Kinetics of the NCO + HCNO reaction

Article abstract of DOI:10.1021/jp066036g

The kinetics of the NCO + HCNO reaction were studied using infrared diode laser absorption spectroscopy. The total rate constant was measured to be k ₁ =(1.58 ± 0.20) × 10^-11 cm³ molecule^-1 s^-1 at 298 K. After detection of products and consideration of secondary chemistry (primarily O + HCNO and CN + HCNO), we conclude that NO + CO + HCN is the major product channel (φ = 0.92 ± 0.04), with a minor contribution (φ = 0.04 ± 0.02) from CO ₂ + HCNN.

Full text of DOI:10.1021/jp066036g

J. Phys. Chem. A 2007, 111, 3831-3835
3831
Kinetics of the NCO + HCNO Reaction†
Wenhui Feng and John F. Hershberger*
Department of Chemistry and Molecular Biology, North Dakota State UniVersity, Fargo, North Dakota 58105
ReceiVed: September 15, 2006; In Final Form: October 25, 2006
The kinetics of the NCO + HCNO reaction were studied using infrared diode laser absorption spectroscopy.
The total rate constant was measured to be k1 ) (1.58 ( 0.20) × 10-11 cm3 molecule-1 s-1 at 298 K. After
detection of products and consideration of secondary chemistry (primarily O + HCNO and CN + HCNO),
we conclude that NO + CO + HCN is the major product channel (æ ) 0.92 ( 0.04), with a minor contribution
(æ ) 0.04 ( 0.02) from CO2 + HCNN.
CN + 18O2 f NC18O + 18
O
(4)
1. Introduction
Fulminic acid, HCNO, is an important intermediate in NO-
reburning process for the reduction of NOx pollutants from fossil
fuel combustion emissions.1 HCNO is formed primarily from
acetylene oxidation followed by the HCCO + NO reaction. The
chemistry of HCNO is of great interest in the overall NO-
reburning mechanism. In our laboratory, we have previously
studied the kinetics of the OH + HCNO and CN + HCNO
reactions.2,3 In this paper, we present a study of the kinetics of
the reaction HCNO with the NCO radical. NCO is an important
intermediate in several combustion environments, including the
RAPRENOx process for reducing NOx emissions.4-6 The title
reaction has several thermodynamically possible product chan-
nels:
Total rate constant and reaction products were measured by
infrared laser absorption spectroscopy using lead salt diode lasers
(Laser Components), as described in previous publications.11,12
IR and UV light passed through a single-pass 143-cm absorption
cell, and the infrared light was detected by a 1-mm diameter
InSb detector (Cincinnati Electronics, ∼1 µs response time) and
the signal was averaged on a digital oscilloscope. To account
for small-probe laser thermal deflection affects, signals were
collected with the diode laser slightly detuned off the spectro-
scopic absorption lines, and such transients were subtracted from
the on-resonant transients. This is only a minor correction
(<7%).
HCNO samples were synthesized as previously described by
flash vacuum pyrolysis of 3-phenyl-4-oximino-isoxazol-5(4H)-
one.13-15 The purity of the HCNO samples was characterized
by Fourier transform infrared (FTIR) spectroscopy and was
typically 95% pure or better, with only small HNCO and CO2
impurities detected. Because HCNO has poor long-term stability,
samples were kept at 77 K except when filling the reaction cell.
In general, HCNO could be allowed to stand at room temper-
ature for ∼5 min in our Pyrex absorption cell with minimal
decomposition.2
NCO + HCNO f NO + CO + HCN
∆H2980 ) -183.69 kJ/mol (1a)
f CO2 + HCNN
∆H2980 ) -232.31 kJ/mol
(1b)
(1c)
(1d)
f CO + HCO + N2
∆H2980 ) -365.62 kJ/mol
f N2O + HCCO
ICN (Aldrich) was purified by vacuum sublimation to remove
dissolved air. O2 (Matheson) and 18O2 (Isotec) were passed
through a glass column filled with Ascarite II to remove CO2.
SF6 and CF4 (Matheson) were purified by repeated freeze-
pump-thaw cycles at 77 K and by passing through a glass
column filled with Ascarite II.
∆H2980 ) -37.99 kJ/mol
Thermochemical information has been obtained from NIST-
JANAF standard tables7 as well as other references for the heats
of formation of NCO,8 HCNO,8 HCNN,9 and HCCO.10
The following molecules were probed using infrared diode
laser absorption spectroscopy:
2. Experimental Section
NCO radicals were generated by 248-nm excimer laser photol-
ysis of ICN/O2 mixtures, using a Compex 200 Excimer laser:
HCNO (0100000) r (0000000)
NCO (V ) 1 r V ) 0)
R(10) at 2203.851 cm-1
at 1939.92 cm-1
ICN + hν (248 nm) f CN + I
CN + O2 f NCO + O
(2)
(3)
In some experiments, 18O2 was used to form isotopically labeled
NC18O:
CO (V ) 1 r V ) 0)
P (10) at 2103.27 cm-1
† Part of the special issue “James A. Miller Festschrift”.
* Author to whom correspondence should be addressed.
NO (V ) 1 r V ) 0)
R(8.5) at 1906.141 cm-1
10.1021/jp066036g CCC: $37.00 © 2007 American Chemical Society
Published on Web 12/05/2006

3832 J. Phys. Chem. A, Vol. 111, No. 19, 2007
Feng and Hershberger
CO2 (0001) r (0000)
P(32) at 2321.134 cm-1
16O12C18O (0001) r (0000)
18O12C18O (0001) r (0000)
N2O (0001) r (0000)
P(13) at 2322.089 cm-1
R(12) at 2322.568 cm-1
P(23) at 2202.744 cm-1
The HITRAN molecular database was used to locate and
identify the spectral lines of NO, CO, CO2, and N2O product
molecules.16 Other published spectral data were used to locate
and identify NCO17 and HCNO18 lines. The spectra lines used
are near the peak of the rotational Boltzmann distribution,
minimizing sensitivity to small heating effects.
Typical experimental conditions were P(HCNO) ) 0.2 Torr,
P(ICN) ) 0.1 Torr, P(O2) ) 4.0 Torr, P(SF6 or CF4) ) 1.5 Torr,
and excimer laser pulse energies of 5 mJ. Under these condi-
tions, we estimate an initial radical yield of [CN]0 ∼ 1.8 × 1013
molecule cm3. CF4 buffer gas was used for CO detection, while
SF6 buffer gas was used for detection of all other product
molecules. The choice of buffer gas was motivated by the desire
to relax any nascent vibrationally excited product molecules to
a Boltzmann distribution.19
Figure 1. Diode infrared absorption signal of NCO as a function of
time. Upper trace: 0.0 Torr HCNO; lower trace, 0.04 Torr HCNO.
Reaction conditions: P(ICN) ) 0.10 Torr, P(O2) ) 4.0 Torr, P(SF6)
) 1.0 Torr.
3. Results
3.1. Total Rate Constants. Figure 1 shows transient infrared
absorption signals of the NCO radical, obtained with and without
HCNO reagent. In the absence of HCNO reagent, an ∼7000 s-1
decay rate over the time range 400 µs is observed. This decay
is attributed to removal of NCO radicals by pathways other than
the title reaction, including NCO + O and NCO + CN reactions
as well as diffusion out of the probed region of the reaction
cell. Upon the addition of HCNO reagent, a large increase in
NCO decay rate is observed. Curves such as shown in Figure
1 were simulated by a single-exponential decay function. Figure
2 shows the resulting decay rates as a function of HCNO
pressure. A linear dependence is observed, as is expected under
the pseudo-first-order kinetics conditions, in which [HCNO] .
[NCO]. As per standard kinetic treatment, the slope of this plot
is the desired bimolecular rate constant k1:
Figure 2. Pseudo-first-order decay rate constant of the NCO radical
as a function of HCNO pressure. Reaction conditions: P(ICN) )
0.10 Torr, P(O2) ) 4.0 Torr, P(SF6) ) 1.0 Torr, P(HCNO) ) variable.
ments were conducted at 298 K. A key point is that many of
the same products produced by the title reaction may also be
produced through 248-nm photolysis of HCNO and subsequent
secondary chemistry. Possible chemistry involved was discussed
in our previous publication.2 To obtain product yields from the
title reaction, product yields obtained from HCNO/O2/buffer gas
mixtures were subtracted from those obtained with the full
HCNO/O2/ICN/buffer gas mixtures. The assumption here is that
the HCNO photolysis and secondary chemistry are not greatly
affected by the presence or absence of the small amount of ICN
precursor.
We observed significant CO and NO transient signals upon
photolysis of HCNO/O2/ICN/SF6. Much smaller CO and NO
transient signals were obtained upon photolysis of HCNO/O2/
SF6 mixture. This result suggests that channels 1a and possibly
1c represent significant product channels. A quantitative analysis
(described below) will demonstrate that most or all of the CO
products arise from 1a rather than from 1c.
k1 (298) ) (1.58 ( 0.20) × 10-11 cm3 molecule-1 s-1
The observed standard deviation (statistical error bar) was quite
small, (0.05 × 10-11 cm3 molecule-1 s-1. The quoted error
bar also includes an estimate of systematic errors, primarily on
the basis of an estimate of the purity of the HCNO samples.
Attempts were made to measure the rate constant at temper-
atures both greater and lower than 298 K, unfortunately without
success. At lower temperatures, the ICN precursor does not have
sufficient vapor pressure to perform these experiments. At
elevated temperatures, NCO absorption signals were weak and
the data were quite scattered, possibly because of HCNO
decomposition effects. The absorption coefficient of NCO is
fairly low (much lower, for example, than CN), and the
sensitivity of the infrared absorption probe decreases with
increasing temperature because of increased Doppler line
broadening, broadening of the rotational and vibrational Boltz-
mann distributions, and so forth.
CO2 products from channel 1b are in principle easily
detectable; however, a small CO2 impurity in the HCNO samples
interferes with this measurement. For that reason, and because
of some other potential secondary chemistry, we used 18O2 to
produce isotopically labeled NC18O via eq 4. Reaction 1b,
correspondingly, can be rewritten as follows:
3.2. Product Yields. Infrared diode laser absorption was used
to attempt detection of NO, CO, CO2, and N2O products upon
248-nm photolysis of HCNO/ICN/O2/buffer gas mixtures. Some
transient signals are shown in Figure 3. All detection experi-
NC18O + HCNO f 16OC18O + HCNN
(1b)

Kinetics of the NCO + HCNO Reaction
J. Phys. Chem. A, Vol. 111, No. 19, 2007 3833
TABLE 1: Product Yields of Reaction NCO + HCNOa
product without ICNb with ICNb differenceb,c relative yieldd
NO
9.15
13.73
0.081
19.53
24.55
0.559
10.38
10.82
0.478
1
CO
1.042
0.046
16OC18O
a Experimental conditions: P(O2) ) 4.0 Torr for detection of NO
and CO, P(18O2) ) 4.0 Torr for detection of 16OC18O, P(ICN) )
0.1 Torr, P(HCNO) ) 0.2 Torr, and P(buffer gas) ) 1.5 Torr. b In units
of 1012 molecule cm-3 c Obtained by subtracting column 3 by column
.
2. d Obtained from difference yields (column 4) being normalized to
[NO] ) 1.00.
ments. The right-hand column of Table 1 shows the resulting
relative product yields.
Examination of Table 1 shows that the yields of NO and CO
molecules are large and almost the same. This is consistent with
NO and CO formation via channel 1a, with little or no
contributions to these product yields from secondary chemistry.
We therefore expect concurrent formation of HCN in channel
1a. HCN is not detectable in our diode laser apparatus because
of the lack of available laser diodes near ∼3300 cm-1. However,
FTIR spectra of both HCNO/ICN/O2 and HCNO/O2 mixtures
before and after extensive photolysis (1000 excimer shots)
clearly show that additional HCN products are produced from
the photolysis of HCNO/ICN/O2 mixture, beyond that formed
by photolysis of HCNO/O2. Although not a quantitative
measurement, this result is consistent with our identification of
channel 1a as a major product.
As shown in Table 1, only a trace of 16OC18O was produced
upon the photolysis of a HCNO/18O2/SF6 mixture, while a
significant yield of 16OC18O yield was obtained by photolysis
of a HCNO/ICN/18O2/SF6 mixture. The difference between the
two measurements represents the yield of CO2 from eq 1b.
However, this yield is much smaller than the yield of 1a,
indicating that 1b is a relatively minor channel.
Table 1 shows nearly identical yields of CO and NO,
suggesting that channel 1c is at most a minor channel. In
addition, the possible presence of channel 1c can be tested by
the following experiment. If 18O2 is used to produce labeled
NC18O radicals, the following reactions are expected:
NC18O + HCNO f C18O + HC16O + N2
HC16O + 18O2 f C16O + H18O2
(1c)
(5)
Figure 3. Transient infrared absorption signals of (a) CO, (b) NO,
and (c) 16OC18O. Reaction conditions: P(ICN) ) 0 Torr (lower traces),
P(ICN) ) 0.10 Torr (upper traces), P(O2) ) P(18O2), for 16OC18O
detection only) ) 4.0 Torr, P(HCNO) ) 0.20 Torr, P(CF4) ) 1.5 Torr
(for CO transient only), P(SF6) ) 1.5 Torr (for NO and 16OC18O
transients only).
or
NC18O + HCNO f C16O + HC18O + N2
HC18O + 18O2 f C18O + H18O2
(1c)
(6)
Therefore, the mixed isotope 16OC18O is expected to be
produced from eq 1b when labeled oxygen is used. The
experimental result is that we observed a significant 16OC18O
transient signal upon photolysis of HCNO/ICN/18O2/SF6 mixture
while only a trace of 16OC18O transient signal was obtained
upon photolysis of HCNO/18O2/SF6 mixture, suggesting that 1b
is a significant product channel.
No N2O transient signal was observed upon photolysis of
HCNO/ICN/O2/SF6. This demonstrates that channel 1d is not a
product channel of the title reaction. We estimate an upper limit
of the branching ratio æ1d < 0.02.
Transient signal amplitudes (peak-peak) of NO, CO, and
16OC18O were converted into absolute concentration using
HITRAN line strengths as described in a previous publication.20
Table 1 shows a typical data set of product molecule yields.
Shown are concentrations obtained both with and without the
ICN precursor and the difference between these two measure-
Regardless of whether the oxygen-18 labeling ends up on the
CO or HCO product in channel 1c, the reaction of the
concurrently produced HCO radicals with oxygen guarantees
that one molecule of C16O and one molecule of C18O are
produced. The experiment result is that many more C18O
molecules than C16O were detected upon photolysis of a HCNO/
ICN/18O2/CF4 mixture: a ratio of [C18O]/[C16O] ) 14 was
observed. This is clearly inconsistent with channel 1c and
provides further evidence that most of the C18O originated from
channel 1a. On the basis of our observed [C18O]/[C16O] ratio,
we estimate an upper limit of the branching ratio æ1c < 0.07.
3.3. Secondary Chemistry. Two important possible second-
ary reactions merit discussion. The first is the reaction of oxygen
atoms produced in eq 3 with HCNO:

3834 J. Phys. Chem. A, Vol. 111, No. 19, 2007
Feng and Hershberger
O + HCNO f HCN + O2
f NCO + OH
∆H2980 ) -285.27 kJ/mol
(7a)
∆H2980 ) -249.56 kJ/mol
(7b)
f CO + HNO
∆H2980 ) -431.36 kJ/mol
(7c)
f NO + HCO
∆H2980 ) -286.62 kJ/mol
(7d)
Miller and co-workers used a large rate constant for the O +
HCNO reaction in their modeling studies of NO-reburning,1,21
but no experimental data or detailed theoretical calculations have
been reported. Although a direct measurement of k7 in our
laboratory is not possible (we have no O-atom detection
capabilities), we have attempted to detect products of eq 7 by
infrared laser absorption upon 248-nm laser photolysis of
HCNO/SO2/buffer gas mixtures. Using experimental conditions
of P(SO2) ) 0.3 Torr and 248-nm laser pulse energy of ∼5 mJ,
only a very small yield (∼1.2 × 1012 molecule cm-3) of CO
was detected, and we were unable to detect NO in this
experiment. Addition of O2 (to convert HCO to CO) did not
result in any detectable increase in the CO yield. This result is
surprising, because under these experimental conditions, we
estimate an initial O atom yield from SO2 photolysis of
∼1.8 × 1013 atom cm-3, using the absorption cross section of
SO2 at 248 nm (8.78 × 10-20 cm2).22 Alternatively, we can
experimentally determine [O]0 by titrating O atoms with NO2:
Figure 4. The dependence of C18O2 product yield on 18O2 pressure.
Reaction conditions: P(ICN) ) 0.10 Torr, P(HCNO) ) 0.20 Torr,
P(SF6) ) 1.5 Torr, P(18O2) ) variable.
TABLE 2: Isotopically Labeled CO2 Product Yieldsa
product without ICNb with ICNb differenceb,c relative yieldd
C16O2
0.447
0.106
0.594
0.572
0.719
2.37
0.125
0.613
1.78
0.067
0.343
1
16OC18O
C18O2
a Experimental conditions: P(18O2) ) 4.0 Torr, P(ICN) ) 0.1 Torr,
P(HCNO) ) 0.2 Torr, P(SF6) ) 1.5 Torr. b In units of 1012 molecule
cm-3 c Obtained by subtracting column 3 by column 2. d Obtained from
.
difference yields (column 4) being normalized to [C18O2] ) 1.00.
O + NO2 f NO + O2
(8)
reported here. One test of whether we have effectively shut down
eq 9 is to use variable pressures of labeled 18O2 (i.e., a HCNO/
18O2/ICN/SF6 mixture is photolyzed). HCCN products of eq 9
then react with 18O2 to produce doubly labeled C18O2:
Through detection of NO upon 248-nm laser photolysis of a
NO/SO2/SF6 mixture, we obtained a yield of ∼1.5 × 1013 atoms
cm-3, which is in excellent agreement with the value calculated
from the SO2 absorption coefficient.
HCCN + 18O2 f 18OC18O + HCN
(10)
As a result of the above experiment, one is tempted to
conclude that 7c and 7d are at most minor channels. We can
further deduce the products in 7b are also produced in very small
amounts at most, because if OH were produced, it would react
with HCNO to produce a significant yield of CO molecules.2
These observations would tend to point to 7a as the major
product. FTIR spectra of HCNO and HCNO/SO2 mixtures
before and after photolysis, however, showed no obvious
differences that can be attributed to HCN formation via 7a.
There are two possible explanations: perhaps 7a is the major
channel, but the FTIR spectrum is not sufficiently sensitive to
observe the HCN produced, primarily because of a large
background signal from HCNO photolysis. Alternately, perhaps
eq 7 is much slower than suspected at room temperature, which
would explain our inability to detect any products in significant
yield. Clearly, more study of eq 7 is warranted, but for the
purposes of the present study, we can conclude that it does not
represent a major interference while determining the products
of eq 1.
where k10 ) 1.8 × 10-12 cm3 molecule-1 s-1 at 298 K.23 CO2
formed in eq 1b will be singly labeled, that is, 16OC18O. Figure
4 shows the yield of C18O2 as a function of 18O2 pressure. As
expected, the yield becomes very small in the limit of high 18O2,
because most of the CN radicals react via 3 rather than 9 under
these conditions. As shown in Table 2, at P(18O2) ) 4.0 Torr,
we detected 2.37 × 1012 molecule cm-3 of C18O2 upon
photolysis of a HCNO/18O2/ICN/SF6 mixture but only 0.59 ×
1012 molecule cm-3 C18O2 upon photolysis of a HCNO/18O2/
SF6 mixture. The difference between the two measurements is
about ∼1.8 × 1012 molecule cm-3, representing the yield of
C18O2 products from eqs 9 and 10. For comparison, we detected
the yield of C16O2 and 16OC18O products along with C18O2 at
P(18O2) ) 4.0 Torr, as shown in Table 2. From Table 2, we
obtain the ratio of [16OC18O]/[C18O2] ) 0.343. Combining this
result with [16OC18O]/[NO] ) 0.046 shown in Table 1, we
obtain the ratio [C18O2]/[NO] ) 0.134. According to the
chemical equivalence relations in eqs 9 and 10, [C18O2] is equal
to [NO] of eq 9. Equation 9, therefore, can account for
approximately 13% of the total NO yield measured at P(18O2)
) 4.0 Torr. This is a rather minor yield and is comparable with
effects of other secondary chemistry, such as O + HCNO. As
a result, we conclude that the effect of eq 9 under the conditions
reported in Table 2 is quite small.
The second potentially serious secondary reaction in our
experiment is that of CN with HCNO. In a previous study in
our laboratory, we found that this reaction is fast (k9 ) 1.04 ×
10-10 cm3 molecule-1 s-1) and has only one major product
channel:3
CN + HCNO f NO + HCCN
(9)
In Figure 4, the yield of C18O2 decreases at high O2 pressure
but not quite to zero. One possible explanation may be a
secondary reaction involving HCNN, produced by the minor
channel 1b:
Equations 3 and 9 compete for CN radicals. In the limit of high
O2 pressure, eq 3 is expected to dominate. This is the reason
for the large O2 pressures used in most of the experiments

Kinetics of the NCO + HCNO Reaction
HCNN + 18O2 f C18O2 + N2 + H
J. Phys. Chem. A, Vol. 111, No. 19, 2007 3835
N-O bond and second N-C bond to produce HCN, NO, and
CO products. Both of these mechanisms are consistent with our
observation that the CO formed upon photolysis of HCNO/ICN/
18O2/SF6 mixture is primarily the C18O isotope.
∆H2980 ) -635.23 kJ/mol (11)
(other product channels may also be possible). No study of this
reaction has been reported in the literature. Equation 11 does
not affect our determination of æ1b, which is based on measure-
ment of [16OC18O] yields.
In summary, we can conclude that channel 1a dominates the
title reaction but that a small yield of 1b exists as well. On the
basis of the quantitative data and after consideration of the
uncertainties present, we estimate æ1a ) 0.92 ( 0.04 and æ1b
) 0.04 ( 0.02.
5. Conclusion
The kinetics and products of NCO + HCNO reaction were
studied using IR diode laser absorption spectroscopy. The
reaction is fast, with k1 ) (1.58 ( 0.20) × 10-11 cm3 molecule-1
s-1 at 298 K. The major product channel is NO + CO + HCN.
Acknowledgment. This work was supported by Division
of Chemical Sciences, Office of Basic Energy Sciences of the
Department of Energy, Grant DE-FG03-96ER14645. Partial
support from ND EPSCoR through NSF grant #EPS-0447679
is also acknowledged.
4. Discussion
Our results represent the first study of the title reaction.
Several experimental artifacts can cause systematic errors in a
pseudo-first-order kinetics experiment. The first, decomposition
of HCNO sample during the experiments, was minimized by
completing a single NCO IR transient signal decay measurement
in about 5 min (typically, this allows 2 min for filling the cell
and 3 min for the reagents to mix). In this amount of time, less
than 10% decomposition occurs. A related issue is the possible
reaction of NCO radicals with decomposition products and
reaction products from the title or secondary reactions. This issue
was discussed in our previous publications2,3 and was shown
to be insignificant when the measured rate constant of the title
reaction is fast, as is the case here. If k1 were several orders of
magnitude slower, such secondary chemistry would be a more
serious issue.
One notable feature of the absorption signals shown in Figure
1 is that the NCO peak transient amplitude is decreased when
HCNO is included in the reaction mixture. This is partly because
in the lower trace of Figure 1, the decay rate is almost as fast
as the rise rate; an extrapolation of the decay to t ) 0 would
yield a substantially increased amplitude. Additional reasons
for this effect include competition for CN radicals (eq 3 vs eq
9) as well as a slight amount of decomposition of ICN in the
presence of HCNO.
No ab initio studies of the potential energy surface of this
reaction have been reported. We can therefore only speculate
regarding details of the reaction mechanism. Our observation
of channel 1a as the major product channel suggests the
following possible mechanisms: NCO attack at the carbon of
HCNO forms a HC(NCO)NO complex, which dissociates via
N-C bond fission to produce a CO molecule and via a further
C-N bond fission to produce NO and HCN products. Alter-
natively, NCO attacks the oxygen of HCNO to form a quasi-
linear complex HCNONCO, followed by fission of the first
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