Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide

Article abstract of DOI:10.1021/jacs.5b06336

Co₃O₄ with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co³⁺ is considered to be the active site, while tetrahedrally coordinated Co²⁺ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H₂ reduction of nanocast Co₃O₄ at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co²⁺ octahedrally coordinated by lattice oxygen in Fm3ˉm symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co²⁺ to Co³⁺ causes the high activity. Other mesoporous spinels (CuCo₂O₄, CoCr₂O₄, and CoFe₂O₄) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co³⁺ highly active but also the octahedrally coordinated Co²⁺ species in CoFe₂O₄ with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co²⁺ species is easily oxidized and shows high catalytic activity for CO oxidation.

Full text of DOI:10.1021/jacs.5b06336

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Article
Highly Ordered Mesoporous Cobalt Containing Oxides: Structure,
Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide
Dong Gu, Chun-Jiang Jia, Claudia Weidenthaler, Hans-Josef
Bongard, Bernd Spliethoff, Wolfgang Schmidt, and Ferdi Schueth
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b06336 • Publication Date (Web): 24 Aug 2015
Downloaded from http://pubs.acs.org on August 25, 2015
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Highly Ordered Mesoporous Cobalt Containing Oxides: Structure,
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Catalytic Properties, and Active Sites in Oxidation of Carbon Monox-
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†
,†, ‡
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†
Dong Gu, Chun-Jiang Jia, *
Claudia Weidenthaler, Hans-Josef Bongard, Bernd Spliethoff,
†
, †
Wolfgang Schmidt and Ferdi Schüth *
†
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.
‡
Key Lab for Colloid and Interface Chemistry, School of Chemistry and Chemical Engineering, Shandong Universi-
ty, Jinan 250100, PR China
Keywords: Mesoporous materials; Nanocasting; Spinel; Cobalt oxide; CO oxidation
ABSTRACT: Co O with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahe-
3
4
3
+
2+
drally coordinated Co is considered to be the active site, while tetrahedrally coordinated Co is assumed to be basically
inactive. In this study, a highly ordered mesoporous CoO has been prepared by H reduction of nanocast Co O at low
2
3
4
2+
temperature (250 °C). The as-prepared CoO material, which has a rocksalt structure with a single Co octahedrally coor-
ꢀ
dinated by lattice oxygen in Fm3m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation
2+
3+
of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co to Co caus-
es the high activity. Other mesoporous spinels (CuCo O , CoCr O , and CoFe O ) with different Co species substituted
2
4
2
4
2
4
with low/non-active metal ions were also synthesized to investigate the catalytically active site of cobalt based catalysts.
3
+
The results show that not only the octahedrally coordinated Co is highly active, but also the octahedrally coordinated
2+
Co species in CoFe O with an inverse spinel structure show some activity. These results suggest that the octahedrally
2
4
2+
coordinated Co species is easily oxidized and shows high catalytic activity for CO oxidation.
22
1
. Introduction
face selective properties of Co O catalysts. However,
3 4
similarly high activity has also been reported for other
Co O systems, where the predominance of specific faces
Ordered mesoporous materials possess many attractive
properties, such as high surface area, uniform and tunable
pore sizes and shapes, various structures and composi-
tions, which make them suitable for applications in catal-
3
4
23
is less obvious, indicating that the complexity in surface
structure of the catalyst makes it very difficult to precisely
identify the active sites for CO oxidation. On the other
hand, it is also questionable to what extent a single crystal
surface reflects the surface structure of the real catalysts
1
-12
ysis, adsorption, battery technology, sensing, and so on.
Moreover, the highly uniform nanopore systems makes
mesoporous materials ideal systems for designing struc-
turally uniform materials as model catalysts for under-
standing the structural characteristics affecting the cata-
24
under reaction conditions.
3
+
Co O has a spinel structure, containing Co on octahe-
3
4
1
3,14
2+
lytic performance.
dral coordination sites and Co on tetrahedral coordina-
2+
tion sites. CoO, contains exclusively Co species in octa-
hedral coordination, and could thus be an ideal system
for the identification of active sites for CO oxidation over
Co-based catalysts. Both in the spinel and the rocksalt
structure, the oxygen sublattice forms a face centered
Oxidation of carbon monoxide (CO) over solid catalysts
is of great interest, not only due to its relevance in practi-
cal applications, such as in CO removal from the hydro-
gen feed of polymer-electrolyte-membrane fuel cells
1
5,16
17,18
(PEMFC),
automotive exhaust gas treatment,
air
2-
2-
1
9
20
cubic (fcc) structure, with ~ 5 % higher O -O packing
cleaning, and sensing, but also due to its importance as
model reaction for the understanding of catalytic mecha-
nisms. Co O is one of the most active non-precious metal
distance in the rocksalt structure than in the spinel struc-
24-26
ture.
All octahedral sites are filled in the rocksalt
3
4
21
structure, but only half are filled in the spinel structure.
The tetrahedral sites, which are completely unoccupied in
the rocksalt structure, are partially filled to one-eighth
occupancy in the spinel structure. In addition to CoO,
catalysts for low temperature CO oxidation. Xie et al.
synthesized Co O nanorods with predominantly exposed
3
4
{
110} faces, which shows very high activity for CO oxida-
tion at – 77 °C. This study revealed the importance of the
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different spinels with specific cobalt occupancies, such as
CuCo O , CoCr O and CoFe O (inverse spinel structure
with Co octahedrally coordinated), are also very inter-
esting systems, which allow studies on the influence of
charge and coordination state of cobalt on catalytic per-
formance.
NaOH solution (2 M, 100 mL, 70 °C). Mesoporous CoO
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was prepared by reducing the nanocast mesoporous
Co O in 4.5 vol.% H at 250 °C for 1.5 h (250 mg Co O , 30
2
4
2
2
4
2
4
+
3
4
2
3
4
-
1
mL·min ). Other spinel oxides (CuCo O , CoCr O and
2
4
2
4
CoFe O ) were prepared accordingly with appropriate
2
4
amounts of the respective metal nitrates as precursors.
The calcination temperatures for CuCo O , CoCr O and
However, in order to be comparable, also the synthetic
pathways have to be comparable, since catalyst prepara-
2
4
2
4
CoFe O were 350, 600, and 600 °C, respectively.
2
4
tion methods have a strong influence on the catalytic ac-
21,23
tivities.
Nanocasting is a powerful method for prepar-
Table 1. Mesoporous metal oxides with different cobalt
species.
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ing high surface area, nanosized, and crystalline cata-
9,27,28
lysts.
In addition, mesoporous materials synthesized
by the nanocasting method can be controlled to have sim-
ilar structural and textural parameters, which makes them
comparable with respect to their catalytic activities. Na-
nosized CoO particles hitherto could only be prepared by
solvothermal methods at critical conditions, and their
tetrahedral
coordination
octahedral
coordination
Sample
Co
2+
3+
O
Co
Co
3
4
2+
CoO
-
Co
surface protection ligands always lead to contaminated
29,30
surfaces.
Our group has successfully synthesized high-
2+
3+
CuCo O
Cu
Co
2
4
ly ordered mesoporous Co O with controllable architec-
3
4
31
2+
3+
Cr
tures, and proven that these materials are active for CO
oxidation at low temperatures. Such Co O4 materials
CoCr O
Co
2
4
32
3
3+
2+
3+
were also successfully transformed into CoO in a pseu-
CoFe O
Fe
Co , Fe
2
4
33
domorphic manner by glycerol reduction. However,
In all the materials, the oxygen sublattice is fcc. The cati-
ons are located in the voids of the oxygen sublattices.
formation of carbonaceous residues, which would affect
catalytic performance, on the surface in this process can-
not be excluded, and the coke is very difficult to remove
due to possible reoxidation of CoO. Thus, in order to ob-
tain directly comparable Co O and CoO, alternative syn-
2.2 Characterization and Measurements.
Powder X-ray diffraction (XRD) patterns were recorded
on a Stoe STADI P θ-θ diffractometer operating in reflec-
tion mode with Cu Kα1,2 radiation and a secondary graph-
ite monochromator. Nitrogen adsorption-desorption
measurements were performed on a NOVA 3200e instru-
ment at 77 K. Prior to the measurements, all samples were
degassed under vacuum for at least 6 h at 150 °C. BET sur-
face areas were calculated from the data in a relative pres-
sure range from 0.05 – 0.20. By using the Barrett-Joyner-
Halenda (BJH) algorithm, the pore volumes and pore size
distributions were derived from the adsorption branches
of the isotherms (normally desorption is recommended,
but the desorption branches could be influenced by the
tensile strength effect). High-resolution scanning electron
microscope (HR-SEM) images and scanning transmission
electron microscope (STEM) images were taken on a Hi-
tachi S-5500 ultrahigh-resolution cold field emission
scanning microscope at an acceleration voltage of 30 kV.
Thermo Scientific NORAN System 7 X-ray Microanalysis
unit was attached to the instrument. Transmission elec-
tron microscopy (TEM) images of the catalysts were rec-
orded with a Hitachi H-7100 microscope at an accelera-
tion voltage of 100 kV, high-resolution transmission im-
ages (HR-TEM) with a Hitachi HF2000 microscope at an
acceleration voltage of 200 kV. The HF2000 microscope
was equipped with a cold field emission gun and a Noran
energy dispersive X-ray (EDX) unit. X-ray photoelectron
spectroscopy (XPS) measurements were performed with a
Kratos HSi spectrometer with a hemispherical analyzer.
3
4
thetic pathways are required.
In the following, we report the successfully preparation
of a series of cobalt-based catalysts (Co O , CoO,
3
4
CuCo O , CoCr O and CoFe O , Table 1) with similar
2
4
2
4
2
4
structures and crystallite sizes by using the same mesopo-
rous silica as hard template. CoO is obtained via pseudo-
morphic transformation from Co O by a low temperature
3
4
reduction process with hydrogen, which avoids the prob-
lem of carbon deposition on the CoO surface. Studies of
the catalytic performance in dependence of the charge
and coordination of the cobalt species then allow to con-
clude on the active species for CO oxidation in such cata-
lysts.
2. Experimental Section
2.1 Catalyst synthesis.
Mesoporous silica SBA-15 was prepared under hydro-
thermal conditions at 110 °C for 24 h, according to estab-
2,34
lished procedures.
Highly ordered mesoporous Co O
3 4
was prepared by the nanocasting method with SBA-15 as a
hard template. The Co(NO ) ·6H O precursor was incor-
3
2
2
porated into the channels of mesoporous silica via an
27,35
evaporation assisted impregnation process.
The ratio
of precursor to template was fixed to 2.0 g of nitrate per
mL of silica pore volume. The subsequent decomposition
of the precursor was carried out in a muffle oven at 600
−
1
°
C for 5 h with a ramp rate of 1 °C·min . Finally, the mes-
oporous silica hard template was leached out with hot
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to −78 °C (2-propanol/dry ice bath) before going into the
The monochromatized Al X-ray source (E = 1486.6 eV)
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was operated at 15 kV and 15 mA. For narrow scans, an
analyzer pass energy of 40 eV was applied. Hybrid mode
was used as lens mode. The base pressure in the analysis
chamber was 4 × 10 Torr. The binding energy scale was
corrected for surface charging by use of the C 1s peak of
contaminant carbon as reference at 284.5 eV. Tempera-
ture-programmed reduction (TPR) experiments were per-
formed with an in-house constructed system, equipped
with a thermal conductivity detector (TCD) to measure
reactor.
For a typical light-off run, in which the temperature was
ramped, the reactor was cooled to −50 °C prior to each
experiment under a flow of He gas, which was then re-
placed by the reaction gas, after the base temperature had
been reached. Then, the temperature was ramped with a
rate of 2 °C·min to the final temperature. The concentra-
tions of CO and CO were analyzed at the outlet of the
reactor with non-dispersive IR spectroscopy on two URAS
3E analyzers (Hartmann and Braun).
−
9
−1
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H consumption. 30 mg sample was reduced with a 4.5
2
−
1
vol.% H /N mixture (30 mL·min ) by heating up to 800
C at a rate of 10 °C·min . CO-TPR experiment was per-
2
2
−
1
The titration experiments were carried out in the same
reactor in which the CO oxidation activities were tested,
with 50 mg of catalyst. The catalysts were pretreated in
°
formed in the same reactor in which the CO oxidation
activities were tested. 50 mg sample was reduced with a 1
−
1
the reaction gas (1 vol.% CO, 20 vol.% O , and 79 vol.%
2
vol.% CO/N gas mixture (50 mL·min ) by heating up to
2
−
1
He) at 300 °C (150 or 200 °C in He for CoO to prevent the
oxidation of CoO to Co O ) for 10 min. Then the gas was
switched to He to remove the O from the reactor. After
the reactor was cooled down to the desired temperature
and kept for 10 min, the catalysts were exposed to a
stream of 1 % CO in N (from Air Liquid, 99.997 % purity,
550 °C at a rate of 5 °C·min .
3
4
Fourier transform infrared (FTIR) spectra of CO adsorp-
tion over CoO and Co O were measured in a diffuse re-
2
3
4
flectance cell (Harrick system) equipped with CaF win-
2
dows on a Bruker Vertex 70 spectrometer using a mercu-
2
−
1
ry-cadmium-telluride (MCT) detector. After pretreatment
H O content ~3 ppm) at a flow rate of 67 mL·min .
2
−1
of the catalysts (30 mg) in a gas flow of N (30 mL·min )
2
Online analysis was performed with non-dispersive IR
analyzers (URSA 3E), that allowed detection of CO and
at 300 °C, the reaction cell was cooled down to 20 °C in
the N stream. Then a background spectrum was collected
2
CO in a nitrogen matrix without interference problems.
2
-1
via 32 scans at 4 cm resolution. The catalyst was exposed
−1
3. Results and Discussion
to 2 vol.% CO in N (30 mL·min ), and diffuse reflectance
2
3.1 Mesoporous CoO catalyst
Fourier transform (DRIFT) spectra were obtained by sub-
tracting the background spectrum from subsequent spec-
tra. The IR spectra for CO adsorption were recorded con-
Mesoporous CoO was synthesized by reducing nanocast
Co O in 4.5 vol.% H at 250 °C. One should note that
3
4
2
tinuously for 30 min. Once switch CO gas to N steam,
mesoporous Co O was not only used as a precursor for
3 4
2
also the corresponding IR spectra were recorded for 30
min, in order to follow the surface changes during the CO
removal process.
the synthesis of CoO with the same architecture, but also
as reference catalyst. Mesoporous silica SBA-15 that was
obtained with an aging temperature of 110 °C was selected
as a hard template, since the pore volume and specific
surface area of mesoporous silica templates change with
aging temperature, and aging at 110 °C was found to be
advantageous for the nanocasting process.
tained SBA-15 silica template with highly ordered
mesostructure (Figure S1, supporting information) has a
large pore volume (1.12 cm ·g ) and high specific surface
2.3 Catalytic Testing.
The catalytic activity of catalysts for CO oxidation was
measured in a plug flow reactor with an inner diameter of
34-37
The ob-
6
mm by use of defined amounts of catalyst (200 mg of
−
1
−1
catalyst, space velocity (SV) = 15,000 mL·h ·g , or 50 mg
of catalyst, space velocity (SV) = 80,000 mL·h ·g
mg of catalyst, (SV) = 600,000 mL·h ·g ) in a gas mix-
cat
−
1
−1
or 10
3
−1
cat
−
1
−1
cat
2
−1
area (728 m ·g ) (Table 1). The low-angle XRD pattern
Figure 1a) of the replica Co O shows five well resolved
ture of 1 vol.% CO, 20 vol.% O , and 79 vol.% He (from Air
2
(
3
4
Liquid, 99.997 % purity, H O content ~3 ppm) or in a gas
2
reflections, which can be indexed as (100), (110), (200),
(210), and (300) reflections of the p6mm symmetry, char-
acteristic for the two-dimensional (2-D) hexagonal struc-
ture, indicating good replication of the silica template.
The mesostructure is retained very well after template
removal, although the template was only impregnated
once with the precursor solution. Wide-angle XRD pat-
tern (Figure 1b) of Co O material shows well resolved
mixture of 2.140 vol.% CO, 0.876 vol.% O and He balance
2
−
1
at a flow rate of 35, 50, 67 or 100 mL·min , respectively.
Catalysts were pressed, crushed, and sieved to the 250 −
5
00 µm size range and diluted with 200 mg of quartz
sand; in the reactor the mixture was supported on a
quartz wool plug. The operation temperature was con-
trolled with a thermocouple and could be adjusted in the
range of −100 to 400 °C. Temperatures measured during
the catalytic tests are always referred to the value meas-
ured with a second thermocouple placed in the catalyst
bed. Before measurement, the catalysts were first activat-
3
4
reflections that can be assigned to the Co O phase with
spinel-type structure (PDF2 entry 42-1467), indicating
well-crystallized nanomaterial.
3
4
H -TPR results (Figure S2) of mesoporous Co O show
ed in situ in the reaction gas or He or H at 200 or 300 °C
2
3
4
2
three typical reduction signals. A small peak is observed
around 200 °C, which can be attributed to the reduction
for 30 min. For the experiments under dry conditions, the
entire feed passed through a molecular sieve trap cooled
3
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nanorods during the structural transformation from
Co O to CoO.
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Figure 1. Low-angle (a) and wide-angle (b) XRD patterns of
ordered mesoporous Co O nanocast from SBA-15 and CoO
3
4
reduced from Co O .
3
4
3
+
of surface Co species. The second one at approximately
3
+
2+
3
30 °C is associated with the reduction of Co ions to Co
(
Eq. (1)) with the corresponding structural change to
3
8-40
CoO.
The third broad peak between 370 − 570 °C
shows two features which are due to the reduction of CoO
to metallic cobalt (Eq. (2)). Based on these results, the
reduction of mesoporous Co O to CoO was performed at
3
4
2
50 °C under a 4.5 vol.% H atmosphere. The low-angle
2
XRD pattern (Figure 1a) of the obtained material shows
similar reflections as observed for the original Co O ma-
3
4
terial, suggesting the well retained mesostructure after
the H reduction process. The wide-angle XRD pattern
2
(
Figure 1b) of the H -treated material shows well resolved
2
reflections that can be assigned to the CoO phase with
rocksalt structure (PDF2 entry 43-1004), indicating com-
plete phase transformation from Co O to CoO.
3
4
Figure 2. HR-SEM (a, b, e, and f) and TEM (c, d, g, and h)
images of ordered mesoporous Co (a-d) and CoO (e-h)
Co O + H → 3CoO + H O
(1)
O
3 4
3
4
2
2
nanocast from SBA-15. CoO is reduced from mesoporous
Co O with 4.5 % H at 250 °C for 1.5 h.
CoO + H → Co + H O
(2)
2
2
3
4
2
HR-SEM and TEM images of the replica Co O are
3
4
shown in Figure 2(a – d) and Figure S3(a, b). Almost all
the obtained Co O particles have rod-like morphology
3
4
3
4
which is replicated from the SBA-15 template, indicating
good replication of the silica template. Each single parti-
cle is composed of ordered arrays of small nano-
rods/nanowires. The HR-SEM (Figure 2b) and TEM (Fig-
ure 2c) images of Co O give a clear indication that the
3
4
rod diameter is in agreement with the pore size of the
silica template, further confirming that the Co O phase
3
4
was formed predominantly inside the porous silica phase.
The HR-TEM (Figure 2d) image clearly reveals the crystal-
line nature of the mesoporous Co O . The HR-SEM (Fig-
3
4
ure 2f) and TEM (Figure 2g and Figure S3d) images of
mesoporous CoO show similar particle morphology and
single nanorod/nanowire width, suggesting that the phase
transformation occurs in a pseudomorphic manner, with-
out reconstruction of the whole framework. The HR-TEM
(Figure 2h) image shows that the structure lattice stretch-
es over several neighboring nanorods/nanowires, suggest-
ing that single crystalline domains are bigger than the rod
diameter. This may due to the interaction between the
Figure 3. Nitrogen adsorption-desorption isotherms and
pore size distribution curves (insert) of mesoporous Co O
3
4
and CoO nanocast from SBA-15.
The nitrogen adsorption-desorption isotherms and pore
size distributions (Figure 3) of the replica Co O and CoO
3
4
materials are rather similar. They are characterized by a
4
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Journal of the American Chemical Society
oxides and the low level of microporosity.
small capillary condensation step at relative pressures
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
around 0.4 – 0.6 that is attributed to adsorption in meso-
pores which were generated by the removal of the silica
walls. Another capillary condensation step at slightly
higher relative pressures, related to larger pores, is most
likely due to the partially incomplete replication of the
silica template. The corresponding BET surface areas (Ta-
The mesoporous CoO was tested for CO oxidation and
the results are shown in Figure 4a and summarized in-
Table 2. Due to the low stability of CoO at high tempera-
ture in air, the catalyst was pre-treated in He flow at 300
°
C for 30 min to remove any molecules adsorbed at the
surface before the catalytic test. Surprisingly, the catalytic
activity of mesoporous CoO under dry gas condition at a
2
−1
ble 2) for Co O and CoO are 87 and 82 m ·g , respective-
3
4
−1
−1
ly, suggesting only minor textural changes during the
space velocity of 80,000 mL·h ·g
(Figure 4a) shows a
cat
reduction process with H . These specific surface areas are
2
CO conversion of around 30 % at –50 °C. With increasing
temperature the CO conversion increases to more than 80
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
relatively high, considering the high density of the metal
Table 2. Physicochemical properties and CO oxidation activities of Co based mesoporous catalysts with different compositions.
Crystalline
Pore size
domain size
T50 (°C)
a
BET surface area
Pore volume
(cm ·g )
Sample name
2
−1
3
−1
b
T50 (°C)
c
(m ·g )
(nm)
(
nm)
Dry
-
SBA-15
728
87
1.12
0.15
0.15
0.19
0.19
0.10
0.25
0.39
8.5
-
-
-
14
Co O (He)
4.9/11.0
4.9/11.0
4.9/12.2
4.9/12.2
4.9
< –50
–25
3
4
14
9
Co O4
87
111
-
3
CoO (He)
CoO
82
–37.7
9
82
96
105
294
223
–9
10
CuCo O4
66
142
143
8 (–4)
279 (285)
265 (177)
2
6
CoCr O4
6.7
2
5
CoFe O4
4.9
2
a
The Co O and CoO sample were treated with He or 1 vol.% CO, 20 vol.% O and 79 vol.% He gas mixture before the catalytic
3
4
2
−
1
−1
−1
−1
tests; for the He treated ones, a SV of 80,000 mL·h ·g
was used. For the other samples 600,000 mL·h ·g
was used.
cat
cat
b
Mean size of crystalline domains, calculated from XRD reflection broadening with Scherrer equation (using NIST Si (640 c)
for accounting for instrumental broadening).
c
the number in brackets is the T50 with the sample weight adjusted to contain the same amount of Co species in all experi-
ments
.
%
below 0 °C and slowly reaches 100 % CO conversion at
around 140 °C. At a temperature of –38 °C a conversion of
0 % CO (T ) is achieved. The wide-angle XRD pattern
–50 °C. With increasing temperature the CO conversion
increases rapidly to 100 %, suggesting a higher activity
than that of mesoporous CoO.
5
5
0
(
Figure S4) of the used CoO catalyst shows reflections
The surface composition and chemical states of the cat-
alysts were analyzed by XPS. The evaluation of the Co 2p
narrow scans allows the assignment of the oxidation state
with positions almost identical to the fresh sample. The
results above indicate that the mesoporous CoO material
has unexpectedly high catalytic activity for CO oxidation,
and that its structure is quite stable during the CO oxida-
tion process at the temperatures tested. As a comparison,
mesoporous Co O catalyst was tested for CO oxidation
2+/3+
of cobalt. As reference materials for mixed Co
oxida-
tion states, bulk Co O was measured. As reference mate-
3
4
2+
3+
rial for Co , CoO was chosen, as reference for pure Co ,
Co(NH ) Cl was analyzed (Figure 5). Pure Co can be
distinguished from mixed Co /Co oxidation state by the
binding energy and the intensities of the shake-up peaks
2+
3
4
3
6
3
2+
3+
under the same conditions. The temperature dependence
curve (Figure 4b) shows a CO conversion of about 50 % at
5
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Page 6 of 15
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
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4
5
5
5
5
5
5
5
5
5
5
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. Temperature dependence of the activity for CO oxidation for mesoporous CoO and Co O catalysts nanocast from
3
4
−
1
−1
−1
SBA-15. Operation conditions: 50 mg catalyst, 67 mL·min ; WHSV = 80,000 mL·h ·g , 1 vol.% CO, 20 vol.% O and He bal-
cat
2
ance. The catalysts were pre-treated in He at 300 °C for 30 min. The feed gas was passed through a molecular sieve trap cooled to
– 78 °C (2-propanol/dry ice bath) before going into the reactor.
3
+
of the Co 2p photopeak. Pure Co shows only a very weak
2+
shake-up peak at 791 eV, whereas pure Co has pro-
nounced shake-up peaks at about 785 eV and 802 eV. If
both oxidation states are present, instead of distinct
shake-up peaks, a kind of plateau is observed. The simi-
larity of the spectra for mesoporous Co O indicates that
3
4
the surface is similar to that of the Co O reference mate-
3
4
rial (Figure 5d), representing mixed oxidation states for
cobalt. For the mesoporous CoO, two obvious shake-up
peaks at about 785 and 802 eV are observed (Figure 5e).
The intensive peak at 785 eV peak indicates a higher frac-
2+
tion of Co species present on the surface as expected for
perfect Co O .
3
4
3+
Generally, Co on the catalyst surface is thought to pro-
2+
vide the active site for CO oxidation, while Co is consid-
41
24
ered as inactive. Langell and coworkers found that the
CoO (100) single crystal surface could be oxidized to form
a thin layer of Co O when annealed in oxygen at 500 °C.
3
4
However, in our case, the sample is only exposed to air at
room temperature and the XPS results show that the cata-
2+
lyst surface is rich in Co species. The high activity of
mesoporous CoO indicates that the oxidation state of the
cobalt species may not be the only reason for the catalytic
activity in CO oxidation.
In order to understand the origin of the catalytic activity
of mesoporous CoO for CO oxidation in more detail, a
series of comparison tests was performed. The mesopo-
rous Co O material was normally pre-activated in situ
Figure 5. Co 2p XPS-spectra of (a) CoO reference material,
b) Co O reference material, (c) Co(NH ) Cl reference ma-
3
4
(
with the reaction gas (1 vol.% CO, 20 vol.% O
vol.% He gas mixture) for 30 min at 300 °C. Since CoO
can be easily oxidized to Co at low temperature in
2
and 79
3
4
3
6
3
terial, (d) mesoporous Co O nanocast from SBA-15, and (e)
3
4
mesoporous CoO reduced from mesoporous Co O by H .
O
3 4
3
4
2
42
air, a lower pretreatment temperature of 200 °C was
used for the comparison with CoO. However, the wide-
6
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Journal of the American Chemical Society
angle XRD data (Figure S5a) show that the CoO material
was partially re-oxidized to Co O even at a low-
origin of the negative apparent activation energies at in-
termediate temperature under normal gas conditions by
in-situ DRIFT measurements. The detrimental effect of
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
4
23
temperature (200 °C) oxidative activation. The CoO con-
tent after the treatment was estimated to be around 10
wt.%, which is very close to 11.6 wt.% as calculated from
the model illustrated in Figure S5b (assuming a rod diam-
eter of the CoO of 6 nm and the first 2 nm of the surface
being oxidized to Co O ). In other words, about 5 layers of
adsorbed water was fostered by DFT studies on H O ad-
2
sorption and its effect on CO oxidation over Co O . H2O
3
4
oct
2f
adsorption at Co sites via hydrogen bonding to the O
site was found to inhibit CO interaction with O sites,
2f
and then further hinders the CO formation by abstract-
3
4
2
2
f
43,44
the CoO nanorods were oxidized to Co O (assuming the
ing O atoms to a certain extent.
Under dry gas con-
3
4
surface is formed by (111) faces, and the thickness of 5 lay-
ers is around 2.33 nm), which is quite similar to the re-
ditions (H O < 1 ppm), the CO conversion at around 75 °C
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
only reaches 98 %. For mesoporous Co O under normal
3
4
24
−1
−1
sults obtained in previous reports. The temperature de-
pendence of the CO conversion for CoO (Co O /CoO
gas condition at a space velocity of 80,000 mL·h ·g , a
cat
“U”-shaped curve with a minimum at around 106 °C (CO
conversion: 97 %) is observed (Figure 6c). However, un-
der dry gas conditions, no region with negative apparent
activation energy was observed. The CO conversions over
CoO under both normal and dry gas conditions are higher
than for Co O material at very low temperature (close to
3
4
composite) and Co O under different conditions is given
3
4
in Figure 6 and additionally summarized in Table 2. The
catalytic activity of mesoporous CoO (Co O /CoO compo-
3
4
site) under normal gas condition (with ~3 ppm H O) at a
2
−
1
−1
space velocity of 80,000 mL·h ·g
(Figure 6a) shows a
cat
3
4
“U”-shaped curve with a minimum at around 72 °C (CO
conversion: 72 %), which had previously been observed
−50 °C), even with the pre-activation temperature being
100 °C lower than for the latter one. The T50 (temperature
for 50 % CO conversion) of both materials are even lower
than −50 °C.
23
for Co O /SiO composite materials, indicating negative
3
4
2
apparent activation energies. Adsorbed H O molecules on
2
the surfaces of the catalysts had been identified as the
Figure 6. Temperature dependence of the activity for CO oxidation for mesoporous CoO (a, b) and Co O (c, d) catalysts nano-
3
4
−
1
−1
−1
cast from SBA-15. (a,c) 50 mg catalyst; reaction gas flow rate, 67 mL·min ; SV = 80,000 mL·h ·g . (b,d) 10 mg catalyst; reaction
cat
−
1
−1
−1
gas flow rate, 100 mL·min ; SV = 600,000 mL·h ·g . The catalysts were pre-treated in 1 vol.% CO, 20 vol.% O and 79 vol.% He
cat
2
gas mixture at 200 (CoO) or 300 °C (Co O ) for 30 min. For measurement under dry conditions, the feed gas was passed through
3
4
a molecular sieve trap cooled to – 78 °C (2-propanol/dry ice bath) before going into the reactor.
7
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In order to investigate the catalytic behavior in more de-
tail, catalysts were evaluated at very high space velocity
ticle size of ~ 325 mesh (Aldrich) was measured under the
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
same conditions for CO oxidation. The wide-angle XRD
pattern (Figure S7) of the commercial CoO catalyst shows
very sharp reflections, indicating large crystallites. The
reflections can be assigned to the rocksalt-type structure
of CoO. A small reflection around 44.2 ° can be assigned
to metallic cobalt. The BET surface area calculated from
−
1
−1
(
600,000 mL·h ·g ). The catalytic results are shown in
cat
Figure 6b, d and summarized in Table 2. The CO conver-
sion over CoO material under normal gas conditions
shows almost no activity at temperature below 50 °C,
whereas under dry gas conditions, an obvious “U”-shaped
curve can be observed with a turning point at 55 °C and 22
2
−1
the nitrogen sorption isotherm is 6 m ·g , which is much
lower than the surface area of the mesoporous CoO cata-
lyst. The light-off CO conversion curves are shown in Fig-
%
CO conversion. For Co O under normal gas condi-
3 4
tions, the CO conversion at temperatures below 150 °C
drops significantly, as expected. The bottom of the “U”
shape curve is around 62 °C with CO conversion of only 12
−
1
−1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ure S8. A space velocity of 80,000 mL·h ·g
was used.
cat
The curves both obtained under normal and dry gas con-
ditions are “U”-shaped, similar to the mesoporous CoO
%
. Even under dry gas conditions, the CO conversion
−
1
−1
does not reach 100 % at 120 °C. The light-off curves for the
first and second run are almost identical for all the sam-
ples (Figure S6), indicating the reproducibility of this ef-
fect. Conversion curves were also found to be reproduci-
ble to within a few percent conversions at identical tem-
perature for separately synthesized batches of catalysts.
material at a high space velocity (600,000 mL·h ·g ),
cat
with slightly higher activity for the mesoporous material.
Considering the about eightfold higher space velocity for
the mesoporous CoO and the about 14 times higher sur-
face area, this suggests that the catalytic properties for
CO oxidation are rather similar for different types of CoO
materials.
For comparison, a commercial CoO material with a par
Figure 7. Temperature dependence of the activity for CO oxidation for H pre-treated mesoporous Co O and CoO catalysts.
2
3
4
−
1
−1
−1
(a,b) Co O . (c,d) CoO. Test conditions: 10 mg catalyst, reaction gas flow rate, 100 mL·min ; SV = 600,000 mL·h ·g . The cata-
3
4
cat
lysts were treated in H at 200 °C for 30 min before tests. For measurement under dry conditions, the feed gas was passed
2
through a molecular sieve trap cooled to – 78 °C (2-propanol/dry ice bath) before going into the reactor.
In order to further explore the nature of the active site
of the mesoporous CoO materials, samples were pretreat-
For comparison, Co O was also treated under the same
3 4
conditions. The results are shown in Figure 7. For the first
runs of the light-off tests for CO oxidation over mesopo-
3
+
ed in situ with H to reduce potential Co surface species.
2
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Journal of the American Chemical Society
tial amounts of CO were still produced (CO concentra-
rous Co O under normal as well as under dry gas condi-
3
4
2
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
tions, the low temperature activity disappeared, indicat-
tion 0.04 %). At 20 °C, a CO response maximum was ob-
2
3
+
ing that the Co surface species play an important role in
the catalytic CO oxidation (Figure 7a,b). However, the
low temperature activity was restored in the second run
for both conditions and is also reproducible for the third
served after 96 s with an even higher CO concentration
2
(0.41 %) than that observed at −20 °C. The CO evolution
2
decreased gradually to the same level observed at −20 °C
after 1000 s. At 120 °C, the CO concentration peaked after
2
3
+
run. This suggests that the Co surface species in Co O
76 s (CO concentration 0.44 %). For mesoporous CoO
3
4
2
can be easily restored below 200 °C. For mesoporous CoO,
the low temperature activity also disappears after the H2
treatment for both conditions (Figure 7c,d). Full CO con-
version in the first run is only archived at 190 °C under
dry gas conditions (Figure 7d). The temperature is much
higher than the temperature of 155 °C observed for Co O .
catalyst, CO evolution with a maximum (CO concentra-
2
2
tion 0.18 %) at 119 s after exposure to CO was observed. At
20 °C, a CO response maximum was observed after 124 s
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
with a maximum of 0.21 %, whereas at 120 °C, a CO re-
2
sponse maximum was observed after 81 s with a maxi-
mum of 0.46 %. From the above results, it can be seen
that the ability of the mesoporous CoO catalyst to supply
reactive oxygen is at the same level as that of the mesopo-
rous Co O one, except at 20 °C. However, the response
3
4
Even in the second run, the low temperature activity of
CoO could not be restored, but the conversion curve
shifts to lower temperature by about 20 °C. The low tem-
perature activity is only restored in the third run. These
3
4
time for mesoporous CoO is slightly slower than for
Co O at all temperature points investigated, suggesting a
3
+
results suggest that also for the CoO catalyst the Co sur-
face species play a crucial role for CO oxidation, and that
these species can be formed during the low temperature
(200 °C) treatment in the reaction gas.
3
4
slower CO adsorption rate and/or CO reaction rate with
the reactive surface oxygen species.
The catalytic activities under O -deficient conditions
2
were also investigated by using a mixed gas with 2.140
vol.% CO, 0.876 vol.% O balanced with He. The activities
2
of the catalysts in O -deficient atmosphere are significant-
2
ly lower than those under O -rich conditions (Figure S9
2
and 6). For Co O , the CO conversion is very similar to
3
4
that of CoO, this is due to (partial) reduction of Co O to
3
4
CoO at 300 °C during the pretreatment step. While pre-
treating the catalysts with pure He, the catalytic activities
are similar to those under O -rich conditions (Figure S10).
2
It is known that the adsorption and activation of molec-
4
1,45-48
ular oxygen are crucial for supported gold catalyst
and Co based catalysts
23,49
for CO oxidation. The for-
mation of adsorbed reactive oxygen species, such as su-
−
peroxide ions (O ), can be correlated to the presence of
2
4
6
Figure 8. CO response in the CO titration experiments of
2
surface oxygen vacancies on the metal oxide support or
at the metal-support interfaces.
45-47
the pretreated (1 vol.% CO, 20 vol.% O and 79 vol.% He gas
2
Different kinds of su-
mixture) mesoporous Co O (a) and CoO (b) catalysts at
3
4
peroxide species have been reported to exist on the sur-
49
temperatures of -20, 20 and 120 °C. All the above experiments
face of CoO-MgO solid solution and Co O /SiO compo-
3
4
2
23
were carried out under normal conditions. 50 mg catalyst,
site materials, and it was thus hypothesized that such
superoxide species may play an important role in CO oxi-
dation and also over the mesoporous materials investigat-
ed here at low temperatures. Therefore, CO titration ex-
periments in the absence of oxygen (1 vol.% CO in N2)
were performed. This allows assessing the extent of the
supply of oxygen stored on the catalyst. Figure 8 shows
−1
reaction gas, 1 vol.% CO in N ; flow rate = 67 mL·min .
2
The integral under the curves reported in Figure 8 al-
lows calculation of the total amount of active oxygen on
the surface of the catalysts. In order to estimate whether
bulk oxygen contributes to the CO oxidation in the titra-
tion experiments, the maximum amount of oxygen that
could be present just on the surface was calculated. This
was done under the assumption that the surface is only
composed of the close-packed (111) plane of Co O or
the transient response of the CO and CO evolution over
2
pretreated mesoporous Co O and CoO at different tem-
3
4
3
4
peratures. For mesoporous Co O , the ability to provide
3
4
CoO. The calculation method was reported in our previ-
reactive oxygen is rather high at low temperature (−20
C). High CO evolution with a maximum (CO concen-
23
ous work. For both Co O and CoO under different tem-
3
4
°
2
2
peratures, the amount of the consumed oxygen are lower
than this maximum amount (Table 3). However, consid-
ering that the surface of the catalysts most probably does
not consist only of the (111) plane, but also other faces
with lower oxygen density, or could contain residual
tration 0.27 %) at 125 s after exposure to CO was observed.
The initial sharp CO response is indicative for a fast reac-
2
tion, proceeding via rapid adsorption of CO followed by
the reaction of CO with the reactive oxygen on the cata-
lyst surface. It is noticed that, even after 1000 s, substan-
9
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Journal of the American Chemical Society
Page 10 of 15
Table 3. Amount of oxygen consumed from mesoporous spinel catalysts in CO titration experiments.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Sample name
Maximum
Temperature
(°C)
O consumption
(µmol)
O consumption
per unit area
2
-
surface O
a
2
(µmol)
(µmol/m )
Co O4
114.7
86.2
58.1
– 20
42.7
52.6
66.8
24.2
35.2
65.5
20.6
36.0
108.1
1.0
9.82
12.09
15.36
5.90
8.59
15.98
6.24
10.91
32.76
0.14
3
20
120
CoO
– 20
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
20
1
20
CuCo O4
– 20
2
20
1
20
CoCr O4
187.2
133.5
– 20
20
2
2.3
0.32
1.23
1
20
8.7
CoFe O4
– 20
11.4
1.59
2
2
0
17.9
49.1
2.50
6.87
120
a
2-
Maximum surface oxygen (O ) capacities of the catalysts are calculated with the assumption that the surface of the
catalysts are composed only of (111) surface. The surface areas of the spinel catalysts used are list in Table 2. The catalyst
amount for CO titration is 50 mg.
amorphous silica (bulk residual silica around 1 wt.%),
the true amount of surface oxygen is probably lower than
the ideally calculated values. The O consumption normal-
ized to unit surface area (Table 3) show that mesoporous
Co O and CoO have very similar numbers at −20 and 120
The experimental results presented above show that
mesoporous CoO has unexpected activity in CO oxida-
tion. The activity may be related to oxidation of surface
2+
Co species in octahedral coordination which results in
3
+
rather similar sites to the Co in the cobalt oxide spinel.
Structurally, at 300 K CoO has a rocksalt structure with a
3
4
°
C. Co O contains more reactive oxygen than CoO per
3 4
2+
unit surface as well as per weight at low temperatures,
corresponding well to the CO oxidation activities shown
in Figure 6b and d.
single Co -site, octahedrally coordinated by lattice oxy-
2
4,25
ꢀ
gen in Fm3m symmetry (Figure 9b).
Co O has a spinel
3 4
3
+
structure with Fd 3ꢀ m symmetry (Figure 9a). The Co cati-
on in the spinel structure is octahedrally coordinated,
In situ DRIFT spectroscopy was also used to detect CO
2+
while Co is tetrahedrally coordinated. The two struc-
tures are strongly related through their oxygen sublattic-
es. Both spinel and rocksalt lattice oxygen are face cen-
adsorbed on the surfaces of CoO and Co O catalysts at
3
4
room temperature. The spectra under CO adsorption
conditions (Figure S11) for both CoO and Co O catalysts
3
4
2−
2−
tered cubic (fcc), with O -O closest packing distance in
the rocksalt structure ~ 5 % larger than in the spinel
structure.
reveal the characteristic gas-phase bands of CO at 2117
−1
δ+
and 2171 cm but no strongly adsorbed CO-Co was
dtected. No obvious differences in CO adsorption over
CoO and Co O were observed. It is noticed that in the
3
4
In rocksalt all octahedral sites are filled, but only half
are filled in the spinel structure. The tetrahedral sites,
which are completely unoccupied in the rocksalt struc-
ture, are partially filled to one-eighth occupancy in the
-
1
range of 1300 – 1700 cm , which corresponds to various
vibrations of carbonates, differences for CoO and Co O
3
4
indeed exist, especially in the CO adsorption process.
However it is rather difficult to assign these bands pre-
cisely as reflected by oppositional reports in the literature.
2+
spinel structure. The Co species in CoO occupy the posi-
3
+
tion which corresponds to the Co species in Co O . The
3
4
octahedrally coordinated sites in CoO are more open than
the tetrahedrally coordinated sites in Co O , which allows
Mesoporous CoO also could be synthesized via reduc-
tion with CO. The respective data are shown in the sup-
porting information (Figures S12-14). The structure of the
material and its CO oxidation activities are similar to that
prepared via reduction with H2.
3
3+
4
2+
easy oxidation of the Co species to Co , resulting inhigh
2+
activity. If all surface Co species in the CoO were oxi-
3
+
dized to Co , their surface density would actually be
higher than in the spinel structure, which could explain
10
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Journal of the American Chemical Society
the even higher catalytic activity at low temperature
compared to the parent Co O material (Figure 6 a and c).
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
4
In addition, the surface oxidized CoO may contain a few
layers of cobalt oxides with a somewhat different struc-
24
ture compared to normal Co O spinel, which could lead
3
4
to the higher catalytic activity of the composite material
compared to the normal mesoporous Co O .
3
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 10. Low-angle (A) and wide-angle (B) XRD patterns
of mesoporous cobalt based spinels nanocast from SBA-15:
(
a) CuCo O , (b) CoCr O and (c) CoFe O .
2 4 2 4 2 4
the notion that the replication of the SBA-15 silica tem-
plate worked well. For CoFe O , only little order is ob-
2
4
served in the mesostructured range as reflected by rather
broad low-angle XRD reflections. For CoCr O , only small
2
4
Figure 9. Three-dimensional structures of metal oxides:
a) Co O , (b) CoO, (c) CoCr O , (d) CuCo O and (e)
nanorods with similar diameters to all the other mesopo-
rous materials can be observed in TEM images, again in
agreement with the results of the low-angle XRD anal-
yses. Nitrogen adsorption-desorption isotherms (Figure
S15) measured for the Co-based spinels show the shape
typical for mesoporous replicated metal oxides. Physico-
chemical properties of the samples are summarized in
Table 2. The specific surface areas differ more strongly
due to the stronger variety of structural ordering and den-
sities among these samples. However, all samples were
synthesized using the same method and the same tem-
plate. The sizes of the individual particles/rods for all
samples are controlled by the pore size of the silica tem-
plate. Thus, the domain sizes of the different catalysts
remain still comparable (Table 2), and thus their catalytic
properties should be comparable as well to some extent,
even if the similarities are less pronounced than for the
Co O /CoO system.
(
3
4
2
4
2
4
CoFe O .
2
4
3
.2 Other Co-based spinel mesoporous catalysts
The structure of CoO is related to the spinel structure.
Several spinels with different Co species substituted by
non/low-active metal ions (CuCo O , CoCr O , and
2
4
2
4
CoFe O ) were synthesized in order to investigate wheth-
2
4
2+
er only the octahedrally coordinated Co species can be
oxidized to Co and thus result in an active catalyst. The
position of each cation is summarized in Table 1. In
3
+
2+
CuCo O , the tetrahedrally coordinated Co is substitut-
2
4
2+
ed by Cu (Figure 9d). In CoCr O , the octahedrally coor-
dinated Co is substituted by Cr (Figure 9c). CoFe O is
an inverse spinel structure, in which half of Fe is tetra-
hedrally coordinated and the other half of Fe is located
together with the Co ions in the octahedrally coordinat-
ed position (Figure 9e).
2
4
3+
3
+
2
4
3
+
3
+
2+
3
4
The catalytic activities of the spinel catalysts for CO ox-
idation were tested in the same reactor as used for the
experiments discussed earlier, using 1 vol.% CO, 20 vol.%
All spinels were synthesized using the same nanocasting
route, starting with SBA-15 as used for the synthesis of
mesoporous Co O . Metal nitrates with the required mo-
3
4
O and 79 vol.% He gas mixture. The temperature de-
2
lar ratios were used as precursors. The low-angle XRD
pattern (Figure 10A(a)) of the CuCo O shows three well
pendence of the CO conversions at a space velocity of
2
4
−1
−1
6
00,000 mL·h ·g
is shown in Figure 12a and b. Meso-
cat
resolved reflections, indicating good replication of the
silica template. The peak positions are similar to those of
mesoporous Co O , indicating that both have similar
porous CuCo O , containing only octahedrally coordinat-
2
4
3+
ed Co species, shows catalytic activity at low tempera-
tures, both under normal and dry gas conditions. The T50
temperature under dry gas conditions was determined to
be 8 °C. As for the Co O and the CoO, a “U”-shaped curve
3
4
structures. In contrast, for CoCr O , only weak reflections
2
4
are observed, indicating lower structural ordering. How-
ever, the wide-angle XRD patterns (Figure 10B) of all the
replica materials show broader reflections than normally
observed for the spinel phases, indicating the formation
of nanomaterials.
3
4
is observed with a minimum around 75 °C, indicating that
the negative apparent activation energy in CO oxidation
is common for Co-based metal oxides. In contrast, meso-
porous CoCr O and CoFe O show very low catalytic ac-
2
4
2
4
TEM images of the spinel replicas are shown in Figure
tivities under both normal and dry gas conditions. The T50
temperatures determined under dry gas conditions are
1
1. CuCo O exhibits well ordered nanorod/nanowire ar-
2 4
rays with rod-like particle morphology. This also supports
294 and 223 °C, respectively. In order to normalize the
samples for cobalt content, measurements for equal
11
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Journal of the American Chemical Society
Page 12 of 15
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 11. TEM images of cobalt based spinels: (a-c) CuCo O , (d-f) CoCr O and (g-i) CoFe O .
2
4
2
4
2
4
amounts of Co species by adjusting the catalyst weight
were carried out under dry gas conditions. 15.3 mg of
CuCo O , 28.4 mg of CoCr O , and 29.3 mg of CoFe O
Co 2p XPS spectrum of mesoporous CoCr O shows a
2 4
2+
strong shake-up peak at 785 eV, which is typical for Co .
For mesoporous CoFe O , two intensive shake-up peaks at
2
4
2
4
2
4
2
4
2+
were used for comparison with 10 mg of Co O . The re-
about 785 and 791 eV are observed, indicating both Co
3
4
3+
sults are shown in Figure 12c. The activity of mesoporous
CuCo O (T = −4 °C) increases and comes close to that
and Co on the surface. Nevertheless, the intensity of the
shake-up peak at 785 eV is slightly higher than expected
2
4
50
2+
3+
of Co O (T = −25 °C). The slightly lower activity may be
for stoichiometric Co -Co distribution, indicating that
this sample contains a slightly higher concentration of
3
4
50
due to the lower specific surface area of CuCo O . The T
50
2
4
2+
2+
of CoFe O containing octahedrally coordinated Co de-
Co species on the surface compared to the reference
2
4
creases from 265 to 177 °C, while that of CoCr O stays
Co O . The results above also indicate that the octahe-
2
4
3
4
2+
almost unchanged (from 279 to 285 °C). These results
drally coordinated Co in CoFe O is easier oxidized than
2 4
3+
2+
suggest that the octahedrally coordinated Co species
that tetrahedrally coordinated Co in CoCr O . The stoi-
2 4
play an important role for the catalytic activity for CO
oxidation, while the tetrahedrally coordinated Co has
chiometric ratios of Co/Cu, Co/Cr, or Co/Fe on the sur-
faces were calculated by XPS to be 34:66, 36:64 and 38:62,
which is more or less identical to the bulk compositions.
2+
almost no activity and can also not be easily oxidized, as
2+
compared to the octahedrally coordinated Co species in
The oxygen storage capacity is related to the metal-
2+
5
2
CoO. CoFe O4 with octahedrally coordinated Co also
2
oxygen bond energy. Metal oxides, such as Co O , have
3 4
shows some activity, which could be related to easy oxida-
lower metal-oxygen bond energy, leading to the abstrac-
tion of oxygen from the surface of the metal oxide to form
surface oxygen vacancies. This finally enhances the ad-
sorption of oxygen for CO oxidation. In order to compare
the oxygen stored on the ion substituted mesoporous spi-
nel catalysts with that of Co O , CO titration experiments
2+
50,51
tion of the Co on such sites.
XPS characterization was performed to investigate the
surface composition and chemical state of the surface
atoms of the mesoporous spinel catalysts. The results are
shown in Figure S16. The Co 2p XPS spectra of mesopo-
rous CuCo O shows peaks characteristic for a mixture of
3
4
in the absence of oxygen (1 vol.% CO in N ) were also per-
formed for these samples. Figure S17 shows the transient
2
4
3+
2
2+
Co and Co oxidation states. The surface seems to con-
2+
3+
sist of Co and Co comparable to reference Co O . The
response of the CO and CO evolution over pretreated
3
4
2
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Journal of the American Chemical Society
ergetic point of view. In a simple ligand field theory pic-
3
+
6
0
1
2
3
4
5
6
7
8
9
ture, octahedral Co (t e ) is energetically substantially
2g
1
g
2+
6
5
2
favored over Co (t2g eg or t2g e ) in a moderate ligand
g
field, such as provided by an oxide environment, while for
tetrahedral coordination the ligand field stabilization of
3
+
3
g
3
2g
2+
4
g
3
2g
Co (e t ) over Co (e t ) is relatively small. There is
2+
thus a higher driving force for oxidation of Co in octa-
hedral environment. Which of the two factors is more
important or both contribute cannot be decided based on
the available data.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4
. Conclusion
In summary, we have synthesized a series of mesoscopi-
cally ordered and crystalline spinel-type metal oxides
Co O , CuCo O , CoCr O and CoFe O ). All materials
3 4 2 4 2 4 2 4
(
were obtained by nanocasting from the same silica tem-
plate. This results in compounds with relatively similar
nanostructures – albeit different degree of mesostructural
order – and textural parameters. Furthermore, these ma-
terials are structurally related on the atomic scale, but
contain cobalt species in different coordination environ-
ments and oxidation states. Thus, these materials are
highly suitable to study the active sites for CO oxidation
over Co-based catalysts. Highly ordered mesoporous CoO
Figure 12. Temperature dependence of the activity for CO
oxidation for mesoporous spinels (Co O , CuCo O , CoCr O
4
3
4
2
4
2
and CoFe O ) nanocast from SBA-15. (a,b) 10 mg catalyst;
2
4
−
1
reaction gas flow rate = 100 mL·min ; SV = 600,000
was prepared by a low temperature H reduction method.
This CoO material shows unexpectedly high activity in
CO oxidation which is attributed to surface oxidation of
−
1
−1
2
mL·h ·g . (c) Catalyst: 10 mg Co O , 15.3 mg CuCo O , 28.4
cat
3
4
2
4
mg CoCr O and 29.3 mg CoFe O ; reaction gas flow rate =
2
4
−
2
4
1
100 mL·min . For measurements under dry conditions, the
feed gas was passed through a molecular sieve trap cooled to
– 78 °C (2-propanol/dry ice bath) before going into the reac-
tor.
2
+
the octahedrally coordinated Co species in the relatively
open CoO rocksalt structure. A series of metal ion substi-
tuted spinels (CuCo O , CoCr O and CoFe O ) was syn-
thesized to further investigate the origin of high activity
in CO oxidation. The experimental data reveal that the
valence state of the cobalt species is important for the
catalytic activity. The coordination environment also
plays a crucial role in that it controls the oxidizability of
the Co species. The octahedrally coordinated Co is
located in a more open framework site than the tetrahe-
drally coordinated Co , and the CFSE of Co is much
lower than that of Co , which makes the former more
accessible to oxygen and it can be easily oxidized to Co .
2
4
2
4
2
4
mesoporous CuCo O , CoCr O and CoFe O at different
2
4
2
4
2
4
temperatures. The corresponding oxygen consumptions
are also summarized in Table 3. From the results, it can
be concluded that the high activity of CuCo O at low
2
4
temperatures is related to a high oxygen storage capacity.
In comparison, CoCr O has almost no oxygen storage
2+
2+
2
4
capacity at temperatures below 120 °C due to its higher
2+
2+
2+
Cr-O bonding energy. For CoFe O , the oxidizable Co
3+
2
4
species is located on the octahedrally coordinated posi-
tion, which is more accessible than the tetrahedrally co-
ordinated one. This leads to a small amount of oxygen
stored.
3+
This is proposed as the origin of the high activity of CoO
material which is thus a true surface effect. In spite of the
reduced bulk material, the surface layer provides the oxi-
3+
For the different Co-substituted spinels, the catalytic ac-
tivities for CO oxidation are related not only to their co-
balt valence state, but also to the position of the cobalt
species in the crystal structure. CuCo O with only octa-
dized Co species which are crucial for activity.
ASSOCIATED CONTENT
2
4
Supporting Information. Additional results of XRD, TEM,
XPS, TPR, nitrogen sorption, DRIFT and CO oxidation re-
sults. This material is available free of charge via the internet
at http://pubs.acs.org.
3+
hedrally coordinated Co species can have a comparable
activity as Co O . CoCr O with only tetrahedrally coordi-
3
4
2
4
2+
3+
nated Co is almost completely inactive. No surface Co
can be detected by the XPS measurements. Co in
CoFe O is octahedrally coordinated, which is similar to
2+
AUTHOR INFORMATION
Corresponding Author
2
4
the coordination of Co in CoO. This site can be oxidized
to some extent and results in moderate activity of
CoFe O .
schueth@mpi-muelheim.mpg.de, jiacj@sdu.edu.cn
Fax: +49 208 3062995; Tel: +49 208 3062373
2
4
While accessibility is a kinetic argument for preferred
2+
oxidation of Co in octahedral sites, oxidation of octahe-
Notes
2+
drally coordinated Co should also be easier from an en-
The authors declare no competing financial interest.
13
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Journal of the American Chemical Society
Page 14 of 15
(
28) Gu, D.; Jia, C.-J.; Bongard, H.; Spliethoff, B.;
ACKNOWLEDGMENT
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
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4
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5
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5
5
5
5
5
5
5
5
6
Weidenthaler, C.; Schmidt, W.; Schüth, F. Appl. Catal. B 2014,
152-152, 11.
We greatly acknowledge the support of this research by the
Alexander von Humboldt-Stiftung, NSFC of China (21301107),
Taishan Scholar Project of Shandong Province (China), Fun-
damental Research Funding of Shandong University
(
29) Ghosh, M.; Sampathkumaran, E. V.; Rao, C. N. R. Chem.
Mater. 2005, 17, 2348.
30) An, K.; Lee, N.; Park, J.; Kim, S. C.; Hwang, Y.; Park, J.-G.;
(
(
2014JC005), in addition to the basic support by the Max-
Kim, J.-Y.; Park, J.-H.; Han, M. J.; Yu, J.; Hyeon, T. J. Am. Chem.
Soc. 2006, 128, 9753.
Plank-Society.
(31) Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Schüth,
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Highly Ordered Mesoporous Cobalt Containing Oxides: Structure, Catalytic Properties, and Active Sites in
Oxidation of Carbon Monoxide
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