High resolution X-ray photoelectron spectroscopic analysis of transformation of surface chromium species on Phillips CrOx/SiO2 catalysts isothermally calcined at various temperatures
High resolution X-ray photoelectron spectroscopic analysis of transformation of surface chromium species on Phillips CrOx/SiO2 catalysts isothermally calcined at various temperatures
复制标题
DOI：
10.1016/j.molcata.2004.05.001
复制
发表时间：
2004-09
影响因子：
--
通讯作者：
Bo-ping Liu;Yuwei Fang;M. Terano
中科院分区：
文献类型：
--
作者：
Bo-ping Liu;Yuwei Fang;M. Terano
菲利普斯 CrOx/SiO2 催化剂每年仍生产数百万吨聚乙烯。然而，对这种重要工业催化剂的表面化学了解不足阻碍了其进一步发展。在这项工作中，采用高分辨率XPS方法对在不同温度下等温煅烧的Phillips CrOx/SiO2催化剂上乙酸铬(III)转化为负载型铬酸盐物种(Cr(VI)Ox,surf)进行了表面分析。高分辨率 XPS 表征阐明了醋酸铬 (III) 转化为大块 CrO3 并随后转化为 Cr(VI)Ox 的特定转化过程，表面是 120 至 800°C 之间的煅烧温度的函数。阐明了各种催化剂样品中的特定表面成分与煅烧温度的关系。很明显，在120℃等温干燥过程中，乙酸铬(III)很容易被氧化并分解成大块的CrO3。大体积的CrO3在200℃时开始转化为负载型铬酸盐物质，并且在400℃时可以作为铬酸盐物质完全稳定在硅胶表面上。由于大块 CrO3 的部分热分解，仅在 200°C 煅烧的样品上观察到大量大块五价 Cr 氧化物 (Cr2O5 或 (Cr2O7)4−)，因为大块 CrO3 在 200°C 下不完全稳定成铬酸盐物质。作为煅烧Phillips催化剂上活性位点的前体，随着催化剂的煅烧温度从200℃升高到800℃，铬酸盐物种(Cr(VI)Ox,surf)逐渐变得越来越缺电子。在高温 (600–800°C) 下仅观察到铬酸盐物质因热诱导部分还原为 Cr(III)Ox,surf。
Phillips CrOx/SiO2catalyst is still producing several million tons of polyethylene each year. However, insufficient understanding of the surface chemistry of this important industrial catalyst hindered its further development. In this work, surface analytical approach on transformation of chromium(III) acetate into supported chromate species (Cr(VI)Ox,surf) of Phillips CrOx/SiO2catalysts isothermally calcined at various temperatures was carried out by high resolution XPS method. High resolution XPS characterizations have elucidated the specific transformation process of chromium(III) acetate into bulky CrO3and subsequently into Cr(VI)Ox,surfas a function of calcination temperature between 120 and 800°C. The specific surface components in various catalyst samples versus calcination temperature were clarified. It was evident that chromium(III) acetate could be very easily oxidized and decomposed into bulky CrO3during 120°C isothermal drying process. The bulky CrO3started to be transformed into supported chromate species at 200°C and could be completely stabilized on silica gel surface as chromate species at 400°C. Significant amount of bulky pentavalent Cr oxide (Cr2O5or (Cr2O7)4−) due to partial thermal decomposition of bulky CrO3was only observed on 200°C-calcined sample due to the incomplete stabilization of bulky CrO3into chromate species at 200°C. As the precursor of active site on calcined Phillips catalyst, the chromate species (Cr(VI)Ox,surf) was found to be gradually becoming more and more electron-deficient with increasing calcination temperature of the catalyst from 200 to 800°C. Only a slight thermal-induced partial reduction of chromate species into Cr(III)Ox,surfwas observed at high temperature (600–800°C).