A surface science investigation of the role of potassium promoters in nickel catalysts for CO hydrogenation

Abstract

The role of potassium promoters in model Ni(100) catalysts for CO hydrogenation has been studied. High-pressure kinetic measurements of the H 2 + CO reaction on Ni(100) 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(100). 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 −1) 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.