Activating cobalt nanoparticles via the Mott-Schottky effect in nitrogen-rich carbon shells for base-free aerobic oxidation of alcohols to esters

Article abstract of DOI:10.1021/jacs.6b10710

Heterogeneous catalysts of inexpensive and reusable transition-metal are attractive alternatives to homogeneous catalysts; the relatively low activity of transition-metal nanoparticles has become the main hurdle for their practical applications. Here, the de novo design of a Mott-Schotfky-type heterogeneous catalyst is reported to boost the activity of a transition-metal nanocatalyst through electron transfer at the metal/nitrogen-doped carbon interface. The Mott-Schottky catalyst of nitrogen-rich carbon-coated cobalt nanoparticles (Co@NC) was prepared through direct polycondensation of simple organic molecules and inorganic metal salts in the presence of g-C₃N₄ powder. The Co@NC with controllable nitrogen content and thus tunable Fermi energy and catalytic activity exhibited a high turnover frequency (TOF)value (8.12 mol methyl benzoate mol^-1 Co h^-1) for the direct, base-free, aerobic oxidation of benzyl alcohols to methyl benzoate; this TOF is 30-fold higher than those of the state-of-the-art transitionmetal-based nanocatalysts reported in the literature. The presented efficient Mott-Schottky catalyst can trigger the synthesis of a series of alkyl esters and even diesters in high yields.

Full text of DOI:10.1021/jacs.6b10710

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Article
Activating cobalt nanoparticles via the Mott-Schottky effect in nitrogen-
rich carbon shells for base-free aerobic oxidation of alcohols to esters
Hui Su, Ke-Xin Zhang, Bing Zhang, Hong-Hui Wang, Qiu-
Ying Yu, Xin-Hao Li, Markus Antonietti, and Jie-Sheng Chen
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Dec 2016
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Journal of the American Chemical Society
Activating cobalt nanoparticles via the Mott-Schottky effect in ni-
trogen-rich carbon shells for base-free aerobic oxidation of alcohols
to esters
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Hui Su1, Ke-Xin Zhang1, Bing Zhang1, Hong-Hui Wang1, Qiu-Ying Yu1, Xin-Hao Li1*, Markus Antonietti2 and Jie-Sheng
Chen1*
1School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China.
2Department of Colloid Chemistry, Max-Planck Institute of Colloids and Interfaces, Wissenschaftspark Golm, 14424
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Potsdam, Germany.
Keywords. Mott-Schottky catalyst, heterojunction, nitrogen-doped carbon, aerobic esterification, base-free.
ABSTRACT: Heterogeneous catalysts of inexpensive and reusable transition-metal are attractive alternatives to homoge-
neous catalysts; the relatively low activity of transition-metal nanoparticles has become the main hurdle for their practical
applications. Here, the de novo design of a Mott-Schottky-type heterogeneous catalyst is reported to boost the activity of a
transition-metal nanocatalyst through electron transfer at the metal/nitrogen-doped carbon interface. The Mott-Schottky
catalyst of nitrogen-rich carbon-coated cobalt nanoparticles (Co@NC) was prepared through direct polycondensation of
simple organic molecules and inorganic metal salts in the presence of g-C3N4 powder. The Co@NC with controllable ni-
trogen content and thus tunable Fermi energy and catalytic activity exhibited a high turnover frequency (TOF) value (8.12
mol methyl benzoate mol-1 Co h-1) for the direct, base-free, aerobic oxidation of benzyl alcohols to methyl benzoate; this
TOF is 30-fold higher than those of the state-of-the-art transition-metal-based nanocatalysts reported in the literature.
The presented efficient Mott-Schottky catalyst can trigger the synthesis of a series of alkyl esters and even diesters in high
yields.
most direct approach to meet the requirements of green
chemistry.
1. INTRODUCTION
Selective oxidation of alcohols is a promising method in
green organic synthesis; it can be used to produce alde-
hydes, acids, acetals and esters1-4. We focus here on esters
because they are needed as bulk chemicals in large
amounts in various fields including the fine chemicals,
commercial products, and polymer industries5-6. In recent
years, noble-metal-based homogeneous catalysts have
been developed that can function as mild catalysts to
promote the chemical transformation from alcohol to
ester7-10. However, heterogeneous catalysts are preferred
industrially for their low cost and ease of separation and
reuse. Therefore, supported noble-metal nanoparticles
have also been used to promote the transformation from
alcohol to ester. However, neither homogeneous catalysts
nor noble-metal nanocatalysts are a preferred choice for
the green production of esters. Heterogeneous catalysts
based on more abundant transition metals promise a
larger potential as low-cost and sustainable heterogene-
ous catalysts for the selective oxidation of alcohols but
currently still suffer from low activity. To improve the
catalytic activity of these heterogeneous catalysts, various
bases have been introduced into reaction systems to acti-
vate the substrate and/or the metal nanoparticles and
lower the activation energy of specific reaction steps,
thereby improving the final conversions11-14. Given the
demand for a sustainable and environmentally friendly
reaction process with fewer or even zero additives and
byproducts, the construction of an environmentally be-
nign base-free oxidation system is highly desirable15.
Again, further promoting the catalytic activity of the tran-
sition-metal nanoparticle-based catalysts should be the
Cobalt and cobalt oxides are earth-abundant and have
been used in many chemical processes, including ni-
troarene reduction, aerobic oxidation of alcohols and the
synthesis of nitriles11, 16-18. The typical strategy using organ-
ic ligands to indirectly control the catalytic performance
of cobalt-based nanocatalysts already leaves many oppor-
tunities to modify the electronic environment of the Co
nanoparticles and boosts their final catalytic activity16.
Recently, we introduced the concept of a Mott-Schottky
photocatalyst to activate the metal sites by using carbon
nitride-based semiconductive materials for H2 generation
reactions, hydrogenation reactions and carbon-carbon
coupling reactions19-21. The manner of activation of the
carbon nitride support is analogous to the function of a
ligand in homogenous catalysts, where the electron densi-
ty at the metal centers can be adjusted by introducing
special organic ligands with electron-donating or -
accepting groups22. As a heterogeneous version of metal-
ligand complexes, the heterojunction of the metal nano-
particle with a semiconductive support is strongly capable
of changing the electron density within the metal, as the
work function of the electrons at the Schottky barrier is
continuous, and the Fermi levels adjust through an effec-
tive flow of charges through the metal-support interface.
This charging or discharging effect can be much stronger
than any ligand effect and finally promotes the activity of
metal particles for specific catalytic reactions23-24
.
Among the majority of semiconductive or semi-metallic
catalyst supports, carbon-based materials with their ad-
justable electron structure and chemical stability have
prominent advantages in stabilizing metal nanoparticles25-
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26. Nitrogen-doped carbon supports usually serve as gen-
Characterizations: The scanning electron microscopy
1
eral promoters to enhance the catalytic activity of various
nanocatalysts for specific reactions27-28. In this work, we
exploit the fact that moderate to high level of nitrogen
dopants allow the modification of the flat band potential
(or “nobility”) of the carbon simplistically, where more
nitrogen creates a more noble support material. In this
way, nitrogen-doped carbon/metallic cobalt based heter-
ogeneous catalysts can be constructed and fine-tuned for
the efficient and sustainable oxidation of alcohols to es-
ters29-31. Herein, we report the construction of an effective
Mott-Schottky catalyst system based on nitrogen-rich
carbon-coated cobalt nanoparticles (Co@NC) for the
base-free oxidation of alcohol to ester under mild condi-
tions. Co@NC samples were prepared via the direct calci-
nation of a mixture of a metal salt, inexpensive organic
molecules and graphitic carbon nitride (g-C3N4) powder
(Figure 1a). Such a simple method proved to be powerful
to tune the nitrogen content of carbon supports without
extensively disturbing the morphology and content of the
Co nanoparticles. The highly integrated structure of the
Co@NC dyads with Co nanoparticles encapsulated inside
carbon shells enables the construction of a rectifying con-
tact at the Co/NC interface and changes the electron den-
sity of the Co nanoparticles. An optimized Co@NC is
shown to promote the catalytic oxidation and subsequent
cross-esterification of alcohols to esters, which was not
observed in the case of bare nitrogen-doped carbon sup-
port or pristine carbon-supported Co nanoparticles. The
observed activity was found to be 30-times higher than
the best current state-of-the-art transition-metal catalyst.
(SEM) observations were performed on a Nova NanoSEM
450 field emission scanning electron microscope (FEI,
USA). The transmission electron microscopy (TEM) and
high-resolution transmission electron microscopy
(HRTEM) observations were carried out on a JEM-2100F
microscope operated at an acceleration voltage of 200 kV.
Powder X-ray diffraction patterns (XRD) were recorded
on a Bruker D8 Advance X-ray diffractometer equipped
with a Cu-Kα radiation source (λ = 1.5418 Å) and operated
at a scan rate of 6° min-1. TGA was performed on a SDT
Q600 thermoanalyzer (DSC-TGA, TA Instruments, USA)
under flowing nitrogen; the heating rate was 10 °C min-1.
The XPS measurements were conducted on a Kratos Axis
Ultra DLD spectrometer using a monochromated Al Kα
radiation. The inductively coupled plasma (ICP) meas-
urements were conducted on a Perkin-Elmer Optima
3300DV (ICP) spectrometer for elemental analysis. Ele-
mental analysis (EA) was carried out on a Vario EL Cube
equipped with an M5X analytical balance. Conversions of
all substrates were determined via gas chromatography
(GC) or gas chromatography-mass spectrometry (GC-MS)
analysis.
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Oxidative esterification of benzyl alcohol under
base-free conditions: Ten milligrams of the catalysts
(Co@NC-x, 5.5 mol % Co), 4 mL of methanol and benzyl
alcohol (0.5 mmol) were added into a 10 mL round-
bottom flask. After the flask was purged 3 times with oxy-
gen gas, the mixture was stirred under 1 bar of oxygen at
60 °C for 1 h (or 4 h, 8 h, 12 h), followed by natural cooling
to room temperature. The reaction temperature was
maintained in oil bath. After the catalysts were collected
by filtration, the remaining liquid mixture was used di-
rectly for GC with a flame ionization detector (FID) detec-
tor and/or for GC-MS analysis.
2. EXPERIMENTAL SECTION
Synthesis of g-C3N4: Urea was placed in a covered cru-
cible, heated over 4h to 550 °C in air and maintained at
this temperature for another 4h. The as-formed yellow
solid was ground into a powder for further use.
Experimental details of oxidative esterification of
benzyl alcohol with base: Ten milligrams of catalysts
(Co@NC-x, 5.5 mol % Co), K2CO3 (0.1 mmol), 4 mL of
methanol and benzyl alcohol (0.5 mmol) were added into
a 10 mL round-bottom flask. After the flask was purged 3
times with oxygen, the mixture was stirred under 1 bar of
oxygen at 60 °C for 1 h followed by natural cooling to
room temperature. The reaction temperature was main-
tained in oil bath. The catalysts were removed by filtra-
tion, and the remaining liquid mixture was used directly
for GC analysis with a FID and/or GC-MS analysis.
Synthesis of Co@NC: The homogeneous cobalt-
containing precursor solution was prepared by dissolving
cobalt
nitrate
hexahydrate
(1.168
g),
1,4-
benzenedicarboxylic acid (2.72 g) and triethylene diamine
(1.92 g) in 150 mL of dimethylformamide (DMF). Then g-
C3N4 (0, 1.39, 9.28, 18.56, or 27.84 g for Co@NC-0,
Co@NC-0.3, Co@NC-2, Co@NC-4 or Co@NC-6, respec-
tively) was added to the mixed DMF solution under mag-
netic stirring. After solvent was removed, the green solid
powder was transferred to a covered crucible, heated to
900 °C at a rate of 1.5 °C/s under an N2 atmosphere and
maintained at this temperature for 1h (Figure S1). The as
obtained black solid samples were used for further char-
acterizations and catalytic reactions.
Synthesis of Co@NC-H+: Fifty milligrams of the as-
obtained Co@NC-4 was placed in 10 M HCl solution (50
mL) and stirred vigorously at 90 °C for 24h. The obtained
black sample was then washed with deionized water and
dried at 60 °C for subsequent characterizations and cata-
lytic reaction.
Quenching reactions for catalytic oxidation of
benzyl alcohol over the Co@NC catalyst: Ten milli-
grams of Co@NC-4 (5.5 mol % Co), KSCN (0.2 mmol) or
butylated hydroxytoluene (0.2 mmol) and 4 mL of meth-
anol were placed into a 10 mL round-bottom flask and
stirred at room temperature for 24 h. Then benzyl alcohol
(0.5 mmol) was added to the flask. After the flask was
purged 3 times with oxygen, the mixture was stirred un-
der 1 bar of oxygen at 60 °C for 12 h followed by natural
cooling to room temperature. After the catalysts were
collected by filtration, the remaining liquid mixture was
used directly for GC analysis with a FID.
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plate and the main nitrogen source at the same time. Con-
Recycling reaction of catalyst for oxidative esterifi-
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sidering the fact that the mixture of cobalt salt and organic
molecules could be carbonized into carbon materials at an
even lower temperature than 680 °C, we speculated that a
confined carbonization process32,33 on the surface of g-C3N4
particles may be a possible path for the formation of pores or
a foam-like structure of Co@NC-x. The morphology of the
nitrogen-doped carbon supports is a carbon foam, as re-
vealed by the scanning electron microscopy (SEM) observa-
tion (Figure S3), and does not change much by adding differ-
ent amounts of the g-C3N4 powder. The X-ray photoemission
spectroscopy (XPS) analysis results (Figure 1b) revealed a
linear relationship between the nitrogen contents of the car-
bon support and the amount of the g-C3N4 powder added.
The real N content of the Co@NC samples was estimated to
be between 0.98 wt. % to 8.72 wt. % for Co@NC-0 and
Co@NC-4, respectively, on the basis of elemental analysis
results (Table S1). Further increasing the weight ratio be-
tween the carbon nitride powder and metal-organic ligand
precursor to 6 (for Co@NC-6) or even higher did not result
in a greater nitrogen content of Co@NC any further, but re-
sulted only in a lower yield of the catalyst materials.
cation of benzyl alcohol: The used catalyst was separat-
ed by centrifugation and washed with 0.1 M NaOH and
pure methanol. The catalyst was then used for the next
catalytic reaction.
3. RESULTS AND DISCUSSION
Preparation and structure of the Co@NC-x catalyst.
To enable a larger-scale production of the Co@NC mate-
rials for possible use as heterogeneous catalysts, we de-
veloped a simple thermal condensation method using a
metal salt (exemplified by Co(NO3)2 here), and low-cost
organic molecules (terephthalic acid and triethylene dia-
mine) as the precursors. Graphitic carbon nitride powder
(bulk phase), as the main nitrogen source for the for-
mation of nitrogen-rich carbon supports, was then added
to the mixed solution of the metal salt and organic mole-
cules in DMF. The solid mixture after the solvent was re-
moved, was carbonized at 900 °C under an N2 atmosphere
(Figure S1). The as-obtained black powder was used di-
rectly for further characterizations and catalytic reactions.
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The XRD patterns (Figure S4) indicate the formation of
metallic cobalt and graphitic carbon as the main components
of the Co@NC-x samples, and also exclude the coexistence of
a detectable amount of crystalline cobalt oxides, nitrides or
carbides. Notably, triethylene diamine molecules were re-
quired to prevent excessive growth (Figure S5) in all of the
Co@NC-x samples. The TEM observations (Figure 1d and S6-
S7) indicate that Co nanoparticles in the Co@NC-x samples
exhibited a similar mean size between 28 and 30 nm. The
contents of Co nanoparticles are all approximately 16 wt.%
for the Co@NC samples, as demonstrated by ICP analysis
(Figure 1c). Again, the involvement of g-C3N4 powder did not
obviously change the structure and composition of the Co
nanoparticles. Furthermore, the HRTEM image of Co@NC-4
(Figure 1e) revealed a highly integrated nanostructure of the
layered carbon-coated Co nanoparticle. A lattice spacing of
0.205 nm was observed (Figure 1f), which is attributed to the
(111) plane of metallic cobalt. Obviously, the carbon layers act
as a shell to protect the metallic Co nanoparticles from ag-
gregation and oxidation during the synthesis process and the
following selective oxidation of alcohols. The X-ray absorp-
tion near-edge structure (XANES) results (Figure S8) re-
vealed the metallic feature of the Co components in all
Co@NC-x samples. The Fourier-transformed X-ray absorp-
tion fine structure spectra (Figure S9) further indicated that
Co-X (X = C, N, O) bonds in the Co@NC-x samples were neg-
ligible.
Figure 1. Synthesis and structure of the Co@NC-x catalyst.
(a) Proposed synthetic protocol for Co@NC from metal-
organic precursors (Co2+ cations, terephthalic acid and tri-
ethylene diamine) and bulk carbon nitride powder. Carbon
atoms are black, nitrogen atoms are green, oxygen atoms are
red and cobalt atoms are purple. (b) XPS spectra and (c) co-
balt contents, as determined by ICP analysis, of the Co@NC-
x samples (x: mass ratios of g-C3N4 and metal-organic precur-
sor). Typical (d) TEM and (e, f) HRTEM images of Co@NC-4.
Catalytic oxidation of benzyl alcohol over the Co@NC-
x catalysts. Direct esterification of alcohol under base-free
condition as a green and sustainable process requires an effi-
cient catalyst to promote the activation of oxygen molecules
and alcohols. As a proof-of-concept application of the Mott-
Schottky catalyst, such a model reaction could directly reflect
the catalytic activity of the activated Co nanoparticles with a
fixed loading amount and constant particles size but differ-
ent concentrations of nitrogen dopants in the carbon sup-
port. Initially, we tested the activity of the catalyst for selec-
tive oxidation of benzyl alcohol using O2 gas as the oxidant
under mild and base-free conditions. As shown in Figure 2a,
aerobic esterification of benzyl alcohol and methanol could
A series of Co@NC samples were obtained at 900 °C in an
N2 atmosphere from the mixture of carbon nitride powder
and metal-organic ligand precursors with different weight
ratios (x = 0, 0.3, 2, 4), where the as-obtained samples were
denoted as Co@NC-x, accordingly. Bulk g-C3N4 decomposes
completely into gaseous, nitrogen-rich fragments at tempera-
tures higher than 680 °C (Figure S2) and acts as a pore tem-
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not proceed in the absence of either catalyst (Entry 1, Table
was replaced with 1 bar N2. The conversion and selectivity
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S2) or O2 (Entry 2, Table S2). Only a trace amount of benzyl
alcohol was oxidized to benzaldehyde over Co@NC-0 (Entry
3, Table S2). These results reveal that the Co-nanoparticle-
based catalysts without the special support could not pro-
mote the aerobic esterification of benzyl alcohol in the ab-
sence of base under mild conditions, whereas Co@NC-x
could considerably prompt the formation of methyl benzoate
(MB) under the identical conditions. The catalytic activity of
Co@NC-x can be adjusted via tuning the concentration of
nitrogen dopants in the carbon support (Entries 4-7, Table S2
and Table S3). Co@NC-0.3 and Co@NC-2 could successfully
trigger the esterification of ben-
were determined by gas chromatography (GC). (b) The con-
version of benzyl alcohol (black) and selectivity for methyl
benzoate (red) and benzaldehyde (green) over Co@NC-4 at
different reaction time intervals.
zyl alcohol and give conversions of 12 % and 31 %, respective-
ly but with poor selectivity (< 52 %). The Co@NC-4 catalyst
exhibited the best performance among all of the samples
tested in this work, giving the highest conversion of benzyl
alcohol (58 %) and selectivity toward methyl benzoate (77
%). Co@NC-6 did not perform better (Entry 7, Table S2 and
Entry 5, Table S3) because a further increase of the amount
of g-C3N4 powder in the precursor did not further elevate the
nitrogen content (Table S1). Considering the relatively low
yield of Co@NC-6, we used Co@NC-4 as the best-in-class
catalyst for the activation of oxygen and alcohol molecules,
in all subsequent experiments. All these results indicate the
key role of the nitrogen-rich carbon support in generating
the catalytic activity of Co nanoparticles in Co@NC for the
aerobic esterification of alcohols using molecular oxygen.
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Further prolonging the reaction time led to a complete
conversion of benzyl alcohol to methyl benzoate over
Co@NC-4 (Figure 2b and Entries 8-10 of Table S2) with a
turnover number (TON) of 18 (Table S4). Removal of the Co
nanoparticles from Co@NC-4 via acid etching significantly
lowered its catalytic activity, leading to only a trace amount
of benzaldehyde formed within 12 h (Figure 3a and Entry 11 of
Table S2). Initially, the sample denoted as Co@NC-H+ was
obtained after etching Co@NC-4 with 10M HCl, leading to a
loss of 53 % of Co components on the basis of ICP analysis.
The residual cobalt particles are mainly those encased in a
thicker carbon shell, where the reactant molecule cannot
easily contact them (Figure S10) for possible catalytic reac-
tions. Moreover, the involvement of SCN− ions, which are
generally known to poison the metal-Nx/C catalyst in oxygen
reduction reactions34, could not completely quench the ester-
ification of benzyl alcohol. A conversion of 30 % and a selec-
tivity of 21 % to methyl benzoate were achieved with
Co@NC-4 even in the presence of 40 mol.% of KSCN (Figure
3b and Entry 12 of Table S2), indicating much higher toler-
ance and stability of the presented catalytic system.
Figure 2. Catalytic oxidation of benzyl alcohol over the
Co@NC-x catalysts. (a) The yields of methyl benzoate from
base-free esterification of benzyl alcohol and methanol over
Co@NC-x (x = 0, 0.3, 2, 4, 6) catalysts. Standard base-free
conditions: 0.5 mmol benzyl alcohol, 4 mL of CH3OH, 5.5
mol.% Co@NC-x, 60 °C, 1 h and 1 bar of O2; for 4*: 1 bar O2
Figure 3. The yields of various products from base-free esterification of benzyl alcohol and methanol (a) over Co@NC-H+ and
over Co@NC-4 in the presence of (b) SCN- or (d) BHT. (c) Base-free esterification of benzaldehyde and methanol over Co@NC-4.
(e) Proposed reaction pathway for aerobic base-free esterification of alcohol over Co@NC-4.
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In the process of the catalytic reaction, benzaldehyde as
ing the reaction process (Figure S11). The fact that benzyl
acid was not detected through GC-MS (Figure S12) further
excluded the formation of free superoxide radical anions in
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the only intermediate product (Figure 2b), further reacts
with methanol to give methyl benzoate (Figure 3c and Entry
13 of Table S2). As shown in Figure 3d and Entry 14 of Table
S2, the conversion of benzyl alcohol over Co@NC-4 largely
decreased after the addition of a free-radical scavenger, bu-
tylated hydroxytoluene (BHT). Such a depressed conversion
suggests that the reaction in our catalytic system progresses
−
the solution. As depicted in Figure 3e, the absorbed •O2 on
the surface of Co@NC-4 bound as a polaron with the coun-
terradical in the Co-nanoparticle ensured the high selectivity
for methyl benzoate through a tandem process, including the
oxidation of benzyl alcohol to benzaldehyde and further de-
hydrogenative oxidation of the hemiacetal with methanol35.
The second oxidation must progress via the hemiacetal, be-
cause benzaldehyde is not further oxidized by the current
catalytic system.
−
via the superoxide radical anion (•O2 ). Furthermore, the
electron paramagnetic resonance (EPR) results obtained us-
ing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the capture
agent directly confirmed the formation of •O2− radicals dur-
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Figure 4. Mott-Schottky effect on the catalytic performance of the Co@NC-x catalysts. (a) Nitrogen (green) and cobalt (red)
elemental-mapping images and the corresponding line profile of a typical Co@NC-4 sample. (b) N 1s and (c) Co 2p XPS spectra
of Co@NC-x. (d) Schematic illustration of Mott-Schottky-type contact of Co@NC. (e) The N/C mole ratio of Co@NC-x samples
as determined by elemental analysis and the corresponding schematic structures (inset of e). (f) TOF values for methyl benzoate
(MB) production via aerobic esterification of benzyl alcohol and methanol over Co@NC-x in this work and the highest TOF val-
ues over a Co-based heterogeneous catalyst with (purple line) or without (orange line) the addition of base reported in the litera-
ture (for detailed information please see Table S5-S6).
Mott-Schottky effect on the catalytic performance of
Co Nanoparticles in Co@NC-x catalysts. Further analysis
of the structure (Figure 4) of the Co@NC-x catalyst was con-
ducted to collect additional evidence on how the nitrogen-
rich carbon support activates the Co nanoparticles for the
selective oxidation of benzyl alcohol. The similar morphology
(Figure S3) of all Co@NC-x samples excludes the obvious
effects of the cobalt content (Figure 1c), particle size (Figure
S7), and nanostructure of Co nanoparticles and carbon sup-
port (Figures S3 and S6) on the final catalytic activity. The
significant effect of surface area (Figure S13) on the final cata-
lytic activity was also excluded here because Co@NC-0 with a
surface area one-third of that of Co@NC-4 was nearly inert
for the base-free aerobic oxidation of alcohols. Elemental-
mapping images and the energy-dispersive X-ray spectrosco-
py (EDS) line-scan profiles (Figure 4a) directly demonstrate
the homogeneous distribution of N atoms in the carbon
framework. The introduced N atoms are mainly graphitic
and pyridinic, as revealed by the N 1s XPS spectra (Figure 4b
and Figure S14). Combined with the EXAFS analysis results
(Figures S8-S9), the O 1s XPS spectra with typical peaks cen-
tered at 531.5 eV, which is attributed to absorbed oxygen-
containing species, further excluded the presence of an obvi-
ous amount of cobalt oxide surface layers (typical XPS peak
at 529.2 eV) in the Co@NC samples36-37 (Figure S15). The
gradual shifts of typical Co 2p XPS peaks (Figure 4c) to high-
er energy further indicate the gradually depressed electron
density of Co particles in Co@NC-x samples with the intro-
duction of more nitrogen dopants. This observation again
demonstrated the key role of the Mott-Schottky effect at the
interface of the heterojunction in modifying the electron
density and thus the final catalytic activity of Co particles.
Theoretical investigation suggested that the electronic
structure of carbon framework could be controlled by intro-
ducing nitrogen atoms to lower the valence band and elevate
the conduction band38-40. Note that the type of nitrogen-rich
carbon, as an ambipolar support41-42 acting in the metal/NC
heterojunction, is mainly dominated by the work functions
of two components. Because the work function of Co is lo-
cated close to that of N-doped carbon, the flow of electrons
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(Figure 4d) sensitively depends on the nitrogen content and
lysts is of key importance to accelerate the whole reaction
process43. Various bases were added in previous works to
facilitate such a deprotonation process. In our case, the elec-
tron-poor Co nanoparticles are sufficiently strong to attract
and activate O2 for the further removal of protons from the
proposed Co-H intermediates.
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the structure. Nitrogen-rich carbon with a relatively higher
flat band potential (or work function) than that of metallic
Co materials, will accept electrons from Co nanoparticles
until their Fermi level reaches equilibrium. This electron
transfer is described as the Mott-Schottky effect in solid-state
physics. Such a Schottky barrier will obviously result in an
electron redistribution at the interface of cobalt and nitro-
gen-doped carbon and enrich the positive charges on the
side of metallic cobalt. The more oxidative area thus forms
on the side of the Co nanoparticles accordingly. Typical XPS
peaks (Figure 4b and 4c) of pyridinic nitrogen and Co gradu-
ally shift to higher binding energy values as the nitrogen
content (Figure 4e) in Co@NC-x increases from Co@NC-0.3
via Co@NC-2 to Co@NC-4. The fact that the electron density
at the nitrogen decreases (according to the XPS results, the
binding energy increased gradually) with increasing nitrogen
content unambiguously supports the electron transfer over
the Mott-Schottky heterojunction at the interface of metallic
cobalt and carbon, as depicted in Figure 4d. We note that the
electron-rich and electron-deficient areas induced by the
Mott-Schottky contact are only several nanometers thick and
are located at the interface, because opposite charges attract
each other and are also bound to each other as a cross-
interface polaron.
More importantly, the nitrogen-doped carbon could also
act as built-in and “strengthened” Lewis base to accelerate
the deprotonation process. Further temperature pro-
grammed desorption (TPD) analysis results (Figure S16) of
typical samples, where CO2 is used as the probe molecule,
directly confirmed the formation of novel and strong basic
sites in the Co@NC-x dyads, because of the enhanced elec-
tron density of nitrogen-doped carbon induced by the Mott-
Schottky effect.
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As a result, the Co@NC-x-based Mott-Schottky catalysts
not only boost the oxidative power of the Co-nanoparticles,
but also enable the reaction to proceed in the absence of
bases to constitute an overall mild and additive-free catalytic
system. As the best catalyst in this work, Co@NC-4 offers a
high turnover frequency (TOF) of 8.12 mol methyl benzoate
mol-1 Co h-1 for base-free oxidation of benzyl alcohol. Such a
TOF value is an order of magnitude higher than the values
(orange line in Figure 4f) reported for current transition-
metal-based heterogeneous catalysts in base-free systems
(Table S5). Furthermore, it is superior to those (purple line in
Figure 4f) obtained in base-containing catalytic systems (Ta-
Besides the oxidation of the substrate molecules, further
oxidative deprotonation of the H-metal intermediates to
regenerate the catalytically active surface of metal nanocata-
ble S6)11-13
.
Reaction conditions a: 0.5 mmol substrates, 4 mL of CH3OH, 5.5 mol % Co catalyst, 60 °C, 12 h and 1 bar O2. Condition b: 0.1
mmol substrates, 4 mL of aliphatic alcohol, 40 mg of Co@NC-4, 70-110 °C, 12 h and 1 bar O2. The conversion (C.) and selectivity
(S.) were determined by GC and/or GC-MS.
Co@NC catalyzed cross-esterification of substituted
eral applicability of Co@NC-4, various substituted benzyl
alcohols with both electron-withdrawing (Br, Cl, NO2) and
aromatic and aliphatic alcohols. To investigate the gen-
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electron-donating (OCH3 and CH3) functional groups were
We report the de novo design of a heterogeneous catalytic
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oxidized under standard conditions. The Co@NC-4 sample
generally gave high conversions (99 %) of these substituted
benzyl alcohols and selectivity (>87 %) toward corresponding
methyl esters under base-free conditions (1a-5a of Table 1).
The preadsorption of the diverse substrates on the surface of
Co@NC-4 was significantly modified by the substitution and
the steric arrangement of functional group. This hypothesis
can be partially confirmed by the presence of a steric effect in
the para, meta and ortho isomers of methoxybenzyl alcohols
with a reactivity trend of para (4a of Table 1) < meta (6a of
Table 1). The moderate selectivity (<57 %) for ortho-
substituted methyl benzoate (Entries 1 and 5, Table S7)
demonstrates the blocking effect of these functional groups
on the esterification reaction of the corresponding alcohols.
The yields of the ortho-substituted methyl benzoate can be
optimized by longer reaction times (7a-8a of Table 1) or add-
ing more catalysts (Entries 4 and 7 of Table S7). Even hetero-
cyclic alcohols including 2-thiophenemethanol (9a, Table 1)
and 2-pyridinemethanol (10a of Table 1) could be oxidized
over Co@NC-4 to their corresponding esters with good con-
version and high selectivity. The advantages of Co@NC-4
over noble-metal-based catalysts with respect to being more
resistant against “poisoning” by N or S atoms are clearly
demonstrated. The aerobic esterification of benzyl alcohol
with other aliphatic alcohols, including ethanol, propanol
and butanol, also proceeds smoothly in the same base-free
catalytic system, further expanding the scope of the synthesis
of esters from methyl to other alkyl esters (11b-13b in Table 1
and Table S8). All these results indicate the high chemoselec-
tivity and functional-group tolerance of Co@NC-4 for base-
free aerobic esterification of alcohols using molecular oxy-
gen.
systems to boost the activity of a transition-metal nanocata-
lyst by creating a Mott-Schottky interface at metal/nitrogen-
doped carbon. Nitrogen-rich carbon shells with controllable
concentration of nitrogen dopants were used to adjust the
electron density within the as-embedded cobalt nanoparti-
cles via enhancing the electron transfer over the Schottky
barrier; this electron transfer substantially enhanced the final
activity of the cobalt/carbon dyad for aerobic esterification of
alcohols to the corresponding esters using molecular oxygen.
Because the synthesis of Co@NC-x catalysts from simple
organic molecules, inorganic metal salts and g-C3N4 powder
is easily scaled up, we expect a more general applicability of
such a series of transition-metal/nitrogen-rich carbon-based
nanocatalysts for specific catalytic reactions. Most im-
portantly, Mott-Schottky electron-transfer-induced en-
hancement of the catalytic activity of metallic nanoparticles
could be used as a general principle to design more efficient
and sustainable heterogeneous catalysts for energy storage,
organic synthesis and synthesis of fine chemicals.
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ASSOCIATED CONTENT
Supporting Information.
Experimental details, more characterization results, and de-
tailed discussions are included in the Supporting Infor-
mation. This material is available free of charge via the Inter-
net at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Dimethyl terephthalate and diethyl terephthalate are two
main monomers for the polymerization of polyethylene ter-
ephthalate (PET), one of most important engineering plas-
tics6, 44. The aerobic esterification of 1, 4-benzenedimethanol
and methanol/ethanol produced dimethyl terephthalate (14b
in Table 1)/diethyl terephthalate (15b in Table 1) in the high-
est conversions (99 %) and selectivity (>96 %) over Co@NC-
4 under base-free conditions. Such a green and efficient cata-
lytic protocol offers new possibilities for the production of
industrially relevant compounds and intermediates in a sus-
tainable manner.
xinhaoli@sjtu.edu.cn (X. H. Li),
chemcj@sjtu.edu.cn (J. S. Chen)
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by National Basic Research Pro-
gram of China (2013CB934102), National Natural Science
Foundation of China (21331004, 21301116, 21673140), SJTU-UM
joint grant, SJTU-MPI partner group, Shanghai Basic Re-
search Program (16JC1401600), Shanghai Eastern Scholar
Program and Shanghai Rising-Star Program (16QA1402100).
Reusability of Co@NC. We also examined the stability
and reusability of the Co@NC-4 catalyst by recovering the
used catalyst for further characterizations and catalytic reac-
tions. The Co@NC-4 catalyst could be reused at least three
times (Figure S17) with only a slight attenuation of activity,
which was mainly attributed to the loss of catalyst after cen-
trifugation. The structure of the used Co@NC-4 catalyst was
not visibly changed as demonstrated by the XRD and TEM
analysis results (Figure S18). Moreover, a deprotonation pro-
cess, where the used Co@NC-4 catalyst was washed with 0.1
M NaOH solution, was carried out to ensure its activity,
again suggesting the deprotonation process is the key step in
the whole aerobic esterification reaction of benzyl alcohol
over the Co@NC heterogeneous catalyst.
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