We present a complete catalytic mechanism describing both the Standard and the Fast selective catalytic reduction (SCR) reactions in their correct stoichiometric form on a vanadia titania (anatase 001 facet) based catalyst model. It consists of two cycles, a NO-activation cycle and a Fast SCR cycle that share the same reduction part but use NO + O-2 and NO2 respectively for the reoxidation. The stoichiometry of the Standard SCR reaction is obtained by coupling the two cycles and the stoichiometry of the Fast SCR reaction is represented by the Fast SCR cycle. We establish structures and energetics for each elementary reaction allowing us to calculate the rate for the two reactions by microkinetic modeling. We find at low temperatures the rate for the Standard SCR reaction is determined by H2O formation and desorption as neither NO nor O-2 reacts exothermically with the reduced site prior to H2O desorption. On the contrary NO2 reacts directly with the reduced site resulting in higher rate for the Fast SCR reaction at low temperatures. The rate for the two reactions is the same at higher temperatures as the rate determining step is in the reduction part which is common to both reactions. (C) 2017 Elsevier Inc. All rights reserved.