Conformity of macroscopic behavior to local properties in the catalytic ammonia synthesis and oscillatory reactions on metal surfaces

Abstract

Unique catalytic potential of metal surfaces has encouraged a great number of basic and applied studies. The manuscript highlights the general regularities in a field on the grounds of strong interrelation between catalytic, kinetic and thermodynamic behaviour of the reaction system. The trials of the catalytic NH3 synthesis and the oscillatory NO+H2 reaction have revealed that the thermodynamics of the local structure determines the properties and multiplicity of the reaction intermediates enabling the peculiar macroscopic kinetics and specific catalytic activity. Structure and activity of catalytic sites are correlated within a realistic model, where total undercoordination of adjacent surface atoms and enthalpy of local reaction is taken as a descriptor for structure and activity, respectively. The model has specified the resonant catalytic centers for NH3 synthesis on metal surfaces in close agreement with experimental data. The basal planes of noble metals are less active than Fe- and Ru-based catalysts, whereas an extraordinary activity of small Pt, Ir and Rh clusters can be expected. A strong advantage of imperfections compared to perfect areas in the surface wave nucleation is evaluated. Isothermal rate oscillations in open heterogeneous catalytic reaction systems are expected under the multiplicity of reaction intermediates fairly different in activity, providing the steady state and reaction rout multiplicity. Switching between active and inactive kinetic brunches gives rise to the explosive coverage changeover that can be visualized as a traveling wave. A single pattern of oscillations in the NO+H2 reaction includes the key role of intermediate NHad species providing the catalytic removal of strongly bound nitrogen. The driving forces, the feedback, and chemical interactions within the traveling waves are clearly understood.