Understanding the Roles of Electrogenerated Co3+ and Co4+ in Selectivity-Tuned 5-Hydroxymethylfurfural Oxidation

Article abstract of DOI:10.1002/anie.202108955

The Co-based electrocatalyst is among the most promising candidates for electrochemical oxidation of 5-hydroxymethylfurfural (HMF). However, the intrinsic active sites and detailed mechanism of this catalyst remains unclear. We combine experimental evidence and a theoretical study to show that electrogenerated Co³⁺ and Co⁴⁺ species act as chemical oxidants but with distinct roles in selective HMF oxidation. It is found that Co³⁺ is only capable of oxidizing formyl group to produce carboxylate while Co⁴⁺ is required for the initial oxidation of hydroxyl group with significantly faster kinetics. As a result, the product distribution shows explicit dependence on the Co oxidation states and selective production of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and 2,5-furandicarboxylic acid (FDCA) are achieved by tuning the applied potential. This work offers essential mechanistic insight on Co-catalyzed organic oxidation reactions and might guide the design of more efficient electrocatalysts.

Full text of DOI:10.1002/anie.202108955

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International Edition: doi.org/10.1002/anie.202108955
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Electrocatalysis
Hot Paper
Understanding the Roles of Electrogenerated Co3+ and Co4+ in
Selectivity-Tuned 5-Hydroxymethylfurfural Oxidation
Xiaohui Deng, Ge-Yang Xu, Yue-Jiao Zhang, Lei Wang, Jiujun Zhang, Jian-Feng Li, Xian-
Zhu Fu,* and Jing-Li Luo*
Abstract: The Co-based electrocatalyst is among the most
promising candidates for electrochemical oxidation of
5-hydroxymethylfurfural (HMF). However, the intrinsic active
sites and detailed mechanism of this catalyst remains unclear.
We combine experimental evidence and a theoretical study to
show that electrogenerated Co3+ and Co4+ species act as
chemical oxidants but with distinct roles in selective HMF
oxidation. It is found that Co3+ is only capable of oxidizing
formyl group to produce carboxylate while Co4+ is required for
the initial oxidation of hydroxyl group with significantly faster
kinetics. As a result, the product distribution shows explicit
dependence on the Co oxidation states and selective production
of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and
2,5-furandicarboxylic acid (FDCA) are achieved by tuning the
applied potential. This work offers essential mechanistic insight
on Co-catalyzed organic oxidation reactions and might guide
the design of more efficient electrocatalysts.
(FFCA) and 2,5-furandicarboxylic acid (FDCA).[2] Compar-
ing with conventional oxidizing methods which operate under
high temperature in the presence of noble metal catalysts and
O2 atmosphere, electrochemical oxidation of HMF catalyzed
by transition metals offers advantages as the upgrading
transformation can be performed under benign conditions
without the use of oxidants.[3] Moreover, it shows the promises
to replace the kinetically sluggish oxygen evolution reaction
and improve the energy conversion efficiency in hydrogen
production from electrochemical water splitting.[1a,c]
So far, Ni and Co-based electrocatalysts have been
intensively studied for electrochemical HMF oxidation. The
close-to-unity Faradaic efficiency towards FDCA, a monomer
towards synthesis of renewable polymer materials, has been
achieved on Ni3S2, Ni2P, CoP and NiFe-LDH, etc.[3a,4]
Mechanistically, it is widely accepted that Ni-based electro-
catalysts catalyze the HMF oxidation following the “Electro-
chemical–Chemical” (E–C) oxidation pathway, in which Ni2+/
Ni3+ redox couple is participating in the sequential oxidation
of HMF to produce HMFCA, FFCA and eventually,
FDCA.[5] The overall process involves the electrochemical
dehydrogenation of the Ni(OH)2 catalyst and spontaneous
nucleophile dehydrogenation of the substrates being oxidi-
zed.[5d] Although an additional pathway involving the hydride
transfer to the Ni4+ site in NiOOH is proposed for the
oxidation of alcohol very recently, its involvement in furfural
oxidation is limited according to experimental data.[6] Never-
theless, due to the reason that HMF and the less-oxidized
intermediates (HMFCA/FFCA) can be further oxidized
under the same potential, the intrinsic selectivity of HMF
oxidation is essentially not being controlled. Consequently,
the selective production of less-oxidized HMFCA or FFCA
has not been realized up to now. On the other hand, the
underlying mechanism of Co-catalyzed HMF oxidation
remains unclear. Although the Co2+/Co3+ was claimed in
literature to exhibit negligible activity towards HMF oxida-
tion while Co4+ might be involved at higher applied potential,
no experimental evidence or detailed mechanistic insights
were provided.[7] In a recent study, Co3+ in the octahedral sites
of Co3O4 was reported to be decisive in selective HMF
oxidation.[8] However, the dynamic variation of Co valence
states during the catalytic turnover are not considered and the
underlying reason for FDCA production with high selectivity
is not adequately revealed. Therefore, research efforts are in
urgent need to answer the fundamental questions such as the
active species and the oxidation mechanism in Co-catalyzed
electrochemical HMF upgrading.
Introduction
As a promising approach to produce value-added chem-
icals, valorization of biomass-derived platform molecules has
gained much attention.[1] 5-Hydroxymethylfurfural (HMF),
which is produced from the dehydration of C-6 monosac-
charides, contains formyl (-CHO) and hydroxyl (-OH) groups
that can be oxidized to produce wide range of chemicals
including 5-diformylfuran (DFF), 5-hydroxymethyl-2-furan-
carboxylic acid (HMFCA), 5-formyl-2-furancarboxylic acid
[*] Dr. X. Deng, Prof. L. Wang, Prof. X.-Z. Fu, Prof. J.-L. Luo
Shenzhen Key Laboratory of Polymer Science and Technology
Guangdong Research Center for Interfacial Engineering of Functional
Materials, College of Materials Science and Engineering
Shenzhen University
Shenzhen (China)
E-mail: xz.fu@szu.edu.cn
jingli.luo@ualberta.ca
G.-Y. Xu, Prof. Y.-J. Zhang, Prof. J.-F. Li
College of Materials, State Key Laboratory of Physical Chemistry of
Solid Surfaces, iChEM, College of Chemistry and Chemical Engi-
neering, Fujian Key Laboratory of Advanced Materials, College of
Energy, Xiamen University
Xiamen (China)
Prof. J. Zhang
Institute for Sustainable Energy/College of Sciences
Shanghai University
Shanghai (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202108955.
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Herein, we present a detailed study on this long-debating
subject by choosing electrodeposited CoOxHy as the model
electrocatalyst. We provide solid experimental evidence to
show that the dynamic valence states of Co are determining
factors in selective oxidation of HMF under alkaline con-
ditions, in which electrochemically generated Co3+ acts as
sluggish chemical oxidants towards the oxidation of formyl
group exclusively, while the turnover limiting Co4+ formation
is responsible for the initial and spontaneous dehydrogen-
ation (oxidation) of hydroxyl groups with significantly faster
oxidizing kinetics. Following the distinct role of Co3+ and
Co4+, the oxidation products can be conveniently tunned
between HMFCA and FDCA by adjusting the potential
applied and the first example of selective HMFCA produc-
tion from electrochemical HMF oxidation is demonstrated
(Scheme 1). The distinct oxidizing capabilities of Co3+ and
Co4+ are further verified by computational investigation and
the results show exceedingly good agreement with the
experimental results.
Figure 1. a) Cyclic voltammogram curves of electrodeposited CoOxHy
in 1 M KOH with (red) and without (black) 0.05 M HMF. b) An
enlarged area showing the variation of the Co2+/Co3+ redox couple.
c) The chronoamperometry response of CoOxHy catalyzing HMF oxida-
tion from 1.1 V to 1.4 V (red). Blue curves show corresponding results
obtained without HMF.
OER process exhibits an onset potential of 1.52 V.
When HMF (0.05 M) is added into the electrolyte,
the anodic current quickly increases after ꢀ1.3 V,
suggesting the catalytic activity towards HMF
oxidation. Quantitively, the potential required to
reach 20 mAcmÀ2 is 190 mV less than that re-
quired for OER, demonstrating the promises of
electrochemical HMF oxidation in replacing OER
as anodic reactions towards energy-efficient hy-
drogen production. Notably, the CV curve shows
Scheme 1. Mechanistic illustration of the potential-dependent oxidation of HMF to
produce HMFCA and FDCA, as mediated by electrogenerated Co3+ and Co4+
species.
Results and Discussion
similar Co2+/Co3+ oxidation region with slightly higher peak
current (Figure 1b). Such phenomena is commonly observed
in Ni-catalyzed alcohol oxidation and is due to the reason that
since NiOOH is reduced back by oxidizing alcohols, more
current is generated for the conversion of Ni(OH)2 to
NiOOH.[6,7] However, the anodic current drops afterwards
but again exhibits sharp increase from 1.3 V. The LSV feature
indicates that the HMF oxidation catalyzed by CoOxHy might
follow two distinct pathways depending on the potential
applied. In the backward scan, two features are worth noting.
The first one is the disappearance of Epc’ which is previously
assigned to the reduction of Co4+ to Co3+. This is in stark
contrast to the CV collected under OER conditions and
indicates the absence of high-valent Co4+ in the cathodic scan
when HMF is present. The second feature is the decreased
current of Epc, illustrating the reduced quantity of Co3+. This is
contrary to the relatively higher Co3+ quantity as suggested by
the change of Epa in the anodic scan and such discrepancy
suggests the participation of Co3+ in HMF oxidation similar to
the case of Ni3+.[12]
CoOxHy is electrodeposited on carbon cloth from nitrate
precursor following a well-established protocol.[9] Conven-
tional characterization including scanning electron microsco-
py (SEM), X-ray diffraction (XRD) and X-ray photoelectron
spectroscopy (XPS) were performed (Supporting Informa-
tion, Figure S1). The nanoarray structure as shown in the
SEM images (Figure S1a) is expected to provide abundant
surface-active sites. The XRD pattern (Figure S1b) can be
assigned to that of a-Co(OH)2 (JCPDS 46-0605) with
characteristic reflections at 34.58 (102) and 59.88 (110) 2 theta
values. The high-resolution XPS spectra of Co 2p (Figure S1c)
shows dominant binding energy at 780.7 eV with two other
peaks at 782.6 eV and 786.5 eV, which is in good agreement
with literature values of Co(OH)2.[10]
The electrochemical behavior of CoOxHy is first studied in
1 M KOH. The potentials reported in this study are referred
to reversible hydrogen electrode (RHE) unless differently
stated. As plotted in Figure 1a, the cyclic voltammogram of
pre-conditioned CoOxHy shows two redox couples with the
oxidation peaks at 1.11 V(Epa)/1.47 V(Epa’) and correspond-
ing reduction peaks at 1.08 V(Epc)/1.46 V(Epc’), which can be
assigned to the Co2+/Co3+ and Co3+/Co4+ redox pairs.[11] The
The variation of CV after HMF addition motivates us to
investigate the potential-dependent activity of CoOxHy to-
wards HMF oxidation. To omit the interference of Co
oxidation on the evaluation of authentic anodic current from
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HMF oxidation, a series of designed-chronoamperometry
tests are performed at potentials from 1.1 V to 1.4 V with an
interval of 0.1 V. Each potential is first applied on CoOxHy
electrode in 1 M KOH for sufficient amount of time in order
to reach the steady state of Co species. Afterwards HMF is
added and the anodic current can be unambiguously assigned
to that responsible for HMF oxidation (detailed experimental
protocol shown in Figure S2). As presented in Figure 1c,
current densities of 1.1 and 2.3 mAcmÀ2 are observed at 1.1 V
and 1.2 V, respectively. Since the oxidation of Co2+ to Co3+
initiates at 0.9 V and reaches peak current at 1.11 V, it can
therefore be concluded that CoOOH is capable of oxidizing
HMF. In addition, significant anodic current is observed at
higher potentials ( ꢀ 29 mA @1.4 V). It was previously
claimed in literature that electrogenerated CoOOH might
be able to directly catalyze HMF oxidation under higher
applied potentials.[7] However, no experimental evidence was
provided and the generation and co-presence of Co4+ in this
potential region adds complex to the catalytic system. Be-
sides, the formyl and hydroxyl groups in HMF molecule
exhibit different redox properties, further complicating the
overall scenario.
With the aim to clarify the correlation between Co species
and oxidation of formyl/hydroxyl groups, the electrochemical
oxidation of HMFCA and FFCA as catalyzed by CoOxHy are
studied. The reason for choosing both substrates is that
HMFCA consists of hydroxyl group and carboxyl group
attached to the furan ring while FFCA consists of formyl
group and carboxyl group instead. Due to the chemical
inertness of carboxyl group under electrochemical conditions,
the oxidation behavior of hydroxyl and formyl groups can
therefore be investigated separately (Figure S3). As plotted in
Figure 2, LSV collected in the presence of HMFCA (2a)
shows nearly identical Epa as OER while the current rapidly
increases after 1.3 V, suggesting that Co3+ is not involved in
the oxidation of hydroxyl group. In comparison, the electro-
chemical oxidation of FFCA (2b) exhibits similar feature as
that of HMF, in which the intensity of Epa shows an obvious
increase apart from the onset at 1.3 V. This indicates that
FFCA can be oxidized by the Co3+ species. Chronoamper-
ometry tests are performed to deconvolute the oxidation
currents from the self-oxidation of Co species. As can be
observed from Figure 2c, at applied potentials of 1.1 and
1.2 V, notable catalytic current is only observed in FFCA
oxidation.
The distinct difference of oxidation behavior towards
formyl and hydroxyl groups can then be related to the valence
states of Co. In contrast to the obvious oxidation peak
exhibited by Co2+/Co3+ couple, Co3+/Co4+ shows a broad peak
within a wide potential range as shown in the CV curve under
OER conditions (Figure 1, black curve). X-ray absorption
spectroscopy (XAS) measurements have revealed that Co4+
species emerges with the applied potential approaching 1.40–
1.45 V under alkaline conditions.[11,13] However, it has to be
noted that XAS as a bulk characterization method can only
reveal the averaged oxidation states. Since not all the Co ions
in the electrode are electrochemically active and only a small
fraction of Co can be oxidized to Co4+, it can be deduced that
Co3+/Co4+ transformation likely occurs at potentials lower
Figure 2. Cyclic voltammograms of electrodeposited CoOxHy in a) 1 M
KOH with (blue)/without (black) 0.05 M HMFCA and b) 1 M KOH
with (red)/without (black) 0.05 M FFCA. c) Comparison of the chro-
noamperometry response of CoOxHy catalyzing HMFCA (blue) and
FFCA (red) oxidation at 1.1, 1.2 and 1.3 V.
than these noted in literature. To provide direct experimental
evidence concerning the presence of Co4+, in situ Raman
spectroscopy was performed and the spectra of electro-
deposited CoOxHy on Pt surface at various applied potentials
were collected and presented in Figure 3a. As can be read, the
as-deposited electrode under OCP exhibits a strong peak at
503 cmÀ1, which can be attributed to the Eg vibrational mode
of Co-O in CoOOH.[14] The presence of CoOOH at OCP is
due to the oxidation of some Co2+ into Co3+ by the OHÀ at
unsaturated surface sites.[11,13] With the applied potential
being increased to 1.2 V and higher, a new peak at 474 cmÀ1
can be unambiguously observed and it is assigned to the Eg
vibrational mode of disordered CoO2.[11,15] In addition, the
broad peak emerging at 560 cmÀ1 corresponds to the A1g
mode of CoO2 lattice as previously demonstrated in delithi-
ated LiCoO2.[15] The peak at 503 cmÀ1 under higher potentials
suggests the dominant presence of CoOOH due to the reason
that only part of the Co3+ species can be oxidized to Co4+.
Overall, the spectroscopic results clearly evidence that Co4+
species starts to form at 1.2 V and this agree well with recent
study where CoOx deposited on Au is studied by Raman
spectroscopy.[16] With similar spectra features, the authors
concluded that the onset potential for the formation of CoO2
containing Co4+ is as low as 1.14 V. Based on the observed
potential-dependent activity towards oxidation of HMFCA
and FFCA, it is therefore proposed that the anodic current
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and S6. We note that the constant potential oxidation at 1.1 V
was performed with an initial HMF concentration of 50 mM
to achieve reasonable anodic current. For other cases, the
HMF concentration was fixed at 10 mM. The HPLC traces
during HMF oxidation for each potentials are presented in
Figure S7. As can be expected, the product selectivity shows
explicit dependence on the oxidation states of Co species. As
plotted in Figure 4a, HMFCA are obtained as the dominant
oxidation products at 1.1 V and this can be attributed to the
exclusive oxidation of formyl to carboxyl group as mediated
by Co3+ only. Due to the absence of Co4+ under this potential,
negligible amount of FDCA, FFCA nor DFF are therefore
detected. In contrast, small quantity of FDCA is observed at
1.2 V (Figure 4b) and this agrees well with Figure 2c and
Figure S4 that hydroxyl groups can be oxidized under this
potential despite the low current density. The selectivity is
further shifted towards FDCA under higher applied poten-
tials (Figure 4c–e), which is reasonable as more Co4+ as active
species are generated. At applied potential of 1.4 V, it is
observed that the relative ratio of HMFCA over FDCA
gradually decreases with increasing amount of charge passed.
This evidences that HMFCA is the key oxidative intermedi-
ate towards FDCA formation in Co-mediated HMF oxidation
as the gradual oxidation of HMFCA results in “selective”
production of FDCA. This pathway is in good agreement with
Ni-mediated “electrochemical” HMF oxidation.[5a] The ab-
sence of FFCA might be due to the readily oxidation of
Figure 3. a) In situ Raman spectroscopy of CoOxHy under increasing
potentials from OCP to 1.5 V in 1 M KOH. b) Plots of HMF-oxidizing
current densities against applied potential in chronoamperometry
measurements and c) derived Tafel plots.
towards HMF oxidation at 1.1 V is due to the indirect
oxidation between Co3+ and formyl group, where Co3+ as
generated electrochemically acts as the chemical oxidants.[6,17]
However, Co3+ is not able to mediate the oxidation of
hydroxyl groups as suggested by Figure 2a. At 1.2 V, Co4+ is
present in small quantity as evidenced by in situ Raman
experiments and this coincides well with the low anodic
current towards HMFCA oxidation (Figure S4). With higher
applied potential, increasing quantity of Co4+ is generated
and participating in the fast oxidation of both hydroxyl and
formyl groups. The mechanism claimed herein differs from
the Ni-involved pathway, in which NiOOH shows no selec-
tivity towards the oxidation of formyl and hydroxyl groups.[18]
The Co3+- and Co4+-mediated HMF oxidation also exhibits
different kinetics, which is further evaluated based on the
chronoamperometry measurements conducted at stepped
potentials (Figure 3b). As can be roughly calculated, Co4+
shows significantly lower Tafel slope (130 mVdecÀ1) over that
of Co3+ (467 mVdecÀ1, Figure 3c), indicating much improved
HMF-oxidation kinetics. It is worth noting that the calculated
Tafel slope exhibited by Co4+ agrees well with literature
where Co-based electrocatalysts are utilized to catalyze
FDCA production from electrochemical HMF oxidation
(CoO 139.5 mVdecÀ1, CoSe2 120.6 mVdecÀ1
,
Co3O4
96 mVdecÀ1).[4f,8] Furthermore, the onset potential between
1.25 and 1.3 V vs. RHE as previously reported is consistent
with the Co4+ formation region as demonstrated herein,
indicating the similar nature of active species.
Given the intimate correlation between oxidized func-
tional groups and valence states of Co, we anticipate the
product selectivity of HMF oxidation can be varied by
changing the applied potential. To determine the concentra-
tion of oxidative products, the electrolyte collected during the
reaction was analyzed by high-performance liquid chroma-
tography (HPLC) and the calibration curves for HMF,
HMFCA, FFCA, DFFand FDCA are presented in Figures S5
Figure 4. Concentration of detected products during electrochemical
HMF oxidation conducted at a) 1.1 V, b) 1.2 V, c) 1.3 V, d) 1.4 V, and
e) 1.5 V. f) The Faradaic efficiency towards HMFCA and FDCA produc-
tion at 10 C against applied potential.
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formyl group comparing with hydroxyl group. The determin-
ing role of Co3+ and Co4+ is further illustrated in Figure 4 f
where the Faradaic efficiency towards HMFCA and FDCA
formation is plotted against applied potentials at a fixed
charge of 10 C (The HMFCA/FDCA ratio at other charges
are presented in Figure S8). As presented, more FDCA is
generated at higher applied potentials due to the increasing
quantity of Co4+. Under lower potentials where the presence
of Co3+ are dominant, HMFCA is obtained as the major
oxidation products.
Notably, the HMF oxidation performed at 1.1 V is the first
demonstration of selective HMFCA production from electro-
chemical HMF oxidation. In previously reported HMF
oxidation catalyzed by Ni-based electrocatalyst, the Ni3+/
Ni2+ redox couple shows no selectivity towards the oxidation
of hydroxyl and formyl groups, therefore producing FDCA as
the dominant product.[6,19] To emphasize the oxidation-state
dependent selectivity of electrogenerated Co species, bench-
mark NiOxHy electrode was prepared following the same
electrodeposition protocol and product selectivity towards
HMF oxidation was examined under various applied poten-
tials (1.4 V, 1.45 Vand 1.5 V vs. RHE). As shown in Figure S9,
FDCA was obtained as dominant oxidation product under all
tested potentials and the concentration of HMFCA was
significantly lower. The strategy of utilizing valence-tuned Co
for selective HMF oxidation as demonstrated herein may also
find applications in the production of other fine chemicals
where the preferential oxidation of formyl groups over
hydroxyl groups is desired. However, we note that the
Faradaic efficiency towards HMF oxidation is relatively low
at 1.1 and 1.2 V (Supporting Information, Table S1). This
Figure 5. a) Experimental protocol to determine if Co4+-catalyzed HMF
oxidation follows the “Electrochemical–Chemical” pathway in which
Co4+ acts as the chemical oxidants. b) HPLC chromatogram traces
obtained after 5, 10, and 15 cycles.
I) followed by the spontaneous dehydrogenation of functional
groups being oxidized (step II).[5d,21] This is similar to the case
of Ni despite the fact that the oxidation of hydroxyl groups
can only be mediated by Co4+. Moreover, Co4+ exhibits
significantly faster HMF oxidation kinetics comparing with
Co3+ as previously suggested in Figure 3c. This can then be
rationalized by the different rate-determining steps (RDS).
For Ni3+ or Co3+-mediated HMF oxidation, step II is the RDS
meaning that the reaction between M3+ and HMF is slower
than the generation rate of active M3+ species. This is
straightforwardly reflected in electrochemical measurements
as the LSV of HMFOR would overlap with that of self-
oxidation of transition metals at the beginning stage of
electrochemical HMF oxidation since excess amount of M3+
species is always generated and only part of them are
participating in HMF oxidation.[3a,7] While in the case of
Co4+, the oxidation current quickly increases after the onset
of Co4+ formation, suggesting the consumption rate of Co4+
well surpasses its generation rate and therefore increasing
quantity of M4+ is generated and participating in the HMF
oxidation. Based on the distinctive feature of LSV in Co3+ and
Co4+-mediated range, it can therefore be concluded that the
RDS for Co4+-mediated HMF oxidation is the electrochem-
ical regeneration of catalytically active Co4+ species.[6]
could be due to the sluggish oxidation kinetics of Co3+
-
mediated oxidation of formyl groups especially with low
HMF concentrations. In comparison, the significantly faster
kinetics of Co4+-mediated HMF oxidation leads to higher FE.
In addition, due to the degradation of HMF under strong
alkaline conditions, the carbon balance is not optimal under
lower applied potential since the duration required to
complete the reaction is significantly longer.[5a,20] The rela-
tively low FE at 1.5 V comparing with 1.4 V is due to the
competing oxygen evolution reaction.
After identifying the distinct role of Co3+ and Co4+ in
electrochemical HMF oxidation, we next sought to probe if
Co4+-enabled HMF oxidation follows the similar “Electro-
chemical–Chemical” pathway as Ni3+/4+ and Co3+, in which
the reaction between electro-oxidized transition metal species
and HMF/intermediates are spontaneous.[5d,6] To perform the
experiments, CoOxHy are pre-oxidized at 1.4 V to reach the
steady state and subsequently transferred to 1 M KOH
containing HMF. The electrode is then allowed to rest
without applied potential (Figure 5a). The cycle was repeated
to allow for the determination of oxidized products, if any. As
can be observed from the HPLC traces presented in Fig-
ure 5b, FDCA, HMFCA, FFCA and DFF in small quantity
are observed as oxidation products. This straightforwardly
evidences that the oxidation of hydroxyl and formyl groups as
catalyzed by high-valence Co species both follows the indirect
“Electrochemical–Chemical” pathway, where the overall
reaction consists of initial catalyst dehydrogenation step (step
With the aim to further compare the intrinsic activity of
Co4+ generated at different potentials, we choose to monitor
the depletion of open circuit potential (OCP) of the electro-
des that are pre-oxidized and subsequently immersed in the
electrolyte containing 10 mM of HMF. As can be observed
from Figure S10, the OCP for CoOxHy pre-oxidized at 1.3 V,
1.35 V and 1.4 V undergoes rapid change in the first few
seconds. Afterwards the depletion rate turns moderate.
As has been intensively discussed, the reaction between
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Co3+/Co4+ and HMF molecules are spontaneous, which can
then rationalize the decay of OCP. In addition, the different
decay rates can be related to the distinctive kinetics of Co3+
and Co4+ species. It is reasonable to assign the fast decay to
the consumption of surface-exposed Co4+ species and the
slow decay to the consumption of Co4+ which are not easily
accessible and Co3+ species. To compare the activity of Co4+
generated at each potential, the OCP depletion rates at the
“fast decay region” can therefore be utilized. As can be
observed, the slope (as marked by parallel black solid lines) is
consistent for all cases, indicating the comparable reaction
kinetics of Co4+ species with HMF. This also agrees well with
the electrochemical generation of Co4+ species being the RDS
and the consistent Tafel slope from 1.3 V to 1.45 V.
Table 1: Calculated changes of electronic energies for HMF oxidation to
HMFCA and DFF, as mediated by Co3+/Co2+ and Co4+/Co3+ redox
couples.
Products
Co states (quantity in model)
DE
Initial
Final
[kcalmolÀ1
]
HMFCA
HMFCA
DFF
Co2+(16), Co3+(2)
Co3+(16), Co4+(2)
Co2+(16), Co3+(2)
Co3+(16), Co4+(2)
Co2+(18)
Co3+(18)
Co2+(18)
Co3+(18)
À4.06
À35.73
+17.87
À18.09
DFF
energy in Co3+/Co2+-mediated DFF formation suggests the
spontaneous oxidation of hydroxyl group in this case is highly
unfavorable. In stark contrast, such process is highly favored
on a surface containing Co4+ sites. Overall the computational
results are consistent with our experimental work, suggesting
that Co4+ is necessary for the initial oxidation of hydroxyl
group in HMF. The presented Co4+-mediated mechanism
differs from that proposed by Taitt et al. previously that
surface CoOOH is able to directly catalyze HMF oxidation
without involving the Co redox couple. In their work, the
presence of Co4+ species prior to the onset of water oxidation
was overlooked.[7] The results presented in Figure 5 shows
Co4+ certainly acts as robust mediator in HMF oxidation
although the possibility of direct HMF oxidation catalyzed by
CoOOH following Taittꢀs pathway cannot be ruled out. It is
interesting to note that at low potential where Co4+ species
are not observed (1.1 V), small quantity of FDCA can be
observed (Figure 4a). Such reactivity exhibited by CoOOH
might be due to the minor contribution of afore-mentioned
pathway.
We also attempted to investigate the effect of basicity on
the products distribution since the Cannizzaro-type trans-
formation of formyl group is expected to be hindered under
less-basic conditions. 0.1 M KOH (pH 13) and 0.1 M KH2PO4/
K2HPO4 (pH 7) buffer solution was employed to ensure
sufficient conductivity of the electrolytes. As can be observed
from Figure S11a, the characteristics of LSV curves measured
in 0.1 M KOH show resemblance with that measured in 1 M
KOH while the current densities measured in chronoamper-
ometry experiments are significantly lower under the same
potential. Under neutral conditions, the oxidation peaks of
Co2+/Co3+ and Co3+/Co4+ redox couples can be hardly differ-
entiated and the HMF oxidation exhibits a much larger onset
potential of ꢀ 1.5 V. From the corresponding chronopotenti-
ometry plots (Figure S11b) it can be read that at 1.35 V where
Co2+ is being oxidized to Co3+, negligible activity is shown
towards HMF oxidation. In terms of oxidation products,
HMFCA was obtained as the dominant product at 1.2 V in
0.1 M KOH, indicating the same Co3+-mediated oxidation
pathway as claimed earlier(Figure S12a,b). When the applied
potential was increased to 1.4 V, significant quantity of FFCA
was observed and the yield of FDCA was much lower
comparing with that under 1 M KOH (Figure S12c,d). This
suggests that the oxidation kinetics of formyl group in formed
HMFCA was severely hindered by the lower concentration of
OHÀ. However, one would expect the presence of DFF in
notable amount. We ambiguously attribute the negligible
amount of DFF to its strong adsorption on the active sites that
Computational investigations are then conducted to
further verify the distinct role of electrogenerated Co3+ and
Co4+ species in HMF oxidation theoretically. As shown in the
optimized structures (Figure 6a), for the reaction mediated
Figure 6. a,b) Simulated catalyst structures employed in DFT calcula-
tions (1. Co18O36H36, 2. Co18O36H34, 3. Co18O36H18, 4. Co18O36H16). Cyan
squares indicate the H sites being electrochemically deprotonated
from the corresponding initial states. c) The structural transformation
of HMF under alkaline condition followed by oxidation/deprotonation
to produce HMFCA.
by Co3+/Co2+ redox couple, the Co(OH)2 electrocatalyst is
first modeled with a (001) surface consisting of 2 stoichio-
metric layers with 18 Co atoms, 36 O atoms and 36 H atoms in
total (1). To introduce the Co3+ species, the pristine Co(OH)2
is partially deprotonated and loses 2 H atoms accordingly (2).
For the calculation of Co4+-mediated HMF oxidation,
CoOOH (001) surface as the initial state is constructed (3)
and the deprotonation of which produces Co4+ sites (4)
accordingly. HMF as the reactant may be oxidized to produce
HMFCA or DFF depending on whether the formyl or the
hydroxyl group is being oxidized. It should be noted that the
oxidation of formyl group under alkaline conditions is
expected to be commenced following the Cannizzaro-type
reaction with base and form an activated alcohol motif (I,
Figure 6b).[22] Afterwards the oxidation of I is mediated by
the redox couples.
Table 1 lists the calculated changes of electronic energies
concerning the oxidation of HMF as mediated by Co3+/Co2+
and Co4+/Co3+ respectively. As shown, the spontaneous
oxidation of formyl group to produce HMFCA can be
mediated by both redox couples as decrease of total energy
is obtained.[6,21] On the other hand, the increase of electronic
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its conversion to FFCA or FDCA occurs successively on the
electrode surface without being released to the solution.[23]
Under neutral condition, DFF and FFCA were obtained as
dominant oxidation products while the amount of produced
HMFCA and FDCAwere much lower (Figure S12e,f). As the
Cannizzaro-reaction of formyl group with base is ceased, the
oxidation of hydroxyl group to produce aldehyde is expected
to be dominant and this would explain the much higher
quantity of DFF. However, the presence of FDCA, HMFCA
and FFCA suggests further oxidation of formyl group to form
carboxylates. A plausible mechanism is proposed (Fig-
ure S13) that the oxidation and deprotonation of formyl
group in HMF or DFF produces radical species in the first
step. Then the carbon atom is further oxidized and then
attacked by the O atom in H2O to produce carboxylate. The
results obtained here agree well with Ni-based catalyst, for
which the oxidation of formyl group under neutral conditions
is also evident and FDCA/FFCA are obtained as correspond-
ing products.[24]
the formyl group is rate limiting. The computational results
agree well with our experimental finding that the spontaneous
oxidation of the alcohol group requires Co4+ to occur. This
work showcases the first example of selectivity-tuned HMF
oxidation by varying the oxidation states of an electrocatalyst.
The mechanistic insights obtained may facilitate rational
design of more efficient electrocatalysts for selective and
value-added transformations of organic substrates.
Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (No.21905181, 21975163) and
Shenzhen Science and Technology Program (No.
KQTD20190929173914967). The authors acknowledge the
Instrumental Analysis Center of Shenzhen University (Xili
Campus) for SEM measurements.
Nevertheless, it is acknowledged that the current density
of Co3+-mediated oxidation of formyl group in this study is
restricted by its sluggish kinetics and low applied potential.
Since the over-oxidation to Co4+ at higher potentials can be
hardly avoided, a straightforward strategy to enhance the
activity is to increase the surface active sites through nano-
structure construction and surface engineering. For instance,
vacancy incorporation has been demonstrated as effective
approaches to improve the supercapacitor performance of
cobalt oxide/hydroxide by increasing the density of redox-
active species.[25] For Co4+-mediated HMF oxidation, one can
aim to improve the kinetics of RDS by facilitating the
generation of Co4+ species. Decorating cobalt-based electro-
catalysts with Au is a well-established protocol in the field of
water oxidation reaction as the high electronegativity of Au
can lead to increasing presence of Co4+ sites.[26] Composition
engineering can also be attempted with non-noble metals as
Fe and Cr doping is able to lower the energy barrier of
Co3+/Co4+ transformation.[27]
Conflict of Interest
The authors declare no conflict of interest.
Keywords: 5-hydroxymethylfurfural · cobalt · electrochemistry ·
reaction mechanisms · selective oxidation
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Manuscript received: July 6, 2021
Accepted manuscript online: July 20, 2021
Version of record online: && &&
,
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Research Articles
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Research Articles
Electrocatalysis
X. Deng, G.-Y. Xu, Y.-J. Zhang, L. Wang,
J. Zhang, J.-F. Li, X.-Z. Fu,*
J.-L. Luo*
&&&& — &&&&
Understanding the Roles of
Electrogenerated Co3+ and Co4+ in
Selectivity-Tuned 5-
A detailed mechanism for cobalt-cata-
lyzed electrochemical 5-hydrox-
ymethylfurfural (HMF) oxidation is
revealed. A combined experimental and
theoretical study shows that a Co3+ spe-
cies is capable of oxidizing the formyl
group to produce carboxylate but remains
inert towards oxidation of the hydroxyl
group. In contrast, a Co4+ species is
required for the initial oxidation of the
hydroxyl group in HMF.
Hydroxymethylfurfural Oxidation
Electrocatalysis
X. Deng, G.-Y. Xu, Y.-J. Zhang, L. Wang,
J. Zhang, J.-F. Li, X.-Z. Fu,*
J.-L. Luo*
&&&& — &&&&
Understanding the Roles of
Electrogenerated Co3+ and Co4+ in
Selectivity-Tuned 5-
A detailed mechanism for cobalt-cata-
lyzed electrochemical 5-hydrox-
ymethylfurfural (HMF) oxidation is
revealed. A combined experimental and
theoretical study shows that a Co3+ spe-
cies is capable of oxidizing the formyl
group to produce carboxylate but remains
inert towards oxidation of the hydroxyl
group. In contrast, a Co4+ species is
required for the initial oxidation of the
hydroxyl group in HMF.
Hydroxymethylfurfural Oxidation
&&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Electrocatalysis
X. Deng, G.-Y. Xu, Y.-J. Zhang, L. Wang,
J. Zhang, J.-F. Li, X.-Z. Fu,*
J.-L. Luo*
&&&& — &&&&
Understanding the Roles of
Electrogenerated Co3+ and Co4+ in
Selectivity-Tuned 5-
A detailed mechanism for cobalt-cata-
lyzed electrochemical 5-hydrox-
ymethylfurfural (HMF) oxidation is
revealed. A combined experimental and
theoretical study shows that a Co3+ spe-
cies is capable of oxidizing the formyl
group to produce carboxylate but remains
inert towards oxidation of the hydroxyl
group. In contrast, a Co4+ species is
required for the initial oxidation of the
hydroxyl group in HMF.
Hydroxymethylfurfural Oxidation
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!