Insights into Adsorption of NH₃ on HKUST‑1 Metal–Organic Framework: A Multitechnique Approach

We report a careful characterization of the interaction of NH₃ with the Cu­(II) sites of the [Cu₂C₄O₈] paddle-wheel cornerstone of the HKUST-1 metallorganic framework, also known as Cu₃(BTC)₂. The general picture emerging from combining XRPD, EXAFS, XANES, mid- and far-IR, DRUV–vis, and EPR techniques is that the presence of traces of water has relevant consequences on the effect of ammonia on the MOF framework. NH₃ adsorption on the dry system results in a strong chemisorption on Cu­(II) sites that distorts the framework, keeping the crystallinity of the material. Perturbation observed upon NH₃ adsorption is analogous to that observed for H₂O, but noticeably enhanced. When the adsorption of ammonia occurs in humid conditions, a time-dependent, much deeper modification of the system is observed by all of the considered techniques. On a methodological ground, it is worth noticing that we used the optimization of XANES spectra to validate the bond distance obtained by EXAFS