Activation energy promoted iron catalyst

There exists comparatively little information on the activation energy of the chemisorption of hydrogen on other metallic surfaces. The only additional data reported are those for doubly promoted iron catalysts by Emmett and Harkness (25). They obtained 10.4 +1.0 kcal./mole for Ae by means of equation (3) for the initial 2 cm. uptake of hydrogen in the temperature range from —78.5° to — 96.5°C. 

Figure 5.12. Potential energy diagram for ammonia synthesis on a promoted iron catalyst with activated absorption.

Fig. XVIII-13. Activation energies of adsorption and desorption and heat of chemisorption for nitrogen on a single promoted, intensively reduced iron catalyst Q is calculated from Q = Edes - ads- (From Ref. 130.)...

It should be noted that the results for the formic acid decomposition donor reaction have no bearing for ammonia synthesis. On the contrary, if that synthesis is indeed governed by nitrogen chemisorption forming a nitride anion, it should behave like an acceptor reaction. Consistent with this view, the apparent activation energy is increased from 10 kcal/mole for the simply promoted catalyst (iron on alumina) to 13-15 kcal/mole by addition of K20. Despite the fact that it retards the reaction, potassium is added to stabilize industrial synthesis catalysts. It has been shown that potassium addition stabilizes the disorder equilibrium of alumina and thus retards its self-diffusion. This, in turn, increases the resistance of the iron/alumina catalyst system to sintering and loss of active surface during use. 

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],... 

Three other observations from the work of Brill and Tauster (241) are noteworthy. The loss of activity of both catalysts after exposure to a pulse of poison was very slow (on the order of 10-100 hr), suggesting that the sulfur was preferentially adsorbed at the entrance to the bed and then diffused slowly through the bed until the poison was uniformly distributed. The activation energy for unpoisoned and partially poisoned catalysts (both promoted and unpromoted) was the same (96 kJ/mol), suggesting that the poisoning involves a blocking of iron sites, rather than a modification of the electronic properties or Fermi level of the metal. Moreover, a linear dependence between the rate constant and the square of the concentration of unpoisoned surface was observed (Fig. 36), suggesting that two iron sites were poisoned by each adsorbed sulfur in the promoted catalyst. 

More recent studies have shown that a magnetic method may reveal the distribution of particle sizes in supported nickel catalysts. The method appears to be effective down to near-atomic dimensions, and it permits independent determination of rates and activation energies for the reduction process as contrasted with the sintering, or particle-growth, process. The structural relationship of impurities or promoters, such as copper, in the nickel is readily determined, and extension of the method to cobalt and iron catalysts seems possible. 

The Mittash catalyst is unsupported because iron is cheap and the alumina promoter warrants a high specific surface area. This situation is different with Ru catalysts, which are usually prepared on either oxide or carbon supports. Loading of zeolites with small Ru particles offers an interesting alternative but with lower activity, however (44,45). Pioneering work was performed by Ozaki and co-workers (46), who presented an alkali-promoted Ru catalyst on a carbon support with an activity superior to that of the conventional Fe catalyst operated at identical conditions. Further development of this type of catalyst (47) led to a material that has recently been installed in an industrial plant (Ocelot, Canada). Apart from lower capital costs, this plant also operates with reduced energy consumption, and replacement of the iron catalysts by Ru-based ones remains an interesting option for the future. 

The relative energies of the reaction intermediates are shown to be completely different. On iron, the initial step is not the hydrogenation of N2, but N2 dissociation. Compared with the adsorbed Natom state, the formation of NHads, NH2ads and NH3 are all thermodynamically endothermic. The industrial catalyst is promoted with oxides such as potassium oxides, which are thought to lower the activation energy Fe in the rate-limiting step, which is the dissociative adsorption of N2. 

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