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The challenge to substitute less expensive Pd for Pt in TWC catalysts is complicated by the fact that Pd is susceptible to fuel poisons. Laboratory studies indicate that while the precious metal support plays an important role in the CO-NOx reaction, sulfur poisoning dominates. In a reaction to probe selectively the Rh metal function within a washcoat, it was found that small levels of Pd can have a deleterious impact on the performance of the Rh metal. Engine aging studies corroborate the work of recent publications showing that conventional Pd/Rh TWC catalysts exhibit poorer performance than standard Pt/Rh catalysts. The more stringent TLEV and LEV emission standards require more robust catalysts than are currently available. To obtain faster light-off in close coupled positions, the catalyst will experience higher exhaust temperatures. A Pd/Rh catalyst, with an engineered washcoat to minimize alloying, can exceed the performance of a current Pt/Rh commercial catalyst.

Three-way catalysts (TWC) to remove the HC, CO, and NOx pollutants from the exhaust of gasoline powered vehicles employ rhodium in combination with platinum and palladium. Of these precious metals, rhodium is by far the most expensive. Since it is so heavily used for its NOx reduction capabilities, the amount per vehicle approaches and sometimes exceeds the naturally occurring mine ratio. A program was conducted to determine the feasibility of a non-rhodium TWC catalyst. It showed that Pt and Pd in conjunction with other washcoat support materials exhibited relatively good TWC characteristics compared to a Pt/Rh catalyst after engine dynamometer aging. In FTP evaluations this new REDOX type catalyst gave comparable HC and CO efficiency and 85% of the NOx efficiency of a Pt/Rh-containing catalyst. Presently the operating window is being defined but comparisons to conventional Pt/Pd and Pt/Rh catalysts have been made under a number of conditions.