Tunneling Spectroscopy for the Study of Adsorption and Reaction on Model Catalysts

Abstract

Inelastic electron tunneling spectroscopy measures the vibrational modes of a small quantity of molecules that are included in or near the insulating layer of a metal-insulator-metal tunnel junction [l, 21. Figure 1 stows two metal electrodes separted by a thin insulating layer. If this layer is thin enough (of order 20 A) and if a voltage V is applied between the two electrodes, an electron current will flow from one electrode to the other. The maximum energy of the tunneling electrons will be of order eV. If there is a molecule in or near the insulating layer with a characteristic vibrational mode energy hv, this vibrational mode may or may not be excited by the tunneling electron. The most important factor turns out to be simply, does the tunneling electron have enough energy? In particular, is eV  hv?     

Theoretical Insight into the Nature of Ammonia Adsorption on Vanadia-Based Catalysts for SCR Reaction

Density functional theory (DFT) calculations were carried out on monomeric and oligomeric vanadium oxide clusters to probe the factors leading to the formation of NH₄ species from the adsorption of ammonia. The interaction of ammonia with monomeric vanadium oxide clusters leads to the formation of hydrogen-bonded NH₃ species, with energy changes for ammonia adsorption near -50 kJ/mol. The interaction of ammonia with oligomeric vanadium oxide clusters leads to the formation of bidentate NH_4 species, where the ammonium cation is coordinated between two V=O groups on adjacent vanadium cations. The energy change for ammonia adsorption in this mode is near -100 kJ/mol. Adsorption of ammonia as NH₄ species was not observed when the oligomeric vanadium oxide clusters were reduced by addition of hydrogen atoms, i.e., in clusters where the formal oxidation state of the vanadium cations was 4+. Based on our findings, a model for the generation of Brönsted acidity through the interaction of vanadium oxide oligomers with the titanium oxide support is proposed.     

Size and Support Effects for CO Adsorption on Gold Model Catalysts

CO adsorption on gold particles deposited on well-ordered alumina and iron oxide films was studied with temperature-programmed desorption. Scanning tunneling microscopy was used to provide correlative structural characterization. The results show that the adsorption of CO on gold exhibits a size effect in that small particles adsorb CO more strongly. For a given particle size (3 nm), CO desorption temperature (at 170 K) is essentially independent of the supports studied. Therefore, support effects seen in CO oxidation on real catalytic systems must arise from the interaction of oxygen rather than CO with these catalysts.     

Photoacoustic Spectroscopy for the Study of the Adsorption of Dyes on Wool Fabrics

Dyes have been applied to different wool fabrics and measurements by photoacoustic spectroscopy showed well-defined absorption bands. Differences were found in the spectral properties of dyes, which appeared visually to be very similar, indicating that photoacoustic spectroscopy may be useful for the identification of dyes on wool fabrics. Photoacoustic spectroscopy was also shown to be capable of measuring two-dye and three-dye combinations and very dark colours. Comparisons with reflectance spectroscopy showed that photoacoustic spectroscopy might usefully complement the results obtained from reflectance measurements.     

酶催化剂的固定化研究及其在催化反应中的应用

酶催化剂具有高效性,多样性,底物专一性,区域选择性、化学选择性、对映选择性以及反应条件温和的特点。而酶的固定化后除了保持原有的特点外,易与反应物和产物分离,可回收重复使用,降低生产成本。本文对酶催化剂的固定化方法以及在有机催化反应中的应用作了部分简述。并对固定化方法进行了比较和评价。     

A New Method for the Study of Adhesion: Application of Inelastic Electron Tunneling Spectroscopy

Inelastic electron tunneling spectroscopy provides an incredibly sensitive and versatile method of detecting and identifying molecular species adsorbed on the surface of a metal oxide. IETS spectra have been measured on the components of the commercial adhesive, Hercules 3501. This epoxy system consists of two molecular components; diamino diphenyl sulfone (DPS) and tetraglycidycl 4,4' diamino diphenyl methane (DPM). IETS spectra of the individual components and of the epoxy mixture adsorbed on aluminum oxide have been obtained and the vibrational modes and frequencies assigned by comparison with computer calculations and existing infrared optical spectra.

IETS is a useful tool for the study of molecular adhesion to an oxide. It is one of the most important methods available for determining the interface physics and chemistry of adhesive bondlines. Its usefulness is greatly enhanced by the fact that in situ studies can be made.

Spectra have been obtained on the components and mixture of the high performance, two component epoxy Hercules 3501. The vibrational modes were identified by comparison with infrared and Raman data, and with computer calculations using force constant information. Evidence for an aging effect was found in the DPS component.

It is suggested that initial studies using IETS should be applied to adhesives of simple molecular structure before proceeding to more complex systems.     

四氢萘在Ni-Mo-W加氢改质催化剂中反应网络研究

利用Ni-Mo-W复合氧化物和分子筛混合制备出非负载改质催化剂,通过四氢萘在该研制催化剂上的产物分布推断出四氢萘加氢开环的反应路径。结果表明,四氢萘加氢开环反应是单分子和双分子反应共存的复杂反应。单分子反应为主要反应,能够生成理想的开环产物丁基环己烷、丁基苯、丁基环己烯。双分子反应则生成大于10个碳原子的重组分,是聚合积碳的主要反应路径。     

Application of Electron Energy-Loss Spectroscopy to the Study of Solid Catalysts

The addition of an electron spectrometer to a transmission electron microscope provides electron energy-loss spectra and energy-filtered images, from which thickness and chemical information can be extracted. This article reviews the techniques and instrumentation required, together with recent applications involving heterogeneous catalysts.     

己二酸二甲酯反应-吸附耦合技术的动力学研究

对己二酸、甲醇在732强酸性阳离子交换树脂的催化和4A分子筛吸附的共同作用下反应-吸附耦合法制备己二酸二甲酯的表观动力学进行了全面的探讨.根据反应的有关理论,建立了其表观动力学方程为:r＝-dcA-dt＝21.0866e-24494-RTCAC2B生成了己二酸二甲酯的表观活化能24.494kJ/mol.     

车轮形催化剂上甲烷水蒸汽转化反应宏观动力学的研究

在3.04×10~6Pa压力下使用KD-1型内循环式无梯度反应器研究了原粒度七筋车轮形催化剂上甲烷水蒸汽催化转化反应的宏观动力学.实验条件为:反应温度600—800℃、H_2O/CH_42.0～5.0、碳空速4320—12420ml/(h·g).采用平行模型来描述甲烷蒸汽转化反应,用电子计算机对实验数据进行回归整理,得到宏观动力学方程为:R_(co)=63475e~(-188536/R_gT)P(CH_4~(0.7)(1-β_1)R_(co_2)=8.464×10~(-8)e~(-136012/R_gT)P_(CH_4~(0.7)P_(H_2O)~(1.5)(1-β_2)结果表明,水煤气变换反应远离平衡;七筋车轮形转化催化剂的内表面利用率比一般环形催化剂提高约一倍.经过对工业转化炉核算,本文所得的宏观动力学方程是可靠的,具有良好的等效性.     

催化裂化催化剂碱氮中毒数学模型的研究

利用几种催化剂的催化裂化反应试验数据,通过碱氮中毒模型方程推导和模型参数的估计,建立了评价裂化催化剂抗碱氮中毒活性稳定性的数学模型。关联研究结果表明,降低催化剂分子筛含量和稀土金属含量,以及提高硅铝比和钠含量,有利于提高催化剂抗碱氮中毒活性稳定性;提高催化剂分子筛含量、硅铝重量比、稀土金属含量,以及降低钠含量,可提高催化裂化反应速率。     

固载杂多酸催化缩醛反应的研究

将固载杂多酸(PW12/C)应用于缩醛反应。以糠醛与乙二醇的缩合为探针反应,详细探讨了影响缩醛反应的各种因素,得出糠醛与乙二醇反应的最佳条件,并成功地应用于糠醛与正己硫醇的缩合反应。糠醛乙二醇缩醛的收率为93.4%,糠醛正己硫醇缩醛的收率为85.6%。     

吡哌酸荷移反应及荧光光谱性质研究

研究了吡哌酸(PPA)与2,5-二羟基-3,6-二氯苯醌(CAA)之间的荷移反应。实验表明,二者在丙酮-甲醇介质中发生荷移反应,在35℃恒温40min即可生成络合比为1∶1的荷移络合物,其荧光发射比吡哌酸(PPA)有显著增强。溶液浓度在0.10~7.0μg·m L-1范围内荷移络合物的荧光强度与浓度线性关系良好,相关系数r=0.9993,检出限为0.10μg·m L-1。该方法用于吡哌酸(PPA)胶囊的测定,加标回收率为97.1%~99.0%,相对标准偏差(RSD)为1.2%~2.9%。     

硅氢加成反应催化剂的研究进展

简要介绍了Pt、Rh、Pd等过渡金属硅氢加成反应催化剂的最新研究进展 ,指出了此类反应的研究前景     

Investigation of Planar Ziegler-Natta Model Catalysts Using Attenuated Total Reflection Infrared Spectroscopy

A sensitive method is presented for the investigation of planar Ziegler-Natta model catalysts by means of attenuated total internal reflection Fourier transform infrared spectroscopy. A solution of MgCl₂ in ethanol, optionally containing an internal donor, was spin-coated directly onto a planar ZnSe ATR crystal. Ethylene and propylene polymerization was monitored in situ and ex-situ, respectively, and interaction of a diester internal donor with MgCl₂ and TiCl₄ was investigated. The isotacticity of the polypropylene produced in the presence and absence of donor was estimated based on characteristic peaks in the infrared spectra.     

苯直接氧化制苯酚反应中苯与苯酚的吸附

本文分别以乙腈,乙酸,乙醇为溶剂,以四种不同的炭基固体酸为吸附剂对苯和苯酚进行了吸附研究。结果表明,以乙醇为溶剂时对苯和苯酚进行吸附时效率最佳;以80%的硫酸磺化后得到的炭基固体酸为吸附剂时对苯和苯酚的吸附效率最佳;以乙腈和乙酸更适合苯直接氧化制备苯酚反应的溶剂。     

Potential of UV–Raman Spectroscopy for Characterization of Sub-monolayer MoOx Model Catalysts at Ambient Pressure

UV–Raman spectroscopy is demonstrated to be a valuable tool to study molybdenumoxide model catalysts at ambient pressure while visible light Raman yields no information. UV–Raman offers the possibility to characterize MoO _x /Al ₂O₃/Si and MoO _x /SiO₂/Si model systems with sub-monolayer loading of MoO _x even though the surface area of a Si-wafer is much lower than that of a bulk support. Anchoring of MoO _x onto Al₂O₃/Si and SiO₂/Si results in similar molybdenum oxide compounds as for bulk catalysts.     

FTIR Spectroscopy Evidence for the Basicity Induced by Adsorption

Basicity of silanol oxygen atom increases after adsorption of ammonia or pyridine, as revealed by FTIR spectra of adsorbed CHF₃ and CO₂. The same effect of “induced basicity” was observed for the dangling hydroxyl groups of ice surface with coadsorbed ethylene and fluoroform. Possible role of the induced acidity and basicity in surface chemistry is discussed.