New In-situ Sampling and Analysis of the Production of CeO<Subscript>2</Subscript> Powders from Liquid Precursors using a Novel Wet Collection System in a rf Inductively Coupled Thermal Plasma Reactor. Part 1: Reactor System and Sampling Probe

A new reactor and a novel in-situ sampling technique were developed for the study of the synthesis of CeO₂ powders produced from dissolved cerium nitrate salts. The conical reactor minimized particle recirculation and provided a highly symmetrical and undisturbed plasma flow suitable for the analysis of the phenomena affecting the formation of CeO₂ powders. Both a calorimetric study of the reactor and a thermodynamic analysis of CeO₂ formation were conducted. The sampling probe is described and near-isokinetic sampling was achieved. The sampled particles were collected using a miniature wet collection system, i.e. a mist atomizer and a custom-made spray chamber. A numerical simulation of the velocity and temperature fields of the plasma gas in the reactor was done using Fluent. A comprehensive droplet-to-particle formation mechanism presented elsewhere is revisited and expanded based on calorimetry, thermodynamics of CeO₂ formation, numerical simulations and collected particles. No traces of other oxidation states other than CeO₂ were found.     

Inductively Coupled Plasma Synthesis of CeO<Subscript>2</Subscript>-based Powders from Liquid Solutions for SOFC Electrolytes

Doped cerium oxide (CeO₂) based electrolytes are attractive alternative materials to replace the existing yttria-stabilized zirconia (YSZ) used as electrolyte for SOFC (solid oxide fuel cells). Cerium oxide electrolytes offer a similar performance to YSZ electrolytes at a lower cell operating temperature (~600--800 ^°C), therefore reducing thermal stresses and solid state reactions among the cell components.Doped Ce_1-xMe_xO_2-x/2(Me = Gd, Sm or Y) fine \hbox{powders} were synthesized from nitrate salts dissolved in water using a radio frequency inductively coupled plasma reactor. It was demonstrated that the relative concentrations of Ce and dopants fed in the solutions were retained in the synthesized powders. The products were all nano-crystalline with the basic crystal structure of CeO₂ and the crystal size of the products was essentially independent of the dopant used. The particle size distributions obtained were multimodal and in most cases trimodal. The results obtained differ from a previously reported mechanism of particle synthesis from liquid precursors.     

Luminescent characteristics and microstructures of Sr<Subscript>2</Subscript>CeO<Subscript>4</Subscript> phosphors prepared via sol–gel and solid-state reaction routes

Blue-light-emitting Sr₂CeO₄ phosphors were synthesized via a sol–gel process and the conventional solid-state method in this study. The developed sol–gel process lowered the synthesis temperature of monophasic Sr₂CeO₄ to as low as 900 °C. In comparison with the solid-state derived powders, the sol–gel derived powders had more uniform morphology and smaller particle sizes. In addition, sol–gel derived Sr₂CeO₄ displayed higher luminescent intensity than that prepared via the solid-state route under the same heating conditions. This is attributed to the improved compositional homogeneity and crystallinity in the sol–gel process. During the heating processes, Sr₂CeO₄ tended to thermally decompose at elevated temperatures. This decomposition reaction resulted in the formation of an impurity phase- SrCeO₃ and thereby a decrease in the luminescent intensity. For obtaining Sr₂CeO₄ phosphors with high luminescent intensity, the heating conditions in both processes need to be well modulated.     

Synthesis of CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> hydrosols via peptization at room temperature

A procedure is developed for the synthesis of concentrated CeO₂-ZrO₂ hydrosols based on the peptization of a precipitate obtained by the hydrolysis of a cerium nitrate-zirconium oxynitrate mixture. The time intervals and optimum [H⁺]/[Me ⁿ⁺] molar ratios giving rise the formation of CeO₂-ZrO₂ hydrosols stable to aggregation with a narrow particle size distribution are established. The size, shape, density, and phase composition of the hydrosol particles are determined.     

Synthesis of CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> hydrosols via peptization at elevated temperature

A method for synthesizing aggregation-stable CeO₂-ZrO₂ hydrosols with different particle compositions is developed based on the peptization of hydrated oxide precipitates at elevated temperature. It is shown that, by varying heat treatment time, sols can be obtained with particles that have different degrees of crystallinity and sizes of no larger than 6 nm.     

Colloidal properties of CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> hydrosols

CeO₂-ZrO₂ hydrosols are synthesized and the size, shape, phase composition, density, and electrophoretic mobility of particles are studied. The pH ranges of the stability of hydrosols and the thresholds of their fast coagulation in the presence of some electrolytes are determined. The nature of the aggregation stability of CeO₂-ZrO₂ hydrosols is discussed.     

CeO<Subscript>2</Subscript>-catalyzed ozonation of phenol

Three different cerium citrate-based precursors were used for synthesizing CeO₂ through thermal treatment. Three morphological types of CeO₂ were obtained. Characterization of these oxides was carried out by XRD patterns, SEM microscopy, N₂ adsorption isotherms, Raman spectroscopy, zeta potential, and UV/Vis luminescence. Ozonation of phenol catalyzed by CeO₂ was studied as a representative reaction of environmental interest. The differences on the catalytic activity showed by these three oxides could be correlated to amounts of Ce³⁺ on CeO₂ surface and, consequently, to the demand for oxygen needed to burn each precursor.     

Underpotential surface reduction of mesoporous CeO<Subscript>2</Subscript> nanoparticle films

The formation of variable-thickness CeO₂ nanoparticle mesoporous films from a colloidal nanoparticle solution (approximately 1–3-nm-diameter CeO₂) is demonstrated using a layer-by-layer deposition process with small organic binder molecules such as cyclohexanehexacarboxylate and phytate. Film growth is characterised by scanning and transmission electron microscopies, X-ray scattering and quartz crystal microbalance techniques. The surface electrochemistry of CeO₂ films before and after calcination at 500 °C in air is investigated. A well-defined Ce(IV/III) redox process confined to the oxide surface is observed. Beyond a threshold potential, a new phosphate phase, presumably CePO₄, is formed during electrochemical reduction of CeO₂ in aqueous phosphate buffer solution. The voltammetric signal is sensitive to (1) thermal pre-treatment, (2) film thickness, (3) phosphate concentration and (4) pH. The reversible ‘underpotential reduction’ of CeO₂ is demonstrated at potentials positive of the threshold. A transition occurs from the reversible ‘underpotential region’ in which no phosphate phase is formed to the irreversible ‘overpotential region’ in which the formation of the cerium(III) phosphate phase is observed. The experimental results are rationalised based on surface reactivity and nucleation effects.     

Co oxidation with oxygen in the presence of hydrogen on CoO/CeO<Subscript>2</Subscript> and CuO/CoO/CeO<Subscript>2</Subscript> catalysts

The catalytic activity of the CoO/CeO₂ and CuO/CoO/CeO₂ systems in selective CO oxidation in the presence of hydrogen at 20–450°C ([CuO] = 1.0–2.5%, [CoO] = 1.0–7.0%) is reported. The maximum CO conversion (X) decreases in the following order: CuO/CoO/CeO₂ (X = 98–99%, T = 140–170°C) > CoO/CeO₂ (X = 67–84%, T = 230–240°C) > CeO₂ (X = 34%, T = 350°C). TPD, TPR, and EPR experiments have demonstrated that the high activity of CuO/CoO/CeO₂ is due to the strong interaction of the supported copper and cobalt oxides with cerium dioxide, which yields Cu-Co-Ce-O clusters on the surface. The carbonyl group in the complexes Co^δ+-CO and Cu⁺-CO is oxidized by oxygen of the Cu-Co-Ce-O clusters at 140–160°C and by oxygen of the Co-Ce-O clusters at 240°C. The decrease in the activity of the catalysts at high temperatures is due to the fact that hydrogen reduces the clusters on which CO oxidation takes place, yielding Co⁰ and Cu⁰ particles, which are inactive in CO oxidation. The hydrogenation of CO into methane at high temperatures is due to the appearance of Co⁰ particles in the catalysts.     

Effect of the microstructure of the supported catalysts CuO/TiO<Subscript>2</Subscript> and CuO/(CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript>) on their catalytic properties in carbon monoxide oxidation

The effect of the microstructure of titanium dioxide on the structure, thermal stability, and catalytic properties of supported CuO/TiO₂ and CuO/(CeO₂-TiO₂) catalysts in CO oxidation was studied. The formation of a nanocrystalline structure was found in the CuO/TiO₂ catalysts calcined at 500°C. This nanocrystalline structure consisted of aggregated fine anatase particles about 10 nm in size and interblock boundaries between them, in which Cu²⁺ ions were stabilized. Heat treatment of this catalyst at 700°C led to a change in its microstructure with the formation of fine CuO particles 2.5–3 nm in size, which were strongly bound to the surface of TiO₂ (anatase) with a regular well-ordered crystal structure. In the CuO/(CeO₂-TiO₂) catalysts, the nanocrystalline structure of anatase was thermally more stable than in the CuO/TiO₂ catalyst, and it persisted up to 700°C. The study of the catalytic properties of the resulting catalysts showed that the CuO/(CeO₂-TiO₂) catalysts with the nanocrystalline structure of anatase were characterized by the high-est activity in CO oxidation to CO₂.     

Three-dimensional hierarchical CeO<Subscript>2</Subscript> nanowalls/TiO<Subscript>2</Subscript> nanofibers heterostructure and its high photocatalytic performance

Combining the versatility of electrospinning technique and hydrothermal growth of nanostructures enabled the fabrication of hierarchical CeO₂/TiO₂ nanofibrous mat. The as-prepared hierarchical heterostructure consisted of CeO₂ nanowalls growing on the primary TiO₂ nanofibers. Interestingly, not only were secondary CeO₂ nanowalls successfully grown on TiO₂ nanofibers substrates, but also the CeO₂ nanowalls were uniformly distributed without aggregation on TiO₂ nanofibers. The photocatalytic studies suggested that the CeO₂/TiO₂ heterostructures showed enhanced photocatalytic efficiency compared with bare TiO₂ nanofibers under UV light irradiation.     

Thermal stability and surface structure of Mo/CeO<Subscript>2</Subscript> and Ce-doped Mo/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

In order to explore the influence of CeO₂ on the structure and surface characteristics of molybdena, an investigation was undertaken by using N₂ adsorption (BET method), thermal analysis and in-situ diffuse reflectance infrared (DRIFT) techniques. In this work, the Mo/CeO₂ and Ce-Mo/Al₂O₃ samples were prepared by impregnation and co-precipitation methods with high Mo loadings. Combining the results one may notice that the presence of ceria led to the increase of polymerized surface Mo species so as to forming Mo-O-Ce linkages besides the formation of coupled O=Mo=O bonds indicative of polymeric MoO₃. From thermal analysis, it can be inferred that Mo/Al₂O₃ is the thermally most stable material in the temperature range used in the experiment (up to 900°C), whereas Ce-Mo/Al₂O₃ and Mo/CeO₂ samples undergo morphological modifications above 700°C resulting in lattice defects, which motivate the mobility of Mo and Ce ions and thus enhance the possibility of interaction between them. Additionally, their activity towards CO adsorption needs reduced ceria and molybdena containing coordinatively unsaturated sites (CUS), oxygen vacancies and hydroxyl groups to form various carbonate species.     

Effects of precipitants and surfactants on catalytic activity of Pd/CeO<Subscript>2</Subscript> for CO oxidation

A series of precipitants and commercial surfactants (soft templates) were employed to synthesize mesoporous/nano CeO₂ by a hydrothermal method. As-prepared CeO₂ was impregnated with palladium and employed for low-temperature catalytic oxidation of CO. It was found that both soft templates and precipitants had significant effects on the morphology, particle size, crystallinity, and porous structure of the CeO₂, having a significant effect on the surface palladium abundance, molar ratios of surface species, and catalytic activity of the final impregnated Pd/CeO₂. Using ammonia as precipitant could facilitate increased surface palladium abundance and surface molar ratios of PdO/Pd _SMSI , Ce³⁺/(Ce³⁺ + Ce⁴⁺), and O_surface/O_lattice. The catalytic activity of the final Pd/CeO₂ catalysts could be enhanced as well. The optimal P123-assisted ammonia-precipitated Pd/CeO₂ catalyst exhibited over 99% catalytic conversion of CO at 50 °C.     

Analysis of preparation of TiO<Subscript>2</Subscript> particles by diffusion flame reactor for photodegradation of phenol and toluene

TiO₂ nanoparticles were produced in the diffusion flame reactor, and the size and anatase/rutile content of TiO₂ were examined by a Particle Size Analyzer and X-ray diffraction, respectively. Increase in fuel/O₂ ratio, initial concentration of TiCl₄ or total gas flow rate causes the larger particle size and the higher rutile composition. The photocatalytic activities of TiO₂ powders were tested on the decompositions of phenol and toluene in the aqueous solution under UV irradiation. The degradation rate increases as the TiO₂ particle size decreases and as the initial concentration of phenol or toluene increases. The photodegradation rate of phenol by TiO₂ particles is higher than that of toluene at the same process conditions. The computational method was used to simulate the gas temperature, velocity and species mass fractions inside the diffusion flame reactor during synthesis of TiO₂ nanoparticles. The measured and simulated temperature results were compared on several positions above the burner and both of them show good agreements. The typical contours of TiCl₄, TiO₂ mass fractions and gas velocities in flame reactor were presented.     

Porous CeO<Subscript>2</Subscript> nanorod-catalyzed synthesis of poly-substituted imino-pyrrolidine-thiones

Porous CeO₂ nanorod has been used as efficient and recyclable heterogeneous catalyst for the synthesis of highly functionalized imino-pyrrolidine-thiones via the reaction of aromatic aldehyde (especially one bearing an electron-donating group), malononitrile, isocyanide and unactivated weakly acidic heterocyclic thiophenol. The high catalytic efficiency of porous CeO₂ nanorod in this reaction was discussed preliminarily based on their morphology and structure.     

Synthesis and antibacterial activity of Ag/CeO<Subscript>2</Subscript> hybrid architectures

Flower-like ceria (CeO₂) architectures consisting of well aligned nanosheets were first synthesized by a glycol solvothermal method. The size of CeO₂ architectures is about 5?μm in width and 10?μm in length, with the nanosheets thickness below 100?nm. Subsequently, the adsorbed Ag ions on the surface of CeO₂ were in situ reduced to form Ag nanoparticles (NPs), leading to the fabrication of Ag/CeO₂ hybrid architectures (HAs). The formed Ag NPs with sizes of 20–40?nm were uniformly loaded on the surface of the CeO₂ sheets. The antibacterial properties of Ag/CeO₂ HAs against Gram-negative E. coli and Gram-positive S. aureus were evaluated by minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and a filter paper inhibition zone method. The results demonstrated that Ag/CeO₂ HAs displayed excellent antibacterial activity toward S. aureus and E. coli, which were attributed to the synergistic antibacterial effect between Ag NPs and CeO₂ in HAs_. Here, CeO₂ nanoflowers as a new substrate could restrict Ag NPs aggregations and improve their antibacterial activities. Therefore, the resulted Ag/CeO₂ HAs would be considered as a promising antibacterial agent.

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ESR Study of the Formation of O<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> Radical Anions on Oxidized CeO<Subscript>2</Subscript> and CeO<Subscript>2</Subscript>/ZrO<Subscript>2</Subscript> Adsorbing a CO + O<Subscript>2</Subscript> Mixture

It is demonstrated by ESR measurements that O ₂ ⁻ (CO + O₂) radical anions result from CO + O₂ adsorption on the oxidized surface of CeO₂. These radical anions are stabilized in the coordination sphere of Ce⁴⁺ cations located in isolated and associated anionic vacancies. This reaction shows an activation behavior determined by CO adsorption. The variation of O ₂ ⁻ (CO + O₂) concentration with CO adsorption temperature suggests that surface carbonates and carboxylates participate in this reaction. In the (0.5– 10.0)%CeO₂/ZrO₂ system, O ₂ ⁻ forms on supported CeO₂ and is stabilized on Ce⁴⁺ and Zr⁴⁺ cations. The stability of O ₂ ⁻ -Ce⁴⁺ complexes is lower on supported CeO₂ than on unsupported CeO₂, indicating a strong interaction between the cerium cations and the support.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 423–429.Original Russian Text Copyright © 2005 by Il’ichev, Kuli-zade, Korchak.     

Effect of CeO<Subscript>2</Subscript> and Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> on the phase transformation and optical properties of photostable titanium dioxide

In the present work, the effect of individual additives calculated as molar fractions of Sb₂O₃ and CeO₂ (x _{Sb 2}O₃ range: 0.03–0.08 %, x _{CeO 2} range: 0.05–0.14 %), on the phase composition, phase transformation, and optical properties of photostable rutile titanium dioxide was studied using selective leaching method, ICP-AES technique, XRD method, spectrophotometric analysis and S _BET measurements. The starting material was hydrated titanium dioxide. It was observed that the addition of Sb₂O₃ to TiO₂ did not influence the anatase-rutile phase transformation, but increasing the CeO₂ addition caused a decrease in the rutilization degree. Thus, CeO₂ acted as an inhibitor of the TiO₂ phase transformation. Sb₂O₃ addition to TiO₂ presumably caused the formation of a co-phase of Sb with Ti. Cerium formed a separate phase, CeO₂, and reacted partly with titanium, probably creating co-phase, Ce_0.8Ti_0.2O₂. Comparing the colour of modified rutile titanium dioxide according to the type of the additive introduced, it was found that TiO₂ with CeO₂ had higher brightness but lower white tone values when compared with TiO₂ modified with Sb₂O₃. The relative lightening power and grey tone of the modified TiO₂ were higher in TiO₂ modified with Sb₂O₃. The values of the photocatalytic activity measured in all TiO₂ samples modified either with Sb₂O₃ or CeO₂ were very similar and varied around the value of 21.     

Synthesis of highly ordered mesoporous CeO<Subscript>2</Subscript> and low temperature CO oxidation over Pd/mesoporous CeO<Subscript>2</Subscript>

Highly ordered mesoporous cerium dioxide (meso-CeO₂) was successfully synthesized using a facile solvent-free infiltration method from a mesoporous silica template, KIT-6. The meso-CeO₂ material, thus obtained, exhibited well-defined mesostructure and high surface area (153 m² g⁻¹). The physicochemical properties of meso-CeO₂ material and Pd-supported on meso-CeO₂ (Pd/meso-CeO₂) were characterized by electron microscopy, X-ray diffraction, N₂ adsorption–desorption, and temperature-programmed experiments. The Pd/meso-CeO₂ catalyst exhibited excellent catalytic activity for CO oxidation compared with those of other Pd/CeO₂ catalysts which were prepared using nanocrystalline CeO₂ and bulk-CeO₂ as the supports. Moreover, a hydrogen pretreatment of the Pd/meso-CeO₂ catalyst resulted in a remarkable increase of catalytic activity (T ₁₀₀ = 52 °C).     

Hydrothermal synthesis,structure and photocatalytic property of nano-TiO<Subscript>2</Subscript>-MnO<Subscript>2</Subscript>

TiCl₄ and MnSO₄· H₂O as raw materials are hydrolyzed stiochiometrically, following the intermediate of oxide hydrating reacts at 150°C, 0.5 MPa in high-pressure reactor, after filtering, washing and drying, nanometric TiO₂-MnO₂ (Ti_1-X Mn _X O₂) is prepared. The effects of the reaction temperature and time on nanometric TiO₂-MnO₂ are also discussed. XRD shows that the product is TiO₂-MnO₂ with amorphous phase. After being sintered at above 780 °C, it transfers into Ti_1-X Mn _X O₂ with a rutile structure. TEM shows that TiO₂-MnO₂ is the spherical particle. And the average diameter of the particles is 20 nm. The optical absorbance was determined by UV-265 spectrophotometer after dispersing the sample in the mixture of water and glycerol with the ratio of 1 : 1 equably. It is found that the nano-material possesses the advantages of both nano-TiO₂ and nano-MnO₂, and it has strong absorption in the UV and visible region. Photodegradation of dyes in an aqueous solution is investigated using nanometricTiO₂-MnO₂ as a photocatalyst. The results show that after 60 min illumination, the decolorization rate of the acidic red B and acidic black 234 dye can be as high as 100%.