Complex CO–potassium interactions on Cu(100): An electron energy loss, thermal desorption, and work function study

Abstract

A detailed examination of the adsorption states of carbon monoxide on a potassium-dosed Cu(100) surface is reported. High resolution electron energy loss spectroscopy (EELS) studies on surfaces with a variety of potassium precoverages indicate the presence of at least seven different C–O vibrational modes ranging in frequency from 900 to 2090 cm−1 with as many as five modes observed simultaneously. Thermal desorption spectroscopy (TDS) demonstrates at least five states of CO coadsorbed with potassium with desorption maxima occuring over a range from 120 to 600 K. Low resolution EELS and work function changes are discussed as well. The complexity of the high resolution EELS and TDS measurements precludes models involving only one or two types of potassium–CO interactions. In this system we suggest that potassium influences CO adsorption by, (1) indirectly increasing 2π* backbonding, (2) indirectly changing bonding sites, and (3) effecting a direct interaction of adsorbed CO with one, two, or possibly three neighboring potassium adatoms. Although we find no conclusive evidence for the reductive coupling of carbon monoxide to form discrete (CO)n–K polyhedral complexes at high potassium surface coverages, we cannot rule out the formation of such species.