Surface hydroxyl site densities

Tamura, H. Tanaka, A. Mita, K.-Y. Furuichi, R. (1999) Surface hydroxyl site densities on metal oxides as a measure of the ion exchange capacity. J. Colloid Interface Sd. 209 225-231... 

Spectroscopic data for Sr(II) were consistent with the selection of outer-sphere reactions to describe adsorption. Two outer-sphere reactions were employed to describe the trends in pH, ionic strength and surface coverage. One of the significant components of the modeling approach was that the TLM parameters used for Cd(II) and Sr(II) were constrained to employ the same model constants derived previously for Co(II). In both cases, it was possible to describe the adsorption behavior of these solutes using the same values of the surface hydroxyl site density, surface protolysis equilibrium constants, and electrolyte binding equilibrium constants. 

Point of zero charge PZC (-) Surface ionization constants ApK (-) Dielectric constant of the oxide e (-) Hydroxyl site density Ns (1/nm2)... 

The a-quartz showed all three kinds of snrface hydroxyl gronps as reported experimentally (de Leeuw et al. 1999b). Geminal hydroxyls were on the 0001 surface. On the (lOll), (lOlI), and 0111 surfaces, isolated hydroxyls were observed. The lOlO surface exhibited vicinal hydroxyls, whereas on the 1120 surface both geminal and vicinal hydroxyl gronps were found. The nnmber of hydroxyl ions on the surface indicates surface reaction site density, and our findings match very well with reported values (Koretsky et al. 1998). 

Of all the aluminas, y-alumina is the most used alumina, due to its high surface area and the high concentration of surface hydroxyl groups. 1 1-alumina is u.sed in i.somerization becau.se it has a higher density of acid sites at the surface. 

By applying Equation 16 to each component of a general compound or mixture of i components capable of hydroxylating and allowing for finite site groups —i.e., of component pure oxides. The total charge density a, comprises the contributions of a, and of each type of hydroxylated site present in the surface at an atomic fraction fa. Thus,... 

The numerical optimization methods do not require additional assumptions of the temporal constancy, or even neglect some physical constants, for example surface potential. Used for the optimization of the edl parameters (surface hydroxyl group reaction constants, capacity and density of adsorption sites) the numerical methods allow us to find the closest values to the experimentally available data (surface charge density, adsorption of ions, zeta potential, colorimetric measurements). Usually one aims to find the parameters, accepted from physical point of view, where a function, that expresses square of the deviation between calculated and measured values will be the smallest. 

Three types of solvent or solute delocalization have now been examined, as summarized in Table III for three different adsorbent types (four, if we distinguish Cig-deactivated silica from silica). The theoretical requirements on the configuration and density of adsorption sites were discussed earlier (Section II,B) for a given type of localization/delocalization to be possible. In each case the nature of adsorption sites is fairly well understood for the four adsorbents of Table III, as disucssed in Ref. / and 17 and shown in Fig. 14. Thus, in the case of alumina, surface hydroxyls do not function as adsorption sites. Although surface oxide atoms are capable of interacting with acidic adsorbate molecules (see below), in most cases the adsorbate will interact with a cationic center (either aluminum atom or lattice defect) in the next layer. As a result, we can say that in most cases adsorption sites on alumina are buried within the surface, rather than being exposed for covalent site-adsorbate interaction. These sites are also rigidly positioned within the surface. Finally, the... 

Spectroscopic evidence for mechanism (2), arsenic adsorption on oxoanion precipitates, has been provided by Myneni (1995) and Myneni et al. (1997) in the arsenate-ettringite system. Arsenate was found to adsorb to different sites as a function of sorption density. At low sorption density, As oxoanions exchange for surface hydroxyl groups bonded to Al atoms and at higher sorption densities As oxoanions exchange for surface hydroxyl groups bonded to Ca atoms, with minor bonding to Al atoms (Table 2). 

It is assumed that this is also a rate determining step for the overall reaction. The activation energy of reaction (4) and the site density of oxygen active centers were the only adjustable parameters of the model. In general, a C-H bond scission for reactants and products of the methane dimerization process occurs by an Eley-Rideal (E-R) type mechanism to form a gas-phase alkyl radical and a hydroxyl surface site (HO ) ... 

Figure 14. Surface protonation and ion exchange equilibria at the kaolinite surfaces. The inset represents the protonation and ion-exchange reactions at the permanent negatively charged surface sites of the siloxane layer (0.1 MNaN03, [Al] = 1.6x 10-4 M, [XO], = 1.46x 10 3 M). The excess proton density, rHV, at the surface hydroxyl group is displayed as a function of pH. Surface protonation is interpreted as a successive protonation of two distinct types of OH groups localized at the gibbsite and edge surfaces. The pHZPC of the edge surface is about 7.5.

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