No abstract

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H+/4 e− process, while oxygen can be fully reduced to water by a 4 e−/4 H+ process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2 −. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

Abstract[NiFe]‐hydrogenase enzymes are efficient catalysts for H2 evolution but their synthetic models have not been reported to be active under aqueous conditions so far. Here we show that a close model of the [NiFe]‐hydrogenase active site can work as a very active and stable heterogeneous H2 evolution catalyst under mildly acidic aqueous conditions. Entry in catalysis is a NiIFeII complex, with electronic structure analogous to the Ni‐L state of the enzyme, corroborating the mechanism modification recently proposed for [NiFe]‐hydrogenases.

Considering the importance of water splitting as the best solution for clean and renewable energy, the worldwide efforts for development of increasingly active molecular water oxidation catalysts must be accompanied by studies that focus on elucidating the mode of actions and catalytic pathways. One crucial challenge remains the elucidation of the factors that determine the selectivity of water oxidation by the desired 4e–/4H+ pathway that leads to O2 rather than by 2e–/2H+ to H2O2. We now show that water oxidation with the cobalt–corrole CoBr8 as electrocatalyst affords H2O2 as the main product in homogeneous solutions, while heterogeneous water oxidation by the same catalyst leads exclusively to oxygen. Experimental and computation-based investigations of the species formed during the process uncover the formation of a Co­(III)–superoxide intermediate and its preceding high-valent Co–oxyl complex. The competition between the base-catalyzed hydrolysis of Co­(III)–hydroperoxide [Co­(III)–OOH]− to release H2O2 and the electrochemical oxidation of the same to release O2 via [Co­(III)–O2 •]− is identified as the key step determining the selectivity of water oxidation.

The protonation state of thiols in self-assembled monolayers (SAMs) on Ag and Au surfaces and nanoparticles (NPs) has been an issue of contestation. It has been recently demonstrated that deuterating the thiol proton produces ostentatious changes in the Raman spectra of thiols and can be used to detect the presence of the thiol functional group. Surface enhanced Raman spectroscopy (SERS) of H/D substituted aliphatic thiols on Ag surfaces clearly shows the presence of S-H vibration between 2150-2200 cm(-1) which shifts by 400 cm(-1) upon deuteration and a simultaneous >20 cm(-1) shift in the C-S vibration of thiol deuteration. Large shifts (>15 cm(-1)) in the C-S vibration are also observed for alkyl thiol SAMs on Au surfaces. Alternatively, neither the S-H vibration nor the H/D isotope effect on the C-S vibration is observed for alkyl thiol SAMs on Ag/Au NPs. XPS data on Ag/Au surfaces bearing aliphatic thiol SAMs show the presence of both protonated and deprotonated thiols while on Ag/Au NPs only deprotonated thiols are detected. These data suggest that aliphatic thiol SAMs on Au/Ag surfaces are partially protonated whereas they are totally deprotonated on Au/Ag NPs. Aromatic PhSH SAMs on Ag/Au surfaces and Ag/Au NPs do not show these vibrations or H/D shifts as well indicating that the thiols are deprotonated at these interfaces.