Kinetics and morphological evolution mechanism of WO3 during non-isothermal hydrogen reduction

Abstract

The hydrogen reduction kinetics of tungsten trioxide (WO3) was investigated via non-isothermal thermogravimetric analysis. Under the local gas–solid reduction conditions, the particle morphology of tungsten powders was found to be consistent with that of raw material WO3. The removal of oxygen from tungsten oxide during hydrogen reduction led to the formation of porous structures between the reduced particles, which were obviously different from the polyhedral single-crystal configuration of tungsten powders obtained via chemical vapor deposition. Moreover, the two-stage hydrogen reduction mechanisms of WO3 under the local gas–solid reduction conditions can be described using the composite autocatalytic function. The activation energies of the first and second stages of the hydrogen reduction of WO3 were determined to be 121 and 135 kJ/mol, respectively.

1. Introduction

Tungsten possesses outstanding properties such as high melting point, excellent hardness, high density, strong corrosion and wear resistance, and good electrical and thermal conductivity [1−3]. For this reason, tungsten is a strategic rare metal that plays an indispensable role in cemented carbides and electronics, as well as in chemical, medical, military, and aerospace industries [4,5]. The particle morphology of tungsten powder is an important characteristic that significantly affects its application scenarios [6−8], which depends on the reaction kinetics during the preparation process [9−11]. In industrial production of tungsten powder, the hydrogen reduction of tungsten oxides has become the mainstream method owing to the mature technic, simple equipment, and cost effectiveness of the process [12−14].

The kinetics analysis and mechanism of the hydrogen reduction of tungsten oxides are the important criteria for gaining insight into the intrinsic laws of the reduction course, which can ensure production efficiency and reduce energy consumption. For instance, SONG et al [15] proposed three mechanisms for the hydrogen reduction of tungsten oxide, namely, the adsorption-autocatalytic reaction, solid-state oxygen migration, and chemical vapor deposition. The current kinetic models describing the redox reactions, including Jander, Ginstling–Brounshtein, and Valensi–Carter models have been shown to provide guidance for practical industrial production [15]. JIANG et al [16] found two reaction mechanisms, namely solid-state oxygen migration and chemical vapor deposition, which were involved in the hydrogen reduction of tungsten oxides. In particular, geometric contraction model is a frequently applied reaction kinetic model for describing the hydrogen reduction of tungsten oxides based on the gas–solid reaction type.

To date, several studies on the morphology control of tungsten powder and kinetics of hydrogen reduction of tungsten oxide have been reported separately [17,18]. However, the correlation between the morphological evolution of tungsten powder and the kinetics of hydrogen reduction process remains unclear. Therefore, in this study, the non-isothermal kinetic study was carried out on the hydrogen reduction of tungsten trioxides (WO3) by using Netzsch Kinetics Neo software. Special attention was paid to gaining an in-depth understanding of the intrinsic laws of the reduction reaction by acquiring the kinetic parameters and reaction mechanisms of the hydrogen reduction of tungsten oxides. These results can supplement the existing knowledge and provide theoretical guidance for industrial production.