Active surface area: definition

In this paper, we presented new information, which should help in optimising disordered carbon materials for anodes of lithium-ion batteries. We clearly proved that the irreversible capacity is essentially due to the presence of active sites at the surface of carbon, which cause the electrolyte decomposition. A perfect linear relationship was shown between the irreversible capacity and the active surface area, i.e. the area corresponding to the sites located at the edge planes. It definitely proves that the BET specific surface area, which represents the surface area of the basal planes, is not a relevant parameter to explain the irreversible capacity, even if some papers showed some correlation with this parameter for rather low BET surface area carbons. The electrolyte may be decomposed by surface functional groups or by dangling bonds. Coating by a thin layer of pyrolytic carbon allows these sites to be efficiently blocked, without reducing the value of reversible capacity. 

There are two conventional definitions in describing the fractality of porous material - the pore fractal dimension which represents the pore distribution irregularity56,59,62 and the surface fractal dimension which characterizes the pore surface irregularity.56,58,65 Since the geometry and structure of the pore surfaces are closely related to the electro-active surface area which plays a key role in the increases of capacity and rate capability in practical viewpoint, the microstructures of the pores have been quantitatively characterized by many researchers based upon the fractal theory. 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

The clay fraction, which has long been considered as a very important and chemically active component of most solid surfaces (i.e., soil, sediment, and suspended matter) has both textural and mineral definitions [22]. In its textural definition, clay generally is the mineral fraction of the solids which is smaller than about 0.002 mm in diameter. The small size of clay particles imparts a large surface area for a given mass of material. This large surface area of the clay textural fraction in the solids defines its importance in processes involving interfacial phenomena such as sorption/desorption or surface catalysis [ 17,23]. In its mineral definition, clay is composed of secondary minerals such as layered silicates with various oxides. Layer silicates are perhaps the most important component of the clay mineral fraction. Figure 2 shows structural examples of the common clay solid phase minerals. 

Figure 7.8 shows the current density of H2 evolution on Pt microcrystals [46]. It is intriguing that the activity increases as the particle size decreases, although the current is referred to unit real surface area. The excess increase in activity is definitely to be attributed to especially active surface atoms emerging in very small particles. 

Correlation between the two observed Mn species and catalytic activity properties was attempted. For this purpose, turnover frequencies (TOF) referred to the bulk content of the different Mn species were derived from the results of catalytic activity tests in CH4 combustion. TOF referred to Mn in Al(2) site was found to be almost constant on varying the overall Mn content. This suggested a possible correlation between catalyst activity and this Mn species. However, an alternative correlation was found by normalizing the catalytic activity to the surface area. Such normalized activity correlated well with the overall Mn content. No further evidence was found in favor of either these two alternatives, so that no definitive conclusion could be drawn. 

Since solid acid catalysts are used extensively in chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have widely different acidic properties from the bulk material this is especially true for well-structured solids like zeolites. It is also not possible to establish a true acid-base equilibrium. 

Variants of preparation have been proposed [135, 248] including sintering [391] or co-electrodeposition of the precursors [138, 407], and aluminization of the surface of Ni at high temperature whose nature has a definite effect on the resulting electrocatalytic activity [408]. The main features of Raney Ni have been evaluated, including the pore size distribution and the real surface area [93, 135]. It has been found that the composition of the precursor alloys and their particle size have important influence on the adsorption properties of the resulting Raney metal, hence on its electrocatalytic properties [409]. 

The final section in this chapter will discuss several areas of current research, which have attempted to use non-crystalline materials such as metallosilicates or supported oxidation catalysts on high surface area solids. The definition of non-crystalline employed here refers to the support system and not necessarily the active catalytic centre. 

There is no precise definition of an active carbon , but it is generally understood to be a carbonaceous material of appreciable specific surface area. If it is to be an effective adsorbent, an active carbon must have a surface area of at least 5 m2 g1. Active carbons used as industrial adsorbents have much larger BET-areas, which may extend well above 2000 m2 g1. In accordance with this broad definition, an active carbon may be porous or non-porous. The term activated carbon has a more specific connotation, however, since it is reserved for a highly porous carbon produced from a carbon-rich material by some form of chemical or physical activation. 

If water was added to the 15% Si02 co-gel to fill the pore voids a partially recrystallized boehmite was formed with a surface area of 464 m /g and with a pore volume of 1.8 oc/g. If water was added to the 15% Si02 co-gel to form a slurry and then dried and calcined at 500 a partially recrystallized bodunite was formed with a surface area of 334 m /g. steam treatment at 760 of this second, small pore, bodunite-like silica-alumina resulted in no change in the surface area. The gas oil cracking activity of the steamed sample was definitely hi er than that for the amorphoias co-gel, i.e., a Micro Activity Test (MAT) Activity Number of 38 (see Table 1.). 

The aim of such conditioning procedure is to reduce V(V) orthophosphate impurity phases in the presence of n-butane, crystallize (V0)2P207in the VPO catalysts and increase their surface area [10]. Several alternative definitions of the equilibrated state of VPO catalysts and models of an active and selective phase and surface sites have been advanced [4,10-13]. Some of the reasons for the reported differences in the catalytic performance of VPO catalysts may be attributed to ... 

For heterogeneous reactions involving fluid and solid phases, the areal rate is a good choice. However, the catalysts (solid phase) can have the same surface area but different concentrations of active sites (atomic configuration on the catalyst capable of catalyzing the reaction). Thus, a definition of the rate based on the number of active sites appears to be the best choice. The turnover frequency or rate of turnover is the number of times the catalytic cycle is completed (or tumed-over) per catalytic site (active site) per time for a reaction at a given temperature, pressure, reactant ratio, and extent of reaction. Thus, the turnover frequency is ... 

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