Magnetite surface area

While the adsorption of nitric oxide is very useful for the measurement of magnetite surface areas, it is not necessarily true that this molecule titrates the active sites for the water-gas shift reaction. (This point will be discussed later in this paper.) For this reason, recent studies have focused on the adsorptive properties of magnetite for other molecules, and in particular, on the... 

The effect of Si substitution on the turnover frequency for WGS is shown in Figure 11. The turnover frequencies plotted in this figure were based on the magnetite surface area as determined by the NO chemisorption technique. The turnover frequencies shown for unsupported Fe O indicate that the factor of 10 decline in activity for the silica-supported catalysts is not a particle size effect, but instead is a consequence of the substitution of Si into the lattice. However, when the adsorption of CO/COo at 663 K was used to titrate the surface sites instead of NO, the resulting turnover frequencies were essentially constant as shown in Figure 12. Accordingly, the CO/CO2 mixture apparently titrates the sites active for WGS. Clearly, the number of active sites is decreased markedly as the particle size decreases in the silica-substituted magnetite catalysts. 

In closing, it is important to note that the CO/CO2 adsorption technique effectively titrates the active sites for WGS on magnetite catalysts which differ in activity by over an order of magnitude. Nitric oxide on the other hand titrates all of the surface cation sites and is unaffected by Si-substitution. Indeed, NO is known to chemisorb strongly on iron oxides and may even be able to reconstruct the surface. Thus, the combined use of NO and CO/CO2 adsorption provides information about the total magnetite surface area and fraction of the magnetite surface which is active for the WGS reaction. 

All deposits contain various ratios of scale and corrosion products, but often one material predominates, such as calcite or magnetite. These materials have different densities and thermal factors that influence the allowable deposit thickness or weight per unit area before cleaning becomes necessary. Practical allowances usually are between the limitations for each of these two materials. These allowances may be perhaps 50 to 100 mg/cm2 of surface area for lower pressure boilers and 25 to 50 mg/cm2 of surface area for higher pressure boilers. (For a more precise allowance, see the information below.)... 

For a boiler of pressure rating below 300 psig, a dirty condition is 10 to 15 mil of dirt thickness, which for calcite equates to between 68.8 and 103.3 mg/cm2 of surface area, and for magnetite is between 131.4 and 197.3 mg/cm2. 

The reduced magnetite with alumina was found to have a N2 BET surface area of 29 m per gram of catalyst. When adsorbing N2 dissociatively it was found that 2.2 mL [standard conditions, i.e. 273 K and 1 bar (= 100000 Pa)] of N2 could be adsorbed per gram of catalyst. Assuming that the atomic nitrogen forms a c(2x2) overlayer on the Fe(lOO) surface determine the iron area per gram of catalyst. The lattice distance of iron is 0.286 nm. 

Catalyst Production. Supported magnetite particles were produced on Grafoll (Union Carbide), a high surface area form of graphite. The nature of Grafoll and the reasons It is convenient to use In MCssbauer spectroscopy experiments eu e described elsewhere (25). Grafoll is also well suited for magnetic susceptibility experiments. 

FeOOCH3 had an area of ca. 60 m g (Morales et ah, 1989). Samples formed from hematite via magnetite by a reduction/oxidation process had length/width ratios of 1-6.3 and corresponding surface areas of 5.S-9.5 m g" (Morales et al., 1994). 

Dos Santos Alfonso and Stumm (1992) suggested that the rate of reductive dissolution by H2S of the common oxides is a function of the formation rate of the two surface complexes =FeS and =FeSH. The rate (10 mol m min ) followed the order lepidocrocite (20) > magnetite (14) > goethite (5.2) > hematite (1.1), and except for magnetite, it was linearly related to free energy, AG, of the reduction reactions of these oxides (see eq. 9.24). A factor of 75 was found for the reductive dissolution by H2S and Fe sulphide formation between ferrihydrite and goethite which could only be explained to a small extent by the difference in specific surface area (Pyzik Sommer, 1981). 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

Of the metal sorbents, amorphous to poorly crystalline iron (oxy)(hydr)oxides are most efficient at sorption because of their large surface areas (Chapters 2,3, and 7). However, as these compounds crystallize into hematite, magnetite, or other minerals, their surface areas decrease. Although the affinity of the iron (oxy)(hydr)oxides to sorb arsenic may not always change very much as a result of crystallization (Dixit and Hering, 2003), the reduction of surface area may lead to the release of surface-complexed arsenic (O Shea, 2006). Smedley and Kinniburgh (2002) provide a detailed list of sorption studies dealing with metal (oxy)(hydr)oxides (Table 6.1). 

The active Fe is formed from the magnetite through a reduction produced by the reactant mixture both A1203 and CaO are structural promoters which preserve the high surface area of the active iron catalyst [5], The K influences the activity per unit area of the Fe by enhancing of the velocity of dissociative nitrogen chemisorption by increments of the adsorption energy [129],... 

The carbon content of both reactors were similar and showed a slight decrease in concentration from the top to the middle of the reactor. Both reactors show an equivalent increase in magnetite concentration through the reactor bed (Figure 6). The relative crystallite size, as well as B.E.T, surface area and pore volumes for the samples from both stage reactors are similar. It is thus apparent that the sulphur plays no part in the magnetite formation,... 

In a batch system the reduction of magnetite becomes thermodynamically favorable at temperatures far above 2,200 K. Experimentally, reduction was demonstrated in an inert atmosphere at ca. 300 K above the melting point of 1,811 K. The low surface area of the resulting product, however, makes mechanical activation of the reduced phase mandatory in order to achieve high reaction rates and productivity and therefore appears as a major obstacle for successful implementation [12]. 

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