In situ Study on Structure of a Diluted Pt/HOPG Model Catalyst System Prepared by the Two-phase Liquid Reduction Method Using a Novel BCLA/HERFD+BI-XAFS Method

The structure of monolayer Pt nanoparticles (Pt NPs) dispersed on a flat highly oriented pyrolytic graphite surface prepared by two-phase reduction method as a model catalyst for fuel cell systems was studied by a bent crystal Laue analyzer enhanced back-illuminated X-ray absorption fine structure (BCLA+BI-XAFS) technique. The BCLA+BI-XAFS technique enables us to investigate the in situ Pt NPs structure in 0.1 M HClO4 solutions while applying a wide range of potentials. We found that Pt NPs have a stable framework of a cuboctahedron with about 309 Pt atoms and a Pt–Pt bond length of 2.76 ± 0.06 Å independent of applied potential. The application of high energy resolution fluorescence detection back-illuminated X-ray absorption near edge structure (HERFD+BI-XANES) successfully elucidated the relationship between surface species and applied potential. We revealed the origin of the high stability of the Pt NPs prepared by the two-phase liquid reduction method. This developed BI-XAFS method provides a new way to investigate Pt NPs on flat substrates as a model catalyst for fuel cell catalyst studies.