Hydroxyl groups on oxide surfaces

The first stages of adsorption, and more specifically the conditions under which water dissociates, turn out to be very important to understanding adhesion processes and more generally the surface reactivity, because the presence of protons and hydroxyl groups induces Brensted acidity. The influence of such factors as temperature, surface coverage and defect concentration - oxygen vacancies, facets, etc., - on water dissociation has been considered carefully. 

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Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

Figure 1. Potential energies surface sections along the reaction pathway for substitution of hydroxyl group on oxide surface by organic residue. Figure taken from [49].

With respect to hydroxyl groups, the situation on oxide surfaces is more comphcated than that on the surface of zeolites. Some examples of the spectral signature of hydroxyl groups on oxide surfaces were already provided in Table 2.2. The M-O bond in metal oxides may possess an ionic, a covalent, or a mixed character, depending on the oxide. Consequendy, the oxides demonstrate acidic, basic, or amphoteric properties. 

Another contribution to die reaction involving steam is thought to be the role of the oxide support in the provision of hydrogen to the surface of adjacent catalyst particles. It is suggested drat die water molecule is adsorbed on the surface of oxides such as alumina, to form hydroxyl groups on the surface, thus... 

Hydroxyl groups on the surface of oxidic supports are extremely important in... 

Hydroxyl groups on oxide and carbon surfaces are often modeled as a one-site, two-pK model as shown in Figure 6.1. Defend this choice of model with the pH shift data for alumina (Figure 6.15). Might a different type of site be envoked for silica (Figure 6.20) and unoxidized carbon (Figure 6.26a) See [21] for more other types of acid-base group models. 

CNTs can be processed such as purification based on oxidation, cutting, and activation by forming carboxylic acid and hydroxyl groups on the surface of CNTs, which can further be linked with other biomolecules to realize special function (Ajayan et al., 1994). As shown in Fig. 9.19, ferritin molecules attached to the surface of CNTs via covalent bond, the nanocomposites with ferritin molecules-functionalized CNTs own better mechanical, thermal, and electronic properties... 

Hydrated iron oxides can adsorb heavy metals. These adsorption properties arise from the presence of structural hydroxyl groups on their surface, which exhibit amphoterism (56) ... 

Inorganic materials, such as y-alumina, molecular sieves (zeolites), and glass, although being essentially metal oxides, have hydroxyl groups on the surface that can be used as the point of attachment. Capka (20) has pioneered the use of these materials by attaching groups, such as... 

Hydroxyl groups on the surface of oxide in aqueous solutions of the electrolyte have an amphoteric character and undergo addition and dissociation reactions of proton according to the following ... 

Knowing the charge accumulated in IHP, one may calculate the concentration of [= SOH An-] = oj ,An/Bff and [= SO Ct+] = ap,ct/Bn respectively. The density of hydroxyl groups on the surface of the oxide, according to Sprycha, is proposed to be calculated, for high concentrations, from the difference between the total amount of hydroxyl group on the surface of the oxide Ns and cation and anion adsorption density in IHP... 

By the transformation of Eq. (61), with equations for the constants [(25) and (26)], and introducing the Nernst potential value as a operation parameter, one can finally present it in the straight line form. Now, this method allows, from the potentiometric titration data of the suspension, us to calculate pH and the density of hydroxyl group on the surface of the oxide. It may... 

With FITEQL numeric procedure Hayes et al. fitted edl parameters to the three models of electric double layer DLM (diffuse layer model), CCM (constant capacity model) and TLM (three layer model) for the following oxides a-FeOOH, AI2O3 and TiC>2 in NaNC>3 solutions [51]. The fitting was performed for surface reaction constants, edl capacity and the densities of the hydroxyl groups on the surface of the oxides. The quality of the fitting was evaluated by the minimization of the function of the sum of the square deviations of the calculated value from the standard error of measured charge. The lower value of the function the better was the fit... 

The determination of the ion reaction constants with hydroxyl groups on the surface of metal oxide, may be done in the similar way, as the determination of the surface hydroxyl group ionization constants by the extrapolation or numerical methods. The example of the first one is a method proposed for the oxide surface by Schindler [16]. According to the previous remarks the surface adsorption of the simple cations may take place on two hydroxyl groups at most. Then it may be described as follows ... 

We consider this to be proof, that the ionic liquids supported on a metal oxide form a covalent bond between the aluminum of the chloroaluminate species and the oxygen of the hydroxyl-groups on the surface of the carrier (Figure 13). This is important for the stability of the immobilised ionic liquids since a leaching of the ionic liquid is prevented. As shown in the case of immobilised ILs based on iron chloride, the lack of strong bond between anion and surface leads to leaching. 

Deuteration of the accessible hydroxyl groups is accomplished with saturated deuterium oxide vapor at room temperature. The extent of deuteration and therefore accessibility can be estimated gravimetrically by infrared spectroscopy. Accessibility rather than crystallinity is measured because deuteration of the hydroxyl groups on crystallite surfaces can also occur. 

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