A direct sulfation method for introducing the transition metal cation Co2+ into ZrO2 with little change in the Bronsted acid sites

Article abstract of DOI:10.1016/j.jcat.2011.01.026

Direct sulfation of Zr-Co hydroxides provides a significant advantage over traditional impregnation by conserving Bronsted acid sites for the introduction of a second active component and also for subsequent catalytic reactions. After Pd loading, sulfated Zr-Co exhibits excellent activity, selectivity, and durability for the selective catalytic reduction of NO _x by methane. Using Co K-edge X-ray absorption spectroscopy, we provide the direct evidence of the entrance of Co into the lattice of cubic zirconia to form a substitutional solid solution.

Full text of DOI:10.1016/j.jcat.2011.01.026

Journal of Catalysis 279 (2011) 301–309
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A direct sulfation method for introducing the transition metal cation Co2+ into
ZrO2 with little change in the Brønsted acid sites
⇑
⇑
Xiangjie Wang, Huayu Wang, Yongchun Liu, Fudong Liu, Yunbo Yu , Hong He
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road,
Haidian District, Beijing 100085, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Direct sulfation of Zr–Co hydroxides provides a significant advantage over traditional impregnation by
conserving Brønsted acid sites for the introduction of a second active component and also for subsequent
catalytic reactions. After Pd loading, sulfated Zr–Co exhibits excellent activity, selectivity, and durability
for the selective catalytic reduction of NOx by methane. Using Co K-edge X-ray absorption spectroscopy,
we provide the direct evidence of the entrance of Co into the lattice of cubic zirconia to form a substitu-
tional solid solution.
Received 12 November 2010
Revised 24 January 2011
Accepted 25 January 2011
Keywords:
Sulfation
Ó 2011 Elsevier Inc. All rights reserved.
Solid acid
Selective catalytic reduction
Solid solution
1. Introduction
of Brønsted acid sites, since these sites are preferentially occupied
by the cations introduced, such as Co [19] and Pd [27,28]. As
Sulfated metal oxides with both Brønsted and Lewis acid sites
are widely used as solid acid catalysts and/or supports in organic
synthesis, transformation reactions, and NOx removal [1–7]. Of
particular interest is sulfated zirconia, which possesses more acid
sites than other sulfated metal oxides [8–13]. Zirconia exists in
several crystalline forms: monoclinic, tetragonal, and cubic. It ex-
ists mainly in the tetragonal phase in the presence of S [14,15].
Using periodic ab initio calculations, Haase and Sauer [16] sug-
gested that the most stable configurations of sulfur species on
the surface of tetragonal zirconia are the tridentate sulfate anion
and the –SO3 complex. Details of the type and number of acid sites
on sulfated zirconia have been identified by 13C NMR, 31P NMR, and
pyridine adsorption [17–19]. To further improve the properties of
sulfated zirconia, active component transition metal cations have
been introduced into sulfated zirconia by impregnation. Li et al.
[19,20] reported that the loading of Co over sulfated zirconia
exhibits high activity for the selective catalytic reduction of NOx
by methane. Mn2+ and Fe3+ show significant catalytic effects on
n-butane isomerization over sulfated zirconia [21–25]. Arata and
co-workers [26] reported that Pt-added sulfated zirconia was ac-
tive for the dehydrogenative coupling of methane. However, this
conventional impregnation method often decreases the number
Brønsted acid sites play a crucial role both in anchoring other ac-
tive components and in catalytic reactions [29–31], such a de-
crease would cause deterioration in the intrinsic properties of
solid acid materials. Thus, designing and implementing new meth-
ods for the introduction of transition metals into sulfated zirconia
while minimizing the consumption of Brønsted acid sites is of par-
ticular importance.
In the present study, the selective catalytic reduction of NOx by
methane (CH4-SCR) was used as a probe reaction because the num-
ber of Brønsted acid sites is very important for this reaction [32–
38]. Sulfated zirconia is commonly used as a support for catalysts
used in CH4-SCR [39–47]. Both Co2+ and Pd2+ are further intro-
duced into sulfated zirconia to enhance its activity and durability
[39,40,42,43]. The Co species in previous studies were introduced
by impregnation at the expense of Brønsted acid sites. Thus, it is
important to find a new method of introducing Co2+ into zirconia
with little change in the Brønsted acid sites. Here, the direct sulfa-
tion of Zr–Co hydroxides is employed to introduce Co species. The
number of Brønsted acid sites on sulfated Zr–Co oxide (denoted as
SZC) prepared by this method was much larger than that on zirco-
nia prepared by the impregnation method. As a result, sufficient
Brønsted acid sites are left available for the introduction of a sec-
ond active component and subsequent catalytic reactions. Specifi-
cally, a SZC-supported Pd catalyst (PdSZC) was further prepared by
the impregnation method. As expected, PdSZC showed high activ-
ity toward CH4-SCR.
⇑
Corresponding authors. Fax: +86 10 6284 9121.
E-mail addresses: ybyu@rcees.ac.cn (Y. Yu), honghe@rcees.ac.cn (H. He).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.01.026

302
X. Wang et al. / Journal of Catalysis 279 (2011) 301–309
of pyridine adsorption were recorded. All reported spectra were ta-
2. Experimental
2.1. Catalyst preparation
ken at a resolution of 4 cmÀ1 for 100 scans.
Powder X-ray diffraction (XRD) analysis was carried out on an
XPERT-PRO diffractometer system operated at an accelerating
voltage of 40 kV and an emission current of 40 mA with Cu K
radiation (k = 1.540598 Å). Data were acquired over the range of
20–90° 2h with a step rate of 4° minÀ1
Raman spectra were obtained at room temperature with a UVR
DLPC-DL-03 UV resonance Raman spectrometer equipped with a
CCD detector cooled with liquid N2. The instrument was calibrated
against the Stokes Raman signal of Teflon at 1378 cmÀ1. A contin-
uous He–Ne laser beam (325 nm) was used as the exciting radia-
a
The Zr–Co hydroxides were prepared by the co-precipitation
method using a mixture of aqueous solutions of Zr(NO3)4 and
Co(NO3)2 (Zr:Co = 9:1 mol ratio) and NH3ÁH2O (26 wt.%) as the pre-
cipitator to maintain a pH of 9. The precipitate was washed thor-
oughly and then dried at 120 °C for 12 h. Subsequently, SZC
samples with different S loadings were obtained by immersing
the above hydroxides into a 0.5 M (NH4)2SO4 solution, drying them
over a water bath at 60 °C, and then calcining them at 550 °C for 3 h
in air in sequence. Sulfated zirconia with 2 wt.% S (SZ) was also pre-
pared according to the above steps. The PdSZC sample (0.2 wt.% Pd)
was prepared by immersing SZC (with 2 wt.% S) in a Pd(NO3)2 solu-
tion using steps similar to those followed for the preparation of SZC.
SZ-supported Co (CoSZ, 4 wt.% Co), SZ-supported Pd (PdSZ, 0.2 wt.%
Pd), and CoSZ-supported Pd (PdCoSZ, 0.2 wt.% Pd) were prepared by
the impregnation method for use as reference materials.
.
tion, and
a source power of 5 mW was used. The spectral
resolution was 2.0 cmÀ1
.
Zr K-edge X-ray absorption spectroscopy (XAS) was measured
at the 1W1B-XAFS beam line at the Beijing Synchrotron Radiation
Facility (BSRF). The storage ring was operated at 2.5 GeV and
200 mA. The spectra were collected at room temperature in trans-
mission mode. Co K-edge XAS was performed at the BL14W1-XAFS
beam line at the Shanghai Synchrotron Radiation Facility (SSRF).
Here, the storage ring was operated at 3.5 GeV and 200 mA. The
spectra were collected at room temperature in fluorescence mode
with an energy resolution of 0.3 eV. The reduction and analysis of
all spectra were performed using the methods of Athena and
Artemis from the ifeffit1.2.11c software package [48], which was
further fitted in R-space with theoretical models constructed based
on the crystal structure of SZC samples from FEFF8 [49]. The k val-
ues used to fit the Zr K-edge ranged from 2 to 12; the range from 2
to 10.2 was used to fit the Co K-edge.
H2 temperature-programmed reduction (H2-TPR) experiments
were carried out with a H2/Ar (4.79 vol.%) flow of 40 ml minÀ1 from
50 to 700 °C with a ramp of 10 °C minÀ1. Before the TPR experi-
ments, all samples were pretreated in an O2 atmosphere at
500 °C for 1 h. H2 consumption was determined by mass spectrom-
etry (Hiden HPR 20), and the production of SO2, H2S, and H2O was
simultaneously determined.
2.2. Catalytic activity tests
The activity tests were carried out in a fixed bed quartz tube
reactor over a temperature range of 250–500 °C. Approximately
1.2 ml (1.7 g) of the catalyst sample (40–60 mesh) was used, and
the reaction conditions were 1500 ppm CH4, 500 ppm NO, 7.5
vol.% O2, N2 balanced, 200 ml minÀ1 total flow rate, and
GHSV = 10,000 hÀ1 (or 3600 hÀ1, 3.3 ml catalyst). Durability tests
of PdSZ and PdSZC for CH4-SCR were carried out at 460 °C under
the same conditions. The concentrations of NO, NO2, N2O, and
CH4 in the inlet and outlet streams were measured by an online
Nicolet 380-FTIR spectrometer equipped with a gas cell of volume
0.2 dm3. To measure the mass balance of nitrogen during the CH4-
SCR reaction over PdSZC, the reaction gas mixture was balanced
by Ar, and an online gas chromatograph (SHIMDZU GC 2014C)
equipped with a TCD detector was used to monitor the formation
of N2. A molecular sieve column (MS-13X) was used to separate
O2 and N2 at 24 °C. Based on the experiment, we can calculate
the nitrogen balance (inlet ([NO] + [NO2]) = outlet ([NO] + [NO2] +
[N2O] + [N2])). Over PdSZC, the nitrogen balance was higher than
97%. The turnover frequency (TOF) was calculated according to
the molar ratio of N2 produced per second to Pd.
3. Results
3.1. BET measurement and elemental composition analysis
The BET-specific surface area, pore volume, and mean pore
diameter of the catalysts are summarized in Table 1. It is clear that
sulfation increased the surface area of the SZC samples. The BET-
specific surface area of SZC (3 wt.% S) is 121 m2/g, nearly twice that
of SZC without S. The pore volumes of SZC changed slightly with
increasing S loading. However, the mean pore diameter of SZC
decreased gradually with increasing S content. CoSZ, PdCoSZ, and
PdSZC exhibited similar surface areas; the PdSZ catalyst showed
the largest surface area among the samples.
2.3. Catalyst characterization
BET (Brunauer, Emmett, and Teller) surface areas and pore diam-
eter distributions were obtained from N2 adsorption and desorption
at 77 K on Quantasorb-18 automatic equipment (Quanta Chrome
Instrument Co.). Before the measurements were obtained, all
samples were first evacuated at 80 °C for 0.5 h and then raised to
300 °C and kept for 5 h. The elemental compositions of the powder
catalysts were determined by a sequential X-ray fluorescence spec-
trometer (XRF-1800, Shimadzu).
Table 1
Physical properties of different samples.
Sample
SBET
Pore volume Mean pore
Elemental
diameter (nm) composition
(wt.%)a
The diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) spectra of pyridine adsorption on different samples were
recorded on a FTIR spectrometer (Thermo Nicolet Corporation
Nexus 670, OMNIC Quantpad software) equipped with a smart col-
lector and a MCT/A detector cooled by liquid nitrogen. The reaction
temperature was controlled by an Omega programmable tempera-
ture controller. The sample was finely ground and then placed in a
ceramic crucible in an in situ chamber. Prior to pyridine adsorp-
tion, the sample was pretreated at 500 °C in a flow of 20 vol.%
O2 + 80 vol.% N2 at a rate of 100 ml minÀ1 for 30 min and then
cooled to the desired temperatures (300 and 30 °C) to obtain a
reference spectrum. The sample was then exposed to saturated
pyridine vapor at 30 °C for 20 min, after which the DRIFTS spectra
(m2/g) (cc/g)
S
0
Co
Zr
SZC 0 wt.% S
SZC 1 wt.% S
SZC 2 wt.% S
SZC 3 wt.% S 121
SZ
CoSZ
PdSZ
PdCoSZ
PdSZC
64
89
90
0.094
0.088
0.078
0.084
0.092
0.086
0.108
0.077
0.083
5.9
3.9
3.5
3.5
3.5
3.4
3.7
3.1
3.3
4.10 68.0
0.97 4.01 66.7
2.08 3.97 65.9
2.96 3.97 65.4
106
100
117
100
99
1.97
1.97 4.22 65.7
1.88 70.3
0
70.0
0
1.79 3.87 66.4
2.13 4.21 65.7
a
Measured by XRF.

X. Wang et al. / Journal of Catalysis 279 (2011) 301–309
303
The elemental compositions of the SZ, CoSZ, SZC, and Pd-added
catalysts were measured by XRF, with the results shown in Table 1.
Clearly, the S and Co content in all samples was almost identical to
the corresponding nominal values. This proves that the two species
do not dissolve into the aqueous solution during the sequential
immersion process. The Pd species was not detected by XRF, likely
due to its low concentration (0.2 wt.%).
considering that the Brønsted acid sites in CoSZ hardly changed
with Pd introduction. In PdCoSZ, Pd may exist in the form of
PdO. To examine the strength of Brønsted acids, samples were
heated from 30 to 300 °C under N2 flow after pyridine adsorption.
The SZC sample with 2 wt.% S maintained nearly the same
Brønsted acidity as SZ, which was also more abundant than that
of CoSZ (Fig. 2b). Thus, we successfully developed a new method
for the introduction of Co into ZrO2 with little change in the num-
ber of Brønsted acid sites. A peak at 1444 cmÀ1 was observed for all
Co-containing samples in Fig. 1b; this peak was assigned to the Le-
wis acid sites related to Co species that were anchored to the
Brønsted sites formed by sulfation.
3.2. Pyridine adsorption experiment
Pyridine adsorption was used to determine the acid sites and
the acidity on SZC samples with different S loadings (Fig. 1a). The
bands at 1439 and 1541 cmÀ1 were attributed to the characteris-
tics of adsorbed pyridine bound to Lewis and Brønsted acid sites,
respectively [19]. The Brønsted acid sites on the SZC samples in-
creased perceptibly as the S content increased from 0 to 2 wt.%,
but they decreased when the S content was further increased to
3 wt.%. Meanwhile, the Lewis acid sites increased significantly with
S addition. The same pyridine adsorption experiment was also per-
formed on SZ, CoSZ, PdSZC, and PdCoSZ samples (Fig. 1b). The num-
ber of Brønsted acid sites on SZC was larger than that on CoSZ with
the same Co and S content. In comparison with SZC (with 2 wt.% S),
the Brønsted acid sites on the PdSZC catalyst decreased sharply.
This trend shows that the introduction of Pd consumes Brønsted
acid sites. In other words, Pd species with the form Pd2+ are an-
chored to Brønsted acid sites [27,28,33,34]. Pd species in the
PdCoSZ catalyst were difficult to anchor onto Brønsted acid sites,
3.3. XRD characterization
The XRD patterns of SZC with different S content and SZ are
shown in Fig. 3. SZ featured the typical tetragonal phase of zirconia
(ICSD 070014). It is well known that surface sites, which adsorb
oxygen at low temperatures, are responsible for causing the tetrag-
onal phase to undergo monoclinic transformation at low tempera-
tures; in contrast, the incorporation of sulfate covers these sites
and maintains the tetragonal phase [15]. After the introduction
of Co via traditional impregnation, tetragonal ZrO2 species in the
SZ sample were also retained. In this case, the Co species was pres-
ent as Co3O4. In contrast to the SZ sample, shoulder peaks at 34.7°
Fig. 2. DRIFT spectra of (a) SZC with different S loadings; (b) samples containing
2 wt.% S prepared by direct sulfation and traditional impregnation methods after
exposed to saturated pyridine vapor at 30 °C and then purged in N2 for 20 min at
300 °C.
Fig. 1. DRIFT spectra of pyridine adsorption at 30 °C over (a) SZC with different S
loadings; (b) samples containing 2 wt.%
S prepared by direct sulfation and
traditional impregnation methods.

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X. Wang et al. / Journal of Catalysis 279 (2011) 301–309
Fig. 3. (a) XRD patterns of SZ and SZC with different S loadings; (b) XRD patterns of SZ, CoSZ, and SZC with 2 wt.% S.
Table 2
Determination of crystalline phase (cubic or tetragonal) of SZC samples with different
S content and SZ from X-ray diffraction analysis. EXPGUI software of GSAS (General
Structure Analysis System) was used.
(0 0 2) and 59.5° (1 0 3) disappeared obviously in the SZC samples,
strongly suggesting that phase transformation occurred due to the
addition of Co and that a cubic phase may be present in the SZC
samples (ICSD 072955).
To examine the index and identify the phase of the samples,
XRD pattern refinement with GSAS was performed using EXPGUI
software [50], the results of which are shown in Table 2. The cu-
bic phase was present in the SZC samples, with lattice constants
rising slowly from 5.100004 to 5.106705 Å with increasing S
loading.
Sample
Crystalline
phase
Lattice constant
(Å)
Rp
(%)
Rwp
(%)
R (F2)
(%)
SZC 0 wt.% S
SZC 1 wt.% S
SZC 2 wt.% S
SZC 3 wt.% S
SZ 2 wt.% S
Cubica
a = 5.100004
a = 5.104160
a = 5.105236
a = 5.106705
a = 3.599640
b = 5.180907
2.22
2.17
2.11
2.89
2.89
3.23
2.86
5.19
2.01
2.75
3.80
1.65
Cubica
Cubica
Cubica
Tetragonalb
2.78
3.82
2.92
a
Space group f m 3 m.
Space group P 42/n m c.
b
3.4. Raman characterization
The Raman spectra of SZ and SZC are shown in Fig. 4. The
Raman bands of SZ were located at 262, 307, 451, and
636 cmÀ1, all of which are characteristic of the typical tetragonal
phase [24,51]. SZC, however, showed no such bands. The XRD
results confirm that SZ samples exist in the tetragonal phase,
and a phase transformation occurs in SZC samples due to the
addition of Co into zirconia.

X. Wang et al. / Journal of Catalysis 279 (2011) 301–309
3.5. XAS measurements
305
3.5.1. XAS for the Zr K-edge
Fig. 5a shows the Zr K-edge XANES spectra of SZ and SZC with
different S loadings. The XANES spectra of all samples are very sim-
ilar, and their half-height edge positions shift to higher energies
with increasing S loading, as shown in the inset of Fig. 5a.
The Fourier transform (k3, 2.0 < k < 12.0) EXAFS for the Zr
K-edge of SZC and SZ are shown in Fig. 5b. The Fourier transforms
of SZC with different S loadings vary minimally compared to the SZ
sample. To obtain additional information, curve-fitting analysis of
EXAFS data was performed. In this case, the cubic phase was se-
lected as a model phase. The lattice constant used for this phase
is listed in Table 2. For the EXAFS simulations, complex backscat-
tering amplitudes were calculated using FEFF 8; the S0 value of
the Zr K-edge, as an amplitude reduction factor, was fixed at 0.66
according to the tetragonal ZrO2 material model, thereby facilitat-
ing the correct determination of the coordination numbers. The fit-
ting results of the first and second shells were listed in Tables 3 and
4. The results of cubic phase fitting are reasonable, further confirm-
ing that the zirconia in SZC samples mainly exists in cubic phase.
Fig. 4. Raman shift of SZ and SZC with different S loadings.
3.5.2. XAS for the Co K-edge
No information about Co species was obtained from the XRD re-
sults of SZC. There are two possible reasons for this: (1) Co could
have formed an interstitial or substitutional solid solution with zir-
conia; or (2) Co could have been isolated and highly dispersed in
the samples. To determine the state of Co species in the SZC sam-
ples, Co K-edge XAS was performed at room temperature. As
shown in Fig. 6a, the pre-edge peak at ca. 7710 eV was ascribed
to the 1s 3d-e.g. transition of Co K in the SZC samples [52]. The
half-height edge positions of SZC were close to that of Co(NO3)2,
but there were 4 eV lower than that of Co3O4, indicating that Co
species in the SZC samples are mainly present as Co2+. It can be
seen from Fig. 6a that the S content (0–2 wt.%) had little effect
on the Co state. When the S content was 3 wt.%, the Co K-edge
absorption energy at half-height shifted to a higher value, with a
shoulder peak at 7731 eV, indicating the formation of Co3O4
(Fig. 6a).
The Fourier transform of k3-weighted EXAFS oscillations for the
Co K-edge of SZC and Co3O4 are shown in Fig. 6b. The SZC samples
with S content from 0 to 2 wt.% showed the same Co–O bond
length; however, the SZC sample with 3 wt.% S loading exhibited
a Co–O bond length similar to that of the Co3O4 species. These re-
sults further confirm the formation of Co3O4 species in SZC when
the S content is 3 wt.%.
In Fig. 6b, a second coordination shell was observed for the SZC
0 wt.% S sample; this requires further verification. Therefore, the EX-
AFS spectrum of this sample was fitted with the assumption that Co
is present in one of the following phases: cubic CoO (a = 4.2495 Å),
substitutional solid solution in cubic zirconia, or interstitial solid
solution in cubic zirconia. For the Co K-edge EXAFS simulations,
complex backscattering amplitudes were also calculated using FEFF
8; the S0 value of the Co K-edge was fixed at 1, according to the model
material of Co3O4, which facilitated the correct determination of the
Fig. 5. (a) Zr K-edge XANES of SZ and SZC with different S loadings; (b) Fourier
transforms of k3-weighted EXAFS oscillation of Zr K-edge of SZ and SZC with
different S loadings (sin window function covering the k range of 2–12 ÅÀ1 was
used).
Table 3
The EXAFS fitting results of different S loading samples at the Zr k edge for the first shell. Sin window function covering the k range 2.0–12.0 ÅÀ1 was used.
Sample
Shell
CNa
Rb (Å)
r2c (10À3 Å2)
D
E (eV)
D
R (Å)
R factor (%)
SZC 0 wt.% S
SZC 1 wt.% S
SZC 2 wt.% S
SZC 3 wt.% S
Zr–O
Zr–O
Zr–O
Zr–O
6.8 1.8
7.2 0.9
6.8 2.2
7.7 1.2
2.11 0.02
2.15 0.01
2.10 0.02
2.11 0.01
7.0 2.0
8.2 1.3
7.8 3.2
8.0 1.5
À12.9 3.6
À9.4 1.7
À13.4 4.6
À12.9 2.2
1.0–2.1
1.0–2.1
1.0–2.1
1.0–2.1
8.5
2.1
10.8
1.94
a
b
c
CN: coordination number.
R: bond distance.
r
2: Debye–Waller factor.

306
X. Wang et al. / Journal of Catalysis 279 (2011) 301–309
Table 4
The EXAFS fitting results of different S loading samples at the Zr k edge for the second shell. Sin window function covering the k range 2.0–12.0 ÅÀ1 was used.
Sample
Shell
CNa
Rb (Å)
r2c (10À3 Å2)
D
E (eV)
D
R (Å)
R factor (%)
SZC 0 wt.% S
SZC 1 wt.% S
SZC 2 wt.% S
SZC 3 wt.% S
Zr–Zr
Zr–Zr
Zr–Zr
Zr–Zr
6.5 5.5
3.58 0.04
3.60 0.02
3.63 0.04
3.61 0.02
1.8 0.7
2.3 0.4
1.3 0.6
1.5 0.2
À9.0 4.8
À8.0 2.1
À3.6 5.6
À5.3 2.0
1.0–3.7
1.0–3.7
1.0–3.7
1.0–3.7
8.5
2.1
10.8
1.9
10.5 6.0
3.9 3.1
6.1 1.9
a
b
c
CN: coordination number.
R: bond distance.
r
2: Debye–Waller factor.
Table 5
The EXAFS fitting results of SZC with 0 wt.% S sample at the Co K edge for the first and
second shells. Sin window function covering the k range 2.0–10.2 ÅÀ1 was used.
Shell
Atom
CNa
7.0 1.2
12.7 5.6
R (Å)b
R0 (Å)c
R factor (%)
1
2
O
Zr
2.18
3.59
2.21
3.61
2.1
2.8
a
b
c
CN: coordination number.
R: bond distance fitted.
R0: bond distance calculated.
N2, while further increases in S content to 3 wt.% decreased
such activity. The SZC with 2 wt.% S was used for the introduction
of a second active species of Pd. The following activity tests were
preformed at a high space velocity of 10,000 hÀ1. As shown in
Fig. 8b, CoSZ prepared by the traditional impregnation method
showed marginal activity for NOx reduction within the temperature
region 250–500 °C, while SZC with 2 wt.% S provided ca. 20%
NOx conversion to N2 at above 400 °C. After the second active
component of Pd was introduced into SZC (PdSZC), NOx conversion
to N2 increased to 82%, three times higher than that of SZC. For
PdCoSZ, however, the conversion of NOx was below 26% in the tem-
perature range 250–500 °C. The TOFs of the PdSZC and PdCoSZ
catalysts for NOx reduction were calculated based on the data at
450 °C. At this temperature, the former exhibits a TOF value of
1.4 Â 10À3 sÀ1, 2.2 times higher than that of the latter catalyst
(4.4 Â 10À4 sÀ1).
N2O as a main byproduct was commonly observed during the
SCR of NOx [53]; this byproduct was also detected during NOx
reduction by CH4 [45,54]. During NOx reduction by methane over
PdSZ, large amounts of N2O were formed above 400 °C. Interest-
ingly, PdSZC significantly lowered the formation of N2O compared
to PdSZ (Fig. 8c), indicating that the introduction of Co enhanced
selectivity to N2 during CH4-SCR.
The PdSZC catalyst exhibited much greater durability than
PdCoSZ, maintaining ca. 84% NOx conversion to N2 at 460 °C until
35 h; in contrast, the latter material showed a gradual decrease
in NOx conversion under the same reaction conditions (Fig. 8d).
The durability of the PdSZC catalyst for NOx reduction was also
evaluated in the presence of 10% water vapor; the results are
shown in Fig. 8d. At 500 °C, NOx conversion to N2 decreased from
80% to 60% after 6.4 h, indicating that the water tolerance of this
catalyst requires improvement in future research.
Fig. 6. (a) Co K-edge XANES of Co3O4, Co(NO3)2Á6H2O and SZC with different S
loadings; (b) Fourier transforms of k3-weighted EXAFS oscillation of Co K-edge of
Co3O4 and SZC with different S loadings (sin window function covering the k range
of 2–10.2 ÅÀ1 was used).
coordination number. Fitting results demonstrate that isomorphous
substitution of Co in cubic zirconia is reasonable (Tables 5 and 6,
Fig. 7). To the best of our knowledge, this is the first time that evi-
dence of Co entering the lattice of cubic zirconia to form a substitu-
tion solid solution was provided. The second coordination shell of
SZC with 3 wt.% S was similar to that of the referential Co3O4 mate-
rial, also confirming the formation of Co3O4.
3.7. TPR
3.6. Activity test
Resasco and co-workers [25] reported that the state of Pd plays
a crucial role in the activity of SZ-supported catalysts. To further
confirm this, H2-TPR experiments were performed, and the results
of which are shown in Fig. 9. H2 consumption was observed be-
tween 100 and 200 °C for PdCoSZ, which can be attributed to
PdO species [55]. The PdSZC sample showed no H2 consumption
over the same temperature range, indicating that Pd2+ was tightly
The catalysts were tested for CH4-SCR, and the results of which
are shown in Fig. 8. Fig. 8a depicts the activity of SZC samples at a
low space velocity of 3600 hÀ1. Obviously, the S content has a great
influence on the activity of SZC during NOx reduction. Increasing the
S loading from 0 to 2 wt.% significantly increased NOx conversion to

X. Wang et al. / Journal of Catalysis 279 (2011) 301–309
307
Table 6
The EXAFS fitting results of SZC with S loading from 0 to 3 wt.% at the Co K edge for the first shell. Sin window function covering the k range 2.0–10.2 ÅÀ1 was used.
Sample
Shell
CNa
Rb (Å)
r2c (10À3 Å2)
D
E (eV)
D
R (Å)
R factor (%)
SZC 0 wt.% S
SZC 1 wt.% S
SZC 2 wt.% S
SZC 3 wt.% S
Co–O
Co–O
Co–O
Co–O
7.0 1.2
7.9 1.2
7.7 1.6
5.7 0.7
2.18 0.01
2.19 0.01
2.20 0.02
1.95 0.04
12.0 2.0
9.0 1.7
9.8 2.3
20.5 2.8
À21.2 2.1
À20.2 1.9
À19.0 2.6
À8.5 5.9
0.6–2.3
0.6–2.3
0.6–2.3
0.6–2.3
1.6
1.3
2.5
8.3
a
b
c
CN: coordination number.
R: bond distance.
r
2: Debye–Waller factor.
As suggested by Xia and co-workers [56], the (1 1 1) surface of
cubic zirconia is energetically favorable, and its stability is closely
equivalent to that of the (1 0 1) surface of tetragonal zirconia [57].
On the latter surface, Haase and Sauer [16] proved that the most
stable configuration of sulfur species adsorbed onto Zr sites is
the tridentate sulfate anion. When the S content was increased
from 0 to 2 wt.%, Zr K-edge XANES shifted to higher energy
(Fig. 5a), while the energy of Co K-edge XANES (Fig. 6b) hardly
changed with increasing S content. This suggests the direct coordi-
nation of Zr atoms to strongly electron-withdrawing SOx centers
[58]. Keeping this in mind, tridentate sulfate anions adsorbed onto
Zr sites were employed in our calculations. Katada et al. adopted an
ingenious way to estimate the surface area occupied by one sulfate
anion [59]. Inspired by this, we calculated S concentrations on the
(1 1 1) surface of cubic ZrO2 on the assumption that one S group
can bond to three Zr atoms.
An area of 1.0819 Â 1.0819 nm on the (1 1 1) surface of pure cu-
bic ZrO2 (a = 5.10 Å), which contains 16 Zr atoms, was adopted to
calculate the surface concentration of S. The one-monolayer S con-
centration over the (1 1 1) surface of pure cubic ZrO2 was esti-
mated to be 5.0 nmÀ2, which was in good agreement with the
results of Li et al. [60]. Similar results were also reported by
Kantcheva [46]. Taking into account the surface area data de-
scribed in Table 1, the S concentrations are estimated to be
2.1 nmÀ2 for the SZC sample with 1 wt.% S and 4.3 nmÀ2 for
the 2 wt.% S sample. For SZC samples, Co species entered the lattice
of cubic zirconia, and a Zr/Co mole ratio of 9 was employed. In this
case, one-ninth of the Zr sites are replaced by Co at the surface and
in the bulk. Hence, the real single monolayer concentration of S in
the Zr–Co cubic phase should be 4.5 nmÀ2. This result suggests that
the surface of SZC samples with 2 wt.% S would be closely covered
by one-monolayer of S.
When the S content was increased to 3 wt.%, the calculated S
concentration was 5.3 nmÀ2. As a result, not enough Zr sites were
available for the adsorption of S species on the SZC samples, and
more than one S monolayer would occupy Co sites. The broadening
of XRD peaks (Fig. 3a) and the formation of Co3O4 in the 3 wt.% S
SZC (Fig. 6, Table 6) clearly demonstrated a reconstruction. In other
words, Co species in the SZC samples with 3 wt.% S were elimi-
nated from the Zr–Co solution and occupied Brønsted acid sites.
Such a transformation would decrease the number of Brønsted acid
sites of SZC with 3 wt.% S compared to the SZC with 2 wt.% S (Figs.
1a and 2a).
Fig. 7. Fourier transforms of Co K-edge k3-weighted EXAFS spectra of SZC with 0
wt.% S (Full line, experimental; dashed line, fitting). (a) R-space; (b) real part in
R-space.
bonded to and highly dispersed onto SZC without the formation of
PdO species [55].
4. Discussion
4.2. Role of acid sites
4.1. Structure analysis of sulfated Zr–Co materials
As described in Table 1, PdSZC and PdCoSZ possessed almost
the same BET surface area, which does not account for their signif-
icant difference in NOx reduction activity. The state of Pd species
had a significant influence on the activity of low-loading Pd cata-
lysts supported on acidic materials (such as sulfated zirconia),
which strongly depended on the metal loading and the acidity of
the supporting material [27]. In our case, the same amount of Pd
was loaded onto SZC and CoSZ, which also does not result in any
The XRD, Raman, and Zr and Co K-edge XAS data confirm that
the Zr–Co solid solution was present in the cubic phase, and this
structure did not change as long as the S loading remained below
3 wt.%. Further increasing the S content to 3 wt.% resulted in the
formation of the Co3O4 phase (Fig. 6 and Table 6). To verify this
observation, the concentration of S species on the surface of ZSC
was calculated.

308
X. Wang et al. / Journal of Catalysis 279 (2011) 301–309
Fig. 8. (a) NOx conversion to N2 over SZC samples with different S content; (b) NOx conversion to N2 over CoSZ, SZC, PdSZ, PdCoSZ, and PdSZC (all sample with 2 wt.% S
loading); (c) NOx conversion to N2O over PdSZ and PdSZC; (d) durability of PdSZ and PdSZC for CH4-SCR without water at 460 °C and with water at 500 °C. Reaction conditions:
1500 ppm CH4, 500 ppm NO, 7.5 vol.% O2, balanced by N2, 200 ml minÀ1 total flow rate, and GHSV = 3600 hÀ1 in (a) and 10,000 hÀ1 in (b–d).
difference in NOx reduction. Consequently, the acid sites on these
materials play a crucial role during CH4-SCR.
materials via the new and conventional method is proposed in
Scheme 1.
The pyridine adsorption results indicated that Lewis and
Brønsted acid sites coexisted on the surface of PdSZC and PdCoSZ
(Fig. 1). The appearance of a peak at 1541 cmÀ1 was attributed to
the presence of Brønsted acid sites, while the peaks at 1444 and
1439 cmÀ1 were attributed to the Lewis sites. These two peaks
exhibited almost the same intensities on the surfaces of the CoSZ,
SZC, PdSZC, and PdCoSZ samples. The four samples possessed dif-
ferent activities for NOx reduction, demonstrating the minor role
of Lewis acid sites in CH4-SCR.
As shown in Fig. 1b, the number of Brønsted acid sites on SZC
decreased noticeably after Pd was loaded, while little change was
observed for Pd-loaded CoSZ. These results strongly suggest that
Brønsted acid sites play a key role in anchoring Pd to form active
species. TPR results (Fig. 9) further confirm that Pd species are
mainly present as highly dispersed Pd2+ on SZC, whereas PdO spe-
cies were predominant on CoSZ. Considering that the PdO species
are active during the combustion of methane, the dispersed Pd2+
was attributed to the selective reduction of NOx to N2 [27,41]. It
is thus reasonable to conclude that PdSZC possesses much higher
activity for CH4-SCR than PdCoSZ.
In the direct sulfation method, a solid solution of Zr–Co oxides is
formed by the co-precipitation of Zr and Co precursors, during
which Co enters the lattice of cubic zirconia by isomorphous sub-
stitution of Zr. When the S content is below 3 wt.%, the sulfation
does not induce phase transformation of cubic zirconia to a tetrag-
onal structure, indicating that the Zr–Co solid solution is not de-
stroyed in this case. As a result, a large number of Brønsted acid
sites remain on the surfaces of the SZC samples, providing suffi-
cient sites for further introduction of Pd2+ and subsequent catalytic
reaction, and relating closely to the high performance of PdSZC for
CH4-SCR. In the traditional impregnation method, Co is preferen-
tially adsorbed onto the Brønsted acid sites of SZ, leading to obvi-
ous decreases in the availability of Brønsted acid sites for Pd
anchoring in the form of Pd2+. The activity of PdCoSZ for NOx reduc-
tion is thus reduced.
5. Conclusions
This study successfully introduced the transition metal Co2+
into a solid acid by the direct sulfation of Zr–Co hydroxides, with
little change in the availability of Brønsted acid sites. XRD, Raman,
and XAS analyses demonstrated that zirconia in SZC was present in
the cubic phase. Furthermore, Co K-edge XAS provided direct evi-
dence of Co entrance into the lattice of cubic zirconia to form a
substitutional solid solution. As a result, sufficient Brønsted acid
sites remained available for the introduction of Pd2+ and catalytic
reaction, which accounts for the high activity of PdSZC during
4.3. Possible mechanism for the introduction of Co into zirconia by
different methods
Based on the above results, we can conclude that the transition
metal Co2+ was successfully introduced into zirconia with little
change in Brønsted acid sites. The preparation process of solid acid

X. Wang et al. / Journal of Catalysis 279 (2011) 301–309
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This work was financially supported by the National Natural
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