Anticorrelation between Surface and Subsurface Point Defects and the Impact on the Redox Chemistry of TiO2(110)

By using a combination of scanning tunneling microscopy (STM), density functional theory (DFT), and secondary‐ion mass spectroscopy (SIMS), we explored the interplay and relative impact of surface versus subsurface defects on the surface chemistry of rutile TiO₂. STM results show that surface O vacancies (V_O) are virtually absent in the vicinity of positively charged subsurface point defects. This observation is consistent with DFT calculations of the impact of subsurface defect proximity on V_O formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O₂ was employed and was observed to be suppressed around them. DFT results attribute this to a perceived absence of intrinsic (Ti), and likely extrinsic interstitials in the nearest subsurface layer beneath inhibited areas. We also postulate that the entire nearest subsurface region could be devoid of any charged point defects, whereas prevalent surface defects (V_O) are largely responsible for mediation of the redox chemistry at the reduced TiO₂(110).     

Dynamical aspects of the adsorption hysteresis phenomenon

Equilibrium and nonequilibrium transport properties of adsorbates in mesoporous Vycor porous glass have been experimentally studied using nuclear magnetic resonance techniques. With the known geometrical characteristics of porous glass and with measured self-diffusivities, transient sorption curves have been quantitatively compared to those predicted within a Fick's law model. This model correctly describes data outside a hysteresis region. In contrast, in the hysteresis region, a two-step mechanism of density relaxation is required to explain the behavior. These two mechanisms are identified as diffusion at early stages and activated density redistribution at later stages of adsorption. The latter mechanism, being intrinsically slow in nature, is anticipated to prevent the system from reaching equilibrium.     

Ceramic-based materials for electrochemical applications

Novel catalytic ceramic-based materials that simultaneously possess high surface area and adsorptive capacity, with proton and/or electron conducting properties, were developed for electrocatalytic and waste-stream treatment processes. These novel inorganic proton conducting membranes were produced by incorporating inorganic low-temperature proton conductors such as polymeric phosphates of polyvalent metals into the porous structure of different active or inert substrates such as ceramics (in the form of tubes, discs and paper), zeolites or carbon cloth. Electrocatalytic activity was obtained by coating electroconductive surface layers that acted both as electrode and catalyst. Bench scale and pilot scale test reactors were built and commissioned. Comparison with existing technologies was undertaken for several applications. Such high surface-area inorganic materials that support nanoscale metal clusters are being tested as electrode materials in anodic oxidation, inorganic fuel cells and hydrogen generation.     

Interpreting early land management through compound specific stable isotope analyses of archaeological soils

Compound specific stable isotope analyses of managed soils using isotope ratio mass spectrometry have been undertaken as a means of determining early land use practices. delta (15)N amino acid signals demonstrate differences between manured grassland, unmanured grassland and continuous cereal cultivation under long-term experimental land use control conditions, with delta (15)N in hydrophobic amino acids providing the most distinctive signals. Analysis of early modern/medieval and of Bronze age anthropogenic soils from Orkney demonstrates that such signals are retained in archaeological contexts. delta (13)C analyses of n- alkanoic acid components of the fossil, Bronze Age, anthropogenic soils suggest a major terrestrial input to these soils, with uniform composition of formation materials. Surficial soils demonstrate the assimilation of isotopically lighter carbon, providing a means of assessing the mobility of the n- alkanoic acids within soils and sediments. Copyright 1999 John Wiley & Sons, Ltd.     

Sliding Presentation of the Jeux de Taquin for Classical Lie Groups

The simple \(GL(n,\mathbb {C})\)-modules are described by using semistandard Young tableaux. Any semistandard skew tableau can be transformed into a well defined semistandard tableau by a combinatorial operation, the Schützenberger jeu de taquin. Associated to the classical Lie groups \(SP(2n,\mathbb {C})\), \(SO(2n+1,\mathbb {C})\), there are other notions of semistandard Young tableaux and jeux de taquin. In this paper, we study these various jeux de taquin, proving that each of them has a simple and explicit formulation as a step-by-step sliding. Any of these jeux de taquin is the restriction of the orthogonal one, associated to \(SO(2n+1,\mathbb {C})\).     

Electron-stimulated production of molecular oxygen in amorphous solid water

The low-energy, electron-stimulated production of molecular oxygen from pure amorphous solid water (ASW) films and ASW films codosed with H(2)O(2) is investigated. Layered films of H(2)(16)O and H(2)(18)O are used to investigate the reaction mechanisms for O(2) production and the spatial profile of the reactions within the films. The O(2) yield is dose-dependent, indicating that precursors are involved in the O(2) production. For temperatures below approximately 80 K, the O(2) yield at steady state is relatively low and nearly independent of temperature. At higher temperatures, the yield increases rapidly. The O(2) yield is enhanced from H(2)O(2)-dosed water films, but the experiments show that H(2)O(2) is not the final precursor in the reactions leading to O(2). Instead, a stable precursor for O(2) is produced through a multistep reaction sequence probably involving the reactions of OH radicals to produce H(2)O(2) and then HO(2). The O(2) is produced in a nonthermal reaction from the HO(2). For relatively thick films, the reactions leading to O(2) occur at or near the ASW/vacuum interface. However, the electronic excitations that initiate the reactions occur over a larger range in the film. A kinetic model that qualitatively accounts for all of the observations is presented.     

Fast magnetic resonance imaging and velocimetry for liquids under high flow rates

We here demonstrate the use of NMR velocity imaging techniques to measure flow in a free falling jet of water at speeds up to and on the order of 1m/s. In particular, we show how to adapt the RARE imaging method, based on a CPMG multiple rf pulse train, so that the real and imaginary parts of the signal may be suitably acquired, enabling pulsed gradient spin echo encoding for flow. We term this method "soft-pulse-quadrature-cycled PGSE-RARE" or SPQC-PGSE-RARE. We further demonstrate the use of a one-dimensional (slice selective) imaging method which takes advantage of the cylindrical symmetry of the flow, and considerably shortens the image acquisition time.     

Nanoporous Glass as a Model System for a Consistency Check of the Different Techniques of Diffusion Measurement

The remarkable differences in the guest diffusivities in nanoporous materials commonly found with the application of different measuring techniques are usually ascribed to the existence of a hierarchy of transport resistances in addition to the diffusional resistance of the pore system and their differing influence due to the differing diffusion path lengths covered by the different measuring techniques. We report diffusion measurements with nanoporous glasses where the existence of such resistances could be avoided. Molecular propagation over diffusion path lengths from hundreds of nanometers up to millimeters was thus found to be controlled by a uniform mechanism, appearing in coinciding results of microscopic and macroscopic diffusion measurement.