摘要
寻找适当的载体,了解载体对于活性组分的分散以及分散态结构的影响,长期以来,一直是催化工业研究中的一项重要工作和催化理论研究中最引人注目的课题之一。由于未能找到一种强有力的手段,直接观察不同载体催化剂中活性组分分散后的结构变化,长期来人们虽然已经做了大量的工作,但是至今催化剂的筛选仍未能完全摆脱'炒菜'的模式,对某些催化体系虽也提出了一些载体的和催化剂的作用机理模型,终因缺乏直接的结构证据,而争议很多。本文中,我们对比了γ-Al_2O_3,SiO_2以及TiO_2三种不同载体上NiO分散后的结构。这些由EXAFS得到的直接结构证据,对于进一步阐述NiO催化剂的分散和作用机理以及筛选新的催化剂,显然是有意义的。
The NiO dispersion phase on three different supports have been studied by EXAFS in combination with XRD quantitative phase analysis. The result of XRD shows the threshold of NiO/γ-Al_2O_3 system is larger than that of NiO/TiO_2 system and the threshold of NiO/SiO_2 system is the least. From the FT functions of EXAFS for samples we can find that in NiO/γ-Al_2O_3 system a part of Ni changes its coordination polyhedra into tetrahedra, the Ni-Ni coordination peaks reduce markedly with the decrease of NiO content in samples. The result of EXAFS fitting and phase isolation shows Ni-Ni coordination number increases with the increase of NiO content in samples of NiO/γ-Al_2O_3 system and reaches a maximum value of 4.0 and remains constant when NiO content reaches the threshold obtained by XRD. It shows the structure of NiO dispersion phase is in better agreement with the NiO monolayer dispersion model than any other models. In FT functions of EXAFS for samples at thresholds of two systems the Ni-Ni coordination peaks of NiO/TiO_2 system is much lower than those of NiO/γ-Al_2O_3 system. But they are similar when their NiO contents both close to the threshold of NiO/TiO_2 system. It shows that though the dispersion capacity of NiO in NiO/TiO_2 system is less than that in NiO/γ-Al_2O_3 system its dispersion performance is better. In NiO/SiO_2 system the coordination peaks are similar to those of pure NiO and the Ni-Ni coordination number approaches 12 of the pure NiO. It shows it is difficult for NiO to disperse on the surface of SiO_2. In fact, for the NiO/SiO_2 system the threshold obtained by XRD is beyond the experiment accuracy so there is no threshold for this system. Therefore above facts indicate that dispersion threshold and reducing of metal-metal coordination number of the dispersion phase is a quite characteristic feature of monolayer dispersion state.
出处
《物理化学学报》
SCIE
CAS
CSCD
北大核心
1994年第1期15-18,共4页
Acta Physico-Chimica Sinica
基金
国家自然科学基金
关键词
分散
氧化镍
硅胶
钛胶
催化剂
Monolayer dispersion, Support effect, EXAFS, NiO, γ-Al_2O_3, SiO_2, TiO_2
