Roles of Zr and Pr in sulfur poisoned Pd/Ce-based catalyst for CO oxidation

Abstract

The diesel engines have drawn interest mainly due to their economic advantages arising from the superior fuel economy. While the regulations on diesel engine emission become more stringent, various catalytic systems are required to effectively remove environmentally harmful gases such as CO, hydrocarbon (HC), soot and NOx. Diesel oxidation catalyst (DOC) plays a key role in converting CO and unburned HCs to CO2 and H2O. DOC system requires low ignition temperature, hydrothermal stability and sulfur resistance. CeO2 is essential support material in DOC for lower light-off temperature because of its excellent oxygen storage capacity (OSC). However, it is easily deactivated by high temperature treatment and sulfur poisoning. In this study, CeZr and CeZrPr mixed-oxide solid solutions are used as catalyst support in comparison with pure CeO2 to overcome these drawbacks Pd 2wt% was loaded by incipient wetness impregnation. After impregnation, the catalysts were calcined at 500 °C and then, hydrothermally treated at 750 °C. Those samples were designated as Fresh and HTA, respectively. In sulfur aging process, the HTA samples were treated with SO2 containing gases. After that, regeneration process was followed. The sulfur aging and regenerated samples were named as SA and DeSOx, respectively. To get information of the physicochemical properties of catalysts, various analyses were carried out by the means of XRD, BET, FT-IR, Elemental analysis, CO-chemisorption, H2-TPR and SO2-TPD experiment. Also, CO oxidation experiments were performed for these differently treated catalysts. Fresh Pd/CeO2 sample exhibits the highest CO oxidation ability because of high dispersion and surface area. After HTA treatment, H2-TPR results indicate the enhancement of the interaction between Pd and support, which results in the improved catalytic activity in all samples. As evidenced by FT-IR, the sulfation treatment on Pd/CeO2 induced the formation of Ce(SO4)2, which causes blocking the Pd active site and the decrease in surface area. In addition, it gives rise to the interruption of oxygen back-spillover from CeO2 to Pd surface as a result of the weak interaction between Pd and CeO2. Consequently, the activity in the low temperature region disappears. Among the SA samples, Pd/CeZrPr shows superior catalytic activity, because Pr complements the interaction between Pd and support through OSC property. After regeneration process, the Pd/CeZr recovers CO oxidation ability most significantly among the samples. The XRD and BET results demonstrate that Pd/CeZr has outstanding sulfur resistance. In case of Pd/CeO2, sulfur adsorption and desorption lead to structural collapse, which causes sintering and reduction of surface area. Therefore, the catalytic activity of Pd/CeO2 is worse than that of sulfated sample. This study reveals the promotional effect of Zr and Pr on Pd/CeO2 catalyst during the sulfur adsorption/desorption process. Pr plays a role in minimizing the decrease in activity during the sulfur aging. The addition of Zr in CeO2 disturbs sulfur adsorption and aids in maintaining the textural property and removing sulfur during DeSOx, thus leading to recover the activity considerably.