Ammonia synthesis formic acid decomposition

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. 

Volcano plots, as in this figure, have been measured for very different reactions, e.g., the formic acid decomposition, the ammonia synthesis reaction, hydrodesulfurization, or hydrodenitrogenation reactions. 

Only modest differences in activity were observed when the decomposition (56) or synthesis (21) of ammonia over Ti02-supported catalysts were compared after LTR and HTR. Similarly, small effects of reduction temperature were observed for the self-hydrogenation of C2H4 (100). On the other hand, the decomposition of formic acid appears to be favored over Rh/Ti02 reduced at high temperature (101). This reaction, like CO hydrogenation and NO reduction, takes place under partially oxidizing conditions that can, at least partially, reverse the SMSI state. 

It is found in many reactions that a particular surface is favored. For example the (111) surface is particularly active in fee and hep metals. A strong dependence is foimd for ammonia synthesis on iron catalysts (Table 5-13) [T35]. Ammonia synthesis is one of the most structurally sensitive reactions. The opposite order was foimd for the decomposition of ammonia on copper, i.e., (111)>(100). In the decomposition of formic acid, the (111) surface is three times more active than (110) or (100). 

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