Characteristics of Pt-BaO/Hydrotalcite Catalysts for NOx Storage-Reduction Reaction

Abstract

Diesel engines are operated under excess of oxygen, so conventional three-way catalyst hardly removes NOx emitted from diesel engines. To remove NOx more effectively, NOx Storage Reduction (NSR) technology is being used as one of the potential methods. The typical NSR catalyst, Pt-BaO/Al2O3, shows good performance

however, there are drawbacks such as inferior thermal stability and narrow active temperature region. This study addresses the use of Mg-Al mixed oxide, known as hydrotalcite material as NSR catalyst support to overcome these disadvantages. In this work, hydrotalcite was synthesized with various Mg/Al molar ratios and used NSR catalyst support on which Pt and Ba were loaded. Hydrotalcite (HT) was prepared by applying co-precipitation method with the Mg/Al ratios of 1:9, 2:8, 3:7, 4:6, 5:5 and 6:4. Hydrotalcite was then calcined at 800 ℃ and used as NSR catalyst support. All prepared catalysts were calcined at 500 ℃, 650 ℃ and 800 ℃, and the NOx storage activity of the catalysts was examined. Then, various characterizations were performed by the means of BET specific surface area, XRD, ICP, FT-IR, CO chemisorption, TEM/ EDS and NO2-TPD. On the samples calcined at 500 ℃, the NOx storage activities of HT-supported catalysts were superior to that of Al2O3 supported one. Also, the range of active temperature region was shifted to high temperature while that of Pt-BaO/Al2O3 catalyst was narrow. Especially, for the case of sample with the Mg/Al ratio of 2:8, the temperature where the activity is maximized is 400 ℃. On the samples calcined at higher temperature (650 ℃ and 800 ℃), the activity of Pt-BaO/Al2O3 catalyst was completely deactivated in all temperature regions due to the sintering of Pt while HT-supported catalysts still show remarkable NOx storage ability especially in high temperature region. It was confirmed that catalysts supported on Mg-Al mixed oxide have better thermal durability such as activity, minimized Pt sintering and less BaAl2O4 formation, thus better NOx storage activity. Especially, Mg/Al ratio is 4:6. This study provides the information on optimal Mg/Al ratio to use hydrotalcite as support for NSR reaction. When Pt and Ba were loaded on the support, HT-supported catalysts showed superior NOx storage performance compared with Al2O3-supported catalyst even after high temperature treatment. Also, we confirmed the positive effect of MgO on the stability of Pt. In summary, it was found that the use of hydrotalcite as NSR catalyst support can aid in improving NOx storage activity and Pt dispersion, and inhibiting sintering of Pt, thus potentially substituting Al2O3 support.