Reaction Mechanism and Kinetics for Ammonia Synthesis on the Fe(111) Surface

The Haber-Bosch industrial process
for synthesis of ammonia (NH3) from hydrogen and nitrogen
produces the millions of tons
of ammonia gas annually needed to produce nitrates for fertilizers
required to feed the earth’s growing populations. This process
has been optimized extensively, but it still uses enormous amounts
of energy (2% of the world’s supply), making it essential to
dramatically improve its efficiency. To provide guidelines to accelerate
this improvement, we used quantum mechanics to predict reaction mechanisms
and kinetics for NH3 synthesis on Fe(111)the best
Fe single crystal surface for NH3 synthesis. We predicted
the free energies of all reaction barriers for all steps in the mechanism
and built these results into a kinetic Monte Carlo model for predicting
steady state catalytic rates to compare with single-crystal experiments
at 673 K and 20 atm. We find excellent agreement with a predicted
turnover frequency (TOF) of 17.7 s–1 per 2 ×
2 site (5.3 × 10–9 mol/cm2/sec)
compared to TOF = 10 s–1 per site from experiment.