Advantages of MgAlOx over 纬-Al2O3 as a Support Material for Potassium-Based High-Temperature Lean NOx Traps

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• 作者：Jinyong Luo ; Feng Gao ; Ayman M. Karim ; Pinghong Xu ; Nigel D. Browning ; Charles H. F. Peden
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：August 7, 2015
• 年：2015
• 卷：5
• 期：8
• 页码：4680-4689
• 全文大小：630K
• ISSN：2155-5435

文摘

MgAlO_x mixed oxides were employed as supports for potassium-based lean NO_x traps (LNTs) targeted for high-temperature applications. Effects of support compositions, K/Pt loadings, thermal aging, and catalyst regeneration on NO_x storage capacity were systematically investigated. The catalysts were characterized by XRD, NO_x-TPD, TEM, STEM-HAADF, and in situ XAFS. The results indicate that MgAlO_x mixed oxides have significant advantages over conventional 纬-Al₂O₃ supports for LNT catalysts, in terms of high-temperature NO_x trapping capacity and thermal stability. First, as a basic support, MgAlO_x stabilizes stored nitrates (in the form of KNO₃) to much higher temperatures in comparison to mildly acidic 纬-Al₂O₃. Second, MgAlO_x minimizes Pt sintering during thermal aging, which is not possible for 纬-Al₂O₃ supports. Notably, combined XRD, in situ XAFS, and STEM-HAADF results indicate that Pt species in the thermally aged Pt/MgAlO_x samples are finely dispersed in the oxide matrix as isolated atoms. This strong metal鈥搒upport interaction stabilizes Pt and minimizes the extent of sintering. However, such strong interactions result in Pt oxidation via coordination with the support so that NO oxidation activity can be adversely affected after aging, which in turn decreases NO_x trapping ability for these catalysts. Interestingly, a high-temperature reduction treatment regenerates essentially full NO_x trapping performance. In fact, regenerated Pt/K/MgAlO_x catalyst exhibits much better NO_x trapping performance than fresh Pt/K/Al₂O₃ LNTs over the entire temperature range investigated here. In addition to thermal aging, Pt/K loading effects were systemically studied over the fresh samples. The results indicate that NO_x trapping is kinetically limited at low temperatures, while it is thermodynamically limited at high temperatures. A simple conceptual model was developed to explain the Pt and K loading effects on NO_x storage. An optimized K loading, which allows balancing between the stability of nitrates and exposed Pt surface, gives the best NO_x trapping capability.