The role of Zr as promoter in the CoZr catalysts for Fischer-Tropsch synthesis

It is reported that Zirconium (Zr) is an effective promoter of cobalt-based catalysts for Fisher-Tropsch synthesis (FTS). Numerous studies have been conducted to explore the nature of the promotion. However, many of these studies focus on oxide-supported cobalt-based catalysts, in which the support effects may have partially enhanced or masked the influences of Zr. In the present work, CoZr catalysts without oxide support have been prepared across a wide range of Zr/Co molar ratio from 0.04 to 0.33 to investigate the nature of the Zr promotion. Comparing with the nano-Co3O4, the promoted catalysts exhibit excellent performance. The reaction temperature required to convert the same amount of syngas on the same amount of cobalt has decreased by about 40 °C. The selectivity of C5+ has increased from 70∼81% to over 85% at the same CO conversion, and even higher than 90% on the CoZr catalysts with high Zr/Co ratio. Evidences from multiple techniques, including in situ XPS, CO-DRIFTs, CO-TPD, NH3-TPD, Py-IR, HRTEM, Raman and XRD, shows that Zr tends to occupy the coordinately unsaturated sites of nano-Co3O4. After reduction, Zr enters the lattice of Co metal, leading to the lattice expansion. Benefitting from the Zr induced structural changes of the Co metal, CO adsorption and dissociation on the CoZr catalysts have been enhanced. Besides, due to the strong interaction between Co and Zr, the growth of the Co metal crystal during H2 reduction and O atoms adsorption on the reduced catalysts surface are inhibited significantly, which make more Co atoms available during FTS reaction. As a result, the catalytic performances are improved obviously.