As a metal-free nitrogen reduction reaction (NRR) photocatalyst, g-C3 N4 is available from a scalable synthesis at low cost. Importantly, it can be readily functionalized to enhance photocatalytic activities. However, the use of g-C3 N4 -based photocatalysts for the NRR has been questioned because of the elusive mechanism and the involvement of N defects. This work reports the synthesis of a g-C3 N4 photocatalyst modified with cyano groups and intercalated K+ (mCNN), possessing extended visible-light harvesting capacity and superior photocatalytic NRR activity (NH3 yield: 3.42 mmol g-1  h-1 ). Experimental and theoretical studies suggest that the -C≡N in mCNN can be regenerated through a pathway analogous to Mars van Krevelen process with the aid of the intercalated K+ . The results confirm that the regeneration of the cyano group not only enhances photocatalytic activity and sustains the catalytic cycle, but also stabilizes the photocatalyst.

Potassium Ion Assisted Regeneration of Active Cyano Groups in Carbon Nitride Nanoribbons: Visible Light Driven Photocatalytic Nitrogen Reduction

Photoreduction of N₂ is among the most interesting and challenging methods for N₂ fixation under mild conditions. In this study, we determined that nitrogen vacancies (NVs) could endow graphitic carbon nitride (g-C₃N₄) with the photocatalytic N₂ fixation ability. Photocatalytic N₂ fixation induced by NVs is free from the interference of other gases. The reason is that NVs could selectively adsorb and activate N₂ because NVs have the same shape and size as the nitrogen atom in N₂. In addition to this, NVs could also improve many other properties of g-C₃N₄ to support photocatalytic N₂ fixation. These new findings could elucidate the design of efficient photocatalysts for photocatalytic N₂ fixation.

Selective photocatalytic N₂ fixation dependent on g-C₃N₄ induced by nitrogen vacancies

Here, we reported that sulfur vacancy-rich O-doped 1T-MoS2 nanosheets (denoted as SV-1T-MoS2) can surpass the activity of Pt as cocatalysts to assist in the photocatalytic nitrogen fixation of CdS nanorods. SV-1T-MoS2 cocatalysts exhibit sulfur vacancies, O-doping, more metallic 1T phase, and high electronic conductivity, thus leading to the exposure of more active edge sites, high Brunauer-Emmett-Teller surface area, enhanced visible light absorption, and improved electron separation and transfer, which are beneficial for photocatalytic nitrogen fixation. Consequently, the optimized 30 wt % SV-1T-MoS2-/CdS composites exhibit an outstanding nitrogen fixation rate of 8220.83 μmol L-1 h-1 g-1 and long-term stability under simulated solar light irradiation, significantly higher than pure CdS nanorods, CdS-Pt (0.1 wt %), and 30 wt % 1T-MoS2/CdS composites. The catalytic mechanism of photocatalytic nitrogen fixation on SV-1T-MoS2 is discussed by density functional theory calculations.

Sulfur Vacancy-Rich O-Doped 1T-MoS2 Nanosheets for Exceptional Photocatalytic Nitrogen Fixation over CdS.

Two-dimensional MXene-based and MXene-derived photocatalysts: Recent developments and perspectives

As an indispensable chemical, ammonia is produced by the Haber–Bosch process using H2 and N2 under severe reaction conditions. Recently, photocatalysis using solar energy has been regarded as a promising technology to fix N2 under mild conditions. However, compared to other photocatalytic applications, photocatalytic N2 fixation suffers from low efficiency because of the difficulties associated with N2 adsorption and activation on the photocatalyst surface, as well as the instability of active sites on the photocatalyst. Herein, we covalently grafted an ethylenediaminetetraacetic acid (EDTA) motif onto graphitic carbon nitride nanosheets (CNNS) by a high-temperature dehydration reaction, and subsequently chelated Fe3+ ions to obtain highly dispersive and stable Fe3+ sites on CNNS. Compared with the un-modified sample, the obtained Fe-EDTA–CNNS has improved visible light absorption and high charge separation and transfer ability. Impressively, a much higher activity for N2 fixation (50 μmol L−1 h−1) than those of the comparative materials and photocatalysts in the literature as well was achieved by the Fe-EDTA–CNNS. Experimental and calculation results demonstrate that the modification of CNNS with an Fe-EDTA unit enhances the reduction ability of photo-generated electrons and decreases the N2 adsorption energy. More importantly, the Fe-EDTA unit with an electron-withdrawing effect can accumulate photo-excited electrons and then activate the adsorbed N2 molecules. Additionally, the strong chelating action results in a relatively high stability for the Fe-EDTA–CNNS photocatalyst. This work not only develops a potential photocatalyst for efficient ammonia production, but also provides a facile surface modification strategy to construct active sites and regulate the charge transfer route on photocatalysts.

Highly dispersive and stable Fe3+ active sites on 2D graphitic carbon nitride nanosheets for efficient visible-light photocatalytic nitrogen fixation

Molten salt assistant synthesis of three-dimensional cobalt doped graphitic carbon nitride for photocatalytic N2 fixation: Experiment and DFT simulation analysis

In this work, three-dimensional macroporous lithium doped graphitic carbon nitride was synthesized. XRD, N2 adsorption, SEM, XPS, UV-Vis spectroscopy, N2-TPD and photoluminescence were used to characterize the prepared catalysts. The result shows that lithium exists as a coordinative Li–N bond, which can chemisorb and activate N2 molecules, and promote the electron transfer from the catalyst to the nitrogen molecules. The as-prepared lithium doped g-C3N4 shows a NH4+ production rate of 4.8 mg L−1 h−1 gcat−1, which is 20 times that of bulk g-C3N4. Density functional theory (DFT) simulations show that lithium doping can not only increase the NN bond length, which activates N2 molecules, but also promote the electron–hole separation efficiency of g-C3N4.

Template free synthesis of lithium doped three-dimensional macroporous graphitic carbon nitride for photocatalytic N2 fixation: the effect of Li-N active sites.

The effect of hydroxyl group grafting on the photocatalytic phenolic compounds oxidation ability of g-C3N4 prepared by a novel H2O2-alkali hydrothermal method

Keyun Wang

Papers

Molten salt assistant synthesis of three-dimensional cobalt doped graphitic carbon nitride for photocatalytic N2 fixation: Experiment and DFT simulation analysis

Template free synthesis of lithium doped three-dimensional macroporous graphitic carbon nitride for photocatalytic N2 fixation: the effect of Li-N active sites.

The effect of hydroxyl group grafting on the photocatalytic phenolic compounds oxidation ability of g-C3N4 prepared by a novel H2O2-alkali hydrothermal method