Controlled synthesis of nanocrystalline CeO2 and Ce1-xMxO2-δ (M=Zr, Y, Ti, Pr and Fe) solid solutions by the hydrothermal method: Structure and oxygen storage capacity

Article abstract of DOI:10.1016/j.jssc.2008.08.018

CeO₂ and Ce_1-xM_xO_2-δ (M=Zr, Ti, Pr, Y and Fe) nanocrystallites of 5-10 nm sizes have been synthesized by hydrothermal method using diethylenetriamine (DETA) and melamine as complexing agents. Compounds have been

Full text of DOI:10.1016/j.jssc.2008.08.018

ARTICLE IN PRESS
Journal of Solid State Chemistry 181 (2008) 3248–3256
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Controlled synthesis of nanocrystalline CeO and Ce M O
2
1ꢀx x 2ꢀ
d
(
M ¼ Zr, Y, Ti, Pr and Fe) solid solutions by the hydrothermal method:
Structure and oxygen storage capacity
Ã
Preetam Singh, M.S. Hegde
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
a r t i c l e i n f o
a b s t r a c t
Article history:
CeO2 and Ce M O
2ꢀ
(M ¼ Zr, Ti, Pr, Y and Fe) nanocrystallites of 5–10 nm sizes have been synthesized
1ꢀx
x
d
Received 9 July 2008
Received in revised form
by hydrothermal method using diethylenetriamine (DETA) and melamine as complexing agents.
Compounds have been characterized by powder X-ray diffraction (XRD), transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis
5
August 2008
Accepted 9 August 2008
Available online 22 August 2008
(EDX) and their structures have been refined by the Rietveld method. All the compounds crystallize in
4
+
cubic fluorite structure. Even up to 50% Zr and Y, 40% Ti, 25% Pr and 15% Fe is substituted for Ce in CeO
by this method. Sizes of crystallites can be tuned by changing the complexing agent and reaction
temperature. Nanocrystalline CeO and Ce1ꢀxZr prepared here have higher or at least competitive
oxygen storage capacity (OSC) than those reported in literature. Ce1ꢀxFe shows higher OSC and
higher percentage of CO oxidation at lower temperature than Ce1ꢀxZr
2008 Elsevier Inc. All rights reserved.
2
Keywords:
Ceria
2
x 2
O
Nanocrystallites
Hydrothermal synthesis
Oxygen storage capacity
CO oxidation
x 2ꢀd
O
O .
x 2
&
1.
Introduction
CeO
2
. Ce1ꢀx
M
x
O
2
(M ¼ Zr, Hf and Ti) have been synthesized by
solution combustion method but sizes of crystallites are in the
20–40 nm range [8,9]. Hydrothermal method has been employed
Controlled synthesis of inorganic nanoparticles has become
one of the important areas of materials chemistry because of their
size- and shape-dependent properties and their potential of self-
to synthesize CeO
using NH OH and NaOH [10–16]. Only about 10% Fe ion could be
substituted in CeO by low-temperature method using NaOH [17].
Crystallite sizes are in the range of 35–50 nm and these lower
valant metal ion-substituted CeO were made to study oxide ion
2
and Ce1ꢀx
M
x
O
2ꢀ
d
(M ¼ La, Pr, Ca, Sm and Bi)
4
assembly as building blocks and mesocrystals [1,2]. Ceria (CeO
2
) is
2
one of the most reactive rare-earth oxides and it has attracted a
great deal of attention due to its unique application in catalysis,
fuel cells, solar cells, gates for metal oxide, semiconductor devices
2
4
+
conductivity only and redox properties of Ce have not been
studied.
2
and phosphor [3–7]. CeO is known to release a high proportion of
its oxygen for CO oxidation while retaining its fluorite structure.
Amount of lattice oxygen that can be reversibly exchanged from
Here, we report hydrothermal synthesis of CeO
2
and
Ce1ꢀx
M
x
O
2ꢀ
d
(M ¼ Zr, Ti, Pr, Y and Fe) nanocrystallites with new
CeO
2
is called oxygen storage capacity (OSC). Reducibility of CeO
2
ligands like diethylenetriamine (DETA) and melamine. Sizes of
crystallites are in the 4–12 nm range, which can be tuned by
varying the temperature and time of the reaction. To see the effect
to the extent of utilization of surface oxygen is dependent on the
size of nanocrystallites: smaller the crystallite size, higher the
reducibility. Electronic and catalytic properties of CeO
modified by isovalent and aliovalant metal ion substitution in
CeO , provided its fluorite structure is retained with higher
2
can be
of metal ions substitution in CeO
properties by measuring OSC of Ce1ꢀx
Lattice oxygen from Ce1ꢀxFe is utilized by CO or H
2
, we have studied redox
nanocrystallites.
at lower
x 2ꢀd
M O
2
x
O
2ꢀ
d
2
thermal stability.
Even though ZrO
shows higher reducibility than CeO
nanocrystalline Ce1ꢀx
x 2
temperature than Ce1ꢀxZr O .
2
cannot be reduced by H
. Therefore, synthesis of
is important. Ti, Hf and transition
can also enhance the reducibility of
2
or CO, Ce1ꢀxZr
x 2
O
2
M
x
O
2
2. Experimental
metal ion substitution in CeO
2
In total, 2.5 mM of ceric ammonium nitrate, Ce(NH
CAN) was dissolved in 45 mL of water and 7.5 mM of melamine
(C ) or DETA (H NCH CH NHCH CH NH ) was added. The
resulting gel was stirred for 10 min. There is no change in the
4 2 3 6
) (NO )
(
Ã
N
6
H
Corresponding author. Fax: +91080 23601310.
3
6
2
2
2
2
2
2
E-mail address: mshegde@sscu.iisc.ernet.in (M.S. Hegde).
0
022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.08.018

ARTICLE IN PRESS
P. Singh, M.S. Hegde / Journal of Solid State Chemistry 181 (2008) 3248–3256
3249
Table 1
Composition and synthesis condition of Ce1ꢀx
x 2ꢀd
M O
Compound
CeO
Metal salts (M) and temperature (1C)
Complexing agent (CA)
Molar ratio CAN:M:CA
Composition
CeO
2
CAN (150 and 200 1C)
CAN (150 and 200 1C)
Melamine
DETA
1:0:2
1:0:3
2
Ce1ꢀxZr
x
O
2
CAN, Zr(NO
CAN, TiO(NO
CAN, Fe(NO
CAN, (Y +HNO
CAN, (Pr 11+HNO
3
)
4
(200 1C)
Melamine
DETA
1ꢀx:x:2
1ꢀx:x:3
1ꢀx:x:3
1ꢀx:x:3
1ꢀx:x:3
x ¼ 0.25, 0.50
x ¼ 0.25, 0.40
Ce1ꢀxTi
Ce1ꢀxFe
x
O
2
3
)
2
from titanium isopropoxide (200 1C)
(200 1C)
x
O
2ꢀ
d
3
)
3
DETA
x ¼ 0.05, 0.10 and 0.15
x ¼ 0.10, 0.30 and 0.50
x ¼ 0.10 and 0.25
a
Ce1ꢀx
Y
x
O
2ꢀ
d
2
O
3
3
) (200 1C)
) (200 1C)
DETA
a
Ce1ꢀxPr
x
O
2ꢀ
d
6
O
3
DETA
a
2 3 6 3
Y O and Pr O11 are dissolved in minimum amount of dilute HNO , evaporated to dryness and redissolved in water.
orange color of CAN solution when DETA or melamine is added;
the solution turned into a gel. The gel was transferred to three
autoclave bombs of 20 mL capacity with 75% of filling and they
were placed in hot air oven at 200 1C for 24 h. The crystalline solid
obtained was filtered and dried in hot air oven at 100 1C for 5 h. To
study the effect of reaction temperature and time, synthesis was
Melamine,150°C
DETA,150°C
6
9
00
0
carried out at 150 and 200 1C for 12 or 24 h. CeO
prepared with NaOH under similar condition. Ratio of metal salts
and ligands are very important for synthesis CeO nanocrystal-
2
was also
00
0
2
lites. When molar ratio of metal to DETA was 1:2, yield was very
low but when ratio was 1:3, 100% yield is obtained. Yield was
1
00% with melamine with 1:2 ratio of metal to melamine.
(M ¼ Zr, Y, Ti, Pr and Fe) were synthesized by taking
CAN and metal salt in required molar ratio with DETA or
melamine. Several complexing agents were tried to make CeO
in a single step. Even though CeO can be prepared by urea
hydrolysis at 100 1C, in hydrothermal condition Ce(OH)CO was
formed [18]. Glycine and ethylene glycol also gave Ce–ligand
complexes instead of CeO . However with DETA and melamine
CeO is precipitated directly. Details of preparation condition are
given in Table 1.
X-ray diffraction patterns of all samples were recorded in a
DETA, 200°C
Ce1ꢀx
x
M O
2ꢀ
d
500
2
2
0
3
Melamine, 200°C
NaOH, 200°C
1400
2
2
0
Phillips X’Pert diffractometer using CuK
a
radiation at scan rate of
range between 101 and
600
0
.251/min with 0.011 step size in the 2
y
8
01. CuK
b
radiations were filtered with a graphite crystal post
monochromator. Structures were refined by the Rietveld method,
using FullProf-fp2k program [19].
0
1
0
20
30
40
50
60
70
80
For transmission electron microscopy (TEM), toluene disper-
sion of the sample was dropped onto the holey carbon-coated Cu
grids, and the grids were allowed to dry in the air. The grids were
examined with FEI Technai 20 at 200 kV. X-ray photoelectron
spectroscopy (XPS) of selected samples was recorded in Thermo
Scientific Multilab 2000 instrument. Binding energies are accurate
within 70.1 eV.
2
θ
2
Fig. 1. XRD pattern of as-prepared CeO with different ligands and temperatures.
with NaOH in similar condition is also given in Fig. 1 for
comparison. Diffraction lines of CeO2 made with melamine and
DETA are much broader compared with CeO2 made with NaOH.
Diffraction lines for CeO2 with melamine at 150 1C are much
To study the redox properties of CeO
2
x 2ꢀd
and Ce1ꢀxM O ,
hydrogen uptake measurements were performed in a microreac-
tor of length 30 cm and internal diameter (0.4 cm) with 5.49%
H
2
/Ar (certified calibration gas mixture from Bhoruka gases Ltd.,
India) flowing at 30 sccm at a linear heating rate of 5 1C/min up to
00 1C. Volume of hydrogen uptake was calibrated against CuO.
broader compared with CeO with DETA at 1501C. Rietveld refined
XRD profiles of CeO2 made with melamine at 150 1C and CeO2
2
6
made with DETA at 200 1C are shown in Fig. 2. The background of
XRD pattern is flat clearly demonstrating that the material is
crystalline. The refined X-ray profiles fit well with the observed
X-ray data. Crystallite sizes were estimated following Scherrer
formula [20]:
3. Results and discussion
3.1. CeO
2
Crystallites size ðdÞ ¼ 0:9
where is the wavelength of X-ray,
maxima (FWHM) in radian and y the diffraction angle. FWHM was
estimated taking the U, V, W values from the Rietveld refinement
data given by the equation [21]
l
=
b
cos
y
(1)
The powder X-ray diffraction patterns of as-prepared com-
l
b
the full width at half
pounds with DETA or melamine at different temperatures are
shown in Fig. 1. The pattern is indexed to fluorite CeO structure
and diffraction lines due to any of Ce–ligand complex are not
observed. CeO is obtained directly with these two ligands and
yield is quantitative. X-ray diffraction (XRD) pattern of CeO made
2
2
2
1=2
2
b
¼ ðU tan
y
þ V tan
y
þ WÞ
(2)

ARTICLE IN PRESS
3
250
P. Singh, M.S. Hegde / Journal of Solid State Chemistry 181 (2008) 3248–3256
where
b values were also obtained by direct measurement of
FWHM from the (111), (200), (220) and (311) diffraction lines.
Crystallite size obtained by both the methods along with lattice
2
parameter, R
f
, Rbrag
,
w
are summarized in Table 2. Crystallite
values from Eq. (2), agree well with
sizes obtained from the
b
b
values obtained from the direct measurements of X-ray diffrac-
tion lines as can be seen from Table 2. Lattice parameter, a,
of CeO
2
is 5.411 A (JCPDS No. 340394). The lattice parameter ‘a’ of
is slightly higher than sintered CeO and
values agree well with literature values correspond to
the nanocrystalline CeO
a
2
2
2 2
nanocrystalline CeO [22]. Average crystallite size of CeO
obtained with NaOH is in 51 nm while with melamine and DETA,
sizes are in the 4–12 nm range under similar experimental
condition.
HRTEM image, bright field image and electron diffraction
pattern of CeO
are shown in Fig. 3(a) and (b). Average crystallite size of CeO
made with melamine at 150 1C is 5 nm and melamine at 200 1C is
nm. Average crystallites size for CeO made with DETA at 200 1C
2
made with melamine at different temperatures
2
8
2
is around 10 nm. The crystallite sizes obtained from TEM given in
Table 2 agree well with those obtained from the X-ray line
broadening. Electron diffraction pattern from these crystallites is
indexed to fluorite structure of CeO
can be made by the hydrothermal synthesis with melamine.
UV absorption spectra of nanocrystalline CeO made with
complexing agents at different temperature are shown in Fig. 4.
CeO made with NaOH, DETA and melamine at 200 and 150 1C
shows absorption edges at 610, 495, 470 and 445 nm, respectively.
Highly crystalline CeO shows absorption edge at 610 nm with a
band gap of 2.03 eV. Blue shift is due to decrease in sizes of CeO
Band gap increased from 2.03 to 2.78 eV with decrease in size of
CeO from 50 to 5 nm. The absorption of CeO in the UV–visible
2 2
. Thus, CeO of 4–5 nm sizes
2
2
2
2
.
2
2
region originates from the charge transfer transition from O (2p)
to Ce (4f) [22–24].
Having made different sizes of CeO
/TPR profiles of CeO of different sizes are shown in Fig. 5(a)
and (b). CeO made with melamine at 200 1C can be reduced to
CeO1.96 which is equivalent to 2.6 cc/g of [O]. But CeO made with
2
, OSC were measured.
H
2
2
2
2
melamine at 150 1C can be reduced to CeO1.86 which is equivalent
to 10.4 cc/g of [O]. Thus, smaller the size of nanocrystallites, higher
is the reducibility and OSC.
Fig. 2. The Rietveld refined XRD profile of CeO
b) with DETA at 200 1C.
2
(a) with melamine at 150 1C and
(
Table 2
Structural parameters of CeO
2
and Ce1ꢀx
M
x
O
2ꢀd
(M ¼ Zr, Ti, Pr, Y and Fe) made with different complexing agents
2
Compound
Lattice parameter
A˚ )
R
f
R
bragg
w
Size from XRD-
plot (nm)
Size from the
Rietveld
Actual size
from TEM
(nm)
(
refinement (nm)
CeO
CeO
CeO
CeO
CeO
2
2
2
2
2
with NaOH, 150 1C
with DETA, 200 1C
with DETA, 150 1C
with melamine, 200 1C
with melamine, 150 1C
5.4147 (6)
5.4156 (7)
5.4130 (7)
5.4135 (13)
5.4164 (12)
5.3437 (9)
5.2714 (2)
5.3975(7)
5.3425 (2)
5.4307 (5)
5.4461 (5)
5.4151 (5)
5.4104 (1)
5.3854 (2)
5.4084 (5)
5.4062 (7)
5.4036 (1)
3.39
3.12
1.21
1.29
1.35
3.21
3.19
1.53
2.00
1.24
1.55
1.12
0.76
0.65
0.6
3.90
2.38
1.47
1.08
0.932
4.20
4.78
2.29
2.56
1.46
2.11
1.19
1.16
1.28
1.17
1.10
1.13
1.10
0.85
0.76
0.90
1.01
0.90
0.75
0.85
0.97
1.14
0.92
51.570.2
12.070.1
9.770.1
10.970.1
3.770.2
10.0
51.5
12.1
9.7
1072
1072
871
10.9
3.8
571
Ce0.75Zr0.25
Ce0.50Zr0.50
O
2
2
10.1
7.2
O
7.1
Ce0.75Ti0.25
Ce0.60Ti0.40
Ce0.90Pr0.10
O
O
O
2
10.2
10.2
7.7
2
7.5
971
871
2ꢀ
d
10.9
11.0
7.1
Ce0.75Pr0.25
O
2ꢀ
1.95
d
7.0
Ce0.90
Ce0.70
Ce0.50
Y
Y
Y
0.10
O
1.58
0.84
0.87
1.38
0.96
1.92
8.6
8.7
0.30
O
O
1.85
6.0
6.2
0.50
1.75
4.9
5.2
671
Ce0.95Fe0.05
O
1.95
8.3
8.0
Ce0.90Fe0.10
O
1.90
1.85
0.61
1.20
8.3
8.0
Ce0.85Fe0.15
O
10.7
11.3
1072

ARTICLE IN PRESS
P. Singh, M.S. Hegde / Journal of Solid State Chemistry 181 (2008) 3248–3256
3251
Fig. 3. (a) HRTEM image and electron diffraction of CeO
HRTEM images of Ce0.75Pr0.25
2
with melamine at 150 1C, (b) bright field image and HRTEM of CeO
2
with melamine at 200 1C and (c, d) bright and
O
2ꢀd
.
10
1
0
0
0
0
0
0
0
0
0
.0
.9
.8
.7
.6
.5
.4
.3
.2
.1
(
x = 0.10)
x = 0.15)
Fe O
5
(
2
3
0
0
3
2
1
(x = 0.25)
0
0
CeO , NaOH,150
°
C
2
(
x = 0.40)
0
0
CeO , Melamine,200°C
2
2
1
(x = 0.50)
x = 0.25)
0
(
CeO , DETA,150°C
2
0
4
CeO , Melamine,150°C
2
2
0
4
2
0
1
00
200
300
400
500
600
3
00
400
500
wavelength (
600
700
800
Temperature (°C)
λ
)
Fig. 5. Hydrogen uptake plot of (a) CeO
(c) Ce1ꢀxZr
2 3
(e) Ce1ꢀxFe O .
2
(8–10 nm), (b) CeO
2
(4–5 nm),
Fig. 4. UV–visible absorption spectra of CeO
2
made with different complexing
x
O
2
(x ¼ 0.25 and 0.50), (d) Ce1ꢀxTi
(x ¼ 0.10 and ¼ 0.15) and Fe
x
O
2
(x ¼ 0.25 and 0.40) and
agent at different temperature and made with NaOH.
x
O
2ꢀ
d

ARTICLE IN PRESS
3
252
P. Singh, M.S. Hegde / Journal of Solid State Chemistry 181 (2008) 3248–3256
Hydrothermal synthesis route is employed here to synthesize a
large number of metal ion-substituted nanocrystalline CeO
results in the formation of oxide nanocrystallites. A mechanism
for formation of nanocrystallites is given below:
2
.
Ce1ꢀx
M
x
O
2ꢀ
d
(M ¼ Zr, Y, Ti, Pr and Fe) nanocrystallites are
CeðNH
4
Þ2ðNO
3
Þ6 ꢁ 6H
2
O þ 3NH
2
CH
2
CH
2
NHCH
2
CH
2
NH
2
synthesized with DETA and melamine as a complexing agents.
These ligands are forming complexes with metal ion in the form of
a gel and in the hydrothermal condition slow base hydrolysis
#
ðGelÞ ) CeðNH
ðHydrolysisÞNH CH CH
4
Þ2ðNH
NHCH
NH
2
CH
2
CH
2
NHCH
2
CH
2
NH
2
Þ3ðNO
2
3
Þ6 þ 6H
CH
2
O
2
2
2
2
CH
2
NH
2
þ 3H
2
O ! 2NH
CH
2
2
OH þ NH
4
OH
þ
ꢀ
4
OH3NH4 þ OH
Ce þ 4OHꢀ ! CeO
þ
þ 2H
2
2
O
4
pH of the gel was 7.5–8 with DETA and was ꢂ7 with melamine
means slower hydrolysis occurs with melamine, which results
slower growth rate of CeO
formed with melamine. Melamine is soluble in hot water;
TiO(NO in hot water hydrolyzes into TiO . Therefore, melamine
could not be used for the preparation of Ce1ꢀxTi . Ce1ꢀxZr
x ¼ 0.25 and 0.50) nanocrystallites form only with melamine as a
complexing agent. When DETA is used for synthesis of Ce1ꢀxZr
ZrO separated out possibly due to faster hydrolysis of Zr–DETA
complex. This is confirmed by the synthesis of ZrO (monoclinic)
with DETA. DETA forms complexes with most of metal salts. That
is why we have used DETA in most of cases and Ce1ꢀx
M ¼ Y, Ti, Pr and Fe) nanocrystallites solid solutions were
synthesized using DETA as a complexing agent.
2
. Thus, smaller size crystallites are
3
)
2
2
x
O
2
x 2
O
(
x 2
O ,
2
2
x 2ꢀd
M O
(
3
.2. Ce1ꢀxZr
x
O
2
Ce1ꢀxZr
x
O
2
(0pxp0.5) could be synthesized only with mela-
mine and not with DETA. The Rietveld refined X-ray diffraction
pattern of Ce0.50Zr0.50 is shown in Fig. 6(a) and it crystallizes in
Fm3m space group of cubic fluorite lattice. Structural data of
Ce1ꢀxZr
O
2
x
O
2
(x ¼ 0.25 and 0.50) obtained by the Rietveld refine-
ment from powder XRD data are summarized in Table 2.
Crystallite sizes are in the range of 7–10 nm. Lattice parameters
agree well with literature value [25].
Hydrogen uptake measurements were carried out for
Ce1ꢀxZr
Ce1ꢀxZr
reduced from Ce0.75Zr0.25
x
O
2
(x ¼ 0.25 and 0.50) samples. A H
(x ¼ 0.25 and 0.50) are shown in Fig. 5(c). Ce0.75Zr0.25
to Ce0.75Zr0.25 1.84 which is equiva-
2
TPR profile of
x
O
2
O
2
O
2
O
lent to 22.41 cc/g. Note that the Y scale in Fig. 5 is different in each
case. Hydrogen uptake measurements are up to 650 1C and total
OSC up to 650 1C are given in Table 3. O
from 650 1C and H uptake is fully reversible. It is important to
note that Ce0.50Zr0.50 made here is of 7–10 nm sizes and the
compound could be reduced to Ce0.50Zr0.50
Ce0.50Zr0.50 cannot be reduced beyond Ce0.50Zr0.50
2
is passed while cooling
2
O
2
O
1.64
.
Normally,
Fig. 6. The Rietveld refined X-ray profile of (a) Ce0.50Zr0.50
O
2
and (b) Ce0.60Ti0.40
O
2
.
O
O1.75 and if
2
Table 3
Composition of metal substituted CeO
2 2
before and after H uptake and OSC value up to 600 1C
Compound
Composition
Oxygen storage capacity (cc/g)
Before reduction
After reduction
4
+
3+
CeO
2
CeO
CeO
2
2
(10 nm)
(5 nm)
Ce0.94Ce0.06
O
1.97
3.9
4
+
3+
Ce0.84Ce0.16
O
1.92
10.4
4
+
4+
4+ 3+
4+
Ce1ꢀxZr
x
O
2
Ce0.75Zr0.25
O
2
2
Ce0.43Ce0.32Zr0.25
O
1.84
22.4
54.6
4
+
4+
Ce0.50Zr0.50
O
Ce0.50Zr0.50
O
1.64
4
+
4+
Ce1ꢀxTi
x
O
2
Ce0.75Ti0.25
O
2
2
Ce0.75Ti0.25
O
O
1.86
21.0
27.3
4
+
4+
Ce0.60Ti0.40
O
Ce0.60Ti0.40
1.84
4
+
3+
4+
3+
3+
Ce1ꢀxPr
x
O
2ꢀx/2
Ce0.90Pr0.10
O
1.95
Ce0.60Ce0.30Pr0.10
O
1.80
19.6
11.0
4
+
3+
4+
3+
3+
Ce0.75Pr0.25
O
1.875
Ce0.59Ce0.16Pr0.25
O
1.795
4
+
3+
4+ 3+ 3+
Ce1ꢀx
Y
x
O
2ꢀx/2
Ce070
Y
0.30
3+
O
1.85
Ce0.58Ce0.12
0.30
4+ 3+
Ce0.46Ce0.04
Y
O
1.79
1.72
8.5
4.5
4
+
Ce0.50
Y
0.50
O
1.75
3+
Y
0.50
O
4
+
3+
4+ 3+
2+
Ce1ꢀxFe
x
O
2ꢀx/2
Ce0.80Ce0.10Fe0.10
O
1.90
1.85
Ce0.64Ce0.26Fe0.10
O
1.76
1.67
18.6
25.6
4
+
3+
3+
4+
3+
2+
Ce0.75Ce0.15Fe0.15
O
Ce0.49Ce0.36Fe0.15
O

ARTICLE IN PRESS
P. Singh, M.S. Hegde / Journal of Solid State Chemistry 181 (2008) 3248–3256
3253
cations are ordered, it can go to Ce
Ce in 3+ and Zr in 4+ states [26]. Here, 7–10 nm crystallites of
can be reduced beyond Ce0.50Zr0.50 The
reduced compound is black in color and conducting in nature.
Tinku et al. in this laboratory have studied H reduction of
prepared by solution combustion method [9].
Nanocrystalline CeO and Ce1ꢀxZr prepared here show higher
or at least competitive OSC than those reported in literature [27].
2
Zr
2
O
7
pyrochlore phase with
nanocrystallites can be decreased further by decreasing the
reaction temperature.
Ce0.50Zr0.50
O
2
O
1.75
.
Hydrogen uptake measurements were carried out for
Ce1ꢀxTi
are shown in Fig. 5(d). Ce0.75Ti0.25
which is equivalent to 21 cc/g of [O]. Similarly, Ce0.60Ti0.40
reduced to Ce0.60Ti0.40 1.84, which equivalent to 27.3 cc/g of [O].
Hydrogen uptake measurements were performed repeatedly by
x
O
2
(x ¼ 0.25 and 0.40) samples and their H
2
/TPR profiles
2
O
2
, reduced to Ce0.75Ti0.25O
1.86
Ce0.50Zr0.50
O
2
O
2
2
x
O
2
O
reducing in H
reversible.
2 2
up to 650 1C and oxidizing them in O and it is fully
x 2
3.3. Ce1ꢀxTi O
Ti-substituted Ce1ꢀxTi
have been synthesized with DETA as a complexing agent by
x
O
2
(x ¼ 0.25 and 0.40) nanocrystallites
x 2ꢀd
3.4. Ce1ꢀxPr O
hydrothermal method at 200 1C. Up to 40% of Ti can be substituted
Ce1ꢀxPr
DETA as a complexing agent in hydrothermal condition. The
Rietveld refined XRD profile of Ce0.75Pr0.25 compounds made
with DETA is shown in Fig. 8(a). A Core level Pr (3d) XPS spectra of
as-prepared Ce0.75Pr0.25 is shown in Fig. 10. Pr (3d5/2) main
peak is observed at 929.3 eV along with intense satellite peak at
x
O
2ꢀ
d
(x ¼ 0.10 and 0.25) has been synthesized with
4+
for Ce by this method. The Rietveld refined XRD profile of
is shown in Fig. 6(b). Decrease in lattice parameter
Ce0.60Ti0.40
O
2
O
2ꢀd
4
+
of Ce0.75Ti0.25
˚
2
O is due to substitution of lower ionic radii of Ti
4+
˚
(0.74 A) compared with Ce (0.97 A). Structural parameters are
O
2ꢀd
summarized in Table 2.
3
+
HRTEM and bright field image of Ce0.60Ti0.40
O
2
is shown in
is 9 nm.
x 2ꢀd
933.3 eV, confirmed Pr present in Ce1ꢀxPr O . The compound
Figs. 7(a) and (b). Average crystallite size of Ce0.60Ti0.40O
2
is red in color. Increase in lattice parameters is observed due to
3
+
˚
Sizes obtained from TEM study agree well with XRD data. Sizes of
substitution of Pr ion (r
i
¼ 1.12 A) in CeO
2
matrix. Structural
a
b
5
0 nm
5 nm
c
d
2
0 nm
10 nm
e
f
20nm
Fig. 7. (a) Bright field and (b) HRTEM image of Ce0.60Ti0.40
O
2
, (c) Bright field image, (d) HRTEM image of Ce0.50Y O1.75, (e) Bright field image and (f) ED pattern of
0.50
2ꢀd
Ce0.85Fe0.15O .

ARTICLE IN PRESS
3
254
P. Singh, M.S. Hegde / Journal of Solid State Chemistry 181 (2008) 3248–3256
0
0
0
0
0
0
0
.12
.10
.08
.06
.04
.02
.00
4
66 cm-1
Ce0.50Y0.50O2-δ
5
71 cm-1
2
00 400 600 800 1000 1200 1400 1600 1800 2000
cm-
1
0.50 1.75
Fig. 9. Raman scattering spectra of Ce0.50Y O .
Pr3+(3d5/2
)
3
1
4000
7000
0
Pr(3d3/2
)
925
930
935
940
945
950
955
960
B.E. (eV)
1.875
Fig. 10. XPS of Pr (3d) in Ce0.75Pr0.25O .
3+
oxide ion vacancy created in the lattice due to Pr
substitution.
ion
3.5. Ce1ꢀx
Y
x
O
2ꢀx/2
Ce1ꢀx
Y
x
O
2ꢀ
d
(x ¼ 0.10, 0.30 and 0.50) have been synthesized
Fig. 8. The Rietveld refined XRD plot of (a) Ce0.75Pr0.25
c) Ce0.85Fe0.15
2ꢀd 0.50 2ꢀd
O , (b) Ce0.50Y O and
with DETA as complexing agent. Up to 50% Y is substituted for
(
2ꢀd
O .
4
+
Ce in CeO
vacancy, Ce0.50
Rietveld refined XRD profile of Ce0.50
x
Fig. 8(b). Lattice parameter, ‘a’ of Ce1ꢀxY O
2
lattice by this method. Even with huge oxide ion
1.75 crystallizes in fluorite structure. The
1.75 is shown in
2ꢀx/2 decreases with
increase in Y substitution in CeO2. Structural parameters of
Y O
0.50
parameters of Ce1ꢀxPr
x
O
2ꢀ
d
(x ¼ 0.10 and 0.25) obtained by the
0.50
Y O
Rietveld refinement from powder XRD data are summarized in
Table 2. Decrease in crystallites size is observed with increase in
substitution of Pr ion in CeO2. Bright field and HRTEM image of
3
+
Ce1ꢀx
Y
x
O
2ꢀx/2 (x ¼ 0.10, 0.30 and 0.50) obtained by the Rietveld
Ce0.75Pr0.25
size of Ce0.75Pr0.25
agree well with XRD data.
uptake study of Ce0.90Pr0.10
Ce0.90Pr0.10 1.80 which is equivalent to 19.6 cc/g of [O] and
Ce0.75Pr0.25 1.875 shows very little OSC of about 11 cc/g up to
00 1C. Thus, OSC value decreases with increase of Pr substitution
O
2ꢀ
d
is shown in Fig. 3(c) and (d). Average crystallite
refinement from powder XRD data are summarized in Table 2.
HRTEM image and electron diffraction pattern of
O
2ꢀd
is 10 nm. Sizes obtained from TEM study
Ce0.50
size of Ce0.50
yttrium is also confirmed by EDAX. Huge oxygen vacancy present
0.50
in Ce0.50Y O1.75 is observed by Raman scattering shown in Fig. 9.
Y
0.50
O
1.75 are given in Fig. 7(c) and (d). Average crystallite
H
2
O
1.95 shows reduction to
0.50
Y O
1.75 is 6 nm. Substitution and composition of
O
O
ꢀ
1
6
High-intensity phonon mode vibration observed at 471 cm
,
2
in CeO . A decrease in OSC can be attributed to large amount of
which is assigned to symmetric breathing mode of oxygen atoms

ARTICLE IN PRESS
P. Singh, M.S. Hegde / Journal of Solid State Chemistry 181 (2008) 3248–3256
3255
around cerium ions and the phonon mode scattering observed at
m
Ce3+(3d5/2
)
(c)
ꢀ1
5
64 cm is characteristic for oxygen vacancy in lattice [28].
s
3+
Y
ion-substituted CeO
2
shows lower OSC and it decreases
with increase in Y content. While Ce0.70 1.85 could be
reduced to Ce0.70 which is equivalent to 8.5 cc/g,
Ce0.50 1.75 could hardly be reduced. Thus, nonreducible
rare-earth ion and Y ion substitution suppresses the OSC of
CeO
3+
Y
0.30
O
Y
0.30 1.79
O ,
Y
0.50
Y
3+
2
.
x 2ꢀd
3.6. Ce1ꢀxFe O
(
b)
There is very little attempt to substitute transition metal ions
in CeO lattice for OSC and other catalytic application. Even
though 10% Fe ion-substituted CeO
et al. [16], redox properties of this material are not evaluated.
Recently, up to 20% Fe-substituted CeO have been prepared by
urea hydrolysis method and best CO oxidation activity is seen
with 10% of Fe ion substituted in CeO [29]. We have synthesized
Ce1ꢀxFe (x ¼ 0.05, 0.10 and 0.15) nanocrystallites with
DETA as the complexing agent. The Rietveld refined XRD profile
compounds made with DETA is shown in
Fig. 8(c). All the peaks were indexed in fluorite structure.
Structural parameters of Ce1ꢀxFe
2
3
+
s
has been synthesized by Li
Ce4+(3d5/2
)
2
m
2
s
2
(a)
x
O
2ꢀ
d
of Ce0.85Fe0.15
O
2ꢀd
x
O
2ꢀ
d
(x ¼ 0.05, 0.10 and 0.15)
obtained by the Rietveld refinement of powder XRD data
are summarized in Table 2. The diffraction peak at half the
angle of (111) reflection at ꢂ14.41 is seen in the XRD pattern
8
80
890
900
910
920
B.E. (eV)
of
Ce0.60Ti0.40
O
2
,
Ce0.75Pr0.25
O
2ꢀd
,
Ce0.50
Y
0.50
O
2ꢀ
d
and
Fig. 11. XPS of Ce (3d) in (a) pure CeO2 (b) as prepared and (c) hydrogen reduced
Ce0.85Fe0.15
O
2ꢀ
d
(Figs. 6b and 8). It can be due to doubling of the
2ꢀd
Ce0.85Fe0.15O .
lattice. But, other diffraction peaks due to doubling of the cell are
not sufficiently intense as seen from the refined pattern. There-
fore, further studies are needed to assign the possible super lattice
formation.
Fe2+(2p3/2
)
(
Color of the compound is red indicting Fe is in 3+ state. When
Fe concentration is increased to 20%, weak reflection of
seen. We believe that 17–18% of Fe can be substituted by this
method. Average crystallite size of Ce0.85Fe0.15 is 10 nm.
Bright field image and diffraction pattern of Ce0.85Fe0.15 is
2 3
g-Fe O is
O
2ꢀd
O
2ꢀd
(
b)
shown in Fig. 7(e) and (f). Sizes obtained from TEM study agree
well with XRD data. Composition is confirmed by EDX analysis.
Fe2+(2p1/2
)
Fe2+sat.)
(Fe3+(2p3/2))
3
+
˚
Due to substitution of Fe (r
i
¼ 0.69 A) in CeO
2
matrix, lattice
parameter decreases. Decrease in lattice parameter is relatively
less than expected. This is possibly due to presence of Ce along
3
+
4+
with Ce in Ce1ꢀxFe
To study the oxidation state of Ce and Fe in Ce1ꢀxFe
of Ce (3d) and Fe (2p) region were recorded. Core level XPS of Ce
3d) of as prepared and H reduced Ce0.85Fe0.15 and CeO are
x 2ꢀd
O .
x
2ꢀd
O , XPS
Fe3+(2p1/2)
(
2
O
2ꢀ
d
2
4
+
shown in Fig. 11(a)–(c). Ce (3d5/2) peak observed at 882.7 eV
(a)
along with satellite peaks at 6.4 and 16 eV below the main peak is
Fe3+ sat.
4+
characteristic of Ce in CeO
2 2 3 2 3 2 3
[30]. La O , Ce O and Pr O shows
3+
similar Ln (3d) XPS spectra as shown for Pr (3d) in Fig. 10. Ce
2 3
O
is characterized by Ce (3d5/2) at 883.3 eV along with an intense
satellite at 887.1 eV [30]. Thus, filling of the valley between
4+
Ce (3d5/2) at 882.7 eV and its satellite at 889.1 eV shows Ce is in
mixed valant state in Ce0.85Fe0.15 . Ce(3d) spectrum was
resolved into Ce and Ce components in Ce0.85Fe0.15 and
5% of Ce is found to be in +3 state. After hydrogen reduction up to
O
2ꢀd
3+
4+
O
2ꢀd
7
05
710
715
B.E. (eV)
720
725
1
6
00 1C 30% of Ce is found in +3 state. Fe (2p3/2, 1/2) core level XPS of
are shown in
Fig. 12(a) and (b). Fe (2p3/2) is at 710.3 eV and a weak satellite at
eV below the main peak in the as-prepared Ce0.85Fe0.15
confirms that Fe is in +3 state. Thus, the formula of as-prepared
as prepared and hydrogen reduced Ce0.85Fe0.15O
2ꢀd
2ꢀd
Fig. 12. XPS of Fe (2p) (a) as-prepared and (b) hydrogen reduced Ce0.85Fe0.15O .
8
O
2ꢀ
d
2
+
0
reduced to Fe state not to Fe after hydrogen reduction up to
600 1C.
4
+
3+
3+
1
(
5% Fe-substituted CeO
2p3/2) at 709.6 eV and satellite peak at 5 eV below the main peak
confirms that Fe is in +2 states in hydrogen reduced
2
can be given as Ce0.70 Ce0.15Fe0.15
O
1.85. Fe
H
2
/TPR profile of Ce1ꢀxFe
equivalent amount of Fe up to 600 1C are shown in Fig. 5(e).
Peak temperature of H reduction of Fe is 400 1C and reduction
x
O
2ꢀ
d
(x ¼ 0.10 and 0.15) and
2 3
O
3
+
Ce0.85Fe0.15
O
2ꢀ
d
[31]. It is important to note that Fe is fully
2
2 3
O

ARTICLE IN PRESS
3
256
P. Singh, M.S. Hegde / Journal of Solid State Chemistry 181 (2008) 3248–3256
melamine and simultaneous hydrolysis in hydrothermal condition
leads to formation of Ce1ꢀx (M ¼ Ti, Pr, Y and Fe)
nanocrystallites. Up to 15% Fe have been substituted in CeO
and it is showing higher OSC and higher percentages of
CO oxidation with lattice oxygen at lower temperature than
x 2ꢀd
M O
3
+
25
20
15
10
5
0
2
(
c)
2
Ce0.75Zr0.25O .
(
d)
Acknowledgments
(
b)
Authors thank Department of Science and Technology, Govern-
ment of India for financial support. Thanks are due to Amit
Mondal of Nano centre, Indian Institute of Science Bangalore and
Viswanath B. of Materials Research Centre, Indian Institute of
Science Bangalore for recording TEM images.
(
a)
References
5
0
100
150
200
250
300
350
400
Temperature (°C)
[1] M. Law, D.J. Sirbuly, J.C. Johnson, J. Goldberger, R.J. Saykally, P.D. Yang, Science
3
05 (2004) 1269–1273.
[2] E.V. Shevehenko, D.V. Talapin, C.B. Murray, S. O’Brien, J. Am. Chem. Soc. 128
2006) 3620–3637.
Fig. 13. The %CO conversion with lattice oxygen from (a) CeO
b) Ce0.75Zr0.25 , (c) Ce0.50Zr0.50 and (d) Ce0.85Fe0.15
2
(5 nm),
(
(
O
2
O
2
2ꢀd
O .
[
[
3] A. Trovareli, Catal. Rev. Sci. Eng. 38 (1996) 439–520.
4] E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649–651.
starts at lower temperature in Fe-substituted CeO
Fe is reduced to Fe 2.5 up to 600 1C. This means all of Fe is
not reduced to Fe state. Reduction temperature of Ce1ꢀxFe
is lower than Ce1ꢀx (M ¼ Zr and Ti). Ce0.90Fe0.10 1.90 can be
reduced to Ce0.90Fe0.10 1.76, which is equivalent to 18.5 cc/g of [O]
and Ce0.85Fe0.15 1.85 is reduced to Ce0.85Fe0.15 1.67, which is
equivalent to 25.6 cc/g. Thus, even at 15% Fe ion substitution
2
than Fe
2
O
3
.
[5] Q. Fu, H. Saltsbarg, M. Flytzani-Stephanopoulos, Science 301 (2003) 935–938.
[6] A. Corma, P. Atienzar, H. Gracia, J.Y. Chane-Ching, Nat. Mater. 3 (2004)
394–397.
3
+
2
O
3
2
O
+
2
O
x 2ꢀd
[
[
[
7] A.H. Morshed, M.E. Moussa, S.M. Bedir, R. Leonard, S.X. Iiu, N. Elmasry, Appl.
Phys. Lett. 70 (1997) 1647–1649.
8] T. Baidya, A. Gayan, M.S. Hegde, N. Ravishankar, L. Dupont, J. Phys. Chem. B
M
x
O
2
O
O
110 (2006) 5262–5272.
O
O
9] T. Baidya, M.S. Hegde, J. Gopalakrishnan, J. Phys. Chem.
5149–5154.
B 111 (2007)
3+
3
+
[
10] W-Q. Han, L. Wu, Y. Zhu, J. Am. Chem. Soc. 127 (2005) 12814–12815.
[11] S. Yang, L. Gao, J. Am. Chem. Soc. 128 (2006) 9330–9331.
12] T. Masui, H. Hirai, R. Hamad, N. Imanka, A. Gin-Ya, T. Sakata, H. Mori, J. Mater.
in CeO
2 2
, OSC is higher than Ce0.75Zr0.25O . This is because Fe is
2+
4+
entirely reduced to Fe in addition to partial reduction of Ce to
[
3+
Ce . Hydrogen uptake data are summarized in Table 3. Even
Chem. 13 (2003) 622–627.
[13] W. Hung, P. Shuk, M. Greenblatt, Chem. Mater. 9 (1997) 2240–2245.
[14] P. Shuk, M. Greemblatt, Solid State Ionics 116 (1999) 217–223.
15] S. Dikmen, P. Shuk, M. Greenblatt, Solid State Ionics 126 (1999) 89–95.
16] G. Li, Y. Mao, L. Li, M. Feng, M. Wang, X. Yao, Chem. Mater. 11 (1999)
1259–1266.
17] G. Li, R.L. Smith, H. Inomata, J. Am. Chem. Soc. 123 (2001) 11091–11092.
18] Z. Guo, F. Du, G. Li, Z. Cui, Inorg. Chem. 45 (2006) 4167–4169.
19] J. Rodriguez-Carvajal, Multi-pattern Rietveld Refinement program Fullprof.
2k, version 3.30, June 2005-LLB.
0
though Fe ions in Fe
2
O
3
and FeO can be reduced to Fe at higher
3+
2+
x 2ꢀd
temperatures, Fe in Ce1ꢀxFe O can be reduced to Fe state
[
[
only. Hydrogen uptake was repeated for several cycles following
reduction and oxidation up to 600 1C, Redox behavior is fully
reversible.
[
[
[
2
Having studied the OSC of these materials by H /TPR, CO
oxidation experiments were performed to see the utilization of
lattice oxygen by CO. CO oxidations were performed in a
microreactor of length 30 and 0.4 cm internal diameter with
[
[
20] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley,
Reading, MA, 1978.
21] G. Caglioti, A. Paoletti, F.P. Ricci, Nucl. Instrum. Methods 3 (1958) 223–228.
1
vol% of CO/He with total flow 100 sccm over 250 mg of catalyst
at a linear heating rate of 10 1C/min up to 500 1C. Ce0.85Fe0.15
shows higher percentages of CO oxidation at lower temperature
and it is shown in Fig. 13. Thus, lattice oxygen
[22] Ho Chunman, C. Jimmy, K. Tszyan, C. Angelo, L. Sukyin, Chem. Mater. 17
2005) 4514–4522.
23] T. Murata, M. Sato, H. Yoshida, K. Morinaga, J. Non-Cryst. Solids 351 (2005)
12–322.
[24] S. Tsunekawa, J. Wang, Y. Kawazoe, A. Kasuya, J. Appl. Phys. 94 (2003)
3654–3656.
(
O
1.85
[
3
than Ce0.75Zr0.25
O
2
x 2ꢀd
is more labile in Ce1ꢀxFe O and it can be utilized as an OSC
material.
[25] G. Dutta, U.V. Waghmare, T. Baidya, M.S. Hegde, K.R. Priolkar, P.R. Sarode,
Catal. Lett. 108 (2006) 165–172.
[26] J.B. Thomson, A.R. Armstrong, P.G. Bruce, J. Solid State Chem. 148 (1999)
56–62.
4
.
Conclusions
Controlled synthesis of CeO
[27] Y. Nagai, T. Yamamoto, T. Tanaka, S. Yoshida, T. Nonaka, T. Okamoto, A. Suda,
M. Sugiura, Catal. Today 74 (2002) 225–234.
[28] A. Mineshige, T. Taji, Y. Moroi, M. Kobune, S. Fujii, N. Nishi, M. Inaba, Z. Ogumi,
2
and Ce1ꢀx
M
x
O
2ꢀ
d
(M ¼ Zr, Ti, Pr, Y
Solid State Ionics 135 (2000) 481–485.
[29] H. Bao, X. Chen, J. Fang, Z. Jiang, W. Huang, Catal. Lett. 124 (2008).
and Fe) nanocrystallites is achieved by hydrothermal synthesis
route using DETA and melamine as a complexing agents. Average
[
30] A. Kotani, H. Ogasawara, J. Electron Spectrosc. Relat. Phenom. 60 (1992)
57–299.
2
sizes of Ce1ꢀx
x 2ꢀd
M O nanocrystallites are in the 5–12 nm range.
[31] C.N.R. RaO, D.D. Sarma, S. Vasudevan, M.S. Hegde, Proc. R. Soc. Lond. A 367
(1979) 239–252.
4
+
Complexation of these metal ions (M) and Ce with DETA or