PNNL

Abstract

The adsorption and reaction of formaldehyde (CH2O) on the oxidized rutile TiO2(110) surface were studied by temperature programmed desorption (TPD), scanning tunneling microscopy (STM), infrared reflection-absorption spectroscopy (IRRAS) and density functional theory (DFT) calculations. The experimental and theoretical data reveal the presence of various species depending on the temperature and coverage. After formaldehyde adsorption on TiO2(110) at 65 K, the multilayer CH2O was detected, which desorbs completely upon heating to 120 K. The isolated CH2O monomer was identified after submonolayer adsorption at low temperatures (45-65 K), in which CH2O is bound to the surface Ti5c sites via ?-donation and adopts a tilted geometry. With heating to higher temperatures the CH2O monomers remain stable up to 70 K and then undergo coupling reactions to form paraformaldehyde (polyoxymethylene, POM) at the Ti5c rows. The POM chain is oriented primarily along the [001] direction in a slightly disordered configuration. POM becomes the predominant species at 120 K and is decomposed releasing CH2O at about 250 K. In addition, dioxymethylene (DOM) was detected as minority species formed via reaction of Ti5c-bound CH2O with both neighboring O2c along the [1-10] direction and oxygen adatoms (Oad) at Ti5c sites along [001] on the oxidized TiO2(110) surface.