Effect of sulfur aging and regeneration on low temperature NO adsorption over hydrothermally treated Pd/CeO 2 and Pd/Ce 0.58 Zr 0.42 O 2 catalysts

Various treatments such as hydrothermal aging (HTA), sulfur aging (SA), and regeneration (DeSOx) were applied to Pd/CeO2 and Pd/Ce0.58Zr0.42O2 (Pd/Ce58) catalysts to understand the effect of sulfur aging and regeneration on the changes in NO adsorption ability and physicochemical properties of catalysts. The addition of Pd to Ce-based supports results in the larger amount of NO adsorbed at higher desorption temperature (250–500 °C) than CeO2. Also, Pd/CeO2 has larger amount of NO adsorbed than Pd/Ce58, indicating that ceria is the active site for NO adsorption at low temperature. However, SA treatment on both catalysts gives rise to the negligible NO adsorption ability since it leads to form thermodynamically more stable Ce(SO4)2. Structural and textural analysis after regeneration shows that Pd/Ce58 maintains the pore size distribution with the similar crystalline size, whereas Pd/CeO2 does not, despite the similar amount of residual sulfur over two samples. In addition, Pd dispersion is recovered completely over the former sample, although the latter does not. However, the fact that both regenerated catalysts do not recover the NO adsorption ability to the level of HTA ones implies that either textural property or Pd dispersion cannot account for the degradation of NO adsorption at low temperature. Instead, H2-TPR analysis indicates that the intimate interaction Pd and ceria is lost for both samples after regeneration, as evidenced by the disappearance of the simultaneous reduction of CeO2 and PdO below 20 °C. Hence, it can be concluded that the exposure of SO2 and the subsequent regeneration significantly affects the interaction between PdO and CeO2, resulting in the irreversible decrease in the NO adsorption ability at low temperature.