Direct measurement of potassium-promoted change in heat of adsorption of CO on Ni{100}

Abstract

THE action of alkali metals and their oxides as promoters for many catalytic reactions has been known for nearly a century¹. Alkali promotion of carbon monoxide reactivity, for example, forms part of the industrially important synthesis of hydrocarbons, and is thought to follow from a weakening of the internal C–O bond resulting from an enhanced binding interaction between the CO molecule and the catalyst². Several spectroscopic techniques have been used to examine this effect for CO adsorption on single-crystal metal surfaces^2–6, but bond energy changes can only be inferred indirectly from these techniques. Here we report direct measurements, using a recently designed single-crystal adsorption microcalorimeter^7,8, of the change in CO adsorption heat on a Ni{100} surface for a range of potassium precoverages. We find that the effect of promotion is unexpectedly large, with the adsorption heat increasing from a clean-surface value of 124 kJ mol⁻¹ to ∼310 kJ mol⁻¹ at high potassium precoverages. We attribute this effect to a combination of stronger CO binding to the surface and increased ionization of the potassium adatoms.