Photocatalytic degradation of typical VOCs on sulfur-doped porous graphene

Abstract

<p indent="0mm">Volatile organic compounds (VOCs), one type of the most harmful pollutants, which mainly from fuel combustion, transportation and chemical industry or other industrial processes, have aroused great concern. Because they are important substances lead to urban haze and photochemical smog, long-term exposure to high concentration of VOCs will stimulate human skin, eyes, and respiratory tract and finally cause a series of diseases even death. Nowadays, a variety of VOCs treatment technologies are available, including biodegradation, combustion, adsorption, and low temperature or photocatalytic oxidation. Among these technologies, the renewable solar-driven multiphase photocatalytic technology has the advantages of rich resources, technical economy, mild reaction conditions, deep mineralized purification, no secondary pollution and so on, becoming an ideal VOCs degradation technology. Due to the large specific surface area and high active center density, semiconductor is often used as a catalyst to realize efficient photocatalysis. Therefore, understanding the performance of semiconductor photocatalyst is of great significance to develop an effective photocatalyst for VOCs degradation from a theoretical perspective. In this study, we calculated the photocatalytic properties of monolayer porous graphene (PG) doped with nonmetallic sulfur using density functional theory (DFT) calculations. The electronic properties of the most stable S-doped PG structure including the energy band structure, the band edge positions, the partial density of states (PDOS), and the highest occupied and the lowest occupied orbitals (HOMO-LUMO) were analyzed first after a comprehensive comparison of doping results. The results manifest that after doping the sulfur atom, the band gap of PG material decreases from <sc>3.050</sc> to <sc>2.127 eV,</sc> which is conducive to the generation of more photogenerated electrons in S-doped PG. At the same time, the reduction of band edge position makes S-doped PG improve the ability of superoxide free radical generation than the original PG monolayer. Then, the adsorption performance of S-doped PG on O₂, H₂O and VOCs molecules was calculated. The results show that the adsorption energy of PG material for O₂ molecule is significantly increased (from −0.590 to <sc>−2.462 eV)</sc> after doping single atom S, the O=O bond length (from 1.218 to <sc>1.418 Å)</sc> and electron transfer number (from 0.024<italic>e</italic> to 0.545<italic>e</italic>) greatly enhanced, all indicating that the O₂ molecule is successfully activated. Combined with the results of band edge position, it can be confirmed that S-doped PG can generate superoxide radicals. In addition, the adsorption performance of S-doped PG on four typical VOCs belongs to the category of chemical adsorption, which provides an important prerequisite for the degradation of VOCs catalyst. Finally, the optical absorption spectrum of S-doped PG was calculated to analyze its optical response properties. The optical absorption curve shifted to the infrared region, the wavelength threshold increased, and according to Compton effect calculation, the light absorption wavelength threshold of S-doped PG and original PG is <sc>583</sc> and <sc>406 nm</sc> respectively, all of these indicating the doped sulfur atom tremendously promoted the absorption response of visible light compared to the original PG material. In summary, our work indicates that S-doped PG is a promising photocatalyst for VOCs degradation, and multidimensional calculation and analysis will provide theoretical guidance for the practical application of porous graphene in photocatalysis.