Enhanced ambient ammonia photosynthesis using nanosheets with light-switchable oxygen vacancies
Researchers have presented a strategy for simultaneously introducing light-switchable oxygen vacancy and doping Mo into Bi5O7Br nanosheets for efficient photocatalytic N2 fixation. The modified photocatalyst has achieved elevated N2 fixation photoactivities by virtue of the optimized conduction band position, enhanced light availability, improved N2 adsorption and charge carrier separation.
The issue to achieve efficient nitrogen (N2) reduction to ammonia (NH3) has posed a significant challenge for decades as the inert N≡N bond could be hardly broken because of the extremely large bond energy of 940.95 kJ mol–1. To date, the industrial fixation of N2 to NH3 is monopolized by the energy-intensive Haber-Bosch process (673-873 K and 15-25 MPa), which unsustainably employs natural gas to make the hydrogen (H2) feedstock with enormous energy consumption from fossil fuels, leading to a large amount of carbon dioxide (CO2) emission. In this context, photocatalytic N2 reduction is regarded as a sustainable alternative way for NH3 synthesis from N2 and water under ambient conditions.
However, the efficiency of most traditional photocatalysts is still far from satisfactory mainly due to the hard bond dissociation of the inert N2, which results from the weak binding of N2 to the catalytic material and further inefficient electron transfer from photocatalyst into the antibonding orbitals of N2. In order to promote efficiency of N2 photofixation, introducing the electron-donating centers as the catalytic activation sites for optimizing the N2 adsorption properties and improving the photoexcited charge transport in the catalysts is a promising strategy.
Oxygen vacancy (OV) represents the most widely and prevalent studied type of surface defect for N2 fixation. On one hand, OV can be facilely created for its relatively low formation energy; on the other hand, OV can assist photocatalysts to gain exciting N2 fixation photoactivity by virtue of its superiority in N2 capture and activation. Therefore, a semiconductor with sufficient OVs may be favorable to improve their N2 fixation performance. Transition metal (TM) doping is another widely investigated effective method to improve the photoactivity of N2 fixation, because the TM species possess the advantageous ability of binding (and even functionalizing) with inert N2 at low temperatures due to their empty and occupied d-orbitals, which can achieve the TM-N2 interaction via "acceptance-donation" of electrons. Mo, as a critical element of the catalytic center in mysterious Mo-dependent nitrogenase, has attracted a lot of attention for the N2 fixation. To this end, OVs-rich and Mo-doped materials would be ideal candidates for N2 photofixation. In addition, layered bismuth oxybromide (BiOBr) materials have attracted numerous attentions because of their suitable band gaps and unique layer structures. For BiOBr-based semiconductors, such as Bi3O4Br and Bi5O7Br, it has been revealed that OV with sufficient localized electrons on their surface facilitates the capture and activation of inert N2 molecules.
Recently, a research team led by Prof. Yi-Jun Xu from Fuzhou University, China reported that the introduction of OVs and Mo dopant into Bi5O7Br nanosheets can remarkably improve the photoactivity of N2 fixation. The modified photocatalysts have showed the optimized conduction band position, the enhanced light absorption, the improved N2 adsorption and charge carrier separation, which jointly contribute to the elevating N2 fixation photoactivities. This work provides a promising approach to design photocatalysts with light-switchable OVs for N2 reduction to NH3 under mild conditions, highlighting the wide application scope of nanostructured BiOBr-based photocatalysts as effective N2 fixation systems. The results were published in Chinese Journal of Catalysis.