Carbon-carbon bond formation on model titanium oxide surfaces: Identification of surface reaction intermediates by high-resolution electron energy loss spectroscopy

The interaction of CH2O with perfect and defective TiO2(110) surfaces (produced by overannealing and Ar ion sputtering methods) was studied by thermal desorption spectroscopy, high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations. Exposing the perfect TiO2(110) surface to CH2O at 100 K leads to the formation of physisorbed CH2O and to polymerization of CH2O, yielding paraformaldehyde. The latter is bound to the 5-fold coordinated surface Ti atoms and is found to decompose and release CH2O at about 270 K. On the defective TiO2(110) surface, CH2O adsorbs more strongly on oxygen vacancy sites, ultimately forming a diolate (-OCH2CH2O-) species, as demonstrated by HREELS. The assignment of the vibrational frequencies was aided by theoretical calculations on the DFT-B3LYP level. Upon heating to higher temperatures, this species undergoes deoxygenation, resulting in ethylene formation.