Highly Efficient MnO2/AlOOH Composite Catalyst for Indoor Low-Concentration Formaldehyde Removal at Room Temperature

Article abstract of DOI:10.1021/acs.inorgchem.0c00852

Indoor formaldehyde from substandard furniture and decorative materials seriously endangers human health. How to remove effectively indoor formaldehyde with low concentration at room temperature is a challenging problem. Using a MnO₂/AlOOH composite by the MnO₂ modification as a catalyst provides an effective approach to solve this challenge. Here, a new type of MnO₂/AlOOH composite catalyst with high ability to remove indoor low-concentration formaldehyde was prepared by redox reaction at room temperature. A MnO₂/AlOOH composite with a homogeneous dispersion of MnO₂ has high specific surface area and a large amount of surface hydroxyl (-OH) which plays a major role in the adsorption of formaldehyde. A partially crystalline structure was observed in the composite, which contains multivalent Mn ions and a large number of vacancy defects. The surface-OH of composite shows strong oxidation activity through the charge exchange of multivalent Mn ions and vacancy defects. The composite has a higher ability to remove indoor low-concentration formaldehyde compared to the birnessite MnO₂ at room temperature. This study proposes a new idea for the improvement of catalytic performance in the structure and composition of the catalyst.

Full text of DOI:10.1021/acs.inorgchem.0c00852

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Highly Efficient MnO /AlOOH Composite Catalyst for Indoor Low-
2
Concentration Formaldehyde Removal at Room Temperature
Zhiyuan Liu, Jingpeng Niu, Weimin Long, Bing Cui,* Kun Song, Fan Dong, and Dong Xu*
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c00852
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sı Supporting Information
ABSTRACT: Indoor formaldehyde from substandard furniture
and decorative materials seriously endangers human health. How
to remove effectively indoor formaldehyde with low concentration
at room temperature is a challenging problem. Using a MnO2/
AlOOH composite by the MnO2 modification as a catalyst
provides an effective approach to solve this challenge. Here, a new
type of MnO /AlOOH composite catalyst with high ability to
2
remove indoor low-concentration formaldehyde was prepared by
redox reaction at room temperature. A MnO /AlOOH composite
2
with a homogeneous dispersion of MnO has high specific surface
2
area and a large amount of surface hydroxyl (−OH) which plays a major role in the adsorption of formaldehyde. A partially
crystalline structure was observed in the composite, which contains multivalent Mn ions and a large number of vacancy defects. The
surface −OH of composite shows strong oxidation activity through the charge exchange of multivalent Mn ions and vacancy defects.
The composite has a higher ability to remove indoor low-concentration formaldehyde compared to the birnessite MnO at room
2
temperature. This study proposes a new idea for the improvement of catalytic performance in the structure and composition of the
catalyst.
1
8−25
INTRODUCTION
performs best.
Birnessite is a kind of layered manganese
■
dioxide built by edge-sharing octahedral MnO . A certain
number of cations like Na and K are located in the layers for
charge balance. Birnessite-type MnO can only partly oxidize
formaldehyde at room temperature. Therefore, many mean-
ingful researches
enhance catalytic activity of birnessite-type MnO
these studies showed that high specific surface area and defects,
which act as adsorption and active sites in catalytic reactions,
are closely related to the high activity of formaldehyde
oxidation.
Nanostructured AlOOH was widely used in the fields of
adsorption and catalysis as a low-cost and environmentally
friendly material. AlOOH has nanosheet structure with high
specific surface area and a large number of surface hydroxyl
−OH) groups. The Pt/AlOOH prepared by dispersing and
depositing nano Pt particles on the AlOOH nanosheets
exhibited a remarkable catalytic activity as well as stability for
elimination of formaldehyde at room temperature. High
catalytic activity does not depend on the amount of noble
Formaldehyde (HCHO) is known as one of the most common
pollutants in the indoor environment. Long-term exposure to
an environment containing a certain concentration of form-
aldehyde will have a serious impact on human health. Indoor
formaldehyde is generally derived from substandard furniture
and decorative materials, of which formaldehyde evaporates
6
+
+
2
26−30
have been carried out in order to further
. Both of
2
1
−3
slowly for up to 10 years.
This requires an efficient and
long-term effective method to remove formaldehyde, especially
the indoor low-concentration formaldehyde. Catalytic oxida-
tion is a promising method which can decompose HCHO into
4
−11
H O and CO with less restriction. In relevant reports,
2
2
noble-metal-based catalysts have the best ability to oxidize
formaldehyde, maintaining high catalytic activity at relatively
low temperatures. Au, Pt, Pd and other noble metal
nanoparticles are dispersed on the surface of (hydro-) oxides
(
such as TiO , FeO , CeO , AlOOH, and MnO . These
2
x
2
2
supports generally have high specific surface area, which
facilitates the dispersion of noble metal particles and the
adsorption of formaldehyde molecules. However, the high cost
of noble metal catalysts has always been a barrier to its
commercialization. It is necessary to develop an inexpensive
room-temperature catalyst.
7,8
Received: March 22, 2020
Another type of catalyst for formaldehyde oxidation is a
transition-metal oxide catalyst, represented by Co O and
3
4
1
2−17
MnO2.
Most transition-metal oxide catalysts require
relatively high temperature (about 80−140 °C) to be
catalytically active, among which the birnessite-type MnO
2
©
XXXX American Chemical Society
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A
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Article
3
1
metal depositing, but rather by higher dispersion. Therefore,
crystal monochromator. The photon energy was calibrated with the
first inflection point of Mn K-edge in Mn metal foil. Data analysis was
performed with the Athena software packages. XPS analysis was
carried out on an X-ray photoelectron spectrometer (AXIS ULTRA,
Kratos, UK) with Al target. The full spectrum of the sample and the
fine spectrum of each element were scanned separately. The binding
energy was calibrated by C 1s peak (284.8 eV). Fourier transform
infrared spectroscopy (FT-IR) was recorded on Nexus (Thermo
Nicolet, U.S.A.) from 450 to 4000 cm . In situ diffuse reflectance
infrared Fourier transform spectroscopy (in situ DRIFTS) was
performed on a Tensor II FTIR spectrometer (Bruker) equipped with
an in situ diffuse reflectance cell (Harrick) to clarify the intermediate
species during HCHO oxidation and its mechanism. The catalyst
samples in powder are loaded in the DRIFT cell. There are three gas
ports and two coolant ports on the reaction chamber. Then, high-
we venture to guess that using MnO particles with high
2
dispersity instead of Pt to complex with AlOOH which has
high specific surface area and a large amount of surface −OH
groups may provide a low-cost and highly efficient form-
aldehyde-removal catalyst. The removal of indoor high-
concentration formaldehyde has been widely studied in recent
However, how to effectively remove indoor low-
concentration formaldehyde has rarely been explored.
1
0,18,26,28
−
1
years.
In this work, we prepared MnO /AlOOH composites by
2
reacting KMnO with a large number of surface −OH groups
from AlOOH and applied it in indoor low-concentration
formaldehyde removal at room temperature. MnO2 was
dispersed uniformly in the MnO /AlOOH composites. The
composite has a heterogeneous structure with partially
crystalline structure containing a large number of surface
4
2
purity O , high-purity He, and 100 ppm of HCHO mixtures were
2
introduced into the cell. Under dark conditions, HCHO adsorption
on the catalysts was carried out for 20 min, and the total gas flow rate
was 100 mL/min. Next, the catalysts were illuminated by a visible
light source (MUA-210) for 30 min under room temperature.
Evaluation of HCHO Removal Activity. To simulate indoor
conditions, a static test of the catalyst’s ability to remove low-
concentration formaldehyde was performed in a self-constructed
reaction facility, as shown in Figure 1. The sealed reaction chamber
−
OH and vacancy defects. This composite catalyst has a
significantly higher activity than the birnessite MnO and has
2
an advantage in indoor low-concentration formaldehyde
removal. We found that −OH on the catalyst surface plays
an important role in the oxidation of formaldehyde, and a new
explanation of its reaction mechanism was given. Unlike most
studies that focus on improving the performance of MnO2
through various methods, this is a new type of catalyst
obtained through MnO modification.
2
EXPERIMENTAL SECTION
Catalyst Synthesis. All chemicals used for preparation were of
■
analytical grade. KMnO and ammonium hydroxide were purchased
4
from Beijing Chemical Reagent Co., Ltd. Other chemicals were all
purchased from Aladdin Reagent Co., Ltd. AlOOH was synthesized
8
by simplified microemulsion-assisted method. The detailed proce-
dure is as follows: A mixture containing 100 mL of cyclohexane and
2
0.46 g of polyethylene glycol (PEG 400) was magnetically stirred
Figure 1. Self-constructed reaction facility for formaldehyde removal.
over 900 rpm in room temperature. After stirring for 10 min, 20 mL
of Al(NO ) solution (0.32 M) and 3.85 mL of NH solution (27 wt
3
3
3
%
) were added stepwise to the above mixture and then aged about 6 h
made of glass has a volume of 5 L and three air holes that can be
switched. First, 0.1 g of composite catalyst powder was spread on a 6
cm diameter glass wafer and placed at the bottom of the reaction
chamber. The wafer was covered by a glass cover with a thin wire
which could pass through the air hole to easily lift the glass cover.
Next, the air was pumped through a certain concentration of
formaldehyde solution into the reaction chamber. The initial
concentration of formaldehyde in the reaction chamber was adjusted
by the pump-down time and the concentration of the formaldehyde
solution. Then, the glass cover was pulled up, and then all the air
holes were closed. The catalyst was contacted with low-concentration
formaldehyde gas in the reaction chamber and started to react at
room temperature. A formaldehyde detector (MEF500, Sensology,
China) was used to record the initial concentration of formaldehyde
in the reaction chamber and the change within 1 h. The formaldehyde
removal rate was calculated as the following:
to make precipitation of aluminum oxyhydroxide. The precipitation
was collected after centrifuging and washing with deionized water
(
four times) to remove possible residual organics.
The AlOOH precipitate was dispersed in 150 mL of deionized
water by ultrasonic. Then a 0.1 M KMnO solution (5 mL, 10 mL, 15
mL, 20 mL, 25 mL, 30 mL) was added to the mixture and stirred
about 12 h until the solution faded. MnO /AlOOH powders were
4
2
obtained by washing and air-drying precipitations at room temper-
ature. With the mole ratio of Al/Mn, the obtained samples were
labeled as A M (for x = 1−6). Powders obtained by air-drying the
10
x
AlOOH without Mn at room temperature were denoted as AlOOH.
As a comparison, pure MnO powder was obtained by the reaction of
methanol with KMnO and air-dried at room temperature, denoted as
MnO2.
2
4
Characterization. X-ray diffraction (XRD) analysis was per-
formed by an X-ray diffractometer (D8 Advance Davinci, Bruker,
Germany) with Cu target. SEM photographs were taken at 10 kV by a
field emission scanning electron microscope (Sirion200, FEI, USA)
and EDS elemental mapping was performed at 20 kV. High-
magnification TEM photographs were taken by transmission electron
microscopy (Tecnai F20, FEI, USA). Samples were all ultrasonically
dispersed in ethanol and then placed on copper grids for TEM
characterization. The specific surface area (SBET) of the samples was
tested by a fully automatic specific surface area and a micropore
analyzer (ASAP2020HD88, Micromeritics, USA). Samples were
degassed at 30 °C.
C − CT
0
PT =
× 100%
C0
where C
is the initial HCHO concentration and C
is the
T
0
concentration at T min.
RESULTS AND DISCUSSION
The crystal structure of the samples was detected by XRD
measurement (Figure 2). The XRD peaks located at 14.5°,
8.1°, 38.3°, 49.2°, 65.0°, and 71.9° of pure AlOOH samples
■
2
The X-ray absorption data at the Mn K-edge of the powder samples
were measured at room temperature by using transmission mode at
beamline BL14W1 of the Shanghai Synchrotron Radiation Facility
assigned to (020), (120), (031), (200), (002), and (251) of
AlOOH (JCPDS no. 21-1307), respectively. It can be seen that
AlOOH synthesized at room temperature by a simplified
(
SSRF), China. The station was operated with a Si (111) double
B
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Table 1. Combined Results of BET and XPS of As-Prepared
Samples
2
samples
SBET (m /g)
Mn/Al (molar)
Osurf/Olatt
Al 2p (eV)
AlOOH
A M
488
424
293
98
−
6.15
6.12
3.04
4.5
74.3
74.4
74.3
−
0.11
0.44
−
10
1
6
A M
10
MnO2
28
g, which is similar to other reports. The samples with
different ratios of AlOOH and MnO still microscopically
2
composed of nanosheets (Figure 3c−f). No obvious MnO
2
Figure 2. (a) XRD patterns of as-prepared AlOOH, A M , and
1
0
x
particles can be found in A M samples. However, it can be
1
0
x
MnO samples and (b) enlarged XRD patterns in the 2θ range of 60−
2
visibly seen that the high MnO ratio samples are denser than
2
7
0°.
the low MnO ratio samples. This was also consistent with the
2
test results of the specific surface area of the samples. Even
though the microstructure appears to be a dense block, the
specific surface area of A M is still far higher than that of
method also has good crystallinity. The XRD peaks located
around 36.5° (100) and 65.5° (110) of pure MnO can be
2
1
0
x
assigned to the birnessite structure (JCPDS no. 80-1098) with
2
6
birnessite MnO . In general, the higher specific surface was
2
poor crystallinity. Poor crystallinity may be due to the low
believed to be beneficial to reduce transfer resistance and
synthesis temperatures and the different raw materials
32
improve catalytic performance. Catalytic oxidation of form-
(
CH OH) used. With the increasing proportion of Mn in
3
aldehyde was usually interpreted as the “adsorption-oxidation”
the compound, the crystallinity of the material appears to be
continuously reduced, which is mainly reflected by the
continuous decrease of the diffraction peaks of AlOOH. The
addition of Mn may damage the ordered structure of AlOOH
or mask the characteristic peaks. Meanwhile, there was no
obvious diffraction peak of MnO found in A M patterns on
33
cycle. Xu et al. studied composites of MnO grown on
2
carbon spheres. Its high specific surface area and abundant
0
surface oxygen played a major role in Hg removal.
In the initial design idea, the synthesized MnO will grow on
2
the AlOOH surface and form MnO nanoclusters. However,
2
2
10
x
the results of the elemental mappings (Figure 4) show that Mn
account of the poor crystallinity of MnO . In order to confirm
2
the presence of MnO in the composite catalyst, the XRD
2
patterns were amplified in the 2θ range of 60−70°, as shown in
Figure 2b. It can be clearly seen that one peak (2θ about 66°)
of A M sample coincides with (110) peak of MnO from the
1
0
6
2
amplified XRD patterns. This indicates that MnO is present in
2
the composite catalyst. However, no peaks of MnO were
2
found in the amplified XRD patterns of A M sample, which
10
1
was due to the lower content of MnO2.
The microscopic morphology of samples was surveyed by
SEM and is shown in Figure 3. The observed powder samples
Figure 4. SEM image and elemental mapping of as-prepared (a−d)
A M sample and (e−h) A M sample.
1
0
1
10
6
and K were in a pretty high dispersion degree as well as Al. Mn
ions were distributed between layers of AlOOH or had already
been incorporated into the bulk phase. Mn and K still maintain
a high dispersion degree in the samples with a high Mn ratio
(
Figure 4e-h).
In order to further explore the specific structure and existing
form of MnO hybridized with AlOOH, TEM characterization
2
was performed (Figure 5). As shown in high-resolution TEM
(
HRTEM) images, composite samples have poor crystallinity
Figure 3. SEM images of as-prepared AlOOH, MnO , and A M
x
2
10
and consist of microcrystals and a noncrystal structure. The
lattice spacing of 0.21 nm observed on all samples was assigned
to the (031) planes of AlOOH, as the content of Mn was too
small and no MnO crystal can be found in A M . In A M
6
samples: (a) AlOOH, (b) MnO , (c) A M , (d) A M , (e) A M ,
2
10
1
10
2
10
5
and (f) A M .
10
6
2
10
1
10
were prepared by hand-grinding after drying and therefore
were all larger particles. Pure AlOOH has a loose layered
structure (Figure 3a) and a high specific surface area of 488
(Figure 5c,d), the lattice spacings of 0.35 and 0.24 nm
correspond to the (002) and (100) lattice planes of
birnessite, which indicate that Mn ions also participated in
the formation of the interface structure with the increase of Mn
ratio. Observed crystallites were small in size and disorderly in
distribution, while amorphous phases account for a large
proportion. This further confirms the Mn ions were dispersed
26
2
m /g (specific surface area data are listed in Table 1).
22
Birnessite MnO was nanospheres composed of nanosheets.
2
The MnO2 sample shown in the Figure 3b appears as
2
nanoclusters. The specific surface area was found to be 98 m /
C
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its Fourier into R space by using IFEFFIT software. A M and
1
0
6
A M did not perform significant Al−Mn peaks, indicating
1
0
5
that manganese oxide and aluminum oxide were phase-
separated. Meanwhile, the peaks of A M and A M are
1
0
6
10
5
slightly lower than pure MnO , which means the O
2
coordination number decreased around Mn, and the Mn
valence state changed. It is consistent with the change of Mn
valence state observed from the result of the XANES (Figure
6
a). Thus, the compounding of MnO with AlOOH can make
2
Mn ions tends to be in different valence states and can also
reduce the grain size of MnO , leading to high catalytic activity
2
for low-concentration HCHO removal. In addition, the
decrease of the O coordination number may also show the
existence of Mn vacancies (V ). Due to the existence of V ,
Mn
Mn
+
K
ion compensates its charge imbalance, facilitating the
formation of surface active oxygen, which in turn acts as the
active site for HCHO oxidation.
26
Figure 5. TEM and HRTEM images of as-prepared A M samples:
In order to further confirm the existence of the multivalent
state of Mn in the composite catalyst, XPS measurements were
also performed to analyze surface chemical compositions and
chemical states, as shown in Figures 7 and 8. The Mn 3s
1
0
x
(
a, b) A M and (c, d) A M .
10 1 10 6
in AlOOH lattice matrix forming a partially crystalline
structure, which encourages the occurrence of the lattice
32,34−36
defects and the exposure of inner atoms.
In addition, it
is interesting to find some annular lattice planes in A M ,
1
0
6
which need further research.
In order to analyze the influence of the fine structure and
chemical valence of the composite catalyst on the catalytic
performance, the as-synthesized samples were performed by X-
ray absorption fine structure (XAFS). Photoelectrons which
were excited by X-ray are scattered by the surrounding
coordinating atoms, leading to X-ray absorption intensity
oscillating with energy. The electronic and geometric local
structures of the system can be obtained by studying these
oscillating signals. XAFS includes two parts, extended X-ray
absorption fine structure (EXAFS) and XANES. The near-edge
X-ray fine structures (XANES) of the samples MnO2,
A M and A M are shown in Figure 6a. The MnO2,
Figure 7. Mn 3s XPS spectra of (a) MnO and (b) A M samples.
2
10
6
spectra of MnO and A M samples are shown in Figure 7. It
2
10
6
is possible to distinguish Mn oxidation states using Mn 3s
37−40
peak.
As we can see from Figure 7, the Mn 3s peaks of
MnO and A M samples have two multiple split components,
1
0
6
10
5
2
10
6
A M and A M lines do not coincide, which means that the
which may be caused by coupling of non-ionized 3s electron
1
0
6
10
5
37,38
chemical valence of Mn has changed due to the loading of
with 3d valence-band electrons.
Magnitude of peak
MnO in AlOOH, and the Mn ions tend to be multivalent. It is
splitting (ΔE) is diagnostic of Mn oxidation state, ΔE for
2
2
+
3+
4+
beneficial to the electron exchange process of manganese ions,
which may activate more active sites to improve the
performance for HCHO decomposition. The EXAFSs of
different samples are shown in Figure 6b. The intensity of the
oscillating peak in the high-energy region is weakened,
MnO (Mn ) 6.0 eV, Mn O (Mn ) ≥ 5.3 eV, MnO (Mn )
2
3
2
37,38
4.7 eV.
sample is 4.79 eV (Figure 7a), which corresponds to the Mn
oxidation state. Magnitude of Mn 3s peaks splitting of A M
6
Magnitude of Mn 3s peaks splitting of MnO
2
4
+
1
0
sample is 5.15 eV (Figure 7b). Compared with the MnO2
sample, the peak splitting of the A M sample is more obvious.
implying AlOOH reduced the grain size of MnO , which
2
10
6
increased the contact of formaldehyde molecules with MnO2.
As we can see from Figure 6c, Mn K-edge XAFS transformed
This indicates that A M sample has a multivalent state of Mn
(Mn , Mn ) in the composite catalyst. It should be noted
10 6
3
+
4+
Figure 6. Fourier-transformed XAFS spectra of different samples (MnO , A M and A M ). (a) The XANES of the different samples. (b) The
2
10
6
10
5
EXAFS of the different samples. (c) The Fourier transform R-space of the Mn K-edge XAFS spectra of different samples.
D
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in AlOOH and its composites was small, while MnO showed
2
strong water absorption. The crystallinity of AlOOH and its
composites was better, so their binding energy of Olatt was
higher than that of MnO . The content of surface adsorbed
2
oxygen species directly reflects the surface −OH group content
of the sample, which plays an important role in the oxidation of
formaldehyde. In all four samples, the peaks of Osurf were
dominant and the Osurf/Olatt ratio was calculated by their peak
areas. AlOOH is a material rich in surface −OH groups. The
ratio of O /O was similarly high in AlOOH and A M but
surf
latt
10
1
small in A M , which was consistent with the previously
1
0
6
described synthesis of MnO by depleting surface −OH. On
2
one hand, −OH can adsorb formaldehyde molecules through
hydrogen bonds to provide sites for reactions. On the other
hand, a large amount of −OH can help accelerate the oxidation
of formaldehyde. It has been pointed out in the literature that
−
OH will directly oxidize the intermediate formate produced
44,45
Figure 8. O 1s XPS spectra of as-prepared samples.
by the initial oxidation of formaldehyde to CO and H O.
2
2
However, there were also some reports that low Osurf/Olatt
ratio was beneficial to Mars−van Krevelen mechanism for
that the Mn 3s spectra for A M sample are absent because
46,47
10
1
formaldehyde oxidation.
It must be pointed out that O
surf
the Mn content in this sample was low and the signal was very
weak. The results of Mn 3s XPS analysis are consistent with
the previous Fourier-transformed XAFS spectra analysis.
Therefore, there must have been some reactions in the
contains a large number of −OH originally present on the
surface of AlOOH, which means that not all of the −OH has a
catalytic reaction activity. In addition, the peak position of Al
2
p remained at 74.3 eV (see Figure S1 and Table 1), indicating
3
+
composite catalyst to produce multivalent Mn ions (Mn ,
that the Al 2p peak did not split and Al remained unchanged as
AlOOH.
4
+
Mn ) and vacancy defects (such as oxygen vacancies). These
reactions may occur in the catalytic process of composites, as
shown in eqs 1−3.
The FT-IR spectra of AlOOH, MnO and A M samples
2
10
x
were measured in order to investigate the surface functional
groups of different samples, as shown in Figure 9. The strong
+
−
A−OH → A + e /−OH
(1)
Mn4 + 2e /−OH ↔ 2Mn + H O + 0.5O
+
−
3+
2
(2)
2
2
+
−
4+
−
4
Mn4 + O → 4Mn + 2e /V + 0.5O
2
O
2
2Mn4 + 2Mn + V + 0.5O
+
3+
→
(3)
O
2
Multivalent Mn ions and a large number of vacancy defects
are conducive to charge exchange, which in turn makes these
chemical reactions possible to finish. According to the eq 3,
oxygen vacancies (V ) were generated in composite catalyst
O
due to the presence of multivalent Mn ions. In addition,
oxygen vacancies may also occur due to the existence of V .
Mn
+
The bond between K and unsaturated oxygen around V is
Mn
not as strong as the bond Mn−O, thus these unsaturated
Figure 9. FT-IR spectra of as-prepared samples.
oxygens are more easily mobile and act as active oxygen
2
6
species. It has been reported that oxygen vacancies were
3+
generated with the presence of Mn in the catalyst to balance
2
7,41
−1
the charge.
The electron exchange of Mn ions involves the
peak at 3412−3424 cm was attributed to the stretching
primary oxidation of formaldehyde and the generation of
vibrations of adsorbed water molecules and structural OH
42
48
reactive oxygen ions. Oxygen vacancies can migrate to the
surface of the catalyst and act as the active site for the
group. The content of −OH reflected by the intensity of
−OH absorption peak of each sample is roughly consistent
43
oxidation of formaldehyde, so its content significantly affects
with the O
content of the samples from the XPS
surf
catalytic performance. In addition, dioxygen molecules in air
measurement. It is noteworthy that the difference between
−OH content of A M samples and AlOOH is large according
−
would be adsorbed on V and dissociated (O + V → O ,
O
2
O
2
10
x
−
O ). The consumed surface −OH is replenished via the
to the FT-IR spectra. However, there is not a big difference
−
reaction with H O (O + H O → 2OH). Surface active
between the Osurf contents of AlOOH and A M samples
2
2
2
10
x
−
−
oxygen (O , O , or −OH groups) has strong catalytic
according to the XPS spectra. There are two main reasons for
this situation: (i) the relative ratio of Osurf/Olatt obtained in
XPS needs to consider the crystallinity of the material and the
2
oxidation ability.
The O 1s XPS patterns shown in Figure 8 can be divided
into three peaks at 529.2−530.4 eV, 531.6−531.8 eV, and
content of the O ; and (ii) the surface adsorbed oxygen of
latt
5
33.0−533.2 eV, corresponding to lattice oxygen (O ),
A M samples in XPS peak contains not only the O but also
latt
10
x
surf
2−
−
43,49
surface adsorbed oxygen species (O ), and adsorbed
a certain amount of active O and O ions.
the small absorption peaks at 1053−1072 cm in AlOOH and
In addition,
surf
3
2
−1
molecular H O. The content of adsorbed molecular H O
2
2
E
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A M samples show that there may be residual alcohols in the
preparation process.
more reactive sites can oxidize the HCHO adsorbed on the
surface in time, making the concentration curve fall faster and
deeper. In fact, a hybrid system combining selective form-
aldehyde adsorption followed by its selective destruction
through catalytic oxidation can be an attractive solution for the
1
0
x
To test the ability of the catalyst to remove indoor
formaldehyde with low concentration at room temperature, a
static reaction for the enclosed space was designed.
Considering the indoor formaldehyde level of ordinary
42,50
formaldehyde removal at low indoor-level concentrations.
3
residents, the initial concentration was 0.5 mg/m (lower
In order to clarify the intermediate species of the catalysts in
the reaction of formaldehyde at room temperature, in situ
DRIFTS spectra of the MnO and A M materials were
concentration), and the degradation curve of formaldehyde
over time is shown in Figure 10. The concentration of
2
10
6
performed. The reaction gas is 100 ppm of formaldehyde gas,
and the equilibrium gas is O and He. The total gas flow rate is
2
1
00 mL/min. The DRIFTS spectra of the MnO and MnO /
2 2
AlOOH composite catalysts after exposure to HCHO for 50
min are shown in Figure 11. A series of infrared absorption
peaks appear on the surface of the birnessite manganese
dioxide during catalytic oxidation. According to previous
studies, the bands located around 1343−1360 (υ (COO−)),
s
−
1
1
542−1592 (υ (COO−)), and 2824−2844 cm (υ(CH))
as
51,52
can be attributed to the formate species,
indicating that the
HCHO adsorbed on the surface of the catalyst is converted
−
1
into a formate species. The band at 1657 cm can be ascribed
53
to water adsorbed on the surface of the catalyst, which may
be derived from water in the reaction gas or products of
Figure 10. Curve of formaldehyde concentration over time with as-
prepared catalysts.
HCHO decomposition. The infrared absorption peaks at 1234,
−1
2
343, and 3684 cm correspond to υas (COO−) of the
carbonate species, carbon dioxide (CO ) adsorbed on the
2
formaldehyde dropped fastest in the first 10 min and tended to
be gentle after 20 min. At 20 min, the formaldehyde removal
rate (P ) of each sample was 32%, 38%, 40% and 52%,
surface, and hydroxyl species on the surface, respectively. In
the dark conditions and the light-on conditions, the infrared
absorption peak of CO appeared, indicating that HCHO is
2
0
2
respectively. A M and A M had similar removal rates and
continuously oxidized after the adsorption process and
adsorption equilibrium. It is worth noting that the intensity
1
0
1
10
4
were higher than MnO , while A M had the best removal
2
10
6
−
1
rate. The change trend of formaldehyde concentration
conforms to the “adsorption−oxidation” formaldehyde remov-
al mechanism. The formaldehyde molecules were first
adsorbed by the catalyst and then degraded on the surface.
After a long time, the degradation curve tends to be flat and
cannot remove completely due to the fact that the catalyst has
poor activity at low temperatures and the formaldehyde cannot
be oxidized in time so that there is a dynamic balance of
of the infrared peak of CO at 2343 cm for the composite
2
catalyst (A M ) is significantly stronger than that of MnO at
1
0
6
2
the adsorption equilibrium (Figure 11b,d). This indicates that
the catalytic performance of the composite catalyst is better
than that of MnO , which is consistent with the analysis result
2
−
1
from Figure 10. The peak around 1475−1479 cm can be
54
ascribed to δ(CH ) in dioxymethylene (DOM). Therefore,
2
in the process of formaldehyde adsorption and oxidation, the
intermediate species of dioxymethylene, formate, and carbo-
“adsorption−desorption” on the catalyst surface. Catalysts with
Figure 11. In situ DRIFTS spectra of (a) HCHO adsorption process of MnO , (b) HCHO adsorption equilibrium of MnO , (c) HCHO
2
2
adsorption process of A M , and (d) HCHO adsorption equilibrium of A M .
1
0
6
10
6
F
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Figure 12. Reaction pathway of HCHO on MnO /AlOOH composite catalyst.
2
nate were produced on the surface of the MnO . A negative
imbalance, facilitating the formation of surface active oxygen,
which in turn acts as the active site for HCHO oxidation.
2
−
1
peak of 3684 cm appeared, indicating that the surface −OH
participates in the catalytic oxidation reaction and gradually
would be consumed as the reaction progresses. The infrared
absorption peak of the intermediate species of A M during
The compounding of MnO
with AlOOH effectively
2
improved the catalytic activity, and the mechanism can be
explained by the following factors. First, the composite catalyst
has a high specific surface area and a large number of surface
10
6
HCHO adsorption oxidation is about the same as that of
−
OH groups, which facilitate the adsorption of formaldehyde
MnO , however, lower accumulation of dioxymethylene
2
molecules and provide a large number of reaction sites.
Adsorption is an important part of the formaldehyde removal
process and exhibits a more pronounced effect at low
temperatures and low HCHO concentrations. Second, the
synthesized complex has good dispersibility and partially
crystalline structure. This amorphous structure produces a
large number of multivalent states of Mn ions and vacancy
defects which facilitates charge exchange (eqs 1−3). The
exchange of charge between Mn ions and vacancy defects
makes the nearby −OH highly oxidizing (eq 2). The
catalytically active −OH serves as the active site for oxidation
of formaldehyde. Mn in A M has more morphology
species appear on in situ infrared spectrum of A M . The
10
6
absorption peak indicates that HCHO has a faster oxidation
rate on the surface of A M . Moreover, compared with
10
6
infrared spectrum of MnO , the accumulation of carbonate
2
species on the surface of A M was also lower, indicating that
10
6
the intermediate species had a faster conversion rate on the
surface of the A M catalyst. In situ infrared spectra of two
1
0
6
catalysts were observed to be gradually consumed by surface
hydroxyl species, which is consistent with the conclusion that
the catalyst surface −OH accelerates catalytic oxidation of
formaldehyde in XPS. A M has a lower negative peak,
10
6
1
0
6
indicating that surface hydroxyl group participates in the
catalytic oxidation reaction at room temperature, A M
6
(
crystalline MnO and amorphous Mn ions), which makes
2
1
0
A M have a higher oxidation activity −OH, explaining its
optimal catalytic activity. The report mentioned that the
combination of active oxygen ions (O , O ) generated by the
1
0
6
catalyst plays a synergistic role by modifying AlOOH, and
surface −OH is activated by the advantage of high specific
surface area of AlOOH. Therefore, the HCHO decompose rate
on the surface of A M becomes high, and the conversion rate
−
−
2
complex exchange of O , oxygen vacancies, and lattice oxygen
2
1
0
6
with H O molecules can generate −OH as a supplement to
2
of the intermediate species also changes accordingly, thereby
facilitating the catalytic oxidation process of formaldehyde.
−
−
OH and self-recovery of the catalyst (Figure 12). The active
OH which can directly oxidize formaldehyde can also be
regarded as an existing form of active oxygen ion. In addition,
The catalytic cycle of HCHO oxidation over MnO /AlOOH
2
composite catalyst with high specific surface area and a large
amount of −OH groups is summarized in Figure 12. HCHO is
first adsorbed on composite catalyst, which is enhanced by
water contained in the composite catalyst via forming
hydrogen bonding with formaldehyde. Adsorbed HCHO and
its hydrate (methanediol, CH (OH) ) would be oxidized by
the existence of multivalent Mn ions and V in the composite
Mn
+
catalyst makes it possible for the K ion to compensate its
charge imbalance, facilitating the formation of surface active
oxygen. These surface active oxygen species in turn act as the
active site for HCHO oxidation.
2
2
Unlike most studies that focus on improving the perform-
−
−
surface active oxygen (O , O , or −OH groups) of composite
catalyst into DOM, formate, carbonate and CO sequentially
2
ance of MnO , this is a new type of highly efficient
2
2
formaldehyde catalyst obtained through MnO modification.
2
according to the analysis results of in situ DRIFTS spectra
AlOOH has a large number of surface −OH but does not have
(Figure 11). Based on the analysis of Mn 3s and O 1s XPS
the ability to oxidize formaldehyde. By combining with MnO ,
2
spectra (Figures 7 and 8), oxygen vacancies were generated in
the composite catalyst with the presence of multivalent Mn
ions and VMn in the catalyst. Then, a dioxygen molecule in air
part of the surface −OH has the ability to oxidize
formaldehyde, and its comprehensive catalytic performance
exceeds MnO2 itself. The A M sample with a partially
10
6
−
would be adsorbed on V and dissociated (O +V → O ,
crystalline structure has better performance. The MnO2/
AlOOH composite catalyst takes advantage of the high specific
surface area of AlOOH and stimulates the activity of surface
−OH, providing a new idea for the improvement of the
O
2
O
2
−
O ). The consumed surface OH is replenished via the reaction
with H O (O + H O → 2OH). Thus, the catalytic cycle is
−
2
2
2
finished. Due to the existence of multivalent Mn ions and VMn
+
in the catalyst, the K ion may compensate its charge
catalytic activity of transition-metal oxides such as MnO .
2
G
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Article
CONCLUSIONS
of Metallurgical Emission Reduction and Resources Recycling,
Anhui University of Technology, Ministry of Education,
Ma’anshan 243002, China
Fan Dong − Research Center for Environmental Science and
Technology, Institute of Fundamental and Frontier Sciences,
University of Electronic Science and Technology of China,
Chengdu 611731, China; orcid.org/0000-0003-2890-9964
■
The MnO /AlOOH composite catalyst was prepared success-
2
fully by redox reaction of potassium permanganate KMnO
4
and surface −OH of AlOOH at room temperature. Various
proportions of MnO were uniformly dispersed in AlOOH,
2
forming a partially crystalline structure. In the static test that
simulated indoor formaldehyde removal at room temperature,
the A M sample showed the best ability to remove indoor
Complete contact information is available at:
1
0
6
low-concentration formaldehyde. Removal of indoor form-
aldehyde should be divided into “adsorption−oxidation”
processes. The composite has a higher specific surface area
and a large amount of surface −OH which facilitates the
adsorption and fixation of formaldehyde. The partially
crystalline structure formed by the complex has a variety of
valence states of Mn ions and a large number of vacancy
defects. Through the charge exchange of Mn ions and vacancy
defects, the nearby −OH can have a strong oxidizing activity.
https://pubs.acs.org/10.1021/acs.inorgchem.0c00852
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by National Natural
Science Foundation of China (grant no. 51572113). The
authors thank beamline BL14W1 (Shanghai Synchrotron
Radiation Facility) for providing the beam time. The authors
also thank Dr. F. Zhuge (Ningbo Institute of Materials
Technology and Engineering, CAS) for his help with
measurement and characterization.
MnO /AlOOH composite catalyst has a strong catalytic effect
2
on indoor formaldehyde with low concentration by a strong
oxidation activity from the nearby −OH. This study proposes
new ideas for the improvement of catalytic performance in the
structure and composition of the catalyst. It is expected that
this modification strategy may be effective for the other
materials, except AlOOH to remove indoor low-concentration
formaldehyde.
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ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00852.
■
*
(
Al 2p XPS spectra of as-prepared samples (PDF)
(
at room temperature using TiO supported metallic Pd nanoparticles.
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