The performance of our hydrophobic photocatalysts in light-driven H2O2 production in a biphasic system was tested using 1-octanol and 1-butanol as water-insoluble organic electron donors. The aim behind the choice of this substrate class is twofold. Firstly, we intentionally avoided the use of aromatic alcohols, such as benzyl alcohol, that are typically applied as model substrates for selective photocatalytic transformations since, in line with the autocatalytic oxidation mechanism under UV light reported recently, we found a conclusive and unambiguous evidence for benzaldehydemediated (auto)photocatalysis of benzyl alcohol oxidation and H2O2 production even under visible (>420 nm) light irradiation. In other words, the benzaldehyde present as an impurity or formed during the photocatalytic reaction can itself absorb light and further oxidize benzyl alcohol to benzaldehyde and simultaneously generate H2O2. Secondly, fatty alcohols, such as 1-octanol, are useful chemical compounds that can be readily obtained from biomass-derived chemicals, and the products of their partial oxidation, such as 1-octanal (caprylaldehyde), octanoic (caprylic) acid or octyl octanoate, represent a valuable feedstock for the synthesis of emulsifiers, pharmaceuticals and cosmetic products. Moreover, due to its non-activated nature, the oxidation of 1-octanol is very challenging even by the standards of conventional thermal catalysis, and has been seldom tackled by photocatalysis. The photocatalytic performance was assessed in H2O2 production under violet LED illumination (406 nm). In order to establish the optimal photocatalyst loading, the preliminary experiments were carried out under monophasic conditions using 50% ethanol solution in water, later the established loadings were further confirmed also for the biphasic conditions. Expectedly, unmodified KPHI, among all other photocatalysts, required the lowest concentration (0.5 g L -1 ) for attaining its optimal performance (see Supplementary Information, Figure ). Other materials, due to dilution of the photocatalytically active PHI phase by silica and grafted hydrophobic functions, need higher loadings for their optimal operation with values of 1.0 g L -1 for KPHI-Si and KPHI-HMDS and 1.5 g L -1 for KPHI-HTS and KPHI-OTS (Supplementary Information, Figure ). Parent KPHI material showed the highest reaction rate of H2O2 production using ethanol as an electron donor, while each subsequent stage of the photocatalyst modification first with silica and then hydrophobization led to a decreased activity (Supplementary Information, Figure ). This can be explained by a partial blockage of PHI with photocatalytically inactive species. However, in a biphasic (i.e., with two immiscible liquids) reaction system and using a more hydrophobic aliphatic alcohol as electron donor, such as 1-butanol, KPHI shows the lowest activity of all the prepared photocatalysts while the higher concentrations of produced H2O2 are observed for KPHI-Si and KPHI-HMDS (see Supplementary Information, Figure ). 1-butanol, although not completely miscible with water, still has a relatively high solubility in it (73 g L -1 ). In order to minimize the concentration of the electron donor in the aqueous phase and thus achieve efficient separation of oxidation and reduction products, 1-octanol was applied as hydrophobic electron donor as its solubility in water is only 0.3 g L -1 . When the pH value of the aqueous phase of the biphasic system was neutral, all the tested samples showed extremely low activity towards H2O2 production (Figure ). However, acidification upon addition of HCl exerted a clearly positive effect on H2O2 formation and its stability in the reaction mixture with time (Figure , see also discussion below). Such pH effects are common in photocatalysis, in particular due to the pH dependence of surface (i.e., acidity/basicity of active sites) and redox (i.e., band edge positions) properties of most photocatalysts. It is noted, however, that protons are not consumed during the reaction since protons for H2O2 production are generated by oxidation of 1-octanol. The hydrophilic KPHI and KPHI-Si samples were capable of producing 0.11 and 0.15 mmols of H2O2, respectively, within 24 h of irradiation when applying 0.5 M HCl as aqueous phase (Figure ). The higher reaction rates demonstrated by the KPHI-Si with respect to that of KPHI might be attributed to a less hydrophilic character of the former rooting in incomplete hydrolysis and elimination of ethoxo-groups of TEOS. The impact of the acid concentration is much more significant in case of hydrophobic materials that show a continuous enhancement of the photocatalytic H2O2 production with increased concentration of the acid in the aqueous layer of the biphasic system (Figure ). The highest amount (0.36 mmols) of H2O2 generated after 24 h irradiation was achieved by KPHI-OTS in 1.0 M HCl/1-octanol system (Figure ). For ensuring that the observed H2O2 production is a photocatalytic reaction promoted by PHI, several control experiments were carried out (Supplementary Information, Figure ). No significant production of H2O2 was observed by irradiation of pure 1-octanol as well as the mixture of 1-octanol and 1-octanal. Conventional melon-type polymeric carbon nitride derived from dicyandiamide also showed only moderate activity in this reaction (See Supplementary Information, Figure ). The oxidation of organic substrates by polymeric carbon nitride photocatalysts is usually a very fast process, while the reduction occurs on a much longer timescale and represents typically the rate-limiting step of the overall photocatalytic reaction. The obtained data show that the concentration of H3O + ions in the aqueous phase of the biphasic system greatly affects the rate of photocatalytic H2O2 production. This is reminiscent of the electrochemical systems studied by the group of Girault and further developed by Opallo 66 and Adamiak, where decamethyl ferrocene was used as lipophilic electron donor in the organic layer of a biphasic mixture, however for H2O2 production it required the presence of a proton donor in the aqueous phase. As the surface activity of hydrated protons at the water/air interface is known to be very high, the strong dependence of the observed H2O2 production rate on HCl concentration is not surprising (Figure ). In other words, we speculate that the concentration of H3O + in the aqueous phase determines the rate of the protonation of the PHI photocatalyst, whichapart from the photocatalytic functionmight also act as a proton shuttle to the organic phase. Indeed, protonation/deprotonation of the hydrophobized PHI in biphasic system takes place upon changing the pH of the aqueous phase and is accompanied by the change of the material's color from yellow (deprotonated form) to white (protonated form) (Supplementary Information, Figure ). This reasoning is in line with the model for photocatalytic H2O2 formation at polymeric carbon nitrides proposed by Shiraishi et al. , and it implies that the formation and stability of the intermediate PHI surface-bound species in the form of endoperoxide and protonated heptazine nitrogen sites is a key step on the way towards H2O2 formation and desorption from the photocatalyst It is also in accord with the results previously published by our group, where we found that the protonation of PHI plays a beneficial role in effective charge extraction from the photoexcited PHI. One of the crucial issues of photocatalytic H2O2 generation is the possibility of separating the H2O2/water mixture from the reaction medium, whereby the maximum concentration of H2O2 in the aqueous phase that can be achieved represents a key figure of merit. In this respect, the great advantage of the application of biphasic system is the immediate separation of H2O2 into the water layer. In order to minimize the possibility of decomposition of the formed H2O2 at the suspended photocatalyst, it is imperative to ensure the stability of the grafted hydrophobic functions towards photogenerated charges and reactive oxygen species produced during irradiation.