The recently discovered UiO-66/67/68 class of isostructural metallorganic frameworks (MOFs) [J. H. Cavka et al. J. Am. Chem. Soc., 2008, 130, 13850] has attracted great interest because of its remarkable stability at high temperatures, high pressures and in the presence of different solvents, acids and bases [L. Valenzano et al. Chem. Mater., 2011, 23, 1700]. UiO-66 is obtained by connecting Zr₆O₄(OH)₄ inorganic cornerstones with 1,4-benzene-dicarboxylate (BDC) as linker resulting in a cubic MOF, which has already been successfully reproduced in several laboratories. Here we report the first complete structural, vibrational and electronic characterization of the isostructural UiO-67 material, obtained using the longer 4,4′-biphenyl-dicarboxylate (BPDC) linker, by combining laboratory XRPD, Zr K-edge EXAFS, TGA, FTIR, and UV-Vis studies. Comparison between experimental and periodic calculations performed at the B3LYP level of theory allows a full understanding of the structural, vibrational and electronic properties of the material. Both materials have been tested for molecular hydrogen storage at high pressures and at liquid nitrogen temperature. In this regard, the use of a longer ligand has a double benefit: (i) it reduces the density of the material and (ii) it increases the Langmuir surface area from 1281 to 2483 m² g⁻¹ and the micropore volume from 0.43 to 0.85 cm³ g⁻¹. As a consequence, the H₂ uptake at 38 bar and 77 K increases from 2.4 mass% for UiO-66 up to 4.6 mass% for the new UiO-67 material. This value is among the highest values reported so far but is lower than those reported for MIL-101, IRMOF-20 and MOF-177 under similar pressure and temperature conditions (6.1, 6.2 and 7.0 mass%, respectively) [A. G. Wong-Foy et al. J. Am. Chem. Soc., 2006, 128, 3494; M. Dinca and J. R. Long. Angew. Chem., Int. Ed., 2008, 47, 6766]. Nevertheless the remarkable chemical and thermal stability of UiO-67 and the absence of Cr in its structure would make this material competitive.

The design of active and durable catalysts for the H₂O/O₂ interconversion is one of the major challenges of electrocatalysis for renewable energy. The oxygen evolution reaction (OER) is catalyzed by SrRuO₃ with low potentials (ca. 1.35 V_RHE), but the catalyst’s durability is insufficient. Here we show that Na doping enhances both activity and durability in acid media. DFT reveals that whereas SrRuO₃ binds reaction intermediates too strongly, Na doping of ~0.125 leads to nearly optimal OER activity. Na doping increases the oxidation state of Ru, thereby displacing positively O p-band and Ru d-band centers, weakening Ru-adsorbate bonds. The enhanced durability of Na-doped perovskites is concomitant with the stabilization of Ru centers with slightly higher oxidation states, higher dissolution potentials, lower surface energy and less distorted RuO₆ octahedra. These results illustrate how high OER activity and durability can be simultaneously engineered by chemical doping of perovskites.

The carbon–carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (−0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials.

High-resolution synchrotron radiation x-ray powder diffraction (HR-XRPD) combined with Hf L3-edge extended x-ray absorption fine structure allowed us to determine the structure of a Hf-UiO-66 metal-organic framework (MOF) showing that it is isoreticular to Zr-UiO-66 MOF [Cavka et al., J. Am. Chem. Soc. 130, 13850 (2008).]. Thermal gravimetric measurements (coupled with mass spectroscopy) and temperature-dependent synchrotron radiation XRPD proved the high thermal stability of the Hf-UiO-66 MOF. The Langmuir surface area (849~m2/g) combined with the high stability of the UiO-66 framework and with the high neutron absorption cross section of Hf suggest that among all microporous crystalline materials the Hf-UiO-66 MOF possesses the physical and chemical requirements for the interim storage of radioactive waste in a much safer way than is currently available. The first results proving the synthesis of a MOF material with UiO-66 topology realized by a B-containing linker are also reported, allowing a further improvement of the neutron shielding power of this class of materials.

Proton exchange membrane water electrolysis is a promising technology to produce green hydrogen from renewables, as it can efficiently achieve high current densities. Lowering iridium amount in oxygen evolution reaction electrocatalysts is critical for achieving cost-effective production of green hydrogen. In this work, we develop catalysts from Ir double perovskites. Sr₂CaIrO₆ achieves 10 mA cm⁻² at only 1.48 V. The surface of the perovskite reconstructs when immersed in an acidic electrolyte and during the first catalytic cycles, resulting in a stable surface conformed by short-range order edge-sharing IrO₆ octahedra arranged in an open structure responsible for the high performance. A proton exchange membrane water electrolysis cell is developed with Sr₂CaIrO₆ as anode and low Ir loading (0.4 mg_Ir cm⁻²). The cell achieves 2.40 V at 6 A cm⁻² (overload) and no loss in performance at a constant 2 A cm⁻² (nominal load). Thus, reducing Ir use without compromising efficiency and lifetime.