Photocatalytic reforming of glycerol for H2 evolution on Pt/TiO2: fundamental understanding the effect of co-catalyst Pt and the Pt deposition route

Abstract

To fundamentally understand the effects of deposited Pt and its deposition route on the photocatalytic reforming (PR) of glycerol for H₂ 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 H₂/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.