Surface titrations

The Pt film, with a surface area corresponding to NG=4.2-10 9 mol Pt, measured via surface titration of oxygen with C2H4,1,4 is exposed to po2 — 4.6 kPa, PC2H4 = 0.36 kPa at 370°C in a continuous flow gradientless (CSTR) reactor of volume 30 cm3. The rate of C02 formation is monitored via an infrared analyzer.1,4... 

For the experiments shown in Fig. 5.30 the ratio Cdi2/Cdi] is on the average 2500, very close to the ratio NG/Ntpb ( 3570)54 where N0 is the gas-exposed electrode surface area and Ntpb is the surface area of the three phase boundaries. These quantities were measured via surface titration and via SEM and the techniques described in section 5.7.2, respectively. Thus once N0 has been measured, AC Impedance spectroscopy allows for an estimation of the three-phase-boundary (tpb) length via ... 

Surface charge of MnCC>3 (rhodochrosite) as a function of pH and pcOo as determined from surface titration curves. The values of pHpzc (point of zero charge as calculated from equilibrium (cf. Eq. 3.12)) are given by arrows. 

Sahai, N. Sverjensky D.A. (1997 a) Evaluation of externally consistent parameters for the triple layer model by the systematic analysis of oxide surface titration data. Geochim. Cosmochim. Acta 61 2801-2826 Sahai, N. Carroll, S.A. Roberts, S. O Day,... 

Glazer and Dogan (4) studied BSA spread as a monomolecular film on water. In general, the surface titration curve resembled that of BSA in solution, with two inflections in the pH ranges 3.5 to 6.5 and 10.0 to... 

Experimental evidence of surface enrichment in Pt3Sn stems from AES (//) and selective chemisorption (14). Both techniques indicate surface enrichment of tin, the element with the lower heat of sublimation. Table I shows that AES yields lower values of surface enrichment than surface titration. This is not surprising because AES scans not only the atoms of the surface layer, but also those of lower-lying layers. However, a quantitative comparison of AES and chemisorption data (14) shows that the results can only be matched if enrichment occurs by inversion of the outer layers, i.e., if depletion of tin atoms occurs in layers next to the outer layer enriched in tin atoms. 

The reaction site at oxide electrodes, the oxide-electrolyte interface, differs from the metal-electrolyte interface in several respects and its structure and properties are of the utmost importance for the understanding of reaction kinetics at oxide electrodes. Most of the information available on the properties of the interfacial region in oxides comes from colloid chemistry, i.e. electrokinetic, or zeta, potentials and surface titration curves. Several models developed by Lyklema, Berube and De Bruyn, and Levine and Smith to explain these experimental results have been reviewed elsewhere [7-9],... 

Using surface titrations to understand lichen-solute interactions... 

Figure 9 shows the pH dependence of electro-osmosis in l mM NaCl for three plasma polymer surfaces having different functional characteristics. The respective surfaces of Fig. 9 are plasma polymerized acrylic acid, hexamethyl-disiloxane (HMDSO), and l, 2-diaminocyclohexane (DACH). It is evident from the figure that these surfaces have very different electrokinetic surface properties. This surface titration clearly distinguishes the acid-base properties of the respective surfaces. 

Double layer studies can also be used to characterize surfaces. Titrations, in combination with electrokinetics, are helpful to Identify charge-determining species and hence establish the relative acldity/basicity of a certain surface. These results may be compared with those from functional gas adsorption. In this way heterogeneous catalysts and pigments have been characterized. Trace... 

There are some studies that did not use in situ measurements. These include surface titration and desorption experiments in which surface cover-... 

The catalytic work on the zeolites has been carried out using the pulse microreactor technique (4) on the following reactions cracking of cumene, isomerization of 1-butene to 2-butene, polymerization of ethylene, equilibration of hydrogen-deuterium gas, and the ortho-para hydrogen conversion. These reactions were studied as a function of replacement of sodium by ammonium ion and subsequent heat treatment of the material (3). Furthermore, in some cases a surface titration of the catalytic sites was used to determine not only the number of sites but also the activity per site. Measurements at different temperatures permitted the determination of the absolute rate at each temperature with subsequent calculation of the activation energy and the entropy factor. For cumene cracking, the number of active sites was found to be equal to the number of sodium ions replaced in the catalyst synthesis by ammonium ions up to about 50% replacement. This proved that the active sites were either Bronsted or Lewis acid sites or both. Physical defects such as strains in the crystals were thus eliminated and the... 

Conclusions. Results of XPS analysis, surface titration, wetting, and CAEDMON analysis of adsorption isotherms are summarized in Table VII. 

Houchm, M.R. and Warren, L.J., Surface titrations and electrokinetic measurements on Austialian cassiterites, Colloids Surf., 16, 117, 1985. 

However, the major recent advance in the modeling of mineral dissolution has been the presentation by Stumm and coworkers (Furrer and Stumm, 1986 Zinder et al., 1986 Stumm arid Furrer, 1987) of a surface coordination approach to explain the proton- and ligand-promoted dissolution of simple oxides (A1203, BeO, a-FeOOH). This approach, which relies on surface titrations and double layer concepts for characterizing the chemical speciation on mineral surfaces, offers several advantages ... 

The treatment of surface titration data within the framework of transition-state theory (TST) thus appears to be one of the most promising tracks for elucidating and unifying the mechanisms of mineral dissolution. The goals of this chapter are to further explore this approach and to show that it can be applied to model the dissolution of complex oxides having several types of surface metal cation sites. 

Figure 3. Surface titration of a basalt glass suspension at 25°C (a) and 50°C (ft).

The prediction by our speciation model that in multiple oxides (as in quartz) Si sites are not susceptible to adsorption by H+ is confirmed by the results of surface titrations of albite, labradorite, and anorthite (Fig. 6). One can see that the net adsorption of H + at the feldspar solution interface increases markedly with the relative number of Al sites to Si sites, which is reflected by the stoichiometry of Al and Si in the different feldspars. These results help clarify the typical features of the dependence on pH of silicate dissolution (Table 2) ... 

Figure 8. Comparison of experimental surface titrations of ground and restored albite (ground albite annealed for 80 h at 1000°C) with theoretical titrations of albite and anorthite N% = 12 adsorption sites nm 2). Note the good superposition of theoretical and experimental curves for restored albite. Possible explanations for the important surface charge exhibited by ground albite are that (I) surface defects produced by prolonged grinding can introduce extra adsorption sites and (2) weakening and breaking of surface Si O bonds due to grinding can increase pKSi, ()11 1, and thus allows proton adsorption on Si sites even in midly acid solutions.

As shown above, surface titration curves of reacted albite exhibit notable differences, particularly in the acid pH range, relative to those obtained with fresh albite (but they are very close to those obtained with silica gels). T hus, one understands that surface titrations performed on fresh silicates can only help elucidate initial dissolution mechanisms. 

Figure 16, Surface titration of shocked labradorite (An60, 1011 dislocations cm-2) (a) unannealed sample (b) sample annealed for 80 h at 1000 C (c) theoretical titration curve for labradorite (JVS= 12 adsorption sites nm 2).

The surface concentration of the particular surface species (which is equal to the concentration of the precursor of the activated complex) (Eq. 3) can usually be determined from the knowledge of the number of surface sites and the extent of surface protonation or surface deprotonation or the surface concentration of ligands. Surface protonation or deprotonation can be measured from alkali-metric or acidimetric surface titrations, and ligands bound to the surface sites can lie determined analytically, from the change in the concentration of ligands in solution. 

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