Spectroscopic study on the formation of CO2- on K-promoted Mo2C/Mo(100) surface

The adsorption of CO2 on clean and potassium-covered Mo2C/Mo(100) surface w as studied by high resolution electron energy loss spectroscopy (HREELS), t hermal desorption spectroscopy (TPD), work function measurements and X-ray photoelectron spectroscopy (XPS). The adsorption of CO2 on Mo2C/Mo(100) cau sed no measurable changes in the work function of Mo2C. CO2 adsorbed weakly on K-free Mo2C at 90 K, producing vibration features at 660, 1260-1340 and 2349 cm(-1). It desorbed in one peak with T-p=190 K. The deposition of pot assium on Mo2C resulted in a maximum work function decrease of 3.3 eV. The bonding strength of potassium sensitively depended on its surface concentra tion. At low coverage it desorbed with T-p=850 K, this peak temperature was 525 K at monolayer, and 355 K at multilayer. The presence of potassium ada toms greatly increased the binding energy of CO2 and led to the formation o f CO2- anion radical characterized by losses at 750-785, 1220-1250 and 1580 -1670 cm(-1). The activated CO2 dissociated to adsorbed CO and O at low pot assium coverage, even at 90-150 K, and disproportionated into adsorbed CO a nd CO3 at and above potassium monolayer. Both compounds were strongly stabi lized on the surface by potassium adatoms and were released from the surfac e coincidentally with a peak temperature of 665-686 K. (C) 2000 Elsevier Sc ience B.V. All rights reserved.