Mobilities of adsorbates

When the temperature of the analyzed sample is increased continuously and in a known way, the experimental data on desorption can serve to estimate the apparent values of parameters characteristic for the desorption process. To this end, the most simple Arrhenius model for activated processes is usually used, with obvious modifications due to the planar nature of the desorption process. Sometimes, more refined models accounting for the surface mobility of adsorbed species or other specific points are applied. The Arrhenius model is to a large extent merely formal and involves three effective (apparent) parameters the activation energy of desorption, the preexponential factor, and the order of the rate-determining step in desorption. As will be dealt with in Section II. B, the experimental arrangement is usually such that the primary records reproduce essentially either the desorbed amount or the actual rate of desorption. After due correction, the output readings are converted into a desorption curve which may represent either the dependence of the desorbed amount on the temperature or, preferably, the dependence of the desorption rate on the temperature. In principle, there are two approaches to the treatment of the desorption curves. 

Transition metal colloids can also be prevented from agglomeration by polymers or oligomers [27,30,42,43]. The adsorption of these molecules at the surface of the particles provides a protective layer. In the interparticle space, the mobility of adsorbed molecules should be reduced decreasing the entropy and thus increasing the free energy (Fig. 2). 

Figure 3.10. Schematic representation of the elementary steps used in microkinetic simulations of the reduction of NO on supported metal particles [23]. The mechanism represented here incorporates adsorption and desorption steps, surface reactions such as NO dimerization and dissociation and N2, N20 and C02 formation, surface oxidation, and mobility of adsorbates. (Figure provided by Professor Libuda and reproduced with permission from Elsevier, Copyright 2005).

Since the most active catalytic sites are usually steps, kinks, and surface defects, atomically resolved structural information including atomic distribution and surface structure at low pressure, possible surface restructuring, and the mobility of adsorbate molecules and of the atoms of the catalyst surface at high temperature and high pressure is crucial to understanding catalytic mechanisms on transition metal surfaces. The importance of studying the structural evolution ofboth adsorbates... 

Chemisorption of benzene at 297°C on Ni(110) occurred in a rather different manner. Several patterns, some streaked, were observed, and they followed the same sequence and showed the same behavior as those obtained when acetylene was chemisorbed on this surface (29). These structures have not been fully elucidated, but the streaked patterns suggest (i) that the mobility of adsorbed species along the "furrows of the (110) face is easier than their mobility across them, and (ii) that dissociation of the carbon skeleton of benzene and the formation of other structures occurs. 

Structure and mobility of adsorbed molecules may exhibit a wide spectrum of features. As a general view it is commonly accepted that layers are formed on increasing abundance. The mobility of molecules in the first layer depends strongly on interactions present in a given case. Local domains of adsorbed molecules may be formed in the case of non-uniform surface. Analysis of deuteron spectra for a series of molecules adsorbed on neutral alumina led to the conclusion that while the exchange of molecules between layers in a given domain is fast, the diffusion rate between domains is slow [5],... 

The rotational mobility of adsorbed molecules is caused by its rotational degree of freedom (resulting from the fact that the molecule is tightly bound to the substrate through the only atom) and by the coupling of molecular vibrations with surface atomic vibrations. The rotational motion intensity is strongly temperature-dependent and affects spectroscopic characteristics. As a result, the rotational mobility of surface hydroxyl groups was reliably detected.200 203... 

Injection of steam or heated air into the subsurface provides large amounts of thermal energy, which speeds the mobilization of adsorbed organic contaminants and results in their removal as either a vapor or liquid phase. Elevated temperature increases the vapor pressure of the chemicals involved and promotes transfer of constituents across the air-water interface, which results in the increased removal of contaminants in high-humidity or nearly saturated soil systems. Additionally, the presence of high-temperature water sometimes results in oxidation or hydration of organic contaminants. 

R. D. Tilton, A. P. Gast, and C. R. Robertson, Surface diffusion of interacting proteins. Effect of concentration on the lateral mobility of adsorbed bovine serum albumin, Biophys. J. 58, 1321-1326 (1990). 

Among the other information which it can provide, the field-emission microscope gives direct evidence of the mobility of adsorbed layers. These results have been reviewed elsewhere by Gomer 118) and will not be further discussed here. 

Decreased mobility of adsorbed chains has been observed and proved in many cases both in the melt and in the solid state [52-54] and changes in composite properties are very often explained by it [52,54]. Overall properties of the interphase, however, are not completely clear. Based on model calculations the formation of a soft interphase is claimed [51], while in most cases the increased stiffness of the composite is explained by the presence of a rigid interphase [55,56]. The contradiction obviously stems from two opposing effects. Imperfection of the crystallites and decreased crystallinity of the interphase should lead to lower modulus and strength and larger deformability. Adhesion and hindered mobility of adsorbed polymer chains, on the other hand, decrease deformability and increase the strength of the interlayer. 

The field emission microscope offers a very clear-cut and basically simple method of determining the mobility of adsorbates quantitatively. If it were possible to evaporate the gas under study from a suitable source (e.g., a heatable CuO filament for oxygen) in such a way that only a portion of the tip became contaminated, one could determine how, and at what temperatures of the tip, migration occurred. If one attempted to evaporate from a gas emitter placed on one side of the tip while the microscope tube was at room temperature, gas rebounding from the walls would instantly contaminate the whole tip and the experiment would fail. 

No cross ozonide was formed from unsymmetrical alkenes. The authors theorized628 that the carbonyl oxide zwitterionic species formed on wet silica gel immediately adds water followed by rapid decomposition of the intermediate hydroxyalkyl hydroperoxide to carboxylic acid and water. It means that water on silica gel acts as participating solvent. In the absence of adsorbed water, rapid recombination of the adsorbed aldehyde and carbonyl oxide due to a favorable proximity effect gives normal ozonide. The low mobility of adsorbed species on the silica surface accounts for the absence of cross ozonides. 

Chen, J-H., Lion, L. W., Ghiorse, W. C. Shuler. M. L. (1995). Mobilization of adsorbed cadmium and lead in aquifer material by bacterial extracellular polymers. Water Research, 29, 421—30. 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

Using silica of a different source, Turro [46] again found that the E/C ratios from irradiation of adsorbed alkanophenones 97 (n = 4, 7,10) at room temperature (3.7-3.9) are close to the values obtained from irradiations in t-butyl alcohol (4.2-6). Consistent with the expected reduction in mobility of adsorbed molecules on silica surfaces at lower temperatures, only cyclization products were isolated from irradiations of 97 on silica at — 125°C. At these... 

Aspects of the distribution of species on surfaces have been reviewed (35) and our understanding of the disposition, composition, and properties of the adsorbed phase is increasing through applications of recently developed high-vacuum techniques, for example, LEED (60, 61). Some information about the mobility of adsorbed material is available (62a-e) and the significance of surface diffusivity in reaction kinetics has been discussed (63). The behavior of supported metal catalysts may be influenced by the transfer of material between the two phases (metal and support) by diffusion (64-66). 

The same conclusion may be obtained from the study of the behavior of many gases adsorbed on charcoal. We shall discuss the mobility of adsorbed molecules in Sec. VII, but we may mention here one of the results of such studies. Many gases, such as A, N2, 02, CO, CH4, etc., when adsorbed on charcoal, behave as two-dimensional nonideal gases (44)- This behavior can be described by a two-dimensional van der Waals equation, from which a two-dimensional van der Waals constant o2 (comparable with the normal three-dimensional van der Waals a) may be derived. The two-dimensional van der Waals constants can also be calculated from the three-dimensional values of a (45). The experimental results show that the actual a2 constants for gases adsorbed on charcoal or on mercury are always far lower than the theoretical ones and are very often even negative (45). The adsorbed molecules tend to repel each other instead of showing a mutual attraction. This behavior also points to a polarization of the adsorbed molecules by the field of the charcoal or of the mercury (47). 

The components of a phase interface have a wide range of mobilities. The mobility of adsorbed particles exceeds that of the atoms in the bulk of a solid. It is usually considered that the mobility of the adsorbed particles is greater than that of the surface atoms of the solid, and when the state of the solid s atoms changes, the adsorbed particles at each instant are distributed practically in equilibrium. The same relation between the mobilities is presumed for absorbed particles and the atoms of a solid in the bulk. This is apparently not always true. In the general case, each component has its own ratio of the migration activation energies, and cases are possible when the mobilities of the surface atoms of a solid and of the adsorbed particles are commensurable. The mobilities of the atoms of a solid and of absorbed particles may also be commensurable. 

To compose equations for adsorption of the 02 molecules with an allowance for the slow surface mobility of adsorbed particles along a surface. Compose the similar equations for adsorption of the CO molecules at low temperatures, when the surface mobility of adsorbed molecules needed be taken into account. 

In the Au/Al203/NiAl(100) system, hemispherical particles occur even at low coverage,7 unlike the situation with titania size distribution was narrow, and particles were stable to 600 K, implying low mobility of adsorbed atoms. Paradoxically, on alumina large particles migrate and coalesce faster than small ones, presumably because the metal-support interaction is weaker but with Au/FeO the diffusivity of atoms is higher due to a lower concentration of surface defects. 

The use of the surface concentration gradient as the driving force for surface diffusion is the most popular approach with which to describe the mobility of adsorbed molecules. When each adsorbed species is assumed to be at adsorption equilibrium and transported along the surface independently of the other species, the molar flux of species i due to surface diffusion can be written as... 

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