Proton adsorption, kinetics

Tab. 10.3 Enthalpies of adsorption and kinetics of proton adsorption and desorption on iron oxides.

Astumian, R.D. Sasaki, M. Yasunga,T. Schelly, Z.A. (1981) Proton adsorption-desorption kinetics on iron oxides in aqueous suspensions, using the pressure jump method. J. Phys. Chem. 85 3832—3835 Atkins, P.W. (1990) Physical chemistry. 4 Ed. 

Onari, S. Arai,T. Kudo, K. (1977) Infrared lattice vibrations and dielectronic dispersion in a- Fe203. Phys. Rev. B16 1717 Onoda, G.Y. de Bruyn, P.L. (1966) Proton adsorption ot the ferric oxide/aqueous solution interface. I. A kinetic study of adsorption. Surface Sd. 4 48—63... 

Ashida, M., Sasaki, M., Kan, H., Yasunaga, T., Hachlya, K, and Inoue, T. (1978). Kinetics of proton adsorption-desorption at Ti02-H20 interface by means of pressure jump technique. J. Colloid Interface Sci. 678, 219-225. 

The kinetics of proton adsorption by alumina was studied in [167,168]. The potential was studied as a function of exposure time (1-14 days) in [169]. 

The kinetics of proton adsorption by silica was studied in [168]. After addition of quartz to a solution [178], the fast stage (the first 4 minutes) was followed by a slower, linear decrease of pH with time. 

The kinetics of proton adsorption/desorption on anatase was studied using a pressure jump technique in [179], and rate constants were calculated. Reference [161] presents the kinetics of proton adsorption for TiO2... 

Direct studies of the effect of pressure on surface charging are rare. The conductance of an anatase dispersion in HCI increased with pressure [179], This suggests a release of pre-adsorbed HCI from the surface at elevated pressure. On the other hand, the pressure effect was negligible in anatase dispersions in water or in NaCl. The experimental setup was designed to study desorption kinetics, and only the sign of the pressure effect could be determined. A similar method was used to study the pressure effect on proton adsorption on alumina dispersions in water [2928,3059], and in NaCl [2928], HNO,. and NaNO, [927] solutions, and the effect was negligible for a pressure of about 10 Pa. On the other hand, the same pressure had a substantial effect on uptake of heavy-metal cations [927] and of anions [2928,3059] on alumina. 

Measurements of the rates of surface reactions on insulator surfaces, such as dissolution, adsorption, and surface diffusion, are possible (Chapter 12). For example, proton adsorption on an oxide surface can be studied using the tip to reduce proton and induce a pH increase near the surface (22). Then, by following the tip current with time, information about proton desorption kinetics is obtained. Studies of corrosion reactions are also possible. Indeed, work has been reported where a tip-generated species has initiated localized corrosion and then SECM feedback imaging has been used to study it (28). In these types of studies, the tip is used both to perturb a surface and then to follow changes with time. 

For the investigation of adsorption/desorption kinetics and surface diffusion rates, SECM is employed to locally perturb adsorption/desorption equilibria and measure the resulting flux of adsorbate from a surface. In this application, the technique is termed scanning electrochemical induced desorption (SECMID) (1), but historically this represents the first use of SECM in an equilibrium perturbation mode of operation. Later developments of this mode are highlighted towards the end of Sec. II.C. The principles of SECMID are illustrated schematically in Figure 2, with specific reference to proton adsorption/desorption at a metal oxide/aqueous interface, although the technique should be applicable to any solid/liquid interface, provided that the adsorbate of interest can be detected amperometrically. 

Proton adsorption/desorption kinetics may be studied by pressure-jump type techniques. Protonation is usually very fast deprotonation may be slower but time scales of a few tens of seconds are not exceeded For practical purposes, the oxide surface charge can be considered as being instantaneously established on contact with the metal-containing solution. 

Adsorption kinetics will be discussed first. Figure 11A shows the equilibration of the alumina system after the addition of HCl [16]. Figure IIB shows that the adsorption of Cu, with liberation of protons [47], takes about 24 h to reach equilibrium. Figure 11C gives kinetic data for the adsorption of Co, Fe, and Ni ions [54], evidently in equilibrium after about 1 h. On the other hand, pressure jump experiments of Cu adsorption used a time scale of 10 -10 s [55]. These results are mentioned to demonstrate that reactions taking place in the adsorption system may have rates that differ by several orders of magnitude. 

When the proton concentration is decreased, the rate of Ity-drogen adsorption is slowed down (Eq. 88). As a result, the simulation of the transients at different pH s allows the characterization of both the hydrogen adsorption kinetics and the thermal coefficient of the donble-layer potential, as explained in Section V.l. 

The catalytic reactions at solid-liquid interfaces of metal oxides have been of great interest to colloid chemists. The adsorption-desorption phenomena of various metal ions on oxides such as Y-AI2O3 have been extensively investigated since adsorption-desorption is a fundamental step in heterogeneous catalytic reactions. Several mechanisms have been proposed by many investigators . Kinetic studies, however, have scarcely been carried out because the reaction is too fast to be measured by ordinary methods. Only a kinetic study on the proton adsorption-desorption at the Ti02-water interface has been reported using the pressure-jump technique by the present authors. ... 

In the ion-association extraction systems, hydrophobic and interfacially adsorbable ions are encountered very often. Complexes of Fe(II), Cu(II), and Zn(II) with 1,10-phenanthro-line (phen) and its hydrophobic derivatives exhibited remarkable interfacial adsorptivity, although the ligands themselves can hardly adsorb at the interface, except for protonated species [19-21]. Solvent extraction photometry of Fe(II) with phen is widely used for the determination of trace amounts of Fe(II). The extraction rate profiles of Fe(II) with phen and its dimethyl (DMP) and diphenyl (DPP) derivatives into chloroform are shown in Fig.9. In the presence of 0.1 M NaC104, the interfacial adsorption of phen complex is most remarkable. The adsorption of the extractable complex must be considered in the analysis of the extraction kinetic mechanism of these systems. The observed initial rate r° shows the relation... 

Spirodela intermedia, L. minor, and P. stratiotes were able to remove Pb(II), Cd(II), Ni(II), Cu(II), and Zn(II), although the two former ions were removed more efficiently. Data fitted the Langmuir model only for Ni and Cd, but the Freundlich isotherm for all metals tested. The adsorption capacity values (K ) showed that Pb was the metal more efficiently removed from water solution (166.49 and 447.95 mg/g for S. intermedia and L. minor, respectively). The adsorption process for the three species studied followed first-order kinetics. The mechanism involved in biosorption resulted in an ion-exchange process between monovalent metals as counterions present in the macrophytes biomass and heavy metal ions and protons taken up from water.112... 

The acidic and adsorptive properties of the samples in gas phase were evaluated in a microcalorimeter of Tian-Calvet type (C80, Setaram) linked to a volumetric line. For the estimation of the acidic properties, NH3 (pKa = 9.24, proton affinity in gas phase = 857.7 kJ.mol-1, kinetic diameter = 0.375 nm) and pyridine (pKa = 5.19, proton affinity in gas phase = 922.2 kJ.mol-1, kinetic diameter = 0.533 nm) were chosen as basic probe molecules. Different VOC s such as propionaldehyde, 2-butanone and acetonitrile were used in gas phase in order to check the adsorption capacities of the samples. 

---