Photooxidation of ethanol and 2-propanol on TiO2(110): evidence for methyl radical ejection

Abstract

The photooxidation of ethanol and 2-propanol was studied under UHV conditions on a single crystal TiO2(110) surface using a combination of temperature programmed desorption (TPD) and pump-probe laser ionization techniques. Previous studies of these reactions have shown that the first step involves photocatalytic dehydrogenation to either an acetaldehyde or acetone intermediate. In this work, we show that when adsorbed alcohols are irradiated with UV light in the presence of molecular oxygen, methyl radicals are ejected from the surface. Furthermore, it is shown that these radicals possess kinetic energy distributions which are remarkably similar to those measured for the photooxidation of acetaldehyde and acetone. This result suggests that methyl radicals are produced during a second photocatalytic step which involves photooxidation of the aldehyde/ketone intermediates.

Unravelling Site-Specific Photo-Reactions of Ethanol on Rutile TiO2(110)

“…In surface science, where single crystalline surfaces can be studied at ultrahigh vacuum (UHV) conditions, particular focus has been on the interaction between EtOH and the rutile TiO 2 (110)–(1 × 1) surface 27 28 29 30 31 32 33 34 35 36 37 38 39 , because this surface is the most stable one of rutile and often serves as a model for transition metal surfaces 1 2 3 4 10 11 12 . The TiO 2 (110)–(1 × 1) surface consists of alternating rows of fivefold-coordinated titanium (5f-Ti) atoms (the Ti troughs) and protruding, twofold coordinated bridge-bonded oxygen (O br ) atoms.…”

“…The adsorption of EtOH and the thermally activated chemistry of EtOH on rutile TiO 2 (110)–(1 × 1) have been studied previously by means of photoelectron spectroscopy (PES) 28 30 44 , temperature-programmed desorption (TPD) 27 28 29 32 33 36 37 38 39 , femtosecond two-photon photoemission spectroscopy (2PPE) 36 , and scanning tunnelling microscopy (STM) 32 33 35 . In addition, the EtOH–TiO 2 (110) interaction has been studied using density functional theory (DFT) calculations 31 32 33 34 35 36 .…”

“…Some studies also addressed the photo-chemistry of EtOH on rutile TiO 2 (110) 30 31 36 37 38 39 . PES and TPD studies suggest that EtOH converts to acetaldehyde (CH 3 CHO) and, finally, acetate (CH 3 COO − ) under UV light illumination in an oxygen background 30 31 .…”

“…A 2PPE study also revealed the formation of CH 3 CHO and reports on a photo-induced excited state ~2.4 eV above E F for EtOH/TiO 2 (110) that was associated with the dissociation of EtOH on 5f-Ti sites 36 . Finally, applying pump–probe laser ionization techniques, it has been suggested that, in the presence of oxygen, methyl radicals are produced during the photo-catalytic oxidation of EtOH 37 .…”

Acetone Formation from Photolysis of 2-Propanol on Anatase-TiO2(101)

“…Not only O−H bond scission, C−C bond can also be broken during the photo-irradiation. Recent results obtained by Kershis and coworkers [17] using temperature programed desorption (TPD) and pump-probe laser ionization techniques have shown that methyl radicals are produced during the photocatalytic oxidation of 2-propanol on R-TiO 2 (110). The result showed that the first step involves photocatalytic dehydrogenation to acetone intermediate, similar to previous studies [7,10,11].…”