Selectively constructing nitrogen vacancy in carbon nitrides for efficient syngas production with visible light

Abstract

Photocatalytic reduction of CO2 and H2O is believed to be a green and sustainable approach for syngas production. However, this reaction typically suffers from moderate efficiency and uncontrollable H2/CO ratio. Herein, we report a general acetonitrile treatment approach to create nitrogen vacancies (NVs) on the surface of polymeric carbon nitride (PCN) photocatalyst. The type and distribution of NVs are analyzed by X-ray photoelectron spectroscopy, positron annihilation spectroscopy, solid-state nuclear magnetic resonance and element analysis. Results confirm that the NVs are mainly stemmed from the selectively breaking of surface N-(C)3 sites of PCN. In-situ diffuse reflectance infrared Fourier transform spectroscopy and DFT calculations determine that the NVs can improve the activation and reduction of CO2, while considerably lowering the formation barrier of COOH* intermediates. Besides, the NVs modification can accelerate the separation and transfer kinetics of photogenerated charge carriers of PCN. As a result, the NVs-PCN exhibits excellent photocatalytic activity. The syngas production rate of the NVs-PCN is almost 10 times higher than that of the pristine PCN under visible light. Importantly, the syngas H2/CO ratio can be tuned between 0.24:1 and 6.8:1 by merely adjusting the concentrations of NVs. Furthermore, we found that this NVs engineering can also promote the visible-light harvesting of PCN, hence realizing efficient syngas production under visible light, even under low-energy red light irradiation (610 nm). Also, this defect implantation strategy can be further extended to synthesize other NVs-functionalized PCN for syngas production with high efficiency.