The effect of the synthesis conditions of aluminum-modified nanosized titanium dioxide on the efficiency of its use in electrorheological dispersions

A comparative study of the physicochemical properties of nanodispersed TiO₂ samples synthesized from TiCl₄ by the sol–gel and coprecipitation methods has shown the advantages of the former from the points of view of both the specific surface area and electrorheological response of the obtained filler in 5% electrorheological dispersions. A correlation has been revealed between the temperature of TiO₂ treatment (600–800°C) and its structure, phase state and electrorheological response. The maximum shear stress increment has been observed for dispersions of TiO₂ samples that contain 7–10 mol % Al and have been thermotreated at 700°C for 3.0–3.5 h.     

Effect of preparation procedure on the properties of CeO2

The effect of preparation procedure on the physicochemical and catalytic properties of CeO₂ was studied. Differences in the electronic and structural characteristics of CeO₂ depending on preparation procedure and treatment temperature were found using X-ray diffraction analysis, transmission electron microscopy, UV-visible electronic spectroscopy, and X-ray photoelectron spectroscopy. With the use of the temperature-programmed reaction with CO, it was demonstrated that CeO₂ samples with a high concentration of point defects—oxygen vacancies caused by the presence of Ce³⁺—were characterized by an increased mobility of bulk oxygen. The samples of CeO₂ with a high concentration of structural defects—micropores of size 1–2 nm and stepwise vicinal faces in crystallites—exhibited a high catalytic activity in the reaction of CO oxidation.     

The study on the preparation methods and the fluidity of large rare earth oxide particles

In this work large CeO₂ particles were prepared using H₂C₂O₄ and NH₄HCO₃ as precipitators. The effects of feeding mode and speed, stirring speed, aging time, precipitate and calcination temperature and precipitation method on physicochemical properties of CeO₂ were studied when the precipitator was H₂C₂O₄. By the NH₄HCO₃ precipitation method, the effects of adding inoculating crystals and additives on particle size, loose density and fluidity of CeO₂ were investigated. Under optimized conditions, large CeO₂ particles with high loose density and good fluidity of can be prepared by either H₂C₂O₄ or NH₄HCO₃ precipitation method. SEM also investigates the morphology of the particles prepared by both methods. The results show that physicochemical properties of CeO₂ particles prepared by the NH₄HCO₃ precipitation method are not as good as those of CeO₂ particles prepared by the H₂C₂O₄ precipitation method. However, both methods are suitable for industrial production due to their simple processes, low equipment investment and ease for industrial development. Our results show that large rare earth particle can be prepared by the optimized conventional methods. This study provided a useful method to prepare of large rare earth particles.     

Synthesis of cerium orthophosphates with monazite and rhabdophane structure from phosphoric acid solutions in the presence of hydrogen peroxide

It has been proposed to conduct the synthesis of cerium(III) orthophosphates by reacting cerium(IV) compounds with hydrogen peroxide in the presence of concentrated orthophosphoric acid at ambient temperature. It has been shown that the reaction of H₂O₂ with CeO₂ suspensions in H₃PO₄ medium produces CePO₄ · xH₂O (rhabdophane structure), while that with CeO₂ solutions in concentrated H₃PO₄ results in CePO₄ (monazite structure).     

Dehydrogenation of Ethylbenzene to Styrene with Carbon Dioxide Over ZrO<Subscript>2</Subscript>-based Composite Oxide Catalysts

We have been exploring various new catalyst systems for the utilization of carbon dioxide as a soft oxidant in the catalytic dehydrogenation of ethylbenzene (EB) to styrene. The utilization of CO₂ as a soft oxidant for the commercially important catalytic dehydrogenation of EB to styrene has received enormous attention recently due to its several attractive features. This review summarizes the results of our most recent findings on zirconia-based composite oxide catalyst systems exploited for this reaction. Under this systematic and comprehensive investigation various zirconia-based composite oxide catalysts namely, TiO₂-ZrO₂, MnO₂-ZrO₂, CeO₂-ZrO₂, K₂O/TiO₂-ZrO₂, B₂O₃/TiO₂-ZrO₂ and CeO₂-ZrO₂/SBA-15 have been synthesized, characterized by various techniques and evaluated for the title reaction. Most of these composite oxide catalysts were found to exhibit very interesting physicochemical characteristics and exceptionally better catalytic properties for this reaction. As revealed by characterization results, a large number of acid–base sites with moderate strength are essential for a high conversion and product selectivity of this reaction with CO₂ as the soft oxidant.     

O<Subscript>2</Subscript>-probe EPR as a method for characterization of surface oxygen vacancies in ceria-based catalysts

Catalytic activity of ceria-based systems is essentially related to the oxygen storage/release characteristics of the surface and, therefore, to the properties of the oxygen vacancies generated upon reduction of CeO₂. EPR analysis of the superoxide species formed upon low temperature oxygen chemisorption on this type of systems is shown to be a very powerful method to characterize such defects. The present work revises results mainly obtained in the authors’ laboratory on this topic and shows the main physicochemical properties of such superoxide species. Situations of practical interest in the field of heterogeneous catalysis are analysed. These include the analysis of defects formed on pure CeO₂, as well as their chemical modification by NO chemisorption or in the presence of chlorine impurities, typically present in supported metal catalysts. Additionally, the characterization of two-dimensional ceria structures in alumina-supported ceria systems with high practical interest is shown to be uniquely provided by this EPR-based method.     

Phase composition and catalytic properties of ZrO2 and CeO2-ZrO2 in the ketonization of pentanoic acid to 5-nonanone

The phase composition, microstructure, and catalytic properties of the samples of ZrO₂ and CeO₂-ZrO₂ calcined in air at 450–500°C in the ketonization reaction of pentanoic acid were studied. It was found that ZrO₂ of tetragonal and monoclinic modifications is characterized by sufficiently high activity and selectivity for 5-nonanone; the yield of 5-nonanone was 66.3–64.9%. The modification of zirconium dioxide with cerium oxide leads to the formation of a substitutional solid solution based on tetragonal ZrO₂. Upon the addition of CeO₂ in an optimum amount of 10 wt % to zirconium dioxide, an increase in the conversion of pentanoic acid was observed with the retention of high selectivity for the target product, which led to an increase in the yield of 5-nonanone to 73.3%. Based on the results of physicochemical studies performed by high-resolution transmission electron microscopy, X-ray diffraction analysis, and X-ray photoelectron spectroscopy, the physicochemical and catalytic properties of the test catalysts were compared.     

Design of anhydrous electrorheological (ER) suspensions based on I2-doped poly(pyridinium salt)

A rigid-rod aromatic poly(pyridinium salt) was synthesized and doped with iodine (I₂) for making anhydrous electrorheological (ER) fluids. The I₂-doped particles were further processed into ones having insulating skins. Dielectric properties and current densities of the suspensions containing these particles were studied to elucidate the roles of conductivity of the dispersed phase in the ER suspension. © 1994 John Wiley & Sons, Inc.     

Influence of Ionizing Radiation on the Properties of a Nanodispersed PdO/CeO<Subscript>2</Subscript> Catalyst in the Reaction of Low-Temperature Carbon Monoxide Oxidation

The effect of electron-beam irradiation on the catalytic properties of PdO/CeO₂ has been examined. It has been found that irradiation in the range of 15–30 kGy results in an increase in the concentration of the radical anion [O^?], thereby enhancing the catalytic activity of PdO/CeO₂.     

Physicochemical and electrorheological properties of titanium dioxide modified with metal oxides

The properties of electrorheological fluids containing dispersed phase of titanium dioxide nanoparticles prepared via the sol-gel method and modified with metal oxides have been studied. Titanium dioxide has the anatase structure with crystallite sizes of 8–10 nm and a specific surface area of 90–140 m²/g. It has been found that the magnitude of the electrorheological response of the filler is determined by the specific surface area and the content of a modifying component. The strongest electrorheological response has been revealed for titanium dioxide modified with aluminum oxide at an Al content of 6.5–7.0 mol % relative to TiO₂.     

CeO2 nanofibers-CdS nanostructures n–n junction with enhanced visible-light photocatalytic activity

In the present work, the n-cerium (IV) oxide (CeO₂)/n-cadmium sulfide (CdS) composite nanofibers were successfully synthesized via a facile electrospinning and hydrothermal synthesis strategy. The physicochemical properties of the synthesized composite nanofibers were investigated by using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), energy Dispersive X-Ray Spectroscopy (EDS), diffuse Reflectance Spectroscopy (DRS), Fourier-transform infrared (FTIR), photoluminescence (PL), Brunauer–Emmett–Teller (BET) and Raman spectroscopy analysis. The activities of the CeO₂/CdS were evaluated through the photocatalytic degradation of Rose Bengal (RB) in an aqueous solution under blue LED light radiation. CeO₂/CdS composites exhibit higher photocurrent density in photocurrent response experiment and smaller charge-transfer resistance in electrochemical impedance spectroscopy (EIS). Overall, the results confirmed higher charge separation efficiency in CeO₂/CdS composites compared to pristine CeO₂ nanofibers, and CdS, which is related to intimately contact among the CeO₂, and CdS. The present work provides a new approach to construct n-n heterojunction photocatalysts based on electrospinning and a deeper insight for the photocatalytic degradation activity. In addition, possible degradation mechanism and pathways were proposed according to the identified intermediates.     

Ordered mesoporous Pt/Fe3O4–CeO2 heterostructure gel particles with enhanced catalytic performance for the reduction of 4-nitrophenol

The unique physicochemical properties of ordered mesoporous transition metal oxides have attracted more and more attention. The hydrolysis process of metal oxide precursors is difficult to control, and it is difficult to synthesize an ordered mesoporous transition metal oxide material using the conventional template method. Ordered mesoporous Pt/Fe₃O₄–CeO₂ heterostructure gel materials with excellent catalytic properties were successfully prepared using aerogel technology and the chemical deposition method. The Pt/Fe₃O₄–CeO₂ material was an n–n combined heterostructured semiconductor material which consisted of a magnetic Fe₃O₄ layer, a CeO₂ core and Pt noble metal doped nanoparticles. A layer of Fe₃O₄ thin film was formed on the surface of ordered mesoporous Pt/CeO₂ gel matrix material using the chemical deposition method. The intriguing heterostructural features could facilitate reactant diffusion and exposure of active sites which could enhance synergistic catalytic effects between the Pt nanoparticles and CeO₂ nanoparticles. Compared with Pt/CeO₂, the prepared Pt/Fe₃O₄–CeO₂ showed enhanced catalytic activity in the reduction of 4-nitrophenol at room temperature. The catalytic activity of the heterostructure catalysts was systematically investigated using 4-nitrophenol reduction as a model reaction. The results showed that the Pt (0.1%)/Fe₃O₄–CeO₂ sample exhibited the optimal catalytic performance toward catalytic reduction of 4-nitrophenol to 4-aminophenol. The study provided a method for the preparation of heterostructure nanocatalysts with high efficiency, which would be effective for application in various catalytic reactions.     

Particulate sol–gel synthesis and optical and electrical properties of CeO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> nanocomposite

CeO₂/TiO₂ nanocomposite was synthesized by particulate sol–gel method. The X-ray diffractogram shows the presence of cubic CeO₂ and anatase TiO₂ in the composite. The high resolution scanning electron micrographs reveal the nanoparticulate nature of the prepared composite. The composite absorbs UV light and exhibits near-band gap emission corresponding to TiO₂ and deep level emission due to crystal defects. The Nyquist plot displays two semicircular arcs indicating the material heterogeneity. The physicochemical characteristics of the synthesized nanocomposite are in favour of its application as an ingredient of sunscreen formulations; under UV light the photocatalytic activity of CeO₂/TiO₂ composite, tested through the degradation of rhodamine B, is very much less than that by pristine anatase TiO₂. Reduced adsorption of moisture by the nanocomposite is a possible reason for the observed very low photocatalytic activity.     

Carbon nanotube assisted synthesis of CeO2 nanotubes

CeO₂ nanotubes have been synthesized facilely using carbon nanotubes (CNTs) as templates by a liquid phase deposition method. The properties of the CeO₂ nanotubes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS) as well as thermogravimetry and differential thermal analysis (TG-DTA). The obtained CeO₂ nanotubes with a polycrystalline face-centered cubic phase have a uniform diameter ranging from 40 to 50 nm. The CeO₂ nanotubes are composed of many tiny interconnected nanocrystallites of about 10 nm in size. The pretreatment of CNTs and calcination temperature were confirmed to be the crucial factors determining the formation of CeO₂ nanotubes. A possible formation mechanism has been suggested to explain the formation of CeO₂ nanotubes.     

Effects of doping with CeO2 and calcination temperature on physicochemical properties of the NiO/Al2O3 system

The effects of doping with CeO₂ and calcination temperature on the physicochemical properties of the NiO/Al₂O₃ system have been investigated using DTA, XRD, nitrogen adsorption measurements at −196°C and decomposition of H₂O₂ at 30–50°C. The pure and variously doped solids were subjected to heat treatment at 300, 400, 700, 900 and 1000°C. The results revealed that the specific surface areas increased with increasing calcination temperature from 300 to 400°C and with doping of the system with CeO₂. The pure and variously doped solids calcined at 300 and 400°C consisted of poorly crystalline NiO dispersed on γ-Al₂O₃. Heating at 700°C resulted in formation of well crystalline NiO and γ-Al₂O₃ phases beside CeO₂ for the doped solids. Crystalline NiAl₂O₄ phase was formed starting from 900°C. The degree of crystallinity of NiAl₂O₄ increased with increasing the calcination temperature from 900 to 1000°C. An opposite effect was observed upon doping with CeO₂. The NiO/Al₂O₃ system calcined at 300 and 400°C has catalytic activity higher than individual NiO obtained at the same calcination temperatures. The catalytic activity of NiO/Al₂O₃ system increased, progressively, with increasing the amount of CeO₂ dopant and decreased with increasing the calcination temperature.     

Electrorheological properties of suspensions of hollow globular titanium oxide/polypyrrole particles

Hollow globular clusters of titanium oxide (TiO₂) nanoparticles were synthesized by a simple hydrothermal method. The prepared particles were consequently coated by in situ polymerization of conductive polymer polypyrrole (PPy) to obtain novel core–shell structured particles as a dispersed phase in electrorheological (ER) suspensions. The X-ray diffraction analysis and scanning electron microscopy provided information on particle composition and morphology. It appeared that PPy coating improved the compatibility of dispersed particles with silicone oil which results in higher sedimentation stability compared to that of mere TiO₂ particles-based ER suspension. The ER properties were investigated under both steady and oscillatory shears. It was found that TiO₂/PPy particles-based suspension showed higher ER activity than that of mere TiO₂ hollow globular clusters. These observations were elucidated well in view of their dielectric spectra analysis; a larger dielectric loss enhancement and faster interfacial polarization were responsible for a higher ER activity of core–shell structured TiO₂/PPy-based suspensions. Investigation of changes in ER properties of prepared suspensions as a function of particles concentration, viscosity of silicone oil used as a suspension medium, and electric field strength applied was also performed.     

Effect of polydimethylsiloxane viscosity on the electrorheological activity of dispersions based on it

The effect the viscosity of a dispersion medium of a polymethylsiloxane fluid (PMS) with a kinematic viscosity over a wide range of values from 5 to 300 cSt has on the electrorheological properties of suspensions based on nanosized titanium dioxide obtained via the sol-gel method is investigated. The investigations are conducted in a wide range of concentrations of suspensions: from 30 to 60 wt % (from 15 to 38 vol %) of the dispersed phase. The role the dispersion medium in two-phase disperse systems plays in the formation of structures of dispersed phase in the presence of an electric field is determined from the dependence of yield points of TiO₂ in PMS with different viscosities on the applied electric field strength.     

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union

This review is focused on the analysis of experimental results on oxygen ion-conducting ceramic materials based on HfO₂, CeO₂, and ThO₂, published in the former Soviet Union. In particular, the physicochemical and transport properties of fluorite-related oxides and the characteristics of electronic conduction in these solid electrolytes are briefly reviewed. Emphasis is given to electrocatalytic and electrochemical properties of cerium-containing oxides, which are promising materials for electrodes of electrochemical cells operating in reducing atmospheres, and mixed-conducting membranes. A comparative analysis of specific features of the solid-electrolyte ceramics based on hafnia, zirconia, ceria, and thoria is performed in order to reveal basic tendencies of oxygen ionic transport in fluorite-type oxides, and to identify the potential applicability of these materials in various high-temperature electrochemical devices. Received: 23 September 1999 / Accepted: 10 October 1999     

Role of step sites on water dissociation on stoichiometric ceria surfaces

The adsorption and dissociation of water on CeO₂(111), CeO₂(221), CeO₂(331), and CeO₂(110) has been studied by means of periodic density functional theory using slab models. The presence of step sites moderately affects the adsorption energy of the water molecule but in some cases as in CeO₂(331) is able to change the sign of the energy reaction from endo- to exothermic which has important consequences for the catalytic activity of this surface. Finally, no stable molecular state has been found for water on CeO₂(110) where the reaction products lead to a very stable hydroxylated surface which will rapidly become inactive.     

Study of the effect of methods for liquid-phase synthesis of nanopowders on the structure and physicochemical properties of ceramics in the CeO<Subscript>2</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Two alternative chemical synthesis methods—cryotechnological coprecipitation of hydroxides and cocrystallization of salts—were used for preparing (CeO₂)_1–x (Y₂O₃) _x nanopowders (x = 0.10, 0.15, 0.20) with a mean coherent scattering domain size of ~7–11 nm and S _sp = 2.1–97.5 m²/g. From these nanopowders, ceramic nanomaterials with mean coherent scattering domain sizes of ~61–85 nm were synthesized. It was studied how the phase composition, microstructure, and electrical transport properties of the produced samples depend on the Y₂O₃ content of a CeO₂-based solid solution and on the synthesis method. It was shown that, in the series (CeO₂)_1–x (Y₂O₃) _x (x = 0.10, 0.15, 0.20), the solid solution (CeO₂)_0.90(Y₂O₃)_0.10 has the highest ionic conductivity with the ion transport number t _i = 0.73 (600°C). In its physicochemical characteristics, this ceramic can be used as a solid electrolyte of intermediate-temperature fuel cells.