动力学控制下焦炭颗粒气化特性的模型研究

焦炭颗粒气化反应属于典型的非催化气固反应,固体结构会随着反应发生变化,颗粒的气化模型包含了体现固体颗粒表面积变化的结构因子。建立了具体的焦炭颗粒与二氧化碳气化过程的扩散 反应模型,并将研究的内容限定在动力学控制区域。该模型的特点在于其反应速率项采用了形式简单的结构因子,模型结果与随机孔模型对比后显示了很好的精确性。利用该模型考察了不同初始孔隙率的焦炭颗粒的转化率变化特性以及固定初始孔隙率下,气化温度和气化剂分压对焦炭颗粒气化的影响。     

粒度对多相反应动力学参数的影响

以纳米氧化锌与硫酸氢钠溶液为反应体系, 研究反应物粒度对动力学参数的影响规律. 讨论了表观活化能降低的原因. 结果表明：当反应物粒径、反应温度和搅拌速率一定时, 纳米氧化锌与硫酸氢钠溶液的反应速率仅与反应物的浓度有关；反应物粒度对多相反应的反应级数、速率常数、表观活化能和指前因子均有较大的影响；随着反应物粒径的减小, 表观活化能和指前因子减小, 而反应级数和速率常数增大, 并且速率常数和表观活化能与反应物粒径的倒数呈线性关系；反应物粒度是通过摩尔表面积、摩尔表面能和摩尔表面熵三个方面影响多相反应的动力学参数的.     

Particle size limitations due to heat transfer in determining pyrolysis kinetics of biomass

A simplified heat transfer model is analyzed in order to determine an upper bound for biomass particle size in conducting experimental pyrolysis kinetics. In determining intrinsic kinetic rates, it is desirable that the entire particle be at reactor temperature for the duration of the chemical reaction. By comparing characteristic times for reaction rates versus heat-up rates, an approximate boundary for particle size can be constructed as a function of temperature; above this boundary, the reaction rate is strongly heat transfer dominated, and below the boundary the reaction rate is kinetically controlled. Using parameters for cellulose pyrolysis, it is estimated that a 200 μm particle will be heat transfer limited due to internal heat transfer at temperatures above 500°C. This boundary applies for conditions where the surface of the particle is brought to reactor temperature instantaneously. Using our specific experimental conditions, it is found that the limitations imposed by external transfer are reached before those predicted by assuming that only internal heat transfer is limiting. In examining wood pyrolysis, where a global reaction rate approximation is insufficient, it is experimentally shown that the decomposition for hemicellulose can be transport limited, while cellulose remains in the kinetic controlled regime.     

Preparation of Nano‐bismuth with Different Particle Sizes and the Size Dependent Electrochemical Thermodynamics

Nano‐bismuth has excellent electrochemical properties. However, it is still unclear how the particle size of nano‐bismuth influences its electrochemical thermodynamic properties. In this paper, spherical bismuth nanoparticles with different particle sizes were prepared by solvothermal method; the electrode potentials, the temperature coefficients of the electrode potentials and the thermodynamic functions of reaction for nano‐bismuth electrodes with different particle sizes at different temperatures were determined; and the effects of particle size on the electrode potential, the temperature coefficient and the thermodynamic functions were discussed. The experimental results show that particle size of bismuth nanoparticles has a significant influences on the electrochemical thermodynamic properties. The standard electrode potential of the nano‐bismuth electrode with a diameter of 39.9 nm was 0.009 V lower than that of the ordinary standard electrode (0.308 V); the temperature coefficient of the electrode potential with a diameter of 39.9 nm was nearly double that of 85.9 nm. With the particle sizes decrease, the standard molar Gibbs energy of reaction, the standard molar enthalpy of reaction, the standard molar entropy of reaction, the molar reversible reaction heat and the temperature coefficient increase; and these quantities are linearly related to the reciprocal of the particle diameter.     

Size-dependent catalytic behavior of platinum nanoparticles on the hexacyanoferrate(III)/thiosulfate redox reaction

Platinum nanoparticles prepared in reverse micelles have been used as catalysts for the electron transfer reaction between hexacyanoferrate(III) and thiosulfate ions. Nanoparticles of average diameter ranging between 10 and 80 nm have been used as catalysts. The kinetic study of the catalytic reaction showed that for a fixed mass of catalyst the catalytic rate did not increase proportionately to the decrease in particle size over the whole range from 10 to 80 nm. The maximum reaction rate has been observed for average particle diameter of about 38 nm. Particles below diameter 38 nm exhibit a trend of decreasing reaction rate with the decrease in particle size, while those above diameter 38 nm show a steady decline of reaction rate with increasing size. It has been postulated that in the case of particles of average size less than 38 nm diameter, a downward shift of Fermi level with a consequent increase of band gap energy takes place. As a result, the particles require more energy to pump electrons to the adsorbed ions for the electron transfer reaction. This leads to a reduced reaction rate catalyzed by smaller particles. On the other hand, for nanoparticles above diameter 38 nm, the change of Fermi level is not appreciable. These particles exhibit less surface area for adsorption as the particle size is increased. As a result, the catalytic efficiency of the particles is also decreased with increased particle size. The activation energies for the reaction catalyzed by platinum nanoparticles of diameters 12 and 30 nm are about 18 and 4.8 kJ/mol, respectively, indicating that the catalytic efficiency of 12-nm-diameter platinum particles is less than that of particles of diameter 30 nm. Extremely slow reaction rate of uncatalyzed reaction has been manifested through a larger activation energy of about 40 kJ/mol for the reaction.     

Ignition of highly reactive metal particles

It was shown that, with increasing reaction rate constant above a certain limit, the bulk ignition mechanism changes to a surface mechanism. In this case, surface layers of a particle undergo local heating and a reaction front forms and propagates into the particle. The time of the frontal burnout of the particle is shorter than the time of its bulk burnout, during which the temperature field relaxes instantaneously.     

The influence of alumina passivation on nano-Al/Teflon reactions

The reaction kinetics of aluminum (Al) and polytetrafluoroethylene (PTFE or Teflon) were recently examined using nanoparticles of both Al and Teflon. Results showed a unique pre-ignition reaction (PIR) associated with the nano-Al/Teflon mixture that was not significant in the micron-Al/Teflon mixture. The PIR is caused by fluorination of the alumina (Al₂O₃) shell passivating the Al particles and reduces the onset temperature of Al ignition for nano compared with micron particle mixtures. Because the alumina shell was found to play a key role in the reaction mechanism, this communication extends our understanding of the interaction between alumina and Teflon by examining the influence of alumina particle size, and therefore surface area, on the fluorination reaction with Teflon. Differential scanning calorimetry analysis show that reaction kinetics vary dramatically as the alumina particle size is reduced from 50 to 15 nm diameter. Specifically, for 15 nm diameter alumina, the first exotherm (corresponding to the PIR) exhibits three times more heat of reaction than for the 30, 40, or 50 nm alumina particles. These results show how particle size and specific surface area affect the Al–Teflon reaction mechanism.     

分散聚合制备粒度均匀的聚甲基丙烯酸环氧丙酯微球

文中描述了粒度均匀的聚甲基丙烯酸环氧丙酯微球的制备,所采用的是分散聚合方法,系统地研究了溶剂体系、单体浓度、引发剂类型与浓度、稳定剂用量、反应温度等各种聚合参数,对聚合产物粒度及其分散性的影响．在优化反应条件的基础上,制备出了微米级（１～８μｍ）粒度均匀性基本呈现单分散的聚合物微球．     

烧结程度对CaO固硫反应转化率及动力学参数的影响

The conversion and kinetic parameters of desulfurizors of CaO of different particle agglomeration degree are investigated with themogravimetric method (TG). The results showed that the CaO particle agglomeration degree increases when CaO calcined temperature or time increases. The dusulfurizors that have higher particle agglomeration degree have low conversion in the desulfurization reaction. The kinetic behavior of desulfurization can be explained by a grain model. The activity energies of suface reaction (Ea) and of product layer diffusion (Ep) were determined by using the grain model. The overall rates of desulfurization are controlled initially by surface chemical reaction, and then shift to product layer diffusion control. The activity energy of surface reaction (Ea) enhances when the CaO particle agglomeration degree increases.     

超声功率对石灰乳活性影响研究

为获得高活性度的石灰乳,在石灰乳化过程引入超声作用,分别采用沉降实验、粒度分析和表观形态观察、中和反应活性实验对超声作用后石灰乳的性质进行了分析,比较机械搅拌和不同超声功率作用下石灰乳的分散稳定性、中和反应活性及粒度分布情况,以考察超声作用对石灰乳化过程和石灰乳性质的影响。结果表明:与机械搅拌相比,在一定功率范围内,超声作用可降低石灰乳的沉降速率,提高其分散稳定性;减小石灰乳体系中未溶解Ca(OH)₂乳颗粒的粒径,并缩小其粒径的分布范围,抑制石灰乳结块的产生;超声作用还增加了Ca(OH)₂颗粒的比表面积和溶解速度,进而提高石灰乳的中和反应活性。     

Nonlinear, electrocatalytic swimming in the presence of salt

A small, bimetallic particle in a hydrogen peroxide solution can propel itself by means of an electrocatalytic reaction. The swimming is driven by a flux of ions around the particle. We model this process for the presence of a monovalent salt, where reaction-driven proton currents induce salt ion currents. A theory for thin diffuse layers is employed, which yields nonlinear, coupled transport equations. The boundary conditions include a compact Stern layer of adsorbed ions. Electrochemical processes on the particle surface are modeled with a first order reaction of the Butler-Volmer type. The equations are solved numerically for the swimming speed. An analytical approximation is derived under the assumption that the decomposition of hydrogen peroxide occurs mainly without inducing an electric current. We find that the swimming speed increases linearly with hydrogen peroxide concentration for small concentrations. The influence of ion diffusion on the reaction rate can lead to a concave shape of the function of speed vs. hydrogen peroxide concentration. The compact layer of ions on the particle diminishes the reaction rate and consequently reduces the speed. Our results are consistent with published experimental data.     

Mesoscale simulation of polymer reaction equilibrium: combining dissipative particle dynamics with reaction ensemble Monte Carlo. I. Polydispersed polymer systems

We present a mesoscale simulation technique, called the reaction ensemble dissipative particle dynamics (RxDPD) method, for studying reaction equilibrium of polymer systems. The RxDPD method combines elements of dissipative particle dynamics (DPD) and reaction ensemble Monte Carlo (RxMC), allowing for the determination of both static and dynamical properties of a polymer system. The RxDPD method is demonstrated by considering several simple polydispersed homopolymer systems. RxDPD can be used to predict the polydispersity due to various effects, including solvents, additives, temperature, pressure, shear, and confinement. Extensions of the method to other polymer systems are straightforward, including grafted, cross-linked polymers, and block copolymers. To simulate polydispersity, the system contains full polymer chains and a single fractional polymer chain, i.e., a polymer chain with a single fractional DPD particle. The fractional particle is coupled to the system via a coupling parameter that varies between zero (no interaction between the fractional particle and the other particles in the system) and one (full interaction between the fractional particle and the other particles in the system). The time evolution of the system is governed by the DPD equations of motion, accompanied by changes in the coupling parameter. The coupling-parameter changes are either accepted with a probability derived from the grand canonical partition function or governed by an equation of motion derived from the extended Lagrangian. The coupling-parameter changes mimic forward and reverse reaction steps, as in RxMC simulations.     

粒度对纳米氧化镍与硫酸氢钠水溶液反应动力学的影响

以纳米氧化镍与硫酸氢钠水溶液为反应体系,研究了不同粒度反应物反应的动力学参数,并讨论了粒度对动力学参数的影响.结果表明,反应物粒度对该反应的速率常数、指前因子和表观活化能均有显著的影响;随着反应物粒径的减小,速率常数增大,指前因子和表观活化能减小,且指前因子的对数和表观活化能分别与反应物粒径的倒数呈线性关系.     

Observation in the Growth of Selenium Nanoparticles

A mild chemical reduction method was developed to synthesize selenium nanoparticles. The observation of the particle growth within a five‐minute time frame at an elevated temperature is achieved, and it was evidenced by analyzing the particle sizes from electron micrographs. The samples at individual growth stages were taken after the reaction was quenched by cooling to stop further particle growth. The nucleation and growth processes dominate different reaction time regimes under the experimental condition. We demonstrate the size evolution of the selenium nanoparticles from 30 nm to about 200 nm.     

Preparation of large monodispersed spherical silica particles using seed particle growth

To obtain large-sized, monodispersed spherical particles of silica by sol precipitation, a seed particle growth method was attempted. The formation of secondary particles during seed particle growth causing a multimodal distribution of particle size was suppressed via fine adjustment of the reaction conditions, such as TEOS, ammonia, and water concentrations, as well as operational conditions such as feeding time and agitation speed. Among the reaction conditions, an increase of TEOS concentration promoted secondary particle formation, resulting in bimodal particle distribution. However, secondary particle formation was depressed with increasing ammonia and water concentrations. In addition, long feeding time (low feed flow rate) and rigorous agitation significantly reduced secondary particle formation because they contributed to the slow generation of supersaturation and rapid seed particle growth, respectively.     

超细Cu-ZnO-ZrO2催化剂上甲醇合成的TPSR和TPD研究

采用ＭＳ－ＴＰＳＲ和ＭＳ－ＴＰＤ技术在不同粒度的超细Ｃｕ－ＺｎＯ－ＺｒＯ２催化上考查了ＣＯ２和ＣＯ加氢合成甲醇的反应过程和吸附活化特征。研究表明,ＣＯ２和ＣＯ都可以直接加氢合成甲醇。     

Particle stability in polymer-assisted reverse colorimetric DNA assays

“Reverse” colorimetric DNA detection by the formation of core-shell particles upon DNA hybridization is described. Specifically, the assay is based on a strategy to covalently link polymer reaction initiators to suspended nanoparticles upon DNA hybridization. These initiators then prompt polymer chain growth to form a thick polymer shell outside of particles, acting as the physical barrier to keep Au particles apart. Particles without DNA hybridization aggregate, accompanied by a pronounced solution color change from red to blue. The focus of this report is to address reaction kinetics of two co-occurring processes: polymer growth and particle aggregation during the reverse colorimetric DNA assay. The results show that Cu ions used as the polymerization catalyst bind strongly to the bases in DNA molecules, resulting in crosslinking of DNA-attached gold nanoparticles and their subsequent precipitation. Both Cu-ion-assisted particle aggregation and polymer growth are found to depend strongly on Cu ion concentration, salt concentration, and reaction temperature. Under the optimized conditions, faster polymer chain growth on the surface overcomes particle aggregation and preserves particle stability via steric stabilization.     

Polydimethylsiloxane Soft Template Assisted Synthesis of Silver Nanoparticles via Vapor‐Solid Reaction Growth

A new route to synthesize silver nanoparticles via vapor‐solid reaction growth (VSRG) is reported. Employing AgCl to react with (Me₃Si)₄Si in a sealed tube (433–673 K) generates silver particles (diameter ≥ 10 μm). Addition of polydimethylsiloxnae (PDMS) to the reaction dramatically reduces the particle size (diameter 60–1200 nm). Moreover, formation of silver nanorods and nanocubes was discovered in some reaction conditions. Adding PDMS and varying the reaction temperature were the key factors influencing the Ag particle size.     

Role of emulsifier on the particle size and polymer particle size distribution using multiwavelength spectroscopy measurements

Latex emulsions depend strongly on the polymer composition, and particle size distribution, which in turn, is a function of the preparation of the latex and on the formulation and composition variables. This study reports measurements of particle size and particle size distribution of latex emulsions as function of the reaction time and the type and concentration of emulsifier by using the multiwavelength spectroscopy technique. Results show changes in the particle size of latex emulsions with the reaction time, obtaining larger particles and broader distributions with increasing of Tween 80 ratio. The steric stabilization provides the sole nonionic emulsifier is not enough to protect the polymer particle, causing the flocculation among the interactive particles, resulting in unstable latex. However, latex emulsions prepared with Tween 80 ratio <70 wt.% can stabilize efficiently the nucleated particles, probably due to the effects provided by both, the electrostatic and steric stabilization mechanisms. The same effect is shown in the curves of conversion (%) as a function of reaction time, resulting in slower polymerization rate for Tween 80 ratio >70 wt.%. On the other hand, smaller polymer particles, in all range of emulsifier mixture, have been obtained to higher emulsifier concentration.     

Functionalization of latex particle surface by using polymeric inisurfs

In this work the emulsion polymerisation of styrene was carried out in presence of the inisurfs (reactive surfactants). The reaction kinetics was followed by dilatometric and calorimetric measurements. The influence of the inisurf concentration in reaction mixture as well as the reaction temperature on the particle properties were studied. Resulting particles were characterised with FFFF‐MALLS and PCS. Viscosity measurements indicate the formation of particle flocks in the solution at certain inisurf concentration in reaction mixture.