Tests of Mechanism for Ammonia Synthesis by a Molybdenum Catalyst at Very Low Pressure
Authors: Moore George E. , Unterwald F. C. 
Published: February 2004

Ammonia has long been synthesized commercially on supported iron catalysts, of complex nature not well understood. Authors usually imply that catalysis occurs on the surface and involves four distinct steps: (1) gaseous N2 and H2 adsorb as atoms; (2) one or both species scurry over the surface as a two-dimensional gas; (3) atomic collisions in this phase form the radicals NH and NH2; (4) these radicals collide with adsorbed hydrogen molecules or atoms to form NH3 which then evaporates. Published experimental evidence for this plausible mechanism is not strong. Present experiments used a metallic molybdenum catalyst, S, under idealized conditions and suggest modifications. S synthesized ammonia only after long induction during which all nitrogen reactant was 28N2. After synthesis began, it was very persistent, but an additional long induction period was required before S could utilize 30N2 to synthesize ammonia. The interior walls also synthesized ammonia but required even longer induction periods. These and other results show that both reactants must penetrate into S before synthesis occurs. Because Papers I and II showed that S split interacting H2 and N2 molecules, successive formation of NH, NH2, and NH3 seems very probable, but one or more of the key precesses apparently occur inside S, rather than on its surface. Formation of NH could well be one key process. Few NH radicals desorbed, possibly because of strong sorption or short life. NH2 desorbed as such but always less abundantly than NH3. ND4 and ND3H occurred but molecules with four hydrogens were always sparse or absent. H2, as such, readily desorbed at low T's, whether N2 was present or not and it also dissolved in S. Ammonia desorbed about as readily. However nitrogen was very tightly bound and dissolved readily, but it desorbed at low T's only as ammonia or amide. Production of ammonia in our experiment requires a considerably more important role for this catalyst than any apparently reported previously. Catalysts presumably merely activate reactant molecules so that they can experimentally achieve an equilibrium already favored thermodynamically at the pressure–temperature conditions of the gaseous reactants; Reactions (I) and (II) are certainly consistent with this conventional role. However, we produced ammonia at pressures much higher than permitted by free-energy calculations for the purely homogeneous gaseous reaction. Production of ammonia at the high observed concentrations therefore implies that the reactants, during their interaction with this catalyst, are in a form equivalent to much higher pressure than in their gaseous phase. The observed stability of the resulting gaseous ammonia after it leaves the catalyst need not surprise, because historic experiments long ago established its stability under very unfavorable thermodynamics. Once the reactants properly impregnate S (or the walls) they tend very strongly to desorb as ammonia, rather than as the separate reactants, although the reactants would be strongly favored thermodynamically. Careful chemical identification of catalytic products was especially necessary when thermodynamic conditions were so unfavorable for synthesis but the designations stated were thoroughly established. D2 and 30N2 as reactants helped in chemical identification and in suggesting mechanisms. Mass spectrometers require pressures less than 10−9 those of commercial synthesis. This, and the complexity of commercial catalysts may limit the relevance of our work for practical synthesis.