Effect of La3+Promotion of Co/SiO2on CO Hydrogenation

Abstract

A study of CO hydrogenation on La3+-promoted Co/SiO2catalysts (La/Co atomic ratios of 0–0.75) has been performed using both global rate measurements and steady-state isotopic transient kinetic analysis (SSITKA). Aqueous impregnation (the means of adding La3+) of Co/SiO2led to a significant decrease in the number of intermediates leading to methane during CO hydrogenation, but it did not alter the nature of the active sites. In a previous characterization paper (19) La3+promotion of Co/SiO2was shown to moderate the strong Co–support interactions brought about by aqueous impregnation of the Co/SiO2and to enhance the reducibility of the Co oxide. La3+addition to the Co/SiO2catalyst was found to lead to a significant increase in the activity for Fischer–Tropsch synthesis (on a per gram Co basis). The SSITKA results showed that the concentration of active intermediates leading to methane increased with increase in La/Co ratio. However, the TOF, the pseudo-first-order rate constant, the reactivity distribution function, and the activation energy were not affected by La3+addition. Thus, it can be concluded that La3+did not change the nature of the active sites, just their concentration. This increase in the concentration of active sites follows exactly the increase in surface Co0. La3+promotion (La/Co≥0.1) also caused the chain growth probability and the olefin/paraffin ratio to increase significantly at 220°C and for H2/CO=2.