Defined Surface Structure Research Articles

On the nature of the active site for the ethylbenzene dehydrogenation over iron oxide catalysts

The dehydrogenation of ethylbenzene to styrene was studied over single-crystalline iron oxide model catalyst films grown epitaxially onto Pt(111) substrates. The role of the iron oxide stoichiometry and of atomic surface defects for the catalytic activity was investigated by preparing single-phased Fe3O4(111) and α-Fe2O3(0001) films with defined surface structures and varying concentrations of atomic surface defects. The structure and composition of the iron oxide films were controlled by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), the surface defect concentrations were determined from the diffuse background intensities in the LEED patterns. These ultrahigh vacuum experiments were combined with batch reactor experiments performed in water–ethylbenzene mixtures with a total gas pressure of 0.6 mbar. No styrene formation is observed on the Fe3O4 films. The α-Fe2O3 films are catalytically active, and the styrene formation rate increases with increasing surface defect concentration on these films. This reveals atomic surface defects as active sites for the ethylbenzene dehydrogenation over unpromoted α-Fe2O3. After 30 min reaction time, the films were deactivated by hydrocarbon surface deposits. The deactivation process was monitored by imaging the surface deposits with a photoelectron emission microscope (PEEM). It starts at extended defects and exhibits a pattern formation after further growth. This indicates that the deactivation is a site-selective process. Post-reaction LEED and AES analysis reveals partly reduced Fe2O3 films, which shows that a reduction process takes place during the reaction which also deactivates the Fe2O3 films.

Cell electrophoretic discrimination of Staphylococcus saprophYticus strains of different origin

Staphylococcus saprophyticus isolates (n = 10) from urinary tract infections (UTIs) could be reproducibly distinguished from S. saprophyticus strains from respiratory tract infections (n = 5) by distinct electrophoretic mobility. Thus, the method of "bacteriopheresis" may offer a new approach to discriminate pathogenic microbial strains of different origin with respect to defined surface structures (hemagglutinins, lectins), generating distinct surface charges.

A comparison of various methods to determine hydrophobic properties of streptococcal cell surfaces

In order to determine whether various methods which are commonly applied to determine the hydrophobicity of bacterial cell surfaces yield similar results, the hydrophobicity of a range of oral streptococcal isolates was measured using various methods including adsorption to hexadecane, hydrophobic interaction chromatography, salt-aggregation and contact angle measurements. In addition the zela potential and the adhesion to saliva-coated hydroxyapatite were determined. To compare the results of the different methods, linear and rank correlation coefficients were calculated. The resulting correlations were weak when applied to the whole range of strains even in those cases where the tests are expected to probe the same surface characteristics. However, generally good correlations were obtained when a set of strains was examined which were derived from a single parent strain and which only differed in defined surface structures. Accordingly it must be concluded from the methods applied here, that if is not possible to define the surface ‘hydrophobicity’ of a bacterium other than on a comparative level with closely related strains. No clear correlation was found between any hydrophobicity test and the adhesion to saliva-coated hydroxyapatite.

Deactivation with silazanes in chromatography, mechanism of the reaction and practical consequences in capillary GC and RP‐HPLC: A29Si CP‐MAS NMR study

AbstractThe reaction of Cab‐O‐Sil, a highly dispersed vitreous quartz, with hexamethyldisilazane (HMDS) was studied in the temperature range 380–500°C, using 29Si solid state NMR and other techniques. Such studies are of importance in view of deactivation procedures of fused silica at high temperatures in capillary GC.The commonly accepted reaction equation: predominates only below ca. 400°C. Above ca. 400°C the intermediate cleavage product Me3SiNH2 reacts with surface silanol groups to form SioSiMe2NH2 + CH4. At higher temperatures these groups may ultimately form (SiO)3SiNH2 groups (analogously to the formation of bi‐and tridentate linkages starting from SiOSiMe3 groups), but (SiO)3SiNH2 groups are also directly formed at lower temperatures, simultaneously with the SiOSiMe3 groups, probably by reaction with siloxane bridges: .The reactions of the trimethylsilylamine part of HMDS with Cab‐O‐Sil were confirmed by an independent series of silylations using N,N‐dimethyltrimethylsilylamine instead of HDMS. The presence of amino groups as SiNH2 was confirmed by FT‐IR. This may be one of the reasons why very high temperature silylation with disilazanes does not provide a satisfactory deactivation of fused silica GC columns: the active SiOH groups are “replaced” by active SiNH2 groups. However, such a material may be of interest in LC. Silylation with silazanes at ca. 350°C in humid atmosphere and/or after extensive hydroxylation (leaching) of the surface should yield sufficiently deactivated surfaces with a rather well defined surface structure.

The Influence of Surface Structure on the Photoelectrochemistry of Polycrystalline n-TiO2-Films

Abstract The relationship between the surface structure, film thickness and the photoelectrochemical behaviour (photocurrent, corrosion and stability) of n-Ti02-films produced by controlled thermal oxidation of titanium foils is carefully investigated. X-ray surveys, optical and scanning electron microscopical observations were performed to elucidate the surface structure. It is established that a special etching procedure of Ti-metal foils, followed by controlled oxidation at 600 to 700 °C for 10 to 30 min in the presence of sufficient oxygen results in n-Ti02 layers with a defined surface structure and high photoactivity. As charge transfer through the Schottky barrier is decisive for the photoefficiency, the formation kinetics of the oxide layer is the most influencing factor.

Characterization of the tubules associated with the replication of three different orbiviruses

During the replication of bluetongue virus (BTV), African horsesickness virus (AHSV), and epizootic hemorrhagic disease virus (EHDV), tubular structures are synthesized in the infected cells. These tubules were purified on sucrose gradients and characterized. BTV and EHDV tubules are large with diameters of 68, and 54 nm, respectively. They have a surface fine structure with a striking 9-nm linear periodicity along the length of the tubules, giving them a ladder-like appearance. AHSV tubules are 18 run wide and have no clearly defined surface structure. The virus tubules are of indeterminate length, contain no RNA, and appear to be empty. They are in each case composed of a virus-specified noncapsid polypeptide of about 54,000 daltons. The AHSV tubule polypeptide is slightly larger than those of BTV and EHDV. Synthesis of the BTV tubules was first observed in the period 2–4 hr postinfection. Later in the infection cycle, they are seen in close association with viral particles and “virus factories.” Virus specificity of the tubules was demonstrated by specific immune precipitation with antisera against purified virus. There is immunological cross-reaction between BTV and EHDV tubules.

The lymphocyte plasma membrane: Locus of control in the immune response

The lymphocyte plasma membrane is the locus of events which control the immune response. T and B lymphocytes, which mediate cellular and humoral immunity respectively, show distinctive plasma membrane morphologies and cell surface receptors. The dynamic state of these plasma components is emphasized by their lateral mobility in the fluid plane of the membrane, as well as variation in their structure or expression as the lymphocyte proliferates and differentiates in response to stimulation by antigen or mitogens. The best understood membrane glycoproteins are surface membrane immunoglobulins that serve as antigen receptors on B cells, and the histocompatability-beta2 microglobulin complex that has an immunoglobulin-like structure. Other less well defined surface structures showing modulation during the cell cycle may affect growth regulation of proliferating lymphocytes. Some of these are shared by fetal and neoplastic cells. Major theories of lymphocyte signaling are discussed, and the early events in lymphocyte activation are reviewed. While a complete model encompassing all these early events is not yet possible, the central issues can be usefully discussed in term of receptor-transducer-effector concepts derived by strong parallels from a knowledge of hormone-membrane interactions.

Inelastic scattering of slow electrons in solids

A theory of the inelastic scattering of slow electrons in solids due to excitation of interband transitions is developed. It is shown that both nondirect and direct transitions occur which can be described by a generalization of the formalism used in solid state optics. Experiments with 30–200 eV electrons scattered from Si (111) surfaces with well defined surface structures as determined by low energy electron diffraction confirm the theoretical predictions. They indicate that the inelastic scattering of slow electrons can be understood in terms of the three-dimensional band structure of solids and suggest the use of inelastic low energy electron scattering as a tool for band structure analysis.

A low energy electron diffraction study of the structural effect of oxygen on the (111) face of a tungsten crystal

A low-energy electron-diffraction study of the structural effect of oxygen adsorption on the (111) face of a clean tungsten monocrystal has been carried out under various conditions of exposure and subsequent heating. The clean tungsten (111) surface is found to be unchanged after heating to temperatures as high as 2650° K. After an oxygen exposure of 5 × 10 −6 Torr · sec, heating to 800° K brings about the rapid development of (211) planes, with a well defined surface structure which is stable up to 1800° K. The clean (111) surface is restored by heating to 1900° K for several minutes or 2100° K for several seconds. Other structures all based on arrangements on (211) surfaces are observed at increased exposure. For exposures in the range 10 −3 to 10 −7 Torr · sec at 300° K and with subsequent heating, three types of patterns are observed separated by disordered transition regions. The inconsistently small quantity of gas measured by the ionization gauge at the high temperature transitions indicates that the strongly bound oxygen leaves as some oxide which condenses on the walls, or that diffusion into the tungsten takes place. Further measurements are required to determine the nature of these high temperature transitions.