纳米氧化铈的制备及其在环境保护中的应用

氧化铈具有立方萤石结构,储放氧能力与价电子构型,被广泛应用于催化剂等领域。对不同纳米结构的氧化铈制备研究现状做了较系统的概述,并分析了制备不同形貌氧化铈的影响因素。着重介绍了纳米氧化铈在汽车尾气净化和工业废水处理中应用进展,并展望了今后纳米氧化铈材料的研究方向。     

Electrical and thermal characterization of Sm doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce_0.9Sm_0.1O_1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10⁻² S cm⁻¹ at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10⁻⁶ m² s⁻¹ and 1.2 W m⁻¹ K⁻¹, respectively.     

Ceria nanocube with reactive {100} exposure planes: Simple and controllable synthesis under moderate conditions

{100} planes of CeO₂ are unstable polar surfaces and important to enhance the oxygen storage capacity. However, a convenient method is still needed for the synthesis of ceria nanocubes which expose six {100} facets. Here, a new and facile synthesis strategy has been developed to synthesize single-crystalline nanocubes of CeO₂. According to transmission electron microscopy, these nanocubes are enclosed by six {100} facets. It is proved that in the synthesis process, the acetate radical ions would adsorb on the positive charged Ce(OH)₃ formed from the reaction of Ce³⁺ and ammonia, then partially inhibit the redox between Ce(OH)₃ and NO₃⁻ and protect the {100} facets during crystal growth. Meanwhile, the Ce⁴⁺ ions would act as crystal seeds to facilitate the formation of single crystals. Both of the addition of Ce⁴⁺ ions and CH₃COO⁻ ions are necessary for the synthesis of monocrystal ceria nanocubes. Throughout the synthesis process, it is moderate reaction conditions that the pressure is atmospheric pressure and temperature is no higher than 80 °C.     

Preparation and characterization of vitrified CeO2 coated cBN composites

The performances of vitrified cBN composites are deeply affected by the wettability of vitrified bonds on cBN particles. CeO₂ coated cBN particles were successfully prepared for the further improvement of the covering and wetting of cBN by vitrified bonds. The microstructure and properties of vitrified cBN composites were characterized by scanning electron microscope (SEM), hot stage microscope (HSM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and flexural strength. Results showed that the prepared CeO₂ coating on the surface of cBN was uniform and dense. Besides, the improved wettability of vitrified bonds on CeO₂ coated cBN particles accompanied with the formation of Ce–O–Al and N–Si confirmed by XPS were supposed to conduce to enhancing the holding power of the vitrified bonds to cBN particles, which resulted in increasing the flexural strength of vitrified cBN composites by 9.16%. Thus, coating cBN with CeO₂ was a potential and effective method to obtain vitrified cBN composites with higher flexural strength.     

Combustion synthesis and characterization of Cu-Sm co-doped CeO2 electrolytes

Nano-sized CSO (Ce_0.80Sm_0.20O_2−δ) and CSCO (Ce_0.79Sm_0.20Cu_0.01O_2−δ) were synthesized by the PVA assisted combustion method, and then characterized by the structure of PVA-cation complexes and nano-powders, as well as mechanical and electrical performance after sintering. The results indicate that the PVA-cation complexes (PVA-(Ce³⁺,Sm³⁺) and PVA-(Ce³⁺,Sm³⁺,Cu²⁺)) were formed by coordinating metal cations to hydroxyl groups, as well as the COO⁻¹ group derived from the oxidation of PVA with NO₃⁻¹. Low temperatures (around 200 °C) caused intense combustion reactions, resulting in the direct crystallization of cubic fluorite nano-CSO (10-20 nm) and nano-CSCO (10-15 nm) crystals with homogeneous element distribution. This slight compositional modification of CSO by co-doping with 1 mol% CuO resulted in a significantly lowered densification temperature, as well as enhanced mechanical and electrical property. The strength improvement can be ascribed to the dense and fine-grained microstructure without normal grain coarsening, resulting in a transgranular-dominant fracture mode during strength testing.     

Polishing behaviors of ceria abrasives on silicon dioxide and silicon nitride CMP

The effects of ceria (CeO₂) abrasives in chemical mechanical polishing (CMP) slurries were investigated on silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) polishing process. The ceria abrasives were prepared by the flux method, using potassium hydroxide (KOH) as the grain growth accelerator. The primary particle size of the ceria abrasives was controlled in the range of ~ 84-417 nm by changing the concentration of potassium hydroxide and the calcination temperature without mechanical milling process. The removal rate of silicon dioxide film strongly depended upon abrasive size up to an optimum abrasive size (295 nm) after CMP process. However, the surface uniformity deteriorated as abrasive size increases. The observed polishing results confirmed that there exists an optimum abrasive size (295 nm) for maximum removal selectivity between oxide and nitride films. In this study, polishing behaviors of the ceria abrasives were discussed in terms of morphological characteristics.     

Aging effects of nanoscale ceria in ceria–platinum composite electrodes for direct alcohol electro-oxidation

Nanocrystalline Pt/CeO₂ composite electrodes were fabricated to study the electrochemical oxidation of methanol and ethanol. The performance of the electrodes was tested as the ceria solutions aged over time. It was observed that the performance oscillated with time, suggesting that the catalytic behavior towards alcohol oxidation was greatly dependent on the aging of the particles. These results point to a great dependence of the catalytic effect on the redox state of the ceria particles.