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

Abstract

In this work, we apply for the first time ambient pressure operando soft X-ray Absorption Spectroscopy to investigate the location, structural properties and reactivity of the defective sites present in the prototypical metal organic framework HKUST-1. We obtain 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 characterization of the surface defects, we demonstrate that CO2 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 into the mechanism involving the decarboxylation of the paddlewheel units at the surface to yield Cu + /Cu 2+ complexes and their reversible restoration upon exposure to gaseous CO2.

CO2 Adsorption in a Robust Iron(III) Pyrazolate‐Based MOF: Molecular‐Level Details and Frameworks Dynamics From Powder X‐ray Diffraction Adsorption Isotherms

“…Different techniques have been proposed to this aim, ranging from nuclear magnetic resonance (NMR) [3,[6][7][8] to inverse gas chromatography (IGC-FC), [9,10] structural and spectroscopic methods and theoretical simulations. [11][12][13][14][15][16][17][18][19] In the recent years, in situ and operando synchrotron radiation high-resolution powder X-ray diffraction (HR-PXRD) experiments have been considered a powerful tool to unveil the main interactions and primary adsorption sites [16,[20][21][22] in a wide range of inorganic, [17,23,24] organic, [25,26] and metal-organic porous materials. [15,16,27,28] Despite these multiple examples, the information obtained so far were limited to the localization of the guest molecules and the modification of the host framework.…”

Two Faces of the Same Coin: Coupling X‐Ray Absorption and NMR Spectroscopies to Investigate the Exchange Reaction Between Prototypical Cu Coordination Complexes

“…Theoretical Cu K‐edge XANES spectra were then calculated starting from the DFT optimized clusters of complexes 1 2 ⋅ Cu 2+ and 2 2 ⋅ Cu + , using the FDMNES [45–48] code in the framework of the finite difference method (FDM).…”

“…The latter CuÀ N distance is in excellent agreement with the previously extended X-ray absorption fine structure (EXAFS) derived one of 2.035(0.002) Å in complex [Cu-(dap) 2 (BF) 4 ] + . [34] Theoretical Cu K-edge XANES spectra were then calculated starting from the DFT optimized clusters of complexes 1 2 • Cu 2 + and 2 2 • Cu + , using the FDMNES [45][46][47][48] code in the framework of the finite difference method (FDM).…”

Caught while Dissolving: Revealing the Interfacial Solvation of the Mg2+ Ions on the MgO Surface

“…On the contrary, the application of XAS in the soft X-ray regime (soft-XAS) to study the interfacial properties of metal ions has been severely hampered by the need of high vacuum conditions. Very recently, specific cells have been designed that allow soft-XAS to be carried out at atmospheric pressure under operando conditions, − a technique referred to as ambient pressure near-edge X-ray fine structure spectroscopy (AP-NEXAFS). In this case soft-XAS is operated in total electron yield (TEY) detection mode which renders the technique surface sensitive due to the low electron escape depth which limits the thickness of the probed sample.…”