Three-dimensionally ordered macroporous spinel-type MCr2O4 (M = Co, Ni, Zn, Mn) catalysts with highly enhanced catalytic performance for soot combustion

Article abstract of DOI:10.1039/c5cy00761e

Three-dimensionally ordered macroporous (3DOM) spinel-type MCr₂O₄ (M = Co, Ni, Zn, Mn) catalysts were successfully prepared by a colloidal crystal template strategy and characterized by means of XRD, FE-SEM, BET, FT-IR, H₂-TPR and O₂-TPD. The 3DOM catalysts exhibited superior activity to their corresponding bulk counterparts, which were used as reference benchmarks in this study, due to the enlarged contact between the catalyst and the soot particle and improved mass transfer caused by the unique well-defined 3DOM structures. Meanwhile, the factors influencing the catalytic activity of the 3DOM catalysts in soot combustion were also optimized, such as metallic cations and feed compositions. Furthermore, the 3DOM catalysts displayed strong durability against structural collapse due to the robust 3DOM structure, demonstrating their promising potential in view of the practical working conditions of diesel engines.

Full text of DOI:10.1039/c5cy00761e

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Three-Dimensionally Ordered Macroporous Spinel-type MCr2O4
(M = Co, Ni, Zn, Mn) Catalysts with Highly Enhanced Catalytic
Performance for Soot Combustion
Received 00th January 20xx,
Accepted 00th January 20xx
Jinguo Wanga, Gaoyang Yanga, Li Chenga, Eun Woo Shinb, and Yong Men*a
DOI: 10.1039/x0xx00000x
Three-dimensionally ordered macroporous (3DOM) spinel-type MCr2O4 (M = Co, Ni, Zn, Mn) catalysts were successfully
prepared by colloidal crystal template strategy and characterized by means of XRD, FE-SEM, BET, FT-IR, H2-TPR and O2-TPD.
The 3DOM catalysts exhibited superior activity compared to their corresponding bulk counterparts, which were used as
reference benchmarks in this study, owning to the enlarged contact between catalyst and soot particle and improved mass
transfer caused by the unique well-defined 3DOM structures. Meanwhile, the influencing factors of catalytic activity for
3DOM catalysts on soot combustion were also optimized such as metallic cations and feed compositions. Furthermore, the
3DOM catalysts displayed strong durability due to the robust 3DOM structure against the structural collapse,
demonstrating their promising potential in view of practical working conditions of diesel engines.
www.rsc.org/
a.College of Chemistry and Chemical Engineering, Shanghai University of
Engineering Science, Shanghai 201620, P. R. China. E-mail: men@sues.edu.cn;
Fax: +86-21-67791214
b.School of Chemical Engineering, University of Ulsan, Daehakro 93, Nam-Gu, Ulsan
680-749, South Korea
†Electronic Supplementary Information (ESI) available
DOI: 10.1039/x0xx00000x
.See
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which display high capability for catalytic oxidation of volatile
organic compounds due to the presence of a higher amount of
surface oxygen related to a higher proportion of Cr6+ species
on the surface of these catalysts.20-22 However, it’s reported
that individual supported chromium catalysts are inadvisable
owning to both the severe deactivation caused by migration
and/or loss of active sites and the formation of extremely toxic
chromium oxychloride at low temperature during reaction
process.23 If chromium species are confined in spinel-type
structure and occupy the octahedral sites, the disadvantages
of chromium catalysts could be probably overcome as well as
it could offer a large amount of surface oxygen species. In total
oxidation of trichloroethylene, Kim and Ihm found that spinel
type CoCr2O4 showed superior performance to other
supported Cr catalysts such as CrOx/Al2O3 and CrOx/MCM-41,
and the authors attributed high CO2 selectivity to the presence
of abundant surface Cr3+ species in the CoCr2O4 oxide.24
Furthermore, the change of A cations also have significant
influence on the catalytic activities of chromite spinels like as
ZnCr2O4 exhibiting higher activity than CuCr2O4 and CoCr2O4
for the combustion of 2-propanol.25
1. Introduction
The emission control of diesel soot has become a research
hotspot in the field of environmental catalysis owning to diesel
soot containing unburned carbonaceous compounds, which
has caused severe environmental and health problems.1-5 One
promising way to eliminate diesel soot is catalytic oxidation of
soot into CO2 by catalysts while two challenges are needed to
improve the catalytic efficiency: one is developing catalysts
with good redox capability, and the other is designing catalysts
with rational structure to enlarge the contact area between
soot and catalyst. Presently, two major kinds of catalysts with
excellent redox capability have been developed to catalytic
oxidation of soot into CO2. One is the noble metal-loaded
catalysts with excellent catalytic activity at low temperature.6-10
But the prohibitively high cost and scarcity of precious metals
have been bottlenecks that impede their widespread use in
soot combustion. The discovery of alternative inexpensive
multicomponent metal oxides catalysts has become a very
attractive subject for the researchers to replace high-cost
precious metal supported catalysts.11-13 However, this kind of
catalysts also has disadvantages in practical applications due to
their pore size below 10 nm smaller than the particle size of
soot above 25 nm.14 So the soot particle couldn’t approach the
active sites on the internal surface of catalysts, only those on
the external surface of catalysts could be utilized for soot
combustion. Thus, one of the key issues is the design of active
catalysts with easily accessible inner active sites by soot
particles.
Herein, we reported the synthesis of spinel-type 3DOM
MCr2O4 (M = Co, Ni, Zn, Mn) catalysts by colloidal crystal
template with the aim to enhance the catalytic activity by
enlarging the contact area between soot and catalyst, and
adjusting the redox capability of catalysts via changing the A
cation. Meanwhile, it is anticipated that the well-defined
3DOM structure could benefit the mass transfer and strength
the durability of catalysts. As expected, the 3DOM catalysts
exhibited high activity during the catalytic oxidation of soot to
CO2. Moreover, the 3DOM catalysts displayed strong durability
due to the well-defined 3DOM structure against the structural
collapse.
Fortunately, three-dimensionally ordered macroporous
(3DOM) materials with large pore size above 50 nm and
interconnected macroporous structure could serve as good
candidates to circumvent the above problems, which
facilitates the contact between soot particles and catalysts
with less diffusion resistance.
The perovskite-type and fluorite-type catalysts with 3DOM
structures such as La1−xKxCoO3, LaMn1−xFexO3 and Ce1−xZrxO2
have been extensively studied for soot combustion.15-18 By
2. Experimental
2.1 Catalyst preparation
contrast, spinel-type oxides AB2O4 with 3DOM structures The 3DOM MCr2O4 (M = Co, Ni, Zn, Mn) catalysts were
especially the components influencing catalytic activity for prepared by the monodispersed polymethyl methacrylate
soot combustion have been rarely reported by far. For normal (PMMA) microspheres with an average size of 360 nm (see
spinel-type oxides AB2O4, A cation is in the tetrahedral Figure S1) as the colloidal crystal template, which were
environment of oxygen atom (M–O) and B cation lies in the synthesized by the modified emulsifier-free emulsion
octahedral (Oh) site. Meanwhile, the surface compositions of polymerization route.26,27 In a typical run of synthesis, 10 mmol
normal spinels do not match with their bulk stoichiometry, and Co(NO3)2·6H2O, 20 mmol Cr(NO3)3·9H2O and 30 mmol citric
almost exclusive octahedral B cations are present on the acid was added into a transparent solution containing 9.0 ml
outermost surface layer of spinels.19 To our knowledge, ethylene glycol and 6.0 ml methanol under stirring at room
chromium oxides also have similar properties to normal spinel, temperature until the metal salt dissolved completely. Then,
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5.0 g PMMA was soaked into the above solution for 5.0 h. inserted to obtain a good contact with the catalyst bed. In a
After being filtered, the solid product was dried at 48oC for 12 typical test, reactant gases containing 5.0 vol% O2 and 2500
h in air, and then calcined at 600oC for 5 h with the heating ppm NO balanced with N2 were passed through the mixture of
rate of 1oC·min-1 and air flow of 70 ml·min-1 in a tube furnace. catalyst and soot at a total actual flow rate of 80 ml·min-1, and
The as-received catalysts were denoted as 3DOM-Co. The the GHSV (Gas Hourly Space Velocity) is 6760 h-1. For the
3DOM ZnCr2O4, NiCr2O4 and MnCr2O4 were also synthesized by experiments containing 10 vol% H2O, deionized water was
the same method except Co(NO3)2·6H2O replaced by introduced into the evaporator (200oC) by a liquid pump and
Zn(NO3)2·6H2O, Ni(NO3)2·6H2O and Mn(NO3)2, and denoted as carried into the reactor by the flowing reactant gases. A self-
3DOM-Zn, 3DOM-Ni and 3DOM-Mn, respectively.
made cold trap was placed before the gas analyzer to
For comparison, the bulk CoCr2O4, ZnCr2O4, NiCr2O4 and condense the moisture. The effluent compositions from the
MnCr2O4 were also prepared by traditional sol-gel method reactor were analyzed by an online automobile emission
with various metal nitrates as precursors, and denoted as BK- analyzer (HPC 500). The selectivity to CO2 formation (SCO2) was
Co, BK-Zn, BK-Ni and BK-Mn, respectively.25
defined as the CO2 outlet concentration (CCO2) divided by the
sum of the CO2 and CO outlet concentrations; i.e.,
SCO2=CCO2/(CCO+CCO2). The catalytic activity was evaluated by
the values of Tm, which was defined as the temperature at
which CO2 content reach the maximum during the course of
the soot TPO procedure. SmCO2 was denoted as the SCO2 at Tm.
The reproducibility of all the activity tests was checked by
repeating each result at least three times and found to be
2.2 Catalyst characterization
The crystal structure and morphology were determined by X-
ray diffraction (BRUKER D2 PHASER with Cu Kα radiation) and
field emission-scanning electron microscopy (FESEM, Hitachi
S4800), respectively. Fourier transform infrared (FTIR) spectra
was on a Nicolet 380 by using the matched gas cell with 10 cm
in length was suitable for NO and NO2 analyzing in ambient
conditions. N2 adsorption-desorption isotherms were
within acceptable limits (
±
2oC).
determined on a Micromeritics ASAP 2460 at 77 K
，from
3. Results and Discussion
which the surface area (SBET), pore volume (VP) and pore
diameter (DP) were calculated by applying Brunauer-Emmett-
Teller (BET) and Barrett-Joyner-Halenda (BJH) models to the
desorption branches. Hydrogen temperature-programmed
reduction (H2-TPR) curves were obtained on a Micromeritics
Autochem II 2920 instrument equipped with a thermal
conductivity detector (TCD). The catalysts were placed in a U-
shaped fixed-bed quartz microreactor, pretreated in Ar
atmosphere at 300oC for 1.0 h, and then subsequently cooled
to 50oC. The pretreated catalysts were heated from 50 to
900oC with a heating rate of 10oC·min-1 under a mixture gas of
10% H2/Ar flow (50 ml·min-1), and the consumed H2 was
determined by TCD. Oxygen temperature-programmed
desorption (O2-TPD) curves were carried out in the same
apparatus to H2-TPR. The catalysts were firstly pretreated at
500oC with an air flow of 50 ml·min-1 for 30 min, cooled to
50oC, and then exposed to pure He for 5.0 min, lastly heated
up to 900oC with a heating rate of 10oC·min-1. The released O2
was determined by TCD.
3.1 Physicochemical characteristics
The XRD patterns in Figure 1 demonstrate that all the MCr2O4
catalysts were present in cubic spinel-type structure with well-
defined peaks, corresponding to 2θ
of 30.3o, 35.7o, 43.4o and
63.1o indicative of (220), (311), (400) and (440) diffractions
(JCPDS No.22-1084 for CoCr2O4, JCPDS No.34-0424 for ZnCr2O4,
JCPDS No.65-3105 for NiCr2O4, and JCPDS No.54-0876 for
MnCr2O4). Interestingly, no peaks other than spinel phase
were detectable for chromite catalysts containing Co and Zn,
indicating the formation of highly pure CoCr2O4 and ZnCr2O4.
However, NiCr2O4 and MnCr2O4 contained a little amount of
Cr2O3 with rhombohedral structure (JCPDS No.38-1479).28,29
Noticeably, the 3DOM catalysts exhibited weaker and wider
diffraction peaks than their bulk counterparts, indicating that
the bulk catalysts owned higher crystal degree. Meanwhile,
the average crystallite sizes of bulk catalysts (see Table 1) are
generally much larger than the 3DOM catalysts, obviously due
to the different preparation methods.
The FESEM images in Figure 2 revealed that all the 3DOM
catalysts were present in 3DOM structures with
2.3 Activity testing
The activity test of the MCr2O4 catalysts for soot combustion interconnected networks of spherical voids while the bulk
was evaluated by a temperature programmed oxidation (TPO) catalysts were present in irregular nano/macroparticles (see
reaction in a fixed-bed tubular quartz microreactor (see Figure Figure S3). The average size of the hexagonally arrayed
S2) from 200 to 700oC with a heating rate of 2.0oC·min-1. 100 macropores about 200 nm was smaller than the original
mg catalyst and 10 mg soot (MA100 Mitsubishi, Japan) were PMMA microsphere template, which could be attributed to
mixed with a spatula by hand to stir over and over again for the framework shrinkage during the calcination process.30
5.0 mins in order to reproduce the loose contact During the high-temperature calcination, the PMMA
conditions.3,6,7 Then the mixture was placed into the tubular microspheres acted as building blocks for the spinel-type oxide
quartz microreactor (Φ= 6.0 mm) and pretreated at 200oC in frameworks and leaded to relatively moderate shrinkage of
N2 for 30 min to eliminate the impact of adsorbed water and the macropores.
hydrocarbons in every test. In order to accurately ensure the
Figure 3 displays that all the 3DOM catalysts exhibited the
desired reaction temperature, a K-type thermocouple was typical type II N2 adsorption-desorption isotherms with H3
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hysteresis loop in the relative pressure (P/P0) range from 0.8 to
1.0 combined with a small H2 hysteresis loop in the range from
0.2 to 0.8.30,31 The H3 hysteresis loop, which does not clearly
demonstrate any adsorption plateau at the relative pressures
close to unity, could be attributed to the existence of slit-
shaped pores in catalysts, indicative of a part of pore size
extending into the macropore range.31-33 Meanwhile, their
corresponding pore size distribution curves demonstrated the
wide pore size range from mesopore to macropore, which
further confirmed the above conclusion. Furthermore, N2
adsorption-desorption isotherms of bulk catalysts were also
presented as shown in Figure S4. Based on N2 adsorption–
desorption isotherms, the SBET, VP and DP could be calculated
by using BET and BJH models, respectively. As shown in Table 1,
the 3DOM catalysts exhibited higher surface area than their
bulk counterparts, which could be attributed to the fact that
using PMMA as the pore-making template could inhibit the
crystal growth during the calcination process and increase the
surface area of catalysts.34
Figure 3. N2 adsorption-desorption isotherms and the
corresponding pore size distribution curves of different
catalysts.
Figure 1. XRD patterns of different catalysts: (a) 3DOM-Co, (b)
BK-Co, (c) 3DOM-Ni, (d) BK-Ni, (e) 3DOM-Zn, (f) BK-Zn, (g)
3DOM-Mn and (h) BK-Mn. Spinel phase labeled with Miller
Figure 4. The H2-TPR (A) and O2-TPD (B) profiles of different
catalysts: (a) 3DOM-Co, (b) BK-Co, (c) 3DOM-Ni, (d) BK-Ni, (e)
3DOM-Zn, (f) BK-Zn, (g) 3DOM-Mn and (h) BK-Mn.
Indicies;
■Cr2O3 phase.
The reducibility of different MCr2O4 catalysts was
investigated by H2-TPR as shown in Figure 4A. It could be seen
that the nature of metal atom M has a great influence on the
reduction profiles. The first reduction peak centered around
190oC for all catalysts could be attributed to the reduction of
CrO3 phase present on the surface of MCr2O4 while the second
reduction peak above 650oC in CoCr2O4 could be assigned to
the reduction of Co2+ ions present in the CoCr2O4 lattice into
Co0.25,35 The reduction profile for NiCr2O4 was more
complicated. Reduction peaks in the range from 300 to 650oC
could be associated with the reduction of Ni2+ and Cr3+ located
on the lattice of NiCr2O4 spinel.36,37 However, the reduction
profiles of ZnCr2O4 and MnCr2O4 exhibited very weak
reduction peaks, indicating only small amount of H2 is
consumed in the range of 100-900°C. The reduction peaks
Figure 2. FESEM images of different catalysts: (a) 3DOM-Co, (b)
3DOM-Ni, (c) 3DOM-Zn and (d) 3DOM-Mn.
4 | J. Name., 2012, 00, 1-3
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above 600°C of MnCr2O4 and ZnCr2O4 could be attributed to
the reduction of minor manganese and zinc oxides phase
which are not incorporated to the spinel lattice or Cr3+ located
on the lattice of MCr2O4 spinel and Cr2O3, respectively.25 By
quantitatively analyzing the reduction peaks of H2-TPR profiles,
the H2 consumption was calculated and listed in Table 1. It
could be seen that the H2 consumption for the 3DOM and bulk
catalysts varied in the same order of ZnCr2O4 < MnCr2O4 <
CoCr2O4 < NiCr2O4, which could be ascribed to the different
natures of metal atom M. Meanwhile, it could be also seen
that the 3DOM catalysts exhibited higher H2 consumption than
their bulk counterparts due to the higher surface area of the
3DOM catalysts providing more active sites and reducible
surface species. Furthermore, O2-TPD spectra in Figure 4B
presented two different desorption peaks. The desorption
peaks below 200oC could be attributed to the removal of
surface oxygen species while the desorption peaks above
600oC could be ascribed to the release of lattice oxygen. From
Table 1, it could be seen that the O2 desorption amount for the
3DOM and bulk catalysts also varied in the order of 3DOM-Mn
< 3DOM-Co < 3DOM-Zn < 3DOM-Ni, and BK-Ni < BK-Mn < BK-
Zn < BK-Co, respectively, indicating the different oxygen
supplying capability of different catalysts.
Figure 5. Comparative study of soot combustion efficiency
over bulk and 3DOM catalysts.
3.2 Catalytic performance
Soot combustion as a typical gas-solid-solid heterogeneous
catalytic reaction takes place at the three-phase contact points
among solid catalyst, solid reactant (soot) and gaseous
reactants (O2, NO), the contact between reactants and catalyst
is vital to determine the soot combustion efficiency. The blank
experiment without catalysts in Figure S5 demonstrated that
the Tm and Sm
for pure soot were 642oC and 55%,
CO2
respectively. In order to investigate the influence of pore size
and specific surface area on soot and catalyst contact
efficiency, a comparative study between bulk and 3DOM
catalysts was conducted as shown in Figure 5. Compared with
Figure 6. Influence of NO (A) and H2O (C, D) on soot
blank experiment, the Tm greatly decreased while the Sm
combustion efficiencies over 3DOM-Co catalyst, and influence
CO2
of NO oxidation on effluent NO and NO2 at 400°C (B).
increased to above 95% after introducing catalysts into soot
combustion. Based on the CO and CO2 concentration plots of
different catalysts as depicted in Figure 5, the Tm and Sm
CO2
Furthermore, it could be also seen that the metal atom M in the
MCr2O4 catalysts is essential in determining the catalytic activity.
The catalytic activity of bulk MCr2O4 catalysts followed the order of
BK-Mn ≈ BK-Ni < BK-Zn < BK-Co while the 3DOM catalysts followed
the slight different order of 3DOM-Mn < 3DOM-Zn < 3DOM-Ni <
3DOM-Co. It could be seen that the variation of A-site element
displayed the remarkable influence of the soot combustion
efficiency. Co was found to be the most favorable element for
improving the catalytic performance of spinel-type oxides. The
superior activity of CoCr2O4 than other catalysts could be ascribed
to the proper and moderate redox property to activate various
reactants. Moreover, many reports shows that Mn doped or
substituted some elements can improve catalytic performance for
soot combustion and decrease the temperature of soot oxidation
due to varied valence state of Mn ion.38-40 However, in our work,
the introduction of Mn element doesn’t show high catalytic activity,
were calculated and listed in Table 1. It could be clearly seen
that the 3DOM catalysts exhibited a remarkable shift of CO2
peak towards lower temperatures than their counterparts due
to the following two aspects. On one hand, the higher surface
area of 3DOM catalysts enlarged the contact area between
catalysts and soot particles, thus providing more active sites
for soot combustion. On the other hand, the well-defined
3DOM structures and interconnected networks of spherical
voids would allow soot particles to access more easily to the
active sites within the inner pore of the catalysts during the
soot combustion, which subsequently promoted the catalytic
activity.16 Meanwhile, it is noted that the 3DOM and bulk
catalysts demonstrated the similar selectivity of CO2
production, which could be attributed to that the selectivity of
CO2 may be more related with the intrinsic chemical nature of
the catalysts other than their pore structure.
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Table 1. Physicochemical parameters and catalytic soot combustion efficiencies of different catalysts.
Pore
O2 desorption
below 650°C
(mmol/g)
/
SBET
VP
DP
(nm)
Crystallite size
(nm)
H2 consumption
(mmol/g)
Tm
Sm
CO2
(%)
Catalysts
size
(nm)
/
/
/
190
/
200
/
190
/
250
(m2/g)
(cm3/g)
(oC)
Soot
BK-Co
/
/
/
14
/
/
642
450
439
420
486
427
473
453
487
459
55.0
96.7
96.4
97.4
97.0
98.5
95.5
95.7
95.0
96.6
40
97
62
18
65
34
62
15
37
0.17
0.19
0.14
0.05
0.14
0.16
0.15
0.05
0.09
32
20
19
23
25
35
15
31
21
3.19
4.40
4.86
4.53
5.64
0.11
0.83
1.68
1.85
0.05
0.07
0.08
0.02
0.12
0.04
0.11
0.03
BK-Co-500
3DOM-Co
BK-Ni
3DOM-Ni
BK-Zn
3DOM-Zn
BK-Mn
3DOM-Mn
6.5
5.3
5.7
5.1
12
5.9
5.2
6.7
0.04
which may be due to the integration of Mn into spinel catalyst could be observed that the Tm and Sm
for 3DOM-Co were
CO2
lattice.
450oC and 94.3% in the absence of NO and H2O while the Tm
The effect of feeding gas composition on soot combustion decreased to 438oC along with Sm of 94.4% in the presence
CO2
efficiencies was also investigated as displayed in Figure 6A. It of H2O. Compared with the above results, the Tm of above two
could be seen that the Tm and Sm for 3DOM-Co were 420oC situations in the presence of H2O was shifted down to 420 and
CO2
and 97.4% in the presence of NO while the Tm increased to 411oC with slight improved Sm
of 97.4% and 96.7%,
CO2
450oC and SmCO2 decreased to 94.3% in the absence of NO. The respectively. The results clearly demonstrated that the feed
experiments of 3DOM-Co catalyzing soot oxidation in the feed gas containing 10 vol.% H2O increased soot combustion
gas NO+O2 or NO indicate that the presence of NO has a activity regardless of the presence of NO or not, which could
obviously positive influence on the catalytic activity, and the be attributed to that H2O may have a role in facilitating the
Tm in NO+O2 is nearly 30°C lower than that only in O2. In decomposition of the highly stable carbon surface species
consideration of the absence of NO2 in feed, additional NO formed by partial oxidation of the reacting carbon surface by
oxidation tests were performed to further understand the NO2-attacking.43
promotion effect of NO2, in a comparative study with and
In the literature, the catalytic combustion of the coke is
without 3DOM-Co as shown in Figure 6B. The results revealed often correlated with the redox properties of the catalyst.38
that the intensity of the peak assigned to NO2 (1625, 1600 cm- The oxygen mobility over supports is considered to be an
1) obviously strengthen compared with that of the peak important factor in coke combustion. Shin et al. has
ascribed to NO (1905, 1844 cm-1) after feed gas passed demonstrated a good correlation between the oxygen mobility
through 3DOM-Co. This phenomenon directly manifested the of the supports and the combustion temperature of the coke
capacity of 3DOM-Co oxidizing NO into NO2. It is therefore deposited on supported Pt catalysts, suggesting that the
suggested that NO2 plays an important role during soot oxygen transfer over supports to coke was a rate determining
combustion as it is a much more effective oxidant than NO and step in the coke combustion process.44 In an attempt to
O2 to oxidize soot at lower temperature. Although NO is the correlate the catalytic activity with textural and structural
main species of NOx existing in diesel exhaust gas and NO2 is properties of the solids, we have represented the Tm and Sm
CO2
only in minor quantity, NO may firstly react with catalyst to versus pore size, BET surface area, H2 consumption and O2
generate a much stronger oxidative NO2, and then NO2 desorption data in Table 1. One can remark that there is no
participate in soot oxidation to accomplish the catalysis cycle satisfactory correlation between single chemical variable such
during soot combustion, which is in line with the NO2 assisted as H2-TPR, O2-TPD and catalytic performance available. In fact,
mechanism reported in the literatures.41,42
the 3DOM-Ni catalyst has the medium activity despite its most
On account of H2O existing in the actual diesel gas exhaust, consumed hydrogen in H2-TPR and released oxygen in O2-TPD.
the experiments of soot combustion over 3DOM-Co were also By considering the difference between soot combustion on
performed in the presence of 10 vol.% H2O with or without NO 3DOM catalyst and the coke oxidation over used supported Pt
in the feed gas, aiming to reveal the possible impact of H2O on catalyst, mass transport of soot towards inner active site is
the catalytic performance as shown in Figure 6C and 6D. It
probably vital for determining the overall activity as soot is in
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loose contact with 3DOM catalyst, while the influence of
contact between catalyst and coke deposited during propane
dehydrogenation is less significant compared to the that in
3DOM since the coke is much more strongly bounded on
catalyst surface in supported Pt catalyst. Very probably, soot
combustion is operating under the intermediate regime both
affected remarkably by the factors due to reaction and mass
transfer, unlike chemical nature is prevailing over supported Pt
catalysts.
Indeed, a good activity correlation with the pore size and
BET surface area could be established. For the catalyst pairs
prepared by two different methods, the higher the surface
area and pore size are, the higher the activity is. It is noted
that 3DOM samples with ordered pore structure and large
pore dimension constantly exhibited superior performance
compared with their bulk counterparts. The different activity
sequences of 3DOM-Ni and BK-Ni were noticed and this odd
performance could be related to the extremely small BET
surface area of BK-Ni sample. To further understand the
impact of specific surface areas and pore size on combustion
efficiency, additional bulk CoCr2O4 sample denoted as BK-Co-
500 with greater specific surface area was deliberately
prepared at lower calcination temperature of 500oC to focus
on the influence of pore structure. The results of BK-Co-500
catalyst characterized by BET, XRD, H2-TPR and O2-TPD and
tested by TPO technology are shown in Figure S6, S7 and Table
1. It can be observed in Figure S7 and Table 1 that 3DOM-Co
catalyst with smaller specific surface area but larger pore
dimension exhibited superior performance to BK-Co-500. Thus,
this result clearly demonstrated that the unique 3DOM pore
structure along with large pore dimension is even more
important than specific surface area in determining the
performance of soot combustion catalyst.
Figure 8. XRD patterns and FESEM images of 3DOM-Co over
fresh and used catalysts: (a) fresh and used 3DOM-Co catalyst,
(b) used 3DOM-Co catalyst.
The root cause of Co as the best choice of A-site cation
remained elusive so far in this study. This could be related to
the moderate redox properties associated with Co and Cr
interaction. Further understanding on the intrinsic nature of
cobalt in 3DOM-Co catalyst is currently under investigation. In
summary, as a complex redox catalytic process, efficiencies of
soot combustion reaction could be affected by many catalyst
structure features. Among them, improved contact imposed
by the unique 3DOM structure and enhanced surface area
along with the intrinsic nature of metal cation are considered
to be the critical factors for enhancing the catalytic efficiencies
of soot combustion.
To examine the durability, the 3DOM-Co catalyst was
recycled after each run of reactions by recharging the soot into
the used catalyst for the subsequent catalytic reaction under the
identical reaction conditions. As shown in Figure 7, the 3DOM-
Co could be used repetitively for at least 4 times without
appreciable deactivation especially the Tm and Sm
almost
CO2
remained unchanged. The excellent durability for the 3DOM-Co
catalyst could be attributed to the robust 3DOM crystalline
framework against the structural collapse, which have been
verified by the XRD pattern and FESEM image (see Figure 8)
since the crystal and morphological structure of the 3DOM-Co
nearly kept intact after the 4th recycling.
4. Conclusions
In summary, a series of spinel-type MCr2O4 (M = Co, Ni, Zn, Mn)
catalysts with and without 3DOM were successfully prepared
by colloidal crystal template and sol-gel methods, respectively.
The 3DOM MCr2O4 catalysts exhibited much higher catalytic
performance for soot combustion than their bulk counterparts
owning to their higher surface area and well-defined 3DOM
structure. Meanwhile, the pore structure, the nature of A-site
ion, feed compositions and H2O were found to have significant
impact on the catalytic activities of MCr2O4 for diesel soot
combustion. Furthermore, the 3DOM catalysts exhibited
strong durability due to the robust 3DOM framework against
the structural collapse. This work might provide deep insight
on the rational designing nano-catalysts for soot combustion
and other heterogeneous catalytic processes.
Figure 7. Stability test of 3DOM-Co catalyst.
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Acknowledgements
This work is supported by The Program for Professor of Special
Appointment (Eastern Scholar) at Shanghai Institutions of
Higher Learning, National Natural Science Foundation of China
(Grant 21443009), The Scientific Research Foundation for the
Returned Overseas Chinese Scholars from State Education
Ministry, and Natural Science Foundation of Shanghai City
(Grant 15ZR1419100).
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