Probing metal-support interactions under oxidizing and reducing conditions: in situ Raman and infrared spectroscopic and scanning transmission electron microscopic-X-ray energy-dispersive spectroscopic investigation of supported platinum catalysts

The strength of PtOx-support and Pt-support interactions was investigated for supported PtOX/SiO2, PtOX/ Al2O3, and PtOx/CeO2 catalysts with time-resolved in situ Raman/IR spectroscopy and scanning transmission electron microscopy (STEM). Raman spectroscopy (and STEM-X-ray energy-dispersive spectoscopy, XEDS) was able to distinguish between crystalline PtO2 (4-7 nm), amorphous PtO2 (similar to 1-1.5 nm), and surface PtOx species (< 1 nm). The domain size of the supported PtOx phase decreased (SiO2 > Al2O3 > CeO2) as the PtOx-support interaction increased (CeO2 > Al2O3 > SiO2). The strength of the PtOx-support interaction also controlled the reducibility of the supported PtOx phase, with the strongly interacting PtOx/CeO2 system being the most difficult to reduce. Corresponding IR spectroscopy showed that the CeO2 support also became reduced by H-spillover during the reduction treatment. Furthermore, Pt redispersion is also related to the PtOx-support interaction, and complete redispersion on CeO2 can be achieved with mild reduction-oxidation treatments. The reduced metallic Pt is reoxidized by bulk lattice oxygen from the CeO2 support and not gas-phase molecular O-2.