Selective Photocatalytic Oxidation of Low Concentration Methane over

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Selectively Photocatalytic Oxidation of Low Concentration Methane over Graphitic Carbon Nitride Decorated Tungsten Bronze Cesium Yuan Li, Jun Li, Gaoke Zhang, Kai Wang, and Xiaoyong Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06270 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 30, 2019

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ACS Sustainable Chemistry & Engineering

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Selectively Photocatalytic Oxidation of Low

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Concentration Methane over Graphitic Carbon

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Nitride Decorated Tungsten Bronze Cesium

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Yuan Li1†, Jun Li1†, Gaoke Zhang1*, Kai Wang1, Xiaoyong Wu1*

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1 State Key Laboratory of Silicate Materials for Architectures, School of Resources and

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Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070,

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China

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†

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Corresponding Author: E-mail: [email protected]；[email protected]

Y. Li and J. Li contributed equally to this paper.

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KEYWORDS：Photocatalytic Methane Conversion, Graphic Carbon Nitride, Tungsten Bronze

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Cesium, Selective Conversion, Low concentration methane

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ACS Paragon Plus Environment

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Although the traditional thermal catalysis is usually used to convert the methane into value-added

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products, its high reaction temperature results in low performance-price ratio in conversion of low

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concentration methane. In this regard, we synthesized a series of mace-like g-C3N4 decorated

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Cs0.33WO3 nanocomposites for photocatalytic conversion of low concentration methane under

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mild conditions. The optimized [email protected] (weight ratio = 3:7) photocatalyst selectively

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converted low concentration methane (1000 ppm) into methanol with yield of 4.38 μmol/h/g under

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light irradiation and at room temperature. Both performance experiments and trapping experiments

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verified that the methane activation and methyl oxidation involved in photocatalytic conversion

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process of methane. The •O2- firstly activated the methane to methyl on the surface of the g-C3N4

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in the composite and then the photogenerated electrons from the Cs0.33WO3 in the composite

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inhibited the peroxidation and increased the generation of methanol. This research provides a new

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route to design and synthesize photocatalysts for converting methane into value-added chemicals.

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Introduction

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The uncontrolled and disordered emission of low concentration methane results in enormous

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energy waste and exacerbates the greenhouse effect, simultaneously.1-4 So, some strategies are

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now being actively sought to reduce the emission of methane and convert it into value-added

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products to improve the utilization efficiency of the low concentration of natural gas.5-10 Among

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of those strategies, the thermal catalysis has been firstly adopted to convert the trace amount of

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methane.11 However, the high energy-consumption, low performance-price ratio and explosion

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risk of the thermal catalysis limited its application. From the viewpoint of the cost and safety, the


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environmentally friendly photocatalysis has been considered as one of the ideal solutions to realize

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the effective conversion of low concentration methane into value-added products.12-17

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The methane converting via the photocatalysis approach is confronted with many fundamental

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technical hurdles at present. As we all known, the methane is very hard to be activated by the

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photocatalysts, because the methane is a kind of non-polar nature molecule with stable C-H bond

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(434 kJ•mol-1).18,19 So, the initial C-H bond cleavage determines the reaction rate of the

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photocatalytic conversion of methane. In terms of reaction process, the generation of strong

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oxidizing free radicals, such as •O2- and •OH, by photocatalysts plays a key role for the C-H bond

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cleavage, implying that the photocatalysts must have higher conduction band (CB) minimum than

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the O2/•O2- (- 0.16 V versus NHE) potential or lower valence band (VB) maximum than the

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•OH/OH- (+ 2.59 V versus NHE) potential.20-22 It is a possibility that the graphitic carbon nitride

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(g-C3N4) nanosheets can effectively activate methane, attributing to its suitable potential of

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conduction band.23-25 However, it is still a challenge to design the photocatalyst, which can realize

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the selectively photocatalytic conversion of methane. Rodriguez et.al thought that some inhibitors

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could be introduced to control the peroxidation process and realize selective photocatalysis.26 A

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hypothesis was proposed that the appropriate amount of semi-metallic semiconductors, such as

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tungsten bronze, which had a special band with rich reducing electrons nearby the Fermi level, 27-

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30

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of low concentration methane.

could composite with the g-C3N4 nanosheets to realize the selectively photocatalytic oxidation

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Herein, we prepared a series of [email protected] nanocomposites using ultrasonic assisted

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synthesis strategy to successfully confirm the above hypothesis. The optimized g-

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[email protected] nanocomposites exhibited superior photocatalytic performance for converting

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low concentration methane into methanol and very high yield of methanol under mild condition.


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The reaction pathway of photocatalytic conversion of methane into methanol by the composite

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was proposed based on a series of characterization techniques. Owing to the suitable band

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structures, the g-C3N4 in the composite effectively photo-activated the methane to intermediate in

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the activation process. Moreover, the Cs0.33WO3 in the composite inhibited the peroxidation of

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intermediate and provided more free electrons to enhance the generation of methanol. This study

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provides a new way for the photocatalytic conversion of low concentration methane into methanol.

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Materials and Methods

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Synthesis of Cs0.33WO3. The Cs0.33WO3 sample was synthesized by the reported water

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controlled-release process.30

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Synthesis of g-C3N4. The bulk g-C3N4 sample was prepared following the previous paper from

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the thermal polycondensation of melamine.25 Then, 0.5 g of the bulk g-C3N4 sample was

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ultrasonicated (3000 Hz) in the 100 ml pure water for 2 h to obtain the homogeneous suspension.

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Then, the suspension was centrifuged at 3000 rpm for 5 min to remove the residual bulk g-C3N4

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nanoparticles. Lastly, the supernatant of centrifuged suspension was dried in a vacuum oven at 60

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oC

to obtain the slightly yellow g-C3N4 nanosheets.

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Preparation of [email protected] nanocomposites. 0.5 g of the g-C3N4 nanosheets was

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dispersed by ultrasonication (3000 Hz) in the 100 ml mixed solution (The volume ratio of ethanol

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and water = 3: 7) for 2 h to obtain the mixture soliquoid. Then, certain amount of Cs0.33WO3 poured

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into the soliquoid with stirring for 12 h. Lastly, the above soliquoid was dried in a vacuum oven at

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60 °C. The nanocomposites, in which the weight ratios of g-C3N4 and Cs0.33WO3 were 1:9, 3:7 and

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7:3, were named as 10CW, 30CW and 70CW, respectively.

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Characterization


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The X-ray diffraction (XRD) pattern, Raman spectra, Fourier transform infrared (FT-IR) spectra

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and chemical state analyses of samples were characterized by a D/MAX-RB diffractometer with

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CuKα radiation, a JY LabRam HR800 Raman microscope with the 514 nm excitation laser beam,

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a Nexus FT-IR spectrometer and an X-ray photoelectron spectroscopy (XPS) utilizing

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monochromatic Mg Kα source and a charge neutralizer, respectively. Transmission electron

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microscopy (TEM) images, high resolution transmission electron microscopy (HRTEM) images

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and selected area electron diffraction (SAED) images were taken on JEOL 2100F (280 kV). A

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UV3500 plus

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spectrofluorometer (Shimadzu, Japan) with emission wavelength of 312 nm and a CHI-660E

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electrochemical workstation (Chenghua, China) was employed, in turn, to measure the absorption

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spectra, photoluminescence (PL) spectra and photoelectrochemical data of the samples. The

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Brunauer-Emmett-Teller (BET) specific surface areas of samples were measured by an ASAP

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2020HD88 nitrogen adsorption apparatus (Micromeritics, American).

UV-Visible-NIR spectrophotometer (Shimadzu, Japan), a

RF-5301PC

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Catalytic Activity Test

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The photocatalytic experiment for conversion of methane was conducted in a home-made vessel

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with a quartz-glass cap using a 300W Xe lamp as light source (Figure S1). The mixture gas, which

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was mixed by 1000 ppm CH4 gas and pure air (O2: N2 = 20: 80), was used as a target gas to simulate

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the emissions of methane from landfills, waste-water treatment, natural gas leakage, livestock

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farming and rice cultivation.4 50 mg of the as-prepared photocatalyst was evenly deposited on the

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bottom of vessel. Then, the target gas was pumped into the vessel for 15 min to remove the residual

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air. Furthermore, the products were qualitatively measured by the gas chromatography

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(Zhongkehuifen 7820, China). Discussion and Results


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As displayed in Figure 1a, the identified peaks in the XRD patterns of the series of

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nanocomposites were well indexed to the peaks of the pure Cs0.33WO3 sample without any

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impurities, but the identified diffraction peaks of the pure g-C3N4 sample cannot be clearly

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observed in the XRD patterns of the three nanocomposites, because of the poor crystallinity and

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high dispersion of the g-C3N4 in the composites.31 Therefore, Raman spectroscopy was employed

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to confirm the existence of g-C3N4 in the composite. Figure 1b demonstrates that four identified

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peaks of the g-C3N4 sample at 470, 710, 980 and 1220 cm−1 are spotted in the Raman spectrum of

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the 30CW sample, verifying the existence of g-C3N4 in the 30CW sample.32 Furthermore, FT-IR

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was used to research the interface between the Cs0.33WO3 and g-C3N4 in the composite. In the FT-

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IR spectra of the pure g-C3N4 and 30CW samples as shown in Figure 1c, the troughs at 800 cm-1,

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the broad bands between 1200 and 1700 cm-1 as well as 3200 cm-1 correspond to the characteristic

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breathing mode of tri-s-triazine units, the stretching vibration of C-N aromatic ring and the N-H

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stretching vibration, respectively.33-36 It can be noted that the W-O bond stretching vibration band

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in the FT-IR spectrum of the 30CW sample displays a slight right shift as compared to that of the

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pure Cs0.33WO3 sample (Figure 1d), indicating that the g-C3N4 in the composite affects the W-O

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bond of the Cs0.33WO3 in the composite.37


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Figure 1. The XRD patterns (a), Raman spectra (b), FT-IR spectra (c) and magnified FT-IR spectra (d) of the different samples.

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Firstly, SEM test was conducted to directly observe the morphology of the samples. The cloud-

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like particles in the Figure S2a attribute to the g-C3N4 samples and the rod-like samples in the

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Figure S2b correspond to the Cs0.33WO3 samples, respectively. Besides, Figure S2c displays that

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the Cs0.33WO3 was wrapped by the g-C3N4 in the composites. Furthermore, as shown in Figure 2a-

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f, HAADF-STEM and mapping analyses directly demonstrate the elements spatial distribution and

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morphology of the 30CW sample. It is not hard to be seen that a large number of blade-like g-C3N4

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samples were loaded on the surface of rod-like Cs0.33WO3 sample to form the mace-like

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nanocomposites. From Figure 2g-h, the lattice fringe spacing of 0.32 nm is observed on Cs0.33WO3

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nanorods, which is assigned to (200) crystallographic plane with 0.32 nm lattice fringe spacing.


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Meanwhile, the SAED pattern displays two closest light spots, which attribute to the (202) and

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(200) planes, respectively (Figure 2i). The above results prove that g-C3N4 nanosheets were

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successfully anchored onto the Cs0.33WO3 nanorods.

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Figure 2. The elements mapping (a-f), TEM image (g), HRTEM image (h) and SEAD pattern (i) of the 30CW sample.

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The survey XPS spectrum in Figure 3a displays that the as-prepared 30CW sample is composed

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of W, C, N, O and Cs elements. As shown in Figure 3b, the C 1s spectrum can be deconvoluted

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into two components of the sp2 carbon (284.6 eV) and N=C-N bond (287.5 eV), respectively.38

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The insert of Figure 3c illustrates that Cs 3d has two binding-energy peaks at 133.2 and 134.8 eV,

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which correspond to Cs 3d5/2, and Cs 3d3/2, respectively.37 The peaks of N 1s at 398.2, 399.3 and


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400.7 eV are attributed to the bonds of C-N=C, N-C3 and N-H, respectively (Figure 3d).39 The O

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1s binding-energy peaks locate at 533.2 eV from the bond of O-H (H2O), 531.5 eV from the

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adsorbed O2 and 529.8 eV from the bond of O-W (Figure 3e).40 In addition, the W 4f binding-

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energy spectrum in Figure 3f demonstrates four peaks. The peaks at 35.5 and 37.4 eV belong to

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W6+, and the other peaks at 34.0 and 36.4 eV correspond to W5+.41

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Figure 3. The survey XPS spectrum of 30CW (a) and its corresponding high resolution XPS spectra of C 1s (b), Cs 3d (c), N 1s (d), O 1s (e) and W 4f (f), respectively.

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Figure 4a shows the methanol yield of the corresponding samples at CH4 (1000ppm) and pure

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air mixed atmosphere under the irradiation of full spectrum light and at the room temperature. The

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pure Cs0.33WO3 sample did not display photocatalytic activity for conversion of methane. Figure

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4a and S3 demonstrate that pure g-C3N4 sample photo-catalyzed methane to CO2 and CO directly.

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In addition, the per gram of the 10CW, 30CW and 70CW samples can convert methane into ca.

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0.66, 17.50 and 7.70 µmol of methanol, respectively, under full spectrum light irradiation for 4 h.


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Importantly, the optimized 30CW sample displayed the best performance of selective

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photocatalysis under the same reaction condition. As shown in Figure 4b, the photocatalytic

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product of the 30CW sample consists of ca. 51.59 % of CH3OH, ca. 12.08 % of CO and ca. 36.33

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% of CO2, deducing that the Cs0.33WO3 in the composite protected the intermediates from

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peroxidation. As shown in Figure S4, the 30CW sample did not demonstrate photocatalytic activity

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for conversion of methane under near infrared light irradiation and displayed nice activity under

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ultraviolet light irradiation. In Figure 4c, the 30CW sample still presented good photocatalysis and

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photostability after 4 times tests (4 h full spectrum light irradiation for each time). In order to

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confirm that the production of methanol was really originated from methane, the reaction of

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photocatalytic conversion of methane was conducted using the 30CW sample in a pure air (N2: O2

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= 80: 20) atmosphere without any carbon gases. It is easily to be seen from Figure S5 that no

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product is generated in the above condition, revealing that the carbon of methanol in the products

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should come from methane. Figure 4d shows the FT-IR spectra of the fresh and reacted 30CW

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samples, respectively. Methane has a special IR vibration mode at 1320 cm-1 which only can be

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observed in the FT-IR spectrum of the reacted 30CW sample.6 Significantly, compared to the FT-

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IR spectrum of the fresh 30CW sample, only the band of chemisorbed methyl radical at 1425 cm-

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1 was detected

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activation process.

in the reacted 30CW sample, implying that methyl is the only product in the methane


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Figure 4. The photocatalytic activities for conversion of methane of the samples at CH4 (1000

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ppm) and pure air atmosphere under the irradiation of full spectrum light: the yield of CH3OH for

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the pure g-C3N4, 10CW, 30CW, 70CW and pure Cs0.33WO3 samples (a), the generation of various

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products during photocatalytic process over the 30CW sample (b), the photostability of the 30CW

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sample (c), respectively, and the FT-IR spectra of the fresh and reacted 30CW samples (d).

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The specific surface area, band structure, immigration of photogenerated carries and reaction

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process were studied to clarify the mechanism of photocatalytic conversion of methane. As shown

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in Table S1, the specific surface areas of all the samples are small (Table S1). DRS measurement

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was carried out to study the light absorption capability and band structure of the photocatalysts.42

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As shown in Figure 5a, the intrinsic absorption edges of all samples are less than 480 nm. In

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addition, the band gap energy of the pure g-C3N4, pure Cs0.33WO3, 10CW, 30CW and 70CW


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samples are calculated to be approximate 2.75, 2.58, 2.61, 2.67 and 2.73 eV, respectively,

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according to the formula:

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𝐸𝑔 = hν𝑚𝑖𝑛 = hc/𝜆𝑚𝑎𝑥 = 1240 ⁄ 𝜆𝑚𝑎𝑥

(1)

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where, h is the Planck constant (6.626×10-34 J S), ν min is the minimum frequency of light, c is the

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speed of light (3×108 m/s), λmax is the maximum absorption edge and Eg is band gap. Furthermore,

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except the g-C3N4 sample, the other samples display the near infrared (NIR) light absorption

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property more or less, because the W5+ atoms in the Cs0.33WO3 based samples and pure Cs0.33WO3

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sample contribute a new band.43,44 Figure 5b demonstrates the VB maximum position of the pure

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g-C3N4, pure Cs0.33WO3 and 30CW samples are located at 1.50, 1.86 and 1.44 eV, respectively.

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Therefore, The CB minimum positions of the pure g-C3N4, pure Cs0.33WO3 and 30CW samples

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are calculated to be -1.25, -0.72 and -1.23 eV, respectively, according to the formula:

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𝐸𝐶𝐵 = 𝐸𝑉𝐵 − 𝐸𝑔

(2)

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where, EVB is the VB maximum potential and ECB is CB minimum potential. In addition, the broad

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peaks nearby the Femi level imply that lots of electrons are localized at this state.45 Therefore, the

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photons could easily drive the electrons from the localization state to CB.44

200


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Figure 5. UV-vis-NIR DRS spectra for the pure g-C3N4, 10CW, 30CW, 70CW and pure

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Cs0.33WO3 samples (a) and valence band XPS measurement plots for the pure g-C3N4, 30CW and

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pure Cs0.33WO3 samples (b).

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PL and photoelectrochemical measurements were conducted to evaluate the separation ability

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of photogenerated carries. Figure 6a displays the two broad photoluminescence peaks between 440

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and 470 nm, implying that the photoinduced electrons and holes in the 30CW sample have lower

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recombination ratio than that in the pure g-C3N4 sample.46 In addition, the transient photocurrent

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responses of the g-C3N4, Cs0.33WO3 and 30CW samples illustrate uniform photocurrent response

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(Figure 6b). Compared to the pure components, the 30CW sample showed the best photocurrent

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response and the lowest electrical impedance (Figure 6c), indicating that the 30CW sample can

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more effectively separate and immigrate the photogenerated carries than the g-C3N4 and Cs0.33WO3

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samples.47

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The radical species trapping experiments were implemented to study the process of

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photocatalytic conversion of methane. 0.2 mmol of K2Cr2O7, para-Quinone, salicylic acid and

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Na2C2O4 were adopted as the scavengers of e-, •O2-, •OH and h+, respectively.48 As shown in Figure

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6d, both K2Cr2O7 and para-Quinone strongly affect the yield of methanol, implying that the •O2-

217

and e- should be main radicals in the process of photocatalytic conversion of methane. Meanwhile,

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both salicylic acid group and Na2C2O4 group have no obvious effect on the methanol yield,

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indicating that the •OH and h+ radicals were not main radicals in the photocatalytic reaction.

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Furthermore, anaerobic condition experiment in Figure S6 verifies that the oxygen in the air

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participated in the photocatalytic conversion reaction of methane. So, it is deduced that the •O2-

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promoted the generation of methoxyl radicals.


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Figure 6. Photoluminescence spectra under 312 nm excitation for the 30CW and g-C3N4

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samples (a), transient photocurrent responses under full spectrum light irradiation (b) and

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electrochemical impedance spectra (c) for the 30CW, pure g-C3N4 and pure Cs0.33WO3 samples,

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the radical species trapping experiments of the 30CW sample (d).

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Figure 7a demonstrates the activation mechanism that the photocatalyst oxidize methane to

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methyl using •O2-. As shown in Figure 7b, the following reaction processes are divided to two

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reaction pathways, including selective oxidation path and peroxidation path. In the selective

231

oxidation pathway, the photogenerated free electron from Cs0.33WO3 transferred rapidly to the

232

decorated g-C3N4 and inhibited the peroxidation of methoxyl radicals. In addition, a little CO2

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produced in the photocatalytic conversion might be attributed to the oxidation of methane over the

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g-C3N4 nanosheets that did not bond with the Cs0.33WO3 nanorods in the composite.


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Figure 7. The activation mechanism for photocatalytic oxidation of methane (a) and proposed

237

reaction scheme for photocatalytic conversion of methane (b).

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Conclusions

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A series of mace-like [email protected] nanocomposites were prepared using ultrasonic

240

assisted synthesis strategy for photocatalytic conversion of low concentration methane into

241

methanol under mild conditions. The optimized [email protected] composite showed superior

242

photocatalytic performance for conversion of methane into methanol, attributing to the synergistic

243

effect of photoinduced •O2- from the g-C3N4 in the composite and photogenerated electrons from

244

the Cs0.33WO3 in the composite. The •O2- dominated the cleavage rate the C-H bond and prompted

245

the generation of the intermediate, methoxyl. The photogenerated electrons protected the methoxyl

246

from the peroxidation and selectively enhanced the conversion ratio from methoxyl into methanol.

247

The strategy of components proportion regulating in the [email protected] nanocomposites

248

controlled the production of •O2- radical and photogenerated electrons, resulting in the effective


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and selective photocatalytic performance. This research supplied a new strategy for selective

250

photocatalytic conversion of methane into added-value products.

251

ASSOCIATED CONTENT

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Supporting Information

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The schematic diagram of photocatalytic experiment for conversion of methane; The

254

photocatalytic performance for conversion of methane for the pure g-C3N4 sample; The

255

photocatalytic performance for conversion of methane for the 30CW sample under pure air

256

condition; The photocatalytic performance for conversion of methane without O2.

257

AUTHOR INFORMATION

258

Corresponding Author

259

* Phone; Fax: +86-27-87887445; E-mail: [email protected] (G.K. Zhang) ;

260

[email protected] (X.Y. Wu).

261

Notes

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The authors declare no competing financial interest.

263

Author Contributions

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†Y.

265

Zhang and X.Y. Wu conceived the project; K. Wang helped to discuss the results and the

266

corresponding analysis.

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ACKNOWLEDGMENT

Li and J. Li contributed equally to this paper; Y. Li and J. Li performed the experiments; G.K.


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This work was supported by the National Natural Science Foundation of China (NSFC

269

No.51472194 and No.51602237) and the NSFC of Hubei Province (2610CFA078).

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Table of Contents (TOC)

419 420

Synopsis

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The synergistic effect of g-C3N4 and Cs0.33WO3 in the composite results in the selectively

422

photocatalytic conversion of methane into methanol.


25

Selective Photocatalytic Oxidation of Low Concentration Methane over

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