The promoted effect of UV irradiation on preferential oxidation of CO in an H2-rich stream over Au/TiO2

The catalytic oxidation of CO over Au/TiO₂ in an H₂-rich stream was performed under UV irradiation. It is found that UV irradiation over Au/TiO₂ promotes the preferential oxidation of CO in the H₂-rich stream. The respective chemisorption of CO, H₂ and O₂ at Au/TiO₂ can be described as a process of forming –OH or H₂O species. UV irradiation over Au/TiO₂ enhances the chemisorption of CO but suppresses the chemisorption of H₂ both at TiO₂ and Au surface. It is proposed that the photogenerated electrons from TiO₂ will cause the change of the chemisorption of CO, H₂ and O₂ at Au/TiO₂, which promotes the preferential oxidation of CO in an H₂-rich stream.     

Activity of Au/ZnO catalysts prepared by photo-deposition for the preferential CO oxidation in a H2-rich gas

In this work, Au supported over ZnO prepared by photodeposition was applied to prepare nano-size Au catalysts by utilizing UV light for the preferential oxidation (PROX) of CO. The results demonstrated that Au can be dispersed homogeneously over ZnO in the size range of 1–2 nm with a narrow size distribution. It was clearly seen that the preparation parameters (i.e. irradiation time, precipitant concentration, calcination, and storage condition) had a significant effect on the catalytic activity. Among the variables studied, low concentrations of precipitant and long irradiation time were by far the most influential on the catalytic activity.     

Effect of synthesis pH and Au loading on the CO preferential oxidation performance of Au/MnOx-CeO2 catalysts prepared with ultrasonic assistance

As a novel and rather convenient method, ultrasonic pretreatment was employed for the preparation of nanostructured Au/MnO_x-CeO₂ (Mn/Ce = 1:1) catalysts which were used for CO preferential oxidation. The effects of synthesis pH (7.0-11.0) and Au loading (0.5-5.0 wt.%) on the performance of these catalysts were systematically investigated. It is found that the Au(1.0)/MnO_x-CeO₂-10.0 with 1.0 wt.% Au prepared at pH = 10.0 exhibits the best catalytic performance, giving not only the highest CO conversion of 90.9% but also the highest oxygen to CO₂ selectivity of 47.8% at 120 °C. The results of XRD, HR-TEM and XPS indicate that this catalyst possesses the highest dispersion of Au species and the largest amount of surface adsorbed oxygen species, which facilitates CO oxidation. The H₂-TPR results reveal that the selectivity of oxygen to CO₂ is mainly determined by the reducibility of Au species in the catalysts. The strong interaction between Au species and the supports in the catalyst Au(1.0)/MnO_x-CeO₂-10.0 decreases its capability for H₂ dissociation, effectively inhibiting the hydrogen spillover, as a result, the selectivity of oxygen to CO₂ is remarkably increased.     

Au/TiO2 catalysts prepared by photo-deposition method for selective CO oxidation in H2 stream

A series of Au/TiO₂ catalysts were prepared by photo-deposition (PD) method. Various preparation parameters, such as pH value, power of UV light and irradiation time on the characteristics of the catalysts were investigated. The catalysts were characterized by inductively-coupled plasma-mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The preferential oxidation of CO in H₂ stream (PROX) on these catalysts was carried out in a fixed-bed micro reactor with a feed of CO: O₂: H₂: He = 1: 1: 49: 49 (volume ratios) and a space velocity of 30,000 ml/g h. Limited amount of O₂ was used to investigate the selectivity of O₂ reacting with CO or H₂. Au/TiO₂ catalysts prepared by PD method showed narrow particle size distribution of gold particles within few nanometers and were found to be 1.5 nm. The particle size of gold nanoparticles deposited on the support depends on irradiation time, UV light source and pH value of preparation. The electronic structure of Au was a function of particle size. The smaller the Au particle size was, the higher the concentration of Au cation was. Using weak power of UV light, appropriate irradiation time and suitable pH value, very fine gold particles on the support could be obtained even in the powder form. The samples prepared with PD method did not need heat treatment to reduce Au cation, UV irradiation could reduce it. Therefore it is easier to have smaller particle size. Au/TiO₂ catalysts prepared by PD method were very active and selective in PROX reaction. In long time test, the catalysts were stable at 80 °C for more than 60 h.     

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.     

Understanding the role of K on PtCo/Al2O3 for preferential oxidation of CO in H2

CO preferential oxidation reaction (CO-PROX) can effectively eliminate CO in H₂ rich atmosphere to avoid CO poison the Pt anode of Proton Exchange Membrane Fuel Cell (PEMFC). To match the operation temperature window for PEMFC, PtCo nanoparticles supported on K modified Al₂O₃ (PtCo/K–Al₂O₃) were prepared to promote CO-PROX activity. The addition of K species weakened the interaction between PtCo nanoparticle and support, which improved the dispersion of Pt particles and redox property of PtCo/Al₂O₃. It also facilitated the formation of Pt₃Co species and active surface ?OH groups, which were involved in CO-PROX reaction. According to in situ DRIFTS spectra, HCO₃^? and HCOO^? were intermediates of PtCo/K–Al₂O₃ catalyzed CO-PROX at low temperature and high temperature, respectively. Thus, the addition of 1 wt% K to PtCo/Al₂O₃ (PtCo/1K–Al₂O₃) could completely oxidize CO in the temperature range of 127–230 °C with O₂ selectivity at 50%. The 100% CO conversion temperature window of PtCo/1K–Al₂O₃ is expanded by 100 °C in comparison of PtCo/Al₂O₃.     

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.     

Hydrogen production from organic fatty acids using carbon-doped TiO2 nanoparticles under visible light irradiation

The photocatalytic production of H₂ using carbon-doped TiO₂ (CTiO₂) nanoparticles has been investigated in single or mixed systems of organic fatty acids (OFAs) under visible light irradiation, including acetic acid, propionate acid, butyric acid and lactic acid. When OFAs were applied at the same electron density (10 e-eq L^?1), the H₂ evolution rates followed the order of propionic acid > butyric acid ≈ acetic acid > lactic acid, whereas at the same molar concentration (0.5 mol L^?1), that order changed to lactic acid > acetic acid > butyric acid ≈ propionic acid. This result implied that the electron transfer efficiency differed from four OFAs, probably due to their different affinity with CTiO₂. O₂^?? and CH₃? partially contributed to OFAs degradation and H₂ production. The quantum dynamics simulations of electron transfer revealed that the dominant mechanism of H₂ production was direct electron transfer from adsorbed OFAs to CTiO₂. This work aims to pursue the synergy of solar energy utilization and conversion of OFAs into H₂.     

H2 evolution under visible light irradiation on La and Cr co-doped NaTaO3 prepared by spray pyrolysis from polymeric precursor

Photocatalysts of Na_1−xLa_xTa_1−xCr_xO₃ and NaTa_1−xCr_xO₃ were prepared by spray pyrolysis from aqueous and polymeric precursor solution. Apart from the contribution of La³⁺ ions co-doped into NaTa_1−xCr_xO₃ on the BET surface area and the surface morphology by preventing crystal growth, this co-doping contributed to the increased Cr³⁺ concentration by partially tuning the electron configuration from A⁺B⁵⁺O₃ to (A⁺A′³⁺)²⁺(B⁵⁺B′³⁺)⁴⁺O₃ in the lattice of the photocatalyst. Na_1−xLa_xTa_1−xCr_xO₃ prepared from polymeric precursor solution reduced the induction period to 33% and enhanced the hydrogen evolution rate 5.6-fold to 1467.5 μmol g⁻¹ h⁻¹ compared with the equivalent values of NaTa_1−xCr_xO₃ prepared from aqueous precursor. The optimum amounts of dopant and additives comprising the polymeric precursor to maximize the hydrogen evolution rate were x = 0.003 and 300 mol%, respectively.     

Effects of charge balance with Na on SrTiO3:Mo prepared by spray pyrolysis for H2 evolution under visible light irradiation

Photocatalyst powders of SrTi_1−xMo_xO₃ and Sr_1−2xNa_2xTi_1−xMo_xO₃ were prepared by spray pyrolysis for hydrogen evolution for the first time from aqueous methanol solution under visible light irradiation. The co-doping of Mo⁶⁺/Na⁺ ions resulted in increase of BET surface area and pore volume, and formation of unique morphology with wrinkled, furrowed and porous surface, without significant distortion of lattice structure of host material. The hydrogen evolution rate of Sr_1−2xNa_2xTi_1−xMo_xO₃ photocatalyst was enhanced up to 1115.8 μmol g⁻¹ h⁻¹ with an induction period of 1 h under visible light irradiation, which was 1.5 times higher than that of SrTi_1−xMo_xO₃. The co-dopant Na⁺ ion contributed to the charge balance in the host material by compromising the excess positive charge of Mo⁶⁺, which was effective for enhancing the hydrogen evolution rate. The optimum composition of photocatalyst corresponding to the maximum hydrogen evolution rate was Sr_1−2xNa_2xTi_1−xMo_xO₃ (x = 0.004).     

Au nanoparticles supported on nanorod-like TiO2 as catalysts in the CO-PROX reaction under dark and light irradiation: Effect of acidic and alkaline synthesis conditions

Gold nanoparticles precipitated-deposited on titania nanostructures (1.0 wt% nominal loading) were studied in the preferential CO oxidation in excess of H₂ at room temperature and atmospheric pressure, both in dark and under simulated solar light irradiation. Titania supports were synthesized by means of two hydrothermal methods markedly acid and basic, giving rise to rutile nanorods and anatase deformed nanorods structures, respectively. Characterization techniques such as N₂ physisorption, XRD, XPS, DRUV-vis, HRTEM and XRF were performed in order to study the chemical, structural and optical properties of the catalysts. Well defined rutile nanorods structures were obtained from the acidic treatment allowing a regular distribution of gold nanoparticles and resulting quite active in the CO-PROX reaction. In particular the sample from the acidic synthetic approach calcined at 700 °C displayed the best results as it was highly selective to CO₂ under both dark and simulated solar light irradiation.     

g-C3N4 coated SrTiO3 as an efficient photocatalyst for H2 production in aqueous solution under visible light irradiation

A highly active photocatalyst based on g-C₃N₄ coated SrTiO₃ has been synthesized simply by decomposing urea in the presence of SrTiO₃ at 400 °C. The catalyst demonstrates a high H₂ production rate ∼440 μmol h⁻¹/g catalyst in aqueous solution under visible light irradiation, which is much higher than conventional anion doped SrTiO₃ or physical mixtures of g-C₃N₄ and SrTiO₃. The improved photocatalytic activity can be ascribed to the close interfacial connections between g-C₃N₄ and SrTiO₃ where photo-generated electron and holes are effectively separated. The newly synthesized catalyst also exhibited a stable performance in the repeated experiments.     

Effect of tri-doping on H2 evolution under visible light irradiation on SrTiO3:Ni/Ta/La prepared by spray pyrolysis from polymeric precursor

Tri-doped photocatalyst, SrTiO₃:Ni/Ta/La, was prepared by spray pyrolysis from aqueous and polymeric precursor solutions. The third dopant, La³⁺, contributed to the BET surface area and porous morphology by preventing crystal growth, and increased the Ni²⁺/Ni³⁺ ratio by affecting the electron configuration in the lattice structure, which is closely related to the hydrogen evolution rate. The hydrogen evolution rate of the tri-doped photocatalyst, SrTiO₃:Ni(0.2 mol%)/Ta(0.4 mol%)/La(0.3 mol%), was increased by about 60%–895.2 μmol g⁻¹ h⁻¹ from the value of 561.2 μmol g⁻¹ h⁻¹ for the co-doped photocatalyst, SrTiO₃:Ni(0.2 mol%)/Ta(0.4 mol%), and was further enhanced to 2305.7 μmol g⁻¹ h⁻¹ when a polymeric precursor was used instead of an aqueous precursor in spray pyrolysis. The optimum additive content for polymeric precursor solution was 300 mol%.     

H2 evolution under visible light irradiation from aqueous methanol solution on SrTiO3:Cr/Ta prepared by spray pyrolysis from polymeric precursor

SrTiO₃:Cr/Ta powders were prepared by spray pyrolysis from polymeric precursors. Effects of the amount of co-dopant and additives on the photocatalytic activity for hydrogen evolution from aqueous methanol solution under visible light irradiation (λ > 415 nm) were investigated. For the photocatalyst prepared by spray pyrolysis from polymeric precursor, the hydrogen evolution rate was increased by a factor of ∼100 and induction period was decreased by a factor of 8 compared with a photocatalyst prepared by solid state reaction. These enhancements result from increased roughness of surface, and the compositional uniformity which are intrinsic characteristics of spray pyrolysis. In addition, photocatalyst prepared by spray pyrolysis from polymeric precursor have large BET surface area and small amount of Cr⁶⁺ ion which is responsible for long induction period. It should be noted that the reduction of Cr⁶⁺ ion was achieved without hydrogen reduction process.     

Effect of visible light on the water contact angles on illuminated oxide semiconductors other than TiO2

Wide band gap semiconductor oxides such as TiO₂, Ta₂O₅, ZnO, indium–tin oxide, InTaO₄ or In₂O₃ are materials with water contact angles in dark between 60° and 130°. The present investigation shows that thin films of these oxides become hydrophilic when they are irradiated with ultraviolet light. This finding indicates that the transformation of the wetting properties of illuminated wide band gap oxides is a common phenomenon not restricted to TiO₂. An additional evidence found for ZnO and InTaO₄ is that the water contact angle decreases by 30°/40° when they are irradiated with visible light.     

Co-doping schemes to enhance H2 evolution under visible light irradiation over SrTiO3:Ni/M (M = La or Ta) prepared by spray pyrolysis

Two photocatalysts, SrTiO₃:Ni/La and SrTiO₃:Ni/Ta, were prepared by continuous spray pyrolysis. The effects of the co-dopants on hydrogen evolution over the uncalcined photocatalysts were evaluated under visible light irradiation. The co-doping of La³⁺ into SrTiO₃:Ni transformed the charge structure and increased the presence of Ni²⁺ at the expense of Ni³⁺ in the host lattice structure. The co-doping of Ta⁵⁺ into SrTiO₃:Ni also increased the Ni²⁺/Ni³⁺ ratio around the Ti⁴⁺ ions. Compared with SrTiO₃:Ni, SrTiO₃:Ni/La showed a 3 times greater rate of hydrogen evolution under visible light irradiation and SrTiO₃:Ni/Ta, a 4 times greater rate. The co-doping levels required for optimized hydrogen evolution over SrTiO₃:Ni/La and SrTiO₃:Ni/Ta prepared by spray pyrolysis were smaller than those prepared by other methods. Spray pyrolysis also produced particles with large surface areas and high roughnesses.     

Enhanced surface electron transfer by fabricating a core/shell Ni@NiO cluster on TiO2 and its role on high efficient hydrogen generation under visible light irradiation

A Ni@NiO core/shell cluster was fabricated on TiO₂ surface (Ni@NiO/TiO₂) and its roles on surface electron transfer and the enhancement on hydrogen evolution under visible light irradiation were investigated. For a comparison, the Ni/TiO₂ and NiO/TiO₂ catalysts were fabricated, respectively. By photosensitization using Eosin Y as an antenna molecule, (1.6 wt%)Ni@NiO/TiO₂ exhibited the highest activity (364.1 μmol h⁻¹) in comparison with (1.6 wt%)Ni/TiO₂ and (1.6 wt%)NiO/TiO₂ and the corresponding apparent quantum efficiency reached 28.6% at 460 nm. The photoluminescence spectra and photoelectrochemical characterization results confirmed that the Ni@NiO core/shell structure could promote the photogenerated electrons transferring from TiO₂ conduction band to Ni@NiO clusters, resulting in the quicker separation of electron–hole pairs. In addition, part of NiO shell can be reduced into metallic Ni during the photoreaction and vice versa. Cyclic voltammogram characterization verified that the transformation between Ni and NiO was a dynamic balance process, which can not only provide reacting channels for electrons and protons but also ensure the photocatalytic hydrogen evolution proceeding continuously. This study discloses structure-dependent effect of non-noble metal cocatalyst on semiconductor photocatalysts in photocatalytic water reduction, and gives an insight into designing high-efficient non-noble metal/semiconductor hybrid photocatalysts.     

Mechanism of CO oxidation over CuO/CeO2 catalysts

A mechanistic study of the CO oxidation reaction over copper–cerium catalysts was performed based on our own results and information available in literature. The fit of parameters was carried out with kinetic information obtained ad-hoc. Two possible mechanisms whose main difference is the role of copper were proposed. The first one postulates changes in the oxidation state of both cations (cerium and copper) while, in the second mechanism, it was assumed that the redox cycle only occurs for the ceria component. The results allow to conclude that the catalytic cycle involves both redox couples (Ce⁴⁺/Ce³⁺ and Cu²⁺/Cu¹⁺). The kinetic expression derived from this mechanism consists of five constants and is able to accurately predict the behavior of the reactor in different reaction conditions.     

Ultrathin hematite films deposited layer-by-layer on a TiO2 underlayer for efficient water splitting under visible light

Ultrathin hematite (α-Fe₂O₃) film deposited on a TiO₂ underlayer as a photoanode for photoelectrochemical water splitting was described. The TiO₂ underlayer was coated on conductive fluorine-doped tin oxide (FTO) glass by spin coating. The hematite films were formed layer-by-layer by repeating the separated two-phase hydrolysis-solvothermal reaction of iron(III) acetylacetonate and aqueous ammonia. A photocurrent density of 0.683 mA cm⁻² at +1.5 V vs. RHE (reversible hydrogen electrode) was obtained under visible light (>420 nm, 100 mW cm⁻²) illumination. The TiO₂ underlayer plays an important role in the formation of hematite film, acting as an intermediary to alleviate the dead layer effect and as a support of large surface areas to coat greater amounts of Fe₂O₃. The as-prepared photoanodes are notably stable and highly efficient for photoelectrochemical water splitting under visible light. This study provides a facile synthesis process for the controlled production of highly active ultrathin hematite film and a simple route for photocurrent enhancement using several photoanodes in tandem.