**3.2 Methane oxidation to methanol using photocatalysts**

The photocatalytic process is a photochemical reaction that is carried out with external energy provided by ultraviolet light radiation that has energy equal to or greater than the energy band gap of a semiconductor. Several of oxidation and reduction processes are involved in the photo-generated electron and hole. TiO2 catalysts have been used as semiconductor photocatalysts for a wide range of environmental applications [67]. In addition, tungsten oxide (WO3) is also a good photocatalyst due to its high chemical stability in aqueous solution under acidic conditions in the presence of an oxidising agent [68]. For example, one study demonstrated that the WO3 photocatalyst produced hydroxyl radicals that react with a methane molecule to produce a methyl radical, which promote the formation of methanol [69]. Another study [70] investigated different experimental parameters for the methane conversion such as catalyst concentration, laser power, laser exposure time, effects of free radical generator (H2O2) and electron capture agent (Fe3+), using visible laser light. Also, this study examined the comparison between WO3 and TiO2, and it was found that the WO3 showed the highest methane conversion [70, 71]. A recent work has studied the introduction of some electron scavengers such as (Fe3+, Cu2+, and Ag+) and H2O2 species to the WO3 catalyst to enhance the selectivity of methanol. They found that WO3/Fe3+ is the most active catalyst with a methanol selectivity of 58.5% [68]. Another photocatalyst for the methane oxidation to methanol is vanadium oxide supported by MCM-41. Nitric oxide (NO) was used as an oxidant for the oxidation of methane under UV irradiation at 295 [72]. **Figure 5** shows an example of methane conversion to methanol via photocatalysis.
