Iron oxides specific surface area

DRI can be produced in pellet, lump, or briquette form. When produced in pellets or lumps, DRI retains the shape and form of the iron oxide material fed to the DR process. The removal of oxygen from the iron oxide during direct reduction leaves voids, giving the DRI a spongy appearance when viewed through a microscope. Thus, DRI in these forms tends to have lower apparent density, greater porosity, and more specific surface area than iron ore. In the hot briquetted form it is known as hot briquetted iron (HBI). Typical physical properties of DRI forms are shown in Table 1. 

Nitrogen adsorption experiments showed a typical t)q5e I isotherm for activated carbon catalysts. For iron impregnated catalysts the specific surface area decreased fix>m 1088 m /g (0.5 wt% Fe ) to 1020 m /g (5.0 wt% Fe). No agglomerization of metal tin or tin oxide was observed from the SEM image of 5Fe-0.5Sn/AC catalyst (Fig. 1). In Fig. 2 iron oxides on the catalyst surface can be seen from the X-Ray diffractions. The peaks of tin or tin oxide cannot be investigated because the quantity of loaded tin is very small and the dispersion of tin particle is high on the support surface. 

The hydrous iron oxide has been characterized to have a specific surface area of 600 m2 g 1 and 0.2 moles of active sites per mol of FeOOH. Then the concentration of the active sites is... 

The large specific surface areas of the Fe solid phases (Fe(II,III)(hydr)oxides, FeS2, FeS, Fe-silicates) and their surface chemical reactivities facilitate specific adsorption of various solutes. This is one of the causes for the interdependence of the iron cycle with that of many other elements, above all with heavy metals, some metalloids, and oxyanions such as phosphate. 

In a similar study, Zhang and Wang (1997) studied the reaction of zero-valent iron powder and palladium-coated iron particles with trichloroethylene and PCBs. In the batch scale experiments, 50 mL of 20 mg/L trichloroethylene solution and 1.0 g of iron or palladium-coated iron were placed into a 50 mL vial. The vial was placed on a rotary shaker (30 rpm) at room temperature. Trichloroethylene was completely degraded by palladium/commercial iron powders (<2 h), by nanoscale iron powder (<1.7 h), and nanoscale palladium/iron bimetallic powders (<30 min). Degradation products included ethane, ethylene, propane, propene, butane, butene, and pentane. The investigators concluded that nanoscale iron powder was more reactive than commercial iron powders due to the high specific surface area and less surface area of the iron oxide layer. In addition, air-dried nanoscale iron powder was not effective in the dechlorination process because of the formation of iron oxide. 

The specific surface area of a solid is the surface area of a unit mass of material, usually expressed as m g . There is an inverse relationship between surface area and particle size. Massive crystals of hematite from an ore deposit (e. g. specularite) may have a surface area 1 m g". As particle size/crystallinity is governed largely by the chemical environment experienced during crystal growth, the surface area of a synthetic iron oxide depends upon the method of synthesis and that of a natural one, upon the environment in which the oxide formed. 

The oxide surface has structural and functional groups (sites) which interact with gaseous and soluble species and also with the surfaces of other oxides and bacterial cells. The number of available sites per unit mass of oxide depends upon the nature of the oxide and its specific surface area. The specific surface area influences the reactivity of the oxide particularly its dissolution and dehydroxylation behaviour, interaction with sorbents, phase transformations and also, thermodynamic stability. In addition, specific surface area and also porosity are crucial factors for determining the activity of iron oxide catalysts. 

The factors which influence the rate of dissolution of iron oxides are the properties of the overall system (e. g. temperature, UV light), the composition of the solution phase (e.g. pH, redox potential, concentration of acids, reductants and complexing agents) and the properties of the oxide (e. g. specific surface area, stoichiometry, crystal chemistry, crystal habit and presence of defects or guest ions). Models which take all of these factors into account are not available. In general, only the specific surface area, the composition of the solution and in some cases the tendency of ions in solution to form surface complexes are considered. 

J. Electrochem. Soc. 114 994-1000 Fontes, M.P.F. Weed, S.B. (1991) Iron oxides in selected Brazilian oxisols I. Mineralogy. Soil Sd. Soc. Am. J. 55 1143-1149 Fontes, M.P.F. Weed, S.B. (1996) Phosphate adsorption by days from Brazilian oxisols relationships with specific surface area and mineralogy. Geoderma 72 37-51 Fontes, M.P.F. Bowen, L.H. Weed, S.B. 

Iron and manganese oxides are characterized by high specific surface areas and high affinity of their surface hydroxyl groups for adsorption of a variety of trace elements. In addition to adsorption processes, oxidation reactions are catalyzed by these surfaces (18-20). The in situ precipitation and dissolution of these oxides are thus significant for the fate of various trace... 

Materials. The titanium dioxide powders were rutile in structure (obtained from the Titanium Division, National Lead Co., Amboy, N. J.), with nominal specific surface areas of 10 and of 100 sq. meters per gram. Chemical analysis by the supplier showed negligible impurities except for 0.8% sodium oxide in the Ti02-100 and traces of iron in both the TiO2-10 and the TiO2-100. The presence of iron was confirmed by the nature of the decay of the neutron irradiation—induced radioactivities. 

Some metal oxides (notably alumina, magnesia and silica) can be readily prepared in a stable state of high specific surface area. Because of their technical importance as adsorbents, they have been featured in many fundamental and applied investigations of adsorption. Other oxides (e.g. those of chromium, iron, nickel, titanium and zinc) tend to give surfaces of lower area, but exhibit specific adsorbent and catalytic activity. These oxides have also attracted considerable interest. 

The previous developed catalyst, consisting of iron oxide on a-alumina of a low specific surface area (6.5 m /g) and wide pores, exhibited a high selectivity. As reported by Berben et al. [1], chromium oxide was added to the iron oxide in order to decrease the rare of deactivation of the catalyst. This catalyst is now used commercially (31. The addition of chromium oxide has, however, some disadvantages. Firstly, the stability of the catalyst strongly depends on the distribution of (he chromium oxide through the iron oxide, which makes the production of the catalyst difficult. Secondly, the toxic nature of chromium causes problems with respect to the handling of the spent catalyst. 

Well dispersed iron oxide on silica of a specific surface area of 50 m /g has proven to be a very suitable catalyst for the selective oxidarion of hydrogen sulfide to elemental sulfur. Under reaction conditions the iron oxide is transformed into ironCII sulfate as revealed by X ray diffraction, Mbssbauer spectroscopy, and wet chemical analysis. The presence of an iron(III) component as observed by ex situ Mossbauer spectroscopy can not be excluded. Although the transfomiation of iTon(]Il) oxide into iron(II) sulfate causes initial deactivation, the increase in selectivity (96%) results in high sulfur yields (up to 94%). 

Properties 87% kaolinite, 3% quartz, 5.4% mica, 1.4% anatase, 1.2% iron oxide, particle size up to 10pm, specific surface area 20 m7g [790]. 

The metal oxides used as supports were AI2O3 (a reference sample from the Catalysis Society of Japan, JRC-ALO-7, specific surface area 180 m /g), TiOz (Degussa., P-25, 60 m /g), and a-FezOz, prepared by calcinations in air at 627 K of the precipitates obtained from an aqueous solution of iron(III) nitrate by neutralizing with sodium carbonate. As an Ir precursor, reagent grade IrC (Kishida Chemicals) and Ir(CH3COCHCOCH3)3 (Tri Chemical Laboratory) were used. (Ir(CH3COCHCOCH3)3 will hereafter be abbreviated Ir(acac)3.)... 

Fig. 5 (b ) and (c) indicated that the particle sizes of iron species in Al-Fe-oxide(urea) and Cr-Fe-oxide(urea) were kept small even after the third oxidation. The changes of particle sizes during redox cycles could be also suggested from the changes in specific surface areas of the samples. Fig. 6 shows specific surface areas of the iron oxide samples before the first reduction and after the third oxidation. The specific surface areas before the first reduction for Al-Fe-oxide(urea) and Cr-Fe-oxide(urea) were larger than that for Fe-oxide(urea), which was consistent with the results of SEM images shown in Fig. 5. The redox cycles decreased the specific surface areas of all the samples. However, the surface areas after the third cycle for AI-Fe-oxide(urea) and Cr-Fe-oxide(urea) were...

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