Atomically-dispersed copper sites in a metal-organic framework for reduction of nitrogen dioxide

Metal-organic framework (MOF) materials provide an excellent platform to fabricate single-atom catalysts due to their structural diversity, intrinsic porosity and designable functionality. However, the unambiguous identification of atomi-cally-dispersed metal sites and elucidation of their role in catalysis are challenging due to limited methods of characteri-zation and lack of direct structural information. Here, we report a comprehensive investigation of the structure and the role of atomically-dispersed copper sites in UiO-66 for the catalytic reduction of NO2 at ambient temperature. The atomic dispersion of copper sites on UiO-66 is confirmed by high-angle annular dark-field scanning transmission electron mi-croscopy, electron paramagnetic resonance spectroscopy and inelastic neutron scattering, and their location identified by neutron powder diffraction and solid-state nuclear magnetic resonance spectroscopy. The Cu/UiO-66 catalyst exhibits superior catalytic performance for reduction of NO2 at 25 °C without the use of reductants. A selectivity of 88% for the formation of N2 at 97% conversion of NO2 with a lifetime of >50 h and an unprecedented turnover frequency of 6.1 h-1 is achieved under non-thermal plasma activation. In situ and operando infrared, solid-state NMR and EPR spectroscopy reveal the critical role of copper sites in the adsorption and activation of NO2 molecules, with the formation of {Cu(I)⋯NO} and {Cu⋯NO2} adducts promoting the conversion of NO2 to N2. This study will inspire the further design and study of new efficient single-atom catalysts for NO2 abatement via detailed unravelling of their role in catalysis.