Photocatalytic Cascade Nitrogen Fixation for Selective Purification of Methane-Rich Coal-Bed Gas Over a Bimetallic...

CH₄-rich coal-bed methane (CBM) is often contaminated with N₂, but their similar properties make conventional separation energy-intensive and inefficient. Herein, we report a photocatalytic strategy for selective N₂ conversion over CH₄, achieving simultaneous CBM purification and nitrogen valorization. A Co-Ni metal-organic framework (CoNi-PYZ) features isolated bimetallic sites, where ligand/Co units donate electrons to Ni, establishing spatially separated reduction and oxidation centers. This structural motif drives a photocatalytic cascade nitrogen fixation that N₂ is first reduced to NH₃ at the Ni sites; subsequently, the generated NH₃ outcompetes CH₄ for the Co sites, leading to its oxidation to NO₃ ^-. This remarkable selectivity is governed by the high polarity and lone-pair electron donation of NH₃, favoring its adsorption over nonpolar CH₄. Notably, the optimal catalyst achieves high activity under mild conditions without sacrificial agents, delivering NH₄ ⁺ and NO₃ ^- production rates of 599.8 and 199.8 µmol g^{- 1} h^{- 1}, respectively. Mechanism analysis reveals that ligand engineering optimizes the d-band center to balance adsorption and desorption, thereby minimizing the rate-determining step barriers for both half-reactions, consistent with the Sabatier principle. This work provides a reaction-driven alternative to traditional phase-separation, establishing a novel paradigm for CBM upgrading coupled with selective N₂ transformation.