Effect of Co doping on mechanism and kinetics of ammonia synthesis on Fe(1 1 1) surface

Abstract

With the aim of improving the efficiency of the Haber-Bosch industrial process for the synthesis of ammonia, here we explore doping the traditional Fe-based Haber-Bosch catalyst with an impurity element. Starting from a previous experimentally-validated theoretical investigation of the reaction mechanism for Haber-Bosch synthesis of ammonia on the Fe bcc(1 1 1) surface, we focus on changes in mechanism and kinetics brought about by substitutional doping of 25% top layer iron with cobalt. The choice of Co is justified by the analysis of the wave functions of the critical reaction steps on the Fe(1 1 1) surface which showed that large changes in the net spin (magnetization) of the Fe atoms are thereby involved, and suggested that dopants with modified spins might accelerate rates. Quantum Mechanics values of free energies and reaction barriers are calculated for the Co-doped system for a set of 20 important surface configurations of adsorbates, and used as input to kinetic Monte Carlo (kMC) simulations to obtain final ammonia production. We find that at T = 673 K, P(H_2) = 15 atm, P(N_2) = 5 atm, and P(NH_3) = 1 atm, target conditions to drastically reduce the extreme energy cost of industrial ammonia synthesis process, top-layer Co doping leads to an acceleration by a factor of 2.3 in reaction rates of ammonia synthesis, and therefore an expected corresponding decrease in production costs.