Enhanced catalytic properties of La doped CeO2 nanopowders synthesized by hydrolyzing and oxidizing Ce46La5C49 alloys Accepted Manuscript Enhanced catalytic properties of La doped CeO2 nanopowders syn[.]
Accepted Manuscript Enhanced catalytic properties of La-doped CeO2 nanopowders synthesized by hydrolyzing and oxidizing Ce46La5C49 alloys Xueling Hou, Qianqian Lu, Xiaochen Wang PII: S2468-2179(17)30018-7 DOI: 10.1016/j.jsamd.2017.02.006 Reference: JSAMD 85 To appear in: Journal of Science: Advanced Materials and Devices Received Date: 14 February 2017 Revised Date: 18 February 2017 Accepted Date: 19 February 2017 Please cite this article as: X Hou, Q Lu, X Wang, Enhanced catalytic properties of La-doped CeO2 nanopowders synthesized by hydrolyzing and oxidizing Ce46La5C49 alloys, Journal of Science: Advanced Materials and Devices (2017), doi: 10.1016/j.jsamd.2017.02.006 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT Enhanced catalytic properties of La-doped CeO2 nanopowders synthesized by hydrolyzing and oxidizing Ce46La5C49 alloys Xueling Hou1, 2,*, Qianqian Lu1,2, and Xiaochen Wang1, Laboratory for Microstructures of Shanghai University, Shanghai, 200072, China RI PT School of Materials Science and Engineering, Shanghai University, Shanghai, SC 200072, China M AN U Abstract: The Ce46La5C49 alloy was first prepared in a 25 kg vacuum induction melting furnace The La-doped CeO2 nanopowders were then prepared by hydrolysis and oxidation of Ce46La5C49 at room temperature These nanopowders were calcinated at different temperatures in order to improve their catalytic activities After the TE D lanthanum ions were used to partially replace the cerium ions in the CeO2 lattice, forming a solid solution of cerium lanthanum Compared to the pure CeO2, the thermal stability of La-doped CeO2 was increased due to the lanthanum doping The AC C EP La-doped CeO2 nanopowders show enhanced CH4 catalytic performance Key words: Cerium lanthanum carbide; La-doped CeO2 nanopowders; Hydrolyzing and-oxidizing; CH4 catalytic performance Corresponding author: flybird1656@163.com or xlhou@staff.shu.edu.cn (X.L Hou) ACCEPTED MANUSCRIPT Introduction As a typical kind of rare earth oxide, ceria (CeO2) has been widely explored in ultraviolet [1], polishing materials [2], gas sensors [3], abrasives [4], solid oxide fuel RI PT cells [5], and catalysts [6-9], where pollutant emissions from internal combustion engines are effectively reduced The catalytic properties of CeO2 are mainly related to SC the following four factors: (i) a large oxygen storage capacity via the redox process Ce4+↔Ce3+; (ii) improvement of the thermal stability of supports; and (iii) promotion M AN U of the water–gas 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, La3+ incorporation into the ceria lattice creates lattice defects due to ionic radius difference between Ce4+ (0.097 TE D nm) and La3+ (0.110 nm) Up to now, La-doped CeO2 nanostructures have been introduced to be synthesized by the co-precipitation [11], sol-gel method [12], and EP hydrothermal process [13] However, it is still challenging to produce large quantities of such materials using these techniques, and a complete understanding of the AC C 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 friendly synthesis approach [14] The methane catalytic performance of La3+-doped CeO2 nanopowders has been tested by methane combustion and compared with that of CeO2 ACCEPTED MANUSCRIPT Experimental 2.1 Synthesis of nanopowders The alloys with nominal compositions of Ce46La5C49 and Ce51C49 were prepared by RI PT 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 SC alloy melt was cast with fast cooling rate in order to obtain the alloys of Ce46La5C49 M AN U and Ce51C49 Then these alloys were crushed into grains ( less than 1.0 mm ) and these powders were immersed into deionized water with 1:10–1:40 mass ratios under agitation at room temperature for 18–30 h until the reaction of hydrolysis and oxidation was completed Subsequently, filtration, washing and drying in the cabinet TE D at 120 oC, the nanopowders of CeO2 and La-doped CeO2 were obtained Finally, they were calcined at 600 oC-800 oC for h in air A schematic of the preparation process of La-doped CeO2 nanopowders is shown in Fig [14] EP 2.2 Analysis of nanopowders AC C The phases of the nanopowders were analyzed by X-ray diffraction (XRD) with Cu Karadiation and Raman spectroscopy (Invia+Plus) Transmission electron microscopy (TEM) and high-resolution trans-mission electron microscopy (HRTEM) were performed to investigate the morphology of nanopowders The grain size was estimated by Scherrer’s equation: D = K k /B cosθ, (1) where D and B are the grain size and the half-width of an XRD peak, respectively, K ACCEPTED MANUSCRIPT is the Scherrer constant, θ is the diffraction angle, and k is the wave length of the x-ray 2.3 Catalytic activity tests RI PT 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 SC 80 ml /min and a space velocity of 24000 mL/(g·h) The reactants of samples were (FID) Results and discussion M AN U analyzed online by gas chromatograph equipped with a flame ionization detector The XRD patterns of La-doped CeO2 and CeO2 nanopowders are shown in Fig TE D 2a All the characteristic lines in the XRD patterns are symmetric which match with that of the standard fluorite-type cubic phase of CeO2 The wide diffraction peaks indicate that the grains of the sample are very fine (see Fig 2(1-2)) After calcining at EP 600 oC for h, the XRD patterns of La-doped CeO2 nanopowders and pure CeO2 AC C nanopowders are shown in Fig 2a(3,4) Compared to pure CeO2 nanopowders, it can also be seen that the XRD peaks of La-doped CeO2 samples shift slightly to lower angles and the 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 La2 O3 corresponding to PDF-ICDD 73-2141 are not observed The Raman spectroscopy of the La-doped CeO2 and CeO2 nanopowders oven ACCEPTED MANUSCRIPT dried at 80 oC are shown in Fig 2b A strong band near 460 cm -1 observed is the F2g Raman active mode of the fluorite structure of CeO2 [15] The occurrence of the bands near 535 cm -1 and 597 cm -1 was found only in La-doped CeO2 These bands RI PT have been attributed to oxygen vacancies and intrinsic or doping defects, which are expected to be beneficial to catalytic performance The absence of the peak near 405 The results are in agreement with the XRD analysis SC cm -1 corresponding to La2O3 indicates the La3+ incorporation into the CeO2 lattice M AN U Figure 3a shows the XRD patterns of samples calcinated from 600 oC to 800 oC for h In comparison with to pure CeO2 nanopowders, 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 temperature of TE D pure CeO2 nanopowders It means that La-doped is beneficial to forming small size of CeO2 The peaks of La2O3 phase not appear This indicates that the La doping increases the thermal stability of CeO2 nanopowders by the host lattice The average EP crystallite size of samples was also estimated by the Debye-Scherrer equation upon all AC C the prominent lines of the XRD data The nanocrystalline size of samples is listed in Table The methane catalytic activity curves of samples after calcining at 600 °C are shown in Fig 3b and Table The catalytic activity is characterized by T10, T50 and T90, in which the reaction temperature is corresponding to 10%, 50% and 90% methane conversions, respectively The T50 and T90 of La-doped CeO2 nanopowders are 502 oC and 652 oC, respectively The T50 and T90 of pure CeO2 nanopowders are ACCEPTED MANUSCRIPT 512 oC and 611 oC, respectively It is very obvious that the T50 and T90 of the La-doped CeO2 nanopowders corresponding to the reaction temperature are lower than those of pure CeO2 This indicates that the La-doped CeO2 nanopowders have RI PT good catalytic activity because La ions are incorporated into the CeO2 lattice to form the La-Ce solid solution, which improves the activity of oxide on the nanopowders surface The catalytic activities of samples prepared by hydrolyzing and oxidizing M AN U synthesized with the aid of glucose and acrylic acid SC Ce-La-C or Ce-C alloys are superior to those reported in Ref [16], which were The excellent catalytic performance of the La-doped CeO2 nanopowders is attributed to the fact that La incorporation into CeO2 refined grain size and increased the thermal stability of CeO2 The TEM image indicates grain size change of the TE D samples during the calcination process The TEM and HRTEM images of the La-doped CeO2 nanopowders are displayed in Fig It is found that the samples oven dried at 80 oC showed some grain agglomerations (Fig 4a,b,c) It can be seen that the EP size of La-doped CeO2 nanopowders is about 3–5 nm, and some interplanar distances AC C are determined to be about 0.309 nm, which corresponds to the (111) plane of the CeO2 phase (see Fig 4d) After calcinating at 600 oC and 800 oC for 1h, the size of La-doped CeO2 nanopowders is about 5–8 nm and 8-15 nm, respectively Some interplanar distances are about 0.312 nm and 0.165 nm, which correspond to the (111) and (311) plane of the CeO2 phase (see Fig 4e,f) These results show that the size of grains’ growth is a little with increasing the calcination temperature and it is a main reason for the La-doped CeO2 nanopowders to possess an excellent catalytic ACCEPTED MANUSCRIPT performance Conclusions La doping into CeO2 can effectively prevent grain growth and it is beneficial for RI PT grain refinement of CeO2 nanopowders Because of the good thermal stability of CeO2 doped by La ion, small size grains increase from about 5.7 to 9.5 when increasing the calcination temperature from 600 oC to 800 oC The samples of La SC dopants have good catalytic activities of methane combustion because La ions are M AN U incorporated into the CeO2 lattice to form Ce-La solid solution, which improves the activities of the ceria nanopowders’ surface by increasing oxygen vacancies and defects Acknowledgements TE D This work was supported by the National Natural Science Foundation of China AC C EP (Grant No U1531120) ACCEPTED MANUSCRIPT References [1] J.H Kang, Y.H Kim, S.M Paek, Surface passivation of CeO2 catalyst and RI PT its ultraviolet screening effect, J Nanosci Nanotechnol 11 (2011) 6448-6452 [2] L.J Mu, Z.Y Zhang, L.Y Zhang, Synthesis of SiO2/CeO2 nanopaticles with core/shell structure and their polishing performance, Adv Mater Res 683 (2013) 30-33 SC [3] N Izu, W Shin, N Murayarna, S Kanzaki, Resistive oxygen gas sensors based on CeO2 fine powder prepared using mist pyrolysis, Sens Actuators, B Chem 87 (2002) 95-98 M AN U [4] X.L Song, D.Y Xu, X.W Zhang, X.D Shi, N Jiang, G.Z Qiu, Electrochemical behavior and polishing properties of silicon wafer in alkaline 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and oxidizing, J Electron Mater 45 [15] J.R Mcbride, K.C Hass, B.D Poindexter, EP W.H Weber, Raman and x-ray studies of Ce1-xRexO2-y, where Re = La, Pr, Nd, Eu, Gd, Tb, AC C J Appl Phys 76 (1994) 2435 [16] H Li，G Lu，Y Wang，Y Guo，Y Guo, Synthesis of flower-like La or Pr-doped mesoporous ceria microspheres and their catalytic activities for methane combustion, Catalysis Communi 11 (2010) 946-950 ACCEPTED MANUSCRIPT Table The nanocrystalline size of samples at different calcination temperatures Crystallite size (nm) La-doped CeO2 Pure CeO2 Oven drying 1.9 600 oC /1 h 5.7 700 oC /1 h 7.3 RI PT Sample 800 oC /1 h 9.5 3.1 11.5 M AN U SC 17.0 23.1 Table The CH4 catalytic activity of La-doped CeO2, CeO2 nanopowders and reference [16] T10(oC) CeO2 AC C T90(oC) 502 562 375 512 611 460 550 600 EP CeLa30 [16] 419 TE D La-doped CeO2 T50(oC) ACCEPTED MANUSCRIPT Figure captions: Fig Schematic of the preparation process for La-doped CeO2 nanopowders [14] RI PT Fig (a) XRD patterns of La-doped CeO2 nanopowders Oven dried at 80 oC: (1) La-doped CeO2 (2) CeO2 Calcinated at 600 oC /1 h: (3) La-doped CeO2, (4) CeO2 (b) Raman spectra of La-doped CeO2 and CeO2 nanopowders, Oven dried at 80 oC: (1) SC La-doped CeO2, (2) CeO2 Fig (a) XRD patterns of samples calcinated at different temperature for h (b) M AN U CH4 catalytic activity of La-doped CeO2 and CeO2 nanopowders at 600 oC /1 h Fig TEM and HRTEM images of La-doped CeO2 nanopowders at different calcination temperatures; (a) (d): dried at 80 oC, (b) (e): 600 oC /1 h, (h) (c) (f): 800 oC AC C EP TE D /1 h AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT ... MANUSCRIPT Enhanced catalytic properties of La- doped CeO2 nanopowders synthesized by hydrolyzing and oxidizing Ce4 6La5 C49 alloys Xueling Hou1, 2,*, Qianqian Lu1,2, and Xiaochen Wang1, Laboratory... stability of La- doped CeO2 was increased due to the lanthanum doping The AC C EP La- doped CeO2 nanopowders show enhanced CH4 catalytic performance Key words: Cerium lanthanum carbide; La- doped CeO2 nanopowders; ... at 600 oC /1 h: (3) La- doped CeO2, (4) CeO2 (b) Raman spectra of La- doped CeO2 and CeO2 nanopowders, Oven dried at 80 oC: (1) SC La- doped CeO2, (2) CeO2 Fig (a) XRD patterns of samples calcinated