Enhancing low-temperature activity and durability of Pd-based diesel oxidation catalysts using ZrO sub(2) supports

We investigated the impact of ZrO sub(2) on the performance of palladium-based oxidation catalysts with respect to low-temperature activity, hydrothermal stability, and sulfur tolerance. Pd supported on ZrO sub(2) and SiO sub(2) were synthesized for a comparative study. Additionally, in an attempt to maximize the ZrO sub(2) surface area and improve sulfur tolerance, a Pd support with ZrO sub(2)-dispersed onto SiO sub(2) was studied. The physicochemical properties of the catalysts were examined using ICP, N sub(2) sorption, XRD, SEM, TEM, and NH sub(3)-, CO sub(2)-, and NO sub(x)-TPD. The activity of the Pd catalysts were measured from 60 to 600 degree C in a flow of 4000 ppm CO, 500 ppm NO, 1000 ppm C sub(3)H sub(6), 4% O sub(2), 5% H sub(2)O, and Ar balance. The Pd catalysts were evaluated in fresh, sulfated, and hydrothermally aged states. Overall, the ZrO sub(2)-containing catalysts showed considerably higher CO and C sub(3)H sub(6) oxidation activity than Pd/SiO sub(2) under the reaction conditions studied. The good performance of ZrO sub(2)-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 ZrO sub(2) was manifested as a greater hydrothermal stability with good oxidation activity even after aging at 800 and 900 degree C for 16 h. In contrast, Pd/SiO sub(2) suffered significant performance loss due to Pd particle coarsening. Although the Pd/ZrO sub(2)-SiO sub(2) catalyst was not more active than Pd/ZrO sub(2), improved tolerance to sulfur was realized. Unlike the bulk ZrO sub(2) support, the ZrO sub(2)-incorporated SiO sub(2) presented only weak basicity leading to a superior sulfur tolerance of Pd/ZrO sub(2)-SiO sub(2). These results confirmed the potential of developing Pd-based oxidation catalysts with enhanced low-temperature activity and durability using ZrO sub(2)-SiO sub(2) supports. Controlling morphology and accessible area of the dispersed ZrO sub(2) layer appeared critical to further maximize the catalytic performance.