Protonic surface hydroxyl groups

It has been found that adsorption/desorption of anions such as Cl and CIO4 on soil constituents is very rapid. In fact, reequilibrium is too rapid to be observed using p-jump relaxation. Fortunately, the electric-field pulse technique can be used for such systems. This method was employed by Sasaki et al. (1983) to study CI and CIO4 adsorption on goethite. Two relaxations on the order of microseconds were observed in acidified aqueous suspensions of a-FeOOH with either NaCl or NaClO4. The fast relaxation was dependent on the applied electric field intensity and was attributed to a physical diffusion phenomenon. The slower relaxation was independent of the applied electric field intensity and was interpreted in terms of the association/dis-sociation reaction of counter ions with protonated surface hydroxyl groups as ion pairs... 

The possibility exists that a surface hydroxyl group on silica can itself function as a proton donor (12),... 

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.

Another key contribution of the Schwarz group was the recognition of the dramatic influence of oxide surfaces on bulk solution pH. In a landmark 1989 paper, Noh and Schwarz [7] demonstrated the method of mass titration, in which successive additions of oxide cause stepwise shifts in solution pH. This procedure is illustrated in Figure 6.7 [7], As indicated in Figure 6.1, the protonation-deprotonation chemistry of the surface hydroxyl groups is coupled to the liquid-phase pH. In mass titration, as the mass (or more appropriately, the surface area) of oxide in solution increases, the solution pH is brought to the PZC of the oxide, at which point no driving force for proton transfer exists... 

Surface protonation at the kaolinite surfaces. The excess proton density, Th.v. 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 pHpzc of the edge surface is about 7.5. 

As mentioned above, an acidic zeolite can provide both protonic (Bronsted) and aprotonic (Lewis) sites. The Bronsted sites are typically structural or surface hydroxyl groups and the Lewis sites can be charge compensating cations or arise from extra-framework aluminum atoms. A basic (proton acceptor) molecule B will react with surface hydroxyl groups (OH ) via hydrogen bonding... 

Charge on the oxide surface is established by dissociation (ionization) of the surface hydroxyl groups. The situation corresponds to adsorption or desorption of protons depending on the pH of the solution (Fig. 10.4). These reactions can be trea-... 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

Experiments conducted using aqueous chloride salts as pretreatment agents prompted Lewandowski and Ollis to adjust this mechanism to reflect the possible role of aqueous protons (H ) in the pretreatment process [69], Based on kinetic studies of the halogenation of alcohols, they suggested that acidic conditions were required to displace surface hydroxyl groups prior to chloride addition to the catalyst ... 

The Bronsted or proton acidity of the surface hydroxyl groups on silica and alumina is weak (51, 52, 53). Evidence for this comes from IR studies of pyridine adsorption, no surface pyridinium species (py H+)... 

Figure I. Potential reaction mechanisms for 3-APTHS (a)-(c) condensation attachment mechanism (d) attachment of molecule via donation of nitrogen lone pair electrons and (e) attachment of molecule via protonation of amine with surface hydroxyl group.

From the XPS data it is not clear if the protonated amine is due to a bicarbonate salt (reaction with moist ambient C02), as suggested by some authors [26, 27], or perhaps due to protonation achieved via surface hydroxyl-groups (Fig. 1(e)). A bicarbonate, if present, represents a rather favorable preformed ion and should lead to a sizable CO, or HCOf ion signal in the negative SSIMS spectrum [25]. While this peak would interfere with the Si02 ion at mlz = 60, or the Si02H ion at mlz = 61, the lack of any sensitivity to surface temperature for these peaks for any of the samples studied, as well as excellent exact mass fits for both peaks, appears to indicate that a bicarbonate is not present. 

The primary bonding mode of 3-APTHS to the Si and Cr surfaces is via the silanol-end of the molecule, leaving the amine end free. A smaller nitrogen component at about 401 eV is associated with protonated amine species. It is proposed that this feature is due to interaction with surface hydroxyl groups. The results indicate that this feature is not due to interaction of the surface layer with moist ambient CO,. 

The amine bicarbonate salt of APS is commonly acknowledged to be present on surfaces treated with APS [2]. The higher BE N Is peak may also be due to a protonated amine by the acidic surface hydroxyl groups, as shown in Fig. 2. This bonding scheme may not be such an unlikely one if one considered the acid-base nature of the two species the Si02 surface is an acidic surface with an isoelectric... 

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