Three-dimensional nitrogen-doped porous carbon anchored CeO2 quantum dots as an efficient catalyst for formaldehyde oxidation

Article abstract of DOI:10.1039/c8ta00076j

Catalytic oxidation of formaldehyde with non-noble metal catalysts is desirable yet still challenging. In this work, we describe a hard template method in combination with an in situ chelating strategy to construct a novel three-dimensionally structured CeO₂-based composite which contains CeO₂ quantum dots of 1.5-2.5 nm size anchored on nitrogen doped hierarchical porous carbon with both macropores and mesopores. When used as an oxidation catalyst, complete oxidation of formaldehyde could be achieved at a temperature 130 °C lower than that of bulk nano-CeO₂ catalyst. The significantly improved catalytic activity could be attributed to the higher amount of oxygen vacancies and enhanced redox properties of CeO₂ quantum dots as well as the three-dimensional porous texture.

Full text of DOI:10.1039/c8ta00076j

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C8TA00076J
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Three-dimensional nitrogen-doped porous carbon anchored CeO2
quantum dots as an efficient catalyst for formaldehyde oxidation†
‡
‡
Dong Luo,ab Bingbing Chen,c Xingyun Li,*ab Zaojin Liu,ab Xiaowei Liu,ab Xuehua Liu, ab Chuan Shi*c
Received 00th January 20xx,
Accepted 00th January 20xx
and Xiu Song Zhaoabd
DOI: 10.1039/x0xx00000x
Catalytic oxidation of formaldehyde with non-noble metal catalyst is desirable yet still challenging. In this work, we
describe a hard template method in combination with an in-situ chelating strategy to construct a novel three-
dimensionally structured CeO2-based composite which contains CeO2 quantum dots of 1.5-2.5 nm in size anchored on the
nitrogen doped hierarchical porous carbon with both macropores and mesopores. When used as an oxidative catalyst, the
complete oxidation of formaldehyde could be achieved at temperature by 130 oC lower than that of bulk nano CeO2
catalyst. The significantly improved catalytic activity could be attributed to the higher amount of oxygen vacancies,
enhanced redox property of CeO2 quantum dots as well as the three-dimensional porous texture.
www.rsc.org/
the active oxygen, leading to a higher conversion of HCHO. By
far, the reports on CeO2 as catalysts for HCHO oxidation are
still limited and the activity still needs to be improved to meet
Introduction
Formaldehyde (HCHO) as a common indoor pollutant exposes
great threat to human health. It is found that long term inhaling
of air with HCHO even at ppm level could lead to severe
diseases.1-3 With the stricter regulations, HCHO abatement has
attracted more and more attention. Catalytic oxidation of
HCHO to CO2 is demonstrated to be a promising technology.
Supported noble metals (such as Pt, Au, Pd, Ag etc.)2, 4-7 are
frequently used as catalysts because they exhibit superior
catalytic activity at low temperature. However, the high cost
and limited reserves of these precious metals arouse the
development of non-noble metal catalysts.8-11 Transition metal
the practical application criteria.
Recently, study on the size effect for catalysis is a hot topic.
Reducing catalyst particles down to sub-size even to clusters or
single atoms could trigger a series of interesting phenomenon,
e.g. high surface to volume ratio, high density of surface
defects, large portion of unsaturated coordination bonds, as well
as quantum size effect etc., which all benefit to catalysis.26-30
For CeO2, the oxidative activity is largely related to defects and
oxygen vacancies.23, 31-33 Therefore, engineering smaller CeO2
quantum dots to enhance its atomic efficiency and boost its
oxidative activity is attractive. In this work, a hard template
method in combination with in-situ chelating strategy was
adapted to prepare a novel three-dimensional (3D) catalyst with
CeO2 quantum dots anchored on nitrogen-doped porous carbon
(3D-CeO2@CN) for HCHO oxidation. To achieve this,
dopamine which is an important mussel inspired mimetic bio-
oxides and rare earth metal oxides such as MnOx12-14, Co3O4
15,
16, MnO2-CeO217, CeO218, and LaMnO319, etc., have been
extensively explored as alternative catalysts. Amongst, CeO2
shows distinguished catalytic oxidation activity because of its
rich active surface oxygen and unique redox property.20-23
Huang et al.24 doped Eu into CeO2 nanosheet to introduce
surface defects, which showed enhanced redox ability and
improved HCHO oxidation performance. Sheng et al.25
prepared a CuO-CeO2 composite catalyst and found that the
presence of CuO could significantly improve the mobility of
34
molecular was used to serve as the CN precursor.10,
Dopamine could self-polymerize in alkaline condition and
adhere onto almost any kind of substrates. And the abundant
catechol and amino groups could offer great opportunity to
35-38
coordinate with inorganic molecular.8,
In the present
synthesis protocol, dopamine could serve as a bifunctional
material to on the one hand complex ceric ammonium nitrate
during the polymerization process and on the other hand
polymerize on the surface of poly-methyl methacrylate
(PMMA) beads which are used as the hard template. After
removal of PMMA spheres by calcination, a unique 3D
nitrogen doped porous carbon stabilized CeO2 quantum dots
(QDs) was obtained. This 3D-CeO2@CN showed superior
catalytic activity in HCHO oxidation reaction. As far as we
know, this is the first report on the application of porous CN
a.Institute of Materials for Energy and Environment, Qingdao University, Qingdao,
266071, China. *E-mail: xingyun_2008@sina.cn
b.College of Materials Science and Engineering, Qingdao University, Qingdao,
266071, China.
c. State Key Laboratory of Fine Chemicals, School of Chemistry, Laboratory of
Plasma Physical Chemistry, Dalian University of Technology, Dalian 116024,
China. E-mail: chuanshi@dlut.edu.cn
d.School of Chemical Engineering, The University of Queensland, St Lucia Campus,
Brisbane, QLD 4074, Australia.
‡
The authors contribute equally to this work.
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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anchored CeO2 quantum dots as an efficient catalyst for the AutoChem
Journal Name
Ⅱ
2920 chemisorption analyzer. 50 mg sample was
View Article Online
o
catalytic oxidation of hydrocarbons.
loaded and pretreated under Ar at 350 CDfOoIr: 110.h10t3o9/rCe8mTAo0v0e07th6eJ
o
adsorbed gaseous species. After cooling to 30 C, TPR profile
was recorded with 5% hydrogen in Ar at a flow rate of 50
mL·min-1 and at a heating rate of 10 oC/min.
Experimental
Catalytic activity test
Material synthesis
Catalytic activity evaluation was performed in a continuous
flow fixed-bed quartz micro-reactor at atmospheric pressure.
The catalyst (0.06 g, 40-60 mesh) was sandwiched between
quartz wool layers in the tubular reactor (i. d. = 4 mm). All the
feed gases used in this work were of high-purity grade (99.99
%). The gas flow rates were controlled by mass flow
controllers. Gaseous HCHO was generated by flowing N2 over
paraformaldehyde (99 %, Aldrich) in a thermostatic bath.
Gaseous H2O was carried into the gas stream by passing N2
through a bubbler in a water bath at room temperature. The
amount of water, expressed as the relative humidity (RH) at 25
oC, was controlled by adjusting the flow rate of N2, while
keeping the total flow constant. The HCHO/N2 and H2O/N2
streams were then mixed with the main gas stream of O2/N2,
leading to a typical feed gas composition of 90 ppm HCHO and
21 vol. % oxygen, balanced by nitrogen with a relative
Synthesis of PMMA. PMMA was synthesized as described
elsewhere.5 Briefly, 0.18 g of potassium persulfate powder
(K2S2O8) was dissolved in 700 mL of deionized water in a
three-necked round-bottom flask. With N2 as the protection
atmosphere, the solution was heated to 70 C. Then 53.7 mL of
methyl methacrylate was dropwise added into the flask under
o
o
continuous stirring at 70 C for 60 min. After cooling down to
room temperature, the suspension was centrifuged and washed
thoroughly with deionized water. PMMA spheres were
obtained after drying at 60 oC overnight.
Synthesis of 3D-CeO2@CN. 1.2 g of the PMMA spheres were
firstly dispersed in a mixed solution of 50 mL ethanol and 50
mL H2O. Then 10 mL ceric ammonium solution (3 g/L) was
added. After stirring for 10 minutes, 0.6 g of dopamine
hydrochloride dissolved in 20 mL of water was added in. Then
tris(hydroxymethyl)aminomethane (Tris-buffer) was used to
tune the pH to 8.5 and the mixture was stirred for 24 h at room
temperature. The dark grey suspension was filtered, washed
o
humidity (RH) of 50% (25 C). The total flow rate was 100
mL∙min-1, corresponding to a gas hourly space velocity
(GHSV) of 100,000 mL∙g-1∙h-1.
o
with plenty of water, dried at 60 C overnight and calcined at
Concentrations of CO and CO2 were measured using an
online non-dispersive infrared analyzer (SICK-MAIHAK-S710,
Germany). In this work, it was not possible to monitor the
HCHO concentration directly by Fourier transform-infrared
spectroscopy (FT-IR) due to the interfering effects of water.
Therefore, the inlet amount of HCHO was measured by
converting it to CO2 in a homemade HCHO-to-CO2 converter
o
650 C in an Ar atmosphere for 3 h. The sample thus obtained
is named as 3D-CeO2@CN. For comparison purposes, three
more samples were prepared. One designated as CeO2@CN,
was prepared as described above but with H2O as the solvent
and without templates. The other one, bulk nano CeO2 was
synthesized as previously reported by homogeneous
precipitation method.39 And sample CeO2/CN was prepared
with polymerized dopamine (PDA) which was pre-calcined at
650 oC as support, and 20 wt. % CeO2 was loaded using
o
(CuO-MnO2/γ-Al2O3 catalyst) at 300 C and determining the
amount of CO2 formed40-43. In typical runs, the reaction data
was collected after HCHO oxidation run time of 30 min at
typical temperature so as to achieve the steady state. No other
carbon-containing compounds except CO2 in the products were
detected for all the tested catalysts.
o
impregnation method followed by calcination at 650 C at Ar
atmosphere for 3 h.
Characterization
The conversion of HCHO was calculated by the following
equation:
Field emission scanning electron microscope (FESEM) images
were obtained on a JEOL JSM-7800F. Transmission electron
microscope (TEM) images were performed on a JEOL JEM-
2100. N2 adsorption-desorption was carried out with a
Quantachrome Autosorb iQ3. The specific surface area was
calculated using the Brunauer-Emmett-Teller (BET) method
and the pore size distribution was estimated with non-local
density functional theory (DFT). Thermogravimetric (TG)
analysis was conducted on a Mettler Toledo TGA-2 thermal
gravimetric analyzer under air atmosphere with a heating rate
[퐶푂2]표푢푡푣표푙. %
퐻퐶퐻푂 푐표푛푣푒푟푠푖표푛 (%) =
× 100%
[퐻퐶퐻푂]푖푛푣표푙. %
where [CO2]out is the CO2 concentration in the product gas
stream (vol. %) and [HCHO]in is the HCHO concentration in
the feed gas (vol. %).
The reaction rate (Rate) based on the mass of CeO2 is
calculated with the following formula:
[ ] (
퐹푙표푤 × 퐻퐶퐻푂 × 퐶표푛푣. 퐻퐶퐻푂
)
o
of 10 C/min. Powder X-ray diffraction (XRD) patterns were
푅푎푡푒 =
( ) (
푀푎푠푠 푐푎푡 × 푤푡. % 퐶푒푂2
)
recorded on a Rigaku Ultima IV X-ray diffractometer with Cu-
Kα radiation (λ=0.15418). X-ray photoelectron spectroscopy
(XPS) was performed on a Thermo Fisher ESCALAB 250Xi
spectrometer with Al Kα X-ray source (1486.6 eV, conducted
at 10.8 mA and 15 kV). H2 temperature programmed reduction
where Flow stands for the flow rate of the inlet gas mixer,
[HCHO] is the concentration of HCHO in the mixed gas,
Conv.(HCHO) represents the conversion of HCHO, Mass(cat)
is the mass of the used catalyst and wt.(CeO2) is the mass
percentage of CeO2 accounts for the whole catalyst.
(H2-TPR) profiles were measured on
a Micromeritics
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(a)
(a)
(b)
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DOI: 10.1039/C8TA00076J
Results and discussion
The synthesis procedure of 3D-CeO2@CN is illustrated in
Scheme 1. To construct the 3D structure, self-assembled
PMMA colloidal spheres were used as the hard template (SEM
image of the PMMA spheres is shown in Fig. S1). The voids of
the self-assembled PMMA spheres were first filled with ceric
ammonium nitrate and dopamine solution. At pH 8.5, dopamine
would self-polymerize and adhere itself on PMMA spheres, and
in the meantime, cerium species were chelated by dopamine.
Upon removal of the spheres by calcination, 3D-CeO2@CN
could be obtained.
200 nm
100 nm
(c)
(d)50
40
30
20
10
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
5 nm
Particle Size (nm)
As the SEM images shown in Fig. 1(a), 3D-CeO2@CN
exhibited honeycomb like structure with interconnected
macropores. The 3D morphology was further confirmed by the
TEM image shown in Fig. 1(b). The macro pore size is about
125 nm, and the wall thickness is around 16 nm. The HRTEM
images in Fig. 1(c) clearly show the presence of CeO2 quantum
dots, which uniformly disperse on the carbon framework. The
lattice spacing is estimated to be about 0.31nm (inset in Fig.
1(c)), corresponding to the (111) planes of CeO2. The mean
particle size of the CeO2 is calculated to be around 1.5-2.5 nm
as shown in Fig. 1(d). If there is no PMMA used in the
synthesis process, the obtained sample CeO2@CN doesn’t
show any regular structure (Fig. S2 (a)), but CeO2 particles of
about 2 nm can be seen (Fig. S2 (b)). As comparison, the
particle size of the bulk CeO2 synthesized by the traditional
method is larger than 10 nm as shown in Fig. S2 (c). This
demonstrates the chelating ability of dopamine to prevent the
agglomeration of CeO2. Additionally, we also prepared sample
CeO2/CN via the traditional impregnation method to load 20 %
CeO2 onto pre-calcined PDA, the size of CeO2 particles was
about 8 nm (Fig. S2 (d)). The above characterization
sufficiently proved the feasibility of the synthesis method by
hard template combined with in-situ chelating strategy, through
which, 3D structured carbon frameworks as well as small size
CeO2 quantum dots could be obtained.
The specific BET surface area of 3D-CeO2@CN is calculated
to be 255.8 m2/g and the pore volume is 0.99 cc/g. The
isotherm in Fig. 2(a) shows typical IV type with hysteresis
loops, indicating that 3D-CeO2@CN is hierarchically structured
possessing not only macro pores but also meso pores. This can
be further proved by the DFT pore size distribution curve as
inset in Fig.2 (a) that the mean pore size located at 2 nm with
some pores of 9.3 nm in diameter. However, sample CeO2@CN
shows a lower specific surface area (68 m2/g) and pore volume
(0.08 cc/g) (see Fig. S3). Comparingly
area of CeO2 is 49.5 m2/g and the pore volume is 0.05 cc/g. (Fig.
S4) The high surface area and unique hierarchical texture of
，the specific surface
(a)
(c)
(e)
(b)
700
600
500
400
300
200
100
0
2 nm
0.08
0.06
0.04
0.02
0.00
100
80
60
40
20
0
82%
9.3 nm
0
3 15 18
Pore si9ze (1n2m)
6
0.0
0.2
0.4
0.6
0.8
1.0
150
300
450
600
750
Temperature (oC)
P/P0
(d)
(f)
C
* CeO
Pyridinic N
Pyrrolic N
2
*
 C
Graphitic N
*
*
394 396 398 400 402 404 406
Binding Energy (eV)
O
*
CeO2
*
*
*
*
*
Ce
N
3D-CeO2@CN

20
40
60
80
0
250 500 750 1000 1250
2θ
Binding Energy (eV)
529.8 eV
520 o
C
CeO2
479 o
C
532.5 eV
CeO2
534.6 eV
3D-CeO2@CN
3D-CeO2@-CN
160 240 320 400 480 560
Temperature (oC)
528
531
534
537
Binding Energy (eV)
Figure 2 a) N2 adsorption-desorption isotherm (inset is the DFT pore size distribution
curve) of 3D-CeO2@CN, b) TG curves of 3D-CeO2@CN, c) XRD patterns of crystalline
CeO2 and 3D-CeO2@CN, d) XPS spectrum of 3D-CeO2@CN (inset is the deconvolution
of N 1s peak), e) O1s XPS spectra CeO2 and 3D-CeO2@CN, f) H2-TPR profiles of CeO2
and 3D-CeO2@CN.
Scheme 1 Schematic illustration of the preparation of 3D-CeO2@CN.
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3D-CeO2@CN would benefit catalytic reactions in terms of
mass transfer and the accessibility of active CeO2 sites to the
reactant. Fig. 2(b) shows the TGA profile of 3D-CeO2@CN.
The mass loading of CeO2 accounts for 18 wt. %, and the value
can be well tuned by adjusting the ratio of precursors (Fig. S5).
The XRD patterns in Fig. 2(c) show that the diffraction peaks
corresponding to CeO2 can hardly be seen from sample 3D-
CeO2@CN which may be due to the extremely small particle
size and/or poor long range crystallinity of CeO2. This suggests
that there are more defects for CeO2 in the composite compared
with pure nano CeO2 with high intensity of diffraction peaks.
From XPS spectrum shown in Fig. 2(d), we can see that C, N,
O and Ce elements present in 3D-CeO2@CN. The N content is
calculated to be 3% with 35% of pyridinic N (398.2 eV), 56.6%
of pyrrolic N (400.8 eV) and 8.4% graphitic N (402.6 eV) as
the deconvolution of N peak inset in Fig. 2(d). It is widely
accepted that oxygen vacancies play significant role in
promoting the oxygen mobility and facilitating the activation of
(a)
(b)
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DOI: 10.1039/C8TA00076J
3D-CeO2@CN (low velocity)
3D-CeO2@CN
CeO2@CN
100
80
60
40
20
0
CeO2
0
50 100 150 200 250 300
Temperature / oC
80
60
40
20
0
3D-CeO2@CN
CeO2
50
100 150 200 250 300
Temperature / oC
oxygen molecular into active superoxide (O2-) species, hence
promoting the oxidation performance.24,
To get the
44, 45
information of oxygen vacancies, we first fit O1s XP spectra as
shown in Fig. 2(e). There are two typical peaks for pure CeO2
with one peak located at 529.8 eV corresponding to lattice O
(Olat) and the other peak at 532.5 eV assigned to surface O
(Osur). While for 3D-CeO2@CN, there also exists a peak at
534.6 eV which could be derived from the oxygen functional
groups on CN surface. It is interesting to find that lattice O is
predominant for CeO2, nevertheless the surface O possess the
temperature than CeO2 indicating the higher reducibility as well
as the enhanced redox property for 3D-CeO2@CN. The result is
in accordance with the more oxygen defects and surface oxygen
on CeO2 quantum dots and may be to some extent due to the
possible electron transfer from the doped N atoms on the
carbon surface.
highest percentage for 3D-CeO2@CN. As reported, the surface
46
O is associated with the oxygen vacancies.24,
This result
The performance of the catalysts for HCHO oxidation was
tested in the fixed bed reactor. By evaluating the influence of
the ratio of cerium to CN precursor as well as the calcination
temperature, it is found that ratio of ceric ammonium nitrate to
demonstrates that downsizing of CeO2 particles could create
more active Osur on CeO2 and lead to more oxygen vacancies.
Meanwhile
，it is well documented that the existence of oxygen
vacancies is expected to correspond to an increase of Ce3+ ratio
compared to Ce4+ so as to maintain the stoichiometric CeO2.47, 48
o
dopamine at 20/1 and calcination temperature at 650 C maybe
the optimized preparation conditions (Fig. S7). From the light
off curve as a function of temperature in Fig. 3 (a), we can see
that 3D-CeO2@CN exhibited the best activity with completely
From the deconvolution of Ce 3d XP spectra shown in Fig. S6
,
we can see that the Ce 3d XP spectra can be fitted into eight peaks49
with peaks labelled as V’ and U’ assigned to Ce3+ while V, V’’, V’’’,
U, U’’ and U’’’ represent the presence of Ce4+. Ce3+ ratio
(Ce3+/(Ce3++Ce4+)) for 3D-CeO2@CN is calculated to be 33.7%
compared to 22.5% for bulk CeO2. This result may provide
additional evidence that there are more oxygen vacancies exist in
3D-CeO2@CN. From the H2-TPR results in Fig. 2 (f) we can see
that 3D-CeO2@CN showed about 41 oC lower reduction
o
o
oxidation of HCHO at 170 C, about 130 C lower than that of
pure CeO2 catalyst at a space velocity of 10000
mL∙g−1∙h−1. In
0
contrast, CeO2/CN showed rather poor activity similar to that of
CeO2 (Fig. S8) which may be due to the larger particles of
CeO2. This result demonstrated the importance of the smaller
size effect for the catalytic reaction. The performance of the
catalyst was also tested at a lower space velocity of 60000
Table 1 Comparison of the performances over representative catalyst for HCHO oxidation
Reaction Rate (mLHCHO∙gcat-1∙h-1)a
Catalyst
T100%
（
oC
）
GHSV (mL∙g −1∙h−1)
Reactant
Ref.
at 130 oC
25
18
24
17
50
15
CeO2-CuO/AC
3D-TiO2/CeO2
4% Eu doped CeO2
MnOx-CeO2
MnO2/cellulose
3D-Co3O4
>250
>250
120
100
140
160
130
---
0.018
1.5
12.2
53
5000
30000
30000
21000
50000
60000
60000
120 ppm HCHO, 6 vol% O2
50 ppm HCHO, 25 vol% O2
50 ppm HCHO, 25 vol% O2
580 ppm HCHO, 18 vol% O2
100 ppm HCHO, 20 vol% O2
400 ppm HCHO, 20 vol% O2
90 ppm HCHO, 21 vol % O2
10
30
3D-CeO2@CN
This work
a Reaction rate is calculated based on the mass of the active species.
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mL∙g−1∙h−1 as shown in Fig. 3(a), it can be seen that the 100%
conversion of HCHO over 3D-CeO2@CN could be obtained at
as low as 130 oC. Furthermore, the converted amount of HCHO
based on the mass of CeO2 and reaction time, namely HCHO
reaction rate, was calculated. From Fig. 3(b), we can see that
the reaction rate for 3D-CeO2@CN at 170 oC is about 5.5 times
that of bulk nano CeO2. This highlights again the superiority of
the composite for the catalytic activity improvement. For the
reference, the performances of some representative metal oxide
catalysts are listed in Table 1. We can see that 3D-CeO2@CN is
among the best non-noble metal catalyst comprehensively
considering the total HCHO conversion temperature as well as
the reaction rate. According to the previous characterization,
the boosted activity for 3D-CeO2@CN could be due to more
oxygen vacancies, abundant active surface oxygen and higher
reducibility of the CeO2 quantum dots. In addition, the CN
surface is responsible to stabilize CeO2 quantum dots and
particularly the hierarchical 3D structure could facilitate the
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A novel method to prepare CeO2 quantum dots anchored on
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hierarchical pore structure, abundant oxygen vacancies and
enhanced redox properties, the composite showed superior
activity towards HCHO oxidation reaction with a reaction rate
o
of 5.5 times that of bulk CeO2 at reaction temperature of 170 C
and GHSV of 100,000 mL∙g−1∙h−1, manifesting itself potent
candidate for the noble metal replacement. This study also
points to a reliable strategy of constructing other 3D structured
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We acknowledge the financial support from Chinese
Postdoctoral Science Foundation (2016M600519), Natural
Science Foundation of Shandong Province, China
(ZR2016BB03) and the Taishan Scholars Advantageous and
Distinctive Discipline Program of Shandong Province, China.
The National Natural Foundation of China (21703113 and
21707015), and the Fundamental Research Funds for the
Central Universities (DUT16RC(4)01). The Australian
Research Council (ARC) is acknowledged for supporting this
research work under the ACR Laureate Fellowship program
(ProjectFL170100101).
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Page 7 of 7
Journal of Materials Chemistry A
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DOI: 10.1039/C8TA00076J
Graphic Abstract:
A unique 3D structured CeO2 quantum dots and CN composite is
successfully constructed by a novel synthesis procedure to serve as an
efficient HCHO oxidation catalyst.