In situ reduction kinetics of NiCl 2 in SiO 2 gel matrix
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K. Biswas, G. C. Das, and S. Mukherjee Department of Metallurgical Engineering, Jadavpur University, Calcutta 700 032, India (Received 25 March 1995; accepted 19 June 1995)
Silica gels containing NiCl2 and dextrose have been reduced by heat-treating the gels under N 2 atmosphere at 800 °C, 850 °C, 900 °C, and 950 °C, respectively. The influence of the volume ratio of ethyl alcohol to tetraethylorthosilicate and the amount of dextrose on the in situ reduction kinetics of NiCl2 in gel matrix have been investigated. The kinetic data on in situ reduction have been analyzed by a reduced time method which indicates that mixed mechanisms are operative. The predominant mechanism of reduction of NiCl2 in SiO2 gel matrix is of nucleation and growth type. The activation energies over different temperatures and fraction converted have been computed by the integration method.
I. INTRODUCTION Glass metal nanocomposites have emerged as a new class of materials and have gained considerable importance due to their remarkable physical properties,1"5 which are functions of shape, size distribution, and the amount of metallic species present in the matrix. These nanocomposites can be prepared through the sol-gel route by reducing metallic salts in the gel matrix either by passing H2 from the outside6 or by the H2 generated in situ.1 Preliminary results on in situ isothermal reduction kinetics of the NiCl2-containing gel have been reported earlier.8 However, it has been reported that the alcohol: methoxy silane ratio has a marked influence on the pore size and porosity of the gel, and both these parameters increase with the increase of alcohol content.9 Therefore, it is expected that the ratio of alcohol to tetraethylorthosilicate (TEOS) ratio R during preparation of the gel and the amount of H2-generating species present in the gel matrix will exert a substantial influence on the reduction rate and mechanism. To date no such studies have been reported. In this paper we report the results of the influence of such parameters on the in situ reduction kinetics of NiCl2 in SiO2 gel matrix. II. EXPERIMENTAL
III. RESULTS AND DISCUSSION During the heat treatment of the gels, dextrose present in the pores of the gel decomposes to carbon TABLE I. Composition of silica gel containing
NiCl2-containing silica gel samples were prepared with different volume ratios of C 2 H 5 OH to TEOS and varying amounts of dextrose as presented in Table I. The methodology of preparation has been reported elsewhere.7 Each silica gel sample contains a fixed amount of NiCl 2 such that complete reduction would yield 5 wt. % metal with respect to SiO2 of the gel matrix. 2938
All the gels were reduced separately by heat-treating them in an electrical heating furnace for 8 min under oxygen- and moisture-free N 2 gas atmospheres at 800 °C, 850 °C, 900 "C, and 950 °C, respectively. Nitrogen gas was made free from oxygen and moisture by bubbling the gas through an alkaline pyrogallol solution, followed by passing it through a packed bed of anhydrous calcium chloride.
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