Adsorption-related strength

Fig. IX-32. The isotherms of adsorption-related strength decrease of naphthalene monocrystals immersed into aqueous solutions of various surfactants 1 - C2H5COOH 2 -C3H7COOH 3 - C4H9COOH 4 - C5H,COOH [9]...

Fig. 2.4. Volcano curve relating strength of adsorption with reaction properties such as the rate of reaction.

One more feature of structural defects that determines their role in the adsorption-induced strength decrease is that in most cases the penetration of liquid phase specifically along the defects facilitates the delivery of adsorption-active medium into a pre-fracture zone, and thus allows the medium to influence the development of cracks. In this sense the role of structural defects is closely related to the role of conditions under which deformation and fracture takes place. With respect to discussed case, these are the conditions under which penetration of active medium into the zone of crack formation and development takes place. 

Kunze, O. R., Choudhury, M. S. U., 1972. Moisture adsorption related to the tensile strength of rice. Cereal Chem.,... 

The grain boundaries present in a polycrystalline solid provide one with the most characteristic example illustrating the effect of real defect structures on the extent of the adsorption-related lowering of strength. (In our discussion, we use the concept of active medium implying both the medium... 

The balance between these different types of bonds has a strong bearing on the resulting ordering or disordering of the surface. For adsorbates, the relative strength of adsorbate-substrate and adsorbate-adsorbate interactions is particularly important. Wlien adsorbate-substrate interactions dominate, well ordered overlayer structures are induced that are arranged in a superlattice, i.e. a periodicity which is closely related to that of the substrate lattice one then speaks of commensurate overlayers. This results from the tendency for each adsorbate to seek out the same type of adsorption site on the surface, which means that all adsorbates attempt to bond in the same maimer to substrate atoms. 

Many chemical elements exhibit catalytic activity (5) which, within limits, is inversely related to the strength of chemisorption of the VOCs and oxygen, provided that adsorption is sufficiently strong to achieve a high surface coverage (17). If the chemisorption is too strong, the catalyst is quickly deactivated as the active sites become irreversibly covered. If the chemisorption is too weak, only a small fraction of the surface is covered and the activity is very low (17) (Fig. 2). 

In this figure, the activation energies of N2 dissociation are compared for the different reaction centers the (111) surface structure ofan fee crystal and a stepped surface. Activation energies with respect to the energy of the gas-phase molecule are related to the adsorption energies of the N atoms. As often found for bond activating surface reactions, a value of a close to 1 is obtained. It implies that the electronic interactions between the surface and the reactant in the transition state and product state are similar. The bond strength of the chemical bond... 

Identification of such universal relations between activation energies and heats of adsorption for particular classes of reaction can be seen as a more precise and more quantitative formulation of Sabatier s Principle. It is promising tool in the search for new materials on the basis of optimized interaction strength between relevant intermediates and the surface. 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

This is the first experimental demonstration of changes in the strength of CO adsorption at Pt-based alloy electrodes. Nprskov and co-workers theoretically predicted a similar linear relation between changes in ads(CO) and shifts in the (i-band center [Hammer et al., 1996 Hammer and Nprskov, 2000 Ruban et al., 1997]. Because the Pt4/7/2 CL shift due to alloying can be more easily measured by XPS than the li-band center can, this should be one of the most important parameters to aid in discovering CO-tolerant anode catalysts among Pt-based alloys or composites. 

Figure 4b shows a quasi linear relation between the activity in alcohol dehydration and the quantity of Bronsted acid sites obtained by DMP adsorption. However a low amount of sites is active in dehydration (on theta alumina) but practically no Bronsted acid sites are detected by DMP adsorption because of their low number and strength. 

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