Direct numerical simulations of rich premixed turbulent n-dodecane/air flames at diesel engine conditions

Rich premixed turbulent n-dodecane/air flames at diesel engine conditions are analyzed using direct numerical simulations. The conditions correspond to a parametric variation of the Engine Combustion Network Spray A (pressure 60 atm; oxidizer oxygen level and temperature 21% and 900 K, respectively; fuel temperature 363 K). Three simulations with equivalence ratios of 3, 5, and 7 are performed with a Karlovitz number (Ka, based on flame time) of order 100 to match the estimated Ka of the rich premixed combustion region in Spray A. At these conditions, the reference laminar flames exhibit a complex structure which involves both low-temperature chemistry (LTC) and high-temperature chemistry over a wide range of length scales. In the presence of turbulence, the flame structure is strongly affected in physical space and the reaction zone exhibits a very complex structure in which broken, distributed, and thin regions co-exist, especially for the leanest case. However, the contribution of the LTC pathway is only weakly affected by turbulence. In progress variable space, the mean flame structure, including the chemical source terms, is found to match remarkably well that of the corresponding unity Lewis number laminar flame, particularly for the $?=$ 3 and 5 cases. This behavior is attributed to the strong turbulent mixing occurring throughout the flames/reaction zones, which suppresses differential diffusion effects. Nevertheless, large conditional fluctuations around the mean chemical source terms are identified. These are found to correlate very well with radical species mass fractions such as OH. In addition, a similar functional dependence is obtained from counterflow laminar flames. As such, it appears from these results that laminar flame models have a potential to be used to represent the thermochemical state of rich premixed turbulent flames under diesel engine conditions.     

Synthesis of 3-fluoro-3-methyl-3-silatetrahydropyran and its conformational preferences in gas and solution by GED,NMR and theoretical calculations

The 3,3-disubstitued 3-silaheterocyclohexane with an electronegative substituent at silicon, 3-fluoro-3-methyl-3-silatetrahydropyran 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED) and low temperature ¹³C and ¹⁹F NMR spectroscopy. Quantum-chemical calculations were carried out both for the isolated species and H-complexes in gas and in polar medium. The predominance of the 1-F_eqMe_ax conformer (1-F_eq:1-F_ax ratio of 65:35, ΔG°?=?0.37?kcal/mol) determined from GED is close to the theoretically estimated conformational equilibrium, especially at the DFT level. In solution, low temperature NMR spectroscopy showed no decoalescence of the signals in ¹³C (down to 95?K) and ¹⁹F NMR spectra (down to 123?K). However, the calculated ¹⁹F chemical shift of ?173.6?ppm for the 1-F_eqMe_ax conformer practically coincides with the experimentally observed value (?173 to ?175?ppm) as distinct from that for the 1-F_axMe_eq conformer (?188.8?ppm), suggesting compound 1 to be anancomeric in solution, in compliance with its theoretical and experimental preference in the gas phase.     

Structural development and kinetic analysis of PbTiO3 powders processed at low-temperature via new sol-gel approach

The synthesis of crystalline lead titanate powder by a generic low-temperature sol-gel approach is developed. Acetoin was added as ligand, instead of the commonly used alkanolamines, to ensure total dissolution of the precursor compounds. The feasibility of the acetoin-Ti isopropoxide complex as a new precursor of PbTiO₃ perovskite particles via sol-gel method has been demonstrated. No excess lead has been introduced. Nanometric PbTiO₃ crystallites have been formed at 400 °C under atmospheric pressure from titanium isopropoxide and lead acetate in alcoholic solution by remarkably low activation energy of crystallization process of 90 kJ mol⁻¹. The powders show tetragonal lattice and dendritic morphology. In addition to the effect of heat-treatment temperature, time, and atmosphere, the sol chemistry particularly influenced the phase composition, particle size, and particle morphology. The use of different ligands significantly modified powder morphology. The extent of the crystallization was quantitatively evaluated by differential thermal analysis and analyzed by Johnson-Mehl-Avrami approach. The crystallization followed two rate regimes depending on the interval of the crystallized fraction.     

Pharmaceutical formulation analysis of thalidomide by solid surface room temperature phosphorimetry

A rapid and simple method of analysis for thalidomide formulations has been reported based on solid surface room temperature phosphorimetry. Thalidomide phosphorescence was enhanced by Hg(II) ions deposited on paper substrates previously treated for background reduction. A calibration curve with a linear dynamic range of three orders of magnitude was obtained (1.37 × 10^–6 M – 10^–3 M) and a 1.8 ng absolute limit of detection was estimated. The recovery of the method was tested in pharmaceutical formulations containing thalidomide as the only active ingredient. The values obtained were 96.3 ± 6.6% (employing the calibration curve procedure) and 98.0 ± 5.3% (employing the standard addition procedure), which show the potential of the technique for the analysis of thalidomide in dosage forms.     

Low-temperature synthetic route based on the amorphous nature of giant species for preparation of lower valence oxides

We examined low-temperature synthetic route based on the amorphous nature of giant species to succeed to prepare Cs blue bronze (Cs_0.3MoO₃), which has never obtained by usual high-temperature methods, at ca. 680 K. Solid solutions (K_1−xRb_x)_0.28MoO₃ and (Li_1−xNa_x)_0.9Mo₆O₁₇ were also obtained at lower temperatures (ca. 670 K). For the latter system consisting of non-isostructural end members, Li_0.9Mo₆O₁₇-structure type solid solution was formed even when 0.25<x<0.70, unlike the case by the usual high-temperature methods. Metastable mixed oxides Ln₂Mo₃O₉ (Ln=La, Gd) were obtained, but not as single phases.     

Photochemistry of Transition-Metal Atoms: Reactions with Molecular Hydrogen and Methane in Low-Temperature Matrices

Reactions of metal atoms with molecular hydrogen and alkanes are prototypical of more complex systems involving metal-mediated reactions in solution, on supports, and on surfaces. The simplicity of metal-atom reactions makes them particularly attractive for fundamental experimental and theoretical studies. Following the first reports of H₂ and CH₄ activation by photoexcited metal atoms in cryogenic matrices, the behavior of a number of transition-metal atoms in their ground and selected electronically excited states has been examined with molecular hydrogen, methane, and ethane. A wealth of structural, electronic, reactivity, and selectivity information is emerging from these studies and it is now becoming possible to recognize the controlling factors at work in these fundamental chemical reactions. In this article, some experimental results for a number of metal-atom systems, in particular, those involving silver, copper, manganese, and iron atoms, are presented along with a discussion of the factors that are believed to be important in the understanding of the observed physical and chemical behavior.     

硫化态Mo/γ-Al2O3催化剂活性位的低温CO-FTIR方法表征及噻吩HDS活性

ＭｏＳ2 是钼基加氢精制催化剂 [Ｃｏ(Ｎｉ)Ｍｏ/γ Ａｌ2 Ｏ3]活性相的主要组份 ,其形态与结构对Ｃｏ(Ｎｉ)Ｍｏ/γ Ａｌ2 Ｏ3的加氢脱硫 (ＨＤＳ)、加氢脱氮 (ＨＤＮ)活性的影响至关重要[1 ,2 ] .目前 ,人们通常使用的ＴＰＲ方法虽可测定过渡金属硫化物的表面活性位 ,并可有效地区分位于催化剂活性相 (ＭｏＳ2 ｓｌａｂ)周边的硫物种 ,但对不同价态的钼尚无法鉴别[3] .本文制备了硫化态Ｍｏ/γ Ａｌ2 Ｏ3催化剂 ,以ＣＯ为探针分子 ,利用ＦＴＩＲ技术在液氮温度 (77Ｋ)下考察了催化剂的表面活性位 (钼物种 )及催化噻吩ＨＤＳ的活性 .1　实…     

双晶型TiO2薄膜的低温制备及表征

采用溶胶-凝胶法低温制备了具有锐钛矿、板钛矿双晶型的TiO2薄膜. 分别利用X射线衍射(XRD)、透射电子显微镜(TEM)、UV-Vis透光率曲线和原子力显微镜(AFM)等手段对所得TiO2溶胶和薄膜进行了性能表征. 采用光催化降解罗丹明B水溶液评价TiO2薄膜的光催化活性. 结果发现, 所得TiO2薄膜具有较高的透明度和光催化活性. 同时, 考察了溶胶回流温度对所制备TiO2薄膜性能的影响, 发现升高溶胶回流温度可以完善薄膜的晶型, 增大薄膜的粗糙度, 从而提高薄膜的光催化活性. 溶胶回流温度为100 ℃时所制备的TiO2薄膜具有最高的光催化活性.     

改性钒钛基SCR催化剂的研究进展

钒钛基选择性催化还原催化剂是目前燃煤电厂应用最为广泛的脱硝催化剂. 由于传统钒钛基催化剂存在低温脱硝效率低?热稳定性差?单质汞氧化效率低?二氧化硫氧化?氨逃逸?碱金属中毒等问题, 人们开始尝试通过对传统 SCR 催化剂进行改性, 以期改善其综合性能. 本文从 (1) 拓宽催化剂的反应温度窗口, 尤其是向低温区扩展, (2) 提高催化剂的热稳定性, (3) 协同氧化单质汞, (4) 控制氨逃逸, (5) 降低 SO2至 SO3的转化率和 (6) 提高催化剂抗碱金属中毒性能等方面综述了改性钒钛基 SCR 催化剂的研究进展, 总结了其催化性能和相关影响机理.研究表明, 某些金属及非金属的改性可以增加钒钛基 SCR 催化剂的表面酸度?活性位点及氧化还原性能, 非金属的掺杂还可以抑制 TiO2载体由锐钛矿向金红石型转化?增加表面氧空位, 从而改善了钒钛基催化剂的低温脱硝性能; 硅?钨?钡和稀土金属等的添加也可抑制 TiO2的金红石化过程, 锆?钾则改变了钒氧化物的存在形态, 抑制其高温聚合, 提高了钒钛基催化剂的热稳定性; 贵金属?过渡金属?金属氯化物及非金属的改性改变了钒钛基催化剂的汞氧化机制, 均可有效促进低氯甚至无氯条件下钒钛基催化剂对单质汞的氧化; 贵金属钌及助剂钼添加的钒钛基催化剂可在维持较高脱硝效率的同时, 实现单质汞及逃逸氨的高效去除, 在 SCR 尾部将逃逸氨选择性氧化生成无害的氮气和水; 被铜?氧化钡?氧化硅等物质改性后, 更多的钒以低价态存在, 使催化剂的氧化还原性能降低, 并抑制了二氧化硫的吸附, 从而减少了三氧化硫的生成;由于具有高储氧能力和氧化还原特性, 还可降低碱金属的吸附量, 铈的掺杂可提高钒钛基催化剂的抗碱金属中毒性能. 此外, 本文还汇总了包括贵金属 (如银?钌)?过渡金属 (如锰?铁?铜?锆等)?稀土金属 (铈?镨) 等金属?金属氯化物 (如氯化铜?氯化钙) 及非金属 (氟?硫?硅等) 改性钒钛基 SCR 催化剂的优缺点. 基于前人研究及作者观点, 改性组分的掺杂有利于进一步提高钒钛基催化剂的综合性能, 具有巨大的发展潜力, 也是在现有基础上实现多污染物控制的方法之一.     

制备方法对MnOx-CeO2催化剂理化性质及低温NH3-SCR脱硝性能的影响

随着人们环保意识的增强,氮氧化物(NOx)的危害引起广泛关注.NOx作为首要的大气污染物之一,主要来源于以燃煤电厂为代表的固定源和以机动车为代表的移动源.它不仅能够导致酸雨和光化学烟雾,而且还是PM2.5的重要前驱体,严重危害人类健康和植物生长.因此,NOx的治理迫在眉睫.研究表明,氨选择性催化还原(NH3-SCR)技术是控制固定源NOx排放最经济有效的方法.商业化V2O5-WO3/TiO2和V2O5-MoO3/TiO2脱硝催化剂的最佳工作温度窗口为300?400℃.因此,NH3-SCR脱硝设施通常安装在除尘器和脱硫装置之前以满足最佳工作温度需要.然而,在这种情况下,脱硝催化剂容易因烟气中的飞灰和含硫化合物堵塞、中毒而失活.此外,对于老电厂增加脱硝设施的改造工程,在除尘器和脱硫装置之前没有足够的空间用于安装脱硝设施.因此,开发环境友好型低温NH3-SCR脱硝催化剂显得尤为重要,因为它可以直接安装在除尘器和脱硫装置之后,从而有效减缓脱硝催化剂失活,有利于改造工程的施工.研究表明,锰基催化剂由于其优异的氧化还原性能和氧迁移能力有利于氧化NO为NO2,促进反应沿着"快速NH3-SCR"途径进行,从而表现出优异的低温脱硝性能.然而,其N2选择性、抗水性能和工作温度窗口还有待改善.因此,开发既具有高催化活性又具有宽工作温度窗口、优异抗水性能以及理想N2选择性的低温脱硝催化剂仍是一个富有挑战性的课题.二氧化铈(CeO2)由于具有优异的氧化还原性能、良好的储/释氧能力、丰富的氧空位以及Ce4+/Ce3+的轻易切换而被广泛用于NH3-SCR反应.因此,将锰氧化物(MnOx)与CeO2相结合而制备的MnOx-CeO2催化剂可能会表现出优异的低温脱硝性能.而催化剂的理化性质和催化性能还强烈地依赖于其制备方法.因此,本文采用不同方法(机械混合法、浸渍法、水热法、共沉淀法以及溶胶-凝胶法)制备了一系列MnOx-CeO2催化剂用于低温NH3-SCR反应,并运用X射线衍射(XRD)、拉曼光谱(Raman)、氮气物理吸附、氢气程序升温还原(H2-TPR)、氨气程序升温脱附(NH3-TPD)、X射线光电子能谱(XPS)以及原位漫反射红外光谱(in situ DRIFTS)等表征技术对催化剂进行了系统分析.重点考察了制备方法对MnOx-CeO2催化剂理化性质和催化性能的影响.结果表明,低温脱硝性能有如下顺序:水热法>溶胶-凝胶法>共沉淀法>浸渍法>机械混合法.这与催化剂表面Ce3+和Mn4+含量、氧空位和表面吸附氧物种浓度以及酸量和酸强度顺序一致.这些性质都与MnOx和CeO2之间的电子相互作用(即Mn3++Ce4+?Mn4++Ce3+)密切相关.特别是水热法制备的MnOx-CeO2催化剂(MnCe-HTM)由于Mnn+掺入到CeO2晶格形成铈基固溶体(含Mn-O-Ce结构)以及水热过程中的高温高压环境加强了MnOx和CeO2之间的电子相互作用,从而表现出优异的理化性质、最佳的低温脱硝性能以及理想的抗水性能.