Identification of Surface Structures in Pt 3 Cr Intermetallic Nanocatalysts
A precise understanding of the catalytic surface of nanoparticles is essential 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 additionally proposes 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%).
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities Division; National Science Foundation (NSF)
- Grant/Contract Number:
- AC02-06CH11357
- OSTI ID:
- 1513233
- Journal Information:
- Chemistry of Materials, Vol. 31, Issue 5; ISSN 0897-4756
- Publisher:
- American Chemical Society (ACS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
In situ observations of the structural dynamics of platinum–cobalt–hydroxide nanocatalysts under CO oxidation
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journal | January 2020 |
Intermetallic Compounds as an Alternative to Single‐atom Alloy Catalysts: Geometric and Electronic Structures from Advanced X‐ray Spectroscopies and Computational Studies
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journal | January 2020 |
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