1,366 research outputs found
First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment
In this paper we investigate by means of first-principles density functional
theory calculations the (111) surface of the Ag-Cu alloy under varying
conditions of pressure of the surrounding oxygen atmosphere and temperature.
This alloy has been recently proposed as a catalyst with improved selectivity
for ethylene epoxidation with respect to pure silver, the catalyst commonly
used in industrial applications. Here we show that the presence of oxygen leads
to copper segregation to the surface. Considering the surface free energy as a
function of the surface composition, we construct the convex hull to
investigate the stability of various surface structures. By including the
dependence of the free surface energy on the oxygen chemical potential, we are
able compute the phase diagram of the alloy as a function of temperature,
pressure and surface composition. We find that, at temperature and pressure
typically used in ethylene epoxidation, a number of structures can be present
on the surface of the alloy, including clean Ag(111), thin layers of copper
oxide and thick oxide-like structures. These results are consistent with, and
help explain, recent experimental results.Comment: 10 pages, 6 figure
Electron attachment to SF6 and lifetimes of SF6- negative ions
We study the process of low-energy electron capture by the SF6 molecule. Our
approach is based on the model of Gauyacq and Herzenberg [J. Phys. B 17, 1155
(1984)] in which the electron motion is coupled to the fully symmetric
vibrational mode through a weakly bound or virtual s state. By tuning the two
free parameters of the model, we achieve an accurate description of the
measured electron attachment cross section and good agreement with vibrational
excitation cross sections of the fully symmetric mode. An extension of the
model provides a limit on the characteristic time of intramolecular vibrational
relaxation in highly-excited SF6-. By evaluating the total vibrational spectrum
density of SF6-, we estimate the widths of the vibrational Feshbach resonances
of the long-lived negative ion. We also analyse the possible distribution of
the widths and its effect on the lifetime measurements, and investigate
nonexponential decay features in metastable SF6-.Comment: 22 pages, 10 figures, submitted to Phys. Rev.
Fragmentation pathways of nanofractal structures on surface
We present a detailed systematical theoretical analysis of the post-growth
processes occurring in nanofractals grown on surface. For this study we
developed a method which accounts for the internal dynamics of particles in a
fractal. We demonstrate that particle diffusion and detachment controls the
shape of the emerging stable islands on surface. We consider different
scenarios of fractal post-growth relaxation and analyze the time evolution of
the island's morphology. The results of our calculations are compared with
available experimental observations, and experiments in which the post-growth
relaxation of deposited nanostructures can be probed are suggested.Comment: 34 pages, 11 figure
The Effect of the Environment on alpha-Al_2O_3 (0001) Surface Structures
We report that calculating the Gibbs free energy of the alpha-Al_2O_3 (0001)
surfaces in equilibrium with a realistic environment containing both oxygen and
hydrogen species is essential for obtaining theoretical predictions consistent
with experimental observations. Using density-functional theory we find that
even under conditions of high oxygen partial pressure, the metal terminated
surface is surprisingly stable. An oxygen terminated alpha-Al_2O_3 (0001)
surface becomes stable only if hydrogen is present on the surface. In addition,
including hydrogen on the surface resolves discrepancies between previous
theoretical work and experimental results with respect to the magnitude and
direction of surface relaxations.Comment: 4 pages including 2 figures. Submitted to Phys. Rev. Lett. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Alloy surface segregation in reactive environments: A first-principles atomistic thermodynamics study of Ag3Pd(111) in oxygen atmospheres
We present a first-principles atomistic thermodynamics framework to describe
the structure, composition and segregation profile of an alloy surface in
contact with a (reactive) environment. The method is illustrated with the
application to a Ag3Pd(111) surface in an oxygen atmosphere, and we analyze
trends in segregation, adsorption and surface free energies. We observe a wide
range of oxygen adsorption energies on the various alloy surface
configurations, including binding that is stronger than on a Pd(111) surface
and weaker than that on a Ag(111) surface. This and the consideration of even
small amounts of non-stoichiometries in the ordered bulk alloy are found to be
crucial to accurately model the Pd surface segregation occurring in
increasingly O-rich gas phases.Comment: 13 pages including 6 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
A Spatially Resolved `Inside-out' Outburst of IP Pegasi
We present a comprehensive photometric dataset taken over the entire outburst
of the eclipsing dwarf nova IP Peg in September/October 1997. Analysis of the
lightcurves taken over the long rise to the peak-of-outburst shows conclusively
that the outburst started near the centre of the disc and moved outwards. This
is the first dataset that spatially resolves such an outburst. The dataset is
consistent with the idea that long rise times are indicative of such
`inside-out' outbursts. We show how the thickness and the radius of the disc,
along with the mass transfer rate change over the whole outburst. In addition,
we show evidence of the secondary and the irradiation thereof. We discuss the
possibility of spiral shocks in the disc; however we find no conclusive
evidence of their existence in this dataset.Comment: 8 pages, 8 figures, to be appear in MNRA
Comparison of Fencing Designs for Excluding Deer from Roadways
We evaluated the efficacy of several fencing designs for restricting movements of 18 captive, female white-tailed deer (Odocoelus virginianus), including standard wovenwire fencing (1.2-m, 1.5-m, 1.8-m, 2.1-m, and 2.4-m tall), opaque fencing (1.2-m, 1.5-m, and 1.8-m tall), and an outrigger fence (i.e., 0.6-m outriggers attached to a 1.2-m-tall wire fence angled at 45º). We recorded the number of successful fence crossings for each deer and characterized behaviors associated with each failed crossing attempt. No deer crossed the 2.4-m fence, whereas all deer crossed the 1.2-m fence. We observed no differences in crossing success between woven-wire and opaque fencing at height
Fibrous Monolithic Ceramics: III, Mechanical Properties and Oxidation Behavior of the Silicon Carbide/Boron Nitride System
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66176/1/j.1151-2916.1994.tb05399.x.pd
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Modeling the impact of sea-spray on particle concentrations in a coastal city
An atmospheric chemistry-transport model is used to assess the impacts of sea-spray chemistry on the particle composition in and downwind of a coastal city--Vancouver, British Columbia. Reactions in/on sea-spray affect the entire particle ensemble and particularly the size distribution of particle nitrate. Urban air quality, and particularly airborne particles, is a major concern in terms of human health impacts. Sea-spray is known to be a major component of the particle ensemble at coastal sites yet relatively few air quality models include the interaction of gases with sea-spray and the fate of the particles produced. Sea-spray is not an inert addition to the particle ensemble because heterogeneous chemistry in/on sea-spray droplets changes the droplets composition and the particle size distribution, which impacts deposition and the ion balance in different particle size fractions. It is shown that the ISOPART model is capable of simulating gas and particle concentrations in the coastal metropolis of Vancouver and the surrounding valley. It is also demonstrated that to accurately simulate ambient concentrations of particles and reactive/soluble gases in a coastal valley it is absolutely critical to include heterogeneous chemistry in/on sea-spray. Partitioning of total particle-NO{sub 3}{sup -} between sea-spray and NH{sub 4}NO{sub 3} is highly sensitive to the amount of sea-spray present, and hence the initial vertical profile, sea-spray source functions [48] and the wind speed. When a fixed wind speed is used to initialize the sea-spray vertical profiles, as expected, the sea-spray concentration decays with distance inland, but the particle-NO{sub 3}{sup -} concentration decays more slowly because it is also a function of the uptake rate for HNO{sub 3}. The simulation results imply model analyses of air quality in coastal cities conducted without inclusion of sea-spray interactions may yield highly misleading results in terms of emission sensitivities of the PM size distribution. The sensitivity of the model results to the initial sea spray profile further suggests there would be great benefit in better definition of the vertical profile of size resolved sea-spray for use in such model studies
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