Work

A Sm(II)-based catalyst for the reduction of dinitrogen, nitrite, and nitrate to ammonia or urea

Year: 2025

Type: article

Abstract: Industrial dinitrogen (N 2 ) reduction to ammonia in the Haber-Bosch synthesis is essential for producing fertilizers and, consequently, food. Methods wherein the energy for nitrogen activation is supplied by light could provide more sustainable alternatives to existing ones. The combination of a photosensitizer and a lanthanide catalyst is reported for an effective >2e − reduction of N 2 in what is the first transition-metal-free molecular photocatalyst for ammonia synthesis. The lanthanide is Earth-abundant Sm. The reaction proceeds at ambient pressure and temperature, with high turnover numbers (up to 98), with visible light irradiation in aqueous solvent mixtures and even pure water, and it uses an environmentally benign non-metallic sacrificial reductant. Nitrite and nitrate were also efficiently reduced to ammonia. Thus, the first photocatalytic co-reduction of nitrite and bicarbonate to urea using an Sm-based photocatalyst was achieved. • A homogeneous Sm-based photocatalyst ( SmL ) reduces N 2 and NO x − to NH 3 • The reaction is driven by visible light and proceeds in the presence of water • NH 3 was obtained with excellent efficiency and selectivity • SmL is the first homogeneous photocatalyst to co-reduce HCO 3 − and NO 2 − to urea Industrial nitrogen fixation is the basis of fertilizer production and of a possible future ammonia-based energy system. Homogeneous catalysts that efficiently and selectively reduce nitrogen to ammonia under mild conditions have been pursued for decades. Nitrogen fixation driven by energy from visible light is an attractive goal. To date, photocatalysts for N 2 reduction to ammonia have been based on transition metals. Despite great strides, even the best existing photocatalytic ammonia syntheses have serious drawbacks, such as the reliance on toxic and expensive stoichiometric chemical reductants and poor selectivity, with hydrogen gas as a prominent by-product. We show that the homogeneous photocatalyst SmL , based on Earth-abundant samarium encapsulated in a ligand carrying a simple light-harvesting chromophore, reduces N 2 to NH 3 upon irradiation with visible light. The reaction proceeds in both aqueous-organic solvent mixtures and pure water, obviating the need for anhydrous organic solvents or for high-molecular-weight proton sources. The terminal reductant is a benign organic amine. Nitrates and nitrites (NO x − ), which are attractive nitrogen sources, were reduced to NH 3 with excellent efficiency and selectivity, and the yield for NH 3 is the highest reported for any homogeneous photocatalytic NO x − -reducing system. The broad utility of the system is demonstrated by its ability to catalyze the co-reduction of bicarbonate and nitrite to urea; SmL is the first homogeneous photocatalyst that yields urea. These results open a new avenue for localized, solar energy-driven, sustainable ammonia production. A homogeneous Sm-based photocatalyst equipped with a light-harvesting coumarin chromophore reduces N 2 to NH 3 upon irradiation with visible light. The reaction is tolerant of water, and the terminal reductant is an organic amine. NO x − were efficiently reduced to NH 3 . NH 3 was obtained with excellent selectivity. The first co-reduction of bicarbonate and nitrite to urea with a homogeneous photocatalyst was demonstrated.

Source: Chem

DOI: 10.1016/j.chempr.2025.102547