The effect of alkali and alkaline earth elements on cobalt based Fischer–Tropsch catalysts

A CoRe catalyst supported on γ-Al2O3 was impregnated with 25–20 000 ppm alkali or alkaline earth elements (Li, Na, K, and Ca). For samples containing less than 1000 ppm impurities catalyst characterization showed that the dispersion as measured by H2-chemisorption was constant (∼7.9%) and that reduction temperatures increased slightly 5–15 K according to temperature programmed reduction experiments. Microcalorimetric measurements with H2 and CO chemisorption did not reveal any significant change for the concentration up to 1000 ppm sodium. However, the catalytic activity at 483 K, 20 bar and H2/CO = 2.1 was 33–43% lower compared with the uncontaminated sample. For samples containing between 10 000 and 20 000 ppm sodium a moderate drop in dispersion and an increase in heats of CO and H2 chemisorption were observed. These samples showed significantly decreased activity in CO hydrogenation in SSITKA experiments (1 bar, 483 K). Our results are in agreement with the idea that small amounts of alkali, below the level where they can be detected by chemisorption, are present on the surface on locations of particular importance for the catalytic activity.