Efficient photocatalytic conversion of CH4into ethanol with O2over nitrogen vacancy-rich carbon nitride at room temperature

Article abstract of DOI:10.1039/d0cc07397k

A record ethanol production rate of 281.6 μmol g^?1h^?1for the photocatalytic conversion of methane over nitrogen vacancy-rich carbon nitride at room temperature was achieved. Systematic studies demonstrate that the CH₄was activated by the highly reactive ˙OH radicals generated,viaH₂O₂, from the photo-reduction of O₂with H₂O.

Full text of DOI:10.1039/d0cc07397k

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Efficient photocatalytic CH4 conversion into ethanol with O2 over
nitrogen vacancy-rich carbon nitride at room temperature
Zhongshan Yang,ab Qiqi Zhang,ab Liteng Ren,ab Xin Chen,ab Defa Wang,ab Lequan Liu,*ab and
Received 00th January 20xx,
Accepted 00th January 20xx
Jinhua Ye abc
DOI: 10.1039/x0xx00000x
A record ethanol production rate of 281.6 μmol g-1 h-1 was achieved CH4 was mainly photochemically converted into CH3OOH, and the
in photocatalytic methane conversion over nitrogen vacancy-rich ·OOH radicals were identified as the active species. As ·OH possesses
carbon nitride at room temperature. Systematic studies higher oxidative potential,20 it is promising that ·OH radicals
demonstrate CH4 was activated by highly active ·OH radical generated from oxygen reduction can be utilized for efficient
generated from O2 photo-reduction with H2O via H2O2.
methane activation. Fan et al. made an attempt and reported that
methane was activated by ·OH radicals in photocatalytic conversion
Methane, as an abundant resource on earth and an important of methane into oxygenates.21 Nevertheless, ·OH radicals were
feedstock for chemicals, its efficient conversion has been attracting mainly generated from H2O oxidation rather than from O2 reduction.
great attentions.1 Due to highly stable regular tetrahedron structure, It is highly desirable to develop efficient system for transforming CH4
traditional indirect transformation of methane, or syngas route, into ethanol with O2 under mild conditions.
bears tremendous energy input.2 It is highly desirable to develop
In this work, highly efficient photocatalytic conversion of CH4 into
efficient route to transform methane into up-market products under ethanol was achieved over the nitrogen vacancy (VN) -rich carbon
mild conditions. Photocatalysis, as a burgeoning technology which nitride with O2 as oxidant. Systematic study indicated that methane
could fulfil water splitting and environmental remediation at room molecules were activated by ·OH radicals photo-generated from O2
temperatures, brings this field new perspectives and feasibility.3-6 reduction with H2O via H2O2. Moreover, the introduction of VN
11
12-14
WO3,7-9 TiO2,10,
and g-C3N4
have been reported for enhanced the light absorption and charge carrier separation
photocatalytic conversion of methane into methanol, formic acid efficiency, then promoted the production of H2O2, which ultimately
and methyl hydroperoxide so on. However, it still faces great facilitated the photocatalytic conversion of methane into ethanol.
challenge to promote the efficiencies, particular for value-added and
more attractive products like ethanol.
Pristine g-C3N4 (CN) was synthesized through
polymerization method by using melamine as the precursor. The
a thermal
The selection of oxidant plays a crucial role in efficient reduced carbon nitride samples (RCN-x) with nitrogen vacancy were
photocatalytic conversion of methane into oxygenates.15 Among synthesized by a modified solid-state chemical reduction method (x
them, water is theoretically an ideal oxidant but suffers from rather = 3, 5, 7, represent the weight ratio of CN:NaBH4 were 1:3, 1:5, 1:7)
low conversion efficiencies.16-18 Hydrogen peroxide is a strong and with the purpose of efficient photocatalytic O2 reduction to oxidative
clean oxidant, while its high cost and difficulties in transportation or radicals.22 Morphology and microstructure of the as-synthesized
storage restrict its application.19 From the perspective of availability samples were characterized by scanning electron microscopy (SEM)
and operating safety, oxygen is an attractive candidate for efficient and transmission electron microscopy (TEM) (Fig. 1a-b, Fig. S1).
photocatalytic conversion of methane into oxygenates. A recent Compared to CN sample，the RCN sample is fluffy and the surface is
work reported by Ye et, al. primarily validates its feasibility.20 While more rough and uneven. Many edges and notches were formed,
which may increase the reactive areas and provide more active sites
for oxygen reduction.23 The crystal structures of CN and RCN samples
were shown in the Fig. S2. Subsequently, series of characterizations
a TJU-NIMS International Collaboration Laboratory, Key Laboratory of Advanced
Ceramics and Machining Technology (Ministry of Education) and Tianjin Key
Laboratory of Composite and Functional Materials, School of Material Science
and Engineering, Tianjin University, Tianjin 300072, China.
E-mail: Lequan.Liu@tju.edu.cn.
were executed to verify the formation of nitrogen vacancies in RCN
samples. As demonstrated in Fig. 1c, RCN samples exhibited obvious
electron spin resonance (ESR) signals with g value of 2.002, which
could be attributed to unpaired electrons on the carbon atoms of the
heptazine rings due to the formation of VN.5 It finds further support
from Fourier transform infrared spectroscopy (FT-IR) spectra, where
b Collaborative Innovation Centre of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China
c International Centre for Materials Nanoarchitectonics (WPI-MANA),
National Institute for Materials Science (NIMS), Japan.
† Electronic Supplementary Information (ESI) available.
See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0CC07397K
Fig. 1 (a) SEM and (b) TEM image for RCN. (c) ESR spectra of CN and RCN samples.
(d) Graphic illustration of formed VN and C≡N groups in RCN samples.
a new vibration bonds emerged at 2180 cm-1 showing the formation
of C≡N functional groups in RCN samples (Fig. S3). Furthermore, an
enhancement of the ESR signal intensity was observed along with the
increase of CN/NaBH4 ratio, indicating the growing concentration of
nitrogen vacancy. Similar trends also manifested in the decreased C-
N=C/N-(C)3 ratio from X-ray photoelectron spectroscopy (XPS)
analysis (Fig. S4). Moreover, the decreased C-N=C/N-(C)3 ratio of CN
(2.68) and RCN (1.93) samples from N 1s XPS spectra indicates that
the VN are majorly at C-N=C sites.5 The graphic illustration of formed
VN and C≡N functional groups are shown in the Fig. 1d.
Fig. 2 (a) Photocatalytic conversion of methane over CN and RCN-x samples in 1 h.
(b) Average production rate of photocatalytic methane conversion over RCN-5
during different reaction time (1 h, 2 h, 3 h, 4 h, 5 h).
The photocatalytic methane conversion reaction was performed
at room temperature (25 ± 1 ℃) under the pressure of 0.1 MPa O2
(purity, 99.99%) and 2 MPa CH4 (purity, 99.999%) in a 50 mL stainless-
steel autoclave equipped with a quartz window to allow light
irradiation (Fig. S5). A 300 W Xe lamp was employed as the light
source, the light intensity was determined to be 100 mW cm-1 (Fig.
S6). The generated products in liquid and gas phase were quantified
reaction time was investigated (Fig. 2b). An average ethanol
production rate in 3 hours reached as high as 281.6 μmol g-1 h-1 over
RCN-5. To the best of our knowledge, it is a new record for C2H5OH
product among all the photocatalytic methane conversion reaction
reported under room temperatures (Table S1). Moreover, the
photocatalysts after reaction remains relatively stable known from
XRD pattern and TEM images in the Fig S12-13. As the reaction time
was prolonged to 4 hours or 5 hours, the C2H5OH production rate
decreased accompanied with the HCOOH and CO2 production rate
increased. This result may indicate the overoxidation of produced
alcohols, which will be discussed later.
To clarify the oxidative intermediates during the photocatalytic
CH4 conversion, H2O2 production over CN and RCN-x samples was
first studied (Fig. 3a, left). As can be seen, H2O2 was detected over all
samples, with C/N atomic ratio ranging from 0.63 to 0.75 as
determined by Elemental Analysis (EA) (Table S2). According to
previous work, the production of H2O2 was mainly generated by two-
electron reduction of O2 over reduced g-C3N4.22 Moreover, H2O2
production rate was obviously accelerated along with the increase of
C/N atomic ratio. Similar trend was observed by XPS analysis (Table
S3). These results agree well with the performance of photocatalytic
CH4 conversion into ethanol, suggesting the route of photo-reduction
of O2 to H2O2. The generation of ·OH radicals was clarified by ESR
1
by H nuclear magnetic resonance (NMR) and gas chromatography
(GC) measurements, respectively (Fig. S7, Fig. S8). After the amount
of catalyst was optimized to 20 mg (Fig. S9), blank experiments were
carried out, and they revealed the indispensable role of oxygen and
light in the photocatalytic conversion of methane (Fig. S10).
Subsequently, as shown in Fig. 2a, photocatalytic conversion of
methane into oxygenates occurred over CN sample, though the
conversion rate was relatively low. Impressively, an C2H5OH
production rate of 211.2 μmol g-1 h-1 with a selectivity of 74.1% was
achieved over RCN-5. These results indicate nitrogen vacancies in g-
C3N4 remarkably promote photocatalytic methane conversion into
ethanol with O2. Moreover, the nitrogen vacancy played an
important role in enhanced alcohols' selectivity as demonstrated in
a control experiment under dark conditions (Fig. S11). As the
concentration of possible H2O2 or ·OH oxidative intermediate varied
with light irradiation, the ethanol production rate as a function of
2 | Chem. Comm., 2020, 00, 1-4
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measurement and photoluminescence (PL) spectra. As shown in molecule (Fig. S14).24 In order to further identify the role played by
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Figure 3b, the four-peak signature with a 1:2:2:1 line shape in the ESR ·OH radicals, a control experiment using tertD-bOuI:ta10n.o10l 3(T9B/DA0)CaCs0t7h3e97K
spectra is ascribed to the presence of ·OH radicals.4 As the valence trapping agent was carried out (Fig. 3a, right). After capturing the ·OH
band (VB) of g-C3N4 is unable to oxidize H2O to form ·OH radicals radicals, no obvious ethanol was detected. These results revealed
(E(H2O/·OH)=2.8 V vs. RHE, pH=7).13 Furthermore，a much higher that ·OH radicals were directly involved in photocatalytic CH4
ESR signal of RCN-5 was detected as compared to CN, indicating VN conversion into ethanol over RCN-5. It is reasonable to draw a
remarkably enhanced the generation of ·OH radicals. Similar trend conclusion that highly active ·OH radicals, mainly generated from
was further confirmed in the PL test using coumarin as a probe photo-reduction of O2 with H2O via H2O2, activate and transform
methane molecules into ethanol. Based on the discussions above,
the proposed reaction route of photocatalytic methane conversion
with oxygen in water over RCN was illustrated in Fig. 3c.
It is worthy to point out that the concentration of generated H2O2
is an important factor influencing the performance of photocatalytic
methane conversion.11, 25 As demonstrated in Fig. 2a, the ethanol
production over RCN-7 decreased accompanied with increased CO2
production as compared with RCN-5, though the H2O2 production
was further accelerated. This could be ascribed to the overoxidation
18
of produced alcohols by excessive ·OH radicals.11,
Similar
phenomenon was also observed in Fig 2b and Fig. S15. That is the
concentration of in-situ generated H2O2 therefore exists an optimal
value,14 which was determined to be 217.1 μmol L-1 in this work.
UV-vis diffuse reflectance spectroscopy (UV-vis DRS), PL
characterization and electrochemical impedance spectroscopy (EIS)
measurement were adopted to study the intrinsic characteristics of
photocatalysts. Compared with CN, RCN-5 exhibited a red shift of the
absorption edge, depicting the enhancement of light absorption in
both UV region and visible region (Fig. 4a). The extended light
absorption could be attributed to the incorporation of C≡N group.26
In addition, as shown in the inset, the results revealed a narrower
bandgap of the RCN-5 sample (2.65 eV) compared to CN (2.81 eV)
due to the introduction of nitrogen vacancy.22 In other words, the
introduction of nitrogen vacancy broadened light absorption and
narrowed the bandgap. The fluorescence decays (Fig. 4b) fitted with
three exponential types (Table S4) demonstrated that RCN-5
possesses a much shorter fluorescence lifetime (4.38 ns) than CN
(10.18 ns), indicating that the charge carrier separation efficiency
was greatly improved.27, 28 A much lower steady state PL intensity
was observed, which further verified the enhanced charge carrier
separation efficiency (Fig. S16). This might be caused by alleviation
of charge carrier recombination due to conduction band (CB)-to-
defect states charge transfer.29, 30 The EIS pattern displays that RCN-
5 has a smaller arc radius than CN, showing a lower charge transfer
resistance (Fig. S17), indicating that the photogenerated electrons
and holes could transfer to the interface and be consumed by the
surface reaction quickly.31 Thus, more efficient electron can localize
on the surface terminal sites of RCN samples, which promoted the
photocatalytic reduction of O2.32
In summary, efficient photocatalytic conversion of CH4 with an
ethanol production rate of 281.6 μmol g-1 h-1 was determined over
RCN-5 at room temperature. Moreover, systematic investigations
and characterizations indicated that methane molecules were
activated by ·OH radicals generated from O2 reduction through H2O2.
The highly active ·OH radicals activate and transform CH4 into
ethanol. The introduction of nitrogen vacancy enhanced the light
absorption and improved the charge carrier separation efficiency,
then promoted the H2O2 production through oxygen reduction with
water, which ultimately boosted the efficient photocatalytic
Fig. 3 (a) Photocatalytic H2O2 production over CN and RCN-x with different
content of nitrogen vacancies in 3 hours ( and represents the C/N
atomic ratio from XPS analysis and EA results, respectively). The right is a
control experiment using tert-butanol (TBA) as the trapping agent for ·OH
radicals. (b) ESR spectra of CN and RCN-5 with 5, 5-dimethyl-1-pyrroline N-
oxide (DMPO) as a trapping agent. (c) The hypothesized reaction route of
photocatalytic methane conversion with Oxygen in water over RCN.
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Fig. 4 (a) UV-vis DRS spectra and transformed Kubelka-Munk function versus
photon energy plots (inset) of CN and RCN. (b) Fluorescence decays of CN and
RCN-5.
methane conversion into ethanol. The discovery reported may
provide a novel and feasible route to promote the efficiency of
photocatalytic methane conversion.
We acknowledge support from the National Natural Science
Foundation of China (21673157, 22072106 and 21633004) and
Beiyang Reserved Academic Program of Tianjin University.
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Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/D0CC07397K
An ethanol production rate as high as 281.6 μmol g-1 h-1 was achieved during photocatalytic CH4
conversion with O2 at room temperature.