The roles of metal co-catalysts and reaction media in photocatalytic hydrogen production: Performance evaluation of M/TiO2 photocatalysts (M = Pd, Pt, Au) in different alcohol–water mixtures

Abstract M/TiO2 photocatalyst activity for H2 production depends on both the metal co-catalyst (M) and the reaction medium. To quantify such effects, we compared the performance of M/TiO2 photocatalysts (M = Pd, Pt, Au) for H2 production in different alcohol–water mixtures under UV excitation. Photocatalysts were prepared using Degussa P25 TiO2 at metal loadings of 0.5 and 1 wt.% for Pd, 1 wt.% for Pt and 1 wt.% for Au. TEM, UV–Vis, XPS and EXAFS analyses confirmed the presence of supported metal nanoparticles of size ∼2, 1.3 and 5.4 nm for Pd/TiO2, Pt/TiO2 and Au/TiO2, respectively. Photoluminescence data established that the metal co-catalysts effectively suppressed electron–hole pair recombination in TiO2 following photo-excitation. The activities of the M/TiO2 photocatalysts for H2 production were evaluated in a wide range of alcohol–water mixtures (alcohol concentration 10 vol.%) under UV (365 nm, 5 mW cm−2). H2 production rates in the alcohol–water mixtures were dependent on (i) the metal co-catalyst; (ii) the co-catalyst loading; and (iii) the alcohol type. Co-catalyst activity followed the order Pd > Pt ≈ Au. The highest H2 production rates were achieved for the 1 wt.% Pd/TiO2 photocatalyst in glycerol–water mixtures (47.5 mmol g−1 h−1) and 1,2-ethanediol–water mixtures (44.5 mmol g−1 h−1). H2 production rates decreased in the order glycerol > 1,2-ethanediol > 1,2-propanediol > methanol > ethanol > 2-propanol > tert-butanol ≫ water. For each M/TiO2 photocatalyst, correlations were established between the rate of H2 production and specific alcohol properties, especially alcohol polarity and the exponential of the alcohol oxidation potential.