F-T合成沉淀铁催化剂中助剂Si作用的穆斯堡尔谱研究

MOSSBAUER SPECTROSCOPIC INVESTIGATION OF THE PROMOTER EFFECTS OF SILICON ON PRECIPITATED IRON CATALYST FOR F-T SYNTHESIS

采用穆斯堡尔谱方法研究了用于F-T合成的Fe-Cu-K-Si沉淀铁催化剂中助剂Si的作用。研究结果表明,Si明显起着分散和稳定铁晶粒的作用,此外,铁与高分散Si之间存在着相互作用,从而导致催化剂在灼烧,氢还原及合成气反应后铁的化学形态的变化。

Mossbauer spectroscopy was used to study the promoter effects of silicon on precipitated iron catalyst for F-T synthesis. It was found that the promoter Si strongly affects the Fe particle sizes. In addition, some evidencefor Fe-Si interaction was observed. These interactions lead to the change of chemical state of Fe at different stages of calcination reduction and carburization.At room temperature the calcined Fe-Cu-K. catalyst mainly consists α-Fe_2O_3 of larger particles as shown by a six-line ferromagnetically split Mossbauer spectra. But the Fe-Cu-K-Si catalyst contains 60% superparamagnetic Fe^(3+) and 22% Fe_3O_4. The ratio of interface Fe^(3+) and bulk Fe in this catalyst was 1.30. The catalyst was shown to have significantly smaller particles of Fe.After reduction and carburization in hydrogen and synthesis gas (H_2/CO=2) the Fe phase of Fe-Cu-K catalyst was completely changed to α-Fe and stable Hugg's carbide X-Fe_5C_2. But the degree of reduction of Fe oxide in the Fe-Cu-K-Si catalyst was strangly influenced by the adding of Si.After reduction under the same condition, the catalyst cantains about 14% α-Fe, 11%Fe_3O_4, 4.5%Fe^(2+) and 69.9% of Fe^(3+), the ratio of interface Fe^(3+) and bulk Fe^(3+) was reduced to 0.43. This fact showns that the Fe crystals grow during the process of hydrogen reduction. The carburization of Fe-Cu-K-Si catalyst in synthesis gas converted about 75% of the Fe to X-Fe_2C_5 with 25% of Fe^(3+) remaining.