Graphene Sheets from Graphitized Anthracite Coal: Preparation ...

Graphene Sheets from Graphitized Anthracite Coal: Preparation,
Decoration, and Application
Quan Zhou, † Zongbin Zhao,* ,† Yating Zhang, ‡ Bo Meng, † Anning Zhou, ‡ and Jieshan Qiu* ,†
† Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals,
School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
‡ School of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, China
ABSTRACT: Coal has been used as an important resource for the production of chemicals, conventional carbon materials, as
well as carbon nanomaterials with novel structures, in addition to its main utilization in the energy field. In this work, we present
the synthesis of chemically derived graphene and graphene−noble metal composites with coal as the starting material by means
of catalytic graphitization, chemical oxidation, and dielectric barrier discharge (DBD) plasma-assisted deoxygenation. It is found
that the graphitization degree of the coal-derived carbon remarkably affects the properties of graphene obtained from chemical
exfoliation, and high crystallinity of coal-derived carbon is essential for the preparation of high-quality graphene sheets (GS). GS
decorated with highly dispersed noble metallic nanoparticles (NP) on their surface (NP/GS) were successfully fabricated via
simultaneous reduction of graphite oxide (GO) and noble metal salts by H 2 DBD plasma technique. The electrochemical
performance of the GS as electrode in supercapacitor and the catalytic activities of NP/GS composites in selective reduction of
nitrogen oxides (NO x) were investigated. This work demonstrates an alternative approach for the fabrication of graphene and its
composites from coal with promising potential in energy storage and environment preservation.
1. INTRODUCTION
As a valuable and abundant solid fuel in nature, coal provides
most of the energy for the economic development and livingused
energy in China. Besides the contribution to energy, coal
can be exploited as an important resource for the production of
chemicals, such as creosote oil, naphthalene, phenol, and
benzene, etc. Coal-based carbon materials, such as activated
carbon, carbon blocks, and carbon electrodes, have been
produced worldwide for a long time. 1,2 With the discovery and
development of carbon nanomaterials, the preparation of
carbon nanomaterials from the cheapest and most abundant
natural carbon source�coal�has attracted increasing attention
in the last few decades. 3 Wilson and his co-workers first
reported the preparation of C60, C70, 4 and CNTs 5 from coal
by using the arc-discharge method with coal-based carbon as
the evaporation anode. Over the past decade, our group has
intensively investigated the possibility and feasibility of
preparing various carbon nanomaterials from Chinese coal. 6
As a novel two-dimensional carbon nanomaterial, graphene
has attracted tremendous scientific attention in recent years due
to its excellent physical and chemical properties. 7−9 To further
broaden the application potential of graphene-based materials,
graphene nanocomposites are being developed and receiving
increased attention, especially the decoration with metal or
metal oxide nanoparticles. Various carbon sources, including
graphite, 10 hydrocarbons (CH4, C2H2), 11,12 polymer
(PMMA), 13 natural biomaterials (food, insects), and even
plastic waste 14 have been widely used for preparing graphene.
However, as the most abundant carbon source in the world,
coal has not been investigated for the production of graphene
up to now. Graphene layers in the structure of coals have been
investigated in molecular level. 15,16 Tomita et al. evaluated the
size of graphene sheets in several Chinese anthracites by a
temperature-programmed oxidation method. 17 Furthermore,
coal possesses abundant polyaromatic structures that are quite
similar to sp 2 bonding characteristics of graphene; therefore, it
is reasonable to expect that coal can be used as starting material
for the fabrication of graphene and graphene-based composite
materials.
Herein, we present the successful synthesis of chemically
derived graphene and composites of graphene sheets (GS)
decorated with noble metal nanoparticles from coal by means
of catalytic graphitization and chemical oxidation combined
with dielectric barrier discharge (DBD) plasma technique. The
graphitization degree of the coal-derived carbon precursor is
controlled by using catalyst, and the correlation between the
graphitization degree of the coal-derived graphite-like carbon
and the resultant graphene quality has been clarified. The
performance of as-prepared chemically derived graphene as
electrode in supercapacitor was investigated, and the graphenebased
composites of GS decorated with noble metal (Pt, Ru,
and PtRu) nanoparticles were used as catalyst in selective
catalytic reduction (SCR) of nitrogen oxides (NO x) with
ammonia.
2. EXPERIMENTAL SECTION
2.1. Preparation of Coal-Derived Graphite Oxide. Taixi coal
(TX), from Shanxi province of China, was used as the starting material
for the preparation of GS. The raw coal was ground and sieved to a
powder, and the obtained particle size of coal powder is less than 0.5
mm. The coal powders were milled for 1.5 h by ball milling (the ratio
of ball and coal is 20:1), and the diameter of more than 90% of them is
less than 15 μm after milling.
Received: October 28, 2011
Revised: July 17, 2012
Published: July 17, 2012
Article
pubs.acs.org/EF
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Energy & Fuels Article
Table 1. Analysis Data of Taixi Coal and Its Demineralization Coal
proximate analysis (wt %) ultimate analysis (daf, wt %)
samples M ad A d V daf C H N S O a
TX 1.38 15.40 8.56 91.10 3.34 0.89 0.36 4.35
TX-de 1.29 2.52 8.08 93.90 3.47 0.77 0.08 1.81
a By difference.
The raw coal was treated with hydrofluoric acid and concentrated
hydrochloride in plastic beakers at 50 °C for 4 h to make deashed coal
(TX-de), and then it was washed with distilled water until no Cl − and
F − were detected in the filtrate.
The transformation of coal into graphite-like carbon was conducted
by heat treatment in the presence of Fe. First, the TX-de and
Fe 2(SO 4) 3 [TX-de:Fe 2(SO 4) 3 = 16:12.6] was well-mixed by ball
milling for 2 min, and then the mixture was subjected to catalytic
graphitization (C-G) at 2400 °C for 2 h under argon, and the product
obtained was named TX-C-G. For comparison, TX was noncatalytically
graphitized (NC-G, without the additional catalyst) under similar
condition to give rise to the product defined as TX-NC-G. The
proximate analysis and ultimate analysis of TX and TX-de are listed in
Table 1. Both TX-C-G and TX-NC-G described above were oxidized
by Hummers method to get their corresponding graphite-like carbon
oxides, 18 TX-C-GO and TX-NC-GO, respectively.
2.2. Preparation of Coal-Derived Graphene by H 2 DBD
Plasma. Synthesis of chemically derived graphene was carried out in a
DBD reactor with H 2 as working gas at ambient atmosphere
pressure. 19 A weighted amount of TX-C-GO or TX-NC-GO powder
was put into a vertical quartz tube (10 mm in diameter, about 100 mm
in length of discharge) with porous plate in the middle. Prior to the
discharge, the reactor was purged with H 2 to exhaust the air in the
quartz tube. Afterward, the DBD plasma was initiated under 50 V ×
1.2 A input ac power at room temperature and atmospheric pressure
for 5 min; significant volume expansion can be observed in a flash. The
product GS derived from TX-C-GO and TX-NC-GO was named TX-
C-GS and TX-NC-GS, respectively.
2.3. Preparation of GS Decorated with Noble Metallic
Nanoparticles. GS decorated with noble metal nanoparticles (NP/
GS) were prepared as follows. 20 The TX-C-GO made as in section 2.1
with a high oxidation degree was used as the starting material for the
support of metal nanoparticles. As a typical procedure to synthesize
the NP/GS composite nanostructures, a certain proportion of TX-C-
GO and precursors of noble metal (H 2PtCl 6·6H 2O, RuCl 3·3H 2O)
were dispersed in ethanol with ultrasonication for 1 h. Subsequently,
the mixed solutions were vacuum-dried at 100 °C for 10 h. The assynthesized
solid powder mixtures were exposed in H 2 plasma with 50
V × 1.2 A ac input power for 10 min to simultaneously reduce the GO
and precursors of noble metal ions to produce NP/GS composites.
2.4. Electrochemical Measurement. For the electrochemical
measurement, the fabrication of working electrodes was carried out as
follows. Typically, the electroactive materials and polytetrafluoroethyene
(PTFE) binder (without any other carbon additive) were
mixed in a mass ratio of 9:1 and dispersed in ethanol, and the resulting
mixture was dried at 60 °C. After drying, it was pressed into a wafer of
8 mm diameter and assembled into an electrode with two pieces of
nickel foam substrate (10 mm in diameter).
The electrochemical properties of chemically derived graphene were
measured in a standard three-electrode system with a Pt sheet as
counter electrode and Hg/HgO electrode as reference electrode in a 6
M KOH aqueous electrolyte. CV curves (scanning rates varying from 2
to 100 mV/s) were measured with an electrochemical workstation
(CHI 660D). Galvanostatic charge/discharge measurements (current
density ranged from 50 to 1000 mA/g) were conducted with a
charge−discharge tester (Arbin BT2000), and the electrochemical
capacitances were obtained from charge−discharge curves.
2.5. SCR Catalytic Activity Measurement. SCR of NO with
NH 3 over the catalysts was carried out in a continuous-flow fixed-bed
quartz reactor under atmospheric pressure. Typically, 50 mg catalyst
was packed inside the reaction zone, the initial gas composition was
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400 ppm NO x (approximately 92% NO, 8% NO 2), 600 ppm NH 3,2%
O 2, and N 2 (balance), and a total gas flow rate of 100 mL/min (STP)
was used throughout this study. NO x concentrations were analyzed
using a flue gas analyzer (KM9106 Quintox, Kane International
Limited) online.
3. RESULTS AND DISCUSSION
The proximate analysis and ultimate analysis results of TX coal
and its demineralization coal TX-de are shown in Table 1. The
high carbon content (91.10%) and low content of volatile
matter (8.56%) suggest that the coal is a typical anthracite coal
with high metamorphic degree. After the treatment with
hydrofluoric acid and concentrated hydrochloride, the ash
content was decreased to 2.52 wt % from an initial value of
15.40 wt %, indicating that the mineral matter in coal has been
efficiently removed.
XRD patterns of samples, including coal-based graphite-like
carbons (TX-NC-G, TX-C-G), their corresponding oxides
(TX-NC-GO, TX-C-GO), and the final product graphene
sheets (TX-NC-GS, TX-C-GS) derived from the coal-based
graphite oxides after DBD plasma treatment, are shown in
Figure 1. There are significant differences in crystallite size
Figure 1. XRD patterns of coal-based graphite (TX-NC-G, TX-C-G),
coal-based graphite oxides (TX-NC-GO, TX-C-GO), and graphene
sheets (TX-NC-GS, TX-C-GS) from DBD plasma.
(La), stacking height (Lc), and degree of graphitization
between TX-NC-G and TX-C-G. The La and Lc of the
crystallite can be determined using the Scherrer equation 21
La = 1.84 λ/Bacos( φa)
Lc = 0.89 λ/Bccos( φc)
where λ is the wavelength of the radiation used, Ba and Bc are
the width of the (100) and (002) peaks, respectively, at 50%
height, and φa and φc are the corresponding scattering angles
or peak positions. The La of TX-NC-G and TX-C-G is 0.23 and
0.92 nm, and the Lc of TX-NC-G and TX-C-G is 0.27 and 0.48
nm, respectively. The graphitization degree (G) was calculated
by using the following equation
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Energy & Fuels Article
Figure 2. (a, b) Digital photos of samples before and after treatment with H 2 discharge plasma. SEM and TEM images of TX-NC-GS (c, d, e) and
TX-C-GS (f, g, h) with different magnification.
Figure 3. (a) FTIR spectra of coal-based graphite oxides and their chemically derived GS. (b) Nitrogen adsorption and desorption isotherms at 77 K
of GS from H 2 discharge plasma (inset: pore-size distribution). (c) Raman spectra of coal-based graphite, GO, and GS.
G = (0.3440 − d )/(0.3440 − 0.3354)
002
where d 002 is the interlayer spacing of (002) calculated from
XRD patterns. From the XRD patterns, it can be calculated that
the graphitizing degree of TX-C-G (91.98%) is much higher
than that of TX-NC-G (66.28%). It is well-known that the
diffraction peaks of (100), (004), and (110) will appear when
high graphitization degree was achieved, and these peaks can be
observed in the XRD pattern of TX-C-G rather than TX-NC-G.
Therefore, it can be concluded that the applied catalyst
substantially promoted graphitization of the coal.
After oxidation of the coal-based graphite, their sharp (002)
peaks around 26.2° basically disappeared, while the as-prepared
TX-NC-GO and TX-C-GO gave rise to (001) peaks located at
11°, corresponding to an increasing interlayer spacing from
0.335 to 0.749 nm, indicating that the TX-NC-G and TX-C-G
were efficiently oxidized and oxygen was bonded to their planar
surface. After DBD plasma treatment, (001) peaks of TX-NC-
GO and TX-C-GO disappeared almost completely to give rise
to TX-NC-GS and TX-C-GS, respectively, without any distinct
peaks in their XRD profiles.
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Figure 2a,b is the optical photos of coal-based graphite oxides
(TX-NC-GO, TX-C-GO) and their corresponding resultant
products (TX-NC-GS, TX-C-GS) after H 2 discharge plasma
under the same experimental conditions. Obviously, the volume
of TX-C-GS increased remarkably compared with its precursor
TX-C-GO, while the volume change from TX-NC-GO to TX-
NC-GS increased only slightly after the DBD plasma treatment.
It can also be observed that TX-C-GS is bulky and fluffy in
appearance while TX-NC-GS is relatively compacted. Taking
into consideration the higher graphitization degree of TX-C-G
compared with TX-NC-G, it can be concluded that the
expansion level of graphite oxide has a close relationship with
the graphitization degree of its precursor. In other words, the
high graphitization degree is of great benefit to the intercalation
of graphite by the oxygen-containing groups during oxidation.
The oxygen intercalated into the interlayer spacing of graphite
will be removed to form gases (H 2O and CO 2) during the
discharge process, and the yielding pressure overcomes the van
der Waals force between the layers, so that the oxides can be
exfoliated immediately during DBD plasma process. For the
low level graphitization carbon, the interaction hardly takes
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Energy & Fuels Article
Figure 4. XPS spectra C1s peaks of coal-based graphite oxides and graphene sheets: TX-C-GO (a), TX-C-GS (b), TX-NC-GO (c), and TX-NC-GS
(d).
place during the oxidation process; therefore, the driving force
resulting from gases (H 2O and CO 2) formed during DBD is
not strong enough to overcome interlayer van der Waals force
for complete expansion.
Typical morphologies of TX-NC-GS and TX-C-GS are
shown in Figure 2. It is revealed that layer structures were
formed after expansion via DBD plasma (Figure 2c,f). Highmagnification
SEM images (Figure 2d,g) of GS show a
wrinkled, thin, sheetlike structure similar to that of graphene
prepared from natural flake graphite by thermal expansion 22
and sonication-assisted chemical reduction. 9 TEM images
(Figure 2e,h) show that these sheet structures have plenty of
ripples and folded regions in their basal planes and seem to be
transparent, possibly owing to their being ultrathin or even
single sheets. It is obvious that the layer of TX-NC-GS appears
to be thicker than that of TX-C-GS due to the incomplete
exfoliation of the former. This is consistent with the volume
change observation during plasma-assisted exfoliation, as shown
in Figure 2a,b.
There are a variety of oxygen-containing groups on the
surface of TX-NC-GO and TX-C-GO (FTIR spectra; see
Figure 3a), which mainly include the C�O, C−O, and O−H.
After H 2 plasma reduction, the peaks for oxygen functional
groups were significantly reduced with fewer C�O groups left
in TX-NC-GS and TX-C-GS compared with their corresponding
precursors. Generally, the wide peak at 1100−1420 cm −1
could mainly be assigned to the �C−H stretch, and the peak
at 1583 cm −1 could be assigned to the aromatic C�C stretch.
According to the FTIR spectra of coal-based graphite oxides,
the C�C bond almost cannot be detected, while there is a
visible C�C peak located at 1583 cm −1 in their GS. The
typical peak of C�O located at 1731 cm −1 of TX-NC-GS is
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stronger than that of TX-C-GS, which may indicate that the
former has more oxygen-containing groups.
The coal-based graphene sheets were further characterized
via N 2 adsorption and desorption at 77 K for deeply
understanding their pore structure. According to the results,
the BET surface area values of TX-C-GS and TX-NC-GS are
306 and 135 m 2 /g, respectively. As shown in Figure 3b, the
isotherms have the characteristics of type IV with a type H3
hysteresis loop at relatively high pressure, revealing that these
materials are composed of aggregated sheets (loose assemblages)
possessing typical mesopore structures, corresponding
with distributions of the most probable pore sizes of 3.69 nm
(TX-NC-GS) and 4.02 nm (TX-C-GS), as shown in the inset
of Figure 3b and the microscopy observations mentioned
above.
The significant structural changes occurring during the
transformation from coal-based graphite to the GS are reflected
in their Raman spectra (Figure 3c). As known, the G band
corresponds to the first-order scattering of the E 2g mode
observed for sp 2 carbon domains, and the pronounced D band
is the disordered band associated with structural defects,
amorphous carbon, or edges that can break the symmetry and
selection rule. 23 The Raman spectra of TX-NC-G and TX-C-G
display a prominent G peak as the feature at 1581 cm −1 . The
strong 2D peak at 2663 cm −1 indicates that the Taixi coal has
been transformed into graphite-like carbon after graphitization
treatment. As expected, the I D/I G ratio of TX-C-G is smaller
than that of TX-NC-G, which indicates that TX-C-G possesses
higher graphitization degree than TX-NC-G, consistent with
the results obtained from XRD. From the Raman spectra of
coal-based graphite oxides (TX-NC-GO and TX-C-GO), it can
be seen that the G bands seem to be broadened compared with
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Figure 5. Electrochemical performance of coal-derived graphene sheets. (a) Cyclic voltammetry curves of GS with the scanning rates of 5 mV/s. (b)
Specific capacitance against different discharge current density with 6 M KOH solution as an electrolyte. (c) Galvanostatic charge/discharge curves
with the current density of 100 mA/g. (d) The cycling performance at a charge/discharge current of 100 mA/g.
their precursors, meanwhile, the D bands at 1337 cm −1 become
much more prominent, indicating the reduction in size of the
in-plane sp 2 domains, possibly due to the extensive oxidation. 10
The Raman spectra of the TX-NC-GS and TX-C-GS also
contain both G and D bands (at 1590 and 1350 cm −1 ,
respectively), however, with a decreased I D/I G intensity ratio
compared to that in TX-NC-GO and TX-C-GO, which
suggests an increase in the average size of the sp 2 domains
and restoration of sp 2 network during reduction of GO in H 2
discharge plasma process. 24,25 It should be noted that no
obvious 2D bands can be observed in the Raman spectra of TX-
NC-GS and TX-C-GS due to the disordered structure (defects,
vacancies, or distortions of the sp 2 domains) caused by the use
of strong acid (H 2SO 4) and strong oxidant (KMnO 4) during
the oxidation stage. We can reasonably believe that it is very
difficult for the graphene sheets to retrieve the graphitic
structures through DBD plasma treatment. Compared with TX-
NC-GS, the D band of TX-C-GS is weaker and wider, which
suggests fewer structural defects in the latter than the former.
As mentioned above, it can be concluded that the crystallinity
of coal-derived carbon remarkably affects the structures and the
properties of the graphene obtained from chemical exfoliation.
26
The oxygen-containing functional groups of TX-NC-GO,
TX-NC-GS, as well as TX-C-GO and TX-C-GS were
characterized by X-ray photoelectron spectroscopy (XPS).
The high-resolution C1s spectrum of the TX-C-GO (Figure
4a) and TX-NC-GO (Figure 4c) reveal that there are three
main components arising from C�C (aromatic rings), C−O
(alkoxy and epoxy), and C�O (carboxyl) groups. After DBD
plasma treatment, carbon with different chemical valences
remains for TX-NC-GS and TX-C-GS in XPS spectra; however,
the peak intensities of them are much smaller than those in TX-
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NC-GO and TX-C-GO. As shown in Figure 4b,d, there are no
O−C�O groups left in TX-NC-GS and TX-C-GS and the
C�C bonds become dominant, while the peak intensities of
C�O and C−O are obviously lower than that of TX-NC-GO
and TX-C-GO, which indicates that most of the oxygencontaining
groups in TX-NC-GO and TX-C-GO have been
removed and the majority of the conjugated graphene networks
are restored 27 by DBD plasma treatment for only 5 min. The
residual oxygen-containing groups on the graphene sheets will
benefit the wettability of the electrode in supercapacitor and
metal or metal oxide catalyst grafting for heterogeneous
catalysis. However, compared with TX-C-GS, it is worth noting
that there are more significant C�O and C−O peaks left in the
XPS spectrum of TX-NC-GS, which is consistent with the
FTIR results.
The electrochemical properties of these coal-derived
graphene were measured in a 6 M KOH aqueous electrolyte
with a three-electrode supercapacitor cell at room temperature
by a CHI 660D electrochemical workstation. Figure 5a shows
the CV measurement results of TX-NC-GS and TX-C-GS at
the scanning rate of 5 mV/s. The CV curve of TX-C-GS
exhibits a more typical rectangular shape than that of TX-NC-
GS, indicating that the TX-C-GS has a better capacitive
behavior and charge propagation at the surface of electrode
following the electric double layer charging mechanism
compared with TX-NC-GS. The specific capacitances of TX-
NC-GS and TX-C-GS against the discharge current density are
shown in Figure 5b, which was calculated from the
galvanostatic charge and discharge curves at different current
density, using the following equation 28
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Figure 6. TEM images of Pt/GS (a), Ru/GS (b), and PtRu/GS (c). (d) XRD patterns of NP/GS composites. (e) NO conversion as a function of
temperature over chemically derived graphene and noble NP/GS.
where I is charge or discharge current, Δt is the time for a full
charge or discharge, m indicates the mass of the active material,
and ΔV represents the voltage change after a full charge or
discharge. As shown in the Figure 5c, there are almost no
voltage drops observed; this indicates that the electrodes can
carry out fast charge/discharge under the experimental current
density and show good rate capability. 29,30 Moreover, lifecycle
is one of the most important performance indexes for
supercapacitor. The TX-C-GS and TX-NC-GS present the
feature of good cycling performance (as shown in Figure 5d)
tested using the galvanostatic charge−discharge technique. The
capacitance of them basically remains invariant during the 1000
cycles under the operation current density of 100 mA/g after
the unstable cycles at the beginning.
It can be found that the specific capacitance of TX-C-GS is
higher than that of TX-NC-GS at any specified discharge
current density used in the present experiment. This may be
attributed to the high-quality of graphene sheets from TX-C-
GS, which has more open structures and higher BET specific
surface area (306 vs 135 m 2 /g) as well as higher conductivity
compared with the TX-NC-GS. These features can significantly
improve the electrical double layer capacitance; thus, TX-G-GS
presented a higher specific capacitance and better electrochemical
performance.
Recently, graphene nanocomposites are receiving tremendously
increasing attention because they possess much more
novel properties compared with pure graphene and therefore
enable more promising potential applications. It is easy to
imagine that the GS can be used as catalyst support due to its
unique two-dimensional basal network plane structure and high
surface area, as well as the presence of surface functional groups
in chemically derived graphene. Noble metal nanoparticles (Pt,
Ru, and PtRu) decorated coal-based graphene sheet (TX-C-
GS) catalysts were successfully fabricated via impregnation and
DBD plasma technique. The loading amounts of Pt, Ru, and
5191
PtRu were kept constant (∼3.5 wt %), and the atomic ratio of
Pt:Ru in PtRu/GS was 3:1.
From the TEM images (Figure 6a−c), it can be clearly seen
that the exfoliated GS was decorated by uniform nanoparticles
less than 2 nm in size. It has been demonstrated that metal
nanoparticles can be deposited on both sides of GS due to their
large surface area and unique basal 2D planar structure, and
furthermore, the attachment of nanoparticles onto the GS
surface can significantly prevent their aggregation and
restacking. 31−33 The highly dispersed metal nanoparticles on
supports with large surface area are favorable to promote their
interfacial contact with the other reactant molecules; therefore,
they have potential advantages in catalytic activity and sensor
sensitivity. 34 The XRD patterns of the NP/GS composites are
shown in Figure 6d. The typical diffraction peaks of the facecentered
cubic (fcc) Pt lattice (111) and (200) in the assynthesized
Pt/GS composites can be clearly observed;
however, the peaks of Ru (Ru/GS) and PtRu alloy (PtRu/
GS) cannot be recognizable in XRD diffraction due to the small
size and low loading. 35,36
Selective catalytic reduction (4NO + 4NH 3 +O 2 =4N 2 +
6H 2O) has been wildly used for the removal of NO in the flue
gas from coal combustion. 37 The NH 3−SCR reaction activities
of GS, Pt/GS, Ru/GS, and PtRu/GS are shown in Figure 6e. It
can be seen that GS itself does not show any catalytic activity in
SCR, while high catalytic activities appeared after the
decoration of the graphene sheets with Pt and PtRu
nanoparticles. Although Ru/GS shows negligible catalytic
activity under the present reaction condition, PtRu/GS
bimetallic alloy catalyst shows the highest catalytic activity at
low temperature among the obtained graphene-based catalysts,
which is similar to the performance of the electrocatalytic
reaction in the fuel cell. 38 The light-off temperature (the
temperature at which the conversion of NO reaches 50%) for
this reaction is as low as 150 °C under present experiment
conditions, and more than 90% NO conversion can be achieved
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Energy & Fuels Article
at 165 °C. Moreover, the reaction temperature window of the
alloy catalyst is relatively wide, ranging from 150 to 230 °C with
80% NO x conversion. The excellent activity for PtRu/GS
catalyst is possibly due to the synergistic effect between Pt and
Ru caused by the Ru that entered into the lattice of Pt in the
PtRu alloy.
4. CONCLUSION
We demonstrate that coal-derived graphene can be successfully
prepared from graphitized coal with the graphite oxide pathway
combined with H 2 DBD plasma technique. Fe can serve as an
effective catalyst to promote the graphitization of coal, which is
critical to the structures, quality, and properties of the final
product of coal-derived graphene. Noble metallic nanoparticles
as small as 2 nm decorated on the surface of GS were
successfully fabricated with high uniformity and dispersity via
the impregnation and H 2 DBD plasma technique. The
electrochemical performance of the produced graphene from
coal presents a mild capacitance and excellent electrochemical
stability in KOH electrolyte. The noble metal/graphene
composites can be used as catalyst, of which PtRu/GS show
high catalytic activity and wide operating temperature window,
as evidenced in SCR of NO x with ammonia under atmospheric
pressure. Our synthesis strategy may lead to an alternative
approach to the preparation of graphene and graphene-based
composites with novel structures and various applications.
■ AUTHOR
INFORMATION
Corresponding Author
*E-mail: zbzhao@dlut.edu.cn (Z.Z.), jqiu@dlut.edu.cn (J.Q.).
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was partly supported by the NSFC (Nos. 51072028,
20876026, 20836002, 20725619) and the Fundamental
Research Funds for the Central Universities (no. DUT
11ZD120).
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dx.doi.org/10.1021/ef300919d | Energy Fuels 2012, 26, 5186−5192