Atomically Dispersed Copper Sites in a Metal-Organic Framework for Reduction of Nitrogen Dioxide.

Yujie Ma,Yichao Zou,Weiyao Li,Christopher Hardacre,Xue Han,Shaojun Xu,Sarayute Chansai,Sarah J Haigh,Marek Nikiel,Zi Wang,Eric J L Mcinnes,Sihai Yang,Svemir Rudić,Pascal Manuel,Yongqiang Cheng,Alena M Sheveleva,Daniel Lee,Martin Schröder,Floriana Tuna,Anibal J Ramirez‐Cuesta ,Iván Da Silva

doi:10.1021/jacs.1c03036

Abstract

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 atomically dispersed metal sites and the elucidation of their role in catalysis are challenging due to limited methods of characterization 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 microscopy, electron paramagnetic resonance spectroscopy, and inelastic neutron scattering, and their location is identified by neutron powder diffraction and solid-state nuclear magnetic resonance spectroscopy. The Cu/UiO-66 catalyst exhibits superior catalytic performance for the reduction of NO2 at 25 °C without the use of reductants. A selectivity of 88% for the formation of N2 at a 97% conversion of NO2 with a lifetime of >50 h and an unprecedented turnover frequency of 6.1 h–1 is achieved under nonthermal 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.

Highlights

Emerging single-atom catalysts show superior selectivity and activity in a variety of catalytic systems due to their unique electronic properties, low-coordination metal sites, and high atom efficiency.[1−4] Metal−organic framework (MOF) materials are a class of crystalline porous materials that are ideal platforms for the fabrication of single-atom catalysts due to their uniform and well-defined structure, ultrahigh porosity, and designable functionality and pore sizes.[5−8] Functional groups, such as hydroxyl bridges, and intrinsic defect sites in MOFs can enable the immobilization of atomically dispersed metal sites on the pore interior
The development and application of new efficient technologies to enable the precise identification of single metal sites and their roles in catalysis are important targets
We have undertaken a comprehensive study of the local structure of the single-atom Cu sites in Cu/UiO-66, enabling elucidation of their critical role in the reduction of NO2

Summary

■ INTRODUCTION

Emerging single-atom catalysts show superior selectivity and activity in a variety of catalytic systems due to their unique electronic properties, low-coordination (or unsaturated) metal sites, and high atom efficiency.[1−4] Metal−organic framework (MOF) materials are a class of crystalline porous materials that are ideal platforms for the fabrication of single-atom catalysts due to their uniform and well-defined structure, ultrahigh porosity, and designable functionality and pore sizes.[5−8] Functional groups, such as hydroxyl bridges, and intrinsic defect sites in MOFs can enable the immobilization of atomically dispersed metal sites on the pore interior. The high-shifted 1H signals (at δ {1H} ∼ 9.6 ppm) can be ascribed to nitric/nitrous acid species.[43,44] The 2D 1H homonuclear dipolar correlation spectrum of Cu/UiO-66 with NO2 (Figure S24) displays correlations between these acidic protons and protons of the ligand but not with water or defect protons This indicates that an intermediate in the reduction of NO2 by Cu/ UiO-66 could be nitric/nitrous acid and that it adsorbs, most likely through hydrogen-bonding, to the ligand. This is corroborated with data from 13C MAS NMR (Figure S25), which shows a shift in the carboxylate 13C signal upon adsorption of NO2.45

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES