The adsorption and decomposition of formaldehyde and formic acid on the clean and modified Fe(100) surface

The adsorption and thermal reaction of formaldehyde and formic acid on the clean and modified Fe(100) surface is studied. CH₂O adsorbs molecularly on the clean Fe(100) surface without the formation of polymer (CH₂O)_x (paraformaldehyde) at exposures below 3.0 L. At 223 K, molecularly adsorbed CH₂O is hydrogenated to form methoxy and dehydrogenated to form surface CO. Coadsorption of H₂O and hydroxy species can oxidize this methoxy species to form formate. Methanol is observed as one of the desorption products for the coadsorption of H₂O or OH and CH₂O. The presence of adatomic C induces the formation of (CH₂O)_x polymers on the surface and changes the mechanism of CH₂O decomposition. The presence of adatomic O decreases CH₂O decomposition and increases molecular CH₂O desorption. Oxygen alters the reactivity of the surface but not the reaction pathway for CH₂O decomposition. At low exposures, formic acid adsorbs to produce a bidentate formate species on the Fe(100) surface at 100 K. A monodentate formate is observed at high coverage, which converts into a bidentate formate with higher symmetry at 253 K. This surface formate further decomposes to either desorb CO or form O_(ad) and C_(ad). On the c(2 × 2)-O and p(1 × 2)-OH surfaces, formic acid transfers carboxylic hydrogen to preadsorbed oxygen or hydroxyl to desorb H₂O during the thermal decomposition reaction.