Praseodymium hydroxide and oxide nanorods and Au/Pr6O 11 nanorod catalysts for CO oxidation

Article abstract of DOI:10.1021/jp055622r

Praseodymium hydroxide nanorods were synthesized by a two-step approach: First, metallic praseodymium was used to form praseodymium chloride, which reacted subsequently with KOH solution to produce praseodymium hydroxide. In the second step the hydroxide was treated with a concentrated alkaline solution at 180°C for 45 h, yielding nanorods as shown by the scanning and transmission electron microscopy images. The results of X-ray diffraction and energy-dispersive X-ray spectroscopy experiments indicate that these nanorods are pure praseodymium hydroxide with a hexagonal structure, which can be converted into praseodymium oxide (Pr₆O₁₁) nanorods of a face-centered cubic structure after calcination at 600°C for 2 h in air. Gold was loaded on the praseodymium oxide nanorods using HAuCl₄ as the gold source, and NaBH₄ was used to reduce the gold species to metallic nanoparticles with sizes of 8-12 nm on the nanorod surface. These Au/Pr₆O₁₁ nanorods exhibit superior catalytic activity for CO oxidation.

Full text of DOI:10.1021/jp055622r

1614
J. Phys. Chem. B 2006, 110, 1614-1620
Praseodymium Hydroxide and Oxide Nanorods and Au/Pr6O11 Nanorod Catalysts for CO
Oxidation
P. X. Huang,† F. Wu,†,‡ B. L. Zhu,§ G. R. Li,† Y. L. Wang,† X. P. Gao,*,† H. Y. Zhu,*,|
T. Y. Yan,† W. P. Huang,§ S. M. Zhang,§ and D. Y. Song†
Institute of New Energy Material Chemistry, Department of Materials Chemistry, Nankai UniVersity, Tianjin
300071, China, School of Chemical Engineering and EnVironmental Engineering, Beijing Institute of
Technology, Beijing 100081, China, Department of Chemistry, Nankai UniVersity, Tianjin 300071, China, and
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland UniVersity of
Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia
ReceiVed: October 3, 2005; In Final Form: December 7, 2005
Praseodymium hydroxide nanorods were synthesized by a two-step approach: First, metallic praseodymium
was used to form praseodymium chloride, which reacted subsequently with KOH solution to produce
praseodymium hydroxide. In the second step the hydroxide was treated with a concentrated alkaline solution
at 180 °C for 45 h, yielding nanorods as shown by the scanning and transmission electron microscopy images.
The results of X-ray diffraction and energy-dispersive X-ray spectroscopy experiments indicate that these
nanorods are pure praseodymium hydroxide with a hexagonal structure, which can be converted into
praseodymium oxide (Pr6O11) nanorods of a face-centered cubic structure after calcination at 600 °C for 2 h
in air. Gold was loaded on the praseodymium oxide nanorods using HAuCl4 as the gold source, and NaBH4
was used to reduce the gold species to metallic nanoparticles with sizes of 8-12 nm on the nanorod surface.
These Au/Pr6O11 nanorods exhibit superior catalytic activity for CO oxidation.
I. Introduction
because there are a number of phases with unique structure and
stoichiometry but without substantial isolated point defects.
In recent years, Au nanoparticles supported on metal oxides
have been found to be active for CO oxidation due to a
synergetic effect at the interface between the metal oxide support
and Au nanoparticles.21,22 One-dimensional nanorod structures
have a very large porosity from the inter-rod voids, and
molecules of fluids can readily diffuse through these voids and
reach the rod surface. Moreover, thin nanorods have large
specific surface areas, on which gold can exist as well-dispersed
metallic nanocrystals. Therefore, gold nanocrystals supported
on thin rare earth metal oxide nanorods, for instance, praseody-
mium oxide nanorods, could be a high-efficiency catalyst. The
aim of the current study is to verify such a supposition
experimentally, which is of significant importance for the design
of catalysts. In this study we synthesized fine praseodymium
hydroxide nanorods by a simple hydrothermal approach in
alkaline solution and converted the obtained hydroxide nanorods
into oxide nanorods by calcination. Gold is then loaded onto
the surface of obtained praseodymium oxide nanorods using
HAuCl4 solution as the gold source and NaBH4 solution as the
reducing agent during the subsequent reduction process. The
catalytic activity of Au/praseodymium oxide nanorods in CO
oxidation is examined.
The oxides of rare earth elements have been widely used in
past decades due to their optic, electric, magnetic, and catalytic
properties. Recently, one-dimensional (1D) nanorods have
attracted remarkable attention owing to their unique properties
and potentials for various novel applications.1,2 Rare earth oxide
nanorods with a high thermal stability, such as Dy2O3,3 Eu2O3,4
La2O3,5 and CeO2,6-9 have been synthesized by various
methods, including hydrothermal reactions, surfactant-assisted
hydrothermal routes, ultrasound irradiation, and AAM templates.
Hydrothermal treatment under moderate conditions is an effec-
tive approach to synthesize nanorods of inorganic oxides and
hydroxides. In many cases, alkaline solution is used in the
hydrothermal synthesis process, in which the shape and size of
the crystals are well controlled.9-15 For example, titanate
nanorods, lanthanide hydroxide, and CuO nanorods were
synthesized successfully via hydrothermal reaction in alkaline
solution.9-15 The nanorods of lanthanide hydroxide or titanate
can be converted into their corresponding oxide nanorods after
a subsequent dehydration process. Maintaining the rodlike
morphology during the phase transition from hydroxide or
polyoxometalate to oxide is a key issue, being as important as
the rod formation in the synthesis.
Praseodymium oxides are important materials as catalysts,
catalyst carriers, promoters and stabilizers in combustion
catalysts, oxygen-storage components of three-way automotive
catalysts, and materials for higher electrical conductivity,16-20
II. Experimental Section
1. Sample Preparation and Characterization. Praseody-
mium metal powder was dissolved in concentrated hydrochloride
acid to form praseodymium chloride solution (0.1 M). To
prepare the praseodymium hydroxide nanorods, an aqueous
solution of 5 M KOH was added dropwise into the 0.1 M
praseodymium chloride solution under stirring until praseody-
mium hydroxide precipitated completely. The precipitate was
* Authors to whom correspondence should be addressed. E-mail:
xpgao@nankai.edu.cn; hy.zhu@qut.edu.au.
† Department of Materials Chemistry, Nankai University.
‡ Beijing Institute of Technology.
§ Department of Chemistry, Nankai University.
|| Queensland University of Technology.
10.1021/jp055622r CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/07/2006

Pr(OH)3 and Pr6O11 Nanorods
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1615
ment in alkaline solution can be indexed to a hexagonal structure
of Pr(OH)3 (Joint Committee on Powder Diffraction Standards
(JCPDS) Card No. 83-2304). The intensities of the XRD peaks
of the praseodymium hydroxide increase remarkably after the
hydrothermal treatment in alkaline solution, indicating that the
average crystallite size increases considerably and better crystal-
linity is achieved by hydrothermal treatment.
Transmission electron microscopy and high-resolution trans-
mission electron microscopy (HRTEM) images of the praseody-
mium hydroxides before and after the hydrothermal treatment
are shown in Figure 2. It can be seen that praseodymium
hydroxides are nanoparticles before the hydrothermal treatment,
which have irregular shapes and consist of a number of small
crystallites (Figure 2b). These irregular nanoparticles are
transformed to nanorods with a high crystallinity after the
hydrothermal treatment in alkaline solution. The Pr(OH)3
nanorods are straight with smooth surfaces (Figure 2c) and have
a diameter of 20-40 nm and a length of several microns, similar
to lanthanide hydroxide nanorods prepared previously.13 This
transformation from irregular nanoparticles to nanorods may
undergo the Ostwald ripening process, in which larger crystal-
lites grow at the expense of smaller crystallites being dissolved.23
The calculated interference fringe spacing of the Pr(OH)3
nanorod is about 0.31 nm (Figure 2d), which is consistent with
the interplanar distance of a (101) plane of the hexagonal
structure in the XRD results. It can be also seen in the SEM
image (Figure 2e) that praseodymium hydroxides are long
nanorods without coexisting particles of other morphologies after
the hydrothermal treatment. There are no impurities (K and Cl)
detected by energy-dispersive X-ray spectroscopy (EDS; Figure
2f), except for copper and carbon elements arising from the Cu
grid and carbon film in TEM measurements.
Figure 1. XRD patterns of praseodymium hydroxides (a) before and
(b) after the hydrothermal treatment.
aged for 15 min in air and then washed repeatedly with distilled
water until the pH became 7 to remove Cl- anions in the
precipitate. The wet precipitate was mixed with 5 M KOH
solution (40 mL) by sonicating in an ultrasonic bath. The
mixture was then maintained at 180 °C for 45 h in a stainless
autoclave with a polytetrafluoroethylene (PTFE) container. The
solid recovered from the autoclaved mixture was rinsed with
deionized water until the pH value approached approximately
7. The so-obtained precipitate was dried at 60 °C for 1 day and
then calcined at designated temperatures for 2 h in air to convert
the precipitate into the oxide. Approximately 0.1 g of the
praseodymium oxide nanorods was dispersed into 40 mL of
HAuCl4 solution (0.1 M) to load gold. The impregnated
praseodymium oxide was separated by centrifugation, and
NaBH4 solution (10 mL, 0.1 M) was mixed with the solid. The
solid was then recovered, rinsed with deionized water to remove
Cl- anions, and dried at 100 °C. The morphology and micro-
structure of the praseodymium hydroxide, praseodymium oxide,
and Au/praseodymium oxide were characterized by scanning
electron microscopy (SEM; Hitachi S-3500N), transmission
electron microscopy (TEM; FEI Tecnai 20) with an accelerating
voltage of 200 kV, and X-ray diffraction (XRD, Rigaku D/max-
2500) with Cu KR radiation (λ ) 1.5418 Å). A dilute suspension
of as-synthesized nanoparticles was prepared and dropped onto
a copper grid covered with a carbon film to make a specimen
for TEM analysis.
2. Catalytic Activity. The catalytic activity measurements
of the catalysts for CO oxidation were carried out in a fixed
bed flow microreactor (of an internal diameter of 7 mm) under
atmospheric pressure using 100 mg of catalyst powder. A
reaction gas mixture consisting of 1% CO balanced with air
was passed through the catalyst bed at a total flow rate of 33.6
mL/min. The reactant and product composition were analyzed
on-line by a GC-508A gas chromatograph equipped with a
thermal conductivity detector (TCD). Hydrogen temperature-
programmed reduction (TPR) was conducted with a conven-
tional apparatus equipped with a TCD. Before the TPR analysis,
the samples were pretreated in argon at 300 °C for 1 h.
Temperature-programmed reduction was performed by heating
the sample (20 mg) at 10 °C/min to 650 °C in a gaseous mixture
of 10% v/v H2 in argon flowing at 40 mL/min.
The praseodymium hydroxide nanorods were calcined at
different temperatures for 2 h in air for dehydration. The XRD
patterns in Figure 3 reveal that the praseodymium hydroxide
with a rodlike morphology has been converted to praseodymium
oxide after the calcination at different temperatures for 2 h in
air. All of the peaks in the XRD patterns are indexed to (1 1 1),
(2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), and (4 2 0)
reflections of the Pr6O11 phase with a face-centered cubic
structure (JCPDS Card No. 42-1121). There are no impurity
phases of hydroxides and other oxides. The Pr6O11 phase rather
than the PrO2 phase is stable at ambient temperature in air.20,24
These patterns indicate a phase conversion from hexagonal
praseodymium hydroxide to face-centered cubic praseodymium
oxide during the calcination process in air. The XRD patterns
in Figure 3 also indicate that the crystallinity of praseodymium
oxides is obviously improved with increasing calcination
temperature. Figure 4 provides SEM images of the praseody-
mium oxides obtained by calcining the praseodymium hydroxide
nanorods at different temperatures. After the calcination at 450
and 600 °C, the resultant praseodymium oxides retain the rodlike
morphology. However, the rodlike morphology is completely
lost when the hydroxide nanorods were calcined at 800 °C owing
to nanorod breakage and aggregation of nanoparticles, although
praseodymium oxides have better crystallinity (which can be
attributed to the larger crystal size of the oxide calcined at 800
°C), similar to the morphological change of titanate nanotubes
to anatase nanoparticles.25,26 Therefore, 600 °C is probably the
optimal calcination temperature to achieve the oxide products,
which maintain the rodlike morphology and have good crystal-
linity.
III. Results and Discussion
As shown in Figure 1, all of the diffraction peaks of the
praseodymium hydroxides before and after hydrothermal treat-
Transmission electron microscopy and HRTEM images of
the obtained praseodymium oxide (Pr6O11) nanorods at 600 °C

1616 J. Phys. Chem. B, Vol. 110, No. 4, 2006
Huang et al.
Figure 2. (a and c) TEM and (b and d) HRTEM images of praseodymium hydroxides (a and b) before and (c and d) after the hydrothermal
treatment. (e) Typical SEM image of bulk praseodymium hydroxide after the hydrothermal treatment. (f) EDS spectrum of the praseodymium
hydroxide nanorods in part c.
are illustrated in Figures 5a and 5b. Clearly, the praseodymium
oxide retains the rodlike morphology with rod diameters between
25 and 40 nm, similar to the parent praseodymium hydroxide,
although the aspect ratio of the oxide nanorods is lower in
comparison with that of the parent praseodymium hydroxide
nanorods. Some rods are broken during the calcination process
in air. The calculated interference fringe spacing of the
individual praseodymium oxide nanorod is approximately 0.32
nm, being consistent with the interplanar distance of the (111)
plane in the face-centered cubic structure of Pr6O11.

Pr(OH)3 and Pr6O11 Nanorods
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1617
Figure 3. XRD patterns of praseodymium oxides converted from
praseodymium hydroxide nanorods after calcination at (a) 450, (b) 600,
and (c) 800 °C.
The TEM images of the Au/praseodymium oxide nanorods
(Pr6O11) are also shown in Figures 5c and 5d. The gold crystals
on the surfaces of the Pr6O11 nanorods have crystallite sizes
between 8 and 12 nm. Moreover, gold nanocrystals are covered
by a thin discrete layer of the praseodymium oxide (1-2 nm),
as indicated in Figure 5d (arrow). This is due to the migration
of the oxide to the gold surface,27 which causes the surface
roughness of praseodymium oxide nanorods. The interplanar
spacing of the gold nanoparticle in HRTEM images (Figure 5d)
is ca. 0.24 nm, corresponding to the interplanar distance of the
(111) plane of face-centered cubic Au nanocrystals found in
Au/TiO2 nanotubes and Au/CeO2 nanorods.28-30 The observa-
tion of the diffraction peaks of metallic gold (JCPDS Card No.
04-0784) in Figure 6b confirms that gold exists as metallic
particles. The gold content in the Au/Pr6O11 nanorods is
estimated from EDS (Figure 5f) to be about 5.5 wt %. For
comparison, a TEM image of the praseodymium oxide nanorods
after treatment with the NaBH4 solution (without the introduc-
tion of Au nanoparticles) is shown in Figure 5e. It is clear that
some small concavities appear on the surface of the Pr6O11
nanorods after the reduction treatment with the NaBH4 solution.
It is reported that the praseodymium oxides possess good
oxygen-storage properties and provide more active surface
oxygen species,31,32 which react readily with NaBH4 solution.
However, there is no surface aggregation and oxide migration
on the surface of the Pr6O11 nanorods. Loading gold nanopar-
ticles in NaBH4 solution enhances the interaction between Au
and Pr6O11 nanorods and promotes the migration of surface
oxides and the formation of local defects on the Pr6O11 nanorods.
Catalytic activities of gold nanoparticles supported on Pr6O11
nanorods and commercial Pr6O11 bulk particles as a function
of reaction temperature are shown in Figure 7. For the Au/Pr6O11
nanorod catalyst, the CO conversion increases slowly with
increasing reaction temperature from 100 to 130 °C, but
complete CO conversion is achieved at 140 °C. In contrast, pure
Pr6O11 nanorods exhibit no activity for CO oxidation until 220
°C. In addition, it is also shown that the catalytic activity of
Au/Pr6O11 bulk particles for CO oxidation is relatively poor as
compared to that of Au/Pr6O11 nanorods at the same condition.
Usually, lattice oxygen atoms and isolated point defects of
Pr6O11 play an important role in CO oxidation.33 The high
crystallinity of Pr6O11 nanorods could explain their poor activity
for CO conversion. The contribution from the lattice oxygen
atoms and the isolated point defects of the nanorods is trivial.
The high catalytic activity of Au/Pr6O11 nanorods is probably
attributable to the synergistic interaction between Au nanopar-
Figure 4. SEM images of praseodymium oxides converted from
praseodymium hydroxide nanorods after calcination at (a) 450, (b) 600,
and (c) 800 °C.
ticles and Pr6O11 nanorods, similar to the situations of Au/TiO2,
Au/CeO2, and Au/CuO as reported recently30,34-36 Furthermore,
it is demonstrated by XRD patterns that the diffraction peaks
of Pr6O11 nanorods are weaken after loading gold nanoparticles,
in comparison with those of the Pr6O11 nanorod support (Figure
6). This means that the migration of the oxide can create defects,
by strong interaction at the interface between Au nanoparticles
and Pr6O11 nanorods during the reduction treatment in HAuCl4
and NaBH4 solution, which may further contribute to the higher
activity for CO conversion of Au/Pr6O11 nanorods.

1618 J. Phys. Chem. B, Vol. 110, No. 4, 2006
Huang et al.
Figure 5. (a and c) TEM and (b and d) HRTEM images of (a and b) praseodymium oxide nanorods converted from praseodymium hydroxide after
calcination at 600 °C (IFFT analysis is in the inset in part b) and (c and d) Au/praseodymium oxide nanorods (intensity line profile is in the inset
in part d). (e) TEM image of praseodymium oxide nanorods after treatment with NaBH4 solution and without the introduction of gold (HRTEM in
the inset). (f) EDS spectrum of Au/praseodymium oxide nanorods.
Temperature-programmed reduction curves of the samples
are shown in Figure 8 to further identify the synergistic
interaction between Au nanoparticles and Pr6O11 nanorods.
There are two reduction peaks in Figure 8a, which are assigned
to the reduction of the surface-active oxygen and bulk oxygen
of Pr6O11, respectively, similar to that of ceria materials.9,37-38
It is obvious that the reduction temperature of Pr6O11 nanorods
is lower than that of the bulk Pr6O11, indicating that Pr6O11
nanorods have a higher activity than bulk materials. When the
TPR curves in Figures 8a and 8b are compared, it can be found

Pr(OH)3 and Pr6O11 Nanorods
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1619
Figure 6. XRD patterns of (a) praseodymium oxide nanorods converted
from praseodymium hydroxide nanorods after the calcination at 600
°C and (b) Au/praseodymium oxide nanorods.
Figure 7. Catalytic activities of Au/Pr6O11 nanorods and Au/Pr6O11
bulk particles at different reaction temperatures
Figure 8. TPR curves of the (a) praseodymium oxides and (b) Au-
supported samples.
that the reduction temperature of praseodymium oxides can be
dramatically decreased by loading Au nanoparticles, reflecting
a strong synergistic interaction between Au nanoparticles and
Pr6O11 materials. In addition, the shoulder peaks at the lower
temperature disappear for Au/Pr6O11 materials due to the loss
of the surface-active oxygen of Pr6O11 materials during the
reduction of gold ions in NaBH4 solution. Similarly, the lower
reduction temperature of Au/Pr6O11 nanorods is also observed
as compared to that of Au/Pr6O11 bulk materials, confirming
further the higher activity of the Au/Pr6O11 nanorods obtained
above.
Acknowledgment. This work is supported by the NCET
(040219), the NSFC (90206043) and the TNSF (033804411),
China. Financial support from the Australian Research Council
(ARC) is also gratefully acknowledged, and H. Y. Zhu is
indebted to ARC for the QE II fellowship.
Supporting Information Available: SEM image of com-
mercial Pr6O11 bulk particles. This material is available free of
charge via the Internet at http://pubs.acs.org.
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