A detailed TPD study of H2O and pre-adsorbed O on the stepped Pt(553) surface

Water molecules desorbing from the bare Pt(553) surface desorb in a three peak structure, associated with, respectively, desorption from step and terrace sites and the water multilayer. Upon pre-covering the step sites with O(ad) we mainly observe OH formation on step sites. When terrace sites are also pre-covered with O(ad), OH(terrace) formation is favored over OH(step) formation, presumably because OH formed at terrace sites is more easily incorporated in a hydrogen bonded network of OH/H(2)O. This is a gradual process: with increasing θ(O) less OH(step) is formed. Thus, in spite of the fact that OH at step sites has a higher binding energy than OH at terrace sites, the possibility of the formation of OH at terrace sites actually inhibits the formation of OH at step sites, leaving O(step) as the most stable water dissociation product on the step.     

V(Ⅳ)/V(Ⅴ)电对在石墨毡与导电塑料复合电极上的反应机理

应用循环伏安、极化曲线和交流阻抗等电化学方法研究了V(Ⅳ)/V(Ⅴ)电对在石墨毡复合电极上反应的速控步骤.结果表明,V(Ⅳ)/V(Ⅴ)电对在石墨毡电极上的反应属准可逆过程,且氧化过程包含有后置化学转化步骤;该过程Tafel斜率的实验值为0.124,而理论计算的,以电化学步骤作为控制步骤的Tafel斜率约0.12,两者吻合很好,表明该氧化过程受电化学步骤控制;以等效电路拟合不同极化电位下的交流阻抗,得出该电化学反应阻抗远大于其他阻抗,意味着电化学过程可能是电极反应的控制步骤,与实验得到的极化曲线分析结果相一致.     

Investigation of the Curing Mechanism in the GPTS/APTS Binder System

The acid catalyzed sol-gel reaction in the mixed binder system, 3-glycidoxypropyltrimethoxysilane (GPTS)/3-aminopropyltriethoxysilane (APTS) was investigated and one step and two step synthesis process were compared. Hydrolysis product was observed using the ¹H, ¹³C NMR and Raman spectra. Especially, based on the Raman spectra, epoxy ring opening was observed, varying the ratio of GPTS to APTS. The two step process made clear sol, while the one step process resulted in a milky suspension. According to the Raman spectra, the epoxy ring opening reaction kinetics proceeded slower in the two step process than one step process. Throughout the two step process, it was possible to apply the binder for the coating of substrate.     

Pd(510)台阶面对氢分子解离吸附的影响

本文应用对势方法构造了Ｈ２－Ｐｄ（１００）和Ｈ２－Ｐｄ（５１０）体系相互作用的ＬＥＰＳ势能面．考察了氢分子在Ｐｄ（１００）面上的非活化前驱态解离吸附特性，理论结果和实验符合得很好．对氢分子在Ｐｄ（５１０）台阶面上的解离吸附行为的研究表明，由于台阶的影响，在台阶下形成了非活化直接解离通道，在台阶上及台阶边棱处，主要存在非活化前驱态解离通道；在台阶面上发现了氢分子的活化吸附．     

Accelerating Effect of Tetramethylthiourea on the Kinetics of Zn2+ Electroreduction

The results of kinetic measurements of Zn²⁺ electroreduction on mercury electrode in the presence of tetramethylthiourea showed the two‐step character of the electrode process. The acquisition of nonlinear wide potential range Tafel plots was found to offer a good opportunity to examine the Zn²⁺ electroreduction mechanism. Three mechanisms including the ion transfer step followed by the electron exchange step (IE mechanism), the adsorption step (IA mechanism) and the chemical step followed by electron transfer (CE) were applied. The accelerating influence of tetramethylthiourea on Zn²⁺ electroreduction was found to result from possible complexes formation in the diffuse layer. The ion transport step is probably combined with the loss or exchange of a ligand. The second step probably consists in the electron transfer or adsorption process connected with partial charge transfer.     

Mono C-alkylation and mono C-benzylation of barbituric acids through zinc/acid reduction of acyl, benzylidene, and alkylidene barbiturate intermediates

Through systematic exploration of reaction conditions, very efficient preparative procedures for obtaining large quantities of substituted 5-alkyl and 5-benzylbarbituric acids were developed. The procedure involves a two step preparation in which the second step is zinc dust/acid reduction. For preparation of 5-alkylbarbiturates, the first step is the preparation of either 5-acyl or 5-alkylidenebarbiturate. If 5-benzylbarbiturate is the target product, then the first step includes the preparation of 5-benzylidene. Regardless of the nature of the first step, all reactions presented synthetic yields around 90% and isolation and purification involves only crystallization.     

Preparation of Porous Alumina Film on Aluminum Substrate by Anodization in Oxalic Acid

Self-ordering of the cell arrangement of the anodic porous alumina was preared in oxalic acid solution at a constant porential of 40V and a temperature of 20℃. The honeycomb structure made by one step anodization method and two step anodization method is different. Pores in the alumina film prepared by two step anodization method were more ordered than those by one step anodization method.     

Kinetic coefficient of steps at the Si(111) crystal-melt interface from molecular dynamics simulations

Nonequilibrium molecular dynamics simulations are applied to the investigation of step-flow kinetics at crystal-melt interfaces of silicon, modeled with the Stillinger-Weber potential [Phys. Rev. B 31, 5262 (1985)]. Step kinetic coefficients are calculated from crystallization rates of interfaces that are vicinals of the faceted (111) orientation. These vicinal interfaces contain periodic arrays of bilayer steps, and they are observed to crystallize in a step-flow growth mode at undercoolings lower than 40 K. Kinetic coefficients for both [110] and [121] oriented steps are determined for several values of the average step separation, in the range of 7.7-62.4 A. The values of the step kinetic coefficients are shown to be highly isotropic, and are found to increase with increasing step separation until they saturate at step separations larger than approximately 50 A. The largest step kinetic coefficients are found to be in the range of 0.7-0.8 m(sK), values that are more than five times larger than the kinetic coefficient for the rough (100) crystal-melt interface in the same system. The dependence of step mobility on step separation and the relatively large value of the step kinetic coefficient are discussed in terms of available theoretical models for crystal growth kinetics from the melt.     

Ni(510)台阶面对氢分子解离吸附的影响

应用对势方法获得Ｈ２／Ｎｉ（１００）和Ｈ２／Ｎｉ（５１０）相互作用体系的ＬＥＰＳ势能面，考察了氢分子在Ｎｉ（１００）面上的解离吸附行为。理论计算结果和实验结果符合很好，对Ｈ２／Ｎｉ（５１０）体系的研究表明，台阶的作用为：（１）在台阶下部，氢分子直接解离的最低能量反应途径密度增大，且平动活化垒降低，（２）在台阶上部及台阶边棱处，前驱态的最低能量反应途径密度增大。因此，台阶处成为氢分子解离和复合的活性     

脱氢枞胺键合交联聚苯乙烯树脂的制备

运用高分子反应法，制备脱氢枞胺键合交联聚苯乙烯树脂，反应分三步进行：醚化反应，双键环氧化和脱氢枞胺与环氧基开环反应，探讨了每一步合成的最佳工艺条件，通过红外光谱对该功能高分子进行了结构表征，并对该合成产物进行对映体分离的初步应用。     

Theoretical investigation on the performance of DNA electrophoresis under programmed step electric field strength: Two‐step condition

Programmed step electric field strength is a simple‐to‐use technique that has already been reported to be effective to enhance the efficiency or speed of DNA electrophoresis. However, a global understanding and the details of this technique are still vague. In this paper, we investigated the influence of programmed step electric field strength by theoretical calculation and concentrated on a basic format named as two‐step electric field strength. Both subtypes of two‐step electric field strength conditions were considered. The important parameters, such as peak spacing, peak width, resolution, and migration time, were calculated in theory to understand the performance of DNA electrophoresis under programmed step electric field strength. The influence of two‐step electric field strength on DNA electrophoresis was clearly revealed on a diagram of resolution versus migration time. Both resolution and speed of DNA electrophoresis under two‐step electric field strength conditions are simply expressed by the shape of curves in the diagram. The possible shapes of curve were explored by calculation and shown in this paper. The subtype II of two‐step electric field strength brings drastic variation on the resolution. Its limitations of enhancement and deterioration of resolution were predicted in theory.     

Effect of local environment on nanoscale dynamics at electrochemical interfaces: anisotropic growth and dissolution in the presence of a step providing evidence for a schwoebel-ehrlich barrier at solid/liquid interfaces

The dynamics of nanoscale islands in the vicinity of a monoatomic step on a metal electrode surface under potential control have been investigated by potential pulse perturbation time-resolved scanning tunneling microscopy (P3 TR-STM). As the metastable islands decay, a clear anisotropy develops in the spatial distribution of islands, relative to the monoatomic step. Islands appear to be stabilized above the step, while a denuded region develops below the step over a distance of about 100 nm. Vacancy islands are observed to be more stable than adatom islands. These results provide evidence for an electrochemical Schwoebel-Ehrlich barrier to diffusion of atoms across the step.     

Investigation of the catalytic activity of a Pd/biphenyl-based phosphine system in the Ullmann homocoupling of aryl bromides

We described Ullmann homocoupling promoted by a Pd/biphenyl-based phosphine system using DMF as solvent. Using Hammett equation it is found that the rate determining step of the reaction depends on the electronic nature of substituents of aryl bromides. Increase the rate of reaction with decreasing the electron donating of the substituent from NH₂ to H suggesting an oxidative addition step as the rate determining step. Decrease the rate of reaction with increasing the electron-withdrawing ability of the substituent from H to NO₂ indicating a reductive elimination step as the rate determining step.     

Anisotropic diffusion of n-butane and n-decane on a stepped metal surface

The diffusion of single n-butane and n-decane molecules on a model stepped surface, Pt655, and on a corresponding flat surface, Pt111, is investigated using molecular-dynamics simulations and anisotropic united atom model. The surface step on Pt655 causes the alkane molecules to adsorb on the lower terrace in all-trans conformations with their long molecular axes adjacent and parallel to the step edge, and to diffuse anisotropically along the surface step via a constant wiggly motion without rotation or marked deviation from the parallel adsorption configuration. At relatively high temperatures, the alkane molecules can temporarily break away from the step edge but cannot migrate across the step edge in either the downstair or upstair direction. In comparison with the diffusion on Pt111, the diffusivity of n-decane is reduced by the surface step but its diffusion barrier is hardly affected. In the case of the shorter n-butane, however, the surface step significantly reduces the diffusion energy barrier and gives rise to higher diffusion coefficients at lower temperatures. Important implications of the simulation results are discussed.     

HNCO热解为CO2和HNCNH的反应机理理论研究

利用从头算ＲＨＦ／３－２１Ｇ方法研究了ＨＮＣＯ二聚后生成ＨＮＣＮＨ和ＣＯ２的反应机理。计算表明,该反应是分步反应,由反应物经第一过渡态生成四元环中间体,再经过第二过渡态分解为产物,与实验得到的结论一致。反应的第一步是速度控制步骤,计算得到的活化位垒为１７２．５５ｋＪ·ｍｏｌ＾－１,与实验上测得的１７６．４０±１６．３０ｋＪ·ｍｏｌ＾－１相吻合。反应的第二位垒为８３．６８ｋＪ·ｍｏｌ＾－１,在实验条     

Oxidation of benzyl alcohol by Cr<Superscript>VI</Superscript> supported on Ambersep 900 (O<Superscript>−</Superscript>H) – a kinetic and mechanistic investigation

The oxidation of benzyl alcohol was found to proceed through ester formation. In a slow step the ester decomposes and produces chromium(IV). Chromium(IV) formed in a slow step, oxidizes another molecule of benzyl alcohol because our oxidant was supported on polymeric material and generates a free radical in a fast step. The free radical subsequently reacts with another oxidant site in a polymeric reagent in a fast step and forms chromium(V). In a last step the intermediate chromium(V) reacts with benzyl alcohol to produce benzaldehyde. The mechanism was predicted and activation parameters were also determined.     

The electrochemical reduction of perezone in the presence of benzoic acid in acetonitrile

An electrochemical study using transient techniques of a quinone-type natural product, perezone, has been performed in acetonitrile and in the presence of benzoic acid. Using linear sweep voltametry and single potential step chronoamperometry, it was possible to establish that the reduction mechanism of perezone involves a monoelectronic charge-transfer step, followed by a protonation step and homogeneous charge transfer due to disproportionation of the protonated intermediate. The mechanism for the homogeneous charge-transfer step was found to be of the type DISP1 (disproportionation order one) from the results of double potential step chronoamperometry experiments. The occurrence of the DISP1 mechanism was provoked by the mildly acidic medium used in this study.     

用热重分析法研究氢氧化镧的热分解过程

在研究固相交换反应过程中,我们对氢氧化镧进行了热重分析,发现氢氧化镧的热分解过程是分两步进行的,由其热重曲线计算了氢氧化镧热分解反应的活化能和反应级数。     

The statistical theory of linear capillary chromatography with uniform stationary phase

Based on the random walk model and probability theory, general relations between the moments of column residence time and the moments of step sojourn time and step displacement are established. And starting from the mass-balances principle of solute molecules in the mobile and stationary phases, the moments of step sojourn time and step displacement are derived and expressed in terms of the basic parameters. Substituting the step moments into the general relations, the moments of column residence time are then obtained. The expression of retention time is completely identical to the well-known, the expressions of second moment or HETP unite and generalize various expressions of stochastic theory and mass balance theory, and the third and forth moments are given in more exact form.     

Analysis of electrocatalytic reaction schemes: distinction between rate-determining and potential-determining steps

This paper discusses the concepts of rate-determining step and potential-determining step in the context of electrocatalytic reaction schemes, illustrates how the simpler concept of potential-determining step often captures the essence of a bottleneck in a reaction scheme, and thereby provides straightforward hints for developing better catalysts.