Ga Adsorption and Desorption Kinetics

During the last few years many studies of the behaviour of sulfur on and with metallic surfaces have been published and have provided a more complete understanding of the interaction processes. They have included gas adsorption and desorption kinetics, surface and grain boundary segregation, embrittlement, sulfidation, corrosion, passivation, catalyst poisoning, among others. 

First, we investigate Ga adsorption and desorption kinetics in vacuum. The sample vras held at a constant temperature of 740 °C. Ga adsorption/desorption isotherms were recorded by monitoring the intensity of the specularly... 

Fig. 4. Adsorption and desorption kinetics of ethane at different bulk gas concentrations in Norit activated carbon of 4.41 mm half-length slab at 30 °C. I atm. — MPSD model-------------Energy distribution model...

Volume 19 is devoted to considering simple processes occurring at the gas-solid interface. Chapter 1 serves as an introduction and deals with the methodology of experimental surface science. Experimental results for metal surfaces on both adsorption and desorption kinetics and surface diffusion are discussed in terms of the current theories of these processes. Chapter 2 deals in the same way with these processes on semi-conductor surfaces. Finally, Chapter 3 is concerned with radiation and photoeffects at gas—solid interfaces. 

In the Langmuir treatment of the kinetics of gas adsorption and desorption, the rate of adsorption is equated to the rate of desorption as follows ... 

To further demonstrate the power of the kinetic lattice gas approach we review briefly the work on precursor-mediated adsorption and desorption [60,61]. We consider an adsorbate in which, in addition to the most strongly bound chemisorbed (or physisorbed) adsorbed state, the adparticles can also be found in intrinsic or extrinsic precursor states. One introduces three sets of occupation numbers, , = 0 or 1, = 0 or 1, and /, = 0 or 1, depending... 

In this section we review a generahzation of the kinetic lattice gas model when surface reconstruction takes place upon adsorption and desorption. 

There are three approaches that may be used in deriving mathematical expressions for an adsorption isotherm. The first utilizes kinetic expressions for the rates of adsorption and desorption. At equilibrium these two rates must be equal. A second approach involves the use of statistical thermodynamics to obtain a pseudo equilibrium constant for the process in terms of the partition functions of vacant sites, adsorbed molecules, and gas phase molecules. A third approach using classical thermodynamics is also possible. Because it provides a useful physical picture of the molecular processes involved, we will adopt the kinetic approach in our derivations. 

Sorption Kinetics. The adsorption and desorption data were analyzed in terms of a model based on the following main assumptions. Micropore diffusion within the sieve crystals is the rate-controlling process. Diffusion may be described by Fick s law for spherical particle geometry with a constant micropore diffusivity. The helium present in the system is inert and plays no direct role in the sorption or diffusion process. Sorption occurs under isothermal conditions. Sorption equilibrium is maintained at the crystal surface, which is subjected to a step change in gas composition. These assumptions lead to the following relation for the amount of ethane adsorbed or desorbed by a single particle as a function of time (Crank, 4). 

Campbell and Paffett reported that adsorbed Cl atoms had no effect on the kinetics of adsorption and desorption of oxygen on Ag(llO) [40]. To an extent, this result was not surprising since Campbell and Paffett dosed the Cl atoms on to the Ag(llO) from CI2 gas at 300 K, producing ordered overlayers of Cl, discemable by LEED, and areas that were Cl free [40]. 

The Langmuir isotherm equation can also be derived from the formal adsorption and desorption rate equations derived from chemical reaction kinetics. In Section 3.2.2, we see that the mass of molecules that strikes 1 m2 in one second can be calculated using Equation (186), by applying the kinetic theory of gases as [dmldt = P2 (MJ2nRT)m], where P2 is the vapor pressure of the gas in (Pa), Mw is the molecular mass in (kg mol ), T is the absolute temperature in Kelvin, R is the gas constant 8.3144 (nT3 Pa mol-K-1). If we consider the mass of a single molecule, mw (kg molecule-1), (m = Nmw), where N is the number of molecules, by considering the fact that (R = kNA), where k is the Boltzmann constant, and (Mw = NAmw), we can calculate the molecular collision rate per unit area (lm2) from Equation (186) so that... 

The Langmuir isotherm is the most widely used in the derivation of kinetic models for gas-solid catalytic reactions. The derivation of a Langmuir isotherm depends on the kinetic method where the equilibrium conditions are determined by the equality of the rates of adsorption and desorption. At equilibrium the following relation holds,... 

In gas-solid chromatography, the kinetics of adsorption and desorption determine the C terms in Figure 13.1. 

Irving Langmuir, the Nobel prize-winning industrial physical chemist who worked at General Electric, built an elegant structure upon this foundation in kinetic theory. He reasoned that not every molecule would adsorb, but only some would do so. Furthermore, one reason for this was that to be adsorbed there should be a site for adsorption to occur. It stands to reason then that on the basis of mass action, the rate of adsorption should be proportional to the concentration of molecules in the gas phase and to the number of sites available on the surface. Additionally, the rate should be related at any time to the number of sites not covered at that time rather than to the total number of sites present per unit area. Conversely, and again by the principle of mass action, the rate of desorption should be proportional to the number of sites currently occupied at that time. Using ka and kd as the proportionality constants (that we will call the rate constant for adsorption and desorption, respectively), we can write the net rate of adsorption for gas phase species i as the difference between the rate of adsorption and the rate of desorption ... 

After initiation, the way an outburst evolves can be strongly influenced by the amount of free gas in the outburst coal. It is therefore important also to understand the kinetics of the adsorption and desorption processes. 

Polymer chemists use DSC extensively to study percent crystallinity, crystallization rate, polymerization reaction kinetics, polymer degradation, and the effect of composition on the glass transition temperature, heat capacity determinations, and characterization of polymer blends. Materials scientists, physical chemists, and analytical chemists use DSC to study corrosion, oxidation, reduction, phase changes, catalysts, surface reactions, chemical adsorption and desorption (chemisorption), physical adsorption and desorption (physisorp-tion), fundamental physical properties such as enthalpy, boiling point, and equdibrium vapor pressure. DSC instruments permit the purge gas to be changed automatically, so sample interactions with reactive gas atmospheres can be studied. 

Kinetic phenomena, such as rates of adsorption and desorption of atoms or molecules, measured as a function of pressure, temperature (substrate or gas), surface structure, etc., contain important information on the energetics of adsorption. Unambiguous relationships between kinetics and energetics are, however, at least in the field of adsorption, frequently not available. The simplest case is the rate of adsorption, rad, of atoms or simple molecules onto a uniform surface [64Hay] ... 

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