Introduction To reduce NOx emissions from lean exhaust gas (e.g.Diesel exhaust) selective catalytic reduction by NH3 (NH3-SCR) has become an established process for passenger cars as well as for heavy duty vehicles [1].In these systems, the injected amount of aqueous Urea solution (generating NH3 onboard via decomposition) must be controlled in order both to maintain a high level of NOx conversion and to prevent NH3 emissions.For this purpose a so called NH3 slip catalyst (ASC) can be placed as a small monolith slice after the SCR converter to oxidize NH3 having left the SCR brick.The combination of a SCR washcoat on top of a PGM containing washcoat in a dual-layer architecture has been proposed as an effective and N2-selective ASC catalyst [2].The precious metal assures in fact high NH3 oxidation activity, while the SCR layer grants higher selectivity to N2: indeed, NOx being formed in the oxidation layer must counter-diffuse through the SCR layer above where they can be selectively converted to N2, further reducing NH3 slip.This contribution presents the development and validation of a chemically and physically consistent mathematical model of a commercial full-scale ASC dual-layer monolithic converter.It follows a systematic approach of growing complexity and emphasizes the beneficial features of the dual-layer configuration.Materials and methods NH3/O2/NO-NO2-N2O steady-state and transient kinetic runs were performed at high space velocities in a representative temperature range (150-550 ℃) over each one of the two ASC components (Fe-zeolite (SCR) and Pt/Al2O3 (PGM), each one in the form of powdered catalyst), and over their mechanical mixtures.Similar tests were also run over core honeycomb samples (≈ 5cm3) of the SCR, PGM and dual-layer ASC catalysts (SCR layer on top).Gas phase products were analyzed by means of Mass Spectroscopy, FT-IR and UV analyzers.Results and Discussion In a first stage, NH3/O2/NO-NO2 steady-state and transient kinetic runs were performed at high space velocities in a representative temperature range (150-550 ℃) over each one of the two ASC components (SCR and PGM) in the form of powders in order to collect intrinsic kinetic information on the prevailing reaction pathways.Original PGM and SCR kinetic models where developed: the first one accounts for the effects of temperature and of NO2/NOx feed ratio (0-1) on NH3 oxidation, the second one describes the SCR reactivity between NH3 and NH3 oxidation products like NOx.The NO2 inhibition effect on NO oxidation was taken into account, as well as a novel NO2 inhibition effect on NH3 oxidation reactions.In a subsequent stage the two powdered catalysts were also tested jointly, both in a sequential dual-bed configuration and in the form of a mechanical mixture, thus acquiring information on the interactions between the SCR and the PGM catalytic chemistries.Data on SCR+PGM combinations confirmed a positive interaction between the two components, which beneficially affects the N2 selectivity.Additionally, the two global kinetic models developed for the SCR and PGM components were suitably combined to generate predictive simulations of the doublebed and of the mechanical mixture runs: the purpose here was particularly to assess the impact of segregation/mixedness of the two catalysts on the process selectivity.