Monodisperse Pt nanoparticles assembled on reduced graphene oxide: Highly efficient and reusable catalyst for methanol oxidation and dehydrocoupling of dimethylamine-borane (DMAB)

Article abstract of DOI:10.1166/jnn.2016.11710

Herein, monodisperse platinum (0) nanocatalyst assembled on reduced graphene oxide (Pt(0)@RGO) was easily and reproducibly prepared by the double solvent reduction method at room temperature. Pt(0)@RGO was characterized by X-ray diffraction (XRD), X-ray photoelectron microscopy (XPS) and transmission electron microscopy (TEM) measurements that verify the formation of monodisperse Pt (0) nanoparticles on RGO. The catalytic and electrocatalytic performances of Pt(0)@RGO in terms of activity, isolability and reusability were investigated for both methanol oxidation and the dehydrocoupling of dimethylamine-borane (DMAB) in which Pt(0)@RGO was found to be highly active and reusable heterogeneous catalyst even at room temperature. The prepared nanoparticles can also electrocatalyze methanol oxidation with very high electrochemical activities (5.64 A/cm² at 0.58 V for methanol,). The activation energy (Ea), activation enthalpy (ΔH^#), and activation entropy (ΔS^#) for DMAB dehydrogenation were calculated to be 59.33 kJ mol-1, 56.79 kJ mol^-1 and -151.68 J mol^-1K^-1, respectively. The exceptional stability of new Pt(0)@RGO nanoparticles towards agglomeration, leaching and CO poisoning allow these particles to be recycled and reused in the catalysis of DMAB dehydrogenation and methanol oxidation. After four subsequent reaction and recovery cycles, Pt(0)@RGO retained ≥75% activity towards the complete dehydrogenation of DMAB.

Full text of DOI:10.1166/jnn.2016.11710

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 5951–5958, 2016
Copyright © 2016 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Monodisperse Pt Nanoparticles Assembled on Reduced
Graphene Oxide: Highly Efficient and Reusable
Catalyst for Methanol Oxidation and
Dehydrocoupling of Dimethylamine-Borane (DMAB)
1
1
1
2
1ꢀ∗
Yunus Yıldız , Esma Erken , Handan Pamuk , Hakan Sert , and Fatih S¸ en
1
Dumlupinar University, Department of Biochemistry, Kütahya, 43100, Turkey
Usak University, Department of Chemical Engineering, Usak, 64100, Turkey
2
Herein, monodisperse platinum (0) nanocatalyst assembled on reduced graphene oxide
Pt(0)@RGO) was easily and reproducibly prepared by the double solvent reduction method at
(
room temperature. Pt(0)@RGO was characterized by X-ray diffraction (XRD), X-ray photoelectron
microscopy (XPS) and transmission electron microscopy (TEM) measurements that verify the forma-
tion of monodisperse Pt (0) nanoparticles on RGO. The catalytic and electrocatalytic performances
of Pt(0)@RGO in terms of activity, isolability and reusability were investigated for both methanol oxi-
dation and the dehydro cDo eu pl i vl i ne gr e odf db iym Ien t gh ye l na mt ai nt oe -: b Uo nr ai vn ee r (sDi tMy Ao Bf )A i un cwk hl ai cnh dPt(0)@RGO was found
to be highly active andI Pr e: u 1s 8a 8b .l e6 8h .e0 t .e2r 3o g5 e On en o: uSs uc na ,t a0 l 5y s Jt ue nv e 2n 0 a1 t6r o0 o1 m: 2 t6e : m0 1p erature. The prepared
nanoparticles can also ele Cc tor op cy ar it ga lhy tz: eA mm ee t rh i ac na on l So xc ii de an t ti oi f ni c wP i ut hb l vi se hr ye r hs igh electrochemical activi-
2
#
ties (5.64 A/cm at 0.58 V for methanol,). The activation energy (Ea), activation enthalpy (ꢀH ꢁ,
#
−1
,
and activation entropy (ꢀS ꢁ for DMAB dehydrogenation were calculated to be 59.33 kJ mol
−
1
−1 −1
5
6.79 kJ mol and −151.68 J mol
K , respectively. The exceptional stability of new Pt(0)@RGO
nanoparticles towards agglomeration, leaching and CO poisoning allow these particles to be recy-
cled and reused in the catalysis of DMAB dehydrogenation and methanol oxidation. After four sub-
sequent reaction and recovery cycles, Pt(0)@RGO retained ≥75% activity towards the complete
dehydrogenation of DMAB.
Keywords: Dehydrogenation, Direct Methanol Fuel Cells (DMFCs), Nanocatalyst,
Electrochemical Activities.
1
. INTRODUCTION
Compared to the pure hydrogen, the use of low-molecular
weight alcohols as energy sources has several advantages,
including the ease with which these materials can be easily
handled, stored and transported.
The general equation for alcohol oxidation reactions
may be written as:
The development of powerful and clean energy sources
currently represents an important issue. With the increas-
ing concern for environmental protection and the depleting
of fossil fuel, there is a pressing need to find cleaner and
greener energy sources. Over the last two decades, there
has been a significant increase in research and develop-
ment in fuel cell due to its ability to convert chemical
energy of fuel into electricity without causing much pol-
lution. Direct alcohol fuel cells could be pivotal in the
future because of the high energy content of low molecular
weight alcohols, easy handling, storage, and transporta-
7
–9
1
–5
C H +1OH+ꢀ2n−1ꢁH O
n
2n
2
+
−
→ ꢀnꢁCO +ꢀ6nꢁH +ꢀ6nꢁe
(1)
2
The direct methanol fuel cell (DMFC), powered by electro-
oxidation of methanol has been studied extensively. DMFC
is one of the most common direct alcohol fuel cells. More-
over, among the various fuel cell technologies, the direct
methanol fuel cell (DMFC), which utilizes methanol as
its fuel, is a promising candidate for its high-efficiency,
6
tion, and their potential application in powering vehicles.
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 6
1533-4880/2016/16/5951/008
doi:10.1166/jnn.2016.11710
5951

Monodisperse Pt Nanoparticles Assembled on Reduced Graphene Oxide: Highly Efficient and Reusable Catalyst
Yıldız et al.
near-zero pollution, ease in refueling and application as
a portable power source such as cellular phones and
Thermo Scientific spectrometer was used for X-ray
Photoelectron Spectroscopy (XPS) measurements and the
X-ray source was Kꢃ lines of Mg (1253.6 eV, 10 mA).
Samples were prepared by depositing the catalyst on Cu
double-sided tape (3 M Inc.). C 1s line at 284.6 eV was
chosen as a reference point and all XPS peaks were fitted
using a Gaussian function and the C 1s line at 284.6 eV
was used as the reference line.
A Panalytical Emperian diffractometer with Ultima +
theta–theta high resolution goniometer, having an X-ray
generator (Cu Kꢃ radiation, k = 1.54056 Å) and operating
condition of 45 kV and 40 mA, were employed in XRD
analysis.
1
0–13
notebook computers, etc.
However, there are sev-
eral limitations associated with the application of direct
methanol fuel cells, including the high price of the finished
devices, low levels of electrocatalytic activity and poor
1
4–17
durability.
fuel cells used as electrocatalysts have contained platinum
Pt) and platinum alloys because of the excellent electro-
To date, the majority of the direct methanol
(
catalytic activities of these materials. Those materials can
also be used other types of reactions like the dehydro-
genation of ammine boranes. Recent studies have shown
that amine-boranes also need to be considered as hydrogen
storage materials among the various kinds of solid hydro-
gen storage materials because of the following advanta-
geous properties:
Electrochemical measurements including Cyclic voltam-
metry (CV) and Chronomperometry (CA) were carried
out in a three-electrode electrochemical cell using a
microcomputer-controlled potentiostat/galvanostat (Gamry
Interface 1000) at room temperature. The reference,
counter and working electrodes were the saturated calomel
electrode (SCE), glassy carbon, and prepared catalysts,
respectively. The working electrode was a 3 mm glassy
carbon disk, polished with Al O paste and washed in dis-
(
(
(
i) having high hydrogen content,
ii) having high stability,
iii) being environmentally benign.
The ammonia-borane can release hydrogen gas either
ꢀ
upon heating to temperatures above 70 C or on the
2
3
hydrolysis under mild conditions. Therefore in this study;
monodisperse platinum (0) nanocatalyst assembled on
reduced graphene oxide (Pt(0)@RGO) were easily and
reproducibly prepared by double solvent reduction method
at room temperature. The prepared platinum nanopar-
ticles have been investigated by a variety of tech-
tilled water and a heterogeneous mixture was prepared by
1
8
our previous studies.
1
1
B NMR spectra were taken on a Bruker Avance DPX
00 MHz spectrometer (128.2 MHz for B NMR).
1
1
4
Delivered by Ingenta to: University of Auckland
.2. Methods
2
2
niques: X-ray diffraction (XRD), transmission electron
microscopy (TEM), and X-ray photoe Cl e oc tpr oy nr i gs hp et :c At r om s ec or ipc ya n Scientific Publishers
IP: 188.68.0.235 On: Sun, 05 Jun 2016 01:26:01
.2.1. The Preparation of Pt(0)@RGO
Pt(0)@RGO has been prepared by double solvent reduc-
tion method. As a summary of this method, superhydride
and ethanol were used to reduce the mixture of 0.25 mmol
(
XPS) (surface characterization); cyclic voltammetry (CV)
and chronoamperometry (CA) (electrochemical) for alco-
hol oxidation and the dehydrogenation of DMAB.
of PtCl dissolved in small amount of anhydrous tetrahy-
4
drofuran and 0.25 mmol of PA ligand. The observation of
a brown-black color in the solution indicates the formation
of platinum nanoparticles. Finally, the solid Pt nanoparti-
cles were dried under vacuum at room temperature. The
prepared platinum nanoparticles were mixed in a 1:1 ratio
with RGO, which is used as a support.
Graphene oxide (GO) was synthesized from graphite
powder using modified Hummer’s method. In brief, 1 g of
graphite and 0.5 g of sodium nitrate were mixed together
followed by the addition of 23 ml of conc. sulphuric acid
2
. EXPERIMENTAL DETAILS
2
.1. Reagents and Instrumentation
PtCl4 (99% Alfa Aesar), tetrahydrofuran (THF) (99.5%,
Merck), methanol (≥99.5%), and HClO4 (60%) were pur-
chased from Merck, lithium triethylborohydride (1.0 M
dissolved in THF), DMAB and Propan-1-amine (PA) were
obtained by Sigma Aldrich). De-ionized water was filtered
by Millipore water purification system (18 Mꢂ) analytical
grade. All glassware and Teflon-coated magnetic stir bars
were cleaned with aqua regia, followed by washing with
distilled water before drying.
Transmission electron microscopy (TEM) images were
obtained on a JEOL 200 kV TEM instrument. Sample
preparation for TEM analysis involves placement of a drop
of 0.5 mg/mL ethanol solution of the prepared catalyst
with a RGO support on a carbon covered 400-mesh copper
grid, the solvent is then allowed to evaporate. Excess solu-
tion was removed with an adsorbent paper and the sample
was dried under vacuum at room temperature before anal-
ysis. More than 300 particles were calculated to get the
integrated information about the overall distribution of Pt-
based catalyst sample.
under constant stirring. After 1 h, 3 g of KMnO was
4
added gradually to the above solution while keeping the
ꢀ
temperature less than 20 C to prevent overheating and
ꢀ
explosion. The mixture was stirred at 35 C for 12 h
and the resulting solution was diluted by adding 500 ml
of water under vigorous stirring. To ensure the comple-
tion of reaction with KMnO , the suspension was further
4
1
9
treated with 30% H O solution (5 ml). The result-
2
2
ing mixture was washed with HCl and H O respectively,
2
followed by filtration and drying, graphene oxide sheets
were thus obtained. RGO was obtained by the hydrazine
reduction that was performed by adding of hydrazine
monohydrate (98%)
5952
J. Nanosci. Nanotechnol. 16, 5951–5958, 2016

Yıldız et al.
Monodisperse Pt Nanoparticles Assembled on Reduced Graphene Oxide: Highly Efficient and Reusable Catalyst
2
.2.2. Catalytic Activities, Reusability Performances and
Mercury (Hg(0)) Poisoning of Pt(0)@RGO in the
Dehydrogenation of DMAB
All reactions and manipulations were performed under dry
nitrogen atmosphere using standard Schlenk techniques
including a vacuum system unless otherwise specified. The
dehydrogenation of DMAB by the help of Pt(0)@RGO
was performed in a typical jacketed, three necked reaction
flask connected to the water-filled cylinder glass tube. The
jacketed reaction flask was kept under vacuum at least for
1
5 min and filled with nitrogen to remove any trace of
oxygen and water present prior to all the catalytic reac-
tions. The catalytic activity of Pt(0)@RGO in the dehy-
drogenation of DMAB was determined by measuring the
rate of hydrogen generation. In a typical set of experiment,
Figure 2. High resolution transition electron micrograph and particle
1
.0 mmol of DMAB was dissolved in a small amount of
size histogram of a Pt(0)@RGO.
THF, and the solution was transferred into the jacketed
reaction flask containing a 1.0 mL THF solution of the
jacketed, three-necked reaction flask, and the mixture was
stirred for 4 h for mercury (Hg(0)) poisoning studies. This
solution was then used in the dehydrogenation of 1.0 mmol
DMAB under the same conditions described above.
ꢀ
prepared catalyst thermostated at 25.0 ± 0.1 C. Hydro-
gen gas release from the catalytic reaction solution was
achieved using a typical water-filled gas burette system
and recording the displacement of the water level in the
column every minute until no more hydrogen evolution
was observed. When no more hydrogen generation was
observed, the experiment was stopped, the reactor was dis-
connected from the water-filled column, and the hydrogen
pressure was released. Next, approximately 0.5 mL of the
reaction solution in the reactor was withdrawn with a glass
3
. RESULTS AND DISCUSSION
3
.1. Characterization of Pt(0)@RGO
The preliminary characterization of Pt(0)@RGO was per-
formed by XRD, TEM, HRTEM and XPS and applications
of the catalyst were examined by CV and CA for methanol
oxidation reaction and hydrogen generation tests for the
dehydrogenation of DMAB.
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Pasteur pipet and added to 1 g of CDCl in a quartz NMR
Copyright: American Scientific Publishers
3
sample tube for NMR analysis.
The analysis of the crystalline structure and the size of
Pt(0)@RGO nanocatalysts were performed by using XRD.
As can be seen in Figure 1, Pt(0)@RGO exhibit distin-
tict diffraction patterns and they can be assigned to Miller
indices (111), (200), (220), (311), and (320), respectively,
planes of the face-centered cubic (fcc) crystal lattice of
platinum.
The lattice parameters (ꢃPt) and average crystallite sizes
of the metal particles was calculated by using the Pt
(220) diffraction peaks of the prepared catalyst. The lattice
For the reusability performance experiments, upon the
complete dehydrogenation of 1.0 mmol of DMAB using
freshly prepared catalyst, the jacketed, three-necked reac-
tion flask was detached from the line and connected to a
vacuum line. After evaporation of the volatile solutions,
the remaining solid residue containing the catalyst was
weighed and used again in the dehydrogenation reaction
under the same conditions (in a small amount of THF
ꢀ
at 25.0 ± 0.1 C). This reaction and recovery cycle was
repeated four times.
A small amount of elemental Hg (300 equiv.) was added
to a THF solution containing the prepared catalyst in a
Figure 1. XRD of Pt(0)@RGO.
Figure 3. Pt 4f electron spectra of Pt(0)@RGO.
J. Nanosci. Nanotechnol. 16, 5951–5958, 2016
5953

Monodisperse Pt Nanoparticles Assembled on Reduced Graphene Oxide: Highly Efficient and Reusable Catalyst
Yıldız et al.
Figure 4. Cyclic voltammogram of Pt(0)@RGO in 0.1 M HClO +
Figure 5. Chronoamperometric curves of Pt(0)@RGO towards
methanol oxidation at 0.6 V (vs. SCE) in 0.1 M HClO +0.5 M CH OH.
4
0
ꢄ5 M methanol (CH OH). Scan rate is 50 mV/s.
3
4
3
constant values for Pt(0)@RGO were found to be 3.917 Å
which is in good agreement with 3.923 Å for pure Pt,
using the following equation.
and (2 0 0) planes of Pt(0)@RGO were observed with a
spacing of 0.226 and 0.195 nm, respectively, which is very
close to the nominal Pt (1 1 1) and (2 0 0) spacing of
2
0
2
2
0
.227 and 0.196 nm, respectively.
X-ray photoelectron spectroscopy has been used to
√
2
2
2
ꢆ h +k +l
Sinꢅ =
(for a cubic structure)
2
a
obtain information about the oxidation state of the
Pt(0)@RGO. As most of the atoms in small particle
clusters are surface atoms, the oxidation state measured as
such would also reflect well the bulk oxidation state. The
From the full width half maximum of the (220) peaks in
XRD, the average crystallite size of Pt(0)@RGO was cal-
2
1
culated as 3.60 ± 0.34 nm using Scherrer equation;
kꢆ
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Lorentzian method and the relative intensities of the
Copyright: American Scientific Publishers
dꢀÅꢁ =
species were estimated by calculating the integral of each
peak, after smoothing, subtraction of the Shirley-shaped
background. As the binding energy (BE) for the line of
zero valent platinum at 284.3 eV is very close to the C1s
line resulting from adventitious carbonaceous species, the
spectrum was used instead for the analysis of Pt oxida-
tion state. It is shown that Pt is mostly in zero oxidation
ꢇcosꢅ
where k = a coefficient (0.9); ꢆ = the wavelength of X-ray
used (1.54056 Å); ꢇ = the full width half-maximum of
respective diffraction peak (rad) ꢅ = the angle at the posi-
tion of peak maximum (rad)
In order to reinforce the results of diffraction measure-
ments, TEM was used. The high resolution electron micro-
graph and particle size histogram of the Pt(0)@RGO are
shown in Figure 2. Uniform and monodisperse distribu-
tion of catalyst particles with a relatively narrow range
was observed. The average particle sizes were found to
be 3.53 ± 0.37 nm for Pt(0)@RGO. This result is in
good agreement with the results from the XRD imag-
ing. The most of the particles are in spherical shape, and
no agglomerations are observed in the prepared catalyst.
Figure 2 also displays the representative atomic lattice
fringes obtained by high resolution transmission electron
microscopy for Pt(0)@RGO. As an example, Pt (1 1 1)
2
3–31
state at about 71.0 and 74.3 eV (71.75%).
As shown
in Figure 3, the other Pt (74.7 and 78.0 eV) doublet has
Table I. The comparison of particle size, ECSA, CSA, Rf and % Pt
utilization for the prepared catalyst with commercial Pt catalyst.
Particle
size (nm)
ECSA
(m /g)
Roughness
factor (Rf)
CSA
(m /g)
Pt utility
(%)
2
2
Figure 6. % conversion versus time graph for the catalytic dehydro-
genation of DMAB in THF at room temperature starting with 7.5% mol
of Pt(0)@RGO.
Pt(0)@RGO
Pt (ETEK)
3ꢄ53
2ꢄ70
46ꢄ63
50ꢄ60
95ꢄ2
103ꢄ2
79ꢄ43
103ꢄ0
58ꢄ70
49ꢄ10
5954
J. Nanosci. Nanotechnol. 16, 5951–5958, 2016

Yıldız et al.
Monodisperse Pt Nanoparticles Assembled on Reduced Graphene Oxide: Highly Efficient and Reusable Catalyst
electrocatalysts for the oxidation reaction of methanol. The
anodic parts of the cyclic voltammogram for the prepared
catalyst showed that Pt(0)@RGO has 9.2 times higher cat-
alytic activities than commercially available Pt-ETEK cat-
alyst. Because alcohol electro-oxidation of this heteroge-
neous catalyst is surface-controlled, the ECSA of the cata-
lyst is an important parameter for determining the catalytic
properties of the electro catalyst. The ECSA of the catalyst
2
−1
in m g could be determined from the integrated charge
in the hydrogen adsorption region of the cyclic voltammo-
3
2–36
gram using the following formula.
Q (mC)
ECSA =
2
Pt loading on electrode×0ꢄ21 mC/cm
Figure 7. % conversion versus time graph for Pt(0)@RGO (7.5% mol)
catalyzed dehydrogenation of DMAB in THF at various temperatures.
where Q is the electric charge for hydrogen desorption
and 0.21 mC/cm represents the charge required to oxi-
2
dize a monolayer of H on platinum. Roughness factor,
Rf (m g Pt cm ꢁ describes the enhancement of the real
2
emerged in higher binding energies due to exposure to the
atmosphere of the type of PtOx.
2
−1
−2
electrochemical surface area (ECSA) in comparison with
2
the geometric area, Ag (cm ꢁ, The Chemical Surface Areas
3
.2. Electrochemical Measurements for Methanol
Oxidation
(
CSA) of the catalysts were calculated using the following
equation, assuming homogenously distributed and spheri-
Cyclic voltammograms of Pt(0)@RGO in 0.1 M HClO4
at room temperature were recorded several times over
a 3 month period, and no change was observed, indi-
cating the stability of the prepared catalysts. Typical
hydrogen and oxygen adsorption and desorption features
were observed on the prepared catalyst. The addition of
34
cal particles.
3
6
×10
CSA =
ꢈ×d
where d is the mean Pt crystalline size in Å (from the
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2
XRD results) and is the density of Pt metal (21.4 g/cm ).
IP: 188.68.0.235 On: Sun, 05 Jun 2016 01:26:01
methanol to the electrolyte resulted inC ao pd yr ar img ha tt i: cA cmh ae nr gi ce an S Tc ihe en tPi ft icu Pt i lui zb al it si oh ne r es fficiency can be calculated using the
3
6–39
in the appearance of the voltammograms due to the alcohol
oxidation reaction taking place on the surface of platinum
nanoparticles as shown in Figure 4. The electrooxidation
of methanol on prepared nanoparticle was characterized by
two well-defined current peaks on the forward and reverse
scans. In the forward scan, the oxidation peak is corre-
sponding to the oxidation of freshly chemisorbed species
which come from methanol adsorption. The reverse scan
peak is primarily associated with removal of carbonaceous
species which are not completely oxidized in the for-
ward scan. The magnitude of the peak current on the for-
ward scan indicates the electrocatalytic activity of the
ratio of the ECSA to CSA.
ECSA
Utilization (% ) = CSA ∗100
Table I indicates the CSA, ECSA, Rf and % Pt utility for
the prepared catalysts. As shown in the table, Pt(0)@RGO
has a higher active surface area, Rf and % Pt utility com-
pared to commercial available Pt (ETEK), which may con-
tribute to higher catalytic activity.
CA was used to examine the long-term stability of
Pt(0)@RGO in the methanol oxidation. Figure 5 shows
the representative CA curves for methanol oxidation on
Figure 8. Arrhenius (a) and Eyring plots (b) for Pt(0)@RGO catalyzed dehydrogenation of DMAB at various temperature.
J. Nanosci. Nanotechnol. 16, 5951–5958, 2016
5955

Monodisperse Pt Nanoparticles Assembled on Reduced Graphene Oxide: Highly Efficient and Reusable Catalyst
Yıldız et al.
Pt(0)@RGO at the potential 0.6 V (vs. SCE) for 3,600 s.
There is a sharp current drop in the first ten minutes due
to poisoning of the Pt surface and then quite a slow decay
of the curve is observed.
3
.3. Catalytic Activity of Pt(0)@RGO in the
Dehydrogenation of Dimethylamine-Borane
(
DMAB)
Pt(0)@RGO was studied in the dehydrogenation of
DMAB by measuring the volume of hydrogen generated
ꢀ
during the reaction at 25 ± 0.1 C and also found to be
highly active for this reaction. The hydrogen evolution
starts immediately after injection of DMAB into the THF
solution of Pt(0)@RGO. The initial turnover frequency
Figure 9. % conversion versus time graph for Pt(0)@RGO catalyzed
dehydrogenation of DMAB in THF at room temperature for 1st and 4th
catalytic runs.
−1
(
TOF) of 20.83 h for Pt(0)@RGO, were obtained from
the slope of the linear portion of the plot given Figure 6
and continues until 1 equivalent of H per mol DMAB is
2
at room temperature. Pt(0)@RGO was isolated as black
powder by vacuum drying and then bottled under a N2
atmosphere after the complete dehydrogenation of DMAB.
Such covered catalyst was found to be still catalyti-
cally active in the dehydrogenation of DMAB. They still
retained ≥75% of its initial activity (Fig. 9) after its fourth
run. This indicates that the prepared Pt(0)@RGO are easily
recoverable and redispersible while remaining catalytically
active. The slight decrease in catalytic activity over multi-
ple runs can be attributed to the formation of a precipitate
released. As shown Figure 6, only ∼8–9% conversion is
observed after an induction period (∼10 min), when PtCl
4
is used as pre-catalyst under the same conditions in the
dehydrogenation of DMAB. After induction period, the
Pt particles precipitate out of solution due to the weakly
coordination of chloride anion of PtCl that is not strong
4
enough to stabilize the Pt (0) nanoparticles for an extended
4
0
period of time. After the dehydrogenation reaction of
DMAB (at 1.0 equiv. H generation), NMR shows the
2
complete conversion of (CH ꢁ NHBH (ꢉ = ∼12ꢄ3 ppm)
3
2
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3
to [(CH ꢁ NBH ] (ꢉ = ∼5.1 pp mI P) :e 1v e8 n8 .a6t 8r .o0 o. 2m 3 5t e mO pn e: r S- un, 05 Jun 2016 01:26:01
3
2
2 2
Table II. Catalysts tested in the dehydrogenation of DMAB under mild
Copyright: American Scientific Publishers
ature. In order to test heterogeneity of Pt(0)@RGO in the
dehydrogenation of DMAB, a series of poisoning experi-
ments were carried out adding of 300 equiv. of Hg (0) per
Pt, entirely stops the reaction, verifying that Pt(0)@RGO
contribute to heterogeneous catalysis in the dehydrogena-
tion of DMAB.
ꢀ
conditions (≤25 C).
Entry
(Pre) Catalysts
Cov. (%)
TOF
Ref.
1
2
3
4
5
6
7
8
Pt(0)@RGO
100
100
100
97
90
100
40
100
95
70
100
95
20
95
25
77
90
100
100
100
5
97
95
20ꢄ83 This study
[(C H -1,3(SiMe ) ) Ti]
2
420ꢄ0
4ꢄ3
[42]
[44]
[45]
[44]
[46]
[33]
[47]
[44]
[43]
[33]
[44]
[44]
[44]
[44]
[43]
[44]
[44]
[48]
[49]
[44]
[45]
[44]
[33]
[50]
[44]
[44]
[34]
5
3
3
2
2
RhCl(PPh ꢁ
3 3
[Cr(CO) (thf)]
13ꢄ4
7ꢄ9
12ꢄ3
1ꢄ6
Rate constants (kobs) for the dehydrogenation of
DMAB were measured from the linear portions of the
plots given in Figure 7 at four different temperatures in
5
RhCl3
Cp Ti
2
Ru(cod)(cot)
Ni(skeletal)
[Rh(1,5-cod)
[Ru(1,5-cod)Cl ]n
Rh(0)NPs
ꢀ
the range of 20–35 C, and they were used to calcu-
3ꢄ2
12ꢄ0
2ꢄ5
−1
late the activation energy of 59.33 kJ mol (Ea), activa-
tion enthalpy of 56.79 kJ mol ( ꢊH ꢁ, and activation
9
10
11
2
]Otf
−1
#
2
−1
−1
#
60ꢄ0
1ꢄ7
0ꢄ2
2ꢄ8
0ꢄ3
entropy of −151.68 J mol K ( ꢊS ꢁ from the Arrhenius
and Eyring plots for Pt(0)@RGO (Fig. 8). Even though
the activation energy values provided by Pt(0)@RGO are
higher than those of the one of the best catalysts, Rh(0)
nanoparticles (34 kJ mol ꢁ, it is smaller than the acti-
vation energies reported in the literature for the same
reaction.35 Furthermore, there is an associative mechanism
in the transition state for the catalytic dehydrogenation of
DMAB between the higher negative value of the activation
entropy and the small value of the activation enthalpy.
1
1
2
3
[Rh(1,5-cod)(dmpe)]PF6
trans-PdCl (P(o-tolyl) ꢁ
2
3 2
14
15
16
Pd/C
IrCl
RuCl .3H O
3
−1
2ꢄ7
8ꢄ2
12ꢄ4
0ꢄ3
1ꢄ5
3
2
17
18
19
Rh(0)/[Noct ]Cl
4
trans-RuMe (PMe ꢁ
2
3 4
(Idipp)CuCl
20
21
[RuH(PMe ꢁ(NC H PPr ꢁ ]
3
2
4
2 2
HRh(CO)(PPh ꢁ
0ꢄ1
19ꢄ9
0ꢄ7
3 3
22
23
24
25
[Cr(CO) (h1-BH NMe ꢁ]
5
3
3
[Ir(1,5-cod)m-Cl]2
Ru(0)/APTS
[RhCl(PHCy ꢁ ]
100
100
100
100
100
55ꢄ0
2ꢄ6
0ꢄ9
12ꢄ5
15ꢄ0
3
.4. Reusability Performance of Pt(0)@RGO in the
Catalytic Dehydrogenation of DMAB
2
3
26
[Cp∗Rh(m-Cl)Cl]
2
2
2
7
8
[Rh(1,5-cod)m-Cl]
Pt (0)/AA
The isolability, recovery and reusability of Pt(0)@RGO
were also evaluated in the dehydrogenation of DMAB
2
5956
J. Nanosci. Nanotechnol. 16, 5951–5958, 2016

Yıldız et al.
Monodisperse Pt Nanoparticles Assembled on Reduced Graphene Oxide: Highly Efficient and Reusable Catalyst
of bulk Pt(0). This becomes visible at the end of the
fourth catalytic run, yielding a clear, colorless (i.e., Pt(0)
nanoparticle free) solution by increasing the concentration
of boron products, which is known to decrease the accessi-
bility of the active sites. The P-XRD pattern of the sample
after the fourth catalytic run verifies the agglomeration of
associative mechanism for Pt(0)@RGO catalyzed dehy-
drogenation of DMAB.
Easy preparation, high stability, catalytic versatility, effi-
cient recovery, prolonged stability, reusability and high
catalytic performance of Pt(0)@RGO offer a promising
candidate to develop highly efficient portable hydrogen
generation systems depending on DMAB as a CO-free,
4
1
Pt(0) nanoparticles.
economical point of view, H storage and generation sys-
2
3
.5. Catalytic Performance Comparison of
Pt(0)@RGO with the Previous Catalyst Systems
Tested in the Catalytic Dehydrogenation of
DMAB
tem for fuel cell applications.
Acknowledgments: The authors would like to thank
to TUBITAK (213 M 448) for the financial support. The
authors gratefully acknowledge DPU-ILTEM and Duzce
University Central Laboratory (DUBIT). The partial sup-
ports by Science Academy are gratefully acknowledged.
˙
As shown in Table II, the apparent initial TOF value of
−1
Pt(0)@RGO (20.83 h ꢁ is higher than most of the other
heterogeneous and homogeneous catalysts reported in the
literature. However, it is still lower than the best known
heterogeneous (60 h ꢁ and homogeneous (420 h ꢁ
catalysts. To the best of our knowledge, our prepared cat-
alysts are one of the a few example of an isolable and
reusable nanocatalyst used in this important catalytic reac-
−1
35
−1 42
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Received: 26 November 2014. Accepted: 26 March 2015.
Delivered by Ingenta to: University of Auckland
IP: 188.68.0.235 On: Sun, 05 Jun 2016 01:26:01
Copyright: American Scientific Publishers
5958
J. Nanosci. Nanotechnol. 16, 5951–5958, 2016