Steady-state isotopic transient-kinetic analysis of iron-catalyzed ammonia synthesis

Steady-state isotopic transient-kinetic analysis of ammonia synthesis catalysis has been conducted over a commercial Fe catalyst (Haldor Topsøe KMIR) permitting, for the first time, direct determinations of coverages in reactive intermediates of the working catalyst surface and assessments of the reaction mechanism and surface heterogeneity at steady-state reaction conditions. The reaction was studied in the temperature range 623–773 K, at a total pressure of 204 or 513 kPa, and for a H2/N2 ratio of 3. Dinitrogen chemisorption in the absence of H2 was also measured at 773 K by the transient kinetic technique. The results suggest that ∗N is the most abundant reactive intermediate. It may be inferred, as expected, that a kinetically significant reaction step exists on the reaction path followed by nitrogen. Hydrogen appears to enhance this step which seems to involve dinitrogen dissociation. The working surface was observed to be heterogeneous with respect to the synthesis reaction. The steady-state isotopic transient curves, at 204 kPa and temperatures of 623 and 673 K, indicate one main pool of nitrogen-containing surface intermediates. At 723 and 773 K, a second less reactive pool of intermediates developed due, perhaps, to the activation of previously dormant or saturated sites or to possible contributions from bulk-phase nitrogen species. Both pools of N-intermediates were kinetically nonuniform. This investigation provides significant new insight into ammonia synthesis catalysis.