Selective reduction of NO2 with acetaldehyde over Co/Al2O3 in lean conditions

Article abstract of DOI:10.1016/j.molcata.2006.07.065

Selective reduction of NO₂ with acetaldehyde (CH₃CHO) was investigated over alumina-supported catalysts. Among the catalysts tested, Co/Al₂O₃ showed the highest activity with a maximum performance at 2 wt.% Co loading. The comparison of the activity of Co/Al₂O₃ and Al₂O₃ for several unit reactions suggested that one of the roles of supported Co is to suppress the combustion of CH₃CHO by O₂, resulting in an enhancement of the selective reaction of CH₃CHO with NO₂. The observation of the adsorbed species by in situ FT-IR spectroscopy showed that enolate species acts as the intermediate for NO₂ reduction by CH₃CHO and that another role of Co is to promote the production of the enolate species.

Full text of DOI:10.1016/j.molcata.2006.07.065

Journal of Molecular Catalysis A: Chemical 261 (2007) 6–11
Selective reduction of NO2 with acetaldehyde
over Co/Al2O3 in lean conditions
Atsushi Takahashi∗, Masaaki Haneda,
Tadahiro Fujitani, Hideaki Hamada
Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received 19 December 2005; received in revised form 6 March 2006; accepted 25 July 2006
Available online 1 September 2006
Abstract
Selective reduction of NO2 with acetaldehyde (CH3CHO) was investigated over alumina-supported catalysts. Among the catalysts tested,
Co/Al2O3 showed the highest activity with a maximum performance at 2 wt.% Co loading. The comparison of the activity of Co/Al2O3 and
Al2O3 for several unit reactions suggested that one of the roles of supported Co is to suppress the combustion of CH3CHO by O2, resulting in an
enhancement of the selective reaction of CH3CHO with NO2. The observation of the adsorbed species by in situ FT-IR spectroscopy showed that
enolate species acts as the intermediate for NO2 reduction by CH3CHO and that another role of Co is to promote the production of the enolate
species.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Acetaldehyde; Co/Al2O3; NO2 selective reduction
1. Introduction
reduction activity by combining catalytic active species accel-
erating certain reaction steps mentioned above. For example,
Hamada et al. [6] investigated Co-supported Al2O3 catalyst for
theselectivereductionofNOwithpropeneandproposedamech-
anism that the supported cobalt promotes NO oxidation to NO2,
which then reacts with propene on Al2O3 to give N2. Misono
et al. [7] reported that the physical mixing of Mn2O3 or CeO2
enhanced considerably the activity of Ce–ZSM-5 for the selec-
tive reduction of NO with propene and that the role of Mn2O3 or
CeO2 is to promote NO oxidation to NO2 without accelerating
propene combustion.
Recently, non-thermal plasma assisted catalysis is attracting
attention as a promising strategy for NOx abatement in diesel
exhaust [8]. In this reaction system, the exhaust gas is first treated
by a plasma reactor before entering the catalytic converter. In
the plasma reactor, NO is oxidized to NO2 and the hydrocarbons
to oxygenates. NO2 and the oxygenates thus formed then react
over the catalyst to produce N2, CO2 and H2O. Since the forma-
tion rate of NO2 and oxygenates can be controlled by adjusting
the power for plasma generation, the development of effective
catalysts for the reduction of NO2 with oxygenate under oxygen-
rich conditions is highly desirable from a practical point of
view.
Selective catalytic reduction of nitrogen oxide (NOx) with
hydrocarbon (HC-SCR) is one of the most desirable technolo-
gies for abatement of NOx in diesel and lean burn engine
exhausts. While a lot of investigations have focused on zeolite-
based catalysts since the report on Cu–ZSM-5 catalyst by Held
et al. [1] and Iwamoto et al. [2], metal oxide-based catalysts
represented by alumina have also drawn considerable attention
[3–5].
The selective reduction of NO with hydrocarbon is a very
complex reaction comprised of several parallel and/or consec-
utive reaction steps. Although there is still disagreement in
the literature regarding the reaction pathway, this reaction is
believed to be a three-step process consisting of: (I) NO oxida-
tion to NO2, (II) oxidative activation of hydrocarbon, and (III)
reaction of NO2 and the activated hydrocarbon leading to the
formation of N2 via N-containing organic intermediates. In this
regard, there have been a number of studies to promote the NO
∗
Corresponding author. Tel.: +81 29 861 9398; fax: +81 29 861 4647.
E-mail address: at-takahashi@aist.go.jp (A. Takahashi).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.07.065

A. Takahashi et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 6–11
7
was passed through the reactor at a gas flow rate of 90 cm3 min−1
(W/F = 0.027 g s cm−3). Steady-state catalytic activity at each
reaction temperature was measured as the reaction temperature
was lowered from 600 to 200 ◦C with a step of 50 ◦C.
The effluent gas was analyzed by gas chromatography (Shi-
madzu, GC-8A TCD detector) with a Molecular Sieve 5A col-
umn to separate N2 and CO and a Porapak Q column to separate
N2O and CO2. The catalytic activity was evaluated in terms of
NO2 conversion to N2 and that of CH3CHO to COx (CO + CO2).
The formation of N2O was found negligible in the present
study. The concentration of NOx (NO + NO2) was continuously
monitored with a chemiluminescence NOx analyzer (Shimadzu,
NOA-7000) to check the stability of catalytic activity.
Kung and co-workers [9] investigated the reaction of NO2
with CH3CHO, and found that anatase TiO2 shows good activity
for this reaction. Yoon et al. [10] investigated the activity of var-
ious catalysts in the system of plasma-assisted NOx reduction,
and reported that Al2O3 and basic zeolites such as NaY are effec-
tive catalysts. Miessner et al. [11] showed that Al2O3 and ZrO2
act as effective catalysts in plasma-assisted NOx reduction. Tran
et al. [12] also reported the activity of In/Al2O3, which strongly
depends on the type of hydrocarbon used and reaction tempera-
ture. However, there have been no comprehensive studies aiming
at the development of effective catalyst for plasma-assisted NOx
reduction.
In the present study, we have investigated the activity of var-
ious alumina-supported catalysts for the selective reduction of
NO2 with CH3CHO, which are major oxidation products by
plasma reaction [13,14], and found that Co/Al2O3 shows the
highest activity for this reaction. The role of supported Co
and the reaction mechanism are discussed based on the cat-
alytic activities for several unit reactions and the behavior of
the adsorbed species observed by in situ FT-IR spectroscopy.
2.3. FT-IR measurements
Diffuse reflectance FT-IR spectra were recorded with a Nico-
let Nexus 670 FT-IR spectrometer, accumulating 64 scans at a
resolution of 4 cm−1. Prior to each measurement, 25 mg of a
catalyst placed in the diffuse reflectance high temperature cell
(Spectra Tech) fitted with CaF2 windows was activated in situ by
heating in flowing 10% O2/He at 600 ◦C, followed by cooling
to the desired temperature (300–400 ◦C). The background spec-
trum of the clean surface was measured for spectra correction.
Various gas mixtures containing 0 or 500 ppm NO2, 1500 ppm
CH3CHO, 10% O2 and He as the balance gas were fed to the
2. Experimental
2.1. Catalyst preparation
Alumina supplied from Mizusawa Chemical Industries
(Neobead GB-45, 185 cm2 g−1) was used as the support for
catalyst preparation. Alumina-supported catalysts (M/Al2O3:
M = Mn, Co, Ni, In, Sn, Ga, Ag, Rh) were prepared by
impregnation method using aqueous solutions of Mn(NO3)2,
(CH3COO)2Co, Ni(NO3)2, In(NO3)3, SnCl4, Ga(NO3)3,
AgNO3 and Rh(NO3)3, respectively, followed by drying at
110 ◦C and calcination in air at 600 ◦C for 5 h. The loading of
metals was set at 1 wt.% except for Co, in which case the loading
was changed from 0.5 to 10 wt.%.
catalyst at a flow rate of 30 cm3 min−1
.
3. Results and discussion
3.1. Activity of alumina-supported catalysts
Table 1 summarizes the catalytic activity of M/Al2O3
(M = Mn, Co, Ni, In, Sn, Ga, Ag, Rh) for the selective reduction
of NO2 with CH3CHO. Al2O3 was found to catalyze effectively
NO2 reduction at temperatures above 300 ◦C with a maximum
NO2 conversion of 46% at 550 ◦C. It can be seen that Co, Ni,
In, Sn, Ga and Ag/Al2O3 catalysts showed higher activity than
Al2O3 with lower effective temperature range. On the other
hand, the addition of Mn and Rh caused a significant decrease of
the activity of Al2O3. The low reduction activity of Mn/Al2O3
2.2. Catalytic activity measurements
The catalytic activity was measured using a fixed bed flow
reactor. A 40 mg of a catalyst grinded to powder was placed in
the quartz tube. The reaction gas mixture consisting of 500 ppm
NO2, 1500 ppm CH3CHO, 10% O2, 6% H2O and He balance
Table 1
Catalytic activity of Metal/Al2O3 for selective reduction of NO2 with CH3CHO
Catalysts
NO2 to N2 conversion (%), CH3CHO to COx conversion (%)
200oC
250 ◦C
300 ◦C
350 ◦C
400 ◦C
450 ◦C
500 ◦C
550 ◦C
600 ◦C
Al2O3
0(0)
0(6)
0(0)
0(1)
1(0)
1(1)
1(1)
1(2)
0(3)
1(4)
0(18)
1(4)
1(3)
1(3)
1(5)
2(4)
1(6)
1(11)
13(15)
0(28)
11(13)
8(11)
10(10)
16(14)
15(13)
5(13)
21(33)
0(45)
27(59)
0(77)
42(83)
9(95)
43(91)
11(97)
72(98)
55(89)
56(97)
53(92)
51(88)
59(96)
0(98)
46(99)
10(99)
58(98)
50(96)
51(99)
53(96)
58(95)
61(99)
0(98)
29(96)
7(99)
Mn/Al2O3
Co/Al2O3
Ni/Al2O3
In/Al2O3
Sn/Al2O3
Ga/Al2O3
Ag/Al2O3
Rh/Al2O3
37(36)
25(26)
28(27)
29(31)
28(29)
15(29)
12(86)
55(62)
40(54)
49(63)
33(58)
34(58)
20(56)
3(97)
73(90)
51(75)
55(86)
47(82)
46(78)
32(85)
1(98)
29(98)
25(100)
26(100)
27(99)
32(97)
33(100)
0(98)
3(27)
Loading of metal: 1 wt.%; reaction conditions: 500 ppm NO2, 10% O2, 1500 ppm CH3CHO, 6% H2O, He balance and W/F = 0.027 g s cm−3
.

8
A. Takahashi et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 6–11
and Rh/Al2O3 can be ascribed to their high activity for CH3CHO
combustion, as can be seen from the CH3CHO conversions.
Among the additives tested here, Co was the best one, since the
maximum NO2 conversion reaches as high as 73% at 450 ◦C.
Consequently, the rest of this paper deals mainly with Co/Al2O3.
3.2. Effect of Co loading on the catalytic activity of
Co/Al2O3
Fig. 1 shows the effect of Co loading on the catalytic activity
of Co/Al2O3 for NO2 reduction with CH3CHO. NO2 conver-
sion over Co/Al2O3 increased with increasing Co loading and
reached a maximum (75%) at 2 wt.% Co loading. However, fur-
ther increase in the Co loading more than 5 wt.% decreased the
NO2 reduction activity. In contrast, the conversion of CH3CHO
to COx monotonously increased with Co loading. Accordingly,
the decrease of NO2 reduction activity at high Co loading can
be explained by predominant CH3CHO combustion. It was
also found that the effective temperature window showing high
Fig. 2. (a) NO2 conversion and (b) CH3CHO conversion for the reaction systems
of NO2 + CH3CHO (ꢂ, ꢄ), CH3CHO + O2 (♦, ꢅ) and NO2 + CH3CHO + O2 (ꢁ,
ꢀ). Open symbols indicated the results obtained on 2 wt.% Co/Al2O3 and the
solid symbol on Al2O3. Reaction conditions: 0 or 500 ppm NO2, 0 or 10% O2,
0 or 1500 ppm CH3CHO, 6% H2O, He balance and W/F = 0.027 g s cm−3
.
NO2 conversion shifts to lower temperature with increasing Co
loading.
3.3. Comparison of NO2 + CH3CHO + O2,
NO2 + CH3CHO and CH3CHO + O2 reactions
In order to get information on the role of supported cobalt
and the reaction mechanism, Al2O3 and 2 wt.% Co/Al2O3 were
examined for catalytic activity for the three reaction systems
(NO2 + CH3CHO + O2, NO2 + CH3CHO and CH3CHO + O2).
Fig. 2(a) shows the NO2 conversion on Al2O3 and 2 wt.%
Co/Al2O3 for the reaction systems of NO2 + CH3CHO + O2 and
NO2 + CH3CHO. Obviously, no NO2 reduction took place in the
reaction system of NO2 + CH3CHO on both Al2O3 and 2 wt.%
Co/Al2O3. Furthermore, no NOx reduction was observed in
an equal mixture of NO and NO2 and absence of oxygen on
Co/Al2O3, suggesting that the presence of O2 is essential for
NO2 reduction to occur. For NO reduction by hydrocarbons, O2
is believed to act as an oxidant for NO oxidation to NO2 and
Fig. 1. Effect of Co loading on the catalytic activity of Co/Al2O3 for the selective
reduction of NO2 with CH3CHO. Reaction conditions: 500 ppm NO2, 10%
O2, 1500 ppm CH3CHO, 6% H2O, He balance and W/F = 0.027 g s cm−3. Co
loading: 0 wt.% (ꢀ), 0.5 wt.% (ꢀ), 1 wt.% (ꢁ), 2 wt.% (ꢁ), 3 wt.% (ꢂ), 5 wt.%
(♦) and 10 wt.% (ꢃ).

A. Takahashi et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 6–11
9
Fig. 3. FT-IR spectra of adsorbed species in flowing CH3CHO (1500 ppm) + O2
(10%) on Al2O3 and 2 wt.% Co/Al2O3 at 300, 350 and 400 ◦C for 60 min.
Fig. 4. FT-IR spectra of adsorbed species in flowing NO2 (500 ppm) + CH3CHO
(1500 ppm) + O2 (10%) on Al2O3 and 2 wt.% Co/Al2O3 at 300, 350 and 400 ◦C
for 60 min.
oxidative hydrocarbon activation. Since NO2 was used instead
of NO in this study, O2 is considered to contribute to the activa-
tion of CH3CHO.
increased up to 400 ◦C, the bands due to formate species
almostcompletelydisappeared, whilethoseduetoacetatehardly
changed.
The reactivity of CH3CHO with O2 was then investigated.
As seen in Fig. 2(b), the CH3CHO conversion on 2 wt.%
Co/Al2O3 is lower than that on Al2O3 for the reaction system
of CH3CHO + O2. This indicates that one of the roles of Co
is to suppress the combustion of CH3CHO by O2, which is a
side reaction. The addition of NO2 to the reaction system of
CH3CHO + O2 slightly increased the conversion of CH3CHO
on Al2O3, while a great increase in the conversion was observed
on 2 wt.% Co/Al2O3. This result strongly suggests that the pres-
ence of Co promotes the reaction of NO2 with CH3CHO to give
N2.
In addition to the bands due to formate and acetate described
above, new IR bands at 1350, 1424 and 1612 cm−1 were detected
on Co/Al2O3 at 300–400 ◦C, as can be seen in Fig. 3. The
intensities ratio of these peaks was constant though the abso-
lute peak intensities changed about 30% with temperature. Yu
et al. [18] observed IR peaks at 1336, 1416 and 1633 cm−1 dur-
ing CH3CHO oxidation over Ag/Al2O3, and assigned them to
enolic species(CH2 CH–O–) on the basis of density functional
theory calculation. Theoretical calculations of the vibrational
frequencies for enolate (CH2 C(CH3)–O) adsorbed on MgO
also indicated that the IR bands due to the symmetric and asym-
metric stretching vibration of the C–C–O group appear at 1690
and 1426 cm−1, respectively [19]. Therefore, the bands at 1350,
1424 and 1612 cm−1 observed here can be assigned to the eno-
late species (CH2 CH–O–).
3.4. FT-IR study
To get further insight into the role of Co and reaction mech-
anism, we observed FT-IR spectra of adsorbed species formed
on Al2O3 and 2 wt.% Co/Al2O3 under the reaction conditions
of CH3CHO + O2 and NO2 + CH3CHO + O2.
3.4.2. Surface species formed in NO2 + CH3CHO + O2
reaction
3.4.1. Surface species formed in CH3CHO + O2 reaction
Fig. 3 shows the IR spectra of the adsorbed species formed
on Al2O3 and Co/Al2O3 after introducing the gas mixture
of CH3CHO + O2 at each temperature. On Al2O3, the forma-
tion of formate and acetate species was confirmed at 300 ◦C
by the absorption peaks at 1393 and 1590 cm−1 (δ(CH) and
νas(COO−) of formate[15–17]) and 1467 cm−1 (νs(COO−) of
acetate[15–17]), respectively. Weak absorption bands due to
acetate and formate species were also observed in the C–H
stretching region (2900–3000 cm−1). When the temperature
The IR spectra measured during the NO2 + CH3CHO + O2
reaction over Al2O3 and Co/Al2O3 at each temperature are
shown in Fig. 4. The IR bands assigned to formate (1376,
1393, 1590 cm−1), acetate (1467 cm−1) and nitrate species
(1301 cm−1 [17]) were observed on both Al2O3 and Co/Al2O3
at 300 ◦C. In addition to these bands, the peaks at 2231 and
2258 cm−1 assigned to isocyanate (–NCO) species formed on
two different Al3+ sites, coordinatively unsaturated tetrahedral
and octahedral Al3+ [20], respectively, were observed. It should
be noted that the bands assigned to enolate species (1350, 1424

10
A. Takahashi et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 6–11
Fig. 6. Time dependence of integrated areas of IR bands due to enolate species
(1417–1428 cm−1) (ꢀ) and –NCO species (2200–2290 cm−1) (᭹) on 2 wt.%
Co/Al2O3 after switching a gas mixture from CH3CHO (1500 ppm) + O2 (10%)
to NO2 (500 ppm) + O2 (10%) at 400 ◦C.
reacts with O2 to form enolate species. This step would be effec-
tively catalyzed by cobalt sites because the formation of enolate
species took place not on Al2O3 but on Co/Al2O3 (Figs. 3 and 4).
Some papers reported that the surface enolate species from car-
bonyl compound such as aldehyde and ketone were formed by
the ␣-hydrogen abstraction of the carbonyl compound, which
was proceeded by O2− ions adjacent to cation sites in the
cation-anion couple site [21–23]. Ulla et al. showed that Co2+ in
CoO–MgO solid solution plays a significant role in the forma-
tion of O2− species [24]. Co/Al2O3 catalyst is known to contain
Co2+ incorporated in aluminium oxide lattice in CoAl2O4-like
structure [25]. We consider that the O2− ion, which is produced
on the sites adjacent to Co2+ sites of Co/Al2O3 in the presence of
oxygen, abstracts ␣-hydrogen of acetaldehyde, resulting in the
formation of surface enolate. It was reported for hydrocarbon
oxidation on transition metal-oxide catalysts that partial oxi-
dation of hydrocarbon effectively occurs on the oxide surface
where enolate anion is formed [26]. This is in agreement with
the result that the conversion of CH3CHO to COx on Co/Al2O3
was lower than that on Al2O3 during CH3CHO + O2 reaction
(Fig. 2). In the second step, the enolate species thus formed
reacts with NO2 to give –NCO species (Fig. 5), which is eventu-
ally converted to N2. It is known that the nitrosation of enolate by
nitrosonium ion (NO+) easily occurs since the enolate is nucle-
ophilic and easily reacts with electrophilic species. It is predicted
by ab initio calculation that the nitorosoaldehyde, which is a
nitrosation product of CH3CHO via enolate species, easily iso-
merizes to the glyoxal monoxime, which is an oxime species
[27]. Obuchi et.al. proposed that –NCO species is formed from
organo-nitroso compound via oxime species formation [28].
Fig. 5. Change in IR spectra of adsorbed species on 2 wt.% Co/Al2O3 after
switching a gas mixture from CH3CHO (1500 ppm) + O2 (10%) to NO2
(500 ppm) + O2 (10%) at 400 ◦C.
and 1612 cm−1) were detected on Co/Al2O3. Since the enolate
species almost completely disappeared at temperatures above
400 ◦C at which NO2 reduction proceeds (Table 1), this would
be an intermediate for NO2 reduction by CH3CHO.
3.5. Reaction pathways of the selective reduction of NO2
with CH3CHO
To confirm the possibility of enolate species as an intermedi-
ate, the reactivity of enolate species toward NO2 was evaluated
by transient response measurement of IR spectra. The Co/Al2O3
catalyst was first exposed to CH3CHO + O2 for 60 min to accu-
mulate enolate species on the surface, and then the reaction
gas mixture was switched to NO2 + O2. This experiment was
performed at 400 ◦C because no formate species, which may
react with NO2, existed on the surface at 400 ◦C (Fig. 3). Fig. 5
shows the change in the IR spectra of adsorbed species after
switching the gas mixture to NO2 + O2. It is obvious that the IR
band for enolate species decreased and those for nitrate species
increased with time on stream. Interestingly, the formation of
–NCO species was observed in a few minutes after introducing
NO2 + O2. Fig. 6 shows the time dependence of the integrated
areas of IR bands due to enolate species (1417–1428 cm−1) and
–NCO species (2200–2290 cm−1). Relatively good correlation
between the consumption of enolate species and the formation of
–NCO species was observed, indicating that the enolate species
can react with NO2 to produce –NCO species. Therefore, the
enolate species would be an intermediate for NO2 reduction by
CH3CHO.
A hypothetical reaction pathway of NO2 reduction by
CH3CHO is illustrated in Scheme 1. In the first step, CH3CHO
Scheme 1. Proposed reaction pathway for the selective reduction of NO2 with
CH3CHO on Co/Al2O3.

A. Takahashi et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 6–11
11
Some researchers also reported the transformation of the oxime
species to –NCO species [29,30]. Therefore, the possible path-
way to –NCO species seems to consist of nitrosation of enolate,
isomerization of nitroso species, and transformation of oxime
species. In Fig. 5, there is a lag time of about 2 min before
NCO band starts to appear, indicating that the induction period is
needed for the appearance of the NCO band in the reaction steps
from enolate to NCO species. The –NCO species thus formed
are rapidly hydrolyzed to NH compounds by reacting with water
in the stream and/or on the surface, and then the NH compounds
react with NOx species to produce N2 [29–32]. Because no water
was supplied in the FT-IR measurements, the hydrolysis reac-
tion of NCO did not occur and consequently NCO band was still
significant while enolate bands have all but disappeared (Fig. 5).
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4. Conclusions
Co/Al2O3 showed the highest activity for the selective reduc-
tion of NO2 with CH3CHO of the alumina-supported catalysts
tested. The optimum Co loading of Co/Al2O3 was 2 wt.%. The
comparison of the activity of Co/Al2O3 and Al2O3 for several
unit reactions suggested that one of the roles of supported Co is
to suppress the combustion of CH3CHO by O2. The observation
of the adsorbed species by in situ FT-IR spectroscopy suggested
that another role of Co is to promote the production of the eno-
late species, which is the reaction intermediate leading to N2
formation.
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