Air pollution by nitrogen oxides, NOx, is a major problem, and new capture and abatement technologies are urgently required. Here we report a metal-organic framework (MFM-520: MFM = Manchester Framework Material) that can efficiently confine dimers of NO2, resulting in a high adsorption capacity of 4.2mmol/g (298K, 0.01bar) with full reversibility and no loss of capacity over 125 cycles. Treatment of NO2@MFM-520 with water in air leads to quantitative
conversion of the captured NO2 into HNO3, an important feedstock for fertilizer production, and fully regenerates MFM-520. The confinement of N2O4 inside nano-pores has been established at a molecular level, and dynamic breakthrough experiments using both dry and humid NO2 gas streams verify the excellent stability and selectivity of MFM-520, and confirm its potential for precious-metal-free deNOx technologies.