高分散CuO/CeO_2-Al_2O_3催化剂中铜物种的精细结构表征

Characterization on the Fine Structures of the Copper Species in the Highly Dispersed CuO/CeO_2-Al_2O_3 Catalysts

采用化学吸附-水解法制备了系列CuO/CeO2-Al2O3催化剂,运用XANES、EXAFS、XRD和H2-TPR等方法对催化剂的结构进行了表征,探讨了不同焙烧温度对催化剂中高分散Cu物种的微观局域结构、分散状态和存在形式的影响,并与样品的CO氧化性能关联。结果表明,500℃焙烧样品中Cu物种主要以高分散的CuO微晶形式存在,与载体及CeO2的作用相对较弱;当焙烧温度提高至650℃,存在3种铜物种,即与Al2O3接触的五配位的、缺角八面体结构的表面尖晶石相,与CeO2接触的小晶粒Cu物种,以及存在于CeO2表面的无定形CuO物种,这些Cu物种与CeO2的作用较强;在更高的焙烧温度(800℃)下,CeO2发生了较严重烧结,Cu与Al2O3作用显著增强,一部分八面体配位的Cu2+离子扩散进入四面体位。CO氧化活性评价结果表明,650℃焙烧的样品,氧化性能最佳,该温度下Cu与CeO2产生有效的相互作用使部分Cu2+取代Ce4+进入CeO2晶格,形成-Cu2+-O-Ce4+-链,导致Cu-O键拉长,而易活化。500℃焙烧样品中Cu与CeO2相互作用不显著,其催化协同性没有完全体现;而800℃焙烧时,Cu与Al2O3发生强相互作用,部分铜物种转化为尖晶石相,致使Cu-O键收缩,加上CeO2的烧结,活性明显下降。

A series of CuO/CeO2-Al2O3 catalysts were prepared by chemisorption-hydrolysis method. The techniques of XANES, EXAFS, XRD and H2-TPR were employed for the structural characterization of the catalysts. The effect of calcination temperature on the fine structures, the dispersion and the existing state of the copper species was investigated carefully. The characterization results reveal that the copper species in the sample calcined at 500 ℃ mainly exist as highly dispersed CuO crystallites, which weakly interact with the support Al2O3 and the promoter CeO2. When the calcination temperature is elevated to 650℃, three kinds of copper species are detected, namely the small crystallites CuO, the amorphous CuO-CeO2 interacting phase and the Cu-Al surface spinel phase. At higher calcination temperature of 800 ℃, CeO2 is obviously sintered, and the interaction between copper species and Al2O3 is remarkably promoted, leading to the diffusion of partial octahedrally coordinated Cu^2＋ into tetrahedral sites. The results of the activity evaluation show that the catalyst calcined at 650 ℃ is the most active one for CO oxidation. In this catalyst, Cu and CeO2 effectively interact with each other, and a part of Cu^2＋ ions have diffused into the lattice of CeO2, leading to the lengthening of Cu-O bond and easier activation of it during the CO oxidation process by forming the -Cu^2＋-O-Ce^4＋- linkage. However, after calcination at 800 ℃, the interaction between Cu and Al2O3 is prominently enhanced, and most Cu species transform into bulk spinel phase, which possesses shorter Cu-O bond length than CuO and Cu-Ce interaction phases. Additionally, the CeO2 phase is seriously sintered at this temperature. All of these make the remarkable decrease of the oxidation activity of the catalyst.