Argon plasma reduced Pt nanocatalysts supported on carbon nanotube for aqueous phase benzyl alcohol oxidation

Article abstract of DOI:10.1016/j.cattod.2013.04.006

Carbon nanotube supported Pt nano-catalyst was prepared by facile glow discharge plasma reduction operated at room temperature. The low-temperature plasma reduced Pt nanoparticles exhibited uniform size distribution and an average particle size of around

Full text of DOI:10.1016/j.cattod.2013.04.006

Catalysis Today 211 (2013) 104–108
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Argon plasma reduced Pt nanocatalysts supported on carbon
nanotube for aqueous phase benzyl alcohol oxidation
Chunmei Zhoua, Hong Chena, Yibo Yana, Xinli Jiaa, Chang-jun Liub,∗∗, Yanhui Yanga,∗
a School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore
b Advanced Nanotechnology Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon nanotube supported Pt nano-catalyst was prepared by facile glow discharge plasma reduction
operated at room temperature. The low-temperature plasma reduced Pt nanoparticles exhibited uni-
form size distribution and an average particle size of around 2.3 nm. Electrochemical measurements
also proved that the plasma reduced Pt catalyst has remarkably higher active surface area comparing to
the conventional hydrogen thermally reduced sample (3.6 nm in diameter). The plasma reduced Pt/CNT
catalysts exhibited significantly higher conversion in benzyl alcohol aerobic oxidation compared to the
hydrogen reduced Pt catalyst. Nonetheless, no obvious difference was found on the catalytic activity
in terms of turn over frequency (TOF) between the plasma and hydrogen reduced Pt/CNT catalysts.
Therefore, low temperature plasma reduction can effectively enhance Pt dispersion without changing
its intrinsic catalytic property.
Received 16 November 2012
Received in revised form 26 February 2013
Accepted 8 April 2013
Available online 13 May 2013
Keywords:
Plasma reduction
Platinum
Electrochemical measurement
Benzyl alcohol aerobic oxidation
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
characterized by its high electron temperature (10,000–100,000 K)
and low gas temperature (near room temperature) [11]. Several
Selective oxidation of benzyl alcohol to benzaldehyde is an
important reaction in fine chemical industry, accounting for many
products serving as the versatile intermediates in organic synthesis
[1]. Supported noble metal catalysts, such as Pd [2,3], Pt [4], and Au
[5], were extensively employed in this reaction due to their superior
catalytic performances. The catalytic properties of these catalysts
were intimately related to the metal particle size and the specific
interaction between metal active sites and the supports [1,6], which
may be largely changed by using different catalyst synthesis meth-
ods. Impregnation followed by hydrogen thermal reduction is a
conventional method, and it has been adopted in numerous catalyst
preparations. However, noble metal nanoparticles may agglomer-
ate substantially under hydrogen thermal reduction at elevated
temperature, resulting in the activity loss and weak interaction
between metal particles and supports [7]. Chemical reductive depo-
sition precipitation is another frequently used method, which
requires reductive chemicals such as polyols [8], NaBH4 [9], and
urea [10].
metallic nanostructures have been reported based on this novel
reduction technique, since the electrons in the glow discharge
plasma can effectively reduce the metal ions into metallic phase at
room temperature [12,13]. Wang et al. showed the platinum nano-
catalyst supported on carbon black via plasma reduction, with a
well-controlled particle size averaging at 1.43 nm for 20 wt.% Pt
loading and 1.50 nm for 40 wt.% Pt loading [14]. Pt/Al2O3 [15] and
Pt/TiO2 [16] catalysts with high Pt dispersion were also prepared by
this method. The synthesis of nanoporous Pt supported on nickel
foam [17] and AuPd bimetallic nanoparticles confined in meso-
pores [18] were also reported via the Ar glow-discharge plasma
reduction. Both catalysts exhibited excellent catalytic activities in
electro-oxidation and solvent-free aerobic oxidation of alcohols. In
this work, carbon nanotube supported Pt nano-catalyst was syn-
thesized via the Ar glow-discharge plasma reduction method, and
the catalytic properties were examined in the aqueous phase ben-
zyl alcohol oxidation. The intrinsic catalytic properties of Pt were
benchmarked against the Pt catalyst prepared by hydrogen thermo-
reduction.
Plasma reduction, the glow discharge in particular, has been
demonstrated as a facile and green method in catalyst prepara-
tion, which is low energy consumption and free of chemical wastes.
Glow discharge is a conventional non-thermal plasma that is
2. Experimental
2.1. Materials
Multi-walled carbon nanotubes (MWCNT) was obtained from
Cnano Technology Ltd. (purity: 97.1%, SBET: 241 m2/g, bulk den-
sity: 0.05 g/cm3). HNO3 (69%), H2SO4 (98%), KOH, H2PtCl4, ethylene
∗
Corresponding author. Tel.: +65 63168940; fax: +65 67947553.
Corresponding author. Tel.: +86 22 27406490; fax: +86 22 27406490.
E-mail addresses: ughg cjl@yahoo.com (C.-j. Liu), yhyang@ntu.edu.sg (Y. Yang).
∗∗
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.04.006

C. Zhou et al. / Catalysis Today 211 (2013) 104–108
105
glycol (EG), benzyl alcohol (98%), benzaldehyde (98%) and benza-
lacide (98%) were provided by Sigma–Aldrich. All chemicals were
used as received without further purification.
Pt (111)
2.2. Catalyst preparation
Pt (200)
The pristine MWCNT was pretreated with concentrated HNO3 at
140 ◦C for 2 h to remove the amorphous carbonaceous and metal-
lic impurities, as well as to introduce surface oxygen functional
groups for anchoring metal precursors. To deposit Pt precursors
onto the functionalized CNT, 0.5 g of CNT was immersed in 7.2 ml
of H2PtCl6 solution (20 mg/ml), then dried at 373 K in vacuum (Pt
loading of 5 wt.%). 0.5 g of dried sample was loaded onto a quartz
boat and placed in a quartz tube with two stainless steel elec-
trodes. The plasma was introduced to reduce the Pt precursors
following the procedures reported in the previous work [18,19].
The entire process was carried out at room temperatures. After 8
plasma treatment cycles (with 10 min per cycle), the obtained sam-
ple was denoted as Pt/CNT-plasma. For the purpose of comparison,
H2PtCl6 impregnated CNT was dried under ambient condition and
reduced by conventional hydrogen thermal reduction at 400 ◦C for
2 h (5% H2/Ar atmosphere), the obtained sample was denoted as
Pt/CNT-IMP.
Pt (220)
Pt/CNT- plasma
Pt/CNT- IMP
50
2 theta / degree
20
30
40
60
70
Fig. 1. XRD patterns of Pt/CNT-plasma and Pt/CNT-IMP catalysts.
condenser was used to condense the vapor of products. Oxy-
gen flow (25 ml/min) was bubbled into the mixture to initiate
the reaction. After the given reaction time, the catalyst pow-
der was filtered off and the reactant and product were extracted
using toluene (10 ml) for three times. The supernatant (mixture of
residual reactant, product) was analyzed using an Agilent gas chro-
matograph 6890 equipped with a HP-5 capillary column (30 m long
and 0.32 mm in diameter, packed with silica-based supel cosil),
and flame ionization detector (FID). Dodecane was the internal
standard. The turnover frequency (TOF) was defined as the number
of benzyl alcohol (BA) converted over one surface-active Pt atom
per hour:
2.3. Catalyst characterizations
Metal loadings were confirmed by inductively coupled plasma
(ICP, AA6800) measurement, hydrofluoric acid (40%) was employed
to dissolve the samples. X-ray diffraction (XRD) patterns were
recorded on a Bruker AXS D8Focus diffractometer using a Ni fil-
tered Cu K␣ radiation (ꢀ = 0.154 nm), operated at 40 kV and 40 mA.
XRD data were collected between 30◦ and 80◦ (2 theta) with a res-
olution of 0.02◦ (2 theta). Particle sizes and size distributions of Pt
were observed by transmission electron microscopy (TEM, Philips
Tecnai G2 F20, operated at 200 kV). The samples were suspended in
acetone and dried on holey carbon-coated Cu grids. Electrochemi-
cal properties of the samples were measured by voltammetries on
an Autolab PGSTAT302 potentiostat. All the electrochemical per-
formance tests were carried out at room temperature (25 ◦C) in
a rotating disk electrode system. The working electrode was pre-
pared by dropping 40 ␮l of the catalyst ink onto a glassy carbon
electrode. The ink was prepared by ultrasonically mixing 2 mg of
catalyst sample with 1 ml of 0.025 wt.% Nafion in deionized water
solution. Pt foil and Ag/AgCl electrode (+0.197 V vs. NHE) were
used as the counter and reference electrodes, respectively. The
cyclic voltammograms were first recorded in nitrogen purged 0.1 M
KOH at a scan rate of 50 mV/s until reproducible voltammograms
were obtained. For the electrochemical active surface area (EAS)
measurement, the average Coulombic charge (QH) of hydrogen
adsorption and hydrogen desorption was used to calculate the Pt
EAS of the electrodes, using the equation EAS = QH/210 (␮C/cm2)/w,
in which ‘w’ represents the content of Pt in the catalysts [20]. Oxy-
gen electro-reduction was measured in 0.1 M KOH saturated with
oxygen by liner potential sweep at 10 mV/s. Benzyl alcohol electro-
oxidation was measured in 0.2 M benzyl alcohol with 0.1 M KOH as
electrolyte at 50 mV/s.
MBA · X
TOF (h−1) =
MPt · D · t
where MBA and MPt are the amount of BA and Pt in feed (mol) respec-
tively, X is the conversion of BA, D is the Pt dispersion calculated
using electrochemical method, and t is the reaction time (h).
3. Results and discussion
3.1. Characterization of catalysts
The XRD patterns for Pt/CNT-plasma and Pt/CNT-IMP (Fig. 1)
show reflections at 2ꢁ = 39.8◦, 46.4◦ and 67.8◦, ascribing to Pt(1 1 1),
Pt(2 0 0) and Pt(2 2 0), respectively, which present the typical char-
acteristics of crystalline Pt fcc pattern (PDF#04-0802). Half peak
width of Pt reflections for Pt/CNT-plasma is remarkably wider than
that of Pt/CNT-IMP, indicating the smaller average particle size for
Pt/CNT-plasma. The particle sizes calculated from Pt(1 1 1) diffrac-
tion peak based on Scherrer’s equation (as listed in Table 1) is
2.1 nm for Pt/CNT-plasma and 3.2 nm for Pt/CNT-IMP. TEM images
of Pt/CNT-plasma and Pt/CNT-IMP are shown in Fig. 2. Pt is found
uniformly dispersed on the surface of CNT for Pt/CNT-plasma, with
an average particle size of 2.3 nm. For Pt/CNT-IMP, agglomerations
of Pt particles obviously occur, and the average particle size of
Pt is 3.6 nm. The TEM observations are well accord with the XRD
results, and both characterizations prove that plasma reduction
results in highly dispersed Pt nanoparticles, which may be due
to the low temperature, while hydrogen thermal reduction leads
to the agglomeration of Pt particles at high temperature. High-
resolution TEM images (inset of Fig. 2) of Pt nanoparticles for both
of Pt/CNT-plasma and Pt/CNT-IMP show the same lattice spacing of
Pt(1 1 1) (approximately 0.228 nm), implying the similar electronic
structures/properties of these two Pt catalysts.
2.4. Catalytic reaction
The selective oxidation of benzyl alcohol (as well as 4-
methylbenzyl alcohol and 4-nitrobenzyl alcohol) was carried out
in a bath-type reactor operated under atmospheric conditions.
Alcohol (1 mmol), deionized water (50 ml), and catalyst (4 mg,
0.001 mmol of Pt) were added into a three-necked glass flask,
stirred continuously (1200 rpm) and heated to 75 ◦C in an oil bath.
The temperature was controlled by a thermocouple, and a reflux
To further characterize the catalysts and probe the elec-
tronic properties of Pt nanoparticles over these two catalysts, the

106
C. Zhou et al. / Catalysis Today 211 (2013) 104–108
Table 1
Properties and catalytic performances of Pt/CNT-plasma and Pt/CNT-IMP catalysts.
c
Catalysts
Da (nm)
EASb (m2/g)
If (A/gPt
)
Conversiond (%)
Selectivitye (%)
TOFf (h−1
)
Pt/CNT-plasma
Pt/CNT-IMP
2.1
3.2
75.2
36.6
638.5
309.4
13.2
6.4
88.7
87.9
1234.9
1235.1
a
Particle sizes calculated from XRD patterns of Pt(1 1 1) based on Scherrer’s equation.
EAS areas deduced from hydrogen adsorption of Fig. 3a.
Max peak current measured from electro-oxidation of benzyl alcohol of Fig. 3b.
b
c
d
e
f
Conversion of benzyl alcohol under the aerobic conditions of: benzyl alcohol 3 mmol, deionized water 15 ml,catalyst 12 mg, T = 75 ◦C, t = 1 h, O2 flow rate 25 ml/min.
Selectivity to benzaldehyde in the aerobic oxidation of benzyl alcohol.
TOF is defined as the number of converted benzyl alcohol molecules in 1 h over one active site.
electrochemical measurements were carried out and the results are
shown in Fig. 3a. CV curves for both Pt/CNT-plasma and Pt/CNT-
IMP display distinct hydrogen desorption/adsorption peaks in the
potential range from −1.0 to −0.6 V. The hydrogen peak area of
Pt/CNT-plasma is significantly larger than that of Pt/CNT-IMP, this
and the calculations (listed in Table 1) together conform that the
EAS of Pt/CNT-plasma (75.2 m2/g) is much larger than that of
Pt/CNT-IMP (36.6 m2/g). The electro-oxidations of benzyl alcohol
are conducted in 0.2 M of benzyl alcohol (in 0.1 M KOH aqueous
solution), and their CV curves are depicted in Fig. 3b. Two oxidation
peaks for benzyl alcohol electro-oxidation are observed for both
catalysts. The first peak (−0.10 V) is ascribed to the oxidation of ben-
zyl alcohol to benzaldehyde, and the second peak (0.25 V) is due to
the deep oxidation of benzaldehyde to benzalacide [4]. The current
for first oxidation peak on Pt/CNT-plasma is about two times larger
than Pt/CNT-IMP, indicating that Pt/CNT-plasma is more efficient
on activation of benzyl alcohol than Pt/CNT-IMP due to the high
dispersion of Pt.
3.2. Catalytic performance of catalysts
Table 1 lists the catalytic performances of Pt/CNT-plasma and
Pt/CNT-IMP in aerobic oxidation of benzyl alcohol in aqueous solu-
tions. The benzyl alcohol conversion over Pt/CNT-plasma catalyst is
nearly doubled as compared to Pt/CNT-IMP when the reaction time
is 1 h, while the selectivity toward benzaldehyde over these two
catalysts is similar. The time course duration of catalysts perfor-
mance in benzyl alcohol oxidation is shown in Fig. 4. Both catalysts
show stable activity, and the benzyl alcohol conversions monoton-
ically increase as the reaction proceeds. After reacting for 5 h, only
16% conversion can be achieved over Pt/CNT-IMP while the con-
version of Pt/CNT-plasma is about 30%. Compared to Pt/CNT-IMP,
Fig. 2. TEM images and particle size distribution of Pt/CNT-IMP (a) and Pt/CNT-plasma (b).

C. Zhou et al. / Catalysis Today 211 (2013) 104–108
107
(a)
(b)
40
20
Pt/CNT-IMP
Pt/CNT-plasma
600
500
400
300
200
100
0
0
-20
-40
-60
PtCNT-IMP
PtCNT-plasma
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
E / V (vs. Ag/AgCl)
-0.8
-0.4
0.0
0.4
E / V (vs. Ag/AgCl)
Fig. 3. Cyclic voltammograms of catalysts in 0.1 M KOH (a) and in 0.1 M KOH + 0.2 M benzyl alcohol (b).
Pt/CNT-plasma exhibits slightly faster deactivation rate, which may
due to the remarkably higher catalytic conversion.
Pt/CNT-plasma
Pt/CNT-IMP
20
15
10
5
For both Pt/CNT-IMP and Pt/CNT-plasma catalysts, the catalytic
activities, represented by TOFs, decrease with reaction time. Inter-
estingly, the activities of Pt nanoparticles over these two catalysts
are identical as demonstrated by the same TOFs. It conforms that
the improved conversion for Pt/CNT-plasma is due to its high dis-
persion and surface area of Pt. Interestingly, different from the
reported results that catalytic activity of noble metal nanoparticles
in the aerobic oxidation of benzyl alcohol significantly depends on
the particle size [21], no observable size effect was found in this
study, at least, over these two catalysts. One of the plausible expla-
nations to the inconsistency is the measurement of Pt dispersions
which would affect the TOF results. In the above-mentioned results,
the dispersions were measured by CO chemisorptions or calculated
using Pd dispersion = 1.12/diameter of Pd particle (from TEM, nm).
Nonetheless, the Pt dispersions were calculated by electrochemi-
cal absorption of H2 and CO in this study. To further address this
issue, alcohol substrates with different electron densities including
4-mthylbenzyl alcohol and 4-nitrobenzyl alcohol were introduced
as probes to examine the catalytic activities of Pt/CNT-IMP and
Pt/CNT-plasma, and the results are shown in Fig. 5. The conver-
sions over Pt/CNT-plasma are constantly higher than Pt/CNT-IMP
for different aromatic alcohols. Compared to benzyl alcohol, 4-
mthylbenzyl alcohol shows a higher conversion, and 4-nitrobenzyl
alcohol shows a lower conversion over both Pt/CNT-plasma and
Pt/CNT-IMP catalysts. Both catalysts show higher catalytic activ-
ity for substituted aromatic alcohols containing electron-donating
group ( CH3) than those containing electro-withdrawing group
0
O
H
O
O
H
H
N
O
Fig. 5. Conversions of different aromatic alcohol substrates (conditions: alco-
hol 1 mmol, deionized water 50 ml, catalyst 4 mg (0.001 mmol Pt), O2 flow rate
25 ml/min, T = 75 ◦C, t = 2 h).
properties of Pt nanoparticles over these two catalysts are the
same. However, based on the previously reported mechanism for Ar
glow-discharge plasma reduction of metal structures [22], the high-
energy species generated by plasma, such as electrons and ions,
should have directly reduce the metal ions through a recombination
process, which would probably afford different Pt atomic arrange-
ments as compare to the high temperature hydrogen reduced Pt
15
Pt/CNT-plasma
(
NO2) [2], implying that the electronic effect and intrinsic
Pt/CNT-IMP
1800
1600
1400
1200
1000
800
40
30
20
10
0
10
TOF
Conversion
Pt/CNT-IMP
Pt/CNT-plasma
5
0
600
fresh
1
2
3
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Reaction time / h
Catalytic cycle number
Fig. 6. Recycle tests for Pt/CNT-plasma and Pt/CNT-IMP in aerobic oxidation of ben-
zyl alcohol (conditions: benzyl alcohol 3 mmol, deionized water 15 ml, T = 75 ◦C,
t = 1 h for each cycle, O2 flow rate 25 ml/min).
Fig. 4. Time course of catalysts for benzyl alcohol oxidation (conditions: benzyl alco-
hol 3 mmol, deionized water 15 ml, T = 75 ◦C, t = 0.25–5 h, O2 flow rate 25 ml/min).

108
C. Zhou et al. / Catalysis Today 211 (2013) 104–108
particles. A reasonable explanation for the disagreement between
reported mechanism and this study is that the catalyst after plasma
reduction may be unstable and has undergone the passivation after
exposing to air and moisture.
20990223) and 111 Project of Ministry of Education of China (Grant
No. B06006).
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bility than Pt/CNT-IMP in the recycle tests, which may due to the
agglomeration of small Pt nanoparticles.
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This study is funded by the Singapore Agency for Science, Tech-
nology and Research (A*STAR, SERC Grant No. 102 101 0020)
and the National Natural Science Foundation of China (Grant No.