## 3 Results and Discussion
### Photocatalytic Activity  
The activity of the synthesized photocatalysts was studied in the reaction of H\({}_{2}\)'s evolution from ethanol aqueous solutions under visible light irradiation. For all the photocatalysts tested, the main reaction product is H\({}_{2}\). The highest rate of H\({}_{2}\) evolution was achieved over the 0.5% Pt/g-C\({}_{3}\)N\({}_{4}\) catalyst, and reached 0.6 mmol h\({}^{-1}\) g\({}_{\text{cat}}\)\({}^{-1}\), which exceeds the activity of unmodified g-C\({}_{3}\)N\({}_{4}\) by 250 times. Among the photocatalysts containing platinum in the oxidized state, 0.5% PtO/g-C\({}_{3}\)N\({}_{4}\) showed the maximum activity in the H\({}_{2}\) evolution reaction, with rate of H\({}_{2}\) evolution of 0.36 mmol h\({}^{-1}\) g\({}_{\text{cat}}\)\({}^{-1}\) (Figure 5a,b). It should be noted that an increase in the content of the cocatalyst in both the case of metallic and oxidized platinum does not lead to an increase in the rate of H\({}_{2}\) formation. This effect may be caused by the weak adsorption of ethanol on the surface of g-C\({}_{3}\)N\({}_{4}\), which limits the rate of hydrogen evolution in the case of composite photocatalysts. The higher activity of the platinum cocatalyst compared to platinum oxide is caused by differences in the energy structure of these compounds. Both during the deposition of platinum and platinum oxide on the surface of g-C\({}_{3}\)N\({}_{4}\), heterojunctions arise at the interface between the two phases; a photogenerated electron migrates from the g-C\({}_{3}\)N\({}_{4}\) surface to the cocatalyst particle [56; 57]. However, charge transfer mechanisms are varied for different states of platinum [58]. Figure 4: (**a**,**c**) HAADF STEM images of 0.5% PtO\({}_{\text{x}}\)/g-C\({}_{3}\)N\({}_{4}\) and (**b**,**d**) HR TEM images of 0.5% Pt/g-C\({}_{3}\)N\({}_{4}\). While using metallic platinum as a occatalyst, an electron from the conduction band of g-C\({}_{3}\)N\({}_{4}\) falls down to the Fermi level of the metal, overcoming the Schottky barrier at the Pt--g-C\({}_{3}\)N\({}_{4}\) contact region [59; 60]. Meanwhile, the photogenerated holes remain in the valence band of g-C\({}_{3}\)N\({}_{4}\); thus, charge carriers--electrons and holes--are spatially separated. As it has one of the highest electron work functions (5.1-5.9 eV), platinum acts as a highly efficient trap for photogenerated electrons, which allows them to significantly increase their lifetime, and, consequently, greatly contributes to increasing the rate of hydrogen evolution [61]. Platinum oxide is a semiconductor with a narrow band gap and a conduction band (CB) position of +1.3 eV, which also allows for heterojunctions of electrons from the CB of g-C\({}_{3}\)N\({}_{4}\) to the CB of PtO [62] (Figure 6). However, in semiconductors with a narrow band gap, electrons and holes can readily recombine, leading to a decrease in photocatalytic activity [58]. Therefore, charge separation in PtO/g-C\({}_{3}\)N\({}_{4}\) photocatalysts is not as efficient as in Pt/g-C\({}_{3}\)N\({}_{4}\); thus, the lifetime of electrons and H\({}_{2}\) evolution rate are decreased. In addition to H\({}_{2}\), C1-C2 products were detected in the gas phase: carbon oxides, methane, ethene, and ethane. It is worth noting that the deposition of a occatalyst--both platinum and its oxide--significantly accelerates the rate of H\({}_{2}\) formation, but not its byproducts, hence allowing a great increase in the selectivity of the target process (Table 2). Figure 5: (a) H\({}_{2}\) evolution rate; (b) kinetic curves of H\({}_{2}\) evolution. Conditions: C(cat.) = 0.5 g L\({}^{-1}\); C\({}_{0}\)(EtOH) = 20%, V = 50 mL, (reaction) = 90 min, \(\lambda_{\text{max}}\) = 409 nm. Figure 6: Scheme of charge transfer mechanisms in PtO/g-C\({}_{3}\)N\({}_{4}\) and Pt/g-C\({}_{3}\)N\({}_{4}\) photocatalysts. Moreover, an increase in the load of the cocatalyst reduces the proportion of carbon oxides and increases the relative content of hydrocarbons (Figure 7a,b).