Trimetallic NiFePd nanoalloy catalysed hydrogen generation from alkaline hydrous hydrazine and sodium borohydride at room temperature

Article abstract of DOI:10.1039/c5ta05814g

A simple, surfactant-assisted, room temperature synthetic route based on the coreduction of Ni, Fe and Pd salts by sodium borohydride has been developed for the synthesis of trimetallic NiFePd nanocatalysts. The catalysts were characterised by XRD, FESEM, EDX, TEM, HRTEM, SAED and XPS, and their catalytic activity and selectivity towards hydrogen generation from hydrous hydrazine and sodium borohydride were studied at room temperature. The support-free nanoparticles possessed good dispersion and a small particle size, revealing excellent catalytic performance for the complete decomposition of hydrous hydrazine and hydrolysis of sodium borohydride at room temperature. The recycle test results of the nanocatalyst showed no significant decrease in catalytic activity over five cycles. It is believed that the trimetallic NiFePd catalyst will provide a process for preparing hydrogen gas in situ, i.e. instantly upon the demand of a selected, hydrogen consuming device or process. The results presented here with a high performance catalyst system with a low noble metal content offer a new prospect for an on-board hydrogen production system.

Full text of DOI:10.1039/c5ta05814g

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Trimetallic NiFePd nanoalloy catalysed hydrogen generation from
alkaline hydrous hydrazine and sodium borohydride at room
temperature
Received 00th January 20xx,
Accepted 00th January 20xx
Debaleena Bhattacharjee and Subrata Dasgupta *
DOI: 10.1039/x0xx00000x
www.rsc.org/
A simple, surfactant-assisted, room temperature synthetic route based on coreduction of Ni, Fe and Pd salts by sodium
borohydride has been developed for the synthesis of trimetallic Ni-Fe-Pd nanocatalysts. The catalysts were characterised
by XRD, FESEM, EDX, TEM, HRTEM, SAED and XPS studies and their catalytic activity and selectivity towards hydrogen
generation from hydrous hydrazine and sodium borohydride were studied at room temperature. The support free
nanoparticles possess good dispersion and small particle size revealing excellent catalytic performance for the complete
decomposition of hydrous hydrazine and hydrolysis of sodium borohydride at room temperature. The recycle test results
of the nanocatalyst show no significant decrease in catalytic activity over five cycles. It is believed that the trimetallic
NiFePd catalyst will provide a process of preparing hydrogen gas in situ, in other words, instantly upon demand of a
selected, hydrogen consuming device or process. The results presented here with a high performance catalyst system
having low noble metal content offer a new prospect for on-board hydrogen production system.
composition 14ꢀ17. Storing and transporting of hydrogen safely
and efficiently is one of the major technological barriers in the
1. Introduction
widespread application of hydrogenꢀfuelled cells (e.g., proton
exchange membrane fuel cells (PEMFCs))18ꢀ24.Recent
developments in this direction have suggested hydrous
hydrazine (H2NNH2.H2O), a liquid over a wide range of
temperature (213ꢀ392 K) with a hydrogen content as high as
8%, easy recharging as a liquid, as a promising material for
hydrogen storage 25ꢀ29, since it will only produce nitrogen in
addition to hydrogen via a complete decomposition reaction
H2NNH2 → N2 + 2H2 (pathway 1). However, to maximise the
usability of hydrazine as a hydrogen storage material, one
should avoid the undesired reaction 3H2NNH2→ 4NH3 + N2
(pathway 2). The reaction pathways are dependent on the
catalyst used and the reaction conditions. Therefore, the
development of highly selective, efficient and economic
catalysts for fast hydrogen release as and when required at low
temperature is urgently important for practical applications.
Aiming at achieving this goal, much effort has been devoted to
the synthesis of metal catalysts for accelerating the hydrogen
generation from N2H4·H2O among which noble metals (e.g.,
Rh, Pt) or their bimetallic alloy catalysts26ꢀ33 are found to be
rather efficient under moderate conditions. Alternatively,
Catalysts are of fundamental importance for the worldwide
production of different chemicals (both bulk and fine
chemicals) and fuels that are sustainable, high in energy density
and efficiency 1ꢀ2. They are often being utilized by the chemical
industries to do reactions near to ambient temperature as is
practical, thus keeping energy usage and costs down. As
hydrogen is an attractive alternative fuel to meet ever growing
energy requirements while reducing the reliance on fossil fuels
along with their associated environmental and resource
problems so, generation of hydrogen from small hydrogen
storage molecules (e.g., hydrazine, sodium borohydride, formic
acid etc.) using new and better catalysts capable of showing
maximum efficiency as well as selectivity at ambient reaction
3ꢀ6
conditions is imperative
. Bi/trimetallic nanoparticles
(BMNPs/TMNPs) formed by incorporation of transition metal
with noble metal are found to be effective in the field of
catalysis due to their unique role of controlling the activity,
selectivity and stability as catalysts 3ꢀ13. The heteroꢀmetallic
bond formation with the introduction of other metals results
from the interꢀmetallic charge transfer or orbital hybridization
of the metals. As a consequence of the bi/trimetallic union, the
electronicꢀstructural modifications drastically influence the
catalytic performance of the mixedꢀmetal catalyst systems
showing enhancement in specific properties at an optimum
composition because of the synergistic effect of the
further
efforts
were
directed
in
developing
supported/unsupported cost effective nonꢀnoble metal
bi/trimetallic nanocatalysts (e.g., Ni, Fe, Mo, Cu, Fe etc.) for
the decomposition of hydrous hydrazine 13,16,34ꢀ39. Our recent
keen studies on bi/trimetallic nanoparticle catalysed hydrous
hydrazine decomposition to hydrogen and nitrogen reveal that
NiFe has emerged as a central component for many of the
active metallic catalysts as these components are inexpensive
and proficient as hydrogen generation catalysts. However, in
the literature, it is reported that although great progress has
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been achieved with NiFe bimetallic nanoparticles, they are minutes. To that 0.1782ꢀ 0.3209 g of NiCl2 was added followed
catalytically inefficient for the decomposition of hydrous by 0.1216ꢀ0.2189 g of FeCl3 and stirred for another 30 minutes.
hydrazine at room temperature (298 K) but are active at The resultant mixture was then added drop wise with constant
elevated temperature i.e, above 70 °C 40. Also, for the supported stirring to 10 mL of 1M NaOH solution containing 0.0896 g of
NiFe nanocatalysts there is one such
temperature hydrogen generation16 where the work describes a added to it; immediately the black colored NPs were formed.
bifunctional NiFeꢀalloy/MgO catalyst obtained via The NPs were separated by centrifugation, severally washed
report on room PVP. Finally, 1mL of 2M sodium borohydride solution was
a
calcination–reduction process of NiFeMgꢀlayered double with deionised water and ethanol and dried at room
hydroxides (LDHs), which exhibits 100% conversion of temperature. Pd, Ni and Fe NPs were synthesized by similar
N2H4·H2O and up to 99% selectivity towards H2 generation at procedure for comparison.
room temperature. Their catalyst synthesis procedure involves
2.3 Physical Characterization
higher temperature, multistep treatment and thus energy
intensive. In that respect efforts are made but by now there is
only one report on a trimetallic system where there is inclusion
of Mo along with NiFe in order to generate hydrogen at 50 °C14
from hydrous hydrazine. So it is evident that there is dearth of
studies regarding the modification of NiFe system in detail, like
studying the performance of catalytic dehydrogenation of
hydrous hydrazine by incorporating noble metal into the NiFe
moiety. This inspired us to fabricate a trimetallic catalyst
system NixFexPd100ꢀ2x (x = 25, 30, 35, 40, 45) which is
composed of nonꢀnoble metals (Ni and Fe) along with a nobel
metal palladium, as a constituent of the active catalyst system
for the purpose of achieving excellent catalytic activity and
overcoming the drawbacks of high temperature hydrogen
generation by NiFe catalyst simultaneously. The present work
demonstrates that in contrast to the catalytic inactivity of NiFe
The crystalline pattern of the synthesized nanopowders were
characterized using Xꢀray diffractometer (XRD, Philips 1710,
USA) with CuKα radiation (α = 1.541 Å).
morphological, compositional and
The spatial,
microꢀstructural
investigations were carried out with fieldꢀemission scanning
electron microscopy (FESEM: model Leo, S430i, U. K.),
transmission electron microscopy (TEM, G2 30ST, FEI
Company, USA operating at 300 kV), highꢀresolution TEM
(HRTEM), energyꢀdispersive Xꢀray spectroscopy (EDX), TEM
energy dispersion spectroscopy (TEMꢀEDS), selected area
electron diffraction (SAED) and Xꢀray photoelectron
spectroscopy (PHI 5000 Versa Probe II, ULVACꢀPHI, INC.,
USA using a monochromatic Al Kα Xꢀray source (1486.6 eV)).
Specific surface area value is calculated from the Adsorptionꢀ
desorption nitrogen isotherms obtained using Quantachrome
nanoparticles,
a significant enhancement in the catalytic
instruments NOVA4000e.
A
gas chromatograph (YL
performance of NixFexPd100ꢀ2x (x = 25, 30, 35, 40, 45,)
nanoparticles has been observed at 298K for hydrous hydrazine
decomposition and also with a slender increase in the reaction
temperature i.e., 3180C.
Instrument 6500GC system, Column: HPꢀPLOT/Q, HPꢀ
Molesieve) connected to a thermal conductivity detector (Valco
Instruments Co. Inc.) was used for gas analysis.
In this work, we focused on the synthesis of trimetallic
NixFexPd100ꢀ2x (x = 25, 30, 35, 40, 45) nano catalysts in a very
simple soft chemical method and its application for the H2
generation from hydrazine decomposition. The activity of the
synthesized NixFexPd100ꢀ2x nanocatalyst was also documented
towards the hydrolysis of alkaline sodium borohydride
(NaBH4) at 298K which stands out as promising storage
materials for delivering H2 gas to fuel cells due to its high
hydrogen storage capacity (10.8 wt%) and potentially safe
operation. Here, we addressed a catalyst system that is capable
of catalysing two very important dehydrogenation reactions at
room temperature exhausting hydrous hydrazine and sodium
borohydride by varying the composition of the active metal,
palladium, in the active site. This versatility is highly desirable
for heterogeneous catalysts and this can start up a new trend for
catalysts with multiple functionalities.
2.4 Decomposition of Hydrous Hydrazine
The catalytic decompositions of Hydrazine were carried out
under Ar atmosphere in a threeꢀnecked impinger. 1.25 ml of
4M Hydrazine was added from the burette to weighed amount
of catalyst dispersed in alkaline aqueous suspension in the
impinger (deaired by Ar purging) and the volume was made up
to 10ml. The gas evolution was monitored by displacement of
water from a burette attached to another neck of the impinger.
The generated gas was collected in Tedlar bags for gas
chromatographic measurements. The molar ratios of catalyst:
Hydrazine (n catalyst / n hydrazine) were kept constant (1:10) for all
cases.
2.5 Recycle test of NixFexPd100-2x catalysts
The NixFexPd100ꢀ2x catalysts used for hydrazine decomposition
reactions were separated by ultracentrifugation, washed several
times with distilled water and then reꢀdispersed in water. The
reꢀdispersed NixFexPd100ꢀ2x suspension was directly used for the
hydrazine decomposition reactions as before to ensure the
recyclability of the NixFexPd100ꢀ2x catalysts.
2. Experimental
2.1 Materials
Analytical grade anhydrous PdCl2, nickel chloride, ferric
chloride, NaOH, hydrazine hydrate, sodium borohydride and
polyvinyl pyrrolidone (PVP) (E. Merck, India) were used
without further purification. MiliꢀQ water was used for the
preparation of all solutions.
3. Results and Discussion
XRD profiles (Fig.1) of Pd, Ni, Fe and NixFexPd100ꢀ2x (x = 30,
35, 40, 45) NPs synthesized under same experimental
conditions reveal face centered cubic (fcc) pattern of the
prepared nanocatalysts. The XRD pattern of the as prepared Pd,
Fe and Ni NPs exactly matches with the literature reported
values (JCPDS, 1995, File no. 05ꢀ0681 for Pd, 52ꢀ0513 for Fe
and 65ꢀ0380 for Ni) indicating the formation of phase pure NPs
2.2 Synthesis of NixPd100-x BMNPs
NixFexPd100ꢀ2x (x
= 25, 30, 35, 40, 45) TMNPs were
synthesized by coꢀreduction of hydroxides of Pd2+ , Ni2+ and
Fe3+ in presence of PVP at 298K. 0.0053 ꢀ 0.2659 g of PdCl2
(depending upon the atomic ratio of Pd in NixFexPd100ꢀ2x) was
dissolved in 20 mL of 0.1M HCl solution and stirred for 30
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111
200
220
g
800
600
400
f
e
d
c
111
110
200
220
b
200
a
20
40
60
80
2 Theta (Degree)
Fig. 1 XRD patterns of (a) Fe, (b) Ni, (c) Ni45Fe45Pd10,
(d) Ni40Fe40Pd20, (e) Ni35Fe35Pd30, (f) Ni30Fe30Pd40 and
(g) Pd NPs.
only.The trimetallic NixFexPd100ꢀ2x NPs show only one visible
diffraction peak corresponding to the (111) plane of palladium.
The absence of characteristic peak of pure Pd, Fe and Ni in the
XRD of NixFexPd100ꢀ2x NPs indicate that the prepared
NixFexPd100ꢀ2x NPs are composed of trimetallic phase rather
than a physical mixture of monometallic Pd, Fe and Ni NPs.
The shift in the 2θ values toward lower angles as compared to
pure Ni and Fe with increasing the Pd content suggest that the
interplanar spacing of the NixFexPd100ꢀ2x NPs changes with the
composition of the feed molar ratio of Pd to Ni and Fe. This
specific trend is indicative of alloy formation where the lattice
expansion occurred (from 3.577 Å for Ni45Fe45Pd10 to 3.684 Å
for Ni30Fe30Pd40) with an increase in Pd content due to the
substitution of larger palladium atoms for smaller nickel and
Fig. 2 (a) TEM bright field, histogram and resolved fringes
(inset) of Ni30Fe30Pd40; (b) TEM dark field and SAED pattern
(inset) images of Ni30Fe30Pd40; (c) HRTEM and FFT images
(inset) of Ni30Fe30Pd40; (d) compositional line profile of
Ni30Fe30Pd40; (e) TEM bright field and histogram of
Ni40Fe40Pd20 and (f) resolved fringes, SAED (inset left) and
dark field (inset right) images of Ni30Fe30Pd40 NPs.
iron atoms.
The TEM bright field (Fig. 2a), dark field images (Fig 2b) and
SAED (inset) of the Ni30Fe30Pd40 revealed the presence of
highly dispersed irregularly shaped NPs with good crystallinity
as evidenced from the clearly resolved fringes in the inset (Fig
2a). The nanoparticles are of extremely small size, viz. 2ꢀ3 nm,
some of them are even less than 2 nm (Fig 2a). The rings in
SAED pattern are very broad (Fig 2b), i.e., 311 and 200 rings
merge to appear as one broad ring. The reason behind such
broadening might be due to the extremely fine size and narrow
size distribution of the synthesized nanocatalysts. The FFT
images (Fig 2c and 2f) display an interꢀplanar spacing of 0.212
nm and 0.206 nm for Ni30Fe30Pd40 and Ni40Fe40Pd20NPs
respectively which is consistent with the data obtained from
XRD measurements (0.213 nm and 0.208 nm) suggesting the
formation of an alloy structure. The compositional line profiles
of Ni, Fe and Pd (Fig 2d) of the Ni30Fe30Pd40 NPs indicate the
uniform alloy composition of the NPs without significant
segregation of each component. Energyꢀdispersive Xꢀray
spectroscopy (EDS) point analyses of randomly chosen NPs
exhibit the presence of Ni, Fe, Pd in Ni30Fe30Pd40 NPs (ESI,
The nitrogen adsorption–desorption isotherms for the
Ni30Fe30Pd40 nanocatalyst following the BET method show that
the room temperature reduction treatment endows the NPs with
a very high surface area of 156 m2gꢀ1 (ESI, Fig.S3). The Xꢀray
photoelectron spectroscopy (XPS) results (Fig.3) of
Ni30Fe30Pd40 NPs show that Pd 3d display doublet which
consists of a low energy band [Pd 3d5/2 at 335.4 eV] and a high
energy band [Pd 3d3/2 at 340.7 eV] of core level of Pd (0).
Meanwhile, signals with binding energies of Ni [2p3/2] at 852.5
eV and 855.87 eV can be attributed to the core level of Ni (0)
and Ni (II) respectively. XPS analysis suggests that the surface
of nickel is partly oxidized. The surface of iron is fully
oxidized, as the peak at 710.55 eV corresponding to Fe2O3 was
found (Fig 3).
The formation of thin oxidised layer on the surface of mixed
metal catalyst system containing Ni and Fe, which are very
sensitive to air, as is evidenced from XPS spectrum, is a very
common phenomenon40. This arises due to the exposure of the
sample in air and can be removed by Ar sputtering40. (ESI,
FigS4). The spectrum obtained after sputtering reveals the Ni
Fig.S1)
and their homogeneous distribution were also
supported by map scan STEMꢀEDX studies (ESI, Fig.S2).
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[2p3/2] and [2p1/2] features with binding energies of 852.5eV and
869.7 eV, respectively which can be attributed to Ni(0) and the
Fe [2p3/2] and [2p1/2] features with binding energies of 707.2eV
and 720.2 eV, respectively for Fe(0). The binding energies for
e
d
c
4b
3.0
2.5
2.0
1.5
1.0
0.5
0.0
41
Pd 3d shifted to higher value and for Ni 2p and Fe 2p shifted
b
30
to lower value in Ni30Fe30Pd40 NPs as compared to the core
level pure Pd , Ni and Fe NPs respectively. These binding
energy shifts of core electrons demonstrate that some electrons
are transferred from Pd to Ni and Fe causing changes in the
bulk charge around the atomic site in Ni30Fe30Pd40 NPs. Thus,
the XPS study firmly supports the alloy structure of
Ni30Fe30Pd40 NPs.
a
0
50 100 150 200 250 300 350 400 450 500
time,min
4 (b) Time profile decomposition of 10 mL of 0.5 M hydrous
hydrazine (hydrazine: catalyst =10:1) using (a) Ni45Fe45Pd10
,
(b)Ni25Fe25Pd50 , (c) Ni40Fe40Pd20 , (d)Ni35Fe35Pd30 , (e)Ni30Fe30Pd40
at 318K.
Catalytic performances of the NixFexPd100ꢀ2x (x = 25, 30, 35, 40,
45) NPs for the decomposition of hydrous hydrazine to
hydrogen were initiated with the introduction of hydrazine
monohydrate into the reactor containing an alkaline suspension
of catalysts kept at 298K. To investigate the dependence of
hydrogen generation on the Ni/Fe/Pd ratios, the NixFexPd100ꢀ2x
nanocatalysts with a range of Pd content (x = 25ꢀ45) have been
studied (Fig.4). The concentration of NaOH (0.4M) is
optimized by taking different amounts of NaOH for the same
catalytic reaction (ESI, FigS5). The resulting gas was identified
by a gas chromatograph connected to a thermal conductivity
detector and the quantity was measured volumetrically by
displacement of water from a burette for the evaluation of
hydrogen selectivity. It can be seen that Pd is the key active
element of all the prepared catalysts. Monometallic Pd, Fe, Ni
and bimetallic NiFe NPs are inactive towards hydrogen
generation from alkaline hydrous hydrazine at 298K. But when
Pd is present along with Ni and Fe in NPs the activities of
NixFexPd100ꢀ2x(x=25ꢀ45) increased appreciably and it is
observed that three of the catalysts namely Ni30Fe30Pd40,
Ni35Fe35Pd30 and Ni40Fe40Pd20 has generated 3 equivalents of
gas at 298K in 75, 153 and 292 minutes respectively (Fig 4a).
The physical mixture of Ni, Fe and Pd NPs was also found to
be inactive toward hydrogen generation from hydrous
hydrazine under similar reaction conditions at 298K. These
further confirm that the presence of three metals on the catalytic
active sites is critical only in the form of nanoalloy
(electronically modified) and not as the separate metals. Also,
the high catalytic performance of Ni30Fe30Pd40 nanocatalysts for
hydrazine decomposition in aqueous solution compared to the
Fig.3 XPS spectra of Ni30Fe30Pd40 NPs
3.1 Catalytic decomposition of hydrous hydrazine
e
d
4a
c
3.0
2.5
2.0
1.5
1.0
0.5
0.0
b
a
0
50
100 150 200 250 300 350 400
Time,min
4 (a) Time profile decomposition of 10 mL of 0.5 M hydrous
hydrazine (hydrazine: catalyst =10:1) using (a) Ni45Fe45Pd10
b)Ni25Fe25Pd50 , (c) Ni40Fe40Pd20 , (d)Ni35Fe35Pd30 , (e) Ni30Fe30Pd40
at 298K.
,
analogously
synthesized
Ni25Fe25Pd50,
Ni35Fe35Pd30,
Ni40Fe40Pd20 and Ni45Fe45Pd10 nanocatalysts indicate that
alloying of Ni and Fe with Pd in a specific combination has
positive effect on the hydrogen selectivity in contrast to the
other compositions despite of the presence of Pd to a larger
extent. It is worth noting that raising the reaction temperature to
318 K, only slightly higher than room temperature, makes it
possible to achieve 100% H2 selectivity by
the same
Ni30Fe30Pd40 nanocatalyst in 37 minutes only (Fig 4b).
However, no further change in H2 selectivity was observed at
higher temperatures, only the reaction completion time was
reduced (i.e., 35minutes at 318K to 27minutes at 323K) for
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Ni30Fe30Pd40 nanocatalyst, (ESI, FigS6). The recycle test intermediate N2H3*. The complete decomposition reaction is
reveals that the Ni30Fe30Pd40 nanocatalyst shows no significant initiated by the coordination of hydrazine to metal surface followed
by an oxidative insertion of metal via N–H bond activation51ꢀ53. An
decrease in catalytic activity over 5 cycles (ESI, FigS8). The
gaseous products were identified as the mixture of H2 and N2
only as analysed by gas chromatography (ESI, Fig.S9). The
absence of trace ammonia as a product of the reaction was
confirmed by Nessler’s test 42 (detection limit 0.025γ NH3; 1γ =
0.001mg) thus excluding the possibility of undesired reaction
pathway 2. The TOF of Ni30Fe30Pd40 is calculated and found to
be 21.5 hꢀ1 at 323 K. This TOF value is much higher than most
of the widely reported nobel metal as well as nobleꢀmetalꢀfree
heterogeneous catalysts for N2H4.H2O decomposition,
abundance of NH adsorbed on the metal surface or the stabilization
of metalꢀbound HN=NH as a transient intermediate is crucial and
leads to the release of N2 and H2 by selective activation of the NꢀH
bond over the NꢀN bond on the metal surface54ꢀ56. The observations
made in the present work make it reasonable to propose that the
alloying of Ni, Fe and Pd leads to a modification of the catalyst
surface through incorporation of considerable intermetallic
electronic interactions and accordingly the catalyst surface
significantly tunes its interactions with hydrazine thereby enhancing
the stability of the transient intermediates on the catalyst surface
preferentially by pathway 1 over pathway 2 for its complete
decomposition to hydrogen and nitrogen at 298K. The above results
strongly support the conclusion that the catalyst performance is
substantially governed by its structure and composition resulting
from the introduction of new components. Considering the poor
catalytic performance of the mono/bimetallic counterparts where Pd,
Fe, Ni and NiFe are inactive for the catalytic decomposition of
alkaline hydrous hydrazine at room temperature, the interaction
between Ni, Fe and Pd in the NiFePd alloy outcomes in significant
improvement in hydrogen generation at ambient reaction conditions.
9,14,28,29,30,33,40,43,44
(ESI, Table S1). This superior activity of
Ni30Fe30Pd40 may be attributed to the electrical modification of
Ni, Fe and Pd in the alloy structure of Ni30Fe30Pd40 and also the
small particle size, good dispersion and very high surface area
of Ni30Fe30Pd40 resulting from the present easy synthetic
method. NaOH actually acts as a promoter for the catalytic
reaction 9,33. Besides promoting the reaction kinetics, the
40
alkaline solution also helps to inhibit the formation of NH3
.
In order to obtain the activation energy (Ea) for the
decomposition of N2H4.H2O catalysed by the Ni30Fe30Pd40 NPs,
the reaction was carried out at different temperatures (298Kꢀ
323K) and the results are shown in ESI, Fig. S6. The Arrhenius
plot of ln TOF vs. 1/T for this catalyst is plotted in ESI, Fig.S7
from which Ea is calculated as 40.0 kJ molꢀ1, which is very
similar to the previously reported values for the catalytic
3.2 Catalytic hydrolysis of sodium borohydride:
NaBH4, a very common promising hydrogen storage material
due to its high hydrogen storage capacity (10.8 wt%) and
potentially safe operation, is widely used to examine the
decomposition of N2H4.H2O.13,44ꢀ45
.
efficiency of newly synthesized catalysts. To evaluate the
efficacy of our synthesized NPs of different compositions, the
catalysts are used towards the hydrolysis of alkaline NaBH4 in
order to generate hydrogen at room temperature.
N H *
2
3
H NNH
HN=NH
2
2
When catalysts are added in alkaline NaBH4 solution, the
release of hydrogen takes place via the following reaction 57
:
H*
Catalyst
N H
H*
2
NaBH4 + 2H2O
NaBO2 + 4H2 ↑
ꢁH = −217 kJ mol−1
H*
(1)
H
2
H*
The reaction is exothermic and the byꢀproduct sodium
metaborate (NaBO2) is nontoxic and can be recycled as the
starting material for synthesizing NaBH458. Since NaBH4
undergoes selfꢀhydrolysis at ambient temperature when stored
in water solution, NaOHꢀstabilized NaBH4 solutions is used for
catalytic hydrogen generation from NaBH4.
N
2
H
2
Fig 5 Mechanistic decomposition of hydrazine by pathway:
H2NNH2 → N2 + 2H2
Catalytic performances of the NixFexPd100ꢀ2x (x = 35, 40, 45)
NPs for the hydrolysis of sodium borohydride to hydrogen were
carried out by adding alkaline NaBH4 (0.1891 g of NaBH4 in
10mL of 5M NaOH) into the reactor containing the measured
quantity of the catalysts (catalyst: borohydride 1:10) of varying
composition kept at 298K.
If mechanistic aspect of hydrazine decomposition is considered46,
then on a catalyst surface, the stabilization of the intermediate N2H3*
determines the fate of the reaction by generating HN=NH species
that further dissociates and ultimately leads to the formation of
hydrogen and nitrogen by pathway: H2NNH2 → N2 + 2H2 (Fig 5).
On the other hand if the intermediate N2H3* dissociates to generate
NH2* radical, by breaking the N–N single bond which gives enough
stability to hydrazine then further cleavage of the high energy N–H
bond becomes difficult, thereby favouring the combination of *NH2
and *H radicals to form ammonia47ꢀ50. As in the present catalyst
system there is no ammonia formation so the reaction must have
followed the desired pathway through the stabilization of the
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ARTICLE
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energies for Pd(0)3d [3d5/2 at 334.7 eV, 3d3/2 at 340.08 eV
]
500
shifted to higher value and for Ni(0) [2p3/2 at 851.9 eV] shifted
41
to lower value as compared to the core level pure Pd(0) and
Ni(0) NPs30 respectively although the Fe [2p3/2 at 709.9eV]
surface seems to be oxidized. These thin oxidized layer formed
during exposure of the sample can be removed by Ar sputtering
as is observed in the previous case of Ni30Fe30Pd40 NPs. These
shifts in electronic spectra evidences change in the bulk charge
around the atomic site in Ni40Fe40Pd20 NPs and play a
significant role in enhancing the catalytic activity due to
synergistic electronic contribution from all the three elements in
the trimetallic system.
400
300
200
100
0
Ni Fe Pd
40 40
Ni Fe Pd
35 35
Ni Fe Pd
45 45
20
30
10
0
10
20
30
4. Conclusion
Tim e,m in
In summary, we report a highly active trimetallic NiꢀFeꢀPd
nanocatalyst prepared by a soft chemical method which exhibits
100% hydrogen selectivity for complete decomposition of
hydrous hydrazine at 298K. The synthesized nanoalloy catalyst
significantly improves the kinetic properties for the catalytic
dehydrogenation of hydrazine at room temperature as well as at
slightly elevated temperature. The catalyst also stimulates
hydrolysis of sodium borohydride within 4 minutes under
alkaline and ambient reaction conditions. It is believed that this
low cost, efficient and selective H2 generation system at room
temperature will promote new opportunities in the development
of highꢀperformance metal bi/trimetallic nanoparticle catalysts
and also encourage the effective application of hydrous
hydrazine as a promising hydrogen storage material.
Fig. 6 Time profile decomposition of 10 mL of 0.5 M sodium
borohydride (sodium borohydride: catalyst =10:1) using
NixFexPd100ꢀ2x (x = 35, 40, 45) NPs at 298K.
Fig 6 presents the comparative study of the volume of hydrogen
generation as function of time using NixFexPd100ꢀ2x (x = 35, 40,
45) catalyst powder at 298K. At first, there is a delay time
(induction time) in which reaction rate is low. The delay or
induction time can be assigned to a surface restructuring time
necessary to transfigure the metal nanoparticles as an active
catalyst. Hence, a rearrangement of the surface atoms seems to
be necessary to create catalytically active sites 59ꢀ60. The
mechanism of hydrolysis of sodium borohydride is clearly
explained in literature61. The mechanism proposed most
recently by PenaꢀAlonso et al.62 involves the reversible
Acknowledgements
ꢀ
chemisorption of BH4 on the catalyst surface, which produces
ꢀ
M–BH3 and M–H intermediate species. The M–H species
The authors like to thank DST, Govt. of India (Project no.
GAP0344), and CSIR, New Delhi (Project no. ESC0104) for
financial support.
reacts with H2O to produce H2 and BH3 (OH)ꢀ. According to the
latest 11B NMR measurements by Guella et al 63ꢀ64, the rateꢀ
.
controlling step of the hydrolysis reaction of NaBH4 is the
cleavage of the water O–H bond. During the course of catalytic
decomposition of NaBH4 using NixFexPd100ꢀ2x (x = 35, 40, 45),
the total hydrogen yield was >99% of the theoretically expected
value based on Eq. (1). Among the NixFexPd100ꢀ2x (x = 35, 40,
45) NPs, it is observed that the hydrogen generation rate is
fastest in the case of Ni40Fe40Pd20 nanocatalyst (within 4
minutes the reaction is complete) at 298K. On the other hand, it
is found that the other compositions Ni35Fe35Pd30 (8 minutes)
and Ni45Fe45Pd10 (11 minutes) takes much more time to
complete the reaction at 298K. For the trimetallic nanocatalyst
the hydrogen generation rate per unit mass could be up to
3343.28, 1555.55, and 1313.78 ml min−1 g−1 catalyst for
Ni40Fe40Pd20, Ni35Fe35Pd30 and Ni45Fe45Pd10 respectively at
298K, which are all superior to crystal state Ni (19.5 ml min−1
g−1 catalyst) and Raney Ni (228.5 ml min−1 g−1)65 . The
comparative data for rate of gas generation for various catalysts
are given66ꢀ76 in ESI, Table S2.
The high catalytic performance of Ni40Fe40Pd20 nanocatalysts
for borohydride hydrolysis in alkaline solution compared to the
analogously synthesized Ni35Fe35Pd30 and Ni45Fe45Pd10
nanocatalysts indicate that alloying of Ni and Fe with Pd in a
specific combination has positive effects on the gas generation
rate. Thus, increase or decrease in Ni, Fe or Pd content from
Ni40Fe40Pd20 results in the decrease of H2 selectivity. Also
electron enriched metal active sites are necessary to facilitate
the catalysis reaction. These electrons are provided by the
alloying Pd to the active metal sites, as observed in the XPS
studies of Ni40Fe40Pd20 (ESI, Fig.S10) where the binding
Notes and references
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Journal of Materials Chemistry A
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Graphical abstract
Room temperature synthesized NixFexPd100-2x nanocatalyst exhibits improved catalytic
activity towards hydrogen generation from hydrazine and borohydride under
ambient reaction conditions