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Some surface and kinetic effects of trace additions of selected metal oxides and halides on the thermodehydration of silica xerogel spheroids
Author(s) -
Ross R. A.,
Henry N.
Publication year - 1974
Publication title -
the canadian journal of chemical engineering
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.404
H-Index - 67
eISSN - 1939-019X
pISSN - 0008-4034
DOI - 10.1002/cjce.5450520416
Subject(s) - halide , oxide , nucleation , metal , chemistry , inorganic chemistry , reaction rate , metal halides , sintering , catalysis , organic chemistry
A kinetic study of the sintering of silica xerogel has been carried out by following the rate of dehydration of xerogel spheroids, 2000 ‐ 841 μ, on a thermal balance in the presence of small additions, usually 5 g. ion percent of selected metal oxides and halides. First order rate constants calculated from the classical kinetic expression were in excellent agreement with those obtained from the Avrami‐Erofeev equation for nucleation and growth in the case of a monomolecular reaction. The surface areas and mercury densities of samples cooled from 900°, 850°, 800° and 500°C have been measured and X‐ray powder diffraction patterns obtained for most samples in an attempt to identify phases formed by solid‐solid interaction between the gel and the additives. The activation energies of the dehydration for the oxide additives, (CuO, NiO, Cr 2 O 3 , BeO, ZnO and Al 2 O 3 ), did not show substantial enough differences to allow any decisive analysis to be made on the basis of established properties of the oxides. CdO was the most active oxide in xerogel dehydration and it is shown that this oxide takes part readily in solid‐solid reactions with the xerogel in the vicinity of the Tamman temperature of silica. In the presence of the transition metal halides, (NiCl 2 , CrCl 3 , CuCl 2 and CoCl 2 ), hydrogen chloride was evolved from the matrix and xerogel dehydration may be accentuated by reaction of this gas with accessible surface hydroxyl groups. The addition of sodium halides showed no significant differences in dehydration rates whereas with lithium salts, the rate and amount of dehydration varied with the melting point of the additive.