In situ synthesis of highly dispersed Co-N-C catalysts with carbon-coated sandwich structures based on defect anchoring

Article abstract of DOI:10.1039/d0nj00213e

Highly dispersed Co-N-C catalysts were successfully prepared via a defect strategy to anchor metal atoms with a carbon coating. The catalysts showed remarkable catalytic performance under mild conditions (a yield of 97.0% for ethylbenzene oxidation). Meanwhile, BET, TEM, XRD, Raman spectroscopy and XPS studies were employed to investigate the as-prepared catalysts. The results revealed that the unique structure was conducive to producing more defects in carbon, increasing the number of active sites and enhancing the dispersion of cobalt. The outstanding catalytic performance of the catalysts was attributed to the synergy effect of the special structure and the Co-N, pyridine nitrogen and graphite nitrogen species.

Full text of DOI:10.1039/d0nj00213e

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In situ synthesis of highly dispersed Co–N–C
catalysts with carbon-coated sandwich structures
Cite this:DOI: 10.1039/d0nj00213e
based on defect anchoring†
Suisheng Li, Lushuang Zhang, Shanshan Jie and Zhigang Liu
*
Highly dispersed Co–N–C catalysts were successfully prepared via a defect strategy to anchor metal
atoms with a carbon coating. The catalysts showed remarkable catalytic performance under mild condi-
tions (a yield of 97.0% for ethylbenzene oxidation). Meanwhile, BET, TEM, XRD, Raman spectroscopy and
XPS studies were employed to investigate the as-prepared catalysts. The results revealed that the unique
structure was conducive to producing more defects in carbon, increasing the number of active sites
and enhancing the dispersion of cobalt. The outstanding catalytic performance of the catalysts was
attributed to the synergy effect of the special structure and the Co–N, pyridine nitrogen and graphite
nitrogen species.
Received 14th January 2020,
Accepted 9th March 2020
DOI: 10.1039/d0nj00213e
rsc.li/njc
high temperature; thus, the metal particle diameter increases
and the distribution of the metal is uneven, which will lead to
Hydrocarbon activation and selective oxidation is one of decreased catalytic performance. Therefore, it is important
the most important reactions in the modern chemical industry; to devise an efficient synthesis method to alleviate metal
it is an important method to obtain pharmaceutical and fine agglomeration when preparing M–N–C catalysts.
1
. Introduction
1
3
1
–4
chemical intermediates with wide applications. However, its
It should be noted that the nitrogen in the M–N–C catalysts
actual application is limited by the low conversion rate of the plays an important part in regulating the properties of materials
substrate and the low selectivity of the target product. At because nitrogen atoms can provide lone pair electrons to form a
1
4
present, most of the catalysts used in industrial production delocalized conjugated system. This system can not only
are inorganic catalysts or noble metal catalysts; however, they change the electronic structure and catalytic properties of the
1
5
still have many shortcomings. For example, inorganic catalysts adjacent carbon atoms, but also coordinate with the empty
1
6
are difficult to recover and produce toxic by-products. More- orbital of the transition metal to anchor metal atoms. Recently,
5
–8
over, Pt and Pd-based catalysts are costly. Hence, it is of great for the process of transforming noble metal nanoparticles
significance to develop a catalyst for the selective oxidation of into thermally stable single atoms via in situ environmental
hydrocarbons with high performance.
transmission electron microscopy, Wei et al. found that the
Very recently, it is worth mentioning that transition metals defects of nitrogen-doped carbon in an inert atmosphere
(e.g., Co, Fe, Ni and Mn) supported by nitrogen-doped carbon can capture movable precious metal atoms and disperse
catalysts (denoted as M–N–C) are one of the most promising metal nanoparticles due to the formation of stable metal–N
4
1
7
alternatives to precious-metal-based catalysts, which is attri- structures. Inspired by this, we proposed a method to prepare
buted to their excellent performance, abundance, economical carbon catalysts with highly dispersed metal species based on
9
–12
applicability and methanol tolerance.
However, M–N–C the defect strategy to anchor metal atoms in nitrogen-doped
catalysts are currently mostly prepared by high-temperature carbon with a carbon coating.
calcination synthesis, in which the metal precursor, N-containing
The specific strategy is shown in Scheme 1. In this project,
ligands and carbon source are one-pot calcined at 600–1000 1C melamine, the nitrogen-rich material, was used as a nitrogen
in an inert atmosphere. Metal atoms tend to agglomerate at a source to obtain nitrogen-doped carbon. We selected metallo-
porphyrin, a metallic macrocyclic compound, as the metal
precursor; it can limit metal ions to the molecular level so that
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry
and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China.
E-mail: liuzhigang@hnu.edu.cn
18
the metal atoms can be uniformly distributed in the catalyst.
A similar metal precursor was reported as an available building
1
9
block precursor for ORR/OER. On account of the strong
coordination ability between chitosan and metal ions, which
†
Electronic supplementary information (ESI) available: Experimental details,
graphics (Fig. S1–S3) and tables (Table S1). See DOI: 10.1039/d0nj00213e
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Subsequently, N,N-dimethyl formamide (100 mL) was dissolved
in 1.0 g of TPP, and CoCl
2
Á6H
2
O (2.5 g, 10.5 mmol) was added to
the flask. Then, the mixture was refluxed for 1 hour at 120 1C.
After cooling overnight, the final product (CoTPP) was prepared
by suction filtration, washing and drying in turn.
2.2.2 Preparation of Co–N–C and related composites. The
Co–N–C catalyst was synthesized from chitosan, melamine and
cobalt porphyrin, which acted as the carbon source, nitrogen
source and metal precursor, respectively. In the process of
synthesis, after adding CoTPP (X mg, X = 5, 10, 20, 50, or 100),
ethyl alcohol (20 mL), chitosan (0.2 g) and melamine (0.8 g) were
added to a 50 mL round-bottomed flask and maintained with
ultrasonic dispersion for half an hour; then, the mixture
was refluxed with magnetic stirring at 60 1C for 3 h. Following
Scheme 1 Schematic of the fabrication process of highly dispersed that, ethyl alcohol was removed by a rotary evaporator, and
Co–N–C-X.
the obtained solid was dried in an oven at 80 1C for 8 h.
Subsequently, the solid was ground into fine powder. The
powder was loaded into a corundum boat and heated in N
2
is conducive to the formation of coating structures by pyrolysis
to slow metal aggregation, we chose this renewable and low-
atmosphere at 700 1C for 2 h. The final product was denoted as
Co–N–C-X (X = 5, 10, 20, 50, or 100). The reference samples,
including N–C (without CoTPP), Co-MEL-20 (without chitosan),
2
0
cost resource as the carbon source. Co–N–C-X catalysts with
small metal particle sizes and high dispersions of Co species
were prepared by pyrolysis of mixtures with different amounts
of CoTPP, melamine and chitosan at 700 1C. We chose 700 1C
as the pyrolysis temperature because the pyrolysis of chitosan
and melamine yielded nitrogen-doped carbon graphene sheets
2
Co-CTS-20 (without melamine) and CoCl –N–C (replacing CoTPP
with 7.1 mg of CoCl O), were prepared via similar methods.
2
Á6H
2
2.3. Tests of catalytic performance
2
1
We explored the catalytic capacity via the selective oxidation of
hydrocarbon under mild conditions as a probe experiment. In
the measurements, after 15 mg of catalyst and DI water (3 mL)
were added in that order, the substrate (1.0 mmol) and TBHP
4
at 700 1C, which facilitated the formation of metal–N struc-
tures to anchor metal atoms and thus disperse the active sites.
We applied ethylbenzene oxidation as a probe experiment and
tert-butyl hydrogen peroxide (TBHP) as an oxidant under mild
reaction conditions to investigate the performance of the catalysts.
Furthermore, the structures and active sites of the prepared
catalysts were detected by nitrogen adsorption–desorption iso-
therms, transmission electron microscopy, X-ray diffraction,
Raman spectroscopy and X-ray photoelectron spectroscopy.
(3.5 mmol, 70 wt% in water) were injected into the reaction
tube by a pipette. The reaction was then maintained at 80 1C
for 12 h, accompanied by magnetic stirring. Following that,
GC analysis was applied to quantitatively detect the sample
extracted with ethyl acetate (9 mL), using n-dodecane (100 mL)
as an internal standard. Meanwhile, the catalyst was recycled
for the next run, denoted as Co–N–C-X-R
2
(X = 5, 10, 20, 50, 100),
2. Experimental section
and Co–N–C-X-R were obtained by the same logic.
3
2.1. Materials
N,N-Dimethyl formamide (DMF), pyrrole, propionic acid, methylene
3
2 2
Cl ), benzaldehyde, cobalt chloride hexahydrate
. Results and discussion
dichloride (CH
CoCl O), ethyl alcohol, melamine and chitosan were commer-
Á6H
3.1. TEM
(
2
2
cially available and were used as received without further purifica-
tion. Deionized (DI) water was homemade.
We carried out TEM and element mapping to determine the
morphology and microstructure of the Co–N–C-20 composite.
As shown in Fig. 1a, the surface topography characteristics of
crumpled and stacked graphene nanosheets were observed
2.2. Catalyst preparation
2.2.1 Synthesis of cobalt(II) porphyrin. The cobalt(II) 5,10,15,20- in Co–N–C-20. In addition, the TEM diagram of Co–N–C-20
tetraphenylporphyrin (CoTPP) was synthesized by referring to a (Fig. 1b) showed uniformly dispersed black spots, which were
2
2
reported technique. Freshly distilled pyrrole (2.62 g) mixed the coating structure formed by the carbonized chitosan coated
with propionic acid (20 mL) was added to a 500 mL three on cobalt atoms. It was reported by Beller’s group that a carbon
2
3
neck flask containing a mixture of benzaldehyde (5.62 g) and coating structure can provide catalytic active sites. This result
propionic acid (200 mL). When the reaction solution turned was consistent with the structure model of the catalyst we
purple-black, it was refluxed for 1 hour at 130 1C. After adding proposed above. We did not find a metal lattice in Fig. 1d; this
100 mL DI water and then cooling overnight, the resulting may be because the carbon-coated sandwich structure based on
precipitate was washed with DI water and purified by column defect anchoring resulted in highly dispersed cobalt nanoclusters.
chromatography, affording tetraphenyl porphyrin (TPP). However, the well-dispersed cobalt species were also illustrated
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Fig. 2 Raman spectral analysis and XRD analysis of Co–N–C-X (X = 5, 10,
2
0, 50, 100).
the catalyst. Among all these Co–N–C-X materials, Co–N–C-20
possessed the highest I /I ratio, indicating that Co–N–C-20
D
G
had the highest defect content. Meanwhile, with respect to
Co–N–C-20, Co–N–C-100, etc., the decrease of ID/IG demon-
strated clearly that the planar extension of ideal graphene
2
7
layers increased. The changing trends of the Raman spectra
agreed with the catalytic activity of the catalysts (Table 2), which
confirmed that the defects had an impact on the catalytic
performance of the catalysts.
X-ray diffraction (XRD) was carried out to study the crystal-
line structure of Co–N–C-X (X = 5, 10, 20, 50, 100), as demon-
strated in Fig. 2b. The XRD patterns of all the samples revealed
a broad peak at 261, which was attributed to the diffraction
2
8
peak of the (002) plane of graphene sheets. This coincided
with the TEM results (Fig. 1a). Unsurprisingly, peaks of metallic
cobalt were not detected in the XRD patterns of Co–N–C-X
(
X = 5, 10, 20, 50, 100); this is probably due to the small size
Fig. 1 (a–d) TEM images of Co–N–C-20 at various resolutions, (e) and (f)
HAADF-STEM images and corresponding element maps of the Co–N–C-
29
and high dispersion of cobalt in the nitrogen-doped carbon. This
result verified that the strategy adopted in this study was conducive
to achieving the synthesis of catalysts with well-dispersed cobalt.
20 catalyst.
3.3. BET
in the element mapping (Fig. 1f). Furthermore, the element
mapping diagram of cobalt was in good agreement with the Nitrogen adsorption/desorption measurements were com-
nitrogen mapping (Fig. 1f), proving that the defects of nitrogen- pleted at 77 K to characterize the porous properties of the
doped carbon could immobilize cobalt atoms and that the samples. As shown in Fig. 3b, the isotherm is similar to type IV
cobalt atoms were successfully combined with nitrogen.
and H3 hysteresis loops in the IUPAC classification; this
indicates that the mesoporous structure originates from mole-
cular stacking, which is consistent with the carbon-coated
3.2. Raman and XRD
3
0,31
To further investigate the graphitization process and hetero- sandwich structure.
The specific surface areas (Table 1)
atomic defects of Co–N–C-X (Fig. 2a), Raman spectroscopy was and pore diameter distributions (Fig. 3a) of the samples were in
À1
conducted. An obvious D band at about 1347 cm (associated line with the results of the Raman analysis, explaining that the
with defects induced by heteroatom doping) and a G band at carbon-coated sandwich structure was favourable to increase
À1
about 1578 cm
(corresponding to graphitic carbon) were the specific surface area.
2
4,25
obtained in the Raman spectra.
sive intensity ratio of the D band versus the G band (I
Generally, the comprehen-
/I ) is a
D G
significant parameter to study the characteristics of carbon
materials, reflecting the defect level in the graphitic carbon
layers. As reported in some research, carbon materials
with high ID/IG ratios are regarded to have more structure
2
6
defects, which can enhance the performance of catalysts.
For Co–N–C-5, Co–N–C-20, etc., the increase of CoTPP content
led to an increase of I /I ; this revealed that the overlapping of
melamine, CoTPP and chitosan synergistically facilitated the
generation of heteroatomic defects from cobalt and nitrogen in (b) nitrogen adsorption–desorption isotherms of Co–N–C-X (X = 5, 20, 100).
D
G
Fig. 3 (a) Pore size distributions of Co–N–C-X (X = 5, 10, 20, 50, 100),
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Table 1 The results of the N
2
adsorption–desorption measurements
Co–N–C-100 (total of 75.00%) and Co–N–C-5 (total of 59.93%)
catalysts (Table S1, ESI†). Meanwhile, the catalytic performance
of Co–N–C-20 was better than those of the other samples,
revealing that pyridinic N and graphitic N may exert influences
on ethyl benzene oxidation.
a
2
À1
b
À1
c
Sample
S
BET (m g
)
V
tot (cc g
)
p
D (nm)
Co–N–C-5
37.7
39.3
39.3
36.7
27.1
0.19
0.15
0.13
0.12
0.09
20.5
18.2
14.3
13.7
13.3
Co–N–C-10
Co–N–C-20
Co–N–C-50
Co–N–C-100
It is worth noting that the content of Co species in Co–N–C-20
was not the highest; however, it possessed the highest content of
a
b
Surface areas calculated using the BET method. Total pore volumes Co 2p3/2. Thus, the Co 2p XPS spectra were acquired to further
c
estimated from the N
2
adsorption isotherm at P/P
0
= 0.98. Mesopore
explore the chemical structures of cobalt element in the samples.
As shown in Fig. 4e, the peaks with representative binding
energies of 780.2 eV and 796.2 eV can clearly be seen, corres-
ponding to the Co 2p3/2 and Co 2p1/2 electrons; meanwhile, the
peaks at 786.3 eV and 802.2 eV are shake-up satellite peaks. The
diameters calculated from the N
method.
2
desorption branches using the BJH
3.4. XPS
In order to further investigate the chemical states and surface high-resolution spectrum of Co 2p3/2 revealed a distribution of
compositions of the N, C, and Co species in the catalysts, we Co 2p3/2 in two species: Co–O and Co–N, observed at 780.6 eV
2
8,37,38
carried out X-ray photoelectron spectroscopy (XPS) measure- and 782.7 eV, respectively.
From Co–N–C-5 to Co–N–C-20,
ments. In Fig. 4a, the elements of Co, C, N and O are displayed the increase of CoTPP content leads to an increase of Co–N and a
in the full survey spectra of Co–N–C-X, revealing the relative decrease of Co–O, indicating that the carbon-coated sandwich
contents of these elements and the changing trend with structure based on defect anchoring promoted the formation of
increasing cobalt porphyrin content. Meanwhile, the peak of Co–N (Table S1, ESI†). However, the proportion of Co–N in
C 1s is highest in Fig. 4a, indicating that the relative contents Co 2p3/2 of Co–N–C-100 was reduced; this may be because
of C were highest in these catalysts. The high-resolution C 1s agglomeration of surplus cobalt destroyed the bonds of Co–N
3
9
spectrum (Fig. 4b) showed three peaks at 284.6 eV, 286.0 eV, so that more Co–O bonds were formed.
and 288.4 eV, which were assigned to CQC, C–N and O–CQO,
3
1,32
3.5. Performance of the catalysts
respectively.
The O 1s spectrum (Fig. 4c) can be deconvo-
luted into two peaks centred at binding energies of 531.1 eV Acetophenone is a significant chemical material that is used to
and 532.3 eV; these were assigned to carboxyl and hydroxyl make perfume and as an intermediate in organic synthesis.
3
3
groups, respectively. The N 1s spectrum (Fig. 4d) presented Catalytically selective oxidation of ethylbenzene is a feasible
four different configurations of N species, including pyridinic N method for the preparation of acetophenone with good prac-
4
0
(
N1, 398.3 eV), pyrrolic N (N2, 399.1 eV), graphitic N (N3, 400.6 eV) tical prospects. Hence, the selective oxidation of ethylbenzene
28,34
and N-oxides (N4, 403.5 eV).
As reported in some research, not with TBHP as an oxidant and water as a solvent was selected
only can pyridine N coordinate with metal ions (Co–N), giving as the probe reaction to evaluate the catalytic properties of the
rise to high catalytic activity, but graphitic N can also be an as-prepared catalysts (Table 2). Co-MEL-20 (without chitosan,
active specie to promote the oxidation reactions of alkanes and conv. 57.5%), Co-CTS-20 (without melamine, conv. 29.8%)
3
5,36
alcohols.
The scale of pyridinic N and graphitic N (total of and N–C (without cobalt, conv. 55.9%), as contrast samples,
86.70%) in the Co–N–C-20 catalyst was higher than those in the exhibited poor catalytic activity, proving that three different raw
materials were essential to the special structure of these
catalysts. It is necessary to note that Co-CTS-20 (less nitrogen,
sel. 72.1%) and N–C (without cobalt, sel. 84.4%) showed lower
selectivity than the other samples, indicating that Co–N is the
main active site of the selective oxidation of ethylbenzene.
2
However, CoCl –N–C (conv. 86.1%), with the same molar
amount of cobalt as Co–N–C-20, afforded lower ethylbenzene
conversion than Co–N–C-20, possibly because the macrocyclic
structure of cobalt porphyrin is beneficial to the high disper-
1
8
sion of cobalt atoms. The influence of the cobalt porphyrin
content on the catalytic performance was explored in order to
further verify the active sites of Co–N–C-X. With increasing
amount of cobalt porphyrin content in the Co–N–C-X catalysts,
the change trend of Co–N content was identical to the change
trend of ethylbenzene conversion (Table S1, ESI† and Table 2),
suggesting once more that the Co–N bond is the active site in
Co–N–C-X for the selective oxidation of ethylbenzene. The
catalytic capability decreased from Co–N–C-20 to Co–N–C-100
Fig. 4 XPS spectra of the as-prepared catalysts. (b) C 1s XPS spectrum
and (c) O 1s XPS spectrum of the sample Co–N–C-20; (a), (d) and (e) XPS
survey spectra, N 1s and Co 2p XPS spectra of Co–N–C-100, Co–N–C-20
and Co–N–C-5.
(
Table 2, entries 7–9), possibly due to aggregation of redundant
3
9
cobalt atoms. Interestingly, the decline in activity was slow,
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Table 2 Catalytic performance of the catalysts for ethylbenzene oxidation bonds and greater number of defects according to the analysis
of the XPS and Raman measurements. Actually, the pyrolysis
temperature has a great influence on the performance of the
catalysts; for example, high temperature is conducive to the
4
1
reduction of cobalt. We prepared Co–N–C (800) at the pyrolysis
Entry
Catalyst
CoCl –N–C
Co-MEL-20
Co-CTS-20
N–C
Co–N–C-5
Co–N–C-10
Co–N–C-20
Co–N–C-50
Co–N–C-100
Co–N–C (800)
Conv./%
Sel./% temperature of 800 1C, in which the other conditions were the
same as for the synthesis of Co–N–C-20. We found that the
1
2
3
4
5
6
7
8
9
2
86.1
57.5
29.8
55.9
74.8
81.7
97.0
93.7
89.0
90.9
84.1
75.0
98.1
92.0
72.1
84.4
95.4
96.9
99.6
98.8
97.3
99.3
95.2
92.2
catalytic capacity of Co–N–C (800) (conv. 90.9%, sel. 99.3%) was
lower than that of Co–N–C-20 because the higher surface energy
42
of cobalt at 800 1C led to more metal agglomeration. However,
the yield of Co–N–C (800) for ethylbenzene oxidation was still
approximately 90%, indicating that our strategy can be useful at
higher temperatures.
1
1
1
0
1
2
Afterwards, we investigated the catalytic performance of
Co–N–C-20 for the oxidation of other hydrocarbons, as shown
in Table 3. Delightfully, the conversions of the examined
Co–N–C-20-R
Co–N–C-20-R
2
3
Reaction conditions: ethylbenzene (1.0 mmol), TBHP (3.5 mmol, hydrocarbons were all good; this shows that Co–N–C-20
7
0 wt% in water), catalyst (15 mg).
possesses a certain level of universality for the catalytic oxidation
of hydrocarbons, especially hydrocarbons with two phenyl groups
illustrating that the carbon-coated sandwich structure can weaken (Table 3, entries 7 and 8). It also suggests that Co–N–C-20 can
the aggregation of metal ions to some extent. Meanwhile, facilitate the activation of C–H bonds. Moreover, compared with
Co–N–C-20 (conv. 97.0%), with higher catalytic competence the conversion of p-ethyltoluene and p-bromoethylbenzene
than the others, was attributed to the higher content of Co–N (Table 3, entries 5 and 6), we found that hydrocarbons with
electron withdrawing groups were more likely to be oxidized.
Table 3 Catalytic oxidation of several substrates by Co–N–C-20
4
. Conclusion
Entry Substrate
1
Product
Conv./% Sel./%
In summary, we successfully prepared highly dispersed metal
nanocluster catalysts via a defect strategy to anchor metal
atoms with a carbon coating. Among these catalysts, Co–N–C-20
not only possessed superior catalytic capacity of selective ethyl-
benzene oxidation, but also had high catalytic activity for other
substrates; this indicates that it possesses wide applicability.
Based on a series of control group experiments and characteriza-
tion results, we discovered that the excellent catalytic performance
of the catalyst is probably due to synergy effects of the special
structure, Co–N bonds, and pyridine nitrogen and graphite
nitrogen species. This new integrated strategy may provide an
additional method for future research.
97
99.6
2
94.1
98.45
3
4
5
6
499
499
90
46.14
63.7
89.12 Conflicts of interest
There are no conflicts to declare.
499
97.5
Acknowledgements
7
8
499
499
499
499
This work was supported by the National Natural Science
Foundation of China (No. 21872045).
Notes and references
9
95.9
70.1
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Reaction conditions: substrate (1.0 mmol), TBHP (3.5 mmol, 70 wt% in
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