The reactivities of TiO2 (rutile and anatase) for the solid-state reactions with BaSO4 and BaCO3

DTA and isothermal kinetic studies were carried out on the reactions of BaSO₄ or BaCO₃ with TiO₂ (anatase and rutile) to BaTiO₃. In the initial steps, the reactions of BaSO₄ with TiO₂ (anatase) proceeded to BaTi₄O₉, of BaSO₄ with TiO₂ (rutile) to BaTi₃O₇, and of BaCO₃ with TiO₂ (anatase and rutile) to Ba₂TiO₄, respectively. These reactions were connected with the formation of binary metal oxide through some intermediates, which are BaSO₄ or BaCO₃ incorporated with TiO₂. The reactivity of anatase was higher than that of rutile in all reaction systems.     

Freeze-drying synthesis and characterisation of Na composites of ZnO,TiO<Subscript>2</Subscript> and ZnTiO<Subscript>3</Subscript> semiconductor oxides

The freeze-drying method of metal oxides synthesis has a number of advantages such as high homogeneity, varying porous structures, morphologies and uniform particle size distribution, etc. Because of these advantages, the binary metal oxides ZnO, TiO₂ and ternary metal oxide ZnTiO₃ were synthesised by the freeze-drying method. The synthesised materials were characterised by X-ray diffraction (XRD), Fourier transform-infra red spectroscopy (FT-IR), UV-VIS spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX). The as-synthesised metal oxides were calcined at different temperatures to study the phase evolution and morphological changes. The crystalline cubic-phase ZnTiO₃ (a = 8.3948 Å) was obtained on calcination of the precursor at 600°C, and decomposed to the cubic phase Zn₂TiO₄ (a = 8.4580 Å) and rutile TiO₂ (a = 4.5955 Å and c = 2.9593 Å) at 1000°C. The band gap of ZnO (3.28?3.10 eV), TiO₂ (3.37?2.97 eV) and ZnTiO₃ (3.92?3.80 eV) calculated using Tauc’s relation was found to vary inversely with calcination temperature and phase transition.     

Photocatalytic Effects of Rutile Phase TiO2 Ultrafine Powder with High Specific Surface Area Obtained by a Homogeneous Precipitation Process at Low Temperatures

The photocatalytic characteristics of nanostructured TiO₂ ultrafine powder with rutile phase produced using the homogeneous precipitation process at low temperatures (HPPLT) were compared with those of commercial P-25 TiO₂ powder by flame hydrolysis. The TiO₂ powder by the HPPLT showed much higher photoactivity in the removal rate, showing lower pH values in the solution than the P-25 powder when eliminating metal ions such as Pb and Cu from the aqueous metal-EDTA solutions. This can be inferred as the more rapid photo-oxidation or -reduction of metal ions from the aqueous solution, together with relatively higher efficiencies in the use of an electron-hole pair formed on the surface of the TiO₂ particles under UV light irradiation. Also, in the view of the TiO₂ particle morphology, compared to the well-dispersed spherical P-25 particles, the agglomerated TiO₂ secondary particles by the HPPLT consist of acicular typed primary particles with a thickness in the range of 3–7 nm and the primary particles radialize in all directions, which would be more effective to photocatalytic reactions without the large electron-hole recombination on the surface of the TiO₂ particle under UV light irradiation. It can be, therefore, thought that the higher photoactivity of the rutile TiO₂ powder by the HPPLT in the aqueous solutions results mainly from having a larger surface area by the acicular shaped primary particles with very thin thickness and radialization in all directions.     

Catalytically Active Rh Sub‐Nanoclusters on TiO2 for CO Oxidation at Cryogenic Temperatures

The discovery that gold catalysts could be active for CO oxidation at cryogenic temperatures has ignited much excitement in nanocatalysis. Whether the alternative Pt group metal (PGM) catalysts can exhibit such high performance is an interesting research issue. So far, no PGM catalyst shows activity for CO oxidation at cryogenic temperatures. In this work, we report a sub‐nano Rh/TiO₂ catalyst that can completely convert CO at 223 K. This catalyst exhibits at least three orders of magnitude higher turnover frequency (TOF) than the best Rh‐based catalysts and comparable to the well‐known Au/TiO₂ for CO oxidation. The specific size range of 0.4–0.8 nm Rh clusters is critical to the facile activation of O₂ over the Rh–TiO₂ interface in a form of Rh?O?O?Ti (superoxide). This superoxide is ready to react with the CO adsorbed on TiO₂ sites at cryogenic temperatures.     

A Tandem Catalyst with Multiple Metal Oxide Interfaces Produced by Atomic Layer Deposition

Ideal heterogeneous tandem catalysts necessitate the rational design and integration of collaborative active sites. Herein, we report on the synthesis of a new tandem catalyst with multiple metal‐oxide interfaces based on a tube‐in‐tube nanostructure using template‐assisted atomic layer deposition, in which Ni nanoparticles are supported on the outer surface of the inner Al₂O₃ nanotube (Ni/Al₂O₃ interface) and Pt nanoparticles are attached to the inner surface of the outer TiO₂ nanotube (Pt/TiO₂ interface). The tandem catalyst shows remarkably high catalytic efficiency in nitrobenzene hydrogenation over Pt/TiO₂ interface with hydrogen formed in situ by the decomposition of hydrazine hydrate over Ni/Al₂O₃ interface. This can be ascribed to the synergy effect of the two interfaces and the confined nanospace favoring the instant transfer of intermediates. The tube‐in‐tube tandem catalyst with multiple metal‐oxide interfaces represents a new concept for the design of highly efficient and multifunctional nanocatalysts.     

The low temperature synthesis of metal oxides by novel hydrazine method

The hydroxide, oxalate and citrate precursors of the metal oxides such as γ-Fe₂O₃, (MnZn)Fe₂O₄, Cu(K)Fe₂O₄, BaTiO₃, La(Sr)MnO₃, La(Sr)AlO₃, La/Gd(Ca/Ba/Sr)CoO₃, and anatase TiO₂ on modifications with the hydrazine decompose at low temperatures give single phase oxides of superior properties, while the complexes without such modification require higher temperatures for achieving the phases. The hydrazine released at lower temperatures reacts with the oxygen in the atmosphere, N₂H₄+O₂→N₂+2H₂O; ΔH=−625 kJ mol⁻¹, and liberates enormous energy that is sufficient for the oxidative decomposition of the complexes now devoid of hydrazine. Such extra energy is not available in the case of the precursors without such modifications. The reaction products of hydrazine oxidation provide desired partial pressure of moisture needed for the stabilization of γ-Fe₂O₃. Also, the nitrogen that is formed in the reaction of hydrazine with oxygen gets trapped in the lattice of TiO₂ giving yellow color nitrogen doped TiO_2−xN_x photocatalyst. Thus, hydrazine method of preparation has many advantages in the preparation of metal oxides of superior properties.     

Effect of surface hydration on the photocatalytic activity of oxide catalysts in the CO oxidation

The effect of the state of hydrated surface of the bulk oxide photocatalysts, TiO₂, CeO₂, and ZnO on the rate of UV-induced oxidation of CO with atmospheric oxygen was studied. The activity of dehydroxylated catalyst samples evacuated at temperatures of >350 °C toward CO photooxidation decreases in the series CeO₂ > ZnO ≈ TiO₂, while that of partially hydrated samples after pretreatment at 20 °C changes in the order TiO₂ > ZnO ≥ CeO₂ ≈ 0. According to the results, the difference in the photocatalytic activity toward CO oxidation on the dehydrated ZnO, TiO₂, and CeO₂ catalysts is attributable to different concentrations of oxygen vacancies, which are formed more readily after high-temperature treatment on ZnO and CeO₂ and thus promote higher rate of CO photooxidation. Using a new technique for recording transmittance IR spectra, it was found that photoirradiation in the presence of adsorbed water and O₂ gives peroxides and hydroperoxides, with their concentrations decreasing in the series TiO₂ >> ZnO >> CeO₂. Most likely, these species are active intermediates of CO photooxidation with oxygen in the presence of adsorbed water. The hydrophobization effect was detected upon TiO₂ modification with zinc, resulting in removal of surface acid sites capable of adsorbing water. The TiO₂ modification with zinc increases the activity of CO photooxidation with respect to the oxidation catalyzed by samples pretreated at low temperatures (20—60 °C).     

Modifications on reduced titanium dioxide photocatalysts: A review

A large variety of reduced titanium dioxide (TiO_2-x) materials have been reported recently. Reduced TiO₂, usually resulting from the removal of oxygen atoms or hydrogen incorporation, is proved to be efficient for achieving highly photocatalytic performance including photodegradation of organic compounds, hydrogen generation from water splitting, CO₂ reduction for CH₄ evolution, solar cells, etc. To further improve the properties and activities of TiO_2-x, a combination of the Ti³⁺ self-doping and other traditional modifications like nonmetals doping has been proposed in the past decades. This paper provides a general and critical review on the further modifications on reduced TiO₂ samples, including non-metal elements (N, B, S, F and I) doping, noble-metal (Au, Pt, Pd and Ag) and iron-group metal (Fe, Co and Ni) grafting, metal oxide compositing, carbon (nanotubes and graphene) and carbon-based-material compositing, special facets exposure (mainly dual {001}-{101} and {111}-{110} facets) of TiO_2-x and ordered structure controlling of TiO_2-x. These modifications enhance the physical and/or chemical properties of the reduced TiO₂, or create new features for the modified TiO_2-x samples, which finally leads to the enhancement of photocatalytic performance. Key examples such as N-doping, Au grafting and graphene-based compositing are discussed carefully, and the mechanisms for solar light enhancement, electron transfer and charge separation are also investigated. Finally, some challenging issues on TiO_2-x catalysts are also proposed to encourage new approaches for preparation of TiO_2-x catalysts with efficiently photocatalytic performance.     

Preparation of LaNiO3 perovskite by oxalate and carbonate precursor method for utilization as catalyst for high-temperature decomposition of N2O

The mixed oxide LaNiO₃ with perovskite structure was prepared by two relatively new and unconventional methods including preparation and thermal decomposition of mixed metal oxalate or carbonate precursors. The intermediates were prepared by reaction in a highly concentrated suspension (paste). The thermal decomposition conditions of these intermediates were described, and the final calcination temperatures were determined, which were done using thermal analysis methods and X-ray diffraction. During the decomposition of mixed carbonates, one-phase LaNiO₃ is produced directly, and in case of decomposition of oxalates, a mixture of LaNiO₃ and La₂O₃ is produced due to the formation of La₂O₂CO₃ during the heating. Catalytic decomposition of nitrous oxide at high temperature (650–930 °C) and high loading (GHSV?=?350,000 h^?1) has shown high LaNiO₃ activity, even at lower temperatures. The results were compared with the same compound obtained by co-precipitation and by solid-state reaction. Methods of preparation based on decomposition of oxalate and carbonate intermediates lead to the preparation of materials with appropriate composition, morphology, specific surface and high catalytic activity.

     

Synthesis and thermoelectric properties of Sr0.95La0.05TiO3−δ-TiO2 solid solutions

Sr_0.95La_0.05TiO_3?δ solid solutions, combined with 10 mol% TiO₂, were prepared at different temperatures. When sintered at a suitable temperature, the Seebeck coefficient presents the same behavior at high temperatures as that of Sr_0.95La_0.05TiO_3?δ ceramic. Otherwise, the Seebeck coefficient would be smaller. After introducing TiO₂, both of the thermal and electrical conductivity are reduced. The sample sintered at 1350 °C exhibits the highest ZT of about 0.2, which slightly larger than that of Sr_0.95La_0.05TiO_3?δ ceramic.     

Environmental remediation by an integrated microwave/UV illumination technique: IX. Peculiar hydrolytic and co-catalytic effects of platinum on the TiO2 photocatalyzed degradation of the 4-chlorophenol toxin in a microwave radiation field

The photocatalyzed degradation of the 4-chlorophenol toxin (4-CP) in aqueous naked TiO₂ and platinized TiO₂ suspensions simultaneously subjected to UV light and microwave radiation was revisited to examine the fate of this toxin in the microwave-assisted photocatalytic process by monitoring loss of total organic carbon (TOC; mineralization), formation of chloride ions (dechlorination of 4-CP), and identification of intermediates using HPLC and electrospray mass spectral (LC–MSD) techniques. Attempts are made to delineate microwave thermal and nonthermal factors that impinge on the degradation by comparing experimental results from microwave-generated heat versus results from a conventional (externally heated) thermally-assisted process, and from results in which the thermal factors were minimized by examining the degradative process at constant ambient temperature (25 °C). Possible microwave radiation effects on the Pt co-catalyst supported on TiO₂ were also probed through comparison of the degradation of 4-CP occurring on Pt/TiO₂ and on naked TiO₂ photocatalysts. Results suggest that, in a microwave radiation field, naked TiO₂ and Pt/TiO₂ particle surfaces interact with the microwaves. The degradation pathway exhibited characteristics of hydrolysis of reactants and intermediates. Nonthermal microwave effects play a role in the overall degradative process occurring in platinized TiO₂ dispersions. The possible nature of these unusual microwave effects is briefly discussed.     

Mechanosynthesis and mechanical activation processes to the preparation of the Sr2[Srn−1TinO3n+1] Ruddlesden–Popper family

A novel mechanochemical activation route has been applied in order to obtain the n=1–4 and ∞ members of the Sr₂[Sr_n−1Ti_nO_3n+1] Ruddlesden–Popper series. The evolution of the (n+1)SrO:nTiO₂ powder mixtures during mechanical treatment was followed by X-ray powder diffraction in all cases. Except for the 2SrO:TiO₂ composition, SrTiO₃ was always mechanosynthesized. High-energy milling of 2SrO:TiO₂ sample resulted in the formation of nanosized Sr₂TiO₄, which is the only K₂NiF₄-type oxide prepared by mechanical treatment until now.The mechanical treatment was followed by annealing at different temperatures to establish the optimized protocol for synthesis of each member of the series. SrTiO₃, Sr₂TiO₄ and Sr₃Ti₂O₇ were obtained with very important decreases in the formation temperatures and reaction times as compared with the traditional ceramic method. Final and milled products were studied by X-ray powder diffraction at room and increasing temperatures, and by thermal analysis and scanning and high resolution transmission electron microscopy.     

Graphene Nanosheets as a Platform for the 2D Ordering of Metal Oxide Nanoparticles: Mesoporous 2D Aggregate of Anatase TiO2 Nanoparticles with Improved Electrode Performance

Graphene nanosheets are successfully applied as an effective platform for the 2D ordering of metal oxide nanoparticles. Mesoporous 2D aggregates of anatase TiO₂ nanoparticles are synthesized by the heat treatment of the uniformly hybridized nanocomposite of layered titanate–reduced graphene oxide (RGO) at elevated temperatures. The precursor layered titanate–RGO nanocomposite is prepared by self‐assembly of anionic RGO nanosheets and cationic TiO₂ nanosols. The calcination of the as‐prepared layered titanate–RGO nanocomposite at 500 °C induces a structural and morphological change of layered titanate nanoplates into anatase TiO₂ nanoparticles without significant modification of the RGO nanosheet. Increasing the heating temperature to 600 °C gives rise to elimination of the RGO component, leading to the formation of sheetlike porous aggregates of RGO‐free TiO₂ nanoparticles. The nanocomposites calcined at 500–700 °C display promising functionality as negative electrodes for lithium ion batteries. Among the present calcined derivatives, the 2D sheet‐shaped aggregate of TiO₂ nanoparticles obtained from calcination at 600 °C delivers the greatest specific discharge capacity with good capacity retention for all current density conditions applied. Such superior electrode performance of the nanocomposite calcined at 600 °C is attributable both to the improved stability of the crystal structure and crystal morphology of titania and to the enhancement of Li⁺ ion transport through the enlargement of mesopores. The present findings clearly demonstrate the usefulness of RGO nanosheets as a platform for 2D‐ordered superstructures of metal oxide nanoparticles with improved electrode performance.     

Photocatalytic reduction of CO2 with H2O on TiO2 and Cu/TiO2 catalysts

Photoinduced reduction of CO₂ by H₂O to produce CH₄ and CH₃OH has been investigated on wellcharacterized standard TiO₂ catalysts and on a Cu²⁺ loaded TiO₂ catalyst. The efficiency of this photoreaction depends strongly on the kind of catalyst and the ratio of H₂O to CO₂. Anatase TiO₂, which has a large band gap and numerous surface OH groups, shows high efficiency for photocatalytic CH₄ formation. Photogenerated Ti³⁺ ions, H and CH₃ radicals are observed as reactive intermediates, by ESR at 77 K. Cu-loading of the small, powdered TiO₂ catalyst (Cu/TiO₂) brings about additional formation of CH₃OH. XPS studies suggest that Cu⁺ plays a significant role in CH₃OH formation.     

Characterization and activity of visible-light-driven TiO2 photocatalyst codoped with nitrogen and cerium

Nitrogen and cerium codoped TiO₂ photocatalysts were prepared by a modified sol-gel process with doping precursors of cerium nitrate and urea, and characterized by X-ray diffraction (XRD), thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray photoelectron spectra (XPS) and ultraviolet-visible light diffuse reflectance spectra (UV-vis DRS). Results indicate that anatase TiO₂ is the dominant crystalline type in as-prepared samples, and CeO₂ crystallites appear as the doping ratio of Ce/Ti reaches to 3.0 at%. The TiO₂ starts to transform from amorphous phase to anatase at 987.1 K during calcination, according to the TG-DSC curves. The XPS show that three major metal ions of Ce³⁺, Ce⁴⁺, Ti⁴⁺ and one minor metal ion of Ti³⁺ coexist on the surface. The codoped TiO₂ exhibits significant absorption within the range of 400-500 nm compared to the non-doped and only nitrogen-doped TiO₂. The enhanced photocatalytic activity of the codoped TiO₂ is demonstrated through degradation of methyl orange under visible light irradiation.     

Thermochemistry of alkali-metal titanates

The molar enthalpies of reaction at 970 K of sodium and potassium titanates have been determined by high-temperature solution calorimetry for the reactions: mM₂TiO₃ + (n-m)TiO₂ = mM₂O·nTiO₂, where (m,n) = (1,1), (4,5), (1,2), (1,3) and (1,6) on M = Na, and (1,1), (2,3), (1,2), (1,4) and (1,6) on M = K. It has been indicated from the resultant data that 4Na₂O·5TiO₂, 2K₂O·3TiO₂ and K₂O·2TiO₂ might be stabilized at high temperatures by significant positive entropies . The enthalpies of formation at 970 K of these compounds have been derived from the combination of the present work with the literature data (ref.1,2). Melting points and degrees of hydrolysis of these compounds are reasonably correlated to their enthalpies of formation.     

Photocatalytic Activity of High Aspect Ratio TiO2 Nanorods

In this article, TiO₂ nanorods (aspect ratio >20) were prepared through a polyol process and doped with metal ions (Cu²⁺, Ni²⁺, Fe³⁺, and Cr³⁺). Compared with TiO₂ nanoparticles, the TiO₂ nanorods displayed relatively higher photocatalytic activity for the degradation of copper sulfophthalocyanine. Moreover, the photocatalytic activity was greatly enhanced when the metal ions were doped in the TiO₂ nanorods.     

Electronic and optical properties of α‐MoO3/TiO2 heterostructures: A DFT study

The coupling of metal oxide semiconductors has become an effective method to improve the separation of photon‐generated carriers and light absorption efficiency. In this study, we explored electronic and optical properties of monolayer and bilayer α‐MoO₃ on TiO₂ (001) surface. It is observed that α‐MoO₃/TiO₂ heterostructures can form a stable Mo‐O‐Ti bonding mode at the interface. Electrons transfer from TiO₂ (001) surface to the α‐MoO₃, leading to the enhancement of the valence band and the optical absorption spectrum in visible light region. In addition, this proper charge transfer generates a built‐in electric field between the interface regions of bilayer α‐MoO₃/TiO₂ heterostructure and forms a favorable type‐II band alignment between the two α‐MoO₃ layers. The α‐MoO₃/TiO₂ heterostructure can prevent the recombination of the electron‐hole pairs; thus, excite electrons can easily move from TiO₂ to the inner layer, and then to the outer layer of α‐MoO₃. These results demonstrate that the bilayer α‐MoO₃/TiO₂ heterostructure, especially the outer layer α‐MoO₃, has efficient photoelectric performance.     

Photoassisted Catalytic Oxidation of Carbon Monoxide at Room Temperature

Summary. The thermal and photoassisted catalytic oxidation of CO at metal oxide supported RuO₂·xH₂O was studied at room temperature. Contrary to neat RuO₂·xH₂O the supported catalysts suffer from fast deactivation attributed to strong adsorption of the reaction product carbon dioxide. The latter can be efficiently removed from the catalyst surface at elevated temperatures. In some cases, i.e. for catalysts supported with selected n-type semiconductors (TiO₂, SnO₂, WO₃), efficient CO₂ desorption and good, constant catalytic activity was observed upon visible light irradiation. Under such conditions the CO to CO₂ conversion observed for RuO₂·xH₂O/TiO₂ was nearly as good and stable as for the unsupported catalyst. It is suggested that light absorption promotes carbon dioxide desorption through positive charging of the catalyst surface.     

Sb modified Fe–Mn/TiO<Subscript>2</Subscript> catalyst for the reduction of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> with NH<Subscript>3</Subscript> at low temperatures

Manganese-based catalysts have attracted much attention due to their excellent performance for NO reduction with NH₃ (NH₃-SCR) at low temperatures. In the current study, the novel metal Sb was modified into Mn/TiO₂ and Fe–Mn/TiO₂, and the NO _x conversion was compared with those of Mn/TiO₂ and Fe–Mn/TiO₂ catalysts to investigate the effect of the Sb. The NO _x reduction activities of the catalysts were evaluated in the temperature range of 100–250 °C at a space velocity of 60,000 h^?1. The physicochemical properties of all the catalysts were characterized by Brunauer–Emmett–Teller surface area, temperature-programmed desorption of ammonia, temperature-programmed reduction, X-ray photoelectron spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy. Interestingly, the Sb-promoted Mn-based catalysts showed significantly higher NO _x conversion than the other catalysts with or without 6 vol% of H₂O. The high performance of the Sb-modified catalysts could be related to the increase of acid sites and redox properties.