arXiv subjects

Karsten Reuter

Publications and source records attributed to Karsten Reuter.

At least 73 records · Page 4Linked to original sources

arXiv2015

Fingerprints of energy dissipation for exothermic surface chemical reactions: O$_2$ on Pd(100)

We present first-principles calculations of the sticking coefficient of O$_2$ at Pd(100) to assess the effect of phononic energy dissipation on this kinetic parameter. For this we augment dynamical simulations on six-dimensional potential energy surfaces (PESs) representing the molecular degrees of freedom with various effective accounts of surface mobility. In comparison to the prevalent frozen-surface approach energy dissipation is found to qualitatively affect the calculated sticking curves. At the level of a generalized Langevin oscillator (GLO) model we achieve good agreement with experimental data. The agreement is similarly reached for PESs based on two different semi-local density-functional theory functionals. This robustness of the simulated sticking curve does not extend to the underlying adsorption mechanism, which is predominantly directly dissociative for one functional or molecularly trapped for the other. Completely different adsorption mechanisms therewith lead to rather similar sticking curves that agree equally well with the experimental data. This highlights the danger of the prevalent practice to extract corresponding mechanistic details from simple fingerprints of measured sticking data for such exothermic surface reactions.

cond-mat.mtrl-sci↗

arXiv2015

First-principles embedded-cluster calculations of the neutral and charged oxygen vacancy at the rutile TiO$_2$(110) surface

We perform full-potential screened-hybrid density-functional theory (DFT) calculations to compare the thermodynamic stability of neutral and charged states of the surface oxygen vacancy at the rutile TiO$_2$(110) surface. Solid-state (QM/MM) embedded-cluster calculations are employed to account for the strong TiO$_2$ polarization response to the charged defect states. Similar to the situation for the bulk O vacancy, the +2 charge state $V_{\rm O}^{2+}$ is found to be energetically by far most stable. Only for Fermi-level positions very close to the conduction band, small polarons may at best be trapped by the charged vacancy. The large decrease of the $V_{\rm O}^{2+}$ formation energy with decreasing Fermi-level position indicates strongly enhanced surface O vacancy concentrations for $p$-doped samples.

cond-mat.mtrl-sci↗

arXiv2015

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 $(\sqrt{5}\times \sqrt{5})R27^{\circ}$ 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.

cond-mat.mes-hall↗

arXiv2014

Temperature-dependent templated growth of porphine thin films on the (111) facets of copper and silver

The templated growth of the basic porphyrin unit, free-base porphine (2H-P), is characterized by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy measurements and density functional theory (DFT). The DFT simulations allow the deconvolution of the complex XPS and NEXAFS signatures into contributions originating from five inequivalent carbon atoms, which can be grouped into C-N and C-C bonded species. Polarization-dependent NEXAFS measurements reveal an intriguing organizational behavior: On both Cu(111) and Ag(111), for coverages up to one monolayer, the molecules adsorb undeformed and parallel to the respective metal surface. Upon increasing the coverage, however, the orientation of the molecules in the thin films depends on the growth conditions. Multilayers deposited at low temperatures (LT) exhibit a similar average tilting angle (30 degree relative to the surface plane) on both substrates. Conversely, for multilayers grown at room temperature a markedly different scenario exists. On Cu(111) the film thickness is self-limited to a coverage of approximately two layers, while on Ag(111) multilayers can be grown easily and, in contrast to the bulk 2H-P crystal, the molecules are oriented perpendicular to the surface. This difference in molecular orientation results in a modified line-shape of the C 1s XPS signatures that is dependent on the incident photon energy, which is explained by comparison with depth-resolved DFT calculations. Simulations of ionization energies for differently stacked molecules show no indication for a packing-induced modification of the multilayer XP spectra, thus indicating that the comparison of single molecule calculations to multilayer data is justified.

cond-mat.mtrl-sci↗

arXiv2014

Thermodynamics of Surface Defects at the Aspirin/Water Interface

We present a simulation scheme to calculate defect formation free energies at a molecular crystal/water interface based on force-field molecular dynamics (MD) simulations. To this end we adopt and modify existing approaches to calculate binding free energies of biological ligand/receptor complexes to be applicable to common surface defects, such as step edges and kink sites. We obtain statistically accurate and reliable free energy values for the aspirin/water interface, which can be applied to estimate the distribution of defects using well-established thermodynamic relations. As a show case we calculate the free energy upon dissolving molecules from kink sites at the interface. This free energy can be related to the solubility concentration and we obtain solubility values in excellent agreement with experimental results.

cond-mat.mtrl-sci↗

arXiv2014

Modeling Heat Dissipation at the Nanoscale: An Embedding Approach for Chemical Reaction Dynamics on Metal Surfaces

We present an embedding technique for metallic systems that makes it possible to model energy dissipation into substrate phonons during surface chemical reactions from first principles. The separation of chemical and elastic contributions to the interaction potential provides a quantitative description of both electronic and phononic band structure. Application to the dissociation of O$_2$ at Pd(100) predicts translationally "hot" oxygen adsorbates as a consequence of the released adsorption energy (ca. 2.6 eV). This finding questions the instant thermalization of reaction enthalpies generally assumed in models of heterogeneous catalysis.

cond-mat.mtrl-sci↗

arXiv2014

Embedded-Cluster Calculations in a Numeric Atomic Orbital Density-Functional Theory Framework

We integrate the all-electron electronic structure code FHI-aims into the general ChemShell package for solid-state embedding (QM/MM) calculations. A major undertaking in this integration is the implementation of pseudopotential functionality into FHI-aims to describe cations at the QM/MM boundary through effective core potentials and therewith prevent spurious overpolarization of the electronic density. Based on numeric atomic orbital basis sets, FHI-aims offers particularly efficient access to exact exchange and second order perturbation theory, rendering the established QM/MM setup an ideal tool for hybrid and double-hybrid level DFT calculations of solid systems. We illustrate this capability by calculating the reduction potential of Fe in the Fe-substituted ZSM-5 zeolitic framework and the reaction energy profile for (photo-)catalytic water oxidation at TiO2(110).

cond-mat.mtrl-sci↗

arXiv2013

Computational screening study towards redox-active metal-organic frameworks

The metal-organic framework (MOF) MFU-4l containing Co(II) centers and Cl- ligands has recently shown promising redox activity. Aiming for further improved MOF catalysts for oxidation processes employing molecular oxygen we present a density-functional theory (DFT) based computational screening approach to identify promising metal center and ligand combinations within the MFU-4l structural family. Using the O2 binding energy as a descriptor for the redox property, we show that relative energetic trends in this descriptor can reliably be obtained at the hybrid functional DFT level and using small cluster (scorpionate-type complex) models. Within this efficient computational protocol we screen a range of metal center / ligand combinations and identify several candidate systems that offer more exothermic O2 binding than the original Co/Cl-based MFU-4l framework.

cond-mat.mtrl-sci↗

arXiv2013

Ready, set and no action: A static perspective on potential energy surfaces commonly used in gas-surface dynamics

In honoring the seminal contribution of Henry Eyring and Michael Polanyi who first introduced the concept of potential energy surfaces (PESs) to describe chemical reactions in gas-phase [Z. Phys. Chem. 12, 279-311, (1931)], this work comes to review and assess state-of-the-art approaches towards first-principle based modeling in the field of gas-surface dynamics. Within the Born-Oppenheimer and frozen surface approximations, the O2-Ag(100) interaction energetics are used as a showcase system to accentuate the complex landscape exhibited by the PESs employed to describe the impingement of diatomics on metal substrates and draw attention to the far-from-trivial task of continuously representing them within all six molecular degrees of freedom. To this end, the same set of ab initio reference data obtained within Density Functional Theory (DFT) are continuously represented by two different state-of-the-art high-dimensional approaches, namely the Corrugation-Reducing Procedure and Neural Networks. Exploiting the numerically undemanding nature of the resulting representations, a detailed static evaluation is performed on both PESs based on an extensive global minima search. The latter proved particularly illuminating in revealing representation deficiencies which affect the dynamical picture yet go otherwise unnoticed within the so-called "divide-and-conquer" approach.

cond-mat.mtrl-sci↗

arXiv2013

Adsorption structure determination of a large polyaromatic trithiolate on Cu(111): combination of LEED-I(V) and DFT-vdW

The adsorption geometry of 1,3,5-tris(4-mercaptophenyl)benzene (TMB) on Cu(111) is determined with high precision using two independent methods, experimentally by quantitative low energy electron diffraction (LEED-I(V)) and theoretically by dispersion corrected density functional theory (DFT-vdW). Structural refinement using both methods consistently results in similar adsorption sites and geometries. Thereby a level of confidence is reached that allows deduction of subtle structural details such as molecular deformations or relaxations of copper substrate atoms.

cond-mat.mtrl-sci↗

arXiv2013

Control of Intermolecular Bonds by Deposition Rates at Room Temperature: Hydrogen Bonds versus Metal Coordination in Trinitrile Monolayers

Self-assembled monolayers of 1,3,5-tris(4'-biphenyl-4"-carbonitrile)benzene, a large functional trinitrile molecule, on the (111) surfaces of copper and silver under ultrahigh vacuum conditions were studied by scanning tunneling microscopy and low-energy electron diffraction. A densely packed hydrogen-bonded polymorph was equally observed on both surfaces. Additionally, deposition onto Cu(111) yielded a well-ordered metal-coordinated porous polymorph that coexisted with the hydrogen-bonded structure. The required coordination centers were supplied by the adatom gas of the Cu(111) surface. On Ag(111), however, the well-ordered metal-coordinated network was not observed. Differences between the adatom reactivities on copper and silver and the resulting bond strengths of the respective coordination bonds are held responsible for this substrate dependence. By utilizing ultralow deposition rates, we demonstrate that on Cu(111) the adatom kinetics plays a decisive role in the expression of intermolecular bonds and hence structure selection.

cond-mat.mtrl-sci↗

arXiv2013

Multi-Doping of Si Cages: High Spin States beyond the Single-Dopant Septet Limit

Density-functional theory based global geometry optimization is employed to systematically scrutinize the possibility of multi-doping of hydrogenated Si clusters in order to achieve high spin states beyond the septet limit of a single-atom dopant. While our unbiased configurational search reveals that the previously suggested Si18H12 double hexagonal prism structure is generally too small to accommodate two dopants in magnetized state, the larger Si24H24 cage turns out to be suitable for such applications. For dimer dopants M2+ = Cr2+, Mn2+ and CrMn+, the structural integrity of the host cage is conserved in the ground-state structure of corresponding M2+@Si24H24 aggregates, as is the unusually high spin state of the guest dopant, which in case of Cr2+ already exceeds the single-atom dopant septet limit by almost a factor of two. Moreover, the possibility of further increasing the cluster spin moment by encapsulating an even larger number of dopants into a suitably sized hydrogenated Si cage is illustrated for the example of a (CrMn+)2@Si28H28 aggregate with a total number of 18 unpaired electrons. These results strongly suggest multi-doping of Si clusters as a viable route to novel cluster-based materials for magneto-optic applications.

cond-mat.mtrl-sci↗

arXiv2013

First-principles thermodynamic screening approach to photo-catalytic water splitting with co-catalysts

We adapt the computational hydrogen electrode approach to explicitly account for photo-generated charges and use it to computationally screen for viable catalyst/co-catalyst combinations for photo-catalytic water splitting. The hole energy necessary to thermodynamically drive the reaction is employed as descriptor for the screening process. Using this protocol and hybrid-level density-functional theory we show that water oxidation on bare TiO2 surfaces is thermodynamically more complex than previously thought. This motivates a screening for suitable co-catalysts for this half-reaction, which we carry out for Au particles down to the non-scalable size regime. We find that almost all small Au clusters studied are better suited for water photo-oxidation than an extended Au(111) surface or bare TiO2 facets.

cond-mat.mtrl-sci↗

arXiv2013

Excited-state potential-energy surfaces of metal-adsorbed organic molecules from Linear Expansion Δ-Self-Consistent Field Density-Functional Theory (ΔSCF-DFT)

Accurate and efficient simulation of excited state properties is an important and much aspired cornerstone in the study of adsorbate dynamics on metal surfaces. To this end, the recently proposed linear expansion ΔSelf-Consistent Field (leΔSCF) method by Gavnholt et al. [Phys. Rev. B 78, 075441 (2008)] presents an efficient alternative to time consuming quasi-particle calculations. In this method the standard Kohn-Sham equations of Density-Functional Theory are solved with the constraint of a non-equilibrium occupation in a region of Hilbert-space resembling gas-phase orbitals of the adsorbate. In this work we discuss the applicability of this method for the excited-state dynamics of metal-surface mounted organic adsorbates, specifically in the context of molecular switching. We present necessary advancements to allow for a consistent quality description of excited-state potential-energy surfaces (PESs), and illustrate the concept with the application to Azobenzene adsorbed on Ag(111) and Au(111) surfaces. We find that the explicit inclusion of substrate electronic states modifies the topologies of intra-molecular excited-state PESs of the molecule due to image charge and hybridization effects. While the molecule in gas phase shows a clear energetic separation of resonances that induce isomerization and backreaction, the surface-adsorbed molecule does not. The concomitant possibly simultaneous induction of both processes would lead to a significantly reduced switching efficiency of such a mechanism.

cond-mat.mtrl-sci↗

arXiv2013

Exploring Morphology-Activity Relationships: Ab Initio Wulff Construction for RuO2 Nanoparticles under Oxidizing Conditions

We present a density-functional theory based Wulff construction of the equilibrium shape of RuO2 particles in an oxygen environment. The obtained intricate variations of the crystal habit with the oxygen chemical potential allow for a detailed discussion of the dependence on the oxidizing pretreatment observed in recent powder catalyst studies. The analysis specifically indicates an incomplete particle shape equilibration in previously employed low temperature calcination. Equilibrated particles could be active CO oxidation catalysts with long-term stability in oxidizing feed and then represent an interesting alternative to the previously suggested core-shell concept.

cond-mat.mtrl-sci↗

arXiv2013

$M$Si$_{20}$H$_{20}$ Aggregates: From Simple Building Blocks to Highly Magnetic Functionalized Materials

Density-functional theory based global geometry optimization is used to scrutinize the possibility of using endohedrally-doped hydrogenated Si clusters as building blocks for constructing highly magnetic materials. In contrast to the known clathrate-type facet-sharing, the clusters exhibit a predisposition to aggregation through double Si-Si bridge bonds. For the prototypical CrSi$_{20}$H$_{20}$ cluster we show that reducing the degree of hydrogenation may be used to control the number of reactive sites to which other cages can be attached, while still preserving the structural integrity of the building block itself. This leads to a toolbox of CrSi$_{20}$H$_{20-2n}$ monomers with different number of double "docking sites", that allows building network architectures of any morphology. For (CrSi$_{20}$H$_{18}$)$_{2}$ dimer and [CrSi$_{20}$H$_{16}$](CrSi$_{20}$H$_{18}$)$_{2}$ trimer structures we illustrate that such aggregates conserve the high spin moments of the dopant atoms and are therefore most attractive candidates for cluster-assembled materials with unique magnetic properties. The study suggests that the structural completion of the individual endohedral cages within the doubly-bridge bonded structures and the high thermodynamic stability of the obtained aggregates are crucial for potential synthetic polymerization routes $via$ controlled dehydrogenation.

cond-mat.mtrl-sci↗

arXiv2013

CO oxidation on Pd(100) vs. PdO(sqrt5xsqrt5)R27^o: First-Principles Kinetic Phase Diagrams and Bistability Conditions

We present first-principles kinetic Monte Carlo (1p-kMC) simulations addressing the CO oxidation reaction at Pd(100) for gas-phase conditions ranging from ultra-high vacuum (UHV) to ambient pressures and elevated temperatures. For the latter technologically relevant regime there is a long-standing debate regarding the nature of the active surface. The pristine metallic surface, an ultra-thin (sqrt5xsqrt5)R27^o PdO(101) surface oxide, and thicker oxide layers have each been suggested as the active state. We investigate these hypotheses with 1p-kMC simulations focusing on either the Pd(100) surface or the PdO(101) surface oxide and intriguingly obtain a range of (T, p)-conditions where both terminations appear metastable. The predicted bistability regime nicely ties in with oscillatory behavior reported experimentally by Hendriksen and coworkers [Catal. Today 105, 234 (2005)]. Within this regime we find that both surface terminations exhibit very similar intrinsic reactivity, which puts doubts on attempts to assign the catalytic function to just one active state.

cond-mat.mtrl-sci↗

arXiv2012

Bistability loss as key feature in azobenzene (non-)switching on metal surfaces

Coinage metal adsorbed azobenzene is investigated as prototypical molecular switch. It is shown that switching capabilities are not just lost due to excited state quenching, but already due to changes in the ground state energetics. Electron demanding coadsorbates are suggested as strategy to regain the switching function.

cond-mat.mtrl-sci↗