A new cobalt metal-organic framework (2D-Co-MOF) based on well-defined layered double cores that are strongly connected by intermolecular bonds has been developed. Its 3D structure is held together by π-π stacking interactions between the labile pyridine ligands of the nanosheets. In aqueous solution, the axial pyridine ligands are exchanged by water molecules, producing a delamination of the material, where the individual double nanosheets preserve their structure. The original 3D layered structure can be restored by a solvothermal process with pyridine, so that the material shows a "memory effect" during the delaminationpillarization process. Electrochemical activation of a 2D-Co-MOF@Nafion-modified graphite electrode in aqueous solution improves the ionic migration and electron transfer across the film and promotes the formation of the electrocatalytically active cobalt species for the oxygen evolution reaction (OER). The so-activated 2D-Co-MOF@Nafion composite exhibits an outstanding electrocatalytic performance for the OER at neutral pH, with a TOF value (0.034 s-1 at an overpotential of 400 mV) and robustness superior to those reported for similar electrocatalysts under similar conditions. The particular topology of the delaminated nanosheets, with quite distant cobalt centers, precludes the direct coupling between the electrocatalytically active centers of the same sheet. On the other hand, the increase in ionic migration across the film during the electrochemical activation stage rules out the intersheet coupling between active cobalt centers, as this scenario would impair electrolyte permeation. Altogether, the most plausible mechanism for the O-O bond formation is the water nucleophilic attack to single Co(IV)-oxo or Co(III)-oxyl centers. Its high electrochemical efficiency suggests that the presence of nitrogen-containing aromatic equatorial ligands facilitates the water nucleophilic attack, as in the case of the highly efficient cobalt porphyrins.

It has been developed the synthesis of a new microporous Metal-Organic Framework (MOF) based on two Secondary Building Units (SBU), with dinuclear cobalt centers. The employing of a well-2 defined cobalt cluster results in an unusual topology of the Co 2 -MOF, where one of the cobalt centers has three open coordination positions, which has not precedent in MOF materials based on cobalt. Adsorption isotherms have revealed that Co 2 -MOF is in the range of best CO2 adsorbents among the carbon materials, with a very high CO2/CH4 selectivity. On the other hand, dispersion of Co 2 -MOF in an alcoholic solution of Nafion gives rise to a composite (Co 2 -MOF@Nafion) with a great resistance to hydrolysis in aqueous media and good adherence to graphite electrodes.In fact, it exhibits a high electrocatalytic activity and robustness for the oxygen evolution reaction (OER), with a TOF value superior to those reported for similar electrocatalysts. Overall, this work has provided the basis for the rational design of new cobalt OER catalysts and related materials employing well defined metal clusters as directing agents of MOF structure.

The structure of UiO-66(Ce) is formed by CeO 2– x defective nanoclusters connected by terephthalate ligands. The initial presence of accessible Ce 3+ sites in the as-synthesized UiO-66(Ce) has been determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR)-CO analyses. Moreover, linear scan voltammetric measurements reveal a reversible Ce 4+ /Ce 3+ interconversion within the UiO-66(Ce) material, while nanocrystalline ceria shows an irreversible voltammetric response. This suggests that terephthalic acid ligands facilitate charge transfer between subnanometric metallic nodes, explaining the higher oxidase-like activity of UiO-66(Ce) compared to nanoceria for the mild oxidation of organic dyes under aerobic conditions. Based on these results, we propose the use of Ce-based metal–organic frameworks (MOFs) as efficient catalysts for the halogenation of activated arenes, as 1,3,5-trimethoxybenzene (TMB), using oxygen as a green oxidant. Kinetic studies demonstrate that UiO-66(Ce) is at least three times more active than nanoceria under the same reaction conditions. In addition, the UiO-66(Ce) catalyst shows an excellent stability and can be reused after proper washing treatments. Finally, a general mechanism for the oxidative halogenation reaction is proposed when using Ce-MOF as a catalyst, which mimics the mechanistic pathway described for metalloenzymes. The superb control in the generation of subnanometric CeO 2– x defective clusters connected by adequate organic ligands in MOFs offers exciting opportunities in the design of Ce-based redox catalysts.

No abstract

The effect of the operational potential on the stability of electrochemical biosensors is particularly relevant in the case of peroxidase biosensors, because these enzymes can catalyze the reduction of hydrogen peroxide via either a high-potential redox cycle [involving Compound I, Compound II, and Fe(III)] or a low-potential redox cycle [involving Fe(III) and Fe(II)]. Herein, it is shown that recombinant tobacco peroxidase immobilized on a graphite surface displays two well-separated electrocatalytic waves, associated with each of these two catalytic cycles. While continuous scanning in the highpotential region does not alter significantly the electrocatalytic current, it is shown that just modest incursions into the low-potential region cause an irreversible loss of the electrocatalytic response. A quantitative analysis of the extent of inactivation as a function of time, potential, and hydrogen peroxide concentration is shown to be consistent with a fast inactivation caused by hydroxyl radicals generated by a Fenton-like mechanism. Accordingly, the inactivation process is shown to slow via the addition of radical scavengers to the solution. Preliminary results indicate that the same inactivation process may also be present in horseradish peroxidase-modified electrodes.