Björn Åkermark

Cobalt^II phthalocyanine in the presence of Ti^III citrate acts as an efficient reducing agent which was able to defluorinate 54.5% of all branched PFOS isomers.

The instability of the molecular electrodes under oxidative/reductive conditions and insufficient understanding of the metal oxide-based systems have slowed down the progress of H₂-based fuels. Efficient regeneration of the electrode’s performance after prolonged use is another bottleneck of this research. This work represents the first example of the bifunctional and electrochemically regenerable molecular electrode which can be used for the unperturbed production of H₂ from water. Pyridyl linkers with flexible arms (-CH₂-CH₂-) on modified fluorine-doped carbon cloth (FCC) were used to anchor a highly active ruthenium electrocatalyst [Ru^II(mcbp)(H₂O)₂] (1) [mcbp²⁻ = 2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine]. The pyridine unit of the linker replaces one of the water molecules of 1, which resulted in RuPFCC (ruthenium electrocatalyst anchored on -CH₂-CH₂-pyridine modified FCC), a high-performing electrode for oxygen evolution reaction [OER, overpotential of ̴ 215 mV] as well as hydrogen evolution reaction (HER, overpotential of ̴ 330 mV) at pH 7. A current density of ̴ 8 mA/cm₂ at 2.06 V (vs RHE) and ̴ -6 mA/cm² at -0.84 V (vs RHE) with only 0.04 wt% loading of ruthenium was obtained. OER turnover of > 7.4 × 10³ at 1.81 V in 48 h and HER turnover of > 3.6 × 10³ at -0.79 V in 3 h were calculated. The activity of the OER anode after 48 h use could be electrochemically regenerated to ̴ 98% of its original activity while it serves as a HE cathode (evolving hydrogen) for 8 h. This electrode design can also be used for developing ultra-stable molecular electrodes with exciting electrochemical regeneration features, for other proton-dependent electrochemical processes.

Vitamin B₁₂ (VB₁₂) has been reported to degrade PFOS in the presence of Ti^III citrate at 70 °C. Porphyrin-based catalysts have emerged as VB₁₂ analogues and have been successfully used in various fields of research due to their interesting structural and electronic properties. However, there is inadequate information on the use of these porphyrin-based metal complexes in the defluorination of PFOS. We have therefore explored a series of porphyrin-based metal complexes for the degradation of PFOS. Co^II-5,10,15,20-tetraphenyl-21H,23H-porphyrin (Co^II-TPP), Co^II-5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphyrin (Co^II-M-TPP), and Co^III-M-TPP exhibited efficient reductive defluorination of the branched PFOS. Within 5-8 h, these compounds achieved the same level of PFOS defluorination as VB₁₂ achieved in 7-10 days. For branched isomers, the specific removal rate of the Co^II-TPP-Ti^III citrate system is 64-105 times higher than that for VB₁₂-Ti^III citrate. Moreover, the Co^II-TPP-Ti^III citrate system displayed efficient (51%) defluorination for the branched PFOS (br-PFOS) in 1 day even at room temperature (25 °C). The effects of the iron and cobalt metal centers, reaction pH, and several reductants (NaBH₄, nanosized zerovalent zinc (nZn⁰), and Ti^III citrate) were systematically investigated. Based on the analysis of the products and previously published reports, a new possible defluorination pathway of branched PFOS is also proposed.

Electrochemical water splitting constitutes one of the most promising strategies for converting water into hydrogen-based fuels, and this technology is predicted to play a key role in the transition towards a carbon-neutral energy economy. To enable the design of cost-effective electrolysis cells based on this technology, new and more efficient anodes with augmented water splitting activity and stability will be required. Herein, we report an active molecular Ru-based catalyst for electrochemically-driven water oxidation (overpotential of ∼395 mV at pH 7 phosphate buffer) and two simple methods for preparing anodes by attaching this catalyst onto glassy carbon through multi-walled carbon nanotubes to improve stability as well as reactivity. The anodes modified with the molecular catalyst were characterized by a broad toolbox of microscopy and spectroscopy techniques, and interestingly no RuO₂ formation was detected during electrocatalysis over 4 h. These results demonstrate that the herein presented strategy can be used to prepare anodes that rival the performance of state-of-the-art metal oxide anodes.

Removing per- and polyfluoroalkyl substances (PFASs) from water by ceric ammonium nitrate (CAN).

This essay for EurJIC's special issue on “Redox Catalysis for Artificial Photosynthesis” introduces the reader to the field of water oxidation using molecular catalysts. The most essential challenge our society must address during the 21st century is perhaps the realization of a system for producing sustainable energy on the global scale. Currently, there exists an urgent need to develop effective and economical carbon‐neutral or carbon‐free energy technologies. The production of solar fuels through water splitting constitutes a key enabling element. The construction of robust and efficient catalysts for oxidation of water is therefore essential. In this essay the progress and mechanistic considerations pertaining to molecular water oxidation catalysts are described and discussed from a personal perspective.

The highly active ruthenium‐based water oxidation catalyst [Ru^X(mcbp)(OH_n)(py)₂] [mcbp²⁻=2,6‐bis(1‐methyl‐4‐(carboxylate)benzimidazol‐2‐yl)pyridine; n=2, 1, and 0 for X=II, III, and IV, respectively], can be generated in a mixture of Ru^III and Ru^IV states from either [Ru^II(mcbp)(py)₂] or [Ru^III(Hmcbp)(py)₂]²⁺ precursors. The precursor complexes are isolated and characterized by single‐crystal X‐ray analysis, NMR, UV/Vis, EPR, and FTIR spectroscopy, ESI‐HRMS, and elemental analysis, and their redox properties are studied in detail by electrochemical and spectroscopic methods. Unlike the parent catalyst [Ru(tda) (py)₂] (tda²⁻=[2,2′:6′,2′′‐terpyridine]‐6,6′′‐dicarboxylate), for which full transformation into the catalytically active species [Ru^IV(tda)(O)(py)₂] could not be carried out, stoichiometric generation of the catalytically active Ru–aqua complex [Ru^X(mcbp)(OH_n)(py)₂] from the Ru^II precursor was achieved under mild conditions (pH 7.0) and short reaction times. The redox properties of the catalyst were studied and its activity for electrocatalytic water oxidation was evaluated, reaching a maximum turnover frequency (TOF_max) of around 40 000 s⁻¹ at pH 9.0 (from foot‐of‐the‐wave analysis), which is comparable to the activity of the state‐of‐the‐art catalyst [Ru^IV(tda)(O)(py)₂].

Catalysts for oxidation of water to molecular oxygen are essential in solar-driven water splitting. In order to develop more efficient catalysts for this oxidatively demanding reaction, it is vital to have mechanistic insight in order to understand how the catalysts operate. Herein, we report the mechanistic details associated with the two Ru catalysts 1 and 2. Insight into the mechanistic landscape of water oxidation catalyzed by the two single-site Ru catalysts was revealed by the use of a combination of experimental techniques and quantum chemical calculations. On the basis of the obtained results, detailed mechanisms for oxidation of water by complexes 1 and 2 are proposed. Although the two complexes are structurally related, two deviating mechanistic scenarios are proposed with metal–ligand cooperation being an important feature in both processes. The proposed mechanistic platforms provide insight for the activation of water or related small molecules through nontraditional and previously unexplored routes.

Herein, we describe the straightforward synthesis and thorough characterization of a magnetically‐separable heterogeneous catalyst comprised of 1–3 nm‐sized Pd nanoparticles immobilized on a mesoporous silica‐magnetite composite (Pd ⁰ ‐AmP‐SMC). Catalytic evaluations were conducted using Suzuki‐Miyaura cross‐couplings as the model reactions, for which this Pd nanocatalyst exhibited high performance in an environmentally‐friendly solvent mixture. Additionally, this Pd nanocatalyst could be re‐used up to five cycles without any observable loss of activity, and separation of the catalyst could be conveniently done by a magnet.