Nanocrystalline MnO2 on an activated carbon fiber for catalytic formaldehyde removal

Article abstract of DOI:10.1039/c6ra15463h

Three different types of nanocrystalline MnO₂, namely, α-MnO₂, γ-MnO₂ and δ-MnO₂, were successfully synthesized via a co-precipitation method, which has the advantages of easy preparation, low cost, size uniformity and excellent crystallinity. It also avoids operating at high temperatures and pressures. The nanocrystalline MnO₂ were then tested for formaldehyde catalytic oxidation at 25 °C. The results suggested that δ-MnO₂ had the highest catalytic activity. Hence, δ-MnO₂ was synthesized using the same method to modify an activated carbon fiber (ACF) substrate. Further tests showed that the as-prepared MnO₂/ACF samples can significantly improve the removal of formaldehyde at room temperature. The MnO₂ content in MnO₂/ACF had great influence on the breakthrough time and we found that the MnO₂ content did not best perform at greater magnitudes but were optimal at 16.12 wt%. The formation method developed in this study may become a promising technique to improve the catalytic activity in formaldehyde removal.

Full text of DOI:10.1039/c6ra15463h

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DOI: 10.1039/C6RA15463H
Journal Name
ARTICLE
Nanocrystalline MnO2 on Activated Carbon Fiber for
Catalytic Formaldehyde Removal
Zijian Dai a, Xiaowei Yu b, Chen Huang a, Meng Li a, Jiafei Su b, Yaping Guo b, He Xu b, Qinfei Ke, *, b, a
Received 00th January 20xx,
Accepted 00th January 20xx
Three different types of nanocrystalline MnO2 namely α-MnO2, γ-MnO2 and δ-MnO2 were successfully synthesized by the
co-precipitated method, which has the advantages of easy preparation, low cost, size uniformity and excellent crystalline.
It also avoids operating at high temperatures and pressures. The nanocrystalline MnO2 were then tested for formaldehyde
catalytic oxidation at 25 oC. The results suggested that δ-MnO2 had the highest catalytic activity. Hence, δ-MnO2 was
synthesized by the same method to modify the activated carbon fiber (ACF) substrate. Further tests showed that the as-
prepared MnO2/ACF samples could significantly improve the removal of formaldehyde at room temperature. MnO2
contents in MnO2/ACF had a great influence on the breakthrough time and we found that the MnO2 contents were not
better at greater magnitudes but rather optimal at 16.12 wt%. The formation method developed in this study might
become a promising technique to improve the catalytic activity in formaldehyde removal.
DOI: 10.1039/x0xx00000x
www.rsc.org/
many chemical catalysts only work in severe condition, e.g., TiO2
Introduction
becomes less effective without UV light and may generate harmful
byproducts during the catalytic processes,9 most of the metal
catalysts have to work at high temperature, and the exorbitant
prices of noble metal catalysts made it impractical to be
popularized.10-12 Plasma technology showed poor performance
under low concentrations and is also easy to generate harmful
byproducts.3 Based on these studies, the combination of adsorption
and catalytic oxidation should be an improved method to remove
indoor formaldehyde at low concentration.13
Among various volatile organic compounds (VOCs), formaldehyde
remains to be a major pollutant, especially in indoor environment.
Formaldehyde is emitted from construction and decoration
materials, and can cause physical and mental illness at elevated
concentrations (WHO 2006). What is worse, exposure to the
formaldehyde environment would result in nasopharyngeal cancer
and other serious diseases in humans.1-3 Thus, the removal of
formaldehyde in indoor environment is of utmost importance.
A lot of methods have been used for formaldehyde removal,
4
6
among which physical adsorption,2, catalytic oxidation5, and
plasma decomposition7, 8 are most commonly used. However, these
methods have certain limitations. For physical adsorption approach
which is most widely used in commercial air-cleaner, short
saturated absorption time and difficult treatment of abandoned
adsorbent make it less effective. For catalytic oxidation approach,
Activated carbon fiber （ACF） has been regarded as the most
promising adsorbent in formaldehyde removal due to the abundant
micro pores, large surface area, low pressure drop and excellent
14, 15
adsorption capacity.4,
The adsorption capacity of ACF for
formaldehyde is not ideal in ambient temperature, because of the
weak interaction between polar formaldehyde and the hydrophobic
surface of porous carbon.16, 17 In order to enhance the adsorption
capacity of ACF, some studies utilized surface modification to
promote the adsorption property, e.g., introducing a functional
substance such as p-aminobenzoic acid,17 CuO18 and TiO2.19 Other
transition metal oxide catalysts have also been proved to possess
high catalytic activity for HCHO complete oxidation, such as CeO2,11,
a.Key Laboratory of Textile Science &Technology (College of Textiles, Donghua
University), Ministry of Education, Shanghai 201620, P. R. China. E-mail:
kqf@dhu.edu.cn
b.Environmental Materials Research Center, Shanghai Normal University, Shanghai
200234, P. R. China
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [Summary of the catalytic
oxidation for formaldehyde removal at ambient temperature in recent literature in
Table S1 and amounts of δ-MnO2 attached in MnO2/ACF in Table S2. The
formaldehyde removal amount was calculated.]. See DOI: 10.1039/x0xx00000x
20-22
23-25
26-28
Co3O4
and MnO2.13,
While most of the previous
research focused on the removal of high-concentration
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formaldehyde (>100 ppm), in indoor environment, however, the
Journal Name
Preparation of δ-MnO2 Modified ACF. Nanocrystalline MnO2
View Article Online
concentration of formaldehyde is generally low. Therefore, the were incorporated in ACF by a co-precipitatiDoOnIm: 1e0t.1h0o3d9./CB6riReAf1ly5,4A63CHF
investigation in catalyst capacities of the formaldehyde at low was first immersed in MnSO4 solution, and then KMnO4 solution
o
concentration levels has practical implications.
was added dropwise and kept stirring violently at 80 C for 2 h.
Recently,
a
multitude of researches have concerned Subsequently, the ACF was taken out and heated to 80 oC for 12 h,
formaldehyde catalytic removal at ambient temperature.6, 13, 27, 29-35 finally dried at 120 oC for 5 h in vacuum. By altering the
Table S1 summarized the reported catalytic materials, concentration of KMnO4 and MnSO4 solution, different MnO2
concentration level, temperature, test methods, and their activities content in MnO2/ACF could be obtained.
on the catalytic oxidation of formaldehyde at ambient temperature
Characterization. Scanning electron microscopy (SEM) images were
over related catalysts. To our knowledge, no research has focused
carried out on a Hitachi S-4800 field emission scanning electron
on formaldehyde removal using MnO2-based ACF substrates.
microscope. X-ray diffraction (XRD) patterns were measured on a
Herein, three kinds of nanocrystalline MnO2 with different phase
D/max-II B, (SHIMADZU，Japan) using Cu Kα radiation (λ = 0.1542
structures (α-MnO2, γ-MnO2 and δ-MnO2) were synthesized by the
nm) operated at 40 kV and 30 mA. The α-MnO2, γ-MnO2, δ-MnO2,
co-precipitated method. MnO2 was selected because no harmful
ACF and MnO2/ACF were scanned from 5o to 90o with a rate of 5o
byproducts were generated during the formaldehyde oxidation
process.36 Catalytic tests at room temperature revealed that δ-
min-1. The δ-MnO2 content of each MnO2/ACF was determined
according to ASTM D2866-11. Ash content in individual ACF (n=5)
MnO2 had the best activity. Based on this result, ACF were modified
was first measured by following methods. The crucible was placed
with δ-MnO2, and the obtained samples showed significant removal
in a furnace at 650 oC for 1 h, cooled down to room temperature to
of formaldehyde.
record the weight (W0). Next, the ACF was added into the crucible,
and its weight was calculated (W1). Then the crucible was
transferred into a preheated muffle furnace at 650 oC for 3 h. Finally,
Experimental
the crucible was cooled to room temperature in a desiccator, and
Materials. KMnO4, MnSO4 (analytical pure reagent) were purchased
its weight was recorded as W2. The ash content was calculated
from Sinopharm Chemical Reagent Co., Ltd and used without
using the Eq (2):
further purification. The Pitch-based ACF was obtained from Osaka
W
2 − W0
(
)
Ash % = W
× 100
(2)
Gas Chemicals Co., Ltd. ACF was first washed with deionized water
for three times to remove ash and dried at 130 oC for 12 h in
vacuum before use.
1 − W0
Then the MnO2/ACF was added into the crucible, total weight
was calculated (W3). The crucible was put into the muffle furnace at
a temperature of 650 oC for 3 h. Finally, the crucible was placed in a
desiccator for conditioning, and its weight was determined (W4).
The weight of ash in MnO2/ACF was defined as W5. The δ-MnO2
content of each MnO2/ACF was derived as follows;
Synthesis of α-MnO2, γ-MnO2 and δ-MnO2. The α-MnO2, γ-MnO2
and δ-MnO2 were prepared by a co-precipitation method. The
redox reaction was listed as follows:37, 38
2KMnO4 + 3MnSO4 + 2H2O → 5MnO2 + 2H2SO4 + K2SO4
(1)
2
(
)
(
)
W5 = W3 − W4 × Ash% + W3 − W4 × Ash%
(3)
(4)
To synthesize α-MnO2, 3.16 g KMnO4 and 4.53 g MnSO4 were
dissolved in 50 mL deionized water respectively and kept stirring at
90 oC. Then the KMnO4 and MnSO4 solution were added dropwise
into 50 mL deionized water, respectively. The obtained solution was
then kept stirring and reacted at 90 oC for 2 h. The products were
washed, centrifuged, and dried at 80 oC for 12 h. For γ-MnO2, 3.16 g
W
4 − W0 − W5
(
)
δ-MnO2 % =
× 100
W
3 − W0
Nitrogen adsorption-desorption isotherms of ACF and MnO2/ACF
were measured on an ASAP 2020 (Micromeritics Instrument, USA)
at 77 K. The surface area of the α-MnO2, γ-MnO2, δ-MnO2, ACF and
MnO2/ACF were obtained based on the Brunauer-Emmett-Teller.2,
o
39
KMnO4 and 18.12 g MnSO4 were synthesized at 80 C for 2 h. For δ-
Pore size distributions of α-MnO2, γ-MnO2, δ-MnO2 , ACF and
o
MnO2, 3.16 g KMnO4 and 2.26 g MnSO4 were synthesized at 80 C
MnO2/ACF were calculated by non-localized density functional
theory (NLDFT).40 X-ray photoelectron spectroscopy (XPS) was
for 2 h.
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performed on a PHI5700 ESCA system equipped with aluminum
The activity tests were also performed in static state to clarify the
anode (Al Kα= 1486.6 eV radiation) at a pressure of 2 × 107 Torr. All mechanism for HCHO removal. The method of static activity tests
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the binding energies were calibrated using contaminated carbon was adapted from previous reports.29, 31 ADsOtIa:i1n0l.e1s0s39s/tCe6eRlAr1e5a4c6t3oHr
(C1s = 284.6 eV). The number of binding energy peaks was with a volume of 0.5 L was covered by a polytetrafluoroethylene
determined by the deconvolution process. Hydrogen temperature- layer on its inner wall. The MnO2/ACF sample was first placed on
programmed reduction (H2-TPR) measurements were performed the bottom of a quartz Petri dish. After putting the dish into the
with Chemisorption Analyzer (AutoChem II 2920), the H2 reactor, 300 ppm of HCHO was emitted into the reactor using an S-
consumption and the mass signal was detected by a thermal 4000 Gas Mixing system (Environics, USA). After the concentration
conductivity (TCD). δ-MnO2, ACF and MnO2/ACF (ca. 50 mg) were of HCHO was stabilized at 150 ppm, the cover of the sample dish
introduced in the U-type quartz microreactor with a flow of 5% was removed to start the adsorption and catalytic reaction of HCHO.
H2/Ar at a rate of 50 mL min-1. The samples were heated from 60 oC HCHO, CO2, CO and water vapor were measured respectively during
to 800 oC at a rate of 10 oC min-1.
test at 25 oC. The yield of CO2 (ΔCO2) and the concentration
variation of HCHO were calculated to analyze the removal ratio.
Adsorption and Catalytic Activity Tests. The room temperature
adsorption and catalytic activity measurements of α-MnO2, γ-MnO2,
δ-MnO2, ACF and MnO2/ACF were performed in a fixed-bed quartz
reactor (length = 500 mm, diameter = 10 mm) at 25 oC. Gas
formaldehyde was generated by flowing pure air (20 ± 2% RH)
through an S-4000 Gas Mixing system (Environics, USA) and the
total gas flow rate was 200 mL min-1, monitored by a mass flow
control system. The concentration of formaldehyde was kept at 15
ppm. For three different MnO2，100 mg samples were used in each
test, corresponding to a gas hourly space velocity (GHSV) of 120 000
mL (gcat h)-1. While 200 mg MnO2/ACF samples with a packing length
of 20 mm were kept in quartz reactor in each test, and GHSV was 60
000 mL (gcat h)-1. HCHO, CO2, CO and water vapor were analyzed
online by a photoacoustic IR multigas monitor (INNOVA AirTech
Instruments Model 1412i). Before each test, the upstream
concentration was first measured for at least 4 h to ensure the
HCHO concentration was constant. During the test, the downstream
gas was also collected hourly and subsequently analyzed by a GC
9800 gas chromatograph equipped with TCD and FID detectors. No
other carbon containing compounds except CO2 in the products
were detected for the tested catalysts. HCHO conversion was
determined by the following equation:
Results and discussion
Morphology Characterization. Fig. 1 depicted the procedure of
MnO2/ACF synthesis. Nanocrystalline MnO2 were incorporated in
ACF by a co-precipitation method. Fig. 2a-f showed the SEM images
of α-MnO2, γ-MnO2, δ-MnO2, ACF, 0.03-MnO2/ACF and 0.05-
MnO2/ACF. The morphology of each MnO2 sample was determined
by different crystal phases. α-MnO2 showed
a needle-like
morphology with the diameters of 50 nm and lengths of 300-900
nm. γ-MnO2 presented an irregular nanoparticle structure, while δ-
MnO2 displayed
a flower-like nanostructure without clear
interparticle boundaries with an average diameter of 200 nm. As
shown in Fig. 2d, the surface of ACF was smooth before loaded with
MnO2. After loading, the surface of ACF became coarser (see Fig.
2e-f). When the concentration of MnSO4 reached at 0.05 mol L-1,
the entire fiber surface was covered by MnO2, suggesting the
maximum MnSO4 feeding capacity was 0.03 mol L-1.
HCHOin− HCHOout
HCHO conversion (%) =
× 100
(5)
HCHOin
Where [HCHO]in (ppm) is the inlet concentration of HCHO before
passing the catalyst, and [HCHO]out (ppm) is the output
concentration of HCHO after passing the catalyst. Such above
parameters of HCHO conversion were used for calculating the
removal capacity of MnO2/ACF samples.
Fig. 1 Scheme of MnO2 /ACF synthesis.
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Journal Name
MnSO4 solution (0.01 mol L-1, 0.02 mol L-1, 0.03 mol L-1, 0.04 mol L-1,
View Article Online
DOI: 10.1039/C6RA15463H
0.05 mol L-1, respectively). The obtained MnO2/ACF samples with
different δ-MnO2 contents were labeled as #X MnO2 /ACF (X
represented the concentration of MnSO4 solution). The detailed
information was shown in Table S2.
Specific surface area and Porosity. Table 1 lists the specific surface
areas (SBET), average pore sizes (Dp) and total pore volumes (Vp) of
α-MnO2, γ-MnO2 and δ-MnO2. The δ-MnO2 exhibited the highest
SBET, Dp and Vp values. Nitrogen adsorption isotherms at 77 K of ACF,
0.03-MnO2/ACF and 0.04-MnO2/ACF were presented in Fig. 4a. The
curve of ACF exhibited type I isotherm, according to the IUPAC
classification,43 which indicated that the sample was microporous.
With the presence of MnO2, a hysteresis loop emerged in the
adsorption-desorption isotherms, which was attributed to the
capillary condensation in mesopores of MnO2. The relevant BET
surface areas of ACF and MnO2/ACF are shown in Fig. 4a. The
results showed that pure ACF had the highest value of 961.20 m2 g-1,
whereas 0.03-MnO2/ACF and 0.04-MnO2/ACF showed the value of
679.12 m2 g-1 and 574.92 m2 g-1, respectively. The results showed
that as more nanocrystalline MnO2 were loaded on ACF, slight
decrease of the surface area was occurred. This is because that
nanocrystalline MnO2 on the surface of ACF blocked the pores,
leading the adsorption amount to decrease.42 Fig. 4(b) showed the
pore size distribution of ACF, 0.03-MnO2/ACF and 0.04-MnO2/ACF.
Besides the micropores, mesoporous values were also observed in
the patterns of 0.03-MnO2/ACF and 0.04-MnO2/ACF, which was
identical to the results of the adsorption-desorption isotherms. The
results also indicated that the pore size increased with the increase
of MnO2 loading amount.
Fig. 2 SEM images of (a) α-MnO2, (b) γ-MnO2, (c) δ-MnO2, (d) ACF, (e) 0.03-MnO2/ACF
and (f) 0.05-MnO2/ACF.
XRD Analysis. XRD patterns were measured to identify the
crystallographic structures of α-MnO2, γ-MnO2, δ-MnO2, ACF and
MnO2/ACF (see Fig. 3).
As shown in Fig. 3, the typical peaks of different MnO2 can be
clearly identified as α-MnO2 (JCPDS 44-0141), γ-MnO2 (JCPDS 14-
0644) and δ-MnO2 (JCPDS 80-1098). The ACF pattern obtained two
broad diffraction peaks at 25o and 43o, respectively, both of them
were characteristics peaks of disordered carbon.41, 42 Compared to
pure ACF pattern, the pattern of MnO2/ACF showed more
diffraction peaks at 12.5o, 37.1o and 66.0o, which were in
accordance to the characteristics peaks of δ-MnO2, suggesting that
the δ-MnO2 has been successfully loaded on the surface of ACF.
Content of δ-MnO2. The initial ash content in pure ACF sample was
~6.72%. The contents of δ-MnO2 in MnO2/ACF samples were
calculated by Eq (5) and the result were ~5.62%, 11.74%, 16.12%,
22.42% and 29.33%, depending on the initial concentration of
Activity Test. The HCHO oxidation activities of three kinds of MnO2
catalysts are shown in Fig. 5. The catalytic activities of δ-MnO2
decreased dramatically at first 5 hours, and the rate of HCHO
conversion maintained at ~25% after 40 hours. The other catalysts,
α-MnO2 and γ-MnO2 showed lower catalytic activities than that of
δ-MnO2. The HCHO conversion using α-MnO2 turned to 20% after
40 hours. Because the pattern of α-MnO2 manifested a similar trait
to δ-MnO2, the α-MnO2 catalyst had an oxidation activity similar
with δ-MnO2. γ-MnO2 displayed the lowest catalytic activity. A low
conversion rate was found at the beginning of the test, after which
it dropped continuously and then stabilized at 15%. Previous
studies28, 44 have found that compared with other properties (such
Fig. 3 XRD patterns of α-MnO2, γ-MnO2, δ-MnO2, ACF and MnO2/ACF.
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DOI: 10.1039/C6RA15463H
Fig.
4
(a) Nitrogen adsorption/desorption isotherms and (b) corresponding pore size distributions for ACF, 0.03-MnO2/ACF and 0.04-MnO2/ACF.
as specific surface area, degree of crystallinity, reducibility and identify the abilities of ACF and MnO2/ACF in catalytic oxidation of
oxidation state of manganese), the tunnel structures of MnO2 is the HCHO. As shown in Fig. 5b, pure ACF had a fast breakthrough time
primary factor to determine catalytic activity. δ-MnO2 normally of less than 60 min, and the 80% breakthrough time was nearly 100
forms a 2D layer structure, while α-MnO2 and γ-MnO2 obtained 1D min. In comparison, both of the breakthrough times of 0.01-
tunnels in their structures. In detail, for α-MnO2, it consists of [2 × 2] MnO2/ACF were prolonged to 50 min and 120 min, respectively.
and [1 × 1] channels and γ-MnO2 contains both [1 × 1] and [1 × 2] More MnO2 contents could also lead to better catalytic activities,
channels.45 Since the effective diameter of the [2 × 2] channel was which could be proved by the results of 0.02-MnO2/ACF and 0.03-
more suitable for the HCHO diffusion and adsorption of HCHO MnO2/ACF. However, when the MnO2 content increased to more
molecules than [1 × 1] channel during the reaction, the α-MnO2 had than 22.42% (critical point), the HCHO oxidation efficiency declined,
higher catalytic activity in HCHO oxidation compared with γ-MnO2. suggesting that overload of MnO2 might cause agglomeration of
For δ-MnO2, the interlayer structure would facilitate the diffusion nanoparticles (see Fig. 2f), which blocked the pores of ACF
and absorption of HCHO molecules to active sites more than the [2 substrates. Therefore, the optimal content of MnO2 was set as
× 2] channel, so the δ-MnO2 showed the best catalytic activity. 16.12%, in which, the 80% breakthrough time and formaldehyde
Therefore, δ-MnO2 was selected in this article to modify ACF in removal capacities at 80% breakthrough point were 550 min and
order to enhance the oxidation of HCHO.
0.74% (see Fig. S1).
According to Pei’s study, a sorbent bed with higher than 80%
breakthrough could be considered ineffective in practice for indoor
air cleaning.46 Hence, the 80% breakthrough point was chosen to
The service life is a critical factor in practical applications for
adsorbents and catalysts. The removal amount of HCHO by
MnO2/ACF at room temperature was tested (n=5) and the results
are shown in Fig. 6. After 5 cycles, the removal capacity showed no
significant difference when compared with the first cycle, while the
yield of CO2 presented a large variation. Besides, it’s encouraging to
see that the MnO2/ACF could be reactivated by being dried at 130
oC for 4 h in vacuum. Based on the results of HCHO oxidation rate
and CO2 generation rate, the reactivated MnO2/ACF was able to
maintain its original function indicating HCHO molecules absorbed
by active carbon fiber and MnO2 could be partially degassed and
removed by high temperature and vacuum pressure. The HCHO
Table 1 Physical parameters of three nanocrystalline MnO2.
Surface areas
SBET (m2 g-1)
Pore diameter
Dp (nm)
Pore volume
Vp (cm3 g-1)
Samples
α-MnO2
γ-MnO2
δ-MnO2
70.11
74.84
87.50
14.48
15.70
19.85
0.13
0.24
0.28
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Fig. 5 (a) HCHO catalytic performance with time on stream over α-MnO2, γ-MnO2 and δ-MnO2 catalysts. Reaction conditions: 25 oC, 15 ppm HCHO, 20 ± 2% RH, GHSV = 120 000 mL
(gcat h)-1. (b) Breakthrough curves of HCHO over ACF and MnO2/ACF. Reaction conditions: 25 oC, 15 ppm HCHO, 20
± 2% RH, GHSV = .
60 000 mL (gcat h)-1
Fig. 6 Concentration variations of HCHO and CO2 with reaction time for 0.03-MnO2/ACF in the static recycle tests.
removal activity by MnO2/ACF was a multiple process (see Fig. 8). Finally, the HCHO removal reached at a stationary state (stage III).
13, 47-49
Based on the previous reported results,
the mechanism of
catalytic oxidation of HCHO with a catalyst was described as an
adsorption-degradation-desorption process, which could also be
used here to explain the oxidation process of HCHO using the
MnO2/ACF sample. As shown in Fig. 7, the concentration of HCHO
decreased sharply in the first 60 min, after which the decreasing
speed became slow. In comparison, the CO2 concentration kept
increasing linearly, regardless of the reaction time. Such trend
indicated that the removal mechanism of HCHO could attribute to
the absorption of ACF at the first 60 min and subsequent oxidization
of MnO2 till 540 min. Initially, most of the HCHO molecules were
absorbed by active carbon and only a tiny proportion of HCHO was
oxidized by MnO2, emitting a small amount of CO2. At the second
stage, HCHO was oxidized by MnO2 thus generating lots of CO2.
Fig.
7 Concentration variations of HCHO and CO2 with reaction time for 0.03-
MnO2/ACF in the static test.
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In general, the ACF substrate and nanocrystalline MnO2 acted
synergistically during the HCHO removal process. The MnO2
provided catalytically active sites, and promoted the effective
degradation of HCHO molecules. The ACF could provide fields for
those active sites and also protect the MnO2 from potential flue gas
poisons. As compared with Table S1, the MnO2 and MnO2/ACF
samples are competitive in HCHO oxidation at room temperature.
Although the catalytic activity for HCHO oxidation of the MnO2/ACF
material is less effective than some noble metal catalysts, it has
great advantage in catalytic HCHO removal at room temperature.
View Article Online
DOI: 10.1039/C6RA15463H
Fig. 8 The proposed mechanism for the catalytic oxidation of HCHO over MnO2/ACF.
Mn4+/Mn3+ also occurred in the HCHO oxidation process. The
deconvolution of O1s XPS spectra were displayed in Fig. 9b. The
spectra of as-prepared sample were consisted of two main peaks at
530.0 and 531.4 eV. The former peak was attributed to the lattice
oxygen (O2-) denoted as Olatt, and the latter peak was assigned to
surface adsorbed oxygen (such as OH) denoted as Oads. From Table
2, it is clear that the surface molar ratios of Olatt/Oads of as-prepared
and after-test samples were 1.71 and 0.55, respectively, suggesting
that when the HCHO oxidation progress completed, the surface
molar ratios of Olatt/Oads declined drastically. Another peak at a
binding energy of 533.0 eV was caused by the presence of adsorbed
water molecule on the surface of ACF and MnO2, which also
appeared at the spectra of after-test sample. Room temperature
oxidation of HCHO follows the Mars and van Krevelen mechanism
as mentioned by previous studies, according to which the
formaldehyde was finally oxidized to H2O and CO2.45 Therefore, it
can be deduced that during the reaction, the formaldehyde was
oxidized by the lattice oxygen of MnO2, and with the consumption
of lattice oxygen, the molecular oxygen was generated in gas phase.
These results proposed that abundant lattice oxygen would
enhance the activity of MnO2 catalysts for HCHO oxidation.
XPS Analysis. The surface electronic state of MnO2/ACF was
investigated by XPS analysis and the result was shown in Fig. 9
(“Before” represented the as-prepared sample while “After”
represented the sample which had used in HCHO oxidation). After
the deconvolution of Mn2p spectrum at 2p3/2, three peaks were
emerged. The peaks at 642.6 eV and 641.5 eV indicated the
presence of Mn4+ and Mn3+, respectively, while another peak at
644.0 eV was assigned to satellite peak.50-52 Compared with the
spectra of as-prepared sample, the spectrum of after-test sample
became broader and both peaks of Mn4+ and Mn3+ had a shift of 0.5
eV to a lower binding energy. This indicated that there was a
significant change for the chemical state of manganese before and
after reaction. MnO2/ACF acted as catalysts in the oxidation process
of HCHO. Table 2 further revealed the molar ratios of Mn4+/Mn3+ on
the surface of as-prepared and after-test samples and the value
decreased from 1.54 to 1.13. Zhang et al,6 recommended that more
surface Mn4+ ions might provide more oxygen vacancies for an
oxide material, which was positive to the adsorption, activation and
migration of oxygen in the gas phase. For this reason, Mn4+ played
an important role in HCHO oxidation and the redox cycle for
Fig. 9 XPS spectra of MnO2/ACF samples: (a) Mn 2p and (b) O 1s.
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ARTICLE
Journal Name
Table 2 Physical parameters of three nanocrystalline MnO2.
View Article Online
DOI: 10.1039/C6RA15463H
Binding energy (eV)
Samples
Molecular ratio
Mn4+/Mn3+
Binding energy (eV)
Molecular ratio
Olatt/Oads
Mn4+
Mn3+
Olatt
Oads
As-prepared
After-test
624.6
642.1
641.5
641.0
1.54
1.13
531.0
531.0
531.4
531.6
1.71
0.55
of nanocrystalline MnO2.54 In addition, since there existed an
intense interaction between ACF and δ-MnO2, MnO2/ACF was more
easily decomposed. To a certain extent, such decomposition could
reflect the oxygen mobility in the samples. Since both δ-MnO2 and
ACF were involved with adequate mobile oxygen during the
reaction, active oxygen would be motivated and enhanced the
reduction of formaldehyde.
Conclusions
The α-MnO2, γ-MnO2 and δ-MnO2 were successfully prepared by
co-precipitation method. Formaldehyde catalytic oxidation results
proved that all nanocrystalline MnO2 had catalytic activities at room
temperature, and the δ-MnO2 obtained the highest activity. The
influence of MnO2 contents in MnO2/ACF on the breakthrough time
has also been studied and the optimal content of MnO2 was found.
The results suggest that the ACF substrate modified by
nanocrystalline MnO2 may be potentially used for formaldehyde
removal in indoor environment.
Fig. 10 H2 -TPR profiles of the δ -MnO2, ACF and MnO2/ACF samples.
Reducibility of Catalyst. H2-TPR measurements were performed to
investigate the reducibility of the related samples. Fig. 10 showed
the TPR profiles of δ-MnO2, ACF and MnO2/ACF. For the δ-MnO2
o
o
catalyst, two reduction peaks were observed at 288 C and 336 C.
It is obvious that the ratio of the lower temperature peak to the
higher temperature peak was about 1, which could lead to the
conclusion that the main product of the δ-MnO2 catalyst could be
Acknowledgments
This work was supported by the Shanghai Municipal Science and
Technology Commission (No. 14520502800), “Chen Guang” project
supported by Shanghai Municipal Education Commission and
Shanghai Education Development Foundation (NO. 14CG34) and
Fundamental Research Funds for the Central Universities (NO.
2232014d3-15).
MnO. The reduction route might be started with MnO2 to Mn2O3,
53
and then to MnO.6,
However, pure ACF displayed only one
reduction peak of 625 oC, which should be attributed to the
decomposition of functional groups, such as oxygenic groups in ACF
sample. When nanocrystalline δ-MnO2 were introduced into the
ACF, the reduction behaviour was differed from that observed on
the ACF. The TPR profiles of the MnO2/ACF sample exhibited three
reduction peaks, two of which shifted to lower temperature at 275
Notes and references
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DOI: 10.1039/C6RA15463H
A synergistic catalytic removal of HCHO was achieved over Nanocrystalline MnO2 modified
activated carbon fiber at room temperature.