Characterization of Undercoordinated Zr Defect Sites in UiO-66 with Vibrational Spectroscopy of Adsorbed CO

The highly studied metal–organic framework (MOF), UiO-66, has been touted for its high surface area, stability, and tunability of structural defects for particular applications in catalysis and gas storage. However, atomic-level characterization and quantification of defects remain challenging for traditional methods. Motivated by this knowledge gap, coordinatively unsaturated zirconium (Zrcus) defect sites within UiO-66 have been determined through infrared spectroscopic studies of the highly sensitive probe molecule, carbon monoxide. A characteristic blue shift of the vibrational frequency for CO adsorbed at nodes within the MOF was used to identify the presence of Zrcus defects. The identification of these coordinatively unsaturated metal sites was further corroborated via exposure of the MOF to the Lewis base, D2O, which was shown to block CO binding to the Lewis acidic Zrcus sites. The infrared spectroscopic signature for CO binding at the Zrcus sites was further employed to track the emergence of missing linker defects that develop upon thermal treatment of the MOF. The concentration of missing linker defects could be characterized over a range from less than 1% (nominally a defect-free MOF) to over 30% with this relatively simple spectroscopic probe.