Platinum Nanosheets between Graphite Layers

Article abstract of DOI:10.1002/tcr.201800098

Two-dimensional platinum nanosheets were obtained by the hydrogen reduction of platinum tetrachloride intercalated between graphite layers, the latter was prepared by the treatment of the mixture of platinum tetrachloride and graphite powder under chlorine atmosphere. The size of platinum nanosheets was 1–3 nm in thickness with a width in the range of 5–300 nm. These nanosheets contain a number of hexagonal holes and edges with an angle of 120°. This review discusses the reduction and oxidation behavior of platinum species and structure of platinum nanosheets between graphite layers. The selective hydrogenation catalyzed by these platinum nanosheets entrapped between the graphite layers is also demonstrated.

Full text of DOI:10.1002/tcr.201800098

Personal Account
DOI: 10.1002/tcr.201800098
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T H E
C H E M I C A L
Platinum Nanosheets between Graphite
Layers
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R E C O R D
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Masayuki Shirai*[a]
Chem. Rec. 2018, 18, 1–10
© 2018 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
Abstract: Two-dimensional platinum nanosheets were obtained by the hydrogen reduction of
platinum tetrachloride intercalated between graphite layers, the latter was prepared by the
treatment of the mixture of platinum tetrachloride and graphite powder under chlorine
atmosphere. The size of platinum nanosheets was 1–3 nm in thickness with a width in the range
of 5–300 nm. These nanosheets contain a number of hexagonal holes and edges with an angle of
1208. This review discusses the reduction and oxidation behavior of platinum species and
structure of platinum nanosheets between graphite layers. The selective hydrogenation catalyzed
by these platinum nanosheets entrapped between the graphite layers is also demonstrated.
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Keywords: Nanoparticles, Intercalations, Layered compounds, Hydrogenation
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1. Introduction
The above reports show that the formation of metal particles
13 Graphite has a layered structure consisting of a regular
14 hexagonal network of carbon atoms which are conjugated
15 with sp2 hybrid orbitals. A variety of chemical substances can
16 be inserted into the interlayer spaces of graphite to produce
17 graphite intercalated compounds (GICs), because graphite
18 layers can accept or donate electrons from the substrates and
19 the interaction between graphite layers is of weak van der
20 Waals force.[1] Metal chloride can be intercalated between
21 graphite layers (MClx-GIC) by the treatment of metal
22 chloride and graphite powder under vacuum or high-pressure
23 chlorine.[1]
between graphite layers had small or plate-like structure with
1–10 nm sizes. There are few reports on the formation of
large two-dimensional metal sheets with nanometer size
thickness between graphite layers.
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The reduction of MClx-GIC is expected to produce
25 nanometal particles intercalated between two-dimensional
26 graphite layers. Several reports are available on the formation
27 of metal particles located in graphite layers e.g. Klotz and
28 Schneider reported that Fe metal between graphite layers
29 could be prepared by the reduction of iron chloride (III)
30 intercalated between graphite layers by sodium in liquid
31 ammonia.[2] Vol’pin et al. also reported that entrapping
32 transition metal particles (Co, Ni, Mn, Cu, Mo, and Cr)
33 between graphite layers could be achieved by the reduction of
34 the corresponding metal chloride intercalated between graph-
35 ite layers.[3] Graphite-intercalated metal particles (Fe, Co, Cu,
36 Ni, and Pd) commercially available in 1980’s (Graphimet)
37 were prepared by the treatment of MClx-GIC with lithium
38 biphenyl at 223 K under helium atmosphere. Graphimets
39 were used as solid catalysts; however, their activities were not
40 so high compared with the conventional catalysts, because
41 active metal sites were encapsulated between graphite layers
42 to which substrate should have an easy access for adsorp-
43 tion..[4–9] Walter et al. reported that small and plate-like Pd
44 metal particles inserted into the graphite layers could be
45 obtained by the reduction of palladium chloride (II)
46 intercalated graphite powder in hydrogen atmosphere.[10,11]
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Scheme 1. Hydrogenation of cinnamaldehyde (CAL).
The preparation, structure and catalysis of two-dimen-
sional platinum nanosheets between graphite layers are
reported in this review. The platinum nanosheets were
prepared by the hydrogen reduction of platinum chloride
intercalated between graphite layers (PtClx-GIC). The size of
platinum nanosheets were 1–3 nm in thickness with 5–
300 nm width. There are a number of hexagonal holes and
edges with 1208 angle, which were formed during reduction
of platinum chloride intercalated between graphite layers by
hydrogen atmosphere. The platinum nanosheets intercalated
between graphite layers (Pt-GIC) showed selective hydro-
genation activity, especially, the selectivities over platinum
nanosheets sandwiched between graphite layers were different
from those over platinum metal particles supported on
graphite layers.
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[a] Prof. M. Shirai
Faculty of Science and Engineering
Iwate University
4-3-5 Ueda, Morioka, Iwate 020-8551
52 E-mail: mshirai@iwate-u.ac.jp
Chem. Rec. 2018, 18, 1–10
© 2018 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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2. Experimental
2.2. Hydrogenation Catalysis of Platinum Nanosheets
2.1. Preparation of Platinum Nanosheets
The catalytic performance of Pt-GIC in organic solvents was
studied in a batch system using stainless-steel reactor (50 ml
capacity). For the hydrogenation in supercritical carbon
dioxide solvent also, a batch system of stainless-steel reactor
was adopted.[14,15] The weighed amounts of catalyst and the
reactant were placed in a reactor and flushed twice with
carbon dioxide. After the required temperature was attained
with a hot air circulating oven, hydrogen and carbon dioxide
were introduced into the reactor. The carbon dioxide pressure
values were determined by the subtraction of hydrogen
pressure from the total pressure. After the appropriate
reaction time, the pressure was released slowly and the
contents were discharged to separate the catalyst by simple
filtration. The products dissolved in acetone were analyzed by
a gas chromatograph.
The treatment with chlorine gas under high pressure and
high temperature conditions is needed for the insertion of
metal chloride into graphite layers. Figure 1 shows the
apparatus for the preparation of GICs, which is made of SUS
tubes and a quartz reactor.
2. Platinum Nanosheets Between Graphite Layers
2.1. Intercalation of Platinum Chloride
Figure 2 shows X-ray diffraction patterns of graphite (KS6
purchased from TIMCAL TIMREX) and the prepared
sample. New peaks at 2q=10.08, 14.68, and 20.38 were
observed for 5PtClx-GIC (Figure 2(b)) in addition to the
characteristic peaks of graphite structure (Figure 2(a)).
Figure 1. Apparatus for preparation of GICs with high pressure chlorine.
The connection between a SUS tube and a quartz reactor
28 was sealed with a Teflon packing. The mixture of graphite
29 powder and platinum chloride (Aldrich, 99.99%) was mixed
30 under high pressure of 0.3 MPa of chlorine to prepare
31 platinum chloride intercalated graphite compound (PtClx-
32 GIC).[12,13] The platinum loading in weight percent is
33 indicated by a value prefixed before the sample name, viz.
34 5PtClx-GIC shows 5 wt% of platinum metal is loaded in the
35 sample.
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The platinum chloride between graphite layers was
37 reduced by 40 kPa of hydrogen at 423–773 K in a batch
38 system to produce platinum metal intercalated between
39 graphite layers (Pt-GIC).
Figure 2. XRD patterns of graphite(a) and 5PtClx-GIC(b).
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These new three peaks are in good agreement with the
diffraction peak positions calculated for (002), (003), and
Masayuki SHIRAI received Ph.D. at University of Tokyo in 1993. After working as an associate professor at
Tohoku University and as a team leader at National Institute of Advanced Industrial Science and Technology
(AIST), he has been a full professor of Iwate University since 2013. His research interest is green chemistry
using supported metal catalysts in carbon dioxide and/or water solvents under high-pressure including
supercritical conditions.
Chem. Rec. 2018, 18, 1–10
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(004) reflections from the repeat distance along the c axis
(c=1.76 nm), which are at 2q=10.08, 15.18, and 20.28,
respectively, indicating that the 5PtClx-GIC sample has a
stage three structure, in which one intercalant layer of
0.75 nm exists between every three graphite layers
(0.336 nm). Platinum tetrachloride, which has a layered
structure,[16] would be intercalated in the graphite layer. A
proposed structure of PtClx-GIC is shown in Figure 3.
Figure 4 shows a TEM image of 5PtClx-GIC, in which small
10 platinum metal particles are formed. The PtClx-GIC samples
11 are not stable in methanol as platinum chloride from graphite
12 dissolved in methanol. The image was obtained with a TEM
13 mesh which was prepared by direct sprinkling of platinum
14 chloride powder. The TEM image shows that small platinum
15 metal particles, which would be formed by the reduction of
16 platinum chloride under the irradiation of electron beam
17 during TEM observation.[17] Figure 4 shows that platinum
18 chloride species were inhomogeneously dispersed. Platinum
19 chloride species would not be homogeneously intercalated
20 and be eccentrically located between the two-dimensional
21 graphite layers, then the platinum metal particles formed by
22 the irradiation would be located in the graphite layers.
Figure 5. The XRD peak intensities of graphite at 26.68 (a) and PtClx-GIC
at 10.08 (b).
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becomes weaker. No other peaks except for the three peaks
and graphite peaks were observed indicating that the PtClx-
GIC samples have two domains; one is “pure” graphite matrix
and the other platinum chloride intercalated graphite with
the stage three structure. The latter should increase with an
increase in the amount of platinum chloride inserted.
2.2. Platinum Nanosheets
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Figure 6 shows TEM images of the 5Pt-GIC sample obtained
by JEOL JEM-2100 (200 keV). XMA analysis confirmed
that the dark images were composed of platinum. Figur-
es 6(a)-(c) are TEM images of 5Pt-GIC. Several rod-like
platinum images in parallel are seen in Figure 6(a) while,
sheet-like platinum images with hexagonal holes are seen in
Figure 6(c). The TEM analysis with the rotation of the Pt-
GIC sample against electron beam confirmed that the rod-
like images shown in Figure 6(a) was a side view and (c) was a
top view of the platinum nanosheets intercalated between
graphite layers. The platinum sheets are arranged in parallel
with graphite layers (interlayer distance of 0.335 nm) in
Figure 6(b), which is the magnified view of Figure 6(a). Void
spaces are also observed at the edge of the platinum sheets.
Figure 6(d) is a drawing of the top view of platinum sheet.
The AB side of the hexagonal holes in the platinum sheet are
straight and parallel to each other. The BC and AC sides in
the hexagonal holes and edges at the sheet are also parallel to
Figure 3. A proposed structure of PtClx-GIC.
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Figure 4. A TEM image of 5PtClx-GIC.
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Figure 5 shows the intensities of a peak at 2q=26.68
49 (graphite (002)) and a peak at 2q=10.08 of the PtClx-GIC
50 samples as a function of platinum loadings. With an increase
51 in amount of platinum tetrachloride inserted, three peaks
52 become stronger, while the diffraction for (002) of graphite
Chem. Rec. 2018, 18, 1–10
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Figure 7. TEM image of 10Pt-GIC.
sional platinum clusters (platinum nanorods) are formed in
the channels of carbon atoms.[20] For the aggregation of
platinum metal species between two-dimensional graphite
layers, the interaction of graphite layer is not so high, hence
the platinum nanosheets could enlarge the space between
graphite layers.
The structure of the graphite layers would reflect on the
peculiar structure (holes an edges) of the platinum nano-
sheets. Platinum atoms or platinum chloride molecules or
both would migrate along with a regular hexagonal net of
carbon atoms during hydrogen reduction at 573 K. Move-
ment during the reduction would determine the morphology
of the resulting platinum nanosheets, which have hexagonal
holes.
The chlorine oxidation and hydrogen reduction behaviors
were studied by the treatment of Pt-GIC with chlorine.[21]
5Pt-GIC was treated with 0.3 MPa of chlorine at 723 K for
1 day to obtain an oxidized sample (5PtClx-GIC2) and
5PtClx-GIC2 was reduced at 40 KPa of hydrogen at 573 K
to produce a reduced sample (5Pt-GIC2). The three peaks at
2q=10.08, 14.68, and 20.38 were also observed in the X-ray
diffraction patterns of 5PtClx-GIC2 and their intensities were
almost the same as that of 5PtClx-GIC, indicating that
platinum metal nanosheets were oxidized and platinum
chloride species were formed between graphite layers with
third stage structure. TEM images for 5Pt-GIC2 showed that
platinum nanosheets with hexagonal holes were also formed.
These results show that hydrogen reduction of platinum
chloride to platinum nanosheets and chloride oxidation of
Figure 6. TEM images of 5Pt-GIC.
40 each other and the angles between two straight sides from
41 three AB, BC, and AC directions are at 1208.
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43 presence of platinum nanosheets which have a number of
44 hexagonal holes and edges at angles of 1208 (Figure 7). The
45 size and thickness of the platinum nanosheets did not depend
46 on the amount of platinum intercalated in the Pt-GIC
47 samples.
TEM images of 10Pt-GIC samples also showed the
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When platinum particles are formed in the pores of
49 supports, the morphology of the metal particles becomes the
50 same as that of pores (a template method). For example,
51 platinum nanowires are formed in the uniform mesotubes of
52 folded-sheet mesoporous material FSM-16.[19] One-dimen-
Chem. Rec. 2018, 18, 1–10
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platinum nanosheets to platinum chloride were reversible
between PtClx-GIC and Pt-GIC samples.
for Pt-GIC. For the application of Pt-GIC as solid catalysts,
the active sites would be edges of the platinum sheets
sandwiched between graphite layers. Compared with graph-
ite-supported platinum catalysts (Pt/G), in which small
platinum metal particles are located on graphite surface, the
number of active sites of platinum nanosheets would be
much smaller than that of small platinum metal particles on
surface, then higher activities of Pt-GIC would not be
expected.
To study the chlorine oxidation process of platinum
nanosheets, 5Pt-GIC was treated under 0.3 MPa of chlorine
from room temperature to 473 K at the rate of 10 K/min.
TEM image of the sample is shown in Figure 8. In addition
to platinum metal sheets, small platinum particles, which are
similar to those observed in the 5PtClx-GIC sample (Fig-
ure 4), are also observed. A part of platinum nanosheets
10 would be oxidized to platinum chloride species, which were
11 reduced to metal by the irradiation of electron beam during
12 TEM observation. Besides the nanosheets, small platinum
13 metal particles are also formed (Figure 8), which indicates
14 that chlorine would intercalate between graphite layers and
15 oxidize the nanosheets from the edge side. On the other
16 hand, small platinum metal particles are also observed in the
17 hexagonal holes and above (or below) of the nanosheets. One
18 probable explanation for the oxidation of nanosheets from
19 the inner side by chlorine is that voids would exist in a
20 graphite layer (graphene) and chlorine molecules would pass
21 through the voids and oxidize from the inner side of platinum
22 sheets. The existence of the voids would be predicted by the
23 reversibility of PtClx-GIC and Pt-GIC. Platinum nanosheets
24 with 1–3 nm thickness could not be produced by the
25 agglomeration of platinum atoms from one intercalant
26 platinum chloride layer with third stage structure during
27 reduction. Many platinum atoms would pass through the
28 voids and aggregate to form platinum nanosheets with 1–
29 3 nm thickness during the hydrogen reduction process. In the
30 chlorine oxidation process, platinum chloride species derived
31 from the nanosheets would pass through the voids to produce
32 third stage structure.
On the other hand, it is expected that Pt-GIC would
show differential selectivities compared with Pt/G. For the
hydrogenation of planer unsaturated compounds over Pt-
GIC, the planer molecules would enter into the two-dimen-
sional expanded graphite layers and adsorb on the edge of the
platinum nanosheets and be hydrogenated. The adsorption
structure of the planer molecules to nanosheets in the two-
dimensional space would be different from that to platinum
particles on graphite surface, indicating that Pt-GIC would
show different selectivities from Pt/G.
3.1. Phenylbenzene Hydrogenation
Benzene hydrogenation activities over 6.3Pt-GIC were
compared with those over a commercially available charcoal
supported platinum catalyst (5Pt/C) in ethanol solvent at
313 K under 4.0 MPa of hydrogen.[22] Cyclohexane was the
main product under the reaction conditions; however, the
activities were different for 6.3Pt-GIC and 5Pt/C catalysts.
5Pt/C showed 33 and 42% conversion after 3 and 15 hours,
respectively. On the other hand, the activity of 6.3Pt-GIC
was very low at 0.4% for 3 hours. The conversion did not
show any appreciable increase with an increase in reaction
time and only 2.4% conversion was obtained over 6.3Pt-GIC
after 15 hours. This result indicates that Pt-GIC is not active
for the aromatic ring hydrogenation in ethanol.
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3. Hydrogenation Catalysis of Pt-GIC
36 Platinum nanosheets, which have 1–3 nm thickness and 5–
37 300 nm width, are located between expanded graphite layers
The hydrogenation of phenylacetylene (PA), which has
two kinds of unsaturated bonds (aromatic ring and carbon-
carbon triple bond), was studied over 6.3Pt-GIC and
commercially available supported platinum catalysts (5Pt/C
and 5 wt% alumina-supported platinum (5Pt/Al2O3, WAKO
Chemicals)) in ethanol solvent at 313 K (Table 1).[22]
Three catalysts showed 100% conversion and the main
product was ethylbenzene (EB) and minor ethylcyclohexane
(EC), indicating that the hydrogenation rate of ethynyl group
was faster than that of the aromatic ring under the present
reaction conditions. Especially, the EC selectivity over 6.3Pt-
GIC was much lower by more than one order of magnitude
compared with the other two supported Pt catalysts for 3 h.
After 15 h, the amounts of EC increased while, that of EB
slightly decreased, indicating that only a small fraction of EB
formed was further hydrogenated to EC. The results of
phenylacetylene and benzene hydrogenation showed that the
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Figure 8. TEM image of 5Pt-GIC treated under 0.3 MPa of chlorine at from
room temperature to 473 K at the rate of 10 K/min.
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P e r s o n a l A c c o u n t
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Table 1. Hydrogenation of phenylacetylene for Pt-GIC and
conventional supported Pt catalysts [a].
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4
catalyst
time/ conversion/ product/mmol
h
%
ethylbenzene ethyl-
5
cyclohexane
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10
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16
6.3Pt-
GIC
5Pt/
Al2O3
5Pt/C
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3
100
100
100
100
100
100
8.97
8.90
8.74
8.62
8.55
8.36
0.027
0.10
0.26
0.38
0.45
0.64
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[a] phenylacetylene 9.0 mmol, 6.3Pt-GIC 38 mg, 5Pt/Al2O3
50 mg, 5Pt/C 50 mg. Reaction temperature 313 K, H2 4.0 MPa,
ethanol solvent 4 cm3.
17 aromatic rings are difficult to be hydrogenated over Pt-GIC
18 compared with the conventional supported Pt particles on the
19 surface. The ethynyl group of phenylacetylene was hydro-
20 genated with Pt-GIC, and so the two substrates, phenyl-
21 acetylene and benzene can get an access to active sites of
22 platinum nanosheets between graphite layers. However, the
23 space between the graphite layers is limited and the periphery
24 of the Pt nanosheets should be active sites; hence aromatic
25 ring would be adsorbed in a mode of mono- or di- s-
26 adsorption while, the adsorption with the p-ring parallel to
27 the Pt surface is unlikely to occur. The attack of hydrogen
28 atoms to the aromatic rings is restricted from the side of p-
29 ring because of the characteristic structure of Pt-GIC and
30 they are difficult to be hydrogenated through such a side
31 attack of hydrogen. In contrast, such effects are unlikely with
32 the carbon-carbon triple bonds.
Figure 9. Cinnamaldehyde hydrogenation over 5Pt-GIC (a) and 5Pt/Gmix
(b) under 5 MPa of hydrogen in n-heptane at 373 K (red: cinnamyl alcohol
(UOL), blue: hydrocinnamaldehyde (SAL), and green: hydrocinnamyl
alcohol (SOL), brown: b-methyl styrene (MS), purple: propylbenzene (PB),
and black: cinnamic acid (CA)).
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3.2. Cinnamaldehyde Hydrogenation
36 Commercially important selective hydrogenation of cinna-
37 maldehyde (CAL) was studied over Pt-GIC in n-heptane and
38 supercritical carbon dioxide (scCO2) solvents. For compar-
39 ison, spherical platinum metal particles on graphite surface
40 were prepared by hydrogen treatment of the mixture of solid
41 chloroplatinic acid hexahydrate and graphite powder (Pt/
42 Gmix) and its activities were studied.
UOL was the main product and the UOL selectivity was
almost constant at around 60% and independent of reaction
rate up to 60 min. The selectivity to dehydroxylated product
of MS and PB was 24% in 60 min. Figure 9(b) shows the
hydrogenation activity over 5Pt/Gmix, which showed high
CAL activity; however, lower UOL selectivity (33%) was
observed over 5Pt/Gmix. The selectivity to dehydrogenated
by-products (MS and PB) over 5Pt/Gmix was higher (50%)
as compared with that over the Pt-GIC sample. The lower
CAL conversion over 5Pt-GIC shows that the lower number
of exposed platinum metal sites of platinum nanosheets
existed between graphite layers. Spherical platinum metal
particles are exposed on graphite surface and CAL molecules
easily accessed the active sites over 5Pt/Gmix. It was hardly
possible to determine the number of Pt atoms located at the
edge of Pt nanosheets; however, it is not difficult to project
that the number of edge sites in Pt nanosheets between
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Figure 9 shows the conversion and the amounts of
44 products for the CAL hydrogenation over 5Pt-GIC and 5Pt/
45 Gmix in n-heptane solvent.[23] Both the catalysts showed
46 hydrogenation activities; however, the hydrogenation rate is
47 not so high over 5Pt-GIC. Hydrocinnamaldehyde (SAL),
48 cinnamyl alcohol (UOL) and hydrocinnamyl alcohol (SOL)
49 were formed after 30 minutes and dehydroxylation products
50 of b-methyl styrene (MS) and small amount of propylbenzene
51 (PB) were formed after 45 min.
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graphite layers (Pt-GIC) would be much smaller than that of
surface atoms of platinum particles on graphite (Pt-Gmix).
The turn number of nanosheets in Pt-GIC may be larger
than that of small particles on graphite in Pt-Gmix.
5Pt-GIC showed higher UOL selectivity compared with
that shown by 5Pt/Gmix. For similar conversion of around
25% (in 60 min over Pt-GIC and 15 min over Pt/Gmix), the
UOL selectivity over Pt-GIC (56%) were also higher than
that over Pt/Gmix (33%). The higher COL selectivity over
4. Summary and Outlook
Platinum nanosheets were synthesized by the hydrogen
reduction of platinum tetrachloride intercalated between
graphite layers. The dimensions of platinum nanosheets were
1–3 nm in thickness with 5–300 nm width. There are a
number of hexagonal holes and edges with 1208 angle in the
platinum nanosheets. The hydrogenation performance (activ-
ity and selectivity) of platinum nanosheets between graphite
layers were different from those of spherical platinum metal
particles supported on graphite layers.
Several noble metal sheets with nanosize thickness could
be synthesized by the reduction of the corresponding noble
metal chloride intercalated between graphite layers. Also,
bimetallic nanosheets could be prepared by the reduction of
binary metal chlorides intercalated between graphite layers.
The noble metal nanosheets and bimetallic nanosheets would
show the different catalysis from those of supported mono-
metal and bimetallic particles.
10 5Pt-GIC shows that the preferential adsorption of carbonyl
11 group of CAL molecule takes place on the exposed Pt metal
12 sites on nanosheets intercalated graphite layers because the
13 adsorption of C=C group of CAL molecule was hindered by
14 the steric factors between phenyl group and graphite layers in
15 the 5Pt-GIC samples. Such hindrance was not possible in the
16 hydrogenation over 5Pt/Gmix because Pt metal particles are
17 arranged on the graphite layers and selectivity to SAL became
18 high and SOL was also formed over 5Pt/Gmix samples. Also,
19 the amounts of dehydroxylated by-product of MS and PB
20 were formed to the larger extent over Pt/Gmix.
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Figure 10 shows the hydrogenation profile over 5Pt-GIC
22 and 5Pt/Gmix under 5 MPa of hydrogen and 10 MPa of
23 scCO2 at 323 K.[18] Both 5Pt-GIC and 5Pt/Gmix catalysts
24 showed higher hydrogenation activities in scCO2 than in n-
25 heptane at 373 K, especially the activity in scCO2 was much
26 higher than in n-heptane over 5Pt-GIC. The higher activities
27 in scCO2 could be explained by the higher hydrogen
28 concentration on platinum sites than in n-heptane under the
29 same hydrogen pressure at 323 K. Hydrogen is completely
30 miscible with scCO2, hence, higher amount of hydrogen
31 atoms would be available on the platinum metal surface in
32 scCO2 and hydrogenation would proceed on both over Pt-
33 GIC and Pt/Gmix catalysts. It is reported that graphite layers
34 could be exfoliated by scCO2 treatment,[24] indicating that
35 carbon dioxide molecules enter into graphite layers. Carbon
36 dioxide molecules under supercritical conditions may enter
37 into graphite layers and also carry CAL molecules to platinum
38 nanosheets between the layers, resulting in higher hydro-
39 genation activities over 5Pt-GIC in scCO2 than in n-heptane.
40 The high UOL selectivity was also obtained over 5Pt-GIC in
41 scCO2. The UOL selectivity was almost constant at 77% and
42 independent of reaction rate up to 240 min over 5Pt-GIC.
43 Dehydroxylation products (MS and PB) were not formed at
44 the beginning of the reaction. Total selectivity for hydro-
45 genated compounds (UOL+SAL+SOL) were more than
46 98% at 360 min. Over Pt/Gmix, the UOL selectivity values
47 were lower than 50% and large amount of the dehydrox-
48 ylation byproducts (MS and PB) was formed.
Figure 10. Cinnamaldehyde hydrogenation over 5Pt-GIC (a) and 5Pt/Gmix
(b) under 5 MPa of hydrogen in 10 MPa of supercritical carbon dioxide
solvent at 323 K (red: cinnamyl alcohol (UOL), blue: hydrocinnamaldehyde
(SAL), and green: hydrocinnamyl alcohol (SOL), brown: b-methyl styrene
(MS), purple: propylbenzene (PB), and black: cinnamic acid (CA)).
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Chem. Rec. 2018, 18, 1–10
© 2018 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Acknowledgements
This work was supported by Grand-in-Aid (18H01782) for
Scientific Research B from Ministry of Education, Culture,
Sports, Science and Technology of Japan.
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Manuscript received: July 2, 2018
Version of record online: November 20, 2018
Chem. Rec. 2018, 18, 1–10
© 2018 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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PERSONAL ACCOUNT
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Prof. M. Shirai*
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Platinum Nanosheets between
Graphite Layers
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In this account, we discuss the prepara-
tion and structure of two-dimensional
platinum nanosheets intercalated between
graphite layers. The size of platinum
nanosheets was 1–3 nm in thickness with
a width in the range of 5–300 nm. These
nanosheets contain a number of
hexagonal holes and edges with an angle
of 1208. The selective hydrogenation
catalyzed by these platinum nanosheets
entrapped between the graphite layers is
also demonstrated.