(769c) Impact of Mixed Metal Oxides Supports on the Durability and Low-Temperature Performance of Pd-Based Diesel Oxidation Catalysts
Future diesel oxidation catalysts (DOC) will need to perform effectively at increasingly low exhaust temperatures; this so-called â??150 Â°C challengeâ? (i.e., achieve over 90% conversion below 150 ÂºC) arises from continued improvements in diesel engine efficiency. Recently, there has been progress made in designing catalytic materials with enhanced low-temperature oxidation performance (e.g., Au and Ag nanoparticles combined with Cu, Mn, or Fe). However, significant technical barriers exist for implementing these novel materials into practice. For instance, these novel materials tend to lose performance rather quickly under harsh reaction environments typical to automotive emissions control due to hydrothermal aging and poisoning. Alternatively, increasing precious metal loading (e.g., Pt) can improve the low-temperature performance of current commercial catalysts, but that approach is cost-prohibitive. We have recently reported the impact of ZrO2 supports on CO and C3H6 oxidation, sulfur tolerance, and hydrothermal stability . Strong interaction between Pd and ZrO2 manifested a greater thermal stability shown by good oxidation performance even after aging at 800 and 900°C for 16 h. On the other hand, Pd/SiO2 suffered significant performance loss due to Pd particle coarsening. A higher and more complete coverage of SiO2 by ZrO2 could have led to a higher dispersion and stability of Pd. This work confirms the potential of developing Pd-based oxidation catalysts with enhanced durability and low-temperature activity using ZrO2-SiO2 and CeZrAl2O3 mixed oxide supports. In this work, the accessible surface area of ZrO2 was enhanced by depositing a layer of ZrO2 on SiO2 spheres, leading to the formation of a core@shell structure, with SiO2 located in the core and ZrO2 on the shell. The SiO2@ZrO2 core-shell structure was verified using TEM, EDX and the surface area of the material was 220 m2/g, much higher than the surface area of bulk ZrO2 (~58 m2/g). Most importantly, the SiO2@ZrO2 support was able to maintain its surface area even after thermal treatments in air at 700 and 800oC. Furthermore, Pd/CeZrAl2O3 catalysts were synthesized by impregnating CeZr aqueous nanoparticles onto Al2O3 via an incipient wetness method followed by a pH controlled impregnation of a Pd(NH3)4(NO3)2 precursor leading to Pd nanoparticles in a close proximity to CeOx. Catalytic performance of both Pd/SiO2@ZrO2 and Pd/CeZrAl2O3 catalysts, with Pd loadings varying from 1 to 2 and 4 wt.%, was measured under the following conditions: temperature increased from 100 to 500 °C at 2 °C/min, total flow rate 333 mL/min, 12% O2, 6% H2O, 6% CO2, 400 ppm H2, 2000 ppm CO, 100 ppm NO, 1667 ppm C2H4, 1000 ppm C3H6 and 333 ppm C3H8 in Ar balance. The performance of Pd catalysts was evaluated in fresh and hydrothermally aged states. Overall, Pd/CeZrAl2O3 catalyst showed considerably higher CO and hydrocarbon oxidation performance than Pd/SiO2@ZrO2 in all states studied. The 2 wt.% Pd/CeZrAl2O3 has shown to have the best low temperature performance and hydrothermal durability. On the other hand, 4 wt.% Pd/CeZrAl2O3 suffered significant performance loss due to Pd particle coarsening. This work confirms the potential of developing Pd-based oxidation catalysts with enhanced durability and low-temperature activity using ZrO2-SiO2 and CeZrAl2O3 mixed oxide supports.
1. Kim, M.-Y.; Kyriakidou, E.A.; Choi, J.-S.; Toops, T.J.; Binder, A.J.; Thomas, C.; Parks II, J.E.; Schwartz, V.; Chen, J.; Hensley, D.K. Appl. Catal. B 187 (2016) 181.