Enhancing low-temperature activity and durability of Pd-based diesel oxidation catalysts using ZrO₂ supports

We investigated the impact of ZrO₂ on the performance of palladium-based oxidation catalysts with respect to low-temperature activity, hydrothermal stability, and sulfur tolerance. Pd supported on ZrO₂ and SiO₂ were synthesized for a comparative study. Additionally, in an attempt to maximize the ZrO₂ surface area and improve sulfur tolerance, a Pd support with ZrO₂-dispersed onto SiO₂ was studied. The physicochemical properties of the catalysts were examined using ICP, N₂ sorption, XRD, SEM, TEM, and NH₃-, CO₂-, and NO_x-TPD. The activity of the Pd catalysts were measured from 60 to 600°C in a flow of 4000 ppm CO, 500 ppm NO, 1000 ppm C₃H₆, 4% O₂, 5% H₂O, and Ar balance. The Pd catalysts were evaluated in fresh, sulfated, and hydrothermally aged states. Overall, the ZrO₂-containing catalysts showed considerably higher CO and C₃H₆ oxidation activity than Pd/SiO₂ under the reaction conditions studied. The good performance of ZrO₂-containing catalysts appeared to be due in part to high Pd dispersion resulting from strong Pd and support interaction. Another beneficial effect of strong interaction between Pd and ZrO₂ was manifested as a greater hydrothermal stability with good oxidation activity even after aging at 800 and 900°C for 16h. In contrast, Pd/SiO₂ suffered significant performance loss due to Pd particle coarsening. Although the Pd/ZrO₂-SiO₂ catalyst was not more active than Pd/ZrO₂, improved tolerance to sulfur was realized. Unlike the bulk ZrO₂ support, the ZrO₂-incorporated SiO₂ presented only weak basicity leading to a superior sulfur tolerance of Pd/ZrO₂-SiO₂. These results confirmed the potential of developing Pd-based oxidation catalysts with enhanced low-temperature activity and durability using ZrO₂-SiO₂ supports. Controlling morphology and accessible area of the dispersed ZrO₂ layer appeared critical to further maximize the catalytic performance.