The Study of Heterogeneous Catalysts by High-Resolution Transmission Electron MicroscoDV

Advances in instrumentation have made it possible in recent years to study the microstructure of inorganic materials at atomic resolution using the technique of high-resolution electron microscopy (HREM). Details of instrumentation have been described elsewhere [l], and applications and trends for HREM have recently been reviewed [2]. Although HREM is primarily a technique for studying bulk defects, it is increasingly also being applied in the profile-imaging mode to derive information about surfaces [3]. The high spatial resolution of the electron microscope makes it a valuable tool for the characterization of heterogeneous catalysts. This is evidenced by the growing number of studies wherein electron micrographs are being used to describe the morphology of a particular catalyst. Profile imaging is proving particularly useful in this regard for following changes in surface structure as a function of treatment conditions [4].     

In Situ Electron Microscopy Studies of Catalyst Particle Behavior

Although transmission electron microscopy (TEM) has been in routine operation for over 25 years, it is somewhat surprising to find that there has only been limited use of the technique by workers in the field of catalysis. Turkevich and his co-workers [1] were among the first investigators to use the electron microscope to study catalytic substances. In general, most applications have centered around the determination of particle size distributions in supported metal catalyst systems [2]. More recently, however, it has been recognized that the technique can yield much more fundamental information on such aspects as the morphology of small particles and the nature of their interaction with a support medium. This advance is the result of the achievement of high resolution TEM (0.25 nm resolution).     

Ti(C,N)基金属陶瓷中陶瓷相芯/壳组织的观察与分析

用ＸＲＤ，ＳＥＭ，ＴＥＭ和ＨＲＥＭ观察、分析和方法研究了Ｔｉ（Ｃ，Ｎ）基金属陶瓷中陶瓷相的芯、壳组织。结果表明，在ＳＥＭ，ＴＦＭ观察中，芯、壳之间存在相界面，而ＨＲＥＭ观察显示陶瓷相的芯／壳组织具有连续相同的点阵结构。     

Structure image of single crystal of polyethylene

High-resolution electron macroscopic images of lamellar single crystal of polyethylene (PE) have been successfully obtained using high-resolution electron microscopy (HREM), although so far the feasibility of obtaining HREM images from such a radiation sensitive polymer is still drastically questioned. The HREM images with a clear two-dimensional periodic structure reported here were recorded in a transmission electron microscope operated at 200 kV. The images consisted of lattice fringes derived from the 〈001〉 zone, and the structure images of different lattice fringes were resolved. To our knowledge, this is the first time that such clear structure images of PE have been reported at a molecular level.     

Direct Observation of Channel Structures in Zeolite Y and A with a Slow-Scan, Charge-Coupled-Device Camera

Zeolite materials are very sensitive to electron beam irradiation. Therefore, there have been only limited attempts to characterize their structures using high-resolution electron microscopy (HREM). The sensitivity to the electron beam depends strongly on the Si/Al ratio in zeolites. A slow-scan, charge-coupled-device (CCD) camera provides the possibility to observe materials using HREM with very low beam doses. In this study, this method was applied to observe sensitive materials such as zeolite type A and type Y, and HREM images were recorded without any damage caused by electron irradiation under a dose density less than 0. 2 A /cm². It was found that fine channel structures, cleavage planes, and the electron irradiation process in the zeolites could be characterized using HREM with the slow-scan CCD camera.     

Pulsed Corona Discharges and Their Applications in Toxic VOCs Abatement

plasma processes are among the emerging technologies for volatile organic compounds (VOCs) sbatoment. Both thermal plasmas and non-equil[brimn plasmas (cold plasmas) are being developed for VOCs clesnup. Particularly, pulsed corona discharges offer several edvantages over conventional VOCs abatement tochniqvee, To optimize the existing technology and to developit further, there is need to understand the mechanlsms involved in plasma chemical reacticms, Furthermore, it is strongly desirable to be able to predict the behavior of new VOCs in non-equillbrlum plasma enviromuent from the data known for a few representative oompounds, Pulsed corona discharge technique is introduced here with dtafion of refevant literature, Fundamental principfes,useful for predicting the VOCs‘ decomposition behavior, have been worked out from the published literature. Latest developments in the area, targeted to minimize the enersy losses, improve the VOCs destruction efficiency and reduce the generation of unwanted organic and inorganic by-products, are presented.     

Visualizing dispersion morphology via ultra-rapid freezing/transmission electron microscopy

Optical microscopy has been a useful technique for visualizing aqueous dispersions, but it lacks the high resolution available with transmission electron microscopy (TEM). In order to withstand the vacuum of the electron microscope, water-based systems must be either dried or frozen, but this distorts morphology. The ultra-rapid freezing technique can be used in such studies to preserve microstructure and also be compatible with the vacuum of the electron microscope. Also known as cryo-TEM, this technique has been used in coatings research to visualize the morphologies of a variety of aqueous dispersions. Besides being consistent with what we already know optically, these complementary techniques of ultra-rapid freezing and transmission electron microscopy now permit us a unique view of fluid microstructure far below the limit of optical microscopy.     

Flammability and thermal stability studies of ABS/Montmorillonite nanocomposite

Acrylonitrile–butadiene–styrene (ABS)/montmorillonite nanocomposites have been prepared using a direct melt intercalation technique by blending ABS and organophilic clay of two different particle sizes: OMTa (5 µm) and OMTb (38 µm). Their structure and flammability properties were characterized by X‐ray diffraction, high resolution electronic microscopy (HREM), thermogravimetric analysis (TGA) and cone calorimeter experiments. The results of HREM showed that ABS/5 wt% OMTa nanocomposite was a kind of intercalated–delaminated structure, while ABS/5 wt% OMTb nanocomposite was mainly an intercalated structure. The nanocomposites showed a lower heat release rate peak and higher thermal stability than the original ABS by TGA and cone calorimeter experiments. Also, the intercalated nanocomposite was more effective than an exfoliated–intercalated nanocomposite in fire retardancy. Copyright © 2003 Society of Chemical Industry     

Microstructural and Microchemical Characterization of Silicon Carbide and Silicon Carbonitride Ceramic Fibers Produced from Polymer Precursors

Several continuous SiC and SiC/N-based ceramic fibers prepared from different polymer precursors have been characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and high-resolution electron microscopy (HREM). Methods to prepare longitudinal as well as cross-sectional thin specimens from brittle ceramic fibers were developed to facilitate HREM and EELS studies. Lattice images clearly showed nanometer-sized crystallites, as well as amorphous regions. Microchemical analysis using EELS permitted study of the form and distribution of the various chemical species within the fibers.     

PLZT陶瓷的晶界结构

根据PLZT透明铁电陶瓷晶界的HREM象的实验结果,提出了可能存在的晶界原子结构模型。这些晶界模型不仅清晰地层现了PLZT晶界的结构特点,而且有助于工艺改进,进一步提高材料的性能。同时,对PLZT陶瓷晶界的研究结果也可供类似结构的陶瓷材料参考。     

HREM imaging of small Pt clusters dispersed in Y-zeolites

High resolution electron microscopy (HREM) has been used to study dispersed Pt clusters in NaY- and USY-zeolites. All the samples contained 0.8% Pt and were reduced at temperatures of 300 ° C, 500 ° C and 650 ° C. The size of the Pt clusters ranged from a few å up to 30 å. When the incident electron beam was sufficiently strong, it caused some of the extremely small metal clusters to sinter. This was usually accompanied by zeolite damage. This in-situ sintering must be taken into consideration when interpreting the particle size distribution results obtained solely by TEM, especially for particles that are smaller than 10 å. The minimum phase contrast imaging condition is demonstrated to be more appropriate than optimum defocus for detecting the extremely small Pt clusters inside the zeolite structures.     

high-resolution electron microscopy of aSr-containing sialon polytypoid phase

A new type of Sr-containing sialon polytypoid phase with the structural formula SrSi_10-xAl_18+xN_32-xO_x (x≈1) has been found in the Sr–Si–Al–O–N system. The phase was characterised by X-ray powder diffraction (XRPD), and its structure was investigated by electron diffraction (ED) and high resolution electron microscopy (HREM). It is considerably disordered, but the average structure has a rhombohedral unit cell with a=5·335(5)≈ ·a_AlN and c=79·1(1)Å≈30·c_AlN. The Sr atoms are located in layers M–Sr–M, M=(Si/Al), at the origin of the unit cell with 12 X=(O,N) atoms around it, at distances of 3 Å, forming a cubo-octahedron. The X atoms that form a hexagon around the Sr atom in the ab plane are corner shared by M=(Si/Al) tetrahedra with opposite polarity in adjacent layers in which 2/3 of the tetrahedra are occupied. The M–Sr–M layers alternate with normally eight-layer-thick AlN type blocks, although the thickness of these blocks frequently varies. The structural model obtained from the HREM images includes a polarity reversal of the tetrahedra in the AlN blocks, similar to that proposed to occur in Si–Al–O–N polytypoid phases. The model with one Sr layer and 10 M=(Si,Al) layers per 1/3 of the repeat unit agrees with the composition of the phase and experimental HREM images.     

Analytical Electron Microscopy of Heterogeneous Catalysts

Conventional transmission electron microscopy (CTEM) is by now a quite well-established technique for the study of heterogeneous catalysts. It is commonly used for the determination of metallic particles shapes and size distribution on various types of carriers [1, 2]. In addition, dark field imaging and diffraction measurements (in selected area or microdiffraction mode) allows advantage to be taken of the diffraction of the electrons in order to identify the active phase on the basis of its crystal structure [3, 4].     

Homogeneous and Heterogeneous Hydrogenation of Nitriles in a Liquid Phase: Chemical, Mechanistic, and Catalytic Aspects

Primary or alkylated amines are important chemicals and intermedi-ates. N-Alkylamines are used without further transformation as surface-active agents. Aliphatic primary diamines polymerize with aliphatic diacids to give linear polyamides which have conquered a large place in textile and mechanical industry [11. Mono- and polyamines are produced by catalytic hydrogenation of the corresponding nitriles [2]. In particular this pathway to hexamethylenediamine has been made easier since the synthesis of adi-ponitrile by hydrocyanation of butadiene was made possible [3]. The spec-ifications for production of amines are often very drastic from the point of view of purity, in particular for the diamines used in the textile industry. Thus the stress is put on selectivity of the reaction and most of industrial processes have a yield approaching stoichiometry. It exists few reviews on reduction of nitriles [4, 51, the literature dealing with this type of reaction being mostly published in patents [6-91, and practical knowledge is at the state of “know-how” [lo]. Up to now, two types of processes have been used industrially for the     

Microstructural Characterization of Graded-Index Antireflective Films

The miscrostructure of graded-index antirefection (GIAR) films was characterized using transmission and scanning electron microscopy, small-angle X-ray scattering, and replication. Transmission electron microscopy and X-ray scattering results agree for the entire coarsening history of the phase-separated substrate borosilicate glass. The complexity of the replication process and the metallization used for scanning electron microscopy caused the effective resolution limits for GIAR films to be much larger than the inherent resolution limits for each instrumentation.     

The role of flow injection analysis within the framework of an automated laboratory

Flow Injection Analysis (FIA) was invented at roughly the same time by two quite dissimilar research groups [1,2]. FIA was patented by both groups in 1974; a year also marked by the publication of the first book on automatic chemical analysis [3]. This book was a major undertaking for its authors and it is hoped that it has added to the knowledge of those analysts attempting to automate their work or to increase the level of computerization/automation and thus reduce staffing commitments. This review discusses the role of FIA in laboratory automation, the advantages and disadvantages of the FIA approach, and the part it could play in future developments.It is important to stress at the outset that the FIA approach is all too often closely paralleled with convention al continuous flow analysis (CFA). This is a mistake for many reasons, none the least of which because of the considerable success of the CFA approach in contrast to the present lack of penetration in the commercial market-place of FIA instrumentation.     

Molecular Transport of Gases,Vapors, and Salt Solutions Through Polymer Membranes

Molecular transport of small molecules through polymer films has been widely studied over the past several years [1–8]. In most of these investigations it has been shown that the penetrant solubility and diffusion follow Henry's law and Fick's law, respectively, and that solubility (S), diffusion (D), and permeability coefficients (P) are independent of penetrant concentration. Several theories of diffusion, permeation, and solution have been developed to relate the experimental data to polymer film morphology, chemical structure, and other physical properties [9–14]. From a practical view point, the phenomenon of transport is very useful in the treatment of hazardous waste in landfill areas. In chemical industries, its importance lies in the separation of organic compounds. Additionally, membrane separation techniques have been used in such industrial applications as desalination of brine, salt manufacturing from seawater, oxygen-enriched air, etc. The pervaporation technique wherein the permeate is removed from a mixture at the opposite side of the membrane as a vapor is now very popular [15].     

Simulation of bubble populations in a gas fluidized bed

The dynamics of bubble populations in a gas fluidized bed have been analyzed using a simulation technique due to the authors [11,12]. It is shown that small bubble populations arising from progressive coalescence may lead to inherent fluctuations which may be considerable. Evidently, such fluctuations could manifest in conversions in fluidized bed reactors. The results obtained were based on a model due to Argyriou, List and Shinnar[1].     

聚醚砜/N，O-羧甲基两性壳聚糖复合微滤膜的研制及其荷电特性

1INTRODUCTIONCharged membranes have been attracted more attention in membrane processes.Firstly,they have strong anti-fouling potential due to their charged groups.Secondly,they can be used as ion-exchange membranes for protein separations.Chitosan is the second abundant biopolymer after cellulose,and chitosan molecules contain a large number of reactive hydroxyl(-OH)and amine(-NH2)groups,therefore it can be an excellent candidate for affinity membranes[1].Until now,chitosan membranes have been used for reverse osmosis[2],pervaporation[3],and ultrafiltration[4].And attempts have been made to improve blood compatibility of chitosan with physical blends,surface modification and synthesis of blood-compatible derivatives[5].The present work reports the preparation of a carboxymethyl amphoteric chitosan[poly(ethersulfone) (PES)/CM-CS]composite MF membrane and its charged characteristics.