Efficient photocatalytic oxidation of methane over β-Ga2O3/activated carbon composites

Article abstract of DOI:10.1039/c7ra05692c

Efficient oxidation of methane over heterogeneous catalysts under ambient conditions is still a challengeable study toward C1 utilization and atmospheric cleansing. In this study, by using a hydrolysis method combined with an impregnation process, β-Ga₂O₃ nanoparticles supported uniformly on activated carbon (AC) can be obtained readily. The as-prepared Ga₂O₃/AC composites show efficient performance for photocatalytic oxidation of CH₄ under ultraviolet irradiation. The experimental results indicate that photocatalytic activity toward CH₄ oxidation is strongly dependent on the weight ratio of Ga₂O₃ to AC. 15%-Ga₂O₃/AC exhibits the highest catalytic activity, which is more than sixfold of P25, a benchmark photocatalyst. The photoluminescence spectra show the decreased recombination centers and the diminished recombination of photo-generated electrons (e^-) and holes (h⁺) when Ga₂O₃ nanoparticles were deposited on AC. Further investigation reveals that the excellent photo-oxidation activity for CH₄ over Ga₂O₃/AC can be ascribed to a synergistic effect involving strong adsorption capacity and improved separation of photo-generated e^-/h⁺ pairs. Moreover, the photocatalytic oxidation of CH₄ obeys pseudo-first-order kinetics and the cycling experiment indicates that Ga₂O₃/AC composites possess stable photocatalytic performance for CH₄ oxidation. The underlying photo-oxidation mechanism is also investigated using electron paramagnetic resonance (ESR) and radical scavenging experiments. This study demonstrates that photocatalysis with Ga₂O₃/AC is a highly efficient method toward the oxidation of low concentrations of CH₄, which provides valuable information for atmospheric environmental cleansing.

Full text of DOI:10.1039/c7ra05692c

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PAPER
Efficient photocatalytic oxidation of methane over
b-Ga2O3/activated carbon composites†
Cite this: RSC Adv., 2017, 7, 37508
a
ab
Zhihui Wen,ab Jun Dai,ab Yao Liab and Banghao Yaoa
*
Jianping Wei, Juan Yang,
Efficient oxidation of methane over heterogeneous catalysts under ambient conditions is still
a
a
challengeable study toward C1 utilization and atmospheric cleansing. In this study, by using
hydrolysis method combined with an impregnation process, b-Ga2O3 nanoparticles supported
uniformly on activated carbon (AC) can be obtained readily. The as-prepared Ga2O3/AC composites
show efficient performance for photocatalytic oxidation of CH4 under ultraviolet irradiation. The
experimental results indicate that photocatalytic activity toward CH4 oxidation is strongly dependent on
the weight ratio of Ga2O3 to AC. 15%-Ga2O3/AC exhibits the highest catalytic activity, which is more than
sixfold of P25, a benchmark photocatalyst. The photoluminescence spectra show the decreased
recombination centers and the diminished recombination of photo-generated electrons (eꢀ) and holes
(h+) when Ga2O3 nanoparticles were deposited on AC. Further investigation reveals that the excellent
photo-oxidation activity for CH4 over Ga2O3/AC can be ascribed to a synergistic effect involving strong
adsorption capacity and improved separation of photo-generated eꢀ/h+ pairs. Moreover, the
photocatalytic oxidation of CH4 obeys pseudo-first-order kinetics and the cycling experiment indicates
that Ga2O3/AC composites possess stable photocatalytic performance for CH4 oxidation. The underlying
photo-oxidation mechanism is also investigated using electron paramagnetic resonance (ESR) and
radical scavenging experiments. This study demonstrates that photocatalysis with Ga2O3/AC is a highly
efficient method toward the oxidation of low concentrations of CH4, which provides valuable
information for atmospheric environmental cleansing.
Received 20th May 2017
Accepted 23rd July 2017
DOI: 10.1039/c7ra05692c
rsc.li/rsc-advances
Intergovernmental Panel on Climate Change, CH4 is respon-
sible for about twenty percent of man-made global warming and
1. Introduction
the greenhouse gas effect of CH4 is nearly 28 times greater than
that of an equivalent mass of CO2.5 More seriously, coal-bed gas
or shale gas exploitation results in the increase of CH4 release
into the atmosphere. Therefore, the conversion of atmospheric
methane into equimolar amounts of carbon dioxide is
extremely important for retarding global warming.
As the major ingredient of natural gas, CH4 is widely employed
as a clean fuel and is also a common carbon source for chemical
production. In view of its effect on improving the living quality
of human beings, the discharge of CH4 has been considered to
be negligible for a long time, which results in an ever-increasing
CH4 concentration in the atmosphere.1,2 Recently, with the
increasing concern surrounding atmospheric contamination
and global warming, the negative effects of CH4 emission
receive more and more attention.3,4 According to a report of the
Due to the high energy of C–H bond (434 kJ molꢀ1) and
nonpolar characteristics of CH4 molecule, oxidation of CH4 has
been mainly studied via thermal catalysis with noble metal and
transition-metal oxide catalysts in the past decades.6,7 However,
the high reaction temperature (ꢁ673 K) and inefficiency in
oxidizing low concentration of CH4 are the defects of conven-
tional thermo-catalysis. Although the microorganism treat-
ments using methane oxidative bacteria could decompose
CH4,8,9 it has been proved to be inefficient for the practical
application because this approach is time consuming and
difficult to control. Wei and his co-workers have recently re-
ported the oxidation of CH4 with hydroxyl radicals (cOH)
generated via Fenton reagent.10 The results demonstrate that
cOH derived from Fenton reagent can drive the oxidation of CH4
under a certain reaction condition. However, the homogeneous
Fenton systems have some limitations because Fe2+ and H2O2
aState Key Laboratory Cultivation Base for Gas Geology and Gas Control, The
Collaborative Innovation Center of Coal Safety Production of Henan, School of
Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, 454000, P.
R. China. E-mail: yangjuanhpu@163.com
bInstitute of Chemical Safety, School of Safety Science and Engineering, Department of
Applied Chemistry, College of Chemistry and Chemical Engineering, Henan
Polytechnic University, Jiaozuo, 454000, P. R. China
† Electronic supplementary information (ESI) available: The schematic diagram
of photocatalytic experimental apparatus; SEM image of bare Ga2O3
nanoparticles; the effect of light intensity on the photocatalytic oxidation of
CH4; XRD pattern and SEM image of as-prepared 15%-TiO2/AC composite; the
amount of produced CO2 during the photocatalytic oxidation of CH4; the
comparison of XRD patterns and XPS spectra between freshly prepared and
recycled 15%-Ga2O3/AC photocatalyst. See DOI: 10.1039/c7ra05692c
37508 | RSC Adv., 2017, 7, 37508–37521
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cannot be separated from the reaction system and reused for methylbenzene) over b-Ga2O3 nanoparticles under ambient
the next cycle. Additionally, iron ions are easy to precipitate conditions and found that the photocatalytic activity of b-Ga2O3
under an alkaline condition, which restricts the CH4 removal particles was much superior to commercial P25.28,29 Porous b-
with Fenton reagent can be only performed in a narrow pH Ga2O3 was highly photoactive for mineralizing benzene to CO2
range.
and no obvious deactivation was observed during the prolonged
Heterogeneous photocatalysis is regarded as an ideal tech- irradiation about 80 h.28 The high activity and long-term
nology for solving the urgent energy and environmental stability of Ga2O3 could be primarily ascribed to its wider
issues,11–13 especially in removing various gaseous pollutants at band gap energy and the resulting stronger oxidative capacity.
ambient pressure and temperature.14,15 Previous studies have
In this study, activated carbon (AC) supported b-Ga2O3
also indicated CH4 molecules can be activated and subse- composites (Ga2O3/AC) were prepared via an in situ synthetic
quently oxidized via TiO2 photocatalytic technology,16,17 method, including facile hydrolysis and impregnation process.
however, the efficiency of CH4 photo-oxidation remains noto- The structure and physical–chemical properties of as-prepared
riously low. Adsorption and photoreaction are two successively Ga2O3/AC were characterized by using various analysis tech-
key steps, which are generally interactional during the hetero- niques. Ga2O3/AC composites demonstrate excellent photo-
geneous photocatalytic process. Surface adsorption and the catalytic activity for methane oxidation under ultraviolet
diffusion of reactants to active sites are the preconditions of irradiation. b-Ga2O3 is chosen because it is an environmental
photocatalytic reactions. Strong adsorption of target reactants friendly semiconductor with wide band-gap satisfying the
facilitates the catalytic reaction between the adsorbed reactant requirement of strong oxidation capability. AC facilitates the
and surface active species. As a result, the efforts toward adsorption of CH4 molecules and the uniform distribution of
improving the adsorption of reactants in photocatalytic system Ga2O3 nanoparticles. The effects of Ga2O3 weight ratios, irra-
have been made through constructing porous materials18,19 or diation intensity, and initial CH4 concentration on the photo-
adsorbent-loaded composites.20,21 Fabricating of the composites catalytic oxidation efficiency were studied systematically. The
with adsorptive support and traditional photocatalyst is results of dark-adsorption experiments and PL spectra indicate
considered as an effective approach to obtain the adsorption- that the superior photocatalytic activity of Ga2O3/AC composites
enhanced photocatalyst. Adsorbent-loaded photocatalysts can be ascribed to a synergistic effect involving strong adsorp-
usually possess excellent synergistic effect because of the tion capacity and improved separation of photo-generated eꢀ/h+
continuous transfer of adsorbed reactants to the supported pairs. Besides, the photocatalytic oxidation of CH4 over Ga2O3/
photocatalytic active sites. Meanwhile, the prolonged retention- AC obeys pseudo-rst-order kinetics. The underlying photo-
time of intermediates on the composite catalyst facilitates oxidation mechanism is also investigated using electron para-
improving the mineralization of pollutants.
magnetic resonance (ESR) and radicals scavenging experi-
Carbon materials, such as graphene oxides (GO),22 carbon ments. The present work provides theoretical support and green
nanotube (CNT)23,24 and activated carbon (AC),25 have been approach to CH4 oxidation and atmospheric environmental
extensively used as one kind of excellent supports for cleansing.
semiconductor-based photocatalyst nanoparticles, which can
efficiently improve the distribution of active components, the
electron-transfer from semiconductor particles to carbon
2. Experimental section
2.1 Materials and agents
supports, and the stability of catalysts. Furthermore, in the eld
of CH4 adsorption and storage, porous carbon materials have
also received wide attention due to their outstanding adsorp-
tion capacity.26,27 Among these carbon-based supports, AC is
commonly used owing to its unique advantages, including
abundant microporous structure, high adsorption capacity, low
production cost, and facile recycling.
b-Gallium oxide (b-Ga2O3) has wider band-gap energy (4.8
eV) than classic photocatalyst TiO2 (3.2 eV). The reported
valence-band potential (ꢀ7.75 eV) of Ga2O3 is lower than that
(ꢀ7.41 eV) of TiO2 (relative to the vacuum energy level) and the
conduction-band potential (ꢀ2.95 eV) of Ga2O3 is higher than
Gallium nitrate hydrate was purchased from Strem Chemicals.
Anhydrous ethanol, tetrabutyl orthotitanate, acetic acid and
ammonia solution were obtained from Aladdin Industrial Inc.
(Shanghai, P.R. China). The standard gas of N2, CH4 and O2
(purity > 99.99%) were obtained from Jinggao Gas Inc. (Beijing,
P.R. China). All chemicals from commercial sources were used
without further purication except AC (AP4-60, purchased by
Suzhou Calgon Carbon Inc.).
2.2 Preparation of photocatalyst
that (ꢀ4.21 eV) of TiO2.28,29 In consequence, the photo-holes and The commercial AC was rstly grinded and ltrated using a 60
photo-electrons generated upon the surface of Ga2O3 possess mesh lter. The obtained AC particles were treated successively
stronger redox capacity than those formed over TiO2. This with 10% NaOH solution and 10% HNO3 solution. The products
contributes to the oxidation of persistent organic pollutants and were collected, washed with deionized (DI) water several times
ꢂ
the mineralization of stable intermediates upon the surface of and dried at 70 C.
Ga2O3.29 In addition, Ga2O3 is also an environmental friendly
Ga2O3/AC composite photocatalysts were fabricated using
material according to Worksafe Australia criteria. Fu and his hydrolysis method combined with impregnation process. In
colleagues have reported the photocatalytic degradation of a typical synthesis, 2 g of treated AC was dispersed into 30 mL
volatile
organic
contaminants
(e.g.
benzene
and DI water, followed by vigorous stirring 30 min to obtain
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a uniform suspension. 1.0 g of gallium nitrate hydrate was
dissolved in 20 mL of anhydrous ethanol to form a homogenous
solution, which was added dropwise into the above suspension
under magnetic stir. Aer 1 h, the mixed solution of ethanol
and ammonia solution (volume ratio 1 : 1) was added slowly
into the obtained suspension, which kept continuous stirring
for 3 h at room temperature.28 The resulting precipitate was
centrifuged, washed with DI water, dried at 80 ꢂC and calcinated
at 600 ꢂC for 2 h to prepare Ga2O3/AC. For simplicity, it is
assumed that all of gallium nitrate hydrate was converted to
Ga2O3. By changing the amount of gallium nitrate, Ga2O3/AC
composites with different weight ratios of Ga2O3 were obtained
and labeled as x-Ga2O3/AC, which x represents the weight ratio
of Ga2O3 in the as-prepared composites. According to the above-
mentioned way, a series of Ga2O3/AC composites with Ga2O3
weight ratio of 5%, 10%, 15%, 20% and 25% were prepared and
denoted as 5%-Ga2O3/AC, 10%-Ga2O3/AC, 15%-Ga2O3/AC, 20%-
Ga2O3/AC and 25%-Ga2O3/AC, respectively.
2.4 Photocatalytic oxidation of CH4
Photocatalytic oxidation of CH4 was implemented with a home-
made xed bed tubular quartz reactor of 500 mL capacity. The
schematic diagram of photocatalytic experimental apparatus
was shown in Fig. S1 (see, ESI†). All of the photocatalytic
experiments were carried out at ambient temperature and
pressure unless indicated otherwise. Firstly, 200 mg of
composite catalyst was added to 10 mL of H2O and ultra-
sonicated for 20 min. The aqueous suspension was then coated
uniformly on the surface of ve glass sheets (each glass sheet is
ꢂ
2 cm ꢃ 8 cm), followed by drying at 60 C until the water was
completely removed. The as-obtained supported catalysts were
horizontally put into the middle of the tubular reactor. Then,
the reactor was vacuumed repeatedly to remove H2O and CO2
that adsorbed on the surface of catalysts and reactor inner-wall.
CH4, O2, and N2 were mixed at a certain percentage via a gas
mixer. The obtained gaseous mixture was used to afford
a reactant stream. The initial concentration of CH4 in the
stream was 1.56 mmol Lꢀ1. Before the irradiation, the reaction
system was kept in the dark for 2 h to ensure an adsorption–
desorption equilibrium state established between photo-
catalysts and reactants. Then, the photocatalysts were irradiated
using a 10 W Philips low-pressure mercury lamp (with a wave-
length centered at 254 nm) located in the middle of the quartz
reactor. At a given time interval, 1.0 mL gas was collected from
the reactor and analyzed using a Thermo gas chromatograph
(GC) with a molecular sieve 13ꢃ column and a HP-Plot/U
For comparison, TiO2/AC nanocomposite sample was also
synthesized via in situ compositing process according to the
recent report.30 Briey, a certain amount of treated AC was
dispersed into 30 mL DI water, which was stirred for 30 min to
ensure the thorough dispersion of AC. Then, 2 mL of tetrabutyl
titanate (TBOT) was mixed with 10 mL anhydrous alcohol and
added dropwise to the above suspension of AC with magnetic
stirring. Aer stirring for 3 h, the suspension was treated under
microwave irradiation at 700 W for 15 min. The precipitates
thus obtained were centrifuged, washed with DI water, and
capillary column equipped with
a thermal conductivity
ꢂ
dried at 80 C in an electric oven to prepare TiO2/AC.
detector and a ame ionization detector. The experimental
temperature was maintained at 25 ꢄ 1 ꢂC by recirculating
a cooling water system. For comparison, the photocatalytic
activity of 15%-TiO2/AC was also investigated under the same
experimental conditions, as those used for 15%-Ga2O3/AC.
In general, photocatalytic activity is determined by the
formation of radical species, such as photo-holes (h+), electrons
(eꢀ), superoxide (O2cꢀ), and cOH. Comparative investigations of
the effect of active species on the photocatalytic activity were
conducted by using scavenger reagents to remove the different
radicals. Potassium iodide (KI), potassium dichromate
(K2Cr2O7), tert-butyl alcohol (TBA), and benzoquinone were
employed as scavengers for h+, eꢀ, O2cꢀ, and cOH, respectively.
Typically, 200 mg of photocatalyst was mixed with 1 mM of the
different trapping reagents in 10 mL of water and ultrasonicated
for 20 min. Then, the aqueous suspensions were coated onto
the glass sheets followed by drying at 60 ꢂC until H2O was
completely removed. The coated sheets were used in the pho-
tocatalytic experiments of CH4 oxidation as described above.
2.3 Characterization of photocatalyst
The crystalline structure of as-synthesized Ga2O3/AC was iden-
tied by X-ray diffraction (XRD) on a Bruker D8 Advance
diffractometer with Cu Ka radiation (l ¼ 0.15405 nm) in the
region 2q ¼ 5–80ꢂ. The morphology of composites was observed
with a scanning electron microscope (SEM, Hitachi, SU8010).
Transmission electron microscopy (TEM) imaging was per-
formed on a FEI Tecnai G2 microscope at an accelerating voltage
of 200 kV. The X-ray photoelectron spectroscopy measurements
were performed on a Thermo scientic ECALAB 250xi system
with Mg Ka source. The binding energies were calibrated using
that of C 1s (284.6 eV). Nitrogen adsorption and desorption
measurements were performed at 77 K using Micromeritics
ASAP2020 equipment. The samples were degassed at 200 ꢂC
before measurements. Photoluminescence spectra (PLs) of the
composite catalysts were recorded using a FLsp 920 lumines-
cence spectrophotometer (Edinburgh) at room temperature with
an excitation wavelength of 220 nm. Electron spin resonance
(ESR) signals were obtained by using a Bruker ER200-SRC
spectrometer at ambient temperature with a Philip lamp of
254 nm as light source. 50 mL of sample solutions were put into
3. Results and discussion
3.1 Characterization of b-Ga2O3/AC composites
quartz capillary tubes and sealed. The capillary tubes were The crystallographic structure of as-synthesized Ga2O3/AC
inserted in the ESR cavity, and the spectra were recorded during composites was investigated by XRD and the results were indi-
the irradiation at a selected time. All the ESR measurements cated in Fig. 1, as well as with the XRD patterns of bare Ga2O3
were performed using the following settings: 20 mW microwave and AC shown for comparison. As can be seen from Fig. 1A, the
power, 100 G scan range and 1 G eld modulation.
XRD pattern of AC showed two weak broad peaks at about 24.7ꢂ
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imaging was carried out and the results were presented in Fig. 2.
Fig. 2a showed that AC had a clean and porous surface, which
was usually employed as the support owing to its huge specic
surface. When the weight ratio of Ga2O3 was low (e.g. 5.0%-
Ga2O3/AC), nano-Ga2O3 particles were primarily dispersed on
the meso-pore or macro-pore mouths of AC support, as indi-
cated in Fig. 2b. With the increase of Ga2O3 weight ratios,
spherical Ga2O3 nanoparticles occupied both the pore mouths
and the outer surface of AC, as observed from Fig. 2c and d. This
is similar to the previously reported TiO2/AC composites.34
Additionally, it was clearly seen from Fig. S2 of ESI† that bare
Ga2O3 was composed of nanoparticles with an average size of 6–
15 nm, whereas the obvious aggregation could be noted for bare
Ga2O3 particles. By comparing SEM images of Ga2O3/AC and
bare Ga2O3, it can be easily found that the loading on AC
facilitates the uniform distribution of Ga2O3 nanoparticles.
To further investigate the microstructure of Ga2O3/AC
composites, the TEM images were provided and shown in Fig. 3.
It is clearly indicated that Ga2O3 nanoparticles (black dots) with
nanosized dimension were uniformly deposited on the surface
of AC (gray region), and the large scale agglomerate was not
detected in 5.0%-Ga2O3/AC and 15%-Ga2O3/AC composites
(Fig. 3a and b). A high-resolution TEM image of 15%-Ga2O3/AC
was shown in Fig. 3d. The lattice fringes of d ¼ 0.282 nm and
d ¼ 0.255 nm matched those of (002) and (111) crystallographic
planes of b-Ga2O3, respectively.35 The intimate contact of acti-
vated carbon with b-Ga2O3 lattice was also evidenced from the
HRTEM image. However, when the weight ratio of Ga2O3
increased up to 25%, the as-formed Ga2O3 particles were
inclined to agglomerate (Fig. 3c). It indicates that the Ga2O3
nanoparticles are easier to agglomerate on the surface of Ga2O3/
AC composites with increasing Ga2O3 weight ratios. It is well
known that the catalytic activities of nanocomposite photo-
Fig. 1 (A) XRD patterns of bare AC, Ga2O3 and 15%-Ga2O3/AC, (B) XRD
patterns of Ga2O3/AC composites with different weight ratios of
Ga2O3.
and 43.8ꢂ, indicating that AC possessed a mainly amorphous
structure. A sharp peak at 26.7ꢂ assigned to (002) crystallo- catalysts strongly depend on the distribution of active compo-
graphic plane of hexagonal graphitic structure was also nents on the surface of support. Compared with Ga2O3 particles
observed,31 which could be due to small regions of crystallinity deposited homogeneously on the surface of 15%-Ga2O3/AC, the
in the commercially-obtained AC substrate. Bare Ga2O3
prepared by the hydrolysis of Ga(NO3)3 in ammonium solution
exhibited the characteristic peaks at 2q values of 31.7ꢂ, 35.4ꢂ,
43.5ꢂ and 64.5ꢂ, which can be indexed as (002), (111), (311) and
(512) planes of monoclinic b-Ga2O3 (JCPDS no. 76-0573).32 It
agglomerated Ga2O3 nanoparticles in 25%-Ga2O3/AC compos-
ites may hamper the light incidence on the photoreactive sites
and consequently reduce the photocatalytic oxidation
efficiency.
The chemical components and surface chemical states of
suggests that the as-prepared Ga2O3 was predominantly nano- Ga2O3/AC composites were conrmed using XPS, and the
sized b-Ga2O3 aer the hydrolysis product was calcined at results were presented in Fig. 4. The typical XPS survey spec-
ꢂ
600 C for 2 h. This result is well consistent with the previous trum with energy ranging from 0 to 1200 eV obtained from 15%-
research.28 Among the four polymorphs of Ga2O3 reported in the
Ga2O3/AC was shown in Fig. 4a, which reveal the peaks of the
eld of catalysis, b-Ga2O3 is the most stable on thermodynamics
and all the other polymorphs can be ultimately transformed
into b-Ga2O3 by calcination.33 As indicated in Fig. 1B, except the
characteristic peaks at 24.7ꢂ, 26.7ꢂ and 43.8ꢂ belonging to
amorphous AC, the diffraction peaks at about 31.7ꢂ, 35.4ꢂ and at 23.6 eV can be ascribed to the presence of gallium in b-Ga2O3
64.5ꢂ corresponding to b-Ga2O3 were also observed, which (Fig. 4a). The O1s XPS signal observed at a binding energy of
indicated that Ga2O3/AC composites were successfully fabri- 532.4 eV (Fig. 4c) corresponded to the characteristic peak of b-
core level from Ga 2p, Ga 3d, Ga LMM Auger peak, C 1s and O
1s. The energy peaks positioned at 1149.2 and 1121.5 eV in
Fig. 4b are known to stem from Ga 2p1/2 and Ga 2p3/2, repre-
senting the Ga–O bonding.36 The energy peak of Ga 3d centered
Ga2O3.37 The XPS analysis conrmed that the as-prepared Ga2O3
was mainly nanosized b-Ga2O3, which accords with the XRD
measurement. Fig. 4d showed the C 1s peak of 15%-Ga2O3/AC
was at 284.6 eV, which was mainly owed to the AC and the
pollutant carbon from the XPS instrument.38 Besides, the
cated via the present experimental method. Additionally, it was
obvious that the diffraction peak intensity of b-Ga2O3 increased
with increasing the amount of Ga2O3 loaded on AC support.
To investigate the morphology of Ga2O3/AC composites and
the distribution of Ga2O3 particles on the surface of AC, SEM
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Fig. 2 SEM images of (a) AC, (b) 5.0%-Ga2O3/AC, (c) 15%-Ga2O3/AC, and (d) 25%-Ga2O3/AC.
Fig. 3 TEM images of (a) 5.0%-Ga2O3/AC, (b) 15%-Ga2O3/AC, (c) 25%-Ga2O3/AC, and (d) high-resolution TEM image of 15%-Ga2O3/AC.
atomic contents of Ga and C in different Ga2O3/AC nano- indicated in Table 1, the calculated weight ratios for Ga2O3 and
composite samples were calculated from XPS peak areas, which AC are closely equal to the adding ratios in the preparation
can be converted to the weight ratios of Ga2O3 to AC. As process, implying that gallium nitrate hydrate can be
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Fig. 4 XPS of 15%-Ga2O3/AC nanocomposite sample: (a) survey spectrum, (b) Ga 2p, (c) O 1s, and (d) C 1s.
Table 1 The calculated weight ratios of Ga2O3 to AC based on XPS measurements and textural characteristics of bare Ga2O3 and Ga2O3/AC
composites obtained from N2 adsorption–desorption experimentsa
Samples
Weight ratio of Ga2O3 to AC
SBET (m2 gꢀ1
)
VT (cm3 gꢀ1
)
VM (cm3 gꢀ1
)
Pore size (nm)
AC
Ga2O3
907
101
836
758
685
652
630
0.618
0.125
0.420
0.302
0.237
0.226
0.219
0.269
0.017
0.120
0.056
0.025
0.023
0.022
1.61
3.85
2.26
3.09
3.48
3.53
3.51
5%-Ga2O3/AC
10%-Ga2O3/AC
15%-Ga2O3/AC
20%-Ga2O3/AC
25%-Ga2O3/AC
5.6%
10.9%
14.3%
21.0%
26.4%
a
SBET, specic surface area obtained by BET equation; VT, total pore volume; VM, micropore volume; average pore size from pore size distribution by
BJH method.
completely converted to Ga2O3 via the as-mentioned hydrolysis 907 m2 gꢀ1 and high total pore volume of 0.618 cm3 gꢀ1
method.
because of the porous property. Aer introducing Ga2O3
Since heterogeneous photocatalysis is generally inuenced particles into AC matrix, SBET, total pore volume and micro-
by the specic surface area of catalysts, the pore structure and pore volume of the obtained Ga2O3/AC composites decreased
BET surface area of the synthesized composites were studied signicantly. Whereas the average pore diameter in the
on the basis of N2 adsorption and desorption measurements. composites increased slightly, compared to bare AC, which
Fig. 5 showed N2 adsorption–desorption isotherms of bare could be due to the formation of meso-pores or macro-pores in
Ga2O3 and 15%-Ga2O3/AC and the corresponding pore size the channels between the deposited Ga2O3 nanoparticles on
distribution was given in the inset of Fig. 5. According to the the surface of AC. The decrease in micro-pore volume of
IUPAC classication, the adsorption isotherms of bare Ga2O3 Ga2O3/AC could be ascribed to the effects of meso-pores
and 15%-Ga2O3/AC were of type IV isotherm, indicating the blocking by deposited Ga2O3, since these meso-pores acted
presence of mesoporous structures. The structural and as the primary passageway to micro porous regions in AC. On
textural characteristics of as-prepared composites can be ob- the other hand, small grain size and uniform distribution of
tained from N2 adsorption–desorption isotherms and the Ga2O3 particles could inhibit the reduction of SBET and pore
corresponding data were summarized in Table 1. For bare volume of Ga2O3/AC composites to some extent. A larger SBET
Ga2O3, the BET specic surface area (SBET) was 101 m2 gꢀ1 and can give rise to more active adsorption sites and photocatalytic
the average pore size was 3.85 nm with a narrow distribution of reaction centers, which means higher SBET will be benecial to
pore size. As indicated in Table 1, bare AC had a large SBET of improving the photocatalytic activity.39
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Fig. 6 The dark adsorption of CH4 over (A) bare AC and Ga2O3, (B)
different Ga2O3/AC composites.
Fig. 5 Nitrogen adsorption–desorption BET isotherms and pore size
distribution curves (inset) of bare Ga2O3 and 15%-Ga2O3/AC.
of AC surface was covered by Ga2O3 nanoparticles. It would
result in a negative effect on the adsorption ability of AC,
3.2 Photocatalytic properties of Ga2O3/AC composites
Surface adsorption is the precondition of photocatalytic reac- especially for the internal adsorption sites in AC because of the
tion and strong adsorption of target reactants facilitates the pore blocking by Ga2O3 particles.
catalytic oxidation between the adsorbed reactants and surface
In the present system, CH4 removal via the gas–solid cata-
active species. Therefore, the dark-adsorption of CH4 on the lytic reaction can be divided into two steps. One is the above-
surface of as-synthesized samples was investigated rstly. The mentioned adsorption process in the darkness, the other step
adsorption capacity of Ga2O3/AC composites as well as bare is photocatalytic oxidation process under UV illumination. For
Ga2O3 and AC was estimated by studying the removal rate of the unmodied AC, only 4.0% of CH4 was further removed aer
CH4 in a xed-bed reactor and the results were presented in UV irradiation for 150 min, as indicated in Fig. 6A. In contrast to
Fig. 6. For bare AC, the removal of CH4 increased gradually with AC, aer dark adsorption for 120 min, CH4 could be signi-
the increase of adsorption time and about 45.1% of the initial cantly decomposed by the photocatalytic oxidation over bare
CH4 was removed until reaching an adsorption equilibrium at Ga2O3. It can be seen from Fig. 6A that CH4 removal increased
the end of 120 min in the dark. Whereas, for bare Ga2O3, only dramatically with prolonging the time of UV illumination and
8.9% of the initial CH4 was removed in the dark-adsorption CH4 removal efficiency reached approximately 36.5% aer
process. In the case of Ga2O3/AC composites, the initial 150 min irradiation, which was mainly attributed to the pho-
removal of CH4 driven by dark adsorption was lower than tocatalytic oxidation process.
unmodied AC. It can be obtained from Fig. 6B that the
To investigate the kinetic behavior of CH4 oxidation, the
adsorption-induced removal percentages of CH4 were 20.4%, concentration data of photocatalytic CH4 degradation were
18.1%, 16.2%, 15.5%, and 14.6% for 5%-Ga2O3/AC, 10%-Ga2O3/ normalized by the initial concentration at the start of illumi-
AC, 15%-Ga2O3/AC, 20%-Ga2O3/AC, and 25%-Ga2O3/AC, nation, which was also regarded as the equilibrium concentra-
respectively. It was clearly found that the adsorption-induced tion of dark-adsorption. The as-obtained degradation
removal of CH4 decreased gradually with the increased Ga2O3 percentage (C/C0) of CH4 as a function of irradiation time was
loading. The results could be explained by SEM images and N2 presented in Fig. 7A. The blank experiment in Fig. 7A referred to
adsorption–desorption experimental results, as indicated in the photocatalytic oxidation of CH4 under UV irradiation in the
Fig. 2 and Table 1. SEM images in Fig. 2 demonstrated that most absence of Ga2O3 or Ga2O3/AC composites. No obvious
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Table 2 First-order kinetic constants and relative coefficients toward
photocatalytic oxidation of methane with bare Ga2O3 and Ga2O3/AC
composites
photocatalytic oxidation of CH4 was observed aer illumination
for 150 min in the absence of catalysts. Careful analysis of CH4
photo-oxidation revealed that the reactions follow pseudo-rst-
order kinetics and the integrated rate equation can be expressed
by:
Samples
k0 (minꢀ1
)
R2
ꢀ
ꢁ
Bare Ga2O3
0.0023
0.0039
0.0081
0.0171
0.0113
0.0067
0.9775
0.9949
0.9959
0.9981
0.9987
0.9970
C0
C
ln
¼ k0t
5%-Ga2O3/AC
10%-Ga2O3/AC
15%-Ga2O3/AC
20%-Ga2O3/AC
25%-Ga2O3/AC
where k0 represents the pseudo-rst-order rate constant
(minꢀ1). It is valid for the comparison of various composite
photocatalysts since the as-obtained photocatalytic activity is
independent of dark-adsorption. Fig. 7B described the liner
relationship between ln(C0/C) and illumination time t, where
the slopes of lines denoted pseudo-rst-order rate constant k0.
The corresponding data were summarized in Table 2.
The photocatalytic oxidation of CH4 primarily proceeded on
the outer surface of catalysts. Thus, the weight proportion and
distribution of Ga2O3 particles on the exterior surface of Ga2O3/
AC are the crucial factors in CH4 oxidation. The effect of Ga2O3
weight proportion on CH4 oxidation rate was shown in Fig. 7.
From Fig. 7A, it can be found the oxidation rate of CH4
increased with the increase of irradiation time, possibly
because of continuous formation of active radicals upon the
surface of irradiated Ga2O3 particles. Photocatalytic CH4
oxidation rates of as-synthesized composites were greatly
dependent on the weight ratio of Ga2O3 to AC. It could be ob-
tained from Fig. 7B, the pseudo-rst-order rate constant of
photocatalytic oxidation CH4 over bare Ga2O3, 5%-Ga2O3/AC,
10%-Ga2O3/AC, 15%-Ga2O3/AC, 20%-Ga2O3/AC, and 25%-
Ga2O3/AC was 0.0023, 0.0039, 0.0081, 0.0171, 0.0113, and 0.0067
minꢀ1, respectively. The photocatalytic oxidation rate rstly
increased with increasing the weight ratio of Ga2O3 to AC, and
then reduced when the weight ratio was higher than 15%. The
similar result has been reported in TiO2/AC photocatalytic
system.39 When the weight ratio of Ga2O3 was low, photo-
catalytic rate of CH4 oxidation was also low, which might be due
to the limited Ga2O3 particles loaded on the composites. With
the increase of Ga2O3 loading, more Ga2O3 were deposited on
the surface of AC. As a result, photo-reactive sites available on
the surface of catalysts increased, which is benecial for the
gas–solid catalytic reaction of CH4 oxidation. It was clearly seen
from Fig. 7 that 15%-Ga2O3/AC exhibited highest oxidation rate
of CH4, revealing that appropriate weight ratio of Ga2O3 to AC
was able to well disperse Ga2O3 nanoparticles and thus improve
the migration of photogenerated electrons. This facilitated the
separation of photoinduced electrons and holes, which could
efficiently promote photocatalytic rate. However, when the
weight proportion of Ga2O3 exceeded 15%, photocatalytic
oxidation rates decreased with the increasing of Ga2O3 weight
proportion. The reason could be due to the fact that a certain
amount of nanosized Ga2O3 particles aggregated on the surface
of Ga2O3/AC, which would reduce the efficient light absorption
and reactive sites for photocatalytic oxidation. Besides, the
reduction of SBET and pore volume (as shown in Table 1)
resulted in the decrease of CH4 adsorption on the surface of
Ga2O3/AC, which partly reduced the photo-activity for CH4
oxidation. The results further indicate that the weight propor-
tion and dispersibility of Ga2O3 particles on the surface of
composite photocatalysts are crucial for the improved photo-
catalytic activity.
To further understand the oxidation process of CH4 removal
over as-prepared Ga2O3/AC samples, the effect of light intensity
on the photocatalytic activity was investigated. Photocatalytic
oxidation of methane upon the optimal composites 15%-Ga2O3/
AC was normalized by the initial concentration at the end of
dark adsorption. As indicated in Fig. S3 of ESI,† the CH4
removal driven by photocatalytic oxidation process on 15%-
Ga2O3/AC was proportional to the irradiation intensity. This
obviously demonstrates that a higher intensity of irradiation
gives rise to a more efficient removal of CH4, suggesting the
Fig. 7 (A) Photocatalytic oxidation of CH4 over bare Ga2O3 and
Ga2O3/AC composites under UV irradiation, (B) pseudo-first-order
reaction kinetics curves of photocatalytic CH4 oxidation over as-
synthesized samples.
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incident light intensity plays an important role in CH4 photo- conditions, the reaction rates are primarily dependent on CH4
catalytic oxidation system. Under UV irradiation, stronger light initial concentration, and proceed more quickly for lower
intensities can excite more Ga2O3 catalysts and generate more concentration. To elucidate the excellent performance of Ga2O3-
charge carriers or active radical species, leading to higher CH4 based photocatalysts for CH4 oxidation, dark-adsorption and
oxidation rates. Meanwhile, the results conrm that the photocatalytic process of commercial P25 and 15%-TiO2/AC
methane removal over Ga2O3/AC composite photocatalyst is were also investigated under the same experimental conditions.
truly driven by a photocatalytic oxidation process.
Commercial P25, as a recognized benchmark photocatalyst, is
Photocatalytic oxidation of CH4 under different initial selected to compare with bare Ga2O3. As shown in Fig. 9A, only
concentrations was also carried out and the results were indi- 5.7% of the initial CH4 was removed over commercial P25 at the
cated in Fig. 8. The photo-oxidation reactions under various end of dark-adsorption, which was slightly lower than that of
initial CH4 concentrations also followed pseudo-rst-order bare Ga2O3 (8.9%). It can be due to the smaller SBET of P25 (ꢁ50
kinetics and the apparent rate constants k0 obtained from m2 gꢀ1), compared to as-prepared Ga2O3 (ꢁ101 m2 gꢀ1). 15%-
Langmuir–Hinshelwood model were 0.0171, 0.0134, 0.0102, TiO2/AC composite was also prepared via in situ synthesis
0.0070 and 0.0051 minꢀ1, respectively, with increasing the strategy, as presented in the Experimental section. Similar to
initial CH4 concentration from 1.56 to 7.90 mmol Lꢀ1 (Fig. 8B). the synthesis of Ga2O3/AC composites, the preparation of
This clearly demonstrates that photocatalytic technique is more nanosized TiO2 and the compositing of as-prepared TiO2 with
competent for eliminating low concentration of CH4. It is totally AC were simultaneously carried out. The XRD pattern and SEM
different from traditional thermal catalysis, which usually image of 15%-TiO2/AC sample were depicted in Fig. S4 and S5 of
exhibits poor catalytic activity when the concentration of reac- ESI.† The characteristic diffraction peaks at 25.4ꢂ, 37.9ꢂ, 48.1ꢂ,
tant is low.
54.7ꢂ and 63.1ꢂ were observed, indicating that as-obtained TiO2
For the photocatalytic methane oxidation, the photo- was anatase phase. SEM images of 15%-TiO2/AC showed that
generated holes or the lattice oxygen activated by photo-holes TiO2 nanoparticles were uniformly dispersed and loaded upon
are considered as the primary active species toward abstract- the surface of AC. It can be found from Fig. 9B that although
ing H from CH4.40 This step is closely related to energy and 15%-TiO2/AC and 15%-Ga2O3/AC exhibited similar adsorption
intensity of irradiation. That is, when xing the irradiation
Fig. 8 (A) Time evolution of methane photocatalytic oxidation over Fig. 9 (A) Dark-adsorption of CH4 over bare Ga2O3, commercial P25,
15%-Ga2O3/AC under UV illumination with various initial CH4 15%-Ga2O3/AC and 15%-TiO2/AC; (B) photocatalytic oxidation of CH4
concentrations, (B) the corresponding pseudo-first-order reaction upon bare Ga2O3, commercial P25, 15%-Ga2O3/AC and 15%-TiO2/AC
kinetics plots of photocatalytic CH4 oxidation.
under UV irradiation.
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capacity for CH4, the photocatalytic oxidation of CH4 over 15%-
Ga2O3/AC was signicantly greater than 15%-TiO2/AC. It
suggests that high catalytic performance of Ga2O3/AC cannot be
simply explained by the surface adsorption. CH4 removal
induced by photocatalytic oxidation over bare Ga2O3 and Ga2O3/
AC was higher than that over P25 and TiO2/AC. It could be
associated with the band gap structures of Ga2O3 and TiO2. For
semiconductor TiO2, the conduction band and valence band
consist of Ti 3d and O 2p orbitals, respectively. In the case of
Ga2O3, the valence band is composed of O 2p orbital, whereas
the conduction band consists of hybridized Ga 4s4p orbital. The
bandgap energy of Ga2O3 is 4.8 eV, much wider than that of TiO2
(3.2 eV). Additionally, the reported valence-band potential of
Ga2O3 is ꢀ7.75 eV, which is lower than that of TiO2 ꢀ7.41 eV.
Whereas, the conduction-band potential of Ga2O3 is ꢀ2.95 eV,
which is higher than that of TiO2 ꢀ4.21 eV.28,29 As a result, the
photo-holes and photo-electrons generated on Ga2O3 possess
stronger redox ability than those formed on TiO2. The intrinsic
energy-band structure of Ga2O3 contributes to the excellent
photocatalytic activity for CH4 oxidation.
Fig. 10 Recycled test of photocatalytic oxidation of CH4 over 15%-
Ga2O3/AC.
To examine the mineralization ratio during photocatalytic
CH4 oxidation, the production of CO2 over 15%-Ga2O3/AC
photocatalysts was detected and the corresponding result was
presented in Fig. S6 of ESI.† Before UV light was turned on, no
CO2 was detected, which indicated the dark-adsorption process
cannot produce CO2. Aer the light was turned on, the
concentration of CH4 decreased signicantly with the irradia-
tion time. Meanwhile, the amount of CO2 increased gradually
and around 0.93 mmol Lꢀ1 of CO2 was produced from CH4
photooxidation under UV irradiation for 150 min. Besides, no
other intermediate products, such as CO or ethane, were
detected by GC during the CH4 photocatalytic oxidation. Carbon
mass balance of 79.0% could be obtained based on the
proportion of the sum of remaining CH4 (0.105 mmol Lꢀ1) and
produced CO2 (0.93 mmol Lꢀ1) to carbon input (1.31 mmol Lꢀ1
CH4 at the end of adsorption process). This suggests that about
79.0% CH4 is mineralized to CO2 aer UV irradiation for
150 min.
The stability or recyclability of a catalyst is an extremely
crucial parameter for practical utilization. To test the stability of
as-prepared Ga2O3/AC photocatalysts, a cycling CH4 photo-
oxidation test over the optimal composite 15%-Ga2O3/AC was
thus performed and the results were indicated in Fig. 10. Aer
six cycles, the activity of 15%-Ga2O3/AC almost remained
unchanged. Moreover, the crystal structure and chemical
composition of recycled 15%-Ga2O3/AC sample were also
investigated by XRD and XPS analysis. No noticeable differences
between the freshly made and the repeatedly used samples were
observed from Fig. S7 of ESI.† The results demonstrate Ga2O3/
AC composites are stable and can be recycled toward CH4
photocatalytic oxidation.
thus the intensity of PL peak can represent the recombination
extent of photogenerated charges. Low PL intensity usually
demonstrates low recombination degree and efficient separa-
tion of h+ and eꢀ. Fig. 11 showed the PL spectra of bare Ga2O3
and various Ga2O3/AC composites. The as-prepared samples
exhibited the same PL peaks and similar curve shapes, which
indicated introduction of AC did not generate new lumines-
cence phenomena but the weight ratio of Ga2O3 indeed affected
the PL intensity. The major luminescence signals were observed
in the wavelength range of 350–480 nm. The peak at around
370 nm and 400 nm could be assigned to the recombination of
self-trapped excitons, which mainly originated from the band
edge emission.41,42 Furthermore, based on the previous reports,
b-Ga2O3 contains two kinds of Ga3+ ions.41,42 One is Ga3+ ion in
the tetrahedral site coordinated by four oxygen atoms and the
other is that in the octahedral site coordinated by six oxygen
atoms. An emission centered at 465 nm might be derived from
the recombination of electrons trapped by oxygen vacancy and
holes generated by either gallium vacancy or gallium–oxygen
3.3 Photocatalytic mechanism of CH4 oxidation over Ga2O3/
AC composites
Photoluminescence (PL) generally arises from the charge
Fig. 11 Photoluminescence spectra of bare Ga2O3 and Ga2O3/AC
recombination process of photoinduced electron and hole and composites.
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vacancy pairs.41,42 These vacancies could be formed by the
thermal treatment during the synthesis process.
More importantly, the incorporation of AC led to the
decrease in the intensity of PL spectra, which suggests that the
composite photocatalysts possess low recombination of h+ and
eꢀ. The intensity of luminescence peaks decreased in the order
of bare Ga2O3 > 5%-Ga2O3/AC > 10%-Ga2O3/AC z 25%-Ga2O3/
AC > 15%-Ga2O3/AC. When the weight ratio of Ga2O3 was low,
Ga2O3 particles were mainly dispersed on the meso-pore or
macro-pore mouths of AC, as presented in Fig. 2b, resulting in
the moderate decrease of PL intensity. As the weight ratio of
Ga2O3 to AC increased, the PL intensity of Ga2O3/AC composites
gradually decreased. A lower PL intensity usually indicates
a slower recombination rate and higher separation efficiency of
photoinduced charge carriers. 15%-Ga2O3/AC sample exhibited
lowest PL intensity, which could be due to the highly dispersed
Ga2O3 nanoparticles supported onto the surface of AC. It was
benecial for the separation of photo-induced electron/hole
pairs and consequently improved the photocatalytic activity.
However, when the weight ratio of Ga2O3 was too high, nano-
sized Ga2O3 particles could aggregate on the surface of AC,
which reduced the efficient light absorption and separation of
photogenerated h+ and eꢀ.
The photocatalytic oxidation of methane generally involves
the surface reactions of photogenerated holes and electrons,
which may also produce oxidizing species, such as O2cꢀ and cOH
to further the oxidation reactions. To identify the generation of
reactive species and investigate the photocatalytic mechanism of
Ga2O3/AC composites toward CH4 oxidation, ESR spectroscopy
with spin trapping and labeling was chosen as an effective
characterization technique. Here, we selected 5,5-dimethyl-1-
pyrroline-N-oxide (DMPO)43 as a spin-trapping agent for O2cꢀ
radicals and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)44 as
a spin-labeling agent for photogenerated electrons.
Fig. 12 ESR spectra obtained from bare Ga2O3 and 15%-Ga2O3/AC
suspensions (A) 40 mM DMPO methanol solution and (B) 0.02 mM
TEMPO aqueous solution. All spectra were recorded after 3 min of UV
irradiation. The blank represented the solution containing spin probes
alone under light illumination.
Photo-holes are considered as the predominant active demonstrate that O2cꢀ can be generated over bare Ga2O3 or
species for photocatalytic oxidation of methane. It is demon- Ga2O3/AC composites under UV irradiation and introduction of
strated that the triplet ESR signal of TEMPO can diminish or AC signicantly enhanced the photogeneration of O2cꢀ.
even vanish if it is reduced by photogenerated electrons from
The photocatalytic oxidation of CH4 over 15%-Ga2O3/AC
semiconductors.44 As a result, the generation of photo-electrons composites was further explored through a series of control
can be monitored accurately by observing the changes in the experiments. As depicted in Fig. 13, the oxidation of CH4 was
intensity of TEMPO signals. ESR spectrum of an aqueous almost unchanged aer adding TBA (a scavenger of cOH45) into
TEMPO solution showed a stable signal having triplet peaks the reaction system, indicating that cOH had little effect on
with intensity of 1 : 1 : 1 (Fig. 12A). The signal intensity photocatalytic oxidation of CH4 over Ga2O3/AC composites.
decreased moderately aer 3 min irradiation in the presence of When KI was added to the reaction system to trap the holes,46
bare Ga2O3; however, a considerable decrease in the ESR signal the CH4 oxidation was signicantly inhibited, revealing that
was observed during the irradiation of 15%-Ga2O3/AC suspen- photogenerated holes were the major oxidative species for the
sion. It indicates that the introduction of AC can enhance the photocatalytic CH4 oxidation. Addition of BQ (a scavenger of
generation of photoelectrons greatly. Furthermore, the pres- superoxide radicals47) clearly suppressed the CH4 oxidation,
ence of photoelectrons is invariably accompanied by the which suggests that O2cꢀ also play a critical role in the photo-
generation of photoholes. Hence, the reduction of ESR signal catalytic oxidation of CH4. Adding K2Cr2O7 (an electron scav-
intensity shown in Fig. 12A can reect the formation of photo- enger48) into the reaction system led to moderate decrease of
holes over Ga2O3/AC composites indirectly. In addition, as photocatalytic oxidation for CH4. All of the above experimental
shown in Fig. 12B, the characteristic peaks of DMPO-O2cꢀ can results were obtained in the presence of 79% N2 gas, 21% O2 gas
be observed in methanol suspensions of bare Ga2O3 upon and 1.56 mmol Lꢀ1 CH4.
irradiation for 3 min.43 For 15%-Ga2O3/AC composites, the
However, when adding electron scavenger K2Cr2O7 in the
signal intensity of DMPO-O2cꢀ increased obviously (about 3 presence of 100% O2 gas and 1.56 mmol Lꢀ1 CH4, the oxidation
times of bare Ga2O3) under the same conditions. These results of CH4 exhibited slight increase. It might be ascribed to the fact
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Fig. 14 Effect of Ga2O3 weight ratios on the enhancement of
photoinduced generation of superoxide radicals and photoelectrons.
Fig. 13 Controlled experiments using different radical scavengers for
the photocatalytic oxidation of CH4 over 15%-Ga2O3/AC catalyst: (a)
no scavenger, (b) TBA, (c) KI, (d) BQ, (e) K2Cr2O7, (f) K2Cr2O7 + 100% O2
gas, and (g) K2Cr2O7 + 100% N2 gas.
photogenerated holes and O2cꢀ radicals play dominant roles in
the photocatalytic oxidation of CH4 over Ga2O3/AC composites,
consistent with the results of active species trapping experiments.
For the photocatalytic oxidation of CH4, Ga2O3/AC catalysts
are thought to act through a dynamic adsorption-photocatalysis
process under UV irradiation. Ga2O3/AC composites exhibit
higher performance for CH4 oxidation compared with bare
Ga2O3, which might be due to the following two aspects. One is
the excellent adsorption capacity of AC for CH4 molecules. The
interface created between AC and Ga2O3 phase facilitates the
adsorption of CH4 onto AC and the transfer of adsorbed CH4 to
catalytic active component Ga2O3. More importantly, AC
support facilitates the dispersion of deposited Ga2O3 nano-
particles (Fig. 2), inhibiting the agglomeration of catalytic active
components. On the basis of above results, the proposed
mechanism for photocatalytic oxidation of CH4 upon Ga2O3/AC
composites is depicted in Scheme 1. Under UV irradiation,
photoinduced electrons and holes are generated upon the
surface of Ga2O3 nanoparticles. The initial step of CH4 activa-
tion primarily involves the reaction with photogenerated holes.
The photogenerated electrons can react with the adsorbed O2
upon the surface of catalysts and generate O2cꢀ radicals. The
obtained methyl radicals via abstracting the hydrogen of CH4
are further oxidized by superoxide anion radicals. The
that trapping of photogenerated electrons with K2Cr2O7 could
benet the generation of photoinduced holes. But the oxidation
of CH4 was signicantly restrained in the presence of K2Cr2O7,
100% N2 gas and 1.56 mmol Lꢀ1 CH4. During the photocatalytic
process of CH4 oxidation, CH4 was activated primarily via
extracting hydrogen by photoinduced holes, consequently
generating methyl radicals (cCH3). These experimental results
imply that high concentration of O2 can also react with cCH3,
although there is no superoxide radicals.
As it is known, CH4 is a highly ammable gas. CH4 is unstable
under high concentration of O2 and can cause combustion and
even explosive reactions. Hence, from the view of experimental
safety, the photocatalytic experiments of CH4 oxidation were
performed in the presence of 79% N2 gas and 21% O2 gas unless
otherwise stated. On the basis of the above experimental results,
it is concluded that the reaction between cCH3 and O2cꢀ is
predominant for the photocatalytic oxidation of CH4 in the
presence of 79% N2 gas and 21% O2 gas.
The ESR results in Fig. 12 indicate that Ga2O3/AC composites
facilitate the generation of photo-induced active species, such as
O2cꢀ and photo-holes. Meanwhile, the weight ratios of Ga2O3
have a decisive role in the photocatalytic activity of Ga2O3/AC
composites (Fig. 7). Therefore, it was critical to investigate
whether the weight ratios of Ga2O3 had similar effects on the
photoinduced generation of active species. The effects of Ga2O3
weight ratios on the ESR signal intensity of DMPO-O2cꢀ and
TEMPO were depicted in Fig. 14. A similar dependence on Ga2O3
weight ratios was observed for O2cꢀ radicals and photoholes. An
initial increase in the ESR signal intensity was noted for Ga2O3/
AC composites with weight ratio of Ga2O3 up to 15%. Dimin-
ishing ESR signal intensities were observed for composite
samples having a higher weight ratio of Ga2O3. Importantly, it is
of interest to note that the changes of ESR signals obtained for
DMPO-O2cꢀ or TEMPO and the photocatalytic activity of CH4
oxidation had a similar dependence on the weight ratios of
Ga2O3. This correlation implied a mechanistic connection
between the generation of photoholes or O2cꢀ radicals and the
Scheme 1 Proposed mechanism for photocatalytic oxidation of CH4
photocatalytic performance. The results also indicate over Ga2O3/AC composites.
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successfully synthesized via
a facile hydrolysis method
combined with impregnation process. Photocatalytic oxidation
of CH4 was systematically investigated over bare Ga2O3 and
Ga2O3/AC composites in a xed-bed tubular quartz reactor. The
adsorption capacity and photocatalytic activity of Ga2O3/AC
were greatly affected by the weight ratios of Ga2O3 to AC. 15%-
Ga2O3/AC composites exhibited the highest photocatalytic
activity and 91.5% of CH4 was decomposed aer 150 min UV
irradiation, which could be ascribed to strong adsorption ability
and efficient separation of photo-generated electrons and holes.
Besides, the photocatalytic oxidation of CH4 obeys pseudo-rst-
order kinetics and the cycled experiment indicates that Ga2O3/
AC composites possess stable photocatalytic performance for
CH4 oxidation. Moreover, the underlying photo-oxidation
mechanism is also proposed on the basis of the results of ESR
measurements and radicals scavenging experiments. The
adsorbed CH4 molecules upon the surface of Ga2O3/AC were
activated primarily via the reaction with photo-generated holes
and the as-obtained methyl radicals were further oxidized by
superoxide anion radicals. The introduction of AC can enhance
the surface adsorption of CH4 molecules, the separation of
photoinduced electron–hole pairs and the generation of active
radical species, thus improving the photocatalytic performance
of CH4 oxidation. Hence, the photocatalytic technique with
Ga2O3/AC composites could be considered as a promising
method toward the removal of low concentration CH4.
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5. Conflict of interest
There are no conicts of interest to declare.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21307027, 51574112, 51404100), the
Program for Innovative Research Team in University of Ministry
of Education of China (IRT_16R22), the Natural Science Foun-
dation of Henan Province (14A440007), the Key Project of
Chinese Ministry of Education (213022A), the Innovation
Scientists and Technicians Troop Construction Projects of
Henan Province (164100510013) and the Funding scheme for
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