Iron nitride, decomposition

Metal nitrides, in particular V and Mo nitrides, seem to accelerate the rate of nitrogen chemisorption and slow hydrogenation rate of adsorbed nitrogen. In fact, N2 chemisorption rate is very fast on pure Mo even at 580 K, but it deceases rapidly with the increase of adsorption nitrogen or nitrides. Therefore, this rate is much less on M02N than Mo. Even for Fe, it was reported that ammonia decomposition rate was two orders smaller on iron nitride (Fe4N) than pure... 

Evidence for a shift in the rate limiting step at low H2 partial pressure [508,652], at high temperatures [664], at low temperatures [663, 666] and far from equilibrium [652] has been reported. Transients in the rate during NH3 cracking suggest that the rate limiting step for NH3 decomposition is different on iron surfaces than on iron nitride surfaces [595, 665]. 

Winter [4] determined the rate of NH3 decomposition on an iron foil at atmospheric pressure in the temperature range 500 to 700 °C. The experiments were carried out in the presence of excess H2 in order to avoid the formation of iron nitride. Many earlier studies had used pure NH3, thereby transforming the catalyst into a nitride phase and making it drastically different from the catalyst under synthesis conditions. 

The reverse reaction to ammonia synthesis, the decomposition to nitrogen and hydrogen, is used in die nitriding of iron and canied out industiially at temperatures around 800 K and atmospheric pressure to produce surfacehardening. This dissolution reaction must also play a part in the synthesis of ammonia by the industiial process. The attempt to ninide non by reaction with nin ogen gas is vety slow under atmospheric pressure, presumably due to the stability of the nitrogen molecule. 

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. 

Thus, ammonia does not reduce magnetite at an appreciable rate at temperatures below 450°C., and it appeal s that at 450°C. and above, the reduction may be accomplished by decomposition products of ammonia rather than by ammonia itself. This contention is based on the fact that the reduction of fused catalysts with ammonia at 450°C. and 550°C. appeared to be an autocatalytic process that is, the rate of reduction increased with time in the initial part of the experiment. Reduction with hydrogen does not appear to be autocatalytic. It may be postulated that a-iron and nitride formed in the reduction are better catalysts for the ammonia decomposition than iron oxide. 

---