Adsorbate-surface bond 644 INDEX

By the use of mainly LEED and lately ion scattering techniques the location of many atomic adsorbates, their bond distances and bond angles from their nearest neighbor atoms have been determined. The substrates utilized in these investigations were low Miller Index surfaces of fee, hep and bcc metals in most cases, and low Miller Index surfaces of semiconductors that crystallize in the diamond, zincblende and wurtzite structures in some cases that could be cleaned and ordered with good reproducibility. 

It is convenient to first establish some definitions, and these are listed in Table 6.2. As discussed in Chapter 5, as an adsorbate approaches a surface, changes occur in the potential energy of the system and one must relate this energy, Epot, to the distance, r, from the surface. It is certainly anticipated that the migration and dissociation of an adsorbate, X, on the surface involve changes in the coordination mode, Mn — X, and the M-X distance, r, where M is a metal atom and n is the coordination number. Assuming quasi-spherical interactions, a two-center M-A bond index, x, in the form of Pauling s bond order [23], is defined as... 

From gas phase measurements CO is known to prefer top sites on all three low index faces, with the CO molecule perpendicular to the surface and bonded through the carbon end of the molecule except at high coverages (27). It is likely that HCOOH and COOH are adsorbed in a similar way. It is not likely that they could "enter the "troughs , which seems to be possible for anions. For Pt(100) on the other hand, upon sweep reversal and gradual oxide reduction, anions are immediately adsorbed on that "flat" surface. They block adsorption of HCOOH. Adsorption of anions decreases as potential becomes more negative. The oxidation of HCOOH commences and the rate increases as at more negative potentials, i.e. at lower overpotential. A competition between anions and HCOOH adsorption explains this apparently anomalous behaviour. The explanation of the "anomalous behaviour of the Pt(110) surface can be also found in the data for stepped surface vicinal to the (100) and (110) orientations. 

The adsorption and ordering characteristics of the various hydrocarbon molecules on the low Miller index platinum surfaces are discussed in great detail elsewhere. These two surfaces appear to be excellent substrates for ordered chemisorption of hydrocarbons, which permit one to study the surface crystallography of these important organic molecules. The conspicuous absence of C-H and C-C bond breaking during the chemisorption of hydrocarbons below 500 K and at low adsorbate pressures (10 9-10-6 Torr) clearly indicates that these crystal faces are poor catalysts and lack the active sites that can break the important C-C and C-H chemical bonds with near zero activation energy. 

The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (5), the Pt(S)-[9(l 11) x (100)], Pt(S)-[6(l 11) x (100)], Pt(S)-[7(lll) x (310)], and Pt(S)-[4(l 11 x (100)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10 9-10-6 Torr). Hydrocarbons on the [9(100) x (100)] and [6(111) x (100)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)] surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by... 

Fig. 9 Top-view on enantiomers and diastereoisomers of adsorbed glycine and alanine. The amino nitrogen and the two oxygen atoms form bonds to the surface and create a chiral footprint configuration. The surface-induced absolute configuration, as indexed with a superscripted i, is specified using CIP-rules and by giving an atom or group closer to the surface a higher priority...

Adsorption on a clean surface shows a strong kinetic isotope effect, implying that the initial chemisorption process involves C-H bond scission. Adsorption appears to be facilitated by the presence of step-edge sites, since adsorbed intermediates are seen at low potentials on both polycrystalline Pt and on high-index Pt(335) surfaces. 

The theories of surface structure and bonding have been reviewed. It should be clear to the reader that surface structural chemistry is indeed a frontier area for both theorists and experimental researchers. From an experimentalists viewpoint the data base of atomic and molecular surface structures is very small at present. Most investigations have been carried out on flat, low Miller index surfaces of monatomic solids, either clean or with atomic or small molecules as adsorbates. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

When heated to 1200 °C they disorder but revert at 800 °C to the structures shown. They may well play an important part in catalysis. In contrast the (510) face transforms to (100) and (210). If one had to generalize about stepped surfaces one might say that they are very reactive compared with low-index faces, breaking H—H, C—H, and C—C bonds. Thus H2 does not readily chemisorb on (111) or (100) faces of Pt, but adsorbs readily on stepped surfaces. 

In any determination of PZC, the ions that occupy the exchange sites and are subjected to measurement, called the index ions, are chosen based on their tendency to adsorb by essentially nonselective electrostatic bonds. These ions may not be in direct contact with the surface, being readily exchangeable by other ions of like charge. Salts of these ions (for example, NaCl, KNO3) are referred to as indifferent... 

Ensemble effects are useful when adsorption requires a special grouping of surface atoms. To explain this, let us examine the simple example of ethylene adsorption on nickel, which occurs in a dt-adsorbed mode. Two nickel atoms, the right distance apart, are needed to bond a pair of carbon atoms. The bonds must be stable, but not too strong or subsequent reaction is difficult. Figure 4.2 shows symmetry and distances for tow index planes of the face centered cubic nickel surface. 

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. 

Typical examples include studies of the underpotential deposition of various metals on metallic substrates. The structure of the upd-layer [33, 34], the position of adsorbed anions and water molecules on top of the upd-layer and the respective bond angles and lengths could be elucidated [35, 36]. Surface reconstruction caused by weakly adsorbed hydrogen [37], surface expansion effects of low-index platinum and gold surfaces correlated with adsorption/desorption of solution species [38] and... 

The adsorption of anions on metal electrodes has been one of the major topics in surface electrochemistry. Specific adsorption of anions occurs when the anion loses aU or part of its solvation shell and forms a direct chemical bond with the substrate. In this situation the surface coverage by anions can be high and the adlayer tends to form a close-packed structure that depends critically on the surface atomic geometry of the underlying substrate and the balance between the anion-metal and anion-anion interaction energies. The structures of halide anions adsorbed onto Au(Jtkl), Ag(hkl), and Pt(hkl) low-index surfaces have been the most widely studied systems by SXS, and a comprehensive review of ordered anion adlayers on metal electrodes is given by Magnussen [57]. 

---