Improvement in hydrogen production activity under visible-light irradiation using Tp-Pa-COF photocatalyst

To achieve a sustainable society, it is essential to have a clean energy system that does not rely on depletable resources. Hydrogen production through water splitting using photocatalysts driven by sunlight is attracting attention as a promising solution. Covalent organic frameworks (COFs) are ideal for this application, as they have a high density of surface-active sites and are water-dispersible, thermally durable, and metal-free. However, current synthesis methods have industrial disadvantages due to the complex and harsh synthesis conditions, such as temperature and atmosphere. In this study, we attempted to synthesize a highly active photocatalyst under simple and mild conditions in order to introduce chlorine substituents into the imine-bonded COF, Tp-Pa-COF.

1,3,5-Trifluoromethyl fluoroglucinol (Tp) and 1,4-phenylenediamine (Pa) or 2,5-dichloro-1,4-phenylenediamine (Pa-Cl₂) were reacted to synthesize Tp-Pa-X-COF (X = H₂ or Cl₂). After 30 minutes of ultrasonic treatment, the mixture was stirred at different temperatures (room temperature to 120 °C) and times (6 to 24 hours) under air atmosphere. Photocatalytic hydrogen evolution was performed using hexachloroplatinic acid as a co-catalyst, sodium ascorbate as a sacrificial agent, and an LED lamp (λ=450 nm, 17.5 mW/cm²) as a light source. UV-vis DRS was used to measure visible-light absorption capacity, and gas chromatography (TCD) was used to quantify hydrogen.

As a result, it was found that even at room temperature and atmospheric pressure, chlorine substitution improved photocatalytic hydrogen production activity, improved visible-light absorption, and suppressed the recombination of electrons and holes. It was revealed that factors such as reaction time, temperature, and solvent affect the crystallization and photocatalytic activity of Tp-Pa-COF.