Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single Atom Sites on Carbon Nitride for Selective Photooxidation of Methane into Methanol

RESEARCH ARTICLE
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Partial Thermal Condensation Mediated Synthesis of
High-Density Nickel Single Atom Sites on Carbon Nitride for
Selective Photooxidation of Methane into Methanol
Pawan Kumar, Peter Antal, Xiyang Wang, Jiu Wang, Dhwanil Trivedi,
Ondřej František Fellner, Yimin A. Wu, Ivan Nemec, Vinicius Tadeu Santana, Josef Kopp,
Petr Neugebauer, Jinguang Hu, Md Golam Kibria,* and Subodh Kumar*
Direct selective transformation of greenhouse methane (CH4) to liquid
oxygenates (methanol) can substitute energy-intensive two-step
(reforming/Fischer–Tropsch) synthesis while creating environmental benefits.
The development of inexpensive, selective, and robust catalysts that enable
room temperature conversion will decide the future of this technology.
Single-atom catalysts (SACs) with isolated active centers embedded in
support have displayed significant promises in catalysis to drive challenging
reactions. Herein, high-density Ni single atoms are developed and stabilized
on carbon nitride (NiCN) via thermal condensation of preorganized
Ni-coordinated melem units. The physicochemical characterization of NiCN
with various analytical techniques including HAADF-STEM and X-ray
absorption fine structure (XAFS) validate the successful formation of Ni single
atoms coordinated to the heptazine-constituted CN network. The presence of
uniform catalytic sites improved visible absorption and carrier separation in
densely populated NiCN SAC resulting in 100% selective photoconversion of
(CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity
can be attributed to the generation of high oxidation (NiIII═O) sites and
selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which
can combine with •OH radicals to generate CH3OH.
P. Kumar, J. Wang, D. Trivedi, J. Hu, M. G. Kibria
Department of Chemical and Petroleum Engineering
University of Calgary
2500 University Drive, NW Calgary, Alberta T2N 1N4, Canada
E-mail: md.kibria@ucalgary.ca
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202304574
© 2023 The Authors. Small published by Wiley-VCH GmbH. This is an
open access article under the terms of the Creative Commons
Attribution-NonCommercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited
and is not used for commercial purposes.
DOI: 10.1002/smll.202304574
1. Introduction
Single-atom catalysts (SACs) are emerging
heterogeneous catalysts with high stabil-
ity and unprecedented catalytic activity be-
cause of their unique metal atom-support
interactions and neighboring environment
(ensemble effect).[1]
Hence, the catalytic
activity and selectivity of the SACs can
be tuned by simply altering the chemical
nature of the support material.[2,3]
How-
ever, achieving uniform distribution and
high density of the single metal atoms
is a challenge.[4]
One of the most ap-
parent problems that occur during the
synthesis of SACs is the coarsening of
single metal atoms into nano/sub-nano
clusters due to high surface energy.[5]
Therefore, various synthesis strategies such
as laser-assisted synthesis,[6]
atomic layer
deposition (ALD),[7]
wet chemistry,[8]
co-
precipitation,[9]
etc. have been applied to
counter these problems. Moreover, various
support materials such as 3D/2D inorganic
materials and doped carbon materials have
also been investigated as support to induce
P. Antal, O. F. Fellner, I. Nemec, S. Kumar
Department of Inorganic Chemistry
Faculty of Science
Palacký University Olomouc
Olomouc 77146, Czech Republic
E-mail: subodh.kumar@upol.cz
X.Wang,Y.A.Wu
Department of Mechanical andMechatronics Engineering
Waterloo InstituteforNanotechnology
Materials InterfaceFoundry
University of Waterloo
Waterloo,Ontario N2L3G1,Canada
V.T.Santana,P.Neugebauer
Central European Instituteof Technology
Brno University of Technology
Purkyňova 123,Brno 61200,Czech Republic
J.Kopp
Department of Experimental Physics Faculty of Science
Palacký University Olomouc
17.listopadu 1192/12,Olomouc 77900,Czech Republic
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diverse chemical and electronic properties. However, 3D inor-
ganic crystals-based SACs suffer from the drawback of lower
concentration, small specific surface area, and labile single
atom pinning sites thus loss of activity under harsh reaction
conditions.[2,10,11]
Although, 2D inorganic materials such as
MoS2, WS2, MXene, and chalcogenides with a tunable chemical
composition and cavity sizes can strongly accommodate a large
fraction of single-atom sites, unfortunately, the non-uniform
chemical composition of the cavity leads to variable coordina-
tion of single atom sites thus compromising chemical selectiv-
ity and/or activity.[12]
On the other hand, heteroatoms doped 2D
carbon material-based SACs can actuate chemical reactions at el-
evated rates due to the chemically uniform coordination sites,
structural tunability, facile synthesis, and strong metal-ligand
interaction.[13]
The most commonly reported SACs are based on
the M─Nx (M = Co, Ni, Pt, Ru, Au, etc.) single-atom active sites
inherited in graphenic/carbonaceous network and mainly func-
tion as an electro-/thermal- catalysts due to the absence of any
bandgap and thus limit their applicability as photocatalyst.[14]
Carbon nitride (g-C3N4, CN) a moderate bandgap semiconductor
constituted of tri-s-triazine (heptazine, C6N7) units has emerged
as an ideal supporting material to decorate single atom sites.[15]
Theoretically, heptazine-constituted N-terminated cavity can ac-
commodate significantly large concentrations of metals at room
temperature (C6N7: metal ratio of 1:1).[4,16]
However, room tem-
perature or thermal synthesis of CN-based SACs using metal
salts and nitrogenous precursors remains uncontrolled, and
metal-metal interaction could lead to agglomeration as the metal
concentration raised above 1%. Preorganized precursors contain-
ing metal centers coordinated to CN constituting units can avoid
agglomeration due to the uniformly distributed metal sites.[17,18]
Partial thermal condensation of nitrogenous precursors can pro-
vide different polymeric units (melem, melam, and melon) con-
taining plenty of ─NH2 groups at a relatively lower temperature
(400–450 °C).[19,20]
We envisioned that NH2- functionality present
in melem units could serve as anchoring points (preorganized
precursor) to tightly hold the metal ions during the thermal con-
densation process to obtain high-density single-atom sites sta-
bilized on carbon nitride. The populated single atom (SA) sites
on a semiconductor material could inevitably enhance the cat-
alytic activity and selectivity for various chemical reactions in-
cluding photo-redox reactions such as the photocatalytic oxida-
tion of methane (CH4).
CH4 is a greenhouse gas and is almost 25 times more potent
than CO2. Natural gas reserves mainly contain CH4 gas, which
is utilized for heating, transport, and generating other forms of
energy important for human daily life.[21]
The development of ef-
ficient technologies for converting CH4 into portable chemicals
(methanol) can reduce global warming while creating economic
benefits.[22]
Currently, CH4 is transformed into Syn gas (CO+H2)
and methanol via energy and capital-intensive dry-reforming and
Fischer–Tropsch requiring high temperatures, pressure, and cat-
alysts regeneration. Direct synthesis of methanol from CH4 can
not only alleviate the expensive processes but also create a facile
route for chemical synthesis.[23]
Various stoichiometric homoge-
neous and heterogeneous catalysts have been explored for par-
tial CH4 oxidation; however, these processes are not selective
and require corrosive oxidants such as oleum, hydrobromic, and
trifluoroacetic acids.[24]
Moreover, these processes produce over-
oxidation products and CO2 as surplus side products, thus im-
posing the barrier to large-scale production. The use of water or
H2O2 as an oxidant and SACs as a selective catalyst could be a
desirable alternative that can achieve a high selectivity under am-
bient conditions.[25]
Additionally, employing a photocatalytic ap-
proach to overcome the energy barrier for C─H bond scissoring
can alleviate the external energy requirement thus making the
process sustainable.[26]
Herein, we have successfully synthesized Ni single atoms sta-
bilized on carbon nitride (NiCN) via thermal annealing of preor-
ganized Ni bonded melem precursor. The catalytic, photophysi-
cal, and optical properties were determined using various tech-
niques including synchrotron-based wide-angle X-ray scattering
(WAXS) and X-ray absorption. The synthesized catalyst exhibited
good to excellent activity for photocatalytic CH4 oxidation into
methanol using H2O2 as an oxidant. The influence of H2O2 dose
and catalytic selectivity was thoroughly investigated.
2. Results and Discussion
2.1. Synthesis and Characterization
The synthesis process for the NiCN SAC is depicted in Figure 1a
(Section 2.0 in Supporting Information). Melamine was used as
C-/N- source because of its low cost, easy availability, and safe na-
ture. The low-temperature condensation of melamine at 400 °C
yielded melem units.[27]
The resulting melem was suspended in
water (1.0 g/100 mL) followed by dropwise addition of nickel
acetate tetrahydrate (NiAc) under continuous stirring. The ob-
tained mixture is then centrifuged, washed many times with wa-
ter to remove the excess unbound Ni salt, and freeze-dried. Fi-
nally, nickel-integrated melem powder was further annealed at
550 °C for 2 h maintaining a heating rate of 3 °C min−1
. The
obtained powdered material was washed with dilute acid solu-
tions followed by washing with water until pH became neutral
and dried under vacuum at 80 °C.
TEM and HR-TEM images of the NiCN catalyst showed ag-
glomerated graphenic nanosheets with a rough surface due to
the condensation of metal complexed melem (Figure 1b; Figure
S1a–d, Supporting Information). In contrast to CN (Figure S2,
Supporting Information), NiCN demonstrated a porous struc-
ture due to the condensation of relatively bigger units, there-
fore, lacking a high condensation degree. The porous structure
of NiCN was also evident from the N2 adsorption–desorption
isotherm, which demonstrated a high specific surface area of
97 m2
g−1
(Figure S3, Supporting Information). The observed
surface area was relatively higher than the reported surface
area of CN demonstrating condensation of melem units pro-
motes porosity in catalyst.[28]
Interestingly, NiCN nanosheets
did not exhibit any sign of nickel-based nano/sub-nano clus-
ters/particle formation suggesting the ultrasmall distribution of
Ni in the CN matrix (Figure S1, Supporting Information). How-
ever, the formation of sub-nanometric Ni species (atomic clus-
ters or single atoms probably in the form of Ni-Nx/Ox/Cx) cannot
be overruled. Therefore, aberration-corrected high-angle annular
dark-field scanning transmission electron microscopy (HAADF-
STEM) images were acquired revealing the atomic dispersion
of individual nickel atoms as bright dots all over the surface of
NiCN sheets (indicated through white arrows and red circles)
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Figure 1. a) Synthetic scheme of NiCN SAC using melem coordinated nickel ion followed by thermal annealing. b) HR-TEM images of NiCN SAC at
20 nm scale bar showing the absence of any nanoparticles dispersion on graphenic sheets. AC-HAADF-STEM images of NiCN SAC at c) 5 nm scale bar
showing the dense population of Ni SA in CN scaffold. d) at 1 nm scale bar showing the bright spots of Ni in the carbonaceous framework. Red circles
show the presence of Ni atoms. e) STEM HAADF image and EDS mapping of the area for f) C g) Ni h) N i) O j) composite of Ni, N, and C.
(Figure 1c,d). Moreover, STEM-EDS mapping disclosed the dis-
tribution of Ni single atoms along with all key elements (C, N,
and O) over the entire carbon nitride scaffold (Figure 1e-i; Figure
S1e, Supporting Information). The EDS signals of Ni and respec-
tive elements were entirely overlapped suggesting the uniform
distribution of Ni on CN support. Melem with three NH2- func-
tional groups provides binding sites to anchor Ni ions forming
Ni complexes that hamper the probability of metal agglomeration
during annealing, therefore, generating Ni single atoms probably
bonded to the N─, O─ in the heptazine network.
X-ray diffraction (XRD) was recorded to determine any reflec-
tions attributed to the Ni species and interlayer distance of the
sheets of the catalyst (Figure 2a). The wide-angle XRD pattern
of the CN exhibits two broad diffraction peaks at 2𝜃 values of
27.48° and 13.64° due to the (100) and (002) plane.[29]
The (002)
diffraction peak with ≈0.33 nm interplanar d-spacing originated
from 𝜋–𝜋 interacted stacked sheets while the weak (100) peak at
13.64° with ≈0.68 nm interplanar distance occurred due to in-
plane packing of heptazine (C6N7) units in CN framework.[30]
Interestingly, for NiCN, the (002) peak was slightly shifted to a
shorter 2𝜃 value (26.44°) with a prominent peak broadening sug-
gesting increased interplanar sheets distance due to incorpora-
tion of Ni in CN scaffold via partial condensation of Ni coordi-
nated melem structure.[31]
Furthermore, the (100) peak almost
disappeared indicating loss of periodicity (long-range ordering of
C6N7 units) which complied with previous reports where conden-
sation of melem produces less ordered condensation compared
to CN.[32,33]
Notably, no pronounced peak related to the metallic
nickel or similar species was observed confirming that the syn-
thesized material contains most of the nickel in atomic form.
The Fourier transform infrared (FTIR) spectroscopy of melem
displayed characteristics vibrational band at 802 cm−1
due to
the bending vibration of the triazine (𝛿C3N3) ring in the tri-s-
triazine (C6N7) units (Figure 2b).[34]
Additionally, a strong band in
3000–3270 cm−1
originated due to the stretch of terminal/strand
─NH2/NH (𝜈N─H) and ─OH (𝜈O─H). The FTIR signals in the fre-
quency range of 1030–1510 cm−1
originated from the stretch-
ing vibration of the triazine ring (𝜈C3N3). The strong bending vi-
bration of surface adsorbed H2O and C═O (𝛿H2O, 𝜈C═O) stretch
at 1534 cm−1
suggests the presence of intercalated water in H-
bonded melem.[35,36]
The FTIR spectra of CN displayed all the
signature peaks for heptazine (C6N7) units with reduced peak
intensity for (𝛿H2O, 𝜈C═O) and (𝜈N─H/(𝜈O─H) peak corroborating
condense C6N7 units in CN scaffold.[37]
After the condensation
of Ni-coordinated melem units, the resulting NiCN displayed an
FTIR spectrum closely matched with CN, suggesting the basic
N-linked heptazine structure of CN remains preserved in NiCN.
In contrast to CN, NiCN displayed increased IR signal intensity
for peaks at 1407, 1321, and 1242 cm−1
that might be due to the
presence of Ni coordination and relatively less condensing struc-
ture changing the vibration pattern. Furthermore, strong vibra-
tion associated with carbonyl stretch and H2O bending were sig-
nificantly reduced suggesting residual C═O and intercalated/H
bonded water was removed during the thermal annealing step.
The UV–vis absorption spectra of pristine CN exhibited a
broad absorption band in the range of 250–400 nm with an
extended band tail up to 450 nm (Figure 2d). The intense
band (250–400 nm) originated from the band-to-band tran-
sition between the hybrid molecular orbitals constituted via
N2p/C2p hybridization.[38,39]
The NiCN also displayed a similar
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Figure 2. a) XRD pattern of CN and NiCN. b) FTIR spectra of melem, CN, and NiCN. c) PL emission spectra CN and NiCN. d) UV–vis spectra of CN
and NiCN.
absorption profile except, overall visible absorption in the range
of 450–800 nm was drastically increased. The increased visible
absorption could be attributed to the coordination of Ni species
to the CN network.[40,41]
On the other hand, the unchanged ab-
sorption pattern in the 250–400 nm range indicates an intact
heptazine-constituted CN scaffold in the NiCN structure.
The charge carrier recombination dynamics that govern the
photocatalytic performance of catalysts were determined using
photoluminescence (PL) spectroscopy under a 380 nm wave-
length excitation source (Figure 2c). The PL spectrum of CN ex-
hibited a broad and intense PL band located at 460 nm due to the
direct band-to-band (CB to VB) recombination of electrons.[42,43]
The NiCN also displayed an almost identical PL recombination
profile, however, the peak intensity was significantly reduced.
The reduced PL intensity in NiCN suggests delayed recombi-
nation due to the partial charge transfer from CN to Ni single
atoms coordinated with the CN framework. The incorporation of
Ni SA species increases the charge separation efficiency due to
the extended conjugation degree. The PL lifetime measurement
clearly demonstrates that Ni-embedded structures (NiCN) have a
relatively higher average lifetime (𝜏avg − 190 μs) compared to CN
(𝜏avg − 72 μs; Figure S4 and Table S1, Supporting Information).
X-ray photoelectron spectroscopy (XPS) was used to deter-
mine the surface and subsurface chemical composition, oxi-
dation state, and possible chemical interactions between con-
stituent elements present in the material (Figure 3a–c; Figure
S5-6, Supporting Information). XPS survey scan of CN and NiCN
showed all the core and sub-core-level peaks of constituting el-
ements (CN:C, N, and O; NiCN:C, N, O, and Ni, respectively;
Figure S5a; Figure S6a, Supporting Information). Ni content of
the NiCN was calculated to be 3.36 at% (2.92 wt.%; Table S2,
Supporting Information). The Ni content of bulk materials deter-
mined using ICP-MS was found to be 0.89 wt.%. Since XPS is a
surface technique that gives the chemical composition of the sub-
surface species (≈10 nm), the calculated Ni content using ICP-
MS was considered a more precise value. The high-resolution
(HR) XPS spectra of NiCN in the C1s region showed three de-
convoluted peaks located at binding energy (BE) values of 284.8,
285.9, and 288.0 eV. The XPS signal at ≈284.8 was assigned to
sp3
C─C turbostratic adventitious carbons while the other two
peaks centered at BE ≈285.9 and 288.0 eV originated from sp2
ter-
tiary (C─(N)3) and secondary (N─C═N) carbons constituting N-
linked heptazine (C6N7) network of carbon nitride (Figure 3a).[44]
The N1s core level HR-XPS of NiCN exhibited three major peak
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Figure 3. Core-level high-resolution XPS spectra of NiCN in a) C1s region, b) N1s region, and c) Ni2p region. EEMS map for N K-edge of d) CN, e)
NiCN, and EEMS map for Ni L-edge of f) NiCl2, g) NiO, and h) NiCN. i) N K-edge NEXAFS spectra of CN (dark gray) and NiCN (dark red). j) Ni L-edge
NEXAFS spectra of NiCl2 (yellow) NiO (blue), and NiCN (dark red).
components at BE ≈398.4, 399.7, and 401.0 eV from sp2
sec-
ondary C═N─C and tertiary N─(C)3 nitrogen in N-bridged hep-
tazine (C6N7) scaffold and sp3
hybridized primary nitrogens
(─NH2/NH) at the edge of sheets (Figure 3b).[45,46]
A very small
peak at 404.7 eV occurs due to the 𝜋–𝜋*
transition in the conju-
gated carbon nitride network. Two O1s peak components at 531.3
and 535.2 eV originated from uncondensed C═O/N─C─O oxy-
gens and surface adsorbed ─OH oxygen/moisture (Figure S6b,
Supporting Information).[47]
XPS spectra of NiCN and CN have
not revealed any major difference suggesting the tri-s-triazine
constituted structure of CN remains intact in NiCN. Ni2p XPS of
NiCN reveals two sharp peak components at 855.7 and 873.5 eV
assigned to Ni2p3/2 and Ni2p1/2 peak components of +2 oxidized
Ni (Figure 3c).[48]
The presence of intense satellite peaks suggests
Ni2+
species were coordinated to N/O in carbon nitride.[49]
Also,
XANES results demonstrated Ni2+
species in NiCN slightly fol-
low the pattern of Ni-O/N species (Figure 4) and therefore cor-
roborate heptazine N coordinated Ni could be present as Ni─O
single atom species.
Synchrotron-based near-edge X-ray absorption fine structure
(NEXAFS) spectroscopy using soft X-ray was performed to eval-
uate the electronic environment and coordination pattern of the
materials. Excitation–emission matrix spectroscopy (EEMS) ob-
tained in the X-ray excitation energy range of 250–2000 eV and
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Figure 4. Synchrotron-based Wide Angle X-ray Scattering (WAXS) 2D images and corresponding Q values of a,b) NiCN and c,d) CN. No additional peaks
for any nanoparticle/nanoclusters were observed. e) XANES spectra and f) FT-EXAFS spectra of Ni metal foil, NiNC, and NiCN. Wavelet transform (WT)
EXAFS map of g) Ni foil, h) NiNC, and i) NiCN.
recording partial fluorescence yield (PFY) demonstrates the ele-
mental compositions of the materials. The EEMS spectra CN dis-
played intense C K-edge and N K-edge fluorescence bands cen-
tered at ≈284 and 400 eV (Figure 3d). In addition to C K-edge
and N K-edge, NiCN displayed an additional band for the Ni L-
edge and O K-edge that suggests the presence of Ni SA species
coordinated to oxygen (Ni─O) (Figure 3e). The comparison of Ni-
specific scans for NiCl2, NiO, and NiCN displayed that not all the
oxygen in NiCN was because of Ni─O single atom species but a
populated concentration was also present in the CN framework
(Figure 3f–h). Similarly, the C K-edge EEMS scan signal for CN
and NiCN displayed a major difference in oxygen fluorescence
(Figure S7a,b, Supporting Information).
The C K-edge NEXAFS spectra of CN and NiCN displayed two
characteristic 𝜋* resonance peaks at 281.6 and 284.3 eV origi-
nating from the 𝜋*C═C resonance of uncondensed/adventitious
carbons and 𝜋*
N─C═N transition in heptazine units (Figure S7c,
Supporting Information).[50]
A broad band in the higher energy
excitation region was observed due to 𝜎*
N─C═N and 𝜎*
C─N excita-
tion of electrons. The NiCN and CN’s N K-edge NEXAFS spectra
also exhibited two 𝜋* resonance peaks at 399.2 and 402.1 eV.[51]
The peak at 399.2 eV originated from 𝜋*
C─N═C excitation of N in
C6N7 moieties while the peak at 402.1 eV originated from 𝜋*
N─C3
resonance of bridging nitrogen (Figure 3i).[52]
The 𝜎*
transition of
sp2
C─N and C─N═C functionalities creates a broad 𝜎* region.
No major difference in C K-edge and N K-edge NEXAFS spec-
tra corroborates the similar structural connectivity in the CN and
NiCN.
The Ni L-edge NEXAFS spectra of NiCl2 and NiO displayed two
intense Ni L3 and Ni L2 peaks for Ni2+
species. Both Ni L3 and Ni
L2 peaks were split into two peaks due to t2g and eg transitions in
the 2+ Ni species (Figure 3j).[53]
In contrast, Ni L-edge NEXAFS
of NiCN displayed a small signal due to the relatively low concen-
tration of Ni compared to pure NiCl2 and NiO. Further, Ni L3 and
Ni L2-edges do not show any splitting suggesting Ni species were
present in atomic state.[54]
To discern the nano-structural attributes, in-plane periodicity
and stacking pattern of the materials synchrotron-based wide-
angle X-ray scattering (WAXS) was utilized. The synchrotron-
based X-ray radiation with a wavelength of 0.8202 Å and detec-
tor distance of 170 mm allows better resolution compared to
CuK𝛼 radiation (1.5418 Å) used for the XRD measurement.[55]
Q value (in Å−1
) that is a wavelength-independent parameter was
reported to compare the 2𝜃 values. The 2D detector image was
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first calibrated using LaB6 (NIST 660b) then measured spectra
were used for the calculation of d spacing. The synchrotron-based
WAXS 2D detector images of NiCN displayed an intense diffrac-
tion ring for the (002) plane with a faint inner ring for the (100)
plane of carbon nitride (Figure 4a).[56]
A few weak fringes for
(300) and (004) planes were also visible in the 2D color map.
The transformation of the 2D detector image to intensity ver-
sus Q plots for CN exhibits (002) and (100) peaks at the Q val-
ues of 1.92 and 0.90 Å−1
with a corresponding d-spacing of 3.2
and 1.6 Å (Figure 4b).[57]
The Q values for the weak (300) and
(004) planes were calculated to be 3.02 and 3.85 Å−1
. Overall, the
WAXS 2D map and Q values of NiCN were almost identical to
CN showing similar crystalline structure (Figure 4c,d). Further-
more, the absence of any peaks related to Ni/Ni-related species
(oxides/nitrides) precludes the possibility of any sub-nanometric
particulates and suggests the atomic distribution of Ni in the CN
matrix.
To understand the chemical oxidation state, electronic charge
distribution, and coordination pattern of Ni single atoms, X-ray
absorption near-edge structure (XANES) spectra of NiCN in the
Ni K-edge region were acquired (Figure 4e). To compare the
chemical environment, XANES of standard Ni foil, NiCl2, NiO,
and standard Ni SACs in graphene (NiNC) were also collected.
As can be seen from Figure 4e, the Ni K-edge absorption energy
of NiCN appeared at a much higher value than Ni foil (Ni0
) and
remained close to Ni2+
(NiCl2 and NiO) demonstrating positively
charged Ni species present in the 2+ oxidation state.[58,59]
A close
evaluation demonstrates that XANES of NiCN matches closely
to NiNC suggesting the presence of isolated Ni─N4 type species
in graphenic carbon nitride framework.[60,61]
Furthermore, NiCN
does not display any pre-edge features, however, a slight hump in
the rising edge suggests Ni might have weak coordination with
O species. The weak coordination might be with O2 and H2O
species and has been previously reported in coordinatively un-
satisfied species.[62,63]
To discern the coordination pattern and neighboring environ-
ment, Fourier-transform extended X-ray absorption fine struc-
ture (FT-EXAFS) spectra of materials were acquired using hard
X-rays (Figure 4f). FT-EXAFS spectra of NiCN showed two in-
tense peaks at 1.51 and 2.75 Å (Figure 4f).[64]
The first sharp peak
at 1.51 Å was originated from Ni─N first shell scattering sug-
gesting strong coordination of Ni in the carbon nitride frame-
work. The second relatively weaker peak at 2.75 Å was assigned
to the second shell scattering for Ni─N─C.[65]
A significantly in-
tense second shell scattering was due to high Ni concentration
and identical numbers of carbon atoms (Ni─N─C) in the scatter-
ing path of the heptazine cavity. Previous reports on high-density
SACs also demonstrate the relatively high intensity of the sec-
ond scattering peak.[66,67]
The Ni─Ni metallic peak at 2.18 Å in Ni
foil was absent in NiCN validating the isolated distribution of Ni
sites.[17]
Furthermore, EXAFS spectra of bulk NiO demonstrate
an intense signal at 2.53 Å, which also remains absent in NiCN
EXAFS and excludes the possibility of any Ni─O isolated species
(Figure S8, Supporting Information).[68]
However, the presence
of weak coordinated Ni─O2/H2O species cannot be completely
excluded.[69]
The EXAFS of NiCN and standard Ni SACs (NiNC)
in graphenic scaffold demonstrates an almost similar scattering
pattern further confirming the presence of Ni in atomic state. EX-
AFS data fitting of NiCN demonstrates two Ni─N first shell coor-
dinations at bond lengths of 1.90 and 2.00 Å due to asymmetric
structure (Figure 4f; Figures S9–S11, Supporting Information).
The calculated coordination number (CN) for Ni─N (1.90 and
2.00 Å) was found to be 1.79 and 2.14 corroborating planar Ni
coordination in the heptazine cavity, similar to graphenic Ni─N4
of NiNC (Table S3, Supporting Information).[61]
Wavelet transform (WT) EXAFS spectra of Ni foil, NiNC,
NiCN, and NiO were plotted to understand the exact coordina-
tion structure of Ni centers. The WT map of Ni foil displays a
sharp contour zone at K = 9.36 Å−1
and R = 2.18 Å due to Ni─Ni
coordination (Figure 4g). Two weak contour zones at K = 7.50
and 11.73 Å−1
, R = 4.30 and 4.40 Å originated due to long-range
metallic scattering. In contrast, NiNC with isolated Ni embed-
ded in graphenic structure exhibited only a single sharp zone
centered at K = 10.31 Å−1
and R = 1.83 Å assigned to Ni─N,
therefore, demonstrating the absence of any Ni─Ni interaction
(Figure 4h).[70]
As expected NiCN also displayed a single zone at
K = 12.38 Å−1
and R = 2.16 Å suggesting the single atom iden-
tity of Ni sites (Figure 4i). The bifurcated zone parallel to R space
value of 1.50 and 2.75 Å and relatively higher K space position for
NiNC implies the contribution of first and second path scattering
originated from single metal centers.[71]
Bulk NiO on the other
hand shows a distinct Ni─O zone at K = 7.53 Å−1
and R = 2.53 Å
with a weak Ni─O─Ni second shell scattering at K = 7.12 Å−1
and
R = 4.20 Å (Figure S8b, Supporting Information). The absence of
such features in the NiCN WT map verifies the planar Ni center
and the absence of any Ni─O interaction.
2.2. Photoelectrochemical and Photocatalytic Performance
The visible-light-induced charge generation efficiency of the
materials was evaluated by recording the photoelectrochemical
(PEC) performance of samples (Figure 5; Figure S12, Supporting
Information). In a three-electrode, setup materials deposited on
FTO were assigned as a photoanode. The photocurrent density of
the materials as a function of time (J–t curve) was determined un-
der AM1.5G solar irradiation (100 mW cm−2
). NiCN displayed a
significantly enhanced photocurrent density at 0.6 V vs Ag/AgCl
(1.23 V vs RHE; water oxidation potential) compared to pristine
CN that demonstrates high photocarrier generation and separa-
tion rate (Figure 5a). The light on–off cycles prove that the gener-
ated current was truly photogenerated. The increased photocur-
rent density of NiCN might be associated with better charge sep-
aration in a metal-coordinated CN framework. The Nyquist plot
obtained from electrochemical impedance spectroscopy (EIS)
demonstrates a smaller semicircle diameter for the NiCN due to
lower charge transfer resistance (Figure S12, Supporting Infor-
mation). Furthermore, extrapolation of the linear region in the
Mott–Schottky plot provides a value of flat band potential value
of CN and NiCN to be −0.321 and −0.324 eV (Figure S13, Sup-
porting Information). Since CN is an n-type semiconductor, the
Fermi level will lie close to the conduction band, therefore the flat
band potential can be considered as a conduction band. After the
addition of Ni SA sites in the CN scaffold, the value of the NiCN
conduction band was slightly increased suggesting partial charge
transfer from CN to Ni centers. Since NiCN displayed improved
visible light-responsive behavior with atomically dispersed cat-
alytic centers, the performance of NiCN was evaluated for the
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Figure 5. a) J--t curve showing the photoresponse of CN and NiCN as a function of time b) Schematic diagram of the photocatalytic reactor used for
the CH4 oxidation reaction c) 1H NMR spectra of reaction product after photocatalytic reaction, Lower to the upper panel: CN using 1000 μL H2O2
(showing the high intensity of CH3OOH), NiCN using 250 μL H2O2 (showing only CH3OH signals), NiCN using 1000 μL H2O2 (showing reduced
CH3OH peak and the emergence of CH3OOH signal) d) Photocatalytic oxidation of CH4 to oxygenates and their distribution using CN, NiCN, and other
control catalysts under various conditions, Time: 2 h, CH4 pressure-20 bar (2mPa), Light Source: AM1.5G e) CH3OH selectivity using various catalysts
f) EPR spectra of the NiCN without H2O2 Conditions: 0.5 mg catalyst, H2O:MeOH (20:1) 200 μL, 200 mM DMPO. g) EPR spectra of the CN with H2O2.
Simulation parameters for component 1: aN = 1.60 mT, aH = 2.26 mT, peak-to-peak linewidth 0.13 mT (assigned to carbon-based DMPO adducts).
component 2: aN = 1.50 mT, aH = 1.50 mT, peak-to-peak linewidth 0.13 mT (assigned to DMPO-OH adduct). Ratio carbon-based adduct and DMPO-
OH adduct: 0.6. Conditions: 0.5 mg catalyst, H2O:H2O2:MeOH (20:1:1) 200 μL, 200 mM DMPO h) EPR spectra of the NiCN with H2O2. Simulation
parameters for component 1: aN = 1.60 mT, aH = 2.26 mT, peak-to-peak linewidth 0.13 mT (assigned to carbon-based DMPO adducts). component 2:
aN = 1.50 mT, aH = 1.50 mT, peak-to-peak linewidth 0.13 mT (assigned to DMPO-OH adduct). Ratio carbon-based adduct and DMPO-OH adduct: 0.3
Conditions: 0.5 mg catalyst, H2O:H2O2:MeOH (20:1:1) 200 μL, 200 mM DMPO i) Plausible mechanism of oxidation of CH4 in the presence of H2O2
and NiCN SAC.
photocatalytic partial oxidation of CH4 to liquid oxygenates. The
reaction was performed in a custom-made reactor with a quartz
window and gas inlet and outlet valve (Figure 5b; Figure S14,
Supporting Information). The catalysts were dispersed in water
(9 mL) followed by the addition of H2O2 (1 mL, 30 wt.%) as an
oxidant and kept the pressure of CH4 at 2 MPa. The photoreac-
tor was irradiated under AM1.5 G solar simulated light using a
class A solar simulator. The reaction products were analyzed by
1
H NMR using D2O/DMSO-d6 mixture as a diluent and internal
standard (Figure 5c; Figures S15–S26, Supporting Information).
The water peak suppression was implemented. The quantiﬁca-
tion was done by adding a known concentration of possible prod-
ucts as an internal standard.
The results of photocatalytic CH4 oxidation using various cat-
alytic components under various conditions are summarized in
Table S4 (Supporting Information). Pristine NiCl2 under visible
irradiation does not yield any trace of liquid oxygenates suggest-
ing bare Ni atoms cannot promote CH4 oxidation (Figure 5d).
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Similarly, NiNC with a zero-bandgap graphenic framework re-
mains photocatalytically silent and does not produce any product.
On the other hand, NiO can oxidize CH4 leading to the formation
of CH3OH and CH3OOH (≈611 μmol g−1
cat after 2 h) corrobo-
rating NiO semiconductor catalysts can promote the CH4 oxida-
tion under visible irradiation. The NiCN under dark conditions
was unable to produce any product demonstrating visible light
is essential for the CH4OR. Furthermore, both CN and NiCN
under photoirradiation conditions and the absence of H2O2 dis-
played extremely small CH3OH and concluded H2O2 was in-
dispensable for the reaction. The small amount of CH3OH un-
der photoirradiation without H2O2 might be due to the forma-
tion of hydroxyl radicals by water oxidation. As expected, NiCN
(24 μmol g−1
cat after 2 h) displayed more product volume com-
pared to CN (116 μmol g−1
cat after 2 h) due to populated charge
carrier generation. In the next step, we evaluated the influence
of H2O2 on product formation. As can be seen from Figure 5d,
the CN catalysts after the addition of 250 μL H2O2 show a sig-
nificant enhancement in product yield reaching a total prod-
uct formation of 980 μmol g−1
cat. CH3OOH (685 μmol g−1
cat)
remains a dominating oxidation product with a small amount
of CH3OH (295 μmol g−1
cat). Further increase of H2O2 con-
centration to 500 μL increased the total oxygenate concentra-
tion up to 1769 μmol g−1
cat. However, the CH3OH yield re-
mains almost identical suggesting non-selective oxidation. When
H2O2 concentration was raised to 1000 μL, the total C1 oxy-
genate yield reached 4645 μmol g−1
cat a dominating concentra-
tion of CH3OOH (3002 μmol g−1
cat) and moderate concentra-
tion of HOCH2OOH and HCOOH. However, methanol yield
(193 μmol g−1
cat) and selectivity (4%) remain unprecedently low
(Table S4, Supporting Information). The CH3OH selectivity us-
ing CN catalysts with 250, 500, and 1000 μL H2O2 was found
to be 28%, 16%, and 4%, respectively, indicating that increased
H2O2 concentration was detrimental (Figure 5e). Interestingly,
NiCN in the presence of 250 μL H2O2 displayed highly selec-
tive oxidation of CH4 to CH3OH reaching a CH3OH yield of
1591 μmol g−1
cat. No product such as CH3OOH or HCOOH was
observed depicting almost 100% CH3OH selectivity (Figure 5e).
Gaseous product analysis after the CH4 photooxidation using GC
does not display any trace of CO2 suggesting CH4 was not overox-
idized to CO2. NiCN with a lower H2O2 concentration ca. 50 and
100 μL displayed nonselective oxidation and CH3OOH was ob-
served as the dominant product. The CH3OH selectivity using
50 and 100 μL H2O2 was found to be 14 and 23%. Raising the
amount of H2O2 to 500 μL increased the total oxygenates yield
to 1822 μmol g−1
cat however CH3OH products remain almost
unchanged. The presence of CH3OOH suggests that increased
H2O2 concentration drives an alternative reaction pathway. In-
terestingly, when the H2O2 amount was increased to 1000 μL in
the presence of NiCN, the CH3OH yield dropped significantly.
However, the overall oxygenate concentration remains almost the
same. Apart from CH3OOH, HCOOH was also observed in the
reaction product demonstrating that NiCN with enhanced H2O2
concentration promotes the overoxidation of the product. Based
on these observations, it can be concluded that NiCN SAC un-
der optimized photocatalytic and H2O2 conditions can lead to al-
most 100% selectivity. The catalytic performance and selectivity
of various previously reported SACs and nanoparticulate-based
photocatalyst has been compared and summarized in Table S5
(Supporting Information). Further to validate that catalysts were
stable under catalytic conditions and Ni atoms do not agglomer-
ate, the recycled NiCN catalysts were analyzed with TEM and AC-
HAADF-STEM (Figure S27, Supporting Information). The TEM
and AC-HAADF-STEM images of recycled catalyst do not show
the presence of any nanoparticulate deciphering the absence of
any agglomeration and high catalyst resiliency. Additionally, af-
ter the reaction, the reaction product was analyzed using ICP-
MS analysis which demonstrated a significantly low Ni content
(0.89 μg L−1
) corresponding to 0.01% metal loss.
To understand the nature of produced reactive oxygen species
(ROS) during the photocatalytic oxidation of CH4, room temper-
ature electron paramagnetic resonance (EPR) analysis was per-
formed under dark and light conditions. For the trapping of gen-
erated radical species, methanolic 5,5-dimethyl 1-pyrroline N-
oxide (DMPO) was used as a spin trap agent. Control experi-
ments, without any catalyst and H2O2 do not give any signal ei-
ther in dark or irradiation conditions (Figure 5f). After adding
NiCN catalysts and light irradiation, without any H2O2, no sig-
nal for any radical can be detected suggesting NiCN alone can
not generate any radical. Interestingly, pristine CN shows weak
EPR sextet (1:1:1:1:1:1) signals under irradiation and the ab-
sence of H2O2 (Figure S28, Supporting Information). The sim-
ulation of EPR spectra reveals the observed spectra correspond
to DMPO-CH3. The appearance of •CH3 radical signals can be
explained as follows: CN under photocatalytic conditions can ac-
tivate O2 to generate superoxide radicals anions (O2
•−
) that con-
comitantly transform to •OOH radicals. The resulting DMPO-
OOH spin trap adduct is extremely short-lived at room tem-
perature (>50 s at r.t.) and immediately degrades and reacts
with methanol, which concomitantly produces a relatively sta-
ble DMPO-CH3 adduct.[72,73]
Therefore, the presence of DMPO-
CH3 signals using CN as a catalyst provides ancillary evidence of
the generation of •OOH radicals. When the CN catalyst was irra-
diated in the presence of H2O2 strong signals corresponding to
DMPO-OH were observed with relatively weak DMPO-CH3 sig-
nals (Figure 5g). These findings corroborate that CN generates
both •OH and •OOH radicals in the presence of H2O2 that leads
to non-selective oxidation of CH4 producing CH3OOH as a dom-
inating product. On the other hand, EPR spectra of NiCN in the
presence of H2O2 and irradiation selectively generate DMPO-OH
with negligible signals of DMPO-CH3 substantiating high selec-
tivity of NiCN toward CH4 oxidation was due to the formation
of only •OH radicals (Figure 5h). The presence of •OH signals
under dark using NiCN catalysts indicates Fenton-type oxidation
of H2O2 under dark conditions.[74]
However, the absence of any
CH4 oxidation product under dark conditions unveils that the
generation of •OH radicals is not enough, and photogenerated
charge carriers are essential to promote C─H bond cleavage. It
should be noted that the •OH signal intensity of NiCN was 2.7
times lower than CN which explains relatively lower product yield
for NiCN compared to CN.
Though the exact mechanism is not clear at this stage, the
increased CH3OH selectivity in the presence of NiCN can
be explained due to the formation of hypervalent Ni species
(Figure 5i).[25]
Under visible light irradiation, isolated Ni atoms
in the NiCN react with H2O2 to form a hypervalent nickel
oxo species (NiIII
═O).[75]
This hypervalent Ni═O can promote
C─H bond cleavage of CH4 resulting in the formation of •CH3
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radical.[63,76]
These •CH3 radicals can either combine with •OH
or •OOH radicals forming CH3OH and CH3OOH, respectively.
Previous reports on Cu SACs and Cu-O-Cu dual single-atom cat-
alysts (DSACs) supported on carbon nitride demonstrate that
Cu sites enable cleavage of H2O2 and CH3OH to create ─OH,
─OOH, and ─CH3 absorbed Cu sites.[77,78]
These sites stabi-
lize •CH3 radicals from further oxidation to produce liquid oxy-
genates after reacting with •OH and •OOH radicals. Based on
EPR studies it was evident that NiCN can produce •OH radi-
cals selectively, which combines with •CH3 radicals and produces
CH3OH selectively.[79]
When an excess of H2O2 was used, more
Ni centers were not available to produce •OH radicals and some
•OOH radicals were also produced on non-metallic CN sites.
Therefore, CH3OOH was also observed as a by-product at higher
concentrations. Furthermore, the observation of a large amount
of CH3OOH in CN was in accordance with the generation of
•OOH radicals, which combined with •CH3 radicals to produce
CH3OOH.
3. Conclusion
In conclusion, an inexpensive Ni single-atom catalyst with high
metal loading was synthesized by thermal condensation of Ni
tethered melem units. The nickel complexation with melem
units prevents the probability of metal coarsening during an-
nealing and affords high Ni single-atom site density. Detailed
characterization, including XAS analysis, reveals unsymmetrical
tetra-coordinated Ni─N4 sites embedded in the heptazine cav-
ity providing robust configuration. When employed as a cata-
lyst for photocatalytic CH4 oxidation, almost 100% selectivity for
methanol formation was observed under visible light irradiation.
NiCN outperformed its analogues corroborating that atomically
isolated Ni species stabilized on the surface of carbon nitride play
a significant role in CH4OR reaction. The improved performance
was attributed to the formation of hypervalent NiIII
═O sites after
the reaction with H2O2 and the stabilization of methyl radicals.
The finding reveals careful control of the coordination environ-
ment and high density of single atom site can activate the C─H
bond of CH4 to selectively transform in liquid oxygenates. Be-
yond CH4OR catalysis, such SACs systems will find application
in various thermal- photo- and electro-catalytic organic transfor-
mations.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
P.A., I.N. and S.K. acknowledges financial support from institutional
sources of the Department of Inorganic Chemistry, Palacký University Olo-
mouc, Czech Republic. Ondrej Tomanec, Ms. J. Stráská, and Martin Petr
are acknowledged for the measurement of HRTEM, TEM, and XPS, respec-
tively. J. Michalička and CzechNanoLab Research Infrastructure supported
by MEYS CR (LM2023051) are also acknowledged for the TEM results. D.
Milde is acknowledged for ICP-MS analysis. P.K., J.H. and M.G.K. would
like to thank the University of Calgary’s Canada First Research Excel-
lence Fund (CFREF) for financial assistance. The authors also acknowledge
Canadian Light Source (project: 35G12344), a national research facility of
the University of Saskatchewan, which is supported by the Canada Founda-
tion for Innovation (CFI), the Natural Sciences and Engineering Research
Council (NSERC), the National Research Council (NRC), the Canadian In-
stitutes of Health Research (CIHR), the Government of Saskatchewan, and
the University of Saskatchewan. Drs. Ning Chen, Adam Leontowich, Beat-
riz Diaz-Moreno, Jay Dynes, Tom Regier, and Zachary Arthur are kindly
acknowledged for XANES/EXAFS, WAXS, and soft X-ray analysis on the
samples.
Conflict of Interest
The authors declare no conflict of interest.
Author Contributions
P.K. and S.K. conceived the research, synthesized and characterized the
catalyst, evaluated the catalytic performance, interpreted the results, and
wrote the initial draft of the manuscript. P.A. performed the FTIR, UV–
vis, and PL characterization. X.W. helped in XAS data interpretation. V.T.S.
and P.N. performed EPR analysis and performed simulations. J.W. and D.T.
assisted in photocatalysis experiments. O.F.F. and I.N. measured the XRD
diffractograms. J.K. performed the BET measurement. J.H., M.G.K., and
S.K. supervised the research and edited the manuscript. All co-authors
read and approved the final version of the manuscript.
Data Availability Statement
The data that support the findings of this study are available from the cor-
responding author upon reasonable request.
Keywords
carbon nitride, heterogeneous catalysts, methane oxidation, Ni single
atom catalysts, photocatalysis
Received: May 31, 2023
Revised: October 30, 2023
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Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single Atom Sites on Carbon Nitride for Selective Photooxidation of Methane into Methanol