The Measure of TiO₂ Photocatalytic Efficiency and the Comparison of Different Photocatalytic Titania

Abstract

The dependence of hydroxyl radical yield on the substrate concentration, pH, oxygen concentration, and light intensity, using different TiO 2 preparations, was investigated. The quantum yields of formaldehyde and carbon dioxide, obtained with the aid of an integrating sphere in the methanol and formate systems, respectively, were used to derive the primary yield of hydroxyl radicals. The limiting yield of • OH ads , achieved at high scavenger concentrations, is independent of the nature of the scavenger (methanol or formate) and is nearly constant in the range 1 < pH < 12. While the presence of air induces a 15-fold increase in product yield, compared to air-free systems, further increase of oxygen concentration has only a small effect. The effect of changing the • OH scavenger concentration, pH, and light intensity corresponds to simple competition between pseudo first-and second-order reactions. Under the conditions of the present work the scavengers CH 3 OH, HCO 2and HCO 2 H react only with the hydroxyl radical, not with its precursor hole. Hydrogen peroxide, which is produced in the above systems upon reduction of oxygen by both TiO 2 electrons and the organic free radicals, is further reduced to • OH radicals, doubling the products yield. A rigorous treatment of the effect of absorbed photon density on the quantum yield, based on the simple square root dependency, enables the derivation of a constant, K d , which is related to the efficiency of surface water oxidation to hydroxyl radicals, relative to the electron hole recombination at the given pH and oxygen concentration. This parameter is a property of the TiO 2 , and is not affected by the experimental conditions such as wavelength and layer thickness. Comparison of K d using different types of TiO 2 shows that in contrast to commonly assumed, preparation methods and sizes of aged titanium dioxide nanoparticles have only a relatively small effect on the basic photocatalytic efficiency. Comparison of the quantum yield in TiO 2 layers to results in aqueous suspensions using similar photon flux shows moderate differences. The results are discussed in view of the much larger absorbed photon density in the suspended nanoparticles combined with much larger volume in the bulk of the solution.

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

“…Comparisons of "apparent quantum efficiencies" are only meaningful if effects due to light scattering and penetration are comparable.) The pH trend and the "apparent quantum efficiency" obtained in this study is similar to what has been reported in previously published works [50,51], where methanol was used as general OH radical probe with a very low tendency to be adsorbed on TiO 2 surfaces. However, our "apparent quantum efficiency" is at variance with the much smaller value reported by Hirakawa et al [26].…”

Photomineralization of phenol on Al2O3: synergistic photocatalysis by semiconductors

“…In aqueous medium, O 2 •2 in turn produces highly reactive species, such as HO • and HO 2 • , which also oxidize the organics. Water is adsorbed on the semiconductor, both molecularly and dissociatively [2,3]; hole-trapping by either the surface hydroxyl group or water molecule results in short-lived HO • radicals, which are the primary oxidizing agents [4][5][6][7]. Reports on the photodegradation of phenol are numerous and TiO 2 in different modifications [8][9][10], including metal-doped [8] and dye-sensitized TiO 2 [11], ZrO 2 [12], MoO 3 [10,12], MoS 2 [13], WO 3 [9], WS 2 [14], PW 12 O 40 3- [15], a-Fe 2 O 3 [10,15], CuO [10], GdCoO 3 [16], ZnO [9,10], ZnS [10], CdO [10], CdS [9,10], CdSe/TiO 2 [17], In 2 O 3 /TiO 2 [18], SnO 2 [13], Bi 2 O 3 [10], Sb 2 O 3 [10], and CeO 2 [10], are the semiconductors used as a catalyst.…”

Semiconductor‐photocatalyzed degradation of carboxylic acids: Enhancement by particulate semiconductor mixture

“…• OH radical are indistinguishable and refer both as hole without discrimination, although in some cases the term localized or trapped hole is implicitly used for • OH ads [47,49]. Hole is quickly converted to…”