Volatile organic compounds (VOCs) are common gaseous environmental pollutants, mainly in the petroleum and petrochemical industry, printing, automobile exhaust, coating manufacturing, and other industries. They can be divided into alkanes, aromatics, olefins, halogenated hydrocarbons, esters, aldehydes and ketones(Sekar, Varghese, & Ravi Varma, 2019; Xinmin Zhang et al., 2017). Alkanes are the main composition of VOCs emitted artificially, accounting for about 40% of the anthropogenic emissions(Wei et al., 2019; Ziemann, 2011). N-hexane (C6H6) is a widely used alkane in industry and the most representative nonpolar solvent. It usually exists in oil in the form of saturated fatty hydrocarbons and can be separated from natural gas and oil fractionation. N-hexane is highly fat-soluble and easy to accumulate in organisms. Long-term exposure can lead to chronic poisoning symptoms such as headache, dizziness and numbness of limbs(Y. Guo et al., 2022; Yang et al., 2019). In serious cases, it can lead to loss of consciousness, cancer and even death. In addition, it can promote the generation of secondary organic aerosols, resulting in photochemical smog, seriously affecting air quality and human health, so it is urgent to deal with it(Thanh Truc et al., 2019; Yu et al., 2020).
Several VOCs control techniques including photocatalytic oxidation (PCO), adsorption, membrane filtration and catalytic combustion have been developed and used to treat alkane (Zhao et al., 2019; Ziemann, 2011). Generally, the membrane separation efficiency is not high. The energy consumption of catalytic combustion is very high. Therefore, there is a great challenge to design novel catalysts with the dual effects of photodegradation and adsorption for removal(Tian, Liao, Ke, Guo, & Guo, 2017).
Adsorption technology is the most effective and has been widely used. Activated carbon, activated alumina, silica gel and zeolite are commonly used adsorbents (T. Guo, Bai, Wu, & Zhu, 2008; Shi, Zheng, Wu, & Ji, 2008). Activated carbon fiber (ACF) is an effective VOCs adsorbent. ACF has many advantages, such as high surface adsorption reactivity, uniform microporous structure, renewability, large specific surface area and no secondary pollution(Das, Gaur, & Verma, 2004; Lin, Cheng, Liu, & Chen, 2012; Z. S. Liu, Peng, & Li, 2014; Miyamoto, Kaneko, & Kanoh, 2005; Yi, Lin, Chen, & Wei, 2008). To improve the adsorption effect and selectivity, it is often necessary to adjust the pore structure of ACF or modify its surface characteristics. At present, the commonly used modification methods are surface oxidation-reduction, supported metal and metal oxide(Yi et al., 2008). Bi et al.(Bi et al., 2021) modified activated carbon fiber (ACF) with Zn(NO3)2, ZnCl2 and Zn(OAc)2, respectively, and then loaded TiO2 on the modified ACF. The experiment showed that TiO2/Zn(OAc)2-ACF had the best toluene degradation performance.
Photocatalytic technology has the advantages of high efficiency, energy-saving, safety, low cost and mild reaction conditions, which is widely used to treat sewage and gas pollution(C. Hou, Liu, & Li, 2021; Mamaghani, Haghighat, & Lee, 2017). However, photocatalytic oxidation technology has the disadvantages of catalyst deactivation, easy recombination of photogenerated electrons and holes, and easy agglomeration of carriers. The commonly used catalysts include TiO2, g-C3N4, bismuth-based materials, graphene and its composites(Fu, Xu, Low, Jiang, & Yu, 2019; Wang et al., 2019; Xie et al., 2019). Among many catalysts, TiO2 has attracted much attention in recent years due to its outstanding advantages such as low energy consumption, simple operation, wide application range and no secondary pollution. However, due to the wide bandgap (3.2eV), pure TiO2 can only absorb ultraviolet light with a short wavelength, and the utilization of solar energy is poor(Raja, Rajasekaran, Selvakumar, Ganapathi Raman, & Swaminathan, 2020). Therefore, TiO2 needs to be modified to improve its visible light response. There are many modification methods for TiO2, such as noble metal modification, semiconductor composite, dye sensitization and transition metal ion doping(Q. H. Li et al., 2021). Graphitic carbon nitride (g-C3N4) is a non-metallic semiconductor catalyst with excellent performance. The raw materials are cheap and easy to get (amino nitrile, urea, melamine, dicyandiamide), narrow bandgap (2.7eV), good thermal and chemical stability(Wen et al., 2015),(Huang et al., 2019). In recent years, g-C3N4 has become a research hotspot of the photocatalyst. However, the photocatalytic activity of g-C3N4 was low because of its small specific surface area and the easy recombination of photogenerated electrons and holes(Fang et al., 2018; Humayun, Fu, Zheng, Li, & Luo, 2018). Accordingly, various methods have been reported to improve its photocatalytic performance, such as doping metal/non-metal elements, constructing heterostructures or optimizing morphology(Gao et al., 2019). Recently, some studies have shown that P doping in g-C3N4 can significantly improve the photocatalytic performance of (Gao et al., 2019; Humayun et al., 2018; Z. Li, Jiang, Zhang, Wu, & Han, 2016; S. Liu, Zhu, Yao, Chen, & Chen, 2018; Wu, Ma, & Hu, 2020). For example, the phosphorus (P) doped g-C3N4 synthesized by a simple sintering method showed an obvious red shift at the absorption edge(Wu et al., 2020). Li et al. (Z. Li et al., 2016) prepared P (x%)-g-C3N4/TiO2 composites showed enhanced light absorption and photocatalytic properties in the visible light region, and had high photocatalytic degradation activity for methyl blue (MB).
Another difficult problem of VOCs photocatalytic oxidation degradation technology is that powder photocatalytic materials are difficult to apply and recover due to the fluidity of gas. To solve the above problems, the powder material needs to be loaded on ACF. Previous studies have shown that the strong adsorption performance of ACF can not only enrich target pollutants, capture intermediate toxic products, promote the photocatalytic performance of nano-TiO2, but also provide support for the renewable performance of TiO2 photocatalytic materials. In addition, metal oxide-based catalysts loaded on ACF can provide a large number of binding sites on the surface of the adsorbent, which significantly improves the adsorption performance of ACF. Moreover, the introduction of catalyst reduces the blockage of ACF microporous structure, promotes the in-situ regeneration of adsorbent, and reduces the risk of adsorbent failure due to the increase of adsorption concentration(T. Guo et al., 2008; Shi et al., 2008; Tran Thi & Lee, 2017).
Under the guidance of the above strategies, TiO2 modified by the P-doped g-C3N4 photocatalyst was prepared by the sol-gel method in this work. PCN/TiO2/Zn(OAc)2-ACF composites were prepared by ultrasonic impregnation on zinc acetate modified ACF. Then, by exploring the effects of different factors (calcination temperature, impregnation times, light intensity, initial concentration of n-hexane, etc.) on PCN/TiO2/Zn(OAc)2-ACF’s degradation of n-hexane, the adsorption and photocatalytic mechanism of the catalyst were further studied.