Trimetallic Three-Way Catalysts

European car manufacturers have traditionally used Pt/Rh or Pd/Rh TWCs with PM loadings of 40-60 g/ft3. New regulations, however have stimulated interest in high Pd loadings (100 g/ft3 or more) in order to drastically reduce HC emissions. Pd is known to have good HC oxidation activity, high thermal stability and is relatively inexpensive. However, it suffers from excessive sensitivity to poisons and is usually associated with poor NO_x conversion.

A research program was initiated with the goal of capturing the benefits of high Pd concentrations while minimizing its disadvantages. It was found that trimetal formulations (Pt/Pd/Rh) could achieve high NO_x conversions provided the loadings of the PMs were optimized based on a Box-Behnken design.

Data showing the high thermal stability and low H₂S emissions of these new “TriMax” catalysts will be presented. Their high performance has led to commercial acceptance by several European car manufacturers.