Resonant active sites in catalytic ammonia synthesis: A structural model

Adsorption sites M-n consisted of n adjacent atoms M, each bound to the adsorbed species, are considered within a realistic model. The sum of bonds Sigma lost by atoms in a site in comparison with the bulk atoms was used for evaluation of the local surface imperfection, while the reaction enthalpy at that site was used as a measure of activity. The comparative study of M sites (n = 1-5) at basal planes of Pt, Rh, Ir, Fe, Re and Ru with respect to heat of N-2 dissociative adsorption Q(N) and heat of N-ad + H-ad -> NHad reaction Q(NH) was performed using semi-empirical calculations. Linear Q(N)(Sigma) increase and Q(NH)(Sigma) decrease allowed to specify the resonant Sigma for each surface in catalytic ammonia synthesis at equilibrium N-ad coverage. Optimal Sigma are realizable for Ru-2, Re-2 and Ir-4 only, whereas other centers meet steric inhibition or unreal crystal structure. Relative activity of the most active sites in proportion 5.0 x 10(-5): 4.5 x 10(-3): 1:2.5: 3.0: 1080: 2270 fora sequence of Pt-4, Rh-4, Fe-4(fcc), Ir-4,Fe2-5(bcc), Ru-2, Re-2, respectively, is in agreement with relevant experimental data. Similar approach can be applied to other adsorption or catalytic processes exhibiting structure sensitivity. (C) 2015 Elsevier B.V. All rights reserved.