An interpretation of the high-pressure kinetics of ammonia synthesis based on a microscopic model

The kinetics of ammonia synthesis at high pressures have been investigated using a recent microscopic model (P. Stoltze and J. K. Nørskov, Phys. Rev. Lett.55, 2502 (1985)). The model is based on the picture of catalysis emerging from quantum mechanical calculations and the results of available ultrahigh-vacuum single-crystal studies of Fe. No reference to measurements of catalytic reaction rates is used in the determination of the input parameters. The model predicts reaction rates in agreement with experiments using the industrial catalyst over large intervals of reaction conditions. This strongly suggests that the model reproduces the essential features of the kinetics of ammonia synthesis. From the model it is found that the largest contribution to the activation enthalpy for the catalytic synthesis of NH3 at high pressures is the energetic cost of creating two free sites on the surface of the working catalyst. The calculated activation enthalpy under typical high-pressure reaction conditions is almost but not completely constant. The predicted values are in good agreement with experiment. It is shown that the reaction orders for N2, H2, and NH3 are directly related to the surface coverages by reaction intermediates. The calculated reaction orders at high pressures are in good agreement with those observed in many previous investigations. The coverages by adsorbed NH, NH2, NH3, and H species are found to be larger than the “coverage by free sites,” i.e., the fraction of unoccupied sites. We suggest that this is the reason why the kinetics of ammonia synthesis cannot be well described by a Langmuir-Hinshelwood expression with only one surface intermediate. It is further shown that the observed high-pressure kinetics may be explained without the assumption of surface heterogeneity used in the usual derivation of the Temkin-Pyzhev or Ozaki-Taylor-Boudart kinetics.