Graphene Sheets from Graphitized Anthracite Coal: Preparation ...

Energy & Fuels Article
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
dx.doi.org/10.1021/ef300919d | Energy Fuels 2012, 26, 5186−5192