Visible-light-responsive hybrid photocatalysts for quantitative conversion of CO2 to highly concentrated formate solutions

Abstract

Photocatalysts can use visible light to convert CO₂ into useful products. However, to date photocatalysts for CO₂ conversion are limited by insufficient long-term stability and low CO₂ conversion rates. Here we report hybrid photocatalysts consisting of conjugated polymers and a ruthenium(II)–ruthenium(II) supramolecular photocatalyst which overcome these challenges. The use of conjugated polymers allows for easy fine-tuning of structural and optoelectronic properties through the choice of monomers, and after loading with silver nanoparticles and the ruthenium-based binuclear metal complex, the resulting hybrid systems displayed remarkably enhanced activity for visible light-driven CO₂ conversion to formate. In particular, the hybrid photocatalyst system based on poly(dibenzo[b,d]thiophene sulfone) drove the very active, durable and selective photocatalytic CO₂ conversion to formate under visible light irradiation. The turnover number was found to be very high (TON = 349 000) with a similarly high turnover frequency (TOF) of 6.5 s⁻¹, exceeding the CO₂ fixation activity of ribulose-1,5-bisphosphate carboxylase/oxygenase in natural photosynthesis (TOF = 3.3 s⁻¹), and an apparent quantum yield of 11.2% at 440 nm. Remarkably, quantitative conversion of CO₂ (737 μmol, 16.5 mL) to formate was achieved using only 8 mg of the hybrid photocatalyst containing 80 nmol of the supramolecular photocatalyst at standard temperature and pressure. The system sustained photocatalytic activity even after further replenishment of CO₂, yielding a very high concentration of formate in the reaction solution up to 0.40 M without significant photocatalyst degradation within the timeframe studied. A range of experiments together with density functional theory calculations allowed us to understand the activity in more detail.