Electrochemical biomass valorization on gold-metal oxide nanoscale heterojunctions enables investigation of both catalyst and reaction dynamics with: Operando surface-enhanced raman spectroscopy

Article abstract of DOI:10.1039/d0sc00136h

The electrochemical oxidation of biomass platforms such as 5-hydroxymethylfurfural (HMF) to value-added chemicals is an emerging clean energy technology. However, mechanistic knowledge of this reaction in an electrochemical context is still lacking and operando studies are even more rare. In this work, we utilize core-shell gold-metal oxide nanostructures which enable operando surface-enhanced Raman spectroelectrochemical studies to simultaneously visualize catalyst material transformation and surface reaction intermediates under an applied voltage. As a case study, we show how the transformation of NiOOH from ～1-2 nm amorphous Ni layers facilitates the onset of HMF oxidation to 2,5-furandicarboxylic acid (FDCA), which is attained with 99% faradaic efficiency in 1 M KOH. In contrast to the case in 1 M KOH, NiOOH formation is suppressed, and consequently HMF oxidation is sluggish in 10 mM KOH, even at highly oxidizing potentials. Operando Raman experiments elucidate how surface adsorption and interaction dictates product selectivity and how the surface intermediates evolve with applied potential. We further extend our methodology to investigate NiFe, Co, Fe, and CoFe catalysts and demonstrate that high water oxidation activity is not necessarily correlated with excellent HMF oxidation performance and highlight catalytic factors important for this reaction such as reactant-surface interactions and the catalysts' physical and electronic structure. The insights extracted are expected to pave the way for a deepened understanding of a wide array of electrochemical systems such as for organic transformations and CO₂ fixation.

Full text of DOI:10.1039/d0sc00136h

View Article Online
View Journal
Chemical
Science
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: N. Heidary and N.
Kornienko, Chem. Sci., 2020, DOI: 10.1039/D0SC00136H.
Volume
Number
9
1
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
7
January 2018
Pages 1-268
Chemical
Science
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
rsc.li/chemical-science
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2041-6539
EDGE ARTICLE
Xinjing Tang et al.
Caged circular siRNAs for photomodulation of gene
expression in cells and mice
rsc.li/chemical-science

Page 1 of 15
Chemical Science
View Article Online
DOI: 10.1039/D0SC00136H
Electrochemical Biomass Valorization on Gold-Metal Oxide Nanoscale
Heterojunctions Enables Investigation of both Catalyst and Reaction
Dynamics with Operando Surface-Enhanced Raman Spectroscopy
1
1*
Nina Heidary and Nikolay Kornienko
1Department of Chemistry, Université de Montréal, Roger-Gaudry Building, Montreal, Quebec
H3C 3J7, Canada.
*
Email: Nikolay.kornienko@umontreal.ca
Abstract:
The electrochemical oxidation of biomass platforms such as 5-hydroxymethylfurfural (HMF) to
value-added chemicals is an emerging clean energy technology. However, mechanistic knowledge
of this reaction in an electrochemical context is still lacking and operando studies are even more
rare. In this work, we utilize core-shell gold-metal oxide nanostructures which enable operando
surface-enhanced Raman spectroelectrochemical studies to simultaneously visualize catalyst
material transformation and surface reaction intermediates under an applied voltage. As a case
study, we show how the transformation of NiOOH from ~1-2 nm amorphous Ni layers facilitates
the onset of HMF oxidation to 2,5-furandicarboxylic acid (FDCA), which is attained with 99%
Faradaic efficiency in 1M KOH. In contrast to the case in 1M KOH, NiOOH formation is
suppressed, and consequently HMF oxidation is sluggish 10 mM KOH, even at high potentials.
Operando Raman experiments elucidate how surface adsorption and interaction dictates product
selectivity and how the surface intermediates evolve with applied potential. We further extend our
methodology to investigate NiFe, Co, Fe, and CoFe catalysts and demonstrate that high water
oxidation activity is not necessarily correlated with excellent HMF oxidation performance and
highlight catalytic factors important for this reaction such as reactant-surface interactions and
catalysts’ physical and electronic structure. The insights extracted are expected to pave the way
for a deepened understanding of a wide array of electrochemical systems such as organic
transformations and CO fixation.
2

Chemical Science
Page 2 of 15
View Article Online
DOI: 10.1039/D0SC00136H
Introduction:
As effects of fossil fuel consumption are increasingly prevalent, renewable energy
technologies and green chemistry are coming more into focus. Of particular interest is the use of
renewable-derived electricity (e.g. from solar, wind, hydro…) to generate fuels and value-added
1
chemicals in a sustainable fashion. Aside from the relatively well-established water electrolysis
and CO reduction reactions, the electrochemical conversion of increasingly abundant biomass
2
platforms into fuels and components of polymers, fertilizers, organic electronics and
2
pharmaceuticals is rather nascent yet growing rapidly. These reactions carry high economic
potential because the products that can be generated have a much higher value than the simple
products generated from water electrolysis and CO
2
reduction (e.g. H , CO, HCOOH…). In
2
addition, biomass oxidation can replace water oxidation in the aforementioned systems to
3
, 4
simultaneously co-generate value-added products on the anodic and cathodic sides.
The oxidation of 5-hydroxymethylfurfural (HMF), an acid-catalyzed dehydration product
of biomass-derived hexose and pentose sugars, to value-added chemicals is one of the most
prominent reactions studied in the field (Fig. 1a).5 A series of different products can be obtained
through sequential electrochemical oxidation of the alcohol and aldehyde groups of HMF, with
-7
2
,5-furandicarboxylic acid (FDCA) being sought after as a substitute for fossil-based terephthalic
acid (typically generated from naptha in an energy demanding fashion) in the fabrication of
polyesters, amongst other polymers. While the conversion of HMF to FDCA has long been
8
established through thermal catalysis, its generation via electrochemical oxidation is much less
known. Both metals and metal oxides have been investigated, with nickel oxides and nickel-
containing materials generally being reported to the most active material in terms of selectivity
9
-18
and efficiency. Despite its potential, mechanistic studies of electrochemical HMF oxidation on
metal oxides to understand surface dynamics and key factors that render certain materials highly
active, especially through spectroscopic means, are few and there are many knowledge gaps left
to fill. In comparison, operando spectroscopy on reactions such as water electrolysis and CO
reduction have greatly enriched the field’s knowledge of these systems and consequently,
2
1
9, 20
accelerated progress.
Recent infrared spectroscopic studies of nickel borate catalysts elucidated that HMF
1
1
oxidation proceeded via the 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) intermediate.
Other infrared spectroscopic studies also hinted at potential- and scan direction-dependent
21
reactivity of glycerol oxidation on Pd-Bi catalysts. Sum frequency generation spectroscopy
similarly revealed that HMF is first oxidized to HMFCA prior to 5-formyl-2-furancarboxylic acid
2
2
(FFCA) and FDCA formation.
Operando surface-enhanced Raman spectroscopic (SERS) studies probed HMF
conversion to 2,5-diformylfuran (DFF) on a Au surface and provided evidence for an oxygen-
1
6
bound surface intermediate. SERS is enabled by enhanced local electromagnetic fields at the
surface of plasmonic materials with a high amount of free electron density (e.g. Au and Ag) and
from charge transfer effects between substrate and chemisorbed molecule, all of which may
increase the resultant Raman intensity by orders of magnitude.2
3, 24
As such, this technique is
uniquely suited for probing heterogeneous catalysis because the signal enhancement decays

Page 3 of 15
Chemical Science
View Article Online
DOI: 10.1039/D0SC00136H
rapidly with distance away from the substrate (~2nm), rendering SERS an inherently surface-
sensitive probe.
Though Au is an excellent model system and SERS substrate, transition metal oxides are
more active for this electrochemical HMF oxidation in alkaline solutions. In this work, we thus set
out to probe HMF oxidation on nickel oxide surfaces with operando SERS. To utilize the SERS
effect on a non-plasmonic substrate, we designed an electrocatalytic system comprised of
electrochemically fabricated plasmonic gold dendrites with a thin (~1-2nm) shell of nickel. This
would then enable the simultaneous detection of catalyst material dynamics and surface reaction
pathways (Fig. 1b). The insights derived through the investigation of this system, and other
transition metal analogues, shed crucial mechanistic information regarding the influence of surface
chemistry on HMF oxidation and how the interplay of HMF and water oxidation dictates
electrocatalyst efficacy.
Figure 1: Possible reaction products of electrochemical oxidation of HMF (a). The designed
system utilizes a plasmonic substrate for SERS enhancement to view catalyst material
transformations as well as the reaction pathway on the surface (b).
Results and Discussion:
As a well-established SERS substrate that can be readily deposited on conductive
substrates, gold dendrites were grown on carbon paper through reductive electrodeposition from
an aqueous gold solution following previously established literature recipes.2
5, 26
Following the
growth of the gold, a sacrificial Zn layer was electrodeposited, which was subsequently converted

Chemical Science
Page 4 of 15
View Article Online
DOI: 10.1039/D0SC00136H
to Ni (and later other metals) through a galvanic exchange reaction by soaking the electrode in a
nickel acetate solution (Fig. 2a). Electron microscopy was then used to characterize the
morphology and composition of the resultant electrodes. Scanning electron microscopy (SEM)
analysis illustrated that rough, dendritic structures were grown on the carbon fibers of the electrode
(
2
Fig. 2b). The Au, as investigated with transmission electron microscopy (TEM), featured a ~1-
nm coating of amorphous nickel (Fig. 2c and S1). Energy-dispersive x-ray spectroscopy (EDS)
showed only peaks arising from Au, Ni, and Cu (with the Cu coming from the Cu TEM grid used)
Fig. 2d). The conformal nature of the Ni coating was further demonstrated with an EDS linescan
(
and elemental map (Fig. 2e-g).
Figure 2: The AuNi catalysts are prepared through a facile electrodeposition and galvanic
exchange method (a). The resultant materials are high surface area Au dendrites (b) with a 1-2 nm
shell of amorphous Ni (c). EDS analysis only shows peaks stemming from Au, Ni and Cu, the
latter originating from the Cu TEM grid employed in this work (d). A catalyst tip (e) was further
evaluated with an EDS linescan (f) and elemental map (g) to show the conformal nature of the Ni
layer.
We next moved to study the electrochemical performance of the AuNi catalysts, using Au
as a comparison, in 10mM KOH and 1M KOH electrolytes. Generally, the reaction kinetics are
faster in highly alkaline solutions, but 10mM KOH was also used as HMF is stable for longer
durations in this environment. When measuring the open circuit potential and injecting 10mM

Page 5 of 15
Chemical Science
View Article Online
DOI: 10.1039/D0SC00136H
HMF into the electrolyte, the open circuit potential for the AuNi rapidly drops from ~0 to -0.7V
vs. Ag/AgCl (and to -0.2V for Au only), indicating a substantial change of the electrode potential
equilibrium (Fig. 3a). This can be caused by the exchange of adsorbates and ions within the
Helmholtz layer possibly hinting to a strong HFM surface adsorption process.
Cyclic voltammetry (CV) measurements show only the redox of the Ni and water oxidation
in 10mM KOH and a broad wave centered at 0V when 10mM HMF is added to the solution (Fig.
3
b). A decrease of current upon progressing to higher potentials may stem HMF fully covering the
-
surface and blocking the OH adsorption, which may act as a reactant in the catalytic process. This
is also evident by the sharp jump in current in the reverse scan at 0V as some of the HMF desorbs
allowing the remainder to react with OH .
-
27
Interestingly, the redox peaks tentatively attributed to NiOOH formation at 0.8V are also
suppressed in the presence of HMF, potentially from its high degree of surface binding (Fig. 3b).
At potentials from 0 to 0.5V, the current typically decayed to 0 within tens of minutes in a
chronoamperometric scan and no reaction products were detected. At higher potentials of 1.2V
DFF is detected in low yields after extended electrolysis (Fig. S6). This indicates that the current
below 0.5V is due to HMF adsorption but not significant catalytic turnover. For reference, the bare
Au shows a Au/AuOx redox peak centered at 0.3V, a catalytic wave at 0.8V, and the same current
jump at 0V as with the Ni (Fig. 3c).
In 1M KOH even larger open circuit potential drops are evident with HMF addition (Fig.
3
d). The Ni shows enhanced currents from -0.8 to 0.2V (Fig. 3e). At 0.4V, the NiOOH formation
is concurrent with a large catalytic wave and the reverse reduction of NiOOH is still seen in the
presence of HMF. At the lower potentials, HMFCA is the most abundant product (Fig. S7),
indicating that the oxidation of the aldehyde group preferentially proceeds first. This is in contrast
to the case in 10mM KOH, where the oxidation of the alcohol group occurred, and a much higher
-
voltage was necessary to generate reaction products. OH availability, both from the electrolyte
and/or on the catalyst surface, may well be the main factor behind this diverging behavior as an
oxygen atom is consumed in the aldehyde to carboxylic acid transformation step. The CVs of Au
in these conditions are also given for reference (Fig. 3f). In contrast to minimal product formation
at lower potentials or at less-alkaline solutions, the reaction with AuNi at 0.4V maintained high
current densities for several hours, until the HMF reactant was used up (Fig. 3g). The reaction
product was mainly FDCA, which, at least in part, proceeded via an HMFCA intermediate as this
species was detected as well (Fig. 3h, i). However, the DFF pathway cannot be completely
excluded just because DFF was not detected in the solution. The Faradaic efficiency for FDCA
formation, calculated from the amount of charge passed and quantity of product formed, was
approximately 99%, as this reaction could be driven on a NiOOH surface prior to the onset of
water oxidation.

Chemical Science
Page 6 of 15
View Article Online
DOI: 10.1039/D0SC00136H
Figure 3: Electrochemical analysis. In 10 mM KOH, strong affinity of HMF to the Ni surface is
evidenced by the change in open circuit potential upon injection of 10 mM HMF into the
electrolyte solution (a). Enhanced current is noted from -0.6 to 0.5V AuNi (b) and from -0.2V to
1
.0V for Au (c). Larger shifts in the open circuit potential are seen in 1M KOH (d) and a current
enhancement is especially pronounced at 0.4V for AuNi (e) and at 0V for Au (f). A
chronoamperometric scan at 0.4V shows a decreasing current as HMF is depleted (g) and
converted into FDCA (h, i).
To obtain a further level of mechanistic insights into the HMF oxidation process on the Ni
surface, we turned to operando SERS, executed with the use of a custom-built
spectroelectrochemical cell and a water-immersion objective (Fig. 4a). The solution spectra of
HMF with the main bands assigned is presented in Fig. 4b. The bands are assigned on the basis of
2
8
established experimental and theoretical studies. In general, surface-enhanced Raman bands shift
upon changes in the molecule’s electron density as, for example, through bonding or charge
transfer processes, and their relative intensities can vary with distance and molecular orientation
2
3, 29
to the surface.
The enhanced intensity of a particular vibrational mode declines rapidly with
distance from the surface and to an extent with an angle-dependent deviation from the direction of
the electric field oscillation. In 10mM KOH, the formation of NiOOH is readily seen beginning at
0
.6V, evidenced by the characteristic marker bands for the Ni-O vibrational modes at 490 and 561

Page 7 of 15
Chemical Science
View Article Online
DOI: 10.1039/D0SC00136H
-1
30
cm of similar intensities (Fig. 4c). This matches the redox wave observed in the CV in Fig. 3b.
However, NiOOH formation is largely suppressed when 10mM HMF is present possible due to
surface HMF binding (Fig. 4d). At the high-frequency region, both, symmetric and asymmetric
-1
-1
-1
-1
C=C bands at 1510 cm and 1570 cm , respectively, are red-shifted by 12 cm and 8 cm relative
to the solution spectra. The symmetric C=C stretching mode of the ring is most pronounced and
-1
-1
broadened. The C-O band of the alcohol group is also red-shifted by 8 cm to 1022 cm (Fig S11).
Due to an HMF adsorption process, the electronic charge redistributes within the molecule, band
position and relative intensity are affected. Hence, this indicates an adsorption motif of the HMF
in which its electron density is reduced by the interaction with the Ni surface through the furan
ring and the alcohol group. Furthermore, the shift to lower frequency may indicate electron density
being transferred from HMF to the Ni surface from the alcohol group in the binding process. The
aldehyde group, however, does not seem to be affected to as significant of an extent. In solution,
3
1
it is commonly detected as a doublet band representing two conformers. Its invariant band
-1
positions centered at 1664 cm , suggests that the carbonyl group is not significantly involved in
3
2
-1
the binding to the Ni surface. The red-shift of 8 cm also suggests that the alcohol group does
not undergo a change in protonation as much larger shifts would be expected if that were the case.
-1
As the potential is poised more positive to 0.4V, the 1022 and 1510 cm bands disappear and a
-
1
-1
new band at 1577 cm emerges, which blue-shifts to 1582 cm beginning at 1.0V, when DFF
-1
formation begins to occur. This band appears as the alcohol 1022 cm band disappears, indicating
that this is a signature of the intermediate en route to DFF, pointing the surface transformation of
the alcohol group to the aldehyde. At this potential, the carbonyl band also blue-shifts from 1664
-1
to 1666 cm .
In 1M KOH, the NiOOH readily forms at 0.4V both with, and without HMF in the solution
-
(
Fig. 4f, g). A higher OH concentration drives the NiOOH formation here, even in the presence
of bound HMF. HMF may also be bound in a different fashion in 1M KOH. However, the ratio of
the two bands is different in the presence of HMF on an identically prepared electrode, indicating
that the physical structure of the NiOOH, such as through the presence of strain or interlayer
3
0, 33, 34
spacing and intercalated, species is altered in these conditions.
At open circuit conditions, a
-1
-1
strong band 1667 cm is seen (Fig. 4h). This vibrational mode at 1667 cm corresponding to the
aldehyde group is very intense and its blue-shift relative to that of the solution spectrum hints at
an interaction between this group and HMF surface or even the formation of an intermediate upon
HMF interaction (possibly in the HMFCA reaction pathway) with the Ni surface. In contrast, the
C-O mode of the alcohol group is found at an identical band position (1027 cm ) relative to the
solution spectrum, pointing to minimal interaction between the alcohol and Ni surface (Fig. S12).
Note, spectral features in the region between 1500-1650 cm are not assigned here, due to strong
overlap of broad features that stem from the underlying carbon support that do not allow further
deconvolution. At potentials >0.3V, the interfering features decrease due to the oxidation of the
carbon surface. As the potential is applied at 0.4V, a new band at 1513 cm emerges. At same
potentials, this band is accompanied by the formation of NiOOH and the disappearance of the band
-1
-1
-1
-1
at 1027 cm representing the alcohol group due to its oxidations. Thus, this band is assigned to a
C=C ring stretching mode of a surface intermediate en route to FDCA that is likely now interacting
with the NiOOH surface via its alcohol group, or at least has this group in close enough proximity

Chemical Science
Page 8 of 15
View Article Online
DOI: 10.1039/D0SC00136H
to be oxidized. For reference, the Raman spectra of FFCA and FDCA in 1M KOH feature strong
-1
bands at 1515 and 1513 cm , respectively (Fig. S10).
Figure 4: Operando SERS. A custom-built reaction cell and water immersion objective were
employed (a). A spectrum of a 500mM aqueous solution of HMF is illustrated in (b) for reference.
In 10mM KOH, NiOOH readily forms beginning at 0.6V (c) but when 10mM HMF is added to
the same solution, its formation is largely suppressed, shown in the low frequency region (d). The
high frequency region illustrates the HMF adsorption and reaction pathway on the surface en route
to DFF (e). In 1M KOH, NiOOH for both without HMF (f) and with HMF (g) present, albeit with
an altered structure. The high frequency spectra portray the surface intermediates as HMF is
converted to HMFCA and to FDCA at potentials of 0.4V and above (h).

Page 9 of 15
Chemical Science
View Article Online
DOI: 10.1039/D0SC00136H
We finally proceeded to extend our investigation to a series of other metal-oxides with
varying water oxidation behavior to see if it correlates with activity for HMF oxidation as well and
to see if HMF oxidation exhibits the same mechanism and kinetics on other transition metal oxide
surfaces. We synthesized Au with Co, CoFe, NiFe and Fe shells through the same procedure as
that for AuNi synthesis, except the galvanic exchange was executed in the different solutions.
Upon HMF addition, the Co exhibited increased current densities beginning at -0.8V and a small
peak at 0.25V (Fig. 5a). The AuCo catalyst produced both HMF and FDCA as detectable products
(Fig. S13). Interestingly, CoFe and NiFe, exhibited the best water oxidation performance but
qualitatively less of a propensity for HMF oxidation, judging from the relatively smaller changes
in the CV upon HMF addition. Though the exact active site of these materials (e.g. Fe or Co/Ni)
and factors affecting their water oxidation performance is still under debate, high water oxidation
performance generally stems from a smooth energy landscape for all steps of the reaction pathway.
As HMF oxidation proceeds through an alternate mechanism, featuring HMF-derived
intermediates, it is not surprising that the criteria for efficient HMF oxidation would be different.
This is in contrast to a study using NiFe layered double hydroxide materials for HMF oxidation,
9
which were found to be highly active for FDCA generation. A potential explanation is that the
active structure of our material is under an applied bias may not be the same and thus, the reactivity
also differs. Fe, on the other hand, featured enhanced currents mostly at potentials more negative
than 0.2V.
Operando SERS measurements on AuCo in 1M KOH both with and without HMF showed
that an amorphous CoOx layer forms on the surface, as opposed to crystalline phases such as Co
3
O
4
-1
or CoOOH, evidenced by a broad spectral feature that is pronounced at 616 cm that interconverts
-1
to 578 cm under applied potential (Fig. 5e). This behavior is similar to that of few-layer Co
3
5
deposited on gold substrates which red-shift as the Co becomes progressively more oxidized.
Like that seen above for AuNi, the AuCo surface oxide is slightly altered in the presence of HMF,
-1
-1
evidenced from a smaller potential-dependent shift of the CoOx band at ~600 cm (88 cm /V
-1
without HMF vs. 63 cm /V with HMF).
The high frequency region of the shows several potential-dependent changes. At open
-1
circuit conditions, peaks at 1027, 1157, 1506, 1533, 1580, 1609, and 1664 cm are visible (Fig.
-1
5
F and S14). Several bands in the 1500-1600 cm region where C=C and C=O modes are typically
found are present in the spectrum at open circuit that are not in the solution spectrum. As in the
-1
case with Ni, the C-O band at 1027 cm is at the same position as that of the HMF solution,
pointing to a probable lack of interaction of the alcohol group and the surface. At 0V, bands at
-1
-1
1
235, and 1296 cm become visible, and the band at 1609 cm grows in intensity. This is coupled
with a peak in the CV as illustrated in Fig. 5a. By 0.3V, this band decreases in intensity and the
-1
spectrum is dominated by the bands at 1335 and 1580 cm . The 1580 band again is likely a C=O
band from a surface bound intermediate in which the C-O bond order is between 1 and 2. At 0.6V,
-1
all bands generally decrease in intensity and mainly, bands at 1440, 1518, 1580 and 1664 cm are
visible. As the primary reaction with CoOx is the oxidation of HMF to HMFCA, these bands likely
correspond to an intermediate in this reaction pathway. The broadening of the C=O band is
-1
indicative of this group interacting with the CoOx surface. The 1518 and 1664 cm bands are

Chemical Science
Page 10 of 15
View Article Online
DOI: 10.1039/D0SC00136H
similar to that spectrum of the AuNi under the same conditions, through the 1580 and 1440 cm-1
bands may indicate a separate intermediate.
Figure 5: Extension to other transition metal thin films. Co (a), CoFe (b), NiFe (c), and Fe (d)
catalysts were screened for their activity towards HMF oxidation. A closer look at AuCo catalysts
with operando Raman spectroscopy in the low frequency region reveals the formation of an
35
amorphous CoO surface under oxidizing conditions (e), and the high-frequency region gives
x
insight on the nature of the surface intermediates (f).
Putting together the sum of the data, several important insights can be obtained regarding
the factors affecting HMF oxidation selectivity and kinetics. A highly alkaline environment both
dictates selectivity and rate of this reaction. In 10mM KOH, DFF is the primary product obtained.
A similar behavior was seen on bare Au surfaces in 10mM KOH. The propensity of oxidizing the
-
alcohol group rather the aldehyde may stem from a relatively lower OH concentration necessary
to oxidize the aldehyde group to the carboxylic acid and from the preferential interaction of the
HMF alcohol group with the catalyst surface (Fig. 6a). Furthermore, the presumable high degree
of HMF surface binding to the Ni surface under these conditions does not allow for the active
NiOOH species to form in substantial quantities, as evidenced by CV and operando SERS
measurements. While it is certainly possible that NiOOH does form as a metastable active species
and is rapidly reduced as it oxidizes HMF, the lack of product accumulation suggests that this is
not highly prevalent in these conditions.
At highly alkaline conditions of 1M KOH, both HMFCA and, to a lesser degree, FDCA,
are generated even prior to NiOOH formation (Fig. 6b). This means that the Ni surface
(
presumably NiOH ) prefers to react with the aldehyde group but as HMFCA is built up, reactions
2
with the alcohol group are still do not occur. This is also evidenced in the Raman spectrum that
hints at an interaction between the aldehyde group and Ni surface. In contrast to the case in 10mM
KOH, NiOOH formation is not suppressed in 1M KOH and once NiOOH forms, FDCA generation

Page 11 of 15
Chemical Science
View Article Online
DOI: 10.1039/D0SC00136H
greatly accelerates. The NiOOH active phase can therefore readily oxidize both alcohol and
aldehyde groups of HMF.
Other transition metals were tested for this reaction as well, and Ni was still the most active
species in our conditions. NiFe and CoFe exhibited the highest activity for water oxidation but
appeared to be rather poor HMF oxidation catalysts. Co also oxidized HMF to HMFCA, though it
did not form the CoOOH phase and remained as a thin, amorphous CoOx layer (Fig. 6c). As such,
the HMFCA was built up over time with only minor FDCA amounts forming at 0.25V, and
substantial FDCA was only generated after prolonged electrolysis (24 hr) at 0.4V (Fig. S15) which
indicates that the oxidation of the alcohol group is also sluggish with this material. However,
previous investigations of CoOOH (though in 0.1M KOH) also demonstrated that FDCA was only
10
formed in limited amounts. Similarities in the Raman spectrum indicate a shared intermediate,
but in contrast to the case with NiOOH, HMFCA is seen building up on the surface to a greater
extent.
Figure 6: Reaction pathways of HMF oxidation on Ni in 10mM KOH (a), 1M KOH (b), and on
Co in 1M KOH (c), as determined from electrochemical and spectroscopic analysis.
Despite the insights unveiled through this work, a plethora of questions remain. The exact
role of solvent alkalinity is not fully solved and precisely, why in 10mM KOH the oxidation of the
alcohol group is favored whereas the aldehyde is preferentially oxidized in 1M KOH. Furthermore,
exact HMF binding motifs to the catalyst surfaces is not fully resolved. While we see preferential
interactions with various functional groups that is surface chemistry and pH dependent, exact
geometries of both the substrate and reactant are not resolved. A previous study indicated that
HMF preferentially adsorbs via its aldehyde group at the water-NiO interface, through there is
3
6
much more to be investigated. The exact role of solvent hydrogen bonding networks, surface
defects and undercoordinated sites, co-adsorbed species, and solvent cations all likely influence

Chemical Science
Page 12 of 15
View Article Online
DOI: 10.1039/D0SC00136H
this to an extent. Site-specific reactivity could perhaps be probed with electrochemical microscopy
and techniques such as operando x-ray absorption could correlate catalyst oxidation state and
structural changes with HMF oxidation performance in a manner complementary to this work.
While NiOOH rapidly oxidizes HMF to FDCA once it is formed, the exact mechanism
3
+
behind this observation is still to be determined, whether is the Ni oxidation state, the NiOOH
layered structure, or both are critical for this. NiOOH did not form in 10mM KOH and did not
yield much FDCA. In addition, Co did not form the CoOOH phase, even at very positive potentials,
3
+
4+
and did not significantly generate FDCA, though it likely did contain Co and Co in these
3
5
conditions
investigations pointed to varying levels of oxygenic intermediates being present on Co, Ni, and Fe
.
Surface chemistry is also postulated to play a role in the catalysts’ activity. Recent
3
7
surfaces. In particular, the high reactivity of adsorbed *OH with methanol led to enhanced
catalytic currents when CH OH was added to the electrolyte. This occurred as Ni(OH) oxidized
3
2
to NiOOH. As we observe HMF oxidation current that initiates at the same potential, a very
plausible scenario is that surface *OH is a driving factor behind efficient HMF oxidation to FDCA.
Methanol oxidation is also found to inhibit NiOOH formation in 10mM KOH, but not in 1M KOH
(Fig. S18). In the latter case, methanol oxidation occurs once NiOOH forms and is likely driven
by the presence of *OH or *O surface-bound species on this material, which are more prevalent in
highly alkaline conditions. The enhanced current observed prior to NiOOH formation stems from
the oxidation of the aldehyde group as HMFCA is the primary product found at 0.1V. Surface
*
OH was also postulated to be found at a higher density on Ni than Co surfaces and could also
help explain the discrepancy between the Ni and Co catalysts’ activity found in this work. While
we do not believe that exposed Au to be the catalytic species in this work because of the much
small reactivity of Au, in terms of steady state current and product generation, as well as because
of several Raman bands in the Au spectra not visible in the AuNi spectra (Fig. S19), there could
indeed be electronic effects imparted by Au onto the overlaying transition metal catalyst. For
example, the activities of Co, Mn, and Ni oxides/oxyhydroxides were enhanced by underlying Au
layers and so could the Ni, Co and Fe here.3
5, 38, 39
Concluding Remarks:
In summary, we present the first operando Raman study of metal oxide - catalyzed HMF
oxidation. The execution of this study was enabled by the use of nanoscopic gold-transition metal
heterojunctions that give surface-enhanced Raman signals from catalysis on non-plasmonic
materials and illustrate both catalyst transformation and surface reactivity under operating
conditions. The results indicate how electrolyte alkalinity surface binding dictates catalytic
mechanism and how the HMF oxidation reaction proceeds transition metal surfaces. While many
questions still remain, the methods developed and insights extracted in this work represent a
significant step forward towards understanding the electrochemical oxidation of HMF, and in a
larger context, the electrocatalytic transformation of organic molecules.
Conflicts of Interest:
There are no conflicts of interest to declare.

Page 13 of 15
Chemical Science
View Article Online
DOI: 10.1039/D0SC00136H
Acknowledgement:
N. K. and N. H. acknowledge the NSERC Grant R-N401- 02839. The authors thank Samir Elouatik
for help with Raman measurements, Philippe Plamondon with SEM measurements, and Jean-
Philippe Masse with TEM measurements.
References:
1
2
3
4
5
.
.
.
.
.
N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 15729-15735.
K. Li and Y. Sun, Chem. - Eur. J., 2018, 24, 18258-18270.
Y. Xu and B. Zhang, ChemElectroChem, 2019, 6, 3214-3226.
S. Verma, S. Lu and P. J. A. Kenis, Nat. Energy, 2019, 4, 466-474.
T. Werpy and G. Petersen, Top value added chemicals from biomass: volume I--results of
screening for potential candidates from sugars and synthesis gas, National Renewable Energy
Lab., Golden, CO (US), 2004.
6
.
R.-J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J. Heeres and J. G. de Vries,
Chem. Rev., 2013, 113, 1499-1597.
7
8
9
1
1
.
.
.
0.
1.
T. Wang, M. W. Nolte and B. H. Shanks, Green Chem., 2014, 16, 548-572.
K. Gupta, R. K. Rai and S. K. Singh, ChemCatChem, 2018, 10, 2326-2349.
W.-J. Liu, L. Dang, Z. Xu, H.-Q. Yu, S. Jin and G. W. Huber, ACS Catal., 2018, 8, 5533-5541.
B. J. Taitt, D.-H. Nam and K.-S. Choi, ACS Catal., 2019, 9, 660-670.
S. Barwe, J. Weidner, S. Cychy, D. M. Morales, S. Dieckhöfer, D. Hiltrop, J. Masa, M. Muhler and
W. Schuhmann, Angew. Chem., Int. Ed., 2018, 57, 11460-11464.
1
1
1
1
2.
3.
4.
5.
K. R. Vuyyuru and P. Strasser, Catal. Today, 2012, 195, 144-154.
S. R. Kubota and K.-S. Choi, ChemSusChem, 2018, 11, 2138-2145.
G. Grabowski, J. Lewkowski and R. Skowroński, Electrochim. Acta, 1991, 36, 1995.
D. J. Chadderdon, L. Xin, J. Qi, Y. Qiu, P. Krishna, K. L. More and W. Li, Green Chem., 2014, 16,
3
778-3786.
1
1
1
6.
7.
8.
N. Heidary and N. Kornienko, Chem. Commun., 2019, 55, 11996-11999.
D.-H. Nam, B. J. Taitt and K.-S. Choi, ACS Catal., 2018, 8, 1197-1206.
X. Deng, X. Kang, M. Li, K. Xiang, C. Wang, Z. Guo, J. Zhang, X.-Z. Fu and J.-L. Luo, J. Mater. Chem.
A, 2019, DOI: 10.1039/C9TA06917H.
1
2
2
9.
0.
1.
N. Heidary, K. H. Ly and N. Kornienko, Nano Lett., 2019, DOI: 10.1021/acs.nanolett.9b01582.
A. D. Handoko, F. Wei, Jenndy, B. S. Yeo and Z. W. Seh, Nat. Catal., 2018, 1, 922-934.
A. Zalineeva, A. Serov, M. Padilla, U. Martinez, K. Artyushkova, S. Baranton, C. Coutanceau and
P. B. Atanassov, J. Am. Chem. Soc., 2014, 136, 3937-3945.
2
2
2.
3.
N. Zhang, Y. Zou, L. Tao, W. Chen, L. Zhou, Z. Liu, B. Zhou, G. Huang, H. Lin and S. Wang, Angew.
Chem., Int. Ed., 2019, 58, 15895-15903.
P. L. Stiles, J. A. Dieringer, N. C. Shah and R. P. Van Duyne, Annu. Rev. Anal. Chem., 2008, 1, 601-
6
26.
2
2
4.
5.
W.-H. Park and Z. H. Kim, Nano Lett., 2010, 10, 4040-4048.
M. Liu, Y. Pang, B. Zhang, P. De Luna, O. Voznyy, J. Xu, X. Zheng, C. T. Dinh, F. Fan, C. Cao, F. P. G.
de Arquer, T. S. Safaei, A. Mepham, A. Klinkova, E. Kumacheva, T. Filleter, D. Sinton, S. O. Kelley
and E. H. Sargent, Nature, 2016, 537, 382.
2
2
6.
7.
M. B. Ross, Y. Li, P. De Luna, D. Kim, E. H. Sargent and P. Yang, Joule, 2019, 3, 257-264.
D. Y. Chung, K.-J. Lee and Y.-E. Sung, J. Phys. Chem. C, 2016, 120, 9028-9035.

Chemical Science
Page 14 of 15
View Article Online
DOI: 10.1039/D0SC00136H
2
8.
T. Kim, R. S. Assary, L. A. Curtiss, C. L. Marshall and P. C. Stair, J. Raman Spectrosc., 2011, 42,
069-2076.
2
2
3
3
3
3
3
3
3
9.
0.
1.
2.
3.
4.
5.
6.
C. L. Haynes, A. D. McFarland and R. P. Van Duyne, Anal. Chem., 2005, 77, 338 A-346 A.
B. S. Yeo and A. T. Bell, J. Phys. Chem. C, 2012, 116, 8394-8400.
M. Rogojerov, G. Keresztury and B. Jordanov, Spectrochim. Acta, Part A, 2005, 61, 1661-1670.
T.-j. Jia, P.-w. Li, Z.-g. Shang, L. Zhang, T.-c. He and Y.-j. Mo, J. Mol. Struct., 2008, 873, 1-4.
M. W. Louie and A. T. Bell, J. Am. Chem. Soc., 2013, 135, 12329-12337.
R. Kostecki and F. McLarnon, 1997, 144, 485-493.
B. S. Yeo and A. T. Bell, J. Am. Chem. Soc., 2011, 133, 5587-5593.
G. Han, Y.-H. Jin, R. A. Burgess, N. E. Dickenson, X.-M. Cao and Y. Sun, J. Am. Chem. Soc., 2017,
1
39, 15584-15587.
H. B. Tao, Y. Xu, X. Huang, J. Chen, L. Pei, J. Zhang, J. G. Chen and B. Liu, Joule, 2019, 3, 1498-
509.
3
3
3
7.
8.
9.
1
Y. Gorlin, C.-J. Chung, J. D. Benck, D. Nordlund, L. Seitz, T.-C. Weng, D. Sokaras, B. M. Clemens
and T. F. Jaramillo, Journal of the American Chemical Society, 2014, 136, 4920-4926.
J. W. D. Ng, M. García-Melchor, M. Bajdich, P. Chakthranont, C. Kirk, A. Vojvodic and T. F.
Jaramillo, Nat. Energy, 2016, 1, 16053.

Page 15 of 15
Chemical Science
View Article Online
DOI: 10.1039/D0SC00136H
3
17x129mm (150 x 150 DPI)