Peroxymonosulfate (HSO and PMS) is an optional bulk oxidant in advanced oxidation processes (AOPs) for treating wastewaters. Normally, PMS is activated by the input of energy or reducing agent to generate sulfate or hydroxyl radicals or both. This study shows that PMS without explicit activation undergoes direct reaction with a variety of compounds, including antibiotics, pharmaceuticals, phenolics, and commonly used singlet-oxygen (O) traps and quenchers, specifically furfuryl alcohol (FFA), azide, and histidine. Reaction time frames varied from minutes to a few hours at pH 9. With the use of a test compound with intermediate reactivity (FFA), electron paramagnetic resonance (EPR) and scavenging experiments ruled out sulfate and hydroxyl radicals. Although O was detected by EPR and is produced stoichiometrically through PMS self-decomposition, O plays only a minor role due to its efficient quenching by water, as confirmed by experiments manipulating the O formation rate (addition of HO) or lifetime (deuterium solvent isotope effect). Direct reactions with PMS are highly pH- and ionic-strength-sensitive and can be accelerated by (bi)carbonate, borate, and pyrophosphate (although not phosphate) via non-radical pathways. The findings indicate that direct reaction with PMS may steer degradation pathways and must be considered in AOPs and other applications. They also signal caution to researchers when choosing buffers as well as O traps and quenchers.

In photosystem II (PSII), photosynthetic water oxidation occurs at the tetramanganese–calcium cluster that cycles through light-induced intermediates (S0–S4) to produce oxygen from two substrate waters. The surrounding hydrogen-bonded amino acid residues and waters form channels that facilitate proton transfer and substrate water delivery, thereby ensuring efficient water oxidation. The residue D1-S169 lies in the “narrow” channel and forms hydrogen bonds with the Mn4CaO5 cluster via waters W1 and Wx. To probe the role of the narrow channel in substrate-water binding, we studied the D1-S169A mutation. PSII core complexes isolated from mutant cells exhibit inefficient S-state cycling and delayed oxygen evolution. The S2-state multiline EPR spectrum of D1-S169A PSII core complexes differed significantly from that of wild-type, and FTIR difference spectra showed that the mutation strongly perturbs the extensive network of hydrogen bonds that extends at least from D1-Y161 (YZ) to D1-D61. These results imply a possible role of D1-S169 in proton egress or substrate water delivery.

The inertness of the C–H bond in CH4 poses significant challenges to selective CH4 oxidation, which often proceeds all the way to CO2 once activated. Selective oxidation of CH4 to high-value industrial chemicals such as CO or CH3OH remains a challenge. Presently, the main methods to activate CH4 oxidation include thermochemical, electrochemical, and photocatalytic reactions. Of them, photocatalytic reactions hold great promise for practical applications but have been poorly studied. Existing demonstrations of photocatalytic CH4 oxidation exhibit limited control over the product selectivity, with CO2 as the most common product. The yield of CO or other hydrocarbons is too low to be of any practical value. In this work, we show that highly selective production of CO by CH4 oxidation can be achieved by a photoelectrochemical (PEC) approach. Under our experimental conditions, the highest yield for CO production was 81.9%. The substrate we used was TiO2 grown by atomic layer deposition (ALD), which features high concentrations of Ti3+ species. The selectivity toward CO was found to be highly sensitive to the substrate types, with significantly lower yield on P25 or commercial anatase TiO2 substrates. Moreover, our results revealed that the selectivity toward CO also depends on the applied potentials. Based on the experimental results, we proposed a reaction mechanism that involves synergistic effects by adjacent Ti sites on TiO2. Spectroscopic characterization and computational studies provide critical evidence to support the mechanism. Furthermore, the synergistic effect was found to parallel heterogeneous CO2 reduction mechanisms. Our results not only present a new route to selective CH4 oxidation, but also highlight the importance of mechanistic understandings in advancing heterogeneous catalysis.

Tandem dye-sensitized photoelectrosynthesis cells are promising architectures for the production of solar fuels and commodity chemicals. A key bottleneck in the development of these architectures is the low efficiency of the photocathodes, leading to small current densities. Herein, we report a new design principle for highly active photocathodes that relies on the outer-sphere reduction of a substrate from the dye, generating an unstable radical that proceeds to the desired product. We show that the direct reduction of dioxygen from dye-sensitized nickel oxide (NiO) leads to the production of HO. In the presence of oxygen and visible light, NiO photocathodes sensitized with commercially available porphyrin, coumarin, and ruthenium dyes exhibit large photocurrents (up to 400 μA/cm) near the thermodynamic potential for O/HO in near-neutral water. Bulk photoelectrolysis of porphyrin-sensitized NiO over 24 h results in millimolar concentrations of HO with essentially 100% faradaic efficiency. To our knowledge, these are among the most active NiO photocathodes reported for multiproton/multielectron transformations. The photoelectrosynthesis proceeds by initial formation of superoxide, which disproportionates to HO. This disproportionation-driven charge separation circumvents the inherent challenges in separating electron-hole pairs for photocathodes tethered to inner sphere electrocatalysts and enables new applications for photoelectrosynthesis cells.

Photoinduced water oxidation at the O 2 -evolving complex (OEC) of photosystem II (PSII) is a complex process involving a tetramanganesecalcium cluster that is surrounded by a hydrogenbonded network of water molecules, chloride ions, and amino-acid residues. Although the structure of the OEC has remained conserved over eons of evolution, significant differences in the chloride-binding characteristics exist between cyanobacteria and higher plants. An analysis of amino-acid residues in and around the OEC has identified residue 87 in the D1 subunit as the only significant difference between PSII in cyanobacteria and higher plants. We substituted the D1-N87 residue in the cyanobacterium Synechocystis sp. PCC 6803 (wild-type) with alanine, present in higher plants, or with aspartic acid. We studied PSII core complexes purified from D1-N87A and D1-N87D variant strains to probe the function of the D1-N87 residue in the water-oxidation mechanism. EPR spectra of the S 2 state and flash-induced FTIR spectra of both D1-N87A and D1-N87D PSII core complexes exhibited characteristics similar to those of wildtype Synechocystis PSII core complexes. However, flash-induced O 2 -evolution studies revealed a decreased cycling efficiency of the D1-N87D variant, whereas the cycling efficiency of the D1-N87A PSII variant was similar to that of wild-type PSII. Steady-state O 2 -evolution activity assays revealed that substitution of the D1-87 residue with alanine perturbs the chloridebinding site in the proton-exit channel. These findings provide new insight into the role of the D1-N87 site in the water-oxidation mechanism and explain the difference in the chloride-binding properties of cyanobacterial and higher-plant PSII.Photosystem II (PSII) is a 700 kDa pigmentprotein complex responsible for water oxidation in photoautotrophic organisms. The site of water oxidation, known as the oxygen-evolving complex (OEC), consists of a µ-oxo-bridged tetramanganese-calcium cluster ligated by a number of amino-acid residues and water molecules (1). The OEC is surrounded by a network of hydrogen-bonded amino-acid residues and water molecules that, along with Cl -ions, play a pivotal role in water oxidation (2,3). This process is initiated by photoinduced charge separation via a chlorophyll molecule called P 680 . The P 680 ⦁ + species thus formed is reduced by oxidation of the tetramanganese cluster, thereby building up oxidizing equivalents that are used for water oxidation (4). The process of water oxidation has been shown to proceed via a fourflash cycle called the Kok cycle with the intermediates formed at each step being referred to as S i states (i = 0 -4) (5). has remained elusive so far while the remaining S states have been studied using many experimental methods, especially electron paramagnetic resonance (EPR), Fourier transform infrared (FTIR), and extended X-ray absorption fine structure (EXAFS) spectroscopy. Information about the S 0 -S 3 states provides valuable insights about water oxidation.Due to the ease of generation an...