Annotation of the results obtained in 2023
As part of the project work, a method was developed to analyze the reaction mixture after photocatalytic experiments on hydrogen production from ultrapure water for the content of dissolved oxygen and hydrogen peroxide, which made it possible to accurately identify the products of water oxidation. Accordingly, the photocatalytic reaction was shown to proceed according to the equation 2H2O = H2O2 + H2. It was shown that for Pt/IrOx/g-C3N4 bimetallic photocatalysts, the H2O2 activity value is at the level of 65-80% of the H2 activity. The partial correspondence of the stoichiometry can be explained by the decomposition of hydrogen peroxide during sample preparation, in particular, by the separation of the photocatalyst from the suspension. In general, photocatalytic splitting of water with formation of hydrogen peroxide looks more promising than decomposition of water with formation of hydrogen and oxygen, since there is no need for separation of H2/O2 mixture, besides, H2O2 is a valuable chemical product. The formation of the active component of selected Pt/g-C3N4 and Pt/IrOx/g-C3N4 samples in the photocatalytic decomposition of water and hydrogen production from aqueous triethanolamine (TEOA) solutions was investigated in detail. It was shown that for Pt/g-C3N4 and Pt/IrOx/g-C3N4 photocatalysts the active component of the photocatalyst - metallic platinum and iridium oxide - is formed during synthesis, the charge state of metals does not change; the structure of the carrier - graphitic carbon nitride - also does not undergo changes during the reaction of hydrogen production or water decomposition. High stability of Pt/IrOx/g-C3N4 photocatalysts in the process of complete decomposition of water into hydrogen and hydrogen peroxide was demonstrated, whereas in the case of hydrogen production using TEOA electron donor in the presence of Pt/g-C3N4 photocatalysts in alkaline medium, the activity after 6 hours of reaction fell several times, and in TEOA suspensions without addition of alkali, the activity drop did not exceed 15-20%. It should be noted that preservation of surface properties of photocatalysts (surface concentration of metals, particle size) was observed only in the process of water decomposition without addition of electron donors. In the process of photocatalytic hydrogen production from aqueous solutions of TEOA, the drop in the activity can be explained by the increase in nanoparticles of the applied metal co-catalyst; in addition, it is possible that the decrease in activity is due to adsorption of products of partial oxidation of TEOA on the surface of the photocatalyst. It was shown that regeneration of Pt/g-C3N4 photocatalysts can be carried out by simple washing with water, which proves that the main cause of deactivation is "contamination" of the surface with TEOA oxidation products. Based on the results of the studies of the 3rd year and previous stages, recommendations on methods of obtaining active and stable bimetallic photocatalysts of water decomposition with ultralow content of noble metals were developed. Thus, a new strategy for the synthesis of Pt/IrOx/g-C3N4 bimetallic photocatalysts with low noble metal content for water decomposition into H2 and H2O2 was proposed. In such systems, platinum acts as a co-catalyst for the reduction of water to form hydrogen, and iridium oxide acts as a co-catalyst for oxidation to form hydrogen peroxide. A distinctive feature of the proposed synthesis method is the combination of two stages: the preparation of g-C3N4 from melamine cyanurate and the sequential precipitation of platinum and iridium oxide from labile Pt and Ir complexes - (Me4N)2[Pt2(OH)2(NO3)8] and ([Ir(H2O)3(NO2)3]). It was shown that by adjusting the precursor deposition order and the conditions of the subsequent thermal treatment, it is possible to directly regulate the state of both metals in the resulting materials. According to literature data, the obtained activities at 100 μmol hydrogen per gram of photocatalyst per hour are among the highest achieved for H2 production without the use of electron donors in the presence of g-C3N4-based photocatalysts. In addition, these photocatalysts are stable during the target process. The proposed synthetic approaches appear to be very promising, since they involve ultra-low content of noble metals (0.1 wt% Pt; 0.01 wt% Ir), and as a "carrier" is used affordable and non-toxic graphitic carbon nitride without any modifications (doping with non-metals, creation of heterostructures, etc.). In 2023, 4 articles were published, 3 in journals indexed in WoS/Scopus databases, 2 belonging to the Q1 quartile; 1 article was accepted for publication and will be published before the end of the year. We participated in 14 conferences, including three plenary, one keynote and two invited papers. As part of popularization of the obtained scientific results, a podcast was recorded for the program "Scientific Council" on the Internet channel SMOTRIM.

 

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Annotation of the results obtained in 2021
Different approaches to the synthesis of graphitic carbon nitride have been successfully tested during the project: a comparison was made between the standard heat treatment of melamine and synthesis methods involving the interaction of melamine with various reagents. It has been shown that the most promising method is the synthesis of g-C3N4 from melamine cyanurate obtained by binding melamine into a complex with cyanuric acid. Photocatalysts prepared by this method exhibit high activity in the reactions of photocatalytic production of hydrogen and oxygen and have a specific surface area of 60 m2/g compared to 20 m2/g for other methods. In addition, it has been shown that this synthesis method makes it possible to obtain materials with reproducible properties. The deposition of metal compounds (Pt, Pd, Rh, Ir) by chemisorption of nitrate (Pt, Pd) and nitroaqua complexes (Rh, Ir) with a mass fraction of metals 0.01-0.1% was studied. These complexes, due to the lability of nitrate and aqua ligands, bind quite easily to the surface of the carrier - g-C3N4. For platinum, standard deposition methods were used as a comparison – photo-reduction and the reduction of precursor H2PtCl6 by sodium borohydride. In total, more than 120 samples of photocatalysts were synthesized within the framework of the 1st stage of work. The activity of synthesized photocatalysts based on g-C3N4 with deposited compounds of iridium, platinum, palladium, and rhodium was studied in the reaction of hydrogen production from aqueous solutions of triethanolamine (TEOA) and oxygen from aqueous solutions of NaIO4 under irradiation of a LED source with a maximum emission at a wavelength of 425 nm. It has been shown that the best co-catalysts for hydrogen and oxygen evolution are platinum in the metallic state and ionic forms of iridium, respectively. The most active Pt/g-C3N4 photocatalysts are formed during the reduction of the precursor (Me4N)2[Pt2(OH)2(NO3)6] with hydrogen at 400 °C. The activity of photocatalysts synthesized by standard methods of soft chemical reduction and photodeposition of platinum over the same sample of g-C3N4 was significantly lower than in the reduction of metal from nitrate complexes. Literature analysis has shown that the photocatalysts proposed during the project show activity and quantum efficiency comparable to the highest values published in the literature. For the 0.1% Pt/g-C3N4 photocatalyst, the quantum efficiency was 5.1% at a wavelength of 425 nm, and the activity was 9 mmol H2 g-1 h-1; at the same time, when calculated per 1 gram of platinum, the activity was 8.8 mol H2 gPt-1 h-1. This value significantly exceeds the activities described in the literature, even with more complex synthetic approaches. Thus, the proposed original method for the synthesis of photocatalysts makes it possible to obtain highly active Pt/g-C3N4 photocatalysts with ultra-low metal content. Samples synthesized by chemisorption of the [Ir(H2O)3(NO2)3] complex were proposed for photocatalytic oxygen production on the surface of graphitic carbon nitride with subsequent heat treatment in air. High activity is achieved even with a mass fraction of 0.005% iridium. It is of principle that with high activity in obtaining oxygen from a solution of the electron acceptor, NaIO4, photocatalysts with precursor treatment in air exhibit very low activity in obtaining hydrogen, that is, they catalyze only the process of oxygen formation. It can be concluded that iridium in oxide form can be a promising co-catalyst for the complete decomposition of water into oxygen and hydrogen in bimetallic systems. In the course of the work, an article was published in Int. J. Hydrogen Energy, (Elsevier, IF = 5.816); authors took part with reports in 4 Russian and international conferences. A note about the work of the group was published in the newsletter of the D.I. Mendeleev Russian Academy of Sciences.

 

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Annotation of the results obtained in 2022
As a part of the project, various approaches were tested for the synthesis of Pt/IrOx/g-C3N4, Pd/IrOx/g-C3N4, Rh/IrOx/g-C3N4, and Ir/IrOx/g-C3N4 bimetallic photocatalysts, which made it possible to split water for hydrogen production without addition of organic substances – electron donors. To obtain graphitic carbon nitride, the method proposed at the first stage consists in thermal decomposition of melamine cyanurate, obtained, in turn, by the interaction of melamine and cyanuric acid in an aqueous solution. The main task was to ensure the processing of photocatalysts in such a way that platinum was reduced to a metallic state, and iridium was in an oxide state. Various approaches were applied, including different sequences of deposition of metals and heat treatment in different atmospheres (air, hydrogen), as well as in different temperature regimes. It was shown that Pt/IrOx/g-C3N4 bimetallic photocatalysts synthesized by successive deposition of platinum and iridium on g-C3N4 exhibited the highest activity in hydrogen evolution from ultrapure water. First, platinum was deposited by the sorption method from a salt solution (Me4N)2[Pt2(OH)2(NO3)8] with hydrogen treatment at 400°C, and then the resulting Pt/C3N4 material was impregnated with an aqueous solution of trinitrotriaquairidium ([Ir(H2O)3 (NO2)3]), dried in a stream of air, and calcined in an air atmosphere at a temperature of 400°C. It was determined that in these photocatalysts, platinum is in two states, metallic and ionic (2+), and iridium is completely represented by the state in the 3+ oxidation state. At the same time, photocatalysts in which iridium is in the metallic state do not show activity in hydrogen evolution. Also, no activity was observed in the production of hydrogen from ultrapure water in the presence of Pd/IrOx/g-C3N4, Rh/IrOx/g-C3N4, and Ir/IrOx/g-C3N4 photocatalysts. It can be concluded that the catalytic effect of platinum in the formation of hydrogen is manifested in two charge states - 0 and 2+, while iridium, in order to function as an active oxidizing center, must be in an oxidized form. Comparing photocatalysts 0.5%Pt/0.01-0.5%IrOx/g-C3N4 and 0.1%Pt/0.01-0.1%IrOx/g-C3N4, it can be noted that the maximum rate of hydrogen evolution is higher (approximately 2 times) for photocatalysts with mass share of platinum 0.5 wt.%. In addition, an interesting observation can be made that at a fixed content of platinum, 0.5 or 0.1 wt.%, the activity increases with a decrease in the proportion of iridium. That is, for complete water splitting, it is necessary that the mass fraction of platinum is at least 2 times higher than the fraction of iridium, while a sufficiently high activity is also observed at a ratio of Pt:Ir = 5-10. It was shown that a significant increase in the rate can be achieved by adding an acid or an alkali to the reaction system: in both cases, an increase in the rate by 6-8 times is achieved. For 0.1% Pt/0.01% IrOx/g-C3N4 and 0.1% Pt/0.05% IrOx/g-C3N4 photocatalysts, the activities in hydrogen production from ultrapure water were 280 µmol H2 per gram of photocatalyst per hour (5 M NaOH) and 200 µmol H2 per gram of photocatalyst per hour (2.5 M H2SO4), respectively. In this case, without the addition of alkali or acid for the 0.5% Pt/0.01% Ir/g-C3N4 photocatalyst, the hydrogen evolution rate was 110 µmol H2 g-1 h-1. An analysis of the literature data showed that the obtained rate values are among the highest values recorded for the process of hydrogen production without the use of electron donors in the presence of photocatalysts based on g-C3N4. Given the low contents of noble metals, the presented results look very promising. In the course of the work, two articles were published in journals belonging to the 1st quartile of WoS and Scopus: Chemical Engineering Journal (IF = 16.744) and ACS Applied Materials and Interfaces (IF = 10.383). According to these articles, press releases were prepared and issued, which were distributed by the Russian media. Participation with reports at ten all-Russian and international conferences was accepted, including two plenary and one keynote lectures.

 

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