Interaction of Formaldehyde with the Rutile TiO2(110) Surface: A Combined Experimental and Theoretical Study

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. Exposure to formaldehyde at 65 K leads to the formation of CH2O multilayers, which desorb completely upon heating to 120 K. After smaller exposures at low temperatures (45-65 K), STM allowed us to identify individual, isolated CH2O monomers. The, theoretical results indicate that these monomers are bound to the surface Ti-5c sites via sigma-donation and adopt a tilted geometry. Upon heating, the CH2O monomers polymerize to form paraformaldehyde (polyoxymethylene, POM) chains, oriented primarily along the Ti-5c rows ([001] direction). Upon further heating, POM is found to decompose around 250 K, releasing CH2O into the gas phase. In addition, dioxymethylene (DOM) was detected as minority species formed via reaction of Ti-5c-bound CH2O with surface O atoms. For all substrate species, the characteristic IR vibrations were measured. Because these are the first IRRAS data for TiO2 macroscopic single crystals exposed to formaldehyde, we have performed DFT calculations to aid the assignment of the various bands.