Efficient conversion of NO2 into N2 and O2 in N2 or into N2O5 in Air by 172-nm Xe 2 excimer lamp at atmospheric pressure

Article abstract of DOI:10.1246/cl.2007.376

Decomposition of NO₂ (200 ppm) in N₂ or air by 172-nm Xe₂ excimer lamp was studied at 1 atm. The NO₂ conversion in N₂ was 99%, and the formation ratios of N₂, O ₂, NO, and N₂O were 47, 98, 0, and 2%, respectively, after 30 min irradiation. The NO₂ in air (5-20% O₂) could be completely converted to N₂O₅ and HNO₃ due to reactions by O₃ and H₂O after only 1.0-1.5 min irradiation. The present results give a new simple photochemical aftertreatment technique of NO₂ in air without using any catalysts. Copyright

Full text of DOI:10.1246/cl.2007.376

376
Chemistry Letters Vol.36, No.3 (2007)
Efficient Conversion of NO2 into N2 and O2 in N2 or into N2O5 in Air
by 172-nm Xe2 Excimer Lamp at Atmospheric Pressure
Masaharu Tsuji,ꢀ1;2;3 Masashi Kawahara,1;2;3 Makoto Senda,1;2;3 and Kenji Noda3
1Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580
2CREST, JST, Kawaguchi 332-0012
3Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences,
Kyushu University, Kasuga, Fukuoka 816-8580
(Received December 19, 2006; CL-061482; E-mail: tsuji@cm.kyushu-u.ac.jp)
Decomposition of NO2 (200 ppm) in N2 or air by 172-nm
After 0.1–30 min photoirradiation, products were analyzed
by using HORIBA gas analysis system (FG-100) equipped with
an FTIR spectrometer and ANELVA gas analysis system (M-
200GA-DTS) equipped with a quadrupole mass spectrometer.
We determined the residual amount of NO2, [NO2]/[NO2]0,
and the formation ratios of N2, O2, and NOx, defined as [N2]/
[NO2]0, [O2]/[NO2]0, and [NOx]/[NO2]0, respectively, from
gas analyses. Here, [NO2]0 is an initial concentration of NO2.
N2 and O2 cannot be detected by FTIR, because these diatomic
molecules are inactive for IR light. If other NOx and O3 are
produced in the photolysis, all of them can be detected. Thus,
the formation ratios of N2 and O2 in N2 were determined from
N and O balance before and after photolysis.
When NO2 was decomposed by a 172-nm excimer lamp for
20 min, the main absorption band of NO2 at 1600 cmꢂ1 reduced
its intensity, as shown in Figure S1 (Supporting Information).8
Figure 1 shows the dependence of NO2 conversion and the for-
mation ratios of N2, O2, NO, and N2O on the irradiation time of
lamp. After 30 min photoirradiation, the residual amount of NO2
decreases to 1%, while the formation ratios of N2 and O2 in-
crease to 47 and 98%, respectively. The formation ratio of NO
initially increases to 9% until 2 min then gradually decreases
to 0% in the 9–30 min range. The formation ratio of N2O can
be kept as low as 3% in the all time range. On the basis of these
results, NO2 can be efficiently decomposed to N2 and O2 keep-
ing NO and N2O emissions at low levels under 172 nm irradia-
Xe2 excimer lamp was studied at 1 atm. The NO2 conversion
in N2 was 99%, and the formation ratios of N2, O2, NO, and
N2O were 47, 98, 0, and 2%, respectively, after 30 min irradia-
tion. The NO2 in air (5–20% O2) could be completely converted
to N2O5 and HNO3 due to reactions by O3 and H2O after only
1.0–1.5 min irradiation. The present results give a new simple
photochemical aftertreatment technique of NO2 in air without
using any catalysts.
We have recently initiated development of a photochemical
method as a new promising removal method of NOx at atmo-
spheric pressure without using expensive catalysts.1–4 An ad-
vantage of photochemical method is that more selective decom-
position is possible than electric discharge method, where ener-
getic electrons are main energy carrier. We have recently studied
decomposition of NO2 into N2 by using 193-nm ArF excimer
laser in N2 at atmospheric pressure.2 Although more than 80%
of NO2 (200 ppm) could be converted into N2, O2, and NO in
N2 at atmospheric pressure, it was difficult to decompose NO2 in
air because after photolysis of NO2 into NO + O (1a), such
backward reaction (1b) occurs significantly.
NO2 þ hꢀ (193 nm) ! NO þ O
NO þ O þ M ! NO2 þ M (M = N2, O2)
ð1aÞ
ð1bÞ
100
When ArF excimer laser photolysis was applied to practical
NOx removal process, there are a lot of severe problems. They
are that excimer laser apparatus is expensive, running cost is
high, and the apparatus is big and heavy including high power
sources. In order to overcome these problems, we used here a
low cost and compact (ꢁ ¼ 128 mm, length = 330 mm) Xe2
lamp as a new VUV right source. The absorption cross section
of NO2 (1:4 ꢁ 10ꢂ17 cm2 moleculeꢂ1) at 172 nm is 20 times
larger than that (7:1 ꢁ 10ꢂ19 cm2 moleculeꢂ1) at 193 nm.5
Therefore, more efficient photolysis of NO2 is expected at
172 nm. We have found for the first time that NO2 could be
efficiently removed not only in N2 atmosphere but also in air
(5–20% O2) at a low NO2 concentration of 200 ppm.
NO2 photolysis chamber used in this study was similar to
that used for ArF laser photolysis1 except for the VUV light
source. Light from an unfocused 172-nm Xe2 lamp (USHIO,
UER20H172:50 mW/cm2, 155–200 nm range) was used to
decompose NO2 at a room temperature. 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
or N2/O2 mixtures was 200 ppm (v/v).
NO2
80
N2
O2
NO
N2O
60
40
20
0
0
5
10
15
20
25
30
Irradiation time / min
Figure 1. Dependence of residual amount of NO2 and the
formation ratios of products on the irradiation time of 172-nm
excimer lamp at a NO2 concentration of 200 ppm in N2 at a total
pressure of 1 atm.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.3 (2007)
377
of N + O2 from NO2 will be unfavorable channel.
100
90
800
700
600
500
400
300
200
100
0
In air, similar photolysis of NO2 must occur until 0.8 min.
The slower photolysis rate in the short range of 0–0.8 min can
be attributed to absorption of 172-nm light by O2 (4:6 ꢁ 10ꢂ19
cm2 moleculeꢂ1)5 and fast backward reaction of NO + O3:
80
70
NO2
60
N2O5
NO þ O3 ! NO2 þ O2 (1:8 ꢁ 10ꢂ14
)
ð3Þ
HNO3
50
Actually, the formation of O3 by the following processes was
observed from N2/O2 mixtures without addition of NO2 after
172-nm irradiation (see Figure S3).8
O3
40
30
20
10
0
O2 þ hꢀ (172 nm) ! O þ O
O þ O2 þ N2 ! O3 þ N2 (5:6 ꢁ 10ꢂ34[N2])
The absence of O3 in the 0–0.8 min range can be attributed to its
fast removal via process (3).
ð4aÞ
ð4bÞ
0
0.5
1
1.5
Irradiation time / min
Figure 2. Dependence of residual amount of NO2 and the
formation ratios of products on the irradiation time of 172-nm
excimer lamp at a NO2 concentration of 200 ppm in air (5%
O2) at a total pressure of 1 atm.
After 0.8 min photoirradiation, the concentration of O3
exceeds the concentration of NO. Then, the following oxidation
reactions of NO2 leading to N2O5 (5) take place:
NO2 þ O3 ! NO3 þ O2 (3:5 ꢁ 10ꢂ17
)
ð5aÞ
ð5bÞ
tion, although it takes a long time to remove NO2.
NO2 þ NO3 þ N2 ! N2O5 þ N2 (3:6 ꢁ 10ꢂ30[N2])
Since a small amount of O2 (5–20%) is involved in N2/O2
mixtures in the practical combustion processes, removal tech-
nique of NO2 must be developed in air. FTIR spectra observed
before and after 172-nm photolysis of NO2 were shown in Figure
S2a–S2c.8 Before photolysis, strong NO2 peak is observed. After
0.5 min photoirradiation, only a small decrease (about 10%) in
NO2 peak intensity was observed. After 1.5 min photoirradia-
tion, the spectrum changed significantly: NO2 peak disappeared
and N2O5, HNO3, and O3 peaks are observed.
In the photolysis chamber, there is a small amount of residual
H2O (ꢃ90 ppm), as observed OH peaks from residual H2O in
FTIR spectra. N2O5 has a high reactivity with H2O leading to
HNO3:
N2O5 þ H2O ! 2HNO3 (2:5 ꢁ 10ꢂ22
)
ð6Þ
Thus, HNO3 peaks arise from the result of reaction (6). Reaction
(6) is an important reaction, because N2O5 can be easily re-
moved as HNO3 by the addition of H2O.
Figure 2 shows the dependence of NO2 conversion and the
formation ratios of N2O5 and HNO3 and concentration of O3
on the irradiation time of lamp. The residual amount of NO2
slowly decreases to 85% in the 0–0.8-min range, suddenly drops
to 3% in the 0.8–1.0-min range, and becomes zero at 1.5 min. At
about 0.8 min, the formation of N2O5, HNO3, and O3 starts, and
their formation ratios increase in the 0.8–1.0-min range. At
1.5 min, the formation ratios of N2O5 and HNO3 are 38 and
32%, respectively. The concentration of O3 rapidly increases
from 0.8 to 1.5 min and becomes 714 ppm at 1.5 min. These re-
sults indicate that NO2 can be more efficiently converted to
N2O5 and HNO3 in air in the presence of O2.
The effects of O2 concentration in N2/O2 mixtures were ex-
amined in the 5–20% range. With increasing the O2 concentra-
tion from 5% to 10 and 20%, the formation of N2O5 starts at
shorter time of 1 min.
In summary, NO2 removal by 172-nm excimer lamp has
been studied. It was found that NO2 in air can efficiently be
converted to N2O5 using O3 generated from 172-nm photolysis
of O2. Since N2O5 can be easily converted to HNO3 by the
addition of H2O, the present results are useful as a new simple
and low cost photochemical aftertreatment technique of NO2
in air without using any catalysts.
The reaction mechanisms of NO2 in N2 and air are briefly
discussed from known photochemical and chemical reactions.4–6
Rate constants shown below in parenthesis are given in
cm3 moleculeꢂ1 sꢂ1 units. NO2 is efficiently decomposed to N2
and O2 under 172-nm light in N2. Detailed photolysis process
of NO2 at 172 nm has not been known to be best of our knowl-
edge. In a similar 172-nm photolysis of NO, fast decomposition
of NO to N2 and O2 in N2 and conversion to NO2 in air were
observed.7 Therefore, not only direct decomposition of NO2 into
N2 and O2 (1) but also multiple step decomposition via NO (2)
probably takes part in the photolysis of NO2 at 172 nm.
This work was partly supported by JST-CREST and Joint
Project of Chemical Synthesis Core Research Institutions.
References and Notes
1
2
3
4
M. Tsuji, J. Kumagae, T. Tsuji, T. Hamagami, J. Hazard. Mater.
2004, 108, 189.
M. Tsuji, K. Noda, H. Sako, T. Hamagami, T. Tsuji, Chem. Lett.
2005, 34, 496.
M. Tsuji, H. Sako, K. Noda, M. Senda, T. Hamagami, T. Tsuji,
Chem. Lett. 2005, 34, 812.
M. Tsuji, H. Sako, M. Senda, K. Noda, T. Hamagami, M.
Kawahara, T. Tsuji, Focus on Hazardous Materials Research,
Nova Science Publishers, New York, 2007, Chap. 4, in press.
a) H. Okabe, Photochemisty of Small Molecules, John Wiley &
Sons, New York, 1978. b) K. Yoshino, W. H. Parkinson, K. Ito,
T. Matsui, J. Mol. Spectrosc. 2005, 229, 238.
IUPAC Gas Kinetic Data Evaluation, Summary Table of Kinetic
Data, June 2006, http://www.iupac-kinetic.ch.cam.ac.uk.
M. Tsuji, M. Senda, M. Kawahara, to be published.
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
NO2 þ hꢀ (172 nm) ! N þ O2
NO2 þ hꢀ (172 nm) ! NO þ O(3P,1D,1S)
NO þ hꢀ (172 nm) ! N þ O
ð1Þ
ð2aÞ
ð2bÞ
5
6
N and O atoms produced in (2b) will be finally converted to N2
and O2 via subsequent reactions in N2 and air.1–6 It seems that
the latter process is more significant, because direct formation
7
8
Published on the web (Advance View) February 3, 2007; doi:10.1246/cl.2007.376