Preparation of Vanadium Nitride by Magnesiothermic Reduction of V 2 O 3 in Nitrogen Atmosphere
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VANADIUM nitride (VN) is regarded as a highly promising material, owing to its excellent physical properties such as extreme hardness, high melting point, high-temperature stability (8.1 9 106 K1), and low thermal conductivity (11.3 W m1 K1).[1–3] Considering these excellent properties, VN has been widely applied in various industrial fields, such as catalysis, superconducting devices, cutting tools.[4–6] Particularly, owing to its excellent activity and selectivity, VN has been used in catalytic hydroprocessing, including hydrodenitrogenation (HDN), hydrodesulfurization (HDS), and hydrodeoxygenation (HDO). Currently, VN is primarily produced by the vacuum carbonitrothermic reduction process. In this process, V2O3 is reduced with carbon under vacuum condition at 1373 K to 1773 K (1100 °C to 1500 °C) and then nitrided to produce VN. The reactions can be described as follows:
V2 O3 þ ð2x þ 3ÞC ! 2VCx þ 3COðx 95 wt pct) was purchased from Alfa Aesar, England. The average powder size of V2O3 and Mg were 2.492 and 135.702 lm, respectively. B. Experiments V2O3 powder, anhydrous MgCl2, and Mg metal powder were weighed and mixed uniformly. The MgCl2/V2O3 molar ratio was about 4, while the Mg/V molar ratio was 1.5, 2.25, or 3. Then, the mixture was placed into a Mo crucible, which was accommodated into the constant-temperature zone of a vertical tube furnace using MoSi2 rods as the heating elements. The furnace temperature was raised to the desired value [948 K to 1073 K (675 °C to 800 °C)] at a heating rate of 5 K/ min and held for 3 hours. After that, the sample was cooled down to room temperature gradually before being taken out from the furnace. During the entire process, N2 gas was introduced into the furnace with a flow rate of 500 mL/min. The schematic diagram of the experimental apparatus is described and shown in Figure 1. After high-temperature reaction, MgO was removed by leaching the as-produced sample in 1M HCl at 298 K (25 °C) for 3 hours. Then, the samples were
filtered and washed several times with deionized water. The wet powder was then dried in the oven and collected for detection. The flowchart of the process is described in Figure 2. During the whole process, except for the molten salt, only high-purity V2O3 (> 95 wt pct, with the other component of high-valence vanadium oxide) and Mg (> 99 wt pct) powders were used. Therefore, the carbon content of the final product should be very low. In order to study the distributions of VN and MgO particles, some samples only washed by deionized (DI) water were also studied. The temperature higher than the melting point of Mg (921 K) can enhance the deoxygenation kinetics. But, if the temperature was too high, it would lead to a large evaporation loss of Mg. MgCl2 salt was added to decrease the local temperature. Therefore, the temperature should be higher than the melting point of MgCl2 (991 K). In order to prepare VN powder at a low temperature, MgCl2-50 wt pct KCl eutectic salt with a melting point of about 760 K (487 °C) was used in some experiments, and the c
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