Photo/electrochemical urea synthesis via CO2 coupling with nitrogenous small molecules: Status and challenges for the development of mechanism and catalysts

Urea (NH2CONH2) is an essential fertilizer for humanity. Nevertheless, industrial urea production and its feedstock NH3 production are energy-extensive and involve largely carbon footprints. Currently, urea production via electro/photochemical routes have shown a significant appeal and potential because only water, environmentally CO2 and nitrogenous small molecules, and solar energy/electricity are involved during the whole process. In the past few years, considerable efforts have been devoted to revealing the mechanism of the C−N coupling processes and the development of advanced electro/photocatalysts for the selective synthesis of urea. In this review, we compare and contrast the two distinct yet inherently closely linked catalytic C−N processes, before we detail recent advanced in the catalytic active sites construction of different urea catalyst systems from the viewpoint of electrochemical and photochemical approaches. We close with a balanced outlook on the current challenges and promising opportunities.