Enhanced chemoselective hydrogenation through tuning the interaction between pt nanoparticles and carbon supports: Insights from identical location transmission electron microscopy and x?ray photoelectron spectroscopy

Article abstract of DOI:10.1021/acscatal.6b02207

Ultrasmall-sized platinum nanoparticles (Pt NPs) (～1 nm) supported on carbon nanotubes (CNTs) with nitrogen doping and oxygen functional groups were synthesized and applied in the catalytic hydrogenation of nitroarenes. The advanced identical location transmission electron microscopy (IL-TEM) method was applied to probe the structure evolution of the Pt/CNT catalysts in the reaction. The results indicate that Pt NPs supported on CNTs with a high amount of nitrogen doping (Pt/H-NCNTs) afford 2-fold activity to that of Pt NPs supported on CNTs with oxygen functional groups (Pt/oCNTs) and 4-fold to that of the commercial Pt NPs supported on active carbon (Pt/C) catalyst toward nitrobenzene. The catalytic performance of Pt/H-NCNTs remained constant during four cycles, whereas the activity of the Pt/oCNTs was halved at the second cycle. Compared with Pt/oCNTs, Pt/H-NCNTs exhibited a higher selectivity (>99%) in chemoselective hydrogenation of halonitrobenzenes to haloanilines due to the electron-rich chemical state of Pt NPs. The strong metal?support interaction along with the electron-donor capacity of nitrogen sites on H-NCNTs are capable of stabilizing the Pt NPs and achieving related catalytic recyclability as well as approximately 100% selectivity. The catalyst also delivers exclusively selective hydrogenation toward nitro groups for a wide scope of substituent nitroarenes into their corresponding anilines.

Full text of DOI:10.1021/acscatal.6b02207

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Article
Enhanced Chemoselective Hydrogenation through Tuning
the Interaction between Pt Nanoparticles and Carbon
Supports: Insights from Identical Location Transmission
Electron Microscopy and X-Ray Photoelectron Spectroscopy
Wen Shi, Bingsen Zhang, Yangming Lin, Qi Wang, Qiang Zhang, and Dangsheng Su
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02207 • Publication Date (Web): 10 Oct 2016
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Enhanced Chemoselective Hydrogenation through
Tuning the Interaction between Pt Nanoparticles and
Carbon Supports: Insights from Identical Location
Transmission Electron Microscopy and X-Ray
Photoelectron Spectroscopy
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Wen Shi†‡, Bingsen Zhang*†‡, Yangming Lin†‡, Qi Wang†‡, Qiang Zhang§, and Dang Sheng Su*†‡
†Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese
‡
Academy of Sciences, Shenyang 110016, China; School of Materials Science and Engineering,
§
University of Science and Technology of China, Hefei 230026, China; Beijing Key Laboratory
of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering,
Tsinghua University, Beijing 100084, China.
* To whom correspondence should be addressed. Tel: 86-24-23971577; Fax: 86-24-83970019;
E-mail: bszhang@imr.ac.cn, dssu@imr.ac.cn.
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Abstract: Ultrasmall sized platinum nanoparticles (Pt NPs) (~1 nm) supported on carbon
nanotubes (CNTs) with nitrogen doping and oxygen functional groups were synthesized and
applied in the catalytic hydrogenation of nitroarenes. Advanced identical location transmission
electron microscopy (IL-TEM) method was applied to probe the structure-evolution of the
Pt/CNT catalysts in the reaction. The results indicate that Pt NPs supported on CNTs with high
amount nitrogen doping (Pt/H-NCNTs) afford 2-fold activity to that of Pt NPs supported on
CNTs with oxygen functional groups (Pt/oCNTs) and 4-fold to that of the commercial Pt NPs
supported on active carbon (Pt/C) catalyst towards nitrobenzene. The catalytic performance of
Pt/H-NCNTs kept constant during 4 cycles while the activity of the Pt/oCNTs was halved at the
second cycle. Compared with Pt/oCNTs, Pt/H-NCNTs exhibited a higher selectivity (>99%) in
chemoselective hydrogenation of halonitrobenzenes to haloanilines due to the electron riched
chemical state of Pt NPs. The strong metal-support interaction along with the electron-donor
capacity of nitrogen-sites on H-NCNTs are capable of stabilizing the Pt NPs and achieving
related catalytic recyclability as well as approximately 100% selectivity. The catalyst also
delivers exclusively selective hydrogenation towards nitro groups for a wide scope of substituent
nitroarenes into their corresponding anilines.
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Keywords: nitrogen doping, IL-TEM, metal-support interaction, platinum nanoparticles,
chemoselective hydrogenation
1. Introduction
Functionalized anilines are important industrial intermediates for a variety of pharmaceuticals,
polymers, herbicides, and dyes. Nowadays, they are mainly obtained by using a vast of reducing
agents, such as sodium hydrosulfite, hydrazine hydrate or some unrecoverable non-noble metal
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ions (e.g., iron, tin or zinc), which always leads to serious environmental issues.1 It is of great
necessity to explore environmental-friendly method to efficiently produce anilines. Catalytic
hydrogenation using molecular hydrogen (H2) is the best choice for the production of
functionalized anilines owning to the clean and efficient production process. Nevertheless, the
classical hydrogenation catalysts usually generate inevitable byproducts (Scheme S1), including
hydroxylamines, nitroso, azo and azoxy derivatives. This is due to the strong adsorption effect
towards intermediated products.2-7 The chemoselective hydrogenation of nitroarenes with other
reducible groups (e.g., -C=C, -OH, -Cl, -C=O and -CN) on the benzene ring is also a great
challenge.
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In most cases, high selectivity is always achieved at the expense of activity by introducing
either transition metal salts or additives that partly poison the active sites.8-10 Au and Ag-based
catalysts with high selectivity require more harsh conditions due to its poor capacity of hydrogen
dissociation.11-15 Pt-based catalysts appeared as promising one for chemoselective hydrogenation
of nitroarenes because of their high dissociative capacity towards H2.16 Furthermore, their
versatile bimetallic structures,17,18 metal-support interaction19 and well-controllable particle
size20,21 also provide electronically modified Pt nanoparticles (Pt NPs), which is suitable for
selective adsorption of nitro-group. Highly-dispersed Pt NPs supported on various supports have
been reported with high performance in the catalytic hydrogenation of nitroarenes (CHN)
reactions.22-25 The role of support is not only to well disperse the active sites but also to modulate
the electronic structure for efficient reactant activation and product desorption.
Carbon nanotubes (CNTs) are promising supports since they have high surface area, abundant
π-electrons and adjustable surface nucleation centers, which are in favor of providing the
anchoring sites as well as regulating the chemical and electronic properties of noble metals.26-31
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The hydrogenation activities of CNTs supported noble metal NPs catalysts are highly dependent
on the nature of functional species on the support surface, which can affect the particle size
distribution (PSD), sintering resistance, and chemical state of metal NPs.32,33 Recently, most of
the investigations focus on the interaction between small Pt NPs and heteroatom doping or
functional species on CNT surface, especially in the electrocatalysis reactions.34,35 For instance,
enhanced performance was assigned to the structural and electronic modification of Pt NPs
induced by the nitrogen participation into the graphitic structure of CNT supports.36-38 However,
the understanding on the critical role of modified CNTs on the supported Pt NPs in different
catalytic reactions is still ambiguous, which strongly hinders the rational design of Pt/carbon
catalyst for vast catalytic applications.
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Herein, Pt NPs supported on pristine, oxygen functionalized, and nitrogen doped CNTs were
synthesized by ethylene glycol (EG) reduction39-41 method. The hydrogenation of nitroarenes is
chosen as probe reaction to explore the interaction between Pt NPs and doping heteroatoms or
functional groups on the support surface. The structural evaluation of the catalysts during the
reaction was studied by the identical location transmission electron microscopy (IL-TEM)
method. By comparing and quantifying a series of TEM images, the structural evolution of the
catalysts was explored in detail, and the corresponding structure-function relationship was
proposed.
2. Experimental section
2.1 Support synthesis. CNTs supplied by Tsinghua University were treated with water-diluted
HCl (volume ratio is 1:1) before use. The average outer diameter range of most CNTs is between
15 and 20 nm. The oxygen-containing CNTs were obtained by treating the pristine CNTs with
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the mixture of concentrated HNO3 and H2SO4 (with the volume ratio of 1:1) at 323 K for 2 h,
labelled as oCNTs. Nitrogen-containing CNTs were obtained from the pre-oxidized CNTs by
thermal treatment at 973 K for 4 h, with a heating rate of 10 K min-1 in a furnace under NH3
atmosphere, marked as L-NCNTs. CNTs with high N content were synthesized by a modified
chemical vapor deposition (CVD) process over FeMo/Al2O3 catalyst and using imidazole as
carbon/nitrogen feedstocks. In the catalytic CVD growth, catalysts (10 mg) uniformly distributed
on a quartz boats were heated to 973 K. Imidazole (5.0 g) were heated to 523 K in a separate
tube furnace connected to that with the catalysts. Then, 10% NH3 (diluted by Ar) was
subsequently introduced to the linked furnace at a flow rate of 100 mL min-1. After these two
furnaces were cooled down to room temperature, the as-obtained raw products were purified by
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concentrated HCl (100 mL
g
-1) under vigorous stirring for 12 h. The products were
product
HCl
collected by filtration, washed with H2O until the pH of the filtrate reach 7 and dried at 393 K.
The obtained N-doped CNTs were flagged as H-NCNTs.
2.2 Catalyst preparation. The preparation of CNTs supported Pt catalyst (Pt/CNTs) was carried
out in EG solution containing H2PtCl6 as precursor salts. Firstly, 100 mg CNTs were dispersed in
50 mL EG in a 100 mL round bottom flask and the mixture was ultrasonically treated to obtain a
homogeneous suspension. Then the suspension was allowed to cool down to room temperature
followed by the addition of 0.5 mL EG solution of chloroplatinic acid (H2PtCl6•xH2O)
containing 2 mg mL-1 Pt and the pH value was adjusted by dropping 1 mol L-1 NaOH to pH=9
with magnetic stirring to ensure the well dispersion. And then the flask was placed in the center
of an oil bath pot refluxing at 471 K for 5 h under the protection of N2 atmosphere. After cooling
down, the pH of the suspension was adjusted to pH=2 by addition of 2 mol L-1 HCl aqueous
solution with vigorous stirring at room temperature. At the end, the product was collected by
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filter and washed several times with deionized water until free of Cl-. The final powders were
washed with ethanol and subsequently dried at 353 K in vacuum oven overnight. Pt supported on
different supports were prepared by the same method, which were denoted as Pt/oCNTs, Pt/L-
NCNTs and Pt/H-NCNTs, respectively. The synthesis process of Pt/H-NCNTs with different
loading amount (0.5 to 3.0 wt%) was similar to the description above, in addition to the
increasing precursor content.
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2.3 Catalyst characterization. Transmission electron microscopy (TEM) and high-angle
annular dark field scanning TEM (HAADF-STEM) images were acquired by using an FEI
Tecnai G2 F20 microscope equipped with HAADF-STEM detector. The TEM and STEM
images obtained from IL-TEM method were operated at 120 kV. X-ray diffraction (XRD)
patterns of CNTs and modified CNTs supported Pt NPs catalysts were characterized by an X-ray
diffractometer (D/MAX-2400) with Cu Kα source at a scan rate of 2° min-1. X-ray photoelectron
spectroscopy (XPS) spectra were carried out by ESCALAB 250 instrument with Al Kα X-rays
(1489.6 eV, 150 W, 50.0 eV pass energy). N2 physisorption was measured at 77 K using a
Micrometrics ASAP 2020 instrument. UV-Raman spectroscopy was performed on powder
samples by using a HORIBA LabRam HR Raman spectrometer. The excitation wavelength was
633 nm with a power of 0.2 mW. The loading amount of Pt was determined by a Leeman Labs
Prodigy inductively coupled plasma mass spectrometry (ICP-MS). The real Pt contents for the
different catalysts were measured by ICP through the method as the following description. 20 mg
catalysts were dispersed in 12 g concentrated nitric acid in an autoclave and kept the autoclave in
the oven at 453 K for 12 h. After acid treatment process, CNTs were completely decomposed.
And then the acid aqueous solution was diluted to a 50 mL volumetric flask.
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2.4 Catalytic reaction. The hydrogenation reaction was carried out in a 60 mL stainless
autoclave with a Teflon inner layer. Before the activity test, 10 mg catalysts were put into the
autoclave and 2 mmol nitrobenzene substrates dissolved in 4 mL ethylbenzene were added into
the autoclave, and then the autoclave was heated to 313 K and kept for 30 min. Following, the
autoclave was flushed with 5 atm hydrogen for three times. After being sealed, the autoclave was
charged with H2 until 5 atm, then it was kept at 313 K for 20 min. Some of the reactions were
also carried out at 313K or 333 K with a certain Pt/substrate ratio under 5 atm hydrogen pressure.
The ambient nitrobenzene hydrogenation reaction was carried out in a 25 mL three-necked round
bottom flask by flowing H2 in the bottom space of an ethylbenzene solution at 353 K. After the
reaction, benzene methyl ether as external standard was added and the reactants were analyzed
using gas chromatography (Agilent 7890A). The results are summarized in Tables 1-3, S3 and
Figures 7, S6-S7.
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2.5 The stability of catalysts investigated by the IL-TEM method. Prior to the reaction, a very
low amount of catalysts were deposited on the top side of a copper grid with alphabetical code.
The grid was thereafter inserted in TEM to capture the initial state of the catalyst and track the
accurate locations of the catalyst using the alphabetical code. The TEM grid acts as a shuttle,
which allows the transfer of Pt/CNT catalysts between the electron microscope and the liquid
reaction environment. Then the TEM grid was transferred to a specimen stage and participated
the nitrobenzene hydrogenation reaction at 353 K under ambient pressure in a 25 mL three-
necked round bottom flask through continuous bubble method (2 mmol nitrobenzene in 10 mL
ethylbenzene with 5 mg catalysts and the activity data are presented in supporting information.
Grid without catalyst tested as reference showed no activity towards nitrobenzene
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hydrogenation). After the reaction, the TEM grid was dried thoroughly, and then transferred to
the electron microscope again to acquire the TEM images at identical locations.
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3. Results and discussion
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3.1 Synthesis and Characterization of the Catalysts
Pristine CNTs, oxygen-containing CNTs, and nitrogen doped CNTs with low- and high-level
nitrogen content were prepared as supports, donated as CNTs, oCNTs, L-NCNTs and H-NCNTs,
respectively. Figure S1 exhibits the low-magnification TEM images of these four supports. H-
NCNTs are typical bamboo-like structures due to the incorporation of large amount of nitrogen
into the bulk CNTs. The features of surface functional groups and doping species on these four
kinds of CNTs are distinct revealed by XPS spectra (Figures S2-S3 and Table S1). The treatment
of concentrated HNO3 and H2SO4 introduces some oxygen functional groups to the graphitic
surface of CNTs, such as carbonyl (531.0 eV), carboxyl (532.1 eV), and hydroxyl (533.3 eV) as
shown in Figure S2a.42 Thermal treatment under NH3 atmosphere at high temperature introduces
low amount of nitrogen species (Figure S2b). High-level nitrogen doped CNTs can be achieved
by using nitrogen containing precursor during the CVD growth.43 The nitrogen species on H-
NCNTs are pyridinic (NPy 398.1 eV), pyrrolic or amine (NAm 399.8 eV), graphitic (NG 400.6-
400.8 eV) and oxidized (404.5) nitrogen as revealed by XPS (Figure S3).44 The supports were
also characterized by Raman spectra and N2 physisorption absorptions, as shown in Figure S4
and Table S2.
A simple ethylene glycerol reduction method was chosen to homogeneously deposit tiny Pt
NPs on the surface modified CNTs, marked as Pt/CNTs, Pt/oCNTs, Pt/L-NCNTs and Pt/H-
NCNTs. The Pt contents were examined as 0.8, 1.1, 1.0, and 1.1 wt%, respectively. It can be
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seen that Pt contents are slightly enhanced by introduction of oxygen groups or nitrogen
heteroatoms on the surface of CNTs, which is ascribed to the more anchor sites for Pt NPs.
Figure 1 shows the representative low-magnification HAADF-STEM images of Pt-based
catalysts. Highly dispersed and ultra-small Pt NPs are observed. Statistical analysis of PSD was
carried out by measuring ca. 350 Pt NPs (insets of Figure 1). The particle sizes are all in the
range of 0.6-2.0 nm. The average sizes are 1.4, 1.4, 1.3, and 1.2 nm regarding Pt/CNTs, Pt/oCNT,
Pt/L-NCNTs and Pt/H-NCNTs, respectively. Pt/H-NCNTs catalyst displays a well dispersion of
Pt NPs despite the smallest surface area. A more detailed analysis on PSD reveals that Pt NPs on
H-NCNTs exhibited a little narrower PSD with 0.6-1.5 nm and a slightly smaller mean size of
1.2 nm. Usually, the particle size of Pt NPs supported on nanocarbons is related to the surface
functional species amount and the interaction between the support and metal NPs.31 It indicates a
strong metal-support interaction (SMSI) effect in Pt/H-NCNTs sample. Energy dispersive X-ray
spectroscopy (EDX) elemental maps over different catalysts (Figure 2) illustrate the well
distribution of O, N, and Pt in each sample.
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X-ray diffraction (XRD) patterns of CNTs and Pt-based catalysts are shown in Figure 3. The
main diffraction peaks at 26.3o and 42.2o are ascribed to (002) and (101) planes of the graphitic-
type lattice, respectively. No obvious diffraction peak assigned to Pt NPs is found in XRD
patterns, indicating that Pt NPs exist as very small size and good dispersion. In local regions, the
selected area electron diffraction (SAED) patterns (Figure 4) show only the diffraction rings of
CNTs. There are no diffraction spots regarding Pt NPs, illustrating ultra-small size Pt NPs with
low crystallinity stabilized on CNT surface, which is in well accordance with the XRD results.
There are several weak peaks in the XRD pattern of H-NCNT besides the main CNT peaks
(Figure 3b), which is attributed to the residual metal NPs capsulated into H-NCNTs. Based on
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aforementioned STEM, XRD, and SAED analysis, it indicates that Pt NPs existed as small
particles with low crystallinity on the surface-modified CNTs.
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XPS is employed to investigate the dispersion of Pt NPs45,46 and the electronic interaction
between supports and Pt NPs based on the intensity ratio and binding energy (BE) shift of Pt
4f.47,48 There is variation of surface Pt ratio on modified CNTs. Pt NPs supported on H-NCNTs
exhibits the highest surface exposure of Pt content, which is mainly attributed to the lowest
specific surface area among the samples (Table S1). The Pt 4f XPS spectra of the series catalysts
(Figure 5) present two main peaks at about 71.6 and 74.8 eV corresponding to the spin–orbit split
doublet of Pt 4f7/2 and 4f5/2, respectively. Pt 4f spectra are deconvoluted into three components,
including the predominant metallic Pt (Pt0), oxide states Pt (Pt2+), and Pt (Pt4+) in detail. The
most intense doublet with BE of 71.5 eV (Pt 4f7/2) and 74.8 eV (Pt 4f5/2) is attributed to metallic
Pt. Peaks at 72.3 eV (Pt 4f7/2) and 75.6 eV (Pt 4f5/2) are assigned to Pt2+ that anchored with the C-
O groups on the surface of CNTs.49,50 The functional species on the CNT surface have great
influence on the distribution of Pt species. Pt NPs supported on H-NCNTs tends to have more Pt0
than that of the other three catalysts, indicating that the introduction of nitrogen on CNTs is in
favor of forming Pt0 species during the EG reduction process. The Pt2+ species on oCNTs and L-
NCNTs can be ascribed to the surface oxygen-containing groups, such as hydroxyl and carboxyl
groups. Due to the weak reduction capability of EG, small amounts of Pt4+ still exist. The BE
shift of Pt 4f is analyzed to unravel the electronic interaction between the distinct functional
species and Pt NPs. Accordingly, it is worthy noticed that Pt 4f BE towards the low-energy side
can be observed obviously in Pt/H-NCNTs sample, but no obvious shift in Pt/L-NCNTs because
of the low N content. In the case of Pt/H-NCNTs, the local electronic structure of Pt NPs can be
significantly modified by the interaction with a large amount of N species. Compared the N 1s
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XPS spectra of L-NCNTs and Pt/L-NCNTs (Figure S2b), a noticeable shift from the lower to the
higher BE of the N 1s spectra was observed after Pt loading, which strongly implies the charge
transfer from N species to Pt NPs. Thus, introduction of nitrogen species into the CNT structures
generates an increased electron density of Pt, indicating a more electron richened chemical state
of Pt NPs.
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However, nitrogen doped CNTs are always a complex system because of the coexistence of
various N species and it is still under debated that which type of N functionality is responsible
for the electronic interaction. Thus, a series of Pt/H-NCNTs with different Pt loading amount
from 0.5 to 3.0 wt% were prepared to investigate the interaction between the distinct N species
and Pt NPs. The Pt contents were determined as 0.2, 0.4, 0.9, 2.2 wt% by ICP. Figure S5 shows
the STEM images of the series Pt/H-NCNTs catalysts, elucidating the homogeneously dispersed
Pt NPs on the support. With the increasing of Pt loading amount, the sizes of Pt NPs show the
tendency of growing up. XPS spectra (Figure 6 and Table S3) revealed that along with the
gradually increasing Pt content, obvious shifting of Pt 4f BE towards low-energy side can be
observed, which is probably due to the size effect of Pt NPs.51,52 Accordingly graphitic N
gradually shifts to the high-energy side while no change to the oxygen functional groups,
strongly implying the electronic interaction is only occurred between NG and Pt NPs. Such an
electronic interaction is demonstrated to partial filling the valence band of Pt, resulting in the
upshift of d-band center.53
3.2 Catalytic Hydrogenation of Nitroarenes to the corresponding Aniline
The hydrogenation of nitroarenes is used as a model reaction to probe the effects of the
different surface-modified CNTs on the supported Pt NPs. The catalytic performance of
nitrobenzene hydrogenation is summarized in Table 1 and Figure S6, with blank experiment and
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the intrinsic activities of the supports (CNTs, oCNTs, L-NCNTs, and H-NCNTs). The
conversion of nitrobenzene hydrogenation over distinct catalysts decreased in the following
order: Pt/H-NCNTs > Pt/L-NCNTs > Pt/oCNTs > Pt/CNTs, and the activity of Pt/H-NCNTs,
Pt/L-NCNTs, Pt/oCNTs, and Pt/CNTs catalysts normalized by Pt loading amount are 189.3,
142.3, 114.3, and 67.1 mol-AN/(mol-Pt min), respectively. Compared with the commercially
available carbon black supported Pt catalyst (5 wt% Pt/C, purchased from Alfa Aesar), the used
catalysts herein all performed much higher reactivity and more than 90% selectivity for aniline,
which is attributed to the well dispersed Pt NPs on the surface-modified CNTs synthesized by
EG method. The reactions at 353 K under ambient condition (Figure S7) revealed that Pt/H-
NCNTs displayed an enhanced activity. The activity (Table S3) of Pt/H-NCNTs catalysts with
different loading amount are 215.8 (0.2 wt%), 209.1 (0.4 wt%), 188.8 (0.9 wt%), and 110.1 (2.2
wt%) mol-AN/(mol-Pt min), respectively. The decreased activity with the increased Pt loading
amount and the liner relationship between the activity of the surface-modified CNTs supported
Pt NPs catalysts and the amount of surface exposed Pt (Figure S6b) indicate that the activity
towards nitrobenzene hydrogenation mainly depends on the dispersion of the active metal and a
better dispersion of Pt can be achieved on H-NCNTs support, which is in accordance with the
reported literatures that the hydrogenation of nitrobenzene over Pt supported catalysts appears to
be size insensitive.54-56
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When the halonitrobenzene is used as a substrate, most Pt-based catalysts suffer from the
hydrogenolysis of weak carbon−halogen (C−X) bond (dehalogenation in this work) reported
previously,57,58 particularly at high conversions. Table 2 displayed the chemoselective
hydrogenation of halonitrobenzene over Pt/oCNTs and Pt/H-NCNTs catalysts. Pt/H-NCNTs
exhibits higher conversion as well as 100% selectivity towards chloronitrobenene hydrogenation
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compared with Pt/oCNTs, avoiding C−Cl bond scissions even at a total conversion and no
further decrease of chloroaniline products was observed over Pt/H-NCNTs even extending the
reaction duration after complete conversion of chloronitrobenene. In any case, the selectivity of
Pt/H-NCNTs catalyst achieved 100% for the halonitroaromatic hydrogenation of nitro groups
into the corresponding amino groups. However, Pt/oCNTs showed lower selectivity (95, 93, and
89% for chloro-, bromo-, and iodo- substrates, respectively) towards nitro group at the total
conversion of substrates and the selectivity decreased following the order of Cl-, Br-, I-, which is
in agreement with the susceptibility to hydrogenlysis induced by the increased atomic number as
well as electronegativity of halogen groups. The electronic enriched chemical state of Pt NPs on
H-NCNTs inhibits dechlorination while the higher binding energy of Pt on oCNTs impels more
active hydrogen species to attack C-X bond or produces specific absorption towards C-X groups,
therefore facilitates the dehalogenation of halonitrobenzene. This hypothesis supported by XPS
results is consistent with the recently publications reported by Iihama et al.59,60 Figure S8
demonstrates that all the catalysts have similar Pt species distribution and the electronic
interaction is also preserved after the reaction. Herein, through tuning the interaction between Pt
NPs and the heteroatoms doping or functional groups on CNT surface, high hydrogenation
capacity with superior selectivity catalyst for halonitrobenzene can be achieved.
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Following the success of nitrobenzene and halonitrobenzene hydrogenation over Pt/H-NCNTs,
it may display good performance when extended to other substituted nitroarenes. The results
indicate that Pt/H-NCNTs catalyst illustrated excellent tolerance to a wide scope of substituted
nitroarens containing electron-donating or electron-withdrawing groups (Table 3). The catalyst
showed high conversion as well as good selectivity to the corresponding anilines with the
presence of -CH3, -F, -CF3, and -O-CH3 groups (entries 2, 3, 6, and 10 in Table 3). Furthermore,
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Pt/H-NCNTs continued to preserve an excellent chemoselectivity towards the hydrogenation of
nitroarene substituent with other reducible groups, such as -C=O, O=C-O, and -CN, which is the
grand challenge for the application of selective hydrogenation of the substituted nitroarenes
(entries 4, 5, 7, 8, and 9 in Table 3).
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The major advantage of heterogeneous catalyst is its stability under reaction conditions
compared with homogeneous catalyst. The conversion of nitrobenzene over Pt/H-NCNTs is
98.6% along with aniline selectivity of 99.8% after four successive runs and can be recovered
from the reaction medium through simple filtration, as shown in Figure 7a. The catalyst stability
was further demonstrated at a low conversion, as shown in Figure 7b. While Pt/oCNTs suffered
from a decrease in activity for the second use (Conv. 47% and Sel. 96%), implying the stronger
anchoring effect of H-NCNTs support than that of oCNTs for Pt NPs. The separated Pt/H-
NCNTs catalyst after the fourth reaction run was examined by ICP, TEM and HAADF-STEM in
sequence. The Pt contents were determined as 1.0 wt% after four cycles, close to the initial Pt
content 1.1 wt%. Comparing with the TEM and HAADF-STEM images of fresh Pt/H-NCNTs
catalyst, the recycled catalysts do not show any obvious changes (Figure S9). The small-sized Pt
NPs are still homogeneously dispersed on the surface of H-NCNTs, and the main PSD is 0.9-1.8
nm, identical to that of the fresh catalyst (0.8-1.6 nm).
3.3 The Stability of the Catalysts studied by IL-TEM method
We use IL-TEM61,62 to follow the structure-evolution and the stability of the catalysts, where
identical locations of a catalyst NP before and after the treatment in liquid reaction conditions
can be investigated and compared. IL-TEM method in liquid reaction for accurate investigation
of the structure-evolution is shown in Figure 8. The acquired TEM images of Pt/oCNTs and
Pt/H-NCNTs catalysts at identical location before and after the reaction for 30 min are shown in
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Figure 9. Pt NPs coalescence due to the particle migration on support surface occurred in
Pt/oCNTs catalyst under reaction conditions, and the same phenomena are not observed in Pt/H-
NCNTs sample at the selected areas. TEM images of several sub-regions for these two catalysts
at increasing magnifications were tracked for a reliable understanding and distinguishing the
structure-evolution between them under reaction conditions (Figure S11 for Pt/oCNTs and
Figure S12 for Pt/H-NCNTs). A quantitative evaluation of the catalysts regions visualized in the
IL-TEM images is shown in Figure 9d and h, which compares the PSD of the catalysts before
and after the participation of the liquid reaction. The average particle size increases from 1.3 nm
to 1.8 nm in Pt/oCNTs, while a slightly change in Pt/H-NCNTs sample and more obvious
particle detachment on Pt/oCNTs than that on Pt/H-NCNTs is observed. Particle detachment and
migration are a consequence of a weak interaction between Pt NPs and support, indicating a
more strong anchoring effect in H-NCNTs than that in oCNTs for Pt NPs. Long term stability on
Pt/H-NCNTs catalyst was also evaluated by this strategy at several selected regions in Pt/H-
NCNTs, suggesting that the particle size and dispersion are barely changed before and after
nitrobenzene hydrogenation with different time at an elevated temperature (353 K) under
ambient atmosphere, as shown in Figures 10, S13 and S14. Pt NPs are well anchored on the
surface of H-NCNTs, exhibiting excellent stability under reaction conditions, which is in
consistence with the good cycling performance.
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Aiming to reveal the structure-function relationship, traditional approaches based on the
comparison between fresh and used catalysts in liquid phase reactions always reach firm
conclusions in conjunction with extensive statistical evaluations by observing different locations
of the sample, which are useful but not always straightforward.63 Some structure changes (e.g.,
particle detachment, dissolution, or agglomeration, particle reconstruction or encapsulation, and
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carbon support corrosion), however, are not directly traced and evidenced, originating from the
reaction or the synthesis procedure. Herein, i) IL-TEM is able to complement the routine
microscopy investigations on the structure-evolution and stability of catalysts, which provide
detailed information of a catalyst at identical locations during liquid reactions; ii) IL-TEM opens
up new opportunities for the visualization of structural evolution on nanoscale and well
establishment of the structure-function relationship; iii) The simple and non-destructive IL-TEM
can be extended to other reaction systems where the conditions or chemical potential can never
be mimicked in environmental TEM or using an environmental cell.64
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4. Conclusions
A series of Pt NPs supported on the surface-modified carbon nanotubes with uniform particle
size distribution were synthesized by ethylene glycol reduction method. The introduction of
nitrogen substantially improved the catalytic performance towards nitrobenzene hydrogenation
under mild conditions, which is ascribed to a better dispersion of Pt NPs. In addition, Pt/H-
NCNTs catalyst exhibited superior stability under reaction conditions revealed by IL-TEM, in
consistent with the good recyclability, implying a strong anchoring effect of N species on CNT
surface. Compared with Pt/oCNTs, Pt/H-NCNTs represented the exclusively selective
hydrogenation towards nitro groups in halonitrobenzene hydrogenation, which is caused by the
electronic interaction between Pt NPs and NG species on CNT surface. The electron-donating
effect of NG species increases the electron density of the supported Pt NPs, such a binding energy
shift resulting in a superior chemoselectivity hydrogenation capacity towards halonitroarenes.
Furthermore, Pt/H-NCNTs catalyst also afforded high selectivity towards nitro group reduction
to a wide scope of substituted nitroarenes (including -CH3, -O-CH3, -C=O, O=C-O and -CN
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groups). The current insight suggests a new route for rational designing high-performance
catalysts through tuning the surface properties of the nanocarbon based supports and thus
enhancing the catalytic hydrogenation performance.
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Supporting Information Available.
The supporting information includes the characterization of CNTs supports and the supported
Pt catalysts, the catalytic performance of the modified CNTs supported Pt NPs catalysts for
nitrobenzene hydrogenation under ambient pressure and the TEM results obtained from the IL-
TEM method. The characterizations include TEM images and corresponding SAED, N2
adsorption-desorption isotherms and pore size distribution, Raman and XPS.
Acknowledgements.
The authors gratefully acknowledge the financial support provided by the National Natural
Science Foundation of China (No.91545119, 21203215, 21133010, 51221264, and
21261160487), the Youth Innovation Promotion Association CAS (2015152), the Joint
Foundation of Liaoning Province National Science Foundation and Shenyang National
Laboratory for Materials Science (2015021011) and Strategic Priority Research Program of the
Chinese Academy of Sciences (XDA09030103).
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Figure 1. HAADF-STEM images of Pt/CNTs (a), Pt/oCNTs (b), Pt/L-NCNTs (c) and Pt/H-
NCNTs (d). The insets are the corresponding PSD histograms.
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Figure 2. EDX elemental maps of Pt/CNTs (a), Pt/oCNTs (b), Pt/L-NCNTs (c) and Pt/H-NCNTs
(d) samples.
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Figure 3. XRD patterns of the pristine CNTs, Pt/CNTs, Pt/oCNTs, and Pt/L-NCNTs (a), and H-
NCNTs and Pt/H-NCNTs (b).
Figure 4. Low-magnification TEM images and corresponding SAED patterns of Pt/CNTs (a),
Pt/oCNTs (b), Pt/L-NCNT (c) and Pt/H-NCNTs (d).
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Figure 5. Pt 4f XPS spectra of Pt/CNTs, Pt/oCNTs, Pt/L-NCNTs and Pt/H-NCNTs catalysts.
Figure 6. Pt 4f (a), N 1s (b) and O 1s (c) XPS spectra of Pt/H-NCNTs with varied Pt contents.
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Figure 7. Recycling test of Pt/H-NCNTs for nitrobenzene hydrogenation to aniline at high
conversion (a) and low conversion (b). (Reaction condition: T = 40 oC, P = 5 atm, Pt/Substrate =
0.025%, 2 mmol nitrobenzene, 4 mL ethylbenzene as solvent. Time = 20 min (a). Reaction
condition: T = 40 oC, P = 5 atm, Pt/Substrate = 0.01%, 4 mmol nitrobenzene, 4 mL ethylbenzene
as solvent. Time = 20 min (b)).
Figure 8. TEM grid with alphabetical code (a), specimen stage (b) and schematic diagram of the
reactor (c). The TEM grid acts as a shuttle, which allows the transfer of the catalysts between the
electron microscope and the liquid reaction environment. TEM images at identical location show
the morphology of catalyst before (d) and after (e) reaction.
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Figure 9. TEM images of Pt/oCNTs (a) and Pt/H-NCNTs (e) catalysts, the enlargement of
selected area (outlined in white) before (b, f) and after the reaction for 30 min (c, g), and the
corresponding PSD histograms (d, h) by quantifying a series of TEM images at identical
locations.
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Figure 10. STEM images at identical location show the morphology of Pt/H-NCNTs catalyst
before (a-c) and at the reaction time of 60 min (d-f) and 120 min (g-i). The insets are the
corresponding PSD.
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Table 1. Nitrobenzene hydrogenation over different Pt-based catalysts.
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Activity data
Entry
Catalyst
Conv. (%) Sel. (%)
(mol-AN/(mol-Pt min))
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11
12
13
14
15
16
17
18
19
20
21
22
23
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30
31
32
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35
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1
2
blank
CNTs
2.5
3.3
3.1
3.4
3.3
37
-
-
-
-
3
oCNTs
-
-
4
L-NCNTs
H-NCNTs
Pt/CNTs
-
-
5
-
-
6
93
93
96
99
72
67.1
114.3
142.3
189.3
53.37
7
Pt/oCNTs
Pt/L-NCNTs
Pt/H-NCNTs
5 wt% Pt/C
63
8
76
9
98
10
38
Reaction condition: T = 40 oC, P = 5 atm, 10 mg catalysts, 2 mmol nitrobenzene, 4 mL
ethylbenzene as solvent. 5 wt% Pt/C commercial catalyst was purchased from Alfa Aesar.
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Table 2. Chemoselective hydrogenation of halonitrobenzenes over different catalysts.
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Conversion
Selectivity
(%)
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9
Entry
Reaction
(%) [t(h)]
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65[0.5][a]
95 [a]
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95[0.5][a]
98[1.5] [b]
100 [a]
95[b]
98[1.5] [b]
99[2] [b]
100[b]
93[b]
99[2] [b]
100[b]
89[c]
100[2] [c]
100[2] [c]
100[c]
Reaction conditions: [a] T = 40 oC, P = 5 atm, Pt/Substrate = 0.025%, [b] T = 40 oC, P = 5 atm, Pt/Substrate = 0.05%,
[c] T = 40 oC, P = 5 atm, Pt/Substrate = 0.1%, ethylbenzene as solvent.
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