Physical surface area

The steady state rates of hydrocarbon synthesis over the carbided iron surface are given in Table I. The reaction rates have been normalized to the physical surface area of the starting iron powder [18 M /g] and are reported in molecules/cm sec. A turnover... 

Cyclic voltammograms of PtSn microelectrodes in 0.5 M sulfuric acid solution are shown in Fig. 15.6. The potential range was -200 to 800 mV (vs. SCE) and the scan rate was 100 mV/s. It can be seen clearly that hydrogen desorption from the PtSn-2 electrode is seriously inhibited compared with that from the PtSn-1 electrode. From the hydrogen desorption peak areas in the CV curves and the Pt single crystallite hydrogen desorption constant of 210 /xC/cm Pt, the electrochemical surface areas (ESA) for PtSn-1 and PtSn-2 were calculated to be 391 and 49 cm /mg, respectively. However, it is evident from XRD and TEM results that the two catalysts have similar particle size and so they should possess the similar physical surface area. The difference... 

From the above experimental results, it can be seen that the both PtSn catalysts have a similar particle size leading to the same physical surface area. However, the ESAs of these catalysts are significantly different, as indicated by the CV curves. The large difference between ESA values for the two catalysts could only be explained by differences in detailed nanostructure as a consequence of differences in the preparation of the respective catalyst. On the basis of the preparation process and the CV measurement results, a model has been developed for the structures of these PtSn catalysts as shown in Fig. 15.10. The PtSn-1 catalyst is believed to have a Sn core/Pt shell nanostructure while PtSn-2 is believed to have a Pt core/Sn shell structure. Both electrochemical results and fuel cell performance indicate that PtSn-1 catalyst significantly enhances ethanol electrooxidation. Our previous research found that an important difference between PtRu and PtSn catalysts is that the addition of Ru reduces the lattice parameter of Pt, while Sn dilates the lattice parameter. The reduced Pt lattice parameter resulting from Ru addition seems to be unfavorable for ethanol adsorption and degrades the DEFC performance. In this new work on PtSn catalysts with more... 

The chemical composition can be measured by traditional wet and instrumental methods of analysis. Physical surface area is measured using the N2 adsorption method at liquid nitrogen temperature (BET method). Pore size is measured by Hg porosimetry for pores with diameters larger than about 3.0 nm (30 A) or for smaller pores by N2 adsorp-tion/desorption. Active catalytic surface area is measured by selective chemisorption techniques or by x-ray diffraction (XRD) line broadening. The morphology of the carrier is viewed by electron microscopy or its crystal structure by XRD. The active component can also be measured by XRD but there are certain limitations once its particle size is smaller than about 3.5 nm (35 A). For small crystallites transmission electron microscopy (TEM) is most often used. The location of active components or poisons within the catalyst is determined by electron microprobe. Surface contamination is observed directly by x-ray photoelectron spectroscopy (XPS). 

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). 

Natural weathering rates, normalized to specific mineral surface areas, are reported in Table 2 for a number of common silicate minerals (after White and Brantley, 2003). Also included in the Table 2 are the approximate pH ranges, ages of the weathering environment and parameters defining the physical surface area of the minerals. 

Optical microscopic study of rock thin sections using dye tracers is a way to determine mineral-water contact (physical surface) areas if the water flows in rock matrix or fine fractures. The surface areas of particulate materials can be computed from particle size and geometry (cf. Sverdrup and Warfvinge 1993) or measured by BET gas adsorption methods. White and Peterson (1990) point out, however, that measured or computed surface areas of geological materials generally exceed their reactive surface areas. The reactive surface area (as de-hned by 5 ) is what we need to model sorption or reaction rates in porous media. 

The simplest method of estimating mineral reactive surface area is to equate it to the physical surface area, usually reported in m /gm of substrate. Specific surface area, Ai/W (m /cm of substrate), will be used here for directly comparing surface areas of minerals of differenf ensities. Geochemical models commonly define the physical surface area in terms of solution mass (m ), as in Equation 1, rather than solid phase volume (V ). The two ratios are related by the expression... 

An equation relating the kinetic reaction rate to the physical surface area is... 

The lack of a direct proportionality between surface area and reaction rate in the latter cases requires a distinction between the physical surface area and that surface area which participates directly in chemical reactions. This paper uses the term reactive surface area to denote the surface containing chemically reactive sites, i.e. the effective surface area as defined by Helgeson et al. (69). 

The method of estimation of physical surface area for each model is included as footnote 1 in Table III. Some models (i, 2, 22, 102) used generic geometric considerations. The surface area of a smooth parallel fracture, A/V, is assumed proportional to 2/w, the fracture width, and for close packed spheres of a given diameter, A/V approximates 17.10/3. An increase in geometric specificity is apparent in models which characterize particle size distributions and system porosity (100. IM, 105). More accurate estimates of physical surface areas are contained in models where the reactive phase was isolated and BET surface measurements were conducted (2S. 106). Some models (22 101 used BET surface areas which were determined on in situ core material where the original particle textures and porosity were maintained. 

Physical Surface Area Reactive Surface Area Geochemical Svstem (n)° % (1) Ref. %... 

Figure 5. Comparison of estimated physical surface areas to calculated reactive surface areas for models tabulated in Table III.

To quantitatively model reaction kinetics of geochemical systems, reliable estimates of the physical and reactive surface areas of the system are needed. The physical surface areas have been measured on the basis of either the macroscopic nature of the surface, i.e. estimates of its bulk geometry, or the microscopic nature, i.e. the areal extent of coverage by atoms or molecules, as in the BET method. In the latter case, comparisons with water sorption isotherms indicate that BET-determincd surface areas produce reliable estimates of the mineral/water interface, except for materials with high microporosity such as expandable clays. 

In a number of kinetic studies, the reaction rate is not directly proportional to the physical surface area. This discrepancy leads to the concept of the reactive surface area which has been closely linked to the reaction site density and the dislocation density. Theoretical analysis indicates that reaction rates will become directly proportional to defect densities only in highly stressed and deformed minerals. However, experiments using high dislocation-density minerals still produce a low correspondence between reaction rates and total defect density. Energies associated with surface dislocations must be highly heterogeneous and the number of dislocations that actually represent potential reaction sites must be extremely low. 

A compilation of available kinetic models shows that, in most cases, the calculated reactive surface areas are one to three orders of magnitude less than the estimated physical surface areas. Commonly, geometric and BET surface areas are used interchangeably in kinetic studies to measure physical surface areas. The models that did produce closer fits were for open systems with short residence times. Comparisons assumed experimentally correct reaction rates and dependent reactive surface areas. In reality, the reaction rate and the reactive surface area are explicitly linked on the basis of surface controlled reactions. The product of these two terms determines the mass transfer for a specific system. 

Fibrous activated carbons, activated carbon fibers, have been prepared recently and developed a new field of applications. They have a number of advantages over granular activated carbons. The principal merit to prepare activated carbon in fibrous morphology is its particular pore structure and a large physical surface area. Differences between ACFs and granular activated carbons are listed in Table 9. 

Although it is difficult to formulate generic design rules for MOPs, a number of criteria are clearly important in terms of producing polymers with high physical surface areas and interconnected micropore structures. 

The high physical surface area in CMPs can be utilized and filled with other conjugated polymers. Such a property of CMPs leads to the synthesis of an interpenetrating network of more than one polymer, which otherwise would be incompatible for blending and would separate out from the phase immediately. 

Measurement of Physical Surface Area and Electrochemical Active Surface Area... 

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