Full text of DOI:10.1021/jp990954f

8906
J. Phys. Chem. A 1999, 103, 8906-8907
COMMENTS
Product Branching Ratios in the NH2 + NO
Reaction: A Re-Evaluation
J. Park and M. C. Lin*
Department of Chemistry, Emory UniVersity,
Atlanta, Georgia 30322
ReceiVed: March 18, 1999; In Final Form:
September 7, 1999
NH2 + NO is a critical reaction in the thermal reduction of
1
NOx by the Exxon process in which NH3 is employed as the
reducing agent. The key reaction responsible for the reduction
is NH2 + NO which can occur by the following two major
channels:
k1a
NH + NO
9
k1b
8 HN + OH
(1a)
(1b)
2
2
98 N2 + H O
2
The reaction has been studied by many investigators2-24 whose
contributions to the kinetic and mechanistic aspects of the
reaction have been cited and reviewed in our earlier publica-
Figure 1. Arrehnius plot of the rate constant for the NH
reaction: solid curve (ref 28); dashed line (ref 32); dotted line (ref
2
2
+ H O
2
3-26
tions.
The key controversy remains in the effect of
3
2
3); dash-dotted curve (ref 34); dash-dot-dotted curve (ref 40); ref
7 (b); ref 35 (4); ref 36 (3); ref 37 (0); ref 38 (×); ref 39 ([).
temperature on the branching ratio for the radical formation
channel (1a), R1a, above 1000 K. This product channel plays
the most important role in the NH3 deNOx reaction because it
generates almost simultaneously the two key chain carriers, OH
and H; the latter can be readily formed by the decomposition
of the transient species HN2 at temperatures above 1000 K.
In our latest paper published in this journal on the NH2 +
(Inset) Arrhenius plot of the OH + NH
3
f NH
2
2
+ H O reaction: solid
line (ref 27); dashed curve (ref 30); dotted line (extrapolation of ref
27).
good correlation between the predicted k-2 and experimental
values, including those for OH + NH3 converted to k-2 with
2
4
NO reaction, we concluded that R1a increased rapidly above
000 K and merged effectively with the large branching ratio
-
2
0.53
the equilibrium constant, K-2 ) 2.68 × 10
T
exp(5846/
1
2
8
21
T). The employment of k-2, instead of Diau’s k2 given above,
reduces the values of R1a by about 20% near 1200 K.
reported by Vandooren et al. from their modeling of the
ammonia flame speed, R1a > 0.5, above 1500 K. Our conclusion
was based on the kinetically modeled NO decay rates measured
by the pulsed laser photolysis/mass spectrometry24 and the NH3
and NO decay rates determined in the pyrolysis/FTIR spec-
Figure 2 summarizes our newly modeled results and the
existing data on R1a, including the most recent result of
29
Votsmeier et al. As shown in the figure, the new values
20
trometry.
obtained by using the theoretically calculated k-2 agree closely
In both modeling studies,20,24 we employed the rate constant
for another influential process which regenerates the NH2
radical,
2
9
with those reported by Votsmeier et al. and the kinetically
3
0
modeled results of Glarborg and co-workers. To test the
generality of the mechanism employed and the sensitivity of
R1a to k2 or k-2, we have also used the forward rate constant
OH + NH f NH + H O
(2)
3
2
2
4
,30
adopted by Glarborg et al. in their modeling,
10 T
k2 ) 2.00 ×
exp(-285/T) cm /(mol s); the values of R1a thus
obtained agree exactly with those evaluated with our k . A
reported by Diau et al.,27 k2 ) 1.98 × 10 exp(-921/T) cm3/
12
6
2.04
3
(mol s), for the temperature range 273-433 K. The absolute
-
2
value of the rate constant, although agreeing closely with others
for the same temperature range, deviates from those determined
at higher temperatures on account of the presence of a strong
upward curvature above 1000 K as illustrated in the inset of
Figure 1. The existence of the curvature is consistent with our
theoretically predicted result for the reverse process, NH2 +
least-squares analysis of R1a based on our data (300-1200 K)
and those of Glarborg and co-workers (1200-1370 K) and
Votsmeier et al. (1330-1670 K) gives rise to the expression,
-3
-4
-7
2
R1a ) 9.69 × 10 - 1.31 × 10 T + 3.96 × 10 T - 9.72
10 T .
-11
3
×
3
2.94
3
On the basis of our total rate constant for the NH + NO
H2O f OH + NH3, k-2 ) 2.33 × 10 T exp(-5566/T) cm /
2
2
4
(mol s), computed with the transition state theory (TST) with
reaction and the values of R1a given above, the absolute rate
constants for the branching channels 1a and 1b can be
represented by
tunneling corrections using the TST parameters obtained by a
G2M/B3LYP/6-311G(d,p) calculation.28 Figure 1 shows the
1
0.1021/jp990954f CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/15/1999

Comments
J. Phys. Chem. A, Vol. 103, No. 44, 1999 8907
(
(
(
7) Caton, J. A.; Siebers, D. L. Combust. Sci. Technol. 1989, 65, 277.
8) Lyon, R. K.; Cole, J. A. Combust. Flame 1990, 82, 435.
9) Miller, J. A.; C. T. Bowman, Int. J. Chem. Kinet. 1991, 23, 289.
(
10) Mertens, J. D.; Chang, A. Y.; Hanson, R. K.; Bowman, C. T. Int.
J. Chem. Kinet. 1991, 23, 173.
(
11) Silver, J. A.; Kolb, C. E. J. Phys. Chem. 1982, 86, 3240.
(12) Stief, L. J.; Brobst, W. D.; Nava, D. F.; Borkowski, R. P.; Michael,
J. V. J. Chem. Soc., Faraday Trans. 2 1982, 78, 1391.
13) Hack, W.; Schake, H.; Schr o¨ ter, H.; Wagner, H. Gg. 17th
Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh,
979; p 505.
14) Diau, E. W. G.; Yu, T.; Wagner, M. A. G.; Lin, M. C. J. Phys.
Chem. 1994, 98, 4034.
15) Lesclaux, R.; Kh eˆ , Pham Van; Dezauzier, P.; Soulignac, J. C. Chem.
Phys. Lett. 1975, 35, 493.
16) Bulatov, V. P.; Ioffe, A. A.; Lozovsky, V. A.; Sarkisov, O. M.
Chem. Phys. Lett. 1989, 161, 141.
17) Atakan, B.; Jacobs, A.; Wahl, M.; Weller, R.; Wolfrum, J. Chem.
Phys. Lett. 1989, 155, 609.
18) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just,
(
1
(
(
(
(
(
Th.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J. Phys.
Chem. Ref. Data 1992, 21, 411.
(
19) Stephens, J. W.; Morter, C. L.; Farhat, S. K.; Glass, G. P.; Curl, R.
F. J. Phys. Chem. 1993, 97, 8944.
20) Halbgewachs, M. J.; Diau, E. W. G.; Mebel, A. M.; Lin, M. C.;
(
Melius, C. F. 26th Symposium (Int.) on Combustion; The Combustion
Institute: Pittsburgh, 1996; p 2109.
Figure 2. Branching ratio for NH
function of temperature: this work’s remodeled result of ref 24 (O);
remodeled result of ref 20 (9); ref 30 (3); ref 29 (0); ref 21 (+); ref
2
+ NO f HN + OH (R1a) as a
2
(
21) Vandooren, J.; Bian, J.; van Tiggelen, P. J. Combust. Flame 1994,
402.
(22) Brown, M. J.; Smith, D. B. 25th Symposium (Int.) on Combustion;
3
2
(4); dash-dotted line (ref 17); dotted line (ref 19); dashed line (ref
4); solid line, the result of a least-squares analysis for R1a based on
The Combustion Institute: Pittsburgh, 1994; p 1011.
(23) Park, J.; Lin, M. C. J. Phys. Chem. 1996, 100, 3317.
(
(
24) Park. J.; Lin, M. C. J. Phys. Chem. A 1997, 101, 5.
25) Park, J.; Lin, M. C. Recent Research DeVelopment in Physical
this work and those of refs 28 and 30.
7
1.40
3
Chemistry; Transworld Research Network: India, 1998; Vol. 2, p 965.
(26) Mebel, A. M.; Lin, M. C. Int. ReV. Phys. Chem. 1997, 16, 249.
k ) 1.43 × 10 T exp(894/T) cm /(mol s)
1
a
(27) Diau, E. W. G.; Tso, T. L.; Lee, Y. P. J. Phys. Chem. 1990, 94,
17
-1.61
3
k ) 1.20 × 10 T
exp(-150/T) cm /(mol s).
5261.
1
b
(28) Mebel, A. M.; Moskaleva. L. V.; Lin, M. C. THEOCHEM 1999,
4
61-462, 223.
It should be noted that our low temperature (T < 1000 K)
data were not affected significantly by the upward curvature of
k2 or k-2 and that the H2O formation data reported by Poole
and Graven31 from their pyrolytic study of NH3 + NO could
no longer be sensitively modeled for R1a.
(29) Votsmeier, M.; Song, S.; Hanson, R. K.; Bowman, C. T. J. Phys.
Chem. A 1999, 103 (11), 1566.
30) Glarborg, P.; Kristensen, P. G.; Dam-Johansen, K.; Miller, J. A. J.
Phys. Chem. A 1997, 101, 3741.
31) Poole, D. R.; Graven, W. M. J. Am. Chem. Soc. 1961, 83, 283.
(
(
(32) Espinosa-Garcia, J.; Corchado, J. C.; Sana, M. J. Chem. Phys. 1993,
0, 1181.
9
(
33) Hack, W.; Hoyermann, K.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys.
Chem. 1974, 78, 386.
34) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
Acknowledgment. The authors are grateful to the Office of
Naval Research for support of this work, under the direction of
Dr. J. Goldwasser through ONR Grant N00014-89-J-1949.
(
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina,
M. J. Chemical kinetics and photochemical data for use in stratospheric
modeling. Evaluation number 12; JPL Publication 97-4, 1997.
References and Notes
(
35) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr,
J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data 1997, 26, 1329-
499.
(36) Pagsberg, P. B.; Eriksen, J.; Christensen, H. C. J. Phys. Chem. 1979,
83, 582.
(37) Zabielski, M. F.; Seery, D. J. Int. J. Chem. Kinet. 1985, 17, 1191.
(
1) Lyon, R. K. Int. J. Chem. Kinet. 1976, 8, 318. U. S. patent
1
3
8
3
,900,554.
(2) Miller, J. A.; Branch, M. C.; Kee, R. J. Combust. Flame 1981, 43,
1.
(
77.
3) Kimball-Linne, M. A.; Hanson, R. K. Combust. Flame 1986, 64,
(38) Hanson, R. K.; Salimian, S. In Combustion Chemistry; Gardiner,
W. C., Jr., Ed.; Springer-Verlag: New York, 1984.
(39) Salimian, S.; Hanson, R. K.; Kruger, C. H. Int. J. Chem. Kinet.
1984, 16, 725.
(4) Glarborg, P.; Dam-Johansen, K.; Miller, J. A.; Kee, R. J.; Coltrin,
M. E. Int. J. Chem. Kinet. 1994, 26, 421.
(5) Perry, R. A.; Siebers, D. L. Nature 1986, 324, 657.
(
6) Wicke, B. G.; Grady, K. A.; Ratcliffe, J. W. Combust. Flame 1989,
(40) Corchado, J. C.; Espinosa-Garcia, J.; Hu, W.-P.; Rossi, I.; Truhlar,
D. G. J. Phys. Chem. 1995, 99, 687.
7
8, 249.