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

[Display omitted] •Pd, Pt and Au activate TiO2 for H2 production in alcohol–water mixtures under UV.•H2 production rates decrease in the order Pd/TiO2>Pt/TiO2≈Au/TiO2≫P25 TiO2.•Metal particle size does not impact H2 production rates in the size range 1–5nm.•Rates follow the order triols>diols>1° alcohols>2° alcohols>3° alcohols.•Rates correlate with alcohol polarity and alcohol standard oxidation potential. 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 1wt.% for Pd, 1wt.% for Pt and 1wt.% for Au. TEM, UV–Vis, XPS and EXAFS analyses confirmed the presence of supported metal nanoparticles of size ∼2, 1.3 and 5.4nm 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 10vol.%) under UV (365nm, 5mWcm−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 1wt.% Pd/TiO2 photocatalyst in glycerol–water mixtures (47.5mmolg−1h−1) and 1,2-ethanediol–water mixtures (44.5mmolg−1h−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.