用固体核磁共振方法研究CeO_2-γ-Al_2O_3混合氧化物中的Al状态(英文)

The ^(27)Al State in CeO_2-γ-Al_2O_3 Mixed Oxides Determined by Solid-State NMR Spectroscopy

用固体核磁共振技术研究了CeO2-γ-Al2O3混合体系中CeO2和γ-Al2O3两相间的相互作用.在混合物的27Al MAS NMR中,除了四配位和六配位的Al位外,有一个尖锐的位于37处的峰,这个峰在高场处有一宽的肩峰.实验证实:位于高场的宽峰来自于γ-Al2O3中的五配位Al ,而位于37处的尖峰则是由CeO2和γ-Al2O3两相间的相互作用产生的,即Al取代了CeO2晶格中八配位的Ce .定量研究表明,这种Al取代的量是极其有限的,整个CeO2晶格只有1 %的Ce能被Al取代.

In this work, CeO2-γ-Al2O3 mixed oxides were studied by solid-state NMR techniques to understand the interaction between CeO2 and γ-Al2O3. The ^27 Al magic angle spinning （MAS） NMR spectra of the mixed oxides showed a sharp peak at ca. 37 ppm and a high-field shoulder, in addition to the signals from tetra- and hexa-coordinated ^27 Al sites. It was demonstrated that the sharp peak at ca. 37 ppm originates from the ^27 Al atoms residing in the lattice of CeO2 with an eight-fold coordination, representing the interaction between CeO2 and γ- Al2O3; the high-field broad shoulder peak arises from penta-coordinated Al sites originally present in γ-Al2O3. It was postulated that four Al^3＋ ions have replaced the three Ce^4＋ ions in three CeO2 crystal cells so that the overall charge balance of the system is maintained. Furthermore, it was shown that the concentration of Al substitution is limited, and only about 1% of the total CeO2 can be replaced by Al^3＋.