稀有金属(英文版) 2021,40(01),31-39

Characterization of CeO₂ microspheres fabricated by an ultrasonic spray pyrolysis method

Shou-Feng Xue Yi-Juan Li Feng-Hua Zheng Xue Bian Wen-Yuan Wu Cheng-Hao Yang

Guangzhou Key Laboratory for Surface Chemistry of Energy Materials,New Energy Research Institute,School of Environment and Energy,South China University of Technology

Institute of Metallurgy,Northeastern University

作者简介：Cheng-Hao Yang e-mail:esyangc@scut.edu.cn;

收稿日期：3 June 2020

基金：financially supported by the National Key R&D Program of China (No.2018YFB1502600);the National Natural Science Foundation of China (Nos.51922042 and 51872098);China Postdoctoral Science Foundation (No. 2019M652888);the Sino-Singapore International Joint Research Institute (SSIJRI),China;

Characterization of CeO₂ microspheres fabricated by an ultrasonic spray pyrolysis method

Shou-Feng Xue Yi-Juan Li Feng-Hua Zheng Xue Bian Wen-Yuan Wu Cheng-Hao Yang

Guangzhou Key Laboratory for Surface Chemistry of Energy Materials,New Energy Research Institute,School of Environment and Energy,South China University of Technology

Institute of Metallurgy,Northeastern University

Abstract：

CeO₂ is one of the main catalysts for solid oxide fuel cell(SOFC).It is critical to find a green and costeffective fabrication method for CeO₂ at scale.In this study,the CeO₂ microspheres were prepared by one-step ultrasonic spray pyrolysis of cerium chloride solution at700℃.Scanning electron microscopy(SEM) and transmission electron microscopy(TEM) study demonstrate that the prepared CeO₂ microspheres exhibit a particle size of0.01-1.08 μm with a mean particle size of 0.23 μm,and more than 94% of the particles have a diameter less than0.5 μm.But the presence of residual Cl in the fabricated CeO₂ microspheres blocks the active sites and leads to the significant degradation of SOFC performance.The formation mechanism and distribution of residual Cl in the fabricated CeO₂ microspheres were systemic ally studied.The water washing method was shown to effectively reduce the residual Cl in the CeO₂ microspheres.Overall,this work provides a clean manufacturing process for the preparation of SOFC electrode/electrolyte materials.

Keyword：

CeO₂; Residual chlorine; Ultrasonic spray pyrolysis;

Received： 3 June 2020

1 Introduction

Ultrasonic spray pyrolysis (USP) is useful to fabricate various advanced materials [ 1] ,which have been widely used in batteries [ 2] ,catalysis [ 3] ,phosphors [ 4] and sensors [ 5] .It is attributed that the morphology,components,and particle size of the materials fabricated by USP process can be controlled [ 6] .There are many advantages of the USP method for the preparation of inorganic materials.First,the products are persified in type,e.g.,metal oxide [ 7] ,metal sulfide [ 8] ,phosphate [ 9] ,halide [ 10] ,and carbon [ 11] materials.Second,the raw materials are accessible and cheap,including inorganic metal salts (e.g.,chloride,nitrates,and phosphate),and organic metal salts(e.g.,oxalates,citrates,and acetates) [ 12] .Third,the USP equipment is integrated,automated,and requires lower energy consumption over the full production process [ 13] .Thus,the USP method has been considered as an environmentally friendly process for the fabrication of metal oxides.

Many kinds of precursors have been used to prepare nano-sized materials by the USP method,but little research has focused on the reaction products of the precursor decomposition and their influence on environmental pollution.For example,when metal nitrate salts are used as USP precursors,severe air pollution will result from nitrogen oxides [ 14] .Recently,many environmentallyfriendly processes have been proposed for the non-ferrous metallurgical industry [ 15, 16, 17, 18] .Ce0₂ is one of the main catalysts for solid oxide fuel cells (SOFCs) [ 19] ,and it is prepared by a typically precipitation method [ 20, 21] .The detailed process is schematically illustrated in Fig.1a,and the main chemical principles are described by Eqs.(1) and(2).Substantial amounts of alkaline compounds (for example,NaHCO₃) are consumed to precipitate Ce³⁺and large amounts of sewage (with high salt) are released.

Fig.1 Preparation of CeO₂ by a precipitation process and b USP method

Figure lb shows the fabrication of CeO₂ using a USP method,and besides the pyrolysis gas of HCl/Cl₂,no other pollutants are generated,as shown in Reaction (3).Moreover,the emissions can be recycled by a water adsorption method and the formed dilute hydrochloric acid could be used for an extraction/leaching process.The USP method is a cleaner process for the preparation of rare earth oxides.Compared with the precipitation method,in practice,the continuous production of the USP method greatly improves the production efficiency and thus reduces the production costs.In theory,all metal ions in the raw materials can be transformed into the target products under suitable pyrolysis conditions,which renders a higher utilization ratio of raw materials by USP method.However,the precipitation method follows the laws of precipitation-dissolution equilibrium,and thus there are portions of metal ions that are discharged with sewage into the environment resulting in heavy metal pollution.

It is critical to remove residual chlorine (Cl) in the fabricated CeO₂ microspheres because the presence of Cl can block active sites and lead to the significant degradation of SOFC performance [ 22] .Herein,the CeO₂microsphere was prepared by a USP method using a cerium chloride solution and the influence of the anions on the morphology of CeO₂ microspheres was studied.Moreover,the effects of precursor temperature,carrier gas velocity,CeCl₃ concentration in the precursor,and pyrolysis temperature on residual Cl concentration in CeO₂ were investigated.The possible formation mechanism of residual Cl was analyzed based on experimental results,and the methods to reduce the residual Cl content in fabricated CeO₂ were studied.

2 Experimental

2.1 Materials fabrication

Cerium chloride heptahydrate (CeCl₃·7H₂O,99.99%) purchased from Jiangsu Guosheng Rare-earth Co.,Ltd.(Province Jiangsu,China) was used to prepare the cerium chloride solution,and deionized (DI) water was used as the solvent.Anhydrous ethanol (analytical purity) was purchased from Sinopharm Chemical Reagent Co.,Ltd.(Beijing City,China).CeCl₃·7H₂O was dissolved in DI water and formed a transparent solution.Then,the obtained transparent solution was transferred into a nebulizer for the generation of mist.Compressed air was used as the carrier gas to bring the mist into the high-temperature pyrolysis zone of a tube furnace oriented horizontally,where the rotameter was used to control carrier gas velocity,the pyrolysis temperature was controlled by a program temperature monitor.The preparation of CeO₂ microspheres is carried out under the following conditions:precursor temperature of 20℃,carry gas velocity of 13 L·min^-1,precursor concentration of 0.01 mo1·L^-1,and a pyrolysis temperature of 800℃,unless stated in otherwise.

To determine the relationships between the residual Cl distribution and various synthetic parameters,e.g.,the effects of precursor temperature,gas velocity of carrier gas,precursor solution concentration,and pyrolysis temperature were investigated.The formation mechanism of the residual Cl,the methods of roasting,and water-washing treatments were used to remove residual Cl from the CeO₂micro spheres.The fresh sample was prepared by USP of0.1 mol.L^-1 CeCl₃ solution at 800℃for 1 h,and the percent of the residual chlorine is 2.2 wt%without further treatment operations.The fresh sample followed the dechlorination operations of roasting treatment at 500℃is labeled as“Roasted”,while washing four times by water is signed as“Washed”.These samples were used to determine the residual Cl distribution by analysis at the atom level.

2.2 Characterization

X-ray diffractometer (XRD,PANaLytical X‘ert Pro,Holland) was applied to identify the sample phase composition.Field emission scanning electron microscopy(FESEM,Hitachi S4800,Japan) and transmission electron microscopy (TEM,FEI Tecnai G2F20,USA) were used to study the morphology and microstructure of the samples.Energy-dispersive X-ray (EDX,Oxford,England) analyzer equipment on the FESEM was used to identify the elemental composition and distribution in the samples.An infrared baking lamp (LP23030-B,Electron Microscopy China,China) was used to the preparation of sample used to the analyses of FESEM and EDX.An electron probes microanalyzer (EPMA,JXA-8530F,Japan) was introduced to determine the concentration of the residual Cl.Nitrogen adsorption and desorption were applied to examine the specific surface area and pore size distribution,and X-ray photoelectric spectroscopy (XPS,Kratos AXIS Supra,Japan) was used to determine the chemical state of the residual chlorine in the prepared CeO₂ microspheres.

3 Results and discussion

3.1 Materials characterization

Figure 2a shows XRD patterns of the obtained samples sintered at different temperatures.The diffraction peaks located at 28.5°,33.0°,47.4°,56.2°,59.1°,69.3°,76.5°,78.9°,and 88.3°were observed,which are matched well with the CeO₂ phase (JCPDS No.34-0394).No other diffraction peaks of impurity were found.The XRD results demonstrated that the pure CeO₂ phase was fabricated using the cerium chloride solution as a precursor by the USP method above 700℃,and the obtained product does not contain any other impure phases because of the incomplete pyrolysis reaction of the raw materials.Morphology,microstructure,element composition,and particle size distribution of the CeO₂ microspheres were studied by using FESEM,TEM,and EDX analysis.As shown in Fig.2,the fabricated CeO2 microsphere has regular microsphere morphology,with diameters of 0.01-1.08μm.The micropores were observed at higher resolution magnification (Fig.2b) and were associated with the escape of pyrolysis gas from the inner layer of the solute microsphere at high temperatures.The hollow and solid structure of the CeO₂ microsphere was observed in dark-field images(Fig.2c-f).The bright dots demonstrate that the prepared CeO₂ microspheres are porous structure (Fig.2e).The CeO₂ microsphere is a secondary particle composed of numerous primary CeO₂ particles with a diameter of～30 nm (Fig.2f).

The diffraction rings of (111),(220),and (222) of CeO₂microspheres were observed in the selected area electron diffraction pattern (SAED,Fig.2g).The rings are composed of numerous bright spots,which verified the polycrystalline characteristic of the prepared CeO₂ powders.The elemental composition of the sample was examined by EDX equipment in the FESEM.Ideally,the prepared CeO₂microsphere was ultrasonically dispersed in ethanol and then dropped onto a silicon wafer.Here,Ce,O,and Cl were detected in the prepared CeO₂ microspheres,with Si was come from the silicon wafer (Fig.2h).The particle size distribution was analyzed based on the number of particles more than 3000,which were counted in the FESEM images.The particle size of the prepared CeO₂microspheres ranged between 0.01 and 1.08μm with a mean particle size of 0.23μm and more than 94%of the particles with a diameter less than 0.5μm (Fig.2i).This indicated that the prepared CeO₂ powders were primarily small particles.The regular morphology is attributed to the soft template of microdroplets that are produced by the nebulizer.

The formation schematic is shown in Fig.2j.First,the microdroplets were introduced into the tube furnace by the carrier gas.At high temperatures,the over-saturation occurred near the surface of the microdroplets,associated with surface precipitation,and rapid evaporation of the solvent.Subsequently,a thin and hard shell formed because of solute precipitation on the microdroplet surfaces,and the hollow structure CeO₂ microsphere was formed.The precipitation happened as soon as super-saturation was reached in all microdroplets.The surface and volume precipitation could be ascribed to the evaporation of the solvent in different droplets.

3.2 Effect of synthetic parameters on residual Cl

The obtained fresh CeO₂,powders were pressed into pellets and the residual Cl concentration was determined by EPMA (Fig.3).To clarify the relationship between residual Cl and precursor temperature,the CeO₂ microspheres were prepared under temperatures of 20,35,50,65,and80℃.As exhibited in Fig.3a,a slight decrease in residual Cl is observed with increasing temperature,the lowest value of Cl content is 0.77 wt%at 65℃.As shown in Fig.3b,the increase of carrier gas velocity from 5 to13 L·min^-1 leads to a decrease of residual Cl in CeO₂microspheres.But when the velocity increases from 13 to20 L·min^-1,residual Cl increases to～2.6 wt%.The gas velocity is playing an important role in the efficiency of gas-solid separation.The pyrolysis product and carrier gas left the USP reactor in a short time and completed the separation process quickly when using a faster carrier gas flow.There was a long contact time in the reactor with a slower carrier gas flow rate applied in the USP process.The carrier gas velocity determines the contact time before the separation of the product from the tail gas.In the range of5-13 L·min^-1,the decreased residual Cl in the CeO₂microspheres is ascribed to the decreased contact time with an increase of carrier gas velocity,so the adsorption of residual Cl decreases.When the carrier gas velocity is faster than 13 L-min^-1,the contact time is further shortened.However,it also shortens the time of the CeO₂microspheres in the high-temperature zone,which will helpless to remove the residual Cl adsorbed on the surface of Ce0₂ microspheres.It needs to be noted that the contact time includes the residence time in the tube furnace and the separation unit during the USP process,and the stay time is the product residence time in the high-temperature zone of tube furnace.Therefore,a desirable gas velocity is13 L·min^-1,which will lead to the lowest residual Cl content (of 0.98 wt%) in Ce0₂ microspheres.

Fig.2 a XRD patterns of CeO₂ sintered at 700-1000℃,b FESEM image,c-g TEM images,h EDX results,and i particle size distribution of CeO₂ microspheres;j schematic illustration of formation of CeO₂ microsphere with hollow and solid structures prepared by USP using a cerium chloride solution

Fig.3 Effect of a precursor temperature,b gas velocity,c precursor concentration and d pyrolysis temperature on residual Cl concentration in CeO₂ microspheres prepared by USP method using cerium chloride solution;e changes of reaction enthalpy AH and Gibbs free energyΔG of Eq.(3),and f changes of Gibbs free energy according to Eqs.(4,5)

The effect of precursor concentration on residual Cl in CeO₂ microspheres is shown in Fig.3c.When the precursor concentration increases from 0.001 to 0.2 mol·L^-1,the residual Cl increases from 0.7 to 3.9 wt%.However,when the precursor concentration further increases to1.0 mol·L^-1,the residual Cl decreases from 3.9 to1.7 wt%.The least amount of residual Cl is observed at high solute concentration.This phenomenon is attributed to the elevated reaction temperature and long residue time in the high-temperature zone during the USP process.

To better understand the relationship of precursor concentration and the content of the residual Cl,the changes of Gibbs free energy (AG) and enthalpy (ΔH) of Reaction (3)were calculated at temperatures 600-1200℃under standard ideal conditions,and the results are shown in Fig.3e.TheΔG from Reaction (3) is less than-50 kJ·mol^-1above 600℃,which demonstrates that the transformation from CeCl₃ to CeO2 is irreversible.TheΔH values are relatively low and decrease from-33 to-194 kJ·mol^-1when the reaction temperature increases from 600 to1200℃.This result demonstrates that the pyrolysis of cerium chloride is an exothermic reaction.The high concentration precursor causes the temperature increase in the CeCl₃ microspheres due to the exothermic effect of the pyrolysis reaction.Thus,only small amount of residual Cl is observed.The gas diffusion rate is described by the diffusion law:k=k₀exp(-E_a/R^T) [ 23] .It presents the relationship of diffusion rate coefficient (k) and temperature (T),where k₀ is pre-exponential constant,E_a is the activation energy,and R is gas constant.The pyrolysis of the cerium chloride solution has an intense exothermic effect with a higher concentration of CeCl₃ in the precursor,which causes the temperature to increase greatly in the solid solute (CeCl₃) microsphere during the pyrolysis.Thus,the diffusion rate of pyrolysis gas from the inner to the outer area of the solid solute microsphere increases at high temperatures.

Figure 3d shows the change of residual Cl in CeO₂microspheres with increasing pyrolysis temperature,the residual Cl is 2.7 wt%at 700℃,and it declines to0.2 wt%at 1000℃.This result indicated that the content of residual Cl in CeO₂ microspheres is reduced at the elevated temperature.Figure 3e shows the pyrolysis reaction of CeCl₃ has large AG and the reaction moved towards the right side of Eq.(3).The thermodynamic calculation of chemical reactions was carried out under standard conditions for qualitative analysis.After the USP process,the CeCl₃ solution was successfully transformed into a pure CeO2 phase (Fig.2a).The metal oxides are soluble in acids and the corrosion phenomenon took place in the acid gas atmosphere.Reactions (4,5) present the possible corrosion of the Ce0₂ microspheres during the USP process.Figure 3f shows the AG from Reactions (4,5).Both AG(4)and AG(5) are larger than 50 kJ·mol^-1 above 600℃.This indicated that the corrosion reactions[Reactions (4,5)]would move towards the left side at the temperature range of 600-1200℃.These results illustrated that the residual Cl in the CeO₂ microspheres does not come from corrosion reaction.

3.3 Distribution and formation mechanism of residual Cl in prepared CeO2 microspheres

Figure 4 shows the elemental composition and distribution determined by EDX analysis.The elements Ce,O,and Cl were identified in CeO₂ microspheres,and a small amount of the residual Cl (～1.33 wt%) was found due to the desorption during ultrasonic dispersion.The samples used for FESEM analysis were prepared by dispersion in ethanol,dropping onto the silicon wafer,and then baking10-15 min.The high oxygen content as shown in EDX results (Fig.4a) can be ascribed to the adsorption of ethanol.In Fig.4b,c,the linear scanning demonstrates that the content of Cl is low and Cl is uniformly distributed along the diameter direction.The distribution of Ce,O,and Cl were analyzed by element mapping operations (Fig.4df) further,and it can be observed that the residual Cl is uniformly distributed in CeO₂ microspheres.

As indicated in Fig.2,the prepared CeO₂ microspheres are identified as a hollow structure with many nanometer pores on the sphere surface.The presence of residual Cl can be ascribed to the physical adsorption of Cl on the porous and hollow structure in CeO₂ microspheres.Our experiment results demonstrate that the preparation temperature has a significant influence on the pore structure and specific surface areas (Fig.5).The nitrogen adsorption/desorption isotherms and the pore size distribution of the CeO₂ microspheres are presented in Fig.5b.Each isotherm adsorption curve has a distinct hysteresis loop,which is a basic characteristic of porous structure materials [ 24, 25, 26] .All isotherm adsorption curves exhibit a typeⅣprofile according to IUPAC2015 [ 27] .The occurrence of the hysteresis loop is generally related to the capillary condensation in the mesoporous structure [ 28] .

Moreover,the hysteresis loop type H4 has a relative pressure (p/p₀) of 0.4-1.0 standard atmospheric pressure(Fig.5a),and the shape of the hysteresis loop has an obvious variation with pyrolysis temperature increasing.This phenomenon implies significant changes in the pore structure.As the pyrolysis temperature increased from 700to 1000℃,the hysteresis loop transformed from H4 to H1,and the steeper slope of the isothermal adsorption curves indicated the presence of more uniform pore structure [ 25] .The pore size distribution of the obtained CeO₂ microsphere samples is shown in Fig.5b.The pore size of the sample ranges between 18 and 50 nm when the product was fabricated at 700℃.It was 20-60 nm and 20-90 nm while the sample fabricated at 800 and 900℃,respectively.For the sample fabricated at 1000℃,a broad pore size distribution ranging between 21 and 140 nm was observed.Macropores larger than 1μm were observed in all the four samples,which can be attributed to the cavities and hollow structures formed during the USP process [ 29] .The changes of the CeO₂ microspheres pore structure resulted in the decrease of Brunauer-Emmett-Teller (BET)specific surface area from 86.5 to 42.1 m²·g-1 (Fig.5c) as the pyrolysis temperature increased from 700 to 1000℃.This indicated that the pyrolysis temperature has an inverse relationship to the specific surface area of fabricated CeO₂microspheres.Therefore,physical adsorption and pyrolysis temperature are important factors responsible for the formation of residual Cl.

The EPMA analysis was employed to determine the concentration of residual Cl in the fabricated CeO₂microspheres.In Fig.6a,the concentration of residual Cl declined to 2.08 wt%,1.98 wt%,1.44 wt%and 0.98 wt%after calcining the CeO₂ microspheres at 200,300,400 and500℃,respectively.The residual Cl decreased quickly when the roast temperature increased to above 300℃.Moreover,the residual Cl was reduced to 1.2 wt%after the one-time water-washing treatment (Fig.6a).These results indicated that calcination and washing methods eliminated the residual Cl in the CeO₂ microspheres prepared by USP method.

XPS measurements were performed to determine the chemical states of residual Cl in the fresh CeO₂microspheres,and in the product after roasting and waterwashing (Fig.6b).The observed Cl 2p XPS spectra can be pided into two peaks located at 199.8-200.4 and198.6-199.2 eV,which are ascribed to binding energies for Cl 2p1/2 and Cl 2p3/2,respectively.Compared with the fresh sample,the Cl 2p peaks shifted to the higher energy state after the roasting treatment.This was attributed to the migration of the partial surface adsorbed Cl^-/Cl (e.g.,HCl or Cl₂) into crystal structure [ 30] ,having the typical lattice defects,such as oxygen vacancy in the CeO₂ crystal structure.Some insignificant peaks shifted to lower energies after washing operations.XPS results indicated that the water-washing method can prevent the formation of crystalline Cl,and therefore,it is more efficient to remove residual Cl in the fabricated CeO₂ microspheres.

Fig.4 a EDX spectrum,b,c linear scanning and d-f EDX element mapping of prepared CeO₂ microspheres

Fig.5 a Nitrogen isothermal adsorption-desorption curves;b pore size distribution of CeO₂ microspheres prepared by USP method at 700,800,900 and 1000℃;c relationship of specific surface area and pyrolysis temperature

4 Conclusion

Ultrafine CeO₂ microspheres were prepared by a facile one-step USP method using a cerium chloride solution.The fabricated CeO₂ microspheres demonstrated a particle size0.01-1.08μm with a mean particle size of 0.23μm,and more than 94%of the particles had a diameter of less than0.5μm.But the presence of residual Cl in the fabricated CeO₂ microspheres blocked the active sites and lead to the significantly degradation of SOFC performance.The experimental conditions of the USP process (including the precursor concentration,precursor temperature,carrier gas velocity,and pyrolysis temperature),formation mechanisms and distribution of residual Cl in the fabricated CeO₂microspheres were systemically investigated.The dechlorination experiments indicated that water-washing is a feasible method to remove residual Cl in CeO₂microspheres.

Fig.6 a Effects of roasting temperature,and water-washing times on residual Cl in CeO₂ microspheres;b high-resolution XPS spectra of Cl 2p for residual Cl before and after being calcined at 500℃and water-washing 4 times

参考文献

[1] Leng J,Wang ZX,Wang JX,Wu HH,Yan GC,Li XH,Guo HJ,Liu Y,Zhang QB,Guo ZP.Advances in nanostructures fabricated via spray pyrolysis and their applications in energy storage and conversion.Chem Soc Rev.2019;48(11):3015.

[2] Zhao YF,Guo JC.Development of flexible Li-ion batteries for flexible electronics.InfoMat.2020;2(5):866.

[3] Krishnaswamy K,Pitchaipillai M,Karunamoorthy S,Karikalan S,Ayyasami K.Photocatalytic degradation of environmental perilous gentian violet dye using leucaena-mediated zinc oxide nanoparticle and its anticancer activity.Rare Met.2019;38(4):277.

[4] Hegazy AR,Salameh B,Alsmadi A.Optical transitions and photoluminescence of fluorine-doped zinc tin oxide thin films prepared by ultrasonic spray pyrolysis.Ceram Int.2019;45(15):19473.

[5] Park SY,Kim Y,Kim T,Eom TH,Kim SY,Jang HW.Chemoresistive materials for electronic nose:progress,perspectives,and challenges.InfoMat.2019;1(3):289.

[6] Patil PS.Versatility of chemical spray pyrolysis technique.Mater Chem Phys.1999;59(3):185.

[7] Napari M,Huq TN,Hoye RLZ,Driscoll JLM.Nickel oxide thin films grown by chemical deposition techniques:potential and challenges in next generation rigid and flexible device applications.InfoMat.2020;2(1):1.

[8] Li X,Qi SH,Zhang WC,Feng YZ,Ma JM.Recent progress on FeS_2 as anodes for metal-ion batteries.Rare Met.2020.https://doi.org/10.1007/s12598-020-01492-4.

[9] Jang H,Taniguchi I.Influence of impregnated carbon on preparation and electrochemical properties of Li_2FeP_2O_7 composite synthesized by spray pyrolysis.J Alloys Compd.2017;709(30):557.

[10] Mao S,Bao R,Fang D,Yi JH.Facile synthesis of Ag/AgX(X=Cl,Br)with enhanced visible-light-induced photocatalytic activity by ultrasonic spray pyrolysis method.Adv Powder Technol.2018;29(11):2670.

[11] Jung DS,Hwang TH,Lee JH,Koo HY,Shakoor RA,Kahraman R,Jo YN,Park MS,Choi JW.Hierarchical porous carbon by ultrasonic spray pyrolysis yields stable cycling in lithium sulfur battery.Nano Lett.2014;14(8):4418.

[12] Okuyama K,Lenggoro IW.Preparation of nanoparticles via spray route.Chem Eng Sci.2003;58(3):537.

[13] Deepa KG,Jampana N.Development of an automated ultrasonic spray pyrolysis system and the growth of Cu_2ZnSnS_4 thin films.J Anal Appl Pyrol.2016;117:141.

[14] Lv L,Wang YL,Cheng PF,Zhang B,Dang F,Xu LP.Ultrasonic spray pyrolysis synthesis of three dimensional ZnFe_2O_4based macroporous spheres for excellent sensitive acetone gas sensor.Sens Actuators Chem.2019:297:126755.

[15] Yin SH,Chen K,Srinivasakannan C,Guo S,Li SW,Peng JH.Enhancing recovery of ammonia from rare earth wastewater by air stripping combination of microwave heating and high gravity technology.Chem Eng J.2018;337:515.

[16] Gao GH,Lai FG,Xu HX,Zhang Q,Guo H,Xiao YF.Enrichment of rare earth from low concentration rare earth sulfate solution by calcium oxide precipitation.Chin J Rare Met.2019;43(4):409.

[17] Huang XW,Long ZQ,Wang LS,Feng ZY.Technology development for rare earth cleaner hydrometallurgy in China.Rare Met 2015;34(4):215.

[18] Kuang ST,Zhang ZF,Li YL,Wei HQ,Liao WP.Extraction and separation of heavy rare earths from chloride medium byα-aminophosphonic acid HEHAPP.J Rare Earths.2018;36(3):304.

[19] Ju JW,Huan DM,Zhang YX,Xia CR,Cui GL.Ionic conductivity of infiltrated Ln(Ln=Gd,Sm,Y)-doped ceria.Rare Met.2018;37(9):734.

[20] Wu WY,Bian X.Rare earth metallurgy technology.Beijing:The Science Publishing Company;2012.216.

[21] Li CQ,Li RX,Cong F,Song XZ,Gu MJ.Solvothermal synthesis of titania/ceria.Rare Met.2011;30(1):544.

[22] Escudero MJ,Serrano JL.Inpidual impact of several impurities on the performance of direct internal reforming biogas solid oxide fuel cell using W-Ni-CeO_2 as anode.Int J Hydrog Energy.2019;44(36):20616.

[23] Pelleg J.Diffusion in ceramics.Solid mechanics and its applications.Basel:Springer;2016.52.

[24] Thommes M,Cychosz KA.Physical adsorption characterization of nanoporous materials:progress and challenges.Adsorption.2014;20(2-3):233.

[25] Shi CW,Liu JJ,Wu WY,Bian X,Chen P,Yang Z,Lu CT.Toward understanding of the effect of nucleation temperature on porous structure of micro-mesoporous composite molecular sieves and related crystallization mechanism.Catalysts.2019;9(9):777.

[26] Li YJ,Xu P,Chen GL,Mou JR,Xue SF,Li K,Zheng FH,Dong QF,Hu JH,Yang CH,Liu ML.Enhancing Li-S redox kinetics by fabrication of a three dimensional Co/CoP@nitrogen-doped carbon electrocatalyst.Chem Eng J.2020;380:122595.

[27] Thommes M,Kaneko K,Neimark AV,Olivier JP,Rodriguez-Reinoso F,Rouquerol J,Sing KS.Physisorption of gases,with special reference to the evaluation of surface area and pore size distribution(IUPAC technical report).Pure Appl Chem.2015;87(9-10):1051.

[28] Chiang YC,Chiang PC,Huang CP.Effects of pore structure and temperature on VOC adsorption on activated carbon.Carbon.2001;39(4):523.

[29] Messing GL,Zhang SC,Jayanthi GV.Ceramic powder synthesis by spray pyrolysis.J Am Ceram Soc.1993;76(11):2707.

[30] Blake PG,Carley AF,Di CV,Roberts MW.Chemisorptive replacement of surface oxygen by hydrogen halides(HCl and HBr)at Pb(110)surfaces.Photoelectron spectroscopic and kinetic evidence for a metastable chloride overlayer.J Chem Soc Faraday Trans Phys Chem Condens Phases.1986;82(3):723.