The Case of Formic Acid on Anatase TiO2 (101): Where is the Acid Proton?

Carboxylic-acid adsorption on anatase TiO₂ is a relevant process in many technological applications. Yet, despite several decades of investigations, the acid-proton localization-either on the molecule or on the surface-is still an open issue. By modeling the adsorption of formic acid on top of anatase(101) surfaces, we highlight the formation of a short strong hydrogen bond. In the 0 K limit, the acid-proton behavior is ruled by quantum delocalization effects in a single potential well, while at ambient conditions, the proton undergoes a rapid classical shuttling in a shallow two-well free-energy profile. This picture, supported by agreement with available experiments, shows that the anatase surface acts like a protecting group for the carboxylic acid functionality. Such a new conceptual insight might help rationalize chemical processes involving carboxylic acids on oxide surfaces.

Quantum tunneling injection of hot electrons in Au/TiO2 plasmonic photocatalysts

Visible light absorption of plasmonic Au nanoparticles supported on semiconductor TiO₂ leads to injection of their photoactivated "hot electrons (e_hot^-)" into the TiO₂ conduction band. This charge separation facilitates several oxidation and reduction reactions. These plasmonic systems, however, suffer from low quantum yields because the Schottky barrier created at the Au-TiO₂ interface suppresses e_hot^- injection. Here we report that Au nanoparticles supported on the anatase particles isolated from Degussa (Evonik) P25 TiO₂ promote e_hot^- injection with much higher efficiency than those supported on other commercially-available TiO₂ and catalyze aerobic oxidation with very high quantum yield (7.7% at 550 nm). Photoelectrochemical and spectroscopic analysis revealed that the number of Ti⁴⁺ atoms located at the Au-TiO₂ interface is the crucial factor. These Ti⁴⁺ atoms neutralize the negative charge of the Au particles and create a Schottky barrier with narrower depletion layer. This facilitates efficient e_hot^- injection by "quantum tunneling" through the Schottky barrier without overbarrier energy. The e_hot^- injection depends on several factors, and loading of 2 wt% Au particles with 3.5-4 nm diameters at around room temperature exhibits the highest activity of plasmonic photocatalysis.

One-pot synthesis of imines from nitroaromatics and alcohols by tandem photocatalytic and catalytic reactions on Degussa (Evonik) P25 titanium dioxide

Photoirradiation (λ > 300 nm) of Degussa (Evonik) P25 TiO2, a mixture of anatase and rutile particles, in alcohols containing nitroaromatics at room temperature produces the corresponding imines with very high yields (80-96%). Other commercially available anatase or rutile TiO2 particles, however, exhibit very low yields (<30%). The imine formation involves two step reactions on the TiO2 surface: (i) photocatalytic oxidation of alcohols (aldehyde formation) and reduction of nitrobenzene (aniline formation) and (ii) condensation of the formed aldehyde and aniline on the Lewis acid sites (imine formation). The respective anatase and rutile particles were isolated from P25 TiO2 by the H2O2/NH3 and HF treatments to clarify the activity of these two step reactions. Photocatalysis experiments revealed that the active sites for photocatalytic reactions on P25 TiO2 are the rutile particles, promoting efficient reduction of nitrobenzene on the surface defects. In contrast, catalysis experiments showed that the anatase particles isolated from P25 TiO2 exhibit very high activity for condensation of aldehyde and aniline, because the number of Lewis acid sites on the particles (73 μmol g(-1)) is much higher than that of other commercially available anatase or rutile particles (<15 μmol g(-1)). The P25 TiO2 particles therefore successfully promote tandem photocatalytic and catalytic reactions on the respective rutile and anatase particles, thus producing imines with very high yields.