464 research outputs found
Examination of the concept of degree of rate control by first-principles kinetic Monte Carlo simulations
The conceptual idea of degree of rate control (DRC) approaches is to identify
the "rate limiting step" in a complex reaction network by evaluating how the
overall rate of product formation changes when a small change is made in one of
the kinetic parameters. We examine two definitions of this concept by applying
it to first-principles kinetic Monte Carlo simulations of the CO oxidation at
RuO2(110). Instead of studying experimental data we examine simulations,
because in them we know the surface structure, reaction mechanism, the rate
constants, the coverage of the surface and the turn-over frequency at steady
state. We can test whether the insights provided by the DRC are in agreement
with the results of the simulations thus avoiding the uncertainties inherent in
a comparison with experiment. We find that the information provided by using
the DRC is non-trivial: It could not have been obtained from the knowledge of
the reaction mechanism and of the magnitude of the rate constants alone. For
the simulations the DRC provides furthermore guidance as to which aspects of
the reaction mechanism should be treated accurately and which can be studied by
less accurate and more efficient methods. We therefore conclude that a
sensitivity analysis based on the DRC is a useful tool for understanding the
propagation of errors from the electronic structure calculations to the
statistical simulations in first-principles kinetic Monte Carlo simulations.Comment: 27 pages including 5 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Effect of a humid environment on the surface structure of RuO2(110)
Combining density-functional theory and thermodynamics we compute the phase
diagram of surface structures of rutile RuO2 (110) in equilibrium with water
vapor in the complete range of experimentally accessible gas phase conditions.
Through the formation of hydroxyl or water-like groups, already lowest
concentrations of hydrogen in the gas phase are sufficient to stabilize an
oxygen-rich polar oxide termination even at very low oxygen pressure.Comment: 7 pages including 5 figures, Submitted to Phys. Rev. B., Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
On the stability of "non-magic" endohedrally doped Si clusters: A first-principles sampling study of MSi16^+ (M =Ti,V,Cr)
Density-functional theory is used to study the geometric and electronic
structure of cationic Si16^+ clusters with a Ti, V or Cr dopant atom. Through
unbiased global geometry optimization based on the basin-hopping approach we
confirm that a Frank- Kasper polyhedron with the metal atom at the center
represents the ground-state isomer for all three systems. The endohedral cage
geometry is thus stabilized even though only VSi16^+ achieves electronic shell
closure within the prevalent spherical potential model. Our analysis of the
electronic structure traces this diminished role of shell closure for the
stabilization back to the adaptive capability of the metal- Si bonding, which
is more the result of a complex hybridization than the orginally proposed mere
formal charge transfer. The resulting flexibility of the metal-Si bond can help
to stabilize also "non-magic" cage-dopant combinations, which suggests that a
wider range of materials may eventually be cast into this useful geometry for
cluster-assembled materials.Comment: 13 pages including 5 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Multi-Lattice Kinetic Monte Carlo Simulations from First-Principles: Reduction of the Pd(100) Surface Oxide by CO
We present a multi-lattice kinetic Monte Carlo (kMC) approach that
efficiently describes the atomistic dynamics of morphological transitions
between commensurate structures at crystal surfaces. As an example we study the
reduction of a PdO(101) overlayer on
Pd(100) in a CO atmosphere. Extensive density-functional theory calculations
are used to establish an atomistic pathway for the oxide reduction process.
First-principles multi-lattice kMC simulations on the basis of this pathway
fully reproduce the experimental temperature dependence of the reduction rate
[Fernandes et al., Surf. Sci. 2014, 621, 31-39] and highlight the crucial role
of elementary processes special to the boundary between oxide and metal
domains.Comment: 19 pages, 10 figure
Towards an exact treatment of exchange and correlation in materials: Application to the "CO adsorption puzzle" and other systems
It is shown that the errors of present-day exchange-correlation (xc)
functionals are rather short ranged. For extended systems the correction can
therefore be evaluated by analyzing properly chosen clusters and employing
highest-quality quantum chemistry methods. The xc correction rapidly approaches
a universal dependence with cluster size. The method is applicable to bulk
systems as well as to defects in the bulk and at surfaces. It is demonstrated
here for CO adsorption at transition-metal surfaces, where present-day xc
functionals dramatically fail to predict the correct adsorption site, and for
the crystal bulk cohesive energy.Comment: slightly revised version: 4 pages including 3 figures; related
publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm
First-principles statistical mechanics study of the stability of a sub-nanometer thin surface oxide in reactive environments: CO oxidation at Pd(100)
We employ a multiscale modeling approach to study the surface structure and
composition of a Pd(100) model catalyst in reactive environments. Under gas
phase conditions representative of technological CO oxidation (~1 atm, 300-600
K) we find the system on the verge of either stabilizing sub-nanometer thin
oxide structures or CO adlayers at the surface. Under steady-state operation
this suggests the presence or continuous formation and reduction of oxidic
patches at the surface, which could be key to understand the observable
catalytic function.Comment: 4 pages including 2 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
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
Monitoring of the “Energiewende” – energy efficiency indicators for Germany
The increasing number of energy and climate targets both at national and international level induces a rising demand for regular monitoring. In this paper, we
analyse the possibilities and limits of using energy efficiency indicators as a tool for monitoring these targets. We refer to the energy efficiency targets of the
German “Energiewende” and calculate and discuss several energy efficiency indicators for Germany both at the level of the overall economy and the main energy consumption sectors. We make use of the energy efficiency indicator toolbox that we have developed within the ODYSSEE database in recent years and find that there is still a considerable gap to close to achieve the overall energy efficiency targets in Germany by 2020. We also show that progress in energy efficiency slowed down between 2008 and 2012, i.e. compared to the base
year of most of the German energy efficiency targets and find that energy efficiency progress in the industrial sector during the last decade has been especially
slow. We conclude that improvements in energy efficiency have to speed up considerably in order to achieve the targets for 2020. Although the use of energy efficiency indicators is limited by data constraints and some methodological problems, these indicators give a deep insight into the factors determining energy consumption and can therefore complement the official monitoring
process of the German “Energiewende” which only relies on highly aggregated indicators for energy efficiency
- …