Synthesis, characterization, photoluminescence and EPR investigations of Mn doped MgAl2O4 phosphors

MgAl₂O₄:Mn phosphors have been prepared at 500 °C by combustion route. Powder X-ray diffraction (XRD) indicated the presence of mono-MgAl₂O₄ phase. Scanning electron microscopy showed that the powder particle crystallites are mostly angular. Fourier transform infrared spectroscopy confirmed the presence of AlO₆ group which makes up the MgAl₂O₄ spinel. Photoluminescence studies showed green/red emission indicating that two independent luminescence channels in this phosphor. The green emission at 518 nm is due to ⁴T₁ → ⁶A₁ transition of Mn²⁺ ions. The emission at 650 nm is due to the charge-transfer deexcitation associated with the Mn ion. EPR spectrum exhibits allowed and forbidden hyperfine structure at g=2.003. The g≈2.00 is due to Mn²⁺ ion in an environment close to tetrahedral symmetry. It is observed that N and χ increase with decrease of temperature obeying the Boltzmann law. The variation of zero-field splitting parameter (D) with temperature is evaluated and discussed.     

Theoretical mechanistic studies on oxidation reactions of some saturated and unsaturated organic molecules

This account describes the results collected by our group during the last years on some themes of environmental/mechanistic interest. Theoretical quantum-mechanical investigations have been carried out to help clarifying the mechanism of some oxidation reactions, which involve mainly unsaturated but also saturated organics as substrates, and, as reactive oxidants, triplet or singlet dioxygen, hydroxyl, ozone, and nitrogen oxides. Depending on the problem, the calculations are either multi-configurational (as CAS-MCSCF, CAS-PT2, MC-QDPT2), or based on the Density Functional Theory for the heavier systems. Research work has thus been developed along the following lines: hydrocarbon oxidations under atmospheric or combustion conditions; definition of a model for soot particles and their interaction with species as HO, O₂, O₃, NO, NO₂, NO₃, etc.; investigation on the reaction mechanism of ¹Δ_g dioxygen with organic unsaturated systems (cycloaddition and ene reactions).     

Pure and doped lanthanum cobaltites obtained by combustion method

We report the synthesis of La_1−xSr_xCoO₃ nanopowders by solution combustion method using metal nitrates and -alanine (alanine method) or urea (urea method) as fuel. The influence of metal nitrates/organic substance molar ratio and the type of fuel was investigated. The isolated complex precursors were characterized by atomic absorption spectroscopy (AAS), FT-IR spectra and DTA–TG analysis. The La_1−xSr_xCoO₃ (x = 0–0.3) powders were characterized by X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive X-ray analysis (SEM–EDX), as well as by specific surface area measurements. XRD patterns indicate the formation of single-phase LaCoO₃ (rhombohedral) when as-synthesized powders were calcined at 873 K, 3 h in the case of the alanine method and at 1073 K, 3 h for urea-based system. Also, strontium doped lanthanum cobaltites obtained by both methods at 1273 K are single phase with rhombohedral perovskite-like structure as XRD data have proved. SEM investigation of pure and doped lanthanum cobaltites reveal that the samples prepared by both methods have fine particles with tendency of agglomerates formation with different shapes, spongy aspect and high porosity. La_1−xSr_xCoO₃ nanopowders obtained by alanine method have larger specific surface area values than those prepared by urea method.     

高碳烃宽温度范围燃烧机理构建及动力学模拟

发动机中燃料点火特性以及燃烧能量的释放对于发动机设计具有非常重要的作用,为了提高燃料的燃烧效率以及减少燃料在燃烧过程中污染物的排放,基于反应动力学机理对燃料燃烧过程的模拟就显得十分必要。因此需要更加深入的认识碳氢燃料的燃烧机理,探索其在燃烧过程中十分复杂的化学反应网络。为了发展能够适用于实际燃料多工况条件(宽温度范围、宽压力范围和不同当量比)燃烧的燃烧机理,基于碳氢燃料机理自动生成程序ReaxGen构建了正癸烷燃烧详细机理(包含1499个物种,5713步反应)和正十一烷燃烧详细机理(包含1843个物种,6993步反应)。详细机理主要由小分子核心机理和高碳烃类(C5以上)机理两部分组成。为了验证机理的合理性与可靠性,本文对于高碳烃燃烧新机理在点火延时时间以及物种浓度曲线进行了动力学分析,并与实验数据及国内外同类机理进行了对比,结果表明本文提出的正癸烷和正十一烷燃烧新机理在比较宽泛的温度、压力和当量比条件下都具有较高的模拟精度,为发展精确航空煤油燃烧模型提供了基础数据。同时考虑到详细机理的复杂性以及机理分析的计算量大和时耗长,本文基于误差传播的直接关系图形(Directed Relation Graph with Error Propagation,DRGEP)方法简化得到的包含709组分2793反应的正癸烷和包含820组分3115反应的正十一烷简化机理,使用DRGEP方法时所采用的数据点选自压力范围从1.0×10~5 Pa到1.0×10~6Pa,当量比范围从0.5到2.0,初始温度范围从600到1400时恒压点火的模拟结果在点火延迟时间附近区域的抽样,同时在正癸烷机理简化中选取正癸烷、O_2和N_2作为初始预选组分,正十一烷的机理简化中主要选取正十一烷、O_2和N_2作为初始预选组分,得到的简化机理在比较宽泛的条件下的预测结果与详细机理吻合很好。最后结合敏感度分析方法分析了正癸烷和正十一烷的点火延迟敏感性,考察了机理中影响点火的关键反应。结果表明:这些机理能够合理描述正癸烷和正十一烷的自点火特性,在工程计算流体力学仿真设计中有很好的应用前景。     

负载型La0.8Sr0.2MnO3燃烧催化剂的载体效应

用浸渍法制备负载型Ｌａ_（０．８）Ｓｒ_（０．２）Ｍｎｏ_３燃烧催化剂,比较了γ－ＡＩ_２Ｏ_３、α－ＡＩ_２Ｏ_３、堇青石（２ＭｇＯ·２Ａｌ_２Ｏ_３、５ＳｉＯ_２）和ＺｒＯ_2四种载体的负载效果．用ＸＲＤ,ＤＰＲ着重研究了γ－Ａｌ_２Ｏ_３载体上由于负载量不同、焙烧温度不同引起的催化剂物相变化和二甲苯完全氧化的催化活性变化．     

负载型La0.8Sr0.2MnO3燃烧催化剂的载体效应

Combustion catalysts La0.8Sr0.2MnO3 supported on γ-Al2O3, α-Al2O3, cordierite (2MgO•2Al2O3•5SiO2) and ZrO2 were compared. Further investigation was focused on LSM/ γ-Al2O3 catalyst. It was observed that LSM/γ－Al2O3 catalyst loaded with 20% (mass fraction) LSM (La0.8Sr0.2MnO3 or corresponding oxides), heated at 750℃ or above, perovskite-type oxides were found by XRD examination, whereas, the same catalyst loaded with 10% or less LSM, perovskite oxides were absent, calcination temperature about 750℃ is necessary for the formation of perovskite structure in LSM/γ-Al2O3 catalysts. High activity of complete oxidation of xylen will be obtained when perovskite-type oxides.
Investigation of TPR showed that neat LSM or LSM/γ-Al2O3(20%) was reduced by H2-N2 mixed gas. Two degradation processes took place. In the first, reduced temperature peak was about 350 - 450℃. If reduction ended at 400℃, perovskite structure was retained, which may be due to the reduction of Mn3+to Mn2+ on the surface of LSM only. In the second process, perovskite structure was destroied, and La2O3, Mn2O3, Mn - Sr - O oxides could be obtained, which took place in the temperature range 685 - 750℃ and ended at 800℃. This was proved by TPR experiments (Fig. 3, 5) and XRD patterns (Fig. 4)
Catalysts LSM/γ-Al2O3(10% or 20%) heated at 500℃ have only one TPR peak, i. e. lower temperature peak. This is due to the absence of perovskite-type oxides in the catalysts. However, neat LSM or LSM/γ-Al2O3(20%) heated 750℃ or above, not only the first low temperature TPR peak but also the second peak, which is contributed by the perovskite-type oxides in these catalysts appeared. Therefore, the second TPR peak, i. e. the higher temperatue peak is a characteristic peak for perovskite-type oxides in the reduced process. When LSM/ γ-Al2O3 (10%) catalys is heated at 750℃, no perovskite-type oxides were detected by XRD, and the second reduction peak was absent also in TPR process. \
The order of the second reduction peak temperature(characteristic peak of perovskite - type ox- ides) is: neat LSM(750℃）> LSM/γ-Al2O3 20% (685-698℃) -deposited LSM/γ-Al2O3 (698℃) > LSM/γ-Al2O3 15% (677 - 680℃) >(LSM/γ-AL2O3 10% 620 - 630℃, for Mn - Al - O medium oxides on surface). It is correleted with the increasing of the effect of support sequentially.
When LSM/γ-Al2O3 catalysts were heated at 900℃, more stable phase, spinel MnAl2O4 appeared, which could be proved by TPR of model catalyst MnAl2O4/γ-Al2O3.     

热力学数据对1, 3-丁二烯燃烧特性的影响

基于G4方法, 计算了1,^{3-丁二烯框架燃烧反应机理中102个物种的热力学数据, 并考察了振动非谐性、 频率校正因子以及受阻内转动对结果的影响. 结果表明, 考虑振动非谐性或采用不同的频率校正因子, 对热力学数据的影响不大; 考虑内转动后, 对热力学数据有较大影响. 而且考虑内转动后, 得到的热力学数据与实验热力学数据吻合得更好. 用所得热力学数据模拟了1,3-丁二烯的绝热燃烧温度以及点火延迟时间, 结果显示, 要得到可靠的绝热火焰温度, 对小分子(如CO和CO₂等)的热力学数据需要采用实验结果. 将用所得热力学数据模拟得到的点火延迟时间, 与机理本身的热力学数据所得点火延迟时间进行对比, 二者差别显著, 表明所得热力学数据主要通过改变一些反应的逆反应速率常数来影响点火延迟时间. 进一步确定了用所得热力学数据对点火延迟时间有显著影响的一些物种.}     

Conventional hydrothermal process versus microwave-assisted hydrothermal synthesis of La1−xAgxMnO3+δ (x = 0, 0.2) perovskites used in methane combustion

A study on the synthesis of La_1−xAg_xMnO_3+δ (x = 0, 0.2) using a microwave process (MWhyd) has been carried out by comparing the heating time and reaction temperature with the same factor under conventional thermal process (CHhyd). Experiments have been conducted using the hydrothermal method at medium pressure (T = 200 °C, P = 20 atm) followed by a thermal treatment of the precursor at 700 °C (10 h).Structural and physico-chemical properties of the catalysts were investigated using X-ray diffraction (XRD), BET sorption, temperature-programmed reduction or desorption, mass spectrometry (TPR-MS and TPD-MS), and X-ray photoelectron spectroscopy (XPS). While CHhyd and MWhyd powder catalysts exhibited the same XRD patterns indexed as pure perovskite structure, their surface physico-chemical properties were found to be strongly influenced by the preparation method. The effect of the nature of oxygen species, their amount and mobility, evidenced by temperature programmed experiments, on the catalytic properties in methane combustion in the presence and in the absence of hydrogen sulphide has been studied. MWhyd-La_0.8Ag_0.2MnO_3+δ catalysts were found to exhibit a much better performance in methane combustion together with higher resistance to sulphur poisoning than CHhyd catalysts.     

流化床锅炉导流板布风装置气动力特性的研究

本文对导流板布风装置气动力特性及其对燃烧工况的改善进行了研究.结果表明,导流板能降低风帽区阻力损失,使近风帽区气流速度衰减变慢,促进床内气流循环,提高流化质量,降低良好沸腾流化数(u/u_(mf)),改善燃烧效率.     

粒状硝胺火药燃烧转爆轰的研究

本文以粒状硝胺火药(编号RDU-4,几何平均粒度δ=68μm)为研究对象,研究了硝胺火药燃烧转爆轰的某些特征.结果表明,该硝胺火药药床在爆轰形成之前有予压区存在,在装填密度为1.0g/cm~3时,最易实现燃烧转爆轰。