Pd and ZnAl2O4 nanoparticles prepared by microwave-solvothermal method as catalyst precursors

Article abstract of DOI:10.1016/j.jallcom.2006.08.077

Poly-N-vinyl-2-pyrrolidone (PVP) stabilized Pd nanoparticles and nanophase zinc aluminate spinel (ZnAl₂O₄) were prepared in different solvents by using microwave-assisted solvothermal method. The obtained materials were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), textural studies (porosity and surface area) and other techniques, and used as precursors of heterogeneous metal colloid catalysts. A Pd/ZnAl₂O₄ system was found to be promising catalyst of light hydrocarbon combustion.

Full text of DOI:10.1016/j.jallcom.2006.08.077

Journal of Alloys and Compounds 439 (2007) 312–320
Pd and ZnAl2O4 nanoparticles prepared by microwave-solvothermal
method as catalyst precursors
Mirosław Zawadzki∗
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, Wrocław 2, Poland
Received 11 April 2006; received in revised form 21 August 2006; accepted 23 August 2006
Available online 2 October 2006
Abstract
Poly-N-vinyl-2-pyrrolidone (PVP) stabilized Pd nanoparticles and nanophase zinc aluminate spinel (ZnAl2O4) were prepared in different solvents
by using microwave-assisted solvothermal method. The obtained materials were characterized by powder X-ray diffraction (XRD), transmission
electron microscopy (TEM), textural studies (porosity and surface area) and other techniques, and used as precursors of heterogeneous metal
colloid catalysts. A Pd/ZnAl2O4 system was found to be promising catalyst of light hydrocarbon combustion.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Microwave-solvothermal method; Nanoparticles; Pd/ZnAl2O4 catalyst
1. Introduction
with metal salt solutions followed by calcinations and subse-
quent reduction with hydrogen. However, it is difficult to obtain
well-dispersed metal nanoparticles with uniform size distribu-
tions. Therefore, considerable effort is focused on develop alter-
native synthesis methods based on electrochemical reduction,
vapor deposition or sonochemistry. In recent years, new method
has been developed for the preparation of heterogeneous metal
colloid catalyst, referred to as the “precursor’ concept [8–10].
This method involves the pre-preparation of nanosized metals
with controlled properties followed by further deposition on the
catalyst carrier material; subsequent heat treatment is then not
required. An obvious advantage of the ‘precursor” concept over
conventional salt impregnation method is fact that structure and
morphology of the nanoparticles can be tailored independently
of the support. By this method fuel cell catalysts have been suc-
cessfully prepared [11].
Mono- or bimetallic nanoparticles are usually prepared by the
use of colloidal methods where metal salts may be reduced using
various agents, in the presence of polymers, microemultions, lig-
ands or appropriate solvents which act as a steric stabilizer by
balancing the van der Waals forces that cause aggregation of the
nanoparticles [12–14]. Tu and Liu [15] applied microwave tech-
niques to the nanoscopic metal colloids synthesis, showing that
PVP stabilized precious colloid metal clusters can be obtained
by microwave heating with alcohol or glycol as a reductant.
Recently, Komarneni [16,17] and researches from other groups
Metal nanoparticles have been attracting much attention for
their intriguing chemical and physical properties and potential
applications[1–3]. Amongthem, Pdnanoparticlesareveryinter-
esting because of their catalytic properties. It is well known
that the catalytic activity of supported metal particle catalyst is
strongly dependent on the shape, size and size distribution of
the particles [4,5]. Nanoparticle catalysts are highly active since
most of the particle surfaces can be available to catalysis because
chemical reactions take place mainly on the surface of the parti-
cles. It could be assumed that approximately 60% of the atoms
exist on the particle surface when its diameter is lower than 3 nm.
These surface atoms behave as the centers where the chemical
reactions could be catalytically activated. It is generally consid-
ered that the optimum functioning of the catalyst takes place
when at least 60% of atoms are surface atoms and only 40%
volumetric atoms are contained inside the nanoparticle.
Conventional synthesis methods of small metal particles on
inorganic supports have been investigated thoroughly and are
well documented [6,7]. Commonly accepted preparation tech-
niquesinvolveionexchangeorimpregnationofasupportsurface
∗
Tel.: +48 71 3435021; fax: +48 71 3441029.
E-mail address: m.zawadzki@int.pan.wroc.pl.
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.077

M. Zawadzki / Journal of Alloys and Compounds 439 (2007) 312–320
313
[18–21] have synthesized by microwave-solvothermal process-
ing route various types of nanophase materials (including metal
nanoparticles) having controlled physical and chemical charac-
teristics. Particularly, nanomaterials with spinel structure were
also prepared in this way [22,23].
The main advantages of microwave-solvothermal methods
are: very fast kinetics, phase purity with better yield and high
reproducibility. These methods seem then to be ideally suited
for precise control of microstructure of nanoparticles. In addi-
tion, it is environment friendly because reactions are carried
out in closed systems at rather low temperatures for a short
time.
microwave-solvothermal reactions were carried out under mild conditions of
temperature 150 ◦C, pressure 10 bars and the hold time of 5 min. As the result,
a dark-black homogeneous colloidal solution of Pd nanoparticles was obtained
without any precipitate. Different Pd colloids were prepared by variation of the
molar ratio of PVP to Pd precursor, the concentration of Pd precursor and the sol-
vent used. As-prepared polyol dispersions of Pd nanoparticles were precipitated
using anhydrous acetone, and then the nanoparticles were separated from the
solution by centrifugation. The resultant precipitates were washed with anhy-
drous acetone to thoroughly remove the polyol residues and dried in vacuum at
room temperature. The dry precipitates were re-dispersed in methanol prior to
TEM characterization and use for catalyst preparation. The re-dispersed PVP
colloids prepared by microwave-solvothermal processing route were very stable
for at least several months.
2.3. Preparation of nanophase ZnAl2O4
The purpose of the present work is to report the synthe-
sis of polymer stabilized Pd nanoparticles under microwave-
solvothermal (MS) conditions using (poly)ethylene glycol as
solvent and reducing agent and PVP as the stabilizing matrix.
Particularly, the effects of the concentration of PVP and palla-
dium salt or the nature of the reducing agent on the morphol-
ogy (shape and size) of the synthesized particles were studied.
The utility of the microwave-hydrothermal processing route
in the presence of urea for the synthesis of high surface area
porous nanophase zinc aluminate ZnAl2O4 with spinel structure
was also demonstrated. ZnAl2O4 is known as very interesting
material for catalytic applications (mainly as catalyst support)
because of its unique combination of desirable properties such
as high thermal stability, high mechanical resistance, low tem-
perature sinterability, better diffusion and low surface acidity
[24,25]. Unfortunately, most of the commonly used preparative
methods (high temperature calcination, coprecipitation, sol–gel)
lead to the low surface area material, usually below 50 m2/g [26]
while conventional catalyst supports require much higher area.
Microwave-solvothermally formed Pd and ZnAl2O4 nanoparti-
cles were used as precursors for the preparation of Pd/ZnAl2O4
system which catalytic properties were tested in hydrocarbon
combustion.
The aluminium precursor for the microwave assisted hydrothermal prepara-
tion of nanosized zinc aluminate was aluminium hydroxide (AlOOH) produced
through hydrolysis of solid aluminium isopropoxide for 20 min in excess of
water heated at 70 ◦C with continuous stirring. The resultant slurry of AlOOH
was still stirred and the proper amount of water solution of zinc acetate was
added to obtain molar ratio of Zn:Al = 1:2; the concentration of the mixture was
always below 10 wt%. The temperature of the mixture was maintained at 70 ◦C
during subsequently stirring for 30 min. After cooling to room temperature, the
controlled amount of urea (0.1 mol/dcm3) was added with stirring. 50 ml of pre-
cursor solution was poured into a reaction vessel held in a microwave reactor.
The microwave hydrothermal reactions were carried out at 150 ◦C, under pres-
sure of 10 bar and for the hold time 5 min. The opalescent sol with no signs
of precipitate was obtained. The former was filtered and washed with water.
The nanoparticles in the sol were concentrated and separated from the sol-
vent by centrifugation. The resulting gel was forced out in a wire from 3 to
4 mm in diameter by extrusion, air-dried overnight to remove any moisture, then
calcined at 500 ◦C for 4 h, crushed and sieved into the 0.8–1.4 mm particles.
Such obtained material was used as support for the preparation of Pd/ZnAl2O4
system.
2.4. Preparation of Pd/ZnAl2O4 catalyst
PVP stabilized Pd colloid which nanoparticles have an average size ∼2 nm
was chosen as catalyst precursor. The catalyst was prepared by dipping the
zinc aluminate support into methanol dispersions of the polymer-stabilized Pd
colloid (∼1 mM metal concentration) at ambient temperature to adsorb the
pre-prepared Pd nanoparticles. The mixture was filtered, and the solid was
washed with methanol and dried under vacuum at room temperature. The
calculated metal weight loading on support was kept at 0.5% {Pd loading
W = WPd/(WPd + WPVP + Wsupport) × 100%}. The metal content of the filtrate was
evaluated by using atomic absorption spectrophotometer.
The syntheses were performed using microwave reactor with
sealed Teflon vessel giving autogenous pressure up to tens bar.
The used methods are very rapid and effective for the preparation
of well-controlled and uniform nanoparticles.
The Pd/ZnAl2O4 system was also prepared by conventional impregnation
method for comparison purposes. The amount of PdCl2 used for impregnation
was adjusted to yield ZnAl2O4 containing 0.5 wt% of Pd. After drying at 80 ◦C
in vacuum, the obtained material was finally heated at 500 ◦C for 4 h with the
heating rate of 5 ◦C/min in dry air and subsequently activated in the presence of
hydrogen at 500 ◦C for 1 h.
2. Experimental
2.1. Materials
Palladium chloride (Alfa Aesar, Germany); poly-N-vinyl-2-pyrrolidone—
PVP K30 (Fluka, Germany): average molecular weight 40,000; zinc acetate
dihydrate (POCh, Poland), aluminium isopropoxide (Aldrich, Germany); urea
(POCh, Poland) were used as received. Organic solvents (ethylene glycol EG,
diethylene glycol DEG, triethylene glycol TEG, iso-propanol, acetone) were
analytical grade reagents and used without further purification.
2.5. Methods
All the microwave assisted solvothermal (hydrothermal) reactions were per-
formed in a microwave accelerated reaction system MW Reactor Model 02-02
(ERTEC, Poland). The system operates at 2.45 GHz frequency with 0–100% of
full power (1000 W). The system is controlled by a temperature (300 ◦C max) or
by pressure (100 bar max). Reaction vessel of 70 ml capacity is double walled
vessel consisting of a Teflon TFM inner liner and cover surrounded by stainless
steel shell.
UV–vis absorption spectral measurements were carried out with a UV–vis
spectrophotometer Cary 5E having a spectral resolution of 0.01 nm between 300
and 700 nm (10 mm path length quartz cuvettes) to confirm the completeness of
the reduction of the palladium colloids (by the disappearance of the absorption
bands for the PdCl2 precursor).
2.2. Preparation of Pd colloids
The PVP stabilized Pd nanoparticles were prepared according to the follow-
ing procedure: H2PdCl4·nH2O was first prepared by treatment PdCl2 water
solution with small amounts of concentrated hydrochloric acid. The appro-
priate amounts of PVP and pre-prepared H2PdCl4·nH2O were added together
to (poly)ethylene glycol (50 ml) under stirring at room temperature. For each
experiment 50 ml of precursor solution was poured in the reaction vessel. The

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M. Zawadzki / Journal of Alloys and Compounds 439 (2007) 312–320
The particle size and morphology of the PVP stabilized palladium nanopar-
ticles and nanosized ZnAl2O4 were characterized using a Philips CM20 Super-
Twin electron microscope (TEM), which at 200 kV provides 0.25 nm resolution;
the crystalline structure was determined by TEM and electron diffraction studies
(SAED). Specimens for TEM were prepared by putting a droplet of the colloidal
dispersion on a copper microscope gird covered with perforated carbon followed
by evaporating off the solvent under IR lamp for 1 h. The mean particle diameter
and size distribution were measured by counting at least 150 particles from the
enlarged micrographs.
The as-prepared and heat treated samples were characterized for their phase
purity and crystallinity by X-ray powder diffraction (XRD) using DRON-3
diffractometer (Ni-filtered Cu K␣ radiation); a scan rate of 0.5◦ min−1 was
used to record the patterns in the 2θ range 20–80◦. The average crystallite
size D was calculated from the broadening of the X-ray line (1 1 1) and (3 1 1),
respectively, for Pd and ZnAl2O4, using Schererr‘s equation, i.e. D = kλ/(β cos θ)
where k = constant, λ = X-ray wavelength (0.15418 nm) and β = full width at
half-maximum (FWHM) intensity of the diffraction line [27]. Elemental analy-
sis and the impurity identification were done by using energy dispersive X-ray
analysis, EDAX 9800, and atomic absorption spectrophotometry.
Thermal analysis was performed with Perkin-Elmer instruments (DTA 7,
TGA 7). Thermogravimetry (TG) and differential thermal analysis (DTA) curves
of the dried (unheated) samples of ZnAl2O4 were recorded over a tempera-
ture range up to 900 ◦C with a heating rate of 10 ◦C/min in the atmosphere
of argon; sintered ␣-Al2O3 was the thermally inert reference material used for
the DTA.
The textural properties of ZnAl2O4 samples (surface area, pore size distri-
bution and pore volume) were determined by nitrogen adsorption–desorption
isotherms at liquid nitrogen temperature by using an automatic volumetric
apparatus (FISONS Sorptomatic 1900). Prior to measurements samples were
degassed at 300 ◦C for some hours and 10−3 Torr. The adsorption of nitrogen was
followed until near saturation (P/P0 = 0.95), then the desorption was followed
until the closure of the hysteresis loop. The adsorption data were interpreted
by the application of the conventional BET method for the determination of
the surface area SBET (m2/g) and by the application of the Horvath–Kavazoe
Fig. 1. Typical UV–vis absorption spectra of solution containing H2PdCl4
before (a) and after (b) microwave solvothermal processing.
absorption without any peaks in the visible range what indicates
that Pd2+ was entirely reduced to Pd0 independently of the sol-
vent used.
The complete reduction of PdCl2 to Pd metal was also con-
firmed by XRD analysis. The X-ray powder diffraction patterns
of the samples prepared in EG solution with various PVP:Pd
molar ratio are shown in Fig. 2, as an instance. As seen in this fig-
ure, several diffraction peaks are observed in the XRD patterns
at 2θ about 40.1◦, 46.6◦ and 68.1◦. These diffraction lines corre-
spond to the (1 1 1), (2 0 0) and (2 2 0) reflections, respectively,
for the face centered cubic (fcc) structure of Pd with space group
Fm3m (according to JCPDS card no. 05-0681). The nanometer
method for the calculation of the micropore volume V [28]. The total pore vol-
␮
ume V (cm3/g) adsorbed near saturation was read from the adsorption isotherm.
The pore size distribution was analyzed following the Dollimore–Heal (D–H)
method [29], which was applied to the desorption branch of each isotherm. The
mean pore size R (nm) for each sample was read from the corresponding pore
size distribution curve.
Activity of Pd catalyst supported on zinc aluminate in total oxidation
of hydrocarbons was evaluated by comparison with conventionally prepared
Pd/ZnAl2O4 in combustion of 0.5 vol% iso-butene in air mixture passed through
a gradient less, circular-flow reactor with a space velocity of 103 h−1. The tem-
perature inside the reactor was increased continuously from 50 to 300 ◦C at
a rate of 2 ◦C/min. Conversion measurements were conducted by using a gas
chromatograph after attaining the reaction temperature.
3. Results and discussion
3.1. Pd nanoparticles
UV–vis absorption spectrophotometry is a convenient
method for evaluation the progress of metal colloid formation
based on the corresponding UV–vis absorption spectra of the
species present in the liquid medium. Relevant here was the
disappearance of the absorption bands for the PdCl2 precur-
sor, located at approximately 320 and 425 nm. Completion of
the reaction was evidenced by a simple continuous absorption
spectrum in the visible range, which is typical for nanosized
palladium colloids [30]. Typical UV–vis spectra of the starting
solution and after microwave solvothermal reactions are pre-
sented in Fig. 1; PdCl2 shows an absorption peak at 430 nm and
product UV–vis spectrum shows an unstructured, continuous
Fig. 2. XRD diffraction patterns of Pd nanoparticles prepared in ethylene glycol
with PVP:Pd molar ratio of: (a) 70:3 and (b) 50:1.

M. Zawadzki / Journal of Alloys and Compounds 439 (2007) 312–320
315
Table 1
MS synthesis conditions and dimensions of PVP-stabilized Pd nanoparticles
PVP:Pd molar ratio
Solvent (reducing agent)
Average size (nm)
Standard deviation (nm)
Relative standard deviation (nm)
50:1
50:1
50:1
50:2
50:3
30:3
70:3
Ethylene glycol
Diethylene glycol
Triethylene glycol
Ethylene glycol
Ethylene glycol
Ethylene glycol
Ethylene glycol
2.0
3.0
3.5
0.18
0.20
0.28
0.85
1.01
2.80
1.12
0.090
0.067
0.080
0.139
0.078
0.176
0.092
6.1
13.0
15.9
11.8
size of the Pd crystallites is evidenced by the broad X-ray reflec-
tions. The average sizes of the nanoparticles in samples A and B
were 2.3 and 11 nm, respectively, as determined from the X-ray
line broadening using the Scherrer’s formula.
centration of PVP or lower content of Pd precursor is used. The
effect of PVP and Pd precursor salt concentration on the mean
diameter of Pd nanoparticles prepared in ethylene glycol, graph-
ically presented in Fig. 3, clearly confirms this relationship. One
can see from Fig. 3, that the average particle size increases with
increasing the concentration of Pd precursor when the amount
of PVP is kept constant. Meanwhile, the average particle size
decreases slightly with increasing the ratio of polymer to Pd.
Similar dependence was reported in the literature for polymer
stabilized Pd colloids prepared by other methods [32].
Figs. 4 and 5 are typical TEM micrographs of different mor-
phologies of microwave-solvothermally prepared Pd nanoparti-
cles with the corresponding particle size distribution histograms.
Fig. 4(a) shows a nearly uniform and high dispersion of Pd
nanoparticles with average size of 2 nm, which is accompanied
by very narrow particle size distribution as shown in Fig. 4(b).
No shape other than spherical or quasi-spherical was observed
for the nanoparticles with PVP:Pd molar ratio of 50:1. It could
be noted that some of them exhibit lattice fringes of distances
about 0.22 nm that well correspond to the (1 1 1) lattice plane of
the fcc structure of Pd metal.
A number of different morphology (shape and size)
Pd nanoparticles were prepared by microwave-solvothermal
method; synthesis conditions and dimensions of obtained par-
ticles are given in Table 1. Standard deviations were calculated
from the Pd particle size distribution. It is known that relative
standard deviation is a useful parameter for evaluation the dis-
persity of colloidal particles with different sizes. As shown in
Table 1, PVP-stabilized Pd particles prepared by MS method
are characterized by very good dispersity independently on the
solvent used. Thus, it may be concluded that MS conditions give
very uniform Pd nanoparticles in contrast to synthesis by con-
ventional chemical reduction methods [31]. The solvents used
have similar, high microwave susceptibilities which manifest
themselves in the rapid temperature and pressure increase up
to fixed values (150 ◦C, 10 bar) under solvothermal conditions.
Therefore, the observed differences (not large) in the average
size of Pd particles obtained by using various glycols may be
affected by their diverse reducing activities at the same condi-
tions. The size of PVP stabilized Pd nanoparticles, as can be
seen from Table 1, is strongly influenced by the Pd precursor
concentration or amount of PVP in reaction solution. It could
be assumed, that the higher concentration of thermodynamically
stable nuclei under reaction conditions leads to the smaller parti-
cle size. Because the reducing agent has to interact with the metal
precursor during the reduction process, the process will be influ-
enced by the ligands surrounding the metal ions and the reducing
agent. Therefore, the size of the polymer stabilized colloidal Pd
particles is governed by the interaction of the polymer with the
Pd precursor and the speed of reduction. As the concentration
of Pd precursor was increased, the rate of the reduction process
is increased leading to an increase of the rate of crystal growth
on initially formed nuclei. As the result, further nucleation is
inhibited and an increase in Pd particle size could be observed.
The amount of PVP added to the reaction solution may be also
expected to affect the reduction of Pd2+ as well as the growth
process of Pd nanoparticles. With the decrease in the PVP to
Pd molar ratio in reaction solution, the reduction of Pd ions
becomes easier and the growth of the particles proceeds faster
due to the lack of protecting groups, thus leading to the larger
particle sizes. The smaller Pd particles, more interesting from
catalytic point of view, could be formed when the higher con-
For the samples prepared with lower PVP:Pd ratios or with
higher Pd precursor concentration, most of the particles have
well-defined geometrical shape like these presented in Fig. 5(a),
formed under PVP:Pd ratio equal 50:3. Hexagonal, triangular,
Fig. 3. Effect of stabilizing polymer (PVP) and Pd precursor salt concentration
on the mean size of Pd nanoparticles prepared in ethylene glycol.

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M. Zawadzki / Journal of Alloys and Compounds 439 (2007) 312–320
Fig. 4. (a and b) TEM image and corresponding particle size distribution his-
togram of PVP stabilized Pd nanoparticles prepared in ethylene glycol with
PVP:Pd molar ratio equal 50:1.
Fig. 5. (a and b) TEM image and corresponding particle size distribution his-
togram of PVP stabilized Pd nanoparticles prepared in ethylene glycol with
PVP:Pd molar ratio equal to 50:3.
rhombohedral and square are the most frequently observed par-
ticle face shapes. Similar crystal morphology has been already
observed for polymer stabilized Pd nanoparticles [33]. These Pd
particlesaredistinctlylarger, haveanaveragesizeofabout13 nm
and are mainly distributed within the range of about 11–15 nm
as shown in Fig. 5(b). The average particle sizes obtained from
TEM studies were in good agreement with corresponding XRD
data what suggest that individual Pd nanoparticles are nanocrys-
tallites.
broad reflections are observed indicating fine particle nature of
the obtained material. The observed diffraction peaks in all the
recorded XRD patterns correspond to those of the standard pat-
terns of cubic zinc aluminate spinel (JCPDS PDF No. 05-0669);
no other phases were detected. These peaks can be indexed as
(2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0), (6 2 0) and
(5 3 3) diffraction lines. EDS analysis showed the presence of
Zn and Al in the 1:2 molar ratio confirming the formation of
single-phase ZnAl2O4 particles. The cubic lattice parameter was
3.2. ZnAl2O4 material
˚
calculated from the powder XRD pattern as a = 7.90 and 8.01 A
The XRD patterns of the powder samples, as-prepared and
after further heating at 500 ◦C are shown in Fig. 6. Extremely
for the as-prepared and heated sample, respectively, what is close
to that of bulk ZnAl2O4 (a = 8.099 A) as described in literature
˚

M. Zawadzki / Journal of Alloys and Compounds 439 (2007) 312–320
317
Fig. 6. XRD diffraction patterns of the as-prepared (a) and heated at 500 ◦C (b)
ZnAl2O4 sample.
Fig. 7. TEM images of the as-prepared (a) and heated at 500 ◦C (b) zinc alumi-
natenanoparticles. Thelatticefringesofdistanceof∼0.24 nmonmarkedparticle
well correspond to the (3 1 1) lattice plane of ZnAl2O4. Inset of a exhibits the
SAED pattern showing rings that match d-spacings for the structure of zinc
aluminate spinel. Inset of b exhibits the particle size distribution histogram for
sample heated at 500 ◦C.
[34]. The smaller value of the lattice parameter is probably due
to the fact that the calculation of this parameter from the XRD
pattern may be underestimated because of the large broadening
of the reflections in the XRD pattern. It could be noticed that the
diffraction peak widths decrease slightly and the intensity of the
diffraction peaks increases after heating at 500 ◦C what is asso-
ciated with an increase in the degree of crystallinity and in the
crystallite size. As evident from XRD analysis, the as-prepared
sample is characterized by high crystallite dispersion—the aver-
age crystallite size is ∼2.2 nm as calculated from Schererr’s
equation. It was found that nanocrystallinity of the microwave-
hydrothermal prepared ZnAl2O4 sample was retained after heat-
ing at 500 ◦C; the average crystallite size increased slightly
to ∼4 nm. It indicates good thermal stability of zinc alumi-
nate obtained by this method. Similar results have been already
obtained by using hydrothermal methods with conventional
heating where much higher increase in the degree of crystallinity
and in the crystallite size of ZnAl2O4 was observed after further
heat treatment only at temperatures above 500 ◦C [35,36].
A representative TEM micrograph of the as-prepared sam-
ple, shown in Fig. 7(a), indicates that the nanosized particles
are similarly spherical and most of them exhibit lattice fringes
of distances about 0.24 nm that well correspond to the (3 1 1)
lattice plane of the cubic zinc aluminate structure. Correspond-
ing selected area electron diffraction pattern (SAED), shown
in inset of Fig. 7(a), exhibits five broad rings with d-spacings
about 0.291, 0.244, 0.203, 0.158 and 0.143 nm, which could be
attributed to (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) reflections
of the cubic zinc aluminate spinel structure, respectively. The
broadening of the diffraction rings suggests that the particles
are small and/or are of low crystallinity. The particle size dis-
tribution obtained from TEM images is very narrow as shown
in Fig. 8. The average diameter of the ZnAl2O4 particles esti-
mated from Gaussian fit of the histogram is about 2.1 nm and is
in very good agreement with that one obtained from XRD data.
This consistency suggests that individual particles obtained are
nanocrystallites. Also, SAED pattern of the as-prepared sample
shows that the material is not fully crystalline and therefore con-
firms the nanocrystalline nature. TEM study of sample heated
at 500 ◦C (Fig. 7(b)) reveals some increase in average particle
size, similar to the observation of crystallite size from XRD, and
some signs of agglomeration but the nanocrystalline nature of
the samples is still preserved. The corresponding particle size
distribution (inset of Fig. 7(b)) indicates the average diameter of
ZnAl2O4 after heat treatment at 500 ◦C as 4.3 nm. It confirmed

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M. Zawadzki / Journal of Alloys and Compounds 439 (2007) 312–320
can be also noticed from the moment in which the filling of
all the pores with condensed nitrogen is achieved, as may be
appreciated from the plateau; it starts beyond P/P0 ∼0.7 and
0.8, respectively, for the as-prepared and heated sample and is
higher for the second one. This means that the pores in the as-
prepared sample are smaller, as demonstrated by the desorption
hystresis loop which is shifted to a lower relative pressure ratio.
The shape of the hysteresis loops suggests that heat treatment
gives rise to a broad pore size distribution in mesoporous region.
Moreover, such a shape of hysteresis loop, as was observed for
the heated sample, is generally characteristic for solids consist-
ing of particles crossed by nearly cylindrical channels or made
by aggregates (consolidated) or agglomerates (unconsolidated)
of spheroidal particles [38].
The pore size distributions were calculated from the corre-
sponding desorption isotherms according to the D–H method
and were shown in Inset of Fig. 9. It indicates that pore size
distributions are monomodal for both samples. In the case of
the as-prepared sample a lot of pores are in the microporous
region and pore size distribution has very sharp curve centered at
∼1.1 nm while a little broad pore size distribution was observed
for heated sample. It should be noticed that the mean pore size
R is only slightly shifted in mesoporous region to ∼2.8 nm.
The as-prepared sample is characterized by high specific
surface area SBET = 210 m2/g, which decreases to 160 m2/g for
sample heated at 500 ◦C. The total pore volume V was sim-
ilar for both samples (∼0.150 cm3/g at P/P0 = 0.95) but the
as-prepared sample had significant contribution of micropore
Fig. 8. Particle size distribution histogram for the as-synthesized ZnAl2O4 sam-
ple.
good thermal stability of ZnAl2O4 nanoparticles obtained under
microwave-assisted hydrothermal conditions.
The typical nitrogen adsorption–desorption isotherms
obtained at temperature of liquid nitrogen for the as prepared
and heated sample, shown in Fig. 9, are of Type IV with H2
hysteresis loop according to IUPAC classification [37]. In the
case of as-prepared sample it should be noted that adsorption
takes place also at very low relative pressures what is charac-
teristic for microporous materials. On the other hand, at the
P/P0 > 0.1 an increase in the adsorbed volume of nitrogen is
observed due to the capillary condensation, which is character-
istic for mesoporous materials. Therefore, the shape of the N2
adsorption–desorption isotherms can be classified as a mixture
between Types I and IV isotherms in this case. Sample heated at
500 ◦C shows typical Type IV adsorption–desorption isotherms
with well-developed hysteresis loop. This means that this sample
is mesoporous with monomodal pore size distribution and a very
low contribution of micropores, responsible for the adsorption
observed at low pressure. The difference between both samples
volume (V = 0.115 cm3/g).
␮
Fig. 10 shows a representative DTA and TGA curves of
ZnAl2O4 sample prepared by microwave assisted hydrothermal
method. The DTA analysis reveals only one significant ther-
mal effect—sharp endothermic peak at around 132 ◦C, which is
associated with the departure of physically adsorbed molecular
water. TheTGAanalysisprovesthatthisthermaleffectisaccom-
panied by weight loss. Additionally, broad and weak endother-
mic peak with its maximum at ∼295 ◦C may be observed in the
DTA curve as the result of the removal of zinc aluminate consti-
tutional water. The lack of any noticeable exothermic effects
Fig. 9. Nitrogen adsorption–desorption isotherms and corresponding pore size
distributions of ZnAl2O4 sample: (a) as-prepared and (b) heated at 500 ◦C.
Fig. 10. DTA–TG curves of ZnAl2O4 sample prepared under microwave
hydrothermal conditions.

M. Zawadzki / Journal of Alloys and Compounds 439 (2007) 312–320
319
4. Conclusions
The proposed microwave-assisted solvothermal method is
very rapid and effective for the preparation of polymer stabilized
Pd nanoparticles with controllable sizes from 1.6 to 15 nm as
well as single-phase zinc aluminate of particles with an average
size ∼2.1 nm and narrow size distribution as were confirmed
by powder X-ray diffraction and electron microscopy studies.
The morphology (shape and particle size) of Pd nanoparticles is
greatly affected by the concentration of the stabilizing polymer
and Pd precursor salt. The nature of reducing agents (EG,
DEG and TEG) used was of less significance under microwave
solvothermal conditions of fixed parameters (temperature,
pressure and time). Liquid nitrogen adsorption–desorption
measurements revealed that nanocrystalline ZnAl2O4 exhibits
interesting textural properties like high specific surface area (up
to 210 m2/g) and microporous structure with an average pore
size about 1.1 nm. After heat treatment at 500 ◦C, some increase
in the degree of crystallinity and a shift of pore size distribution
towards mesoporous region was observed. Such properties
make it a promising material for use in the development of
catalyst carriers. Pd colloid nanoparticles supported on zinc
aluminate spinel was found to be more active in iso-butane com-
bustion than corresponding system prepared by conventional
impregnation method. This indicates that nanostructured Pd
colloid and porous ZnAl2O4 prepared under microwave assisted
solvothermal conditions are very promising catalyst precursors.
Fig. 11. Combustion of iso-butene over 0.5 wt% Pd/ZnAl2O4 systems obtained
from pre-prepared Pd nanoparticles and by impregnation method.
suggests that no recrystallization of the dispersed ZnAl2O4
spinel occurs at temperature up to 900 ◦C.
3.3. Pd/ZnAl2O4 system
The catalytic activity of PVP stabilized palladium colloid
supported on porous high surface area zinc aluminate, both
prepared by using microwave solvothermal processing route,
was evaluated in total oxidation of iso-butane in comparison
with Pd/ZnAl2O4 system obtained by conventional impregna-
tion method. The catalytic activities of both systems were inves-
tigated at a temperature of 50–250 ◦C and compared in terms
of their light-off temperatures corresponding to 50% conver-
sion. Fig. 11 presents typical iso-butene conversion curves as a
function of increasing reaction temperature (2 ◦C/min). Conver-
sion started at 90 ◦C and 110 ◦C, respectively, for the “colloid”
and “impregnated” catalyst, and increased more or less rapidly
with the increasing reaction temperature. Conversions of 100%
could be obtained above 170 and 210 ◦C, respectively. It should
be stated that both catalyst showed relatively high activity but
reaction temperature over “colloid” catalyst was always lower
by at least 20 ◦C for 50% conversion than over convention-
ally prepared catalyst. This indicates that ZnAl2O4 obtained by
microwave hydrothermal method is promising catalyst support
material in both cases. However, probably due to the high surface
area of support as well as the high dispersion of polymer stabi-
lized nanoparticles, the temperatures of 50 and 100% conversion
were noticeable lower for the catalyst prepared from colloid
nanoparticles. It is well known that the surface of the support
on the molecular scale, and therefore that surrounding the active
metal sites, is not well defined so it is usually difficult to obtain,
by conventional impregnation method, monodispersed metals
because the interaction of the individual metal atoms is high
compared with the interaction between the reduced metal and
the support. Additionally, active sites formed by conventional
methods can be significantly contaminated with the support.
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