Surface chemical bond formation

Electronic interactions are very different when surface chemical bond formation is dominated by interaction with adsorbate orbitals that do not overlap with the d-valence electron band energy. This becomes relevant when molecular fragments dominated by bonding with strong o bonds adsorb. We discuss this explicitly in Section 10.3.4 for adsorption of ammonia. In this case, the average d-valence electron band position is found to be important... 

Chemical Bond Formation (Chemisorption). This is the mechanism that leads to the formation of the strongest bonds between coUectors and mineral surfaces. Chemically adsorbed reagents usuaUy form surface compounds at the active waU sites. The flotation of calcite (CaCO ) and... 

Anodic dissolution reactions of metals typically have rates that depend strongly on solution composition, particularly on the anion type and concentration (Kolotyrkin, 1959). The rates increase upon addition of surface-active anions. It follows that the first step in anodic metal dissolution reactions is that of adsorption of an anion and chemical bond formation with a metal atom. This bonding facilitates subsequent steps in which the metal atom (ion) is tom from the lattice and solvated. The adsorption step may be associated with simultaneous surface migration of the dissolving atom to a more favorable position (e.g., from position 3 to position 1 in Fig. 14.1 la), where the formation of adsorption and solvation bonds is facilitated. 

SAMs of alkanethiols on an Au(l 11) surface are widely used to control surface properties, electron transfer processes and to stabilize nano-clusters [6, 7]. SAMs are formed by chemical bond formation between Sand Au when an Au(l 11) substrate is immersed in a solution containing several mM of alkanethiols for hours to days. Various functions have been realized by using SAM s of alkanethiols on Au substrates as listed in Table 16.1. 

Although valence band spectra probe those electrons that are involved in chemical bond formation, they are rarely used in studying catalysts. One reason is that all elements have valence electrons, which makes valence band spectra of multi-component systems difficult to sort out. A second reason is that the mean free path of photoelectrons from the valence band is at its maximum, implying that the chemical effects of for example chemisorption, which are limited to the outer surface layer, can hardly be distinguished from the dominating substrate signal. In this respect UPS, discussed later in this chapter, is much more surface sensitive and therefore better suited for adsorption studies. 

The adsorption of gas can be of different types. The gas molecule may adsorb as a kind of condensation process it may under other circumstances react with the solid surface (chemical adsorption or chemisorption). In the case of chemiadsorption, a chemical bond formation can almost be expected. On carbon, while oxygen adsorbs (or chemisorbs), one can desorb CO or C02. Experimental data can provide information on the type of adsorption. On porous solid surfaces, the adsorption may give rise to capillary condensation. This indicates that porous solid surfaces will exhibit some specific properties. Catalytic reactions (e.g., formation of NH3 from N2 and Hj) give the most adsorption process in industry. 

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... 

Figure 2.58. Schematic illustrations of the five different types of chemical bond formation on metal surfaces.

Fig. 6. Self-assembled monolayers are formed by immersing a substrate into a solution of the surface-active material. Necessary conditions for the spontaneous formation of the 2-D assembly include chemical bond formation of molecules with the surface, and intermolecular interactions.

Adsorbents. See Adsorption and Adsorbents Adsorption and Adsorbents. Adsorption may be defined as the ability of a substance (adsorbent) to hold on its surface, including inner pores or cracks, thin layers of gases, liquids or dissolved substances (adsorbates). Adsorption is a surface phenomenon and should not be confused with absorption (qv). Adsorption may be divided into physical and chemical (also called chemisorption). In physical adsorption the forces are those betw the adsorbing surface and the molecules of the adsorbate, and are similar to Van der Waals forces. In chemisorption, which in eludes ion exchange, the forces are much stronger than those of physical adsorption and depend on chemical bond formation. 

Until this point, we have focused on cases in which we could neglect chemical bond formation between the sorbate and materials in the solid phase. However, at least two kinds of surface reactions are known to be important for sorption of some chemicals (referred to as chemisorption). Simply, some organic substances can form covalent bonds with the NOM in a sediment or soil (see Fig. 9.2) other organic sor-bates are able to serve as ligands of metals on the surfaces of inorganic solids (Fig. 11.le). We discuss these processes below. 

Adsorption of uncharged organic molecules without clear indication of chemical bond formation occurs by replacement of solvent (water) at the interface at potentials close to the potential of zero charge (pzc) because the surface energy of the adsorbate is less than that of the polar solvent (water). At very negative and positive electrode potentials with respect to the pzc, highly polar water molecules are more stable at the interface in the presence of high electric fields. 

In recent work by Arkles el al. [4, 5], it has been proposed that, in comparison with monomeric silanes, polymeric silanes may react with substrates more efficiently. A typical polymeric silane is shown in Fig. la, in which pendant chains of siloxanes are attached through methylene chain spacers to a polyethyleneimine backbone. The film-forming polymeric silane thus provides a more continuous reactive surface to the polymer matrix in the composite. In this case, the recurring amino groups on the polymeric silane backbone can react with an epoxy resin matrix through chemical bond formation. 

FIGURE 3.1. (a) Schematics illustrating self-assembly. Self-assembled monolayers are formed by immersing a substrate (e.g.. a piece of metal) into a solution of the surface-active material. The functional end groups of molecules chemically react with the substrate material spontaneously, forming a two-dimensional assembly. Its driving force includes chemical bond formation of functional end groups of molecules with the substrate surface and intermolecular interactions between the backbones, (b) Cross-sectional schematic of self-assembled monolayers formed on a substrate. 

Adsorption desorption. Gas can stick to surfaces either by physisorption or chemisorption. Generally, in physisorption, there is a weak Van der Waals interaction between the surface and the adsorbed species. The enthalpy of adsorption is about the same as the enthalpy of condensation, e.g. the maximum observed values of the enthalpy of physisorption of H2, N2 and H20 are -84, -21 and 57kJmol-1, respectively. (In chemisorption, the adsorbing species sticks to the surface as a result of chemical bond formation. The energies involved are much greater than in physisorption.)... 

Plasma. A plasma is used to clean the electrode surface, leaving unbonded surface atoms and, thus, an activated surface. Carbon is much used for this subsequent exposure to amines or ethenes, for example, results in chemical bond formation. Plasma discharge in the presence of radical monomers in solution, leading to polymer formation on the surface, is equivalent to chemical activation. The use of lasers in this area may be interesting, but has been little exploited as yet. 

The term chemisorption was coined in order to classify the interaction between a particle in the gas phase and a solid surface, i.e. the result of the adsorption process [1]. If the interaction leads to the formation of a chemical bond the adsorbate formed is called a chem-isorbate. Where chemical bond formation is not important the process is classified as physisorption. There are several conceptual problems with such a differentiation which we briefly address in the following, and which indicate that a more detailed look at the entire process of adsorbate formation is needed before a reliable classification may be carried out. In fact, as it turns out, for a conclusive classification one would need the full theoretical and experimental understanding of the system under investigation. Such an approach must include the static aspects, i.e. the energies involved, as well as the dynamic aspects, i.e. the processes involved in the formation of the adsorptive interactions. 

New experimental results on specific polymer material problems are presented in the last nine chapters. Several cases involve the study of polymers from commercial sources. The topics include (1) surface chemistry as induced by (a) outdoor weathering, (b) chemical reactions, and (c) plasma exposure (2) chemical bond formation at the polymer -metal interface and (3)biomaterials characterization and relationship to blood compatibility. 

In considering the vast literature on this topic, the reader should also be aware that different authors may define their surface charge differently (some Include the specifically adsorbed charge) or define the notion "chemically" differently. (Stumm restricts "chemical" to real chemical bond formation corresponding to our chemisorption.)... 

Every molecule is capable of weakly interacting with any solid surface through van der Waals forces. The enthalpy change associated with this weak adsorption mode, called physisorption, is typically 40 kJ moF or less, which is far lower than the enthalpy of chemical bond formation. Even though physisorbed molecules are not activated for catalysis, they may serve as precursors to chemisorbed molecules. More than one layer of molecules can physisorb on a surface since only van der Waals interactions are involved. The number of physisorbed molecules that occupy... 

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