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

[Image: see text] 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(2+) dimeric complexes arising from the temperature-induced dehydration/decarboxylation of the pristine Cu(2+)/Cu(2+) paddlewheel units is reported. In addition to characterizing the surface defects, we demonstrate that CO(2) may be reversibly adsorbed and desorbed from the surface defective Cu(+)/Cu(2+) 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(2+) complexes and their reversible restoration upon exposure to gaseous CO(2.)