Addressing the characterisation challenge to understand catalysis in MOFs: the case of nanoscale Cu supported in NU-1000

Abstract

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 a 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 appears 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.