Boosting the photocatalytic hydrogen production of TiO2 by using copper hexacyanocobaltate as co-catalyst

The development of cheap and efficient co-catalysts is crucial to improve the performance of well-known photocatalysts towards the hydrogen evolution reaction. Here, copper hexacyanocobaltate was evaluated for the first time as a potential candidate to be used as co-catalyst coupled with conventional TiO₂. Copper hexacyanocobaltate was formed by chemical precipitation in the presence of TiO₂, without needing further treatments. The composite exhibited paramount performance towards hydrogen generation, surpassing by up to 16 times the behavior reached with bare TiO₂. This composite also overcome the performance of conventional TiO₂ modified with copper and cobalt oxides derived from copper hexacyanocobaltate. The enhanced behavior of TiO₂/Cu₃[Co(CN)₆]₂ composite was promoted by the efficient separation of photogenerated charge carries, and the faster charge transfer from photocatalyst towards species in solution, as it was proved by the photoelectrochemical characterization of the materials. Furthermore, the composite experienced a slight detriment (15%) in its hydrogen production rate after four consecutive photocatalyst tests. This variation was attributed to the slow leaching of copper in the co-catalyst caused by its partial transformation into metal hydroxides, as it was suggested by the ex-situ XPS characterization. Nevertheless, the structural characterization evinced the presence of the Cu₃[Co(CN)₆]₂ in the composite after long-term use. This study should be considered a proof of concept on a reliable route to obtain appropriate composites for hydrogen production using light as primary energy source.     

Photoelectrochemical hydrogen production from water/methanol decomposition using Ag/TiO2 nanocomposite thin films

Though less frequently studied for solar-hydrogen production, films are more convenient to use than powders and can be easily recycled. Anatase TiO₂ films decorated with Ag nanoparticles are synthesized by a rapid, simple, and inexpensive method. They are used to cleave water to produce H₂ under UV light in the presence of methanol as a hole scavenger. A simple and sensitive method is established here to monitor the time course of hydrogen production for ultralow amounts of TiO₂. The average hydrogen production rate of Ag/TiO₂ anatase films is 147.9 ± 35.5 μmol/h/g. Without silver, it decreases dramatically to 4.65 ± 0.39 μmol/h/g for anatase TiO₂ films and to 0.46 ± 0.66 μmol/h/g for amorphous TiO₂ films fabricated at room temperature. Our method can be used as a high through-put screening process in search of high efficiency heterogeneous photocatalysts for solar-hydrogen production from water-splitting.     

Photocatalytic hydrogen production in the water/methanol system using Pt/RE:NaTaO3 (RE = Y,La, Ce,Yb) catalysts

Perovskite type RE-doped NaTaO₃ (where RE represents Rare Earths elements like Y, La, Ce, Yb) catalysts, derived from solid state synthesis method, have been used for the photocatalytic hydrogen production from water using methanol as sacrificial agent. The photocatalytic activity of NaTaO₃ in H₂ production is improved on its doping with Y, La or Yb elements. In contrast, Ce-doped sample shows a worsening of the photoactivity, which is even lower than the un-doped catalyst. The characterization of the materials reveals that the monoclinic structure and the presence of Ce⁴⁺ do not favour to increase the performance of NaTaO₃. Previous reports proved that La-doping boosted the water splitting photoactivity of NaTaO₃. However the results of this work reveal an increase in the H₂ production by the use of Y as dopant. This enhancement in the photoactivity could be related with the modification of the opto-electronic properties of NaTaO₃ with the inclusion of RE elements (Y, La, Yb) in the orthorhombic structure of this perovskite. Moreover, the catalytic activity of all the RE-doped samples is further increased on their modification with Pt nanoparticles (NPs) loading that act as photo-generated electron scavenger facilitating the reduction reactions.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

Photocatalytic activity of TiO2 nanotube layers coated with Ag and Pt

Abstract

Abstract

Thin films of anatase TiO₂ nanotube arrays (TiO₂ NTs) were prepared in this study. Pt and Ag were coated on the TiO₂ NTs films, which intend to increase the photocatalytic activity under ultraviolet-visible (UV-vis) irradiation. The phase and structure of the films were investigated by X-ray diffraction and scanning electron microscopy. Photocatalytic activity was tested by UV-vis absorption spectroscopy and showed that UV-vis light absorption of the films was remarkably improved by coated Ag and Pt by 72% and 183% respectively. The photocatalytic activities of the films towards degraded methyl orange and HCHO were compared and were all found to follow the sequence Pt/TiO₂ NTs>Ag/TiO₂ NTs>TiO₂ NTs. It was also found that the kinetics of HCHO photocatalytic degradation by the films fits the first order reaction model better and has higher efficiency than that of the methyl orange photocatalytic degradation by the same films.     

Biomass-derived hierarchical porous CdS/M/TiO2 (M = Au,Ag, pt,pd) ternary heterojunctions for photocatalytic hydrogen evolution

We demonstrate a general method for the synthesis of biomass-derived hierarchical porous CdS/M/TiO₂ (M = Au, Ag, Pt, Pd) ternary heterojunctions for efficient photocatalytic hydrogen evolution. A typical biomass—wood are used as the raw sources while five species of wood (Fir, Ash, White Pine, Lauan and Shiraki) are chosen as templates for the synthesis of hierarchical porous TiO₂. The as-obtained products inherited the hierarchical porous features with pores ranging from micrometers to nanometers, with improved photocatalytic hydrogen evolution activity than non-templated counterparts. Noble metals M (M = Pt, Au, Ag, Pd) and CdS are loaded via a two-step photodeposition method to form core (metal)/shell (CdS) structures. The photocatalytic modules—CdS(shell)/metal (core)/TiO₂ heterostructures, have demonstrated to increase visible light harvesting significantly and to increase the photocatalytic hydrogen evolution activity. The H₂ evolution rates of CdS/Pd/TiO₂ ternary heterostructures are about 6.7 times of CdS/TiO₂ binary heterojunctions and 4 times higher than Pd/CdS/TiO₂ due to the vertical electron transfer process. The design of such system is beneficial for enhanced activity from morphology control and composition adjustment, which would provide some new pathways for the design of promising photocatalytic systems for enhanced performance.     

Design,modelling and optimal power and hydrogen management strategy of an off grid PV system for hydrogen production using methanol electrolysis

Hydrogen used as an energy carrier and chemical element can be produced by several processes such as gasification of coal and biomass, steam reforming of fossil fuel and electrolysis of water. Each of these methods has its own advantage and disadvantage. Electrolysis process is seen as the best option for quick hydrogen production. Hydrogen generation by methanol electrolysis process (MEP) gained much attention since it guarantees high purity gas and can be compatible with renewable energies. Furthermore, due to its very low theoretical potential (0.02 V), MEP can save more than 65% of electrical energy required to produce 1 kg of hydrogen compared to water electrolysis process (WEP). Electrolytic hydrogen production using solar photovoltaic (PV) energy is positioned to become as one of the preferred options due to the harmful environmental impacts of widely used methane steam reforming process and also since the prices of PV modules are more competitive.In this paper, hydrogen production by MEP using PV energy is investigated. A design of an off grid PV/battery/MethElec system is proposed. Mathematical models of each component of the system are presented. Semi-empirical relationship between hydrogen production rate and power consumption at 80 °C and 4 M concentration is developed. Optimal power and hydrogen management strategy (PHMS) is designed to achieve high system efficiency and safe operation. Case studies are carried out on two tilts of PV array: horizontal and tilted at 36° using measured meteorological data of solar irradiation and ambient temperature of Algiers site. Simulation results reveal great opportunities of hydrogen production using MEP compared to the WEP with 22.36 g/m² d and 24.38 g/m² d of hydrogen when using system with horizontal and tilted PV array position, respectively.     

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.     

Zinc oxide nanorods based catalysts for hydrogen production by steam reforming of methanol

Zinc oxide (ZnO) nanorods were epitaxially grown on porous cordierite support by a hydrothermal process and utilized for catalyzing methanol steam reforming (MSR) reaction. Catalytic activity of ZnO nanorods for MSR process was correlated to the terminated surfaces of ZnO crystallites. Copper (Cu), palladium (Pd) and gold (Au) nanoparticles infused ZnO nanorods were prepared by in-situ precipitation of the metals on the nanorods. 28% hydrogen selectivity was observed with Cu/ZnO nanorods (Cu/10Zn), while Pd/ZnO nanorods and (Pd/10Zn) showed slightly lower activity. Higher catalytic activity of copper and palladium impregnated ZnO nanorods can be attributed to the synergistic combination of bimetallic oxides. In contrast, Au/ZnO nanorods (Au/10Zn) showed very high activity for methanol dehydrogenation and higher than 97% methanol conversion was achieved for operating temperatures as low as 200 °C.     

Heterostructured SnS2/SnO2 nanotubes with enhanced charge separation and excellent photocatalytic hydrogen production

SnO₂, a promising candidate for photocatalytic water splitting, displays poor activity due to insufficient light utilization and rapid electron-hole recombination of charge carriers. Herein, one-dimensional heterostructures of SnO₂/SnS₂ nanotubes was designed and synthesized through a facile electrospinning followed by vulcanized method. The unique heterostructured SnO₂/SnS₂ could simultaneously promote photocarrier transport and suppress charge recombination through the uniquely coupled SnO₂/SnS₂ heterogeneous interface. Additionally, the optimized type-II heterostructure could also improve light absorption and weak the barrier of photocharge transfer. As a result, the SnO₂/SnS₂ exhibited excellent photocatalytic H₂ evolution performance under simulated light irradiation with high H₂ production rate of 50 μmol h^?1 without the use of any noble metal co-catalyst, which is 4.2 times higher than that of pure SnO₂ under the same condition.     

Catalytic properties of Ag promoted ZnO/Al2O3 catalysts for hydrogen production by steam reforming of ethanol

Ag promoted ZnO/Al₂O₃ catalysts were prepared by using the incipient wetness impregnation method. The catalytic properties of steam reforming reaction for hydrogen production on the prepared catalysts were evaluated with H₂O:C₂H₅OH molar ratios of 3:1 at 450 °C and atmospheric pressure. Ag promoted ZnO/Al₂O₃ catalysts show higher SRE catalytic activity than ZnO/Al₂O₃ catalysts. H₂ and CH₃CHO are the major products on Ag promoted catalysts, and C₂H₄ is also produced probably due to acid sites on Al₂O₃. SRE mechanism on Ag promoted ZnO/Al₂O₃ catalysts, which contains C-C scission, is different from that on ZnO/Al₂O₃ catalysts. A method based on thermogravimetry (TG), differential scanning calorimetry (DSC) and mass spectrometry (MS) was used to analysis the coking behavior on catalyst surface. The surfaces of Ag promoted ZnO/Al₂O₃ catalysts show two different types of coking, and suffer higher coke deposition during the steam reforming reaction.     

Photocatalytic hydrogen production with aid of simultaneous metal deposition using titanium dioxide from aqueous glucose solution

The photocatalytic hydrogen production with aid of simultaneous metal deposition using TiO₂ was investigated in biomass glucose solution. Because the hydrogen production was very trace with pure TiO₂, the simultaneous metal deposition was applied into the glucose solution. The photocatalytic H₂ production activity with TiO₂ was significantly enhanced by simultaneous metal deposition for Au and Pd. The experimental factors such as glucose concentration, metal ion concentration and reaction temperature were investigated. The photocatalytic hydrogen production increased with increasing the concentration of glucose, and it followed Langmuir–Hinshelwood mechanism. Under the optimal conditions, the photocatalytic hydrogen generations from aqueous glucose solution with in-situ Au and Pd deposited TiO₂ were about 203 and 362 times larger compared with those observed with pure TiO₂. The enhanced photocatalytic activity could be explained in terms of reduced electron hole recombination via electron transfer from conductance band of TiO₂ to metal.     

Enhanced photocatalytic hydrogen production from aqueous methanol solution using ZnO with simultaneous photodeposition of Cu

The enhanced photocatalytic hydrogen production from aqueous methanol solution using ZnO was investigated with aid of simultaneous metal deposition. The simultaneous deposition for such metals as Ag, Au, Cu, Ni, Pd, Pt, and Rh was evaluated for the H₂ production from aqueous methanol solution. As a result, the addition of Cu ion was effective improvement in photocatalytic hydrogen evolution. The photocatalytic hydrogen production using ZnO photocatalyst with aid of simultaneous deposition of Cu was approximately 130 times better than those obtained with bare ZnO. The Cu-deposited ZnO had the response to the visible light for the hydrogen formation. After the photocatalytic hydrogen production, the in-situ Cu-photodeposited ZnO sample was characterized by X-ray diffraction (XRD), UV–visible diffuse reflectance spectrometry (UV-DRS), and photoluminescence (PL) spectroscopy.     

Thermal behavior and hydrogen production of methanol steam reforming and autothermal reforming with spiral preheating

The present study aims to investigate the thermal behavior and hydrogen production characteristics from methanol steam reforming (MSR) and autothermal reforming (ATR) under the effects of a Cu-Zn-based catalyst and spiral preheating. Two different reaction temperatures of 250 and 300 °C are taken into account. Meanwhile, the O/C ratio (i.e. the molar ratio between O₂ and methanol) and S/C ratio (i.e. the molar ratio between steam and methanol) are controlled in the ranges of 0-0.5 and 1-2, respectively. The condition of O/C = 0 represents the reaction of MSR. By monitoring the supplied power into the reactor with a fixed gas hourly space velocity (GHSV) of 72,000 h⁻¹, the experimental results indicate that an exothermic reaction from ATR can be attained once the O/C ratio is as high as 0.125. Increasing O/C ratio causes more heat released from the reaction, this results in the decrease in the frequency of supplied power, especially at O/C = 0.5. It is noted that the concentration of CO in the product gas is quite low compared to that of CO₂. An increase in O/C ratio abates the concentration of H₂ from the consumption of per mol methanol; however, the H₂ yield in terms of thermodynamic analysis is increased. On account of the utilization of spiral preheating on the reactants, within the investigated operating conditions the methanol conversion and hydrogen yield were always higher than 95 and 90%, respectively. A comparison suggests that the methanol conversion from ATR of methanol with spiral preheating is superior to those of other studies.     

Manganese-promoted nickel/alumina catalysts for hydrogen production via auto-thermal reforming of ethanol

The manganese-promoted nickel-based catalysts were prepared via wet-incipient impregnation, and tested in auto-thermal reforming (ATR) of ethanol for hydrogen production. The Ni/Al₂O₃ catalyst, in which Ni existed as NiAl₂O₄, produced a low H₂ yield near 1.85 mol H₂/mol ethanol. The Ni–Mn/Al₂O₃ catalyst showed a better performance in a 30-h test: the conversion of ethanol reached 100%, and a higher H₂ yield remained stable near 3.1–3.2 mol H₂/mol ethanol. This improvement can be attributed to the promotion of Mn: With Mn, the ilmenite-type NiMnO₃ was formed, the reducibility of Ni–Mn/Al₂O₃ was thus improved, and there were more Ni⁰ over the surface of catalysts. Moreover, these Ni⁰ species were stable in the ATR test, as indicated by XRD and XPS.     

The evaluation of autothermal methane reforming for hydrogen production over Ni/CeO2 catalysts

Nanocrystalline Ni/CeO₂ catalysts with various loadings of Ni (10, 15, 20, and 25%) were synthesised by a facile solvent deficient precipitation method for methane autothermal reforming process. The characterisation techniques such as XRD, BET, TPH, H₂-TPR were carried out on fresh and spent samples to investigate the catalytic properties of the Ni/CeO₂. On the basis of characterisation results, the 20% Ni/CeO₂ performs the best activity among the catalysts with different Ni contents. The optimal reaction conditions for autothermal methane reforming has been investigated by evaluating the effect of reaction parameters including the reactivity temperature, the gas hourly space velocity (GHSV) and H₂O/CH₄ (S/C) and O₂/CH₄ (O/C) molar ratios. The stability of 20 wt% Ni/CeO₂ catalyst at 700 °C is examined for 20 h on-stream reaction. It reveals that the methane conversion starts a graduate decrease trend from the second 10 h, which is found to be because of the sintering of Ni nanoparticles by TPH and BET analysis.     

The enhanced photocatalytic hydrogen production of the fusiform g-C3N4 modification CaTiO3 nano-heterojunction

The fusiform g-C₃N₄/CaTiO₃ nano-heterojunctions is synthesized via simple preparation using the hydrothermal co-deposition method. The results of morphology and structure imply that the g-C₃N₄ has deposited on the surface of fusiform CaTiO₃ successfully, and the photocatalytic activity of the g-C₃N₄/CaTiO₃ nano-heterojunctions exhibits a remarkably enhancement of 18 times than that of the unmodified sample. Further, proved by the transient photocurrent, PL, EIS and Motty-Scotty plots, the photocatalytic hydrogen production enhancement could be ascribed to the nano-heterojunction at the interface.     

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.     

The optical absorption and hydrogen production by water splitting of (Si,Fe)-codoped anatase TiO2 photocatalyst

The electronic and optical properties are studied using the density functional theory in (Si,Fe)-codoped anatase TiO₂. The calculated results suggest that the synergistic effects of (Si,Fe) codoping can effectively induce the redshift of optical absorption edge, which leads to higher visible-light photocatalytic activity for hydrogen production by water splitting than pure anatase TiO₂. To verify the reliability of our calculated results, nanocrystalline (Si,Fe)-codoped TiO₂ is synthesized by a sol-gel-solvothermal method, and excellent absorption performance and photocatalytic activity for hydrogen production by water splitting are observed in our experiments.     

Giant enhancement of photocatalytic H2 production over KNbO3 photocatalyst obtained via carbon doping and MoS2 decoration

This paper was designed for the first time to improve the photocatalytic activity of KNbO₃ via carbon doping and MoS₂ decoration simultaneously. The efficient photocatalytic hydrogen production was realized on the MoS₂/C-KNbO₃ composite under simulated sunlight irradiation in the present of methanol and chloroplatinic acid. The optimal composite presents a H₂ production rate of 1300  μmol·g^?1·h^?1, which reaches 260 times that of pure KNbO₃. Characterization results of the synthesized composite indicates that the introduction of a small amount of carbon into the KNbO₃ lattice greatly hinders the recombination of electron-hole pairs. The decoration of MoS₂ further induces the separation of charge carriers via trapping the electron in the conduction band of C-KNbO₃, which is proven by the EIS and transient photocurrent response analyses. The remarkably enhanced separation efficiency of electron-hole pairs is believed to be the origin of the excellent photocatalytic performance, though other changes in surface area and optical property may also contribute the photocatalytic process. This study provides a feasible way for the design and preparation of novel photocatalysts with high efficiency.