Photocatalytic Methane Conversion to C1 Oxygenates over Palladium and Oxygen Vacancies Co-Decorated TiO 2

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Introduction
As the principal constituent of natural/shale gases, methane (CH 4 ) is a promising industrial feedstock for manufacturing value-added chemicals. [1]However, efficient CH 4 conversion is still of a great challenge owing to its high C-H bond energy (439 kJ mol À1 ), low electron affinity (À1.9 eV), and high ionization energy (12.6 eV). [2]The current industrial CH 4 conversion via dry/steam-reforming [3] and subsequent Fischer-Tropsch synthesis [4] is an energy-intensive and indirect route, where high temperature (>700 °C) is required. [5]Accordingly, direct CH 4 conversion under mild conditions is highly desired.
Photocatalysis has emerged as the green pathway to activate CH 4 under mild conditions through the injection of a photoinduced charge carrier instead of thermal energy. [6]The key to efficient photocatalytic CH 4 conversion lies in the development of a suitable photocatalyst.Recently, ZnO loaded with noble metal was reported to convert CH 4 into liquid oxygenates, with oxygen (O 2 ) as the oxidant. [7]Au 1 -BP promoted CH 4 conversion into CH 3 OH with the reactive hydroxyl radicals (OH), which are formed by O 2 with the assistance of water under light irradiation. [8]It is clear that the predominant challenge lies in simultaneous regulation of both activation of CH 4 and selectivity of desired products.Suitable co-catalysts like Au and Pd were reported to be the hole/ electron acceptors to promote charge separation, [9] as well as accelerating H 2 O oxidation and O 2 reduction to generate reactive oxygen species.Such encouraging advances then provide to some extent understanding of both charge dynamics and surface kinetics during photocatalytic CH 4 activation.Besides co-catalysts modification, surface engineering is the other way to promote charge dynamics. [10]10c,11] Moreover, interfacial resistance could also be regulated through surface engineering. [12]In parallel, surface kinetics could also be optimized by the introduction of surface defects by providing additional chemical adsorption and reactive sites. [13]Given these aforementioned attractive potentials of co-catalyst and OVs modification, the synergy of both would largely promote charge separation and surface reactions.
Besides the design of suitable photocatalysts, reaction conditions including oxidant, solvent, pressure, and reaction time during CH 4 conversion are also important taking into account the reaction kinetics.Though it is difficult to gain an efficient activity due to the low solubility of CH 4 in H 2 O, H 2 O oxidation into •OH radicals was reported to be essential in the activation of CH 4 . [14]eanwhile, H 2 O could also promote the desorption of the oxygenate products and avoid over-oxidation of CO 2 . [15]In parallel, a high pressure would increase the concentration of reactants, and a long reaction time might result in deep oxidation.Moreover, the oxidants are also important.Compared with H 2 O 2 , O 2 is much more benign and economically available, which is beneficial for future industrial application. [16]Therefore, it is critical to study the influence of the reaction conditions on CH 4 conversion.
Herein, Pd nanoparticles and OVs co-modified TiO 2 photocatalyst were designed to drive CH 4 conversion with O 2 as the oxidant in an aqueous solution.The optimized production rate of C1 oxygenate products reached 54 693 μmol g À1 h À1 with %98.6% selectivity.Mechanism investigations proved that Pd and OVs acted as the hole and electron acceptors, respectively, making a synergetic contribution to inhibit charge recombination and activate both methane and oxygen gas.Furthermore, the reaction pathway and the oxygen source including O 2 and H 2 O were discussed according to the isotopic experiments.

Structural Identification
X-ray diffraction (XRD) patterns were conducted to study the phase of crystals.As shown in Figure 1a, XRD patterns displayed the anatase TiO 2 structure (PDF#21-1272) of TiO 2 , def-TiO 2, and Pd 0.5 -def-TiO 2 .The characteristic diffraction peaks remained the same for all samples.No Pd-associated diffraction peak was observed, indicating the low loading amount or uniform distribution of Pd species.Electron paramagnetic resonance (EPR) spectra were applied to study the unpaired electrons of the catalysts.In Figure 1b, both TiO 2 and def-TiO 2 exhibited a similar EPR signal at g ¼ 2.003, which was attributed to OVs. [17] The improved EPR intensity on def-TiO 2 indicated the successful introduction of OVs into def-TiO 2 during calcination with urea.In addition, Pd 0.5 -def-TiO 2 showed the largest intensity of OVs, which implied that the Pd species loading might be beneficial to more OVs formation by reducing the formation energy of OVs. [18]As shown in Figure S1, Supporting Information, the corresponding low-magnification transmission electron microscope (TEM) images of TiO 2 and def-TiO 2 exhibited that the pristine TiO 2 and def-TiO 2 photocatalysts were nanoparticles.The lattice fringe of d ¼ 0.36 nm was indexed to the (101) facet of anatase TiO 2 .Moreover, the high-resolution TEM (HRTEM) image of def-TiO 2 showed an amorphous layer, indicating the successful introduction of OVs.HRTEM image of Pd 0.5 -def-TiO 2 (Figure 1c) showed a 3 nm amorphous layer, which was consistent with the existence of OVs as proven by EPR spectra.The lattice fringe of d ¼ 0.36 nm was indexed to the (101) facet of anatase TiO 2 .Figure 1d displayed the fast Fourier transform (FFT) image of Pd 0.5 -def-TiO 2 , in which the diffraction rings could be indexed as (101), (200) facet of TiO 2, and (111) facet of Pd.Energy dispersive spectroscopy (EDS)-mapping images showed the elemental distribution of O, Pd, and Ti (Figure 1e), which further proved the successful introduction of Pd nanoparticles.Moreover, specific surface area (S BET ) was measured by the nitrogen adsorption-desorption isotherms (Figure S2, Supporting Information).No obvious difference in S BET was detected, implying that surface area would not be the main factor for the improvement of catalytic CH 4 conversion investigated later.

Photocatalytic CH 4 Conversion
Photocatalytic activity was evaluated by CH 4 conversion conducted in a stainless-steel autoclave reactor with the top irradiation.The detailed oxygenate production was summarized in Table S2, Supporting Information.The targeted C1 products included CH 3 OH, CH 3 OOH, and HCHO, while CO 2 was regarded as the overoxidation product.The effect of noble metal species was studied by loading Pt, Au, Ag, and Pd (0.5 wt%) on def-TiO 2 (Figure 2a).Under the same reaction condition, Pt 0.5 -def-TiO 2 , Au 0.5 -def-TiO 2, and Ag 0.5 -def-TiO 2 exhibited a similar yield of C1 products at 10 664, 11 705, and 14 885 μmol g À1 h À1 , respectively.While a much higher performance was observed over Pd 0.5 -def-TiO 2 (54 693 μmol g À1 h À1 ).For the other noble-metal modification including Au, Pt, and Ag, no significant improvements were observed, which is because they were reported as electron acceptors and could not efficiently trap holes for CH 4 oxidation. [19]Such results indicated that Pd was a more suitable cocatalyst compared with other noble metals to drive CH 4 conversion.
The effect of OVs and the loading amount of Pd on CH 4 conversion were then studied.As observed from Figure 2b, in the absence of OVs, pristine TiO 2 presented a low C1 yield of 3994 μmol g À1 h À1 .The relatively low activity was attributed to the severe charge recombination of pristine TiO 2 .After modifying with OVs, the yield of C1 products improved to 13 765 μmol g À1 h À1 for def-TiO 2 , 3.4 times higher than that of pristine TiO 2 , indicating the critical role of OVs in promoting CH 4 conversion.Further modification with Pd cocatalyst resulted in a dramatic enhancement of CH 4 conversion.The C1 yield increased from 21 951 to 54 693 μmol g À1 h À1 as the Pd loading varied from 0.1 to 0.5 wt%.The highest yield of C1 products reached 54 693 μmol g À1 h À1 over the optimal photocatalyst Pd 0.5 -def-TiO 2 , almost 14 and 4 times that of pristine TiO 2 and def-TiO 2 .Further increasing Pd content led to the declined yield of C1 products, which might be caused by the enlarged particle size of Pd cocatalysts. [20]11b] Reaction conditions including reaction time, the molar ratio of CH 4 to O 2 , total pressure, and dosage of H 2 O were then investigated on Pd 0.5def-TiO 2 .As prolonging reaction time, oxygenates produced and gradually occupied the adsorption sites of CH 4 .Therefore, further prolonging the reaction time contributed little to the formation of oxygenates while the improved concentration of oxygenates on the surface easily led to the overoxidation of CO 2 , and exhibited the improved production rate of CO 2 from 525 to 1172 μmol g À1 h À1 (Figure 2c).Therefore, the short reaction time was beneficial to obtain higher C1 products.The molar ratio of CH 4 to O 2 was optimized on Pd 0.5 -def-TiO 2 at a total pressure of 2.1 MPa (Figure 2d).At CH 4 /O 2 ¼ 6/15, a relatively low yield of C1 products reaches only 15 483 μmol g À1 h À1 .A higher ratio at CH 4 /O 2 ¼ 11/10 led to the increased yield to 27 247 μmol g À1 h À1 .The yield of C1 products increased to the highest (54 693 μmol g À1 h À1 ) at CH 4 /O 2 ¼ 20/1.Besides, the yield of CO 2 increased from 311 to 768 μmol g À1 h À1 with the gradually increased molar ratio of CH 4 to O 2 .Compared with CH 4 , the solubility of O 2 in H 2 O is higher, [21] and when the partial pressure of CH 4 increased, the dissolved CH 4 in water increased as well.Therefore, with the increase of dissolved CH 4 in H 2 O, the yield of oxygenates generated also increased.Maintaining a constant molar ratio of CH 4 to O 2 (20:1) and lowing the amount of CH 4 and O 2 by lowing the total pressure, the yield of C1 products decreased from 54 693 μmol g À1 h À1 at 2.1 MPa to 3290 μmol g À1 h À1 at 0.6 MPa (Figure 2e).It is clear that the solubility of CH 4 and O 2 plays a crucial role in CH 4 conversion.The dosage of H 2 O was then investigated and exhibited in Figure 2f.Along with the increase in H 2 O dosage, the yield of C1 products increased from 22 466 to 54 693 μmol g À1 h À1 , which is probably attributed to the enhanced mass transfer by water. [22]The wavelength-dependent AQY of C1 oxygenate products was then measured as 1.05% at 365 nm for Pd 0.5 -def-TiO 2 (Table S3, Supporting Information).

Mechanism Investigation
UV-vis diffuse reflectance spectra (UV-DRS) spectra were conducted to determine the light absorbance of the photocatalysts.Compared to TiO 2 , def-TiO 2 exhibited a slight improvement in absorption between 395 nm and 540 nm due to oxygen vacancies (Figure 3a).In addition, TiO 2 , def-TiO 2, and Pd 0.5 -def-TiO 2 showed a similar adsorption edge at 380-390 nm, indicating the identical structure of the as-prepared TiO 2 -based photocatalysts.
To study the charge transfer of the as-prepared photocatalysts, in situ XPS under light were conducted.Pd 3d XPS spectrum of Pd 0.5 -def-TiO 2 displayed two main peaks located at 340.02 and 334.78 eV, and two minor peaks at 340.87 and 335.82 eV (Figure 3b).18a] The content of Pd 2þ species increased from 18.8% in the dark to 31.5% under irradiation, meanwhile, the content of Pd 0 species decreased from 81.2% to 68.5%, indicating the role of Pd cocatalyst as the hole acceptor.
The band structure of photocatalysts was measured to figure out whether the catalysts can generate reactive oxygen species (ROS).The bandgap energy (E B ) was calculated to be 3.10 eV of TiO 2 , 3.04 eV of def-TiO 2, and 3.00 eV of Pd 0.5 -def-TiO 2 by the Tauc plots (Figure S3, Supporting Information).As shown in Figure S4, Supporting Information, Mott-Schottky plots were used to measure the flat band potential, which is located below the conduction band (CB) by 0.1 V for n-type semiconductor. [23]he positive slopes of the three samples indicated that three samples were n-type semiconductors.The correlative CB position worked out at À0.97 V of TiO 2 , À0.88 V of def-TiO 2 and À1.05 V of Pd 0.5 -def-TiO 2 vs Ag/AgCl (pH ¼ 7).Accordingly, the energy level of the valence band (E V ) was attained by and E C are the energy level of the valence band, bandgap, and CB, respectively).Therefore, the corresponding valence band worked out at 2.74 V of TiO 2 , 2.77 V of def-TiO 2, and 2.56 V of Pd 0.5 -def-TiO 2 vs RHE (pH ¼ 0).The band positions with respect to RHE at pH ¼ 0 are shown in Figure S5, Supporting Information.It suggested the band potentials of the catalysts are theoretically sufficient for the generation of ROS.
In situ solid-state EPR spectra were conducted to further elucidate the photogenerated charge dynamics of def-TiO 2 under light (Figure 3c).Under dark condition, the signal at g ¼ 2.004 was observed, which was attributed to the OVs. [24]Under light illumination for 30 s, the signal intensity of OVs enhanced significantly, indicating that after excitation by light, OVs played a vital role in capturing the migrated photo-generated electrons. [25]owever, after illumination for 120 and 240 s, the signal intensity of OVs decreased, which might be the recombination of photogenerated electrons and holes. [26]teady-state PL spectra were conducted to study charge separation behavior (Figure 3d).Pristine TiO 2 exhibited a strong PL emission peak at 475 nm, which was corresponding with the severe charge recombination. [23]11c] The weakest PL peak intensity of Pd 0.5 -def-TiO 2 showed the highest carrier separation efficiency, which was ascribed to the synergistic effect of the OVs and Pd nanoparticles loading.Photocurrent density further confirmed the charge separation efficiency of the photocatalyst (Figure S6, Supporting Information).A low photocurrent density of 113 μA cm À2 was observed over pristine TiO 2 .After OVs decoration, the photocurrent density for def-TiO 2 showed 1.4 times improvement compared with pristine TiO 2 .The highest photocurrent intensity was found to be 227 μA cm À2 for Pd 0.5 -def-TiO 2 , nearly 2.0 times that of pristine TiO 2 , indicating the most efficient charge separation efficiency, consistent with the PL analysis.Electrochemical impedance spectroscopy (EIS) showed the smallest radius of the Pd 0.5 -def-TiO 2 compared with others, representing the lowest polarization resistance, which was more favorable for interfacial charge transfer (Figure S7, Supporting Information).
In situ EPR spectra were used to study the ROS under light over Pd 0.5 -def-TiO 2 with DMPO as the radical trapping agent.Figure 4a exhibited DMPO-OH and DMPO-OOH signals in the presence of Pd 0.5 -def-TiO 2 under light irradiation, which indicated that •OH and •OOH were the ROS during the CH 4 conversion. [27]ROS generation was further evaluated by using COU and NBT as the •OH and •OOH radicals probes, respectively.Figure 4b exhibited the fitted kinetic curves of NBT photodegradation, which were used to evaluate the production rate of •OOH radicals. [28]Pristine TiO 2 showed a low kinetic constant at 0.036 min À1 , and the constant of def-TiO 2 was improved to 0.10 min À1 .The highest constant was 0.18 min À1 for Pd 0.5def-TiO 2 .Such results demonstrated that Pd 0.5 -def-TiO 2 exhibited the strongest for •OOH radicals generation.As shown in Figure 4c, TiO 2 and def-TiO 2 showed the similar intensity of 7HC after 10 min irradiation, and the relatively low intensity indicated the moderate ability of TiO 2 and def-TiO 2 to generate •OH radicals.The strongest PL intensity of 7HC was observed over Pd 0.5 -def-TiO 2 , indicating its strongest ability to form the •OH radicals after 10 min irradiation.
Isotopic labeling experiments over Pd 0.5 -def-TiO 2 were applied to investigate the oxygen sources for oxygenating production.In the presence of H OH was the majority one.It was further confirmed that O 2 was the main oxygen source.Based on the aforementioned analysis, the mechanism of photocatalytic CH 4 conversion over the Pd 0.5 -def-TiO 2 was proposed in Scheme 1. Electrons were excited to the CB and holes settled on the valence band of Pd 0.5 -def-TiO 2 under light irradiation (Equation 1).Then, the holes transferred to the Pd nanoparticle which was confirmed by the in situ XPS spectra and in situ solid-state EPR spectra, activating H 2 O to form •OH radicals (Equation 2 and 3).The existence of •OH radicals was proved by the in situ EPR spectra and COU probe detected by PL spectra.In parallel, electrons transferred to the oxygen vacancies and then reduced the adsorbed O 2 to produce •OOH (Equation 5). [29]The as-formed •OH radicals next activated CH 4 into •CH 3 radicals (Equation 4).As the oxygen source for CH 3 OH came from both H 2 O and O 2 , it was accordingly concluded that •CH 3 radicals reacted with both •OH radicals and •OOH radicals (Equation 6 and 7), which were generated from H 2 O oxidation and O 2 reduction, respectively.The generation of HCHO [30] was derived from further oxidation of the as-produced CH 3 OOH and CH 3 OH. [7]The overoxidation of C1 oxygenates to CO 2 might be caused by •OH radicals.

Conclusion
In summary, efficient and selective oxidation of CH 4 to form oxygenates over Pd 0.5 -def-TiO 2 has been achieved under very mild reaction conditions with O 2 and H 2 O as the oxidants at room temperature.The C1 products including CH 3 OH, CH 3 OOH, and HCHO reach a high yield of 54 693 μmol g À1 h À1 with %98.6% selectivity.Pd and OVs have been proved to act as the hole and electron acceptors, as confirmed by in situ XPS and EPR under light irradiation, respectively.Consequently, the enhanced charge separation efficiency is achieved over the optimized Pd 0.5 -def-TiO 2 .In addition, both O 2 and H 2 O provide the oxygen sources for CH 3 OH formation through H 2 O oxidation and O 2 reduction, with O 2 as the predominant one.This work thus provides effective guidance for the synergistic effect of metal cocatalysts and OVs on direct CH 4 conversion under mild conditions.
Synthesis of OVs-Modified TiO 2 : OVs-modified TiO 2 photocatalyst was prepared through the two-step thermal calcination of the mixture of urea and anatase. [31]Typically, a certain amount of urea and anatase TiO 2 were uniformly grinded in a mortar, and then underwent calcination under a cover at 550 °C for 4 h in ultrapure argon (99.999 vol%) with a heating rate of 2 °C min À1 .Afterward, the obtained yellow powder was further calcinated in air at 550 °C for 2 h with a ramp rate of 5 °C min À1 .The as-prepared faint yellow product was named as def-TiO 2 .
Synthesis of Pd and OVs Co-Modified TiO 2 Photocatalyst: Pd and OVs co-modified TiO 2 photocatalysts were synthesized by the photodeposition method with def-TiO 2 as the substrate. [32]In a typical experiment, 250 mg def-TiO 2 was suspended in a 30 mL methanol aqueous solution (10 vol%).After being stirred for 5 min, a certain amount of K 2 PdCl 4 solution was dropped into the suspension while stirring.After purging with argon, the suspension was sealed and irradiated for 3 h in a multichannel reactor.Photocatalyst was then collected after being centrifuged, washed, and dried at 60 °C.The as-prepared photocatalysts were denoted as Pd x -def-TiO 2 .x wt% (x ¼ 0.1, 0.3, 0.5, 0.7, 1.0) represented the mass percent of Pd to def-TiO 2 .Au, Pt, and Ag were also deposited on def-TiO 2 by the same procedures, except 125 μL HAuCl 4 solution (1 wt%), H 2 PtCl 6 solution (1 wt%) or AgNO 3 solution (1 wt%) was dropped into the suspension of def-TiO 2 instead of K 2 PdCl 4. Actual metal contents (Table S1, Supporting Information) were measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES).
Characterizations: XRD patterns were measured by a D8 ADVANCE diffractometer (Bruker Co., Ltd) using Cu Kα radiation.HRTEM and EDS-mapping images were recorded on the Talos F200X instrument (FEI Co., Ltd).Nitrogen physical adsorption-desorption isotherms were measured on TRII Star 3020 gas adsorption analyzer at À196 °C.Before measurement, the samples were degassed at 150 °C overnight and backfilled with nitrogen.X-ray photoelectron spectroscopy (XPS) spectra were measured on the PHI5000Versa ProbeIII instrument (ULVAC-PHI Co., Ltd) with an Al Kα excitation source.Taking BaSO 4 as a reference, UV-DRS were taken on a UV-3600 plus spectrometer (Shimadazu Co., Ltd).In situ XPS results under light irradiation were obtained on the Thermo Scientific ESCALAB 250Xi with an Al Kα radiation source.Steady-state photoluminescence (PL) spectra were obtained on the F-4500 spectrofluorometer with the excitation wavelength at 330 nm.Timeresolved PL spectra were acquired on the FLSP920 spectrofluorometer.Solid-state EPR curves were measured on the ELEXSYS II instrument (Bruker Co., Ltd).Photoelectrochemical properties were measured on the CHI660E electrochemical workstation in 0.1 M Na 2 SO 4 solution.Ag/AgCl electrode and platinum sheet electrode were used as the reference electrode and counter electrode, respectively.The working electrodes were made from 0.2 g photocatalysts, 0.1 mL Nafion solution, and ethanol.The specific procedure was to mix the powder and the solution evenly and then cover the ITO electrodes by scraping.300 W Xe lamp (PLS-SXE300D, Beijing Perfectlight Technology Co., Ltd.) was used as the light source during measurement.
Photocatalytic CH4 Conversion: Photocatalytic CH 4 conversion reaction was conducted in a 200 mL stainless-steel autoclave reactor equipped with a top quartz window.LED lamp (365 nm, PLS-LED100B, Beijing Perfectlight Technology Co., Ltd.) was used as the light source.In a typical test, 10 mg catalyst was dispersed uniformly in 100 mL water through ultrasonication, then the reactor was sealed and purged with ultrapure O 2 (99.999 vol%) for 20 min.Afterward, 2.0 MPa CH 4 (99.999vol%) and 0.1 MPa O 2 (99.999 vol%) were injected into the reactor.The reaction was conducted for 20 min at 25 °C with a circulating cooling device.Gaseous products, as well as CH3OH in the reactant, were detected by gas chromatograph (GC-2014, Shimadazu Co., Ltd.) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID).CH 3 OOH was measured by the 1 H nuclear magnetic resonance ( 1 H NMR) spectroscopy (AVANCE III JEOL Ltd).As CH 3 OOH and CH 3 OH have the same amount of methyl, the molar ratio of CH 3 OOH/CH 3 OH should be regarded as the area ratio in 1 H NMR results.HCHO was quantified by the colorimetric method. [33]Typically, 100 mL color reagent was first prepared by the mixture of 15.0 g ammonium, 0.3 mL acetic acid, and 0.2 mL diacetylmethane.Then, 1.0 mL liquid product was mixed with 4.0 mL distilled water and 1 mL of the above color reagent, which was then maintained at 35°C for 1 h.The absorbance of the solution was then measured by UV-vis absorption spectroscopy (UV-3600 plus, Shimadazu Co., Ltd) and used for the quantification of HCHO.
Isotope Labeling Experiment: For the detection of oxygen-source in the products using isotopic labeled H 2 18 O: 20 mg Pd 0.5 -def-TiO 2 photocatalyst was dispersed in 2 mL H 2 18 O (99%).The reactor was then degassed for 30 min to completely remove air, and then was refilled with 2.0 MPa CH 4 (99.999vol%) and 0.1 MPa 16 O 2 (99.999 vol%).The reaction was carried out at 25 °C for 6 h.The products were measured by gas chromatographymass spectrometer (GC-MS) (QP2020, Shimadzu Co., Ltd) equipped with a Cap WAX column.
For the detection of oxygen-source in the products using isotopically labeled 18 O 2 : 20 mg Pd 0.5 -def-TiO 2 photocatalyst was dispersed in 2 mL H 2   16   O.The reactor was then degassed for 30 min to completely remove air, and then was refilled with 2.0 MPa CH 4 (99.999vol%) and 0.1 MPa 18 O 2 (98%).The reaction was carried out at 25 °C for 6 h.The products were measured by GC-MS (QP2020, Shimadzu Co., Ltd) which was equipped with the Cap WAX column.
Measurement of Apparent Quantum Yield: The apparent quantum yield (AQY) was measured over TiO 2 , def-TiO 2, and Pd 0.5 -def-TiO 2 under 365 nm irradiation with the Xe lamp equipped with a band-pass filter as the light source.Light intensity was measured as 74.0 mW cm À2 by the light intensity meter (PL-MW2000, Beijing Perfectlight Technology Co., Ltd).As the formations of CH 3 OOH, CH 3 OH, and HCHO need 1, 3, and 5 photogenerated charges, respectively, [7] AQY was calculated by the following equation.

Figure 2 .
Figure 2. Photocatalytic direct methane conversion over a) different cocatalyst-def-TiO 2 for photocatalytic activity.b) Pd x -def-TiO 2 for Pd loading content optimization.Investigations on: c) the reaction time, d) the molar ratio of CH 4 to O 2 , e) the total pressure, and f ) dosage of H 2 O over Pd 0.5 -def-TiO 2 .Reaction conditions: 10 mg catalyst, 100 mL H 2 O, 2 MPa CH 4 and 0.1 MPa O 2 for 20 min irradiation with 365 nm light emitting diode (LED) light and maintained at 25 °C.