The effects of high temperature lean exposure on the subsequent HC conversion of automotive catalysts

Degreened samples of monolithic catalysts containing Pd-only, Pt-only, Pt/Rh (5/0/1), and Pt/Pd/Rh (1/13/1 and 1/4/1) were reduced in rich exhaust and then evaluated on consecutive lean temperature ramps where the maximum temperature was increased from test to test, in order to assess the effects of the recent thermal-chemical history on the conversion of CH4, C3H8, and C3H6. A highly loaded Pt-only catalyst displayed relatively consistent HC conversion on the consecutive lean temperature ramps. However, the HC conversion of the other catalysts degraded from run to run as the maximum temperature on the previous test increased. For the Pd-only catalyst, this degradation was attributed to the increasing oxidation of metallic Pd to Pd oxide. While previous studies suggested that Pd oxide is more active than metallic Pd for CH4 and C3H8 conversion, the continued oxidation increasingly depleted the catalyst of metallic Pd sites, which may be involved in the dissociative adsorption of the HC. Alternatively, the continued oxidation may have decreased the phase boundary between Pd metal and Pd oxide, which has been reported to enhance CH4 combustion. When the Pd-only sample was exposed to ca. 800 °C under lean conditions, where Pd oxide is known to decompose, the CH4 conversion on the subsequent test improved dramatically and was similar to that after the rich treatment.