Morphology-dependent properties of CeO2 nano-catalysts on CH2Cl2 oxidation

Article abstract of DOI:10.1039/c6ra14699f

Four types of CeO₂ nanoparticles with different morphologies (nanorods, nanocubes, nanopolyhedra, and bulk CeO₂ nanoparticles) were synthesized and used in dichloromethane (DCM) oxidation. Their detailed physicochemical properties were investigated by X-ray diffraction, N₂ physisorption, H₂ temperature-programmed reduction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Results indicated that DCM oxidation over CeO₂ nanoparticles has significant morphology-dependent effects. CeO₂ nanorods showed the best activity among all the investigated samples (T₉₀ is only 323 °C). The main products were CO₂ and HCl, although trace amounts of CHCl₃, CCl₄, CO, and Cl₂ could be detected. The high performance of CeO₂ nanorods in DCM oxidation may be related to the abundant surface defects, increased amount of adsorbed active oxygen species, and good reducibility of the catalyst.

Full text of DOI:10.1039/c6ra14699f

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Morphology-dependent Properties of CeO2 Nano-catalysts on
CH2Cl2 Oxidation
Xinhua Zhang, a, b Zhiying Pei, a Hanfeng Lu c and Haifeng Huang a,*
Received 00th January 20xx,
Accepted 00th January 20xx
Four types of CeO2 nanoparticles with different morphologies (nanorods, nanocubes, nanopolyhedra, and bulk CeO2
nanoparticles) were synthesized and used in dichloromethane (DCM) oxidation. Their detailed physicochemical properties
were investigated by X-ray diffraction, N2 physisorption, H2 temperature-programmed reduction, high-resolution
transmission electron microscopy, and X-ray photoelectron spectroscopy. Results indicated that DCM oxidation over CeO2
nanoparticles has significant morphology-dependent effects. CeO2 nanorods showed the best activity among all the
investigated samples (T90 is only 323 °C). The main products were CO2 and HCl, although trace amounts of CHCl3, CCl4, CO,
and Cl2 could be detected. The high performance of CeO2 nanorods in DCM oxidation may be related to the abundant
surface defects, increased amount of adsorbed active oxygen species, and good reducibility of the catalyst.
DOI: 10.1039/x0xx00000x
www.rsc.org/
resolved.
1 Introduction
CeO2-based catalysts have been extensively studied in CVOC
The chlorinated volatile organic compounds (CVOCs) emitted
by numerous industries are highly toxic, carcinogenic, and
oxidation for their unique redox properties and high C–Cl bond
dissociation ability [2, 12-15]. However, the strong adsorption
of chloride species on the surface of CeO2 during CVOC
destruction will cause the partial deactivation of the catalysts
[12, 15]. A conventional approach to resolve this problem is to
incorporate CeO2 with other transition metal oxides [2, 13-15].
However, deactivation could be still observed. Recently, CeO2
nanoparticles with anisotropic shapes at the nanometer scale
have attracted much interest. These materials can significantly
improve the reaction performance when their specific facets
are selectively exposed [16-19]. The morphology effects of
ceria nanoparticles have been identified in CO [20-22]
oxidation and the WGS reaction [23, 24]. However, the
morphology-dependent properties of CeO2 in CVOC oxidation
have not been fully studied, although this oxide is widely used
as catalyst in CVOC destruction. Herein, three types of low-
dimensional CeO2 nanoparticles (nanorods, nanocubes, and
nanopolyhedra) were synthesized by a facile hydrothermal
method and used in dichloromethane (DCM) oxidation. This
study aimed to explore the structure–activity relationship of
CeO2 nano-catalysts during DCM oxidation.
genotoxic to humans, even at very low concentrations [1]
.
Thus, various countries set stringent emission ceilings to these
harmful compounds. In addition, several abatement
techniques have been developed to meet the increasingly
strict legislations. Catalytic combustion is one of the most
promising approaches to destroy CVOCs because it can be
operated at a relatively low temperature range as compared
with the thermal incineration process [2]. The formation of
NOx can also be restrained.
Undoubtedly, the catalyst is a key factor in CVOC oxidation.
A suitable catalyst is highly active for the target CVOCs and
should have good selectivity for the desired products (CO2 and
HCl). Supported noble metals [3-5], transition metal oxides [2,
6], and zeolites [7, 8] are usually reported in CVOC oxidation.
Supported noble metal catalysts show relatively higher activity
than the other two counterparts in a low-temperature range
but are easily poisoned by chlorinated intermediates from
CVOC destruction [9, 10]. Although the transition metal oxides
are less active than the supported noble metals in most cases,
these catalysts are believed to be more resistant to chlorine
poisoning [2, 11]. Notably, transition metal oxides are also
more abundant and much cheaper than noble metals.
Therefore, the use of transition metal oxides to substitute
noble metals in CVOCs oxidation is highly significant. However,
their activity in the low-temperature range still needs to be
2 Experimental
2.1 Catalyst preparation
Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O), sodium
hydroxide (NaOH), and DCM were analytical grade reagents.
All chemicals were purchased from Aladin Reagents and used
as received without further purification. The synthesis of CeO2
nanorods and nanocubes followed the revised procedure of
Mai et al [25]. Typically, Ce(NO3)3·6H2O (3.4720 g) and NaOH
(33.6 g) were dissolved in 20 mL and 140 mL of deionized
water, respectively. The two solutions were subsequently
mixed in a Teflon bottle, and the mixed solution was stirred for
30 min at room temperature to form a slurry. The Teflon bottle
a.College of Biological and Environmental technology, Zhe Jiang University of
Technology, Hang Zhou 310032, PR China.
b.College of Chemical and Environmental engineering, Jiu Jiang University, Jiu
Jiang 332005, PR China
c. College of Chemical Engineering, Zhe Jiang University of Technology, Hang
Zhou 310032, PR China
.
.
† Corresponding Author: Haifeng Huang; Email Address: hhf66@zjut.edu.cn; Tel:
+86-571-88320385
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was tightly sealed and placed in a stainless-steel autoclave. 2.3 Activity test
The tightly sealed autoclave was hydrothermally treated at
DCM oxidation experiments were conducted in a fixed bed
tubular reactor (6 mm i.d.) under atmosphere pressure
between 150 °C and 450 °C. 200 mg catalysts (40-60 mesh)
was loaded in the center of the reactor and sandwiched with
quartz wool. 1.8 g of quartz sand was placed on the inlet-end
to preheat the feed gas. DCM feed concentration was kept
constant at 1000 ppm by controlling the flow rate of bubbling
air (20.8% O2 and 79.2% N2) through an ice bathed saturator,
and the gas hourly space velocity (GHSV) in the reactor bed
was set to 15000 mL g-1 h-1 by adjusting the flow rate of
balance air in all experiments. The effluent gases were
analyzed online with a GC-1620 gas chromatograph equipped
with a flame ionization detector (FID) and a TCD. A further
detailed analytic process was described in our previous work
[6].
100 °C for 24 h. After cooling, the obtained white precipitates
were collected by centrifugation, washed with deionized water
for 3–4 times, and dried at 60 °C in air overnight. The acquired
yellow powder was calcined in air at 450 °C for 5 h in a muffle
furnace to obtain the CeO2 nanorods. The synthesis of CeO2
nanocubes was similar to that of CeO2 nanorods, except the
hydrothermal treatment temperature was set to 180 °C. The
synthesis of CeO2 nanopolyhedra mainly followed the
procedure of Si et al [24], but the calcination temperature was
revised to 450 °C. Typically, Ce(NO3)3·6H2O (3.2567 g) was
dissolved in 150 mL of NaOH solution (0.1 M). The mixture was
stirred for 10 min at room temperature and transferred into a
Teflon-lined stainless-steel autoclave. The autoclave was
tightly sealed and hydrothermally treated at 180 °C for 24 h.
The subsequent steps were followed to produce CeO2
nanorods. For comparison, bulk CeO2 nanoparticles were also
prepared by directly calcinating Ce(NO3)3·6H2O in air at 450 °C
for 5 h. For convenience, the CeO2 nanorods, nanocubes,
nanopolyhedra, and bulk nanoparticles were denoted as CeO2-
r, CeO2-c, CeO2-p, and CeO2-b, respectively.
3 Results and discussion
3.1 Catalytic activity results
100
80
60
40
20
0
CeO2-r
CeO2-c
CeO2-p
CeO2-b
quartz sand
2.2 Catalyst characterization
Powder X-ray diffraction patterns (XRD) were obtained on an
X’Pert Pro powder diffract meter with the scanning rate of 0.1°
per minute, using Cu Ka radiation (λ= 0.154056 nm). The phase
compositions were analyzed using jade 6.5.
N2-physical adsorption and desorption were recorded on
Micromeritics ASAP 2020 at 77 K. The specific surface area of
the samples was calculated from adsorption isotherms using
the BET equation.
150
200
250
300
350
400
450
H2 temperature-programmed reduction (H2-TPR) was
measured on FINE SORB-3010 E instrument equipped with a
thermal conductivity detector (TCD). 200 mg of sample was
heated from 50 to 900 °C in a 10 vol% H2/Ar flow (30 mL min-1)
at a ramp of 10 °C/min.
Temperature (
°
C)
Fig.
1 Light-off curves of DCM over CeO2 with different
-1
morphologies (1000 ppm DCM，air balance, GHSV: 15000 mL · gcat
· h-1)
Fig. 1 shows the DCM conversion over CeO2 nanoparticles with
different morphologies. The pure quartz sand had almost no
catalytic activity under the given testing conditions. The complete
conversion of DCM could be achieved at 350–450 °C for all
investigated samples. The values of T50 and T90 (temperatures with
50% and 90% DCM conversion achieved, respectively) are listed in
Table 1. At T90, the activity of the investigated samples was CeO2-r >
CeO2-b > CeO2-p > CeO2-c. The CeO2 nanorods showed the best
activity; its T50 and T90 were only 278 and 323 °C, respectively. By
contrast, the CeO2 nanocubes showed the worst activity. Compared
with the CeO2 nanorods, the T50 and T90 of CeO2 nanocubes
increased 81 and 90 °C, respectively. DCM oxidation showed the
different activities of CeO2 nanoparticles with different
morphologies.
High Resolution Transmission Electron Microscope (HR-TEM)
images of the samples were obtained using Philips-FEI (Tecnai
G2 F30 S-Twin). All samples were ultrasonically dispersed in
absolute alcohol for 1 h, then the suspension was deposited on
an amorphous carbon-coated circular copper grid before
measurement.
X-ray photoelectron spectroscopy (XPS) spectra of the
samples were recorded on
a Kratos AXIS Ultra DLD
photoelectron spectrometer (Shimadzu scientific instruments,
Japan) and using a monochromatic radiation source Al K (45
W, 3×10-9 mbar). The charging of samples was corrected by
setting the binding energy (BE) of adventitious carbon (C 1s) at
284.8 eV.
Laser Raman spectra were obtained on LabRAM HR-800
spectrometer (Horiba Jobin Yvon, France) equipped with an air-
cooled multichannel CCD detector, Ar+ ion (531.95) was employed
as an excitation source (Nd: YAG 20 mW) to obtain visible Raman
spectra.
Fig. 1 also shows that DCM conversion experienced temporary
deactivation in the temperature range of 200–250 °C, which could
be attributed to the chlorine species adsorption on the catalyst’s
active sites in relative low temperature range during DCM
destruction. Similar phenomena were also observed by Dai et al
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100
[26]. However, the gradual removal of Cl adsorption on the oxygen
vacancies would be done thanks to the elevated temperature and
high ratio of O/Cl in the feed gas [11]. Consequently, the activity of
ceria was gradually recovered as the temperature increased.
CeO2-p
DCM
TCM
CCl4
2000
1500
1000
500
0
80
60
40
20
0
Cl2
HCl
The desired products of DCM oxidation are CO2 and HCl.
However, the poor selectivity may produce more toxic by-products.
The main products of DCM oxidation over the investigated samples
are CO2 and HCl, with a small amount of trichloromethane (TCM),
carbon tetrachloride (CCl4), CO, and Cl2, depending on the catalysts
used.
100
CeO2-r
DCM
TCM
CCl4
2000
1500
1000
500
0
150
200
250
300
350
400
450
80
60
40
20
0
Temperature (
°C)
Cl2
100
80
60
40
20
0
HCl
DCM
TCM
CCl4
CeO2-b
2000
Cl2
HCl
1500
1000
500
0
150
200
250
300
350
C)
400
450
Temperature (
°
100
80
60
40
20
0
2000
1500
1000
500
0
CeO2-c
150
200
250
300
350
400
450
DCM
TCM
CCl4
Temperature (°C)
Cl2
Fig.2 Selectivity of chlorinated species during DCM oxidation over
CeO2 with different morphologies (1000 ppm DCM, air balance,
GHSV: 15000 mL · gcat-1 · h-1)
HCl
Fig. 2 shows the selectivity of chlorinated species during DCM
destruction. The maximum selectivity of HCl over CeO2-r, CeO2-c,
CeO2-p, and CeO2-b was 65.46%, 56.61%, 72.56%, and 66.22%,
respectively. For TCM, the order of maximum selectivity was CeO2-p
(350 °C) > CeO2-c (400 °C) > CeO2-b (350 °C) > CeO2-r (325 °C).
(Henceforth, the numbers in brackets indicate the corresponding
temperature.) For CCl4, the order of maximum selectivity was CeO2-
p (375 °C) > CeO2-c (425 °C) > CeO2-b (350 °C) > CeO2-r (375 °C).
CeO2-r showed the worst selectivity for TCM and CCl4, with the
maximum concentrations of 118 and 116 mg/Nm3, respectively; its
selectivity was much less than when CeO2-p was used as a catalyst
(the maximum selectivity of TCM and CCl4 was 374 and 390
mg/Nm3, respectively). Meanwhile, a small amount of Cl2 was also
detected on all the investigated samples in the high-temperature
range (above 300 °C), which could be related to the Deacon
150
200
250
300
350
400
450
Temperature (°C)
reaction (2HCl + 1/2O2 = H2O + Cl2) [26, 27]
.
Table 1 Textual properties and activity data of the investigated
samples
②
T50/T90
/°C
H2 consumption
Crystallite Sizes
BET area
/m2 g-1
Total pore volume
/cm3 g-1
Pore sizes
/nm
Samples
ID/IF2g
①
/umol g-1
/nm
CeO2-r
CeO2-c
CeO2-p
CeO2-b
10.3
40.9
12.9
12.9
86.4
20.5
68.0
67.6
0.33
0.15
0.10
0.23
15.3
29.3
6.1
278/323
359/413
308/346
286/331
722
0.058
0.023
0.033
0.036
74
413
13.8
495
①
②
The value were estimated by Scherrer equation, applied to the (1 1 1) reflection of CeO2.
The values were calculated from the reduction of standard CuO.
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surface layer [28]. These reduced oxygen species are usually
attributed to the chemical adsorbed oxygen on the vacancies of
CeO2 [22]. For peak β, characterized with some connected
reduction peaks, usually involved the reduction of subsurface layer
and deep interior of CeO2 nanoparticles [28]. The more complex
reduction profiles of CeO2-c, CeO2-p and CeO2-b indicate some
kind of structural inhomogeneities of these samples as revealed in
the next TEM characterizations. The peak γ at the high-
temperature range (ca. 800 °C) is attributed to the reduction of
bulk lattice oxygen anions of CeO2 [12, 29]. It should be
mentioned that the initial reduction temperature of capping
oxygen ions was changed with the testing samples: CeO2-r (145 °C)
< CeO2-b (154 °C) < CeO2-p (168 °C) < CeO2-c (173 °C), thereby
indicating that the morphologies of CeO2 influenced the activity of
capping oxygen. The H2 consumption of the investigated samples
in the low-temperature range was calculated; the results are listed
in Table 1. The order of H2 consumption was CeO2-r > CeO2-b >
CeO2-p > CeO2-c, which is mainly consistent with their activity in
DCM oxidation (Fig. 1).
3.2 X-ray diffraction
The X-ray diffraction patterns of as-prepared samples are shown in
Fig. 3, and the corresponding crystallite sizes are listed in Table 1.
The diffraction peaks in 28.5°, 33.1°, 56.3°, and 59.1° can be
clearly observed on all testing samples, which could be indexed to
the cubic fluorite CeO2 crystal phase (PDF# 89-8436, space group:
Fm-3m). The diffraction peaks of CeO2-c were much narrower than
those of other samples, which was consistent with the larger
crystallite sizes of this sample, as listed in Table 1. The results were
also consistent with those of high-resolution transmission electron
microscopy (HR-TEM) characterization (see below).
(111)
(220) (311)
CeO2-b
(200)
(222)
(331)
(400)
CeO2-p
γ
β
α
CeO2-c
CeO2-r
380
487
CeO2-b
451
154
315
CeO2-p
CeO2-c
CeO2-r
168
173
10
20
30
40
50
60
70
80
520
374
2
θ /°
479
Fig. 3 The XRD patterns of investigated samples
3.3 N2 physisorption by the Brunauer–Emmett–Teller method
145
The Brunauer–Emmett–Teller (BET) areas and corresponding pore
volume and pore sizes of the investigated samples are listed in
Table 1. CeO2-r and CeO2-c have the largest and smallest BET area
as well as the best and worst activity during DCM oxidation,
respectively. However, the activity of CeO2-p was much inferior to
that of CeO2-b, although their BET areas were relatively
comparable. These discrepancies could be also observed between
the pore volume, pore sizes, and their activity in DCM oxidation.
Therefore, the BET areas, pore volume, and pore sizes of nano-
CeO2 are not key factors of DCM oxidation. Other reasons may
influence the activities of CeO2 nanoparticles with different
morphologies.
0
200
400
600
800
Temperature / °C
Fig. 4 H2-TPR of investigated samples
3.5 HR-TEM
HR-TEM micrographs of the as-prepared samples are shown in Fig.
5. The morphologies of nanorods, nanocubes, and nanopolyhedra
can be clearly observed, which are consistent with the
experimental designs. The CeO2-b nanoparticles were severely
aggregated but no specific shapes were exhibited. Fig. 5a shows
that the CeO2 nanorods have a diameter distribution of 6–7 nm
and a length distribution of 40–65 nm; their surfaces are rich in
micropores. Fast Fourier-transform (FFT) revealed that the growth
of CeO2 nanorods are oriented in the [1 1 0] direction to expose
the {1 1 0} and {1 1 1} planes (Fig. 5b). The CeO2 nanocubes had a
less uniform size distribution, with side lengths ranging from
50 nm to 100 nm, but all were cubic in shape (Fig. 5c). FFT
revealed that this sample had selectively exposed {1 0 0} and
{1 1 0} planes (Fig. 5d). The CeO2 nanopolyhedra had a size
distribution of 10–12 nm and selectively exposed {1 1 1} planes
(Figs. 5e and 5f, respectively). Similar results were reported in [17,
24, 25]. Micrographs showed that the CeO2 nanocubes had the
3.4 H2 temperature-programmed reduction
The H2 temperature-programmed reduction (H2-TPR) of
investigated samples is shown in Fig. 4. All tested samples
displayed two or more reduction peaks involved different
positions and relative intensities in the temperature range of 50–
900 °C. Three main reduction peaks could be identified in the
temperature range of 300-450 °C (α), 450-700 °C (β), and >700 °C
(γ), respectively. The low-temperature reduction peak α involved a
stepwise surface reduction process with the formation of
nonstoichiometric CeO2-x phase and oxygen vacancies in the
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largest sizes, which is consistent with their having the smallest BET
area and narrowest XRD peaks, as revealed in Table 1 and Fig. 3,
respectively.
are results of Ce 3d94f1 O 2p5 and Ce 3d94f2 O 2p4 [30]. No obvious
bands corresponding to the Ce3+ states could be indexed, thereby
indicating that the surfaces were almost complete oxidized over
all the investigated samples or that the amount of Ce3+ is very low
but similar in the four types of ceria [31]. By the deconvolution
method, the O 1s core level peak in CeO2 could be indexed to
three bands. The BE in 529.3–529.5 eV could be assigned to lattice
oxygen (Olatt
ascribed
surface-adsorbed oxygen (Oads) [15, 32, 33]. The high BE in 533.0–
)
[15, 32], whereas the BE in 531.0–531.4 eV is
to
533.5 eV is attributed to the surface hydroxyl species and/or
adsorbed water [32-34]. The ratios of each surface oxygen species
were calculated, and the results are listed in Table 2. This table
shows that the amount of oxygen species was significantly
different for all investigated samples. The order of adsorbed
oxygen species was CeO2-r > CeO2-b > CeO2-p > CeO2-c. Surface
adsorbed oxygen species are known to have high mobility and
usually regarded as active oxygen species in CVOC combustion
[33, 35, 36]. This could explain the high performance of CeO2-r in
DCM oxidation. The activity of the investigated samples was
basically consistent with the amount of absorbed oxygen species
on their surface (Fig. 1).
(a)
CeO2-b
CeO2-p
CeO2-c
CeO2-r
u′′
v
′′
v
u′′′
v′′′
u
930
920
910
900
890
880
870
B. E. / eV
Olatt
(b)
Oads
OH-/H2O
CeO2-b
CeO2-p
CeO2-c
CeO2-r
536
534
532
530
B.E.(eV)
528
526
Fig. 5 HR-TEM of nano-CeO2 with different morphologies (a, b:
Fig. 6 XPS profiles of CeO2 with different morphologies: Ce 3d (a)
CeO2-r; c, d: CeO2-c; e, f: CeO2-p; g: CeO2-b)
and O 1s (b).
3.5 X-ray photoelectron spectroscopy
The Ce 3d and O 1s X-ray photoelectron spectroscopy spectra of
the as-prepared samples are given in Fig. 6(a). The six
characteristic bands of Ce4+ can be clearly identified. The BE states
of u′′′ (916.9 ± 0.1 eV) and v′′′ (898.3 ± 0.1 eV) are the result of the
Ce 3d94f0 O 2p6 final state. The lowest BE states of u′′, v′′, u, and v
are located at 907.3, 888.5, 901.3, and 882.7 ± 0.1 eV; these states
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Table 2 Amount of each surface oxygen species
Samples
Lattice oxygen
Adsorbed oxygen
OH- and/or H2O
80
60
40
20
0
%
BE (eV)
%
BE (eV)
531.0
531.0
531.0
531.0
%
BE (eV)
533.1
533.1
533.3
533.0
CeO2-r
CeO2-c
CeO2-p
CeO2-b
34.3
67.3
69.3
65.1
529.5
529.5
529.4
529.4
36.1
28.0
28.9
32.2
29.7
4.7
1.8
2.7
3.6 Raman spectroscopy results
0
20
40
60
Time / h
80
100
Raman spectroscopy, known to be highly sensitive for detecting
the concentrations of the oxygen vacancies of CeO2, was
performed here for CeO2 with anisotropic shapes. As shown in
Fig. 7, a strong band in about 463 cm-1 and a weak band in
about 598 cm-1 were clearly observed on all samples, which
could be indexed to the F2g and defect-induced (D) models of
cubic CeO2 fluorite phase[20, 31], respectively. Also the relative
intensity ratio of ID (598 cm-1)/IF2g (463 cm-1) was used to estimate
the defect concentration on CeO2, and results were listed in Table
1. The ID/IF2g ratio of all tested samples followed the order CeO2-r
> CeO2-b > CeO2-p > CeO2-c, indicating that the CeO2-r have more
defect concentrations than the others. The results were also
consistent with their activity in DCM oxidation.
Fig. 8 Stability test of CeO2-r in DCM oxidation at 325 °C (1000
ppm DCM, air balance, GHSV: 15000 mL · gcat-1 · h-1)
5 Discussion
CeO2 nanorods, nanocubes, and nanopolyhedra were well
synthesized by a facile hydrothermal method. Using DCM
oxidation as
a model reaction, the morphology-dependent
properties of CeO2 nano-catalysts were investigated. CeO2
nanorods showed much higher activity and better selectivity than
CeO2 nanocubes, CeO2 nanopolyhedra and bulk CeO2
nanoparticles in DCM oxidation. XRD results showed that the four
synthesized samples exhibited cubic fluorite CeO2 crystallite phase
with different crystallite sizes. No other crystallite phases can be
identified. The discrepancies of the tested activity with their
crystallite sizes, BET surface areas, total pore volume, and pore
sizes imply that the morphologies properties of ceria may play
important roles in DCM oxidation. H2-TPR results showed the
reducibility and H2 consumption of CeO2 followed the order: CeO2-
r > CeO2-b > CeO2-p > CeO2-c, which is mainly consistent with their
activity in DCM oxidation. HR-TEM showed that CeO2 nanorods
selectively exposed {1 1 0} and {1 1 1} planes, CeO2 nanocubes
selectively exposed {1 0 0}, {1 1 0} planes, and CeO2
nanopolyhedra selectively exposed {1 1 1} planes, while the bulk
CeO2 nanoparticles presented undifferentiated shapes. It is well
known that the CeO2 {1 1 0} planes and {1 1 1} planes are both
nonpolar surfaces composed with stoichiometric Ce4+ and O
atoms, while the {1 0 0} surfaces of CeO2 are polar planes with O
atom layers terminated [20]. The nonpolar {1 1 0} and {1 1 1}
surfaces may benefit the adsorption of DCM with nonpolar
properties. Given the deconvolution of O 1s core level spectra, the
amount of adsorbed oxygen species on the surface of CeO2
followed the order: CeO2-r > CeO2-b > CeO2-p > CeO2-c. It is
generally accepted that the adsorbed oxygen species are active in
CVOC destruction; thus, the activity of ceria with anisotropic
shapes was mainly consistent with the order of their adsorbed
oxygen species. Generally, the adsorbed oxygen species are
usually related to oxygen vacancies on the surface of CeO2. Raman
spectra showed the defect concentration on CeO2 also followed
the order: CeO2-r > CeO2-b > CeO2-p > CeO2-c. Since the {1 1 0}
surfaces are more exposed Ce cations than the {1 0 0} and {1 1 1}
surfaces, the electrostatic interactions between the adsorbed
oxygen species and Ce cations are stronger on CeO2 nanorods than
on CeO2 nanocubes and CeO2 nanopolyhedra; thus, it is rational
that the stability and amount of adsorbed oxygen species on the
CeO2 {1 1 0} plane are more than those on the {1 0 0} and {1 1 1}
463
598
CeO2-b
CeO2-p
CeO2-c
CeO2-r
200
400
600
800
1000
1200
Raman Shift / cm-1
Fig. 7 Raman spectra of CeO2 with different morphologies
4 Stability test of CeO2 nanorods in DCM
oxidation
Since CeO2-r was highly active in DCM oxidation among all
investigated samples, the stability of this sample in DCM oxidation
was investigated in 325 °C for 100 h continuous testing, the result
is shown in Fig. 8. It could be observed that CeO2-r experienced a
slowly deactivation in the initial 7 h testing, then stabilized. The
conversion of DCM could be maintained at 83%–85% hereafter.
The initial deactivation of CeO2-r could be attributed to the
adsorption of chlorine species on the surface of CeO2 as discussed
above.
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ARTICLE
planes. The results are also consistent with the order of oxygen
vacancy formation energy of CeO2 surfaces: {1 1 0} < {1 0 0} <
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{1 1 1}, as demonstrated by DFT calculations [20, 24, 31]
.
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6 Conclusions
CeO2 nanorods, nanocubes, and nanopolyhedra and bulk CeO2
nanoparticles were well synthesized and used in DCM destruction.
Characterization revealed that CeO2 nanorods selectively exposed
{1 1 0} and {1 1 1} planes with better reducibility, as well as more
surface adsorbed oxygen species and oxygen vacancies
concentration than the other samples. Therefore, CeO2 nanorods
showed the best activity and good stability during long-term
testing in DCM oxidation.
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Acknowledgement
The authors thank the financial support of Natural Science
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Morphology-dependent properties of CeO2 nano-catalysts on CH2Cl2 oxidation:
A
diagrammatic drawing of dichloromethane oxidation over CeO2 nanorods, Xinhua Zhang, Zhiying
Pei, Hanfeng Lu, Haifeng Huang, RSC Advances.
O2(g)
CO2+HCl
CH2Cl2
CeO2 nanorods with {1 1 0} planes exposed