Potassium promoted C–O bond weakening on Ru(001). I. Through-metal interaction at low potassium precoverage

Abstract

The interaction of carbon monoxide and submonolayer coverages of potassium on Ru(001) has been investigated with high resolution electron energy loss spectroscopy,LEED, Auger electron spectroscopy, thermal desorption, and work function measurements. The adsorption of potassium on ruthenium at submonolayer coverages (θ k ≤0.15) is characterized by ionic, mutually repulsive potassium atoms as evidenced by a strong work function decrease (−4.3 eV), various LEED patterns and a large desorption energy (65 kcal/mol) at low coverage. The adsorption of CO on a potassium precovered surface (θ k =0.10) is nondissociative and reversible with an initial increase in the activation energy for desorption from 40 kcal/mol for clean Ru(001) to 50 kcal/mol on the potassium precovered surface. The C–O bond is anomalously weak as evidenced from vibrational spectroscopy (EELS), where C–O stretch frequencies in the range of 1400 to 1970 cm− 1 are found. Observation of the first vibrational overtone indicates a strong anharmonicity of the C–O potential well, similar to that of molecularly adsorbed oxygen on Pt(111), thus indicating extremely weak C–O bonds. This is further supported by thermal desorption experiments, where complete isotopic scrambling between 1 2C1 6O and 1 3C1 8O is observed at low CO coverage. Vibrational, work function, and thermal desorption data support a through‐metal interaction, where the potassium induced charge redistribution in the metal surface inreases backdonation of metal electrons into the antibonding 2π*‐MO of CO, thus causing a C–O bond weakening and an increase in the M–CO bond strength. The observation of multiple states in the vibrational spectra indicate a finite range of the K–CO interaction. Both strength and range of the interaction are found to vary with the ratio of CO:K coverages. At low CO coverage, a strong, short‐range interaction between K and CO is observed which causes the CO to adsorb in a side‐on bonding mode. With increasing CO coverage (at constant θ k ) a weakening of the K–CO interaction occurs, which results in CO molecules adsorbed in metal bridge sites, with their molecular axis oriented presumably perpendicular to the surface. Structure models proposed for various potassium and CO coverages suggest a long range for this weak interaction.