Direct investigations of the NH2+NO reaction by laser photolysis at different temperatures

Nineteenth Symposium (International) on Combustion/The Combustion Institute, 1982/pp. 11-22

DIRECT

INVESTIGATIONS OF THE NH~ + NO REACTION PHOTOLYSIS AT DIFFERENT TEMPERATURES

BY L A S E R

P. ANDRESEN, A. JACOBS, C. KLEINERMANNS AND J. WOLFRUM

Max-Planck-lnstitut fiJr Strfmungsforschung D3400 GiJttingen, W. Germany The reaction NHe + NO was studied directly in an isothermal (300-1150 K) flow reactor. NH2-radicals were generated by exciplex laser photolysis of NHs. Reaction products were monitored by time-resolved quantitative atom resonance absorption, spectral resolved infrared fluorescence and laser induced fluorescence. From these measurements the contributions of the various pathways NH2 + NO --->N2 + H20

(3a)

--->NzO + H2

(3b)

--> N2 + H + OH

(3c)

--->NzOH + H

(3d)

--9 N~H + OH

(3e)

were determined. No hydrogen atoms (-<0.05) were found as reaction products in 290-900 K temperature range. From the measured concentration-time profiles of vibrationally excited H20 molecules the rate constant ks = (1.0 +- 0.3). 1013cm3 mol -I at 295 K is obtained in good agreement with previous measurements, Laser induced fluorescence measurements showed (3e) to be the most important channel.

Introduction

tance of various product channels is measured directly by time resolved detection of H-atoms, OHradicals and H~O after initiation of the NH z + NO Detailed inf~rmations on absolute rates as well reaction by laser photolysis of NH 3 at different as on the primary product and energy distribution of free radical reactions are of basic interest for a 9 temperatures. complete quantitative description of combustion processes. The present paper describes experimenExperimental tal investigations using laser photolysis in an isothermal flow reactor to study directly the reaction of NH.2-radicals with nitric oxide. Atom Resonance Absorption and Infrared In an earlier paper 1 we have shown that the hoFluorescence Diagnostics mogeneous reaction of NO with NH2-radicals can The experiments were performed using a therbe used for a selective reduction of NO to form mostated flow reactor (290-1150 K) coupled to an Nz. This reaction is also the basis for NO reduction exciplex laser system (Lambda Physik EMG 500) in large scale combustion processes by the addition as shown in Fig, la, The quartz reactor was heated of ammonia in the presence of oxygen near 1200 via a solid silver tube covering the reactor, Through K,2,a A key point in attempts4 to produce a quanthe excellent thermal conductivity of the silver titative model of this selective NO reduction protube together with an electronic temperature concess is the question on the absolute reaction rate trol unit the gas temperature in the reactor was and the primary product composition of the NO stabilized better than ---1 K in the 290 to 1150 K + NH 2 reaction. In the present paper the impor11

12

ELEMENTARY REACTIONS I

He.H2

J 1,4W D ~

',+'';'-'"~-, "--'?'- "-S',, I

t~,

,i

//-_~=_~'

,,

~ pump

+-/Z,L-~co+,,n9/A_ e.,-

I

~I

-"

tL;l

~'~t~m ~ _ __ ,-~=j_2_2_2---_-

exciplex laser

"Ill

,~+,,~

//+o++e Ag tube tube- oven

absorption /

cell

r~

/

/

reactants

mo+++

"~"

~

~ 02 pump

PM

FIC. la. Thermostated flow reactor (295-1150 K) for exciplex laser photolysis and time-resolved atom resonance absorption.

I

He(l) -Cooled

~

--t t t t Ge Hg Detector J ~ I,

copeI

-v-I

.~(.-Coo,e~ Circular Variable IR-Filter--

~

i

<~. I ~I

.L

~ ~ / / / I/ / / / / / / / / / / / / / / ! i

Control

I

NH 3. NO _

He

h'ump Flow Control

' EXCIPLEX

I ~;

I

LASER

..'~' ,

--1

Signal i JAverager]

mator

I Pump I

XY Recorder

FIG. lb. Schematic of the experimental arrangement with time and spectral resolved visible and infrared emission spectroscopy.

NH2 + NO REACTION BY LASER PHOTOLYSIS

13

reactants

I~L

J energy detector

\ __

l_ 3~oo~,

pump

I

Lp~4l'J Ar F

OH-fluorescence

-

Excimer-

~..~.

'

I~,

/

laser 193 nm

193nmscattered light

__l~

310nmscattered light

A

5300

~oBooL. {

I pulse -

generator variabledelay

Pretrigger

FIG. 2. Schematic of the experimental set up for OH radical detection by laser induced resonance fluorescence. range. For detection of H-atoms formed by exciplex laser photolysis or subsequent chemical reactions water cooled MgF2-windows are attached to the flow tube in the observation region. Lyman-~ emission at 121.6 nm was generated in a microwave discharge (MWD). After passing through the reaction vessel and an O2-filled absorption baffle the intensity of the radiation was monitored with a 0.3 m vacuum monochromator (McPherson 218) and a solar blind photomultiplier (EMIt 614-J-08-18). The photon induced current was amplified by a fast current-to-voltage converter (time constant 0.7 Ixs). The signals were accumulated by a transient recorder (Biomation 8100) coupled to a signal averager (Tracor Northern NS-575) and monitored by a x-y-recorder (ilohde and Schwarz ZSK 2). As shown schematically in Fig. lb the observation region could also be equipped for detection of time resolved infrared fluorescence by a Ge-Hg detector (SBIIC) combined with a cooled variable wavelength infrared filter (OCLI) and for visible and near infrared fluorescence by a monochromator (McPherson 218) and photomultiplier (RCA). Laser Induced Fluorescence

As shown in Fig. 2a a flow reactor is equipped with a baffle system to reduce the scattered light from the ArF-exciplex laser (Lambda Physik EMG 200) photolysis pulse at 193 nm (energy max.: 600

mJ/pulse, 15 ns pulse duration) and a Nd-YAG pumped dye laser (Quantel) analysis pulse. The dye laser operates with Rhodamine 640 and a frequency doubling KDP crystal to generate a pulse in the 306-311 nm region (0.2 cm -1 line width, 12 ns pulse duration, 1-10 mJ pulse energy) to probe OH (2II)-radicals by laser induced fluorescence (LIF). The fluorescence light is detected by a photomultiplier (EMI 9659 QB) through imaging optics and a filter transmitting between 240 and 390 nm. The photomultiplier current is measured by a boxcar integration system (PAR 162/165). The timing sequence of the laser pulses originates with a signal pulse emitted by the probe laser, which starts the exciplex laser discharge and the Q-switch of the probe laser. The time delay is continuously adjustable with jitter -<15 ns. Gases of the highest purity commercially available further purified by cooled trap purification were used: He (99,9996%), 0 2 (99,998%), NO (99,8%, NO~ < 3-10-4), NO z (98%), NH 3 (99,998%, 10% in He), H N Q (99,5%).

Experimental Results and Discussion The Reaction NH~ + NO

NHz-radicals were produced by ArF-laser photolysis of NH 3

14

ELEMENTARY REACTIONS I

NH3 (X 1AI) + hv L (k = 193 nm) ---> NHz (2B1) + H (2S)

(1)

Since the photon energy 619.8 kJ mo1-1 exceeds significantly N--H bond energy (456 kJ mo1-1) vibrational as well as electronic excitation (2A1) of the NH2-radicals formed is possible. In the ArFLaser photolysis generation of NH 2 (ZA1) occurs to less than 2,5%s of the NHe (2B1) formed. As shown in Fig. 3a a rapid quenching of the NH 2 (2A1) radicals with a rate constant higher than 10145 cmz mo1-1 s -1 is observed in agreement with the literature value. 6 Also the vibrational relaxation of NHz (2B1) is very fast. As shown in Fig. 3b for the N--H stretching vibration the addition of He as carrier gas in the torr range ensures vibrational deactivation of NHz (2B1) in times short compared to the chemical reaction rate used in the present experiments. The time resolved absorption signal from the H (ZS)-atoms formed in the NH 3 photolysis is shown in Fig. 4a. The concentration remains nearly constant over several milliseconds showing the uniformity of the laser photolysis and the small amount of H-atom recombination due to the low radical concentrations used. Variation of the laser pulse energy in the region 10-100 mJ gives no indication for nonlinear processes in the photolysis. The addition of small amounts of NO 2 to the NH 3/He mixture results in a rapid decay i

i

Considering now the reaction

the measured negative temperature dependence for the decay rate of NHz-radicals9-11 suggests the initial formation of a NHzNO adduct complex. Geometries and energies for different isomeric and tautomeric forms of this complex as well as for the various dissociation channels have been estimated by empirical 4 and ab initio 12 methods. In view of the high value of k3 at low temperatures only exothermic decomposition channels of the adduct will be considered here -AH~ kJ mol-1 NHz + NO ---> N~ + H20

527

(3a)

---> N20 + H 2

199

(3b)

24

(3c)

---) Nz + H + OH I

[He]

I

5

I

10

(3)

NH~ + NO ---) Products

[NH 3]

~ LASER

(2)

NO2 + H ---) NO + OH

i

I NH2~I

0

of the hydrogen atoms as illustrated in Fig. 4b. The measured decay rate at 290 K corresponds to a rate of k2 = 6.6.1013 cm3 mo1-1 s -I in good agreement with literature values 7'8 of k2 for the reaction

2 mTorr

18mTorr

I 15

I

20

TIMEtlas]

FIG. 3a. Decay of the NH~ (2A~) formed in ArF-Laser photolysis of NH 3 at T = 293 K.

NH2 + NO REACTION BY LASER PHOTOLYSIS I

I

I

15 I

[HelTorr

022 "--"

I mox

INH2+

I:NH 31 = 24mTorr 0.38

-

078

-

0

I

I LASER

1

50

100

I

150

I

200 TIME[psi

FIG. 3b. Vibrational deactivation of NH2 (zB~,v) radicals formed in ArF-Laser photolysis of NH 3 at T = 293 K. [NH~] o = 6 • 10 11 mol cm -3. NzOH + H

15

(3d)

N2H + OH

13

(3e)

In experiments using the photolysis of NH 3 / N O / He mixtures with the ArF-Laser in a static cell at

H_~ absorption

concent ration

[%]

(x I0 .,2)frnol.cm3]

10

~

time [ m s l

FIG. 4a. Formation of H-atoms in the ArF-Laser photolysis of NH a. T = 295 K. [NHa]o = 1,2 x 10 -u mol cm -a. [He] = 5,5 x 10 -8 tool cm -a. Laser pulse energy 90 mJ. Loser

I La absorption [%]

1

0

b) 0

5

2

H - atom concentration [ 10-12 rnol,cm-~

'~

6 ol

15

1

8

o'.s

; ~time

[ms]

FIG. 4b. Concentration-time profiles of H-atoms. T = 570 K, P = 1 Torr, [NH3] o = 1,1 • 10 -11 mol cm -3. a) [NO2]o = 0. b) [NO2]o = 3 • 10 -u mol cm-3.

room temperature combined with mass spectrometric (Typ 500 Varian) stable end product analysis no NzO but a quantitative reduction of NO to N 2 was detected. Thus, since NzO reacts slowly with H-atoms 13 and other radicals the pathway (3b) can be ruled out (k3b - 0.01 k3) under these conditions. Hydrogen atoms can be formed in (3c) and (3d). Since the photolytic process (1) generates initially [NH2] o -- [H]o the importance of these channels can be quantitatively measured by comparing the concentration of hydrogen atoms formed in (1) with the additiQnal amount of H formed in (3c) and (3d). As shown in Fig. 5 and Table I no additional hydrogen atom formation is observed due to the presence of NO in the temperature range 290 to 900 K. This is in agreement with our earlier experiment at room temperature 1 using ESR detection for H-atoms. In the present experiments the low concentration of NH 3 (see Table I) and consequently of H-atoms and NH2-radicals (see Fig. 3) coupled with a high time resolution ensures that subsequent reactions of radicals formed in the primary process cannot take place during the relevant observation time. From the measurements it is concluded that also the pathways (3c) and (3d) give no significant contribution (k3c + k3d <-- 0.05 k3) in the shown temperature range. In the earlier experiments1 the formation of water in channel (3a) could not be monitored with nozzle beam sampling mass spectroscopy due to the interference from the OH + NH 3 ~ HzO + NH 2

(4)

reaction. However, a strong stationary infrared

ELEMENTARY REACTIONS I

16

=

NH 2 + NO

Products

----N2+H+OH[I_ OCH ] N2OH+H 1.5

t a(H(T)

0.5

0

I

I

I

300

i

500

Temperature

FIG.5. Measurements on the temperature

I

700

[K]

I

900

--

dependence of the amount of H-atoms formed in reaction

(3). chemiluminescence was observed which could be attributed to different fundamental and overtone vibrations of H20. The present experiments allow now a direct time-resolved observation of the vibrationally excited H 2 0 formed in channel (3a). Measurements in the O - - H stretch vibration region around 3 txm are shown in Fig. 6 for various NO excess concentration. From a number of experiments in which the NH 3 concentration was var-

ied between 10 - l ~ and 10 -8 mol cm -3 and the carrier gas pressure between 10 p~bar with an [NO]o / [NH2] o ratio between 2 and 200 the rate constant k 3 = (1.0 --+ 0,3)- 1013 cm 3 mol -I s -1 at 295 K is obtained. This value is in good agreement with the results of Lesclaux et al., z Welge et al., 14 and Sarkisov et al. 15 Our earlier result was a factor of

TABLE I Measurements on the amount of H-atoms formed in the NH2-NO reaction (3) T - 0,5 (K)

P - 0,03 (mbar)

[NH3] -+ 1% (10 -12 tool. cm -3)

[NO] -+ 5% (10 -9 mol. crn -3)

~H (s. Fig. 5)

293 300 572 583 590 600 877 877 895 898

2,6-50,0 0, 9~3, 2 2,6 -20, 0 1,3-2, 7 0,5 1,3-40,1 2,6-45,3 0,06 0,9-2,7 6,7

11,3 7, 5 8,5 12, 2 12,1 6,3 10,5 10,5 10,2 8,0

6,2-40 0, 5-20 2, 5-23 0, 5-10 0,03 1,5~50 4,8-43 0,5 0,9-13 0,09

0,973 0,993 0,972 0,958 1,035 0,968 1,014 0,930 1,063 1,014

NH 2 + NO REACTION BY LASER PHOTOLYSIS I

]

H20* IRF

I

I

17 I

2 Torr NO .

I

0

~

50

100

I

150

. . . . .

I

200

/

TIME [IJS]

ArF- LASER

FIG. 6. Time-resolved infrared emission (2,5-3,5 txm) of H20 formed in reaction (3). T = 295 K, [NHz]o = 1,4 • 10 -1~ tool cm -3, [He] = 10 Torr. two lower and based on the measured absolute NO consumption and N 2 formation in a flow system by a combination of reaction (2) (4) and (3). We have repeated this modelling and used the rate k2 obtained here and a more recent measurement for k4. With these data we obtain excellent agreement with the measured NO and N2 profiles given in 1 by using the rate k3 = 1.0.1013 cm3 mol -I s-I derived above. It is very difficult to extract an absolute value for the contribution of channel (3a) from the infrared chemiluminescence experiments. Therefore the distinction between the remaining pathways was made by measuring the absolute amount of OH-radicals formed in (3) by LIF-spectroscopy. The high sensitivity and time resolution of the set up shown in Fig. 2a allows the detection of reaction products down to single collision conditions excluding effects from secondary or wall reactions. The fraction to which reaction (3) branches to OH-radicals was determined by comparing the amount of [OH]o~ formed in reaction (2) with the concentration [OH]on from reaction (3). Fig. 7 shows a comparison of the OH-radical spectra (R]-branch, K = 1-7) obtained from reaction (2) and (3) with a 130 i~s delay between photolysis and probe laser so that both reactions are completed and the OH LIF-signal intensity has reached a plateau. The absolute intensity (S/N -> 100) of the various lines shows

a very good reproducibility, because the OH absorption can be driven to saturation so that the LIF intensity is independent of variations of the laser power. The LIF-signals were proportional to the intensity of the ArF-photolysis pulse and to the NH 3 concentration as long as the NHa absorption is not saturated by a high intensity photolysis laser pulse. [OHio2 and [OH] 03 are obtained by- summing over the population of all rotational states both in v = 0 and v = 1 level. This summation was necessary, since both reactions (2)17 and (3) produce a non thermal population in the OH rotational levels. The OH spectrum shown in Fig. 7 for reaction (2) exhibits a rotational temperature of Tn -~ 1100 K even with 130 Ixs analysis pulse delay and 10 torr Argon as buffer gas. This can be explained by the slow OH rotational relaxation (AK = 1) for the high rotational states with their increasing energy gaps.lS The OH radicals formed in reaction (3) are rotationally less excited (s. Fig. 7). The summation results in a ratio of [OH]o3 _ 0.7 + 0.2 [OH]o~ Complications from NO 2 impurities in NO, NO~ photolysis and subsequent reactions of oxygen atoms with NH 3 and NH z could be ruled out by sep-

18

ELEMENTARY REACTIONS [ NH 2 +

NO ~

OH (2IT) + N2H

R,/, R15 R16

R1 7

~

/

306./,9

R1 3

R1 2

I

Rt 1

~, [nrn] 30"/.20

H +NO2 - - ' " OH (211) + NO

1

[

J

h [nm]

307.20

306./,9

FIG. 7. Laser induced resonance fluorescence spectra of OH-radicals formed in reaction (3) and (2) at 130 Ixs delay between photolysis and analysis laser pulse. [NH~]o = 5,4 • 10 -H mol cm-Z; [NO], [NO2] = 5,4 • 10 9 mol em-3; [Ar] = 5 • 10 _7 mol cm-Z; T = 295 K.

arate experiments and the concentrations of NH 3 and NO z used. Also in separate experiments it was shown that the fast reaction I~ NHz + NO2 ~ Products

(5)

In conclusion the pathways for reaction (3) can be formulated as: NH2 + NO--~ N 2 + H20

->0.29

(3a)

--~ N20 + H z -<0.01 (3b) forms no significant amounts (-<0.1) of H atoms and --* N 2 + H + OH (3c) OH radicals. The obtained branching ratio agrees within the combined experimental errors with the ---* N2OH + H ---0.05 (3d) value found by Silver et al, 11 by a different method ---* NzH + OH ->0.65 (3e) using NH 2 generation by reaction F + NH 3 ----> NH 2 + HF in a discharge flow system. In contrast However, a number of additional experiments are to these experiments the LIF-OH-spectra from reaction (3) could be obtained near to single collision in progress to obtain further information on the vibrational energy distribution of N~ and H20 conditions (1 Ixs delay between pulses, 30 mtorr NH3, 80 mtorr NO), excluding effects from sec- formed in (3a) and of N2 formed in (3e) followed by ondary and wall reaction. Determination of the rate constants for the reactions (2) and (3) from the meaN2H + NO--} N 2 + HNO (6) sured OH-concentration versus time profiles is hampered by the fact that the rotational relaxation interferes strongly with the OH production by the by CARS spectroscopy. Further investigations on chemical reaction. Under the conditions of Fig. 7 the direct detection of N2H and HNO by CARS k 3 = (7 +-- 4)-1012 cm 3 mo1-1 s 1 at 298 K is deand LIF-spectroscopy, the temperature depenrived from the OH appearance profile. dence of the OH-radical formation and spectro-

NH~ + NO REACTION BY LASER PHOTOLYSIS scopic studies of the (NHzNO) addition complex in a low temperature matrix will be performed.

19

Chem. Soc. Faraday Trans. II 73, 98 (1977). 8. MICHAEL, J. V., NAVA, D. F., PAYNE, W. A., LEE, J. H. AND STRIEF, L. J.: J. Phys. Chem.

83, 2818 (1979).

Acknowledgment T h e authors wish to thank Prof. H. Gg. Wagner for his continuous interest and helpful discussions in this work. The financial support of the Deutsche Forschungsgemeinschaft (Sonder forschungsbereich 93) is gratefully acknowledged.

REFERENCES 1. GEHRING, M., HOYERMANN, K., SCHACKE, H., AND WOLFRUM, J.: Fourteenth Symposium (International) on Combustion, p. 99, The Combustion Institute, 1973. 2. LYON, R. K.: Method for the Reduction of the Concentration of NO in Combustion Effluents Using Ammonia, U.S. Patent No. 3,900,554 (1975). 3. LYON, R. K. AND BENN, D. J.: Seventeenth Symposium (International) on Combustion, p. 601, The Combustion Institute, 1979. 4. MILLER, J. A., BRANCH, M. C., AND KEE, R. J.: Combustion and Flame 43, 81 (1981). 5. DONNELLY, V. M., BARONAVSKI, A. P. AND McDONALD, J. R.: Chem. Phys. 43, 271 (1979). 6; DONNELLY, V. M., BARONAVSKI, A. P., AND MCDONALD, J. R.: Chem. Phys. 43, 283 (1979). 7. WAGNER, H. GG, WELZBACHER, U. AND ZELLNER, R.: Bet. Bunsenges. Phys. Chem. 80, 1023 (1976). CLYNE, M. A. A. AND MONKHOUSE, P. B.: J.

9. LESCLAUX, R., KHE, P. V., DEZANZlER, P. AND SOULIGNAC, J. C.: Chem. Phys. Lett. 35, 493 (1975). 10. HACK, W., SCHACKE, H., SCHROTER, M. AND WAGNER, H. GG.: Seventeenth Symposium (International) on Combustion, p. 505, The Combustion Institute, 1979. 11. SILVER,J. A. AND KOLB, C. E.: J. Chem. Phys. (in press). 12. CASEWIT, C. J. AND CODDARD III, W. A.: J. Am. Chem. Soc. (in press). 13. ALBERS, E. A., HOYERMANN, K., SCHACKE, H., SCHMATJKO, K. J., WAGNER, H. GG AND WOLFRUM, J.: Fifteenth Symposium (International) on Combustion, p. 765, (1974). 14. HANCOCK, G., LANGE, W., LENZl, M. AND WELGE, K. H.: Chem. Phys. Lett. 33, 168 (1975). 15. SARKISOV,O. M., CHESKIS, S. G. AND SVIRIDENKOV, E. A.: Isv. Akad. Nauk SSSR 27, 2612 (1978). 16. SILVER, J. A. AND KOLB, C. E.: Chem. Phys. Lett. 75, 191 (1980). 17. SILVER, J. A., DIMPFL, W. L., BaoenY, J. H. AND KINSEY, J. L.: J. Chem. Phys, 65, 184 (1976). MARIELLA, R. e., LANTZSCH, B., MAXSON, V. T. AND LUNTZ, A. C.: J. Chem. Phys. 69, 5411 (1978). 18. KLEY, D. AND WELGE, K. H.: J. Chem. Phys. 49, 2870 (1968).

COMMENTS Sidney W. Benson, University of Southern California, USA. I am concerned about an experimental question. Since the primary photolysis produces H atoms in concentration equal to or greater than NH~, the observation of additional H atoms from secondary reactions may be very difficult.

Author's Reply. With initially [NH2]o = [H]o the importance of reactions (3c) and (3d) can be measured by comparing [H]o with the additional amount of hydrogen atoms formed by reactions (3c) and (3d), which is less than [H]o with [NO] in excess of [NH2]o. We checked our sensitivity for observing a change in H concentration by adding different amounts of NO 2 to the NH3/He mixture and observing the

decrease of H concentration due to the H + NO2 OH + NO reaction. Our result k~ + k3a -< 0.05 k3 is in good agreement with the value found by Silver et al. (J. Chem. Phys. 86, 3240 (1982)) with a different method generating NH~ radicals chemically without H atoms as byproducts.

R. K. Lyon, Exxon Research & Engineering Co., USA. It's important to specify the temperature when you quote the product distribution. For example, your results show NzO formation to be less than 1%. Our work at higher temperature suggests that it is 1% and Hanson's work at still higher temper-

20

ELEMENTARY REACTIONS I

atures suggests that it is a major channel.

Author's Reply. Our measurements of the N20 yield less than 1% are performed at room temperature, so that result is not contradictory to yours.

D. R. Crosley, SRI International, USA. The excitation scan for the OH formed in the NH~ + NO reaction looked quite cold. If colder than ambient, and if it represents the rotational distribution for the nascent product, it might say something about the reaction pathway. For example, a complex might be expected to equilibrate among modes. What is the distribution and does it vary with temperature? Have you looked at the pressure dependence to see if it's sensitive to nascent OH? Have you looked for vibrationally excited OH in this reaction? Author's Reply. The high time resolution and sensitivity of our LIF Detection method enables us to observe OH spectra from the NH~ + NO reaction near to single collision conditions (1 p,s delay between laser pulses, nearly 100 m torr total pressure). However, at these conditions, i.e. without adding high amounts of inert gas, we do not quench electronically and vibrationally excited NH 2 formed by the NHs photolysis at 193 nm. Because we cannot exclude significant distortions of the measured nascent OH rotational distributions from the reaction of these excited NH~ species with NO, we decided to perform these measurements in a crossed molecular beam experiment with internally cold NHz and NO beams. We could not find vibrationally excited OH from the NH2 + NO reaction.

M. Farber, Space Sciences, Inc. Monrovia, CA, USA. I agree with your reaction mechanism for formation of H20. However, since you did not determine the N2H radicals experimentally, but decided that the mechanism followed the OH radical measurements, it would be speculative to state that the reaction N2H + OH = Nz + H20 is 65% and that the reaction NH 2 + NO ~ N2 + H20 is 25%. The OH radicals are involved in numerous reactions. Is it not probable that some NH radicals which are highly reactive also appear? To obtain a primary mathematical distribution of the reactive steps would definitely require a quantitative measurement experimentally of the major reaction species involved: i.e., N2H as well as OH, H and O species, as well as the NH2 produced by the laser beam. Until this is done I think that presenting a quantitative distribution of the mechanism involved is unwarranted.

Author's Reply. First of all, because of the high time resolution of our measurements and the fact that we obtained the NH 2 radicals via photolysis, i.e. without contact to the walls, we can safely say, that OH is a primary product of the NH2 + NO reaction and does not result from secondary or wall reactions. As I have shown in my talk, we quantitatively determined the H-yield of the NH2 + NO reaction to be less than 5% over a broad temperature range from 300-1000 K. We determined the OH yield to be 70% at room temperature. From energetic reasons there are only two reaction pathways in the NH~ + NO reaction to form OH at room temperature; they are the Nz + H + OH and the N~H + OH channels. From that the N2H + OH channel has to be greater than 65% and the NzH radical has to be an important product of the reaction. We have no indication that significant amounts of NH are produced at our laser energy fluences, i.e. without focussing the photolysis laser.

J. A. Miller, Sandia National Laboratories, Livermore, USA. First of all, I am very pleased to see that these investigators, as well as others, have acted on our suggestionII/ of a few years ago and looked at the product distribution of the NHz + NO reaction directly. I am even more pleased that our hypothesis that the primary products are NNH + OH appears to be confirmed. With regard to this, I would like to make two comments: 1) It is essential to mention that ab initio calculations for the reaction show that the NNH + OH products lie a few kcal/mole (6 (Melius and Brinkley in our group), 12 (Casewit and Goddard)) above the NH~ + NO reactants. However, I think it is a mistake to weigh the electronic structure calculations more heavily than direct product measurements, particularly when the latter have been confirmed in a second laboratory (Silver, et al. at Aerodyne), in making such a close call as whether NHz + NO ~ NNH + OH is endothermic or exothermic. The mechanism proposed by Casewit and Goddard as an alternative to our Thermal De-NOx model, I" mentioned by Professor Kaufman in his plenary lecture, can be eliminated immediately. It implies second order dependence on [NO] and/or pressure dependence for k(NH2 + NO). Both of these dependences have already been ruled out experimentally. Consequently, the ab initio calculations have offered no new alternatives. 2) The important point to consider now is, if NNH is formed as a product, what happens to it in secondary reactions? (If NNH is a product 65% of the time at room temperature, it is likely to be a product close to 100% of the time at tempera-

NHa + NO REACTION BY LASER PHOTOLYSIS tures of interest!). Interesting possibilities exist for reaction with O2 and NO. Reaction of NNH with NO should yield HNO, which should be detectable spectroscopically. Reaction with O2 might yield HO2, which would lead to NO2 via HO~ + NO --~ NO2 + OH. Another possible reaction is NNH + O2 N~O + OH. A careful investigation of the NH2 + NO reaction with added quantities of O2, looking for HNO, NO~ and N20 as secondary products, then should be very illuminating.

REFERENCE 1. J. A. MILLER, M. C. BRANCh, AYD R. J. KEE, Combustion and Flame 43, 81 (1981); Sandia National Laboratories Report SAND 80-8635 (1980).

Author's Reply. I agree with your comment. We have started to perform a crossed molecular beam study of the NH 2 + NO reaction to measure the unrelaxed product energy distribution of OH via LIF. From the highest occupied OH-rotational states we will be able to determine a lower limit for the enthalpy of the N2H + NO pathway and so also for the heat of formation of N2H. The species NOH, possible product of the secondary reaction N2H + NO, is a very suitable candidate for laser induced fluorescence so that we are able to directly investigate that reaction in our apparatus.

21

system. So we have an experimental disagreement with your results which I am simply at a loss to explain. Secondly, work on this chemical reaction is quite old. Bamford~ in 1939 suggested from product measurements in a stationary photochemical experiment that only N2 and H20 were formed in the reaction. Work by Serewicz and Noye s4 and Srinivasan 5 corroborated Bamford's suggestion and showed that Nz was formed from nitrogen atoms which were originally in NH3 and NO, respectively. Subsequently, Jayanty, et al. 6 measured N2 quantum yields at room temperature and showed that qbN2 was between 0.7 and 1.0 under all conditions. When NH 3 was 7 torr or less, no quenching of excited NH3 occurred, and then qb~2 = 1.0 -+ 0.1. Since the primary quantum yield to form ground state NH z + H is unity ~ at the wavelength used by Jayanty, et al., 6 the direct reaction,

NH2 + N O ~

N~ + H~O,

was considered to be the best explanation for their result (i.e.; qbN2 = 1). If the branching ratio is as high as indicated in your work, then there is a good possibility, in very low light level photochemical experiments where radical-radical reactions become less important, that a chain reaction could occur as follows: NH3 + h v---~ NH~ + H NH2 + NO---~ N2 + HzO

N~H + OH

J. v. Michael, Brookhaven National Laboratory, USA. Your result that OH is formed in the reaction of NH2 + NO with a branching ratio of about 0.7 seems quite convincing. I would like to mention some points which seem to me to be quite puzzling if the ratio is that high. In a recent paper on the kinetics of this reaction, my colleagues and I measured the rate constants by the FP-LIF technique over the temperature range, 216-480 K. l The results were well represented by, k = (2.77 -+ 0.89) • 10 -7 T-L6r•1761763 molecule-is-~ in best agreement with Lesclaux, et al. 2 In the course of this work we looked for OH product at room temperature with resonance fluorescence, and we were able to observe it, but only in a static system. If the reagents were flowed through the reaction cell then no OH could be observed. The OH in the static experiments was subsequently traced to H20 outgassing from the vacuum system. Since I had been in contact with Professor Wolfrum on this issue, I presume that our experience contributed to your decision to do your work in a flowing

d~=l -> 0.29 -> 0.65.

Then, OH + NH3--~ HzO + NHz, which has a relatively large rate constant, 8 would regenerate NH2 radicals. In the presence of excess NO, more OH would be formed, etc. The chain length would of course be regulated by radical termination reactions. The point I wish to make is that the observation of qbN2 = 1 in this photochemical system is a solid observation and points to the need for chemically modeling the photochemical system in addition to the practical systems such as the "De-NO~" process. The unit quantum yield may be explainable, but any explanation will surely have implications with regard to NzH and chain center (OH and NH2) homogeneous or heterogeneous reactivities. I might remark further that in the Jayanty, et al experiments, 6 if a large pressure of He was added to the system, the nitrogen quantum yield decreased (to 0.7) rather than increased, and this has implications regarding the diffusion and heterogeneous wall loss of chain centers. Thirdly, Professor Wolfrum presented tentative results last year, 9 which we referred to in our pa-

22

ELEMENTARY REACTIONS I

per,~ that suggested a lower value for the branching ratio. These results were based on H concentration increases of the type you presented here when CO was added to the system. It was presumed that if OH were a direct product of the title reaction then H could be used as a tracer of OH through the reaction, OH+CO~CO~+H. As I recall the expected increases were not observed. Has this situation now been successfully resolved? Lastly, justification for the direct reaction forming N~H and OH rests in part on thermochemical grounds, and you have suggested that the reaction is exothermic by about 3 keal mole -~. On what basis was this value calculated? I note that the ab initio calculations of Casewit and Goddard 1~ suggest an endothermicity of 12 kcal mole 1.

REFERENCES 1. L. J. SaIEE, W. D. BROBST, D. F. NAVA, R. P. BORKOWSKI, AND J. V. MICHAEL, J. Chem. Soc. Faraday Trans. 2, in press. 2. R. LESCLAUX, P. V. KHE, P. DEZAUZIER, AND J. C. SOUIaGNAC, Chem. Phys. Lett. 35, 493 (1975). 3. C. H. BAMFORD, Trans. Faraday Soc. 35, 568 (1939). 4. A. SEREWlCZAND W. A. NOYES, JR., J. Phys. Chem. 63, 843 (1959). 5. R. SRINIVASAN,J. Phys. Chem. 64, 679 (1960). 6. R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J. Phys. Chem. 80, 433 (1976). 7. See U. SCHURATH, P. TIEDEMANN, AND R. N.

SCHINDLER, J. Phys. Chem. 73, 457 (1969), and references cited therein. 8. See J. A. SILVER, AND C. E. KOLB, Chem. Phys. Lett. 75, 191 (1980), and references. 9. J. WOLFRUM, paper 4 presented at the 182nd National Meeting of the American Chemical Society, New York, NY, August, 1981. 10. C. J. CASEWITAND W. A. GODDARD III, J. Am. Chem. Soc., in press.

Author's Reply. We have performed our measurements on the reaction NH2 + NO in a flow system. We could not detect any OH with the probe laser at a short time before the photolysis pulse ensuring sufficient flow velocity and complete exchange of the gases in the cell before the next photolysis pulse. Also we found k3 = (7 +-- 4) 9 1012 cm3mol-ls -I at 298 K from the OH appearance profile in good agreement with various other authors for the overall reaction (3) ensuring OH to be a primary product of the NH2-NO reaction. W e agree, that finally all NH2 is converted to N2, so that ~N2 = 1.0. Our results refer to the primary product of reaction (3) which may be converted to N2 and H20 by different secondary reactions. The reason why the indirect determination of OH via OH + CO ~ CO2 + H did not show the expected increase in H-atom concentration is not cleat'. There are different guesses and calculations of the thermodynamics of the NHz + NO ~ N2H + OH reaction between 12 kcal/mol endothermic (C. J. Casewitt, W. A. Goddard, J.A.C.S. 1982, 104 (3280-3287)) and approximately t h e r m o n e u t r a l (Baird, N. C., J. Chem. Phys. 62, 300 (1975)). Our direct product measurements show that reaction (3e) cannot be strongly endothermic.