A facile dissolution strategy facilitated by H2SO4 to fabricate a 2D metal-free g-C3N4/rGO heterojunction for efficient photocatalytic H2 production

A 2D g-C₃N₄(pPCN)/rGO heterojunction for photocatalytic hydrogen production is fabricated by a facile dissolution strategy facilitated by H₂SO₄. The bulk g-C₃N₄ (CN) can be directly exfoliated into ultrathin protonated g-C₃N₄ (PCN) nanosheets under the assistance of H₂SO₄, and PCN can be further modified by rGO in a dissolved state under the electrostatic self-assembly process. The nanocomposite exhibits a large surface area (146.47 m²/g) and intimate contact interfaces between pPCN and rGO due to the specific synthesis method. Based on the DRS, PL and photoelectrochemical analyses, the introduction of rGO can greatly improve the light absorption and photogenerated charge carrier separation and transfer of g-C₃N₄. The optimal pPCN/2 wt% rGO nanocomposite shows an efficient photocatalytic H₂ evolution rate of 715 μmol g^?1 h^?1 under visible light irradiation, which is 2.6 and 13 times higher than those obtained on pPCN and CN. In addition, a photocatalytic mechanism over a 2D pPCN/rGO heterojunction is proposed. This work offers a new effective strategy for fascinating gC₃N₄based nanocomposites with promising hydrogen generation.     

Catalytic transformation of H2S for H2 production

Hydrogen sulfide (H₂S) gas is a by-product from natural gas refining, hydrodesulfurization of various fossil fuels, and syngas cleaning from pyrolysis and gasification. Catalytic pyrolysis of H₂S provides an alternative and effective pathway to recover both H₂ and sulfur. Catalysts from hydrotalcite of ZnAl, ZnNiAl, and ZnFeAl were employed for H₂S pyrolysis and compared with TiO₂ and MoS₂ at atmospheric pressure and temperatures in the range of 923–1123 K. Kinetic analysis was carried out in a packed bed reactor which revealed the effect of H₂S partial pressures to be of the order of 0.8–1 with respect to H₂S. The developed novel catalysts showed improved performance with significantly reduced activation energy compared to TiO₂ by 30 kJ/mol as well as higher H₂S conversion during pyrolysis (17% at 1173 K) than with MoS₂ catalyst, even at high H₂S partial pressure which is necessary for viable hydrogen production. The new approach showed an alternate economical and efficient pathway of catalyst design to obtain high activity and stability for simultaneous H₂ energy and pure sulfur recovery from unwanted H₂S resources.     

S-doped ZnO nanorods on stainless-steel wire mesh as immobilized hierarchical photocatalysts for photocatalytic H2 production

S-doped ZnO nanorods were grown on stainless steel mesh as immobilized hierarchical photocatalysts for hydrogen production. Properties of the photocatalysts were investigated by field-emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), photoinduced current, and photocatalytic hydrogen evolution test. Effects of polymer additive and doping on the surface texture, surface property, and H₂ production performance of the photocatalysts were studied. Polyethyleneimine helps the growth of nanorods on the entire surface of wire mesh. Photocatalytic H₂ production activity of the photocatalysts changes with dopant content and surface texture modification. Due to increased surface area of the hierarchical photocatalyst, enhanced light trapping and liquid flow among wire-mesh, the highest hydrogen evolution rate of 3640 μmol g⁻¹ h⁻¹ is obtained. The photocatalytic activity of photocatalyst remained at 87% of its original performance after five cycles.     

Enhanced H2 fermentation of organic waste by CO2 sparging

This study aimed to improve the productivity of dark fermentative hydrogen production from organic waste. An anaerobic sequencing batch reactor was used for hydrogen fermentation and it was fed with food waste (VS 4.4 ± 0.2% containing 27 g carbohydrate-COD/L) at various CO₂ sparging rates (40–120 L/L/d), hydraulic retention times (HRTs; 18–42 h), and solid retention times (SRTs; 18–160 h). CO₂ sparging increased the H₂ productivity by 5–36% at all the examined conditions, confirming the benefit of the replacement of headspace gas by CO₂. The maximum H₂ production was obtained by CO₂ sparging at 80 L/L/d, resulting in the H₂ productivity of 3.18 L H₂/L/d and the H₂ yield of 97.3 mL H₂/g VS_added. Increase of n-butyrate and isopropanol yields were concurrent with the enhanced H₂ yield by CO₂ sparging. Acidogenic efficiency, the sum of H₂, organic acid, and alcohol, in the CO₂-sparged reactor ranged from 47.9 to 56.0%, which was comparable to conventional acidogenesis. Thermodynamic analysis confirmed that both CO₂ sparging and CO₂ removal were beneficial for H₂-producing reactions, but CO₂ sparing has more profound effect than CO₂ removal on inhibiting H₂-consuming reactions.     

Photocatalytic H2 production from acetic acid solution over CuO/SnO2 nanocomposites under UV irradiation

The CuO/SnO₂ composites have been prepared by the simple co-precipitation method and further characterized by the XRD, FESEM and Raman spectroscopy. The photocatalytic H₂ production from acetic acid (HAc) solution over CuO/SnO₂ photocatalyst has been investigated at room temperature under UV irradiation. Effects of CuO loading, photocatalyst concentration, acetic acid concentration and pH on H₂ production have been systematically studied. Compared with pure SnO₂, the 33.3 mol%CuO/SnO₂ composite exhibited approximately twentyfold enhancement of H₂ production. The H₂ yield is about 0.66 mol-H₂/mol-HAc obtained under irradiation for prolonged time. The Langmuir-type model is applied to study the dependence of hydrogen production rate on HAc concentration. A possible mechanism for photocatalytic degradation of acetic acid over CuO/SnO₂ photocatalyst is proposed as well. Our results provide a method for pollutants removal with simultaneous hydrogen generation. Due to simple preparation, high H₂ production activity and low cost, the CuO/SnO₂ photocatalyst will find wide application in the coming future of hydrogen economy.     

High capacity potassium-promoted hydrotalcite for CO2 capture in H2 production

This paper presents an experimental study for a newly modified K₂CO₃-promoted hydrotalcite material as a novel high capacity sorbent for in-situ CO₂ capture. The sorbent is employed in the sorption enhanced steam reforming process for an efficient H₂ production at low temperature (400–500 ^°C). A new set of adsorption data is reported for CO₂ adsorption over K-hydrotalcite at 400 °C. The equilibrium sorption data obtained from a column apparatus can be adequately described by a Freundlich isotherm. The sorbent shows fast adsorption rates and attains a relatively high sorption capacity of 0.95 mol/kg on the fresh sorbent. CO₂ desorption experiments are conducted to examine the effect of humidity content in the gas purge and the regeneration time on CO₂ desorption rates. A large portion of CO₂ is easily recovered in the first few minutes of a desorption cycle due to a fast desorption step, which is associated with a physi/chemisorption step on the monolayer surface of the fresh sorbent. The complete recovery of CO₂ was then achieved in a slower desorption step associated with a reversible chemisorption in a multi-layer surface of the sorbent. The sorbent shows a loss of 8% of its fresh capacity due to an irreversible chemisorption, however, it preserves a stable working capacity of about 0.89 mol/kg, suggesting a reversible chemisorption process. The sorbent also presents a good cyclic thermal stability in the temperature range of 400–500 °C.     

Efficiency and efficacy of pre-treatment and bioreaction for bio-H2 energy production from organic waste

Two energy conversion parameters that are able to evaluate and score the pre-treatments and biohydrogen conversion processes of organic waste refuses have been introduced and applied using original experimental data. The parameters can be considered a suitable tool to score and select processes using rich lignocelluloses materials. The first efficiency (ξ) takes into account the quantity of energy that the process is able to extract as hydrogen, compared to the available amount of energy embedded in the refuse; the second efficacy (η) compares the energy conversion efficiency of the bioprocess using the refuse with the same energy conversion parameter obtained using glucose as a lignin-cellulose free substrate. Both the efficiency and efficacy have been applied in several experimental tests carried out with different kinds of experimental apparatus: an Erlenmeyer flask and bench bioreactor (2 L stirred-batch reactor STR), using mechanical (kitchen blade mixer) and chemical (HCl or NaOH for 24 h at 30 °C) pre-treated Organic Waste Market (OWM) refuse. The alkaline pre-treatment is the most efficient. A comparison of OWM efficiency with that of a glucose test under the same bench bioreactor experimental conditions, shows that the efficacy of energy production is 45%, which is equivalent to 7.3 L H2/kg as the gross material i.e. at its original undiluted strength. The paper shows that the two parameters are able to quantify the efficacy of energy production of such a bioprocess, including the pretreatment, using lignin-cellulose refuses, and to score different processes against glucose.     

Photocatalytic H2 production from water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled Pt/TiO2 nanocrystal photocatalyst

Sensitized photocatalytic production of hydrogen from water splitting is investigated under visible light irradiation over mesoporous-assembled titanium dioxide (TiO₂) nanocrystal photocatalysts, without and with Pt loading. The photocatalysts are synthesized by a sol–gel process with the aid of a structure-directing surfactant and are characterized by N₂ adsorption–desorption analysis, X-ray diffraction, UV–vis spectroscopy, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray analysis. The dependence of hydrogen production on the type of TiO₂ photocatalyst (synthesized mesoporous-assembled and commercial non-mesoporous-assembled TiO₂ without and with Pt loading), the calcination temperature of the synthesized photocatalyst, the sensitizer (Eosin Y) concentration, the electron donor (diethanolamine) concentration, the photocatalyst dosage and the initial solution pH is systematically studied. The results show that in the presence of the Eosin Y sensitizer, the Pt-loaded mesoporous-assembled TiO₂ synthesized by a single-step sol–gel process and calcined at 500 °C exhibits the highest photocatalytic activity for hydrogen production from a 30 vol.% diethanolamine aqueous solution with dissolved 2 mM Eosin Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic activity for hydrogen production are 3.33 g dm⁻³ and 11.5, respectively.     

The effects of micellar media on the photocatalytic H2 production from water

The photocatalytic H₂ production efficiency of a multi-component system, containing [Ir(ppy)₂(bpy)]⁺ as photosensitizer, [Co(bpy)₃]²⁺ as H₂-evolving catalyst, and triethanolamine (TEOA) as sacrificial electron donor, was investigated in water/acetonitrile (8:2, v/v) solution in the presence of different kinds of surfactants, such as cetyl trimethyl ammonium bromide (CTAB), Triton X-100, and sodium lauryl sulfonate (NaLS). The H₂ evolution rate is found to follow the order of cationic micellar media > nonionic micellar media > anionic micellar media > water/acetonitrile solution. The underlying mechanism was discussed.     

Experimental and numerical investigation of low-pressure laminar premixed synthetic natural gas/O2/N2 and natural gas/H2/O2/N2 flames

The oxidation of laminar premixed natural gas flames has been studied experimentally and computationally with variable mole fractions of hydrogen (0, 20, and 60%) present in the fuel mixture. All flames were operated at low pressure (0.079 atm) and at variable overall equivalence ratios (0.74<?<1.0) with constant cold gas velocity. At the same global equivalence ratio, there is no significant effect of the replacement of natural gas by 20% of H₂. The small differences recorded for the intermediate species and combustion products are directly due to the decrease of the amount of initial carbon. However, in 60% H₂ flame, the reduction of hydrocarbon species is due both to kinetic effects and to the decrease of initial carbon mole fraction. The investigation of natural gas and natural gas/hydrogen flames at similar C/O enabled identification of the real effects of hydrogen. It was shown that the presence of hydrogen under lean conditions activated the H-abstraction reactions with H atoms rather than OH and O, as is customary in rich flames of neat hydrocarbons. It was also demonstrated that the presence of H₂ favors CO formation.     

Two-step steam pretreatment of softwood by dilute H2SO4 impregnation for ethanol production

Fuel ethanol can be produced from softwood through hydrolysis in an enzymatic process. Prior to enzymatic hydrolysis of the softwood, pretreatment is necessary. In this study two-step steam pretreatment by dilute H₂SO₄ impregnation to improve the overall sugar and ethanol yield has been investigated. The first pretreatment step was performed under conditions of low severity (180°C, 10 min, 0.5% H₂SO₄) to optimise the amount of hydrolysed hemicellulose. In the second step the washed solid material from the first pretreatment step was impregnated again with H₂SO₄ and pretreated under conditions of higher severity to hydrolyse a portion of the cellulose, and to make the cellulose more accessible to enzymatic attack. A wide range of conditions was used to determine the most favourable combination. The temperatures investigated were between 180°C and 220°C, the residence times were 2, 5 and 10 min and the concentrations of H₂SO₄ were 1% and 2%.

The effects of pretreatment were assessed by both enzymatic hydrolysis of the solids and with simultaneous saccharification and fermentation (SSF) of the whole slurry, after the second pretreatment step. For each set of pretreatment conditions the liquid fraction was fermented to determine any inhibiting effects. The ethanol yield using the SSF configuration reached 65% of the theoretical value while the sugar yield using the SHF configuration reached 77%. Maximum yields were obtained when the second pretreatment step was performed at 200°C for 2 min with 2% H₂SO₄. This form of two-step steam pretreatment is a promising method of increasing the overall yield in the wood-to-ethanol process.     

Small stationary reformers for H2 production from hydrocarbons

This paper describes the activities performed in ENEA (Italian National Agency for New Technologies, Energy and Environment) during the last years in order to investigate the hydrogen production from largely available hydrocarbons, such as natural gas [1] and LPG (Liquefied Petroleum Gas), aimed at feeding Polymer Electrolyte Fuel Cells (PEFC) in the 1–5 kW electric power range.     

H2 production from CH4 decomposition: Regeneration capability and performance of nickel and rhodium oxide catalysts

Nickel–lanthanum (LaNiO₃) and nickel–rhodium–lanthanum (LaNi_0.95Rh_0.05O₃) perovskite-type oxide precursors were synthesized by different methodologies (co-precipitation, sol–gel and impregnation). They were reduced in an H₂ atmosphere to produce nickel and rhodium nanoparticles on the La₂O₃ substrate. All samples were tested in the catalytic decomposition of CH₄. Methane decomposed into carbon and H₂ at reaction temperatures as low as 450 °C—no other reaction products were observed. Conversions were in the range of 14–28%, and LaNi_0.95Rh_0.05O₃ synthesized by co-precipitation was the most active catalyst. All catalysts maintained sustained activity even after massive carbon deposition indicating that these deposits are of the nanotube-type, as confirmed by transmission electron microscopy (TEM). The reaction seems to occur in a way that a nickel or rhodium crystal face is always clean enough to expose sufficient active sites to make the catalytic process continue. The samples were subjected to a reduction–oxidation–reduction cycle and in situ analyses confirmed the stability of the perovskite structure. All diffraction patterns showed a phase change around 400 °C, due to reduction of LaNiO₃ to an intermediate La₂Ni₂O₅ structure. When the reduction temperatures reach 600 °C, this structure collapses through the formation of Ni⁰ crystallites deposited on the La₂O₃. Under oxidative conditions, the perovskite system is recomposed with nickel re-entering the LaNiO₃ framework structure accounting for the regenerative capability of these solids.     

H2 production by methane decomposition: Catalytic and technological aspects

Ni and Co supported on SiO₂ and Al₂O₃ silica cloth thin layer catalysts have been investigated in the catalytic decomposition of natural gas (CDNG) reaction. The influence of carrier nature and reaction temperature was evaluated with the aim to individuate the key factors affecting coke formation. Both Ni and Co silica supported catalysts, due to the low metal support interaction (MSI), promotes the formation of carbon filament with particles at tip. On the contrary, in case alumina was used as support, metals strongly interact with surface thus depressing both the metal sintering and the detachment of particles from catalyst surface. In such cases, carbon grows on metal particle with a “base mechanism” while particles remain well anchored on the catalyst surface. This allowed to realize a cyclic dual-step process based on methane decomposition and catalyst oxygen regeneration without deactivation of catalyst. Technological considerations have led to conclude that the implement of a process based on decomposition and regeneration of catalyst by oxidation requires the development of a robust catalytic system characterized by both a strong MSI and a well defined particle size distribution. In particular, the catalyst should be able to operate at high temperature, necessary to reach high methane conversion values (> 90%), avoiding at the same time the formation of both the carbon filaments with metal at tip or the encapsulating carbon which drastically deactivate the catalyst.     

Cu2ZnSnS4 decorated CdS nanorods for enhanced visible-light-driven photocatalytic hydrogen production

Design and preparation of high performance photocatalysts are always the keys for photocatalytic hydrogen production by using green and unlimited solar energy. In this work, we present the synthesis of Cu₂ZnSnS₄ (CZTS) decorated CdS nanorods and their use for visible-light-driven photocatalytic hydrogen production. The as-synthesized CZTS decorated CdS nanorods exhibit much higher visible-light-driven photocatalytic hydrogen production performance than that of individual CdS nanorods and individual CZTS nanoparticles. Specifically, the hydrogen production rate of representative CZTS decorated CdS nanorods was 48-times and 165-times higher than that of individual CdS nanorods and individual CZTS nanoparticles. The enhanced photocatalytic hydrogen production performance may be contributed by the p-n heterojunction as well as the synergistic effect between CdS nanorods and CZTS particles. The present work not only reported new low-cost and highly efficient photocatalysts for visible-light-driven photocatalytic hydrogen production, but also provided new method for the design and preparation of high performance visible-light-driven heterostructured photocatalysts for photocatalytic hydrogen production.     

Effect of iron concentration on continuous H2 production using membrane bioreactor

The effect of FeSO₄ on continuous H₂ production in a membrane bioreactor (MBR) was investigated using anaerobic mixed microflora under mesophilic condition. The H₂ production of 41.6 l/day was obtained at 10.9 mg FeSO₄/l, which was 1.59 times higher than that at 2.7 mg FeSO₄/l. Between 2.7 and 13.7 mg FeSO₄/l, the H₂ production rate increased in parallel with the H₂ yield under high-cell-density condition. For the same amounts of FeSO₄, increases in butyric acid together with decreases in lactic acid promoted a reduction of the number of protons and the resultant release of H₂. The hydrogenase activity of 1.08 mg methylene blue (M.B) reduced/min at 10.9 mg FeSO₄/l was about sixfold higher than at 2.7 mg FeSO₄/l. These results suggest that the addition of iron and sulfur to an MBR is an important key factor in the enhancement of H₂ production.     

In situ growth of a-few-layered MoS2 on CdS nanorod for high efficient photocatalytic H2 production

An ultrathin MoS₂ was grown on CdS nanorod by a solid state method using sulfur powder as sulfur source for photocatalytic H₂ production. The characterization result reveals that the ultrathin MoS₂ nanosheets loaded on CdS has a good contact state. The photoelectrochemical result shows that MoS₂ not only are beneficial for charge separation, but also works as active sites, thus enhancing photocatalytic activity. Compared with pure CdS, the photocatalytic activity of MoS₂ loaded CdS was significantly improved. The hydrogen evolution rate on m(MoS₂): m(CdS) = 1: 50 (m is mass) reaches 542 μmol/h, which is 6 times of that on pure CdS (92 μmol/h). This work provides a new design for photocatalysts with high photocatalytic activities and provides a deeper understanding of the effect of MoS₂ on enhancing photocatalytic activity.     

Performance of Na2O promoted alumina as CO2 chemisorbent in sorption-enhanced reaction process for simultaneous production of fuel-cell grade H2 and compressed CO2 from synthesis gas

The performance of a novel thermal swing sorption-enhanced reaction (TSSER) concept for simultaneous production of fuel-cell grade hydrogen and compressed carbon dioxide as a by-product from a synthesis gas feed is simulated using Na₂O promoted alumina as a CO₂ chemisorbent in the process. The process simultaneously carries out the water gas shift (WGS) reaction and removal of CO₂ from the reaction zone by chemisorption in a single unit. Periodic regeneration of the chemisorbent is achieved by using the principles of thermal swing adsorption employing super-heated steam purge.     

A multi-criteria assessment of six energy conversion processes for H2 production

The study of Very Large Complex systems (“VLCS”), of which modern energy conversion systems are an important subset, requires a holistic approach to analyze the system itself and all of its “external” and “internal” interactions. The view taken in this paper is that the VLCS should be considered as an “extended” (in a sense specified below) thermodynamic system. The evaluation of the flows of matter and energy sustaining a VLCS and the knowledge of the transformations therein can be used to describe the rate of exploitation of the available natural resources, to assess the efficiency of the conversion process, and to provide a quantitative estimate of the impact of the system on the environment. This kind of information is an important part of the essential database of any Decision Support System for both the internal and global policy planning and for resources management. Several assessment methods are in use at present, and each one of them provides a different insight in the “performance” of the conversion chain under examination.     

Homogeneous combustion of fuel-lean H2/O2/N2 mixtures over platinum at elevated pressures and preheats

The gas-phase combustion of H₂/O₂/N₂ mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gas-phase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase conversion. Simulations in channels with practical geometrical confinements of 1 mm indicated that gas-phase combustion was altogether suppressed at atmospheric pressure, wall temperatures as high as 1350 K and preheats up to 773 K. While homogeneous ignition chemistry controlled gaseous combustion at atmospheric pressure, flame propagation characteristics dictated the strength of homogeneous combustion at the highest investigated pressures. The decrease in laminar burning rates for p ? 8 bar led to a push of the gaseous reaction zone close to the channel wall, to a subsequent leakage of hydrogen through the gaseous reaction zone, and finally to catalytic conversion of the escaped fuel at the channel walls. Parametric studies delineated the operating conditions and geometrical confinements under which gas-phase conversion of hydrogen could not be ignored in numerical modeling of catalytic combustion.