Impossible catalyst for impossible reaction

02 November 2012 - Ammonia synthesis

klik hier voor de afbeelding op ware grootteOne percent (!) of the world's energy output is spent on the Haber-Bosch process for the production of ammonia Kitano et al. (DOI) remind us in a recent contribution to Nature Chemistry. Good thing they invented a new experimental catalytic system just a little more efficient than the previous one.

Converting nitrogen and hydrogen to ammonia is hugely energy demanding because of the N:::N triple bond with regular catalysts (Fe or Ru) requiring temperatures up to 600°C and high pressures. Their secret weapon is combining ruthenium metal with the very unusual electride (Ca24Al28O64)4+(e-)4, a zeolite-like compound that has trapped electrons where it should have counterions!

This electride can be produced by annealing monocalcium aluminate (12CaO.7Al2O3), a constituent found in common cement (also see Mayenite). Heating chases away two equivalents of oxygen leaving the electrons behind in the crystal's interconnected cavities. With the electrons freely and easily available, the electride is a great electron donor for the reduction of nitrogen. Ruthenium was added mixing the electride with Ru3(CO)12 and heating at 450°C.

Compared to commercially used Ru-Ba/activated carbon the new catalyst has a much higher ammonia synthesis rate (8-fold) even at a high operating temperature of 400°C with cutting the activation energy in half.

In regular catalyst systems hydrogen absorbs on the metal surface first and the resulting adatoms hinder absorbtion of nitrogen. Catalyst poisoning in the new system is limited because hydrogen can absorb as hydride ions into the crystals. When an hydride ion reacts with an activated nitrogen species on the metal surface, electrons are again left behind.