Sonochemical synthesis of Ce1-xFexO 2-δ (0 ≤ x ≤ 0.45) and Ce0.65Fe 0.33Pd0.02O2-δ nanocrystallites: Oxygen storage material, CO oxidation and water gas shift catalyst

Article abstract of DOI:10.1039/c0dt00480d

Nanocrystalline Ce_1-xFe_xO_2-δ (0 ≤ x ≤ 0.45) and Ce_0.65Fe_0.33Pd_0.02O _2-δ of ～ 4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce _1-xFe_xO_2-δ (0 ≤ x ≤ 0.45) and Ce_0.65Fe_0.33Pd_0.02O_2-δ crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe³⁺ ion in CeO ₂, lattice oxygen is activated and 33% Fe substituted CeO ₂i.e. Ce_0.67Fe_0.33O_1.835 reversibly releases 0.31[O] up to 600 °C which is higher or comparable to the oxygen storage capacity of CeO₂-ZrO₂ based solid solutions (Catal. Today2002, 74, 225-234). Due to interaction of redox potentials of Pd^2+/0(0.89 V) and Fe^3+/2+ (0.77 V) with Ce ^4+/3+ (1.61 V), Pd ion accelerates the electron transfer from Fe ²⁺ to Ce⁴⁺ in Ce_0.65Fe_0.33Pd _0.02O_1.815, making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce_0.65Fe _0.33Pd_0.02O_1.815 is found to be as low as 38 kJ mol^-1. Ce_0.67Fe_0.33O_1.835 and Ce_0.65Fe_0.33Pd_0.02O_1.815 have also shown high activity for the water gas shift reaction. CO conversion to CO ₂ is 100% H₂ specific with these catalysts and conversion rate was found to be as high 27.2 μmoles g^-1 s^-1 and the activation energy was found to be 46.4 kJ mol^-1 for Ce _0.65Fe_0.33Pd_0.02O_1.815. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00480d

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Sonochemical synthesis of Ce Fe O (0 £ x £ 0.45) and
1
-x
x
2-d
Ce Fe Pd O nanocrystallites: oxygen storage material, CO
0
.65 0.33
0.02 2-d
oxidation and water gas shift catalyst†
Preetam Singh and M. S. Hegde*
Received 17th May 2010, Accepted 5th July 2010
DOI: 10.1039/c0dt00480d
x
Nanocrystalline Ce1-xFe O2-d (0 £ x £ 0.45) and Ce0.65Fe0.33Pd0.02O2-d of ~ 4 nm sizes were synthesized by
a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were
characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and
x
transmission electron microscopy (TEM). Ce1-xFe O2-d (0 £ x £ 0.45) and Ce0.65Fe0.33Pd0.02O2-d crystallize
in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of
3
+
smaller Fe ion in CeO
2
, lattice oxygen is activated and 33% Fe substituted CeO
2
i.e. Ce0.67Fe0.33
O
1.835
◦
reversibly releases 0.31[O] up to 600 C which is higher or comparable to the oxygen storage capacity of
CeO
2
–ZrO based solid solutions (Catal. Today 2002, 74, 225–234). Due to interaction of redox
2
2
+/0
3+/2+
4+/3+
potentials of Pd (0.89 V) and Fe
(0.77 V) with Ce
1.815, making it a high oxygen storage material as well as a
highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO
(1.61 V), Pd ion accelerates the electron
2
+
4+
transfer from Fe to Ce in Ce0.65Fe0.33Pd0.02
O
-
1
oxidation with Ce0.65Fe0.33Pd0.02
Ce0.65Fe0.33Pd0.02
O
1.815 is found to be as low as 38 kJ mol . Ce0.67Fe0.33
1.815 have also shown high activity for the water gas shift reaction. CO conversion to
specific with these catalysts and conversion rate was found to be as high
O1.835 and
O
CO
2
2
is 100% H
2
7.2 mmoles g s-1 and the activation energy was found to be 46.4 kJ mol-1 for Ce0.65Fe0.33Pd0.02
-
1
O
.
1.815
1
7
1
. Introduction
than Ce1-xTi
x
2
x y
O but Ce1-x-yTi Pd O2-d is reported to show higher
activity for CO oxidation and NO reduction by CO compared
Synthesis of ceria-based solid solutions with controllable particle
size and dopant concentration is of fundamental significance
because of potential applications in oxide ion conductivity,
exhaust-catalysis, water gas shift (WGS) catalysis, fuel cells,
solar cells, semiconductor devices and phosphor. CeO
24
25,26
to Ce1-x-yZr
Ce1-x-yTi
tween Pd , Ti
x
PdyO2-d and Ce1-xPd
O
x
O
2-d
.
Higher activity of
x
2+/0
Pd
y
2-d is attributed to the synergistic interaction be-
1
4+/3+
4+/3+
17,24
and Ce
redox couples.
(0.77 V) is lower compared
(1.61 V), Fe ion substituted ceria can be another
2,3
4
5
3+/2+
27
As redox potential of Fe
6
7,8
4+/3+
23
3+
2
is
to Ce
utilized as a promoter in exhaust catalysis as it can release a high
proportion of its lattice oxygen for CO and hydrocarbon oxidation
under fuel rich conditions. Thus a multi-component catalyst with
promising candidate for higher OSC material as well as three-
way exhaust catalyst in the presence of noble metal ions. Indeed
Ce0.89Fe0.1Pd0.01
O
2-d synthesized by solution combustion method
noble metals Pt, Pd, Rh impregnated on silica or Al
oxygen storage materials forms an auto exhaust catalyst.
amount of lattice oxygen that can be reversibly exchanged from
2
O
3
along with
28
showed high OSC and catalytic activity for CO oxidation. 15–
9
–12
The
2
0% Fe substituted CeO
2
also synthesized by template route
29
3+
showed high conversion for ethanol steam reforming. Fe ion
could be the cheapest substituent for Ce ion in CeO . Thus
higher substitution of Fe ion for Ce in ceria will reduce the
cost of the OSC material and will increase the OSC of ceria. But
only up to 10% Fe could be substituted by solution combustion
method, up to 15% by hydrothermal method
Fe ion is substituted by nitrate-citrate gel method. Therefore we
considered it worthwhile to find an alternate method to substitute
13
CeO
2
is called oxygen storage capacity (OSC). However, the OSC
4+
2
of ceria can be enhanced by isovalent and aliovalant metal ion
3+
4+
1
4,15
substitution in CeO
2
, in the form of solid solutions Ce1-xZr
x
O ,
2
16,17
18,19
20
21
Ce1-xTi
x
O
2
,
Ce1-xSn
x
O
2
,
Ce1-xCr
x
O
2+d and Ce1-xRu
x
O
2-d
.
20
21
Higher OSC of Ce2/3Cr1/3
O
2.11
and Ce0.9Ru0.1
O
2-d
nano-
28
30,31
and up to 20%
22
crystallites than Ce0.5Zr
to interaction of redox couples of Cr
in their respective solid solutions. For example in Ce2/3Cr1/3
5
O
2
at lower temperature is attributed
3+
32
3
+/4+/6+
4+/3+/0
4+/3+
, Ru
with Ce
O
2.11
,
higher % of Fe ion in CeO
2
even to the extent of 50%. We
4
+/6+
3+
in the H
2
reduction step first Cr
is reduced to Cr and due
to lower redox potential of Cr /Cr (1.33 V) compared to
Ce /Ce (1.61 V), Cr is oxidized to Cr by Ce reduction
to Ce state. Ce ion is oxidized to Ce by feed oxygen to
is higher
have designed a new low temperature sonochemical method to
6
+
3+
23
3+
2+
synthesize higher % Fe and Pd substituted ceria. An earlier
4
+
3+
23
3+
6+
4+
sonication route was employed by Yin et al. to synthesized CeO
nanoparticles.
2
3
+
3+
4+
33
regenerate Ce2/3Cr1/3
O2.11. Similarly OSC of Ce1-xZr
x
O
2
Here we report synthesis, OSC, CO oxidation and WGS
activity of Ce1-xFe
x
O2-d (0 £ x £ 0.45) and Ce0.65Fe0.33Pd0.02O
2-d
Solid State and Structural Chemistry Unit, Indian Institute of Science,
Bangalore, 560012, India. E-mail: mshegde@sscu.iisc.ernet.in; Fax: +91-
nanocrystallites. Compounds were synthesized by sonochemical
method using diethylenetriamine (DETA) as complexing agent.
8
0-2360 1310; Tel: +91-80-2293 2614
Ce0.67Fe0.33O2-d shows higher OSC at lower temperature than
†
Electronic supplementary information (ESI) available: Additional data.
22
2+
3+
Ce0.5Zr0.5
O
2
and both Pd and Fe ion substituted CeO
2
i.e.
See DOI: 10.1039/c0dt00480d
1
0768 | Dalton Trans., 2010, 39, 10768–10780
This journal is © The Royal Society of Chemistry 2010

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Ce0.65Fe0.33Pd0.02
activity.
O
2-d show high CO oxidation and WGS reaction
total gas flow of 100 sccm over 200 mg of oxide sample, at a
◦
-1
◦
linear heating rate of 10 C min up to 600 C. CO oxidation
over the catalysts are carried out with 200 mg of the catalyst at
-
1
a gas space hourly velocity (GSHV) of 43000 CC h with 1 : 1
2
. Experimental
◦
-1
volume% of CO and O
Gaseous products were analyzed by means of a quadrupole mass
spectrometer. For WGS study, 5cc/min CO and 30 cc/min H
vapor were passed along with N over 200 mg of catalyst keeping
2
with linear heating rate of 10 C min .
Ce1-xFe
x
O
2-d (x = 0, 0.15, 0.25, 0.33 and 0.45) solid solutions were
synthesized by sonication method from the starting materials,
NH Ce(NO (CAN), Fe(NO O and DETA by taking
·9H
them in a molar ratio of 1-x : x : 3. For the synthesis of
2-d , 6.7 mmol of CAN is dissolved in 20 ml of distilled
water and 3.3 mmol Fe(NO O was dissolved in 5 ml of
2
O
(
4
)
2
3
)
6
3
)
3
2
2
-
1
the total gas flow at 130 cc/min and GSHV of 55900 h . Gaseous
products obtained from the WGS reaction were analyzed by online
gas chromatograph (GC). Rate and activation energy of catalyst
for CO oxidation and WGS reaction were carried out with different
weights (25, 50, 100, 150 to 200 mg) of the catalyst keeping the
total flow and GSHV the same.
Ce0.67Fe0.33
O
3
)
3
·9H
2
water, both the solutions were mixed and 30 mmol (3.4 ml) of
DETA was added to it. The solution turned into brown colored
gel immediately. The gel was kept under sonication (Oskar O U
9
1
equipped with high intensity piezoelectric resonator, 30 kHz,
-
2
00 W cm ) for 8 h. The temperature of the solution reached
◦
up to 80 C during sonication. The precipitated brown colored
solid was filtered, washed in hot distilled water and dried in a
hot air oven at 110 C for 3 h. Similarly, for the preparation
3
. Results and discussion
◦
(
a) Synthesis and characterizations
of Ce0.65Fe0.33Pd0.02
O
2-d , 6.5 mmol of CAN and 0.2 mmol of
O were dissolved in 20 ml of distilled water and
.3 mmol Fe(NO O was dissolved in 5 ml of water. The
Pd(NO
3
3
)
2
·4H
2
Only up to 15% Fe ion substituted CeO , Ce1-xFe O2±d (0 £ x £
0.15) were synthesized by hydrothermal method using NaOH or
2
x
3
0
3
)
3
·9H
2
3
+
solutions were mixed and 30 mmol or 3.4 ml of DETA was added.
The resulting gel after DETA addition was kept under sonication
for 8 h. The resulted solid obtained after the filtration and drying
was brown in color.
DETA as precipitating agent. Higher % of Fe ion substitution
and simultaneous substitution of Pd and Fe in ceria could not
be achieved by the hydrothermal method. In Ce1-xFe
0.15) synthesised by the hydrothermal method with DETA or
NaOH as a precipitating agent, gFe impurity phase was
precipitated along with Ce1-xFe 2-d crystallites were
x
O2±d , (x >
X-ray diffraction (XRD) patterns of powders were recorded in
a Phillips X’Pert diffractometer using Cu-Ka radiation at scan
2
O
3
x
x
O2-d . Ce1-xFe O
◦
-1
◦
rate of 0.2 min with 0.01 step size in the 2q range between
1
not precipitated with DETA or NaOH directly without sonication.
Amorphous gel was formed by simultaneous complexation of
◦
0 to 90 . Cu-K
b
radiation was filtered with a graphite crystal
post monochromator. Structures were refined by Rietveld method,
using FullProf-fp2k program
CAN and Fe(NO
of sonication, quantitative precipitation of Ce1-xFe
2-d were obtained. Compounds
were brown in color. Even under sonication, precipitation with
NaOH instead of DETA results in the formation of g-Fe and
2-d . Under hydrothermal condition, the temperature is
3
)
3
·9H
2
O solutions by DETA. Only after 8 h
34
x
O2-d (0 £
For transmission electron microscopy (TEM), a toluene disper-
sion of the sample was dropped onto the holeycarbon coated Cu
grids and images were recorded with FEI Technai 20 at 200 kV.
EDX analysis was also carried out with the probe connected
with TEM instrument. X-ray Photo-electron Spectra (XPS) of
selected samples were recorded in a Thermo Scientific Multilab
x £ 0.45) and Ce0.65Fe0.33Pd0.02O
2
O
3
x
Ce1-xFe O
◦
around 150–200 C and very high pressure is generated inside
3
0,31
the autoclave bombs.
This facilitates the faster hydrolysis of
phase is separated out. In the
2
000 instrument using Al-Ka at 150 W. Binding energies reported
here are with reference to C(1 s) at 284.5 eV and are accurate within
0.1 eV. UV-visible spectra of solid samples were recorded in by
Fe-DETA complex and Fe
2
O
3
sonochemical method presented here, the temperature reached
◦
±
is 80 C after 6–8 h of sonication. Thus simultaneous hydrolysis
Perkin Elmer’s LAMBDA 35 UV-visible spectrometer. BaSO
4
was
of Ce-DETA and Fe-DETA complexes under sonication results
used for baseline correction.
x
in the precipitation of Ce1-xFe O2-d . Similarly hydrolysis of Ce-
x
Oxygen storage/release properties or OSC of Ce1-xFe O2-d were
DETA, Fe-DETA complex and Pd-DETA complexes under 8 h of
studied by hydrogen intake measurements carried out in a mi-
croreactor (30 cm length and 0.4 cm internal diameter) employing
x y 2-d
sonication results in qualitative precipitation of Ce1-x-yFe Pd O
crystallites.
5
.49% H
and at a linear heating rate of 10 C min , up to 550 C. Amount
of H consumption or equivalent [O] release or OSC of material is
2
/Ar (Chemix, Bangalore, India) with 30 sccm flow rate
Powder XRD pattern of as prepared pure CeO
2-d solid solutions (x = 0.15, 0.25, 0.33 and 0.45) are
shown in Fig. 1(a–e) No impurity peaks, corresponding to Ce-
DETA complex or Fe-DETA complex or Fe were observed
2
and
◦
-1
◦
x
Ce1-xFe O
2
calibrated against hydrogen intake/consumption against known
weight of CuO. OSC of material was also determined by direct
oxidation of CO with compound in absence of external/feed
oxygen. Powder catalyst was made into pressed granules of 40–
8
0
ultra high pure UHP N
Bangalore India) were used for CO oxidation and WGS reaction.
Direct CO oxidation from the materials were investigated in a
2
O
3
in powder XRD pattern. All the peaks could be indexed to
cubic fluorite and structures were refined by Rietveld method
34
using FullProf program Because of the substitution of smaller
3
+
˚
35
4+
0 mesh sizes to load in the micro reactor of length 30 and
.4 cm internal diameter. Calibrated 10% CO/N , 10% O /N
, (UHP) H , and UHP CO (Chemix,
Fe (ionic radii in 8 coordination = 0.78 A) for Ce (ionic
35
˚
2
2
2
,
radii = 0.97 A) in CeO
2-d (0 £ x £ 0.45) is observed with increasing
Fe ion substitution (Table 1). Decrease in lattice parameter
2-d (0 £ x £ 0.45) is almost linear with increasing
substitution (x) and follows Vegard’s law.
2
, systematic decrease in lattice parameter
2
2
2
x
of Ce1-xFe O
3
+
x
of Ce1-xFe O
similar microreactor employing 1 vol% CO balanced in N with
2
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 10768–10780 | 10769

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Table 1 Structural parameters of Ce1-xFe
x
O
2-d (x = 0, 0.15, 0.25, 0.33 and 0.45) and Ce0.65Fe0.33Pd0.02O
2-d
Lattice
Parameter (a)
Crystallite sizes/nm from
XRD & (TEM)
2
Compound
R
f
R
bragg
R
p
R
wp
R
exp
c
CeO
2
5.413(1)
5.384(1)
5.346(2)
5.339(1)
5.319(2)
5.327(2)
5.378(1)
5.346(1)
5.368(1)
5.374(2)
0.50
0.76
0.42
0.40
0.86
0.98
0.37
0.60
0.21
0.56
0.65
1.05
0.70
0.70
1.44
1.38
0.65
0.89
0.39
0.88
6.32
5.23
5.50
6.08
7.4
5.65
5.52
7.85
5.86
6.99
7.12
6.41
6.16
7.19
8.9
7.07
6.80
9.91
7.02
8.47
6.63
5.33
5.72
6.38
8.1
6.67
6.31
9.13
6.65
8.60
1.14
1.43
1.15
1.27
1.20
1.12
1.16
1.17
1.12
0.97
3.5
3.1
3.1
3.2 (~4)
3.1
4.5
3.6
3.7
3.1 (~4)
4.2
Ce0.85Fe0.15
Ce0.75Fe0.25
Ce0.67Fe0.33
Ce0.55Fe0.45
Ce0.55Fe0.45
Ce0.67Fe0.33
Ce0.67Fe0.33
O
O
O
O
O
O
O
1.925
1.875
1.835
1.775
◦
1.775 heated in air up to 600 C for 5 h
1.835 reduced in H
1.835 heated in air up to 600 C for 5 h
◦
2
up to 500 C
◦
Ce0.65Fe0.33Pd0.02
Ce0.65Fe0.33Pd0.02
O
O
1.815
◦
1.815
2
H reduced up to 450 C
sizes are found to be in the range of 3 to 5 nm. Crystallite size
obtained for Ce1-xFe
x
O
2-d from powder XRD data along with
2
lattice parameters, R
in Table 1.
f
, Rbragg, R
p
, Rwp
Rexp and c are summarized
Bright field and HRTEM images of Ce0.67Fe0.33O2-d are shown
in Fig. 3(a, b) Electron diffraction pattern and enlarged view of
lattice fringes are shown in the inset of Fig. 3(a, b) respectively.
Ring pattern is assigned to fluorite structure and width of lattice
˚
fringes are 3.10 A which agrees well with d111 planes of fluorite
lattice (Ce0.67Fe0.33
O2-d ). Absence of diffraction lines and lattice
fringes due to Fe
2
O
3
phase suggest that Fe substituted ceria
is crystallized to fluorite structure only. Bright field image and
HRTEM image also showed that the compound is porous and
individual crystallites of 3–4 nm are well interconnected with
each other. The SEM image of Ce0.67Fe0.33
crystallites are well interconnected and contain pores of 50–100 nm
Fig. 4(a, b)). BET Surface area and porosity measurements
2-d confirmed that the compound is
porous and the surface area and average pore diameter of the
O1.85 also showed that
(
of as prepared Ce0.67Fe0.33
O
2
-1
compound were found to be 167 m g and ~2.5 nm respectively
Supporting Figure S1 (a)†). Thus Ce0.67Fe0.33 2-d crystallites from
(
O
the sonication method are porous in nature and hierarchically
structured with individual crystallites of 3–4 nm.
Thermal stability of materials was studied and the compounds
◦
(
Ce1-xFe
x
O
2-d ) are found to be stable up to 600 C in all composi-
tions. Rietveld refined powder XRD profiles of Ce0.55Fe0.45
O
2-d and
◦
Ce0.67Fe0.33
O2-d heated in air at 600 C for 10 h are shown in Fig. 2(b,
x
Fig. 1 Powder XRD pattern of Ce1-xFe O2-x/2 (x = 0.0, 0.15, 0.25, 0.33
and 0.45) in (a–e) respectively.
c) Reflections corresponding to any of the Fe
2
3
O phases are not
detected in the XRD patterns. Structural parameters derived from
3
+
We have been able to synthesize up to 45% of Fe ion substi-
tution in CeO by the sonication method. We have focused our
Rietveld analysis are summarized in Table 1. Bright field and
◦
2
HRTEM images of Ce0.67Fe0.33
O
2-d heated at 600 C for 10 h are
2-d
studies on Ce0.67Fe0.33O2-d and 2% Pd substituted Ce0.67Fe0.33O ,
shown in Fig. 3(c, d). The compound retained the porous structure
and individual crystallites of 6–7 nm are well connected with each
other. Electron diffraction pattern and enlarged view of lattice
fringes are shown in the insets of Fig. 3(c, d) respectively. The
ring pattern is assigned to fluorite structure and the lattice fringes
Ce0.65Fe0.33Pd0.02
O
2-d . The Rietveld refined powder XRD pattern of
2-d is given in Fig. 2(a). The refined X-ray profiles are
well fitted with the observed data. Crystallite sizes were estimated
Ce0.67Fe0.33
O
36
by following the Scherrer formula,
˚
are of 3.11 A which agrees well with d111 planes of fluorite lattice
Crystallite size (d) = 0.9l/bcosq
(1)
(Ce0.67Fe0.33
O2-d ). Absence of diffraction lines and lattice fringes
where l is the wavelength of X-ray, b is the full width at half
maxima (FWHM) in radian and q is the diffraction angle. FWHM
was estimated by the equation,
due to any of the Fe O phases suggest that material is stable up
2
3
◦
to 600 C. BET Surface area and porosity measurements of as
prepared Ce0.67Fe0.33O2-d confirmed that the compound is porous
and the surface area and average pore diameter of the compound
37
2
1/2
b = (Utan q + Vtanq + W)
(2)
2
-1
were found to be 53 m g and ~3.5 nm respectively (Supporting
By taking the U, V, W values from the Rietveld refinement data
for (111), (200), (220) and (311) diffraction lines, the crystallite
Figure S2 (b)†). Further, the SEM image of Ce0.67Fe0.33
at 600 C for 10 h also showed that crystallites remain well
O
2-d heated
◦
1
0770 | Dalton Trans., 2010, 39, 10768–10780
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◦
Fig. 2 Rietveld refined XRD profile of (a) as prepared Ce0.67Fe0.33
O
1.835, (b) Ce0.55Fe0.45
O
1.775 heated in air at 600 C for 10 h, (c) Ce0.67Fe0.33O1.835 heated in
◦
◦
air at 600 C for 10 h and (d) Ce0.67Fe0.33
O
1.835 reduced in H
2
up to 500 C.
3
+
interconnected with microscopic pores of ~50 nm (Fig. 4(c)). Thus
of Ce0.67Fe0.33O2-d samples also confirm Fe ion substitution in the
the hierarchical structure of the compound remains stable even
CeO
2
lattice.
◦
after sintering at 600 C for 10 h.
Rietveld refined powder XRD pattern of Ce0.65Fe0.33Pd0.02O2-d is
3
+
Substitution of Fe ion in CeO
2
was also examined by
shown in Fig. 6(a) All the diffraction peaks could be indexed
to fluorite structure only and no impurity peaks or residual
intensities are observed corresponding to Pd metal or PdO in the
Rietveld refined XRD pattern. Bright field and HRTEM images
UV-visible spectroscopy. UV-visible spectra of (a) CeO
2
, (b)
◦
Ce0.67Fe0.33
O
2-d (as prepared), (c) Ce0.67Fe0.33
O2-d heated at 550 C
◦
for 48 h, (d) Ce0.67Fe0.33
O2-d heated at 800 C for 5 h and (e) g-Fe
2
O
3
are shown in Fig. 5. g-Fe
near to 865 nm, 661 nm and 540 nm. CeO
sonication method showed a strong absorption band near 358 nm
2
O
3
(curve e) shows three distinct bands
of Ce0.65Fe0.33Pd0.02O2-d are shown in Fig. 3(e, f) Crystallites are
2
made by the same
porous structured with individual crystallites of 3–4 nm, which is
close to the sizes obtained from the Rietveld refined XRD data
(Table 1). The inset of Fig. 3(e) shows the ring type diffraction
pattern and the inset of Fig. 3(f) shows an enlarged view of
30
and adsorption edge near to 462 nm. UV-visible absorption of
2-d (curve b) shows a broad absorption with band edge
Ce0.67Fe0.33
O
at 801 nm and no distinct peaks were seen due to g-Fe
heating at 550 C for 48 h also, there were no change in spectra
2
O
3
. On
the lattice fringes of Ce0.65Fe0.33Pd0.02O2-d . The electron diffraction
pattern was indexed to fluorite structure and lattice fringes of
˚
3.11 A corresponding to d111 of fluorite Ce0.65Fe0.33Pd0.02O .
2-d
◦
(
curve c) and an absorption edge was observed at 750 nm. Since
3
+
Fe is substituted in fluorite lattice, distinct Fe
2
O
3
type absorption
Absence of diffraction rings and lattice fringes, corresponding
to Fe and PdO or Pd suggest that Fe and Pd are substituted
in CeO . EDX study has been carried out over the lattice fringes
38
bands were not observed, only broad absorption is observed.
2
O
3
◦
UV-visible spectra of Ce0.67Fe0.33
O
2-d heated at 800 C for 5 h
2
(
5
curve d) showed distinct absorption bands near 876 nm, 669 nm,
(111 planes) of nanocrystallites by the EDX probe in the TEM
instrument (Fig. 3(g)) and indeed Ce, Fe and Pd were found
in the ratio of 0.65 : 0.32 : 0.03, which agrees well with the
composition taken for the preparation. BET surface area and
porosity measurements of as prepared Ce0.65Fe0.33Pd0.02O2-d showed
that surface area and average pore diameter of the compound are
31 nm similar to g-Fe
2
O
3
(curve e). This because Ce0.67Fe0.33
O
2-d
◦
◦
◦
is stable up to 600 C only, when heated at above 600 C (800 C
in this case) it decomposes to Ce1-xFe
why the UV-visible spectra of Ce0.67Fe0.33
contain the absorption band for g-Fe
x
O
O
2-d + gFe
2-d heated at 800
2
O
3
. That is
◦
C
2
O
3
. Thus UV-visible spectra
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The
Ce0.65Fe0.33Pd0.02
electronic
2-d were investigated by X-ray photo-electron
spectroscopy. Core level Fe(2p) spectra of Ce0.67Fe0.33
2-d are shown in Fig. 7(a, b) In both cases
structure
of
Ce0.67Fe0.33
O
2-d
and
O
O
2-d and
Ce0.65Fe0.33Pd0.02
O
binding energy of Fe(2p3/2) is observed at 710.4 eV and a weak
satellite at 8 eV from the main peak confirms that Fe is in +3
39
state. Ce(3d) spectra of Ce0.67Fe0.33
O2-d and Ce0.65Fe0.33Pd0.02O
2-d
are shown in Fig. 8(a, b). Ce (3d5/2) peak was observed at 882.7 eV
along with satellite peaks at 6.4 and 16 eV from the main peak,
4
+
40
which are the characteristic peaks of Ce in CeO
in +4 state in Ce0.67Fe0.33
2
.
Thus Ce is
2-d . Core level
2-d is shown in Fig. 9(a).
Pd(3d5/2) binding energy is observed at 337.6 eV, which is higher
O2-d and Ce0.65Fe0.33Pd0.02O
Pd(3d) spectrum of Ce0.65Fe0.33Pd0.02
O
2
+
41
than the binding energy of Pd (3d5/2) in PdO but lower than
2
+
19
the binding energy of Pd (3d5/2) in PdCl
2-d . Accordingly formulae of the compounds
can be written as Ce0.67Fe0.33
2
So Pd is in +2 state
in Ce0.65Fe0.33Pd0.02
O
O
1.835 and Ce0.65Fe0.33Pd0.02
O
1.815
.
An estimate of relative concentrations of Ce and Fe and Ce : Fe :
Pd were carried out from the intensities of Fe(2p) and Ce(3d) and
Pd (3d) peaks in Ce0.67Fe0.33
surface concentration is calculated from the formula:
O
2-d and Ce0.65Fe0.33Pd0.02
O
2-d . Relative
4
2
Relative concentration C
/l .s .D )/R(I /l .s
M
=
.D
(
3)
(
I
M
M
M
M
M
M
M
M
)
where I
Fe(2p) and Pd (3d)), l
photoelectrons, s is the photoionization cross section, and D
is the geometric factor. The photoionization cross-section values
M
is the integrated intensity of the core levels (M = Ce(3d),
M
is the mean escape depth of the respective
M
M
43
were taken from Scofield’s data and mean escape depths were
44
taken from Penn’s data. The geometric factor was taken as 1,
because the maximum intensity in this spectrometer is obtained
◦
at 90 . Surface concentrations of Ce and Fe are found in the
ratio of 0.68 : 0.32 in Ce0.67Fe0.33
O2-d and surface concentrations
of Ce, Fe and Pd were found in the ratio of 0.67: 0.30 : 0.03 in
Ce0.65Fe0.33Pd0.02
O2-d . Thus surface compositions of Ce0.67Fe0.33
O
2-d
and Ce0.65Fe0.33Pd0.02
O2-d are almost same as the bulk composition.
(
b) OSC
Redox properties or OSC of Ce1-xFe
x
O2-d and Ce0.65Fe0.33Pd0.02
O
1.815
were studied by H /TPR or hydrogen uptake. Hydrogen up-
2
take measurements were performed repeatedly by reducing the
◦
compound in H
2
up to 550 C followed by oxidation in O
2
at
◦
350 C. H
2
/TPR plots of Ce0.67Fe0.33
O2-d for 6 consecutive cycles
Fig. 3 (a) Bright field image and (b) HRTEM image of Ce0.67Fe0.33
and indexed electron diffraction pattern and enlarged view of lattice fringes
O
2-d
are given in Fig. 10(a-f) and OSC is fully reversible. OSC of all
compositions of Ce1-xFe O are given in Table 2 and increase
x
2-d
in the inset of (a, b) respectively. (c) Bright field image and (d) HRTEM
with increase in Fe substitution. Ce0.67Fe0.33
O2-d releases ~0.31
◦
image of Ce0.67Fe0.33
O
2-d heated at 600 C for 10 h and indexed electron
[
O]/mole of compound or OSC equivalent to 2205 mmol of [O]/g.
diffraction pattern and enlarged view of lattice fringes in the inset of (c, d)
respectively. (e) Bright field and (f) HRTEM image of Ce0.65Fe0.33Pd0.02
and indexed electron diffraction pattern and enlarged view of lattice fringes
in the inset of (e, f) respectively. (g) EDX analysis pot of Ce0.65Fe0.33Pd0.02
nanocrystallites.
◦
OSC of Ce0.67Fe0.33O2-d heated at 600 C for 10 h was also studied
O
2-d
and OSC was found to ~0.3 [O]/mole of compound (Fig. 10(g)).
Maximum OSC expected for Ce0.5Zr0.5 is 0.25 mol of [O]/mol
or 1693 mmol g and the highest OSC realized experimentally for
O
2
O
2-d
-
1
◦
-1 22
Ce0.5Zr0.5
O up to 600 C is 0.22 mol of [O]/mol or 1500 mmol g .
2
2
-1
1
69 m g and ~3 nm respectively (Supporting Figure S1(c)†). The
The high OSC (0.31 [O]/mole) of Ce0.67Fe0.33
Ce0.67 Fe0.33 O1.835 is reduced to Ce0.38 Ce0.29 Fe0.33 O1.525 state.
O
2-d suggests that
4
+
3+
4+
3+
2+
SEM image of as prepared Ce0.65Fe0.33Pd0.02
O2-d also showed that
45
the crystallites are interconnected and contain microscopic pores
of 50–100 nm (Fig. 4(d)). Thus TEM, SEM and BET surface
area and porosity measurements confirmed that the compound is
porous and hierarchically structured with individual crystallites of
OSC of g-Fe
2
O
3
synthesized by solution combustion method
equivalent to amount of
2-d was given in Fig. 10(a) Fe
was also studied and OSC plot of gFe
2
O
3
3
+
Fe ions present in Ce0.67Fe0.33
O
2
3+
O
3
◦
is reduced to Fe
2
O2.65 state up to 600 C. It means not all Fe is
2
+
3+
3
–4 nm.
reduced Fe state in Fe
2
3 x
O whereas in Ce1-xFe O2-d , all Fe ions
1
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◦
Fig. 4 (a, b) SEM images of as prepared Ce0.67Fe0.33
O
2-d , (c) SEM image of Ce0.67Fe0.33
O
2-d heated at 600 C for 10 h and (d) SEM images of as prepared
Ce0.67Fe0.33Pd0.02
2-d
O .
2
+
4+
are reduced to Fe state along with partial reduction of Ce to
Ce state.
3
+
Electronic states of H reduced sample (Ce0.67Fe0.33O1.525) were
2
examined by X-ray photo-electron spectroscopy. Core level Fe(2p)
spectrum of reduced sample is shown in Fig. 7(c). Fe(2p) binding
energy is observed at 709.9 eV and an almost negligible satellite
3
9
peak 8 eV from the main peak confirms that Fe is in +2 state.
4
+
Ce(3d) core level spectrum is shown in Fig. 8(c). The Ce (3d5/2
)
peak observed at 882.7 eV along with satellite peaks at 6.4 and
4
+
40
1
6 eV below the main peak are characteristic of Ce in CeO .
2
3
+
Ce (3d5/2) in Ce
2
O is characterized by Ce(3d5/2) at 883.3 eV
along with an intense satellite at 887.1 eV. Thus filling of the
valley between Ce (3d5/2) at 882.7 eV and its satellite at 889.1 eV
Fig. 8(c)) confirmed that Ce is in a mixed valence (+4 and +3)
state in H 2-d . Thus XPS of Ce(3d) and
3
40
4
+
(
2
reduced Ce0.67Fe0.33O
3
+
4+
Fe(2p) also confirmed that both Fe and Ce were reduced in H
2
◦
uptake experiments at such a low temperature (~ 400 C). Rietveld
refined powder XRD pattern of H
1.525 is shown in Fig. 2(d). FeO or Fe
out during H reduction. Due to the presence of Fe and Ce
state in H
2
reduced Ce0.67Fe0.33O1.835 viz
Ce0.67Fe0.33
O
2
O is not separated
3
2
+
3+
2
2
reduced sample, an increase in lattice parameter of
Fig. 5 UV-visible spectra of (a) CeO
2
, (b) as prepared Ce0.67Fe0.33
O
2-d , (c)
˚
◦
◦
Ce0.67Fe0.33
Ce0.67Fe0.33
O
1.525 (a = 5.378 A) is observed compared to as prepared
Ce0.67Fe0.33
O
2-d heated at 500 C, (d) Ce0.67Fe0.33
O
2-d heated at 800 C and
˚
O1.835 (a = 5.339 A).
(
2 3
e) g-Fe O .
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Table 2 OSC data for Ce1-xFe
x
O
2-d (x = 0, 0.15, 0.25, 0.33 and 0.45) and Ce0.65Fe0.33Pd0.02O
2-d
Composition
-1
◦
Compound
Before H
2
reduction
After H
2
reduction
OSC/mmol g up to 550 C
4
+
4+
3+
CeO
2
Ce O
2
Ce 0.74Ce 0.26
O
187
2+
745
4+
3+
4+
3+
Ce0.85Fe0.15
Ce0.75Fe0.25
Ce0.67Fe0.33
Ce0.55Fe0.45
O
O
O
O
1.925
1.875
1.835
1.775
Ce0.85 Fe0.15
Ce0.75 Fe0.25
Ce0.67 Fe0.33
Ce0.55 Fe0.45
O
O
O
O
1.925
1.875
1.835
Ce 0.59Ce 0.26Fe 0.15
O
O
O
O
1.71
1353
1883
2205
2286
4+
3+
3+
3+
3+
4+
3+
2+
2+
Ce0.45 Ce0.30 Fe0.25
1.595
1.525
1.475
4+
4+
3+
Ce0.38 Ce0.29 Fe0.33
4+ 3+ 2+
4+
1.775
2+
Ce 0.40Ce 0.15Fe 0.45
4+ 3+ 3+
4+
◦
Ce0.65Fe0.33Pd0.02
O
1.815
Ce0.65 Fe0.33 Pd 0.02
O
1.815
Ce 0.26Ce 0.39Fe 0.33Pd0.02
O
1.60
1526 up to 200 C
Fig. 7 Core level Fe(2p) spectra in (a) as prepared Ce0.67
0.33
F O2-d , (b) as
prepared Ce0.65Fe0.33Pd0.02
O
2-d , (c) Ce0.67
F
0.33
O
2-d reduced in hydrogen up to
◦
◦
5
00 C, (d) Ce0.65Fe0.33Pd0.02
O
2-d reduced in hydrogen up to 500 C and (e)
Ce0.65Fe0.33Pd0.02O2-d after chronoamperometry experiment at 1.2 V.
3
+
→ 2Ce + O2-
4+
2
Ce +O
2
(5)
So redox couples play a crucial role in reversible OSC of
Fig. 6 Rietveld refined Powder XRD pattern of Ce0.65Fe0.33Pd0.02
O
2-d (a)
◦
Ce0.67Fe0.33O1.835
.
as prepared and (b) reduced in H
2
up to 500 C.
Having studied the reversible OSC of Ce0.67Fe0.33
1.815 was also studied by H /TPR to see
the effect of Pd ion substitution on OSC. H /TPR plot of
1.815 is shown in Fig. 11(a) and plots for same
1.835 are also given in Fig. 11(b,
O1.835, OSC
of Ce0.65Fe0.33Pd0.02
O
2
Reversibility of OSC of Ce0.67Fe0.33
O1.835 can be explained
2
+
3
+
2+
2
on the basis of redox potentials of Fe /Fe (0.77 V) and
4
+
3+
Ce0.65Fe0.33Pd0.02O
Ce /Ce (1.61 V). In general transition metal ion substituted
CeO are reduced at a lower temperature than pure CeO
Preferential absorption of H or CO on transition metal ions
amount of CeO and Ce0.67Fe0.33O
2
2
2
.
c) respectively for comparison. Interestingly a single sharp
2
◦
20,28,30
hydrogen uptake occurs below 150 C with a reduction peak at
seems to be the reason for this.
Thus in the presence of H
2
,
◦
3
+
3+
125 C. Hydrogen uptake of the material, Ce0.65Fe0.33Pd0.02
O
1.815
due to preferential adsorption of hydrogen on Fe , first Fe will
◦
2
+
3+
2+
heated at 600 C for 24hwas alsostudiedandthere was no decrease
be reduced to Fe and due to lower redox potentials of Fe /Fe
◦
4
+
3+
in OSC except the sharp reduction peak is shifted to 165 C from
than the redox potentials of Ce /Ce the following redox reaction
occurs:
◦
1
25 C (Supporting Figure S2†). OSC of Ce0.65Fe0.33Pd0.02
O
1.815
-
1
◦
is found 1526 mmol g up to 200 C or equivalent to release of
2
+
4+
3+
3+
0.215 [O]/mol of compound. H/Pd ratio is found to be as high as
Fe + Ce → Fe + Ce
(4)
2
+
◦
2
1.5. Ideally it should be equal to 2 if Pd is reduced to Pd . This
3
+
2+
The reduced Ce is oxidized by feed oxygen:
higher H/Pd ratio suggests hydrogen spill-over from Pd ion to
1
0774 | Dalton Trans., 2010, 39, 10768–10780
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Fig. 8 Core level Ce(3d) spectra in (a) as prepared Ce0.67
0.33
F O2-d , (b) as
Fig. 10 (a–f) H
2
/TPR or hydrogen uptake profile of Ce0.67Fe0.33
O
1.835 for
prepared Ce0.65Fe0.33Pd0.02
O
2-d , (c) Ce0.67
F
0.33
O
2-d reduced in hydrogen up to
◦
◦
6 consecutive cycles and (g) hydrogen uptake of Ce0.67Fe0.33
O
1.835 heated at
5
00 C, (d) Ce0.65Fe0.33Pd0.02
O
2-d reduced in hydrogen up to 500 C and (e)
◦
6
00 C for 24 h. H
2
/TPR of equivalent mol of Fe
2
O
3
is also given in (a)
Ce0.65Fe0.33Pd0.02O2-d after chronoamperometry experiment at 1.2 V.
for comparison.
Fig. 11
b) CeO
H
2
/TPR or hydrogen uptake profile of (a) Ce0.65Fe0.33Pd0.02
1.815
O ,
(
2
and (c) Ce0.67Fe0.33
1.835
O .
3
+
4+
neighboring Fe and Ce ions in Ce0.65Fe0.33Pd0.02
O
1.815. Though
Fe and Ce reduction occurs at ~400 C in Ce0.67Fe0.33
no separate reduction peak is observed for Ce0.65Fe0.33Pd0.02
3
+
4+
◦
O
O
1.835
,
1.815
◦
3+
4+
up to 500 C. This clearly indicates Fe and Ce ions are also
reduced along with Pd ions in Ce0.65Fe0.33Pd0.02
2
+
2+
O1.815. Thus Pd
ion substitution in Ce1-xFe 2-d induces the synergistic effect by
x
O
4
+
3+
bringing down the H
ions.
2
reduction temperature of Ce and Fe
Hydrogen reduced Ce0.65Fe0.33Pd0.02
O1.815 i.e. Ce0.65Fe0.33Pd0.02
O
1.6
was black in color but as soon it was exposed to air, it became
hot and regained the original brown color suggesting that the
compound is oxidized in air. Therefore XRD and XPS of the
Fig. 9 XPS of Pd(3d) core level in Ce0.65Fe0.33Pd0.02
O
2-d (a) as prepared,
◦
(
b) reduced in hydrogen up to 500 C and (c) after chronoamperometry
experiment at 1.2 V.
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2
+
3+
hydrogen reduced samples were recorded by first cooling it by
liquid N followed by covering the surface with liquid paraffin
to avoid air oxidation as much as possible. Rietveld refined
exposed to air due to fast oxidation of Fe to Fe . Thus due
2
+/0
2
to synergistic interaction of redox couples of Pd (0.89 V) with
3
+/2+
4+/3+
Fe
(0.77 V) and Ce
(1.61 V), such a high OSC was observed
powder XRD pattern of H
2
reduced Ce0.65Fe0.33Pd0.02
O
1.815 up to
at lower temperature in Ce0.65Fe0.33Pd0.02
Further to observe the synergistic redox interaction of
O
1.815
.
◦
500 C is shown in Fig. 6(b) and impurity peaks corresponding
2
+/0
3+/2+
4+/3+
to Pd metal, PdO, Fe metal or FeO
increase in lattice parameter of H
x
were not observed. An
Pd (0.89 V), Fe
(0.77 V) and Ce
(1.61 V) redox couples
1.815, electrochemical O
evolution experiments were carried out with Ce0.67Fe0.33
1.815 electrodes. Electrochemical O evolution was
studied by cyclovoltammetry and chronoamperometry experi-
ments in 0.5 M H SO using Pt as counter electrode and saturated
2
reduced Ce0.65Fe0.33Pd0.02
˚
O
1.815
in Ce0.67Fe0.33
O
1.835 and Ce0.65Fe0.33Pd0.02
O
2
viz Ce0.65Fe0.33Pd0.02
O1.60 (a = 5.374 A) was observed compared to
O1.835 and
˚
as prepared Ce0.65Fe0.33Pd0.02
O
1.815 (a = 5.368 A). Core level Pd (3d)
Ce0.65Fe0.33Pd0.02
O
2
spectrum of the H
resolved into Pd and Pd states. Binding energy for Pd (3d5/2
is observed at 335.1 eV and binding energy for Pd (3d5/2) was
observed at 337.2 eV. Thus Pd is present in mixed valence 0 and
2
reduced sample is shown in Fig. 9(b) and is
◦
2+
◦
)
2
4
19
2+
calomel electrode (SCE) as a reference electrode. A working
electrode was made by mixing a known amount of catalyst
with 45 wt% of graphitic carbon and 5 wt% of PVDF (poly-
vinyledenedifluoride) binder followed by depositing a thin layer of
+
2 states in the ratio of 35 : 65 in the H reduced sample. Core level
2
Fe(2p) spectrum of the reduced sample is shown in Fig. 7(d). The
Fe(2p) main peak is observed at 710.4 eV and a satellite peak 8 eV
from the main peak confirms that Fe was in 3+ state. Ce(3d) core
level spectrum is shown in Fig. 8(d) and cerium ion was present in
mixed valance +3 and +4 oxidation states.
2
this slurry on a graphite paper over an area of 1 cm . The cyclic
voltammograms for CeO
2
, Ce0.98Pd0.02
O2-d , Ce0.67Fe0.33O1.835 and
Ce0.65Fe0.33Pd0.02
in Fig. 13(a–d). Ce reduction starts over ~1 V in pure CeO
O
1.815 in 0.5 M H
2
SO
4
from 0.0 V to 1.2 V are shown
. In the
case of Ce0.98Pd0.02 2-d a reduction peak is observed at ~0.8 V, in the
case of Ce0.67Fe0.33O1.835 and Ce0.65Fe0.33Pd0.02O1.815 a reduction peak
4
+
2
Hydrogen uptake measurements were performed repeatedly by
O
◦
◦
reducing in H
to be fully reversible. Hydrogen uptake for five consecutive cycles
are shown in Fig. 12(a–e). Reversible OSC of Ce0.65Fe0.33Pd0.02
2
up to 200 C and oxidizing in O
2
at 50 C and found
is observed at ~0.7 V. No separate reduction peak is observed with
2
+
O
1.815
Ce0.65Fe0.33Pd0.02
O
1.815 electrode suggesting that reduction of Pd ,
2
+/0
3+
4+
can be explained on the basis of redox potentials of Pd (0.89 V),
Fe and Ce is occurring simultaneously which is also reflected
3
+/2+
4+/3+
Fe
(0.77 V) and Ce
(1.61 V). Due to the lower reduction
in terms of a higher current for the Ce0.65Fe0.33Pd0.02
O1.815 electrode
3
+/2+
2+/0
4+/3+
potential of Fe
compared to Pd
and Ce
, it is prone to
compared to Ce0.98Pd0.02 1.835 and CeO
O
2-d , Ce0.67Fe0.33
O
2
. Further
evolution is confirmed
by chronoamperometry. Chronoamperometry plots of oxygen
evolution at 1.2 V for CeO
faster oxidation and Pd can act as bridge between Fe and Ce in
terms of fast electron transfer; the redox reaction can be written
as:
higher electro-oxidation or higher O
2
2
(a), Ce0.98Pd0.02
O
2-d (b), Ce0.67Fe0.33
O
1.835
(
c) and Ce0.65Fe0.33Pd0.02
O
1.815 (d) in 0.5 M H
2
SO
shown in the inset of Fig. 13. Higher current density is ob-
1.815 compared to the same amount
1.835. So similar to H
uptake measurements (H/Pd = 21.5 for Ce0.65Fe0.33Pd0.02
much higher current density in electrochemical O evolution for
Ce0.65Fe0.33Pd0.02O1.815 electrode is observed compared to other
CeO 1.835 electrodes. Thus the
4
solution are
2
+
2+
3+
◦
2
Fe + Pd → 2Fe + Pd
(6)
(7)
served with Ce0.65Fe0.33Pd0.02
of CeO
O
0
4+
2+
3+
Pd + 2Ce → Pd + 2Ce
and Ce is further oxidized by feed oxygen
2
, Ce0.98Pd0.02 2-d and Ce0.67Fe0.33
O
O
2
3
+
O1.815),
2
3
+
→ 2Ce + O2-
reduced sample (Fig. 6(d)) also suggests
4+
2
Ce + O
2
(8)
, Ce0.98Pd0.02O2-d and Ce0.67Fe0.33O
2
+
2
The absence of Fe in H
2
that Pd ion is enhancing or accelerating the electron transfer from
2
+
4+
Fe to Ce and that is why the compound also gets hot when
2
Fig. 13 Cyclic voltammogram for oxygen evolution with (a) CeO , (b)
Ce0.98Pd0.02
O
2-d , (c) Ce0.67Fe0.33
O
1.835 and (d) Ce0.65Fe0.33Pd0.02
, (b) Ce0.98Pd0.02
1.815 in 0.5 M H
O
1.815 and, in
Fig. 12 Hydrogen uptake of Ce0.65Fe0.33Pd0.02
O
1.815 for five consecutive
inset, chronoamperometry plot with (a) CeO
2
O
2-d , (c)
cycles in (a–e).
Ce0.67Fe0.33
O
1.835 and (d) Ce0.65Fe0.33Pd0.02
O
2 4
SO .
1
0776 | Dalton Trans., 2010, 39, 10768–10780
This journal is © The Royal Society of Chemistry 2010

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3
+
2+
absence of separate reduction peaks for Fe and Pd in the
cyclic voltammogram and highest current density for O evolution
1.815 electrode can be attributed to synergistic
2
with Ce0.65Fe0.33Pd0.02
O
2
+/0
3+/2+
4+/3+
interaction of Pd
couples in Ce0.65Fe0.33Pd0.02
redox couples with Fe
1.815. Oxygen evolution reaction with
1.815 electrode can be written as
and Ce
redox
O
Ce0.65Fe0.33Pd0.02
O
4
+
3+
2+
Ce 0.65Fe 0.33Pd 0.02
O
1.815 + H O ¤
2
4
+/3+
3+/2+
2+/0
+
Ce
0.65Fe
0.33Pd
0.02
O
1.815-z + 2H +2 e¯ + 1/2(1 + z)O
2
(9)
(
z ~ 0.2)
Once the steady state with z = 0.2 is reached, further O
occurs due to the water splitting:
2
evolution
(10)
+
-
2
H
2
O → O
To observe the high reducibility of Ce0.65Fe0.33Pd0.02
states of Ce0.65Fe0.33Pd0.02 1.815 electrode after chronoamperometry
2
+ 4H + 4e
O
1.815, electronic
O
experiment at 1.2 V were examined by X-ray photo-electron spec-
troscopy. The electrodes were taken out from the electrochemical
◦
cell, dried at 60 C in a hot air oven and mounted directly inside
the X-ray photo-electron spectroscopy chamber. The core level
Fe(2p) spectrum shown in Fig. 7(e) indicates the presence of Fe
in a mixed valence (+2/+3) state. Ce(3d) core level spectrum is
shown in Fig. 8(e) and Ce was also present in mixed a valence
(
+3/+4) state. Pd(3d) core level spectrum is shown in Fig. 9(c) and
Pd is present in both +2 and 0 oxidation states. Since the catalyst
was coated with graphite powder and deposited on graphite paper
and binder coating, the electrochemically reduced oxide remained
in a reduced state even on exposure to air. Thus XPS study also
confirms the higher reducibility of Ce0.65Fe0.33Pd0.02
in electrochemical oxygen evolution.
O2-d electrode
(
c) CO oxidation
To see the applicability of high OSC in exhaust catalysis, CO
oxidations were carried out with the catalysts both in the presence
of external oxygen and direct utilization of lattice oxygen was
also studied by CO oxidation in absence of feed oxygen. %CO
conversions both in the presence and in the absence of feed
oxygen over Ce0.65Fe0.33Pd0.02
O1.815 and Ce0.67Fe0.33O1.835 are shown
in Fig. 14(a). Complete conversion of CO to CO
2
was achieved
◦
◦
below 85 C with Ce0.65Fe0.33Pd0.02
O1.815 and below 250 C with
Ce0.67Fe0.33
O
1.835 in the presence of feed oxygen. Utilization of
lattice oxygen was shown with both catalysts for CO oxidation
in the absence of feed oxygen. In the case of Ce0.67Fe0.33 ,
O
1.835
CO conversion in the absence of feed oxygen is observed in a
◦
broad temperature range (150 to 550 C) and it increases with
increasing temperature suggesting that lattice oxygen is activated
by Fe ion substitution in ceria. The amount of lattice oxygen
utilized for CO oxidation is also calculated by taking the area
under the CO oxidation plot and the amount of lattice oxygen
utilized directly by CO was found to 0.31 [O]/mol of compound
which is in close agreement with the OSC observed by H
2
/TPR. In
Fig. 14 (a) CO oxidation with Ce0.67Fe0.33O1.835 and Ce0.65Fe0.33Pd0.02O1.815
the case of Ce0.65Fe0.33Pd0.02
O
1.815, high CO conversion is observed
both in presence and absence of O
with Ce0.65Fe0.22Pd0.02O1.815 at different temperatures and (c) rate (R) vs.
temperature plot and Arrhenius plot (Ln(r) vs. 1000/T/K in the inset
of (c).
2
, (b) % CO conversion vs. W/F
◦
in a narrow temperature range (30–150 C) with a reduction peak
◦
around 90 C. The area under the reduction peak is also calculated
and the amount of lattice oxygen utilized for CO oxidation was
found to be 0.23 [O]/mol of compound which is almost equal to
the OSC of compound observed by H
be reversibly exchanged with lattice oxygen in Ce0.67Fe0.33
1.815 for CO oxidation.
2
/TPR. Thus feed O
2
can
Keeping the flow rate and GSHV same, CO oxidation was also
carried out with different weights of the catalysts to obtained the
actual rates from the equation,
O1.835 and
Ce0.65Fe0.33Pd0.02
O
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Dalton Trans., 2010, 39, 10768–10780 | 10777

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Table 3 Rate and activation energies for CO oxidation and wgs reaction
with different catalysts
-1
Rate/mmol g Ea/kJ
-1
◦
-1
Reaction Catalyst
CO + O 5 wt% Ru/SiO
s
(T/ C)
mol
Reference
2
2
1.00 (110)
0.316 (143)
0.32 (115)
3.9 (120)
94
103
56
9
9
9
25
5
5
wt% Pd/SiO
wt% Pt/SiO
2
2
Ce0.98Pd0.02
O
1.98
121
0
.57(100)
Ce0.95Ru0.05
Ce0.90Ru0.10
O
O
1.97
1.94
2.05(100)
3.3(100)
92.2
43
21
21
Ce0.73Ti0.25Pd0.02
Ce0.83Sn0.15Pd0.02
Ce0.65Fe0.33Pd0.02
O
O
O
2-d
2-d
53.5
82.4
38.0
17
19
1.815 4.1(80)
Present study
Rate(R) = (F/W).x
(11)
-
1
where F is flow of gaseous molecules in mole s , W is the weight
of the catalyst (g) and x is fractional conversion. Fractional
conversion with W/F was plotted at different temperatures as
shown in Fig. 14(b) for Ce0.65Fe0.33Pd0.02
O1.815. The weight of the
catalysts, W, were varied from 25 to 200 mg, whereas F, the
-
1
flow rate (mol s ), was kept constant. The plot is linear up
to nearly 70% conversion, and the reaction rates at different
temperatures determined from the slopes of the linear region are
shown in Fig. 14(c). The Arrhenius plot is also shown in the
inset of Fig. 14(c). Activation energy was also calculated from
-
1
the rates derived from W/F plots and it is equal to 38 kJ mol .
A comparison of rate and activation energy with different catalyst
for CO oxidation is given Table 3. Ce0.65Fe0.33Pd0.02O1.815 is found
Fig. 15 (a) % CO consumption, CO
2
and H
2
formation plots with
Ce0.65Fe0.33Pd0.02
O
1.815, % CO conversion to CO
2
by H
2
O vapors with
CeO
2
, Ce0.85Fe0.15
O
1.925, Ce0.67Fe0.33
O
1.835 and Ce0.65Fe0.33Pd0.02
O
1.815 in (b–e)
superior to the other noble metal ion substituted and metal
respectively. (f) % CO conversion, H
2
and CO
2
formation plots with
and CO
9,17,19,21,25
impregnated catalysts.
Ce0.65Fe0.33Pd0.02
O
1.815 in presence for externally fed H
2
2
.
(
d) WGS reaction
◦
was achieved at 375 C and almost 98% conversion was achieved
◦
3+
at 480 C with Ce0.67Fe0.33
O1.835. Thus with increasing Fe ion
The WGS reaction is an important reaction which produces H
from oxidation of CO with water and the reaction is written as:
2
substitution in CeO
CO by water) was observed which suggests that higher OSC
is beneficial for high conversion in WGS reaction. About 97%
2
, higher WGS activity (CO conversion to
2
-
1
CO + H
2
O → CO
2
+ H
2
(DH = -41.1 kJ mol , DG =
1
(
12)
-
-
28.6 kJ mol )
conversion of CO to CO
observed at 375 C with Ce0.65Fe0.33Pd0.02
exothermic reaction, complete (100%) conversion is not reached
and equilibrium conversion is reached to 97% only with these
catalysts at high temperature (≥375 C). Activation energy for
2
and equivalent H production was
2
◦
Essentially the WGS reaction is a two step reaction, the first
step involves oxidation CO by utilizing the lattice oxygen of
the catalyst and in the second step catalyst is regenerated by
O1.835. Since WGS is an
◦
H
2
O releasing H
showed high OSC both by H
absence of external oxygen they can be good catalysts for H
2
. Since Ce0.67Fe0.33
O
1.835 and Ce0.65Fe0.33Pd0.02
O
1.815
2
/TPR and by CO oxidation in
CO + H
2
O → CO
2
+ H
2
were estimated from the rates for <15%
2
conversion by using Arrhenius’ equation. Activation energies
are found to be 52.1 and 61.3 kJ mol for Ce0.65Fe0.33Pd0.02
-
1
production via WGS reaction. Therefore WGS reactions were
O
1.815
studied with these catalysts. In the product stream, volumes of
and Ce0.67Fe0.33
O1.835 catalysts respectively. To get the actual rate
CO, CO
formation (CO oxidation to CO
consumption plots with Ce0.65Fe0.33Pd0.02
in Fig. 15(a). % CO conversion was over 97% at ≥375 C. Possible
by-products of the reaction such as CH
other hydrocarbons were not detected in the product stream in
the detection limit of 3–5 ppm. CO conversion is almost 100%
2
and H
2
were measured independently in GC and CO
by H O), H formation and CO
2
and activation energy of catalyst (Ce0.65Fe0.33Pd0.02
O1.815), WGS
2
2
2
reactions were carried out with different weights (25, 50, 100, 150
to 200 mg) of the catalyst, keeping the total flow and GSHV the
same. Fractional conversion vs. W/F plot is given in Fig. 16(a).
Rates (R) were derived from slopes of the plot Fig. 16(a) and
O
1.815 catalyst are shown
◦
3
OH, H
2
CO, CH , and
4
◦
rate vs. T/ C is given in Fig. 16(b). Ln(R) vs. 1000(T/K) is given
in the inset of Fig. 16(b) The rate was found to be as high as
-
1
-1
◦
H
2
specific over this catalyst. % CO conversion profiles with
CeO
are shown in Fig. 15(b–e) respectively for comparison. Practically
no conversion observed with pure CeO , almost 10% CO con-
version to CO with equivalent amount of H production was
1.925 and ~74% conversion
27.2 mmol g
s
at 375 C and activation energy was found to be
-
1
2
, Ce0.85Fe0.15
O1.925, Ce0.67Fe0.33
O1.835 and Ce0.65Fe0.33Pd0.02
O
1.815
46.4 kJ mol . To see the viability of the catalyst, WGS reaction
was also studied in the presence of externally fed H
with a gas mixture flow (CO (5 cc/min), CO (10cc/min) and H
(10 cc/min balance in N
same with 200 mg of catalyst. Up to 35% CO conversion to CO
2
and CO
2
2
2
2
2
2
2
) keeping total gas flow and GSHV the
◦
observed at 375 C with Ce0.85Fe0.15
O
2
1
0778 | Dalton Trans., 2010, 39, 10768–10780
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couples is also reflected in high current density in electrochemical
O evolution.
2
Acknowledgements
Authors thank Dept. of Science and Technology, Govt. of India
for financial support. We also thank Amit Mondal and Sharad
Kumar of Nano centre, Indian Institute of Science Bangalore for
recording TEM images
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