Study on single step solid state synthesis of WC@C nanocomposite and electrochemical stability of synthesized WC@C & Pt/WC@C for alcohol oxidation (methanol/ethanol)

Article abstract of DOI:10.1016/j.jallcom.2015.09.098

WC@C nano composite was prepared by a single step solid-state reaction through in situ reduction and carburization of WO₃ in the presence of Mg and activated charcoal. The XRD results and thermodynamics analysis showed that the optimization of reaction temperature facilitates the reduction as well as carburization of tungsten oxide(s) at different reaction temperature. Thermogravimetric analysis of the product was done to assess the thermal stability in air. The Raman spectroscopy was used to find out the nature (amorphous/graphitic) of carbon in the obtained phase. The N₂ adsorption-desorption measurement showed a narrow pore size distribution from 3 to 4 nm with BET surface area of up to 522.5 m²/g. TEM/HRTEM images confirmed formation of the WC nano particles with spherical morphology. Electrochemical stability of pure and platinized carbide sample (Pt/WC) has been investigated using cyclic voltammetry in acidic media for alcohol (methanol and ethanol) oxidation.

Full text of DOI:10.1016/j.jallcom.2015.09.098

Accepted Manuscript
Study on single step solid state synthesis of WC@C nanocomposite and
electrochemical stability of synthesized WC@C & Pt/WC@C for alcohol oxidation
(methanol/ethanol)
Mr. Gourav Singla, Graduate Student, Dr. K. Singh, Professor, Dr. O.P. Pandey,
Senior Professor
PII:
S0925-8388(15)31074-4
10.1016/j.jallcom.2015.09.098
JALCOM 35373
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 30 July 2015
Revised Date: 7 September 2015
Accepted Date: 12 September 2015
Please cite this article as: G. Singla, K. Singh, O.P. Pandey, Study on single step solid state synthesis
of WC@C nanocomposite and electrochemical stability of synthesized WC@C & Pt/WC@C for
alcohol oxidation (methanol/ethanol), Journal of Alloys and Compounds (2015), doi: 10.1016/
j.jallcom.2015.09.098.
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Graphical abstract

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Study on single step solid state synthesis of WC@C nanocomposite and
electrochemical stability of synthesized WC@C & Pt/WC@C for alcohol
oxidation (methanol/ethanol)
Gourav Singla, K. Singh and O. P. Pandey*
School of Physics and Materials Science, Thapar University, Patiala 147004, India
*Corresponding author:-
Dr. O.P. Pandey
Senior Professor
School of Physics and Materials Science, Thapar University, Patiala (India)
Tel.: +91 175 2393130; fax: +91 175 2393020, E-mail address:- oppandey@thapar.edu
Mr. Gourav Singla (First Author)
Graduate Student,
School of Physics and Materials Science, Thapar University, Patiala (India)
E-mail address: gsinghla@gmail.com
Dr. K. Singh (Co- Author)
Professor
School of Physics and Materials Science, Thapar University, Patiala (India)
E-mail address: kusingh@thapar.edu
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Study on single step solid state synthesis of WC@C nanocomposite and
electrochemical stability of synthesized WC@C & Pt/WC@C for alcohol
oxidation (methanol/ethanol)
Gourav Singla, K. Singh and O. P. Pandey*
School of Physics and Materials Science, Thapar University, Patiala 147004, India
ABSTRACT
WC@C nano composite was prepared by a single step solid-state reaction through in situ
reduction and carburization of WO3 in the presence of Mg and activated charcoal. The XRD
results and thermodynamics analysis showed that the optimization of reaction temperature
facilitates the reduction as well as carburization of tungsten oxide(s) at different reaction
temperature. Thermo gravimetric analysis of the product was done to assess the thermal stability
in air. The Raman spectroscopy was used to find out the nature (amorphous/graphitic) of carbon
in the obtained phase. The N2 adsorption–desorption measurement showed a narrow pore size
distribution from 3 to 4 nm with BET surface area of up to 522.5 m2/g. TEM/HRTEM images
confirmed formation of the WC nano particles with spherical morphology. Electrochemical
stability of pure and platinized carbide sample (Pt/WC) has been investigated using cyclic
voltammetry in acidic media for alcohol (methanol and ethanol) oxidation.
Keywords: Nano composite, thermodynamic, reduction, carburization, cyclic voltammetry
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1. Introduction
In the current era of 21st century, a lot of emphasis is given to enhance the performance of fuel
cell to reduce the petroleum consumption which is in diminishing stage. Among the non
conventional energy resources, fuel cell is an environmental friendly energy conversion system.
This is an electrochemical device similar to battery, which consists of anode, cathode, catalysts
and electrolyte. In fuel cell, electricity is generated by electrochemical reaction between
hydrogen and oxygen through charge movement and also generates pure water (2H++O2-→H2O)
[1]. The performance of this device mainly depends upon the properties of materials used in it.
To enhance the charge transport through the cell in this electrochemical device, a necessary
support (electrocatalyst) is required. This electrocatalyst should be electrically conductive but at
the same time must be resistant to corrosion in the extreme electrochemical environment in the
fuel cell. Although platinum (Pt) nanoparticles have been regarded as the best electrocatalyst for
energy conversion in fuel cells, but it has been restricted because of high cost, CO deactivation
and limitation in resources [2].
Now, WC has emerged as a new electrocatalyst which can replace Pt as WC has similar
properties to that of noble metals like Pt, Pd, Au, etc. [3-5]. Hence, the cost of fuel cell can be
minimized by reducing the usage of noble metals while maintaining the electrochemical
performance of electrocatalyst. The catalytic activity of WC depends upon the preparation
conditions like the reactions by which the carbides are produced. Apart from this, the reaction
temperature, composition of the reaction atmosphere, carbon deficiency and surface composition
of synthesized powder also play an important role to improve its chemical activity [6-8]. Apart
from the electrochemical activity, the mixture of WC and Co is used as cermet (WC-Co)
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for machining, cutting, drilling, mining, and wear resistant parts, due to their excellent
high hardness, wear resistance and improved toughness [9, 10].
Because of unusual properties of materials at nano range different methods like chemical
processing method [9], thermal processing [10], mechanical alloying method [11], combustion
synthesis method [12], chemical vapor condensation (CVC) [13], solvothermal route [14], ion
exchange method [15], single step pyrolysis method [16] etc. have been adopted to prepare nano
WC by taking tungsten, tungsten oxide, tungsten hexacarbonyl, tungsten hexachloride,
ammonium metatungstate (AMT) as the starting materials. Among different tungsten sources,
tungsten oxide has been found to be better one for single step synthesis of tungsten carbide [14].
Moreover, the carbon supporting materials, such as carbon nanotubes (CNT), carbon nanofibers
(CNFs), mesoporous carbon (MC) and graphene are also frequently used as catalyst supports to
improve the electro catalytic reaction in the fuel cells [17-20]. So, the stability and activity of
WC can be further improved by formation of WC@C nanocomposite prepared by microwave
heating method [21], solution synthesis [22], surface decoration and in situ reduction
carburization [23], solution plasma synthesis method [24], direct carburization method [25] etc.
However, the existing difficulties for each process have put the limitations of above-mentioned
methods for producing highly active electrocatalyst in a more economical way. Moreover,
synthesis of highly pure WC nano powder at low reaction temperature will further cut the
cost of production to considerable extent. This has led to explore the new processes. In this
work, we report a new efficient single step solid state synthesis route to develop WC@C by
taking tungsten oxide (WO3), activated charcoal and magnesium (Mg). Because of fast thermite
reaction of magnesium (Mg) during synthesis [26], temperature as well as time for getting pure
transition metal is minimized. It makes Mg as better reducing agent and is widely used in the
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synthesis of carbides [14, 27-32]. The developed process is single step, economical and of
commercial importance.
2. Experimental Procedure
2.1. Preparation
2.1.1 Synthesis of tungsten carbide
For the synthesis of nano tungsten carbide (WC), WO3 (2.314 g), Mg (2.304 g) and activated
charcoal AC (2 g) were added into a specially designed stainless steel autoclave. The autoclave
was sealed from the top, and the entire assembly was put inside the electric furnace at room
temperature. The temperature of the furnace was raised to different temperature (500, 600, 700
and 800 °C) slowly at a heating rate 5 °C/minute and maintained at these temperatures for 10 h
in the absence of inert atmosphere. After that the autoclave was allowed to cool to room
temperature. The resultant product was collected and washed by dilute HCl (1:1) to remove
MgO. After leaching, the powder was washed with distilled water several times to remove HCl.
Finally, the obtained product was washed with acetone and then dried in vacuum at 100 °C. The
sample so obtained are denoted as C5, C6, C7 and C8 corresponding to their reaction
temperature of 500, 600, 700 and 800 °C.
2.1.2 Deposition of Pt
Pt nano particles were deposited on synthesized WC by dispersing 40 mg of WC powder in 20
ml of ethanol in ultrasonic bath and then 100 µL hexachloroplatinic acid (H2PtCl6) aqueous
solution was added to ultrasonicated solution. In the mixture of metal salt and WC support,
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appropriate amount of 0.05 M NaOH/ethanol was added to maintain the pH~10. The precipitate
was washed with high purity water and then dried at 80 °C.
2.2 Characterization of synthesized nanopowder
2.2.1 Physical characterization of synthesized powder
The X-ray diffraction study of the product phase was done to identify crystalline phases. The
XRD of the samples was performed using PANalytical Xpert-Pro with CuKα radiation (λ = 1.54
Å) obtained from the copper target using an inbuilt Ni filter. During XRD experiment, scan
speed was 5 °C min-1. Crystallite size of synthesized WC was determined by XRD profile
technique. The Williamson-Hall equation as given in equation (1) was used to calculate the
crystallite size.
ꢀꢁꢂꢃcosꢄꢁꢂꢃ = [Kꢅ/ꢆ] + [4ꢇsinꢄꢁꢂꢃ
]
(1)
Where βhkl is the integral breadth (A/Io, A: area of the peak, Io: maximum intensity), λ is the X-ray
wavelength, t is the crystallite mean size and ε is strain. Thermo gravimetric analysis (TGA) was
done to find out the thermal stability of the samples. The TGA experiment was done at heating
rate of 5 °C min-1 from room temperature to 800 °C in air atmosphere. Raman spectra of powder
sample was recorded using Renishaw via Raman spectrometer with the 514.5 nm line of an Ar+
laser at 20 mW power. The morphology, particle size and distribution of the samples were
studied by high-resolution transmission electron microscope (HRTEM, JEOL 2100F) operating
at 200 kV. The energy dispersive X-ray spectra (EDS) attached to the (JEOL JSM-6510 LV;
INCA) were also used to investigate the surface chemical composition of the powder sample.
The N2 adsorption/desorption studies to determine the BET surface area were conducted using at
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TriStar 3000 V6.05 A. Before the measurement, all the samples were degassed at 200 °C for 2 h
in vacuum.
2.2.2 Electrochemical characterization of WC@C powder
The electrochemical measurement was performed at Bio-Logic EC Lab SP300. A standard three
compartment electrochemical cell was used with glassy carbon electrode (GCE), Pt wire as the
counter electrode and saturated calomel electrode (SCE) as the reference electrode. All the
measurement was carried out at room temperature in 0.5 M H2SO4.
The top surface of GCE rod having 3 mm diameter was pre cleaned with emery paper. Typically
4 mg of synthesized powder sample was mixed with 1 mL of ethanol. The mixture was treated
ultrasonically for 20 min for uniform dispersion. 10 µL of mixture was then dropped onto the
surface of the GCE. Finally a drop of 10 µL of Nafion® 117 solution (Sigma Aldrich) was
dropped onto the top surface and left in air for overnight drying. Before the CV measurement the
electrolyte solution is purged with argon gas for 5 minute to deoxygenate the solution. The
electrochemical testing of the sample has been done by cycling the potential from -0.2 to 1.2 V at
a sweeping rate of 30 mV s−1 in the presence of 0.5 M H2SO4. Thereafter, the alcohol (methanol
and ethanol) electrolyte was added and cyclic voltammogram tests were carried out for oxidation
from -0.2 to 1.2 V at sweeping rates of 30 mV s−1. The influence of alcohol concentration on the
catalyst catalytic performance was tested with concentrations 0.4 M, 0.6 M and 0.8 M.
3. Results and discussion
3.1 X-ray diffraction analysis (XRD)
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The X-ray powder diffraction data were collected for all the samples at room temperature
between 20°≤2θ≤80°. Fig. 1 shows the XRD pattern of carbon source (AC: activated charcoal),
and the product phase(s) synthesized at different temperatures for 10 h after leaching the
samples. The appearance of WO3, WO2.72, WO2 and W peaks in XRD pattern at 500 °C gives a
path way for the reduction of WO3 as (WO3→WO2.72→WO2→W). The intermediate lower oxide
(WO2.72) known as Magneli phase clearly indicates that this kind of reduction will be very fast
and multi step reaction [33]. The broadened nature of XRD peaks of tungsten carbide indicates
that the particles are of nanometer scale. The peak position (2θ), integrated intensity, the peak
maximum position and the peak width of the individual reflections was obtained through fitted
Gaussian function. The result of the curve fit routine for the (100) peak for sample C8 is shown
in Fig. 2(a). The strain and crystallite size for samples synthesized at different temperatures are
obtained using Williamson-Hall equation (1) by plotting the value of βhkl cosθhkl as a function of
4 sinθhkl, which is shown in Fig. 2 (b). The value of strain ε and the crystallite size (t) are
estimated from the slope of the line as well as from the intersection with the vertical axis
respectively and is listed in Table 1. From the calculated value of ε, it is clear that the
synthesized samples are in the strained form. It arises due to the transformation of lower ionic
radii ion (0.6 nm) W6+ to higher ionic radii (0.66 nm) W4+ during the formation of WC nano
powder, as discussed in our earlier report [32]. The higher value of strain for sample C7 may be
because of the occurrence of reversible reaction process which followed the path
(WC→W2C→W→W1.09) at 700 °C. Due to the higher strain value in sample C7 larger value of
crystallite size could be expected in such typical case which is also reported in one of our earlier
report [30]. With further increase in temperature (800 °C), the defected tungsten oxide (WO1.09
)
because of high reactivity carbonized easily [34] and undergoes to relax state with the formation
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of WC as is evident from the lower strain value given in Table 1. This indicates that a sample
will attain more stability at higher synthesis temperature as compared to other samples, which is
obtained at lower temperatures. On the other hand the up shift of XRD peak for free carbon in
sample C6 at 25.90°, for sample C7 at 26.43°, and for sample C8 at 26.44° clearly represents the
transformation of disorder to order nature with the increasing temperature [35].
3.2. Thermodynamic analysis on series of reduction and carburization reactions
It is essential to consider the various reactions that eventually take place while heating the
autoclave for different temperatures. Kinetics of the carburization process essentially involves
the diffusion of carbon into reduced tungsten oxide. Generally, the change in Gibbs-free energy
ꢀG gives an idea for the spontaneous direction of the process. After heating the mixture of
powder tungsten oxide, magnesium and carbon to certain temperature, several reactions will
occur during the reduction as well as carburization of oxide. To analyze the transformation, the
heat of reaction associated for each reaction has been considered and the change in free energy
(∆G) for respective reaction has been calculated by the following equation:
∆ꢈ = ∆ꢊ + ꢉ ꢍꢎ dꢏ − ꢏ[∆ꢐꢉ +
dꢏ]
(2)
(3)
ꢌꢉ
ꢑ
ꢒ
ꢌ
ꢉ
ꢉ
ꢋ
ꢋ
ꢉ
ꢉ
ꢉ
ꢋ
ꢋ
Where ꢍꢎ = ꢓ + ꢔꢏ + ꢕꢏꢖꢗ + ꢘꢏꢖꢙ.ꢚ
Where Cp is specific heat capacity and a, b, c and d are the thermodynamic data of reactants and
products at the standard state.
In the reduction and carburization reactions of WO3 it is reported by many authors [9, 10, 32, 36]
that different tungsten oxide phases form as intermediate product before the final W phase.
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Thermodynamically different solid state reactions may occur during reduction of WO3 with C
and Mg as the temperature is increased from low to higher range. The corresponding reduction
equations as given below:
WOꢛ + 0.28 C → WOꢗ.ꢜꢗ + 0.28CO
WOꢛ + 0.14 C → WOꢗ.ꢜꢗ + 0.14COꢗ
WOꢛ + 0.28 Mg → WOꢗ.ꢜꢗ + 0.28Mg O
WOꢗ.ꢜꢗ + 0.72 C → WOꢗ + 0.72CO
WOꢗ.ꢜꢗ + 0.36 C → WOꢗ + 0.36COꢗ
WOꢗ.ꢜꢗ + 0.72 Mg → WOꢗ + 0.72MgO
WOꢗ + 2 C → W + 2CO
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
WOꢗ + C → W + COꢗ
WOꢗ + Mg → W + 2MgO
2W + C → WꢗC
WꢗC + C → 2WC
W + C → WC
The values of ꢀG for all the above possible reactions can be calculated using equations (2) at
different temperatures by using the thermodynamic values (Table S1) mentioned in Ref. [37].
Most of the authors [9, 36] have reported the reduction of tungsten oxide by carbon at higher
temperature range (600-900 °C). Magnesium (Mg) being a highly reactive substance also reacts
with WO3 to yield lower intermediate tungsten oxide phases as a solid-state reaction product of
magnesium and WO3. Because of lower ꢀG (ꢀG<0) value of reactions (6, 9, 12) through Mg
reactant at room temperature as compared to another carbon reducing reactions, it act as a best
catalyst to enhance the reduction rate at lower reaction temperature range. The thermodynamic
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analysis of magnesium reduced tungsten oxide reactions is shown in Fig. 3(a-c). From
thermodynamic study it is clear that at 500 °C synthesis temperature both magnesium as well as
carbon play vital role in reduction of tungsten oxide (WO3) into lower intermediate oxide
(WO2.72) in 10 h synthesis time and makes the reduction temperature lower as compared to
reported by Koc et al. [9]. Because of higher ꢀG value of carbon reduced reactions at 600 °C, the
magnesium induced reduction process will dominate which leads to intermediate oxide (WO2.72
)
and finally into pure tungsten (W) through the formation of WO2. Due to the greater affinity of C
with the in-situ produced fresh W atoms as compared to other tungsten products like W2C [38], it
will diffuse at faster rate through unreacted W core at 600 °C which is also confirmed by lower
ꢀG value of carburization reaction (15) as compared to reaction (14) at 600 °C.
Moreover, before the carburization process at 700 °C and 800 °C, carbon also start to contribute
in the reduction process of oxide(s) with the emission of CO2 and CO gases as byproduct which
depend upon the temperature. From ꢀG-T curve it has been observed that at temperature ≈700
°C the gas product for carbon reduced reaction will be CO2 which transform to CO with the
increase in temperature up to 800 °C. It means that at distinct reaction temperatures, the same
reactants can have different byproducts [39]. The ratio of CO and CO2 plays very important role
to decide the direction of reaction process. Hence for a given amount of reactant, a decrease of
reaction rate is observed which results to move the reaction process backward with the formation
of W2C phase. Moreover, the reoxidation can even take place at a specific temperature in CO2
rich reaction (CO2/CO>0.1) [36, 40]. Hence the appearance of WO1.09 in XRD profile (Fig. 1)
indicates that the reaction at 700 °C occurs in the presence of CO2 gas. With further increase in
temperature the ratio of CO2/CO is going to decrease which may push the reaction in forward
direction at 800 °C and increase the ratio of WC/W2C. From thermodynamic analysis, it is clear
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that the reduction of tungsten oxide by magnesium corresponds to rise in ꢀG-T curve which may
lead to the production of lower intermediate oxides. Therefore, the lower intermediate oxide
produced from magnesium reducing activity is unstable at higher temperatures.
From above discussion, it can be said that, all the three tungsten oxide (WO3, WO2.72 and WO2)
can be reduced by magnesium as well as carbon at all temperature range giving rise to the
byproducts MgO, CO and CO2 at different temperatures. The detailed mechanism of reduction as
well as carburization is also shown in Fig. 4. The sequence of the initial reaction temperature of
the tungsten oxide is WO2, WO2.72 and WO3 from the low to the high temperature. All the above-
mentioned reactions generate a lot of heat and results in an increase in local temperature as
indicated by lower calculated ꢀG value. So on the basis of XRD and thermodynamic analysis the
route of reductions as well as carburization of tungsten oxide will be
WO3→WO2.72→WO2→W→W2C→WC that may occur in a wide range of temperatures.
3.3 Thermal Analysis
Thermal analysis of the product phase synthesized at different reaction temperatures is shown in
Fig. 5. It is clear from the TGA plot (Fig. 5) that the sample weight decreases continuously
during heating. The rate of weight loss is quite low below 425 °C and high loss rate occurs in the
range of 425–600 °C; beyond this range the loss of weight slows down again. The reduction in
weight loss below 425 °C may be due to the protective carbon coating layer around the
synthesized nano particle. The oxidation of this outer carbon layer decreases the sample weight
sharply. After wards the oxidation of tungsten carbide begins at about 650 °C which transforms
WC to WO3 with the formation of CO2 [41]. However, the plateau after 650 °C indicates that all
carbon and WC have been oxidized, leaving only the stable WO3. Such kind of TGA curve
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nature has been observed by many of the authors in their own carbide systems [31, 32, 41-46].
The presence of lower carbide (W2C) as well as W affects the nature of the thermogram as
shown in Fig. 5. Due to the presence of carbon deficient elements W and W2C, thermogram of
sample C6 and C7 show higher weight loss as compared to sample C8. Moreover, the presence
of these elements also affects the thermal stability of synthesized product with the shifting of
onset temperature of weight loss towards lower side. The loading of WC in C6, C7 and C8 are
mentioned in Table 1, which is much greater than the synthesized WC@C nano composites
reported by Yan et al. [43]. The sample composition was calculated from the final mass change
by assuming the presence of WC and elemental carbon in the sample. The fraction of carbon
present after the final synthesized powder is calculated as
ꢠꢡ
% free C = 100 × (1 − ꢝꢝ
×
)
ꢢꢣ
ꢞ
ꢟ
ꢠꢡ
ꢢꢤ
ꢥ
Where mi and mf are the initial and final masses observed from TGA experiment, respectively;
MWWC and MWWO3 are the molecular weights of WC and WO3, respectively. The calculated
results given in Table 1 clearly indicates that the amount of free carbon is minimum at 800 °C
corresponding to higher percentage of WC loading in the obtained product.
3.3. Raman Spectroscopy
The as obtained samples were further characterized by Raman spectroscopy in detail to confirm
the presence of amorphous/ graphitic carbon during the synthesis of nano particles. Fig. 6
displays the Raman spectra of samples C6, C7 and C8 in which the most common and important
D-band around 1350 cm-1 and G-band around 1580-1600 cm-1 [47, 48] are observed. According
to Ferrari and Robertson [47] the D-band is often attributed to disorder scattering, and this mode
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remains absent in perfect graphite. The G-band is related to the bond stretching of all pairs of sp2
atoms in both rings and chains. The peak intensity ratio between D-band and G-band (ID/IG)
provides information about the crystalline as well as amorphous nature of synthesized sample.
The higher intensity of D band for 600 °C synthesized sample clearly represents the formation of
disordered amorphous carbon phase which decreased with the increasing in synthesis
temperature from 600°C to 800 °C. This indicates the progressive crystallization of disordered
amorphous carbon phase, which is also observed in XRD pattern. The higher intensity of G band
as compared to D band clearly represents the ordering.
3.4. Microstructure analysis
Fig. 7(a) shows the TEM micrograph of synthesized sample produced at 800 °C for 10 h. It
indicates that the resulting powder consist of particles with spherical morphology. The TEM
image clearly shows that most of the prepared nano particles have core-shell morphology as has
been observed by many authors in their work [32, 49, 50]. The detail mechanism for the
formation of core shell is discussed in our earlier report [32].
The HRTEM of two particles with core-shell structure shown in Fig. 7(b) also reveals that the
inner core is well crystallized tungsten carbide, which is further confirmed by the plane distance
of 0.28 nm and 0.24 nm corresponding to [001] and [100] plane of WC. The absence of any kind
of fringes around the WC nanoparticles in the shell component is clearly evident to amorphous
nature of carbon. The sphere like nano particles are not uniformly dispersed but are arranged into
chains like nanostructure as seen in Fig. 7(a). These nano chains consist of two regions: internal
region called core with dark shade (WC nano particle) encapsulated by external region called
shell with light shade (amorphous carbon) and named as a core-shell nanostructures. According
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to Xi et al. [29] there are two factors, which are responsible for such a kind of core-shell
nanostructures. One is the template effect of the tungsten carbide nanoparticles and the other is
the activated effect of intermediately formed magnesium oxide nanoparticles.
3.5. BET surface area analysis
The pore structure of carbon coated C6 and C8 samples was further investigated by N2
adsorption–desorption isotherms as shown in Fig. 8. Since the sample is synthesized in the
presence of in-situ produced CO gas [51, 52], so it will provide an excess carbon layer on WC
nano particles. These amorphous carbon layers will have suitable porous structure and high
surface area, which exhibit typical IV curve with a distinct hysteresis loop as shown in the N2
adsorption/desorption isotherm (Fig. 8a) for both samples. It clearly indicates the mesoporous
nature of the prepared samples. Moreover, it is evident from the Fig 8b that both C6 and C8
samples have narrow pore size distribution in the range of 3 to 4 nm, while the pore size less than
2 nm is also showing the presence of micropores in the synthesized sample. The pore size,
specific surface areas and pore volumes of synthesized nano samples from N2 adsorption are
calculated and are listed in Table 2.
The observed value of BET specific surface area for samples seems to be higher than that of
estimated from particle size analysis which is also observed by Li et al. [53]. This is attributed
due to the combined effect of morphology and porous structure of outer carbon layer on WC
nanoparticles which significantly increases the specific surface area of WC@C [54]. The BET
surface area of WC@C powder sample obtained from current synthesis method is comparable to
value as reported by other methods [19, 46, 55], which makes it suitable materials for
electrochemical catalysts. It is clear from the Table 2 that the sample C8 shows higher BET
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surface area (522.5 m2 g-1) and larger pore volume (0.34 cm3 g-1) which got increased from 310.6
m2 g-1 and 0.24 cm3 g-1 for C6 when the synthesis temperature is increased from 600 °C to 800
°C. These results are consistent with the fact that the micropore wall of sample collapse, which
leads to increase the pore diameter, pore volume and graphitization at the higher temperatures
which is also confirmed from our XRD and Raman analysis [56].
3.5. Electrochemical stability of WC and Pt/WC in 0.5 M H2SO4
Cyclic voltammogram (CV) of synthesized samples was recorded by normalizing with electrode
area in acidic solution within the potential range between -0.2 V and +1.2 V by defining the
anodic current to be positive as shown in Fig. 9. In the first cycle, a large anodic current was
observed at potentials > 0.9 V vs. SCE for sample C6. As observed from Fig 9(a), the onset
potential for anodic current is shifted to lower side for samples which is obtained at synthesis
temperature varying from 600 °C to 800 °C. The shifting of onset potential, suggested that the
stability of synthesized sample towards electrochemical reaction decreases with the increase in
the synthesis temperature. Moreover, the effect of synthesis temperature on the response of
samples towards the electrochemical activity is clearly visible from the Fig. 9(a) in which
sample C8 exhibits higher value of anodic current. It is due to the higher surface area
(SBET=522.5 m2 g-1) of sample C8 as discussed in last section, which attributes more active
catalytic site for the electrochemical reactions [19, 53]. Hence the response of catalytic activity
in terms of anodic current density in acidic media for our samples (WC@C) seems to be better
than the response reported by other authors [19, 57]. These CV peaks decrease with the increases
in CV scans as shown in Fig. 9(b) for sample C6. Despite that, at the same time, the redox peaks
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appear between 0.0 to 0.4 V vs SCE on the both anodic and cathodic sweep and becomes more
prominent with the increase in CV scans.
The electrochemical reactions on the sample surface may take place in two stages. In the
potential range -0.2―0.4 V, the peaks express the adsorb/desorb process of hydrogen on the
sample. During the forward scan, the anodic current above ~0.7 V vs. SCE is due to the
electrochemical oxidation of WC, as already reported by many authors [58-60]. With the
successive increase in the CV scans, WC was oxidized into WO3. Obviously, in this condition,
an inactive surface layer of WO3 having very low solubility in the electrolyte [61] is formed on
the WC surface, resulting in decreases in the current gradually. In the reverse scan in the
cathodic region between -0.2 and 0.4 V vs SCE hydrogen atoms are adsorbed on the cathodic
sweep and desorbed on the anodic sweep. Due to the adsorption and desorption of hydrogen on
WO3 a reversible redox process of WO3/HxWO3 (Hydrogen tungsten bronzes; x=0.1–0.5) is
attributed in cyclic voltammograms [7, 58, 62-64]. Moreover, the existence of peaks due to
hydrogen intercalation/de-intercalation into/out of the samples in the potential region 0−0.55 V is
in agreement with Jayaraman et al. [65]. According to Zhu et al. [58], the consistent increase in
the intensities of these anodic and cathodic peaks with CV scans (Fig. 9(b)) is due to the more
WO3 productions. The comparison of the CV curve (Fig. 9(c)) for the WC and Pt/WC shows that
Pt modified C8 sample (Pt/WC) displayed enhanced catalyst stability in H2SO4 by shifting the
onset potential for oxidation to higher side in comparison to pure WC electrode. Moreover, the
effect of platinum nano particle deposition is also seen during the negative sweep of CV, which
shows clearly dominated peak as well as it leads to shift the onset potential of reduction ~0.07V
towards higher side. Such kind of shifting attributes the contribution of platinum which enhance
the catalytic activity which is not observed by Weigert et al. [57] in their work. The effect of
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platinum deposition on the electrochemical stability of sample is also observed by investigating
the reduction in anodic current value with cycle which is ~38% for sample C8 and it remains
only ~25% for sample C8/Pt in between 5th and 25th cycle. Electrochemically active surface areas
(ECAs) of deposited Pt electrocatalysts on C8 sample is determined from the charge densities
involved in the under potential deposition (UPD) processes of hydrogen adsorption/desorption
⁄
by using relation ꢦꢍꢧ = ꢨꢩ ꢨꢪ [67, 68]. In our experiment the value of QH for hydrogen
adsorption on Pt/WC measured is 90 µC by integrating I-V curve appearing in the hydrogen
adsorption potential region in cyclic voltammograms. By dividing the value of QS (210 µC cm-2)
[68], the electroactive surface area of C8/Pt is calculated to be 0.42 cm2. Fig. 10(a) shows the
XRD pattern of C8/Pt in which the diffraction peaks close to 39.5°, 46.1° and 67.2°can be
assigned to Pt, demonstrating that the platinum precursor has been successfully reduced to Pt
nanoparticles. Moreover, the broad XRD diffraction peaks indicate a high dispersion of the Pt
nanoparticles, which have a size of approximate 4 nm as calculated by using the Debye–Scherrer
formula. The EDS elemental spectrum shown in Fig. 10(b) also indicates the presence of carbon,
oxygen, tungsten and platinum in the Pt/C8 sample. The elemental analysis clearly indicates that
the electrode sample prepared by the current method consists of platinum along with tungsten
carbide and carbon. The O peak is due to a partial surface oxidation of WC nanoparticles during
sampling [69].
Herein we study the electrocatalytic performance of Pt catalysts for methanol/ethanol oxidation
in the 0.5 mol L−1 H2SO4 containing solution. As shown in Figure 11(a), the broad feature of the
peak observed in the CV curve after the addition of fuel indicates that the methanol/ethanol is
oxidized within this potential range. Hence it is clear that the catalyst shows good catalytic
behavior for the electrooxidation of alcohol. Figure 11(a) shows the methanol and ethanol
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oxidation of WC in the H2SO4 electrolyte environment. The CV curve of ethanol oxidation on
the WC surface shows an onset beginning at ~0.9 V. In addition, it was also observed that onset
potential of methanol oxidation with WC has a 30 mV negative shift compared with ethanol
oxidation Moreover, the WC catalyst also contribute in the negative sweep of CV curve which is
attributed by the appearance of reduction peak.
Figure 11(b & c) shows the results of alcohol oxidation in the presence of platinum deposition
with different concentrations. The Pt/WC catalyst show similar catalytic characteristics towards
methanol and ethanol oxidation with decreasing in catalytic current with increasing the
concentration. As it is observed, from the figure that the current density for both the alcohol
oxidation decreases with the increase in concentrations. In additions, the lower onset potential
for methanol oxidation as compared to ethanol oxidation suggests that Pt/WC catalyst has higher
catalytic activity for methanol oxidation.
In the CVs of alcohol electrooxidation, the anodic peak in the reverse scan can be attributed to
the removal of the incompletely oxidized carbonaceous species formed in the forward scan.
Thus, the If/Ib ratio suggests a tolerance to carbonaceous species accumulation of catalyst during
alcohol electrooxidation [69]. A low If/Ib ratio indicates poor oxidation of both alcohols
(methanol and ethanol) to carbon dioxide during the anodic scan and excessive accumulation of
carbonaceous residues on the catalyst surface. The If/Ib ratio of Pt/WC is higher in methanol
oxidation. Therefore it shows that Pt/WC exhibits less accumulation of intermediate
carbonaceous species and good anti-poisoning ability while using methanol as fuel. The higher
value of current density in Figure 11(d) by using methanol as fuel clearly attributes the weak
interaction carbonaceous species. Pt surface area loss as well as decline in oxidation activity of
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Pt/W-C catalyst was examined at the constant potential of 0.6 V by plotting current density as a
function of the cycle number shown in Figure 11(e). As seen from Figure 11(e) the catalyst
show almost similar behavior for both the fuel in which at the beginning of the test the activity of
Pt/WC increased upto 30th scan and then decreased with further increasing scan count. In
addition the rate of increment as well as decrement of activity of catalyst with alcohol is related
directly to the interaction as well as response of catalyst with the fuel.
4. Conclusion
In summary, WC@C nano powders with average size of 16 nm have been synthesized by a
simple in situ reduction and carburization reactions of the WO3, Mg and activated charcoal at
different temperatures. The thermal kinetics of different reactions having same reactants with the
increasing temperature were analyzed thermodynamically and observed that the gas products
will be CO2 at low temperature and CO at high temperature. The thermal analysis indicates that
the synthesized samples are thermally stable below 650 °C in air atmosphere. From thermal
analysis, it is also confirmed that the current method used here is simple, rapid and economical
for the mass production of nanomaterials. The powder samples obtained from this method have
high surface area which is more prominent at higher synthesis temperature (800 °C). Moreover,
the resultant synthesized samples (WC@C) exhibited higher anodic current value which
increased with the increasing in the synthesis temperature. Modification by low coverage of Pt
on the surface of synthesized WC (C8 sample) powder leads to its enhanced stability in H2SO4
which is clear from CV curve by observing slight shifting in oxidation potential on higher side.
Moreover, the catalytic performance of Pt deposited WC towards methanol oxidation is greatly
promoted as compared to ethanol oxidation.
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Acknowledgments
Authors are grateful to the Department of Science and Technology (DST), New Delhi, India for
providing the financial grant vide letter no. SR/S3/ME/0037/2009-SERC Dated 15/07/2010 to
carry out this research and Dr. Akshay Kumar for valuable suggestions.
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