Highly Active Visible Light-Promoted Ir/g-C₃N₄ Photocatalysts for the Water Oxidation Reaction Prepared from a Halogen-Free Iridium Precursor

Abstract

A combination of the exceptional stability of fac-[Ir­(H2O)3(NO2)3] together with thermolability of nitro and aqua ligands and high solubility in various solvents makes it promising as a brand-new chlorine-free precursor of iridium for the preparation of heterogeneous catalysts. In the current work, a new technique of fac-[Ir­(H2O)3(NO2)3] preparation based on hydrothermal treatment of (NH4)3[Ir­(NO2)6] was developed. For this purpose, the influence of reaction parameters such as the reaction time, temperature, and pH of the solution on the process of hexanitroiridate salt hydrolysis was investigated. The synthesized fac-[Ir­(H2O)3(NO2)3] solution in this optimized way was used for the preparation of the series of Ir/g-C3N4 catalysts, which were evaluated in the water oxidation reaction with NaIO4 utilized as a sacrificial reagent. A 20-fold enhancement of the oxygen evolution reaction (OER) activity was found to take place under visible light (λ = 411 nm) illumination of the systems. The highest rate of the photoinduced OER per iridium center was achieved by the Ir0.005/g-C3N4 (air, 400°C) catalyst with an exceptional turnover frequency value of 967 min–1 approaching the activity of known homogeneous iridium OER catalysts. The leaching experiments have shown that aquated Ir species are generated in a solution after prolonged functioning of the catalysts. Despite this, in the closed system the photodriven OER activity persists at a steady-state level evidencing an equilibrium achieved between dissolved and anchored Ir species forming catalytic tandem with the g-C3N4.

Selectivity Control of CO2 Reduction over Pt/g-C3N4 Photocatalysts under Visible Light

“…Absorption edges slightly decrease from 2.82 to 2.75 eV with the increase in calcination temperature (Figure 1b), which corresponds to the band gap of g-C 3 N 4 [28,32]. X-ray diffraction pattern (Figure S1) demonstrates a dominant peak at 2θ≈28 • , that is attributed to the (002) plane of g-C 3 N 4 [33].…”

Bimetallic Pt-IrOx/g-C3N4 Photocatalysts for the Highly Efficient Overall Water Splitting under Visible Light

“…In order to increase the surface area and obtain materials with higher catalytic activity, various methods are proposed. For example, in our group, a new synthetic technique, based on thermolysis of a melamine cyanurate complex, has been proposed for the formation of g-C 3 N 4 with a high surface area and photocatalytic activity [30][31][32].…”

“…For the first process, 0.01-0.5% Pt/g-C 3 N 4 photocatalysts were prepared by thermal treatment of melamine-cyanuric acid supramolecular complex and further sorption of platinum nitrato complexes as a precursor of Pt cocatalyst [30]. For the second process, 0.005-0.5% IrO x /g-C 3 N 4 photocatalysts were prepared via deposition of Ir aqua/nitro complex onto the surface of g-C 3 N 4 with further thermal treatment in air [32]. It was shown that at high activity in the production of oxygen from an electron acceptor solution, NaIO 4 , the IrO x /g-C 3 N 4 photocatalysts show very low activity in the production of hydrogen, i.e., they catalyze only the process of oxygen formation [32].…”

Photoinduced Hydration Boosts O2 Evolution on Co-Chelating Covalent Organic Framework

“…18,19 It is challenging to explore the semiconductor as supporter that can provide synergistic effects to mimic the synergy of multinuclear CaMn 4 O 5 for oxygen evolution in natural photosynthesis. 20 In recent years, organic semiconductors such as C 3 N 4 , 21,22 conjugated microporous polymer, 23−27 and covalent organic frameworks 28−30 have been explored to support atomic metal sites for photocatalytic water oxidation, and they have demonstrated promising activities in O 2 evolution. However, the key synergy from the supporter for water activation and transformation on atomic metal sites is still elusive.…”

“…Photocatalysts with atomic metal sites exhibit unique structure and maximum atom efficiency and have been attracted continuous interest. − In the photocatalytic water oxidation, the activity of metal sites strongly relies on the chemical microstructures composed of metal ions and the surrounding coordination environment. , The isolated metal site should be less active for dissociation of water and/or O–O bond formation, which is generally rate-limiting. , It is challenging to explore the semiconductor as supporter that can provide synergistic effects to mimic the synergy of multinuclear CaMn 4 O 5 for oxygen evolution in natural photosynthesis . In recent years, organic semiconductors such as C 3 N 4 , , conjugated microporous polymer, − and covalent organic frameworks − have been explored to support atomic metal sites for photocatalytic water oxidation, and they have demonstrated promising activities in O 2 evolution. However, the key synergy from the supporter for water activation and transformation on atomic metal sites is still elusive.…”