Insights into the solar light driven thermocatalytic oxidation of VOCs over tunnel structured manganese oxides

Article abstract of DOI:10.1039/c6cp02776h

Different tunnel structured manganese oxides (1?1, 2?2, and 3?3) have been synthesized via a facile hydrothermal strategy. The three catalysts exhibit high photothermal performance, resulting in a considerable increase of temperature above the light-off temperature for VOC oxidation. On this point, aerobic oxidation reactions of propane and propylene under simulated sunlight and infrared light irradiation were selected as probe reactions to explore their light driven thermocatalytic activity. Furthermore, the light-off curves of the manganese oxides for propane and propylene were carefully investigated, which clearly explained the possibility of combining both the efficient photothermal effect and excellent thermocatalytic activity of the manganese oxides. Results show that the catalytic effects follow the order of 1?1 < 3?3 < 2?2. 2?2 exhibited the best catalytic properties due to better low-temperature reducibility, suitable tunnel structure and the presence of more Mn⁴⁺. This work suggests new applications for traditional catalysts with intense photoabsorption and provides insights into the overall utilization of solar energy.

Full text of DOI:10.1039/c6cp02776h

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Insights into the solar light driven thermocatalytic oxidation of
VOCs over tunnel structured manganese oxides
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a,b
a
a,b
a,b
Yali Zheng, Wenzhong Wang,* Dong Jiang, Ling Zhang, Xiaoman Li and Zhong Wang
DOI: 10.1039/x0xx00000x
Different tunnel structured manganese oxides (1*1, 2*2, and 3*3) have been synthesized via a facile hydrothermal
strategy. The three catalysts exhibit high photothermal performance, resulting in considerable increase of temperature
above the light-off temperature for VOCs oxidation. On this point, aerobic oxidations of propane and propylene under
simulated sunlight and infrared light irradiation were selected as probe reactions to explore the light driven
thermocatalytic activity. Furthermore, the light-off curves of the manganese oxides for propane and propylene were
carefully investigated, which clearly explained the possibility of combining both the efficient photothermal effect and
excellent thermocatalytic activity of the manganese oxides. Results show that the catalytic effects follow the order of 1*1
www.rsc.org/
<
3*3 < 2*2. 2*2 exhibited the best catalytic properties due to better low-temperature reducibility, suitable tunnel
4
+
structure and the presence of more Mn . This work suggests new applications for traditional catalysts with intense
photoabsorption and provides the insights into the overall utilization of solar energy.
infrared part) to thermal energy, resulting in a considerable
increase of temperature, which is just enough to stimulate the
1
. Introduction
7
, 8
9
thermocatalytic oxidation of VOCs.
Recently, Meng et al.
Volatile organic compounds (VOCs) as major air pollutants are
not only hazardous to human health but also harmful to the
1
environment. Among the strategies for the elimination of
investigated the Group VIII nanocatalysts, which showed
effective energy utilization over the whole range of the solar
spectrum, excellent photothermal performance, and unique
VOCs, catalytic oxidation using transition metal oxides has
been highly promising because of its technical and economic
2
feasibility. However, high operating temperature in this
activation abilities, thus realizing a two-step water-based CO
2
conversion driven by solar energy. Furthermore, we have
previously shown that the maximum utilization of
photothermal conversion can be realized through amorphous
process means low energy efficiencies, and will compromise
the long-term stability of catalysts, decrease the selectivity for
1
0
3
, 4
MnO
Co
x
modified Co
4
systems.
3
O
4
or conductive substrate supported
the desired products. At present, The efficient removal of
VOCs at mild temperature is becoming a crucial but
challenging issue in the context of increasingly serious air
3
O
4
Manganese oxides are among the prominent candidates for
catalytic reactions due to their outstanding structural
5
pollution.
1
1, 12
uniformity, low cost, and environmental compatibility.
The recently emerged photothermal effect of catalysts
suggests a new means for harvesting solar energy. The infrared
light, accounting for almost half of the solar energy, has not
been utilized effectively by traditional semiconductor-based
solar energy conversion technology that is mainly active in the
More importantly, they possess strong photoabsorption ability
and exhibit excellent photothermal effect. Manganese oxides
have very diverse structures, depending on the connectivity
1
3
6
between the [MnO ] units via sharing corners or edges.
6
Among them, considerable attentions have been paid to the
tunnel structured manganese oxides due to their distinguished
region from ultraviolet to visible light. As most light sources
supply full spectrum irradiation, suitable catalysts can
efficiently transform the absorbed solar energy (typically the
1
4
catalytic performances. Typically, three types of square
tunnel structures of manganese oxides including pyrolusite,
cryptomelane and todorokite are showed in Fig. 1. Pyrolusite
a.
(β-MnO ) is featured for its corner-shared MnO6 in its
2
State Key Laboratory of High Performance Ceramics and Superfine
Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences,
structural unit and forms a small tunnel structure (1 * 1, ca.
12
1
295 Dingxi Road, Shanghai 200050, P. R. China. E-mail: wzwang@mail.sic.ac.cn
2
b.
0.23 × 0.23 nm ) absent of extra water and cations. The
accessibility to β-MnO is limited due to small voids in the
University of Chinese Academy of Science, Beijing 100049, People’s Republic of
China.
2
1
5
Electronic Supplementary Information (ESI) available: [Fig. S1-S6 displayed the N
adsorption-desorption isotherm, pore size distributions, blank experiments for
and C oxidation, C and C oxidation under Xe lamp and IR lamp
2
structure, which further deteriorates the activity. The crystal
structure of cryptomelane consists of two double edge-sharing
C
3
H
8
3
H
6
3
H
8
3 6
H
irradiation, UV-Vis-IR spectra and HCHO degradation for the 1*1, 2*2, and 3*3 MnO octahedral chains, which are corner-connected to form
6
catalysts during the three cycles]. See DOI: 10.1039/x0xx00000x
2 16
one-dimensional tunnels (2 * 2, ca. 0.46 × 0.46 nm ). Because
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temperature. The resulting black slurry was filtered, washed
with deionized water and then freeze-dried.
The todorokite manganese oxide (3*3) was prepared
through two steps. Briefly, buserite was initially prepared by
the co-precipitation method. MnCl
MgCl ·6H O (0.5083 g) were dissolved into 25 ml deionized
water to form a mixed solution, to which the 8.8 M NaOH (25
ml) was rapidly added. Then 0.025 M KMnO (25 ml) solutions
were successively added to the above solution with a speed of
2.5 mL/h. The mixture was aged at 60 C for 24 h and then
2 2
·4H O (0.2969 g) and
2
2
4
6
°
washed by the deionized water to obtain buserite. 3*3 was
prepared by the addition of the buserite above into 40 mL
deionized water, and charged into a 50 mL Teflon-lined
Fig. 1 Crystal structure models of three manganese
oxides with different square tunnel sizes.
autoclave at 160 °C for 24 h. The system was allowed to cool
down to room temperature. The resulting black slurry was
of its unique structural characteristics such as porous
structure, mixed valency of Mn, easy release of lattice oxygen,
filtered, washed with deionized water and then freeze-dried.
etc., cryptomelane has been extensively investigated for
1
diverse applications of catalytic oxidation. The todorokite 2.2 Characterization
crystals possess a specific oxide-type structure with a
The purity and crystallinity of the manganese oxides was
measured by X-ray diffraction (XRD) using a Rigaku D/MAX
framework consisting of triple chains of edge-sharing MnO
octahedra that corner-share to form tunnels of ca. 0.69 × 0.69
6
2
250 V diffractometer with monochromatized Cu Kα radiation
2 17, 18
nm (3 * 3).
The todorokite shows significantly larger
(λ=0.15418 nm) under 40 kV, 100 mA and with the 2θ ranging
adsorptive capacity for molecules with size dimensions close to
9
from 10° to 80°. The morphologies and structures of the
samples were characterized by transmission electron
microscope (TEM), high-resolution transmission electron
microscopy (HRTEM) (TecnaiG2 F20 S-Twin, accelerating
1
6 4
its tunnel size, e.g. C H12 and CCl .
Specifically, herein we report three kinds of tunnel
structured manganese oxides via hydrothermal synthesis,
which show strong absorption in the entire solar spectrum
region and transform the absorbed solar energy to thermal
voltage 200 kV), and N adsorption-desorption measurements
2
using a V-sorb 2800P surface area analyzer. The specific
surface areas were calculated by the Brunauer–Emmett–Teller
energy. Aerobic oxidations of propane (C
3 8
H ) and propylene
3 6
(C H
) under simulated sunlight and infrared light irradiation
(BET) method. Pore volume and pore size distribution plots
were selected as probe reactions to explore the light driven
thermocatalytic activity for the first time. The surface area,
were obtained by the Barrett–Joyner–Halenda (BJH) method.
X-ray photoelectron spectroscopy (XPS) were obtained by
irradiating every sample with a 320 μm diameter spot of
monochromated aluminum Kα X- rays at 1486.6 eV under
ultrahigh vacuum conditions (performed on ESCALAB 250,
THERMO SCIENTIFIC Ltd.). Charging effects were corrected by
adjusting the binding energy of C 1s to 284.6 eV. UV-vis diffuse
reflectance spectra (DRS) of the samples were measured using
a Hitachi UV-3010PC UV-vis spectrophotometer. Hydrogen
4
+
tunnel structure, degree of crystallinity, Mn species, and
reducibility of the catalyst has been integrally discussed. In
addition, this work offers a new way for the overall utilization
of solar energy, by combining both the efficient photothermal
effect and excellent thermocatalytic activity of the tunnel
structured manganese oxides.
temperature-programmed reduction (H -TPR) measurements
2
2
. Experimental
were performed (for each sample, 50 mg) on a ChemiSorb
2750 instrument equipped with a thermal conductivity
2
.1 Material preparation
Preparation of pyrolusite (1*1). For a typical synthesis,
MnCl ·4H O (8 mmol) and an equal amount of (NH were
added to 40 mL distilled water at room temperature to form a 2.3 Light driven catalytic oxidation of C H and C H
3 6
-
1
2
0
2
detector, under a 4 vol% H /Ar flow (25 mL min ) at a heating
o
−1
rate of 10 C min .
2
2
4 2 2 8
) S O
3
8
homogeneous solution, which was then transferred into a 50
mL Teflon-lined stainless steel autoclave, sealed and
The catalytic activities of all the samples were evaluated by the
gas-phase aerobic oxidation of C (50 ppm) and C (50
22
3
H
8
3 6
H
maintained at 140
°C for 12 h. The system was allowed to cool
ppm). The apparatus used was described in earlier papers .
down to room temperature. The resulting black slurry was
filtered, washed with deionized water and then freeze-dried.
The cryptomelane (2*2) sample was prepared according to
Experiments were operated in a gas-closed vitreous reactor
−2
(capacity 600 mL) with a quartz window, and a 500 mW cm
Xe lamp was lighted to simulate the optical excitation and
thermal activation. Before the catalytic test, a thermocouple
was placed on the catalyst to measure the temperature of the
catalyst under the irradiation of the Xe lamp. During the
oxidation process, a GC analysis (GC 7900, Techcomp)
equipped with two detective channels was used to detect the
2
1
the similar procedure reported by Hou et al.: MnCl
2 2
·4H O
(1.25 mmol) and KMnO (2.5 mmol) were added to 40 mL
4
distilled water at room temperature to form a homogeneous
solution, which was then transferred into a 50 mL Teflon-lined
stainless steel autoclave, sealed and maintained at 100 °C for
4 h. The system was allowed to cool down to room
2
2
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decrease of C
The channel for C
Al /S capillary column and a flame ionization detector (FID).
Another channel for CO
detection consists of a TDX-01, Fig. 2 gives the XRD patterns of the manganese oxides. The
0−100 mesh packed column followed by a methane diffraction patterns of the 1*1 prepared by the reaction of
conversion furnace and a FID. In addition, an infrared (IR) lamp MnCl with (NH match standard diffraction for
was also used as the light source to simulate the thermal pyrolusite (β-MnO , JCPDS 24-0735). The 2*2 obtained from
activation.
the reaction of MnCl with KMnO showed intense diffraction
peaks at 2θ = 12.8 , 17.9 , 28.7 , 37.7 , 41.9 , 49.9 , and
3
H
6
, C
3
H
8
and the increase of CO
2
simultaneously.
3
. Results
3
H
8
and C detection consists of a TM plot-
3 6
H
3
.1 Textural characterization of the catalysts
2 3
O
2
8
2
4 2 2 8
) S O
2
2
4
°
°
°
°
°
°
2
.4 Thermocatalytic oxidation of C
3 8 3 6
H and C H
5
6.1°, which can be attributed to the crystalline phase of pure
The thermocatalytic activities of all the samples for C H and
tetragonal cryptomelane (KMn O , JCPDS 29-1020). XRD
2
3
3
8
8
16
C H oxidation were tested in a quartz tube reactor with an
3
6
pattern for the synthesized 3*3 is in agreement with the
1
2, 24
inner diameter of 10mm at atmospheric pressure. 200 mg of
the manganese oxides were used as catalyst. The reagent gas
consisted of 100 ppm C H + 50 ppm C H +20% O + the
previous reports.
The diffraction peaks can be indexed to
(JCPDS 38-
the orthorhombic phase of the todorokite MnO
2
3
6
3
8
2
0475). To take the intensities and half-widths of the diffraction
peaks into account, the degree of the crystallinity decreases in
the sequence: 1*1> 2*2 > 3*3.
balance made up of N at a steady flow rate of 200 SCCM, thus
2
-1 -1
giving a weight hourly space velocity (WHSV) of 60 L h g . The
reactants and products were analyzed online by gas
chromatograph (GC) equipped with a TCD. The activity of the
catalysts was characterized by T , T , and T , which
The morphology and size of the as-prepared manganese
oxides were estimated by TEM. Fig. 3 shows the TEM images of
the manganese oxides with different magnifications. It can be
clearly seen that both 1*1 and 2*2 can be characterized by the
morphology of nanorods with lengths of ca. 0.5~2 μm and
diameters of ca. 15~50 nm. Generally, the diameter of 1*1 is
much larger than 2*2. 3*3 displays a completely different
morphology. As shown in Fig. 3, 3*3 is characterized by the
morphology of hexagon nanosheets with side lengths of ~30
nm. More notably from the high resolution TEM image,
1
0
50
90
represent the temperature of target gas conversion at 10, 50,
and 90%, respectively.
2
.5 Room temperature oxidation of formaldehyde (HCHO)
The catalytic oxidation activity of the as-prepared samples (50
mg) for HCHO degradation (260 ppm) was tested in a gas-
closed vitreous reactor (capacity 650 mL) with a double-walled
jacket with water for temperature control. During the
homogeneous
worm-like
pores
distributed
among
experiment, the reaction temperature was kept at 25 °C by
nanoparticulates can be observed within every nanosheet,
2
circulating water. The activity of the catalysts on HCHO
oxidation was estimated by the concentration change of HCHO
before and after the 60 min oxidation process. HCHO
concentration in the reactor was analyzed by phenol
spectrophotometric method. 10 mL gas sample containing
trace HCHO was absorbed by 15 mL phenol reagent solution
5
suggesting the mesoporous structure of 3*3.
adsorption-desorption isotherms of different manganese
N
2
oxides catalysts and their corresponding pore size distribution
-
4
(
1×10 wt %) for 5 min. Then, 1.2 mL (1 wt %) ammonium
ferric sulfate solution was added as the coloring reagent. After
being shaken and waiting for 15 min in the dark, HCHO
concentration was determined by measuring light absorbance
at 630 nm with a spectrophotometer (Hitachi U-3010 UV-vis
spectrophotometer). CO and CO
analysis (GC 7900, Techcomp).
2
were monitored by GC
Fig. 3 TEM images of 1*1 (A1-3), 2*2 (B1-3), and 3*3
(C1-3).
Table 1. Pore structural parameters of the manganese
oxides.
catalysts
2
BET surface area(m /g)
3
pore volume(cm /g)
1*1
8.7
2*2
3*3
61.9
35.8
0.109 0.163
0.532
Average pore
diameter(nm)
25.5 16.6
26
Fig. 2 XRD patterns of the as-prepared manganese
oxides.
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are shown in. Fig S1 (supporting information (SI)). BET surface ratio. Different surface oxygen species have been identified by
2
-1
areas of the 1*1, 2*2, and 3*3 are 8.7, 35.8, and 61.9 m g , the deconvoluted O 1s spectrum. Fig. 4B revealed the fitted
respectively (Table 1). The surface area increases with the O1s peaks. In detail, the peak at 529.4–529.8 eV (O1) is
2
7
increase of tunnel sizes of the three samples. 1*1 possesses originated from lattice oxygen, the peak at 531.1–531.8 eV
the lowest surface area, which may be due to its smallest (O2) is attributed to a high number of defect sites with a low
8
tunnel size as a result of the densely packed manganese oxygen coordination, and the peak at 532.8 eV (O3) is
2
6
octahedra.
associated with hydroxyl species of surface-adsorbed water
2
8
molecules. As reported in Table 2, the percentages of
different surface oxygen species are nearly the same for 1*1
and 2*2. The area of the peak of O2 is the largest for the 3*3
among the three samples, which indicates that the 3*3 with
mesoporous structure possess more oxygen vacancies than
the other counterparts. These oxygen vacancies are related to
3
.2 Surface composition and reducibility of the catalysts
XPS was performed to further investigate the surface
compositions and elementary oxidation states. The XPS
spectra of Mn 2p and O 1s for 1*1, 2*2, and 3*3 are displayed
in Fig. 4A, 4B. The oxidation state of Mn and the relative levels
4
+
3+
of the Mn /Mn atomic ratio are analyzed by studying the
Mn 2p3/2 spectra (Table 2). Fig. 4A shows that Mn 2p3/2 has two
components displayed in binding energy at 642.1 and 643.2
3+
4+
20
the coexistence of Mn and Mn ions. XPS can only quantify
elements on the surface of a material. However, if the channel
structure and specific surface area in the bulk phase of
mesoporous 3*3 is considered, 3*3 should possess many more
surface adsorbed oxygen species (typically O2).
3
+
4+
eV, which are due to the surface Mn and Mn ions,
2
7
4+
3+
respectively. From Table 2, the surface Mn /Mn molar
ratios of 1*1, 2*2, and 3*3 are 0.877, 0.961, and 0.639,
respectively. The 2*2 catalyst possess the highest ratio of
Reducibility of the manganese oxide catalysts, an important
factor known to be correlated with their redox activity, was
4
+
3+
Mn /Mn on the surface, and the 3*3 displays the lowest
investigated by means of H -TPR (Fig. 5). The TPR profile of
2
manganese oxides usually presented two well-resolved
reduction peaks, which indicated a two-step reduction for
2
9, 30
MnO : MnO →Mn O , Mn O → MnO.
Among all the
samples, 1*1 showed a low reduction temperature of 288
with a two-step reduction (the second locates at 389 C). 3*3
also showed a two-step reduction (at 359 and 476 C) and the
2
2
3
4
2 3
°C
°
°
ratio of the lower temperature peak to the higher temperature
peak was around 2. The lower temperature reduction was
attributed to the reduction of MnO to Mn O and the higher
2
3 4
2
Fig. 5 H -TPR profiles of the 1*1, 2*2, and 3*3 tunnel
manganese oxides.
Fig. 4 (A) Mn 2p XPS spectra and (B) O 1s XPS spectra of
1*1, 2*2, and 3*3.
Table 2. Summary of area percentages of different
elemental components obtained from the deconvoluted
spectra
4
+
3+
sample
O1
O2
O3
Mn /Mn
eV
529.9 69.9 531.6 30.1
530 66.6 531.1 33.4
529.9 33.6 531.1 60.2 532.8 6.2
%
eV
%
eV
%
1
2
3
*1
*2
*3
0.877
0.961
0.639
Fig. 6 C H oxidation over 1*1, 2*2, and 3*3 under
3
6
simulated sunlight (Xe lamp).
4
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temperature peak was attributed to the reduction of Mn
3
O
4
to
C
3
H
6
contributed to less than 14% of C
3
H
6
decrease under Xe
MnO. Similar reduction profiles were previously observed for lamp irradiation, while changes in C
3 8
H content were not
28, 31
MnO
2
which supports our assignments.
On the other hand, distinct, indicating the indispensable role of the catalyst. The
concentration variations (C /C ) of C against the irradiation
What’s time over various catalysts are shown in Fig. 6, while all the
(results not
concentration was
are compared based on their lowest temperature reduction observed in the presence of the 2*2 catalyst. About 78.1% C
peaks and terminated temperature ranges, the enhanced oxidation was achieved within 20 min. Meanwhile, after the Xe
reducibility of the samples is in the order 1*1 < 3*3< 2*2. lamp irradiation for 20 min, the C oxidation of 1*1 and 3*3
.3 Catalyst activity evaluation
The light-off temperature for C H oxidation is significantly
2
,
*2 only showed one broad reduction peak centered at 306
°C
t
0
3 6
H
3
1, 32
which is typical for large and nonporous particles.
more, the temperature where the reduction peak of 2*2 starts catalysts display no catalytic activity towards C
is lower than that of 1*1. When the three manganese oxides shown). A rapid decrease of the C
3 8
H
3 6
H
3 6
H
3 6
H
is only 28.2% and 55.4%, respectively. In general, 2*2 exhibits
the highest light driven catalytic activity in comparison to the
behavior of the other two. Cutoff filter (> 690 nm) had been
used to determine the catalytic activity of the manganese
oxides under the infrared irradiation of the Xe lamp, related
3
3
8
3
3, 34
higher than that for C H
3
, which can be easily understood
6
on the basis of the generally higher activity of alkenes
compared to alkanes. The light driven catalytic activity of the
manganese oxides were investigated by evaluating the aerobic
oxidation of C H (50 ppm) and C H (50 ppm) under the
results were shown in Fig. S3. For 2*2, about 50% C H6
3
oxidation were achieved within 20 min, which accounted for
nearly 60% of the catalytic activities under the irradiation of Xe
lamp with full spectrum. Specifically, much enhanced catalytic
activities were realized for all the catalysts when an extra IR
lamp was used for the same experiments (Fig. S2 and S4, SI),
especially the 2*2, 20.3% C H and 98% C H oxidation was
3
8
3 6
simulated solar irradiation from the Xe lamp. The C H and
3
8
C H catalytic oxidation cannot proceed at room temperature
3
6
without being irradiated. Besides, blank tests in the absence of
any catalyst were performed (Fig. S2, SI). The photolysis of
3
8
3 6
achieved within 20 min.
Moreover, C H and C H were chosen for the individual
3
8
3 6
thermocatalytic oxidation tests to determine the temperature
when the catalytic activity of the manganese oxides starts (Ts).
Fig. 7A, 7B show the light-off curves of the manganese oxides
for C H and C H . The reaction temperatures of T , T , and
3
8
3
6
10
50
T90 (corresponding to the target gas conversion = 10%, 50%,
and 90%, respectively) for the catalysts is compared (Table 3).
As shown in Fig. 7A, 7B and Table 3, all the measured T , T ,
1
0
50
and T90 temperatures follow the order of 1*1 > 3*3 > 2*2. The
performance of 3*3 is relatively close to 1*1. For the same
conversion (C H ), the temperature 3*3 needed is just ca. 15
3
6
o
C lower than that of 1*1. Remarkably, as compared to 3*3
and 1*1, 2*2 shows a tremendous decrease (for C H , ΔT
=
10
3
6
o
o
o
5
1-66 C, ΔT = 42-55 C, ΔT = 40-58 C) at the reaction
50 90
temperature of T , T , and T .
90
1
0
50
4
. Discussion
4
.1 Photothermal effect
The photoabsorption ability is quite important for efficient
photothermal conversion. We measured the UV-Vis-IR spectra
of 1*1, 2*2, and 3*3. All the three samples show strong
absorption in the entire solar spectrum region (Fig. S5). Before
the catalytic tests, the photothermal effect was measured. The
temperature evolution profiles of 1*1, 2*2, and 3*3 under the
irradiation of the Xe lamp are shown in Fig. 8A. Generally, the
temperature of the samples quickly increases from room
Fig. 7 (A) (B) The light-off curves of the manganese
oxides for C H and C H .
3 8 3 6
Table 3. Reaction Temperatures at which the C
C H conversions are 10, 50, and 90%.
3
H
8
and
3
6
o
o
o
T
10( C)
T
50( C)
T
90( C)
sample
ID
C
3
H
8
C
3
H
6
C
3
H
8
C
3
H
6
C
3
H
8
3 6
C H
o
temperature to more than 100 C in 120 s with the lamp on.
1
*1
*2
251
191
233
185 300 223 350
119 227 168 263
170 276 210 312
263
205
245
When the temperature reaches a plateau, an equilibrium is
established between the absorption of light energy and the
2
7
energy dissipation from the catalyst to the surroundings . The
3*3
o
) of 1*1, 2*2, and 3*3 are 157 C, 142
plateau temperatures (T
o
p
o
and 165 C, revealing their efficient photothermal
C
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4
+
structure and possessed more Mn , which may be the major
factors that lead to its excellent activity.
2
Firstly, 2*2 shows the highest reducibility (H -TPR, Fig. 5). In
general, more readily reduced surface species have higher
redox activities because their reduction occurs at lower
temperature. It is well known that low-temperature
reducibility is one of the critical factors for the oxidation
reaction. Secondly, despite its medium degree of crystallinity
and surface area, the tunnel structure of 2*2 takes a great
effect for its catalytic activity. The manganese oxides with the
1
2
tunnel structure show molecular-sieve properties.
The
accessibility to 1*1 is limited due to small voids in the
1
5
structure, while 3*3 shows significantly larger adsorptive
capacity for molecules with size dimensions close to its tunnel
1
9
size. Considering of the size dimensions of the C H and C H
3 6
3
8
molecules (0.40~0.43 nm), tunnel structure of 2*2 is suitable
for the adsorption of C H and C H . The tunnel diameter of
3
8
3 6
2
*2 was reported to be a major factor that gave rise to its high
1
2
catalytic activity. Similar results occurred for the catalytic
oxidation of HCHO without light or heat source (fig. S6), while
2
*2 showed the best performance. Thirdly, higher oxidation
4
+
state of manganese species (Mn ) was previously found to be
preferable for oxidation reactions over the manganese-
Fig. 8 Temporal change of the temperature on the
catalysts under the irradiation of the Xe lamp (A) and IR
lamp (B).
29
36
containing catalysts. Kapteijn et al. have investigated the
reduction of NO by NH3 over pure manganese oxides and
found that the NO conversions decreased in the order of MnO2
conversion. When IR lamp was used as the light source, the
temperature of the samples quickly increases from room
>
Mn O > Mn O > Mn O . For α-MnO (2*2, 3*3) catalysts,
5 8 2 3 3 4 2
3
+
4+
o
temperature to more than 150 C in 120 s (Fig. 8B). The values
either a small amount of Mn or a large amount of Mn (or
both together) are important for high activity of CO oxidation
o
o
o
p
of T are 226 C (1*1), 258 C (2*2), and 244 C (3*3), much
3
7
catalysts, which is consistent with the present study. For 3*3
with the lowest degree of crystallinity, highest surface area,
and more surface adsorbed oxygen species (table 2), its
catalytic activity is clearly not as expected. In this regard, the
larger than that under the Xe irradiation.
The manganese oxides did not exhibit photocatalytic activity
via the traditional photocatalytic route at room temperature.
The efficient catalytic activity of the three tunnel structured
manganese oxides under the Xe or IR lamp irradiation
originates from the light (typically the infrared part) driven
2
larger 0.69 × 0.69 nm tunnel size of 3*3, the poorer
3
+
reducibility, and the presence of higher Mn content is
considered to be responsible for diminished activity.
7
thermocatalysis. Integrating the results of Fig. 7B and Fig. 8A,
o
) of 1*1 (~150 C), 2*2
apparently, the light off temperature (T
s
o
o
(
~80 C), and 3*3 (~120 C) for C
their corresponding T no matter under Xe or IR lamp
irradiation. Thus, the high induced by efficient
photothermal conversion is enough to trigger off
thermocatalytic C oxidation for the catalysts in the light
irradiation experiments. As for the C oxidation, the
situation is opposite. Considering the much higher T when the
3 6
H oxidation is lower than
5
. Conclusions
p
In summary, tunnel structured manganese oxides (1*1, 2*2,
and 3*3) have been synthesized through a facile hydrothermal
method. The three kinds of catalysts exhibit strong absorption
in the entire solar spectrum and shows strong solar heating
effect, resulting in a considerable increase of temperature
above the light-off temperature for VOCs oxidation. Aerobic
T
p
3 6
H
3 8
H
p
IR lamp is used, there is no doubt for the much higher catalytic
activity.
oxidations of C
3
H
8
and C
3
H
6
under simulated sunlight were
concentration
performed, while rapid decreases of the C
3 6
H
4
.2 Mechanism discussion
were observed. Enhanced catalytic activities were realized for
all the catalysts when the IR lamp was used for the same
It is believed that catalytic activities are associated with the
surface area, tunnel structure, degree of crystallinity, oxygen
defect, and transition-metal ion redox ability (reducibility) of
experiments. Much higher T
p
can be realized under the
infrared light irradiation, resulting in better catalytic activities.
Furthermore, the light-off curves of the manganese oxides for
3
5
the catalyst. So the question is why the 2*2 catalyst shows
much higher activities. It can be concluded that the influence
induced by the photothermal effect can be ignored on account
3 8 3 6
C H and C H clearly explained the mechanism of the light
driven thermocatalytic oxidation. The catalytic activities under
all the test conditions followed the order 1*1 < 3*3 < 2*2. The
excellent catalytic property of the 2*2 catalyst is primarily due
to better low-temperature reducibility, suitable tunnel
p
of the smaller T of 2*2 compared to that of 1*1 and 3*3
under Xe irradiation. From the characterization results, 2*2
exhibited higher low-temperature reducibility, suitable tunnel
6
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6CP02776H
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ARTICLE
4
+
structure and the presence of more Mn . On the basis of this 25 Z. Shu, W. Huang, Z. Hua, L. Zhang, X. Cui, Y. Chen, H. Chen,
C. Wei, Y. Wang, X. Fan, H. Yao, D. He and J. Shi, J. Mater.
study, a brand new strategy for catalyst design toward the fully
Chem. A, 2013, 1, 10218.
6 X. Zheng, T. Lin, G. Cheng, B. Lan, M. Sun and L. Yu, Adv.
Powder Technol., 2015, 26, 622-625.
7 C. Shi, Y. Wang, A. M. Zhu, B. B. Chen and C. Au, Catal.
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use of both the efficient photothermal effect and excellent
thermocatalytic activity is suggested.
2
2
Acknowledgements
28 N. D. Wasalathanthri, A. S. Poyraz, S. Biswas, Y. Meng, C.-H.
Kuo, D. A. Kriz and S. L. Suib, J. Phys. Chem. C, 2015, 119
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,
We acknowledge the financial support from the National Basic
1
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