Efficient decomposition of NO2 into N2 and O 2 by 193-nm ArF laser in N2 atmosphere

Article abstract of DOI:10.1246/cl.2005.496

Decomposition of NO₂ in N₂ by 193-nm ArF excimer laser was studied at atmospheric pressure. The NO₂ conversion was 86% and the formation ratios of N₂, O₂, and NO were 32, 75, and 22%, respectively, at a low NO₂ concentration of 200 ppm. The lower NO yield than that predicted from simple NO₂ → NO + O photolysis was explained as the second absorption of 193-nm light by NO (X ²Π:ν″ > 0) followed by N + O predissociation and subsequent secondary reactions leading to N₂ and O₂. Copyright

Full text of DOI:10.1246/cl.2005.496

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Chemistry Letters Vol.34, No.4 (2005)
Efficient Decomposition of NO into N and O by 193-nm ArF Laser in N Atmosphere
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yy
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Masaharu Tsuji, Kenji Noda, Hiroshi Sako, Taro Hamagami, and Takeshi Tsuji
Institute for Materials Chemistry and Engineering, Kyushu University, and CREST, JST, Kasuga, Fukuoka 816-8580
y
CREST, JST
Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences,
Kyushu University, Kasuga, Fukuoka 816-8580
yy
(Received January 11, 2005; CL-050049)
Decomposition of NO2 in N2 by 193-nm ArF excimer laser
was studied at atmospheric pressure. The NO2 conversion was
6% and the formation ratios of N2, O2, and NO were 32, 75,
and 22%, respectively, at a low NO2 concentration of
00 ppm. The lower NO yield than that predicted from simple
22% at 180 s, respectively.
The observed data are compared with calculated values from
a kinetic model, which was similar to that used for the decompo-
8
1
sition of N2O by 193-nm laser light. Four photolysis processes
and ten two-body and six three-body reactions are taken into ac-
2
2
NO2 ! NO þ O photolysis was explained as the second absorp-
count in the simulation. Among them, 11 major photolysis and
subsequent reaction processes are given in Table 1. Photolysis of
2
00
tion of 193-nm light by NO (X ꢀ:ꢀ > 0) followed by N þ O
predissociation and subsequent secondary reactions leading to
N2 and O2.
NO2 by 193-nm ArF laser starts from process (1) leading to
3
1
NO þ O( P, D) with a large absorption cross section.
The observed NO2 conversion below 90 s is higher than cal-
culated values, while it is lower than that in the 90–180 s region.
It should be noted that there are significant discrepancies be-
tween the observed and calculated values for the formation ratios
of N2, O2, and NO. According to the model calculation, the for-
mation ratio of NO is nearly equal to the NO2 conversion and no
N2 formation is expected. This is explained by the fact that the
major reaction pathway of NO2 under 193-nm laser light is sim-
We have recently initiated development of a photochemical
method as a new promising removal method of NOx at atmo-
spheric pressure. An advantage of photochemical method is that
more selective decomposition is possible than electric discharge
method, where energetic electrons are main energy carrier. We
have recently succeeded in efficient decomposition of N2O into
N2 and O2 by using 193-nm ArF excimer laser in N2 and air at
3
1
ple photolysis of NO2 into NO þ O( P, D) followed by reactions
1
3
1
atmospheric pressure. In the present study, decomposition of
of O( P, D) atoms with NO2 and NO.
NO2 by 193-nm laser was investigated. We have found for the
first time that the product distribution in ArF laser photolysis
of NO2 strongly depends on the concentration of NO2 and that
the NO yield can be greatly suppressed at a low NO2 concentra-
tion of 200 ppm.
When NO2 in a high concentration of 5% was decomposed
by 193-nm ArF laser, NO2 conversion was only 27%, and the
formation ratios of N2, O2, and NO were 3, 16, and 21%, respec-
tively, after laser irradiation for 30 min. These results indicate
that the branching ratio of N2 decreases, while that of NO in-
creases and it is close to the NO2 conversion. This result is con-
sistent with the above simple decomposition model. The low
conversion at the high NO2 concentration is attributed to the
contribution of oxidation of NO via processes (5) and (9).
An outstanding finding of this study that the decomposition
of NO2 under 193-nm ArF laser depends strongly on the NO2
N2O photolysis chamber used in this study was identical to
1
that reported previously except for the use of higher sensitive
analytical systems of products. Light from an unfocused ArF ex-
cimer laser (Lambda Physik, COMPex102) was used to decom-
pose NO2 at a room temperature. The laser pulses had a duration
of 15 ns and delivered an output energy of 80 mJ per pulse at a
repletion rate of 5 Hz. All experiments were carried out in a
closed batch system. The total pressure was kept at atmospheric
pressure and the NO2 concentration diluted in N2 was 200 ppm
or 5% (v/v).
1
00
80
NO : Obs.
2
After 10–180-s laser irradiation, products were analyzed by
using HORIBA gas analysis system (FG-100) equipped with an
FTIR spectrometer and ANELVA gas analysis system (M-
N2 : Obs.
O2 : Obs.
NO: Obs.
6
0
200GA-DTS) equipped with a quadrupole mass spectrometer.
NO : Calc.
2
We determined the NO2 conversion, 1-[NO2]/[NO2]0, and the
formation ratios of N2, O2, and NO, defined as [N2]/[NO2]0,
40
N2 : Calc.
O2 : Calc.
NO: Calc.
[
O2]/[NO2]0, and [NO]/[NO2]0, respectively, from gas analy-
2
0
0
ses. Here, [NO2]0 is an initial concentration of NO2.
Figure 1 shows the dependence of NO2 conversion and the
formation ratios of N2, O2, and NO on the irradiation time of
ArF laser at a NO2 concentration of 200 ppm. The NO2 conver-
sion and the formation ratios of N2, O2, and NO increase to 83,
0
60
120
180
Laser irradiation time / s
31, 72, and 21%, respectively, in the 0–90 s range and level off
Figure 1. Dependence of NO conversion and the formation ra-
2
above that. The maximum conversion of NO2 is 86% and the
maximum formation ratios of N2, O2, and NO are 32, 75, and
tios of N , O , and NO on the laser irradiation time at a NO con-
2
centration of 200 ppm.
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Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.4 (2005)
497
Table 1. Major photolysis and subsequent reaction processes
under 193-nm ArF excimer laser irradiation
Not only radiative process, but also predissociation into N þ
O will occur significantly in such high states, as known for high
vibrationally excited states of NO (A, C, D) states.
NO (A, D, E) ! NO (X, A) þ hꢀ (radiative decay) ð13aÞ
! N þ O (predissociation) ð13bÞ
1
2,13
Coefficients
(Refs. 3–6)
Processes
NO2 þ hꢀ ! NO þ O ( P)
Number
3
ꢂ19
(1a)
(1b)
(2)
1:6 ꢁ 10
1:3 ꢁ 10
8:95 ꢁ 10
6:5 ꢁ 10
3:0 ꢁ 10
1:5 ꢁ 10
1:0 ꢁ 10
4:0 ꢁ 10
1:2 ꢁ 10
2:6 ꢁ 10
2:98 ꢁ 10
1:93 ꢁ 10
2:9 ꢁ 10
1:21 ꢁ 10
1
ꢂ19
ꢂ20
ꢂ12
ꢂ10
ꢂ10
ꢂ31
ꢂ11
ꢂ10
ꢂ11
ꢂ33
ꢂ38
ꢂ11
ꢂ11
!
NO þ O ( D)
Subsequent fast reactions of N atoms with NO and NO2,
10) and (11), give N2 and N2O. N2O is selectively decomposed
1
N2O þ hꢀ ! O ( D) þ N2
(
3
O ( P) þ NO2 ! NO þ O2
(3)
1
1,8
into N2 þ O ( D) by 193-nm light. On the basis of above
facts, it is reasonable to assume that efficient decompostion of
NO and NO2 into N2 and O2 observed here at the low NO2 con-
1
O ( D) þ NO2 ! NO þ O2
(4a)
(4b)
(5)
(6a)
(6b)
(7)
(8)
(9)
(10)
(11)
ꢂ1
3
!
O ( P) þ NO2
3
O ( P) þ NO þ N2 ! NO2 þ N2
4
centration results fom the N ( S) producton via predissociation
13b).
It is known that vibratinal relaxation is fast for excited states
1
3
O ( D) þ NO ! O ( P) þ NO
(
4
!
N ( S) þ O2
3
1
O ( D) þ N2 ! O ( P) þ N2
00
11
of NO (X:ꢀ > 0) by collisions with NO2. Thus, at a high NO2
concetration of 5%, vibrational relaxtion by collisions with NO2
becomes significant before second laser absorption. In such a
case, excitation into high-energy predissociation states becomes
insignificant and the product distribution reflects simple photol-
ysis of NO2 into NO þ O.
3
3
O ( P) þ O ( P) þ N2 ! O2 þ N2
NO þ O2 ! 2NO2
2
4
3
N ( S) þ NO ! N2 þ O ( P)
4
3
N ( S) þ NO2 ! N2O þ O ( P)
2
3
Units of coefficients are cm molecule for (1) and (2), cm
ꢂ1 ꢂ1
6
ꢂ2
molecule
s
s
for three-body reactions.
for two-body reactions and cm molecule
In summary, NO2 photolysis at 193-nm ArF laser was stud-
ied. At a low NO2 concentration of 200 ppm, NO2 was decom-
posed into N2, O2, and NO. The high formation ratios of N2
ꢂ1
concentration and that it does not follow a simple decomposition
model at a low NO2 concentration of 200 ppm. The observed for-
mation ratios of N2 and O2 are much larger than calculated val-
ues, while the formation ratio of NO is much smaller than the
calculated value. These results suggest that NO produced from
photolysis of NO2 is further efficiently converted into N2 and
O2 at the low NO2 level through some processes, which are
not included in our model calculation.
and O and low formation ratio of NO than those expected from
2
known absorption and kinetic data were explained as the second
2
00
absorption of laser light by NO (X ꢀ:ꢀ > 0) leading to N þ O
via predissociation. Actual NO emission in the industrial proc-
2
ess occurs at low level below 1000 ppm. Thus, the photolysis by
193-nm ArF laser is effective for the decomposition of NO into
2
N and O . A further detailed study for NO photolysis by 193-
2
2
2
nm ArF laser in N and air, including process (13b) in the model
2
2
00
It is known that NO (X ꢀ:ꢀ ¼ 0) molecules are selectively
calculation, is in progress in order to obtain optimum experimen-
tal conditions for NO removal.
2
0
excited into the excited NO (B ꢀ:ꢀ ¼ 7) states under 193-nm
2
7
ArF laser excitation. On the other hand, photoexcitation of
2
NO (X ꢀ) in the vibrationally excited levels gives emitting ex-
2
The authors acknowledge financial support from a Grant-
in-Aid for Scientific Research Number 15310059 from the
Japanese Ministry of Education, Culture, Sports, Science and
Technology.
þ
2
þ
cited states of NO (D ꢁ , E ꢁ ) in the photolysis study of N2O
þ
8
2
by 193-nm ArF laser. Similar NO emissions from NO (A ꢁ ,
2
þ
2
þ
9
D ꢁ , E ꢁ ) have been observed by Haak and Stuhl in the
NO2 photolysis by 193-nm ArF laser at the maximum energy
of 150 mJ/pulse. These excited states must be also produced
by second absorption of 193-nm ArF laser light by NO
References and Notes
1
2
3
4
5
M. Tsuji, J. Kumagae, T. Tsuji, and T. Hamagami, J. Hazard.
Mater., 108, 189 (2004).
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published.
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NIST Chemical Database on the Web, Public Beta Release 1.2,
Standard Reference Database 17, Version 7.0 (http://kinetics.
nist.gov/index.php).
2
00
10
2
00
(
X ꢀ:ꢀ > 0), because Gong et al. detected NO (X ꢀ:ꢀ ¼
1{6) after 193-nm ArF laser photolysis of NO2 by using FTIR
spectrometer. In addition to initial photolysis of NO2, such sub-
sequent secondary reactions as processes (3), (4a), and (6a) will
2
00
provide NO (X ꢀ:ꢀ > 0) levels. The above experimental ob-
servation and our detailed model calculation lead us to conclude
2
2
00
that some NO (X ꢀ:ꢀ > 0) molecules produced from photoly-
sis (1a) and (1b) absorb a second photon within a laser pulse
width of 15 ns. On the other hand, those produced from (3),
6
IUPAC Gas Kinetic Data Evaluation, Summary Table of Kinetic
Data, July 2004 (http://www.iupac-kinetic.ch.cam.ac.uk).
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J. Chem. Phys., 74, 6787 (1981).
(
4a), and (6a) absorb subsequent laser photons, because colli-
2
00
sional relaxation of NO (X ꢀ:ꢀ > 0) by N2 is slow at a low
NO2 concentration. For example, V–V transfer rate of NO
(X ꢀ:ꢀ ¼ 1) for N2 is slower than that for NO2 by a factor
of 2300. After a second absorption of 193-nm ArF laser light,
the NO (X ꢀ:ꢀ > 0) molecules are excited to high energy
states above dissociation limit of N–O.
7
8
11
2
00
11
9
1
H. K. Haak and F. Stuhl, J. Photochem., 17, 69 (1981).
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2
00
1
1
2
00
NO (X ꢀ:ꢀ > 0) þ hꢀ (193 nm)
327 (1975).
ð12Þ
2
þ þ þ
2
2
!
NO (A ꢁ , D ꢁ , E ꢁ )
13 J. Luque and D. R. Crosley, J. Chem. Phys., 112, 9411 (2000).
Published on the web (Advance View) March 5, 2005; DOI 10.1246/cl.2005.496