Quantum yield of formaldehyde formation in the presence of colloidal TiO2-based photocatalysts: Effect of intermittent illumination, platinization, and deoxygenation

Colloidal TiO2 (2.4 nm average particle diameter) has been prepared and modified by photodeposition of Pt (PtTi-S1) and by mixing with Pt nanoparticles (PtTi-S2). Transmission electron microscopy reveals particle aggregation in colloidal TiO2 and large networks of agglomerated particles in the platinized samples. The photocatalytic activity of the samples (0.1 g L-1) has been investigated by measuring the quantum yield, phi(HCHO), of HCHO formed from aqueous methanol at pH 3.5 under different conditions. In CW photolysis of the oxygenated suspensions (300-400 nm UV light, 8 x 10(-7) einstein L-1 s(-1) photon absorption rate) the platinized photocatalysts (1 wt % Pt) enhance phi(HCHO) by a factor of 1.5-1.7 with respect to neat colloidal TiO2 where phi(HCHO) is 0.02. In addition to the action of Pt as an electron sink, the strong promotion of photocatalytic methanol oxidation by PtTi-S2 at the small Pt/TiO2 particle ratio of ca. 1: 1060 is proposed to arise from transfer of excitation energy or of photogenerated charge carriers through the particle network. Repetitive laser-pulse illumination of the oxygenated samples (351 nm, 0.5 Hz repetition frequency) increases phi(HCHO) by ca. 50% in comparison with CW illumination at the same average photon absorption rate of ca. 8 X 10(-7) einstein L-1 s(-1). As a tentative explanation an increase of the photocatalyst surface by laser-pulse stimulated deaggregation of colloidal TiO2 and fragmentation of the networks in the platinized samples is suggested. HCHO is also produced in the deoxygenated suspensions under repetitive laser-pulse illumination. PtTi-S1 and PtTi-S2 increase phi(HCHO) by factors of 1.8 and 1.2, respectively, in comparison with neat colloidal TiO2 where phi(HCHO) is 0.027. Possible mechanisms are discussed. The higher activity of PtTi-S1 is attributed to stronger electrocatalysis of H-2 formation by highly dispersed Pt in PtTi-S1 as compared with the Pt particles in PtTi-S2.