Compared to the pure CeO2, the thermal stability of La-doped CeO2was increased due to the lanthanum doping.. The catalytic properties of CeO2 are mainly related to the following three fa
Trang 1Original Article
Xueling Houa,b,*, Qianqian Lua,b, Xiaochen Wanga,b
a Laboratory for Microstructures of Shanghai University, Shanghai, 200072, China
b School of Materials Science and Engineering, Shanghai University, Shanghai, 200072, China
a r t i c l e i n f o
Article history:
Received 14 February 2017
Received in revised form
18 February 2017
Accepted 19 February 2017
Available online 27 February 2017
Keywords:
Cerium lanthanum carbide
La-doped CeO 2 nanopowders
Hydrolyzing and-oxidizing
CH 4 catalytic performance
a b s t r a c t
The Ce46La5C49alloy wasfirst prepared in a 25 kg vacuum induction melting furnace The La-doped CeO2 nanopowders were then prepared by hydrolysis and oxidation of Ce46La5C49at room temperature These nanopowders were calcinated at different temperatures in order to improve their catalytic activities The lanthanum ions were used to partially replace the cerium ions in the CeO2lattice, forming a solid so-lution of cerium lanthanum Compared to the pure CeO2, the thermal stability of La-doped CeO2was increased due to the lanthanum doping The La-doped CeO2nanopowders show enhanced CH4catalytic performance
© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
As a typical kind of rare earth oxide, ceria (CeO2) has been
widely explored in ultraviolet[1], polishing materials[2], gas
sen-sors[3], abrasives[4], solid oxide fuel cells[5], and catalysts[6e9],
where pollutant emissions from internal combustion engines are
effectively reduced The catalytic properties of CeO2 are mainly
related to the following three factors: (i) a large oxygen storage
capacity via the redox process Ce4þ4 Ce3 þ; (ii) improvement of
the thermal stability of supports; and (iii) promotion of the
wateregas shift reaction [10] The addition of different metal
dopants into CeO2 lattice leads to formation of defects in crystal
structure enhancing oxygen storage/release capacity and oxygen
conductivity In particular, La3þincorporation into the ceria lattice
creates lattice defects due to ionic radius difference between Ce4þ
(0.097 nm) and La3þ(0.110 nm) Up to now, La-doped CeO2
nano-structures have been introduced to be synthesized by the
co-precipitation[11], sol-gel method[12], and hydrothermal process
[13] However, it is still challenging to produce large quantities of
such materials using these techniques, and a complete
under-standing of the relationship between the structure and properties
of the material has thus not been reached
In this paper, we report a mass synthesis of La-doped CeO2 nanopowders by hydrolyzing and oxidizing cerium lanthanum carbide alloys, which represents an environmentally and friendly synthesis approach [14] The methane catalytic performance of
La3þ-doped CeO2nanopowders has been tested by methane com-bustion and compared with that of CeO2
2 Experimental 2.1 Synthesis of nanopowders The alloys with nominal compositions of Ce46La5C49and Ce51C49 were prepared by induction melting furnace During the melting, the graphite crucible was used Ce, La and C melting at a high power
of 35 Kw for a certain period of time (3 min) was to make carbon saturated in the alloy After carbon was fully dissolved in the alloy, the melt alloy was cast with fast cooling rate in order to obtain the alloys of Ce46La5C49and Ce51C49 Then these alloys were crushed into grains (less than 1.0 mm) and these powders were immersed into deionized water with 1:10e1:40 mass ratios under agitation at room temperature for 18e30 h until the reaction of hydrolysis and oxidation was completed Subsequently, the nanopowders of CeO2 and La-doped CeO2were obtained with furtherfiltrating, washing and drying in the cabinet at 120C Finally, they were calcined at
600C-800C for 1 h in air A schematic of the preparation process
of La-doped CeO2nanopowders is shown inFig 1 [14]
* Corresponding author Laboratory for Microstructures of Shanghai University,
Shanghai, 200072, China.
E-mail addresses: flybird1656@163.com , xlhou@staff.shu.edu.cn (X Hou).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
http://dx.doi.org/10.1016/j.jsamd.2017.02.006
2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Journal of Science: Advanced Materials and Devices 2 (2017) 41e44
Trang 2Fig 1 Schematic of the preparation process for La-doped CeO 2 nanopowders [14]
Fig 2 (a) XRD patterns of La-doped CeO 2 nanopowders Oven dried at 80 C: (1) La-doped CeO 2 (2) CeO 2 Calcinated at 600 C/1 h: (3) La-doped CeO 2 , (4) CeO 2 (b) Raman spectra
of La-doped CeO 2 and CeO 2 nanopowders, Oven dried at 80 C: (1) La-doped CeO 2 , (2) CeO 2
X Hou et al / Journal of Science: Advanced Materials and Devices 2 (2017) 41e44 42
Trang 32.2 Analysis of nanopowders
The phases of the nanopowders were analyzed by X-ray
diffraction (XRD) with Cu K-alpha radiation and Raman
spectros-copy (InviaþPlus) Transmission electron microscopy (TEM) and
high-resolution transmission electron microscopy (HRTEM) were
performed to investigate the morphology of nanopowders The
grain size was estimated by Scherrer's equation:
where D and B are the grain size and the half-width of an XRD peak,
respectively K is the Scherrer constant,qis the diffraction angle,
and k is the wave length of the X-ray
2.3 Catalytic activity tests
The catalytic activity testing for the methane combustion was
carried out in a quartz reactor The catalyst particles (200 mg) were
placed in the reactor The reactant gases (1.0% CH4, 20% O2, balanced
with argon) went through the reactor at a rate of 80 ml/min and a space velocity of 24000 mL/(g$h) The reactants of samples were
ionization detector (FID)
3 Results and discussion The XRD patterns of La-doped CeO2and CeO2nanopowders are shown inFig 2a All the characteristic lines in the XRD patterns are
cubic phase of CeO2 The wide diffraction peaks indicate that the grains of the samples are veryfine (seeFig 2(1e2)) After calcining
at 600C for 1 h, the XRD patterns of La-doped CeO2nanopowders and pure CeO2nanopowders are shown inFig 2a(3,4) Compared to pure CeO2nanopowders, it can also be seen that the XRD peaks of
FWHM of the XRD peaks becomes broader with low intensity The XRD peak's changes were attributed to the grain decrement with La doping into CeO2 nanopowders The XRD peaks of La2O3 corre-sponding to PDF-ICDD 73-2141 are not observed
nanopowders oven dried at 80C are shown inFig 2b A strong band near 460 cm1observed is due to the F2g Raman active mode
of thefluorite structure of CeO2[15] The occurrence of the bands near 535 cm1and 597 cm1was found only in La-doped CeO2 These bands have been attributed to oxygen vacancies and intrinsic
or doping defects, which are expected to be beneficial to catalytic
corre-sponding to La2O3indicates the La3þincorporation into the CeO2
lattice The results are in agreement with the XRD analysis
Fig 3a shows the XRD patterns of samples calcinated from
600 C to 800C for 1 h In comparison with pure CeO2 nano-powders, it can also be seen that the XRD peaks of La-doped CeO2
nanopowders shift slightly to lower angles and the FWHM of diffraction peaks becomes broader at the same calcinating
beneficial to forming small size of CeO2 The peaks of La2O3phase
Table 1
The nanocrystalline size of samples at different calcination temperatures.
Table 2
The CH 4 catalytic activity of La-doped CeO 2 , CeO 2 nanopowders and reference [16]
T 10 ( C) T 50 ( C) T 90 ( C)
X Hou et al / Journal of Science: Advanced Materials and Devices 2 (2017) 41e44 43
Trang 4do not appear This indicates that the La doping increases the
thermal stability of CeO2 nanopowders by the host lattice The
average crystallite size of samples was also estimated by the
Debye-Scherrer equation upon all the prominent lines of the XRD data The
nanocrystalline size of samples is listed inTable 1
The methane catalytic activity curves of samples after calcining
at 600C are shown inFig 3b andTable 2 The catalytic activity is
characterized by T10, T50and T90, in which the reaction temperature
is corresponding to 10%, 50% and 90% methane conversions,
respectively The T50and T90of La-doped CeO2nanopowders are
502 C and 652 C, respectively The T50 and T90 of pure CeO2
nanopowders are 512C and 611C, respectively It is very obvious
that the T50and T90of the La-doped CeO2 nanopowders
corre-sponding to the reaction temperature are lower than those of pure
CeO2 This indicates that the La-doped CeO2 nanopowders have
good catalytic activity because La ions are incorporated into the
CeO2lattice to form the La-Ce solid solution, which improves the
activity of oxide on the nanopowders surface The catalytic
activ-ities of samples prepared by hydrolyzing and oxidizing Ce-La-C or
Ce-C alloys are superior to those reported in Ref.[16], which were
synthesized with the aid of glucose and acrylic acid
nanopowders is attributed to the fact that La incorporation into
CeO2refined grain size and increased the thermal stability of CeO2
The TEM image indicates grain size change of the samples during
the calcination process The TEM and HRTEM images of the
La-doped CeO2nanopowders are displayed inFig 4 It is found that
the samples oven dried at 80C showed some grain agglomerations
(Fig 4a,b,c) It can be seen that the size of La-doped CeO2
determined to be about 0.309 nm, which corresponds to the (111)
plane of the CeO2phase (seeFig 4d) After calcinating at 600C and
800C for 1 h, the size of La-doped CeO2nanopowders is about
5e8 nm and 8e15 nm, respectively Some interplanar distances are
about 0.312 nm and 0.165 nm, which correspond to the (111) and
(311) plane of the CeO2phase (seeFig 4e and f) These results show
that the size of grains' growth is a little with increasing the
calci-nation temperature and it is a main reason for the La-doped CeO2
nanopowders to possess an excellent catalytic performance
4 Conclusion
La doping into CeO2can effectively prevent grain growth and it
is beneficial for grain refinement of CeO2nanopowders Because of
the good thermal stability of CeO2doped by La ion, small size grains
increase from about 5.7 nm to 9.5 nm when increasing the
dopants have good catalytic activities of methane combustion
because La ions are incorporated into the CeO2lattice to form Ce-La solid solution, which improves the activities of the ceria nano-powders' surface by increasing oxygen vacancies and defects Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No U1531120)
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