Reaction rates of NH2-radicals with NO, NO2, C2H2, C2H4 and other hydrocarbons

REACTION

RATES O F N H 2 - R A D I C A L S WITH NO, NO2, C2He, Cell 4 AND OTHER HYDROCARBONS W. HACK, H. SCHACKE, M. S C H R O T E R AND H. Gg. WAGNER

Max-Planck-Institut fiir Stro'mungsforschung, 34-Go'ttingen, W. Germany Absolute rate constants for gas phase reactions of NH 2 radicals with nitrogen dioxide, nitric oxide, acetylene and ethylene have been determined in a discharge flow system with laser-induced resonance fluorescence detection of NH 2. The radicals were produced by the fast reaction F + NH 3. The kinetic measurements were performed in the temperature range 209 <-- T ~ 505 K at pressures of one torr He as the main carrier gas. The following rate constants were obtained: kNO2+NHz (T) = 1.9 9 1020 - T -3"~ [cma/mol sec] kNO+NH~ (T) = 2.7

9 1017

"

T -t's5 [cma/mol sec]

kC2n~+NHz (T) = 1.42 - 1016 9 T -z7 [cm3/mol see] The rate constant for the reaction of CzH 4 with NH~ shows little temperature dependence kc~n4+Nn2 (T) = 1.3 " 108 [cma/mol see]. The reactions of NHz with allene and propylene are slow. The measurements lead to the upper limits k C 3 H 4 + ~ H 2 --.< 5 " l 0 8 [cm3/mol sec] and kCaH~+NHZ<-- 6 " 10 s [cm3/mo1 sec] at room temperature.

Introduction It was a great success w h e n Haber and Bosch first broke the N - - N b o n d to produce NHa from N 2 molecules. Nowadays the destruction of N z and the resulting production of NOx leads to severe problems since the production of energy by fuel-air combustion has increased so rapidly in the last twenty years. On the other hand, products with N - - N bonds are formed in the oxidation of nitrogenhydrogen compounds. ~ It has been pointed out by several authors 2~ that the NHz radical may be the most important species to produce N - - N bonds. The reactions of N H 2 with NO and NO~ may be of special interest for the removal of N O in internal combustion engines through the addition of N H 2producing compounds to the system at the right place. The NH 2 present in the atmosphere and stratosphere produced by photolysis of NH~ is one of the other reasons why the elementary reactions of NH~ radicals are of interest. There are many publications on the isoelectronic OH- and CH3-radicals, whereas little work has been done on the kinetics of NH 2 reactions. Only a few elementary reactions 505

O + NH 2,3) NO + NH~ 3) a n d NH 2 + NH24'5'6~ have been studied in detail. In this work, the rate constants of reactions of NH 2 with the nitrogen oxides NO and NO d and the two unsaturated Ca-hydrocarbons were studied. The reaction: F + NH 3 ~ NH z + HF is a rather clean source for NH~, suitable for kinetic measurements, if at low NH~-eonstruetions all reactions of NH~ with itself are suppressed and formation of NH4F is avoided.

Experimental The experimental set up is s h o w n in fig. 1. It consists of an isothermal flow reactor (id. 2.0 cm or 3.9 cm) which ends in a laser resonance fluorescence cell (black anodized AI). The fluorine atoms are produced in a movable quartz injection probe in an electrodeless microwave discharge of a F 2/ H e mixture. The reactant is admixed through another movable quartz injection probe coaxial to the first

506

KINETICS

~icrowave

___----movable injection probe

discharge

F2/He ~ "~- movable injection ~:~ probe /

temperature bath

--NH~

/

intake

- -

Ar-ion laser

dye- laser

,/•- ~ v ~ chopper

pump

~ pump

manometer

-

-

. 1impedance transformer pm~f ,

lock in amplifier

plotter

pump ~oscilloscope FIG. 1. Experimental set-up,

one. To avoid the effect of pressure dependence on the fluorescence intensity, fluorescence was measured in some experiments in a second chamber separated from the first one by a probe with a 2.5 mm i.d. at the top. The flow velocity in the probe is always larger by a factor of two than the velocity in the reactor. The pressure in the flow system varies between 0.6 and 10 torr with He as the main carrier gas and the linear flow velocity between 5 and 40 [m/sec]. The wall temperature of the flow reactor can be changed between 200 and 510 K with an accuracy of +2 K. The NH~ radicals are excited by the beam of a ew-dye laser (Coherent Radiation 490) in the rhodamine 6(; region (5978 ~ . The beam is modulated with 3.7 kHz before entering the fluorescence cell. Fluorescence is observed perpendicular to the exciting beam with a photomultiplier tube (RCA C 31034 A 01) via a lock-in amplifier. The dye-laser intensity

is controlled during the experiment by a second photomultiplier. Reaction products are checked by mass spectrometer in another device. Gases of the highest purity available are used, some without further purification. Helium, however, passes a liquid nitrogen trap, and F2 passes a NaF(s) and a N2(/q) trap. The flow reactor was cleaned with HF and rinsed with distilled water.

Analysis In our experiments only two reactions are essential to describe the NH~ source if the conditions are I N H a l e > > [F]oand [NH2] -< 9.. 10-~z [moi/cm~] F + NH3 --) NHz + HF

(1)

NH 2 + wall--* products

(9,)

REACTION RATES OF NHz-RADICALS The measured increase of NH2 with time and the time at which NHz (t) reaches a maximum lead to a value of k~, while for long reaction times NH~ decay yields the rate constant ks and the wall activity 7 = k~2R/tb (R = radius of the reactor, u3 = mean molecular velocity. Diffusion time of the particles to the wall is short compared to their characteristic time of consumption at the wall). In the presence of a reactant, the radical NH~ is additionally consumed by

(1) is a suitable NH~ source for kinetic measurements, if [NH~] ~< 5 - 10 - ~ tool/era a and if the reactant is admixed 15 m sec after reaction (1) has started ([NH31 = 10 - ' ~ mol/cma).

The reaction of NH~ with NO~ For the reaction of NO~ with NH2, two exothermic reaction channels are to be considered: NH b +

NH~ + R ~ products

(Sa)

(I)

Is = fluorescence signal in the present of reactant Io = fluorescence signal when the reactant is replaced by He [R] = concentration of the reactant At = change in reaction time

Results and discussion The radical source for NH z The production of NH z via

F + NH 3"* NH2 + H F AH = - l l l . 6 [ k J / m o l ]

(1)

was measured at pressures of 0.5 -< p -< 6 torr. In all cases the NH3 concentration (2.9 9 1 0 - " <[NH,]o -< 1.1 9 10 -8 tool/era a) was in large excess over the F-atom concentrations (20 -< [NH3] o / I F ] o -< 650). The concentration of H F has to be less than 3 9 10 -~1 m o l / c m ~ to be sure that the formation of solid NH 4 F is avoided. Based on NH2 production, the rate constant k~ at room temperature was measured to be:

NO 2

--, N 2 0 + H 2 0

With [R] > > [NH~] reaction (Sa) is pseudo first order in NH~ and k~ can be evaluated simply by

a In (IJIo) = k,.. [R] At

507

AH = - 3 8 5 . 5 [ k I / m o l l

(Sa)

--* N b + HbO b AH = -365.0 [kJ/mol]

(Sb)

The products of reaction (5) were analyzed by mass spectrometer in another flow system at a pressure of 2.3 torr and a flow velocity of 39 m/sec. There the NO 2 at a concentration of 3.1 9 10 -~ [ m o l / c m ~] was in excess over NH~ by a factor of up to 65. The mass spectra with NO b present or absent in the system and microwave discharge on and off were recorded in order to get the production due to reaction (5). An increase was found for the mass peaks m / e = 44 and 18, while m / e = 16 and 46 were consumed and the other mass peaks especially m / e = 28, 33, 34, and 47, remained unchanged. The reaction channel NH~ + NOb--* NzO + H~O (5a) is the main path while the contribution of channel (Sb) to reaction (5) ought to be less than 5% (taking into account the large background for m / e = 28). N H z decay at 298 K for several NO~ concentrations between 4.6 9 10 -t~ and 1.4 9 10 -~~ [ m o l / c m z] are given in fig. 5. NO 2 itself shows fluorescence and influences the fluorescence of NHb. If NO2 is present in the system at room temperature the wall activity is changed. This effect is found by measuring the NH b decay several times, after having

k, (598 K) = (5 + 3) 10 *~ lcmZ/mol see] This value is one order of magnitude larger than that reported by Pollock et al. ~ but is in reasonable agreement with the lower limit k~ > 10 t3 [cm3/mol see] given by Warnatz. 8~ For long reaction times, a plot of In I vs. t gives a straight line with [NHb] = 5 9 10 - ~ m o l / c m 3, This means that first-order wall decay dominates the NH~ profile. The activity of the wall under our experimental conditions is

.z. s

."~1~ . . .

~

" [~] [~,,o.~] .....~,

[ms]

7(598 K, glass~H~ ) < 3

" 10 -3

if no reactant is present in the system. Thus reaction

FIc. 2. NH 2 decay due to reaction with NO~ at various NO2 concentrations (4.6 9 10 -12 s | N O e l --< 1.4 9 10 -w mol/cma).

508

KINETICS TABLE I Experimental data for the rate constant of the reaction NH 2 + NO 2 ---) products (5)

T [k]

p [torr]

t~ [m/sec]

[NH~]/F] o

250. 250. 250. 295. 295. 295. 295. 295. 381. 500. 500. 500.

1.0 1.0 2.0 1.0 1.0 1.0 1.0 1.2 1.0 1.0 1.0 1.0

6.4 19.4 27.1 7.7 21.3 21.1 24.6 23.6 23.2 13.7 14.1 14.4

520 520 520 520 520 520 60 28 280 500 500 500

[NO21101' Imol/cm3l

k~,p [see- ]

I. 1 4.0 1.5 1.5 2.0 3.4 9.7 14.1 3.9 1.1 3.0 4.4

129. 478. 154. 74. 116. 168. 615. 942. 171. 13. 49. 62.

8.0 8.0 11.0 20.0 7.5 7.5 18.0 16.0 13.0 11.0 11.0 10.0

stopped the NO2 flow. The plot of A In Is~At vs. [NO2] leads to a finite intercept for [NOa] ---) 0. The rate constants are corrected due to this effect. The temperature dependence of ks was measured in the range 250 -< T -< 503 K at a pressure of one torr in an excess of NO2 (1.1 9 10 -H -< [NO2] -< 1.4 9 10 -1~ [mol/cm3]). The experimental data for the rate constants of reaction (5) are given in table I. The rate constant as a function of temperature is shown in fig. 3. The temperature dependence of k~ in the interval given above is expressed by ks(T) = 1.9 9 10 ~9 T -3~ [ c m 3 / m o l sec]

~3.s

~3o o ~.

--%

~zo 2.3

24

2 5 - -

i 2.6

2.7

28

togto T [K]

For the narrow temperature range 615-660 K, Bedford et al. 9~ give ks/ks (T) = 10 -2.3 exp(26.78 [k]/mol]/RT), which combined with k o (T) of this work gives ks(T ) = 1.34 9 10 TM exp ( - 2 4 7 0 / T ) . The absolute values of k s measured by Bedford et al. 9~ at 600 K differ by a factor of two from our results at that temperature.

The reaction of NH 2 with NO I n the main channel of the reaction of NO with NH2 NO + NHa "-') N~ + H 2 0 AH = - 5 2 4 . 5 [ k J / m o l l

[NH~] o 1 0 1 3 [ m o l / c m a]

(6)

a long-lived complex NH2NO is formed, as had already been detected by Gehring et al. 3~at a pressure of 2.4 torr at 298 K, also using a mass spectrometer. The formation of the complex is expected to become faster at low temperature. The reaction was studied in the temperature range 210-500 K. The NO-concentration was 2 . 0 ' 10 -11 < - [NO] ~ 7 " 10 -1~ [ m o l / c m 3] at pressures between 0.6 and 4.0 torr.

FIG. 3. Temperature dependence of the rate constant for the reaction NH 2 + NO= ---* products (5).

The experimental data to determine k s (T) are given in table II and for 332 K in fig. 4 as a first order rate constant vs. the NO-concentration plot, which gives a straight line through the origin. The temperature dependence is given by: ks (T) = 2.7 9 1017 T - ' s 5 [ c m 3 / m o l see] in the temperature range 209 <- T ~ 500 K. This result is in good agreement with that of Lesclaux et al. ~~ for b o t h temperature dependence and absolute value. Gordon et al. 4) give a rate constant at 298 K which is larger by a factor of two in the pressure range 250 -< p ~ 1000 torr. The k 6 (298 K) = 1.25 - 1013 [cm3/molsec] r e p o r t e d b y Hancock et al. TM is also larger. The value of Gehring et al. 3~ k 6 (298 K) = 5 9 1012 [cm3/mol sec] is in agreement with the room temperature value reported here. The temperature dependence of ks and the results of the other authors are s h o w n in fig. 5.

R E A C T I O N R A T E S O F NH~-RADICALS

509

TABLE II Experimental data for the rate constant of the reaction N H 2 + NO ---* p r o d u c t s (6). The data for 332 K are given in fig. 4 T [KI

P [torr]

t5 [m/see]

INH~] o 10'2 [tool/emil

[NO] 10 '~ [ t o o l / e r a 3]

k p [see-' ]

210 210 243 243 284 284 284 284 284 284 293 293 293 293 293 293 298 298 371 371 371 371 418 418 418 418

1,0 1.0 4.0 0.85 1.2 1.3 1.3 0.6 4.0 4.0 1.0 1.0 1.0 1.0 1.0 1.0 2.1 1.2 0.6 0.6 1.0 0.6 1,0 1.0 1.0 1.0

9.3 9.5 22. 43. 79. 75. 70. 37. 31. 31. 23. 15, 16. 23. 23. 24. 50. 55. 40. 40. 66. 41. 36. 36. 36. 36.

1.6 1.6 1.8 0.75 14. 18. 16. 0.22 11. 11. 2.9 4,4 3.0 2.9 2.1 2.9 27. 9.3 0.24 0.24 6.6 0.26 1.2 1.2 1.2 1.2

0.18 0.12 0.92 2.70 0.38 0.76 1.20 1.50 2.10 6,9 0.34 0.37 0.40 0.60 0.68 2.5 1.3 2.5 1.2 1,2 2.2 4.0 0.16 0.41 0.97 1.50

258. 170. 520. 2100. 418. 988. 1320. 1110. 1009. 5380. 230. 250. 213. 408. 510. 1690. 715. 1550. 750. 930. 860. 2280. 53. 198. 467. 650.

503 503 503

1.0 1.0 1.0

48, 48. 48.

0.95 0.95 0.95

0.20 0.46 0.88

75. 148. 470.

13.5 3000

--

-

-

I

i

i

,--2

o

13,0 2000 m

o

4 1000

.J

~

12.5

*

120

,o

~'o [ N O ] [10 -It r r ~ t / c m

%

,'o

I 23

- -

i

2,/,

I 2,5

I 2.6

I 27

5o

3]

FIG. 4. First-order rate c o n s t a n t of the reaction NH2 + N O as function of the NO concentration at 332 K.

FIG. 5, T e m p e r a t u r e d e p e n d e n c e of the rate constant for the reaction N H 2 + N O ~ p r o d u c t s (6): 9 T h i s work, O G e h r i n g et al.a>; & G o r d o n et al.,4~ [] H a n c o c k et al.; TM - 9 - Lesclaux et al.J ~

510

KINETICS TABLE I l l Experimental data for the rate constant of the reaction NH 2 + Coil 2 ---* products (3)

T [k]

P [ton"l

6 [m/secl

[NHal / [F]o

[NHzl o10 ~a [mol/cmal

210 250 250 295 295 295 295 295 295 295 295 295 295 380 380 380 505 505

1.0 1.0 1.0 0.35 1.0 0.45 0.6 1.0 0.6 1.0 1.0 1.0 1.2 1.0 1.0 1.0 1.0 1.0

4.7 5.6 8.7 5.8 23.0 5.6 8.8 5.3 13.6 11.9 28.5 53.2 50.3 7.2 12.0 16.8 9.4 22.0

520 520 520 100 45 75 23 280 75 280 8 45 45 220 220

33, 27. 18, 95. 21, 85. 180. 60. 230. 26. 46. 9.0 9.5 47. 28.

220

20.

220 220

36. 15.

[C2H=] 10 ~ [mol/cmal 11.0 9.2 29.0 2.4 6.1 8.7 10.0 15.0 17.0 37.0 40.0 41.0 43.0 5.4 20.0 27.0 4.1 20.

k~, [sec-ll 99. 54. 166. 5.9 16.

22. 18. 49. 37. 105. 109. 120. 119. 8.1 21. 45, 3.4 16.5

The reaction of NH= with C=H~ The rate constant for the reaction:

t

10.0

NH~ + C~H z ~ products

(3) ~ E

was measured at one tort in the temperature range 210 ~ T <- 505 K. The experimental data for the rate constant k a are given in table III. The concentration of the reactant was varied between 2.4 9 I0 -a and 4.3 9 10 -a [ m o l / c m a] leading to a large excess 1.25 9 103-< [CzH~]o/[NH~]o <- 1.6 9 104. The rate constant k a as a function of temperature is given in fig. 6 as a log k vs. log T plot. This leads to the expression

95 o

9.0

2!~

2!4

2!5--- ~

2~

10910 Y |K)

FXG. 6. Temperature dependence of the rate constant for the reaction NH~ + C2H ~ ~ products (3).

ka (T) = 1.42 9 10 '~ T . . . . [ema/mol see] The observed negative temperature dependence of ka indicates that the reaction of NH~ with acetylene proceeds via the initial addition of NH 2 radicals to C2H2, followed by stabilisation or decomposition of the NH 2 - C~H 2 adduct. NH 2 + C~H 2 ~ NH~ C2H 2" NH2C=H2" + M ~ N H 2 C2H 2 + M This mechanism is supported by preliminary measurements of ka as a function of pressure up to 10 torr. The mechanism suggested is very similar to that for the reaction of H atoms TM and of O H t4~ with acetylene.

The reaction of NH 2 with C~H4 For the reaction: NH 2 + C2H 4 ~ products

(4)

NH~ decays were measured at flow velocities of 4.7 <-- 6 <- 37.2 [m/sec] at one torr. The ethylene concentration was varied in the range: 7 9 10 -~~ -< [C2H,] <- 3.8 9 10 -8 m o l / c m ~ which leads, as in the case of acetylene, to a large excess of the reactant over NH~-radical concentrations. The experimental data for the rate constant of reaction (4) are given in detail in table IV. The second-order rate constant varies only slightly

REACTION RATES O F NHa-RADICALS

511

T A B L E IV Experimental data for the rate constant of the reaction NH 2 + Call 4 ---* products (4) T [k]

P [torr]

t5 [m/sec]

[NHa] / [F] o

[NH2] 10 '2 [ m o l / c m 3]

[C~H,1109 [ m o l / c m 3]

k~.o [sec - l ]

295 295 295 295 295 295 295 295 399 399 399 399 399 505 505 505 505

1.0 0.85 1.0 1.0 0.9 1.0 0.9 1.0 1.0 1.0 1.0 1.0 1.0 0.4 1.0 1.0 1.0

25.4 20.5 4.7 26.2 24.9 8.3 11.4 12.1 32.0 34.8 34.1 36.2 37.2 18.7 7.5 13.7 20.9

22 80 280 22 80 280 27 280 15 15 15 15 15 220 220 220 220

4.4 1.6 6.5 4.6 1.3 3.8 15.0 26.0 4.7 4.3 4.4 4.2 4.1 1.8 4.5 2.5 1.6

3.2 4.5 10.0 13.0 14.0 29.0 31.0 37.0 0.72 3.2 4.4 6.5 13.0 1.8 4.6 17.9 22.

5.2 8.4 13. 16. 11. 37. 41. 44. 1.1 6.2 3.4 9.3 13.5 1.5 11. 19. 51.

with temperature in the range 295 -< T -< 505 K so that a constant value of k,(T) = (1.3 + 0.3) 9 l 0 s [ c m 3 / m o l see] fits the measured data within experimental error. The temperature dependence given by Lesclaux et al. TMwith an activation energy of 16.5 k J / m o l and the room temperature value of 1.6 9 10 a [cma/mol see] should not be compared to our results without keeping in mind that Lesclaux et al. ~2~ made their measurements at pressures around one atmosphere. This high pressure will change the absolute value and especially the temperature dependence if the same mechanism is assumed for the reaction N H 2 + C2H4 as suggested for N H 2 + acetylene. T h e first step in the reaction of NH 2 with ethylene: NH2 + C2H 4 "-* NH~ C2H 4

The Call4 concentration was varied between 6.6 ' 10 -~ and 2.3 9 10 -8 [mol/cm3]. At a flow velocity of t~ = 7.7 m / s e e the first-order rate constant along a distance of 50 cm is less than k -< 10 [see-~]. These measurements lead to an u p p e r limit k 7 -< 5 9 108 [cm3/mol sec] at one torr and 298 K. For propylene, the reactant concentrations were varied in the range 3.5 9 10 - 8 - < [Calls] <- 4.6 9 10 -8 [mol/cm3], resulting in a large excess of C3 H8 over N H 2 (1.8 9 104 to 2.3 - 104). Over a 50 cm reaction length, NH a decay was typically less than 50%, leading to an upper limit of k8 -< 6 9 108 [cm3/mol sec] at one torr and room temperature. The rate constant given by Lesclaux et al. ~2~ k 8 (300 K) = 2.2 " I08 [ c m a / m o l sec] is in agreement with the upper limit given in this work.

(4) REFERENCES

is determined by Schindler et al. tS)

The reactions of NH~ with Ca-hydrocarbons The reactions of allene a n d propylene with N H 2 : NH2 + CH~ = C = C H 2---> products

(7)

NH~ + CH~ = CH - CH a ~ products

(8)

and

are both very slow.

1. MACLEAN,D. I. AND WACr~Ea, H. Go. Eleventh Symposium (International) on Combustion, p. 871 (1967). 2. see e.g. WACNEa, H. GC. Fourteenth Symposium (International) on Combustion, p. 27 (1972); BAr~FO~O, C. H. Trans Faraday Soe. 35, 568 (1939). 3. GEHRING, M., HOYERMANN, K., SCHACKE, a . J., WOL~RUM, 1" Fourteenth Symposium (International) on Combustion, p. 99 (1972). 4. GO~DON, S., MVLAC, W. AND NANGI^, P. J. Phys. Chem. 75, 2087 (1971).

512

KINETICS

5. SALZMANN,J. D. AND BArn, E. J. J. C h e m . Phys. 41, 3654 (1964). 6. KHE, P. V., SOOLIGNAC, ]. C., LESCLAUX, R. J. P h y s . C h e m . 81, 210 (1977). 7. POLLOCK, T. L. AND JONES, W. E. Can. J. C h e m . 51, 2041 (1973). 8. WARNATZ,J. D i s s e r t a t i o n G 6 t t i n g e n 1971. 9. BEDFORD, G. AND THOMAS, J. H. ]. C h e m . Soc. F a r a d a y T r a n s I, 68, 2163 (1972). 10. LESCLAUX, R., KHE, P. V., DEZAUZIEtL P. AND SOULIGNAC, ], C h e m . P h y s . Lett. 35, 493 (1975). 11. HANCOCK, G., LANCE, W., LENZI, M. AND WELCE, K. H. C h e m . P h y s . Lett. 33, 168 (1975); HANCOCK, G., LANCE, W., LENZi, M. A~O WW'LCE, K. H. Int.

J. C h e m . Kinet. S y m p . 1, 508 (1975). 12. LESCLAUX, R., KHE, P. V. 12th I n f o r m a l Confere n c e on P h o t o c h e m i s t r y P 3-1 (1976); LESCLAUX, R., SOUmGNAC, J. S. AND KHE, P. V. C h e m . Phys. Lett. 43, 520 (1976). 13. HOYERMANN,K., WAGNER,H. GG. AND WOLFIqUM, ]. Ber. B u n s e n g e s . P h y s . C h e m . 72, 1005 (1968), HOYEaMANN, K., WACNER, H. Gc;., WOLFRUM,J. AND ZELLNER, R. Ber. B u n s e n g e s . P h y s , C h e m . 75, 22 (1971). 14. PEBRY, R. A., ATKINSON, R. AND PITTS JR., J. N. J. C h e m . P h y s . 67, 5577 (1977). 15. SCHURATH,M., TIEDEMANN, P. AND SCmNDLER,R. N. J. P h y s . C h e m . 73, 456 (1969).

COMMENTS R. 1. Santoro, Princeton University, USA. W o u l d y o u c o m m e n t on t h e effect of third b o d y collisional q u e n c h i n g effects d u e to reactant s p e c i e s (NO, NO2, C z H a a n d C 2 H 4 ) on t h e laser f l u o r e s c e n c e determin a t i o n of the o b s e r v e d rate c o n s t a n t s ? Authors" Reply. T h e v a r i o u s reactants do influence the radiative l i f e t i m e s of N H 2. T h e rate constants, however, are m e a s u r e d u n d e r p s e u d o first order c o n d i t i o n s , i.e. the reactant c o n c e n t r a t i o n a n d t h u s the q u e n c h i n g effects d u e to reactant s p e c i e s do not c h a n g e w i t h reaction time. Since we n e e d to m e a s u r e only relative N H z c o n c e n t r a t i o n s to d e t e r m i n e the rate c o n s t a n t s , their v a l u e s are n o t i n f l u e n c e d by t h o s e q u e n c h i n g effects.

K. Schofield, University of California, Santa Barbara, USA. A l t h o u g h there are e x p e r i m e n t a l indications in the literature of the direct reaction c h a n n e l N O + N H 2 - - * N~ + H 2 0 a n d p o s s i b l y also NO 2 + NH2--* N20 + H20 p e r s o n a l l y I find it d i f f i c u l t to accept that s u c h acrobatic r e a r r a n g e m e n t s w h e r e i n 3 b o n d s form a n d 3 are b r o k e n occur in s u c h one step m e c h a n i s m s . T h e s e interactions are a n a l o g o u s to 4-center reactions w h i c h are k n o w n to be g e n e r a l l y inefficient.

9

R. Lyon, Exxon Research & Eng. Co., U,S.A. T h e reaction o f N H 2 a n d N O is e x o t h e r m i c w h e t h e r the p r o d u c t s are N 2 + H z O or N 2 + H + O H . Did y o u i n v e s t i g a t e w h i c h p a t h is f o l l o w e d ? Authors' Reply. T h e reaction: N O + N H 2 was s t u d i e d in reference (1) a n d (2). F u r t h e r e x p e r i m e n t s with E S R are in progress,

REFERENCES 1. Poss, R.: T h e s i s , G 6 t t i n g e n , 1971. 2. GEHHING, M., HOYERMANN, K., SCHACKE, H. ANO WOLFRUM, J., Fourteenth Symposium (International) on Combustion, p. 99, T h e C o m b u s t i o n I n s t i t u t e , 1973.

1. Hilliard, Imperial College, U.K. T h e a u t h o r m e n t i o n s the p o s s i b l e m o d e of r e d u c i n g N O x emiss i o n s f r o m c o m b u s t i o n p r o d u c t s b y the injection of NH~ type species, specifically N H 2. I n the r~action o f N H 2 w i t h t h e C 2 u n s a t u r a t e d h y d r o c a r b o n s he does n o t s p e c i f y w h a t p r o d u c t s h e gets, or expects. (1) D o e s the a u t h o r c o n s i d e r the p r o d u c t i o n of h y d r o g e n c y a n i d e to be likely in the N H 2 / H C system? (2) D o e s h e c o n s i d e r the p r o d u c t i o n of n i t r o s a m i n e s to be likely in the N H 2 / N O / N O z / H C s y s t e m that w o u l d exist in a practical application of injection o f NH~ type s p e c i e s ?

Authors" Reply. T h e reaction: N H 2 + N O --* N z

REACTION RATES OF NH2-RADICALS + PI20 is the fastest known NO removing reaction that can be used to reduce NO emission from combustion systems. The reaction products of N i t / C H - - s y s t e m s have beeo investigated in detail

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by Prof. Fetting (Technische Hochschule Darmstadt). Studies of the variations in products of NH z reacting with C 2 unsaturated hydrocarbons with experimental conditions are under way.