Determination of photonic efficiency and quantum yield of formaldehyde formation in the presence of various TiO2 photocatalysts

Illumination of aqueous TiO2 suspentions yields hydroxyl radicals, which ca n be trapped by methanol producing formaldehyde (HCHO). In this work, the p hotonic efficiency and quantum yield of HCHO formation in colloidal TiO2 so lutions and, P25 and UV 100 suspensions have been determined. Differences i n photocatalytic activity of the three photocatalysts have been found and a re discussed. The photonic efficiency of HCHO formation in the presence of P25 and UV 100 depends on the concentration of TiO2 and the pH. The critica l concentration is 2.5 g/L. Below this, the photonic efficiency with P 25 i s higher than with UV 100, and vice versa. Optimum pH values for P25 and UV 100 giving the maximum photonic efficiency are 7.7 and 10.4, respectively. Compared to P25 and UV 100, the true quantum yield of HCHO formation in co lloidal TiO2 solution varies a little with pH and virtually does not change with the amount of loading of TiO2. The true quantum yield varies as the i nverse square root of light intensity. The quantum yield increases from 0.0 2 to 0.08 when the absorbed photon flux decreases from 8.1 x 10(-7) Ein/L s to 4.9 x 10(-8) Ein/L s. A simple model is presented to explain the experi mental observation.