The structure of zinc and copper oxide species hosted in porous siliceous matrices

Abstract

Microporous and mesoporous siliceous phases (MFI, FAU, MCM-48, MCM-41, and SBA-15) were modified with zinc via aqueous and organometallic routes and characterized with techniques that reflect the structure of the matrix, the distribution of zinc over the matrix, and the structure of the zinc oxide species (XRD, nitrogen physisorption, IR spectroscopy, XPS, X-ray absorption fine structure (XAFS), UV-Vis spectroscopy). For comparison, MFI and MCM-48 were modified with copper alone and characterised by XPS/X-ray induced Auger electron spectroscopy and XAFS. From the results, it can be concluded that zinc interacts strongly with the siliceous surfaces, which prevents the formation of ZnO aggregates even when the coverage exceeds the monolayer limit. While zinc could be well distributed over FAU and mesoporous matrices by treatment with diethyl zinc, it remained on the external surface of MFI, possibly due to pore entrance narrowing by adsorbed species. For MCM-48 and MCM-41, the formation of a thin, probably monolayer surface zinc silicate coating was concluded from the data. With an aqueous impregnation technique, strong changes in the pore system of MCM-48 were noted, which indicate rupture of pore walls and partial structural damage. Different from zinc, copper forms small oxide aggregates upon introduction into micro and mesoporous siliceous matrices.