Iron surface poisoning

A. S. Joy Fuel Research Station, London) It is well known that sulfur is a poison for many metallic catalyses. Consequently, I would like to ask Dr. Uhlig to give further details concerning the antidote effect of copper on iron surfaces poisoned with sulfur. 

In order to improve the selectivity toward the formation of 1,3-PDO, we studied the influence of metal salt additives. While the addition of calcium or copper salts exhibited a moderate influence, the presence of iron salts played a significant role on the rate and selectivity of the reaction (Figure 35.1). The metal additives reduced noticeably the activity of the rhodium catalysts suggesting that they acted as a surface poison, but they modified the selectivity of the glycerol hydrogenolysis, probably through selective diol chelation. 

Deactivation by sulfur has been explained by the withdrawing of electrons from the catalyst surface. It has also been shown that sulfur inhibits the dissociation of CO on iron surfaces l]. The deliberate partial poisoning of iron/manganese cataly.sts with sulfur has been used to shift the product selectivity towards short-chain hydrocarbons. At higher sulfur concentrations (0.7 mg S/g catalyst) the activity is significantly decreased and the olefin selectivity increased [82]. Sulfur poisoning of nickel catalysis has recently been shown to inhibit the chemisorption of hydrogen 83.84). 

With regard to the sulfur bound on the catalyst surface, differences exist between the various types of ammonia catalysts, especially between those that contain alkali and alkaline earth metals and those that are free of them. Nonpromoted iron and catalysts activated only with alumina chemisorb S2N2 and thiophene. When treated with concentrations that lie below the equilibrium for the FeS bond, a maximum of 0.5 mg of sulfur per m2 of inner surface or free iron surface is found this corresponds to monomolecular coverage [382], [383], The monolayer is also preserved on reduction with hydrogen at 620 °C, whereas FeS formed by treatment above 300 °C with high H2S concentrations is reducible as far as the monolayer. For total poisoning, 0.16-0.25 mg S/m2 is sufficient. Like oxygen, sulfur promotes recrystallization of the primary iron particle. 

For pure Fe304, at 444°C, when Kp = [H2]/[H20] = 5, Fe and Fes04 coexist. However, it is known from the experiment that at 444°C, [H2]/[H20] a 2,000, there are still measurable oxygen remained on the iron surface. It is confirmed from poisoning experiments on ammonia synthesis catalyst that a very low concentration of water vapor in synthesis gas causes the oxygen to be retained on the iron and reduces the catal3+ic activity. 

Sulfur is chemisorbed very strongly by the iron surface, as discussed in Stoltze s chapter. Studies of sulfur poisoning of the synthesis catalyst have been carried out [5-8], whereas in [3,4], the interaction between single crystal surfaces of iron and sulfur was discussed. 

Rates of precipitation. The rate of precipitation of iron from bismuth in a pure iron steel crucible is very rapid. Iron precipitated from bismuth, saturated at 615 C, as rapidly as the temperature could be lowered to 425°C. The addition of Zr plus Mg to liquid metal did not change the rapid precipitation of most of the iron from the bismuth under these same conditions, but produced a marked delay in the precipitation of the last amount of iron in excess of equilibrium solubility. An apparently stable supersaturation ratio of 2.0 was observed for more than 7 hr at 425°C in a pure iron crucible containing Bi - - 1000 ppm Mg - - 500 ppm Zr, and 1.7 for more than 48 hr at 450°C. In a 5% Cr steel crucible, a supersaturation ratio of iron in Bi -f- Mg-f Zr of 2.9 was observed after 24 hr at 425°C. This phenomenon may be due to the ability of the formed surface deposits to poison the effectiveness of the iron surface as a nucleation promotor or catalyst, the different supersaturations observed being due to the relative abilities of a Zr-Fe intermetallic compound or of ZrN to promote nucleation of iron. This observed. supersaturation suggests that mass transfer should be nearly eliminated in a circulating system in which the solubility ratio due to the temperature gradient docs not exceed the measured "stable supersaturation at the cold-leg temperature. 

The formation of ammonia on the reduced iron surface is extremely structure sensitive and the 111 and 211 planes are by far the most reactive of the five possible crystal surfaces. The close-packed 111 plane, for example, at the base of each plate-like crystal can expose three layers of iron atoms. Potential active sites may, therefore, have seven near neighbour iron atoms that are more active and less easily poisoned than sites with fewer near neighbour atoms. The most active sites are known as C7 sites. " ... 

The goal of Haber s research was to find a catalyst to synthesize ammonia at a reasonable rate without going to very high temperatures. These days two different catalysts are used. One consists of a mixture of iron, potassium oxide. K20, and aluminum oxide. Al203. The other, which uses finely divided ruthenium, Ru. metal on a graphite surface, is less susceptible to poisoning by impurities. Reaction takes place at 450°C and a pressure of 200 to 600 atm. The ammonia... 

While chlorine is a poison for the ammonia synthesis over iron, it serves as a promoter in the epoxidation of ethylene over silver catalysts, where it increases the selectivity to ethylene oxide at the cost of the undesired total combustion to C02. In this case an interesting correlation was observed between the AgCl27Cl ratio from SIMS, which reflects the extent to which silver is chlorinated, and the selectivity towards ethylene oxide [16]. In both examples, the molecular clusters reveal which elements are in contact in the surface region of the catalyst. 

Under FCCU operating conditions, almost 100% of the metal contaminants in the feed (such as nickel, vanadium, iron and copper porphyrins) are decomposed and deposited on the catalyst (2). The most harmful of these contaminants are vanadium and nickel. The deleterious effect of the deposited vanadium on catalyst performance and the manner in which vanadium is deposited on the cracking catalyst differ from those of nickel. The effect of vanadium on the catalyst performance is primarily a decrease in catalyst activity while the major effect of nickel is a selectivity change reflected in increased coke and gas yields (3). Recent laboratory studies (3-6) show that nickel distributes homogeneously over the catalyst surface while vanadium preferentially deposits on and reacts destructively with the zeolite. A mechanism for vanadium poisoning involving volatile vanadic acid as the... 

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