278
Bull. Chem. Soc. Jpn. Vol. 82, No. 2 (2009)
Removal of NOx and N2O by 146-nm Photolysis
0.3
0.2
0.1
0.0
0.2
0.1
0.0
Table 1. Absorption Cross Sections of NO, NO2, N2O, N2,
and O2 at 146 and 172 nm (Ref. 19) and Their Ratios
(a) Before photoirradiation
Cross sections
Cross sections
at 172 nm (·172
NO
Molecules at 146 nm (·146
)
)
·
146/·172
/cm2 moleculeÕ1 /cm2 moleculeÕ1
NO
NO2
N2O
N2
4 © 10Õ18
1.4 © 10Õ17
6.5 © 10Õ18
3.3 © 10Õ21
1.5 © 10Õ17
5 © 10Õ20
1.5 © 10Õ17
1.0 © 10Õ19
0
80
0.93
65
NO2
(b) After photoirradiation
O2
4.6 © 10Õ19
33
NO
Experimental
4000 3500 3000 2500 2000 1500 1000
Wavenumber / cm-1
The VUV photolysis apparatus used in this work was the
same as those reported previously.14Í17 The inside volume of
the photolysis chamber was 185 cm3. Lights from an unfocused
146 nm Kr2 lamp (USHIO, UER20H146: 25 mW cmÕ2, 120Í
165 nm range, full width at half maximum (FWHM): 16 nm) and
172 nm Xe2 lamps (USHIO, UER20H172: 50 mW cmÕ2 or USHIO
trial product: 300 mW cmÕ2 155Í200 nm range, FWHM: 14 nm)
were used to remove NO, NO2, and N2O at room temperature. The
diameter and length of the low power commercial Kr2 and the Xe2
lamps were 30 and 200 mm, whereas those of the high power Xe2
lamp were 70 and 240 mm, respectively.
Figure 1. FTIR spectra of NO (1000 ppm) in N2 (a) before
and (b) after 25 mW cmÕ2 Kr2 excimer lamp irradiation for
30 min.
The following gases were used without further purification: N2
(Taiyo Nissan Inc.: purity >99.9998%), NO (Taiyo Sanso Inc.:
2.02% in high purity N2), NO2 (Nippon Sanso Inc.: 3630 ppm in
high purity N2), N2O (Nippon Sanso Inc.: 959 ppm in high purity
N2), and O2 (Nippon Sanso Inc.: >99.99995%). NO and NO2 in N2
and N2O in N2 or N2/O2 mixtures were diluted before use.
Experiments were carried out in a closed batch system. The total
pressure was maintained at atmospheric pressure. The concen-
tration of NO and NO2 in pure N2 was 1000 ppm (v/v), whereas
that of N2O in N2 was 100 ppm. They were introduced through
mass flowmeters. Before and after photoirradiation, outlet gases
were analyzed using a HORIBA gas analysis system (FG122-LS)
equipped with an FTIR spectrometer and an ANELVA gas analysis
system (M-200GA-DTS) equipped with a quadrupole mass
spectrometer. The detection limit of gases by the mass spectrom-
eter was µ0.1%. The lowly sensitive mass spectrometer was used
for the determination of N2/O2 ratios of buffer gases, whereas the
highly sensitive FTIR system was used for the detection of NOx,
N2O, and O3. The light path length and the volume of analyzing
chamber in FTIR were 2.4 m and 300 cm3, respectively. The
spectra were measured in the 900Í5000 cmÕ1 region with an
optical resolution of 4 cmÕ1. The reliable calibration curves of
NO, NO2, and N2O for FTIR measurements were supplied by
HORIBA Inc. The detection limits of NO2, NO, and N2O were, 1,
1, and µ0.5 ppm, respectively, in our FTIR spectrometer. The
concentration of O3 was evaluated from the absorbance of O3 by
reference to standard spectral data supplied by HORIBA Inc. We
determined the residual amount of a reagent gas A, [A]/[A]0, and
the formation ratio of a product gas B, [B]/[A]0, from gas analysis.
Here, [A]0 is an initial concentration of A. The fluxes of photons in
our 25 mW cmÕ2 146 nm and 50 mW cmÕ2 172 nm lamps were
Results and Discussion
NO Removal in N2. Figures 1a and 1b show FTIR spectra
of NO (1000 ppm) before and after 146 nm photoirradiation.
After photoirradiation for 30 min, an NO peak at µ1900 cmÕ1
decreases in intensity by µ50%, and an NO2 peak at 1620 cmÕ1
appears. The intensity of the NO2 peak is µ4 times stronger
than that of the NO one in Figure 1b because the absorption
peak of NO2 at 1620 cmÕ1 is about one order stronger than that
of NO at µ1900 cmÕ1 at the same concentration. Similar FTIR
spectra of NO were obtained after 172 nm photoirradiation.
Figures 2a and 2b show the dependence of the residual amount
of NO and the formation ratios of NO2, N2, and O2 on the
irradiation time of 25 mW cmÕ2 146 nm Kr2 and 50 mW cmÕ2
172 nm Xe2 lamps, respectively. The residual amount of NO at
146 nm decreases to 60, 48, and 34% with increasing the
irradiation time to 10, 20, and 30 min, respectively. On the
other hand, that at 172 nm decreases to 37, 12, and 9% for the
same irradiation times. The formation ratio of NO2 at 146 nm is
about 20% in the 10Í30 min range, whereas that at 172 nm is
less than 10% in the same time range.
The removal rates of NO under 146 and 172 nm photo-
irradiation were determined assuming that they obey the
following simple first-order decay of molecules.
estimated to be 1.8 © 1016 and 4.3 © 1016 photons cmÕ2 sÕ1
,
respectively, indicating that the photon flux of the 146 nm lamp
is smaller than that of the 172 nm one by a factor of 2.4. When the
removal rates of NO2, NO, and N2O using the two lamps are
compared, they are normalized to these photon fluxes of the lamps.
IR inactive N2 and O2 molecules could not be detected using the
FTIR spectrometer. Therefore, no information could be obtained
about concentrations of N2 and O2 from FTIR measurements.
However, it was possible to evaluate the formation ratios of N2 and
O2 from mass balance of N or O atom between reagent and
products on the basis of gas-phase reactions shown later, when no
products other than N2O, NO, and NO2 were obtained.
k1
NO þ h¯ ꢀꢀ! N þ O
½NOꢁ ¼ ½NOꢁ0 expðꢀk1tÞ
ð1Þ
ð2Þ
The k1 values of NO were evaluated to be 5.4 © 10Õ4 sÕ1 at
146 nm and 1.4 © 10Õ3 sÕ1 at 172 nm from slopes of plots of
ln([NO]/[NO]0) vs. irradiation time in the 0Í30 min range. The
correlation kinetic coefficients (R2) of k1 values in the linear
least square plots were 0.949 at 146 nm and 0.962 at 172 nm. It
is therefore reasonable to assume that the removal rates of NO
obey a simple first order. Taking photon fluxes of the two lamps