Atomistic understanding of enhanced selectivity in photocatalytic oxidation of benzyl alcohol to benzaldehyde using graphitic carbon nitride loaded with single copper atoms

Abstract

The loading of graphitic carbon nitride (gCN) with transition metals has received significant attention for efficient light-driven catalysis. However, the contribution of the loaded metals to enhanced performance remains unclear. In this study, Cu is loaded onto gCN to understand how photocatalytic activity is regulated by the loaded metals. Loading gCN with 3 wt% of Cu increases the electron population by 8.1 and 4.6 times under UV (λ < 370 nm) and visible light (390 < λ < 740 nm), respectively. This sample shows nearly 100% selectivity for oxidizing benzyl alcohol to benzaldehyde and a high yield-to-power ratio, reaching 0.35 mmol g⁻¹ h⁻¹ W⁻¹. The loaded Cu species exist as single atoms with a +1-oxidation state. Each Cu⁺ cation is coordinated to two (at 3 wt% Cu) or four (at 6 wt% Cu) N atoms within the cavity of the gCN framework. Doubling the Cu loading results in a smaller electron population and coordinatively more saturated Cu⁺ cations, making it catalytically less reactive. Ab initio molecular dynamics simulations show that Cu⁺ cations produce filled mid-gap states above the valence band, which function as hole traps and hence oxidation centers. The Cu⁺ cation and the neighboring N atoms are electron-depletion and electron-accumulation sites due to Cu → N electron transfer, making it highly reactive for oxidative transformations via the hole transfer pathway. The role of Cu as a hole-transfer site updates the received understanding that surface-loaded Cu serves as an electron-accumulation site. A strong correlation is observed between the electron population at steady-state and the product yield, indicating that it could serve as a promising performance indicator for the design of future photocatalysts.