Chemical reactions at the liquid-solid interface in meso/microporous inorganic gels and glasses

Abstract

Interfacial reactions between concentrated aqueous solutions of iron nitrate and alumina or aluminosilicate meso- or microporous monolithic gels and glasses have been studied by IR absorption, Raman scattering, N 2 adsorption measurements, X-ray absorption (EXAFS) spectroscopy and scanning electron microscopy. Comparison is made with monoliths impregnated with cobalt nitrate solution. For the former, the reaction occurring at the monolith surface is described by the formation of an FeOOH-like hydroxide at the pore opening, whereas the nitrate ions diffuse into all the pores. A diffusion of iron ions inside the AlOSi framework, to give an Fe 1− x (Al, Si) x OOH phase, is assumed to explain the vibrational and EXAFS spectra as well as the formation of a brown crust around the optically clear NO 3 − impregnated core. Further heating of the outer crust under H 2 leads to metallic iron, αFe as well as spinel formation [γFe 2O 3 (maghemite) or Fe 3O 4]. These precipitates give rise to αFe 2O 3 (hematite) in air. A 1340cm −1 band seems to indicate the formation of amorphous diamond-like precipitates between 1000 °C and 1300 °C.