Dual-Active-Sites Design of Co@C Catalysts for Ultrahigh Selective Hydrogenation of N-Heteroarenes

Article abstract of DOI:10.1016/j.chempr.2020.07.023

The dual-active-sites Co@C catalyst provides a general powerful strategy to break the limitation of scaling relation on traditional metal surfaces and thus affords unprecedentedly selective hydrogenation of various N-heteroarenes as well as high activity and stability. A porous carbon shell not only allows H₂ diffusion to Co sites for activation but also blocks accessibility of N-heteroarenes, and the hydrogenation of N-heteroarenes is achieved on carbon by the spilled hydrogen from Co sites. In addition, the presence of surface/subsurface carbon at the Co sites shows high anti-sulfur poisoning and anti-oxidant capability. Ideal heterogeneous metal hydrogenation catalysts are featured by simultaneously high activity, selectivity, and stability. Herein, we report a general yet powerful strategy to design and fabricate dual-active-sites Co@C core-shell nanoparticle for boosting selective hydrogenation of various N-heteroarenes. It can break the limitation of scaling relation on traditional metal surfaces, and thus afford unprecedentedly high selectivity, activity, and stability. Combining kinetics analysis and DFT calculations with multiple techniques directly unveil that the critical porous carbon shell with a pore size of 0.53 nm not only allows H₂ diffusion to Co sites for activation and blocks accessibility of N-heteroarenes but also catalyzes hydrogenation of N-heteroarenes via hydrogen spillover from Co sites. In addition, the presence of surface/subsurface carbon at the Co sites shows high anti-sulfur poisoning and anti-oxidant capability. This work is valuable for guiding the design and manipulation of cost-effective and robust hydrogenation catalysts. Our research can provide an environmentally friendly approach to afford unprecedentedly selective N-heteroarenes hydrogenation, which will greatly reduce the resource and energy consumption and decrease the amount of waste discharge and water pollution. Therefore, these results could help in achieving the “Clean water and sanitation” goal in the 10 UN Sustainable Development Goals. Meanwhile, the products of N-heteroarenes hydrogenation are the core structural motifs in both fine and bulk chemicals, which will make our life more beautiful. Thus, our research also benefits the “Good health and well-being” goal.

Full text of DOI:10.1016/j.chempr.2020.07.023

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Article
Dual-Active-Sites Design of Co@C Catalysts
for Ultrahigh Selective Hydrogenation of N-
Heteroarenes
Sai Zhang, Jie Gan, Zhaoming
Xia, Xiao Chen, Yong Zou,
Xuezhi Duan, Yongquan Qu
xzduan@ecust.edu.cn (X.D.)
yongquan@mail.xjtu.edu.cn (Y.Q.)
HIGHLIGHTS
The Co@C catalyst with dual
active sites
Breaking the limitation of scaling
relation on traditional metal
surfaces
Ultrahigh selective N-
heteroarenes hydrogenation
Excellent anti-sulfur poisoning
and anti-oxidant ability
The dual-active-sites Co@C catalyst provides a general powerful strategy to break
the limitation of scaling relation on traditional metal surfaces and thus affords
unprecedentedly selective hydrogenation of various N-heteroarenes as well as
high activity and stability. A porous carbon shell not only allows H2 diffusion to Co
sites for activation but also blocks accessibility of N-heteroarenes, and the
hydrogenation of N-heteroarenes is achieved on carbon by the spilled hydrogen
from Co sites. In addition, the presence of surface/subsurface carbon at the Co
sites shows high anti-sulfur poisoning and anti-oxidant capability.
Zhang et al., Chem 6, 1–13
November 5, 2020 ª 2020 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2020.07.023

Please cite this article in press as: Zhang et al., Dual-Active-Sites Design of Co@C Catalysts for Ultrahigh Selective Hydrogenation of N-Hetero-
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Article
Dual-Active-Sites Design of Co@C
Catalysts for Ultrahigh Selective
Hydrogenation of N-Heteroarenes
Sai Zhang,1,4 Jie Gan,2,4 Zhaoming Xia,3 Xiao Chen,3 Yong Zou,3 Xuezhi Duan,2,
*
and Yongquan Qu1,3,5,
*
SUMMARY
The Bigger Picture
Our research can provide an
environmentally friendly
Ideal heterogeneous metal hydrogenation catalysts are featured by
simultaneously high activity, selectivity, and stability. Herein, we
report a general yet powerful strategy to design and fabricate
dual-active-sites Co@C core-shell nanoparticle for boosting selec-
tive hydrogenation of various N-heteroarenes. It can break the lim-
itation of scaling relation on traditional metal surfaces, and thus
afford unprecedentedly high selectivity, activity, and stability.
Combining kinetics analysis and DFT calculations with multiple tech-
niques directly unveil that the critical porous carbon shell with a
pore size of 0.53 nm not only allows H2 diffusion to Co sites for acti-
vation and blocks accessibility of N-heteroarenes but also catalyzes
hydrogenation of N-heteroarenes via hydrogen spillover from Co
sites. In addition, the presence of surface/subsurface carbon at the
Co sites shows high anti-sulfur poisoning and anti-oxidant capa-
bility. This work is valuable for guiding the design and manipulation
of cost-effective and robust hydrogenation catalysts.
approach to afford
unprecedentedly selective N-
heteroarenes hydrogenation,
which will greatly reduce the
resource and energy consumption
and decrease the amount of waste
discharge and water pollution.
Therefore, these results could
help in achieving the ‘‘Clean water
and sanitation’’ goal in the 10 UN
Sustainable Development Goals.
Meanwhile, the products of N-
heteroarenes hydrogenation are
the core structural motifs in both
fine and bulk chemicals, which will
make our life more beautiful.
Thus, our research also benefits
the ‘‘Good health and well-being’’
goal.
INTRODUCTION
Development of highly efficient yet stable solid metal catalysts is an ideal but chal-
lenging topic in the selective hydrogenation.1–6 Generally, heteroatomic substrates,
e.g., N-heteroarenes especially substituted with halogen substituents and their par-
tial hydrogenation products being core structural motifs in both fine and bulk chem-
icals7–9 have strong adsorption with the catalysts and then greatly weaken the
adsorption of other reactant molecules. These lead to a trade-off between the cat-
alytic activity and selectivity. The principle behind this phenomenon mainly arises
from the limitation of Brønsted-Evans-Polanyi (BEP) scaling relations on various
metal surfaces with similar active sites structures.10–15 Hence, there is a significant
need to re-design the microstructures of metal catalysts, separating the reaction
steps to dual active sites, for breaking the scaling relations to accelerate catalytic
hydrogenation.
In the commercial process, the widely used noble catalysts (Pd, Pt, etc.) undergo
serious deactivation, which mainly arises from the serious metal leaching due to
the unselectively hydrogenated dehalogenation of various halogenated N-heteroar-
enes.16–23 Meanwhile, the dehalogenation also leads to serious equipment corro-
sion in the strong acidic reaction system. In addition, the existence of trace sulfur-
containing contaminants in industrial-grade N-heteroarenes, primarily extracted
from the coal tar, can cause serious catalyst poisoning. It is, therefore, highly
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desirable to address these urgent yet unresolved issues and develop cost-effective
and robust metal catalysts for the selective hydrogenation of N-heteroarenes.
Herein, we developed a general yet powerful strategy to fabricate a porous carbon
encapsulated Co nanoparticles (Co@C) for the sequential H2 dissociation and N-het-
eroarenes hydrogenation at the Co and C dual active sites, respectively. It is found
that the well-designed pores of carbon shell with the size of 0.53 nm allow small H2
access to Co sites for H2 activation, followed by promoting hydrogen spillover to the
surface carbon sites for catalytic hydrogenation of the adsorbed N-heteroarenes.
Such breaking of the scaling relation limitation and competitive adsorption afforded
ultrahigh selective hydrogenation of N-heteroarenes with a wide scope. Our devel-
oped cost-effective Co@C catalysts also exhibited high durability to sulfur-contain-
ing poison and oxidative atmosphere, which are potential alternative catalysts for in-
dustrial applications.
RESULTS AND DISCUSSION
Exemplified with 6-chloroquinoline to produce 6-chloro-1,2,3,4-tetrahydroquinoline,
adsorption behaviors of the reactant and its primary hydrogenation product on three
typical metal surfaces were theoretically studied by DFT calculations, and the results
are shown in Figures 1A and S1A–S1F. Clearly, the Co(111) surface against the
Pd(111) and Pt(111) surfaces shows appropriate adsorption of reactant yet weak adsorp-
tion of intermediate products, suggesting that the Co catalyst has a strong ability to sup-
press the occurrence of over-hydrogenation. As expected, experimental observations of
6-chloroquinoline hydrogenation over mono-active metal sites, traditional supported
Pd/C, Pt/C, and Co/C catalysts (Figures S1G–S1J) are well consistent with the trend of
the theoretical prediction. Notably, although the mono-active-sites Co catalyst shows
the highest selectivity to the targeted primary hydrogenation product, the unselectively
hydrogenated dehalogenation still occurs, especially under the conditions with high 6-
chloroquinoline conversions.
Considering that the carbon materials have also shown good hydrogenation activity
in nitrobenzene24,25 and easy hydrogen spillover26 together with weakly adsorbed 6-
chloroquinoline and 6-chloro-1,2,3,4-tetrahydroquinoline compared to the above
three metal surfaces, as also seen in Figures 1A and S2, fabricating dual-sites
metal-carbon hybrid could be a promising strategy for the sequential H2 dissociation
and 6-chloroquinoline hydrogenation proceeding at the Co and C sites, respec-
tively. For instance, in an ideal scenario, schematically shown in Figure 1B, the
Co@C core-shell catalysts are fabricated with the critical, well-designed pore size
of the carbon shell to only allow the interaction between small H2 and Co sites for
H2 activation and then the spillover of the activated H* to the surface carbon sites
for hydrogenating the adsorbed 6-chloroquinoline. Such a strategy can in principle
break the scaling relation limitation and competitive adsorption, which would be ex-
pected to achieve ultrahigh selective hydrogenation of 6-chloroquinoline.
1School of Chemistry and Chemical Engineering,
Northwestern Polytechnical University, Xi’an
710072, China
2State Key Laboratory of Chemical Engineering,
East China University of Science and Technology,
Shanghai 200237, China
Along this line, carbothermal reduction of Co-doped polycarbonate precursor was
proposed as the strategy to synthesize the well-designed carbon encapsulated Co
catalysts, i.e., Co@C, where the porous carbon shell acts as a ‘‘sieve’’ with the smaller
pore diameter than the size of the reactants and products (Figure 1B). Initially, the
precursor was fabricated by the copolymerization of CO2 and styrene oxides by us-
ing Co2+ as Lewis acidic catalysts (Figures 2A and S3A). During the carbothermal
reduction process at 500ꢀC, the in situ generated CO and H2 can gradually reduce
Co2+ to metallic Co, while the CO2 and/or H2O can create a microporous structure
3Frontier Institute of Science and Technology,
Xi’an Jiaotong University, Xi’an 710049, China
4These authors contributed equally
5Lead Contact
*Correspondence: xzduan@ecust.edu.cn (X.D.),
yongquan@mail.xjtu.edu.cn (Y.Q.)
https://doi.org/10.1016/j.chempr.2020.07.023
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Figure 1. Theoretical Design of the Dual-Active-Sites Co@C Catalysts for Hydrogenation of 6-
chloroquinoline
(A) Adsorption behavior of the reactant (6-chloroquinolines) and primary product (6-chloro-1,2,3,4-
tetrahydroquinoline) on various metals and carbon.
(B) Schematic diagram of dual-active-sites Co@C-catalyzed selective hydrogentaion of 6-
chloroquinoline.
of carbon shell (Figures S3B and S3C).1,27,28 X-ray diffraction (XRD) (Figure S3C), dark
field transmission electron microscopy (TEM) (Figures 2A and S3D) and energy-
dispersive X-ray spectroscopy (EDS) (Figures S3E and S3F) analyses reveal that the
as-synthesized cobalt nanoparticles are in the form of metallic Co with an average
size of 19.6 G 3.4 nm and encapsulated with ꢁ 5 nm graphite-like carbon shell.
The measured lattice fringes of nanoparticle core and carbon shell are 0.189 and
0.335 nm from the high-resolution TEM of Co@C (Figures S3G and S3H), respec-
tively, which are consistent with the standard metallic Co and the spacing of graphite
carbon. More importantly, the irregular but connective defects with sizes of
0.3ꢁ0.6 nm in carbon shell are also observed from the high-resolution TEM image,
indicating the presence of pores in carbon layer (Figure S3H). N2 physisorption pro-
file of the Co@C catalysts further reveals the main pore size of 0.53 nm (Figures 2A
and S3I), which is large enough for efficient H2 diffusion29 but smaller than the size of
6-chloroquinoline molecule (0.5 3 0.8 nm, Figure S2H). Thus, 6-chloroquinoline mol-
ecules cannot diffuse through the pores, avoiding their direct interaction with Co.
Impressively, for the hydrogenation of 6-chloroquinoline, the Co@C catalysts deliv-
ered not only a much higher catalytic activity than the reference Co/C, Pd/C and Pt/C
catalysts but also an unprecedented high selectivity of >99.5% to the primary prod-
uct of 6-chloro-1,2,3,4-tetrahydroquinoline at the end of the hydrogenation under
the same reaction conditions (Figures 2B and 2C). The bare hydrogenation of 6-
chloroquinoline in the absence of H2 (Figure S4A) and successful selective hydroge-
nation to 6-chloro-1,2,3,4-tetrahydroquinoline in toluene (Figure S4A) indicated that
hydrogenation of 6-chloroquinoline catalyzed by Co@C was not a hydrogen transfer
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Figure 2. Characterizations and Catalytic Performance of the Co@C Catalyst
(A) Schematic diagram for the synthesis of Co@C catalysts using styrene oxides and CO2 as raw materials.
(B) Time course of conversion of 6-chloroquinolines catalyzed by various catalysts.
(C) Conversion course of selectivity of 6-chloro-1,2,3,4-tetrahydroquinoline for various catalysts.
(D) Catalytic performance of Co@C for the hydrogenation of 6-chloroquinolines in the presence of thiourea. Reaction conditions: catalysts (10 mg of
Co@C, Pd/C and Pt/C, and 50 mg of Co/C), 6-chloroquinolines (2 mmol), isopropanol (2 mL), 80ꢀC, and 2 MPa H2.
process. Inductively coupled plasma (ICP) analysis showed no Co leaching of Co@C
after the catalytic reaction, indicating their catalytic robustness. However, severe
metal leaching happened on the three traditionally supported catalysts, i.e., 180,
750, and 150 ppm for the Co/C, Pt/C, and Pd/C, respectively. Meanwhile, the
Co@C catalyst also exhibited extraordinary ability against sulfur poisoning. When
the molar ratio of thiourea as poison to metal was 1:10, the catalytic activities of
the Pd/C, Pt/C, and Co/C catalysts were completely quenched (Figures 2D and
S4B). In contrast, the catalytic activity of the Co@C catalyst was slightly reduced
with the preserved chemoselectivity of 6-chloro-1,2,3,4-tetrahydroquinoline under
the same operations (Figures 2D and S4C). Even at the equal molar proportion of
thiourea and metallic Co, the Co@C catalysts could still completely hydrogenate
6-chloroquinolines with the preserved selectivity through the prolonged reaction
time of 21 h. Also, both the activity and selectivity of the Co@C catalyst in the pres-
ence of thiourea could be well preserved during cycling (Figure S4D), further sug-
gesting their excellent anti-sulfur poisoning capability.
Because the outstanding catalytic performance and the satisfactory anti-sulfur
poisoning ability of the Co@C catalyst are extremely different from the traditionally
supported metal catalysts, we speculate that the alternated catalytic pathway should
occur on this well-defined catalyst. The carbon shell with subnano pore on the sur-
face of Co nanoparticles suggests that the hydrogenation of 6-chloroquinolines
may occur at dual active sites, i.e., hydrogen dissociation on the Co surfaces fol-
lowed by hydrogenation of 6-chloroquinolines adsorbed on the C surfaces through
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Figure 3. Catalytic Mechanism Investigations
(A) EDS mapping of the used Co@C catalysts.
(B) XPS analysis of the used Co@C catalysts.
(C) The catalytic performance of various M@C catalysts for hydrogenation of 6-chloroquinlines. Reaction conditions: catalysts (10 mg of Pd@C, Pt@C,
and Ni@C), 6-chloroquinolines (2 mmol), isopropanol (2 mL), 80ꢀC, and 2 MPa H2.
(D) The pore size distribution of the various Co@C-T catalysts.
(E) Ea and selectivity of various Co@C-T catalysts for hydrogenation of 6-chloroquinlines.
hydrogen spillover process. Therefore, the prerequisite for such a dual-active-site
process is that the porous carbon layer with small pores can thoroughly block the
accessibility of the relatively lager reactants to Co surfaces.
Then, the used Co@C catalysts were tested by EDS analysis to get the visual
information (Figure 3A). The signal of Cl element from the 6-cloroquineline and/
or 6-cholor-1,2,3,4-tetrahydroquinoline was consistent with the signal of C element
but lager than the signal of Co element, indicating the presence of 6-chloroquino-
lines on the surface of carbon shell of the Co@C catalyst. To experimentally
distinct the locations of the adsorbed reactants on carbon shell or Co, the time-de-
pended X-ray photoelectron spectroscopy (XPS) analysis on the spent Co@C cata-
lysts by an in situ Ar plasma etching was carried out to examine the possible
adsorption of reactants on the catalysts. As shown in Figure 3B, the adsorption
of reactants on the used Co@C catalysts was evidenced by the presence of the
Cl 2p profile. As increasing the times of the Ar plasma treatments, the increased
Co 2p peaks and reduced C 1s peaks could indicate that the carbon shell had
been gradually etched but not completely removed along with the Ar plasma
process (Figure 3B). However, the Cl 2p peaks completely disappeared after
20 min etching (Figure 3B), illustrating that the 6-cloroquineline and/or 6-cholor-
1,2,3,4-tetrahydroquinoline molecules had been removed from the surface of the
used Co@C catalysts along with the etching carbon shell process. These results
provide direct evidence to support the dual-site process herein, in which the
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reactants do interact with the surface of the carbon layer instead of the metal Co
surface.
When the Co nanoparticles were etched by HCl solution, the residual Co@C catalyst
(Figures S5A and S5B) did not exhibit legible activity for the hydrogenation of 6-chol-
orquinolines at 80ꢀC and 2 MPa H2 even after 24 h, indicating that the H2 cannot be
activated by only porous carbon shell. Also, DFT calculation results deliver a high re-
action heat (Er) of +1.62 eV for H2 splitted into the activated H* on graphite-like car-
bon than on the Co(111) surface (À0.90 eV), and much weaker adsorption of H2
molecule on graphene (+0.31 eV) than on Co(111) surface (À0.70 eV), further sug-
gesting the thermodynamically facilitated process of H2 dissociation on the Co
rather than on the graphite-like carbon (Figures S5C and S5D). Fortunately, H2 mol-
ecules are small enough to easily diffuse through the porous carbon shell, which can
be naturally activated and dissociated on the Co surfaces. DFT calculations show
that the energy barriers of H2 dissociation on the surface of Co(111) or at the Co
with carbon atom are only 0.066 and 0.073 eV, respectively (Figure S5E).
This evidences strongly supported that the hydrogen activation and hydrogenation
of 6-chloroquinlines occur on the dual active sites of the metallic Co and surface
porous carbon shell, respectively. In this case, the activated H* must be spilled
from the metallic Co to the surface carbon shell for hydrogenating substrates. Pre-
vious reports have also proved that the activated H* species can easily diffuse on
the surface of carbon materials at low temperatures, even below room tempera-
ture.30–32 To confirm the hydrogen spillover, a physical mixture of WO3 and Co@C
catalyst was obtained for the reaction. The results showed the successful reduction
of the bright yellow WO3 into light blue WO3-x and experimentally confirmed the
occurrence of hydrogen spillover (Figure S5F).33,34
Meanwhile, the hydrogen spillover for hydrogenation at carbon surface can be
further verified by the anti-sulfur poisoning capability of the Co@C catalysts. Theo-
retically, the relatively small thiourea molecule can diffuse through the subnano
pores of carbon layer, and then access and occupy the surface of Co, leading to
the sulfur-poisoned Co surface and consequently impairing the H2 activation ability.
However, thiourea is too large to block the interface of Co and carbon shell, where
H2 can still diffuse into the interface and be activated thereby.35,36 Overall, the hy-
drogenation activity is decreased due to the thiourea-poisoning of the exposed
Co surface active sites but still can be activated owing to the existence of the inter-
face between carbon shell and Co for hydrogen activation (Figure 2D). Therefore,
the successful hydrogenation by Co@C strongly confirmed the existence of
hydrogen spillover from the interface for hydrogen activation to the carbon surface
for hydrogenation 6-chloroquinolines.
This strategy of the dual active sites for the well-defined metal core@porous carbon
shell configuration was also extended to other metal@C catalysts to realize the high-
ly selective hydrogenation of 6-chloroquinolines. Pt, Pd, and Ni nanoparticles
encapsulated in porous carbon shell (M@C) were synthesized through the identical
protocol, where (NH3)4Pt(NO3)2, Pd(NO3)2, and Ni(CH3COO)2 were used as the
metal precursors, respectively (Figures S5G–S5I). Compared with the catalytic per-
formances of their traditionally supported catalysts (Figures 2B and 2C), the M@C
catalysts presented the significantly improved catalytic selectivity for hydrogenating
6-chloroquinolines into 6-chloro-1,2,3,4-tetrahydroquinoline, reflecting the benefits
of such metal@carbon structures with subnano pores in the carbon shell for the se-
lective hydrogenation of N-heteroarene against over-hydrogenation as well as
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dehalogenation (Figure 3C). The spent M@C catalysts also exhibited the barely
changed electronic structures from the XPS analysis, also suggesting the selective
adsorption of 6-chloroquinlines on the carbon (Figures S5J, S5K, and S5M). There-
fore, the outstanding catalytic performances of those M@C catalysts strongly prove
the success of the dual active sites for the selective hydrogenation of the substituted
N-heteroarenes.
From the above analysis, the precise control on the pore size of the porous carbon
shell is a prerequisite for selective hydrogenation by constructing the dual active
sites for the hydrogen activation on the metal surface and hydrogenation on the car-
bon surface. If the pore size is larger than that of 6-chloroquinoline molecules, the
dual active sites will not work efficiently enough even with the similar structural fea-
tures. To verify this assumption, the Co-polymer precursors were pyrolyzed at
various temperatures to modulate the pore sizes of Co@C catalysts (Co@C-T, T rep-
resents pyrolysis temperature). The pore size of carbon layer via carbothermal reduc-
tion can be controlled by the treatment temperatures and the high temperature
generally results in the increased pore size of the carbon layer. As shown in Figure S6,
all synthesized Co@C catalysts have similar core@shell structural features but
different pore sizes in the porous carbon shell (Figure 3D). The Co@C-600 catalyst
showed a larger main pore size of 0.63 nm compared with Co@C-500 catalyst
(0.53 nm) from the N2 physisorption (Figures S6H and S6I), which can allow the
possible diffusion of 6-chloroquinolines through those pores. Both subnano
(0.65 nm) and large pores (3ꢁ4 nm) in the carbon shell of the Co@C-700 catalyst indi-
cated the effective interaction between the reactants and the Co surface
(Figure S6J).
Then, the pore-size-dependent catalytic hydrogenation of 6-chloroquinolines was
evaluated on the Co@C-T catalysts (Figures 3E, S6K, and S7). Among them, the
Co@C-500 catalyst delivered the highest catalytic activity and selectivity. Kinetically,
the Co@C-500 catalyst also exhibited the smallest activation energy of 41.9 kJ molÀ1
compared with 59.4 kJ molÀ1 for the Co@C-600 catalyst and 82.9 kJ molÀ1 for the
Co@C-700 catalyst. Based on DFT calculations and previous studies, the hydrogen
dissociation and subsequent hydrogen spillover easily occur in the Co@C catalyst.
Thus, the calculated activation energy can directly reflect the energy barrier of 6-
chloroquinoline hydrogenation, which could be determined by their activation de-
gree. As shown in Figures S7K and S7N, when the 6-chloroquinline molecule ad-
sorbs on the surface of carbon and Co(111) surface, the N atom gains electrons of
1.16e and 0.1e, respectively, indicating that the carbon surface can activate reactant
more efficiently than Co surface. Meanwhile, the carbon shell with small pore reveals
a similar phenomenon for the activation of 6-chloroquinlines (Figure S7M). For the
Co@C-500 catalyst, the small pores of carbon shells can ensure the efficient
hydrogen activation and hydrogenation through the hydrogen spillover process
on the surface of the carbon layer. The more effective activation of 6-chloroquinline
on the surface of carbon shell results in a lower activation energy for Co@C-500 cata-
lyst. However, the Co@C-700 catalyst with the direct interaction between 6-chloro-
quinline and Co nanoparticles leads to a higher activation energy.
Meanwhile, the relatively low selectivity of the Co@C-600 and Co@C-700 catalysts
could also be attributed to the accessibility of reactants on the Co surface through
the large pores in the carbon shell. However, the small pore size of Co@C-500 effec-
tively prevents the diffusion of 6-chloroquinolines through the carbon shell, resulting
in the high selectivity of 6-chloro-1,2,3,4-tetrahydroquinoline (Figure 3E). Therefore,
both the catalytic activity and selectivity of the Co@C-T catalysts exhibit a strong
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correlation with the pore size in the carbon shell, demonstrating the importance of
the pore size for such a dual-site strategy.
Based on the above analysis, the dual reactive pathway and hydrogen spillover
mechanism are illustrated and strongly clarified for hydrogenation of 6-chloroquino-
lines using Co@C as catalysts. With the segregation of subnano porous carbon shell,
the competitive adsorption of reactants/products on cobalt could be broken, result-
ing in the selective hydrogenation of N-heteroarenes under high conversions (Fig-
ure 1B). Next, the substrate scope of the Co@C-500 catalyst was extended to various
substituted N-heteroarenes (including quinolines and indoles) containing
halogen and other reducible groups. As summarized in Table 1, the catalyst ex-
hibited excellent catalytic performances both in the activity and selectivity for 20
substrates.
In addition, the Co@C catalyst also displayed the preserved catalytic stability at least
for 8 cycles, in which the catalyst could easily separated by a magnet and reused
without any treatments. The conversion of reactants was 71.7% at 6 h for the first cy-
cle, then gradually increased to 85.7% at the fourth cycle and reached equilibrium in
the range of 85.1% and 86.9% afterward (Figures 4A and S8A). In contrast, all the Pd/
C, Pt/C, and Co/C catalysts exhibited the different degrees of the decayed catalytic
activity during three recycles (Figures 4A and S8B), indicating that the strong coor-
dination of N atoms with the surface metal species leads to the metal leaching. Also,
the stepwise increased conversion would be attributed to the increased surface Co0
fraction for the used Co@C catalysts compared with fresh catalysts after exposure to
air (Figure S8C). Importantly, the chemoselectivity to the target product was well
preserved over 99% for each cycle (Figure S8A). After cycling, the Co@C catalyst ex-
hibited the unchanged morphology features (Figure S8D), proving their robust struc-
tural durability.
Markedly different from any previously reported Co-based catalysts that are easily
oxidized in air, the Co@C catalyst also exhibited excellent anti-oxidant capability.
X-ray absorption near-edge structure spectra (XANES) and external X-ray absorp-
tion fine structure (EXAFS) spectra of the stored Co@C catalyst after 6 months sug-
gested their unchanged lattice structure and electronic structure (Figures S8E and
S8F). After exposure in the air for 6 months, the Co@C catalyst delivered the near-
identical catalytic activity and selectivity for the hydrogenation of 6-chloroquinolines
under the same conditions (Figures 4B and S8G). In contrast, the supported Co/C
catalyst completely lost the catalytic activity after exposure in the air for only 24 h
(Figures 4B and S8G), which was attributed to the significant oxidization of cobalt
in the air from the XPS analysis (Figure S8H) and consequent loss of ability for H2
activation.
The excellent anti-oxidation capability of the Co@C catalyst is derived from its
unique structural features. For the carbothermally reduced Co@C catalyst, it is
easy to form CoCx species at the interface of Co and carbon layer under high-tem-
perature treatment.37,38 As shown in Figure 4C, the Co L2/L3 edge near edge X-ray
absorption fine structure (NEXAFS) spectra of the Co@C catalysts confirmed that the
catalysts were mainly composed of metallic Co, while the C element in Co@C cata-
lysts retained the feature of the graphite. The photon energy slightly shifted from the
referenced 285.6 eV (graphite) to 285.2 eV (Co@C), indicating an electronic interac-
tion between Co and the carbon layer. To examine the surface species of cobalt
nanoparticles, an in situ Ar plasma etching process was used to remove the graphite
layer and expose the interface. The Co L2/L3 edge of Co@C was unchanged
8
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Table 1. Hydrogenation of Various Quinolines/Indoles by the Co@C Catalysts
Entry
Substrate
Product
Temp. (oC)
Time (h)
Conv. (%)
Sel. (%)
1
80
4
98.6
>99.9
2
3
80
80
4
2
>99.9
>99.9
>99.9
95.3
4
5
6
7
8
9
80
80
80
100
80
80
7
4
7
8
3
3
98.6
99.9
94.7
53.6
89.4
>99.9
>99.9
98
>99.9
95.5
96.6
>99.9
10
11
80
8
8
86.3
91.5
99.9
92.0
100
12
80
8
93.5
>99.9
(Continued on next page)
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Article
Table 1. Continued
Entry
Substrate
Product
Temp. (oC)
Time (h)
Conv. (%)
Sel. (%)
13
80
2
>99.9
>99.9
14
15
80
80
2
4
>99.9
>99.9
>99.9
93.6
16
17
100
100
8
92.6
96.5
99.6
99.3
12
18
19
20
100
100
100
10
14
6
98.3
90.6
83.9
99.9
98.3
97.2
Reaction conditions: Co@C catalysts (10 mg), substrate (0.2 mmol), isopropanol (2 mL) and H2 pressure (2 MPa).
(Figure 4C), indicating the unchanged electronic structure of Co during this process.
Although the NEXAFS of C K-edge was verged on the standard Co2C, the Co L2/L3
edge of Co@C was similar to metallic Co rather than Co2C. The results illustrated the
possible dissolution of a very small amount of C atom in the lattice of Co nanopar-
ticles at the interface of metal and carbon layer. With the co-existence of the surface
carbon layer, the presence of CoCx in the interface can efficiently prevent the Co
nanoparticles from oxidation after exposure to oxygen, resulting in their excellent
anti-oxidant capability. Compared to the adsorption energies of oxygen on the sur-
face of Co(111), the weaker adsorption energy of O atom on the Co(111) surface with
one surface/subsurface C atom indicates that the metallic Co is hard to be oxidized
by oxygen in the presence of the dissolved carbon from the DFT calculation results
(Figure 4D). Therefore, the excellent anti-oxidation of the Co@C catalyst meets the
demands for their long-term storage under ambient conditions.
Conclusion
In summary, we have demonstrated the dual-active-sites design of metal@C core-
shell catalysts as the general and powerful strategy to break the scaling relation
and competitive adsorption for ultrahigh selective hydrogenation of N-heteroarenes
even under the high conversions. The Co@C catalyst structure-performance
10
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Article
Figure 4. Catalytic Stability of the Co@C Catalyst
(A) The cycling stability of Co@C catalysts for the hydrogenation of 6-chloroquinlines. Reaction
conditions: catalysts (10 mg of Co@C, Pd/C and Pt/C, and 50 mg of Co/C), 6-chloroquinolines
(2 mmol), isopropanol (2 mL), 80ꢀC, 2 MPa H2, and 6 h reaction.
(B) Reaction rate of the Co@C and Co/C after exposure in the air for 6 months and 24 h, respectively.
(C) Co L2/L3 edge and C k-edge NEXAFS spectra of Co@C catalysts.
(D) The stable adsorption configurations and calculated adsorption energy of O atom on Co(111)
and Co(111) with one carbon atom.
relationship has been established by combining kinetics analysis and DFT calcula-
tions with multiple techniques. The well-designed pores of carbon shell with the
size of 0.53 nm has shown to only allow small H2 to contact with the Co sites for
H2 activation and then the spillover of H atoms to the surface carbon sites for cata-
lytic hydrogenation of the adsorbed 6-chloroquinoline. The surface/subsurface car-
bon at the Co sites has also been revealed to exhibit high anti-sulfur poisoning and
anti-oxidant capability. The synthetic strategy and design concept make the hydro-
genation catalysts efficient, robust, selective, storable, and easily separated, which
would open a new avenue for designing other heterogeneous catalytic systems and
offer the intriguing opportunities for industrial applications.
EXPERIMENTAL PROCEDURES
Resource Availability
Lead Contact
Further information and requests for resources and reagents should be directed to and
will be fulfilled by the Lead Contact, Yongquan Qu (yongquan@mail.xjtu.edu.cn).
Materials Availability
The related experiments in this study did not generate new unique reagents.
Data and Code Availability
The study did not report the availability of any unpublished custom code,
software, or algorithm that is central to supporting the main claims of the
paper.
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arenes, Chem (2020), https://doi.org/10.1016/j.chempr.2020.07.023
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Article
Synthesis of Co@C as well as Reference Pd/C, Pt/C, and Co/C Catalysts
The Co-polymer precursor was synthesized through copolymerization of CO2 and sty-
rene oxides by using Co2+ as Lewis acidic catalysts. Initially, 2 mmol Co(CH3COO)2
and 0.5 mL of styrene were added into 2 mL N,N-dimethylformamide (DMF) with stirring.
Then, the polymerization was performed at 1 MPa CO2 pressure and 100ꢀC by stainless
steel autoclave for a certain time. Finally, the cooling reaction mixture was injected into
water to wash the undesired DMF solvent. The Co-doped polycarbonate precursor
could be obtained with centrifugation and dried overnight.
The well-designed Co@C catalysts were synthesized by carbothermal reduction of
Co-doped polycarbonate precursor. Under the protection of argon gas, the precur-
sor was calcined at various temperatures for 2 h to obtain the Co@C-T catalysts,
where T represented the calcination temperatures.
The reference Pd/C, Pt/C, and Co/C catalysts were synthesized through the impreg-
nation method. Typically, 300 mg of carbon black supports was dispersed in 15 mL
of ethanol with the desired amount of Na2PdCl4 (Pd, 1 wt %), H2PtCl4 (Pt, 1 wt %), and
Co(NO3)2 (Co, 15 wt %), respectively. Then, the mixture was continuously stirred for
2 h at room temperature. Afterward, ethanol was evaporated at 80ꢀC for another 2 h.
Finally, the obtained solid catalysts were treated in 5% H2/Ar at 300ꢀC in order to
obtain the metallic catalysts.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2020.07.023.
ACKNOWLEDGMENTS
We acknowledge the financial support from the Natural Science Foundation of China
(21872109 and 21922803). Y.Q. is also supported by the Cyrus Tang Foundation
through the Tang Scholar program. S.Z. is supported by the Natural Science Basic
Research Program of Shaanxi (2019JQ-039). The soft X-ray adsorption experiment
(NEXAFS) was performed at the BL10B beamline in the National Synchrotron Radi-
ation Laboratory (NSRL) in Hefei. The hard X-ray adsorption experiment (XANES
and EXAFS) was performed at BL14W1 beam line in the Shanghai Synchrotron Radi-
ation Facility (SSRF). This work was supported by the National Key Research and
Development Program of China (2016YFB0700404). We alsoacknowledge the dis-
cussion and recommendations from Dr. Zheng Jiang in SSRF.
AUTHOR CONTRIBUTIONS
S.Z., X.D., and Y.Q. designed the studies and wrote the paper. S.Z., Z.X., X.C., and
Y.Z. performed most of the experiments. J.G. carried out the DFT calculations. S.Z.,
J.G., Z.X., X.D., and Y.Q. performed data analysis. All authors discussed the results
and commented on the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: April 7, 2020
Revised: May 27, 2020
Accepted: July 21, 2020
Published: August 17, 2020; corrected online: September 17, 2020
12
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Article
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