Invited Paper SS+AS+HC-MoA3
Photochemistry of CO, Acetone and O₂ on Reduced Rutile TiO₂(110)

Monday, October 30, 2017, 2:20 pm, Room 25

TiO₂ is an important photocatalyst with many practical applications. However, a fundamental understanding of the thermal and non-thermal reactions on TiO₂ surfaces is still lacking. We use ultrahigh vacuum (UHV) surface science techniques to investigate in detail representative reactions on single crystal rutile TiO₂(110). For this talk, I will focus on the ultraviolet (UV) photon-stimulated reactions of CO, acetone and oxygen adsorbed on TiO₂(110). For CO co-adsorbed with O₂, CO₂ is produced during UV irradiation. The CO₂ preferentially desorbs in the plane perpendicular to the bridge-bonded oxygen (BBO) rows at an angle of 45°. Furthermore, the production rate of CO₂ is zero when the UV irradiation starts and reaches a maximum value at intermediate times before decaying at longer times. The results demonstrate that the photooxidation of CO is a multi-step reaction that proceeds through a metastable intermediate state that is oriented perpendicular to the BBO rows. This state is consistent with an O-O-C-O state found via density functional theory. For acetone co-adsorbed with oxygen, previous research suggests that a thermal reaction between acetone and adsorbed oxygen to form an acetone diolate precedes the photochemistry. During UV irradiation, a methyl radical is ejected leaving acetate on the surface. Using infrared reflection absorption spectroscopy (IRAS), we have identified the acetone diolate. We have also measured the angular distribution of the photodesorbing methyl radicals. Consistent with its ejection from acetone diolate, we observe a peak in the distribution at ~45° in the plane perpendicular to the BBO rows. However, a second photodesorption peak normal to the surface indicates that a second, previously unidentified, reaction channel is available. We attribute this second channel to the photo-induced formation of an enolate intermediate. When only O₂ is adsorbed on TiO₂(110), its photochemistry depends on the coverage. For small coverages, only ~14% desorbs while the rest either dissociates during UV irradiation, or remains molecularly adsorbed on the surface. For the maximum coverage of chemisorbed oxygen, the fraction of O₂ that photodesorbs is ~45%. While photo-generated holes are responsible for the O₂ photodesorption, photo-generated electrons are responsible for the photo-induced O₂ dissociation. In general, these studies provide new insights into mechanisms responsible for the photochemistry of small molecules on TiO₂.