Ag-doped TiO2 photocatalysts with effective charge transfer for highly efficient hydrogen production through water splitting

The development of efficient metal doped semiconductors for solar energy harvesting to produce hydrogen has attracted significant attention. Herein, the H₂ generation over Ag-doped TiO₂ photocatalyst, synthesized using a simple and cost-effective method based on chemical reduction, was reported. The Ag/TiO₂ exhibited an absorption peak in the visible region and the reduction of the bandgap to 2.5 eV due to surface plasmonic resonance (SPR). X-ray photoelectron spectroscopy revealed the presence of oxygen vacancies and 11% of Ag in Ti–Ag–O phase. The effect of reaction time and photocatalyst loading in the absence and presence of sacrificial reagents (alcohols and sulfur) on water splitting was studied and compared the activity of Ag/TiO₂ with that of bare TiO₂. The H₂ production rate of 23.5 mmol g⁻¹ h⁻¹ (with an apparent quantum yield of 19%), over 1.5Ag/TiO₂, was the highest ever reported so far. The observed higher activity could mainly be attributed to the existence of oxygen vacancies and the Ti–Ag–O phase. The photocatalyst was stable for three consecutive cycles in both the presence and absence of sacrificial reagents. This study offers new insights into the rational design of metal-support hybrid structures for hydrogen production through photocatalytic water splitting.     

A novel dual-bed photocatalytic water splitting system for hydrogen production

A novel dual-bed system was designed to produce hydrogen through photocatalytic water splitting. The system was comprised of a photocatalytic reaction bed and a regeneration bed. Aqueous KI solution and Pt-loaded TiO₂ constituted the photocatalytic reaction bed where hydrogen was produced; meanwhile the hole scavenger iodide ion was oxidized into I_2. The effluent containing I₂ from the photocatalytic bed entered the regeneration bed and passed through a Cu₂O layer where I₂ was reduced to I⁻. The regeneration bed effluent was then recycled to the photocatalytic reaction bed. Since the hole scavenger KI in the photocatalytic bed was constantly kept at a high level through the continuous reduction of I₂ in the regeneration bed, steady production of hydrogen was achieved in the dual-bed system for a much longer period as compared to a single-bed system without regeneration.     

Photocatalytic water splitting for hydrogen production on Au/KTiNbO5

Gold nanoparticles were deposited on potassium titanoniobate, KTiNbO₅ using deposition-precipitation (DP), conventional impregnation (IMP) and photodeposition method in order to improve photocatalytic hydrogen production from water splitting. The effect of synthesis pH value of a HAuCl₄ aqueous solution used in the DP process on the morphology of gold nanoparticles, optical property and photocatalytic activity of water splitting under UV light irradiation was investigated. These catalysts were characterized by powder X-ray diffraction patterns (XRD), inductively coupled plasma mass spectrometry (ICP-MS), UV–visible spectroscopy (UV–vis), and Transmission Electron Microscopy (TEM). The Au/KTiNbO₅ catalysts prepared by the DP method consisted of a good metal–semiconductor interface which allowed for a much higher efficient electron-hole separation. The 0.63 wt% Au/KTiNbO₅ catalyst prepared by the DP method at pH = 10 showed a uniform dispersion of gold nanoparticles with an average gold particle size of 4.2 nm and exhibited an ultra-high photocatalytic water splitting activity (3522 μmol g⁻¹ h⁻¹), about 47 times higher than that exhibited by the KTiNbO₅ photocatalyst.     

Generation-IV reactors and nuclear hydrogen production

There are dozens of hydrogen production methods and techniques from many sources such as fossil fuels, renewable energy sources and nuclear energy in the literature. Thermo-chemical methods are more efficient at higher temperatures to produce large quantities of hydrogen. In this study, a comparative overview of Generation VI nuclear reactor types for major hydrogen production methods have been researched in the literature and suggestions have been carried out.This research work is addressing that both electric power cycle and hydrogen production based on nuclear technologies need to be developed. Generation IV nuclear reactors can provide hydrogen for a worldwide hydrogen economy. Both thermo-chemical and electrolysis (hybrid) processes in hydrogen production have a promising future, especially when integrated with Generation IV nuclear power plants. Efficient heat transfer is required for both high temperature thermodynamic cycles and the high temperature steam electrolysis. Hence, highly efficient heat exchanger designs are one of the key technologies for that purpose.     

SiI2 monolayer as a promising photocatalyst for water splitting hydrogen production under the irradiation of solar light

The feasibility of SiI₂ monolayer as the candidate for photocatalytic water splitting for hydrogen generation under the irradiation of the solar light is explored. The geometrical structure, the electronic and optical properties, the mobility of carrier and strain engineering of the monolayer are investigated based on the first-principles calculations. The results demonstrate SiI₂ monolayer possesses an indirect gap of 2.33 eV (HSE06), and both the band edge and the bandgap match the redox potential conditions of the water splitting for hydrogen generation. There is an obvious optical absorption in the visible light and near-ultraviolet region and can be enhanced by the compressive strain. Moreover, the mobility of the electron is significantly different from that of the hole, implicating that the effective spatial charge separation is expectable and the ratio of the recombination of the photogenerated charge pairs is low. The primary adsorption site of the water molecule is identified. The Gibbs free energy and the adsorption energies are calculated to demonstrate the H₂ generation from the water molecule splitting on the monolayer. All the considered properties support that SiI₂ monolayer can be achieved as a promising candidate for the photocatalytic water splitting for hydrogen production under the irradiation of the solar light.     

A review on frontiers in plasmonic nano-photocatalysts for hydrogen production

Nanoparticles have plenty of active sites in each particle and short travel distance for photoexcitons proved to be beneficial characteristics for photocatalytic hydrogen generation. Noble metal (Au, Ag, Pt, Pd, Ru, and Rh) attached with semiconductor photocatalyst displayed plasmonic oscillations results in localized surface plasmon resonance (LSPR) effect and schottky junction formed to pump the photoelectrons to surface for reactions. Literature reports evidenced that the choice of synthesis method and experimental conditions directly affects the catalytic and optical properties. Hence, this review discusses comprehensive information on chemical methods reported for the preparation of plasmonic photocatalyst that resulted in co-catalyst and visible light sensitizer properties. Recent developments and achievements on plasmonic photocatalyst for hydrogen production in pure water and sacrificial agent containing water are discussed and highlighted.     

Solar water splitting for hydrogen production with monolithic reactors

The present work proposes the exploitation of solar energy for the dissociation of water and production of hydrogen via an integrated thermo-chemical reactor/receiver system. The basic idea is the use of multi-channelled honeycomb ceramic supports coated with active redox reagent powders, in a configuration similar to that encountered in automobile exhaust catalytic aftertreatment.Iron-oxide-based redox materials were synthesized, capable to operate under a complete redox cycle: they could take oxygen from water producing pure hydrogen at reasonably low temperatures (800 °C) and could be regenerated at temperatures below 1300 °C. Ceramic honeycombs capable of achieving temperatures in that range when heated by concentrated solar radiation were manufactured and incorporated in a dedicated solar receiver/reactor. The operating conditions of the solar reactor were optimised to achieve adjustable, uniform temperatures up to 1300 °C throughout the honeycomb, making thus feasible the operation of the complete cycle by a single solar energy converter.     

Modeling of fluidized bed membrane reactors for hydrogen production from steam methane reforming with Aspen Plus

Hydrogen production via steam methane reforming with in situ hydrogen separation in fluidized bed membrane reactors was simulated with Aspen Plus. The fluidized bed membrane reactor was divided into several successive steam methane sub-reformers and membrane sub-separators. The Gibbs minimum free energy sub-model in Aspen Plus was employed to simulate the steam methane reforming process in the sub-reformers. A FORTRAN sub-routine was integrated into Aspen Plus to simulate hydrogen permeation through membranes in the sub-separator based on Sieverts' law. Model predictions show satisfactory agreement with experimental data in the literature. The influences of reactor pressure, temperature, steam-to-carbon ratio, and permeate side hydrogen partial pressure on reactor performances were investigated with the model. Extracting hydrogen in situ is shown to shift the equilibrium of steam methane reactions forward, removing the thermodynamic bottleneck, and improving hydrogen yield while neutralizing, or even reversing, the adverse effect of pressure.     

Synthesis of novel CoxMo1-xS-Cd0.5Zn0.5S composites with significantly improved photocatalytic hydrogen evolution performance under visible-light illumination

Recently, MoS₂ incorporates with Co²⁺ (or Ni²⁺) was found to increase the photocatalytic performance of semiconducting materials more effectively. In this study, novel Co_xMo_1-xS was effectively deposited on the surface of Zn_0.5Cd_0.5S semiconductors as an efficient promotor using in-situ hydrothermal process. The as-prepared Co_xMo_1-xS-Zn_0.5Cd_0.5S composites are examined by the following techniques: XRD, TEM, DRS, XPS, PL and TRPL. The photocatalytic hydrogen evolution performance under visible illumination over Zn_0.5Cd_0.5S is remarkably increased by adding cheap Co_xMo_1-xS as promotor. The Co_xMo_1-xS-Zn_0.5Cd_0.5S hybrid specimen with 10% molar amount illustrates the best catalytic performance with a homologous hydrogen generation rate of 188.65 μmol h⁻¹, which is estimated to be 14.5 folds than that of unmodified Zn_0.5Cd_0.5S specimen in the presence of visible light. The apparent quantum yield of Co_0.3Mo_0.7Zn_0.5Cd_0.5S sample is determined to be 16.72% at monochromatic light of 420 nm. The experimental outcomes indicate that the synergistic action between Co_xMo_1-xS and Zn_0.5Cd_0.5S obviously promotes transfer of photo-induced charge carriers in the hybrid sample. A reasonable catalytic mechanism for the increased photocatalytic performance of Co_xMo_1-xS promotor was presented and authenticated by TRPL measure, which would present a new notion for the design of ideal semiconductors with plummy photocatalytic capability.     

Solar hydrogen production and its development in China

Because of the needs of sustainable development of the mankind society and natural environment building a renewable energy system is one of the most critical issues that today's society must address. In the new energy system there is a requirement for a renewable fuel to replace current energy carrier. Hydrogen is an ideal secondary energy. Using solar energy to produce hydrogen in large scale can solve the problems of sustainability, environmental emissions, and energy security and become the focus of the international society in the area of energy science and technology. It has also been set as an important research direction by many international hydrogen programs. The Ministry of Science and Technology of China supported and launched a project of National Basic Research Program of China (973 Program) – the Basic Research of Mass Hydrogen Production using Solar Energy in 2003 for R&D in the areas of solar hydrogen production. The current status of solar hydrogen production research is reviewed and some significant results achieved in the project are reported in this paper. The trends of development and the future research directions in the field of solar hydrogen production in China are also briefly discussed.     

Hydrogen production from methane using vanadium-based catalytic membrane reactors

The application of vanadium-based membranes as the hydrogen separation membrane for a catalytic membrane reactor system was investigated for the direct production of hydrogen from methane. The methane conversion and hydrogen production rates of the catalytic membrane reactor system with Pd-coated 100 μm-thick vanadium-based membranes were comparable with the reactor using 50 μm-thick Pd–Ag alloy membrane at all temperatures examined. The methane conversion rates of the catalytic membrane reactor with the Pd-coated vanadium-based membranes were approximately 35% and 62% at 623 K and 773 K, respectively. The hydrogen production rates were around 660  μmol min⁻¹ at 623 K, and reached over 1710  μmol min⁻¹ at 773 K. The relationship between the methane conversion rates and hydrogen permeation fluxes of the catalytic membrane reactor confirmed that the removal of hydrogen from the reaction site enhances the methane decomposition reaction. Further, the vanadium based membrane exhibited good stability against Fe in a hydrogen containing atmosphere.     

Pristine and Se-/In-doped TlAsS2 enhance the solar energy-driven water splitting for hydrogen generation

The enhanced photocatalytic performance of Se-/In-doped TlAsS₂ to generate hydrogen from water splitting is investigated based on the first-principle density functional theory calculation with meta-GGA + TPSS. Three structures, namely, pristine TlAsS₂ and substitutions of S with Se and Tl with In, are considered. Their geometrical lattices are fully optimized and their electronic and optical properties are calculated to evaluate the photocatalytic efficiency for hydrogen generation. Results show that the three structures can be used for solar energy photocatalysis to generate hydrogen from water splitting. Moreover, the Se- and In-doped atoms can strengthen the absorption coefficient within the visible light range. Therefore, these structures are promising catalysts for generating hydrogen from water splitting through solar energy photocatalysis.     

Synthesis of carbon-doped SnO2 nanostructures for visible-light-driven photocatalytic hydrogen production from water splitting

Environmental issues: global warming, organic pollution, CO₂ emission, energy shortage, and fossil fuel depletion have become severe threats to the future development of humans. In this context, hydrogen production from water using solar light by photocatalytic/photoelectrochemical technologies, which results in zero CO₂ emission, has received considerable attention due to the abundance of solar radiation and water. Herein, a single-step thermal decomposition procedure to produce carbon-doped SnO₂ nanostructures (C–SnO₂) for photocatalytic applications is proposed. The visible-light-driven photocatalytic performance of the as-prepared materials is evaluated by photocatalytic hydrogen generation experiments. The bandgaps of the photocatalysts are determined by ultraviolet–visible diffused reflectance spectroscopy. The crystallinity, morphological features (size and shape), and chemical composition and elemental oxidation states of the samples are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The proposed simple thermal decomposition method has significant potential for producing nanostructures for metal-free photocatalysis.     

Membrane reactors for green hydrogen production from biogas and biomethane: A techno-economic assessment

This work investigates the performance of a fluidized-bed membrane reactor for pure hydrogen production. A techno-economic assessment of a plant with the production capacity of 100 kg_H2/day was carried out, evaluating the optimum design of the system in terms of reactor size (diameter and number of membranes) and operating pressures. Starting from a biomass source, hydrogen production through autothermal reforming of two different feedstock, biogas and biomethane, is compared.Results in terms of efficiency indicates that biomethane outperforms biogas as feedstock for the system, both from the reactor (97.4% vs 97.0%) and the overall system efficiency (63.7% vs 62.7%) point of views. Nevertheless, looking at the final LCOH, the additional cost of biomethane leads to a higher cost of the hydrogen produced (4.62 €/kg_H2@20 bar vs 4.39 €/kg_H2@20 bar), indicating that at the current price biogas is the more convenient choice.     

Reprint of: Solar hydrogen production and its development in China

Because of the needs of sustainable development of the mankind society and natural environment building a renewable energy system is one of the most critical issues that today's society must address. In the new energy system there is a requirement for a renewable fuel to replace current energy carrier. Hydrogen is an ideal secondary energy. Using solar energy to produce hydrogen in large scale can solve the problems of sustainability, environmental emissions, and energy security and become the focus of the international society in the area of energy science and technology. It has also been set as an important research direction by many international hydrogen programs. The Ministry of Science and Technology of China supported and launched a project of National Basic Research Program of China (973 Program) – the Basic Research of Mass Hydrogen Production using Solar Energy in 2003 for R&D in the areas of solar hydrogen production. The current status of solar hydrogen production research is reviewed and some significant results achieved in the project are reported in this paper. The trends of development and the future research directions in the field of solar hydrogen production in China are also briefly discussed.     

Recent advances in carbon nitride-based nanomaterials for hydrogen production and storage

There are different types of materials comprising of carbon and nitrogen elements. Typical materials are the cyanogen family, beta carbon nitride, graphitic carbon nitride, azafullerenes, and heterofullerenes, N-containing heterocycles. Except cyanogen (C₂N₂) is a gas, most others are solid. Among these solids, the graphitic carbon nitride, with the general chemical formula of C₃N₄, is widely studied in heterogeneous catalysis and energy storage. Such applications exploit the resiliency of the material in different environments due to its labile protons and Lewis acid functionalities, as well as its layered structure. The structure of graphitic C₃N₄ allows it to store a significant amount of hydrogen. Furthermore, it offers the space for dopants, which are used purposely for tuning the band gap and the electronic properties of C₃N₄ to make it suitable for water splitting using sun light, or many other applications in waste water treatment under radiation. We think that the material is important and it is not being exploited at its highest capability, especially in hydrogen production via water splitting technique. This review aims to summarize recent outcomes using the carbon nitride material in hydrogen production, and a brief about hydrogen storage. We also highlight future research directions which might worth being persuaded.     

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.     

A process dynamic modeling and control framework for performance assessment of Pd/alloy-based membrane reactors used in hydrogen production

In the present study a comprehensive, insightful and practical process dynamic modeling framework is developed in order to analyze and characterize the transient behavior of a Pd/alloy-based (Pd/Au or Pd/Cu) water-gas shift (WGS) membrane reactor. Furthermore, simple process control ideas are proposed aiming at enhancing process system performance by inducing the desirable dynamic characteristics in the response of the controlled process during start-up as well as in the presence of unexpected adverse disturbances (process upset episodes) or operationally favorable set-point changes that reflect new hydrogen production requirements. Finally, the proposed methods are evaluated through detailed simulation studies in an illustrative example involving a Pd/alloy-based WGS membrane reactor that exhibits complex dynamic behavior and is currently used for lab-scale pure hydrogen production and separation.     

Mathematical modeling of an anion-exchange membrane water electrolyzer for hydrogen production

A mathematical model that considers coupled mass transport, charge transport, and electrochemical reactions in an anion-exchange membrane (AEM) water electrolyzer is developed. Validations against the literature experimental data show that the present model can accurately predict the performance of the water electrolyzer. Numerical results show that the voltage loss in the electrolyzer is majorly due to the activation polarizations of hydrogen and oxygen evolution reactions. The effects of the exchange current density, the membrane thickness, and the liquid saturation on the performance are also studied; it is shown that the performance of the water electrolyzer improves with an increase in the exchange current density and liquid saturation, but with a decrease in the membrane thickness.     

Optofluidic microreactor for the photocatalytic water splitting to produce green hydrogen

The microfluidic devices can effectively be used for the renewable energy conversion, such as solar to chemical (e.g., H₂) energy, to meet the global energy demand. The microchannel design plays a vital role in improving the mass transfer in photocatalytic processes. In this study, a simple, rapid, and inexpensive adhesive tape-based method was used to fabricate the serpentine, planar and micropillared optofluidic microreactors with sharp edges without any wall irregularities. The sol-gel method was used for the CdS catalyst coating in the microreactors. The effect of liquid flow rate (0.05–1 mL min^?1) and sacrificial reagent (Na₂SO₃/Na₂S) concentration (0.05–0.5 M) on the hydrogen generation under visible light was studied. A higher H₂ production rate was observed in the serpentine microreactor as compared to that in planar and micropillared microreactors. The serpentine microreactor, having higher surface-to-volume ratio, induced the micromixing that enhanced the mass transfer of the sacrificial reagent and formed H₂ gas. A maximum H₂ production rate of 2.65 μmol h^?1 cm^?2 was observed at a flow rate of 1.0 mL min^?1 and a sacrificial reagent concentration (Na₂SO₃/Na₂S) of 0.5 M. The new approach developed in this study is a step forward in fabricating highly efficient and inexpensive optofluidic microdevices for the hydrogen production from solar energy.