Electrochemical studies of titanium in fluoride-chloride molten salts

The interaction between titanium and Ti⁴⁺ ions (K₂TiF₆), the electroreduction reaction of Ti⁴⁺ ions and the anodic reaction of Ti in KCl–NaCl–KF melts with K₂TiF₆ at 973 K were studied by means of electrochemical and physical measurements. It was found that the fluoride ions played a very important role in these reactionsIn KCl–NaCl-3 wt % K₂TiF₆ molten salts with less than 3 wt % KF, the interaction reaction was considered to proceed as Ti⁴⁺+Ti=2Ti²⁺. If the bath contained more than 10 wt% KF, the reaction 3Ti⁴⁺+Ti=4Ti³⁺ occurred.The electrochemical reduction of Ti⁴⁺ (K₂TiF₆) ions in the molten salts with less fluoride ions was observed to proceed according to three reaction steps, i.e. Ti⁴⁺+e=Ti³⁺, Ti³⁺+e=Ti²⁺, Ti²⁺+2e=Ti. In the case of the fluoride ion concentration being higher, two reduction steps, i.e. Ti⁴⁺+e=Ti³⁺, Ti³⁺+3e=Ti were suggested.     

Effect of TiO2 particle size on the photocatalytic reduction of CO2

Pure TiO₂ anatase particles with a crystallite diameters ranging from 4.5 to 29 nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO₂ photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14 nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.     

Effect of molar ratio of TiO2/SiO2 on the properties of particles synthesized by flame spray pyrolysis

Titania (TiO₂)–silica (SiO₂) nanoparticles were synthesized from sprayed droplets of a mixture of TEOS and TTIP by flame spray pyrolysis (FSP). The effect of molar ratio between TEOS and TTIP in the mixture on the particle properties such as particle morphology, average particle diameter, specific surface area, crystal structure, etc., were determined using TEM, XRD, BET, and FT-IR. A UV-spectrometer was also used to measure the absorption spectrum and the band gap energy of the product particles. As the molar ratio of TEOS/TTIP increased by increasing TEOS concentration at the fixed TTIP concentration, the average particle diameter of the mixed oxide nanoparticles increased with maintaining uniform dispersion between TiO₂ and SiO₂, and crystal structure was transformed from anatase to amorphous. The band gap energy of the TiO₂–SiO₂ nanoparticles increased with respect to the increase of the molar ratio due to the decrease of width of UV-absorption spectrum. Photocatalytic activity of TiO₂–SiO₂ composite particles decreased with the concentration of TEOS.     

Selective oxidation of benzaldehyde derivatives on TiO2 photocatalysts modified with fluorocarbon group

Fluorocarbon groups were introduced onto surfaces of SiO₂-covered TiO₂ particles (SiO₂–TiO₂). Oxidation of pentafuluorobenzaldehyde on the surface modified TiO₂ powders proceeded much efficiently than that on SiO₂-covered TiO₂ particles without surface modification. In addition, no enhancement of activity level of surface-modified SiO₂–TiO₂ for oxidation of benzaldehyde was observed. The enhancement of the surface-modified SiO₂–TiO₂ is due to the interaction of F atoms between the substrate and fluorocarbon groups introduced on SiO₂–TiO₂.     

Pd nanoparticles prepared by “controlled colloidal synthesis” in solid/liquid interfacial layer on silica. I. Particle size regulation by reduction time

Controlled colloidal synthesis (CCS) was developed to prepare monodisperse palladium particles in the nano-scale range on suspended SiO₂ particles in an ethanol–toluene mixture. On colloidal SiO₂ an about 1 nm thick ethanol-rich adsorption layer was produced in adsorption equilibrium with the liquid mixture. Ethanol served as a reducing agent for the Pd(II) ions diffusing from a toluene-rich liquid solution into the interfacial layer. The low reduction rate ensures the dominancy of particle growth over the nucleation of palladium during the reduction process after the initial nucleation. The relation between the reduction time and the particle size produced was studied. XRF, XPS, TEM, CO chemisorption, and benzene hydrogenation as catalytic test were employed to characterize the samples prepared using different reduction time.     

Kinetics and Mechanism of the Interaction of Alkali Calcium Silicate Glasses with Salt Melts in the KNO3–Pb(NO3)2 System

The interaction of alkali calcium silicate glasses with salt melts in the KNO₃–Pb(NO₃)₂ system is investigated at temperatures of 420–520°C. The chemical composition of crystalline coatings formed upon treatment contains both components of the initial glass (SiO₂, 9–12 wt %; CaO, 0.8–1.2 wt %) and components of the salt melt (PbO, 82–89 wt %). The treatment temperature is the main factor affecting the structure of the modified surface layer. The mechanism of the interaction of alkali calcium silicate glasses with salt melts is analyzed. According to this mechanism, the interaction involves the ion exchange (with the participation of Na⁺, K⁺, Ca²⁺, and Pb²⁺ ions), crystallization of modified surface layers, and incorporation of Pb _x O _y nanoparticles (formed in the salt melt) into the coating structure.     

Physical and electrochemical characterization of nanostructured composites formed by TiO2 templates and PEDOT-PPS films

We report the physical and electrochemical characterization of nanostructured composites formed by TiO₂ templates and PEDOT-PPS films. TiO₂ templates were prepared by sol-gel techniques using TiCl₄ and TiF₄ as the titania precursors, and also by adding a soluble polymer at the initial stages of the TiCl₄ sol-gel synthesis to induce porous oxide networks. The effect of different precursors and annealing environments in the crystallite and particle size of the templates was followed by XRD and SEM studies, and correlated to the electrochemical properties of TiO₂ templates dip-coated in a conducting polymer solution (PEDOT-PSS 1.3%). We found that the microstructure had a strong influence in the adsorption of the conductive polymer, which shows superior electrochemical activity in matrices with abundant surface hydroxylation, broad particle size distribution and residual carbon. Cyclic voltammetry and electrochemical impedance data revealed the differences between the several TiO₂/PEDOT-PSS systems, with the best performance observed in systems based on TiO₂ templates obtained from the TiCl₄ sol-gel route and containing residual mesoporous carbon.     

Structural Evolution of Highly Active Ir-Based Catalysts for the Selective Reduction of NO with Reductants in Oxidizing Conditions

NO _x reduction over Ir-based catalysts in the presence of excess oxygen with hydrocarbon as a reductant was investigated in the focus on observing microstructure of Ir particle supported on various carriers and structural evolution of highly active Ir-based catalysts in the NO _x reduction. Characterization of Ir-based catalysts using SEM, TEM, XRD, CO chemisorption and XPS, and reaction studies on various Ir-based catalysts have proved that the formation of relatively large Ir metal particle with 40–60 nm of nanocrystal size carried on inert supports has been a prerequisite for the evolution of high activities in the NO _x reduction rather than the formation of Ir metal state on the catalyst. Furthermore, in inert gas conditions in a high temperature range of 850–950°C, Ir metal was easily formed by using the support such as TiO₂ and ZrO₂ that drastically decreased its specific surface area in the temperature range.     

Effect of Heat-Treatment on the Mechanism and Kinetics of the Hydrogen Evolution Reaction on Ni—P + TiO2 + Ti Electrodes

Composite Ni—P + TiO₂ + Ti layers were prepared by codeposition of Ni—P alloy with TiO₂ and Ti powders from a solution containing suspension of TiO₂ and Ti particles. The electrodeposition was carried out under galvanostatic conditions at room temperature. The layers exhibited an amorphous Ni—P matrix in which crystalline TiO₂ and Ti were embedded. On the deposit surface, the nonstoichiometric Ti oxide, Ti₁₀O₁₉, and intermetallic compounds, NiTi, formed during the electrodeposition, were also present. The heat treatment of these layers in argon leads to the crystallization of Ni—P matrix and formation of nonstoichiometric Ti oxides, detected by XRD. Electrolytic activity towards the hydrogen evolution reaction (HER) was studied on these electrode materials before and after heat treatment. The mechanism of the HER was also studied, and the kinetic parameters were determined using steady-state polarization and electrochemical impedance spectroscopy (EIS). An increase in activity occurring after heating of Ni—P + TiO₂ + Ti layers is related to TiO₂ reduction and formation of nonstoichiometric Ti oxides: Ti₁₀O₁₉(400 °C), Ti₇O₁₃(500 °C) and Ti₄O₇(800 °C). It is postulated that the increase in electrochemical activity is related to the properties of these oxides and a facility for H reduction/adsorption on their surface, as well as to the presence of NiTi intermetallics as compared with the Ni—P + TiO₂ + Ti electrode.     

Oxidative carbonylation of methanol to dimethyl carbonate over CuCl/SiO2–TiO2 catalysts prepared by microwave heating: The effect of support composition

Copper(I) chloride catalysts with a loading of 20 wt%, supported on silica–titania mixed oxides with Si/Ti ratios of 1, 5, 10 and 50 were prepared by conventional and microwave heating methods and tested in the oxidative carbonylation of methanol to dimethyl carbonate (DMC). X-ray diffraction (XRD), nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA) were used to examine the bulk and surface properties of the CuCl/SiO₂–TiO₂ catalysts. Quantum-chemical calculations were performed to explore the interaction of CuCl with the silica–titania support. Microwave heating showed some significant advantages over the conventional heating method, with markedly reduced preparation temperature and time, and provided improved catalytic activity in the oxidative carbonylation of methanol. The catalytic behavior of CuCl/SiO₂–TiO₂ in the test reaction studied was strongly dependent on the support composition. Incorporation of tetrahedral Ti(IV) species into the silica matrix could enhance the interaction of copper species with the oxide support. The improved catalytic performance of CuCl/SiO₂–TiO₂ in the DMC synthesis can be understood by the existence of the strong coordination interactions between the Cu⁺ centers of CuCl and the bridging oxygen atoms at the Si–O–Ti bonds in the silica–titania support.     

Electrochemical properties of Composites based on Nanocrystalline Anatase (TiO2) and Copper compounds (CuBr, CuO)

The electrochemical properties in aqueous solution of composite materials made from nanocrystalline anatase TiO₂ with CuBr and CuO are reported. CuO–TiO₂ composite samples are prepared by a novel route based on oxidation of CuBr–TiO₂. The corrosion of CuBr–TiO₂ composite electrodes prevents a detailed electrochemical analysis. The data on CuO–TiO₂ composites are consistent with the presence of a surface layer of TiO₂ nanoparticles. A Mott–Schottky analysis gives a flat-band potential of –0.5 V/NHE (pH = 6) and a low carrier density of 10¹⁴cm^–3.     

Hybrid photocatalysts for the degradation of trichloroethylene in air

Hybrid photocatalysts based on an adsorbent SiMgOx and a photocatalyst TiO₂ were developed in a plate shape. The ceramic surface was coated with TiO₂ by the slip-casting technique. The effect of the support in the photocatalytic degradation of trichloroethylene (TCE) was analyzed by modifying TiO₂ loading and the layer thickness. Photocatalysts were characterised by N₂ adsorption–desorption, mercury intrusion porosimetry, SEM, UV–vis spectroscopy and XRD. A direct relationship between the TiO₂ content and the photocatalytic activity was observed up to three layers of TiO₂ (0.66 wt.%). Our results indicate that intermediate species generated on the TiO₂ layer can migrate through relatively long distances to react with the OH⁻ surface groups of the support. By increasing the TiO₂ loading of the photocatalyst two effects were observed: trichloroethylene conversion is enhanced, while the efficiency of the oxidation process is decreased at expenses of increasing the selectivity to COCl₂ and dichloroacetylchloride (DCAC). The results are discussed in terms of the layer thickness, TiO₂ amount, TCE conversion and CO₂, and COCl₂ selectivity.     

Inorganic gels as precursors of TiO2 photocatalysts prepared by low temperature microwave or thermal treatment

A simple procedure for preparing active TiO₂ photocatalysts is presented. The starting materials were unusual TiO₂ gels formed from TiCl₄. The use of microwaves for a very short time enhanced the TiO₂ crystallinity preventing an increase of particle size and minimizing the decrease of specific surface area. This result makes this preparation very attractive. The formation of the gels was monitored through measurements of viscosity. All the samples were characterized by means of X-ray diffraction, diffuse reflectance spectroscopy and BET specific surface area measurements. The photoactivity of the samples was evaluated using the photodegradation of 4-nitrophenol in liquid–solid regimen and gaseous 2-propanol as probe reactions. Commercial TiO₂ Degussa P25 was used for the sake of comparison.     

Oxidation of CH4 over Pd supported on TiO2-doped SiO2: Effect of Ti(IV) loading and influence of SO2

Titania-modified silicas with different weight% of TiO₂ were prepared by sol–gel method and used as supports for Pd (1 wt%) catalysts. The obtained materials were tested in the oxidation of methane under lean conditions in absence and in presence of SO₂. Test reactions were consecutively performed in order to evaluate the thermal stability and poisoning reversibility. Increasing amounts of TiO₂ improved the catalytic activity, with an optimum of the performance for 10 wt% TiO₂ loading. Moreover, the titania-containing catalysts exhibited a superior tolerance towards SO₂ by either adding it to the reactants or feeding it as a pure pretreatment atmosphere at 350 °C. Catalysts were characterized by XPS, XRD, FT-IR and BET measurements. According to the structural and surface analyses, the mixed oxides contained Si–O–Ti linkages which were interpreted as being responsible for the enhanced intrinsic activity of supported PdO with respect to PdO on either pure SiO₂ or pure TiO₂. Moreover, the preferential interaction of the sulfur molecule with TiO₂ and the easy SO_x desorption from high surface area silica were the determining factors for the superior SO₂ tolerance of the TiO₂-doped catalysts.     

NOx storage behavior and sulfur-resisting performance of the third-generation NSR catalysts Pt/K/TiO2–ZrO2

The NO_x storage and reduction (NSR) catalysts Pt/K/TiO₂–ZrO₂ were prepared by an impregnation method. The techniques of XRD, NH₃-TPD, CO₂-TPD, H₂-TPR and in situDRIFTS were employed to investigate their NO_x storage behavior and sulfur-resisting performance. It is revealed that the storage capacity and sulfur-resisting ability of these catalysts depend strongly on the calcination temperature of the support. The catalyst with theist support calcined at 500 °C, exhibits the largest specific surface area but the lowest storage capacity. With increasing calcination temperature, the NO_x storage capacity of the catalyst improves greatly, but the sulfur-resisting ability of the catalyst decreases. In situ DRIFTS results show that free nitrate species and bulk sulfates are the main storage and sulfation species, respectively, for all the catalysts studied. The CO₂-TPD results indicate that the decomposition performance of K₂CO₃ is largely determined by the surface property of the TiO₂–ZrO₂ support. The interaction between the surface hydroxyl of the support and K₂CO₃ promotes the decomposition of K₂CO₃ to form –OK groups bound to the support, leading to low NO_x storage capacity but high sulfur-resisting ability, while the interaction between the highly dispersed K₂CO₃ species and Lewis acid sites gives rise to high NO_x storage capacity but decreased sulfur-resisting ability. The optimal calcination temperature of TiO₂–ZrO₂ support is 650 °C.     

The pH-dependence of thermally-coated Ti/TiO2 electrodes in the absence and presence of nitrobenzene

Cyclic voltammetric measurements on thermally coated Ti/TiO₂ electrodes in 0.05–3.0 m H₂SO₄, Na₂SO₄ and NaOH media were carried out to evaluate the cause of pH sensitivity of these electrodes. The electrode activity decreased in the order H₂SO₄ > NaOH > Na₂SO₄. Ionic strength had a significant influence on the electrode activity. In the redox catalysis of nitrobenzene reduction the pH dependence increased significantly. The electrode activity again decreased in the same order. In acid media the TiO₂ surface is protonated. In alkaline media OH- ions are specifically adsorbed on the TiO₂ surface. The concentration of these charged species, as well as the ease of counter ion transport through the charged layers during surface redox reactions in the oxide layer, is identified as the cause of the pH sensitivity of the electrodes. In acid media proton transport in the film occurs through a surface Grothus-like mechanism. In alkaline medium proton abstraction by OH^– ions from the trapped H₂O molecules may facilitate proton transport.     

Synergistic effect of TiO2 and iron oxide supported on fluorocarbon films: Part 2: Long-term stability and influence of reaction parameters on photoactivated degradation of pollutants

The degradation of hydroquinone (HQ) and nalidixic acid (NA) mediated by TiO₂ and iron oxide immobilized on functionalized polyvinyl fluoride films (PVF^f–TiO₂–Fe oxide) in the presence of H₂O₂ under simulated solar light has been examined. The results show that the contribution of homogeneous photo-Fenton oxidation to the initial mineralization process was low. The degradation rates were not dependant of initial pH. Heterogeneous photocatalytic activity of PVF^f–TiO₂–Fe oxide was enhanced by increasing temperature, NaCl addition and by long-term utilization.The PVF^f–TiO₂–Fe oxide surface was characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) at different states of utilization. Correlations between the catalyst surface composition and degradation kinetics are discussed. Long-term stability evaluated by repetitive pollutant degradations was outstanding. The presence of TiO₂ seems to (i) limit contact between polymer film and highly reactive radicals in the solution and (ii) act as a charge trap. Moreover, during the photocatalysis mediated by PVF^f–TiO₂–Fe oxide, some leaching of supported iron increased the amount of the top TiO₂ layer exposed to the light increasing the synergistic effects between the two oxides leading to enhanced pollutant degradation.     

Study on the formation process of Al2O3-TiO2 composite powders

Fine particles of anatase were suspended in solutions of ammonium alum with Al₂O₃/TiO₂ molar ratios from 0.1:1 to 7:1. By spray drying the suspensions and calcining the spray-dried powders, Al₂O₃-TiO₂ composite particles were obtained. The results show that after the spray drying, coatings of ammomium alum are formed on the surface of the anatase particles, leading to composite precursor powders (CCPs) with larger particle sizes. Upon calcining the CCPs, ammomium alum pyrolyzes to amorphous Al₂O₃ and anatase transforms into rutile. Both are mainly responsible for the observed particle size reductions as well as the densification of each composite particle. The in-situ formed α-Al₂O₃ and rutile may have higher reactivities, forming aluminum titanate at 1150 °C, about 130 °C lower than the theoretical temperature for the formation of Al₂TiO₅ by solid reaction. The reaction between α-Al₂O₃ and rutile starts from the interface between the anatase and the alum coating and mainly takes place in the single particles formed by spray drying. The molar ratio of Al₂O₃ to TiO₂ influences the final crystalline phases in the composite powders, but not stoichiometrically.     

Synergistic effect of TiO2 and iron oxide supported on fluorocarbon films. Part 1: Effect of preparation parameters on photocatalytic degradation of organic pollutant at neutral pH

Iron oxide and TiO₂ were immobilized on modified polyvinyl fluoride films in a sequential process. Synergic effects of iron oxide and TiO₂ on the polymer film were observed during the heterogeneous degradation of hydroquinone (HQ) in the presence of H₂O₂ at pH close to neutrality and under simulated solar irradiation. Within the degradation period, little iron leaching (<0.5 mg/L) was observed.The surface of commercial polyvinyl fluoride (PVF) film was modified by TiO₂ under light inducing oxygen group (C–OH, CO, COOH) formation on the film surface. During this treatment, TiO₂ nanoparticles simultaneously bind to the film, leading to PVF^f–TiO₂. The possible mechanistic pathway for the TiO₂ deposition and the nature of the polymer–TiO₂ interaction are discussed. Furthermore PVF and PVF^f–TiO₂ were immersed in an aqueous solution for the deposition of iron oxide layer by hydrolysis of FeCl₃, leading to PVF–Fe oxide and to PVF^f–TiO₂–Fe oxide respectively.HQ degradation and mineralization mediated by PVF^f–TiO₂, PVF–Fe oxide and PVF^f–TiO₂–Fe oxide were investigated under different conditions. Remarkable synergistic effects were observed for PVF^f–TiO₂–Fe oxide possibly due to Fe(II) regeneration, accelerated by electron transfer from TiO₂ to the iron oxide under light.