氧化钼在CH_4/H_2气氛中还原碳化机理研究

Study on the carburizing mechanism of molybdenum trioxide in CH_4/H_2 mixture

采用TG-DTA技术研究了MoO3在CH4/H2气氛中的还原碳化行为,考察了程序升温速率和还原碳化终点温度对氧化钼还原碳化行为的影响,并探索适宜的还原碳化条件。结果表明,在1℃/min的程序升温条件下,MoO3在CH4/H2气氛中经三段失重过程被还原碳化为Mo2C,相应的反应历程为MoO3→MoO2→MoOxCy→Mo2C,适宜的还原碳化终点温度为675℃;程序升温速率升至2℃/min以上时,MoO3在CH4/H2气氛中的反应历程为MoO3→MoO2→Mo+MoOxCy→Mo2C,且随程序升温速率的增大,第二段失重过程中金属Mo的生成量增大,还原碳化反应的始、终点温度升高。提高还原碳化终点温度,MoO3在CH4/H2气氛中的还原碳化反应规律相同,但过高的还原碳化温度会引起有机烃类分解生炭反应的发生,沉积在催化剂的表面,导致制备的碳化钼催化剂表面积炭增多,比表面积降低,从而引起催化活性的下降。

The carburizing reaction of molybdenum trioxide was investigated in CH4/H2 mixture using in-situ TG- DTA. The effects of ramping rate and carburization temperature on the carburizing reaction of molybdenum trioxide were also studied. The results indicate that molybdenum carbide can be prepared by temperature-programmed reaction （TPR） to carburize molybdenum trioxide through the pathway of MoO3 → MoO2 → MoOx Cy→ Mo2 C under CH4/H2 atmosphere at ramping rate of 1 ℃/min with the optimal carburization temperature of 675 ℃. When the ramping rate is above 2 ℃/min ,the carburizing reaction process of molybdenum trioxide changes to MoO3℃MoO2℃ Mo ＋ MoOxCy℃Mo2C. In the mean time, the amount of Mo element generated in the second weight loss stage increases with the increase of the ramping rate. The beginning and the finishing temperature of carburizing reaction also increase with in the increase of the ramping rate. Despite the fact that the carburizing reaction pathway of MoO3 in CH4/H2 mixture is similar in a range of carburizing temperatures, high reaction temperature could lead to the coking reaction of the hydrocarbon and deposit of free carbon on the catalyst surface, which could induce the BET surface area decrease and hence reduce the catalytic activity of the Mo2C.