Defect-Engineered Metal-Organic Frameworks: A Thorough Characterization of Active Sites Using CO as a Probe Molecule

Defect engineering (DE) has been recognized as a powerful approach to tune the structural, optical, and chemical properties of metal-organic framework (MOF) materials. Here, a detailed characterization using ultrahigh-vacuum Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy was performed for defect-engineered NOTT-100 (Cu-2(BPTC), BPTC = biphenyl-3,3',5,5'-tetracarboxylates). Defects were introduced either via thermal defect engineering (TDE) or the defective linker approach (synthetic defect engineering, SDE). A quantitative analysis of the spectroscopic results revealed the formation of reduced, undercoordinated Cu+/Cu-2+ dimer defects via both TDE and SDE approaches in a controlled manner. Exposure of the MOFs to CO led to various (CO)(a)Cu+ and (CO)(a)Cu2+ (a = 1,2) species. The binding energies of these species as determined by temperature-dependent experiments showed strong variations. The type and doping concentration of defective linkers as well as the annealing temperatures played a crucial role in tuning the structural and electronic properties of DE-NOTT-100 MOFs. The Bronsted acid sites exposed by protonated carboxylic acids were unambiguously identified by both the characteristic vibrational frequency of adsorbed CO and the corresponding red shift of the acidic OH group.

Automated summary

Defect engineering is a powerful method for tuning the properties of MOF materials, with experiments on NOTT-100 showing controlled formation of Cu+/Cu2+ dimer defects through thermal defect engineering and synthetic defect engineering. Exposure to CO results in the formation of various Cu+ and Cu2+ species, with binding energies showing strong variations. The structural and electronic properties of DE-NOTT-100 MOFs are influenced by the type and doping concentration of defective linkers, as well as annealing temperatures.