Reduction of chromia–silica catalysts: A molecular picture
Reduction of chromia–silica catalysts: A molecular picture
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DOI：
10.1016/j.jcat.2016.04.022
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发表时间：
2016-08
影响因子：
7.3
通讯作者：
M. Gierada;P. Michorczyk;F. Tielens;J. Handzlik
中科院分区：
文献类型：
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作者：
M. Gierada;P. Michorczyk;F. Tielens;J. Handzlik
二氧化硅上还原铬物种的性质仍在争论中。本文采用密度泛函理论和原位紫外-可见漫反射光谱相结合的方法研究了氧化铬-二氧化硅催化剂的还原过程。先进的周期和集群模型的无定形二氧化硅再现表面铬物种的异质性。几何参数，振动频率，激发能，和Cr网站的相对稳定性进行了测定。据预测，还原单体Cr（VI）物种的Cr（IV）网站是高度放能的，而深度还原为Cr（II）是不太degenically青睐。二聚Cr（VI）物质最终可被还原为Cr（III）或Cr（II）。原位紫外-可见漫反射光谱研究表明，在干燥条件下，在CO作用下，Cr（VI）物种被选择性地转化为Cr（II），而用干燥H2还原则导致Cr（III）和Cr（II）的形成。对于这两种还原剂，水的存在阻碍了深度还原为Cr（II），这可以通过再氧化/水解过程来解释。
The nature of reduced Cr species on silica is still under debate. In this work, combined DFT and in situ UV–vis DRS investigations of reduction of chromia–silica catalyst have been done. Advanced periodic and cluster models of amorphous silica are applied to reproduce the heterogeneity of surface Cr species. Geometrical parameters, vibrational frequencies, excitation energies, and relative stabilities of Cr sites are determined. It is predicted that reduction of monomeric Cr(VI) species to Cr(IV) sites is highly exergonic, whereas deep reduction to Cr(II) is less thermodynamically favored. Dimeric Cr(VI) species can finally be reduced to Cr(III) or Cr(II). In situ UV–vis DRS studies indicate that Cr(VI) species are selectively converted to Cr(II) under CO in dry conditions, whereas reduction with dry H2leads to formation of Cr(III) and Cr(II). For both reducing agents, the presence of water hampers deep reduction to Cr(II), which can be explained by reoxidation/hydrolysis processes.