Resistance, promoted iron effects

Bulatnikov et al. (340) studied the effects of promoters on sulfur resistance of iron by measuring the amount of radioactive H2S adsorbed on iron catalysts promoted with A1203 and/or K20. They reported irreversible deactivation of Fe promoted with A1203, A1203 + K20, or KzO after 0.8, 1.5, and 5 monolayers of sulfur had been adsorbed. In other words, the presence of K20 was responsible for increasing sulfur adsorption capacity, although it was not clear upon which portion of the surface sulfur had adsorbed. It was also reported that A1203 was necessary to prevent volatilization of K20. 

The activity of Fe is decreased by addition of Ni [78, 79], and a Mn-Fe alloy of 30-40 percent Mn gives a high activity [80]. The conventional promoted iron catalyst is further promoted by the addition of Co. The catalysts are prepared by burning a Fe-Co alloy in O2 followed by the addition of promoters [81]. The alloying effect of Fe-Co, and Fe-Ni was studied in detail [82]. The addition of 3-7 wt% RUO2 to a catalyst composed of Co ferrite (25-35%), Mg ferrite (20-25%), K2O (0.5-2%), and Fe oxide (rest%) increases the ammonia activity and heat resistance of the catalyst [83]. 

The ammonia synthesis catalyst problem could be considered solved when the catalytic effectiveness of iron in conversion and its onstream life were successfully and substantially improved by adding reduction-resistant metal oxides [232] (Table 15). The iron catalysts promoted with aluminum and potassium oxides proved to be most serviceable [238]. Later, calcium was added as the third activator. Development work in the United States from 1922 can be found in [239]. 

Previous investigators have drawn attention to the beneficial effect of lime when added in small quantities to asphaltic bitumen. The lime helps retard oxidative hardening (13) and reduces the tendency towards water-stripping (4,11,12). Most asphalts are slightly acidic because of the presence of phenolic or carboxylic substituents and would therefore react with basic oxides to form insoluble salts. For example, Fromm (10) has described the use of iron salts of naphthenic acids as adhesion promoters to improve the water resistance of asphalt concretes. This promising approach is now undergoing commercial trials. The literature also describes methods of chemically modifying asphalt with maleic anhydride or acrylic acid (14), sulfur trioxide (15), sulfur dioxide (16), acetyl sulfate (17-21), and sulfuric acid (20). (For a recent review of the interfacial phenomena in asphaltic compositions see Ref. 4.)... 

Chlorine attack on reverse osmosis membranes is believed to be catalyzed by transition metal ions such as iron, manganese and cobalt. Figure 5.14 shows the effect of ferric ion in promoting chlorine attack, using in this case FT-30 membranes preimpregnated with ferric chloride. The presence or absence of these heavy metal ions may explain the discrepancies in chlorine resistance that have been reported for basically identical membranes such as NS-300 and NTR-7250. 

Phosphate pre-treatments may be either zinc phosphate (from zinc dihydrogen phosphate solutions) or an iron phosphate (fi om alkali phosphate solutions) (see Conversion coating and Pre-treatment of steel). The conversion reactions are promoted by accelerators (depolarizers), for example, bromates or molybdates in alkali phosphate baths or chlorates in zinc phosphate baths (with Ca or Ni grain-refining additions). Iron phosphate pre-treatment coatings are often described as amorphous . In practice, however, they are usually crystalline deposits of iron oxides and iron phosphate. Zinc phosphate pre-treatment coatings are always crystalline. A fine, dense crystal pattern of zinc phosphate on the metal surface is the ideal, as it improves both paint adhesion and corrosion resistance most effectively. 

Although atmospheric corrosion of ferrous metals is related first to the amount of moisture in the air (relative humidity), atmospheric moisture alone has no influence on the corrosion of stainless steel. Of primary importance are the effects of such atmospheric contaminants as particulate chlorides and iron-based dust. Sulfur-based acids will promote corrosion while oxides of nitrogen improve the resistance. Contaminants such as hydrocarbons, ammonia, and oxides of carbon have no effect. 

The roles of alkali, alkali earth and rare earth metal oxides seem different from the structural promoters. These oxides are able to increase the specific activity per unit surface area, while decrease the heat-resisting and anti-toxic ability. Thus, they are called as electronic promoters. Because the diameter of K+ ions is quite large, it is not possibly for K to enter into the lattice of magnetite. After reduction, K2O diffuses to the surface of crystallite. The surface potassium is able to accumulate with various forms during reduction and operations, to accelerate the recrystallization effect, but due to the electron, negative alkali metals decrease the effusion work of iron atoms, and accelerate the adsorption of dinitrogen or desorption of ammonia and finally are able to increase the specific activity per unit surface area. 

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