G.D. Moggridge

An in situ XRD investigation of singly and doubly promoted manganese oxide methane coupling catalysts

In situ X-ray diffraction (XRD) and concurrent measurements of catalytic performance have been used to characterize the solid phases present during various stages in the history of working methane coupling catalytic systems. Three such systems were studied: unpromoted, K-promoted, and KCl-promoted manganese oxide. In each case the effect of pulses of CHCl{sub 3} on the activity, selectivity, and catalyst structure was determined. Depending on the conditions, various oxides or mixed oxides of Mn were present. Of these, Mn{sub 3}O{sub 4} is an unselective total oxidation catalyst; K-promotion stabilizes this phase, drastically reducing CO{sub 2} formation and raising C{sub 2} activity. Chloride ions (added as KCl) prevent the formation of highly unselective potassium manganese oxides. Chlorine introduced from the gas phase has an additional effect: C{sub 2} activity is further enhanced leaving CO{sub 2} production unaffected. This form of chlorine promotion is greatly augmented by the presence of KCl. There are strong indications that the effects of chlorine involved substantial modification of the surface chemistry and are not purely due to Cl atom-induced homogeneous chemistry.

What determines the probing depth of electron yield XAS

Abstract Comparative Ni K-edge (8332.8 eV) total electron-yield (TEY) and conversion electron-yield (CEY) XAS spectra were collected from Ni wafers covered by NiO of varying thickness. In contrast to previous predictions, the surface sensitivity of TEY detection was found to be actually higher than for CEY detection. Examination of signal amplification factors upon switching from TEY to CEY detection indicates that the role of the surface insensitive inelastic KLL Auger electrons has been underestimated in the past. These energetic electrons constitute a major component of the emitted flux which is selectively amplified in CEY mode. Analysis of the TEY attentuation characteristics can be carried out solely in terms of Auger electron penetration ranges.

Oxygen surface species on lithium nickelate methane coupling catalysts and their interaction with carbon oxides

The surface chemistry of a LiNiO2 methane coupling catalyst has been studied with respect to adsorption/desorption behaviour and interaction with carbon oxides. The results indicate that anion vacancies induced at the surface at high temperature are readily replenished by interaction with both O2 and N2O. The O2− lattice oxygen species (α) so formed are thought to be responsible for methane coupling. A chemisorbed dioxygen species (β) appears to be formed at defect sites by adsorption of O2; it is not formed by N2O adsorption. Experiments with 18O2 indicate that this chemisorbed β-O2 desorbs by a dissociative recombination mechanism. These findings are in agreement with catalytic and electron spectroscopic studies. On the clean surface, C02 interacts strongly with O2−, destroying sites for β-O2 adsorption; it also removes preadsorbed β-O2. CO, however, does not adsorb at all on the clean surface; it does react with β-O2 to form a species (probably CO32−) that evolves CO2 on subsequent heating. This strongly supports the conclusion that the β species is a form of dioxygen.

Environmental cells for in situ X-ray diffraction and X-ray absorption spectroscopic studies of heterogeneous catalysts

Abstract Application of synchrotron radiation for in situ studies of catalysis requires construction of suitable environmental cells. We describe here examples of cells for extremes of temperature, pressure and chemical conditions. These cells are suitable for XAS and XRD and have proved reliable over a decade of use. For the first time we show in situ XAS cells for catalysis studies based on conversion electron yield (CEY) detection.

In situ probing of the near-surface properties of heterogeneous catalysts under reaction conditions: An introduction to total electron-yield XAS

Abstract Over the last two years, our understanding of gas-flow total electron-yield (TEY) detection for in situ X-ray absorption spectroscopy (XAS) has rapidly progressed. A short summary of recent developments is presented with a particular view to applications in the field of heterogeneous catalysis. For the first time, true in situ TEY data (of Ni- and Cu-based systems) acquired in reactive gas atmospheres and at pressures up to 40 atm are presented. The salient physical principles underlying the formation of the TEY signal are briefly introduced. Previously unexplained ‘self-absorption’ distortions in the TEY signal are explained by photoelectrons excited by fluorescent photons. The possibilities and limitations of in situ TEY XAS studies are highlighted.

Electron spectroscopic study of the growth, composition and stability of GeSx films prepared in ultra-high vacuum

Growth morphology, composition and thermal behaviour of ultra-thin GeSx films deposited on Ni under UHV conditions have been studied by X-ray photoelectron spectroscopy and X-ray Auger electron spectroscopy. The flux emitted by the GeSx evaporation source consisted of GeS and S2 molecules which condensed on the substrate to yield a film in which the first layer was S-rich, subsequent layers growing with a composition close to GeS2. The Ge and S Auger spectra and the Ge 3d X-ray photoelectron spectra clearly discriminate between the contact layer and subsequent layers. Ge Auger parameters indicate that Ge atoms in the first layer are in contact with metallic Ni and are consistent with GeS2 formation in the higher layers. The growth mode and agglomeration behaviour of the evaporated film are described and discussed.

Electron-yield x-ray absorption spectroscopy with gas microstrip detectors

It is shown that gas microstrip detectors developed for high-energy physics applications afford a considerable improvement of the surface sensitivity of electron-yield x-ray absorption fine-structure spectroscopy at ambient pressures. Data with excellent signal-to-noise characteristics can be collected using photon fluxes typical of bremsstrahlung x-ray sources and of synchrotron storage rings operating with reduced (“single bunch”) currents. Possible applications for this detection technology are discussed.

The effect of photo-oxidation on the sticking and reactivity of Ag on amorphous GeS2

Photo-oxidation of amorphous films illuminated by band-gap radiation drastically alters the growth mode and reactivity of subsequently deposited Ag. In the former case (monolayer/simultaneous multilayer growth) the Ag reacts with both Ge and S sites. In the latter case (Stranski - Krastanov growth) Ge sites are selectively oxidized and film growth proceeds by Ag nucleation at the unoxidized S sites. The behaviour is very different from that reported earlier for Zn deposition on , where photo-oxidation results in very large changes in metal sticking probability. XPS, XAES and EXAFS data provide the basis for understanding both this phenomenon and the very different photodiffusion behaviour of Zn and Ag in .

A NEXAFS STUDY OF THE ORIENTATION OF BENZOATE INTERCALATED INTO A LAYER DOUBLE HYDROXIDE

NEXAFS is shown to be an excellent technique, of potentially widespread application, for the determination of the orientation of organic molecules intercalated in preferentially oriented thin films of polycrystalline, layered minerals. A NEXAFS study of [Mg2Al(OH)6]+C7H5O2 · nH2O, a layered anionic clay, is described. This material shows a transition from a layer spacing of 15.4 A to only 9 A at a remarkably low temperature (below 100°C). This is shown to be accompanied by a change in the angle of the plane of the benzoate molecule to the 00l planes from 35° ± 10° to 0° ± 10°. The tilt of the benzoate anion in the room temperature structure demonstrates the presence of an interaction between the phenyl ring and the positively charged, brucite-like layers. Furthermore it is suggestive of the importance of hydrogen bonding in determining the interlayer spacing and stability.

Hemocompatibility of styrenic block copolymers for use in prosthetic heart valves

Certain styrenic thermoplastic block copolymer elastomers can be processed to exhibit anisotropic mechanical properties which may be desirable for imitating biological tissues. The ex-vivo hemocompatibility of four triblock (hard–soft–hard) copolymers with polystyrene hard blocks and polyethylene, polypropylene, polyisoprene, polybutadiene or polyisobutylene soft blocks are tested using the modified Chandler loop method using fresh human blood and direct contact cell proliferation of fibroblasts upon the materials. The hemocompatibility and durability performance of a heparin coating is also evaluated. Measures of platelet and coagulation cascade activation indicate that the test materials are superior to polyester but inferior to expanded polytetrafluoroethylene and bovine pericardium reference materials. Against inflammatory measures the test materials are superior to polyester and bovine pericardium. The addition of a heparin coating results in reduced protein adsorption and ex-vivo hemocompatibility performance superior to all reference materials, in all measures. The tested styrenic thermoplastic block copolymers demonstrate adequate performance for blood contacting applications.