The catalytic reduction of NO in combination with the oxidation of CO over a catalytic surface has been analyzed using density functional theory (DFT) calculations. In the light of this prototypical reaction the modeling of a complex catalytical reaction on the basis of DFT calculations is demonstrated. In particular activation energies, pre-factors and rate constants for all reaction steps are obtained. Reaction barriers and rates agree well with experimental estimates, providing a basis for kinetic modeling studies.