Combustion synthesis and characterization of porous perovskite catalysts

Porous perovskite-type complex oxides LaCoO₃ and La_0·95Sr_0·05Ni_0·05Co_0·95O₃ were produced by combustion method. The properties of these porous materials such as crystal structures, particle sizes, surface patterns, pore size, surface area and pore volume were characterized by X-ray diffraction( XRD), scanning electron microscopy(SEM) and BET measurements. The results indicated that all porous materials are of the perovskite-type complex oxides. Doping Sr²⁺ ions on site A and doping Ni²⁺ ions on site B entered the crystal lattices of LaCoO₃ in the place of La³⁺ and Co³⁺, respectively, and the maximum peak of XRD patterns of doping sample was weaken and broaden. Morphological microscopy demonstrated agglomerates involved mostly thin smooth flakes and layers perforated by a large number of pores and its lamella decreased with the introduction of Sr²⁺ and Ni²⁺. Hysteresis loop in the N₂ adsorption-desorption isotherm of samples indicated its porous structures and the doping effect on its pore size, surface area and pore volume were improved. The porous catalysts have been tested for methane catalytic combustion and the results showed that these catalysts possessed high catalytic activity.     

Preparation of nanocrystalline CuAlO<Subscript>2</Subscript> through sol–gel route

Nanocrystalline powders of CuAlO₂ were synthesized through sol–gel method using nitrate-citrate route and also through solid state reaction method. We used a new set of precursor materials for the synthesis of CuAlO₂ through sol–gel route which were not reported in the past. A little lowering of the synthesis temperature (1,000 °C) was observed in case of sol–gel process compared to the solid state reaction method (1,100 °C) and also at shorter time duration. The particle size of the synthesized powders was determined through small angle X-ray scattering. It has been observed that the particle size prepared by nitrate-citrate technique is less than the particle size prepared by the solid-state reaction method. Chemical states of the atomic species were determined by X-ray photoelectron spectroscopy. The formation of phase pure CuAlO₂ were also confirmed by Fourier transformed infrared spectroscopy. A number of solvents were also used for finding the best possible combinations for obtaining phase pure CuAlO₂ at 1,000 °C and it was observed that only the combination of nitrate salts, citric acid and ethanol resulted phase pure CuAlO₂.     

温度对三元正极材料前驱体Ni0.65Co0.1Mn0.25(OH)2理化特性的影响

以类球形的Ni_(0.65)Co_(0.1)Mn_(0.25)(OH)_2前驱体为研究对象,系统地研究了处理温度对该前驱体的比表面积、形貌、粒径、金属含量等理化指标的影响规律。通过X射线衍射、热重、X射线光电子能谱等检测手段揭示了高温处理过程中前驱体物相演变行为、热稳定性以及过渡金属元素的价态变化规律。     

Recent developments in the preparation of high surface area metal fluorides

An overview of the main procedures for the preparation of fluorides with very high surface areas is given. Three processes are outlined: (i) plasma fluorination, (ii) sol–gel route and (iii) oxidative decomposition of inorganic precursors. From all three processes nanostructured metal fluorides with 100–400 m² g⁻¹ can be obtained. Prevention of the local overheating during fluorination seems to be the key factor to obtain the high surface area fluorides. TEM investigations of AlF₃ and CrF₃ obtained by oxidative decomposition revealed considerable differences in their morphologies and crystallinity. CrF₃ is completely amorphous and unstable under beam. AlF₃ contains an amorphous phase and nanocrystalline phases of α-AlF₃ and β-AlF₃. Nanocrystals are uniformly distributed within the amorphous phase. Also present are the rod-like nanostructures that consist of β-AlF₃ and are 5–10 nm wide.     

Mechanochemical-hydrothermal synthesis of calcium phosphate powders with coupled magnesium and carbonate substitution

Magnesium- and carbonate-substituted calcium phosphate powders (Mg-, CO₃-CaP) with various crystallinity levels were prepared at room temperature via a heterogeneous reaction between MgCO₃/Ca(OH)₂ powders and an (NH₄)₂HPO₄ solution using the mechanochemical-hydrothermal route. X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis were performed. It was determined that the powders containing both Mg²⁺ and CO₃²⁻ ions were incorporated uniformly into an amorphous calcium phosphate phase while in contrast, the as-prepared powder free of these dopants was crystalline phase-pure, stoichiometric hydroxyapatite. Dynamic light scattering revealed that the average particle size of the room temperature Mg-, CO₃-CaP powders was in the range of 482 nm-700 nm with a specific surface area between 53 and 91 m²/g. Scanning electron microscopy confirmed that the Mg-, CO₃-CaP powders consisted of agglomerates of equiaxed, ≈20-35 nm crystals.     

Influence of synthesis route on physicochemical properties of nanostructured electrolyte material La0.9Sr0.1Ga0.8Mg0.2O3−δ for IT-SOFCs

Nanosized La_0.9Sr_0.1Ga_0.8Mg_0.2O_3?δ (LSGM) has been synthesized by both gel-combustion method and solid state reaction method as an electrolyte material for IT-SOFCs. The effect of synthesis route on phase purity of the samples has been studied by X-ray diffraction technique. In the gel-combustion method, perovskite structure was formed at 1,200 °C with only trace amount of impurity and has an average crystallite size of 27 nm obtained by Scherrer’s equation. In solid state route, phase pure product was obtained only at the calcination temperature of 1,500 °C. The characteristics of the samples were also studied using FTIR, TG/DTA, Small angle X-ray scattering, BET surface area, thermal expansion measurements, and electrochemical impedance spectroscopy. The activation energy for oxide ion conduction of LSGM samples derived from Arrhenius plot is ~1.01 and 1.09 eV for gel-combustion and ceramic route, respectively. Linear increment of thermal expansion obtained by Dilatometry shows that there is no phase change at higher temperature in the sample. Sintered densities and microstructural features of the samples were also studied. The chemical compatibility of this electrolyte material has been studied with the perovskite oxide-based cathode material La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ and NiO.     

Oxidation reactions catalysed by molybdenum(VI) complexes grafted on UiO‐66 metal–organic framework as an elegant nanoreactor

A heterogeneous catalyst was synthesized by immobilizing Mo(CO)₃ in a UiO‐66 metal–organic framework. The benzene ring of the organic linker in UiO‐66 was modified via liquid‐phase deposition of molybdenum hexacarbonyl, Mo(CO)₆, as starting precursor to form the (arene)Mo(CO)₃ species inside the framework. The structure of this catalyst was characterized using X‐ray diffraction, and chemical integrity was confirmed using Fourier transform infrared and diffuse reflectance UV–visible spectroscopic methods. The metal content was analysed with inductively coupled plasma. Field emission scanning electron microscopy was used to measure particle size and N₂ adsorption measurements to characterize the specific surface area. This catalytic system was efficiently applied for epoxidation of alkenes and oxidation of sulfides. The Mo‐containing metal–organic framework was reused several times without any appreciable loss of its efficiency.     

A new reduction route for the synthesis of nanoscale metals and metal oxides with ascorbic acid at low temperature

Reduction of the transition metal complexes in aqueous solution has been investigated systematically by ascorbic acid as the reducing agent without the assistance of any surfactant. Nanoparticles of α-Mn₂O₃, Ag and Cu were synthesized directly through aqueous phase reduction at room temperature. Nanoscale metal oxides such as Co₃O₄, α-Fe₂O₃ and MoO₂ were obtained through ascorbic acid reduction in alkali medium at 40°C. All the products were characterized on their structure and micro-morphology by the X-ray diffraction (XRD) and atomic force microscopy (AFM). The particle size of metal and metal oxides was about 10–50 nm. The reaction details and features were described and discussed.     

Photocatalytic Activity of Microwave Plasma-Synthesized TiO2 Nanopowder

Nanocrystalline TiO₂ was synthesized using the microwave plasma technique and characterized using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, laser particle size analyzer, UV–vis spectroscopy and BET surface area analyzer. The synthesized TiO₂ powder crystallized in anatase phase and the crystallite sizes were in nanometers. The photocatalytic activity of the compound was determined and compared against the activity of the commercial Degussa P-25 TiO₂ catalyst. The degradation rates of the dyes were found to be higher over the synthesized TiO₂ as compared to that over commercial Degussa P-25 TiO₂.     

Amorphous Boron Dispersed in LaCoO3 with Large Oxygen Vacancies for Efficient Catalytic Propane Oxidation

Unsatisfactory oxygen mobility is a considerable barrier to the development of perovskites for low-temperature volatile organic compounds (VOCs) oxidation. This work introduced small amounts of dispersed non-metal boron into the LaCoO₃ crystal through an easy sol-gel method to create more oxygen defects, which are conducive to the catalytic performance of propane (C₃H₈) oxidation. It reveals that moderate addition of boron successfully induces a high distortion of the LaCoO₃ crystal, decreases the perovskite particle size, and produces a large proportion of bulk Co²⁺ species corresponding to abundant oxygen vacancies. Additionally, surface Co³⁺ species, as the acid sites, which are active for cleaving the C−H bonds of C₃H₈ molecules, are enriched. As a result, the LCB-7 (molar ratio of Co/B=0.93:0.07) displays the best C₃H₈ oxidation activity. Simultaneously, the above catalyst exhibits superior thermal stability against CO₂ and H₂O, lasting 200 h. This work provides a new strategy for modifying the catalytic VOCs oxidation performance of perovskites by the regulation of amorphous boron dispersion.     

Influence of Ce substitution in LaMO3 (M = Co/Ni) perovskites for COx-free hydrogen production from ammonia decomposition

The La based perovskite type LaMO₃ (M = Ni, Co) oxides were prepared by combustion synthesis method using citric acid as organic fuel. These catalyst precursors tested for ammonia decomposition. The LaNiO₃ and LaCoO₃ catalysts showed good activity for NH₃ decomposition. The LaNiO₃ catalyst displayed greater activity than LaCoO₃. This due to high surface area and easily reducibility of Ni species. A 50% of La was substituted by Ce in both LaNiO₃ and LaCoO₃ catalysts. A remarkable effect on catalytic performance was observed with the partial substitution of La by Ce in perovskite catalyst especially at lower temperatures. The La_0.5Ce_0.5NiO₃ catalyst exhibited highest activity among all prepared samples. The achieved superior activity is due to boost in surface area, reducibility and suitable basicity. The SEM elemental mapping of La_0.5Ce_0.5NiO₃ catalyst concluded that metal oxide constituents dispersed homogeneously. The La_0.5Ce_0.5NiO₃ catalyst showed excellent stable catalytic performance during 50 h time on study at 550 °C.     

Self‐Assembling of Er2O3–TiO2 Mixed Oxide Nanoplatelets by a Template‐Free Solvothermal Route

An easy solvothermal route has been developed to synthesize the first mesoporous Er₂O₃–TiO₂ mixed oxide spherical particles composed of crystalline nanoplatelets, with high surface area and narrow pore size distribution. This synthetic strategy allows the preparation of materials at low temperature with interesting textural properties without the use of surfactants, as well as the control of particle size and shape. TEM and Raman analysis confirm the formation of nanocrystalline Er₂O₃–TiO₂ mixed oxide. Mesoscopic ordered porosity is reached through the thermal decomposition of organic moieties during the synthetic process, thus leading to a template‐free methodology that can be extended to other nanostructured materials. High specific surface areas (up to 313 m² g^?1) and narrow pore size distributions are achieved in comparison to the micrometric material synthesized by the traditional sol–gel route. This study opens new perspectives in the development, by solvothermal methodologies, of multifunctional materials for advanced applications by improving the classical pyrochlore properties (magnetization, heat capacity, catalysis, conductivity, etc.). In particular, since catalytic reactions take place on the surface of catalysts, the high surface area of these materials makes them promising candidates for catalysts. Furthermore, their spherical morphology makes them appropriate for advanced technologies in, for instance, ceramic inkjet printers.     

Observation of unusual Griffith’s phase behavior in quadruple perovskite oxide CaCu3Mn4O12 (CCMO) synthesized through chemical route

The CaCu₃Mn₄O₁₂ (CCMO) ceramic was successfully synthesized through the chemical route. The phase formation was confirmed by the X-ray diffraction pattern. Thermogravimetric (TGA), FT-IR, SEM, TEM, EDX, and XPS analysis were performed for investigation of the thermal behavior, phase identification, microstructural analysis, elemental analysis and oxidation state of the CCMO ceramic respectively. FT-IR spectra confirmed the existence of MnO₆ octahedral in body-centered cubic (BCC) complex perovskite oxide that resembles the CaCu₃Ti₄O₁₂ structure. The average particle size was observed by TEM in the range between 100 and 200 nm. AFM shows the average roughness of the surface was found to be in the range of 30 ± 5 nm. XPS and EDX studies confirmed the purity and oxidation state of the CCMO ceramic. The synthesized material shows very interesting unique Griffith’s phase (T_G) results that arise disorder in magnetic susceptibility in which ferromagnetic transition was to be observed in the paramagnetic region.     

Low Temperature Solvent Evaporation-induced Crystallization Synthesis of Nanocrystalline TiO2 Photocatalyst

A novel and efficient methodology for obtaining highly active photocatalyst of bi-phase TiO2 with small particle size and high specific surface area was developed by solvent evaporation-in-duced crystallization (SEIC) method at low temperature. The prepared TiO2 powder was characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET surface areas. The photocatalyfic activity was evaluated by the photocatalyflc oxidation of acetone in air. The results showed that the photocatalytic activity of the TiO2 powder preDared by this method approached that of Degnssa P25. This may be atotributed to the fact that the predated TiO2 powder had larzer specific surface areas (265 m2. g- 1 ) and smaller crystallite size (about 5 nm), but relatively low crystallinity, as compared with Degussa P25.     

Sol-Gel Synthesis of Nanoscaled Spinels Using Propylene Oxide as a Gelation Agent

Spinel nanoparticles of CoAl₂O₄ (blue), CoCr₂O₄ (bluish green), ZnFe₂O₄ (brown) and CuCr₂O₄ (black) were synthesized by a sol-gel route with propylene oxide as a gelation agent. This method has proven to be an effective route to synthesize mixed oxide nanoparticles, especially for that with one of metal ion having a formal charge of less than +3. Transmission electron micrographs show the small particle size (less than 60 nm) of the four pigments and their narrow particle size distributions.     

Production of Carbon Nanotubes over Pre‐reduced LaCoO3 by Using Fluidized‐bed Catalytic Reactor

A technique has been developed to grow carbon nanotubes by flowing acetylene over pre‐reduced LaCoO₃ catalyst in a fluidized‐bed catalytic reactor. Carbon nanotubes were characterized by means of SEM and TEM. The pre‐reduced LaCoO₃ catalyst was found to be effective in producing carbon nanotubes with even diameter. The effects of reduction temperature of LaCoO₃ on the growth of carbon nanotubes were investigated. This process can easily be scaled up.     

Growth and properties of particulate Fe films vapor deposited in UHV on planar alumina substrates

An integrated experimental approach was used to prepare and characterize well-defined particulate deposits of Fe on a variety of planar Al₂O₃ supports. The Fe particles were vapor deposited in UHV on in-situ cleaned and characterized amorphous and single crystal alumina supports at controlled substrate temperatures and deposition rates. Integrated support and deposit properties were monitored in-situ by AES, XPS, and LEED; particle number densities and sizes were monitored by standard TEM. It was found thata) metal exposure is an insufficient and often misleading measure of particle size when the support surface properties are unknown or poorly controlled,b) judicious combination of depostion temperature and impingement flux lead to size- and number density-controlled particulate deposits of good thermal stability, which can be improved by annealing. Preliminary results of XPS and of CO-TPD and H₂-TPD measurements exhibiting particle size effects are also reported.     

Ovalbumin mediated synthesis of Mn3O4

By using Mn²⁺ and Mn³⁺ salts, and freshly extracted ovalbumin, Mn₃O₄ nanocrystals have been synthesized successfully. The X-ray diffraction results indicated that the synthesized nanoparticles have only the spinel structure without the presence of any other phase impurities. As the ovalbumin–water mixture was highly basic, the process did not require any use of base to increase the pH where hydrolysis took place. A gel formed where water soluble ovalbumin proteins served as a perfect matrix for entrapment of metal ions (Mn²⁺ and Mn³⁺). Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Mn₃O₄. The discrepancy between the crystallite size from XRD and particle size SEM analysis reveals polycrystalline nature of the synthesized particles with this route. EPR analysis of Mn₃O₄ shows a narrow and symmetric line indicating the absence of hyperfine splitting.     

Conformal Coverage of ZnO Nanowire Arrays by ZnMnO3: Room-temperature Photodeposition from Aqueous Solution

Compositionally and structurally complex semiconductor oxide nanostructures gain importance in many energy-related applications. Simple and robust synthesis routes ideally complying with the principles of modern green chemistry are therefore urgently needed. Here we report on the one-step, room-temperature synthesis of a crystalline–amorphous biphasic ternary metal oxide at the ZnO surface using aqueous precursor solutions. More specifically, conformal and porous ZnMnO₃ shells are photodeposited from KMnO₄ solution onto immobilized ZnO nanowires acting not only as the substrate but also as the Zn precursor. This water-based, low temperature process yields ZnMnO₃/ZnO composite electrodes featuring in 1 M Na₂SO₄ aqueous solution capacitance values of 80–160 F g⁻¹ (as referred to the total mass of the porous film i. e. the electroactive ZnMnO₃ phase and the ZnO nanowire array). Our results highlight the suitability of photodeposition as a simple and green route towards complex functional materials.     

Thermal Stability of Pt/Al2O3 Catalysts Prepared by Sol-Gel

A series of Pt/Al₂O₃ catalysts were prepared using a sol-gel method. The influence of several parameters used in the synthesis including: metal content, identity of the metal precursor, and the water/alkoxide ratio on the structural properties of the fresh (dried) and calcined samples were studied. It was found that the BET surface area decreased with an increase in the platinum content. A surface area of 500 m²/g was obtained following calcination at 773 K. The structure of fresh samples as determined by FTIR corresponded to that of a pseudoboehmite structure. Samples prepared using a water/alkoxide ratio (H₂O/ATB) of 9 showed a well-defined, uniform pore size distribution following calcination at 773 K. Metal dispersions comparable to those obtained using impregnation methods were obtained. Aging studies (calcination at 873 K for 24 h) performed on these catalysts, exhibited sintering behavior which were similar to Pt/Al₂O₃ catalysts prepared by other methods. The sample prepared using a H₂O/ATB ratio of 9 had the highest surface area and was more thermally resistant towards metal sintering. A bimodal metal particle size distribution was observed: some particles exhibited sintering while others of similar size showed a greater thermal stability to sintering. The sample having the largest surface area and the highest thermal stability following thermal treatment was a consequence of a more condensed structure and a higher pore roughness obtained after drying the gel. This enabled the formation of an alumina structure which was more amorphous and limited aggregation of platinum particles due to surface diffusion within the pore structure.