Ammonia synthesis activation energy for

Figure 7.26. The activation energy for ammonia synthesis on Fe(lll) and K/Fe(lll). Within experimental error there is no change, suggesting that potassium does not change the reaction mechanism of ammonia synthesis [59].

Finally, the overall activation energy for ammonia synthesis predicted by this potential energy diagram is 17kJmor which is considerably lower than the experimental values of 45 kJ mol of Nielsen or the 81 kJ mol" of Somorjai et... 

Addition of too much K increases the activation energy for ammonia synthesis [176, 242]. The optimum concentration of K [67, 68] depends on the nature and concentration of the structural promoters [291], and on the preparation method [612]. 

Apparent Activation Energy of Ammonia Synthesis Catalysts for Formic Acid... 

H2S is very poisonous for NH3 synthesis [14, 18, 721, 722] and N2 adsorption [513]. The poisoning is irreversible [14, 590]. The poisonous effect is due to S surface blockage [590, 723]. The activation energy of ammonia synthesis is unchanged during H2S poisoning [723] After exposure to large partial pressures of H2S, FeS may be observed by X-ray powder diffraction [590]. 

Figure 4.1 Activation energy for catalysed and uncatalysed ammonia synthesis...

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. 

Effects of Potassium on Ammonia Synthesis Kinetics Extensive research has been completed in which the effects of potassium on ammonia synthesis over iron single-crystal surfaces of (111), (100), and (110) orientations [59] have been determined. The apparent order of ammonia and hydrogen for ammonia synthesis over iron and K/Fe surfaces has been determined in addition to the effect of potassium on the apparent activation energy Ef) for the reaction. In all the experiments, potassium was coadsorbed with oxygen because only about 0.15 ML of potassium coadsorbed with oxygen is stable under ammonia synthesis conditions (20... 

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