Nitrogen adsorption and dissociation on Fe(111)

Nitrogen adsorption and dissociation on Fe(111)
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by J. J. Mortensen, L. B. Hansen, B. Hammer, and J. K. Nørskov.

J. Catal. **182**, 479 (1999).

Abstract
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We report extensive density functional calculations of the energetics of
N2 adsorption and dissociation on a Fe(111) surface. From the
calculations we can present a detailed picture of the rate limiting step
in the ammonia synthesis which is consistent with available experimental
observations. Four different molecularly adsorbed states are identified,
including a new state not seen by experiment. The new state is a true
precursor to dissociation. We find that there are two dissociation
channels, one going through all the molecular states sequentially with a
low energy barrier, but a high entropy barrier, and the other a direct
channel into the new precursor, which is highly activated. In this way
we can explain both the measured sticking probability for a thermal gas
of N2 above a Fe(111) surface and the molecular beam scattering
experiments. During ammonia synthesis conditions the low barrier channel
is expected to dominate, but at the highest synthesis temperatures, the
high barrier channels may become the most effective. The origin of the
alkali promotion of the N2 dissociation process is also discussed.