Adsorption on stepped surfaces

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. 

It is worth noting that for both systems the observed AUWr value corresponding to the onset of the formation of the ordered Na adlattice is practically the same, which strongly supports the idea that this AUwr value is characteristic of the chemical potential of this structure. The fact that a small but not negligible Na coverage (0ga < 0.015) preceeds the formation of the ordered Na structure on the surface of polycrystalline Pt samples (Fig. 5.54) may indicate preferential Na adsorption on stepped surfaces before Na adsorption on Pt(lll) starts taking place. 

The surface structures observed for gas adsorption on stepped surfaces are listed in Table 5.6. In this table the stepped surfaces are denoted by either their Miller index label or stepped surface designation, depending on which system was used by the original author. By using Table 5.5 one may convert back and forth between these two systems. It is interesting to compare the surface structures formed on stepped surfaces to those... 

In particular, reactions in heterogeneous catalysis are always a series of steps, including adsorption on the surface, reaction, and desorption back into the gas phase. In the course of this chapter we will see how the rate equations of overall reactions can be constructed from those of the elementary steps. 

Such analytes require carefully chosen extraction conditions in terms of pH, solvent composition and technique. Also, these analytes tend to become lost by adsorption on (glass) surfaces or undergo conjugation so that a chemical or enzymatic deconjugation step may be required. Often only the use of radiotracers... 

Hydrogen adsorption and oxidation of formic acid show a pronounced dependence on the structure of single crystal surfaces. The influence of the terrace and step orientation and step density is reflected in both reactions on step surfaces. The multiple states of hydrogen adsorption can be correlated with the nature of adsorption sites. 

There is a negligible effect of adsorbate-adsorbate interaction on step surfaces. Some lateral repulsion of hydrogen adsorbed on Pt(lll) could be inferred. A strong adsorption of bisulphate and sulphate anions on the (111) oriented terraces and step sites considerably affects both reactions. These data show that each crystallographic orientation of the electrode surfaces gives a different electrochemical entity. 

The interpretation of voltammetry curve for the Pt(100) surface poses some problems, e.g. the origin of the peak at E=—0.15 V (Fig. 1). Markovifi et al. (12) ascribed this peak to hydrogen adsorption on particular surface imperfections, the (111)-oriented step sites. The height of this peak varies from one set of data to another, indicating a lack of control of the surface structure. Further support of this view will be shown below with the data for stepped surfaces. 

A database of molecularly adsorbed species on various surfaces is also included (see Table 4.3). In all cases, the chemisorption energies have been calculated on stepped surfaces using density functional theory (see [56] for details). The metals have been modeled by slabs with at least three close-packed layers. The bcc metals are modeled by the bcc(210) surface and the fee and hep metals have been modeled by the fcc(211) surface. A small discrepancy between the adsorption on the hep metals in the fcc(211) structure is thus expected when the results are compared to the adsorption energies on the correct stepped hep structure instead. When mixing... 

Standard mechanisms for chain reactions generally miss out the surface termination steps, but these should be included. Such terminations are written as first order in radical since diffusion to the surface or adsorption on the surface are rate determining, rather than the second order bimolecular step of recombination of the two radicals adsorbed on the surface. A complete mechanism will also include the need for a third body in any unimolecular initiation or propagation steps, and in any gas phase termination steps. 

The conclusion drawn from Worked Problem 6.14 is that changing the type of termination step from gas to surface alters the kinetics. This is because the order with respect to the radical differs between the second order recombination of the gas phase termination and surface termination where diffusion to the surface or adsorption on the surface is rate determining and first order. If, however, the rate-determining step in surface termination were bimolecular recombination on the surface, the order would not change between gas and surface termination. This is because both recombinations would now have the same order, i.e. 2 4[R ]2 and 2 7[R ]2, with the total rate of termination if both contributed being 2(k + 7)[R ]2. 

In Fig. 14 we show HREEL spectra of ethylene adsorbed at Ag(4 1 0) and at Ag(2 1 0) at T = 105 K and compare them with the spectra recorded for Ag(l 00). On stepped surfaces (upper two spectra) C2H4 was dosed with a pure beam. Non-activated adsorption is witnessed by the loss in the 121-125 meY region. No adsorption takes place, on the other hand, on the extended (100) terraces of Ag(l 00) [90] up to much higher energies (see spectrum recorded at Ex = 0.31 eV in Fig. 14). Chemisorption on Ag(l 0 0) is observed when the ethylene exposure is performed with Ex — 0.35 eV. Adsorption on the flat surface is therefore translationally activated for extended (100) terraces and non activated for stepped surfaces. Physisorbed molecules do not contribute to the HREEL spectra since desorption takes place within a few seconds after the end of the dose at 105 K (as evident from Fig. 2) and recording a spectrum requires many minutes. 

It is important to concentrate special attention on the first step of the process, viz on the study of polyelectrolyte adsorption on charged surface. The time dependence of layer growing and structural reconstruction can give the information of formation mechanism of the multilayer. 

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. 

Generally, there are four steps are included in the mass transfer mechanisms of the adsorption process. These steps are fluid-film transfer, pore diffusion, surface adhesion, and surface diffusion. The rate of surface adhesion for physical adsorption on the surface of porous adsorbents is very rapid, enough to be assumed instantaneous relative to the other transfer rates [5]. 

M. J. Bojan and W. A. Steele, Computer Simulations of the Adsorption of Xenon on Stepped Surfaces, Mol. Phys., in press. 

However, species-1 formed by the C 02 adsorption on the surface carrying species-3 and -4, which was previously formed by the C 02 adsorption at the room temperature, is desorbed in a complex manner [Two-step adsorption. Figure 2(B)]. For example, the isotopic composition of species-1 is considerably diluted by 0, and C 02 species which... 

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