简介概要

Synthesis and characterization of Nd³⁺-doped Ce_0.6Zr_0.4O₂ and its doping significance on oxygen storage capacity

来源期刊：Rare Metals2021年第1期

论文作者：Chandramohan Somayaji S.Kanagaraj

摘要：Cerium and cerium-based oxides are found to be an important element in three-way catalytic converter（TWC）.The effective utilization of TWC is found to be reduced due to thermal loading which results in structural deformation of ceria,Doping Zr4+into the rare earth element can increase the oxygen storage capacity and thermal stability.Hence,an attempt was made to study the oxygen storage capacity and thermal stability of ceria by doping Zr⁴⁺ and Nd³⁺.Cerium-based nanocrystallite in the composition of Ce_0.6Zr_0.4-xNd_{1.3 x}O₂（0≤x≤0.4） was prepared by sol-gel synthesize technique with citric acid as a gel-forming agent.X-ray diffraction（XRD） result shows that doping Nd3+into ceria lattice forms homogenous solid solution of cubic fluorite structure up to 25 % of substitute only.Doping higher amount of Nd3+into ceria lattice leads to the formation of Nd₂ O₃.Raman spectrum study confirms that oxygen storage capacity band is present in Ce_0.6Zr_0.4O₂ and Ce_0.6Zr_0.3Nd_0.13O₂.The oxygen storage capacity was calculated through weight loss of the sample during the second heating cycle with cyclic heating from30 to 800℃ in thermogravimetric analysis（TGA）.The TGA study reveals that the oxygen storage capacity of Ce_0.6Zr_0.4O₂ decreases after the substitution of Nd³⁺,which is due to the larger ionic radius of Nd³⁺ compared with that of Zr⁴⁺and CeO₂.

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稀有金属(英文版) 2021,40(01),231-236

Synthesis and characterization of Nd³⁺-doped Ce_0.6Zr_0.4O₂ and its doping significance on oxygen storage capacity

Natesan Shanmuga Priya Chandramohan Somayaji S.Kanagaraj

Siddaganga Institute of Technology

Indian institute of Technology

作者简介：Natesan Shanmuga Priya e-mail:shanmuga@iitg.ernet.in;

收稿日期：18 August 2015

Synthesis and characterization of Nd³⁺-doped Ce_0.6Zr_0.4O₂ and its doping significance on oxygen storage capacity

Natesan Shanmuga Priya Chandramohan Somayaji S.Kanagaraj

Siddaganga Institute of Technology

Indian institute of Technology

Abstract：

Cerium and cerium-based oxides are found to be an important element in three-way catalytic converter(TWC).The effective utilization of TWC is found to be reduced due to thermal loading which results in structural deformation of ceria,Doping Zr4+into the rare earth element can increase the oxygen storage capacity and thermal stability.Hence,an attempt was made to study the oxygen storage capacity and thermal stability of ceria by doping Zr⁴⁺ and Nd³⁺.Cerium-based nanocrystallite in the composition of Ce_0.6Zr_0.4-xNd₍1.3 x)O₂(0≤x≤0.4) was prepared by sol-gel synthesize technique with citric acid as a gel-forming agent.X-ray diffraction(XRD) result shows that doping Nd3+into ceria lattice forms homogenous solid solution of cubic fluorite structure up to 25 % of substitute only.Doping higher amount of Nd3+into ceria lattice leads to the formation of Nd₂ O₃.Raman spectrum study confirms that oxygen storage capacity band is present in Ce_0.6Zr_0.4O₂ and Ce_0.6Zr_0.3Nd_0.13O₂.The oxygen storage capacity was calculated through weight loss of the sample during the second heating cycle with cyclic heating from30 to 800℃ in thermogravimetric analysis(TGA).The TGA study reveals that the oxygen storage capacity of Ce_0.6Zr_0.4O₂ decreases after the substitution of Nd³⁺,which is due to the larger ionic radius of Nd³⁺ compared with that of Zr⁴⁺and CeO₂.

Keyword：

Cerium oxide; Sol-gel technique; Thermogravimetric analysis; Ionic radius; Homogeneous solid solution; Oxygen storage capacity;

Received： 18 August 2015

1 Introduction

In recent years,ceria and its oxides have attracted considerable amount of attention as important oxygen storage materials because of its advantages in the field of catalysts.The ability of absorbing/releasing the lattice oxygen reversible with oxygen-rich/oxygen-lean environment is called oxygen storage capacity (OSC).The OSC of ceria makes it not only be used as a catalyst,but also be applied in various fields such as fuel cell [ 1] ,oxygen sensor [ 2] ,ultraviolet (UV) shielding [ 3] ,luminescence [ 4] ,surface lapping,and polishing material [ 5] .The reversible reduction of Ce⁴⁺to Ce³⁺in CeO₂ is denoted as redox property,which was found to be responsible for OSC and it would be high at low redox temperature.The cubic fluorite structure of ceria,one of the desirable properties for high OSC,was found to be retained only up to 900 K under reducing atmosphere [ 6] .The cubic ceria was observed to change its phase to hexagonal at a temperature greater than 1273 K.Aneggi et al. [ 7] observed that the conversion process of Ce⁴⁺to Ce³⁺was reduced with reduction temperature increasing.In addition,ceria has a resource limitation in terms of availability.In order to overcome the above-said problems,some modifications on ceria lattice with the substitution of Zr0₂ were widely accepted by the science community [ 8] .

Kawamoto et al. [ 9] suggested that doping ZrO₂ into the lattice of rare earth elements like ceria led to the increase in oxygen vacancy.Suda et al. [ 10] synthesized CeO_2-ZrO₂by co-precipitation method and stated that the maximum total OSC value occurred with the composition of 50 mol%CeO_2-50 mol%ZrO_2.Zhang et al. [ 11] synthesized CeO_2-ZrO₂ and confirmed that the higher OSC was found with the combination of Ce_0.6Zr_0.4O_2.Our earlier studies confirmed that the OSC of Ce_0.6Zr_0.4O₂ was found to be higher than that of other compositions [ 12] .The hindered thermal stability of ceria due to Zr doping was favored for high oxygen mobility which resulted in high OSC [ 13] .

Jia et al. [ 14] noted that the addition of+3 valence elements into ceria lattice could enhance the oxygen storage property.Since doping ZrO₂ into the lattice of rare earth elements leads to the increase in oxygen vacancy [ 9] ,the OSC of ceria can be further increased by the combined doping of Zr⁴⁺and Nd³⁺.Doping oxygen vacancy trivalent rare earth cations into ceria can stabilize the fluoriterelated structure and introduce lattice defects,which leads to the improvement of OSC [ 15] .Mikulova et al. [ 16] prepared Zr-Ce-Nd-O compounds and found that the neodymium cations in the valence state of 3+reduced the OSC of the mixture.Owing to the controversy result,doping Nd³⁺with ceria need to be further explored.However,according to our knowledge,CeO₂ doped with Zr⁴⁺and Nd³⁺for oxygen studies has been rarely investigated in detail up to now.

Nanostructured ceria and ceria-based oxides have been synthesized by various routes including co-precipitation [ 17] ,sol-gel [ 18] ,microemulsion [ 19] ,ultrasonic-assisted membrane reaction [ 20] ,thermal hydrolysis [ 21] ,spray pyrolysis [ 22] ,spray freezing method [ 23] ,surfactant-assistant method [ 24] ,and combustion emulsion methods [ 25] .Sol-gel method among them was found to be the suitable method to control the shape,morphology,and textural properties of nanostructured end products [ 26] .Early synthesizing route in the publication showed that the sol-gel was suitable method to derive less crystallite size and high OSC nanoparticles compared to co-precipitation method [ 27] .In order to increase the OSC of CeO₂ further,the combined doping of Zr⁴⁺and Nd³⁺was explored.The purpose of this present article was to check the feasibility of increasing OSC by doping Nd³⁺,Zr⁴⁺into ceria lattice,and to study the chemical compositions and its structural properties in the form of Ce_0.6Zr_0.4-xNd_1.3xO₂(0≤x≤0.4)(CZN).

2 Experimental

The following chemicals were used to prepare Ce_0.6Zr_0.4O₂(CZ) and Ce_0.6Zr_0.4-xNd_1.3xO₂ (0.1≤x≤0.4)(CZN):cerium (Ⅲ) nitrate hexahydrate (Ce(NO₃)₃·6H₂O)(Aldrich,99%and Alfa Aesar),zirconyl (Ⅳ) oxynitrate hydrate powder (ZrO(NO₃)₃.H₂O)(Otto),neodymium (Ⅲ)nitrate hexa hydrate (Nd(NO₃)3·6H₂O)(Alfa Aesar),lobachemie (C₆H₈O₇),and ammonium hydroxide(NH₄OH)(Merck,30%GR).These chemicals were used without any further purification.

2.1 Nanoparticles preparation

The precursor gel of Ce_0.6Zr_0.4＿xNd_1.3xO2 was prepared through sol-gel synthesis technique as suggested by Zhang et al. [ 6] .In this typical synthesis,required amounts of CeNO₃-6H₂O,ZrO(NO₃)3·6H₂O and Nd(NO₃)·3H₂O were dissolved in distilled water.Citric acid was added into the precursor solution as a gel-forming agent.Ammonium hydroxide was added in order to increase the pH to 6.The gel-forming reaction was carried out at 70℃under continuous stirring condition.The obtained gel was ignited in muffle furnace at 500℃,followed by agitation to get a fresh nanostructured CZN.Following the ignition,the CZN was calcined at the temperature of 700℃for 8 h in order to obtain the aged samples.CZ samples were obtained in the same procedure without using Nd(NO₃)·3H₂O.The prepared samples were named as Ce_0.6Zr_0.3Nd_0.13O₂(CZN1),Ce_0.6Zr_0.2Nd_0.26O₂ (CZN2),Ce_0.6Zr_0.1Nd_0.39O₂(CZN3),Ce_0.6Nd_0.52O₂ (CZN4),and Ce_0.6Zr_0.4O₂ (CZ).

2.2 Evaluation of oxygen storage capacity of CZ and CZN

The OSC of CZN samples was evaluated by cyclic heating using thermogravimetric analysis (TGA).The samples were initially heated from room temperature to 800℃in N₂ environment and cooled down to 150℃in air environment followed by heating the sample up to 800℃,as suggested by Ozawa et al. [ 28] .The rates of heating and cooling were maintained at 10℃·min^-1,where the initial mass of the sample was kept at 0.5 g.A commercial Netzsch thermogravimetric-differential thermal analyzer (TG-DTA,STA 409,Germany) was employed for the same.

2.3 Characterization techniques

The structural analysis of the sample was studied using a Bruker D8 advance powder X-ray diffractometer (XRD)with Cu Ka (λ=0.15406 nm) radiation.The intensity data were collected in a 2θrange varying from 10°to 75°with the scanning rate of 0.02 (°)·s^-1 and step time of 1 s.The mean crystallite size of the catalyst was calculated using Scherrer's equation,where the particle shape factor was taken as 0.894.

Raman spectra from 100 to 800 cm^-1 were obtained at room temperature for a powder sample using Jovin Yvon,Triax 550 spectrometer.The Raman scattering was excited by a laser source with the wavelength of 514.0 nm.A200-kV transmission electron microscope (TEM,JEOL JEM 2100) was used to confirm the crystallite size and shape of the solid solution.

Specific surface area of CZN samples was performed using surface area analyzer (Autosorb-IQ MP,Quantachrome).Prior to analysis,samples of approximately0.4-0.5 g were heated to 200℃under vacuum(1.33×10^-3 Pa) for at least 24 h to remove adsorbed species.Nitrogen adsorption data were taken at five relative pressures from 0.05 to 0.20 at 77 K,and the surface area was calculated using Brunauer-Emmett-Teller (BET) theory.

3 Results and discussion

3.1 Structural analysis

Figure 1 shows the XRD patterns of freshly prepared Ce_0.6Zr_0.4-xNd_1.3xO2 (CZN)(0≤x≤0.4) solid solutions.The diffractograms of CeO₂,ZrO₂,and Nd₂O₃ were also plotted for comparison purpose where the respective data were obtained from JCPDS Nos.34-0394,49-1642,and83-1356,respectively.The peaks are observed to be around29°,33°,48°,and 57°for the planes of (111),(200),(220),and (311),respectively,in all the CZN samples.A shoulder peak at 60°is also observed in all CZN samples.The formation of solid solution is confirmed with the peak shift in CZ,CZN1,and CZN2,where the respective peaks corresponding to Nd₂O₃ and ZrO₂ are not observed.Though the characteristic peaks for cubic fluorite structure of CeO₂ are found in all CZN samples,the peaks corresponding to Nd₂O₃ are observed in CZN3 and CZN4,confirming the presence of cubic Nd₂O_3.It might be due to the saturation limit of Nd³⁺in ceria lattice.

The XRD patterns of aged Ce_O.6Zr_0.4-xNd_1.3xO₂(0≤x≤0.4) solid solutions are shown in Fig.2 along with aged samples of CeO₂ for comparison.It is observed that the full width at half maximum decreases,which results in a higher crystallite size compared to that of fresh samples.The development of cubic Nd₂O₃ is evident after aging in case of CZN3 and CZN4,just as that of fresh samples.The crystallite size of aged CZN increases compared to that of fresh CZN samples and varies from 7.5 to8.5 nm,which is about 10%-19%enhancement in comparison with fresh samples,and it might be due to the sintering effect of the doping element in CZN.

Fig.1 XRD patterns of fresh Ce0.6Zr_0.4-xNd_1.3xO2 (CZN)(0≤x≤0.4) solid solutions

Fig.2 XRD patterns of aged Ce_0.6Zr₀.4-xNd_1.3xO₂ (CZN)(0≤x≤0.4) solid solutions

Table 1 gives the crystallite size,d-spacing,and lattice constant of fresh and aged samples for Ce_0.6Zr_0.4-xNd_1.3xO₂ (0<x<0.4) solid solutions.The crystallite size and d-spacing of aged CZN were calculated from their major peak (111).Lattice constant of CZN increases from 0.53 to 0.54 nm in fresh CZN samples with the replacement of Zr by Nd ions in ceria lattice.The lattice expansion is due to the larger ionic radius of Nd³⁺(0.1109 nm) compared to that of Ce⁴⁺and Zr⁴⁺according to Wu et al. [ 29] .Kawamoto et al. [ 9] observed that the lattice constant increased with the substitution of larger ionic radii element into CeO₂ lattice.This phenomenon is also observed in aged CZN lattice.

3.2 Raman spectroscopy study

Figure 3 depicts the Raman spectra of fresh Ce_0.6Zr_0.4-xNd_1.3xO₂ (CZN)(0≤x≤0.4) solid solutions.The peaks observed for CZ,CZN 1,CZN2,CZN3,and CZN4are 471.12,469.40,467,30,461.20,and 463.40 cm^-1,respectively.The main peak shifts toward lower wavelength side with the increase in Nd³⁺substitution.The Raman spectra for cubic CZ and CZN are characterized by a broad peak near 530-670 cm^-1 and additional poorly defined structures related to the disordered oxygen sublattice.Though the peak related to tetragonal phase is not observed in all CZN samples,the OSC band from 604.00 to650.00 cm^-1 is present only in CZ and CZN1.The absence of OSC band in CZN2,CZN3,and CZN4 might be due to the formation of Nd₂O₃,which is also confirmed through XRD study.

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Table 1 Crystallite size,d-spacing,and lattice constant of fresh and aged Ce_0.6Zr_0.4-xNd₁.3xO₂ (0≤x≤0.4)(CZN) solid solutions

Increase in crystallite size of aged samples with respect to fresh samples

Fig.3 Raman spectra of fresh Ce_0.6Zr_0.4-xNd_1.3xO₂ (CZN)(0≤x≤0.4) solid solutions

3.3 TEM analysis

Figure 4 shows TEM images of fresh Ce₀.6Zr_0.3Nd_0.13O₂and Ce_0.6Zr_0.4O₂ samples.TEM images are presented only for the above samples because of the presence of OSC band.The shape and size of the test samples are uniform in both cases.It is noted that the particles are densely packed in CZNl,but in case of CZ it is packed loosely.The shape of the particles is spherical in nature.The crystals are less than 10 nm in size and appear to be agglomerated.In both cases,the results obtained from TEM image in terms of the crystallite size are in good agreement with XRD results.

3.4 TGA oxygen storage capacity

The solid solution of Ce_0.6Zr_0.4-xNd_1.3xO₂ (0≤x≤0.4)was heated from 30 to 800℃and cooled down to 150℃,and the second heating cycle was followed up to 800℃.The weight loss during the second heating cycle was used to plot the curve,which is shown in Fig.5,and these data were used to calculate the OSC of Ce_0.6Zr_0.4-xNd_1.3xO2(0≤x≤0.4) solid solution,as described by Ozawa et al. [ 28] .It is observed that the change in weight loss is higher in CZ than in CZN1-CZN4 composition.CZN1 has more weight loss than CZN1-CZN4 samples.

Fig.4 TEM images of fresh Ce₀.6Zr_0.3Nd_0.13O2 a and Ce_0.6Zr_0.4O₂ b

The weight loss data during second heating cycle were used to calculate the OSC of the ceria-based solid solution using the following equations [ 28] .

Table 2 shows the comparison of OSC and specific surface area of CZN1,CZN2,CZN3,and CZN4 solid solutions with respect to Ce_0.6Zr_0.4O₂ (CZ).The OSC of Ce_0.6Zr_0.4-xNd_1.3xO₂ solid solutions is less than that of Ce_0.6Zr_0.4O2.In CZN1-CZN4 composition,the reduced OSC might be due to the formation of Nd₂O₃ after substitution of 25%Nd³⁺,which is confirmed by XRD technique.The appearance of Nd₂O₃ peak in CZN3 and CZN4 prevents the formation of solid solution,which is favored for higher OSC.Though the lattice expansion is more significant in CZN1-CZN4 than in CZ,which is due to the larger ionic radius of Nd³⁺(0.1109 nm) [ 29] ,the Zr⁴⁺ionic radius (0.084 nm) is found to be lower than that of Ce⁴⁺(0.097 nm) [ 30] .The OSC of CZN1 solid solutions is 15%less than that of CZ.The lattice expansion is caused by doping second and third elements in the ceria lattice,where the third element is Nd³⁺and the second element is Zr⁴⁺.Since the OSC of CZN1 is high in CZN1-CZN4 samples,the specific surface area was analyzed only for CZ and CZN1.The specific surface area of pure CeO₂is also considered here for comparison,and it increases with the substitution of Zr⁴⁺in ceria lattice.However,the same for CZN1 solid solution is 38%less compared to that of Ce_0.6Zr_0.4O₂ samples.The OSC of CZN1 reduces compared to that of Ce_0.6Zr_0.4O₂,and it is correlated with reduced specific surface area of the solid solution.It is also observed that the decrease in specific surface area of CZN 1also leads to the decrease in OSC compared to Ce_0.6Zr_0.4O_2.

Fig.5 Weight loss of Ce0.6Zr_0.4-xNd_1.3xO₂ (0≤x≤0.4) solid solutions during second heating cycle (m mass after heating,mo mass at room temperature)

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Table 2 OSC and specific surface area for Ce⁰_.6Zr_0.4-xNd_1.3xO₂(0≤x≤0.4) solid solutions

4 Conclusion

In this study,the role of Nd³⁺,Zr⁴⁺,and combination of these two ions in ceria lattice for the OSC properties and structural characterization was investigated.The ceriabased oxides,doped with Zr⁴⁺and Nd³⁺,were synthesized through sol-gel technique,in the form of Ce_0.6Zr_0.4-xNd_0.13xO₂ (0≤x≤0.4).The formation of homogeneous solid solution occurs only in CZ and CZN1through XRD study.The average crystallite size is in the range of 6.5-7.7 nm for all CZN samples.OSC band appears only in Ce_0.6Zr_0.4O₂ and CZN 1 through Raman lattice vibration study.The OSC of the sol-gel prepared CZN1-CZN4 samples is less than that of Ce₀.6Zr_0.4O₂,which is due to the higher ionic radius and its lower specific surface area of Nd³⁺compared to that of Zr⁴⁺.