Catching the Reversible Formation and Reactivity of Surface Defective Sites in Metal-Organic Frameworks: An Operando Ambient Pressure-NEXAFS Investigation.

In this work, we apply for the first time ambient pressure operando soft X-ray absorption spectroscopy (XAS) to investigate the location, structural properties, and reactivity of the defective sites present in the prototypical metal-organic framework HKUST-1. We obtained direct evidence that Cu defective sites form upon temperature treatment of the powdered form of HKUST-1 at 160 °C and that they are largely distributed on the material surface. Further, a thorough structural characterization of the Cu/Cu dimeric complexes arising from the temperature-induced dehydration/decarboxylation of the pristine Cu/Cu paddlewheel units is reported. In addition to characterizing the surface defects, we demonstrate that CO may be reversibly adsorbed and desorbed from the surface defective Cu/Cu sites. These findings show that ambient pressure soft-XAS, combined with state-of-the-art theoretical calculations, allowed us to shed light on the mechanism involving the decarboxylation of the paddlewheel units on the surface to yield Cu/Cu complexes and their reversible restoration upon exposure to gaseous CO