Catalytic reduction of N2O by various hydrocarbons over Fe-ZSM-5: Nature and reactivity of carbonaceous deposits

Article abstract of DOI:10.1016/j.molcata.2004.09.083

The catalytic reduction of dinitrogen monoxide (N₂O) by various hydrocarbons (CH₄, C₂H₄, C₂H ₆, C₃H₆, C₃H₈) in the absence and presence of O₂ has been studied over Fe-ZSM-5 catalysts. These hydrocarbon reductants are phenomenologically divided into three groups, namely CH₄, C₃H₆, and others (C ₂H₄, C₂H₆, C₃H ₈), referred to as the C₂H₄ group. Two types of carbonaceous deposits (Cα, Cβ) are formed on Fe-ZSM-5 during the reduction of N₂O by C₂ and C₃ hydrocarbons in the absence of O₂. In both cases of C₂H₄ and C₃H₆, the catalytic activity of Fe-ZSM-5 decreases with an increase in the amount of Cα, while it is not affected by the presence of Cβ. The Cα species is formed on Fe sites and the Cβ is mainly accumulated on the support. The formation of these carbonaceous deposits from the C₂H₄ group is suppressed by the presence of O ₂ in the feed gas, and this promotes the catalytic reduction of N₂O. The amount and the chemical nature of Cα formed in the cases of the C₂H₄ group and C₃H₆ are similar, while those of Cβ are significantly different. The reactivity of Cβ with O₂ should be different and this may be responsible for the difference in the effects of O₂ addition on the reduction of N₂O between the C₂H₄ group and C ₃H₆. In the case of CH₄, a high stable conversion of N₂O is obtained irrespective of the presence and absence of O₂ because the carbonaceous deposits are scarcely accumulated on the catalyst.

Full text of DOI:10.1016/j.molcata.2004.09.083

Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
Catalytic reduction of N O by various hydrocarbons over Fe-ZSM-5:
2
nature and reactivity of carbonaceous deposits
a
a
b
a,∗
T. Chaki , M. Arai , T. Ebina , M. Shimokawabe
a
Division of Materials Science and Engineering, Graduated School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
b
National Institute of Advanced Industrial Science and Technology, Sendai 983-8551, Japan
Received 28 June 2004; received in revised form 17 September 2004; accepted 30 September 2004
Available online 2 December 2004
Abstract
The catalytic reduction of dinitrogen monoxide (N
of O has been studied over Fe-ZSM-5 catalysts. These hydrocarbon reductants are phenomenologically divided into three groups, namely
CH , C
Fe-ZSM-5 during the reduction of N
2 4 2 4 2 6 3 6 3 8
O) by various hydrocarbons (CH , C H , C H , C H , C H ) in the absence and presence
2
4
3
H
6
, and others (C
2
H
4
, C
2
H
6
, C
O by C
3
H
8
), referred to as the C
2
H
4
group. Two types of carbonaceous deposits (C␣, C␤) are formed on
. In both cases of C and C , the catalytic activity
2
2
and C hydrocarbons in the absence of O
3
2
2
H
4
3 6
H
of Fe-ZSM-5 decreases with an increase in the amount of C␣, while it is not affected by the presence of C␤. The C␣ species is formed on Fe
sites and the C␤ is mainly accumulated on the support. The formation of these carbonaceous deposits from the C group is suppressed by
the presence of O in the feed gas, and this promotes the catalytic reduction of N O. The amount and the chemical nature of C␣ formed in the
cases of the C group and C are similar, while those of C␤ are significantly different. The reactivity of C␤ with O should be different
and this may be responsible for the difference in the effects of O addition on the reduction of N O between the C group and C . In the
case of CH , a high stable conversion of N O is obtained irrespective of the presence and absence of O because the carbonaceous deposits
are scarcely accumulated on the catalyst.
2004 Elsevier B.V. All rights reserved.
2
H
4
2
2
2
H
4
3
H
6
2
2
2
2
H
4
3 6
H
4
2
2
©
Keywords: Fe-ZSM-5; N2O reduction; Hydrocarbon; Carbonaceous deposit
1
. Introduction
and C3H as reductants in the presence and absence of O2
6
was studied with various metal ion-exchanged ZSM-5 cata-
lysts. Pronounced activities were observed with Fe-ZSM-5,
Pd-ZSM-5, and Pt-ZSM-5 catalysts, and no significant de-
activation was detected with Fe-ZSM-5 in the presence of
O2, whereas the activities of Pt- and Pd-ZSM-5 decreased
drastically by the presence of 5% O2. Segawa et al. [10–12]
Dinitrogen monoxide (N2O) is well known to be a green-
house gas component and contribute to the catalytic destruc-
tion of ozone in the stratosphere [1]. Therefore, the removal
of N2O by suitable catalytic methods has been a very im-
portant subject in order to protect the global environment.
Recently, several research groups have reported high cat-
alytic performance of various metal ion-exchanged zeolites
and other metal containing catalysts for decomposition [2–7]
and reduction of N2O with hydrocarbons [8–20], activated
carbon [21–23], CO [24–27], and NH3 [27,28]. In our pre-
vious work [13], the catalytic reduction of N2O using CH4
studied the selective reduction of N2O using C3H as a reduc-
6
tant and reported high reaction rates over Fe/MFI even in the
presence of O2 and H2O. These authors assumed that the ad-
sorption and protonation of C3H were important steps in the
6
reduction of N2O by C3H over Fe/MFI [11]. Kunimori and
6
his co-workers [14–16] reported that ion-exchanged Fe-BEA
and Fe/MFI show good performance in the selective catalytic
reduction of N2O by CH4 and C3H in the presence of excess
O2. These authors [17,18] stated that the CHxOy(a) species
∗
6
Corresponding author. Fax: +81 11 706 7556.
E-mail address: shimo@proc-ms.eng.hokudai.ac.jp
(M. Shimokawabe).
such as methoxy and formate species formed at initial steps of
1
381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.09.083

1
88
T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
the reduction of N2O by CH4, and may play an important role
in the activation/oxidation of CH4. Zhu et al. [20] indicated
that the activity of Cu- or Co-loaded activated carbon in N2O
conversion was higher than that of Cu/ZSM-5 or Co/ZSM-5,
respectively. They [22,23] also investigated the role of O2 in
NO– and N2O–carbon reactions and reported that the pres-
ence of O2 greatly enhanced the NO–carbon reaction while it
suppressed the N2O–carbon reaction. However, little funda-
mental study has been concluded pertaining to the reduction
of N2O by carbon species including reaction intermediates
in the presence of O2 over metal-loaded zeolites.
Several research groups have reported the nature and the
role of oxygen species in the oxidation of hydrocarbons over
metal-exchanged zeolites, Li and Armor [29] studied Pd-
exchanged zeolites (ZSM-5, mordenite, and ferrierite) cat-
alysts for CH4 oxidation. They reported that Pd supported on
these zeolites showed much higher activities than Pd/Al2O3
and suggested that extra-lattice oxygen atoms could be very
active in the complete oxidation of CH4 at low temperature.
Panov et al. [30] proposed that a surface oxygen species,
dant (O2, N2O) has been also investigated to elucidate the
role of O2 in the oxidation of carbonaceous deposits.
2. Experimental
2.1. Catalyst preparation
Fe-ZSM-5 catalysts were prepared by a conventional ion-
exchange method using Na-ZSM-5 (SiO2/Al2O3 = 23.8) sup-
plied from Tosoh Co. Ltd. Na-ZSM-5 (4 g) was added to
3
−2
100 cm of FeSO4·7H2O aqueous solution of 2.45 × 10
M
and the resulting mixture was then stirred at 343 K for 12 h.
After filtration, the metal-supported zeolites were washed
with distilled water, dried at 383 K for 24 h, and calcined
in air at 773 K for 3 h. The content of Fe loaded was fixed at
3.4 wt.%, corresponding to 100% ion-exchangeable level.
2.2. N2O reduction
␣
-oxygen, generated in Fe-ZSM-5 under N2O decomposi-
The reaction was carried out in a conventional flow re-
−
3
tion exhibited high reactivity in oxidation of methane, ben-
zene and CO. They concluded that (1) ␣-oxygen was formed
on Fe-ZSM-5 but not on the other iron catalysts and that
actor at a W/F of 0.06 g s cm and at 598 K. The reactor
was made of 9 mm diameter Pyrex glass tubing in which a
catalyst sample of 0.05 g was mounted on loosely packed
quartz wool. Prior to the runs, the catalyst was treated in
a stream of He at 773 K for 2 h and cooled to the reaction
temperature.
(2) it was produced upon N2O decomposition but not upon
O2 adsorption. Sachtler et al. [31] identified the oxygen de-
posited by N2O decomposition on Fe/MFI by means of iso-
topic exchange technique. They suggested that [Fe2O2]2+
type ions should be considered in addition to the mononu-
clear [Fe O]2 ion as the oxo-species of dissociative N2O
adsorption on iron ions in partially pre-reduced and dehy-
drated Fe/MFI. However, little fundamental study has been
made concerning the nature and the role of surface oxygen
in the oxidation of hydrocarbons over Fe-zeolite catalysts.
Recently [32], we have reported that two types of carbona-
ceous deposits (C␣, C␤) are formed on Fe-ZSM-5 catalysts
during the N2O reduction by C2H4 in the absence of O2,
and the catalytic activity decreases with an increase in the
amount of C␣ formed. The C␣ species may be formed on the
Fe sites. It was found that the catalytic reduction of N2O by
C2H4 is promoted by O2, which prevents the accumulation
of carbonaceous deposits on Fe sites. These carbonaceous
deposits were scarcely accumulated by supplying only C2H4
over Fe-ZSM-5 or N2O–C2H4 mixture over Na-ZSM-5 par-
ent zeolite; therefore, these carbonaceous deposits are formed
through the reaction between N2O and C2H4 over Fe sites.
In the present study, the reduction of N2O by various hydro-
The reactant gases used were N2O and a hydrocarbon of
CH4, C2H4, C2H , C3H , or C3H8 diluted by He. The con-
6
6
+
centration used for CH4, C2H4, C2H , C3H , C3H8, and N2O
6
6
was 3000, 2000, 2000, 1300, 1300, and 2000 ppm, respec-
tively. In our previous work [32], a 2000 ppm concentration
of C2H4 was used. One mole of C2H4 is equivalent to six
moles of O atoms if the following stoichiometry is assumed:
C + 2O → CO2 and H2 + O → H2O. The concentration of the
other hydrocarbons was also adjusted to approximately have
the same equivalent number of reducing C and H atoms. To
examine the influence of O2, it was introduced into the feed
gas in different quantities up to 5%. The concentrations of
N2O, N2, O2, CO, CO2, and hydrocarbons in the outflow gas
were determined using gas chromatographs (Hitachi 663-50
and 063) with porapak Q and molecular sieve 5A columns.
The concentration of NO2 was monitored using a UV–vis
spectrophotometer (Hitachi Model 100-10). Because of low
concentrations of N2O and hydrocarbons used, the total flow
rate was practically constant throughout the catalyst bed.
carbons (CH4, C2H , C3H , C3H8) in addition to C2H4 in
2.3. Catalyst characterization
6
6
the presence and absence of O2 has been studied over Fe-
ZSM-5. The reducing ability of the hydrocarbons has been
discussed in terms of the accumulation of carbonaceous de-
posits on the catalyst. The nature and the structure of the
carbonaceous deposits have been investigated by means of
temperature-programmed oxidation (TPO) and X-ray photo-
electron spectroscopy (XPS) analysis. The reactivity of the
carbonaceous deposits and gaseous product (CO) with oxi-
Temperature programmed oxidation experiments were
carried out in the same reactor as used for the N2O reduction.
After the reaction, the reactor was cooled to room tempera-
ture, the stream of reactants was switched to He, and it was
allowed to flow through for 60 min, and then programmed
heating was started from room temperature to 773 K at a
−
1
rate of 5 K min in the stream of 5000 ppm O2 in He. The

T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
189
amounts of gases (CO, CO2) evolved were determined from
the peak areas of TPO curves.
depending on the reductants used. As described in our previ-
ous report [32], the reduction of N2O by C2H4 was strongly
inhibited by formation of the carbonaceous deposits on the
catalyst. The decrease in the conversion of N2O observed
with the other hydrocarbons could be also explained on the
basis of the accumulation of carbonaceous deposits on the
catalyst. When O2 (5000 ppm) was added (Fig. 1b), the N2O
conversion rapidly decreased from about 95 to 75% and re-
mained at around 75% in the N2O reduction by C2H4, C2H6
X-ray photoelectron spectroscopy (XPS)analysis was per-
formed in a ULVAC PHI ESCA 5600 instrument. Al K␣ ra-
diation (14.0 kV, 400 W) was used to excite photoelectrons,
which were detected with an analyzer operated at 1253.6 eV
constant pass energy. Correction of the energy shift due to
steady state charging was accomplished by taking the C1s
line from adsorbed carbons at 284.5 eV as an internal stan-
dard.
and C3H8 in addition to CH4. For C3H , the N2O conversion
6
to N2 also decreased rapidly at the initial stage of reaction but,
in contrast, it then decreased gradually with time on stream.
The influence of O2 is significantly different between C3H6
and the other hydrocarbons.
3
. Results and discussion
3
.1. N2O reduction by various hydrocarbons
3.2. Formation and influence of carbonaceous deposits
Fig. 1 shows the conversion of N2O to N2 as a func-
on the activity of catalyst
tion of reaction time in the reduction of N2O (2000 ppm) by
various hydrocarbons, CH4 (3000 ppm), C2H4 (2000 ppm),
C2H (2000 ppm), C3H (1300 ppm), or C3H8 (1300 ppm),
TPO was used to characterize chemical species formed
on the surface of catalyst after N2O reduction. Fig. 2 shows
TPO profiles after N2O reduction over Fe-ZSM-5 by hydro-
carbons, CH4, C2H4, C2H6, C3H6, or C3H8, in the absence
of O2 at 598 K for 2 h. CO and CO2 were observed to evolve
along with a trace amount of H2O. When CH4 was used, CO
and CO2 were scarcely evolved. When C2H4, C2H6, C3H6,
or C3H8 was used, the evolution peak of CO appeared at ap-
proximately 570 K and CO2 peaks appeared at approximately
570 and around 670 K. The peaks centered at temperatures of
570 and 670 K are henceforth referred to as ␣- and ␤-peak,
respectively. These TPO results show the formation and accu-
mulation of carbonaceous deposits on the catalyst during the
N2O reduction by C2 and C3 hydrocarbons. In our previous
work [32], we classified the carbonaceous deposits formed
during N2O reduction by C2H4 into two types by their reac-
tivity with O2 in TPO, and the carbonaceous deposits evolved
6
6
in the absence and presence of O2 (5000 ppm) over Fe-ZSM-
at 598 K. These hydrocarbon reductants are phenomeno-
5
logically divided into three groups, namely CH4, C3H , and
6
others (C2H4, C2H , C3H8), referred to as the C2H4 group.
6
When CH4 was employed for the reductant in the absence of
O2 (Fig. 1a), a stable conversion higher than 90% was ob-
served during the reaction. On the other hand, when C2H4,
C2H , C3H and C3H8 were used, the conversions of N2O
6
6
decreased rapidly with time on stream at the initial stage of
reaction and then changed gradually to steady-state values
Fig. 1. The catalytic reduction of N2O (2000 ppm) by hydrocarbons in the
absence (a) and presence (b) of O2 (5000 ppm) over Fe-ZSM-5 at 598 K.
Fig. 2. TPO curves of carbonaceous deposits accumulated over Fe-ZSM-5
during N2O (2000 ppm) reduction by hydrocarbons at 598 K for 2 h in the
absence of O2 ((ꢀ) CO; (ꢁ) CO2).
(ꢀ) CH4 (3000 ppm); (᭹), C2H4 (2000 ppm); (ꢀ), C2H6 (2000 ppm); (ꢁ),
C3H6 (1300 ppm); (ꢁ), C3H8 (1300 ppm).

1
90
T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
regardless of the presence of O2, since the carbonaceous de-
positswerescarcelyformedfromCH4. WhenC3H wasused,
6
a considerable amount of C␤ was accumulated exceptionally
even in the presence of O2.
Fig. 2 and Table 1 indicate that two types of carbonaceous
deposits were formed on the catalyst irrespective of the kinds
of hydrocarbons used expect for CH4. The saturated amounts
of C␣ formed during the N2O reduction in the absence of O2
−
1
for 2 h were similar, 1.0–1.4 mmol g , for C2 and C3, while
those of C␤ were different. Previously, it was found that the
composition of C␣ is represented by approximately CH and
that the amount of C␣ formed on the Fe-ZSM-5 catalyst dur-
ing N2O reduction by C2H4 was saturated after 2 h (around
−
1
1
.3 mmol g ). This saturated value, which corresponds to
the C/Fe ratio of 2, did not depend on the reaction temper-
ature and partial pressure of reactants [32]. Thus, it is pre-
sumed that the amount of C␣ formed during N2O reduction
by C2 and C3 hydrocarbons in the absence of O2 are simi-
lar irrespective of the kinds of hydrocarbons and the reaction
conditions employed.
Fig. 3. TPO curves of carbonaceous deposits accumulated over Fe-ZSM-5
during N2O (2000 ppm) reduction by hydrocarbons at 598 K for 2 h in the
presence of O2 (5000 ppm) ((ꢀ) CO; (ꢁ) CO2).
as CO2 and/or CO at ␣- and ␤-peak regions were named C␣
and C␤, respectively.
Fig. 4 shows the changes in the conversion of N2O to N2
and the amounts of C␣ and C␤ accumulated during N2O re-
The influence of addition of O2 into the feed gas was also
investigated by TPO. Fig. 3 shows TPO curves of the car-
bonaceous deposits accumulated over Fe-ZSM-5 during the
reduction of N2O by hydrocarbons at 598 K for 2 h in the
presence of O2. In this study, as described in Section 2.2, the
quantitiesofhydrocarbonsusedwereapproximatelythesame
with respect to the quantities of such reducing species as C
and H included; for example, 2000 ppm C2H4 and 1300 ppm
duction by CH4, C2H4, and C3H in the absence of O2. In
6
the case of CH4, a stable N2O conversion higher than 90%
was observed as shown in Fig. 1a and Table 1, and little de-
position of carbonaceous material was observed during the
reaction. In both cases of C2H4, and C3H , the conversion of
6
C3H wereused. Theconcentrationoftheotherhydrocarbons
6
was also adjusted to approximately have the same equivalent
number of reducing atoms. If these inlet gaseous reactants
react with each other completely, the stoichiometry of the
reaction, for C2H4 for example, would be:
N2O + C2H4+5/2O2→N2+2CO2+2H2O
(1)
In this case, for example, 2000 ppm N2O, 2000 ppm C2H4
and 5000 ppm O2 give the stoichiometric composition and
no accumulation of carbonaceous deposit may occur as ex-
pected by the Eq. (1). Under these conditions, when CH4,
C2H4, C2H and C3H8 were employed as the reductant, the
6
evolution of CO and CO2 was scarcely observed in the TPO
runs. On the other hand, when C3H was used, ␤-peaks of
6
evolution of CO and CO2 were observed.
Table 1 summarizes the conversion of N2O to N2 obtained
in the reduction of N2O in the absence and presence of O2
shown in Fig. 1 and the amounts of carbonaceous deposits
(C␣, C␤) determined by TPO shown in Figs. 2 and 3. As
described previously [32], the accumulation of the carbona-
ceous deposits in the reduction of N2O by C2H4 was sup-
pressed and the catalytic reduction of N2O was promoted
by the presence of O2. In the cases of C2H and C3H8, in a
6
similar manner to C2H4, the accumulation of carbonaceous
deposits scarcely occurred and the high stable values of N2O
conversion were obtained in the presence of O2. When CH4
was used, the high values of N2O conversion were obtained
Fig. 4. Conversion of N2O to N2 and amounts of C␣ and C␤ accumu-
lated over Fe-ZSM-5 during the reduction N2O (2000 ppm) by: (a) CH4
(
3000 ppm); (b) C2H4 (2000 ppm); and (c) C3H6 (1300 ppm) at 598 K ((ꢀ),
conversion of N2O to N2; (᭹), C␣; (ꢁ), C␤).

T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
191
Table 1
The amount of carbonaceous deposits accumulated over Fe-ZSM-5 during the reduction of N2O by hydrocarbons at 598 K for 2 h
Reactant (ppm)
Conversion (%)
Amount of carbonaceous deposits
−
1
(mmol g cat.)
Hydrocarbon
CH4 (3000)
N2O
2000
000
O2
N2O to N2
C␣
C␤
0
93.6
78.1
60.0
–
0.14
Trace
0.013
0
0.04
Trace
0.005
0
2
5000
12000
0
0
0
C2H4 (2000)
2000
0
5000
0
38.7
74.0
14.5
–
1.39
0.11
1.26
0.07
1.85
0.07
2.93
0.09
2000
12000
0
0
C2H6 (2000)
C3H6 (1300)
2000
0
5000
0
58.7
68.9
–
1.00
0.01
0.002
1.41
0
0.005
2000
0
2000
000
0
5000
0
28.6
27.1
6.3
–
1.46
0
1.19
0.63
2.12
4.38
4.00
0.18
2
12000
0
0
C3H8 (1300)
2000
0
5000
0
47.2
69.6
–
1.24
0.12
0.03
1.99
0.20
0.01
2000
0
Fe (3.4 wt.%)-ZSM-5: 0.05 g.
N2O decreases with time on stream, and this change corre-
sponds well with an increase in the amount of C␣. However,
the amounts of C␤ increase linearly with time on stream for
Previously [32], it was suggested that C␣ and C␤ formed
over the Fe-ZSM-5 catalyst during the N2O reduction by
C2H4 were the carbonaceous species with isolated carbon
and those with carboxylic groups, respectively, from XPS
measurements. Furthermore, as described in Section 3.2, the
both C2H4 and C3H . Thus, it is probable that the reduction
6
of N2O is inhibited by the presence of C␣; the deactivation
of Fe-ZSM-5 catalyst in the absence of O2 is caused by the
accumulation of C␣ on its surface.
amounts of C␣ formed in the cases of C2H4 and C3H were
6
similar. Then, the chemical nature of C␣ obtained from C3H6
and C2H4 may be similar. On the other hand, in the case of
C␤ from C2H4, little amount of carbide was observed by XPS
and, thus, theremaybeasignificantdifferenceinthechemical
3
.3. Characterization of carbonaceous deposits
The nature of C␣ and C␤ deposits was characterized by
means of XPS. Fig. 5 shows XPS spectra of C1s for Fe-ZSM-
used for the reduction of N2O with C3H in the absence of
5
6
O2 at 598 K. Fig. 5 (1) shows the XPS spectra of the Fe-ZSM-
catalyst before reaction as a background. The dotted and
5
broken curves shown in Fig. 5 (2) and (3) represent decon-
voluted spectra of the solid curves. Fig. 5 (2) shows the C1s
spectrum after the reduction of N2O for 15 min, in which the
carbonaceous deposits formed on the catalyst were mostly
C␣ along with a small quantity of C␤. A peak at 283.6 eV
displayed by a broken line may indicate the formation of an
isolated carbon [33], and C␣ can be regarded as carbona-
ceous species with the isolated carbon. Another small peak
at 286.0 eV may be assigned to alcoholic groups [33] and
could be due to the accumulation of C␤. Fig. 5 (3) shows the
C1s spectrum after the reduction of N2O for a longer time of
1
20 min. The peak at 286.0 eV obtained for 120 min is larger
than that for 15 min. A shoulder peak appearing at 282.0 eV
may be assigned to carbide [33]. C␤ species are formed on
the catalyst for longer reaction time and, therefore, C␤ can
be regarded as the carbonaceous species with carbide and
alcoholic groups.
Fig. 5. XPS spectra of C1s for Fe-ZSM-5 catalyst before and after the re-
duction of N2O (2000 ppm) by C3H6 (1300 ppm) in the absence of O2 at
598 K. (1) Before reaction; (2) after 15 min; (3) after 120 min.

1
92
T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
Table 2
Surface composition of Fe-ZSM-5 catalyst used for the reduction of N2O
2000 ppm) by C3H6 (1300 ppm) in the absence of O2 at 598 K as measured
(
by XPS
Reaction time (min)
Type of C deposit
Content (at.%)
C
Fe
Si
Before reaction
–
C␣
C␣, C␤
9.4
12.1
18.2
1.2
0.84
0.68
24.4
24.8
22.5
1
1
5
20
nature of C␤ between C2H4 and C3H . The reactivity of C␤
6
with O2 should be different and this may be responsible for
the difference in the effects of O2 addition on the reduction
of N2O between C3H and C2H4 (Fig. 1).
6
The elemental compositions of surface of the catalyst were
also examined by means of XPS. The relative amounts of sur-
face carbon, iron and silicon after the reduction of N2O by
C3H are listed in Table 2. It is seen that the C content in-
6
creased with an increase in the reaction time and, in contrast,
the Fe content decreased. The C␣ species, which was mainly
formed at the initial stage of N2O reduction, may be accu-
mulated on the Fe site, since the Fe content decreased by the
formation of C␣ while the Si content remained almost unal-
tered. On the other hand, C␤ species may be accumulated on
the support, ZSM-5 zeolite, since the Si content decreased
by the appearance of C␤ for a longer time of 120 min. Con-
cerning the site for the formation of C␣ and C␤, these results
with C3H agree with our previous results with C2H4 [32].
6
Fig. 6. The effects of partial pressure of O2 on the conversion of N2O to N2
andtheamountofthecarbonaceousdepositsformedonFe-ZSM-5duringthe
reductionofN2O(2000 ppm)by:(a)CH4 (3000 ppm);(b)C2H4 (2000 ppm);
and (c) C3H6 (1300 ppm) at 598 K for 2 h. ((ꢀ), partial pressure of N2; (ꢀ),
partial pressure of COx; (᭹), the amount of the carbonaceous deposit).
3
.4. Influence of the partial pressure of O2 on N2O
reduction
As described in Section 3.2, the deactivation of the Fe-
but a considerable quantity of the carbonaceous deposit still
remained even at 5% O2. The conversion of N2O increased
with an increase in the concentration of O2 between 0.5 and
ZSM-5 catalyst in the absence of O2 was caused by the ac-
cumulation of C␣ on its surface and the catalytic reduction
of N2O was promoted by the presence of O2. The influence
of the partial pressure of O2 on N2O reduction was further
investigated. In Fig. 6, the amount of carbonaceous deposit
and the partial pressure of N2, COx (CO and CO2) formed in
2%, and a maximum value of 49.2% was obtained at 2% O2.
The partial pressure of COx increased with an increase in the
concentration of O2 up to 2%, and the value did not change
notably with the concentration of O2 above 2%.
the reduction of N2O with CH4, C2H4, and C3H at 598 K for
6
2
h are plotted against the concentration of O2 added. When
CH4 was employed as the reductant, a little amount of car-
bonaceous deposit was formed in the absence of O2 but not
in the presence of O2. The partial pressure of COx formed
did not change markedly with the concentration of O2. When
C2H4 was employed, the amount of the carbonaceous deposit
decreased rapidly with an increase in the concentration of O2
between 0 and 0.5% and then it disappeared at O2 concentra-
tions above 1%. N2O conversion obtained at O2 concentra-
tions below 0.1% was about 36%, while the value increased
with an increase in the O2 concentration up to 0.5%, at which
a maximum value of 87.4% was obtained. The conversion of
N2O did not change with the concentration of O2 above 1%.
3.5. Reactivity of CO with O2 and N2O over Fe-ZSM-5
In the reduction of N2O by various hydrocarbons, CO and
CO2 were produced. The conversion of CO and CO2 was
measured at various W/F (weight of catalyst/total flow rate
of reactant gas) values to examine whether the formation of
CO and CO2 is either in parallel or in a consecutive manner.
It was found that CO2 might be mainly formed through CO
consecutively. Then, the reactivity of CO with N2O and O2
was further investigated over Fe-ZSM-5 at 598 K to elucidate
the role of CO in the reduction of N2O in the presence of
O2. In Fig. 7, the reactivity of CO with N2O in the absence
and presence of O2 is compared with that of CO with O2.
When N2O was reacted with CO in the absence of O2, N2O
When C3H was employed, the amount of the carbonaceous
6
deposit decreased with an increase in the concentration of O2,

T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
193
Table 3
The amount of carbonaceous deposits accumulated over Fe-ZSM-5 during
the reaction between O2 and hydrocarbons at 598 K for 2 h
Reactant (ppm)
Amount of carbonaceous
deposits (mmol g cat.)
−
1
Hydrocarbon
CH4 (3000)
O2
C␣
C␤
1000
0
0
0
0
6
000
1000
000
1000
000
C2H4 (2000)
C3H6 (1300)
0
0.07
0.99
0.12
6
0
0
6.29
6.38
6
Fe (3.4 wt.%)-ZSM-5: 0.05 g.
Fig. 7. The influence of partial pressure of O2 on the conversion of N2O
to N2 and CO to CO2 in the reaction between N2O (2000 ppm) and CO
carbonaceous deposits (C␣, C␤) scarcely occurred without
oxidants by providing hydrocarbons except C3H . It is pre-
6
(
2000 ppm) or O2 and CO (2000 ppm) at 598 K for 2 h. ((ꢀ), conversion of
sumed that oxidants are necessary to accumulate the carbona-
ceous deposits on the catalyst. Most of carbonaceous deposits
formed without gaseous oxidants might be produced by the
reaction between hydrocarbon and the surface oxygen of cat-
N2O to N2; (ꢁ), conversion of CO to CO2 in the reaction between N2O and
CO; (ꢁ), conversion of CO to CO2 in the reaction between O2 and CO).
conversion was about 53%, while the value decreased about
1
0% by addition of 0.1% O2 and further decreased gradually
3
6
alyst. Only in the case of C H , one-half of saturated value
with an increase in the concentration of O2 up to 5%. The total
conversion of CO to CO2 obtained in the reaction between
CO and N2O in the presence of O2, however, did not change
so much, since the decrease in the CO oxidation contributed
by N2O was compensated by addition of O2. On the other
hand, the conversion of CO to CO2 in the reaction between
CO and O2 without N2O was considerably low and slightly
increased with an increase in the partial pressure of O2. On
the basis of these findings, it is presumed that N2O is reduced
easily by CO even in the presence of O2 over Fe-ZSM-5.
of C␣ may be formed by adsorption without oxidants.
The role of O in the formation and the oxidation of
2
carbonaceous deposits (C␣, C␤) were further investigated
by reactions between hydrocarbons and O without N O,
2
2
and the results are shown in Table 3. In these experiments,
1000 or 6000 ppm of O was introduced in place of 2000 or
2
12,000 ppm N O. When CH was used, no carbonaceous de-
2
4
posit formed on Fe-ZSM-5. On the other hand, when C H
3
6
was employed, the formation of a considerable extent of C␤
was observed, while little amount of C␣ was formed. It is
presumed that the presence of N2O is necessary for the for-
mation of the C␣ type carbonaceous deposit over Fe-ZSM-5
or the C␣ type carbonaceous deposit is reactive with O2. Ac-
tually Kameoka et al. [16,17] stated that the presence of N2O
is necessary for the initial activation of hydrocarbons.
Table 4 summarizes the amount of reactants (hydrocar-
bon, N2O, and O2) consumed and those of products (CO,
3
.6. Reactivity of O2 and N2O for various hydrocarbons
As previously noted in Table 1 and Figs. 1 and 2, most
of the carbonaceous deposits were consumed by the reaction
with stoichiometric composition of O2. In order to compare
the reactivity of N2O with that of O2 on the oxidative con-
sumption of carbonaceous deposits, the stoichiometric com-
position of N2O (12,000 ppm) was introduced in the place of
the mixture of N2O (2000 ppm) and O2 (5000 ppm) for the
CO2, C2H4, and C3H ) formed in the reduction of N2O by
6
CH4, C2H4, C2H , C3H , or C3H8, over Fe-ZSM-5 at 598 K.
6
6
These values were obtained at the steady state in the same
experiments as shown in Tables 1 and 3 and summarized to
elucidate the influence of the partial pressure of N2O and
O2 on the basis of the carbon balance values between the
gaseous reactants and products. The last column of Table 4
shows the contribution of N2O in the total amount of the
reacted agents (N2O + O2). Yoshida et al. [19] reported the
value of (consumed N2O)/(3CO + 4CO2) to represent the ra-
tio of the consumption rate of N2O to the formation rate
of (3CO + 4CO2) for the reduction of N2O by CH4. In this
case, (3CO + 4CO2) represents the total amount of oxidizing
agents assuming the equations: CH4 + 4[O] → CO2 + 2H2O,
CH4 + 3[O] → CO + 2H2O. [O] is oxygen atom originat-
ing from N2O and O2. Similarly the contribution of
N2O in the total amount of the oxidizing agents reacted
(N2O + O2) are represented by the values of (consumed
reaction with CH4, C2H4, and C3H , and the results are also
6
shown in Table 1. When C2H4 and C3H were used, a simi-
6
lar amount of carbonaceous deposits accumulated in spite of
high partial pressures of N2O (2000 and 12,000 ppm). This
suggests that the carbonaceous deposits are unlikely to re-
act with N2O even at 12,000 ppm. In both cases, however, it
was observed that C␣ decreased slightly and C␤ increased
apparently in the reaction with higher partial pressure N2O.
Particularly, in the case of C3H , a large quantity of C␤ ac-
6
cumulated in the reaction with 12,000 ppm N2O. Then, only
hydrocarbon was provided over Fe-ZSM-5 to investigate the
role of oxidant in the formation of carbonaceous deposits on
the catalyst surface. Table 1 also summarizes the amount of
carbonaceous deposits formed by providing of CH4, C2H4,
C2H , C3H , or C3H8 without oxidants. The formation of
6
6

1
94
T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
Table 4
The reduction of N2O by hydrocarbons over Fe-ZSM-5 at 598 K for 2 h
Reactant (ppm)
Amount of reactants consumed (ppm)
Amount of products formed (ppm)
Contribution
of N2Oa
Hydrocarbon
CH4 (3000)
N2O
2000
000
O2
Hydrocarbon
N2O
O2
CO
348
219
588
0
CO2
C2H4
C3H6
–
501
549
1944
0
1872
1562
7200
–
–
153
330
1329
0
0
0
0
0
0
0
0
0
0
0
1.13
0.79
1.02
–
2
5000
–
1000
6000
605
–
12000
–
–
0
0
–
0
0
0
–
C2H4 (2000)
2000
–
5000
–
1000
6000
394
1942
638
384
946
774
1480
1740
–
56
2232
128
348
1176
104
1660
308
236
572
0
0
0
0
0
1.83
0.17
1.47
–
2000
4040
–
804
2034
–
–
–
–
12000
–
–
–
–
C2H6 (2000)
C3H6 (1300)
2000
–
510
1290
1174
1378
–
3175
72
1280
100
1240
164
30
0
0
222
0.18
2
000
2000
000
5000
–
5000
–
1000
6000
484
1114
229
635
1210
572
542
756
–
–
2510
–
1000
3050
27
749
39
105
958
47
878
82
174
901
33
Trace
0
17
14
–
–
–
–
–
2.93
0.13
2.33
–
2
12000
–
–
–
–
C3H8 (1300)
2000
–
378
1192
944
1392
–
3350
43
1576
66
1287
12
0
120
0
2.95
0.17
2000
5000
Fe (3.4 wt.%)-ZSM-5: 0.05 g.
a
Normalized contribution of N2O in the reaction between N2O and hydrocarbon in the presence of O2 [19]; CH4: (consumed
N2O)/(3CO + 4CO2); C2H4: (consumed NO)/(2CO + 3CO2); C2H6: (consumed N2O)/(5/2CO + 7/2CO2); C3H6: (consumed NO)/(2CO + 3CO2); C3H8: (con-
sumed N2O)/(7/3CO + 10/3CO2).
N2O)/(2CO + 3CO2), (consumed N2O)/(5/2CO + 7/2CO2),
and (consumed N2O)/(7/3CO + 10/3CO2) for the reduction
posits may be formed on the surface of catalyst when C2 and
C3 hydrocarbons were used for the reduction of N2O in the
absence of O2.
of N2O by C2H4 and C3H , C2H , and C3H8, respectively.
6
6
When CH4 was used, no reaction occurred with O2 (1000
or 6000 ppm) over Fe-ZSM-5 and most of CO and CO2
was formed by the reaction between CH4 and N2O (2000
or 12,000 ppm). The total amounts of COx (CO and CO2)
were slightly increased by addition of O2 (5000 ppm), while
the amount of CO2 formation increased apparently according
to the oxidation of CO by O2. Actually the contribution of
N2O obtained in the reduction of N2O by CH4 in the absence
of O2 is almost unity, while it slightly decreased to 0.79 in
the presence of O2. This suggests that most of CO and CO2
was formed by the reaction between CH4 and N2O (2000
or 12,000 ppm) in the absence of O2 and CO2 was further
formed by the oxidation of CO with additional O2.
3.7. Reaction scheme for SCR of N O by hydrocarbons
over Fe-ZSM-5
2
Onthebasisofthefindingsmentionedabove, hydrocarbon
reductants are phenomenologically divided into three groups,
namely CH , C H , and others (C H , C H , C H ), referred
4
3
6
2
4
2
6
3
8
to as the C H group and we propose the possible reaction
2
4
scheme for each group as shown in Scheme 1.
In the initial stage, N O decomposes to produce N and
2
2
Os, active nascent oxygen, Eq. (2). When CH is employed,
4
CH is hardly adsorbed on Fe-ZSM-5. Kunimori and his co-
4
workers [17–19] have reported that nascent oxygen originat-
On the other hand, when C2H4, C2H , and C3H8 were
ing from N O decomposition could play an important role in
6
2
used, N2O and O2 were well reacted over Fe-ZSM-5 with
hydrocarbons and produced equivalent amount of gaseous
carbon oxides. However, when only N2O was used as ox-
idant, the amount of gaseous carbon oxides was less than
that of hydrocarbons consumed and the differences of car-
bon species could be due to the formation of carbonaceous
the activation of methane. In this case, Os is thus reduced by
∗
∗
CH to produce CH s and H O, Eq. (3), and CH s may be
4
2
2
2
very active and reacted instantaneously with N O or O to
2
2
produce CH Os and N or CO and H O, respectively, Eqs.
2
2
2
(4) and (5). As shown in Fig. 1, the steady state conversion
of N O to N decreased from about 95 to 75% by addition
2
2
deposits. When C3H was used, considerable amounts of car-
2 4 2
of O , while no reaction occurred between CH and O over
6
bon species remained on the catalyst even in the presence of
O2. Actually the contribution of N2O obtained for the reduc-
tion of N2O by C2 and C3 hydrocarbons in the absence of
O2 is 1.83–2.95, and only 0.13–0.18 in the presence of O2.
This suggests that significant amounts of carbonaceous de-
Fe-ZSM-5, as shown in Table 4. For these reasons, it is pre-
∗
sumed that the active CH s species may react partially with
2
O to produce CO and H O. Nobukawa et al. [17,18] has
2
2
2
reported that the reaction intermediates of methoxy and for-
mate species were observed over Fe-BEA during the SCR of

T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
195
sites generated by the ion-exchange. It can be presumed that
significant amounts of C␤ are formed during the oxidation
of the adsorbed propylene species with O2. When C3H is
6
employed for reductant, Os reacts with C3H to produce
6
(
CHδO)3s, H2O and N2, through very active hydrocarbon
*
adsorbed species, (CHδ)3 s, Eqs. (13) and (14), where, δ is
calculated at 4/3 = 1.33. In fact, Kameoka et al. [15] reported
that carbon-, hydrocarbon- and/or oxygen containing species
such as CxHy(a) and CxHyOz(a) are produced on the Fe-ZSM-
5
surface by the reaction of the N2O–O2–C3H mixture and
6
the average composition ratio of CxHy(a) and CxHyOz(a) on
the Fe ion site is roughly estimated to be FeC3H4O3. Then,
(CHδO)3s may react with N2O to produce N2, CO, H2O, and
carbonaceous species, and carbonaceous species, C2s, which
may further accumulate as C␣ in the absence of O2, Eq. (16).
In the presence of O2, (CHδO)3s partially reacts with O2 to
produce CO2, H2O and an active site (s), Eq. (17), while con-
siderable amount of O2 need to promote Eq. (17), and thus
carbonaceous species may remain as C␤.
On the basis of the findings from XPS as described in
Section 3.3, the chemical nature of C␣ obtained from C3H6
and C2H4 may be similar, while that of C␤ is significantly
Scheme 1. Reaction scheme for N2O reduction by various hydrocarbons.
N2O with CH4. They observed by FT-IR experiments that the
Fe-OH species is present on the Fe-BEA catalyst during the
SCR of N2O with CH4 and it plays an important role in the
reaction. In a similar manner to this, the oxygenated inter-
mediate species, CH2Os, further reacted with N2O or O2 to
produce N2, CO, H2O, Eq. (6) and (7). Gaseous or adsorbed
CO may be oxidized consecutively with Os or N2O to CO2
different and C␤ from C3H can be regarded as carbide rich
6
carbonaceous species. The reactivity of C␤ with O2 should
be different and this may be responsible for the difference in
the effects of O2 addition on the reduction of N2O between
C3H and C2H4.
6
(Fig. 7, Table 4). In this case, CH4 itself is scarcely oxidized
with gaseous O2 (Table 3).
4. Conclusions
When C2H4 is employed, a little amount of C2H4 is ad-
sorbed on the catalyst without oxidant. Os, active nascent
oxygen, reacts with gaseous or adsorbed C2H4 to produce
The nature and reactivity of carbonaceous deposits have
been studied over Fe-ZSM-5 in the catalytic reduction of
dinitrogen monoxide by various hydrocarbons (CH4, C2H4,
(
CHO)2s, carbonaceous deposit precursor, H2O and N2,
*
through very active hydrocarbon adsorbed species, (CH)2 s,
Eqs. (8) and (9). (CHO)2s may react with N2O to produce
N2, CO, H2O and carbonaceous adsorbed species, Cs, which
may further accumulate as C␣ in the absence of O2, Eq. (11),
while (CHO)2s reacts with O2 above the stoichiometric com-
position and an active site (s) is regenerated, Eq. (12) (Fig. 1,
C2H , C3H , C3H8) in the absence and presence of O2.
6 6
The hydrocarbon reductants used are phenomenologically
divided into three groups, namely CH4, C3H , and others
6
(C2H4, C2H , C3H8), referred to as the C2H4 group. Two
6
types of carbonaceous deposits (C␣ and C␤) as classified by
TPO are formed on the catalyst during the reduction of N2O
in the absence of O2 irrespective of the kind of hydrocarbons
used except for CH4. The C␣ species is formed on Fe sites
and the C␤ is mainly accumulated on the support.
Table 1). The reaction scheme for C2H and C3H8 could
6
be represented phenomenologically in a similar manner to
C2H4.
On the other hand, the influence of O2 is significantly dif-
In both cases of C2H4 and C3H , the catalytic activity of
6
ferent between C3H and the other hydrocarbons (Fig. 1).
Fe-ZSM-5 decreases with an increase in the amount of C␣,
while it is not affected by the presence of C␤. The chemical
nature and the amount of C␣ obtained from the C2H4 group
6
C3H is adsorbed readily on Fe-ZSM-5 without oxidants
6
and C␣ with one-half of the saturated value accumulated
(Table 1). Furthermore, significant amounts of C␤ were
and C3H is similar, while those of C␤ is significantly dif-
6
formed on Fe-ZSM-5 during N2O reduction by C3H even
ferent and C␤ from C3H can be regarded as carbide rich
6
6
in the presence of O2 (Table 1) as well as the reaction be-
carbonaceous species. The formation of C␣ is suppressed by
the presence of O2, and this promotes the catalytic reduction
of N2O. The reactivity of C␤ with O2 should be different and
this may be responsible for the difference in the effects of O2
addition on the reduction of N2O between the C2H4 group
and C3H6.
tween C3H and O2 (Table 3). Yamada et al. [12] suggested
6
on the basis of the observations of FT-IR spectroscopy that
the bands attributed to CH2 group, CH3 group, and hydro-
carbon oligomers are obtained by the adsorption of C3H6
and SCR of N2O by C3H occurs mainly on Lewis acid
6

1
96
T. Chaki et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 187–196
In the case of CH4, a stable high conversion of N2O is
[15] S. Kameoka, K. Yazaki, T. Takeda, S. Tanaka, S. Ito, T.
Miyadera, K. Kunimori, Phys. Chem. Chem. Phys. (2001)
56.
3
obtained irrespective of the presence and absence of O2 be-
cause the carbonaceous deposits are scarcely accumulated
on the catalyst. It is concluded that CH4 is the most effective
reductant for the selective reduction of N2O.
2
[16] S. Kameoka, K. Kita, S. Tanaka, T. Nobukawa, S. Ito, K.
Tomishige, T. Miyadera, K. Kunimori, Catal. Lett. 79 (2002)
63.
[
[
[
17] S. Kameoka, T. Nobukawa, S. Tanaka, S. Ito, K. Tomishige, K.
Kunimori, Phys. Chem. Chem. Phys. 5 (2003) 3328.
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Kunimori, J. Phys. Chem. 108 (2004) 4071.
19] Y. Yoshida, T. Nobukawa, S. Ito, K. Tomishige, K. Kunimori, J.
Catal. 223 (2004) 454.
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1
(