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.

The incorporation of Ti ions within the framework of aluminophosphate zeotype AlPO-5 and their chemical reactivity is studied by means of CW-EPR, HYSCORE, and UV-vis spectroscopies. Upon reduction, Ti(3+) ions are formed, which exhibit large (31)P hyperfine couplings, providing direct evidence for framework substitution of reducible Ti ions at Al sites.

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Abstract.We have systematically used Electron Paramagnetic Resonance (EPR) to understand the nature of excess electron centers in Titanium dioxide and to classify their spectroscopic features. Excess electrons in TiO 2 (probably the most important photoactive oxide) have been generated either by photoinduced charge separation or by reductive treatments and are stabilized in the solid by titanium ions which reduce to paramagnetic Ti 3+ . These are monitored by EPR and classified on the basis of their g tensor values in order to amend a certain confusion present in the literature about this subject. In the previous paper of this series (S. Livraghi et al. J. Phys. Chem. C 2011, 115, 25413) excess electron centers in anatase were investigated while the present one is devoted to rutile and brookite, the two other TiO 2 polymorphs, in the aim of providing a thorough and consistent guideline to researchers working in the wide area of titanium dioxide applications.2