Surface protonation reaction

Fig. 5.38 Reduction of 10-3m phenylglyoxylic acid at the mercury streaming electrode in acetate and phosphate buffers containing 1 m KN03 (1) pH 5.02, (2) pH 5.45, (3) pH 5.85, (4) pH 6.25. The curves 2, 3 and 4 are shifted by 0.2 V, 0.4 V and 0.6 V with respect to curve 1. The first wave is controlled by the surface protonation reaction while the second is a direct reduction of the acid anion. (According to J. Koryta)...

Compounds 1-8 have been investigated. 4-Pyridylcarbinol, 1, is of interest because it is one of the electrochemical reduction products of 4-pyridinecarboxaldehyde, the species for which surface protonation reactions were discovered and characterized in earlier work (6). 

It may be assumed that hydration of an oxide surface results in surface hydroxide groups bound to metal ions (MOH), and that these groups have amphoteric character [4,20.22,23]. The following equilibria are assumed surface protonation (reaction p)... 

Using these new standard states for the sorbent sites and sorbate species, the equilibrium constant for the surface protonation reaction, Eq. (6.6), is given by... 

Sverjensky and Sahai (1996) proposed that the standard Gibbs free energy of the vth surface protonation reaction (AG ,) can be broken into three terms according to... 

Reconstitution of a secondary Al phase. The pH dependence of the Al detachment (Fig. 13) may not be explained simply by surface protonation reactions. Aluminum(III) may adsorb on the siloxane layer (inset in Fig. 14) and reconstitute a secondary precipitate. Hence, the pH dependence of Al detachment reflects the release of Al from this Al-rich precipitate rather than the dissolution process at the kaolinite surface. In the presence of oxalate, the Al phase is dissolved and the dissolution process occurs stoichiometrically at low pH. The accumulation of Al on mica surfaces has already been postulated (t Serstevens et al., 1978). Figure 15 reveals that the Al center occurring at the surface mainly affects the dissolution characteristic of kaolinite and muscovite. 

Figure 2. The charging of silica as function of pH for various concentrations of indifferent electrolyte ions. Data are from Bolt (1957). The lines are calculated using the one step surface protonation reaction SiO + <=> SiOH ...

An increase of the white noise proportional to the square of the stationary current has been also reported for some nanopores [28], Since the thermal noise is independent of the current and the shot noise increases linearly with the current, this additional white noise has been attributed to surface protonation reactions inside the nanopore that induce surface charge fluctuations [28], It is an important term for long solid-state nanopores (few tens of nanometre) and low-frequency range (less than 10 kHz). 

Platinum catalysts were prepared by ion-exchange of activated charcoal. A powdered support was used for batch experiments (CECA SOS) and a granular form (Norit Rox 0.8) was employed in the continuous reactor. Oxidised sites on the surface of the support were created by treatment with aqueous sodium hypochlorite (3%) and ion-exchange of the associated protons with Pt(NH3)42+ ions was performed as described previously [13,14]. The palladium catalyst mentioned in section 3.1 was prepared by impregnation, as described in [8]. Bimetallic PtBi/C catalysts were prepared by two methods (1) bismuth was deposited onto a platinum catalyst, previously prepared by the exchange method outlined above, using the surface redox reaction ... 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

The surface proton adsorption which occurs after Step 2, however, complicates the determination of the heat content change resulting from anion adsorption. In order to make this correction, the heat associated with proton adsorption must be determined from the previous potentiometric-calorimetric titrations. Proton adsorption on goethite is exothermic, and Figure 1 provides an average value of -29.6 kj/mol near pH 4. This value, when multiplied by the moles of protons required to return to pH 4 after anion adsorption, allows correction for the heat associated with proton adsorption. This correction, however, is based on the assumption that the proposed two-step anion adsorption mechanism described above represents the only surface reactions which occur during anion adsorption. As such, the results obtained by this procedure are model dependent and are best used for comparative purposes. 

To what extent is the macroscopic proton release the direct expression of the metal/surface site reactions Table V compares the macroscopic proton coefficients (Xp ) ) with the coefficient expected if only the Cd(II) surface reactions are considered is the proton coefficient determined by considering the mole fraction of Cd(II) surface species and their formation reactions (Figure 14b). For example, when pSOH is 2.84, y = 0.11 x 1 + 0.89 x 2 = 1.89. At high alumina concentrations pSOH 2.14-2.53) the single surface reaction required to fit the data sets a limiting proton release of 2.0. 

Earlier suggestions that the two uncoordinated and invariant residues His35 (inaccessible to solvent and covered by polypeptide) and His83 (remote and 13 A from Cu) are, from effects of [H ] on rate constants (and related pKg values), sites for electron transfer may require some re-examination. Thus, it has been demonstrated in plastocyanin studies [50] that a surface protonation can influence the reduction potential at the active site, in which case its effect is transmitted to all reaction sites. In other words, an effect of protonation on rate constants need not necessarily imply that the reaction occurs at the site of protonation. His35 is thought to be involved in pH-dependent transitions between active and inactive forms of reduced azurin [53]. The proximity of... 

Intrinsic surface charge density, defined by the number of Coulombs per square meter bound by surface functional groups, either because of isomorphic substitutions, or because of dissociation/protonation reactions. 

The protonation mechanism includes Coulomb electrostatic forces resulting from charged surfaces. The development of surface acidity by the solid phase of the subsurface offers the possibility that solutes having proton-selective organic functional groups can be adsorbed through a protonation reaction. 

These reactions take place via electrophilic attack of a surface proton on the ligand atom bonded to the metal center with liberation of an alkane, an alcohol (or a phenol), HX (X = halide) or NHR2 (11.1) ... 

Surface protonation/deprotonation reactions at the edge of the silanol and aluminol sites (>SOH) of montmorillonite, which can be exemplified by the following reactions ... 

It is the proton gradient that causes the enzyme to release the ATP formed on its surface. The reaction coordinate diagram of the process (Fig. 19-22) illustrates the difference between the mechanism of ATP synthase and that of many other enzymes that catalyze ender-gonic reactions. 

Addition of HC1 to 1,3-butadiene in the gas phase was shown to be a surface-catalyzed reaction occurring at the walls between a multilayer of adsorbed HC1 and gaseous or weakly adsorbed 1,3-butadiene.131 The initial proton transfer to the terminal carbon and the chloride attack occur almost simultaneously. Chloride... 

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