Combustion synthesized crystalline La-Mn perovskite catalysts: Role of fuel molecule on thermal and chemical events

Solution combustion synthesis (SCS) technique was applied to produce LaMnO_3+δ with the aim to investigate the effect of the chemical nature of a series of six fuel molecules (glycine, maleic acid, succinic acid, citric acid, acetic acid, urea) on the combustion reaction mechanism and physicochemical properties of the as-prepared powders. The whole SCS process was found to involve two types of combustion reactions depending on the used sacrificial molecules. Type I (with glycine, maleic acid and succinic acid) was characterized by a one-step exothermic reaction implying a semi-decomposed mixed nitrate-fuel complex and NO₂ arising from manganese nitrate decomposition. The heat emission allows reaching the temperature suitable for well crystallized as-prepared perovskite powders. Type II (with citric acid, acetic acid and urea) was typified by a multi stage process in which intermediate decomposition reactions occurred before the formation of a mixed nitrate-fuel complex. In this case, the heat emission became lower than that expected from stoichiometric reaction, thus limiting the completion of the direct reaction for perovskite production. Consequently, part (with citric acid and acetic acid) or totally (with urea) of lanthanum and manganese remained distinctly combined in two amorphous phases (La(OH)₂NO₃, MnO_x) that were intimately mixed. With respect to other fuels, combustion synthesis, using glycine, produced better crystallized, more defective and performant catalytic perovskite phase toward deep ethanol oxidation.     

Influence of combustion reagent and microwave drying method on the characteristics of nano-sized Nd<Superscript>3+</Superscript>:YAG powders synthesized by the gel combustion method

Nanosized Nd³⁺ doped Y₃Al₅O₁₂ (Nd³⁺:YAG) powders have been synthesized by the gel combustion method using different combustion reagent such as citric acid, ethylene diamine tetraacetic acid (EDTA), glycine, glycol and the combination of citric acid and EDTA with different ratio. The pure YAG phase was obtained at relatively low temperature around 950 °C for citric acid or EDTA and 1,050 °C for glycine or glycol as combustion reagent, respectively by the gel combustion method. It was found that citric acid and EDTA are the better combustion reagents and yield rather homogeneous and well dispersed club-shape Nd³⁺:YAG samples, and the particle size synthesized by EDTA is larger than that by citric acid. Moreover, the particle size of Nd:YAG was enlarged when the ratio of EDTA was increased in the combination reagent, and the better dispersion of YAG was obtained when the ratio of citric acid to EDTA was 1:1 compared to that of other ratios and pure one as combustion reagent. On the other hand, the results showed that the microwave assisted in drying process of gel to xerogel produced more smaller Nd³⁺:YAG particles in size, and more homogeneous dispersion of the Nd³⁺:YAG particles than that of the traditional dry method.     

Nanostructured composite materials of cerium oxide and barium cerate

Nanosized powders with a composition of (1-x)Ce_0.8Sm_0.2O_2-δ-xBace_0.8Sm_0.2O_3-δ (x = 0, 0.3, and 1) were obtained by self-ignition combustion synthesis (SICS) from the appropriate nitrates and various organic fuels (glycine, glycerol, citric acid, and a mixture of citric acid and ethylene glycol). The most finely dispersed powders formed when the concentration of the perovskite phase in the system decreased or when glycerol or citric acid-enthyleneglycol mixture was used as a fuel during SICS. A procedure for the preparation of powders and nanostructured ceramics was developed and their electric properties were studied.     

Effects of experimental parameters on characterization of AgNb1?xTaxO3 materials by a modified sol–gel route

Nano-sized AgNb_1?xTa_xO₃ (x?=?0.4) ceramic powders were synthesized by a modified citrate sol?Cgel route. Homogeneous Ag?CNb?CTa precursor gel was prepared with silver citrate, niobium citrate, and tantalum citrate as source of Ag, Nb and Ta, respectively. Citrate acid and ethylene glycol were used as the complexing agents. The structural variation of the AgNb_1?xTa_xO₃ powder with annealing temperature was studied by TG?CDTA and X-ray diffraction. The precursor gel calcinated at 680?°C produced a pure perovskite phase. The effects of experimental parameters including pH value of the solution and the proportion of citric acid to the metal ions on the formation of homogeneous and microstructure of the powders were also investigated. The results indicated that a homogeneous Ag?CNb?CTa precursor gel with no precipitate was formed at about pH?=?6 and the optimum molar ratio of citric acid and the metal ions were 2:1?C3:1. The XRD data was analysis and the correlation between dielectric properties and (Nb,Ta)O₆ octahedra was discussed.     

Physical and electrochemical properties of (La0.3Sr0.7)0.93TiO3–δ synthesized by Pechini method as an anode material for solid oxide fuel cells

The lanthanum strontium titanate (LST) has to be calcined at significantly high temperature (above 1,300 °C) to obtain its pure perovskite structure when synthesized by conventional solid-state method, which is main reason for reducing active surface area. In this study, A-site deficient (La_0.3Sr_0.7)_0.93TiO₃ was synthesized by Pechini method. Although the prepared powders were calcined at 600 °C, the pure perovskite structure can be obtained without any secondary phase such as TiO₂. Moreover, the porosity and surface area are 6 times and one order of magnitude higher in the LST powders synthesized by Pechini method than in the powders synthesized by solid-state method. Based on these results, the LST electrode (Pechini) leads to two times lower electrode resistance than the LST electrode (solid-state). Thus, the LST powders synthesized by Pechini can contributes to saving the energy needed for calcination process as well as increasing the porosity and active surface area, enhancing physical and electrochemical properties in SOFC anode.     

Rapid synthesis of DyFeO<Subscript>3</Subscript> nanopowders by auto-combustion of carboxylate-based gels

EDTA and citric acid as two typical chelating reagents with multi-carboxyl groups were used to prepare DyFeO₃ nanopowders, respectively. The experimental results show that all of the carboxylate-based gels exhibited auto-propagating combustion behaviors. The XRD results indicate that DyFeO₃ single phase can be formed directly with CA/MN (citric acid to metal nitrate mole ratio) = 1 when the calcination temperature was above 700 °C. The specimen with EA/MN (EDTA to metal nitrate mole ratio) = 1 had the minimum crystallite size of 33 nm. The SEM images show that the as-burnt powders prepared with EDTA had more excellent dispersibility feature and clearer grain boundaries than that of citric acid. The magnetic measurement results show that DyFeO₃ nanopowders displayed antiferromagnetism characteristics at low temperatures due to the strong exchange interaction between Fe sublattice. As the ambient temperature increased, there was a transition from antiferromagnetism to paramagnetism in DyFeO₃ nanopowders.     

Combustion synthesis of some iron oxides used as adsorbents for phenol and p-chlorophenol removal from wastewater

The effect of the fuel nature and of the reaction atmosphere (in air/in the absence of air) on the synthesis of iron oxides by the combustion method was investigated. Working in air, using urea with ammonium chloride as fuel, the final product of reaction is α-Fe₂O₃. Working in the absence of air, using oxalic, tartaric, respectively, citric acid as fuel, the single phase resulted in combustion reaction was Fe₃O₄. From the synthesized iron oxides investigated as potential sorbents for the removal of phenol and p-chlorophenol (PCP) from wastewater, only the sorbents obtained using tartaric acid as fuel (S3) and those obtained using citric acid as fuel (S2) show adsorption capacity for the two pollutants. The sorbent S3 shows better adsorption capacity for both phenol and for PCP compared with sorbent S2.     

Optimization of synthesis conditions and the study of magnetic and dielectric properties for MgFe2O4 ferrite

Nano-sized magnesium ferrites were synthesized by the sol-gel auto-combustion method using a variety of chelating/combustion agents: tartaric acid, citric acid, cellulose, glycine, urea and hexamethylenetetramine. The original purpose of this work was the synthesis of nano-sized magnesium ferrite by using, for the first time, cellulose and hexamethylenetetramine as chelating/combustion agents. Synthesized samples were subjected to different heat treatments at 773 K, 973 K and, respectively 1173 K in air. The disappearance of the organic phase and nitrate phase with the spinel structure formation was monitored by infrared absorption spectroscopy. Spinel structure, crystallite size and cation distribution were evaluated by X-ray diffraction data. The morphology of as-prepared powders was studied using scanning electron microscopy. The magnetic and dielectric properties were studied for the obtained samples.      

Combustion synthesis and properties of nanocrystalline zirconium oxide

Nanocrystalline tetragonal zirconia powders have been synthesized by aqueous combustion using glycine (Gly) as a fuel and zirconyl nitrate (ZN) as an oxidizer. The effect of the fuel-to-oxidant molar ratio on the structural and morphological properties of nanocrystalline zirconia powders was studied. Thermodynamic modeling of the combustion reaction showed that the increase in the Gly:ZN molar ratio leads to the increase in theoretical combustion temperature, heat of combustion and amount of produced gases. Powder properties were correlated with the nature of combustion and results of thermodynamic modelling. The increase in the Gly:ZN molar ratio produces more agglomerated powders characterized by a lower degree of uniformity, a lower specific surface area and a slightly bigger crystallite size. On the other hand, the presence of hard agglomerates suppresses the volume expansion, stabilizing tetragonal zirconia, as confirmed by Rietveld refinement. The absence of cubic zirconia was confirmed by FTIR and Raman Spectroscopy. The increase in the calcination temperature led to more agglomerated, compact and less uniform powders. The nanocrystalline nature of zirconia is the reason for the formation of bigger crystallites, the increase in the relative amount of monoclinic phase and sample sintering after calcination at high temperature. The highest measured specific surface area of zirconia was 45.8 m²·g⁻¹, obtained using a fuel-lean precursor.     

Characterization and preparation of nanocrystalline MgCuZn ferrite powders synthesized by sol–gel auto-combustion method

Nanocrystalline Mg–Cu–Zn ferrite powders were successfully synthesized through nitrate–citrate gel auto-combustion method. Characterization of the nitrate–citrate gel, as-burnt powder and calcined powders at different calcination conditions were investigated by using XRD, DTA/TG, IR spectra, EDX, VSM, SEM and TEM techniques. IR spectra and DTA/TGA studies revealed that the combustion process is an oxidation–reduction reaction in which the NO₃ ⁻ ion is oxidant and the carboxyl group is reductant. The results of XRD show that the decomposition of the gel indicated a gradual transition from an amorphous material to a crystalline phase. In addition, increasing the calcination temperature resulted in increasing the crystallite size of Mg–Cu–Zn ferrite powders. VSM measurement also indicated that the maximum saturation magnetization (64.1 emu/g) appears for sample calcined at 800 °C while there is not much further increase in M _s at higher calcination temperature. The value of coercivity field (H _c) presents a maximum value of 182.7 Oe at calcination temperature 700 °C. TEM micrograph of the sample calcined at 800 °C showed spherical nanocrystalline ferrite powders with mean size of 36 nm. The toroidal sample sintered at 900 °C for 4 h presents the initial permeability (μ _i) of 405 at 1 MHz and electrical resistivity (ρ) of 1.02 × 10⁸ Ω cm.     

Structure and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> ferrites synthesized by sol–gel and microwave calcination

CoFe₂O₄ ferrites were synthesized sol–gel with cobalt chloride, ferric chloride and citric acid as the main raw material. X-ray diffraction, vibrating sample magnetometer and simultaneous thermal analysis were applied to character the structure and magnetic properties of traditional and microwave calcined samples. The samples with pH 5 and molar ratio of citric acid to metal nitrate 1–1.2 showed the optimal structure and magnetic properties. Microwave calcination reduces the synthesis time from 2 h for conventional calcination to 15–30 min. The saturation magnetization (σ _s ) for sample microwave-calcined at 550 °C for 30 min reaches to 75.89 emu/g, much higher than that of conventional-calcined samples.     

Synthesis of terbium doped calcium phosphate nanocrystalline powders by citric acid sol–gel combustion method

Terbium doped calcium phosphate (Tb-doped CaP) nanocrystalline powders were synthesized by the citric acid sol–gel combustion method. The phase composition, morphology and luminescent property of Tb-doped CaP nanocrystalline powders were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, fluorescence spectrophotometer and fluorescence microscopy. At 700 °C, Tb-doped CaP nanocrystalline powders are composed of HAP (main phase) and β-TCP (minor phase) with Tb doping content of 0.5–4%. SEM and TEM observations show that the 4% Tb-doped CaP nanocrystalline powders are about 50–150 nm spherical particles. The 4% Tb-doped CaP nanocrystalline powders exhibit the strongest emission at 548 nm (λ_excitation = 240 nm) and show strong green fluorescence under fluorescence microscopy.     

Synthesis of visible-light reactive TiO2−xNy photocatalyst by mechanochemical doping

Yellowish TiO_2−xN_y was prepared by a novel mechanochemical nitrogen-doping method. The samples were prepared by a high-energy ball milling of P25 titania with different nitrogen sources such as hexamethylenetetramine, urea or ammonium carbonate, followed by calcination in air at 400 °C. The high mechanical energy accelerated the phase transformation of anatase to rutile, while the existence of the nitrogen reagents tended to block the transformation. The calcination treatment slightly increased the crystallinity of the prepared titania. The prepared powders possessed two absorption edges at around 400 and 540 nm and showed an excellent photocatalytic ability for the oxidation of nitrogen monoxide under visible light irradiation. Under the irradiation of visible light with wavelengths of >510 nm, nitrogen monoxide could be continuously removed by the nitrogen doped titania prepared from the P25 titania-hexamethylenetetramine mixture, while the powders prepared using urea and ammonium carbonate as nitrogen sources showed lower activities. This mechanochemical technique might be widely useful for doping oxides with nonmetallic elements.     

Nanocrystalline SrCexFe12?xO19 (x?=?0.00, 0.02, 0.04, 0.06, 0.08) microfibers by sol–gel method

The SrCe_xFe_12?xO₁₉ (x????0.08) ferrite microfibers were prepared via sol?Cgel method from starting reagents of metal nitrate salts and citric acid. The obtained microfibers were characterized by TG-DSC, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The XRD patterns show that the main phase is M-type strontium hexaferrite without other impurity phases when calcined at 750?°C for 1?h, and the Ce³⁺ ions doping has not resulted in crystal structural changes of the Sr ferrite magnetoplumbite type structure. The SEM and TEM images indicate that samples with different calcination temperature and various values of x possess the diameters of below 10???m and homogeneous hexagonal plate-like morphology. The VSM results show that saturation magnetization (Ms) gradually increases with increase of calcination temperature while decreases with increase of addition of Ce³⁺ ions, and coercive force (Hc) reveals an increase at first and then decreases with increase of calcination temperature and Ce³⁺ ions content.     

The thermal behavior of some polymeric precursors used in CaAl12O19 synthesis

CaAl₁₂O₁₉ was synthesised using three different precursors: (a) a polyesteric type precursor resulted from the traditional Pechini method; (b) a polyesteric type precursor resulted from the reaction between citric acid and calcium and aluminum nitrates; and (c) a polymeric type precursor resulted from the reaction between acrylic acid and calcium an aluminum nitrates. The thermal behavior of the three precursors used in the CaAl₁₂O₁₉ synthesis was monitored to underline the thermal effects associated to the CaAl₁₂O₁₉ formation. Thermal analyses performed on precursors do not reveal clear differences regarding the thermal effects assigned to calcium aluminates formation, at temperatures over 800?°C. In contrast, thermal analysis of samples pre-fired at 200?°C, and especially at 600?°C, show clear differences between samples obtained in different ways. It is noted that in samples obtained from acrylic acid and nitrates, and citric acid and nitrates, CA₆ is practically single phase after calcination at 1,200?°C. However, in the sample obtained from citric acid, ethylene glycol, and nitrates, calcined at 1,200?°C, CA₆ is present along with CA₂ and ??-Al₂O₃.     

Fe/Sr ratio and calcination temperature effects on processing of nanostructured strontium hexaferrite thin films by a sol?Cgel method

Nanostructured single-phase strontium hexaferrite, SrFe₁₂O₁₉ (SrM), thin films have been synthesized successfully for the first time on silicon substrates using a spin-coating sol?Cgel process. The precursor solution was prepared from metal nitrates, citric acid, and ethylene glycol. The thin films with various Fe/Sr molar ratios of 8?C12 were heat treated at different temperatures from 700 to 900???C. The composition, microstructure, and magnetic properties of the SrM thin films were also characterized. The results showed that the optimum molar ratio for Fe/Sr was 10, at which the lowest calcination temperature for obtaining the single-phase strontium hexaferrite thin films was 800???C. The crystallite size of the resultant thin films was below 50?nm.     

CuO-ZnO-ZrO2的柠檬酸燃烧法制备及其催化CO2加氢合成甲醇的性能

采用柠檬酸燃烧法制备了CuO-ZnO-ZrO2(CZZ)催化剂,并将其用于CO2加氢合成甲醇反应.按推进剂化学原理对燃烧反应进行了分析,并采用热重-差热分析(TG-DTA)技术记录了其燃烧行为.采用X射线衍射(XRD)、氮吸附、程序升温还原(TPR)及氧化亚氮(N2O)反应吸附技术对制得的催化剂进行了表征.结果表明:柠檬酸燃烧法的燃烧过程比较温和,燃料用量对催化剂物化和催化性能的影响不大,并结合燃烧反应的特点进行了解释.此外,还对三种燃料(柠檬酸、尿素和甘氨酸)的用量与CZZ性能之间的关系进行了比较,表明柠檬酸作燃料具有更好的工艺可控性.柠檬酸燃烧法是一种简单、快速且有效的制备CZZ催化剂的方法.     

Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> powders by the complex polymerization method: synthesis,characterization and morphology

Lanthanum-modified bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂, BLTO) powders were prepared by the complex polymerization method. The structure and morphology of BLTO powders were investigated by X-ray diffraction and scanning electron microscopy. The complexation of citric acid with the metallic cations was detected by Fourier transformed infrared (FT-IR). The thermal analyses of obtained gels were investigated by differential thermal gravimetric (DTG). The pure and normally stoichiometric phase of BLTO powders could be obtained at relatively low temperature of 550–700 °C even if the bismuth content is not excess in the starting precursors, while the secondary phase could be detected at lower and higher calcination temperatures. The shape of the BLTO grains is similarly to platelet in Bi-layer structure and stoichiometry BLTO was detected by the analysis of energy dispersive spectrometry.     

Preparation and magnetic properties of RFeO<Subscript>3</Subscript> nanocrystalline powders

The rare-earth orthoferrites RFeO₃ (R, rare-earth element) crystallize in an orthorhombic distorted perovskite structure. RFeO₃ compounds exhibit interesting physical and chemical properties because of their ionic and electronic defects. Polycrystalline nano-sized RFeO₃ powders were synthesized by the sol–gel combustion method. X-ray powder diffraction indicated that nanocrystalline powders were single ReFeO3 phase, which are agglomerated with average crystallite size of 60–90 nm estimated with the Scherrer’s equation. Magnetic measurements were carried out using a superconducting quantum interference device magnetometer. The influence on hysteresis curve of electronic structure of rare-earth element was investigated for R = Y, La and Nd. Varied magnetic behaviors were observed in these compounds, which are believed to be associated with the different interactions of Fe and rare earths.     

Influence of N sources on the photocatalytic activity of N-doped TiO2

Influence of nitrogen precursors urea, semicarbazide and N,N’-dimethyl urea on the photocatalytic activity of the N-doped TiO₂ were studied by a simple decomposition method. The nano N-TiO₂ catalysts were synthesized via two different modified approaches by calcination at 500 °C. The synthesized samples were characterized by IR, UV-DRS, Raman, TG-DTA, XRD, EDX, XPS, SEM, TEM and BET analysis. Of the synthesized six samples of N-TiO₂ five samples showed better photocatalytic activity towards direct sunlight photo-degradation of methylene blue (MB) and rhodamine B (RhB) than Degussa P25. The catalysts obtained using semicarbazide samples F3 and F4 having large surface area of 76 and 85.8 m²/g displayed maximum photocatalytic activity. The sample F4 was 1.5 times more active than Degussa P25 for the decolourisation of MB and 1.9 times more active for the decolourisation of RhB. The presence of nitrogen, large surface area and coupling of rutile-anatase phases were found to be the main responsible factors for the enhanced photocatalytic activity. The exclusive formation of the anatase phase in the case of urea precursor is attributed to the slow evaporation of urea due to the formation of melamine derived products. The calcination temperature is the deciding factor responsible for the photocatalytic activity of the N-TiO₂ samples prepared from precursors which can potentially form the melamine and its oligomerized products on the surface of TiO₂.