Ga-Doped and Pt-Loaded Porous TiO2-SiO2 for Photocatalytic Nonoxidative Coupling of Methane

Article abstract of DOI:10.1021/jacs.8b13858

Photodriven nonoxidative coupling of CH₄ (NOCM) is a potential alternative approach to clean hydrogen and hydrocarbon production. Herein, a Mott-Schottky photocatalyst for NOCM is fabricated by loading Pt nanoclusters on a Ga-doped hierarchical porous TiO₂-SiO₂ microarray with an anatase framework, which exhibits a CH₄ conversion rate of 3.48 μmol g^-1 h^-1 with 90% selectivity toward C₂H₆. This activity is 13 times higher than those from microarrays without Pt and Ga. Moreover, a continuous H₂ production (36 μmol g^-1) with a high CH₄ conversion rate of ～28% can be achieved through a longtime irradiation (32 h). The influence of Ga on the chemical state of a surface oxygen vacancy (Vo) and deposited Pt is investigated through a combination of experimental analysis and first-principles density functional theory calculations. Ga substitutes for the five-coordinated Ti next to Vo, which tends to stabilize the single-electron trapped Vo and reduce the electron transfer from Vo to the adsorbed Pt, resulting in the formation of a higher amount of cationic Pt. The cationic Pt and electron-enriched metallic Pt form a cationic-anionic active pair, which is more efficient for the dissociation of C-H bonds. However, the presence of too much cationic Pt results in more C₂₊ product with a decrease in the CH₄ conversion rate due to the reduced charge-carrier separation efficiency. This study provides deep insight into the effect of the doping/loading strategy on the photocatalytic NOCM reaction and is expected to shed substantial light on future structural design and modulation.

Full text of DOI:10.1021/jacs.8b13858

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Article
Ga-doped and Pt-loaded Porous TiO2-SiO2 for
Photocatalytic Nonoxidative Coupling of Methane
wu shiqun, Xianjun Tan, Juying Lei, Haijun Chen, Lingzhi Wang, and Jinlong Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 Mar 2019
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Ga-doped and Pt-loaded Porous TiO2-SiO2 for Photocatalytic
Nonoxidative Coupling of Methane
Shiqun Wua, Xianjun Tanc, Juying Leia, Haijun Chenb, Lingzhi Wang,a* Jinlong Zhanga*
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a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular
Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237 (P.R. China)
b Department of Electronics and Tianjin Key Laboratory of Photo-Electronic Thin Film Device and Technology,
Nankai University, Tianjin, 300071 (P. R. China)
c Key Lab of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese
Academy of Sciences, Shanghai 201210 (P. R. China)
Keywords: methane conversion• photocatalysis • gallium doping• oxygen vacancy • Pt loading
ABSTRACT: Photodriven nonoxidative coupling of CH4 (NOCM) is a potential alternative approach to clean hydrogen
and hydrocarbon production. Herein, a Mott-Schottky photocatalyst for NOCM is fabricated by loading Pt nanoclusters
on a Ga-doped hierarchical porous TiO2-SiO2 microarray (HGTS) with an anatase framework, which exhibits a CH4
conversion rate of 3.48 µmolg-1h-1 with 90% selectivity towards C2H6. This activity is 13 times higher than those from
microarrays without Pt and Ga. Moreover, a continuous H2 production (36 µmolg-1) with a high CH4 conversion rate of
approx. 28% can be achieved through a longtime irradiation (32 h). The influence of Ga on the chemical state of a surface
oxygen vacancy (Vo) and deposited Pt is investigated through a combination of experimental analysis and first-principles
density functional theory calculations. Ga substitutes for the five-coordinated Ti next to Vo, which tends to stabilize the
single-electron trapped Vo and reduce the electrons transfer from Vo to the adsorbed Pt, resulting in the formation of a
higher amount of cationic Pt. The cationic Pt and electron-enriched metallic Pt form a cationic-anionic active pair, which
is more efficient for the dissociation of C-H bonds. However, the presence of too much cationic Pt results in more C2+
product with a decrease in the CH4 conversion rate due to the reduced charge-carrier separation efficiency. This study
provides deep insight into the effect of the doping/loading strategy on the photocatalytic NOCM reaction and is expected
to shed substantial light on future structural design and modulation.
INTRODUCTION
reported an efficient CH4 combustion on
composite, which is also active for NOCM with
a Ag-ZnO
a
Methane is a promising energy source with huge reserves
and is considered as one of the alternatives to nonrenewable
petroleum resources, since it can be converted to valuable
hydrocarbon feedstocks and hydrogen through appropriate
reactions.1-6 Nonoxidative coupling of methane (NOCM) has
proven to be a promising approach to methane conversion
for C2 products and hydrogen.7-10 However, a high activation
temperature is required to trigger this reaction owing to the
tremendous thermodynamic barrier, which often causes coke
deactivation of the catalyst and low selectivity.11 Therefore,
there is an urgent demand to develop new strategies for
NOCM under mild conditions.
comparatively low conversion rate of 0.35%.23 The surface
defects including Vo and Zn+ play a vital role in the CH4
photooxidation, and Ag endows a visible-light activity due to
the surface plasmon effect. Meanwhile, Long et al. revealed
the high efficiency of Au/ZnO for NOCM benefitting from
the surface plasmon effect of Au, while the effect from
defects is excluded through EPR analysis.24 The above studies
have
demonstrated
the
feasibility
of
traditional
doping/loading strategies on enhancing the photocatalytic
NOCM efficiency. However, compared with those for other
C1 conversions, the available active structures for
photocatalytic CH4 conversion are still in short supply.
Moreover, the influence of doping/loading on the band
structure, surface atomic arrangement and electronic
characteristics of semiconductors and their relation with C-H
activation and NOCM selectivity remain quite ambiguous,
requiring a long-standing effort to explore.
Photocatalysis is a green technology for C1 conversion,
which can significantly lower the reaction temperature.12-15
Until now, several photocatalysts have been developed for
NOCM reactions including silica-alumina-titania,16, silica-
17
supported oxides,18, 19 and ceria-based9, 20 and zeolite-based
photocatalysts.21 For examples, Yoshida et al. achieved an
effective C2H6 production (0.69 µmolg-1h-1) on the ternary
oxide SiO2-Al2O3-TiO2 under Xe lamp irradiation.16 Chen et al.
achieved an extremely high CH4 conversion rate of 29.8
µmolg-1h-1 for NOCM under UV irradiation using Ga3+-
modified ETS-1 as the photocatalyst.22 Very recently, Yi et al.
Herein,
a
Ga-doped TiO2-SiO2 microarray with
a
hierarchical macro-mesoporous structure (HGTS) is
developed for NOCM. Pt nanoclusters, as a commonly used
cocatalyst for the photocatalytic hydrogen evolution reaction,
are further deposited (Pt/HGTS) to explore their effect on
NOCM. A high CH4 conversion rate of 3.48 µmolg-1h-1 for the
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selective production of C2H6 is achieved at room temperature.
Preparation of Pt/HTS and Pt/HGTS: Pt/HTS and
Moreover, a steady H2 yield of 36 µmolg-1 within 32 h is
attained and is accompanied by a high CH4 conversion
percentage of 28% without loss of structural stability. The
successive influence of Ga doping on the characteristics of
surface Vo, Pt deposition and NOCM efficiency are explicitly
unraveled by combining analyses of XPS, EPR and first-
principles density functional theory (DFT) calculations.
Pt/HGTS were prepared through a photodeposition method.
Typically, 0.2 g of HTS or HGTS was dissolved in 20 g of H2O
and 15.8 g of EtOH. After that, 6.64 mL of a H2PtCl6•6H2O
aqueous solution (1 g/L) (corresponding to a 1 wt% loading
amount) was added to the mixture. The mixture was stirred
for 30 min and then irradiated by a 300 W xenon lamp for 1 h.
The product was obtained by centrifuging the crude product
with EtOH and H2O 3 times and was then dried in a vacuum
overnight.
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EXPERIMENTAL SECTION
Chemicals. Sodium dodecyl sulfate (SDS, AR), styrene
(AR), potassium persulfate (KPS, AR) and Pluronic triblock
copolymer P123 (EO20PO70EO20, Mav=5800) were purchased
from Sigma-Aldrich. Titanium isopropoxide (TTIP, AR) was
purchased from Adamas-beta. Gallium nitrate hydrate
(Ga(NO3)3•xH2O, AR) was purchased from Sinopharm
chemical reagent Co., Ltd. Titanium tetrachloride (TiCl4, AR),
tetraethyl orthosilicate (TEOS, AR) and ethanol (EtOH, AR)
were obtained from Aladdin. Ultrahigh purity CH4 (99.99%)
was purchased from Shanghai Shenkai Gases Technology
Company. Chloroplatnic acid hexahydrate (H2PtCl6•6H2O,
Pt>37.5%) and ultrapure water were used without any further
purification.
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NOCM Reaction Test and Calculation of Conversion
and Selectivity: A 0.2-g solid sample was added to a closed
quartz reaction vessel (45 cm3), which was then evacuated for
10 min to remove air. Afterwards, 44.6 µmol of pure methane
(99.99%) was injected into the vessel by a gas injection
needle and stirred for 1 h to achieve an adsorption-
desorption balance. The vessel was irradiated by a 300-W Xe
lamp for 4 h. Before the photocatalytic NOCM reaction, all
the samples had been evacuated in a tube furnace at 393 K
for 4 h to remove the adsorbed water and other molecules.
The temperature of the reactor under photoirradiation was
measured to be approximately 333 K. The hydrocarbon
products were obtained by heating the reaction vessel at 523
K for 30 min for desorption and then analyzed by gas
chromatography (GC) with a flame-ionization detector (FID).
Hydrogen was analyzed by GC with a high-sensitivity
thermal conductivity detector (TCD). The relative deviation
of detection was less than 5% for GC.
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Preparation of PS: Generally, polystyrenes were
employed as hard templates to generate macropores. First,
0.45 g of SDS and 0.6 g of KPS were dissolved in 118.5 g of
EtOH and 270 g of H2O in a 500-mL three-neck flask and
stirred for 30 min. Then, the flask was evacuated and charged
with N2 to 1 bar. The flask was heated to 344 K, and 32.7 g of
fresh washed styrene was injected. The reaction was finished
after 19 h. The polystyrene opals were obtained by drying the
polystyrene emulsion in a 343 K oven for 3 days.
Calculation of Conversion and Selectivity: The
conversion of methane refers to the moles of methane
consumed in reaction to the initial total moles of methane
before reaction (Equation 1). The selectivity for ethane refers
to the moles of obtained ethane to the total moles of all
obtained hydrocarbon products in terms of carbon (Equation
2).
Preparation of HTS, HGTS and MGTS: The hierarchical
ordered TiO2-SiO2 composites (HTS) were synthesized by the
evaporation-induced self-assembly (EISA) method. Typically,
the Ti-Si precursor was synthesized by mixing together 2 g of
P123, 23.7 g of EtOH, 0.83 g of TEOS, 0.92 g of TiCl4 and 3.2 g
of TTIP. The precursor was stirred at 298 K for 4 h, and then
the polystyrene opals were immersed into the precursor,
which was evaporated at 341 K and 55% relative humidity
over 3 days and then dried at 340 K for 3 days. The obtained
blocks were calcined at 773 K for 5 h with a heating rate of 1
K/min. After that, the samples were collected and washed
with water 2 times for further use; the samples are denoted
as HTS. For the preparation of HGTS, the process was the
same except that different quantities of Ga(NO3)3•xH2O were
added to the Ti-Si precursor. For the preparation of MGTS,
the process was the same except that there was no PS
template.
Equation 1:
Consumed methane
100% ×
Added methane
Conversion =
Equation 2:
Obtained ethane
100% ×
All hydrocarbon products
Selectivity =
Density functional theory calculations: Calculations for
the total energy and electronic structure were carried out
using the CASTEP program within the framework of density
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functional theory (DFT).
The ultrasoft pseudopotential
was used for electron-ion interactions, and the Perdew-
Burke-Ernzerhof (PBE) form of the generalized gradient
approximation (GGA) was employed to describe the
exchange-correlation functional.27 For the C-H dissociation
and TS search calculations, the DMol3 program was used.28
For the spin-density polarization calculation, DFT+U was
adopted with a U of 3.3 eV.29, 30
Preparation of TS-1: Microporous titanosilicate (TS-1),
with a BET surface of 423 m2/g, was prepared by combining
11.3 g of TEOS and 0.61 g of TBOT and stirring for 1 h. H2O (5
g) was dropped into 5 g of tetrapropylammonium hydroxide
(40 wt%) and stirred for 1 h. The titanium silicate was slowly
dropped into the tetrapropylammonium hydroxide solution,
and within 1 h, 9.7 g of H2O was dropped into it; the solution
was stirred for 2 h, and then the temperature was increased
to 333 K for 0.5 h. The temperature was increased to 348 K
and kept for 3 h to remove the ethanol while 10 g of H2O was
slowly dropped into it. The above products were placed in a
hydrothermal kettle and held at 573 K for 2 days. The
product was obtained by centrifuging and washing with
EtOH and H2O 3 times and then dried in a vacuum overnight.
Instruments and Characterization: The morphology
was characterized by transmission electron microscopy (TEM,
JEM1400) and high-resolution transmission electron
microscopy (HRTEM, JEM2100). Powder X-ray diffraction
(XRD) characterization of all samples was carried out on a
Rigaku D/MAX 2550 diffractometer (Cu K radiation, λ=1.5406
Å) operating at 40 kV and 40 mA; data were collected in the
range of 10-80o (2 theta). The scanning electron microscopy
(SEM) analysis was performed using a TESCAN Nova III
scanning electron microscope. The Brunauer-Emmett-Teller
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Figure 1. (a) Schematic illustration of the synthesis process for Pt/HGTS. (b) and (c) TEM images of Pt/HGTS (2%). (d) and (e)
HAADF-STEM images of Pt/HGTS (2%). (f) HAADF-STEM image of Pt/HGTS (2%) and the corresponding elemental maps for Pt,
Ga, Ti, Si, and O (scale bar: 50 nm). The theoretical Pt loading is 1 wt%, and the inset of Figure 1c is the corresponding HRTEM
image of Pt/HGTS (2%).
(BET) surface area measurement was carried out by N2
adsorption at 77 K using an ASAP2020 instrument. X-ray
photoelectron spectroscopy (XPS) was performed on a
Perkin-Elmer PHI 5000C ESCA system with Al Kα radiation
operated at 250 W. The shift in the binding energy owing to
the relative surface charging was corrected using the C 1s
level at 284.4 eV as an internal standard. Electro-spin
resonance spectrometry (ESR) was performed on a 100G-
18KG/EMX-8/2.7 instrument. The UV-vis absorbance spectra
of the dry-pressed disk samples were acquired using a Scan
UV-vis spectrophotometer (Varian, Cary 500) equipped with
an integrating sphere assembly, using BaSO4 as the
reflectance sample. The photoluminescence (PL) emission
spectra of the solid catalysts were obtained using
luminescence spectrometry (Cary Eclipse) at room
temperature at an excitation wavelength of 350 nm. The
electrochemical measurement was performed on an
electrochemical analyzer (Zahner, Zennium) at room
temperature. The standard three-electrode system was
composed of a working electrode, a Pt wire counter electrode
and a saturated calomel reference electrode. The working
electrode was prepared by depositing a sample film on a F-
lamp without the filter. The Mott-Schottky measurement
was conducted in the potential range from -1.0 to 0.5 V (vs
RHE) at a frequency of 1.0 kHz under the 300-W Xe lamp
irradiation. The contents of Pt and Ga on the catalysts were
analyzed by inductively coupled plasma atomic emission
spectrometry (ICP-AES, Varian 710-ES, Agilent). Raman
spectra were recorded on a Renishaw in Via-Reflex Raman
microprobe system equipped with Peltier charge-coupled
device detectors and a Leica microscope. A 532-nm laser was
used as the excitation light source. High-angle annular dark-
field scanning transmission electron microscopy (HAADF-
STEM) imaging and elemental mapping were carried out on
a JEM-2100F electron microscope operated at 200 kV.
RESULTS AND DISCUSSION
Figure 1a illustrates the synthesis process for Pt/HGTS.
First, monodisperse polystyrene (PS) spheres with an average
diameter of 355 nm (Figure S1) are used to form PS opals.
Then, a Ga-Ti-Si precursor, with a fixed Ti/Si molar ratio of 4
and containing mesoporous template P123, is impregnated
into the void of the PS opals (Scheme 1). The Ga-modified
sample is marked as HGTS (x%), where x% represents the
theoretical molar ratio of Ga/Ti. During the solvent
evaporation and the subsequent aging process, the
mesopores templated from P123 micelles are formed in the
voids between the PS opals. PS and P123 are removed
through calcination, forming the hierarchical macro-
mesoporous HGTS. Pt nanoparticles are then loaded via a
doped SnO2-Coated (FTO) glass.
A 5-mg sample was
dissolved in 0.5 mL of ethanol, and then 20 µL of solution
was dropped on the FTO glass within an area of 1 cm2 and
dried at room temperature. The transient photocurrent
response of the different samples was determined in a 0.5 M
Na2SO4 aqueous solution under irradiation of a 300-W Xe
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photodeposition method using H2PtCl6•6H2O as the
precursor.
show any diffraction peak for Pt, possibly due to the small
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particle size and good dispersion (Figure S7). The average
TiO2 crystallite size in HGTS (2%) is approx. 8.6 nm, as
calculated from the Scherrer equation. The small crystallite
size is attributed to the restricted growth of TiO2 embedded
in the mesoporous framework. The Raman spectra (Figure S8)
show five representative peaks of anatase, where the peaks at
144, 196 and 636 cm-1 are attributed to the Eg mode and
where the peaks at 396 cm-1 and 517 cm-1 are assigned to the
B1g mode and doublet of A1g/B1g.33, 34 No peak attributed to Ga
oxide species is found, further confirming that Ga is
successfully doped into HTS.35 The UV-Vis DRS spectra show
that Ga doping has no effect on the bandgap of HTS (Figure
2b and Figure S9). The visible-light absorption intensity
increases and the absorption edge of HTS slightly shifts to a
longer wavelength after Pt loading (Figure 2b and Figure S10).
Two capillary condensation steps can be seen in the N2
adsorption-desorption isotherms (Figure 2c). The step at
P/P0=0.4-0.8 is attributed to mesopores, while the hysteresis
loop at higher pressure (P/P0=0.8-0.995) is from the
macroporous cavities. The pore size distributions (Figure 2d)
derived from the adsorption branch of the isotherms
utilizing the BJH model show that the mesopore sizes are
approximately 4.8 nm for HGTS (2%) and 4.9 nm for
Pt/HGTS (2%). The specific surface areas of HGTS (2%) and
Pt/HGTS (2%) are calculated to be 195 and 191 m2g-1, which
indicate that Pt loading has no clear effect on the specific
surface area and pore size distribution. Moreover, the tuning
of the Ga percentage in the range of 1-5% does not cause
evident variations in the absorption bands (Figures S9-S10)
and porous characteristics of HGTS and Pt/HGTS (Figures
S11-S12). The actual Pt loading percentage is approx. 0.25%
for all Pt/HGTS (x%) samples, and the real Ga amount is
approx. 50% of the theoretical Ga/Ti ratio (Table S1).
The transmission electron microscopy (TEM) image of
Pt/HGTS (2%) (Figure 1b) clearly displays a typical inverse
opal structure, demonstrating the successful replication of
the PS opals. The hexagonally arranged macropores are well
interconnected and have an average diameter of approx. 260
nm, which is favorable for the diffusion and transportation of
molecules. The enlarged image shows that Pt nanoparticles
(NPs) with a uniform size of approx. 4.4 nm are well
dispersed on the framework surface (Figure 1c and Figure S2).
The HRTEM image (Inset, Figure 1c) shows the lattice fringes
of 0.27 nm and 0.35 nm, indexing to the (111) planes of Pt and
the (101) planes of anatase.31 High-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM)
verifies the uniform distribution of Pt particles (Figure 1d-e).
The elemental maps of the macroporous framework reveal
the homogeneous distribution of Pt, Ti, O, Si and Ga (Figure
1f and Figure S3). Moreover, all of the Pt/HGTS samples with
various Ga/Ti ratios (1-5%) possess an ordered hierarchical
porous arrangement and similar Pt sizes (Figure S4),
indicating that Ga doping does not cause the deterioration of
the porous structure or variation in the Pt size.
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Figure 2. (a) XRD patterns, (b) UV-Vis DRS spectra, (c)
nitrogen adsorption-desorption isotherms and (d) pore size
distributions of HTS, HGTS (2%) and Pt/HGTS (2%).
Figure 3. (a) Methane conversion and the corresponding
selectivity towards ethane obtained in the photodriven
NOCM reaction over various samples and under direct
irradiation from a 300-W Xe lamp for 4 h. Samples from left
to right: HTS, HGTS (2%), Pt/HTS, and Pt/HGTS (2%). (b)
The ethane, propane, and hydrogen yields and the
corresponding methane conversion from the photodriven
NOCM reaction over Pt/HGTS (2%) under direct irradiation
from a 300-W Xe lamp for 32 h. The error bars arise from
values extracted from several measurements on multiple
catalysts.
The XRD patterns for HTS, HGTS (2%) and Pt/HGTS (2%)
all have diffraction peaks at 25.3, 37.8, 48.0, 55.1, 62.7 and 75.0°
(Figure 2a), which can be indexed to the (101), (004), (200),
(211), (204) and (215) crystal facets of anatase phase (JCPDS
No. 21-2172), respectively. Compared with the pattern for
HTS, HGTS (2%) shows no other diffraction peaks,
suggesting that Ga is possibly doped into the framework of
HTS or forms a highly dispersed oxide species on the pore
surface. Other Ga-modified samples show the same
diffraction peaks (Figure S5). Compared with the pattern for
pure HTS, the enlarged image indicates that the highest (101)
diffraction peak shifts toward small angles with increasing Ga
amount (Figure S6); this result suggests the substitution of
Ga3+ for Ti4+ causes a lattice expansion since Ga3+ (0.076 nm)
is larger than Ti4+ (0.068 nm).32 Compared with the pattern
for HTS, Pt/HGTS (2%) and other Pt-loaded samples do not
The photocatalytic NOCM performance of Pt/HGTS was
tested at room temperature upon irradiation with a 300-W
Xe lamp. No hydrocarbon product can be detected without
photoirradiation or methane. Thermal desorption was used
to ensure complete molecular desorption after the reaction,
and its influence on the NOCM efficiency was excluded
through a series of control experiments (Figure S13). All
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Pt/HGTS samples are active for NOCM (Table S2). The
highest CH4 conversion percentage of approx. 6.24% is
achieved within 4 h of irradiation of sample Pt/HGTS (2%)
530.1 eV, which are assigned to oxygen atoms from -OH
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bonds, neighboring to Vo and from the perfect lattice. The
peak related to Vo shows negligible improvement upon Ga
doping (Figure 4b), implying that the introduction of Ga
does not readily alter the Vo concentration. This signal
disappears after Pt loading
with 90.1% selectivity to ethane, corresponding to
a
conversion rate of 3.48 µmolg-1h-1 (Figure 3a). The catalyst
shows a high efficiency among those of reported catalysts
(Table S3). An apparent quantum yield (AQY) of approx. 10-4
is achieved on Pt/HGTS (2%) (Table S4). Decreasing or
increasing the Ga content leads to a decrease in NOCM
activity, while a higher Ga content deteriorates the selectivity
to C2H6 (Table S2). Pt/HTS without Ga was prepared to
understand the origin of the activity, and it shows a lower
conversion efficiency of 3.92% (Figure 3a). The activity
further decreases to 1.06% on Pt/P25, using commercial P25
as the carrier (Table S2). Moreover, P25 and the titanosilicate
TS-1 are inactive for NOCM, while HTS, with the hierarchical
pore structure, shows a low conversion rate of 0.41% (Figure
3a). To further understand the effect of hierarchical pores, a
mesoporous GTS loaded with the same amount of Pt was
synthesized, and it shows a lower activity than that of its
hierarchical porous counterpart (Table S2), verifying the
contribution from the interconnected hierarchical porous
structure. The above results suggest that the loading of Pt is
key to the NOCM activity, and the ordered hierarchical
porous microarray, together with Ga doping, helps to
improve the conversion efficiency. Furthermore, the stability
of Pt/HGTS and the changes in product distribution were
investigated by prolonging the irradiation time, using
Pt/HGTS (2%) for the demonstration. The conversion
percentage improves with the irradiation time and reaches
approx. 28% after continuous irradiation for 32 h. The
selectivity towards C2H6 decreases to 63.3% due to the
formation of propane and a tiny amount of butane. Besides
the hydrogen atoms in the hydrocarbon products, almost all
of the remaining hydrogen atoms from CH4 are released in
the form of H2, with a production rate of 36 µmolg-1. The
hierarchical morphology and crystal structure of Pt/HGTS
remain intact, as verified by the HRTEM image (Figure S14),
demonstrating the excellent structural stability of the
catalyst. We also carried out a recycling test of Pt/HGTS (2%)
by venting and re-injecting the reactants after 4 h of
irradiation. With repeated experiments, the conversion of
methane and selectivity towards ethane decrease gradually
(Figure S15), and after 4 reaction cycles, they are reduced to
52.4% and 81.7% of the original values obtained by the first
NOCM reaction, respectively. Overall, the photocatalytic
%
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Figure 4. High-resolution XPS spectra of O 1s for (a) HTS and
(b) HGTS and of Pt 4f for (c) Pt/HTS and (d) Pt/HGTS (2%).
(Figure S18), suggesting that Pt prefers to occupy Vo sites.
Moreover, the Pt 4f spectrum (Figure 4c) indicates that both
metallic and cationic Pt species are present on the surface of
HTS; however, most Pt species on the catalyst surface are
present in the metallic state. Furthermore, the presence of
Ga results in a higher amount of cationic Pt (Table S5).
stability remains sound compared with that of previous
24
reports.22,
We assume that Pt accepts the photoinduced
electrons from the semiconductor TiO2 under light
irradiation and is responsible for the initial CH4 activation.
However, the accumulated holes in TiO2 may alter the Pt
characteristics and gradually lead to the decreased
conversion efficiency for methane and selectivity towards
ethane.
To uncover the mechanism of Ga doping in enhancing
NOCM activity, XPS spectroscopy was applied to study the
surface atomic information. The refined Ti 2p XPS spectra of
HGTS (x%) (Figure S16) reveal bimodal peaks at
approximately 458.8 and 464.5 eV, which correspond to Ti
2p1/2 and Ti 2p3/2, respectively.36 The Ga 2p2/3 spectrum is
mainly characterized by a peak at 1118.3 eV (Figure S17),
proving that the majority of surface gallium is in the Ga3+
state.37 Figure 4a-d shows three O 1s peaks at 532.6, 531.5 and
Figure 5. (a) EPR results for different samples. (b) Mott-
Schottky plots for samples at frequencies of 1.0 kHz. (c)
Transient photocurrent responses of different samples (300-
W Xe lamp is used as the light source, and 0.5 M Na2SO4 is
used
as
the
electrolyte).
(d)
Room-temperature
photoluminescence (PL) emission spectra of different
samples (excitation wavelength is 350 nm).
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EPR (electron paramagnetic resonance) was further
electron trapped by Vo (Figure 5a). Since the concentration
of surface Vo does not show an apparent variation with Ga
doping,
1
2
3
employed to explore the influence of Ga doping on the
electronic structure of Vo in TiO2. The intense signal at
g=2.002 for sample HGTS (2%) originates from the unpaired
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Figure 6. (a) Structures and polaron analysis of the anatase TiO2 (101) surface with Vo (top left) and the anatase TiO2 (101) surface
with Vo and Ga substitution (top right). The structures and corresponding Mulliken charge analyses of surface atoms on
different structures, from bottom left to bottom right, are: the anatase TiO2 (101) surface with Vo, anatase TiO2 (101) surface, and
anatase TiO2 (101) surface with Vo and Ga substitution. (b) Energies of the two routes for Ga substitutions of Ti5c and Vo
formation, black route: Ga substitutes for Ti5c first and then forms Vo in TiO2; red route: Ga substitutes for the Ti5c neighboring
the original Vo. (c) Adsorption energies for a Pt dimer adsorbed on defective TiO2 with (red line) and without (black line) Ga
substitution. (d) Optimized geometries of four models for C-H cleavage: (model 1) -CH3 and -H are adsorbed on Pt1 and Pt2,
(model 2) -H and -CH3 are adsorbed on Pt1 and Pt2, (model 3) -H and -CH3 are adsorbed on Pt2 and the O neighboring Ga, and
(model 4) the same as model 1 except there is no Ga substitution. (e) Reaction energy profiles of four models, where the black
line refers to model 1, red line refers to model 2, blue line refers to model 3, and green line refers to model 4. Ti, O, Pt, Ga, C and
H atoms are shown in yellow, red, royal, pink, gray and white colors, respectively. (f) The proposed molecular mechanism of the
photocatalytic NOCM reaction.
according to the XPS spectra, it is highly possible that Ga
doping alters the chemical state of the original Vo. As is
known, the excess electrons formed from Vo in anatase TiO2
tend to be localized on neighboring Ti. The above results
suggest that the presence of Ga may be in favor of stabilizing
the single-electron trapped Vo. The signal at g = 2.002
disappears after Pt loading, indicating the transfer of the
excess electrons from Vo to Pt. This result is consistent with
the XPS analysis that Pt nanoparticles prefer to grow on Vo
sites.38
uncover the carrier characteristics. According to the Mott-
Schottky plots (Figure 5b), all of the catalysts show positive
slopes, indicating the n-type character of their electronic
band structures. The electron carrier concentrations of HTS
and HGTS (2%) are approx. 5.051020 cm-3 and 9.401020 cm-3
(Table S6), respectively, which indicate that Ga doping is
beneficial for increasing the concentration of free excess
electrons. Generally, Ga3+, as a p-type dopant in the perfect
TiO2 lattice, is expected to form excess holes and decrease
the concentration of excess electrons in TiO2. Therefore, for
the sample HGTS, Ga may prefer to substitute the Ti
neighboring Vo over that in the perfect lattice, which
Photoelectrochemical
and
room-temperature
photoluminescence (PL) analyses were further carried out to
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obstructs the electron transfer from Vo to neighboring Ti.
adsorption energy of a Pt dimer at the surface Vo next to Ga
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The electron left at Vo may be more delocalized than that
pinned at Ti and is responsible for the improved total density
of free electron carriers. The deposition of metallic Pt causes
a more significant increase in the free electron concentration,
as observed in Pt/HTS (1.591021 cm-3); however, this value is
reduced to 1.541021 cm-3 on Pt/HGTS (2%). Further
increasing the Ga percentage leads to a continuous decrease
in the carrier concentration, while a lower Ga content
(Pt/HGTS (1%)) results in a higher concentration of 1.671021
cm-3 (Figure S19). Similarly, the transient photocurrent
response indicates that the loading of Pt on HTS causes a
considerable signal improvement, which decreases on
Pt/HGTS (2%) (Figure 5c). The response to the Ga doping
content is similar to the result from the Mott-Schottky
analysis, where 1% Ga doping improves the current intensity
of Pt/HGTS but additional Ga doping leads to an intensity
decrease (Figure S20). The PL spectra of the above samples
also show the same tendency (Figure 5d and Figure S21). The
peak located at 388 nm is attributed to the band-gap
transition, the peaks at approximately 451 nm and 470 nm
are from the band-edge free excitons, and the other two
is 1.18 eV positive to that without Ga (Figure 6c), but both are
thermodynamically favorable. The neutral Pt2 and electron-
enriched Pt1 are formed on TiO2. In contrast, both Pt1 and Pt2
become more positively charged in the Ga-doped anatase
TiO2(1 0 1) slab, resulting in cationic Pt2 and electron-
enriched metallic Pt1; this determination agrees well with the
XPS result that Ga doping helps to form cationic Pt. We
assume that Ga doping reduces the electron transfer from Vo
to the adsorbed Pt. Therefore, the decrease in
photoelectrochemical efficiency with increasing Ga should be
related to the formation of a higher amount of cationic Pt.
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Further DFT calculations were performed to understand
the effect of Ga doping and the Pt state on the NOCM
conversion. Four models for C-H cleavage were calculated
(Figure 6d). First, the dissociated -CH3 and -H are adsorbed
on Pt1 and Pt2. Second, the dissociated -H and -CH3 are
adsorbed on Pt1 and Pt2. Third, -H and -CH3 are adsorbed on
Pt2 and the O neighboring Ga. Fourth, the model is the same
as model 1 except there is no Ga substitution (Pt/HTS). The
C-H dissociation on Pt1 and Pt2 (models 1 or 2) is more
thermodynamically favorable than on Pt2 and O (model 3)
according to the reaction energy (Figure 6e). Furthermore,
model 1 with –H on cationic Pt2 and –CH3 on metallic Pt1 is
more kinetically favorable than models 2 and 3 according to
the reaction barrier. We infer that the electron-deficient Pt2
and electron-enriched Pt1 result in a cationic-anionic active
pair, which is more efficient for the dissociation of C-H
bonds. Specifically, CH4 can be more easily activated by
metallic Pt1, and the cationic Pt helps to abstract H.
Additionally, compared with model 4, model 1 decreases the
C-H dissociation energy by approx. 0.41 eV but slightly
increases the reaction barrier. The decreased reaction barrier
on Pt/HTS should be ascribed to the more negatively
charged Pt1 on TiO2. However, considering the low reaction
barriers on both Pt/HTS and Pt/HGTS, the reaction should
be thermodynamically controlled, and Ga substitution helps
to retard the reverse reaction. Furthermore, an excessive
addition of Ga is found to not only reduce the photocatalytic
conversion efficiency but also lead to the formation of more
C2+ species (Table S2). This result verifies our assumption
about the effect of Ga doping on the formation of cationic Pt
that facilitates C-H cleavage but retards the photoinduced
carrier separation. Based on the above results, the effect of
Ga doping is summarized as follows: Ga tends to substitute
Ti5c neighboring Vo sites, which alters the electronic
characteristics of Vo by blocking the electron transfer,
leading to the formation of Vo with an unpaired electron.
Furthermore, Ga doping reduces the electron transfer from
Vo to the adsorbed Pt, resulting in the formation of a higher
amount of cationic Pt. The cationic-metallic Pt pairs provide
active sites for C-H dissociation. However, when a higher
amount of cationic Pt is formed from the increasing Ga
doping, the NOCM efficiency is decreased due to the reduced
carrier separation efficiency. Therefore, an appropriate
amount of Ga doping next to Vo is required to achieve the
cooperation between charge separation and C-H activation.
Modulation of the Vo concentration is expected to further
enhance the cooperation effect. Based on the results and
peaks at approximately 483 nm and 493 nm are related to
40
bound excitons.39,
The improved photoelectrochemical
performance obtained after Pt loading, as demonstrated
above, is attributed to the formation of a Mott-Schottky
junction between Pt and TiO2,41 which can significantly
accelerate the carrier separation. The electron-enriched Pt
accounts for the enhanced NOCM activity of Pt/HTS.
However, it should be noted that although Pt/HGTS (1%)
with the lowest Ga doping presents the best carrier
separation performance, the highest NOCM efficiency is
achieved on Pt/HGTS (2%). The inconsistency between the
photoelectric and NOCM efficiencies indicates that the
photocatalytic NOCM is not simply determined by the
separation efficiency of photoinduced charge carriers.
The DFT calculation was carried out to more deeply
understand the effect of Ga doping on the chemical states of
Vo and the adsorbed Pt (Figure 6). For the model, (1 0 1)
facet-exposed TiO2 slabs with and without a surface Vo
formed from the 2-coordinated O (O2c) are adopted. The slab
with a Ti5c next to Vo substituted by Ga is used to represent
sample HGTS. The DFT+U calculation indicates that a
polaron is formed at the surface Vo, where excess electrons
are pinned at the neighboring Ti6c and Ti5c and accompanied
by the lattice distortion (Figure 6a).42 In contrast, the
electron transfer from Vo to Ti6c is blocked in the presence of
Ga. Only one polaron is observed at the Ga site, with a more
delocalized electron cloud (Figure 6a). The Mulliken charge
analysis further verifies that the excess electrons from Vo
tend to be localized at Ti5c and Ti6c on the (1 0 1) surface,
decreasing the charge of Ti5c and Ti6c (Figure 6a). The
electron transferred from Vo is reduced after the substitution
of Ga for Ti5c. Moreover, the energy required for Ga to
substitute Ti5c decreased approx. 3 eV in the presence of Vo,
suggesting that Vo indeed favors the doping by Ga (Figure
6b). The above results are accordant with the results from
XPS and EPR, further confirming that Ga doping next to Vo
results in Vo with a trapped electron. Furthermore, the Pt
dimer is used to explore both metal-metal and metal-support
discussion above, we propose
a mechanism for the
photocatalytic NOCM reaction on Pt/HGTS (Figure 6f). First,
CH4 is activated by the electron-enriched metallic Pt, and
then the C-H bond is dissociated under light irradiation. The
interactions, where Pt1 occupies the Vo site and Pt2 is
44
adsorbed near a bridging O2c site away from Vo.43,
The
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(5) Shimura, K.; Kawai, H.; Yoshida, T.; Yoshida, H. Bifunctional
resultant -CH3 and -H are adsorbed on Pt1 and Pt2,
respectively. Second, another CH4 is activated and
dissociated in the same way as the first CH4 (Figure S22).
Then, the two -H intermediates generate a H2 molecule, and
the two -CH3 intermediates generate a C2H6 molecule. Finally,
the formed C2H6 and H2 are desorbed to complete a cycle
reaction.
Rhodium Cocatalysts for Photocatalytic Steam Reforming of
Methane over Alkaline Titanate. ACS Catal. 2012, 2, 2126-2134.
(6) Chanmanee, W.; Islam, M. F.; Dennis, B. H.; Macdonnell, F.
M. Solar Photothermochemical Alkane Reverse Combustion. P.
Natl. Acad. Sci. USA. 2016, 113, 2579-2584.
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X.; Deng, D.; Wei, M.; Tan, D.; Si, R.; Zhang, S.; Li, J.; Sun, L.;
Tang, Z.; Pan, X.; Bao, X. Direct, Nonoxidative Conversion of
Methane to Ethylene, Aromatics, and Hydrogen. Science 2014,
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in Sol-gel Prepared Silica-alumina for Photoinduced Non-
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2459-2465.
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Nonoxidative Coupling of Methane over Supported Ceria
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Non-oxidative Coupling, Dry Reforming and Steam Reforming of
Methane. Catal. Surv. Asia. 2014, 18, 24-33.
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CONCLUSION
In summary, Pt-loaded and Ga-doped macro-mesoporous
TiO2-SiO2 composites were fabricated and applied to the
photodriven NOCM reaction. Ga doping leads to the
formation of a higher amount of cationic Pt but decreases the
carrier separation efficiency. Through the appropriate tuning
of the Ga doping amount, a high methane conversion rate of
3.48 µmolg-1h-1 can be achieved due to the promoted C-H
cleavage by cationic Pt and the effective separation of photo-
induced charge carriers by metallic Pt. It is expected that this
work will offer significant guidance to the future design of
efficient photocatalysts toward methane conversion, as
achieved through modulation of the surface atomic
arrangement and electronic characteristics of semiconductor
composites.
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(12) Yuliati, L.; Itoh, H.; Yoshida, H. Photocatalytic Conversion
of Methane and Carbon Dioxide over Gallium Oxide. Chem.
Phys. Lett. 2008, 452, 178-182.
(13) Teramura, K.; Iguchi, S.; Mizuno, Y.; Shishido, T.; Tanaka,
T. Photocatalytic Conversion of CO2 in Water over Layered
Double Hydroxides. Angew. Chem. Int. Ed. 2012, 124, 8132-8135.
(14) Dong, C.; Xing, M.; Zhang, J. Economic Hydrophobicity
Triggering of CO2 Photoreduction for Selective CH4 Generation
on Noble-Metal-Free TiO2-SiO2. J. Phys. Chem. Lett. 2016, 7,
2962-2966.
ASSOCIATED CONTENT
Supporting Information. HAADF-STEM, TEM and SEM
images, XRD patterns, Raman spectra, UV-Vis DRS spectra,
BET result, HRTEM image, transient photocurrent responses,
room-temperature photoluminescence (PL) emission
spectra, XPS results, and a comparison of the photodriven
methane conversions of different samples.
AUTHOR INFORMATION
Corresponding Author
(15) Dong, C.; Xing, M.; Zhang, J. Double-cocatalysts Promote
Charge Separation Efficiency in CO2 Photoreduction: Spatial
Location Matters. Mate. Horiz. 2016, 3, 608-612.
* wlz@ecust.edu.cn
* jlzhang@ecust.edu.cn
(16) Yoshida, H.; Matsushita, N.; Yuko Kato, A.; Hattori, T.
Synergistic Active Sites on SiO2-Al2O3-TiO2 Photocatalysts for
Direct Methane Coupling. J. Phys. Chem. B 2003, 107, 8355-8362.
(17) Kato, Y.; Matsushita, N.; Yoshida, H.; Hattori, T. Highly
Active Silica-alumina-titania Catalyst for Photoinduced Non-
oxidative Methane Coupling. Catal. Commun. 2002, 3, 99-103.
(18) Yuliati, L.; Hattori, T.; Yoshida, H. Highly Dispersed
Magnesium Oxide Species on Silica as Photoactive Sites for
Photoinduced Direct Methane Coupling and Photoluminescence.
Phys. Chem. Chem. Phys. 2005, 7, 195-201.
(19) Yoshida, H.; Chaskar, M. G.; Kato, Y.; Hattori, T. Active
Sites on Silica-supported Zirconium Oxide for Photoinduced
Direct Methane Conversion and Photoluminescence. J.
Photochem. Photobiol. A. 2003, 160, 47-53.
(20) Yuliati, L.; Hamajima, T.; Hattori, T.; Yoshida, H. Highly
Dispersed Ce(III) Species on Silica and Alumina as New
Photocatalysts for Non-oxidative Direct Methane Coupling.
Chem. Commun. 2005, 38, 4824-4826.
(21) Kato, Y.; Yoshida, H.; Satsuma, A.; Hattori, T.
Photoinduced Non-oxidative Coupling of Methane over H-
zeolites Around Room Temperature. Micropor. Mesopor. Mat.
2002, 51, 223-231.
ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (21673073, 21677048 and 5171101651), the
Science and Technology Commission of Shanghai
Municipality (18520710200, 16JC1401400, 17520711500),
Shanghai Pujiang Program (18PJD012), the PetroChina
Innovation Foundation (2015D-5006-0402) and the
Fundamental Research Funds for the Central Universities
(222201717003, 22221818014).
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