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.