Carbon-based metal-free catalysts are promising green catalysts for photocatalysis and electrocatalysis due to their low cost and environmental friendliness. A key challenge in utilizing these catalysts is identifying their active sites, given their poor crystallinity and complex structures. Here we demonstrate the key structure of the double-bonded conjugated carbon group as a metal-free active site, enabling efficient O 2 photoreduction to H 2 O 2 through a cascaded water oxidation − O 2 reduction process. Using ethylenediaminetetraacetic acid as a precursor, we synthesized various carbon-based photocatalysts and analyzed their structural evolution. Under the polymerization conditions of 260 °C to 400 °C, an N -ethyl- 2 -piperazinone-like structure was formed on the surface of the catalyst, resulting in high photocatalytic H 2 O 2 photoproduction (2884.7 μmol g −1 h −1 ) under visible light. A series of control experiments and theoretical calculations further confirm that the double-bond conjugated carbonyl structure is the key and universal feature of the active site of metal-free photocatalysts.
Understanding photocatalytic processes of nanoclusters at the nanoscale remains a challenge. Here, we synthesized two kinds of gold nanoclusters stabilized by thiol ligands (AuNCs) and carbon dots (AuCDs) to study their interfacial kinetics in photocatalytic H 2 O 2 production. Three operando testing methods, including transient photovoltage (TPV), transient potential scanning (TPS) and photo-induced current (TPC), are used to investigate the influence of carbon dots (CDs) on catalytic processes. While AuNCs and AuCDs show similar O 2 adsorption abilities, AuCDs exhibit a lower reaction rate due to less surface effective photoelectrons. The AuNCs utilize their own ligands to consume photo-generated holes while the presence of CDs in AuCDs changes this way and providing stable catalytic sites for water oxidation reaction. Additionally, with the addition of CDs, the oxygen reduction reaction pathway changes from a two-step one electron process to a one-step two electron process. This work provides new insights for understanding the photocatalytic processes.
Solar-driven photocatalysis is a promising approach for renewable energy application. H2O2 photocatalysis by metal-free graphitic carbon nitride has been gaining attention. Compared with traditional thermal catalysis, metal-free graphitic carbon nitride photocatalysis could lower material cost and achieve greener production of H2O2. Also, to better guide photocatalyst design, a fundamental understanding of the reaction mechanism is needed. Here, we develop a series of model cost-effective metal-free H2O2 photocatalysts made from graphitic carbon nitride (melem) and common imide groups. With 4,4′-oxydiphthalic anhydride (ODPA)-modified g-C3N4, a H2O2 yield rate of 10781 μmol/h·g·L could be achieved. Transient absorption and ex situ Fourier transform infrared (FTIR) measurements revealed an ultrafast charge transfer from the melem core to water with ∼3 ps to form unique N–OH intermediates. The electron withdrawing ability of the anhydride group plays a role in governing the rate of electron transfer, ensuring efficient charge separation. Our strategy represents a new way to achieve a low material cost, simple synthesizing strategy, good environment impact, and high H2O2 production for renewable energy application.
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