Role of Pt Loading in the Photocatalytic Chemistry of Methanol on Rutile TiO2(110)

As a cocatalyst, Pt is well-known for accepting photoexcited electrons and lowering the overpotential of hydrogen production in photocatalysis, being responsible for the enhanced photocatalytic efficiency. Despite the above existing knowledge, the adsorption of reactants on the Pt/photon-absorber (for example, Pt/TiO2) interface, a prerequisite to understand the photocatalytic chemistry, is extremely difficult to investigate mainly because of the complexity of the powdered material and solution environment. Combining ultrahigh vacuum and well-ordered single crystals, we study the photocatalytic chemistry of methanol on Pt-loaded rutile TiO2(110) using temperature-programmed desorption (TPD) and ultraviolet photoelectron spectroscopy (UPS). Despite the same photocatalytic chemical products (i.e., formaldehyde and surface hydrogen species) as on Pt-free TiO2(110), the subsequent chemistry of surface hydrogen species and the photocatalytic reaction rate are much different. The bridging hydroxyls desorb as water molecules around 500 K on the Pt-free TiO2(110) surface, and by contrast, this desorption channel disappears completely and water and molecular hydrogen desorb at much lower temperature (<300 K) after Pt deposition, which can prevent the recombination of hydrogen species with formaldehyde. More importantly, methanol dissociates into methoxy at the Pt/TiO2(110) interface, which is crucial in the photocatalytic chemistry of methanol on TiO2 surfaces because methoxy is a more effective hole scavenger than methanol itself. The photocatalytic chemical reaction rate is increased by nearly 1 order of magnitude after 0.12 monolayer Pt deposition. This work suggests that Pt loading can promote the dissociation of methanol into methoxy and lower the desorption barrier of molecular hydrogen, which may work cooperatively with separating photoexcited charges to enhance the photocatalytic efficiency. Our work implies the importance of the cocatalysts in affecting the surface structure and adsorption of reactants and products and then improving the photoactivity, in addition to the well known role in charge separation.