Facet-controlled CeO2 nanocrystals for oxidative coupling of methane

Article abstract of DOI:10.1166/jnn.2016.11623

Whether the catalysts of the high temperature reaction such methane oxidation coupling has a structure-sensitive catalytic behavior or not, it is discussed and confirmed the shape-specific impact on methane activity by designing the catalysts with different crystal facets exposed. CeO₂ nanowires enclosed by {110} and {100} planes show the higher CH₄ conversion and higher C₂ hydrocarbons (C₂H₄ and C₂H₆) selectivity, compared with particle CeO₂ rounded by {111} and {100} planes, suggesting that CeO₂ (110) surface favors the activation of CH₄. Encouraged by the result, to control facet-controlled synthesis of catalysts for tailoring the catalytic properties at high temperature, the CeO₂ (110) surface is chosen as doped sites to form the doped catalyst such as Ca doped CeO₂ nanowires for OCM reaction, enhancing C₂ hydrocarbons selectivity dramatically and suppressing the deep oxidation product (CO and CO₂) selectivity.

Full text of DOI:10.1166/jnn.2016.11623

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 4692–4700, 2016
Copyright © 2016 American Scientific Publishers
All rights reserved
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Printed in the United States of America
Facet-Controlled CeO2 Nanocrystals for
Oxidative Coupling of Methane
Yongnan Sun1ꢀ2, Yue Shen1, Jianjun Song2, Rongbin Ba2, Shuangshuang Huang2,
Yonghui Zhao2, Jun Zhang2, Yuhan Sun2, and Yan Zhu2ꢀ∗
1Department of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
2CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute,
Chinese Academy of Sciences, Shanghai 201210, China
Whether the catalysts of the high temperature reaction such methane oxidation coupling has a
structure-sensitive catalytic behavior or not, it is discussed and confirmed the shape-specific impact
on methane activity by designing the catalysts with different crystal facets exposed. CeO2 nanowires
enclosed by {110} and {100} planes show the higher CH4 conversion and higher C2 hydrocarbons
(C2H4 and C2H6ꢀ selectivity, compared with particle CeO2 rounded by {111} and {100} planes,
suggesting that CeO2 (110) surface favors the activation of CH4. Encouraged by the result, to control
facet-controlled synthesis of catalysts for tailoring the catalytic properties at high temperature, the
CeO2 (110) surface is chosen as doped sites to form the doped catalyst such as Ca doped CeO2
nanowires for OCM reaction, enhancing C hydrocarbons selectivity dramatically and suppressing
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2
the deep oxidation product (CO and CO ꢀ selectivity.
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2
Copyright: American Scientific Publishers
Keywords: Oxidation Coupling of Methane, Crystal Planes, Facet-Dependent, Doped Active
Sites.
ꢀ
1. INTRODUCTION
and ignition temperature of methane >550 C,18ꢁ19 which
leads to the undesirable surface reactions and gas-phase
combustion reactions which suppress hydrocarbons for-
mation and increase the formation of COx, e.g., CO and
CO2.20–22 Catalysis based on crystal facets of nanocrystal
catalysts has attracted increasing research interest and the
structurally motifs have been studied,23–26 however, little
attention has been paid to the catalytic reactions carried
out at high temperatures, since all most structure of cat-
alysts shows sensitive at high temperatures. In particular,
the dependence of the activation of C–H bond of methane
with crystal facets of catalysts arouses no interest, since
it is difficult for C–H bond to be activated at mild tem-
perature, and the high temperatures lead to the crush of
well-defined crystal facets. Thus it is difficult to precisely
correlate the catalytic properties with the exact structure
of the catalysts, and as well as to learn what control the
surface active site structure. It is not easy to guide the syn-
thesis and modification of catalysts to tailor the catalytic
performance like activity and selectivity without the cor-
relation properties to structure. Therefore the study that
what surface can give an enhanced activity towards CH4
The effective conversion of methane, as the major com-
ponent of natural gas, for more valuable products under
mild conditions is of great challenge.1–4 Though there
is strong desirable for direct and indirect utilization of
methane, the activation of C–H bond in methane is always
inefficient which requires high temperatures and multi-
step processes.5–7 A variety of processes have been devel-
oped to activate the C–H bond and one promising route
of direct conversion of CH4 into hydrocarbons is oxida-
tive coupling of methane (OCM), in which products (C2H4
and C2H6ꢀ could be converted to gasoline as well as
employed as feedstocks.8–11 Many catalysts including alka-
line earth metal oxides, rare earth metal oxides, and
transition-metal oxides have exhibited a certain activity
for OCM reaction at high temperatures.12–17 However,
the structural stability of these catalysts at high temper-
atures remains an outstanding question, since the cata-
lysts reported require feed-gas temperature of 700–850 ꢀC
∗Author to whom correspondence should be addressed.
4692
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 5
1533-4880/2016/16/4692/009
doi:10.1166/jnn.2016.11623

Sun et al.
Facet-Controlled CeO2 Nanocrystals for Oxidative Coupling of Methane
conversion is valuable and important for a desirable goal
to design and synthesize the catalysts, which would be to
increase the more reactive planes so as to optimize the
structure of the active sites and improve the catalytic effect
on the oxidative coupling of methane.
2.2. OCM Catalytic Testing
The catalytic performance for oxidative coupling of
methane was evaluated at atmospheric pressure in a fixed-
bed quartz tubular reactor. All the catalysts used for the
OCM reaction were pelletized, crushed, and sieved to
40–80 mesh. 200 mg catalyst and 800 mg quartz sands
as diluent were placed in the reactor. Before the reac-
tion, the catalyst in the reactor was heated to the reaction
Herein, to improve the atomic efficiency and attain the
correlation of structure with properties, the well-defined
active sites for catalytic reactions could be achieved by
controlling the morphologies of CeO2 nanostructure.27ꢁ28
We compared the influence of the different crystal facets
of CeO2 on oxidative coupling of methane reaction, which
presented the strong correlation with the activity and selec-
tivity of structure-sensitive OCM reaction. Optimization
of surface structure of CeO2 nanocrystals was performed
in the sites on the (110) surface for high CH4 conversion
and C2 selectivity. Based on the correlation of properties
with structure, the modification of (110) surface by dop-
ing moderate alkaline-earth metal such as Ca significantly
indeed improved its catalytic behavior on oxidative cou-
pling of methane reaction.
ꢀ
temperature (400 C) in 40 mL/min N2 flow. The reac-
tant gases CH4 and O2 went through the reactor at a rate
of 180 mL/min with n(CH4ꢀ:n(O2ꢀ = 3 and the gas hour
space velocity (GHSV) was fixed in 72000 mL/g/h. The
ꢀ
OCM reaction temperature was controlled from 400 C
to 800 ꢀC. A cold trap was placed at the reactor out-
let of the quartz tube to separate any condensed water
vapor from the exit gas stream. The reaction products
(CO, CO2, C2H6, C2H4ꢀ were analyzed by an online gas
chromatograph (GC) equipped with a thermal conductivity
detector.
2.3. Characterization
X-ray diffraction (XRD) patterns of the catalysts were
recorded on a Japanese Rigaku UltimaIV powder diffrac-
tometer using Cu Kꢂ radiation with the wavelength of
1.54056 Å at 40 kV and 40 mA. Rietveld refinement
with MDI-Jade software was used to determine the lat-
2. MATERIAL AND METHODS
2.1. Synthesis of Catalysts
Preparation of CeO2 nanowires: 2 mM of Ce(NO3ꢀ3 ·
6H2O was dissolved in 5 mL nanopure water. When com-
pletely dissolved, the above solution was transferred to
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tice constants from the XRD diffraction analysis. The
35 mL of 6 mol/L sodium hydroxide solution, and this
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morphology and size of nanomaterials were characterized
using JEOL JEM-2100 Electron Microscope (JEOL, TEM,
2011, Japan) operated at 200 kV. The BET specific surface
area was measured by nitrogen adsorption, using Quanta
chrome Autosorb-iQ2-MP analyzer, which was performed
Copyright: American Scientific Publishers
mixture was stirred for 30 min. Subsequently, the above
mixture was held in the Teflon bottle and sealed tightly
by the autoclave. The autoclave was put in a furnace
at 100 ꢀC for 24 h. Light purple sediment was sep-
arated by centrifugation, washed with deionized watꢀer.
The yellow powders were obtained by drying at 80 C
for 12 h.
ꢀ
after degassing the sample at 80 C for 6 h under vac-
uum. The specific surface areas were calculated through
the Brunauer-Emmett-Teller (BET) method. Pore volume
and pore size distribution were determined by the Barrett-
Joyner-Halenda (BJH) method. The surface basicity/base
strength distribution of the prepared catalysts were mea-
sured by the CO2-TPD (Temperature-programmed desorp-
tion). TPD studies for the catalysts were performed in a
U-shaped tubular quartz reactor heated by an electrical fur-
nace and the reactor was loaded with 0.15 g of catalyst.
Prior to each TPꢀD run, the catalyst was pretreated in the
He flow at 800 C in a quartz reactor. After the reactor
Preparation of CeO2 nanoparticles: 5 mL of 2 mM
of Ce(NO3ꢀ3 · 6H2O solution and 35 mL of 0.01 mol/L
sodium hydroxide solution were mixed and stirred for
30 min. Then the above mixture was held in the Teflon
bottle and sealed tightly by the autoclave. The autoclave
ꢀ
was put in a furnace at 100 C for 24 h. The product
was separated by centrifugation and washed with deion-
ized water, and dried at 80 ꢀC for 12 h. The powders were
obtained.
Preparation of Ca-doping CeO2 catalysts: the calcium
doped cerium oxide catalysts were synthesized by adding
Ca(NO3ꢀ2 solution to the initial solution with different
doping amount. 2 mM of Ce(NO3ꢀ3 ·6H2O and an appro-
priate amount of Ca(NO3ꢀ2 were dissolved in 5 mL nanop-
ure water and then transferred to 35 mL of 6 mol/L or
0.01 mol/L sodium hydroxide solution. Then the above
mixture was held in the Teflon bottle, and sealed tightly
ꢀ
temperature was lowered to 700 C, the catalyst adsorbed
CO2 for 30 min under CO2 flow of 50 mL/min. And after
the temperature down to room temperature, He gas was
fed into the reactor at 50 mL/min for 30 min to purge aꢀny
residual oxygen. The catalyst was then heated to 750 C
ꢀ
at a constant heating rate of 10 C/min under He flow of
50 mL/min. The X-ray Photoelectron Spectroscopy (XPS)
of the samples were determined on a RBD upgraded PHI-
5000C ESCA System in a high-vacuum chamber with the
base pressure below 1 ×10−8 Torr.
ꢀ
by the autoclave, and put in a furnace at 100 C for 24 h.
The pꢀroduct was washed with deionized water, and dried
at 80 C overnight.
J. Nanosci. Nanotechnol. 16, 4692–4700, 2016
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Facet-Controlled CeO2 Nanocrystals for Oxidative Coupling of Methane
Sun et al.
2.4. Analytic Methods
3. RESULTS AND DISCUSSION
The methane conversion (XCH
ꢀ
and C2 selectiv-
3.1. Comparison of CeO2 Nanowires and
4
ity (SC ꢀ were calculated according to the following
Nanoparticles for OCM Reaction
2
Eq.:
The CeO2 nanoparticles enclosed by {111} and {100}
planes as well as wire-like CeO2 rounded by {110}
and {100} have been synthesized, and the surface struc-
ture and composition are studied by high-resolution the
transmission electron microscopy (HRTEM) and XRD.
Figure 1(a) shows XRD patterns of synthesized CeO2
samples. The diffraction peaks of samples are typically
indexed as the (111), (200), (220), (311), (222), (400),
F
−F
CH4ꢁin
CH4ꢁout
XCH
=
4
F
CH4ꢁin
2F
C2ꢁout
SC
=
F
2
−F
CH4ꢁin
CH4ꢁout
Where Fi is the molar flow rate of component i.
(a)
(b)
CeO2 nanowires
CeO2 nanoparticles
10
20
30
40
50
60
70
80
2θ (degree)
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(c)
(d)
Copyright: American Scientific Publishers
(e)
(f)
Figure 1. (a) X-ray diffraction patterns of CeO2 nanocrystals. (b) HRTEM image of CeO2 nanoparticles. (c) TEM and (d) HRTEM images of CeO2
nanowires. The inset of figure (d) is a fast Fourier transform (FFT) analysis. (e) TEM image of CeO2 nanowires after OCM reaction. (f) TEM image
of CeO2 nanoparticles after OCM reaction.
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Sun et al.
Facet-Controlled CeO2 Nanocrystals for Oxidative Coupling of Methane
(a)
nanocrystals were indicated by TEM studies. Figure 1(b)
reveals that particles are in 11 nm size and the crystal
lattice fringes are 0.32, 0.27, and 0.19 nm apart enclosed
by {111} and {100} planes. Nanowires with a uniform
diameter in about 10 nm and length up to about 200 nm
are exhibited in Figure 1(c). HRTEM result reveals that
the exposed planes of the CeO2 nanowires are predomi-
nately {110} and {100}, indicated by a fast Fourier trans-
form (FFT) analysis (Fig. 1(d) and the inset). The clear
(111), (200), and (220) lattice fringe directions with inter-
planar spacing of 0.32, 0.28, and 0.19 nm reveal that the
nanowires grow along with a preferred direction [110] and
are enclosed by {110} and {100} planes.
25
20
15
10
5
CeO2 nanoparticles
CeO2 nanowires
0
400
500
600
700
800
Temperature (ºC)
By comparison CeO2 nanoparticles with CeO2
nanowires for OCM reaction, the nanowires enclosed by
{110} and {100} planes achieved higher CH4 conversion
and C2 hydrocarbons selectivity than the nanoparticles
enclosed by {111} and {100} planes, as shown in Figure 2.
15
(b)
CeO2 nanoparticles
CeO2 nanowires
10
5
ꢀ
CH4 over CeO2 nanowires initially converted at 450 C,
while CeO2 particles could give rise to an obvious con-
version of methane when the temperature was higher
than 500 ꢀC. With increase of the reaction teꢀmpera-
ture, CH4 conversion was about 20% for at 450 C and
ꢀ
near to 25% at 800 C for nanowires, however particles
0
ꢀ
gave 3% conversion of CH4 at 450 C and rose to 18%
400
500
600
700
800
CH4 conversion at 800 ꢀC. From 450 ꢀC to 750 ꢀC,
C2 hydrocarbons selectivity over nanowires was always
Temperature (ºC)
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higher than that on particles (Fig. 2(b)), although the
Figure 2. Catalytic performances of CeO2 catalysts with different
shapes for OCM reactions: (a) CH4 conversion and (b) C2 selectivity.
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complete combustion product (CO2ꢀ is major product
Copyright: American Scientific Publishers
on CeO2 catalyst. The results indicate that the surface
structure of CeO2 nanowires favors the activation and
conversion of CH4. Importantly, the structure of CeO2
nanowires was saved and only the length of nanowires did
undergo a change uniform after OCM reaction, implying
(331) and (420) diffractions of a cubic fluorite structure.
The lattice constant, determined from a Rietveld refine-
ment of XRD, of nanoparticles and nanowires is 5.41
and 5.42 Å, respectively. The defined structure of CeO2
Figure 3. Top view of the calculated adsorption structures of the key surface reaction intermediates on the peroxided (110) surface (A) and (111)
surface (B), respectively. (a) CH4 atop O with H closer to the surface; (b) H atop O; (c) CH3 atop with C down; (d) CH3 +H atop co-adsorption. The
white and gray spheres represent C and H atoms. The distances (d, Å) between the surface oxygen and surface adsorbates, adsorption energies of the
reaction intermediates are also denoted.
J. Nanosci. Nanotechnol. 16, 4692–4700, 2016
4695

Facet-Controlled CeO2 Nanocrystals for Oxidative Coupling of Methane
Sun et al.
(a)
that nanowires are stable during OCM reaction under
high temperatures (Fig. 1(e)). The edges and corners of
nanoparticles got round and became sphere after catalytic
reaction (Fig. 1(f)).
25
20
15
10
5
DFT calculation (VASP package) also verifies that
methane activation is preferred over the peroxided (110)
surface. Here, U value of 5 eV is used as the theoret-
ical tests like the literatures reported.29–31 As suggested
in experimental findings that oxygen vacancies in ceria,
particularly at the (111) and (110) surfaces, play a signif-
icant role on ceria’s catalytic properties. It is found that
there is little difference in CH4 activation on perfect and
reduced (111) and (110) surfaces. O2 might also fall into
the surface oxygen vacancy to form peroxide and the cal-
culated energy to form peroxided surface. Many adsorption
configurations are considered and the adsorption energies
for CH4, CH3, H and coadsorption of CH3 and H are given
in Figure 3. It can be seen that the adsorption strength
follows the order peroxided CeO2 (110) > (111) for all
the intermediates (−0.02 vs. −0.00 eV for CH4, −2.47
vs. −2.22 eV for CH3, and −3.40 vs. −3.16 eV for H).
The interatomic distances between O and H, O and C on
the peroxided CeO2 (110) are all shorter than the (111)
surface (vs. 2.39 Å for CH4, 0.97 vs. 0.98 Å for H, and
1.40 vs. 1.43 Å for CH3ꢀ. The same trends are observed
in the CH3 +H co-adsorption, with adsorption energies of
−7.03 eV and −6.46 eV, respectively. Thus, the corre-
CeO2 nanowires
10% Ca-CeO2 nanorods
20% Ca-CeO2 nanorods
0
400
500
600
700
800
Temperature (ºC)
(b) 35
CeO2 nanowires
30
25
20
15
10
5
10% Ca-CeO2 nanorods
20% Ca-CeO2 nanorods
0
400
500
600
700
800
Temperature (ºC)
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sponding reaction energies of CH activation on the per-
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Figure 4. Catalytic performances of CeO2 nanowires, Ce0ꢃ9Ca0ꢃ1O1ꢃ9
,
4
Copyright: American Scientific Publishers
and Ce0ꢃ8 Ca0ꢃ2O1ꢃ8 nanorods catalysts: (a) CH4 conversion and (b) C2
selectivity.
oxided CeO2 (110) and (111) surfaces are −0.65 and
−1.13 eV, respectively. According to the Brønsted–Evans–
Polanyi relationship, such significant difference (0.47 eV)
in reaction energy suggests that methane activation is pre-
ferred over the peroxided (110) surface, even when taking
the actual reaction conditions into account.
CH4 conversion and achieve 24.6% CH4 conversion at
650 ꢀC. Interestingly, C2 selectivity increased dramatically
over doped CeO2 nanowires. Temperature is 600 C as
ꢀ
an example, compared with 7.8% for C2 selectivity over
pure CeO2 nanowires, 17.4% towards C2 selectivity can
be obtained over Ce0ꢃ9Ca0ꢃ1O1ꢃ9 nanorods, and 24.5% was
achieved over Ce0ꢃ8Ca0ꢃ2O1ꢃ8 nanorods. For understanding
further the catalytic properties of doped nanorods cata-
lyst for OCM reaction, the hysteresis ring was noted as
CH4 conversion and C2 selectivity with a temperature
3.2. Catalytic Performance of CeO2 (110) Surface
Doped with Ca
This initial study opens up a new opportunity for precisely
correlation the catalytic properties with the exact structure
of the catalysts for OCM reaction. If the (110) surface is
doped by a moderate dopant, the catalytic performance of
doped catalyst should be improved dramatically. To ver-
ify this, CeO2 nanowires doped with Ca were synthesized
and used as catalysts for OCM reaction. The catalytic per-
formances of rod-like Ce0ꢃ9Ca0ꢃ1O1ꢃ9 (10 wt% Ca–CeO2ꢀ
and Ce0ꢃ8Ca0ꢃ2O1ꢃ8 (20 wt% Ca–CeO2ꢀ nanocatalysts for
OCM reaction are shown in Figure 4. With the level of
Ca dope, the conversion of CH4 increased and C2 selec-
tivity went up, although the initial reaction temperature
of methane conversion rose a little. Methane over both
of pure CeO2 nanowires and Ce0ꢃ9Ca0ꢃ1O1ꢃ9 nanorods ini-
ꢀ
programmed heating–cooling cycle (from 400 C up to
ꢀ
ꢀ
ꢀ
800 C and then from 800 C down to 400 C or lower).
In Figure 5, on decreasing the temperature, for undoped
nanowires, the CH4 conversion and C2 selectivity (red line)
are a bit higher than the corresponding ones (black line) on
increasing the reaction temperature, indicating the stability
of nanowires catalyst even at high temperature. For doped
nanorods, in contrast, the CH4 conversion and C2 selec-
tivity on decreasing temperature are always lower than
those on increasing temperature, revealing the undoped
nanowires is more stable than the doped nanorods. The Ca
doped CeO2 nanoparticles made a sharp comparison on
the catalytic results for OCM reaction. Although the sur-
face structure and shape of nanoparticles are unchanged,
ꢀ
tially converted at 450 C, while Ce0ꢃ8Ca0ꢃ2O1ꢃ8 nanorods
could give an obvious conversion ꢀof methane when the
ꢀ
temperature was higher than 500 C. At 600 C, CeO2
nanowires doped 10 wt% Ca can give a rise to 21.8%
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Sun et al.
Facet-Controlled CeO2 Nanocrystals for Oxidative Coupling of Methane
30
25
20
15
10
25
20
15
10
5
CeO2 nanowires, 400 to 800 ºC
CeO2 nanowires, 800 to 300 ºC
10% Ca-CeO2 nanorods, 400 to 800ºC
5
20% Ca-CeO2 nanoparticles
20% Ca-CeO2 nanorods
10% Ca-CeO2 nanorods, 800 to 400 ºC
20% Ca-CeO2 nanorods, 400 to 800 ºC
20% Ca-CeO2 nanorods, 800 to 400 ºC
0
400
500
600 700 800
0
Temperature (ºC)
300
400
500
600
700
800
Temperature (ºC)
30
25
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15
10
5
CeO2 nanowires, 400 to 800 ºC
CeO2 nanowires, 800 to 300 ºC
10% Ca-CeO2 nanorods, 400 to 800 ºC
10% Ca-CeO2 nanorods, 800 to 400 ºCo
20% Ca-CeO2 nanorods, 400 to 800 ºC
20% Ca-CeO2 nanorods, 800 to 400 ºC
40
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30
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20% Ca-CeO2 nanoparticles
20% Ca-CeO2 nanorods
0
400
500
700
800
600
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600 Copy7r0i0ght: A8m00erican SFciigeunretif6i.c Publishers
Temperature (ºC)
0
300
400
500
Catalytic results of 20 wt% Ca doped CeO2 nanorods and
20 wt% Ca doped CeO2 nanoparticles for OCM reaction, respectively.
Temperature (ºC)
Figure 5. Catalytic performance of CeO2 nanowires and Ca doped
CeO2 nanorods catalyst with reaction temperature programmed heating–
cooling cycle.
decreases slightly with the calcium doped, suggesting the
formation of surface defects of Ca-doped CeO2 resulting
in the distortion of the lattice. The crystalline size of (110)
surface of CeO2 is no change with doped Ca. After cat-
alytic reaction, doped catalyst remains the rod-like shape
(Fig. 7(d)) and the lattice structure is still similar to the
sample before catalytic reaction, which points to the sta-
bility of doped nanorods during the catalytic process at
high temperatures.
The large surface area could give better catalytic perfor-
mance, whereas here the surface area cannot account for
the better catalytic performance of the nanowire/nanorods
for OCM reaction. As shown in Figure 8, the surface area
of CeO2 nanoparticles (110.2 m2/g) measured by N2 sorp-
tion is the largest among these catalysts (82.7 m2/g for
CeO2 nanowires, 88.5 m2/g for Ce0ꢃ9Ca0ꢃ1O1ꢃ9 nanorods
and 80.9 m2/g for Ce0ꢃ8Ca0ꢃ2O1ꢃ8 nanorods), but it gave the
lowest CH4 conversion and C2 selectivity. Besides the sur-
face structure of catalysts is proposed to play a key role
on the catalysis, to gain further insight into the influence
of facet-dependent catalytic behavior, the surface oxygen
species and surface basicity/base properties should be also
Ca doped CeO2 nanorods always achieved higher CH4
conversion and C2 selectivity below 700 C (Fig. 6). It
clearly confirmed that the CeO2 (110) surface was cho-
sen as doped sites can enhance C2 selectivity and CH4
conversion.
ꢀ
More amount of Ca doped in the CeO2 nanorods such
as 25 wt% Ca brought about the appearance of Ca(OH)2,
revealed by the XRD (Fig. 7(a)). Ce0ꢃ9Ca0ꢃ1O1ꢃ9 (10 wt%
Ca) and Ce0ꢃ8Ca0ꢃ2O1ꢃ8 (20 wt% Ca) have a similar diffrac-
tion pattern with pure CeO2, suggesting that small amount
of Ca doped did not change the inferior type structure
of the catalyst, since the ion radius of Ca2+ (0.099 nm)
is close to one of Ce4+ (0.097 nm) and Ca2+ is easy to
go through the crystal lattice of CeO2 nanocrystal.32ꢁ33
The lattice constants of Ce0ꢃ9Ca0ꢃ1O1ꢃ9 and Ce0ꢃ8Ca0ꢃ2O1ꢃ8
are 5.39 and 5.37 Å, respectively, inferring that dope
with calcium leads to slightly distorted structures com-
pared with the pure CeO2. It is also obtained from TEM
studies (Figs. 7(b)–(c)) and the following example of
Ce0ꢃ8Ca0ꢃ2O1ꢃ8 illustrates the above statement. The lattice
distance of (111) and (200) surface of CeO2 nanowires
J. Nanosci. Nanotechnol. 16, 4692–4700, 2016
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Facet-Controlled CeO2 Nanocrystals for Oxidative Coupling of Methane
Sun et al.
Ce0.75Ca0.25O1.75 nanorods
(a)
(b)
Ce0.8Ca0.2O1.8 nanorods
Ce0.9Ca0.1O1.9 nanorods
10
20
30
40
50
60
70
80
2θ (degree)
(c)
(d)
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Figure 7. (a) X-ray diffraction patterns of the doped CeO2 nanocrystals. (b) TEM image of Ce0ꢃ8Ca0ꢃ2O1ꢃ8 nanocrystals. (c) HRTEM image of
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Ce0ꢃ8Ca0ꢃ2O1ꢃ8 nanocrystals and the inset is a fast Fourier transform analysis. (d) TEM image of Ce0ꢃ8
O1ꢃ8 nanorods after OCM reaction.
Copyright: American Scientific Publishers 0ꢃ2
taken into account for the different catalytic performance
of catalysts.
binding energy spectra of pure CeO2 and Ca doped CeO2
catalysts (Fig. 9). It is known that one of the most essen-
tial reaction step of the OCM reaction is the generation of
active oxygen species from gas-phase oxygen on the cat-
alyst surface. Four different oxygen species can be obvi-
ously observed from the O1s spectra of CeO2 catalysts,
i.e., superoxide ions O−2 , carbonate CO23−, peroxide ions
O− and lattice oxygen O2−. Electrophilic oxygen species
(O− + O−2 ꢀ on the surface of catalysts are main active
species for OCM and are effective for the C2 selectivity,
compared with lattice oxygen specie which is easier to
cause methane oxidation completely.34 The direct corre-
lation can be drawn between the ratio of (O− + O2−ꢀ to
O2− calculated using the 80% Gaussian-20% Lorentizian
peak shape (Table I) and the catalytic selectivity, that is
higher the ratio value and higher C2 selectivity. In detail,
the ratio value of (O− +O2−ꢀ/O2− of CeO2 nanoparticles is
0.66, whereas the ratio from nanowires is 0.69. The high
C2 selectivity can be achieved over CeO2 nanowires and
high complete oxidation products are obtained over CeO2
nanoparticles, which is good agreement with the experi-
mental results obtained. Note the ratio of (O + O−2 ꢀ/O2−
values of the Ca doped CeO2 nanorods are significantly
higher than the pure CeO2 nanowires, such as 0.99 for
Ce0ꢃ9Ca0ꢃ1O1ꢃ9 and 1.55 for Ce0ꢃ8Ca0ꢃ2O1ꢃ8, which gives a
It has been reported that the formation energy of oxygen
vacancies for {110} and {100} is lower than that of {111}
for CeO2, and the oxygen vacancies are easier to form
on {110} and {100} planes.23 Different surface structure
generates different oxygen species, revealed by the O1s
700
CeO2 nanoparticles
CeO2 nanowires
600
500
400
300
200
100
0
Ce0.9Ca0.1O1.9 nanorods
Ce0.8Ca0.2O1.8 nanorods
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Figure 8. Nitrogen absorption isotherms of CeO2 nanoparticles, CeO2
nanowires, Ce0ꢃ9Ca0ꢃ1O1ꢃ9, and Ce0ꢃ8Ca0ꢃ2O1ꢃ8 nanorods.
4698
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Sun et al.
Facet-Controlled CeO2 Nanocrystals for Oxidative Coupling of Methane
(a)
(b)
O 1s
O 1s
1
1
2
2
3
3
526
528
530
532
534
526
528
530
532
534
536
Binding Energy (ev)
Binding Energy (eV)
(c)
(d)
O 1s
O 1s
1
1
2
2
3
3
4
4
526
528
530
532
534
536
538
526
528
530
532
534
536
Delivered by Ingenta to: Purdue University Libraries
Binding Energy (eV)
Binding Energy (eV)
IP: 46.161.57.48 On: Fri, 27 May 2016 06:44:06
Figure 9. O1s binding energy spectra of (a) CeO nanoparticles, (b) CeO nanowires, (c) Ce Ca
O
nanorods, and (d) Ce0ꢃ8Ca0ꢃ2O1ꢃ8 nanorods,
Copyright: American Scientific Publi0sꢃ9hers
2
2
0ꢃ1 1ꢃ9
respectively. (1: O2−, 2: O−; 3: CO32−; 4: O ꢀ.
−
2
ꢀ
good explain that Ce0ꢃ8Ca0ꢃ2O1ꢃ8 achieved the highest C2
selectivity.
peak at 100 C are larger than those of CeO2 nanopar-
ticles, showing the larger amount of basic sites on the
{110} planes. The addition of Ca results in the appear-
ance of intermediate strength basic sites at middle tem-
The surface basicity/base properties of the prepared cat-
alysts are measured by the CO2-TPD (Fig. 10), reflect-
ing that the different surface structure has the different
strength of basicity/base. One desorption peak of CeO2 is
ꢀ
perature (450–550 C), which is due to the chemisorbed
CO2.32ꢁ35 Compared with Ce0ꢃ9Ca0ꢃ1O1ꢃ9 nanorods, the
ꢀ
located at low temperature (100 C), which is attributed
middle strength peak of Ce0ꢃ8Ca0ꢃ2O1ꢃ8 nanorods slightly
to the desorption of weakly adsorbed CO2. The inten-
sity and the peak area of CeO2 nanowires desorption
CeO2 nanorods
Table I. Curve-fitting results from XPS data on O1s binding energies,
the corresponding FWHM (eV), and relative amount of oxygen species
in CeO2 and doped CeO2 nanocrystals.
10% Ca-CeO2 nanorods
20% Ca-CeO2 nanorods
CeO2 nanoparticles
FWHM (eV) and Relative
amount of oxygen species
Catalyst
CeO2
O2−
O−
CO23−
O2−
(O− +O−2 )/O2−
529.0
530.2
531.6
–
0.66
nanoparticles 46.5% 30.5% 23.0%
CeO2 528.8 530.4 531.3
nanowires 46.0% 31.6% 22.4%
528.6 530.1 531.3
–
–
–
0.69
0.99
1.55
Ce0ꢃ9 Ca0ꢃ1O1ꢃ9
nanorods
533.6
0
200
400
600
800
42.2% 26.8% 15.8% 15.2%
Temperature (ºC)
Ce0ꢃ8 Ca0ꢃ2O1ꢃ8
nanorods
528.7
530.0
531.6
533.9
8.8%
30.8% 39.0% 21.4%
Figure 10. CO2-TPD profiles of CeO2 and doped CeO2 nanocrystals.
J. Nanosci. Nanotechnol. 16, 4692–4700, 2016
4699

Facet-Controlled CeO2 Nanocrystals for Oxidative Coupling of Methane
Sun et al.
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In summary, CeO2 nanowires enclosed by {110} and {100}
planes exhibited a lower ignition temperature of CH4,
higher conversion of CH4 and higher selectivity towards
C2 hydrocarbons, compared with the CeO2 nanoparticles
rounded by {111} and {100} planes. Meanwhile, CeO2
nanowires doped with calcium significantly improved CH4
conversion and C2 selectivity. Optimization of surface
structure of ceria-based nanocrystals is performed in the
sites on the (110) surface for better OCM performance
at lower ignition temperature and lower feed-gas tempera-
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{100} planes synthesized here might not be practical as an
industrial catalyst, however, the distinctly functional inter-
face of CeO2 (110) surface makes a breakthrough in fur-
ther understanding the role of the surface atomic structure
playing on catalytic nature of OCM. The novel design of
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to gain a fundamental understanding of the mechanism of
methane conversion to synthesize C2 hydrocarbons.
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Received: 2 February 2015. Accepted: 22 April 2015.
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