TEMPERATURE-PROGRAMMED STUDIES OF ALKALI-PROMOTED NICKEL/SILICON DIOXIDE CATALYSTS (FISCHER-TROPSCH, POTASSIUM)

The role of potassium as an alkali promoter in the methanation and Fischer-Tropsch synthesis has been studied. Promoted catalysts were prepared in a manner such that the potassium present on the catalytic metal could be distinguished from that present on the support.; Potassium addition had the effect of decreasing the steady-state rate of methane formation as well as higher hydrocarbon formation. The activation energy for methanation increased upon potassium promotion. However, significant increases in the selectivity for higher hydrocarbon and olefin formation occurred as the potassium coverage of the nickel metal was increased.; The role of potassium in bringing about the above effects was investigated through temperature-programmed studies. Temperature-programmed desorption studies showed that the bonding strength of hydrogen to nickel decreased upon promotion. Also, the activation energy for CO dissociation decreased and the bonding strength of active surface carbon to nickel increased upon promotion. Temperature-programmed surface reaction studies indicated a depression of the rate of hydrogenation of active surface carbon as a result of the addition of potassium. It was therefore concluded that active surface carbon hydrogenation was rate-determining for methanation on nickel. The active surface carbon species could be transformed into an inactive carbon species, which deactivated the nickel catalyst. Potassium promotion slowed the transformation of active to inactive carbon, thereby prolonging the effective lifetime of the Ni/SiO(,2) catalyst.