The Role of Carbonate in Catalytic Oxidations

Redox Properties of CeIVDOTA in Carbonated Aqueous Solutions. A Radiolytic and an Electrochemical Study

The redox chemistry of Ce^IIIDOTA in cage in carbonate solutions was studied using electrochemistry and radiolysis techniques (continuous radiolysis and pulse radiolysis). Spectroscopic measurements point out that the species present in the solutions at high bicarbonate concentrations are [Ce^IIIDOTA(CO₃)]^3- (or less plausible [Ce^IIIDOTA(HCO₃)]^2-) with the carbonate (bicarbonate) anion as the ninth ligand versus [Ce^IIIDOTA(H₂O)]^- present in the absence of bicarbonate. Electrochemical results show a relatively low increase in the thermodynamic stabilization of the redox couple Ce^IV/III in the presence of carbonate versus its aqueous analogue. [Ce^IVDOTA(CO₃)]^2- and [Ce^IVDOTA(H₂O)], prepared electrochemically, decompose photolytically. However, kept in the dark, both are relatively long lived; [Ce^IVDOTA(H₂O)], though, is orders of magnitude kinetically more stable (a considerably longer half-life). Thus, one concludes that the carbonate species have a different mechanism of decomposition depending also on the presence of dioxygen after its preparation (in deaerated/aerated solutions). The [Ce^IVDOTA(CO₃)]^2- species is produced radiolytically by oxidation of the trivalent species by CO₃^•- with a rate constant, measured using pulse radiolysis, of 3.3 × 10⁵ M^-1 s^-1. This rate constant is at least 1 order of magnitude smaller than most of the rate constants so far reported for the reaction of CO₃^•- with transition metal/lanthanide (cerium)/actinide complexes. This result together with the bulkiness of the reactants might suggest an outer-sphere electron transfer rather than the inner-sphere one so far proposed. The lifetime of the tetravalent cerium species obtained radiolytically in the presence of carbonate is shorter than the electrochemical one, suggesting a different conformer involved.

Surface plasmon-driven photocatalytic activity of Ni@NiO/NiCO3 core-shell nanostructures

Ni@NiO/NiCO3 core-shell nanostructures have been investigated for surface plasmon driven photocatalytic solar H2 generation without any co-catalyst. Huge variation in the photocatalytic activity has been observed in the pristine vs. post-vacuum annealed samples with the maximum H2 yield (∼110 μmol g-1 h-1) for the vacuum annealed sample (N70-100/2 h) compared to ∼92 μmol g-1 h-1 for the pristine (N70) photocatalyst. Thorough structural (X-ray diffraction) and spectroscopic (X-ray photoelectron spectroscopy and transmission electron microscopy coupled electron energy loss spectroscopy) investigations reveal the core Ni nanoparticle decorated with the shell, a composite of crystalline NiO and amorphous NiCO3. Significant visible light absorption at ∼475 nm in the UV-vis region along with the absence of a peak/edge corresponding to NiO suggest the role of surface plasmons in the observed catalytic activity. As per the proposed mechanism, amorphous NiCO3 in the shell has been suggested to serve as the dielectric medium/interface, which enhances the surface plasmon resonance and boosts the HER activity.

Redox-inactive metal single-site molecular complexes: a new generation of electrocatalysts for oxygen evolution?

Bypassing the metal-based oxidation in a Cu-containing water oxidation catalytic system.