Electrocatalytic Upgrading of Lignin-Derived Bio-Oil Based on Surface-Engineered PtNiB Nanostructure

Article abstract of DOI:10.1002/adfm.201807651

The development of robust electrocatalysts for electrocatalytic hydrogenation (ECH) of guaiacol and related lignin model monomers is necessary for the stabilization or upgrading of bio-oil. Additionally, the efficiency of biomass conversion to bio-oil products remains below the minimum requirements for its implementation at scale. Herein, a PtNiB/CMK-3 catalyst with pronounced ECH performance in the conversion of guaiacol and related model lignin monomers to bio-oil under optimally mild conditions, through a modulation strategy that modified the electronic structure of PtNi via boron alloying, is prepared. Notably, the optimized PtNiB/CMK-3 exhibited an inspiring high faradaic efficiency of 86.2%, which is significantly higher (13.7 times) than that of the PtNi/CMK-3 without B-doping (6.3%). Experimental results and theoretical calculations showed that the B-doping optimized the PtNiB alloy surface electron structure, simultaneously promoting substrate and intermediate adsorption and the ECH process. In addition, the uniform dispersion of PtNiB nanoparticles embedded within the mesoporous channels of CMK-3 ensures an enhanced utilization efficiency, leading to improvements in stability and bio-oil product generation. The lab-scale ECH experiment of guaiacol also certified the scale-up potential. This work opens a promising avenue to the rational design of advanced and highly efficient electrocatalysts for biomass upgrading.

Full text of DOI:10.1002/adfm.201807651

FULL PAPER
Electrocatalysis
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Electrocatalytic Upgrading of Lignin-Derived Bio-Oil Based
on Surface-Engineered PtNiB Nanostructure
Yulin Zhou, Yijing Gao, Xing Zhong,* Wenbin Jiang, Yulin Liang, Pengfei Niu,
Meichao Li, Guilin Zhuang, Xiaonian Li, and Jianguo Wang*
technology for energy production.[1] Thus,
The development of robust electrocatalysts for electrocatalytic hydrogena-
tion (ECH) of guaiacol and related lignin model monomers is necessary for
the stabilization or upgrading of bio-oil. Additionally, the efficiency of biomass
conversion to bio-oil products remains below the minimum requirements for
its implementation at scale. Herein, a PtNiB/CMK-3 catalyst with pronounced
ECH performance in the conversion of guaiacol and related model lignin
monomers to bio-oil under optimally mild conditions, through a modulation
strategy that modified the electronic structure of PtNi via boron alloying, is
prepared. Notably, the optimized PtNiB/CMK-3 exhibited an inspiring high
faradaic efficiency of 86.2%, which is significantly higher (13.7 times) than that
of the PtNi/CMK-3 without B-doping (6.3%). Experimental results and theo-
retical calculations showed that the B-doping optimized the PtNiB alloy surface
electron structure, simultaneously promoting substrate and intermediate
adsorption and the ECH process. In addition, the uniform dispersion of PtNiB
nanoparticles embedded within the mesoporous channels of CMK-3 ensures an
enhanced utilization efficiency, leading to improvements in stability and bio-oil
product generation. The lab-scale ECH experiment of guaiacol also certified the
scale-up potential. This work opens a promising avenue to the rational design
of advanced and highly efficient electrocatalysts for biomass upgrading.
lignin valorization in a cost-effective way
has drawn increasing attention from both
the scientific and industrial communi-
[
2]
ties. KA oil (a mixture of cyclohexanol
and cyclohexanone) and its derivatives
are used as high-volume industrial feed-
stock for the manufacture of adipic acid
and as essential industrial precursors in
the pesticide, pharmaceutical, and food
[
3]
industries. However, given the chal-
lenges associated with the low conversion
rates, a significant proportion of undesir-
able N O emissions, and the high energy
2
consumption of the established petro-
chemical routes, identification of alterna-
tive new processes for KA oil production
from a more abundant and versatile bio-
mass feedstock would be a significant
development.
Electrocatalytic hydrogenation (ECH)
of model lignin monomers is a facile,
mild condition route to upgrade bio-oils.
During ECH, the adsorbed hydrogen
(
Hads) is electrochemically generated in
situ on the electrode surface rather than through the dissocia-
1
. Introduction
[
4]
tion of molecular H2. However, phenols are more resistant to
hydrogenation compared to other bio-oil components, such as
aldehydes and ketones, due to the high energy requirements
for CO bond cleavage in carbonyl groups and hydrogenation
The conversion of widely available, underutilized lignin bio-
mass into high value-added chemical products and bio-oils
is regarded as an ideal alternative, environmentally friendly
[
5]
of the aromatic ring. Therefore, the large-scale application
of bio-oil upgrading requires the development of powerful
ECH electrocatalysts. Several groups have reported the ECH
of biomass-based phenolic compounds to cyclohexanone and
Y. Zhou, Dr. Y. Gao, Prof. X. Zhong, W. Jiang, Dr. P. Niu, Prof. M. Li,
Prof. G. Zhuang, Prof. X. Li, Prof. J. 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, P. R. China
E-mail: zhongx@zjut.edu.cn; jgw@zjut.edu.cn
[
6]
[7]
[6,8]
[9]
cyclohexanol using Pt/C, Rh/C, Pd/C,
Raney nickel
Ru/C, and
[
10]
catalysts. Nevertheless, the performance of
ECH remains poor to fulfill specific requirements and the
metal nanoparticles (NPs) are prone to dissolution or aggrega-
tion during ECH, affecting the electrochemical stability. Thus,
there is considerable room for improvement by discovery of
new electrocatalyst systems. The modulation of Pt with suit-
able concomitants at the atomic level may lead to optimization
of its intrinsic electronic structure. Furthermore, the presence
of concomitants can provide versatility with regard to surface-
Dr. Y. Liang
Shanghai Institute of Applied Physics
Chinese Academy of Sciences
Shanghai 201800, P. R. China
Dr. Y. Liang
University of Chinese Academy of Sciences
Beijing 100049, P. R. China
[
11]
structure tuning, yielding unexpected benefits. As promising
catalysts, amorphous metal–boron (B) alloys have a wide range
of applications in petrochemical production, energy conversion,
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201807651.
[
12]
DOI: 10.1002/adfm.201807651
and environmental catalysis. Metal alloying with B may lead
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(
(
Supporting Information). Transmission electron microscopy
TEM) of CMK-3 showed a highly ordered mesoporous struc-
ture with uniform parallel porous channels (Figure 2b,c). The
relatively bright spots (Figure 2d,e) observed correspond to
PtNiB NPs successfully loaded and dispersed on the CMK-3
substrate, showing no obvious agglomeration. The PtNiB NPs
were shown to be approximately 2.98 nm in diameter; the cor-
responding size distribution image is shown in the top right
corner of Figure 2d. Furthermore, the high-angle annular dark-
field scanning TEM image (Figure 2f) revealed well-defined
lattice fringes, with a lattice spacing of 0.225 nm, as expected
for the (111) planes of the PtNiB alloy. Energy-dispersive X-ray
spectroscopy elemental mapping analysis (Figure 2g–j) clearly
revealed a uniform distribution of Pt, Ni, and B within the NPs,
confirming the successful alloying of PtNi and B. Moreover, the
inductively coupled plasma (ICP) confirmed the Pt/Ni mole
ratio to be 0.5, and the molar mass fraction of B to be 20% of
that of Pt in PtNiB/CMK-3, while no B was detected in PtNi/
CMK-3 or Pt/CMK-3 (Table S1, Supporting Information). TEM
images of PtNi/CMK-3 and NiB/CMK-3 were taken for compar-
ison (Figures S2 and S3, Supporting Information). NiB/CMK-3
had thick NiB flakes located on the external surface of CMK-3,
similar to the morphology of NiB particles reported in previous
Figure 1. Schematic of ECH of guaiacol into KA oil integrated with O2
production, equipped with PtNiB/CMK-3 and IrO /C as the cathode and
2
anode, respectively.
to a boost in electrochemical performance. However, to date, no
effective approach has been proposed to control the interfacial
electronic effects on Pt-based NPs modulated by B-doping to
promote ECH performance.
Herein, we developed a facile, one-step reduction synthetic
strategy to prepare a PtNiB/CMK-3 hybrid electrocatalyst by suc-
cessfully tailoring the electronic properties of PtNi NPs by via
B-doping. The resulting PtNiB/CMK-3 electrocatalysts exhibited
remarkable electrocatalytic performance for ECH of guaiacol
and related model lignin monomers into KA oil at the cathode,
with high conversion, high faradaic efficiency (FE), and robust
[12,17]
studies.
Therefore, embedding of PtNiB NPs on the carbon
walls or mesoporous channels of CMK-3 will potentially lead to
good stability due to the inherent constraints in particle aggrega-
tion and growth, which are vital for electrocatalysis applications.
X-ray powder diffraction (XRD) of PtNiB/CMK-3, PtNi/
CMK-3, Pt/CMK-3, NiB/CMK-3, and CMK-3 (Figure S4, Sup-
porting Information) showed the retention of the ordered chan-
nels of CMK-3 in the prepared catalysts. The wide-angle broad
peak at 41° and 48° corresponded to the (111) and (200) plane of
PtNi in PtNi/CMK-3 whose Pt and Ni precursors are reduced by
durability under mild conditions. Additionally, O production
2
[18]
was obtained at the anode using the IrO /C simultaneously
dimethylformamide, while PtNiB/CMK-3 exhibited a broader
peak at 38°–48°; it should be noted that all diffraction peaks were
slightly shifted due to B alloying. In addition, the weak reflec-
tions and low intensity also demonstrated that the PtNiB NPs
were well dispersed over the CMK-3 surface, in agreement with
2
13]
(
Figure 1).[
Notably, experimental and theoretical studies
attributed the superior ECH efficiency of PtNiB/CMK-3 to the
alteration of the nanocatalyst electronic structure by B-doping,
thereby modulating the adsorption of guaiacol and related inter-
mediates on the PtNiB surface. The mesoporous CMK-3 sub-
strate also provided low-resistance diffusion channels, which
facilitate electrolyte diffusion and ion transport.[ Moreover, the
encapsulation of PtNiB NPs into CMK-3 also proved a feasible
strategy to prevent NPs agglomeration and corrosion during the
ECH process, leading to enhanced stability.
[19]
TEM characterization.
Nitrogen adsorption isotherms were
obtained to assess the porous structure of PtNiB/CMK-3 and
CMK-3 (Figure S5, Supporting Information). Both materials
displayed a typical hysteresis loop in the relative pressure range
from 0.4 to 0.9, indicating type-IV isotherms, and showed a
narrow Barrett, Joyner, and Halenda (BJH) pore size distribution
centered at 4.3 nm. The BET surface areas of CMK-3 and PtNiB/
14]
2
−1
CMK-3 were calculated as 965.43 and 853.39 m g , respectively,
3
−1
with a respective total pore volume of 1.32 and 1.17 cm g .
These results follow from the presence of PtNiB NPs within
the interior walls of the CMK-3 leading to a decrease in specific
2
. Results and Discussion
The PtNiB/CMK-3 catalyst was prepared by the formation of
PtNiB NPs within the pores of ordered mesoporous carbon
[20]
surface area. Nevertheless, the specific surface area and pore
[21]
(CMK-3) by impregnation of chloroplatinic acid and NiCl2,
volume of PtNiB/CMK-3 remained adequate for mass transfer.
and subsequent reduction by NaBH (Figure 2a). NaBH acts
The surface valence state of elements and electron transfer
between PtNi and B in PtNiB/CMK-3 were acquired by X-ray
photoelectron spectroscopy (XPS). Four clear peaks, located at
4
4
not only as a reducing agent for the alloy, but also as a B pre-
cursor for B-doping.[ The reaction mechanism is described in
15]
[
16]
0
Equation (1) as below (M is on behalf of metal elements)
MCl + 4NaBH + 9H O → M B + 4NaCl + 12.5H + 3H BO
Details on the morphology and structural characterization
:
71.8, 72.7, 75.3, and 76.9 eV (Figure 3a), represent Pt 4f7/2,
Pt 4f7/2, Pt0 4f5/2, and Pt 4f5/2, respectively.
2
+
2+
[22]
Compared
2
2
4
2
2
2
3
3
(1)
with Pt/CMK-3 and PtNi/CMK-3, the binding energy of Pt 4f
in PtNiB/CMK-3 was slightly positively shifted toward a higher
energy. The Ni 2p spectrum (Figure 3b) was further analyzed,
of PtNiB/CMK-3 are available in Figure 2 and Figure S1
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Figure 2. a) Schematic fabrication process of the PtNiB/CMK-3. b,c) TEM and high-resolution TEM images of as-synthesized PtNiB/CMK-3. d,e) STEM
images of PtNiB/CMK-3 (inset: size distribution image). f) Atomic-resolution high-angle annular dark-field scanning TEM image of PtNiB/CMK-3. The
corresponding energy-dispersive X-ray spectroscopy mapping of g) Pt, h) Ni, i) B, and j) overlap. Scale bar: 1 nm.
revealing two main peaks at 857.5 and 874.2 eV, together with
satellite peaks at 863.1 and 881.5 eV, corresponding to Ni 2p
Ni B (Figure 3c). The Ni absorption edge and the main transi-
4 3
tion energies of PtNiB/CMK-3 and NiB/CMK-3 were between
3/2
2
+
[15c]
and Ni 2p1/2 of Ni , respectively.
Moreover, relative to the
those of Ni foil and Ni(OH) , suggesting that Ni carries a posi-
2
binding energy of NiB/CMK-3 and PtNi/CMK-3, that of Ni 2p
for PtNiB/CMK-3 was shifted toward a lower energy. PtNiB/
CMK-3 and NiB/CMK-3 both showed a minor peak at approxi-
tive charge and is in an intermediate valence state. In addition,
compared with NiO, the pre-edge region of PtNiB/CMK-3 and
NiB/CMK-3 exhibited a small peak at approximately 8333 eV,
0
[23]
mately 853 eV, corresponding to Ni . In addition, the surface
very similar in shape and position to the Ni(OH) reference
2
of B particles is spontaneously oxidized when exposed to air or
spectra. Moreover, the spectra of NiB/CMK-3 and PtNiB/CMK-3
showed a higher white line intensity, with their maxima shifted
to higher energies similar to that of Ni B ; these results indi-
water, forming surface oxides/hydroxides[
16a,24]
; this was con-
firmed from the observed O 1s spectrum (Figure S6, Supporting
Information). The O peaks of NiB/CMK-3 and PtNiB/CMK-3
curves are in 535.0 eV, which is different from the O peaks
in PtNi/CMK-3, Pt/CMK-3, and CMK-3. Simultaneously, we
found the oxidized boron in B 1s XPS spectra (Figure S7, Sup-
porting Information), which is also consistent with Figure S6
4
2
3
+
3+
cate the presence of B with Ni in the Ni or Ni state due to
the formation of PtNiB alloy. Ni EXAFS spectra (Figure 3d) for
NiB/CMK-3 and PtNiB/CMK-3 showed backscattering events
2+
3+
characteristic of Ni or Ni compounds, namely Ni-O at
1.59 Å and Ni-Ni2 at 2.73 Å (uncorrected for phase shift). How-
ever, the intensity of the Ni-Ni peak decreased considerably in
the PtNiB/CMK-3 sample, revealing a decreased number of
neighboring Ni atoms in PtNiB/CMK-3 due to the incorpora-
tion of Pt atoms. Furthermore, the presence of an additional
peak emerging at 2.27 Å (uncorrected) on the EXAFS spectrum
of NiB/CMK-3 and PtNiB/CMK-3 indicated the presence of B
in a local Ni environment. The EXAFS data therefore suggest
that the close arrangement of Pt, Ni, and B atoms in the NPs
affects their chemical state, leading to a new hybridized state
+
[
15a,c]
in the Supporting Information and the previous articles.
Therefore, the structural formula of as-prepared PtNiB NPs can
be approximately formulated as PtNiBO3.
In order to provide further insight into the electronic struc-
ture and coordination environment of Ni and Pt atoms in
PtNiB/CMK-3, X-ray absorption near-edge structure (XANES)
and extended X-ray absorption fine structure (EXAFS) spectra
were performed. The Ni K-edge XANES of PtNiB/CMK-3 was
very different compared with those of the Ni foil, NiO, and
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Figure 3. a) Pt 4f XPS spectrum of Pt/CMK-3, PtNiB/CMK-3, and PtNi/CMK-3. b) Ni 2p XPS spectrum of NiB/CMK-3, PtNiB/CMK-3, and PtNi/CMK-3.
c) Ni K-edge XANES spectra and d) EXAFS spectra of NiB/CMK-3, PtNiB/CMK-3, Ni foil, NiO, and Ni B . e) Pt L3-edge XANES spectra and f) EXAFS
3
4
spectra of PtNiB/CMK-3, Pt/CMK-3, and reference Pt foil.
that enhances the catalytic activity. Further evidence of this was
obtained by the Pt L3-edge XANES spectra (Figure 3e) and the
corresponding Fourier transform (Figure 3f), wherein the Pt
L3-edge for PtNiB/CMK-3 showed a distinctly higher white line
intensity compared with Pt/CMK-3 and standard Pt foil, con-
firming that PtNiB/CMK-3 possesses a relatively higher oxida-
tion state due to electron transfer. Furthermore, the intensity
of PtNiB/CMK-3 Pt L3-edge in Fourier-transformed EXAFS was
obvious oxidation peak at 0.43 V for PtNiB/CMK-3 and NiB/
CMK-3. Such anodic peaks are commonly observed in Ni-based
oxygen evolution reaction (OER) and assigned to the oxidation
conversely, PtNi/CMK-3 showed no such
oxidation peak. Collectively, the above characterization results
described the successful formation of PtNiB NPs on a CMK-3
of Ni2 to Ni ;
+
3+ [15c]
substrate with PtNiBO structure, and the B alloying process
3
led to the modification of the surface electronic structure of
PtNiB/CMK-3.
2
.0 Å, much higher than those of Pt/CMK-3 and Pt foil, and
the scattering intensity ranged down to 1.7 Å (uncorrected),
demonstrating the presence of light neighbors. These results
also indicate that the white line intensity changes observed are
an effect of PtNiB alloy formation.[ Cyclic voltammetry and
linear scan voltammogram (LSV) measurements of the as-pre-
pared PtNiB/CMK-3, PtNi/CMK-3, and NiB/CMK-3 composites
in 1 m KOH (Figure S8, Supporting Information) showed an
The ECH performance of the as-prepared materials was
assessed by LSV measurements under acidic conditions.
For comparison, the H2 evolution reaction (HER) in the
absence of guaiacol was also performed. PtNiB/CMK-3 exhib-
ited a high catalytic current density of 10 mA cm beyond
−0.41 V versus reversible hydrogen electrode (RHE) for HER
(Figure 4a). Upon the addition of 10 mm guaiacol, the potential
25]
−
2
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Figure 4. a) HER and ECH polarization curves of PtNiB/CMK-3. b) HER and ECH polarization curves of PtNi/CMK-3. c) Content (%) of guaiacol and
its hydrogenation products during ECH by PtNiB/CMK-3. d) Conversion curves of guaiacol on ECH reaction for different electrocatalysts. e) Columnar
composite diagram of conversion, selectivity, and FE. f) Conversion, selectivity, and FE under 20 successive cycles of PtNiB/CMK-3.
of PtNiB/CMK-3 dramatically left shifted to only −0.26 V, which
was at least 150 mV smaller than that of HER. LSV for PtNi/
CMK-3, Pt/CMK-3, and NiB/CMK-3 in the presence or absence
of guaiacol was also performed, yet such obvious shifts were
not observed (Figure 4b; Figure S9, Supporting Information),
suggesting that PtNiB/CMK-3 possesses a maximum potential
shift. The above phenomenon implies that more favorable and
economically attractive for guaiacol ECH than HER on PtNiB/
CMK-3 occurred. Moreover, polarization curves of PtNiB/
CMK-3 electrocatalyst, with or without of iR correction, were
also investigated (Figure S10, Supporting Information). The
corrected iR is slightly better than uncorrected HER perfor-
mance. Following from the above results, we assessed the influ-
ence of different reaction conditions on ECH based on PtNiB/
CMK-3. A H-type cell (10 mL per chamber) with a Nafion-117
membrane was utilized for ECH of guaiacol equipped with
the prepared samples and IrO /C as the cathode and anode,
respectively (Figure S11, Supporting Information). The cathode
2
compartment was filled with 0.2 m HClO containing 10 mm
4
guaiacol, whereas the anode compartment had 0.2 m HClO4.
Due to the solvent effect vital influence to the ECH reaction,
several different solvents have been investigated in the ECH
of guaiacol to KA oil. The high concentration of acid (0.5 m
HClO ) in electrolyte reduced the ECH activity, while the use
4
of 0.2 m HClO as the electrolyte led to the highest activities
4
for PtNiB/CMK-3; however, 0.2 m HCl and 0.2 m H SO led to
2
4
moderate conversions (Figure S12, Supporting Information). In
addition, 0.2 m HClO achieved optimal conversion and selec-
4
tivity for KA oil. The ideal conditions for ECH of guaiacol were
shown to be at 60 °C (Figure S13, Supporting Information), with
a current of 20 mA (Figure S14, Supporting Information), and
10 mm guaiacol (Figure S15, Supporting Information). For
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investigating the effect of NaBH4 to the ECH reaction, the
an obviously inferior ECH activity (less than 18% conversion)
over 120 min under the same reaction conditions compared to
PtNiB/CMK-3, with an unsatisfactory KA oil yield (Figure 4d,e).
Furthermore, the FE of PtNiB/CMK-3 (86.2%) was almost 13.7
times higher than that of PtNi/CMK-3 (6.3%) within 1 h (Table
S2, Supporting Information). The advantages of PtNiB/CMK-3
over other tested catalysts were further highlighted following
comparisons of the turnover frequency (TOF). The TOF value
different mass of NaBH were used to prepare PtNiB/CMK-
4
3
-B0.5 and PtNiB/CMK-3-B2, with the similar method as for
PtNiB/CMK-3. (0.5 and 2 means the mass of NaBH are 0.5:1
4
and 2:1 compared with PtNiB/CMK-3.) The ECH performance
of PtNiB/CMK-3-B0.5 and PtNiB/CMK-3-B2 electrocatalysts
was almost with no difference compared with PtNiB/CMK-3
(Figure S16, Supporting Information), which indicated that the
−
1
mass of NaBH may not further influence the ECH activity of
of PtNiB/CMK-3 reached 291.28 h , which was dramatically
4
−
1
PtNiB/CMK-3. Furthermore, the Pt/Ni molar ratio in PtNiB/
CMK-3 (Figure S17, Supporting Information) was shown to
have no beneficial effect in ECH performance at higher (1:1)
or lower (1:4) molar ratios. Therefore, a Pt/Ni molar ratio of
higher than that for PtNi/CMK-3 (35.83 h ) and other electro-
catalysts. We also examined the effect of NiB on the ECH reac-
tion; following NiB reduction by NaBH loaded on Pt/CMK-3
4
(Pt/NiB/CMK-3), there was only a minor improvement in ECH
performance compared to Pt/CMK-3 alone. Additionally, when
only NiB/CMK-3 was used in ECH, no product was detected.
These observations are consistent with HER behavior, wherein
NiB alone does not obviously affect ECH. Moreover, in order to
1
:2 was found to be optimum to achieve a high yield of KA oil
within 60 min (Table S2, Supporting Information). The electro-
chemical active surface area (ECSA) was measured to reveal the
effect of intrinsic electrocatalysis activity of the PtNiB/CMK-3
(
Figure S18, Supporting Information). The PtNiB/CMK-3 had
assess whether Ni(OH) on the surface of PtNiB may influence
2
−2
larger double-layer capacitance C (27.67 mF cm ) than the
the ECH of guaiacol, commercial Ni(OH)2 NPs were added
dropwise onto a carbon cloth and used as the cathode in the
same conditions. The results showed that no product could be
dl
−2
−2
Pt/CMK-3 (19.57 mF cm ) and PtNi/CMK-3 (12.84 mF cm ),
suggesting the much high ECSA of the PtNiB/CMK-3, which
further explains why PtNiB/CMK-3 has the better ECH per-
formance. The electrochemical impedance spectroscopy (EIS)
also was used to investigate the electrode kinetics, and the cor-
detected, confirming that Ni(OH) also had no effect on ECH
2
of guaiacol (Table S2, Supporting Information). Meanwhile,
the interaction between Pt and Ni(OH) was also researched,
2
responding Nyquist plots are shown in Figure S19 in the Sup-
and the Pt/Ni(OH)2 showed much poor ECH performance
compared with PtNiB/CMK-3. Additionally, PtNiB/CMK-3 was
shown to have a better electrocatalytic activity than PtNiB/
XC-72 (Figure S21, Supporting Information), indicating that the
performance of ECH is strongly correlated to the high surface
area and confined space provided by the mesoporous structure
of CMK-3. Furthermore, chemical etching of PtNi/CMK-3 with
2
porting Information. The R is estimated to be ≈5.3 Ω cm . The
s
2
Rct of PtNiB/CMK-3 is estimated to be ≈20.2 Ω cm , close to
that of PtNi/CMK-3 (21.1 Ω cm ) and lower than that of Pt/
CMK-3 (≈25.2 Ω cm ) and CMK-3 (≈28.9 Ω cm ). The smaller
Rct of the PtNiB/CMK-3 indicated the faster electron transfer
process, which further confirms that the B-doping in PtNiB/
CMK-3 is vital to the promotion of the ECH activity.
2
2
2
a fresh NaBH solution (PtNi/NiB/CMK-3) increased the per-
4
To understand the reaction mechanism in the ECH of guai-
acol to KA oil, the reaction was performed at optimum con-
ditions using PtNiB/CMK-3. The evolution of the reactant
and product concentrations with reaction time is depicted
in Figure 4c. ECH of guaiacol was easily conducted and the
product distributions changed in a complex manner with the
reaction progress. The reaction was accompanied by a rapid
increase in cyclohexanone and a decrease in guaiacol over
formance of ECH compared to that of PtNi/CMK-3, indicating
that B-doping indeed improved the ECH activity (Figure S21,
Supporting Information). In addition, the CMK-3 (NaBH ) pre-
4
pared by adding the NaBH solution into the CMK-3 showed
4
no activity, which indicated that B dispersing on CMK-3 has
no obvious effect on ECH performance (Table S1, Supporting
Information; Figure 2i). Moreover, the ECH conversion of
guaiacol of PtNiB/CMK-3 was about 2.5 times higher than that
of 10% commercial Pt/C. As shown in Table S3 in the Sup-
5
0 min, indicating that guaiacol is electrocatalytically hydro-
genated to cyclohexanone in the first step. The cyclohexanone
yield reached a maximum at 57.7%, then decreasing with
time. In a consecutive step, cyclohexanone was hydrogenated
to cyclohexanol, with the cyclohexanol yield increasing slowly
for the first 30 min, subsequently increasing and reaching a
maximum. The optimization of reaction time suggested that
porting Information, the remarkable J normalized value also
0
further suggested the excellent ECH activity of PtNiB/CMK-3.
According to the above results, the addition of B in PtNiB leads
to a reconfigurable surface electronic structure, facilitating the
balance of chemisorption and desorption of reaction interme-
diates in PtNiB/CMK-3, thereby optimizing the electrocatalytic
activity of ECH.
6
0 min seemed to be suitable, achieving a 98.9% guaiacol con-
version with 90.3% selectivity for KA oil. Intermediates, such
as phenol, 2-methoxycyclohexanone, and cyclohexanone, were
also detected. The conversion of these intermediates were very
low (<6%) over the entire reaction, with their concentrations
gradually increasing initially and then decreasing, suggesting
that hydrogenation of the benzene ring skeleton and direct
CO cleavage occurred simultaneity during ECH. Additionally,
cyclohexanone was hydrogenated to cyclohexanol along with
the formation of a minor amount of cyclohexane, following a
A recyclability study of the PtNiB/CMK-3 electrocatalyst was
performed by repeating the ECH of guaiacol for reaction cycles
(Figure 4f). Impressively, PtNiB/CMK-3 was highly recyclable,
maintaining high conversion (≥95%) and selectivity (≥90%), with
an average FE of 81.8%. Additionally, the TEM analysis of PtNiB/
CMK-3 after recycling measurements (Figure S22, Supporting
Information) revealed that the size distribution of PtNiB NPs
was preserved, with no obvious evidence of NPs agglomeration.
The XPS data after ECH reaction cycles were also investigated
(Figure S23, Supporting Information), and found that the struc-
ture of PtNiB/CMK-3 was well maintained after the ECH reaction.
first-order reaction on PtNiB/CMK-3 (Figure S20, Supporting
Information).[
26]
However, the PtNi/CMK-3 catalyst showed
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Table 1. ECH of other phenolic compounds using PtNiB/CMK-3.
To assess the applicability of PtNiB/CMK-3
for ECH, other lignin-derived phenolic com-
pounds with a wide range of functional
groups were also evaluated under 20 mA
at 60 °C (Table 1). For phenol, 3-methoxy-
phenol, 2-methoxy-4-methyl phenol, eugenol,
isoeugenol, and 2,6-dimethoxyphenol, PtNiB/
CMK-3 showed a high catalytic activity, with
conversion rates higher than 95%, FEs of
Entry
Structural formula
t (min) Conv. (%) FE (%)
Product (sel. %)
1
50
60
97.5
95.8
90.8
92.6
8
0–93%, and a total yield of corresponding
ketone and alcohol products of >85%. In
particular, PtNiB/CMK-3 was also active in
the hydrogenation of branched, unsaturated
CC bonds such as eugenol and isoeugenol,
further confirming the wider universality of
the catalyst. All of the above results demon-
strated that the PtNiB/CMK-3 catalyst has
an excellent ECH activity for multifarious
phenolic compounds under mild reaction
conditions.
2
3
60
95.1
88.1
In order to gain an in-depth understanding
on the reaction intermediates as well as
greater insight into the reaction mechanisms
of the ECH process of guaiacol, electrode
potential-dependent in situ Fourier-transform
infrared spectroscopy (FTIR) spectra were per-
formed (Figure 5a). The appearance of new
characteristic peaks continuously increasing
from approximately −0.3 V was observed,
indicating that the ECH reaction occurred
at a very low potential on PtNiB/CMK-3.
The two peaks located at approximately
4
90
98.7
84.3
−1
3450 and 3550 cm were assigned to the
5
90
97.5
86.2
OH stretching of cyclohexanol and phenol,
respectively, the peaks of which overlap and
[
27]
cannot be easily separated.
Additionally,
−1
the peak at 1650 cm was attributed to the
CO stretching band of ketone-based inter-
mediates or cyclohexanone. Similarly, the new
band at 1300–1500 cm corresponded to the
disappearance of CC double bond in the
phenolic ring, indicating the hydrogenation of
phenolic compounds. In addition, an upward
−1
6
90
98.6
80.2
−1
sharp peak near 1240 cm was observed, cor-
responding to the cleavage of CO bond in
Furthermore, the cathode electrolyte after the ECH reaction was
analyzed by ICP. The Pt in cathode electrolyte was almost inex-
istent, and very few of Ni leached, which could be owing to the
nano-confined effect of CMK-3, and such little loss of Ni may
not have notable influence on the ECH performance (Table S5,
Supporting Information). The above results indicate the strong
stability of PtNiB/CMK-3 in the ECH reaction and the B content
indeed critically affected the electronic properties of PtNiB NPs,
allowing interfacial electron or charge transfer to easily occur.
Moreover, the mesoporous carbon framework of CMK-3 sup-
presses the agglomeration/dissolution of NPs and provides low
resistance diffusion channels for the electrolyte solution, facili-
tating mass and charge transport and allowing PtNiB/CMK-3 to
acquire superior activity and boost ECH performance.[12]
guaiacol, reflected by the disappearance of the methoxyl group
and its further hydrogenation to the final product, indicating
[28]
that PtNiB NPs were highly active for CO bond cleavage.
The ECH of guaiacol can theoretically proceed through three
different mechanisms, namely (i) demethoxy to phenol,
(ii) hydrogenation of aromatic ring to 2-methoxycyclohexanone,
and (iii) direct dehydroxylation to methoxybenzene. Herein, the
presence of methoxybenzene was not observed.
Density functional theory (DFT) calculations were per-
formed to assess the ECH reaction mechanism and pathway.
The adsorption energies of different sites of guaiacol on the
surface of PtNiB and PtNi were studied to illustrate the fea-
sibility of hydrogenation (Figure S24, Supporting Informa-
tion). The calculated adsorption energies on PtNiB were
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Figure 5. a) In situ FTIR spectra of the electrocatalytic guaiacol hydrogenation reaction on PtNiB/CMK-3 catalysts in 0.2 mol L−1 HClO and 10 mm
4
guaiacol electrolyte. b) The structure of adsorbed guaiacol on PtNiB and PtNi. c) The most stable structure of adsorbates (phenol, cyclohexanone,
cyclohexanol, and 2-methoxycyclohexanone) on PtNiB and their corresponding charge distribution. d) Probable ECH pathways for guaiacol and main
intermediates/products. e) The distribution of transferred charges between guaiacol and catalyst (PtNiB and PtNi). The green and yellow color rep-
−
3
resent the charge accumulation and depletion regions, respectively. The isovalue is all the same, equal to 0.003 eÅ . f) Electron localization function
plots for adsorbed guaiacol on PtNiB and PtNi. g) Electron localization function plots for PtNiB and PtNi.
carbon ring (CRing; Ead = −0.91 eV), followed by oxygen in the
methoxy group (O ; E = −0.62 eV), oxygen in the hydroxy
propose that the ECH of guaiacol may proceed via paths (i)
and (ii) as the dominant reaction pathway over PtNiB/CMK-3
(Figure 5d).
1
ad
group (O ; E = −0.15 eV), and carbon in the methoxy group
2
ad
(
C ; E = −0.07 eV) (Figure 5b). The adsorption energies of
The role of B in the ECH process was also shown to be sig-
nificant. The adsorption energy on PtNi (−0.59 eV) was not as
strong as that on PtNiB (−0.91 eV) (Figure 5b), indicating that
the B-doped PtNiB surface is more favorable than PtNi for ECH
reactions. The charge density differences were induced by the
adsorbed guaiacol on PtNiB or PtNi (Figure 5e). For a given
catalyst, the stronger the adsorption, the greater the charge
transferred. Indeed, the amount of charge transfer on PtNiB
was shown to be greater than that on PtNi on the same adsorp-
tion site, indicating that B plays a vital role in the ECH process.
The electronic structure, as analyzed by electron localization
function (ELF) plots, also demonstrated a stronger localiza-
tion of electrons near the adsorbed guaiacol on PtNiB than on
7
ad
the CRing and O1 sites represented chemisorption, whereas
those of O2 and C7 indicated weak physical adsorption. In
other words, ECH is most likely to occur either on the ben-
zene ring (CRing; path (i)) or on the methoxy group (O ; path
1
(
ii)). In addition, weak physical adsorption means that O and
2
C are hard to activate, leading to subsequent difficulties in
7
hydrogenation. These results are consistent with the experi-
mentally observed results of ECH and the product distribution
on PtNiB/CMK-3.
The most stable adsorption structures and corresponding
charge distributions for each species were calculated for
both paths (i) and (ii) (Figure 5c). The charge density dif-
ference analysis and the large negative energy (Figure 5d)
represent a strong interaction between species and PtNiB,
allowing optimal adsorption of intermediates and thus high
ECH activities.[ Therefore, with the observation of inter-
mediates during the ECH of guaiacol by in situ FTIR anal-
ysis and experiments, as well as theoretical calculations, we
[
30]
PtNi (Figure 5f). In addition, the localized electrons around
the metal atoms slightly shifted toward the B atoms, which
may promote reactant activation in PtNiB (Figure 5g). Overall,
these results clearly confirmed that the superior ECH electro-
catalytic performance of PtNiB/CMK-3 can be attributed to the
effective optimization of the PtNiB surface electron structure by
29]
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Figure 6. a) The photograph of ECH electrolyzer cell. b) The CCM in ECH electrolyzer cell. c) The structural components of ECH electrolyzer cell.
d,e) Schematic and photograph of ECH reaction system. f) The gram-scalable KA oil in ethyl acetate solution prepared by the ECH reaction system.
B-doping, which simultaneously promotes substrate adsorption
and ECH.
100 mL 0.2 m HClO , cathode electrolyte: 100 mL 0.2 m HClO
4 4
with 10 mm guaiacol) in the two electrolytic tanks were cir-
To assess the practical application of the developed catalyst,
an electrolyzer was employed in the ECH of guaiacol and OER
culated in the system by a peristaltic pump with flow rate of
−
1
50 mL min , and the temperature was maintained at 60 °C by
a constant temperature water bath to achieve isothermal reac-
tion conditions. A constant current meter was employed to con-
trol the current. Guaiacol was converted to KA oil by ECH reac-
tion with a constant current of 40 mA for 5 h at a voltage range
of 1.7–2.0 V (Figure S25, Supporting Information). The con-
version rate of KA oil was as high as 80.1% with an excellent
selectivity of KA oil (96.7%) with K/A ratio of 0.7, and the KA
oil could be obtained via extraction by ethyl acetate (Figure 6f).
Efforts directed toward the future process optimization have
been initiated. Such encouraging results highlight prospects for
the use of PtNiB/CMK-3 in large-scale implementation of the
electrosynthesis of bio-oil.
equipped with PtNiB/CMK-3 and IrO /C coated on a Nafion
2
1
17 membrane as the cathode and anode, respectively. Photo-
graph and structural components of ECH electrolyzer cell
are shown in Figure 6a,c. And the catalyst-coated membrane
(CCM) is shown in Figure 6b. In detail, 40 mg of PtNiB/CMK-3
with 40 µL Nafion (5%) was sprayed onto Nafion 117 (3.5 cm ×
3
.5 cm) to form a 2.0 cm × 2.0 cm square cathode catalyst layer,
while 40 mg IrO /C with 40 µL Nafion (5%) was sprayed on
2
the other side of Nafion 117 as the anode catalyst layer in the
same way. The electrolyzer was then combined with the peri-
staltic pump and the electrolyte tank contained the ECH reac-
tion system (Figure 6d,e). The electrolytes (anodic electrolyte:
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. Conclusion
In summary, we elaborately designed a novel hybrid nano-
composite, PtNiB/CMK-3, by surface engineering of PtNi NPs
with B-doping. The present strategy can be used to promote the
electrocatalytic activity for ECH of guaiacol and related lignin
model monomers. The optimized surface electron structure
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free energy of intermediates on PtNiB. Additionally, the large
specific surface area of CMK-3, which has a uniform porous
structure, maximized the number of exposed active sites and
provided low resistance diffusion channels. Thus, PtNiB/
CMK-3 exhibited a superior ECH performance with a high FE
and robust stability, consequently capable of converting model
lignin monomers into KA oil and other valuable chemicals
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confirmed to be feasible. The current work sheds new insight
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