Adsorption sites, characterization

Protons bound to a magnetic particle with adsorption sites characterized by different Larmor angular frequency shifts (8c0i) will progressively become out of phase, and the signal S(t) will decay according to... 

The results show that the nature of the adsorption sites characterized by their gzz values depend on both the type and the total dose of radiation and also on whether it is carried out in oxygen or in vacuo followed by contact with oxygen. It is possible that some features in the spectra might belong to V-type centers which are not broadened because of their location in sites not accessible to oxygen. This is clearly the case of the V-type center at g = 2.046 observed by Wang and Lunsford (263). To add to the complexity of the problem, other oxygen species such as Of can also be formed by irradiation (Section V,A). 

Fig. 12.1 Schematic representation of the activation energy profiles for the adsorption-desorption process studied in the experiments of Drazer and Zanette [17]. The different activation energy profiles correspond to adsorption sites characterized by different adsorption heats. All energy profiles start from the same value f7iiquid which corresponds to the liquid phase, increase up to the same maximum value f/max. and then decrease to various final values f7site(l) site(2) f/site(3), ., corresponding to surface sites characterized by various adsorption heats. (From [6].)...

In the case of high (infinite) spatial correlations, adsorption sites characterized by the same value of adsorption energy are grouped into "patches" that are so large that interaction between molecules adsorbed on different patches can be neglected. The whole adsorption system can then be considered to be a collection of independent subsystems in thermal and material contact. For such patchwise surface topography, the function takes the form [37]... 

HREELS and TFD have played a unique role In characterizing the surface chemistry of systems which contain hydrogen since many surface techniques are not sensitive to hydrogen. We have used these techniques to characterize H2S adsorption and decomposition on the clean and (2x2)-S covered Ft(111) surface (5). Complete dissociation of H,S was observed on the clean Ft(lll) surface even at IlOK to yield a mixed overlayer of H, S, SH and H2S. Decomposition Is primarily limited by the availability of hydrogen adsorption sites on the surface. However on the (2x2)-S modified Ft(lll) surface no complete dissociation occurs at IlOK, Instead a monolayer of adsorbed SH Intermediate Is formed (5) ... 

As the reader might have noticed, many conclusions in electrocatalysis are based on results obtained with electrochemical techniques. In situ characterization of nanoparticles with imaging and spectroscopic methods, which is performed in a number of laboratories, is invaluable for the understanding of PSEs. Identification of the types of adsorption sites on supported metal nanoparticles, as well as determination of the influence of particle size on the adsorption isotherms for oxygen, hydrogen, and anions, are required for further understanding of the fundamentals of electrocatalysis. 

In the past two decades, 129Xe NMR has been employed as a useful technique for the characterization of the internal void space of nanoporous materials. In particular, the xenon chemical shift has been demonstrated to be very sensitive to the local environment of the nuclei and to depend strongly on the pore size and also on the pressure [4—6], Assuming a macroscopic inhomogeneity resulting from a distribution of adsorption site concentrations, 129Xe NMR spectra of xenon in zeolites have been calculated, and properties such as line widths, shapes as well as their dependence on xenon pressure can be reproduced qualitatively. A fully quantitative analysis, however, remains difficult due to the different contributions to the xenon line shift. (See Chapter 5.3 for a more detailed description of Xe spectroscopy for the characterization of porous media.)... 

Resistance changes upon benzene chemisorption have also been used by Gryaznov et al. (20) in an attempt to characterize adsorption sites at the surfaces of thin (< 10, 10-20, and 20-30 A) platinum films. 

Sander M, Pignatello J (2005) Characterization of charcoal adsorption sites for aromatic compounds insights drawn from single-solute and bi-solute competitive experiments. Environ Sci Technol 39 1606-1615... 

The mobility of 1-butene molecules decreases from the gas (AH = 5 Hz) to the adsorbed state (AH = 35 Hz) at room temperature The latter depends on surface coverage as previously shown for benzene (33). At the low surface coverage (0 = 0.5) where not all the available Sjj sites are occupied - there are only 3 molecules for 4 different positions (4 ) - the mobility is smaller than in the higher surface coverage sample (0 = 0.9) and this effect is even more important at lower temperaure (250 K). In the latter, characterized by 4.6 molecules for 4 Sji sites - the molecules are exchanged between the strong adsorption sites and the inner cavity, hence the smaller linewidth. 

Sorption processes are influenced not just by the natures of the absorbate ion(s) and the mineral surface, but also by the solution pH and the concentrations of the various components in the solution. Even apparently simple absorption reactions may involve a series of chemical equilibria, especially in natural systems. Thus in only a comparatively small number of cases has an understanding been achieved of either the precise chemical form(s) of the adsorbed species or of the exact nature of the adsorption sites. The difficulties of such characterization arise from (i) the number of sites for adsorption on the mineral surface that are present because of the isomorphous substitutions and structural defects that commonly occur in aluminosilicate minerals, and (ii) the difference in the chemistry of solutions in contact with a solid surface as compound to bulk solution. Much of our present understanding is derived from experiments using spectroscopic techniques which are able to produce information at the molecular level. Although individual methods may often be applicable to only special situations, significant advances in our knowledge have been made... 

The L-type (Langmuir) isotherm is characterized by a decreasing slope as concentration increases since vacant adsorption sites decrease as the adsorbent becomes covered. Such adsorption behavior could be explained by the high affinity of the adsorbent for the adsorbate at low concentrations, which then decreases as concentration increases. 

Coluccia, S., Marchese, k., and Martra, G. (2000) Molecular probes for the characterization of adsorption sites in micro-and mesoporous materials, in Photofunctional Zeolites (ed. M. Anpo), NOVA Science, New York, pp. 39-74. 

Our failed — at least insofar as precise transition state characterization is concerned — attempt was nonetheless instructive i) contrary to what found in ref 30, the surface version of reaction (1) does show a barrier, although small, thus Supporting the view that both desolvation of the Cl ion and weak hydrogen bonding to the nitrate group contribute to the barrier ii) reaction (1) appears to be faster than (1), if it is verified that the proton has transferred away from the adsorption site. Concerning the latter point, as noted in Sec.II., there is experimental support for the view that such a transport does not occur. On the other hand, there is other experimental support for the view that it does, so that it seems fair to say that the situation remains ambiguous from an experimental viewpoint. 

In general, all these cases of adduct formation are characterized by a low thermal stability consistent with a weaker interaction with the adsorption site than is found where an electron transfer occurs to form a largely ionic bond. 

Of the different oxygen species, the main interest has been in O-, Oj, and O3. Relatively little attention has been paid to the characterization and reactivity of singlet oxygen, O -, lattice or adsorbed O2- species, and most importantly polynuclear species. The work on dioxygen species is likely to be related to the studies of oxygen carriers. Features of special interest in the future on the characterization side are likely to be the detailed geometry of the adsorption site, how the oxygen species is bonded to the surface, and its mobility. 

The ease of movement has been expressed by Inglezakis et al. (2004) by die term exchange site accessibility, while the available sites for exchange/adsorption have been expressed by the term exchange site availability. In the framework of the so-called exchange site accessibility concept, an exchange or an adsorption site is characterized by two factors (Inglezakis et al., 2004) ... 

To investigate the hydration and dehydration processes of H-SAPO-34 and H-SAPO-37, H and Al MAS NMR spectroscopy was applied under CF conditions with the equipment shown in Fig. 12 (217). The chemical behavior and the change of the silicoaluminophosphate framework were monitored as nitrogen loaded with water or dry nitrogen was injected into the MAS NMR rotor filled with the silicoaluminophosphates. By this approach, the primary adsorption sites of water in silicoaluminophosphates and the variation of the aluminum coordination were observed. Furthermore, the formation of framework defects and the conditions of water desorption were characterized. 

Many variations on this theme that allow repetitive, direct injection and chromatographic analysis of untreated sample matrices including tissue homogenates, milk, plasma, and saliva (107-112) have appeared since the original publication. These packings have been described generically as restricted-access media because they are generally characterized by a limited accessibility of macromolecular compounds to the adsorption sites of the porous supports. 

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