Energy diagram, ammonia synthesis

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... 

Fig. 13. Schematic potential energy diagram for the catalytic synthesis and decomposition of ammonia on iron. The energies are given in kj/mol to convert...

Fig. 4.2 Mechanism and energy diagram for ammonia synthesis on iron. (Energies in l

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. 

Figure 4.15. Calculated potential energy diagram for ammonia synthesis over a close-packed and a...

Fig. 3. Schematic energy diagram for the progress of ammonia synthesis on Fe (energies in kJ/mol). From Ref. (21).

TABLE 5.1. Energetics of the Elementary Reactions Derived from the Potential Energy Diagram (Fig. 5.1) of Ammonia Synthesis"... 

Therefore the energetics listed in Table 5.1 are the exact re-expression, in elementary reaction terms, of the potential energy diagram published by Ertf for ammonia synthesis. 

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

TABLE 5.4. Energetics of the Elementary Reactions of a High-Coverage, Activated Nitrogen Adsorption Model of Ammonia Synthesis Used to Construct the Potential Energy Diagram of Fig. 5.12... 

The potential energy diagram (Fig. 5.11), derived from the detailed kinetics of ammonia synthesis obtained from a considerable body of work by Ertl and co-workers " in combination with experimentally derived A-factors, predicts rates of ammonia synthesis within a factor of 10 of those found experimentally when the kinetics are extrapolated by calculation to pressures (107 bar) and temperatures (723 K) used in the industrial manufacture of ammonia. Furthermore, the overall activation energy predicted by the model (62kJmoP ) is also in good agreement with that (81 kJ moP found by Somorjai and co-workers " at 20 bar. 

The potential energy diagram was therefore modified following suggestions by Ertl, on the basis that, under the conditions of temperature and pressure required for industrial ammonia synthesis, the dissociative adsorption of nitrogen becomes an activated process. The potential energy diagram is reported in Fig. 4.13. While there is no direct experimental evidence for this postulate, it is noteworthy that the same basic assumption was taken by Temkin for the development of his kinetic equation. ... 

Figure 9.5. One-dimensional potential energy diagrams for ammonia synthesis and decomposition on platinum and iron (a) Polycrystalline platinum (b) iron/ ...

FIGURE 3.9 Gibbs free energy diagram for ammonia synthesis on the stepped Ru(OOOl) surface at 1,10, and 100 har. The numbers correspond to the six different reaction steps (see also Figs. 2.6 and 3.9) as defined in the preceding text. Data was obtained from the CatApp and corrected for ZPE and entropy contrihutions. The entropy of adsorbed species was assumed to be zero. Reaction conditions are as follows T = 700 K, = 1 3, conversion to ammonia = 10%. 

If the pressure term is included for varying pressures as described in Equation (3.17), a quantitative picture of the pressure dependence of the different reaction steps is achieved. Figure 3.9 shows the free energy diagram of ammonia synthesis at 700 K and 10% conversion toward ammonia. At ambient pressures (Ibar total pressure). 

We shall now take the ammonia synthesis reaction to exemplify the approach for an industrially relevant reaction. Ammonia synthesis has been discussed in Chapters 2 and 3, and the free energy diagrams have been shown in Figures 3.8 and 3.9 as a function of reaction temperature and pressure, respectively. As has been discussed in Chapter 2, synthesis of ammonia from and involves the following steps ... 

We will now analyze more complex reactions than the simple generic catalytic reaction discussed earlier, starting with the ammonia synthesis reaction. The free energy diagram of ammonia synthesis has been discussed in Chapter 3 (see Figs. 3.9 and 3.10), and a microkinetic model for this reaction has been developed in Chapter 5. The microkinetic model deals with ammonia synthesis on a stepped Ru surface, and we wiU now use the same model and extend it to all possible (stepped) metal surfaces through the scaling relations for adsorbates and for transition states as introduced in Chapter 6. 

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