2,651 research outputs found

    CO oxidation at Pd(100): A first-principles constrained thermodynamics study

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    The possible formation of oxides or thin oxide films (surface oxides) on late transition metal surfaces is recently being recognized as an essential ingredient when aiming to understand catalytic oxidation reactions under technologically relevant gas phase conditions. Using the CO oxidation at Pd(100) as example, we investigate the composition and structure of this model catalyst surface over a wide range of (T,p)-conditions within a multiscale modeling approach where density-functional theory is linked to thermodynamics. The results show that under the catalytically most relevant gas phase conditions a thin surface oxide is the most stable "phase" and that the system is actually very close to a transition between this oxidic state and a reduced state in form of a CO covered Pd(100) surface.Comment: 13 pages including 7 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

    Novel Reconstruction mechanisms: A comparison between group-III-nitrides and "traditional" III-V-semiconductors

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    We have studied the driving forces governing reconstructions on polar GaN surfaces employing first-principles total-energy calculations. Our results reveal properties not observed for other semiconductors, as for example a strong tendency to stabilize Ga-rich surfaces. This mechanism is shown to have important consequences on various surface properties: Novel and hitherto unexpected structures are stable, surfaces may become metallic although GaN is a wide-bandgap semiconductor, and the surface energy is significantly higher than for other semiconductors. We explain these features in terms of the small lattice constant of GaN and the unique bond strength of nitrogen molecules.Comment: 13 pages, 5 figure

    Soil moisture-runoff relation at the catchment scale as observed with coarse resolution microwave remote sensing

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    International audienceMicrowave remote sensing offers emerging capabilities to monitor global hydrological processes. Instruments like the two dedicated soil moisture missions SMOS and HYDROS or the Advanced Scatterometer onboard METOP will provide a flow of coarse resolution microwave data, suited for macro-scale applications. Only recently, the scatterometer onboard of the European Remote Sensing Satellite, which is the precursor instrument of the Advanced Scatterometer, has been used successfully to derive soil moisture information at global scale with a spatial resolution of 50 km. Concepts of how to integrate macro-scale soil moisture data in hydrologic models are however still vague. In fact, the coarse resolution of the data provided by microwave radiometers and scatterometers is often considered to impede hydrological applications. Nevertheless, even if most hydrologic models are run at much finer scales, radiometers and scatterometers allow monitoring of atmosphere-induced changes in regional soil moisture patterns. This may prove to be valuable information for modelling hydrological processes in large river basins (>10 000 km2. In this paper, ERS scatterometer derived soil moisture products are compared to measured runoff of the Zambezi River in south-eastern Africa for several years (1992?2000). This comparison serves as one of the first demonstrations that there is hydrologic relevant information in coarse resolution satellite data. The observed high correlations between basin-averaged soil moisture and runoff time series (R2>0.85) demonstrate that the seasonal change from low runoff during the dry season to high runoff during the wet season is well captured by the ERS scatterometer. It can be expected that the high correlations are to a certain degree predetermined by the pronounced inter-annual cycle observed in the discharge behaviour of the Zambezi. To quantify this effect, time series of anomalies have been compared. This analysis showed that differences in runoff from year to year could, to some extent, be explained by soil moisture anomalies

    Limitations of rapid myelin water quantification using 3D bSSFP

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    Object: Imaging of the myelin water fraction (MWF) is conventionally performed using a multi-echo spin-echo sequence. This technique requires long acquisition times and therefore often suffers from a lack of volume coverage. In this work, the application of 3D balanced steady-state free precession (bSSFP) sequences to extract high-resolution myelin water maps is discussed. Materials and Methods: Based on a two-pool water exchange model, an approximate bSSFP signal equation is derived and applied to fit the flip angle dependence of the invivo bSSFP signal. Thereby, the MWF and signal amplitude are fitted, while a priori assumptions are made for the other parameters of the two-pool system. Results: The effects of magnetization transfer, finite RF pulses, B 0 and B 1 inhomogeneities, as well as variation of the constant fit parameters, are investigated. Acquisition and calculation of quantitative, high-resolution MWF maps from white matter of healthy volunteers based on bSSFP is feasible and averaged MWF fit results agree with literature. However, results from numerical simulations indicate a severe dependence of the derived MWF values on the constant two-pool parameters. Conclusion: The demonstrated dependence of the MWF on the two-pool parameters considerably impairs the applicability of the proposed metho

    Nanostructures at surfaces from substrate-mediated interactions

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    CO oxidation on Pd(100) at technologically relevant pressure conditions: A first-principles kinetic Monte Carlo study

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    The possible importance of oxide formation for the catalytic activity of transition metals in heterogenous oxidation catalysis has evoked a lively discussion over the recent years. On the more noble transition metals (like Pd, Pt or Ag) the low stability of the common bulk oxides suggests primarily sub-nanometer thin oxide films, so-called surface oxides, as potential candidates that may be stabilized under gas phase conditions representative of technological oxidation catalysis. We address this issue for the Pd(100) model catalyst surface with first-principles kinetic Monte Carlo (kMC) simulations that assess the stability of the well-characterized (sqrt{5} x sqrt{5})R27 surface oxide during steady-state CO oxidation. Our results show that at ambient pressure conditions the surface oxide is stabilized at the surface up to CO:O2 partial pressure ratios just around the catalytically most relevant stoichiometric feeds (p(CO):p(O2) = 2:1). The precise value depends sensitively on temperature, so that both local pressure and temperature fluctuations may induce a continuous formation and decomposition of oxidic phases during steady-state operation under ambient stoichiometric conditions.Comment: 13 pages including 5 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm
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