Adsorption from the Gas Phase

In the simplest interpretation, the dipole moment is caused by a partial charge q sitting at a distance d from the metal surface p = qd. However, the image charge does not reside on the geometrical surface separating the substrate from the vacuum, but is spread out in front of it and may partially surround the adatom.69 Therefore, it is difficult to infer the partial charge from the dipole moment - just as, in 

At small coverages, the change A t in the work function is proportional to the surface concentration. At higher coverages, the variation of t with Ns becomes less rapid in some cases it may even pass through a maximum before reaching the value for a saturated monolayer. This apparent decrease of the dipole moment is caused by the Coulomb interaction between the adsorbed atoms, which makes large dipole moments unfavorable.62 

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Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

Figure 5.39. Characterization of the spillover species by photoelectron spectra of the Ols region taken from a 0.02 pm2 spot on the Pt surface (a) The residual O Is spectrum after the cleaning cycles (b) The Ols spectrum measured in 02 atmosphere (pO2=lxI0 6 mbar) (c) The Ols spectrum obtained during electrochemical pumping in vacuum with UWr = 1.1 V. R1 and R2 are the components which are formed by adsorption from the gas phase and by electrochemical pumping. The fitting components of the residual oxygen are shown with dashed lines. Photon energy = 643.2 eV, T 350-400°C.67 Reprinted with permission from Elsevier Science.

Fig. 2.3. Thermal desorption spectra after adsorption from the gas phase (a) adsorbed CO on Pt (b) H2 on Pt.

Adsorbed carbon monoxide on platinum formed at 455 mV in H2S04 presents a thermal desorption spectrum as shown in Fig. 2.4b. As in the case of CO adsorption from the gas phase, the desorption curve for m/e = 28 exhibits two peaks, one near 450 K for the weakly adsorbed CO and the other at 530 K for the strongly adsorbed CO species. The H2 signal remains at the ground level. A slight increase in C02 concentration compared to the blank is observed, which could be due to a surface reaction with ions of the electrolyte. Small amounts of S02 (m/e = 64) are also observed. 

The statements made hitherto are all based upon Greenler s paper. If the parallel light interacts with surface and the solution, but the vertical light only with the solution. In the case of adsorption from the gas phase, the adsorbed phase is sharp and consists essentially only of molecules actually in contact with the surface. In electrochemical situations, however, substantial amounts of "absorbed" solute are in the layer near the electrode. A careful examination of the Greenler paper shows that the net signal from the parallel and vertical components of the light does carry information from the solution phase as well as from the electrical phase. 

For the purposes of the derivation below, we will consider the process of adsorption from the gas phase. A simple example of a system involving adsorption of gases is the Haber process, in which N2(g) and H2(g) adsorb to the surface of metallic iron. 

It is often convenient to think of adsorption as occurring in three stages as the adsorbate concentration increases. Firstly, a single layer of molecules builds up over the surface of the solid. This monolayer may be chemisorbed and associated with a change in free energy which is characteristic of the forces which hold it. As the fluid concentration is further increased, layers form by physical adsorption and the number of layers which form may be limited by the size of the pores. Finally, for adsorption from the gas phase, capillary condensation may occur in which capillaries become filled with condensed adsorbate, and its partial pressure reaches a critical value relative to the size of the pore. 

Most early theories were concerned with adsorption from the gas phase. Sufficient was known about the behaviour of ideal gases for relatively simple mechanisms to be postulated, and for equations relating concentrations in gaseous and adsorbed phases to be proposed. At very low concentrations the molecules adsorbed are widely spaced over the adsorbent surface so that one molecule has no influence on another. For these limiting conditions it is reasonable to assume that the concentration in one phase is proportional to the concentration in the other, that is ... 

Equation 17.2 has been developed for adsorption from the gas phase. It is convenient to also express it in terms of partial pressures, which gives ... 

Adsorption from the gas-phase Desorption to the gas-phase Dissociation of molecules at the surface Reactions between adsorbed molecules Reactions between gas and adsorbed molecules. 

Since we are concerned here with adsorption from the gas phase, the chemical potential may be related to the pressure of the gas by... 

It should be apparent — since an adsorption isotherm can be derived from a two-dimensional equation of state —that an isotherm can also be derived from the partition function since the equation of state is implicitly contained in the partition function. The use of partition functions is very general, but it is also rather abstract, and the mathematical difficulties are often formidable (note the cautious in principle in the preceding paragraph). We shall not attempt any comprehensive discussion of the adsorption isotherms that have been derived by the methods of statistical thermodynamics instead, we derive only the Langmuir equation for adsorption from the gas phase by this method. The interested reader will find other examples of this approach discussed by Broeckhoff and van Dongen (1970). 

Example 1 Vapor Pressure and Molecular Interactions in the Pure Liquid Compound Example 2 Air-Solvent Partitioning Examples of Adsorption from the Gas Phase Example 3 Air-Solid Surface Partitioning... 

The atomic species can arise by adsorption from the gas phase but it can also have an origin in the formation of ethylene oxide, as suggested by Worbs in 1942 (see ref. 84 in Voge and Adams [343]), viz. 

Figure 9.2 Schematic plot of eight types of adsorption isotherms commonly observed. If adsorption from the gas phase is studied, the abscissa is the partial pressure P. For adsorption from solution the concentration c is used.

In this chapter, we have so far discussed the adsorption of gases in solids. This section gives a brief description of the adsorption process from liquid solutions. This adsorption process has its own peculiarities compared with gas-solid adsorption, since the fundamental principles and methodology are different in almost all aspects [2,4,5], In the simplest situation, that is, a binary solution, the composition of the adsorbed phase is generally unknown. Additionally, adsorption in the liquid phase is affected by numerous factors, such as pH, type of adsorbent, solubility of adsorbate in the solvent, temperature, as well as adsorptive concentration [2,4,5,84], This is why, independently of the industrial importance of adsorption from liquid phase, it is less studied than adsorption from the gas phase [2],... 

Instead of c, for adsorption from the gas phase, it is custom to use the partial pressure. For that isotherm it has to be assumed (a) that a localized adsorption (i.e., finite and defined number of adsorption sites) takes place at an isotropic surface, (b) that the adsorbed particles do not interact with each other, and (c) that the maximum coverage is a monolayer of the adsorbed particles [iii]. About the importance of the Langmuir isotherm in electrochemistry see - adsorption isotherm. 

Adsorption from liquid solution is almost a new world in comparison with adsorption from the gas phase the fundamental principles and methodology are different in almost all respects (Gregg, 1961). 

In the method developed by Exerowa, Cohen and Nikolova [144] the insoluble (or slightly soluble) monolayers are obtained by adsorption from the gas phase. A special device (Fig. 2.28) was constructed for the purpose a ring a in the measuring cell of Scheludko and Exerowa for formation of microscopic foam films at constant capillary pressure (see Section 2.1.2.). The insoluble (or slightly soluble) substance from reversoir b is placed in this ring. Conditions for the adsorption of the surfactant on either surface of the bi-concave drop are created in the closed space of the measuring cell. The surfactant used was n-decanol which at temperatures lower than 10°C forms a condensed monolayer. Thus, it is possible to obtain common thin as well as black foam films. The results from these studies can be seen in Section 3.4.3.3. 

Adsorption from the gas phase can be measured by either gravimetric or volumetric techniques. In the gravimetric method, the weight of adsorbed gas is measured by observing the stretching of a helical spring from which the adsorbent is himg (see Fig. 2).f Alternatively,... 

For adsorption from the gas phase onto nonporous, impermeable surfaces, phys-isorption and nonactivated chemisorption are governed largely by gas-phase kinetics and are instantaneous on the time scale of chemical sensor measurements. 

In the study of adsorption from the gas phase, one sometimes finds differences between the extent or mechanism of adsorption of gases on metals and semiconductors. Hydrogen, for example, is dissociatively adsorbed on many metals but must be dissociated before it is adsorbed on semiconductors. 

An obvious way to determine the amount of a substance adsorbed on the surface is to measure the resulting change in its bulk concentration. This is equivalent to measuring adsorption from the gas phase by determining the decrease in partial pressure of the relevant gas. 

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