Morphological investigations on mesostructured metal oxides
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Morphological investigations on mesostructured metal oxides V. Weidenhof a) Institute of Experimental Physics, University of Saarbr¨ucken, P.O. Box 151150, D-66041 Saarbr¨ucken, Germany
F. Gropper and U. M¨uller ZAK/Z, BASF AG, D-67059 Ludwigshafen, Germany
L. Marosi and G. Cox ZAA/S, BASF AG, D-67059 Ludwigshafen, Germany
R. Houbertz and U. Hartmann Institute of Experimental Physics, University of Saarbr¨ucken, P.O. Box 151150, D-66041 Saarbr¨ucken, Germany (Received 23 August 1996; accepted 24 January 1997)
The synthesis and characterization of mesostructured zirconia and titanium oxides are presented. The samples were investigated by x-ray powder diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). XRD and TEM revealed only lamellar structures for both materials, whereas AFM could detect locally restricted initial stages of cubic or hexagonal phases in a globally lamellar Ti oxide.
I. INTRODUCTION
In the early nineties, a family of silicates was discovered which exhibits a highly regular pore system like zeolites, but with pore sizes that are up to an order of magnitude larger than in zeolitic materials.1,2 Such mesoporous solids with pore sizes in the range between 2 and 10 nm are of interest because of their potential relevance in heterogeneous catalysis and in host-guest chemistry.3,4 The synthesis leading to this “M41S” family of mesoporous silicates is based upon the cooperative assemblage of periodic inorganic and surfactant-based structures.2,5 Ordered patterns of surfactant molecules were used for the three-dimensional polymerization of the silicates. Materials with lamellar, cubic, and hexagonal symmetry were described.1 A variety of studies concerning the formation of mesoporous silicates have been published.6–10 The most prominent model presented so far is that of Monnier et al.7 According to this model, anionic silicate oligomers develop multidentate bindings to cationic surfactant head groups. At the interface the inorganic polyanions then condense to build up stable walls. The symmetry of the occurring mesoporous phase is ultimately determined by the matching of the charge distributions at the surfactant-silicate interface. It was recognized quite some time ago that the synthesis pathway should also be applicable to other inorganic species, a)
Present address: Institute of Surface Research and Vacuum Physics, Research Center J¨ulich, D-52425 J¨ulich, Germany.
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J. Mater. Res., Vol. 12, No. 6, Jun 1997
if they involve a pronounced polyion chemistry enabling multidentate binding prior to oxide formation.6,7 A variety of novel mesoporous oxides with promising catalytic properties should evolve. Transition metal-based materials are the subject of considerable attention.4,6,11–13 In the present work, the synthesis and the structural characterization of mesoporous Zr and Ti oxides are presented. Analytical data were mainly obtained from x-ray powder diffraction (XRD), transmission electron microscopy (TEM),
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