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Radiolysis of ammonia in nitrogen; Effects of nitrogen monoxide and oxygen on decomposition of ammonia
Inremarional Journul a/ Applwd Rudbrion and Isotoprr Printed in Great Brilak. All rights reserved
Vol.32, pp. 567 to 572. 1981
CO2~7CU?X/81/080567-06SO2.00,‘0 Copyright 0 1981 Pcrgamon Press Ltd
Radiolysis of Ammonia in Nitrogen; Effects of Nitrogen Monoxide and Oxygen on Decomposition of Ammonia OKIHIRO
TOKUNAGA,’
TSUTOMU and
SEKINE,2 MASANOBU
NOBUTAKE
SkKANO~E2
SUZUKI’
‘Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment, Watanuki-Machi, Takasaki, Gunma 370-12, and “Department of Chemistry, Faculty of Science, Kanazawa University, Otemachi, Kanazawa 920, Japan (Receioed 5 March 1980; in reuisedjorm
15 December 1980)
The effects of NO and O2 on the decomposition of NHs were studied at low concentrations (3@4I-900ppm) in N, under irradiation by electron beams of 1.5 MeV energy at 120°C. The NH3 WBS decomposed to H2 and N2 in N2, with a G-value of 1.3. In the presence of NO (800 ppm) or O2 (5X,), the NH, decomposition was much enhanced (G-values were 3.1 and 3.0, respectively), the Hs formation was suppressed, and N20 formation was observed with a high yield. The enhancement of the NH3 decomposition in the presence of NO or Q2 is attributed to the reactions of H and NH2 radicals with NO or 02, leading to the formations of N20, N2, H20, etc. In the presence of NO, the reaction of NO+ with NH3 and that of NH: with NO may also be important for the NH3 decomposition.
Introduction A SERIES of fundamental research on the radiation treatment of exhaust gases has been carried out in our laboratory, in order to elucidate the mechanism of removing NO, and S02. The addition of NH3 to exhaust gas before irradiation was found to make it possible to collect NO, and SO, as solid ammonium nitrate and ammonium sulfate, respectively, and to enhance the reaction of removing NO, and SOl.(i) For these advantages, a small amount of NH3 was added before irradiation in the pilot scale test for the radiation treatment of exhaust gas from a sintering furnace.‘2) However, the reaction mechanism of NH3 in the exhaust gas was still not completely understood. The radiolysis of NH3 has been studied extensively to estimate the primary yields of active species and to clarify the mechanism of the NH3 decomposition.‘3) PAGSBWG et a1J4i and EYRE and SMITHIES(~)have studied the effects of O2 and NO on the radiolysis of NHa, and proposed the reactions of O2 and NO with H and NH2 radicals for the NH, decomposition. But, the radiolysis of NH3 has not yet been studied at low concentration in N,. In the radiolysis of low concentration of NH3 in N2, it is presumed that the active species, produced by the radiolysis of N,, may contribute to the reactions of NH,. In the present paper, the effects of NO and 0, (as components of exhaust gases) on the radiolysis of NH3 are studied at low concentrations in N,. The 567
mechanism of reactions including charge transfers from Ni and Ni ions to NHS, NO and O1 are proposed for the NH, decomposition.
Experimental The experiments were carried out using the flow experimental apparatus described in the preceding paper.(6) The commercially available gases, N,, 01, NH, and NO (all above 99.9%) were used without further purification. The flow rate of NHs was automatically regulated by a mass-flow controller (Okura Denki Co., MFC-1 type).‘The gaseous mixtures were irradiated with electron beams at dose rates in the range 0.06437 Mrad . s- ’ . An electron accelerator by Radiation Dynamics, Inc. (Dynamitron type; maximum energy, 3 MeV; maximum current, 25 mA) was used for the irradiation. The analysis of NH3 was carried out with ultraviolet derivative spectrometry.“) The second derivative spectrometer used (Yanagimoto, UO-1 type) has a single light beam which passes through an analyzing cell (SOcm pass length). The absorption spectrum in the wavelength region of 195-215 nm was used for the analysis. The Hz and N20 were analyzed by gas chromatographs (Shimadzu, GC-2AL and GC3BT type) with thermal conductivity detectors. For the analysis of Hz, a .column (2 m x 5 mm) packed with Molecular Sieve 5A was used at 100°C with Ar as a carrier gas,
0. Tokunaga et al.
568
and for the analysis of N,O, a column (3 m x 5 mm) packed with Porapak .Q was used at 35°C with Ar. The G-values were calculated on the basis of the energy absorbed by the whole mixture.
$2
Results
0
1. Decrease in NH3 concentration by irradiation When NH3 (42&930ppm)-N, mixtures were irradiated by electron beams at 120°C the NH3 concentration, [NHJ, decreased linearly with dose in all initial NH3 concentrations, as shown in Fig. 1. The G-values of the decrease in [NH,], obtained from the slopes of the straight lines, were shown by circles in Fig. 2. The G-value* (1.3) was almost independent of the initial NH3 concentrations in the range from 420 to 930 ppm. 2. Effects of NO and O2 on decrease in NH3 concen-
1 OO-
500 1000 Nti3 initial cont. @pm)
FIG. 2. Initial G-value of the decrease in NH, concentration as a function of initial NHs concentration: the units of G-value in this paper are molecules, ions or radicals per unit energy (1OOeV) absorbed. 0 = mixture of NH3 and N1; A = mixture of NH,, NO (800ppm) a?d N,; 13 = mixture of NH,, O2 (5%) and N2. Irradiation temperature: 120°C.
tration
In Fig. 3, [NH,] in the NH3-N2 mixture containing NO (800ppm) and in the mixture containing 02 (5%) were shown as a function of dose by triangles
* The units of G-value in this paper are molecules, ions or radicals per unit energy (1OOeV)absorbed.
0
0
1
I
I
1
2
3.
I
I
4.
5
Dose Wrod)..
and squares, respectively. The [NHJ decreased linearly with dose up to about 2 Mrad for all initial NHp concentrations. The G-values of the decrease in [NHJ for each initial NH. concentration, calculated from the slope of the straight part at low doses, were shown in Fig. 2 as a function of the initial NH3 concentration. In the figure, the G-values in the mixture containing NO (800ppm) and in that containing O2
6
__
FIG. I. NH, concentration as a function of dose ii the mixture of NH3 and N, for various initial NH3 concentrations. Irradiation temperature: 120°C.
FIG. 3. NH, concentration as mixture of Nk, and N, with A = with NO (800ppm); 0 = temperature:
a function of dose in the and without NO or 0,: with O2 (5%). Irradiation 120°C.
Radiolysis
ofammoniain
nitrogen
569
(5%) were shown by triangles and squares, respectively. The G-values were almost independent of the initial NH3 concentrations ranging from 290 to 92Oppn-1, amounting to 3.1 for the mixture with NO (800 ppm) and 3.0 for that with O2 (5%). 3. Effects of NO and O2 on formation of H2 When NHJ-NI mixtures were irradiated, Hz was formed, as shown in Fig. 4. In the figure, the concentrations of H,, [Hz], for the initial NH, concentrations of 670ppm and of 930ppm were shown as a function of dose by open triangles and circles, respectively. The H2 concentration increased linearly, with dose for both initial NH3 concentrations. From the slopes of the straight lines shown in the figure, the G-values of the Hz formation were calculated to be 2.0 for both initial NH3 concentrations. In Fig. 4, the values of [HJ in the NO (8OOppm)added mixture of NH3 (670ppm) and N,, and in the NO (800 ppm)-added mixture of NH3 (930 ppm) and N2 were also shown as a function of dose by closed triangles and closed circles, respectively. The H2 formation was markedly suppressed by the addition of NO. For example, [Hz] for the dose of 5.4 Mrad was decreased by the addition of NO (800ppm) from 320 ppm to 70 ppm in the mixture with 930 ppm NH3 and to 30 ppm in the mixture with 670 ppm NHJ. The G-values of the H, formation in the NO-added mixtures were calculated to be 0.7 from the slopes of the straight parts of the curves at low doses in Fig. 4.
Y ‘0
I
,
I
1
I
I
1
2
3
4
5
6
Dose (Mad )
FIG. 5. N20 concentration as a function of dose in the mixture of NH3 (800 ppm), NO and N1 for various initial NO concentrations. Initial concentration; NO: (0 = 2OOppm;A = 4OOppm;(3 = 600ppm; 0 = 800ppm and V = 15OOppm); NH3:(800ppm).
The H, formation in the NHI-N2 mixture was also suppressed effectively by addition of Oz. The H, concentration in the NH, (920ppm)-N, mixture irradiated to a dose of 5.4Mrad was decreased from 320ppm to a value less than the limit of detection (100 ppm) by the addition of 5% Oz. 4. Effect of NO on formation of NJ0 In Fig. 5, the concentration of N20, [NIO], was shown as a function of dose for NO initial concentrations of 200, 400, 600, 800 and 1500ppm in NH3 (8OOppm)-N, mixtures. The values of [N,O] increased linearly with dose up to about 0.5 Mrad and the rates of the increases were reduced gradually with dose. The N,O concentrations increased gradually to the maximum concentration of about 35 ppm (at about 2 Mrad), 70 ppm (at about 4 Mrad) and 95 ppm (at about 4 Mrad) for the NO initial concentrations of ,200, 400 and 600 ppm, respectively, and then decreased gradually in all cases. On the other hand, [N,O] increased monotonically with dose for the NO initial concentrations of 800 and 1500ppm. The G-values of the N,O formation in the initial stage of irradiation were calculated to be 2.1 from the slopes of the straight part of the curves at low doses, for all NO initial concentrations. 5. Effect of NH3 on decrease in NO concentration
6
FIG. 4. H, concentration as a function of dose in the mixture of NH, and N, with and without NO. Without NO; A = NH, (67Oppm) in N,; o = NH3 (93Oppm) in N2 with NO (800 ppm); A = NH, (670 ppm) in N,; l = NH, (930 ppm) in N2. Irradiation temperature: 120°C. A&I. 32/s--c
Figure 6 shows the NO concentration, [NO], in the NO (8OOppmtN, mixtures as a function of dose, with and without NHS. The [NO] in the mixture without NH3 decreased with dose as shown by a dotted line in the figure. The G-value of the decrease in [NO] at the initial stage of irradiation was found to be 3.3, using the slope of the straight part of the curves at low doses. The rate of the decrease in [NO]
570
0. Tokunaga et al.
effectively to the formations cals:“”
of NH2 and H radi-
N: + NH3+NH;
+ 2N2
N:+NHs--,NH:+N+N, NH: + NH,--rNH; NH:
+e--NH,
(4) (5)
+ NH,
(6)
+ H
(7)
Since the G-values of Ni and N+ ions formed by the primary process (1) are reported to be 2.27 and 0.69, respectively, (*)both the G-values of NH2 and H radicals, produced through reactions (2-7), are estimated to be as much as 2.96. The observed NH3 decomposition accompanying the H, formation is attributed to charge transfer reactions from N; and N; ions to NHS, followed by radical reactions of NH1 and H radicals’“) which are formed through reactions (2-7). ‘0
1
2 3 4 Dose (Wad 1
5
NH, + NH, + N,H, I NH, + NH
6
FIG. 6. NO concentration as a function of dose in the mixture of NO and N2 with and without NH,. Initial concentration; NH3: (----- without NH,; 0 = 420ppm: A = 620ppm; 0 = WOppm), NO: (8OOppm). Irradiation temperature: 120°C.
NH, + H+NHs I NH+H, H+H+H2 NH + NH-N2
was greatly promoted by the addition of NH,, as shown by solid lines in the figure. The G-values of the
decreases in [NO] at low doses were enlarged from 3.3 for the mixture without NHs to 5.6, 7.0 and 7.0 by the additions of NH3 of 42Oppm, 620 ppm and 890 ppm, respectively.
Discussions 1. Decomposition of NH3 in Nz When NH3 was irradiated in Ns, the values of [NH,] decreased accompanying the formation of Hs, as shown in Figs 1 and 4. The resulting G-values of the decrease in [NH,] and of the H2 formation were 1.3 and 2.0, respectively. That is, the ratio of the former to the latter is about 2:3. This ratio indicates that the radiolysis of the low concentration of NH3 in N, is decomposed mainly to H, and N,. The primary process induced by the energy absorbed in N, can be written simply as follows:(s)
(8)
+ H2
NH + NH2 + N2H3 N2HJ + N2H3 -
N, + 2NH,
The observed suppressions of H, formation and of NH3 reproduction by NO or O2 through scavenging NH2 and H radicals confirm the occurrence of these radical reactions, being described in the following sections. The observed G-value of the NH3 decomposition, 1.3, is much smaller than the estimated G-values of H radical and of NH2 radical, 2.96. This finding indicates that the NH3 reproduction reactions and the NHs decomposition reactions take place simultaneously in the radiolysis of NH3 in N2. The N atom is also formed in the radiolysis of N2 containing NH3 through reactions (1) and (S), and is consumed not only by the recombination reaction with itself, but also by the reaction with NHJ:(13*14’ N + N + N2-+2N2 N + NH3-+NH
(9) + NH2
(10)
The G-value of N atom formed by reaction (5) is equal to that of\N+ ion, 0.69, and the initial G-value The N; and N+ ions will cluster immediately with of N atom formed by the process (1) is reported to be 2”‘) The total G-value of N atom is, therefore, as N, to form N: and N: ions, respectively.‘9’ large as 2.69. Using the G-value of N atom (2.69), N; + 2N2-N: + N2 c-3 the rate constant of reaction (9) (1.8 x 10’” cc2 N+ + 2N2-* N; + N2 (3) molm2 sec-‘)06), that of reaction (10) ($10’ cc mol- ’ s-‘)04), the N2 concentration (3.1 x 10-s molcc- ‘) In the presence of NHJ, N: and N; ions, formed and the NI$ concentration (0-lOOOppm), the ratios through reactions (2) and (3), undergo rapid charge ’ of the rate of reaction (9) to that of reaction (10) can transfers to NH3 to give NH: ions”‘), which lead be estimated for various NH3 concentrations. The N2 --\\++ N;, N+, N, Nf, e-
(1)
571
Radiolysis of ammonia in nitrogen
estimated rate-ratios are greater than lo6 for the 1 ppm NH,, and greater than 10 for the 1OOOppm NH,. These results indicate that N atom is mainly consumed by the recombination reaction with itself [reaction (9)] in the radioiysis of N, containing NH, up to 1OOOppm.That is, both the yields of NH and NH, radicals formed by reaction (10) are not more than the G-value of 0.3 even for the NHs concentration as much as 1000 ppm. and the reaction with N atom is expected to be a minor contribution to the observed NH5 decomposition. 2. Decomposition of NH3 in the presence of NO In the presence of NO in the mixture of NHs and Nz, Nf and N; ions will undergo charge-transfer not only to NHs [reactions (4) and (511, but also to NO (17,181
Nf + NO-NO+
+ 2N2
N; + NO+NO+ + N + N, I N20+ + N2 NsO+ + NO-N20
(11) (12a) (12b)
+ NO+
(13)
NsO+ + NHs + N20 + NH:
(14)
The G-value of the NHs decomposition was enhanced from 1.3 to 3.1 by the presence of NO. The resulting G-value in the presence of NO was approximately equal to the sum of the initial G-values of N: ion, 2.27, and of N+ ion, 0.69, which convert to NO+ and NH; ions through reactions (11-14). This suggests, therefore, that almost all NO+ and NH; ions are consumed for the NHs decomposition. We propose the following reactions of NH: and NO+ ions to lead the NHs decomposition. These ionic reactions were observed in a mass spectrometric study of the mixture of NO and NH,!i9’ NO+ + NHs--rNO+NHs
(15)
NH; + NO+NO+NHs
(16)
NO+NHs
+ NHs + NH;
+ NHINO
(17)
The NH2N0, produced by reaction (17), decomposes thermally to N2 and H20.(20) The H and NH2 radicals are formed by reactions of NH: and NH: ions [reactions (6) and (7)], also in the presence of NO. From the observed suppression of the Hz formation, shown in Fig. 4, it is considered that H and NH2 radicals formed through the reao tions of NH: and NH: ions are scavenged effectively by NO as follows: H + NO-+HNO NH, + NO+
NH,NO
HNO -i- HNO--r N,O + Hz0
consumed by the following reactions:‘2z~ N+NO-+N*+O
(21)
0 + NO-+NOz
(22)
The NO1 formed by reaction (22) reacts quickly with H and NH2 radicals, producing NO and OH, and NHINO1, respectively.‘23*24’In the presence of NO, H and NH2 radicals are scavenged not only by NO, but also by NO*, leading to suppression of the radical reactions of H and NH2 [reactions (8)]. Therefore, the H2 formation and the NHs reproduction is suppressed by the presence of NO. A part of NH2N02, produced by the reaction of NO2 with NH, radical, decomposes thermally to N,O and H20.‘25) The resulting large G-value of N20 formation, 2.1, confirms the NzO formation by the decomposition of NH2N02, in addition to the N20 formation by reactions (14) and (20). Considering the proposed reaction scheme [reactions (1 l-22)] for the NHs decomposition in the presence of NO, NO is also decomposed by radical reactions (l&22), in addition to the ionic reactions (11-l 7). Of these reactions, the rate of the ionic reactions depends strongly on the NHs concentration. The yield of NH: ion, formed by reactions (4). (5) and (14), becomes higher with the NH3 concentration, and the rate of the reaction of NH; ion with NH3 [reao tion (611also become higher with NH3 concentration. That is, the yields of NH: ion and NH, radical, whose reactions are derived from the NO decomposition, become higher with NHs concentration. Therefore, the NO decomposition is enhanced by the presence of NHB, as shown in Fig. 6. Both the NHs decomposition and the NO decomposition are greatly enhanced in the presence of NO and NHs. _ _.
_ __ _.__ .
3. Decomposition of NH3 in the presence of O2 In the mixture of 5% O2 and 95% N2, 0; ion is formed by the direct ionization of 4 (G-value of O.l), by charge transfer from N: [reaction (23)] (G-value of 2.16) and by other reactions such as the reaction of N+ ion with O2 (G-value of 0.70)!**9) N: + 02-+2N2
+ 0;
(23)
(18)
The. total G-value of 0: ion formation is, therefore, about 2.96. Since the effective charge transfer from 0: ion to NH3 takes place,“O) followed by reaction (6), G-values of NH: ion and of NH, radical are approximately equal to that of 0: ion in the presence of NHs. The NH: ion, formed by reaction (6). is neutralized with 0; ion to produce NHs and HO2 radical :
(19)
NHf + 0; --r NH3 + HO2
(20)
Therefore, almost all 0: ions will convert to NH2 and HO2 radicals in the mixture of NHs, O2 and N,, and G-values of these radicals are calculated acwrdingly to be about 2.96.
In the presence of NO, the N atom formed by the primary process (1) and reactions (5) and (I2a) reacts rapidly with NO,“l) producing an 0 .atom, which is
(24)
572
0. Tokunaga et al.
If NH2 radical were consumed by the reactions with itself [reactions (8)] or with HO2 radical [reaction (25)], the formation of H, and the reproduction of NH3 would be observed in the presence of Oz. NH2 + HOz + NH, + O2
(25)
The reproduction of NH3 results in a reduction of the G-value of the NH, decomposition. AS has been described in the Results section, the remarkable suppression of the H, formation and the close agreement between the G-value of the NH3 decomposition, 3.0, and that of 0: ion, 2.96, have been observed in the presence of Oz. These facts indicate that the Hz formation reactions and the NH3 reproduction reactions [reactions (911are suppressed effectively in the presence of Oz. From the above considerations, it can be concluded that almost all NH, radicals are scavenged by as much as 5% O2 in the mixture, though the reactivity of O1 with NH2 radical is not very large, i.e. k f 5 x lo9 mol- ’ s- 1!20) The NH3 decomposition in N2 is induced also by the energy absorbed in NHS, but the extent of the energy is calculated to be less than that absorbed in N2 by a factor of about 10-j in the mixture containing NH3 of lOOOppm concentration. Therefore, the extent of the NH3 decomposition, induced by the energy absorbed in NH,, can be neglected in the radiolysis of low concentrations of NH, in N,.
Conclusions Radiolysis of NH3 was studied at low concentrations (430-930ppm) in N2 with and without NO or 02. and following conclusions were drawn: 1. The NH3 is decomposed to H2 and N, in Nz by irradiation. The G-values of the NH3 decomposition and of the H, formation are 1.3 and 2.0, respectively. The NH3 decomposition accpmpanying with the H2 formation is attributed to charge transfer reactions from N: and N: ions to NH3, followed by radical reactions of NH2 and H radicals. A part of these radical reactions lead to the NH, reproduction. 2. The NH3 decomposition is strongly enhanced, the Hz formation is greatly reduced, and the N20 formation is observed with a high yield by addition of NO or 02. The G-values of the NH3 decompositions are 3.1 and 3.0 in the presence of NO (800 ppm) and 02 (5x), respectively, and the G-value of the H2 formation is 0.7 in the presence of NO. 3. In the presence of NO, NH, is decomposed by the reactions of NH; and NO+ ions, produced through the charge transfers from N: and N; ions to NH3 and NO, in addition to the reactions of H and NH2 radicals with NO. The
NH, reproduction reaction is suppressed by scavenging these radicals by NO. 4. In the presence of 02, NH3 is decomposed by the reaction of O2 with NH, radical, produced by charge transfers from N:, N; and 0: ions to NHJ. The effective scavenging of NH2 radical by O2 suppresses the recombination reaction of NH, radical, leading to the NH, reproduction. authors wish to thank Mr M. Washino of the Japan Atomic Energy Research Institute
Acknowledgement-The
for his support to this work.
References 1. TOKUNAGA0.. NISHIMURAK., SUZUKI N., MACHI S. and WA~HINOM. Radiat. Phys. Chem. 11,299 (1978). 2. MACHI S. Radiat. Phys. Chem. 14, 155 (1979). 3. PETERSON D. B. NSRDS-NBS44 (1974). 4. PAGSBERGP. B. and ERIKSENJ. J. Phys. Chem. 83, 582 (1979). 5. EYRE J. A. and SWITHIPSD. Trans. Faraday Sot. 66, 2199 (1970). 6. TOKUNAGAO., NISHIMURAK., SUZUKI N. and WASHIN0 M. Radiat. Phys. Chem. 11, 117 (1978). 7. HAGERR. N. JR. CLARKSOND. R. and SAVORYJ. Anal.
Chem. 42, 1813 i1970). 8. WILLISC. and BOYDA. W. Int. J. Radiat. Phys. Chem. 8, 71 (1976). 9. MCDANIEL E. W., CERMAKV., DALGARNOA., FERGUSON E. E. and FRIEDMANL. Ion-Molecule Reaction (Wiley, New York, 1970). 10. WATANABEK., NAKAYAMAT. and M~TTL J. J. Quant. Spectrosc. Radiat. Transfer 2, 369 (1962).
11. RYAN K. R. J. Chem. Phys. 53, 3844 (1970). 12. BOYD A. W., WILLIS C. and MILLER 0. A. Can. J. Chem. 49, 2283 (1971). 13. KISTIAKOWSKYG. B. and VOLPI G. G. J. Chem. Phys. 28, 665 (1958). 14. BR~CKLEHURSTB. and JENNINGSK. R. Progress in Reaction Kinetics Vol. 4 (Pergamon Press, London, 1976). 15. NISHIMURA K.. TOKUNAGA O., WASHINO M. and SUZUKIN. J. &cl. Sci. Tech. 16, 596 (1979). 16. CLYNEM. A. A. and STEDMAND. H. J. Phvs. Chem. 71. 307 1 (1967). 17. LINDINGE~W. J. Chem. Phys. 64, 3720 (1976). 18. BOYDA. W. and MILLER0. A. Can. J. Chem. 56. 147 (1978). 19. P~JCKETTL. J. and TEAGUEM. W. J. Chem. Phys. 54, 4860 (1971). 20. JAYANTYR. K. M., SIMONAITIS R. and HEICKLENJ. J. Phys. Chem. 80, 433 (1976). 21. PHILLIPSL. F. and SCHIFFH. I. J. Chem. Phys. 36, 1509 (1962). 22. BECKER K. H., GROTHW. and THRAND. Chem. Phys. Lett. 15, 215 (1972). 23. PHILLIPS L. F. and SCHIFFH. I. J. Chem. Phys. 37, 1233 (1962). 24. KURASAWAH. and LESCLAUXR. Chem. Phys. Lett. 66, 602 (1976). 25. VASTP. Bull. Sot. Chem. Fr. 2136 (1970).
Vol.32, pp. 567 to 572. 1981
CO2~7CU?X/81/080567-06SO2.00,‘0 Copyright 0 1981 Pcrgamon Press Ltd
Radiolysis of Ammonia in Nitrogen; Effects of Nitrogen Monoxide and Oxygen on Decomposition of Ammonia OKIHIRO
TOKUNAGA,’
TSUTOMU and
SEKINE,2 MASANOBU
NOBUTAKE
SkKANO~E2
SUZUKI’
‘Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment, Watanuki-Machi, Takasaki, Gunma 370-12, and “Department of Chemistry, Faculty of Science, Kanazawa University, Otemachi, Kanazawa 920, Japan (Receioed 5 March 1980; in reuisedjorm
15 December 1980)
The effects of NO and O2 on the decomposition of NHs were studied at low concentrations (3@4I-900ppm) in N, under irradiation by electron beams of 1.5 MeV energy at 120°C. The NH3 WBS decomposed to H2 and N2 in N2, with a G-value of 1.3. In the presence of NO (800 ppm) or O2 (5X,), the NH, decomposition was much enhanced (G-values were 3.1 and 3.0, respectively), the Hs formation was suppressed, and N20 formation was observed with a high yield. The enhancement of the NH3 decomposition in the presence of NO or Q2 is attributed to the reactions of H and NH2 radicals with NO or 02, leading to the formations of N20, N2, H20, etc. In the presence of NO, the reaction of NO+ with NH3 and that of NH: with NO may also be important for the NH3 decomposition.
Introduction A SERIES of fundamental research on the radiation treatment of exhaust gases has been carried out in our laboratory, in order to elucidate the mechanism of removing NO, and S02. The addition of NH3 to exhaust gas before irradiation was found to make it possible to collect NO, and SO, as solid ammonium nitrate and ammonium sulfate, respectively, and to enhance the reaction of removing NO, and SOl.(i) For these advantages, a small amount of NH3 was added before irradiation in the pilot scale test for the radiation treatment of exhaust gas from a sintering furnace.‘2) However, the reaction mechanism of NH3 in the exhaust gas was still not completely understood. The radiolysis of NH3 has been studied extensively to estimate the primary yields of active species and to clarify the mechanism of the NH3 decomposition.‘3) PAGSBWG et a1J4i and EYRE and SMITHIES(~)have studied the effects of O2 and NO on the radiolysis of NHa, and proposed the reactions of O2 and NO with H and NH2 radicals for the NH, decomposition. But, the radiolysis of NH3 has not yet been studied at low concentration in N,. In the radiolysis of low concentration of NH3 in N2, it is presumed that the active species, produced by the radiolysis of N,, may contribute to the reactions of NH,. In the present paper, the effects of NO and 0, (as components of exhaust gases) on the radiolysis of NH3 are studied at low concentrations in N,. The 567
mechanism of reactions including charge transfers from Ni and Ni ions to NHS, NO and O1 are proposed for the NH, decomposition.
Experimental The experiments were carried out using the flow experimental apparatus described in the preceding paper.(6) The commercially available gases, N,, 01, NH, and NO (all above 99.9%) were used without further purification. The flow rate of NHs was automatically regulated by a mass-flow controller (Okura Denki Co., MFC-1 type).‘The gaseous mixtures were irradiated with electron beams at dose rates in the range 0.06437 Mrad . s- ’ . An electron accelerator by Radiation Dynamics, Inc. (Dynamitron type; maximum energy, 3 MeV; maximum current, 25 mA) was used for the irradiation. The analysis of NH3 was carried out with ultraviolet derivative spectrometry.“) The second derivative spectrometer used (Yanagimoto, UO-1 type) has a single light beam which passes through an analyzing cell (SOcm pass length). The absorption spectrum in the wavelength region of 195-215 nm was used for the analysis. The Hz and N20 were analyzed by gas chromatographs (Shimadzu, GC-2AL and GC3BT type) with thermal conductivity detectors. For the analysis of Hz, a .column (2 m x 5 mm) packed with Molecular Sieve 5A was used at 100°C with Ar as a carrier gas,
0. Tokunaga et al.
568
and for the analysis of N,O, a column (3 m x 5 mm) packed with Porapak .Q was used at 35°C with Ar. The G-values were calculated on the basis of the energy absorbed by the whole mixture.
$2
Results
0
1. Decrease in NH3 concentration by irradiation When NH3 (42&930ppm)-N, mixtures were irradiated by electron beams at 120°C the NH3 concentration, [NHJ, decreased linearly with dose in all initial NH3 concentrations, as shown in Fig. 1. The G-values of the decrease in [NH,], obtained from the slopes of the straight lines, were shown by circles in Fig. 2. The G-value* (1.3) was almost independent of the initial NH3 concentrations in the range from 420 to 930 ppm. 2. Effects of NO and O2 on decrease in NH3 concen-
1 OO-
500 1000 Nti3 initial cont. @pm)
FIG. 2. Initial G-value of the decrease in NH, concentration as a function of initial NHs concentration: the units of G-value in this paper are molecules, ions or radicals per unit energy (1OOeV) absorbed. 0 = mixture of NH3 and N1; A = mixture of NH,, NO (800ppm) a?d N,; 13 = mixture of NH,, O2 (5%) and N2. Irradiation temperature: 120°C.
tration
In Fig. 3, [NH,] in the NH3-N2 mixture containing NO (800ppm) and in the mixture containing 02 (5%) were shown as a function of dose by triangles
* The units of G-value in this paper are molecules, ions or radicals per unit energy (1OOeV)absorbed.
0
0
1
I
I
1
2
3.
I
I
4.
5
Dose Wrod)..
and squares, respectively. The [NHJ decreased linearly with dose up to about 2 Mrad for all initial NHp concentrations. The G-values of the decrease in [NHJ for each initial NH. concentration, calculated from the slope of the straight part at low doses, were shown in Fig. 2 as a function of the initial NH3 concentration. In the figure, the G-values in the mixture containing NO (800ppm) and in that containing O2
6
__
FIG. I. NH, concentration as a function of dose ii the mixture of NH3 and N, for various initial NH3 concentrations. Irradiation temperature: 120°C.
FIG. 3. NH, concentration as mixture of Nk, and N, with A = with NO (800ppm); 0 = temperature:
a function of dose in the and without NO or 0,: with O2 (5%). Irradiation 120°C.
Radiolysis
ofammoniain
nitrogen
569
(5%) were shown by triangles and squares, respectively. The G-values were almost independent of the initial NH3 concentrations ranging from 290 to 92Oppn-1, amounting to 3.1 for the mixture with NO (800 ppm) and 3.0 for that with O2 (5%). 3. Effects of NO and O2 on formation of H2 When NHJ-NI mixtures were irradiated, Hz was formed, as shown in Fig. 4. In the figure, the concentrations of H,, [Hz], for the initial NH, concentrations of 670ppm and of 930ppm were shown as a function of dose by open triangles and circles, respectively. The H2 concentration increased linearly, with dose for both initial NH3 concentrations. From the slopes of the straight lines shown in the figure, the G-values of the Hz formation were calculated to be 2.0 for both initial NH3 concentrations. In Fig. 4, the values of [HJ in the NO (8OOppm)added mixture of NH3 (670ppm) and N,, and in the NO (800 ppm)-added mixture of NH3 (930 ppm) and N2 were also shown as a function of dose by closed triangles and closed circles, respectively. The H2 formation was markedly suppressed by the addition of NO. For example, [Hz] for the dose of 5.4 Mrad was decreased by the addition of NO (800ppm) from 320 ppm to 70 ppm in the mixture with 930 ppm NH3 and to 30 ppm in the mixture with 670 ppm NHJ. The G-values of the H, formation in the NO-added mixtures were calculated to be 0.7 from the slopes of the straight parts of the curves at low doses in Fig. 4.
Y ‘0
I
,
I
1
I
I
1
2
3
4
5
6
Dose (Mad )
FIG. 5. N20 concentration as a function of dose in the mixture of NH3 (800 ppm), NO and N1 for various initial NO concentrations. Initial concentration; NO: (0 = 2OOppm;A = 4OOppm;(3 = 600ppm; 0 = 800ppm and V = 15OOppm); NH3:(800ppm).
The H, formation in the NHI-N2 mixture was also suppressed effectively by addition of Oz. The H, concentration in the NH, (920ppm)-N, mixture irradiated to a dose of 5.4Mrad was decreased from 320ppm to a value less than the limit of detection (100 ppm) by the addition of 5% Oz. 4. Effect of NO on formation of NJ0 In Fig. 5, the concentration of N20, [NIO], was shown as a function of dose for NO initial concentrations of 200, 400, 600, 800 and 1500ppm in NH3 (8OOppm)-N, mixtures. The values of [N,O] increased linearly with dose up to about 0.5 Mrad and the rates of the increases were reduced gradually with dose. The N,O concentrations increased gradually to the maximum concentration of about 35 ppm (at about 2 Mrad), 70 ppm (at about 4 Mrad) and 95 ppm (at about 4 Mrad) for the NO initial concentrations of ,200, 400 and 600 ppm, respectively, and then decreased gradually in all cases. On the other hand, [N,O] increased monotonically with dose for the NO initial concentrations of 800 and 1500ppm. The G-values of the N,O formation in the initial stage of irradiation were calculated to be 2.1 from the slopes of the straight part of the curves at low doses, for all NO initial concentrations. 5. Effect of NH3 on decrease in NO concentration
6
FIG. 4. H, concentration as a function of dose in the mixture of NH, and N, with and without NO. Without NO; A = NH, (67Oppm) in N,; o = NH3 (93Oppm) in N2 with NO (800 ppm); A = NH, (670 ppm) in N,; l = NH, (930 ppm) in N2. Irradiation temperature: 120°C. A&I. 32/s--c
Figure 6 shows the NO concentration, [NO], in the NO (8OOppmtN, mixtures as a function of dose, with and without NHS. The [NO] in the mixture without NH3 decreased with dose as shown by a dotted line in the figure. The G-value of the decrease in [NO] at the initial stage of irradiation was found to be 3.3, using the slope of the straight part of the curves at low doses. The rate of the decrease in [NO]
570
0. Tokunaga et al.
effectively to the formations cals:“”
of NH2 and H radi-
N: + NH3+NH;
+ 2N2
N:+NHs--,NH:+N+N, NH: + NH,--rNH; NH:
+e--NH,
(4) (5)
+ NH,
(6)
+ H
(7)
Since the G-values of Ni and N+ ions formed by the primary process (1) are reported to be 2.27 and 0.69, respectively, (*)both the G-values of NH2 and H radicals, produced through reactions (2-7), are estimated to be as much as 2.96. The observed NH3 decomposition accompanying the H, formation is attributed to charge transfer reactions from N; and N; ions to NHS, followed by radical reactions of NH1 and H radicals’“) which are formed through reactions (2-7). ‘0
1
2 3 4 Dose (Wad 1
5
NH, + NH, + N,H, I NH, + NH
6
FIG. 6. NO concentration as a function of dose in the mixture of NO and N2 with and without NH,. Initial concentration; NH3: (----- without NH,; 0 = 420ppm: A = 620ppm; 0 = WOppm), NO: (8OOppm). Irradiation temperature: 120°C.
NH, + H+NHs I NH+H, H+H+H2 NH + NH-N2
was greatly promoted by the addition of NH,, as shown by solid lines in the figure. The G-values of the
decreases in [NO] at low doses were enlarged from 3.3 for the mixture without NHs to 5.6, 7.0 and 7.0 by the additions of NH3 of 42Oppm, 620 ppm and 890 ppm, respectively.
Discussions 1. Decomposition of NH3 in Nz When NH3 was irradiated in Ns, the values of [NH,] decreased accompanying the formation of Hs, as shown in Figs 1 and 4. The resulting G-values of the decrease in [NH,] and of the H2 formation were 1.3 and 2.0, respectively. That is, the ratio of the former to the latter is about 2:3. This ratio indicates that the radiolysis of the low concentration of NH3 in N, is decomposed mainly to H, and N,. The primary process induced by the energy absorbed in N, can be written simply as follows:(s)
(8)
+ H2
NH + NH2 + N2H3 N2HJ + N2H3 -
N, + 2NH,
The observed suppressions of H, formation and of NH3 reproduction by NO or O2 through scavenging NH2 and H radicals confirm the occurrence of these radical reactions, being described in the following sections. The observed G-value of the NH3 decomposition, 1.3, is much smaller than the estimated G-values of H radical and of NH2 radical, 2.96. This finding indicates that the NH3 reproduction reactions and the NHs decomposition reactions take place simultaneously in the radiolysis of NH3 in N2. The N atom is also formed in the radiolysis of N2 containing NH3 through reactions (1) and (S), and is consumed not only by the recombination reaction with itself, but also by the reaction with NHJ:(13*14’ N + N + N2-+2N2 N + NH3-+NH
(9) + NH2
(10)
The G-value of N atom formed by reaction (5) is equal to that of\N+ ion, 0.69, and the initial G-value The N; and N+ ions will cluster immediately with of N atom formed by the process (1) is reported to be 2”‘) The total G-value of N atom is, therefore, as N, to form N: and N: ions, respectively.‘9’ large as 2.69. Using the G-value of N atom (2.69), N; + 2N2-N: + N2 c-3 the rate constant of reaction (9) (1.8 x 10’” cc2 N+ + 2N2-* N; + N2 (3) molm2 sec-‘)06), that of reaction (10) ($10’ cc mol- ’ s-‘)04), the N2 concentration (3.1 x 10-s molcc- ‘) In the presence of NHJ, N: and N; ions, formed and the NI$ concentration (0-lOOOppm), the ratios through reactions (2) and (3), undergo rapid charge ’ of the rate of reaction (9) to that of reaction (10) can transfers to NH3 to give NH: ions”‘), which lead be estimated for various NH3 concentrations. The N2 --\\++ N;, N+, N, Nf, e-
(1)
571
Radiolysis of ammonia in nitrogen
estimated rate-ratios are greater than lo6 for the 1 ppm NH,, and greater than 10 for the 1OOOppm NH,. These results indicate that N atom is mainly consumed by the recombination reaction with itself [reaction (9)] in the radioiysis of N, containing NH, up to 1OOOppm.That is, both the yields of NH and NH, radicals formed by reaction (10) are not more than the G-value of 0.3 even for the NHs concentration as much as 1000 ppm. and the reaction with N atom is expected to be a minor contribution to the observed NH5 decomposition. 2. Decomposition of NH3 in the presence of NO In the presence of NO in the mixture of NHs and Nz, Nf and N; ions will undergo charge-transfer not only to NHs [reactions (4) and (511, but also to NO (17,181
Nf + NO-NO+
+ 2N2
N; + NO+NO+ + N + N, I N20+ + N2 NsO+ + NO-N20
(11) (12a) (12b)
+ NO+
(13)
NsO+ + NHs + N20 + NH:
(14)
The G-value of the NHs decomposition was enhanced from 1.3 to 3.1 by the presence of NO. The resulting G-value in the presence of NO was approximately equal to the sum of the initial G-values of N: ion, 2.27, and of N+ ion, 0.69, which convert to NO+ and NH; ions through reactions (11-14). This suggests, therefore, that almost all NO+ and NH; ions are consumed for the NHs decomposition. We propose the following reactions of NH: and NO+ ions to lead the NHs decomposition. These ionic reactions were observed in a mass spectrometric study of the mixture of NO and NH,!i9’ NO+ + NHs--rNO+NHs
(15)
NH; + NO+NO+NHs
(16)
NO+NHs
+ NHs + NH;
+ NHINO
(17)
The NH2N0, produced by reaction (17), decomposes thermally to N2 and H20.(20) The H and NH2 radicals are formed by reactions of NH: and NH: ions [reactions (6) and (7)], also in the presence of NO. From the observed suppression of the Hz formation, shown in Fig. 4, it is considered that H and NH2 radicals formed through the reao tions of NH: and NH: ions are scavenged effectively by NO as follows: H + NO-+HNO NH, + NO+
NH,NO
HNO -i- HNO--r N,O + Hz0
consumed by the following reactions:‘2z~ N+NO-+N*+O
(21)
0 + NO-+NOz
(22)
The NO1 formed by reaction (22) reacts quickly with H and NH2 radicals, producing NO and OH, and NHINO1, respectively.‘23*24’In the presence of NO, H and NH2 radicals are scavenged not only by NO, but also by NO*, leading to suppression of the radical reactions of H and NH2 [reactions (8)]. Therefore, the H2 formation and the NHs reproduction is suppressed by the presence of NO. A part of NH2N02, produced by the reaction of NO2 with NH, radical, decomposes thermally to N,O and H20.‘25) The resulting large G-value of N20 formation, 2.1, confirms the NzO formation by the decomposition of NH2N02, in addition to the N20 formation by reactions (14) and (20). Considering the proposed reaction scheme [reactions (1 l-22)] for the NHs decomposition in the presence of NO, NO is also decomposed by radical reactions (l&22), in addition to the ionic reactions (11-l 7). Of these reactions, the rate of the ionic reactions depends strongly on the NHs concentration. The yield of NH: ion, formed by reactions (4). (5) and (14), becomes higher with the NH3 concentration, and the rate of the reaction of NH; ion with NH3 [reao tion (611also become higher with NH3 concentration. That is, the yields of NH: ion and NH, radical, whose reactions are derived from the NO decomposition, become higher with NHs concentration. Therefore, the NO decomposition is enhanced by the presence of NHB, as shown in Fig. 6. Both the NHs decomposition and the NO decomposition are greatly enhanced in the presence of NO and NHs. _ _.
_ __ _.__ .
3. Decomposition of NH3 in the presence of O2 In the mixture of 5% O2 and 95% N2, 0; ion is formed by the direct ionization of 4 (G-value of O.l), by charge transfer from N: [reaction (23)] (G-value of 2.16) and by other reactions such as the reaction of N+ ion with O2 (G-value of 0.70)!**9) N: + 02-+2N2
+ 0;
(23)
(18)
The. total G-value of 0: ion formation is, therefore, about 2.96. Since the effective charge transfer from 0: ion to NH3 takes place,“O) followed by reaction (6), G-values of NH: ion and of NH, radical are approximately equal to that of 0: ion in the presence of NHs. The NH: ion, formed by reaction (6). is neutralized with 0; ion to produce NHs and HO2 radical :
(19)
NHf + 0; --r NH3 + HO2
(20)
Therefore, almost all 0: ions will convert to NH2 and HO2 radicals in the mixture of NHs, O2 and N,, and G-values of these radicals are calculated acwrdingly to be about 2.96.
In the presence of NO, the N atom formed by the primary process (1) and reactions (5) and (I2a) reacts rapidly with NO,“l) producing an 0 .atom, which is
(24)
572
0. Tokunaga et al.
If NH2 radical were consumed by the reactions with itself [reactions (8)] or with HO2 radical [reaction (25)], the formation of H, and the reproduction of NH3 would be observed in the presence of Oz. NH2 + HOz + NH, + O2
(25)
The reproduction of NH3 results in a reduction of the G-value of the NH, decomposition. AS has been described in the Results section, the remarkable suppression of the H, formation and the close agreement between the G-value of the NH3 decomposition, 3.0, and that of 0: ion, 2.96, have been observed in the presence of Oz. These facts indicate that the Hz formation reactions and the NH3 reproduction reactions [reactions (911are suppressed effectively in the presence of Oz. From the above considerations, it can be concluded that almost all NH, radicals are scavenged by as much as 5% O2 in the mixture, though the reactivity of O1 with NH2 radical is not very large, i.e. k f 5 x lo9 mol- ’ s- 1!20) The NH3 decomposition in N2 is induced also by the energy absorbed in NHS, but the extent of the energy is calculated to be less than that absorbed in N2 by a factor of about 10-j in the mixture containing NH3 of lOOOppm concentration. Therefore, the extent of the NH3 decomposition, induced by the energy absorbed in NH,, can be neglected in the radiolysis of low concentrations of NH, in N,.
Conclusions Radiolysis of NH3 was studied at low concentrations (430-930ppm) in N2 with and without NO or 02. and following conclusions were drawn: 1. The NH3 is decomposed to H2 and N, in Nz by irradiation. The G-values of the NH3 decomposition and of the H, formation are 1.3 and 2.0, respectively. The NH3 decomposition accpmpanying with the H2 formation is attributed to charge transfer reactions from N: and N: ions to NH3, followed by radical reactions of NH2 and H radicals. A part of these radical reactions lead to the NH, reproduction. 2. The NH3 decomposition is strongly enhanced, the Hz formation is greatly reduced, and the N20 formation is observed with a high yield by addition of NO or 02. The G-values of the NH3 decompositions are 3.1 and 3.0 in the presence of NO (800 ppm) and 02 (5x), respectively, and the G-value of the H2 formation is 0.7 in the presence of NO. 3. In the presence of NO, NH, is decomposed by the reactions of NH; and NO+ ions, produced through the charge transfers from N: and N; ions to NH3 and NO, in addition to the reactions of H and NH2 radicals with NO. The
NH, reproduction reaction is suppressed by scavenging these radicals by NO. 4. In the presence of 02, NH3 is decomposed by the reaction of O2 with NH, radical, produced by charge transfers from N:, N; and 0: ions to NHJ. The effective scavenging of NH2 radical by O2 suppresses the recombination reaction of NH, radical, leading to the NH, reproduction. authors wish to thank Mr M. Washino of the Japan Atomic Energy Research Institute
Acknowledgement-The
for his support to this work.
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11. RYAN K. R. J. Chem. Phys. 53, 3844 (1970). 12. BOYD A. W., WILLIS C. and MILLER 0. A. Can. J. Chem. 49, 2283 (1971). 13. KISTIAKOWSKYG. B. and VOLPI G. G. J. Chem. Phys. 28, 665 (1958). 14. BR~CKLEHURSTB. and JENNINGSK. R. Progress in Reaction Kinetics Vol. 4 (Pergamon Press, London, 1976). 15. NISHIMURA K.. TOKUNAGA O., WASHINO M. and SUZUKIN. J. &cl. Sci. Tech. 16, 596 (1979). 16. CLYNEM. A. A. and STEDMAND. H. J. Phvs. Chem. 71. 307 1 (1967). 17. LINDINGE~W. J. Chem. Phys. 64, 3720 (1976). 18. BOYDA. W. and MILLER0. A. Can. J. Chem. 56. 147 (1978). 19. P~JCKETTL. J. and TEAGUEM. W. J. Chem. Phys. 54, 4860 (1971). 20. JAYANTYR. K. M., SIMONAITIS R. and HEICKLENJ. J. Phys. Chem. 80, 433 (1976). 21. PHILLIPSL. F. and SCHIFFH. I. J. Chem. Phys. 36, 1509 (1962). 22. BECKER K. H., GROTHW. and THRAND. Chem. Phys. Lett. 15, 215 (1972). 23. PHILLIPS L. F. and SCHIFFH. I. J. Chem. Phys. 37, 1233 (1962). 24. KURASAWAH. and LESCLAUXR. Chem. Phys. Lett. 66, 602 (1976). 25. VASTP. Bull. Sot. Chem. Fr. 2136 (1970).