具有不同活性中心的固体催化剂的失活模式

 An approach is suggested to distinguish different types of active sites responsible for different reactions on bifunctional catalysts. The model assumes a non-uniform vulnerability of active sites that depends on their location. Problems on the relationship between the dispersion of the active phase and selectivity are discussed. The effect of coke formation on the activity change of different sites is analyzed.     

O2和CO表面催化反应的活性位分布模型

对于O_2和CO表面催化反应，建立了一个新的不可逆Monte Carlo模拟模型。在 二维格子中，引进了表面活性位和非活性位的要领。模型假设，一定浓度的活性位 随机分布在非活性位上，形成了活性位分布的二维格子模型反应器，并在ZGB模型 的基础上，考虑了氧原子和CO分子的表面扩散，特别是引进了吸附粒子的定向表面 扩散。其中，活性位和活性位最近邻是表面吸附物质反应的活性中心，而非活性位 的作用是通过表面扩散传质。当活性位浓度C_a = 1且考虑扩散时，模型还原为增 加了扩散的ZGB模型。当活性位浓度C_a = 1且只考虑氧的扩散时，模拟结果表明， 扩散几率达到某一数值（0.3）时，二级相变点完全消失。当活性位浓度C_a逐渐减 小时，单位活性位产生的CO_2的速率不断增大，这表明活性位的利用率提高了。     

纳米Pd和Au催化CO2还原粒径效应的密度泛函理论计算研究

以可再生能源为能量来源,在水溶液中进行的光(电)催化CO₂还原生成高附加值化学品和燃料是解决能源危机与环境污染的有效途径之一.CO是一种简单却很重要的CO₂还原产物,它可以作为水煤气变换反应与费托合成的重要原料.具有较高CO选择性的贵金属纳米颗粒催化剂(如Au和Pd)一直受到研究者的广泛关注.一般来说,金属颗粒催化剂的催化性能与粒径大小密切相关,即所谓的粒径效应.然而在实际的理论计算研究中,由于受到计算能力的限制,催化剂模型都仅局限于简单的周期性模型或小的金属团簇模型,无法准确描述真实颗粒上复杂的反应位点的性质,导致了对催化行为的误解.因此,建立更加真实的颗粒模型对探究纳米颗粒催化剂上活性位点的性质,解释其粒径效应至关重要.本文旨在阐述Au与Pd纳米颗粒催化剂不同活性位点上CO₂还原反应与产H2副反应的竞争机制,并解释Au与Pd纳米颗粒催化剂在CO₂电还原中表现出不同粒径效应的原因.本文基于密度泛函理论,采用VASP软件,BEEF-vdW泛函进行计算.分别建立了原子数为55,147,309和561的颗粒模型和高CO^*覆盖度模型,避免了传统周期性模型的局限性,探究了金属颗粒催化剂不同反应位点上的CO选择性.结果表明,对于颗粒模型来说,(100)位点对CO的选择性优于边缘位点;但对于周期性模型来说,Au(211)对CO的选择性则优于Au(100).产生这种反差的主要原因在于Au颗粒的边缘位点对H*的吸附过强.通过对比,我们直观地展现了颗粒模型上平面位点和Edge位点与相对应的周期性模型上CO选择性的区别,突出了模型选择对揭示活性位点性质的重要性.在此基础上,通过计算理论CO法拉第效率,发现Au颗粒随着粒径的减小,CO选择性降低,与实验的趋势一致.对于Pd催化剂来说,低覆盖度模型无法正确预测活性位点的性质;而高CO覆盖度的情况下,Pd颗粒的边缘位点对COOH^*吸附能更强,这是导致边缘位点上CO选择性更高的主要原因.同样通过计算理论CO法拉第效率,发现随着粒径的减小,Pd颗粒上CO选择性升高.本文不仅成功揭示了Au与Pd颗粒催化剂上活性位点的性质,对粒径效应做出了合理解释,也强调了合理的计算模型是理论研究的基础.     

Kinetic inhomogeneity of copolymerization sites of butadiene and isoprene on titanium catalyst

Copolymerization of butadiene and isoprene on the titanium catalyst proceeds on three types of active sites, and this correspond to the standard set of homopolymerization of isoprene. Sites responsible for the preparation of low-molecular-mass fractions of macromolecules are more active in the copolymerization as compared with homopolymerization of butadiene and isoprene. Change in the hydrodynamic regime in the reaction region is accompanied by an increase in the copolymerization rate due to an increase in the over-all concentration of active sites but it does not affect the typical set of growth sites of macromolecules. Distribution curve of active sites with respect to their reactivity is shifted to the high-molecular-mass region with increasing average weight molecular mass with decreasing average number molecular mass when the corresponding molecular-mass distribution becomes broader.     

氮、 硼掺杂石墨烯活性位点及其在氧化反应中作用的研究进展

随着绿色化学的逐渐推广,碳材料作为最有前途的绿色无金属催化剂而备受关注。通过对石墨烯引入杂原子进行化学掺杂是目前最常用于改良碳材料催化活性的有效方法。从结构上看,掺杂石墨烯内特定活性物种在催化过程中起到活性位点的作用,且催化剂的催化活性随活性位点含量增加而增强。且其内部活性位点含量可通过改变制备方法中制备条件实现调控,这有助于开发具有高催化活性的掺杂石墨烯催化剂。本文综述了氮掺杂石墨烯和硼掺杂石墨烯内可作为活性位点的官能团,提出制备方法对活性位点含量的影响,并讨论了内部活性位点在氧化反应中的作用。最后对未来研究方向提出了建议和展望,为开发更高效掺杂石墨烯催化剂提供了思路。     

Stripping voltammetry of carbon monoxide oxidation on stepped platinum single-crystal electrodes in alkaline solution

The electrochemical oxidation of a CO adlayer on Pt[n(111)x(111)] electrodes, with n = 30, 10, and 5, Pt(111), Pt(110) as well as a Pt(553) electrode (with steps of (100) orientation) in alkaline solution (0.1 M NaOH) has been studied using stripping voltammetry. On these electrodes, it is possible to distinguish CO oxidation at four different active oxidation sites on the surface, i.e. sites with (111), (110) and (100) orientation, and kink sites. The least active site for CO oxidation is the (111) terrace site. Steps sites are more active than the (111) terrace sites, the (110) site oxidizing CO at lower potential than the (100) site. The CO oxidation feature with the lowest overpotential (oxidation potential as low as 0.35 V vs. RHE) was ascribed to oxidation of CO at kink sites. The amount of CO oxidized at the active step or kink sites vs. the amount of CO oxidized at the (111) terrace sites depends on the concentration of the active sites and the time given for the terrace-bound CO to reach the active site. By performing CO stripping on the stepped surfaces at different scan rates, the role of CO surface diffusion is probed. The possible role of electronic effects in explaining the unusual activity and dynamics of CO adlayer oxidation in alkaline solution is discussed.     

纳米Ni－B非晶态合金的催化性质研究

纳米Ｎｉ－Ｂ非晶态合金的催化性质研究杨军,崔黎丽,邓景发（复旦大学化学系,第二军医大学药学院,上海,２００４３３）关键词纳米材料,非晶态合金,催化性质纳米非晶材料融合了纳米晶与非晶材料的特性,既有很高的表面原子比,又呈高度几何无序状态。与普通非晶材料...     

Selectivity effects related to the diversity of active sites operating on nanostructured multifunctional catalysts

The goal of the present study was to gain a better understanding of the selectivity of processes over multifunctional catalysts exhibiting diversity of operating active sites. The concept is that the concurrent performance of different types of active sites may provoke effects on the process selectivity comparable to the effects resulting from the kinetic regularities and activation energies of the occurring reactions. Accordingly, in the kinetic model the authors introduce specific parameters reflecting the contribution of distinct types of active sites, facilitating different reaction routes. Reasons are adduced how suchlike parameters serve to account the impact of various reaction routes occurring on different types of sites. The suggested approach links the deactivation-caused selectivity changes to dissimilarities in the vulnerability of different types of active sites. This work relates the probabilities for action of different types of sites to the size of active-phase islands. Various reaction mechanism patterns are modeled to examine relevant selectivity effects.     

Kinetic evaluation of various isospecific active sites on MgCl2-supported Ziegler catalysts

The formation, variation and conversion of isospecific active sites were investigated, based on the isotacticity distribution of the polypropenes analyzed by the temperature rising elution fractionation (TREF) method. Stopped-flow polymerization of propene was carried out with a MgCl₂-supported Ziegler catalyst in the absence or presence of an internal or external electron donor so that the effects and roles of the electron donors could be clarified. The results showed that various kinds of active sites with different isospecificities, including the highest isospecific active sites responsible for producing the highest isotactic fraction (elution temperature: > 112°C) existed, even in the electron donor-free catalyst system. The isospecificity of the active sites in the donor-free catalyst might have originated from a surface monolayer multinuclear titanium species, namely an “island” of titanium species. The addition of the external electron donor converted a part of the aspecific and/or low isospecific active sites into the second highest isospecific active sites, but showed no effect on the formation of the highest isospecific active sites, whereas the addition of an internal donor played an important role in creating the highest isospecific active sites as well as suppressing the formation of the aspecific active sites.     

Estimation of the energetic nonhomogeneity of the surface of oxide and carbon materials

The molecular statistical method for evaluating the distribution of active sites of various adsorbents relative to their energies has been improved. This method is used not only for the treatment of experimental data on the adsorption of hydrocarbons on various adsorbents, which is the usual procedure, but also data on the adsorption of polar water and methanol molecules on the active sites of adsorbent surfaces. Two types of active sites differing in energy have been shown to exist on the surface of graphitized carbon black, the complex shungite carbon/mineral adsorbent, and modified Silochrom. Chromatographic, calorimetric, and structural adsorption data were used to establish the relationship between the observed maxima of the energy distribution function of the adsorption sites with concrete adsorption sites or pores of the surface, on which the molecules are adsorbed. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 5, pp. 315–320, September–October, 2008.     

Formation, deactivation and transformation of stereospecific active sites on TiCl4/dibutylphthalate/Mg(OEt)2 catalyst induced by short time reaction with Al-alkyl cocatalyst

Formation, deactivation and transformation of stereospecific active sites on TiCl₄/dibutylphthalate (DBP)/Mg(OEt)₂ Ziegler–Natta catalyst induced by short time reaction with triethylaluminum (TEA) cocatalyst (with TEA pretreatment time from 0 to 600 s) were investigated by stopped-flow propene polymerization combined with temperature rising elution fractionation (TREF) and GPC methods. It was demonstrated that both formation and deactivation of active sites with broad multiplicity in isospecificity on the catalyst are slow reactions with an induction period of ca. 0.2 s. It was most important to find that the formation of active sites with the highest isospecificity strongly depends on the interaction between the catalyst and cocatalyst (up to 60 s of pretreatment) even in the presence of internal donor. This newly observed phenomenon (according to our knowledge) suggested that the transformation of monometallic active sites (aspecific or less isospecific) into bimetallic active sites (highly isospecific) through reversible complexing with TEA cocatalyst (or its reaction product diethylaluminum chloride (DEAC)) in Ziegler–Natta catalysts cannot be overlooked even in the presence of internal electron donor. The existence of –OC₂H₅ ligand in the catalyst most probably gave birth to a new group of active titanium species. The stability of active sites increases with increasing isospecificity in the early stage of pretreatment (up to 60 s of pretreatment). While all the active sites became relatively stable in the later stage of pretreatment (from 60 to 600 s of pretreatment). The extraction of internal donor DBP by TEA from the catalyst within the pretreatment procedure is found to initiate from 60 s of pretreatment resulting in slight transformation of isospecific active sites into aspecific sites.     

甲烷在氧化镧上C-H断键活性中心的研究

在不同条件下制备了一系列La~2O~3样品,以CH~4与CD~4间的氢原子交换反应作为测试反应来测量La~2O~3对CH~4分子中C-H键的断键能力.采用NH~3-TPD,CO~2-TPD和NH~3,CO~2对催化剂的中毒反应来研究La~2O~3表面的酸中心和活性中心.La~2O~3的表面组成用XPS测量.实验发现尽管酸中心在活性中心只占较小的部分. 但它与碱中心一样对于CH~4分子的C-H断键是必不可少的.本文提出, 组成活性中心的酸中心各碱中心很可能分别是一部分具有较强酸性的晶格La^3^+和具有较强碱性的晶格O^2^-,而La~2O~3晶体中晶格缺陷的形成能使更多的晶格离子成为活性中心.     

Co-Mo/γ-Al2O3催化剂的原位红外光谱表征研究

采用CO和NO作为探针分子,应用原位红外光谱法(in-situ FT-IR)和程序升温还原(H₂-TPR)对Mo/γ-Al₂O₃和Co-Mo/γ-Al₂O₃加氢催化剂进行表征,并对催化剂进行了加氢脱硫(HDS)活性评价。实验结果表明,在Co-Mo/γ-Al₂O₃催化剂表面存在三个吸附位;在Mo/γ-Al₂O₃催化剂中加入助剂钴对钼吸附位起到显著的改性作用,并且引入新的活性中心,提高了催化剂的催化活性;随着钼含量的增加,活性中心数目逐渐增多;用CO-NO共吸附原位红外光谱研究了Co-Mo/γ-Al₂O₃催化剂表面活性中心的信息,证明不同的Mo中心分别吸附CO和NO,并将它们区分开来,解决了不同活性中心的光谱互相重叠的问题。     

Oxidative Dehydrogenation on Nanocarbon: Intrinsic Catalytic Activity and Structure–Function Relationships

Physical and chemical insights into the nature and quantity of the active sites and the intrinsic catalytic activity of nanocarbon materials in alkane oxidative dehydrogenation (ODH) reactions are reported using a novel in situ chemical titration process. A study on the structure–function relationship reveals that the active sites are identical both in nature and function on various nanocarbon catalysts. Additionally, the quantity of the active sites could be used as a metric to normalize the reaction rates, and thus to evaluate the intrinsic activity of nanocarbon catalysts. The morphology of the nanocarbon catalysts at the microscopic scale exhibits a minor influence on their intrinsic ODH catalytic activity. The number of active sites calculated from the titration process indicates the number of catalytic centers that are active (that is, working) under the reaction conditions.     

Probing redox photocatalysis of trapped electrons and holes on single Sb-doped titania nanorod surfaces

We used a fluorogenic reaction to study in conjunction the photocatalytic properties for both active sites (trapped photogenerated electrons and holes) on individual Sb-doped TiO(2) nanorods with single-molecule fluorescence microscopy. It was found that active sites around trapped holes show higher activity, stronger binding ability, and a different dissociation mechanism for the same substrate and product molecules in comparison with the active sites around trapped electrons. These differences could be elucidated by a model involving the charged microenvironments around the active sites.     

Bridging the Surface Charge and Catalytic Activity of a Defective Carbon Electrocatalyst

Electrocatalysis is dominated by reaction at the solid–liquid–gas interface; surface properties of electrocatalysts determine the electrochemical behavior. The surface charge of active sites on catalysts modulate adsorption and desorption of intermediates. However, there is no direct evidence to bridge surface charge and catalytic activity of active sites. Defects (active sites) were created on a HOPG (highly oriented pyrolytic graphite) surface that broke the intrinsic sp²‐hybridization of graphite by plasma, inducing localization of surface charge onto defective active sites, as shown by scanning ion conductance microscopy (SICM) and Kelvin probe force microscopy (KPFM). An electrochemical test revealed enhanced intrinsic activity by the localized surface charge. DFT calculations confirmed the relationship between surface charge and catalytic activity. This work correlates surface charge and catalytic activity, providing insights into electrocatalytic behavior and guiding the design of advanced electrocatalysts.     

Kinetic heterogeneity of the active sites of titanium-containing catalytic systems in the stereospecific polymerization of isoprene

The kinetic heterogeneity of the active sites of titanium-containing catalytic systems in the stereospecific polymerization of isoprene was studied based on solving inverse problems for the molecular-weight distribution of polyisoprene with the use of the Tikhonov regularization method. It was found that from two to four types of active sites can occur depending on the nature of the organoaluminum compound used in the catalytic system. The rate constants of elementary steps of the polymerization process for particular types of active sites were obtained for the first time by solving inverse kinetic problems.     

Modeling Steroid 5alpha-reductase and Characterizing Its Potential Active Sites

Steroid 5alpha-reductase of human is an enzyme in the biosynthetic pathway from testosterone (T) to dihydrotestosterone (DHT). Up to now, no crystal structure of this enzyme has been reported. However, knowledge of the tertiary structure and possible active sites is essential for understanding the catalysis mechanism and for the design of inhibitors. A model with putative active sites has been created and evaluated by using homology modeling and molecular docking techniques based on the bioinformatics knowledge. The homology model is optimized in Swiss PDB Viewer with MM method and substrate structures before docking are also optimized on HF/6-31G. The active site for the docking of NADP, T, DHT and Finasteride is located near the N-terminus of enzyme. Four active amino acids in the active site are identified as Ala26, Arg53, Arg176 and Lys177. Reaction procedure, binding pattern of active sites, the types of weak interaction and so on are also discussed.     

Diffusion-influenced reactions of particles with several active sites

The main concern of the present work is consideration of sterically specific liquid-phase reactions in the case where one of the reactants has several active sites. In kinematic approximation we derived compact formulas for partial rate constants of individual active sites. Reaction rates are expressed via the kinetic rate constants and convolutions of the free Green functions over reaction zones. These results are valid for arbitrary geometry of reactants and active sites. Effects of mutual influence of the active sites were studied, their simple estimate was proposed.