Carbon−Carbon Bond Formation on Model Titanium Oxide Surfaces: Identification of Surface Reaction Intermediates by High-Resolution Electron Energy Loss Spectroscopy

Abstract

The interaction of CH 2 O with perfect and defective TiO 2 (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 TiO 2 (110) surface to CH 2 O at 100 K leads to the formation of physisorbed CH 2 O and to polymerization of CH 2 O, yielding paraformaldehyde. The latter is bound to the 5-fold coordinated surface Ti atoms and is found to decompose and release CH 2 O at about 270 K. On the defective TiO 2 (110) surface, CH 2 O adsorbs more strongly on oxygen vacancy sites, ultimately forming a diolate (-OCH 2 CH 2 O-) 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.

Tracking Site-Specific C–C Coupling of Formaldehyde Molecules on Rutile TiO2(110)

“…We have also considered the possibility of paraformaldehyde, which was suggested as the dominant species from HREELS study on perfect surfaces at low temperature (110 K). 8 The diolate species that we observe in STM images is formed from one V O -bound CH 2 O and one Tibound CH 2 O. Thus, it is unlikely to be the paraformaldehyde identified on perfect surfaces.…”

“…9,13,20,21 Experimentally, temperature programed desorption (TPD) results show that formaldehyde adsorbs more strongly on bridge-bonded oxygen vacancy (V O ) sites and undergoes the carbon-carbon bond coupling to produce C 2 H 4 via a diolate intermediate (−OCH 2 CH 2 O−) identified by high resolution electron energy loss spectroscopy (HREELS). 8 The formation of C 2 H 4 is further supported by other groups via X-ray photoemission spectroscopy (XPS) and TPD studies. 13,22 Similarly, reductive coupling of acetaldehyde to butene occurs preferentially on reduced TiO 2 (110).…”

Vibrational spectroscopic studies on pure and metal‐covered metal oxide surfaces

“…On the basis of the HREELS data for r-TiO 2 (110) and UHV-FTIRS data for r-TiO 2 nanoparticles, this reaction occurs only at surface O vacancy sites forming a diolate species (-OCH 2 CH 2 O-). Upon heating to about 600 K, this species undergoes deoxygenation yielding ethylene as product [68,70].…”

Formamide reactions on rutile TiO2(011) surface

“…Numerous important reactions have been seen on their surfaces, including dehydration of alcohols and carboxylic acids, oxidation/ reduction and carbon-carbon bond forming reactions with simple organic molecules through condensation and carbonyl coupling reactions [5]. TiO 2 has also been linked to chemical processes relevant to the beginning of life [6,7], where, under photon irradiation, nucleic acid bases were formed from formamide (NH 2 CHO) as well as HCN.…”