The relative importance of external and internal transport phenomena in three way catalysts

The conversion efficiency of three way catalyst (TWC) depends on chemical reaction and transport limitations. This paper reports a quantitative analysis of the relative importance of these limiting processes based on experimental data obtained under real world vehicle operating conditions. The main conclusions are as follows: (i) above light-off temperature the mass transport phenomena overlaps the kinetic limitation but pure external mass transfer control is hard to attain in TWC operating under real world automotive conditions, even when both space velocity and operating temperature are very high; (ii) above light-off temperature the automotive TWC operates in a mixed regime with both internal and external mass transfer playing important roles; (iii) the internal mass transfer limitation is more important to control conversion in the TWC than the external mass transfer limitation but the relative importance of the external mass transfer increases as the temperature rises; (iv) the internal mass transfer limitation cannot be disregard in TWC modeling studies and (v) the results show that the current generation of TWC is over designed from an external mass transport viewpoint so that future improvements of these devices can be achieved with porous washcoat with improved transport proprieties.