Passive NO_x reduction with CO using Pd/TiO₂/Al ₂O₃ + WGSR catalysts under simulated post-euro IV diesel exhaust conditions

This study deals with diesel DeNO_x catalysis by physically mixed Pd/TiO₂/Al₂O₃ and water-gas-shift-reaction (WGSR) catalysts under CO-rich conditions at a CO/NO_x ratio of 16. In the post-Euro IV era, many diesel engines are expected to produce CO-rich exhaust emissions due to the trend toward emission regulation and limitations in related technologies. Under these circumstances, the passive DeNO_x strategy utilizing CO as a single reductant can be considered appropriate. The Pd/TiO₂/Al₂O₃ catalyst is well known for its high DeNO_x performance when CO and H₂ are available as NO_x reductants. However, when CO is used as a single reductant, the catalyst shows poor DeNO_x activity, even if the CO concentration is exceptionally high (CO/NO_x = 16). In this study, the DeNO_x activity of Pd/TiO₂/Al₂O₃ was noticeably improved by physically mixing it with a proper WGSR catalyst (Cu/ZnO/Al ₂O₃), which is capable of producing H₂ and feeding it to Pd/TiO₂/Al₂O₃ successfully even in the presence of O₂. The NO_x conversion exceeded 70% at 400-450 °C using a Pd/TiO₂/Al₂O₃ + Cu/ZnO/Al₂O₃ catalyst mixture at a weight ratio of 33:67. The reaction gas consisted of 500 ppm NO, 8000 ppm CO, 8 vol.% O₂, 5 vol.% CO₂ and 10 vol.% H₂O and w/f was 0.1 g s/cm ³.