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/SiO 2, PtOx/Al 2 O 3, and PtOx/CeO 2 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 PtO 2 (4−7 nm), amorphous PtO 2 (∼1−1.5 nm), and surface PtOx species (<1 nm). The domain size of the supported PtOx phase decreased (SiO 2 > Al 2 O 3 > CeO 2 ) as the PtOx−support interaction increased (CeO 2 > Al 2 O 3 > SiO 2 ). The strength of the PtOx−support interaction also controlled the reducibility of the supported PtOx phase, with the strongly interacting PtOx/CeO 2 system being the most difficult to reduce. Corresponding IR spectroscopy showed that the CeO 2 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 CeO 2 can be achieved with mild reduction−oxidation treatments. The reduced metallic Pt is reoxidized by bulk lattice oxygen from the CeO 2 support and not gas-phase molecular O 2 .