H2 Oxidation Research Articles

Kinetic interactions between H2 and CO in catalytic oxidation over PdO

Kinetic interactions between H2 and CO over PdO, a widely used catalyst in combustion systems, were studied experimentally and numerically. Global reaction parameters of H2 and CO oxidation over PdO were extracted from wire microcalorimetry experiments at atmospheric pressure in the temperature range 380–800 K, based on which a full catalytic reaction mechanism was developed. Comparison of ignition temperatures and heat release rates of different H2/CO blends along with density functional theory (DFT) simulations revealed complex physicochemical coupling of the H2 and CO catalytic oxidation pathways. The coupling evolves from an inhibiting effect of one fuel component onto the other due to their competition for surface adsorption sites and a direct repelling mechanism between the co-adsorbed H(s) and CO(s), to a promoting effect at sufficiently high temperatures caused by alleviated O(s) surface blocking. Implications of the H2CO kinetic coupling to the operation of practical power generation systems are outlined.

The Feasibility of Electrochemical Ammonia Synthesis in Molten LiCl-KCl Eutectics.

Molten LiCl and related eutectic electrolytes are known to permit direct electrochemical reduction of N2 to N3- with high efficiency. It had been proposed that this could be coupled with H2 oxidation in an electrolytic cell to produce NH3 at ambient pressure. Here, this proposal is tested in a LiCl-KCl-Li3 N cell and is found not to be the case, as the previous assumption of the direct electrochemical oxidation of N3- to NH3 is grossly over-simplified. We find that Li3 N added to the molten electrolyte promotes the spontaneous and simultaneous chemical disproportionation of H2 (H oxidation state 0) into H- (H oxidation state -1) and H+ in the form of NH2- /NH2 - /NH3 (H oxidation state +1) in the absence of applied current, resulting in non-Faradaic release of NH3 . It is further observed that NH2- and NH2 - possess their own redox chemistry. However, these spontaneous reactions allow us to propose an alternative, truly catalytic cycle. By adding LiH, rather than Li3 N, N2 can be reduced to N3- while stoichiometric amounts of H- are oxidised to H2 . The H2 can then react spontaneously with N3- to form NH3 , regenerating H- and closing the catalytic cycle. Initial tests show a peak NH3 synthesis rate of 2.4×10-8 mol cm-2 s-1 at a maximum current efficiency of 4.2 %. Isotopic labelling with 15 N2 confirms the resulting NH3 is from catalytic N2 reduction.

The Feasibility of Electrochemical Ammonia Synthesis in Molten LiCl–KCl Eutectics

AbstractMolten LiCl and related eutectic electrolytes are known to permit direct electrochemical reduction of N2 to N3− with high efficiency. It had been proposed that this could be coupled with H2 oxidation in an electrolytic cell to produce NH3 at ambient pressure. Here, this proposal is tested in a LiCl–KCl–Li3N cell and is found not to be the case, as the previous assumption of the direct electrochemical oxidation of N3− to NH3 is grossly over‐simplified. We find that Li3N added to the molten electrolyte promotes the spontaneous and simultaneous chemical disproportionation of H2 (H oxidation state 0) into H− (H oxidation state −1) and H+ in the form of NH2−/NH2−/NH3 (H oxidation state +1) in the absence of applied current, resulting in non‐Faradaic release of NH3. It is further observed that NH2− and NH2− possess their own redox chemistry. However, these spontaneous reactions allow us to propose an alternative, truly catalytic cycle. By adding LiH, rather than Li3N, N2 can be reduced to N3− while stoichiometric amounts of H− are oxidised to H2. The H2 can then react spontaneously with N3− to form NH3, regenerating H− and closing the catalytic cycle. Initial tests show a peak NH3 synthesis rate of 2.4×10−8 mol cm−2 s−1 at a maximum current efficiency of 4.2 %. Isotopic labelling with 15N2 confirms the resulting NH3 is from catalytic N2 reduction.

A new combined electrolysis and catalytic system for removal of VOCs

A new combined electrolysis and catalytic system is proposed to remove volatile organic compounds (VOCs). The catalytic activity and reaction properties of a Pt/TiO2 catalyst were investigated for the oxidation of H2 and gas-phase VOCs. The test results of the catalytic activity suggest that H2 oxidation can lead to the generation of heat from an exothermal reaction of the catalyst and plays an important role in the oxidation of toluene at low temperatures. To apply a catalyst to a pilot-scale plant, a catalyst coating was applied to a honeycomb-type structure; in addition, the catalyst system was designed and the crucial factors were derived from activity evaluation under various operating conditions. The flow properties of the combined process system were confirmed by a computational fluid dynamics (CFD) simulation and the catalyst performance of the combined pilot-scale system was studied. The honeycomb-type catalyst together with the combined VOCs removal system developed in this study provides a practical and safe approach for the removal of liquid- and gas-phase VOCs.

Oxygen caught on film

Thin films of hydrogenase enzymes dispersed in a redox-active gel effect rapid anodic oxidation of H2 even in the presence of O2.

Bismuth(III) interactions with Desulfovibrio desulfuricans: inhibition of cell energetics and nanocrystal formation of Bi2S3 and Bi0.

Sulfate-reducing bacteria have been suggested to have an etiological role in the development of inflammatory bowel diseases and ulcerative colitis in humans. Traditionally. bismuth compounds have been administered to alleviate gastrointestinal discomfort and disease symptoms. One mechanism by which this treatment occurs is through binding bacterial derived hydrogen sulfide in the intestines. With the addition of bismuth-deferiprone, bismuth-citrate and bismuth subsalicylate to reactions containing cells of D. desulfuricans ATCC 27774, the oxidation of H2 with sulfate as the electron acceptor was inhibited but H2 oxidation with nitrate, nitrite and sulfite was not reduced. Our research suggests that a target for bismuth inhibition of D. desulfuricans is the F1 subunit of the ATP synthase and, thus, dissimilatory sulfate reduction does not occur. At sublethal concentrations, bismuth as Bi(III) is precipitated by hydrogen sulfide produced from respiratory sulfate reduction by D. desulfuricans. Nanocrystals of bismuth sulfide were determined to be Bi2S3 through the use of high resolution transmission electron microscopy imaging with X-ray energy-dispersive spectroscopy analysis. In the absence of sulfate, D. desulfuricans oxidizes H2 with the reduction of Bi(III) to Bi0 and this was also established by X-ray energy-dispersive spectroscopy analysis.

Energy Conservation and Hydrogenase Function in Methanogenic Archaea, in Particular the Genus Methanosarcina.

The biological production of methane is vital to the global carbon cycle and accounts for ca. 74% of total methane emissions. The organisms that facilitate this process, methanogenic archaea, belong to a large and phylogenetically diverse group that thrives in a wide range of anaerobic environments. Two main subgroups exist within methanogenic archaea: those with and those without cytochromes. Although a variety of metabolisms exist within this group, the reduction of growth substrates to methane using electrons from molecular hydrogen is, in a phylogenetic sense, the most widespread methanogenic pathway. Methanogens without cytochromes typically generate methane by the reduction of CO2 with electrons derived from H2, formate, or secondary alcohols, generating a transmembrane ion gradient for ATP production via an Na+-translocating methyltransferase (Mtr). These organisms also conserve energy with a novel flavin-based electron bifurcation mechanism, wherein the endergonic reduction of ferredoxin is facilitated by the exergonic reduction of a disulfide terminal electron acceptor coupled to either H2 or formate oxidation. Methanogens that utilize cytochromes have a broader substrate range, and can convert acetate and methylated compounds to methane, in addition to the ability to reduce CO2 Cytochrome-containing methanogens are able to supplement the ion motive force generated by Mtr with an H+-translocating electron transport system. In both groups, enzymes known as hydrogenases, which reversibly interconvert protons and electrons to molecular hydrogen, play a central role in the methanogenic process. This review discusses recent insight into methanogen metabolism and energy conservation mechanisms with a particular focus on the genus Methanosarcina.

Complete Protection of O2-Sensitive Catalysts in Thin Films.

Energy conversion schemes involving dihydrogen hold great potential for meeting sustainable energy needs, but widespread implementation cannot proceed without solutions that mitigate the cost of rare metal catalysts and the O2 instability of biological and bioinspired replacements. Recently, thick films (>100 μm) of redox polymers were shown to prevent O2 catalyst damage but also resulted in unnecessary catalyst load and mass transport limitations. Here we apply novel homogeneous thin films (down to 3 μm) that provide protection from O2 while achieving highly efficient catalyst utilization. Our empirical data are explained by modeling, demonstrating that resistance to O2 inactivation can be obtained for nonlimiting periods of time when the optimal thickness for catalyst utilization and current generation is achieved, even when using highly fragile catalysts such as the enzyme hydrogenase. We show that different protection mechanisms operate depending on the matrix dimensions and the intrinsic catalyst properties and can be integrated together synergistically to achieve stable H2 oxidation currents in the presence of O2, potentially enabling a plethora of practical applications for bioinspired catalysts under harsh oxidative conditions.

Investigating the Kinetic Competency of CrHydA1 [FeFe] Hydrogenase Intermediate States via Time-Resolved Infrared Spectroscopy.

Hydrogenases are metalloenzymes that catalyze the reversible oxidation of H2. The [FeFe] hydrogenases are generally biased toward proton reduction and have high activities. Several different catalytic mechanisms have been proposed for the [FeFe] enzymes based on the identification of intermediate states in equilibrium and steady state experiments. Here, we examine the kinetic competency of these intermediate states in the [FeFe] hydrogenase from Chlamydomonas reinhardtii (CrHydA1), using a laser-induced potential jump and time-resolved IR (TRIR) spectroscopy. A CdSe/CdS dot-in-rod (DIR) nanocrystalline semiconductor is employed as the photosensitizer and a redox mediator efficiently transfers electrons to the enzyme. A pulsed laser induces a potential jump, and TRIR spectroscopy is used to follow the population flux through each intermediate state. The results clearly establish the kinetic competency of all intermediate populations examined: Hox, Hred, HredH+, HsredH+, and Hhyd. Additionally, a new short-lived intermediate species with a CO peak at 1896 cm-1 was identified. These results establish a kinetics framework for understanding the catalytic mechanism of [FeFe] hydrogenases.

Carbon dioxide fixation by a hydrogen-oxidizing bacterium: Biomass yield, reversal respiratory quotient, stoichiometric equations and bioenergetics

A strain of Cupriavidus necator was grown on a gas mixture of hydrogen, oxygen and carbon dioxide to measure the cell mass formation and uptake rates of individual gases. The cell mass yield on consumed hydrogen and reversal respiratory quotient (RRQ) changed with cell density, indicating dynamic growth of the autotrophic microbes. A general stoichiometric equation was formulated by combining three half reactions of electron donor, electron accepter and cell mass synthesis. The only unknown parameter was the mole fraction of electrons (free energy) used in cell mass synthesis and determined with the measured cell mass yield or RRQ. The stoichiometric coefficients depended on the time duration of the measurements because of maintenance requirements. The energy efficiency of biomass formation from CO2 declined from 25% to 14% with increased cell density. The efficiency decline was not caused by wasteful oxygenase activity of Rubisco, but other mechanisms that decoupled the proton pump and ATP synthesis. Under the conditions of fast autotrophic growth with a low RRQ, the coupling was quite efficient, generating ca. 3 mol ATP per mole H2 oxidation (P/O ≈ 3), and the energy cost of CO2 reduction to carbohydrate via the CBB cycle was ca. 263 kJ (C-mol CO2)−1.

Phosphorus and Iron Doped-Porous Carbon As an Efficient Electrocatalyst for Oxygen Reduction Reaction in Alkaline Medium

The U.S. Energy Information Administration has projected a 27% growth in energy consumption by the year 2050.1 The continuous increase in the energy demands worldwide and the environmental concerns reaching critical levels, has required researchers to make the development of clean and sustainable energy conversion devices their paramount challenge. Fuel cells and metal-air batteries with high efficiency and low cost are the prime candidates. Platinum loaded carbon substrate is the catalyst of choice in fuel cells for anodic oxidation of H2 and cathodic reduction of O2 but large-scale commercial production is restricted by its prohibitive cost, limited supply, and weak durability.2 Platinum’s vulnerability to potential drift over time and easy loss of activity through methanol poisoning, questions its competitive catalytic activity. Electrocatalyst that are easily synthesized, have low cost and are earth-abundant is still highly desired. Heteroatom doped carbon electrocatalysts are highly considered candidates and are well researched because of their high abundance and low cost, with transition metal and nitrogen doped carbon material showing competitive result towards the oxygen reduction reaction (ORR).3 , 4 Among those heteroatoms phosphorus doped carbon is an emerging catalyst with high potential. The attraction towards phosphorus arises from the fact that it has a larger atomic radius- hence larger covalent radius- and higher electron-donating ability than nitrogen, making it a promising dopant with an ability to modify the electron transport properties. The Introduction of phosphorus, an atom with lower electronegativity, changes the charge density distribution and electronic properties of a high surface area carbon structure. Phosphorus doping increases the affinity towards an acceptor molecule such as oxygen5, hence a better interaction with the oxygen intermediate and enhances their electrocatalytic activities towards ORR. Such heteroatoms are typically covalently bonded within a carbon framework ensuring long-term stability. The advantages of a heteroatom doped porous carbon are twofold, stabilization of the non-precious metal in the alkaline medium and generation of active sites for facilitating the oxygen reduction reaction. Electrocatalytic activity of phosphorus doped and iron treated phosphorus doped carbon has only been covered by few groups and needs more improvements.6,7 The purpose of the study of electron transfer processes is to understand the reactivity of a relatively new, doped porous carbon complexes incorporating phosphorus as a heteroatom bonded to a metal center. According to DFT calculations, O2 and other ORR intermediates can be stably adsorbed on to Fe sites8, making phosphorus and iron doped carbon a prime candidate for ORR. Using a single precursor as a source of phosphorus, we propose a simple one-step chemical activation, with anhydrous ZnCl2 and anhydrous FeCl2 as the agents of carbonization, yielding P and Fe-doped porous carbon (PFeC). The electrocatalytic activity towards ORR was studied using a rotating ring-disk electrode (RRDE) technique and rotating disk electrode (RDE) connected to the CHI 600 potentiostat. The electrochemical activity of PFeC was compared with the commercially available 20 wt% Pt/C. PFeC showed high activity towards ORR, with comparable onset potential and a small negative shift in half-wave potential. PFeC electrocatalyst selectively catalyzes oxygen to water via a direct four-electron pathway. PFeC showed to be inert toward alcohol oxidation and had superior long-term stability. References (1) U.S. EIA. Annual Energy Outlook 2018 with Projections to 2050. Annu. Energy Outlook 2018 with Proj. to 2050 2018, 44 (8), 1–64. (2) Steele, B. C.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414 (November), 345–352. (3) Lin, L.; Zhu, Q.; Xu, A. W. Noble-Metal-Free Fe-N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions. J. Am. Chem. Soc. 2014, 136 (31), 11027–11033. (4) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electroctalytic Activity for Oxygen Reduction. Science (80-. ). 2009, 323 (FEBRUARY), 760–764. (5) Cruz-Silva, E.; Lopez-Urias, F.; Munoz-Sandoval, E.; Sumpter, B. G.; Terrones, H.; Charlier, J. C.; Meunier, V.; Terrones, M. Phosphorus and Phosphorus-Nitrogen Doped Carbon Nanotubes for Ultrasensitive and Selective Molecular Detection. Nanoscale 2011, 3 (3), 1008–1013. (6) Razmjooei, F.; Singh, K. P.; Bae, E. J.; Yu, J.-S. A New Class of Electroactive Fe- and P-Functionalized Graphene for Oxygen Reduction. J. Mater. Chem. A 2015, 3 (20), 11031–11039. (7) Singh, K. P.; Bae, E. J.; Yu, J. S. Fe-P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137 (9), 3165–3168. (8) Lee, D. H.; Lee, W. J.; Lee, W. J.; Kim, S. O.; Kim, Y. H. Theory, Synthesis, and Oxygen Reduction Catalysis of Fe-Porphyrin-like Carbon Nanotube. Phys. Rev. Lett. 2011, 106 (17), 8–11.

Evidence for microbial iron reduction in the methanic sediments of the oligotrophic southeastern Mediterranean continental shelf

Abstract. Dissimilatory iron reduction is probably one of the oldest types of metabolisms that still participates in important biogeochemical cycles, such as those of carbon and sulfur. It is one of the more energetically favorable anaerobic microbial respiration processes and is usually coupled to the oxidation of organic matter. Traditionally this process is thought to be limited to the shallow part of the sedimentary column in most aquatic systems. However, iron reduction has also been observed in the methanic zone of many marine and freshwater sediments, well below its expected zone and occasionally accompanied by decreases in methane, suggesting a link between the iron and the methane cycles. Nevertheless, the mechanistic nature of this link (competition, redox or other) has yet to be established and has not been studied in oligotrophic shallow marine sediments. In this study we present combined geochemical and molecular evidences for microbial iron reduction in the methanic zone of the oligotrophic southeastern (SE) Mediterranean continental shelf. Geochemical porewater profiles indicate iron reduction in two zones, the uppermost part of the sediment, and the deeper zone, in the layer of high methane concentration. Results from a slurry incubation experiment indicate that the deep methanic iron reduction is microbially mediated. The sedimentary profiles of microbial abundance and quantitative PCR (qPCR) of the mcrA gene, together with Spearman correlation between the microbial data and Fe(II) concentrations in the porewater, suggest types of potential microorganisms that may be involved in the iron reduction via several potential pathways: H2 or organic matter oxidation, an active sulfur cycle, or iron-driven anaerobic oxidation of methane. We suggest that significant upward migration of methane in the sedimentary column and its oxidation by sulfate may fuel the microbial activity in the sulfate methane transition zone (SMTZ). The biomass created by this microbial activity can be used by the iron reducers below, in the methanic zone of the sediments of the SE Mediterranean.

Discriminating changes in intracellular NADH/NAD+ levels due to anoxicity and H2 supply in R. eutropha cells using the Frex fluorescence sensor

The hydrogen-oxidizing “Knallgas” bacterium Ralstonia eutropha can thrive in aerobic and anaerobic environments and readily switches between heterotrophic and autotrophic metabolism, making it an attractive host for biotechnological applications including the sustainable H2-driven production of hydrocarbons. The soluble hydrogenase (SH), one out of four different [NiFe]-hydrogenases in R. eutropha, mediates H2 oxidation even in the presence of O2, thus providing an ideal model system for biological hydrogen production and utilization. The SH reversibly couples H2 oxidation with the reduction of NAD+ to NADH, thereby enabling the sustainable regeneration of this biotechnologically important nicotinamide cofactor. Thus, understanding the interaction of the SH with the cellular NADH/NAD+ pool is of high interest. Here, we applied the fluorescent biosensor Frex to measure changes in cytoplasmic [NADH] in R. eutropha cells under different gas supply conditions. The results show that Frex is well-suited to distinguish SH-mediated changes in the cytoplasmic redox status from effects of general anaerobiosis of the respiratory chain. Upon H2 supply, the Frex reporter reveals a robust fluorescence response and allows for monitoring rapid changes in cellular [NADH]. Compared to the Peredox fluorescence reporter, Frex displays a diminished NADH affinity, which prevents the saturation of the sensor under typical bacterial [NADH] levels. Thus, Frex is a valuable reporter for on-line monitoring of the [NADH]/[NAD+] redox state in living cells of R. eutropha and other proteobacteria. Based on these results, strategies for a rational optimization of fluorescent NADH sensors are discussed.

Cysteine SH and Glutamate COOH Contributions to [NiFe] Hydrogenase Proton Transfer Revealed by Highly Sensitive FTIR Spectroscopy

AbstractA [NiFe] hydrogenase (H2ase) is a proton‐coupled electron transfer enzyme that catalyses reversible H2 oxidation; however, its fundamental proton transfer pathway remains unknown. Herein, we observed the protonation of Cys546‐SH and Glu34‐COOH near the Ni–Fe site with high‐sensitivity infrared difference spectra by utilizing Ni‐C‐to‐Ni‐L and Ni‐C‐to‐Ni‐SIa photoconversions. Protonated Cys546‐SH in the Ni‐L state was verified by the observed SH stretching frequency (2505 cm−1), whereas Cys546 was deprotonated in the Ni‐C and Ni‐SIa states. Glu34‐COOH was double H‐bonded in the Ni‐L state, as determined by the COOH stretching frequency (1700 cm−1), and single H‐bonded in the Ni‐C and Ni‐SIa states. Additionally, a stretching mode of an ordered water molecule was observed in the Ni‐L and Ni‐C states. These results elucidate the organized proton transfer pathway during the catalytic reaction of a [NiFe] H2ase, which is regulated by the H‐bond network of Cys546, Glu34, and an ordered water molecule.

Cysteine SH and Glutamate COOH Contributions to [NiFe] Hydrogenase Proton Transfer Revealed by Highly Sensitive FTIR Spectroscopy.

A [NiFe] hydrogenase (H2 ase) is a proton-coupled electron transfer enzyme that catalyses reversible H2 oxidation; however, its fundamental proton transfer pathway remains unknown. Herein, we observed the protonation of Cys546-SH and Glu34-COOH near the Ni-Fe site with high-sensitivity infrared difference spectra by utilizing Ni-C-to-Ni-L and Ni-C-to-Ni-SIa photoconversions. Protonated Cys546-SH in the Ni-L state was verified by the observed SH stretching frequency (2505 cm-1 ), whereas Cys546 was deprotonated in the Ni-C and Ni-SIa states. Glu34-COOH was double H-bonded in the Ni-L state, as determined by the COOH stretching frequency (1700 cm-1 ), and single H-bonded in the Ni-C and Ni-SIa states. Additionally, a stretching mode of an ordered water molecule was observed in the Ni-L and Ni-C states. These results elucidate the organized proton transfer pathway during the catalytic reaction of a [NiFe] H2 ase, which is regulated by the H-bond network of Cys546, Glu34, and an ordered water molecule.

A Hydrophilic Channel Is Involved in Oxidative Inactivation of a [NiFeSe] Hydrogenase

Hydrogenases are metalloenzymes that catalyze the redox conversion between H2 and protons. The so-called [NiFeSe] hydrogenases are highly active for both H2 production and oxidation, but like all hydrogenases, they are inhibited by O2. In the [NiFeSe] enzyme from Desulfovibrio vulgaris Hildenborough this inhibition results from the oxidation of an active site cysteine ligand. We designed mutations that constrict a hydrophilic channel which connects the protein surface to this active site cysteine. Two of the variants show markedly increased tolerance to O2 inactivation, while they retain high catalytic activities in both directions of the reaction, and structural studies confirm that these mutations prevent the oxidation of the cysteine. Our results indicate that the diffusion of O2 or ROS to the active site can occur through a hydrophilic water channel, in contrast to the widely held assumption that only hydrophobic channels are involved in active site inactivation. This provides an original strategy for optimizing the enzyme by protein engineering.

Engineered Enzymes and Bioinspired Catalysts for Energy Conversion

Metalloenzymes play critical roles in the environment by catalyzing redox reactions in biogeochemical cycles. The elucidation of structure–function relationships in these biocatalysts has provided a foundation for the development of bioinspired catalysts for fuel production and small-molecule activation. In this Perspective, we highlight developments in engineered biomolecular and bioinspired catalysts for the reduction of H+, O2, and CO2 and for the oxidation of H2 and H2O. The roles of proton transfers and second-sphere interactions in particular are highlighted.

Probing the geological source and biological fate of hydrogen in Yellowstone hot springs.

Hydrogen (H2 ) is enriched in hot springs and can support microbial primary production. Using a series of geochemical proxies, a model to describe variable H2 concentrations in Yellowstone National Park (YNP) hot springs is presented. Interaction between water and crustal iron minerals yields H2 that partition into the vapour phase during decompressional boiling of ascending hydrothermal fluids. Variable vapour input leads to differences in H2 concentration among springs. Analysis of 50 metagenomes from a variety of YNP springs reveals that genes encoding oxidative hydrogenases are enriched in communities inhabiting springs sourced with vapour-phase gas. Three springs in the Smokejumper (SJ) area of YNP that are sourced with vapour-phase gas and with the most H2 in YNP were examined to determine the fate of H2 . SJ3 had the most H2 , the most 16S rRNA gene templates and the greatest abundance of culturable hydrogenotrophic and autotrophic cells of the three springs. Metagenomics and transcriptomics of SJ3 reveal a diverse community comprised of abundant populations expressing genes involved in H2 oxidation and carbon dioxide fixation. These observations suggest a link between geologic processes that generate and source H2 to hot springs and the distribution of organisms that use H2 to generate energy.

Photocatalytic Oxygenation Reactions Using Water and Dioxygen.

Water (H2 O) is the most environmentally benign reductant and is oxidized to evolve dioxygen (O2 )-the greenest oxidant-in photosystem II. This Minireview focuses on photocatalytic oxygenation of substrates with H2 O as an oxygen source and O2 as an oxidant. Metal complexes can be oxidized by two molecules of one-electron oxidants with H2 O to produce high-valent metal-oxo complexes, which act as active oxidants for oxygenating organic substrates. When an appropriate oxidant is employed for the substrate oxidation, the reduced oxidant can be oxidized by dioxygen to regenerate the oxidant when water and dioxygen are used as an oxygen source and an oxidant, respectively. Photoinduced electron transfer from a substrate (S) to the excited state of complex [(L)MIII ]+ produces a substrate radical cation (S.+ ), accompanied by the regeneration of [(L)MII ]. S.+ then reacts with H2 O to produce an OH adduct radical that is oxidized by [(L)MIII ]+ to yield an oxygenated product (SO), in which the oxygen atom originates from H2 O, accompanied by regeneration of [(L)MII ]. Photocatalytic oxidation of H2 O by O2 to produce H2 O2 is combined with the catalytic oxygenation of substrates with H2 O2 to produce the oxygenated products, in which the oxygen atom originates from O2 at the beginning but later from water. This Minireview provides a promising strategy for oxygenation of substrates by using H2 O as an oxygen source and O2 as the greenest oxidant.

Unravelling the sulphur cycle of marine sediments.

In this invited paper, I give a personal description of my start as a young scientist studying the sulphur cycle of marine sediments. The initial objective was to quantify the processes experimentally using 35 S as a tracer. The results showed an unexpected large contribution of sulphate reduction to the oxidation of sediment organic matter. This implied that sulphate reducing bacteria can perform complete oxidation of their substrates to CO2 . A quantification of sulphate reducing bacteria compared to measured rates of sulphate reduction revealed that the viable counting techniques available at the time vastly underestimated the true cell numbers. Through our introduction of microsensors for oxygen, sulphide and pH, the microbial oxidation of H2 S could be analysed at high spatial resolution. Studies of the large, filamentous sulphur bacteria revealed surprising adaptations to sulphide oxidation based on anaerobic respiration with nitrate stored in vacuoles.