DIRECT MEASUREMENT OF POTASSIUM-PROMOTED CHANGE IN HEAT OF ADSORPTION OF CO ON NI(100)

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 surfaces2-6, but bond energy changes can only be inferred indirectly from these techniques. Here we report direct measurements, using a recently designed single-crystal adsorption microcalorimeter7,8, 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 approximately 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.