Noble metal catalysts supported on nanofibrous polymeric membranes for environmental applications

Article abstract of DOI:10.1016/j.cattod.2014.03.040

The effect of preparation method on the particle size of palladium and platinum nanoparticles supported on poly(2,6-dimethyl-1,4-phenylene) oxide electrospun membranes by the wet impregnation technique was investigated. Catalysts with similar metal loadin

Full text of DOI:10.1016/j.cattod.2014.03.040

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Contents lists available at ScienceDirect
Catalysis Today
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Noble metal catalysts supported on nanofibrous polymeric
membranes for environmental applications
a
a
b
c
a
K. Soukupa,∗, P. Topka , V. Hejtmánek , D. Petrásˇ , V. Valesˇ , O. Solcová
ˇ
a Institute of Chemical Process Fundamentals of the ASCR, v. v. i., Rozvojová 135, CZ-165 02 Prague 6, Czech Republic
b Centre of Polymer Systems, Polymer Centre, Tomas Bata University in Zlín, nám. T. G. Masaryka 5555, CZ-760 01 Zlín, Czech Republic
c Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, CZ-121 16 Prague, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of preparation method on the particle size of palladium and platinum nanoparticles supported
on poly(2,6-dimethyl-1,4-phenylene) oxide electrospun membranes by the wet impregnation technique
was investigated. Catalysts with similar metal loading (0.63–0.78 wt.%) possessing various mean metal
particle size (2.5–8.7 nm) were prepared employing different impregnation times and nominal metal
loadings. The catalysts were tested in the total oxidation of methanol (1000 ppm in air). The catalytic
activity of platinum catalysts increased with increasing size of Pt nanoparticles while the effect of the plat-
inum loading was not observed. On the other hand, the catalytic activity of palladium catalysts increased
with the increasing palladium loading and did not correlate with the nanoparticle size.
© 2014 Elsevier B.V. All rights reserved.
Received 6 August 2013
Received in revised form 9 January 2014
Accepted 19 March 2014
Available online xxx
Keywords:
Electrospinning
Nanofibrous polymeric membrane
Poly(2,6-dimethyl-1,4-phenylene) oxide
Noble metals
Volatile organic compounds
Catalytic oxidation
1. Introduction
the reaction mixture. Application of nanofibers as excellent sup-
ports is connected to the fact that a variety of polymers can be
Metal nanoparticles play an irreplaceable role in many fields of
chemistry and physics owing to their high dispersion, large concen-
tration of highly under-coordinated surface sites, and the presence
of quantum confinement effects, which can improve their reac-
tivity [1,2]. Nevertheless, the major difficulty with nanoparticles
lies in their undesirable agglomeration to form larger particles.
To prevent the formation of such agglomerates, surfactants [3],
ligand exchange materials [4] or polymeric carriers [5] are exten-
sively applied. One of the new routes seems to be the deposition of
nanoparticles from solution directly on electrospun fibers [6–8];
however, the polymeric nanofibers do not exhibit the quantum
effects usually associated with nanoparticles. The reason lies in
the fact that the nature of the electronic states of the organic
compounds including polymeric materials, which can control both
electronic and optical properties, does not correspond to the nature
of semiconductors or conductors [9].
prepared by electrospinning. They meet the various requirements
on supports; moreover, their high porosity and the interconnec-
tivity as the electrospun supports endow them with a low mass
transport resistance [10].
A number of different techniques including template synthesis,
phase separation, self-assembly, drawing, etc. [11] can be generally
used for preparation of the polymeric nanofibrous mats and mem-
branes. However, electrospinning currently represents not only the
most straightforward but also the cheapest way to the fabrication
of nanofibers. Fiber formation during the electrospinning process
is based on electrical forces and takes place via a very peculiar self-
organization process driven by the repulsive electrostatic forces
[12].
The general electrospinning setup consists of two major parts—a
high voltage power supply and two opposite charged electrodes.
During the electrospinning process, the polymer solution or the
molten polymer precursor goes through a hollow electrode that is
usually in the form of a thin nozzle or a syringe and receives some
quantum of the charge from this electrode. A strong electrostatic
field is applied between the electrode and the grounded collector.
The electric field draws a solution droplet into the micro-object,
commonly known as the Taylor cone [12]. As soon as the elec-
tric forces overcome the surface tension of the polymer solution
or melt, the polymer is attracted to the collector and a polymer jet
Especially in the area of catalysis, the polymeric electrospun
supports reveal the distinctive characteristics and superiority.
Nanofibrous supports allow the control of the specific surface area,
the catalyst accessibility, as well as the catalyst regeneration from
∗
Corresponding author. Tel.: +420 220 390 283; fax: +420 220 920 661.
E-mail address: soukup@icpf.cas.cz (K. Soukup).
http://dx.doi.org/10.1016/j.cattod.2014.03.040
0920-5861/© 2014 Elsevier B.V. All rights reserved.
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is created. As the jet dries out in flight, it is elongated by a whip-
ping process caused by the electrostatic repulsion initiated at small
bends in the fiber until it is finally deposited on the surface of the
grounded collector. The elongation of the fibers leads to the forma-
tion of uniform fibers with the nanometer scale diameters which
can be collected in various orientations to create unique structures
with different composition and mechanical properties.
electric conductivity to 150 ␮S/cm by adding tetraethylammonium
bromide to the prepared polymeric solution, which was found as
optimum for the electrospinning process.
Nanofibrous membranes were prepared immediately from the
fresh solution by the electrospinning technique. The used commer-
cial equipment (Elmarco Inc., Czech Republic) depicted in Fig. 1
consists of the steel multi-jet spinning electrode and the steel
plate as the collecting electrode. The high voltage electrospinning
electrode (multi-jet head) with +80 kV of DC, electrode distance
180 mm, temperature 21 2 ◦C, relative humidity 25 2.5% and
solution flow rate of 0.4 ml/min was used for the multi-jet elec-
trospinning process. The steel multi-jet spinning electrode was
immersed into the polymeric solution during the whole electro-
spinning process.
The utilization of the polymeric electrospun nanofibrous sup-
ports loaded with monometallic nanoparticles (such as platinum,
palladium or rhodium) in heterogeneous catalysis has already been
reported in the literature [6–8]; however, a systematic study con-
cerning the effect of deposition conditions on nanoparticle size
distribution and catalytic activity has occurred only rarely.
Generally, two routes exist for the deposition of the metal-
lic nanoparticles on the electrospun nanofibrous support. Within
the first approach, the polymeric nanofibrous support is typically
electrospun directly from the solution containing metal salts as
appropriate precursors. Demir et al. [6] focused on the prepa-
ration of palladium nanoparticles on electrospun copolymers of
acrylonitrile and acrylic acid. The nanofibrous catalyst was electro-
spun directly from the homogeneous solution containing polymers
and PdCl2 precursor. Afterwards, Pd salt was reduced to zero-
valent catalytic active Pd nanoparticles by means of the treatment
in an aqueous hydrazine solution at room temperature. Its cat-
alytic activity was then tested in the selective hydrogenation of
dehydrolinalool in toluene at 90 ◦C. The Authors found that the
Pd nanoparticle size depends mainly on the amount of functional
groups, acrylic acid and PdCl2 concentration in the spinning solu-
tion since the increasing concentrations of acrylic acid and PdCl2
on polymer chains lead to larger Pd nanoparticles.
The other approach of the metal deposition on nanofibrous
supports is represented by the impregnation technique. Prepared
electrospun support is generally immersed into the impregna-
tion solution containing the appropriate metal salt. During the
next step a catalyst precursor is reduced either purely thermally
in air or in the presence of the reducing agents such as H2 or
hydrazine. Ebert et al. [7] investigated the catalytically active
poly(amideimide) nanofibrous membranes in the liquid-phase
hydrogenation of methyl-cis-9-octadecenoate. The nanofibrous
supports were immersed into the activation solution for 1 min, then
removed from the bath and fixed in a metal frame and heated in
air. Pd nanoparticles with an average size of 2.4 nm homogeneously
dispersed on the surface of the individual polymeric fibers were
obtained. The prepared electrospun catalytic system showed the
almost seven times higher catalytic activity than the commonly
used Pd/Al2O3 catalyst.
Prepared membranes were dried overnight in air at 140 ◦C (nor-
mal boiling point of chlorobenzene is 131 ◦C) to a constant weight.
2.2. Characterization of parent electrospun membranes
Three standard methods including physical adsorption of nitro-
gen, high-pressure mercury porosimetry, and helium pycnometry
were utilized for the determination of the texture characteristics of
prepared catalysts.
The basic texture characteristics involving the BET surface area
SBET, the mesopore surface area Smeso and the micropore volume Vꢀ
were evaluated from the nitrogen physical adsorption–desorption
isotherms measured at 77 K by means of the ASAP 2020M instru-
ments (Micromeritics, USA).
The specific surface area, SBET
, was evaluated from the
nitrogen adsorption isotherm in the range of relative pressure
p/p0 = 0.05–0.25 (p is the adsorbate pressure and p0 is the adsorbate
vapour pressure at the measuring temperature) using the standard
Brunauer–Emmett–Teller (BET) procedure [13].
The high-pressure mercury porosimetry performed on AUTO-
PORE III instrument (Micromeritics, USA) was used for the
determination of the intrusion volume Vintr, the apparent density
ꢁHg and the pore-size distribution curves. Washburn equation was
used for the evaluation of the pore sizes:
ꢀ ꢁ
1
p
d = −
4ꢂcos
ϕ
(1)
where d is the pore diameter, p the applied pressure, ꢂ the surface
tension and ϕ the contact angle. The skeletal density ꢁHe was deter-
mined on the AccuPyc 1920 instrument (Micromeritics, USA) and
the porosity ε of all studied samples was determined according to
ꢀ
ꢁ
ꢁHg
ꢁHe
ε = 1 −
(2)
The main objective of the present study is to investigate the
effect of the impregnation method on the particle size of palla-
dium and platinum catalysts supported on polymeric electrospun
membranes based on poly(2,6-dimethyl-1,4-phenylene) oxide. The
catalysts prepared by the wet impregnation technique were char-
acterized by the electron microscopy, textural analysis and X-ray
diffraction. The correlations between the noble metal particle size
and the catalytic activity of the prepared nanofibrous catalysts were
studied through the model gas-phase reaction—total oxidation of
methanol in air.
Morphology of the virgin membranes was studied by scanning
electron microscope (TESCAN, Vega II LSH, Czech Republic). Mem-
branes were sputtered with Au/Pt in plasma (approximately 5 nm)
and analyzed for their fiber diameter and the pore-network topol-
ogy.
2.3. Electrospun catalysts preparation
Both palladium and platinum nanoparticles were deposited on
the parent poly(2,6-dimethyl-1,4-phenylene oxide) electrospun
support (denoted as PPO) by means of a wet impregnation route.
Acetone and methanol were chosen as the appropriate indifferent
solvents towards the parent PPO support. Palladium(II) acetate
(99.9% purity, Aldrich, USA) and platinum(II) acetylacetonate (98%
purity, Stream Chemicals, USA) were used as precursors. Both
metal salts were dissolved separately in the mixture of acetone
and methanol (volumetric ratio 2:1). Finally, citric acid was added
into the impregnation solution since it was found [14,15] that the
2. Experimental
2.1. Electrospun support preparation
Poly(2,6-dimethyl-1,4-phenylene oxide) (Aldrich, powder of
p.a. grade) was dissolved in a mixture of chlorobenzene (Penta
Chemikalie, Czech Republic, p.a.) and tetrachloroethane (Penta
Chemikalie, Czech Republic, p.a.) with volume ratio 1:1. The poly-
mer weight concentration was adjusted to 15% (w/v) and the
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3
Fig. 1. Scheme of the electrospinning setup.
dispersion of the noble metal catalysts deposited on the surface of
the polymeric membrane can be positively affected by the addition
of citric acid, especially in the low-temperature calcination. The
main reason lies in the fact that citric acid improves the dispersion
by serving as anion and by creating a reducing atmosphere during
the calcination.
a carbon foil. The X-ray diffraction patterns were measured by
the PANalytical-MPD diffractometer in the conventional focusing
Bragg–Brentano geometry with variable slits using Ni-filtered Cu
K␣ characteristic radiation. PIXcel PSD detector was used to collect
the scattered intensity in the diffraction angle range 5–100◦ with
the step of 0.015◦.
The impregnation conditions were varied as shown in Table 1.
The initial concentrations of metal salts as well as citric acid in the
impregnation solution were the same in all cases (0.34 wt.% and
3.5 wt.%, respectively). Before the impregnation procedure, the PPO
support was cut into 1–5 mm pieces that were washed with acetone
and dried in static air at 150 ◦C for 12 h. After that, nanofibrous
supports were immersed into the stirred activation solution (the
stirring rate was set at 200 rpm) and filtered out after 15 or 60 min
of impregnation duration (see Table 1). Catalysts were dried in air at
room temperature for approximately 3 h. After drying, reduction by
heat treatment (calcination) was performed in a convection oven
at the temperature of 175 ◦C for 6 h. During the calcination process
the catalysts color turned from yellowish to gray. Prepared samples
are referred to as XPd/PPO and XPt/PPO, where X is the palladium
and platinum content, respectively, in wt.% according to ICP-MS
analysis (see Table 1).
2.5. Determination of metal nanoparticle size
The image processing and the image analysis of the TEM micro-
graphs were performed with the public domain software ImageJ
1.44 (National Institutes of Health, USA). Due to small nanofiber
dimensions, all TEM images were 8-bit (256 gray-level values) and
high-quality (resolution of 1600 × 1400 pixels) with appropriate
magnification (70,000–100,000×) (Fig. 2a). Each raw image was
subjected to a sequence of operations that consists of nonlinear fil-
tering, contrast enhancement, and segmentation [16]. To remove
the random noise from images, the median filter with kernel mask
of 3 × 3 pixel size was used. This filter provides a good capability
of noise reduction and induces considerably less blurring of images
in comparison with linear filters (e.g. averaging filter). The filtered
images were very light and had a low dynamic range. Therefore,
the grayscale intensities of the images were enhanced by the tech-
nique of unsharp masking, which tends to have higher contrast
even for little speckles (Fig. 2b). The dark shadowed objects were
recognized as metal deposits. These were extracted from grayscale
images by the segmentation method [17] which is partitioning of
images into subjects (metal deposits as foreground, other ones as
background) that are meaningful and easier to analyze. The isola-
tion of the individual metal deposits in images was performed by
the morphological operation (Fig. 2c). In this way, the deposit con-
tours were smoothed and thin protrusions were eliminated to the
separately standing metal deposits. These are located on the orig-
inal image (Fig. 2d). The area of the individual metal deposits on
images was calculated by the image scale and transformed to the
2.4. Catalysts characterization
The noble metal loading of prepared catalysts was determined
by the inductively coupled plasma mass spectrometry (ALS, Czech
Republic). The mean metal nanoparticle size, together with the
particle size distribution, was determined by means of the trans-
mission electron microscopy (Philips EM 201, 80 kV, W filament,
The Netherlands) and the X-ray diffraction method (XRD). The used
TEM resolution was set at appropriate value to enable the reli-
able determination of the metal nanoparticle sizes. Before the TEM
analysis, electrospun samples were dispersed in isopropanol solu-
tion and put directly onto a copper grid of 300 mesh coated with
Table 1
Impregnation conditions.
Sample
Metal salt* concentration [wt.%]
Citric acid concentration [wt.%]
Nominal loading** [wt.%]
Real loading*** [wt.%]
Impregnation time [min]
0.63Pd/PPO
0.78Pt/PPO
0.67Pd/PPO
0.67Pt/PPO
0.71Pd/PPO
0.65Pt/PPO
0.34
0.34
0.34
0.34
0.34
0.34
3.5
3.5
3.5
3.5
3.5
3.5
9.0
9.0
4.5
4.5
9.0
9.0
0.63
0.78
0.67
0.67
0.71
0.65
15
15
15
15
60
60
*
Palladium(II) acetate (XPd/PPO) or platinum(II) acetylacetonate (XPt/PPO).
Computed from the initial amount of either Pd or Pt in the impregnation solution.
Determined from ICP-MS analysis.
**
***
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Fig. 2. TEM micrographs of Pd nanoparticles on nanofiber. (a) Original image (magnification 70,000×, 0.85 nm/pixel), (b) filtered image after unsharp masking, (c) thresholded
image after removing very thin protrusions (<1 nm2), (d) original image with white outlined metal deposits.
equivalent deposit diameter. The size distribution of metal deposits
was considered as lognormal and the median was computed for
every metal loading.
the average temperature was taken as the catalyst temperature.
The inlet concentration of methanol in the air was 1000 vol-
ume ppm. The catalysts were used without any pretreatment.
Before the experimental data were taken, the catalytic bed was
kept under the feed stream until the methanol outlet concen-
tration became constant. The reaction products were analyzed
by a gas chromatograph and an infrared analyzer; temperatures
T50 and T90 (temperature of 50 and 90% conversion of methanol,
respectively) and selectivity to CO2 at 90% methanol conver-
sion S90 were employed as a measure of activity and selectivity
of the catalysts. Detailed information can be found elsewhere
[18].
2.6. Catalytic test
Total oxidation of methanol was carried out in the fixed-bed
glass reactor (inner diameter 5 mm) in the temperature range
from 50 to 230 ◦C (the furnace temperature linearly increased
with the rate of 2 ◦C /min). 0.2 g of the catalyst was tested
at the 20 m3/kg/h space velocity. Temperature was measured
inside the reactor in front of and behind the catalyst bed and
(a)
(b)
60
50
40
30
20
10
0
25
75
125 175 225 275 325 375
Fiber diameter [nm]
Fig. 3. SEM image (a) and fiber diameter distributions (b) for the parent support.
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Table 2
5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Textural properties of parent electrospun support.
SBET [m2/g] Vintr [cm3/g] ꢁHe [g/cm3] ꢁHg [g/cm3] ε [−]
PPO support 4.3
0.8
1.12
0.22
0.80
appears that nanofibrous support exhibits maximum on PSD curve
corresponding to the pore radius of approximately 850 nm. High
porosity, together with the prevailing wide pores, is essential for
the effective catalyst support having the low fluid transport resis-
tance.
3.2. Characterization of the electrospun catalysts
Three pairs of Pd and Pt catalysts were prepared using differ-
ent impregnation conditions. The precursor concentrations were
kept constant for all six prepared catalysts (0.34 wt.%). Table 1
summarizes the impregnation conditions together with the real
Pd and Pt loadings on the nanofibrous support. Contents of Pd in
the prepared catalysts were 0.63, 0.67, and 0.71 wt.%; contents of
Pt were detected as follows: 0.65, 0.67, and 0.78 wt.%. It appears
from Table 1 that neither Pd nor Pt real loadings depend on the
impregnation time or the nominal metal loading. On the basis of
our investigation, it seems that the dominant parameter affecting
the final concentration of the noble metal catalyst supported on the
nanofibrous mat is the precursor concentration in the impregnation
solution. This observation might be explained by the equilibrium
between adsorbed cations and cations in the impregnation solu-
tion. The rate of the metal cation adsorption on nanofibers is so fast
that the equilibrium between adsorbed cations and bulk cations
in the solution is established during the first 15 min. Therefore,
a longer impregnation time did not affect the resulting catalyst
loading.
Fig. 5 illustrates the representative X-ray diffraction patterns for
Pd and Pt catalysts. It can be easily recognized that the characteristic
diffraction lines are rather weak, which is in a good agreement with
the high dispersion of the noble metals on electrospun supports.
Both X-ray diffractograms revealed the broad reflection peaks that
could be assigned to the amorphous phase of the parent PPO. The
main line appeared around 2Â = 40.1◦, which corresponds to the
(1 1 1) line of the nanocrystalline Pd crystallized in a face-centered
cubic (fcc) structure. For Pt catalyst, the presence of the weak
diffraction lines at 39.6◦ (1 1 1), 46.2◦ (2 0 0) and 80.3◦ (1 1 3) cor-
responds to the metallic nanocrystalline Pt in the same cubic (fcc)
structure.
100
101
102
103
104
105
106
r [nm]
Fig. 4. Pore-size distribution of the parent support evaluated from the high-pressure
mercury porosimetry.
3. Results and discussion
3.1. Characterization of the parent electrospun membrane
The poly(2,6-dimethyl-1,4-phenylene) oxide was selected as
an appropriate catalyst support owing to its good microstruc-
ture properties [8] as well as the sufficient thermal stability (the
experimentally determined glass transition temperature was found
to be 215 ◦C). The areal density of the parent nanofibrous mem-
brane was 223 g/m2. The SEM image shows the typical morphology
of the prepared PPO membrane (Fig. 3a). It is evident that the
membrane predominantly contains the straight fibers with the
mean diameter 150 nm (Fig. 3b). For statistical evaluation, more
than 40 randomly distributed fibers over the SEM picture were
taken into account. However, some beads can be also observed
on the straight nanofibers. Texture properties of the electrospun
support are summarized in Table 2. It can be seen that the par-
ent support revealed the rather low inner surface area and the
high porosity. Pore-size distribution (PSD) obtained from the high-
pressure mercury porosimetry measurement is depicted in Fig. 4. It
50000
26000
Pd/PPO
Pt/PPO
45000
111
21000
16000
11000
6000
111
200
40000
35000
30000
25000
20000
15000
113
0
20
40
60
80
100
0
20
40
60
80
100
2Θ [degrees]
2Θ [degrees]
Fig. 5. X-ray diffractograms of the typical Pd and Pt catalyst.
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Table 3
Dependence of metal nanoparticle size on impregnation conditions.
Sample
Real loading [wt.%]
Impregnation time [min]
TEM particle size* [nm]
XRD particle size** [nm]
0.63Pd/PPO
0.78Pt/PPO
0.67Pd/PPO
0.67Pt/PPO
0.71Pd/PPO
0.65Pt/PPO
0.63
0.78
0.67
0.67
0.71
0.65
15
15
15
15
60
60
3.0
6.1
5.5
2.5
2.5
7.9
3.8
11.1
3.9
–
–
–
*
Determined from TEM micrographs by image analysis method.
Determined from X-ray line broadening using Scherrer equation.
**
Characterization of the size of metal nanoparticles deposited
X-ray diffraction lines broadening using the Scherrer equation
[19]:
on individual nanofibers was performed by XRD and TEM.
However, XRD calculation could be reliably carried out only for
some samples (see Table 3) due to the very weak diffraction lines
of noble metals. The mean particle size was determined by the
Kꢃ
L =
(3)
2Âcos
Â
Fig. 6. TEM micrographs of electrospun catalysts.
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30
30
0.63Pd/PPO
0.78Pt/PPO
25
20
15
10
5
25
20
15
10
5
0
0
1
3
5
7
9
11 13 15
2
6
10 14 18 22 26 30
Nanoparticle diameter [nm]
Nanoparticle diameter [nm]
50
40
30
20
10
0
80
60
40
20
0
0.67Pt/PPO
0.67Pd/PPO
2
8
14 20 26 32 38 44 50
Nanoparticle diameter [nm]
0
4
8
12 16 20 24 28
Nanoparticle diameter [nm]
60
50
40
30
20
10
0
50
40
30
20
10
0
0.71Pd/PPO
0.65Pt/PPO
0
4
8
12 16 20 24 28 32 36 40
4
6
8
10 12 14 16 18 20
Nanoparticle diameter [nm]
Nanoparticle diameter [nm]
Fig. 7. Distribution of palladium and platinum particle size on electrospun support.
where K is the geometrical factor, ꢃ is the wavelength of Cu K␣
radiation, 2Â is the difference of the width of diffraction lines and
 represents the diffraction angle. The calculated average sizes of
Pd and Pt nanocrystals are summarized in Table 3.
Except for the well-established XRD method, the size of the
catalytically active particles, together with their dispersion on the
outer surface of nanofibers, can be simply evaluated from the TEM
micrographs. Fig. 6 shows the TEM images of supports loaded with
Pd and Pt nanoparticles in the bright contrast. Metal nanoparti-
cles appear as black spots on the lighter background corresponding
to nanofibers. The nanoparticles appear to be rather spherical for
all samples independently on impregnation conditions at the used
magnification.
Based on the TEM micrographs, the area of the individual metal
nanoparticles on images was calculated by the image scale and
transformed to the equivalent diameter (see Table 3 and evalu-
ated histograms in Fig. 7). The size distribution of the metal deposit
was considered as lognormal and the median was computed for
various metal loadings. It can be seen that the nanoparticle-size
distribution for the 0.63Pd/PPO and 0.78Pt/PPO catalysts is gen-
erally broader than for the other samples. The 0.63Pd/PPO catalyst
revealed broader maximum of a histogram in the range of 1.5 nm to
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100
100
(b)
(a)
80
60
40
20
0
80
60
40
0.63Pd/PPO
20
0.78Pt/PPO
0.67Pt/PPO
0.65Pt/PPO
0.67Pd/PPO
0.71Pd/PPO
0
40
80
120
160
200
240
40
80
120
160
200
240
T [oC]
T [oC]
Fig. 8. Catalytic activity of electrospun catalysts based on palladium (a) and platinum (b) in the total oxidation of methanol.
3.5 nm, while the 0.78Pt/PPO catalyst showed a maximum between
the T50 decreases with the increasing of the mean Pt particle sizes.
This is in agreement with the generally observed trend in the oxi-
dation of volatile organic compounds over Pt and Pd catalysts—the
oxidation rates increase with increasing of the metal particle sizes
[20].
3 nm and 7 nm. The 0.67Pd/PPO and 0.67Pt/PPO catalysts exhibited
a narrow nanoparticle-size distribution with the maxima ran-
ging from 2.5 nm to 7.5 nm and at 3 nm, respectively. Finally, the
0.71Pd/PPO catalyst showed the most frequented distribution of
the nanoparticle size between 1 nm and 3 nm and the 0.65Pt/PPO
catalyst revealed the nanoparticle-size distribution corresponding
to the bigger crystallites in the range from 5 nm to 13 nm.
It appears from comparison of the results summarized in Table 3
that the mean Pt particle size is larger if the nominal loading (i.e.
molar amount of Pt precursor in the impregnation solution) is
higher. Moreover, the mean Pt particle size remains almost the
same in the case of the different impregnation times, but the same
molar amount of Pt in the impregnation solution is employed.
In contrast, none of these trends was observed for Pd catalysts.
However, it should be noted that the calculated values of the mean
Pd particle size were affected by the statistically significant pop-
ulation of the large Pd particles (tens of nanometers—compare
Figs. 6 and 7), which were detected in the case of 0.67Pd/PPO and
0.71Pd/PPO catalysts.
In contrast, Hinz et al. [21] reported that the catalytic activity of
Pt/Al2O3 catalysts with a similar Pt dispersion in methanol oxida-
tion increased with increasing Pt loading. A similar behavior was
observed with our Pd/PPO catalysts (see Fig. 10). Their catalytic
activity increased with increasing Pd loading (the T50 tempera-
tures were 155, 143 and 130 ◦C for the 0.63Pd/PPO, 0.67Pd/PPO and
0.71Pd/PPO catalysts, respectively) while the effect of the mean Pd
particle size was not observed (the mean particle sizes were 3.0,
5.5 and 2.5 nm, respectively, for the above mentioned catalysts).
It appears from the literature [22] that the activity of the cat-
alysts in the oxidation of volatile organic compounds depends on
the type of the noble metal. The different activity trends observed
for the Pt and Pd catalysts have been rationalized in terms of the
relative adsorption–desorption strengths of reactants [23] or differ-
ent reaction mechanisms [24] on the different metals. Therefore,
it may be assumed that the different nature of the investigated
3.3. Catalytic tests
160
140
120
100
80
The methanol conversion curves obtained over investigated cat-
alysts are depicted in Fig. 8 and corresponding temperatures of 50
and 90% methanol conversion are summarized in Table 4. For all
tested catalysts, the selectivity to CO2 was 100% and no by-products
were detected during the experiments. With the 0.65Pt/PPO cata-
lyst, no deactivation was observed during the steady-state test at
153 ◦C that lasted for 8 h.
The catalytic activity of the Pt catalysts decreased in the order of
0.65Pt/PPO > 0.78Pt/PPO > 0.67Pt/PPO as the T50 temperatures were
88, 128 and 152 ◦C, respectively. While the catalytic activity was not
affected by the platinum loading, a significant effect of the mean
size of Pt nanoparticles was observed. From Fig. 9 it is seen that
Table 4
Activity of the catalysts in total oxidation of methanol.
Catalyst
T50 [◦C]
T90 [◦C]
0.63Pd/PPO
0.78Pt/PPO
0.67Pd/PPO
0.67Pt/PPO
0.71Pd/PPO
0.65Pt/PPO
155
128
143
152
130
88
200
161
200
226
199
126
2
4
6
8
10
Pt mean particle size [nm]
Fig. 9. Dependence of catalytic activity on the mean size of Pt nanoparticles.
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catalyst preparation and the catalytic nanoparticle size distribu-
tion, as well as the catalytic activity, were obtained on the basis of
a limited set of experimental data. Despite this fact, we believe that
the present study brings a new insight into the issue of the noble
metal-immobilized electrospun polymer nanofibers. Additionally,
it may help us understand the fundamental relationships between
the electrospun catalyst preparation conditions and the catalytic
activity.
155
145
135
Acknowledgments
The financial support of Grant Agency of the Czech Republic
(grant no. P106/11/P459 and grant no. 13-24186P) is gratefully
acknowledged.
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Please cite this article in press as: K. Soukup, et al., Noble metal catalysts supported on nanofibrous polymeric membranes for environ-
mental applications, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.040