Role of the heat-transfer coefficient in determining the overall activation energy of thermokinetic oscillations

Thermokinetic oscillations obtained during the heterogeneous catalytic oxidation of methanol on Pd/LiAl₅O₈ in a dynamic calorimeter were characterized by an overall activation energy. This parameter was determined using the minimum and maximum values of the temperature oscillations. Using bifurcation diagrams with oxygen or methanol as bifurcation parameters. E values for all heating loss laws were calculated which are within the range for heterogeneous catalytic oxidation reactions. The obtained results have been discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

活化能与温度关系图的计算机辅助释疑

作者使用计算机绘制了气体分子能量分布曲线图 ,较全面的阐明了活化能与温度的关系。是对无机化学中活化能与温度关系图的改进与补充。     

峰宽法快速测定程序升温脱附活化能

TPD技术^[1]已成为研究固体表面性质的重要手段,本文在总结前人工作的基础上考察了TPD谱峰峰宽和相应参数与脱附活化能(E_d)之间的变化规律,证明用任意峰宽法或半峰宽法测定E_d值有很好的实用性,该法直接运用TPD谱图参数,无需另外作图,故十分快速、简便,与其它方法比较,结果吻合。     

最小二乘法计算苯、噻吩和正辛烷在NaY上程序升温脱附活化能

采用程序升温脱附(TPD)技术测定了苯、噻吩和正辛烷在NaY上以不同升温速率升温时的TPD谱图. 利用TPD谱图的峰形和其微分曲线判断了程序升温脱附过程中的脱附级数. 提出了一种利用最小二乘法计算吸附剂/催化剂的脱附活化能及其动力学参数的方法. 以这些TPD谱图为基础, 分别采用传统TPD计算模型、最小二乘法以及一阶微分曲线法计算了苯、噻吩和正辛烷在NaY上的脱附活化能和动力学参数. 结果表明, 最小二乘法对在不同线性升温速率时的程序升温脱附活化能的计算结果是一致的.     

对活化能相关问题的探析

在介绍阿仑尼乌斯速率方程基础上,从碰撞理论、过渡态理论和托尔曼统计等视角诠释活化能概念,认为仅以活化能大小作依据判断反应速率大小不严密,并从教学层面和试题角度进行了反思。     

化学反应活化能测定实验

通过测定反应温度和碘化钾浓度对反应速率的影响计算活化能,根据公式推导结果消除了试剂浓度对实验结果的影响。用Origin软件对实验数据进行拟合,从图形中得到反应速率、反应浓度、反应温度和活化能的直观对应关系。改进后的实验由单纯的验证性实验变成了自主设计综合性实验。     

利用MGI方法寻找锂离子扩散激活能的材料基因

锂离子电池的应用涉及了正负极和电解质等关键材料,材料中的锂离子扩散属于最核心的微观过程。通过实验测量和理论计算可以找到锂离子扩散激活能小的优秀材料,但是通过一个好的激活能参数,我们不能明确它的影响因素是什么,也无法实现优化现有材料和发现新的锂电材料的目的。本文利用材料基因组计划(MGI)的研究思路,利用第一性原理计算得到的40多种典型体系的激活能参数,结合能带结构特征的计算结果,并通过数据分析,明确了晶格结构中影响锂离子扩散激活能的因素,确定出价带宽度和价带中d电子轨道比例数等基因参数影响激活能的规律。这些结果反映了锂离子材料能带整体特征研究的必要性,也体现了MGI研究方法的优势和特点。     

Differential Non-Linear Isoconversional Procedure for Evaluating the Activation Energy of Non-Isothermal Reactions

A differential isoconversional non-linear procedure for evaluating activation energy from non-isothermal data is suggested. This procedure was applied to model reactions (simulations) and to the dehydration of CaC₂O₄⋅H₂O. The results were compared with those obtained by other isoconversional methods. This revised version was published online in August 2006 with corrections to the Cover Date.     

溯源辨析国内大学化学教材关于碰撞理论中活化能的定义

目前国内化学基础课教材关于碰撞理论中活化能的定义存在严重分歧,简单总结了目前中文教材中的3种定义。通过对碰撞理论形成过程中部分历史背景的梳理,总结出碰撞理论所具备的2个基本要素即气体分子运动论的应用以及碰撞活化假设,然后溯源3种不同定义来源的历史文献,从2个基本要素的角度分析3种定义。研究发现中文教材广泛采用的Lewis活化能解说不承认碰撞活化假设,采用了辐射活化假说,同时表述笼统不严谨,Tolman活化能解说是单纯的统计力学处理,没有应用气体分子运动论,不属于碰撞理论范畴。而简单碰撞理论提出的有效碰撞的最低能量作为活化能的定义更能承载碰撞理论的基本要素。     

气相双原子分子自交换电子转移过程中重组能与活化能的相关分析

基于电子转移过程中的基本特征，提出了标度电子转移过程活化能和重组能的两种精确确定方案，并利用有关实验光谱数据拟合的精确势函数对气相双原子分子自交换过程的能量指标进行了确定．分析表明势能面的非谐性修正是重要的，该方案是合理的，所得结果吻合较好，并证明了重组能与活化能并不存在简单的４倍关系．     

Two Types of Uncertainty in the Values of Activation Energy

The activation energies of the same process are often reported to have different values, which are usually explained by the differences in experimental conditions and sample characteristics. In addition to this type of uncertainty, which is associated with the process (ΔE _process) there is an uncertainty related to the method of computation of the activation energy (ΔE _method). For a method that uses fitting single heating rate data to various reaction models, the value of ΔE _method) method is large enough to explain significant differences in the reported values of the activation energy. This uncertainty is significantly reduced by using multiple heating rate isoconversional methods, which may be recommended for obtaining reference values for the activation energy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of Spontaneous Electrochemical Oscillations by the Wavelet Transformation Method

A new way of analyzing current oscillations that accompany anodic metal dissolution is presented. A wavelet transformation is used for separation and spectral analysis of singular courses—elementary components of electrochemical oscillation recordings. A method of obtaining oscillation energy distribution analogous to the spectral power density function is presented. It is shown that the chaotic component (Shil'nikov chaos) can be separated from the deterministic component of the described process.     

动力学、热力学和活化能在花生壳吸附铅离子实验中的应用

以花生壳为吸附剂、Pb2+为模拟污染物研究了物理化学中固体表面的吸附过程,考察了Pb2+的初始浓度、吸附温度的影响,研究了固体表面吸附反应动力学、热力学和活化能。结果表明:在不同Pb2+初始浓度下,Pb2+在花生壳表面的吸附动力学符合准二级动力学模型,表明该吸附过程是化学吸附为控制步骤的吸附过程。热力学结果表明Pb2+在花生壳表面的吸附是一个自发的放热过程,因为Pb2+由三维运动转变为二维运动导致系统熵减小。活化能Ea=31.35 kJ·mol-1再次证明Pb2+在花生壳表面的吸附是化学吸附过程,这些结果较好地验证了物理化学教学中固体表面的吸附行为。通过该拓展实验加深了学生对固体表面吸附过程的理解,同时,培养了学生的科技创新能力。     

Variation of the Effective Activation Energy Throughout the Glass Transition

Summary: An advanced isoconversional method has been applied to determine the effective activation energies (E) for the glass transition in polystyrene (PS), poly(ethylene terephthalate) (PET), and boron oxide (B₂O₃). The values of E decrease from 280 to 120 kJ · mol⁻¹ in PS, from 1 270 to 550 kJ mol⁻¹ in PET, and from 290 to 200 kJ mol⁻¹ in B₂O₃. It is suggested that a significant variation in E should be observed for the fragile glasses that typically include polymers.

Variation in the effective activation energy of PS, PET, and B₂O₃ with temperature.     

Determination of Activation Energy for Glass Transition of an Epoxy Adhesive Using Dynamic Mechanical Analysis

The activation energy associated with the glass transition relaxation of an epoxy system has been determined by using the three-point bending clamp provided in the recently introduced TA Instruments DMA 2980 dynamic mechanical analyzer. A mathematical expression showing the dependency of modulus measurements on the sample properties and test conditions has also been derived. The experimental results showed that the evaluation of activation energy is affected by the heating rate and test frequency, as well as the criterion by which the glass transition temperature (T _g) is established. It has been found that the activation energy based on the loss tangent (tanδ) peak is more reliable than on the loss modulus (E ₂) peak, as long as the dynamic test conditions do not cause excessive thermal lags. This revised version was published online in August 2006 with corrections to the Cover Date.     

Concentration Dependence of the Activation Energy of Conductivity in Aqueous Sodium Selenite and Potassium Tellurite

The temperature and concentration dependences of conductivity have been studied for aqueous solutions of sodium selenite and potassium tellurite. The corresponding empirical equations have been derived. The activation energies of conductivity in both electrolytes are calculated for different solution concentrations. The ascertained dependence is interpreted in the light of Samoilov's theory on positive and negative hydration of ions. Analysis of the results suggests that the larger the positive hydration of ions, the higher the activation energies of conductivity for the salts.     

一种评估阳离子交换吸附活化能的方法

Ion exchange adsorption is an important physicochemical process at solid/liquid interfaces. In this study, an approach was established to estimate the activation energy of cation exchange reaction on the charged surface, considering Hofmeister effects. The experimental results showed that Hofmeister effects strongly affect the ionic adsorption equilibrium on the charged particle surface. The position of the adsorbed counterion in the diffuse layer was predicted according to the established model, and the ion exchange activation energies for different bivalent cations were estimated via the cation exchange experiments. The activation energy decreases with increasing ion concentration, and the adsorption saturation of cations is a function of the activation energy at different concentrations. The established model of cation exchange adsorption in the present study has universal applicability in solid/liquid interface reactions.     

Activation energy of Se2Ge0.2Sb0.8 chalcogenide glass by differential scanning calorimetry

Results of differential scanning calorimetry (DSC) at different heating rates on Se₂Ge_0.2Sb_0.8 chalcogenide glass are reported and discussed. As the heating rate () changed, also the glass transition temperature (T _g) and onset temperature of crystallization (T _c) changed. As the value of the transition activation energyE _t changed, the crystallization fraction (), heat flow (_q and the crystallization peak temperature (T _p) also changed. The value of the effective activation energy of crystallizationE _c was calculated by means of six different methods. The Se₂Ge_0.2Sb_0.8 chalcogenide glass has two crystallization mechanisms, a one-dimensional and an other surface crystallization growth. The average value ofE _t for Se₂Ge_0.2Sb_0.8 is equal to 194.95±3.9 kJ·mol^–1 and the average value ofE _c is equal to 164±3.3 kJ·mol^–1.     

Comparison of the Activation Energies as Determined by DSC and by Conventional Methods

The results of determination of activation energies (E_A) of polymeric cable insulations obtained by conventional methods (usually based on the evaluation of changes of mechanical properties of insulations after their ageing in thermal chamber at different temperatures) have been compared with results obtained by methods employing the differential scanning calorimetry (DSC). Three DSC methods have been tested: the method according the ASTM E 698; measuring of DSC characteristics in the isothermal mode at several different temperatures; and the method based on evaluation of DSC characteristics of insulations after their thermal ageing in thermal chamber. The last method — which can be called as a modified conventional method, because instead of mechanical properties, the DSC characteristics are determined — has been found as most acceptable and giving similar values of E_A as the other conventional methods. This revised version was published online in August 2006 with corrections to the Cover Date.