K-doping of Co/Al2O3 low temperature Fischer–Tropsch catalysts

The effect of potassium doping on Co/Al2O3 Fischer–Tropsch catalyst has been investigated using in situ IR spectroscopic measurements and catalytic activity tests in flow reactor. The data confirm the inhibiting effect of K doping on reducibility and catalytic activity, together with a stabilization of the catalytic activity, a reduction of methane selectivity and a slight increase of CO2 selectivity. CO adsorbs over the Co/Al2O3 catalyst forming on-top surface carbonyls, responsible for a Infrared CO stretching band at a position more typical of fcc 1 0 0 face than of hcp 0 0 0 1 face. In the presence of hydrogen CO adsorption shifts in part from terminal to bridging sites. Potassium doping significantly increases the electron donating ability of cobalt metal particles, as evidenced by the shift down of 15 cm−1 of the CO stretching frequency of adsorbed carbon monoxide. CO adsorbed on surface defects is also evident. After CO adsorption at 250 °C even lower frequency carbonyls are observed only on the K-doped catalyst (2000 and 1825 cm−1) providing evidence of a stronger interaction of CO with the surface. At 250 °C in the presence low pressure of CO + H2, methane, ethylene and other lower olefins form in the IR cell. The ethylene/methane molar ratio produced over the K-doped catalyst is definitely higher than over the K-free sample, suggesting that potassium favours the first chain growth step in the Fischer–Tropsch process thus allowing decreased selectivity to methane. Reasons for the increased catalyst stability are briefly discussed.