Synthesis and performance of nano silver coated ZSM-5/SBA-15

Article abstract of DOI:10.1166/jnn.2017.12853

A new synthetic approach were chosen to prepare the desired small Ag nanoparticles on a mesoand microporous ZSM-5/SBA-15 system. By the functionalization of ZSM-5/SBA-15 surface with the amine groups, it was possible to obtain small coated silver nanoparticles (in the range of 2 to 5 nm) formed both inside and outside the mesoporous system. The size and distribution of AgNPs can be shown by transmission Electron Microscopy and by the decrease of mesoporous volume of Ag/ZSM-5/SBA-15 sample. The catalytic activity of AgNPs and the effect of mesopore system were proved by antibacterial and benzene oxidation applications. AgNPs inside the mesoporous system of Ag/ZSM-5/SBA-15 limited its antibacterial performance. The improved performance of this catalytic system could be shown in the benzene oxidation, for which a total benzene oxidation temperature of 300 °C could be observed.

Full text of DOI:10.1166/jnn.2017.12853

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Nanoscience and Nanotechnology
Vol. 17, 1813–1819, 2017
Copyright © 2017 American Scientific Publishers
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Synthesis and Performance of Nano Silver
Coated ZSM-5/SBA-15
Le Thi Hoai Nam1ꢀ∗, Tran Quang Vinh1ꢀ∗, Nguyen Thi Bich Hong1, Pham Minh Duc1,
Nguyen Van Quyen1, Bui Quang Hieu1, Nguyen Thi Nhiem1, Le Kim Lan1, and Joerg Radnik2
1Institute of Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Street,
Cau Giay District, Hanoi, 10000, Vietnam
2Leibniz-Institute of Catalysis, University of Rostock (LIKAT), Albert-Einstein-Str. 29a, 18059 Rostock, Germany
A new synthetic approach were chosen to prepare the desired small Ag nanoparticles on a meso-
and microporous ZSM-5/SBA-15 system. By the functionalization of ZSM-5/SBA-15 surface with
the amine groups, it was possible to obtain small coated silver nanoparticles (in the range of 2 to
5 nm) formed both inside and outside the mesoporous system. The size and distribution of AgNPs
can be shown by transmission Electron Microscopy and by the decrease of mesoporous volume
of Ag/ZSM-5/SBA-15 sample. The catalytic activity of AgNPs and the effect of mesopore system
were proved by antibacterial and benzene oxidation applications. AgNPs inside the mesoporous
system of Ag/ZSM-5/SBA-15 limited its antibacterial performance. The improved performance of
this catalytic system could be shown in the benzene oxidation, for which a total benzene oxidation
ꢀ Delivered by Ingenta to: Purdue University Libraries
temperature of 300 C could be observed.
IP: 5.8.37.237 On: Wed, 07 Dec 2016 04:03:13
Copyright: American Scientific Publishers
Keywords: Nano Silver, ZSM-5/SBA-15, Benzene Oxidation, Modification.
1. INTRODUCTION
system of ZSM-5 (5.5 Å). Therefore, its catalytic activity
could be limited.
ZSM-5 zeolite has been well known as a benzene and
its derivatives adsorbate. A Benzene molecule is roughly
around 6 Å in size. It can be adsorbed on the 5.5 Å micro-
pore in the zeolite pore framework by porous adsorption.
In addition, benzene and its derivatives can be adsorbed
on the bronsted sites of ZSM-5 by an interaction between
benzene molecule and proton and/or Na+ cation of bron-
sted site.1ꢀ2
Silver particles at nanosize (AgNPs) have been well
known as an excellent catalyst for oxidation because of the
subsurface oxygen species formation at low temperature.3
By coating AgNPs on ZSM-5, it showed that AgNPs were
formed with very small size (2–3 nm).4 Ag/ZSM-5 could
be a effective catalyst for benzene and its derivatives oxi-
dation. However, the activity of AgNPs mainly depends
not only on their size but also on their content. Because
the bronsted sites number is limited, ZSM-5 could not load
high Ag content by ion exchange method. In addition, ben-
zene molecules (6 Å) could not diffuse into the micropore
The synthesis and application of ZSM-5/SBA-15 com-
posite material (Z5S15) have been much interested in
recent times.5–11 Z5S15 material has similar micropore
system and bronsted sites to ZSM-5 zeolite, so it can
well absorb benzene and its derivatives. AgNPs also can
be loaded at bronsted site positions of Z5S15 material.
In addition, this composite material also has SBA-15 struc-
ture which has a lots of silanol groups (–OH) on the
surface. Therefore, it is the same as SBA-15, Z5S15 mate-
rial can be functionalized to produce surface functional
groups which can help it loading higher AgNPs content
than ZSM-5 zeolite.
Furthermore, Z5S15 material has a multiporous struc-
ture of micro- and mesopores which has a beneficial influ-
ence on the catalytic performance.10ꢀ11 Benzene molecules
could diffuse through the mesopore system of Z5S15.
Therefore, it may help AgNPs coated Z5S15 (Ag/Z5S15)
material having high catalytic activity in benzene or its
derivatives oxidation.
In this study, the Ag/Z5S15 preparation was investi-
gated to optimize the content and size of the formed
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 3
1533-4880/2017/17/1813/007
doi:10.1166/jnn.2017.12853
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Synthesis and Performance of Nano Silver Coated ZSM-5/SBA-15
Nam et al.
ꢀ
AgNPs. The presence, content, size, and valence state of
AgNPs were characterized by X-ray diffraction (XRD),
atomic absorption spectroscopy (AAS), transmission elec-
tron microscopy (TEM), and X-ray photoelectron spec-
troscopy (XPS) methods. Before the coating process,
Z5S15 material was functionalized via two steps:
(1) P123 template removal using H2O2 oxidation agent to
remain silanol groups and
(2) functionalization with (3-Aminopropyl)triethoxysilane
(APTES) to produce functional amine groups on the sur-
face of Z5S15.
The obtained materials were characterized by infrared
spectroscopy (IR), XRD, and thermo gravimetric analysis
(TGA) methods.
90 C for 24 h in Argon flow (2 ml/min) to avoid water
moisture absorption from the air. The solid was filtered
and washed with 100 ml anhydrous toluene. After that, it
was dried at 100 ꢀC in vacuum for 12 h. The solid product
was signed as Z5S15-APTES.
2.5. Synthesis of Ag/Z5S15 Material
The nominal loading Ag content was fixed at 8 wt%.
Ag/Z5S15 sample was synthesized by dispersing Z5S15-
APTES sample in 10−3 M AgNO3 solution. The exchange
reaction was performed by stirring mixture in the dark for
24 h. Ag+ ion was reduced by adding a sufficient amount
of 0.05 M NaBH4 solution into resulting mixture. This
mixture was further stirred for 24 h. The solid was filtered
andꢀwashed by distilled water. Then, the solid was dried at
60 C for 24 h in vacuum. The solid product was signed
as Ag/Z5S15 sample.
2. EXPERIMENTAL DETAILS
2.1. Chemicals
The chemicals used for the preparation of Ag/Z5S15
are AgNO3 (>99%, Sigma-Aldrich); H2O2 solution
(30% w/w, Sigma-Aldrich); ethanol (>99.8%, china);
APTES (99%, Sigma-Aldrich); Toluene (anhydrous,
99.8%, Sigma-Aldrich); Distilled water.
2.6. Performance Tests
In order to evaluate the effect of mesopore system of
Ag/Z5S15 on its activity, Ag/Z5S15 sample was evaluated
in two applications, including E. coli bacterial elimination
and benzene oxidation. The activity of Ag/Z5S15 sample
was compared to that of Ag/ZSM-5 material that was syn-
thesized and characterized in our previous article.4
2.2. Modification of the Z5S15 Composite Material
The Z5S15 composite material (Si/Al molar ration of 50)
was successfully synthesized and characterized accord-
2.7. E. coli Bacterial Elimination
The procedures of antibacterial experiment include medi-
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ing to our previous report.8 The modification of Z5S15
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was performed by using the as-synthesized Z5S15 (signed
as Z5S15-AS). Z5S15-AS sample was the Z5S15 sample
that was received after the hydrothermal treatment. It had
not undergone templates removal. Z5S15-AS sample was
modified by two stages. In the first stage, Z5S15-AS sam-
ple was P123 copolymer removed to remain the silanol
(–OH) groups by H2O2 oxidizing agent. After that, sample
was functionalized with amine groups by using APTES
linker in the second step. These amine groups enabled
the coating of Ag+ ion onto Z5S15 sample via proton
exchange reaction.
Copyright: American Scientific Publishers
ums preparation, initial cells suspension preparation,
antibacterial assay by contact time between E. coli bacteria
and sample, and the analysis of the E. coli concentration
by colony counter method. All these procedures are the
same as one reported in our previous article.4 The initial
concentration of E. coli was 107 cfu/ml. The experimental
contact time was varied at 10 min, 20 min, 30 min, and
60 min.
2.8. Benzene Oxidation
Benzene oxidation measurements were performed in a
fixed-bed microreactor under atmospheric pressure. An
amount of 100 mg catalyst was used for each exper-
iment. The feed gas rate was 50 ml/min with oxy-
gen/benzene molar ratio of 15 and space velocity (WHSV)
of 30,000 ml · h−1 · g−1. Reaction temperature was con-
trolled by a k-type thermocouple that directly contacted
with the catalꢀyst bed. Before reaction, catalyst was cal-
cined at 550 Cꢀ for 5 h in air flow. Then, catalyst was
reduced at 450 C in a 5% H2 gas flow. The feed and
product compositions were analyzed by a gas chromatog-
raphy (GC) equipped with a TCD detector to determine
CO2, and a FID detector to determine hydrocarbon.
2.3. P123 Co-Poplyme Removal from
Z5S15-AS Material
The procedure of P123 removal from Z5S15-AS sample
was performed as follows: 1 g Z5S15-AS composite mate-
rial and 150 ml H2O2 solution (30% v/v) were put into an
autoclave with stirring and being heated at 100 ꢀC for 24 h.
The solid was filtered and washed with distilled water and
ethanol. Then, it was dried at 100 ꢀC in vacuum. The solid
product was signed as Z5S15-PR.
2.4. Functionalization of Z5S15-PR Material
1 g Z5S15-PR sample was dispersed in a mixture of 2.5 ml
APTES and 100 ml anhydrous toluene. The mixture was
homogenized in a ultrasonic bath for 30 min at room tem-
perature. Then, the mixture was stirred and refluxed at
The CO2 product yield (YCO , %) was calculated via
2
amount of CO2 actual which was analyzed by GC-TCD
of product (SCO actualꢁ, and amount of CO2 which was
received in the 2 case of oxidation reaction of benzene
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Nam et al.
Synthesis and Performance of Nano Silver Coated ZSM-5/SBA-15
Figure 3. High-angle XRD patterns of Ag/Z5S15 sample (A) before
and (B) after AgNPs coating.
radiation (operating at 40 kV, 40 mA, wavelength ꢄ =
0ꢅ154 nm). The patterns were recorded at room tem-
perature (298 K); 2ꢆ values ranged from 0.1ꢀ to 5ꢀ
and 5ꢀ to 50ꢀ, and the scanning speeds were 1ꢀ min−1
and 2ꢀ min−1. Transmission electron microscopy (TEM)
was performed using a Philips Tecnai-10 microscope at
100 kV. AAS method was performed with an ICE 3300
AA Spectrometer. XPS investigation was performed with
a ESCALAB 220iXL (ThermoFisher) using monochro-
matic Al Kꢃ radiation (E = 1486ꢅe eV). For charge com-
Figure 1. IR spectra of samples: (A) Z5S15-AS; (B) Z5S15-CA;
(C) Z5S15-PR; and (D) Z5S15-APTES.
reaches fully conversion (SCO 100%ꢁ. At that time, there was
2
not any benzene or hydrocarbon signals could be detected
by GC-FID. YCO value was calculated by the equation as
2
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follows:
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Copyright: American Scientific Publishers
pensation a flood gun was used. The binding energy
YCO = ꢂSCO actual/SCO 100%ꢁ∗100%
2
2
2
was referenced to the C1s peak of adventitious carbon
at 284.8 eV. The binding energy of the peaks and the
surface composition were obtained after Shirley back-
ground subtraction and fitting the peaks with a mixed
Gaussian-Lorenttzian function. The BET surface area and
pore volume were measured by using nitrogen adsorp-
tion/desorption isotherms by using ASAP2010 equipment
(Micrometrics-USA). The sample was treated in vacuum
of 10−6 mmHg, at 120 ꢀC for 4 h and at 350 ꢀC
for 9 h.
2.9. Characterization Method
Characterization of samples was performed by using the
following methods: FT-IR spectra were recorded using
an IMPACT-410 (Germany) Infrared Spectrophotome-
ter at room temperature in the range 400–1300 cm−1
on thin wafer of KBr in which 1% (w/w) of sample
was dispersed. Powder X-ray diffraction patterns were
recorded on a HUT-PCM-Bruker D8 Advance instrument
diffractometer system equipped with Ni-filtered Cu Kꢃ
Figure 2. XRD patterns of samples: (A) Z5S15-CA; (B) Z5S15-PR;
Figure 4. Low-angle XRD patterns of Ag/Z5S15 sample (A) before
(C) Z5S15-APTES.
and (B) after AgNPs coating.
J. Nanosci. Nanotechnol. 17, 1813–1819, 2017
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Synthesis and Performance of Nano Silver Coated ZSM-5/SBA-15
Nam et al.
range of 2900–3000 cm−1 as well as C–H (CH2 and CH3ꢁ
bending vibrations in the range of 1370–1470 cm−1 13
ꢅ
The
C–N stretching vibration, a typical bond of TPABr, is nor-
mally observed in the range of 1000–1200 cm−1 14
How-
ꢅ
ever, this peak is not resolved due to the overlay with
the IR absorptions of Si–O–Si asymmetric stretching and
Si–CH2-R deformation in the ranges of 1130–1000 cm−1
and 1250–1200 cm−1, respectively.
The disappearance of the typical peaks of templates
(P123 and TPABr) in the Z5S15-PR infrared spectrum,
and the agreement of IR spectra between Z5S15-PR and
Z5S15-CA samples (Z5S15-CA sample is the Z5S15 sam-
ple tꢀhat is undergone templates removal by calcination at
550 C for 5 h in our previous report8) indicate that P123
and even TPABr templates have been completely removed
from Z5S15-AS sample. IR spectrum of Z5S15-PR sam-
ple shows the appearance of silanol groups in term of
the higher intensity of the O–H (silanol groups) stretching
vibrations in the range 3200–3700 cm−1 compared with IR
spectra of Z5S15-CA and Z5S15-AS.
Figure 5. XPS spectrum of Ag/Z5S15 sample.
The samples were calcined at 550 ꢀC for 5 h before the
TEM and BET analysis.
The success of the functionalization can be proven by
the IR spectrum of Z5S15-APTES sample (Fig. 1(d)) that
shows the appearance of C–H (CH2 and CH3ꢁ bending
and stretching vibrations because of APTES presence. The
incorporation of amine groups in the silicate framework
can be confirmed. At the wavenumber of 693 cm−1, a
weak peak can be seen due to the be+nding vibrations
of N–H bonds, and the symmetrical NH3 bending vibra-
tions at 1560 cm−1. That weak peak indicates the exis-
tence of amine groups.13 In addition, the IR spectrum
of Z5S15-APTES sample also shows the dramatically
decrease of the O–H stretching vibrations in the range of
3200–3700 cm−1. This may be due to the loading of amine
groups that take the place of surface silanol groups.
A comparison between the small angle XRD patterns
of Z5S15-APTES sample and Z5S15-CA samples (Fig. 2)
indicates that the functionalization has an effect on the
3. RESULTS AND DISCUSSION
3.1. Result of Z5S15 Functionalization
The as-synthesized Z5S15 (Z5S15-AS) sample consists of
P123 and TPABr templates. The conventional method of
removing these templates is calcination in an air atmo-
sphere. However, the calcination can cause the disappear-
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ance of silanol groups of Z5S15-AS material during the
condensation.12 So as to prevent these problems, P123 oxi-
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Copyright: American Scientific Publishers
dation method using H2O2 oxidation agent is a good alter-
native. TPABr template also can be partially extracted by
this method.
The result of template removal by H2O2 from Z5S15-
AS sample is demonstrated in IR spectra (Fig. 1). The IR
spectrum of Z5S15-AS sample shows the peaks attributed
to the bonding vibration of P123 and TPABr templates.
They are C–H (CH2 and CH3ꢁ stretching vibrations in the
Figure 6. TEM images of Ag/Z5S15 sample.
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Nam et al.
Synthesis and Performance of Nano Silver Coated ZSM-5/SBA-15
long range ordering of the mesostructure of Z5S15 mate-
rial. The slight shift to lower angle and the decrease in
reflection intensity of the peak around 2-theta value of
1ꢀ may be explained by the presence of grafted APTES
molecules. This presence may cause a change of XRD
pattern of Z5S15-APTES sample due to the phase cancel-
lation between X-rays scattered from the walls and pores
regions.
3.2. The Characterization of Ag/Z5S15 Sample
Ag/Z5S15 sample are characterized by XRD, TEM, XPS
and BET methods. XRD patterns in Figures 3 and 4 show
that the microporous and mesoporous structure of Z5S15
substrate are strongly affected by too much of AgNPs
formed both inside and outside the mesoporous system of
Ag/Z5S15 sample. The peaks at around 2-theta values of
38ꢀ, 44ꢀ, and 65ꢀ (Fig. 3) correspond respectively to (111),
(200), (220) reflections of pure silver metal with face cen-
tered cubic symmetry (a = b = c = 4ꢅ08570ꢁꢅ15
The valence state of AgNPs on the Ag/Z5S15 samples
is further characterized by XPS method. The XPS spec-
trum of Ag/Z5S15 sample in Figure 5 shows the binding
energies of 368.0 eV for the Ag 3d5/2 peak and 374.0 eV
for the Ag 3d3/2 peak that correlate to zerovalent Ag.16ꢀ17
The binding energies of 366 eV for the Ag 3d5/2 peak and
372 eV for the Ag 3d3/2 peak could be correlated to the
Ag(np)-APTES bonds.18
Figure 8. Pore distribution of Z5S15-CA sample.
sample may be due to the exchange reaction of Ag+ ions
with both amine groups and bronsted sites of Z5S15-
APTES sample. This result also indicate that the bronsted
sites may be existed inside the mesoporous channels of
Z5S15-APTES.
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The N2 adsorption–desorption isotherms, pore distribu-
The size of AgNPs on Ag/Z5S15 sample are character-
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tion of Z5S15-CA sample in Figures 7 and 8 were com-
pared with that of Ag/Z5S15 sample in Figures 9 and 10.
The results show that the mesoporous system of Z5S15
substrate was considerably changed. This may be because
of the entering of AgNPs into it.
Copyright: American Scientific Publishers
ized by TEM images that are shown in Figure 6. TEM
images of Ag/Z5S15 sample show very small AgNPs with
average nanosize of 2–5 nm. There are only few AgNPs
with size of 6–17 nm. AgNPs are distributed both inside
and outside the mesoporous channels of Ag/Z5S15 sam-
ple. The nanosize range formation of AgNPs in Ag/Z5S15
Properties of samples in Table I show a high AgNPs
loading content of Ag/Z5S15 sample (3.734 wt%).
Figure 7. N2 adsorption/desorption isotherms of Z5S15-CA sample.
J. Nanosci. Nanotechnol. 17, 1813–1819, 2017
Figure 9. N2 adsorption/desorption isotherms of Ag/Z5S15 sample.
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Synthesis and Performance of Nano Silver Coated ZSM-5/SBA-15
Nam et al.
Table II. Antibacterial performance of samples.
E. coli concentration, cfu/ml
10 min 20 min 30 min
Samples
0 min
60 min
Ag/Z5S15 2ꢅ7×107 2ꢅ85×106 2ꢅ70 ×106 2ꢅ65×106 4ꢅ45×105
Ag/ZSM-5 2ꢅ7×107 1ꢅ10 ×106 6ꢅ10 ×105 2ꢅ50 ×105 2ꢅ90 ×104
But in the case of Ag/ZSM-5, the E. coli concentration
after 60 min is 2ꢅ90 ×104 cfu/ml with the same initial
concentration. This is due to the fact that the AgNPs
(2–5 nm) may be formed inside the mesoporous system
of Z5S15 substrate (7–8 nm). These AgNPs can not con-
tact with E. coli bacteria which have much bigger size
(0ꢅ5 ×5 ꢇm). This limits the antibacterial performance of
Ag/Z5S15.
3.5. Catalytic Performance of Samples in
Figure 10. Pore distribution of Ag/Z5S15 sample.
Benzene Oxidation
The size of one benzene is around 6 Å. So benzene
molecules may easily diffuse through the mesopore sys-
tem of Ag/Z5S15 sample. Benzene oxidation reaction
overꢀcatalysts aꢀre performed by temperature ranging from
100 C to 500 C. CO2 product yield of reaction by tem-
perature over Ag/Z5S15 sample is compared with that
over Ag/ZSM-5 sample. The results are demonstrated in
The loading of AgNPs causes the low specific surface area
of Ag/Z5S15 (267 m2/g) in comparison to that of Z5S15-
CA sample (541 m2/g). In addition, the mesopore size
of Ag/Z5S15 sample is 10–25 nm with very low inten-
sity (dV/dlog(w) pore volume value) (Fig. 10). Z5S15-
CA sample has the mesopore size of 7–8 nm with high
intensity (Fig. 8). These results may prove the pore fill-
Figure 11.
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ing effect of AgNPs on the porous system of Ag/Z5S15
sample.
The catalytic result of samples show that the con-
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Copyright: American Scientific Publishers
tent of AgNPs decides the oxidation performance
of samples. Because of the considerable increase of
AgNPs content from 0.253 wt% (Ag/ZSM-5 sample)
to 3.734 wt% (Ag/Z5S15 sample), and the increase of
contact ability between benzene, oxygen and AgNPs
inside the mesoporous system of Ag/Z5S15 sample,
the CO2 product yield (Y value) is strongly increased
3.3. Performance Evaluation
Performance of Ag/Z5S15 sample was measured in com-
parison to that of Ag/ZSM-5 material. These materials
have near similar size of AgNPs (around 2–3 nm). How-
ever, Ag/Z5S15 sample has much higher AgNPs content
(3.734 wt%) than Ag/ZSM-5 sample (only 0.253 wt%).4
3.4. Antibacterial Performance of Samples
The E. coli antibacterial performance of samples are
presented in Table II. A very interesting result can be
seen that Ag/ZSM-5 sample (0.253 wt% AgNPs) has
better antibacterial performance than Ag/Z5S15 sample
(3.734 wt% AgNPs) after all experimental contact times.
The E. coli concentration after the contact time of 60 min
is decreased from 2ꢅ7 × 107 cfu/ml (initial concentra-
tion) to 4ꢅ45 ×105 cfu/ml in the case of Ag/Z5S15.
Table I. Physico-chemical properties of samples.
Pore volume,
cm3/g
Ag content by SBET
AAS, wt%
,
Average pore
width, nm
Sample
m2/g Vmeso
Vmicro
Z5S15-CA
Ag/Z5S15
0
541
267
0ꢅ43
0ꢅ35
0ꢅ11
0ꢅ07
5ꢅ3
10ꢅ1
Figure 11. Product efficiency of benzene oxidation by temperature over
catalysts.
3.734
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Nam et al.
Synthesis and Performance of Nano Silver Coated ZSM-5/SBA-15
from 37.55 % (over Ag/ZSM-5 sample) to 100%
(over Ag/Z5S15 sample). In addition, the temperature that
benzꢀene molecules are started to be totally oxidized is only
300 C. This result may prove that in this application, the
AgNPs inside the mesoporous system work. Benzene and
oxygen molecules can diffuse into mesoporous system of
Ag/Z5S15 to contact with AgNPs. This result also shows
the effective support ability of mesoporous system in term
of increasing the contact ability between the feed gas (ben-
zene and oxygen molecules) with AgNPs.
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4. CONCLUSIONS
Ag/Z5S15 material based on a Z5S15 composite material
was successfully prepared. In comparison to Ag/ZSM-5
material (AgNPs content was only 0.253 wt%), the
AgNPs content of Ag/Z5S15 is considerably increased
to 3.734 wt%. The small AgNPs 2–5 nm were formed
both inside and outside the mesoporous system of Z5S15
substrate.
The performance test results showed that the combi-
nation of high content, small size of AgNPs and the
mesoporous system of Z5S15 substrate helped improving
catalytic activity of Ag/Z5S15 material in benzene oxi-
dation. It showed very good catalytic activity in term of
ꢀ
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Acknowledgments: This research is funded by
Vietnam National Foundation for Science and Tech-
nology Development (NAFOSTED) under grand num-
ber 104.03-2012.41.
Delivered by Ingenta to: Purdue University Libraries
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Received: 1 January 2016. Accepted: 23 February 2016.
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