Photocatalytic activity of hydrothermally synthesized tantalate pyrochlores for overall water splitting

Tantalate and niobate pyrochlores, Ca₂M₂O₇ (M = Nb, Ta) and A₂Ta₂O₆ (A = Na, K), were prepared by a hydrothermal method. These oxides were crystallized directly from tantalum or niobium alkoxide in the presence of sodium, potassium or calcium cation in basic solutions at temperatures between 373 and 448 K, and their crystallinity was improved by calcination in air. Stoichiometric liberations of H₂ and O₂ were achieved over tantalate pyrochlores loaded with a small amount of nickel oxide (NiO) as a catalyst for H₂ production, while the NiO-loaded niobate pyrochlore showed no photocatalytic activity. The photocatalytic functions of the tantalate pyrochlores were attributed to their energy levels of the bottom of conduction bands being much more negative than those of the niobate pyrochlores.     

Synthesis and characterization of mesoporous Ta2O5–TiO2 photocatalysts for water splitting

Two series of Ta₂O₅–TiO₂ photocatalysts (Ta:Ti = 4:1, 1:1 and 1:4) were prepared by sol–gel technique applying triblock copolymer of Pluronic P123 and were tested in platinized form (0.3 wt.%) in photodecomposition of water under ultraviolet and visible light (λ > 300 nm). It was found the mesoporous character of tantalum containing catalysts with relatively high surface area (100–130 m² g⁻¹) of these samples. However, higher concentration of TiO₂ in mixed oxides leads to the destruction of mesoporous character of synthesized photocatalysts. All samples were characterized with thermogravimetry, XRD, N₂ physisorption, DR-UV–vis and FTIR spectroscopy. The mixed oxides of Ta₂O₅–TiO₂ system showed much lower band-gap than pure Ta₂O₅ and relatively high activity in platinized state in photocatalytic hydrogen generation under visible. Doping of pure oxides and mixed systems with sulfur resulted in lowering of the band-gap values below 3 eV and much better activity in H₂ evolution reaction. Non-platinized photocatalysts showed activity in liquid phase cyclohexene photooxidation at 305 K.     

Photocatalytic reduction of selenium ions using different TiO2 photocatalysts

Comparative performance studies between different photocatalysts for reduction reactions are scarce. Here, Millennium PC500 and PC50 photoreactivities were investigated for selenite (Se(IV)) and selenate (Se(VI)) reduction to their elemental form (Se(0)) and compared with that of Degussa P25 TiO₂. Millennium PC500, with the highest surface area demonstrated the fastest photoreduction of Se(IV) and Se(VI), compared to P25 and Millennium PC50. Millennium PC50 and P25, which have comparable surface areas, showed similar photoreactivities. UV-Vis reflectance measurements and XRD characterisation revealed that the Se(0) deposits underwent phase transformation from amorphous to stable crystalline Se(0) during the drying and ageing process. Overall, the Millennium PCs appear to be promising photocatalysts for the photoreduction process.     

Highly efficient decomposition of pure water into H2 and O2 over NaTaO3 photocatalysts

NaTaO₃ with 4.0 eV band gap showed a high activity for the decomposition of pure water into H₂ and O₂ without co-catalysts when it was prepared in the presence of excess Na in order to prevent the formation of defects by the volatilization of Na. When NiO was supported on the NaTaO₃ powder, the activity was drastically increased. The quantum yield of the NiO(0.05 wt%)/NaTaO₃ photocatalyst was 28% for the water splitting at 270 nm. This revised version was published online in November 2006 with corrections to the Cover Date.     

可见光分解水制氢催化剂研究进展

可见光分解水制氢研究主要集中在新型光催化剂的研制以及对传统光催化剂的改性。本文概述了近年来可见光分解水制氢技术的重要进展及最新研究成果,深入研究和分析了拓展光催化剂响应范围、提高其反应活性的几种技术路线,主要包括阴阳离子掺杂技术、固溶体技术、半导体复合技术以及加入助催化剂等;总结出了传统光催化剂存在可见光利用率低、光化学转化效率低等问题,提出了开发新型高效光催化剂并加强机理研究的必要性和可行性。     

Recent progress of photocatalysts for overall water splitting

Recent progress of photocatalysts for H₂O decomposition into H₂ and O₂ was briefly reviewed with the emphasis on the results concerning the use of several ion-exchangeable layered materials. Very recent results concerned with the first successful example of overall water splitting induced by visible light irradiation was also presented.     

Generation of H2O2 from H2 and O2 over zeolite beta containing Pd and heterogenized organic compounds

The catalytic generation of H₂O₂ from H₂ and O₂ has been studied over zeolite beta-supported Pd and zeolite beta-adsorbed organic compounds such as 1,4-benzoquinone (BQ), hydroquinone (HQ), azobenzene (AB) and hydrazobenzene (HAB). According to catalytic results, zeolite beta-supported Pd catalysts display effective performance relative to those prepared from other types of zeolites reported and Pd-loaded zeolite beta-adsorbed HQ catalysts show enhanced activity compared to zeolite beta-supported Pd catalysts. In situ UV–Vis spectroscopic study indicates that HQ can readily be converted to BQ reversibly under H₂ and air inside zeolite beta only in the presence of Pd. The results suggest that HQ acts as a strong hydrogen transfer agent to promote the production of H₂O₂ from H₂ and O₂ in cooperation with a Pd catalyst. By contrast, adsorption of BQ, AB and HAB induces suppression of the catalytic properties of Pd/zeolite beta.     

Photocatalytic reduction of CO2 with H2O on Ti-MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts

Titanium oxide species included within the framework of mesoporous zeolites (Ti-MCM-41 and Ti-MCM-48) prepared by a hydrothermal synthesis exhibited high and unique photocatalytic reactivity for the reduction of CO₂ with H₂O at 328 K to produce CH₄ and CH₃OH in the gas phase. In situ photoluminescence, diffuse reflectance absorption, ESR and XAFS investigations indicated that the titanium oxide species are highly dispersed within the zeolite framework and exist in tetrahedral coordination. The charge transfer excited state of the highly dispersed titanium oxide species played a significant role in the reduction of CO₂ with H₂O exhibiting a high selectivity for the formation of CH₃OH.     

Oxidation of dilute H2 and H2/O2 mixtures by hydrogenases and Pt

Hydrogenase enzymes that allow micro-organisms to gain energy from oxidation of H₂ undergo efficient electrocatalysis of H₂ oxidation or production when adsorbed on a graphite rotating disk electrode [K.A. Vincent, A. Parkin, F.A. Armstrong, Chem. Rev. 107 (2007) 4366]. Combining potential sweeps or steps with precisely controlled gas exchanges is enabling us to build up a detailed understanding of the many factors that control the chemistry of nickel-iron membrane-bound hydrogenase (MBH) enzymes. The observation that the MBH enzymes from Ralstonia strains have extremely high affinity for H₂ and continue oxidising H₂ in the presence of O₂ and CO has relevance for selective fuel cell catalysis [K.A. Vincent, J.A. Cracknell, J.R. Clark, M. Ludwig, O. Lenz, B. Friedrich, F.A. Armstrong, Chem. Commun. (2006) 5033; K.A. Vincent, J.A. Cracknell, O. Lenz, I. Zebger, B. Friedrich, F.A. Armstrong, Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 16951], and this has led us to compare the ability of hydrogenases and platinum to oxidise low levels of H₂ and mixtures of H₂ and O₂. We show that Pt is a poor catalyst for oxidation of sub-atmospheric levels of H₂ compared to the MBH from Ralstonia eutropha H16, and that at a platinised electrode, H₂ oxidation competes less favourably with reduction of O₂ compared to the situation at hydrogenase-modified graphite. This should have implications for development of future selective energy catalysts able to concentrate the energy available from dilute H₂.     

太阳能光解水制氢催化剂研究进展

介绍了直接利用太阳能光解水制氢的几种新型光催化剂：钽酸盐、铌酸盐、钛酸盐、多元硫化物,阐述了提高光催化剂反应活性的途径：光催化剂纳米化、离子掺杂、半导体复合、染料光敏化、贵金属沉积、电子捕获剂、表面螯合及衍生作用、外场耦合。展望了该领域未来的研究方向。     

Photocatalytic water splitting over ZrO2 prepared by precipitation method

ZrO₂ prepared by the precipitation method of zirconium oxychloride with various hydrolyzing agents was studied for photocatalytic water splitting reaction under UV light irradiation. The crystal structure as well surface properties were varied with the hydrolyzing agent of KOH, NH₄OH, and NH₂CONH₂. Especially, the surface area of the prepared ZrO₂ calcined at the same temperature of 750 °C for 6 h was dependent highly on the hydrolyzing agent, and thus the highest photocatalytic activity was obtained for ZrO₂ with the highest surface area when KOH was used as a hydrolyzing agent, In the presence of Na₂CO₃, the photocatalytic activity of ZrO₂ increased by 2–3 fold, which was ascribed to the physically adsorbed carbonate species on the ZrO₂ surface. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Kinetics of propylene epoxidation using H2 and O2 over a gold/mesoporous titanosilicate catalyst

The oxidation of propylene to propylene oxide (PO) with hydrogen–oxygen mixtures was studied on gold supported on the mesoporous titanium silicate, Ti-TUD. The catalyst gave stable activity at low conversions of propylene (<6%) and high selectivity to PO (>95%). Kinetic data were fit to a power-rate law and gave the following expression: r_PO = k(H₂)^0.54(O₂)^0.24(C₃H₆)^0.36. The fractional orders in hydrogen, oxygen, and propylene indicated that these reactants interacted with the catalyst to form species that led to the final PO product. The catalyst likely operated by the commonly accepted mechanism of hydrogen peroxide production on gold sites, and epoxidation on titanium centers. Carbon dioxide was formed primarily from further oxidation of PO rather than the oxidation of propylene, while water was produced from the reaction of hydrogen and oxygen.     

Direct formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in aqueous acidic reaction medium over halide-containing Pd/SiO2 catalytic system

Formation of H₂O₂ from H₂ and O₂ and decomposition/hydrogenation of H₂O₂ have been studied in aqueous acidic medium over Pd/SiO₂ catalyst in presence of different halide ions (viz. F⁻, Cl⁻ and Br⁻). The halide ions were introduced in the catalytic system via incorporating them in the catalyst or by adding into the reaction medium. The nature of the halide ions present in the catalytic system showed profound influence on the H₂O₂ formation selectivity in the H₂ to H₂O₂ oxidation over the catalyst. The H₂O₂ destruction via catalytic decomposition and by hydrogenation (in presence of hydrogen) was also found to be strongly dependent upon the nature of the halide ions present in the catalytic system. Among the different halides, Br⁻ was found to selectivity promote the conversion of H₂ to H₂O₂ by significantly reducing the H₂O₂ decomposition and hydrogenation over the catalyst. The other halides, on the other hand, showed a negative influence on the H₂O₂ formation by promoting the H₂ combustion to water and/or by increasing the rate of decomposition/hydrogenation of H₂O₂ over the catalyst. An optimum concentration of Br⁻ ions in the reaction medium or in the catalyst was found to be crucial for obtaining the higher H₂O₂ yield in the direct synthesis.     

Effect of TiO2 supporting layer on Fe2O3 photoanode for efficient water splitting

For an electrochemical water splitting system, titanate nanotubular particles with a thickness of ∼700 nm produced by a hydrothermal process were repetitively coated on fluorine-doped tin oxide (FTO) glass via layer-by-layer self-assembly method. The obtained titanate/FTO films were dipped in aqueous Fe solution, followed by heat treatment for crystallization at 500 °C for 10 min in air. The UV–vis absorbance of the Fe-oxide/titanate/FTO film showed a red-shifted spectrum compared with the TiO₂/FTO coated film; this red shift was achieved by the formation of thin hematite-Fe₂O₃ and anatase-TiO₂ phases verified using X-ray diffraction and Raman results. The cyclic voltammetry results of the Fe₂O₃/TiO₂/FTO films showed distinct reversible cycle characteristics with large oxidation–reduction peaks with low onset voltage of I–V characteristics under UV–vis light illumination. The prepared Fe₂O₃/TiO₂/FTO film showed much higher photocurrent densities for more efficient water splitting under UV–vis light illumination than did the Fe₂O₃/FTO film. Its maximum photocurrent was almost 3.5 times higher than that obtained with Fe₂O₃/FTO film because of the easy electron collection in the current collector. The large current collection was due to the existence of a TiO₂ base layer beneath the Fe₂O₃ layer.     

NO reduction with H2 in the presence of excess O2 over Pd/MFI catalyst

The NO SCR (selective catalytic reduction) activity with H₂ in the presence of excess O₂ was investigated over Pd/MFI catalyst prepared by sublimation method. With GHSV=90?000 h⁻¹, a very high steady-state conversion of NO to N₂ (70%) is achieved at 100 °C. Significant reorganizations take place inside the catalyst upon its first contact with all reactants and products at the reaction temperature. Pd⁰, which has a significant role in the NO-H₂-O₂ reaction, is possibly the active site for NO reduction. The formation of Pd-β hydride deactivates the catalyst for NO reduction. Throughout the entire NO-H₂-O₂ reaction, no N₂O or NO₂ is formed; N₂ is the only N-containing product. The presence of O₂ inhibits the formation of undesirable NH₃. The rate of the NO+H₂ reaction is fast or comparable to that of the H₂+O₂ reaction. The oxidation of Pd⁰ and subsequent agglomeration of PdO are responsible for the decreased NO reduction activity at high temperature.     

Mechanistic aspects of N2O and N2 formation in NO reduction by NH3 over Ag/Al2O3: The effect of O2 and H2

A mechanistic scheme of N₂O and N₂ formation in the selective catalytic reduction of NO with NH₃ over a Ag/Al₂O₃ catalyst in the presence and absence of H₂ and O₂ was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N₂ and N₂O are formed via direct or oxygen-induced decomposition of surface NH₂NO species. These intermediates originate from NO and surface NH₂ fragments. The latter NH₂ species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N₂O and O₂. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgO_x particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N₂O production in the presence of H₂ is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N₂O decomposition.     

羟基磷灰石负载钯催化剂用于H2和O2直接合成H2O2的动力学

H₂和O₂直接合成H₂O₂过程绿色环保,反应具有原子经济性,是最有潜力的H₂O₂合成新方法之一。采用等量浸渍法,将Pd负载于羟基磷灰石（HAp）载体上,得到了高分散的Pd/HAp纳米催化剂,Pd平均粒径2.5 nm。运用幂指数模型,研究该催化剂在H₂O₂加氢及H₂和O₂直接合成反应中的动力学,计算得到H₂O₂加氢、H₂O₂和H₂O的生成反应的表观活化能及O₂、H₂表观反应级数。结果表明低温及高O₂分压有利于H₂O₂的生成,而高H₂分压则有利于H₂O的生成。     

Photocatalytic reactions of imazamox at TiO2, H2O2 and TiO2/H2O2 in water interfaces: Kinetic and photoproducts study

The photocatalytic transformation of imazamox, a herbicide of imidazolinone family, is investigated in aqueous solution containing titanium dioxide, hydrogen peroxide or the combination of TiO₂/H₂O₂ under simulated sunlight irradiation. The effect of parameters such as the amount of catalysts, the concentration of herbicide, and the pH were investigated by measurement of the rate constant of degradation. Experimental data obtained under different conditions describe the dependency of degradation rate on the above mentioned parameters. Consequently, kinetic parameters were experimentally determined and a pseudo-first-order kinetic was observed. Mulliken charge distributions calculated by the DFT method B3LYP/6–31+G(d) level of theory for key cationic, anionic and neutral structures of imazamox give interpretation for the dependency of photodegradation rate constant on pH. The degradation rate constants were always higher for the heterogeneous catalysis in reactions (TiO₂/UV, TiO₂/UV/H₂O₂) compared to the homogeneous systems (UV alone, H₂O₂/UV). In parallel, five photoproducts could be tentatively identified using Electrospray ionization Fourier transform ion cyclotron resonance mass spectroscopy based on precise chemical formula assignments.     

Kinetics of CO and H2 oxidation over CuO-CeO2 catalyst in H2 mixtures with CO2 and H2O

The kinetics of CO and H₂ oxidation over a CuO-CeO₂ catalyst were simultaneously investigated under reaction conditions of preferential CO oxidation (PROX) in hydrogen-rich mixtures with CO₂ and H₂O. An integral packed-bed tubular reactor was used to produce kinetic data for power-law kinetics for both CO and H₂ oxidations. The experimental results showed that the CO oxidation rate was essentially independent of H₂ and O₂ concentrations, while the H₂ oxidation rate was practically independent of CO and O₂ concentrations. In the CO oxidation, the reaction orders were 0.91, −0.37 and −0.62 with respect to the partial pressure of CO, CO₂ and H₂O, respectively. In the H₂ oxidation, the orders were 1.0, −0.48 and −0.69 with respect to the partial pressure of H₂, CO₂ and H₂O, respectively. The activation energies of the CO oxidation and the H₂ oxidation were 94.4 and 142 kJ/mol, respectively. The rate expressions of both oxidations were able to predict the performance of the PROX reactor with accuracy. The independence between the CO and the H₂ oxidation suggested different sites for CO and H₂ adsorption on the CuO-CeO₂ catalyst. Based on the results, we proposed a new reaction model for the preferential CO oxidation. The model assumes that CO adsorbs selectively on the Cu⁺ sites; H₂ dissociates and adsorbs on the Cu⁰ sites; the adsorbed species migrates to the interface between the copper components and the ceria support, and reacts there with the oxygen supplied by the ceria support; and the oxygen deficiency on the support is replenished by the oxygen in the reaction mixture.     

Effect of pH, inorganic ions, organic matter and H2O2 on E. coli K12 photocatalytic inactivation by TiO2: Implications in solar water disinfection

The effect of different chemical parameters on photocatalytic inactivation of E. coli K12 is discussed. Illumination was produced by a solar lamp and suspended TiO₂ P-25 Degussa was used as catalyst. Modifications of initial pH between 4.0 and 9.0 do not affect the inactivation rate in the absence or presence of the catalyst. Addition of H₂O₂ affects positively the E. coli inactivation rate of both photolytic (only light) and photocatalytic (light plus TiO₂) disinfection processes. Addition of some inorganic ions (0.2 mmol/l) like HCO₃⁻, HPO₄²⁻, Cl⁻, NO₃⁻ and SO₄²⁻ to the suspension affects the sensitivity of bacteria to sunlight in the presence and in absence of TiO₂. Addition of HCO₃⁻ and HPO₄²⁻ resulted in a meaningful decrease in photocatalytic bactericidal effect while it was noted a weak influence of Cl⁻, SO₄²⁻ and NO₃⁻. The effect of counter ion (Na⁺ and K⁺) is not negligible and can modify the photocatalytic process as the anions. Bacteria inactivation was affected even at low concentrations (0.2 mmol/l) of SO₄²⁻ and HCO₃⁻, but the same concentration does not affect the resorcinol photodegradation, suggesting that disinfection is more sensitive to the presence of natural anions than photocatalytic degradation of organic compounds. The presence of organic substances naturally present in water like dihydroxybenzenes isomers shows a negative effect on photocatalytic disinfection. The effect of a mixture of chemical substances on photocatalytic disinfection was also studied by adding to the bacterial suspension nutrient broth, phosphate buffer and tap water.