Reactivity of surface nitrate species in the selective reduction of NO with propene over Na-H-mordenite as investigated by dynamic FTIR spectroscopy

Article abstract of DOI:10.1039/a705292h

The reactivity of surface adsorbed species over Na-H-mordenite has been examined by using dynamic in situ IR spectroscopy and the mechanism of the selective reduction of NO with C₃H₆ discussed. When Na-H-mordenite was exposed to a flow of NO-C₃H₆-O₂ at 573 K, a strong absorption band assignable to NO₃^- was observed at 1394 cm^-1. The intensity of the band significantly decreased when the flowing gas was switched to C₃H₆-O₂. The rate of decrease in NO₃^- was ca. 3.0 × 10^-8 mol g^-1 s^-1 which was equivalent to the rate of NO conversion to N₂ in the NO-C₃H₆-O₂ flow reaction. Thus, it was indicated that NO₃^- is a reaction intermediate for the formation of N₂ and the surface reaction of NO₃^- is the rate determining step of the formation of N₂ in the selective reduction of NO with C₃H₆. When the flowing gas was switched to C₃H₆, the rate of decrease in NO₃^- species was twice as fast as that in C₃H₆-O₂, i.e., the reactivity of NO₃^- species strongly depended on the presence of O₂. Since NO was also formed simultaneously with N₂ in the case of NO₂-C₃H₆ reaction, it was suggested that NO₃^- species oxidize C₃H₆ accompanying not only the conversion into N₂ but also the dissociative decomposition into NO. Weak bands assignable to adsorbed hydrocarbon, carbonyl, nitrite and isocyanate were also observed. On the basis of the changes in the intensity of these bands, the consecutive reaction of surface organic species with NO₃^- species over Na-H-mordenite was suggested.

Full text of DOI:10.1039/a705292h

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Reactivity of surface nitrate species in the selective reduction of NO
with propene over Na–H-mordenite as investigated by dynamic FTIR
spectroscopy
Atsushi Satsuma,* Takayuki Enjoji, Ken-ichi Shimizu, Kazuhiro Sato, Hisao Yoshida and
Tadashi Hattori
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-01, Japan
The reactivity of surface adsorbed species over NaÈH-mordenite has been examined by using dynamic in situ IR spectroscopy
and the mechanism of the selective reduction of NO with C H discussed. When NaÈH-mordenite was exposed to a Ñow of
3
6
NOÈC H ÈO at 573 K, a strong absorption band assignable to NO ~ was observed at 1394 cm~1. The intensity of the band
3
6
2
3
signiÐcantly decreased when the Ñowing gas was switched to C H ÈO . The rate of decrease in NO ~ was ca. 3.0 ] 10~8 mol
g~1 s~1 which was equivalent to the rate of NO conversion to N in the NOÈC H ÈO Ñow reaction. Thus, it was indicated that
3 6
NO ~ is a reaction intermediate for the formation of N and the surface reaction of NO ~ is the rate determining step of the
3
6
2
3
2
2
3
2
3
formation of N in the selective reduction of NO with C H . When the Ñowing gas was switched to C H , the rate of decrease in
3 6 3 6
NO ~ species was twice as fast as that in C H ÈO , i.e., the reactivity of NO ~ species strongly depended on the presence of O .
3 6
Since NO was also formed simultaneously with N in the case of NO ÈC H reaction, it was suggested that NO ~ species
3 6
oxidize C H accompanying not only the conversion into N but also the dissociative decomposition into NO. Weak bands
2
3
2
3
2
2
2
3
3
6
2
assignable to adsorbed hydrocarbon, carbonyl, nitrite and isocyanate were also observed. On the basis of the changes in the
intensity of these bands, the consecutive reaction of surface organic species with NO ~ species over NaÈH-mordenite was sug-
3
gested.
is known that the activation of hydrocarbons results in the
The reduction of nitrogen oxides has been widely studied, and
formation of various organic nitrogen compounds on the
the selective catalytic reduction (SCR) of NO with hydrocar-
bons in excess oxygen over ion-exchanged zeolites is one of
catalyst surface. Tanaka et al.16 suggested that nitro and
nitrite species are reaction intermediates over Pt/SiO .
the promising methods for NO removal.1h5 Currently, clari-
2
x
Tabata et al.17 observed the formation of nitrite and nitrate
Ðcation of the reaction mechanism is one of the important
species over Cu-MFI by in situ IR spectroscopy.
concerns. The main questions for discussion of the reaction
mechanism are as follows.6h8 (1) How does the selective cata-
lytic reduction begin? (2) What is/are the nature of the surface
intermediates which react with hydrocarbons? (3) What is/are
the Ðnal intermediates?
Isocyanate (NCO) and nitrile (CN) species are thought to
be the Ðnal intermediates in the formation of nitrogen. The
formation of isocyanate over copper-containing catalysts was
reported by Ukisu and workers.18h20 Takeda and Iwamoto21
demonstrated that isocyanate species, formed through the
It is often claimed that the formation of NO through the
2
decomposition of cyanuric acid, react with NO and NO .
oxidation of NO is the initial step for the selective reduction
2
Aylor et al.22 suggested that nitrile species are the reactive
of NO. Hamada and co-workers reported that the reduction
of NO over Al O exhibits higher activity than that of NO,
intermediates which produce nitrogen and carbon dioxide
over Co-ZSM-5.
2
2 3
and suggested that the formation of NO through the oxida-
2
As mentioned above, the roles of various surface species in
the SCR of NO have been well investigated. However, most of
these studies are limited in the qualitative discussion and the
reactivity of these surface species is not well correlated to the
reaction rate of NO in the SCR reaction. In order to clarify
the reaction mechanism, it is necessary to measure the reacti-
vity of the surface species, or, in other words, the reaction rate
of surface species and gas phase molecules should be carefully
compared. From this point of view, we have focused on the
kinetics and dynamics of surface species at the reaction tem-
perature of the SCR of NO. Previously, we have reported that
the alkaline-exchanged zeolites exhibit higher activity per acid
site at 573 K than acidic zeolites for the selective reduction of
tion of NO is the key step in the selective NO reduction. The
key role of NO was also suggested in the case of Ga-ZSM-5
2
by Kikuchi and Yogo10 and for Co-ZSM-5 by Hall and co-
workers.11
As for the surface species, it is suggested mainly from IR
spectra that various types of adsorbed nitrogen oxides react
with hydrocarbons. Adelman et al.12 indicated that adsorbed
nitrate (NO ~) and nitro (NO ) species over Cu-ZSM-5 react
3
2
with propane and then N is formed. Sadykov et al.13 report-
2
ed a good agreement in the temperature range of the catalytic
activity and the desorption temperature of adsorbed nitrite
(
ONO) and nitrate after Ñowing NO species over the catalysts,
such as Cu-ZSM-5, Ca-Fe-ZSM-5 and CuO/Al O , and con-
2
3
NO with C H , which can be correlated to the formation of
cluded that nitrite and nitrate are the reaction intermediates.
3 6
NO ~ species in zeolite channels.23 NO ~ is also suggested to
Hoost et al.14 observed a strong interaction of NO with
3
3
2
be the active species for the selective reduction of NO over
Cu-ZSM-5 as reported by Adelman et al.,12 Hadjiivanov et
al.15 and Centi et al.24 Although the activity of NaÈH-
mordenite is signiÐcantly lower than Cu-ZSM-5, the behavior,
acetone adsorbed on Cu-ZSM-5 according to IR spectroscopy
and suggested that NO and O-containing adsorbed species
2
are the reaction intermediates while Hadjiivanov et al.15 indi-
cated that adsorbed NO over Cu-ZSM-5 converts to NO ~
3
reactivity and role of the adsorbed NO ~ species on NaÈH-
which exhibits very high oxidation ability towards propane. It
3
mordenite can be expected to reÑect the behavior of those on
J. Chem. Soc., Faraday T rans., 1998, 94(2), 301È307
301

View Article Online
Cu-ZSM-5. Here, the reactivity and role of NO ~ species in
3
the SCR of NO over NaÈH-mordenites were examined on the
basis of a kinetic study using dynamic in situ IR spectroscopy.
Experimental
Na-mordenite was supplied from the Committee of the Refer-
ence Catalysts of Catalysis Society of Japan (JRC-Z-Ml0,
SiO /Al O ratio: 9.9). Na-mordenite was at Ðrst fully ion-
2
2 3
exchanged with Na` in an aqueous solution of NaNO (1.0
3
mol dm~3) at 353 K for 24 h and this ion-exchange procedure
was repeated three times. Partially protonated Na-mordenites
(
NaÈH-mordenites) were prepared from fully exchanged Na-
mordenite by ion-exchange in an aqueous solution of
NH NO (1.2È4.8 ] 10~3 mol dm~3) at 353 K for 24 h. The
4
3
ion-exchange levels of Na` calculated from loaded protons
were 95 and 90%, and these catalysts were denoted Na(95)È
H(5)-mordenite and Na(90)ÈH(10)-mordenite, respectively.
The amount of protons determined from chemical analysis
and NH3 temperature programmed desorption23 was in the
range 1È5% for Na(95)ÈH(5)-mordenite and 7È10% for
Na(90)ÈH(10)-mordenite.
Fig. 1 IR spectra of adsorbed species over Na(90)ÈH(10)-mordenite
(a) in Ñowing NOÈC H ÈO for 120 min and (b) followed by He
3
6
2
purge for 20 min at 573 K
In situ IR spectra were measured with JASCO FT/IR-300
instrument equipped with a quartz cell connected to a conven-
tional Ñow reaction system. A self-supporting catalyst disc (20
mm diameter) was set in the center of the quartz cell. All the
spectra were measured at 573 K. The concentrations of NO,
1
630 cm~1 are due to stretching vibration of NxO covalent
bond26 and can be assigned to bidentate NO ~ (1600
3
cm~1)12,17 and NO or NO ~ (1630 cm~1),12,14,15 respec-
2
3
tively. The band at 1862 cm~1 can be attributed to l(NO) of
coordinated NO and/or N O species.14,15,26,29 The band
ads
2 3
C H
and O2
were Ðxed at 940 ppm, 288 ppm and 6.6%,
at 1903 cm~1 can be assigned to gaseous NO 25 or weakly
adsorbed nitrosonium species, i.e., NOd`.15,26,30,31
3
6
respectively. The feed gas was diluted with He and fed to the
catalyst disc at a Ñow rate of 42.2 cm3 min~1.
The Ñow reactions of NOÈC H ÈO and NO ÈC H were
In a separate experiment, very weak and broad absorption
bands were observed in Ñowing C H at 1350È1450, 1600È
3
6
2
2
3 6
3
6
carried out in a conventional continuous-Ñow apparatus at
atmospheric pressure. The composition of feed gas was
NO : C H : O : He\940 ppm : 288 ppm : 6.6% : balance and
1
700 and 2800È3000 cm~1, which can be assigned to
d (CwH), l(CxC) and l(CwH) of C H , respectively.14,25,32
In the Ñowing C H ÈO mixture, the same bands were
s
3 6
3
6
3 6
2
3
6
2
NO : C H : He \ 972 ppm : 288 ppm : balance, respectively.
observed at the same positions.
2
The mixture gases were fed to 1.5 g of a catalyst at a Ñow rate
of 42.2 cm3 min~1 (space velocity \ 1200 h~1), and a steady
state activity was measured after the conÐrmation of a good
carbon balance. The products were analyzed by gas chroma-
Table 1 summarizes the assignment of the bands in Ñowing
tri- and bi-molecular gas mixtures. The bands in the spectrum
of propionic acid adsorbed on Na(90)ÈH(10)-mordenite are
also indicated. As assigned above, the bands at 1394, 1600,
tography and by an NO analyzer (Best BCL-100 uH).
1
630, 1862 and 2435 cm~1 observed in Ñowing NOÈC H ÈO
x
3
6
2
are attributed to adsorbed NO species. Since adsorbed C H
3 6
gave only very weak and broad bands at around 1350È1450,
1
2222 and 2273 cm~1 can be attributed to oxygenated and/or
organic nitrogen compounds. The bands at 1726 cm~1 was
also observed in the spectrum of adsorbed propionic acid, and
can be assigned to l(CxO) stretching of carbonyl.26,32 The
x
Results
600È1700 and 2800È3000 cm~1, the bands at 1682, 1726,
Formation of adsorbed species
Fig. 1(a) shows IR spectra of adsorbed species over NaÈH-
mordenite in Ñowing NOÈC H ÈO . A strong band was
3
6
2
clearly observed at 1394 cm~1 while weak bands were also
observed at 1600, 1630, 1682, 1726, 1862, 1903, 2222, 2273 and
2
435 cm~1. Fig. 1(b) shows a spectrum after the purging with
He. Only the weak band at 1903 cm~1 disappeared, while the
other bands remained.
Fig. 2 shows IR spectra of adsorbed species on NaÈH-
mordenite in Ñowing NOÈO . A strong band was also
2
observed at 1394 cm~1, and weak bands were observed at
1
600, 1630, 1862, 1903 and 2435 cm~1. The intensity of these
bands increased with time on stream of NOÈO and attained
a maximum after 120 min. After an He purge, the bands at
2
1
394 and 2435 cm~1 remained. These bands are attributed to
NO species adsorbed on NaÈH-mordenite. The strong band
x
at 1394 cm~1 has been observed in the IR spectrum of
NaNO , and assigned to l (NO ) of unidentate NO ~.25h28
3
a
2
3
Since the weak band at 2435 cm~1 was also observed in the
IR spectrum of NaNO , the band may be due to the overtone
3
of l (NO ) of NO ~ around 1250È1290 cm~1.25 The weak
s
2
3
bands observed at 1600, 1630, 1862 cm~1 disappeared after an
He purge. These bands can be attributed to weakly adsorbed
NO species on NaÈH-mordenite, since the observed wave-
Fig. 2 IR spectra of adsorbed species over Na(90)ÈH(10)-mordenite
x
numbers were di†erent from those of gaseous NO (770, 1361,
in Ñowing NOÈO
2 at 573 K for (a) 5 min, (b) 15 min, (c) 60 min and
2
1
689 cm~1) and NO (1904 cm~1). The bands at 1600 and
(d) 120 min.
302
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

View Article Online
Table 1 Wavenumber and assignment of absorption bands in in situ IR spectra
trimolecular bimolecular
adsorbed
propionic
acid
Ñowing
after
He purge
Ñowing
Ñowing
wavenumber/cm~1
NOÈC H ÈO
NOÈO
C H ÈO
assignment
ref.
3
6
2
2
3
6
2
1
350È1450
È
s
È
s
È
m
m
È
w
w
w
È
w
w
È
s
w
m
È
m
È
È
È
È
m
È
È
È
È
È
m
CwH
26, 32
25È28
1394
1450
1600
1630
È
È
È
È
w
È
È
È
È
È
È
È
w
NO ~
3
È
m
m
È
w
w
w
w
w
w
w
È
È
m
m
È
È
È
w
CH
14, 32
2
NO ~
12, 17, 27, 28
12, 14, 15
26, 32
16, 17, 25
26, 32
3
NO , NO ~
2
3
1
550È1650
CxC
1682
1726
1862
1903
2222
2273
2435
wONO
Œ
CxO
†
NO , N O
14, 15, 26, 29
15, 25, 26, 30, 31
22, 30, 33, 25, 32
18, 20È22, 25, 30, 33
È
ads
2 3
w
NOd`, gaseous NO
È
È
w
CN, Cy C
NCO
w
È
NO ~ (overtone)
3
2
800È3000
È
CwH
26, 32
s \ Strong (absorbance [ 0.1), m \ medium (0.1 [ absorbance [ 0.01), w \ weak (0.01 [ absorbance), È \ not observed.
band at 1682 cm~1 can be attributed to nitrite (ONO).16,17,25
A weak band at 2273 cm~1 has been observed in various
types of catalysts, especially copper-containing catalysts, and
attributed to isocyanate (NCO) species.18,20h22,30,33 It is pos-
sible that a shoulder at 2222 cm~1 is assignable to both
l(Cy N) of nitrile22,30,33 and Cy C stretching mode of
alkyne.25,32
was estimated from the molar absorption coefficient of NO ~
3
calculated from the spectra of NaNO measured using the
3
KBr method. As seen in Fig. 4, NO ~ species decreased only
3
slightly in Ñowing He, indicating that NO ~ species are rela-
3
tively stable in an inert gas. On the other hand, the concentra-
tion of NO ~ species rapidly decreased in the presence of
3
C H . The rate of the decrease in NO ~ species was deter-
3
6
3
mined as the negative of the slope of the depletion curve,
Dynamics of adsorbed NO species
x
([[NO ~]/dt) , and these are listed in Table 2. For
3
t/0
Fig. 3 shows IR spectra of adsorbed species on NaÈH-
mordenite. The spectrum in Fig. 3(a) was measured after
Na(90)ÈH(10)-mordenite, the rate of the decrease in NO ~
3
species in Ñowing C H ÈO was ca. 3.0 ] 10~8 mol g~1 s~1,
3
6
2
pretreatment in Ñowing NOÈO for 120 min followed by an
and that in Ñowing C H was 7.0 ] 10~8 mol g~1 s~1, i.e.
3 6
2
He purge at 573 K. Then, the Ñowing gas was switched to
about twice that in the presence of oxygen. A similar result
C H ÈO , and the spectra of Fig. 3(b)È(d) were obtained. The
was also obtained for Na(95)ÈH(5)-mordenite. However, for
3
6
2
weak bands disappeared, while the band at 1394 cm~1 signiÐ-
cantly decreased in Ñowing C H ÈO . In these spectra, the
3
6
2
shift or broadening of the band at 1394 cm~1 was not
observed. Since the band assignable to CwH bending mode
around 1394 cm~1 was negligibly small in Ñowing C H ÈO
3
6
2
(
Table 1), the decrease in intensity of the band at 1394 cm~1
should simply reÑect the consumption of the adsorbed NO ~
species. On the other hand, the intensity of the bands around
3
1
600È1700 cm~1 increased, indicating the increase in surface
species containing nitrite and/or carbonyl groups.
Fig. 4 shows the depletion of the band at 1394 cm~1 in
Ñowing He, C H and C H ÈO . The concentration of NO ~
3
6
3 6
2
3
Fig. 4 Depletion of the band at 1394 cm~1 over Na(90)ÈH(10)-
mordenite in various atmospheres at 573 K as a function of time.
Before the measurement, the catalyst disc was treated in Ñowing
NOÈO for 120 min followed by He purge for 20 min.
2
Table 2 Rate of decrease in NO ~ species on NaÈH-mordenites at
3
5
73 K
ion-exchanged (%)
rate of decrease in NO ~/10~8 mol g~1 s~1
3
Na`
H`
in C H ÈO
in C H
in He
Fig. 3 IR spectra of adsorbed species over Na(90)ÈH(10)-mordenite
3
6
2
3
6
at 573 K. The catalyst disc was treated in Ñowing NOÈO for 120
1
05
È
5
10
0.0
2.5
3.0
0.0
4.3
7.0
0.0
0.6
1.2
2
min. The spectra were measured (a) after He purge for 20 min, fol-
9
9
5
0
lowed by switching to Ñowing C H ÈO for (b) 30 min, (c) 120 min
3
6
2
and (d) 240 min.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
303

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Na(105)-mordenite containing excess Na`, the concentration
of NO ~ did not decrease at all.
3
Dynamics of adsorbed organic species
Fig. 5 shows IR spectra of adsorbed species measured in
Ñowing NOÈO . After the exposure of the catalyst disc to
2
Ñowing C H for 60 min followed by purging with He, broad
3
6
bands assignable to l(CwH) and l(CxC) stretching of
adsorbed C H were observed at 2800È3000 and 1550È1650
3
6
cm~1, respectively [Fig. 5(a)]. The bands at 2800È3000 cm~1
immediately decreased after the Ñowing gas was switched to
NOÈO [Fig. 5(b)È5(d)], while bands around 2200È2300 and
2
1
550È1903 cm~1 appeared. As shown in Table 1, these bands
can be attributed to adsorbed isocyanate (2273 cm~1), carbon-
yl (1726 cm~1), nitrite (1682 cm~1), NO and/or NO ~ (1600,
2
3
1
630 cm~1) and coordinated NO species (1903, 1862 cm~1),
x
respectively. The intensities of these bands varied with time on
stream, as shown in Fig. 6. The band assignable to l(CÈH) at
2
800È3000 cm~1 rapidly decreased. The bands attributed to
Fig. 6 Intensity of the bands of the adsorbed propene (L), carbonyl
carbonyl (1726 cm~1) and nitrite (1682 cm~1) initially
increased and then decreased. The maximum intensities of
these bands were observed at 9.5 min. For the band assignable
to isocyanate (2273 cm~1), an inductive period was observed
and the maximum intensity of the band was observed at 23
min. Since the contribution of external CO cannot be
neglected, the bands at 2300È2400 cm~1 attributable to
gaseous CO were not discussed.
(K), nitrite (|) and isocyanate ()) species on NaÈH-mordenite in
NOÈO as a function of time. Before the measurement, the catalyst
2
disc was treated in Ñowing C H for 60 min followed by He purge for
2
3
6
0 min.
rate of formation of N , CO and CO, the selectivity can be
2
2
2
estimated as the contribution of NO and O to the oxidation
2
of C H .34
3
6
2
Generally, the NOÈC H ÈO reaction proceeds as a com-
3
6
2
petitive oxidation of C H with NO and O . When oxidant
Catalytic runs of NO–C H –O and NO –C H reaction
3
6
2
3
6
2
2
3 6
from NO and/or O is represented as (O) the competitive
2
Table 3 shows the results in the NOÈC H ÈO Ñow reaction
over NaÈH-mordenite. For both Na(90)ÈH(10)- and Na(95)È
H(5)-mordenite, the reaction rate of NO in the Ñow reaction,
reaction can be simpliÐed into the following components:
3
6
2
NO ] 1/2N ] (O)
(1)
(2)
(3)
(4)
2
2
.5È2.8 ] 10~8 mol g~1 s~1, was equivalent to the rate of
O ] 2(O)
decrease in NO ~ in Ñowing C H ÈO . On the basis of the
2
3
3 6
2
1
/3C H ] 3(O) ] CO ] H O
3
6
2
2
1
/3C H ] 2(O) ] CO ] H O
3
6
2
Eqn. (1) and (2) illustrate the reduction of NO and O with
2
C H which eliminates the oxidant (O), while eqn. (3) and (4)
3
6
describe the oxidation of C H with NO and/or O . From
3
6
2
eqn. (3) and (4), the amount of total (O) atoms supplied from
NO and O is equivalent to (3CO ] 2CO). Thus, the fraction
2
2
of (O) atoms supplied from NO, i.e., the selectivity, is
2
N /(3CO ] 2CO).
2
2
As shown in Table 3, the value of the selectivity, 2N /(3CO
2
2
]
2CO), was 31% over both Na(90)ÈH(10)- and Na(95)ÈH(5)-
mordenite. This value indicates that the contribution of NO
and O as oxidant of C H was close to NO : O \ 1 : 1
2
3 6
2
(
NO/O in Table 3).
2
Table 3 also shows that Na(105)-mordenite exhibited no
activity in the NOÈC H ÈO Ñow reaction. Since Na(105)-
3
6
2
mordenite contains excess Na` ions but no residual acid sites,
this result indicates that the presence of acid sites are also
essential for the selective reduction of NO over NaÈH-
Fig. 5 IR spectra of adsorbed species over Na(90)ÈH(10)-mordenite
mordenite.9,10,34,35 Since NO ~ species were also formed on
at 573 K. The catalyst disc was treated in Ñowing C H for 60 min.
3
3
6
The spectra were measured (a) after He purge for 20 min, followed by
Na(105)-mordenite, the acid sites would be mainly e†ective for
switching to Ñowing NOÈO for (b) 9.5 min, (c) 23 min and (d) 60 min.
the activation of C H .
2
3 6
Table 3 Results in NOÈC H ÈO Ñow reaction over NaÈH-mordenites at 573 K
3
6
2
rate/10~8 mol g~1 s~1
ion-exchanged (%)
Na` H`
05
reaction
NO
formation
(
2N /3CO ] 2CO)
2
2
N2
0.0
1.2
1.4
CO2
0.0
2.3
2.5
CO
(%)
NO/O a
xb
2
1
È
0.0
2.5
2.8
0.0
0.7
0.8
0
30.5
31.1
0.0
2.5/2.8
2.8/3.2
0
1.53
1.56
9
9
5
0
5
10
a Ratio of reaction rate of NO and O . b See text.
2
304
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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Table
4
shows the production distribution in the
NO ~ undergoes di†erent reactions in the presence of oxygen
3
NO ÈC H Ñow reaction over NaÈH-mordenite. for the
from that in the absence of oxygen. The product distribution
2
3 6
NO ÈC H reaction, NO was also produced together with
3 6
N . The similar behavior of NO was reported by Lukyanov
in the NOÈC H ÈO Ñow reaction (Table 3) indicates that the
2
3 6
2
contribution of NO and O as oxidant was NO : O \ 1 : 1,
2
2
2
2
et al. 11 that NO is converted into NO in NO ÈCH reaction
suggesting the stoichiometry of the NOÈC H ÈO reaction
can be represented as follows:
2
2
4
3 6
2
over Co-ZSM-5. The dissociation of adsorbed NO over Cu-
2
ZSMÈ5 into NO and oxygen has been reported by Li and
1
3
Armor36 and Adelman et al.37 in the absence of hydrocarbon.
Since the formation of NO was not observed in the empty
reactor, Table 4, NO must be produced during the reaction of
NO with C H over the catalyst. It should be noted that O
NO ] O ]
(C H ) ] 1/2N ]
(CO ] H O)
x
2
x ] 1 3 6
2
x ] 1
2
(
7)
2
3 6
2
was not formed in the NO ÈC H Ñow reaction. From the
The parameter x was ca. 1.5 in these experiments. The contri-
2
3 6
product distribution, the reaction of NO
as follows:
2 can be represented
bution of NO and O in eqn. (7) can be extracted as follows:
2
NO ] O ] 1/2N ] 3(O)
(8)
2
2
2
NO ] 1/2N ] NO ] 3(O)
(5)
2
2
where (O) represents oxygen atoms in produced carbon oxides
and water.
where (O) represents oxygen atom in produced CO, CO and
2
H O.
For the NO ÈC H reaction (Table 4), NO was formed
2
2
3 6
together with N . The fact indicates that the oxygen atoms
2
Discussion
(O) are incorporated into carbon oxides and water from both
the conversion of NO
into NO.
2 into N
and the dissociation of NO
2
2
Reactivity of NO — species
3
In the IR spectra of adsorbed species on NaÈH-mordenite
under the reaction conditions, a strong band was observed at
NO ] 1/2N ] 2(O)
(9)
2
2
NO ] NO ] (O)
(10)
1
394 cm~1 which was assigned to NO ~ species and was in
2
3
accord with l (NO ) of NaNO .26,32 Since NO ~ was not
a
2
3
3
Since the ratio of the rate of formation of N and NO was
observed over H-mordenite,23 NO ~ species should be
2
3
N : NO \ 1 : 2 the contribution of eqn. (9) and 10) is equiva-
adsorbed around Na` ions. In Ñowing NOÈO , the rate of
2
2
lent. On the other hand, the oxidation of NO to NO [eqn.
formation of NO ~ species was 4.7 ] 10~7 mol g~1 s~1 (Fig.
2
(11)] should be the Ðrst step for the production of N for the
2
3
2
). NO ~ is known to be formed by the adsorption of NO on
3
2
NOÈC H ÈO reaction, and the produced NO would adsorb
metal oxides, such as alumina,27 titania38,39 and chromia.40
3 6
2
2
rapidly around Na` sites to form NO ~ species.
Parkyns27 reported that the adsorption of NO on alumina
3
2
results in the formation of NO ~ being coordinated via
2NO ] O ] 2NO
(11)
3
2
2
oxygen to Al3`. The formation of NO ~ species over
3
Cu-ZSM-5 by the adsorption of NO was also reported by
2
Consequently, the combination of eqn. (9)È(11) leads to
Hadjiivanov et al.15 Adelman et al.37 revealed that NO is oxi-
NO ] O ] 1/2N ] 3(O)
(12)
2
2
dized over Na-zeolites by O to form NO adsorbed species.
2
2
Eqn. (12) obtained mainly from the NO ÈC H reaction is the
Thus, NO ~ species should originate from adsorbed NO
species and form through the reaction of NO and basic
oxygen sites coordinated with Na` in zeolite channel as
2
3 6
3
2
same as eqn. (8) obtained from the product distribution in the
2
NOÈC H ÈO reaction. The agreement of NO and O
3 6
2
2
balance in the NOÈC H ÈO and NO ÈC H reactions indi-
follows:
3 6
2
2
3 6
cates that a similar mechanism is occurring in both the reac-
tions.
NO ] O~¼Na` ] NO ~¼Na`
z }
(6)
2
3
Scheme 1 summarizes the above reaction sequence of NO ~
formation and conversion. The di†erence in the depletion rate
of NO ~ in the presence and absence of oxygen (Fig. 4) can be
well explained by this mechanism. In the presence of oxygen,
3
Since NO ~ species adsorbed on NaÈH-mordenite were
3
depleted in the presence of C H , as shown in Fig. 3 and 4, it
3
6
3
was demonstrated that the NO ~ is highly reactive with
3
3
C H . The rate of decrease in NO ~ species in the presence of
NO formed through eqn. (10) will be easily reoxidized to NO
3
3
6
6
2
C H and O measured by the dynamic method was 2.5È
which is in turn adsorbed as NO ~ [eqn. 11(c)]. Thus, NO ~
2
3
3
3
.0 ] 10~8 mol g~1 s~1, which was lower than the rate of
decreases only through eqn. (9). In the absence of oxygen, on
formation of NO ~ species and equivalent to the reaction rate
the other hand, NO ~ decreases through both the pathways
3
3
of NO in the SCR Ñow reaction, i.e., 2.5È2.8 ] 10~8 mol g~1
of eqn. (9) and (10), because NO formed via process (10) can
s~1 (Tables 2 and 3). The good agreement in these rates
not be reoxidized.
clearly indicates that NO ~ is an intermediate of the SCR of
One might suspect from the stoichiometry of eqn. (7), i.e.,
3
NO and is related to the rate determining step for the forma-
the contribution of NO and O as oxidant being equal to
2
tion of N .
NO : O \ 1 : 1, that half of oxygen oxidizes NO to NO and
2
2
2
the other half directly oxidizes C H . This is because the stoi-
Role of NO — species for oxidation of C H
3 6
3
3 6
chiometry of the former reaction should be NO : O \ 2 : 1 on
2
As shown in Table 2, the rate of decrease in NO ~ species
the assumption that oxygen takes part in the reaction only
3
depended on the presence of oxygen, i.e., the rate in Ñowing
through the oxidation of NO into NO , [eqn. (11)]. It should
2
C H was twice that in Ñowing C H ÈO . This suggests that
be noted, however, that NaÈH-mordenite was not active for
3
6
3 6
2
Table 4 Product distribution in NO ÈC H reaction over NaÈH-mordenite at 573 K
2
3 6
rate of reaction
10~8 mol g~1 s~1
rate of formation
/10~8 mol g~1 s~1
/
catalyst
NO2
C H
NO
N2
CO2
CO
3
6
Na(95)ÈH(5)-mordenite
18.5
0.0
5.8
0.1
6.6
0.0
3.5
0.0
8.4
0.0
2.7
0.0
blank
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
305

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only oxidize organic species with dissociative decomposition
into NO and (O) (oxidant). Since the rate of formation of N
2
agreed well with the rate of depletion of NO ~ species, the
3
reaction steps of NO ~ with the organic species would be rate
3
determining. This may be due to very small number of acid
sites (\10%) relative to Na` ions for the adsorption of
NO ~, or, in other words, the reaction rate of NO ~ and the
3
3
organic species may be suppressed because of the low surface
concentration of organic species adsorbed on acid sites.
Conclusion
In this study, the reactivity of surface NO ~ species in the
Scheme 1 Proposed mechanism of the selective reduction of NO
with propene over NaÈH-mordenite. Numbers in parentheses corre-
spond to the equations in the text.
3
selective reduction of NO with C H over NaÈH-mordenite
3
6
was investigated by using dynamic in situ IR spectroscopy.
The depletion of surface NO ~ species in the presence of
the C H ÈO reaction at 573 K.23 The proposed mechanism
3
C H strongly indicated that NO ~ is the reaction interme-
3
6
2
indicates that the stoichiometry of NO : O \ 1 : 1 is possible
3 6
3
diate and reacts with surface organic species. Because of the
2
even when oxygen does not directly react with C H . The
good agreement with the rate of depletion of NO ~ and the
3
6
determinative step is a redox cycle of NO shown by eqn. (10)
and (11c) in Scheme 1. Since step (11a) corresponds to an initi-
ation step in the chain reaction, its contribution to the conver-
3
reaction rate of NO in the NOÈC H ÈO Ñow reaction, it was
3 6
2
clearly conÐrmed that the reaction of NO ~ with surface
organic species is the rate determining step in the formation of
3
sions of NO and O can be neglected. Therefore, NO is
N . The behavior of NO ~ species in the presence and
2
converted only through step (11b), and oxygen through steps
2
3
absence of oxygen and the stoichiometry in the Ñow reactions
(
11b) and (11c). The stoichiometry of NO : O2
must be 1 : 1,
suggested that NO ~ species react to produce N as a Ðnal
because step (11b) is identical to step (11c). Thus, it is clearly
3
2
product and dissociate to NO. It was also found that NO ~
indicated that oxygen cannot directly oxidize C H on NaÈH-
3
acts as a carrier of oxygen into surface organic species in the
3
6
mordenite, but that it participates in the oxidation of C H
latter reaction. C H consequently is converted into adsorbed
3
6
only through the formation of NO at the stoichiometry of
3 6
carbonyl, nitrite and isocyanate species. Finally, the reaction
2
NO : O \ 1 : 1.
mechanism of the selective reduction of NO with C H
NaÈH-mordenite has been proposed.
6 over
2
3
Consecutive reaction of surface species
The changes in the intensity of the adsorbed species in Fig. 5
suggested that the surface organic species react consecutively
over the catalysts. As shown in Fig. 6, adsorbed C H con-
This work was partly supported by a Grant-in-Aid from the
Ministry of Education, Science and Culture, Japan (No.
07242105 and 06555242).
3
6
verts to carbonyl and/or organic nitrite, and Ðnally iso-
cyanate. Surface carbonyl and nitrite were also observed when
the Ñowing gas was switched from NOÈO to C H ÈO (Fig.
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