A simplified approach to modeling of dual-layer ammonia slip catalysts

Dual-layer (SCR+PGM) ammonia slip catalysts have been proposed as promising solutions to attain high DeNO(x) efficiencies at minimal NH3 breakthrough in mobile urea-SCR catalytic converters. Mathematical models of such catalytic systems involve in principle the description of coupled reaction/diffusion processes in both SCR and PGM catalytic layers, resulting in computationally demanding modeling tools. In the present contribution we show that assuming only the surface of the PGM washcoat to be effectively active is a reasonable approximation in the range of operative conditions typical of exhausts after treatment systems. On the contrary, simulation results point out that the presence of the bottom PGM catalyst layer causes the onset of significant NH3 and NOx concentration gradients within the SCR layer. Thus, a rigorous description of coupled reaction/diffusion phenomena, which is often avoided in modeling of washcoated single-layer SCR monolith converters, becomes strictly necessary for dual-layer ammonia slip catalysts. Based on these results, a simplified Layer+Surface Model (LSM) is developed, which accounts for coupled reaction/diffusion in modeling of the SCR catalytic layer only. (c) 2012 Elsevier Ltd. All rights reserved.