Platinum nanoparticles on carbon nanomaterials with graphene structure as hydrogenation catalysts

Article abstract of DOI:10.1134/S1070363209040057

Carbon nanomaterials with graphene structure (single- and multiwall nanotubes and nanofibers) after oxidizing by a mixture of sulfuric and nitric acids and presumable introducing of carboxyl groups can be used as carrying agents of hydrogenation catalysts. Platinum in a concentration which should not exceed 10 wt % can be fixed using H₂PtCl₆ as a precursor in presence of an organic base. Catalysts based on these nanomaterials with the average size of platinum particles 6-8 nm exceed in activity the Pt/C catalyst with the size of platinum particles 65-70 nm, but are inferior to catalysts based on fullerene black with the average size of platinum particles 3-4 nm.

Full text of DOI:10.1134/S1070363209040057

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 4, pp. 706–710. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © S.D. Kushch, N.S. Kujunko, B.P. Tarasov, 2009, published in Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 4, pp. 542–546.
Platinum Nanoparticles on Carbon Nanomaterials
with Graphene Structure as Hydrogenation Catalysts
S. D. Kushch, N. S. Kujunko, and B. P. Tarasov
Institute of Chemical Physics Problems, Russian Academy of Sciences,
Pr. Akademika Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia
e-mail: ksd@icp.ac.ru
Received June 19, 2008
Abstract―Carbon nanomaterials with graphene structure (single- and multiwall nanotubes and nanofibers)
after oxidizing by a mixture of sulfuric and nitric acids and presumable introducing of carboxyl groups can be
used as carrying agents of hydrogenation catalysts. Platinum in a concentration which should not exceed 10 wt %
can be fixed using H2PtCl6 as a precursor in presence of an organic base. Catalysts based on these
nanomaterials with the average size of platinum particles 6–8 nm exceed in activity the Pt/C catalyst with the
size of platinum particles 65–70 nm, but are inferior to catalysts based on fullerene black with the average size
of platinum particles 3–4 nm.
DOI: 10.1134/S1070363209040057
Carbon nanomaterials with graphene structures
draw attention as potential carrying agents for catalysts
or electrocatalysts. Principal causes of such attention
are connected with the fact that they contain graphene
structures in their constitution, which impair a high
conductance (comparable with that of graphite) and
satisfactory mechanical characteristics to the materials.
A priori the conductance of a carrying agent is of great
importance for catalysts of electron-transfer reactions
[1], in particular, for the catalysts of hydrogenation,
dehydrogenation, oxidation, etc.
At the same time, nanofibers NF 1 containing
amorphous carbon are oxidized more difficultly than
NF 2 (Fig. 1).
We noted earlier [2] that it is difficult to introduce
functional groups into carbon nanomaterials to the
extent sufficient for the formation of covalent bonds
with platinum. In fact, substitution reactions involving
aromatic ring protons are impossible in this case
because of the absence of protons and extremely small
(of the order 10–4 g–1) amount of broken bonds. The
presence of mainly conjugated aromatic bonds, as
differentiated from non-conjugated double bonds
present in fullerenes or fullerene black, is the principal
In the present work we have prepared platinum-
containing catalysts based on carbon nanomaterials
with a graphene structure, in particular, single- (SNT)
and multiwall nanotubes (MNT) and nanofibers (NF),
and have studied their catalytic properties in the liquid-
phase hydrogenation of organic compounds containing
a double С=С bond or an aromatic nitro group.
1
100
2
80
4
60
Carbon nanomaterials with graphene structure,
unlike fullerene black, are characterized by high
temperatures of oxidation beginning (from 350°С) and
by clearly pronounced graphite structure (Figs. 1
and 2). Of all materials studied in this work, only
nanofibers NF 1 contain amorphous carbon which
manifests itself as a characteristic halo (Fig. 2). The
presence of amorphous carbon seems to be a reason for
a higher specific surface area of NF 1 compared to NF 2.
3
40
20
0
200
400
600
800
1000 T, °C
Fig. 1. Oxidative thermograms of carbon nanomaterials:
(1) MNT, (2) NF 1, (3) NF 2, and (4) SNT.
706

PLATINUM NANOPARTICLES ON CARBON NANOMATERIALS
707
T, °C
100
300
m/z 32
500
700
900
10
8
100
98
m/z 18
6
6 × m/z 44
4
0.1 × m/z 28
m/z 17
2
4
96
m/z 16
200
3
2
1
0
10 20 30 40 50 60 70 80 90
400
600
T, °C
800
1000
2θ, deg (CuKα)
Fig. 3. Thermogram of oxidized nanofibers combined with
mass-spectral analysis of gaseous products of NF 2 oxida-
tion in argon atmosphere.
Fig. 2. Diffractograms of carbon nanomaterials: (1) NF 2,
(2) NF 1, (3) MNT, and (4) graphite.
distinction of carbon nanomaterials with graphene
structure from fullerene-like materials. The oxidation
by sulfuric or nitric acid or by their mixtures (see, for
example, [3–19]), or by a K2Cr2O7 solution in sulfuric
acid [20], or by hydrogen peroxide [21] widely used in
the graphite functionalisation makes it possible to
introduce carboxylic, anhydride, and lactone, phenolic
and carbonyl, or quinoid groups into carbon nano-
materials, but results in destruction of a conjugation
system in the site of adding these groups. As the
amount of defects and ends with fullerene-like
fragments in carbon materials with graphene structures
is insignificant and aromatic structures are highly
stable, it is clear that the amount of functional groups
which can be introduced into these materials by a non-
destructive oxidation is insignificant. In fact, the gross
formula of these carbon nanomaterials after oxidation,
calculated from the elemental analysis data, appro-
aches С27О. An additional oxidation of NF 1 after the
catalyst removal by nitric acid does not change the
gross formula. In view of unaffected internal carbon
atoms, the gross formula points to a high concentration
of surface functional groups in the nanomaterial.
Nevertheless, when one platinum atom is coordinated
with two oxygen atoms, it is possible to fix no more
than (195×0.5×100)/(27×12 + 16 +195×0.5) = 22.28%
of platinum on these nanomaterials. Taking into
account incomplete accessibility of functional groups
and possible steric hindrances, this amount should be
considered as being overestimated.
the course of thermal analysis the available functional
groups are decomposed to evolve СО2 (m/z 44) and
water (Fig. 3). Oxygen (m/z 32 and 16) and CO are
absent from the thermolysis products of carbon
nanomaterials.
The comparison of the thermal analysis data of
oxidized carbon materials with graphene structures
points to the similar nature of functional groups
introduced by the oxidation with nitric and/or sulfuric
acids. Probably these are carboxylic groups as follows
from the low temperatures of decarboxylation and the
absence of water, except for physically sorbed water.
Reactions of carboxylic groups with PtCl62– were
carried out with adding an organic base–amine (Am)
that converts H2PtCl6 to [Pt(Am)2]Cl4 [22]. Conditions
of the ligand substitution promote the formation of the
only possible cis-fragment in the course of the
complex formation of platinum with carboxylic groups
of oxidized nanomaterials.
After platinum fixation with participation of
carboxylic groups on oxidized nanomaterials and
reducing by formiate ions, platinum particles of size 6–
8 nm (Fig. 4) were obtained. The resulting products
based on nanomaterials catalyze the hydrogenation of
individual 1-decene (Fig. 5) and nitrobenzene in 2-
propanol (Fig. 6). In the both cases the catalysts based
on NF 1 are more active than catalysts based on other
studied carbon nanomaterials including also the
traditional catalyst of 10.0% Pt/C based on active
carbon AC-3. Therewith no correlation between the
catalytic activity and the specific surface area of the
carrying agent was observed.
The amount of oxygen-containing groups in
oxidized carbon nanomaterials with graphene
structures is extremely small (no more than 6 wt %). In
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KUSHCH et al.
25
20
15
10
5
1
1
2
3
2
3
5
4
4
0
1
3
5
7
9
11
t, min
64
66
68
70
72
2θ, deg (CuKα)
Fig. 5. Initial sections of hydrogen absorption curves for
the hydrogenation of individual 1-decene over catalysts
based on carbon nanomaterials: (1) 55.2% Pt/NF 1, (2)
10.3% Pt/NF 1, (3) 12.6% Pt/NF 2, (4) 9.3% Pt/MNT, and
(5) 11.4% Pt/SNT.
Fig. 4. Pt(220) line in diffractograms of catalysts: (1) 10.3%
Pt/NF 1, (2) 12.6% Pt/NF 2, (3) 9.3% Pt/MNT, and (4) 11.4%
Pt/SNT.
decene and nitrobenzene in 2-propanol are practically
equal and comprise 45.87 mol Н2/mol Pt min (Figs. 8
and 9). It is not true for other carrying agents, i.e.
normally the rate of hydrogen adsorption at the 1-
decene hydrogenation is higher than in the case of
nitrobenzene.
When 1-decene is hydrogenated the rate of
hydrogen adsorption for catalysts with a platinum
content of 5-55% is not proportional to a platinum
content on NF 1 (Fig. 7). The catalytic activity of 5.2%
Pt/NF in the 1-decene hydrogenation is 105.43,
whereas for the 55.2% Pt/NF 1 catalyst it is only
31.74 mol Н2/mol Pt·min (Fig. 5). Catalysts with high
platinum contents are often deactivated, since a
considerable part of platinum is precipitated on the
walls of a reaction vessel.
The catalysts containing platinum on carbon
nanomaterials with graphene structure as carrying
agents catalyze liquid-phase hydrogenation of alkenes
and aromatic nitro compounds. The activity of such
catalysts is not record-breaking high, but when the
carrying agents are nanofibers obtained by the
Specific activities of catalysts based on NF 1 (for
example, 14.45% Pt/NF 1) in the hydrogenation of 1-
2.5
2.0
1.5
1.0
0.5
0.0
1
2
9
7
5
3
1
3
4
5
0
10
20
30
40
50
60
0
2
4
6
8
10
t, min
Fig. 6. Initial sections of hydrogen absorption curves for
the hydrogenation of nitrobenzene in 2-propanol over
catalysts based on carbon nanomaterials: (1) 10.3% Pt/NF 1,
(2) 11.6% Pt/NF 2, (3) 11.4% Pt/SNT, (4) 9.3% Pt/MNT,
and (5) 10.0% Pt/C.
Pt content, wt %
Fig. 7. Dependence of the rate of hydrogen absorption for
the hydrogenation of individual 1-decene on the platinum
content in Pt/NF 1 catalyst.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 4 2009

PLATINUM NANOPARTICLES ON CARBON NANOMATERIALS
40
709
40
35
30
25
20
15
10
5
1
30
1
20
2
2
10
0
0
0
10
20
30
40
50 t, min
0
10
20
30
40
50
60 t, min
Fig. 8. Kinetic curve of hydrogen absorption for the
hydrogenation of two [(1) total curve and (2) the second
portion] portions of 1-decene in 2-propanol over 14.45%
Pt/NF 1. The moment of the input of a fresh 1-decene
portion is shown by an arrow.
Fig. 9. Kinetic curve of hydrogen absorption for the
hydrogenation of two [(1) total curve and (2) the second
portion] portions of nitrobenzene in 2-propanol over
14.45% Pt/NF 1. The moment of input of a fresh
nitrobenzene portion is shown by an arrow.
pyrolysis of the propane-butane fraction over a Ni/
MgO catalyst, it surpasses the activity of the traditional
catalyst: 10.0% Pt/C based on active carbon AC-3.
Unlike catalysts on fullerene black, the hydrogenation
of individual 1-decene for catalysts on NF 1 prac-
tically does not differ from the hydrogenation of 1-
decene in 2-propanol. The catalysts are not poisoned
by hydrogenation products (Figs. 8 and 9).
of nickel and yttrium hydrides with a subsequent
refining [23]. Carbon nanofibers NF 1 were obtained
by a pyrolysis of the propane-butane fraction over the
Ni/MgO catalyst with a subsequent removal of the
catalyst by nitric acid. The resulting nanofibers NF 1
had a specific surface area of 130 m2 g–1. Carbon
nanofibers NF 2 with a specific surface area of 59 m2 g–1
were obtained by a pyrolysis of a mixture of ethene
with hydrogen (1:1.75 vol) in presence of argon
(18 vol %) as a diluent over a powder of the LaNi5
alloy at 700°С with a subsequent removal of the metals
by hydrochloric acid.
However catalysts based on NF 1 and AC-3 coal
are inferior in activity to catalysts based on fullerene
black, which have much lower conductance than
graphite and a specific surface lower approximately by
an order of magnitude than that of AC-3. Possibly the
rather high activity of catalysts based on NF 1 is
connected with a high content of amorphous carbon
(likewise fullerene black), which is seen from Figs. 1
and 2.
A suspension of multiwall nanotubes in a mixture
of concentrated nitric and sulfuric (1:1 vol) acids was
subjected to an ultrasonic treatment at 40°С within 4 h.
Single-wall nanotubes NF 1 and NF 2 were oxidized
by concentrated nitric acid at a long-term (24–72 h)
heating (70°С).
Thus, no influence of conductance and specific
surface area of the carrying agent on the activity of
catalysts based on it was found, but, similarly to the
use of fullerene black as the carrying agent, the activity
of catalysts is defined by the size of platinum clusters.
Platinum was deposited on oxidized carbon
nanomaterials from an aqueous solution of chloro-
platinic acid H2PtCl6·xH2O (~40 wt % Pt). An ordinary
procedure consisted in the dropwise addition of an
H2PtCl6·xH2O aqueous solution (concentration of
approximately 1–4 g l–1) under stirring and heating
within 3–5 h to a suspension of an oxidized carbon
nanomaterial in water (approximately 4–5 g l–1) with
0.5 ml of pyridine added. Then the reaction mixture
was boiled within 0.5–1 h, an aqueous solution of
sodium carbonate was added, and 1 ml of formic acid
was added dropwise. The mixture was held overnight,
the prepitate was separated by filtration, washed with
water up to a neutral reaction, and dried.
EXPERIMENTAL
Multiwall nanotubes were obtained by electric arc
graphite vaporization from a loose part of the cathode
deposit, which was purified from amorphous carbon
[23]. Single-wall nanotubes were synthesized under
500 torr pressure of helium and 130 A strength of
current by the electric arc vaporization of a composite
graphite rod of 6 mm in diameter containing a mixture
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KUSHCH et al.
6. Li, W., Liang, C., Zhou, W., Qiu, J., Zhou, Z., Sun, G.,
The elemental analysis was carried out by a
and Xin, Q., Carbon, 2004, vol. 42, p. 436.
semimicromethod. The X-ray patterns were recorded
on a DRON-3 instrument. Average sizes of platinum
particles were calculated by Selyakov–Scherrer’s
equation for the line Pt(220). Thermograms were
recorded on an STA-409 PC Luxx thermoanalyzer
combined with a QMS-403 C (NETZSCH corporation,
Germany) mass-spectrometer. Catalytic hydrogenation
was carried out in a temperature-controlled shaken
(≥600 min–1) flask at atmospheric pressure of
hydrogen and temperature of 45°С. 1-Decene was
hydrogenated in a mass or in 2-propanol (5 ml of 1-
decene per 20 ml of 2-propanol). A weighted sample
of a catalyst in all cases was 0.03 g. Specific catalytic
activity was calculated from initial sections when a
conversion did not exceed 1–2%. The second portion
of 1-decene (5 ml) was introduced in a hydrogen flow
after noticeable (more than tenfold) decrease in the rate
of hydrogen absorption.
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ACKNOWLEDGMENTS
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This work was financially supported by the Russian
Foundation for Basic Research (grant no. 08-03-
01117).
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