Activation of nitrogen by alkali metal-promoted transition metal: VI. Hydrogen effect on isotopic equilibration of nitrogen and rate-determining step of ammonia synthesis on potassium-promoted ruthenium catalysts

Abstract

The effect of hydrogen on isotopic equilibration of nitrogen was studied on ruthenium catalysts with and without potassium at P N 2 = 40–150 mm Hg, P H 2 = 7–180 mm Hg and 300–450 °C. The equilibration rate is increased on pure Ru but is decreased on RuK by the addition of hydrogen. The rate of ammonia synthesis is correspondingly faster than equilibration on pure Ru but is slower on RuK. The equilibration rate on RuK is decreased by condensing the ammonia produced in the presence of hydrogen and by increasing the H 2 N 2 ratio and approaches negligible value at H 2 N 2 = 3 , demonstrating that the rate-determining step of ammonia synthesis is the dissociation of nitrogen molecule. The equilibration rate on RuK in the presence of ammonia also decreases as hydrogen pressure increases, whereas the reaction order with respect to nitrogen pressure is higher than that in the absence of hydrogen, suggesting that the adsorbed hydrogen exhibits a retarding effect on the adsorption of nitrogen. The surface area of Ru-K was found to increase on hydrogen treatment of nitrogen covered surface, suggesting a corrosive adsorption of nitrogen to form a nitride on RuK.