Room-temperature synthesis with inert bubble templates to produce clean PdCoP alloy nanoparticle networks for enhanced hydrazine electro-oxidation

Article abstract of DOI:10.1039/c4ra14423f

PdCoP alloy nanoparticle networks (ANN) were prepared by simultaneous reduction of PdCl₂, CoCl₂ and NaH₂PO₂ with NaBH₄ using N₂ bubbles as soft-templates at room temperature. The PdCoP ANN porous structure was formed at the inert gas bubble surface, and could be obtained with varying ratios of precursors to produce catalysts with different compositions and catalytic activity. PdCoP ANN produced markedly higher hydrazine oxidation activity than PdCo ANN and PdCoP grain aggregates, and catalytic activity varied with different ratios of Pd, Co and P. We conclude that PdCoP ANN are promising hydrazine oxidation catalysts and that the developed method provides a facile and convenient mean of producing mesoporous catalysts with high-surface area.

Full text of DOI:10.1039/c4ra14423f

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PAPER
Room-temperature synthesis with inert bubble
templates to produce “clean” PdCoP alloy
nanoparticle networks for enhanced hydrazine
electro-oxidation†
Cite this: RSC Adv., 2015, 5, 9837
a
a
a
b
b
Yuanyuan Ma, Rongfang Wang,* Hui Wang, Julian Key and Shan Ji*
PdCoP alloy nanoparticle networks (ANN) were prepared by simultaneous reduction of PdCl
2 2
, CoCl and
NaH PO with NaBH using N bubbles as soft-templates at room temperature. The PdCoP ANN porous
2
2
4
2
structure was formed at the inert gas bubble surface, and could be obtained with varying ratios of
precursors to produce catalysts with different compositions and catalytic activity. PdCoP ANN produced
markedly higher hydrazine oxidation activity than PdCo ANN and PdCoP grain aggregates, and catalytic
activity varied with different ratios of Pd, Co and P. We conclude that PdCoP ANN are promising
hydrazine oxidation catalysts and that the developed method provides a facile and convenient mean of
producing mesoporous catalysts with high-surface area.
Received 13th November 2014
Accepted 6th January 2015
DOI: 10.1039/c4ra14423f
www.rsc.org/advances
architecture can be modied to improve the catalytic perfor-
1
. Introduction
1
1
mance of metal-based alloys.
In particular, three-
Direct hydrazine fuel cells (DHFCs) are attractive candidate dimensional porous or network-like structures are of great
power sources for both portable power applications and electric interest due to their high surface area-to-volume ratio, low
vehicles. Compared to other types of fuel cells, DHFCs have the density, high gas permeability (compared to their solid
1
2
following advantages: (i) their open-circuit cell voltage of 1.61 V counterparts), and high density of atomic scale defects.
is higher than that of formic acid and direct methanol fuel cells; Such structures are oen synthesized using hard templates or
ii) they produce higher energy and power density than the chemical/electrochemical dealloying from a binary or multi-
(
ꢀ1
13–15
hydrogen fuel cell (5400 W h L ); and (iii) the products of component alloy.
However, these methods are rather
hydrazine oxidation (N
2
and H
2
O) are neither harmful to the uneconomical for mass production due to their sacricial
1
environment nor carbon-containing. Over the past few components. Recently, Zhang's group synthesized a series of
2,3
4,5
6
16
17
18
decades, noble-metals such as Pt, Pd, and Au, have received noble alloy networks, including PtNi, PtCo and PdP alloy
much attention as hydrazine electrooxidation catalysts. networks, by reducing metal precursors with NaBH in water
4
However, like many other types of fuel cells, the high cost of using different so surfactant templates. The resultant cata-
noble metals hinders the commercial success of DHFC's. lysts had high catalytic activity for either oxidation of meth-
Development of lower cost and highly efficient electrocatalysts anol or formic acid, as well as for the oxygen reduction
for electro-oxidation of hydrazine therefore remains a key reaction. However, reports also showed that effective removal
research goal.
of adsorbed surfactant from the surface of metals or alloys, to
1
9
Bimetallic and multimetallic alloy catalysts (compared to obtain a “clean” surface, was a difficult process. At the same
catalysts comprising a single noble metal element) have been time, the sol–gel approach without the help of templates has
shown to produce higher catalytic activity due to synergistic the potential for the formation of porous structures to over-
7
–10
effect between their different components.
such as morphology, composition, and nano-structural prepared a series of Pd-based alloys by the sol–gel approach
Various factors come the above-mentioned drawbacks. Herrmann et al.
2
0
using supercritical drying; however, the process is neither
economical nor simple. Meanwhile, Lu et al. prepared PdCoP
alloy networks by reducing K PdCl /K Co(CN) cyanogel
a
College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou 730070, China. E-mail: wangrf@nwnu.edu.cn; Fax: +86 931 7971533; Tel:
2
4
3
6
2
1
without any surfactants, but the precursor, K
3
6
Co(CN) , is
+
86 931 7971533
b
strongly toxic. Therefore, the search for a facile, green, low-
South African Institute for Advanced Materials Chemistry, University of the Western
Cape, Private Bag X17, Bellville, Cape Town 7535, South Africa. E-mail: sji@uwc.ac. cost and universal synthetic processes for making “clean”
za; Fax: +27-21-9599316; Tel: +27-21-9599316
surface, porous or network-like nanomaterials appears to
remain challenging.
†
Electronic supplementary information (ESI) available: SEM, XRD, isotherms and
electrochemical performance. See DOI: 10.1039/c4ra14423f
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The use of gas bubbles as so template, developed by Li's
group, offers a novel, simple and effective approach. In this
method, gas–liquid interface can be acted as the
agglomeration/nucleation center of the nanocrystals with
xPd + yCo + zP / Pd Co P
x y z
The resultant precipitate was collected by ltration, washed
ꢀ
1
2
2
special morphology. In general methods, the surfactants with 6 mol L ammonia solution, then washed with deionized
and capping agents were introduced to act as templates for water several times and stored in ethanol solution. To obtain
special shape, but these reagents are hard to completely alloy networks of different composition, we altered the
remove from the nal products in the case of maintaining the precursor ratio as listed below in Table S1.†
2
3
morphology of the products. The presence of capping
agents around the products may severely limit their chemical 2.2 Characterization
2
4
activity. That gas bubbles serve as templates avoiding the
introduction of surfactant or capping agents e.g. impurities.
Li's group used gas bubbles generated during the reaction
process as a template to prepare CoTe and NiTe nanocluster
wires and ZnSe hollow microspheres. In our previous work,
we introduced inert bubbles, which directly served a similar
role. In the present work, we have extended this green and
facile synthesis method to prepare PdCoP alloy nanoparticle
networks (ANN) as catalysts for hydrazine oxidation, without
the use of a strongly toxic precursor. We report PdCoP ANN
prepared by a one-step co-reduction of metal precursors
XRD patterns were recorded on a Shimadzu XD-3A (Japan) using
ltered Cu-K
radiation (l ¼ 0.15418 nm) generated at 40 kV
and 30 mA. Scans for 2q values were recorded at 4 min

a
ꢂ
ꢀ1
.
Scanning electron microscopy (SEM) images were obtained
using a Carl Zeiss Ultra Plus. Transmission electron micro-
graphs (TEM), high angle annular dark eld scanning trans-
mission electron microscopy (HAADF-STEM) images of the
catalysts were taken on a JEOL (JEM-2000 FX) microscope
operating at 200 kV. The specic surface area was determined
using the Brunauer–Emmett–Teller (BET) method and the
density functional theory (DFT) method was used to analyze the
full range of the pore size distribution. The average chemical
compositions of the catalysts were obtained using an IRIS
advantage inductively coupled plasma atomic emission spec-
troscopy (ICP-AES) system (Thermo, America). X-Ray photo-
electron spectra (XPS) was acquired with a VG Escalab210
spectrometer tted with Mg 300 W X-ray source.
2
5
2
using N bubbles as template, and show the composition of
the PdCoP alloys can be modulated by adjusting the initial
molar ratio of the metal precursors, while preserving the
unique network structure. The method offers a mean for
developing highly efficient catalysts with optimal chemical
composition.
2.3 Electrochemical measurements
2. Experimental
Electrochemical measurements were carried out using an elec-
trochemical work station (CHI 650D). A common three-
electrode electrochemical cell was used for the measurements.
The counter and reference electrodes were a platinum wire and
an Ag/AgCl (3 mol L KCl) electrode respectively. The working
electrode was prepared on a 5 mm diameter glassy carbon disk,
2
.1 Synthesis
All reagents were of analytic grade and ultrapure water was used
throughout the experiments. To prepare PdCoP nanoparticle
alloy networks with a molar ratio of Pd : Co : P ¼ 1 : 10 : 14, we
ꢀ
1
2
6

rst made three solutions as described in our previous work:
(
1) 270 mg of CoCl
dissolved in 30 mL water bubbled continuously with N
0.6 mg of NaBH
was transferred to a constant funnel, and (3) 20 mg of PdCl
2 2 2 2 2
$6H O and 160 mg of NaH PO $H O were
2
mg of catalyst was dispersed ultrasonically in 0.4 mL Naon–
2
, (2)
ethanol (0.25% Naon) solution, and 16 mL was pipetted and
dried in air on the surface of glassy carbon.
8
4
was dissolved in 20 mL water and the solution
2
was
ꢀ1
dissolved in 10 mL water and adjusted to pH ꢁ9 using 6 mol L
NaOH solution with vigorous stirring. Solution (2) was added 3. Results and discussion
drop-wise into solution (1) under the stirring conditions
and bubbled with N . Then, solution (3) was rapidly added to
the mixture and stirred vigorously for another 4 h with contin-
uously bubbled N
NaH PO $H O were reduced to produce PdCoP and NaBH
the following reactions:
PdCoP ANN (alloy nanoparticle networks) were prepared by co-
reduction of PdCl , CoCl , and NaH PO in the presence of
NaBH as a reducing agent and N bubbles as a template at
2
2
2
2
2
4
2
2
. In the reaction scheme, PdCl
2
, CoCl
2
, and
via
room temperature (see ESI† for details). Low magnication
scanning electron microscopy (SEM) (Fig. 1a) showed the
PdCoP-3 comprised an extensive network-like structure on a
large scale. High magnication (Fig. 1b) showed that the
network-like structure was constructed from sub-10 nm aligned
nanoparticles, which were also visible using transmission
electron micrographs (TEM) (Fig. 1c). High resolution TEM
displayed in Fig. S1 (ESI†) presented clear lattice planes, indi-
cating PdCoP-3 possessed a crystalline structure. The chemical
microstructure of PdCoP-3 nanoparticles was examined using
electron energy loss spectroscopic mapping. Fig. 1d shows the
high angle annular dark eld scanning transmission electron
2
2
2
4
NaBH4
2
þ
Pd ƒƒƒ ƒƒ ! Pd
NaBH4
2
þ
Co ƒƒƒ ƒƒ ! Co
NaBH4
ꢀ
H
2
PO
2
ƒƒƒ ƒƒ ! P þ H
2
O
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h reaction time produced the extensive network structure (ESI,
Fig. S5b†). Therefore, we hypothesize, according to our previ-
26
ously proposed N2 bubble so-template model, that the
formed PdNiP particles aggregate at the interface between the
bubble and aqueous solution. With increased reaction time,
sufficiently high aggregation produces the network structure
(Scheme 1). In addition, we successfully extending the method
to synthesis of PdCo alloy nanoparticle networks (PdCo ANN)
(ESI, Fig. S6†).
Here, it is benecial to do the comparison between the
bubble-templates synthesis and solvothermal synthesis since
solvothermal synthesis with surfactants is a widely used tech-
nique. We prepared PdCoP samples by solvothermal method,
and their SEM images were present in Fig. S7 (ESI Table S2†).
We used water and ethanol as solvent and CTAB and SDS as
surfactants, respectively. Among four samples, only one sample
prepared using ethanol as solvent and CTAB as surfactant pre-
sented network structure. However, there is a few big particles
in the sample. Based on the above facts, the bubble-templates
synthesis is a facile method to form network structure.
Fig. 1 (a and b) SEM images at different magnifications, (c) TEM image,
(
d) HAADF-STEM image, and the elemental mapping of Pd (e), Co (f),
and P (g) of PdCoP-3.
Phase analysis of PdCoP ANN with various compositions was
carried out using XRD. PdCoP-3 and PdCo ANN (Fig. 2a)
ꢂ
produced diffraction peaks at around 40, 46, 68, and 82 cor-
microscopy image of PdCoP-3 where the area circled by the
orange box was spectroscopically imaged. Fig. 1e–g shows the
distribution of Pd, Co and P elements in PdCoP-3 respectively.
Overlapping positions of the Pd (green) Co (purple), and P (blue)
distributions indicates formation of PdCoP alloy structure.
Using ICP analysis (ESI, Table S1†) the atomic ratio of
Pd : Co : P in the bulk composition of PdCoP-3 was 1 : 5.6 : 11.
It is well established that the activity of multimetallic alloy
electrocatalysts can vary according to their compositional ratio
of elements. Therefore, synthesis of PdCoP ANN samples with
varying ratios of elemental composition, while preserving the
same morphology, would allow a further mean of optimizing
PdCoP ANN catalytic activity. To obtain variations in PdCoP
ANN composition, we adjusted the molar ratio of PdCl /CoCl
responding to (111), (200), (220), and (311) diffractions respec-
tively, which t the typical pattern of polycrystalline Pd face-
centered cubic (fcc) phase (JCPDS 89-4897: marked with the
dot black vertical lines). The diffraction peaks of PdCoP-3 and
PdCo ANN shied to a slightly higher 2q degree compared to
pure Pd, implying that the lattice parameter had shrunk
because Pd atoms were replaced by smaller atoms. The peaks of
PdCoP-3 had a slightly positive shi compared to PdCo ANN,
indicating a shrinkage of the Pd lattice in PdCoP-3 compared to
pure PdCo perhaps due to replacement of Pd atoms by P atoms.
In addition, upon the increasing content of Co in PdCoP ANN,
the diffraction peaks in Fig. S8 (ESI†) shied towards high 2q
angle from PdCoP-1 to PdCoP-4, indicating that the lattice
parameter became smaller with the increasing amount of Co
atoms inserted into the Pd lattice.
The porosity and the surface area of PdCoP ANN were eval-
uated by nitrogen physisorption. High surface areas for the
PdCoP ANN samples were expected owing to their network-like
structure. Fig. 2b shows N adsorption–desorption isotherms
for PdCoP-3 and PdCo ANN. Fig. 2 shows a hybrid II/IV isotherm
according to IUPAC classication) was predominant, indi-
cating multilayer adsorption behavior, and the absence of a
plateau for high relative pressure values revealed the presence
2
2
precursors from 1/6, 1/8, 1/12, to 1/14. Fig. S2 (see ESI†) shows
PdCoP ANN samples with different compositions shared similar
network-like structure with nanoparticle frameworks between
5–9 nm in diameter. The actual Pd/Co/P atomic ratios were
determined by ICP (listed in Table S1†), and were close to the
initial ratios of their reactant precursors.
2
To elucidate the role of N gas bubbles in the formation of
2
(
porous structure, a series of control experiments was carried out
during PdCoP ANN preparation. Firstly, to demonstrate the
importance of the bubbles in network formation, synthesis in
the absence of N
PdCoP GA) instead of a network structure (ESI, Fig. S3†).
Secondly, to determine if any specic chemical affinity to N gas
2
bubbles produced PdCoP grain aggregates
(
2
played a role in forming network structure, synthesis was
carried out using two other inert gasses, argon and helium.
Fig. S4† shows that all three gasses produced no obvious
difference in network structure. Thirdly, time-dependent
experiments was carried out and shown in Fig. S5.† We found
that a reaction time 0.5 h produced many aggregate clusters
consisting of small nanoparticles (ESI, Fig. S5a†), whereas a 1.5 Scheme 1 Schematic process of PdCoP ANN formation on inert gas
bubbles.
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Fig. 2 (a) XRD patterns, (b) nitrogen isotherms and (inset) the corresponding pore size distribution of PdCoP-3 and PdCo ANN. (c) Pd 3d of
PdCoP-3 and PdCo ANN. (d) P 2p XPS spectrum of PdCoP-3.
27
of large mesopores and macropores inside the material. The positively shied ca. 0.3 eV compared to that of PdCo ANN. This
insets of Fig. 2b show the corresponding cumulative pore suggests that the donation of electrons from Pd to P may results
volumes calculated by applying DFT (density functional theory). in a decrease of the 3d electron density of the Pd atom in PdCoP-
In the evaluated range between ca. 0.8 and 36 nm, the distri- 3. The result indicates that P atoms in the PdCo alloy modify the
bution was broad in the mesopore range, did not reach a electronic structure due to the chemical interactions between
plateau, and no prevalent pore size was observed. This corre- Pd, Co and P atoms. This was further demonstrated by the
lates to the SEM characterization of open pores and tunnels. positive shi of Co 2p BE of PdCoP-3 relative to that of PdCo
This hierarchical pore system should greatly favor the transport (ESI Fig. S10†). For the case of the P 2p spectrum of PdCoP-3
of reactants and reactant products to and from the catalyst (Fig. 2d), the P 2p peaks at 129.7 and 133.4 eV was assigned
surface respectively. Based on the N
2
sorption data, the BET to elemental state phosphorus and oxidized phosphorus, which
2
ꢀ1
surface area and total pore volume of PdCoP-3 were 73.1 m g
is indicative of PdCoP alloy formation.
3
ꢀ1
and 0.446 cm g respectively, and were higher than those of
Cyclic voltammetry (CVs) was rst carried out on PdCoP-3,
2
ꢀ1
3
ꢀ1
ꢀ1
PdCo at 49.5 m g and 0.390 cm g respectively. The result PdCo ANN and PdCoP GA in N
2
saturated 0.1 mol L KOH to
2 4
could relate to the addition of P which reduces the particle evaluate their electrochemical behavior in the absence of N H
18
size. PdCoP-3 also had a large the surface area and pore (Fig. 3a). All three materials produced a strong oxidation peak
volume than PdCoP GA (ESI, Fig. S9 and Table S3†), suggesting from 0.15 to 0.5 V correlating to the oxidation of Co(0) to Co(II)
that the network-like structure would provide a larger surface and Co(II) to Co(III). On the reverse sweep, the dened peak near
area for catalysis than solid particle aggregated structure. In 0.2 V was characteristic of the reduction of Co(II) to Co(0) and
8
addition, the surface area of PdCoP ANNs did not change in a Co(III) to Co(II). The broad and weak peaks at ꢁꢀ0.78 V in the
regular way with the molar ratio of PdCo, suggesting that forward scan and at ꢁꢀ0.45 V in reverse scan were character-
multiple factors affect the surface area, which should be further istic of Pd oxidation/reduction. The results suggest that both Pd
studied in future.
and Co were present in all three catalysts. For PdCoP ANN of the
The surface composition and the chemical state of the other prepared molar ratios, similar electrochemical behavior
elements in PdCoP-3 and PdCo ANN were analyzed by XPS. In was also observed (ESI, Fig. S11†).
ꢀ
1
the Pd 3d spectrum of two samples (Fig. 2c), the Pd 3d signals
tted into two pairs of doublets with a spin–orbit separation of + 0.1 mol L KOH + 0.1 mol L KCl solution. The current was
.4 eV, which corresponded to Pd(0) and Pd(II) respectively. The normalized to the Pd mass on the electrodes. In the positive
binding energy (BE) and the composition are listed in Table S4 scan direction, the hydrazine onset oxidation potential for the
ESI†). Pd in the two samples existed predominately in the three catalysts was poorly dened by curve slope, thus proving
Fig. 3b shows CVs of the three catalysts in 0.1 mol L N H
2 4
ꢀ
1
ꢀ1

5
(
oxidized state, and the percentage of Pd(0) in PdCoP-3 was difficult to compare their catalytic activity on this basis.
higher than that in PdCo ANN. The BEs of Pd 3d XPS in PdCoP-3 However, the maximum peak current on PdCoP-3, e.g. mass
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ꢀ1
ꢀ1
Fig. 3 (a) CVs on PdCoP-3, PdCoP GA and PdCo ANN electrodes in N
2
-saturated 0.1 mol L KOH solution at a scan rate of 20 mV s , the inset
shows an enlargement of the forward scan between ꢀ0.9 V and ꢀ0.6 V. (b) CVs on PdCoP-3, PdCoP GA and PdCo ANN electrodes in N
ꢀ1
+ 0.1 mol L KCl + 0.1 mol L KOH solution at a scan rate of 100 mV s . (c) CVs on PdCoP-3 electrode in 0.1 mol L
2
-
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
saturated 0.1 mol L
N
2
H
4
ꢀ
1
ꢀ
1
ꢀ
1
N H + 0.1 mol L KOH + 0.1 mol L KOH solution at scan rates of 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140 and 160 mV s (from bottom to
top); the top inset: I vs. v plot. The bottom inset: E vs. log(v) plot. (d) Percent of the oxidation peak current of the 1000 cycle compared to
p p
2 4
1
/2
th
the initial cycle.
ꢀ
1
activity, was 483 mA mg
PdCoP GA (308 mA mg
at ca. ꢀ0.30 V, i.e. 1.6 fold that of potential (E ) shied slightly positively. The I increased
1/2
Pd
Pdꢀ
p
p
1
at ca. ꢀ0.34 V), conrming that the linearly with the square root of the scan rate (v ) in the range
ꢀ1
network-like structure was favorable for oxidation of hydra- from 20 to 160 mV s (the top inset), indicating the process
zine. The mass activity on PdCoP-3 was 2.9 times as that of was inuenced by diffusion rather than by surface controlled
ꢀ
1
28
PdCo ANN (168 mA mgPd at ca. ꢀ0.28 V), which veried the electron transfer kinetics. Additionally, a linear relationship
promotion of P for the catalytic of PdCo alloy. Therefore, both between E and log(v) was obtained (bottom inset Fig. 3c),
introduction of P and formation of network structure were demonstrating that hydrazine electrooxidation on PdCoP-3
p
2
9
benecial to hydrazine oxidation activity. While introduction was an irreversible process. We inferred that the overall
of P modied the electron structure of Pd and Co atoms (as electro-oxidation of hydrazine (i.e., N H + OH / N + H O +
demonstrated by XPS analysis), porous network structure 4e ) occurred on PdCoP-3 electrode in alkaline solution.
favors mass transfer and possibly provides more active sites Thus, no peak arose on the negative potential scan in CV test
ꢀ
2
4
2
2
ꢀ
for catalysis. The effect of Co ratio in PdCoP ANN on catalytic due to the stability of N
2
molecule.
activity for hydrazine oxidation was also investigated (ESI, To evaluate the stability of PdCoP-3, PdCoP GA and PdCo
Fig. S12†). Among ve catalysts of different Co mass ratio, ANN, the three samples were subject to accelerated electro-
PdCoP-3 produced the highest catalytic activity for hydrazine chemical degradation tests through repeating potential
oxidation in terms of oxidation peak current density. This cycles (ESI, Fig. S14†). Decrease in catalytic activity was
indicates that the optimal ratio for highest mass activity was measured by recording the peak current loss against the cycle
1
: 9.2 : 11 of Pd : Co : P. To further investigate the activity of number (Fig. 3d). Aer 1000 cycles, the peak current for
the all samples, chronoamperrometry measurements were hydrazine oxidation on PdCoP-3 was ꢁ56% of the initial peak
carried out, (Fig. S13) (ESI†). As expected, PdCoP-3 exhibited current. While, the peak current on PdCoP GA and PdCo ANN
the highest current density for hydrazine oxidation at 1000 s. electrodes retained 65% and 55% of the original value
This result corresponds well to the CV result. To further respectively, indicating that the stability of PdCoP-3 isn't
evaluate hydrazine oxidation on PdCoP-3, CV was carried out better than that of PdCoP GA. The degradation of PdCoP-3
at different scan rates. Fig. 3c shows that with increased scan may result from the network structure leaching P in alka-
2
6
rate, the hydrazine oxidation peak current (I ) and peak line medium.
p
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9
F. Xu, L. Zhao, F. Zhao, L. Deng, L. Hu and B. Zeng, Int. J.
Electrochem. Sci., 2014, 9, 2832–2847.
4
. Conclusions
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sized using a N -bubble-template method at room temperature
Hydrogen Energy, 2013, 38, 16264–16268.
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Chem. Phys., 2014, 16, 3593–3602.
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2
2
1
1
9 J. Wang, X.-B. Zhang, Z.-L. Wang, L.-M. Wang, W. Xing and
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Acknowledgements
2
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