A Durable Nickel Single-Atom Catalyst for Hydrogenation Reactions and Cellulose Valorization under Harsh Conditions

Article abstract of DOI:10.1002/anie.201802231

Hydrothermally stable, acid-resistant nickel catalysts are highly desired in hydrogenation reactions, but such a catalyst remains absent owing to the inherent vulnerability of nickel under acidic conditions. An ultra-durable Ni-N-C single-atom catalyst (S

Full text of DOI:10.1002/anie.201802231

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Accepted Article
Title: A Durable Nickel Single-Atom Catalyst for Hydrogenation
Reactions and Cellulose Valorization under Harsh Conditions
Authors: Wengang Liu, Yinjuan Chen, Haifeng Qi, Leilei Zhang,
Wensheng Yan, Xiaoyan Liu, Xiaofeng Yang, Shu Miao,
Wentao Wang, Chenguang Liu, Aiqin Wang, Jun Li, and Tao
Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802231
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10.1002/anie.201802231
Angewandte Chemie International Edition
COMMUNICATION
A Durable Nickel Single-Atom Catalyst for Hydrogenation
Reactions and Cellulose Valorization under Harsh Conditions
Wengang Liu,+ Yinjuan Chen,+ Haifeng Qi,+ Leilei Zhang, Wensheng Yan, Xiaoyan Liu, Xiaofeng Yang,
Shu Miao, Wentao Wang, Chenguang Liu, Aiqin Wang,* Jun Li,* Tao Zhang
Abstract: Hydrothermally stable and acid-resistant nickel catalysts
are highly desired in industrially important hydrogenation reactions.
Yet, such catalyst remains absent due to the inherent vulnerability of
nickel under acidic reaction media. In this work, we have developed
an ultra-durable Ni−N−C single-atom catalyst (SAC) that possesses
a remarkable Ni content (7.5 wt%) required for practical usage. This
SAC shows not only high activities for hydrogenation of various
unsaturated substrates but also unprecedented durability for the
one-pot conversion of cellulose under very harsh conditions (245 oC,
60 bar H2, presence of tungstic acid in hot water). Using integrated
spectroscopy characterization and computational modeling, we have
identified the active site structure as (Ni−N4)…N, where significantly
considerable activity. Unfortunately, most of the nickel catalysts
cannot survive under acidic reaction media; the inherent
dissolution of low valence state Ni0/Niδ+ species in acids will
result in leaching and aggregation of catalytically active Ni
species, thus significant loss of activity.[6] This vulnerability of
nickel catalysts has become
a vast barrier for practical
applications in biomass transformations. Therefore, it is keenly
desired to develop a robust and acid-resistant Ni catalyst for
hydrogen-assisted biomass valorizations.
Enormous efforts were made towards improving the stability
of Ni catalysts, including encapsulating Ni nanoparticles (NPs)
with a porous carbon layer or a carbon nitride layer,[7] as well as
alloying with a noble metal.[8] However, physical isolation by an
inert layer will cause mass transfer limitations in treating bulky
biomass molecules, and the addition of a noble metal can only
alleviate the leaching of nickel to a certain extent. Interestingly,
single-atom catalysis offers a new strategy to fabricate highly
stable active center.[9] The recently developed M-N-C (M refers
to transition metals typically like Fe, Co, Ni) single-atom
catalysts (SACs) with M bonding strongly with the surrounding N
atoms have shown promising performances in various
electrochemical reactions[10] as well as organic synthesis.[11]
Benefited from the strong covalent chemical bonding between M
and N atoms, the M-N-C SACs might be resistant against acid
leaching and heat-induced aggregation. Indeed, as shown in our
previous work about Fe-N-C and Co-N-C SACs prepared with
pyrolysis of N-containing precursors followed by acid etching,
the survived Fe and Co atoms after the acid etching remain
quite stable during reduction and oxidation reactions.[12]
Compared to Fe and Co, Ni tends to have a higher intrinsic
activity for hydrogen activation, but the synthesis of Ni-N-C
SACs with a practically required Ni loading (e.g., ~10%) is yet to
be achieved. Very recently, a few Ni-N-C SACs targeted to
electrochemical reactions have been reported although the low
Ni loadings (1~2 wt%) preclude practical hydrogenation
reactions for biomass valorization.[11a, 11b, 13]
distorted octahedral coordination and pyridinic
N constitute a
frustrated Lewis pair for the heterolytic dissociation of dihydrogen,
and the robust covalent chemical bonding between Ni and N atoms
accounts for its ultrastability.
Biomass, as the most abundant renewable carbon source on the
earth, is being explored as an alternative feedstock for
production of liquid fuels and chemicals to reduce CO2
emissions.[1] Different from the traditional petroleum chemical
processes that are mainly operated in gas phase, the highly
functionalized feature of biomass molecules (O/C ratio as high
as 1.0) necessitates the chemical transformations in liquid phase
and often at harsh reaction conditions,[2] e.g., strong acid/basic
media, high temperature and pressures. Such tough reaction
environment raises a significant challenge for the catalyst
durability. For example, nickel catalysts were reported to be
highly active in the hydrogen-involved biomass conversion
reactions such as hydrogenation,[3] hydrogenolysis[4] and
hydrodeoxygenation reactions,[5] and nickel would be a favorable
choice to noble metals due to its low cost, earth abundance and
[]
W. Liu,+ H. Qi,+ L. Zhang, X. Liu, X. Yang, S. Miao, W. Wang, Prof.
Dr. A. Wang, Prof. Dr. T. Zhang
State Key Laboratory of Catalysis, and Collaborative Innovation
Center of Chemistry for Energy Materials, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences
457 ZhongShan Road, Dalian, 116023, China
E-mail: aqwang@dicp.ac.cn
Herein, we report the first synthesis of high-loading Ni-N-C
SAC (Ni content 7.5 wt%) and its application for the reaction of
cellulose conversion to important polyols (ethylene glycol and
acetol). In contrast to the poor stability of Ni NPs supported on
active carbon (Ni/AC), the Ni-N-C SAC exhibits unprecedentedly
W.Liu; H. Qi
University of Chinese Academy of Sciences, Beijing, 100049, China
Y. Chen,+ Prof. Dr. Jun Li
o
durability under very harsh reaction conditions (245 C, 60 bar
Key Laboratory of Organic Optoelectronics & Molecular Engineering
of the Ministry of Education, Department of Chemistry, Tsinghua
University, Beijing 100084 (China)
H2, presence of tungstic acid in hot water) and it can be reused
for at least 7 times without any deactivation or single atom
aggregation. Extensive experimental characterizations in
conjunction with theoretical studies revealed the active-center
structure of (Ni−N4)…N and an unusual heterolytic dihydrogen
activation mode on a frustrated Lewis pair (FLP) site constituted
by isolated Ni cation and the adjacent non-coordinated pyridinic
N atom.
E-mail: junli@tsinghua.edu.cn
Y. Chen+, Chenguang Liu
State Key Laboratory of Heavy Oil Processing, College of Chemical
Engineering, China University of Petroleum (East China), Qingdao,
Shandong, 266580, China
W. Yan
National Synchrotron Radiation Laboratory, University of Science
and Technology of China
To prepare the Ni−N−C SAC, a Ni(phen)3 complex
precursor was first dispersed on MgO and then the mixture was
Hefei, 230029, China
[+]
† These authors contributed equally to this work.
o
submitted to pyrolysis at 600 C in N2 for 2 h, after which the
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201802231
Angewandte Chemie International Edition
COMMUNICATION
state (between NiII and NiIII).[11b] The N 1s spectrum could be
deconvoluted into three peaks centered at 399.0, 400.9, and
405.9 eV, which were corresponding to pyridinic N, graphitic N,
and N-oxide (N+-O-), respectively.[15] The relatively high
percentage of pyridinic N (72.2%) could justify the high loading
of Ni single atoms since the pyridinic N is generally believed to
serve as the coordinating sites for transition metal species.[12]
The atomic ratio of pyridinic N to Ni, determined by XPS, was
4.4, suggesting that each Ni atom is coordinated with four or five
pyridinic N atoms. This will be verified by the following X-ray
absorption spectroscopy characterizations.
Figure 1. (a) Schematic illustration for preparation of Ni−N−C catalyst; (b)
HAADF-STEM image and (c) Fast Fourier transform; (d) Intensity profiles
along the lines at positions 1 and 2 in the HAADF-STEM image (c); (e)
Elemental mapping of Ni/N/O/C in Ni−N−C catalyst. The white dots in (b, c)
are Ni single atoms.
MgO support was removed by acid etching (Figure 1a, more
details in supporting information). ICP analysis revealed that the
Ni loading was 7.5 wt%, which was several times higher than
[11a, 11b,
those of the Ni-N-C SACs prepared by other techniques.
13] However, no any Ni or Ni oxide peaks were present in XRD or
Raman spectra of the Ni-N-C samples (Figures S1, S2),
indicating that the Ni species is highly dispersed.[14] On the other
hand, the pyrolysis temperature imposed a significant effect on
both the textural properties and the dispersion of Ni species. The
BET surface areas reached the highest (500 m2/g) at 600 oC and
Figure 2. Comparison of experimental and calculated XANES spectra of the
Ni-N-C-600, and the inset shows the structure model of Ni-N5.
The structure of the Ni-N-C SAC was resolved by X-ray
absorption near-edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) spectroscopy at the Ni K-edge.
As shown in Figure S8, the XANES spectra of the three Ni−N−C
samples show similar features that are quite different from Ni foil,
Ni2O3 and Ni(II)Pc. The E0 values (the first inflection point on the
edge; the higher E0, the higher oxidation state, see Table S1)
followed the order of Ni foil < Ni-N-C-700 < Ni(II)Pc < Ni-N-C-
600 < Ni-N-C-500 < Ni2O3 (inset of Figure S8), confirming that
the Ni single atoms in the Ni-N-C-600 are between NiII and NiIII,
which is in good agreement with the XPS result. Moreover, the
XANES spectrum of the Ni−N−C-600 catalyst presents four
distinct features (Figure 2, black line): (i) peak A, pre-edge at
8333 eV; (ii) peak B, pre-edge at 8339 eV; (iii) peak C,
absorption edge at 8351 eV; (iv) peak D, post-edge at 8366 eV.
The first pre-edge peak A can be attributed to the 1s → 3d
electronic transition of Ni cations in an octahedral symmetry,[16]
although this transition is generally forbidden in octahedral
ligand field due to orbital symmetry mismatch. Only slight (p-d)
orbital mixing in distorted local symmetry can give some
probability for the 1s→3d transitions.[16b] Therefore, we speculate
that the Ni single atoms in the Ni-N-C-600 might be located in a
distorted octahedral coordination geometry. The second pre-
edge peak B is due to the 1s→4pz electronic transition, which is
often regarded as the fingerprint of square-planar M−N4
moiety.[12a, 17] Compared to Ni(II)Pc (Figure S8), the remarkably
o
then decreased markedly to 376 m2/g at 700 C, indicating the
loss of pores due to structural collapse at higher temperatures
(Figure S3). This phenomenon, however, was not observed for
either Fe-N-C or Co-N-C materials prepared with a similar
procedure,[12] implying that the metal species participated in and
probably catalyzed the carbonization process. If Ni NPs were
formed at 700 oC, they would catalyze the methanation of
carbon and then resulted in collapse of porous structure. This
speculation was confirmed by low-magnification STEM images
(Figure S4). For both the Ni-N-C-500 and Ni-N-C-600 samples
we did not observe any metal particles. In contrast, Ni particles
with sizes of 10-20 nm were clearly observed in the Ni-N-C-700
sample, demonstrating that higher pyrolysis temperature led to
considerable aggregation of Ni species. To identify the invisible
Ni species in the Ni-N-C-600 sample, we resorted to aberration-
corrected subångström-resolution HAADF-STEM technique. As
shown in Figure 1b and Figure S5, a high density of Ni single
atoms (1.6 Ni atoms/nm2) were uniformly dispersed in the
Ni−N−C-600 sample, and the intensity profiles along the lines at
position 1 and 2 (Figure 1c, d) further confirmed that the white
dots in Figure 1b are individual Ni atoms. The EDX analysis
attached to electron microscope revealed a Ni content of 6.55 wt%
(Figure S6), which was close to the value determined by ICP.
The elemental mapping showed totally overlapping images of Ni,
C, O and N (Figure 1e), suggesting that single atoms of Ni might
be chemically bonded with N, C or O.
weakened peak
B of the Ni-N-C-600 indicates also a
significantly distorted Ni-N4 structure. In addition, the main peak
C at the absorption edge corresponds to transition from 1s to 4p
continuum states and the fourth peak D occurring ~15 eV above
the main peak C is due to the multiple scattering.[16a, 18]
The chemical states of Ni, N and C in the Ni−N−C sample
were probed with XPS (Figure S7). The Ni 2p spectra showed
two peaks at binding energies of 855.3 eV (2p3/2) and 872.7 eV
(2p1/2), suggesting that Ni single atoms are in a high oxidation
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10.1002/anie.201802231
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Based on these features, we constructed using density
functional theory (DFT) calculations various models with
different coordination geometries (planar or nonplanar, nitrogen
or oxygen atoms, distinct Ni−N4, Ni−N5 and Ni−N6 structures).
For computational details see supporting information. We
calculated XANES spectrum of each model and compared it with
the experimental spectrum of the Ni−N−C-600 catalyst (Figure
Ni centre, it is activated via the interaction with Ni and the
adjacent pyridinic N atoms. As a result, the H2 inserts into
Ni-N5 with the H-H bond elongated by 0.237 Å (from 0.760 Å in
gas phase H2 to 0.997 Å in TS1) and the weak Ni-N3 bond is
cleaved (Figure 3a, TS1). It can be viewed as the coordination
exchange of the pyridine ligand and hydrogen atom during H2
activation. The activation barrier for this step is 0.58 eV (△H),
S9). It turned out that the theoretical XANES spectrum of (Ni−N4) which is 0.87 eV lower than that on Au(111) surface.[19] The
┅ N configuration (dotted line and inset in Figure 2) could
perfectly reproduce the main features of the experimental
spectrum of Ni−N−C-600 catalyst. In this model, the Ni center
atom is coordinated with four pyridinic N atoms (Ni−N1N2N3N4) to
form a significantly distorted quasi-planar Ni-N4 structure, while
the fifth pyridinic N atom coordinates very weakly to the centre
Ni atom in the perpendicular direction. As a result, the whole Ni-
N5 structure can be viewed as substantially distorted octahedral
electron density difference of TS1 (Figure 3c) shows a
significant electron density decrease on Hδ+ and an increase on
Hδ-. From the ELF and LOL maps the covalent interaction
between N and Hδ+ is clearly seen, and the high localization of
electron density around Hδ- and delocalization between Hδ- and
Ni are also observed. Consequently, the H2 molecule is
heterolytically dissociated to form a meta-stable Hδ+-N and Hδ--Ni
configuration (Figure 3b and Figure S13). The localized
molecular orbitals (LMOs) and the localized orbital bonding
analysis (LOBA) clearly show the oxidation state of each split H
atoms and Ni center (Figure 3b), Hδ+ +1, Hδ- -1 and Ni(II).[20]
Such a heterolytic dissociation manner is reminiscent of the FLP
in homogeneous catalysis, which can perform well the activation
of dihydrogen in a similar way.[21] Indeed, the Ni-N5 structure
possesses both Lewis acid and basic sites as revealed by CO2-
TPD and NH3-TPD process (Figure S14). It can be anticipated
that our Ni-N-C catalyst with well-defined Ni-N5 structure will be
active in a series of hydrogenation reactions.
coordination geometry with
a ligand-missing. The good
agreement between this XANES-optimized Ni-N5 structure and
the XPS-determined pyridinic N/Ni atomic ratio (4.4) implies the
homogeneity of the structure throughout the catalyst.
The above structure was corroborated with EXAFS result at
the Ni K-edge. As shown in Figure S10, the Fourier transformed
k2-weighted EXAFS spectrum of the Ni−N−C-600 presents only
one prominent peak at ca. 1.40 Å, similar to the feature of
Ni(II)Pc. This peak should be attributed to the coordination
between Ni and light C/N elements. The absence of Ni-Ni bond
agrees with HAADF-STEM result while the fitting result of the
EXAFS data showed two distinct Ni-N coordination shells, with a
coordination number of 3.4 and 1.7 at a distance of 1.90 and
2.14 Å (Figure S11, S12 and Table S2), respectively. This result
agrees well with the DFT and XANES calculation result (Table
S3) within the error limit.
As expected, the catalyst showed excellent activity for the
hydrogenations of various biomass-bearing unsaturated groups
including nitroarenes (-NO2), aldehyde (-C=O), styrene (-C=C-),
phenylacetylene (-C≡C-) and quinline (-C=N-). Remarkably,
nearly 100% yields of the corresponding products were achieved
under mild conditions (Figure S15). To identify the FLP
mechanism, we made two control experiments. First, we
deliberately added pyrrole molecule as a Lewis acid into the
reaction system of nitrobenzene hydrogenation, and found that
the yield of aniline drastically declined from 99.0% to 39.4%
(Figure S16), suggesting that Lewis basic sites are necessary
for the reaction. Second, we added SCN− ion to poison the Ni
cations (Lewis acid sites) and the yield decreased to 11.1%.
Based on these control experiments, we propose that the Ni
cations (Lewis acid sites) and the neighbouring non-coordinated
pyridinic N atoms (Lewis basic sites) constitute a FLP that act as
the active sites for hydrogenations. This FLP mechanism is
proved by DFT calculations. As shown in Figure 3a and Figure
S17, upon the formation of Hδ+ and Hδ- via FLP, the
hydrogenation of unsaturated substrate could proceed facilely
(from a2 to FS in Figure 3a). For example, for the
glycolaldehyde hydrogenation, our DFT calculation revealed that
an energy barrier of only 0.55 eV is required (Figure 3a, Figure
S17 and Table S4), which means that the hydrogenation can
proceed even at room temperature.
Figure 3. (a) The proposed H2 activation and glycolaldehyde hydrogenation
energy change profile on Ni-N-C-600 catalyst. (b) The energy level of H2
heterolytic splitting species a1 and localized molecular orbitals (LMOs, see
also Figure S13a). (c) Maps of electron density difference, electron localization
function (ELF), and localized orbital locator (LOL) of TS1 (from left to right,
relevant maps of a1 see Figure S13b).
Not only with high activity for hydrogenation of a variety of
unsaturated substrates, the Ni-N-C SACs also possess
outstanding stability inherited from the strong covalent bonding
between Ni and N atoms. In fact, the high temperature pyrolysis
as well as the subsequent acid-etching processing during the
synthesis of Ni-N-C SACs has already indicated that the Ni-Nx
sites are highly resistant against sintering and acid-leaching. To
study the stability of the Ni-Nx during hydrogenation reactions,
Such a well-defined Ni-N5 structure is able to activate
hydrogen in spite of relatively high oxidation state of central Ni
atoms. The DFT calculations show that H2 molecule can be
activated heterolytically. When dihydrogen molecule approaches
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10.1002/anie.201802231
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Figure 4. (a) Durability of the Ni−N−C-600 in comparison with Ni/AC for the
one-pot conversion of cellulose. Reaction condition: 0.3 g cellulose, 50 mg
catalyst, 30 mg tungstic acid and 30 mL water in 100 mL autoclave, 245 oC,
6.0 MPa H2, 1 h 45 min, 1000 rpm. (b, c) HAADF-STEM images of the reused
Ni-N-C-600 and Ni/AC catalysts. EG (black): ethylene glycol; HA (pink):
Hydroxyacetone; Iso-sor (blue): Iso-sorbitol; Man (magenta): mannitol; Gly
(orange): glycerol; 1,2-PG (navy): 1,2-propylene glycol; 1,2-BDO (olive): 1,2-
Butanediol; others (light violet); Ery (wine): erythritol.
here we apply the catalyst to the one-pot conversion of cellulose
to ethylene glycol, which is a key reaction in the valorization of
6, 22]
renewable lignocellulose.[1a,
To degrade the recalcitrant
cellulose, a binary catalyst composed of tungstic acid and Ni-N-
C-600 was used, in which the tungstic acid promotes C-C
cleavage of the cellulose-derived sugars via a retro-aldol
pathway while the Ni-N-C-600 catalyzes the hydrogenation of
aldehyde intermediates (Figure S18).[1a]
In summary, we have synthesized a high-loading Ni−N−C
single-atom catalyst, which not only is active for hydrogenation
of various unsaturated substrates, but also manifests
extraordinary durability for cellulose conversion under harsh
reaction conditions. The high activity and ultrastability arise from
the significantly distorted (Ni−N4)…N structure, in which the
isolated nickel cation and the adjacent non-coordinated pyridinic
N atoms constitute FLP sites for heterolytic dissociation of
dihydrogen. The atomic resolution of the active sites will help to
design and explore more efficient and robust catalysts towards
biomass and other energy-related reactions.
Based on our earlier studies,[1a, 6, 22] cellulose hydrolysis as
well as the C-C cleavage reaction had a high activation energy
o
and thus required a very harsh reaction condition (245 C, 60
bar H2, stongly acidic hydrothermal conditions). Cogently, most
of the base metal catalysts could not survive under such severe
conditions. As expected, our Ni-N-C-600 catalyst shows
promising activity (Figure 4a, left), with cellulose completely
converted and the main products as EG (36.8%) and acetol
(17.4%). The formation of acetol suggests the occurrence of
isomerization of glucose to fructose, which is usually catalyzed
by Lewis acid or basic sites (Figure S19), again proving the FLP
feature of the Ni-N-C-600 catalyst. Furthermore, the Ni-N-C-600
catalyst has unprecedented durability under aforementioned
harsh reaction conditions; the total yield of EG and acetol only
has a slight decrease from 54.2% in the first run to the 47.4% in
the 7th run. Further inspection of the reused catalyst reveals that
this slight loss is not caused by the catalyst deactivation; instead,
it results from the decreased feed of the Ni-N-C-600 catalyst
during reuse test. We note that approximately 20% tungsten is
deposited on the Ni-N-C-600 catalyst after seven reuses, which
is not considered during the reuse test.
In remarkable contrast to the excellent stability of the Ni-N-
C SAC, the Ni NPs supported on active carbon (Ni/AC) lost half
of the initial activity in the second run (Figure 4a, right) although
the initial activity of the Ni/AC was much higher (TOF 663.9 h-1
for glycolaldehyde hydrogenation on Ni/AC while 57.3 h-1 for Ni-
N-C). The characterizations of the reused catalysts show that
the Ni-N-C-600 structure remains intact; single atoms of Ni are
still highly dispersed and no aggregation is observed (Figure 4b,
Figure S20a). Morevoer, the XPS, EXAFS and XANES spectra
of the Ni-N-C-600 catalyst keep unchanged before and after the
reaction (Figure S21, S22), indicating that the partially reduced
Ni species can be completely restored to the high oxidation state
and stay bonded to N atoms. In contrast, significant sintering of
Ni NPs ocurs as a result of leaching and redeposition of active
Ni species (Figure 4c, Figure S20b), which is ubiquitous when
nickel metal catalyst is used at acidic or hydrothermal reaction
conditions.
Acknowledgements
The authors are grateful to the National Natural Science
Foundation of China (21690080, 21690084, 21673228,
21721004, 21590792, 91645203,11435012) and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDB17020100), the National Key Projects for Fundamental
Research and Development of China (2016YFA0202801), the
Fundamental Research Funds for the Central Universities
(15CX06037A), and the BL 14W beamline at the Shanghai
Synchrotron Radiation Facility (SSRF). The calculations were
performed using supercomputers at Tsinghua National
Laboratory for Information Science and Technology, and the
Computational Chemistry Laboratory (Xuetang) of the
Department of Chemistry, Tsinghua University.
Keywords: Ni−N−C • Single-atom catalyst • active site • XANES
simulation • biomass conversion
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10.1002/anie.201802231
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
A durable and high loading nickel
single-atom catalyst is developed for
hydrogenation reactions and one-pot
conversion of cellulose to ethylene
glycol under harsh conditions. The
catalyst is featured with a well-defined
active site with a significantly distorted
(Ni−N4) ┅ N structure, in which the
isolated nickel cation and the adjacent
Wengang Liu, Yinjuan Chen, Haifeng Qi,
Leilei Zhang, Wensheng Yan, Xiaoyan
Liu, Xiaofeng Yang, Shu Miao, Wentao
Wang, Chenguang Liu, Aiqin Wang,*
Jun Li,* Tao Zhang*
Page No. – Page No.
A Durable Nickel Single-Atom Catalyst
for Hydrogenation Reactions and
Cellulose Valorization under Harsh
Conditions
non-coordinated pyridinic
N
atom
constitute a Frustrated Lewis Pair for
heterolytic dissociation of dihydrogen.
This article is protected by copyright. All rights reserved.