In-situ SHINERS Study of the Size and Composition Effect of Pt-based Nanocatalysts in Catalytic Hydrogenation

Article abstract of DOI:10.1002/cctc.201901747

Understanding the structure-activity relationship of catalytic reactions at a molecular level still remains a great challenge. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is employed to in-situ study the catalytic hydrogenation of para-nitrothiophenol (pNTP) on Pt-based nanocatalysts with different size and composition. The nanocatalysts are assembled on pinhole-free shell-isolate nanoparticles (SHINs), which work as Raman amplifiers to enhance the Raman signals of species on the catalysts, allowing the in-situ monitoring of catalytic reactions carried out on the catalysts. Using this strategy, we find that the catalytic activity of the Pt nanocatalysts shows a volcanic trend with the Pt size, and that PtM (M=Ni or Cu) bimetallic nanocatalysts display much higher performance compared with Pt due to better activation of the nitro group. This work demonstrates that SHINERS is a promising technique for in-situ study of nanocatalysis thus can deepen the understanding of the behaviors of different catalysts.

Full text of DOI:10.1002/cctc.201901747

Accepted Article
Title: In-situ SHINERS study of the size and composition effect of Pt-
based nanocatalysts in catalytic hydrogenation
Authors: Chen Wang, Xing Chen, Tian-Ming Chen, Jie Wei, Si-Na Qin,
Ju-Fang Zheng, Hua Zhang, Zhong-Qun Tian, and Jian-Feng
Li
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To be cited as: ChemCatChem 10.1002/cctc.201901747
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10.1002/cctc.201901747
ChemCatChem
COMMUNICATION
In-situ SHINERS study of the size and composition effect of Pt-
based nanocatalysts in catalytic hydrogenation
Chen Wang,[a] Xing Chen,[a] Tian-Ming Chen,[a] Jie Wei,[a] Si-Na Qin,[a] Ju-Fang Zheng,*[b] Hua
Zhang,*[a] Zhong-Qun Tian,[a] and Jian-Feng Li*[a],[c]
Abstract: Understanding the structure-activity relationship of
catalytic reactions at a molecular level still remains a great challenge.
Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy
(SHINERS) is employed to in-situ study the catalytic hydrogenation
of para-nitrothiophenol (pNTP) on Pt-based nanocatalysts with
different size and composition. The nanocatalysts are assembled on
pinhole-free shell-isolate nanoparticles (SHINs), which work as
Raman amplifiers to enhance the Raman signals of species on the
catalysts, allowing the in-situ monitoring of catalytic reactions carried
out on the catalysts. Using this strategy, we find that the catalytic
activity of the Pt nanocatalysts shows a volcanic trend with the Pt
size, and that PtM (M = Ni or Cu) bimetallic nanocatalysts display
much higher performance compared with Pt due to better activation
of the nitro group. This work demonstrates that SHINERS is a
promising technique for in-situ study of nanocatalysis thus can
deepen the understanding of the behaviors of different catalysts.
Au or Ag nanostructures,[9] allowing in situ study the catalytic
reactions carried out on their surfaces.[10] The intermediate
species and kinetics of the reaction, as well as the effect of the
morphology and composition of the catalysts on them, can be
studied in depth using in-situ SERS.[11] However, only a few
metals (gold, silver and copper, etc.) have strong SERS
enhancement. Though a borrowing strategy has been developed
to expand SERS to other transition metals,[12] it still remains a
great challenge to in-situ monitor reactions catalyzed by
transition-metal nanocatalysts.
To overcome this limitation, our group developed a universal
spectroscopic technique named shell-isolated nanoparticle-
enhanced Raman spectroscopy (SHINERS).[13] In this technique,
the Au core works as the Raman signal amplifier and is coated
with an ultrathin but pinhole-free silica shell to isolate the
interference of the Au core. SHINERS overcomes the long-
standing limitations of SERS in materials and morphologies, so
it has been widely used in electrochemistry, materials science,
life science, energy storage and conversion, etc.[14]
In this work, SHINERS has been used to in-situ study the
size and composition effect of Pt-based nanocatalysts towards
catalytic hydrogenation. The nanocatalysts are uniformly self-
assembled around the shell-isolated nanoparticles (SHINs)
through electrostatic interaction to form SHINERS-satellite
nanocomposites. Using such nanocomposites, the influence of
the size and composition of the platinum nanocatalyst on the
hydrogenation of para-nitrothiophenol (pNTP) to para-
aminothiophenol (pATP) are investigated by in-situ SHINERS.
This work demonstrates that the SHINERS-satellite strategy can
be a general strategy for various catalytic reactions.
In-situ study of catalytic reaction processes is essential for
the fundamental understanding of reaction mechanism and the
structure-activity relationship thus would promote the rational
design of efficient catalysts.[1] The development of various
characterization techniques has led to the rapid growth of this
field. For example, X-ray photoelectron spectroscopy(XPS)[2]
and scanning tunneling microscopy(STM)[3] have been used for
the in-situ characterization of the fine structure of catalysts under
working conditions. With the help of these techniques, the
influence of the size,[4] morphology,[5] and composition[6] of
nanocatalysts on the catalytic performance have been
extensively investigated.[7]
Recently, surface-enhanced Raman spectroscopy (SERS)
has been employed to in-situ monitor the catalytic reactions.[8]
SERS has ultrahigh surface sensitivity due to the
electromagnetic and chemical enhancement generated by the
[a] C. Wang, X. Chen, T. M. Chen, J. Wei, S. N. Qin, Dr. H. Zhang, Prof.
Z. Q. Tian, Prof. J. F. Li
College of Materials, Fujian Key Laboratory of Advanced Materials,
MOE Key Laboratory of Spectrochemical Analysis and
Instrumentation, State Key Laboratory of Physical Chemistry of Solid
Surfaces, iChEM, College of Chemistry and Chemical Engineering,
Xiamen University
Xiamen 361005, China
E-mail: Li@xmu.edu.cn; zhanghua@xmu.edu.cn
[b] J. F. Zheng
Key Laboratory of the Ministry of Education for Advanced Catalysis
Materials, College of Chemistry and Life Sciences
Zhejiang Normal University
Jinhua 321004, China
E-mail: jfzheng@zjnu.cn
[c] Prof. J. F. Li
Figure 1. (a) Schematic illustration of the SHINERS-satellite nanocomposite
and its application in the in-situ study of the hydrogenation of pNTP. (b) TEM
image of the Pt-SHIN nanocomposite. (c) Comparison of the Raman signals of
pNTP adsorbed on the Pt-SHIN nanocomposites and silica supported Pt
nanoparticles.
Shenzhen Research Institute of Xiamen University, Shenzhen 518000,
China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201901747
ChemCatChem
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The schematic illustration of the SHINERS-satellite strategy
is shown in Figure 1a. The nanocatalysts are uniformly self-
assembled around the SHIN, which consists of Au cores and
pinhole-free silica shells. The fabrication of the SHINERS-
satellite nanocomoposite is achieved through a charge-induced
self-assemble method,[15] in which the surface of the
nanocatalysts are modified by NOBF4 to be positivity charged
while the SHIN is negtively charged. Figure 1b shows TEM
image of the as-prepared Pt-SHIN nanocomposites. The size of
the Au cores is ~120 nm, while the thickness of the silica shell is
~2 nm. Pt nanoparticles with a mean diameter of ~3.7 nm are
uniformly assembled on the surface of the Au@SiO2 SHINs.
Since the nanocatalysts are directly located within the
electromagnetic field enhancement range of the SHINs, the
Raman signal on the nanocatalysts surface can be greatly
enhanced. This can be verified through the comparison of the
Raman signals of pNTP adsorbed on the Pt-SHIN
nanocomposites and Pt nanoparticles supported on silica. As
dispalyed in Figure 1c, strong Raman signals of pNTP can be
clearly observed for the Pt-SHIN nanocomposites, while such
signals completely disappear for the silica supported Pt
nanoparticles.
The SHINERS-satellite nanocomposite is used to study the
size and composition effect of Pt-based nanocatalysts in the
hydrogenation of pNTP to pATP. To achieve this goal, the
nanocatalysts with different size and composition were first
prepared. The Pt nanocatalysts with a size of ~3.7 nm was first
prepared (Figure 2a and Figure S1a) and used as the seed for
the growth of Pt nanoparticles with larger sizes. Figures 2b-d and
Figures S1b-d display the TEM imanges and correponding size
distributions of Pt nanoparticles with larger sizes, which clearly
exhibit that homogeneous Pt nanoparticles with mean sizes of
~5.0 nm, ~6.8 nm, and ~8.0 nm are successfully prepared via
such a seed-mediated growth method. Furthermore, we also
prepared PtCu and PtNi nanocatalysts to investigate the
composition effect on the hydrogenation of pNTP. Figures 2e
and 2f show that PtCu and PtNi bimetallic nanoparticles with
near-spherical morphology are obtained.
The Pt nanoparticles with different sizes are then assembled
on SHINs and used in the hydrogenation of pNTP. The Pt-SHIN
nanocomposites with pNTP were deposited on a silicon wafer,
which was then put in a home-made Raman cell for the in situ
SHINERS study. Figures 3a-d display the in-situ SHINERS
spectra of the hydrogenation of pNTP on Pt nanocatalysts with a
size of 3.7, 5.0, 6.8, and 8.0 nm, respectively. In the Raman
spectra, the band at 1331 cm-1 is assigned to the –NO2 stretching
vibration of pNTP, and the band at 1571 cm-1 and 1586 cm-1 are
C-C stretching vibration of the benzene ring of pNTP and pATP,
respectively.[8],[16] Such assignments are also confirmed by the
Raman spectra of bulk pNTP and pATP (Figure S2). From the
result, we can see that as time goes on, the band at 1571 cm-1
gradually decreases while the band at 1586 cm-1 increases for
all the catalysts (as shown Figure S3a, such a trend can also be
observed for 3.7 nm Pt when the reaction time is prolonged to
1400 s). Further control experiments performed under Ar
atmosphere show that the C-C stretching vibration of the
benzene ring of pNTP remain almost unchanged (Figure S3b).
Such results indicate that pNTP can be hydrogenated to pATP
under the catalysis of Pt nanoparticles. Furthermore, no Raman
peaks for p,p‘-dimercaptoazobenzene (DMAB) (at ~1140, 1390
and 1435 cm-1) are observed, suggesting that the silica shell can
isolate the influence of the Au cores on the reaction.[16]
The SHINERS spectra of Pt nanoparticles with different sizes
clearly show that the reaction rate of pNTP hydrogenation is
strongly dependent on the Pt size. As the Pt size increases form
3.7 nm to 8.0 nm, the conversion diplays a volcanic trend, and
6.8 nm Pt nanocatalysts exhibits the best catalytic performance
(Figure S4). Such results imply that the hydrogenation of pNTP
is a structure sensitive reaction. As pNTP is adsorbed on Pt via
the Pt-S bond, the nitro group would be far away from the Pt
surface, if the Pt size is too small, thus cannot participate in the
hydrogenation reaction. On the other hand, Pt with too large size
are well coordinated thus cannot efficiently activate hydrogen.
Figure 2. TEM images of Pt nanocatalysts and the corresponding size
histograms of (a) ~ 3.7 nm, (b) ~ 5.0 nm, (c) ~ 6.8 nm, (d) ~ 8.0 nm. Scale bar,
20 nm. (e)TEM image of PtCu bimetallic nanocatalyst. Scale bar, 50 nm. (f)
PtNi bimetallic nanocatalyst. Scale bar, 20 nm.
Figure 3. In-situ SHINERS spectra of the hydrogenation of pNTP on Pt-SHIN
nanocomposites with different Pt size. (a) 3.7 nm. (b) 5.0 nm. (c) 6.8 nm. (d)
8.0 nm. Reaction conditions: 25 C and H2 at a flow rate of 10 mL/min.
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Figure 4. In-situ SHINERS spectra of the hydrogenation of pNTP under using (a) Pt-SHIN, (b) PtCu-SHIN, and (c) PtNi-SHIN, respectively. (d) The corresponding
conversion of pNTP as a function of reaction time. (e) The O-N-O stretching vibration band of the pNTP absorbed Pt, PtCu and PtNi.
Bimetallic nanocatalysts usually tend to have better catalytic
activity compared with pure metals.[17] Therefore, we have also
fabricated PtCu-SHINs and PtNi-SHINs nanocomposites, and
further investigated the catalytic performance of the bimetallic
nanocatalysts towards the hydrogenation of pNTP. As shown in
Figure S5, both PtCu and PtNi nanoparticles are successfully
attached on the surface of SHINs, forming homogeneous
electronic structure changes after introducing Cu or Ni,[18] which
promotes the activation of the –NO2 group in the pNTP and
weakens the strength of the N-O bond.
In summary, Pt and bimetallic Pt nanocatalysts are
assembled on SHINs through electrostatic interactions to form
the SHINERS-satellite nanocomposite, thus allowing the in-situ
monitoring the reaction processes occurring on the catalysts.
Using such nanocomposites, the size effect of Pt nanocatalysts
on the hydrogenation of pNTP has been studied. As the size
increases, the catalytic activity first increases then decreases,
leading to an optimized size of ~6.8 nm. The reaction kinetics of
PtCu and PtNi bimetallic nanocatalysts have also been in-situ
investigated using SHIENRS and compared with the pure Pt
nanocatalyst. It is found that the bimetallic nanocatalysts display
much higher activity than that of Pt. According to the SHINERS
study, such an improvement should result from the more efficient
activation of the –NO2 group on the bimetallic nanocatalysts. This
work not only uncover the kinetics of the hydrogenation of pNTP
on Pt-based nanocatalysts with different size and composition,
but also extends the SHINERS for the in-situ study on various
catalysts.
SHINERS-satellite
nanocomposites.
The
time-resolved
SHINERS spectra of Pt, PtCu, and PtNi nanocatalysts for
hydrogenation of pNTP are displayed in Figures 4a-c. The
bimetallic nanocatalysts show much higher activity than that of
Pt. The hydrogenation of pNTP to pATP on the bimetallic
nanocatalysts almost completes in 100 s (the activity of PtNi is
slightly higher than that of PtCu), while the conversion for Pt is
less than 20% (Figure 4d).
To reveal the reason for the higher activity of the bimetallic
nanocatalysts compared the Pt, the adsorption of pNTP on these
catalysts were carefully studied. As shown in Figure 4e, the O-
N-O stretching vibration band of the pNTP absorbed the
bimetallic nanocatalysts display a significant red shift compared
with the pure Pt nanocatalyst, indicates the strengthen of the O-
N-O bond of pNTP on these catalysts follows the order of
Pt>PtCu>PtNi. This indicates that the superior performance of
the bimetallic nanocatalysts may result from the surface
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Research Projects of Shenzhen Research
& Development Fund
Experimental Section
(JCYJ20170306140934218), the Fundamental Research Funds for the
Central Universities (20720190044 and 20720190018), and the Planned
Science and Technology Project of Zhejiang Province (2018C37048).
Synthesis of SHINs
The SHINs were synthesis according to the previous work done by
our group.[13] Briefly, 120 nm Au nanoparticles were first prepared with a
seed-mediated growth method (see Supporting information for details).
Then, 0.4 mL of 1 mM fresh (3-aminopropyl) trimethoxysilane (APTMS)
and 3.2 mL 0.54 wt% sodium silicate were added to 120 nm Au
nanoparticles sequentially. The mixed solution was stirred at room
temperature for 15 min and then heated to 90 C to coat the silica shell
on the Au nanoparticles.
Keywords: SHINERS • in-situ study • catalytic hydrogenation •
platinum-based nanocatalysts • size and composition effect
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In-situ Raman study
The as-prepared silicon wafer was soaked in a 10 mM pNTP/ethanol
solution for 30 min, allowing pNTP to be adsorbed on the Pt-based
nanoparticles. The silicon wafer was then washed with clean ethanol for
at least 3 times to remove the physically adsorbed pNTP, and dried at
room temperature. The treated silicon wafer was put into a home-made
Raman cell for in-situ study. First, the gas in the pipeline was purged with
Ar, then H2 is introduced to start the hydrogenation reaction, while the
Raman signals were recorded in real time. The conversion of pNTP is
calculated according to the change of the Raman intensity of pNTP (the
Raman band at 1331 cm-1). The Raman experiments were carried out by
using the Jobin-Yvon Horiba Xplora confocal Raman system with an
objective that has a numerical aperture of 0.55. The laser power is ~ 1.5
mW, and the laser was defocused ~25 μm.
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Acknowledgements
This work was supported by the National Natural Science Foundation
of China (21703181, 21972117, 21775127, and 21703180), Natural
Science Foundation of Guangdong Province (2016A030308012), Basic
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Entry for the Table of Contents
COMMUNICATION
Shell-isolated nanoparticles enhanced
Raman spectroscopy (SHINERS) has
been employed to reveal the size and
composition effect of Pt-based
nanocatalysts towards the
Chen Wang, Xing Chen, Tian-Ming
Chen, Jie Wei, Si-Na Qin, Ju-Fang
Zheng,* Hua Zhang,* Zhong-Qun Tian,
and Jian-Feng Li*
In-situ SHINERS study of the size and
composition effect of Pt-based
nanocatalysts in catalytic
hydrogenation
hydrogenation of pNTP.
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