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

Abstract

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 (∼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 O2.