Activation energy ammonia synthesis

Tabie 6.60 shows the reaction activation energy calculated by the methane peak height data of methanation and the activity of ammonia synthesis at different temperatures. 

From Table 6.60, it suggests that the activation energy is inversely proportional to the initial temperature of methanation. The higher the activation energy, the higher the initial temperature of methanation is which is consistent with the general kinetic character. It shows that Sm, Sr, Ce, Mg and Ba can not only inhibit the methanation reaction but also increase the activity of ammonia synthesis reaction. 

Nitrogen adsorption has been especially studied on iron, which is the basis of the industrial catalyst for ammonia synthesis. This catalytic reaction is now relatively well understood [21]. The main function of iron is to activate the very stable N2 molecule by dissociative adsorption. Results in Table 3 show that the Fe-N binding energy is approximately the same on the three main low-index planes. However, the rate of N2 dissociation decreases by many orders of magnitude when going from Fe(lll), the most active for ammonia synthesis, to the less active Fe(llO). 

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

In general, TPR measurements are interpreted on a qualitative basis as in the example discussed above. Attempts to calculate activation energies of reduction by means of Expression (2-7) can only be undertaken if the TPR pattern represents a single, well-defined process. This requires, for example, that all catalyst particles are equivalent. In a supported catalyst, all particles should have the same morphology and all atoms of the supported phase should be affected by the support in the same way, otherwise the TPR pattern would represent a combination of different reduction reactions. Such strict conditions are seldom obeyed in supported catalysts but are more easily met in unsupported particles. As an example we discuss the TPR work by Wimmers et al. [8] on the reduction of unsupported Fe203 particles (diameter approximately 300 nm). Such research is of interest with regard to the synthesis of ammonia and the Fischer-Tropsch process, both of which are carried out over unsupported iron catalysts. 

To describe catalytic reactions on a metal surface, adsorption energies of the reactants, intermediates and products are essential and so are the activation energies separating different intermediate steps. Figure 4.15 illustrates a full potential energy diagram for a catalytic reaction the synthesis of ammonia N2+3H2 — 2NH3. 

The above are equilibrium reactions, and their successful exploitation requires that they be carried out under conditions in which the equilibrium favors the product. Specifically, this requires that the adsorbed species in Reactions (D)-(I) not be held so tightly on the catalyst surfaces as to inhibit the reaction. On the other hand, strong interaction between adsorbate and catalyst is important to break the bonds in the reactant species. Optimization involves finding a compromise between scission and residence time on the surface. Although we are especially interested in metal surfaces, those constituents known as promoters in catalyst mixtures are also important. It is known, for example, that the potassium in the catalyst used for the ammonia synthesis shifts Equilibrium (F) to the right and also increases the rate of Reaction (D) by lowering its activation energy from 12.5 kJ mole to about zero. 

Data on the rate of synthesis or decomposition of ammonia on a number of metals give activation energies of ammonia decomposition, E, close to 40 kcal/mol, as in the case of iron catalysts, and m = 0.5 (107). 

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. 

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

---