Addressing the Characterisation Challenge to Understand Catalysis in MOFs: The Case of Nanoscale Cu Supported in NU-1000
We explore the dynamic structure and reactivity of Cu-species supported on NU-1000. By combining pair distribution function (PDF) analysis and difference envelope density (DED) analysis of in-situ synchrotron-based X-ray scattering data, we simultaneously probe the local structure of supported Cu-species, their distribution within NU-1000 and distortions of the NU-1000 lattice under conditions relevant to catalysis and catalyst activation. These analyses show that atomic layer deposition (ALD) of Cu in NU-1000 (Cu-AIM) leads to the formation of Cu-oxo clusters within the small pores that connect the triangular and hexagonal channels. Exposure of Cu-AIM to reducing atmosphere at 200 °C produces metallic Cu0 of two distinct particle sizes: ~4 nm nanoparticles and small sub-nanometer clusters. The size of these nanoparticles appear to be constrained by NU-1000 pore dimensions, with evidence of the sub-nanometer clusters being bound within the triangular channels flanked by pyrene rings. This supported Cu0-NU-1000 system is catalytically active for gas-phase ethylene hydrogenation. Exposure of the catalyst to oxidative atmosphere re-oxidises the Cu species to a Cu2O cuprite phase. The dynamic restructuring of the system in different chemical environments underscores the importance of probing these systems in-situ.