Catalyst layer performance modeling limitation

We first assessed the impact of internal diffusion limitations in the PGM layer. For this purpose a simulation study was performed progressively increasing the PGM washcoat thickness, from 10 pm up to 71 pm. From the analysis of NH3 concentration profiles a dramatic impact of dififiisional limitations was apparent indeed only the surface of this layer is effectively active due to the extremely high reactions rates of the PGM catalyst. For this reason, we developed a Layer -I- Surface Model (LSM) of dual-layer ASC where we treat the PGM layer as a surface, while we retain the rigorous description of coupled reaction/dififusion in the SCR layer, based on the previous ID -I- ID model of SCR monolithic converters [12,25,26]. Indeed, avoiding the description of diffusion phenomena in the PGM layer enables the direct inclusion of the PGM reactivity in the SCR converter model by simply modifying the inner boundary conditions of the species differential mass balances in the SCR layer, i.e., those now at the interface with the PGM phase. Treating the PGM layer as a surface thus enabled a simple extension of the ID -I- ID SCR converter model to simulate dual-layer catalytic systems too. 

---