Identification of Surface Structures in Pt₃Cr Intermetallic Nanocatalysts

A precise understanding of the catalytic surface of nanoparticles is critical for relating their structure to activity. For silica-supported Pt-Cr bimetallic catalysts containing nominal Cr/Pt molar ratios of 0, 1.9, and 5.6, a fundamental difference in selectivity was observed as a function of composition for propane dehydrogenation, suggesting different surface structures. The formation of bimetallic catalysts and the phases present were confirmed by synchrotron in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) of the nanoparticle as a function of reduction temperature. With the increasing reduction temperature, there is a systematic increase in the Pt L_III edge X-ray absorption near edge structure (XANES) energy, which is consistent with the incorporation of more metallic Cr into the nanoparticles. Pt L_III edge extended X-ray absorption fine structure (EXAFS) shows that the nanoparticles are Pt rich regardless of the reduction temperature, and XRD shows the presence of both Pt and Pt₃Cr phases at temperatures below about 700 °C. For the latter, a full Pt₃Cr intermetallic alloy forms after reduction at 800 °C. This work also presents a method for the characterization of the catalytic surface by the analysis of XAS difference spectra and XRD difference patterns of the (reduced and oxidized) catalysts. The surface analysis suggests that Pt₃Cr formation begins at the surface, and at low reduction temperatures, a core-shell morphology is formed containing a Pt core with a Pt₃Cr surface. By combining the XAS and XRD analyses with transmission electron microscopy (TEM) particle sizes, the thickness of the shell can be approximated. All evidence indicates that the shell thickness increases with the increasing reduction temperature until a full alloy is formed after reduction at about 800 °C but only if there is enough Cr₂O₃ available near Pt nanoparticles to form Pt₃Cr. Catalysts containing a full monolayer coverage of Pt₃Cr have higher olefin selectivity (>97%) compared with partially covered Pt surfaces (88%).