Can microwave assisted in situ reduction of supported Pt nanoparticles challenge the chemical method in controlling the dispersion profile-catalytic performance relationship?

Article abstract of DOI:10.1039/c5ra08116e

In this study, Pt (of 0.3, 0.6 and 0.9 wt% loadings) was supported on a mesoporous silica surface via a microwave-assisted solution (MAS) method or rotary chemical evaporation (RCE) method in the in situ reduction step. The as-synthesized Pt nanocatalysts were characterized through XRD, XRF, TGA/DSC, TEM, N₂-adsorption-desorption, H₂-pulse titration and electrical conductivity techniques. The samples prepared by MAS method exhibited higher surface area and a better dispersion profile of Pt NPs, of average sizes not exceeding 10 nm with increasing the Pt loading. In contrast, although RCE method showed higher efficacy in decomposing the used precursor, an uneven distribution of larger Pt nanoparticles (≥33 nm) was determined. Electrical properties in terms of AC conductivity and dielectric constant confirmed the enhancement of an even distribution of smaller Pt NPs with a higher concentration of grain boundaries effected by microwave electromagnetic radiations. Highly mobile electrons and lattice vibrations (phonons) were favored, as compared with aggregated NPs produced during RCE method. The TOF values calculated for reactions to selectively produce ethylene (from ethanol) or benzene (from cyclohexane) decreased with Pt loading on the catalyst samples synthesized by MAS method. The highly dispersed NPs (of 3-7 nm) seemed to be responsible for the activity in both reactions, probably tending to be structure insensitive. However, samples reduced by RCE method, with enlarged average sizes of surface Pt NPs (approaching 15.5 nm), exhibited increasing TOF values with Pt loading, i.e., causing the reactions most probably to be structure sensitive.

Full text of DOI:10.1039/c5ra08116e

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Can microwave assisted in situ reduction of supported Pt nanoparticles be
challenging with chemical method in controlling the dispersion profile-
catalytic performance relationship
1
2*
3
2
2
Salah A. Hassan , H. M. Gobara , M. M. Gomaa , R. S. Mohamed , F. H. Khalil
1
Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt.
2
Catalysis Department, Refining Division, Egyptian Petroleum Research Institute, 1 Ahmed
3
ElꢀZomor St., Nasr City, 11727, Cairo, Egypt. Geophysical science Department, Physics
building, National Research Center, Dokii 12311, Cairo, Egypt.
*
0
Corresponding author; email: hebagobara@ymail.com , Tel: +2(02) 01223898196, +2(02)
1091446345, Fax: +2(02)227727433.
Abstract
In this study, Pt (of 0.3, 0.6 and 0.9 wt % loadings) was supported on mesoporous silica
surface via microwaveꢀassisted solution (MAS) method or rotary chemical evaporation
(RCE) method in the in situ reduction step. The asꢀsynthesized Pt nanocatalysts were
characterized through XRD, XRF, TGA/ DSC, TEM, N ꢀadsorptionꢀdesorption, H ꢀpulse
2
2
titration and electrical conductivity techniques. The samples prepared by MAS method
exhibited higher surface area and a better dispersion profile of Pt NPs, of average sizes not
exceeding 10 nm with increasing the Pt loading. In contrast, although RCE method showed
higher efficacy in decomposing the used precursor, uneven distribution of larger Pt
nanoparticles (≥ 33 nm) was displayed. Electrical properties in terms of AC conductivity and
dielectric constant confirmed the enhancement of even distribution of smaller Pt NPs with
higher concentration of grain boundaries affected by microwave electromagnetic radiations.
Highly mobile electrons and lattice vibrations (phonons) were favored, as compared with
aggregated NPs produced during RCE method. The TOF values calculated for reactions to
selectively produce ethylene (from ethanol) or benzene (from cyclohexane) decreased with Pt
loadings on catalyst samples synthesized by MAS method. The highly dispersed NPs (of 3ꢀ7
nm) seemed to be responsible of the activity in both reactions, tending probably to be
structure insensitive. However, samples reduced by RCE method, with enlarged average sizes
of surface Pt NPs (approaching 15.5 nm), exhibited increasing TOF values with Pt loading,
i.e., turning the reactions most probably to be structure sensitive.
Keywords: PtꢀSiO
2
nanocatalysts; Microwaveꢀassisted solution method; Rotary chemical
evaporation method; Dispersion profile; Dielectric properties; Catalytic performances
1
. Introduction
Transition metal nanoparticles supported on metal oxides have been used as catalysts for
a variety of organic reactions, including hydrogenation, dehydrogenation, oxidation, etc.
They have advantages such as enhanced activity, selectivity and dispersion of metallic
species during immobilization on support surface. Metal oxides supports usually exhibit high
surface areas, high thermal stabilities and organized pore structures. Commonly used
supports are alumina [1] zeolite [2], titania [3], carbon [4] and silica [5]. Of these supporting
materials, silica has a potential to be used in a wide variety of applications ranging from
microelectronics to medical applications [6]. The silica supports used include silica gel [7],
silica monoliths [8], mesoporous silica [9] and others.
Supported nanocatalysts are typically prepared in two steps. In the first step, the loading
of metal precursor on support surface can be achieved by several techniques, including
1

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impregnation, where dispersion property depends on the metal, metal loading and support,
deposition/precipitation or coꢀprecipitation, where metal dispersion depends on the nature of
support, solution pH and concentration of the precursor solution [10], and grafting involving
formation of covalent bond between the metal precursor and the support functional groups
[
11]. The second step includes calcination and/or reduction at different conditions, which
controls the metalꢀsupport interaction, mode of metal dispersion and consequently the
average particle size as well as the dispersion behaviourꢀcatalytic performance relationship
[
12]. The two steps together significantly affect the overall properties of the resultant catalyst.
Among the applied catalyst preparation techniques, microwaveꢀassisted irradiation is now
growing in popularity, as a time saving and energy efficient process [13]. In this technique, it
is important that the synthesis medium has the ability to absorb the electromagnetic radiations
and to efficiently convert them to heat. This property is related to the material's loss factor or
loss tangent [14]. Ethylene glycol (EG), frequently used in microwave irradiation, has a high
loss tangent of 1.35 at 2.45 GHz, which is a high value when compared to the other solvents
[
13]. It serves not only as the reducing agent but also provides the heating environment.
In this work, a comparative study is presented to follow the effect of preparation conditions to
optimize the dispersion properties of Pt nanoparticles / SiO , as synthesized by impregnation
2
technique and via applying microwaveꢀassisted solution method (MAS) or rotary chemical
evaporation (RCE) method in the in situ reduction step. The comparative study was extended
to include the electrical properties within the various characteristics in relation to the catalytic
performances of the studied systems in ethanol and cyclohexane conversions.
Electrical properties of such highly dispersed systems and related mesoporous structures
have attracted the attention of several researchers, as the understanding of charge transport
mechanism in support/guest materials is important both from fundamental and technological
points of view. Dielectric measurements are acceptable means for studying the dynamic
properties (capacitance, conductance, permittivity and loss factor) of dielectrics. The previous
works [15, 16] have focused on mobility of the exchangeable cations (Pt and Ni) loaded on
different supports. Recently, the enhancement of electrical conductivity of the hybrid
prepared from Pt NPs and reduced graphine oxide (rGO) using the aniline stabilizer was
reported to originate from the small particle size, uniform dispersion of Pt NPs and good
interfacial interaction between the Pt NPs and rꢀGO hybrid [17]. In the present study, the
A.C. conductivity and dielectric constants were studied in the frequency range from 42 Hz to
5
MHz, at 300 K and 1 V, with an attempt to explore how the charge transport and relaxation
can be affected by the support/guest interaction raised in both synthesis methods applied.
2
. Experimental
.1. Materials
2
All the chemicals used were of analytical grade (99.9 % purity). Hexa chloroplatinic acid
PtCL .6H O] (Merck) was used as a precursor for preparation of Pt nanoparticles. Sodium
hydroxide (NaOH), ethylene glycol (C H O EG), hydrazine monohydrate (N H .H O),
[
H
2
6
2
2
6
2,
2
4
2
cyclohexane and ethanol (absolute) were used as received.
.2. Preparation of nanocatalysts
In the typical synthetic procedure, a calculated amount of SiO
the solution of H PtCl .6H O of certain concentration in deionized water such that the
2
2
support was impregnated with
2
6
2
loadings of Pt were 0.3, 0.6 and 0.9 wt %. Two different reduction techniques were applied to
get the Pt nanoparticles, namely;
i) In the microwaveꢀassisted solution (MAS) method, EG (50 ml) with drops of NaOH was
added first to the impregnated Pt precursor solution, after which 5 ml of hydrazine
monohydrate were added drop wise. The mixture was then irradiated for 2 min in a
2

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commercial microwave oven (Kenwood Model MW 867, 1100 W, 2.45 GHz, λ =12.2 cm,
with electronic digital system). The filtered solids were washed with deionized water and
acetone for several times and then washed with absolute ethanol. Finally, the obtained
°
catalyst samples were dried for 24 h at 70 C in a vacuum oven.
ii) In the rotary chemical evaporation (RCE) method, EG (50 ml) was added to the
impregnated Pt precursor solution heated at 60° C for 30 min in an oil bath (SBꢀ1000),
designed for the rotary evaporator, EYELA Model N1001 SꢀW, in a standard 1000 ml pear
shaped flask. Sodium hydroxide solution was added drop wise. When the colour of the
solution changed from dark yellow to light yellow, the temperature of rotary evaporator (with
standard diagonal condenser glassware nozzle OD 3/8’’ (10 mm), equipped with a digital
°
temperature display) was raised to 75 C and kept for 20 min. 5 ml of hydrazine hydrate was
°
added drop wise and the mixture was kept at 75 C for 1 h. The precipitate was washed with
copious deꢀionized water and acetone for 3 times, then with absolute ethyl alcohol and dried
°
at 70 C for 24 h in a vacuum oven.
2.3. Physicochemical Characterization
The textural characteristics of the prepared nanoꢀcatalysts were investigated by the aid of
o
N
2
adsorptionꢀdesorption isotherms measured at ꢀ196 C using a NOVA 3200 apparatus,
ꢀ
4
o
USA. All samples were preꢀtreated under vacuum (10 Torr) at 300 C for 24 h. Surface
areas (SBET) were calculated from the adsorption isotherms by applying BET equation using
multi–points method at relative pressure P/Po range 0.05 ꢀ 0.3. Pore size distribution was
estimated BJH method from the desorption branch of the isotherms.
The powder Xꢀray diffraction (XRD) patterns were recorded adopting a Brucker D8 advance
Xꢀray diffractometer, using Cu–Kα radiation (λ=1.5418 Å) with 30 mA and 40 kV in the 2θ
o
o
ꢀ1
from 10 to 80 range with a scan rate of 10 min .
The elemental composition of the samples was determined by Xꢀray fluorescence (XRF)
spectrometry in a Panalytical AXIOS PW4400 spectrophotometer with Rh tube as source of
radiation. The measurements were performed onto pressed pellets containing 6 wt % wax.
Thermal gravimetric and differential scanning calorimetric (DSCꢀTGA) analyses were
performed using simultaneous DSCꢀTGA devise of model SDTQ 600, USA, under N
atmosphere, with a heating rate of 10 C min .
2
o
ꢀ1
The degree of surface dispersion and average size of supported Pt nanoparticles was assessed
o
2
through H ꢀchemisorption. The sample was maintained at 300 C for 2 h, flushed in a pure
3
−1
dry nitrogen gas flow (20 cm min ) for 30 min and then cooled to room temperature.
Thereafter, the sample was subjected to hydrogen chemisorption on Chembet 3000 apparatus,
3
Quantachrome USA, using a pulse titration (pulse volume = 0.1 cm ) and 10 % (v/v) H /N .
2
2
The hydrogen uptake was expressed in µLH at STP per g of catalyst.
2
The morphology of different samples were investigated by the aid of high resolution
transmission electron microscope, model JEM 2100 (JEOL, Japan). It offered high resolution
electron imaging up to 0.143 nm and magnification as high as 1.5 million times.
2.4. Catalytic activity measurement
Catalytic activity of SiO and supported (PtꢀSi) nanocatalysts were tested in cyclohexane and
2
ethanol conversions as two model reactions using a micro catalytic pulse reactor. The reactor
effluent was passed through a chromatographic column for separation and determination of the
products using flame ionization detector (FID) connected to computerized data acquisition
station. The column of 200 cm length and 0.3 cm internal diameter was used containing acidꢀ
washed PW chromosorb (60ꢀ80 mesh size) loaded by 15% by weight squalane. The reactions
o
were carried out under atmospheric pressure in the temperature range, 250 – 450 C. Hydrogen
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ꢀ1
flow rate was kept constant at 50 ml min . Prior to catalytic activity runs, the reduced catalyst
samples were heated in H flow for 2 h at 450 C for activation. Few doses of reactants were
2
o
injected first to reach the steady state of the reaction. The chromatographic column temperature
o
was adjusted and controlled at 50 C.
The catalytic activity experiments for ethanol and cyclohexane conversions were expressed in the
terms of yield, total conversion, % selectivity and the turnover frequencies calculated as follows:
Ethanol or cyclohexane conversion (wt %) =
Σ
yield % of each component in the products
Component yield (wt %) = Quantity of the component / Total quantity of the produced components
Fractional selectivity of each component (wt %) = Yield (wt %) / total conversion X 100.
Turnover frequencies = moles of product produced (or substrate consumed)/ surface metal
per unit time
2.5. Electrical properties measurement
The geometry (area and thickness) of the samples for electrical measurements was chosen to
reduce errors due to stray capacitance. The sample faces were polished to be parallel using
Hioki 3522ꢀ 50 LCR Hitester Impedance Analyzer. Samples were collected to cover all
Jurassic formations present in the area. Data were measured in the frequency range from 42
Hz to 5 MHz. The samples were connected to nonꢀpolarizing electrodes (Cu/CuSO
4
) [15, 16]
and the oscillation amplitude was set at 1 volt (small signal).
Electrical properties of various samples under study could be expressed in either the series or
parallel configuration. The measured parameters involved parallel capacitance (C p ),
conductance (G p ) and series impedance (Z) at different frequencies. The complex relative
dielectric constant
where, the real part of the complex relative dielectric constant (
ε
*
could be expressed as:
ε
*=
ε
'−i
ε
"
(6),
ε
'
) before correction is
(7)
ε
'= Cpd ε0 A
and the imaginary part of the complex relative dielectric constant (
"= Gpd ωε0
A is the crossꢀsectional area of the sample, d is its thickness, ε0 is the permittivity of free
ε") is
ε
A
(8)
−
12
space (8.85×10 F/m) and
ω
is the angular frequency. The conductivity was thus
calculated from the following equation:
σ
'= Gp
(d A
)
=
ε
"
ωε0
(9)
σ'
where, is the real part of the conductivity.
The measurements were performed in an isolated chamber at room temperature (~27° C) and
atmospheric relative humidity of (~40%).
3. Results and Discussion
2
3.1. Physicochemical characteristics of various Pt/SiO nanocatalyst samples synthesized
through different reduction methods
3.1.1. XRD and XRF analyses
The XRD patterns obtained for silica support and supported Pt/SiO nanocatalyst samples of
2
different Pt loadings, viz., 0.3, 0.6 and 0.9 wt %, prepared by the MAS reduction method are
shown in Fig. 1(a). The corresponding patterns of the same catalyst samples prepared by RCE
2
method are shown in Fig. 1(b). It is obvious that the amorphous pattern of SiO , characterized
o
by the broad peak located at 2θ of 15ꢀ30 [22], is maintained in all studied catalyst samples
by varying the reduction methods. Moreover, no diffraction lines could be observed for Pt
phases, referring to the well dispersed state of Pt NPs of average sizes less than 30 Å in all
cases.
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Fig.1: XRD patterns of (a) parent silica support and various samples of Pt/SiO
2
nanocatalyst
synthesized via MAS reduction method and (b) the corresponding samples prepared by RCE
method
On the other hand, the atomic percentages of loaded Pt determined by XRF in all catalyst
samples synthesized by the mentioned reduction methods (MAS and RCE) are included in
Table 1. The obtained atomic percentages, normalized as a ratio to SiO rather than to the
2
other components of the catalyst system, are very close to the targeted ratios for this study.
However, the relatively lower ratios of Pt, than the nominal ones, exhibited in case of RCE
reduction method seems to be related to the type of longer term rotary chemical treatment.
This assists the efficient decomposition of the precursor; deeper penetration of a fraction of Pt
NPs in pore system and some loss upon evaporation and purification. During MAS reduction,
exposing the synthesis system to electromagnetic radiations, even for only short time not
sufficient for complete decomposition of Pt precursor, seems to expel a fraction of Pt out of
pores to the external surface, with some loss of magnetic SiO NPs (i.e., leading to some
2
relatively higher Pt ratios than the nominal ones). In all cases, some sort of strong interaction
between Pt and SiO support seemed to exist without detectable leaching of Pt species.
2
3.1.2. Textural investigation
The N
2
adsorptionꢀdesorption isotherms over the surface of silica support and various
prepared by MAS reduction method as well as their pore size
samples of supported Pt/SiO
2
distribution curves are illustrated in Fig. 2 (a and b). Also, the measured isotherms and pore
size distribution curves over the catalyst samples of the same Pt loadings prepared through
the RCE method are shown in Fig. 2 (c and d). The isotherms obtained for the asꢀsynthesized
catalyst samples by both methods are similar in behavior to that of silica support, probably
due to the nanosized metal particles and/or their high dispersion. All the presented isotherms
are of type IV, characteristic mainly of the mesoporous silica [18], being associated with
hysteresis loops of H2 type in the relative pressure (p/p ) range of 0.4 – 0.8, usually related to
o
the inkꢀbottle pores with narrower orifice of the border inner parts [19].
Table 1 summarizes the different surface data derived from the adsorption isotherms. It is
clear that the surface area of pure silica support has suffered a marked decrease by loading Pt
nanoparticles, reduced by either MAS or RCE methods. The catalyst samples synthesized by
MAS reduction method had relatively higher surface areas, especially for diluted samples, as
compared with those samples synthesized by RCE method. Although the pore volume
exhibited only slight changes in both sets of synthesized nanocatalysts, the average pore radii
showed a considerable decrease, especially for lower Pt loadings, viz., 0.3 and 0.6 wt %,
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(
becoming more pronounced with MAS reduction) most likely due to incorporation of a
fraction of Pt nanoparticles near the orifices of the silica pore system.
For the 0.9 PtꢀSiO sample prepared by MAS method, a slight increase in pore volume with
2
almost retained pore radius as that of pure silica may suggest the transfer of a fraction of Pt
nanoparticles to the top surface. In contrast, for the same catalyst sample of 0.9 % Pt,
prepared by RCE method, the increase in surface area with nearly the same pore size
parameters as 0.3PtꢀSiO
2 2
and 0.6PtꢀSiO samples may be related to deeper incorporation of
Pt nanoparticles in SiO pore system.
2
As in pure silica support, relatively narrower unimodal pore size distributions (PSD) of most
probable hydraulic pore diameter D = 3.60 nm were shown for the catalyst samples
synthesized by MAS method (Fig. 2, b). Their population seemed to decrease with the
increase in metal loading, due probably to the transfer of some of Pt NPs out of the pore
system. However, for catalyst samples synthesized by RCE method, the slightly broadened
PSD curves were shown with smaller most probable hydraulic pore diameter D = 3.39 nm.
2 2 2
Fig. 2: N adsorptionꢀdesorption isotherms for bare SiO support and Pt/SiO nanocatalysts
synthesized by: (a) MAS method and (c) RCE method, as well as the pore size distribution
curves (PSD) of SiO support and Pt/SiO nanocatalysts synthesized by: (b) MAS method
2
2
and (d) RCE method.
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Table 1 Surface parameters of SiO
2
2
and various Pt–SiO nanocatalyst samples prepared by
MAS and RCE method and atomic percentages of supported Pt determined by XRF in all
cases
.
Pt loading
Catalyst
sample
S
V
rp
(m g ) (cm g ) (nm)
444.0 0.430 8.52
BET
P
a
2
ꢀ1
3
ꢀ1
(wt %)
SiO
Pt/SiO
.3 PtꢀSi
2
ꢀꢀꢀꢀꢀꢀ
2
samples synthesized by MAS method
0
0
0
0.341
0.610
0.913
352.5
362.2
356.1
0.427
0.429
0.479
3.88
3.94
8.26
.6 Ptꢀ Si
.9 Ptꢀ Si
2
Pt/SiO samples synthesized by RCE method
0
.3 PtꢀSi
0.289
0.591
0.897
335.0
265.2
382.4
0.406
0.384
0.443
4.74
4.19
4.39
0.6 Ptꢀ Si
.9 Ptꢀ Si
0
a
Wt % of Pt from elemental analysis by XRF
3
.1.3. Platinum dispersion profile in various Pt/SiO nanocatalyst samples
2
The degrees of surface dispersion of Pt in the various Pt/SiO samples, synthesized via MAS
2
2
and RCE reduction methods, were evaluated from the H chemisorption data, summarized in
Table 2, assuming the adsorption stoichiometry of H: Pt = 1: 1. It is clear that the degree of Pt
dispersion decreased by increasing the Pt loading in both the applied reduction methods. The
dispersion values (D) were higher by using MAS method, where the dispersion profile
seemed more regular. The dispersion parameters for higher Pt loadings (0.6 % and 0.9 %)
were better by applying MAS method than those obtained by RCE method, i.e, more fraction
e
of exposed surface atoms of Pt (cf., smaller average particle sizes, a in nm, and higher
2
ꢀ1
surface areas of supported Pt, SPt, m g ). For diluted catalyst samples (of 0.3 % Pt), the
dispersion parameters, D, a and S , obtained by using RCE method, were comparable with,
e
Pt
or even more favorable than those parameters resulted in MAS method.
e
Table 2 Degree of Pt dispersion (D), average size of Pt nanoparticles (a ) and metallic surface area of
supported Pt nanoparticles in Pt/SiO catalysts synthesized by MAS or RCE method
2
Catalyst
sample
Degree of
Dispersion
Av. Particle
size (a
Metallic surface
2 ꢀ1
e
,
nm)
(SPt) (m g )
(
D)
Pt/SiO
2
samples synthesized by MAS method
0.3 PtꢀSi
0.6 Ptꢀ Si
0.9 Ptꢀ Si
0.763
0.607
0.368
3.71
4.66
7.70
754.3
599.9
363.3
2
Pt/SiO samples synthesized by RCE method
0
0
0
.3 PtꢀSi
.6 Ptꢀ Si
.9 Ptꢀ Si
0.966
0.316
0.129
2.30
15.50
15.50
725.2
474.4
358.5
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3.1.4. Thermal analysis
The TGA and DSC analyses of various Pt/SiO nanocatalysts synthesized by MAS or
2
o
RCE inꢀsitu reduction methods were performed at temperatures ranging between 25 to 1000
C under N atmosphere (Fig. 3, aꢀd). Almost the same thermal trend was displayed in both
2
o
cases, exhibiting reasonable thermal stability up to 1000 C. As seen in this figure, the main
weight loss was occurred below 100 C accompanied with an endothermic peak representing
o
the removal of physically adsorbed water from the catalyst surface. The intensity of this
endothermic peak becomes more pronounced with increasing the metal loading, especially in
the 0.9 Pt –Si sample reduced by RCE method. Furthermore, the TG curves for the diluted
samples (0.3Pt and 0.6 Pt loaded samples) reduced by MAS method showed a slight weight
o
loss in the second step in the range, >200 – 550 C. This step seemed to be linked with the
progressive decomposition of the residual precursors, probably without heat changes.
Moreover, the lower losses in weights in this region for the samples prepared by RCE method
(2.1 % vs. 5.3 % in MAS method) can be related to the relatively higher efficacy of this
method in decomposing the used precursor.
2 2
Fig. 3: TGA (a) and DSC (b) of SiO support, Pt/SiO nanocatalysts with different Pt
loadings synthesized through MAS reduction method. TGA (c) and DSC (d) of Pt/SiO2
nanocatalysts synthesized through RCE method
3.1.5. Morphological investigation
The TEM images obtained for pure SiO
2
2
support and the various Pt/SiO nanocatalyst
samples (of 0.3, 0.6 and 0.9 wt % Pt) synthesized via MAS reduction method are depicted in
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Fig. 4. For silica support, the image (a) shows very fine, homogenous and well distributed
nanoparticles of an average size ≤ 2 nm. On the other hand, for the diluted catalyst sample (of
0
3
.3 % Pt), image (b) reveals even dispersion of surface Pt nanoparticles with average sizes ≤
nm. By increasing the Pt loading to 0.6 wt %, the Pt nanoparticles approaching average
sizes of 5ꢀ6 nm seem to be embedded in the nanosilica matrix (image c). By further
increasing the Pt loading to 0.9 wt %, an increase in Pt particle sizes is occurred (image d),
but still not exceeding 10 nm. This sample exhibited a distinguished dispersion profile,
compared with the other samples in this series, which may run in harmony with the
suggestion of transfer of a fraction of incorporated Pt nanoparticles in silica pore system to
top surface of the support (cf. textural results).
As well, the TEM images of the studied catalyst system with the same Pt loadings
synthesized by RCE method are represented in Fig. 5. The 0.3% Pt/SiO
homogeneously distributed larger Pt particles of average sizes in the range of 18ꢀ25 nm
image a). By increasing the Pt loading to 0.6 %, some enlarged particles of average sizes ≥
3 nm are occurred (image b), as embedded in silica matrix in less homogeneous surface
2
sample shows
(
3
distributions. For the 0.9 % PtꢀS, the well dispersion of Pt nanoparticles with average particle
sizes of 3ꢀ5 nm in some sheet structure becomes evident (image c). This observation may
confirm the deeper incorporation of Pt nanoparticles in silica pore system, associated with
some restructuring effect (cf. textural results).
Generally, it is clear that the applied MAS reduction method achieves more organized
distribution of less sized Pt nanoparticles on the outer surface of silica support, compared to
the RCE method.
Reproducibility of characterization results obtained for the samples produced by applying
both MAS and RCE methods was inspected as described in the electronic supporting
information (Fig S1 and Fig S2).
Fig. 4 TEM images of parent silica support (a) and various Pt/SiO
2
nanocatalyst samples of:
(b) 0.3 wt % Pt, (c) 0.6 wt % Pt and (d) 0.9 wt % Pt, synthesized via MAS reduction method
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2
Fig. 5 TEM images of various Pt/SiO nanocatalyst samples of: (a) 0.3 wt % Pt, (b) 0.6 wt %
Pt and (c) 0.9 wt % Pt, synthesized via RCE method
2
3.2. Electrical properties of various Pt/SiO nanocatalyst samples
This part focuses on studying the A. C. electrical properties of Pt/silica nanocatalysts
prepared via the two reduction methods (MAS and RCE) in the frequency range from 42 Hz
to 5 MHz, at 300 K and 1 V. Assuming that Pt nanoparticles are optimally dispersed (as
guest) over the surface of silica support (the host), this may give rise to electrical behavior
which differs from those of the individual constituents. The obtained results showed direct
dependences of the conductivity and dielectric constant on the Pt loading as well as the
frequency applied, as shown in Figs. 6 and 7 for samples synthesized by RCE method and
MAS method, respectively. In both cases, the conductivity increases while the dielectric
constant decreases with the increase of frequency. Such changes are fast at low frequency as
compared to higher frequency that may be due to the electrode polarization effects [20].
It is generally accepted that, the electrical conductivity and dielectric constant are directly
related to the amount of free or bound charge carriers in the surface of the samples, as well as
their mobility. The ionic conduction of samples results from the migration or exchange of
cations along the channels and cavities of the grains according to an ionꢀhopping mechanism.
The activation energy of ion hopping depends on the temperature, water content and
conductor content (module) [15].
Referring to Figs 6 and 7 at low frequencies (<10 KHz), the behavior of the conductivity may
reflect the production of charge carriers at the interfaces and their transportation to reach the
measuring electrode where they at last discharge. At higher frequencies (> 10 KHZ), such
charge carriers cannot reach the electrodes, as they are collected around grains and undergo
local motion which gives rise to frequency dependent conductivity.
It is worth to recall here that such highly dispersed metal systems under study, of nanoꢀsized
particles, contain a high concentration of grain boundaries and a large internal surface in the
case of porous systems, compared to the bulk material. So, the electrical conductivity is
largely affected by the microstructure and mainly by the grain boundaries.
’
Table 3 A.C. conductivity
σ
AC (at 100 Hz) and dielectric constant
ε
(at 5MHz) for the
Pt/SiO
2
nanocatalys samples synthesized via MAS and RCE reduction methods
’
Catalysts
sample
σ
AC (S/m)
ε
1
00 Hz
5 MHz
Samples synthesized by MAS method
ꢀ
8
5
4
3
SiO
2
3 x 10ꢀ
10
12
15
20
0
0
0
.3PtꢀSi
.6PtꢀSi
.9PtꢀSi
5 x 10ꢀ
2 x 10
1 x 10
ꢀ
Samples synthesized by RCE method
ꢀ
5
5
4
0
0
0
.3PtꢀSi
.6PtꢀSi
.9PtꢀSi
2 x 10ꢀ
2
9
15
5 x 10ꢀ
2 x 10
.
’
As given in Table 3, the derived conductivity
σ
AC (S/m) at 100 Hz and dielectric constant
ε
at 5 MHz increase with Pt loading. It is clear that the guest Pt NPs create improved electrical
contact between the silica host nanoparticles. It is consequently evident that the concentration
of charge carriers increases with the increase of Pt loading, probably with much transport of
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diffused charge carriers to reach the electrodes and discharge there. Such process dominates
all over the applied frequency range.
Both the electrical parameters,
σAC and ε’, are higher for the samples synthesized by MAS
reduction method, which seems to be linked with the more homogeneously distributed Pt
NPs, produced with higher concentration of grain boundaries. The response of a material to
electromagnetic radiation as in MAS method contains important information on highly
mobile (conduction) electrons, bound (valence) electrons, and lattice vibrations, i.e., phonons.
The highest conductivity and dielectric constant obtained are shown for the 0.9PtꢀSi sample.
Inspection of Fig 6 for pure silica and supported samples synthesized by RCE reveals that the
conductivity values for pure silica exhibit nearly one slope with the frequency (~0.94). For
supported PtꢀSiO
2
samples, the conductivity values have two slopes; the low frequency slope
’
is nearly zero while the high frequency slope is (~0.66). Also, the dielectric constant
show a decreasing slope of ~0.46 with the frequency for the silica support and two slopes for
the supported samples (viz., ~0.46 to ~0.92 at low frequencies and ~0.06 at high frequencies).
ε
values
1
1
1
1
1
1
1
E-2
E-3
E-4
E-5
E-6
E-7
E-8
0
0
0
.3 PSP
.6 PSP
.9 PSP
Pure Silica
Frequency (Hz)
1E+6 1E+7
1E+1
1E+2
1E+3
1E+4
1E+5
Fig.6 Conductivity and dielectric constant as a function of frequency for pure silica support as well as
different samples of Pt/SiO
2
of 0.3, 0.6 and 0.9 wt % Pt synthesized via RCE method
1
1
1
1
1
1
1
E-2
E-3
E-4
E-5
E-6
E-7
E-8
1E+4
0
0
0
.3 PSM
.6 PSM
.9 PSM
1E+3
Pure silica
1E+2
0
0
0
.3 PSM
.6 PSM
.9 PSM
Pure silica
Frequency (Hz)
1E+1
1E+1
1E+2
1E+3
1E+4
1E+5
1E+6
1E+7
1E+1
1E+2
1E+3
1E+4
1E+5
1E+6
1E+7
Frequency (Hz)
Fig.7 Conductivity and dielectric constant as a function of frequency for pure silica support as well as
different samples of Pt/SiO of 0.3, 0.6 and 0.9 wt % Pt synthesized via MAS method.
2
From Fig. 7 illustrating the variation of conductivity and dielectric constant with frequency
for the various PtꢀSiO samples synthesized via MAS reduction method, it can be noticed that
2
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both parameters are markedly higher in values than those of the samples prepared by RCE
method, especially at low frequencies (cf., also Table 2). At low frequencies up to 10 KHz,
the conductivity shows nearly no dispersion with frequency (slope ~0.01), while at high
frequencies, >10 KHz, the conductivity shows a dependence on frequency (of slope~0.6).
In conclusion, it is evident that the electromagnetic radiations in MAS method enhance the
even distribution of smaller in size Pt NPs, with higher concentration of grain boundaries,
generating highly mobile electrons and lattice vibrations (phonons), as compared with the
nanoparticles produced during RCE method. These radiations seem also to shoot the smaller
nanoparticles from internal pore system to outer surface in more organized style, specifically
for the higher Pt loading (viz., 0.9 wt %). These actions reflect thereby on the electrical
properties and catalytic performances in several petrochemical processes.
3
.3. Catalytic performances of various Pt/SiO
.3.1. In ethanol conversion
2
nanocatalyst samples
3
The catalytic activity parameters of Pt/ SiO
2
nanocatalyst samples (of 0.3 wt %, 0.6 wt % and
0
.9 wt % Pt loadings), synthesized via MAS or RCE reduction method, were investigated in
o
ethanol conversion, in the temperature range 250 – 450 C, as illustrated in Fig. 8. All catalyst
samples under study exhibited the main activity toward production of ethylene, i.e., toward
dehydration pathway of the reaction, with much lower activity toward acetaldehyde
production (dehydrogenation pathway). The % conversion of ethanol increased markedly
with rising the reaction temperature and decreased slightly with the Pt loading. This seemed
to be related to the dispersion behavior of supported Pt nanoparticles [21]. However, the
o
selectivity toward ethylene, achieved at 450 C for the 0.6 wt % and 0.9 wt % Pt loaded
samples synthesized via MAS method, was somewhat higher (viz. 50ꢀ53 %) than those
values obtained for the samples prepared by RCE method (∼47 %). For the diluted catalyst
sample (of 0.3 wt % Pt) prepared by RCE method, the ethylene yield % (27%) and selectivity
o
to ethylene (53%) at 450 C were higher than those values noticed for the samples prepared
via MAS method (23% and 50%, respectively). These findings run in harmony with the
dispersion parameters of Pt NPs, highlighting the role of nanosilica support in this reaction.
On the other hand, the produced acetaldehyde, detected only in low yields in this reaction,
was found to increase with temperature and with % loading of Pt nanoparticles, achieving its
o
maximum yield at 400 C for the 0.9 wt % loaded sample prepared by RCE method (22 % vs.
19 % for the corresponding sample synthesized via MAS method).
3.3.2. In cyclohexane conversion
The same catalyst samples under study were examined in cyclohexane conversion in the
o
o
temperature range, 300 C – 450 C, the results of which are represented in Fig. 9. It is evident
that all samples of different loadings, synthesized by both MAS and RCE reduction methods,
display the main activity toward dehydrogenation pathway of the reaction, achieving almost
100 % selectivity to benzene all over the temperature range in all cases [22, 23]. Virtually, no
activity for the undesired hydrogenolysis pathway, producing light hydrocarbons of low value
could be observed. This may be acknowledged to the probable maximized hydrogen spillover
effect under the applied experimental conditions. The % conversion of cyclohexane as well as
the benzene yield (%) increased gradually with rising the reaction temperature and with the Pt
loading, especially from 0.3 % to 0.6 % Pt content prepared by both reduction procedures
[
22ꢀ24].
This could be referred to the dispersion behavior of the optimized nanostructure of supported
Pt particles; related mostly to the interaction profile of Pt species with surface hydroxyls of
nanosilica support. These two activity parameters are clearly higher for the samples prepared
by RCE method, characterized by preservation of major fraction of Pt nanoparticles in silica
pore system. However, the 0.9 wt % Pt loaded sample prepared by MAS method could be
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distinguished as exhibiting some hydroisomerization activity to methylcyclopentane (mcp), at
the expense of benzene production. This may be interpreted in view of the suggestion that
MAS procedure could shoot a fraction of supported Pt NPs outside the pore system, as
mobile surface species that seemed to expose some hydroxyl groups, presumably Brønsted
acid sites, to function in producing mcp. In contrast, the identical sample prepared by RCE
method, of deeply incorporated fraction of Pt NPs, showed the same activity pattern as in the
remaining samples.
Table 4: TOF for ethanol and cyclohexane conversions on different Pt/SiO
synthesized through MAS or RCE reduction methods
2
catalyst samples
Catalyst
TOF (benzene)
TOF (ethylene)
(10 s )
ꢀ3
ꢀ1
ꢀ3 ꢀ1
(
10 s )
Samples synthesized by MAS method
0
0
0
.3 PtꢀSi
.6 PtꢀSi
.9 PtꢀSi
11.9
10.7
7.1
13.2
9.6
5.3
Samples synthesized by RCE method
0.3 PtꢀSi
0.6 PtꢀSi
0.9 PtꢀSi
17.2
20.6
21.8
11.1
11.8
19.3
The turnover frequencies (TOF) were calculated using the Pt loading, measured reaction rate
o
(
ethanol and cyclohexane conversions) at the end of the reaction at 400 C and the number of
exposed Pt atoms on the sample surface estimated from the degree of dispersion (Table 2)
related directly to the average size of Pt NPs. The results summarized in Table 4 indicate that
TOFs of the reactions to selectively produce ethylene (from ethanol) or benzene (from
cyclohexane) on the catalyst samples synthesized by MAS reduction decreases with Pt
loading, i.e., with the number of exposed Pt atoms, especially on the surface of the 0.9 PtꢀSi
sample, where MAS irradiation encouraged the transfer of smaller Pt NPs from wider fraction
of mesopores to the outer top surface. The dispersed nanoparticles (3ꢀ7 nm, Table 2) seem to
be responsible of the activity in both reactions tending probably to be structure insensitive.
On the other hand, the samples reduced by RCE method exhibit increasing TOF values with
Pt loading (in agreement with the total conversion data). The decreased number of exposed
atoms related to the enlarged average sizes of the surface Pt NPs (reaching to 15.5 nm, Table
2) seems to control the reactions in these cases, being most probably structure sensitive.
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2
Fig.8: Catalytic Activity of Pt/ SiO nanocatalyst samples of 0.3, 0.6 and 0.9 wt % Pt
loadings synthesized via MAS reduction method (A, B, C) and by RCE method (C, D, F) in
ethanol conversion.
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2
Fig.9: Catalytic Activity of Pt/ SiO nanocatalyst samples of 0.3 wt %, 0.6 wt % and 0.9 wt % Pt
loadings synthesized via MAS reduction method (A, B, C) and by RCE method (C, D, F) in
cyclohexane conversion.
4
. Conclusion
Silica nanopowder used in this study seemed to have high surface area and consisted of fine,
2
homogenous and well distributed nanoparticles of an average size ≤ 2 nm. The SiO ꢀ
supported Pt nanocatalysts (of 0.3, 0.6 and 0.9 wt % loadings) were synthesized via
microwaveꢀassisted solution (MAS) method or rotary chemical evaporation (RCE) method in
the in situ reduction step. The samples synthesized via MAS method exhibited higher surface
areas, with decreased pore dimensions, especially for diluted samples, compared with those
samples synthesized by RCE method. With increasing the metal loading, probably a transfer
of some of Pt NPs out of the pore system to top silica surface by MAS effect was suggested.
The MAS reduction method achieved also more organized distribution of less sized Pt NPs,
not exceeding 10 nm, on silica surface, compared with the RCE method, resulting in uneven
distribution of larger Pt NPs (≥ 33 nm). The results of AC conductivity and dielectric
constants confirmed that the electromagnetic radiations in MAS method enhance the even
distribution of smaller in size Pt NPs with higher concentration of grain boundaries,
generating highly mobile electrons and lattice vibrations (phonons), as compared with
nanoparticles produced during RCE method. The radiations encouraged shooting the smaller
NPs from the inner pore system to the outer surface in more organized style, in the higher
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metal loaded samples (viz., 0.9 wt % Pt). TOFs of the reactions to selectively produce
ethylene (from ethanol) or benzene (from cyclohexane) decreased with Pt loading on catalyst
samples synthesized by MAS method, i.e., with the increase in number of exposed surface Pt
atoms. The dispersed NPs (of 3ꢀ7 nm) seemed to be responsible of the activity in both
reactions, tending to be structure insensitive. However, the samples reduced by RCE method
exhibited increasing TOF values with Pt loading. The decreased number of exposed atoms
related to the enlarged average sizes of the surface Pt NPs (approaching 15.5 nm) seemed to
control the reactions in these cases, tending most probably to be structure sensitive.
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