Potassium promotion of Ni/Al 2O 3 catalysts

Abstract

Potassium promotion of Ni/Al 2O 3 catalysts was studied using steady-state CO hydrogenation, temperature-programmed reaction (TPR) of adsorbed CO with flowing H 2, and temperature-programmed desorption (TPD) of CO and H 2. Two series of catalysts were used; the potassium concentration was varied in one series and the nickel concentration in the other series. Potassium promotion decreased the specific activity for methane formation from both CO adsorbed on nickel and the H CO complex adsorbed on the Al 2O 3, and it increased the CO dissociation rate on nickel. In addition, potassium inhibited the transfer of CO from nickel to Al 2O 3 and poisoned the adsorption sites on Al 2O 3. Much of the potassium was on the Al 2O 3 surface and thus the number of adsorption sites on nickel did not change significantly. Potassium decreased the steady-state rate of CO hydrogenation on Ni/Al 2O 3 catalysts, but the decrease was much smaller than seen on Ni/SiO 2 catalysts because much of the potassium was on the Al 2O 3. Potassium increased selectivity to olefins and higher hydrocarbons. Because potassium was on both the support and the nickel, the K:(Ni+Al 2O 3) ratio appears to be a better measure of the catalytic activity than the K:Ni ratio. Good agreement was obtained between steady-state kinetics and TPR.