Adsorption on crystal surfaces

Until now, our treatment in this chapter of the solid-gas interface has been very one-sided, focusing almost entirely on the adsorbed layer(s). To be sure, we have extracted some information about the solid, e.g., the specific surface area and the presence or absence of pores. In addition, we have extracted values for the energy of interaction between the solid adsorbent and the adsorbate molecules but, other than this, the influence of the solid has been ignored. Likewise, the solids we have considered have been high-surface-area powders, presenting 

In this section we discuss the adsorption on crystal surfaces. First, we begin with low-energy electron diffraction (LEED)—an experimental method for examining crystal surfaces—and introduce some basic crystallographic concepts needed to interpret the experimental measurements. Then we look at the implication of the adsorbate structure to adsorption and the structure of adsorbed layers using LEED measurements. 

Diffractometry provides an excellent tool for examining structure so we turn now to low-energy electron diffraction to study the order at a specific face of a single crystal, with and without adsorbed molecules. For the remainder of the chapter, we focus attention on the faces of the metal crystals. There are several reasons for this choice  

Many metals crystallize in the relatively simple cubic (either primitive, face-centered, or body-centered) structures. 

Metal surfaces have been studied extensively by LEED in the context of a number of practical applications such as corrosion, friction, and semiconductor devices. 

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Another special case of weak heterogeneity is found in the systems with stepped surfaces [97,142-145], shown schematically in Fig. 3. Assuming that each terrace has the lattice structure of the exposed crystal plane, the potential field experienced by the adsorbate atom changes periodically across the terrace but exhibits nonuniformities close to the terrace edges [146,147]. Thus, we have here another example of geometrically induced energetical heterogeneity. Adsorption on stepped surfaces has been studied experimentally [95,97,148] as well as with the help of both Monte Carlo [92-94,98,99,149-152] and molecular dynamics [153,154] computer simulation methods. 

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. 

Adsorption is the preferential concentration of a species at the interface between two phases. Adsorption on solid surfaces is a very complex process and one that is not well understood. The surfaces of most heterogeneous catalysts are not uniform. Variations in energy, crystal structure, and chemical composition will occur as one moves about on the catalyst surface. In spite of this it is generally possible to divide all adsorption phenomena involving solid surfaces into two main classes physical adsorption and chemical adsorption (or chemisorption). Physical adsorption arises from intermolecular forces... 

We observe that for the bonds depicted in Figs, lb and le, an atom M or an atom R, to which the chemisorbed particle C is attached, are more weakly bound to the lattice than the normal ions M+ or, respectively, Rr. As a result, in some cases we can expect that the molecule CM or CR may evaporate that is, the particle C upon desorption may carry off with it an atom of the lattice, thereby violating the stoichiometric composition of the crystal. In all cases such adsorption should facilitate surface creep which plays such an important role in the sintering, recrystallization, and disintegration of solids in reaction. This may also explain the well-known influence of adsorption on the surface mobility of the adsorbent atoms. 

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. 

Binding forces in the process of surfactant adsorption on the surface of ionic crystals were studied in detail by Richter and Schneider74) who analyzed the adsorption conditions for low 0 values and values of 0 in the range of a monomolecular layer. For low values the following energetic contributions may play a role ... 

The influence of the crystallite size of catalysts upon such reactions as hydrogenation or dehydrogenation over platinum or nickel has been investigated by Rubinshtein and others (376). Roginskil s school has applied mathematical statistics to systems formed by primary monocrystals of a catalyst the cracks and pores of varying dimensions created by these crystals predetermine the nature of the resulting porosity. The application of the statistical method to the theory of adsorption and catalysis was recently described by V. I. Levin (200) and an equation for adsorption on nonuniform surfaces derived by Ya. Zel dovich and S. Z. 

Factors that influence growth of sucrose crystals have been listed by Smythe (1971). They include supersaturation of the solution, temperature, relative velocity of crystal and solution, nature and concentration of impurities, and nature of the crystal surface. Crystal growth of sucrose consists of two steps (1) the mass transfer of sucrose molecules to the surface of the crystal, which is a first-order process and (2) the incorporation of the molecules in the crystal surface, a second-order process. Under usual conditions, overall growth rate is a function of the rate of both processes, with neither being rate-controlling. The effect of impurities can be of two kinds. Viscosity can increase, thus reducing the rate of mass transfer, or impurities can involve adsorption on specific surfaces of the crystal, thereby reducing the rate of surface incorporation. 

Many applications of AFM to pillared clays or zeolites have not specifically addressed the porosity characteristics, but rather the occurrence of adsorbed surface Al species in, e.g., pillared montmorillonite [41], or the crystal growth processes, adsorption on porous surfaces and the surface structure of natural zeolites [42]. Sugiyama et al. [43] succeeded to reveal the ordered pore structure of the (001) surface of mordenite after removal of impurities that clogged the pores. The authors indicated that resolution in AFM imaging of zeolites is significantly affected by the magnitude of the periodical corrugation on the crystal surface, so that if the surface contains deep pores only the pore structure, but not the atomic structure, can be resolved. 

The quartz crystal microbalance (QCM) is an excellent tool for these investigations since the frequency change produced by the adsorption on the surface of a piezoelectric crystal can be used to assess the mass (to a few ng/cm ) of the adsorbent using the Sauerbrey equation. Since the adsorbed protein layers can have some degree of structural flexibility or viscoelasticity that is undetectable by the determination of the resonance frequency alone, the energy loss, or dissipation factor (D), due to the shear of the adsorbent on the crystal in aqueous solution must also be determined.The technique is termed QCM-D and as well as representing an improvement in the study of biomolecular-surface interactions, it presents an opportunity to observe the adsorption of AFP and PVP, on a model nucleator with a hydrophilic surface. 

Melting of the part of stationary phase changes the retention mechanism the n-octane retention in the temperature range below 58°C is the result of adsorption on the surface of monolayer and crystals of solid n-octadecanol above this temperature the main process responsible for the retention of n-octane becomes its dissolution in liquid n-octadecanol. 

Jiang, J., Klauda, J.B., and Sandler, S.l. (2005). Hierarchical modelling N2 adsorption on the surface of and within a Cgg crystal from quantum mechanics to molecular simulation. J. Phys. Chem. B, 109, 4731-7. 

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