Study On Phase Transitions And Kinetic Mechanism During Reduction Of Molybdenum Oxides

Molybdenum and its alloys are the important materials for powder metallurgy. Mo has the advantages of high melting point, large hardness, high modulus of elasticity, low expansion coefficient, high thermal and electrical conductivities as well as large corrosion resistance. Therefore, Mo products have been widely applied. China has the abundant Mo resource, but has fallen behind many countries like Germany, Japan and Austria in producing metallic Mo powders. The industrial production of Mo still relies on the experience; The mechanism and kinetic behavior of hydrogen reduction of MoO3 to MoO2 are a long debated and still unresolved question; There has so far been no detailed paper to report the morphological evolutions during the reduction of MoO3 and MoO2; The kinetic parameters are still not obtained; No detailed paper has been published to report the reduction of molybdenum oxides by other reductant such as CO.Therefore, the main aim of this study is to further solve these problems using FactSage software, thermo-gravimetric (TG) analyzer. X-ray Diffraction (XRD). Scanning Electron Microscope (SEM) and Brunauer-Emmett-Teller (BET) method. The obtained conclusions are as follows:1) At 678 K, no intermediate product was observed. Above 713 K, an intermediate product, Mo4O11, was formed, while other phases (Mo8O23, Mo9O26) were not detected. The formation of Mo4O11 was found to obey the consecutive mechanism instead of other mechanisms. The dual reactions model was established. The rate controlling step for the reduction of MoO3 to Mo4O11 is the chemical reaction, while that for the reaction of Mo4O11 to MoO2 changes with temperatures:in the temperature range of 735 to 773 K, it obeys the nucleation and growth model; in the range of 797 to 829 K, it obeys the diffusion model. The kinetic parameters for every reaction were obtained by using the developed model. The morphological evolutions during reduction of MoO3 to MoO2 are also investigated:in the temperature range from 713 to 733 K. MoO3 was firstly reduced to large spherical or oval Mo4O11 grains then to spherical or oval MoO2 ones; While in the temperature range from 793 to 829 K, most of MoO3 particles were firstly reduced to large plate-shaped Mo4O11 ones, subsequently to small plate-shaped MoO2 ones.2) Mo4O11 was used as the raw material to investigate the reduction mechanism and kinetics. In low tamperatures (791 K,816 K,831 K), the reduction curves can be separated to three periods (incubation period, acceleration period and deceleration period) and the lower the temperature is, the longer the incubation period will be. For the reduction of MoO2 to Mo:at 791 K,816 K and 831 K three periods (incubation period, acceleration period and deceleration period) also appear, however, at high temperatures, the reduction extent vs time presents a linear relationship. In the reduction of Mo4O11 to MoO2, the reduction mechanism at lower temperatures and higher temperatures were nucleation and growth and chemical reaction controlling mechnism, respectively. In the reduction of MoO2 to Mo, the reduction obeys chemical reaction mechanism, except in the beginning at low temperatures, at which it obeys nucleation and growth mechanism. The morphological evolutions are:large Mo4O11 particles were first reduced to small plate-shaped MoO2, and then reduced to Mo; The final product Mo crystals was kept the same shape as MoO2 ones.3) The reduction of MoO2 by hydrogen was studied under both isothermal and non-isothermal conditions. The kinetic model has been proposed, which incorporates various factors such as time, temperature, partial hydrogen content, etc. Good agreements have been achieved between the experimentally measured and theoretically calculated results. It was found that the hydrogen reduction of MoO2 was controlled by the chemical reaction at the reaction interface with the apparent active energy of 90.6-92.5kJ/mol. At low temperatures (blow 1173 K), the reduction proceeded through pseudomorphic transformation mechanism, and the product of molybdenum kept the same platelet shape as the initial MoO2. However, at a high temperature of 1393 K, the produced molybdenum was formed as perfect spherical-shaped particles, which indicated that the reduction obeyed the chemical vapor transport (CVT) mechanism. When the temperature is between 1173 K and 1393 K, both mechaisms occur. The influence of dew point in the atmosphere on the morphological changes of Mo powders was also important. It was beneficial for the nucleation and growth of Mo particles following CVT mechanism with increasing the dew point of hydrogen. However, the influence of hydrogen content (25% H2-100% H2) on the morphological changes could be neglected at low temperature or high temperature when one reduction mechanism was dominated.4) A new and simple route for preparing Mo2C by reducing MoO2 powders with CO was also proposed in this work. The reduction mechanism at lower temperatures was quite different from that at higher temperatures under the conditions tested:at low temperatures. MoO2 was reduced and carburized to Mo2C in one step; At higher temperatures, MoO2 was first reduced to metallic Mo, and then metallic Mo was carburized to Mo2C. The grain size and the surface area for Mo2C in this study are similar to those obtained in the reference. Therefore, reduction of MoO2 by CO was also a possible way to produce Mo2C catalyst.