The formation of detected intermediates can be explained by the action of OH•, O2 •− and/or h+ (Fig. 12). The opening of an aromatic cycle is the major route of transformation. There is still some uncertainty about which of these radicals
reacts to form quinones such as 1,4-naphthoquinone. The first hypothesis is a successive addition of two hydroxyl radicals
on a cycle, leading to a naphthohydroquinone, which is next oxidized to the corresponding naphthoquinone [10,13,15]. The second possibility is an attack of naphthalene by a superoxide radical, leading to the formation of an unstable endoperoxide, which dissociates into a quinone [33]. However, since superoxide radical has a less reactivity, direct attack by superoxide is possible but rare. The last possibility is a direct oxidation of adsorbed naphthalene molecule by photogenerated holes to form naphthalene cation radical, and then the cation radical react with superoxide radical to form peroxide species which, in turn, dissociate into 1,4-naphthoquinone and/or 2-formylcinnamaldehyde.