- Email: [email protected]
On the rate of the reaction of ammonia with oxygen atoms
Volume 106. number 3
CHEMICAL
PHYSICS
LEl-l-ERS
20
April 1984
ONTHERATEOFTHEREACTlONOFAMMONlAWITHOXYGENATOMS* Robert A. PERRY Sandia
ffariotzal
Laboratories.
Livermore.
California
945.50,
USA
Received 2 December 1983; in tinal form 25 January 1984
Absolute rate constanls for the reaction of 0c3P) with ammonia wcrc measured over rhe tcmperaturr ran_ee 4-18-841 K by means of a lnscr photolysis-chrmilumincscence technique. The data were fitted with the following Arrhenius expression: k = 3.42 x lo-” cxp( -(9000 + 600 cal/mole)/RT] cm3 moIccule~ 5’ _Corn parisons with literarure data and predictions based upon ihc extrapolations 10 higher tcmpcraiures round in combusiion arc presentcd.Thc cstmpolations 3re in agreement with recent shock-tube data and indicllle that rhis reaction is more important in high-temperature ammonia oyidsrion than previously believed.
l_ Introduction Major progress has been made in modeling nitrogen chemistry to explain phenomena such as the Exxon “Thermal De-NOx” process and ammonia oxidation in combustion systems [l--5]. Srill, many uncertainties remain regarding of the reactions which are utilized in the models. Based upon empirical or theoretical arguments one might expect an important reaction in hightemperature lean ammonia/oxygen systems to be:
O+NH,-+OH+NH,,
AH = 6.6 kcal/mole.
(1)
However, previous experimental results indicate that this reaction is slow even at moderately high temperatures [6-IO]. Although various absolute rate measurements have been made for reaction (1) [6-g], all of these measurements required subsequent modeling of the reaction mechanism in order to correct for stoichiometry effects in the experiments. While the various experimental values obtained have apparently been reconciled [7], a disparity still exists between the experimental results and calculations based upon theoretical or empirical considerations [6]. Also, given that the heat of the reaction is 6.6 kcal/mole, it is difficult to reconcile the past results which give an activation enQ Work supported by Office partment of Energy.
of Basic Energy Science,
US De-
0 009-2614/84/S 03.00 0 Eiscvier Science Publishers (North-Holland Physics Publishing Division)
B.V.
ergy which is less than the endothermicity of the reaction. In recent high-temperature shock-rube esperimenus [ 1 I] the reaction was found to proceed substantially faster than extrapolations from low-temperature experiments would indicate. In order to resolve the order of magnitude discrepancy between calculated and experimental values for the rate of reactions, and to resolve fully the difference between recent shock-tube experimenrs and the literature, experiments were performed to measure this reaction rate using a laser photolysis-NO1 chemiiuminescence technique. The results of these esperiments are described and the implications for the nCechanism of ammonia oxidation are discussed_
2. ExperimentsI
O(jP) atoms were produced in a low-pressure reacrion vessel by the pulsed laser photolysis of 0, (=0.030.04 Torr) and NO (-0.09-0.26 Torr) u&~g a F., estimer laser (157 nm). The laser was operated at pulse energies of =I rnJ/pulse with the laser intensity in the reactor being adjusted by varying the flow rate of the nitrogen purge gas situated between the laser and the reaction vessel. O(jP) atom concentrations were monitored as a function of time after the pulse by using an EM1 9789@A photomultiplier fitted with an interfer223
Volume 106, number 3
CHEMICAL PHYSICS LETTERS
ence filter (center wavelength 5 145 A with a full-width half-maximum bandpass of 100 A) to monitor NO2 chemiluminescence from the reaction O+NO+M+NO;+M, NO; + NO2 + Iw.
(2)
Decays of the NO, chemiluminescence, and hence of O(!P) atom concentrations, were accumulated from 25-800 laser pulses, depending on signal strengths at a repetition rate of about 03 Hz. O(3P) atom half-lives ranged from 2.86 to 18.2 ms, and in all cases the O(3P) decay was monitored over at least 3 half-lives. Signals were obtained by photon counting in conjunction with multichannel scaling. The reactor consisted of a quartz reaction vessel. enclosed in a ceramic furnace which could be heated to 1200 K. The temperature of the furnace was monitored by chromel/alumel thermocouples mounted inside the reaction vessel with the temperature of the vessel maintained to 5 K over the entire temperature range. All experiments were carried out under slow flow conditions so that the premixed reactant gas mixture could be replenished between laser pulses, thereby avoiding the accumulation of photolysis or reaction products. The gases used had the following purity levels according to the manufacturer: Ar > 99.995%; O2 > 995%; NO > 99.0%; NH, > 99.998%. NO was passed through a Matheson 13X gas filter to remove any H,O or NO, present.
3. Results
20 April 1984
ties, k. is the first-order rate coefficient for removal of 0(3P) atom in absence of added ammonia and NO (due to diffusion out of viewing zone and reaction with impurities and 02) and k 1 and k, are the rate constants for the reactions (1) and (2). respectively. The decay was analyzed following a xl ms delay after the excimer laser pulse to minimize interference from laser induced emission from the quartz vessel and to ensure that secondary reactions of other atomic species formed during the photolysis event, i.e. N(4S) and 0( 1D), would not interfere with the reaction of interest. Under the conditions used in this experiment N(4S) has a half-life of ~220 ps due to the reaction with NO and O,, while O(1D) is quenched by Ar with a halflife of =3 j.~s_ In all experiments exponential decays of the NO, chemiluminescence signals were observed. The data were analyzed by numerical least-squares fitting of the decays using a VAX 1 l/780, which was directly interfaced to a Nuclear Data (ND66) multichannel analyzer. The measured decay rates were observed to depend linearly on the concentration of added ammonia for fvted total pressure and fixed NO concentration. The absolute rate constants were determined by plotting the measured decay rate against ammonia concentration and performing a least-squares fit to obtain the slope, k, . Figs. 1 and 2 are the plots of the data for 0 + NH, at the indicated temmperatures. Table 1 lists the measured rate constants while fig. 3 shows the corresponding Arrhenius plot of the data. Least-squares analysis of the data yields the 400
1
The reaction of O(3P) atoms with ammonia was studied over the temperature range 448-841 K with 75 Torr of argon as a diluent gas. In all the experiments conditions were chosen so that the decay oi NO, chemiluminescence, and consequently O(3P) atom concentrations, was exponential. Rates were determined using the expression
w[0~3p)l(J/P~3p~lr) = ‘“P&) = W. +$ [NH31‘-$ [NOI WI) (I - loh where [O(3P)]o and [0(3P)]t are the concentration of O(3P) atoms at t&s to and c respectively, So and S, are the corresponding NO? chemiluminescence intensi224
0
l !& 1
&,I (molecule
crZ$
Fig. 1. Plot of the O(“P) atom decay rate against NH3 concentration at 448 and 541 K.
Volume
106. number
CHEMICAL
3
PHYSICS
20 April
LETTERS
Arrhenius
1984
expression
k, = 3.42 X 10-l’ cm3 molecule-’
exp[-(9000
_+600 cal/mole)/f?T]
s-l,
where the indicated error limits include the standard deviation (415 cal) from the least-squares analysis and the estimated uncertainty due to gas composition (GS%).
01 2
0
8
10
4. Discussion
-10” Fig. 2. Plots of the O(3P) atom centration at 662 and 8: 1 K.
decay
rate against
NH3 con-
Table 1 hleasurcd rate constanr,k, for the reaction of 0(3P) \vith ammonia. Indicated errors arc estimates to overall error limits which include the least-squares standard deviation, l-2.54,
as well as estimated accuracy limits due to gas composition uncertainty lot4
k (cm3
molecules’
s-l)
0.159(~0.012)
Temperature(K) 448
0.673(+0.040)
541
3.59(+0.25)
662
17.1(+ 1.0)
831
lo '
lo-‘” 1 1 Fig. 3. Arrhcnius
I
,
,
125
150
/ 1.75
I 2
225
1000/T (K) plot of log X-1 against 1000/T(K)
2.50
for NH3.
Table 2 lists various literature values for k, obtained by both direct and indirect measurement techniques. It should be noted that all previous measurements were made under conditions where detailed modeling of the reaction was necessary to derive the rate constant. In the present experiment, however, sufficiently low 0 atom concentrations were utilized so that secondary reactions
were
unimportant
and
no correction
for
stoichiometry was necessary. Disagreement exists between previous absolute rate measurements and our present results over the temperature rsnge of this study. For example at 841 K using the data of Albers et al. [6], a value of X-t = 69 X IO-l4 is derived. This value is a factor of 7-5 less than the present value, which is significantly outside the estimated combined error limits of both experiments_ In the case of Kurylo et al. [7], an extrapolation of their data to 841 K gives a value which is 35% lower than the present value. Although the difference is within the combined error limits of both studies at 840 K, the discrepancy is more pronounced at 448 K where a value derived from the data of Kurylo et al. is a factor of 25 times higher than the present value. It is worth noting that the Arrhenius expression (table 2) reported by Aganesyan [ 131, is in resonable agreement with the present data. They utilized an indirect method for dere rmining the rate of reaction (l), and tie agreement with the present results is quite remarkable considering the complexity of their analysis. Although the other major low-pressure techniques [ :-51 give reasonably consistent rates for reaction (l), calculations for the hear of the reaction [ 141 give AH = 6.6 kcal, nearly 3 kcal below the activation energy obtained in these studies. Thus, the accuracy of these earlier measurements of the activation energy for reaction (1) must be questioned based on the thermochemistry.
Volume
106, number
CHEMLCAL PHYSICS
3
Table 2 Comparison of pre-exponential data with this work
factors,A,
and activation
lot* A (cm3 molecule-’
energies,
Ea (cai/mole)
S-’ )
20 April 1984
LETI-ERS
Ea. for reaction
of 0(3P)
with ammonia
for selected
literature
Technique ‘)
Ref.
34.2 2.5
9000
present work
6000
I61
6.6
6600
I71
8.2
6140
I81
DF hlS
1.7
4800
191
SR MS
17
6600
IlO1
ST
32
10000
IllI
ILM DF PA ST
DF TOFMS DF ESR/hIS DF ESR
5.0
0.0184 k(1950)
K) =
ai DF TOFMS: discharge flow-time~f-flight troscopy,
DF MS: discharge
flow-mass
3.50 x lo-‘*
-’
1121 I131
mass spectroscopy, DF ESR/MS: discharge flow-electron spectroscopy. ST: shock tube, ILhI: ignition limit modeling.
spin resonance/mass specDF PA: discharge flow-
product analysis. An alternative
monia reaction
for oxygen atom-amcan also be written:. mechanism
0(3P) + NH, + ONH;*, ONH;*
+ M + ONH, + M,
ONH;*
+ NH20H*,
NH20H*
+ NH, + OH,
NH,OH*
+ M + NH,OH
LYI = -27
kcal/mole,
AH = 6.6 kcal/mole, + M,
AH =
-58
kcal/mole.
This mecmm, which is analogous to the mechanism proposed for oxygen atom attack on substituted amines by Atkinson et al. [15]-and confirmed by Slagle et al. [ 161, involves a spin change to form the ground-state singlet NH,0 or the tautomer hydroxylamine (NH?OH). The triplet-NH30 formed initially is calculated to lie on a repulsive surface [ 171, and hence it is unlikely that an addition complex is formed for the reaction of O(3P) with NH,, although such a complex may well form for substituted amines. Note that it is not possible to exclude this alternative reaction mechanism solely on thermodynamic arguments without further information on the thermochemistry of the triplet NH30 species. Calculations indicate, however, that a significantly higher activation energy would be anticipated 1181. 226
It is important in any measurement of elementary reaction rates to avoid interfering effects of other competing reactions. In this study we have considered the possibility that other atom-radical or atom-molecule reactions other than reaction (1) might contribute to the observed decay of O(3P) atoms. In particular, at low temperatures where the rate of reaction (1) is slow, other competing reactions such as O(3P) + NH, + products
(3)
might become important if the NH? concentration becomes significant_ In order to consider the importance of reaction (3) one needs to estimate the NH-, concentration in the mixture due to the photolysis Gf ammonia_ The initial NH, concentration can be related to the initial 0 atom concentration by the following expression:
[NH,]0
= [0(3P)]o
where [NH&,
[02]i’02
iNH3]i ‘NH3 @NH3 eo2 + [NOIi‘NO@NO ’
W(3P)1~ are the NH, and O(3P) concentration immediately after the laser pulse at time 0; [O,]i. [NO]i, and [NH3]i are the initial concentrations of &, NO, and NH,; eo2, ENH, , ENO are the respective extinction coefficients and do,, hoI hH3 are
Volume
106. number
3
CHEMICAL
PflYSICS
the quantum efficiencies for photodissociation at the laser frequency. From this equation the NH, concentration can be estimated to be as high as IOf? at 448 K for 0(3P) concentrations equal to 1011 molecule cm-x. This is a significant concentration of NH,, i.e. under these conditions reaction (3) would contribute up to 17% to the loss of 0 atoms at 448 K if these initial 0(3P) concentrations are utilized. Care was exercised to work at low 0(3P) atom concentrations, i.e. low laser intensities, to reduce the importance of reaction (3). It may be possible that the slight curvature in the Arrhenius plot is the result of interference from photolysis of ammonia, although no dependence on laser intensity was observed in the measured rate coefficient for the experimental conditions used. Note that photolysis of NH, along with the decreasing rate for reaction (1) preclude additional experiments at lower temperatures. At higher temperatures, where the ammonia concentration is significantly lower, initial NH, concentrations are produced, and reaction (3) is not important. Although at higher temperatures reactions such as H+O1+M+HO,
+M,
(4)
H+O,+OH+O,
(5)
O+HO, - +O, _ +OH,
(6)
potentially become important, at the temperatures, pressures and partial pressures of H atoms and 0, expected it is readily shown that reactions (4)-(6) will cause negligible impact upon reaction of interest. In particular, the reaction H+NO+M+HNO+M,
A./Y = -50
kcal/mole,
(7)
dominates reaction (5) (4 : 1) for the conditions found in this experiment at 840 K, i.e. nitric oxide and oxygen concentrations of 3 X lOI and 4 X !O14 molecule crns3, respectively. Those oxygen atoms regenerated are formed on a time scale which is long in comparison with the experiment and, hence, would cause negligible error in the measured high-temperature rate constant_ This is also supported by the linearity of the plots shown in figs. 1 and 2 for variations of factors of 5 in the ammonia concentration and by the observation that a factor of 10 variation in the laser intensity produced no change in the rate constants within the experimental errors.
20.4pril
LE’lTERS
1984
One major difference between this experiment and past experiments is that no stoichiometric correction is required in the present experiment_ As indicated in table 2, a stoichiometric correction based upon assumptions about reaction mechanisms is required to determine /cl from experimental loss of 0 atoms in past esperiments. Problems with determining the temperature dependence of the stoichiometric correction factor could lead to erroneous conclusions regarding the activation energy of the reaction in past experiments. Using the Arrhenius expression derived from the present data one fiids that there is reasonable agreement with the recently reported high-temperature shock-tube data of Salimian et al. [ 13]_ Using the present data to estrapolate to higher temperatures indicates that at 2100 K as much as 25% of the consumption of ammonia in lean ammonia/oxygen flames results from reaction (1) (the other 75% occurring throug the reaction of OH with ammonia) [ 191.
5. Conclusions Laser photolysis-chemiluminescence provides a powerful technique for studying reactions which have been difficult to study previously using conventional discharge flow techniques. The rate constant for the reaction of O(3P) with ammonia has a higher activation ener,7 and pre-esponential factor than previous measurements indicate. Agreement esist between this work and predictions based upon empirical arguments. Also, the present work supports the thermochemistry for reaction (1) (Lw = 6.6 kcal/mole) and the recent shock tube results [ 13]_ Finally. it appears that reaction (1) must be considered in high-temperature. lean ammonia oxidation where 0 atoms atrack on NHj \viU compete with the OH radicals attack on ammonia.
References [ 11 R.R. Lyon and DJ. Ban,
17tb Symposium (lnrcrnaCombustion (The Combusrion Institute. Pittsburgh, 1979) p_ 601. [Z] R.K. Lyon, Intern. J.Chcm. Kim% 8 (1976) 315. (31 JA. Blikr. MC. Branch and R J. Kee, Cornbust. Flame -I3(1961) 81. (-I] C. hlorlcy, 1Sth Symposium (International) on Combustion (The Combustion In~rirurs, PirtsburFh, 198 1) p. 23. tiond)
on
227
Volume 106, number 3
CHEMICAL PHYSICS LETTERS
(51 J.A. Miller, M.D. Smooke. RM. Green and R.J. Kee, Cornbust. Sci. Tech., (1983), to be published; Sandia Report SAND82-8609. [6] EA. A1bers.K. Hoyermann.H.Gg. Wagner and J. Wolfrum. 12th Symposium (l’nternational) on Combustion (TheCombustion Institute. Pittsburgh. 1969) p_ 313. [7] M J; Kurylo,CA. Hollinden, H.F. LeFevre and R.B. Timmons. J.Chem.Phys. Sl(l969) 4497. [ 81 K. Kirchner, N. Merget and C. Schmidt,Chem. Ing. Tech. 46 (1974) 661; K. Kirchner, Intern. J. Chem. Kine!. Symp. 1 (1975) 103. [ 91 EL. Wong and AE. Potter, J. Chem. Phys. 39 (1963) 2211:43(1965) 3371. (IO] J.E. Dove and WS. Nip.Can. J.Chem. 52 (1974) 1171. [ 111 K.T. Aganesyan and AB. Nalbandyan. Dokl. Akad. Nauk SSSR 160 (1965) 162.
228
20 .April 1984
1121 L_i. Avramcnko. R.V. Kolesnikova and NJ_. Kuznctsova. Izv. Akad. Nauk SSSR Otd. Khim. Nauk. 6 (1962) 983. 113) S. Salimian. R.K. Hanson and CH. Kruger. prcsenled at Wcstem States Section/Combustion Institute. paper No. WSS/Cl83-38. Pasadena.Califomia (1983). ( 141 S.W. Benson. Thermochemical kinetics. 2nd Ed. (Wiley. New York, 1976). 1151 R. Atkinson and IN. PittsJr., J.Chem. Phys. 68 (1978) 911. 1161 IX. S1agle.J.R. Dudich and D. Gutman. J. Phys. Chcm. 83 (1979) 3065. 1171 BJ.Hart,AustralianJ.Chcm.29(1976)231. [ 181 C.F. hlelius. private communication. 1191 JA. Miller. private communicdon.
CHEMICAL
PHYSICS
LEl-l-ERS
20
April 1984
ONTHERATEOFTHEREACTlONOFAMMONlAWITHOXYGENATOMS* Robert A. PERRY Sandia
ffariotzal
Laboratories.
Livermore.
California
945.50,
USA
Received 2 December 1983; in tinal form 25 January 1984
Absolute rate constanls for the reaction of 0c3P) with ammonia wcrc measured over rhe tcmperaturr ran_ee 4-18-841 K by means of a lnscr photolysis-chrmilumincscence technique. The data were fitted with the following Arrhenius expression: k = 3.42 x lo-” cxp( -(9000 + 600 cal/mole)/RT] cm3 moIccule~ 5’ _Corn parisons with literarure data and predictions based upon ihc extrapolations 10 higher tcmpcraiures round in combusiion arc presentcd.Thc cstmpolations 3re in agreement with recent shock-tube data and indicllle that rhis reaction is more important in high-temperature ammonia oyidsrion than previously believed.
l_ Introduction Major progress has been made in modeling nitrogen chemistry to explain phenomena such as the Exxon “Thermal De-NOx” process and ammonia oxidation in combustion systems [l--5]. Srill, many uncertainties remain regarding of the reactions which are utilized in the models. Based upon empirical or theoretical arguments one might expect an important reaction in hightemperature lean ammonia/oxygen systems to be:
O+NH,-+OH+NH,,
AH = 6.6 kcal/mole.
(1)
However, previous experimental results indicate that this reaction is slow even at moderately high temperatures [6-IO]. Although various absolute rate measurements have been made for reaction (1) [6-g], all of these measurements required subsequent modeling of the reaction mechanism in order to correct for stoichiometry effects in the experiments. While the various experimental values obtained have apparently been reconciled [7], a disparity still exists between the experimental results and calculations based upon theoretical or empirical considerations [6]. Also, given that the heat of the reaction is 6.6 kcal/mole, it is difficult to reconcile the past results which give an activation enQ Work supported by Office partment of Energy.
of Basic Energy Science,
US De-
0 009-2614/84/S 03.00 0 Eiscvier Science Publishers (North-Holland Physics Publishing Division)
B.V.
ergy which is less than the endothermicity of the reaction. In recent high-temperature shock-rube esperimenus [ 1 I] the reaction was found to proceed substantially faster than extrapolations from low-temperature experiments would indicate. In order to resolve the order of magnitude discrepancy between calculated and experimental values for the rate of reactions, and to resolve fully the difference between recent shock-tube experimenrs and the literature, experiments were performed to measure this reaction rate using a laser photolysis-NO1 chemiiuminescence technique. The results of these esperiments are described and the implications for the nCechanism of ammonia oxidation are discussed_
2. ExperimentsI
O(jP) atoms were produced in a low-pressure reacrion vessel by the pulsed laser photolysis of 0, (=0.030.04 Torr) and NO (-0.09-0.26 Torr) u&~g a F., estimer laser (157 nm). The laser was operated at pulse energies of =I rnJ/pulse with the laser intensity in the reactor being adjusted by varying the flow rate of the nitrogen purge gas situated between the laser and the reaction vessel. O(jP) atom concentrations were monitored as a function of time after the pulse by using an EM1 9789@A photomultiplier fitted with an interfer223
Volume 106, number 3
CHEMICAL PHYSICS LETTERS
ence filter (center wavelength 5 145 A with a full-width half-maximum bandpass of 100 A) to monitor NO2 chemiluminescence from the reaction O+NO+M+NO;+M, NO; + NO2 + Iw.
(2)
Decays of the NO, chemiluminescence, and hence of O(!P) atom concentrations, were accumulated from 25-800 laser pulses, depending on signal strengths at a repetition rate of about 03 Hz. O(3P) atom half-lives ranged from 2.86 to 18.2 ms, and in all cases the O(3P) decay was monitored over at least 3 half-lives. Signals were obtained by photon counting in conjunction with multichannel scaling. The reactor consisted of a quartz reaction vessel. enclosed in a ceramic furnace which could be heated to 1200 K. The temperature of the furnace was monitored by chromel/alumel thermocouples mounted inside the reaction vessel with the temperature of the vessel maintained to 5 K over the entire temperature range. All experiments were carried out under slow flow conditions so that the premixed reactant gas mixture could be replenished between laser pulses, thereby avoiding the accumulation of photolysis or reaction products. The gases used had the following purity levels according to the manufacturer: Ar > 99.995%; O2 > 995%; NO > 99.0%; NH, > 99.998%. NO was passed through a Matheson 13X gas filter to remove any H,O or NO, present.
3. Results
20 April 1984
ties, k. is the first-order rate coefficient for removal of 0(3P) atom in absence of added ammonia and NO (due to diffusion out of viewing zone and reaction with impurities and 02) and k 1 and k, are the rate constants for the reactions (1) and (2). respectively. The decay was analyzed following a xl ms delay after the excimer laser pulse to minimize interference from laser induced emission from the quartz vessel and to ensure that secondary reactions of other atomic species formed during the photolysis event, i.e. N(4S) and 0( 1D), would not interfere with the reaction of interest. Under the conditions used in this experiment N(4S) has a half-life of ~220 ps due to the reaction with NO and O,, while O(1D) is quenched by Ar with a halflife of =3 j.~s_ In all experiments exponential decays of the NO, chemiluminescence signals were observed. The data were analyzed by numerical least-squares fitting of the decays using a VAX 1 l/780, which was directly interfaced to a Nuclear Data (ND66) multichannel analyzer. The measured decay rates were observed to depend linearly on the concentration of added ammonia for fvted total pressure and fixed NO concentration. The absolute rate constants were determined by plotting the measured decay rate against ammonia concentration and performing a least-squares fit to obtain the slope, k, . Figs. 1 and 2 are the plots of the data for 0 + NH, at the indicated temmperatures. Table 1 lists the measured rate constants while fig. 3 shows the corresponding Arrhenius plot of the data. Least-squares analysis of the data yields the 400
1
The reaction of O(3P) atoms with ammonia was studied over the temperature range 448-841 K with 75 Torr of argon as a diluent gas. In all the experiments conditions were chosen so that the decay oi NO, chemiluminescence, and consequently O(3P) atom concentrations, was exponential. Rates were determined using the expression
w[0~3p)l(J/P~3p~lr) = ‘“P&) = W. +$ [NH31‘-$ [NOI WI) (I - loh where [O(3P)]o and [0(3P)]t are the concentration of O(3P) atoms at t&s to and c respectively, So and S, are the corresponding NO? chemiluminescence intensi224
0
l !& 1
&,I (molecule
crZ$
Fig. 1. Plot of the O(“P) atom decay rate against NH3 concentration at 448 and 541 K.
Volume
106. number
CHEMICAL
3
PHYSICS
20 April
LETTERS
Arrhenius
1984
expression
k, = 3.42 X 10-l’ cm3 molecule-’
exp[-(9000
_+600 cal/mole)/f?T]
s-l,
where the indicated error limits include the standard deviation (415 cal) from the least-squares analysis and the estimated uncertainty due to gas composition (GS%).
01 2
0
8
10
4. Discussion
-10” Fig. 2. Plots of the O(3P) atom centration at 662 and 8: 1 K.
decay
rate against
NH3 con-
Table 1 hleasurcd rate constanr,k, for the reaction of 0(3P) \vith ammonia. Indicated errors arc estimates to overall error limits which include the least-squares standard deviation, l-2.54,
as well as estimated accuracy limits due to gas composition uncertainty lot4
k (cm3
molecules’
s-l)
0.159(~0.012)
Temperature(K) 448
0.673(+0.040)
541
3.59(+0.25)
662
17.1(+ 1.0)
831
lo '
lo-‘” 1 1 Fig. 3. Arrhcnius
I
,
,
125
150
/ 1.75
I 2
225
1000/T (K) plot of log X-1 against 1000/T(K)
2.50
for NH3.
Table 2 lists various literature values for k, obtained by both direct and indirect measurement techniques. It should be noted that all previous measurements were made under conditions where detailed modeling of the reaction was necessary to derive the rate constant. In the present experiment, however, sufficiently low 0 atom concentrations were utilized so that secondary reactions
were
unimportant
and
no correction
for
stoichiometry was necessary. Disagreement exists between previous absolute rate measurements and our present results over the temperature rsnge of this study. For example at 841 K using the data of Albers et al. [6], a value of X-t = 69 X IO-l4 is derived. This value is a factor of 7-5 less than the present value, which is significantly outside the estimated combined error limits of both experiments_ In the case of Kurylo et al. [7], an extrapolation of their data to 841 K gives a value which is 35% lower than the present value. Although the difference is within the combined error limits of both studies at 840 K, the discrepancy is more pronounced at 448 K where a value derived from the data of Kurylo et al. is a factor of 25 times higher than the present value. It is worth noting that the Arrhenius expression (table 2) reported by Aganesyan [ 131, is in resonable agreement with the present data. They utilized an indirect method for dere rmining the rate of reaction (l), and tie agreement with the present results is quite remarkable considering the complexity of their analysis. Although the other major low-pressure techniques [ :-51 give reasonably consistent rates for reaction (l), calculations for the hear of the reaction [ 141 give AH = 6.6 kcal, nearly 3 kcal below the activation energy obtained in these studies. Thus, the accuracy of these earlier measurements of the activation energy for reaction (1) must be questioned based on the thermochemistry.
Volume
106, number
CHEMLCAL PHYSICS
3
Table 2 Comparison of pre-exponential data with this work
factors,A,
and activation
lot* A (cm3 molecule-’
energies,
Ea (cai/mole)
S-’ )
20 April 1984
LETI-ERS
Ea. for reaction
of 0(3P)
with ammonia
for selected
literature
Technique ‘)
Ref.
34.2 2.5
9000
present work
6000
I61
6.6
6600
I71
8.2
6140
I81
DF hlS
1.7
4800
191
SR MS
17
6600
IlO1
ST
32
10000
IllI
ILM DF PA ST
DF TOFMS DF ESR/hIS DF ESR
5.0
0.0184 k(1950)
K) =
ai DF TOFMS: discharge flow-time~f-flight troscopy,
DF MS: discharge
flow-mass
3.50 x lo-‘*
-’
1121 I131
mass spectroscopy, DF ESR/MS: discharge flow-electron spectroscopy. ST: shock tube, ILhI: ignition limit modeling.
spin resonance/mass specDF PA: discharge flow-
product analysis. An alternative
monia reaction
for oxygen atom-amcan also be written:. mechanism
0(3P) + NH, + ONH;*, ONH;*
+ M + ONH, + M,
ONH;*
+ NH20H*,
NH20H*
+ NH, + OH,
NH,OH*
+ M + NH,OH
LYI = -27
kcal/mole,
AH = 6.6 kcal/mole, + M,
AH =
-58
kcal/mole.
This mecmm, which is analogous to the mechanism proposed for oxygen atom attack on substituted amines by Atkinson et al. [15]-and confirmed by Slagle et al. [ 161, involves a spin change to form the ground-state singlet NH,0 or the tautomer hydroxylamine (NH?OH). The triplet-NH30 formed initially is calculated to lie on a repulsive surface [ 171, and hence it is unlikely that an addition complex is formed for the reaction of O(3P) with NH,, although such a complex may well form for substituted amines. Note that it is not possible to exclude this alternative reaction mechanism solely on thermodynamic arguments without further information on the thermochemistry of the triplet NH30 species. Calculations indicate, however, that a significantly higher activation energy would be anticipated 1181. 226
It is important in any measurement of elementary reaction rates to avoid interfering effects of other competing reactions. In this study we have considered the possibility that other atom-radical or atom-molecule reactions other than reaction (1) might contribute to the observed decay of O(3P) atoms. In particular, at low temperatures where the rate of reaction (1) is slow, other competing reactions such as O(3P) + NH, + products
(3)
might become important if the NH? concentration becomes significant_ In order to consider the importance of reaction (3) one needs to estimate the NH-, concentration in the mixture due to the photolysis Gf ammonia_ The initial NH, concentration can be related to the initial 0 atom concentration by the following expression:
[NH,]0
= [0(3P)]o
where [NH&,
[02]i’02
iNH3]i ‘NH3 @NH3 eo2 + [NOIi‘NO@NO ’
W(3P)1~ are the NH, and O(3P) concentration immediately after the laser pulse at time 0; [O,]i. [NO]i, and [NH3]i are the initial concentrations of &, NO, and NH,; eo2, ENH, , ENO are the respective extinction coefficients and do,, hoI hH3 are
Volume
106. number
3
CHEMICAL
PflYSICS
the quantum efficiencies for photodissociation at the laser frequency. From this equation the NH, concentration can be estimated to be as high as IOf? at 448 K for 0(3P) concentrations equal to 1011 molecule cm-x. This is a significant concentration of NH,, i.e. under these conditions reaction (3) would contribute up to 17% to the loss of 0 atoms at 448 K if these initial 0(3P) concentrations are utilized. Care was exercised to work at low 0(3P) atom concentrations, i.e. low laser intensities, to reduce the importance of reaction (3). It may be possible that the slight curvature in the Arrhenius plot is the result of interference from photolysis of ammonia, although no dependence on laser intensity was observed in the measured rate coefficient for the experimental conditions used. Note that photolysis of NH, along with the decreasing rate for reaction (1) preclude additional experiments at lower temperatures. At higher temperatures, where the ammonia concentration is significantly lower, initial NH, concentrations are produced, and reaction (3) is not important. Although at higher temperatures reactions such as H+O1+M+HO,
+M,
(4)
H+O,+OH+O,
(5)
O+HO, - +O, _ +OH,
(6)
potentially become important, at the temperatures, pressures and partial pressures of H atoms and 0, expected it is readily shown that reactions (4)-(6) will cause negligible impact upon reaction of interest. In particular, the reaction H+NO+M+HNO+M,
A./Y = -50
kcal/mole,
(7)
dominates reaction (5) (4 : 1) for the conditions found in this experiment at 840 K, i.e. nitric oxide and oxygen concentrations of 3 X lOI and 4 X !O14 molecule crns3, respectively. Those oxygen atoms regenerated are formed on a time scale which is long in comparison with the experiment and, hence, would cause negligible error in the measured high-temperature rate constant_ This is also supported by the linearity of the plots shown in figs. 1 and 2 for variations of factors of 5 in the ammonia concentration and by the observation that a factor of 10 variation in the laser intensity produced no change in the rate constants within the experimental errors.
20.4pril
LE’lTERS
1984
One major difference between this experiment and past experiments is that no stoichiometric correction is required in the present experiment_ As indicated in table 2, a stoichiometric correction based upon assumptions about reaction mechanisms is required to determine /cl from experimental loss of 0 atoms in past esperiments. Problems with determining the temperature dependence of the stoichiometric correction factor could lead to erroneous conclusions regarding the activation energy of the reaction in past experiments. Using the Arrhenius expression derived from the present data one fiids that there is reasonable agreement with the recently reported high-temperature shock-tube data of Salimian et al. [ 13]_ Using the present data to estrapolate to higher temperatures indicates that at 2100 K as much as 25% of the consumption of ammonia in lean ammonia/oxygen flames results from reaction (1) (the other 75% occurring throug the reaction of OH with ammonia) [ 191.
5. Conclusions Laser photolysis-chemiluminescence provides a powerful technique for studying reactions which have been difficult to study previously using conventional discharge flow techniques. The rate constant for the reaction of O(3P) with ammonia has a higher activation ener,7 and pre-esponential factor than previous measurements indicate. Agreement esist between this work and predictions based upon empirical arguments. Also, the present work supports the thermochemistry for reaction (1) (Lw = 6.6 kcal/mole) and the recent shock tube results [ 13]_ Finally. it appears that reaction (1) must be considered in high-temperature. lean ammonia oxidation where 0 atoms atrack on NHj \viU compete with the OH radicals attack on ammonia.
References [ 11 R.R. Lyon and DJ. Ban,
17tb Symposium (lnrcrnaCombustion (The Combusrion Institute. Pittsburgh, 1979) p_ 601. [Z] R.K. Lyon, Intern. J.Chcm. Kim% 8 (1976) 315. (31 JA. Blikr. MC. Branch and R J. Kee, Cornbust. Flame -I3(1961) 81. (-I] C. hlorlcy, 1Sth Symposium (International) on Combustion (The Combustion In~rirurs, PirtsburFh, 198 1) p. 23. tiond)
on
227
Volume 106, number 3
CHEMICAL PHYSICS LETTERS
(51 J.A. Miller, M.D. Smooke. RM. Green and R.J. Kee, Cornbust. Sci. Tech., (1983), to be published; Sandia Report SAND82-8609. [6] EA. A1bers.K. Hoyermann.H.Gg. Wagner and J. Wolfrum. 12th Symposium (l’nternational) on Combustion (TheCombustion Institute. Pittsburgh. 1969) p_ 313. [7] M J; Kurylo,CA. Hollinden, H.F. LeFevre and R.B. Timmons. J.Chem.Phys. Sl(l969) 4497. [ 81 K. Kirchner, N. Merget and C. Schmidt,Chem. Ing. Tech. 46 (1974) 661; K. Kirchner, Intern. J. Chem. Kine!. Symp. 1 (1975) 103. [ 91 EL. Wong and AE. Potter, J. Chem. Phys. 39 (1963) 2211:43(1965) 3371. (IO] J.E. Dove and WS. Nip.Can. J.Chem. 52 (1974) 1171. [ 111 K.T. Aganesyan and AB. Nalbandyan. Dokl. Akad. Nauk SSSR 160 (1965) 162.
228
20 .April 1984
1121 L_i. Avramcnko. R.V. Kolesnikova and NJ_. Kuznctsova. Izv. Akad. Nauk SSSR Otd. Khim. Nauk. 6 (1962) 983. 113) S. Salimian. R.K. Hanson and CH. Kruger. prcsenled at Wcstem States Section/Combustion Institute. paper No. WSS/Cl83-38. Pasadena.Califomia (1983). ( 141 S.W. Benson. Thermochemical kinetics. 2nd Ed. (Wiley. New York, 1976). 1151 R. Atkinson and IN. PittsJr., J.Chem. Phys. 68 (1978) 911. 1161 IX. S1agle.J.R. Dudich and D. Gutman. J. Phys. Chcm. 83 (1979) 3065. 1171 BJ.Hart,AustralianJ.Chcm.29(1976)231. [ 181 C.F. hlelius. private communication. 1191 JA. Miller. private communicdon.