Photonic efficiency and quantum yield of formaldehyde formation from methanol in the presence of various TiO2 photocatalysts

UV illumination of aqueous TiO(2) suspensions yields hydroxyl radicals which can be trapped by methanol yielding formaldehyde (HCHO). The photonic efficiency of HCHO formation (345 +/- 35 nm ew illumination) in aqueous, oxygenated TiO(2) suspensions (2.4 nm diameter TiO(2) particles, Degussa P25 and Sachtleben Hombikat UV 100) containing methanol at pH ca. 1-12 has been determined as well as the respective quantum yield in case of 2.4 nm TiO(2). Differences in the activity of the three photocatalysts have been found and are discussed. The photonic efficiency in the presence of P25 and Hombikat UV 100 depends on the catalyst loading (g l(-1)) and the pH. Below 2.5 g l(-1) the photonic efficiency is higher for P25 than for Hombikat UV 100 and vice versa at above 2.5 g l(-1), Optimum pH values for P25 and Hombikat UV 100 resulting in maximum photonic efficiencies (ca. 0.13 for P25 and 0.07 for Hombikat UV 100) are 7.7 and 10.4, respectively. Other than with P25 and Hombikat UV 100, which scatter light strongly, the quantum yield of HCHO formation in the colloidal 2.4 nm TiO(2) suspension varies but little with the pH and virtually does not change with the photocatalyst loading (0.1-1.0 g/l). Employing these colloidal particles as photocatalysts the quantum yield varies as the inverse square root of light intensity. It increases from 0.02 to 0.08 when the absorbed photon flux decreases from 8.1 x 10(-7) to 4.9 x 10(-8) Einstein l(-1) s(-1). A simple model is presented to explain this observation. (C) 2002 Elsevier Science B.V. All rights reserved.