Bioinspired multimetal electrocatalyst for selective methane oxidation

Chemical Engineering Journal 474 (2023) 145827
Available online 4 September 2023
1385-8947/© 2023 Elsevier B.V. All rights reserved.
Bioinspired multimetal electrocatalyst for selective methane oxidation
Tareq Al-Attas a,1
, M.A. Khan b,1
, Tiago J. Goncalves c,1
, Nael G. Yasri a
, Soumyabrata Roy d
,
Ali Shayesteh Zeraati a
, Pawan Kumar a
, Kristen A. Miller d
, Pulickel M. Ajayan d
, Ian D. Gates a
,
Jinguang Hu a
, Venkataraman Thangadurai c
, Samira Siahrostami c,*
, Md Golam Kibria a,*
a
Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
b
Department of Chemical and Materials Engineering, University of Alberta, 116 St & 85 Ave, Edmonton, AB T6G 2R3, Canada
c
Department of Chemistry, University of Calgary, 2500 University Drive NW, AB T2N 1N4 Calgary, Canada
d
Department of Materials Science and NanoEngineering, Rice University, 6100 Main St., Houston, TX 77005, USA
A B S T R A C T
Selective partial electrooxidation of methane (CH4) to liquid oxygenates has been a long-sought goal. However, the high activation energy of C–H bonds and
competing oxygen evolution reaction limit product selectivity and reaction rates. Inspired by iron (IV)-oxo containing metalloenzymes’ functionality to activate the
C–H bond, here we report on the design of a copper-iron-nickel catalyst for selective oxidation of CH4 to formate via a peroxide-assisted pathway. Each catalyst serves
a specific role which is confirmed via electrochemical, in situ, and theoretical studies. A combination of electrochemical and in situ spectroelectrochemical studies
revealed that H2O2 oxidation on nickel led to the formation of active oxygen species which trigger the formation of iron (IV) at low voltages. Density functional
theory analysis helped reveal the role of iron (IV)-oxo species in reducing the activation energy barrier for CH4 deprotonation and the critical role of copper to
suppress overoxidation. Our multimetal catalyst exhibits a formate faradaic efficiency of 42% at an applied potential of 0.9 V versus a reversible hydrogen electrode.
1. Introduction
For decades, natural gas lagged coal and oil as an energy source, but
today its consumption is growing rapidly as countries seek to lower
greenhouse gas emissions by displacing coal for heating and power
generation [1]. The recent discoveries of vast shale gas reserves in the
United States and widespread use of hydraulic fracturing have led to a
decrease in natural gas prices and today it supplies ~22% of our global
energy needs [2]. In its 2020 report, the International Energy Agency
(IEA) projected global demand for natural gas to increase by 30% by
2040, making it the largest energy source among fossil fuels [3]. In
addition, the main component of natural gas i.e., methane (CH4) is a
well-established and widely available feedstock to produce several
important commodity chemicals such as methanol (CH3OH), hydrogen
(H2), ammonia (NH3), and formaldehyde (HCHO) [4]. Typical valori
zation of CH4 involves a combination of steam methane reforming and
Fischer-Tropsch synthesis [5]. However, these processes are highly
endothermic requiring high temperatures (300–1100 ◦
C), pressures
(10–40 bars) and suffer from a trade-off between conversion and
selectivity [5]. Moreover, they are highly capital-intensive requiring
centralized infrastructure, long-term commissioning, thus hindering
their implementation on a small scale for distributed production of high-
value products [6]. Modular production at smaller scales is appealing for
integration with renewable energy sources. Thus, there is an incentive to
develop low-cost, modular, and sustainable processes for the direct
partial oxidization of CH4 to useful chemicals and fuels under ambient
conditions [7].
In this context, partial electrochemical oxidation of CH4 (eCH4OR) to
liquid oxygenates is particularly attractive [8]. The ability to control
reaction selectivity via electrode potential is a key advantage of elec
trochemistry. Furthermore, the modular design, ease of automation, and
ability to operate using renewable electricity under ambient conditions
mean electrochemical technologies could be deployed for producing
both specialty and commodity chemicals at small and large scales [7,9].
There have been attempts to develop catalysts for eCH4OR using
metal/metal-oxides in various reaction conditions, however with
limited success. A major difficulty arises due to the stable non-polar
tetrahedral molecular geometry of CH4 and high C–H bond energy
(ΔHC–H = 439.3 kJ mol− 1
) [10,11]. Once this high activation energy is
attained, the reaction proceeds to the terminal and thermodynamically
favorable pathway of CO2 production [8,12]. In aqueous media, the
reported partial current densities or reaction rates are often low due to
the low solubility of CH4 (of the order of µA cm− 2
to 1 mA cm− 2
)
[13–17]. Furthermore, the competitive oxygen evolution reaction (OER)
* Corresponding authors.
E-mail addresses: samira.siahrostami@ucalgary.ca (S. Siahrostami), md.kibria@ucalgary.ca (M.G. Kibria).
1
These authors contributed equally to this work.
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
https://doi.org/10.1016/j.cej.2023.145827
Received 25 May 2023; Received in revised form 4 August 2023; Accepted 1 September 2023

2
poses additional challenges to attaining high selectivity towards
eCH4OR products. Conventional alkaline water electrolyzers operate at
low temperatures (40–90 ◦
C), with the hydroxide ion (OH−
) generally
functioning as the oxidant [14]. Nevertheless, OH−
possesses negligible
activity towards proton abstraction from CH4 under mild conditions
[18].
In nature, members of the iron-containing soluble methane mono
oxygenase (sMMO) and cytochrome P450 enzymes have shown the
ability to activate the C–H bond at mild conditions [19,20]. The sMMO is
a three-component water-soluble protein complex consisting of hy
droxylase (MMOH), which performs CH4 hydroxylation on its nonheme
diiron catalytic site (Fig. 1a) [19]. The diiron center is activated by
molecular oxygen to form high-valent ferryl-oxo species (FeIV
O) (known
as intermediate Q). On the other hand, the active sites for P450 are
obtained by activating its heme iron (intermediate I) into the FeIV
–
–O
species (Fig. 1b) [19,20]. Cytochrome P450 enzymes have been reported
to obtain the FeIV
–
–O through the peroxide shunt pathway in which
peroxides, such as hydrogen peroxide (H2O2), serve as the oxygen source
instead of molecular oxygen.
Inspired by the catalytic site of P450 enzymes, we reasoned that C–H
bond dissociation could be triggered by FeIV
O active sites, which could
be generated via a peroxide-assisted pathway. We exploited the
peroxide-assisted pathway for C–H bond dissociation and selective
oxidation of CH4 using a multimetal Cu-Fe-Ni catalyst. The FeIV
O active
sites are generated via active oxygen species, formed during H2O2
oxidation on the nickel (Ni) surface. Although our Fe catalyst was
capable of oxidizing CH4, only CO2 was produced. To address this issue,
we introduced copper as a promoter to prevent overoxidation and
enable selective conversion of CH4 to formate by increasing the activa
tion energy of the intermediate step. Our multimetal (Cu-Fe-Ni) catalyst
exhibits eCH4OR to formate at high current density (31 mA cm− 2
) and
faradaic efficiency (42%) using a low applied potential (0.9 VRHE) with
50 mM H2O2.
2. Results
2.1. Electrochemical CH4 oxidation reaction (eCH4OR)
We began studying the eCH4OR on our hydrothermally grown Fe on
nickel foam (NF) catalyst in a 3-electrode H-cell setup. The catalyst is
hereafter referred to as Fe/NF and details of the catalyst preparation and
H-cell setup are provided in Methods and Supplementary Fig. 1. We
carried out chronoamperometry (CA) experiments at 1.3 VRHE, using 1
M KOH as the electrolyte (Fig. 2a) and under different purge gases i.e.,
argon (Ar) and CH4. At 1.3 VRHE, the catalyst demonstrated negligible
currents irrespective of purge gas as the applied voltage was below the
onset of the oxygen evolution reaction (OER). With the addition of 50
mM H2O2, we observed a jump in initial current density to 110 mA cm− 2
while bubbling the anolyte with Ar. The current can be assigned to H2O2
oxidation on NF, which we confirmed by carrying out identical experi
ments using a titanium foam scaffold instead of NF that led to negligible
current densities (<1 mA cm− 2
; Supplementary Fig. 2). The continuous
oxidation or consumption of H2O2 is also the reason for the decrease in
electrochemical current density as observed in Fig. 2a.
More importantly, we observed an increase in current density when
switching the purged gas in the anolyte from Ar to CH4. However, the
observed current density with the use of CH4 did not correlate to liquid
product formation. Proton nuclear magnetic resonance (1
H NMR)
spectroscopy of the liquid phase did not detect any soluble liquid
product (Supplementary Fig. 3). We detected only O2 as a gaseous
product, which we generated from H2O2 electrooxidation. Since the
reaction was done in alkaline conditions, we hypothesized that any CO2
produced from CH4 oxidation would be captured to form carbonate
(CO3
2−
). We used the total alkalinity method to quantify CO3
2−
in the
electrolyte (details in Methods, Supplementary Table 3, and Supple
mentary Figs. 4–6). The product quantification revealed that the reacted
CH4 completely oxidizes to CO2 (Fig. 2b). The results also indicate that
selectivity to CH4 oxidation decreases with increasing voltage as the
faradaic efficiency (FE) towards CO3
2−
formation decreases from 40 % at
1.0 VRHE to 24% at 1.3 VRHE. O2 was the only other product detected in
the reaction. However, since it was not quantified it is assigned as
missing FE in Fig. 2b.
Earlier methane oxidation studies have indicated that copper pro
motion of iron-based catalysts is crucial to prevent the overoxidation of
oxygenates to CO2 [23,24]. Hence, we introduced Cu as a promoter
(CuFe/NF) and carried out CA experiments at 1.0 VRHE for 1 h while
purging with CH4. The electrolyte was identical to previous experiments
i.e., 1 M KOH with the addition of 50 mM H2O2. The analysis of the
liquid products after the CA experiments using 1
H NMR surprisingly
showed the selective production of formate (HCOO–
) (Fig. 2c). Impor
tantly, HCOO−
was only detected with the CuFe/NF electrode and none
of the control electrodes i.e., NF, Fe/NF, Cu/NF yielded formate (Fig. 2c
and Supplementary Figs. 3, 7–9). This indicated a synergetic role of the
different metals i.e., Ni, Fe, and Cu to selectively oxidize CH4 to formate.
Product analysis using isotopically labelled 13
CH4 and the 13
C NMR
spectroscopy (inset of Fig. 2c) confirmed the origin of carbon in HCOO−
was from CH4.
We then carried out further experiments using our CuFe/NF elec
trode under different electrochemical potentials (Fig. 2d). Irrespective of
the voltage applied, HCOO−
was the only soluble product detected. With
increasing voltage, there was a trade-off between current density and FE
for HCOO–
production. The current density increased from 31 to 164
mA cm− 2
while the FE decreased from 42% to 13% when the voltage was
increased from 0.9 to 1.2 VRHE. On the flip side, FE for CO3
2−
production
increased from a negligible amount at 0.9 VRHE to 32% at 1.2 VRHE.
To reveal the role of electrochemical potential and H2O2 in eCH4OR,
we performed control experiments using our CuFe/NF electrode under
an open circuit potential (OCP) and without the addition of H2O2. The
1
H NMR results revealed that no HCOO−
was generated without
applying a bias (Supplementary Fig. 10), i.e., the reaction is driven by an
electrochemical potential. When we performed eCH4OR without the
addition of H2O2 (Supplementary Fig. 11), we identified CO3
2−
as the
only oxidation product. While this confirmed the possibility to oxidize
CH4 without the use of H2O2, it also revealed the undesirable over
oxidation of CH4 due to the high voltages required (1.8 to 2.6 VRHE).
The harsh oxidizing conditions warranted stability testing of our
CuFe/NF catalyst. We confirmed the stability of our catalyst by con
ducting CA experiments at 0.9 VRHE (Fig. 2e) and an initial concentra
tion of 50 mM H2O2 in 1 M KOH. The initial current density of 31 mA
cm− 2
decreases due to H2O2 consumption. When the current density
decreased to almost zero, we added the same amount of H2O2 which led
Fig. 1. Biological examples for CH4 hydroxylation. a, Diiron diamond-core
structure of the intermediate Q proposed in sMMO and a proposed CH4 hy
droxylation reaction via a radical rebound mechanism [21,22]. Glu, glutamate
amino-acid residue. b, Heme structure of the intermediate I proposed in cyto
chrome P450 and a proposed CH4 hydroxylation reaction [20]. Cys, cysteine
thiolate ligand.
T. Al-Attas et al.

3
to a jump in the current density to its initial value. The FE for HCOO−
production remained steady at ~42% throughout the stability tests for
around 500 min (>8 h).
We summarized the electrochemical performance metrics of our
CuFe/NF catalyst for eCH4OR in a radar chart (Fig. 2f), wherein we
compared our results with previously reported literature. Our catalyst
demonstrated a current density of ~31 mA cm− 2
and FE of 42% at an
applied potential of 0.9 VRHE. There are only a limited number of studies
Fig. 2. Electrochemical CH4 oxidation reaction, eCH4OR. a, Chronoamperometry of Fe/NF electrode at 1.3 VRHE in 0 mM and 50 mM H2O2 in 1 M KOH while
switching between argon and methane. The continuous drop in current throughout the chronoamperometry is because of the consumption of H2O2. b, eCH4OR
products FEs and corresponding current densities obtained at different applied potentials on the Fe/NF in 1 M KOH. c, 1
H NMR spectra after CH4 electrolysis
performed using NF, Fe/NF, Cu/NF, and CuFe/NF electrodes at a constant voltage of 1.0 VRHE for 1 h in 1 M KOH with 50 mM H2O2 (inset: 13
C NMR spectrum of
13
CH4 electrolysis performed using the CuFe/NF electrode at a constant voltage of 1.0 VRHE for 1 h in 1 M KOH with 50 mM H2O2) (full 1
H NMR spectra are available
in Supplementary Figs. 3, 7–9). d, eCH4OR products faradaic efficiencies (FEs) and corresponding initial current densities obtained at different applied potentials on
the CuFe/NF in 1 M KOH with 50 mM H2O2. e, Chronoamperometry of CuFe/NF at 0.9 VRHE showing the consumption of H2O2 after 260 min and the effect of adding
the same amount of H2O2 in restoring the current. f, Radar chart showing a comparison of eCH4OR FE, selectivity towards the target product, liquid oxygenates
production rate, applied potential, and current density against selected best reports from the literature at ambient conditions.
Fig. 3. Structural characterization of the catalyst. a, XRD pattern of the CuFe/NF in comparison with standard XRD patterns. b,c, Fe L3,2-edge and Cu L3,2-edge of the
CuFe/NF electrode, respectively. d, HRTEM image of the CuFe/NF electrode. e,f, Core-level spectra for Fe 2p and Cu 2p3/2 of the CuFe/NF electrode, respectively.
Black empty circles: experimental points, solid lines: fitted data.
T. Al-Attas et al.

4
wherein a high FE for eCH4OR has been reported. However, the high FE
reported are with systems that operate at very low current densities (µA
cm− 2
) and/or high applied potentials (>1.4 VRHE) (Supplementary
Table 4) [13,15,17,25,26].
2.2. Catalyst characteristics
To investigate the reason behind the active and selective nature of
our catalyst, we characterized the bulk and surface structure. The
crystalline structure of our CuFe/NF electrode was analyzed using X-ray
diffraction (XRD). The XRD pattern (Fig. 3a) shows that three large
diffraction peaks at 44◦
, 52◦
, and 76◦
are due to the (111), (200), and
(220) facets of the Ni scaffold, while the ones at 30◦
, 35◦
, and 37◦
match
the (112), (103), and (211) facets of tetragonal CuFe2O4 (JCPDS: 34-
0425). We also observed additional diffraction peaks which can be
assigned to CuO and Fe2O3. A previous report by Inamdar et al.
confirmed that the hydrothermally grown CuFe on nickel foam at 105 ◦
C
forms a bimetallic composite of crystalline Cu matrix incorporated with
Fe [27]. Our XRD data confirms that our CuFe/NF catalyst is a composite
system of tetragonal CuFe2O4 [27,28] and CuFe oxides. XRD patterns of
NF, Cu/NF, and Fe/NF are provided in Supplementary Fig. 12. It is
worth mentioning that XRD patterns of the CuFe/NF electrode, after
conducting the eCH4OR, reveal the presence of the metal oxides (Sup
plementary Fig. 13). This finding serves to emphasize the electrode’s
relative stability under reaction conditions.
We then conducted soft X-ray absorption spectroscopy (sXAS) to gain
more understanding of the electronic and oxidation states of each
element in the electrode. The sXAS spectrum of the Fe L3-edge presents
two main peaks at 708 and 711 eV whereas the Fe L2-edge shows peaks
at 720 and 722 eV (Fig. 3b) [29]. The peak at ~711 eV confirms the
presence of both octahedral and tetrahedral sites of FeIII
as compared to
the spectrum of Fe2O3 reference. The suppression of the peak at 708 eV
(transition of 2p electrons to 3d t2g orbitals) is attributed to the contri
butions from CuI
cations [30]. The Cu L3,2-edge spectrum of the CuFe/
NF coincides with the absorption spectrum of CuO, which shows that
copper predominantly exists as CuII
on the electrode (Fig. 3c) [31].
However, the peak at 935 eV also signifies the presence of CuI
species.
The field emission scanning electron microscopy (FE-SEM; Supple
mentary Fig. 14) of the CuFe/NF electrode revealed that the synthesized
CuFe consists of randomly interconnected compact nanoflakes covering
the NF substrate. Energy dispersive X-ray (EDX) spectroscopy proved the
existence of Fe, Cu, and O in the CuFe/NF electrode with an atomic ratio
for Cu/Fe at 1.65 (Supplementary Fig. 15). This observation confirms
that the CuFe composite is Cu-rich even though an equimolar amount of
Cu and Fe precursors were used during the hydrothermal synthesis. A
high-resolution transmission electron microscopy (HRTEM) image of the
CuFe/NF electrode (Fig. 3d inset) reveals the lattice fringes with a dis
tance of 2.4 Å, which is associated with the (311) facet of CuFe2O4 while
the fringes with lattice distances of 2.1 Å and 1.8 Å correspond to the
(111) and (200) facets of Fe2O3 and CuO, respectively (Fig. 3d)
[27,32].
Seeking to further characterize the chemical state of the surface
species on CuFe/NF electrode, we conducted X-ray photoelectron
spectroscopy (XPS) (Supplementary Fig. 17). The high-resolution spec
trum of Fe 2p (Fig. 3e) reveals two peaks at 721.4 and 710.75 eV which
correspond to Fe 2p1/2 and Fe 2p3/2, respectively [33,34]. This obser
vation confirms the presence of the FeIII
state in the CuFe/NF electrode.
The Cu 2p spectrum (Fig. 3f) shows two satellite shake-up peaks at 954.4
and 951.0 eV and two peaks at 932.6 and 931.2 eV, confirming the
presence of a combination of CuI
and CuII
on the surface of CuFe/NF
electrode [35,36]. The core-level O 1s further confirms the presence of
the metal oxides on the catalyst surface (Supplementary Fig. 18) [27].
2.3. Mechanistic study of eCH4OR
The characterization reveals that the catalyst is CuO and Fe2O3 with
a relatively small amount of CuFe2O4 on the NF surface. Considering
these findings, we sought to understand the reaction mechanism for
eCH4OR to HCOO–
on our CuFe/NF catalyst. We hypothesized that C–H
bond dissociation of CH4 is triggered by the high-valent metal oxo
species (FeIV
O), formed on our CuFe/NF catalyst, during the oxidation
reaction in the presence of H2O2 (since FeIV
O was reported to be the
catalytic site of iron-containing cytochrome P450 and soluble methane
monooxygenase enzymes) [20–22]. The possible routes of C–H bond
dissociation on FeIV
O are further discussed in Supplementary Note 1. To
confirm the presence of FeIV
O, we performed in situ spectroelec
trochemical measurements which allowed us to monitor changes in the
absorption spectra as a function of applied voltage. Specifically, we
performed in situ X-ray absorption near-edge structure (XANES) spec
troscopy of Fe K-edge in the presence of H2O2. We noticed a positive
shift of the white line (sharp intense peak) by ~0.7 eV when the applied
potential was held at 1.0 VRHE (Fig. 4a) versus OCP. Similarly, when the
applied potential was decreased to 0.9 VRHE, a comparable behaviour
was observed (Supplementary Fig. 19). This feature illustrates the
presence of FeIV
as reported in earlier electrochemical water oxidation
studies [37,38]. This small edge shift is due to the high covalency of
metal–ligand bonding in ferryl-oxo species [38–41].
In situ spectroelectrochemical measurements were also conducted
using ultraviolet–visible (UV–Vis) spectroscopy (Supplementary
Fig. 20). An intense integrated absorption (ΔAbs) peak centred at 600
nm is observed in the absence of H2O2, which grew in at applied po
tentials ≥1.4 VRHE where OER is prevalent. The potential-dependent
peak is assigned to the electronic absorption bands of the d-d ligand-
field transition of FeIV
O [42–47]. Upon addition of H2O2, the absorp
tion potential-dependent peak from FeIV
O species was observed at a
lower voltage of <1.4 VRHE. Further details on the in-situ UV–Vis
spectroelectrochemistry are available in Supplementary Note 2 and
Supplementary Figs. 20 and 21.
Our in situ spectroelectrochemical studies indicated that the for
mation of the high-valent FeIV
O species is facilitated by H2O2 oxidation
at lower voltages where OER is not prevalent (<1.5 VRHE). The in situ
studies in conjunction with literature reports led us to hypothesize that
the electrooxidation of H2O2 would generate appreciable reactive
hydroperoxyl radicals (•OOH), leading to the formation of FeIV
O (Eqs.
(1)–(2)) [48–50]:
H2O2 → •OOH + H+
+ e−
(1)
2FeIII
+ •OOH → 2FeIV
O + H+
+ e−
(2)
To confirm this hypothesis, we performed electron paramagnetic
resonance (EPR) spin-trapping with 5,5-dimethyl-1-pyrroline-N-oxide
(DMPO), which indicated that •OOH is the most populous radical pro
duced during the electrooxidation of H2O2 on nickel (Supplementary
Figs. 21 and 22). Further discussion on the EPR results is presented in
Supplementary Note 3.
We also used spin-polarized density function theory (DFT) to study
the role of FeIV
O in reducing the thermodynamic and activation energy
barrier of the CH4 deprotonation step on the most stable ⍺-Fe2O3 (110)
facet [51–58]. Fig. 4b shows that FeIV
O species are able to reduce the
activation energy barrier of CH4 dehydrogenation by 0.16 eV compared
to the FeIII
species via homolytic dissociation to produce CH3 (step 3)
[59]. Despite the seemingly small reduction in the activation barrier,
such a difference in the free energy leads to kinetic rates around 450
times faster than those for FeIII
species. Hydroperoxyl ligands (− OOH)
(step 5) can be formed via direct dissociation of H2O2 on FeIV
(step 4)
and/or due to the presence of abundant •OOH free radicals (1). The CH3
will eventually combine with the adjacent –OOH to form CH3OOH (step
6) as confirmed by conducting the eCH4OR in a neutral electrolyte
(Supplementary Fig. 23). After desorption of CH3OOH, the initial active
site (step 1) reforms and the catalytic cycle is closed by proton
abstraction of H2O2 or via reacting with •
OOH (2). The elementary steps
of the reaction pathway are summarized in Fig. 4c (transition steps are
T. Al-Attas et al.

5
shown in Supplementary Fig. 24).
Previous thermocatalytic studies indicate that the CH3OOH is sub
sequently oxidized to HCOOH and CO2 on zeolite-confined Fe sites
(Supplementary Table 5) [48,60]. However, when we performed
eCH4OR on Fe/NF, we only observed CO2 with an increment in current
density (Fig. 2a,b) indicating that HCOO−
could be further oxidized to
yield CO2 in the absence of Cu centers [24,48]. DFT analysis was also
used to investigate the role of Cu in inhibiting the overoxidation of
HCOO−
to CO2. Tetragonal CuFe2O4 was adapted to best mimic the real
Cu-containing surfaces of the catalyst used in this study. We used formic
acid as the reference to calculate the adsorption-free energies of in
termediates and activation barriers in the free energy diagrams. Fig. 4d
compares the pathway of the complete oxidation of formate to CO2 on
⍺-Fe2O3 and on CuFe2O4. CuFe2O4 showed a considerably high formate-
adsorption free energy on the surface and decomposition barrier to CO2
(ΔG‡
= 1.32 eV) compared to ⍺-Fe2O3 (ΔG‡
= 0.98 eV). As the Gibbs free
energy barrier corresponding to the formation of CO2 from formate is
increased by 0.34 eV, we conclude that Cu hinders the formation of CO2
by preventing formate from being adsorbed and decomposed on the
surface. A schematic illustration summarizing the proposed reaction
mechanism of eCH4OR on the CuFe/NF electrode is shown in Fig. 4e.
3. Conclusions
Taken together, we demonstrate a pathway for selective partial
oxidation of methane at ambient conditions. We reasoned that the CH4
oxidation could happen at FeIV
O species by looking at relevant metal
loenzymes. However, these FeIV
O can be electrocatalytically obtained at
high overpotentials in which liquid oxygenates would go through un
wanted overoxidation to CO2. In situ potential-controlled spectroelec
trochemistry and DFT calculations showed that FeIV
can be obtained
with the help of reactive oxygen species generated via the partial
electrooxidation of H2O2 at lower overpotentials. Moreover, we show
that Cu is found to have a crucial role in protecting the produced liquid
oxygenates from overoxidation to CO2. These led us to synthesize the
trimetallic catalyst of CuFeNi that report faradaic efficiencies of 42% at
current densities of 31 mA cm− 2
with 50 mM H2O2. Future mechanistic
studies via in situ/operando spectroelectrochemistry and computational
modelling will allow steering the selectivity towards other targeted
products e.g., methanol. Translating the multimetal electrocatalyst of
eCH4OR to a gas diffusion electrode (GDE) should also be considered in
future studies to eliminate the mass transport limitation of CH4 and in
turn target industrially relevant current densities.
Declaration of Competing Interest
The authors declare the following financial interests/personal re
lationships which may be considered as potential competing interests:
Md Golam Kibria has patent pending to Innovate Calgary.
Data availability
No data was used for the research described in the article.
Acknowledgments
This work was financially supported by the Canada First Research
Excellence Fund (CFREF) at the University of Calgary. We thank the
Canadian Light Source (CLS) synchrotron for the general access pro
vided under project number 35G12344 ~ Kibria. We thank T. Regier, J.
Dynes, and Z. Arthur for technical support at the 11ID-1 (SGM) beamline
in CLS. We thank N. Chen and W. Chen for technical support at the 06ID-
1(HXMA) beamline in CLS. We thank W. White from the Department of
Chemistry at the University of Calgary for NMR support. We thank H.
Fig. 4. Mechanistic understanding of the eCH4OR. a, In situ Fe K-edge XANES spectra of CuFe/NF at open circuit potential (OCP) conditions and 1.0 VRHE with the
addition of 50 mM H2O2 in 1 M KOH. b, Gibbs free energy profile of CH4-to-CH3OOH pathway on FeIII
(in dark red) and FeIV
O (in blue) surfaces at room temperature,
investigated on ⍺-Fe2O3. c, Proposed reaction scheme of the CH4-to-CH3OOH pathway on FeIV
O. We additionally evaluated different magnetic moments and spin
configurations for ⍺-Fe2O3 and CuFe2O4, and performed a comparison study between PBE and PBE + U functionals, which is available in Supplementary Note 4 and
Supplementary Figs. 25–27. d, Gibbs free energy profile of formate deprotonation on ⍺-Fe2O3 (in blue) and CuFe2O4 (in dark red) at room temperature. e, A schematic
illustration summarizing the proposed reaction mechanism of eCH4OR on the CuFe/NF electrode. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
T. Al-Attas et al.

6
Shaker Shiran for the help during in situ XANES experiments. T.A.
thanks CFREF, Alberta Innovates, and the Government of Alberta for
their support through graduate scholarships.
Author contributions
M.G.K. and S.S. supervised the project. T.A. performed the catalyst
preparation, characterization, and catalytic tests. M.A.K. and N.G.Y.
participated in the catalyst synthesis and catalytic tests. T.J.G. carried
out and discussed the DFT simulation. S.R. and K.A.M. carried out XPS
and HRTEM analyses. P.K. and A.S.Z. carried out and discussed EPR
analysis. P.M.A., I.D.G., J.H., and V.T. provided helpful discussion.
Competing interests
T.A., M.A.K., N.G.Y., and M.G.K. have filed provisional patent
application no. 63/229, 188 regarding multi-metal electrocatalytic
system for methane oxidation.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.cej.2023.145827.
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