| The objective of this research is to study the nitrogen effect on silica supported iron catalyst in the Fischer-Tropsch synthesis and to compare the effect with potassium promotion. Four silica supported catalysts were utilized in the Fischer-Tropsch process at pressures of 1, 7.8 and 14 atmospheres, and at 250(DEGREES)C. The catalysts used were reduced iron (Fe-carbide), reduced and nitrided iron (Fe-nitride), potassium-promoted reduced iron (Fe-K-carbide) and potassium-promoted reduced and nitrided iron (Fe-K-nitride). Both Fe-carbide and Fe-nitride catalysts were characterized using x-ray diffraction and Mossbauer spectroscopy. No obvious changes in particle size after 12 hours of nitridation at 490(DEGREES)C were observed. In the first 3 hours of reaction in the syngas environment, the Fe-nitride catalyst became increasingly iron rich, indicating that the nitrogen atoms removed by reduction were not simultaneously replaced by carbon atoms. Mossbauer spectroscopy and reaction studies showed that the Fe-nitride catalyst became stable after 18 and 25 hours, respectively, in the syngas environment. The composition of the stabilized Fe-nitride catalyst was Fe(,2.18)C(,0.87)N(,0.13) from Mossbauer spectra analysis and Fe(,2.18)C(,0.75)N(,0.25) from the x-ray diffraction method.; Synthesis reactions indicated that Fe-carbide catalyst was more active at lower pressures and less active at higher pressures than Fe-nitride. Moreover, it was found that potassium promoted catalysts were less active than catalysts without potassium. Both Fe-nitride and potassium promoted catalysts showed higher olefin selectivities, higher water-gas shift activity, lower alcohol yields and larger proportion of higher molecular weight products than their unpromoted counterparts.; The Schulz-Flory product distribution plots are presented for all catalysts at different pressures and conversion levels. The significance of the incorporation of alcohols into the Schulz-Flory plot is discussed and a mechanism for alcohol formation is proposed.; The effect of nitrogen on the catalytic reaction is proposed based on the donor model theory--nitrogen atoms donate electrons to the iron d band. From the product distribution of Fe-nitride and potassium promoted catalyst, it is concluded that nitrogen and potassium have a similar promoting function on the Fischer-Tropsch reactions and that an additive effect is observed in Fe-K-nitride catalyst. |
