CH4 conversion and NH3 formation Model Assessment over a Pd-Rh Three-Way Catalyst for automotive application

The growing diffusion of Heavy-Duty engines powered by Natural Gas (NG) has made it necessary to investigate specific Three-Way Catalysts (TWC), with the consequent development of reliable numerical models. This reactor allows a high conversion efficiency exclusively in a limited range of Air-to-Fuel Ratio (AFR) around the stoichiometric. In highly dynamic conditions, its behavior can differ from Steady-State ones due to numerous phenomena of Oxygen Storage and oxide formation on palladium surface, not fully detected from Engine Test Bench (ETB) data. Goal of the present research activity was to develop a 1D chemical reaction model of a Pd/Rh TWC for NG application with the use of a Synthetic Gas Bench (SGB) through an extensive and innovative experimental characterization. Specific Steady-State experiments have shown a dynamics in the methane conversion, not only to ? variation but also at fixed ? and temperature conditions, linked to the reversible Pd ? PdO transformation. Through dedicated temperature-programmed oxidation and reduction (TPO / TPR) tests, hysteresis phenomena in the methane oxidation rate were highlighted. Furthermore, with fast Rich-Lean ? transitions tests, it has been also demonstrated that the presence of NO reduces the methane oxidation reaction rate. A dedicated experimental protocol has shown the high impact of Steam Reforming reaction on methane conversion, partly responsible of a particular decrease in CO conversion efficiency at high temperatures in rich and stoichiometric conditions. In addition, given the high reliability of the experimental data and the possibility of managing the chemical composition of the gas entering the catalyst, important aspects related to the NH3 formation and decomposition were analyzed. The proposed reactions kinetic scheme describes in an acceptable way the behavior of a TWC for NG application.