Isolated or contiguous, the surface distributions of Pd atoms in the bimetallic (Pd-M) nanoparticle (NP) catalysts often influence activity and selectivity towards specific reactions. In this study, we used a Pd hydride formation upon H2 exposure as a probe of presence of contiguous Pd regions in bimetallic NPs. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) contain information about Pd-Pd bond expansion resulting from H2 absorption. If revealed during the EXAFS and/or XANES measurements, it can be used to characterize the H/Pd ratio and the size of Pd regions. For demonstrating this method, we prepared supported monometallic Pd and bimetallic Pd-Au and Pd-Ag NPs, and used XANES, EXAFS, scanning transmission electron microscopy (STEM) and infrared (IR) spectroscopy to detect and quantitatively analyze the Pd hydride regions. In the case of Pd-Ag, poor Z contrast of Pd and Ag, which are neighboring elements in the periodic table, prevents the detection of Pd regions by STEM imaging and by analysis of Pd-Pd and Pd-Ag coordination numbers by EXAFS. The observation of the Pd-Pd bond expansion, detected in the case of Pd-Ag and Pd-Au alloys under H2, provides a viable alternative to conventional methods for characterizing intra-particle heterogeneities within a broad class of bimetallic NPs at ambient temperature and pressure.

New role of Pd hydride as a sensor of surface distributions of Pd atoms in bimetallic nanocatalysts

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.