PROMOTER ACTION OF ALKALI NITRATE IN RANEY RUTHENIUM CATALYST FOR ACTIVATION OF DINITROGEN

Alkali nitrate promoted Raney Ru catalysts were prepared by decomposition of alkali nitrates (CsNO3, RbNO3, KNO3, and NaNO3) with hydrogen over Raney Ru. These catalysts were as active as Raney Ru promoted with metallic potassium at 573 K in N2 activation (ammonia synthesis and especially isotopic equilibration reaction (IER) of N2). The promotional behavior of alkali nitrates on Raney Ru was different from that on the supported Ru catalysts. The alkali was estimated to work as a metallic on Raney Ru, whereas it was estimated to be hydroxide on supported Ru. The more reduced form on Raney Ru-CsNO3 was considered to give a higher turnover frequency of IER of N2 than that over alumina-supported Ru-CsNO3. Since the rate of IER of N2 is a rate of tracer atom moving from N2 to adsorbed N under the condition of adsorption equilibrium, it should be slower than the rate of ammonia synthesis whose adsorption step is rate-determining in a dynamic condition. To the contrary, the rates of ammonia synthesis were slower than IER rates of N2 over Raney Ru-CsNO3, suggesting hydrogen inhibition in the N2 activation process. Indeed, the IER of N2 over Raney Ru-CsNO3 was proved to be retarded by the presence of hydrogen. A kinetic analysis disclosed that N(a) and H(a) compete with each other on the Ru surface where H(a) adsorption is stronger than N(a) adsorption at 473-523 K. The heats of adsorption of N2 and H-2 were estimated from the kinetics.