RETRACTED ARTICLE: Monodispersed palladium-cobalt alloy nanoparticles assembled on poly(N-vinyl-pyrrolidone) (PVP) as a highly effective catalyst for dimethylamine borane (DMAB) dehydrocoupling

Article abstract of DOI:10.1039/c6ra00536e

Herein we report the fabrication of monodispersed poly(N-vinyl-2-pyrrolidone) supported palladium-cobalt nanomaterials (3.45 ± 0.36 nm) and their outstanding efficiency as catalysts in dimethylamine-borane dehydrogenation. By the use of an ultrasonic double reduction method, palladium and cobalt cations were co-reduced in PVP solution and then the prepared nanocatalysts were characterized by UV-Vis, XRD, XPS and HR-TEM-EDX analyses. The nanocatalysts could be easily reused, and at extremely low concentrations and temperature they showed record catalytic activity, giving the best catalytic performance yet with a very high turnover frequency (330.94 h^-1) and a low E_a value of 50.78 ± 2 kJ mol^-1 for DMAB dehydrocoupling.

Full text of DOI:10.1039/c6ra00536e

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Monodispersed palladium–cobalt alloy
nanoparticles assembled on poly(N-vinyl-
Cite this: RSC Adv., 2016, 6, 24097
pyrrolidone) (PVP) as a highly effective catalyst for
dimethylamine borane (DMAB) dehydrocoupling†
a
a
ab
a
a
Betul Çelik, Yunus Yıldız, Hakan Sert, Esma Erken, Yagmur Ko ¸s kun
¨
a
and Fatih S¸ en*
Herein we report the fabrication of monodispersed poly(N-vinyl-2-pyrrolidone) supported palladium–
cobalt nanomaterials (3.45 ꢀ 0.36 nm) and their outstanding efficiency as catalysts in dimethylamine-
borane dehydrogenation. By the use of an ultrasonic double reduction method, palladium and cobalt
cations were co-reduced in PVP solution and then the prepared nanocatalysts were characterized by
UV-Vis, XRD, XPS and HR-TEM-EDX analyses. The nanocatalysts could be easily reused, and at extremely
Received 7th January 2016
Accepted 21st February 2016
low concentrations and temperature they showed record catalytic activity, giving the best catalytic
DOI: 10.1039/c6ra00536e
ꢁ1
performance yet with a very high turnover frequency (330.94 h ) and a low E
a
value of 50.78 ꢀ 2 kJ
ꢁ1
www.rsc.org/advances
mol for DMAB dehydrocoupling.
shown that DMAB is really a model substrate since the products
are relatively easy to understand compared to those of the other
Introduction
Hydrogen is a gas which has been considered as a clean and ABs (Scheme 1).
environmentally benign energy carrier for electricity generation.
Furthermore, various homogeneous and heterogeneous
However, the storage of hydrogen is the main obstacle for the catalysts have been tested for the dehydrocoupling of DMAB
2
1
hydrogen economy. Among the tested hydrogen sources, NaBH4 such as Ru, Rh, Pd, and Ir complexes, Ru(H)(PMe
3
)(PNP) and
H
22
23–25
(
SBH) looks like one of the most suitable energy carriers due to trans-Ru(H)
2
(PMe
3
)(PNP ),
[Rh(1,5-cod)(m-Cl)] ,
2
26,27
2
2
2
a high hydrogen content (10.8%), high stability, availability, [Cp
Ti],
(PCy
ammonia boranes (ABs) can also be thought of as one of the laurate-stabilized
[RuH
(h -H
)
(PCy
)
2
]
and [RuH
, colloidal Rh/[Oct4N]Cl and Rh/Al
(h :h -H
B-
2
2
2
2
3
2
2
28
23
easy handling and lack of ammability. From recent studies, NMe
)
2
)
], RhCl
O
,
30
2
3
2
3
2
3
2
9
Rh(0),
hexanoate-stabilized
Rh(0),
31
32
most convenient hydrogen energy sources due to a high aminopropyltriethoxysilane-stabilized Ru(0), Re complexes,
3
3
34
hydrogen content (19.6%), and high solubility and stability in Rh4–6 clusters, RhCl(PHCy
2
)
3
,
Ru/ZIF-8, [Ru(p-Cym)(bipy)Cl]
Pt(0)/amylamine,
reaction of ABs at room temperature is very simple when TPA@AC, and Pt(0)@CNT. Although record activity has been
1,2
35
36
37
38
water at room temperature. Hydrogen generation through the Cl,
Pd(0)/MOF,
Pt(0)/TBA,
Pt(0)/
Ti]
39
40
5
a convenient catalyst is present. For the dehydrogenation of achieved using
a
homogeneous [h (C
H
-1,3(SiMe
3
)
2
)
5
3
2
2
41
ABs, different types of catalysts have been employed until now. catalyst, the current research has focused on the development
Due to a smaller size and better catalytic performances, nano- of new metal nanoparticle catalysts because of their signicant
3–20
catalysts are preferred for the hydrogen evolution reaction.
advantages in product isolation, catalyst recovery and reus-
Recently, dimethylamine-borane, DMAB, one of the derivatives ability. We report, for the rst time, the preparation and char-
of ABs, has attracted much attention as it has various advan- acterization of highly efficient and monodispersed palladium–
tages such as being nontoxic, a crystalline solid at room cobalt nanomaterials stabilized by poly(N-vinyl-2-pyrrolidone)
temperature, stable in air and water, and environmentally for DMAB dehydrogenation. A few papers have recently re-
friendly. Of particular interest, recent studies have already ported different PdCo bimetallic alloys that enhance the Pd
42–46
electrocatalytic activity and stability for chemical reactions.
a
Sen Research Group, Department of Biochemistry, Faculty of Arts and Science,
Dumlupınar University, Evliya Çelebi Campus, 43100 K u¨ tahya, Turkey. E-mail:
fatih.sen@dpu.edu.tr
b
Department of Chemical Engineering, Faculty of Engineering, Usak University, Usak,
Turkey
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra00536e
Scheme 1 The catalytic dehydrocoupling of dimethylamine-borane
((CH NHBH , DMAB).
1
)
3 2
3
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Herein, the use of Pd–Co@PVP nanocatalysts in the dehydro-
genation reaction of DMAB and their outstanding performance
is explained. The production of the nanocatalyst was performed
using a special method in which both of the metals were co-
reduced by an ultrasonic double reduction method, preferred
for its stability for nanoparticle dispersion, and analytical
techniques such as UV-Vis, XRD, TEM and XPS were carried out
for the characterization. Nanocatalysts containing noble metals
like Pd are known to be expensive, however, their exceptional
performances as catalysts give them a special position in
compact hydrogen production systems employing DMAB as
a
hydrogen containing compound eliminating worries
regarding the high catalyst charge.
Experimental methods
Pd–Co nanomaterials stabilized by PVP production
In the preparation of Pd–Co@PVP (poly(N-vinyl-2-pyrrolidone))
47,48
NPs, an ultrasonic double reduction method
PVP was utilized for stabilization and reduction. Firstly, PVP
2.5 mmol), CoCl (0.25 mmol) and K PdCl (0.25 mmol) were
was used and
(
2
2
4
stirred together under ultrasonic conditions. Later, reuxing of
ꢂ
the resulting mixture was performed at 90 C for 2 h. At room
temperature, the color of the palladium–cobalt nanomaterials
stabilized by PVP is brownish black and they have good stability.
Analytical investigations of the prepared Pd–Co@PVP NPs, their
catalytic and reusability performances for DMAB dehydroge-
nation and investigation of the E of Pd–Co@PVP NPs have been
a
examined in detail in the ESI.†
Fig. 1 HR-TEM, particle size histogram (a) and EDX (b) of Pd–Co@PVP
(1 : 1 ratio) NPs.
Results and discussion
Production and analytical investigation of the Pd–Co
nanomaterials stabilized by PVP
shape, and no agglomerations were observed in the synthesized
catalyst. Furthermore, the HRTEM image for the mono-
dispersed Pd–Co@PVP NPs has also been used to see atomic
lattice fringes in Fig. 1. As a result of these fringes, a Pd (111)
plane was observed with spacing of 0.22 nm on the prepared
The monodispersed Pd–Co@PVP NPs have been characterized
with the use of UV-VIS, XRD, TEM, HRTEM and XPS techniques.
The fabrication of the Pd–Co@PVP NPs was carried out by an
ultrasonic double reduction method in which in the presence of
poly(N-vinyl-2-pyrrolidone), CoCl2 and K PdCl were reduced
catalyst, which is exactly the same as a nominal Pd (111) spacing
2
4
49,50
of 0.22 nm.
Besides, an EDX spectrum of the Pd–Co@PVP
together under ultrasonic conditions. The function of PVP is
stabilizing and reducing the compound. Aer this, the resulting
mixture was reuxed for 2 h, and a brownish black colored
NPs conrmed the alloy structure and 1 : 1 ratio of Pd : Co. The
: 1 ratio of Pd : Co was also conrmed by ICP analysis
Pd Co ).
1
(
2
+
2+
53
47
solution was observed, which implied that the Pd and Co
XRD was used to dene the crystal structure and average
ions were reduced to Pd(0) and Co(0) to form bimetallic nano-
particles. In order to see this change, UV-VIS data were used.
The UV-VIS results (Fig. S1†) demonstrate the conversion of the
Pd and Co salts by poly(N-vinyl-2-pyrrolidone) to Pd–Co nano-
crystallite size of the Pd–Co@PVP NPs. The crystal structures of
Pd–Co@PVP NPs are determined to be face centered cubic
structures, as shown in Fig. 2. As can be seen from this gure,
the diffraction peaks for the catalyst containing palladium and
cobalt were slightly shied to higher 2q values compared to
those of pure Pd, which indicates the formation of the Pd–
2
+
2+
materials. Because of the d–d transitions of the Pd and Co
ions, absorption lines vanish at the end of the reux, showing
that all these cations have been reduced.
Furthermore, the size, morphology and composition of the
Pd–Co@PVP NPs were investigated by TEM-EDX analyses as
shown in Fig. 1 and the average particle size was found to be
ꢂ
Co@PVP NP alloy. Furthermore, the peak at around 24.8 is
attributed to PVP. The XRD pattern of the pure Co sample
2
+
prepared by reduction of Co with PVP exhibited a single broad
ꢂ
peak around 2q ¼ 44.3 indicative of the amorphous structure of
3
.45 ꢀ 0.36 nm. The high resolution electron micrograph
the Co species (JCPDS no. 15-0806). Due to the relatively strong
signal for the Pd species, no signicant diffraction peaks
characteristic of the Co species were detected from the XRD
(HRTEM) of the catalyst is also illustrated in Fig. 1, which shows
the morphology of the prepared catalyst as well. From HRTEM
data, it was observed that most of the particles were a spherical
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Fig. 2 XRD of Pd@PVP, Co@PVP and Pd–Co@PVP NPs.
pattern for the Pd–Co samples, which is likely to be related to
their amorphous structure. In addition to the presence of the
weak Co (111) diffraction peak in the curve of the Pd–Co@PVP
ꢂ
ꢂ
ꢂ
ꢂ
NPs, peaks at 2q ¼ 40.3 , 46.7 , 68.3 and 82.2 correspond to Pd
(
111), (200), (220) and (311) crystal planes, respectively. More-
over, the average crystallite particle size of the monodispersed
Pd–Co@PVP NPs have been calculated to be about 3.71 ꢀ 0.43
49,51,52
nm using the following equation:
ꢀ
ꢁ
kl
ꢀ
d A ¼
b cos q
where k ¼ a coefficient (0.9); l ¼ the wavelength of X-ray used
Fig. 3 Co 2p (a) and Pd 3d (b) XPS spectra of Pd–Co@PVP NPs.
ꢀ
(
1.54056 A); b ¼ the full width half-maximum of the respective
diffraction peak (rad); q ¼ the angle at the position of the peak
maximum (rad).
the binding energy of Co, the shi of the 2p3/2 peak to lower
The Pd (220) diffraction peak of the prepared catalyst is used
for calculating the lattice parameter (aPd) values. The lattice
parameter value of the highly monodispersed Pd–Co@PVP NPs
49–51
energy indicates the alloying of Co with Pd.
These results
indicate that Co and Pd are present as elements in the Pd–
Co@PVP NPs rather than oxygen-containing compounds due to
the PVP wrapping of Pd(0) and Co(0) metals during the catalyst
fabrication. The Pd(II) and Co(II) peaks in Fig. 3 may be caused
by the surface oxidation and/or chemisorption of environ-
mental oxygen during the preparation process. Since the
sensitivity factor of Pd-3d is 2–3 times higher than that of Co-2p
the peak area of Pd is larger than that of Co. Furthermore, by
carrying out some comparative studies in which the catalytic
performances during the dehydrogenation of DMAB of several
types of compositions of the metals (Pd and Co nanoparticles,
physically mixed with a ratio of 1 : 1, and Pd–Co bimetallic
nanomaterials (1 : 1)) were investigated, the production of the
alloy character of Pd–Co@PVP was proven. The turnover
˚
was obtained as 3.88 A using the following equation and is in
49–51
˚
good agreement with the 3.89 A value for pure Pd.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2
l h þ k þ l
sin q ¼
ðfor a cubic structureÞ
2a
The surface compositions and chemical oxidation states of
Pd and Co in the monodispersed Pd–Co@PVP NPs have been
investigated using X-ray photoelectron spectroscopy (XPS). For
this purpose, the Pd 3d and Co 2p regions of the spectrum were
evaluated by the Gaussian–Lorentzian method and the estima-
tion of the relative intensity of the species have been performed
by calculating the integral of each peak, aer smoothing, and
subtracting the Shirley-shaped background. In the XPS spec-
trum, accurate binding energies (ꢀ0.3 eV) have been deter-
ꢁ1
frequencies (TOFs) of 42, 18, 64 and 330.94 h for Pd@PVP
nanoparticles, Co@PVP nanoparticles, physical mixture of
Pd@PVP–Co@PVP nanoparticles, and PdCo@PVP bimetallic
nanoparticles, respectively, were obtained. The comparatively
larger catalytic value of the Pd–Co@PVP nanoparticles to
a simple mixture of Pd and Co nanometals determined that the
fabricated catalyst didn’t contain a Pd and Co nanograin
51
mined by referencing to the C 1s peak at 284.6 eV.
XPS spectra (Fig. 3a) show that the surface Pd and Co are
found to be metallic, not oxides, by comparing experimental
binding energies (Pd-3d5/2: 335.2 eV, Co-2p3/2: 778.1 eV) to those
in the literature (Pd-3d5/2: 334.4 eV, Co-2p3/2: 781.5 eV). As for
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mixture, rather it consisted of a Pd–Co alloy in Pd–Co@PVP
bimetallic nanomaterials.
The performance examination of Pd–Co nanomaterials
stabilized by PVP
The experiments revealed that the as-fabricated Pd–Co@PVP NP
catalysts had high efficiency for the catalytic dehydrogenation
of DMAB. The graph of nDMAB/nH2 versus time (s) (Fig. 4)
shows DMAB dehydrogenation in the existence of nanocatalysts
with various amounts (0.1, 0.2, 0.3, 0.4 and 0.5 mM) at 25.0 ꢀ
ꢂ
0
.1 C. Sudden hydrogen evolution begins linearly without any
initiation time (induction period) and goes on until the catalytic
dehydrogenation reaction of DMAB nishes. NMR data proves
the complete conversion of (CH ) NHBH (d ¼ ꢃ12.7 ppm) to
3
2
3
Fig. 5 Plot of % conversion versus time for the Pd–Co@PVP NP (7.5%
mol) catalysed dehydrocoupling of DMAB in THF at various
temperatures.
[
(CH ) NBH ] (d ¼ ꢃ5 ppm) which shows the dehydrocoupling
3
2
2 2
reaction of DMAB (at 1.0 equiv. H generation) even at room
2
ꢂ
temperature. At different temperatures (20, 25, 30 and 35 C),
the results obtained from the catalytic dehydrogenation of
DMAB with Pd–Co@PVP NPs are shown in Fig. 5. This gure
was used to calculate the rate constants of the fabricated
hydrogen during the dehydrogenation of DMAB at the ve
various temperatures. The rate constants were also employed to
record catalytic activity has been obtained using the Pd–
ꢁ
1
Co@PVP NPs (330.94 h ) for DMAB dehydrocoupling. This
TOF data is almost 5.52 times larger than the literature data of
ꢁ
1
ꢁ
1
60 h for Rh(0) NPs which was the best ever catalyst until

nd the E to be 50.78 ꢀ 2 kJ mol from the Arrhenius graph
a
53
#
ꢁ1
now. In order to compare the TOF data of different catalysts
for the dehydrogenation reaction of DMAB, Table 1 was
prepared. As shown from this table, it is obvious that the initial
(
Fig. 6a) and also the activation enthalpy (DH ¼ 47.01 kJ mol
)
#
ꢁ1 ꢁ1
and activation entropy (DS ¼ ꢁ139.95 J mol
K ) from the
Eyring plot (Fig. 6b) for dehydrogenation.
ꢁ1
TOF value of the Pd–Co@PVP NPs (330.94 h ) is higher than all
Moreover, it is understood that there is an associative
mechanism in the transition state for the catalytic dehy-
drocoupling of DMAB between the higher negative value of the
activation entropy and the small value of the activation
enthalpy.
other heterogeneous and homogeneous catalysts listed in Table
In summary, it is valuable to note that in a very short time
2
hydrogen gas (1.0 mol H /mol DMAB) is entirely emitted,
ꢁ
1
analogous to a highest approximate TOF number of 330.94 h
ꢂ
at 25.0 ꢀ 0.1 C, in the catalytic dehydrogenation of DMAB with
Pd–Co@PVP nanoparticles. To the best of our knowledge,
2
Fig. 4 Plot of nDMAB/nH versus time (s) for the DMAB dehy-
drocoupling in the presence of Pd–Co@PVP NPs in different catalyst Fig. 6 (a) Arrhenius and (b) Eyring plots for the Pd–Co@PVP NP cat-
ꢂ
concentrations at 25.0 ꢀ 0.1 C.
alysed dehydrocoupling of DMAB at various temperatures.
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Table 1 The data of turnover frequencies of the different investiga-
tions for DMAB dehydrogenation
Conclusions
In conclusion, several signicant points related to the fabrica-
tion, analytical examination and utilization of PVP-stabilized
Entry
(Pre) catalysts
Conv. (%)
TOF
Ref.
1
2
3
4
5
6
7
8
9
Pd–Co@PVP NPs
100
90
95
100
100
95
95
100
95
100
25
97
330
7.9
2.8
12.4
12.3
0.7
This study palladium–cobalt nanomaterials as efficient catalysts for the
RhCl
Pd/C
3
23
catalytic dehydrogenation of DMAB are listed as follows:
23
23
26
ꢄ By an ultrasonic double reduction method, Pd–Co@PVP
trans-RuMe
Cp Ti
[Ir(1,5-cod)m-Cl]
[Rh(1,5-cod)(dmpe)]PF
[Rh(1,5-cod)m-Cl]
[Rh(1,5-cod) ]Otf
[RuH(PMe )(NC
IrCl
[Cr(CO)
Rh(0)/[Noct
[RhCl(PHCy
RhCl(PPh
2
(PMe
3
)
4
nanopieces can be fabricated following a facile route, in which
2
palladium and cobalt salts are co-reduced together by the help
of PVP under ultrasonic conditions. The ultrasonication
method was employed to increase the Pd–Co NP dispersion on
PVP. The data show that the route is very effective for the
uniform distribution of Pd–Co NPs on PVP and for the
agglomeration problem of Pd–Co NPs.
2
23
23
23
23
22
23
54
23
34
23
55
56
21
54
31
31
41
31
23
23
38
38
53
37
39
40
6
1.7
2
12.5
12.0
1.5
0.3
13.4
8.2
2.6
4.3
0.3
3.2
0.9
19.9
1.6
2.7
420.0
2.5
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
3
2
H
4
PPr
2
)
2
]
3
5
(thf)]
]Cl
4
90
ꢄ Pd–Co@PVP nanopieces have an extraordinary catalytic
performance in the dehydrogenation of DMAB, with the best
ever catalytic effectiveness.
2
)
3
]
100
100
100
100
100
97
40
77
100
70
5
3
)
3
(Idipp)CuCl
Ni(skeletal)
ꢁ
1
ꢄ A record TOF (330.94 h ) is achieved for the dehy-
drocoupling of DMAB.
[Cp*Rh(m-Cl)Cl]
2
1
ꢁ1
[Cr(CO)
Ru(cod)(cot)
RuCl $3H
[(C -1,3(SiMe
[Ru(1,5-cod)Cl
HRh(CO)(PPh
5
(h -BH
3
NMe
3
)]
ꢄ A 50.78 ꢀ 2 kJ mol
a
E was obtained for the catalytic
reaction of DMAB in the presence of the Pd–Co@PVP
nanocatalysts.
3
2
O
5
H
3
3
)
2
)
2
Ti]
2
ꢄ Pd–Co@PVP nanograins can be considered as promising
alternative catalysts possessing the greatest performance for
simple implementation to provide hydrogen gas from the
catalytic reaction of DMAB for fuel cells.
2 n
]
3
)
3
0.1
0.2
trans-PdCl
Pt(0)/BA
2
(P(o-tolyl)
3
)
2
20
100
100
100
100
100
100
24.88
31.24
60.0
15.0
34.14
57.32
Pt(0)/TBA
Rh(0) NPs
Pt(0)/AA
Pt(0)/TPA@AC
Pt(0)@CNT
Acknowledgements
The authors would like to thank TUBITAK (115 Z 615) for the

nancial support.
4
1
ꢁ1
1
, except for the prior homogeneous catalyst (420 h ) which is
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