Photocatalytic vs. photosynthetic reduction of nitrogen into ammonia: The importance of the sacrificial reducing agent

A review on the photochemical processes capable of producing ammonia from dinitrogen. A clear distinction is made between those processes that are energetically downhill (photocatalytic transformations) and those that are energetically uphill (photosynthetic transformations). The latter can be considered more promising as they are generally based on the use of less energetic sacrificial electron donors that are widely available (e.g. water). They are currently most commonly found among systems incorporating heterogeneous / semiconductor materials, albeit with a low or modest energetic efficiency (10 −4 –10 −2 ). In contrast, homogeneous systems reported to date rely on energetic sacrificial electron donors such as amines or alcohols. The global production of ammonia represents a formidable effort to supplement natural nitrogen fixation pathways in order to meet the ever-growing needs of the human population. Growing research effort is aimed at reducing the high energetic and environmental cost of the Haber-Bosch process and the latest advances are highlighted. • Review focusing on photochemical reduction of nitrogen to produce ammonia. • Differentiation between approaches that are energetically uphill (photosynthesis) or downhill (photocatalysis). • Includes section on practical aspects of ammonia photoproduction and quantification.