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in Table 1, the Pt exposed surface area and dispersion of cata-
lyst also depend on the type of support. The Pt amounts of ex-
posed surface area and dispersion have the following order:
Pt/RGO>Pt/SiO2 >Pt/AC>Pt/CNTs>Pt/G. The catalyst with
greater exposed surface area and dispersion of Pt usually
shows higher conversion in hydrogenation reaction.[31,37]
Normally, the metallic catalyst with small metal particle size
possesses high dispersion and high catalytic activity. Why are
the Pt dispersion and the catalytic activity for Pt/G are so low?
The low catalytic activity of Pt/G catalyst may be due to the
high oxidation states of the metal catalyst on G support. As
shown in Figure S3, Pt4f spectra of Pt/G show that the main
valences of Pt are Pt2+ and Pt4+. However, the Pt0 is the active
site in the cellobiose conversion reaction (Scheme 1). The oxi-
dation state of Pt can be attributed to the reoxidation of
oxygen in air. The high oxidation state may be also the reason
that the Pt dispersion on Pt/G catalyst is so low. Compared
with Pt/AC, the Pt/CNTs catalyst with similar Pt dispersion has
better catalytic performance, which may be due to the follow-
ing reasons: Firstly, most pores in AC are micropores, and are
too small for cellobiose to enter.[32,38] Secondly, as in Figure 2,
CNTs with lots of surface functional groups formed by acid pre-
treatment have high sorbitol yield and low glucose yield, indi-
cating that the CNTs have higher catalytic activity than AC in
this reaction. With high Pt dispersion, Pt/SiO2 has the lowest
catalytic performance and this may be due to the same reason
as for Pt/AC. Therefore, the high catalytic activity is attributed
to the synergistic effects of supports and the supported Pt
nanoparticles. As in Figure 4, Pt loaded on RGO shows the
highest catalytic performance, which indicates that RGO is the
best support for the conversion of cellobiose to sorbitol.
Figure 4. Catalytic performance of Pt loaded on different supports for cello-
biose conversion to sorbitol. Reaction conditions: catalyst weight=0.050 g
(Pt nanoparticles, 0.0025 g); cellobiose weight=0.171 g; reaction time=3 h;
temperature=463 K; pressure=5 MPa; H2 as reaction gas.
method, the yield of sorbitol is improved from 41.9% to 91.5%
when RGO is used as the Pt support. Note that not all support
materials could improve the catalytic performance. From
Figure 4, it is clear that equal amounts of Pt loaded, the levels
of catalytic activity have the following order: Pt/RGO>Pt/
CNT>Pt nanoparticles> Pt/AC>Pt/SiO2 >Pt/G.
The XRD patterns of Pt nanoparticles loaded on different
supports are displayed in Figure S1. The strong diffraction
peaks at 2q=39.68, 46.28, and 67.58 can be assigned to the
characteristic (111), (200), and (220) crystalline planes of Pt. The
average crystallite sizes were calculated by the Scherrer equa-
tion at 39.68, which is shown in Table 1. Moreover, the mor-
phology of Pt nanoparticles and Pt nanoparticles loaded on
different supports was investigated by TEM. As in Figure S2,
the dispersion and particle sizes of Pt are dependent on sup-
port type. The particle size distribution in Figure S2 and Table 1
shows that the average size of Pt nanoparticles is about
3.5 nm for Pt nanoparticles, 3.6 nm for Pt/AC, 3.1 nm for Pt/
CNTs, 1.2 nm for Pt/G, 3.6 nm for Pt/RGO, and 3.1 nm for Pt/
SiO2. The dispersion and exposed surface area of the Pt nano-
particles were determined by chemisorption of CO. As shown
The effect of Pt particle size on the catalytic performance
To disclose the effect of Pt particle size on the catalytic activity,
Pt/RGO catalysts with different Pt particle sizes were prepared
through the same reduction method. In this microwave-assist-
ed method, Pt particle size can be controlled with different
temperature treatments. The XRD patterns of GO and Pt/RGO
after different heat treatments are shown in Figure 5. It can be
seen that the typical diffraction peak (002) of GO at 11.68 shifts
to higher angle after the loading of Pt nanoparticles on RGO
with a treatment temperature above 403 K; this indicates that
the GO is converted to RGO. The strong diffraction peaks at
2q=39.68, 46.28, and 67.58 can be assigned to the characteris-
tic (111), (200), and (220) crystalline planes of Pt, respectively.
Pt possesses face-centered-cubic (fcc) structure. The diffraction
peak of Pt (111) is used to estimate the Pt particle size by the
Scherrer equation. The calculated average particle sizes of Pt
on GO sheets are 2.0 nm for Pt/RGO-403 K, 2.7 nm for Pt/RGO-
413 K, 3.6 nm for Pt/RGO-433 K, and 4.3 nm for Pt/RGO-453 K.
Figure 6 shows the TEM images and Pt particle size distribu-
tions of Pt/RGO-T. All TEM images demonstrate that graphene
nanosheets are uniformly decorated by distributed Pt nanopar-
ticles with few aggregations, indicating a strong interaction be-
tween the graphene support and Pt particles. The mean size of
Table 1. The physicochemical properties of different catalysts.
Catalyst Exposed Pt surface Pt dispersion[a] Particle size[b] Crystallite sizes[c]
area[a] [m2 gÀ1
]
[%]
[nm]
[nm]
Pt/SiO2 65.2
26.4
13.7
12.8
3.5
3.1
3.6
3.1
1.2
3.6
2.9
7.7
4.0
–
3.6
Pt/AC 33.9
Pt/CNTs 31.6
Pt/G 8.7
Pt/RGO 79.2
[d]
32.3
[a] The surface area was determined by CO chemisorption. [b] The average
size of nanoparticles was evaluated from counting and averaging TEM
images. [c] The average crystallite sizes were calculated from XRD pattern
by using the Scherrer equation at 39.68. [d] The diffraction peak was too
low to estimate.
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ChemSusChem 2014, 7, 1398 – 1406 1401