Luca Braglia

XAS on Rh and Ir metal sites in post synthetically functionalized UiO-67 Zirconium MOFs

We synthesized UiO-67 metal-organic-frameworks (MOFs) functionalized with different transition metals (Rh, Ir). Using EXAFS we verified that the synthesis has been successful. Furthermore, we observed the change of local environment while varying of metal site. XAS spectroscopy is the most informative technique to characterize these kind of materials and to study the local environment around the metal site.

A XAFS study of the local environment and reactivity of Pt- sites in functionalized UiO-67 MOFs

We synthesized UiO-67 Metal Organic Frameworks (MOFs) functionalized with bpydcPt(II)Cl2 and bpydcPt(IV)Cl4 complexes (bpydc = bipyridine-dicarboxylate), as attractive candidates for the heterogenization of homogeneous catalytic reactions. Pt L3-edge XAFS experiments allowed us to thoroughly characterize these materials, in the local environment of the Pt centers. XAFS studies evidenced the rich reactivity of UiO-67-Pt(II) MOFs, including reduction to bpydcPt(0) under H2 flow in the 600-700 K range, room-temperature oxidation to bpydcPt(IV)Br4 through oxidative addition of liquid Br2 and ligand exchange between 2 Cl- and even bulky ligands such as toluene-3,4-dithiol. Preliminary XANES simulations with ADF code provide additional information on the oxidation state of Pt sites.

Probing Structure and Reactivity of Metal Centers in Metal–Organic Frameworks by XAS Techniques

In this chapter, we introduce MOFs as the new class of crystalline porous materials of remarkable potentialities. We underline the flexibility in the realization of different MOFs and the fact that they are ideal materials for performing X-ray absorption experiments at the metal K or L3 edges in transmission mode. A selection of relevant results appearing in the 2010-2016 period follows, highlighting the relevant role of both EXAFS and XANES in determining the structural and electronic configuration of metal centers inside MOFs.

Insights into Cr/SiO2 catalysts during dehydrogenation of propane: an operando XAS investigation

In situ and operando XAS spectroscopic methods were applied to monitor the variations in the oxidation state and in the local structure of the chromium sites in a 2.0Cr/SiO2-DHS catalyst during propane dehydrogenation under non-oxidative and different oxidative conditions. The spectroscopic data have been compared to the catalytic performances. Although the majority of the specialized literature ascribes the propane dehydrogenation activity mainly to CrIII species and only a few works report the presence of small amounts of CrII species, our data unequivocally demonstrate the co-presence of both CrIII and CrII species during the propane dehydrogenation reaction. The relative amount of the two chromium phases has been estimated by analyzing both the XANES and EXAFS spectra with a two-phase fitting approach. It was found that the CrIII-phase prevails when oxygen is present in the reaction mixture and when the reaction is performed at low flow or under static conditions. The amount of the CrIII-phase correlates well with the propane conversion. In contrast, the selectivity to propene correlates with the amount of the CrII-phase, at least when propane dehydrogenation is performed under oxidative conditions. Although other phenomena surely contribute to the overall catalytic performances of Cr/SiO2 catalysts, our results indicate that the relative proportion of the CrII and CrIII phases plays a role in influencing the selectivity of the propane oxidative dehydrogenation reaction.