Surface Modification of TiO₂ by Phosphate:  Effect on Photocatalytic Activity and Mechanism Implication

Abstract

Phosphate modified TiO2 photocatalysts were prepared by phosphoric acid treatment before or after TiO2 crystallization. Substrates with different structures were chosen to explore the photocatalytic activity of as-modified TiO2 under UV irradiation. It was found that the effect of phosphate modification is definitely attributed to the surface-bound phosphate anion, and the modification by phosphate can affect both the rates and pathways of photocatalytic reactions, which are of great dependence on the structures and properties of substrates. The degradation of substrates (such as 4-chloropehenol, phenol, and rhodamine B) with weak adsorption on the pure TiO2 was markedly accelerated by phosphate modification, while substrates (such as dichloroacetic acid, alizarin red, and catechol) with strong adsorption exhibited a much lower degradation rate in the phosphate modified system. A much higher amount of hydroxyl radical was produced in phosphate modified system. All of the experimental results imply that phosphate modification largely accelerates the hydroxyl radical attack, but hinders the direct hole oxidation pathway. A common operating mechanism for the phosphate modification, which can be applicable to other inert anions, is also discussed from the viewpoint of an anion-induced negative electrostatic field in the surface layer of TiO2 and the hydrogen bond between modification anion and H2O molecule.

Singlet oxygen photogeneration and 2,4,6-TCP photodegradation at Pt/TiO2 under visible light illumination

“…OH attack on the para or ortho positions of the aromatic ring. 27,28 These intermediates, produced from ? OH attack, do not display a pink color in aqueous solution.…”

Generation of Free OHaq Radicals by Black Light Illumination of Degussa (Evonik) P25 TiO2 Aqueous Suspensions

“…It is known that anions such as phosphate show a strong tendency to be bound on TiO 2 surfaces [45]. When adsorption of phthalic hydrazide to TiO 2 at different pH values was measured in presence of 5 mM phosphate by the spectrophotometric method described in 3.2.1, no significant adsorption could be detected.…”

“…Recently, additions of fluoride and phosphate ions were also shown to result in increased photocatalytic activities [45,47,48].…”

Assessment of solar driven TiO2-assisted photocatalysis efficiency on amoxicillin degradation

“…Chen et al (2003) have reported a similar phenomenon of increased adsorption of certain dye-constituent aromatics on the surface of TiO2 in the presence of phosphate anions. The negatively charged phosphate anions strongly adsorbed onto the TiO2 surface may favor not only the adsorption of AMX species bearing a positive (Chen et al 2003) or neutral (Guillard et al 2003) charge on the -NH2 group, but also the formation of free hydroxyl radicals via the enhancement of the separation of the photogenerated hole and electron facilitated by an inner-sphere surface complex, as suggested by Zhao et al (2008). The same authors thus concluded that phosphate modification accelerates the degradation of pollutants either more prone to hydroxyl radical attack or with weak adsorption on pure TiO2 particles.…”