Jesper Kleis

Scaling Relationships for Adsorption Energies on Transition Metal Oxide, Sulfide, and Nitride Surfaces**

There has been substantial progress in the description of adsorption and chemical reactions of simple molecules on transition-metal surfaces. Adsorption energies and activation energies have been obtained for a number of systems, and complete catalytic reactions have been described in some detail. Considerable progress has also been made in the theoretical description of the interaction of molecules with transition-metal oxides, sulfides, and nitrides, but it is considerably more complicated to describe such complex systems theoretically. Complications arise from difficulties in describing the stoichiometry and structure of such surfaces, and from possible shortcomings in the use of ordinary generalized gradient approximation (GGA) type density functional theory (DFT). Herein we introduce a method that may facilitate the description of the bonding of gas molecules to transitionmetal oxides, sulfides, and nitrides. It was recently found that there are a set of scaling relationhips between the adsorption energies of different partially hydrogenated intermediates on transition-metal surfaces. We will show that similar scaling relationships exist for adsorption on transition metal oxide, sulfide, and nitride surfaces. This means that knowing the adsorption energy for one transition-metal complex will make it possible to quite easily generate data for a number of other complexes, and in this way obtain reactivity trends. The results presented herein have been calculated using self-consistent DFT. Exchange and correlation effects are described using the revised Perdew–Burke–Ernzerhof (RPBE) GGA functional. It is known that GGA functionals give adsorption energies with reasonable accuracy for transition metals. It is not clear, however, whether a similar accuracy can be expected for the oxides, sulfides, and nitrides, although there are examples of excellent agreement betweenDFT calculations and experiments, for example, with RuO2 surfaces. [9] In our study we focused entirely on variations in the adsorption energies from one system to another, and we expected that such results would be less dependent than the absolute adsorption energies on the description of exchange and correlation. For the nitrides, a clean surface and a surface with a nitrogen vacancy were studied. For MX2-type oxides or sulfides, an oxygenor sulfur-covered surface with an oxygen or sulfur vacancy was studied. The structures of the clean surface considered in the present work and their unit cells are shown in Figure 1. The adsorption energies given below are for the adsorbed species in the most stable adsorption site on the surface. By performing calculations for a large number of transition-metal surfaces of different orientations, it was found that the adsorption energy of intermediates of the type AHx is linearly correlated with the adsorption energy of atom A (N, O, S) according to Equation (1):

Prediction of solid oxide fuel cell cathode activity with first-principles descriptors

In this work we demonstrate that the experimentally measured area specific resistance and oxygen surface exchange of solid oxide fuel cell cathode perovskites are strongly correlated with the first-principles calculated oxygenp-band center and vacancy formation energy. These quantities are therefore descriptors of catalytic activity that can be used in the first-principles design of new SOFC cathodes.

Potassium intercalation in graphite : A van der Waals density-functional study

Potassium intercalation in graphite is investigated by first-principles theory. The bonding in the potassium-graphite compound is reasonably well accounted for by traditional semilocal density functional theory (DFT) calculations. However, to investigate the intercalate formation energy from pure potassium atoms and graphite requires use of a description of the graphite interlayer binding and thus a consistent account of the nonlocal dispersive interactions. This is included seamlessly with ordinary DFT by a van der Waals density functional (vdW-DF) approach [Phys. Rev. Lett. 92, 246401 (2004)]. The use of the vdW-DF is found to stabilize the graphite crystal, with crystal parameters in fair agreement with experiments. For graphite and potassium-intercalated graphite structural parameters such as binding separation, layer binding energy, formation energy, and bulk modulus are reported. Also the adsorption and sub-surface potassium absorption energies are reported. The vdW-DF description, compared with the traditional semilocal approach, is found to weakly soften the elastic response.

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.

Rate Control and Reaction Engineering

A concept for evaluating the relative importance of steps in complex reactions may guide the development of better catalysts. A simple explanation for the acceleration of reactions by catalysts is that they speed up the rate-determining step. However, many reactions of practical importance are complex, multistep processes, and assessing the role that each step may play in controlling the overall rate of the reaction is not trivial. In a recent paper, Stegelmann et al. (1) have introduced a useful concept for characterizing such reactions: the general degree of rate control. Identifying the elementary steps that exert the greatest control on the overall rate of a chemical reaction offers the possibility of finding an appropriate catalyst or improving existing ones.

Influence of van der Waals forces on the adsorption structure of benzene on silicon studied using density functional theory

Two different adsorption configurations of benzene on the

Binding of polycyclic aromatic hydrocarbons and graphene dimers in density functional theory

\mathrm{Si}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)

Electronic shell structure and chemisorption on gold nanoparticles

surface, the tight-bridge and butterfly structures, were studied using density functional theory. Several exchange and correlation functionals were used, including the recently developed van der Waals density functional (vdW-DF), which accounts for the effect of van der Waals forces. In contrast to the Perdew-Burke-Ernzerhof (PBE), revPBE, and other generalized-gradient approximation functionals, the vdW-DF finds that, for most coverages, the adsorption energy of the butterfly structure is greater than that of the tight-bridge structure.

Nature and strength of bonding in a crystal of semiconducting nanotubes: van der Waals density functional calculations and analytical results

An early van der Waals density functional (vdW-DF) described layered systems (such as graphite and graphene dimers) using a layer-averaged electron density in the evaluation of nonlocal correlations. This early vdW-DF version was also adapted to approximate the binding of polycyclic aromatic hydrocarbons (PAHs) (Chakarova S D and Schroder E 2005 J. Chem. Phys. 122 054102). In parallel to that PAH study, a new vdW-DF version (Dion M, Rydberg H, Schroder E, Langreth D C and Lundqvist B I 2004 Phys. Rev. Lett. 92 246401) was developed that provides accounts of nonlocal correlations for systems of general geometry. We apply here the latter vdW-DF version to aromatic dimers of benzene, naphthalene, anthracene and pyrene, stacked in sandwich (AA) structure, and the slipped-parallel (AB) naphthalene dimer. We further compare the results of the two methods as well as other theoretical results obtained by quantum-chemistry methods. We also compare calculations for two interacting graphene sheets in the AA and the AB structures and provide the corresponding graphene-from-graphite exfoliation energies. Finally, we present an overview of the scaling of the molecular–dimer interaction with the number of carbon atoms and with the number of carbon rings.

van der Waals interaction of simple, parallel polymers

We use density functional theory (DFT) to investigate the electronic structure and chemical properties of gold nanoparticles. Different structural families of clusters are compared. For up to 60 atoms we optimize structures using DFT-based simulated annealing. Cluster geometries are found to distort considerably, creating large band gaps at the Fermi level. For up to 200 atoms we consider structures generated with a simple EMT potential and clusters based on cuboctahedra and icosahedra. All types of cluster geometry exhibit jellium-like electronic shell structure. We calculate adsorption energies of several atoms on the cuboctahedral clusters. Adsorption energies are found to vary abruptly at magic numbers. Using a Newns-Anderson model we find that the effect of magic numbers on adsorption energy can be understood from the location of adsorbate-induced states with respect to the cluster Fermi level.