Effect of potassium and copper promoters on reduction behavior of precipitated iron catalysts

Abstract

Precipitated iron catalysts were characterized using temperature-programmed methods, isothermal reduction, and X-ray photoelectron spectroscopy to study the influence of copper and potassium promoters on reduction behavior and catalytic properties for carbon monoxide hydrogenation. TPR results showed that the promotional effect of copper on the reduction of iron increased with increasing copper content up to ~3 wt% Cu. The time required for completion of iron reduction at 300°C in either H2 or CO decreased with increasing copper content. The presence of potassium influenced the reducibility of the precipitated iron catalysts both with and without copper. A possible blockage of nucleation sites by potassium may inhibit the reduction of iron in H2. However, the time required for completion of iron reduction in CO decreased in the presence of potassium. The inhibition effect of SiO2 and AI2O3 supports on the reduction of iron increased with increasing support content. The inhibition effect was less pronounced for the alumina support than for the silica support. XPS results showed that H2 and CO treatments of the precipitated iron catalysts at 300°C resulted in differences in the surface composition of catalysts. The surfaces of H2-reduced samples were largely covered by a layer of iron oxide; however, the surfaces of the CO-reduced samples possessed zero-valent iron. A marked enrichment of potassium was found on the surfaces of H2-reduced samples compared to those of CO-reduced samples. No significant change in catalytic selectivity for CO hydrogenation at 250°C was observed in the presence of copper promoter. However, significant increases in olefin/paraffin ratios and hydrocarbon product chain lengths were observed for the potassium-promoted iron catalysts in the CO hydrogenation results. CO2 production was higher for H2-reduced catalysts than for CO-reduced catalysts. Lower percentages of methane formation and higher hydrocarbon chain lengths were observed for H2-reduced iron catalysts compared to those observed for CO-reduced catalysts.