Potential energy diagram for ammonia

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

FIGURE 6.7. Mechanism and potential energy diagram for ammonia synthesis on iron (energies in kJ/mol) [8]. 

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

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

Figure C2.7.1. Schematic potential energy diagram for tire catalytic syntliesis and decomposition of ammonia on iron. The energies are in kJ mol tire subscript ads refers to species adsorbed on iron [i].

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 subsequent activation energies of the surface hydrogenation steps [reactions (5.5), (5.6), and (5.7)] are predetermined by the shape of the reactant and product parts of the potential energy diagram. At the product end of the potential, the overall reaction is exothermic by 46 kJ moP and the desorption activation energy of ammonia from Fe(lll) is 50 kJ moP which is the activation energy for reaction (5.8) (Es.g) of Table 5.1. 

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. 

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... 

The striking feature of the potential energy diagram so produced (Fig. 5.12) is the shallowness of the well (136 kJ moP ) for one nitrogen atom (42 kJ moP ) and three hydrogen atoms (94 kJ moP ) adsorbed on the surface. From this point now only 90 kJ moP is required to produce ammonia in the gas phase. 

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. ... 

Let us consider Fig. 3 in which a double-minimum function represents the Born-Oppenheimer potential for the electronic ground-state of ammonia. This is an energy versus internal inversion coordination diagram. Every value of the inversion coordinate corresponds to a particular nuclear framework for ammonia. The two minima, for example, correspond to pyramidal structures, whereas the maximum corresponds to a planar... 

Lewis acids are electron acceptors and Lewis bases electron donors. This means that the former, as we can see using BH3 as an example, have at least one low-lying unoccupied orbital. In the case of BH3, this is the LUMO, a pure boron p-orbital. The AMI-calculated energy for this MO is +1.6 eV, a low value for the LUMO of a neutral compound. This means that BH3 can accept an extra electron to form the BH3 radical anion. When the extra electron is added, the BH3 moiety becomes pyramidal, as shown in Sect. 2.5 (Walsh diagrams). We can now use ammonia as an example of a Lewis base. The lone pair HOMO has a calculated energy of -10.4 eV (i.e. a Koopmans theorem ionization potential of 10.4 eV). We can remove an electron from this MO to form the ammonia radical cation, which has a planar trigonal structure. 

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