Axel Groß

Reactions at surfaces studied by ab initio dynamics calculations

Abstract Owing to the development of efficient algorithms and the improvement of computer power it is now possible to map out potential energy surfaces (PESs) of reactions at surfaces in great detail. This achievement has been accompanied by an increased effort in the dynamical simulation of processes on surfaces. The paradigm for simple reactions at surfaces — the dissociation of hydrogen on metal surfaces — can now be treated fully quantum dynamically in the molecular degrees of freedom from first principles, i.e., without invoking any adjustable parameters. This relatively new field of ab initio dynamics simulations of reactions at surfaces will be reviewed. Mainly the dissociation of hydrogen on clean and adsorbate covered metal surfaces and on semiconductor surfaces will be discussed. In addition, the ab initio molecular dynamics treatment of reactions of hydrogen atoms with hydrogen-passivated semiconductor surfaces and recent achievements in the ab initio description of laser-induced desorption and further developments will be addressed.

Experimental analysis of charge redistribution due to chemical bonding by high-resolution transmission electron microscopy

The electronic charge density distribution or the electrostatic atomic potential of a solid or molecule contains information not only on the atomic structure, but also on the electronic properties, such as the nature of the chemical bonds or the degree of ionization of atoms. However, the redistribution of charge due to chemical bonding is small compared with the total charge density, and therefore difficult to measure. Here, we demonstrate an experimental analysis of charge redistribution due to chemical bonding by means of high-resolution transmission electron microscopy (HRTEM). We analyse charge transfer on the single-atom level for nitrogen-substitution point defects in graphene, and confirm the ionicity of single-layer hexagonal boron nitride. Our combination of HRTEM experiments and first-principles electronic structure calculations opens a new way to investigate electronic configurations of point defects, other non-periodic arrangements or nanoscale objects that cannot be studied by an electron or X-ray diffraction analysis.

Adsorption of small aromatic molecules on the (111) surfaces of noble metals: A density functional theory study with semiempirical corrections for dispersion effects

The adsorption of benzene, thiophene, and pyridine on the (111) surface of gold and copper have been studied using density functional theory (DFT). Adsorption geometries and energies as well as the nature of bonding have been analyzed and compared to experimental results. Dispersion effects between neighboring molecules and between molecules and the surface have been taken into account via a semiempirical C(6)R(-6) approach. The C(6) coefficients for metal atoms have been deduced using both atomic properties and a hybrid QM:QM approach. Whereas the pure DFT calculations underestimate the adsorption energies significantly, a good agreement with experimental results is obtained using the DFT-D method based on the QM:QM hybrid approach.

Properties of metal–water interfaces studied from first principles

Properties of the metal–water interface have been addressed by periodic density functional theory calculations, in particular with respect to the electronic and geometric structures of water bilayers on several transition metal surfaces. It will be demonstrated that the presence of the metal substrate leads to a significant polarization of the water bilayer. This causes a substantial water-induced reduction of the work function in spite of the weak water–metal interaction, but it is not associated with a significant change of the electronic structure of the metal substrates. The structure and the vibrational spectra of water bilayers at room temperatures have been studied performing ab initio molecular dynamics simulations. The simulations suggest that the water bilayer structure on noble metals is not stable at room temperature, whereas on more strongly interacting metal surfaces some ordering of the water layer persists. In addition, metal–water interfaces under electrochemical conditions, i.e. for charged metal substrates, are addressed. Our simulations show that the charging of the surface leads to characteristic changes in the wall–oxygen distribution and the vibrational spectra.

Theoretical Surface Science

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Local reactivity of thin Pd overlayers on Au single crystals

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Dissociative adsorption of hydrogen on strained Cu surfaces

The local reactivity of thin pseudomorphic Pd overlayers on Au(111) and (100) single crystal surfaces has been studied by periodic density functional theory calculations within the generalized gradient approximation. We have determined the adsorption energies of atomic hydrogen and of CO as a microscopic probe of the reactivity. We demonstrate that both surface strain effects and substrate interaction effects contribute to the modification of the reactivity of the overlayer system. While there is no unique trend in the adsorption energies as a function of the lattice strain, at all considered adsorption sites we find a maximum of the binding energies of both H and CO on two Pd overlayers on Au thus providing a microscopic explanation for recent experiments on the electrochemical reactivity of flat Pd nanoparticles on Au(111). Furthermore, we address the initial stages of Pd deposition on Au. # 2003 Elsevier Science B.V. All rights reserved.

Dispersive interactions in water bilayers at metallic surfaces: A comparison of the PBE and RPBE functional including semiempirical dispersion corrections†

The adsorption and dissociation of hydrogen on strained clean and oxygen-covered Cu surfaces have been studied by calculations based on density functional theory within the generalized gradient approximation. On all surfaces we find an upshift of the surface d-band center upon lattice expansion. Still there is no general trend in the hydrogen adsorption energies at the high-symmetry sites and the dissociation barrier heights as a function of lattice strain for the low-index Cu surfaces in contrast to the predictions of the d-band model. It turns out that the adsorbate-induced change of the Cu local d-band density of states has to be taken into account in order to rationalize these results. As far as the oxygenprecovered Cu(1 00) surface is concerned, the strain-induced change in the hydrogen adsorption energies and dissociation barriers can simply be related to the increased hydrogen–oxygen distance upon lattice expansion. 2002 Elsevier Science B.V. All rights reserved.

The virtual chemistry lab for reactions at surfaces: Is it possible? Will it be useful?

The accuracy and reliability of the density functional theory (DFT)‐D approach to account for dispersion effects in first‐principles studies of water–metal interfaces has been addressed by studying several water–metal systems. In addition to performing periodic DFT calculations for semi‐infinite substrates using the popular PBE and RPBE functionals, the water dimer and water–metal atom systems have also been treated by coupled‐cluster calculations. We show that indeed semiempirical dispersion correction schemes can be used to yield thermodynamically stable water bilayers at surfaces. However, the actual density functional needs to be chosen carefully. Whereas the dispersion‐corrected RPBE functional yields a good description of both the water–water and the water–metal interaction, the dispersion‐corrected PBE functional overestimates the energies of both systems. In contrast thereto, the adsorption distances predicted by the PBE functional is hardly changed due to the additional dispersion interaction, explaining the good performance of previous DFT‐PBE studies of water–metal systems.

Dispersion corrected RPBE studies of liquid water

Ab initio total-energy calculations based on electronic structure theory have tremendously enlarged our knowledge about the geometrical and electronic structure of clean and adsorbate-covered low-index surfaces and reactions on these surfaces. In technological applications, however, extended flat surfaces are very rarely used. Hence the applicability of the theoretical results for the technological surfaces are indeed questionable. In this review I will reflect on the question whether ab initio calculations of reactions at surfaces can contribute to the development of, e.g., better catalysts. Simulations alone will not be able to lead to new products but it will be demonstrated that they can contribute enormously to the development process. Thus the virtual chemistry lab is indeed possible and helpful.