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Journal of Science: Advanced Materials and Devices (2017) 41e44 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Enhanced catalytic properties of La-doped CeO2 nanopowders synthesized by hydrolyzing and oxidizing Ce46La5C49 alloys Xueling Hou a, b, *, Qianqian Lu a, b, Xiaochen Wang a, b a b Laboratory for Microstructures of Shanghai University, Shanghai, 200072, China School of Materials Science and Engineering, Shanghai University, Shanghai, 200072, China a r t i c l e i n f o a b s t r a c t Article history: Received 14 February 2017 Received in revised form 18 February 2017 Accepted 19 February 2017 Available online 27 February 2017 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 The 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 La-doped CeO2 nanopowders show enhanced CH4 catalytic 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/) Keywords: Cerium lanthanum carbide La-doped CeO2 nanopowders Hydrolyzing and-oxidizing CH4 catalytic performance 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 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 Ce4ỵ Ce3ỵ; (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, La3ỵ incorporation into the ceria lattice creates lattice defects due to ionic radius difference between Ce4ỵ (0.097 nm) and La3ỵ (0.110 nm) Up to now, La-doped CeO2 nanostructures have been introduced to be synthesized by the coprecipitation [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 understanding of the relationship between the structure and properties of the material has thus not been reached * 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 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 La3ỵ-doped CeO2 nanopowders has been tested by methane combustion and compared with that of CeO2 Experimental 2.1 Synthesis of nanopowders The alloys with nominal compositions of Ce46La5C49 and 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 Ce46La5C49 and 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 CeO2 were obtained with further filtrating, washing and drying in the cabinet at 120 C Finally, they were calcined at 600 C-800 C for h in air A schematic of the preparation process of La-doped CeO2 nanopowders is shown in Fig [14] 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 (http://creativecommons.org/licenses/by/4.0/) 42 X Hou et al / Journal of Science: Advanced Materials and Devices (2017) 41e44 Fig Schematic of the preparation process for La-doped CeO2 nanopowders [14] Fig (a) XRD patterns of La-doped CeO2 nanopowders Oven dried at 80 C: (1) La-doped CeO2 (2) CeO2 Calcinated at 600 C/1 h: (3) La-doped CeO2, (4) CeO2 (b) Raman spectra of La-doped CeO2 and CeO2 nanopowders, Oven dried at 80 C: (1) La-doped CeO2, (2) CeO2 Fig (a) XRD patterns of samples calcinated at different temperature for h (b) CH4 catalytic activity of La-doped CeO2 and CeO2 nanopowders at 600 C/1 h X Hou et al / Journal of Science: Advanced Materials and Devices (2017) 41e44 Table The nanocrystalline size of samples at different calcination temperatures Sample Crystallite size (nm) Oven drying 600 C/1 h 700 C/1 h 800 C/1 h La-doped CeO2 Pure CeO2 1.9 5.7 7.3 9.5 3.1 11.5 17.0 23.1 T10 ( C) T50 ( C) T90 ( C) 419 375 460 502 512 550 562 611 600 2.2 Analysis of nanopowders The phases of the nanopowders were analyzed by X-ray diffraction (XRD) with Cu K-alpha radiation and Raman spectroscopy (InviaỵPlus) 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: D ¼ K k=B cos q; 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 analyzed online by gas chromatograph equipped with a flame ionization detector (FID) Results and discussion Table The CH4 catalytic activity of La-doped CeO2, CeO2 nanopowders and reference [16] La-doped CeO2 CeO2 CeLa30 [16] 43 (1) where D and B are the grain size and the half-width of an XRD peak, respectively K is the Scherrer constant, q is 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 The XRD patterns of La-doped CeO2 and CeO2 nanopowders are shown in Fig 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 samples are very fine (see Fig 2(1e2)) After calcining at 600 C for h, the XRD patterns of La-doped CeO2 nanopowders and pure CeO2 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 La2O3 corresponding to PDF-ICDD 73-2141 are not observed The Raman spectroscopy of the La-doped CeO2 and CeO2 nanopowders oven dried at 80 C are shown in Fig 2b A strong band near 460 cmÀ1 observed is due to the F2g Raman active mode of the fluorite structure of CeO2 [15] The occurrence of the bands near 535 cmÀ1 and 597 cmÀ1 was 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 performance The absence of the peak near 405 cmÀ1 corresponding to La2O3 indicates the La3ỵ incorporation 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 800 C for h In comparison with 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 pure CeO2 nanopowders It means that La-doped is beneficial to forming small size of CeO2 The peaks of La2O3 phase Fig TEM and HRTEM images of La-doped CeO2 nanopowders at different calcination temperatures; (a) (d): dried at 80 C, (b) (e): 600 C/1 h, (h) (c) (f): 800 C/1 h 44 X Hou et al / Journal of Science: Advanced Materials and Devices (2017) 41e44 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 DebyeScherrer equation upon all 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 C and 652 C, respectively The T50 and T90 of pure CeO2 nanopowders are 512 C and 611 C, 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 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 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 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 samples during the calcination process The TEM and HRTEM images of the Ladoped CeO2 nanopowders are displayed in Fig It is found that the samples oven dried at 80 C showed some grain agglomerations (Fig 4a,b,c) It can be seen that the size of La-doped CeO2 nanopowders is about 3e5 nm, and some interplanar distances 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 C and 800 C for h, the size of La-doped CeO2 nanopowders 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 CeO2 phase (see Fig 4e and 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 performance Conclusion La doping into CeO2 can effectively prevent grain growth and it is beneficial for 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 nm to 9.5 nm when increasing the calcination temperature from 600 C to 800 C The samples of La dopants have good catalytic activities of methane combustion because La ions are incorporated into the CeO2 lattice to form Ce-La solid solution, which improves the activities of the ceria 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C: (1) La- doped CeO2 (2) CeO2 Calcinated at 600 C/1 h: (3) La- doped CeO2, (4) CeO2 (b) Raman spectra of La- doped CeO2 and CeO2 nanopowders, Oven dried at 80 C: (1) La- doped CeO2, (2) CeO2 Fig... XRD patterns of La- doped CeO2 nanopowders and pure CeO2 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... T90 of La- doped CeO2 nanopowders are 502 C and 652 C, respectively The T50 and T90 of pure CeO2 nanopowders are 512 C and 611 C, respectively It is very obvious that the T50 and T90 of
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