Photocatalytic selective oxidation of methane by quantum sized bismuth vanadate

Yingying Fan (  ccyyfan@gzhu.edu.cn ) Guangzhou University Wencai Zhou National Center for Nanoscience and Technology Xueying Qiu National Center for Nanoscience and Technology Hongdong Li National Center for Nanoscience and Technology Yuheng Jiang National Center for Nanoscience and Technology Zhonghui Sun Guangzhou University Dongxue Han Guangzhou University Li Niu Guangzhou University Zhiyong Tang National Center for Nanoscience and Technology https://orcid.org/0000-0003-0610-0064


Introduction
Methane (CH 4 ), approximately occupying 70% to 90% in natural gas 1,2 , has been broadly recognized as an indispensable feedstock in the manufactures of fuels and chemicals with high added-values 3,4 .
Traditional industrial technology has succeeded in implementing large-scale transformation of CH 4 to liquid chemicals via an indirect mode, involving the rst syngas (H 2 and CO) formation and the second downstream process 5,6 . Obviously, the divided reaction steps and required high operation temperature (800 -900 o C) 7 are the formidable challenges for modern sustainable development.
Taking light illumination instead of high-temperature heating, photocatalytic CH 4 oxidation holds the great promise to transform CH 4 into many types of high valued liquid chemicals at room temperature 8, 9 . Amongst the investigated liquid products, methanol (CH 3 OH) and formaldehyde (HCHO) are two major targets because of their roles as the basic and widely used chemicals 10,11,12 . Until now, a few photocatalytic CH 4 conversions with satis ed yield of CH 3 OH and HCHO have been reported; for instance, photocatalytic oxidation of CH 4 over 0.1 wt% Au/ZnO and 0.1 wt% Ag/ZnO achieved 41.2 μmol CH 3 OH (15.7% selectivity) and 112.1 μmol HCHO (42.6% selectivity) 12 , respectively, under moderate reaction condition.
Noteworthily, there has been always a serious dilemma between the productivity and selectivity for CH 4 oxidation, which is a complicated and energy-downhill process of CH 4 →CH 3 OOH→CH 3 OH→HCHO→CO 2 13, 14, 15, 16, 17 (Fig. 1), and the undesired intermediates and overoxidation always exist. To be speci c to the formation of CH 3 OH, optimizing its selectivity needs maximizing the reduction of CH 3 OOH while preventing the overoxidation to HCHO and CO 2 . Similarly, to promote the selectivity of HCHO, CH 3 OH intermediates must be further oxidized whilst avoiding overoxidation to CO 2 .
Herein, we suggest a highly selective aerobic conversion of CH 4 to CH 3 OH and HCHO at room temperature using quantum-sized BiVO 4 nanoparticles (q-BiVO 4 ) as photocatalyst. On the basis of the maximum yield (1.1 mmol g -1 CH 3 OH or 13.1 mmol g -1 HCHO), the selectivity of CH 3 OH and HCHO reaches 96.6% and 86.7%, respectively. The q-BiVO 4 is characteristic of high kinetic energy of charge carrier and large speci c surface, enabling effective conversion of CH 4 into CH 3 (Fig. S1). Subsequent high-pressure hydrothermal reaction made silica template fully dissolve, which broke BiVO 4 shell into quantum sized particles. Transmission electron microscopy (TEM) images reveal that the spherical aggregates (Fig. 2b, c) are composed of point-like q-BiVO 4 with an average particle size of ~ 4.5 nm (Fig. 2d, e). It is known that the Bohr radius of BiVO 4 is about 2 nm 18,19 which is close to the radius of q-BiVO 4 (2.25 nm). Thus, q-BiVO 4 should exhibit a strong quantum con nement effect. X-ray diffraction (XRD) pattern indicates that as-synthesized q-BiVO 4 is of monoclinic scheelite structure without any impurity crystal phase ( Fig. 2g and Fig. S2). Note that the absence of broad XRD shoulder corresponding to amorphous silica manifests the successful removal of template (Fig. S2). The characteristic XRD peaks are also used to analyze the average aggregation size of q-BiVO 4 (10.5 nm) within Debye-Scherrer method (Fig. S3). The clear lattice fringe in high-resolution TEM (HRTEM, Fig. 2f) and the selected area electron diffraction (SAED) pattern both con rm the high crystalline nature of q-BiVO 4 . Element mapping images based on high-angle annular dark eld scanning transmission electron microscopy (HAADF-STEM, Fig. S4) display that all the Bi, V and O elements are homogeneously distributed over q-BiVO 4 , and the quantitative estimation by energy dispersive X-ray (EDX) spectroscopy presents the stoichiometric composition of Bi, V and O elements in q-BiVO 4 (Fig. S5).
Photocatalytic performance. The photocatalytic CH 4 oxidation was investigated in a high-pressure stainless-steel vessel with a transmittance quartz glass window on the top plate (Fig. S6). The adoption of high-pressure reactor is based on below two considerations. One is to economically utilize the available pressure energy from the practical transportation and storage condition of natural gas, where the pressure is as high as 70-200 bar in main pipelines and tanks 20,21 . The speci c analysis is brie y summarized in supplementary information (Fig. S7). The other is to promote the yield of desired products through dissolving more CH 4 reactants in the solvent at high pressure. In a typical reaction, 10 mg q-BiVO 4 photocatalysts were dispersed in certain amount of H 2 O, while the total pressure of CH 4 and O 2 mixed gas was xed to be 20 bar. After irradiation with Hg lamp (excitation wavelength of 300-600 nm) at room temperature (surface temperature of catalyst = 28.5±0.5 o C, Fig. S8), the yield of liquid organic products such as CH 3 OH and C 2 H 5 OH was evaluated by nuclear magnetic resonance spectroscopy (NMR) and gas chromatography (GC) with ame ionization detector. Note that liquid HCHO is known to exist in the form of methanediol (HOCH 2 OH) in aqueous solution 22 , which 1 H NMR signal overlaps with the broad peak of water solvent. So, the quantity of HCHO was determined by acetylacetone colordeveloping method (Fig. S9, S10, S11). Other gaseous products (CO 2 , C 2 H 6 , O 2 and H 2 ) were monitored by GC spectra with ame ionization and thermal conductivity detectors.  Fig. S12). As shown in Fig. 3a, about 4 folds in the oxygenated products by q-BiVO 4 (2.3 mmol g -1 CH 3 OH, 1.9 mmol g -1 HCHO and 0.3 mmol g -1 CO 2 ) are acquired with respect to s-BiVO 4 (0.5 mmol g -1 CH 3 OH, 0.5 mmol g -1 HCHO and 0.1 mmol g -1 CO 2 ). The corresponding increment is also distinguished in conversion of CH 4 and O 2 reactants (Fig. S13a). These enhancements by q-BiVO 4 are ascribed to both the raised charge carrier kinetics ( Fig.   S14) and the improved Brunauer-Emmett-Teller (BET) speci c surface area (226.9 m 2 g -1 , Fig. S15). This deduction is supported by comparing the BET surface area normalized CH 4 oxidation activity (Table S1), and q-BiVO 4 still exhibits higher performance than s-BiVO 4 . Except for s-BiVO 4 , commercial TiO 2 nanoparticle (P25) is also employed as the contrast sample and tested under the same reaction condition, and only HCHO (48.9 μmol g -1 ) and CO 2 (48.9 μmol g -1 ) are found in the products (Fig. S16).
Optimizing the yield of both CH 3 OH and HCHO is carried out through varying O 2 amount ( Fig. 3b) 12,23 .
The highest productivity of CH 3 OH and HCHO is 2.3 and 1.9 mmol g -1 at the O 2 pressure of 10 bar, respectively, and the corresponding conversion percentage of O 2 is 0.23% (Fig. S13b). The gradual increase of oxygenates at the O 2 partial pressure of less than 10 bar implies that O 2 involves the ratedetermining step of CH 4 oxidation. When the O 2 pressure surpasses 10 bar, the reduced productivity of CH 3 OH and HCHO is ascribed to the lowered CH 4 partial pressure and their overoxidation to CO 2 . Hence, the partial pressure of O 2 is controlled to be 10 bar.
The inspection of reaction time discloses that 3 h is the best for the yield of CH 3 OH (2.3 mmol g -1 ) though its selectivity continuously decreases (Fig. 3c). It is noticed that when the reaction time arrives 7 h, the productivity of HCHO ascends to 5.6 mmol g -1 with 69.8% selectivity, demonstrating that the prolonged reaction promotes oxidation of both CH 4 reactants and CH 3 OH intermediates. The corresponding conversion percentage of CH 4 at 7 h is 0.40% ( Fig. S13c) with turnover number (TON) of 2.6. Evidently, extending the reaction time enables improving the HCHO selectivity simultaneously ensuring its output.
The balance between the consumption rate of feed gas (CH 4 and O 2 ) and the formation rate of products (CH 3 OH, HCHO and CO 2 ) is explored by Fig. S17 and S18. The relationship between photocatalytic performance and input photon number or light energy is also examined. As indicated in Fig Based on the above results, we conclude the favorable reaction condition for production of HCHO and CH 3 OH, respectively. Since HCHO is the further oxidation product of CH 3 OH, increasing the oxidation capacity would be an e cient strategy to elevate the selectivity of HCHO. As demonstrated in Fig. 4a, utilization of short-wavelength UV irradiation (300-400 nm) with high intensity (170 mW cm -2 ) not only promotes the CH 4 conversion but also accelerates the oxidation of CH 3 OH to HCHO. Signi cantly, upon a long time oxidation of 7 h, HCHO product is achieved with a good selectivity and yield of 86.7% and 13.1 mmol g -1 (TON = 4.7), respectively. The corresponding conversion of both CH 4 and O 2 reaches 0.73% (Fig.   S13g). On the contrary, reducing the oxidation degree of reaction system could be conducive to production of CH 3 Fig. 4b, S20). The corresponding conversion of both CH 4 and O 2 is 0.06% (Fig. S13h).
Evidently, the diluted concentration of CH 3 OH generated in large solvent volume gives rise to the enhanced selectivity from 92.8% to 96.6% through depressing the overoxidation (Fig. S21) (Table S3). To preclude the in uence of temperature, thermocatalytic reaction (30 o C) under similar condition is also conducted and no products is observed (Fig. S22). It deserves to be stressed that the highly selective generation of CH 3 OH or HCHO from CH 4 oxidation over the single photocatalyst is unattained in previous works (Table S4).
Photocatalytic mechanism. Fig. 5A illustrates the proposed radical mechanism for CH 4 oxidation on q-BiVO 4 (Fig. 5). Under light irradiation, q-BiVO 4 is excited to induce • OH generation via two routes: oxidation of H 2 O by valence band holes and reduction of O 2 by conduction band electrons (step 1). The • OH cleaves C-H bond for producing the methyl radical ( • CH 3 ) (step 2), which is a rate-determining step.
Also as an oxidant, O 2 rapidly binds to • CH 3 and then reacts with as-formed H + and electron to generate methylhydroperoxide (CH 3 OOH, step 3), which will be reduced by electrons 23,24 or decomposed under UV irradiation to form CH 3 OH (step 4) 25,26 . Upon oxidation by holes from valence band of q-BiVO 4 , asformed CH 3 OH is further activated to • CH 2 OH that is combined with • OH to produce HCHO in the form of HOCH 2 OH (step 5). In the following parts, we validate this reaction mechanism step by step.
To solidly prove that the oxygenated products results from the conversion of CH 4 , 13 CH 4 was used as reactant instead of 12 CH 4 . Gas chromatography-mass spectrometry (GC-MS) with isotopically labelled 13 CH 4 discloses that CH 4 is the carbon source of CH 3 OH with the appearance of 13 CH 3 OH peak at m/z = 33 (red curve in Fig. 6a). The obvious 13 C NMR peaks of CH 3 OH and HOCH 2 OH (HCHO) also verify that the C1 oxygenated products are derived from CH 4 (black curve in Fig. S23a). The carbon type of C1 oxygenated products is identi ed from the 13 C DEPT-135 (distortionless enhancement by polarization transfer) spectrum, where up and down signals represent -CH 3 and -CH 2 groups from CH 3 OH and HOCH 2 OH (HCHO), respectively (red curve in Fig. S23a). Furthermore, as displayed in Fig. S23b generation.
According to the mechanism illustration, • OH is responsible for removing the H atom from CH 4 molecule, which is the key step for CH 4 activation. To explore the ability of q-BiVO 4 towards the generation of • OH in step 1, its band structure is established through UV-Vis diffuse re ectance spectrum 27 (Fig. S26a), transformed Kubelka-Munk function plot (Fig. S26b) 28 , Mott-Schottky plot (Fig. S26c, S27) 29 and ultraviolet photoelectron spectroscopy (Fig. S28). As shown in Fig. 6b, the conduction and valence bands of q-BiVO 4 are tested to be 0.075 and 2.555 V vs. normal hydrogen electrode (NHE) at pH=0, respectively.  31 . In order to assess the generation of • OH from both hole oxidation and electron reduction of q-BiVO 4 , we carried out the uorescence detection of coumarin solution without or with O 2 (Fig. S29). In the absence of O 2 (Fig.   S29a), an enhanced uorescence signal is achieved with q-BiVO 4 photocatalyst under visible light, indicating that the holes from valence band can successfully oxidize H 2 O into • OH. After the solution is saturated with O 2 (Fig. S29b), the uorescence intensity with q-BiVO 4 exhibits 2.3 times increase than that in absence of O 2 , con rming that O 2 also greatly contributes to the generation of • OH. Note that the quantity of O 2 solely generated from H 2 O decomposition by q-BiVO 4 is too low to make a detectable yield of oxygenates (Fig. S30). Moreover, the energy band structure of s-BiVO 4 is likewise constructed (Fig. S31 and S32) with the conduction and valence bands at 0.084 and 2.474 V vs. NHE at pH=0, respectively. With respect to the energy band structure of s-BiVO 4 , the more negative conduction band and positive valence band of q-BiVO 4 lead to more • OH species for CH 4 oxidation (Fig. S33). We notice that such uorescence signal comes from 7-hydroxycoumarin that is the • OH trapping product of coumarin ( Fig.  S34 and S35).
Step 2 refers to the activation step of CH 4 by • OH, which is recognized as the rate-determining step in this work. To verify that • OH is the initiator for CH 4 activation on the surface of q-BiVO 4 rather than the photoinduced hole, we performed a thermocatalytic reaction containing 10 mg q-BiVO 4 , 5 mL H 2 O 2 , 5 mL H 2 O, 10 bar O 2 and 10 bar CH 4 at 60 o C in absence of light irradiation. The H 2 O 2 is decomposed to provide • OH at 60 o C, and the dark condition prevents the hole formation on the surface of q-BiVO 4 . After reaction, CH 4 is found to be oxidized to CH 3 OOH, which is not further reduced to CH 3 OH due to the lack of photogenerated electrons under dark condition (Fig. 6c). In absence of H 2 O 2 (Fig. S36) or q-BiVO 4 ( Fig.   6d), no oxygenated product is distinguished. Thus, we deduce that • OH activates CH 4 for oxygenate generation on the surface of q-BiVO 4 . Besides, the solid evidence that CH 4 is not directly oxidized by photoinduced holes from q-BiVO 4 is supported by electron spin resonance (ESR) spectroscopy (Fig. 6e).
Under Xenon lamp irradiation in argon (Ar) atmosphere, the ESR spectrum of q-BiVO 4 shows a g value of 2.006 32 , corresponding to the active hole center O - (Fig. 6e, blue curve); subsequently, atmosphere is switched from Ar to CH 4 , no distinct intensity decrease is discerned on Osignal (Fig. 6e, green 34 . Taking 420 nm monochromatic light for irradiation, the KIE value is estimated to be 8.3 (larger than 6) 35 based on the reaction rate ratio (k H / k D ) using CH 4 or CD 4 as reactants, respectively ( Fig. 6f and S37). This large value reveals the C-H bond cleavage in the rate-determining step.

Incorporation of O 2 with • CH 3 to form CH 3 OH in step 3 was proved by 18 O 2 and H 2 18 O isotope tests as
well as the reusability of q-BiVO 4 photocatalyst, considering that these three oxygen sources possibly involved the catalytic reaction. GC-MS analyses (Fig. 6g)  preserves its initial catalytic activity (Fig. S38). Therefore, O 2 is the mere oxygen source for CH 3 OH formation (step 3). TEM images (Fig. S39), XRD patterns (Fig. S40), XPS (Fig. S41), Raman (Fig. S42a), UV-Vis diffuse re ectance spectra (Fig. S42b) and inductively coupled plasma optical emission spectrometer tests (Fig. S43) were carried out to prove only slight change in the morphologies and chemical states of q-BiVO 4 before and after photocatalytic reaction.
According to step 4, CH 3 OOH is rst formed and then reduced to CH 3 OH. Since this reduction process easily happens under high photoinduced electron density 36 , the NMR signal of CH 3 OOH is di cult to be detected. Therefore, we increased the amount of q-BiVO 4 (50 mg) and reduced the amount of H 2 O solvent (0.5 mL) in order to increase the concentration of CH 3 OOH intermediates. By prolonging the reaction time to 7 h, a weak CH 3 OOH peak appeared at 3.78 ppm, providing the clear evidence on CH 3 OOH production ( Fig. 6h). As-formed CH 3 OH would be further oxidized to HCHO (HOCH 2 OH) in step 5. Based on the Gibbs free energy values ( Fig. 1) 22, 37, 38 , oxidation of CH 3 OH to HCHO is thermodynamically favorable. To con rm this transformation process from the experimental aspect, CH 3 OH was taken as reactants in a similar photocatalytic system with argon (Ar) replacing CH 4 . The 13 C NMR spectrum of CH 3 OH oxidation products veri es that CH 3 OH can be oxidized to HOCH 2 OH using q-BiVO 4 as catalysts under light illumination (Fig. S6i). Photocatalytic oxidation of CH 4 . Photocatalytic oxidation of CH 4 was performed in a stainless-steel autoclave with a quartz glass window on the top (Supplementary Fig. S5). Photocatalysis experiments were carried out at a xed pressure of 20 bar and room temperature along with 25 o C cooling water unless otherwise stated. 10 mg photocatalyst sample was placed at the center inside reactor, and then certain amount of deionized water was added. The mixed CH 4 and O 2 at different ratio (8:12, 10:10, 12:8, 14:6) were lled, and high-pressure mercury lamp (excitation wavelength of 300-600 nm, CEAULIOHT), Xenon lamp (excitation wavelength of 400-780 nm and irradiation intensity of 170 mW cm -2 , Perfect Light) or LED monochromatic light source (Perfect Light) was used the light source for photocatalytic reactions. Acetylacetone color-developing method. 3 mL product solution was mixed with 2 mL as-prepared 0.25% (V/V) acetylacetone solution, and then heated for 3 min in boiling water. Afterward, the mixed solution became yellow-color. Through absorbance detection at 413 nm, the HCHO content was obtained. Speci c reaction mechanism was shown in Fig. S6.

Summary
Preparation of 0.25% (V/V) acetylacetone solution. 25 g ammonium acetate was dissolved in 10 mL water, and then 3 mL acetic acid and 0.25 mL acetylacetone were added in sequence. Afterward, the solution volume was diluted to 100 mL. With pH adjusted to 6, the solution was stored at 2 o C -5 o C, which could stay stable for one month. The sample was pressed into lm and bonded to the surface of conductive tape, while the other surface of conductive tape was xed on the stage. Electrical contact between the stage and sample was made using a copper tape. Subsequently, the sample was stored within a vacuum chamber for further test. During the UPS measurement, He(I) emission line provided an illumination at 21.22 eV from a helium discharge lamp, and the partial gas pressure of He was adjusted to 2 × 10 -8 mbar. The stage was biased at -10.0 eV in order to accurately guarantee the low-kinetic energy cutoff and the collection of electron emission at 0 o from normal. Cutoff energy was measured from the intersection between the linear extrapolation of the cutoff region and the baseline.
Hydroxyl radical detection. Coumarin was taken as a probe to detect production of • OH through monitoring formation of 7-OH-coumarin. Typically, 8 mg photocatalyst was dispersed in 15 mL 1 mM coumarin aqueous solution under stirring, and then either argon gas was injected to evacuate the air to produce an anaerobic environment or O 2 was bubbled to create a saturated O 2 condition under the sealed condition. After irradiation for 1 h, a certain amount of reaction solution was taken out and centrifuged to examine the uorescence spectrum. was employed for irradiation, while 25°C cooling water was used to keep the reaction at room temperature. After reaction for 3, 5 or 7 h, the yield of oxygenates (CH 3 OH, HCHO and CO 2 ) was tested from CH 4 and CD 4 reactant systems, respectively. According to the corresponding yield under different reaction time, we calculated the product formation rate constant in CH 4 (k H ) and CD 4 (k D ) systems, respectively. As a result, the rate-determining step was veri ed through KIE = k H / k D .
Date availability. The data that support the ndings within this study are available upon request.