Identification of a Pt3Co Surface Intermetallic Alloy in Pt-Co Propane Dehydrogenation Catalysts

Bimetallic Pt-Co nanoparticles (NPs) were prepared and characterized by scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, in situ synchrotron X-ray diffraction, and catalytic conversion for propane dehydrogenation with and without added H-2. In addition, the surface extended X-ray absorption fine structure (EXAFS) obtained by fitting the difference spectrum between the fully reduced and room-temperature-oxidized catalysts suggest that the surface structure remains Pt3Co, although the core changes from Pt to Pt3Co and to PtCo. At low Co loading, the bimetallic nanoparticles form a Pt3Co intermetallic surface alloy with Pt-rich core. With increasing Co loading, a full alloy forms where both the surface and NP compositions are Pt3Co. A further increase in Co loading leads to a Co-rich NP core, likely PtCo, with a surface of Pt3Co. Although Pt-Co intermetallic alloys form two different phases and several morphologies, the surface structures are similar in all catalysts. Although both monometallic Pt and Co are active for alkane dehydrogenation, all bimetallic Pt-Co catalysts are significantly more olefin selective than either single metal. The turnover rates of the bimetallic catalysts indicate that Pt is the active atom with little contribution from Co atoms. The high olefin selectivity is suggested to be due to Co acting as a less active structural promoter to break up large Pt ensembles in bimetallic NPs.