Three-way catalyst emissions control technologies for spark-ignition engines - recent trends and future developments

Exhaust gas emissions from passenger cars with traditional gasoline engines have been markedly reduced over the three decades since the introduction of catalysts. The first oxidation catalysts were superseded by two-catalyst three-way systems that were followed by single three-way catalysts (TWCs). The first Pt/Rh-based TWCs formulations were refined by incorporating increasingly thermally stable ceria-based components to keep the surface composition close to stochiometric during exhaust gas composition fluctuations. Substituting Pd for Pt gave further improvement in some applications, as can zoning catalyst type or loading on a single substrate. Optimisation of cell density, geometric area, and thermal capacity of the substrate provided additional benefits. Modern advanced TWCs are very thermally stable so they can be mounted directly on the exhaust manifold. Here, with a rapid engine start-up strategy catalyst operating temperatures are quickly achieved after the engine is started. As a result extremely low emissions can be achieved, and computer models are used to predict the performance of different catalyst loadings and layouts. This is a powerful tool in the design of TWC systems for new engines/vehicles to predict what will be optimal, and today high performance from relatively very low PGM loadings is possible, in spite of increasingly stringent tailpipe emissions limits and distances over which they must be maintained. Now the performance of TWCs is monitored during actual on road driving, and this is done via measurement of the catalyst oxygen storage capacity.