Atomically Dispersed Pt-N3C1Sites Enabling Efficient and Selective Electrocatalytic C-C Bond Cleavage in Lignin Models under Ambient Conditions

Article abstract of DOI:10.1021/jacs.1c02328

Selective cleavage of C-C linkages is the key and a challenge for lignin degradation to harvest value-added aromatic compounds. To this end, electrocatalytic oxidation presents a promising technique by virtue of mild reaction conditions and strong sustainability. However, the existing electrocatalysts (traditional bulk metal and metal oxides) for C-C bond oxidative cleavage suffer from poor selectivity and low product yields. We show for the first time that atomically dispersed Pt-N₃C₁sites planted on nitrogen-doped carbon nanotubes (Pt₁/N-CNTs), constructed via a stepwise polymerization-carbonization-electrostatic adsorption strategy, are highly active and selective toward C_α-C_βbond cleavage in β-O-4 model compounds under ambient conditions. Pt₁/N-CNTs exhibits 99% substrate conversion with 81% yield of benzaldehyde, which is exceptional and unprecedented compared with previously reported electrocatalysts. Moreover, Pt₁/N-CNTs using only 0.41 wt % Pt achieved a much higher benzaldehyde yield than those of the state-of-the-art bulk Pt electrode (100 wt % Pt) and commercial Pt/C catalyst (20 wt % Pt). Systematic experimental investigation together with density functional theory (DFT) calculation suggests that the superior performance of Pt₁/N-CNTs arises from the atomically dispersed Pt-N₃C₁sites facilitating the formation of a key C_βradical intermediate, further inducing a radical/radical cross-coupling path to break the C_α-C_βbond. This work opens up opportunities in lignin valorization via a green and sustainable electrochemical route with ultralow noble metal usage.

Full text of DOI:10.1021/jacs.1c02328

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Article
Atomically Dispersed Pt−N3C1 Sites Enabling Efficient and Selective
Electrocatalytic C−C Bond Cleavage in Lignin Models under
Ambient Conditions
Tingting Cui,¶ Lina Ma,¶ Shibin Wang,¶ Chenliang Ye, Xiao Liang, Zedong Zhang, Ge Meng,
Lirong Zheng, Han-Shi Hu, Jiangwei Zhang,* Haohong Duan,* Dingsheng Wang,* and Yadong Li
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ABSTRACT: Selective cleavage of C−C linkages is the key and a
challenge for lignin degradation to harvest value-added aromatic
compounds. To this end, electrocatalytic oxidation presents a
promising technique by virtue of mild reaction conditions and
strong sustainability. However, the existing electrocatalysts (tradi-
tional bulk metal and metal oxides) for C−C bond oxidative
cleavage suffer from poor selectivity and low product yields. We
show for the first time that atomically dispersed Pt−N3C1 sites
planted on nitrogen-doped carbon nanotubes (Pt1/N-CNTs),
constructed via a stepwise polymerization−carbonization−electro-
static adsorption strategy, are highly active and selective toward
Cα−Cβ bond cleavage in β-O-4 model compounds under ambient
conditions. Pt1/N-CNTs exhibits 99% substrate conversion with
81% yield of benzaldehyde, which is exceptional and unprecedented compared with previously reported electrocatalysts. Moreover,
Pt1/N-CNTs using only 0.41 wt % Pt achieved a much higher benzaldehyde yield than those of the state-of-the-art bulk Pt electrode
(100 wt % Pt) and commercial Pt/C catalyst (20 wt % Pt). Systematic experimental investigation together with density functional
theory (DFT) calculation suggests that the superior performance of Pt1/N-CNTs arises from the atomically dispersed Pt−N3C1 sites
facilitating the formation of a key Cβ radical intermediate, further inducing a radical/radical cross-coupling path to break the Cα−Cβ
bond. This work opens up opportunities in lignin valorization via a green and sustainable electrochemical route with ultralow noble
metal usage.
century.6,7 A series of homogeneous and heterogeneous
catalytic systems have been reported with promising
1. INTRODUCTION
Lignin is an abundant, low-cost, and underutilized renewable
biomass. Its valorization to produce fuels and small molecular
conversion and C−C bond cleavage selectivity for lignin
model compounds under thermal conditions.8−13 However,
aromatic compounds is a promising strategy to diminish
reliance on fossil fuel resources.1 However, this is severely
these conventional thermal catalytic processes have encoun-
tered the drawbacks of harsh reaction conditions like elevated
temperature and pressure, requirements for costly catalysts, as
well as sometimes a long reaction time, which may hinder
large-scale applications.14 Therefore, it is highly appealing to
develop efficient alternative strategies for the selective oxidative
cleavage of the C−C bond within lignin under mild conditions.
Electrocatalytic oxidation is a potentially promising
technique for lignin depolymerization.15−18 It features eminent
hindered by its recalcitrant polymeric structures consisting of
methoxylated phenylpropane subunits connected through
various C−O and C−C linkages.2 Particularly, C−C bonds
generally have higher dissociation energy than those of C−O
bonds in lignin, rendering selective cleavage of C−C bonds the
key and a challenge for lignin degradation. In this regard,
several strategies including hydrolysis, pyrolysis, reduction, and
oxidation have been developed.3,4 Among these, selective
catalytic oxidation has attracted most of the attention since
oxidation could cleave the C−C linkage while preserving the
aromatic ring structure, transforming lignin into highly
functionalized monomeric aromatic compounds, such as
phenolic aldehydes, ketones, acids, and acid derivative
products, which can be used directly as fine chemicals or as
platform chemicals.5 The oxidative depolymerization of lignin
has been under investigation since the first half of the last
Received: March 1, 2021
Published: June 17, 2021
https://doi.org/10.1021/jacs.1c02328
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Figure 1. Electrocatalytic oxidation cleavage of the Cα−Cβ bond in lignin β-O-4 model compounds. (a)−(c) Previously reported electrocatalytic
systems, including bulk Ni, Pt, and carbon electrode combined with ABTS mediator. (d) We show in this work the highly efficient and selective
Cα−Cβ bond oxidative cleavage using the Pt1/N-CNTs electrocatalyst under ambient conditions.
Figure 2. Synthetic strategy and characterization of the Pt1/N-CNTs catalyst. (a) Schematic illustration of the preparation of Pt1/N-CNTs. (b)
TEM image of Pt1/N-CNTs. (c) HAADF-STEM image of Pt1/N-CNTs. (d) Representative AC HAADF-STEM image of Pt1/N-CNTs. (e) EDX
elemental mapping analysis of Pt1/N-CNTs.
advantages over conventional thermocatalytic processes, such
as mild reaction conditions, enhanced sustainability by
utilization of renewable electricity as a power source, and the
simultaneous generation of valuable H2 at the cathode as a
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supererogatory product.19−21 Nevertheless, research on elec-
trocatalytic lignin oxidation is still at the infancy stage,
especially for selective C−C bond cleavage.22,23 Pardini and
co-workers first reported electrocatalytic oxidation of a lignin
β-O-4 dimeric model compound targeting Cα−Cβ bond
cleavage using bulk Ni electrode at 150−160 °C, but only
20.9% yield of the Cα−Cβ bond cleaved products (aromatic
aldehyde and carboxylic acid) was obtained (Figure 1a).24
Later, they further found that when utilizing Pt foil as the
anode, Cα−Cβ bond oxidative cleavage could proceed at
ambient temperature while delivering an improved yield of
aromatic aldehyde (Figure 1b).25 Alternatively, Rochefort and
co-workers described an indirect electrooxidation methodology
by using 2,2′-azinobis(3-ethylbenzthiazoline-6-sulfonate)
(ABTS) as a redox mediator to facilitate Cα−Cβ bond
cleavage. A slightly higher yield of aldehyde product (32.5%)
% Pt). The reaction mechanism was revealed by radical
trapping, isotope labeling, as well as DFT calculations. The
strategy of constructing single-site active centers with unique
configuration represents a significant advance in boosting Cα−
Cβ bond cleavage while reducing the noble metal usage.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Structural Characterizations of the
Pt1/N-CNTs Catalyst. The Pt1/N-CNTs catalyst was
fabricated via a facile three-step strategy as illustrated in
Figure 2a. First, the carbon-based conjugated polymer
precursors (PPy-co-PANI) were constructed by copolymeriza-
tion of pyrrole and aniline using MnO2 nanowires (Figure S1)
as the oxidant and reactive template66,67 (Step 1). Notably, the
redox potential of MnO2 (1.224 V vs SHE for MnO2/Mn2+)68
is higher than the redox polymerization potential of pyrrole
and aniline (<0.7 V vs SHE).69 Thus, MnO2 could effectively
initiate the polymerization of monomers. Furthermore, MnO2
nanowires also acted as the template for the growth of PPy-co-
PANI since the polymerization occurred at the MnO2/
monomers interface. During this process, MnO2 was consumed
via the reaction of MnO2 + 4H+ + 2e → Mn2+ + 2H2O,70
consequently leading to the formation of hollow-structured
PPy-co-PANI. Then the resultant PPy-co-PANI was converted
into N-CNTs via carbonization at 920 °C under N2
atmosphere (Step 2). The transmission electron microscopy
(TEM) images in Figure S2 clearly show that N-CNTs possess
a well-defined hollow cavity with an outer diameter of 40−70
nm and length of 0.2−1 μm. Note that abundant micropores
with sizes of 0.5 and 1.2 nm were generated across the CNT
walls during carbonization (Figure S3), benefiting from the
robust framework of PPy-co-PANI. A high N-doping level of
7.8 wt % was achieved in N-CNTs as revealed by X-ray
photoelectron spectroscopy (XPS) characterization. In addi-
tion, N-CNTs exhibit a much higher D/G intensity ratio (ID/
IG) of 3.5 than that of commercial CNTs (0.15) in the Raman
spectra (Figure S4), indicating the presence of numerous
defects on the walls, which may be caused by the microporous
structure and abundant N-doping.71−73 The content of
residuary Mn in N-CNTs is 0.0033 wt % as determined by
inductively coupled plasma-optical emission spectrometry
(ICP-OES). Afterward, Pt4+ species were uniformly deposited
on N-CNTs via electrostatic adsorption, and the composites
were reduced at 160 °C in 5 vol % H2/Ar atmosphere for 0.5 h,
through which the Pt1/N-CNTs catalyst was produced (Step
3) (for more details, see the Supporting Information).
TEM and HAADF-STEM images reveal that there are no Pt
NPs or small clusters throughout the entire region of the
sample (Figure 2b, c). This is also in good agreement with the
absence of obvious diffraction peaks of Pt in the X-ray
diffraction (XRD) profile (Figure S5). Energy-dispersive X-ray
spectroscopy (EDS) mapping (Figure 2e) shows a homoge-
neous distribution of C, N, and Pt. To identify the dispersion
status of Pt species on N-CNTs at the atomic scale,
subangstrom resolution HAADF-STEM was used. As shown
in Figure 2d, a large number of ultrasmall white dots (∼1−2
Å) are densely planted on N-CNTs, ascribable to single Pt
atoms on account of the atomic number contrast in the image
(Figure S6). By further examining numerous low/high-
magnification HAADF-STEM images obtained from different
regions of the sample, we concluded that the as-synthesized
Pt1/N-CNTs contained only isolated Pt atoms. The Pt loading
amount was determined to be 0.41 wt % by ICP-OES analysis.
was reported in this system (Figure 1c).26 In addition, a series
27−33
of metal oxides (PbO2
and TiO234) and mixed metal
oxides based on precious metals (RuO235−41 and IrO2
)
35,37−41
were also reported for native and technical lignin depolyme-
rization. However, these catalysts suffered from poor
selectivity, resulting in highly complex product mix-
tures.27,28,33,36,39−46 The lack of selectivity and low product
yields represent the critical bottlenecks in the electrocatalytic
lignin oxidation process. In this context, developing a catalyst
that can effectively steer Cα−Cβ bond selective cleavage is
highly desired but challenging.
Recently, single-atom catalysts (SACs), referring to hetero-
geneous catalysts comprised of spatially isolated metal atoms
stabilized by neighboring surface atoms such as carbon,
nitrogen, or oxygen, etc., on appropriate hosts, have stood
out and become a brand-new research frontier in heteroge-
neous catalysis.47,48 They possess well-defined active centers
with 100% atomic utilization in theory. Moreover, SACs
usually exhibit unique electronic structures due to unsaturated
coordinated environments, strong metal support interactions,
and quantum size effects.49 Thus, relative to nanoscale metal
counterparts, SACs often give rise to a different reaction
pathway, leading to superior catalytic performance, as has been
demonstrated in various thermocatalysis,50−57 photocataly-
sis,58−61 and electrocatalysis62−65 conversions. Moreover, given
the highly uniform active sites brought about by similar spatial
and electronic interaction with substrates, SACs have
tremendous potential to enhance the selectivity in lignin
electrochemical oxidative cleavage. However, to the best of our
knowledge, SACs have never been exploited as an electro-
catalyst for lignin oxidation.
Intrigued by the merits of SACs and scanty research on
electrocatalytic C−C bond cleavage, herein we design a single-
atom Pt catalyst anchored on N-doped carbon nanotubes (Pt1/
N-CNTs) for selective Cα−Cβ bond cleavage. The atomic
structure was carefully examined by aberration-corrected high-
angle annular dark-field scanning transmission electron
microscopy (AC HAADF-STEM) and X-ray absorption
spectroscopy (XAS), revealing isolated sites with the Pt−
N3C1 configuration. Pt1/N-CNTs exhibits unprecedented high
Cα−Cβ bond cleavage activity and selectivity in β-O-4 model
compounds under ambient conditions, achieving 99%
conversion and 81% C−C bond cleavage selectivity, which
greatly exceed previously reported electrocatalysts. Moreover,
Pt1/N-CNTs using only 0.41 wt % Pt achieved a much higher
benzaldehyde yield than those of the state-of-the-art bulk Pt
electrode (100 wt % Pt) and commercial Pt/C catalyst (20 wt
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Figure 3. Structural identification of a Pt single atom in the Pt1/N-CNTs catalyst. (a) The Pt L3-edge XANES spectra of Pt1/N-CNTs (red), along
with Pt foil (black) and PtO2 (cyan) for comparison. (b) FT of the k2-weighted EXAFS spectra of Pt1/N-CNTs (red), along with reference samples
Pt foil (black) and PtO2 (cyan). WT for the k2-weighted EXAFS signals of (c) Pt foil, (d) PtO2, and (e) Pt1/N-CNTs. (f) FT of the k2-weighted
EXAFS spectrum and fit in R space of Pt1/N-CNTs with the magnitude (red) and real component (blue). (g) Comparison between the
experimental Pt L3-edge XANES spectrum of Pt1/N-CNTs and the theoretical spectrum calculated for Pt−N3C1. The FT are corrected for the
phase shift.
By contrast, isolated Pt atoms as well as well-crystalline
particles of 1−2.5 nm were formed when commercial CNTs
were used as the support with a similar Pt loading amount
(Figure S7 and Table S1), and the resulting sample was
denoted as Pt1+NPs/CNTs. This result highlights the distinct
superiority of N-CNTs in anchoring single atoms by high-level
N-doping and abundant micropores on the walls. In addition,
another control sample, consisting of 0.44 wt % Pt NPs on N-
CNTs (Pt NPs/N-CNTs), was also prepared by mixing Pt
NPs of 2.5−5 nm with N-CNTs for investigating the effect of
the Pt states on the catalytic performance (Figure S8).
The electronic structures of C and N species in Pt1/N-
CNTs were detected by XPS and synchrotron-based soft X-ray
absorption near-edge structure (XANES) spectroscopy. As
shown in Figure S9, the N 1s high-resolution XPS spectrum
can be well fitted with three peaks at the binding energies of
398.5, 401.0, and 403.7 eV, assigned to pyridinic-N, graphitic-
N, and oxidized-N, respectively (Table S2). In good
accordance with the XPS result, three N 1s−π* transitions
peaks (N1, N2, N3) corresponding to pyridine-like, graphite-
like, and NO structures were found in the N K-edge XANES
spectrum (Figure S10).74 Notably, the relative intensity of
peak N1 in Pt1/N-CNTs is obviously weaker than that of N-
CNTs, suggesting that Pt single atoms are trapped at pyridine-
like N sites, highlighting the anchoring effect of pyridinic-N for
single atoms. The C K-edge spectrum of Pt1/N-CNTs showed
three obvious peaks at 286.0 eV (peak A), 289.1 eV (peak B),
and 292.7 eV (peak C), which derive from the dipole transition
of the C 1s core electron into the antibonding orbitals of π*
CC, mixed states of π* and σ* C−N/O, and σ* C−C,75
respectively (Figure S11). No obvious change can be observed
in the C K-edge spectra during deposition of Pt SAs on N-
CNTs.
Pt1/N-CNTs as well as those of Pt NPs/N-CNTs and Pt1+NPs/
CNTs for comparison. Pt1/N-CNTs exhibits a single doublet
(4f7/2 and 4f5/2) at 72.9 and 76.2 eV, implying that Pt exhibits
the valence state close to +2. In contrast, two sets of peaks for
Pt1+NPs/CNTs can be observed with 4f7/2 at 71.5 and 73.1 eV,
which can be assigned to Pt0 and Ptδ+ (2 < δ < 4), respectively,
due to the coexistence of NPs and SAs.76 Pt NPs/N-CNTs
consists almost entirely of Pt0 species with a small contribution
from Pt2+ due to the surface oxidation of NPs. In order to
decode the electronic structure and coordination configuration
of Pt, XANES and extended X-ray absorption fine structure
(EXAFS) spectroscopy measurements were performed at the
Pt L3-edge. In the XANES spectra, the intensity of the white
line peak of Pt species in Pt1/N-CNTs is much lower than that
of PtO2 but higher than that of metallic Pt foil (Figure 3a),
indicating that the Pt species in Pt1/N-CNTs exhibit a positive
valence, which is well consistent with the XPS result. Figure 3b
shows the phase-corrected Fourier transform (FT) curves at
the R space of the Pt L3-edge EXAFS spectra for Pt1/N-CNTs
in comparison with the references of Pt foil and PtO2. Notably,
k2-weighted FT was adopted to achieve an intuitive
comparison between different samples. Pt1/N-CNTs has a
dominant peak at 1.95 Å corresponding to the Pt−N/C
scattering path with the absence of a peak at 2.78 Å from Pt−
Pt contribution, categorically confirming the isolated state of
Pt atoms, consistent with the HAADF-STEM observation.
EXAFS wavelet transform (WT) analysis, which can provide
not only radial distance resolution but also k-space resolution,
was employed for further investigation of the coordination
conditions of Pt centers in Pt1/N-CNTs (Figure 3c−e). A WT
intensity maximum near 5.7 Å−1 arising from the light atom
coordinator is well resolved at 1.95 Å for Pt1/N-CNTs,
whereas an intensity maximum at 8.5 Å−1 associated with the
Pt−Pt coordination was not detected. This result not only
further confirms that Pt exists as a mononuclear center in the
The chemical state of Pt SAs in Pt1/N-CNTs was examined
by XPS. In Figure S12, we present the Pt 4f XPS spectra of
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a
Table 1. Catalytic Conversion of 2-Phenoxy-1-phenylethanol over Different Catalysts
c
product yield (%)
b
c
entry
catalyst
Pt loading amount (wt %)
conversion 1a (%)
2a
3a
4a
1
2
3
4
N-CNTs
Pt NPs/N-CNTs
Pt1+NP/CNTs
Pt1/N-CNTs
Pt/C
0
0.44
0.38
0.41
20
<10
62
77
99
21
ND
<5
ND
<5
ND
ND
ND
ND
25
36
81
19
55
55
ND
12
21
56
11
35
<5
d
5
e
6
Pt/C
Pt electrode
20
100
100
88
7
a
Reaction conditions: 1a (0.1 mmol), nBu4NOH (0.2 mmol), TBHP (0.5 mmol), MeCN (1.0 mL), RT, 5 h, under air, electrolysis under current
of 20 mA and potential of 5.0 V vs Ag/AgCl, unless otherwise noted. Catalyst loading on GCE of 0.5 mg/cm2. Glassy carbon (GC) as the counter
b
c
electrode. Ag/AgCl as the reference electrode. Pt loading amounts in the catalysts were determined by ICP-OES. Conversions of substrates were
d
determined by 1H NMR using CH2Br2 as an internal standard. Yields of products were the isolated yields. The mass loading of Pt on the GCE was
e
kept the same as entry 4. The mass loading of the total catalyst on the GCE was kept the same as entry 4.
absence of metallic Pt species in Pt1/N-CNTs but also
substantiates the assignment of the major peak at ∼1.95 Å to
Pt−C/N bonding. We also carried out least-squares EXAFS
fitting analysis to extract quantitative structural results for the
Pt moiety in Pt1/N-CNTs. The best-fitting analysis reveals that
each Pt atom is coordinated with one C atom (CN = 0.9) and
three N atoms (CN = 3.1) at distances of 1.92 and 1.99 Å,
respectively (Figure 3f, Figure S14, and Table S3). Considering
that the XANES spectrum possesses higher sensitivity to the
local coordination environment of the absorbing species,
XANES simulations were further carried out. Different
structure models with various Pt−N/C coordinating con-
ditions were established by DFT calculations (Figure S15), and
the corresponding XANES spectra were calculated. As shown
in Figure 3g, the simulated spectrum based on the proposed
Pt−N3C1 model can well reproduce the experimental XANES
features of Pt1/N-CNTs. Moreover, the average Pt−C and Pt−
N bond lengths predicted by DFT are 1.92 and 1.97 Å,
respectively, which are close to the structural parameters given
by EXAFS fitting (Table S3). However, the calculated spectra
for the structures of Pt−N4, Pt−N1C3, Pt−N2C2-1, and Pt−
N2C2-2 models, which exhibit either more positive or more
negative absorption edges than the experimental result, are
distinctly different (Figure S16). Taken together, the
combination of EXAFS and XANES results clearly revealed
the formation of isolated Pt−N3C1 moieties in Pt1/N-CNTs.
Additionally, spin-polarized calculations further reveal that the
Pt atom in the Pt−N3C1 model exhibits a very low magnetic
momentum value of 0.02 μB, and the Bader charge value of the
Pt atom was calculated to be +0.57 |e|, suggesting that the Pt
valence state is close to +2 with an electronic configuration of
6s05d8, which is well consistent with the above experimental
evidence.
along with H2 generation at the cathode under ambient
conditions. Results with different catalysts for catalytic
oxidation of 1a are summarized in Table 1. For the support
N-CNTs only, there was hardly any activity toward the
conversion of the substrate (less than 10%, Table 1, entry 1),
indicating that the residuary trace Mn is inactive for this
reaction. When loading 0.44 wt % Pt NPs on N-CNTs (Pt
NPs/N-CNTs), a medium activity with 62% conversion of 1a
(Table 1, entry 2) was detected. However, the yield of the
desired product of benzaldehyde (3a) from Cα−Cβ bond
oxidative cleavage is very low. When Pt1+NPs/CNTs was used,
which contains both Pt SAs and NPs, with a total Pt loading of
0.38 wt %, the conversion was promoted to 77% with a higher
yield of 36% for 3a (Table 1, entry 3). This result suggests the
prominent advantage of Pt SAs over NPs in oxidative cleavage
of 1a. Inspired by this, we further examined the performance of
Pt1/N-CNTs, which contains only SA Pt with 0.41 wt %
loading under the same conditions. To our delight, Pt1/N-
CNTs catalyst confers remarkably higher activity than Pt NPs/
N-CNTs and Pt1+NPs/CNTs, achieving 99% substrate con-
version with 81% yield and 5.6% faradaic efficiency for 3a,
further corroborating the superior catalytic performance of the
Pt SAs (Table 1, entry 4). In addition, we also investigated the
catalytic performance of the benchmark commercial Pt/C
catalyst decorated with 20 wt % Pt NPs. As shown in Table 1,
entry 5, Pt/C only delivered a conversion of 21% with 19%
yield of 3a when the Pt loading amount on the glassy carbon
electrode (GCE) was kept the same as for Pt1/N-CNTs. Even
in the case of the same catalyst loading as Pt1/N-CNTs on
GCE, Pt/C still exhibited a much lower yield of 55% for 3a,
although its conversion was good enough (Table 1, entry 6). It
is worth noting that the Pt1/N-CNTs catalyst even has a
significant advantage over the bulk Pt electrode with 100% Pt
content, which delivered a much lower yield of 55% for 3a
(Table 1, entry 7). Compared with previously reported
electrocatalytic systems (Figure 1), an overwhelming advant-
age for Pt1/N-CNTs can be seen. We further compared the
catalytic performance of Pt1/N-CNTs with previously reported
typical thermocatalytic and photocatalytic systems (Table S4).
Apparently, the present Pt1/N-CNTs electrocatalyst surpasses
most of the previously reported catalytic systems in terms of
2.2. Electrocatalytic Oxidation of 2-Phenoxy-1-phe-
nylethanol. To study the C−C bond cleavage regularity of
the as-prepared Pt−N3C1 catalyst, a representative β-O-4
dimeric model compound, 2-phenoxy-1-phenylethanol (1a),
containing Cα−OH, Cα−Cβ, and Cβ−O bonds, was used as the
substrate in this study. Bulk electrolysis of 1a at the anode was
performed using tert-butyl hydroperoxide77 (TBHP, 70% aq.
soln) as the oxidant under a constant current of 20 mA for 5 h
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Figure 4. Reaction pathway based on experiments and DFT calculations. (a) Proposed mechanism of Pt1/N-CNTs-catalyzed conversion of 1a. (b)
DFT-calculated potential energy surface for 1a conversion on the Pt1/N-CNTs surface.
Cα−Cβ bond cleavage performance (i.e., the yield of 3a, 5a,
and 6a). Furthermore, from the point of energy utilization, the
electrocatalytic process is more sustainable than the traditional
one by virtue of direct integration with renewable energy.
Notably, the high-temperature electrocatalytic process deserves
further exploration to achieve energy integration with lignin
pretreatment processes as well. On the other hand, considering
the challenge associated with the separation of lignin cleavage
products, the oxygen-containing aromatics obtained here may
be further upgraded via hydrodeoxygenation to aromatic
hydrocarbons and used directly as biofuels. Alternatively,
convergent transformation of lignin derivatives into a relatively
single commodity via the funneling approach is also viable to
fundamentally solve the separation issue, as we reported in
another work recently.78
To investigate the generality of the Pt1/N-CNTs catalyst, a
variety of lignin model compounds with different substituents
were evaluated (Table S5). It is known that β-O-4 dimeric
model compounds with methoxy groups are structurally closer
to the true nature lignin, which may affect the activation of the
Cα−Cβ bond. The catalytic oxidative cleavage of the β-O-4
compound with methoxy groups in both benzene rings (red
and blue colored part) over Pt1/N-CNTs was investigated
(Table S5, entry 1). The substrate was nearly completely
converted, producing the corresponding aldehyde in a
moderate yield. It has been reported that the Cγ−OH will
make the Cα−Cβ bond more resistant to oxidation.79 Hence,
lignin models with Cγ−OH were also investigated. Moderate
to high yields of aromatic aldehydes were obtained under the
present mild conditions (Table S5, entries 2, 3). In addition,
electrocatalytic oxidation of more challenging β-1 models with
different substituents such as methoxy, halogen, and hydroxyl
was also examined (Table S5, entries 4−7). The 1-phenyl-
ethanol derivative with a halogen substituent at the para-
position of the phenyl group (Table S5, entry 5) generated the
corresponding aldehyde product in 78% yield. The reaction
efficiency was affected by steric hindrance when a Cl group was
located at the meta-position (Table S5, entry 6), which
delivered a decreased aldehyde yield of 59% under standard
reaction conditions. Notably, hydroxyl-substituted 1-phenyl-
ethanol derivative (Table S5, entry 7) can be completely
converted while affording 36% yield of aldehyde product. The
relatively low yield is because the hydroxyl group easily causes
a side reaction such as being oxidized to benzoquinone.
Nevertheless, these data demonstrate the wide applications of
Pt1/N-CNTs in electrocatalytic oxidative cleavage of the Cα−
Cβ bond in lignin model compounds. Moreover, the high
recycling stability and reutilization of the Pt1/N-CNTs catalyst
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was shown by carrying out the catalytic cleavage of lignin
model compound 1a five times without loss in catalytic activity
(Figure S17). The content of Pt in the recovered Pt1/N-CNTs
was determined to be 0.39 wt %, indicating no Pt leaching.
EDX mapping and AC HAADF-STEM characterizations show
that Pt atoms are still atomically dispersed with no aggregation
observed (Figure S18), further corroborating the robust nature
of the Pt1/N-CNTs catalyst.
CNTs anode surface (2). It is noteworthy that the abstraction
of a hydrogen atom to produce the radical intermediate to
initiate Cα−Cβ bond cleavage was also reported on the Ni(III)
anode surface.24 Lastly, tBuO·/tBuOO· reacts with the unstable
Cβ radical B· to generate intermediate C via a radical/radical
cross-coupling reaction (3). The reaction is followed by
electron transferring in C,84 inducing the cleavage of the Cα−
Cβ bond and the generation of aromatic aldehyde, phenol, and
CO2.85 H2 is simultaneously produced at the cathode via H2O
reduction.
2.3. Mechanism Study of Electrocatalytic Cα−Cβ Bond
Cleavage. We were intrigued about the mechanism of
electrocatalytic Cα−Cβ bond oxidative cleavage, as it has rarely
been addressed in the literature.24,25 A series of control
experiments were conducted over the Pt1/N-CNTs catalytic
system (Scheme S1). The ketone 2a was hardly converted with
very low yields of products under the standard reaction
conditions (Scheme S1, eq 1), revealing that 2a is not the
intermediate for 1a conversion. This phenomenon is in direct
contrast to those found in conventional thermocatalytic
systems,80−83 where Cα−OH was oxidized to CαO prior
to the Cα−Cβ bond cleavage. Thus, a distinct reaction
mechanism may take place in our electrocatalytic system.
Phenyl formate as the other product of Cα−Cβ bond cleavage
was not detected in the reaction.84 Further studies with phenyl
formate as the substrate (Scheme S1, eq 2) indicate that it was
unstable under reaction conditions, with considerable decom-
position to phenol,85 which illustrated the origin of phenol
produced during this electrocatalytic reaction. Notably, the
reaction was greatly inhibited when a radical scavenger, 2, 2, 6,
6-tetramethyl-1-piperidinyloxy (TEMPO)/butylated hydroxy-
toluene (BHT), was added into the reaction system (Scheme
S1, eq 3), indicating the reaction proceeded via radical
intermediates. An electron paramagnetic resonance (EPR)
spin-trap technique was further employed to identify the
generated radical species with 5,5-dimethyl-1-pyrroline N-
oxide (DMPO) as the trapping agent. As shown in Figure S19,
six EPR peaks can be clearly observed over Pt1/N-CNTs under
the electrocatalytic conditions, indicating the formation of a
carbon-centered radical intermediate during the electro-
catalytic process.86
To further investigate the detailed reaction mechanism of
tBuOOH/1a activation and the radical cross-coupling reaction
mentioned above, DFT calculations were also performed. The
results are shown in Figure 4b. Two possible reaction pathways
t
were proposed in this work. Within the BuOO· pathway,
tBuOOH dehydrogenates on the Pt1/N-CNTs surface, and the
Gibbs reaction energy and energy barrier were calculated to be
1.24 and 1.53 eV, respectively. Within the chemisorption
structure of A2*+H*, the hydrogen atom adsorbs on the C site
of the Pt−N3C1 center, and the adsorption energy was
calculated to be 0.33 eV lower than that on the Pt site. For the
H* configuration (A2·+H*), the catalyst could be regenerated
via the hydrogen evolution reaction, with the Gibbs reaction
energy for this step calculated to be exothermic of −0.36 eV.
After the formation of A2· radical, the reaction is followed by
1a dehydrogenation. The corresponding reaction energy and
energy barrier were calculated to be 1.28 and 2.05 eV,
respectively. The significant energy barrier and reaction energy
reveal that 1a direct dehydrogenation to form B· radical is the
rate-determining step of this reaction pathway, and the
apparent activation energy (Ga) for this reaction pathway
was calculated to be 3.56 eV. After the formation of A2· and B·
radicals, the reaction is followed by the radical coupling step to
form the C2 intermediate, and this step was calculated to be
highly exothermic of −2.32 eV. The C2 intermediate further
forms the benzaldehyde, phenyl formate, and tBuOH products
via one-step dissociation. The reaction was calculated to be
highly exothermic of −3.08 eV, with a moderate energy barrier
of 1.21 eV.
For an in-depth understanding of the catalytic mechanism of
the Cα−Cβ bond cleavage, the experiments with two
deuterated substrates 1a′−D and 1a″−D were performed
(Scheme S2). The substrate 1a′−D with Cα−D can be
effectively converted as that of 1a, meaning that Cα−H
oxidation is not the rate-determining step. Moreover, the
presence of Cα−D in deuterated aldehyde (Figure S22)
indicates that the benzylic hydrogen is not scrambled or
exchanged during the electrocatalytic C−C bond cleavage
reaction. In contrast, the conversion of 1a″−D with Cβ−D
decreased dramatically compared with that of 1a, indicating
that Cβ−H abstraction is the rate-determining step for Cα−Cβ
bond cleavage. The cyclic voltammetry (CV) curves of Pt1/N-
CNTs show that TBHP exhibited a lower onset oxidation
potential of 0.20 V vs Ag/AgCl than that of 1a (0.80 V vs Ag/
AgCl) (Figure S20); thus, TBHP decomposed first prior to 1a
oxidation under the electrocatalytic conditions.
Additionally, we investigated the possible reaction pathway
t
t
of BuOOH dehydroxylation to BuO· radical (A1·). The
reaction energy and energy barrier were calculated to be −0.12
and 0.63 eV, respectively. This shows that for the first step
tBuOOH activation, it is much more favorable to form A1·
radical on the Pt1/N-CNTs catalyst. After the formation and
desorption of A1· radical, the reaction is then followed by 1a
dehydrogenation. In this step, the H atom from 1a transfers to
the OH* on the surface to form H2O, and the B* intermediate
adsorbs on the Pt1 site with the C−Pt bond length of 2.20 Å.
The detailed structures for each elementary step are shown in
Figure S21. The reaction energy and energy barrier for this
step were calculated to be −0.51 and 0.44 eV, respectively.
After the formation of A1· and B· radicals, the reaction is
followed by the radical coupling step to form the C1
intermediate. This step was also calculated to be highly
exothermic of −2.85 eV. The C1 intermediate further forms
the benzaldehyde, phenyl formate, and tBuH products via one-
step dissociation. The reaction was calculated to be exothermic
of −1.01 eV, with a moderate energy barrier of 0.56 eV. The
apparent activation energy (Ga′) for this reaction pathway was
calculated to be 1.57 eV. By comparing the two reaction
On the basis of the above results and literature reports,77,87
the possible mechanism of electrocatalytic transformation of
the lignin model molecule over Pt1/N-CNTs is proposed in
Figure 4a. First, the tert-butoxyl radical (tBuO·) A1· and tert-
butyl peroxyl radical (tBuOO·) A2· are generated from
TBHP77,88 (1). Subsequently, Cβ radical B· is produced via
Cβ−H abstraction of lignin model compound 1a on the Pt1/N-
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t
pathways, we are able to conclude that the BuO· pathway is
Dingsheng Wang − Department of Chemistry, Tsinghua
University, Beijing 100084, China; orcid.org/0000-0003-
0074-7633; Email: wangdingsheng@mail.tsinghua.edu.cn
t
much more favorable than the BuOO· pathway.
Remarkably, Cα ketone13 or Cα radical intermediates89 were
usually involved in previously reported catalytic systems for 1a
oxidation. Both the two intermediates possess decreased bond
dissociation energy (BDE) for Cβ−O but increased BDE for
the Cα−Cβ bond; thus, only β-O-4 bond cleavage could be
evoked while the Cα−Cβ bond remains intact. In contrast, the
present Pt1/N-CNTs electrocatalyst featured with the
formation of the Cβ radical intermediate, which is capable of
Authors
Tingting Cui − Department of Chemistry, Tsinghua
University, Beijing 100084, China
Lina Ma − State Key Laboratory of Chemical Resource
Engineering, Beijing University of Chemical Technology,
Beijing 100029, China
Shibin Wang − Institute of Industrial Catalysis, State Key
Laboratory Breeding Base of Green-Chemical Synthesis
Technology, College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou 310032, China
Chenliang Ye − Department of Chemistry, Tsinghua
University, Beijing 100084, China; orcid.org/0000-0002-
0111-6748
Xiao Liang − Department of Chemistry, Tsinghua University,
Beijing 100084, China
Zedong Zhang − Department of Chemistry, Tsinghua
University, Beijing 100084, China
t
coupling with BuO· to induce Cα−Cβ bond cleavage, is
significantly distinguished from previously reported catalysts
rendering its high selectivity for Cα−Cβ bond cleavage.
3. CONCLUSIONS
In summary, we present a new type of electrocatalyst for
selective Cα−Cβ bond oxidative cleavage in lignin models,
namely atomically dispersed Pt−N3C1 sites anchored on N-
doped CNTs. The electrocatalytic measurements suggest that
Pt1/N-CNTs possesses unprecedented high Cα−Cβ bond
cleavage activity and selectivity, delivering 81% yield of
benzaldehyde, which greatly exceeds previously reported
electrocatalysts. Moreover, Pt1/N-CNTs also demonstrates
an overwhelming advantage over the state-of-the-art Pt
electrode and Pt/C benchmark catalyst. Experimental mech-
anism investigation combined with DFT calculation suggests
that the reaction proceeds via a key Cβ radical intermediate,
which facilitates the specific Cα−Cβ bond cleavage during the
following radical/radical cross-coupling process. This work
represents a significant advancement in developing high-
performance and cost-effective catalysts for lignin degradation
and valorization via the green and sustainable electrochemical
route.
Ge Meng − Department of Chemistry, Tsinghua University,
Beijing 100084, China
Lirong Zheng − Beijing Synchrotron Radiation Facility,
Institute of High Energy Physics, Chinese Academy of
Sciences, Beijing 100049, China
Han-Shi Hu − Department of Chemistry, Tsinghua University,
Beijing 100084, China; orcid.org/0000-0001-9508-1920
Yadong Li − Department of Chemistry, Tsinghua University,
Beijing 100084, China; orcid.org/0000-0003-1544-1127
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c02328
Author Contributions
¶T.C., L.M., and S.W. contributed equally to this work.
ASSOCIATED CONTENT
■
Notes
sı
* Supporting Information
The authors declare no competing financial interest.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c02328.
ACKNOWLEDGMENTS
■
Experimental procedures for the preparation, character-
ization of materials, and the test of catalytic perform-
ances; details of DFT calculations; additional TEM,
HAAD-STEM, physisorption, ICP-OES, Raman, XRD,
This work was supported by the National Natural Science
Foundation of China (22005173, 21890383, 21871159,
21978147, 21935001), the China Postdoctoral Science
Foundation (2019M660421), the National Key R&D Program
of China (2018YFA0702003, 2020YFA0406101), and the
Science and Technology Key Project of Guangdong Province
of China (2020B010188002). We gratefully acknowledge the
BL14W1 beamline of Shanghai Synchrotron Radiation Facility
(SSRF), 1W1B beamline of Beijing Synchrotron Radiation
Facility (BSRF), and BL11U beamline of the National
Synchrotron Radiation Laboratory (NSRL) for providing
beam time.
1
XPS, XANES, EXAFS, EPR, and CV data; copies of H
and 13C NMR spectra; data of recycling stability,
substrate scope extension, and mechanism experiments
for Pt1/N-CNTs; comparison of the catalytic perform-
ance of Pt1/N-CNTs with previously reported thermo-
catalytic and photocatalytic systems for the oxidative
cleavage of 1a (PDF)
AUTHOR INFORMATION
■
REFERENCES
■
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