Adsorption sites, distinguishing between

Adsorption environments, TSLS complex, 120,12 Adsorption measurements, procedures, 90 Adsorption sites, distinguishing between, 93-94 Aging, effect on particle size of titanium dioxide sols, 200-201,203/... 

The lateral interactions in the adsorbate can enhance or diminish the interaction energy in the surface. If the adsorption sites at the boundary between reconstructed and unreconstructed areas of surface are further distinguished from those inside these patches, we can introduce more interactions such as... 

Definitions of the most common adsorption sites are shown in Fig. 5.5. They are named on-top site, bridge sites (long or short bridge), and hollow sites, which may be three-fold or four-fold in character. In case of three-fold adsorption on the fcc(lll) surface it is also necessary to distinguish between hep and fee sites, having an atom just below the site or not. 

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

In order to accurately determine the chemisorbed amount from the overall adsorption isotherm, the sample can be further outgassed at the same temperature to remove the physically adsorbed amount, after which a new adsorption procedure is carried out to obtain isotherm II. The difference between the first and second isotherm gives the extent of irreversible adsorption ( ) at a given temperature (Figure 13.5b), and can be considered as a measurement of the amount of strong sites in the catalyst. However, in the first approximation, the magnitude of the heat of adsorption can be considered as a simple criterion to distinguish between physical and chemical adsorption. 

Fig. 14 Schematic view of the association between morphological arrangements of carbon crystallites and energetic characteristics of carbon black surfaces. Four different types of adsorption sites are distinguished that refer to the de-convolution shown in Fig. 13...

Ai (140) measured the acid site concentration by the adsorption of ammonia. No correlation was found between the P/V ratio, the acidity, and the catalytic activity. This result has been attributed to the use of ammonia as a probe molecule that cannot distinguish between Lewis and Br0nsted acidity. Comaglia et al. (137) measured the acid sites using pyridine and acetonitrile as probes. However, the pyridine results showed no correlation between the Lewis to Br0nsted acid site ratio or the Lewis acid site concentration and the activity and selectivity of the catalyst for MA formation. 

Theoretical. In deriving a theoretical expression for k, we have developed a reaction mechanism model for calcite dissolution which expands on the adsorption layer heterogeneous reaction model of Mullin ( ). We assume that a thin (possibly only a few molecules thick) "adsorption layer" (or "surface layer") exists adjacent to the crystal surface, between the crystal surface and the hydrodynamic boundary layer. Species in the adsorption layer are loosely bound to the crystal surface and have relatively low mobility, particularly in comparison with species mobility in the boundary layer. The crystal surface is believed to be sparsely covered by reaction sites at discontinuities in the surface ( 3). To distinguish between species activities in the bulk fluid, at the base of the boundary layer (near the crystal surface), and in the adsorption layer, we use the subscripts (B), (o), and (s), respectively. 

This situation Is typical for solid-liquid interfaces where, during adsorption, the area A remains constant. In lattice models this is Interpreted as the constancy of the total number of sites. With adsorption from solution at liquid-liquid Interfaces this restriction Is often absent then one can distinguish between adsorption at given A and that at constant Interfaclal tension y or. for that matter, at constant x. 

In a more recent study. Nelson and Yang [494] pre.sented a surface complex-ation model to describe the effect of pH on adsorption equilibria of chlorophe-nols, i.e., the electrostatic effect they also discussed the potential importance of 7t-7t interactions and donor-acceptor complex formation but could not distinguish between the two and concluded, somewhat vaguely, that [t]hese proposed mechanisms provide plausible explanations for the surface complexation reactions between chlorophenols (neutral or anionic forms) and the surface of activated carbon (acidic or basic sites). ... 

Surface properties of Ti-containing adsorbents such as Ti-silica gel, Ti-Al beta zeolite, two samples of TS-1 zeolite prepared in different ways, as well as of pure silicalite were studied by means of IR spectroscopy of CO adsorbed at low temperatures Surface silanol groups, acidic bridged hydroxyls, coordinatively unsaturated Ti and A1 atoms were characterized as adsorption sites. The capability of IR spectroscopy of adsorbed CO to distinguish between TS-1 samples of different origin is demonstrated. 

In the first part of this chapter, we investigate the desorption of NO and CO molecules from a platinum surface to leam about energy flow between the substrate and the adsorbate. By using a stepped surface we unravel how the chemical dynamics are affected by surface defects [2], and demonstrate that the chemical dynamics depend critically on the precise adsorption site. In the second part we will follow diffusion of CO over the surface in real-time by using the adsorbate vibration-sensitive SFG method in a pump-probe manner. The stepped surface allows us to distinguish between molecules on the step and the terrace sites and therefore makes the diffusion from step to terraces sites visible [1]. 

The type-I sites have, in protein based CSPs, identical behavior toward the two enantiomers, and cannot distinguish between them. Many columns contain mostly type-I sites. On type-I sites all possible molecular interactions, between the analyte molecules and atoms or groups of atoms belonging to the adsorbent surface, take place. These interactions can originate from the nonchiral parts of the protein and/or from the adsorbent (silica) matrix. The energies of each interaction on type-I sites are small. The other type of adsorption sites have, in protein based CSPs, much higher adsorption energy and are enantioselective (chiral). These sites, type-II sites, are responsible for the enantiomeric separations. On most CSPs the type-II sites are relatively few. 

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