AMMONIA SYNTHESIS AND DECOMPOSITION ON MOLYBDENUM AND ITS INTERSTITIAL ALLOYS

The rate of ammonia synthesis from a stoichiometric hydrogen-nitrogen mixture was measured in an integral flow packed-bed catalytic reactor at atmospheric pressure. The unsupported catalyst powders which were prepared consisted of molybdenum and its interstitial alloys, molybdenum carbide, nitride, and oxycarbide. The steady-state catalytic activity was reported as a site time yield, STY, or average number of ammonia molecules produced in the bed per surface metal atom as measured by titration with carbon monoxide after reaction. Values of STY were the same within a factor of three, even though the materials adopted different crystalline forms, body-centered cubic, face-centered cubic, and closed-packed hexagonal. The carbide and oxycarbide approached steady-state quickly whereas molybdenum took a considerably longer time. The amount of nitrogen taken up by the catalysts as soon as they reached steady-state was measured by flowing hydrogen over the beds and measuring how much ammonia was produced. This amount was found to be that required to nitride only a few topmost layers of the molybdenum catalysts.; At the onset the nitrogen uptake at the entrance of a catalytic bed will be larger than at the exit and the total uptake will be overestimated. To avoid this problem a series of experiments was undertaken in which the amount of catalyst used was reached to one tenth of a gram compacted into a wafer. The wafer was loaded onto a microbalance which permitted the measurement of the nitrogen uptake simultaneously with the rate as well as the gravimetric determination of the BET surface areas of the catalysts. It was found that the carbide and oxycarbide reached steady-state when approximately one monolayer of nitrogen was taken up, a monolayer defined as one nitrogen atom per surface molybdenum atom. On pure molybdenum metal the steady-state rate was reached when the same monolayer was taken up in addition to two subsurface layers of dimolybdenum nitride. Thus, the steady-state catalytic rate in ammonia synthesis is reached after only one or at the most two or three layers of nitride are taken up by molybdenum or its interstitial alloys.; The equilibrium absorption of nitrogen and the steady-state rate of ammonia decomposition on a polycrystalline molybdenum ribbon were studied between one thousand and twelve hundred Kelvin in a low pressure continuous stirred tank reactor. Surface coverage by nitrogen during adsorption and reaction was measured by Auger electron spectroscopy. Adsorption isotherms of nitrogen yielded isosteric heats of adsorption which decreased linearly with coverage indicating a broadly non-uniform surface. Nitrogen coverage measured during reaction was found to be always higher than the corresponding equilibrium coverage at the same dinitrogen pressure. There was no effect on the steady-state rate of the partial pressures of dinitrogen and dihydrogen. This confirmed that dinitrogen was not in equilibrium with surface nitrogen and indicated that the ammonia adsorption step was also non-equilibrated. Kinetic treatments based on two irreversible steps involving adsorption of ammonia and desorption of nitrogen gave good fits of the rate data and confirmed the thermodynamic indication of surface nonuniformity. Transient experiments permitted the independent measurement of the rates of the two steps and demonstrated that the steady-state rate occurred when those rates were equal.