Highest H2 Production Rate Research Articles

Efficient visible-light-driven photocatalytic hydrogen production using CdS@TaON core–shell composites coupled with graphene oxide nanosheets

Large-scale hydrogen production through water splitting using photocatalysts with solar energy can potentially produce clean fuel from renewable resources. In this work, photocatalytic evolution of H2 with a high efficiency was achieved using graphene oxide (GO) nanosheets decorated with CdS sensitized TaON core–shell composites (GO–CdS@TaON). The CdS@TaON core–shell nanocomposites were prepared by an ion-exchange route with assistance from a hydrothermal process on GO as the support. The TaON core–shell composites containing 1 wt% CdS nanocrystals showed a high rate of H2-production at 306 μmol h−1 with an apparent quantum efficiency (QE) of 15% under 420 nm monochromatic light. The rate of hydrogen formation was 68 times faster in comparison with the rate observed on pure TaON. The rate was further increased to 633 μmol h−1 with a high quantum efficiency of 31% when the GO–CdS@TaON hybrid composite was coupled with 1 wt% of graphene oxide and 0.4 wt% of Pt (about 141 times higher than that of the pristine TaON). This high photocatalytic H2-production activity is ascribed firstly to the presence of CdS nanocrystals that alter the energy levels of the conduction and valence bands in the coupled semiconductor system; secondly to the involvement of graphene oxide that serves as an electron collector and transporter to efficiently lengthen the lifetime of the photogenerated charge carriers from CdS@TaON composites. This investigation can open up new possibilities for the development of highly efficient TaON-based photocatalysts that utilize visible light as an energy source.

Enhanced visible-light photocatalytic H2-production performance of multi-armed CdS nanorods

Visible light photocatalytic H2 production from aqueous solutions using solar light is of great importance from the viewpoint of solar energy conversion and storage and environment protection. In this study, a novel visible-light-driven photocatalyst, multi-armed CdS nanorods, was synthesized using a solvothermal method with dodecylamine as solvent. The prepared CdS nanorod samples showed especially high and stable photocatalytic H2-production activity with aqueous lactic acid solution as sacrificial reagent and Pt as co-catalyst under visible light irradiation. The CdS nanorod sample prepared at 140 °C for 12 h exhibited a high H2-production rate of 1.21 mmol h−1 (about 3.2 times higher than that of CdS nanoparticles (NPs) formed in water) with a very small amount of Pt (0.23 wt%) and a quantum efficiency (QE) of 51% at a wavelength of 420 nm. This high photocatalytic H2-production activity can be attributed to the synergistic effects of several factors such as the hexagonal phase structure, high surface area, great pore volume and good crystallization. Furthermore, the prepared CdS nanorod sample was photostable and no photocorrosion was observed after photocatalytic recycling. Our work demonstrated that multi-armed CdS nanorods were a promising candidate for the development of high-performance visible-light photocatalysts in photocatalytic H2 production and that the morphology of the CdS nanocrystals had a great effect on the photocatalytic H2-production activity.

Bioreactor design for continuous dark fermentative hydrogen production

Dark fermentative H 2 production (DFHP) has received increasing attention in recent years due to its high H 2 production rate (HPR) as well as the versatility of the substrates used in the process. For most studies in this field, batch reactors have been applied due to their simple operation and efficient control; however, continuous DFHP operation is necessary from economical and practical points of view. Continuous systems can be classified into two categories, suspended and immobilized bioreactors, according to the life forms of H 2 producing bacteria (HPB) used in the reactor. This paper reviews operational parameters for bioreactor design including pH, temperature, hydraulic retention time (HRT), and H 2 partial pressure. Also, in this review, various bioreactor configurations and performance parameters including H 2 yield (HY), HPR, and specific H 2 production rate (SHPR) are evaluated and presented.

Simultaneous Biohydrogen and Bioethanol Production from Anaerobic Fermentation with Immobilized Sludge

The effects of organic loading rates (OLRs) on fermentative productions of hydrogen and ethanol were investigated in a continuous stirred tank reactor (CSTR) with attached sludge using molasses as substrate. The CSTR reactor with attached sludge was operated under different OLRs, ranging from 8 to 24 kg/m3·d. The H2 and ethanol production rate essentially increased with increasing OLR. The highest H2 production rate (10.74 mmol/h·L) and ethanol production rate (11.72 mmol/h·L) were obtained both operating at OLR = 24 kg/m3·d. Linear regression results show that ethanol production rate (y) and H2 production rate (x) were proportionately correlated and can be expressed as y = 1.5365x − 5.054 (r2 = 0.9751). The best energy generation rate was 19.08 kJ/h·L, which occurred at OLR = 24 kg/m3·d. In addition, the hydrogen yield was affected by the presence of ethanol and acetic acid in the liquid phase, and the maximum hydrogen production rate occurred while the ratio of ethanol to acetic acid was close to 1.

Influence of solids retention time on continuous H2 production using membrane bioreactor

H2 production Solids retention time Specific loading rate Membrane bioreactor Butyric acid Extracellular polymeric substances abstract The influence of solids retention time (SRT) on continuous H2 production in a submerged membrane bioreactor (MBR) was investigated using mixed mesophilic microflora. The bioreactor was continuously operated at the four SRTs of 2, 4, 12.5 and 90 d on a glucose medium under the hydraulic retention time (HRT) of 9 h and the mesophilic condition of 35 � C � 0.5. Stable biogas production with H2 content of 50.8%-60% was achieved at SRTs ranging from 2 to 12.5 d. No methane gas was observed in monitoring the experimental conditions. The H2 production increased from 17.62 to 26.1 l-H2/d when the SRT increased from 2 to 12.5 d, but decreased to 9.1 l-H2/d at the 90 d SRT. The best H2 yield, 1.19 mol-H2/ mol-glucose, was observed at the SRT of 2 d and the highest H2 production rate, 5.8 l-H2/l/ d, was obtained at the SRT of 12.5 d. Stable H2 production was achieved by maintaining the SRT in the range of 2 - 12.5 d, regardless of the fermentative pathway related to higher lactate production. The decrease in H2 yield was observed at long SRTs due to the low volatile suspended solids/total suspended solids (VSS/TSS) as well as the high extracel- lular polymeric substances (EPS) concentrations. These results suggest that the SRT is the key factor enabling sustainable H2 fermentation in MBR, and that an SLR value of around 1.6 kg-DOC/kg-VSS/d might be the specific condition for achieving optimum H2 production.

Ethanol Steam Reforming for Hydrogen Production over Bimetallic Pt–Ni/Al2O3

Ethanol steam reforming was studied at 673–823 K over Pt–Ni/δ-Al2O3. Results indicate that bimetallic catalyst is resistant to coke deposition at steam-to-carbon ratios as low as 1.5 and higher ratios are beneficial for both ethanol conversion and hydrogen formation. About 773 K is the optimum since high H2 production rates are accompanied by low CO and CH4 production rates. A power-function rate expression obtained on the basis of intrinsic rates at 673 K gives reaction orders of 1.25 (±0.05) and −0.215 (±0.015) for ethanol and steam, respectively; the apparent activation energy is calculated as 39.3 (±2) kJ mol−1 between 673 and 723 K.

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Catalytic decomposition of methane (CDM) for COx-free hydrogen production is carried out over 20 wt% Ni supported on H-ZSM-5/MCM-41 composite material, synthesized from alkaline desilication of a conventional H-ZSM-5(Si/Al = 40) and surfactant-recrystallization process. The composite material is used, for the first time, as support of Ni-catalyst in CDM. Tested for 6 h at 620 °C under atmospheric pressure, the 20 wt% Ni/H-ZSM-5/MCM-41 exhibits relatively higher H2 production rate than 20 wt% Ni supported on the parent H-ZSM-5(Si/Al = 40) or on the pure MCM-41 in the order H-ZSM-5/MCM-41 > H-ZSM-5 > MCM-41. All catalysts display a remarkable catalytic stability. The higher catalytic activity of Ni/composite could be related to an improvement in both the access to active Ni metal atoms and the dispersion of the latter as evidenced by H2-chemisorption measurement. The fresh catalysts are characterized by several techniques while the reacted ones are studied by the Raman spectroscopy.

Dark Hydrogen Fermentation from Hydrolyzed Starch Treated with Recombinant Amylase Originating from Caldimonastaiwanensis On1

Starch is one of the most abundant resources on earth and is suited to serve as a cost-effective feedstock for biological hydrogen production. However, producing hydrogen from direct fermentation of starch is usually inefficient, as the starch hydrolysis is often the rate-limiting step. Therefore, in the present work, enzymatic starch hydrolysis was conducted to enhance the feasibility of using starch feedstock for H2 production. The amylase (with a molecular weight of ca. 112 kDa) used for starch hydrolysis was produced from a recombinant E. coli harboring an amylase gene originating from Caldimonas taiwanensis On1. Using statistical experimental design, the optimal pH and temperature for starch hydrolysis with the recombinant amylase was pH 6.86 and 52.4 degrees C, respectively, at an initial starch concentration of 7 g/L. The hydrolyzed products contained mainly glucose, maltotriose, and maltotetrose, while a tiny amount of maltose was also detected. The enzymatically hydrolyzed products of soluble starch and cassava starch were used as the substrate for dark hydrogen fermentation using Clostridium butyricum CGS2 and Clostridium pasteurianum CH4. The highest H2 production rate (vH2) and yield (YH2) of C. butyricum CGS2 was 124.0 mL/h/L and 6.32 mmol H2/g COD, respectively, both obtained with the hydrolysate of cassava starch. The best H2 production rate (63.0 mL/h/L) of C. pasteurianum CH4 occurred when using hydrolyzed cassava starch as the substrate, whereas the highest yield (9.95 mmol H2/g COD) was obtained with the hydrolyzed soluble starch.

05/00618 Tri-reforming of CH 4 using CO 2 for production of synthesis gas to dimethyl ether

Steam reforming (SR) of dimethyl ether (DME) was investigated for the production of hydrogen forfuel cells. The activity of a series of solid acids for DME hydrolysis was investigated. The solid acid catalysts wereZSM-5 [Si/Al = 25, 38 and 50: denoted Z(Si/Al)] and acidic alumina (γ-Al2O3) with an acid strength order that was Z(25)>Z(38)>Z(50)>γ-Al2O3. Stronger acidity gave higher DME hydrolysis conversion. Physical mixtures containing a CuO-ZnO-Al2O3-ZrO2 catalyst and solid acid catalyst to couple DME hydrolysis and methanol SR wereused to examine the acidity effects on DME SR. DME SR activity strongly depended on the activity for DME hydrolysis. Z(25) was the best solid acid catalyst for DME SR and gave a DME conversion>90% [T = 240°C, n(H2O)/n(DME) = 3.5, space velocity = 1179 ml·(g cat)−1·h−1, and P = 0.1MPa]. The influences of the reaction temperature, space velocity and feed molar ratio were studied. Hydrogen production significantly depended ontemperature and space velocity. A bifunctional catalyst of CuO-ZnO-Al2O3-ZrO2 catalyst and ZSM-5 gave a high H2 production rate and CO2 selectivity.

The biotechnology of hydrogen production by Nostoc flagelliforme grown under chemostat conditions

The potential of using N2-fixing cyanobacteria to produce hydrogen photobiologically has stimulated research on the physiology and biotechnology of species exhibiting high H2 production rates over long periods of time. In this work Nostoc flagelliforme, a terrestrial N2-fixing cyanobacterium, has been examined to establish its physiology and potential for H2 production under controlled conditions. Cell filaments of N. flagelliforme were purified and grown in liquid culture to optimize its H2 metabolism. In batch-grown cultures the activity of nitrogenase, the key enzyme for H2 production in N2-fixing organisms, was found to be high only during a short phase of exponential growth. A chemostat system was thus constructed for long-term experiments using continuous cultures, with the aim of exploiting the exponential growth phase. The dilution rate (D) and environmental factors, such as N2 concentration in the gas phase and temperature, significantly influenced H2 production. Cells grown continuously under the optimized conditions of D = 0.022 h−1, 34 °C and 5.1 kPa N2 in the gas phase exhibited H2 production rates that were more than four times higher than the maximal rates under standard batch growth conditions.

Increased H2 production by immobilized microorganisms

Viable cells of H2-producers (Bacillus licheniformis and a mixed microbial culture) were immobilized on brick dust and in calcium alginate beads. In batch culture, cells of the mixed culture in the free state yielded 8.2 l H2/mol glucose utilized, whereasB. licheniformis evolved 13.1 l H2. Immobilized cells, however, gave 4-fold more H2 than the free bacteria. Highest yields were from the cells immobilized on brick dust. High H2-production rates continued over two rounds of re-use of the immobilized cells.

Increased Nitrogenase-Dependent H 2 Photoproduction by hup Mutants of Rhodospirillum rubrum

Transposon Tn5 mutagenesis was used to isolate mutants of Rhodospirillum rubrum which lack uptake hydrogenase (Hup) activity. Three Tn5 insertions mapped at different positions within the same 13-kb EcoRI fragment (fragment E1). Hybridization experiments revealed homology to the structural hydrogenase genes hupSLM from Rhodobacter capsulatus and hupSL from Bradyrhizobium japonicum in a 3.8-kb EcoRI-ClaI subfragment of fragment E1. It is suggested that this region contains at least some of the structural genes encoding the nickel-dependent uptake hydrogenase of R. rubrum. At a distance of about 4.5 kb from the fragment homologous to hupSLM, a region with homology to a DNA fragment carrying hypDE and hoxXA from B. japonicum was identified. Stable insertion and deletion mutations were generated in vitro and introduced into R. rubrum by homogenotization. In comparison with the wild type, the resulting hup mutants showed increased nitrogenase-dependent H(2) photoproduction. However, a mutation in a structural hup gene did not result in maximum H(2) production rates, indicating that the capacity to recycle H(2) was not completely lost. Highest H(2) production rates were obtained with a mutant carrying an insertion in a nonstructural hup-specific sequence and with a deletion mutant affected in both structural and nonstructural hup genes. Thus, besides the known Hup activity, a second, previously unknown Hup activity seems to be involved in H(2) recycling. A single regulatory or accessory gene might be responsible for both enzymes. In contrast to the nickel-dependent uptake hydrogenase, the second Hup activity seems to be resistant to the metal chelator EDTA.

Hydrogen photoproduction by algae and higher plants: Optimal conditions for polarographic determination

Photosynthetic H2 production capacity has been studied using a H2 sensitive electrode in green and blue-green algae, higher plant cells and chloroplasts and photosynthetic bacteria. Among the cell types tried,Anacystis showed the maximum rate of H2 production. All the cell types require either an electron acceptor or a reductant (sodium dithionite) for H2 production. In the presence of an electron acceptor or reductant H2 production was observed even in unadapted cells. Cultures ofAnacystis grown for a maximum period of 70 days showed a high rate of H2 production up to 40 days. The light saturation curves and pH optima for H2 production were similar to that of O2 evolution in the cell types studied.