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Rate Constant and Branching Fraction for the NH2 + NO2 Reaction
Stephen J. Klippenstein,t Lawrence B. Harding,t Peter Glarborg,*'* Yide Gaoj Huanzhen Huj
and Paul Marshalls
tChemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
tDTU Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark
Department of Chemistry, University of North Texas, 1155 Union Circle #305070, Denton, Texas 76203-5017, United States
0 Supporting Information
ABSTRACT: The NH2 + NO2 reaction has been studied experimentally and                 roaming OH+cik-HNNO
theoretically. On the basis of laser photolysis/LIF experiments, the total rate  10NH -+N  v H NO+NO  OH+crans-HNNO
constant was determined over the temperature range 295-625 K as ki,exp(T) = 9.5  -10
X 10-7(T/K)-2.05 exp(-404 K/T) cm3 molecule-1 s-1. This value is in the upper  -20
range of data reported for this temperature range. The reactions on the NH2 +  j -30
NO2 potential energy surface were studied using high level ab initio transition state  m  -40  /NONO
theory (TST) based master equation methods, yielding a rate constant of       -50            ''cis-HNN(0)OH
kl,theory(T) = 7.5 X 10-2(T/K)-0'72 exp(687 KIT) cm3 molecule-1 s1, in good  ;  -60  H,\    trans-HNN(O OH
agreement with the experimental value in the overlapping temperature range. The  co-70
two entrance channel adducts H2NNO2 and H2NONO lead to formation of N20      "64
+ H20 (RIa) and H2NO + NO       (Rib), respectively. The pathways through     -9
H2NNO2 and H2NONO are essentially unconnected, even though roaming may
facilitate a small flux between the adducts. High- and low-pressure limit rate
coefficients for the various product channels of NH2 + NO2 are determined from the ab initio TST-based master equation
calculations for the temperature range 300-2000 K. The theoretical predictions are in good agreement with the measured overall
rate constant but tend to overestimate the branching ratio defined as 6 = kia/(kia + klb) at lower temperatures. Modest
adjustments of the attractive potentials for the reaction yield values of kia = 4.3 X 10-6(T/K)-2'191 exp(-229 K/T) cm3
molecule-1 s-1 and klb = 1.5 X 10-2(T/K)0032 exp(761 K/T) cm3 molecule-1 s-1, in good agreement with experiment, and we
recommend these rate coefficients for use in modeling.

INTRODUCTION
The formation and consumption of nitrogen oxides (NOr) at
high temperatures continue to be an important area of research.
Of particular interest is the chemistry of amine radicals.
Ammonia is formed in significant quantities in devolatilization
of solid fuels' and the selectivity in oxidation of amine radicals
to form NO or N2 is important for the yield of NOx.2 Reactions
of amine radicals are also important for the performance of
selective noncatalytic reduction (SNCR) of NO using amine-
based additives such as ammonia or urea. In the past, both NH3
oxidation2-6 and SNCR27-11 have been studied extensively
within combustion.
Following the detection of NO2 as an important intermediate
in SNCR,' the reaction of NH2 with NO2 was identified as a
key step.9' This reaction has two major product channels:
NH2 + NO2 = N20 + H20                            (Ria)
NH2 + NO2 = H2NO + NO                            (Rib)
The prior analysis of Glarborg et al.'3 provides an overall
picture of the kinetics for this reaction. The NH2 radical can
add to either the N atom of NO2 to form H2NNO2 or one of
the 0 atoms to form H2NONO. The H2NNO2 species
ultimately produces N20 + H20 (RIa) after a number of
ACS Publications      2013 American Chemical Society

isomerizations. Meanwhile, the H2NONO species directly
dissociates to H2NO + NO (Rib). Both of these sets of
products are exothermic relative to reactants, and the transition
states on the pathway to forming N2O + H2O are all below the
entrance channel. Thus, at least at low temperature, the rate
coefficients are governed by the rates of adding the NH2 to
either the N or the 0 sites of NO2-
Values for both the overall rate constant-22 and the
branching fraction p, defined as kia/(kia + kib),13,14,20-2s have
been reported over a wide range of conditions, but the scatter
in the data is considerable. Measurements of the overall rate
constant vary by an order of magnitude at 500 K.14,7'18 In
addition, there appears to be some inconsistency in the low and
high temperature measurements of the rate constant, with the
recent high temperature data of Song et al.22 being substantially
greater than expected from a simple extrapolation of the low
temperature data. Also, the reported branching fraction values
vary greatly; most determinations indicate a value of
approximately 20%,13,21-25 but values of 65-95% have also
been reported.4,20

Received:
Revised:
Published:

July 10, 2013
August 20, 2013
August 22, 2013

dx.doi.org/10.1021 /jp4068069 I J. Phys. Chem. A 2013, 117, 9011-9022

9011