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.     

Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO

TiO₂ photocatalyst with deposited CuO (CuO-TiO₂) was synthesized by the impregnation method using P25 (Degussa) as support, and exhibited high photocatalytic hydrogen generation activity from methanol/water solution. A substantial hydrogen evolution rate of 10.2 ml min⁻¹ (18,500 μmol h⁻¹ g⁻¹_catalyst) was observed over this efficient CuO-TiO₂ with optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol; this improved hydrogen generation rate is significantly higher than the reported Cu-containing TiO₂, including some Pt and Pd loaded TiO₂. Optimal Cu content of 9.1 mol% provided maximum active sites and allowed good light penetration in TiO₂. Over this efficient CuO-TiO₂, the hydrogen generation rate was accelerated by increasing the methanol concentration according to Freundlich adsorption isotherm. However, the photocatalytic hydrogen generation rate was suppressed under long time irradiation mainly due to accumulation of by-products, reduction of CuO and copper leaching, which requires further investigation.     

Photocatalytic water splitting with In-doped H2LaNb2O7 composite oxide semiconductors

The In-doped HLaNb₂O₇ oxide semiconductors synthesized by solid-state reaction followed by an ion-exchange reaction were found to be a novel composite photocatalyst system with enhanced activity for water splitting. Pt was incorporated in the interlayer of In-doped HLaNb₂O₇ by the stepwise intercalation reaction. The In-doped HLaNb₂O₇ powder samples were characterized with X-ray diffraction (XRD) and UV-vis diffuse reflectance spectrometry. The photocatalytic activities of Pt-loaded In-doped HLaNb₂O₇ and individual precursor materials were evaluated by H₂ evolution from aqueous CH₃OH solution under UV light irradiation. It was found that the composite In-doped HLaNb₂O₇ showed a higher H₂ evolution rate in comparison with individual materials. The hydrogen production activity of In-doped HLaNb₂O₇ was greatly enhanced by Pt co-incorporation. The In content in the In-doped HLaNb₂O₇ system was discussed in relation to the photophysical and photocatalytic properties. As In content equal 5 mol%, the HLaNb₂O₇:In/Pt showed a photocatalytic activity of 354 cm³ g⁻¹ hydrogen evolution in 10 vol% methanol solution under irradiation from a 100 W mercury lamp at 333 K for 3 h.     

Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water

Efficient Cu incorporated TiO₂ (Cu–TiO₂) photocatalysts for hydrogen generation were fabricated by four methods: in situ sol–gel, wet impregnation, chemical reduction of Cu salt, and in situ photo-deposition. All prepared samples are characterized by good dispersion of Cu components, and excellent light absorption ability. Depending on the preparation process, hydrogen generation rates of the as-prepared Cu–TiO₂ were recorded in the range of 9–20 mmol h⁻¹ g_catalyst⁻¹, which were even more superior to some noble metal (Pt/Au) loaded TiO₂. The various fabrication methods led to different chemical states of Cu, as well as different distribution ratio of Cu between surface and bulk phases of the photocatalyst. Both factors have been proven to influence photocatalytic hydrogen generation. In addition, the Cu content in the photocatalyst played a significant role in hydrogen generation. Among the four photocatalysts, the sample that was synthesized by in situ sol–gel method exhibited the highest stability. High efficiency, low cost, good stability are some of the merits that underline the promising potential of Cu–TiO₂ in photocatalytic hydrogen generation.     

Efficient photocatalytic hydrogen generation by Pt modified TiO2 nanotubes fabricated by rapid breakdown anodization

Photo-assisted hydrogen generation studies of platinum loaded titanium (IV) oxide nanotubes suspended in ethanol–water mixture were carried out at room temperature. The TiO₂ nanotubes synthesized by rapid breakdown anodization technique were loaded with Pt nanoparticles by chemical reduction of aqueous chloroplatinic acid solution using sodium borohydride. The chemisorption (active) surface area of the synthesized nanocomposites for hydrogen was measured by pulse chemisorption method using temperature programmed desorption reduction oxidation equipment and found to decrease with increase in platinum loading in the range 1–10 wt%. The platinum supported nanotube composites were characterized for phase and morphology by XRD, TEM and SEM. The hydrogen generated by the photocatalytic reduction of water from water–ethanol mixture at different wavelengths of incident light, using the Pt-TiO₂ nanocomposite photocatalyst, was determined by using a proton exchange membrane based hydrogen meter. The highest hydrogen generation efficiency was observed at 1–2.5 wt% of Pt loading. The maximum photocatalytic hydrogen generation of 0.03 mol/h/g of Pt-TiO₂ was observed with a 64 W UV light source (λ = 254 nm). The photoluminescence property of the Pt loaded TiO₂ has been correlated with the hydrogen generation efficiency and the reaction mechanism briefly discussed.     

Hydrogen production from water splitting under UV light irradiation over Ag-loaded mesoporous-assembled TiO2-ZrO2 mixed oxide nanocrystal photocatalysts

Hydrogen production from the photocatalytic water splitting reaction is very attractive because it is an environmentally friendly process, where hydrogen is produced from two abundantly renewable sources, i.e. water and solar energy, with the aid of photocatalysts. TiO₂ is the most widely investigated photocatalyst; however, it alone still exhibits low performance to photocatalytically produce hydrogen. Hence, the aim of this work focused on the enhanced photocatalytic hydrogen production over Ag-loaded mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalysts under UV light irradiation. The TiO₂-ZrO₂ mixed oxides with various TiO₂-to-ZrO₂ molar ratios were synthesized by a sol-gel process with the aid of a structure-directing surfactant, followed by Ag loading via a photochemical deposition method. The influences of photocatalyst preparation parameters, i.e. calcination temperature, phase composition, and Ag loading, were studied. The results revealed that the mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalyst with a TiO₂-to-ZrO₂ molar ratio of 93:7 calcined at 500 °C exhibited the highest photocatalytic hydrogen production activity, and the Ag loading of 0.5 wt.% further greatly enhanced the photocatalytic activity of such TiO₂-ZrO₂ mixed oxide photocatalyst.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Photocatalytic activity of N, S co-doped and N-doped commercial anatase TiO2 powders towards phenol oxidation and E. coli inactivation under simulated solar light irradiation

Nitrogen and sulfur co-doped and N-doped TiO₂ anatase TKP 102 (Tayca) were prepared by manual grinding with thiourea and urea, respectively, and annealing at 400 °C. Both materials showed visible-light absorption as measured by Diffuse Reflectance Spectroscopy (DRS). Interstitial N-doping, anionic and cationic S-doping was found when the TiO₂ was doped with thiourea while TiO₂ doped with urea showed only the presence of interstitial N-doping as measured by X-ray Photo-electron Spectroscopy (XPS). The N content on the surface of N-doped TKP 102 photocatalyst was 2.85 at.% and higher than the N content in the N, S co-doped TiO₂ photocatalyst (0.6 at.%).The photocatalytic activity of the doped catalysts was tested using phenol and Escherichia coli as chemical and biological targets, respectively, using N, S co-doped, N-doped TiO₂, undoped Degussa P-25 and undoped TKP 102 powders under simulated solar light. It was found that undoped Degussa P-25 was the photocatalyst with the highest photocatalytic activity towards phenol oxidation and E. coli inactivation. N, S co-doped powders showed almost the same photocatalytic activity as undoped TKP 102 while N-doped TKP 102 was the less active photocatalyst probably due the N impurities on the TiO₂ acting as recombination centers.     

Cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal synthesis of ZnIn2S4 as an efficient visible-light-driven photocatalyst for hydrogen production

A series of ZnIn₂S₄ photocatalysts was synthesized via a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal method. These ZnIn₂S₄ products were characterized by X-ray diffraction (XRD), UV–visible absorption spectra (UV–vis) and scanning electron microscopy (FESEM). The effects of hydrothermal time and CTAB on the crystal structures, morphologies and optical properties of ZnIn₂S₄ products were discussed in detail. The photocatalytic activities of the as-prepared samples were evaluated by photocatalytic hydrogen production from water under visible-light irradiation. It was found that the photocatalytic activities of these ZnIn₂S₄ products decreased with the hydrothermal time prolonging while increased with the amount of CTAB increasing. The highest quantum yield at 420 nm of ZnIn₂S₄ photocatalyst, which was prepared through the CTAB (9.6 mmol)-assisted hydrothermal procedure for 1 h, was determined to be 18.4%. The optimum amount of Pt loaded for the ZnIn₂S₄ photocatalyst was about 1.0 wt%, under the present photocatalytic system.     

Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light

The advantage of copper doping onto TiO₂ semiconductor photocatalyst for enhanced hydrogen generation under irradiation at the visible range of the electromagnetic spectrum has been investigated. Two methods of preparation for the copper-doped catalyst were selected – complex precipitation and wet impregnation methods – using copper nitrate trihydrate as the starting material. The dopant loading varied from 2 to 15%. Characterization of the photocatalysts was done by thermogravimetric analysis (TGA), temperature programmed reduction (TPR), diffuse reflectance UV-Vis (DR-UV-Vis), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Photocatalytic activity towards hydrogen generation from water was investigated using a multiport photocatalytic reactor under visible light illumination with methanol added as a hole scavenger. Three calcination temperatures were selected – 300, 400 and 500 °C. It was found that 10 wt.% Cu/TiO₂ calcined at 300 °C for 30 min yielded the maximum quantity of hydrogen. The reduction of band gap as a result of doping was estimated and the influence of the process parameters on catalytic activity is explained.     

Preparation and photocatalytic properties of HLaNb2O7/(Pt,TiO2) perovskite intercalated nanomaterial

A novel perovskite intercalated nanomaterial HLaNb₂O₇/(Pt, TiO₂) is fabricated by successive intercalated reaction of HLaNb₂O₇ with [Pt(NH₃)₄]Cl₂ aqueous solution, n-C₆H₁₃NH₂/C₂H₅OH organic solution and acidic TiO₂ colloid solution, followed by ultraviolet light irradiation. The gallery height and the band gap energy of HLaNb₂O₇/(Pt, TiO₂) is less than 0.6 nm and 3.14 eV, respectively. The photocatalytic activity of HLaNb₂O₇/TiO₂ is superior to that of unsupported TiO₂ and is enhanced by the co-incorporation of Pt. The photocatalytic hydrogen evolution based on HLaNb₂O₇/(Pt, TiO₂) is 240 cm³ h⁻¹ g⁻¹ using methanol as a sacrificial agent under irradiation with wavelength more than 290 nm from a 100-W mercury lamp. High photocatalytic activity of HLaNb₂O₇/(Pt, TiO₂) may be due to the host with rare earth La element and perovskite structure, the quantum size effect of intercalated semiconductor and the coupling effect between host and guest.     

Visible light induced hydrogen evolution on new hetero-system ZnFe2O4/SrTiO3

The physical properties and photoelectrochemical characterization of the spinel ZnFe₂O₄, elaborated by chemical route, have been investigated for the hydrogen production under visible light. The forbidden band is found to be 1.92 eV and the transition is indirectly allowed. The electrical conduction occurs by small polaron hopping with activation energy of 0.20 eV. p-type conductivity is evidenced from positive thermopower and cathodic photocurrent. The flat band potential (0.18 V_SCE) determined from the capacitance measurements is suitably positioned with respect to H₂O/H₂ level (−0.85 V_SCE). Hence, ZnFe₂O₄ is found to be an efficient photocatalyst for hydrogen generation under visible light. The photoactivity increases significantly when the spinel is combined with a wide band gap semiconductor. The best performance with a hydrogen rate evolution of 9.2 cm³ h⁻¹ (mg catalyst)⁻¹ occurs over the new hetero-system ZnFe₂O₄/SrTiO₃ in Na₂S₂O₃ (0.025 M) solution.     

Understanding the effect of TiO2, VCl3, and HfCl4 on hydrogen desorption/absorption of NaAlH4

The main objective of this work was to investigate the different effects of transition metals (TiO₂, VCl₃, HfCl₄) on the hydrogen desorption/absorption of NaAlH₄. The HfCl₄ doped NaAlH₄ showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl₃ or TiO₂ desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH₄ doped with TiO₂ reduced to 140 °C. On the contrary, in the case of NaAlH₄ doped with HfCl₄ or VCl₃, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH₄ better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5 wt% by doping with TiO₂, while the amount of restored hydrogen was lower for HfCl₄ and VCl₃ doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO₂ doped samples. In contrast, there was evidence of Al–V alloy in the VCl₃ doped sample and Al–Hf alloy in the HfCl₄ doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH₄ showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.     

Study on photocurrent of bilayers photoanodes using different combination of WO3 and Fe2O3

Bilayer photoanodes were prepared onto glass substrates (FTO) in order to improve generated photocurrents using UV-vis light by water splitting process. A comparative study of photocatalytic was performed over the films surface using Fe₂O_3, WO₃ and mixture of bicomponents (Fe₂O₃:WO₃). Different types of films were prepared using Fe₂O_3, WO₃ and bicomponents (mixture) on FTO substrates. The films were grown by sol gel method with the PEG-300 as the structure-directing agent. The photo-generated of the samples were determined by measuring the currents and voltages under illumination of UV-vis light. The morphology, structure and related composition distribution of the films have been characterized by SEM, XRD and EDX respectively. Photocurrent measurements indicated surface roughness as the effective parameter in this study. The deposited surfaces by bicomponents or mixture are flat without any feature on the surface while the deposited surfaces by WO₃ appears rough surface as small round (egg-shaped particles) and cauliflower-like. The surface deposited by Fe₂O₃ show rough no as well as WO₃ surface. The deposited surfaces by WO₃ reveal the higher value of photocurrent measurement due to surface roughness. Indeed, the roughness can be effective in increasing contact surface area between film and electrolyte and diffuse reflection (light scattering effect). The solution (Fe₂O₃:WO₃) shows the low photocurrent value in compare to WO₃ and Fe₂O₃ hat it may be due to decomposition the compound at 450 ± 1 °C to iron-tungstate Fe₂(WO₄)₃.     

Physical and photoelectrochemical studies for hydrogen photo-evolution over the spinel ZnCr2O4

The present work deals with the photoelectrochemical hydrogen production over the spinel ZnCr₂O₄. The photoactivity is dependent on the synthesis conditions and the oxide has been prepared by nitrate way in order to produce homogeneous powder with large active surface. The transport properties indicate p-type conductivity with activation energy of 0.21 eV. A corrosion potential of 0.404 V_SCE and an exchange current density of 50 μA cm⁻² have been determined from the semi logarithm plot. The photocurrent onset potential, assimilated to the flat band potential, was found to be −0.39 V_SCE. ZnCr₂O₄/S₂O₃²⁻ is a self driven system where absorption of light promotes electrons into the conduction band with a potential (−1 V) sufficient to reduce water into hydrogen. The activity shows a tendency toward saturation whose deceleration is the result of the competitive reductions of end products namely S₂O₆²⁻ and S₂O₄²⁻ with water. A comparative study with CuCr₂O₄ is reported.     

Photocatalytic H2 evolution on a novel CaIn2S4 photocatalyst under visible light irradiation

A novel visible-light-driven photocatalyst CaIn₂S₄ was synthesized using a facile hydrothermal method followed by a post-calcination process. The influence of the calcination temperature and time on the activities of the photocatalyst was investigated. CaIn₂S₄ exhibits optical absorption predominantly in visible region with an optical band gap of 1.76 eV. Considerable activity for hydrogen evolution from pure water was observed without any sacrificial agents or cocatalysts under visible light irradiation. The maximum hydrogen evolution rate achieved was 30.92 μmol g⁻¹ h⁻¹ without obvious deactivation of the photocatalytic activity for four consecutive runs of 32 h.     

Preferential oxidation of CO in H2 stream on Au/CeO2–TiO2 catalysts

A series of Au catalysts supported on CeO₂–TiO₂ with various CeO₂ contents were prepared. CeO₂–TiO₂ was prepared by incipient-wetness impregnation with aqueous solution of Ce(NO₃)₃ on TiO₂. Gold catalysts were prepared by deposition–precipitation method at pH 7 and 65 °C. The catalysts were characterized by XRD, TEM and XPS. The preferential oxidation of CO in hydrogen stream was carried out in a fixed bed reactor. The catalyst mainly had metallic gold species and small amount of oxidic Au species. The average gold particle size was 2.5 nm. Adding suitable amount of CeO₂ on Au/TiO₂ catalyst could enhance CO oxidation and suppress H₂ oxidation at high reaction temperature (>50 °C). Additives such as La₂O₃, Co₃O₄ and CuO were added to Au/CeO₂–TiO₂ catalyst and tested for the preferential oxidation of CO in hydrogen stream. The addition of CuO on Au/CeO₂–TiO₂ catalyst increased the CO conversion and CO selectivity effectively. Au/CuO–CeO₂–TiO₂ with molar ratio of Cu:Ce:Ti = 0.5:1:9 demonstrated very high CO conversion when the temperature was higher than 65 °C and the CO selectivity also improved substantially. Thus the additive CuO along with the promoter and amorphous oxide ceria and titania not only enhances the electronic interaction, but also stabilizes the nanosize gold particles and thereby enhancing the catalytic activity for PROX reaction to a greater extent.     

Catalytic effect of Gd2O3 and Nd2O3 on hydrogen desorption behavior of NaAlH4

This study investigated the effect of Nd₂O₃ and Gd₂O₃ as catalyst on hydrogen desorption behavior of NaAlH₄. Pressure-content-temperature (PCT) equipment measurement proved that both two oxides enhanced the dehydrogenation kinetics distinctly and increasing Nd₂O₃ and Gd₂O₃ from 0.5 mol% to 5 mol% caused a similar effect trend that the dehydrogenation amount and average dehydrogenation rate increased firstly and then decreased under the same conditions. 1 mol% Gd₂O₃–NaAlH₄ presented the largest hydrogen desorption amount of 5.94 wt% while 1 mol% Nd₂O₃–NaAlH₄ exerted the fastest dehydrogenation rate. Scanning Electron microscopy (SEM) analysis revealed that Gd₂O₃–NaAlH₄ samples displayed uniform surface morphology that was bulky, uneven and flocculent. The difference of Nd₂O₃–NaAlH₄ was that with the increasing of Nd₂O₃ content, the particles turned more and more big. Compared to dehydrogenation behavior, this phenomenon demonstrated that small particles structure were beneficial to hydrogen desorption. Besides, the further study found that different catalysts and addition amounts had different effects on the microstructure of NaAlH_4.     

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.     

Li3V2(PO4)3 cathode material synthesized by chemical reduction and lithiation method

The monoclinic-type Li₃V₂(PO₄)₃ cathode material was synthesized via calcining amorphous Li₃V₂(PO₄)₃ obtained by chemical reduction and lithiation of V₂O₅ using oxalic acid as reducer and lithium carbonate as lithium source in alcohol solution. The amorphous Li₃V₂(PO₄)₃ precursor was characterized by using TG–DSC and XPS. The results showed that the V⁵⁺ was reduced to V³⁺ by oxalic acid at ambient temperature and pressure. The prepared Li₃V₂(PO₄)₃ was characterized by XRD and SEM. The results indicated the Li₃V₂(PO₄)₃ powder had good crystallinity and mesoporous morphology with an average diameter of about 30 nm. The pure Li₃V₂(PO₄)₃ exhibits a stable discharge capacity of 130.08 mAh g⁻¹ at 0.1 C (14 mA g⁻¹).