Bonding of adsorbed molecules

The development of ab-initio self-consistent field (SCF) wave functions for clusters by Bagus et al. provides a powerful basis for interpreting the experimental observations related to the localized bonding of adsorbed molecules (43-47). In these... 

Electron tail ( spillover electrons, Fig. 3.17) can be also involved in bonding of adsorbed molecules (atoms). Adsorption is discussed further in Chapter 10. 

Atoms and molecules frequently adsorb on surfaces, where they may decompose and/or react with other adsorbed species. Modern technology is increasingly dependent on surface chemistry which underlies many industrially important processes as well as destructive processes such as corrosion. It is useful to distinguish two types of adsorption physical adsorption, or physisorption, and chemical adsorption, or chemisorption. Physisorption is similar in nature to condensation and involves little chemical interaction with the surface, being associated with van der Waals forces. Chemisorption involves a tme chemical interaction with the surface, with the formation of a chemical bond. Thus, the enthalpy of physisorption is usually of the order of 20 kJ mol while for chemisorption values are in the region of200 kJ mol . A chemisorbed molecule may either remain intact in molecular chemisorption, or fall apart in dissociative chemisorption. In an important recent development, it is now possible to identify individual molecular bonds of adsorbed molecules using STM... 

Temperature dependent changes of bonding of adsorbed molecules on solid surfaces... 

Vibrational energy states are too well separated to contribute much to the entropy or the energy of small molecules at ordinary temperatures, but for higher temperatures this may not be so, and both internal entropy and energy changes may occur due to changes in vibrational levels on adsoiption. From a somewhat different point of view, it is clear that even in physical adsorption, adsorbate molecules should be polarized on the surface (see Section VI-8), and in chemisorption more drastic perturbations should occur. Thus internal bond energies of adsorbed molecules may be affected. 

Insight into chemisorption bonds is important for the interpretation of photoemission and vibrational spectra of adsorbed molecules, as well as for the understanding of catalytic reactions. If we have a feeling for how electrons rearrange over orbitals when a molecule adsorbs, we may be able to understand why molecules dissociate or not, to what extent they are activated, or also how a promoter influences the reactions on the surface. 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

Bombardment of solid surfaces with electrons can cause desorption of ground-state neutrals (both atoms and molecules), ions, and metastable species. In addition, dissociation of adsorbed molecules with the resulting fragments remaining attached to the surface can be induced by electron bombardment. Conversion of one bonding mode to another can also occur. 

In physisorption the bond is a van der Waals interaction and the adsorption energy is typically 5-10 kJ/mol. This is much weaker than a typical chemical bond and the chemical bonds in the adsorbing molecules remain intact. However, the van der Waals interactions between adsorbed molecules is not much different from the van der Waals interaction between the molecules and the surface. For this reason many layers of adsorbed molecules may be formed. 

It is essential to have tools that allow studies of the electronic structure of adsorbates in a molecular orbital picture. In the following, we will demonstrate how we can use X-ray and electron spectroscopies together with Density Functional Theory (DFT) calculations to obtain an understanding of the local electronic structure and chemical bonding of adsorbates on metal surfaces. The goal is to use molecular orbital theory and relate the chemical bond formation to perturbations of the orbital structure of the free molecule. This chapter is complementary to Chapter 4, which... 

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