Effect of additives on the structure characteristics,thermal stability,reducibility and catalytic activity of CeO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript> solid solution for methane combustion

M _y O _x -modified CeO₂–ZrO₂ (M = Al, Ba, Cu, La, Nd, Pr, Si) solid solutions with the atomic ratio of Zr/Ce = 1 were prepared by the reverse microemulsion method, and the effect of different additives on the structure characteristics, thermal stability, reducibility, and catalytic activity of CeO₂–ZrO₂ solid solution for methane combustion were investigated. According to their different effects, M _y O _x can be classified into three groups. The first group includes SiO₂ and Al₂O₃ which do not vary the crystalline phase of CeO₂–ZrO₂ solid solution but distort the crystal lattice obviously. They are the most effective additives for improving the surface area, thermal stability, and reducibility of CeO₂–ZrO₂, and they can also promote the catalytic activity of Pd/CeO₂–ZrO₂ for methane combustion. The second group includes La₂O₃, Pr₂O₃, and Nd₂O₃, which can also keep the same crystalline phase, distort the crystal lattice, and improve the surface area and thermal stability of the solid solution, but their effects are much weaker and they decrease the reducibility of the solid solution. The third group includes BaO and CuO, whose effects on the property of CeO₂–ZrO₂ are much different. BaO and CuO, especially CuO, can decrease the thermal stability, and reduction extent of CeO₂–ZrO₂. CuO-modified CeO₂–ZrO₂ calcined at 550 °C shows the comparable high activity for the methane combustion, but after being calcined at 900 °C, CuO-modified CeO₂–ZrO₂ would separate into three phases as CeO₂, ZrO₂, and CuO, resulting in the much lower activity for the methane catalytic combustion.     

Morphology and crystal structure of CeO2-modified mesoporous ZrO2 powders prepared by sol–gel method

Mesoporous ZrO₂ powder modified by CeO₂ has been prepared by sol–gel method combined with novel heat-treatment without any templates. The morphology and crystal structure were characterized by Fourier transform infrared spectrophotometer (FT-IR), thermal gravimetry/differential thermal analysis (TG/DTA), X-ray diffraction (XRD), Raman spectroscope, N₂ absorption–desorption, scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). It was demonstrated that the morphologies and crystal structures were varied with the CeO₂ content and calcination temperatures. Some circular grooves with diameter about 300–500 nm and thickness of 150–200 nm were observed for 30 mol% CeO₂ modified samples. Tetragonal ZrO₂ and cubic (Ce,Zr)O₂ solid solution were in turn observed with CeO₂ content increasing. The oxygen storage capacity (OSC) of the mesoporous powders was determined by thermalgravimetry (TG) under cyclic thermal treatments.     

Oxygen release–absorption properties and structural stability of Ce0.8Fe0.2O2−x

Oxygen release–absorption properties and structural stability of Ce–Fe mixed oxides (Ce_0.8Fe_0.2O_2?x) with different calcination temperatures (600–1000 °C) were investigated and correlated to their oxygen storage capacity. Iron ions could be incorporated into the CeO₂ lattice to form a solid solution after calcination at low temperatures, but such solid solution was unstable under high-temperature thermal treatments. High-temperature (≥800 °C) calcination resulted in the appearance of exposed Fe₂O₃ phases on the surface of the solid solution, and this structural evolution finally affected the reduction behavior. The Fe³⁺ reduction from the Ce–Fe oxide solid solution was easier than the bulk Fe₂O₃ particles, while the small Fe₂O₃ particles in close contact with CeO₂ could enhance the reducibility of cerium oxides. The strong interaction between the exposed small Fe₂O₃ particles and the solid solution made the Ce–Fe mixed oxides possess good reduction stability and high oxygen storage capacity (OSC) even after repeated redox treatments. Such interactions were absent toward the physically mixed sample. An unusual enhancement on the reducibility of Ce–Fe mixed oxides was observed after a successive redox treatment. Large oxygen evolution appeared at around 600 °C for the recycled samples, and the OSC rose to 1.31 mmol-O₂/g after six redox cycles. The XRD, Raman, and TEM analyses revealed that the material structure of the mixed oxides was stabilized to have an inter-region between the Fe₂O₃ particles and the solid solution after the redox treatment. It was concluded that such microstructural evolutions of composite particle from solid solution under redox conditions brought beneficial property to the OSC of the Ce–Fe mixed oxides.     

Preparation of porous spherical ZrO2–SiO2 composite particles using templating and its solid acidity by H2SO4 treatment

Porous ZrO₂–SiO₂ composite sphere particles were prepared by impregnating precursor solutions into organic monolith particles, with subsequent calcination in air. The porous spheres possessed uniformly sized pores of around 10 nm. Addition of SiO₂–ZrO₂ decreased the ZrO₂ crystallinity and increased the specific surface area. The acid amount on the surface of the composite spheres was increased by treatment with H₂SO₄. The acid strength and its amount, including the Lewis/Br?nsted acid ratio, depended on the SiO₂/ZrO₂ ratio and the H₂SO₄ concentration. The powder treated under an optimum condition exhibited higher solid acidity than the reference solid acid catalyst. The prepared porous SO₄ ²⁻/ZrO₂–SiO₂ spheres showed higher saccharization activity than the reference solid acid catalyst did.     

Influence of metal particles on the reduction properties of ceria-based materials studied by TPR

Temperature programmed reduction (TPR) investigations on ceria-based catalytic materials are presented. Pure ceria shows two major reduction regions around 790 K and 1100 K due to surface capping oxygen ions and bulk oxygen ions. The extent of reduction in the low temperature region depends greatly on the surface area of the sample. The remnant reduction features that appear below 700 K are ascribed to O²⁻ ions located at various low coordination sites on the oxide crystallites. The substitution of ZrO₂ (40–60%) in CeO₂ lattice decreases the overall reduction temperature of high surface area samples. Metal particles supported on low surface area CeO₂-ZrO₂ solid solutions induce bulk reduction of the support at low temperatures to a great extent. These observations provide a clue that the CeO₂-based oxide supports, irrespective of their surface area, can perform as good oxygen exchangers in the presence of metal particles, contributing indirectly to the overall catalytic activity.     

Preparation and visible-light photocatalytic activity of Au-supported porous CeO2 spherical particles using templating

Porous spherical CeO₂ particles were prepared by impregnation of a cerium precursor solution into organic monolith sphere particles, with subsequent firing at 500 °C in air. The single-phase CeO₂ powder had specific surface area of greater than 140 m²/g. Photodeposition with UV illumination loaded Au onto the CeO₂ particle surface, which changed from yellowish to purple because of localized surface plasmon resonance (LSPR). The Au-loading increased photocatalytic decomposition activity of the CeO₂ powder for gaseous 2-propanol (IPA) under visible light. Thermal desorption of IPA, which was adsorbed to all porous spheres, provided flux to the photocatalytic reaction field of the sphere outer surface.     

Effects of atomic oxygen exposure on the tribological performance of ZrO2-reinforced polyimide nanocomposites for low earth orbit space applications

The effects of atomic oxygen exposure on pure polyimide and nano-ZrO₂ reinforced polyimide composites were investigated in a ground-based simulation facility. The experimental results indicated that the surface structure of both pure polyimide and ZrO₂/polyimide composites were destroyed by atomic oxygen attack, but the addition of nano-ZrO₂ particles in polyimide could obviously decrease the mass loss, which showed that ZrO₂ could enhance the atomic oxygen resistance. The results of ZrO₂/polyimide composites before and after atomic oxygen exposure showed that atomic oxygen irradiation aggravated the friction and wear of the ZrO₂/polyimide composites. The wear mechanism was mainly abrasive particles wear arising from the ZrO₂-rich layer on the surface of composites. The ZrO₂/polyimide composites with 1 wt% nano-ZrO₂ owns the lowest varying rate of the friction coefficient and wear rate before and after atomic oxygen exposure, which showed stable friction and wear properties and was expected to become a kind of potential tribological materials for practical spacecraft designation.     

Novel synthesis of nanocrystalline CeO2 by mechanochemical and water-in-oil microemulsion methods

Nanocrystalline cerium dioxide (CeO₂) had been synthesized by two different methods which were mechanochemical and water-in-oil microemulsion. Effects of synthesis conditions on properties of nanocrystalline cerium dioxide were investigated. X-ray diffraction (XRD) was used to characterize the phase and crystallite size of synthesized cerium dioxide nanoparticles. XRD results showed that face centered cubic CeO₂ nanoparticles with crystallite size in nanometer scale were formed. The crystallinity increased with increasing annealing temperature. The average specific surface area of the particles was probed using gas adsorption-desorption measurements. The average particles size was calculated from the specific surface area and was determined to be 5.2 nm for microemulsion samples and 6.9 nm for mechanochemical samples. These results showed that properties of synthesized cerium dioxide could be tailored by adjusting the synthesis conditions.     

Single step synthesis of nanosized CeO2–MxOy mixed oxides (MxOy?=?SiO2, TiO2, ZrO2, and Al2O3) by microwave induced solution combustion synthesis: characterization and CO oxidation

Various CeO₂ –M _x O _y (M _x O _y  = SiO₂, TiO₂, ZrO₂, and Al₂O₃) mixed oxides were prepared by microwave induced solution combustion method and analyzed by different complimentary techniques, namely, X-ray diffraction (XRD), Raman spectroscopic (RS), UV–Vis diffuse reflectance spectroscopy (UV-DRS), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG-DTA), and BET surface area. XRD analyses revealed that CeO₂ –SiO₂ and CeO₂ –TiO₂ mixed oxides are in slightly amorphous form and exhibit only broad diffraction lines due to cubic fluorite structure of ceria. XRD lines due to the formation of cubic Ce_0.5Zr_0.5O₂ were observed in the case of CeO₂ –ZrO₂ sample. RS results suggested defective structure of the mixed oxides resulting in the formation of oxygen vacancies. The UV-DRS measurements provided valid information about Ce⁴⁺ ← O²⁻ and Ce³⁺ ← O²⁻ charge transfer transitions. XPS studies revealed the presence of cerium in both Ce³⁺ and Ce⁴⁺ oxidation states. The ceria–zirconia combination exhibited better oxygen storage capacity (OSC) and CO oxidation activity when compared to other samples. The significance of present synthesis method lays mostly on its simplicity, flexibility, and the easy control of different experimental factors.     

Catalytic combustion of methane over Pt and PdO-supported CeO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript>–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

Catalytic combustion of methane was investigated on Pt and PdO-supported CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ catalysts prepared by a wet impregnation method in the presence of polyvinylpyrrolidone. The catalysts were characterized by X-ray fluorescence analysis, X-ray powder diffraction, X-ray photoelectron spectra, transmission electron microscopy, and BET specific surface area measurements. The Pt/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ and PdO/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ catalysts were selective for the total oxidation of methane into carbon dioxide and steam, and no by-products such as HCHO, CO, and H₂ were obtained. The catalytic activities of the PdO/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ catalysts were relatively higher than those of the Pt-supported catalysts, due to the facile re-oxidation of metallic Pd into PdO based on lattice oxygen supplied from the CeO₂–ZrO₂–Bi₂O₃ bulk. A decrease in the calcination temperature during the preparation process was found to be effective in enhancing the specific surface area of the catalysts, whereby particle agglomeration was inhibited. Optimization of the PdO amount and calcination temperature enabled complete oxidation of methane at temperatures as low as 320 °C on the 11.6 wt% PdO/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ catalyst prepared at 400 °C.     

Solid solutions of metastable tetragonal ZrO2 and Ce3ZrO8 in the system ZrO2-CeO2

In the system ZrO₂-CeO₂, metastable t-ZrO₂ solid solutions containing up to 30 mol% CeO₂ crystallize at temperatures of 385–430 °C from amorphous materials prepared by the hydrazine method. Crystalline Ce₃ZrO₈ solid solutions are formed in as-prepared powders between 30–75 mol % CeO₂. The variation of the lattice parameters of both solid solutions is determined as a function of CeO₂ content. The value of the lattice parameter of pure Ce₃ZrO₈ (cubic) is a = 0.5342 nm. Detailed characterization of the Ce₃ZrO₈ powder has been performed. Crystallite size and particle size are strongly dependent on the heating temperature. Specific surface areas do not drop below 40 m²g^–1 until the heating temperature is above 1000°C.     

Synthesis of micro-sized hierarchical TiO2 particles of nano-scale effectiveness and their photocatalytic activities at various surface hydroxyl concentrations

The 3-dimensional hierarchical TiO₂ particles of micro-sized diameter were synthesized through modified sol–gel process with polyethylene glycol (PEG) as a structure-controlling agent. The anatase crystal structure was obtained after calcination at 450 °C. The size and specific surface area of particles were 1.0–1.8 μm and 96.85 m² g⁻¹, respectively. The specific surface area of the TiO₂ particles corresponded to that of the spherical nanoparticles with average size of 15.9 nm. Although the size of synthesized TiO₂ was micro-scale, they had the specific surface area similar to that of nano-scale particles due to the effect of PEG on the formation of particles. Subsequently, the surface modification with various concentration of ammonia solution was carried out for the preparation of hydroxyl-rich TiO₂ particles at surfaces. As the concentration of ammonia solution was increased, the amount of chemically adsorbed hydroxyl groups on the TiO₂ surface was increased. As an application of prepared TiO₂ for water treatment, their catalytic performances for the degradation of methylene blue (MB) were examined by using UV–Vis spectrophotometer with the assistance of UV lamp. After hydroxylation treatment, the micro-sized TiO₂ particles showed the higher performance of MB degradation than that of nano-sized P25 particles because of their large specific surface area and hydroxyl-rich surface.     

Aqueous co-precipitation of Pd-doped cerium oxide nanoparticles: chemistry, structure, and particle growth

Nanoparticles of palladium-doped cerium oxide (Pd–CeO₂) have been prepared by aqueous co-precipitation resulting in a single phase cubic structure after calcination according to X-ray diffraction (XRD). Inhomogeneous strain, calculated using the Williamson–Hall method, was found to increase with palladium content, and the lattice contracts slightly, relative to nano-cerium oxide, as palladium content is increased. Moreover, high resolution transmission electron microscopy reveals some instances of defective microstructure. These factors combined imply that palladium is in solid solution with CeO₂ in these nanoparticles, but palladium (II) oxide (PdO) peaks in the Raman spectra indicate that solid solution formation is partial and that highly dispersed PdO is present as well as the solid solution. Nevertheless, the addition of palladium to the CeO₂ lattice inhibits the growth of the 6% Pd–CeO₂ particles compared to pure CeO₂ between 600 and 850 °C. Activation energies for grain growth of 54 ± 7 and 79 ± 8 kJ/mol were determined for 6% Pd–CeO₂ and pure CeO₂, respectively, along with pre-exponential Arrhenius factors of 10 for the doped sample and 600 for pure cerium oxide.     

Thermal stability and photoluminescence of Zr1−xCexO2 (0 ≤ x ≤ 1) nanoparticles synthesized in a non-aqueous process

Complete range Zr_1−xCe_xO₂ (0 ≤ x ≤ 1) nanoparticles are synthesized by the thermal decomposition of metal-organic precursors (zirconium acetylacetonate and cerium acetylacetonate) in oleylamine. XRD and HRTEM indicate that all of the as-prepared nanoparticles are single-crystal, and the crystallinity becomes better with increasing Ce content. The Zr_1−xCe_xO₂ nanoparticles with Ce content larger than 1/8 crystallize in cubic fluorite phase. XRD measurements on the as-prepared and calcinated ZrO₂ samples reveal that the tetragonal ZrO₂ nanoparticles are stable below 600 °C, and the lattice parameters of ZrO₂ nanoparticles decrease with decreasing particle size. The thermal stability of the cubic phase increases with increasing Ce content. The UV–vis absorption spectra reveal that the band gap energy increases with increasing cerium content. Room-temperature photoluminescence (PL) spectra of pure ZrO₂ nanoparticles show strong emission peaks centered at about 441 nm at room temperature, which is attributed to the ionized oxygen vacancies in the nanoparticles. On the other hand, room-temperature PL spectra of the as-prepared CeO₂ nanoparticles shows two peaks at 417 and 436 nm, which might arise from the transition from the cerium 4f and to the oxygen 2p band (valence band) in CeO₂ and the presence of oxygen vacancies, respectively.     

The influence of preparation conditions on ZrO2 nanoparticles with different PEG–PPG–PEG surfactants by statistical experimental design

In this study mesoporous Zirconia powder with high surface area was prepared by using PEG–PPG–PEG new block copolymer as the non-ionic surfactant. The preparation conditions were optimized by Taguchi method of experimental design and Minitab Software to synthesize high surface area tetragonal-ZrO₂ nanoparticles. The BET surface area of powders was 114–175 m²/gr and the particles size calculated by Deby–Sherrer equation was 5–9 nm. pH = 11, aging time 38 h, Zr molarity 0.03, Surfactant/Zr mole ratio 0.04 and molecular weight 8400 were the best conditions to manufacture ZrO₂ with higher surface area. The sample prepared under optimized conditions was compared to that synthesized by PEG surfactant. XRD patterns of two ZrO₂ samples, hysteresis loop, pore size distribution, BET surface area and SEM results are similar.     

Catalytic performance of ceria fibers with phosphatase-like activity and their application as protein carriers

Ceria (CeO₂) synthesized by cerium nitrate hexahydrate in alkaline solution under hydrothermal treatment produces a fiber structure that allows high O-P bond cleavage activity. Brunauer–Emmett–Teller (BET) analysis revealed that fiber-morphological CeO₂ with high surface area (73.9 m²/g) and pore volume (0.42 cm³/g) showed better hydrolytic activity than nanopolyhedral and cubic morphologies. The CeO₂ fiber displayed hydrolytic activity in a tris(hydroxymethyl)aminomethane (Tris) buffer; however, no reaction occurred in a phosphate buffer. From analysis by based on scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX), the phosphate group in the buffer was seen to be immediately adsorbed on the surface of CeO₂ particles; therefore, the CeO₂ catalysts could not attack the phosphoric esters as a substrate. In addition, the CeO₂ fiber showed hydrolytic activity for deoxyribonucleic acid (DNA). Moreover, enzymes loaded on the CeO₂ fiber particles retained activity levels equivalent to free-solution enzymes. It is thought that the findings of the present study regarding the properties of CeO₂ fiber will have a significant impact in the fields of not only antibacterial and antimicrobial reagents, but also biosensors and biocatalysts.     

Room temperature co-precipitation of nanocrystalline CeO2 and Ce0.8Gd0.2O1.9−δ powder

This paper describes a simple method to co-precipitate CeO₂ and Ce_0.8Gd_0.2O_1.9−δ with ammonium hydroxide from solvents such as: water, ethylene glycol, ethyl alcohol and isopropyl alcohol. Characterization by Raman spectroscopy and XRD evidenced the formation of a solid solution of gadolinium-doped ceria at room temperature. Nanometric particles with crystallite size of 3.1 nm were obtained during synthesis using ethyl alcohol as solvent. This is a promising result compared with those mentioned in the literature, in which the smallest crystallite size reported was 6.5 nm.     

Spin coating of hybrid suspensions using infrared-irradiation to increase layer thickness

This paper describes an increase in thickness of spin coated layers by using IR-irradiation. Hybrid suspensions with 5 wt.% solid content (80% BaTiO₃, 20% ZrO₂) are spin coated on platinized silicon wafers. The IR-irradiation increases the evaporation rate and accelerates the aging of the solution. This leads to an increase in viscosity and a faster increase of the solid content, which results in an earlier percolation of the hybrid suspension with more particles remaining on the substrate. Using IR-irradiation, an increase in layer thickness from 71 nm to 242 nm could be achieved in one coating step.     

Preparation and characterization of α-Fe2O3–CeO2 composite

In our previous study we attempted to see the effect of cerium doping (Ce/Fe ratio 0.015 to 0.074) on goethite matrix and conversion of doped goethite to hematite. In the present communication, nano-structured α-Fe₂O₃–CeO₂ composite with Fe/Ce weight ratio as 1.1 has been synthesized by calcination of goethite-cerium hydroxide precursor prepared by co-precipitation method. It was observed that co-precipitation of cerium along with iron in hydroxide medium resulted in hindering the formation of crystalline order as the precursor formed showed poorly crystallized goethite and almost no crystallinity in Ce(OH)₄. Calcination of the precursor at 400 °C showed the formation of hematite together with a broad peak corresponding to cerium oxide whereas at 800 °C, two distinct phases of α-Fe₂O₃ and CeO₂ were observed. The Mössbauer spectra showed the presence of a paramagnetic component both for the precursor as well as for the sample calcined at 400 °C but on raising the calcination temperature to 800 °C, the paramagnetic component disappeared and the spectrum corresponding to pure α-Fe₂O₃ phase was observed. The microstructure of the product obtained by calcining at 800 °C showed rod like structure (30 to 50 nm width and 300 to 500 nm length) of α-Fe₂O₃ having equi-dimensional CeO₂ particles on and around the surface. Besides the rods, equi-dimensional particles and agglomerates corresponding to CeO₂ were also observed. The results show that co-precipitation followed by calcinations gives nanorods hematite with CeO₂ particles bonded to its surface.