Intrinsic reduction kinetics of cobalt- and nickel-titanates by hydrogen
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Intrinsic reduction kinetics of cobalt- and nickel-titanates by hydrogen I. Arvanitidis, A. Kapilashrami, Du Sichen, and S. Seetharaman Division of Theoretical Metallurgy, Royal Institute of Technology, S-100 44 Stockholm, Sweden (Received 11 June 1999; accepted 11 November 1999)
The isothermal reduction of synthetic CoTiO3 and NiTiO3 in hydrogen (1 atm) was investigated using thermogravimetric analysis technique in the temperature range, 928–1287 K (CoTiO3) and 884–1387 K (NiTiO3). Shallow beds of fine titanate powders were reduced by hydrogen at a high flow rate. Quenched samples were analyzed by scanning electron microscopy. The rates of the reaction of the titanates with H2 were very fast during the reduction of Co2+, Ni2+, or Fe2+ ions into metals. The reduction of the remaining titanium oxide was very slow. The activation energy for the reduction of CoTiO3 by hydrogen to Co and TiO2 was evaluated to be 151 ± 1 kJ/mol, and the activation energy for the reduction of NiTiO3 by hydrogen to Ni and TiO2 was evaluated to be 153 ± 1 kJ/mol. The study was complemented by hydrogen reduction of synthetic TiO2 (rutile). The results were also compared with the hydrogen reduction of FeTiO3.
I. INTRODUCTION
Solid-state reduction-powder metallurgy is an extremely attractive method for production of specialized metallic or metal–matrix composites. There is a widespread use of transition metal titanates. They are used as substrates1 for screen printing of high-temperature YBa2Cu3O7−x superconductors. Strontium titanate, surface treated with various transition metal oxide additives, has a photocatalytic effect on the reduction of aqueous carbon dioxide.2 Metal fillers and fibers have been used in polypropylene composites of higher electrical conductivities. In order to improve the mechanical properties of filled plastics, the surface is treated with metal fillers or aluminum alloy flakes with titanate coupling agents.3 The presence of both rutile and trivalent titanium in the SYNROC phase assemblage has been found to be responsible for the accommodating nature of the ceramics to incorporate high-level waste.4,5 In this context, the use of hydrogen or natural gas as reductants appears to be attractive from an environmental view point. In the case of the reduction of complex oxides, it is important to have an understanding of the mechanisms underlying such reduction reactions so that the conditions can be optimized to achieve maximum efficiency. With this in mind, a series of studies of the hydrogen reduction of metallic oxides was carried out at the Department of Metallurgy and a number of pure oxides as well as tungstates, chromates, and aluminates6–9 involving transition metals were studied. The present work is part of this series and deals with the reduction of transition metal titanates. 338
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J. Mater. Res., Vol. 15, No. 2, Feb 2000 Downloaded: 07 Feb 2015
Previous studies6–9 indicate that the kinetics of reduction of complex metal oxides are str
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