Hydrogen peroxide (H2 O2 ) has received increasing attention because it is not only a mild and environmentally friendly oxidant for organic synthesis and environmental remediation but also a promising new liquid fuel. The production of H2 O2 by photocatalysis is a sustainable process, since it uses water and oxygen as the source materials and solar light as the energy. Encouraging processes have been developed in the last decade for the photocatalytic production of H2 O2 . In this Review we summarize research progress in the development of processes for the photocatalytic production of H2 O2 . After a brief introduction emphasizing the superiorities of the photocatalytic generation of H2 O2 , the basic principles of establishing an efficient photocatalytic system for generating H2 O2 are discussed, highlighting the advanced photocatalysts used. This Review is concluded by a brief summary and outlook for future advances in this emerging research field.

Production of Hydrogen Peroxide by Photocatalytic Processes

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

Recent advances in g-C3N4-based heterojunction photocatalysts

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

Summary Polymeric carbon nitride (g-C3N4)-based Z-scheme photocatalysts have attracted widespread attention owing to their notable light-harvesting capability, spatially separated reductive and oxidative active sites, and strong redox ability. This review article presents a panorama of the latest developments of g-C3N4-based Z-scheme photocatalysts for various applications. The development status and basic working principle of g-C3N4-based Z-scheme photocatalysts is introduced. After that, the characterization technologies for g-C3N4-based Z-scheme photocatalysts are discussed. Later, a series of representative g-C3N4-based Z-scheme photocatalysts for various photocatalytic applications, including water splitting, CO2 reduction, N2 fixation, and H2O2 production are exemplified. Finally, this review offers some perspectives on the major challenges and opportunities for future research in this emerging field. There is little doubt that advanced g-C3N4-based Z-scheme nanostructured materials could act as highly efficient photocatalysts to achieve new breakthroughs for diverse photocatalysis applications.

Emerging polymeric carbon nitride Z-scheme systems for photocatalysis

Over the past few decades, near infrared light (NIR), as an important part of sunlight, has seldom been utilized in photocatalytic reactions. In this work, a full-spectrum-response Cu2(OH)2CO3/g-C3N4 photocatalyst with outstanding photocatalytic H2O2 production performance was synthesized. XRD, UV-Vis, N2 adsorption, XPS, PL, EIS and EPR are used to characterize the as-prepared catalysts. As a light absorber from UV to NIR, Cu2(OH)2CO3 can form more photogenerated electrons to recombine the holes in g-C3N4 through the "Z-scheme" mechanism. The as-prepared Cu2(OH)2CO3/g-C3N4 photocatalyst shows the H2O2 equilibrium concentration of 8.9 mmol L-1, over 16 and 26.9 times higher than that of neat g-C3N4 and Cu2(OH)2CO3. According to the Z-scheme mechanism, a "two channel route" to form H2O2 is proposed for the Cu2(OH)2CO3/g-C3N4 heterojunction catalyst.

Fabrication of a full-spectrum-response Cu2(OH)2CO3/g-C3N4 heterojunction catalyst with outstanding photocatalytic H2O2 production performance via a self-sacrificial method.

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.

Nanni Xiong

Papers

Fabrication of a full-spectrum-response Cu2(OH)2CO3/g-C3N4 heterojunction catalyst with outstanding photocatalytic H2O2 production performance via a self-sacrificial method.

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