Hydrogen spillover and structural defects in a PdO/zirconia nanophase synthesized through a surfactant-templated route

Article abstract of DOI:10.1016/j.jallcom.2005.08.070

Nanosized zirconia with structural defects was synthesized by using surfactant-templated method. The average crystallite size, phase composition, cationic defect concentration and structural microstrain in the monoclinic phase of zirconia annealed at 800 °C were quantitatively determined by the Rietveld refinement. An unusual behaviour of hydrogen reversibly storing into/releasing from the PdO/ZrO₂ nanophases was observed, which related to formation/decomposition of a hydride phase β-PdH_0.76 and to hydrogen storage in the cationic defects of zirconia nanophases by means of hydrogen spillover from Pd metals to zirconia support.

Full text of DOI:10.1016/j.jallcom.2005.08.070

Journal of Alloys and Compounds 417 (2006) 220–223
Hydrogen spillover and structural defects in a PdO/zirconia nanophase
synthesized through a surfactant-templated route
a
b,∗
b
c
c
c,1
L.F. Chen , J.A. Wang , M.A. Valenzuela , X. Bokhimi , D.R. Acosta , O. Novaro
a
Departamento de Ciencias B a´ sicas, Universidad Aut o´ noma Metropolitana-A, Av. San Pablo, No. 180, Col. Reynosa-Tamaulipas, 02200 M e´ xico D.F., Mexico
b
Laboratorio de Cat a´ lisis y Materiales, ESIQIE, Instituto Polit e´ cnico Nacional, Unidad PALM, Col. Zacatenco, 07738 M e´ xico, D.F., Mexico
c
Instituto de F ´ı sica, Universidad Nacional Aut o´ noma de M e´ xico, A.P. 20-360, 01000 M e´ xico, D.F., Mexico
Received 20 May 2005; accepted 1 August 2005
Available online 16 November 2005
Abstract
Nanosized zirconia with structural defects was synthesized by using surfactant-templated method. The average crystallite size, phase composition,
◦
cationic defect concentration and structural microstrain in the monoclinic phase of zirconia annealed at 800 C were quantitatively determined by
the Rietveld refinement. An unusual behaviour of hydrogen reversibly storing into/releasing from the PdO/ZrO nanophases was observed, which
2
related to formation/decomposition of a hydride phase ␤-PdH0.76 and to hydrogen storage in the cationic defects of zirconia nanophases by means
of hydrogen spillover from Pd metals to zirconia support.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Zirconia; Surfactant-templated synthesis; Structural defects; Hydrogen spillover
1
. Introduction
structure of zirconia may be strongly modified, for example, pro-
ducing some cationic or anionic defects in the crystals, which
is very interesting for catalytic reactions or other applications
like hydrogen storage [13]. However, details of the surface and
bulk defective structure of nanosized zirconia prepared by using
surfactant-templated method have not been reported yet. This
work attempts to prepare zirconia nanophases via a surfactant-
templated route in a base condition and to study the possible
relationship between the structural defects and the surface reduc-
tion properties of a PdO loaded zirconia nanophases, aiming to
developing new functional material.
Zirconium oxide is an important catalyst support used for
many catalytic reactions, such as for methanol conversion
to hydrocarbons, Fischer–Tropsh synthesis and hydrocracking
[
1,2]. Sulfated-promoted zirconia (SO42 /ZrO2) was recog-
−
nized as superacid, and is an active catalyst for n-butane iso-
merization at room temperature [3,4]. Zirconia has a defective
crystalline structure that makes it possible to be used as solid
oxide fuel cell [5–7]. Because of the attractive applications, a
considerable amount of work has been devoted to zirconia syn-
thesis and characterization [8,9].
An interesting synthesis route on the basis of mesoporous
MCM-41 synthesis was reported to obtain zirconia in acidic
conditions by using surfactant as template or pseudo-template
2. Experimental
The 51.9 g of ZrOCl2·8H2O was dissolved in one container with 500 ml
deionized water to form a zirconium solution. This solution was, drop by
drop, added into a 3500 ml vessel that contains a solution of 10 g of surfac-
tant (cetyltrimethylammonium bromide, CH3(CH2)15N(CH3)3Br, as molecular
template). The ratio of surfactant/Zr was about 1.2 and the pH of the mixture was
controlled at 11 by adding ammonia (28%). During the addition, a proper agi-
tation was operated to disperse the droplets before local concentrations become
excessive. After aging for 24 h, the precipitate was filtered and washed with
[
10–12]. As a result, a mesostructured zirconia with enhanced
surface area was obtained. Since the surfactants are incorporated
into the network of the solid in the synthesis procedure, when the
surfactant cations are removed by calcination, the surface or bulk
◦
water and methanol, and then dried at 80 C for 2 days and calcined at 200, 400,
∗
◦
Correspondingauthor. Tel.:+525557296000x55124;fax:+525555862728.
600 or 800 C for 4 h in air.
E-mail addresses: jwang@ipn.mx, wang j a@yahoo.com (J.A. Wang).
Member of El Colegio Nacional de Mexico.
The PdO loaded ZrO2 sample was prepared by impregnating the ZrO2 sup-
port which was calcined at 800 C, with an aqueous solution of Pd(NO3)2·2H2O.
1
◦
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.08.070

L.F. Chen et al. / Journal of Alloys and Compounds 417 (2006) 220–223
221
◦
The 3 wt% PdO/ZrO2 material was dried at 120 C for 5 h and the dried sample
◦
◦
was calcined at 800 C in air for 4 h with a heating rate of 10 C/min. Finally,
the 3 wt% PdO/ZrO2 was obtained.
The X-ray power diffraction patterns of the samples packed in a glass holder
were obtained at room temperature with Cu K␣ radiation in a Bruker Advanced
D-8 diffractometer having a θ–θ configuration and a graphite secondary beam
◦
◦
monochromator. Diffraction intensities were measured between 10 and 110 ,
◦
with a step of 0.02 for 2.3 s per point. Crystalline structures were refined with
the Rietveld technique by using FULLPROF98 code [14]. For refinement of
crystalline structure by the Rietveld method, the zirconia structures were fitted
with a tetragonal unit cell with space group P42/nmc and a monoclinic unit cell
with a symmetry described by space group P21/c. Peak profiles modeled with
pseudo-Voigt functions contained average crystalline size as one of its character-
istic parameters [15,16]. The standard deviations of the structural refinements,
which show the last figure variation of a number, are given in parentheses.
High resolution transmission electron microscopy (HRTEM) observations
of ZrO2 crystals were carried out in a JEOL 4000 EX electron microscope
equipped with a pole piece with spherical aberration coefficient of Cs = 1.00 mm.
The ZrO2 powder sample was grounded softly in an agate mortar and dispersed
in isopropylic alcohol in an ultrasonic bath during several minutes. Some drops
were deposited on 200 mesh copper grids covered with a holed carbon film.
Electron micrographs were recorded in electron negative films and in a digital
PC system attached to the electron microscope.
◦
Fig. 2. A Rietveld refinement plot of ZrO2 solid calcined at 800 C. Experi-
mental data are indicated by crosses while the calculated curve obtained after
the refinement is indicated with a continuous line. Upper tick masks correspond
to monoclinic zirconia. The lower one to the tetragonal phase. The continuous
curve under the tick marks represents the difference between the experimental
and the calculated data.
The hydrogen reduction behaviour of the PdO/zirconia sample was evaluated
by using temperature-programmed reduction (TPR) technique in a temperature
range between 20 and 800 C on a TPD/TPR-2900 analyser (Micromeritics). A
◦
Below 200 C of calcination, the zirconia were armorphous.
◦
◦
When the calcination temperature was increased to 400 C,
1
0% H2 in Ar was used as reducing agent with 99.9% He as carrier. The 0.1 g of
the crystallization of the solid took place, producing 69 wt%
monoclinic zirconia and 31 wt% tetragonal zirconia. Further
increasing the calcination temperature lead to the phase trans-
formation from tetragonal to monoclinic zirconia, for example,
the solid annealed at 800 C consisted of 91 wt% monoclinic
and 9 wt% tetragonal zirconia phases, being the monoclinic
one the main phase. The average crystallite size of the mon-
sample placed in a quartz tube was first thermally treated under a current of He
◦
at 400 C for 1 h to remove the adsorbed species on the sample, and then it was
◦
cooled down to 20 C. Afterwards, the 10% H2 in Ar was allowed into the U-
type tube with a flow rate of 75 ml/min for the TPR experiment. The temperature
◦
was increased at a rate of 10 C/min. The TPR signal was recorded by a thermal
◦
conductivity detector (TCD).
◦
3
. Results and discussion
oclinic phase annealed at 800 C was approximately 27 nm
(Table 1).
The XRD patterns of the zirconia solid calcined at various
The Rietveld refinement reveals that not only the zirconium
temperatures are shown in Fig. 1 and a Rietveld refinement plot
of the zirconia sample annealed at 800 C is shown in Fig. 2.
occupancy number in the crystalline structure of monoclinic
phase is smaller than the value 1.000 that corresponds to cation
occupancy number in a perfect monoclinic crystal of zirconia,
but also the ratio of occupancy number of zirconium/oxygen is
less than the stoichiometric value. These are clear indications
that the monoclinic phase contains cationic lattice defects in its
◦
◦
crystalline structure. After 800 C of calcination in monoclinic
crystal, the zirconium occupancy number is 0.8325 which cor-
responds to 0.63 cationic defects per lattice cell. Formation of
the structural defects was also directly observed in the HRTEM
micrograph (Figs. 3 and 4). Electron diffraction recorded from
Table 1
Data of average crystallite size, phase concentration, cation occupancy number,
and zirconium defect number per unit cell in the monoclinic phase of zirconia
◦
(
m-ZrO2) annealing at 800 C
Average crystallite size (nm)
Monoclinic concentration (%)
Average crystal microstrain (%)
Zr atom occupancy number
27 (0.5)
91(6)
1.2 (1)
0.8325 (1)
0.63
Number of Zr defect per lattice cell
Cell parameter
a (nm)
b (nm)
0.51315 (5)
0.5316 (1)
0.53143 (7)
c (nm)
Fig. 1. XRD patterns of the ZrO2 solid calcined at different temperatures.

2
22
L.F. Chen et al. / Journal of Alloys and Compounds 417 (2006) 220–223
Fig. 5. A profile of temperature programmed reduction of the PdO/ZrO2
◦
nanophases calcined at 800 C.
◦
Fig. 3. A HRTEM micrograph of the zirconia solid calcined at 800 C. Dislo-
cations and structural defects like the white arrowed are commonly found in
the sample. The Fourier transform in the inset comes from the whole HRTEM
micrograph presented.
◦
after 800 C of calcination, the average crystallite microstrain
reached 1.2%.
The surface reduction properties of the PdO loaded zirco-
nia nanophase were studied by using temperature-programmed
reduction technique. Fig. 5 shows a TPR profile of the 3 wt%
the upper section in Fig. 3 displays a spotted pattern that corre-
sponds to the monoclinic phase of zirconia. Lattice line defects
in the m-ZrO2 structure were observed, as indicated with arrows
in Fig. 3. Many lattice defects appeared in the boundaries of the
crystals as shown in Fig. 4.
◦
PdO/ZrO2 solid calcined at 800 C. A sharp, negative peak with
◦
a minimum at approximately 75 C was observed. In the tem-
◦
perature between 100 and 300 C, a wide, negative peak was
also detected. We also observed a similar TPR behaviour in
the PdO/ZrO2 samples prepared by using other surfactants like
myristyltrimethylammonium bromide and octadecytrimethy-
lammonium bromide as templates and in the materials of
PdO/CeO2 and PdO/CexZr1 − xO2 (0.2 < x < 0.8) prepared by
using the same surfactant as template (cetyltrimethylammo-
nium bromide) [17,18]. The negative peak in the TPR pro-
file indicates that molecular hydrogen is delivered during the
TPR procedure. Ozkan et al. once reported that PdO sup-
ported on TiO2 could be reduced even at subambient tem-
perature to form palladium hydride which was decomposed
Creation of the lattice defects in the structure is suggested to
mainly associate to the incorporation of the cationic surfactant in
the solid during the synthesis by strong interactions between the
deprotonated hydroxyl groups and positively charged surfactant
headgroups. At a proper calcination temperature, these incorpo-
rated surfactant species were combusted, generating some lattice
defects in the corresponding locations. The zirconia structures
showstrongdistortiondue to the presence ofcationic defects that
gives rise to microstrain generation in the crystals, for example,
◦
to release hydrogen at 95 C [19]. By using in situ X-ray
absorption spectroscopy, McCaully also observed the forma-
tion of ␤-PdHx phase on the PdO/C and PdO/␥-Al2O3 cat-
alysts when being treated by hydrogen [20]. Based on those
◦
studies, the first negative peak centered at 75 C in Fig. 5 is
assigned to hydrogen generation due to decomposition of palla-
dium hydride (␤-PdHx) where x = 0.76. It is likely that palladium
oxide was first reduced to palladium atoms at a temperature
◦
2
0–50 C, simultaneously, some hydrogen was dissociatively
adsorbed on the palladium crystals to form a new compound
Pd hydride phase (␤-PdHx), which was unstable, and it then
decomposed to release molecular hydrogen when the tempera-
◦
ture was raised to about 70 C, thus a sharp, negative peak in
the TPR profile was produced. These steps can be expressed by
Eqs. (i)–(iv):
PdO reduction by hydrogen : PdO + H2 = Pd + H2O
Hydrogen dissociation on Pd metals : H2 = 2H
Pd-hydride phase formation : xH + Pd = ␤-PdHx
(i)
(ii)
◦
Fig. 4. A HRTEM micrograph of the zirconia solid calcined at 800 C. Many
(iii)
structural defects in the boundaries as indicated by white arrows are observed.

L.F. Chen et al. / Journal of Alloys and Compounds 417 (2006) 220–223
223
defects in its crystalline structure. Hydrogen may be stored into
or released from the PdO loaded zirconia nanophases. This is
postulated to be related to the formation and decomposition of
␤
-PdHx hydride and hydrogen, which stores into the cationic
defects of zirconia nanophases by means of hydrogen spillover
from Pd clusters to the zirconia support. The nanophases of
palladium supported zirconia with cationic lattice defects and
ability to reversibly store/release hydrogen would be potentially
used as hydrogen storage media at nearly room temperature, or
used as a material for solid fuel cells and membrane reactors; it
can also be used as catalyst for many catalytic reactions involv-
ing hydrogen transport.
Fig. 6. Mechanism of hydrogen spillover, storage and release from the
PdO/ZrO2 nanophases.
Acknowledgments
This work is finally supported by projects of CONACyT of
Mexico (Grant No. 31282-U), FIES-IPN-IMP (Grant No. 98-
Thermal decomposition of ␤-PdHx to release H2 :
29-III) and CGPI-IPN (Grant No. 20021187). The authors thank
␤
-PdHx = Pd + 0.5xH2
(iv)
Miss Y. Mart ´ı nez for her technical assistance. L.F. Chen is grate-
ful to the doctorate fellowship sponsored by the CONACyT,
Mexico.
According to Eq. (i), there should appear one TPR peak cor-
responding to hydrogen consumption by the reduction of the
PdO crystals, and this positive TPR peak must appear before the
negative one. However, since the reaction (i) occurs near room
temperature, it is difficult, in some time, to be observed by TPR
starting from room temperature. Indeed, in our TPR experiment,
one may find one positive peak located at about 35 C in the TPR
spectrum, which characterizes PdO reduction. This observation
further consolidates the above postulation.
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