NO_x储存催化剂Pt/BaAl_2O_4-A1_2O_3的XAFS研究

The Study of XAFS for the NO_x Storage Catalyst Pt/BaAl_2O_4-Al_2O_3

采用共沉淀－浸渍法在不同载体焙烧温度下，制备了不同Ａｌ／Ｂａ原子比的Ｐｔ／ＢａＡｌ２Ｏ４－Ａ１２Ｏ３系列样品。用ＸＲＤ，ＸＡＮＥＳ，ＥＸＡＦＳ，以及ＮＳＣ（ＮＯ_ｘ ｓｔｏｒａｇｅ ｃａｐａｃｉｔｙ）测定等手段对样品的微观结构和ＮＯ_ｘ储存性能进行了详细的表征．样品中Ｂａ物种是以ＢａＡｌ２Ｏ４和ＢａＣＯ３两种混合物相的形式存在，且伴随着载体焙烧温度和Ｂａ含量的降低，ＢａＡｌ２Ｏ４物相的分散度变高，ＮＯ_ｘ储存活性也随之提高，这表明ＢａＡｌ２Ｏ４相的分散度与样品的ＮＯ_ｘ储存性能密切相关，小颗粒的ＢａＡｌ２Ｏ４相是ＮＯ_ｘ的主要储存活性中心．在样品中，Ｐｔ物种以金属原子簇形式存在．分散度很高，其Ｐｔ—Ｐｔ壳层配位数较标样Ｐｔ粉有显著下降，Ｐｔ—Ｐｔ键长变短，出现了纳米收缩现象．高分散的小颗粒金属Ｐｔ原于簇为捕获和氧化ＮＯ_ｘ的主要活性中心．

A series of Pt/BaAl2O4-Al2O3 NOx trap samples were prepared by coprecipitation-impregnation method. The supports BaAl2O4-Al2O3 were calcined at 800 degreesC and 1000 degreesC, respectively. X-ray Diffraction (XRD), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure(EXAFS) were used to characterize the microcosmic structure of barium and platinum species in the samples. It is shown that the barium exists in the forms of BaAl2O4 and BaCO3 crystallites. With the decreasing of calcination temperature and barium content, the dispersion of BaAl2O4 phase becomes higher. Platinum in the samples mainly exists as small atomic clusters, which possess very high dispersion. In comparison with the standard compound of platinum, the magnitude for Pt-Pt coordination shell is significantly decreased, and the coordination distance is also shortened about 0.01 nm. All the results suggest that the BaAl2O4 crystallites should be the centers for NOx storage, while the fine-grained platinum species with high dispersion are deduced as the centers for the capture and oxidation of NOx species.