MECHANISM AND ISOTOPE EFFECT IN AMMONIA SYNTHESIS OVER MOLYBDENUM NITRIDE

The mechanism of ammonia synthesis over molybdenum nitride has been studied by means of 15N tracer, deuterium isotope effect, and kinetics. The kinetic data as well as the deuterium isotope effect indicates that the rate-determining step is the chemisorption of nitrogen retarded by adsorbed nitrogen atoms, which is the same situation as concluded with the unpromoted iron. In agreement with this conclusion, it is found that no isotopic mixing takes place in the reactant nitrogen during the synthesis. The subsequent step of the synthesis, i.e., the hydrogenation of the adsorbed nitrogen, is evidently equilibrated. The deuterium isotope effect in the rate of hydrogenation of preadsorbed nitrogen is caused thermodynamically. © 1969.