Photocatalytic nitrogen reduction to ammonia: Insights into the role of defect engineering in photocatalysts

Abstract

Engineering of defects in semiconductors provides an effective protocol for improving photocatalytic N2 conversion efficiency. This review focuses on the state-of-the-art progress in defect engineering of photocatalysts for the N2 reduction toward ammonia. The basic principles and mechanisms of thermal catalyzed and photon-induced N2 reduction are first concisely recapped, including relevant properties of the N2 molecule, reaction pathways, and NH3 quantification methods. Subsequently, defect classification, synthesis strategies, and identification techniques are compendiously summarized. Advances of in situ characterization techniques for monitoring defect state during the N2 reduction process are also described. Especially, various surface defect strategies and their critical roles in improving the N2 photoreduction performance are highlighted, including surface vacancies (i.e., anionic vacancies and cationic vacancies), heteroatom doping (i.e., metal element doping and nonmetal element doping), and atomically defined surface sites. Finally, future opportunities and challenges as well as perspectives on further development of defect-engineered photocatalysts for the nitrogen reduction to ammonia are presented. It is expected that this review can provide a profound guidance for more specialized design of defect-engineered catalysts with high activity and stability for nitrogen photochemical fixation.

Robust Porous WC‐Based Self‐Supported Ceramic Electrodes for High Current Density Hydrogen Evolution Reaction

“…[31] The existence of a large number of anion vacancies was also confirmed by electron paramagnetic resonance (EPR) spectroscopy, where the WC had a very weak signal, while a strong resonance line appeared at g of 2.0026 in the WC-N/W-1200 electrode (Figure S8, Supporting Information). [32] These defects can effectively alter the periodic structure of the crystal and affect the chemical properties and the electronic structure of the catalyst, improving the intrinsic catalytic activity and exposing more active sites, thereby the HER performance is improved. [33] These findings confirm that NH 3 gradually destroyed the surface crystal lattice of WC grains, and N was doped into the WC crystal lattice, which is in good agreement with the XPS and Raman spectra.…”

Photocatalytic Ammonia Synthesis: Mechanistic Insights into N2 Activation at Oxygen Vacancies under Visible Light Excitation

“…Photocatalytic ammonia (NH 3 ) synthesis using air and water under mild conditions is an appealing alternative to the traditional Haber–Bosch scheme as an intensive energy consumption and massive carbon dioxide emission process. − The ideal scenario often requires effectively initial N 2 activation to overcome the first proton-coupled electron transfer (PCET) process, that is, N 2 + e – → N 2 – , or N 2 + e – + H + → N 2 H, which is generally regarded as the rate-determining step. − Many oxide photocatalysts, such as bismuth-based oxide, including BiOBr, , BiO, BiOCl, Bi 5 O 7 Br, layered-double-hydroxide (LDH), , TiO 2 , − and WO 3 , expose the feasibility of N 2 fixation. The catalytic activity is often attributed to surface oxygen vacancy (OV), which can effectively activate the strong NN triple bond and facilitate the photogenerated electrons transfer from the photocatalyst to N 2 .…”

Challenges and Opportunities for Renewable Ammonia Production via Plasmon‐Assisted Photocatalysis

“…[40] However, they may not be associated exclusively with beneficial effects as they may also act as recombination centers or undesired trap states, especially in the case of bulk defects, thus reducing the photocatalytic activity. [41] As such, improving our understanding of the structure and properties of defects would help the rational design of highperformance semiconductor photocatalysts. Oxygen vacancies are responsible for N 2 photofixation in In(OH) 3 /C 3 N 4 catalysts, [42] bismuth oxyhalide-based materials, [43] TiO 2 -containing catalysts, [40] In 2 O 3 /In 2 S 3 , [44] W 18 O 49 -based materials, [45] or Fe-doped SrMoO 4 .…”