Synthesis, structure and oxygen-storage capacity of Pr1-xZrxO2-δ and Pr1-x-yPdyZrxO2-δ

Article abstract of DOI:10.1016/j.materresbull.2007.10.033

Nanocrystalline Pr_1-xZr_xO_2-δ (0 ≤ x ≤ 1) and Pr_1-x-yPd_yZr_xO_2-δ (x = 0.50, y = 0.02) solid solutions have been synthesized by a single step solution combustion method. The whole range of solid solution compositions crystallize in cubic fluorite structure. The lattice parameter 'a' linearly varied up to x = 1.0. Oxygen-storage capacity (OSC) and redox properties of Pr_1-xZr_xO_2-δ (0.0 ≤ x ≤ 0.8) solid solutions have been investigated by temperature-programmed reduction (TPR) and are compared with those of Ce_1-xZr_xO₂. Pr_1-xZr_xO_2-δ exhibited H₂ uptake and CO oxidation at a lower temperature than Ce_1-xZr_xO₂. Small amount of Pd ion (y = 0.02) substitution was found to bring down the temperature of oxygen release-storage significantly.

Full text of DOI:10.1016/j.materresbull.2007.10.033

Materials Research Bulletin 43 (2008) 2658–2667
www.elsevier.com/locate/matresbu
Synthesis, structure and oxygen-storage capacity of
Pr1ꢀxZrxO2ꢀd and Pr1ꢀxꢀyPdyZrxO2ꢀd
Manjunath B. Bellakki a, C. Shivakumara a, Tinku Baidya a,
A.S. Prakash b, N.Y. Vasanthacharya a, M.S. Hegde a,
*
a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
b Functional Materials Division Central Electrochemical Research Institute, Karaikudi 630006, Tamil Nadu, India
Received 25 April 2007; received in revised form 18 August 2007; accepted 29 October 2007
Available online 4 November 2007
Abstract
Nanocrystalline Pr1ꢀxZrxO2ꢀd (0 ꢁ x ꢁ 1) and Pr1ꢀxꢀyPdyZrxO2ꢀd (x = 0.50, y = 0.02) solid solutions have been synthesized by
a single step solution combustion method. The whole range of solid solution compositions crystallize in cubic fluorite structure. The
lattice parameter ‘a’ linearly varied up to x = 1.0. Oxygen-storage capacity (OSC) and redox properties of Pr1ꢀxZrxO2ꢀd
(0.0 ꢁ x ꢁ 0.8) solid solutions have been investigated by temperature-programmed reduction (TPR) and are compared with those
of Ce1ꢀxZrxO2. Pr1ꢀxZrxO2ꢀd exhibited H2 uptake and CO oxidation at a lower temperature than Ce1ꢀxZrxO2. Small amount of Pd
ion (y = 0.02) substitution was found to bring down the temperature of oxygen release–storage significantly.
# 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides; B. Chemical synthesis; C. X-ray diffraction; D. Catalytic properties
1. Introduction
The solid solutions of ceria with zirconia, Ce1ꢀxZrxO2 are a well known oxygen storage material for exhaust
catalysis. The unique feature of oxygen storage material is to release or store oxygen under fuel rich and fuel lean
conditions, respectively. The loosely bound lattice oxygen of Ce1ꢀxZrxO2 would oxidize either CO or CnHy
(hydrocarbons) and the vacant oxide ion sites created would be filled by the stream oxygen under fuel lean conditions:
Ce0:7Zr0:3O2 þ dCO ! Ce0:7Zr0:3O2ꢀd þ dCO2
d
Ce0:7Zr0:3O2ꢀd þ 2O2 ! Ce0:7Zr0:3O2
Reversible deintercalation–intercalation of oxygen to the extent of d is defined as the oxygen storage capacity, OSC
[1]. For pure CeO2, d is ꢂ0.01 and for Ce0.7Zr0.3O2, d is ꢂ 0.2. ZrO2 cannot be reduced by H2 or CO whereas Ce4+ ion
can be reduced to Ce3+ in the Ce1ꢀxZrxO2 solid solution at a lower temperature (<500 8C) and to a large extent. The
reason for such a behavior is due to destabilization of oxygen sublattice of the fluorite structure of Ce1ꢀxZrxO2.
Coordination of both Ce4+ and Zr4+ in the ideal fluorite is 8 but in the solid solution, both Ce and Zr ions have 4 + 4
* Corresponding author. Tel.: +91 80 2293 2614; fax: +91 80 2360 1310.
E-mail address: mshegde@sscu.iisc.ernet.in (M.S. Hegde).
0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.10.033

M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
2659
coordination leading to four Zr(Ce)–O short and four Zr(Ce)–O long bands. The four Ce–O bonds in Ce1ꢀxZrxO2 are
longer than the Ce–O bonds in CeO2 and hence they can be reduced more easily [2,3].
Pr is the next element in the lanthanide series and Pr6O11 is its most stable oxide. Extensive studies have been
done by Eyring on the defect fluorite structure of PrnO2nꢀ2 [4]. Pr6O11 crystallizes in defect fluorite structure.
Pr6O11 can be more easily reduced to Pr2O3 than CeO2 to Ce2O3. Hence, Pr1ꢀxZrxO2ꢀd solid solutions are
expected to be a better oxygen storage material. There are a few reports in the literature on the synthesis, structure
and properties of PrOy–ZrO2 solid solution. Narula et al. have studied solid solutions of CeO2–ZrO2, PrOy–CeO2
and PrOy–ZrO2 and the compounds were synthesized by sol–gel method. The synthesized materials were
calcined up to 900 8C to obtain crystalline phases and it resulted in low oxygen-storage capacity [5]. PrOy–ZrO2
with Pr upto 15% is shown to crystallize in tetragonal ZrO2 structure and study was aimed at stabilizing
tetragonal ZrO2 by Pr ion [6]. PrOy–ZrO2 solid solution by hydroxide coprecipitation method has also been
reported and is shown that Pr upto 17% could be substituted in ZrO2 and these solid solutions crystallize in cubic
fluorite structure [7]. Defect fluorite Re0.6Zr0.4ꢀxYxO2ꢀd (Re = Ce, Pr) have been reported to show high oxygen
mobilities due to oxide ion defects induced by Pr and Y ion in Pr0.6Zr0.4ꢀxYxO2ꢀd [8]. However, there is no report
on the formation of complete solid solution of Pr1ꢀxZrxO2ꢀd (0 ꢁ x ꢁ 1) and their structure and oxygen-storage
capacity.
Here we report the synthesis of nano Pr1ꢀxZrxO2ꢀd (0 ꢁ x ꢁ 1) solid solution by single step solution combustion
method and their structure, oxygen-storage capacity from H2/TPR and CO/TPR reactions. Further, oxygen-storage
capacity is demonstrated by the oxidation of CO by the utilization of the lattice oxygen to form CO2 and its
replacement by O2. Furthermore, we show that substitution of Pd ion in Pr1ꢀxZrxO2ꢀd enhances OSC at a lower
temperature.
2. Experimental
Pr1ꢀxZrxO2ꢀd (0 ꢁ x ꢁ 1) solid solutions were prepared by taking stoichiometric amounts of praseodymium oxide
(Pr6O11 99.99% Indian Rare Earths) dissolved in dilute nitric acid, zirconyl nitrate, ZrO(NO3)2ꢃ6H2O (Aldrich) and
oxalyl dihydrazide fuel (ODH, prepared by the reaction of 1 mol of diethyl oxalate and 2 mol of hydrazine hydrate). In
a typical preparation of Pr0.5Zr0.5O2ꢀd, 4.89 mmoles of praseodymium oxide (Pr6O11), 29.40 mmoles of zirconyl
nitrate (in solution) and 73.42 mmoles of ODH, were taken in a borosilicate dish of 130 cm3 capacity. The reactants
were dissolved in 20 ml water and introduced into a preheated muffle furnace at 500 8C. The solution boiled with
frothing, foaming and ignited to burn with a flame (ꢂ1000 8C) yielding a voluminous solid product. Similarly pure
ZrO2 and PrO2ꢀd were also prepared by the combustion method taking ZrO(NO3)2ꢃ6H2O or praseodymium oxide,
respectively. The chemical reaction for the formation of a solid solution by a combustion process, for example
Pr0.5Zr0.5O2, is as follows:
2PrðNO3Þ3ðaqÞ þ 2ZrOðNO3Þ2ðaqÞ þ 5C2H6N4O2ðaqÞ þ 2xO2
! 4Pr0:5Zr0:5O1:75þxðSÞ þ 10CO2ðgÞ þ 15N2ðgÞ þ 15H2OðgÞðx ꢂ 0:166Þ
Pr0.48Pd0.02Zr0.5O2ꢀd was synthesized by taking Pr6O11, PdCl2, ZrO(NO3)2 (in solution) and ODH in the mole ratio
of 0.48:0.02:0.5:1.22. In typical reaction, 4.89 mmoles of praseodymium oxide (Pr6O11), 30.58 mmoles of zirconyl
nitrate (in solution), 1.223 mmoles of palladium chloride and 74.65 mmoles of ODH were used.
X-ray diffraction (XRD) patterns of all the oxides synthesized were recorded on a Philips X’Pert X-ray
diffractometer with Cu Ka source (l = 1.5418 Å) at a scan rate of 0.5 8/min with 0.02 step size in the 2u range 10–808.
Rietveld refinement of the structures were carried out using Fullprof program.
Hydrogen uptake studies were carried out using a temperature-programmed reduction (TPR) system with 5% H2/
Ar. About 100 mg of sample in a granular form (40–80 mesh powder) was placed in a fixed bed tubular reactor, over
which 5% H2 in Ar was continuously passed at a heating rate of 10 8C minꢀ1. The volume of hydrogen uptake,
calibrated against a known amount of CuO, was measured using a TCD detector.
To understand the activity of lattice oxygen, CO oxidation over these oxides in absence of feed oxygen was done in
a temperature-programmed reaction system equipped with a quadrupole mass spectrometer SX200 (VG Scientific
Ltd., England) for product analysis in a packed bed tubular quartz reactor (dimension 25 cm ꢄ 0.4 cm) at atmospheric
pressure. Typically, 250 mg of the catalyst (40/80 mesh size) diluted with SiO2 (30/60 mesh size) was loaded in the

2660
M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
reactor to get a column length of 2.2 cm and endings were plugged with ceramic wool. For all the reactions, the total
flow was kept fixed at 100 sccm to achieve a gas hourly space velocity (GHSV) of 10,000 hꢀ1. Before the catalytic
test, the as-prepared catalyst was heated in O2 flow at 200 8C for 1 h followed by degassing in He flow to the
experimental temperature. The reactions were carried out as a function of temperature with a linear heating rate of
10 8C minꢀ1
.
Fig. 1. (a) Powder XRD pattern of Pr1ꢀxZrxO2ꢀd (0 ꢁ x ꢁ 1) (in ZrO2, asterisk indicates monoclinic impurity phase). (b) Plot of lattice parameter ‘a’
vs. composition of Pr.

M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
2661
3. Results and discussion
Powder XRD pattern of Pr1ꢀxZrxO2ꢀd (0 ꢁ x ꢁ 1) solid solutions are shown in Fig. 1a. The XRD patterns can be
indexed to fluorite structure with space group Fm3m (2 2 5). As can be seen from the figure, solid solution of
Pr1ꢀxZrxO2ꢀd is formed between PrOy and ZrO2 in the entire range x = 0–1. The structural parameters are refined by
Rietveld method using Fullprof program. Typical refined XRD patterns of (a) Pr0.5Zr0.5O2ꢀd and (b) Pr0.4Zr0.6O2ꢀd
compounds are given in Fig. 2. Oxygen content was fixed to the value obtained from the valencies of Zr = +4 and
Pr = +3.66. In Table 1, we have summarized refined parameters. Lattice parameter and cell volume increases with
increase in Pr content. Lattice parameter as a function of Pr content is given in Fig. 1(b). Oxygen estimation of PrO2ꢀd
was done by idiometric method and the composition is found to be Pr6O11ꢅ0.02. However, Pr0.5Zr0.5O2ꢀd does not
dissolve in HCl and so oxygen could not be estimated by idiometric titrations. In the Pr1ꢀxZrxO2ꢀd solid solution,
oxidation state of Pr is assumed to be same as in Pr6O11 since the combustion synthesized Pr oxide also gave the
composition Pr6O11 (JCPDS No. 42-1121). Further, a plot of lattice parameter of Pr1ꢀxZrxO2ꢀd versus composition of
Pr was linear for x = 0.3–1 (Fig. 1b); x = 1 being Pr6O11. Therefore, oxidation state of Pr in the solid solution is the
same as in Pr6O11. There are no literature values for lattice parameter for whole range of composition of Pr1ꢀxZrxO2ꢀd
.
This is the first study on the formation of complete solid solution of Pr1ꢀxZrxO2ꢀd (0 ꢁ x ꢁ 1) and their structure and
oxygen-storage capacity.
Fig. 2. Retvield refined XRD pattern of (a) Pr0.5Zr0.5O1.916 and (b) Pr0.4Zr0.6O1.933
.

2662
M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
Table 1
Rietveld refined parameters and selected bond lengths for Zr1ꢀxPrxO2ꢀd (0 ꢁ x ꢁ 1)
Compounds
Lattice parameter a (Å)
Cell volume (Å3)
Rf (%)
Bond distance Zr/Pr–O (Å)
ZrO2
5.113(2)
5.199(1)
5.328(2)
5.361(1)
5.378(2)
5.401(1)
5.416(2)
5.419(1)
5.472(1)
134.102
140.507
151.325
154.123
155.552
157.632
158.902
159.103
163.851
5.129
5.557
5.273
4.359
5.099
4.907
6.757
1.127
5.115
2.214
2.251
2.307
2.321
2.328
2.338
2.345
2.346
2.369
Pr0.2Zr0.8O1.966
Pr0.3Zr0.7O1.949
Pr0.4Zr0.6O1.933
Pr0.5Zr0.5O1.916
Pr0.6Zr0.4O1.899
Pr0.7Zr0.3O1.883
Pr0.8Zr0.2O1.866
PrO1.833
Space group: Fm-3m; Zr/Pr (4a)-(0, 0, 0); O (8c)–(1/4, 1/4, 1/4).
In Fig. 3, the Rietveld refined XRD pattern of Pr0.48Pd0.02Zr0.5O2ꢀd, is shown. The profile fits well to fluorite
structure. The lattice parameter ‘a’ is 5.297(2) Å which is lower than Pr0.5Zr0.5O2ꢀd (a = 5.378(2)) indicating
substitution of Pd ion in Pr0.5Zr0.5O2ꢀd. In the XRD of Pr0.48Pd0.02Zr0.5O2ꢀd (d ꢂ 0.165), diffraction lines due to Pd
metal or PdO could not be detected.
Oxygen release/uptake of Pr0.5Zr0.5O2ꢀd was investigated by TPR in both reduction and oxidation cycles and we
characterized the reduction product of Pr0.5Zr0.5O2ꢀd by XRD. As prepared Pr0.5Zr0.5O2ꢀd is cubic fluorite with
a = 5.378(2) Å (Fig. 4a). The reduction of Pr0.5Zr0.5O2ꢀd upto 800 8C in the TPR shows weak reflections due to a
pyrochlore structure with lattice parameter, a = 10.63 Å (Fig. 4b) which is close to reported value of 10.71 Å [9].
Complete ordering of Pr3+, Zr4+ ions does not seem to occur and hence the pyrochlore lines are weak. On reoxidation
in air, the reduced yellowish white compound turns back to brown colour and the structure is restored to cubic fluorite
(Fig. 4c).
Transmission electron microscopy image of typical Pr0.5Zr0.5O2ꢀd is shown in Fig. 5(a). The average particle sizes
are in the range of 25–30 nm. The corresponding electron diffraction pattern shown in Fig. 5(b) is composed of dark
diffraction rings, indexed to fluorite structure.
3.1. Hydrogen uptake studies
Fig. 6(a–e) shows the H2-TPR profile of Pr1ꢀxZrxO2ꢀd (0 ꢁ x ꢁ 0.8). TPR profile of Ce0.5Zr0.5O2 (Fig. 6f) is given
for comparison. Notice the Y scales are different in the plots for different compositions. Total reduction from 30 to
800 8C has been carried out for all the compositions. Total hydrogen uptake is obtained from the integrated values
Fig. 3. Retvield refined XRD pattern of Pr0.48Pd0.02Zr0.5O1.898
.

M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
2663
Fig. 4. Powder XRD pattern of (a) Pr0.5Zr0.5O1.916, (b) reduced product of Pr0.5Zr0.5O1.916 (pyrochlore structure) and (c) oxidized product of
pyrochlore structure (b).
from 30 to 800 8C and they are given in Table 2. As can be seen from the curve (a) of Fig. 6, Pr6O11 gets reduced below
550 8C and the volume of H2 uptake corresponds to reduction of Pr6O11 to Pr2O3. Indeed, light green Pr2O3 is formed.
Contrary to this, only the surface oxygen can be removed from CeO2 between ꢂ400 and 500 8C and complete
reduction does not occur below 550 8C. As the Zr substitution is increased from 20 to 80% in Pr6O11, H2 uptake
profiles give reduction peaks, one at lower temperature ꢂ350 8C and the other at higher temperature ꢂ650 8C.
Compositions of the reduced oxides derived from the total oxygen removed upto 800 8C are summarized in Table 2.
OSC measured from the H2 uptake study shows that Pr ion gets reduced to +3 state and the resulting reduced oxide was
Table 2
Integrated TPR results; comparison of oxygen-storage capacities of Zr1ꢀxPrxO2ꢀd (0 ꢁ x ꢁ 1) with the reported values
Sample
Composition after H2 uptake
OSC mm/g (30–800 8C)
OSC mm/g (30–900 8C)a
Pr0.2Zr0.8O1.966
Pr0.25Zr0.75O1.957
Pr0.3Zr0.7O1.949
Pr0.4Zr0.6O1.933
Pr0.5Zr0.5O1.916
Pr0.6Zr0.4O1.899
Pr0.7Zr0.3O1.883
Pr0.75Zr0.25O1.873
Pr0.8Zr0.2O1.866
PrO1.833
Pr0.2Zr0.8O1.94
–
187
–
–
185
–
Pr0.3Zr0.7O1.85
Pr0.4Zr0.6O1.82
Pr0.5Zr0.5O1.73
Pr0.6Zr0.4O1.78
Pr0.7Zr0.3O1.68
–
705
781
1254
1116
1312
–
–
597
1430
–
978
–
Pr0.8Zr0.2O1.62
PrO1.53
Pr0.48Pd0.02 Zr0.5O1.73
1544
1777
1141
1387
–
Pr0.48Pd0.02 Zr0.5O1.899
a
From Refs. [5] and [8].

2664
M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
Fig. 5. (a) Transmission electron microscopy image and (b) the corresponding electron diffraction pattern of Pr0.5Zr0.5O1.916
.
pale green. For example, Pr0.5Zr0.5O1.916 is reduced to Pr0.5Zr0.5O1.73 where the entire Pr ion is reduced to Pr3+ state
(see curve (c)). Ce0.5Zr0.5O2 is reduced to Ce0.5Zr0.5O1.75 in the H2-TPR upto 800 8C (see curve (f) of Fig. 6), but the
temperature at which major peaks obtained is at ꢂ500 8C. Pr0.5Zr0.5O2ꢀd gets reduced at a lower temperature than
Ce0.5Zr0.5O2. Thus, the oxide ion is more labile in Pr1ꢀxZrxO2ꢀd compared to Ce0.5Zr0.5O2. OSC as measured from the
H2 uptake increases with Pr content (see Table 2). We have compared the total OSC from integrated H2/TPR of
Pr1ꢀxZrxO2ꢀd with the values reported by Narula et al. [5]. They have reported OSC values from the integrated H2/TPR
upto 900 8C and our values are upto 800 8C. The OSC values are generally higher than the reported values except for
Pr0.6Zr0.4O2ꢀd composition.
With 2 at% Pd substitution, the temperature of reduction is decreased significantly (100–200 8C) (see curve (g) of
Fig. 6). PdO itself gets reduced at around ꢂ45 8C and so the first reduction peak is not due to PdO reduction. The other
two peaks at 350 and 575 8C are due to the reduction of Pr ion. The H/Pd molar ratio obtained from the first peak is 6.5.
If the first peak is due to Pd2+ ion reduction to Pd0, H/Pd ratio should have been 2 due to PdO + H2 ! Pd + H2O. The
higher H/Pd ratio observed here demonstrates that part of Pr ion also gets reduced at a temperature much below Pr6O11
or Pr ion in the Pr0.5Zr0.5O1.916. The second and the third peak of the H2 uptake can be attributed to the reduction of
bulk Pr3.66+ ion which also occurs at lower temperature than the Pr ion in ZrO2.

M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
2665
Fig. 6. TPR profiles of pure Pr6O11 (curve a), Pr1ꢀxZrxO2ꢀd (x = 0.3, 0.5, 0.6 and 0.8) (curves b–e) solid solutions, and Ce0.5Zr0.5O2 (curve f),
(Pr0.48Pd0.02Zr0.5O1.899) (curve g). Notice difference in Y scale for different samples.
3.2. CO oxidation study
To understand the reactivity of lattice oxygen, CO-TPR was carried out in absence of feed oxygen. Fig. 7(a) shows
the CO oxidation profile over Pr0.7Zr0.3O2ꢀd, Pr0.5Zr0.5O2ꢀd, Pr0.48Pd0.02Zr0.5O2ꢀd and Ce0.5Zr0.5O2ꢀd without feed
oxygen. In Fig. 7(b) CO oxidation profile over Pr0.7Zr0.3O2ꢀd, Pr0.3Zr0.7O2ꢀd and PrO2ꢀd without feed oxygen are
given. % CO conversion is a function of both temperature and Pr content in the Pr1ꢀxZrxO2ꢀd solid solutions. The CO
conversion begins in the following order: Pr0.5Zr0.5O2ꢀd at 150 8C, Pr0.7Zr0.3O2ꢀd at 250 8C and Pr0.3Zr0.7O2ꢀd at
350 8C. The % CO conversion temperature is lowest at x = 0.5. However, final % CO conversion increases with the Pr
content. The % CO conversion is ꢂ91% with Pr0.7Zr0.3O2ꢀd and CO conversion over PrO2ꢀd starts at ꢂ200 8C and
maximum conversion is only 74% (see Fig. 7(b)). Thus, lability of oxygen is higher in Pr1ꢀxZrxO2ꢀd. After one cycle
of CO/TPR in absence of oxygen, the compound gets reduced partially. The compound is oxidized by passing oxygen
at 300 8C. Lattice oxygen utilized to form CO2 is replenished and CO/TPR repeated in the second cycle as can be seen
in the case of Pr0.5Zr0.5O2ꢀd
.
With Pd ion substitution, at temperature as low as 100 8C, 20% CO conversion is observed and 80% conversion of
CO occur below 250 8C. The catalyst regains oxygen upon O2 passed over the catalyst at 250 8C and lattice oxygen can
be utilized again. The Pd substituted compound is a true OSC material at much lower temperature than Pr0.5Zr0.5O2ꢀd
.
In the case of Pr0.48Pd0.02Zr0.5O2ꢀd, CO oxidation starts at 50 8C. In our earlier studies, we have shown that Pt, Pd, or
Rh ions substituted CeO2 catalyst is far more reactive than the respective metal particles impregnated on CeO2 [10–
12]. We have also observed that substitution of noble metal ions in CeO2, TiO2, Ce1ꢀxTi2O2, do not change the total
reducibility of the support compared to the pure oxide except the decrease in the temperature of reduction. That CO
adsorption occurs on Pt2+, Pd2+ ions are shown by IR studies in our earlier paper [13]. The advantage of the presence of
Pd ion in the support is to introduce a site for the adsorption of CO which is absent in the pure oxide. Hence,
Pr0.48Pd0.02Zr0.5O2ꢀd shows much higher activity than Pr0.5Zr0.5O2ꢀd. Study of CO oxidation to CO2 without stream

2666
M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
Fig. 7. % CO conversion as a function of temperature over (a) Pr0.5Zr0.5O1.916, Pr0.48Pd0.02Zr0.5O1.898 and Ce0.5Zr0.5O2 (b) Pr0.3Zr0.7O1.494,
Pr0.7Zr0.3O1.916 and PrO1.833 oxides under the reaction condition: CO = 0.25 cm3 minꢀ1; 250 mg cat; GSHV = 10,000 hꢀ1; ramp rate = 15 K minꢀ1
.
oxygen (O2) is to find the availability of lattice oxygen in the oxides at a lower temperature. Of course Pd2+ ion will be
reduced under (reducing) CO atmosphere. But the Pd0 gets reoxidized in O2. Hence, the CO to CO2 reaction repeated
after replacing the oxygen as seen from Fig. 7(a).
4. Conclusion
We have shown that nano Pr1ꢀxZrxO2ꢀd (x = 0–1) crystallizing in fluorite structure can be prepared by single step
solution combustion method and H2/TPR as well as CO/TPR occur at a lower temperature than Ce1ꢀxZrxO2. Pd ion
substitution in Pr1ꢀxZrxO2ꢀd enhances OSC property.
Acknowledgement
Financial support from the Department of Science and Technology, Govt. of India is gratefully acknowledged.

M.B. Bellakki et al. / Materials Research Bulletin 43 (2008) 2658–2667
2667
References
[1] A. Trovarelli, Catalysis by Ceria and Related Materials, Imperial College Press, London, 2002, p. 103.
[2] G. Dutta, U.V. Waghmare, T. Baidya, M.S. Hegde, K.R. Priolkar, P.R. Sarode, Catal. Lett. 108 (2006) 165.
[3] G. Dutta, U.V. Waghmare, T. Baidya, M.S. Hegde, K.R. Priolkar, P.R. Sarode, Chem. Mater. 18 (2006) 3249.
[4] L. Eyring, in: O. Toft Sorensen (Ed.), Nonstoichiometric Oxides, Academic Press, London, 1981, p. 344.
[5] C.K. Narula, P. Haack, W. Chun, H.-W. Jen, G.W. Graham, J. Phys. Chem. B 103 (1999) 3634.
[6] M.C. Caracoche, J.A. Marinez, P.C. Rivas, A.M. Rodriguez, Chem. Mater. 16 (2004) 4319.
[7] Y.-Q. Yao, Y.-F. Ying, M.-F. Luo, Y.-J. Wang, J.-M. Ma, Mater. Lett. 61 (2007) 192.
[8] H. He, H.X. Dai, K.W. Wong, C.T. Au, Appl. Catal. A 251 (2003) 61.
[9] M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Proc. Solid State Chem. 15 (1983) 55.
[10] P. Bera, K.C. Patil, V. Jayaram, G.N. Subbanna, M.S. Hegde, J. Catal. 196 (2000) 293.
[11] P. Bera, K.R. Priolkar, A. Gayen, P.R. Sarode, M.S. Hegde, S. Emura, R. Kumashiro, V. Jayaram, G.N. Subbanna, Chem. Mater. 15 (2003)
2048.
[12] A. Gayen, K.R. Priolkar, P.R. Sarode, V. Jayaram, M.S. Hegde, G.N. Subbanna, S. Emura, Chem. Mater. 16 (2004) 2317.
[13] P. Bera, A. Gayen, M.S. Hegde, N.P. Nalla, L. Spadaro, F. Frusteri, F. Arena, J. Phys. Chem. B 107 (2003) 6122.