Integrating hydrogen production with anodic selective oxidation of sulfides over a CoFe layered double hydroxide electrode

Article abstract of DOI:10.1039/d0sc05499b

Replacing the sluggish oxygen evolution reaction (OER) with oxidation reactions for the synthesis of complex pharmaceutical molecules coupled with enhanced hydrogen evolution reaction (HER) is highly attractive, but it is rarely explored. Here, we report an electrochemical protocol for selective oxidation of sulfides to sulfoxides over a CoFe layered double hydroxide (CoFe-LDH) anode in an aqueous-MeCN electrolyte, coupled with 2-fold promoted cathodic H₂productivity. This protocol displays high activity (85-96% yields), catalyst stability (10 cycles), and generality (12 examples) in selective sulfide oxidation. We demonstrate its applicability in the synthesis of four important pharmaceutical related sulfoxide compounds with scalability (up to 1.79 g). X-ray spectroscopy investigations reveal that the CoFe-LDH material evolved into amorphous CoFe-oxyhydroxide under catalytic conditions. This work may pave the way towards sustainable organic synthesis of valuable pharmaceuticals coupled with H₂production.

Full text of DOI:10.1039/d0sc05499b

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Integrating hydrogen production with anodic
selective oxidation of sulfides over a CoFe layered
double hydroxide electrode†
Cite this: DOI: 10.1039/d0sc05499b
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
a
Lina Ma, ‡a Hua Zhou,‡b Ming Xu,a Peipei Hao,
Xianggui Kong
b
*
and Haohong Duan
Replacing the sluggish oxygen evolution reaction (OER) with oxidation reactions for the synthesis of
complex pharmaceutical molecules coupled with enhanced hydrogen evolution reaction (HER) is highly
attractive, but it is rarely explored. Here, we report an electrochemical protocol for selective oxidation of
sulfides to sulfoxides over a CoFe layered double hydroxide (CoFe-LDH) anode in an aqueous-MeCN
electrolyte, coupled with 2-fold promoted cathodic H2 productivity. This protocol displays high activity
(85–96% yields), catalyst stability (10 cycles), and generality (12 examples) in selective sulfide oxidation.
We demonstrate its applicability in the synthesis of four important pharmaceutical related sulfoxide
compounds with scalability (up to 1.79 g). X-ray spectroscopy investigations reveal that the CoFe-LDH
material evolved into amorphous CoFe-oxyhydroxide under catalytic conditions. This work may pave the
way towards sustainable organic synthesis of valuable pharmaceuticals coupled with H2 production.
Received 5th October 2020
Accepted 11th November 2020
DOI: 10.1039/d0sc05499b
rsc.li/chemical-science
production of valuable chemicals and H2, only a handful of
simple molecules were studied.6f,g The anodic synthesis of
Introduction
functional and complex organic molecules, especially pharma-
ceuticals with medicinal signicance, has rarely been explored.
In the family of pharmaceuticals, organosulfur compounds
play essential roles in many biological processes linked with
human disease therapy.7 For example, sulforaphane offers
efficient chemoprotection against various cancers (e.g. prostate,
lung, breast, and colon cancer).7g Direct oxidation of their
sulde precursors is a straightforward and atom-economical
synthetic method, and thus has been widely applied for the
synthesis of sulfoxides.8 However, current synthetic methods
oen require homogeneous catalysts (e.g. Mn- and Fe-based
catalysts) and strong oxidizing agents such as H2O2.9 Addi-
tional oxidants such as peroxy acids, prevalent iodine reagents
or oxone are also needed.10 In 2012, He and co-workers reported
a catalyst-free protocol for the selective oxidation of suldes
with an inorganic oxidant oxone in ethanol.11 However, the
utilization of oxone may result in purication issues and envi-
ronmental contamination. More importantly, over-oxidation of
sulfoxide to sulfone or co-oxidation of other functional groups
in the sulfoxide molecule would occur and generate medically
useless products. Recently, Xu and co-workers reported an
efficient heterogeneous polyoxovanadate catalyst for sulde
oxidation; nevertheless the reaction was assisted by a strong
oxidant (tert-butylhydroperoxide (TBHP)) at elevated tempera-
tures.12 Jiang's group reported a photocatalysis strategy to ach-
ieve sulde selective oxidation driven by visible light under mild
conditions, but the use of homogeneous catalysts (UO2(OAc)2-
$2H2O) would make separation difficult.12 Therefore, the
Electrochemical water splitting is considered to be a promising
hydrogen production approach to deal with the increasing
global energy demand and environmental problems associated
with fossil fuel utilization.1 Water electrolysis involves two half-
reactions, including H2 and O2 evolution reactions (HER and
OER).2 The overall reaction rate is oen restricted by the anodic
OER because of its more sluggish kinetics.2c As a result, much
higher overpotential is needed for the OER to match the H2
production rate, thereby undermining the overall energy
conversion efficiency.3 Although advanced non-noble-metal
OER electrocatalysts have been developed, accomplishing an
OER efficiency comparable with that of the HER still remains
a challenge.4 In addition, the economic value of O2 is far inferior
to that of H2. Recently, electrooxidation of low-cost organic
agents has emerged as an alternative strategy to replace the
OER, achieving lower overpotential for total water splitting and
at the same time producing high-value chemicals.5 Electro-
oxidation has been exploited for converting biomass-derived
platform chemicals, e.g., ethanol, glycerol, and 5-hydrox-
ymethyfurfural (HMF).6 Despite the advanced concept of co-
aState Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, China
bDepartment of Chemistry, Tsinghua University, Beijing 100084, China. E-mail:
hhduan@mail.tsinghua.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0sc05499b
‡ These authors contributed equally.
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Fig. 1 (a) Electrocatalytic oxidation of sulfides 1 coupled with the HER.
(b) Oxidation of sulfides 1 to form sulfoxides 2.
selective oxidation of suldes using heterogeneous catalysts
under ambient conditions remains challenging.
Electrochemical oxidation by using heterogeneous electro-
catalysts shows promise to achieve selective sulde oxidation
under mild conditions while avoiding separation issues.
Recently, an environmentally benign electrochemical oxidation
of suldes was reported by Laudadio and colleagues, who
evaluated the electrocatalysis with continuous reactors that
shows great promise for practical applications.13 Non-noble
metal-based layered double hydroxides (LDHs), a class of inor-
ganic layered materials with unique 2D structures in which di-
and tri-valence cations are dispersed within the layers at the
atomic level,14 have emerged as efficient electrocatalysts for the
OER15 and anodic organic agents for electrooxidation, for
example converting HMF to value-added 2,5-furandicarboxylic
acid (FDCA).16 However, there is no report of developing LDHs
as the electrocatalyst for sulde selective oxidation with the goal
of synthesis of sulfoxide-containing pharmaceuticals.
In this regard, we report an electrochemical approach for
selective oxidation of suldes to replace the OER over CoFe
layered double hydroxides supported on carbon cloth (CoFe-
LDH/CC) as the anode in an aqueous-MeCN electrolyte,
coupled with 2-fold promoted cathodic H2 productivity (Fig. 1a).
A variety of aryl, heteroaryl and alkyl suldes at the CoFe-LDH/
CC anode can be selectively converted into the corresponding
sulfoxides with 85–96% yields under ambient conditions
(Fig. 1b). Importantly, this method was successfully extended to
the synthesis of complex pharmaceutical compounds with
sulfoxide moieties from their sulde precursors in good yields,
including ricobendazole (78%), omeprazole (70%), sulindac
(63%) and amino acid methionine (89%). Moreover, the catalyst
CoFe-LDH/CC was used in the gram-scale synthesis of diphenyl
sulfoxide with 83% yield (up to 1.63 g) and amino acid methi-
onine with 86% yield (up to 1.79 g). Preliminary investigations
suggest that the in situ formed amorphous metal oxyhydroxide
acts as the active species for selective oxidation of suldes to
sulfoxides via a radical process.
Fig. 2 (a) Schematic illustration of the preparation of the CoFe-LDH
nanoarray supported on carbon cloth. (b) SEM, (c) HAADF-STEM, (d)
EDS mapping, (e) high-resolution TEM, (f) SAED pattern, and (g) XRD
pattern of the CoFe-LDH/CC material.
electron microscope (SEM) and transmission electron micro-
scope (TEM) images (Fig. 2b and c, ESI, Fig. S1a†), the cross-
linked LDH nanosheets are homogeneously grown on CC with
an average thickness of 9.4 nm (ESI, Fig. S1b and c†). Mean-
while, the atomic Co/Fe ratio is 3.01 as determined by induc-
tively coupled plasma-atomic emission spectrometry (ICP-AES,
ESI, Table S1†), which is consistent with the stoichiometry of
the introduced values. The energy dispersive spectrometry
(EDS) mapping technique reveals the uniform dispersion of Co
and Fe species in the nanosheet (Fig. 2d). A high-resolution
TEM image (HRTEM, Fig. 2e) shows that the interplanar
spacing is approximately 0.26 nm, which is assigned to the (012)
plane of the CoFe-LDH phase.18 The characteristic (003), (006),
(012), and (110) planes of the LDH structure were also observed
in the selected area electron diffraction (SAED) pattern of the
nanosheet (Fig. 2f). Consistently, these diffractions were
observed in the X-ray diffraction (XRD) pattern of the CoFe-
LDH/CC (Fig. 2g).
The selective electrooxidation of diphenyl sulde (1a) was
chosen as a model reaction to evaluate the activity of CoFe-LDH/
CC and to establish the optimal reaction conditions. In the
reaction, diphenyl sulfoxide (2a) was the desired product, and
diphenyl sulfone (3a) was the over-oxidized byproduct. To
investigate the effect of the applied voltage, the potential was
increased from 2.0 to 3.5 V vs. Ag/AgCl, resulting in the yield of
over-oxidized byproduct 3a increasing from trace to 30%. This
result suggests that the relatively low potential was a key factor
for inhibiting the formation of over-oxidized byproducts, in
agreement with the previous literature.13 Aer extensive opti-
mization of the electrolyte, solvent, cathode and atmosphere
(ESI, Table S2†), we found that upon using MeCN/H2O (1 : 1 v/v)
as the mixed solvent and nBu4NH2PO4 as the electrolyte, the
desired product 2a was obtained in 85% yield and 85% faradaic
efficiency (FE) in 2 h at room temperature (entry 1 of Table 1),
Results and discussion
Initially, CoFe-LDH/CC was fabricated by a facile electrodepo-
sition method (see the ESI† for details).17 As illustrated in
Fig. 2a, Co and Fe nitrate precursors were converted to CoFe
hydroxide by electrolysis with the formation of a LDH nanoarray
grown on a carbon cloth cathode. As shown in the scanning
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Table 1 Optimization of reaction conditions
Entry
Deviation from standard conditions
Conv.a (%)
Yield 2aa (%)
Yield 3aa (%)
1
2
3
4
5
6
None
>99
39
12
>99
39
<5
85(81)
35(33)
Trace
81(81)
11(10)
nd
Trace
2
nd
Trace
Trace
nd
H2O as solvent
MeCN as solvent
N2 atmosphere
CC as the anode
No electric current
a
Conversion and yields were determined by 1H NMR using CH2Br2 as an internal standard. nd ¼ not detected. The isolated yields are given in
parentheses. Standard reaction conditions and variations on the electrochemical oxidation of diphenyl sulde: 1a (0.5 mmol), nBu4NH2PO4 (0.5
mmol), MeCN/H2O (2.0 mL, 1 : 1 v/v), RT, 2 h, CoFe-LDH anode (working area: 1 cm2), Pt cathode, applied potential: 2.0 V vs. Ag/AgCl (passing
charge of about 96C), under air, unless otherwise noted.
which represents a high performance compared to existing CoFe-LDH/CC anode. Aer adding 1a, the current density
homo- and heterogeneous catalysts (ESI, Table S3†). Speci- signicantly increases, and the required potential for gaining
cally, H2O was initially tested as the solvent for its wide use in current density of 5 mA cmꢀ2 dramatically decreased from
the OER and anodic oxidations, but moderate conversion and 1.90 V (without 1a) to 1.39 V (with 1a). The desired product 2a
yield of 2a were obtained, accompanied by overoxidation (entry increased with the time, and to our delight, the overoxidized
2, Table 1). Replacing it by MeCN or other organic solvents product (sulfone 3a) was rarely observed (<5% yield, Fig. 3b).
showed even lower activities (entry 3 in Table 1, entries 1–4 in More importantly, 2a can be obtained on a gram scale with 83%
the ESI, Table S2†). We found that using a mixed solvent of yield (up to 1.63 g) (for details, see Experimental procedures in
MeCN and H2O could furnish signicantly higher efficacy, the ESI†). We then assembled a two-electrode setup in which
which is likely due to MeCN showing higher conductivity and CoFe-LDH/CC was used as the anode for 1a oxidation and Pt as
capacity to dissolve the electrolyte, substrate, and reactants the cathode for the HER. Impressively, it shows two-fold higher
compared with other employed solvents, which may lead to
catalytic enhancement (entries 5–6 in the ESI, Table S2†).19 The
use of electrolytes other than nBu4NH2PO4 showed lower
activity (entries 7–9 in the ESI, Table S2†), which is due to the
promotion effect of nBu4NH2PO4 for the oxidation reaction.12b
In addition, the yield of 2a didn't show obvious change when
the reaction atmosphere was varied from air to N2 (entry 4 in
Table 1), indicating that H2O in the electrolyte could likely be
the source of oxygen in 2a. It should be noted that under our
reaction conditions in air, it remains possible that oxygen in the
product can be from the air. A blank experiment using CC as the
anode showed much lower conversion for sulde oxidation,
suggesting CoFe-LDH/CC was essential for catalytic activity of
sulde oxidation (entry 5 in Table 1). Finally, no reaction
occurred in the absence of electric current, demonstrating the
nature of electrocatalysis (entry 6 in Table 1). Pt ions were not
observed in the electrolyte aer electrooxidation by using an
Inductive Coupled Plasma Emission Spectrometer (ICP), indi-
cating that Pt was not leached into the electrolyte. According to
the conversion of 1a and total yields of products 2a (or with 3a),
there remained other byproducts while their structures can't be
recognized as no other signals were observed by crude NMR
analysis.
Fig. 3 (a) LSV curves of the CoFe-LDH/CC anode at a scan rate of
In the following catalytic studies, the catalytic reactions were 50 mV sꢀ1 in MeCN/H2O (2.0 mL, 1 : 1 v/v) with and without 0.5 mmol
1a. (b) Tracking experiments of the sulfoxidation procedure. (c) H2
production with and without 1a. (a–c) CoFe-LDH/CC anode and Pt
cathode. (d) Cycle-dependent conversions for (methylsulfinyl)
benzene.
examined over the as-synthesized CoFe-LDH/CC under the
optimized conditions. Fig. 3a shows the linear sweep voltam-
metry (LSV) curves in the presence and absence of 1a over the
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Fig. 4 Substrate scope for the formation of sulfoxides. Yields were
determined by 1H NMR using CH2Br2 as an internal standard. The
isolated yields were given in parentheses.
Fig. 5 Synthesis of existing pharmaceuticals: showcasing the appli-
cability of this protocol by integrating with current procedures. Yields
were determined by 1H NMR using CH2Br2 as an internal standard. The
isolated yields are given in parentheses. 2m, ricobendazole. 2n,
omeprazole. 2o, sulindac. 2p, amino acid methionine. The sulfoxide
moieties in the pharmaceuticals are highlighted in pink color.
activity for H2 production compared with the one without 1a
that follows the OER process (Fig. 3c), indicative of enhanced
energy efficiency. To a certain extent, the organic solvent MeCN
has some slight effect on the faradaic efficiency of H2 (90%),
lower than the faradaic efficiency of 96% in aqueous electrolyte.
Aer ten-cycle runs, no apparent decrease of activity was
observed (Fig. 3d), indicative of promising catalytic stability.
The structural stability of the used CoFe-LDH/CC anode was
also demonstrated (ESI, Fig. S2†).
Subsequently, the generality of the electrocatalytic selective
oxidation using CoFe-LDH/CC was studied by using 12 exam-
ples (Fig. 4). In general, the suldes and their derivatives with
functional groups including –H, –F, –Cl and –Br worked very
well to produce sulfoxides (2a–2g) under the optimized condi-
tions. The reaction efficiency was not affected by steric
hindrance (2d, 2f and 2g). A multi-substituted sulfoxide (2h) was
also produced efficiently. Aliphatic and benzyl groups could be
tolerated, and target products 2i and 2j were furnished in 91%
and 86% yields, respectively. Notably, nitrogen-containing
hetero-aromatics can be selectively oxidized to form the corre-
sponding sulfoxide (2k) without N-oxide formation. Finally,
alkyl sulfoxide (2l) was well tolerated under our reaction
conditions, given that it is oen sensitive to oxygenation
conditions.13
The late-stage oxygenation of complex molecules to
construct sulfoxides is oen accompanied by the existence of
various functional groups, and maintaining these functional
groups is very important but remains challenging. By using our
selective method, we expect that complex pharmaceuticals can
be precisely modied at the last stage, in which sulde is
oxidized to sulfoxide while the side reactions on other func-
tional groups are avoided. As a proof-of-concept, we focused on
four important drug molecules with sulfoxide moieties (Fig. 5).
Specically, the corresponding suldes were synthesized rst by
the reported procedures (for Experimental procedures see the
ESI†), which was followed by electrochemical oxidation to the
targeted sulfoxides by using CoFe-LDH/CC as the anode under
the optimized conditions. The catalytic results show that, ric-
obendazole20 (2m), an effective drug for a potential anticancer
agent, was obtained by selective oxidation in 78% yield. Ome-
prazole20 (2n), a well-known drug for treating gastroesophageal
reux, was also obtained in 70% yield. Sulindac20 (2o), another
anticancer reagent, was generated with 63% yield even in the
presence of oxygen-sensitive sequential conjugated alkenes.
Amino acid methionine (2p),20
a
biologically relevant
compound, could be efficiently synthesized in good yield.
Moreover, this electrochemical protocol could be used in the
gram-scale synthesis of sulfoxides (2p) with 1.79 g and high
yield of 86% was still obtained. Therefore, this protocol poten-
tially enables access to electrochemical synthesis of valuable
pharmaceutical related sulfoxides without external oxidants. In
addition, the CoFe-LDH/CC electrocatalyst potentially over-
comes disadvantages, such as catalyst reusability, which are
oen encountered by using homogeneous catalysts in conven-
tional methods.
In order to understand the reaction mechanism, a set of
control experiments were carried out (Fig. 6). When the stan-
dard reaction was performed in the presence of 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) or butylated hydrox-
ytoluene (BHT) as the radical scavengers,21 the reaction was
completely inhibited (Fig. 6a). According to a study on oxygen-
ation of suldes,12b,21 the sulde radical (Ph2Sc) or persulfoxide
radical (Ph2SOOc) was generally present in the process. The EPR
experiments were designed to detect radical intermediates by
adding the radical trapping agent DMPO (5,5-dimethyl-1-
pyrroline N-oxide). No obvious signals could be observed in
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the used CoFe-LDH/CC (1st recycled CoFe-LDH/CC) shows only
two broad diffraction peaks at 24.5ꢁ and 44.0ꢁ, which are
attributed to carbon cloth, indicating that CoFe-LDH undergoes
dramatic reconstruction to form an amorphous structure under
reaction conditions. Consistently, the SAED further conrmed
the amorphous nature of the used CoFe-LDH (ESI, Fig. S3†).
We then investigated the structure of CoFe-LDH before and
aer reaction using XPS and XAFS techniques. As shown in
Fig. 7b, the XPS signal of Co2p3/2 in the starting material shows
a typical satellite peak at 787.7 eV and a peak at 783.0 eV
attributed to CoII in LDHs. Aer the reaction, the intensity of
the peak of Co2+ decreased with the appearance of a new peak at
780.1 eV which could be attributed to Co3+, suggesting the
oxidation of Co2+ to Co3+ in the electrooxidation process.24
Concomitantly, the deprotonation of hydroxyl on CoFe-LDH
occurred, giving rise to a peak at 529.1 eV in the O1s XPS
spectrum of the used CoFe-LDH that can be attributed to the
typical M-O-M species in metal oxyhydroxides (Fig. 7c).24,25
Meanwhile, the Fe2p XPS spectra reveal a stable FeIII state in the
fresh and the used CoFe-LDH (ESI, Fig. S4†). In addition, X-ray
absorption near-edge structure (XANES) analysis in Fig. 7d
shows that the Co K edge prole of the used CoFe-LDH/CC
moved to a higher energy and next to that of the crystalline
CoFe-oxyhydroxide reference (c-CoFeOOH, obtained from CoFe-
LDH/CC by anodization in 1 M KOH, ESI, Fig. S5†), indicating
that some CoII ions in starting CoFe-LDH/CC were oxidized to
CoIII during the reaction, which is consistent with the XPS data.
In addition, the extended XAFS spectra reveal that the length of
Fig. 6 (a) Radical trapping experiments, (b) EPR spectra of the DMPO-
adduct under different conditions, (c) mass spectra of 2a from isotope
labelling experiments with H2O or H218O, respectively.
the absence of 1a (Fig. 6b, black line). Under the standard
conditions, a typical signal of the DMPO-radical adduct was
determined by the EPR during reaction (Fig. 6b, red line). We
speculated that it might be a sulde radical (Ph2Sc) or persulf-
oxide radical (Ph2SOOc) generated in the reaction with the
DMPO adduct.22 These ndings suggest that the electro-
oxidation reaction over CoFe-LDH/CC may involve a radical
process. To conrm the source of the oxygen atom in the
formed product, oxidation of 1a was carried out in MeCN–H218O
electrolyte, and the GC mass spectrum analysis shows that one
18O atom was introduced into 2a (Fig. 6c). This result suggests
that water could likely supply oxygen for the sulfoxide oxidation
under our reaction conditions.
˚
the rst coordination shell C–O (2.03 A) and the second coor-
˚
dination shell Co–Co(Fe) (3.13 A) in the initial CoFe-LDH was
˚
˚
decreased to 1.88 A and 2.83 A, respectively (Fig. 7e), which are
identical to those of c-CoFeOOH, reecting its metal oxy-
hydroxide nature. This structural transformation during elec-
trooxidation was induced by oxidation of CoII ions to CoIII.26
Notably, the intensities of both Co–O and Co–Co(Fe) in the used
CoFe-LDH are lower than those of fresh CoFe-LDH and c-
CoFeOOH in the spectra of wavelet transformed EXAFS (ESI,
Fig. S6†), owing to its amorphous structure. Moreover, the
EXAFS analysis show that the as-formed oxyhydroxide structure
was stable aer four cycle reactions (Fig. 7f). We can conclude
that the starting CoFe-LDH/CC material evolved into the cor-
responding amorphous oxyhydroxide during electrooxidation
as the real active species. In addition, SCNꢀ, oen adopted to
poison metal sites,27 was used as an indicator for active sites. As
shown in Fig. S7,† a remarkable decrease of 2a yield was
observed by adding SCNꢀ, thus indicating that the metals in
CoFe-LDH contribute to the catalytic activity. As an electro-
chemical reaction takes place at the interface between the
catalyst and electrolyte, we evaluated the stability of CoFe-LDH
for 7 h using chronoamperometry (CA) at 2.0 V versus the Ag/
AgCl electrode (Fig. S8†). A current densityof ꢂ11 mA cmꢀ2
was retained, indicating the stability of the catalyst over long
reaction times.
To gain an insight into the active species in this catalytic
system, the structural evolution of the CoFe-LDH/CC anode was
in-depth investigated using XRD, TEM, SAED, X-ray photoelec-
tron spectroscopy (XPS), and X-ray absorption ne structure
spectroscopy (XAFS).23 As shown in Fig. 7a, the XRD pattern of
On the basis of our results and the previous literature,6d,28
a reaction mechanism of this catalytic system is proposed in
Fig. 8. In the neutral aqueous-organic electrolyte (MeCN as the
organic phase), water molecules undergo Volmer and Heyrovsky
Fig. 7 (a) XRD patterns, (b) Co 2p XPS spectra, and (c) O1s XPS spectra
of fresh and 1st recycled CoFe-LDH/CC. (d) Normalized Co K-edge
XANES spectra and (e and f) Fourier-transform Co K-edge EXAFS
spectra of reference samples, fresh and used CoFe-LDH/CC.
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sulindac (63%) and amino acid methionine (89%). Moreover,
CoFe-LDH/CC was used in the gram-scale synthesis of diphenyl
sulfoxide with 83% yield (up to 1.63 g) and amino acid methi-
onine with 86% yield (up to 1.79 g). In addition, the CoFe-LDH/
CC electrocatalyst could be reused, maintaining the yield for
more than ten cycles. Mechanistic studies indicate that the
reaction pathway proceeds through a radical process, and the in
situ formed CoFe-oxyhydroxide may serve as the active species
for the sulde oxidation. The efficiency of this reaction system
may pave the way for the electrochemical synthesis of valuable
organic molecules by using heterogeneous catalysts without
external oxidants under ambient conditions.
Conflicts of interest
Fig. 8 Proposed reaction mechanism.
There are no conicts to declare.
reactions to produce H2 and hydroxide ions (OHꢀ) over the Pt
cathode under work potential bias. Meanwhile, the starting
CoFe-LDH was oxidized and evolved into amorphous CoFe-
oxyhydroxide via the deprotonation process with the assis-
tance of OHꢀ very fast in a few minutes (<5 minutes), and it may
complete as soon as the reaction starts, as demonstrated by the
anode color change and XRD and XPS results (Fig. S9†),
ensuring the maintenance of its activity during the reaction
time course. We synthesized reference materials (Co(OH)2/CC
and Fe(OH)3/CC) and compared their catalytic activity with that
of CoFe-LDH/CC. As shown in Fig. S10a,† Co(OH)2/CC showed
much higher activity than Fe(OH)3/CC, suggesting that the
Co(OH)2-derived structure is the main active phase. The CoFe-
LDH/CC exhibits the highest performance among the three
catalysts, indicating that a synergistic effect exists between Co
and Fe in this reaction. The Co2p3/2 XPS spectra of the used
CoFe-LDH/CC, the derived CoFeOOH structure, exhibits
a 0.43 eV shi to higher binding energy compared with that of
the used Co(OH)2/CC (Fig. S10b†), indicating the modulation of
the local electronic structure of Co cations in the presence of Fe
species. Because of the modulated electronic structure, the
redox behavior of Co cations could probably be manipulated by
Fe incorporation, leading to the active phases in sulde elec-
trooxidation with higher activity, which is in agreement with
recent reports.29 This amorphous oxyhydroxide might act as the
active phase for selective sulde oxidation using water as the
oxygen source.
Acknowledgements
The work was nancially supported by the National Natural
Science Foundation of China (21978147, 21935001), and the
China Postdoctoral Science Foundation (2019M660421). We
greatly thank Profs. Duan Xue and Yadong Li for long-term
support.
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