Coverage dependence

A more dramatic type of restmctiiring occurs with the adsorption of alkali metals onto certain fee metal surfaces [39]. In this case, multilayer composite surfaces are fomied in which the alkali and metal atoms are intemiixed in an ordered stmcture. These stmctiires involve the substitution of alkali atoms into substrate sites, and the details of the stmctiires are found to be coverage-dependent. The stmctiires are influenced by the repulsion between the dipoles fomied by neighbouring alkali adsorbates and by the interactions of the alkalis with the substrate itself [40]. 

An experienced candidate toller probably carries the basic insurance required for the industry but contracts are typically dependent upon proof of adequate coverage. Depending on the anticipated contract terms and the financial assessment of both firms, other insurance or bonds may need to be evaluated. These may address business interruption, third party liability, or other identified loss potential. 

Transfer matrix calculations of the adsorbate chemical potential have been done for up to four sites (ontop, bridge, hollow, etc.) or four states per unit cell, and for 2-, 3-, and 4-body interactions up to fifth neighbor on primitive lattices. Here the various states can correspond to quite different physical systems. Thus a 3-state, 1-site system may be a two-component adsorbate, e.g., atoms and their diatomic molecules on the surface, for which the occupations on a site are no particles, an atom, or a molecule. On the other hand, the three states could correspond to a molecular species with two bond orientations, perpendicular and tilted, with respect to the surface. An -state system could also be an ( - 1) layer system with ontop stacking. The construction of the transfer matrices and associated numerical procedures are essentially the same for these systems, and such calculations are done routinely [33]. If there are two or more non-reacting (but interacting) species on the surface then the partial coverages depend on the chemical potentials specified for each species. 

In addition to the temperature dependence of Sq T) is that of the correlation functions, which also determine the coverage dependence. 

For a nonassociative desorption, evidently = 1 and fcd" is configuration-independent. In other cases, the functions gi(6 y conf) and (conf y) can be combined into a single coverage-dependent function... 

Let us consider that Ed corresponding to a peak on the desorption curve is coverage dependent, while kd (and thus the adsorption entropy) remains constant. (For the variability of kd see Section II.A.) When seeking the required function Ed (6) we refer to Eq. (8) in which the term exp (— Edf RT) exhibits the greatest variability. A set of experimental curves of the desorption rate with different initial populations n,B must be available. When plotting ln(— dn,/dt) — x ln(n ) vs 1/T, we obtain the function Ed(ne) from the slope, for the selected n, as has been dealt with in Section V. In the first approximation which is reasonable for a number of actual cases, let us take a simple linear variation of Ed with n ... 

It should be clear that, as well known from the surface science literature (Chapter 2) and from the XPS studies of Lambert and coworkers with Pt/(3"-A1203 (section 5.8), the Na adatoms on the Pt surface have a strong cationic character, Nas+-5+, where 5+ is coverage dependent but can reach values up to unity. This is particularly true in presence of other coadsorbates, such as O, H20, C02 or NO, leading to formation of surface sodium oxides, hydroxides, carbonates or nitrates, which may form ordered adlattices as discussed in that section. What is important to remember is that the work function change induced by such adlayers is, regardless of the exact nature of the counter ion, dominated by the large ( 5D) dipole moment of the, predominantly cationic, Na adatom. 

In general Pj is coverage-dependent and also the Helmholtz equation has to be written in its general form ... 

As long as there are no important steric contributions to the transition-state energies, the elementary rate constant of Eq. (1.22) does not sensitively depend on the detailed shape of the zeolite cavity. Then the dominant contribution is due to the coverage dependent term 9. 

However, in subsequent studies [23-25,88-90] it was demonstrated that in reality the particle deposition is not a purely geometric effect, and the maximum surface coverage depends on several parameters, such as transport of particles to the surface, external forces, particle-surface and particle-particle interactions such as repulsive electrostatic forces [25], polydispersity of the particles [89], and ionic strength of the colloidal solution [23,88,90]. Using different kinds of particles and substrates, values of the maximum surface coverage varied by as much as a factor of 10 between the different studies. 

Hence, the activation energy contains coverage-dependent, second-order terras, which are usually ignored. This is only allowed in three cases. The trivial cases are when the kinetic parameters are constant, or when the coverage does not change... 

Because the Arrhenius plots of both TPD experiments are straight lines over a large portion of the data points, the reaction between CO and O is, most likely, an elementary step, with an activation energy of 103 5 kj mol and a pre-exponential factor of s . This analysis is again only valid if coverage dependencies play... 

Carbon Monoxide Oxidation on Platinum Coverage Dependence of the Product Internal Energy... 

The term atomic layer is used here to indicate, in general, a layer of atoms on the surface, where all the atoms are in contact with the surface. The term atomic layer does not specify a coverage, just that the layer is no more then one atom thick, probably less then a ML, relative to the number of substrate surface atoms. There can be several structures formed at different coverages, all under a ML, but all are one atom thick, and all would correspond to an atomic layer. Thus a statement that an atomic layer was formed suggests only that no bulk atoms were deposited. Where as, the statement that a monolayer was formed suggests a coverage, dependent on the ML definition in use. 

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