Synthesis of Pt dendritic nanocubes with enhanced catalytic properties

Article abstract of DOI:10.1039/c4ra16920d

Pt dendritic nanocubes (DNCs) were successfully synthesized in oleylamine under a 1 bar H₂ atmosphere. The catalytic activity of Pt DNCs was much greater than conventional Pt nanoparticles (NPs) for the methanol electrooxidation reaction. Pt DNCs also showed excellent catalytic performance in the hydrogenation of unsaturated hydrocarbons and nitrobenzene.

Full text of DOI:10.1039/c4ra16920d

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Synthesis of Pt dendritic nanocubes with enhanced
catalytic properties†
Cite this: RSC Adv., 2015, 5, 16497
Ruigang Xie, Yue Pan* and Hongwei Gu*
Received 23rd December 2014
Accepted 30th January 2015
2
Pt dendritic nanocubes (DNCs) were successfully synthesized in oleylamine under a 1 bar H atmosphere.
The catalytic activity of Pt DNCs was much greater than conventional Pt nanoparticles (NPs) for the
methanol electrooxidation reaction. Pt DNCs also showed excellent catalytic performance in the
hydrogenation of unsaturated hydrocarbons and nitrobenzene.
DOI: 10.1039/c4ra16920d
www.rsc.org/advances
Noble metal nanomaterials have been extensively studied for under 1 bar hydrogen atmosphere, maintained at this temper-
1
–3
catalytic and electrocatalytic applications recently. In addi- ature for several hours, and then cooled to room temperature.
tion to large surface area-to-volume ratios, the morphology of The nal product was precipitated out via centrifugation at 4000
nanomaterials has been widely pursued because the size and rpm for 10 min. The products was dispersed in hexane for
shape exercises a great inuence on their surface area and further use.
exposed crystalline facets, and consequently their catalytic
Fig. 1A shows transmission electron microscopic (TEM)
4–7
reactivity. As one of the most important noble metals, Pt images of the as-synthesized Pt DNCs. The high-resolution TEM
based nanomaterials are major catalysts for many industrial image (Fig. 1B) revealed that the as-prepared Pt DNCs were well
applications, including fuel-cell technologies, ne chemical crystallized. The interfringe distances are measured to be 0.19
8–12
synthesis and gas sensing etc.
A lot of studies have been nm and 0.23 nm, corresponding to (200) and (111) lattice
devoted to the shape control of Pt nanomaterials such as spacing of the face-centered cubic Pt. The crystal structures
nanoparticles, nanorods, nanowires, nanocubes, nanoframes, were further determined by X-ray powder diffraction (XRD)
13–23
dendritic nanostructures, branched-NPs and multipods.
analysis (Fig. S1†), where the pattern positions and relative
Among these morphologies, the dendritic nanostructures intensities for (111), (200), (220), (311) and (222) matched well
possess high surface area with open pores and rich edge/corner with standard patterns of face-centered cubic Pt.
atoms, which make them highly favorable for low-cost catalytic
As shown in Fig. 2, the TEM images of the nanostructures
24–27
applications.
still requiring multiple steps or harsh conditions.
However, the existing synthetic procedures are were taken as a function of reaction time, which elucidate the
14,28
Therefore, formation process for the dendritic nanocubes. When the
ꢀ
single step methods for synthesizing Pt DNCs are highly reaction temperature was raised to 90 C for 1 min, the nano-
desired. Herein, we report a facile approach for the synthesis of seeds appeared (Fig. 2A), and rst-generation Pt branches were
Pt DNCs under mild conditions. The formation mechanism was grown on the surface of the seeds (Fig. 2B and C) in the rst 10
investigated and the electrocatalytic methanol oxidation activity minutes. Aer a further 20 minutes, second and third genera-
of resulting Pt DNCs was much greater than conventional Pt tion Pt branches grew on the arms of the rst-generation metal
NPs. Subsequently, Pt DNCs showed excellent catalytic perfor-
mance in the hydrogenation of unsaturated hydrocarbons and
nitrobenzene.
As illustrated in Scheme 1, the synthesis route was relatively
straightforward: the Pt DNCs were synthesized via reduction of
Pt(acac)
2
in oleylamine (OAm) under 1 bar hydrogen atmo-
sphere. Typically, Pt(acac)
necked round-bottom ask. The mixture was heated to 90 C
2
was dissolved in OAm in a three-
ꢀ
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Collaborative Innovation Center of
Suzhou Nano Science and Technology, Soochow University, Suzhou, 215123, China.
E-mail: hongwei@suda.edu.cn; panyue@suda.edu.cn; Fax: +86-65880905; Tel: +86-
6
5880905
†
Electronic supplementary information (ESI) available: Details of general
Scheme 1 The illustration of the formation of Pt DNCs.
experimental procedures. See DOI: 10.1039/c4ra16920d
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Fig. 1 TEM and HRTEM images of Pt DNCs.
Fig. 3 The plot showing the molar ratio of octadecylamine varies with
the reaction time. The ratio of each time point is calculated from the
1
HNMR data during the hydrogenation of oleylamine over Pt nano-
crystals to octadecylamine in different reaction time.
1
detection on the solvent by HNMR characterizations. Octade-
cylamine is detected aer a very short reaction time and nally
replace the original solvent of oleylamine at round 24 h. These
data of molar ratio of octadecylamine is calculated of the
chemical shi of hydrogen in different chemical environments
from the NMR characterization (Fig. S2†) as well as the growth
manner of Pt nanocrystals (Fig. 2 and S3†). We also conducted a
series of experiments that solely utilizes pure octadecylamine,
pure octadecene as solvents to grow the Pt nanocrystals, no
nanobranches were formed aer 2 hours (Fig. 4). These results
reveal that the both the alkenyl and amino groups take crucial
role in tuning the nanobranches, which could be degradation
with reaction time due to the Pt-catalyzed hydrogenation.
We further examined the electrocatalytic behaviors of the Pt
DNCs and found the electrocatalytic activity could be enhanced
by introducing the dendritic structures onto Pt nanocubes.
Fig. 5A shows a typical cyclic voltammogram (CV) of Pt DNCs
and Pt NPs (Fig. S4†) in an N
2
-saturated solution of 0.5 M
. It is evident that the cathodic and anodic peaks between
0.25 V and 0.05 V (vs. SCE) were attributed to the adsorption
H
2 4
SO
ꢁ
and desorption of a monolayer of hydrogen in the acidic
Fig. 2 Time dependent TEM images of Pt nanocrystals obtained at (A)
1
min, (B) 5 min, (C) 10 min, (D) 30 min, (E) 2 h and (F) 5 h.
branches to form the dendritic nanostructures (Fig. 2D). The
nanostructure formed uniform dendritic nanocubes aer 2 h
reaction (Fig. 2E).
According to the tracking analysis in the process of Pt DNCs
formation, the dynamic composition of oleylamine is the key
29–31
factor for Pt DNCs.
Interestingly, we found the growth
manner is closely related to the transition of the solvent from
oleylamine to octadecylamine under the Pt-catalyzed hydroge-
nation (Fig. 3). Such conclusion arise from the time-lapse
Fig. 4 TEM image of Pt nanocrystals prepared in pure octadecylamine
(A) and octadecene (B) in hydrogen atmosphere.
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Table 1 Hydrogenation of olefin, alkynes and nitrobenzene over Pt
a
DNCs
b
Enter Substrate
Product
Yield (%) Time (h)
1
2
>99
>99
1
1
2
Fig. 5 (A) CVs of Pt DNCs and Pt NPs in a N -saturated aqueous
ꢁ
1
solution of 0.5 M H
SO
2 4
. Scan rate: 50 mV s and (B) CVs of the
-saturated
methanol oxidation catalyzed by Pt DNCs and Pt NPs in N
.5 M H SO + 0.5 M CH OH. Scan rate: 50 mV s .
2 4 3
2
3
4
5
>99
>99
>99
1
1
2
ꢁ1
0
medium. The peaks at around 0.3–0.7 V were reduction peak of
oxides. It is observed that three peaks of hydrogen desorption is
characterized for Pt DNCs, which corresponds the facets of Pt
(111), (110) and (100), respectively. In contrast, only one peak
that corresponds to Pt (110) was observed for Pt NPs. Further,
we utilize the electrochemically active surface area (ECSA) to
characterize the catalytic activity of two samples, which can be
calculated according to the peak area of hydrogen desorption:
6
7
8
>99
>99
>99
3
3
3
Q
H
ECSA ¼ q
ꢂ Lmetal
H
ꢁ
2
where Q
desorption, q
H
(mC cm ) is the charge associated with hydrogen
ꢁ2
H
(0.21 mC cm ) is a monolayer of hydrogen on a
Pt surface, Lmetal is the metal loading (mg cm ) on the GC
ꢁ2
9
>99
3
electrode (3 mm in diameter). The calculated values of ECSA
2
ꢁ1
(
Table S1†) shows that Pt DNCs (38.5 m g Pt) possess more
2
ꢁ1
a
than two fold enhanced ECSA than that of Pt NPs (18.52 m g
All reactions were carried out with 1 mg Pt dendritic nanocubes, 1.0
ꢀ
Pt). This means that DNCs have more catalytic activity. Fig. 5B mmol substrates and 2 mL methanol at 50 C in hydrogen balloon
shows a typical cyclic voltammogram (CV) of Pt DNCs and Pt
b
condition. GC Yield.
NPs in an N
2 2 4
-saturated solution of 0.5 M H SO and 0.5 M
CH OH. Both the two catalytic reactions show two peaks, which
3
corresponds to oxidation of methanol (ꢃ0.62 V) and reduction
of adsorbed OH or Pt (ꢃ0.42 V). Besides, the forward scanning
peak of Pt DNCs is about 5.3 fold higher than that of Pt NPs,
showing a higher catalytic activity, which is also consistent to
the calculated ECSA results.
As Pt based materials is an excellent catalyst used in hydro-
genation, the chemical catalytic activity of Pt dendritic nano-
cubes was investigated with alkenes, alkynes and nitrobenzene
as substrates. As shown in Table 1, styrene, p-methyl styrene
and p-methoxy styrene were converted completely to form C–C
single bond in 1 h when 0.5 mol% of catalyst was carried out. In
the view of the hydrogenation of alkynes, 1-ethynylbenzene, p-
methyl and p-methoxy ethynylbenzene provided 100% conver-
sion to corresponding ethylbenzene products, respectively.
With Pt dendritic nanocubes catalysts, nitrobenzene, p-methyl
nitrobenzene and p-methoxy nitrobenzene were converted to
the corresponding aniline products in 3 h.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (no. 21373006, 51402203) and the
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD).
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catalytic performance in the hydrogenation of unsaturated
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