Synthesis of CeO2 by thermal decomposition of oxalateand kinetics of thermal decomposition of precursor

Yongni Li • Xuehang Wu • Wenwei Wu •

Kaituo Wang • Liqin Qin • Sen Liao •

Yanxuan Wen

Received: 14 August 2013 / Accepted: 5 January 2014 / Published online: 18 March 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract CeO2 was synthesized by calcining

Ce2(C2O4)3�8H2O above 673 K in air. The precursor and

its calcined products were characterized using thermo-

gravimetry and differential scanning calorimetry, Fourier

transform infrared spectra, X-ray powder diffraction,

scanning electron microscopy, and UV–Vis absorption

spectroscopy. The result showed that cubic CeO2 was

obtained when the precursor was calcined above 673 K in

air for 2 h. The UV–Vis absorption spectroscopy studies

showed that superfine CeO2 behaved as an excellent UV-

shielding material. The thermal decomposition of the pre-

cursor in air experienced two steps, which are: first, the

dehydration of eight crystal water molecules, then the

decomposition of Ce2(C2O4)3 into cubic CeO2. The values

of the activation energies associated with the thermal

decomposition of Ce2(C2O4)3�8H2O were determined

based on the Starink equation.

Keywords CeO2 � Chemical synthesis � UV absorbency �Nonisothermal kinetics � Thermal decomposition

Introduction

CeO2 has many unique properties, such as high-mechanical

strength, oxygen ion conductivity, oxygen storage capacity,

strong UV absorption, and luminescent properties. There-

fore, CeO2 and/or doped CeO2 has widely been used in

catalysis [1], fuel cell [2], gas sensors [3], solid state

electrolytes [4], glass polishing material [5], cosmetics [6],

and UV-shielding materials [7, 8]. The quality of CeO2

powders highly depends on the synthesis method and

conditions, which determine particle size, morphology of

CeO2 associated with performances [9–21]. Preparations of

high-quality samples with superfine particle size and/or

dopant have generally been considered to improve the

performances of CeO2.

To date, various methods of synthesizing CeO2 have

been developed, including the precipitation method [9, 10],

the microemulsion method [11–13], the hydrothermal

method [14–16], the microwave assisted method [17],

carbon nanotube assisted synthesis [18], solvothermal

synthesis [19], the sol–gel method [20], spray pyrolysis

using polymeric precursor solution [21], azeotropic distil-

lation processing [22], and thermal decomposition of

oxalate [23–26]. In the synthesis of CeO2, the crystallite

diameter, morphology, and crystalline phases of CeO2

associated with its performances highly depend on the

synthesis method and conditions. Chen et al. [14] synthe-

sized cubic CeO2 nano-rods using the hydrothermal

method using Ce(NO3)3�6H2O, Na3PO4, and H2O2 as the

starting materials. Mercadelli et al. [17] obtained cubic

CeO2 with spheric, flower-like, and needle-like shape

morphologies by means of the microwave assisted method

using diethylene glycol as the template. Flower-like and/or

needle-like shape CeO2 was obtained when reaction tem-

perature was above 170 �C. The flower-like and/or needle-

like shape CeO2 behaved with higher catalytic activity at

relatively low temperature, compared to spherical shape

CeO2. Almeida et al. [23] obtained rod-like CeO2 via

thermal decomposition of hydrated cerium oxalates with

rod-like morphology. Most researchers attempt to obtain

cubic CeO2 with high performance at the lowest possible

cost. However, many CeO2 synthesis methods are complex

Y. Li � X. Wu � W. Wu (&) � K. Wang � L. Qin � S. Liao �Y. Wen

School of Chemistry and Chemical Engineering, Guangxi

University, Nanning 530004, People’s Republic of China

e-mail: gxuwuwenwei@aliyun.com; gxuwuwenwei@126.com

123

J Therm Anal Calorim (2014) 117:499–506

DOI 10.1007/s10973-014-3651-4

processes with high cost, so preparing the product on a

large scale is difficult. Therefore, new synthesis methods

for cubic CeO2 are needed to be studied and innovated.

This study aimed to prepare nano-sized ceria powder

with spherical shape by calcining a precursor,

Ce2(C2O4)3�8H2O, and study the kinetics of thermal

decomposition of precursor. The latter objective was con-

ducted using thermogravimetry and differential scanning

calorimetry (TG/DSC). Nonisothermal kinetics of thermal

decomposition of precursor was interpreted by means of

the Starink equation [27–31]. The kinetic parameters (Ea,

A) and mechanisms of thermal decomposition of

Ce2(C2O4)3�8H2O were discussed.

Experimental

Preparation of CeO2

The CeO2 precursor was prepared by solid-state reaction at

low-heating temperatures [8, 27] using Ce2(SO4)3�8H2O

and Na2C2O4 as starting materials. All chemicals were of

reagent-grade purity ([99.9 %). In a typical synthesis,

Ce2(SO4)3�8H2O (62.10 g), Na2C2O4 (36.78 g), and sur-

factant polyethylene glycol-400 (4.0 mL, 50 vol%) were

placed in a mortar, and the mixture was thoroughly ground

by hand with a rubbing mallet for 40 min. The grinding

velocity was about 210 circles min-1, and the strength

applied was moderate. The reactant mixture gradually

became damp, and then a paste formed quickly. The

reaction mixture was kept at 303 K for 2 h. The mixture

was washed with deionized water to remove soluble inor-

ganic salts until SO42- ion could not be visually detected

with a 0.5 mol L-1 BaCl2 solution. The solid was then

washed with a small amount of anhydrous ethanol and

dried at 353 K for 5 h. The resulting material was subse-

quently determined to be Ce2(C2O4)3�8H2O. Nano-sized

CeO2 with cubic structure was obtained via calcining

Ce2(C2O4)3�8H2O above 673 K in air for 2 h.

Physical characterization

Thermogravimetry and differential scanning calorimetry

(TG/DSC) measurements were conducted using a Netzsch

Sta 409 PC/PG thermogravimetric analyzer under a con-

tinuous flow of air (40 mL min-1). The sample mass was

approximately 13 mg. X-ray powder diffraction (XRD) was

performed using a Rigaku D/Max 2500 V diffractometer

equipped with a graphite monochromator and a Cu target.

The radiation applied was Cu Ka (k = 0.15406 nm), oper-

ated at 40 kV and 50 mA. XRD scans were made from 5� to

70� in 2h with a step size of 0.02�. Fourier transform infrared

(FT-IR) spectra of the precursor and its calcined products

were recorded on a Nexus 470 FT-IR instrument. The

morphologies of the calcined products were observed on a

S-3400 scanning electron microscope (SEM). The ultravi-

olet visible (UV–Vis) spectra of the calcined products were

recorded on a TU-1901 double beam UV–Vis

spectrophotometer.

Method of determining kinetic parameters

and mechanism functions

Determination of activation energy by the Starink

equation

Activation energy of thermal decomposition of the solid

compound can be obtained using the Starink equation

(Eq. 1)

lnbi

T1:92a;i

!¼ Const� 1:0008

Ea

RTa

� �ð1Þ

where bi is the heating rate (K min-1), Ta is the reaction

temperature (K) corresponding to degree of conversion (a)

in TG curve, Ea is the activation energy (kJ mol-1) of

thermal decomposition corresponding to degree of con-

version (a), and R is the gas constant

(8.314 9 10-3 kJ mol-1 K-1).

The conversion degree (a) can be expressed as Eq. (2):

a ¼ mi � mt

mi � mf

ð2Þ

where mi, mf, and mt are the initial, final, and current

sample mass at the moment t, respectively. The depen-

dence of ln (bi/Ta,i1.92) on 1/Ta must give rise to a straight

line. Thus, reaction activation energy Ea can be obtained

from linear slope (-1.0008 Ea/R, Eq. (1).

Determination of the most probable mechanism

functions

The following equation was used to estimate the most

probable reaction mechanism of thermal decomposition of

Ce2(C2O4)3�8H2O, i.e., g(a) function [30–34]:

ln gðaÞ ¼ lnAEa

Rþ ln

e�x

x2þ ln hðxÞ

� �� ln b ð3Þ

where x = Ea/(RT), hðxÞ ¼ x4þ18x3þ86x2þ96xx4þ20x3þ120x2þ240xþ120

, and b is

the heating rate (K min-1). The conversions a corre-

sponding to multiple rates at the same temperature are

placed into the left of Eq. (3), combined with 31 types of

mechanism functions [33, 35], the slope k and correlation

coefficient r2 are obtained from the plot of ln g(a) vs. ln b.

The probable mechanism function is that for which the

500 Y. Li et al.

123

value of the slope k is near -1.00000, and correlation

coefficient r2 is better.

Calculation of pre-exponential factor A

The pre-exponential factor was estimated from Eq. (4)

[29]:

A ¼ bgðaÞEa

RT2max

expEa

RTmax

� �ð4Þ

where A is the pre-exponential factor (s-1), b is the heating

rate (K min-1), g(a) is the most probable mechanism

function determined using Eq. (3), Ea is the activation

energy (kJ mol-1) of thermal decomposition, R is the gas

constant (8.314 9 10-3 kJ mol-1 K-1), and Tmax is the

most rapid decomposition temperature [i.e., peak temper-

ature in differential thermogravimetry (DTG) curve, K].

Results and discussion

Composition analysis of the precursor

0.0300 g the precursor was dissolved in 15.0 mL 50 vol%

HCl solution, and then diluted to 100.00 mL with distilled

water. Cerium (Ce) in the solution was determined by

inductively coupled plasma atomic emission spectrometry

(Perkin Elmer Optima 5300 DV). The result showed that

the mass percentage of Ce in the precursor was 40.73 %,

which was in good agreement with 40.71 % mass per-

centage of Ce in the Ce2(C2O4)3�8H2O.

TG/DTG/DSC analysis of the precursor

Figure 1 shows the TG/DTG/DSC curves of the precursor.

The TG/DTG/DSC curves show that thermal decomposi-

tion of Ce2(C2O4)3�8H2O below 1,000 K occurs in two

well-defined steps. For heating rate of 10 K min-1, the first

step starts at 340 K, ends at 451 K, and is characterized by

a DTG peak at about 423 K and an endothermic DSC peak

at 435 K, which can be attributed to dehydration of eight

crystal water molecules from Ce2(C2O4)3�8H2O (mass loss:

observed, 21.95 %; theoretical, 20.94 %). The second

decomposition step starts at 451 K, ends at 673 K, and is

characterized by a strong DTG peak at 591 K and a

stronger DSC exothermic peak at 595 K, attributed to

reaction of Ce2(C2O4)3 with two O2 molecules into CeO2,

and of six CO2 molecules (mass loss: observed, 28.01 %;

theoretical, 29.06 %). From Fig. 1a, no other exothermic

DSC peak that was ascribed to crystallization of CeO2

between 573 and 1,000 K except exothermic DSC peak at

595 K, which can be attributed that exothermic DSC peak

of crystallization of CeO2 overlapped with exothermic

DSC peak from reaction of Ce2(C2O4)3 with two O2 mol-

ecules into CeO2. The observed mass loss differs from the

theoretical mass loss, attributed to the error of the appa-

ratus measurements.

IR spectroscopic analysis of the precursor and its

calcined products

The FT-IR spectra of Ce2(C2O4)3�8H2O and its calcined

products are shown in Fig. 2. The Ce2(C2O4)3�8H2O

exhibited a strong and broad band at about 3,353 cm-1 that

can be assigned to symmetric and asymmetric stretching

modes of water molecules. The bending mode of water

expected around 1,618 cm-1 overlapped with the intense

oxalate band which was around 1,616 cm-1 [36–42]. The

bands at 1,313, 1,126, 801, 659, 594, and 497 cm-1 can be

assigned to either the appearance of new Ce–OC2O3 bonds

and/or to the combinations of OH group vibration and

lattice modes [30, 31]. FT-IR spectra of the precursor and

product obtained at 473 K are similar, implying that the

structure of the oxalates is still stable up to 473 K. How-

ever, FT-IR spectra of product obtained above 573 K has a

great difference in comparison with those of the precursor

and the product obtained at 473 K. Such as, the bands at

about 1,313, 1,126, 801, 659, 594, and 497 cm-1 became

weak and/or disappeared. When Ce2(C2O4)3�8H2O was

calcined above 673 K, the weak-absorption band at about

3,353 cm-1 was attributed to absorption water from air.

The spectrum of the calcined products above 573 K agrees

with that of cubic CeO2 from literature [25].

300 400 500 600 700 800 900 1000

50

60

70

80

90

100 10 K min–1

20 K min–1

15 K min–1

10 K min–1

595 K

Temperature/K

TG

/%

435 K

5 K min–1

a

–6.0

–4.5

–3.0

–1.5

0.0

DS

C/m

W m

g–1

300 400 500 600 700 800 900 1000–1.6

–1.4

–1.2

–1.0

–0.8

–0.6

–0.4

–0.2

0.0

0.2

DT

G/m

g m

in–1

Temperature/K

b

a

b

cd

a 5 K min–1

b 10 K min–1

c 15 K min–1

d 20 K min–1

Fig. 1 TG/DTG/DSC curves of

Ce2(C2O4)3�8H2O at different

heating rates in air

Synthesis of CeO2 by thermal decomposition of oxalate 501

123

XRD analysis of the precursor and its calcined products

Figure 3 shows the XRD patterns of the precursor and its

calcined products. From Fig. 3, the results show that

Ce2(C2O4)3�8H2O is a crystalline compound. When

Ce2(C2O4)3�8H2O was calcined at 473 K for 2 h, the char-

acteristic diffraction peaks of crystalline compound disap-

peared, implying that the calcined product at 473 K was an

amorphous compound. When the precursor was calcined

between 573 and 673 K, part weak characteristic diffraction

peaks of cubic CeO2 were observed. With the increase of the

calcination temperature, a wide and low-diffraction pattern

of thermal decomposition product was observed. All the

diffraction peaks in the pattern of products obtained at 873

and 1,023 K were in agreement with those of cubic CeO2

with space group Fm-3m(225) from PDF card 65-5923. The

lattice parameters of CeO2 obtained at 1,023 K were

a = b = c = 0.5389 nm, a = b = c = 90�, and density =

7.31 g cm-3, respectively.

The crystallite diameter of CoO2 was estimated using

the following Scherrer formula [36, 37]:

D ¼ Kk=ðbcoshÞ ð5Þ

where D is crystallite diameter, K = 0.89 (the Scherrer

constant), k = 0.15406 nm (the wavelength of the X-ray

used), b is the width of line at the half-maximum intensity,

and h is the corresponding angle. The resulting crystallite

sizes of the products from calcining precursor at 673, 873,

and 1,023 K in air for 2 h were 9.9, 19.5, and 42.7 nm,

respectively. The crystallinity of cubic CeO2 can be cal-

culated via MDI Jade 5.0 software, the results showed that

crystallinities of cubic CeO2 obtained at 673, 873, and

1,023 K were 85.7, 95.5, and 99.8 %, respectively.

SEM analyses of the precursor and its calcined products

The morphologies of Ce2(C2O4)3�8H2O and its calcined

products are shown in Figure 4. From Fig. 4a, it can be

seen that Ce2(C2O4)3�8H2O sample is composed of platelet

grains, which contain particles having a distribution of

small particles (150–450 nm) and large particles (450 nm–

1.3 lm). From Fig. 4b–d, the calcined products obtained at

473, 573, and 673 K can still keep platelet morphology of

Ce2(C2O4)3�8H2O. However, the thickness of these pro-

ducts becomes thinner. With the increase of the calcination

temperature, the calcined products are split into smaller

particles further. Figure 4e, f shows that the SEM images

of products obtained at 873 and 1,023 K, respectively. The

two products have been changed into approximatively

spherical particles, and there is soft agglomeration phe-

nomenon among one particle of product. The observed

average particle sizes of products obtained at 873 and

1,023 K are about 70 and 100 nm, respectively, which

were significantly smaller than the values determined by

SEM. This difference can be attributed to the fact that the

values observed using SEM technique have the size of the

secondary particles, which are composed of several or

many crystallites by means of soft reunion. In addition, the

X-ray line broadening analysis disclosed only the size of

single crystallite.

UV–Vis analyses of the calcined products

The UV–Vis absorption spectra of the calcined products

are presented in Fig. 5. It can be seen that all products

obtained at different calcination temperatures have strong

UV absorption at the wavelength between 250 and 360 nm,

and their absorbency exceeds 1.30. To compare the UV-

absorbing abilities of products obtained at different calci-

nation temperatures quantificationally, the dependence of

UV absorbency peak area between 250 and 400 nm on

calcination temperature is given in Fig. 6. The result shows

that the UV absorbency peak area of the product obtained

at 673 K between 250 and 400 nm has the largest value.

The larger is the UV absorption peak area, the stronger is

the UV absorbency, and thus, the product obtained at

673 K has the strongest ultraviolet absorption ability.

4000 3500 3000 2500 2000 1500 1000 500

1023 K

873 K

673 K

573 K

Tra

nsm

ittan

ce/%

Precursor

Wavenumbers/cm–1

473 K

Fig. 2 FT-IR spectra of Ce2(C2O4)3�8H2O and its calcined products

10 20 30 40 50 60 70 80

Inte

nsity

/a.u

.

573 K

873 K

1023 K

673 K

473 K

CeO2

2θ /°

Precursor

Unknown

Fig. 3 XRD patterns of Ce2(C2O4)3�8H2O and its calcined products

at different temperatures for 2 h

502 Y. Li et al.

123

Kinetics of thermal decomposition of the precursor

In accordance with TG/DTG/DSC, IR, and XRD analyses

of the precursor and its calcined products mentioned above,

thermal decomposition of the precursor below 1000 K

consists of two steps, which can be expressed as follows.

Ce2ðC2O4Þ3 � 8H2OðcrÞ ! Ce2ðC2O4Þ3ðamÞ þ 8H2O(g)

ð6Þ

Ce2ðC2O4Þ3ðamÞ þ 2O2ðgÞ ! 2CeO2ðcÞ þ 6CO2ðgÞ ð7Þ

According to nonisothermal method, the basic data of aand T were collected from the TG curves of the thermal

decomposition of Ce2(C2O4)3�8H2O at four heating rates

(5, 10, 15, and 20 K min-1). According to Eq. (1), the

isoconversional calculation procedure of the Starink

equation was used. The corresponding Starink lines for

Fig. 4 SEM images of the

products synthesized at different

temperatures for 2 h: a 353,

b 473, c 573, d 673, e 873, and

f 1023 K

300 400 500 600 700 800

0.0

0.3

0.6

0.9

1.2

1.5

1.8

1023 K873 K673 K

Abs

orbe

ncy

Wavelength/nm

573 K

Fig. 5 UV–Vis absorption spectra of calcined products

220

230

240

250

260

270

Pea

k ar

ea

Temperature/K600 700 800 900 1000

Fig. 6 Effect of calcination temperature on the UV absorption peak

area

Synthesis of CeO2 by thermal decomposition of oxalate 503

123

different decomposition steps were obtained at different

conversion degrees a and different heating rates b at first,

and then reaction activation energy Ea can be obtained

from linear slope (-1.0008 Ea/R). The results are shown in

Fig. 7. The results show that values of average activation

energies for steps 1 and 2 are 104.36 ± 20.89 and

99.41 ± 9.51 kJ mol-1, respectively. Average Ea value for

step 1 is close to that obtained using Ozawa method, and

that for step 2 is close to value obtained using Coats–

Redfern method [25].

The activation energies changes in step 1 with a are

higher than 10 %, whereas those in step 2 with a are lower

than 10 %. Hence, the dehydration of eight crystal water

molecules from Ce2(C2O4)3�8H2O (step 1) could be mul-

tistep reaction mechanisms. By contrast, the reaction of

Ce2(C2O4)3 with two O2 into CeO2 (step 2) is simple

reaction mechanisms [43–46].

Figure 8 shows curves of a vs. t and da/dt vs. t at heating

rate of 10 K min-1 for step 2. From Fig. 8a, step 2 is

sigmoidal model (sometimes also called autocatalytic)

[29], which represents the process of step 2 the initial and

final stages of which demonstrate the accelerating and

decelerating behavior, respectively, so that the process rate

reaches its maximum at some values of the extent of

conversion. The result shows that rate for step 2 reaches its

maximum at 13. 9 min (Fig. 8b).

We randomly chose several temperatures corresponding

conversions 0.10 \ a\ 0.90. The conversions corre-

sponding to the temperature for b = 5, 10, 15, and

20 K min-1 were assigned to 31 types of mechanism

functions [33, 35]. The slope k, correlation coefficient r2,

and intercept B of linear regression of ln g(a) vs. ln b were

obtained. Two probable mechanism functions with better

correlation coefficient r2 were determined. Several tem-

peratures were randomly chosen to calculate the slope k,

correlation coefficient r2, and intercept B of the two

probable mechanism functions using the same method.

Mechanism function in which the value of k is the closest

to -1.00000 and the correlation coefficient r2 is higher was

chosen as mechanism function of thermal decomposition

of Ce2(C2O4)3�8H2O. The probable mechanism func-

tion integral form of thermal decomposition of

Ce2(C2O4)3�8H2O for step 2 was g(a) = a. The rate-

determining mechanism for step 2 was contracting disk.

The pre-exponential factor was obtained from Eq. (4),

inserting the most probable mechanism function g(a), b,

Ea, R, and Tmax values. The pre-exponential factor ln

A(s-1) of thermal decomposition of Ce2(C2O4)3�8H2O for

step 2 was determined to be 10.27.

In order to prove the validity of the kinetic mechanism

for step 2 of the decomposition of Ce2(C2O4)3�8H2O, the

comparisons were drawn between experimental data and

results of the kinetic mechanism for every heating rate. The

results are shown in Fig. 9. The model-predicted plots

agree with the experimental plots. Therefore, the mecha-

nism function for step 2 of thermal decomposition of

Ce2(C2O4)3�8H2O is reliable.

Conclusions

We have successfully synthesized nano-sized CeO2 with

spherical shape by calcining Ce2(C2O4)3�8H2O. XRD

60

80

100

120

140

Step 2

α

Step 1E

a/kJ

mol

–1

0.3 0.4 0.5 0.6 0.7 0.8

Fig. 7 The dependence of Ea on a at different thermal decomposition

steps

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

t /min

13.9 min

Step 2 a

b 0.0

0.3

0.6

0.9

1.2

0 10 20 30 40 50

d α/d

t

α

Fig. 8 Curves of a vs. t and da/dt vs. t at heating rate of 10 K min-1

0.3

0.4

0.5

0.6

0.7

0.8

0.9

a 5 K min–1

b 10 K min–1

c 15 K min–1

d 20 K min–1

Step 2

T/K

α

a b c d

570 585 600 615 630 645

Fig. 9 Comparison of model results (dash line) with the experimen-

tal data (solid line) of the thermal decomposition of Ce2(C2O4)3�8H2O

for step 2 at different heating rates

504 Y. Li et al.

123

analysis suggested that cubic CeO2 was obtained when

Ce2(C2O4)3�8H2O was calcined above 673 K in air for 2 h.

The UV–Vis absorption spectroscopy studies showed that

CeO2 powder behaved as an excellent UV-shielding

material. The thermal decomposition of the precursor in air

experienced two steps, namely, the dehydration of eight

crystal water molecules and the decomposition of

Ce2(C2O4)3 into cubic CeO2. The values of average acti-

vation energies associated with the thermal decomposition

of Ce2(C2O4)3�8H2O were determined to be

104.36 ± 20.89 and 99.41 ± 9.51 kJ mol-1 for the first

and second thermal decomposition steps, respectively.

Thermal decomposition of Ce2(C2O4)3�8H2O for step 1

could be multistep reaction mechanisms, whereas that for

step 2 is simple reaction mechanisms.

Acknowledgements This study was financially supported by the

National Nature Science Foundation of China (Grant no. 21161002)

and the Guangxi Nature Science Foundation of China (Grant no.

2011GXNSFA018036).

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