The ammonia synthesis over rhenium single-crystal catalysts: Kinetics, structure sensitivity, and effect of potassium and oxygen

The ammonia synthesis from nitrogen and hydrogen has been investigated over model single-crystal and polycrystalline foil rhenium catalysts at 20 atm reactant pressure and in the temperature range 720-900 K. The reaction rate is remarkably sensitive to the catalyst surface structure. A reactivity ratio of 1:94:920:2820 was found for the Re(0001), Re(101̄0), Re(112̄0), and Re(112̄1) crystal faces, respectively. The catalytic activity of rhenium for ammonia synthesis is higher the more open and rough the surface is. An apparent activation energy of 19.4 ± 1.1 kcal/mol was observed, regardless, of the catalyst surface structure. Kinetic data, reactant pressure dependence, and deuterium isotope effect indicate that, as in the case of iron, the rate-determining step is the dissociative chemisorption of the nitrogen molecule. Potassium metal adsorbed on the clean rhenium surface does not have any effect on the reaction rate. The coadsorption of potassium with oxygen does not suppress the reactivity of the catalyst and stabilization of potassium on the surface occurs.