A Surface Science Investigation of the Role of Potassium Promoters in Nickel Catalysts for Co ~drogenation *

The role of potassium promoters in model Ni(100) catalysts for CO hydrogenation has been studied. High-pressure kinetic measurements of the H, +CO reaction on Ni(lOO) containing well-controlled submonolayer quantities of potassium adatoms have been combined with detailed surface analysis performed before and after reaction. Potassium addition decreases the steady-state rate of methane formation and increases that for higher hydrocarbons relative to clean Ni(lOO). These same results are reported for supported, high-surface-area Ni catalysts, indicating that metal/support interactions are not necessary in achieving the promoter effect. The activation energy for methanation ( 25 kcal mole‘) does not depend upon potassium coverage, suggesting that K changes neither the reaction mechanism nor the rate-limiting step. Surface carbide, a vital reaction intermediate, increases sharply in coverage upon the addition of 0.10 monolayer potassium. This is shown to result from a marked decrease in the activation energy for CO dissociation effected by potassium. The catalyst activity and selectivity are discussed in light of these results.