Adsorption of Nitrogen on Iron Catalysts

REACTION MODELING ON THE BASIS OF THE PUBLISHED ADSORPTION/DESORPTION KINETICS OBTAINED BOTH ON SINGLE CRYSTALS AND ON SINGLY AND DOUBLY PROMOTED CATALYSTS 

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

Reactions (5.3) and (5.4) describe the overall dissociative adsorption of hydrogen. The activation energy for desorption of atomic hydrogen (E 5 4) is 109 kJ moP (32) having zero activation energy to dissociative adsorption (E5 3F5 4). 

From the energetics of reactions (5.1)-(5.4) listed in Table 5.1, the potential energy minimum for one adsorbed nitrogen atom (113 kJ moP and three hydrogen atoms (163 kJ moP is 276 moP This is the energy minimum shown in Fig. 5.11, and also corresponds to the value reported by Ertl.  

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. 

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Type 2 Adsorption of nitrogen on iron catalysts at -195 C (many solids fall into this type). 

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