New Polymer Colloidal and Carbon Nanospheres: Stabilizing Ultrasmall Metal Nanoparticles for Solvent-Free Catalysis

Article abstract of DOI:10.1021/acs.chemmater.7b00710

Herein, we report the synthesis of new colloidal polydiaminopyridine (PDAP) nanospheres with uniform particle size tuned from 76 to 331 nm. The polymer colloidal nanospheres and thin films assembled by the nanospheres exhibit amphiphilic or superhydrophilic properties, originated from the different polymer surfactants. In addition, the polymer nanospheres can be easily carbonized into microporous carbon nanospheres with high N content up to 24 wt %, thus enabling a preferred basicity for CO₂ adsorption. As-synthesized PDAP show good compatibility that can not only encapsulate typical nanoparticles to form core-shell composite nanospheres but also stabilize noble metal ions to obtain ultrasmall metal nanoparticles (e.g., Pd and Au) during the thermal reduction process by the nitrogen sites from sphere frameworks. The nanocomplex Pd/PDAP-500 shows exceptional activity and high selectivity in the solvent-free oxidation of alcohols with O₂.

Full text of DOI:10.1021/acs.chemmater.7b00710

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Article
New Polymer Colloidal and Carbon Nanospheres: Stabilizing
Ultra-Small Metal Nanoparticles for Solvent-Free Catalysis
Tao Wang, Pengfei Zhang, Yan Sun, Bing Liu, Yunling Liu, Zhen-An Qiao, Qisheng Huo, and Sheng Dai
Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017
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Chemistry of Materials
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New Polymer Colloidal and Carbon Nanospheres: Stabilizing Ultra-
Small Metal Nanoparticles for Solvent-Free Catalysis
†
Tao Wang,† Pengfei Zhang,‡ Yan Sun,† Bing Liu,† Yunling Liu,† Zhen-An Qiao,*, Qisheng Huo†
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and Sheng Dai‡
†State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130012,
China
‡Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
ABSTRACT: Herein we report the synthesis of new colloidal polydiaminopyridine (PDAP) nanospheres with uniform
particle size tuned from 76 to 331 nm. The polymer colloidal nanospheres and thin films assembled by the nanospheres
exhibit amphiphilic or superhydrophilic properties, originated from the different polymer surfactants. In addition, the
polymer nanospheres can be easily carbonized into microporous carbon nanospheres with high N content up to 24 wt%,
thus enabling a preferred basicity for CO2 adsorption. As-synthesized PDAP show good compatibility that can not only
encapsulate typical nanoparticles to form core-shell composite nanospheres, but also stabilize noble metal ions to obtain
ultra-small metal nanoparticles (e.g., Pd and Au) during the thermal reduction process by the nitrogen sites from sphere
frameworks. The nanocomplex Pd/PDAP-500 shows exceptional activity and high selectivity in the solvent-free oxidation
of alcohols with O2.
Because of the lacking of cross-linkable organic groups
for a stable network, traditional polymer colloidal nano-
spheres like polystyrene, poly(methyl methacrylate), and
poly(hydroxyethyl methacrylate) were typically used as
high monodispersed templates to synthesize porous or
INTRODUCTION
The development of novel methodologies and the de-
sign of highly functional systems for the production of
polymer and carbon colloidal nanospheres stand out as
appealing goals enthusiastically pursued by the chemical
community, because polymer and carbon nanospheres
with uniform size and tunable architecture are vibrant
materials for catalysis, sustainable energy, environment
hollow materials, but the nanospheres themselves failed
to convert to carbon nanospheres.2,
Phenol-
18
formaldehyde resin has a highly crosslinked structure
with a good thermal stability and could be carbonized to
carbon nanospheres without deformation of spherical
shape,8 thus several methods have been developed to syn-
thesize uniform microporous or mesoporous carbon nan-
ospheres based on phenol-formaldehyde resin.2, 8, 19-21 Re-
cently, our group developed a facile “silica-assisted” syn-
thesis for diverse carbon spheres-a category that covers
mesoporous carbon nanospheres, hollow mesoporous
carbon nanospheres, and yolk-shell mesoporous carbon
nanospheres.[19] Compared to phenol-formaldehyde resin,
carbon nanospheres based on polydopamine22 and poly-
aniline-co-polypyrrole[6] are nitrogenous carbon materials
and further developed to achieve highly efficient oxygen-
reduction catalysts. However, the synthesis of polydopa-
mine and polyaniline-co-polypyrrole nanomaterials is
relatively expensive, time consuming or temperature-
dependent. Hence, the synthesis of new uniform polymer
colloidal and porous N-doped carbon nanospheres is
indeed difficult and remains a grand challenge.11, 23
2
protection, and biomedical applications.1, In the past
several years, scientists have made a lot of efforts in the
synthesis, characterization, and application of polymer
nanospheres and a variety of polymers have been success-
fully used to synthesize uniform nanospheres, such as
polystyrene,
poly(hydroxyethyl
poly(methyl
methacrylate),
methacrylate),
polyaniline-co-
polypyrrole, polydopamine, and phenol-formaldehyde
resin.3-9 The conversion of polymer nanospheres into car-
bon nanospheres under inert gas by pyrolysis process is
an important way to achieve higher stability, larger sur-
face area, higher catalytic activity and so on.10, 11 In par-
ticular, N-doped carbon materials as emerging carbon
materials have gained much attention because their en-
hanced electrical conductivity, chemical reactivity, specif-
ic capacitance and adsorption affinity towards CO2.12-16
Toward the design of high quality polymer/carbon nano-
spheres, a few issues still remain such as poor thermal
stability, long synthesis time, extreme conditions and
high cost of raw material, which limit their large scale
synthesis and industrial applications.17
Herein, we report a facile strategy to rapidly synthesize
a new type of polymer colloidal nanospheres with 2,6-
diaminopyridine (DAP) as precursor, polymer surfactants
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(poly-vinyl pyrrolidone (PVP K-30), Brij-58, Pluronic P123, tions on the same sample. The X-ray photoelectron spec-
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or Pluronic F127) as the protective agent and ammonium
peroxydisulfate (APS) as the oxidizer. The obtained
polydiaminopyridine (PDAP) nanospheres have a uniform
particle size distribution (76~331 nm) and the size of
colloidal PDAP can be precisely adjusted by two tunable
factorsthe concentration of APS and the amount of
ethanol. The surfactant around the PDAP surface
influences the hydrophilicity of PDAP nanospheres (am-
phiphilic or superhydrophilic properties), which can be
used to paint PDAP thin films with different wettability.
PDAP nanospheres were carbonized under protection of
an inert gas to produce microporous N-doped carbon
nanospheres, which showed very high CO2 areal capaci-
ties (~9.8 μmol m-2). The as-synthesized PDAP provides a
new choice for coating oxides or metal particles to form
core-shell nanostructures, and stabilizing ultra-small
noble metal nanoparticles by nitrogen sites to fabricate
nanocomposites (Scheme 1). Pd nanoparticles stabilized
by PDAP show excellent catalysis activity for solvent-free
aerobic oxidation of diverse alcohols with high turnover
frequencies (TOFs), high selectivity, and good
recyclability, which show great potential for prospective
applications in industrial heterogeneous catalysis for
petroleum refining and fine chemical production.
troscopy (XPS) measurements were performed on a
ESCALAB250 system with Al Ka radiation (1486.6 eV).
Powder X-ray diffraction (XRD) patterns were collected
by using a Rigaku 2550 diffractometer with Cu Kα radia-
tion (λ = 1.5418 Å). N2 adsorption-desorption isotherms
o
were obtained at -196 C on a Micromeritics 2420 instru-
ment. CO2 gas adsorption measurements were performed
on a Micromeritics ASAP 2020 instrument. Samples were
degassed at 150 oC for a minimum of 12 h prior to analysis.
The thermal gravimetric analyses (TG) were performed on
TGA Q500 thermogravimetric analyzer used in N2 with a
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heating rate of 10 C min-1. The CHN element analyses
were Perkin-Elmer 2400CHN analyzer. Raman spectra of
PMO was performed on a Renishaw inVia Raman micro-
scope. The catalytic products were analyzed with a gas
chromatograph system (Shimadzu GCMS-QP 2010 Plus)
with a flame ionization detector (FID) using tetralin as
the internal standard.
Preparation of PDAP nanospheres. Typically, PDAP-V
nanospheres with particle sizes around 200 nm were syn-
thesized as below. DAP (0.15 g), PVP K30 (0.075 g, Mw ca.
40000), 0.045 g of NaOH, and 9.2 mL of deionized water
were combined and stirred to form a clear solution at
ambient temperature. Then, 0.8 mL of APS solution (1.46
M) were added under stirring. The color of solution
changed immediately, and the reaction finished in 5
minutes. PDAP-V nanospheres with different average
sizes were obtained by modulating the addition of APS
solution or ethanol. PDAP-B, PDAP-P, or PDAP-F nano-
spheres were obtained by replacing PVP with the same
weight of Brij-58 (Mw ca. 1123), P123 (Mw ca. 5800) or F127
(Mw ca. 12600), respectively. The product was collected
by centrifugation, washed with water and ethanol several
times.
Scheme 1. Synthesis and applications of PDAP
colloidal nanospheres.
Fabrication of PDAP Films. 5 mg of PDAP-V nano-
spheres was homogeneously dispersed in 20 mL of etha-
nol by ultrasonication. Then, a clean glass slide was im-
mersed into the solution, pulled up and dried in air sever-
al times until a thin film had been coated on the slide.
The fabrication of PDAP-B, PDAP-P or PDAP-F film was
the same as the fabrication of PDAP-V film.
EXPERIMENTAL SECTION
Materials and Methods. DAP, Brij-58, Pluronic P123 and
Pluronic F127 were purchased from Sigma-Aldrich. PVP
K-30, NaOH, glacial acetic acid, ethanol and K2PdCl4 were
purchased from Sinopharm Chemical Reagent Co.
(Shanghai, China). Ce(NO3)3∙9H2O, APS, benzyl alcohol,
1-phenylethanol, cinnamic alcohol, 1-octanol, 2-octanol
and 3-octanol were purchased from Aladdin. All of the
chemicals were used without further purification.
Preparation of CeO2 nanospheres. CeO2 nanospheres
were synthesized according to the literature.24 Briefly, 3.0
g of Ce(NO3)3∙9H2O, 3 mL of glacial acetic acid, and 2.5
mL of water were combined and stirred to form a clear
solution at ambient temperature. Then, the solution was
o
transferred to an autoclave and heated at 140 C for 12 h.
The structure and morphology of PDAP nanospheres
were analyzed with FEI Tecnai G2 F20 s-twin D573 field
emission transmission electron microscope operated at
200 kV and JEOL JSM-6700F field-emission scanning elec-
tron microscope operated at 5 kV. The sizes of PDAP
nanospheres were measured by photon correlation spec-
troscopy employing a Nano ZS90 laser particle analyzer
(Malvern Instruments, UK) at 25 oC. Water contact angles
(CAs) on PDAP films were measured using a DSA 100
drop shape analyzer at ambient temperature and each
was obtained by measuring more than five different posi-
The product was collected by centrifugation, washed with
water and ethanol several times. Then, the product was
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dried at room temperature, and calcined at 600 C for 3 h
under air.
Preparation of CeO2@PDAP core-shell nanospheres. The
as-synthesized CeO2 nanospheres (0.06 g) and PVP (0.1 g)
were homogeneously dispersed in 9.7 mL of water by ul
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Figure 1. (a) SEM images of PDAP-V nanospheres with different sizes. (b) Photographs of PDAP-V and PDAP solutions. (c)
PDAP-V nanospheres with different sizes ajusted by the concentration of APS and the amount of ethanol. (d) Relationship
between the concentration of APS and nanosphere sizes of PDAP-V. (e) Relationship between the amount of ethanol and
nanosphere sizes of PDAP-V. Scale bar: 200 nm.
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trasonication. Then, 0.15 g of DAP and 0.045 g of NaOH
were added under stirring. 0.8 mL of APS solution (1.46
M) was added under stirring after the dissolution of DAP.
The product was collected by centrifugation, washed with
water and ethanol several times.
reaction. Then, 50 μL of the liquid mixture and 50 μL of
internal standard was added to 5 mL of ethanol for GC-
MS analysis.
Catalyst recycling test. After the first oxidation, the cat-
alyst was collected by centrifugation, washed with ethanol
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Carbonization of PDAP nanospheres. The obtained
PDAP-V nanospheres were dried in air under room tem-
perature and heated under a nitrogen atmosphere at 2
for twice, and dried in a vacuum oven at 65 C for 12 h.
Finally, the recovered Pd/PDAP-500 was used in the sub-
sequent reaction.
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oC/min from room temperature to 350 C and kept at 350
oC for 2 h. The temperature was then raised at 5 oC/min to
800 oC and kept at 800 oC for 1 h. The product was denot-
ed as PDAP-800. PDAP-700 and PDAP-600 were obtained
RESULTS AND DISCUSSION
Synthesis of PDAP nanospheres
o
Under the protection of PVP, PDAP nanospheres with
different average sizes (76~331 nm) can be obtained by
modulating the concentration of APS and the amount of
ethanol, and characterized by dynamic light scattering
method (DLS, Fig. 1c) and scanning electron microscopy
(SEM, Figure 1a and S1). The size of PDAP-V nanospheres
increased from 76 nm to 255 nm with the increase con-
centration of APS (Figure 1d) in the absence of ethanol.
Varying the ethanol/water volume ratio from 0 to 1.5 still
has a dramatic effect upon the size of PDAP nanospheres,
and the average diameter of PDAP nanospheres can be
tailored from 132 to 331 nm at the constant concentration
of APS (0.07 M, Figure 1e). On the basis of these observa-
tions, a good linear function of size vs the concentration
of APS (Figure 1d) and a good quadratic function of size vs
the amount of ethanol (Figure 1e) were obtained through
calculation. Compared to traditionally single-variable
control system, the method we reported has two tunable
variables that provide more composite modes and pre-
dictable nanosphere sizes. After centrifugation of the
reaction mixture, the obtained PDAP nanospheres
attaching centrifuge tubes show green luster (Figure 2a),
by changing the final temperature to 700 or 600 C while
PDAP-350 was obtained without the second heating step.
The carbonization of CeO2@PDAP core-shell nanospheres
had the same heating steps as PDAP-800.
Preparation of Pd/PDAP-500. 1.2 g of PDAP-V was dis-
persed in 50 mL of water. Then 1.7 mL of K2PdCl4 solution
(10 g/L) was added and then stirred at room temperature
for 12 h. The powdery solid was collected by centrifuga-
tion, washed with water, and dried at room temperature.
Subsequently, the Pd-loaded PDAP-V was thermally
treated at 500 °C for 2 h in reducing atmosphere (H2/N2 =
1/99, volume ratio). The palladium loading in terms of
total weight of the obtained catalyst (Pd/PDAP-500) is
~0.76 wt%.
Catalytic oxidation of hydrocarbons and alcohols. In a
typical oxidation, 10 g of benzyl alcohol and 20 mg of
Pd/PDAP-500 catalysts were added into a 25 mL three-
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neck glass reactor. The reaction was performed at 120 C
in an oil bath with magnetic stirring. A stream of O2 was
introduced into the reaction mixture at a constant flow
rate (20 mL/min). The liquid phase of the reaction mix-
ture was collected by filtration after completion of the
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which indicates the existence of locally ordered packing spheres show the similar amphiphilic behavior as PDAP-F
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structure (Figure 2b), owing to the uniform particle size
of PDAP nanospheres. It is worth noting that PDAP-V
nanospheres can be rapidly formed within 5 minutes un-
der a wide range of other experimental conditions such as
nanospheres, mainly because of the highly similar chemi-
cal composition of the protective agents of F127 and P123.
PDAP-V and PDAP-B nanospheres are not amphiphilic,
which can be dissolved in most of the solvents except
hexane (Figure S3c). Fourier transform infrared (FTIR)
spectroscopy was used to identify the polymer surfactants
coated on PDAP nanospheres. The principal absorption
peaks of PDAP-V, PDAP-B, PDAP-P and PDAP-F (Figure
S3e) between 2850 and 3000 cm-1 are assigned to C-H
stretching vibrations of PVP, Brij-58, P123 and F127, re-
spectively, 25 which indicates the amphiphilic or hydro-
philic behavior of the PDAP nanospheres originated from
the different polymer surfactants on the nanospheres.
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pH (3 to 10) and temperature (0 to 50 C, Figure S1).
About 11 g of PDAP powder can be obtained from 750 mL
of the reaction solution, still with high yield of ~95 %
(Figure 2e). Thus, the synthesis approach is considered to
be robust under complex conditions and very suitable for
industrial production.
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Carbonization of PDAP nanospheres
PDAP nanospheres show good thermal stability that can
be directly converted to N-doped carbon nanospheres
with a yield of 37 wt% at 800 oC by a carbonization
process, according to the thermogravimetry (TG) curve of
PDAP-V in the atmosphere of N2 (Figure S4a). The
pyrolysis product of PDAP-V is denoted as PDAP-x, where
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x is the calcination temperature (in C). As an example,
PDAP-800 nanoparticles performed a linear shrinkage of
25 % on SEM images (Figure 3a and b) and disordered
micropores on TEM images (Figure 3d) after thermal
treatment. The pore characteristics of PDAP-x were
examined by nitrogen adsorption experiment (Figure 3c).
The Brunauer-Emmett-Teller surface areas (SBET) of
PDAP-600, PDAP-700 and PDAP-800 are 311, 360 and 412
m2 g-1 based on the N2 adsorption isotherm, respectively.
The nitrogen adsorption-desorption isotherms of PDAP-
600, PDAP-700 and PDAP-800 exhibit characteristics of
type I according to the classification of the International
Union of Pure and Applied Chemistry (IUPAC),
indicating the microporous frameworks of the carbon
nanospheres. The N content of the pyrolysis product
decreased with the increase of pyrolysis temperature
based on the results of CHN elemental analysis, 23.72 wt%
for PDAP-600, 18.50 wt% for PDAP-700, and 13.08 wt% for
PDAP-800, which are very high values among N-doped
carbons according to the summary of the reported
synthesis methods of doped carbons.26
Figure 2. Photograph (a) and SEM image (b) of PDAP-V
nanospheres after centrifugation. (c) Photographs of PDAP-F
dispersed in various solvents. (d) Photograph of PDAP-V
film. (e) Photograph of 11 g of PDAP-V powder. (f) Water CA
images of PDAP films.
Various polymer surfactants can be used as the
protective agent in the synthesis of PDAP nanospheres,
such as PVP, Brij-58, Pluronic P123, and Pluronic F127.
These nanospheres stabilized by different surfactants are
correspondingly denoted as PDAP-V, PDAP-B, PDAP-P
and PDAP-F (Figure S3a), and can be used to paint films
on glass surface by easy evaporation of suspension (Figure
2c and S3b). As shown in Figure 2f, all the PDAP films are
hydrophilic and the water apparent contact angles (CAs)
are ranging from 3o to 30o. The PDAP-V film shows
superhydrophilicity, on which the water apparent CA is
less than 5o. PDAP-F nanospheres show their unique
ability to dissolve in most of the common solvents, in-
cluding such extremes as hexane and pure water (Figure
2c), and can be easily moved to the water/hexane
interface to form a black thin film at the interface by
shaking or sonication (Figure S3d), which demonstrates
the amphiphilic nature of nanospheres. PDAP-P nano-
Figure 3. SEM images of PDAP-V before (a) and after (b)
carbonization at 800 oC. (c) N2 adsorption-desorption
isotherms of PDAP-x at 77 K. TEM images (d) XPS spectra (e)
and (f) Raman spectra of PDAP-800.
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Chemistry of Materials
X-ray photoelectron spectroscopy (XPS) was used to
273 K— higher than that of PDAP-700 (~8.4 μmol m-2),
and more than double that of PDAP-800 (~4.5 μmol m-2),
which indicates the beneficial effect of nitrogen content
on CO2 uptake. To determine the strength of the interac-
tion between CO2 molecules and the pyrolysis PDAP, the
isosteric heat (Qst) of CO2 adsorption was calculated using
CO2 adsorption isotherms at 273 K and 298 K based on
the Clausius−Clapeyron equation (Figure 4d). The calcu-
lated Qst for PDAP-600 is in the range of 46.3−32.7 kJ mol-
1, substantially higher than the Qst for PDAP-700, though
their CO2 adsorptions are quite close. The Qst for PDAP-
800 is the lowest among these three materials, mainly due
to its least nitrogen content.
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confirm the presence of nitrogen and the binding
between carbon and nitrogen in PDAP-800. Typically, the
C 1s peak is observed at ca. 284.8 eV (consistent with sp2
graphitic carbon) and displays a slightly asymmetric
nature (Figure S4b). The N 1s spectra (Figure 3e)
demonstrates four types of nitrogen atoms and the peaks
located at 398.3, 400.5, 401.2 and 402.9 eV in a percentage
of 48.7, 30.5, 14.9 and 5.8 %, related to the pyridinic
nitrogen, pyrrolic nitrogen, graphitic nitrogen and
oxidized nitrogen, respectively.27 Powder X-ray diffraction
(PXRD) pattern exhibits two broad low intensity peaks at
2θ = 24.3° and 43.8° that correspond to the (002) and (100)
planes of graphite, indicating the amorphous carbon
structures in PDAP-800 (Figure S4c). The Raman spectra
of PDAP-800 (Figure 3f) shows two bands at ca. 1345 cm-1
(D band) and ca. 1575 cm-1 (G band), which further
verified amorphous carbon nature of the pyrolysis
product.28
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PDAP-600 was activated by KOH to enhance CO2 ad-
sorption capacity. SBET of KOH-activated PDAP-600 (de-
noted as PDAP-KOH) is 2200 m2 g-1 (Figure 4b), almost
seven times SBET of PDAP-600. CO2 adsorption of PDAP-
KOH is improved significantly to 6.23 mmol g-1 (273 K, 1
bar), more than double that of PDAP-600 (Figure 4c). The
calculated Qst for PDAP-KOH is in the range of 37.5−24.4
kJ mol-1, and the N content is 8.47 wt%. The CO2 adsorp-
tion capacity of PDAP-350 is 1.15 mmol g-1 because of its
very low SBET (~22 m2 g-1). Obviously, the enhanced CO2
adsorption of PDAP-KOH benefits from the enlargement
in surface area, due to the creation of additional mi-
cropores during the KOH activation process. That mains
larger SBET is beneficial for improving CO2 adsorption, and
N content determines Qst of CO2 adsorption. It is worth
mentioning that CO2 areal capacities of PDAP-600 (~9.8
μmol m-2) is higher than those of previously reported po-
rous carbons, and CO2 adsorption of PDAP-KOH (6.23
mmol g-1) is a very high value among those excellent car-
bon materials (Table S1).32
PDAP based nanocomposites
The tunable structural characteristics and controllable
chemical functionalities of the polymer nanospheres
provide an exceptional platform for the fabrication of new
types of polymer and carbon nanocomposites. The as-
synthesized PDAP has been used to encapsulate other
nanoparticles, such as ceria , silica or Au nanoparticles, to
form multifunctional core-shell nanocomposites. The
core-shell structure can be clearly identified in TEM
images (Figure 5a, b and S9), and the shell thickness can
be precisely tailored from 4 to 86 nm by the concentra-
tion of APS, like the control of the size of PDAP nano-
spheres (Figure S7). Similarly, porous carbon shelled CeO2
was generated by heating the core-shell nanocomposites
Figure 4. (a) N2 adsorption-desorption isotherms of PDAP-
KOH at 77 K. (b) Pore size distribution analysis for PDAP-
KOH according to DFT model. (c) CO2 adsorption isotherms
at 273 K of PDAP-x and PDAP-KOH. (d) The isosteric heat of
CO2 adsorption on PDAP-x and PDAP-KOH calculated from
the experimental adsorption isotherms at 273 and 298 K.
CO2 adsorption capacities of N-doped carbons
Extensive research efforts have been undertaken
worldwide to develop feasible materials for CO2 capture,
for the reason that the CO2 emission associated with in-
dustrial production has serious impact on the global
warming and climate change.29 We have then investigated
CO2 adsorption capacity of the obtained N-doped carbon
nanospheres, because nitrogen-containing porous car-
bons with extensive basicity are potential materials for
CO2 capture.13, 30, 31 Firstly, the CO2 adsorption capacities
of PDAP-800, PDAP-700, and PDAP-600 were investigat-
ed at 273 K and 298 K (Figure 4c and Figure S4d). Though
SBET of PDAP-600 is the lowest, CO2 adsorption of PDAP-
600 is 3.05 mmol g-1, the largest among these three mate-
rials at 273 K and 1 bar. Through calculation, PDAP-600
exhibits very high CO2 areal capacities (~9.8 μmol m-2) at
o
at 800 C under N2, and denoted as CeO2@PDAP-800
(Figure 5c). The pore size distribution of CeO2@PDAP-
800 was calculated according to DFT model based on the
N2 adsorption isotherm (Figure S8). The micropores
below 2 nm are attributed to the carbon shell, and the
mesopores above 3 nm are attributed to the inner cores.
The porous carbon shell may serve as molecular sieve—
not only the mass transfer channel but also the protector
of the inner core.
Benefiting from abundant amino groups and nitrogen
sites,33 the as-synthesized PDAP polymer spheres can be
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Figure 5. TEM images of Au@PDAP (a), CeO2@PDAP (b), CeO2@PDAP-800 (c); the inset shows the magnified TEM image of
CeO2@PDAP-800. (d) TEM image of Pd/PDAP-500; the inset shows Pd particle size distribution. (e) XPS spectra and (g) PXRD
patterns for Pd/PDAP-500. (f) The reusing tests of Pd/PDAP-500 in the benzyl alcohol oxidation. Reaction conditions: 10 g of
benzyl alcohol, 20 mg of catalyst (recovered), O2 1 atm, 120 oC, 11 h.
directly used as adsorbent and complexing agent for
metal ions, stabilizing metal nanoparticles (e.g., Pd and
Au) during the thermal reduction process. The PXRD
pattern of obtained Pd/PDAP-500 (Figure 5g) shows a
series of broad reflection peaks at 2θ = 39.3°, 45.8° and
66.9°, assigned to (111), (200) and (220) reflection of the
cubic (fcc) palladium lattice, respectively. The mean size
of Pd nanoparticles is 3 nm, calculated by the Scherrer
equation using full-width-at-half- maximum (FWHM). Pd
nanoparticles on Pd/PDAP-500 can be identified with the
average size about 4 nm by TEM (Figure 5d), which is in
agreement with the XRD pattern calculation. The Pd 3d
XPS spectra in Figure 5e consists of two asymmetric peaks
related to Pd 3d5/2 and Pd 3d3/2 core levels, which can be
fitted using two doublets. The peaks around 335.7 and
341.0 eV are attributed to metallic Pd0, and those around
337.1 and 342.4 eV correspond to Pd2+ species.34
Furthermore, other metal nanoparticles with ultra-small
sizes, such as Au and Pt, can also be stabilized on PDAP
by
a similar loading process (Figure S12). These
nanocomposites verified the excellent compatibility of
collidal PDAP.
Solvent-Free Oxidation Using Pd/PDAP-500 Catalyst
To investigate the performance of the Pd/PDAP-500
catalyst, solvent-free oxidation of benzyl alcohol under
atmospheric O2 pressure, an important step in the
synthesis of fine chemicals, was selected as a model
reaction. The Pd/PDAP-500 catalyst with the Pd loading
amount 0.76 wt% (Figure S10) exhibited turnover
frequencies (TOFs) of 4582 h-1 and a 42.5 % benzyl alcohol
conversion with very high selectivity (sel.: 99 %) to
benzaldehyde in 33 h over the reaction period (Entry 3,
Table 1). The conversion of benzyl alcohol and the
selectivity to benzaldehyde are comparable to the
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Table 1． Oxidation of alcohols catalysed by Pd/PDAP-500 catalysts with O2.♦
1
2
3
4
5
6
7
8
9
Conv.
Entry
Substrate
Time (h)
Product
Sel. (%)
TOF (h-1)†
4582
TON‡
(%)
28.3
36.0
42.5
24.5
19.0
1
11
99
99
99
92
82
18281
23310
27518
15826
12273
2
22
33
33
33
OH
O
3
4♣
5※
1292
388
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OH
O
6
11
6
39.7
52.3
99
35
2179
22774
27521
OH
O
7
7938
8◊
9◊
11
11
3.6
3.2
99
98
375
473
965
857
OH
O
OH
O
O
OH
10◊
11
9.5
99
650
2550
Conv., conversion; Sel., selectivity.
♦Reaction conditions: 10 g of substrate, 20 mg of Pd/PDAP-500, O2 1 atm, 120 oC.
♣The catalyst is Pd loaded on PDAP bulk and treated the same as Pd/PDAPꢀ500.
※The catalyst is Pd/PDAPꢀ600.
†TOF = [reacted mol substrate]/[(total mol metal)×(reaction time)]. The TOFs were measured after first 0.5 h of reaction.
‡TON = [reacted mol substrate]/[total mol metal].
◊The oxidation was performed in 5 g of alcohols with 20 mg of Pd/PDAP-500 as catalyst.
reported excellent catalysts under the similar conditions
(Table S2),34-38 such as 74.5 % and 91.6 % using Au-
Pd/TiO2 catalyst reported by Hutchings et al. The Pd-
catalysed oxidation of 1-phenylethanol also proceeded
very selectively to give acetophenone as product (Entry 6,
Table 1). The oxidation of cinnamyl alcohol exhibited a
high conversion (52.3 %) with selectivity (sel.: 35 %) to
cinnamaldehyde in 6 h (Entry 7, Table 1). Pd/PDAP-500
catalyst is also suitable for the selective oxidation of 1-
octanol, 2-octanol, 3-octanol to octyl aldehyde, 2-
octanone and 3-octanone (Entry 8, 9 and 10, Table1),
which is inactive with the Au-Pd/TiO2 catalyst.35 As a
comparison, Pd loaded on PDAP bulk did not perform as
well as Pd/PDAP-500 in solvent-free oxidation of benzyl
alcohol (Entry 4, Table 1), which verified the
enhancement of uniform nanospheres. Pd/PDAP was
S11). Thus, Pd/PDAP-500 is the optimal catalyst for
solvent-free oxidation of benzyl alcohol. The recycling
test (Figure 3f) reveals that the Pd/PDAP-500 catalyst
worked in a heterogeneous manner with negligible Pd
leaching. The TEM image (Figure S13) and XRD pattern
(Figure 5g) of recovered Pd/PDAP-500 verified the good
morphology and structure stability. Therefore, Pd
nanoparticles stabilized by PDAP bring about a new
heterogeneous catalyst for solvent-free oxidation of
alcohols with high activity and high selectivity.
CONCLUSION
In summary, we have developed a programmable and
robust method to synthesis uniform PDAP nanospheres
with
superhydrophilic PDAP nanospheres are directly
synthesized by using the corresponding polymer
surfactant. Calcined PDAP nanospheres are porous
carbon with high N content. As-synthesized PDAP can
not only encapsulate other nanoparticles to form core-
shell composite nanospheres, but also stabilize noble
different
average
sizes.
Amphiphilic
or
o
carbonized under higher temperature (600 C) to obtain
Pd/PDAP-600. The performance of Pd/PDAP-600 con-
trasted unfavorably with that of Pd/PDAP-500 in solvent-
free oxidation of benzyl alcohol (Entry 5, Table 1), because
the mean Pd particle size grew up to about 8 nm
according to the PXRD pattern of Pd/PDAP-600 (Figure
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(9)
Guo, S.-R.; Gong, J.-Y.; Jiang, P.; Wu, M.; Lu, Y.; Yu, S.-
metal ions to obtain uniform noble metal nanoparticles,
which show good activity and high selectivity in the
solvent-free oxidation of alcohols. These excellent
properties enable PDAP nanospheres great potential in
many areas, such as surface modification, stabilizing
emulsions, CO2 adsorption, composite materials and
catalysis.
H. Biocompatible, Luminescent Silver@Phenol Formaldehyde
Resin Core/Shell Nanospheres: Large-Scale Synthesis and Appli-
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ASSOCIATED CONTENT
Molecular-based design and emerging applications of nanopo-
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(12)
Lin, J.; Lou, X. W. Nitrogen-containing microporous carbon
nanospheres with improved capacitive properties. Energy Envi-
ron. Sci. 2011, 4, 717-724.
Supporting Information
Su, F.; Poh, C. K.; Chen, J. S.; Xu, G.; Wang, D.; Li, Q.;
SEM images of PDAP-V nanospheres under different synthe-
sis conditions. TG curve of PDAP-V. XPS spectra of PDAP-
800. CO2 adsorption isotherms at 298 K of PDAP-x and
PDAP-KOH. TEM images of CeO2@PDAP with different
thickness of shells. SEM-EDX spectra of Pd/PDAP-500. Char-
acterization of Au/PDAP-200. This material is available free
of charge via the Internet at http://pubs.acs.org.
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Wickramaratne, N. P.; Xu, J.; Wang, M.; Zhu, L.; Dai,
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tractive Materials for Supercapacitor Electrodes and CO2 Ad-
sorption. Chem. Mater. 2014, 26, 2820-2828.
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Titirici, M.-M.; White, R. J.; Brun, N.; Budarin, V. L.;
AUTHOR INFORMATION
Su, D. S.; del Monte, F.; Clark, J. H.; MacLachlan, M. J. Sustaina-
ble carbon materials. Chem. Soc. Rev. 2015, 44, 250-290.
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Wang, X. Bioinspired hollow semiconductor nanospheres as
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Corresponding Author
Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.;
* qiaozhenan@jlu.edu.cn
Wang, Y.; Kong, A.; Chen, X.; Lin, Q.; Feng, P. Efficient
Notes
The authors declare no competing financial interests.
Su, D. S.; Perathoner, S.; Centi, G. Nanocarbons for the
ACKNOWLEDGMENT
This work was supported by the Young Thousand Talented
Program and the National Natural Science Foundation of
China (Grant No. 21671073, 21671074, 21371067, 21621001 and
21373095).
Agrawal, M.; Gupta, S.; Stamm, M. Recent develop-
ments in fabrication and applications of colloid based composite
particles. J. Mater. Chem. 2011, 21, 615-627.
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