Abstract
The industrial production of tungsten powder is carried out by the reduction of tungsten oxide powder via hydrogen. In this process, the size of the W particles is limited to particle sizes larger than 100 nm. To get below this limit, alternative processes are needed. In the current work, the possibility of preparing W powder below 100 nm via a vapour phase reduction of volatile WO2(OH)2 by hydrogen was investigated. The process consists of two stages. In the first stag,e WO2(OH)2 is formed by reacting WO3 with water vapour at temperatures of 1000–1100 °C. In the second stage, WO2(OH)2 is reduced by hydrogen at about 1000 °C to form metallic tungsten. The influence of process parameters such as furnace temperature, humidity and gas flow on the WO2(OH)2 evaporation and formation of tungsten powder was investigated. The characterization of the resulting powders was performed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). By optimization of the reaction conditions, powder with a metallic tungsten content of about 70 at% besides tungsten oxides was produced with metal particle sizes down to 5 nm. Further optimization should lead to a high tungsten content and a high product yield. Due to the small particle size, applications in catalysis might be possible, although an industrial realization of the process seems unrealistic at moment.
Highlights
IntroductionReduction of tungsten oxides by hydrogen is a well-established industrial process for the manufacture of well-defined and high-quality tungsten powder [1,2,3,4,5,6,7]
Reduction of tungsten oxides by hydrogen is a well-established industrial process for the manufacture of well-defined and high-quality tungsten powder [1,2,3,4,5,6,7].The overall reaction can be summarized by the chemical equation shown in Eq (1).WO3 + 3H2 ⇌ W + 3H2O. (1)In detail, the reduction is divided into several steps characterized by different intermediate oxides: WO3 → WO2.9 → WO2.72 → WO2 → W [1,2,3,4, 8, 9]
At least the last three reduction steps are driven by chemical vapour transport (CVT) processes based on the formation of volatile W O2(OH)2 [10, 11]
Summary
Reduction of tungsten oxides by hydrogen is a well-established industrial process for the manufacture of well-defined and high-quality tungsten powder [1,2,3,4,5,6,7]. The reaction rates at the different reduction steps are determined by the H 2O/H2 ratio in the vapour phase surrounding the oxide particles. The approach is to generate W O2(OH) via the CVT reaction and reduce it directly in the vapour phase to metallic W powder. The approach should lead to the formation of tungsten powder with particle sizes in the lower nm range, with no intergrowths and low agglomeration, which are not achievable by the established industrial processes. Only the results of the variation of the furnace temperature and temperature of the H 2O-bubbler are described in detail in this paper
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