Adsorption cluster approach

Staemmler V (2005) The Cluster Approach for the Adsorption of Small Molecules on Oxide Surfaces. 12 219-256... 

One of the most efficient approaches allowing us to investigate in a reasonable time a catalytic cycle on non-periodic materials in combination with reliable DFT functional is a cluster approach. The present study is devoted to the investigation of the effect of the cluster size on the energetic properties of the (p-oxo)(p-hydroxo)di-iron metal active site. As a first step, we have studied the stability of the [Fen(p-0)(p-0H)Fen]+ depending on the A1 position and cluster size. Then, we compared the energetics for the routes involving the first two elementary steps of the N20 decomposition catalytic process i.e. the adsorption and dissociation of one N20 molecule. 

Adsorption energies on metals calculated in a cluster approach often show considerable oscillations with size and shape of the cluster models because such (finite) clusters describe the surface electronic structure insufficiently [257-260]. These models may yield rather different results for the Pauli repulsion between adsorbate and substrate, depending on whether pertinent cluster orbitals localized at the adsorption site are occupied or empty. The discrete density of states is an inherent feature of clusters that may prevent a correct description of the polarizability of a metal surface and thus hinders cluster size convergence of adsorption energies [257]. Even embedding of metal clusters does not offer an easy way out of this dilemma [260,261]. Anyway, the form of conventional moderately large cluster models may be particularly crucial. Such models are inherently two-dimensional with substrate atoms from two or three crystal layers usually taken into accormt thus, a large fraction of atoms at the cluster boundaries lacks proper coordination. 

Abstract The correct description of the weak interaction between small molecules and oxide surfaces is still a challenge for theory. In the present review, the current status of the cluster approach to the calculation of adsorption geometries and energies by means of quantum-chemical ab initio methods is discussed. In the first part, the physical and chemical contributions to the bonding mechanism are briefly characterized and the different clusters models currently used for treating molecule/surface interactions are presented free clusters, hydrogen saturated clusters and embedded clusters. We continue with a description of the... 

In the present review we will discuss the current status of the quantum-chemical treatment of the adsorption of small molecules on oxide surfaces. We will limit our attention to oxide surfaces, because the problems encountered here are quite different from those connected with the treatment of metal surfaces. There are essentially two approaches to deal with a system that consists of a small molecule and an extended solid surface, i.e., a local process on a semi-infinite substrate. One way is the cluster approach described in the following in which a small cluster of atoms is cut out of the surface and the system molecule and cluster is treated as a supermolecule with the methods of molecular quantum chemistry. The alternative way is the supercell approach , in which the adsorbed molecule is repeated periodically on the surface, and the system surface with an ordered overlayer of adsorbed molecules is treated by means of periodic calculations. 

We will only treat the cluster approach in the present review. We further refer the reader to a series of text bool [ 1-4] and review articles [5-11 ] in which the geometrical and electronic properties of oxide surfaces and the theoretical treatment of the adsorption on such surfaces are covered in more detail. 

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