Effects of Surface Coverage

There are two issues that are relevant here. First, the adsorbates in a supercell calculation necessarily have a long-range pattern that is repeated exactly as the supercell is repeated. With periodic boundary conditions, it is impossible to model any kind of truly random arrangement of adsorbates. The good news is that in nature, it happens that adsorbates on crystal surfaces often do exhibit long-range ordering, especially at low temperatures, so it is possible for calculations to imitate real systems in many cases. 

The second issue is that the size of the supercell controls the distance between adsorbates. If the supercell is small, it defines a surface with a high density (or coverage) of adsorbates. If the supercell is large, surfaces with lower coverages can be defined. When there is one adsorbate per surface atom, the adsorbed layer is often said to have a coverage of 1 monolayer (ML). If there is one adsorbate per two surface atoms, the coverage is 0.5 ML, and so on.  

As you might imagine, a system of naming has evolved for discussing the symmetry of overlayers. Instead of delving into the rationale behind all the names, we simply give some representative examples. 

In Fig. 4.17, several examples of H adsorption on Cu(100) are depicted. It may be helpful to start with three examples. Going from Fig. 4.17(a) to (b) to 

TABLE 4.4 Energy of Adsorption of H on Cu(100) for Three Different Surface Overlayers 

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For the purpose of demonstrating the effects of surface coverage by Pd, 0pd, on the rate of electro-oxidation of formic acid and the ORR, Fig. 8.17 reveals that the i versus 0Pd relationship again has a volcano-like form, with the maximum catalytic activity being exhibited for 1 ML of Pd. The examples that we have given indicate that volcano relationships are the rule rather than the exception, emphasizing the importance of a systematic evaluation of the catalyst factors that control catalytic activity. A thorough... 

Srinivasan, G, Sander, L.C., and Muller, K., Effect of surface coverage on the conformation and mobility of C-18-modified silica gels, AnaZ. Bioanal. Chem., 384, 514, 2006. 

Lippa, K.A., Sander, L.C., and Mountain, R.D., Molecular dynamics simulations of alkylsilane stationary-phase order and disorder. 1. Effects of surface coverage and bonding chemistry, Anal. Chem., 77, 7852, 2005. 

The details of the sample preparation and studies of the nature of the supported-metal samples have been described in a paper dealing with the effect of surface coverage on the spectra of carbon monoxide chemisorbed on platinum, nickel, and palladium (1). The samples consist of small particles of metal dispersed on a nonporous silica which is produced commercially under the names Cabosil or Aerosil.f This type of silica is suitable as a support because it is relatively inert and has a small particle size (150-200 A.). The small particle size is important because it reduces the amount of radiation which is lost by scattering. A nonporous small particle form of gamma-alumina, known as Alon-C, is also available. This material is not so inert as the silica and will react with gases such as CO and CO2 at elevated temperatures. 

Fig. 9. Effect of surface coverage on the (X) absorbance arid (B) wavelength of CO chemisorbed on Pt.

Fig. 47. Effect of surface coverage of chlorine on ethylene oxidation. EtO stands for ethylene oxide, the product of partial oxidation. From Campbell and Paflfett (1984).

Here, we will examine the photochemistry of Fe(CO)s adsorbed on the surface of porous silica (4,5). Using IR and UV-visible spectroscopy to monitor photoproduct formation, we find that surface functional groups play a key role in determining the outcome of photochemical reactions in this system. The effects of surface coverage and surface temperature are particularly important. We will discuss these effects in detail, and we will propose a mechanism for the participation of silica surface groups in the photochemical reactions of Fe(CO)s. Finally, the results of our experiments on porous silica will be compared to the results of recent experiments on the photochemistry of Fe(CO)s adsorbed onto other surfaces. 

As the surface of the catalyst plays an important role in the catalytic reaction the effect of surface coverage especially on the heat of adsorption is discussed in the second part. 

FIGURE 2.2 Effect of surface coverage on activation energies. 

Luckham PF, Vincent B, Tadros TF (1983) The controUed fiocculation of particulate dispersions using smaU particles of opposite charge. IV. Effect of surface coverage of adsorbed polymer on heterofiocculation. CoUoids Surf 6(2) 119-133... 

Polarization curves for the CL demonstrating the effect of surface coverage and pore blockage due to liquid water. 

The heterogenicity of catalyst surfaces makes problems complicated. If the kinetic data were obtained in a narrow range of experimental conditions, only Langmuir kinetics could be applied. If using the wide range of experimental conditions, micro-kinetic model may involve non-Langmuir effect of surface coverage state. 

Figure 1.17 The effect of adsorbate-adsorbate interactions on the CO oxidation volcano, (a) Adsorbate interactions characteristic for Pt were included in the generation of the volcano plot, (b) The standard activity volcano is shown, but the positions of the metals have been corrected to account for the effect of surface coverage.

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