GM R&D Develops and Demonstrates Strontium-Doped Perovskite Catalysts Rivaling Platinum Catalysts for NOx Control in Diesel Exhaust; Lower Cost Could Be a Boost for Diesel

.

Many reports have suggested that NO oxidation to NO₂ is an important step in lean NO_x reduction, because NO₂ enhances the activities of ammonia SCR and LNT. For SCR catalysts, a NO:NO₂ ratio of 1:1 is most effective for NO_x reduction at lower temperatures (<250 °C). For LNT catalysts, NO must be oxidized to NO₂ before adsorption on the storage components. Because NO₂ constitutes less than 10% of NO_x in the diesel engine-out exhaust, an oxidation catalyst is required to increase the NO₂ fraction. Platinum has been found to be especially active for NO oxidation; thus, Pt-based diesel oxidation (DOC) and LNT catalysts have been widely used for diesel exhaust aftertreatment. However, they suffer from issues such as high cost and poor thermal durability. Consequently, there is substantial interest in the development of better-performing, low-cost, and more durable NO oxidation catalysts.

—Kim et al.

The catalysts developed by the GM team are based on perovskite oxides, La_1–xSr_xCoO₃ and La_1–xSr_xMnO₃. Under realistic conditions, La_1-xSr_xCoO₃ catalysts achieved higher NO-to-NO₂ conversions than a commercial platinum-based DOC catalyst. Similarly, a La_0.9Sr_0.1MnO₃-based LNT catalyst achieved NO_x reduction performance comparable to that of a commercial platinum-based counterpart.

These perovskite catalysts are prone to deactivation by sulfur, a contaminant present in fuel. However, the oxidation activity of the catalyst can be improved in the presence of sulfur by adding palladium.

The potential use of perovskites for automotive applications is
hindered by the fact that the perovskites alone are susceptible to deactivation by S. However, the NO_x-treating performance of Pd/perovskite-based DOC and LNT catalysts in simulated diesel exhaust demonstrated the potential of Pd/perovskite catalysts as a viable substitute for Pt in diesel aftertreatment catalysts. This substitution could
drastically reduce the cost of diesel aftertreatment systems for mobile applications. Lean-burn gasoline engines will also benefit from this technology.

—Kim et al.

In an accompanying Perspective in Science, James E. Parks, II from Oak Ridge National Laboratory noted that:

The catalyst developed by Kim et al. greatly reduces the amount of PGM in LNTs while still maintaining their effectiveness for NO_x reduction from lean engines. This alternative technology will allow engineers greater flexibility as they work to develop better catalysts in a market where volatile PGM prices have made commercial introduction of fuel-efficient lean vehicles challenging. It is possible that these catalysts may allow lean-burn technology to be used with minimal added cost compared to conventional engines.

Resources

• Chang Hwan Kim, Gongshin Qi, Kevin Dahlberg, Wei Li (2010) Strontium-Doped Perovskites Rival Platinum Catalysts for Treating NO_x in Simulated Diesel Exhaust. Science Vol. 327. no. 5973, pp. 1624 – 1627
  doi: 10.1126/science.1184087

• James E. Parks, II (2010) Less Costly Catalysts for Controlling Engine Emissions. Science Vol. 327. no. 5973, pp. 1584 – 1585 doi: 10.1126/science.1187154