The Solvent Effect on H2 O2 Generation at Room Temperature Ionic Liquid|Water Interface

H₂ O₂ is a versatile chemical and can be generated by the oxygen reduction reaction (ORR) in proton donor solution in molecular solvents or room temperature ionic liquids (IL). We investigated this reaction at interfaces formed by eleven hydrophobic ILs and acidic aqueous solution as a proton source with decamethylferrocene (DMFc) as an electron donor. H₂ O₂ is generated in colorimetrically detectable amounts in biphasic systems formed by alkyl imidazolium hexafluorophosphate or tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids. H₂ O₂ fluxes were estimated close to liquid|liquid interface by scanning electrochemical microscopy (SECM). Contrary to the interfaces formed by hydrophobic electrolyte solution in a molecular solvent, H₂ O₂ generation is followed by cation expulsion to the aqueous phase. Weak correlation between the H₂ O₂ flux and the difference between DMFc/DMFc⁺ redox potential and 2 electron ORR standard potential indicates kinetic control of the reaction.

Photo-recycling the Sacrificial Electron Donor: Towards Sustainable Hydrogen Evolution in a Biphasic System

H₂ may be evolved biphasically using a polarised liquid|liquid interface, acting as a "proton pump", in combination with organic soluble metallocenes as electron donors. Sustainable H₂ production requires methodologies to recycle the oxidised donor. Herein, the photo-recycling of decamethylferrocenium cations (DcMFc⁺ ) using aqueous core-shell semiconductor CdSe@CdS nanoparticles is presented. Negative polarisation of the liquid|liquid interface is required to extract DcMFc⁺ to the aqueous phase. This facilitates the efficient capture of electrons by DcMFc⁺ on the surface of the photo-excited CdSe@CdS nanoparticles, with hydrophobic DcMFc subsequently partitioning back to the organic phase and resetting the system. TiO₂ (P25) and CdSe semiconductor nanoparticles failed to recycle DcMFc⁺ due to their lower conduction band energy levels. During photo-recycling, CdS (on CdSe) may be self-oxidised and photo-corrode, instead of water acting as the hole scavenger.

Catalytic Activity of Alkali Metal Cations for the Chemical Oxygen Reduction Reaction in a Biphasic Liquid System Probed by Scanning Electrochemical Microscopy

Chemical reduction of dioxygen in organic solvents for the production of reactive oxygen species or the concomitant oxidation of organic substrates can be enhanced by the separation of products and educts in biphasic liquid systems. Here, the coupled electron and ion transfer processes is studied as well as reagent fluxes across the liquid|liquid interface for the chemical reduction of dioxygen by decamethylferrocene (DMFc) in a dichloroethane-based organic electrolyte forming an interface with an aqueous electrolyte containing alkali metal ions. This interface is stabilized at the orifice of a pipette, across which a Galvani potential difference is externally applied and precisely adjusted to enforce the transfer of different alkali metal ions from the aqueous to the organic electrolyte. The oxygen reduction is followed by H2 O2 detection in the aqueous phase close to the interface by a microelectrode of a scanning electrochemical microscope (SECM). The results prove a strong catalytic effect of hydrated alkali metal ions on the formation rate of H2 O2 , which varies systematically with the acidity of the transferred alkali metal ions in the organic phase.

Mechanistic Study on the Photogeneration of Hydrogen by Decamethylruthenocene

Detailed studies on hydrogen evolution by decamethylruthenocene ([Cp*₂ Ru^II ]) highlighted that metallocenes are capable of photoreducing hydrogen without the need for an additional sensitizer. Electrochemical, gas chromatographic, and spectroscopic (UV/Vis, ¹ H and ¹³ C NMR) measurements corroborated by DFT calculations indicated that the production of hydrogen occurs by a two-step process. First, decamethylruthenocene hydride [Cp*₂ Ru^IV (H)]⁺ is formed in the presence of an organic acid. Subsequently, [Cp*₂ Ru^IV (H)]⁺ is reversibly reduced in a heterolytic reaction with one-photon excitation leading to a first release of hydrogen. Thereafter, the resultant decamethylruthenocenium ion [Cp*₂ Ru^III ]⁺ is further reduced with a second release of hydrogen by deprotonation of a methyl group of [Cp*₂ Ru^III ]⁺ . Experimental and computational data show spontaneous conversion of [Cp*₂ Ru^II ] to [Cp*₂ Ru^IV (H)]⁺ in the presence of protons. Calculations highlight that the first reduction is endergonic (ΔG⁰ =108 kJ mol^-1 ) and needs an input of energy by light for the reaction to occur. The hydricity of the methyl protons of [Cp*₂ Ru^II ] was also considered.