Effect of Ce on the structural features and catalytic properties of La(0.9−x)CexFeO3 perovskite-like catalysts for the high temperature water-gas shift reaction

La_(0.9−x)Ce_xFeO₃ perovskite-like catalysts were investigated for the production of hydrogen from simulated coal-derived syngas via the water-gas shift reaction in the temperature range 450-600 °C and at 1 atm. These catalysts exhibited higher activity at high temperatures (T ≥ 550 °C), compared to that of a commercial high temperature iron-chromium catalyst at 450 °C. Addition of a low Ce content (x = 0.2), has little influence on the formation of the LaFeO₃ perovskite structure, but enhances catalytic activity especially at high temperatures with 19.17% CO conversion at 550 °C and 40.37% CO conversion at 600 °C. The LaFeO₃ perovskite structure and CeO₂ redox properties play an important role in enhancing the water-gas shift activity. Addition of a high Ce content (x = 0.6) inhibits the formation of the LaFeO₃ perovskite structure and decreases catalyst activity.     

Thermodynamic analysis of novel vanadium redox materials for solar thermochemical ammonia synthesis from N2 and CH4

Owing to the wide applications of ammonia in hydrogen field and high energy consumption of the Haber-Bosch process, developing economic and environmentally benign ammonia synthesis process has attracted great interests. This work focuses on the moderately high two-step solar water-splitting of VN to produce ammonia, thus avoiding the reliance of fossil-fuel based heating source and pure hydrogen. Based on the equilibrium composition analysis, we find that V₂O₃, CH₄ and N₂ with mole ratio of 1:3:1.5 at T_H = 1050 °C is enough for complete methane reforming and sufficient nitrogen fixation of V₂O₃. As for the water-splitting of VN, the production of NH₃ is only possible at T_L ≤ 400 °C, and inputting excessive water vapors is found to exert little effect on ammonia production at H₂O:AlN>3:2. At the temperature range of full conversion between V₂O₃ and VN, the cycle efficiency, η_cycle, and solar-to-fuel efficiency, η_{solar-to-fuel,} under different operating temperatures are compared, in which the highest η_cycle and η_{solar-to-fuel} are 31.9% and 35.3% respectively. Moreover, efficiencies could be increased up to more than 37% with consideration of heat recuperation, demonstrating the great solar energy storage and fuel production potential of the proposed system.     

La(1−x)SrxMnO3−δ perovskites as redox materials for the production of high purity hydrogen

Materials of the perovskite structure and of the general formula La_1−xSr_xMnO₃ (x = 0, 0.3, 0.7) are investigated as redox catalysts for the two-step steam reforming of methane towards the production of high purity hydrogen. During the activation step, methane is oxidized with lattice oxygen to carbon dioxide and carbon monoxide, while oxygen is withdrawn from the material until a maximum deficiency level which depends on the strontium content and the reaction temperature. During the reaction step water is splitted to gaseous hydrogen and lattice oxygen that fills the oxygen vacancies. It appeared that, after the achievement of a characteristic oxygen deficiency level, La_1−xSr_xMnO₃ materials exhibit good activity for the water-splitting reaction. The activity is further found to be proportional to the oxygen vacancy concentration. At high activity levels, initial water conversions per 15 μmol pulse of up to 70% are achieved at 1000 °C. The cumulatively produced hydrogen during the water-splitting step, per injected water, increases with increasing strontium content, reaching a production of 60 μmol H₂ per 500 μmol water passed over 200 mg La_0.3Sr_0.7MnO₃ at 1273 K and no coke formation. The materials exhibit stable behavior after eight successive oxidation–reduction cycles. The relations between the redox behavior and the material defect chemistry are discussed. Finally the energy efficiency of the process, future prospects and ways for its optimization are discussed.     

Fabrication of nanocrystalline LaFeO3: An efficient sol–gel auto-combustion assisted visible light responsive photocatalyst for water decomposition

Synthesis of nano photocatalysts, LaFeO₃ with orthorhombic perovskite structure by sol–gel auto-combustion method was demonstrated. The samples were characterized by PXRD, SEM, HRTEM, XPS and optical absorption studies. Photocatalytic water decomposition over LaFeO₃ nanoparticles activated at various temperatures without any co-catalyst were investigated under visible light irradiation (λ >> 420 nm). Highest amount of H₂ and O₂ evolved in 180 min over the LaFeO₃ activated at 500 °C was recorded to be 1290 μmol and 640 μmol, respectively having apparent quantum efficiency (AQE) 8.07%. The pronounced activity of nano LaFeO₃ samples towards water decomposition was consistent with BET-surface area and particle size analyses.     

Experimental and DFT studies on catalytic reforming of acetic acid for hydrogen production over B-doped Co/Al2O3 catalysts

Steam reforming of bio-oil for hydrogen production is a promising green technology. Acetic acid was used as the bio-oil model compound. Experimental and density functional theory calculations were carried out to study the performance of Co/Al₂O₃ catalysts doped with boron (B) with a 1 wt.%–5 wt.% content. Catalyst characterization by BET, XRD, XPS, NH₃-TPD, H₂-TPR, TEM, and TG-DTG was performed. We found that the catalyst performance improved significantly by B doping. Under the reaction conditions of T = 500 °C, steam-to-carbon ratio (S/C) = 5, and liquid hourly space velocity (LHSV) = 4.3 h^?1, the catalyst with a B doping ratio of 1 wt.% had the highest hydrogen yield of 85% and a maximum acetic acid conversion rate of 95%. The corresponding hydrogen productivity was 0.8 mmol/min. The stability of this catalyst exceeded 29 h. Density functional theory calculations showed that the interactions between the reaction intermediates and the surface were strengthened with B addition.     

Limiting and achievable efficiencies for solar thermal hydrogen production

For reversible conversion, we derive theoretical Sun-to-H₂ (STH₂) efficiencies for water-splitting processes harnessing solar energy as heat predominantly. At solar concentration ratios (C) of 2,000–10,000, the derived STH₂ efficiency limits are 72.4–80.1%. For real processes with irreversibilities, we conceptualize direct and two-stage thermal water-splitting processes to estimate the achievable STH₂ efficiency, the favorable operating conditions and design challenges that must be overcome. For direct thermal water-splitting, achievable STH₂ efficiencies between 35 and 50% are possible at reaction temperatures of 1300–2000 K, and C = 2,000–10,000. This STH₂ efficiency range is greater than the estimates of achievable values available for low and high temperature water electrolysis or single bandgap methods for generating H₂. The direct process requires efficient heat integration, and high temperature membranes for H₂(g) and O₂(g) separation to surpass reaction equilibrium limitations. Alternatively, for two-stage water-splitting using Fe₃O₄/FeO with solar heat recovered at 1600–2300 K, the calculated estimates for the achievable STH₂ efficiency are 38–54%.     

Thermochemical two-step water-splitting for hydrogen production using Fe-YSZ particles and a ceramic foam device

Fe₃O₄ supported on cubic yttria-stabilized zirconia (Fe₃O₄/c-YSZ) is proposed as a promising redox material for the production of hydrogen from water via a thermochemical two-step water-splitting cycle. In this study, the evolution of oxygen and hydrogen during the cyclic reaction was examined using Fe₃O₄/c-YSZ particles in order to demonstrate reproducible and stoichometric oxygen/hydrogen production through a repeatable two-step reaction. Subsequently, a ceramic foam device coated with Fe₃O₄ and c-YSZ particles was prepared and examined as a thermochemical water-splitting device in a directly irradiated receiver/reactor hydrogen production system. The Fe₃O₄/c-YSZ system formed a Fe-containing YSZ (Fe-YSZ) by high-temperature reaction between Fe₃O₄ and the c-YSZ support at 1400 °C in an inert atmosphere. The reaction mechanism of the two-step water-splitting cycle is associated with the redox transition of Fe²⁺–Fe³⁺ ions in the c-YSZ lattice. The Fe-YSZ particles exhibit good reproducibility for reaction with a hydrogen/oxygen ratio of approximately 2.0 throughout repeated cycles. The foam device coated with Fe-YSZ particles was also successful for continual hydrogen production through 32 repeated cycles. A 20–27% ferrite conversion was obtained using 10.5 wt% Fe₃O₄ loading over an irradiation period of 60 min.     

Preparation,characterization and electronic properties of LaFeO3 perovskite as photocatalyst for hydrogen production

Perovskite-type LaFeO₃ was successfully used as photocatalyst for the H₂ production. It was prepared by a conventional sol-gel technique via the nitrate route. X-Ray fluorescence/Energy Dispersive X-Ray fluorescence (XRF), X-Ray Diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), UV–Visible diffuse reflectance, FTIR spectroscopy, Scanning Electron Microscopy (SEM/EDX) and specific surface area were deployed to characterize the synthetized material after heat-treated at 850 °C. LaFeO₃ crystallizes in a cubic structure (Space Group: Pm3 m) with a crystallite size of 16 nm and BET surface area of 12 m²/g. Field-dependent magnetization was measured at 300 K in the region (±25 kOe) and the perovskite exhibits a high magnetism with a saturation magnetization (0.15 emu/g). Such result indicates that the Fe-3d are in localized high spin state. LaFeO₃ has a narrow band gap of 2.08 eV determined by diffuse reflectance resulting to the crystal field splitting of 3d orbital of Fe³⁺ octahedrally coordinated with an internal d-d transition. Cyclic voltammetry showed the reduction of adsorbed H₂O molecules to gaseous hydrogen at −0.7 V_SCE, a potential less cathodic than the conduction band (−0.45 V_SCE). The latter was determined from the capacitance measurements in alkaline electrolyte (NaOH 0.1 M) where the perovskite exhibits a stability with an exchange current density of ∼0.4 mA cm⁻². The photocatalytic activity reveals an optimal H₂ production of 99 μmol after 20 min at 50 °C in NaOH medium and a catalyst mass of 50 mg, under visible light irradiation (13 W) in the presence of thiosulfate S₂O₃²⁻ as hole scavenger.     

Hydrogen production through an ocean thermal energy conversion system operating at an optimum temperature drop

Storage of electrical energy produced from an ocean thermal energy conversion (OTEC) system is considered to be extremely essential, since the conversion process could take place in a remote offshore area and distant from the actual utilization sites. Energy conversion from an OTEC system into hydrogen energy, which is used for power generation through fuel cells, is an important approach of storing such energy for further utilizations. In this paper, a technical analysis of hydrogen production through an OTEC system coupled with a polymer electrolyte membrane electrolyser (PEM), which is developed by the Japanese international clean energy network using hydrogen conversion (WE-NET), is performed. The analysis is conducted at an optimum temperature drop between the working fluid and seawater, δT_op. Furthermore, the analysis is carried out at various temperature differences between the surface and deep sea water, ΔT. The calculated results demonstrated the significance of temperature drop and temperature difference on the electrical power output and conversion efficiency. Moreover, the actual rate of hydrogen production varied from 2.5 N m³/h to 60 N m³/h as ΔT raised from 5 °C to 25 °C, respectively.     

Glycerol oxidation in solid oxide fuel cells based on a Ni-perovskite electrocatalyst

An investigation of the electrochemical oxidation of glycerol as alternative to hydrogen and methane in solid oxide fuel cells (SOFCs) based on a noble metal-free anode catalyst was carried out. The anode electrocatalyst consisted of a Ni-modified La_0.6Sr_0.4Fe_0.8Co_0.2O₃ (LSFCO) perovskite. After thermal activation, air treatment at 1100 °C followed by reduction at 800 °C in H₂, Ni was mainly present as ultrafine La₂NiO₄ particles homogeneously dispersed on the perovskite surface. The thermal activation also caused a modification of perovskite into a lanthanum depleted structure. The thermal reduction at 800 °C determined the occurrence of metallic Ni on the surface. These results were corroborated by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). A suitable power density (327 mW cm⁻²) was achieved for the electrolyte supported SOFC fed with chemical-grade glycerol in almost dry condition, i.e. steam to carbon ratio (S/C) of 0.2. The highest electrical efficiency (voltage efficiency) approached 50% at the peak power under mild humidification (S/C = 0.2). Whereas an increase of water to glycerol ratio, caused a progressive decrease of voltage efficiency at the peak power down to 44% for S/C = 2.     

Exploration of 1D-2D LaFeO3/RGO S-scheme heterojunction for photocatalytic water splitting

Sustainable energy innovation is spearheading the way to achieve decarbonisation through commercially viable and highly competitive renewable technologies for green hydrogen. Photocatalytic water splitting has received global attention, as it promotes the direct conversion of solar energy to chemical energy and hydrogen production. Lanthanum orthoferrite (LaFeO₃) has been selected due to its narrow bandgap perovskite-oxides (ABO₃) type nature, low cost and high chemical stability but it is limited with fast charge recombination. To circumvent its constraint of fast charge recombination, an efficient graphene-based nanocomposite has been prepared by employing reduced graphene oxide (RGO) nanosheets as charge separators for visible light driven photocatalytic water splitting. Here, we present a thorough physical and spectroscopic characterization of the Lanthanum orthoferrite/Reduced Graphene oxide (LaFeO₃/RGO) nanocomposites, and investigate its photocatalytic and photoelectrochemical performance. The photocurrent density of the nanocomposites demonstrated ∼21 times higher in comparison to pure LaFeO₃. The as-prepared nanocomposites have been successfully used as photocatalysts for H₂ generation through water reduction under visible light. A significant enhancement in H₂ generation has been recorded for nanocomposites (∼82 mmol g⁻¹ h⁻¹) as compared to that of bare LaFeO₃ (∼9 mmol g⁻¹ h⁻¹) which is among the highest values obtained using noble-metal-free graphene-based photocatalytic nanocomposites. This work offers a facile approach for fabricating highly efficient 1D-2D heterostructure for photocatalysis application.     

Enhanced alkaline water splitting on cobalt phosphide sites by 4d metal (Rh)-doping method

The construction of effective water-splitting electrocatalysts in alkaline conditions is challenging due to lower water dissociation efficiency than in acidic conditions. In this study, we investigated the effect of doping 4d and 5d metals into the 3d metal active site of cobalt phosphide (Co_xP) on the water-splitting reaction. Introducing Ru slightly improved hydrogen evolution efficiency, but Rh doping significantly enhanced the catalytic parameters with an overpotential of 0.03 V at 10 mA/cm². Rh regulated the electronic structure of Co_xP to improve proton reduction. The Rh-Co_xP electrode showed a comparable catalytic efficiency to that of a Pt/C standard. Ir doping slightly improved catalytic reactivity, but not as much as Rh. Our results showed that doping 4d metal from the same group as Co maximizes the doping effect during hydrogen evolution. A lab-scale water electrolyzer built with Rh-Co_xP successfully demonstrated catalytic water splitting in alkaline electrolyte.     

Hydrogen production from hydrolysis of ammonia borane with limited water supply

Effect of limited water supply to hydrolysis of ammonia borane for hydrogen evolution is studied over the cases in which the initial molar ratio of water to ammonia borane (H₂O/AB) is set at 1.28, 2.57 and 4.50. The conversion efficiency of ammonia borane to hydrogen is estimated from the accumulated volume of produced hydrogen gas and the quantitative analysis of hydrolysate by solid-state ¹¹B NMR. Characteristics of hydrogen evolution are significantly influenced by both water dosage and injection rate of water. In the case that water is a limiting agent, namely, H₂O/AB = 1.28, less hydrogen is produced than that predicted stoichiometrically. In contrast, conversion efficiency of ammonia borane reaches nearly 100% for the case with H₂O/AB = 4.50. Injection rate of water to ammonia borane also affect profoundly the produced volume and production rate of hydrogen, if water is used as a limiting agent in the hydrolysis of ammonia borane. Nonetheless, boric acid and metaboric acid are found to be the dominant products in the hydrolysate from XRD, FT-IR and solid-state ¹¹B NMR analysis. The hydrogen storage capacity using limited water supply in this work could reach as high as about 5.33 wt%, based on combined mass of reactants and catalyst.     

Thermodynamic investigation on gasification performance of sewage sludge-derived hydrochar: Effect of hydrothermal carbonization

Compared with the conventional thermal drying process, hydrothermal carbonization (HTC) can reduce the energy cost of water removal from sewage sludge prior to its steam gasification. However, less attention is paid on the interactions between HTC and gasification. In this study, the thermodynamic evaluation on hydrochar gasification performance under different operating conditions including HTC duration (τ), HTC temperature (T_HTC), gasification temperature (T_g), and steam/hydrochar mass ratio (S/C ratio) is performed. Two indicators including carbon conversion rate (CC) and cold gas efficiency (CGE) are used to assess the gasification performance. The results show that elevating both gasification temperature and S/C ratio can enhance the H₂ production, which also result in the increase of CC and CGE. The content and gasification activity of fixed carbon increase under moderate HTC duration and temperature, favoring the H₂ formation despite of the apparent loss of volatiles species in the hydrochar. Longer HTC duration or higher HTC temperature declines the H₂ production due to the sharp reduction of carboxyl and hydroxyl groups, weakening water gas reaction and on-site reforming reaction of tar occurred on the hydrochar surface. In terms of the values of CC = 93.9% and CGE = 64.38%, the optimum HTC conditions of τ = 30min and T_HTC = 200 °C can be determined. The data provided here favor guiding HTC treatment of sewage sludge targeting gasification and thus promoting the development of this promising waste-to-energy technology.     

Hydrogen desorption from a hydride container under different heat exchange conditions

The desorption behavior of a hydrogen storage prototype loaded with AB₅H₆ hydride, whose equilibrium pressure makes it suitable for both feeding a PEM fuel cell and being charged directly from a low pressure water electrolyzer without need of additional compression, was studied. The nominal 70 L hydrogen storage capacity of the container (T = 20 °C, P = 101.3 kPa) suffices for ca. 2.5 h operation of a 50 W hydrogen/oxygen fuel cell stack. The hydride container is provided with aluminum extended surfaces to enhance heat exchange with the surrounding medium. These surfaces consist of internal disk-shaped metal foils and external axial fins. The characterization of the storage prototype at different hydrogen discharge flow rates was made by monitoring the internal pressure and the temperatures of the external wall and at the center inside the container.     

Multiple-phases LaFeO3 decorated with sea-urchin-like Au nanoparticles for photoelectrochemical hydrogen generation from bio-ethanol and 1-butanol

Narrow-band-gap LaFeO₃ (approximately 2.1 eV) has good thermal stability, and applications for light-driven water splitting. Thus, heterojunction catalyst containing LaFeO₃ and Au nanoparticles (NPs) are investigated for photoelectrochemical hydrogen production. The use of a citric acid or KOH modifier and calcination yields different LaFeO₃ particle morphologies and phases (La(OH)₃ alone or La(OH)₃ and La₂O₃, respectively), which affect the Au NPs coverage ratio. In addition, the formation of a heterojunction, the surface-plasma resonances (SPRs) effect, and the unique nanospikes on the Au NPs, red-shift happens in the light wavelength and reduce the photonic extinction to less than 2.1 eV. Further, in ethanol and l-butanol under AM 1.5G irradiation, more hydrogen is generated in the former than the latter at all tested temperatures. In addition, the activation energy for the formation of hydrogen is lower in ethanol than in 1-butanol. Finally, an amorphous structure is beneficial for photocatalysis.     

Optimization of hydrogen production by filtration combustion of natural gas by water addition

The effect of adding steam during filtration combustion of natural gas–air mixtures was studied with the aim to evaluate the optimization of hydrogen production. Temperature, velocity, chemical products of combustion waves, and conversion from fuel to H₂ and CO were evaluated in the range of equivalence ratio (φ) from stoichiometric (φ = 1.0) to φ = 3.0 and steam content in the mixture from 0% to 39%, at filtration velocities from 12 to 25 cm/s. Numerical simulation was carried out using GRI-MECH 3.0. Results suggest that H₂ and CO concentrations, dominant for rich and ultrarich combustion, are products from partial oxidation and steam natural gas reforming processes. Experimental and numerical results show that hydrogen yield increase with an increase of steam content in the natural gas–air mixtures.     

Separation of hot electrons and holes in Au/LaFeO3 to boost the photocatalytic activities both for water reduction and oxidation

Construction of plasmon-based nanostructures is an effective way to enhance the photocatalytic activities of semiconductor photocatalysts for water-splitting. However, the synergistic effect of plasmon-related hot electrons and holes for water splitting in the plasmon-hybrid photocatalyst is rarely considered. Herein, we construct a plasmon-based Au/LaFeO₃ composite photocatalyst to investigate the complex roles of hot electrons and holes for solar water splitting. Benefiting from the formation of Schottky junction and surface plasmon resonance effect of the Au nanoparticles, the synthesized photocatalyst exhibits an excellent photocatalytic activity for each half-reaction of water splitting, and the rates for H₂ and O₂ generation are obtained as high as 202 μmol g⁻¹ h⁻¹ and 23 μmol g⁻¹ h⁻¹, respectively. Moreover, an in-depth investigation reveals that the improved hydrogen evolution is caused by the hot electron injection from Au to LaFeO₃, and the hot holes in Au induced by the separation of hot charges can initiate the water oxidation directly on the surface of gold. Thus, this work provides a new insight into the synergistic effect of plasmon-related hot electrons and holes for boosting the photocatalytic reactions.     

Diesel fuel reforming over catalysts derived from LaCo1−xRuxO3 perovskites with high Ru loading

Catalysts derived from LaCo_1−xRu_xO₃ perovskite precursors with high Ru loading (x = 0.2 and 0.4) were studied in the oxidative reforming of diesel for hydrogen production. High partial substitution of Co by Ru in LaCoO₃ perovskite modifies its physicochemical characteristics. It was observed that the high degree of Co substitution in the perovskite changes the rhombohedral crystalline perovskite particles to particles with orthorhombic structure, lower size and higher specific surface area. These modifications affected the structure and morphology of the catalysts derived from the reduction of the perovskite precursors. Catalysts derived from LaCo_1−xRu_xO₃ perovskite precursors with high Ru loading shown particles of La₂O₃ and Co⁰ with small particle size and high surface concentration of Ru⁰ particles. The modifications in the structural characteristics of the catalysts induced by the addition of Ru in the LaCo_1−xRu_xO₃ perovskite precursor had influence on their catalytic behaviour in the oxidative reforming of diesel. The catalyst derived from the perovskite with higher degree of Co substitution (x = 0.4) showed the higher activity and stability for the production of hydrogen for long periods of time-on-stream. The greater Ru exposition achieved in this catalyst was responsible of the increase in the stability observed in this sample taking into account the lower tendency to form carbonaceous deposits of the Ru particles formed.     

Renewable hydrogen from ethanol by a miniaturized nonthermal arc plasma-catalytic reforming system

Ethanol–water mixtures were reformed directly into H₂-rich gas without extra heat source with conversion rates of 69.8% and 88.0% by nonthermal arc plasma and plasma-catalytic reactors, respectively. The plasma reactor consists of a Laval nozzle electrode and a central electrode. The ethanol, water and air mixtures were mixed by a spray nozzle, and then introduced into the Laval nozzle. In terms of energy efficiency, the optimal reforming condition was determined to be O/C ∼ 0.5 and S/C ∼ 1.0 with an ethanol input rate of ∼0.10 g s⁻¹. Furthermore, it is also found that applying Ni/γ-Al₂O₃ catalyst just at the downstream of the discharge region contributed to a better conversion extent and a higher hydrogen production rate, while the power consumption increased slightly, thus the specific energy required for hydrogen production reduced from 68.5 to 40.1 kJ mol⁻¹ at O/C = 0.44, S/C = 1.28 and inlet ethanol = 0.10 g s⁻¹. This reforming technology has promising prospects not only for low-cost hydrogen generation and efficiency improvement for inner combustion engine, but also for many other potential chemical applications, such as nanophase material preparation and solar fuel cell manufacturing.