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The Photochemistry of Dinitrogen Pentoxide Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory Title THE PHOTOCHEMISTRY OF DINITROGEN PENTOXIDE Permalink https://escholarship.org/uc/item/5dn1m436 Author Connell, Peter Steele Publication Date 1979-05-01 eScholarship.org Powered by the California Digital Library University of California LBL-9034 THE. PHOTOCH£MfSTRY OF DlNITROGEN PENTOXIDE Peter Steele Connell (Ph.D. thesis) May 1979 Prepared for the U. S. Department of"' Energy under Contract W-7405-ENG-48 TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Diu is ion, Ext 6782 LBL-9034 THE PHOTOCHEMISTRY OF DINITROGEN PENTOXIDE by Peter Steele Connell Materials and Molecular Research Division Lawrence Berkeley Laboratory and Department of Chemistry University of California, Berkeley, California 94720 iii THE PI-mOCHEMISfRY OF DINITROGEN PENTOXIDE Peter Steele Connell Table of Contents ABSTRACT • • • • • • vii L INTRODUCTION . 1 A. THERMAL DEC(]I.1p()SITION 2 B. PHOTOCHEMISTRY OF N 0 •• 6 2 5 C. 1HE STRUCTIJRE AND THERMJCHBVliSTRY OF N 7 2o5 I I. EXPERIMENTAL METillDS AND APPARATUS 9 A. METHODS . • • • • • • • • 9 1. Thermal Decomposition . 9 2. Photochemistry 11 B. APPARATUS • • • • 21 1. Reaction Cell and Temperature Control . 21 2. Optical System ' ' . 22 3. Detector ' . ' . 24 4. Signal Processing ' . ' . 25 5. Photolytic Source . 32 c. GASES AND FLOW SYSTE!v! . 35 lV IIL EXPERIMENTAL PROCEDURES AND DATA . 38 A. INFRARED ABSORPTION CROSS SECTIONS 38 B. THERMAL DECOMPOSITION EXPERIMENTS . 46 C. CLOSED CELL PHOTOLYSES ' ' . 61 D. CONSTANT INTENSI1Y PHOTOLYSIS OF FLOWING SYSTEMS . ' . 62 E. MJDULATION EXPERIMENTS . 65 F. ACTINOMETRY . 68 IV. RESULTS AND DISCUSSION . 73 A. THERMAL DECOMPOSITION 73 B. PHOTOCHEMISTRY . 105 1. UV Spectrum of N2o5 . 105 2. Closed Cell Photolyses . 106 3. Constant Illumination in Flowing Systems . 116 4. Intermittent Photolysis in Flowing Systems . 116 5. Summary of Photolysis Results . ' ' . 121 v. STRATOSPHERIC IMPLICATIONS • . ' ' . 124 A. EXT'RAPOLATION OF THE THERMAL DISSOCIATION RATE CONSTANT . 124 B. NI CHIT IME STRATOSPHERE MJDEL. 129 c. N2o5 PHO'I'Q.tlEMISTRY IN THE STRATOSPHERE 136 v APPENDIX A: MULTIPARAMETER NON-LINEAR LEAST SQUARES FITTING PROGRAM • • • • • • • • • • 141 APPENDIX B: COMPLETE REACTION SET FOR PHOTOCHEMICAL M)DELING 149 ACKNOWLEDGMENTS 150 BIBLIOGRAPHY 151 vii 1HE PtmOCHEMISTRY OF DINITROGEN PENTOXIDE Peter Steele Connell Materials and Molecular Research Division, Lawrence Berkeley Laboratory and Department of Chemistry, University of California, Berkeley, California 94720 ABSTRACT This study examined the gas phase photochemistry and thermal decomposition kinetics of dinitrogen pentoxide, N2o5. The products of photodecomposition and the quantum yield as a function of pressure have been determined at a wavelength of 254 nm. The rate of thermal decompo­ sition of N2o5 in the presence of excess NO has been observed between 14 19 3 4 x 10 to 2.8 x 10 molecules cm- and 262 to 307 K. The investigation was conducted by monitoring the infrared ab­ sorptions of N2o5, N02 and HN0 3. Absorption cross sections for these species were experimentally determined. The techniques used included closed cell decay profiles, with and without photolyzing light, constant illumination of flow systems and modulation of species in a flow system with intermittent photolytic illumination. The thermal decomposition of N in the presence of NO proceeds 2o5 by the mechanism viii for which the rate is controlled by the rate of the first step, the unimolecular decomposition of N2o5. At low pressure the reaction be­ comes second-order with respect to the total pressure. Measurements 16 -3 made in nitrogen, for which [N 2) < 2 x 10 molecule em , give an expression for the second-order low pressure limit 6 3 1 1 268 to 307K k' = 8.05 x 10- exp(-9630 ± 200/T) cm molec- sec- . 0 At sufficiently high pressure, the reaction is strictly first-order. The experimental indication obtained for the high pressure limit of the rate constant is 17 k 1. 78 x 10 exp(-12540 ± 200/T) sec-l 262 to 272 K 00 = The behavior of the rate constant for thermal decomposition in the fall- off region between the high and low pressure limits is adequately de­ scribed over the temperature range 262 to 307 K by the expression 2 1 2 -(k I [M]+k) + ((k ! [M]+k ) + 22k k I [M]) 1 . 0 00 0 00 ooo k = 11 These expressions can be combined with the equilibrium constant, measured by Graham, 27 to give the third and second-order limits of the reverse reaction The values are and ix The low pressure limit obtained is in good agreement with the earlier work of Johnston and Perrine7 and Johnston.8 The activation energy in the high pressure limit is larger than the estimate of Mills and Johnston6 of 10570 K and implies a small positive activation energy for the recombination reaction of N0 AND N0 at high pressure. The 2 3 predicted high pressure limit of the dissociation rate constant at 300 K about one half the value reported by Mills and Johnston. Experiments in which N was photolyzed by 254 nm radiation ln 2o5 the presence of nitrogen or oxygen buffer showed the primary photolytic step to be The quantum yield is a function of both N2o5 and buffer gas pressure and of the rates of secondary reactions. The suggested mechanism in the absence of secondary reactions is N2o5* + N2o5 zN2o5 ~ ZNo + 2 o The ratio of self-quenching to collisionally activated decomposition is 2.2. If the sum of these two processes is considered collisional, the collision-free lifetime of the excited N 0 * is 6 x 10-6 sec. Quenching 2 5 by nitrogen and oxygen is about 104 less efficient than quenching by X From experlinental upper llinits to N concentrations in the 2o5 stratosphere and standard air density values, the quantum yield for N o photolysis in the stratosphere is calculated to be greater than 2 5 0.9 above 20 km and essentially unity throughout the region of maximum N concentration. 2o5 1 I. INTRODUCTION Recent interest in nitrogen oxides as tropospheric1 and strato­ spheric2 pollutants has created a need for laboratory measurements of kinetic and photolytic parameters associated with the reactions of these species (NO, N0 , N0 , N o , ... ).The thermal decomposition of dinitrogen 2 3 2 5 pent oxide (1) has historically been studied as an example of a true tmimolecular re­ action with which to test various kinetic theories. 3 These measured decomposition rate constants can also be used in an atmospheric model to predict distributions of N o and N0 in the stratosphere. The atmos­ 2 5 3 pheric role of N o is to provide a temporary sink for the more active 2 5 forms of NOX, which participate in controlling ozone concentrations. In addition to thermal decomposition, N o exhibits a continuous 2 5 absorption in the ultraviolet spectral region, which begins at around 380 nm and increases smoothly with decreasing wavelength, showing no maximum above 210 nm. The photochemistry of this absorption may con­ tribute to limiting the lifetime of N2o5 in the stratosphere and to pos- sible reduction of ozone by the mechanism N02 + 03 N03 + 02 M~ N02 + N0 Nzos 3 (2) + hv NO + N0 Nzos 2 + oz NO+ 0 N0 + 3 2 net: 203 302 2 A. THERMAL DECOMPOSITION The technique used to study this process originated in the ob- servation a reaction between nitric oxide and dinitrogen pent- oxide to yield nitrogen dioxide. 4 Smith and Daniels5 proposed the mechanism. (3) in which the NO acts as a fast scavenger for the N0 produced in the 3 decomposition of N2o5, preventing its recombination with N02. Thus, at least initially, the observed rate of N2o5 disappearance in the presence of NO is that of the slow unimolecular decomposition of N . Mills and 2o5 Johnston6 employed this scheme, monitoring the appearance of the product, N02, optically to study the reaction over a range of pressures from 0.011 to 933 kPa at 300 K, and at a few pressures at 273, 277 and 313 K. Their work was refined at low pressures by Johnston and Perrine,7 8 9 by Johnston and Wilson and Johnston, again by optical detection of N0 2. Johnston and Perrine extended the study of the rate of decomposition down to a total pressure of 6.7 Pa at 300, 323 and 344 K, obtaining a low pressure activation energy. Further work by Wilson and Johnston at 324 K in the presence of a variety of buffer gases clarified the relative activational efficiencies of N , NO, Ar, N , and SF • Hisatsune, 2o5 2 co2 6 Crawford and Ogg10 followed the decomposition at 7.6 and 53.3 kPa total pressure in nitrogen. Using a fast scanning infra-red spectrometer, they were able to monitor both the disappearance of N2o5 and the appear­ ance of N02 at a few temperatures between 293 and 303 K. 3 Recently Viggiano, et a1. 11 have applied their technique of chemical ionization mass spectrometry to observe the decomposition of N in the presence of NO and N in a flowing system. They have ex~ 2o5 2 tended the temperature range of observations from 268 to 379 K, for pressures between 1.87 and 46.7 kPa. These are the first data for which experimental conditions approach those characteristic of the stratosphere. Representative data of the various workers are presented in Fig. 1 as log kml . plotted against log [M], the total effective number den­ sity. If necessary for the sake of comparison, data have been adjusted to common temperatures using reported activation energies. The total effective number density is calculated as the effective N2 concentration, using the measured relative efficiencies of Johnston.
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