Relationship between molecular hydrogen production,proton transport and the F0F1-ATPase activity in Rhodobacter sphaeroides strains from mineral springs

This article aims to study hydrogen production and proton transport in two strains of purple non-sulfur bacterium Rhodobacter sphaeroides isolated from mineral springs of Armenia. This bacterium is able to grow and produce molecular hydrogen (H₂) in anaerobic conditions upon illumination. Along with H₂ production, a marked decrease in redox potential and the alkalization of the medium have been observed; the latter might be the evidence of proton influx. H₂ production and alkalization of the medium by whole cells both are suppressed by the F₀F₁-ATPase inhibitors – N,N′-dicyclohexylcarbodiimide (DCCD), sodium azide (NaN₃) and protonophore – carbonyl cyanide m-chlorophenylhydrazone (CCCP). Membrane vesicles of two strains of R. sphaeroides demonstrate ATPase activity, inhibited by DCCD and NaN₃, but not by CCCP. These results indicate a relationship between H₂ production, proton transport and the F₀F₁-ATPase activity that might be a pathway to regulate bacterial activity under anaerobic conditions.     

Concentration-dependent effects of metronidazole,inhibiting nitrogenase,on hydrogen photoproduction and proton-translocating ATPase activity of Rhodobacter sphaeroides

Rhodobacter sphaeroides MDC6522 is able to produce hydrogen (H₂) during photofermentation. Metronidazole (2-methyl-5-nitroimidazole-1-ethanol) has been shown to affect bacterial growth under anaerobic nitrogen-limited conditions in a concentration-dependent manner (within the range of 0.1–2 mM) by increasing lag phase duration and decreasing growth rate. The addition of metronidazole into the growth medium resulted in a delayed decrease of redox potential (E_h): by the addition of 0.1 mM metronidazole E_h decreased to −590 ± 25 mV, whereas in the presence of 2 mM E_h drop down was to −175 ± 15 mV. H₂ production during R. sphaeroides growth disappeared in the presence of metronidazole. By addition of 0.5 mM metronidazole H₂ yield was ∼8 fold lower in comparison with control; whereas the bacterium was unable to produce H₂ in the presence of 1–2 mM. The effects of metronidazole on nitrogenase-dependent H₂ production by R. sphaeroides might be explained by change of general photosynthetic electron transport with metronidazole as an alternative electron acceptor instead of nitrogenase. ATPase activity of membrane vesicles was determined with and without N,N'-dicyclohexylcarbodiimide (DCCD), an inhibitor of the F₀F₁-ATPase. It was revealed that DCCD-inhibited ATPase activity increased in the presence of metronidazole. It is possible that this effect may be resulted in either by direct affect of metronidazole on the F₀F₁-ATPase, or by its effect on E_h regulating ATPase activity.     

Growth characteristics and hydrogen production by Rhodobacter sphaeroides using various amino acids as nitrogen sources and their combinations with carbon sources

Some amino acids (alanine, asparagine, glutamate, glycine, proline, and tyrosine) were used as nitrogen sources in combination with carbon sources (succinate and malate) to study growth properties and H₂ production by purple non-sulfur bacterium Rhodobacter sphaeroides strains A-10 and D-3. Both strains produced H₂ in succinate–glutamate and malate–glutamate media. Succinate was a better carbon source than malate. In comparison with strain D-3, strain A-10 was able to utilize proline, alanine or tyrosine as nitrogen sources in succinate medium and to produce H₂. Both strains were unable to produce H₂ in the presence of asparagine or glycine as nitrogen sources. N,N′-dicyclohexylcarbodiimide, the F₀F₁-ATPase inhibitor, led to marked inhibition of H₂ production activity of R. sphaeroides. The results suggest that the R. sphaeroides cells growth can be achieved by the use of a large diversity of substrates but only some of them can increase the H₂ production rate.     

Enhancement of Escherichia coli bacterial biomass and hydrogen production by some heavy metal ions and their mixtures during glycerol vs glucose fermentation at a relatively wide range of pH

Escherichia coli growth and H₂ production were followed in the presence of heavy metal ions and their mixtures during glycerol or glucose fermentation at pH 5.5–7.5. Ni²⁺ (50 μM) with Fe²⁺ (50 μM) but not sole metals stimulated bacterial biomass during glycerol fermentation at pH 6.5. Ni²⁺+Fe³⁺ (50 μM), Ni² +Fe³⁺+Mo⁶⁺ (20 μM) and Fe³⁺+Mo⁶⁺ (20 μM) but not sole metals enhanced up to 3-fold H₂ yield but Cu⁺ or Cu²⁺ (100 μM) inhibited it. At pH 7.5 stimulating effect on biomass was observed by Ni²⁺+Fe²⁺+Mo⁶⁺. H₂ production was enhanced 2.7 fold particularly by Ni²⁺+Fe³⁺+Mo⁶⁺ at the late stationary growth phase. Whereas at pH 5.5 increased biomass was when Fe²⁺+Mo⁶⁺ or Mo⁶⁺ were added. H₂ yield was decreased compared with that at pH 6.5, but metal ions again enhanced it. During glucose fermentation at pH 6.5 biomass was increased by the mixtures of metal ions, and 1.2 fold increased H₂ yield was observed. At pH 7.5 Ni²⁺+Fe²⁺ increased biomass but Cu⁺ or Cu²⁺ had suppressing effect; Fe³⁺+Mo⁶⁺ stimulated H₂ production. At pH 5.5 biomass also was raised by Ni²⁺+Fe²⁺+Mo⁶⁺; H₂ yield was increased upon Mo⁶⁺ and Mo⁶⁺+Fe²⁺ or Mo⁶⁺+Fe³⁺ additions. The results point out the importance of Ni²⁺, Fe²⁺, Fe³⁺ and Mo⁶⁺ and some of their combinations for E. coli bacterial growth and H₂ production mostly during glycerol but not glucose fermentation and at acidic conditions (pH 5.5 and 6.5). They can be used for optimizing fermentation processes on glycerol, controlling bacterial biomass and developing H₂ production biotechnology.     

Effect of changes in the level of light harvesting complexes of Rhodobacter sphaeroides on the photoheterotrophic production of hydrogen

Increase in levels of photosynthetic spectral complexes by maintaining the plasmids harboring DNA encoding puhA, pufBA, or pucBAC in trans in Rhodobacter sphaeroides resulted in decrease of the photoheterotrophic production of H₂. However, removal of B875 or B800–850 light-harvesting (LH) complexes affected H₂ production differently. Lack of B875 complex following in-frame deletion of pufBA (mutant PUF1) not only slowed photoheterotrophic growth but also decreased H₂ production, indicative of the essential requirement of the complex for LH process. However, the pucBA-deleted mutant, PUC1 lacking of B800–850 complex, increased H₂ production in comparison with its parental cell by approximately twofold, given irradiated with light (10 W/m²) saturating the growth of wild type. The H₂ production of PUC1 did not increase in proportion to the light intensity, which is also observed with wild type. Thus, we suggest that light is not limited for the H₂ production of the cells under the experimental conditions employed in this work, but the cellular energy to be used for the formation of B800–850 complex may flow into the metabolism leading to the H₂ production in PUC1.     

Electrochemical properties of LiFe0.9Mn0.1PO4/Fe2P cathode material by mechanical alloying

LiFePO₄, olivine-type LiFe_0.9Mn_0.1PO₄/Fe₂P composite was synthesized by mechanical alloying of carbon (acetylene back), M₂O₃ (M = Fe, Mn) and LiOH·H₂O for 2 h followed by a short-time firing at 900 °C for only 30 min. By varying the carbon excess different amounts of Fe₂P second phase was achieved. The short firing time prevented grain growth, improving the high-rate charge/discharge capacity. The electrochemical performance was tested at various C/x-rate. The discharge capacity at 1C rate was increased up to 120 mAh g⁻¹ for the LiFe_0.9Mn_0.1PO₄/Fe₂P composite, while that of the unsubstituted LiFePO₄/Fe₂P and LiFePO₄ showed only 110 and 60 mAh g⁻¹, respectively. Electronic conductivity and ionic diffusion constant were measured. The LiFe_0.9Mn_0.1PO₄/Fe₂P composite showed higher conductivity and the highest diffusion coefficient (3.90 × 10⁻¹⁴ cm² s⁻¹). Thus the improvement of the electrochemical performance can be attributed to (1) higher electronic conductivity by the formation of conductive Fe₂P together with (2) an increase of Li⁺ ion mobility obtained by the substitution of Mn²⁺ for Fe²⁺.     

Effect of H2O2 on Nafion properties and conductivity at fuel cell conditions

During PEM fuel cell operation, formation of H₂O₂ and material corrosion occurs, generating trace amounts of metal cations (i.e., Fe²⁺, Pt²⁺) and subsequently initiating the deterioration of cell components and, in particular, PFSA membranes (e.g., Nafion). However, most previous studies of this have been performed using conditions not relevant to fuel cell environments, and very few investigations have studied the effect of Nafion decomposition on conductivity, one of the most crucial factors governing PEMFC performance. In this study, a quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to fuel cell environments (30-100%RH and 80 °C) was performed. Nafion membranes were pre-ion-exchanged with small amounts of Fe²⁺ ions prior to H₂O₂ exposure. The degradation degree (defined as loss of ion-exchange capacity, weight, and fluoride content), water uptake, and conductivity of H₂O₂-exposed membranes were found to strongly depend on Fe content and H₂O₂ treatment time. SEM cross-sections showed that the degradation initially took place in the center of the membrane, while FTIR analysis revealed that Nafion degradation preferentially proceeds at the sulfonic end group and at the ether linkage located in the pendant side chain and that the H-bond of water is weakened after prolonged H₂O₂ exposure.     

Culture conditions affecting H2 production by phototrophic bacterium Rhodobacter sphaeroides KD131

The effect of culture conditions on photo-H₂ production was investigated using the photosynthetic bacterium Rhodobacter sphaeroides KD131. When the initial cell concentrations were either below or above a threshold of 0.56 g-dcw/L, the H₂ production decreased due to an imbalance between the biomass and the substrate. Malate- and succinate-fed cultures exhibited the highest substrate conversion to H₂ production, whereas more than 85% of the substrate was utilized for cell growth in acetate- and butyrate-fed cultures. Compared with (NH₄)₂SO₄, glutamate as a nitrogen source was more appropriate for the initial H₂ production, but inhibited H₂ evolution during extended cultivation due to released NH₄⁺ ion. Even though the KD131 strain grew well under slightly acidic conditions, the pH value should be maintained in a neutral range in order to enhance H₂ production. The highest H₂ yield of 3.65 mol-H₂/mol-succinate was achieved when the KD131 strain grew in the succinate–glutamate medium with an initial cell concentration of 0.56 g-dcw/L and the pH level controlled to 7.5.     

Response of H2 production and Hox-hydrogenase activity to external factors in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803

The effects of external factors on both H₂ production and bidirectional Hox-hydrogenase activity were examined in the non-N₂-fixing cyanobacterium Synechocystis PCC 6803. Exogenous glucose and increased osmolality both enhanced H₂ production with optimal production observed at 0.4% and 20 mosmol kg⁻¹, respectively. Anaerobic condition for 24 h induced significant higher H₂ase activity with cells in BG11₀ showing highest activities. Increasing the pH resulted in an increased Hox-hydrogenase activity with an optimum at pH 7.5. The Hox-hydrogenase activity gradually increased with increasing temperature from 30 ^○C to 60 ^○C with the highest activity observed at 70 ^○C. A low concentration at 100 μM of either DTT or β-mercaptoethanol resulted in a minor stimulation of H₂ production. β-Mercaptoethanol added to nitrogen- and sulfur-deprived cells stimulated H₂ production significantly. The highest Hox-hydrogenase activity was observed in cells in BG11₀-S-deprived condition and 750 μM β-mercaptoethanol measured at a temperature of 70 °C; 14.32 μmol H₂ mg chl a⁻¹ min⁻¹.     

Photo-catalytic hydrogen production over Fe2O3 based catalysts

The hydrogen photo-evolution was successfully achieved in aqueous (Fe_1−xCr_x)₂O₃ suspensions (0 ≤ x ≤ 1). The solid solution has been prepared by incipient wetness impregnation and characterized by X-ray diffraction, BET, transport properties and photo-electrochemistry. The oxides crystallize in the corundum structure, they exhibit n-type conductivity with activation energy of ∼0.1 eV and the conduction occurs via adiabatic polaron hops. The characterization of the band edges has been studied by the Mott Schottky plots. The onset potential of the photo-current is ∼0.2 V cathodic with respect to the flat band potential, implying a small existence of surface states within the gap region. The absorption of visible light promotes electrons into (Fe_1−xCr_x)₂O₃-CB with a potential (∼−0.5 V_SCE) sufficient to reduce water into hydrogen. As expected, the quantum yield increases with decreasing the electro affinity through the substitution of iron by the more electropositive chromium which increases the band bending at the interface and favours the charge separation. The generated photo-voltage was sufficient to promote simultaneously H₂O reduction and SO₃²⁻ oxidation in the energetically downhill reaction (H₂O + SO₃²⁻ → H₂ + SO₄²⁻, ΔG = −17.68 kJ mol⁻¹). The best activity occurs over Fe_1.2Cr_0.8O₃ in SO₃²⁻ (0.1 M) solution with H₂ liberation rate of 21.7 μmol g⁻¹ min⁻¹ and a quantum yield 0.06% under polychromatic light. Over time, a pronounced deceleration occurs, due to the competitive reduction of the end product S₂O₆²⁻.     

Studies on B sites in Fe-doped LaNiO3 perovskite for SCR of NOx with H2

A series of LaNi_1−xFe_xO₃ (x = 0.0, 0.2, 0.4, 0.7, and 1.0) perovskites were synthesized and characterized by X-ray diffraction (XRD), N₂ physisorption, scanning electron microscopy (SEM), H₂-temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The perovskites were investigated for selective catalytic reduction of NO_x by hydrogen (H₂-SCR). It is shown that Fe addition into LaNiO₃ leads to a promoted efficiency of NO_x removal, as well as a high stability of perovskite structure. Moreover, easy reduction of Ni³⁺ to Ni²⁺ with the aid of appropriate Fe component mainly accounts for the enhanced activity. Meanwhile, deactivation of the sulfated catalysts is due to that sulfates mainly deposit on active Ni component while doping of Fe can protect Ni to some extent at the expense of partial sulfation.     

Synthesis and electrochemical properties of Li1−xFe0.8Ni0.2O2–LixMnO2 (Mn/(Fe + Ni + Mn) = 0.8) material

A new type of Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ (Mn/(Fe + Ni + Mn) = 0.8) material was synthesized at 350 °C in air atmosphere using a solid-state reaction. The material had an XRD pattern that closely resembled that of the original Li_1−xFeO₂–Li_xMnO₂ (Mn/(Fe + Mn) = 0.8) with much reduced impurity peaks. The Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell showed a high initial discharge capacity above 192 mAh g⁻¹, which was higher than that of the parent Li/Li_1−xFeO₂–Li_xMnO₂ (186 mAh g⁻¹). We expected that the increase of initial discharge capacity and the change of shape of discharge curve for the Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell is the result from the redox reaction from Ni²⁺ to Ni³⁺ during charge/discharge process. This cell exhibited not only a typical voltage plateau in the 2.8 V region, but also an excellent cycle retention rate (96%) up to 45 cycles.     

Evaluation of hydrogen production by Rhodobacter sphaeroides O.U.001 and its hupSL deficient mutant using acetate and malate as carbon sources

Rhodobacter sphaeroides O.U.001 is one of the candidates for photobiological hydrogen production among purple non-sulfur bacteria. Hydrogen is produced by Mo-nitrogenase from organic acids such as malate or lactate. A hupSL in frame deletion mutant strain was constructed without using any antibiotic resistance gene. The hydrogen production potential of the R. sphaeroides O.U.001 and its newly constructed hupSL deleted mutant strain in acetate media was evaluated and compared with malate containing media. The hupSL⁻R. sphaeroides produced 2.42 l H₂/l culture and 0.25 l H₂/l culture in 15 mM malate and 30 mM acetate containing media, respectively, as compared to the wild type cells which evolved 1.97 l H₂/l culture and 0.21 l H₂/l culture in malate and acetate containing media, correspondingly. According to the results, hupSL⁻R. sphaeroides is a better hydrogen producer but acetate alone does not seem to be an efficient carbon source for photoheterotrophic H₂ production by R. sphaeroides.     

Ni (II) and Mg (II) ions as factors enhancing biohydrogen production by Rhodobacter sphaeroides from mineral springs

Various metal ions play a key role in biohydrogen (H₂) production by phototrophic bacteria through incorporation into or stimulating the responsible enzymes and/or related pathways. The Ni (II) and Mg (II) ions effects on growth and H₂ production by Rhodobacter sphaeroides strain MDC6521 isolated from mineral springs in Armenia were established. The highest growth specific rate was obtained with 4–6 μM Ni²⁺ and 5 mM Mg²⁺. pH of the growth medium changed from 7.0 to 9.2–9.4 during the bacterial growth up to 72 h in spite of Ni²⁺ added but pH increased in different manner with Mg²⁺. In the presence of 2–4 μM Ni²⁺ external oxidation-reduction potential (ORP) decreased to more negative values (−800 ± 15 mV). This decrease of ORP indicated ∼2.7-fold enhanced H₂ yield (9.80 mmol L⁻¹) with Ni²⁺ compared with the control (without Ni²⁺). The H₂ yield determined in the medium with Mg²⁺ was ∼2.2 fold higher than that with 1 mM Mg²⁺. These results reveal new regulatory ways to improve H₂ production by R. sphaeroides those were depending on Ni²⁺ and Mg²⁺ of different concentrations.     

Effect of Co doping on the electrochemical properties of Sr2Fe1.5Mo0.5O6 electrode for solid oxide fuel cell

Co is doped to Sr₂Fe_1.5Mo_0.5O₆ to enhance its electrochemical activity as the cathode for intermediate-temperature solid oxide fuel cells. Pure cubic perovskites of Sr₂Fe_1.5−xCo_xMo_0.5O₆ (SF_1.5−xC_xM, x = 0, 0.5, 1) are synthesized using a glycine-nitrate combustion progress. The average thermal expansion coefficient varies from 15.8 to 19.8 × 10⁻⁶ K⁻¹. The electrical conductivity increases while its activation energy decreases with increasing Co content. X-ray photoelectron spectroscopy analysis demonstrates mixed valences of Fe, Co and Mo, suggesting small polaron hopping mechanism. Electrical conductivity relaxation (ECR) measurement shows that the surface exchange coefficient increases about two orders of magnitude when the content increases from x = 0 to x = 1.0, i.e. from 2.55 × 10⁻⁵ to 2.20 × 10⁻³ cm s⁻¹ at 750 °C. ECR also exhibits that chemical diffusion coefficient increases with Co content. Density Functional Theory calculation demonstrates that oxygen vacancy formation energy decreases with Co content, suggesting high oxygen vacancy concentration at high Co content. Impedance spectroscopy on symmetric cells consisting of SF_1.5−xC_xM electrodes and La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ electrolytes shows that Co doping is very effective in reducing the interfacial polarization resistance, from 0.105 Ω cm² to 0.056 Ω cm² at 750 °C. These results suggest that Co doping into Sr₂Fe_1.5Mo_0.5O₆ can substantially improve its electrochemical performance.     

Photocatalytic hydrogen production over CuGa2−xFexO4 spinel

Recovery of hydrogen from industrial H₂S waste using spinel photocatalyst was studied. Spinel metal oxide photocatalysts (CuGa_2−xFe_xO₄ for x = 0.8, 0.6 and 0.4) were synthesized by ceramic route. They were loaded with 0.5 and 1 wt% noble metal oxide, RuO_2. Their XRD pattern revealed a single phase cubic spinel crystalline structure for all the catalysts. SEM displayed small size cubic particles with the particle size decreasing with the decrease in iron content. 1 wt% RuO₂ loaded CuGa_1.6Fe_0.4O₄ decomposed H₂S in aqueous 0.5 M KOH solution under visible light (λ ≥ 420 nm) irradiation and generated H₂ to the tune of 10,045 μmol/h, giving rise to a high quantum efficiency of 21% at 510 nm.     

Effect of Y substitution for Nb in Li5La3Nb2O12 on Li ion conductivity of garnet-type solid electrolytes

We report the effect of Y substitution for Nb on Li ion conductivity in the well-known garnet-type Li₅La₃Nb₂O₁₂. Garnet-type Li₅La₃Nb_2−xY_xO_12−δ (0 ≤ x ≤ 1) was prepared by ceramic method using the high purity metal oxides and salts. Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), ⁷Li nuclear magnetic resonance (Li NMR) and AC impedance spectroscopy were employed for characterization. PXRD showed formation of single-phase cubic garnet-like structure for x up to 0.25 and above x = 0.25 showed impurity in addition to the garnet-type phases. The cubic lattice constant increases with increasing Y content up to x = 0.25 in Li₅La₃Nb₂−_xY_xO12−δ and is consistent with expected ionic radius trend. ⁷Li MAS NMR showed single peak, which could be attributed to fast migration of ions between various sites in the garnet structure, close to chemical shift 0 ppm with respect to solid LiCl and which confirmed that Li ions are distributed at an average octahedral coordination in Li₅La₃Nb₂−_xY_xO₁₂−_δ. Y-doped compounds showed comparable electrical conductivity to that of the parent compound Li₅La₃Nb₂O₁₂. The x = 0.1 member of Li₅La₃Nb₂−_xY_xO₁₂−_δ showed total (bulk + grain-boundary) ionic conductivity of 1.44 × 10⁻⁵ Scm⁻¹ at 23 °C in air.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.