Pt/TiO2 photocatalysts were prepared by incipient wetness impregnation followed by oxidative and/or reductive thermal treatments. By varying the TiO2 form (commercial P25 and P90, and homemade shape-controlled), the Pt loading (0.2-1 wt% Pt) and the treatment temperature (200-600 degrees C), it has been possible to tune the Pt cluster size. An increase in the ethanol dehydrogenation rate under ultraviolet irradiation as the Pt cluster average diameter decreases from 17 to 9 angstrom is suggested by our data. Whereas pre-reduction in H-2 leads to Pt clusters, pre-calcination in air leads to atomically dispersed cationic Pt species. The former are more active and stable than the latter. This conclusion is valid both in gas- and liquid-phase reaction conditions for given TiO2 type and Pt loading. The activity results are consistent with a recent theoretical work showing that 1 nm is an optimal Pt cluster size for favoring both photoelectron transfer from TiO2 to Pt and hydrogen coupling on Pt. The best catalytic performance is obtained in gas phase for pre-reduced 0.2% pt/TiO2-p90, with an H-2 production rate of 170 mmol h(-1) g(cat)(-1). (C) 2019 Elsevier Inc. All rights reserved.