Internally Supported Metal-Oxide Nanocatalyst for Hydrogenation of Nitroaromatics

Article abstract of DOI:10.1021/acs.langmuir.7b04200

The uncalcined but highly dispersive oxide-supported metal catalyst for liquid phase reactions may suffer from the agglomeration of metal nanoparticles and the drop of metal catalyst in solution, which will decrease the activity and shorten their life in

Full text of DOI:10.1021/acs.langmuir.7b04200

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Internally Supported Metal-oxide Nanocatalyst
for Hydrogenation of Nitroaromatics
Bo Wei, Kefa Sheng, and Jianping Ge
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04200 • Publication Date (Web): 28 May 2018
Downloaded from http://pubs.acs.org on May 28, 2018
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Internally Supported Metalꢀoxide Nanocatalyst for
Hydrogenation of Nitroaromatics
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Bo Wei, Kefa Sheng and Jianping Ge*
Shanghai Key Laboratory of Green Chemistry and Chemical Processes School of Chemistry and
Molecular Engineering, East China Normal University, Shanghai, 200062, China. Eꢀmail:
jpge@chem.ecnu.edu.cn.
Keywords: metal, catalyst, internally dispersed, mesoporous, hydrogenation
Abstract: The uncalcined but highly dispersive oxide supported metal catalyst for liquid phase
reactions may suffer from the agglomeration of metal nanoparticles and the drop of metal
catalyst in solution, which will decrease the activity and shorten their life in catalysis. Here, a
oneꢀpot successive polyol reaction was developed to prepare MꢀExOy colloidal particles as
heterogeneous nanocatalysts, which merge the controlled synthesis of metal catalysts and oxide
supports, the inꢀsitu loading of catalyst and even the mesopore amplification into a highly
integrated process. Unlike the traditional surface deposited catalysts, the noble metal
nanoparticles even with a large amount of loading are internally dispersed in the mesoporous
oxide particles, which shows higher activity and stability in the hydrogenation of nitroaromatics
compared to the isolated nanocatalysts or surface deposited catalysts. The improved activity and
stability comes from the physical confinement of metal nanoparticles and good mass
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transportation of substrate/product within the support particles. This work proposed a novel
method to prepare highly dispersed metal catalysts, which could be potentially useful to
heterogeneous catalytic reactions with high throughput and long life demands.
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Introduction
Noble metal is one kind of most important nanocatalysts in heterogeneous catalysis, and its
size effect1ꢀ7, separability8ꢀ10, stability8, 11, crystal facet effect12ꢀ15 and localized surface plasmon
resonance16ꢀ18 have been widely investigated in the past decades. With the breakthrough of
synthetic method, great success has also been achieved in the utilization of these metal
nanoparticles in catalysis. For instance, Tatsuya Tsukuda’ group had synthesized a series of Au
nanoclusters with wellꢀdefined sizes, in which a sample with an optimal size exhibited highest
activity and selectivity in the oxidation of cyclohexane to cyclohexanol and cyclohexanone.4 Li
et al. had reported an oleylamineꢀassisted synthesis of Pd nanocrystals with various shapes,
which exhibited good separability and reusability in MiyauraꢀSuzuki reaction compared to
traditional homogeneous catalysts because of their different solubility in polar and nonpolar
organic solvent.8, 9 Based on similar amineꢀassisted synthesis, Zheng et al. have prepared
concave polyhedral Pt nanocrystals, which showed excellent electrocatalytic activity comparing
with the commercial Pt black due to the explosion of highꢀindex crystal facets.13
In practical heterogeneous catalysis, the metal catalysts are generally supported by various
metal oxides due to their contributions in the enhancement of activity and improvement of
catalyst life.19ꢀ22 For example, Xiao et al. have reported Au nanocatalysts supported by MgOꢀ
Al2O3 layer double oxide (LDO) for CO oxidation to CO2, where the strong metal−support
interaction renders the catalyst sinter resistance at high temperature and long life in reaction.21
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For another instance, Guo et al. have prepared flowerꢀlike Pd/MgOꢀZnO composites, which
exhibit excellent catalytic activity for CO oxidative coupling to dimethyl oxalate compare to
Pd/ZnO catalyst, because the doping of MgO can increase the number of Lewis basic sites and
facilitate the donation of electron from Pd active sites.23 Furthermore, metal oxides with large
surface area usually provide adequate positions for loading of metal nanoparticles, which is
favorable to the high dispersion of active sites. For example, Pt catalyst loaded on mesoporous
CeO2 show outstanding activity and high selectivity for CO oxidation in the presence of
hydrogen due to large surface area of catalyst support.24 Traditionally, the metalꢀoxide
heterogeneous catalysts are prepared by deposition or inꢀsitu generation of metal catalysts on the
surface of metal oxides, which directly expose the active sites to the substrate molecules and lead
to a high activity in catalysis. However, when these catalysts are used in liquid phase reactions,
they usually are not calcined to insure the good dispersion in solution and sufficient interaction
with the substrate molecules. Without calcination, the interaction between metal catalyst and
oxide support is relatively weak so that the metal catalysts may drop from the oxide support and
agglomerate in the solution, which decrease the activity and shorten the life of nanocatalysts.
Therefore, a heterogeneous nanocatalyst for liquid phase reaction which possesses the high
activity and good stability at the same time will be highly desired.
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In this work, we shall report a oneꢀpot successive polyol reaction to synthesize metal
nanocatalysts supported within the mesoporous oxide, which integrated the controlled synthesis
of metal catalysts and oxide supports, the inꢀsitu loading of catalyst and even the mesopore
amplification into a compatible process. Unlike the traditional surface deposited catalysts, the
noble metal nanoparticles are internally dispersed inside the mesoporous oxide particles, which
shows higher activity and stability in the hydrogenation of nitroaromatics. The improved activity
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and stability comes from the physical confinement of metal nanoparticles and good mass
transportation of substrate/product within the mesoporous oxide particles. Moreover, we shall
also investigate the promotion of catalyst activities by amplification of mesopores in oxide
particles through protective etching.
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Experimental
Materials: Palladium(II) acetylacetonate (Pd(acac)2, 99%), platinum(II) acetylacetonate
(Pt(acac)2, 98%), gold (III) chloride trihydrate (AuCl3·H2O, 99%) and ethylene glycol (EG, 99%)
were purchased from J&K Co. Ltd. Zinc acetate (ZnAc2, 99%) and cerium(III) nitrate
hexahydrate (Ce(NO3)3·6H2O, 99.5%) were obtained from Aladdin CO. Ltd. Polyvinyl
pyrrolidone (PVP, Mw = 29,000) and diethylene glycol (DEG, 99%) were purchased from Sigma
Aldrich. Copper(II) acetate monohydrate (CuAc2·H2O, 98%), hydrochloric acid (HCl, 36ꢀ38
vol%), sulfuric acid (H2SO4, 95ꢀ98%) and ethanol (EtOH, 99.7%) were obtained from
Sinopharm Chemical Reagent Co. Ltd. All chemicals were directly used without further
treatments.
Synthesis of Pd, Au and Pt nanoparticles: Monodisperse Pd nanoparticles (NPs) were
synthesized through pyrolysis of Pd(acac)2 in DEG at high temperature with PVP as surfactants.
Typically, Pd(acac)2 (80 mg) was first dispersed in DEG (5 mL) by sonication to generate a
yellow suspension. At the same time, PVP (400 mg) was added to DEG (15 mL), which was
heated to 160 °C to form a transparent and pale yellow solution. Once the Pd precursor was
injected to the PVP solution, it turned to black mixture rapidly in 1 min, which was maintained at
160 °C for 1 hour to produce Pd nanoparticles. The reaction solution was cooled down to room
temperature and directly kept as a stock solution of Pd NPs (1.39 mg/mL) for the following
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synthesis. The Pt nanoparticles were prepared by similar polyol process except that Pt(acac)2 (80
mg) was used as precursor. The Au nanoparticles could be synthesized by this method, where
HAuCl4 (80 mg) and oleyamine (0.5 mL) were used as precursor and surfactant, respectively.
Their reaction solutions were also kept as Au NPs (1.96 mg/mL) and Pt NPs (1.98 mg/mL) stock
solutions for the following synthesis.
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Synthesis of Pd-ExOy (E = Zn, Cu, Ce) composite nanostructures: For the synthesis of Pdꢀ
ZnO nanocomposites, the aforementioned stock solution of Pd NPs (6.17 mL) was mixed with
ZnAc2 (2 mmol) and DEG (13.83 mL) to form a black homogeneous solution. Then, it was
heated to 160 °C and kept at that temperature for 2 hours to produce PdꢀZnO composite
nanoparticles. After the reaction solution was cooled down to room temperature, the PdꢀZnO
particles were separated by centrifugation and washed by ethanol for several times. The
theoretical loading amount of Pd was calculated to be 5% according to the dosage of Pd and Zn,
and it could be adjusted from 1% to 15% by changing the volume of Pd NPs stock solutions in
the synthesis.
For the synthesis of PdꢀCu2O nanocomposites, the Pd NPs stock solution (5.43 mL) was
mixed with CuAc2·H2O (2 mmol), PVP (4 mmol) and DEG (14.57 mL) to form a greenish black
solution. Then, it was heated to 155 °C under N2 protection and kept at that temperature for 1
hour to produce PdꢀCu2O composite nanoparticles. After the reaction solution was cooled down
to room temperature, the PdꢀCu2O particles with 5% of Pd were separated by centrifugation and
washed by ethanol for several times.
For the synthesis of PdꢀCeO2 nanocomposites, the Pd NPs stock solution (39.16 mL) was
mixed with Ce(NO3)3·6H2O (6 mmol), PVP (10 mmol) and DEG (0.84 mL) to form a black
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solution. Then, it was heated to 155 °C under N2 protection and kept at that temperature for 1
hour to produce PdꢀCeO2 composite nanoparticles with 5% of Pd. After the reaction solution was
cooled down to room temperature, the PdꢀCeO2 particles were separated by centrifugation and
washed by ethanol for several times.
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Synthesis of M-ZnO (M = Pd, Au, Pt) composite nanostructures: The PtꢀZnO and AuꢀZnO
nanocomposites were prepared by similar reaction and postꢀtreatments as described in the
synthesis of PdꢀZnO. The Pt NPs stock solution (4.32 mL) and the Au NPs stock solution (4.37
mL) were used as precursors respectively, which generate composite particles with theoretical
loading amount of noble metals to be 5%.
Synthesis of porous Pd-ZnO nanocomposites by protective etching: The porous PdꢀZnO
could be directly prepared by etching the freshly generated PdꢀZnO particles in their original
reaction solution. After the growth of PdꢀZnO particles was performed at 160 °C for 2 hours, the
original reaction solution was cooled down to 120 °C and maintained at that temperature for 10
min. Then, the aqueous solution of HCl (0.1 mL, 12 mol/L) was quickly injected to the above
dispersion to initiate the etching. After 20 min etching, the reaction solution was cooled down to
room temperature and the porous PdꢀZnO nanocomposites could be obtained by centrifugation,
ethanol washing and drying in oven.
Characterization: Transmission electron microscope images (TEM) and highꢀangle annular
darkꢀfield (HAADF) scanning transmission electron microscope (STEM) images were captured
by a FEI Tecnai G2 F30 microscope operated at 300 kV. Powder Xꢀray diffraction (XRD)
patterns were measured on Rigaku Ultima IV Xꢀray diffractometer operated at 35 kV and 40 mA
with Niꢀfiltered CuꢀKα radiation as Xꢀray beam source. Nitrogen adsorptionꢀdesorption
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isotherms and BET (BrunauerꢀEmmettꢀTeller) surface areas were measured by a BelsorpꢀMax
analyzer at 77 K. The precise mass percentages of Pd content were measured by Thermo IRIS
Intrepid II XSP inductively coupled plasma atomic emission spectrometer (ICPꢀAES). The
conversion of substrate in catalysis were analyzed by Agilent 7890A gas chromatography (GC).
Xꢀray photoelectron spectroscopy (XPS) analyses on the surface of the samples were performed
in AXIS Ultra DLD. Metal dispersion were measured by chemical absorption of CO on
Micromeritics AutoChem II chemisorption Analyzer.
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Catalytic hydrogenation of Nitroaromatics: Typically, nitrobenzene (51 ꢁL, 0.5 mmol) and
the PdꢀExOy nanocatalyst (Pd, 0.5 mg) were dispersed in methanol (5 mL) in a reaction tube. The
reaction tube was vacuumed and filled with H2 for 3 times, and then it was connected to H2
source (1 atm) during the reaction. With the presence of nanocatalyst, the hydrogenation of
Nitroaromatics were performed at room temperature. The reaction solution was sampled at
specific times and the conversion of Nitroaromatics can be analyzed by gas chromatography.
Results and Discussions
The PdꢀZnO composite nanostructures were prepared by a polyol synthesis of ZnO in the
presence of freshly made Pd nanoparticles (NPs). (Figure 1) In a typical process, monodisperse
Pd NPs were first synthesized through pyrolysis of Pd(acac)2 in DEG at high temperature with
PVP as surfactants. Without any post treatment, the reaction solution containing the Pd NPs were
directly mixed with the polyol solution of ZnAc2 to form a new homogenous suspension. After
the temperature being raised to 160 °C, zinc oxide nanocrystals began to nucleate and then
agglomerate with Pd NPs to form a small and heterogeneous nanocrystal cluster. TEM images of
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PdꢀZnO particles produced by 30 min of reaction clearly confirmed that the Pd NPs had been
attached to the ZnO particles since the beginning of reaction. As the polyol reaction proceeded,
the heterogeneous agglomeration of Pd NPs and ZnO nanocrystals continued, so that the small
clusters eventually grew into uniform composite nanostructures with Pd NPs embedded inside
them.
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The asꢀprepared PdꢀZnO composite nanostructures were composed of a large number of
internally dispersed Pd NPs and the porous ZnO framework, which formed useful structural
characteristics for heterogeneous nanocatalysts. In order to locate the distribution of Pd, a typical
PdꢀZnO sample with theoretically 5% of Pd were embedded in resin and cut by quartz knife for
TEM observation. (Figure 2) The crossꢀsection TEM images of almost every composite particle
indicated that Pd NPs were distributed inside the ZnO particle but not on its surface. Highꢀangle
annular darkꢀfield scanning transmission electron microscope (HAADFꢀSTEM) image as well as
the energy dispersive Xꢀray spectroscopy (EDS) mapping of a single PdꢀZnO particle also
confirmed the above TEM observations, and the measured Zn/Pd ratio (97.6/2.4) was consisted
with the ICPꢀAES results as well. The STEM images of the partially etched PdꢀZnO particle
showed the encapsulated Pd NPs more clearly. The high dispersity of Pd NPs not only ensured
the synergistic interaction between catalyst active sites and supporting materials, but also
inhibited the loss or merging of Pd NPs during the catalytic reaction. On the other hand, the
crossꢀsection TEM images suggested that the ZnO particles should have porous characteristics
since they were composed of small ZnO nanocrystals. The clusterꢀlike structure of ZnO actually
agreed with many colloidal nanoparticles synthesized in polyol process, and the induced porous
structure would effectively contribute to the transportation of substrate molecules and their
access to the Pd nanocatalyst deeply embedded inside the ZnO particles. In a word, an interesting
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combination of internally dispersed and confined Pd and a slightly porous ZnO as catalyst
supports might form an ideal nanocatalyst for effective and longꢀlife catalysis.
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The oneꢀpot successive polyol reactions reported in this work provided a convenient and much
simplified method for the synthesis of supported nanocatalysts, which integrated the controllable
synthesis of metal catalysts and support materials, the inꢀsitu loading of catalyst and even the
mesopore construction in a compatible reaction system. Thanks to the fast development of nano
synthesis in polyol solution, metal nanostructures with various chemical compositions, shapes
and exposed facets could be obtained by polyol reaction. Without any post treatment, the reaction
solution of metal nanostructures could be directly introduced to the preparation of oxide
supporting materials, which produced highly dispersed nanocatalyst within oxide particles. The
addition of appropriate acid or base could controllably etch the oxide particles to create more
mesopores and thereby increase the reactivity. Although the total synthesis was composed of
several steps, these reactions actually could be performed in a same flask and no separation or
purification were required between each steps, which effectively simplified the procedures for
preparation of composite nanostructures.
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In addition to the simplification of procedures in synthesis, the current oneꢀpot reactions also
provided good tunabilities and flexibilities to the choice of metal catalyst and oxide supports. For
example, PdꢀZnO, PdꢀCu2O and PdꢀCeO2 composite particles could be prepared in the presence
of Pd NPs, when ZnAc2, CuAc2 and Ce(NO3)3 were used as reactants respectively in polyol
reactions. (Figure 3) TEM images of these particles showed that they all had similar structures,
where Pd NPs were internally dispersed inside the oxide particles. The only difference lies in the
small particle size of PdꢀCu2O (40 nm) compared to the other two samples (150 nm), which
might be caused by the relatively fast nucleation and growing speed of Cu2O in poly reaction.
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The XRD patterns suggested that the composite nanostructures were majorly composed of
crystalline oxides, while the diffraction peak around 40° corresponding to the (111) plane of Pd
indicated that each of them contained a small amount of Pd, both of which were well consistent
with the formation of PdꢀExOy composite particles. For another instance, MꢀZnO with different
metallic active sites could be obtained when different metal NPs were introduced to the polyol
synthesis of ZnO. (Figure 4) For demonstration, premade monodisperse Pd NPs (10 nm), Au
NPs (10 nm) and Pt (5 nm) were coupled to the pyrolysis of ZnAc2 at high temperature, which
produced PdꢀZnO, AuꢀZnO and PtꢀZnO composite particles, respectively. Similarly, TEM
images showed that the metal nanoparticles were dispersed inside ZnO, and the XRD patterns
confirms the formation of composite nanostructures. The great flexibility in tuning the metals
catalysts and oxide supports not only provided ideal materials for the systematic study of
catalytic mechanism but also lay a solid foundation for the screening of highly active
nanocatalysts.
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In the following discussion, we shall demonstrate the structural advantages of the asꢀprepared
nanocatalysts through hydrogenation of nitroaromatics, which was widely used as a probe
reaction to evaluate the activity and stability of metal catalysts. (Table S1) In a typical reaction,
the substrate (0.5 mmol) and PdꢀZnO catalyst (10 mg) were dispersed in methanol to form a
homogeneous suspension under stirring. The reaction tube was vacuumed and filled with H2, and
the reaction was then performed under atmospheric H2 at room temperature. After the removal of
nanocatalyst by centrifugation, the products were analyzed by gas chromatography. For the
catalytic hydrogenation of nitrobenzene, the yield of aniline was 64.8% in 1 hour and the
substrate were completely converted in 2 hour. For hydrogenation of 4ꢀnitroaniline and 1,3ꢀ
dinitrobenzene, the yields of pꢀphenylenediamine and mꢀphenylenediamine are only 5.3% and
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8.5% in 1 hour, and they increased to 30.0% and 48.2% in 6 hours as the hydrogenation
continued. For the hydrogenation of 4ꢀnitroacetophenone, the substrate completely reduced to 4ꢀ
aminoacetophenone in 1 hour. The PdꢀZnO showed different catalytic activities for different
nitroaromatics (Table 1), which proved that it was a universal catalyst for a broad range of
substrate in hydrogenation. Although the confinement of Pd within mesoporous ZnO does
suppress the activity to some extent, the TOF value (1.15 minꢀ1) for the hydrogenation of
nitrobenzene using 1ꢀatm H2 and 10ꢀnm Pd at room temperature was comparable to most
literature results under similar conditions (Table S2), which confirms the good accessibility to
internally dispersed metal catalysts.
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We have prepared Pd nanoparticles (NPs), ZnO particle, internally dispersed Pd(i)ꢀZnO
particles and surface deposited Pd(s)ꢀZnO particles and studied the function of Pd and ZnO in
reaction by absorption experiments and comparison of catalytic activities. Generally, H2
molecules are adsorbed on the surface of Pd nanoparticles and disassociated to active H to
reduce the nitrobenzene. The mesoporous ZnO particles serve as supporting materials to stabilize
the Pd catalysts. In the previous research works, transition metal oxide with oxygen vacancies,
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such as WOx , can behave as active sites for catalysis due to the adsorption of nitrobenzene.
However, the substrate molecules are not adsorbed to ZnO particles in this work, which is
concluded from the similar UVꢀVis absorption of nitrobenzene with and without the presence of
ZnO particles. (See supporting information Figure S1) When the Pd nanoparticles (NPs), ZnO
particle, Pd(i)ꢀZnO particles and Pd(s)ꢀZnO particles were used as catalyst for hydrogenation, the
corresponding conversion of nitrobenzene at 1 hour were determined to be 99.9%, 0%, 56.8%
and 99.4%, which proved that ZnO has no catalytic activity for hydrogenation. Therefore, the
high activity of Pd(i)ꢀZnO catalyst comes from the metal active site (Pd), and the mesoporous
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ZnO particle serve as a good supporting material to stabilize the Pd catalysts.
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The current MꢀExOy nanocatalysts provided a good opportunity to understand the influence of
supporting materials upon the electron structure and thereby the activity of catalytic centers, as
these composite particles were prepared under similar conditions and were similar in structure
either. Here, PdꢀZnO, PdꢀCu2O, PdꢀCeO2 nanocomposites (10 mg) containing the same amount
of Pd (0.5 mg) were used as catalysts for the hydrogenation of nitrobenzene (0.5 mmol) in
methanol (5 mL) at room temperature with 1ꢀatm H2. (Figure 5) The kinetic curves for the
production of aniline showed that the conversion of nitrobenzene catalyzed by PdꢀZnO was
completed in 2 hours and the conversions catalyzed by PdꢀCu2O, PdꢀCeO2 only reached 56.4%
and 35.9% at the same time. The corresponding TOF values for these hydrogenation reactions
are 0.89, 0.49 and 0.33 minꢀ1, respectively.
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In order to explain the different activities, XPS analysis for the doublet state (3d5/2 and 3d3/2)
of Pd were performed to characterize the electronic structures of Pd.23, 26, 27 The XPS spectrum
for PdꢀExOy catalysts before hydrogenation can be explained by the chemical environment in
synthesis, because they were not calcined or treated by H2 at high temperature. According to the
Figure 5, the Pd2+/Pd0 ratio for PdꢀZnO, PdꢀCu2O and PdꢀCeO2 before catalysis were determined
to be 0.14, 0 and 11.8. (Table S3) For PdꢀZnO particles, most of the Pd content are kept as Pd0
species, and only a small amount of Pd was oxidized to Pd2+ due to the oxygen in reaction
system. For PdꢀCu2O particles, only Pd0 species exist in the product because the oxidation is
suppressed by the reductant species (Cu+). It is just the opposite situation for PdꢀCeO2 particles,
where most of the Pd content are Pd2+ species since the oxidation of Pd0 is greatly promoted by
the oxidant species (Ce4+, NO3 ) in synthesis. After catalytic hydrogenation, the PdꢀZnO, Pdꢀ
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Cu2O and PdꢀCeO2 catalysts were analyzed by XPS again (Figure S2), and their Pd2+/Pd0 ratio
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were determined to be 0.246, 0.165 and 0.596, respectively. It can be concluded that a small
amount of Pd0 in PdꢀZnO and PdꢀCu2O were oxidized to Pd2+ in hydrogenation, while a net 60%
of Pd2+ in PdꢀCeO2 were reduced to Pd0 in hydrogenation. Combined with the reaction
mechanism of hydrogenation of nitroaromatics, it is not difficult to understand that PdꢀCu2O and
PdꢀZnO have better activity than PdꢀCeO2, since Pd0 was the active center for hydrogenation.
However, we cannot clarify the function of Pd2+ due to the lack of inꢀsitu characterizations in
catalysis. The catalytic activity of PdꢀZnO is higher than that of PdꢀCu2O, probably because the
electron density of Pd on Cu2O was decreased during hydrogenation due to the reduction of
surface Cu2O to Cu. The decreased electron density was unfavorable to the dissociation of H2
into active H species on Pd, so that the catalytic activity decreased accordingly.
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One advantage for the current nanocatalysts with internally dispersed noble metals was the
high stability and good reusability compared to isolated nanocrystals and surface deposited
catalyst. Here, three kinds of nanocatalysts (10 mg) with same amount of Pd (0.5 mg), including
Pd nanoparticles, Pd(s)ꢀZnO with surfaceꢀdeposited Pd (prepared by impregnation) and Pd(i)ꢀ
ZnO with internally dispersed Pd, were used for successive hydrogenation of nitrobenzene, and
its conversion was measured at 1 hour in each cycle. (Figure 6) The experimental results showed
that the initial conversions of nitrobenzene catalyzed by Pd and Pd(s)ꢀZnO were much higher
than that of Pd(i)ꢀZnO, because the isolated Pd nanoparticles dispersed in solution and the
surface deposited Pd nanoparticles had better accessibility to the substrate. The asꢀprepared
Pd(i)ꢀZnO particles only had about half of the activity in the first round probably because the Pd
nanoparticles were embedded inside the porous ZnO, which slow down the transportation of
substrate and product. However, when these catalysts were recycled for 5 successive reactions,
the conversion of nitrobenzene decreased from 99.9% to 1.6% for isolated Pd nanoparticles and
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from 99.4% to 33.2% for Pd(s)ꢀZnO, respectively. For Pd(i)ꢀZnO with internally dispersed Pd,
the conversion steadily decreased from 56.8% to 44.2%, which suggest it had higher stability
than both of the other two catalysts.
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We have also examined the structure and composition of Pd(i)ꢀZnO catalyst after
hydrogenation to further confirm its high stability in reaction. TEM images show that the Pd
nanoparticles were dispersed inside mesoporous ZnO particle, which was almost the same as
those before the reaction. (Figure S3) ICPꢀAES analysis indicated that the practical Pd content
was 4.83%, which was also close to 4.85% before the reaction. After 5 continuous reactions, we
noticed a loss of catalyst from 10.2 mg to 7.75 mg, which corresponded to a loss of 24% in mass.
This loss was inevitable in separation since the catalysts used here was only several milligrams
in weight. Meanwhile, the conversion of nitrobenzene catalyzed by Pd(i)ꢀZnO decreased from
56.8% to 44.2% after 5 cycles, which corresponded to a decline of 22% in conversion. Therefore,
it can be concluded that the decline of catalytic activity was majorly caused by the loss of
catalyst during separation. If the loss is counted in evaluation, one can find that the activity of
Pd(i)ꢀZnO are almost unchanged after 5 cycles. These results indicated that the porous ZnO
framework effectively prevents the agglomeration and drop of metal nanocatalysts during the
violent reactions. Although the initial activity of the internally dispersed nanocatalysts is lower
than that of the isolated or exposed catalyst, it still will be a preferred model for supported
catalyst when long life is required in practically reactions.
Another advantage for the MꢀExOy nanocatalysts was the high loading of metallic active sites
and thereby a good conversion efficiency in practical catalytic reactions with high throughput
demands. Similar to the impregnation process, the loading of noble metals can be precisely
controlled by adjusting the dosage of metal nanoparticle stock solution in synthesis. For the case
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of PdꢀZnO, we can prepare a series of nanocatalysts with practical Pd weight percentage of
0.7%, 4.8%, 9.7% and 13.1% through the oneꢀpot successive polyol reactions, where the Pd
contents were originally set to be 1%, 5%, 10% and 15% according to the dosage of Pd
nanoparticles. The deviation of ICPꢀMS results from the theoretical values was caused by the
incomplete encapsulation of Pd nanoparticles by ZnO, especially when the loading of Pd content
is high. It should be noted that the highest loading of Pd that can be reached by current synthesis
is about 13%, because excessive Pd nanoparticles can be found outside the composite particles
when the dosage of Pd is further increased to 20%. (Figure S4) With the increase of Pd content,
the aqueous solution of PdꢀZnO nanocatalysts turned from greyish white to black, and their XRD
patterns showed an increased peak around 40° which was attributed to the diffraction of (111)
crystal plane of elemental Pd. (Figure 7) When these PdꢀZnO nanocatalysts (10 mg) were used
for hydrogenation of nitrobenzene in methanol, they presented an accelerated conversion along
with the increasing of Pd loading, but the TOF values were measured to be 1.19 minꢀ1, 0.86 minꢀ
1, 0.48 minꢀ1 and 0.57 minꢀ1 accordingly. The low TOF value for high loading Pd(i)ꢀZnO might
be attributed to the aggregation of Pd nanoparticles inside ZnO framework, which can be
confirmed by the decreased metal dispersion for Pd catalyst. Through chemical absorption of
CO, the metal dispersions were measured to be 3.04%, 3.14%, 1.26% and 1.29% with the
increasing of Pd content. As a reference, we had also measured the metal dispersion and TOF
value of hydrogenation for Pd(s)ꢀZnO catalyst (5% Pd), which turned out to be 5.6% and 1.9
minꢀ1, respectively. The comparison of Pd(i)ꢀZnO and Pd(s)ꢀZnO catalyst suggested that the
encapsulation of Pd within mesoporous ZnO did suppress the metal dispersion and decrease the
TOF value to some extent. However, Pd(i)ꢀZnO catalysts with Pd loading of 4.8% might be the
optimal one with both good stability and catalytic activity. (Table S4) From the viewpoint of
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economy of Pd species, the internally dispersed PdꢀZnO catalysts with extremely high loading of
Pd was less valuable compared to the general catalysts. However, they could be very useful in
high throughput conversions with limited space for catalysts only if their activity were
maintained. Our experiments proved that the PdꢀZnO nanocatalyst with 13% of Pd helped to
complete almost every conversion of nitrobenzene for 10 continuous reactions, in which the
steady conversion rates around 96% showed the valuable stability of this catalyst even with
dense distribution of metals active sites.
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The activities of these MꢀExOy nanocatalysts can be further improved by amplification of
mesopores through a oneꢀpot protective etching reaction which is conveniently integrated into
the synthesis of MꢀExOy particles. (Figure 8) For the original PdꢀZnO, its BET surface area and
BJH average pore size were measured to be 22.49 m2/g and 4.77 nm by N2 adsorptionꢀdesorption
test. It indicated that the original PdꢀZnO colloidal nanocrystal clusters composed of small and
aggregated nanoparticles already had mesoporous characteristics. Once these particles were
etched by HCl solution at 120 °C in the presence of PVP as protective agent, their mesopore
diameters were found to increase to 4.92 nm, 5.13 nm and 6.82 nm with the increasing of HCl
concentration, and their surface areas increased to 37.4 m2/g, 46.1 m2/g and 47.0 m2/g
accordingly. (Table S5) The positive correlation between pore size and surface area suggested
that the acidic etching amplified the mesopores and increase the surface area. Once the etched
particles are washed and dried, their weight loss from the original PdꢀZnO are measured to be
5.3%, 11.3% and 18.4%, which was consistent with the etching reaction. TEM image of the
etched PdꢀZnO showed that many small and individual Pd nanoparticles were uniformly
dispersed inside the ZnO particle (Figure 2d), which might be favorable to the mass
transportation of reactants and products.
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The etched PdꢀZnO were proved to have higher catalytic activity due to the easy access of
substrate molecules to the Pd catalysts. Here, 4 catalysts including PdꢀZnO (10 mg), PdꢀZnO E1
(9.5 mg), PdꢀZnO E2 (8.9 mg) and PdꢀZnO E3 (8.3 mg) were used for hydrogenation of
nitrobenzene. All the catalysts contained the same amount of Pd (0.5 mg) according to the ICPꢀ
AES analysis of Pd percentage. In the parallel experiments, the conversion of nitrobenzene at
any specific time increased when the PdꢀZnO catalyst is further etched. (Figure 9a) In the
recycle experiments, the initial conversion for PdꢀZnO, PdꢀZnO E1, PdꢀZnO E2 and PdꢀZnO E3
were determined to be 50.9%, 54.6%, 59.4% and 62.3%, which proved again that the activity
increased with the etching of catalyst support. (Figure 9b) According to the N2 adsorptionꢀ
desorption test, the mesopores within the PdꢀZnO particles were amplified along with the etching
of ZnO by HCl. These amplified mesopores were certainly favorable to both the transportation of
nitrobenzene molecules towards the Pd catalyst and the leaving of aniline molecules from the
catalyst. In other words, the Pd catalysts became more accessible when the supporting ZnO
particles were gradually etched. The etching process address the concerns of relatively low initial
activity for internally dispersed PdꢀZnO catalyst due to the embedding of Pd nanoparticles inside
ZnO. After five repeated catalytic reactions, the activities of these 4 catalysts decreased 24.8%,
22.2%, 27.1% and 23.3% respectively due to the loss of catalyst in separation, which suggested
that the catalysts’ stability was not affected by the etching treatment.
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Conclusions
In summary, MꢀExOy composite nanostructures were prepared by polyol synthesis of ExOy in
the presence of freshly made noble metal nanoparticle. Unlike the traditional surface deposited
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catalysts, the noble metal nanoparticles are internally dispersed inside the porous ExOy particles.
The oneꢀpot successive polyol reactions provided a simplified method to synthesize supported
nanocatalysts, which integrated the flexible and controlled synthesis of metal catalysts and oxide
supports, the inꢀsitu loading of catalyst and even the mesopore amplification into a compatible
process. Using the hydrogenation of nitroaromatics as probe reaction, the internally dispersed
noble catalysts have been proved to possess higher activity and stability compared to the isolated
nanocatalysts or surface deposited catalysts. At the same time, they are also capable of loading
large amount of metallic active sites without aggregation, which is useful to practical reactions
with high throughput demands. Moreover, the catalytic activities can be further improved by
amplification of mesopores through protective etching reaction and easier access of substrate
molecules to the active sites. This work proposed a novel way to prepare highly dispersed metal
catalysts supported by oxide nanoparticles, which could be potentially useful to other
heterogeneous catalytic reactions after changing the chemical composition of active center and
supporting material.
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Acknowledgement
This work is supported by the National Key Research and Development Program of China
(2016YFB0701103), National Natural Science Foundation of China (21471058, 21671067) and
ShuGuang Program supported by Shanghai Education Development Foundation and Shanghai
Municipal Education Commission (15SG21).
Supporting Information.
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Conditions for all catalytic reactions, comparison of activity with literature results, metal
dispersion and TOF values for Pd(i)ꢀZnO catalysts with different Pd loading, BET results for
etched Pd(i)ꢀZnO catalyst, TEM and XPS analysis of Pd(i)ꢀZnO catalysts before and after
catalysis were supplied as Supporting Information.
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Figure 1. a) Schematic illustration to the formation of PdꢀZnO composite nanostructures. TEM
images of b, e) Pd nanoparticles, c, f) small clusters of PdꢀZnO nanocrystals and d, g) PdꢀZnO
composite particles.
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Figure 2. a, b) Crossꢀsection TEM images of PdꢀZnO composite particles. STEM image and
EDS mapping of c) a typical PdꢀZnO particle and d) an etched particle.
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Figure 3. TEM images of a, d) PdꢀZnO, b, e) PdꢀCu2O and c, f) PdꢀCeO2 composite particles
and g) their XRD patterns. The JCPDF card numbers for ZnO (*), Cu2O (#) and CeO2 (&) are
36ꢀ1451, 05ꢀ0667 and 43ꢀ1002 respectively.
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Figure 4. TEM images of a) Pd, b) Au and c) Pt nanoparticles. TEM images of d) PdꢀZnO, e)
AuꢀZnO and f) PtꢀZnO composite particles and g) their XRD patterns. The JCPDF card numbers
for Pd (*), Au (#) and Pt (&) are 46ꢀ1043, 04ꢀ0784 and 04ꢀ0802 respectively.
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Figure 5. a) Kinetic curves for the hydrogenation of nitrobenzene catalyzed by PdꢀZnO, Pdꢀ
Cu2O and PdꢀCeO2. bꢀd) The XPS spectra of Pd 3d for the above three catalysts.
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Figure 6. Conversion of nitrobenzene in 5 successive hydrogenation reaction catalyzed by
isolated Pd nanoparticles, surfaceꢀdeposited Pd catalyst (Pd(s)ꢀZnO) and internally dispersed Pd
catalyst (Pd(i)ꢀZnO).
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Figure 7. a) Solution of PdꢀZnO particles with different loading of Pd NPs, b) their XRD
patterns and c) corresponding kinetic curves in hydrogenation when using these particles as
catalysts. d) Conversion of nitrobenzene in 10 continuous reaction catalyzed by Pd(13%)ꢀZnO
particles.
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Figure 8. N2 adsorptionꢀdesorption of a) porous PdꢀZnO particles and bꢀd) the corresponding
products etched by HCl with increased concentration.
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