Abstract
The size and shape of monometallic Rh and Pt and bimetallic Rh−Pt clusters on zeolite NaY were determined by EXAFS. In the monometallic as well as in the bimetallic samples, the first coordination numbers were independent of the metal loading (1.5−4.8 wt % for Pt and 0.8−2.5 wt % for Rh in monometallic samples, and 0.8−2.4 wt % for Pt and 0.4−1.2 wt % for Rh in the bimetallic samples). All first coordination numbers were around 7, demonstrating that all monometallic and bimetallic particles have similar dimensions. The second- and third-shell coordination numbers of the monometallic clusters are in accordance with a spherical shape of the metal clusters which just fit into the supercages of the Y zeolite structure. Both Rh and Pt have the tendency to bind preferentially to their own kind. The observed coordination numbers, as well as the EXAFS spectra obtained after CO absorption, showed that no substantial surface enrichment is present in the bimetallic RhPt/NaY samples. Since EXAFS data are averaged ove...
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