Atomic adsorption on metals

Turning to metal substrates, in most cases of atomic adsorption on metal surfaces where the adsorption geometry has been determined (cf. Table 6.1), only one adsorption site is involved, i.e., all adatoms have identical surroundings [the exceptions are Ni(l 11)... 

This chapter is organized as follows. First, in sect. 2, we consider the surfaces of metals. In sect. 2.1 we describe the structure of unreconstructed clean metal surfaces and then proceed, in sect. 2.2, to consider the reconstructed surfaces. The surface structure of ordered and disordered metallic alloys is described in sect. 2.3. In sect. 2.4 we describe the surface structures associated with atomic adsorption on metals and in sect. 2.5 we consider molecular adsorption on metals. The structure of semiconductor surfaces is... 

The atomic adsorption on metals is not caused by van-der Waals forces, but depends on forces similar to those postulated by Slater for explaining metallic cohesion... 

The effect of the presence of alkali promoters on ethylene adsorption on single crystal metal surfaces has been studied in the case ofPt (111).74 77 The same effect has been also studied for C6H6 and C4H8 on K-covered Pt(l 11).78,79 As ethylene and other unsaturated hydrocarbon molecules show net n- or o-donor behavior it is expected that alkalis will inhibit their adsorption on metal surfaces. The requirement of two free neighboring Pt atoms for adsorption of ethylene in the di-o state is also expected to allow for geometric (steric) hindrance of ethylene adsorption at high alkali coverages. 

The possibility of adsorption on a virtual exciton was indicated by E. L. Nagayev (.14) on the simplest example of the adsorption of a one-electron atom. This problem is an example of the many-electron approach in chemisorption theory. Recently, V. L. Bonch-Bruevich and V. B. Glasko (16) have treated adsorption on metal surfaces by the many-electron method. 

For the purposes of this chapter, which focuses on comparisons of isocyanide binding in transition metal complexes and isocyanide adsorption on metal surfaces, we first summarize known modes of isocyanide binding to one, two and three metals in their complexes. In such complexes, detailed structural features of isocyanide attachment to the metals have been established by single-crystal X-ray diffraction studies. On the other hand, modes of isocyanide attachment to metal atoms on metal surfaces are proposed on the basis of comparisons of spectroscopic data for adsorbed isocyanides with comparable data for isocyanides in metal complexes with known modes of isocyanide attachment. 

Based on the first-principles study of helium adsorption on metals (Zaremba and Kohn, 1977), Esbjerg and Nprskov (1980) made an important observation. Because the He atom is very tight (with a radius about 1 A), the surface electron density of the sample does not vary much within the volume of the He atom. Therefore, the interaction energy should be determined by the electron density of the sample at the location of the He nucleus. A calculation of the interaction of a He atom with a homogeneous electron distribution results in an explicit relation between the He scattering potential V r) and the local electron density p(r). For He atoms with kinetic energy smaller than 0.1 eV, Esbjerg and Nprskov (1980) obtained... 

Another difficulty is met with in adsorption on metallic surfaces. Metals, or rather conducting bodies, are considered as adsorbents with an ideal polarizability. Accepting this view as true does not make it clear whether the metallic properties leading to this ideal polarizability should be assumed to start at the outer peripheries of the surface atoms of the metal or whether we must assume these properties to be found from a plane through the centers of the surface atoms. The choice of the outer boundary of the region of conducting electrons is very important, however, for the assessment of the distance of the adsorbed atom to the metal. 

As we shall sec in Secs. V,7,8 and VI,1, the forces that cause adsorption on metal surfaces are exercised by the adsorbent body as a whole, rather than by the constituent atoms. It is, therefore, important to know the... 

The free molecules of metallic oxides can best be described as having covalent links, possessing dipoles (58). Similarly oxygen atoms adsorbed on metallic surfaces form covalent bonds, sharing two pairs of electrons with the metal (59) or with one specific atom or two atoms of the metal. Their dipoles point with the negative ends away from the metal. We may, to give an example, express the situation of the adsorption of oxygen on silver by... 

The influence of substrate structure is briefly as follows. The active entity (H in hydrogenation and dehydrogenation reactions) must be adsorbed, even in the surface layer, in sites which make crystallographic sense, as these proper sites will be those of lowest potential energy on the surface. For example, atoms chemisorbed on metals will be located in interstitial surface sites which fall on the lattice of interstitial sites for the crystal as a whole. Adsorption immediately above surface atoms is improbable, adsorption in surface interstices probable. Diffusion into the solid from the surface is then by a path similar to the subsequent stages in the bulk solid. The oxygen atoms adsorbed on oxides will occupy proper anion lattice sites and will either create or destroy defects. The defects created will be able to diffuse immediately beneath the surface and appear at the site where their properties are required. Molecular reactants of any size cannot usually penetrate the catalyst surface, but atoms or radicals may be separated from them. The molecule itself is chemisorbed with efficiency only if certain structural inter-relations are satisfied. Beeck and coworkers (62) have demonstrated this adequately for the adsorption of ethylene on various metals and on various crystal planes of the same metal. The adsorption of gases... 

The case of triangular lattice is particularly interesting since it corresponds to adsorption on graphite and on the (111) plane of several fee metal crystals [15,102,103,135]. The distance between adjacent potential minima for the graphite basal plane is equal to 2.46A and hence is too small to allow for their mutual occupation by even very small atoms of light noble gases. The same is true for adsorption on metals. Experimental studies have demonstrated that for rare gas atoms and simple molecules adsorbed on the graphite basal plane as well as on the (111) faces of fee crystals the ordered state corresponds to either the /3 X %/3 [102] or to the 2x2 phase [103,136,137] shown in Fig. 8. 

The adsorption geometry of alkali-metal atoms chemisorbed on metal surfaces. The alkali metal to substrate bond length is derived from the determined coordinates. The adatont radius is obtained by subtracting the metallic Tadius of the substrate atom from the determined bond length. The adatom radius is expressed as the ratio of the adatom radius to the mcLallic radius of the adatom. 

Oxygen is the most extensively studied of all atomic adsorbates. In table 9 the Structural results obtained for oxygen adsorption on metals are summarized. 

There are a few, relatively early, studies of Se and Te adsorption on metals. Selenium is found to adsorb at the high coordination (hollow) sites on the low Miller index surfaces Ni(100) and Ag(lOO). On the most open surface to have been examined, Ni(J10), the bond distance to the Ni atom in the second substrate layer (2.35 A) is slightly shorter than that to the top layer (2.42 A), suggesting the formation of a Ni-Se bond to the second substrate layer. However, it should be noted that the LEED studies of Se adsorption on metals originate before 1975, whilst more recent studies (1982) were by photo-electron diffraction only. Consequently, detailed substrate distortions, of the type seen in more recent studies of O and S adsorption on metals, have not been searched for. 

Fig. 6. The determined C-metal bond length for CO adsorption on metallic surfaces plotted as a function of the covalent radius of the substrate atom. The dashed line is a guide to the eye through the data, the error bars are those of the determination. Note that although the covalent radius has been used to compile this figure, for all metals shown the metallic radius is a uniform 0.09 A larger.

The quantitative analysis of adsorption on metal clusters, as discussed earlier, can be sensitive to the cluster chosen to model the chemisorption site. In a systematic series of studies, we examined the adsorption of various atomic and molecular adsorbates on different metal surfaces. We found that it was important to optimize 1) size, 2) structural configuration, 3) spin state, and... 

---