Coverage multilayer

Type II isotherms are typical of physical adsorption on nonporous solids. In contrast to type I, the adsorbate molecules in these cases also have relatively strong mutual interactions, which leads to the tendency for multilayer formation. The initial rapidly rising part of the isotherm corresponds to the equivalent type I adsorption. Point B on the curve is identified with complete mono-layer coverage. Multilayer formation then begins which may lead to surface condensation. Type II isotherms are sometimes encountered for microporous solids, in which case point B would correspond to both completion of mono-layer coverage and filling of the micropores by capillary condensation. The rest of the curve would then correspond to normal multilayer formation. 

The accepted explanation for the minimum is that it represents the point of complete coverage of the surface by a monolayer according to Eq. XVII-37, Sconfig should go to minus infinity at this point, but in real systems an onset of multilayer adsorption occurs, and this provides a countering positive contribution. Some further discussion of the behavior of adsorption entropies in the case of heterogeneous adsorbents is given in Section XVII-14. 

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]. 

This review is structured as follows. In the next section we present the theory for adsorbates that remain in quasi-equilibrium throughout the desorption process, in which case a few macroscopic variables, namely the partial coverages 0, and their rate equations are needed. We introduce the lattice gas model and discuss results ranging from non-interacting adsorbates to systems with multiple interactions, treated essentially exactly with the transfer matrix method, in Sec. II. Examples of the accuracy possible in the modehng of experimental data using this theory, from our own work, are presented for such diverse systems as multilayers of alkali metals on metals, competitive desorption of tellurium from tungsten, and dissociative... 

To derive an explicit expression of the rate of desorption we restrict ourselves to nondissociative adsorption, listing references to other systems— such as multicomponent and multilayer adsorbates with and without precursors—for which such a treatment has been given, later. We look at a situation where the gas phase pressure of a molecular species, P, is different from its value, P, which maintains an adsorbate at coverage 6. There is then an excess flux to re-establish equilibrium between gas phase and adsorbate so that we can write [7-10]... 

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

The following are now expected first, in the CO oxidation electrode potential range on Pt/Ru surfaces (i.e., at low potentials), entrapped oxygen should be found second, the entrapped (subsurface) oxygen concentration between the Ru layers should increase with increasing multilayer character (and coverage) of the Ru deposit on Pt. Research focusing on these two issues is planned. 

Conventional bulk measurements of adsorption are performed by determining the amount of gas adsorbed at equilibrium as a function of pressure, at a constant temperature [23-25], These bulk adsorption isotherms are commonly analyzed using a kinetic theory for multilayer adsorption developed in 1938 by Brunauer, Emmett and Teller (the BET Theory) [23]. BET adsorption isotherms are a common material science technique for surface area analysis of porous solids, and also permit calculation of adsorption energy and fractional surface coverage. While more advanced analysis methods, such as Density Functional Theory, have been developed in recent years, BET remains a mainstay of material science, and is the recommended method for the experimental measurement of pore surface area. This is largely due to the clear physical meaning of its principal assumptions, and its ability to handle the primary effects of adsorbate-adsorbate and adsorbate-substrate interactions. 

Therefore, recent interest has been focused prevailingly on electrodes modified by a multilayer coverage, which can easily be achieved by using polymer films on electrodes. In this case, the mediated electron transfer to solution species can proceed inside the whole film (which actually behaves as a system with a homogeneous catalyst), and the necessary turnover rate is relatively lower than in a monolayer. 

The AES results indicate that the aniline coverage is more than two times greater than the maximum coverage based on van der Waals radii. The TPR results show this species is too stable to be a condensed multilayer. Hence, we conclude that aniline polymerized forming a very stable polymer layer. In addition, the absence of infrared bands corresponding to C=C stretches or ring vibrations indicated that the poly(aniline) film was formed with the phenyl rings parallel to surface. The infrared results also indicated that the poly(aniline) film had N-H bonds which were oriented perpendicular relative to the surface. 

A comparison of CO desorption from ruthenium (6), and from multilayer (10 ML) and monolayer copper covered ruthenium is shown in Figure 1. The CO coverage is at saturation. The TPD features of the 1 ML copper (peaks at 160 and 210 K) on ruthenium are at temperatures intermediate between those found for adsorption on surfaces of bulk ruthenium and copper, respectively. This suggests that the copper monolayer is perturbed electronically and that this perturbation is manifested in the bonding of CO. An increase in the... 

A Ru(0001) sample, with vacuum deposited Cu, has been characterized by cyclic voltammetry by transferring to an electrochemical cell (16). Figures 4e-4h shows the anodic stripping curves for four different coverages of Cu. A single stripping peak was observed at +110 mV for 0.6 ML Cu and shifted to +145 mV for 5.2 ML Cu. This peak represents the removal of the first monolayer of Cu or Cu in direct contact with the Ru surface. The curve for 5.2 ML Cu shows an additional peak at -20 mV for the stripping of multilayer Cu. 

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