Dendrimer-encapsulated bimetallic pt-ni nanoparticles as highly efficient catalysts for hydrogen generation from chemical hydrogen storage materials

Article abstract of DOI:10.1002/cctc.201300143

Bimetallic Pt-Ni dendrimer-encapsulated nanoparticles (DENs) with different Pt/Ni ratios have been successfully synthesized through the co-complexation of Pt²⁺ and Ni²⁺ cations to the internal tertiary amine of fourth-generation hydr

Full text of DOI:10.1002/cctc.201300143

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DOI: 10.1002/cctc.201300143
Dendrimer-Encapsulated Bimetallic Pt-Ni Nanoparticles as
Highly Efficient Catalysts for Hydrogen Generation from
Chemical Hydrogen Storage Materials
Kengo Aranishi,[a, b] Ashish Kumar Singh,[a] and Qiang Xu*[a, b]
Bimetallic Pt-Ni dendrimer-encapsulated nanoparticles (DENs)
with different Pt/Ni ratios have been successfully synthesized
through the co-complexation of Pt2+ and Ni2+ cations to the in-
ternal tertiary amine of fourth-generation hydroxyl-terminated
poly(amidoamine) dendrimers (G4-OH) followed by coreduc-
tion by NaBH4 in aqueous solution. The catalytic activities for
the decomposition of hydrous hydrazine and the hydrolysis of
ammonia borane over the Pt-Ni DENs have been studied in
aqueous solution, and the Pt-Ni DENs show high catalytic per-
formances for these hydrogen-generation reactions. TEM meas-
urements reveal that the size-controlled Pt-Ni NPs, which have
an average particle size of 1.1ꢁ0.3 nm, are encapsulated in
the G4-OH dendrimer.
Introduction
Dendrimers, which have highly branched structures, are very
important for the encapsulation of metal salts and the forma-
tion of nanoparticles (NPs) and clusters of desired sizes.[1–4] Re-
cently, hydroxyl-terminated poly(amidoamine) (PAMAM) den-
drimers have been used extensively as a macroligand to stabi-
lize metallic NPs of different sizes.[5–8] PAMAM dendrimers act
as a flexible membrane around metallic NPs, and the applica-
tion of PAMAM dendrimer-encapsulated NPs (DENs) in catalytic
reactions offers promising new opportunities to create tailored
catalytic systems.[9–14] Dendrimers offer the control of the
number of metal complexes in an assembly through stepwise
complexation, which allows the complexes to accumulate in
discrete nanocages.[15–17]
The only byproduct of this reaction (N2) does not need to be
collected for recycling. Moreover, N2 can be transformed into
ammonia by the Haber–Bosch process, homogeneous catalytic
processes, or an electrolytic process[33–36] and subsequently to
hydrazine on a large scale[37–39] or perhaps transformed directly
to hydrazine by using an electrolytic process similar to that for
ammonia synthesis.[35] However, the key to effectively exploit
the hydrogen storage properties of hydrazine is to avoid its in-
complete decomposition by the undesired reaction pathway
shown in Equation (2).
3 H2NNH2 ! N2þ4 NH3
ð2Þ
A great deal of work has been devoted to the development
of effective hydrogen storage materials for hydrogen fuel cells
and hydrogen-based energy systems.[18–22] Recently, our work
has suggested that hydrous hydrazine (e.g., H2NNH2·H2O) is
a promising hydrogen storage material because it is a liquid
over a wide temperature range (213–392 K) and contains as
much as 8 wt% hydrogen available for hydrogen generation,
which can be released by the complete decomposition of hy-
drazine to hydrogen and nitrogen [Eq. (1)].[23–32]
A number of metal catalysts have been investigated for the
decomposition of hydrazine, and the reaction pathways for hy-
drazine decomposition strongly depend on the catalyst used
and the reaction conditions.[23–32] However, the development of
highly active and efficient catalysts is of significant importance
for the practical use of hydrous hydrazine as a potential hydro-
gen storage material.[23–32]
Another attractive candidate for chemical hydrogen storage
is ammonia borane (NH3BH3, AB), which has a hydrogen ca-
pacity as high as 19.6 wt%, exceeding that of gasoline.[40–46]
Hydrogen can be generated by the thermal decomposition of
AB,[40] and catalytic hydrolysis provides an alternative route for
hydrogen generation under milder conditions [Eq. (3)].[41–46]
H2NNH2 ! N2þ2 H2
ð1Þ
[a] K. Aranishi, Dr. A. K. Singh, Prof. Dr. Q. Xu
National Institute of Advanced Industrial Science and Technology (AIST)
Ikeda, Osaka 563-8577 (Japan)
NH3BH3þ2 H2O ! NH4þþBO2ꢀþ3 H2
ð3Þ
Fax: (+81)72-751-9629
E-mail: q.xu@aist.go.jp
[b] K. Aranishi, Prof. Dr. Q. Xu
Graduate School of Engineering
Kobe University
In this work, we have prepared Pt-Ni DENs with different Pt/
Ni ratios by the co-complexation of Pt2+ and Ni2+ cations to
the internal tertiary amine of fourth-generation hydroxyl-termi-
nated PAMAM dendrimers (G4-OH) followed by coreduction by
NaBH4 and investigated their catalytic performances for hydro-
Nada Ku, Kobe, Hyogo 657-8501 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300143.
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drazine in aqueous solution at 343 K for G4-OH(Pt12Ni48) is
4.0 molH molmetalꢀ1 minꢀ1, which is 18 times the value of Pt-Ni
2
alloy NPs stabilized by hexadecyltrimethylammonium bromide
(CTAB) at 343 K.[27] This result indicates that for the catalytic de-
composition of hydrous hydrazine, the activity of G4-OH-
(Pt12Ni48) DENs is much higher than that of CTAB-stabilized Pt-
Ni alloy NPs, which is because of the controlled size of the en-
capsulated Pt-Ni NPs in dendrimers (see below). Moreover, the
TOF value for G4-OH(Pt12Ni48) is among the highest for hydrous
hydrazine decomposition catalysts reported to date.[24–32]
Scheme 1. Synthesis of dendrimer-encapsulated Pt-Ni nanoparticles and
their use as catalysts for the decomposition of hydrous hydrazine and hy-
drolysis of AB.
G4-OH(Pt12Ni48) DENs exhibit a 100% hydrogen selectivity,
whereas the other bimetallic Pt-Ni DENs (G4-OH(Pt6Ni54), 79%;
G4-OH(Pt24Ni36), 86%; G4-OH(Pt36Ni24), 70%; G4-OH(Pt48Ni12),
63%) and monometallic Ni DENs (G4-OH(Ni60), 18%) show
lower hydrogen selectivities, and G4-OH(Pt60) DENs are inactive
(Figure 2). These results indicate the involvement of the bimet-
gen generation from the complete decomposition of hydrous
hydrazine and the hydrolysis of AB (Scheme 1).
Results and Discussion
Catalytic decomposition of hydrous hydrazine
The catalytic decomposition of hydrous hydrazine to hydrogen
has been performed over G4-OH(PtxNi60ꢀx) DEN catalysts (x=
60, 48, 36, 24, 12, 6, and 0) at 343 K in the presence of NaOH
(0.4m). The catalytic activities and hydrogen selectivities of the
G4-OH(PtxNi60ꢀx) DENs depend on the Pt/Ni ratio (Figure 1).
Figure 2. H2 selectivities in the decomposition of hydrous hydrazine (0.2m)
to H2 in the presence of G4-OH(PtxNi60ꢀx) DENs, x=60, 48, 36, 24, 12, 6, and
0, with NaOH (0.4m) at 343 K (metal/H2NNH2 =0.1).
allic phase as the active species on the catalyst surface and
that the synergistic interaction between Ni and Pt[47] is neces-
sary for the selective decomposition of hydrazine to hydrogen
and nitrogen, which is consistent with our previous work.[25] In
addition to the volumetric results, the complete decomposi-
tion of hydrazine to hydrogen and nitrogen over the bimetallic
G4-OH(Pt12Ni48) DENs has been further confirmed by MS (H2/
N2 =2; Figure 3).
Figure 1. Decomposition of hydrous hydrazine (0.2m; n=moles) to H2 over
time in the presence of G4-OH(PtxNi60ꢀx) DENs, x=a) 60, b) 48, c) 36, d) 24,
e) 12, f) 6, and g) 0, with NaOH (0.4m) at 343 K (metal/H2NNH2 =0.1).
Monometallic G4-OH(Pt60) DENs are catalytically inactive (Fig-
ure 1a), and G4-OH(Ni60) DENs show a poor activity and low
hydrogen selectivity (18%; Figure 1g) for hydrazine decompo-
sition in aqueous solution. These observations are in agree-
ment with our previous report on the decomposition of hy-
drous hydrazine with monometallic Pt and Ni catalysts, which
show no activity and poor activity, respectively.[25] However, bi-
metallic Pt-Ni DENs exhibit high catalytic activities and hydro-
gen selectivities for this reaction. Among the bimetallic DENs
investigated, G4-OH(Pt12Ni48) DENs show the highest activity,
and the reaction reached completion in 5 min (metal/
H2NNH2 =0.1) with (N2+H2)/H2NNH2 =3.0, which corresponds
to 100% hydrogen selectivity (Figure 1e). The turnover fre-
quency (TOF) for the catalytic decomposition reaction of hy-
Catalytic hydrolysis of ammonia borane
The catalytic activities for the hydrolysis of AB were investigat-
ed over the G4-OH(PtxNi60ꢀx) DENs at room temperature. Both
monometallic Pt and Ni DENs and bimetallic Pt-Ni DENs cata-
lyze AB hydrolysis at room temperature with the release of
3.0 equivalents of hydrogen (Figure 4), which indicates com-
plete hydrogen generation from AB according to Equation (3).
The catalytic AB hydrolysis reactions were completed in 1.0,
1.5, and 4.3 min, which corresponds to TOF values of 48, 32,
and 11 molH molmetalꢀ1 minꢀ1 over G4-OH(Pt60), G4-OH(Pt12Ni48),
2
and G4-OH(Ni60) DENs, respectively (metal/AB=0.1). Although
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tertiary amines, which arises from ligand-to-metal charge trans-
fer (LMCT). Notably, the intensities of the bands increase with
increasing Pt content. After the reduction by NaBH4, two signif-
icant spectral changes were observed (Figure S3). First, the in-
tensity of the LMCT bands at 250 nm decrease, and second,
there is an increased absorbance at longer wavelengths. These
observations are consistent with the partial reduction of the
Pt2+/dendrimer complex by NaBH4.[8,17,50] However, dendrimer/
Ni2+ complexes (G4-OH(Ni2+60)) have a featureless absorbance
(Figures S1 and S2), although they exhibit an increase in ab-
sorbance toward lower wavelengths after reduction (Fig-
ure S3), which is consistent with previous spectroscopic obser-
vations for Ni DENs.[51,52]
Figure 3. Mass spectrum of gases released from the decomposition of hy-
drous hydrazine (0.2m) over the G4-OH(Pt12Ni48) DENs with NaOH (0.4m)
under Ar at 343 K.
To understand the states of Pt and Ni that coexist in the G4-
OH(Pt12Ni48) DENs, X-ray photoelectron spectroscopy (XPS) with
Ar sputtering was performed, which revealed that the Pt-Ni
DENs were composed of Pt0 and Ni0 (Figure S4). After Ar sput-
tering, the Ni2p3/2 and Ni2p1/2 peaks for the G4-OH(Pt12Ni48)
DENs were observed with binding energies of 853.3 and
870.4 eV, respectively, which corresponds to metallic Ni0 (Fig-
ure S4a).[53] No shifts of these peaks were observed during Ar
sputtering (240 min). However, the Pt4f7/2 and Pt4f5/2 peaks
(73.5 and 76.7 eV, respectively), which correspond to Pt2+,
before Ar sputtering, shifted to 71.6 and 74.9 eV, respectively,
which correspond to Pt0, after Ar sputtering (4 min; Fig-
ure S4b).[53] These results suggest that the G4-OH(Pt12Ni48)
DENs contain both Pt0 and Pt2+, in which Pt2+ bonded to the
amide N atoms of G4-OH PAMAM dendrimer and complexed
Pt2+ cannot be reduced completely by NaBH4 owing to the for-
mation of a coordination sphere through a strong stabilizing
chelate effect.[8,50] No change in the Pt/Ni intensity ratio was
observed after Ar sputtering (240 min), which indicates the ho-
mogeneity of the composition of the bimetallic Pt-Ni DENs.
To investigate the microstructure of the Pt-Ni DENs, TEM
(high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) and energy dispersive spectroscopy
(EDS)) and SEM measurements were performed. The typical
TEM, HAADF-STEM, and SEM images of the G4-OH(Pt12Ni48)
DENs (Figures 5 and S5–S7) reveal that size-controlled Pt-Ni
NPs with an average particle size of 1.1ꢁ0.3 nm are encapsu-
lated in the G4-OH dendrimer. EDS analyses of points selected
randomly (Figure S6c and S6d) and the bulk area (Figure S6b)
indicate uniform bimetallic Pt-Ni NPs. These results are in
accord with the observation that the bimetallic Pt-Ni DENs
show enhanced catalytic performance, whereas monometallic
Pt DENs and Ni DENs show lower performances in the catalytic
decomposition of hydrous hydrazine and hydrolysis of AB (Fig-
ures 1, 2, and 4).
Figure 4. H2 generation from an aqueous AB solution (0.32m; n=moles) cat-
alyzed by a) G4-OH(Ni60), b) G4-OH(Pt12Ni48), and c) G4OH(Pt60) DENs at RT
(metal/AB=0.062).
the TOF of G4-OH(Pt60) DENs is slightly higher than that of G4-
OH(Pt12Ni48) DENs, the latter contains only 20% Pt instead of
100% Pt in the former. Additionally, the TOF of G4-OH(Pt12Ni48)
DENs is approximately 6 and 1.5 times higher than that of Pt-
Ni catalysts reported previously (Pt12Ni88 and Pt65Ni35, respec-
tively).[48,49] This indicates that the high performances of bimet-
allic G4-OH(Pt12Ni48) DENs are not only a result of the synergis-
tic effect of Pt and Ni but also because of the controlled size
of the encapsulated Pt-Ni NPs in the dendrimers (see below).
Catalyst characterization
All the catalysts were characterized by UV/Vis spectroscopy.
The UV/Vis spectra of G4-OH(Pt2+xNi2+60ꢀx) before and after re-
duction as well as spectra of the starting materials (G4-OH,
NiCl2, K2PtCl4) are shown in Figures S1–S3. The G4-OH PAMAM
dendrimer exhibits a very weak absorption band at 285 nm,
which corresponds to intraligand transitions. Fresh dendrimer/
Pt2+-Ni2+ complexes with various Pt/Ni ratios exhibit a band at
Conclusion
We have demonstrated that highly dispersed bimetallic Pt-Ni
nanoparticles encapsulated in G4-OH PAMAM dendrimers ex-
hibit excellent performances as catalysts for the selective de-
composition of hydrous hydrazine and the hydrolysis of AB.
The synergistic effect of Ni and Pt coupled with the dendri-
mer-controlled small size of the nanoparticles account for the
approximately 215 nm, which is a result of the presence of
2ꢀ [17]
PtCl4
.
These bands disappear after stirring for 1 week (Fig-
ure S2), and another band appears at approximately 250 nm
owing to the complexation of the metals with the dendrimer
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Catalytic reaction of hydrous
hydrazine decomposition
The catalytic decomposition of hy-
drous hydrazine was performed ac-
cording to our procedure reported
previously.[23–30]
Typically,
H2NNH2·H2O (1.0 mmol) was inject-
ed to a glass reactor that con-
tained an aqueous suspension of
Pt-Ni DENs and NaOH (0.4m) to ini-
tiate the catalytic decomposition
reaction of hydrous hydrazine. The
reaction temperature was kept at
343 K by using a water bath. The
gas released during the reaction
was passed through a HCl solution
(1.0m) before it was measured
Figure 5. TEM image and the corresponding particle-size distribution of the G4-OH(Pt12Ni48) DENs.
volumetrically. Recently, we reported that the catalytic performance
for the decomposition of hydrous hydrazine is strongly dependent
on the reaction temperature and the presence of a strong base
(NaOH).[27–29] In this work, we employed reaction conditions similar
to those used for Ni-Fe nanocatalysts (0.4m NaOH, 343 K).[29] For
the preparation of samples for MS, the HCl trap was not used. The
selectivity toward H2 generation (y) was evaluated by using Equa-
tion (4).[23]
excellent catalytic performance. Although monometallic Ni
DENs have a very low activity and hydrogen selectivity and Pt
DENs are inactive for the decomposition of hydrous hydrazine,
bimetallic G4-OH(Pt12Ni48) DENs efficiently catalyze the decom-
position of hydrous hydrazine to hydrogen and nitrogen with
100% selectivity at 343 K in the presence of NaOH (0.4m). Ad-
ditionally, bimetallic Pt-Ni DENs show excellent catalytic perfor-
mance for the hydrolysis of AB at room temperature. The bi-
metallic Pt-Ni DENs present a step toward a high-performance
catalyst system that will open the door to exploit hydrous hy-
drazine and AB as promising, practical materials for hydrogen
storage.
3 H2NNH2 ! 4ð1ꢀyÞ NH3þ6y H2þð1þ2yÞ N2
ð4Þ
As NH3 was absorbed by the water and HCl trap, the gas volume
measured at the end of the reaction contained only N2 and H2,
from which the molar ratio of n(N2+H2)/n(N2H4) (l) was obtained.
Therefore, the selectivity was calculated by using Equation (5):
Experimental Section
y ¼ ð3lꢀ1Þ=8
ð5Þ
Chemicals
in which l=n(N2+H2)/n(H2NNH2) (1/3ꢂlꢂ3).
Commercial chemicals were used as received for catalyst prepara-
tion and the catalytic reactions. Ammonia borane (AB, 97%) was
purchased from JSC Aviabor. Hydrazine monohydrate (98%), NaBH4
(98.5%), and fourth-generation hydroxyl-terminated PAMAM den-
drimers (G4-OH, 10 wt% in methanol, Mw =14279) were purchased
from Sigma–Aldrich. NiCl2·6H2O (98.0%) and NaOH (98%) were
purchased from Kishida Chemical. K2PtCl4 (46.8ꢁ0.1% Pt) was pur-
chased from Mitsuwa Chemicals.
Catalytic reaction of AB hydrolysis
The catalytic hydrolysis of AB was performed following our proce-
dure reported previously.[41–43] Typically, AB (51.5 mg) dissolved in
water (1 mL) was injected to a glass reactor that contained an
aqueous suspension of Pt-Ni DENs to initiate the hydrolysis of AB.
The reaction was performed at RT in air. The evolution of H2 was
monitored by using a gas burette.
Syntheses of dendrimer-encapsulated Pt-Ni nanoparticles
Pt-Ni DENs were prepared according to a procedure reported by
Crooks and co-workers.[9,16] Briefly, an aqueous solution that con-
tained K2PtCl4 and NiCl2·6H2O was added to an aqueous solution
of G4-OH (0.336 mm, 5 mL), which was prepared in advance by
drying to remove methanol from the solution. The metal-to-dendri-
mer molar ratio was fixed at 60:1 and the Pt/Ni molar ratio was ad-
justed to x:(60ꢀx) in which x=60, 48, 36, 24, 12, 6, and 0. The
metal complex and dendrimer solution was stirred for 72 h in
a 30 mL two-necked round-bottomed flask to allow the Pt2+ and
Ni2+ ions to fully complex with the interior amines of the dendrim-
ers. NaBH4 (20 mg) and hydrazine monohydrate (1.0 mmol) were
added to the solution with vigorous shaking, which caused the
color of the solution to change from pale yellow to black. The re-
sulting bimetallic G4-OH(PtxNi60ꢀx) DENs were characterized and
used for the catalytic reactions.
Catalyst characterization
UV/Vis absorption spectra were recorded by using a Shimadzu UV-
2550 spectrophotometer in the wavelength range of 190–600 nm.
TEM (FEI TECNAI G2), HAADF-STEM, SEM (Hitachi S-5000), and EDS
were used to determine the detailed microstructure of the Pt-Ni
DENs. The TEM (HAADF-STEM, EDS) and SEM samples were pre-
pared by depositing one or two droplets of the NPs suspended in
a solution onto amorphous carbon-coated Cu grids, which were
then dried under Ar. MS of the generated gases were collected by
using a Balzers Prisma QMS 200 mass spectrometer.
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