigan 48202 (Received March 10, 1978) Publication costs assisted by Wayne State UniversiTy
The presence of vibrationallyexcited OH in the products of the H + NO2 reaction has been confirmed by detailed spectroscopic analysis of the IR emission. The observations were carried out in a fast flow discharge reactor at room temperature and at total pressures near 1Torr. Although diffusion limited mixing was evident in our system, the presence of vibrationally excited OH restricts the range over which the H + NO2 reaction can be used as a source of ground state OH for subsequent reaction studies.
Introduction and Recent results of Silver et al.,l Spencer and Polanyi and Sloan3 have indicated that the exothermic reaction, H + NO2 OH + NO, produces OH in vibrationally excited states. While the percent exothermicity which is channeled into vibrational modes of OH as determined by each of these investigations is somewhat different, all report that at least 25% of the 28-kcal/mol exothermicity is used to populate OH u = 1,2, and 3. This value is in substantial disagreement with earlier work by Kaufman and Del G r e ~ owho , ~ reported less than 2% (the limit of their detectivity) excitation of OH(u = 1)in a flow discharge system. Since many have used the H + NOz reaction as a source of OH for subsequent reaction studies, it is important to quantify the presence of vibrationally excited species prior to subsequent reactions. Recent results8 on 0 + HC1 have indicated that the presence of HCl(u = 2) can increase the rate by 2 X lo4 over the ground state rate at room temperature. Spencer and Glass9 report an enhancement of from 2 to 3 in the fast 0 + OH reaction due to OH(u = 1). Theoretical predictionslO on the O(3P) + H2 reaction indicate an increase in the rate of about lo4 at room temperature due to the presence of H2(u). While the results of Silver et aL,I Spencer and Glass,z and Polanyi and Sloan3 were conducted under experimental conditions which would minimize OH(u) loss processes, we report here the presence of OH(u = 1,2) from the H + NO2 reaction in a fast flow tube reactor at observation times of from 1to 3 ms. The presence of OH(v = 1, 2) was confirmed by comparing the experimentally observed infrared emission with a detailed line by line calculation of the OH spectra, accounting for the split ground state and using the detailed A coefficients computed by Mies.ll
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Experimental Section The flow tube is a 3.98-cm i.d. stainless steel tube with General Electric Type 125 quartz windows located at 13, 17, 24.5, 32.5,40, and 60 cm from the downstream flange. Pumping is provided by a Leybold-Heraeus Model 1100 1147 cfm Roots type blower, backed by a Leybold-Heraeus Model E-225 mechanical pump. The system is maintained at a pressure of less than 10 pm between experiments by a Sargent Welch Model 1376 mechanical pump. Partial pressures of additive gases were measured to an accuracy of 1 pm in situ by a MKS Type 77 Baratron. A Santa Barbara Research Model 9735 PbS detector operating at 193 K was used in conjunction with either a Jarrel-Ash NRC/NRL Postdoctoral Fellow, 1977-present. 0022-3654/78/2082-2139$01 .OO/O
82-410 0.25-m grating monochromator (gratings blazed at 2.2 and 5.4 pm) or various filters to measure OH(u) emission. A modulated microwave discharge, Raytheon Model PGM-10-X2, was used to produce atomic hydrogen from a mixture of about 100-200 pm of Hz and 400-600 pm of either He or Ar. The modulated portion of the discharge signal (usually operated between 20 and 100 Hz) was used as the reference signal for a PAR Model Hr-8 lock-in amplifier. This method of pulsing the discharge produced pulsed reaction products which were amplified and detected. The pulsed signal also provided a means to measure velocities in the system by determining the phase shift of the reference signal relative to the emission signal at two points in the flow tube.12J3 A complete description of the apparatus can be found e1~ewhere.l~ Gases were used directly from the manufacturer and were not purified further. The following Linde gases were used: helium (99.995%), argon (99.998%), hydrogen (99.99%), extra dry oxygen (99.6%), nitric oxide (98.5%), and nitrogen dioxide (99.5%).
Results and Discussion In order to verify that the emission was from vibrationally excited OH, a line-by-line artificial spectra was generated using the vibronic A coefficients of Miesll and the spectroscopic parameters from Herzberg.14 Rotational term values of the components of the OH doublet states (2~3/2, z) were calculated using the formulas first This detailed artificial suggested 6y Hill and VanV1e~k.l~ spectra was integrated over a triangular slit function to yield a low resolution spectra comparable to the experimental results. A full discussion of this calculation inA cluding the computer code can be found e1~ewhere.l~ typical calculated spectra is shown in Figure 1based on a vibrational and rotational temperature of 5000 and 500 K, respectively. Shown in Figure 2 is an experimental spectra taken at a reaction time of about 1ms with H atom concentrations of 3 X lOI4 and 6-9 X 1013molecule/cm3 of NOz. Hydrogen atom concentrations were determined by comparing intensities from the H + NO reaction ( I c: [H][N0])16 the case where H2dissociation was found to be complete (0.1% H2 in He)12J3with the intensity of the same reaction under operating conditions. Typically 5-10% dissociation of Hz was observed. As shown in Figure 2, the spectra have not been corrected for system response, however, we have also included in the figure the response curve for the monochromator-PbS detector combination. The three prominent peaks in this spectra correspond to the Q(1-0) branch at 2.8 pm, the P~-~(1-0), and overlap from the RNe2,d2-1) at 2.874 pm. The peak at 2.939 ,urn corresponds to overlap between @ 1978 American Chemical Society
2140
The Journal of Physical Chemistry, Vol. 82, No. 19, 1978 8T
WAVELENGTH
(micron)
Figure 1. Calculated infrared emission from OH(v = 1, 2) based on the A coefficients of Mies" with an assumed vibrational temperature of 5000 K and a rotational temperature of 500 K.
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G. K. Smith and E. R. Fisher
The decay of the OH emission down the tube was found to be pseudo-first order with an apparent rate constant of 2.5 X cm3/molecule s under conditions of excess H atoms. Under these same conditions, Spencer and Glass2report that the major loss of vibrationally excited OH should be relaxation by H atoms occurring with rate constants of 3.3 X and 2.7 X cm3/molecule s for OH(u = 2) and OH(u = l), respectively. Although their results depend strongly on the relative population of OH(u = 1) to OH(u = 2) the results reported here cannot be reconciled with those of Spencer and Glass2 without assuming poor reactant mixing. Poor reactant mixing would prolong the source of OH(u) down the tube, resulting in an observed loss rate for OH(u) slower than the true bimolecular loss rate, as specified by Spencer and Glass.2 Simple calculations based on a characteristic diffusion time, R 2 / D ( D = diffusion coefficient, R = characteristic length), suggest that in flow systems, where R 2 / D lies in the range of 10-L10-3 s, reaction processes with first-order half-lives shorter than this will be diffusion limited. Under conditions of 3.2 X 1014molecule/cm3 of H atoms (excess HI, the half-life for the H + NO2 reaction is about 10" s. Detailed modeling of this system is described elsewhere.ls Although the presence of the observed OH emission was, in part, due to inhomogeneities in the mixing of the reactants, our results suggest that the H NO2 reaction should be used judiciously in the future as a source of OH due to the possible presence of OH(u) and its subsequent effect on measured reaction rates. This conclusion is reached even though the 0 NO2reaction has not shown any mixing effects in our system12due to the fact that the 0 + NO2reaction is about two orders of magnitude slower than the H + NO2 reaction. While the experimental conditions of this study are not representitive of systems with slow flow (-5-20 m/s) or low reactant concentrations ( 1010-1012molecule/cm3), we feel that careful consideration should be given to mixing lengths and observation times to ensure that even small amounts of OH(u) do not affect kinetic measurements.
+
iV
2.9 3.1 3.3 WAVE LEN GTH (m icr on 1
3,5
+
Figure 2. Experimental spectra of OH(v = 1, 2) from the H NOp reaction taken at a reaction time of 1 ms with H = 3 X IOi4 molecule/cm3 and NOp = 6-9 X 1013 molecule/cm3.
the PN&-0) and Q(2-1) branch. The R branch of the 1-0 transition, which is free from overlap of the 2-1 emission, shows emission from rotational levels as high as N = 10. Corresponding emission in the P branch should occur out to 3.24 pm. As seen in the spectra, emission is observed at wavelengths well past 3.3 pm. With correction for falling system response, these peaks would be increased by at least a factor of 2, further evidence for a contribution to the spectra from 2-1 transitions. Several functional forms for the vibrational population distributions (ramps, linear, Boltzmann) were used to best fit the artificial spectra with the experimental spectra. While we cannot characterize the spectra exactly with any one distribution, best agreement between experiments and calculated spectra occurred when the vibrational and rotational temperatures used in the calculation were 5000 and 500 K, respectively. Under these conditions the ratio OH(u = 2)/OH(u = 1)was about 0.35 f 0.1. This suggests lower limits to formation rates for these vibrationally excited states consistent with those of Polanyi and Sloan, i.e., k(u = 1) = 1.00; k(u = 2) = 0.35 f 0.1 relative to k(u = 1). Based on a simple set of reaction kinetics for this system, we estimate that vibrationally excited OH(u = 1,2) is about 1% of OH(u = 0) at reaction times of 1ms. Whether this is a significant fraction depends entirely on the effect vibrational energy has on subsequent reactions. In general, if a ground state reaction has an activation barrier greater than about 3.0 kcal/mol, then the vibrationally excited OH observed in our system may have an impptant effect on overall observed rates. An example of such a system (recently investigated by Blackwell, Polanyi, and Sloan)" is the OH(u) + C1 HCl(u) + 0 reaction where the barrier height is 5.0 kcal/mol.
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Acknowledgment. The authors thank Professor Ralph Kummler for his comments and discussions.
References and Notes (1) J. A. Silver, W. L. Dimpfl, J. H. Brophy, and J. L. Kinsey, Chem. phys., 65, 1811 (1976). (2) J. E. Spencer and G. P. Glass, Chem. Phys., 15, 35 (1976). (3) J. C. Polanyi and J. J. Sloan, Jnt. J. Chem. Kinet., Symp. I, 51 (1975). (4) F. Kaufman and F. P. Del Greco, J . Chem. Phys., 35, 1895 (1961). (5) C. J. Howard and K. M. Evenson, J. Chem. Phys., 61, 1943 (1974). (6) K. H. Becker, D. Haaks, and T. Tatarczyk, Chem. Phys. Lett., 25, 564 (1974). (7) J. S. Chang and F. Kaufman, J . Chem. Phys., 66, 4989 (1977). (8) J. E. Butler, J. W. Hudgens, M. C. Lin, and G. K. Smith, Chem. phys. Left ., submitted for publlcation. (9) J. E. Spencer and G. P. Gbss, Int. J. Chem. Kinet., IX, 111 (1977). (10) P. A. Whitkxk, J. T. Muckerman, and E. R. F W , RIES Report 76-114, Wayne State University, Nov., 1976. (11) F. H. Mies, J . Mol. Specfrosc., 53, 150 (1974). (12) B. B. Krieger, Ph.D. Dissertation, Wayne State Unlversity, 1975. (13) G. K. Smith, Ph.D. Dissertation, Wayne State Unlversity, 1977. (14) G. Herzberg, "Spectra of Diatomic Molecules", 2nd ed, Van Nostrand-Reinhold, New York, N.Y., 1950. (15) E. L. Hill and J. H. VanVleck, Phys. Rev., 32, 250 (1923). (16) M. A. A. Clyne and 8. A. Thrush, Discuss. Faraday Soc., 33, 139 (1962). (17) B. A. Blackwell, J. C. Polanyi, and J. J. Sloan, Chem. Phys., 24, 25 (1977). (18) 0. K. Smith, B. 8. Krieger, and P. M. Herzog, AIChE J., submitted for publication. (19) B. B. Krieger, M. Malki, and R. H. Kummler, Environ. Sci. Techno!., 6, 472 (1972).
