Platinum Nanosheets Intercalated into Natural and Artificial Graphite Powders

Article abstract of DOI:10.1002/asia.202100504

Insertion of sheet-type platinum particles (platinum nanosheets) between graphite layers was achieved by a thermal treatment of a mixture of platinum chloride (IV) and graphite powder (natural graphite or artificial graphite) under 0.3 MPa of chlorine at 723 K, followed by the treatment under 40 kPa of hydrogen pressure. Similar platinum nanosheets, which were 1–3 nm in thickness and 100–500 nm in width and had a number of hexagonal holes and edges with 120° angle, were formed between the layers of both natural graphite or artificial graphite; however, their location in the graphite layers depended on the type of graphite used. A number of platinum nanosheets were observed in the edge region of natural graphite particles which have flat surface. On the other hand, a number of platinum nanosheets were found inside and away from the edge of the artificial graphite particles especially in the vicinity of the cracks. Both the platinum nanosheet-containing artificial and natural graphite samples showed high selectivity to cinnamyl alcohol in cinnamaldehyde hydrogenation under supercritical carbon dioxide conditions, while spherical platinum particles, which were located on the surface of natural and artificial graphite, showed lower selectivity.

Full text of DOI:10.1002/asia.202100504

CHEMISTRY
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Accepted Article
Title: Platinum Nanosheets Intercalated Natural and Artificial Graphite
Powders
Authors: Masayuki Shirai, Kohei Kubo, Mika Sodeno, and Hidetaka
Nanao
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Platinum Nanosheets Intercalated Natural and Artificial Graphite
Powders
[a]
[a]
[a]
[a]
Masayuki Shirai*, Kohei Kubo, Mika Sodeno, and Hidetaka Nanao
This manuscript dedicates Prof. Andy Hor for his 65th birthday.
[a]
Prof. Dr. M. Shirai, K. Kohei, M. Sodeno, Dr. H. Nanao
Chemistry course, Faculty of Science and Technology
Iwate University
4-3-5, Ueda, Morioka,
E-mail: mshirai@iwate-u.ac.jp
Abstract: Insertion of sheet-type platinum particles (platinum
nanosheets) between graphite layers was achieved by a thermal
treatment of a mixture of platinum chloride (IV) and graphite powder
disk-shaped iridium particles with 1-3 nm thickness and 1-14 nm
[5]
diameter (iridium nanodisks) , and spherical palladium particles
with 3 nm size (palladium nano particles)[6] between the layers of
artificial graphite by the insertion of the corresponding noble metal
chloride between layers and following the hydrogen reduction.
However, the location of platinum nanosheets in the graphite
particles is not yet well understand. We also have reported that
the noble metal particles between graphite layers showed high
selectivity and activity than those on graphite layers for
cinnamaldehyde (CAL) hydrogenation. Platinum nanosheets
were highly selective to cinnamyl alcohol (COL) in supercritical
(natural graphite or artificial graphite) under 0.3 MPa of chlorine at 723
K, followed by the treatment under 40 kPa of hydrogen pressure.
Similar platinum nanosheets, which were 1-3 nm in thickness and 100
-
500 nm in width and had a number of hexagonal holes and edges
with 120º angle, were formed between the layers of both natural
graphite or artificial graphite; however, their location in the graphite
layers depended on the type of graphite used. A number of platinum
nanosheets were observed in the edge region of natural graphite
particles which have flat surface. On the other hand, a number of
platinum nanosheets were found inside and away from the edge of
the artificial graphite particles especially in the vicinity of the cracks.
Both the platinum nanosheet-containing artificial and natural graphite
samples showed high selectivity to cinnamyl alcohol in
cinnamaldehyde hydrogenation under supercritical carbon dioxide
conditions, while spherical platinum particles, which were located on
the surface of natural and artificial graphite, showed lower selectivity.
[7]
carbon dioxide , and palladium nano particles were highly active
[6]
in organic solvent than particles on graphite surfaces .
One
probable explanation for the different selective catalysis of metal
particles intercalated between graphite layers from those on
graphite support is the adsorption characteristics of the reactant
on active sites.
There are two types of graphite, natural and artificial varieties.
Both have the same layered structure; however, several local
structure such as, surface roughness and crystalline nature vary
due to the difference in their preparation and generation
processes, hence it is expected to that platinum particles would
get intercalated with different insertion structures in natural and
artificial graphite samples. In this work, we studied the formation
of platinum nanosheets between the layers of natural and artificial
graphite samples, which have similar particles size of 5 m. The
location of platinum nanosheets in graphite particles and their
behaviors in cinnamaldehyde hydrogenation are also discussed.
Introduction
Nanomaterials having ultrathin structure show good properties
than three-dimensional structure.[1] Platinum loaded carbon
materials are used in several fields of chemical processes, such
as heterogeneous catalysts and electrodes, and the morphology
of platinum metal particles are critical for their properties.[ Two-
dimensional platinum nano particles are expected to exhibit
peculiar properties. Graphite has a layered structure and each
layer (graphene) consists of carbon atoms which are two-
2]
Results and Discussion
2
dimensionally connected through sp hybrid orbitals. Because
Natural and artificial graphite samples
In this study, we used two types of graphite samples, artificial
graphite layer can donate and accept electrons and bound to each
other with a weak van der Waals force, various compounds can
be introduced between graphite layers to produce graphite-
graphite (average particle size 5 m, BET surface area 16.2
2
-1
m g ) (hereinafter we refer to as SGP5) and natural graphite
[3]
intercalated-compounds (GIC) . Several metal chloride species
can be inserted between graphite layers by mixing solid metal
chloride with graphite powder and following thermal treatment
2
-1
(
average particle size 5 m, BET surface area 9.6 m g , which
were provided by SEC CARBON, Ltd. (hereinafter we referred to
as SNO5). Diffraction peaks of (002) and (004) from the stacking
of graphene layers were observed at 2 = 26.5º and 54.6º in the
X-ray diffraction patterns of both graphite samples (Supporting
information). Diffraction peaks attributed to (100), (101), and
[3]
under a vacuum or a chlorine atmosphere . It is expected that
metal particles with nano size order can be formed by the
reduction of the corresponding metal chloride intercalated
between two-dimensional graphite layers because of the steric
hindrance of the upper and lower graphite layers. We have
succeeded to produce sheet-shaped platinum particles with 1-3
(
4
102) of hexagonal structure were also observed at 2 = 42.3º,
4.5º, and 54.6º for both samples, respectively. Besides,
diffraction peaks attributed to (101), (012), and (104) of
[4]
nm thickness and 100-300 nm width (platinum nanosheets) ,
1
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rhombohedral structure were observed at 2 = 43.2º, 46.1º, and
5
Platinum nanosheets between layers of artificial and natural
graphite
6.6º, respectively, in natural graphite SNO5 [8]
.
A crack was observed in the artificial graphite particle, on the
Figure 2 shows TEM images of a sample obtained by the
other hand, the surface of the natural graphite particle was flat as
observed in the SEM images of SGP5 and SNO5 samples
4
reduction of the PtCl -SGP5-GIC sample at 573 K under 40 KPa
of hydrogen for 1 hour (we refer Pt-SGP5-GIC). XMA analysis
confirmed that black images are of platinum. Figure 2(a) shows
a sheet-like platinum particle with a number of hexagonal holes.
The sizes of the hexagonal holes were not uniform; however, the
directions of the sides of the hexagonal holes (AB, BA, and CA)
were parallel to each other and angles between two sides (ABC,
BCA, and CAB) of holes and edges were 120º (Figure 2(a) and
2(c)). Figure 2(b) is another image of Pt-SGP5-GIC sample, in
which several rod-like platinum images are seen in parallel with
graphite layers. Void spaces are also observed in the edges of
the platinum sheets. Figure 2(a) is a top view and 2(b) is a side
view of platinum nanosheets intercalated between graphite layers.
We have reported that similar platinum nanosheets (1-3 nm
thickness and 100-300 nm width) were observed for platinum
(
Supporting information). Natural graphite is obtained by crushing
and flotation of mineral ore, while artificial graphite is
manufactured by the treatment of the mixture of pitch and
additives such as iron oxide at 1273 K and graphitization at 3273
K. Cracks are formed and rhombohedral structure is partially
broken during graphitization for the artificial graphite sample,
indicating that the in-plane crystallinity of the graphite layer of the
artificial graphite sample is lower than that of the natural graphite
sample.
Insertion of platinum chloride (IV) between graphite layers
The mixture of both graphite samples and platinum chloride was
treated at 723 K under 0.3 MP of chlorine atmosphere for 1 week
[4]
to produce platinum chloride intercalated compounds (PtCl
4
-
intercalated artificial graphite KS6 . Figure 2 shows that similar
platinum nanosheets were formed in different artificial graphite
sample SGP5. When platinum chloride was reduced and
agglomerated to metal particles between graphite layers, sheet-
like structure is formed because it aggregates between a two-
dimensional space sandwiched by two graphene layers. The
interaction between platinum chloride and carbon atoms in two
SGP5-GIC and PtCl -SNO5-GIC). Their XRD patterns are shown
4
in Figure 1. In addition to the graphite peaks, new peaks were
observed at 2 = 9.8º, 14.6º, and 20.3º in both samples,
respectively, which were also seen in the platinum chloride
[4]
intercalated artificial graphite KS6 (Timrex) . The positions of
three peaks are in good agreement with the calculated values for
(
002), (003) and (004) reflections from the repeat distance along
graphene layers, which are hexagonal, would affect the
orientation of sides of holes and edges .
[9]
[4]
with the c axis (c=1.76 nm). From the size of platinum chloride
and graphite layer (0.3354 nm), we propose that both PtCl
4
-
5
SGP5-GIC and PtCl4-SNO -GIC samples have third stage
structure, in which a periodicity of one platinum chloride and three
[4]
graphene layers .
(a)
PtCl -SGP5-GIC
4
SGP5
1
0
20
30
40
50
60
2
θ/ degree
(b)
Figure 2. TEM images of Pt-SGP5-GIC. (a) top view, (b) side view, (c) enlarged
of (b) and (d) Schematic of platinum nanosheets within graphite layers.
4
PtCl -SNO5-GIC
A Pt-SGP5-GIC image at low magnification in Figure 3(a) shows
the insertion position of platinum nanosheets in the artificial
particles. Since the thickness of the platinum nanosheets is of
few nanometers, the contrast of platinum to carbon is low. Figure
3(b) is a magnified image of Figure 3(a). It can be seen that
SNO5
platinum nanosheets are located inside from the edge of an
artificial graphite particle and platinum nanosheet in Pt-SGP5-GIC
has hexagonal holes and edges at an angle of 120º.
1
0
20
30
40
50
60
Another image of Pt-SGP5-GIC at low magnification in Figure
(a) shows the overlayer of graphite and cracks in the artificial
2
θ/ degree
4
Figure 1. XRD patterns of graphite and platinum chloride intercalated graphite
samples (a) SGP5 and PtCl -SGP5-GIC(b) SNO5 and PtCl -SNO5-GIC.
4
4
particles. The magnified image (Figure 4(c)) shows that platinum
nanosheets are located in the vicinity of cracks and at the edges
2
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of graphene overlayers. Figures 3 and 4 show that platinum
nanosheets located at cracks and edges of overlayers in an
artificial graphite sample.
3 nm thickness and hexagonal holes were formed between
natural graphite layers.
Figure 6(a) shows a Pt-SNO5-GIC image at low magnification.
The position of the platinum nanosheets is marked in Figure 6(b).
It is shown that platinum nanosheets are present around the outer
edge of a GNO5 graphite particle. The enlarged image Figure
6(b) shows that platinum nanosheets having hexagonal holes and
edges with 120º angle are densely packed at the outer portion of
natural graphite particles.
Figure 3. TEM images of Pt-SGP5-GIC. (a) top view, (b) side view, (c) enlarged
of (b), and (d) schematic of platinum nanosheets within graphite layers.
Figure 5. TEM images of Pt-SNO5-GIC. (a) top view and (b) enlarged of (a).
Figure 4. TEM images of Pt-SGP5-GIC. (a) top view and (b) enlarged of (a).
Figure 5 shows a TEM image of a sample obtained by the
reduction of PtCl -SNO5-GIC at 573 K for 1 hour under hydrogen
4
atmosphere of 40 KPa (We refer Pt-SNO5-GIC). The platinum
sheets having a number of hexagonal holes and edges with 120º
angle were densely packed at the outside of the graphite particle
Figure 6. TEM images of Pt-SNO5-GIC. (a) tip view, (b) side view, and (c)
enlarged of (b).
(
Figure 5(a). Another image shows that rod-like platinum particle
with 3 nm thickness are located parallel to graphite layers (Figure
(b)). This figure shows that similar platinum nanosheets with 1-
5
Platinum nanosheets with hexagonal hole and edges with 120º
angle were intercalated between both artificial and natural
3
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graphite samples by the thermal treatment of the mixture of
platinum chloride and graphite powder samples under chlorine
atmosphere followed by the reduction with hydrogen gas.
However, the locations of platinum nanosheets in graphite
particles were different in artificial and natural graphite samples.
The platinum nanosheets were located on inner portion of artificial
graphite particles while, they were located densely at the outer
portion of natural graphite particles. The different locations of Pt
nanosheets would be due to the difference in surface structure of
two graphite samples. Artificial graphite sample has a lot of
cracks in the surface and platinum chloride would intercalate from
the cracks. Platinum nanosheets were formed by the reduction of
platinum chloride intercalated near the cracks. On the other
hand ,the surface of natural graphite sample is smooth and flat.
Only the edge of layers is the location where platinum chloride
can enter into the layers and platinum nanosheets are formed by
the reduction of metal chloride located near the edge of graphite
layers.
Hydrogenation of cinnamaldehyde over Pt-GIC and Pt/Gmix
Cinnamaldehyde (CAL) is an unsaturated compound and has
three types of unsaturated bonds: aromatic ring, carbon-carbon
double bond (C=C), and aldehyde group (C=O). The CAL
hydrogenation is a structure sensitive reaction. The selectivity to
cinnamyl alcohol (COL), which is produced by the hydrogenation
of C=O bond in CAL molecules, and to 3-phenyl propionaldehyde
(HAL), which is a product by the hydrogenation of C=C bonds,
depends on metal species and the structure of metal particles.
Supported platinum catalysts are active for the CAL
hydrogenation and form hydrogenation products as cinnamyl
alcohol, 3-phenyl propionaldehyde, and 3-phenyl-1-propanol
(HCOL), which is produced by both C=C and C=O bonds[10]. The
catalytic behavior of supported platinum catalysts for the CAL
hydrogenation, especially the difference of selectivity to COL is
closely related to the structure of platinum sites. The CAL
hydrogenation over platinum nanosheets between graphite layers
and particles on graphite surface in a supercritical carbon dioxide
solvent was studied with an expectation that different selectivities
would be observed for platinum nanosheets intercalated between
graphite layers and spherical platinum particles located on
graphite surface. Conversion and selectivity were determined as
follows, conversion (%) = (total amount of products) / ((total
amount of products) + (amount of unreacted cinnamaldehyde)) x
100, selectivity (%) = (quantity of each product) / (total quantity of
each product) x 100.
Figures 8 and 9 show the conversions and product yields of the
CAL hydrogenation in supercritical carbon dioxide with graphite
supported platinum catalyst with the same platinum metal loading
of 5wt%. All the catalysts were active for the hydrogenation and
COL, HAL, and HCOL were formed. However, COL selectivity
depended on the catalyst. The platinum nanosheets intercalated
between graphite catalysts (Pt-SGP5-GIC and Pt-SNO5-GIC)
showed high COL selectivity of 70% or more (Figure 8) regardless
the reaction time and conversion. On the other hand, the COL
selectivity for the platinum supported on graphite surface
(Pt/SGP5-Gmix and Pt/SNO-Gmix) was not so high and was
about 55% and dehydroxylation products such as -methyl
styrene and propyl benzene) were formed (Figure 9) regardless
reaction time and conversion. The higher UOL selectivity over
platinum intercalated (Pt-GIC) samples than that over Pt/Gmix
samples would be explained by the preference adsorption of C=O
bond in CAL molecules on platinum nanosheets compared with
spherical platinum particles. For the system with Pt-GICs
Platinum metal particles on artificial and natural graphite
surface
Figures 7 shows the TEM images of Pt-SGP5-Gmix and Pt-
SNO5-Gmix in which 5wt% of platinum metal particles were
supported on SGP5 or SNO5 by the impregnation of platinum
precursor followed by hydrogen reduction. Spherical platinum
metal particles less than 20 nm were loaded on the artificial (a and
b) and natural (c and d) graphite surfaces. The size of platinum
metal particle on SGP5 were a little smaller than that of SNO5.
On SGP5 surface, a number of platinum particles of less than 10
nm diameter were observed, on the other hand, several larger
particles of more than 10 nm were seen on SNO5 (Figures 7(c)
and (d)). The artificial graphite sample SGP5 has cracks and the
2
-1
surface area value is 16.2 m g . The natural graphite SNO5
2
-1
sample has plain surface and the area value (9.6 m g ) was
smaller than that of SGP5. The larger platinum metal particles
were formed on SNO5 surface because of the larger surface area
and much smooth surface than SGP5 surface.
(
platinum nanosheets between graphite layers), supercritical
carbon dioxide solvent would transport CAL molecules between
the graphite layers. CAL molecules interact with the upper and
lower graphene layers, and the benzene ring exists parallel to the
layers, resulting in the adsorption of end functional aldehyde
group of CAL molecules at the edge of platinum nanosheets. On
the other hand, in the Pt/Gmix (platinum particles on graphite
surface), platinum particles existed on the graphite surface, and
the CAL molecules dissolved in supercritical carbon dioxide are
adsorbed on the platinum particles without any spatial preference,
which led to generate both HAL and COL leading to low COL
selectivity.
Figure 7. TEM images of Pt/SGP5-Gmix (a, b) and Pt/SNO5-Gmix (c, d).
The initial conversion over Pt-SGP5-GIC (50% for 30 min) was
higher than that over Pt-SNO5-GIC (43%). Similar platinum
nanosheets were formed in SGP5 and SNO5 layers; however, the
number of active platinum sites, on which CAL molecules could
adsorb, would be larger in Pt-SGP5-GIC than in Pt-SNO5-GIC
4
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(
Figures 3 and 6) because the artificial SGP5 graphite sample had Conclusion
a lot of cracks and platinum nanosheets were located near these
cracks. Further study to determine the number of active sites are
needed.
By the treatment of a mixture of graphite powder (natural graphite
and artificial graphite) and platinum chloride under 0.3 MPa of
chlorine at 723 K, followed by the reduction with 40 kPa of
hydrogen, intercalation of platinum nanosheets between the
graphite layers was achieved. In both natural and artificial
graphite samples, platinum nanosheets were of 1-3 nm thickness
and 100-500 wide and had a number of hexagonal holes and
edges at an angle of 120º. The natural graphite particles had a
flat surface and platinum nanosheets were located on the edges
of the graphite particles. The artificial graphite particles had
cracks and platinum nanosheets were located in the vicinal area
of the cracks and the edge of graphene overlayers on surface.
The platinum nanosheets intercalated in natural and artificial
graphite (Pt-SGP-GIC and Pt-SNO5-GIC) samples were active
for CAL hydrogenation and more selective to COL than platinum
particles located on the surface of natural and artificial graphite
2
1
1
.0
.5
.0
1
00
(a)
8
0
64
6
0
5
0
4
0
7
6
0
.5
0
7
2
2
0
0
0
60
120
Time/ min
2
1
1
.0
.5
.0
100
(b)
8
0
(
Pt/SGP5-Gmix and Pt/SNO5-Gmix) samples. Pt-SGP5 was
6
1
6
0
more active than Pt-SNO5 and both samples showed high
cinnamyl alcohol selectivity.
4
3
4
0
0
.5
74
7
0
20
0
0
0
60
120
Time/ min
Experimental Section
Figure 8. Cinnamaldehyde hydrogenation (a) Pt-SGP5-GIC and (b) Pt-SNO5-
GIC (products: cinnamyl alcohol (COL), 3-phenyl propionaldehyde (HAL), and
Two types of graphite powder samples (natural graphite powder (SNO5,
SEC CARBON, Ltd.) and artificial graphite powder (SGP5, SEC CARBON,
3-phenyl-1-propanol (HCOL)).
4
Ltd.) were used in this study. Solid platinum chloride (PtCl (IV), Aldrich)
and graphite powder (SGP5 or SNO5) was mixed in a round flask and the
mixture was treated at 723 K for 1 week under 0.3 MPa chlorine to give
2
1
1
.0
.5
.0
100
4 4
platinum chloride intercalated compounds (PtCl -SGP5-GIC or PtCl -
(
a)
SNO5-GIC). The platinum chloride insertion compounds were reduced at
573 K for 1 hour under 40 kPa of hydrogen to produce a platinum metal
intercalation compound (Pt-SGP5-GIC or Pt-SNO5-GIC) in a batch system.
The platinum metal loadings in Pt-SGP5-GIC and Pt-SNO5-GIC were
5wt%, respectively.
8
0
5
9
6
0
4
4
4
0
0
5
3
0
.5
0
5
6
2
Platinum metal supported on graphite samples were prepared by an
impregnation method. The slurry of aqueous solution of chloroplatinic acid
hexahydrate (H PtCl ・6H O, Wako Chemicals) are mixed in a round flask,
2 6 2
and the mixture is treated at 573 K for 1 hour under 40 kPa of hydrogen to
obtain platinum metal particles supported on graphite surface (Pt/SGP5-
Gmix or Pt/SNO5-Gmix). The platinum metal loadings in Pt/SGP5-Gmix
and Pt/SNO5-G-mix were 5wt%, respectively.
0
0
60
120
Time/ min
2
1
1
.0
.5
.0
100
(b)
8
0
Hydrogenation of cinnamaldehyde was carried out in a SUS batch reactor
6
0
(inner volume space 50 mL). 20 mg of catalyst, 1.98 mmol of trans-
cinnamaldehyde (Wako Chemical) are placed in the reactor, 0.5 MPa of
argon is repeatedly introduced, and it is released three times. It was
purged by and then heated to 373K. Next, hydrogen was introduced up to
4
4
4
0
3
6
0
.5
0
2
0
5
6
5
3
5
MPa. After a predetermined reaction time, the reactor was submerged
0
in an ice bath and rapidly cooled to room temperature. The mixture of
liquid products and catalysts were recovered with acetone and filtered to
remove the catalyst. Products dissolved in acetone were analysed by gas
chromatography on Agilent Technologies, model HP-6890N, using a DB-
WAX capillary column equipped with a flame ionization detector (GC-FID).
0
60
120
Time/ min
Figure 9. Cinnamaldehyde hydrogenation (a) Pt/SNO5-Gmix and (b) Pt/SNO5-
Gmix. (products: cinnamyl alcohol (COL), 3-phenyl propionaldehyde (HAL), 3-
phenyl-1-propanol (HCOL), -methyl styrene and propyl benzene).
5
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Acknowledgements
The graphite powder samples used in this study (SGP5 and
SNO5) were provided by SEC CARBON, Ltd. This work was
supported by Grand-in-Aid (18H01782) for Scientific Research B
from Ministry of Education, Culture, Sports, Science and
Technology of Japan.
Keywords: Graphite intercalated compounds • Platinum
nanosheet • Cinnamaldehyde hydrogenation • Graphite •
Supercritical carbon dioxide
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6
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Table of Contents
Platinum nanosheets having 1-3 nm in thickness and 100 - 500 nm in width and a number of hexagonal holes and edges with 120º
angle were prepared between layers of graphite (natural and artificial). Platinum nanosheets were located in the edge region of
natural graphite particles which have flat surface. On the other hand, they were found inside the artificial graphite particles especially
in the vicinity of the cracks.
7
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