Methane Conversion and Ammonia Formation Model over a Pd-Rh Three-Way Catalyst for CNG Heavy-Duty Engines

Research activities in the development of reliable computational models for aftertreatment systems are constantly increasing in the automotive field. These investigations are essential in order to get a complete understanding of the main catalytic processes which clearly have a great impact on tailpipe emissions. In this work, a 1D chemical reaction model to simulate the catalytic activity of a Pd/Rh Three-Way Catalyst (TWC) for a Natural Gas heavy-duty engine is presented. An extensive database of tests carried out with the use of a Synthetic Gas Bench (SGB) has been collected to investigate the methane abatement pathways, linked to the lambda variation and oxide formation on palladium surface. Specific steady-state tests have shown a dynamics of the methane conversion even at fixed ? and temperature conditions, essentially due to the Pd/PdO ratio. Furthermore, combining the results of such test with dedicated Rich-Lean ? transitions it has been demonstrated that the presence of NO reduces the rate of the methane oxidation reaction. 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 were analyzed. In the proposed kinetic scheme, NH3 decomposition phenomena are also present at high temperature due to the presence of Rh in the catalyst. Reactions involving Cerium for oxygen storage and release characterization are included in the proposed model in addition to the surface reaction mechanism, reasonably determining the TWC conversion efficiency of the main species