Unprecedentedly high efficiency for photocatalytic conversion of methane to methanol over Au-Pd/TiO2-what is the role of each component in the system?

Article abstract of DOI:10.1039/d1ta00420d

Direct and highly efficient conversion of methane to methanol under mild conditions still remains a great challenge. Here, we report that Au-Pd/TiO2 could directly catalyze the conversion of methane to methanol with an unprecedentedly high methanol yield of 12.6 mmol gcat-1 in a one-hour photocatalytic reaction in the presence of oxygen and water. Such an impressive efficiency is contributed by several factors, including the affinity between Au-Pd nanoparticles and intermediate species, the photothermal effect induced by visible light absorption of Au-Pd nanoparticles, the employment of O2 as a mild oxidant, and the effective dissolution of methanol in water. More importantly, for the first time, thermo-photo catalysis is demonstrated by the distinct roles of light. Namely, UV light is absorbed by TiO2 to excite charge carriers, while visible light is absorbed by Au-Pd nanoparticles to increase the temperature of the catalyst, which further enhances the driving force of corresponding redox reactions. These results not only provide a valuable guide for designing a photocatalytic system to realize highly efficient production of methanol, but also, highlight the great promise of thermo-photo catalysis. This journal is

Full text of DOI:10.1039/d1ta00420d

Journal of
Materials Chemistry A
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PAPER
Unprecedentedly high efficiency for photocatalytic
conversion of methane to methanol over Au–Pd/
Cite this: J. Mater. Chem. A, 2021, 9,
0796
1
TiO – what is the role of each component in the
2
system?†
Xiaojiao Cai, Siyuan Fang and Yun Hang Hu *b
a
b
Direct and highly efficient conversion of methane to methanol under mild conditions still remains a great
challenge. Here, we report that Au–Pd/TiO
2
could directly catalyze the conversion of methane to
catꢀ
1
methanol with an unprecedentedly high methanol yield of 12.6 mmol g
in a one-hour photocatalytic
reaction in the presence of oxygen and water. Such an impressive efficiency is contributed by several
factors, including the affinity between Au–Pd nanoparticles and intermediate species, the photothermal
effect induced by visible light absorption of Au–Pd nanoparticles, the employment of O
oxidant, and the effective dissolution of methanol in water. More importantly, for the first time, thermo-
photo catalysis is demonstrated by the distinct roles of light. Namely, UV light is absorbed by TiO to
2
as a mild
2
Received 15th January 2021
Accepted 31st March 2021
excite charge carriers, while visible light is absorbed by Au–Pd nanoparticles to increase the temperature
of the catalyst, which further enhances the driving force of corresponding redox reactions. These results
not only provide a valuable guide for designing a photocatalytic system to realize highly efficient
production of methanol, but also, highlight the great promise of thermo-photo catalysis.
DOI: 10.1039/d1ta00420d
rsc.li/materials-a
Other than traditional thermal catalysis, photocatalysis
Introduction
shows great promise for molecular activation due to the facile
9
Methane, as the main constituent of natural gas, has been production of free radicals. So far, TiO
2
, ZnO, WO
3
, BiVO
4
,
widely used as a gaseous fuel and an important raw material to Bi WO , black phosphorus, etc. have been employed as semi-
2 6
1
,2
5,10–16
produce methanol, hydrogen, acetylene, etc. Conversion of conductor photocatalysts for methane conversion.
methane to methanol is especially desired and has been However, bare semiconductors generally show poor photo-
regarded as the holy grail reaction in C1 chemistry because catalytic activities due to the fast recombination of photo-
methanol is an essential feedstock in the chemical and energy generated charge carriers, which can be suppressed by
5,14,17,18
industries. This can be achieved either directly or indirectly via loading appropriate cocatalysts.
The cocatalysts exploited
3,4
syngas or methyl esters with catalysts. Direct conversion in the for photocatalytic methane conversion include metals such as
ꢁ
5,19
gas phase generally requires a high temperature (200–500 C) to Pt, Pd, Au, and Ag, and metal oxides such as FeO
activate the oxidant and accelerate the desorption of as- Ag
generated methanol, whereas overoxidation generally rates are in the range from 2 to 4300 mmol gcat
x
, CoO
x
, and
14,20,21
O.
2
However, currently reported methanol production
ꢀ1
ꢀ1
h
(Table S1†),
5–7
occurs. This can be effectively addressed by carrying out the which are far below the requirement for scalable production.
catalytic reaction in the liquid phase with hydrogen peroxide This encourages us to explore multi-component cocatalysts
6,7
2 2 2 2
(H O ) as the oxidant. However, the cost of H O
is even such as alloys where the synergetic effect of different compo-
higher than those of the oxygenated products, making this nents is expected.
8
process uneconomical. Direct utilization of O2 to convert
Alloy cocatalysts can promote photocatalysis by extending
22
methane to methanol under mild conditions still remains the light absorption range, balancing the distribution of
2
3
a huge challenge.
electron density,
enhancing interfacial charge transfer
24
25
kinetics, providing distinct active sites for redox reactions,
etc. Au, as an excellent electronic conductor, can facilitate the
transfer of photo-excited electrons, and more importantly,
hinders the oxidation of other coupling metals such as Ag, Ni,
a
School of Environmental Science and Engineering, Shanghai Jiao Tong University,
Shanghai 200240, People's Republic of China
26
b
Department of Materials Science and Engineering, Michigan Technological University, Pd, and Cu during photocatalysis as Au is less oxophilic. Pd is
Houghton, Michigan 49931-1295, USA. E-mail: yunhangh@mtu.edu
an ideal candidate to couple with Au because Pd can not only
strongly absorb both UV and visible light via inter-band
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ta00420d
1
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Journal of Materials Chemistry A
electronic transitions but also effectively trap photo-excited Aer 2–3 min of stirring, 6.4 mL freshly prepared 0.1 M NaBH
4
27,28
electrons.
So far, Au–Pd containing catalysts have been aqueous solution was added twice such that the molar ratio of
23,27,28
studied mainly in photocatalytic hydrogen production
and NaBH -to-metal was 5.0. The produced dark-brown colloid was
4
29
pollutant degradation, whereas their application in photo- stirred for 30 min to ensure all the metal precursors were
catalytic methane conversion has not been explored yet. reduced to the metallic form. Aerwards, the colloid was
Furthermore, the plasmon resonance absorption of noble immobilized by adding 2.0 g TiO under vigorous stirring, fol-
metal nanoparticles (NPs) is well known to induce the photo- lowed by quick acidication to pH ¼ 1 with 1 M sulfuric acid to
2
30
thermal effect, which results in a locally high temperature.
achieve more homogeneous deposition of metal NPs. The
This leads to a unique thermo-photo catalytic system without supernatant became clear aer 1 h, indicating that the depo-
additional thermal energy. In our previous studies, with the sition process was complete. The sol-immobilized catalyst was
simultaneous supply of thermal and photo energies via then ltered and washed thoroughly with 2.0 L distilled water
ꢁ
a furnace and lamp, photocatalytic efficiency was found to be and dried at 110 C for 16 h. Monometallic catalysts containing
signicantly enhanced because thermal energy could increase Au or Pd NPs and metal-free catalysts were prepared following
the driving force for photocatalytic reactions and thereby break the same procedure but with a single metal precursor or without
2,31–36
the kinetic limitations in photocatalysis.
However, thermo- metal precursors. The total mole number of decorated metal(s)
photo catalysis under mild conditions without any external heat was kept the same for bimetallic and monometallic catalysts.
source has not been explored yet. Here, the Au–Pd containing
system is ideal for studying this kind of thermo-photo catalysis
Catalyst characterization
due to the notable photothermal effect.
The crystal structure of the samples was determined in air by
using an X-ray diffractometer (Bruker/Siemens D5000 system)
In this work, we synthesized a Au–Pd/TiO catalyst via a sol-
2
immobilization method and conducted photocatalytic methane
conversion over it in the presence of oxygen and water.
Impressively, an unprecedentedly high methanol yield of
˚
with a Cu Ka radiation source (l ¼ 1.54056 A) in the range from
ꢁ
ꢁ
ꢀ1
1
0 to 80 and a step size of 2 min . The morphology of the
samples was investigated by using a high-resolution trans-
mission electron microscope (HR-TEM) (FEI Tecnai G2 F20)
equipped with a Gatan digital camera operated at an accelera-
tion potential of 200 kV with a FEG eld effect. The optical
properties of the samples were recorded by using a UV-visible
absorption spectrophotometer (Shimadzu UV-2550), scanning
from 200 to 1400 nm with two detection shis at 300 and
ꢀ
1
1
2.6 mmol gcat was obtained in just one hour. This is due to
the affinity between Au–Pd NPs and CH OOH which is bene-
3
cial for its further conversion, the synergetic effect of thermal
and photo energies realized by visible light absorption by Au–Pd
NPs and UV light absorption by TiO , the employment of O as
2
2
a mild oxidant, and the efficient dissolution of the oxygenated
products by water. Furthermore, the reaction mechanism was
proposed.
900 nm, respectively. Steady-state photoluminescence spectra
were obtained using a Varian Cary-Eclipse 500 spectrometer
with an excitation wavelength at 380 nm. The chemical state of
the samples was analyzed using an X-ray photoelectron spec-
trometer (Thermo Fisher 250Xi) with a full-spectrum pass
energy of 100 eV and a step size of 0.6 eV at a 15 kV acceleration
voltage. All binding energies were calibrated with respect to the
C1s line of surface adventitious carbon at 284.8 eV. Electron
paramagnetic resonance (EPR) spectra were recorded on
a Bruker Elexsys E500 spectrometer with 2,2-dimethyl-3,4-
dihydro-2H-pyrrole-1-oxide (DMPO) as the spin probe. Mott–
Schottky measurements were conducted in a three-electrode
Experimental
Materials
Hydrogen tetrachloroaurate trihydrate (HAuCl
Au basis, Sigma-Aldrich) and palladium chloride (PdCl
4 2
$3H O, $49.0%
2
,
$
99.9%, Sigma-Aldrich) were used as metal precursors.
2
Commercial titanium dioxide (submicron TiO , $99.9%,
Shanghai Macklin Biochemical Co., Ltd) was used as the cata-
lyst support. Sodium borohydride (NaBH , 99.99%, Sigma-
Aldrich) and polyvinyl pyrrolidone (PVP, MW ¼ 1 300 000,
Sigma-Aldrich) were used as the reducing agent and stabilizer
for catalyst preparation, respectively. Methane (99.999%, Air
Liquide Shanghai Co., Ltd) and oxygen (99.999%, Air Liquide
Shanghai Co., Ltd) were used as the feedstock gases. Hydrogen
4
2 4
cell in 0.5 M Na SO solution in the dark, where the catalyst-
coated uorine-doped tin oxide glass, Pt, and saturated
calomel served as the working, counter, and reference elec-
trodes, respectively.
2 2
peroxide (H O , 3.0 wt%, Sigma-Aldrich) was used for compar-
ison. Double-distilled water was used in all experiments.
Photocatalytic conversion of methane
Methane conversion was carried out in a 100 mL stainless steel
autoclave reactor with a Teon vessel, gas inlet and outlet, and
a quartz window on the top for light irradiation. A xenon lamp
Catalyst preparation
1
wt% Au–Pd/TiO
2
(atomic ratio of Au and Pd ¼ 1) was prepared with full-spectrum irradiation was employed as the light source
ꢀ
2
using a sol-immobilization method. 1.08 mL HAuCl aqueous and the light intensity was controlled at 250 mW cm . To
4
solution (59 mM) and 3.2 mL PdCl aqueous solution (20 mM in investigate the respective roles of UV and visible light, a UV
2
0
.5 M HCl) were dissolved simultaneously in 800 mL distilled reector and a UV lter were employed for UV and visible light
3
+
water to give a precursory solution containing 0.08 mM Au
and 0.08 mM Pd . 0.023 g PVP was then added to the solution. The light intensities of UV and visible light were 91 and 165 mW
irradiation, respectively, with 425 nm as the cutoff wavelength.
2
+
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ꢀ
2
cm , respectively. The temperature of the suspension was
measured using a thermocouple placed in the reactor. The
reactor can be heated by using an electric furnace.
In a typical test, 5 mg catalyst and 30 mL distilled water were
added into the reactor. Then the autoclave was sealed and
purged with oxygen three times, followed by pressurization with
3
.0 MPa methane and 1.0 MPa oxygen. Prior to the photo-
catalytic reaction, the suspension was magnetically stirred at
000 rpm in the dark for 20 min to reach the adsorption–
1
desorption equilibrium. The reaction lasted for 1 h under stir-
ring. Aer reaction, the autoclave reactor was cooled in ice to
ꢁ
a temperature below 10 C to minimize the loss of volatile
2
products. Then, CO and CO were quantied by using a gas
chromatograph (GC) equipped with a thermal conductive
detector and a TDX-01 column. Subsequently, the aqueous
3
mixture was ltered for analysis. CH OH was analyzed by GC
with a ame ionization detector and a Porapak Q packed
column. CH O was measured by acetylacetone spectropho-
2
tometry at 413 nm. CH OOH and HCOOH were quantied by
3
high-performance liquid chromatography (HPLC) with a 250 m
C18 column.
Results and discussion
Catalyst characterization
Au/TiO
2
, Pd/TiO
2
, and Au–Pd/TiO
2
were prepared via a sol-
immobilization approach. TiO
2
existed in the anatase form as
demonstrated by XRD patterns (Fig. S1†), while no diffraction
peaks corresponding to Au and Pd were detected, which might
be caused by their high dispersion and low concentration.
According to TEM images, metal NPs were well dispersed on
Fig. 1 TEM images and corresponding size distributions of metal NPs
in (a) Au/TiO , (b) Pd/TiO , and (c) Au–Pd/TiO . (d) Elemental maps of
2 2 2
TiO
found to have an average size of 2.65 ꢂ 0.87 and 4.91 ꢂ 1.37 nm, Au–Pd/TiO2.
respectively, while the mean size of Au–Pd NPs in Au–Pd/TiO
2 2 2
(Fig. 1a–c). Au NPs in Au/TiO and Pd NPs in Pd/TiO were
2
was ꢃ3.8 nm. The small and uniform size of metal NPs could
enhance the separation of photo-generated charges and supply
more active sites. Moreover, the obtained Au–Pd NPs exhibited
lattice spacings of 0.235 and 0.225 nm, which were consistent
2
from TiO to metal NPs, which would be benecial for multiple-
electron reduction to produce oxygenates.
Furthermore, the optical properties of the catalysts were
comparatively analysed. The UV-visible spectrum of Au–Pd/TiO2
showed strong visible light absorption but without the charac-
teristic Au plasmon absorption peak at ꢃ530 nm (Fig. 3a), which
indicated a strong interaction between Au and Pd and further
implied the formation of a Au–Pd alloy. Moreover, there was no
signicant difference in the band gaps of these catalysts (ꢃ3.18
with those of Au(200) and Pd(111) planes as observed in Au/TiO2
2
and Pd/TiO , demonstrating the formation of a polycrystalline
37
Au–Pd alloy. Furthermore, element mapping (Fig. 1d) revealed
that the distribution of Au agreed well with that of Pd, especially
for relatively large NPs. This kind of alloy-like structure enabled
2
effective interaction between the two metals and TiO , thus
forming both metal–metal and metal–titania heterojunctions.
To reveal the electronic interactions and chemical states of the
samples, XPS analysis was performed. O1s XPS spectra (Fig. 2a)
could be deconvoluted into three peaks, corresponding to lattice
2
eV), revealing that TiO and metal NPs were responsible for UV
and visible light absorption, respectively. The photo-
luminescence (PL) spectra are shown in Fig. 3b, where the
photoluminescence intensity decreased in the order of TiO >
2
oxygen (O
respectively.
(
L
), surface hydroxyl (O
H
), and absorbed water (O
The proportion of O reached the highest
14.53%) in Au–Pd/TiO among these catalysts (Table S2†), which
W
),
Au–Pd/TiO > Pd/TiO > Au/TiO , suggesting the easier recom-
2
2
2
38,39
H
bination of the charge carriers in Au–Pd/TiO compared with
2
2
2 2
Au/TiO and Pd/TiO .
3
+
would contribute to $OH production. Ti was not found in the
surface layer of all the samples (Fig. 2b). Moreover, compared with
the monometallic counterparts, Au–Pd/TiO2 showed a slightly
promoted oxidation of Au but a signicantly inhibited oxidation of
2
Activity comparison between Au–Pd/TiO and its
monometallic counterparts
Pd (Fig. 2c and d). The binding energies of both metallic Au and Pd Photocatalytic methane oxidation was carried out in a batch
shied to lower values, indicating the accelerated charge transfer reactor with a quartz window on the top for light irradiation.
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Fig. 4 Photocatalytic methane oxidation over TiO
and Au–Pd/TiO under UV-visible light irradiation (5 mg catalyst,
0 mL water, 3.0 MPa CH , 1.0 MPa O , 1 h reaction).
2 2 2
, Au/TiO , Pd/TiO ,
2
3
4
2
2 2 2 2
Fig. 2 XPS spectra of bare TiO , Au/TiO , Pd/TiO , and Au–Pd/TiO .
(
a) Ti 2p, (b) O 1s, (c) Au 4f, and (d) Pd 3d XPS spectra.
5
mg catalyst was suspended in 30 mL water and the reaction
took place under vigorous magnetic stirring in the presence of
.0 MPa CH4 and 1.0 MPa O2 for one hour. Under sunlight
3
ꢀ
2
irradiation (UV-visible light, 250 mW cm ), all the catalysts,
namely, TiO , Au/TiO , Pd/TiO , and Au–Pd/TiO , showed
excellent photocatalytic activities with large amounts of
CH OOH, CH OH, and CH O detected, together with scarce
amounts of HCOOH, CO, and CO (Fig. 4). The selectivity of the
total liquid oxygenates even reached 98% for all the catalysts.
The total yields obtained over TiO , Au/TiO , Pd/TiO , and Au–
Fig. 5 Reaction mechanism of photocatalytic methane conversion
over Au–Pd/TiO
2
2
2
2
2
.
3
3
2
2
and a high selectivity of 42.8% were obtained, in great contrast
ꢀ
1
ꢀ1
to those of 0.6 mmol g and 4.9% over TiO
2
, 4.6 mmol g and
2
2
2
ꢀ1
22.3% over Au/TiO
2
, and 5.6 mmol g and 18.0% over Pd/TiO
2
ꢀ1
Pd/TiO were 12.4, 20.8, 30.8, and 20.0 mmol g , respectively.
2
(
Fig. 4). In general, there are three destinies of as-produced
The relatively low methane conversion efficiency over Au–Pd/
CH
3
OOH on the catalyst surface: (1) being reduced to CH
3
OH
TiO
carriers as demonstrated by the PL spectra (Fig. 3b).
However, though Au/TiO and Pd/TiO could convert
methane more efficiently, the dominant product was CH OOH
2
was attributed to the high recombination rate of charge
(eqn (1)), (2) being oxidized to CH O (eqn (2)), and (3) dissolving
2
5,20
in water (Fig. 5).
CH OOH + 2H + 2e / CH
CH OOH / CH O + H O
2
2
3
+
ꢀ
3
3
OH + H
2
O
(1)
(2)
with high selectivities of 60.1% and 66.0%, which was just an
intermediate species in methane conversion with limited
3
2
2
5
value. In other words, further conversion of CH OOH to
3
CH
degrees was suppressed over Au/TiO
over Au–Pd/TiO , an impressive CH
3
OH, CH
2
O, and other products with higher oxidation
and Pd/TiO . In contrast,
OH yield of 8.6 mmol g
2
2
The affinity between CH OOH and Au–Pd NPs was benecial
for its further conversion compared with dissolution, and the
3
ꢀ
1
2
3
abundant electrons on Au–Pd NPs (demonstrated by XPS and PL
spectra) promoted the reduction of CH OOH to CH OH. This
3 3
was further evidenced by measuring the activities of Au–Pd/TiO2
catalysts with various Au–Pd loading amounts (Fig. S2†).
Namely, the increased Au–Pd loading amount led to the
decrease of the selectivity for CH
3
OOH while increasing the
selectivity for CH OH. Furthermore, TEM images revealed that
3
there was no evident difference in the sizes of Au–Pd NPs in
samples with different loading amounts, while the quantity of
Au–Pd NPs increased with the loading amount (Fig. S3†). This
implied that more Au–Pd NPs could provide more active sites
Fig. 3 Optical properties of TiO
2 2 2 2
, Au/TiO , Pd/TiO , and Au–Pd/TiO .
for the conversion of CH OOH to CH OH. Based on these
(
a) UV-visible diffuse reflectance spectra. (b) Photoluminescence
3
3
spectra.
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results, Au–Pd/TiO
Paper
was considered as an ideal catalyst for Table 1 Temperature of the catalyst coated on a SiO
2
substrate under
2
UV, visible, and UV-visible light irradiation (background temperature:
highly efficient methane conversion to CH OH.
3
ꢁ
42
C, presented temperature: the average temperature during the
illumination period from 15 to 60 min)
The role of UV and visible light
Substance UV
Visible
UV-visible
In order to clarify the respective roles of UV and visible light,
which were mainly absorbed by TiO and Au–Pd NPs respec- SiO2
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
2
44.8 C
45.3 C
45.1 C
ꢁ
ꢁ
Au/TiO
2
on SiO
2
43.7 C
55.8 C
56.0 C
tively, photocatalytic methane conversion was carried out under
ꢁ
ꢁ
ꢀ2
ꢀ2
Pd/TiO on SiO2
45.0 C
49.3 C
50.1 C
2
individual UV (91 mW cm ) and visible (165 mW cm ) light
ꢁ
ꢁ
Au–Pd/TiO
2
on SiO
2
47.5 C
51.7 C
52.3 C
irradiation with the water temperature xed at the same
temperature as that under full-spectrum illumination (42 C). It
ꢁ
is interesting that though visible light itself made little contri-
bution to methane conversion, the addition of visible light to
UV light evidently promoted the production of all products,
revealed via EPR measurements of the catalyst suspension with
5
,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin probe. As
ꢀ
shown in Fig. S5,† the co-existence of $OH and $O2 was
especially CH OH, whose yield increased from 6.2 to 8.6 mmol
3
ꢀ1
demonstrated over Au–Pd/TiO under light irradiation since the
2
g
(Fig. 6). Herein, as-absorbed visible light by Au–Pd NPs
DMPO–OH spin adduct (with a quartet EPR signal) and the
DMPO–OOH spin adduct (with a sextet EPR signal) were
could provide additional heat for the catalyst due to the pho-
40
tothermal effect. As an indirect approach to measuring the
temperature of the catalyst, we respectively dispersed Au–Pd/
41
ꢀ
detected. This indicated the processes of O
2
reduction to $O
2
by photo-excited electrons and H O (or surface hydroxyl)
2
TiO
2
, Au/TiO
2
, and Pd/TiO
2
onto a SiO
2
substrate, heated them
ꢁ
oxidation to $OH by photo-excited holes. The generation of $OH
could promote the activation of C–H bonds, during which Au
played an important role due to its strong electron-accepting
to 42 C, turned on the light, and then measured their
temperature by thermocoupling aer 15 min stabilization. It
was clearly found that visible light irradiation could induce an
evident thermal effect during the illumination period from 15 to
42
ability.
Based on the above observations, the oxidation driving force
(energy difference between the redox potential of $OH/H O (ref.
3) and the valence band maximum) was found to be 0.89 V,
6
0 min (Table 1). The addition of visible light to UV light led to
ꢁ
2
an ꢃ5 C temperature rise of the Au–Pd/TiO
2 2
and Pd/TiO
4
catalysts, while the Au/TiO
2
catalyst showed a much more
ꢁ
while the reduction driving force (energy difference between the
signicant increase of the temperature to ꢃ12 C. This implied
that Au was the main contributor to the photothermal effect,
owing to its localized surface plasmon resonance, whereas this
could be suppressed to some extent by forming an alloy.
conduction band minimum and the redox potential of dis-
ꢀ
solved O /$O (ref. 44)) was even as small as 0.16 V (Fig. 5). As
2
2
revealed in our previous work, insufficient potential driving
31–34
force would suppress the photocatalytic reaction.
Here, the
That is to say, in this case, the absorption of UV light by TiO
2
heat resulting from the photothermal effect could make the
reactants adsorbed on the catalyst more active, enhancing the
kinetic driving force and thus promoting photocatalysis.
was responsible for the excitation of charge carriers to drive
corresponding reactions, while visible light absorbed by Au–Pd
NPs could provide heat to accelerate these reactions. This leads
to a unique process of thermo-photo catalysis realized by the
synergetic effects of UV and visible light. In order to elucidate
this unique process, the band structure of Au–Pd/TiO2 was
depicted based on the Mott–Schottky plot (Fig. S4†) and UV-
visible spectrum (Fig. 3a). Moreover, active species were
Moreover, the more notable promotion in the production of
ꢁ
CH
3
OH than that of other products might be because the ꢃ5 C
temperature rise showed more signicant contribution to
electron donation from the conduction band of TiO through
2
Au–Pd NPs to CH OOH to form CH OH (eqn (1)) by enhancing
3
3
its driving force. This is the rst time we demonstrate thermo-
photo catalysis achieved by the distinct roles of light.
The role of oxidants
Furthermore, the role of oxidants was comparatively investi-
ꢀ
1
gated (Fig. 7). In the absence of O and H O , 5.1 mmol g
2
2 2
CH OOH was generated as the dominant product. In this case,
3
water itself and/or dissolved O
a mild oxidation process. Adding 10 mM H
promoted the further conversion of CH
2
served as the oxidant, leading to
into water mainly
OOH into CH OH
O (3.0 mmol g ), and HCOOH (4.0 mmol
), while the total yield showed a slight enhancement from 6.3
2 2
O
3
3
ꢀ
1
ꢀ1
(
g
1.2 mmol g ), CH
2
ꢀ1
ꢀ
1
3
to 9.1 mmol g . The selectivity of CH OH was as low as 13.3%.
In contrast, introducing 1.0 MPa O which possesses a weaker
2
2
Fig. 6 Photocatalytic methane oxidation over Au–Pd/TiO under UV-
oxidizing ability than H O was advantageous for the produc-
2
2
visible, UV, and visible light irradiation with the water temperature
ꢀ1
ꢀ1
ꢁ
3 3
tion of CH OOH (6.2 mmol g ), CH OH (8.6 mmol g ), and
controlled at 42 C (5 mg catalyst, 30 mL water, 3.0 MPa CH
4
, 1.0 MPa
ꢀ
1
O
2
, 1 h reaction).
CH
2
O (4.5 mmol g ), with a scarce amount of HCOOH
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CH
3
OOH and CH
2
O production, indicating the key role of water
as the solvent to remove the oxygenated products from the
catalyst surface. Moreover, though the selectivity of CH OH
3
remained stable, the selectivity of CH OOH increased while that
3
of CH
that the dissolution of CH
conversion to CH O. However, it should be mentioned that in
such a three-phase system containing the catalyst (solid), water
liquid), and CH and O (feed gases), the contact between the
2
O decreased with increasing water volume, suggesting
3
OOH in water inhibited its further
2
(
4
2
catalyst and feed gases can be suppressed by water. Therefore,
relatively high pressures of feed gases are desirable, especially
ꢀ
1
for CH whose solubility in water is as low as 22.7 mg L under
4
Fig. 7 Photocatalytic methane oxidation over Au–Pd/TiO
2
without or standard conditions. This was demonstrated by the experi-
with O
2
or H O
2 2
under UV-visible light irradiation (5 mg catalyst, 30 mL
, 10 mM H or 1.0 MPa O , 1 h reaction).
mental results; a higher CH
4
pressure led to a higher methane
water, 3.0 MPa CH
4
O
2 2
2
conversion efficiency (Fig. S6†).
ꢀ
1
(
0.4 mmol g ). Besides, the total yield was signicantly Conclusions
ꢀ
1
enhanced by more than three fold from 6.3 to 20.0 mmol g . In
terms of CH OH production, the selectivity and productivity
obtained with O were 3 and 7 times those with H . From this
perspective, the use of O rather than H O was not only cost-
In conclusion, an unprecedentedly high efficiency for CH
3
OH
production was realized, reaching 12.6 mmol g in a one-hour
photocatalytic reaction over Au–Pd/TiO . This is due to each
component in the photocatalytic system. While TiO absorbed
3
ꢀ
1
2
2 2
O
2
2
2 2
2
effective but also highly-efficient for CH OH production.
3
UV light to generate electrons and holes, Au–Pd nanoparticles
not only helped to transfer photo-generated electrons, but also
absorbed visible light to increase the catalyst temperature by
The role of water
ꢁ
ꢃ5 C. The temperature rise enhanced the driving force for H O
2
Moreover, it is of vital importance to investigate the role of water
in photocatalytic methane conversion. As shown in Fig. 8, the
yield of CH OH (mmol g ) signicantly increased by four fold,
3
from 3.3 to 12.6 mmol g , with the increase of the water
volume from 10 to 50 mL. Here, the water volume could not be
further increased due to the limitation of the reactor. Never-
theless, such a remarkable yield is three times higher than the
highest value reported so far (Table S1†). In contrast to the rapid
ꢀ
oxidation into $OH, O
2
reduction into $O
2
, and more impor-
3 3
tantly, the reduction of CH OOH into CH OH. Moreover, by
ꢀ
1
increasing the water volume, the production efficiency could be
further enhanced as water could effectively dissolve as-
produced oxygenates adsorbed on the catalyst surface.
ꢀ
1
Furthermore, in this case, cost-effective O
2
was surprisingly
found to be an even more efficient oxidant than H O . The
2
2
above results provide a valuable guide for the design of a pho-
tocatalytic system to realize highly efficient conversion of CH to
CH OH.
ꢀ
1
increase of the yield, the concentration of CH OH (mmol L
)
3
4
showed slight reduction with water volume. The above obser-
vations implied that water was an effective solvent for CH OH
and there was a balance between the adsorbed and dissolved
CH
more CH
3
3
3
OH. Therefore, more water was benecial for obtaining Author contributions
3
OH. A similar phenomenon was also observed in
Xiaojiao Cai: investigation and validation. Siyuan Fang: inves-
tigation and writing – original dra. Yun Hang Hu: conceptu-
alization, supervision, and writing – review & editing.
Conflicts of interest
There are no conicts to declare.
Notes and references
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8 Photocatalytic methane oxidation over Au–Pd/TiO with
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