Catalytic Role of TiO₂ Terminal Oxygen Atoms in Liquid‐Phase Photocatalytic Reactions: Oxidation of Aromatic Compounds in Anhydrous Acetonitrile

Abstract

On the basis of experiments carried out with controlled amounts of residual oxygen and water, or by using oxygen-isotope-labeled Ti(18) O2 as the photocatalyst, we demonstrate that (18) Os atoms behave as real catalytic species in the photo-oxidation of acetonitrile-dissolved aromatic compounds such as benzene, phenol, and benzaldehyde with TiO2 . The experimental evidence allows a terminal-oxygen indirect electron-transfer (TOIET) mechanism to be proposed, which is a new pathway that involves the trapping of free photogenerated valence-band holes at Os species and their incorporation into the reaction products, with simultaneous generation of oxygen vacancies at the TiO2 surface and their subsequent healing with oxygen atoms from either O2 or H2 O molecules that are dissolved in the liquid phase. According to the TOIET mechanism, the TiO2 surface is not considered to remain stable, but is continuously changing in the course of the photocatalytic reaction, challenging earlier interpretations of TiO2 photocatalytic phenomena.

Laser-flash-photolysis-spectroscopy: a nondestructive method?

“…These ndings are essential for the evaluation of the underlying photocatalytic reactions. Although there are many studies which present such structural changes of the TiO 2 nanoparticles upon illumination, 32,57,58,[67][68][69][70][71] this effect has rarely been considered for the interpretation of the photoinduced photocatalytic processes. 72 Based upon these possible reactions it is certainly highly advisable to study the role of the surface reorganization on the reactions induced upon illumination.…”

Unveiling the active sites of graphene-catalyzed peroxymonosulfate activation

“…Generally, the catalytic properties of metalbased catalysts are very sensitive to the size and shape of the metal crystal, due to the specifically exposed crystal facets and ill-defined surface with edges, corners exhibit better excess and lower binding energy for adsorption with the reactants [4]. Meanwhile, the transition metal oxides possess variable valent states, different types of bridging or terminal lattice oxygen, and oxygen vacancies, which play vital roles in interacting with the reactants especially in a redox process [5][6][7]. Distinctly, it has been widely accepted that pristine carbocatalysis might be originated from the "active sites'' such as structure defects, functional groups, or heteroatom-doping induced modulations to the carbon network, which usually exhibit characteristic electronic states and spin cultures.…”

What Controls Photocatalytic Water Oxidation on Rutile TiO2(110) under Ultra-High-Vacuum Conditions?

“…Some studies on TiO 2 point to free or adsorbed hydroxyl radicals generated from water molecules as the active catalytic species. − However, the formation of adsorbed hydroxyl radicals from adsorbed water photooxidation by photogenerated valence band (VB) free holes has been questioned on the basis of the electron photoemission spectra of water adsorbed at the rutile TiO 2 (110) surface under ultra-high-vacuum (UHV) conditions, − whose analysis indicates that the reaction is thermodynamically not possible because the water highest occupied orbital lies below the TiO 2 valence band maximum (VBM) . Alternatively, other studies assign the catalytically active species to O radical centers which originate from the trapping of the h at terminal 2-fold-coordinated substrate O atoms, O 2s (bridging O atoms in TiO 2 (110)). − One of the main questions in this discussion is whether the O of the active radical species is associated with an O atom from a water molecule or an O atom of the TiO 2 lattice.…”