The mechanism of water photooxidation reaction at atomically flat n-TiO2 (rutile) surfaces was investigated in aqueous solutions of various pH values, using photoluminescence (PL) measurements. The ...PL bands, which peaked at around 810 and 840 nm for the (110) and (100) surfaces, respectively, were assigned to radiative transitions between conduction-band electrons and surface-trapped holes (STH), Ti−O=Ti2s +, formed at triply coordinated (normal) O atoms at the surface lattice. The PL intensity (I PL) decreased stepwise with increasing solution pH, namely, it sharply decreased at around pH 4, near the point of zero charge of TiO2 (about 5), and then rapidly decreased to zero near pH 13. The first sharp decrease around pH 4 is attributed to the increased rate of nucleophilic attack of a water molecule to a hole at a site of surface bridging oxygen (Ti−O−Ti), the density of which increases with increasing pH. The nucleophilic attack is regarded as the main initiating step of the water oxidation reaction in low and intermediate pH. The high PL intensity at low pH is ascribed to slow nucleophilic attack owing to a very low density of Ti−O−Ti by its protonation at the low pH. The second sharp decrease near pH 13 is attributed to formation of surface anionic species like Ti−O- which can be readily oxidized by photogenerated holes. Interrelations between reaction intermediates proposed in this work and those reported by time-resolved laser spectroscopy are discussed.
Both inorganic fertilizer inputs and crop yields have increased globally, with the concurrent increase in the pollution of water bodies due to nitrogen leaching from soils. Designing agroecosystems ...that are environmentally friendly is urgently required. Since agroecosystems are highly complex and consist of entangled webs of interactions between plants, microbes, and soils, identifying critical components in crop production remain elusive. To understand the network structure in agroecosystems engineered by several farming methods, including environmentally friendly soil solarization, we utilized a multiomics approach on a field planted with Brassica rapa. We found that the soil solarization increased plant shoot biomass irrespective of the type of fertilizer applied. Our multiomics and integrated informatics revealed complex interactions in the agroecosystem showing multiple network modules represented by plant traits heterogeneously associated with soil metabolites, minerals, and microbes. Unexpectedly, we identified soil organic nitrogen induced by soil solarization as one of the key components to increase crop yield. A germ-free plant in vitro assay and a pot experiment using arable soils confirmed that specific organic nitrogen, namely alanine and choline, directly increased plant biomass by acting as a nitrogen source and a biologically active compound. Thus, our study provides evidence at the agroecosystem level that organic nitrogen plays a key role in plant growth.
Shewanella is an electrogenic microbe that has significant content of c type cytochromes (ca. 0.5 mM). This feature allows the optical absorption spectra of the cell-membrane-associated proteins to ...be monitored in vivo in the course of extracellular respiratory electron-transfer reactions. The results show significant differences to those obtained in vitro with purified proteins.
Protein power grids: A metal‐reducing bacterium, Shewanella loihica PV‐4, has the ability to self‐organize an electrically conductive network using outer‐membrane proteins and semiconductive minerals ...as a long‐distance electron transfer conduit.
Molybdenum sulfide (MoS2) is the most widely studied transition-metal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward ...desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS2 significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS2 has a protonation site with a pKa of ∼5.5, where the proton is located ∼3.26 Å from redox-active Mo site. This protonation site is unique to 1T-MoS2 and induces sequential proton−electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS2, expanding the application of TMDs to selective electrocatalysis.
The first photocatalysis operated by the visible-light induced metal-to-metal charge transfer (MMCT) has been demonstrated for the Ti(IV)/Ce(III) bimetallic assemblies synthesized on the pore of ...mesoporous silica. The Ce LIII-edge XANES measurements combined with 18O-isotopic labeling as well as photoelectrochemical experiments proved the ability of Ti(IV)/Ce(III) assemblies to drive the site-specific photocatalytic reaction under visible-light irradiation. Photocatalytic experiments using the oxidative decomposition of 2-propanol showed that the quantum efficiency for Ti(IV)/Ce(III) bimetallic photocatalysts was remarkably higher than that for the one of the most active visible-light sensitive photocatalysts, nitrogen-doped TiO2. It was also found that the grafting of Ce(III) ions onto nanocrystalline TiO2 particles led to the appearance of intense MMCT band in the visible-light regions, thus verifying it as the generally applicable strategy to obtain visible-light photocatalysts. To date, the grafting of metal cations on the pore of mesoporous silica has been established for about one-fourth of the elements in the periodic table. Therefore, the abilities of hetero-bimetallic assemblies to drive photocatalysis with a high quantum efficiency and their high flexibility in metal combination provide an opportunity to design and fabricate the wide variety of molecular-based inorganic photocatalysts according to the needs from the target reactions as well as their operation environments.
The oxygen evolution reaction (OER; 2H2O → O2 + 4H+ + 4e–) is being intensively studied to generate fossil fuel-independent energy carriers. As 4d/5d rare metal catalysts, such as amorphous iridium ...oxide (IrO x ), display higher activity than 3d metal catalysts, elucidating the critical mechanistic differences between these materials is important for the synthesis of cost-effective OER catalysts. Although most studies of OER catalysts have focused on O–O bond formation energetics, here, we examined the OER mechanism of IrO x based on charge accumulation, which was recently shown to determine the OER activity for Mn and Fe oxides. Kinetic analysis using Tafel and trumpet plots, along with the difference in the pH dependence between the OER onset potential and that of iridium valence change, showed that the valence change of iridium is more favorable than O–O bond formation. In situ evanescent wave spectroscopy revealed that an intermediate assignable to Ir5+ with oxygen ligands in opposite spin serves as the precursor of OER regardless of pH. As the generation of this species is not related to valence changes of iridium, these results confirm that charge accumulation is not rate-limiting for OER on IrO x , which is a key mechanistic difference between IrO x and less-efficient 3d metal electrocatalysts.
Pore genius: A nanoporous composite of graphite felt and polyaniline was developed and used as the anode of a microbial fuel cell, resulting in an order of magnitude increase in power output. The ...hierarchical conductive anode is a promising strategy for constructing highly efficient microbial fuel cells (see figure).
Prebiotic organic synthesis catalyzed by Earth-abundant metal sulfides is a key process for understanding the evolution of biochemistry from inorganic molecules, yet the catalytic functions of ...sulfides have remained poorly explored in the context of the origin of life. Past studies on prebiotic chemistry have mostly focused on a few types of metal sulfide catalysts, such as FeS or NiS, which form limited types of products with inferior activity and selectivity. To explore the potential of metal sulfides on catalyzing prebiotic chemical reactions, here, the chemical diversity (variations in chemical composition and phase structure) of 304 natural metal sulfide minerals in a mineralogy database was surveyed. Approaches to rationally predict the catalytic functions of metal sulfides are discussed based on advanced theories and analytical tools of electrocatalysis such as proton-coupled electron transfer, structural comparisons between enzymes and minerals, and in situ spectroscopy. To this end, we introduce a model of geoelectrochemistry driven prebiotic synthesis for chemical evolution, as it helps us to predict kinetics and selectivity of targeted prebiotic chemistry under "chemically messy conditions". We expect that combining the data-mining of mineral databases with experimental methods, theories, and machine-learning approaches developed in the field of electrocatalysis will facilitate the prediction and verification of catalytic performance under a wide range of pH and Eh conditions, and will aid in the rational screening of mineral catalysts involved in the origin of life.
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