Excellent low temperature performance for total benzene oxidation over mesoporous CoMnAl composited oxides from hydrotalcites

Article abstract of DOI:10.1039/c6ta02593e

Mesoporous CoMnAl mixed metal oxide catalysts with various Co/Mn atomic ratios have been obtained by calcination at 450 °C of layered double hydroxide (LDH) precursors prepared by the NH₄OH co-precipitation-hydrothermal method without distinct MnCO₃ peaks. The catalysts exhibited high efficiency for total oxidation of volatile organic compounds (VOCs). The physicochemical properties of the catalysts were characterized using several analytical techniques. Among them, CoMn₂AlO shows the optimal activity and the temperature required to achieve a benzene conversion of 90% (T₉₀) was about 238 °C, with a reaction rate and activity energy (E_a) of 0.24 mmol g_cat^-1 h^-1 and 65.77 kJ mol^-1 respectively. This temperature was 47 °C lower than that on the Co₃AlO sample with a lower reaction rate of 0.19 mmol g_cat^-1 h^-1 and a higher E_a 130.31 kJ mol^-1 at a high space velocity (SV = 60000 mL g^-1 h^-1). The effects of calcination temperature on the textural properties and catalytic activity of the CoMn₂AlO catalyst were further investigated. The as-prepared CoMn₂AlO-550 sample displayed superior catalytic activity, with T₉₀ at 208 °C, compared CoMn₂AlO-450. The formation of a solid solution with high surface area, rich oxygen vacancies, high Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios and low-temperature reducibility made a great contribution to the significant improvement of the catalytic activity.

Full text of DOI:10.1039/c6ta02593e

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Materials Chemistry A
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Excellent low temperature performance for total
benzene oxidation over mesoporous CoMnAl
composited oxides from hydrotalcites†
Cite this: DOI: 10.1039/c6ta02593e
Shengpeng Mo,ab Shuangde Li,a Wenhui Li,ab Jiaqi Li,ab Jiayuan Chenab
a
*
and Yunfa Chen
Mesoporous CoMnAl mixed metal oxide catalysts with various Co/Mn atomic ratios have been obtained by
calcination at 450 ꢀC of layered double hydroxide (LDH) precursors prepared by the NH4OH co-
precipitation–hydrothermal method without distinct MnCO3 peaks. The catalysts exhibited high
efficiency for total oxidation of volatile organic compounds (VOCs). The physicochemical properties of
the catalysts were characterized using several analytical techniques. Among them, CoMn2AlO shows the
optimal activity and the temperature required to achieve a benzene conversion of 90% (T90) was about
ꢁ1
238 ꢀC, with a reaction rate and activity energy (Ea) of 0.24 mmol gcat hꢁ1 and 65.77 kJ molꢁ1
respectively. This temperature was 47 ꢀC lower than that on the Co3AlO sample with a lower reaction
rate of 0.19 mmol gcat hꢁ1 and a higher Ea 130.31 kJ molꢁ1 at a high space velocity (SV ¼ 60 000 mL
ꢁ1
gꢁ1
h
ꢁ1). The effects of calcination temperature on the textural properties and catalytic activity of the
Received 29th March 2016
Accepted 9th May 2016
CoMn2AlO catalyst were further investigated. The as-prepared CoMn2AlO-550 sample displayed superior
catalytic activity, with T90 at 208 ꢀC, compared CoMn2AlO-450. The formation of a solid solution with
high surface area, rich oxygen vacancies, high Mn4+/Mn3+ and Co3+/Co2+ ratios and low-temperature
reducibility made a great contribution to the significant improvement of the catalytic activity.
DOI: 10.1039/c6ta02593e
www.rsc.org/MaterialsA
method for VOC abatement, due to its lower operating temper-
ature and less harmful reaction by-products than thermal
oxidation.5 Noble metals and transition metal oxides are
1. Introduction
Volatile organic compounds (VOCs) are one of the main air
pollutants emitted from industrial processes and the combus-
tion of fossil fuels; they may pollute the atmosphere directly or
indirectly as secondary pollutants, such as ozone formation and
photochemical smog.1 Indoor VOCs emitted from cement
concrete, furniture and paint also have become an increasingly
common concern over the past several years, as they cause bad
indoor air quality (IAQ) together with indoor microbes and
particulates which can be found at higher concentrations
indoors than outdoors.2,3 Lowering the concentration of VOCs
and their associated photochemical impacts on human health
has become a major issue.4 Therefore, it is highly desirable to
efficiently and cost-effectively remove hazardous VOCs. Many
technologies for VOC abatement (e.g. adsorption, thermal
oxidation and catalytic oxidation) have been developed in the last
few decades: catalytic oxidation is regarded as an effective
commonly used for the catalytic oxidation of VOCs. Though
supported noble metal catalysts (Pt, Pd, Au and Ag) exhibit higher
activity for deep oxidation of VOCs at very low temperatures, their
high cost, easy sintering and coking, and susceptibility to
poisoning have limited their wide and abundant usage. The key
issue is the availability of a low-cost catalyst with high activity in
catalytic oxidation technology. Thus, substantial efforts are
currently being made to develop transition metal oxide catalysts
(Co3O4, NiO, Cr2O3, CuO and MnO2) which may be expected to be
better alternatives to the precious metal catalysts.
Previous studies revealed that mixed metal oxides (e.g. Mn–
Co–O,6,7 Mn–Ce–O,8,9 Mn–Ni–O,10 Co–Ce–O,11,12
)
possess
improved catalytic activities compared to single-component
materials due to synergistic effects. There are frequently weaker
interactions between mixed metal oxides, which decrease their
catalytic activity and stability, and their resistance to water
vapor. Thus, there are still challenges and opportunities in
developing synergistic effects for mixed metal oxide catalysts
with high catalytic activity. Recently, many reports have
contributed to the synthesis, characterization, and catalytic
applications of mixed oxides produced by thermal decomposi-
tion of layered double hydroxides (LDHs), such as Mn–Al,13 Cu–
Co–Al,14 and Co–Mg–Al.15
aState Key Laboratory of Multi-phase Complex Systems, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing, 100190, China. E-mail: yfchen@
mail.ipe.ac.cn; chenyf@ipe.ac.cn; Fax: +86 10 8254 4896; Tel: +86 10 8254 4896
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
† Electronic supplementary information (ESI) available: SEM image, element
distribution and EDX data of the Co3AlO, Co2MnAlO and Mn3AlO sample, CO2
conversion and analysis of exhausted gas. See DOI: 10.1039/c6ta02593e
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Layered double hydroxides, known as hydrotalcite-like Firstly, a solution of Co(NO3)2$6H2O, Mn(NO3)2 (Co2+ + Mn2+
compounds or anionic clays, are a class of two-dimensional (2D) 15 mmol) and Al(NO3)3$9H2O (5 mmol) dissolved in 70 mL of
¼
layered inorganic materials, which can be represented by the deionized water was produced. Secondly, NH4OH (water solu-
general chemical formula [M1ꢁx2+Mx3+(OH)2](Anꢁ
)x/n$mH2O, tion 25%) was added dropwise to the mixed nitrate salt solution
where M2+ and M3+ represent metallic cations, Anꢁ indicates with constant agitation, keeping at a constant pH value of 9.0.
interlayer anions such as NO3ꢁ or CO32ꢁ, x is the molar ratio of Thirdly, the obtained precipitate was transferred to a Teon
M3+/(M2+ + M3+) and m represents the amount of water.16,17
A
autoclave at 120 C for 6 h. Subsequently, the obtained precip-
ꢀ
unique structural characteristic of LDH materials is that the M2+ itate was centrifuged,ꢀwashed with deionized water and ethanol,
and M3+ are distributed in an ordered and uniform manner in and then dried at 80 C overnight.
brucite-like layers. Mixed oxide catalysts obtained by controlled
thermal decomposition of LDHs have two advantages: (1) they
are effective supports for the immobilization of catalytically
2.2 Preparation of Co3ꢁxMnxAlO samples
active species on the surface and (2) the uniform dispersion of The Co3ꢁxMnxAl LDHs samples wꢁer1e calcined in air at 450 ꢀC for
M2+ and M3+ cations in the layers can act as excellent solid base 4 h at a heating rate of 2 C min (denoted as Co3ꢁxMnxAlO).
ꢀ
catalysts. Co–Mn–Al mixed oxides with high specic surface The effect of calcination in air at different temperatures (350 or
areas synthesized using co-precipitation techniques improved 550 ꢀC) on the catalyst performance was then investigated
the catalytic activity towards toluene oxidation, and can be an (resulting samples denoted as Co3ꢁxMnxAlO-350, Co3ꢁxMnxAlO-
excellent alternative to noble metal catalysts.18 Forming hybrid 550, respectively).
oxide catalysts with hierarchical structure from the calcination
of their LDH precursors offers the benets of synergistic effects
between the metal oxide materials and further enhances the
2.3 Preparation of monolithic CoMnAlO lm catalyst
catalytic activity and stability. Consequently, the rational design CoMnAl LDH lm was prepared by in situ crystallization on
and controllable preparation of mixed metal oxide catalysts a pure aluminum substrate. In a typical procedure, 10 mmol of
based on LDH materials with simultaneously enhanced activity Mn(NO3)2 and 5 mmol of Co(NO3)2$6H2O were dissolved in
and stability is of great interest.17
deionized water to form a clear solution with a total volume of
The use of LDH powdered catalysts presents problems 70 mL, the pH value was adjusted to 9.0 by adding diluted
because of mass/heat transfer resistance, as well as the difficulty ammonia (25 wt% NH4OH). Three pieces of Al substrate (4 ꢂ 15
in recycling. An approach of growing an LDH support on cm) were rolled into a tubular shape and immersed vertically in
aluminum foil19 has been attracting considerable attention: it the solution, and were then cleaned with acetone, ethanol and
affords a structured catalyst with excellent mass ow properties deionized water in sequence before use. They were transferred
ꢀ
and recyclability, as well as high thermal conductivity. to a Teon autoclave placed in an oven at 120 C for 12 h. The
Kovanda20 synthesized layers of Co–Mn–Al mixed oxides sup- substrates were taken out of the autoclave, rinsed with deion-
ported on anodized aluminum foil which showed slightly ized water and dried at room temperature. Then the as-prepared
ꢁ1
ꢀ
ꢀ
higher specic activity in the total oxidation of ethanol. We CoMnAl LDH lm was heated at 550 C for 4 h at 2 C min
previously fabricated a monolithic CuCoAl MMO lm catalyst (denoted as CoMnAlO lm).
by an in situ growth–calcination method, which displayed
comparable catalytic activity to CuxCo3ꢁxAl MMO powder for
2.4 Characterization of the catalysts
the catalytic oxidation of benzene.14 In the present work, we
consider that Mn and Co mixed oxides are valuable materials in The powder X-ray diffraction (XRD) patterns were measured on
oxidation reactions due to their ability to exist in multiple a Panalytical X'Pert PRO system within the range of scattering
oxidation states and optimum oxygen storage capacity. To angle 2q of 5–90ꢀ. N2 adsorption/desorption isotherms were
investigate systematically the effect of Co (or Mn) substitution, measured via N2 adsorption at 77 K on an automatic surface
we have prepared CoMnAlO powdered mixed metal oxide analyzer (SSA-7300, China), and the BET specic surface areas
catalysts derived from LDHs with high synergistic effect, which and pore size distributions were obtained according to the
signicantly promoted the activity towards benzene combus- Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda
tion, and are resistant to large amounts of water vapor. More- (BJH) methods, respectively. The morphology and microstruc-
over, we have successfully fabricated a monolithic highly tures were recorded on a scanning electron microscope (SEM,
dispersed CoMnAlO lm catalyst by an in situ growth–calcina- JEOL JSM-6700F, Japan, 15 kV, 10 mM) and transmission elec-
tion method with outstanding performance to replace powder tron microscope (TEM, JEOL JEM-2010F, 200 kV). Hydrogen
samples.
temperature programmed reduction (H2-TPR) with 20 mg
catalyst (40–60 mesh) was carried out in a U-shaped quartz
reactor under a gas ow (10% H2 balanced with Ar, 25 mL
minꢁ1) at a constant rate of 10 ꢀC minꢁ1. Surface species of the
as-prepared catalysts were determined by X-ray photoelectron
2. Experimental
2.1 Synthesis of Co3ꢁxMnxAl LDHs
The synthesis of LDH precursor samples (with Co/Mn molar spectroscopy (XPS) using an XLESCALAB 250Xi electron spec-
ratios of 3/0, 2/1, 1.5/1.5, 1/2, 0/3 and M2+/M3+ molar ratio of 3/1) trometer from VG Scientic with monochromatic Al Ka
was based on the co-precipitation–hydrothermal method. radiation.
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2.5 Catalytic activity measurement
The catalytic activity was evaluated in a continuous-xed-bed
quartz microreactor (4 ¼ 6 mm) at a space velocity (SV) of 60 000
mL gꢁ1 hꢁ1. The catalysts (100 mg, 40–60 mesh) mixed with 200
mg of quartz sand (40–60 mesh) were loaded in the quartz
reactor with quartz wool packed at both ends of the catalyst bed.
The reactant gas was composed of 100 ppm gaseous benzene
balanced with air (20% O2 + balance N2), which was purged into
the reactor at a continuous ow of 100 mL minꢁ1. The
concentration of benzene was analyzed using a gas chromato-
graph (Shimadzu GC-2014) equipped with a ame ionization
detector (FID) and the concentration of CO2 in the outlet gas
was detected by another FID with a conversion furnace for
converting CO2 to CH4. The catalytic evaluation of the CoMnAlO
lm catalyst on the Al substrate was performed on pre-tailored
fragments (1 ꢂ 5 cm) which were rolled into a tubular shape.
The complete conversion value of benzene (hbenzene) was
calculated according to the following equations:
Fig. 1 (A and B) (a–e) XRD patterns of Mn3Al, CoMn2Al, Co1.5Mn1.5Al,
Co2MnAl, Co3Al LDH precursors; (C and D) (a–g) XRD patterns of
Mn3AlO, CoMn2AlO, Co1.5Mn1.5AlO, Co2MnAlO, Co3AlO, CoMn2AlO-
350 and CoMn2AlO-550 catalysts.
Cbenzene;in ꢁ Cbenzene;out
hbenzene
¼
ꢂ 100
Cbenzene;in
for Mn2+, respectively) in an octahedral environment.23 The XRD
C
CO2;out
hCO
¼
ꢂ 100
2
results also indicate that the single phase of the NO3ꢁ/Co3ꢁx
-
6 ꢂ Cbenzene;in
MnxAl LDHs can be synthesized by the NH4OH co-precipitation
method without any distinct peaks corresponding to the MnCO3
(rhodochrosite) admixture commonly produced with the NaOH
and Na2CO3/K2CO3 co-precipitation method.15
where Cbenzene,in (ppm), Cbenzene,out (ppm) and CCO ,out (ppm) are
the concentrations of benzene in the inlet and outlet gases, and
CO2 in the outlet gas, respectively.
To investigate the effect of water vapor on the catalytic
activity, the on-stream benzene oxidation experiment was
carried out in the presence and absence of 7.2 vol% water vapor.
Typically, an air ow containing gaseous benzene (100 mL
minꢁ1) was used for bubbling through water in a 500 mL conical
ask at 40 ꢀC to give a water vapor amount of 7.2 vol%, and
benzene (100 ppm) was used for catalytic testing.
2
ꢀ
Aer calcination in air at 450 C, the LDH precursors were
completely transformed to a mixture of oxide and spinel oxide
phases. The XRD patterns of Co3ꢁxMnxAlO are shown in Fig. 1(C
and D). It can be clearly seen that the positions and relative
intensities of the main diffraction peaks of the samples are
rather similar. The strongest reection at 2q ¼ 36.8ꢀ corresponds
to a Co3O4 spinel phase (PDF card no. 42-1467) in Fig. 1(B)(e),
and the Co3AlO sample is observed with other weak diffraction
peaks located at 31.3ꢀ, 44.9ꢀ, 59.4ꢀ and 65.4ꢀ, corresponding to
the (220), (400), (511) and (440) planes, respectively. Further-
more, the main diffraction peaks of the Co3AlO sample are
rather similar to those of other spinel oxide phases (Co2AlO4,
PDF card no. 38-0814; CoAl2O4, PDF card no. 44-0160), which are
hard to distinguish by X-ray diffraction.24,25 The formation of
Co3O4 is due to the easy oxidation potential of Co2+ ions and the
thermodynamic stability of Co3O4, higher than that of CoO in
air.18 As the cobalt content is decreased, the intensities of the
Co3O4 peaks decrease and the peaks become weak. The XRD
pattern of Mn3AlO (Fig. 1(C)(b)) shows complex diffraction
peaks, which are ascribed to a mixture of lower-crystalline
Mn5O8 (JCPDS 18-0801) and Mn3O4 (JCPDS 02-1062). Aer
addition of cobalt species into MnAl precursors, a typical
diffraction peak of their mixed oxides around 36.5ꢀ appears
which may be ascribed to the formation of cobalt manganese
oxide (JCPDS 018-0410) with spinel structure or a new cobalt
aluminum manganese oxide (CoMnAlO4, JCPDS 036-0269)
homogeneous crystal phase. The XRD patterns of CoMn2AlO
samples at different calcination temperatures are shown in
3. Results and discussion
3.1 Textural study of the precursors and catalysts
The XRD patterns of Co3ꢁxMnxAl LDHs precursors are shown in
Fig. 1(A and B). In the material prepared by co-precipitation, the
XRD patterns clearly exhibit the characteristic reections of
LDH materials with a series of (00l) peaks, indicating the
generation ofꢁa layered structure with 3R symmetry. In the case
of using NO3 as a guest anion, the lattice parameter a ¼ 2 ꢂ
d(110) around 2q ¼ 58.5ꢀ, intense diffraction peaks (d(003) ¼
0.86 nm and d(006) ¼ 0.44 nm) were observed with Mn3Al LDH
structure, while c ¼ 3 ꢂ d(003) around 2q ¼ 10.3ꢀ, which is
a function of the interlayer distance.21 For Co3Al LDH, diffrac-
tion peaks located at 10.8ꢀ and 21.6ꢀ are observed, corre-
sponding to the (003) and (006) planes, respectively.22 In
addition, cobalt ammine azide formation is possible even if
NH4OH solution was employed during the preparation. As
shown in Fig. 1(B), the low angle shi of the peaks indicates that
the increase in the lattice distance results from doping with
2+
Mn2+ which possesses a large radius (0.74 A for Co vs. 0.83 A
˚
˚
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Fig. 1(D)(b, f and g), the intensities of the peaks increase with
increasing calcination temperature. When the calcination
temperature is increase to 550 ꢀC, the formation of a solid
solution can obviously improve the catalytic activities due to the
synergistic effect of the components.26 The average crystalline
sizes calculated by using the Scherrer equation are listed in
Table 1. With increasing the Co species into the MnAl lamellar
structure, the grain size of crystallites is signicantly increased.
The size of CoMn2Al is 14.6 nm which isꢀsmallest among other
samples prepared by calcination at 450 C. This result further
indicates that the formation of a solid solution between Co and
Mn oxides indeed can decrease the size of crystallites compared
to the single Co3AlO (24.9 nm). Small-sized particles will produce
more crystal defects which are benecial to catalytic activity.
Fig. 2(A) displays the N2 adsorption/desorption isotherms
and the pore size distribution patterns for the Co3ꢁxMnxAlO
catalysts. The isotherms are consistent with type IV isotherms
with a H2-type hysteresis loop in the relative pressure range of
0.4–1.0, which are associated with capillary condensation
taking place in mesopores and conrmed by the distribution of
pore size in Fig. 2(B). The surface area, pore volume and pore
diameter of all the samples are shown in Table 1. When Co2+ is
incorporated more efficiently into the layered structure, they
Fig. 2 (A) N2 adsorption/desorption isotherms and (B) pore-size
distributions for the Co3ꢁxMnxAlO catalysts.
with high aspect ratio akes. The as-prepared Mn3Al LDH
product is of higher quality in terms of morphology, size,
uniformity, and crystallinity, which is much superior to those of
Co3Al LDH. These results are consistent with the X-ray diffrac-
tion peak intensity between Co3Al LDH and Mn3Al LDH
samples. As shown in Fig. 3(B), the particle size of CoMn2Al
LDH is slightly different from the samples including alternating
Co and Mn metal ions, indicating that when Co2+ is incorpo-
rated efficiently into the layered structure, the morphology and
size of particles are changed. The SEM images of CoMn2AlO
aer calcination in air at different temperatures are shown in
Fig. 3(D–F). The morphology of CoMn2Al LDH can maintain
a relatively complete shape aer calcination in air at 350 ꢀC,
have larger surface areas around 120–130 m2 gꢁ1 for Co3ꢁx
-
MnxAlO catalysts compared with those of Co3AlO and Mn3AlO
catalysts (about 84.63 and 36.77 m2 gꢁ1). It is noticeable that the
average pore size distributions become narrower for CoMn2AlO
with 7.1 nm than other samples with 8–24 nm. The accessibility
of nitrogen gas to the particle interior varied depending upon
the particle size. For large-sized particles, nitrogen gas cannot
easily access the active sites of the particles with a low surface
area.27,28 These results indicate that Co2+ substitution in the
brucite layer leads to changes in the surface properties of
Mn3AlO and can lead to improvements in the catalytic activity
for benzene oxidation. The effect of calcination temperature on
CoMn2AlO samples was also examined. The surface area grad-
ually decreased from 143.8, 128.0 to 102.4 m2 gꢁ1, together with
increased pore size distribution, ꢀwith enhanced calcination
temperature from 350, 450 to 550 C.
3.2 Morphology, microstructure and element distribution
Fig. 3 (A–C) SEM images of Co3Al, CoMn2Al and Mn3Al LDHs and (D–
F) SEM images of the CoMn2AlO samples calcined at different
The SEM pictures of the Co3ꢁxMnxAl LDHs particles are shown
in Fig. 3. These shapes approximate to thin hexagonal platelets temperatures (350, 450 and 550 ꢀC).
Table 1 Physical characterization, catalytic activities, Ea values and surface elemental compositions of the samples
BET surface
area (m2 gꢁ1
Pore diameter Total pore volume
Ea
XPS
Co/Mn
Sample
)
(nm)
(cm3 gꢁ1
)
D (nm) T10 (ꢀC) T50 (ꢀC) T90 (ꢀC) (kJ molꢁ1
)
EDX
Co3AlO
Co2MnAlO
Co1.5Mn1.5AlO 127.46
CoMn2AlO
Mn3AlO
CoMn2AlO-350 143.81
CoMn2AlO-550 102.43
CoMnAlO lm
84.63
121.12
10.7
8.1
9.0
7.1
24.2
9.6
0.226
0.246
0.287
0.196
0.222
0.346
0.328
0.037
24.9
21.6
18.1
14.6
22.1
13.8
14.8
—
216
193
178
159
184
156
142
152
250
232
220
206
224
199
185
205
285
268
250
238
260
227
208
240
130.31
96.03
81.91
65.77
93.07
—
Co/Al ¼ 2.86
—
1.87
1.02
—
—
128.04
36.77
Co/Mn ¼ 0.72 0.47
Mn/Al ¼ 2.16
—
—
—
—
0.60
0.43
—
12.8
6.7
—
—
21.95
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while the structures of the other two samples (at 450 ꢀC and 550 that of bulk Co3O4, and CoAl2O4 spinel gives the reduction peak
ꢀC) have decomposed to many ne particles. at temperatures as high as >800 ꢀC.31 For the Co3AlO catalyst, it
The microstructures of different oxides are shown in Fig. 4. is observed that there are three separate reduction peaks with
The TEM images of the catalysts indicate the layered structure the maximum rates of H2 consumption occurring at 255, 360
ꢀ
of hydrotalcite composed of a number of tiny particles, these and 712 C, respectively. Obviously, the low-temperature peak
catalysts present attened surfaces and embedded pores in the (150–400 ꢀC) consists of two peaks representing the reduction of
plate matrix. In addition, many plates have been broken into Co(III) to Co(II) and Co(II) to Co0.29 The third separate reduction
smaller pieces. From the high-resolution TEM image (Fig. 4(D)) peak is ascribed to the reduction ofꢀthe CoO or cobalt alumi-
of the Co3AlO catalyst, the surface lattice spacing is measured to nates at high temperature (500–800 C),32 caused by the diffu-
be 0.29 nm corresponding to the (220) crystal plane of Co3O4 sion of Co2+ ions into the Al2O3 support where they may occupy
spinel phase. The HRTEM image of the CoMn2AlO catalyst tetrahedral or octahedral lattice sites.33 The Mn3AlO sample is
exposes two identied fringes with lattice spacings of about 0.29 reduced in two main temperature regions and the reduction of
nm and 0.48 nm, rather close to the (220) crystal phase of the manganese oxides can be described by the successive reduction
Co3O4 spinel phase with good crystallization and the (101) plane processes: MnO2 / Mn2O3 / Mn3O4 / MnO.26,34 The
of the Mn3O4 phase with slightly poor crystallization. The maximum of the rst one (200–350 ꢀC) is around 314 ꢀC, which
microstructures demonstrate that the CoMnAl composite oxide can be attributed to the reduction of MnO2 to Mn3O4. The
is formed simultaneously during the calcination, which is in second reduction peak (350–500 ꢀC) is ascribed to the reduction
agreement with XRD results. In can therefore be stated that the of Mn3O4 to MnO. Except for the Mn3AlO catalyst, the reduction
calcination has a great impact on the as-prepared samples and peaks of the other catalysts are rather similar to the reduction
is advantageous for the formation of a solid solution.
peak of the Co3AlO catalyst. It is obvious that the peaks in the
To obtain information about the element dispersions, the range 150–500 ꢀC move to lower temperatures, but the third
samples (Co3AlO, CoMn2AlO and Mn3AlO) were tested by SEM– peak shis to higher temperature with increasing the content of
EDX analysis (Fig. S1–S3†). The EDX mapping of samples cobalt species (Fig. 5(A)). The reduction of catalyst is becoming
demonstrates uniform distributions of various elements (O, Al, easier and the strong interaction of Co and Mn species
Mn or Co) in the particle region; the related peaks can be seen improves the low temperature reducibility of the Co3ꢁxMnxAlO
clearly in the spectra and the proportions of the various catalysts. From the reduction peaks (150–500 ꢀC), the Co1.5
-
elements are listed. The dispersion of Co and Mn further Mn1.5AlO and CoMn2AlO catalysts exhibit lower reduction
conrms that the results are in agreement with the XRD and temperature and higher peak intensity, indicating the improved
TEM analysis.
higher catalytic activity towards VOC oxidation.
Fig. 5(B) presents the H2-TPR proles of the CoMn2AlO
samples calcined at different temperatures (350, 450 and 550
ꢀC). The CoMn2AlO-350 catalyst exhibited clear peaks in the
lower temperature zone, which can be attributed to the reduc-
tion of higher metal ions. However, the reduction temperatures
of the CoMn2AlO-550 sample slightly shi to higher values,
which means an increase of oxygen mobility on the catalyst. The
oxygen mobility of the catalyst will be an important factor for
the catalytic oxidation of benzene.
3.3 Temperature-programmed reduction
The reduction behavior and the interaction of various catalysts
were examined by using the temperature programmed reduc-
tion (TPR) technique in the temperature range from 100 to 800
ꢀC (Fig. 5). Some researchers suggested that the low tempera-
ture peak is ascribed to the two-step reduction of relatively large
crystalline Co3O4 particles.29 The reduction of Co3O4 usually
proceeds via the sequence Co3O4 / CoO / Co0.30 The reduc-
tion temperature of Co3O4 supported on g-Al2O3 is higher than
3.4 X-ray photoelectron spectra (XPS)
The surface elemental compositions and the oxidation state of
surface species were investigated by X-ray photoelectron
Fig. 4 (A–C) TEM images of the Co3AlO, CoMn2AlO (450 ꢀC) and Fig. 5 (A) H2-TPR profiles of Co3ꢁxMnxAlO catalysts and (B) H2-TPR
Mn3AlO samples; (D–F) the HRTEM images corresponding to A–C, profiles of CoMn2AlO calcined at different temperature (350, 450 and
respectively.
550 ꢀC).
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spectroscopy (XPS). The Mn 2p, Co 2p and O 1s XPS spectra of ascribed to Co3+ in octahedral sites, and the binding energy at
the Co3ꢁxMnxAlO catalysts are shown in Fig. 6. As shown in 781.8 eV and the satellite structure are ascribed to Co2+ in
Fig. 6(A), it can be seen that the peaks corresponding to Mn 2p tetrahedral sites.39 As summarized in Table 2, the surface Co3+
/
XPS of all samples are asymmetric toward the high binding Co2+ ratios are calculated from the areas by quantitatively
energy side, and can be broken down into three components. analyzing the XPS spectra of the samples. From the areas of the
According to the literature,35 the absence of a satellite peak at +5 different spin–orbit doublets, the surface Co3+/Co2+ ratio tends
eV from the Mn 2p3/2 peak suggests that no Mn2+ is present. to increase with decreasing cobalt. The calcination temperature
Hence these three components corresponding to binding of catalysts increases the distribution of surface Co3+ species.
energies of 641.2, 642.5 and 644.1 eV can be assigned to the According the TPR proles of the catalysts, the reduction of
surface Mn3+ and Mn4+ species and to the satellite of Mn3+ higher oxidation states is preferred to take place at lower
species, respectively.36,37 Moreover, the presence of the two Mn temperature.
species (Mn4+ and Mn3+) is in agreement with the H2
The XPS spectra of O 1s are shown in Fig. 6(E and F). The O 1s
consumption peaks in the TPR results. As summarized in Table peak is curve-tted with two components at binding energy
2, one can obtain the surface Mn4+/Mn3+ atomic ratios by values of 530.1 and 531.7 eV. The former is due to the surface
quantitatively analyzing the XPS spectra of the samples. The lattice oxygen (Olatt) species, whereas the latter is due to the
appropriate doping cobalt method has a remarkable inuence surface adsorbed oxygen (Oads) species.40 The surface molar
on the surface Mn4+/Mn3+ atomic ratio which increased in the percentages of Oads/Olatt are summarized in Table 2. The surface
following order: CoMn2AlO > Co1.5Mn1.5AlO > Mn3AlO ¼ Co2-
Oads/Olatt molar ratio (0.554) of the CoMn2AlO-550 catalyst is
MnAlO. Apparently, the calcination temperature of catalysts has much highest than those of the other samples, indicating that
a great impact on the distribution of surface higher Mn4+ the catalyst has a greater amount of electrophilic oxygen species
species, increased apparently with increasing the calcination which can be benecial for the enhancement in catalytic
temperature (Fig. 6(B)).
performance for deep oxidation reactions.
As shown in Fig. 6(C and D), Co 2p XPS spectra are tted
assuming a theoretical value of 2 for the ratio of Co 2p3/2 and
2p1/2 and a spin–orbit splitting of 15.2 eV. Generally, the spectra
3.5 Evaluation of the catalytic behavior
can be broken down into two main components together with The catalytic performances of the Co3ꢁxMnxAlO catalysts shown
four shake-up satellites.38 The binding energy at 780.2 eV is in Fig. 7(A) were evaluated using the temperatures of 10% (T10),
50% (T50), and 90% (T90) benzene conversion at a space velocity
(SV) of 60 000 mL gꢁ1 hꢁ1. As summarized in Table 1, the order
of activity is concluded to be CoMn2AlO > Co1.5Mn1.5AlO >
Mn3AlO > Co2MnAlO > Co3AlO. Apparently, the CoMn2AlO
catalyst is the most active among all the catalysts achieving T90
benzene conversion at 238 ꢀC, while the Mn3AlO and Co3AlO
catalysts exhibit relatively low activity with only 71% and 35%
oxidation of benzene. The reaction rate at T90 ¼ 238 ꢀC on
CoMn2AlO is about 0.24 mmol gcatꢁ1 hꢁ1 which is clearly greater
ꢁ1
than that of Mn3AlO with 0.19 mmol gcat hꢁ1, and Co3AlO
ꢁ1
with 0.09 mmol gcat hꢁ1. The CO2 selectivity was more than
99.0% (Fig. S4†). The off-gas was collected using the organic
solvent methanol (GC) at different reaction temperatures, and
characterized by mass spectrometry (Shimadzu GCMS-QP2010)
(Fig. S5†). There was no detection of products of incomplete
oxidation, indicating that benzene can be completely oxidized
over these catalysts. These results indicate the strong interac-
tion of Co and Mn species obviously promotes the catalytic
activities of the catalysts compared with Mn3AlO and Co3AlO
catalysts aer the formation of Co, Mn and Al mixed oxides.
The catalytic activities of the CoMn2AlO catalysts with
different calcination temperature (350, 450 and 550 ꢀC) were
also characterized, as shown in Fig. 7(B). It is obvious that the
CoMn2AlO-550 catalyst exhibits the highest catalytic activity
among all catalysts, achieving complete benzene conversion,
ꢀ
and the T50 and T90 values were 185 and 208 C which were 21
and 30 ꢀC lower than those achieved over the CoMn2AlO-450
catalyst. It is worth pointing out that the catalytic performance
over the CoMn2AlO-350 catalyst is also higher than that over the
CoMn2AlO-450 catalyst. Meanwhile, the catalytic performance
Fig. 6 The Mn 2p, Co 2p and O 1s XPS spectra of the Co3ꢁxMnxAlO
catalysts.
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Table 2 Surface compositions and oxidation states
Mn 2p
Co 2p
Co3+
O 1s
Catalysts
Mn3+
Mn4+
Mn4+/Mn3+
Co2+
Co3+/Co2+
Olatt
Oads
Oads/Olatt
Co3AlO
—
—
—
0.514
0.526
0.556
0.568
—
0.558
0.579
0.486
0.474
0.443
0.432
—
0.442
0.421
1.06
1.11
1.25
1.31
—
1.26
1.37
0.728
0.732
0.725
0.703
0.709
0.671
0.708
0.272
0.268
0.275
0.297
0.291
0.329
0.179
0.41
0.37
0.38
0.42
0.37
0.49
0.55
Co2MnAlO
Co1.5Mn1.5AlO
CoMn2AlO
Mn3AlO
CoMn2AlO-350
CoMn2AlO-550
0.492
0.485
0.452
0.492
0.431
0.471
0.508
0.515
0.548
0.508
0.569
0.530
1.03
1.06
1.21
1.03
1.31
1.12
follows rst-order and zero-order kinetics with respect to the
benzene concentration and oxygen concentration, respec-
tively.46 It is reasonable to assume that in the case of excess
oxygen the reaction should obey rst-order kinetics, and the
equations are as follows:
NC H ꢂ hC H
6
6
6
6
gC H
¼
6
6
Wcat
ꢀ
ꢁ
ꢂꢃ
Ea
gC H ¼ ꢁkc ¼ ꢁ A exp ꢁ
c
6
6
RT
where NC H is the C6H6 gas ow rate (mol sꢁ1), Wcat is the
6
6
catalyst weight (g), and g, k, A, and Ea are the reaction rate (mmol
gꢁ1 sꢁ1), rate constant (sꢁ1), pre-exponential factor, and
apparent activation energy (kJ molꢁ1), respectively. The k values
can be calculated from the reaction rates and reactant
conversions.
Fig. 7 (A) Benzene conversion as a function of reaction temperature
over Co3ꢁxMnxAlO catalysts; (B) benzene conversion over CoMn2AlO
catalysts calcined at different temperatures (350, 450 and 550 ꢀC); (C)
Arrhenius plots for the oxidation of benzene over the Co3ꢁxMnxAlO
catalysts; (D) effect of water vapor on the activities of CoMn2AlO-550
catalysts at 190 ꢀC and 230 ꢀC. Benzene concentration ¼ 100 ppm,
The Arrhenius plots for the oxidation of benzene at
a benzene conversion <20% are displayed in Fig. 7(C). The plots
of ln(g ꢂ 105) versus 1000/T perform excellent linear relation-
ships with the correlation coefficients (R2) were rather close to 1
and the activation energies (Ea) based on the slopes of the
Arrhenius plots are listed in Table 1. The results indicate that
the Ea values decreased in the following order: Co3AlO > Co2-
MnAlO > Mn3AlO > Co1.5Mn1.5AlO > CoMn2AlO, which conrms
the promoting effect of Co species.
To examine the stability and effect of water vapor on the
catalytic performances of the CoMn2AlO-550 catalyst, the on-
stream reactions at different temperatures (190 and 230 ꢀC)
were carried out in the absence and presence of 7.2 vol% water
vapor as shown in Fig. 7(D). When 7.2 vol% H2O was introduced
into the stream at 190 ꢀC, the benzene conversion decreased
sharply from 57.8 to 41.5% within 8 h. When the water vapor
was cut off, the benzene conversion at 190 ꢀC nearly recovered.
Then the on-stream reaction was continued, adjusting the
temperature from 190 ꢀC to 230 ꢀC. The benzene conversion was
unchanged with 99.8% with or without the water vapor. These
results suggest that in spite of the competitive adsorption of
water and benzene molecules on the active sites, the CoMn2AlO-
550 catalyst can tolerate the presence of water vapor and
maintain excellent catalytic activity at high temperature. The
catalyst with stronger oxygen mobility can more easily activate
water concentration ¼ 7.2 vol% and SV ¼ 60 000 mL gꢁ1 hꢁ1
.
of the CoMn2AlO-550 catalyst is similar to the three-dimen-
sional manganese oxides prepared by the oxalate route with T90
around 209 ꢀC, probably due to 2D mesoporous metal oxides
being convenient for rapid and homogeneous oxygen diffusion
throughout the whole surface of the catalysts, which can
provide many active sites for catalytic reactions.27,41 Obviously,
the catalytic activity over the CoMn2AlO-550 catalyst is much
better than other reported literature,9,42–45 such as MnCe(18 : 1)/
Al-PILC (T90 ¼ 290 ꢀC), Mn/Al-PILC (T90 ¼ 340 ꢀC), Co5AlO (T90
¼
300 ꢀC) Co/CeO2/SBA-15 (T90 ¼ 265 ꢀC), CeO2–MnOx (T90 ¼ 375
ꢀ
ꢀC) and 0.5% Pt/Al2O3 (T90 ¼ 260 C). Obviously, the relatively
lower reaction temperature among the reported catalysts makes
CoMnAlO a promising catalyst for benzene combustion.
The catalytic performance can also be evaluated by
comparing the Ea values of different catalysts, and the sample
with lower Ea value will exhibit superior catalytic activity. The
catalytic oxidation of benzene in the presence of excess oxygen
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oxygen molecules at a relatively higher temperature, where the demonstrate the electronic transfer between cobalt and
adsorption of oxygen is stronger than that of water.47
manganese to provide a redox mechanism: Co3+–Mn3+ 4 Co2+–
It is noticeable that the catalytic activity of a transition metal Mn4+ which is critical to catalytic reaction aer the formation of
oxide is associated with several factors, such as the distribution Co and Mn mixed oxides.
of surface species, oxygen vacancies, reducibility, surface area,
and morphology. A catalyst with a higher surface area should
show better catalytic activity. Thus, the CoMn2AlO-350 catalyst
3.6 The structure and catalytic performance of the
monolithic CoMnAlO lm catalyst
exhibited higher activity at lower temperature than the
Taking into account the practical application of catalysts, the
CoMn2AlO-450 catalyst due to its higher surface area. However,
monolithic catalyst on the Al substrate was used to replace
it seems that the CoMn2AlO-550 catalyst with slightly lower
powder samples with better heat conduction. As we can see, the
CoMnAl LDH lm crystallized as thin curved and hydrotalcite-
like platelets with the ab plane in a perpendicular orientation to
the support (Fig. 8(D)), and two sharp diffraction lines corre-
surface area exhibits higher specic activity, which is similar to
other report.8 The homogeneous distributions of various
elements could greatly improve the dispersion of active species.
Amorphous Al2O3 dispersed in the particle region makes the
structure difficult to damage at high temperature, making the
dried sample (Fig. 8(A)). The XRD patterns of the monolithic
active elements remain very well dispersed. It is well known that
an increased amount of structural defects, which is benecial
for the activation of oxygen molecules to active oxygen adspe-
cies, and low temperature reducibility result in better catalytic
regular and adhesive to the Al substrate (Fig. 8(C)). The amount
performance.48 The structural defects of the spinel Co3O4 lattice
could play a key role in creating oxygen vacancies on the surface,
is more than 0.08 g. As shown in Fig. 8(B), the catalytic activity of
which might be favorable for accelerating the adsorption and
dissociation of oxygen molecules, hence resulting in the
formation of highly active electrophilic oxygen species. Mean-
sponding to aluminum are found in the XRD pattern of the
CoMnAlO lm catalyst and its LDH precursor show a series of
diffraction peaks, which are similar to those of their powder
samples. The photograph of the sample shows that the lm is
of CoMnAlO lm catalyst grown on the Al substrate (4 ꢂ 15 cm)
the CoMnAlO lm is a little reduced for benzene conversion
with T90 at 240 ꢀC compared with the corresponding CoMn2AlO
powder catalyst with T90 at 208 ꢀC. However, the monolithic
while, when the content of high valence metal ions increases,
catalyst with more active sites exposed exhibits a reaction rate of
the chemical potential and reactivity of oxygen adjacent to the
ꢁ1
1.19 mmolꢁ1gcat hꢁ1 at T90, much higher than that of 0.24
metal ions will be promoted. Gabrovska49 proposed an active
mmol gcat
hꢁ1 over the CoMn2AlO-550 powder sample.
xꢁ
assembly formed by the redox couple Co3+/Co2+ and O2 radi-
cals stabilized on Al3+ ion for CO oxidization:
Compared with lms prepared by colloidal deposition tech-
niques, thin lms growing directly from the Al substrate can
have better adhesion and mechanical stability. The mechanism
of oxidation supposes adsorption of organic compounds on the
catalyst surface, and the activation of adsorbed oxygen
Co2+
+
,Al3+ + O2 4 Co3+ + O2xꢁ/Al3+
where ,Al3+ represents an anion vacancy close to an Al3+ ion.
The adsorbed oxygen can be promoted by increasing the
content of higher manganese oxidation state (Mn4+) based on
the principle of electro-neutrality. A lower manganese state
(Mn3+) existing in the crystal will produce oxygen vacancies (VO),
which will promote the mobility of lattice oxygen. Surface
oxygen vacancies will be generated to maintain electrostatic
balance according to the following process:
4Mn4+ + O2ꢁ / 4Mn4+ + 2eꢁ/VO + 1/2O2 /
2Mn4+ + 2Mn3+ + VO + 1/2O2
Based on XPS spectra results, the catalytic activity of benzene
oxidation is inuenced by various factors (Mn4+/Mn3+, Co3+/
Co2+, Oads/Olatt and synergetic effect of Co and Mn). The sample
with higher Mn4+/Mn3+ molar ratio with the lower thermal
treatment temperature will contain more adsorbed oxygen
species, and the physically adsorbed oxygen and chemically
adsorbed oxygen species are relatively easier to desorb. The
adsorbed surface oxygen is responsible for the high catalytic
activity of CoMn2AlO-550 in the oxidation of benzene due to the
high crystallinity of samples with a high content of lattice
Fig. 8 (A) XRD patterns of the monolithic CoMnAl LDH film and
CoMnAlO film catalyst on the Al substrate, (B) the comparison of the
defects and the good synergistic effect between cobalt and benzene conversion over CoMnAlO film and CoMn2AlO-550 powder
catalyst, (C) photograph and (D) SEM image of the CoMnAl LDH film
precursor on the Al substrate.
manganese at high temperature. Furthermore, the XPS results
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´
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This research was supported by the National Natural Science
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