In Situ-Generated Co0-Co3O4/N-Doped Carbon Nanotubes Hybrids as Efficient and Chemoselective Catalysts for Hydrogenation of Nitroarenes

Article abstract of DOI:10.1021/acscatal.5b00737

The earth-abundant nanohybrids Co⁰/Co₃O₄@N-doped carbon nanotubes were fabricated via an efficient thermal condensation of d-glucosamine hydrochloride, melamine, and Co(NO₃)₂·6H₂O. The hybrids furnish excellent catalytic activity and perfect chemoselectivity (>99%) for a wide range of substituted nitroarenes (21 examples) under relatively mild conditions. The high catalytic performance and durability is attributed to the synergistic effects between each component, the unique structure of graphene layers-coated Co⁰, and the electronic activation of doped nitrogen. Density functional calculations indicate that the inner Co⁰ core and N species on the carbon shell can significantly decrease the dissociation energies of H₂, giving evidence of the ability of carbon shell in the hybrids to enable H₂ activation. These results open up an avenue to design more powerful low-cost catalysts for industrial applications.

Full text of DOI:10.1021/acscatal.5b00737

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Article
In Situ Generated Co0-Co3O4/N-Doped Carbon Nanotubes Hybrids as
Efficient and Chemoselective Catalysts for Hydrogenation of Nitroarenes
Zhongzhe Wei, Jing Wang, Shanjun Mao, Diefeng Su,
Haiyan Jin, Yihe Wang, Fan Xu, Haoran Li, and Yong Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00737 • Publication Date (Web): 03 Jul 2015
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In Situ Generated Co ꢀCo O /NꢀDoped Carbon
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Nanotubes Hybrids as Efficient and Chemoselective
Catalysts for Hydrogenation of Nitroarenes
‡
‡
Zhongzhe Wei , Jing Wang , Shanjun Mao, Diefeng Su, Haiyan Jin, Yihe Wang, Fan Xu, Haoran
Li, Yong Wang*
Advanced Materials and Catalysis Group, ZJUꢀNHU United R&D Center, Department of
Chemistry, Zhejiang University, Hangzhou 310028, P. R. China.
0
ABSTRACT: The earthꢀabundant nanohybrids Co /Co O @Nꢀdoped carbon nanotubes were
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fabricated via an efficient thermal condensation of Dꢀglucosamine hydrochloride, melamine and
Co(NO ) ·6H O. The hybrids furnish excellent catalytic activity and perfect chemoselectivity
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(>99%) for a wide range of substituted nitroarenes (21 examples) under relatively mild
conditions. The high catalytic performance and durability is attributed to the synergistic effects
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between each component, the unique structure of graphene layersꢀcoated Co and the electronic
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activation of doped nitrogen. Density functional calculations indicate that the inner Co core and
N species on the carbon shell can significantly decrease the dissociation energies of H , giving
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evidence of the ability of carbon shell in the hybrids to H activation. These results open up an
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avenue to design more powerful lowꢀcost catalysts for industrial applications.
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KEYWORDS: chemoselective hydrogenation; heterogeneous catalyst; metallic cobalt;
nitroarenes; nitrogen
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1. INTRODUCTION
Catalytic reduction reactions at nanoparticle surfaces constitute one of the most important
operations for the conversion of chemical raw materials and in the production of highly valuable
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,2
chemicals. Development of new materials holds the key to fundamental advances in catalysis,
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both of which are vital in order to meet the challenge of environmental and energy crises.
Functionalized amines are industrially important intermediates for the manufacture of
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,7
pharmaceuticals, agrochemicals, dyes, and fine chemicals. The general method for the
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synthesis of anilines is the reduction of aromatic nitro compounds. However, such processes
cause environmental issues or selectivity problems. Undoubtedly, selective catalytic
hydrogenations with hydrogen as the reductant represent one of the most critical technologies in
chemical industry.
In terms of industrial applications, the careful design of heterogeneous catalysts instead of
homogeneous ones is prevailing for the ease of their separation and recycling. However, the
noble metals are often not chemoslective and they must be modified with suitable additives to
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improve selectivity, but mostly at the cost of activity.
Moreover, the additives also cause
environmental issues. Breakthrough in supported Au nanoparticles (NPs) provided new
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opportunities for the selective hydrogenation of nitroarenes.
Even so, it remains a grand
challenge to realize commercialization because of the risk in supply, and volatile price. As a
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response, intensive researches have been devoted to designing heterogeneous catalysts based on
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Earthꢀabundant metals, such as Fe, Co and Ni.
Although base metals are abundant and
inexpensive, sintering or leaching of base metals (e.g., Co) cause irreversible deactivation of the
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catalysts under liquidꢀphase conditions. Therefore, new strategies to develop stable,
handleꢀconvenient, highly active and selective baseꢀmetal catalysts without the use of any
additives are highly necessary.
Nitrogenꢀcontaining nanostructured carbon materials, which possess enhanced chemical,
electrical and functional properties, are potentially of great technological interest for the
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development of catalytic system.
Though nitrogen doped graphene and carbon nanotubes
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(
CNTs) can be applied into the reduction of 4ꢀnitrophenol and nitrobenzene , strong reductants
such as NaBH , hydrazine hydrate and etc. rather than H were inevitably used, still achieving
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inferior activity and limited reaction scope. The reason may come from that carbon material with
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highly graphited sp carbon is rather chemically inert to H .
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To overcome the limitations mentioned above, we speculate that the combination of Nꢀdoped
carbon nanotubes (NCNTs) and firstꢀrow transition metals may lead to a successful integration
of the properties of the two components in the new hybrid materials, which may present
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important features in catalysis.
Herein, we report a novel and straightforward method to
prepare cobalt and NCNTs hybrids through selfꢀassembly of inexpensive starting materials,
Dꢀglucosamine hydrochloride (GAH), melamine and Co(NO ) ·6H O. The nanohybrids serve as
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highly active, selective and stable catalysts for the hydrogenation of nitro compounds. Full
conversion of various nitro substrates with excellent selectivity (>99% for most of the cases)
toward the respective amines were achieved. Density functional theory (DFT) calculations
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indicated that the introduction of Co and N dopants can simultaneously tune the inert graphene
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layer to be active in H activation by lowering its dissociation energy, thus promoting the
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considered to be inert in H activation.
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2. RESULTS AND DISCUSSION
2.1 Fabrication and Characterization of Catalysts
The overall synthetic strategy was illustrated in Figure 1. Briefly, the hybrids were achieved
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by simple thermal condensation of GAH, melamine,and Co(NO ) ·6H O at 900 C. A holding
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sequence at 600 C resulted in a sandwichꢀlike structure, during which melamine thoroughly
polymerized to form graphitic carbon nitride and GAH was condensed to form carbon skeleton
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in the interlayer of the carbon nitride.
The generated metal NPs were confined within the
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sandwich layers. High temperature (>700 C) annealing led to the thermal decomposition of
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carbon nitride, and cobaltꢀbased NPs catalyzed the growth of NCNTs , thus producing the
hybrids (denoted as CoO @NCNTs). For comparisons, Co(NO ) /GAH, Co(NO ) /Melamine,
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Co(NO ) /Glucose, and Co(NO ) /commercialꢀCNTs were treated under identical conditions,
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and the composites obtained were named CoO @GAH, CoO @M, CoO @G, and CoO @CNTs,
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Figure 1. The overall synthetic procedure of CoO @NCNTs hybrids.
x
The morphology and microstructures of CoOx@NCNTs were unambiguously characterized
by scanning electron microscopy (SEM) and highꢀsolutiontransmission electron microscopy
(HRTEM). The hybrids are made of several ꢁm long and 50ꢀ100 nm thick multiꢀwalled carbon
nanotubes (Figure 2a, b). Most NPs dispersed effectively with a mean size of 12.9 nm by
counting >300 NPs. In addition, a few particles are aggregated in the endpoint of the NCNTs. A
preliminary analysis of the phenomenon is that the nanocrystals may be dispersed effectively in
the polymerization process of melamine and GAH. The doped nitrogen atoms will anchor the
NPs and protect them from agglomeration. The lattice spacing outside of NCNTs were measured
to be 0.242, and 0.283 nm, which are in good agreement with that of crystalline Co O (Figure
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2c). Interestingly, the structure of nitrogen doped carbonꢀcoated NPs can be observed in Figure
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Figure 2. Representative SEM image (a), TEM image (b), and HRTEM images (c, d) of
CoO @NCNTs. Inset (a) is magnification of the SEM image and inset (b) is metal particle size
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distribution histogram.
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The presence of Co O , Co and crystalline carbon was further corroborated by powder Xꢀray
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diffraction (XRD). A pronounced diffraction peak at 26 and a weak reflection around 43
appeared in Figure 3b, which were ascribed to the (002) and (100) reflection of the graphiticꢀtype
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lattice. The diffraction peaks at 31.3 , 36.9 could be assigned to the (220) and (311) planes of
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Co O nanocrystal (PDF 42ꢀ1467). Besides, the diffraction peaks at 44.2 and 51.5 were
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recorded, which are the characteristic (111) and (220) reflections of cubic Co, respectively (PDF
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5ꢀ0806). Xꢀray photoelectron spectroscopy (XPS) investigations confirmed that CoO @NCNTs
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contained C, N, Co, and O as the main elements (Figure S1 in the Supporting Information). As
manifested in Figure 3c, two coreꢀlevel signals located at ~780 and 796 eV were attributed to
Co2p3/2 and Co2p , respectively. The peaks around 780 and 795.6 eV were assigned to Co O
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1/2
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phase, while those around 778.4 and 793.2 eV were in accordance with the formation of Co .
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Particularly worth mentioning was that a higher abundance of Co species on the surface of
CoO @NCNTs compared with CoO @G (Figure S2), implying that a strong interaction exists
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between Co O and nitrogenꢀdoped carbon skeleton through CoꢀOꢀC and CoꢀNꢀC bonds, and
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accordingly leads to a lower electron density at the Co site. The textural properties of
CoO @NCNTs were measured by N adsorptionꢀdesorption analysis. The adsorption isotherm
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resembled type IV with a hysteresis loop, corresponding to the existence of mesopores (Figure
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a). The specific surface areas were calculated to be 254 m g . Further insights into the
structural and electronic properties of the nanohybrids were gained from Raman spectroscopy.
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Two wellꢀdefined peaks located at 1350 and 1580 cm in Figure 3d could be assigned to typical
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Dꢀ and Gꢀbands of carbon, respectively. Other peaks were recorded and can be assigned to the
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Ramanꢀactive modes (A , E , F ) of Co O .
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The obvious N 1s, C 1s spectra and elemental analysis demonstrated that N has been
incorporated into the system successfully (Table S1, Figure S3). Scanning TEM (STEM) and
elemental mapping analyses of CoO @NCNTs disclosed that C, N, Co and O species were
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uniformly distributed in the entire skeletal framework (Figure S4). All the investigations show
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that in situ generated Co , Co O , and NCNTs together constitute CoO @NCNTs through
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oneꢀpot synthesis.
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Figure 3. a) Adsorption / desorption isotherms, b) XRD patterns, c) XPS spectrum, d) Raman
spectrum of the parent CoO @NCNTs. The inset in a) is the pore size distribution.
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2.2 Screening of Catalysts
To gain insight into the synergistic effect of each component, a series of control experiments
based on the hydrogenation of the benchmark substrate nitrobenzene were performed.
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Commercial Co worked in the hydrogenation of nitrobenzene but showed poor selectivity and
the bulk Co O was completely inactive for the reaction (Table 1, entries 1ꢀ2). Commercially
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available CoO @CNTs acquired negligible yield, implying the existence of notable cooperative
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effect of the components in CoO @NCNTs (Table 1, entry 3). What is more, the catalysts,
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entries 4ꢀ6). We speculated that melamine, GAH, and Co(NO ) ·6H O are all indispensible in
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the fabrication of active catalysts and directly related to the catalytic performance. The surface
area and total pore volume of CoO @M, CoOx@GAH and CoO @G decreased dramatically
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compared with CoO @NCNTs, which may be a reason for the conspicuous loss in activity
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(Table S2, Figure S6). Moreover, it is generally accepted that the dispersion and the mean size of
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NPs may affect the catalytic activity greatly. TEM images (Figure S7) validate that the
supported NPs of above catalysts are larger than CoO @NCNTs, verifying the confinement
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effect of the sandwichꢀlike structure derived from the polymerization of GAH and melamine. It
must be noted that the nitrogenꢀfree catalyst (CoO @G) exhibits the worst catalytic activity
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(Table 1, entry 7), which may be due to the much larger particle size (Table S2). These
observations appear to manifest the doped nitrogen atoms play a crucial role in our system. The
versatile nitrogen can provide more surface nucleation sites and promote the formation of some
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individual sections around the Nꢀrich sites, allowing efficient anchoring of metal NPs.
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Table 1. Catalytic hydrogenation of nitrobenzene
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b
Entry
Catalyst
Time (h)
2.5
Conv. (%)
Yield (%)
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Co
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CoO @NCNTs
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>99
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CoO @NCNTs
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Reaction conditions: 1 mmol nitrobenzene, 4.0 mol% Co, 5 mL CH CH OH, 110 C, 3 MPa
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b
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H , 2.5 h. Determined by GC (internal standard: nꢀdodecane). Carbon is obtained from the
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calcination of GAH and melamine at 900 C. The catalyst was pyrolyzed at 1000 C for 1 h.
Apart from the structural beneficial effects, the nitrogen functional groups also greatly affect
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the chemical state of the supported NPs. XRD results show that the diffraction peaks of Co are
evident in CoO @M and CoO @GAH but almost invisible for CoO @G (Figure S8). Instead,
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more characteristic peaks of Co O occurred in the nitrogenꢀfree catalyst, suggesting that the
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nitrogen species can promote the formation of Co . Briefly, the promoting effect of the nitrogen
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atoms leading to smaller particle size and more Co may be responsible for the higher catalytic
activity.
Gratifyingly, the kinetic curves of the reaction (Figure S9) reveal that there is no significant
formation of nitroso or polymerized products in our system, which suggests that the reaction may
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involve the direct hydrogenation of nitrobenzene to aniline. The outstanding performance
points to the high potential of the catalyst in industrial applications. Additionally, the reaction
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conditions were systematically optimized through variation of the pressure, temperature, and
solvent with the results listed in Table S3.
2.3 The Reusability of the Catalyst and Catalytic Active Sites Analysis
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Catalyst stability and the feasibility of reuse is a key attraction to robust heterogeneous catalysts.
To that end, we tested the catalytic activity of CoO @NCNTs in successive runs and compiled
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the results in Figure 4a. Unfortunately, the yield of aniline decreased gradually then level off
despite maintenance of perfect chemoselectivity throughout. Nevertheless, the activity can
th
recover through extending the reaction time (for the 11 cycle). TEM studies of the catalyst after
11 time recycles revealed the particle size of ~13.6 nm (Figure S10), almost the same as the fresh
one, which indicated aggregationꢀfree after reuse. The decreased activity of CoO @NCNTs was
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primarily due to metal leaching as manifested in Table S4 during the recycle reactions. In a
o
separated test, 51 % conversion of nitrobenzene was achieved within 1.5 h at 110 C. Then we
collected the liquid phase by centrifugation. The amount of nitrobenzene remained almost
unchanged after a further 1.5 h reaction, hinting that the leached species in the filtrate made no
contribution to the conversion of nitrobenzene. More importantly, the variation of the metal state
in our system also had a significant effect on the catalytic activity. XRD result of the catalyst
0
after the first use showed that only CoO and Co reflections were recorded (Figure S11). The
results suggested that Co O in the fresh catalyst was totally converted to CoO in the first use.
3
4
0
Besides, the Co content in the fresh and recycled catalyst was determined by
0
semiquantitative analysis of XPS. The Co content increased slightly to 22.5 % from 16.3 % after
0
1
1 recycles (Table S2, S5). This suggests that CoO may be partly converted to Co , which makes
the catalytic activity level off despite existence of leaching. To explain it clearly, hydrogen
temperatureꢀprogrammed reduction (H ꢀTPR) was conducted. Two representative reduction
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peaks of Co O were detected in CoO @NCNTs catalyst, which was extraordinarily lower than
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x
that of commercial Co O (Figure S13), indicating the phase of Co O in our system was very
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likely to be reduced by hydrogen under the reaction conditions. The results match well with the
conclusion that Co O can be readily converted to CoO. Meanwhile, the transformation from
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CoO to Co is also possible.
th
th
Figure 4. a) Reuse of CoO @NCNTs (4 mol% Co), 3 h up to 10 recycle and 12 h for 11 cycle
x
0
o
and b) Reuse of Co @NCNTs (2.4 mol% Co), 3 h for each recycle. Reaction conditions: 110 C,
3 MPa H2.
0
0
The unexpected stability of Co inspired us to investigate the catalyst in detail. Co covered
0
with nitrogenꢀdoped graphene layers (denoted as Co @NCNTs) can be easily obtained by acid
treatment of CoO @NCNTs. The HRTEM images (Figure S14) showed the complete coverage
x
0
0
of Co with graphene layers. Full yield of aniline was also achieved with Co @NCNTs (3 mol%
o
0
Co) in 3 h (110 C, 3 MPa H ). It shows that Co @NCNTs perform better than CoO @NCNTs
2
x
0
(
Table 1, entry 9), indicating that Co is even more effective than CoO for the hydrogenation of
x
o
nitrobenzene. Upon raising the pyrolysis temperature to 1000 C, there is an apparent decline in
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Co content (determined from XPS analysis) (Table S2). SEM images showed that part of
NCNTs decomposed and carbon spheres appeared (Figure S15). Meanwhile, the mean size of the
cobaltꢀbased NPs increased (Table S2). Correspondingly, the activity dropped dramatically, only
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furnishing 48% yield (Table 1, entry 10), reconfirming that Co NPs and their uniform
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morphology gave a big boost to the performance of catalysts. What is more, Co @NCNTs can be
recycled up to 5 times with unconspicuous loss of activity (Figure 4b), overcoming the poor
durability of cobaltꢀbased catalyst for the hydrogenation of nitrobenzene in liquid phase. Very
0
clear cubic Co reflections were recorded for Co @NCNTs after 5 times use (Figure 5b),
0
verifying the improved resistance of Co against oxidation with the aid of nitrogen doped
0
graphene layers. Co @NCNTs exhibits superior heterogeneous catalytic performance in terms of
selectivity, stability, and recyclability.
Figure 5. XRD patterns of the fresh and recycled catalysts, a) CoO @NCNTs and b)
x
0
Co @NCNTs.
2
.4 DFT Calculations of H Activation
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Generally, graphitic carbon deposited on transition metal is considered as catalyst poison due
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to the chemical inertness and physical blockage of surface active sites. In contrast, the excellent
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activity of Co @NCNTs in this work shows a unique phenomenon to the activation of H on the
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carbon shell outside, which is possibly attributed to the modification of the inner Co core and N
species on the carbon shell. To confirm the deduction, a firstꢀprinciple calculation was
0
performed. We studied the dissociation of H on perfect graphene (GP), Co with covered
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0
0
0
graphene (Co @GP), and Co with covered graphitic N doped graphene (Co @NGP),
respectively. As is illustrated in Figure 6, graphene, i.e. the carbon basal surface alone is rather
0
inert to H , giving a dissociation energy of 1.85 eV. Once the graphene is deposited on Co , the
2
0
dissociation energy is extremely lowered down, which implies that Co can improve the activity
0
of graphene layer in H activation. What is more, the activity of Co @GP is further improved by
2
N doping with a dissociation energy of 0.40 eV. The optimized dissociative structures of the
three catalysts are shown in Figure 6. The calculated results match well with the experimental
phenomenon. To illustrate the origin of the improved catalytic activity, the electronic structures
0
of the catalysts were performed. There is an electron transfer of net 0.05 e from the coated Co to
each carbon atom of the carbon shell in average, making the carbon shell more electronꢀrich,
which would be beneficial to dissociate H since the activation process involves the electron
2
0
injection to the H 1s antiꢀbonding state. For Co @NGP, though the carbon atom gets no net
electron transfer in average due to the bigger electronegativity of N, the electron distribution of
the carbon shell changes obviously. As shown in Figure S16, some of the carbon atoms in N
doped carbon shell get more electrons than the unꢀdoped one. This may explain the reason why
0
N doping further enhances the catalytic activity. Note that only the influence of Co and N to the
activation of H on the graphene is considered since the real active sites on the catalyst are still
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debated.
A more inꢀdepth work on the reaction mechanism is to be further investigated.
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Figure 6. The dissociation energies of H on different catalysts. Inserts are the optimized
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dissociative structures. Color code: cobalt is yellow, carbon is gray, nitrogen is blue, and
hydrogen is white.
2.5 Hydrogenation of Substituted Nitroarenes
With the optimized reaction conditions in hand, we were keen to test the general scope of
CoO @NCNTs in the hydrogenation of nitroarenes with diverse substituent groups. Pleasingly,
x
the catalyst was found to be a highly active and almost exclusively selective catalyst for the
hydrogenation of substituted nitroarenes. Apart from nitrobenzene (Table 2, entry 1), the
substituted nitrobenzene having electronꢀdonor or electronꢀacceptor groups were also furnished
excellent yields (Table 2, entries 2ꢀ8). Notably, halogenꢀsubstituted nitroarenes proceeded
smoothly to the respective haloaromatic amines without any dehelogenation (Table 2, entries
9ꢀ14).
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Moreover, CoO @NCNTs also showed impressive chemoselectivity in the hydrogenation of
x
the mostꢀchallenging substrates bearing other easily reducible groups. The reducible functional
groups in aromatic nitro substrates such as nitrile, aldehyde, keto, amide and alkene (Table 2,
entries 15ꢀ21) remained totally unaffected, thus giving the corresponding amines selectively.
0
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0
These results highlight again the distinctive difference between CoO @NCNTs and the noble
x
metal catalysts in the selective hydrogenation of substituted aromatic compounds, with the cobalt
catalyst displaying superior selectivity in general.
a
Table 2. Chemoselective hydrogenation of various substituted nitroarenes
Time
Selectivity
Yield
Entry
Substrate
Product
b
b
(h)
(%)
(%)
1
2
3
>99
>99
>99
>99
3
6
c
3
>99
99
>99
99
4
8
c
5
8
6
>99
>99
>99
>99
>99
>99
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7
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>99
>99
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>99
>99
>99
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>99
>99
>99
>99
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>99
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>99
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95
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>99
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c
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98
98
a
o
Reaction conditions: 1 mmol nitroarene, 4.0 mol% Co, 5 mL CH CH OH, 110 C, 3 MPa H ,
>99% conversion observed in all cases. Determined by GC (internal standard: nꢀdodecane). 0.5
mmol substrate.
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b
c
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0
3
. CONCLUSIONS
In summary, we developed a scalable, environmentally benign, and straightforward strategy
0
for the synthesis of Co /Co O @NCNTs. The hybrids display excellent catalytic performance for
3
4
the hydrogenation of nitroarenes into the corresponding anlines. This work represents a clear
demonstration of the cooperative effect of each component in the ternary catalyst, where the
sandwichꢀlike structure derived from the polymerization of melamine and GAH plays a crucial
role in the uniform dispersion of NPs and the doped nitrogen atoms can promote the formation of
0
0
Co . DFT calculations indicate that the Co and N atoms in the hybrids can simultaneously tune
the inert graphene layer to be active in H activation by lowering its dissociation energy. The
2
finding opens up an avenue to design more powerful catalysts through coating of metal NPs with
graphene layers. Moreover, the synthesis strategy provides a versatile platform to introduce
various metal species on nitrogenꢀdoped carbon with targeted and improved properties for
diverse catalytic reactions. Work along this line is currently underway.
4
. EXPERIMENTAL SECTION
4
.1 Materials and Catalysts Preparation
Dꢀglucosamine hydrochloride, melamine, glucose, Co(NO ) ·6H O, cobalt powder and
3
2
2
Co O were purchased from Aladdin. Commercial CNTs were used as received from Shenzhen
3
4
Nanotech Port Co., Ltd. Unless otherwise stated, all solvents and chemicals used were of
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commercially available analytical grade and used without further treatment. All gases (H , N )
2
2
used for catalyst preparation and reaction process were ultrahigh purity.
The synthesis of CoO @NCNTs involves selfꢀassembly of GAH, Melamine, and
x
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Co(NO ) ·6H O. In a typical synthesis, mixture solid of 1g GAH, 40 g melamine and 0.95 g
3
2
2
Co(NO ) ·6H O was grinded into powder. Then the homogeneous mixture was transferred into a
3
2
2
o
o
ꢀ1
o
crucible, directly heated to 600 C at a rate of 2.5 Cmin and kept at 600 C for 1 h by flowing
ꢀ1
o
o
ꢀ1
N of 400 mlmin . The material was further raised to 900 C (2.5 Cmin ) for 1 h. After that, the
2
samples were cooled to room temperature under N ambient. Finally, the products were collected
2
from the crucible. For comparison, CoO @GAH, CoO @M, CoO @G, and CoO @CNTs were
x
x
x
x
0
obtained through the same operating. Co @NCNTs was acquired by acid treatment of
o
CoO @NCNTs (1 M HNO for 48 h at room temperature). The sample pyrolyzed at 1000 C was
x
3
o
o
ꢀ1
o
first heated at 600 C at a rate of 2.5 Cmin and kept at 600 C for 1 h. Then the material was
o
o
ꢀ1
ꢀ1
further raised to 1000 C (2.5 Cmin ) for 1 h by flowing N of 400 mlmin .
2
4.2 Characterization Analyses
SEM (LEO 1550) was applied to visualize the morphology of the catalysts. TEM images were
obtained from a Hitachi H7700 transmission electron microscope with CCD imaging system on
an acceleration voltage of 100 kV. HRTEM, STEMꢀHAADF and STEMꢀEDX were performed
on Tecnai G2 F30 SꢀTwin at an acceleration voltage of 300 kV. Nitrogen adsorption analysis
was performed at 77 K using a Micromeritics ASAP 2020 to access the surface areas and pore
distributions. The BrunauerꢀEmmettꢀTeller (BET) method was used to calculate the specific
surface area (SBET). Total pore volumes (Vtotal) were calculated from the amount of nitrogen
adsorbed at a relative pressure, P/P of 0.97. The pore size distribution (PSD) plot was recorded
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from the desorption branch of the isotherm based on the BarrettꢀJoynerꢀHalenda (BJH) model.
o
o
XRD patterns were collected at room temperature with 2 θ scan range between 5 and 90 using a
wideꢀangle Xꢀray diffraction (Model D/texꢀUltima TV, Rigꢀaku, Japan) equipped with Cu Ka
radiation (40 kV, 30 mA, 1.54 Å). The Co O sizes were calculated from the Scherrer formula
0
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0
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0
3
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according to the 311 (2θ=36.9) diffraction lines in wideꢀangle XRD. XPS information was
obtained with an ESCALAB MARK II spherical analyzer using an aluminum anode (Al 1486.6
eV) Xꢀray source. The Raman spectra were collected on a Raman spectrometer (JY, HR 800)
using 514ꢀnm laser. The element analysis was measured on a Vario El elemental analyzer. The
Co content was determined by ICPꢀAES (PerkinElmer Optima OES 8000) and aqua regia was
used to dissolve the sample. H ꢀTPR was conducted with a FINESORBꢀ3010 apparatus equipped
2
with a thermal conductivity detector (TCD). Before a TPR run, catalysts were pretreated in Ar at
4
23 K for 1 h. TPR was performed under a flow of 10% H /Ar gas mixture at a flow rate of 10
2
sccm with a temperature ramp of 10 °C/min.
4.3 Catalytic Procedure and Recycling
The highꢀpressure hydrogenation reactions were carried out in a 50 mL stainlessꢀsteel
autoclave with an external temperature and stirring controller. In a typical experiment, the
autoclave was charged with nitrobenzene (123 mg, 1 mmol), the internal standard (dodecane,
1
10 mg), the cobalt catalyst (CoO @NCNTs, 8.7 mg, 4 mmol %) and the green solvent ethanol
x
(5 mL). The reactor was purged with H to remove the air for 3 times and then sealed tight and
2
pressurized to 3 MPa H . The autoclave was placed into an oilꢀbath (placed 15 minutes before
2
o
counting the reaction time in order to obtain reaction temperature) preheated at 120 C and the
o
reactions were stirred at 1000 rpm at 110 C. After the reaction, the reactor was placed into a
water bath and cooled to room temperature. The remaining H was carefully vented and the
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reaction mixture was removed for further analysis. The mixture was diluted with 25 mL ethanol
followed by centrifugation. The contents of the mixture were analyzed on GC (Shimadzu,
GCꢀ2014) equipped with a Rtxꢀ1071 column and the products were identified by GCꢀMS
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(
Agilent Technologies, GC 6890N, MS 5970).
For recycling study, the hydrogenation reaction was performed with 2.3 mmol scale (282 mg
nitrobenzene, 230 mg dodecane, 20 mg CoO @NCNTs, and 10 mL ethanol) maintaining the
x
same reaction conditions as mentioned above except using the recovered catalyst. The catalyst
o
was recovered by centrifugation, washed 3 times with ethanol and dried at 70 C overnight and
then used for the next run without any reactivation or purification.
4
.4 DFT Calculations
The calculations are performed by using periodic, spinꢀpolarized DFT as implemented in
50,51
Vienna ab initio program package (VASP)
with an electronꢀion interaction described by the
5
2
projector augmented wave (PAW) method which was proposed by Blöchl and implemented by
5
3
54
Kresse. RPBE functional is used as exchangeꢀcorrelation functional approximation with an
energy cutoff of 400 eV for a plane wave basis. A 3×3×1 kꢀpoints is used for the Brillouin
zone sampling for structure optimizing, and a 5×5×1 kꢀpoints for the electronic structure
calculation. The graphene sheet (GP) was modelled by a 3×3 unit cell with 9 carbon atoms and
0
a vacuum space of 13 Å between adjacent slabs was set. Co @GP was modelled from GP with 3
0
0
(111) Co layers under covered. Co @NGP was modelled from Co @GP with a graphitic doped
nitrogen on the graphene layer (Figure S17). For GP catalyst, all the atoms in the cell were
0
0
allowed to relax during the structure optimization. For Co @GP and Co @NGP catalysts, the
outer two layers were allowed to relax and the two bottom layers were fixed. The optimization is
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stopped when the force residue on the atom is smaller than 0.02 eV/Å. The dissociation energy is
defined as:
E = E – E – E ;
(1)
dis
tot
slab
H2
0
1
2
3
4
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0
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Where E is the total energy after the dissociation of H on the catalyst; E is the energy of the
tot
2
slab
clean slab alone; EH2 is the energy of H in the gas phase.
2
ASSOCIATED CONTENT
Supporting Information. Detailed analysis of the fresh catalysts and recycled catalysts,
optimization parameters for the hydrogenation of nitrobenzene, kinetic curves for nitrobenzene
hydrogenation, and supplementary figures. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel.: 86ꢀ571ꢀ88273551. Fax: +86ꢀ571ꢀ87951895. Eꢀmail: chemwy@zju.edu.cn.
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENT
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Financial support from the National Natural Science Foundation of China (21376208), the
Zhejiang Provincial Natural Science Foundation for Distinguished Young Scholars of China
(
LR13B030001), the Specialized Research Fund for the Doctoral Program of Higher Education
J20130060), the Fundamental Research Funds for the Central Universities, the Program for
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Zhejiang Leading Team of S&T Innovation, and the Partner Group Program of the Zhejiang
University and the MaxꢀPlanck Society are greatly appreciated.
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