7,083 research outputs found
Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?
During the exothermic adsorption of molecules at solid surfaces dissipation
of the released energy occurs via the excitation of electronic and phononic
degrees of freedom. For metallic substrates the role of the nonadiabatic
electronic excitation channel has been controversially discussed, as the
absence of a band gap could favour an easy coupling to a manifold of
electronhole pairs of arbitrarily low energies. We analyse this situation for
the highly exothermic showcase system of molecular oxygen dissociating at
Pd(100), using time-dependent perturbation theory applied to first-principles
electronic-structure calculations. For a range of different trajectories of
impinging O2 molecules we compute largely varying electron-hole pair spectra,
which underlines the necessity to consider the high-dimensionality of the
surface dynamical process when assessing the total energy loss into this
dissipation channel. Despite the high Pd density of states at the Fermi level,
the concomitant non-adiabatic energy losses nevertheless never exceed about 5%
of the available chemisorption energy. While this supports an electronically
adiabatic description of the predominant heat dissipation into the phononic
system, we critically discuss the non-adiabatic excitations in the context of
the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed
V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin
asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)
Effect of surface nanostructure on temperature programmed reaction spectroscopy: First-principles kinetic Monte Carlo simulations of CO oxidation at RuO2(110)
Using the catalytic CO oxidation at RuO2(110) as a showcase, we employ
first-principles kinetic Monte Carlo simulations to illustrate the intricate
effects on temperature programmed reaction spectroscopy data brought about by
the mere correlations between the locations of the active sites at a
nanostructured surface. Even in the absence of lateral interactions, this
nanostructure alone can cause inhomogeneities that cannot be grasped by
prevalent mean-field data analysis procedures, which thus lead to wrong
conclusions on the reactivity of the different surface species.Comment: 4 pages including 3 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
The place of space technology in economic development: Reflections on present and future aspects
The effects of the development of satellite applications on the orientation of the space effort were examined. The gap between available and exploited technology, the impact of the current economic climate and future trends are discussed. Europe's low level of public funding for its space effort, in comparison to other space powers, and the dangers of complacency regarding Europe's competitiveness in the space market are illustrated. A proposal for the general direction which Europe's future strategy must take if European independence in this field is to be preserved is presented
Oxide formation at the surface of late 4d transition metals: Insights from first-principles atomistic thermodynamics
Using density-functional theory we assess the stability of bulk and surface
oxides of the late 4d transition metals in a ``constrained equilibrium'' with a
gas phase formed of O2 and CO. While the stability range of the most stable
bulk oxide extends for ruthenium well into gas phase conditions representative
of technological CO oxidation catalysis, this is progressively less so for the
4d metals to its right in the periodic system. Surface oxides could
nevertheless still be stable under such conditions. These thermodynamic
considerations are discussed in the light of recent experiments, emphasizing
the role of (surface) oxides as the active phase of model catalysts formed from
these metals.Comment: 7 pages including 3 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
kmos: A lattice kinetic Monte Carlo framework
Kinetic Monte Carlo (kMC) simulations have emerged as a key tool for
microkinetic modeling in heterogeneous catalysis and other materials
applications. Systems, where site-specificity of all elementary reactions
allows a mapping onto a lattice of discrete active sites, can be addressed
within the particularly efficient lattice kMC approach. To this end we describe
the versatile kmos software package, which offers a most user-friendly
implementation, execution, and evaluation of lattice kMC models of arbitrary
complexity in one- to three-dimensional lattice systems, involving multiple
active sites in periodic or aperiodic arrangements, as well as site-resolved
pairwise and higher-order lateral interactions. Conceptually, kmos achieves a
maximum runtime performance which is essentially independent of lattice size by
generating code for the efficiency-determining local update of available events
that is optimized for a defined kMC model. For this model definition and the
control of all runtime and evaluation aspects kmos offers a high-level
application programming interface. Usage proceeds interactively, via scripts,
or a graphical user interface, which visualizes the model geometry, the lattice
occupations and rates of selected elementary reactions, while allowing
on-the-fly changes of simulation parameters. We demonstrate the performance and
scaling of kmos with the application to kMC models for surface catalytic
processes, where for given operation conditions (temperature and partial
pressures of all reactants) central simulation outcomes are catalytic activity
and selectivities, surface composition, and mechanistic insight into the
occurrence of individual elementary processes in the reaction network.Comment: 21 pages, 12 figure
First-principles kinetic Monte Carlo simulations for heterogeneous catalysis, applied to the CO oxidation at RuO2(110)
We describe a first-principles statistical mechanics approach enabling us to
simulate the steady-state situation of heterogeneous catalysis. In a first step
density-functional theory together with transition-state theory is employed to
obtain the energetics of all relevant elementary processes. Subsequently the
statistical mechanics problem is solved by the kinetic Monte Carlo method,
which fully accounts for the correlations, fluctuations, and spatial
distributions of the chemicals at the surface of the catalyst under
steady-state conditions. Applying this approach to the catalytic oxidation of
CO at RuO2(110), we determine the surface atomic structure and composition in
reactive environments ranging from ultra-high vacuum (UHV) to technologically
relevant conditions, i.e. up to pressures of several atmospheres and elevated
temperatures. We also compute the CO2 formation rates (turnover frequencies).
The results are in quantitative agreement with all existing experimental data.
We find that the high catalytic activity of this system is intimately connected
with a disordered, dynamic surface ``phase'' with significant compositional
fluctuations. In this active state the catalytic function results from a
self-regulating interplay of several elementary processes.Comment: 18 pages including 9 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
CO oxidation at Pd(100): A first-principles constrained thermodynamics study
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
Ionic conductivity and relaxation dynamics in plastic-crystals with nearly globular molecules
We have performed a dielectric investigation of the ionic charge transport
and the relaxation dynamics in plastic-crystalline 1-cyano-adamantane (CNA) and
in two mixtures of CNA with the related plastic crystals adamantane or
2-adamantanon. Ionic charge carriers were provided by adding 1% of Li salt. The
molecules of these compounds have nearly globular shape and, thus, the
so-called revolving-door mechanism assumed to promote ionic charge transport
via molecular reorientations in other PC electrolytes, should not be active
here. Indeed, a comparison of the dc resistivity and the reorientational
alpha-relaxation times in the investigated PCs, reveals complete decoupling of
both dynamics. Similar to other PCs, we find a significant mixing-induced
enhancement of the ionic conductivity. Finally, these solid-state electrolytes
reveal a second relaxation process, slower than the alpha-relaxation, which is
related to ionic hopping. Due to the mentioned decoupling, it can be
unequivocally detected and is not superimposed by the reorientational
contributions as found for most other ionic conductors.Comment: 9 pages, 7 figure
CO oxidation on Pd(100) at technologically relevant pressure conditions: A first-principles kinetic Monte Carlo study
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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