Reactive-surface-area characterization

White A. F. and Peterson M. L. (1990) Role of reactive-surface-area characterization in geochemical kinetic models. In Chemical Modelling of Aqueous Systems II (eds. D. C. Melchior and R. L. Bassett). American Chemical Society, Washington, DC, vol. 416, pp. 461-475. 

Role of Reactive-Surface-Area Characterization in Geochemical Kinetic Models... 

WHITE PETERSON Reactive-Surface-Area Characterization... 

Watson EB, Cherniak DJ (1997) Oxygen diffusion in zircon. Earth Planet Sci Lett 148 527-544 Wendlandt RW (1991) Oxygen diffusion in basalt and andesite melts Experimental results and discussion of chemical versus tracer diffusion. Contrib Mineral Petrol 108 463-471 West AR (1984) Solid State Chemistry and Its Applications. John Wiley and Sons, New York Whipple RTP (1954) Concentration contours in grain boundary diffusion. Phil Mag 45 1225-1236 White AF, Peterson MI (1990) Role of reactive-surface area characterization in geochemical kinetic models. In Melchior DC, Bassett RL (eds) Chemical Modeling of Aqueous Systems. II. Am Chem Soc Symp 416 461-475... 

White AF, Peterson ML (1990) Role of reactive-surface area characterization in geochemical knietic models. In Melchior DC, Bassett RL (eds) Chemical modeling of aqueous systems II. Am Chem Soc Symp Ser 416, Washington DC, pp 461-477... 

Local corrosion sites are typified by (1) local chemistries that are commonly only loosely related to the bulk exposure environment, (2) the separation of anodic and cathodic sites, and (3) the localization of corrosion damage sites (i.e., within pits, crevices, and cracks). Since, within a local corrosion site, the reactive surface area to available solution volume can be very large, extreme environments (in terms of concentration, concentration gradients, pH) are often encountered. For the same geometric reasons, these environments are difficult to characterize. Extremely high corrosion current densities can be sustained within the local site by the presence of much lower cathodic current densities over a much larger available surface area outside the corrosion site. Finally, the existence of ionic and concentration gradients between the local corrosion site and the external environment introduces complex transport scenarios. 

The more fundamental approach to addressing the physical dimensions involved in weathering is to characterize the surface areas of the individual minerals, i.e., the specific mineral surface area S (m g )- The extent to which this specific surface area scales directly with the reactive surface areas in natural environments is a matter of considerable debate, particularly in regard to the accessibility of water. For unsaturated environments, such as those in most soils, the wetted surface area may be considerably less than the physical surface area of contained mineral grains (Drever and Clow, 1995). In addition, surface areas of microscopic features such as external pits and internal pores may be associated with stagnant water that is thermodynamically saturated and not actively involved in weathering reactions (Oelkers, 2001). 

Equation 6 can be shown to correspond in mathematical form to a model predicated on a continuous spectrum of sorption interaction energies. If this interpretation is imposed on equation 6, the variable n can be said to reflect both the level and distribution of sorption energies, and KF the sorption capacity. For most natural solids, n generally ranges in value between 0.5 and 1.0, the upper limit characterizing a linear isotherm. As defined, KF would logically incorporate the specific reactive surface area, SH, of the sorbent, which can be abstracted to yield a capacity term, KFh, expressed per unit surface area (KFh = KF/SH). A logarithmic transform of equation 6 can be used to facilitate evaluation of both KVu and n from observed equilibrium sorption data. 

Subsequently one plots InNo vs tHe and extrapolates to tHe=0. This plot provides the 02 desorption kinetics at the chosen temperature T. The intersect with the N0 axis gives the desired catalyst surface area NG (Fig. 4.8) from which AG can also be computed. More precisely NG is the maximum reactive oxygen uptake of the catalyst-electrode but this value is sufficient for catalyst-electrode characterization. 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

Different modifications of hydrous oxides, even if present in solution with the same surface area concentrations, are characterized by significantly different reactivities (e.g., dissolution rate). This depends above all on the different coordination geometry of the surface groups. For a given pH (on surface protonation) the reactivity of a Fem-center is likely to increase with the number of terminal ligands (Wehrli et al., 1990), i.e., groups such as -Fe-OH are less acid and react faster than... 

Recent applications of relaxation dispersion measurements to concrete or cement-based materials are promising for characterizing reactive nanopor-ous materials, the structure of which may evolve over time (75-78). The MRD profiles have provided, for the first time, a direct means for characterizing the specific surface area, Sp, of a hydrated cement-based material (79), without exposing the sample to extremes of temperature or pressure (80-83). The interest in such a surface area is to provide information on the microsctruc-ture and its impact on macroscopic or structural properties. The method is based on a clear separation of surface and bulk contributions of the overall... 

In dispersed-metal catalysts, the metal is dispersed into small particles, on the order of 5 to 500 A in diameter, which are generally located in the micropores (20-1000 A) of a high surface area support. This provides a large metal surface area per gram for high, easily measurable reaction rates, but hides much of the structural surface chemistry of the catalytic reaction. The surface structure of the small particles is unknown only their mean diameter can be measured and the pore structure could hide reactive intermediates from characterization. Some of the same difficulties also hold for thin films. However, we can accurately characterize and vary the surface structure of our single-crystal catalysts, and in our reactor the surface composition can also be readily measured both are prerequisites for the mechanistic study of the catalysis on the atomic scale. 

Mesoporous silicas are characterized by surface areas 10 to 20 times larger than that of zeolites, typically around 1000 cm2 g. The pores can be adjusted in a range between 2 and 30 nm, and the pore volumes range between 0.5 and 2.5 cm3 g . The mesopore sizes and volumes are ideal for chemical reactions to take place in their interior, therefore the ability to block the pore entrances with stimuli responsive gates enables a control in chemical reactivity suitable for a variety of applications, including selective sensing, catalysis, and delivery, among others. 

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