The Effect of Pt-Pd Ratio on Oxidation Catalysts Under Simulated Diesel Exhaust 2011-01-1134
With a tighter regulatory environment, reduction of hydrocarbon emissions has emerged as a major concern for advanced low-temperature combustion engines. Currently precious metal-based diesel oxidation catalysts (DOC) containing platinum (Pt) and palladium (Pd) are most commonly used for diesel exhaust hydrocarbon oxidation. The efficiency of hydrocarbon oxidation is greatly enhanced by employing both Pt and Pd together compared to the case with Pt or Pd alone. However, there have been few systematic studies to investigate the effects of the ratio of platinum to palladium on catalytic oxidation over the DOC. The present study illustrates the relationship between the Pt-Pd ratio and catalyst activity and stability by evaluating a series of catalysts with various Pt to Pd ratios (1:0, 7:1, 2:1, 1:2, 1:5, 0:1). These catalysts were tested for their CO and hydrocarbon light-off temperatures under simulated conditions where both unburned and partially burned hydrocarbons were present. All bimetallic Pt-Pd catalysts showed better hydrocarbon light-off activity and thermal stability than the Pt- or Pd-only catalyst. NO oxidation to NO₂; however, was found to always depend directly on platinum content. If NO₂ production becomes less important over the DOC as advanced NOx reduction technology becomes less sensitive to the NO to NO₂ ratio, it may be possible to substantially reduce the platinum content of the DOC while maintaining stability and a low-temperature hydrocarbon light-off. These and other results will be discussed.