Effect of acid functionalised carbon supports for Pd-Ni-Sn catalyst on ethanol oxidation reaction

Article abstract of DOI:10.1039/c5ra07508d

The sodium borohydride reduction method was employed to prepare a ternary catalyst with an atomic ratio of 20%Pd10%Ni10%Sn impregnated on four carbon supports, Vulcan XC-72R carbon black (CB), functionalised carbon black (CB_sn), multi-walled carbon nanotubes (MWCNTs) and functionalised MWCNTs (MWCNT_sn), to improve the catalytic activity for direct ethanol fuel cells (DEFCs). The acid functionalisation was performed using H₂SO₄ and HNO₃. The addition of Ni and Sn metals helped to increase catalytic activity as well as catalyst stability in the ethanol oxidation reaction (EOR). The XRD and XPS results confirmed the presence of Ni(OH)₂ and SnO₂ compounds. The crystalline size was in the range of 6.4 to 7.4 nm for all samples. TEM images of the catalyst on MWCNT_sn confirmed that the catalyst particles were uniformly distributed and dispersed with a particle size in the range of 6 nm. This result was consistent with the highest number of active sites investigated by CO chemisorption. This was due to the highly crystalline structure of MWCNT_sn. Moreover, the catalyst on MWCNTs exhibited the highest current density (291 mA cm^-2) for EOR, great catalyst stability, the highest electrochemical surface area (77.155 m² g^-1 Pd) and excellent CO tolerance.

Full text of DOI:10.1039/c5ra07508d

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Effect of acid functionalised carbon supports for
Pd–Ni–Sn catalyst on ethanol oxidation reaction†
Cite this: RSC Adv., 2015, 5, 61298
c
bde
S. Jongsomjit,ab K. Sombatmankhong and P. Prapainainar
*
The sodium borohydride reduction method was employed to prepare a ternary catalyst with an atomic ratio
of 20%Pd10%Ni10%Sn impregnated on four carbon supports, Vulcan XC-72R carbon black (CB),
functionalised carbon black (CBsn), multi-walled carbon nanotubes (MWCNTs) and functionalised
MWCNTs (MWCNTsn), to improve the catalytic activity for direct ethanol fuel cells (DEFCs). The acid
functionalisation was performed using H2SO4 and HNO3. The addition of Ni and Sn metals helped to
increase catalytic activity as well as catalyst stability in the ethanol oxidation reaction (EOR). The XRD and
XPS results confirmed the presence of Ni(OH)2 and SnO2 compounds. The crystalline size was in the
range of 6.4 to 7.4 nm for all samples. TEM images of the catalyst on MWCNTsn confirmed that the
catalyst particles were uniformly distributed and dispersed with a particle size in the range of 6 nm. This
result was consistent with the highest number of active sites investigated by CO chemisorption. This was
due to the highly crystalline structure of MWCNTsn. Moreover, the catalyst on MWCNTs exhibited the
highest current density (291 mA cmꢀ2) for EOR, great catalyst stability, the highest electrochemical
surface area (77.155 m2 gꢀ1 Pd) and excellent CO tolerance.
Received 24th April 2015
Accepted 29th June 2015
DOI: 10.1039/c5ra07508d
www.rsc.org/advances
types of fuel cells being developed for stationary and portable
applications, since it offers several benets, for example, low
1 Introduction
operating temperature and pressure, low catalyst loading, and
high durability.1 There are various types of fuels being used in
PEMFCs, such as hydrogen gas, formic acid, methanol, ethanol,
etc. Commercially viable fuel cell systems use hydrogen or direct
methanol as a fuel depending on the application. The conven-
tional hydrogen fuel cell is usually used to deliver high power
density, whereas the direct methanol fuel cell (DMFC) is more
suitable for low power applications.2 However, ethanol appears
to be the preferred candidate fuel over methanol as the energy
density and specic energy of ethanol are much higher.3,4
Moreover, ethanol can be produced at low cost and in large
quantities by fermentation of domestic biomass. For these
reasons, a direct ethanol fuel cell (DEFC) has become a prom-
ising renewable and clean energy device with less issues with
catalyst poisoning and fuel crossover when compared to a
DMFC.5
The kinetic rate of ethanol oxidation reaction in alkaline
DEFCs is higher than that of DEFCs in acid media.6 Thus, non-
noble metal catalysts such as nickel and silver can be utilized
along with low-cost noble metals such as palladium (Pd) and
ruthenium (Ru) as anode catalysts.4 However, electrode
poisoning oen associated with these anode catalysts is a crit-
ical issue in a DEFC. The catalyst instability is caused by the
adsorption of CO intermediate generated during the ethanol
oxidation reaction (EOR). A considerable amount of work has
been carried out to nd alternative alloys of non-noble and
noble metals which yield high EOR activity and durability.7–9
Nowadays, the world’s demand for energy consumption is
dramatically increasing, while the major source of energy
coming from fossil fuels is limited and depleting at a fast rate.
The use of fossil fuels also raises serious environmental
concerns; hence the development of alternative and renewable
energy technologies is needed. Among others, fuel cell tech-
nology has the potential to contribute to a sustainable and
emission-free transport and energy system. A fuel cell produces
electricity via electrochemical reaction but without combustion
as in conventional combustion processes. The proton exchange
membrane fuel cell (PEMFC) is one of the most widely used
aInterdisciplinary Graduate Program in Advanced and Sustainable Environmental
Engineering, Kasetsart University, Bangkok 10900, Thailand
bNational Center of Excellence for Petroleum, Petrochemicals and Advanced Materials,
Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, 50
Ngamwongwan Road, Ladyao, Jatujak, Bangkok 10900, Thailand. E-mail: fengpwn@
ku.ac.th; exclusive_boat@hotmail.com; korakots@mtec.co.th; Fax: +66-82-561-4621;
Tel: +66-88-020-2494
cNational Metal and Materials Technology Center, 114 Thailand Science Park,
Phahonyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand
dCenter for Advanced Studies in Nanotechnology and Its Applications in Chemical Food
and Agricultural Industries, Kasetsart University, 50 Ngamwongwan Road, Ladyao,
Jatujak, Bangkok 10900, Thailand
eDepartment of Chemistry and NANOTEC, Center for Nanoscale Materials Design for
Green Nanotechnology, Kasetsart University, 50 Ngamwongwan Road, Ladyao,
Jatujak, Bangkok 10900, Thailand
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra07508d
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High selectivity of the anode catalyst is also important as chemisorption analysis. Additionally, several modes of voltam-
complete oxidation of ethanol to CO2 is desired to produce the metry were performed to explore their catalytic activity, CO
most electrons from one molecule of ethanol.10
tolerance, durability, and electrochemical surface area.
Pd, one of the most abundant noble metals in nature and
cheaper than platinum (Pt) by 50%, has been recognized as the
most efficient catalyst for EOR in alkaline media.11–14 Thus, the
metal and its alloys serve as substitutes for Pt in catalytic
reactions. However, its catalytic stability and activity need to be
improved. This can be achieved by introducing binary or ternary
catalysts such as Au, Ni, Ru, Sn, Ir, Bi, etc.15–28 It has been
reported that Ni, Au, or Ru could be used to increase the activity
and the stability of Pd while Sn, Ir, or Bi have been used to
facilitate the desorption of CO-intermediate during the EOR.29
The type of catalyst support is also a key factor to enhance
the catalytic activity.17,30 In general, carbon black is the most
widely used catalyst support in DEFCs.12,31–33 The average size of
the primary particles obtained from pyrolysis or incomplete
combustion of carbon compounds is normally in the range of
10–50 nm.34 The BET surface area (SBET) is in the range of 10–
1500 m2 gꢀ1, depending on the method to produce the carbon
black. There are many types of carbon black commercially
available, such as acetylene black, Vulcan XC-72, and Ketjen-
black. Apart from carbon black supports, varieties of carbon
nanotubes are also exploited, including single-walled (SWCNTs)
and multi-walled carbon nanotubes (MWCNTs). The unique
structure and particular morphologies of nanotubes make them
more attractive as catalyst supports for fuel cells. They have high
surface area, corrosion resistance, good electro-conductivity,
high stability, meso-porosity, and high degree of cry-
tallinity.35–38 As a result, they have exhibited excellent perfor-
mance when used as catalyst supports. Treatment with HNO3
and H2SO4 was carried out following Osorio et al. procedure.39
They treated carbon nanotubes in different acids and found
that the carbon nanotubes treated by nitric–sulfuric acid and
nitric–sulfuric–hydrochloric acid showed good support
characteristics.
Based on the preceding, Pd-based catalysts with Ni and Sn
co-catalysts were developed in this present work to increase the
catalyst efficiency for DEFC for the rst time using the sodium
borohydride reduction method and also to reduce the cost of
the catalysts. Moreover, the developed catalysts were prepared
on functionalised and non-functionalised carbon based
supports. Ternary catalysts with an atomic ratio of 20%Pd10%
Ni10%Sn were initially synthesized on different types of carbon
supports: (i) Vulcan XC-72R carbon black, (ii) MWCNTs, (iii)
functionalised Vulcan XC-72R carbon black, and (iv) function-
alised MWCNTs. The catalyst ratio was studied rst to select the
most suitable ratio. Physical properties of these carbon
supports were examined using temperature program reduction
(TPR), chemisorption analysis, Raman spectroscopy, Fourier
transform infrared spectroscopy (FT-IR), thermogravimetric
analysis (TGA) and scanning electron microscopy-energy
dispersion spectroscopy (SEM-EDX). In order to access their
suitability for use in DEFC, 20%Pd10%Ni10%Sn deposited on
2 Materials and methods
2.1 Materials
The metal precursors for the anode electro-catalysts were
prepared from palladium chloride (PdCl2), nickel chloride
(NiCl2), and tin chloride (SnCl2), which were purchased from
Merck. Sodium borohydride (NaBH4) was purchased from
HiMedia. Carbon black (Vulcan XC-72R) was supplied by Cabot
Corporation. Ethanol was purchased from RCI, and potassium
hydroxide (KOH) was purchased from Ajex. HNO3 and H2SO4
were purchased from J. T. Backer and Merck, respectively.
2.2 Functionalisation of carbon supports
Firstly, 1 g of carbon support (either Vulcan XC-72R or
MWCNTs) was mixed with 50 ml of a mixed concentrated H2-
SO4 : HNO3 (3 : 1) solution in a closed vessel under sonication,
at room temperature for 2 h. Aer that, the reaction was le
undisturbed for another 15 h. The suspension was washed with
deionised water until the effluent had a pH of 4–5. Finally, the
treated support was dried in a convection oven at 100 ꢁC
overnight.
2.3 Physical characterisation of carbon supports
Characterisation of surface area and characterisation of pore
size distribution of different carbon supports were carried out
by TPR analysis (BELCAT-B). The sample was initially degassed
at 300 ꢁC for 2 h under vacuum prior to the N2 adsorption
measurement. Raman spectroscopy (Renishaw Raman Spec-
troscopy System 2000) was used to investigate the resulting
chemical structure and the defects of the carbon supports aer
functionalisation. Raman spectra were obtained at ambient
temperature with a green laser wavelength of 532 nm, radiated
from an argon laser with a power of 2 mW. In addition, FT-IR
(Thermo Nicolet 6700) was used to diagnose the functional
groups obtained from the functionalisation. Each sample was
mixed with KBr and was then analysed at wave numbers of 400
to 4000 cmꢀ1. In order to analyse the quantity of functional
groups on the supports, TGA (Mettler Toledo TGA/SDTA 851e)
was performed at scanning temperatures from 70 to 850 ꢁC, at a
ramp rate of 5 C minꢀ1, and under air ow conditions. Brau-
ꢁ
ner, Emmet and Teller (BET) surface area was also investigated
using an Autosorp-I to nd specic surface areas of the
prepared supports. The samples were degassed at 300 ꢁC for 2 h
under vacuum before the nitrogen adsorption measurement.
Finally, SEM-EDX (FEI Quanta 400 & EDAX) was employed to
assess the surface morphology and elemental compositions of
supports.
2.4 Catalyst preparation
different supports were investigated using X-ray diffraction The 20%Pd10%Ni10%Sn by weight was prepared from the
(XRD), transmission electron microscopy (TEM), SEM-EDX, X- metal precursors – PdCl2, NiCl2 and SnCl2 – on carbon supports.
ray photoelectron spectroscopy (XPS), and CO-pulse and TPR Firstly, the metal precursors were added to deionised water and
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vigorously stirred. One gram of carbon support was then sus- and subsequently dried at 60 ꢁC for 10 min. Ag/AgCl and Pt rods
pended in the precursor solution for 5 min using a sonicator. were used as a reference electrode (RE) and a counter electrode
Aer that, an excess amount of NaBH4 solution was added (CE), respectively. The potential across the WE and the CE was
under vigorous stirring for 6 h. The mixture was then centri- scanned from ꢀ0.75 to 0.25 V.
fuged and washed with deionised water several times. The
The durability of the prepared catalyst was tested in a
ternary catalyst impregnated on tꢁhe carbon support was nally mixture of 1 M KOH and 1 M ethanol solution using chro-
dried in a convection oven at 70 C for 20 h. This method was noamperometry (CA). A potential of ꢀ0.3 V was constantly
applied to all types of prepared catalysts.13
applied to the three-electrode system for 1000 seconds, with an
active surface area the same as that in the activity test. The
electrochemical surface area was analysed from a cyclic vol-
tammogram recorded from ꢀ1.2 to 0.5 V at a scan rate of 100
mV sꢀ1, in 1 M KOH solution purging with nitrogen gas with an
active surface area of 0.40 cm2 (Pd metal loading 0.0952 mg
cmꢀ2). The method for measuring CO tolerance was rather
similar to that of electrochemical surface area. Initially, the CO
adsorption on the ternary catalyst system was performed in 1 M
KOH solution using CA under CO feeding conditions with an
active surface area the same as that in the ESA test. Aer that,
the electrolyte solution was purged with nitrogen gas to remove
all the remaining dissolved CO gas. Finally the potential was
scanned from ꢀ1.2 to 0.5 V (at 100 mV sꢀ1) to remove the pre-
adsorbed CO molecules from the catalyst surface.
2.5 Physical and chemical characterisation of catalyst
impregnated on carbon support
Firstly, XRD (Philips X’pert) was used to investigate the crys-
tallization and phase of the catalysts. The XRD patterns were
obtained using a Cu Ka source operating at 45 kV and 30 mA
with a scan rate of 0.04ꢁ sꢀ1. Additionally, TEM (JEOL 2010) was
used to investigate the particle size and the dispersion of cata-
lyst particles using a LaB6 lament at 200 kV. SEM and EDX
were also employed to nd the morphology of the catalyst
surface and metal loading ratio, respectively. The form of the
resulting ternary catalyst was determined using XPS (AXIS Ultra
DLD) operating in the lens-dened mode swept 3 times, at a
pass energy of 20 eV, a maximum X-ray source power of 150 W,
and an energy step size of 100 meV. A gas sorption chemi-
sorption analyser was employed to perform TPR and CO pulsing
analysis. The starting and ending temperatures of hydrogen
reduction were rstly dened by TPR analysis using 5% H2 in
argon media at a controlled ow rate of 50 ml sꢀ1 and at a wide
range of temperatures (from 0 to 650 ꢁC). Next, the CO-pulse
method was used to provide the number of active sites and
metal dispersion. The CO-pulse steps include the following: (i)
preheating at 400 ꢁC for 25 min, (ii) oxidation with 99.999% O2
for 15 min, (iii) purging with He for 15 min, (iv) reduction with
pure H2 gas for 15 min, (v) purging with He for 15 min, (vi)
3 Results and discussion
The acid-treated carbon supports and the ternary catalysts were
characterised by several methods to assess their suitability for
use in DEFCs. The characterisation of carbon supports are
detailed in the rst subsection. The physical, chemical, and
electrochemical characterisations of the catalyst are later
explained in more detail in Section 3.2.
3.1 Functionalisation of carbon-based supports
ꢁ
cooling to 50 C, and (vii) injection of 0.89 ml of 10% CO–He
Acid functionalisation is known to be a process of removing
impurities and inserting acid functional groups on the surface
of carbon supports. The hydroxyl groups (–OH) of carbon
supports were replaced herein by carboxylic groups
(–COOH).40,41 Fig. S1† shows the SEM images of CB and
until the change in adsorption peaks was below 2%.
2.6 Electrochemical characterisation of catalyst
impregnated on carbon support
The electrochemical properties of 20%Pd10%Ni10%Sn MWCNTs before and aer functionalisation. The modied
impregnated on different carbon supports were investigated surface can be observed to be a rougher surface when compared
using a potentiostat and galvanostat (Autolab PGSTAT100) to to the non-functionalised one. Additionally, the particle sizes of
perform the following measurements: catalytic activity, dura- both CBsn and MWCNTsn were found to be decreased aer the
bility, CO tolerance, and electrochemical surface area. All the acid functionalisation. This may be due to residues or impuri-
measurements employed a three-electrode system operating at ties being removed from the surface supports.
room temperature. For the characterisation of catalytic activity,
TGA results are shown in Fig. S2.† It can be seen that both
cyclic voltammetry (CV) was carried out in a mixture of 1 M functionalised supports started to decompose at a lower
ethanol and 1 M sodium hydroxide aqueous solution. The temperature when compared to the non-functionalised ones
working electrodes (WEs) were composed of 20%Pd10%Ni10% (i.e., untreated samples). This indicated that the existence of
Sn impregnated on different carbon supports coated on a gas acid functional groups in the supports caused a decrease in the
diffusion layer (Toray Carbon Paper, TGP-H-60) with an active thermal stability of the supports. The weight loss of these
surface area of 0.28 cm2 (Pd metal loading 0.0680 mg cmꢀ2). In functionalised supports was located at approximately 230 ꢁC, a
the process of WE fabrication, the catalyst ink was prepared temperature associated with the decomposition of carboxylic
from 5 mg of catalyst powder mixed with 1 ml of 5% wt Naon® groups.39 It was also found that the decomposition temperature
117 solution and 1 ml of 5 M ethanol solution. The ink was of MWCNTsn was higher than that of CBsn. The decomposition
sonicated for 1 h to provide a homogeneous suspension. The temperature of carbon-based supports was in the range of 400–
resulting catalyst ink was deposited on the Toray carbon paper 700 ꢁC. The residues obtained at the end of TGA measurement
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indicated the impurity level of the samples.39 Aer the thermal
decomposition, both functionalised supports contained less
residue than that contained on the non-functionalised ones.
This can imply that the acid functionalisation helped to remove
some impurities despite causing the reduction of thermal
stability. The weight percents of acid functional groups were
5.85% and 5.42% for CBsn and MWCNTsn, respectively.
Raman spectroscopy was used to verify the existence of
–COOH functional groups from the change of bond structure
among the functionalised carbon supports. Fig. S3† shows
Raman spectra of all carbon supports. The D band at 1350 cmꢀ1
appears as a shoulder to the G band (located at around 1580–
1620 cmꢀ1). It was suggested that the D band originated from a
hybridised vibration associated with graphene edges, which
usually indicates the presence of disorder in the graphene
structure. The high intensity of the D band in carbon materials
refers to a higher order of defects or disorder in the graphene
structure. The high intensity of the G band indicated less
amorphous carbon and highly crystalline structures of the
untreated MWCNTs. The G0 band (the overtone of D band at 2D
position) corresponds to the stacking order of the graphitic
walls and the crystallinity of the graphitic planes.42–44 The ratios
of D band and G band intensity (ID/IG) of all supports are given
in Table S1.† The ID/IG ratio of MWCNTsn was higher than that
of MWCNTs as a result of covalent functionalization of the
MWCNT sidewall being increased aer acid treatment.45
However, the ID/IG ratio of CBsn was slightly lower than that of
CB, indicating that the degree of disorder of the microcrystal-
line structure of CB was not as badly effected as that of
MWCNTs by acid treatment.33
FTIR was used to conrm the functional groups attached to
the original structures. The stretching and bending vibrations
of atoms were used to determine the functional groups in the
structures. As explained previously, the acid treatment enabled
the purication and/or functionalisation of carbon supports to
occur. The process was related to the oxidation of the double
bond in the graphene wall yielding sulfonated groups (SO3),
carboxylic groups (COOH), and hydroxyl groups (OH) on the
carbon surface.46,47 Fig. 1 shows FTIR spectra of all the carbon
supports. It can be seen that the characteristic peaks of CBsn
and MWCNTsn were located at the same positions, but the peak
intensity of CBsn was weaker than that of MWCNTsn. The
ngerprint region below 1400 cmꢀ1 could hardly be used to
identify any compounds because of its complexity. The peaks at
around 3400 cmꢀ1 and at around 1640 cmꢀ1 refer to the H-bond
of hydroxyl groups and the H-bond of carboxyl groups (C]O),
respectively. These H-bonds were conjugated with C]C bonds
(i.e., at approximately 1600 cmꢀ1) in the graphene wall of
Fig. 1 The FT-IR spectra of CB compared with CBsn (top) and
MWCNTs compared with MWCNTsn (bottom) revealing the effect of
acid functionalisation.
method was used to analyse the surface area of the supports.
According to IUPAC isotherms, all the support samples showed
type-II adsorption isotherms, a normal form of isotherms
shown by a nely divided non-porous adsorbent. This repre-
sents an unrestricted monolayer–multilayer adsorption.50 The
conversion of the isotherm graph to a BET graph resulted in a
linear relationship between 1/W[(P/P0) ꢀ 1] and P/P0, as shown
in Fig. S4.†
Fig. S4† reveals an increase in the slopes of the functional-
ised supports when compared to the non-functionalised ones.
This means that the surface area of the functionalised supports
was much higher than that of the non-functionalised supports.
The values of BET surface area (SBET) were calculated as given in
Table S2.† The SBET values of the CB and MWCNTs were 427.4
and 221 m2 gꢀ1, respectively which agree very well with the
values reported in the literature.51,52 The SBET of the pore
MWCNTsn.
48 In addition, the peaks at approximately 2900 cmꢀ1
and 620 cmꢀ1 corresponding to the C–H bond were also
obtained. It was believed to undergo an oxidation reaction due
to the presence of C–O bonds at approximately 1475 cmꢀ1
.
˚
diameters for all samples were below 250 A or 25 nm, indicating
Moreover, a peak at around 1100 cmꢀ1 can be observed, indi-
cating the formation of sulfonated groups and corresponding to
C–O vibration.49
the formation of mesopores. Interestingly, the SBET values of
MWCNTsn and CBsn decreased by 42% and 62%, respectively,
which was also observed by Paulina et al. (2012). The BET
surface area of the two functionalised carbon supports
The specic surface areas of supports strongly affected the
activity of the catalyst. The multilayer adsorption model or BET
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decreased due to the increasing functional group content. This crystal lattice, and d-spacing is the spacing between the planes.
can be related to the blocking of the inter-bundle galleries and The results are given in Table 1. It was found that the crystalline
intra-bundle interstitial channels by the functional groups.53
size of catalysts was in the range of 6.4–7.4 nm, and similar
values of d-spacing and lattice parameter were obtained.
SEM images of all samples are shown in Fig. 3. The metal
particles were dispersed uniformly on the surface of the carbon
supports. The actual compositions of the as-prepared catalyst
samples were examined using SEM-EDX with 5 sampling points
per sample. It was found that the actual compositions were
quite similar to the desired compositions as shown in Table 2.
TEM images of 20%Pd10%Ni10%Sn on various supports
(see Fig. 4) show that the metal catalysts were well dispersed on
all the carbon supports. The histograms of catalyst particle size
distribution on various supports (see Fig. 5) reveal a narrow
average particle size of 6.0–6.7 nm. The catalyst particle sizes
greater than 14 nm shown in Fig. 5 may come from overlapping
of the supports which was not averaged.
The metal dispersion (Dm) and the number of active sites (Ns)
of 20%Pd10%Ni10%Sn on various supports assessed by
chemisorption are given in Table 3. It was found that the cata-
lyst on both MWCNTsn and CBsn exhibited high levels of Dm and
Ns implying that well-dispersed and distributed metal particles
were achieved with the use of acid functionalised carbon
supports. The Dm result was consistent with the catalyst distri-
bution results observed by TEM.
3.2 Properties of electrocatalysts on different carbon
supports
3.2.1 Physical properties. 20%Pd10%Ni10%Sn was
impregnated on four different carbon supports: CB, CBsn
,
MWCNTs, and MWCNTsn. The selected catalyst composition
showed the maximum current density for EOR as reported in
our previous study.54 The physical properties of 20%Pd10%
Ni10%Sn deposited on different carbon supports were analysed
using XRD, SEM-EDX, TEM, CO-pulse chemisorptions, and XPS.
Then, the electrochemical properties were characterised using a
potentiostat/galvanostat to explore the activity, durability, CO
tolerance and electrochemical surface area (ESA) of those
samples.
XRD was used to identify the forms of the metal catalysts as
demonstrated in Fig. 2. All catalysts revealed similar peak
positions, indicating the identical forms of metal catalysts
deposited on different supports. The rst peak at around 25–30ꢁ
was the (0 0 2) plane of carbon structure in CB or MWCNTs. The
characteristic peaks of Pd included the peaks at 39.901ꢁ,
46.209ꢁ, 67.618ꢁ, 81.364ꢁ, and 86.342ꢁ, which referred to (1 1 1),
(2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes, respectively. All peaks of
Pd represented the face-centred cubic (FCC) structure. The
peaks at 33.193ꢁ and 59.303ꢁ refer to the (1 0 0) and (1 1 0)
planes of Ni(OH)2, respectively, whereas the peaks at 26.511ꢁ,
33.769ꢁ, 51.612ꢁ, and 65.752ꢁ refer to the (1 1 0), (1 0 1), (2 1 1),
and (3 0 1) planes of SnO2, respectively. The SnO2 peaks were
difficult to observe as three peaks of SnO2 were overlapped with
the peaks of amorphous carbon (25–30ꢁ), Ni(OH)2 (33.193ꢁ), and
Pd (67.618ꢁ).
XPS was carried out to evaluate the oxidation number of the
metal catalysts. Fig. 6a shows the XPS survey scan of 20%Pd10%
Ni10%Sn catalyst on CBsn and MWCNTsn supports. The survey
scan contained the atomic peaks of carbon (C 1s), oxygen (O 1s),
nickel (Ni 2p), tin (Sn 3d), and palladium (Pd 3d). The XPS area
ratios of chemical state of Pd, Ni and Sn species and their
oxidation states are detailed in Table 4. The XPS spectrum of the
Pd 3d peak consisted of two dominant peaks and three sub-
peaks, corresponding to Pd metal (Pd0) and PdO (Pd2+) forms,
respectively. The XPS spectrum of the Ni 2p peak was composed
of two dominant peaks and four sub-peaks associated with Ni,
NiO, Ni(OH)2, and NiOOH with the oxidation states of Ni0, Ni2+,
Ni2+, and Ni3+, respectively. The XPS spectrum of Sn 3d included
two main peaks and three sub-peaks associated with Sn–SnO
and SnO2 with the oxidation states of Sn0–Sn2+ and Sn4+,
respectively. In both catalyst samples, the highest ratios of
Ni(OH)2 and SnO2, which are known to promote the EOR and
CO oxidation, respectively, were obtained.11,13,17,18 Fig. 6b–d
show XPS spectra of Pd 3d, Ni 2p, and Sn 3d, respectively, of
catalyst on MWCNTsn (top) and MWCNT (bottom). The carbon
support showed the interaction between metal loading by the
shi of metal peak. The XPS results show a positive shi of 0.1
eV for Pd 3d and a negative shi 0.4 eV for Sn 3d on catalyst on
The peak of Pd (2 2 0) at 67.618ꢁ was used to calculate
spacing, lattice parameter, and crystalline size in all samples.
The lattice parameter is the physical dimension of unit cells in a
MWCNTsn.
55,56 In the case of Ni on MWCNT, the peak contained
much noise and was of a low intensity making it hard to identify
the position of peak for the peak shi.
3.2.2 Activity, stability, CO tolerance, and electrochemical
surface area (ESA). The cyclic voltammograms of the catalyst
samples for EOR were recorded in a mixture of 1 M ethanol and
1 M KOH aqueous solution and are shown in Fig. 7. The EOR
can be observed on the anodic peak which was located between
ꢀ0.2 and 0.1 V. It can be seen that the onset potentials were
Fig. 2 XRD patterns of all prepared catalysts at 2q ¼ 20–90ꢁ ( carbon,
Ni(OH)2, SnO2, Pd).
61302 | RSC Adv., 2015, 5, 61298–61308
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Table 1 d-Spacing, lattice parameter, and crystalline size calculated from the Pd peak at (2 2 0) plane
Lattice parameter
(nm)
Crystalline
size (nm)
Composition
2q
d-Spacing (nm)
20%Pd10%Ni10%Sn/CB
67.845
67.911
67.916
67.813
0.13802
0.13791
0.13790
0.13808
0.39038
0.39007
0.39004
0.39054
6.8
6.6
6.4
7.4
20%Pd10%Ni10%Sn/CBsn
20%Pd10%Ni10%Sn/MWCNT
20%Pd10%Ni10%Sn/MWCNTsn
Fig. 3 SEM images (ꢂ30 000) of 20%Pd10%Ni10%Sn catalyst on (a)
CB, (b) CBsn, (c) MWCNTs, and (d) MWCNTsn
.
Fig. 4 TEM images at (ꢂ50 000) of 20%Pd10%Ni10%Sn catalyst on (a)
CB, (b) CBsn, (c) MWCNTs, and (d) MWCNTsn
.
relatively similar for all samples. Table 5 shows the corre-
sponding anodic peak potentials and current densities for EOR.
The 20%Pd10%Ni10%Sn/MWCNTsn exhibited the maximum
current density or activity (291 mA cmꢀ2) and mass activity
(4288 mA mgPdꢀ1). It has been suggested that the catalytic
activity towards EOR was more favourable when the catalyst
exhibited a higher magnitude of anodic current density but a
lower value of the onset potential.57 Compared to other works, it
was found that the catalyst activity Pd2Ni3/C prepared by Shen
et al. showed lower current density (about 80 mA cmꢀ2) and the
Pd/C catalyst prepared by Yi et al. also showed a current density
about 50% lower than that obtained with the catalysts prepared
in this work.13,57 The highest mass activity of the catalyst on
MWCNTsn (about 2–3 times that of others) suggested that Pd
supported on MWCNTsn has a more effective utility value.9 The
results also conrmed that the 20%Pd10%Ni10%Sn deposited
on functionalised supports generated higher current density
than those on non-functionalised supports. This may be due to
the high crystallinity and strong covalent bond of the support,
leading to a good dispersion and a nanoparticle size of
metal.58,59 In terms of catalyst activity, MWCNTsn was found to
be a promising catalyst support for EOR in DEFC.
The stability of 20%Pd10%Ni10%Sn catalyst deposited on
different carbon supports was determined by chro-
noamperometry. The test method reported by Yaojuan et al. and
Table 2 Elemental compositions of 20%Pd10%Ni10%Sn catalyst on different carbon supports
Desired composition (% wt) Actual composition using EDX (% wt)
Catalyst/carbon support
Pd
Ni
Sn
Pd
Ni
Sn
20%Pd10%Ni10%Sn/CB
50
50
50
50
25
25
25
25
25
25
25
25
49.21 ꢃ 2.92
52.11 ꢃ 4.69
53.78 ꢃ 3.99
45.37 ꢃ 6.47
23.58 ꢃ 5.45
16.45 ꢃ 2.00
16.81 ꢃ 2.38
29.25 ꢃ 6.33
27.22 ꢃ 2.58
31.44 ꢃ 3.34
29.41 ꢃ 2.09
25.37 ꢃ 1.83
20%Pd10%Ni10%Sn/CBsn
20%Pd10%Ni10%Sn/MWCNT
20%Pd10%Ni10%Sn/MWCNTsn
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Fig. 5 Particle size distribution of 20%Pd10%Ni10%Sn catalyst on (a) CB, (b) CBsn, (c) MWCNTs, and (d) MWCNTsn
Table 3 Metal dispersion (Dm) and number of active sites (Ns) of 20%
.
range of ꢀ0.368 to ꢀ0.480 V. The metal promoters (Sn and Ni)
facilitated the removal of CO poison on the Pd surface and
refreshed the reaction sites for ethanol oxidation. As regards to
the onset potential of the CO oxidation peak, a lower value
indicates well facilitated CO removal on the Pd surface for the
route of ethanol oxidation. The catalyst on MWCNT and
MWCNTsn supports showed lower onset potentials than the
catalyst on CB and CBsn supports. This might be due to the
better effect of Ni and Sn oxides on the catalyst surface of
MWCNT and MWCNTsn, or it could be due to the effect of Pd–Ni
and Pd–Sn alloys on CO removal reaction.63 The reaction path-
ways to remove CO poisoning of metal oxide (MtOx: SnOx, NiOx)
are shown in eqn (1)–(3), with the metal oxide forms revealed by
XPS and XRD results. Moreover, the catalyst on functionalised
support exhibited a lower onset potential (the lowest onset
potential was the catalyst on MWCNTsn) than the non-
functionalised one, as shown in Fig. 9. The onset and peak
potentials for the CO-stripping process extracted from Fig. 9 are
detailed in Table 5. The peak potential was in the range of
ꢀ0.162 to ꢀ0.179 V. The catalyst on MWCNT and MWCNTsn
had lower onset potentials than those of the catalyst on CB and
CBsn, indicating the higher efficiency for CO tolerance obtained
when using MWCNT supports.
Pd10%Ni10%Sn on different carbon supports
Catalyst
Dm (%)
Ns (site)
20%Pd10%Ni10%Sn/CB
1.4474
2.1705
0.9251
2.8822
1.64 ꢂ 1019
2.46 ꢂ 1019
1.05 ꢂ 1019
3.26 ꢂ 1019
20%Pd10%Ni10%Sn/CBsn
20%Pd10%Ni10%Sn/MWCNT
20%Pd10%Ni0%Sn/MWCNTsn
Changwei et al. was followed.60,61 The stability was found
unchanged aer 1000 s. Therefore, a stability test of the catalyst
was carried out and the results are demonstrated in Fig. 8. The
percentages of current drop of MWCNTsn, MWCNT, CB, and
CBsn were 2.58, 1.15, 0.76, and 0%, respectively. Thus, 20%
Pd10%Ni10%Sn/MWCNTsn exhibited the greatest catalyst
stability. The reason for this was because the covalent bonds of
carboxylic functional groups attached to the MWCNTsn surface
enabled a good interaction between metals and support58 and
the high degree of crystallinity of the MWCNTsn support resul-
ted in a good electrical connection at a molecular level.62
A CO-stripping test was also used to determine the catalytic
ability in the presence of CO poisoning in the system. It is also a
useful method for determination of electrochemical surface
area, especially for Pd-based catalysts. CO-stripping voltam-
mograms of 20%Pd10%Sn10%Ni on various supports are
demonstrated in Fig. 9; the inset shows the onset potential and
the peak potential for CO oxidation. The peak for CO oxidation
occurred at around ꢀ0.1 to ꢀ0.2 V with the onset potential in a
2Pd + CH3CH2OH 0 2Pd–COads + 6H+ + 6eꢀ
MtOx + H2O 0 MtOx–OHads + H+ + eꢀ
(1)
(2)
Pd–COads + MtOx–OHads 0 Pd + MtOx + CO2 + H+ + eꢀ (3)
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Fig. 6 (a) Survey XPS spectra of 20%Pd10%Ni10%Sn on CBsn and MWCNTsn; (b) XPS spectra of Pd 3d of catalyst on MWCNTsn and MWCNT; (c)
XPS spectra of Ni 2p of catalyst on MWCNTsn and MWCNT; (d) XPS spectra of Sn 3d of catalyst on MWCNTsn and MWCNT.
where MtOx is a metal oxide such as SnOx, NiOx.
cathodic peak, it was found that the reduction reaction of
Furthermore, another anodic peak which is associated with NiOOH to Ni(OH)2 was at about 0.4–0.2 V, as shown in eqn (4)
Ni metal and, in particular, the oxidation of Ni(OH)2 to NiOOH and (5). These peaks are associated with the increase of activity
was found at approximately 0.4–0.5 V. As regards another of the catalyst. Ni metal on the support catalyst can work
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Table 4 XPS area ratios of chemical state of Pd, Ni, and Sn species
XPS area ratio (%)
Form of metal
Metal on CBsn
Metal on MWCNTsn
Pd(0)
PdO(2+)
Ni(0)
87.5
87.5
9.0
12.5
12.5
5.7
NiO(2+)
15.3
60.0
15.7
4.5
9.2
Ni(OH)2(2+)
NiOOH(3+)
Sn–SnO(0,2+)
SnO2(4+)
51.8
33.3
95.5
96.7
3.3
Fig. 8 Chronoamperometric curves of 20%Pd10%Ni10%Sn deposited
on different catalyst supports recorded in a mixture of 1 M ethanol and
1 M KOH solution at an applied potential of ꢀ0.3 V.
Fig. 7 Voltammograms of 20%Pd10%Ni10%Sn deposited on different
catalyst supports recorded in a mixture of 1 EOH and 1 M KOH at a scan
rate of 50 mA sꢀ1
.
similarly to Pd (on catalyst activity) and reduce CO-poisoning in
the same manner as Sn.
Fig. 9 CO stripping voltammograms of 20%Pd10%Sn10%Ni on various
carbon supports and the corresponding forward scan of CO stripping
voltammograms (inset).
Ni(OH)2 + OHꢀ 4 NiOOH + H2O + eꢀ
NiOOH + H2O + eꢀ 4 Ni(OH)2 + OHꢀ
(4)
(5)
ESAs of the 20%Pd10%Ni10%Sn on various supports were
determined by hydroxyl (OHꢀ) adsorption in 1 M KOH solution.
Fig. 10 shows the cyclic voltammograms of these catalyst
Table 5 The data extracted from cyclic voltammograms (Fig. 10) of 20%Pd10%Ni10%Sn deposited on different catalyst supports recorded in a
mixture of 1MeOH and 1 M KOH
Cyclic voltammetry of ethanol oxidation
CO-stripping
ESA
Onset
potential
(V)
Anodic
peak
voltage (V)
Anodic peak current
density or activity
Mass
activity
(mA mgPd
CO stripping
onset
potential (V)
CO stripping
peak
potential (V)
Area of desorbed
OHꢀ peak
Catalyst
on
ESA
(mA cmꢀ2
)
)
(mC cmꢀ2
)
(m2 gꢀ1 Pd)
ꢀ1
CB
CBsn
MWCNT
MWCNTsn
ꢀ0.414
ꢀ0.419
ꢀ0.472
ꢀ0.396
ꢀ0.077
ꢀ0.141
ꢀ0.182
0.096
146
177
107
291
2151
2608
1576
4288
ꢀ0.478
ꢀ0.480
ꢀ0.415
ꢀ0.368
ꢀ0.162
ꢀ0.168
ꢀ0.174
ꢀ0.179
22.856
23.791
19.711
29.760
59.255
61.680
52.615
77.155
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samples. OHꢀ desorption was used to calculate the ESA values narrow size distribution and good dispersion of catalyst parti-
(given in Table 5). The anodic peak appeared in the forward cles on the MWCNTsn support with an average particle size of
scan, and the cathodic peak showed in the backward scan. In 6.08–6.65 nm. The acid functionalisation was found via char-
the forward scan, the oxidation peak was at ꢀ0.65 V, associated acterisation by CO chemisorption to increase the number of
with the formation of the OHꢀ group adsorption (OHads) on the active sites. Moreover, the crystalline forms of Pd, Ni(OH)2, and
Pd surface which was then converted to Pd–OHads. Aer that, SnO2 were conrmed by the XRD and XPS results. For the
the backward scan caused the electrodesorption of the pre- electrical properties of 20%Pd10%Ni10%Sn deposited on
adsorbed OHads from the Pd surface. From Table 5 it can be different supports, it was found that the 20%Pd10%Ni10%Sn/
seen that the use of functionalised supports exhibited higher MWCNTsn exhibited outstanding electrical properties,
ESAs than those obtained with the non-functionalised supports. including a maximum current density of 291 mA cmꢀ2 for EOR,
MWCNTsn showed the highest ESA which may be because the high catalyst stability and ESA, and excellent CO tolerance. This
metal was well attached to and dispersed in this support. This may be due to high crystallinity and strongly covalent bonds of
agreed well with the TEM results. Moreover, the result was MWCNTsn and the metals, leading to a good dispersion of
consistent with the number of active sites measured from the nanoparticles of metal.
CO chemisorption test. Among the samples, MWCNTsn was
found to be the most suitable support for the deposition of 20%
Pd10%Ni10%Sn as a ternary catalyst in DEFC, as it had excel-
Acknowledgements
lent catalytic activity and stability, great performance for CO This work was nancially supported by the Kasetsart University
tolerance, and the highest ESA.
Research and Development Institute (KURDI) and Center for
Nanocale Materals Design for Green Nanotechnology, Kastsart
Universty. Thanks are also to National Center of Excellence for
Petroleum, Petrochemical, and Advanced Materials and
National Metal and Materials Technology Center (P1550433)
Thailand.
4 Conclusions
The 20%Pd10%Ni10%Sn tri-metallic electrocatalyst was syn-
thesised on four different carbon supports (i.e., CB, CBsn
,
MWCNT, and MWCNTsn) to investigate the most suitable
support for DFEC electrodes. The catalyst samples were
produced using a NaBH4 reduction method, which is a simple
and a versatile technique. Then, their physical and electro-
chemical properties were characterised using several tech-
niques. The results obtained by TGA diagram, Raman spectra,
and FTIR spectra veried the existence of acid functional groups
on both functionalised supports: CBsn and MWCNTsn. However,
TGA and BET results suggested that the thermal stability and
SBET were decreased aer acid functionalisation. The actual
catalyst compositions in all samples were investigated by EDX,
and the results agreed very well with the desired composition of
20%Pd10%Ni10%Sn. TEM micrographs demonstrated the
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