The e4ect of the addition of alkali ions to commercial amorphous silica, generally used as support for heterogeneous catalysts, has been investigated from the point of view of morphological and structural changes. Samples of alkali-doped silica were prepared by impregnation and subsequent calcination at various temperatures. The structural e4ect of Li, Na, K, and Cs was determined by use of techniques such as wide-angle (WAXS) and small-angle X-ray scattering (SAXS). The WAXS di4ractograms, analyzed with the Rietveld method using the GSAS program, allowed qualitative and quantitative identi5cation of the fraction of the di4erent silica polymorphs like quartz, tridymite, and cristobalite. SAXS measurements, using the classical method based on Porod:s law, yielded the total surface area of the systems. The calculated areas were compared with the surface areas determined by the nitrogen adsorption technique using the analytical method of Brunauer+Emmett+Teller. The results are explained in terms of sizes of the alkali ions and cell volume of the di4erent crystalline phases.
Effect of alkali ions on the amorphous to crystalline phase transition of silica
Venezia;La Parola V;
2001
Abstract
The e4ect of the addition of alkali ions to commercial amorphous silica, generally used as support for heterogeneous catalysts, has been investigated from the point of view of morphological and structural changes. Samples of alkali-doped silica were prepared by impregnation and subsequent calcination at various temperatures. The structural e4ect of Li, Na, K, and Cs was determined by use of techniques such as wide-angle (WAXS) and small-angle X-ray scattering (SAXS). The WAXS di4ractograms, analyzed with the Rietveld method using the GSAS program, allowed qualitative and quantitative identi5cation of the fraction of the di4erent silica polymorphs like quartz, tridymite, and cristobalite. SAXS measurements, using the classical method based on Porod:s law, yielded the total surface area of the systems. The calculated areas were compared with the surface areas determined by the nitrogen adsorption technique using the analytical method of Brunauer+Emmett+Teller. The results are explained in terms of sizes of the alkali ions and cell volume of the di4erent crystalline phases.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
