Adsorption kinetics role

In Chapter 1 we consider the physical and diemical basis of the method of semiconductor chemical sensors. The items dealing with mechanisms of interaction of gaseous phase with the surface of solids are considered in substantial detail. We also consider in this part the various forms of adsorption and adsorption kinetics processes as well as adsorption equilibria existing in real gas-semiconductor oxide adsorbent systems. We analyze the role of electron theory of chemisorption on... 

The study of adsorption kinetics of a surfactant on the mineral surface can help to clarify the adsorption mechanism in a number of cases. In the literature we found few communications of this kind though the adsorption kinetics has an important role in flotation. Somasundaran et al.133,134 found that the adsorption of Na dodecylsulfonate on alumina and of K oleate on hematite at pH 8.0 is relatively fast (the adsorption equilibrium is reached within a few minutes) as expected for physical adsorption of minerals with PDI H+ and OH". However, the system K oleate-hematite exhibits a markedly different type of kinetics at pH 4.8 where the equilibrium is not reached even after several hours of adsorption. Similarly, the effect of temperature on adsorption density varies. The adsorption density of K oleate at pH 8 and 25 °C is greater than at 75 °C whereas the opposite is true at pH 4.8. Evidently the adsorption of oleic acid on hematite involves a mechanism that is different from that of oleate or acid soaps. 

Selective adsorption may also have dramatic consequences for crystal growth if certain crystal planes can be blocked ). Trace amounts of Impurities may in this way determine the eventual crysted shape. This feature plays an Important role in the preparation of (tailor-made) colloids (Volume IV) obviously adsorption kinetics plays an Important role here. Certain substances may of course work the other way around In that they "attack the solid. Examples are some sulfur-containing compounds with silver halides, and chelating agents with haematite (a-Fe O ). 

Role of Pore Size Distribution in the Binary Adsorption Kinetics of Gases in Activated Carbon... 

Role of pore size distribution in the binary adsorption kinetics of gases in activated carbon... 

Kose R, Brown WA, King DA (1999) Role of lateral interactions in adsorption kinetics CO/Rh 100. J Phys Chem B 103 8722... 

The support plays an important effect in the adsorption kinetics of CO on supported clusters. Indeed CO physisorbed on the support is captured by surface diffusion on the periphery of the metal clusters where it becomes chemisorbed. The role of a precursor state played by CO adsorbed on the support is a rather general phenomenon. It has been observed first on Pd/mica [173] then on Pd/alumina [174,175], on Pd/MgO [176], on Pd/silica [177], and on Rh/alumina [178]. This effect has been theoretically modeled assuming the clusters are distributed on a regular lattice [179] and more recently on a random distribution of clusters [180]. The basic features of this phenomenon are the following. One can define around each cluster a capture zone of width Xg, where is the mean diffusion length of a CO molecule on the support. Each molecule physisorbed in the capture zone will be chemisorbed (via surface diffusion) on the metal cluster. When the temperature decreases, Xg increases, then the capture zone increases to the point where the capture zones overlap. Thus the adsorption rate increases when temperature decreases before the overlap of the capture zones that occurs earlier when the density of clusters increases. Another interesting feature is that the adsorption flux increases when cluster size decreases. It is worth mentioning that this effect (often called reverse spillover) can increase the adsorption rate by a factor of 10. We later see the consequences for catalytic reactions. 

Although large number of studies have been reported on the equilibrium adsorption of ionic surfactants at the interfaces, very little attention has been paid to the adsorption kinetics. Only a few attempts have been made to follow the time evolution of the process from the initial adsorption to the equilibrium configuration and to understand the role of the diffusion [24,25,31]. 

The role of adsorption kinetics and the diffusion of surfactants is especially important in controlling capillary impregnation. According to studies by N.N. Churaev, the solution impregnating the capillary quickly loses its dissolved surfactant due to adsorption of the latter on capillary walls, so the rate of impregnation may be limited by the diffusional transport of surfactant from the bulk of the solution to the menisci in the pores. 

In this chapter specific theories and experimental set-ups for interfacial relaxation studies of soluble adsorption layers are presented. A general discussion of relaxation processes, in bulk and interfacial phases, was given in Chapter 3. After a short introduction, in which the important role of mechanical properties of adsorption layers and the exchange of matter for practical applications are discussed, the main differences between adsorption kinetics studies and relaxation investigations are explained. Then, general theories of exchange of matter and specific theories for different experimental techniques are presented. Finally, experimental setups, based on harmonic and transient interfacial area deformations, are described and results for surfactant and polymer adsorption layers discussed. 

In a recent review Malysa (1992) discussed the role of effective dilational elasticity and exchange of matter as a contribution to the resistance of wet foam films against disturbances. The stability of wet foams results as a complex process, in which surface activity, adsorption kinetics and effective elasticity of present surfactants have a complementary effect. A qualitative correlation between wet foam stability and effective dilational elasticity was found for some simple surfactants. 

The ratio Tj/Tj characterises the role of a non-equilibrium DL on the adsorption kinetics of ionic surfactants. If for example vj7s,g 8-10, z=2 and C(, Cj the deviation of c(x, t) from equilibrium can retard the adsorption kinetics by two orders of magnitude. However, an addition of electrolyte can suppress this effect. 

In chapter 4 we discussed the physical properties of chromatographic adsorbents. In this chapter we will discuss their chemical properties. The most important aspect of the chemistry of a packing is the character of the adsorbing surface. But the chemistry of the packing also plays a role with respect to its hydrolytic stability or whether and to what degree it shrinks and swells in various solvents. In addition, the chemistry of the packing influences its physical strength. The chemistry of the surface influences adsorption kinetics and mass transfer. 

The last two chapters have addressed the adsorption kinetics in homogeneous particle as well as zeolitic (bimodal diffusion) particle. The diffusion process is described by a Fickian type equation or a Maxwell-Stefan type equation. Analysis presented in those chapters have good utility in helping us to understand adsorption kinetics. To better understand the kinetics of a practical solid, we need to address the role of surface heterogeneity in mass transfer. The effect of heterogeneity in equilibria has been discussed in Chapter 6, and in this chapter we will briefly discuss its role in the mass transfer. More details can be found in a review by Do (1997). This is started with a development of constitutive flux equation in the presence of the distribution of energy of interaction, and then we apply it firstly to single component systems and next to multicomponent systems. 

The effect of heterogeneity was accounted for by the use of the energy distribution as shown in previous sections. For adsorption of paraffins onto activated carbon, the source of the energy distribution is assumed to be due to the micropore size distribution. This heterogeneity is called the micropore size-induced heterogeneity. The energy of interaction between the micropore and the adsorbate molecules is a strong function of the size of the adsorbate as well as the size of the micropore. The effect of micropore size distribution on adsorption equilibrium has been addressed in Chapter 6. Here we address its role in the adsorption kinetics. 

The adsorption kinetics are found to be completely different for the two molecules, AHS and DHS. The rate of dissolution of molecules into solution from fully formed monolayers at room temperature is negligible, so equilibrium carmot be established by desorption and readsorption of monolayer components in the complete mono-layer. Equilibration could proceed through the physisorbed thiol. Rapid equilibration between the physisorbed molecule and the molecules in solution would be followed by relatively slow conversion of the physisorbed thiols to chemisorbed thiolates. If the rate constant for conversion of thiol to surface thiolate is independent of the structure of the thiol, which is likely, a chemisorbed layer would be kinetically trapped. The adsorption rate constant would then be determined by the equilibration between the physisorbed thiol and the thiols in solution, which might explain the observed large differences between AHS and DHS. A similar argument has been used by Bain et al. to explain the observed composition of mixed monolayers. This would explain the major role of the solvent in the adsorption kinetics and possibly also in the resulting film structure. A further consideration is the expected hy-... 

In considering isotherm models for chemisorption, it is important to remember the types of systems that are involved. As pointed out, conditions are generally such that physical adsorption is not important, nor is multilayer adsorption, in determining the equilibrium state, although the former especially can play a role in the kinetics of chemisorption. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

The mesopores make some contribution to the adsorptive capacity, but thek main role is as conduits to provide access to the smaller micropores. Diffusion ia the mesopores may occur by several different mechanisms, as discussed below. The macropores make very Htde contribution to the adsorptive capacity, but they commonly provide a major contribution to the kinetics. Thek role is thus analogous to that of a super highway, aHowkig the adsorbate molecules to diffuse far kito a particle with a minimum of diffusional resistance. 

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