Characterization of hydrogen production by the co-culture of dark-fermentative and photosynthetic bacteria

Apprehension over exhaustion of fossil fuels and global warming, due to increasing amounts of CO₂, has generated a lot of attention for the subject of renewable energy. Renewable energy has an intermittency problem and its output fluctuates depending on natural conditions. Biohydrogen is one of the promising renewable energy sources. Hydrogen produced by photosynthetic bacteria depends on the intensity of light irradiation and also fluctuates with the daily variation of sunlight. The co-culture system of dark-fermentative and photosynthetic bacteria is one solution for reducing the dependency of hydrogen production on light intensity. Because these two strains of bacteria have different processes of hydrogen production, it is possible to combine different outputs so far as the co-culture system works well. This study performed hydrogen production by the co-culture system composed of agar gels embedded with both dark-fermentative bacteria, Clostridium butyricum MIYAIRI, and photosynthetic bacteria, Rhodobacter sphaeroides RV, under a fluctuating light-irradiation. The time-course of hydrogen production was determined for the different conditions of co-culture in the mixing ratios of the two bacterial strains and light-irradiation patterns. As a result, the co-culture system succeeded in producing hydrogen exceeding that in the case of a single culture system and improved its stability against light fluctuation. Hydrogen production by the co-culture system would be applicable to the reduction of intermittency in renewable energies.     

Spiral tubular bioreactors for hydrogen production by photosynthetic microorganisms

Spiral tubular bioreactors were constructed out of transparent PVC tubing for H2 production applications. Both a cyanobacterialAnabaena variabilis mutant that lacks uptake hydrogenase activity and the photosynthetic bacteriumRhodobacter sp. CBS were tested in the bioreactors. Continuous H₂ photoproduction at an average rate of 19 mL min^-2.h^-1 was observed using theA. variabilis mutant under an air atmosphere (without argon sparging or application of a partial vacuum). The cyanobacterial photobioreactor was run continuously for over one month with an average efficiency of light energy conversion to H₂ of 1.4%. Another H₂-producing approach employed a unique type of activity found in a strain of photosynthetic bacteria that shifts CO (and H₂O) into H₂ (and CO₂) in darkness. Continuous dark H₂ production byRhodobacter sp. CBS from CO (in anticipation of using synthesis gas as the future substrate) at rates up to 140 mL . g cdw^-1 . h^-1 was observed in a bubble-train bioreactor for more than 10 d.

     

The time-series evaluation of biohydrogen production by photosynthetic bacteria under fluctuating illumination pattern

In recent years, world climate change and global warming have been big issues. One of the solutions is to use renewable energies; however, renewable energies have an intermittent nature. In the case of photovoltaic arrays, the intermittency is mainly caused by fluctuating irradiation from sunlight due to clouds. In this study, biohydrogen production from photosynthetic bacteria was focused for the use of fluctuating sunlight irradiation. Previous researches have revealed some characteristics of biohydrogen production, and these results enable one to expect that photosynthetic bacteria have fluctuating light tolerance of biohydrogen production, in which the bacteria are able to produce biohydrogen continuously under fluctuating light irradiation. There have been quite a few studies to evaluate time-course changes of biohydrogen production under fluctuating irradiation, and therefore time-course evaluations have been performed. A 10-min light/dark illumination pattern was set for the fluctuating irradiation and the magnitude of the fluctuation was used to evaluate the fluctuation of the hydrogen production rate and irradiation light. The results indicated that the fluctuation was 0.22 times smaller through the photosynthetic bacteria. The results of this study indicate that photosynthetic bacteria have fluctuating light tolerance. Biohydrogen production, having fluctuating light tolerance, would be useful for realistic use of sunlight energy as renewable energy.     

The heliobacteria, a new group of photosynthetic bacteria

A review is given of the photosynthetic properties of the heliobacteria, a new group of photosynthetic bacteria, discovered only 14 years ago. These bacteria contain a “new” pigment, bacteriochlorophyll g, and they have a relatively simple pigment system, consisting of a core-reaction center complex only. Like the green sulfur bacteria, they have a Photosystem I-type reaction center, with a chlorophyll a derivative as primary electron acceptor. Because of the absence of an extensive peripheral antenna system, the reaction center processes in these bacteria are much easier to study than those in the green sulfur bacteria.     

Theoretical calculations of RYDMR effects in photosynthetic bacteria

A detailed quantitative analysis of low-field RYDMR (reaction yield detected magnetic resonance) spectra of the photosynthetic bacterium Rhodobacter sphaeroides is presented. Reaction centres, in which the first stable electron acceptor (a quinone) had been pre-reduced, show pronounced temperature dependence around 280 K. This is interpreted in terms of a change in the separation of the constituents of the primary radical pair formed by photoinduced electron transfer.     

A C alpha-H...O hydrogen bond in a membrane protein is not stabilizing

Hydrogen bonds involving a carbon donor are very common in protein structures, and energy calculations suggest that Calpha-H...O hydrogen bonds could be about one-half the strength of traditional hydrogen bonds. It has therefore been proposed that these nontraditional hydrogen bonds could be a significant factor in stabilizing proteins, particularly membrane proteins as there is a low dielectric and no competition from water in the bilayer core. Nevertheless, this proposition has never been tested experimentally. Here, we report an experimental test of the significance of Calpha-H...O bonds for protein stability. Thr24 in bacteriorhodopsin, which makes an interhelical Calpha-H...O hydrogen bond to the Calpha of Ala51, was changed to Ala, Val, and Ser, and the thermodynamic stability of the mutants was measured. None of the mutants had significantly reduced stability. In fact, T24A was more stable than the wild-type protein by 0.6 kcal/mol. Crystal structures were determined for each of the mutants, and, while some structural changes were seen for T24S and T24V, T24A showed essentially no apparent structural alteration that could account for the increased stability. Thus, Thr24 appears to destabilize the protein rather than stabilize. Our results suggest that Calpha-H...O bonds are not a major contributor to protein stability.     

The role of an interface in stabilizing reaction intermediates for hydrogen evolution in aprotic electrolytes

By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electro-reduction reactions of HF, water and methanesulfonic acid (MSA) on the single crystal (111) facets of Au, Pt, Ir and Cu in organic aprotic electrolytes, 1 M LiPF₆ in EC/EMC 3:7W (LP57), the aprotic electrolyte commonly used in Li-ion batteries, 1 M LiClO₄ in EC/EMC 3:7W and 0.2 M TBAPF₆ in 3 : 7 EC/EMC. In our previous work, we have established that LiF formation, accompanied by H₂ evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li⁺ adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having adsorbed Li⁺. Li⁺ is needed to facilitate the dissociation of HF which is the source of protons. Further experiments on other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F⁻ or HO⁻, a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy.

By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electroreduction reactions of HF, water and methanesulfonic acid on the single crystal (111) facets of Au, Pt, Ir and Cu in a variety of aprotic electrolytes.     

Steam-reforming of ethanol for hydrogen production

Production of hydrogen by steam-reforming of ethanol has been performed using different catalytic systems. The present review focuses on various catalyst systems used for this purpose. The activity of catalysts depends on several factors such as the nature of the active metal catalyst and the catalyst support, the precursor used, the method adopted for catalyst preparation, and the presence of promoters as well as reaction conditions like the water-to-ethanol molar ratio, temperature, and space velocity. Among the active metals used to date for hydrogen production from ethanol, promoted-Ni is found to be a suitable choice in terms of the activity of the resulting catalyst. Cu is the most commonly used promoter with nickel-based catalysts to overcome the inactivity of nickel in the water-gas shift reaction. γ-Al₂O₃ support has been preferred by many researchers because of its ability to withstand reaction conditions. However, γ-Al₂O₃, being acidic, possesses the disadvantage of favouring ethanol dehydration to ethylene which is considered to be a source of carbon deposit found on the catalyst. To overcome this difficulty and to obtain the long-term catalyst stability, basic oxide supports such as CeO₂, MgO, La₂O₃, etc. are mixed with alumina which neutralises the acidic sites. Most of the catalysts which can provide higher ethanol conversion and hydrogen selectivity were prepared by a combination of impregnation method and sol-gel method. High temperature and high water-to-ethanol molar ratio are two important factors in increasing the ethanol conversion and hydrogen selectivity, whereas an increase in pressure can adversely affect hydrogen production.     

Imino- and aminomethylphenylboronic acids: stabilizing effect of hydrogen bonds

ortho-Iminomethylphenylboronic acids were synthesized from the reaction of 2-formyl–phenylboronic acid with primary aromatic amines. Reduction of these compounds yielded the corresponding aminomethylphenylboronic acids. For both types of the compounds, the crystal structure was determined by single crystal X-ray diffraction method. Hydrogen-bonded dimers with an additional intramolecular B–O–H…N hydrogen bond have been observed. Calculations at MP2/6–31+G** level proved that the most stable form is that with the above-mentioned intramolecular hydrogen bond while the form with dative N→B bond is less favoured. Since the calculated energy difference is small, the competition between possible forms was analyzed in terms of substituent effect stabilization energy (SESE). In the case of p-iminomethylphenylboronic acid, both hydroxyl groups are engaged in intermolecular O–H…O interactions resulting in a supramolecular ribbon motif.     

Energy-saving seawater electrolysis for hydrogen production

The change in the polarization potentials of anode and cathode due to pH change on electrode surfaces during galvanostatic polarization was examined in 0.5 M NaCl solutions of different pH. On the basis of these results, feeding of the anolyte after oxygen evolution to the cathode compartment for hydrogen production was examined for energy-saving seawater electrolysis. This was assumed to prevent the occurrence of a large pH difference on cathode and anode in electrolysis of neutral solution if sufficient H⁺ is permeated through the membrane. The cell performance was examined using Nafion 115 or Selemion HSF membranes for separation of anode and cathode compartments. The permeation fraction of H⁺ with Nafion 115 was 45–65% in 0.5 M NaCl and was about 90% in 0.25 M Na₂SO₄. These values were smaller than 97% necessary for prevention of the occurrence of pH difference on cathode and anode. The permeation fraction of H⁺ with Selemion HSF became more than 97% during electrolysis of 0.025 M Na₂SO₄, and the cell voltage was kept at low values. These results indicate the effectiveness of our seawater feeding system if the 97% H⁺ permeation fraction through the membrane is attained. Contribution to the Fall Meeting of the European Materials Research Society, Symposium D: 9th International Symposium on Electrochemical/Chemical Reactivity of Metastable, Warsaw, 17th-21st September, 2007.     

Nanostructured materials for photocatalytic hydrogen production

Photocatalytic hydrogen (H₂) production represents a very promising but challenging contribution to a clean, sustainable and renewable energy system. The photocatalyst material plays a key role in photocatalytic H₂ production, and it has proven difficult to obtain corrosion resistant, chemically stable, visible light harvesting and highly efficient photocatalysts, which have their band edges matching the O₂ and H₂ production levels. Nanoscience and nanotechnology are opening a new vista in the development of highly active, nanostructured photocatalysts with large surface areas for optimized light absorption, minimized distances (or times) for charge-carrier transport, and further favorable properties. Our focus here is on recently developed nanostructured photocatalysts. In particular, the particle size, chemical composition (including dopants), microstructure, crystal phase, morphology, surface modification, bandgap and flat-band potential of the nanophotocatalysts have shown a visible effect on photocatalytic H₂ production rates, which may be further increased by adding sensitizers, cocatalysts or scavengers. Finally, potential directions required to push this research field a step further are highlighted.     

Photocatalytic water splitting for hydrogen production

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Thermodynamic analysis of the copper production step in a copper–chlorine cycle for hydrogen production

The hybrid copper–chlorine (Cu–Cl) thermo/electrochemical cycle for decomposing water into its constituents is a novel method for hydrogen production. The process involves a series of closed-loop chemical reactions. The cycle is assumed driven in an environmentally benign manner using nuclear energy. The cycle involves five steps of which three are thermally driven chemical reactions and one has an electrochemical reaction. In the present study, the electrochemical reaction, copper (Cu) production step, is described with its operational and environmental conditions, and analyzed thermodynamically. Various parametric studies are carried out on energetic and exergetic aspects of the step, considering variable reaction and reference-environment temperatures. At a reaction temperature of 45 °C, the reaction heat of the Cu production step is 140,450 kJ/kmol H₂. At a constant reaction temperature of 45 °C, the exergy destruction of the step varies between 50 kJ/kmol H₂ and 7000 kJ/kmol H₂ when the reference-environment temperature increases from 0 °C to 30 °C. At a reaction temperature of 45 °C and a reference-environment temperature of 25 °C, the exergy efficiency of this step is 99% and decreases with increasing reference-environment and/or reaction temperatures.     

Hydrogen sulfide as a source of hydrogen production

Potentialities and perspectives of using the known processes of hydrogen sulfide decomposition (thermal, plasmochemical, electrochemical, and photochemical) to produce hydrogen are examined. The results of theoretical and experimental studies of hydrogen sulfide dissociation on the surface of single crystals are presented. The data on the low-temperature decomposition of H₂S on the sulfide and metal catalysts are discussed. The electronic structure of diatomic sulfur and thermodynamics of its formation in the processes of H₂S decomposition are considered. The decomposition of hydrogen sulfide on the heterogeneous catalysts placed under the solvent layer is shown to be promising. The mechanism of assimilation of hydrogen sulfide by colorless sulfur bacteria is proposed.     

Evaluation of the complexity of charge recombination kinetics in photosynthetic bacteria

Abstract Improvements in instrumentation and methodology have allowed us to collect data of high signal to noise and reliability on the kinetics of recovery of both light-induced absorbance changes and ESR signals at 95 K. The results obtained by the two methods are identical and can not be fit with a single exponential curve. The decay kinetics can be fit well with three exponential components which represent 85, 9 and 6% of the total change with rate constants of 29 s⁻¹, 69 s⁻¹ and 2.3 s⁻¹, respectively. An interesting effect by molecular oxygen on the relaxation time of the donor cation radical was found by ESR measurements at low temperatures and higher microwave power. This interaction with oxygen could be blocked by addition of small amounts (e.g. 0.05%) of organic solvents such as ethanol. A variety of systems were examined including R. rubrum whole cells and chromatophores prepared from R. rubrum and Rps. sphaeroides. R. rubrum chromatophore samples were examined at high and low light intensities, at pH values from 6 to 10, in the presence and absence of air and after equilibration in D₂O media. In all cases, the same decay kinetics were observed. It seems possible that the observed complex decay may be a characteristic of phototraps of all photosynthetic material and reflect fundamental structural and functional features yet to be uncovered.     

Optically detected zero-field triplet state magnetic resonance in photosynthetic bacteria

Triplet state transitions of the photosynthetic bacteria Rhodospirillum Rubrum, Rhodopseudomonas Spheroides and Chromatium Vinosum in chemically reduced preparations have been observed by zero-field optical detection of magnetic resonance at 2 K. For each bacterial preparation two sharp, structureless, zero-field EPR transitions were observed as microwave-induced decreases in the fluorescence intensity of the frozen cellular preparations. The depopulating rate constants for the spin sublevels of the triplet states observed in R Rubrum and R Spheroides were also measured. The similarities of the triplet state frequencies, spectral features and intersystem crossing rates suggest a common structure for the reaction centers in the photosynthetic bacteria.     

Microbial control of hydrogen sulfide production in a porous medium

Applied Biochemistry and Biotechnology - The ability of a sulfide- and glutaraldehyde-tolerant strain ofThiobacillus denitrificans (strain F) to control sulfide production in an experimental system...