World Journal of Nano Science and Engineering, 2012, 2, 117-125
http://dx.doi.org/10.4236/wjnse.2012.23015 Published Online September 2012 (http://www.SciRP.org/journal/wjnse)
117
Preparation, Characterization and Catalytic Activity of
Palladium Nanoparticles Embedded in the Mesoporous
Silica Matrices
Nadiia A. Ivashchenko1*, Wojciech Gac2, Valentyn A. Tertykh1, Viktor V. Yanishpolskii1,
Sergei A. Khainakov3, Alla V. Dikhtiarenko3, Sylwia Pasieczna-Patkowska2, Witold Zawadzki2
1Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, Kyiv, Ukraine
2Faculty of Chemistry, Maria Curie-Skłodowska University, Lublin, Poland
3Department of Organic and Inorganic Chemistry, Faculty of Chemistry, University of Oviedo, Oviedo, Spain
Received April 9, 2012; revised May 4, 2012; accepted August 21, 2012
ABSTRACT
Novel in-s itu reduction approach was applied for the synthesis of palladium nanoparticles in the pores of mesoporous
silica materials with grafted silicon hydride groups. Matrices possessing different structural properties (MCM-41,
SBA-15 and Silochrom) were used. Samples were studied by nitrogen adsorption-desorption method, low-angle X-ray
diffraction, transmission electron microscopy (TEM) and FT-IR/PAS spectroscopy. The temperature-programmed oxi-
dation (TPO) and reduction (TPR) methods were applied to examine reducibility of palladium species. Palladium con-
taining catalysts were tested in methane oxidation reaction. It was demonstrated that relatively large pores in SBA-15
type silica facilitated formation of well-dispersed palladium nanoparticles confined in the pores channels. In the case of
MCM-41 support, metallic palladium nanoparticles were formed on the external surface. The obtained materials
showed high catalytic activity. Lower activity of the samples containing small crystallites located in the pore volume at
high temperatures was related to worse accessibility of active sites to the reation mixture.
Keywords: Palladium Nanoparticles; Siliconhydride Groups; Mesoporous Ordered Silica; MCM-41; SBA-15;
Palladium-Containing Nanocomposites; Methane Oxidation
1. Introduction
Metal nanoparticles have attracted considerable attention
in modern technology. The emerging physicochemical
properties of materials are often observed when the par-
ticle size reaches nanometer range [1]. Palladium nano-
particles are expected to show high activity and selec-
tiveity in numerous catalytic processes.
The application of unsupported metal nanoparticles is
often limited due to aggregation processes which occur
in the activation stage or catalytic reaction conditions.
Nowadays significant number of research studies has
been devoted to the synthesis of such objects with
searching for possible ways to prevent undesirable sin-
tering processes [2-4]. The use of advantageous porous
supports, which can decrease the growth of particles is
one of the approaches for the control of metal nano-
clusters during synthesis and remaining them separated
from each other, even at high temperatures [2,4-8].
However, application of common reducing reagents,
such as sodium borohydride, hydrazine [5,9] or hydrogen
[6] usually leads to considerable clusters aggregation in
the synthesis conditions.
Immobilization of groups possessing pronounced re-
ducing properties on the surface of silica allow one to
solve the problem of nanoparticles sintering. In this case,
formation of nanoparticles on the silica surface occurs
due to reduction of metal ions immediately in a place of
reducer attachment [10]. Reduction process is caused by
properties of surface silicon hydride groups, accom-
panied by their hydrolysis and formation of high-disperse
metal particles of nanometer size [11,12].
Recently silica materials with grafted silicon hydride
groups were successfully applied for the synthesis of
gold and silver nanoparticles [13-15]. Direct reduction of
ions on the surface of hydridesilica, in contrast to com-
mon reduction in the hydrogen atmosphere, allowed one
to regulate the size of the formed metal particles within
certain limits, by varying concentration of metal salt
taken for reduction and time of reduction.
Thereby in present work we combine two advanta-
geous approaches. Silicon hydride groups, possessing
reducing properties, grafted to the surface of silica allow
*Corresponding author.
C
opyright © 2012 SciRes. WJNSE

N. A. IVASHCHENKO ET AL.
118
obtaining metal nanoparticles immediately in the place of
reducer attachment. Another consists in application of
ordered mesoporous silicas as supports confining palla-
dium nanoparticles and preventing particles aggregation.
The palladium/silica nanocomposites were characterized
by different physico-chemical methods and tested as
catalysts in methane oxidation reaction.
Methane is a relatively potent greenhouse gas. About
8% of the world’s anthropogenic methane emissions
come from coal mines around the world. The catalytic
oxidation of methane may be considered as a promising
solution of methane-poor gas mixtures utilization. The
process allows not only to achieve methane complete
oxidation in its low-concentrated mixtures, but it also
may be carried out at moderate (depended on the catalyst
used) temperatures [16]. Noble metals, especially palla-
dium, placed on the supports with large specific surface
areas and good thermal stability, demonstrated the high-
est activity for complete oxidation of methane among
other [17,18].
2. Experimental
2.1. Samples Preparation
Silicon hydride groups immobilized on the surface of
mesoporous ordered silica (MCM-41, SBA-15) and si-
lochrom type silica were used for in situ reduction of
palladium ions. Silochrom is a porous material obtained
from concentrated water suspensions of fine pyrogenic
silica (Aerosil).
For the preparation of mesoporous ordered silica ma-
terials (MCM-41 and SBA-15) following procedures
were applied. In the synthesis of MCM-41 material reac-
tion mixture with the mole ratio 1.0TEOS:0.153CTM-
AB:1.57NH4OH:148H2O was used according to the
recommendations given by Grun et al. [19]. In the syn-
thesis 8.3 g of hexadecyltrimethylammonium bromide
(CTAB) was dissolved in 270 cm3 of distilled water. The
mixture was stirred at the temperature 40˚C, and then 24
cm3 of tetraethylortosilicate (TEOS) was introduced.
Next 24 cm3 of ammonium hydroxide (25%) was slowly
introduced. The obtained white gel was stirred for 1 h,
and then filtered and washed with distilled water. A sam-
ple was dried for 9 hours at 90˚C, and then initially cal-
cined at the static air at 550˚C for 6 h.
SBA-15 mesoporous silica powder was prepared ac-
cording to the procedure reported in the study made by
Zhao et al. [20]. 20 g of poly(ethylene oxide-propylene
oxide-ethylene oxide) triblock copolymer with the gen-
eral formula EO20PO70EO20 (Pluronic Р123) was dis-
solved in 600 cm3 of distilled water and 162 cm3 of 2M
HCl. The mixture was stirred for 5 hours at the tempera-
ture 38˚C. Next 39.4 ml of TEOS was poured to the flask
and the solution was stirred for 20 more hours at the
same temperature. The resulting mixture was then heated
in thermostat at 100˚C for 24 h. The white product was
filtrated, washed, dried, and calcinated at 550˚C for 6 h.
Modification of silica matrix was carried out under
anhydrous conditions according to the procedure pre-
viously described in [21]. Triethoxysilane was added
dropwise to glacial acetic acid. After stirring, mixture
was poured to the flask containing silica. After impreg-
nation, the obtained mixture was transferred to the ther-
mostat and heated there for 2 h at 90˚C. Samples of
modified silica were dried in an air at 90˚C during one
hour and then heated in the oven at 150˚C for 2 h.
Introduction of palladium was performed by impre-
gnating of modified silica with palladium nitrate solution
with pH = 1.6. Samples were then washed with distilled
water, and dried at 100˚C for 3 hours. Metal nanoclusters
were formed due to reduction of palladium ions immedi-
ately in a place of surface  SiH groups attachment.
Palladium loading in as-prepared composites was the
same 0.5%, however samples were prepared using dif-
ferent silica supports, so they were denoted as Pd/Si-
lochrom, Pd/MCM-41, and Pd/SBA-15. Control of the
metal content in the equilibrium solution was carried out
by spectrophotometric method with the use of thiourea
[22]. Measurements were carried out by spectropho-
tometer SF 46 (LOMO, Russia) at the wavelength 460
nm using cuvettes with layer thickness 1 cm. Content of
palladium in one gram of Pd/Silica nanocomposite (wt%)
was determined as it was reported in [21,23].
In order to remove the residual acetic acid used for
modification and nitrate species, all the samples before
analyses were heated up to 370˚C in the helium flow (50
cm3/min).
2.2. Samples Characterization
The analysis of samples low-temperature adsorption/
desorption of nitrogen were obtained using analyser of
porosity and specific surface area Micromeritics ASAP
2020, USA at-196˚C. Total surface area was calculated
from BET method and pore volume as well as pore size
distribution was obtained using BJH method.
X-ray powder diffraction analysis was carried out with
DRON-4-07 X-ray diffractometer (Burevestnik, Russia)
using a Ni-filtered CuK

radiation.
TEM observations were performed on MET JEOL-
2000 EX-II (Japan) transmission electron microscope.
For the TEM analysis, the materials were dispersed in
ethanol and a drop of this suspension was deposited onto
a carbon-coated copper grid.
FT-IR/PAS spectra of the samples were recorded by
means of the Bio-Rad Excalibur 3000 MX spectrometer
equipped with photoacoustic detector MTEC300 (in the
helium atmosphere in a detector) over the 4000 - 400
cm−1 range at the resolution of 4 cm−1 and maximum
source aperture. The spectra were normalized by comput-

N. A. IVASHCHENKO ET AL.
119
ing the ratio of a sample spectrum to the spectrum of a
MTEC carbon black standard. A stainless steel cup (di-
ameter 10 mm) was filled with powder samples (thick-
ness < 6 mm). Interferograms of 1024 scans were aver-
aged for each spectrum.
with the 0.05 g of catalyst. The reaction mixture was
composed of 1% CH4 and 99% air. The total flow rate of
the mixture was 100cm3/min. Temperature was increased
stepwise at 20˚C every 10 min from 150˚C up to 700˚C.
3. Results and Discussion
In the temperature-programmed oxidation (TPO) ana-
lysis, the sample was introduced to the quartz flow reac-
tor, then the temperature of the reactor was increased
with the rate 10˚C/min. The total flow of 5% O2/He mix-
ture was 30 cm3/min. Evolved gases were analyzed by
mass spectrometer HAL201RC (Hiden Analytical).
The nitrogen adsorption/desorption isotherms for MCM-
41, SBA-15 and palladium-supported materials are dis-
played in Figure 1. The isotherms of the samples exhib-
ited a highly ordered mesoporous nature, whereas the
sharp capillary condensation step in the range p/p0 = 0.2
- 0.4 for MCM-41 and p/p0 = 0.6 - 0.8 for SBA-15 sug-
gests cylindrical pores with narrow pore size distri- bu-
tion. The isotherms of palladium-containing nano-
composites (Pd-MCM-41 and Pd-SBA-15) indicate that
metal introduction doesn’t affect the ordered mesopor-
ous structure of the supports; however, volume of ad-
sorbed nitrogen is lower than in the case of initial silica
matrix.
Reducibility of palladium species was determined by
temperature-programmed reduction method (TPR) using
apparatus AMI-1 (Altamira Instruments Inc.). The analy-
sis was carried out by placing 0.05 g of the sample in a
quartz reactor. Reduction was performed in the mixture
6% H2/Ar at the flow rate 30 cm3/min, the linear tem-
perature increase was 10˚C/min. The samples before re-
duction were pre-treated in the flow of argon (30
cm3/min). Samples were heated up to 370˚C, kept at this
temperature for 30 minutes and then cooled down to
−70˚C. Water vapor formed during reduction was removed
in a cold trap (immersed in liquid nitrogen-methanol
slush). The signal of thermal conductivity detector (TCD)
was calibrated by injecting 55 µl of argon to the carrier gas.
Specific surface area (SBET) and pore volume (Vpores) of
palladium containing nanocomposite decreases in com-
parison with initial silica matrix SBA-15. The textural
properties of the initial ordered silicas and palladium-
containing composites are summarized in Table 1.
The location palladium nanoparticles on the external
surface or in the pore channels of silica matrices may
The activity of the catalysts in the complete oxidation
of methane was determined in the quartz reactor filled
(a) (b)
Figure 1. Isotherms of low-temperature ad(de)sorption of nitrogen at −196˚C for initial silicas (а: МСМ-41; b: SBA-15) and
palladium-containing composites (Pd/МСМ-41, Pd/SBA-15).
Table 1. Structural characteristics of initial silica matrices and palladium-containing nanocomposites based on them.
Parameter Silochrom Pd/Silochrom SBA-15 Pd/SBA-15 MCM-41 Pd/MCM-41
dpores, nm 17 20 6.2 6.2 2.3 2.3
SBET, m2/g 120 101 540 473 939 832
Vpores, cm3/g 0.975 0.722 0.574 0.497 0.530 0.483

N. A. IVASHCHENKO ET AL.
120
block nitrogen flow into the pores, leading to the de-
crease of adsorbed gas quantity causing a shortage of
specific surface area and pore volume. Thus, in the case
of Pd/SBA-15 pores volume is reduced by 13.4% and the
shortage of specific surface area is 12.4%, for the
Pd/MCM-41 those values are 9 and 11% correspondingly,
and 26% and 16% for the Pd/Silochrom sample.
Figure 2(a) shows the small-angle XRD pattern of the
MCM-41 mesoporous silica powder and Pd/MCM-41
nanocomposite. The small angle XRD pattern for MCM-
41 shows four well-resolved peaks which can be indexed
as (100), (110), (200) and (210) reflections and are char-
acteristic for hexagonal porous structure. TEM image for
MCM-41 type silica matrix (Figure 3(a)) demonstrates
parallel porous channels typical for it. Four reflections
displayed in the small-angle region of XRD spectrum
for Pd/MCM-41 nanocomposite indicate that hexagonal
phase of silica matrix remains intact after impregnation
of MCM-41 support with grafted SiH groups with palla-
dium nitrate solution Figure 2(a). This can be explained
by the formation of palladium nanoparticles on the ex-
ternal surface of support, as it can be seen from TEM
image for Pd/MCM-41 nanocomposite (Figure 3(b)).
Formed particles have quite large size and broad distri-
bution (from 10 up to 50 nm).
Diffraction pattern in the small-angle region of syn-
thesized silica SBA-15 type Figure 2(b) demonstrates tree
well-resolved peaks assigned to (100), (110) and (200)
reflections, which suggest hexagonal ordered structure of
support. Transmission electron microscopy data confirm
ordered pore array in the synthesized matrices, honey-
comb porous structure characteristic for SBA-15, is easily
(a) (b)
Figure 2. Small-angle X-ray diffraction pattern of initial ordered mesoporous silica matrices (а: МСМ-41; b: SBA-15) and
palladium-containing composites (Pd/МСМ-41, Pd/SBA-15).
Figure 3. TEM images of silica support MCM-41 type (a) and palladium nanoparticles supported on it (b).

N. A. IVASHCHENKO ET AL. 121
seen on the image (Figure 4(a)). XRD investigations for
Pd/SBA-15 composite revealed the decrease of diffract-
tion peak intensity comparing to supports diffraction
pattern (Figure 2(b)). This change can be related to a
partial distortion of the regular structure caused by metal
incorporation in the pore channels of SBA-15, however,
small content of palladium in composite (0.5%) does not
lead to complete destruction of structure and intensity
decrease is quite small.
Transmission electron microscopy data confirm the
formation of palladium nanoparticles inside the pore
channels of SBA-15, as well as retention of porous
structure (Figures 4(b) and (c)). Metal particles in the
Pd/SBA-15 composite have narrow size distribution with
the maximum near 5 nm. Growth of nanoparticles can be
limited by pores of ordered mesoporous silicas, leading
to the formation of small particles with narrow size dis-
tribution.
The effects caused by palladium introduction can be
also observed in the FT-IR results presented in the Fig-
ure 5. A sharp peak at 3743 cm−1 is connected with vi-
bration of the isolated Si-OH silanol groups. Slight
shoulder with the center located at around 3650 cm−1 is
often ascribed to the bridging hydroxyls (SiOH-OSi)
vibrations. Stretching vibrations νOH (Si-O-H) are visi-
ble in the broad peak with the center at around 3440 cm−1.
Small peaks located at the shoulder of the broad band are
assigned to the asymmetric CH3, CH2 and symmetric
CH2 + CH3 vibrations. The absorption bands at around
1030 and 1080 cm−1 are due to asymmetric stretching
vibrations of Si-O-Si bridges. The absorption band at 960
- 970 cm−1 is connected with the stretching vibrations of
the Si-OH groups, while the bands at 780 - 800 cm−1, and
540 - 560 cm−1 are the symmetric stretching vibrations of
Si-O-Si bridges. The bands located at 450 - 460 cm−1 are
assigned to the Si-O bending vibrations.
Large intensity of the peaks in the 2800 - 4000 cm−1
region for SBA-15 (Figure 5(b)) sample reveals a sub-
stantial amount of hydroxyl groups located in the pores.
While the broadening of the peaks located between 1000
and 1300 cm−1 in SBA-15 support, indicates less ordered
structure of materials.
An introduction of palladium does not strongly influ-
ence the number of hydroxyl groups nor the silica struc-
ture for Pd/MCM-41 composite (Figure 5(a)), however
in the case of Pd/SBA-15 sample content of hydroxyl
groups is obviously decreased comparing to initial SBA-
15 silica (Figure 5(b)).
In the previous research [21], it was demonstrated that
silicon hydride groups grafted on the silica support may
induce in-situ reduction of palladium species to metallic
particles. Thus, adsorption bands at 2240 cm−1 in the
FTIR spectrum of the modified silica provide evidence
for the fact of silicon hydride groups’ attachment. The
(a)
20nm
(a)
Figure 4. TEM images of silica support SBA-15 type (a) and
palladium-containing nanocomposite Pd/SBA-15 (b, c).
presence of such groups in the confined space in pores of
that materials as SBA-15 can lead to the formation of
small palladium nanoparticles.
For the comparison, our previous investigation showed
[21] that in the case of Pd/Silochrom composite suppor-

N. A. IVASHCHENKO ET AL.
122
(a)
(b)
Figure 5. FT-IR spectra of the silica matrices and palladium
containing composite based on it: a: MCM-41 and Pd/MCM-
41; b: SBA-15 and Pd/SBA-15 samples.
ted nanoparticles have size about 10 - 20 nm.
Structural characteristics of supports can serve as an
explanation of differences in nanoparticles arrangment in
the porous structure of MCM-41 and SBA-15. Small
pore diameter on MCM-41 type silica (2.3 nm in com-
parison to 6.2 for SBA-15) can be the reason why in
Pd/MCM-41 nanocomposite palladium nanoparticles are
formed mainly on the external surface of silica and none
of them are incorporated in porous channels of silica ma-
trix. Pore openings seem to be small for penetration of
modificator (triethoxysilane), which at low concentra-
tions in acetic acid media exists predominantly in the
form of oligomers, into the pore channels, which leads to
impossibility of particles formation directly within the
pore channels.
Study of fresh composite Pd/MCM-41 by temperature-
programmed oxidation (TPO) method reveal desorption
and decomposition of compounds confined in the porous
material (Figure 6). The curves indicate that the sample
contains a large amount of water. Water is desorbed in
the wide range of temperatures, ranged from room tem-
perature to around 250˚C. A few overlapped peaks can
be distinguished. The maxima on the m/e = 44 curve lo-
cated at 210˚C and 290˚C evidence the presence of oxi-
dation of carbon containing compounds towards CO2.
Small maxima on the curves with the pattern m/e = 30
may indicate on desorption of the traces of residual ni-
trogen-containing compounds from palladium precursor.
The treatment conditions may be crucial for develop-
ment of active phase in catalysts. Figure 7 show TPR
results of the samples heated up in the flow of oxygen
and argon. TPR curves of the nanocomposite Pd/MCM-
41 after treatment in oxygen reveal a reduction of palla-
dium oxide species. The maximum of hydrogen con-
sumption peak is observed at temperature ca. 60˚C (Fig-
ure 7). Hydrogen participates in the reduction reaction of
palladium oxide species (Equation (1)) [24,25]:
2
H + PdOPd + HO2 (1)
It is often assumed, that irregular shape of reduction
peak may results from overlapping palladium hydride
decomposition at around 80˚C. In the case of the same
sample (Pd/MCM-41) which was treated in argon at
400˚C one can observe negative peak at 80˚C due to hy-
drogen release from the decomposition of palladium hy-
dride [25]. The absence of intensive peak at 60˚C for
samples treated in the argon flow confirms the formation
of metallic palladium. The same curve was obtained
Figure 6. TPO curves of the Pd/MCM-41 sample.

N. A. IVASHCHENKO ET AL. 123
Figure 7. TPR curve of Pd/MCM-41 nanocomposite treated
in oxygen at 400˚C and argon flow at 400˚C respectively.
for the sample Pd/SBA-15 calcined in argon flow at
400˚C.
Our previous investigations [21] suggested that pre-
sented method with the use of hydridesilica led to the
formation of metallic palladium supported on silica ma-
trix. This statement was assumed from the X-ray diffract-
tions studies of palladium-containing nanocomposites in
the wide-angle region (10˚ - 70˚). XRD pattern revealed
three well-defined (111), (200) and (220) reflexes typical
for the palladium face-centered cubic lattice [26]. How-
ever those reflexes were observed only for composite
containing 1.5% Pd. For the samples with lower metal
content (0.5%) reflexes have very small intensity and
almost invisible.
Palladium-containing nanocomposites synthesized us-
ing different supports, but the same metal content (0.5%)
were applied as catalysts in the oxidation of methane in
the excess of oxygen.
The properties of catalysts are presented in the Table 2
and Figure 8. The best activity at low temperature can be
observed for Pd/SBA-15 samples. While Pd/Silochrom
samples shows higher activity at elevated temperatures.
It can be assumed from presented results, that, among
the series of catalysts, the sample Pd/Silochrom seems to
be the most active catalyst, upon which the complete
conversion of methane was reached at the lowest tem-
perature, i.e. 456˚C. It can be a result of accessibility of
palladium nanoparticles for the reaction mixture. In the
case of Pd/SBA-15 composite, there are spatial restric-
tions related with the fact that palladium nanoparticles
are situated inside the pore channels of silica matrix.
Should be noted that, despite the small content of metal,
all the samples exhibit good catalytic activity.
Table 2. Characteristics of prepared palladium containing
composites and their catalytic activity in methane oxidation
reaction.
Sample Diameter of Pd
nanoparticles, nm
Temperature of
full methane
conversion, ˚C
Pd/Silochrom 7 - 17 456
Pd/MCM-41 10 - 50 477
Pd/SBA-15 5 516
Figure 8. The activities of catalysts in the complete oxida-
tion of methane.
4. Conclusion
Peculiarities of palladium nanoparticles synthesis on the
surface of hydridesilica with different structural charac-
teristics were studied. It was shown that modification of
silica mesoporous support with silicon hydride groups
may leads to the formation of small metallic palladium
species. Their size can be related to the structural proper-
ties of the support. Thus, in the case with the usage of
SBA-15 silica as support, formed nanoparticles are in-
corporated in the porous channels of matrix and have
narrow pore size distribution. TPR studies suggest the
formation of metallic palladium nanoparticles supported
on silica. Synthesized palladium-containing nanocompo-
sites were found to be effective catalysts in the reaction
of methane oxidation. However, the activity of the cata-
lysts, in spite of the presence of the small crystallites may
be at a low level due to location and partial encapsulation
of the active species in the pore channels.
5. Acknowledgements
This work was supported by European Community,
seventh Framework Programm (FP/2007-2013), Marie

N. A. IVASHCHENKO ET AL.
124
Curie International Research Staff Exchange Scheme
(grant no. 230790) and project MEC 06 MAT2006
01997.
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