REACTIONS OF FORMIC-ACID AND FORMALDEHYDE ON RH(111) AND RH(111)-(2X2)O SURFACES

HREELS was used to study the adsorption and decomposition of formic acid and formaldehyde on the Rh(111) surface. Vibrational spectra showed that formic acid adsorbed on the clean surface via the lone pair electrons on the hydroxyl oxygen. The softening of the vibrational modes of the O-H bond also indicated hydrogen bonding among the molecules in the adlayer. At ca. 140 K a formate intermediate was formed, fingerprinted by its strong O-C-O vibrational modes. This formate intermediate decomposed to carbon dioxide, carbon monoxide, hydrogen, and water via two channels, with CO2 peaks at 235 and 275 K in TPD experiments. The addition of 1/4 monolayer of oxygen atoms to the Rh(111) surface shifted the formate decomposition temperature to 330 K, as previously reported. On the clean Rh(111) surface formaldehyde dehydrogenated completely to adsorbed carbon monoxide and hydrogen atoms below 130 K. Paraformaldehyde was identified in vibrational spectra obtained after formaldehyde adsorption on the clean surface at high dosing rates. Exposure of the oxygen-predosed surface to formaldehyde resulted in the formation of adsorbed paraformaldehyde, carbon monoxide, formate and eta-1-formaldehyde, but the major carbon-containing desorption product was carbon dioxide. This CO2 was formed via three different channels: formate decomposition at 320 K, CO and atomic oxygen direct reaction at 400 K, and oxidation of surface carbon at 530-1000 K.