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 [ Time:2019/9/18 ]
A new insight into nitrogen reduction pathway of carbon nitride photocatalyst
Author :WANG Weikang

Researchers from the Institute of Solid State Physics, Hefei Institutes of Physical Science developed a model photocatalyst of carbon nitride to gain the mechanistic insight of photocatalytic nitrogen reduction reaction (NRR) involving cyano type active sites. Their findings were published in Angewandte Chemie-International Edition.

In recent years, enormous research efforts have been devoted to explore alternative ammonia (NH3) production processes with high energy efficiency and low CO2 emission. Among various explored alternatives, the visible light driven photocatalytic NRR to produce NH3 has attracted an increasing attention because it can be powered by sunlight and operated under ambient conditions, and does not generate CO2.

To date, different types of NRR photocatalysts, such as TiO2, ZnO, BiOBr, WO3 and graphitic carbon ntride (g-C3N4) have been widely reported. As a class of 2D metal-free NRR photocatalysts, g-C3N4 possesses advantageous features of scalable-synthesis at low cost, stable chemical properties and an appropriate band structure.

However, due to the photocatalytic NRR mechanism and whether the decomposition of nitrides in the NRR process contributes to the NH3 synthesis need to be clarified, the NRR reaction path of g-C3N4 with rich nitrogen atoms has become a hot issue.

Herein, the group synthesized a cyano group modified g-C3N4 nanoribbons (mCNN) photocatalyst capable of yielding NH3 at an impressive rate of 3.42 mmol g-1 h-1 under visible-light irradiation.

Importantly, the 15N isotopic labelling experiments confirms a cyano group (每C√N) nitrogen replacement NRR pathway, analog to Mars-van Krevelen (MvK) mechanism. The DFT calculations indicate that during a NRR cycle, the N in 每C√N is firstly and directly photocatalytically reduced to NH3 when attacked by the proton-coupled photoelectrons, while 每C√N is simultaneously converted into the unsaturated carbon sites when the yielded NH3 is detached. With the aid of K+, these unsaturated carbon sites is converted to C2N4 rings by reacting with the adsorbed N2, which is subsequently regenerated back to 每C√N by reacting with proton-coupled photoelectrons. The obtained results also confirm that the regeneration of 每C√N not only enhances activity and sustains the catalytic cycle, but also stabilizes the catalyst.

The reported findings in this work address key questions for applying g-C3N4-based photocatalysts for NRR that is highly beneficial for future development of high performance g-C3N4-based NRR photocatalysts by defects-engineering.

Figure 1 (a) FE-SEM and TEM (inset) images of mCNN; (b) UV-vis diffuse reflectance spectra of   g-C3N4 and mCNN; (C) 1H NMR spectra of photocatalytic reaction solvent (0.02 g/mCNN) with 15N2 as feed gas for different time; standard (14NH4)2SO4 and labeled (15NH4)2SO4; (d) Schematic illustration of NRR pathway of mCNN.ㄗImage by WANG Weikangㄘ




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