Fe/Fe3C Boosts H2O2 Utilization for Methane Conversion Overwhelming O2 Generation

Article abstract of DOI:10.1002/anie.202016888

H₂O₂ as a well-known efficient oxidant is widely used in the chemical industry mainly because of its homolytic cleavage into ^.OH (stronger oxidant), but this reaction always competes with O₂ generation resulting in H₂O₂ waste. Here, we fabricate heterogeneous Fenton-type Fe-based catalysts containing Fe-N_x sites and Fe/Fe₃C nanoparticles as a model to study this competition. Fe-N_x in the low spin state provides the active site for ^.OH generation. Fe/Fe₃C, in particular Fe₃C, promotes Fe-N_x sites for the homolytic cleavages of H₂O₂ into ^.OH, but Fe/Fe₃C nanoparticles (Fe⁰ as the main component) with more electrons are prone to the undesired O₂ generation. With a catalyst benefiting from finely tuned active sites, 18 % conversion rate for the selective oxidation of methane was achieved with about 96 % selectivity for liquid oxygenates (formic acid selectivity over 90 %). Importantly, O₂ generation was suppressed 68 %. This work provides guidance for the efficient utilization of H₂O₂ in the chemical industry.

Full text of DOI:10.1002/anie.202016888

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 8889–8895
International Edition: doi.org/10.1002/anie.202016888
German Edition: doi.org/10.1002/ange.202016888
Methane Oxygenation
Hot Paper
Fe/Fe3C Boosts H2O2 Utilization for Methane Conversion
Overwhelming O2 Generation
Yicheng Xing, Zheng Yao, Wenyuan Li, Wenting Wu,* Xiaoqing Lu, Jun Tian, Zhongtao Li,
Han Hu, and Mingbo Wu*
Abstract: H2O2 as a well-known efficient oxidant is widely
used in the chemical industry mainly because of its homolytic
cleavage into COH (stronger oxidant), but this reaction always
competes with O2 generation resulting in H2O2 waste. Here, we
fabricate heterogeneous Fenton-type Fe-based catalysts con-
taining Fe-Nx sites and Fe/Fe3C nanoparticles as a model to
study this competition. Fe-Nx in the low spin state provides the
active site for COH generation. Fe/Fe3C, in particular Fe3C,
promotes Fe-Nx sites for the homolytic cleavages of H2O2 into
COH, but Fe/Fe3C nanoparticles (Fe0 as the main component)
with more electrons are prone to the undesired O2 generation.
With a catalyst benefiting from finely tuned active sites, 18%
conversion rate for the selective oxidation of methane was
achieved with about 96% selectivity for liquid oxygenates
(formic acid selectivity over 90%). Importantly, O2 generation
was suppressed 68%. This work provides guidance for the
efficient utilization of H2O2 in the chemical industry.
competitive process is ignored. Like iron porphyrin, there is
a general and ambiguous agreement that heterogeneous Fe-
based catalysts containing Fe-Nx structure can act as Photo-
Fenton catalyst for the reaction in the presence of H2O2.[2]
However, up to now, almost no clear evidence or investigation
was discussed in detail about the influence of various other Fe
species (unavoidable species when large-scale preparation) in
Fe-Nx based catalysts for H2O2 decomposition.
As the main component of extremely abundant natural
gas and shale gas, methane is an economical chemical
feedstock to be converted into its oxygenated derivatives,
easily transportable and storable liquid chemicals.[3] Tradi-
tionally, it is an energy-intensive and indirect way to its
industrials synthesis that entails such as steam-reforming and
subsequent Fischer–Tropsch synthesis.[4] The direct selective
oxidation of methane to oxygenates is more attractive but
À
remains great challenges because of methaneꢀs high C H
bond strength (439.3 kJmolÀ1), negligible electron affinity,
and low polarizability.[5] Furthermore, the value-added oxy-
genates such as methanol, formaldehyde and formic acid are
more active than methane molecules and are prone to deep
mineralization.[6] Among various activating reagents of meth-
ane, hydroxyl radicals (COH), mainly from H2O2 decomposi-
Introduction
H2O2, a well-known efficient and environmentally friend-
ly oxidant, has been regarded as one of the 100 most
important chemicals in the world due to its wide applications
especially in chemical synthesis.[1] However, the excess H2O2,
great deviation in product composition from the stoichiom-
etry, are always involved in these reactions, which greatly
raise production costs and decrease the product yield and
selectivity. Actually, O2 evolution always competitively ac-
companied with H2O2 itself utilization or the homolytic
cleavage of H2O2 to generate COH (stronger oxidant), but this
À
tion, is green and mild to achieve C H bond cleavage.
Hutchings et al.[7] utilize H2O2 and O2 to oxidize CH4 into
methanol using Au-Pd colloidal catalysts, resulting in meth-
anol selectivity of around 73% (a total organic compound
selectivity of 92%). Deng et al.[8] reported that inexpensive
catalyst containing graphene-confined single iron atoms can
convert CH4 to C1 oxygenated mixture (CH3OH, CH3OOH,
HOCH2OOH and HCOOH) using H2O2 as an oxidant, and
the highest selectivity of the total C1 oxygenated products was
around 94% (selectivity of CH3OOH, 34%; selectivity of
HCOOH, 29%). Therefore, the selective oxidation of meth-
ane was selected as the model reaction to study in this work,
because there is still great challenge in developing highly
active and inexpensive catalysts to efficient utilize H2O2 in
this selective oxidation reaction.
[*] Y. Xing, Z. Yao, W. Li, Prof. W. Wu, Z. Li, H. Hu, Prof. M. Wu
State Key Laboratory of Heavy Oil Processing
Institute of New Energy, College of Chemical Engineering
China University of Petroleum (East China)
Qingdao 266580 (P. R. China)
E-mail: wuwt@upc.edu.cn
wumb@upc.edu.cn
Prof. X. Lu
Herein, we fabricate a model catalyst, that is FeNx/C
catalyst with Fe-Nx sites and graphene-encapsulated Fe/Fe3C
nanoparticles through a one-step pyrolysis, and these active
sites were finely tuned via Fe-imidazole coordination com-
pound (Fe-ZIF, Fe source) and melamine (N source to
stabilize Fe atom and to balance Fe content) under different
temperature. These catalysts were used in the direct methane
conversion into oxygenated products with the presence of
H2O2 at mild condition (room temperature and atmospheric
pressure). The methane conversion rate of optimized catalyst
College of Materials Science and Engineering
China University of Petroleum (East China)
Qingdao 266580 (P. R. China)
Prof. J. Tian
State Key Laboratory of Molecular Engineering of Polymers and
Department of Macromolecular Science, Fudan University
Shanghai 200433 (P. R. 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.202016888.
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(FeNx/C-5-700) can reach 18% with a liquid oxygenates
selectivity of about 96% (formic acid selectivity over 90%,
4659 mmolgcatÀ1). At the same time, the O2 generation was
suppressed 68%, greatly reducing H2O2 waste. Through
a series of characterization methods, it shows that Fe-Nx is
essential to COH generation for further methane conversion,
and proper Fe/Fe3C nanoparticles can dramatically facilitate
this process due to its good electron transport ability.
However, further tuning various Fe species make catalyst
processing more electrons, which in turn promote the O2
generation and overwhelm COH generation. This finding
provides guidance for the rational design catalysts to effi-
ciently utilize H2O2 and its application in selective oxidation
reactions.
prepare the catalysts. Chemical structures of this series of
FeNx/C-R-T were first characterized by X-ray diffraction
(XRD, Figure S1). For FeNx/C-5-800, the primary diffraction
peak at 26.18 can be assigned to (002) planes of graphitic
carbon. And five diffraction peaks at 37.78, 42.88, 43.78, 45.88
and 44.68 in the XRD pattern of FeNx/C-5-800 can be
attributed to (210), (211), (102), (112) planes of Fe3C
(cohenite, JCPDS No. 35-0772), and (110) planes of cubic
Fe (JCPDS No. 06-0696), respectively. When reducing the
synthesis temperature to 7008C, weak broad peaks can be
observed at relative positions (FWHM, full width at half
maximum, Table S1). It indicates that the content of Fe/Fe3C
nanoparticles decrease as reducing pyrolysis temperature. For
FeNx/C-5-600, there is almost no peaks from 358 to 708,
indicating that there is little Fe/Fe3C species.
The transmission electron microscopy (TEM) images of
FeNx/C-5-600 (Figure 1a) show that it has a distinct layered
porous structure. Fe, N and C elements were further identified
and mapped out by Energy dispersive X-ray spectrometer
(EDS) (Figure 1b). The C and N elements are uniformly
distributed and the overlay of Fe and N signals in the
elemental mapping images (Figure 1c) clearly discloses that
Fe atoms are adjacent to N atoms at atomic level, suggesting
the presence of Fe-Nx coordination (Figure 1d). As the
pyrolysis temperature increases (7008C), Fe/Fe3C nanoparti-
Results and Discussion
The synthesis of FeNx/C-R-T (R represents the mass ratio
of melamine over Fe-ZIF, T represents the pyrolysis temper-
ature.) were carried out via a one-step pyrolysis of Fe
precursors and melamine (See Supporting Information for
the details). R at 5 was selected in this work according to their
catalytic performance (vide infra). In order to study the roles
of various Fe species, Tat 600, 700 and 8008C were selected to
Figure 1. a) TEM images, b,c) HADDF-STEM EDS mapping, d) schematic illustration of model systems of FeNx/C-5-600. e) TEM images,
f,g) HADDF-STEM EDS mapping, h) schematic illustration of model systems of FeNx/C-5-700. i) TEM images, j,k) HADDF-STEM EDS mapping,
l) schematic illustration of model systems of FeNx/C-5-800. (c), (g), and (k) are enlarged figures from the yellow square frames in (b), (f) and (j),
respectively. Yellow, blue, and white spheres in (d), (h), and (l) depict Fe, N, and C atoms, respectively.
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cles with a lattice distance of 0.204 nm (the d-spacing of (110)
shown in Fe K-edge XANES (dipole transition from 1s to
planes of a-Fe or (220) planes of Fe3C) are encapsulated with
well crystallized graphene layers (lattice distance of 0.36 nm)
(Figure 1e), and these graphite may stabilize the nanoparti-
cles.[9] EDS of FeNx/C-5-700 shows that uniformly distributed
C and N elements still exist, but some of Fe elements
aggregate (Figure 1 f,g), suggesting that there is Fe-Nx coor-
dination and it overlaps with Fe/Fe3C nanoparticles (Fig-
ure 1h). In addition, as the pyrolysis temperature increases to
8008C, most Fe seems to aggregate and left more exposed Fe/
Fe3C active sites (Figure 1i). At the same time, N content
reduces obviously, indicating that Fe-Nx sites were reduced
and may transformed into the exposed Fe/Fe3C nanoparticles
(Figure 1j, k, l).
In X-ray photoelectron spectroscopy (XPS) of these FeNx/
C-5-T catalysts (Figure 2a, S2 and S3), signals corresponding
to the elements Fe, C, N, O and almost no S are observed in
the wide spectrum survey. The Fe content of these samples
increases as the temperature increase from 600 to 8008C
(Table S2). The signal at around 711 eV can be assigned to Fe
in Fe-Nx configuration, which can be observed in all
catalysts.[10] The signal at 707.1 eV in Fe 2p spectra of FeNx/
C-5-700 and FeNx/C-5-800 can be assigned to zero-valance Fe
(metallic iron or carbide).[11] The N content shows the
opposite trend (Table S3), and it decreases from 46.9%
(FeNx/C-5-600) to 5.5% (FeNx/C-5-800), which are consistent
with STEM results.
4p),[12] Fe K-edge spectrum of FeNx/C-5-600 is close to that of
Fe(TPPCl)(II), suggesting that its valance state is close to FeII
divalent state. The absorption edge of FeNx/C-5-700 shifts to
lower energy, indicating that the valence state of Fe species in
FeNx/C-5-700 is lower than FeII divalent state. Associated with
TEM and XPS, it is probably because of the increasing
content of Fe/Fe3C nanoparticles, resulting in the higher
electron density than that of FeNx/C-5-600. Similarly, FeNx/C-
5-800 processing the lowest energy among these catalysts,
means that it has the highest electron density among these
catalysts.
The k3-weighted Fourier transformation of Fe K-edge
from extended X-ray absorption fine structure (EXAFS)
spectra for these catalysts were further studied as shown in
Figure 2c, and the relative fitting results were summarized in
Table S4. For FeNx/C-5-600, the first shell peaks at around
1.45 Š can be assigned to Fe-N, confirming the existence of
Fe-N4.[13] For FeNx/C-5-700, it shows a strong peak around
2.09 Š, which can be attributed to Fe-Fe shell. There is
a shoulder peak around 2.70 Š, which may be assigned to
Fe-C shell. Compared with FeNx/C-5-700, FeNx/C-5-800 show
a sharp and strong peak at around 2.09 Š. It can be seen that
the intensity of Fe-Fe shell in R space increases and Fe-Nx
reduces as increasing the pyrolysis temperature (Figure 2c).
The Fe-Fe coordination number increases from 1.9 (FeNx/C-
5-700) to 5.5 (FeNx/C-5-800). Combined with the STEM
results, it can be inferred that Fe-Nx sites overlap with Fe/Fe3C
nanoparticles in FeNx/C-5-700. Further increasing pyrolysis
temperature to 8008C, more accumulated Fe/Fe3C nano-
particles intensely overlap with the Fe-Nx sites in FeNx/C-5-
800. This overlap structure can promote the increasing
electron density of Fe-Nx seizing from Fe/Fe3C sites.
In order to further reveal the structure-property relation-
ship of these catalysts, X-ray absorption spectroscopy at Fe K-
edge was further conducted to analyze the valence states and
coordination environment of Fe species (Figure 2b). As
In order to carefully analysis Fe species function, the
contents of various Fe species were further distinguished by
Mçssbauer spectroscopy (Figure 2d). The Mçssbauer fitting
parameters and the relative areas of different Fe species are
shown in Table S5. The curve of FeNx/C-5-600 was fitted to
three components. Doublet D1 and D2 are both assigned to
high-spin states of FeII-N4 coordination. Ferric high spin
complexes have relatively small quadrupole splitting values as
only the lattice term contributes.[14] D3 can be ascribed to
middle-spin states of FeII-N4 coordination.[15] The curve of
FeNx/C-5-700 was fitted to five components. Concretely,
doublet D4 is assigned to low-spin states of FeII-N4 coordi-
nation with a relative content of 11.9%.[16] Sextet 1 and sextet
2 both are assigned to Fe3C with a total relative content of
68%.[14] Sextet 3 and singlet 1 are assigned to Fe0 that is a-Fe
and Superparamagnetic Fe, with the total relative content of
20.1%. The curve of FeNx/C-5-800 is similar to FeNx/C-5-700,
but the contents of Fe0 and Fe3C are different. The total
relative contents of Fe0 and Fe3C in FeNx/C-5-800 are 49.9%
and 42.7%, respectively. Combined with ICP measurements,
the content of each Fe species was calculated and listed in
Table S6. The Fe-Nx content decreased from 10.9% (FeNx/C-
5-600) to 1.1% (FeNx/C-5-800). FeNx/C-5-800 has a little
more content Fe than that of FeNx/C-5-700, but FeNx/C-5-700
has more Fe3C and less Fe0, which may cause the electron
density FeNx/C-5-700 lower than that of FeNx/C-5-800.
Figure 2. a) Fe 2p XPS spectra of FeNx/C-5-T. b) K-edge XANES spectra
of FeNx/C-5-T, Fe foil, Fe2O3, Fe(TPPCl). c) Fourier transforms of the k3-
weighted K-edge EXAFS spectra of FeNx/C-5-T, Fe foil, Fe(TPPCl).
d) 57Fe Mçssbauer spectra of FeNx/C-5-T.
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In order to study structure-property of catalysts for H2O2
assisted catalysis performance, catalysts prepared at different
pyrolysis temperature and ratios between metal precursors
and melamine in the methane conversion with the assistant of
H2O2 are carefully studied (Figure 3a, S4, S5 and Table S7).
the mass spectra, the peak at m/z = 48 can be observed
(Figure S9a), which could be attributed to 13CH313CH2OH
(Calcd for 13C2H6O ([M]+): m/z 48. Found: m/z 48.). Similarly,
the peak at m/z = 47 was observed (Figure S9b), which
indicates the generation of H13COOH (Calcd for 13CH2O2
([M]+): m/z 47. Found: m/z 47.). These results confirm that the
products are derived from CH4 rather than the catalyst.
The UV-vis absorption spectra of FeNx/C-5-T shows that
these catalysts have good adsorption in the whole UV-vis
range (Figure S10). Photocatalytic methane conversion ex-
periments were performed under different monochromatic
light irradiation (350, 400, 450, 500, 550, 600 and 650 nm)
(Figure S11) to calculate the apparent quantum efficiency
(AQE). For HCOOH production, AQE of FeNx/C-5-700 at
500 nm was calculated at 2.76% with the best methane
conversion activity.
Figure 3. a) Product yields and methane conversion rates of FeNx/C-5-
T. b) Three runs of formic acid production by FeNx/C-5-700. Reaction
conditions: 8 mg catalysts, 1 mL 0.5 M H2O2, 4 mL deionized water,
1 atm CH4, 1500 WmÀ2 Xe lamp, 258C, 4 h reaction time.
It is well known that H2O2 can be decomposed into COH
with high oxidation ability through Photo-Fenton reaction
pathway, but it cannot be ignored that H2O2 can also be
decomposed into O2 and H2O, then subsequently may convert
to CO2À, 1O2 or other reactive oxygen species with less
oxidation ability.[19] In order to determine the role of H2O2
in methane conversion, formic acid as the main product was
taken as an example to compare the catalysis performance,
and COH quencher (Isopropyl alcohol, IPA),[20] CO2À quencher
FeNx/C-5-700 shows the highest methane conversion (18%)
as the pyrolysis temperature is 7008C. With the assistant of
H2O2, formic acid as the main products can reach
À1
4659 mmolgcat with over 90% selectivity, ethanol as the
major by-product has around 6% selectivity, leading to 96%
in total for liquid oxygenates selectivity, and CO2 as the only
gas oxygenates has only 4% selectivity without CO, hydro-
carbons and other liquid oxygenates produced. FeNx/C-5-800
is second, and the lowest is FeNx/C-5-600. For the precursor
ratio on the catalytic activity, it can be clearly found that the
ratio at 5 (FeNx/C-5-700) greatly benefits the methane
conversion. Compared with the previous reports, FeNx/C-5-
700 shows significant higher activity and selectivity for photo-
or thermocatalytic direct oxidation of CH4 with H2O2
(Table S8).[7,8,17] In addition, FeNx/C-5-700 still has good
catalytic ability after 3 recycle tests, indicating its excellent
stability (Figure 3b). According to the ICP measurements,
the content of Fe species was almost unchanged after 3
recycle tests (Table S2). And the XAS reveals that FeNx/C-5-
700 almost retains the same chemical state and coordination
after reaction (Figure S6). To further support the above
results, Mçssbauer spectroscopy were used to distinguish the
change of Fe species in FeNx/C-5-700 and FeNx/C-5-800 after
reaction (Figure S7). The Mçssbauer fitting parameters as
well as the relative areas of various Fe species are shown in
Table S5. It can be seen that the Mçssbauer spectra of FeNx/
C-5-700 has little change, which confirms the stability of FeNx/
C-5-700. But FeNx/C-5-800 changes obviously, and it is
severely oxidized to form Fe3O4,[18] which may be due to the
instability of the exposed Fe/Fe3C nanoparticles of FeNx/C-5-
800 without the graphene protection (Figure S7).
1
(p-Benzoquinone, PBQ)[21] and O2 quencher (2,2,6,6-Tetra-
methylpiperidine, TEMP)[19b] were selected and added to the
methane conversion system catalyzed by FeNx/C-5-700 under
experimental conditions (Figure 4a). It can be easily seen that
IPA can sharply reduce the yield of formic acid from 4659 to
504 mmolgcatÀ1, which means that COH from the H2O2 decom-
position plays a decisive role for the methane conversion.
In order to evaluate the degree of H2O2 decomposition,
the H2O2 reduction potentials of FeNx/C-5-T were measured
on glassy carbon rotating ring-disk electrodes (Figure 4b).
FeNx/C-5-800 show the most negative onset potential of H2O2
reduction, indicating that it is much easier to decompose H2O2
than those of FeNx/C-5-700 and FeNx/C-5-600. But it doesnꢀt
seem to be consistent with the photocatalytic results. The
abilities to generate COH from H2O2 were further studied
through the coumarin fluorescence test, wherein coumarin
can selectively react with COH in solution and form the strong
fluorescent substance 7-hydroxycoumarin with a PL signal at
around 456 nm.[22] The solution of coumarin and H2O2
photocatalyzed by FeNx/C-5-700 shows the strongest fluores-
cence emission, the next is FeNx/C-5-800, and the last one is
FeNx/C-5-600 (Figure 4c). It indicates that FeNx/C-5-700 has
the strongest ability to generate COH, consistent with the
photocatalytic results. Electron paramagnetic resonance
(EPR) experiments were used to further confirm the COH
generation (Figure 4d). The signal with the intensity ratio of
1:2:2:1 is observed, which could be attributed to COH.[23] And
the signal intensity increases as prolonging the light irradi-
ation time, which is consistent with the coumarin fluorescence
test (Figure S12). It indicates that the light can promote the
COH generation, and it is Photo-Fenton like reaction.
Associated with the degree of H2O2 decomposition by these
catalysts, it is speculated that there is another way to compete
with the COH generation, especially for FeNx/C-5-800. Then,
Control experiments without catalysts or visible light were
carried out, exhibiting a very low methane conversion rate
(Figure S8), indicating that catalysts and light irradiation are
essential for this methane conversion. There are no products
observed for the control experiments only without the
addition of CH4, indicating that the products are not derived
from the catalyst itself (Figure S8). Isotope labelling experi-
ment of 13CH4 was further conducted with FeNx/C-5-700. In
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To further support the above speculation, two control
experiments were carried out. Firstly, SCNÀ can coordinate
with Fe-Nx sites and poison them, so KSCN was added into
the photooxidation methane reaction system.[25] The methane
conversion rate and formic acid yield of FeNx/C-5-700
À1
decreases significantly to only 2% and 540 mmolgcat
,
respectively (Figure S13), which can be attributed to the
poison of Fe-Nx sites by KSCN. Secondly, FeNx/C-5-T were
soaked in 2 M HCl at 608C for 3 h to remove or substantially
diminish Fe/Fe3C nanocrystals rather than Fe-Nx sites. The
XPS image and XRD image in Figure S14 confirms that
almost no Fe/Fe3C nanoparticles were found after the acid
leaching. Then the experiment of oxygen evolution was
carried out. As shown in Figure 4e, after leaching these three
catalysts with acid, the amount of oxygen evolution of FeNx/
C-5-600 is unchanged, FeNx/C-5-700 decreases slightly and
FeNx/C-5-800 decreases significantly. In addition, the experi-
ment of methane conversion was also carried out. As shown in
Figure 4 f, the methane conversion rate of FeNx/C-5-600
decreases slightly after leached with acid, but FeNx/C-5-700
and FeNx/C-5-800 decrease significantly to 11% and 6%,
respectively. Compared with FeNx/C-5-600, FeNx/C-5-700 has
less Fe-Nx active sites (Table S5), but it still shows higher
catalytic performance than that of FeNx/C-5-600 after acid
leaching. Thus, Fe-Nx at the low spin state is more important
for the catalysis. Associated with the analysis of Fe3C and Fe0
(Table S5), it is speculated that Fe/Fe3C nanoparticles (FeNx/
C-5-700), especially for Fe3C, boost Fe-Nx to produce COH for
methane conversion, while Fe/Fe3C nanoparticles (FeNx/C-5-
800) with more Fe0 promote O2 evolution and reduce the
H2O2 utilization efficiency.
Figure 4. a) Formic acid production by FeNx/C-5-700 with different
quenchers. b) Voltammograms recorded with glassy carbon rotating
disk electrodes surface-coated with FeNx/C-5-T in 0.1 M H2O2 and
0.1 M Na2SO4 aqueous solution. c) The COH-trapping PL spectra of
suspensions containing FeNx/C-5-T/H2O2 (0.1 M) and coumarin.
d) EPR spectra of DMPO-trapped FeNx/C-5-T and an H2O2-aqueous
system under light irradiation. e) Oxygen evolution of as-prepared and
acid-leached FeNx/C-5-T in 0.1 M H2O2. f) Yields of liquid products
and methane conversion rates of as-prepared and acid-leached FeNx/
C-5-T.
Density functional theory (DFT) calculations were used
to further understand the mechanism for the competing
reactions of H2O2 decomposition. Four possible models with
various active sites of FeNx/C-5-T are determined in Fig-
ure S15. H2O2 molecules can be easily absorbed on the active
Fe-Nx sites to form Fe-OOH, then COH can be generated by
the homolytic cleavage of FeO-OH.[26] Therefore, the adsorp-
tion configurations (Figure S15) and the corresponding dis-
sociation Gibbs free energies (eV) of the reaction intermedi-
ate OOH* (DGOOH) were studied (Table S9). As for FeNx/C-
5-700, DGOOH of Fe-Nx (1.985 eV) is lower than that of Fe0
(3.306 eV), indicating that FeNx is more suitable as an active
site to decompose Fe-OOH. The DGOOH of FeNx/C-5-800
(1.137 eV) and FeNx/C-5-700 (1.985 eV) is lower than that of
FeNx/C-5-600 (2.440 eV), indicating that Fe/Fe3C can reduce
DGOOH value of FeNx/C-5-700 and FeNx/C-5-800 and easily
decompose Fe-OOH. In addition, COOH can also be ionized
the O2 evolution in the above system was carried out
(Figure 4e). FeNx/C-5-800 has the highest oxygen evolution
(50 mmol), while FeNx/C-5-700 has only 16 mmol O2 evolution
(Video in SI), suppressing 68% O2 generation. From the
aspect of the product generation, gain factor is another
evaluating criterion for the efficiency of H2O2 utilization
(Table S7). Compared with FeNx/C-5-800 (Gain factor=
0.23), the gain factor of FeNx/C-5-700 comes to 0.58. This
result is consistent with that of O2 evolution, and it also reveal
that O2 evolution is main way for the H2O2 waste.
In order to confirm the active sites, control experiments
with FeCl2 (9 mmol, the same Fe amount of FeNx/C-5-700,
Figure S8) instead of FeNx/C-5-T was added into the catalytic
system, but there is almost no catalytic effect, indicating that
the active sites for this reaction are Fe-Nx and/or Fe/Fe3C
rather than Fe ions. In addition, after simply mixing the FeNx/
C-5-600 and FeNx/C-5-800, the catalytic reaction was carried
out, and the catalytic effect is less than FeNx/C-5-700 (Fig-
ure S8), indicating that there is a synergistic effect between
Fe-Nx and Fe/Fe3C. Taking into account the catalytic activity
(Figure 3 and Figure 4) and structure of these three catalysts
(Table S5), it is speculated that Fe-Nx, especially for the low
spin state, is the active site for COH generation. The proper
amount of Fe/Fe3C nanoparticles, especially for Fe3C species,
may promote the catalysis process.[24]
À
into O2 through electron transfer process, then the formed
À
COOH/O2 are prone to reacting with COH/H2O2 to produce
O2.[27] For the central Fe atom of Fe-Nx, the Bader charges of
FeNx/C-5-800 (7.221) with more Fe atom is higher than that of
FeNx/C-5-700 (7.152) with less Fe atom. It indicates that Fe/
Fe3C nanoparticles of FeNx/C-5-800 (Fe0 as the main compo-
nent) provide more electrons to its Fe-N4 active sites, which
promote the decomposition of H2O2 into O2 instead of useful
COH. Therefore, Fe/Fe3C (FeNx/C-5-700, Fe3C as the main
component) with proper electron is essential for the efficient
utilization of H2O2.
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It is interesting that the main product from this methane
Acknowledgements
conversion system is formic acid instead of methanol. In order
to verify the reaction mechanism of methane activation, the
process of methane activation to generate methyl radicals in
the reaction was first tested by EPR (Figure S16). The signal
of methyl radicals was clearly observed after 5 mins light
irradiation, and the signal is enhanced as prolonging the
irradiation time.[28] In situ infrared (IR) of FeNx/C-5-700 show
that two distinct IR bands attributed to the adsorption of
methane molecules on the catalyst can be observed at 3009
and 1295 cmÀ1 (Figure S17),[29] shifting toward lower wave-
numbers than that of free methane molecules (3020 and
1306 cmÀ1). It indicates that FeNx/C-5-700 can adsorb meth-
ane, which benefits to activate methane. Associated with the
EPR (Figure 4d) and the coumarin fluorescence test (Fig-
ure S12), FeNx/C-5-700 with the assistant of light irradiation
can catalyze the homolytic cleavage H2O2 to produce COH
We thank Prof. Min Jiang (Hangzhou Normal University),
Prof. Bo Jiang (Qingdao University of Technology), and Dr.
Hui Ning, Dr. Qingshan Zhao, Dr. Yang Wang, and Dr.
Wengang Xu (China University of Petroleum) for useful
discussions and suggestions. This work was financially sup-
ported by NSFC (51672309, 51172285 and 51372277) and the
Fundamental Research Funds for Central Universities
(18CX07009A). We also acknowledge the Young Taishan
Scholars program of Shandong province (tsqn20182027) and
technological leading scholar of 10000 talent project
(W03020508).
Conflict of interest
(Photo-Fenton like reaction),[30] which can easily attack C H
À
The authors declare no conflict of interest.
bond of methane absorbed on FeNx/C-5-700 and form CCH3.
Then the generated CCH3 and COH can produce methanol and
other deep oxidation or conversion products (e.g. formic acid
and ethanol).[8,29] Actually, formic acid is the major product,
and ethanol is the minor product without the generation of
deeper oxidized product (acetic acid). It indicates that
methanol is the major intermediate product. But in the actual
reaction, the methanol content is extremely low because
methanol may be further oxidized as an intermediate product
to produce formic acid and ethanol, and it is much easier than
methane oxidation. A series of experiments were carried out
to verify the conjecture. 250 mL methanol was added into the
methane conversion system, and a significant increase of
formic acid and ethanol was observed (Table S10). In
addition, introducing 250 mL methanol into the system with-
out methane, and formic acid and ethanol were observed in
the products. For the hypothetic mechanism for photocata-
lytic methane conversion is shown in Figure S18.[31] Therefore,
the efficient utilization of H2O2 promote the further con-
version of intermediate methanol into formic acid.
Keywords: hydrogen peroxide ·iron ·methane oxygenation ·
nanomaterials ·photochemistry
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Manuscript received: December 20, 2020
Revised manuscript received: January 21, 2021
Accepted manuscript online: February 4, 2021
Version of record online: March 10, 2021
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