Photocatalytic reforming of glycerol for H-2 evolution on Pt/TiO2: fundamental understanding the effect of co-catalyst Pt and the Pt deposition route

To fundamentally understand the effects of deposited Pt and its deposition route on the photocatalytic reforming (PR) of glycerol for H-2 evolution over Pt/TiO2, several 1 wt% Pt/P25 (PT) samples were prepared by photo-deposition (PD, glycerol as hole scavenger) and impregnation-reduction deposition routes (IRD, NaBH4 or H-2/Ar as reductant) using H2PtCl6 as the precursor. The samples were characterized by XRD, UV-Vis DRS, TEM and XPS, and the PR activities were examined and compared under ambient conditions. The formation of photo-induced charge carriers (CCs) over PT was measured by a photoluminescence technique using terephthalic acid as a probe molecule. The results indicated that the reforming activity depends on both the nature of the light harvesting of P25 and the characteristics of the co-catalyst Pt, including its chemical state, size, and the interaction with P25; Pt particles serve as the active sites for H2 evolution. Uniform Pt particles could be selectively and intimately deposited on P25 in the Pt(0) state via an in situ PD process (PT-S), while by IRD routes, Pt particles were randomly loaded on P25 with the surface in Pt(0) and the bulk in Pt(II/IV) states. Unlike the Pt chemical state, the Pt sizes were less impacted by the deposition routes and were about 2 nm. Compared to P25, a low generation efficiency of CCs was observed on platinized samples due to the covering of the photo-active sites by Pt. Pt(0) exhibits higher light shielding effect than Pt(II/IV). Meanwhile, the separation of CCs was promoted by Schottky barriers formed at the Pt-TiO2 interface. Photo-induced electrons could be trapped by the barriers and the process was favored by well-contacted Pt(0) and obstructed by the bulk Pt(II/IV) component. The promotion effect of Pt(0) prevails over its adverse effect. Thus, PT-S exhibited the highest PR activity as it only possesses Pt(0), demonstrating the advantage of the PD process. A control test suggests Pt with this kind of feature can only be achieved in a dilute suspension by the PD route.