Iron surface addition

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

Silvery, shiny, and hard. Unique metal, gives off an odor as it forms volatile 0s04 on the surface (oxidation states 81). Osmium is the densest element (22.6 g cm3 record ). Was replaced in filaments (Osram) by the cheaper tungsten. Used in platinum alloys and as a catalyst. Haber s first catalyst in ammonia synthesis was osmium, which fortunately could be replaced by doped iron. The addition of as little as 1 to 2 % of this expensive metal increases the strength of steel (e.g. fountain-pen tips, early gramophone needles, syringe needles). 

Partial transformation of rust into ferric-tannates was found to occur with the addition of mangrove tannins leading to a low inhibitive behaviour. The mechanism of adsorption of catechin onto the iron surface has been established via theoretical... 

Although the fine structure hypothesis of the promoter action of alumina on iron appears well founded, it is conceivable that, in addition to the conservation of the iron surface, the beneficial effect of alumina involves also a modification of the valence forces at the borderlines between alumina (or the iron-alumina spinel A1203 FeO), and the crystallites of metallic iron (49). 

In order to explain the degradation kinetics of TCE and PCE, for which the adsorption onto the nonreactive sites is significant (Burris et al., 1995), a two-site model is developed. The basic assumption for the single-site model, i.e., pre-adsorption equilibrium followed by reductive dechlorination, is still valid here. In addition, the two-site model assumes that there are both reactive and nonreactive sites on the iron surface, and while the adsorption of TCE and coadsorbate can occur on both types of sites, reductive dechlorination of TCE only takes place on the reactive sites. Coadsorbate is not involved in redox reactions. The reaction scheme for this model is ... 

Return to the Fe dissolution experiment discussed above, altering the solution to contain 5 pM M Fe2+. In addition, allow hydrogen evolution to occur on the iron surface with an exchange current density of 1(T5 A/cm2, whereas the exchange current density for the iron reaction is 1CT6 A/cm2. Assume that both reactions have Tafel slopes of 100 mV/decade. These conditions are illustrated graphically in the Evans diagram, named in honor of its creator, U. R. Evans, shown in Fig. 25. The lines represent the reaction kinetics of the two reactions considered. 

An additional interpretation issue involves the presence of oxidation reactions that are not metal dissolution. Figure 28 shows polarization curves generated for platinum and iron in an alkaline sulfide solution (21). The platinum data show the electrochemistry of the solution species sulfide is oxidized above -0.8 V(SCE). Sulfide is also oxidized on the iron surface, its oxidation dominating the anodic current density on iron above a potential of approximately -0.7 V(SCE). Without the data from the platinum polarization scan, the increase in current on the iron could be mistakenly interpreted as increased iron dissolution. The more complex the solution in which the corrosion occurs, the more likely that it contains one or more electroactive species. Polarization scans on platinum can be invaluable in this regard. 

The rale of the Boudouard reaction is mcreased by struciuial promoters proportional to the increase of the iron surface area (85). Fleetronic promoters not only enhance the catalyst activity but atscarbon deposition (57). This effect can be controlled by the addition of SiO . Thus, in order to minimize carbon depcKirion during Fischer Tropsch synthesis, it is necessary to control the catalyst basicity (851. 

Brunauer and Emmett (70) concluded from an adsorption study of carbon monoxide, carbon dioxide, and nitrogen on doubly promoted iron catalysts that 1 wt. % of potassium oxide contained in the catalyst covers more than 50% of its total surface. Additional observations in agreement with these findings were made by Matsui (71). 

CO2 and H2 chemisorption studies were performed on supporting materials and iron supported on HY KY zeolite catalysts to determine the relative basicity. The results were listed in Table 2. No chemisorption of CO2 was observed on the HY and KY zeolite. However, the chemisorbed amount of CO2 increased with increasing the iron content on the supports. By the way, iron supported on potassium ions in zeolite-Y catalyst showed a much higher chemisorption capacity of CO2. From these results, it is concluded that CO2 appears to chemisorb on the free iron surface and on the iron surface on the potassium present in the zeolite matrix. The addition of potassium into Fe/HY and Fe/KY catalysts slightly increased the chemisorption amount of CO2 due to the electron donating ability of potassium to neighboring surface iron atoms. On the other hand, the chemisorbed amount of H2 did not show considerable difference in all samples. 

A coating of aluminum oxide is tough and does not flake off easily, as iron oxide rust does. When rust flakes fall off a surface, additional metal is exposed to air and becomes corroded. 

Baker, R.T.K. Chludzinski, Jr., J.J. "Filamentous Carbon Growth on Nickel-Iron Surfaces The Effect of Various Oxide Additives" J. Catalysis 1980, 64 464. 

In addition, Grabke (95) has determined the rate of dissolution of nitrogen in iron and the rate of the reverse reaction at temperatures between 700 and 1000°C. A thin iron foil was exposed to N2-H2 mixtures at various compositions and pressures, and the resistance of the iron foil was measured as a function of time. In essence, the resistivity of iron is proportional to the nitrogen content and, therefore, can he used as a measure of the nitrogen concentration. Thoroughly dried N2-H2 mixtures rather than pure N2 were used in order to ensure an iron surface practically free of oxygen, which easily blocks the surface (81, 92, 93, 96, 97). At 700 to 900°C and low H2 partial pressures, the rate of the forward reaction... 

Catalyst Preparation The industrial catalyst is prepared by the reduction of iron oxide, Fe304 (94 wt%). It is in the shape of small porous particles with a surface area in the range of 10-15 m /g. Additives that improve its performance include AUO (2.3 wt%), K2O (0.8 wt%), and often CaO (1.7 wt%), MgO (0.5 wt%), and Si02 (0.4 wt%). Al, Mg, Ca, and Si oxides stabilize the pore structure and the surface structure of the iron catalyst K2O, although decreases the iron surface area somewhat still greatly increases the ammonia yield at 613 K from 0.2 mol% to 0.34 mol%. 

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