A. Eichler

CO adsorption on close-packed transition and noble metal surfaces: trends from ab initio calculations

We have studied the trends in CO adsorption on close-packed metal surfaces: Co, Ni, Cu from the 3d row, Ru, Rh, Pd, Ag from the 4d row and Ir, Pt, Au from the 5d row using density functional theory. In particular, we were concerned with the trends in adsorption energy, geometry, vibrational properties and other parameters derived from the electronic structure of the substrate. The influence of specific changes in our set-up, such as choice of the exchange correlation functional, the choice of pseudopotential, size of the basis set and substrate relaxation, has been carefully evaluated. We found that, while the geometrical and vibrational properties of the adsorbate–substrate complex are calculated with high accuracy, the adsorption energies calculated with the gradient-corrected Perdew–Wang exchange–correlation energies are overestimated. In addition, the calculations tend to favour adsorption sites with higher coordination, resulting in the prediction of the wrong adsorption sites for the Rh, Pt and Cu surfaces (hollow instead of top). The revised Perdew–Burke–Erzernhof functional (RPBE) leads to lower (i.e. more realistic) adsorption energies for transition metals, but to the wrong results for noble metals—for Ag and Au, endothermic adsorption is predicted. The site preference remains the same. We discuss trends in relation to the electronic structure of the substrate across the periodic table, summarizing the state-of-the-art of CO adsorption on close-packed metal surfaces.

Hydrogen adsorption on palladium : a comparative theoretical study of different surfaces

Abstract The interaction of atomic hydrogen with the Pd(111), Pd(100) and Pd(110) surfaces is studied by ab-initio density functional calculations within the generalized gradient approximation (GGA). For the three surfaces, we have determined the preferred adsorption sites, the adsorption structures, the work function changes and the surface diffusion barrier, including relaxation effects. This comparative study allows some common features to be seen, in particular in the adsorption energies and geometries for both surface and subsurface H-atoms, and some significant differences such as the surface diffusion and the dispersion of the H-induced surface state. The origin of these differences is explained by a detailed analysis of the electronic structures of both clean and hydrogen-covered surfaces. Our study leads to an interesting correlation between the hydrogen diffusion barrier and the surface roughness since it plays an important part in the catalytic activity of the respective surfaces.

CO oxidation on transition metal surfaces: reaction rates from first principles

In this study, density function theory calculations are applied to the simulation of CO oxidation reactions over platinum, palladium and rhodium surfaces. On the basis of these calculations alone the detailed reaction scenario together with activation energies, pre-factors and rate constants can be derived. Such studies allow a systematic analysis of trends due to exactly identical conditions. The comparison with observed reaction rates demonstrates that such an approach gives reliable results and provides further insight into the reaction mechanism.

Ab initio density-functional study of NO on close-packed transition and noble metal surfaces: I. Molecular adsorption

Ab initio density-functional calculations have been used to investigate the molecular adsorption of NO on the close-packed surfaces of late transition metals (Co, Ni, Ru, Rh, Pd, Ir, Pt) and noble metals (Cu, Ag, Au). The energetics, geometry, and vibrational properties of the adsorbate–substrate complex have been calculated. With the exception of Ir and Au, adsorption in a hollow-site is always preferred. On Ir(111) the potential-energy surface for NO adsorption is very flat, with a slight preference for a linear on-top geometry. On Au(111), where NO adsorption is only very weak, bridge-adsorption with a strong tilting of the NO molecule relative to the surface normal is predicted. Among the different hollows on a (111) surface preference changes from hcp on Co, Ni, Ru, Rh to fcc on Cu, Pd, Pt, and Ag. However, not only on Ir, but also on Co, Ru, Rh and Pt are the site-dependent differences in the adsorption energies small enough to allow a coexistence of NO adsorbed on different sites. A careful comparison of the calculated vibrational eigenmodes with the available experimental data leads to full agreement between the predicted site preference and the observed NO stretching frequencies. This leads to a redefinition of the characteristic frequency intervals to be used for the site assignment. The trends in the adsorption energies and in the vibrational spectra are compared to those derived from studies of CO adsorption on the same surfaces and discussed in terms of the filling of the d-band of the substrate. In a forthcoming publication, these studies will be extended to NO dissociation on these substrates.

Structural, electronic and magnetic properties of nickel surfaces

Abstract The structural, electronic and magnetic properties of the low-index surfaces of Nickel have been investigated via fully self-consistent ab-initio local-spin-density-functional (LSDF) calculations. Our technique is based on ultrasoft pseudopotentials, residuum minimization techniques for the calculation of the electronic ground-state and of the Hellmann–Feynman forces and stresses, and on a conjugate-gradient technique for the optimization of the atomic structure. The calculations were performed for nine-layer symmetric slabs, allowing for the relaxation of the upper three layers. We also present a detailed analysis of electronic surface states.

Structural and electronic properties of rhodium surfaces : an ab initio approach

Abstract Structural and electronic properties of the low-index surfaces of rhodium have been investigated via fully self-consistent ab initio local density functional (LDF) calculations. Our technique is based on ultrasoft pseudopotentials, a preconditioned conjugate-gradient technique for the calculation of the electronic ground-state and of the Hellmann-Feynman forces and stresses, and on a conjugate-gradient technique for the optimization of the atomic structure. The calcualtions were performed for five- to ten-layer slabs in symmetric and asymmetric geometries, allowing for the relaxation of up to seven surface layers. For the (111), (100), and (110) surfaces an inward relaxation of the top layer by −1.7±0.2, −3.8±0.2, and −9.8±0.6% is predicted, the surface energies increase parallel to the inward relaxation. The analysis of the electronic structure shows that the inward relaxation is caused by the de-population at the surface of anti-bonding states at the top of the d-band. We also present a detailed analysis of electronic surface states.

Ab-initio calculations of the 6D potential energy surfaces for the dissociative adsorption of H2 on the (100) surfaces of Rh, Pd and Ag

Abstract Detailed investigations of the six-dimensional potential energy surface (PES) for the dissociative adsorption of a hydrogen molecule on the (100) surface of Rh, Pd and Ag are presented. The calculations are based on local density functional theory with generalized gradient corrections to the exchange-correlation functional, and have been performed using the Vienna ab-initio simulation package vasp . vasp works in a plane-wave basis and uses ultrasoft pseudopotentials. We show that adsorption on Rh(100) and Pd(100) is in general non-activated, but barriers exist along certain reaction channels. The “adiabatic” minimum-energy channel has been determined by a five-dimensional minimization of the total energy at a fixed height of the molecule. The variation of the covalent hydrogen-metal bond along this channel is studied using crystal orbital overlap populations and the electron localization function. On Ag(100), H 2 adsorption is strongly activated, with a pronounced variation of the barrier over the surface cell.

Ab initio density-functional study of NO adsorption on close-packed transition and noble metal surfaces: II. Dissociative adsorption

Following our investigation of molecular NO adsorption (Gajdos et al 2006 J. Phys.: Condens. Matter 18 13), the dissociation of NO molecules on the close-packed surfaces of late transition (Co, Ni, Ru, Rh, Pd, Ir, Pt) and noble (Cu, Ag, Au) metals has been studied using first-principles density functional calculations. The nudged-elastic-band method has been used for the determination of the transition states. Our results demonstrate that the transition-state energies show a linear dependence on the dissociative chemisorption energies according to the Bronsted–Evans–Polanyi rule. The validity of this linear relationship is shown to arise from the geometrical similarity of the transition states on all metals, which are very close to the final state geometries.

Hydrogen adsorption on the (100) surfaces of rhodium and palladium: the influence of non-local exchange - correlation interactions

We report ab initio investigations of the adsorption of atomic hydrogen on the (100) surfaces of Rh and Pd in the local-density-functional and generalized-gradient approximations. Our calculations have been performed using a plane-wave basis, using optimized ultrasoft pseudopotentials for describing the electron - ion interactions. Detailed results are reported for the adsorption energies, the stabilities of various adsorption geometries, and the adsorption-induced changes in the surface relaxations and in the work-functions. We find that the adsorption of a monolayer of hydrogen changes the inward relaxation of the top layer of the substrate into an outward relaxation. However, the change of the substrate relaxation has only a very small influence on the adsorption energy and geometry. For both metals the stable adsorption sites are the fourfold hollows. The site preference has its origin in a maximum gain of covalent bonding energy resulting from the overlap of the hydrogen s and the metal orbitals and from a minimal Pauli repulsion. Non-local exchange - correlation corrections have only a small influence on the atomic adsorption process and on the relaxation of the substrate, but influence the adsorption energy through corrections to the binding energy of the hydrogen molecule. Relativistic effects, however, turn out to be quite important.

Adsorption of CO on Rh(100) studied by ab initio local-density functional calculations

Ab initio local-density functional studies of the adsorption of CO on the (100) surface of Rh have been performed. We show that although adsorption in the bridge site is always energetically more favorable than adsorption in either the on-top or the hollow sites, two different mechanisms can lead to a relatively high occupation of the on-top sites: (i) At higher coverage the interactions between the adsorbates stabilize a pseudohexagonal coincidence lattice with the experimentally observed p(4√2×√2) structure with a bridge/on-top ratio of 2:1 (all adsorbates being slightly shifted from their high-symmetry positions). (ii) At lower coverages there seems to be a contradiction between the energetic preference for bridge-site adsorption and the mixed top/bridge adsorption reported in the experiments. This could simply be dismissed as a failure of density-functional theory. However, we speculate about a possible way to reconcile the calculated potential energy surface and the experimental observations: At dist...