Absence of CO dissociation on Mo(112)

Abstract

We revisit the problem of CO adsorption and thermal dissociation on the Mo(112) surface by means of density-functional calculations of binding energies, local densities of states, and CO vibrational frequencies for various configurations of equilibrated adlayers. The bridge-on-row adsorption sites on the Mo(112) surface are found to be the most favorable and CO molecules will occupy less stable in-furrow sites only after the completing of the first monolayer. At low coverages, CO molecules are tilted by approximately 40 degrees with respect to the normal to the surface (the beta state), but with increasing coverage, due to lateral interactions, attain an upright orientation with the carbon end down (the alpha state). The tilting of CO results in a significant elongation of the C-O bond (to 1.20 A) and, consequently, the C-O stretching vibration frequency decreases to 1159 cm(-1). Nonetheless, the beta state cannot be attributed to the precursor to CO dissociation, because the estimated potential barrier for the dissociation (approximately 2.8 eV) substantially exceeds the chemisorption energy (2.1 eV), which makes the thermally induced CO dissociation on Mo improbable. With estimated chemisorption energies, Monte Carlo simulations have shown that the two-peak shape of TPD spectra can be explained without involving the CO dissociation. We predict also that the lack of dissociation can be detected in photoemission studies for CO on Mo(112) by the presence of the -23 and -7 eV peaks, characteristic of chemisorbed CO, and absence of the -18 and -5 eV peaks characteristic of adsorbed O atoms.