Carbon monoxide hydrogenation using cobalt-manganese oxide catalysts: The influence of potassium as a promoter

The effect of potassium promoter on the CO hydrogenation reaction over precipitated cobalt manganese oxide catalysts was studied in a plug flow laboratory microreactor (220°C, 540 kPa, GHSV = 250 h −1, CO/H 2-ratio 1/1). On addition of low levels of the promoter (0.01-0.5%), significant increases in alkene/alkane ratios and higher hydrocarbon formation were observed, coupled with decreased methanation activity. Furthermore, the results showed that the presence of promoter favored the formation of longer chain alcohols, accompanied by decreases in the C 1OHC 4OH fraction. Maximum C 5+-hydrocarbon selectivity commensurate with high CO conversion and low methane formation was observed at between 0.1 and 0.2% potassium. C 2- and C 3-olefin selectivity was studied as a function of reactor operating conditions, and the selectivity was found to decrease with increasing temperature, pressure (constant conversion), reciprocal space velocity (space-time), and conversion. Bulk catalyst characterization by XRD and DSC suggested that the promoter did not influence the bulk structure of the catalyst. The potassium promoter concentrated on the surface as demonstrated from BET surface area measurements and XPS studies. BET measurements showed the catalyst surface area to initially decrease at low levels of promoter (0% potassium, 13.2 m 2/g; 0.01% potassium, 10.1 m 2/g), and then to increase with increased promoter loadings (1% potassium, 17.0 m 2/g). XPS studies on a 0.25% potassium promoted catalyst showed a surface aggregation of the potassium promoter from the bulk to occur during stages of calcination (6%), reduction (10%) and subsequent CO hydrogenation (2–4%). The data from the study suggest that the role of potassium is to modify the concentration of cobalt active sites and simultaneously to assist in the breakdown of the CO reactant.