The homogeneity range and the microwave dielectric properties of the BaZn2Ti4O11 ceramics

Ceramics with a composition close to BaZn₂Ti₄O₁₁ were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn₂Ti₄O₁₁ was formed in the homogeneity range corresponding to the formula BaZn_2 − xTi₄O_11 − x (0 < x < 0.1). Densely sintered BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) ceramics exhibited a dielectric constant around 30, τ_f = −30 ppm/K and high Q × f values, which increased from 68,000 GHz at x = 0 to 83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) resulted in a slight decrease in the unit-cell volume. The influence of secondary phases in the BaZn₂Ti₄O₁₁-based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO₃, hollandite-type solid solutions (Ba_xZn_xTi_8 − xO₁₆) and BaTi₄O₉ caused a decrease in Q × f values.     

Ce1−xCuxO2−x/Al2O3/FeCrAl catalysts for catalytic combustion of methane

A series of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts (x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H₂-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts the surface phase structure were the Ce_1−xCu_xO_2−x solid solution, -Al₂O₃ and γ-Al₂O₃. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce_1−xCu_xO_2−x solid solution. The Cu component of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce_1−xCu_xO_2−x solid solution and the Al₂O₃ washcoats and the FeCrAl support.     

Effect of glass additions on the sintering and microwave properties of composite dielectric ceramics based on BaO–Ln2O3–TiO2 (Ln = Nd, La)

Ten weight percent BBZS (Bi₂O₃, B₂O₃, ZnO and SiO₂) glass was added to x(Ba₄Nd_9.333Ti₁₈O₅₄) − (1 − x)(BaLa₄Ti₄O₁₅) (BNLT, 0 ≤ x ≤ 1) composite dielectric ceramics to lower their sintering temperature whilst retaining microwave properties useful for low temperature co-fired ceramic and antenna core technology. With the addition of 10 wt% BBZS glass, dense BNLT composite ceramics were produced at temperatures between 950 and 1140 °C, depending on composition (x), an average reduction of sintering temperature by 350 °C. X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy studies revealed that there was limited inter-reaction between BLT/BNT and the BBZS glass. Microwave property measurement showed that the addition of BBZS glass to BNLT ceramics had a negligible effect on _r and τ_f, although deterioration in the measured quality factor (Qf) was observed. The optimised composition (xBNT − (1 − x)BLT)/0.1BBZS (x = 0.75) had _r  61, τ_f  38 ppm/°C and Qf  2305 GHz.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Synthesis, characterization and catalytic activity for NO–CO reaction of Pd–(La, Sr)2MnO4 system

La_xSr_2−xMnO₄ (0 ≤ x ≤ 0.8) oxides were synthesized and single-phase K₂NiF₄-type oxides were obtained in the range of 0.1 ≤ x < 0.5. The catalytic activity of La_xSr_2−xMnO₄ for NO–CO reaction increased with increasing x in the range of solubility limit of La. La_0.5Sr_1.5MnO₄ showed the highest activity among La_xSr_2−xMnO₄ prepared in this study, but its activity was inferior to perovskite-type La_0.5Sr_0.5MnO₃. Among the Pd-loaded catalysts, however, Pd/La_0.8Sr_1.2MnO₄ showed the higher activity and the selectivity to N₂ than Pd/La_0.5Sr_0.5MnO₃ and Pd/γ-Al₂O₃. The excellent catalytic performance of Pd/La_0.2Sr_1.2MnO₄ could be ascribable to the formation of SrPd₃O₄ which was detected by XRD in the catalyst but not in the other two catalysts.     

Electronic and structural properties of SnxTi1−xO2 solid solutions: a periodic DFT study

The structural and electronic properties of selected compositions of Sn_xTi_1−xO₂ solid solutions (x=0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard’s law in the Sn_xTi_1−xO₂ system. Our results also account for the fact that chemical decomposition in Sn_xTi_1−xO₂ system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of Sn_xTi_1−xO₂ solid solutions. Sn substitution for Ti in TiO₂ increases the oxidation–reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron–hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in Sn_xTi_1−xO₂ solid solutions.     

Preparation of CexZr1−xO2 (x = 0.75, 0.62) solid solution and its application in Pd-only three-way catalysts

Nanoparticles of Ce_xZr_1−xO₂ (x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H₂O₂ as an oxidant, and characterized by N₂ adsorption, XRD and H₂-TPR. Ce_xZr_1−xO₂ prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of Ce_xZr_1−xO₂ increased, but thermal stability of Ce_xZr_1−xO₂ decreased. The surface area of Ce_0.62Zr_0.38O₂ was 41.2 m²/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO₂, and the reduction properties of Ce_xZr_1−xO₂ were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce_0.75Zr_0.25O₂ was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C₃H₈ was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C₃H₈ was induced by the steam reform and dissociation adsorption reaction of C₃H₈. Pd-only catalyst using Ce_0.75Zr_0.25O₂ as active washcoat showed high light off activity, the reaction temperatures (T₅₀) of 50% conversion of CO, C₃H₈ and NO were 180, 200 and 205 °C, respectively. However, the conversions of C₃H₈ and NO showed oscillation with continuously increasing the reaction temperature. The presence of La₂O₃ in washcoat decreased the light off activity and suppressed the oscillation of C₃H₈ and NO conversion. After being aged at 900 °C for 4 h, the operation windows of catalysts shifted slightly to rich burn. The presence of La₂O₃ in active washcoat can enhance the thermal stability of catalyst significantly.     

Reductive decomposition of nitrate ion to nitrogen in water on a unique hollandite photocatalyst

Photocatalysis of a hollandite compound K_xGa_xSn_8−xO₁₆ (x = ca. 1.8) was examined for the reduction of nitrate ion with a reducing agent of methanol in water under UV irradiation. Hollandites have a characteristic one-dimensional tunnel structure. The hollandite powder, which was prepared by the sol–gel method and unloaded with any additives like metals, was used as the photocatalyst and its photocatalytic reaction was analyzed quantitatively by using ion chromatography and on-line mass spectrometry, and its reaction mechanism was analyzed by in-situ FT-IR. The hollandite photocatalyst showed a significant activity for the formation of N₂ from NO₃⁻. The nitrate was reduced to N₂ and NO₂⁻, while the reducing agent methanol was partly oxidized to change to formic acid. The conversion of NO₃⁻was proportional to the yields of N₂, NO₂⁻, and HCOO⁻. The present photocatalyzed decomposition of NO₃⁻ to N₂ would be a useful photocatalysis for the environmental protection of water.     

Regeneration of thermally aged Pt-Rh/CexZr1−xO2-Al2O3 model three-way catalysts by oxychlorination treatments

Pt-Rh/Ce_xZr_1−xO₂-Al₂O₃ with 0.6 and 1.0 wt.% noble metal loadings were prepared and characterized for their metal dispersion with respect to Ce_xZr_1−xO₂-free Pt-Rh/Al₂O₃ in fresh, thermally aged and oxychlorinated states. Thermal ageing at 973 K led to loss of metal dispersion in all cases but to negligible effect on the dispersion of the Ce_xZr_1−xO₂ component where present. Oxychlorination was able to fully recover metal dispersion in all cases but led to different effects on the redox properties of Ce_xZr_1−xO₂ which appeared to be related to the metal loadings. Despite showing improved dispersion following regeneration, higher loaded catalyst showed no improvement in light-off performance for either NO reduction or CO oxidation and showed poorer oxygen storage (OSC) ability, particularly at higher temperatures. Lower loaded catalyst showed improved dispersion, improved OSC and reduced light-off temperatures for NO reduction and CO oxidation after oxychlorination compared to that in the thermally aged state.     

Superior performance of Ir-substituted hexaaluminate catalysts for N2O decomposition

Novel Ir-substituted hexaaluminate catalysts were developed for the first time and used for catalytic decomposition of high concentration of N₂O. The catalysts were prepared by one-pot precipitation and characterized by X-ray diffraction (XRD), N₂-adsorption, scanning electronic microscopy (SEM) and temperature-programmed reduction (H₂-TPR). The XRD results showed that only a limited amount of iridium was incorporated into the hexaaluminate lattice by substituting Al³⁺ to form BaIr_xFe_1−xAl₁₁O₁₉ after being calcined at 1200 °C, while the other part of iridium existed as IrO₂ phase. The activity tests for high concentration (30%, v/v) of N₂O decomposition demonstrated that the BaIr_xFe_1−xAl₁₁O₁₉ hexaaluminates exhibited much higher activities and stabilities than the Ir/Al₂O₃-1200, and the pre-reduction with H₂ was essential for activating the catalysts. By comparing BaIr_xFe_1−xAl₁₁O₁₉ with BaIr_xAl_12−xO₁₉ (x = 0–0.8), it was found that iridium was the active component in the N₂O decomposition and the framework iridium was more active than the large IrO₂ particles. On the other hand, Fe facilitated the formation of hexaaluminate as well as the incorporation of iridium into the framework.     

Enhanced dielectric tunability properties of Ba(ZrxTi1−x)O3 thin films using seed layers on Pt/Ti/SiO2/Si substrates

The compositionally graded and homogeneous Ba(Zr_xTi_1−x)O₃ (BZT) thin films were fabricated on LaNiO₃ (LNO) buffered Pt/Ti/SiO₂/Si and Pt/Ti/SiO₂/Si substrates by a sol–gel deposition method, respectively. These films crystallized into a single perovskite phase. The BZT thin films deposited on LaNiO₃/Pt/Ti/SiO₂/Si substrates had a highly (1 0 0) preferred orientation and exhibited a preferred (1 1 0) orientation when the thin films were deposited on Pt/Ti/SiO₂/Si substrates. The LNO and Ba(Zr_0.30Ti_0.70) served as seed layer on Pt/Ti/SiO₂/Si substrates and analyze the relationship of seed layer, microstructure and dielectric behavior of the thin films. The compositionally graded thin films from BaTiO₃ to BaZr_0.35Ti_0.65O₃ were fabricated on LNO/Pt/Ti/SiO₂/Si substrates. The tunability behavior of compositionally graded films was analyzed in order to produce optimum effective dielectric properties. The dielectric constant of BaZr_xTi_1−xO₃ compositionally graded thin films showed weak temperature dependence. This kind of thin films has a potential in a fabrication of a temperature stable tunable device.     

Oxidation of unsymmetrical dimethylhydrazine over heterogeneous catalysts Solution of environmental problems of production, storage and disposal of highly toxic rocket fuels

The catalytic oxidation of unsymmetrical dimethylhydrazine (UDMH) by air has been studied in a vibro-fluidized catalyst bed laboratory kinetic setup over catalysts Cu_xMg_1−xCr₂O₄/γ-Al₂O₃, 32.9%Ir/γ-Al₂O₃ and β-Si₃N₄ in a temperature range 150–400 °C. The catalyst Cu_xMg_1−xCr₂O₄/γ-Al₂O₃ was found to be optimal regarding high yields of CO₂ and low yields of NO_x. A probable mechanism of UDMH heterogeneous catalytic oxidation is proposed. Catalyst Cu_xMg_1−xCr₂O₄/γ-Al₂O₃ has been further used in the pilot plant specially designed for the destruction of UDMH. Results of testing the main fluidized bed catalytic reactor for UDMH oxidation and the reactor for selective catalytic reduction of NO_x with NH₃ are presented. These results prove that the developed UDMH destruction technology is highly efficient and environmentally safe.     

Nanocolumnar titania thin films uniquely incorporated with carbon for visible light photocatalysis

Carbon-doped or carbon-covered titania can enhance photocatalytic performance under visible light. Here we report the first instance of synergistic effect of the carbon incorporation in anatase titania (TiO_2−xC_x:C) films with both substitutional doping and surface covered characteristics, by reactively co-sputtering Ti metal and graphite targets. The nature of incorporated carbon is characterized by Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The absorption edge of the carbon incorporated titania thin films shifted from ultraviolet to visible region and directly depended on the carbon content. The photocatalytic performance, e.g., photodegradation of methylene blue (MB), photo-reduction of silver-ions and super hydrophilicity, was greatly enhanced with increasing carbon content. The best photocatalytic activity is obtained in the TiO_2−xC_x:C film of the most carbon concentration about 9.3 at.% with a degradation rate-constant of 0.108 h⁻¹ for MB under visible-light illumination.     

Kinetics of the water–gas shift reaction over nanostructured copper–ceria catalysts

Water–gas shift reaction was studied over two nanostructured Cu_xCe_1−xO_2−y catalysts: a Cu_0.1Ce_0.9O_2−y catalyst prepared by a sol–gel method and a Cu_0.2Ce_0.8O_2−y catalyst prepared by co-precipitation method. A commercial low temperature water–gas shift CuO–ZnO–Al₂O₃ catalyst was used as reference. The kinetics was studied in a plug flow micro reactor at an atmospheric pressure in the temperature interval between 298 and 673 K at two different space velocities: 5.000 and 30.000 h⁻¹, respectively. Experimentally estimated activation energy, E_af, of the forward water–gas shift reaction at CO/H₂O = 1/3 was 51 kJ/mol over the Cu_0.1Ce_0.9O_2−y, 34 kJ/mol over the Cu_0.2Ce_0.8O_2−y and 47 kJ/mol over the CuO–ZnO–Al₂O₃ catalyst. A simple rate expression approximating the water–gas shift process as a single reversible surface reaction was used to fit the experimental data in order to evaluate the rate constants of the forward and backward reactions and of the activation energy for the backward reaction.     

Methane combustion and CO oxidation on LaAl1−xMnxO3 perovskite-type oxide solid solutions

Structural, redox and catalytic deep oxidation properties of LaAl_1−xMn_xO₃ (x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl_1−xMn_xO₃ samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m² g⁻¹ ranges, respectively. Redox properties and the content of Mn⁴⁺ were derived from temperature programmed reduction (TPR) with H₂. Two reduction steps are observed by TPR for pure LaMnO₃, the first attributed to the reduction of Mn⁴⁺ to Mn³⁺ and the second due to complete reduction of Mn³⁺ to Mn²⁺. The presence of Al in the LaAl_1−xMn_xO₃ solid solutions produces a strong promoting effect on the Mn⁴⁺→Mn³⁺ reducibility and inhibits the further reduction to Mn²⁺. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO₃, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.     

Enhanced high-potential and elevated-temperature cycling stability of LiMn2O4 cathode by TiO2 modification for Li-ion battery

The surface of spinel LiMn₂O₄ was modified with TiO₂ by a simple sol–gel method to improve its electrochemical performance at elevated temperatures and higher working potentials. Compared with pristine LiMn₂O₄, surface-modification improved the cycling stability of the material. The capacity retention of TiO₂-modified LiMn₂O₄ was more than 85% after 60 cycles at high potential cycles between 3.0 and 4.8 V at room temperature and near to 90% after 30 cycles at elevated temperature of 55 °C at 1C charge–discharge rate. SEM studies shows that the surface morphology of TiO₂-modified LiMn₂O₄ was different from that of pristine LiMn₂O₄. Powder X-ray diffraction indicated that spinel was the only detected phase in TiO₂-modified LiMn₂O₄. Introduction of Ti into LiMn₂O₄ changed the electronic structures of the particle surface. Therefore a surface solid compound of LiTi_xMn_2−xO₄ may be formed on LiMn₂O₄. The improved electrochemical performance of surface-modified LiMn₂O₄ was attributed to the improved stability of crystalline structure and the higher Li⁺ conductivity.     

Catalytic oxidation of methane over hexaaluminates and hexaaluminate-supported Pd catalysts

An aqueous (NH₄)₂CO₃ coprecipitation method, based on that of Groppi et al. [Appl. Catal. A 104 (1993) 101–108] was used to synthesize Sr_1−xLa_xMnAl₁₁O₁₉₋ hexaaluminates. These materials were first synthesized by alkoxide hydrolysis. This synthesis route requires special handling of the starting materials and is not likely to be commercially practical. The materials prepared by (NH₄)₂CO₃ coprecipitation have similar surface areas as those prepared by the alkoxide hydrolysis method. Their CH₄ oxidation activity, measured as the temperature needed for 10% conversion of methane, is higher than those prepared by alkoxide hydrolysis. The La-substantiated material, LaMnAl₁₁O₁₉₋, shows high surface area with 19.3 m²/g after calcination at 1400°C for 2 h. It is active for CH₄ oxidation with T_10% at 450°C using 1% CH₄ in air and 70 000 cm³/h g space velocity. The stability and activity of LaMnAl₁₁O₁₉₋ prepared by (NH₄)₂CO₃ coprecipitation method is a simple and important step forward for the application of CH₄ catalytic combustion for gas turbines.     

磁性光催化剂Fe3O4/SiO2/TiO2的制备及光催化降解苯酚

采用撞击流-旋转填料床辅助沉淀法连续制备Fe₃O₄磁性纳米颗粒,然后采用溶胶-凝胶法制备了具有磁性核壳结构Fe₃O₄/SiO₂/TiO₂颗粒,通过傅里叶变换红外光谱（FTIR）、X射线衍射仪（XRD）、透射电子显微镜（TEM）、热重分析仪（TGA）、振动样品磁强计（VSM）和紫外-可见漫反射光谱（UV-vis）等对材料的微观结构、形貌及性能进行了表征。考察了正硅酸乙酯添加量、铁钛比对Fe₃O₄/SiO₂/TiO₂磁性催化剂催化性能的影响。结果表明：制得的Fe₃O₄/SiO₂/TiO₂结晶度好,磁响应性高。当正硅酸乙酯添加量为20mL、铁钛比为1∶8时,制备的Fe₃O₄/SiO₂/TiO₂纳米光催化剂表现出较高的光催化活性和磁性能,紫外光照射2h后,苯酚水溶液降解效率高达86.7%。     

Performance of La1−xSrxAl1−y−y′FeyMgy′O3−δ perovskites in methane combustion: effect of aluminum and magnesium content

Perovskites of different La_1−xSr_xAl_1−y−y′Fe_yMg_y′O_3−δ compositions (x=0, 0.1, 0.15, 0.2 and y=0.1, 0.3, 0.5, 0.8) were prepared from a reactive precursor slurry of hydrated oxides. Each sample was aged between 16 and 26 h up to 1473 K. Activity in methane combustion (1%/air) was determined in a plug-flow reactor, with 1 g catalyst and 24 l/h flowrate. Gradual decrease in activity due to thermal aging was observed, the degree of activity loss being composition dependent. Nevertheless, activity of samples aged at 1370 K was nearly independent of composition. The best thermal stability showed LaAl_0.65Fe_0.15Mg_0.2O₃ perovskite. None of the magnesium substituted perovskites performed better than a La_0.85Sr_0.15Al_0.87Fe_0.13O₃ reference sample.     

Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC

Supported LaCoO₃ perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce_1−xZr_xO₂ (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N₂ adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T₅₀ than the corresponding Ce_1−xZr_xO₂ supports. Twenty weight percent LaCoO₃ catalysts presented lower T₅₀ than bulk LaCoO₃. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO₃ by the surface area and the oxygen mobility.