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⁻.     

The influence of hydrogen production on the formation of metabolic pathways and regulation of ΔpH in Escherichia coli

Hydrogen (H₂) metabolism in Escherichia coli occurs via reversible membrane-associated hydrogenase enzymes (Hyd). Hyd-3 and Hyd-4 with formate dehydrogenase H (FDH-H) form formate hydrogen lyase complexes. The changes of metabolic pathways and ΔpH (pH_in-pH_ex) regulation during fermentation of glucose, glycerol and formate in non H₂-producing hypF (lack of all Hyds) and fdhF (lack of FDH-H) mutants at pH 7.5 were investigated. It was shown that specific growth rate was higher by ～23% in hypF and fdhF, compared to wild type (wt), suggesting the negative effect of H₂ on bacterial growth. Moreover, it was shown that H₂ generation did not have a vital role in glucose and glycerol utilization rate at 0–72 h. The utilization of external formate was detected in wt (～2.6 mM) and hypF (～0.68 mM), but not in fdhF, due to the absence of enzyme responsible for formate metabolism. Nevertheless, the changes in ΔpH were not evident at 3 h. The ratio of generated end-products and regulation of ΔpH at late log (6 h) and exponential phase (24, 72 h) were various in hypF and fdhF due to formate disproportionation in hypF and proton generation, therewith absence of H₂ generation. Taken together it can be concluded that bacteria regulate generation of fermentation end-products via balancing the concentration of acids and ethanol to maintain ΔpH and redox potential values. The results obtained are important for development and regulation of H₂ production technology when applying mixed carbon sources.     

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.     

Evaluation of continuous biohydrogen production from enzymatically treated cornstalk hydrolysate

Enzymatically treated cornstalk hydrolysate was tested as substrate for H₂ production by Thermoanaerobacterium thermosaccharolyticum W16 in a continuous stirred tank reactor. The performance of strain W16 to ferment the main components of hydrolysate, mixture of glucose and xylose, in continuous culture was conducted at first, and then T. thermosaccharolyticum W16 was evaluated to ferment fully enzymatically hydrolysed cornstalk to produce H₂ in continuous operation mode. At the dilution rate of 0.020 h⁻¹, the H₂ yield and production rate reached a maximum of 1.9 mol H₂ mol⁻¹ sugars and 8.4 mmol H₂ L⁻¹ h⁻¹, respectively, accompanied with the maximum glucose and xylose utilizations of 86.3% and 77.6%. Continuous H₂ production from enzymatically treated cornstalk hydrolysate in this research provides a new direction for economic, efficient, and harmless H₂ production.     

Photocatalytic water splitting into hydrogen and research on synergistic of Bi/Sm with solid solution of Bi–Sm–V photocatalyst

Photocatalyst Bi_1−xSm_xVO₄ were prepared by solid phase reaction and characterized by XRD, UV–visible DRS, BET, and SEM. Bi_1−xSm_xVO₄ showed two structures with the component content. When the composition was above x = 0.3, Bi_1−xSm_xVO₄ were of single phase with tetragonal type and can be regarded as solid solutions of was BiVO₄ and SmVO₄. When Bi_1−xSm_xVO₄ was loaded with 0.3 wt% Pt, the samples showed photocatalytic activities for water decomposition to hydrogen under UV light. Among these catalysts, Bi_0.5Sm_0.5VO₄ showed the best photocatalytic activity for water splitting, which indicated synergistic of Bi/Sm enhanced the photocatalysis efficiency. What's more, Bi_0.5Sm_0.5VO₄ loaded with other co-catalysts was found to act as a photocatalyst for water decomposition to hydrogen and oxygen under UV light irradiation, and the photocatalyst loaded with Pt/Cr₂O₃ had the best photocatalytic property. The amounts of the produced hydrogen and oxygen, respectively, were about 188.25 μmol h⁻¹ g⁻¹ and 95.90 μmol h⁻¹ g⁻¹. This study indicated that the formation of solid solution was the feasible method to adjust energy band and synergistic of Bi/Sm can enhance the photocatalytic activities of water decomposition.     

Hydrogen production from C1 compounds by a novel marine hyperthermophilic archaeon Thermococcus onnurineus NA1

A novel marine hyperthermophile, Thermococcus onnurineus NA1, was found to grow on C1 carbon compounds, such as formate and carbon monoxide (CO), and produce hydrogen (H₂). In the present study, the growth and H₂ production of NA1 were examined to determine its potential as H₂ producer. NA1 showed relatively high specific growth rates, 0.48 h⁻¹ and 0.40 h⁻¹ with CO (20%, v/v) and formate (100 mM), respectively, when cultivated in batch mode in a minimal salt medium fortified with 1.0 g L⁻¹ yeast extract. On the other hand, cell growth in both cases stopped at approximately 6 h and the final cell densities were extremely low at 18.2 and 12.1 mg protein L⁻¹ with CO and formate, respectively. The maximum final cell density could be improved greatly to 36.0 mg protein L⁻¹ by optimizing CO content (50%, v/v) and yeast extract concentration (4.0 g L⁻¹), but it was still very low. During the cell growth, formate and CO were used as energy source rather than carbon source. In the resting cell experiments, NA1 exhibited remarkably high H₂ production activities as 385.0 and 207.5 μmol mg protein⁻¹ h⁻¹ for CO and formate, respectively. When formate (100 mM) or CO (100%, v/v) was added repeatedly at 30–35 h intervals, NA1 showed consistent H₂ production for 3 cycles with a yield of approximately 1.0 mol H₂ mol⁻¹ for both CO and formate. This study suggests that T. onnurineus NA1 has a high H₂ production potential from formate or CO but a method for achieving a high cell density culture is needed.     

Ag3PO4 photocatalyst: Hydrothermal preparation and enhanced O2 evolution under visible-light irradiation

Visible-light-driven semiconducting photocatalysts of Ag₃PO₄ were prepared by a hydrothermal method, and were optimized by adjusting reaction conditions, i.e., temperature, pH of reaction solution, concentration of feedstock, and time of hydrothermal process. The obtained photocatalysts were then systematically characterized by different instruments, such as XRD, UV–vis, FESEM, and BET, to reveal the physicochemical properties. Furthermore, activities of photocatalysts for visible-light-driven O₂ evolution were evaluated, demonstrating that the photocatalytic activity of Ag₃PO₄ prepared by hydrothermal reaction (initial rate of O₂ evolution, 1156 μmol g⁻¹ h⁻¹) was more than two times as that of sample prepared by room-temperature reaction (initial rate of O₂ evolution, 533 μmol g⁻¹ h⁻¹), which could be attributed to its better ability to utilize visible light and more regulated morphology.     

A mixed electronic and protonic conducting hydrogen separation membrane with asymmetric structure

In this work, highly doped ceria with lanthanum, La_0.5Ce_0.5O_2−δ (LDC), are developed as hydrogen separation membrane material. LDC presents a mixed electronic and protonic conductivity in reducing atmosphere and good stability in moist CO₂ environment. LDC separation membranes with asymmetrical structure are fabricated by a cost-saving co-pressing method, using NiO + LDC + corn starch mixture as substrate and LDC as top membrane layer. Hydrogen permeation properties are systemically studied, including the influence of operating temperature, hydrogen partial pressure in feed stream and water vapor in both sides of the membrane on hydrogen permeating fluxes. Hydrogen permeability increases as the increasing of temperature and hydrogen partial pressure in feed gas. Using 20% H₂/N₂ (with 3% of H₂O) as feed gas and dry high purity argon as sweep gas, an acceptable flux of 2.6 × 10⁻⁸ mol cm⁻² s⁻¹ is achieved at 900 °C. The existing of water in both sides of membrane has significant effect on hydrogen permeation and the corresponding reasons are analyzed and discussed.     

An efficient ZnS-UV photocatalysts generated in situ from ZnS(en)0.5 hybrid during the H2 production in methanol–water solution

Highly active ZnS-UV was obtained in situ from ZnS(en)_0.5 hybrid during the hydrogen formation using a methanol–water solution under UV irradiation. X-ray diffraction patterns and UV spectroscopy for both ZnS-UV and ZnS-400 obtained from the calcination of the ZnS(en)_0.5 hybrid showed similar structural and photophysical properties; however, the efficiency of the ZnS-UV semiconductor was 7 times higher (4825 μmol h⁻¹ g⁻¹) compared to the ZnS-400. The highest H₂ production was obtained using a UV lamp of very low intensity (2.2 mW cm⁻¹) and it is attributed to a quantum size effect caused by the slow elimination of ethylenediamine (en) in the structural ZnS layer during the UV irradiation.     

Pd-Gardenia-TiO2 as a photocatalyst for H2 evolution from pure water

The photocatalytic activity for H₂ evolution from pure water over Pd loaded TiO₂ prepared by gardenia extract (Pd-Gardenia-TiO₂) is systematically investigated. The as-prepared photocatalysts are characterized by X-ray diffraction, high resolution transmission electron microscopy, Fourier transform infrared spectra, and X-ray photoelectron spectroscopy. Gardenia extract functions as reducing and stabilizing agents simultaneously. The mean size of the as-prepared Pd nanoparticles is in the range of 2.3 ± 0.5 nm based on TEM images. The Pd-Gardenia-TiO₂ catalyst exhibits good photocatalytic activity for H₂ evolution (93 μmol · h⁻¹ · g⁻¹), which is much higher than that of Pd photodeposited on TiO₂. Possible factors for its photocatalytic activity from pure water are also investigated.     

Metabolic engineering of Escherichia coli strains for co-production of hydrogen and ethanol from glucose

The co-production of H₂ and ethanol from glucose was studied to address the low H₂ production yield in dark fermentation. Several mutant strains devoid of ackA-pta, pfkA or pgi were developed using Escherichia coli BW25113 ΔhycA ΔhyaAB ΔhybBC ΔldhA ΔfrdAB as base strain. Disruption of ackA-pta eliminated acetate production during glucose fermentation but resulted in the secretion of a significant amount of pyruvate (0.73 mol mol⁻¹ glucose) without improving the co-production of H₂ and ethanol. When pfkA or pgi was further disrupted to enhance NAD(P)H supply by diverting the carbon flux from Embden-Meyerhof-Parnas (EMP) pathway to the pentose phosphate pathway (PPP), the cell growth of both strains was severely impaired under anaerobic conditions, and only the ΔpfkA mutant could recover its growth after adaptive evolution. The production yields of the ΔpfkA strain (H_2, 1.03 mol mol⁻¹ glucose and ethanol, 1.04 mol mol⁻¹ glucose) were higher than those of the pfkA⁺ strain (H₂, 0.69 mol mol⁻¹ glucose and ethanol, 0.95 mol mol⁻¹ glucose), but pyruvate excretion was not reduced. The excessive excretion of pyruvate in the ΔpfkA mutant was attributed to an insufficient NAD(P)H supply caused by the diversion of carbon flux from the EMP pathway to the Entner-Doudoroff pathway (EDP), rather than the PPP as intended. This study suggests that co-production of H₂ and ethanol from glucose is possible, but further metabolic pathway engineering is required to fully activate PPP under anaerobic conditions.     

Cobalt sulfide quantum dots modified TiO2 nanoparticles for efficient photocatalytic hydrogen evolution

Cobalt sulfide quantum dots (CoSx QDs) modified TiO₂ nanoparticles are prepared with a precipitation-deposition method using TiO₂, cobalt acetate and sodium sulfide as the precursors. CoSx QD acts as an effective cocatalyst, which accelerates the transfer of the photo-generated electrons and serves as the active site for the reaction between electrons and H₂O, thus enhancing the separation of the e⁻/h⁺ pairs and the photocatalytic H₂ production activity of TiO₂. The amount of CoSx exhibits an optimum value at about 5% (mole ratio to TiO₂), at which the H₂ production rate achieves 838 μmol h⁻¹ g⁻¹ using ethanol as the sacrificial reagent. This exceeds that of the pure TiO₂ by more than 35 times.     

Sulfate-reducing bacteria as new microorganisms for biological hydrogen production

Sulfate-reducing bacteria (SRB) have an extremely high hydrogenase activity and in natural habitats where sulfate is limited, produce hydrogen fermentatively. However, the production of hydrogen by these microorganisms has been poorly explored. In this study we investigated the potential of SRB for H₂ production using the model organism Desulfovibrio vulgaris Hildenborough. Among the three substrates tested (lactate, formate and ethanol), the highest H₂ production was observed from formate, with 320 mL L⁻¹_medium of H₂ being produced, while 21 and 5 mL L⁻¹_medium were produced from lactate and ethanol, respectively. By optimizing reaction conditions such as initial pH, metal cofactors, substrate concentration and cell load, a production of 560 mL L⁻¹_medium of H₂ was obtained in an anaerobic stirred tank reactor (ASTR). In addition, a high specific hydrogen production rate (4.2 L g⁻¹_dcw d⁻¹; 7 mmol g⁻¹_dcw h⁻¹) and 100% efficiency of substrate conversion were achieved. These results demonstrate for the first time the potential of sulfate reducing bacteria for H₂ production from formate.     

Photocatalytic hydrogen evolution from aqueous solutions of glycerol under visible light irradiation

Multiphase photocatalysts Pt/Cd_1−xZn_xS/ZnO/Zn(OH)₂ and single-phase photocatalysts Pt/Cd_1−xZn_xS were prepared by a two-step technique. The photocatalysts were characterized by a wide range of experimental techniques: X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) combined with energy-dispersive X-ray (EDX) spectroscopy, low-temperature N₂ adsorption/desorption, and UV/VIS spectroscopy. The photocatalytic activity was tested in a batch reactor in the reaction of hydrogen evolution from aqueous solutions of glycerol under visible light irradiation (λ > 420 nm). The highest achieved photocatalytic activity was 449 μmol H₂ per gram of photocatalyst per hour; the highest quantum efficiency was 9.6% (λ > 420 nm). The activity of the multiphase catalysts was shown to exceed that of the single-phase catalysts by a factor of 2.1, likely because of the heterojunctions between sulfides, oxides and hydroxides.     

Hydrogen production and consumption of organic acids by a phototrophic microbial consortium

Alternative fuel sources have been extensively studied. Hydrogen gas has gained attention because its combustion releases only water, and it can be produced by microorganisms using organic acids as substrates. The aim of this study was to enrich a microbial consortium of photosynthetic purple non-sulfur bacteria from an Upflow Anaerobic Sludge Blanket reactor (UASB) using malate as carbon source. After the enrichment phase, other carbon sources were tested, such as acetate (30 mmol l⁻¹), butyrate (17 mmol l⁻¹), citrate (11 mmol l⁻¹), lactate (23 mmol l⁻¹) and malate (14.5 mmol l⁻¹). The reactors were incubated at 30 °C under constant illumination by 3 fluorescent lamps (81 μmol m⁻² s⁻¹). The cumulative hydrogen production was 7.8, 9.0, 7.9, 5.6 and 13.9 mmol H₂ l⁻¹ culture for acetate, butyrate, citrate, lactate and malate, respectively. The maximum hydrogen yield was 0.6, 1.4, 0.7, 0.5 and 0.9 mmol H₂ mmol⁻¹ substrate for acetate, butyrate, citrate, lactate and malate, respectively. The consumption of substrates was 43% for acetate, 37% for butyrate, 100% for citrate, 49% for lactate and 100% for malate. Approximately 26% of the clones obtained from the Phototrophic Hydrogen-Producing Bacterial Consortium (PHPBC) were similar to Rhodobacter, Rhodospirillum and Rhodopseudomonas, which have been widely cited in studies of photobiological hydrogen production. Clones similar to the genus Sulfurospirillum (29% of the total) were also found in the microbial consortium.     

Novel Ni–ZrO2 catalyst doped with Yb2O3 for ethanol steam reforming

A type of Yb₂O₃ doped Ni–ZrO₂ catalyst for ethanol steam reforming was developed, and displayed excellent catalyzing performance for the selective formation of H₂ and CO₂. Over a Ni_1.25Zr₁Yb_0.8 catalyst, STY(H₂) can maintain stable at the level of 0.396 mol h⁻¹ g⁻¹ (data taken 120 h after the reaction started) under the reaction conditions of 0.5 MPa and 723 K, which was 1.6 times that (0.247 mol h⁻¹ g⁻¹) of the Yb-free counterpart Ni_1.25Zr₁. Characterization of the catalyst revealed that dissolution of an appropriate amount of Yb³⁺ ions in the zirconia host resulted in the formation of the Zr–Yb composite oxide with cubic-ZrO₂ structure, c-(Zr–Yb)O_z, which inhibited effectively the transformation of c-ZrO₂ to thermodynamically more stable m-ZrO₂, thus avoiding sintering of the (Zr–Yb)O_z composite. It was demonstrated that the doping of Yb₂O₃ to Ni–ZrO₂ changed also the valence states or the micro-environments of the Ni-species at the quasi-active surface of the tested catalyst, which was conducive to inhibiting agglomeration of the Ni_x⁰–Niⁿ⁺ species active catalytically, with resulting in maintaining the high metallic nickel dispersion and inhibiting coking. The aforementioned two factors both contributed to improving the activity and operating stability as well as heat-resistant quality of the catalyst.     

Biohydrogen production from algal biomass (Anabaena sp. PCC 7120) cultivated in airlift photobioreactor

The present study deals with the optimization of pretreatment conditions followed by thermophilic dark fermentative hydrogen production using Anabaena PCC 7120 as substrate by mixed microflora. Different airlift photobioreactors with ratio of area of downcomer and riser (A_d/A_r) in range of 0.4–3.2 were considered. Maximum biomass concentration of 1.63 g L⁻¹ in 9 d under light intensity of 120 μE m⁻² s⁻¹ was observed at A_d/A_r of 1.6. The mixing time of the reactors was inversely proportional to A_d/A_r. Maximal H₂ production was found to be 1600 mL L⁻¹ upon pretreatment with amylase followed by thermophilic fermentation for 24 h compared to other methods like sonication (200 mL L⁻¹), autoclave (600 mL L⁻¹) and HCl treatment (1230 mL L⁻¹). The decrease of pH from 6.5 to 5.0 during fermentation was due to the accumulation of volatile fatty acids. Amylase pretreatment gave higher reducible sugar content of 7.6 g L⁻¹ as compare to other pretreatments. Thermophilic fermentation of pretreated Anabaena biomass by mixed bacterial culture was found suitable for H₂ production.     

Enhancement of hydrogen production by the filamentous non-heterocystous cyanobacterium Arthrospira sp. PCC 8005

The efficiency of hydrogen production by different cyanobacterial species depends on several external factors. We report here the factors enhancing hydrogen production by filamentous non-heterocystous cyanobacterium Arthrospira sp. PCC 8005. Cells adapted to dark-anaerobic conditions produced hydrogen consistent with increased hydrogenase activity when supplemented with Fe²⁺. Stimulation of hydrogen production could be achieved by addition of reductants, either dithiothreitol or β-mercaptoethanol with higher production observed with the latter. Additionally, Fe²⁺ and β-mercaptoethanol added to nitrogen- and sulphur-deprived cells significantly stimulated H₂ production with maximal value of 5.91 ± 0.14 μmol H₂ mg Chla⁻¹ h⁻¹. Glucose and a small increase of osmolality imposed by either NaCl or sorbitol enhanced hydrogen production. High rates of hydrogen production were obtained in cells adapted in nitrogen-deprived medium with neutral and alkaline external pH, significant decrease of hydrogen production occurred under acidic external pH.     

Sustained hydrogen production from formate using immobilized recombinant Escherichia coli SH5

Biohydrogen is an ideal energy carrier for mobile chemical fuel cells, but its use is often limited by unavailability of sustained H₂ production system(s). Here, we developed a compact system for H₂ production from formate based on immobilized cells of recombinant Escherichia coli SH5. Three different matrices were tested as immobilization medium, among which agar showed the best performance in mechanical stability and permeability of substrate(s) and/or gaseous products (H₂ and CO₂). To explore and optimize the H₂ production capability of the immobilized cells, the conditions for cell immobilization including cell loading and agar concentration as well as the factors affecting H₂ production rate such as temperature, pH, and substrate concentration were studied in detail. A maximum volumetric production rate of 2.4 L H₂ L⁻¹ h⁻¹ was obtained when the immobilized cells were incubated with 350 mM sodium formate at pH 6.5 and 37 °C. Periodic supplementation of 200 mM formate with 20 mM glucose at pH 6.5 maintained the high H₂ production rate for a prolonged period of 10 h. We believe that our process can be developed for sustained H₂ production and is applicable to the operation of fuel cells in small-scale.