Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis

Nanocrystalline TiO₂, CeO₂ and CeO₂-doped TiO₂ have been successfully prepared by one-step flame spray pyrolysis (FSP). Resulting powders were characterized with X-ray diffraction (XRD), N₂-physisorption, Transmission Electron Microscopy (TEM) and UV-Vis spectrophotometry. The TiO₂ and CeO₂-doped TiO₂ nanopowders were composed of single-crystalline spherical particles with as-prepared primary particle size of 10-13 nm for Ce doping concentrations of 5-50 at%, while square-shape particles with average size around 9 nm were only observed from flame-made CeO₂. The adsorption edge of resulting powder was shifted from 388 to 467 nm as the Ce content increased from 0 to 30 at% and there was an optimal Ce content in association with the maximum absorbance. This effect is due to the insertion of Ce^3+/4+ in the TiO₂ matrix, which generated an n-type impurity band.     

Influence of flame conditions on the dispersion of Pd on the flame spray-derived Pd/TiO2 nanoparticles

The Pd/TiO₂ nanoparticles containing 5 wt.% Pd were synthesized by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) results, the average particle sizes of Pd/TiO₂ were increased from 9.7 to 24.6 nm with increasing the precursor concentration and the feed flow rate as well as reduction of the O₂ dispersing gas during FSP synthesis. Although the BET surface area and %anatase phase content were decreased with increasing Pd/TiO₂ particle size, %Pd dispersion as determined from the amounts of CO chemisorption were higher on the larger size FSP-made Pd/TiO₂ nanoparticles. It is suggested that the shorter residence time in flame and/or the lower combustion energy (enthalpy density) resulted in more coverage of Pd surface by the formation of Ti-O groups, rendering lower CO chemisorption ability of the smaller size Pd/TiO₂.     

Characteristics and catalytic properties of Pt–Sn/Al2O3 nanoparticles synthesized by one-step flame spray pyrolysis in the dehydrogenation of propane

The Pt–Sn/Al₂O₃ catalysts with 0.3 wt% Pt and 0.5–1.5 wt% Sn loading were prepared by one-step flame spray pyrolysis (FSP). Unlike the catalysts prepared by conventional impregnation method, the FSP-derived catalysts were composed of single-crystalline γ-alumina particles with the as-prepared primary particle size of 10–18 nm and contained only large pores. The FSP catalysts exhibited superior catalytic activity and better stability than the ones made by impregnation in the dehydrogenation of propane, while they did not alter the selectivity to propylene (in all cases, propylene selectivity ≥96%). The presence of large pores in the flame-made catalysts not only facilitated diffusion of the reactants and products but could also lessen the amount of carbon deposited during reactions. As revealed by CO chemisorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), the metal particles appeared to be partially covered by the alumina matrix (Al–O) due to the simultaneous formation of particles during FSP synthesis. Such phenomena, however, were shown to result in the formation of active Pt–Sn ensembles for propane dehydrogenation as shown by higher turnover frequencies (TOFs).     

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.     

TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

Reduction of NO by CO on Cu/ZrO2/Al2O3 catalysts: Characterization and catalytic activities

ZrO₂, γ-Al₂O₃ and ZrO₂/γ-Al₂O₃-supported copper catalysts have been prepared, each with three different copper loads (1, 2 and 5 wt%), by the impregnation method. The catalysts were characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) with H₂, Raman spectroscopy and electronic paramagnetic resonance (EPR). The reduction of NO by CO was studied in a fixed-bed reactor packed with these catalysts and fed with a mixture of 1% CO and 1% NO in helium. The catalyst with 5 wt% copper supported on the ZrO₂/γ-Al₂O₃ matrix achieved 80% reduction of NO. Approximately the same rate of conversion was obtained on the catalyst with 2 wt% copper on ZrO₂. Characterization of these catalysts indicated that the active copper species for the reduction of NO are those in direct contact with the oxygen vacancies found in ZrO₂.     

Onset coarsening-coalescence kinetics of ZrO2 and less refractory TiO2 nanoparticles: Effect of homologous temperature and phase transformation

Specific surface area change of ZrO₂ (predominant tetragonal - (t) symmetry, 30-50 nm) and less refractory TiO₂ anatase nanoparticles (20-50 nm) upon isothermal firing at 700-1000 °C in air was determined by N₂ adsorption-desorption hysteresis isotherm. The nanoparticles underwent onset coarsening-coalescence within minutes without appreciable phase transformation for TiO₂, but with extensive transformation into monoclinic (m-) symmetry for ZrO₂. The apparent activation energy of such a process being not much higher for ZrO₂ (77 ± 23 kJ/mol) than TiO₂ (56 ± 3 kJ/mol) nanoparticles can be attributed to transformation plasticity. The minimum temperature for coarsening/coalescence of the present ZrO₂ and TiO₂ nanoparticles was estimated as 710 and 641 °C, respectively.     

Dopant-free, polymorphic design of TiO2 nanocrystals by flame aerosol synthesis

A dopant-free aerosol synthesis of highly crystalline TiO₂ nanoparticles (20–35 nm) with tunable polymorphic content is demonstrated by rapid flame spray pyrolysis. By controlling precisely the total ambient oxygen partial pressure of the combustion in a quartz tube enclosure, anatase content as high as 96 wt% (4 wt% rutile) was obtained at high oxic flame conditions, while rutile content as high as 94 wt% (6 wt% anatase) was obtained under anoxic flames. The polymorphic variability lies within a narrow range of combustion equivalence ratios, that is, 1.0<Φ<1.5. Unlike any other flame aerosol syntheses, the anatase and rutile crystallite sizes were similar within each sample. Under highly oxic flame conditions (Φ<1.0), twinnings between anatase {0 1 1} planes could be observed, inferring oriented attachment taking place. Such mechanism could not, however, be seen under anoxic flame (Φ>1.0) possibly due to physical hindrance by surface carbonaceous content (typically <2 wt%). The carbon content can be easily removed by short calcination without significantly affecting the surface areas and crystallite properties of the original TiO₂ nanocrystals, preserving hence its pristine state.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Laser stereolithography of ZrO2 toughened Al2O3

Ceramic laser stereolithography is a manufacturing process suitable candidate for the production of complex shape technical ceramics. The green ceramic is produced layer by layer through laser polymerisation of UV curable ceramic suspensions. A number of critical issues deserve attention: high solid loading and low viscosity of the suspensions, high UV reactivity, prevention of interlayer delamination in the green and in the sintered body, good mechanical performance. In this work, ZrO₂-reinforced Al₂O₃ components have been obtained from an acrylic modified zircon loaded with alumina powders. The zircon compound is effective as organic photoactivated resin and allows the dispersion of a high volume fraction of Al₂O₃ powder (up to 50 vol.%) while keeping viscosity at reasonable low values. The zircon compound also represents a liquid ceramic precursor that converts to oxide after burning out of the binder. Thank to the good dispersion of the alumina powder in the zircon acrylate, a uniform dispersion of ZrO₂ submicron particles is obtained after pyrolysis. These are located at the grain boundaries between alumina grains. Formation of both monoclinic and tetragonal ZrO₂ occurs as evidenced by XRD. No delamination occurs in bending tests as evidenced by SEM fractography, satisfactory modulus and strength values were concurrently found.     

Preparation of ZrO2/Gd2O3 composite ceramic materials by coprecipitation method

High burnup is a goal for further development of advanced nuclear power in the future. However, along with the increase of burnup, it becomes more diffidult to control reactor reactivity, which affects the operation safety of the nuclear reactor. Al₂O₃/B₄C burnable poison materials widely used in pressurized water reactor currently will not meet the requirements of burnable poison materials in high burnup nuclear power. Because of the better performance of ZrO₂/Gd₂O₃ burnable poison materials than that of Al₂O₃/B₄C, this paper studies the preparation of ZrO₂/Gd₂O₃ composite ceramic materials by the coprecipitation method. The experimental results show that at the sintering temperature of 1500–1650 °C, ZrO₂/Gd₂O₃ composite ceramic grains are small, compact and uniform with the generation of homogeneous solid solution. At 1600 °C, ZrO₂–10%Gd₂O₃ has the highest density and mechanical strength.     

Preparation and characterization of ZrO2/PES hybrid ultrafiltration membrane with uniform ZrO2 nanoparticles

In current or next-generation reuse systems, the development of new classes of antifouling membranes is needed before viable membrane-based approaches for wastewater reclamation can be achieved. In this study, ZrO₂/PES hybrid ultrafiltration membranes with uniform ZrO₂ nanoparticles were prepared by combining an ion-exchange process with a traditional immersion precipitation process. Hydrous ZrO₂ sol was synthesized by addition of anion-exchange resin in N,N-dimethylformamide solvent containing zirconyl chloride. Homogeneous ZrO₂/PES casting solution was then obtained by dissolving PES polymer in the ZrO₂ sol. The existence and dispersion states of ZrO₂ nanoparticles in the resultant membrane matrix were examined by X-ray photoelectron spectrometer, thermogravimetric analysis, scanning electron microscope and transmission electron microscope. The results indicate that the ZrO₂ nanoparticles were well dispersed throughout the PES matrix and the diameter of the formed nanoparticles was about 5–10 nm. The hydrophilicity of the membranes was determined by measuring the contact angles. The antifouling property was determined by antifouling experiments and atomic force microscopy. The results confirm that the existence of ZrO₂ nanoparticles improves the hydrophilicity and reduces protein adsorption of membranes.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts prepared via a route of solid-state reaction

Cu/ZnO/ZrO₂ catalysts were prepared by a route of solid-state reaction and tested for the synthesis of methanol from CO₂ hydrogenation. The effects of calcination temperature on the physicochemical properties of as-prepared catalysts were investigated by N₂ adsorption, XRD, TEM, N₂O titration and H₂-TPR techniques. The results show that the dispersion of copper species decreases with the increase in calcination temperature. Meanwhile, the phase transformation of zirconia from tetragonal to monoclinic was observed. The highest activity was achieved over the catalyst calcined at 400 °C. This method is a promising alternative for the preparation of highly efficient Cu/ZnO/ZrO₂ catalysts.     

烧结温度对漂珠/ZrO2多孔轻质材料性能的影响

以漂珠为成孔材料,以ZrO2为结合剂,采用凝胶注模的方法制备出多孔轻质材料。通过分析材料的微观形貌、物相组成、显气孔率、体积密度、抗弯强度等性能参数,研究了烧结温度1200℃~1500℃对材料性能的影响。结果得出,烧结温度达到1400℃时,漂珠外壳与氧化锆的界面结合不再明显,ZrO2颗粒由点接触扩展到面接触;烧结温度1400℃的试样中锆英石的峰最强,说明漂珠和氧化锆已烧结熔融;1400℃下制备试样的显气孔率最大(68%),体积收缩率(20.8%)最低,而抗弯强度可达7.5MPa;烧结温度控制在1400℃,可使材料处于烧结中期,保证材料的各项性能满足其应用需求。     

Enhancement of oxygen storage capacity by reductive treatment of Al2O3 and CeO2–ZrO2 solid solution nanocomposite

Oxygen storage capacity (OSC) of CeO₂–ZrO₂ solid solution, Ce_xZr_(1−x)O₄, is one of the most contributing factors to control the performance of an automotive catalyst. To improve the OSC, heat treatments were employed on a nanoscaled composite of Al₂O₃ and CeZrO₄ (ACZ). Reductive treatments from 700 to 1000 °C significantly improved the complete oxygen storage capacity (OSC-c) of ACZ. In particular, the OSC-c measured at 300 °C reached the theoretical maximum with a sufficient specific surface area (SSA) (35 m²/g) after reductive treatment at 1000 °C. The introduced Al₂O₃ facilitated the regular rearrangement of Ce and Zr ions in CeZrO₄ as well as helped in maintaining the sufficient SSA. Reductive treatments also enhanced the oxygen release rate (OSC-r); however, the OSC-r variation against the evaluation temperature and the reduction temperature differed from that of OSC-c. OSC-r measured below 200 °C reached its maximum against the reduction temperature at 800 °C, while those evaluated at 300 °C increased with the reduction temperature in the same manner as OSC-c.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.     

Amorphous-to-crystalline transition of silicon-incorporated anodic ZrO2 and improved dielectric properties

Sputter-deposited zirconium and Zr-16 at.% Si alloy have been anodized to various voltages at several formation voltages in 0.1 mol dm⁻³ ammonium pentaborate electrolyte at 298 K for 900 s. The resultant anodic films have been characterized using X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy, and electrochemical impedance spectroscopy. The anodic oxide films formed on Zr-16 at.% Si are amorphous up to 30 V, but the outer part of the anodic oxide films crystallizes at higher formation voltages. This is in contrast to the case of sputter-deposited zirconium, on which the crystalline anodic oxide films, composed mainly of monoclinic ZrO₂, are developed even at low formation voltages. The outer crystalline layer on the Zr-16 at.% Si consists of a high-temperature stable tetragonal phase of ZrO₂. Due to immobile nature of silicon species, silicon-free outermost layer is formed by simultaneous migrations of Zr⁴⁺ ions outwards and O²⁻ ions inwards. An intermediate crystalline oxide layer, in which silicon content is lower in comparison with that in the innermost layer, is developed at the boundary of the crystalline layer and amorphous layer. Capacitances of the anodic zirconium oxide are highly enhanced by incorporation of silicon due to reduced film thickness, even though the permittivity of anodic oxide decreases with silicon incorporation.     

Reduction and oxidation properties of Fe2O3/ZrO2 oxygen carrier for hydrogen production

Fe₂O₃ is a promising oxygen carrier for hydrogen production in the chemical-looping process. A set of kinetic studies on reduction with CH₄, CO and H₂ respectively, oxidation with water and oxygen containing Ar for chemical-looping hydrogen production was conducted. Fe₂O₃ (20 wt.%)/ZrO₂ was prepared by a co-precipitation method. The main variables in the TGA (thermogravimetric analyzer) experiment were temperatures and gas concentrations. The reaction kinetics parameters were estimated based on the experimental data. In the reduction by CH₄, CO and H₂, the reaction rate changed near FeO. Changes in the reaction rate due to phase transformation were observed at low temperature and low gas concentration during the reduction by CH₄, but the phenomenon was not remarkable for the reduction by CO and H₂. The reduction rate achieved using CO and H₂ was relatively faster than achieved using CH₄. The Hancock and Sharp method of comparing the kinetics of isothermal solid-state reactions was applied. A phase boundary controlled model (contacting sphere) was applied to the reduction of Fe₂O₃ to FeO by CH₄, and a different phase boundary controlled model (contacting infinite slab) was fit well to the reduction of FeO to Fe by CH₄. The reduction of Fe₂O₃ to Fe by CO and H₂ can be described by the former phase boundary controlled model (contacting sphere). This phase boundary controlled model (contacting sphere) also fit well for the oxidation of Fe to Fe₃O₄ by water and FeO to Fe₂O₃ by oxygen containing Ar. These kinetics data could be used to design chemical-looping hydrogen production systems.     

Carbon dioxide reforming of ethanol over Ni/Y2O3–ZrO2 catalysts

Supported nickel catalysts of composition Ni/Y₂O₃–ZrO₂ were synthesized in one step by the polymerization method and compared with a nickel catalyst prepared by wet impregnation. Stronger interactions were observed in the formed catalysts between NiO species and the oxygen vacancies of the Y₂O₃–ZrO₂ in the catalysts made by polymerization, and these were attributed to less agglomeration of the NiO during the synthesis of the catalysts in one step. The dry reforming of ethanol was catalyzed with a maximum CO₂ conversion of 61% on the 5NiYZ catalyst at 800 °C, representing a better response than for the catalyst of the same composition prepared by wet impregnation.