Photocatalytic hydrogen peroxide production by anthraquinone-augmented polymeric carbon nitride

We describe the exploitation of the selective catalytic property of anthraquinone (AQ) for solar photocatalytic synthesis of hydrogen peroxide (H₂O₂) as a green, sustainable alternative to organic-solvent-based and energy-intensive industry-benchmark processes that also rely on AQ catalysis. We accomplished this by anchoring AQ onto polymeric carbon nitride (C₃N₄), a metal-free visible light photocatalyst (band gap energy = 2.7 eV), that has been previously demonstrated for selective H₂O₂ synthesis. A net H₂O₂ production rate of 361 μmol g⁻¹ h⁻¹ and an apparent quantum yield (AQY) of 19.5% at 380 nm excitation were achieved using AQ-augmented C₃N₄ under simulated 1-sun illumination in the presence of an organic electron donor (2-propanol); these results were 4.4-fold and 8.3-fold higher than those reported for bare C₃N₄, respectively. A suite of experimental analyses confirmed the unique roles of AQ co-catalysis in (i) capturing electrons from the conduction band of C₃N₄, thereby reducing futile exciton recombination, which is otherwise prevalent in bare C₃N₄; (ii) effectively mediating electron transfer to drive hydrogenation reaction to form anthrahydroquinone (AQH₂) from AQ; and (iii) catalyzing oxygen reduction to H₂O₂ through the dehydrogenation of AQH₂ back to AQ, resulting in the facile and selective formation of H₂O₂. In addition, the reduced decomposition of produced H₂O₂ by the C₃N₄/AQ composite photocatalysts, when compared to bare C₃N₄ or C₃N₄ composited with common metallic co-catalysts such as Pt and Ag, was found to contribute to the significant enhancement in H₂O₂ production through the oxidation of both organic and water.