Mechanochemical synthesis, characterization and catalytic activity of Bi2WO6 nanoparticles in CO, n-hexane and methane oxidation reactions

Article abstract of DOI:10.1016/j.jallcom.2013.03.157

This article represents data on the mechanochemical synthesis, characterization and catalytic properties of Bi₂WO₆ nanoparticles. The synthesis of Bi₂WO₆ nanopowder (10 nm) started after 90 min of milling time interval and it was completely finished after 4 h when using 2.5 g batch of the initial oxides. Longer milling time intervals (8 h) were needed to synthesize the target nanophase (4 nm), when using a 4.0 g batch of precursor oxides. The Bi₂WO₆ powders manifest low absorbance ability in the visible range with a band gap ranging from 2.85 to 2.88 eV. The catalytic activity was investigated in the reactions of CO, n-hexane and methane oxidation, respectively. The oxidation of CO on Bi₂WO₆ proceeds at temperatures higher than 200 C, while the complete oxidation of n-hexane was found to occur at temperatures higher than 250 C. A remarkable increase in the catalytic activity was achieved in the case of Bi₂WO₆ supported with 0.14 wt.% Pd. The promotional effect of Bi on the catalytic activity of the Pd loaded sample could be explained by the formation of alloy or inter-metallic phases (α-Bi₂Pd and β-BiPd), which is considered as perspective for the development of a catalyst for treatment of methane emissions.

Full text of DOI:10.1016/j.jallcom.2013.03.157

Journal of Alloys and Compounds 570 (2013) 34–40
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Mechanochemical synthesis, characterization and catalytic activity
of Bi2WO6 nanoparticles in CO, n-hexane and methane oxidation
reactions
a
b
Maria Gancheva a, , Reni Iordanova , Yanko Dimitriev , Diana Nihtianova a,c, Plamen Stefanov a,
⇑
Anton Naydenov a
a Institute of General and Inorganic Chemistry, Bulgarian Academy of Science, ‘‘Acad. G. Bonchev’’ Str., bl. 11, 1113 Sofia, Bulgaria
b University of Chemical Technology and Metallurgy, ‘‘Kl. Ohridski’’ bl. 8, 1756 Sofia, Bulgaria
c Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, ‘‘Acad. G. Bonchev’’ Str., bl. 107, 1113 Sofia, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history:
This article represents data on the mechanochemical synthesis, characterization and catalytic properties
of Bi2WO6 nanoparticles. The synthesis of Bi2WO6 nanopowder (10 nm) started after 90 min of milling
time interval and it was completely finished after 4 h when using 2.5 g batch of the initial oxides. Longer
milling time intervals (8 h) were needed to synthesize the target nanophase (4 nm), when using a 4.0 g
batch of precursor oxides. The Bi2WO6 powders manifest low absorbance ability in the visible range with
a band gap ranging from 2.85 to 2.88 eV. The catalytic activity was investigated in the reactions of CO,
n-hexane and methane oxidation, respectively. The oxidation of CO on Bi2WO6 proceeds at temperatures
higher than 200 °C, while the complete oxidation of n-hexane was found to occur at temperatures higher
than 250 °C. A remarkable increase in the catalytic activity was achieved in the case of Bi2WO6 supported
with 0.14 wt.% Pd. The promotional effect of Bi on the catalytic activity of the Pd loaded sample could be
Received 15 January 2013
Received in revised form 6 March 2013
Accepted 17 March 2013
Available online 28 March 2013
Keywords:
Mechanochemical synthesis
Nanoparticles
Catalytic activity of CO
n-Hexane and methane oxidation
explained by the formation of alloy or inter-metallic phases (
perspective for the development of a catalyst for treatment of methane emissions.
Ó 2013 Elsevier B.V. All rights reserved.
a-Bi2Pd and b-BiPd), which is considered as
1. Introduction
been reported in the current literature: solid state reaction
[3,9,26,27], hydrothermal method [10–19], sol–gel method
[20,21], solvothermal synthesis [22,23], ultrasound-assisted syn-
thesis [24], low-temperature combustion method [25] and mecha-
nochemical treatment [28–30]. A. Castro et al. [28] used
mechanochemical activation for the synthesis of Bi2Mo1ÀxWxO6 so-
lid solution with ferroelectric properties. Maczka et al., applied this
method to the synthesis of Bi2WO6 and Eu3+-Bi2WO6 powders
[29,30]. The authors showed that these powders possess good pho-
tonic and luminescence properties. The mechanochemical tech-
nique possesses some advantages to produce nanocrystalline
materials with a wide range of properties [33–38]. There are many
parameters, which influence the synthesis results such as: milling
speed and time, ball to powder mass ratio and the nature of precur-
sors. Changes in the milling parameters appear to be a decisive fac-
tor for the preparation and the properties of the final product. Our
group performs investigations on mechanochemical synthesis of
different inorganic materials using pure oxides as precursors
[39–42]. This work is a continuation of the efforts in this direction
and accumulation of additional knowledge about the mechano-
chemical synthesis of Bi2WO6.
Bi2WO6 belongs to the Aurivillius family with a general formula
AnÀ1BnO3n+3 (where A = Ca, Sr, Ba, Pb, Na, K; B = Ti, Nb, Ta, Mo, W,
Fe), which possess layered structure [1]. This compound exists in
three polymorphous forms. At room temperature the Bi2WO6 has
orthorhombic structure with space group Pca21, which consists of
alternating Bi2O2 layers and perovskite-like WO4 layers, containing
corner-linked WO6 ochatedra [1–6]. This phase was transformed
into the same symmetry with space group B2cb at 660 °C. This
structural change involves the loss WO6 tilt mode of perovsike-like
WO4 layer of the structure [5,6]. Above 940–960 °C this phase
undergoes phase transition into a high-temperature form having
monoclinic symmetry with space group A2/m [3–7]. The ortho-
rhombic Bi2WO6 founds application mainly in different fields of
catalysis: as photocatalyst for water splitting [8], degradation of
organic compounds under visible light irradiation [9–24] and in
the CO oxidation reaction [26,27]. Moreover Bi2WO6 possesses
some piezoelectric, ferroelectric, luminescence and phonon prop-
erties [28–32]. Different methods for the synthesis of Bi2WO6 have
On other hand the thermal stability of Bi2WO6 could be an
advantage for other applications in catalytic reactions, especially
⇑
Corresponding author. Tel.: +359 979 3588; fax: +359 870 5024.
E-mail address: mancheva@svr.igic.bas.bg (M. Gancheva).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.03.157

M. Gancheva et al. / Journal of Alloys and Compounds 570 (2013) 34–40
35
Fig. 1. (A) X-ray diffraction patterns of the sample A (2,5 g batch) mechanochemically activated at: (a) 0 min; (b) 30 min; (c) 60 min; (d) 90 min; (e) 2 h; and (f) 4 h. (B) X-ray
diffraction patterns of the sample B (4 g batch) mechanochemically activated at: (a) 0 min; (b) 2 h; (c) 4 h; (d) 6 h; and (e) 8 h.
determined using a TEM JEOL 2100 at an accelerating voltage of 200 kV. The spec-
imens were prepared by grinding the samples in an agate mortar and disintegrating
them as ethanol suspension by ultrasonic treatment for 6 min. A droplet of the sus-
pension was coated on standard carbon films on Cu grids. Additional data for crystal
when the practice requires the possibility for stable operation at
high temperatures (above 500 °C), for example in catalytic com-
bustion of volatile organic compounds (VOCs), including methane.
Therefore it is of interest to investigate the catalytic properties of
Bi2WO6 in complete catalytic oxidation of CO and n-hexane as a
first step in the development of catalytic system, capable of resist-
ing the negative changes due to thermal sintering effects.
The aim of the present work was to study the mechanochemical
synthesis of Bi2WO6 from pure oxides at constant milling speed
varying the amount of the precursors and also to test its catalytic
activity in CO, n-hexane and methane oxidation?
structure were obtained by selected area electron diffraction (SAED) method. The
optical properties were measured at room temperature using Evolution 300 UV–
Vis Spectrophotometer in the range 200–1000 nm wavelength. The catalytic tests
were carried out in a continuous-flow type reactor (quartz-glass, inner diameter
of 6.0 mm) at atmospheric pressure. The following testing conditions were chosen:
catalyst particles sieve fraction: 0.31–0.63 mm; GHSV: 30,000 hÀ1. For compensa-
tion of the adiabatic effect of the reaction, the reaction temperature was kept within
the limits of 1 °C, measured both at the inlet and at the outlet of the catalyst
bed. The inlet concentrations of reactants were fixed as follows: CO: 1500 ppm;
n-hexane: 250 ppm; oxygen: 21 vol.%. All feed gas mixtures were balanced to
100% with nitrogen (5.0). The gas analysis was performed by on-line analyzers of
CO/CO2/O2 (Maihak) and THC (total hydrocarbon content, Horiba, with a flame ion-
ization detector, on-line). After the catalytic test the sample was additionally sup-
ported by Pd-nitrate solution (to obtain 0.14 wt.% Pd) and heated at 400 °C for
2 h in air. The presence of the Pd particles on the surface of sample was established
by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy
(XPS). The catalytic test of this sample was performed under same above mentioned
conditions. The morphology and size distribution of palladium particles on the sam-
ple were examined by scanning electron microscopy using a JEOL JSM 6390 electron
microscope (Japan), equipped with ultrahigh resolution scanning system (ASID-3D),
in regimes of secondary electron image (SEI). The accelerating voltage was 20 kV,
2. Experimental
A batch containing stoichiometric mixture of Bi2O3 (Merck, purity 99.9%) and
WO3 (Merck, purity 99.9%) in a 1:1 M ratio was subjected to intensive mechanical
treatment using a planetary ball mill (Fritsch – Premium line – Pulversette No. 7).
Two samples charged as different batch amounts were chosen: sample A containing
2.5 g batch of pure oxides and sample B containing 4 g batch of pure oxides. The fol-
lowing milling parameters were used:
– ZrO2 chambers and balls, with ball diameter of 5 mm/
– Air atmosphere and milling speed of 500 rpm/
– Ball to powder mass ratio of 10:1/
– The degree of phase transformation was determined after different time inter-
vals of treatment (30 min,1,2,3,4,5,6,7 and 8 h)/
– In order to avoid excessive temperature rise within the grinding chamber,
15 min ball milling duration was followed by a pause of 5 min.
I ꢀ 65
spectroscopy (XPS) measurements were performed in a VG ESCALAB II electron
spectrometer using Al K radiation with energy of 1486.6 eV. The binding energies
l
A. The pressure was of the order of 10À4 Pa. The X-ray photoelectron
a
(BE) were determined with an accuracy of 0.1 eV utilizing the C1s line at 285.0 eV
(from an adventitious carbon) as a reference.
Powder XRD patterns were registered on a Bruker D8 Advance diffractometer
3. Results and discussion
using Cu K
resulting from X-ray diffraction broadening were determined using the Scherrer’s
formula: D = Kk/bcos , where D is the mean crystallite size in nm, K is Scherrer’s
constant, conventionally accepted to be 1.0, k the Cu K wavelength (1.5406 Å), b
the full width at half maximum (FWHM) of the diffraction line, in radians, and
a radiation in the 2H range 10–80°. The average crystallite sizes (D)
Fig. 1A and B shows the XRD patterns of mechanochemically
synthesized Bi2WO6 using different batch amounts (sample A con-
tains 2.5 g and B contains 4 g batch) depending on milling time
interval. There is no interaction between oxides after 1 h mechan-
ical treatment of sample A (Fig. 1A-c). The observed broadening
and lower intensity of the diffraction lines is an indication of the
formation of defects and decrease in the crystallites size. New
peaks characteristic of orthorhombic Bi2WO6 (JCPDS-79-2381)
H
a
H
is the corresponding diffraction peak. The calculations were carried out using all
diffraction peaks and the ‘‘PowderCell’’ program. Infrared spectra were registered
in the range 1200–400 cmÀ1 on a Nicolet-320 FTIR spectrometer using the KBr pel-
let technique. Differential thermal analysis was performed on DTA, Stanton Red-
croft at a heating rate of 5 °C minÀ1. The specific surface area of the samples was
measured using a modified BET method. The particle size and morphology were

36
M. Gancheva et al. / Journal of Alloys and Compounds 570 (2013) 34–40
(sample B) led to crystallization of orthorhombic Bi2WO6 (average
crystallite size was 4 nm). The thermal treatment of both phases
did not lead to increase in the crystallites size. With Fig. 4 we pres-
ent evidence for polycrystalline nature of the particles, but some of
them are amorphous also. The main purpose of our TEM investiga-
tion was to confirm the phase composition of the sample A ob-
tained by mechanochemical synthesis. The poorly crystalline
sample makes the investigation by TEM (SAED) very difficult be-
cause the particles are thicker and non-transparent in electron
microscope. From XRD we receive information for crystallite size,
but in bright field micrographs, obtained by TEM we have probably
the particles which consist of small grains size. These particles can
be considered like agglomerates. With Fig. 4 we present only vari-
ations in particles size (20–100 nm) in the sample. The selected
area electron diffraction (SAED) pattern showed that only one crys-
tal phase of orthorhombic Bi2WO6 has been obtained. The observed
strongest peak at (131) as well as the peaks at (141) and (152)
planes are in good agreement with the XRD data (Fig. 1A-f). The
specific surface area of this sample is 5m2/g. A similar value range
for the surface area was reported for this compound, prepared by
solid state reaction, as well as for some other inorganic com-
pounds, obtained by us applying this technique [26,27,39–42].
Fig. 2. DTA analysis of the mechanochemically synthesized Bi2WO6 (samples A)
and amorphous phases (sample B).
were detected after 90 min of milling time (Fig. 1A-d). But still
some small amount of WO3 (JCPDS-83-0951) remained after 2 h
milling time intervals (Fig. 1A-e). The reaction was completed after
4 h of milling time and broad diffraction lines characteristic only of
orthorhombic Bi2WO6 were registered (Fig. 1A-f). The calculated
average crystallite size of the Bi2WO6 is 10 nm. Upon using higher
batch amount (4 g) there is no interaction between oxides after 2 h
mechanical treatment of the sample B (Fig. 1B-b). An intermediate
stage of amorphization was observed after 4 h of milling time
(Fig. 1B-c). The amorphous phase is transformed into single ortho-
rhombic Bi2WO6 after 8 h of milling time (Fig. 1B-e). The calculated
average crystallite size of Bi2WO6 under these experimental condi-
tions is 4 nm. This fact is probably due to the formation of the
amorphous phase, which is a favorable prerequisite for the formation
of the particles of very small size. Maczka et al. [29] realized a solid
state metathesis reaction between Bi(NO3)3Á5H2O, Na2WO4Á2H2O
and NaOH by mechanochemical activation at 350 rpm for 1 h,
forming BiWO precursor, which was washed, milled and treated
thermally. We achieved a faster synthesis technique of the Bi2WO6
(after 4 h activation) using small amount (2.5 g) of pure oxides.
Activation of higher amount (4 g) of pure oxides led to longer time
interval of the synthesis (8 h).
Fig. 2 represents DTA curves of the mechanochemically synthe-
sized Bi2WO6 (sample A) and amorphous phase (sample B), respec-
tively. Two exothermal peaks of low and high intensity were
observed in the DTA curves. The peaks at 440 °C and 470 °C can
be attributed to the process of crystallization (Fig. 1A and B). Com-
paring with the DTA data, reported by Maczka et al. [29], the crys-
tallization temperature of amorphous phase in our case is higher
by 60 °C. This is probably because of the different nature of the pre-
cursors and different synthesis stage. The first low intensive exo-
thermal peaks at 400 °C (sample A) and 440 °C (sample B) can be
attributed to the formation of an intermediate fluorite-like phase
before the crystallization of the Bi2WO6 having Aurivillius type of
structure [28]. The endothermal peaks within the range 870–
920 °C are due to phase transformation of Bi2WO6 from ortho-
rhombic into monoclinic form [3–7]. Taking into account the above
given DTA data we performed thermal treatment of these samples
(Fig. 3A and B). After the thermal treatment of mechanochemically
synthesized Bi2WO6 (sample A) the diffraction lines become shar-
per with higher intensity, which indicates that an additional crys-
tallization process was occurring (average crystallite size was
11 nm). The heat treatment at 470 °C of the amorphous phase
Fig. 3. (A) XRD patterns of the mechanochemically synthesized Bi2WO6 and heat-
treated at 440 °C (sample A). (B) XRD patterns of the amorphous phase and heat-
treated at 470 °C (sample B).

M. Gancheva et al. / Journal of Alloys and Compounds 570 (2013) 34–40
37
bands are characteristic of vibration of the WO6 octahedra building
up the orthorhombic crystalline Bi2WO6 structure. The band at
540 cmÀ1 is superposition between bending vibration of WO6
and stretching and bending vibrations of the Bi–O polyhedra
[12,19,20,29,30,45]. The IR spectrum of amorphous phase, ob-
tained by us, is similar to that obtained by Maczka et al. [29,30]
although they used different precursors. The IR spectra of Bi2WO6
as a function of batch amount showed that the spectra exhibit sig-
nificant changes upon decreasing the particle size. Broadening of
the absorption band, centered at 740 cmÀ1 with lower intensity,
is observed in the spectra of Bi2WO6 containing smaller particles
size (4 nm).
The UV–Vis absorption spectra of the mechanochemically syn-
thesized Bi2WO6 (samples A and B) and amorphous phase (sample
B) are represented in Fig. 6A. The samples show strong absorption
in the UV to visible light range at wavelength shorter than 450 nm,
which was in good agreement with other reports [10–14,
18–25,29,30]. The amorphous phase possesses slightly higher
absorption compared to the two crystalline phases. This fact could
be related to the distortion structure of the amorphous phase. For
crystallite semiconductors, the optical absorption band follows the
equation
is a constant, h is Plank’s constant,
a
h
m
= A(h
m
À Eg)n, where
a
is the absorption coefficient, A
m the photon frequency, Eg the
optical band gap and n1/2 in case of direct semiconductor. An
extrapolation of the linear region of a plot of (
1/2 on the y-axis
versus photon energy (h ) on the x-axis gives the value of the opti-
Fig. 4. TEM micrograph and SAED pattern (inset) of the mechanochemically
synthesized Bi2WO6 (sample A).
ahm)
m
We monitored by IR spectroscopy (Fig. 5A and B) the phase for-
mation of Bi2WO6 using different batch amounts (sample A and B)
depending on the milling time interval. The intensity and the num-
ber of absorption bands of Bi2O3 [43] and WO3 [44] decrease after
mechanical treatment due to destruction of the long range order
and partial amorphization of these oxides (Fig. 5A-b, c and B-b).
The appearance of new absorption bands at 880, 740, 710 and
540 cmÀ1 is the result of formation of new bonds between the ini-
tial oxides during mechanochemical activation. According to the IR
and Raman studies of Bi2WO6 made by Maczka et al. [29,30] these
cal band gap Eg (Fig. 6B) [46]. The calculated band gaps of
mechanochemically synthesized Bi2WO6 (sample A with crystal-
lites size of 10 nm), amorphous and crystalline phase (sample B
with crystallites size of 4 nm) were 2.85, 2.81 and 2.88 eV. The
band gap energies decrease with the increase in the crystallites
size. A similar result was reported in Refs. [12,16,29,30].
The above described properties of mechanochemically synthe-
sized Bi2WO6 enable the option to develop a thermally stable
material with potential application in the environmental
catalysis. In order to investigate this possibility, tests on catalytic
Fig. 5. (A) IR spectra of the sample A (2.5 g batch) mechanochemically activated at: (a) 0 min; (b) 30 min; (c) 60 min; (d) 90 min; (e) 2 h; and (f) 4 h. (B) IR spectra of the
sample B (4 g batch) mechanochemically activated at: (a) 0 min; (b) 2 h; (c) 4 h; (d) 6 h; (e) 8 h.

38
M. Gancheva et al. / Journal of Alloys and Compounds 570 (2013) 34–40
oxidation of CO and n-hexane were performed. Fig. 7 shows the
results from catalytic activity tests obtained with the mechano-
chemically synthesized Bi2WO6 (sample A). It can be seen that
the CO oxidation reaction is proceeding at temperatures higher
than 200 °C and it reaches 98% conversion degree at 395 °C, while
the complete oxidation of n-hexane was found to occur at tem-
peratures higher than 250 °C, a significant conversion degree (of
the order of 70%) was measured only at about 500 °C. It should
be pointed out that the obtained results are in good agreement
with the catalytic activity in CO oxidation reaction on Bi2WO6,
prepared by solid state reaction [26,27]. It should be noted that
there are no data in the literature on the complete oxidation of
n-hexane over Bi2WO6. The catalytic performance of pure Bi2WO6
cannot be characterized as very satisfactory, comparing with the
sample having 0.14 wt.% of palladium supported additionally.
Although the palladium content is very small the effect on the
catalytic activity is remarkable – the decrease in reaction temper-
ature is with about 150 °C in the case of CO oxidation and 130 °C
Fig. 7. Dependencies of the conversion on the temperature during the catalytic
oxidation of CO, n-hexane and methane on the mechanochemically synthesized
Bi2WO6 (sample A) and same sample contains 0.14 wt.% Pd.
Fig. 8. SEM image of the mechanochemically synthesized Bi2WO6 (samples A)
contains 0.14 wt.% Pd.
Table 1
Surface composition (at.%) estimated from the XPS intensities.
Surface composition (at.%)
Fig. 6. (A) UV–Vis absorption spectra of the mechanochemically synthesized
1/2
O
Pd
W
Bi
Bi2WO6 (samples A and B) and amorphous phase (sample B). (B) Plot of the (
versus photon energy (h
ples A and B) and amorphous phase (sample B).
ahm)
m) for the mechanochemically synthesized Bi2WO6 (sam-
71.8
12.5
3.4
12.3

M. Gancheva et al. / Journal of Alloys and Compounds 570 (2013) 34–40
39
Fig. 9. X-ray photoelectron spectra of the mechanochemically synthesized Bi2WO6 (sample A) contains 0.14 wt.% Pd: on the Bi4f and Pd3d levels.
for the complete oxidation of n-hexane (at conversion degree of
50%). Additionally, the presence of palladium on the catalytic sur-
face leads to about fourfold decrease in the selectivity of the cat-
alyst towards undesirable products of incomplete oxidation (in
the case of CO it declined from 6% in the absence of palladium
to less than 1.5% of the totally converted n-hexane, when
0.14 wt.% palladium was applied). The reaction of complete oxi-
dation of methane on Pd/Bi2WO6 was also investigated at temper-
atures up to 505 °C. The maximum conversion degree of methane
was 7.1% and this result shows the need of remarkable improve-
ment of the catalyst in order to apply it in the process of prevent-
ing the global warming by elimination of methane emissions into
the atmosphere. The SEM analysis was performed to study the
distribution and morphology of the palladium particles on the
mechanochemically synthesized Bi2WO6 (sample A). Two areas
with a smooth and rough surface were observed (Fig. 8). The high
magnification image shown in the inset of Fig. 8 clearly proves
that Pd particles are in the range of nanometer size. The SEM im-
age shows that the Pd nanoparticles are not uniformly distributed
on the surface. The surface state of mechanochemically synthe-
sized Bi2WO6 loaded with 0.14 wt.% Pd was analyzed by X-ray
photoelectron spectroscopy. Table 1 represents the surface com-
position of the Pd/Bi2WO6 sample. The observed atomic intensity
ratio Bi/W is much higher than the theoretical value. This means
that the surface layers of tungstate are enriched in bismuth. Fur-
ther, the XPS analysis of Pd/Bi2WO6 sample revealed that a con-
siderable fraction of Pd was in a close contact with Bi as
indicated by both Pd 3d and Bi 4f photoelectron spectra in
Fig. 9. The Bi 4f core level spectra can be fitted by two doublets,
assigned to Bi0 (158.9 eV) and Bi3+ (160.1 eV), respectively. The Pd
3d core level spectra also display two doublets, which correspond
to Pd0 (336.2 eV) and Pd2+ (337.4 eV). The zerovalent Bi is proba-
bly stabilized in association with Pd0 in the form of Bi–Pd alloy
during the pretreatment of Pd/Bi2WO6 at 400 °C. According to
promotional effect of Bi on the catalytic activity of the Pd loaded
sample [48].
4. Conclusion
This study shows the opportunity to synthesize nanostructured
Bi2WO6 by mechanochemical activation of pure oxides. It was
established that the amount of the batch is a decisive factor for
the synthesis time interval. Single Bi2WO6 phase was obtained
after 4 h of milling time at room temperature using a small batch
amount (2.5 g). The increase in the batch amount (4 g) preserving
the other milling parameters constant is a prerequisite for the for-
mation of amorphous phase and it led to prolongation of the syn-
thesis time interval (8 h of milling time). The amorphous phase
favors the obtaining of particles of smaller size (4 nm). The
mechanochemically synthesized Bi2WO6 was successfully investi-
gated in the reaction of complete oxidation of CO, n-hexane, which
was observed at relatively high temperatures (above 200 °C for CO
and above 250 °C for n-hexane). The catalytic activity was in-
creased remarkably when the sample is modified by 0.14 wt.%
Pd, supported on the surface of Bi2WO6. The promotional effect
of Bi on the catalytic activity of the Pd loaded sample could be ex-
plained by the formation of alloy or inter-metallic phases (a-Bi2Pd
and b-BiPd), which is a basis for the development of a stabile cat-
alyst for treatment of methane emissions. The reported results
should be considered as a beginning of the process of catalyst
development and many new tasks related to the optimization of
the composition appear now, which will be the aim of further
investigations.
Acknowledgment
the Bi–Pd phase diagram [47] both the a-Bi2Pd and b-BiPd phases
could be formed at temperatures below 380 °C. Thus the forma-
tion of alloy or inter-metallic phase could explain the
The financial support of the National Science Fund, Ministry of
Education and Science, Bulgaria (Contract No. DFNI – T01/6) is
greatly acknowledged.

40
M. Gancheva et al. / Journal of Alloys and Compounds 570 (2013) 34–40
[27] R. Rangel, P. Bartolo-Perez, E. Martinez, X.A. Trejo-Cruz, G. Diaz, D.H. Galvan,
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