Probing into the catalytic nature of Co/sulfated zirconia for selective reduction of NO with methane

Article abstract of DOI:10.1016/j.jcat.2004.04.026

In this work, the structural and surface properties of Co-loaded sulfated zirconia (SZ) catalysts were studied by X-ray diffraction (XRD), N₂ adsorption, NH₃-TPD, FT-IR spectroscopy, H₂-TPR, UV-vis diffuse reflectance spec

Full text of DOI:10.1016/j.jcat.2004.04.026

Journal of Catalysis 225 (2004) 307–315
www.elsevier.com/locate/jcat
Probing into the catalytic nature of Co/sulfated zirconia
for selective reduction of NO with methane
Ning Li, Aiqin Wang, Mingyuan Zheng, Xiaodong Wang, Ruihua Cheng,
∗
and Tao Zhang
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, Dalian 116023, China
Received 12 February 2004; revised 12 April 2004; accepted 19 April 2004
Abstract
In this work, the structural and surface properties of Co-loaded sulfated zirconia (SZ) catalysts were studied by X-ray diffraction (XRD),
N adsorption, NH -TPD, FT-IR spectroscopy, H -TPR, UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy
2
3
2
(
XPS), and NO-TPD. NH -TPD and FT-IR spectra results of the catalysts showed that the sulfation process of the support resulted in the
3
generation of strong Brønsted and Lewis acid sites, which is essential for the SCR of NO with methane. On the other hand, the N adsorption,
2
2
−
H -TPR, UV/vis DRS, and XPS of the catalysts demonstrated that the presence of the SO
2
species promoted the dispersion of the Co
4
species and prevented the formation of Co O . Such an increased dispersion of Co species suppressed the combustion reaction of CH by
3
4
4
O and increased the selectivity toward NO reduction. The NO-TPD proved that the loading of Co increased the adsorption of NO over SZ
2
catalysts, which is another reason for the promoting effect of Co.

2004 Elsevier Inc. All rights reserved.
Keywords: NO reduction; Methane; Co; Sulfated zirconia
1
. Introduction
catalysts for the SCR reaction [8,27–30]. Therefore, the de-
velopment of new catalysts with higher tolerance toward the
poisoning of water and SO2 is very important.
Oxides of nitrogen, as by-products of high-temperature
combustion, cause serious environmental problems, such as
acid rain and photochemical smog. Therefore, strict legisla-
tion around the world has led to the development of efficient
DeNOx technologies in order to preserve the global environ-
ment. Over the last several years, the use of hydrocarbons
as a substitute for ammonia in the selective catalytic reduc-
tion (SCR) of NO from oxygen-rich streams has drawn much
attention [1–11]. Among various hydrocarbons, methane is
the most preferable, because it is the main component of
natural gas [5–11]. Many kinds of catalysts such as Co-
Recently, we first investigated the catalytic performance
of a series of nonnoble metal (Co, Mn, In, Ni)-loaded sul-
fated zirconia (SZ) for the SCR of NO with methane and
found that Co exhibited the highest promoting effect on the
activity of SZ catalysts among the investigated metals [31].
The optimum Co content of Co/SZ catalyst is about 4 wt%.
Sulfation of support is necessary for the high activity and
selectivity of Co/SZ catalysts. One of the most attractive
features of Co/SZ catalysts is that they are less sensitive to
water and SO2 poisoning than Co/HZSM-5 catalysts and ex-
hibited higher reversibility after removal of water or SO2. In
this work, to gain a deeper insight into the essence of the
Co/SZ catalyst which is responsible for the good catalytic
performance of the catalyst, and to understand the promoting
[
5,12–14], Ni- [13], Mn- [15–17], Ga- [18], In- [19–24],
and Pd- [25,26] loaded ZSM-5 have been reported to be
active for the catalytic reduction of NO with methane. How-
ever, water vapor and sulfur dioxide are typically present in
combustion exhausts and may cause severe deactivation of
effect of Co and SO42 for the SCR of NO with methane, we
used a series of techniques to characterize the Co/SZ cata-
lysts.
−
*
Corresponding author. Fax: +86-411-84691570.
E-mail address: taozhang@dicp.ac.cn (T. Zhang).
0021-9517/$ – see front matter  2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2004.04.026

308
N. Li et al. / Journal of Catalysis 225 (2004) 307–315
◦
2
. Experimental
activated by heating in vacuum at 500 C for 1 h, cooled
down to ambient temperature, and saturated with pyridine
◦
2
.1. Catalyst preparation
vapor (at 0 C) for 15 min. The spectra of the activated
samples (taken at ambient temperature before adsorption of
pyridine) were used as a background reference. Then, the
samples were heated in vacuum at different temperatures for
1 h. The spectra of the samples that had been subjected to
elevated temperatures were recorded after the IR cell had
been cooled to ambient temperature. All of the spectra pre-
sented were obtained by subtraction of the corresponding
background reference.
Sulfated zirconia was prepared as described previously
31] by impregnating the precursor Zr(OH)4 (AR grade,
[
Shanghai Agent Company, China) with an aqueous solution
of 0.5 M (NH4)2SO4 in the ratio of 15 ml/g. The slurry was
stirred slowly for half an hour, filtrated without washing, and
◦
dried at 110 C overnight. Co/SZ was prepared by incipient
wetness impregnation of the SZ with an aqueous solution of
Co(NO3)2. For comparison, Co/ZrO2 was prepared by the
same method with Zr(OH)4 as the support precursor. All the
2.2.5. H2-TPR measurements
◦
samples were dried at 110 C for 8 h and calcined in air at
Temperature-programmed reduction (TPR) experiments
were carried out with a Micromeritics AutoChem II 2920
automated catalyst characterization system using an H2/Ar
mixture (10% H2). A catalyst charge of 150 mg was used.
Before each experiment, the catalyst sample was pretreated
◦
6
00 C for 6 h. The Co/ZrO2 catalyst as prepared was black
in color, while the colors of Co/SZ catalysts varied with their
Co contents: the colors of 2 wt% Co/SZ and 4 wt% Co/SZ
were pink, the colors of 6 wt% Co/SZ and 8 wt% Co/SZ
were, respectively, brown and black.
◦
◦
at 600 C in Ar flow for 0.5 h and then cooled to 40 C. Sub-
sequently, the Ar flow was switched to the H2/Ar mixture,
◦
2
2
.2. Characterization methods
and the reduction was performed from 40 to 600 C at a
heating rate of 10 C/min. The consumption of H2 was de-
◦
.2.1. XRD measurements
XRD patterns were obtained with a Rigaku (D/MAX-
βB) diffractometer equipped with an on-line computer. Ni-
filtered Cu-Kα radiation was used.
termined with a TCD detector. Before the detector, a cold
trap was used to remove the water generated in the reduc-
tion.
2
.2.6. UV–vis DRS measurements
2
.2.2. BET surface area measurements
DRS spectra were recorded in air in the wavelength range
The specific surface areas of the catalysts were deter-
200–800 nm on a JASCO V-550 spectrometer. The data
were acquired and analyzed by a computer at the rate of
100 nm/min.
mined by nitrogen adsorption at 77 K using a Micromeritics
ASAP 2010 apparatus. Before each experiment, the samples
◦
were evacuated at 150 C for 3 h.
2
.2.7. XPS measurements
2
.2.3. NH3-TPD measurements
XPS spectra were recorded with a VG ESACALAB
NH3-TPD of the catalysts was conducted with Micro-
MK-2 X-ray photoelectron spectrometer, using Al-Kα radi-
ation (1486.6 eV, 12.5 kV, 250 W). Samples were ground in
an agate mortar and pressed onto a gold-decorated tantalum
plate attached to the sample holder. The analysis chamber
was evacuated at pressures lower than 10 Pa. The com-
puter collected sequentially the kinetic energy region of Co
2p1/2 and Co 2p3/2 (665–710 eV) and Zr 3d3/2 and Zr 3d5/2
(1290–1310 eV). Binding energy (BE) values were refer-
enced to the Zr 3d5/2 peak taken as 182.5 eV.
meritics AutoChem II 2920 automated catalyst characteri-
zation system. A sample of 150 mg was placed in a quartz
◦
reactor and heated at 600 C for 0.5 h in He flow to remove
−7
physically absorbed water. The adsorption of ammonia was
◦
performed at 100 C. After saturation, the sample was heated
◦
◦
linearly with a rate of 10 C/min, from 100 to 650 C under
a constant He flow of 30 ml/min. The desorbed amount of
ammonia was detected with a TCD detector.
2
.2.4. FT-IR spectroscopy measurements
2.2.8. NO-TPD measurements
The FT-IR spectra with pyridine as a molecular probe
NO-TPD was conducted on a flowing reaction system us-
ing a mass spectrometer (Omini-star, GSD-300) as detector.
Before the TPD experiment, the catalysts (300 mg) were
were recorded on a Nicolet Nexus 470 equipped with a
liquid–nitrogen-cooled MCT detector at a resolution of
−1
◦
4
cm (64 scans). An adsorption IR cell allowed recording
pretreated at 600 C in flowing He for 1 h, cooled to room
of the spectra at ambient temperature and catalyst activa-
tion at higher temperatures. The cell was equipped with a
heater and connected to a vacuum system. The temperature
was monitored with a thermocouple placed in direct contact
with the sample. Self-supporting wafers (ca. 0.020 g/cm )
of the samples were used for FT-IR studies. Before the
surface characterization was performed, the samples were
temperature, and exposed to a gas stream of 1% NO in He at
50 ml/min to achieve saturated adsorption. Then, the cata-
lysts were purged with He at 50 ml/min to remove gas-phase
NO and weakly adsorbed NO. As the NO level, monitored
by the mass spectrometer, returned to the background of
the mass spectrometer, the TPD run was started from room
2
◦
◦
temperature to 600 C at a ramping rate of 15 C/min. The

N. Li et al. / Journal of Catalysis 225 (2004) 307–315
309
effluent gases from the reactor were continuously monitored
as a function of temperature.
3
. Results
3
.1. XRD characterization
In this work, we investigated the effect of sulfation on
the XRD patterns of catalyst. The XRD patterns of ZrO2,
wt% Co/ZrO2, SZ, and 4 wt% Co/SZ catalysts are shown
4
2
−
in Fig. 1. All the samples that contain SO4 , such as SZ
and 4 wt% Co/SZ, have the same XRD patterns as that of
tetragonal zirconia [32]. However, for the ZrO2 and 4 wt%
Co/ZrO2 which have not been sulfated, the diffraction lines
of monoclinic zirconia can also be observed [32]. These re-
sults demonstrate that the sulfation process is helpful for the
stabilization of tetragonal zirconia. In addition, no distinct
peaks of cobalt oxide can be observed after loading ZrO2 or
SZ with 4 wt% Co.
Fig. 2. XRD patterns of the Co/SZ catalysts with different Co contents.
Table 1
BET surface areas of the investigated catalysts
Furthermore, we compared the XRD patterns of Co/SZ
catalysts with different Co contents. As shown in Fig. 2, the
XRD patterns of the 2 wt% Co/SZ and 4 wt% Co/SZ cata-
lysts are similar to that of SZ. While, in the XRD patterns of
2
Catalyst
BET surface area (m /g)
ZrO
2
SZ
45
94
33
83
4
4
wt% Co/ZrO
wt% Co/SZ
6
wt% Co/SZ and 8 wt% Co/SZ, a weak peak of Co3O4 can
2
be observed, respectively.
3
.2. BET characterization
Table 1 lists the BET surface areas of different cata-
lysts. It is noticeable that sulfated samples such as SZ and
Co/SZ have much greater surface areas than those of unsul-
fated ones (ZrO2 and Co/ZrO2), indicating that the sulfation
process largely increased the surface areas of the catalysts.
The difference in BET surface area between sulfated and
unsulfated samples can be interpreted by the crystal phase.
From the XRD result, it can be seen that the introducing of
2
−
SO4 can restrain the phase transformation of the catalyst
from a tetragonal phase to a denser monoclinic phase. This
is the reason for the larger surface areas of sulfated samples.
On the other hand, the loading of Co on the supports caused
a slight decrease of the BET surface areas, as that of 4 wt%
Co/ZrO2 and 4 wt% Co/SZ compared with that of ZrO2 and
SZ, respectively.
3
.3. NH3-TPD and FT-IR spectroscopy characterization
It is generally accepted that the acid sites of catalysts play
an important role in the SCR of NO with methane [33–39].
Therefore, in this work, we compared the NH3-TPD spec-
tra of different catalysts. From Fig. 3, it was evident that the
amount of ammonia desorbed from pure zirconia was sig-
nificantly increased by the sulfation. The spectrum of pure
zirconia contained a very broad signal in the temperature
◦
Fig. 1. XRD patterns of ZrO , SZ, 4 wt% Co/ZrO , 4 wt% Co/SZ, and
range between 100 and 450 C. Due to the sulfation, the
2
2
Co O .
3
4
intensity of the signal increased significantly and the tem-

310
N. Li et al. / Journal of Catalysis 225 (2004) 307–315
Fig. 3. NH -TPD spectra of (a) ZrO , (b) SZ, (c) 4 wt% Co/ZrO , and (d)
3
2
2
4
wt% Co/SZ.
◦
perature window of ammonia desorption widened to 600 C.
Corma et al. [40] observed on sulfated zirconia an NH3-TPD
◦
peak at 540 C which they assigned to ammonia desorbed
from strong acid sites. In this work, an evident peak can also
be observed in the NH3-TPD spectrum of our SZ at about
◦
5
50 C. Thereby, the conclusion can be drawn that sulfation
of our zirconia led to the formation of strong acid sites as
well. Moreover, it is very interesting that after loading SZ
with Co a significant increase in the intensity of ammonia
desorbed at higher temperature occurred, while the inten-
sity of ammonia desorbed at lower temperature remained the
same as its initial value. This result indicated that the intro-
duction of Co increased the number of strong acid sites of
SZ catalyst.
Fig. 4. The FT-IR spectra of pyridine adsorbed on ZrO (a) and 4 wt%
2
Co/ZrO (b) after evacuation at different temperatures.
2
SZ catalysts. According to the literature [43,44], these new
strong Lewis acid sites may be caused by the Co ions whose
It is well known that the pyridine chemisorbed on Lewis
and Brønsted acid sites will lead, respectively, to the ad-
Lewis acid strength was increased owing to the electron-
−1
sorption bands at 1450 and 1540 cm in the infrared spec-
tra [41,42], which is widely used to distinguish the type and
relative quantity of the acid sites on the surface of catalysts.
Therefore, in order to gain a deeper insight into the nature
of the acid sites on different catalysts, we studied their FT-
IR spectra with pyridine as a molecular probe. As shown in
Fig. 4, the FT-IR spectra of both ZrO2 and Co/ZrO2 con-
sisted only of bands of the Lewis acid–pyridine complex at
2−
withdrawing effect of SO4
.
Miller and co-workers [33,34] have investigated the role
2+
of acid sites in Co -exchanged zeolite catalysts for selec-
tive catalytic reduction of NOx. According to their opinion,
the selective reduction of NOx with methane under lean con-
ditions includes the following two steps:
2NO + O → 2NO ,
2
2
(1)
−1
about 1450 cm . The intensities of such bands decreased
with the evacuation temperatures and vanished after evacua-
2
NO2 + CH4 → CO2 + 2H2O + N2.
(2)
◦
tion at 450 C. It is clear that the ZrO2 and Co/ZrO2 possess
predominantly weak Lewis acid sites. While, in the spectra
of SZ and Co/SZ (cf. Fig. 5), evident bands of both Lewis
and Brønsted acid can be observed; these bands still ex-
NO oxidation is catalyzed predominantly by cobalt ions
and cobalt oxides. By contrast, the formation of N occurs at
2
the strong Brønsted acid sites of catalysts by first protonation
of the methane and then the reaction of adsorbed carbenium
◦
isted even after evacuation at 450 C. These results indicate
that the presence of SO42 leads to the generation of strong
Lewis and Brønsted acid sites. On the other hand, it is noted
that the Lewis–Brønsted ratio of the 4 wt% Co/SZ is higher
than that of SZ. Such phenomenon is more pronounced af-
−
ions with gas-phase NO leading to organo–nitrogen inter-
2
mediates and the eventual formation of N . The rate of N
2
2
formation increases linearly with the number of strong Brøn-
sted acid sites, and the specific rate increased with Brønsted
acid strength. A similar conclusion was drawn by Kung and
co-workers [35] based on the fact that the conversion of NO
over a physical mixture of Co/Al2O3 and H-zeolite catalysts
◦
ter evacuation at 450 C. Combining these results with those
of NH3-TPD, the conclusion can be drawn that the loading
of Co increased the number of strong Lewis acid sites of

N. Li et al. / Journal of Catalysis 225 (2004) 307–315
311
Fig. 6. NH -TPD spectra of (a) 2 wt% Co/SZ, (b) 4 wt% Co/SZ, (c) 6 wt%
3
Co/SZ, and (d) 8 wt% Co/SZ.
catalyst is strongly dependent on the Co loading. With the
increase of Co content, the maximum NO conversion over
Co/SZ increased initially, reached the highest value when
the Co content is about 4 wt%, and then decreased. More-
over, the optimum temperature at which the maximum NO
conversions were reached shifted to lower values. These re-
sults can be interpreted by Miller and co-workers [34] by
the cooperation between Co and strong Brønsted acid sites
of Co/SZ catalysts. When the Co content is low or at a low
temperature, NO oxidation is slow and the rate-limiting step.
Therefore, increasing the Co content will lead to the for-
Fig. 5. The FT-IR spectra of pyridine adsorbed on SZ (a) and 4 wt%
Co/SZ (b) after evacuation at different temperatures.
mation of more NO and the increase of activity of Co/SZ
2
catalysts. On the other hand, when the Co content is high or
at a high temperature, the NO oxidation is in equilibrium,
and excess Co will facilitate the nonselective reduction of
NO2 and decrease the selectivity of methane,
was found to be twice that expected if these components
acted independently.
In our case, it can be seen from the results of NH3-
TPD and FT-IR that the sulfation of zirconia will lead to
the generation of strong Brønsted acid sites which can act
as those of zeolites to protonate methane and facilitate the
SCR of NO with methane by catalyzing the formation of
N2. Furthermore, we do not exclude the possibility that sul-
fation of support led to the generation of strong Lewis acid
sites which may also play an important role in the SCR
of NO with methane. Recently, Kantcheva et al. [45] stud-
ied the interaction between the CH4 and the adsorbed NOx
species over Co/SZ and Co/ZrO2 with FT-IR spectra. Ac-
cording to them, methane can be adsorbed dissociatively
over strong Lewis acid sites with the generation of strongly
bound metal–alkyl or methoxy species [43,45]. Then, the
formate species (formic acid) can be produced by fast ox-
idation of the methoxide. Such formate species plays a role
of intermediates that are capable of selectively reducing NO.
Furthermore, we investigated the effect of Co content on
the NH3-TPD spectra of the Co/SZ catalysts as well. As
shown in Fig. 6, the acidities of Co/SZ catalysts varied little
when the Co content increased from 2 to 8 wt%. In our pre-
vious work [31], we have found that the activity of Co/SZ
4
NO2 + CH4 → 4NO + CO2 + 2H2O,
2O2 + CH4 → CO2 + 2H2O.
3.4. H2-TPR characterization
2
H -TPR curves of different catalysts are presented in
Fig. 7. Both pure ZrO and SZ have no H consumption
2
2
◦
peak at temperatures below 550 C, indicating that the two
supports were difficult to be reduced. After they were loaded
with Co, different results were observed. For 4 wt% Co/ZrO2
2
catalyst, a very wide H -TPR peak was observed in the tem-
◦
perature range between 300 and 450 C. According to the
literature [46–48], this wide peak can be attributed to the se-
quential reduction of Co O to CoO and CoO to Co. In con-
trast, 4 wt% Co/SZ has only a peak at temperatures higher
than 500 C, which indicates that the presence of SO
vented the formation of Co O . This result can be explained
by those of BET, NH -TPD, and FTIR because the sulfation
process greatly increased the surface areas of the catalysts
and generated strong Brønsted acid sites, such a dual ef-
fect of sulfation improved the dispersion of Co species and
3
4
◦
2−
4
pre-
3
4
3

312
N. Li et al. / Journal of Catalysis 225 (2004) 307–315
Fig. 7. H -TPR curves of (a) ZrO , (b) SZ, (c) 4 wt% Co/ZrO , and
2
2
2
(d) 4 wt% Co/SZ.
Fig. 9. UV/vis DRS spectra of (a) ZrO , (b) SZ, (c) 4 wt% Co/ZrO , and
2
2
(d) 4 wt% Co/SZ.
content of the catalysts, which indicates the presence of in-
creasing amounts of Co3O4.
3
.5. UV–vis DRS characterization
Fig. 9 shows the DRS spectra of different catalysts. It can
be seen that the spectra of ZrO2 and SZ samples only con-
sisted of a broad absorption at wavenumbers greater than
−1
2
5,000 cm . After loading them with 4 wt% Co, the re-
sults were completely different for ZrO2 and SZ. Unlike
pure ZrO2, the spectrum of 4 wt% Co/ZrO2 consisted of a
−1
weak band at about 14,000 cm , indicating the presence of
Co3O4 [44,49]. In contrast, the spectrum of 4 wt% Co/SZ
consisted of a wide band in the wavenumber range between
−1
1
6,000 and 21,000 cm , absent on pure SZ. According to
the literature [44,50], this peak can be attributed to typical
2
+
Co in octahedral sites. These results can explain the great
difference in the activities of Co/ZrO2 and Co/SZ for the
SCR of NO with methane from the nature of Co species. It
is generally accepted that the Co2 is highly active for the
+
2
+
SCR of NO with methane [51–56]. On one hand, Co can
promote the oxidation of NO [33–35] and the formation of
surface nitrates like that previously observed by Sachtler and
co-workers on Co zeolites [54]. These nitrates are highly re-
active toward methane [54,57]. On the other hand, as shown
Fig. 8. H -TPR curves of (a) 2 wt% Co/SZ, (b) 4 wt% Co/SZ, (c) 6 wt%
2
Co/SZ, and (d) 8 wt% Co/SZ.
2
+
in the results of NH -TPD and IR, the presence of Co
3
increases the number of strong Lewis acid sites of SZ cat-
alysts. These strong Lewis acid sites may play a very im-
prevented the formation of Co3O4 which is more easily re-
ducible than dispersed Co species.
portant role for the partial oxidation of CH and the SCR
4
of NO [45]. Co O is also very active for the oxidation of
3
4
The next comparison that we have made is the H2-TPR
curves of Co/SZ catalysts with different Co contents. It can
be seen from Fig. 8 that the maximum ability of SZ to dis-
perse Co is about 4 wt%, when the Co content of Co/SZ
catalyst is greater than this value, the peak of Co3O4 re-
duction will appear and its intensity increases with the Co
NO [33,56]. However, due to its high catalytic activity for
the total oxidation of methane, it is unexpected for the SCR
of NO [49,51,52].
Subsequently, we compared the DRS spectra of Co/SZ
catalysts with different Co contents, and the results are
shown in Fig. 10. It is clear that the DRS spectra of Co/SZ

N. Li et al. / Journal of Catalysis 225 (2004) 307–315
313
Fig. 11. XPS spectra of 4 wt% Co/ZrO and 4 wt% Co/SZ catalysts.
2
Fig. 10. UV/vis DRS spectra of (a) 2 wt% Co/SZ, (b) 4 wt% Co/SZ,
c) 6 wt% Co/SZ, and (d) 8 wt% Co/SZ.
Table 2
(
XPS parameters from Co 2p in Co/ZrO and Co/SZ catalysts and relevant
2
reference compounds
Material
CoO
BE of Co 2p
(eV) 2p
3/2
− 2p1/2 gap (eV) Reference
3/2
catalysts with lower Co content (ꢀ 4 wt%) only consisted of
780.5
780.9
780.7
15.9
16
15.0
15.5
15.9
[58]
[59]
[58]
a wide band in the wavenumber range between 16,000 and
Co(OH)
2
−1
2+
2
1,000 cm , indicating that Co exists in the form of Co
Co O
3
4
ions [44,50]. While, in spectra of the Co/SZ catalysts with
4 wt% Co/ZrO 780.7
4 wt% Co/SZ
2
−1
Co content greater than 4 wt%, a band at about 14,000 cm
780.5
appeared, indicating that these samples contained Co3O4.
These results are consistent with those of H2-TPR and can
interpret the reason why the optimum Co content of Co/SZ
is 4 wt% in another way: the maximum ability of SZ to
anchor Co2 ions is about 4 wt%; when the Co content is
beyond this value, Co will aggregate to Co3O4 and decrease
the selectivity of CH4 to NO reduction which will lead to the
decrease of NO SCR activity of the catalyst.
+
3
.6. XPS characterization
To further investigate the difference of Co species in
Co/ZrO2 and Co/SZ, we compared the XPS of 4 wt%
Co/ZrO2 and 4 wt% Co/SZ. As shown in Fig. 11 and Ta-
ble 2, the XPS spectra of 4 wt% Co/ZrO2 and 4 wt% Co/SZ
samples are similar in shape and consist of two main bands
at about 780 eV (Co 2p3/2) and 796 eV (Co 2p1/2), but there
are still some differences in the BE for Co 2p3/2 and Co
Fig. 12. NO-TPD profiles of SZ and 4 wt% Co/SZ catalysts.
2
p3/2 − Co 2p1/2 gaps. For the Co/SZ sample, the BE value
formation of Co O which is unexpected for the SCR of NO.
3
4
of Co 2p3/2 was found to be 780.5 eV, and the difference be-
tween the BE of Co 2p3/2 and Co 2p1/2 is 15.9 eV. These
values are consistent with the literature values for CoO [58],
indicating that the Co species in the 4 wt% Co/SZ mainly ex-
ists in a bivalent state. In contrast, for Co/ZrO2, the BE value
of Co 2p3/2 is 780.7 eV and the difference between the BE
of Co 2p3/2 and Co 2p1/2 is 15.5 eV (between the value of
CoO (15.9 eV) and that of Co3O4 (15.0 eV) [58]), indicating
that a part of Co exists in the form of Co3O4. These results
are in good agreement with those of H2-TPR and DRS and
3.7. NO-TPD characterization
Fig. 12 shows the NO-TPD profiles of SZ and 4 wt%
Co/SZ samples after the standard pretreatment. The NO-
TPD profile of a SZ support has a wide peak range from
◦
room temperature to 350 C. After loading it with 4 wt%
Co, the intensity of the NO desorption peak of SZ was in-
creased significantly, indicating that the introduction of Co
increased the NO adsorption of SZ catalysts. This result can
give further proof that the presence of SO42 did prevent the
−

314
N. Li et al. / Journal of Catalysis 225 (2004) 307–315
elucidate the promoting effect of Co to the activity of SZ
from an adsorption point of view.
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(
[
[
In addition, it can be seen from Fig. 12 that the NO-
TPD profile of a 4 wt% Co/SZ catalyst contains two peaks.
According to the literature [60], the first peak at low temper-
3
4
+
ature can be attributed to the NO desorption from Zr –NO
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+
[
n+
assigned to the NO desorption from the Co –NO (n = 2, 3)
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species. Furthermore, it is found that the NO-TPD profile
of our 4 wt% Co/SZ seems very similar to that obtained by
Hadjiivanov et al. [60] on their highly dispersed Co/SZ sam-
ple (relative Co dispersion is 1.00), which reflects the high
Co dispersion in our 4 wt% Co/SZ catalyst in another way.
[
[
[
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[
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[
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termining the catalytic performance of Co/sulfated zirconia
catalysts. The sulfation process improved the dispersion of
Co species and provided the strong Brønsted and Lewis acid
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1
549.
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Acknowledgments
[
[
Prof. Can Li, Pinliang Ying, Dr. Zhimin Liu, and Wei-
cheng Wu are acknowledged for their great help in FT-IR
spectroscopy characterization.
[
[
353.
[
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