Received 23rd February 2022 Accepted 28th March 2022 DOI: 10.1039/d2sc01140a rsc.li/chemical-science

Jianxin Zhai,a Baowen Zhou,*b Haihong Wu, *a Shuaiqiang Jia, a Mengen Chu,a Shitao Han,a Wei Xia,a Mingyuan He*a and Buxing Han *ac

Introduction

   Highly efficient and selective transformation of CH4 is of great significance for the sustainable development of our society because of the revolution of shale gas.1 However, CH4 activation is a great challenge for catalysis because it is a very stable and inert molecule, and the unique symmetrical tetrahedral structure gives it four strong identical C–H bonds, which leads to a low electron and proton affinity, weak acidity, and low polarizability.2 Many efforts have been made on CH4 transformation by energy-intensive thermocatalytic routes.3 Carbon monoxide, which is a key component of syngas, is a valuable feedstock for manufacturing many products, such as alcohols, aldehydes, and hydrocarbon fuels. However, the mature industrial approach to producing syngas is mostly required to be done at high temperatures above 1000 K through the steam reforming of methane where high temperature is essential owing to the high C–H bonding energy (434 kJ mol 1 ) of the CH4 molecule.4 Hence selective oxidation of methane into carbon monoxide under ambient conditions paves a new avenue for syngas production but remains a grand challenge.

     As a promising green strategy, photocatalytic CH4 oxidation can be carried out under mild conditions. The formation of an oxygen species which is reactive and electrophilic can initiate the dissociation of the C–H bond of CH4 at room temperature when excited by photons with several eV of energy.5 For this purpose, some materials have been used as photocatalysts in the gas or liquid phase, such as ZnO,6 WO3,7 SrTiO3,8 heteropolyacids,9 BiVO4, 10 zeolite,11 etc. However, low selectivity to target products, and often abundant CO2 production are still major problems. The activation energy of methane conversion is usually higher than that for products of high value-added C1 platform molecules such as carbon monoxide, methanol, formaldehyde etc., resulting in the overoxidation of products and irreversible carbon loss. To date, research on photocatalytic conversion of CH4 to CO is relatively scarce.¹²

    Among various photocatalysts, Ag@AgX (X ¼ Cl, Br) possess excellent catalytic activity owing to the filled d10 electronic conguration of Ag+ ions, which can have a hand in the formation of the energy band structure or hybridization and strong absorptivity, which have attracted considerable attention.13,14 However, for a single component Ag@AgCl photocatalyst, the photogenerated electrons and holes easily recombine owing to the strong coulombic force between electrons and holes. It is known that the construction of hybrid heterostructures can effectively improve the photocatalytic performance. According to previous reports, as an emerging guest component, SiO2 has been used to build hybrid heterostructures for photocatalysis due to its low cost, excellent surface properties, UV-visible-IR optical transparency, and high stability/inertness.^15–17 Based on the pioneering attempts, it is rational to improve the photocatalytic activity of Ag/AgCl by modification with SiO2. ^18,19

     Herein, a method for preparing a Ag/AgCl@SiO2-x photocatalyst (x stands for wt% of SiO2 in the catalysts) is proposed, which used ionic liquid 1-octyl-3-methylimidazolium chloride ([Omim]Cl) as a Cl source. The catalysts were used in direct selective photocatalytic conversion of methane into carbon monoxide under ambient conditions for the first time. A series of characterization studies revealed that modification using SiO2 improved the photocatalytic activity. An appreciable carbon monoxide production of 2.3 mmol h  1 , was achieved with a high selectivity of 73% under ambient conditions, presenting a green and viable route for the transformation of CH4 to carbon monoxide. In addition, the success in utilization of real sunlight to catalyse the valuable transformation indicated the potential for practical application.

Experimental

Photocatalytic tests

The photocatalytic conversion of methane was carried out in a glass reactor (250 mL) with a quartz window under atmospheric pressure. A 300 W Xe lamp (Beijing China Education Au-Light Co., Ltd) equipped with a 300–800 nm cut-off filter was used as the light source to get light of the desired wavelength (300–800 nm). In a typical process, 10 mg of the catalyst powder was dispersed in 10 mL water in a reactor, and the reactor was heated at 333 K overnight to volatilize the solvent and a thin film of the catalyst was formed for illumination reaction. Prior to the illumination reaction, this reactor was evacuated by using a vacuum pump, and then filled with the mixture gas (0.2% CH4/1% O2/98.8% Ar) at atmospheric pressure. This evacuation- filling operation was repeated three times. During the whole experiment, the reactor was wrapped in aluminium foil to avoid light interference from the surroundings and the temperature was controlled by using a water bath set at 298 K. After a desired reaction time, the gas products were detected by using a gas chromatograph (Agilent GC-8860) and calibrated with a standard gas mixture. After the reaction, 10 mL of distilled water was injected into the reaction chamber; and the suspended photocatalyst was removed by filtration. The liquid mixture was subsequently characterized by using a nuclear magnetic resonance spectrometer (Bruker Avance III HD 500).

Materials characterization

The morphology of the samples was characterized by using a Zeiss Sigma HD scanning electron microscope (SEM). A Rigaku Ultima VI X-ray diffractometer was used to record the X-ray diffraction patterns with a scanning speed of 5 min 1 between 10 and 90 , which was operated at 25 kV and 35 mA with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) data were obtained on an AXIS Supra surface analysis instrument using a monochromatic Al Ka X-ray beam (1486.6 eV). BET measurements were carried out using N2 at  196  C in Quadrasorb evo equipment. UV-vis diffuse reflectance spectra were obtained by using a UV-Visible spectrophotometer with a diffuse reflectance unit, (UV-2700, Shimadzu, Japan) where BaSO4 was used as the internal reflectance standard. A liquid nitrogen cooled charge coupled device (CCD) spectrometer (Princeton Instruments) was used to detect the steady-state PL spectra under 375 nm excitation. The surface photovoltage measurements were carried out on a surface photocurrent spectroscope (CEL-SPS1000). The transient-state surface photovoltage measurements were carried out on a CEL-TPV2000 device. Semi in situ FTIR spectra were recorded with a NICOLET iS50 FTIR spectrometer (Thermo SCIENTIFIC, USA) equipped with a high-temperature reaction chamber and a mercury cadmium telluride (MCT) detector at a resolution of 4 cm 1 and 32 scans per spectrum. The background spectrum was scanned before the mixture gas (0.2% CH4/1% O2/98.8% Ar) was introduced. The Si contents were quantified by using an inductively coupled plasma emission spectrometer (ICP-OES) on an Optima 8300. The in situ electron paramagnetic resonance (EPR) measurement was carried out using an EMXplus- 10/12 to detect 1O2 radicals by adding 2,2,6,6-tetramethylpiperidine (TEMP) as a spin-trapping reagent in the photocatalytic reaction after the mixture gas (75% CH4/25% O2) was introduced. A 300 W Xe lamp (Beijing China Education Au-Light Co., Ltd) equipped with a 300–800 nm cut-off filter was used as the light source to get light of the desired wavelength (300–800 nm).