T H E
J O U R N A L
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PHYSICAL CHEMISTRY Registered in U.5. Patent O$ce
@ Copyright, 1974, by the American Chemical Society
VOLUME 78, NUMBER 1 JANUARY 3, 1974
Photolysis of Nitric Acid Vapor Harold S. Johnston,* Shih-Ger Chang, and Gary Whitten Department of Chemistry, University of California, Berkeley, and inorganic Materials Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 (ReceivedJuly 76, 1973)
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The primary reaction in the photolysis of nitric acid vapor by ultraviolet radiation is " 0 3 hv HO NOz and the primary quantum yield is 1. This article gives experimental evidence in favor of these two conclusions for the wavelengths 200, 255, 290, and 300 nm. The photolysis of nitric acid vapor in laboratory apparatus is subject to several unwanted side reactions, and conditions must be carefully selected to eliminate the effect of such reactions.
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Introduction
Experimental Section
Nitric acid has been observed in the stratosphere by Murcray, et al.l It is presumably formed from hydroxyl radicals and nitrogen dioxide
Materials. Anhydrous nitric acid was prepared by vacuum distillation from a 50-50 slurry of concentrated sulfuric acid and sodium nitrate. The high temperature should not exceed 30" in order to avoid the thermal decomposition of liquid nitric acid to form nitrogen dioxide, and the low temperature should not be below -40" to avoid distillation of water from the concentrated sulfuric acid. The pure nitric acid was completely colorless, and it remained pure indefinitely when stored at Dry Ice temperature and in the dark. The oxygen (Matheson, research grade) was passed through traps at Dry Ice temperature. Carbon monoxide (Matheson UHP grade) was passed through a 5-ft long column of activated charcoal on glass wool in order to remove iron carbonyl. Apparatus. The glass vacuum apparatus was of conventional design. Stopcocks were lubricated with Kel-F stopcock grease, which is inert to nitric acid, or were of the nonlubricated variety with a Teflon plug and Viton 0 rings. Pressures were measured by a Pace transducer, which we calibrated against an oil manometer. The reaction cells were cylindrical, 100 mm in length, and 35 mm in diameter. Two grease-free stopcocks with Viton 0 rings were sealed to the cell with quartz-to-glass graded seals. The silica windows were fused to the cell. Several different light sources were used: a 50-W deuterium arc at 200 and 215 nm with a Bausch and Lomb high-intensity grating monochromator; a 200-W high-pressure mercury arc with the same monochromator; a 1600-W xenon arc with a 500-mm Bausch and Lomb monochromator. Light intensities were measured by two methods: (1) potassium ferrioxalate actinometry5 above 255 nm and HBr photolysis actinometrys at 200 and 215 nm and (2) a Hewlett-Packard 8330-A radiant fluxmeter. The intensi-
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and it may also be formed by gaseous dinitrogen pentoxide abstracting water from aqueous sulfuric acid droplets N,Ob+H@
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The maximum mole fraction of nitric acid vapor is found between 20 and 24 km. The decreasing mole fraction above the maximum is probably caused by the photolysis of nitric acid vapor. Recently the absorption spectrum of nitric acid vapor has been obtained.z However, quantitative modeling calculations of the photolysis of nitric acid in the stratosphere require knowledge of the products of the primary photochemical reaction and the quantum yield as a function of wavelength. The primary products could be HNO3 hv HO NO, H+NO,
+
--
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-+ 0 +HN02 Berces and Forgeteg3 reported a quantum yield of about 0.1 a t 265 nm, but their conclusions involved assumed values of other rate constants that have since been shown by Morris and Niki4 to be in error by several orders of magnitude. The purpose of this study was to identify the primary products and obtain the primary quantum yield for the photolysis of nitric acid vapor by ultraviolet radiation.
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Photons Absorbed ( ~ r n - ~ )
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Figure 1. Ultraviolet absorption spectra of "03, NO2, and N205: In la I uNL, where u has units cm2, L cm, and N molecules cm- .
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ties obtained by these methods were in good agreement, but all quantitative data are based on the chemical actinometry. The optical band width (full width a t half-maximum) was 2.2 nm at 300 nm, 6.5 nm a t 290 nm, 2.5 nm a t 255 nm, and 6.5 nm below 225 nm. Procedure. Three types of runs were carried out: (1) with pure nitric acid alone, (2) with nitric acid plus carbon monoxide, and (3) with nitric acid plus carbon monoxide and oxygen. Nitrogen dioxide in the cell was measured by stopping the photolysis, by transferring the reaction cell to a Beckman DU or a Cary 14 spectrophotometer, and by measuring optical density at 405 nm ( u = 6.24 X cm2 molecule-I, compare Figure 1). The carbon dioxide produced was measured in a Consolidated Electronics Corporation 21-110 high-resolution mass spectrometer. Appropriate blanks were measured with no photolysis. Most runs were carried to only very small degrees of conversion (1 or 2%), and thus it was not practicable to follow the course of the reaction in terms of disappearance or of CO. The course of the reaction was folof " 0 3 lowed from time to time by optical analysis of NO2 formed; or it was followed by ending the run, freezing out the nitric acid, and analyzing the CO and COS with the mass spectrometer. All runs were made at 25". Results and Discussion The photolysis of nitric acid vapor was studied at 25" with initial nitric acid pressures between 5 and 40 Torr. it During the course of the photolysis at 40 Torr of "03, was noticed that liquid droplets (presumably aqueous nitric acid) condensed out on the walls of the reaction cell, and all runs at 40 Torr have been rejected. At 5 Torr of HN03 the rate of formation of products was exceedingly slow. Most runs were made a t 15 or 30 Torr. Photolyses were carried out at wavelengths between 200 and 315 nm. The absorption spectrum of NO2, "03, and N2Os is given as Figure 1. At all wavelengths the absorption cross section of NzOs is substantially greater than that of HN03, and thus secondary photolysis of N205 is a complicating feature of certain experiments. The absorption spectrum of NO2 occurs in two bands, above and below 250 nm. Between about 250 and 400 nm the product of the photolysis of NO2 is N O and ground-state oxygen atoms, O(3P),Below 250 nm the product of the photolysis of NO2 is N O and an excited singlet oxygen atom, O(ID).? The Journal of Physical Chemistry, Vol. 78, No. 1, 1974
Figure 2. Photolysis of pure nitric acid vapor with 290-nm ultraviolet radiation. The straight line corresponds to unit quantum yield. The formation of NO2 from pure "03 occurs with a quantum yield much less than 1.
The excited singlet oxygen atom reacts very rapidly with H20, H2, CH4, etc.8 and presumably it would react very rapidly with " 0 3 . On the other hand, the ground-state oxygen atom, O(3P), reacts very slowly (if at all) with nitric acid vapor.4 Thus the reactions following the secondary photolysis of the product NO2 are quite different above and below 250 nm. Most runs were made at 255 nm (where the cross section for light absorption by HN03 is greater than that for NO2) or at 290 or 300 nm (where the cross section of NO2 greatly exceeds that of HN03). At 200 nm the cross section of " 0 3 greatly exceeds that of NOz, and successful runs were made at this wavelength. A series of photolyses was carried out at 290-nm radiation, 30 Torr of pure HN03, and the progress of the reaction was followed by light absorption by NO2. The results are given in Figure 2 where the logarithm of concentration of NO2 is plotted against the logarithm of photons absorbed per cm3. The initial nitric acid concentration is indicated at the top of the figure. The primary quantum yield 9 is defined as number of molecules of HN03 destroyed @= (1) number of photons absorbed by "03 The quantum 9 ( x ) with respect to some product x is defined as number of molecules of x formed +(x) = (2) number of photons absorbed by HN03 If NO2 were produced with a quantum yield of 1, the experimental points would lie on the 45" line given on the figure; points above the line would correspond to a quantum yield greater than 1; and points below the line correspond to quantum yields less than 1. The first four experimental points represent quantities of NO2 which are less than 1% of the initial H N 0 3 and the quantum yield for formation of NO2 is about 0.1, in rather close agreement with the results of Berces and F ~ r g e t e g .Our ~ interpretation of the results, however, is quite different from that of Berces and Forgeteg. These experimental results were interpreted by a model of 42 reactions (Table I) carried out by the complete Gear r ~ u t i n e modified ,~ for this photochemical study. This discussion focuses on the dominant reactions to give the reader a qualitative understanding of what is involved; quantitative conclusions are based on the integration of
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Photolysis of Nitric Acid Vapor
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8. NOz NO 0 9.0 NOz + N O 0 2 10.0 NO M +NOz M ~ ~ . O + O Z + M + O ~ + M 12. NO 0 3 NOz OZ 13. H O CO + H COz 14. H NOz HO NO 0 2 M HOO M 15. H 16. H "01 Hz NO3 + HO HNOz 17. HOO NO + NO2 HO 18. HO HO HzO 0 19. HO HOO -+ HzO 0 2 20. HOO HOO + HzOz 0 2 21. NO NO Oz NOz NOz 22. HO NO M + HNOz Mc 23. HNOz HNOz -+ HzO NO NOz 24. HzOz HO -+ Hz0 HOO
+ + + + + + + + + + + + + + + + + + ++ + + + + + + + 25. HZOZ HO + HO HOO + HO 26. HzOz + 0 27. HzOz + 0 HzO + Oz 28. HzOz + H Hz + HOO 29. HzOz + H HzO + HO 30. + NOz NO3 + On hv 31. NzOs 0 + 2N02 32. 0 + HNOs HO + NOa 33. HO + HNOz HzO + NOz 34. 0 + NzOj Oz + 2N02 35. HO + 0 H + 36. HOO + H Hz + Oz 37. HOO + H HO + HO 38. NO + 2HNOa HzO + 3NOz -+
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