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 over all particles, no conclusion could be drawn about the compositional homogeneity of the particles.

The Quick EXAFS technique was used to study the formation and growth of bimetallic PtRh particles in zeolite Nay during reduction. Before reduction, only metal-oxygen signals were detectable. While increasing the temperature at a rate of S°C/min and flowing H, through the in-situ cell, 1 minute EXAFS scans were recorded at the Rh and Pt edges. At 40°C, an intense Rh-Rh signal appeared within few degrees, indicating the formation of monometallic Rh clusters (20-25 atoms). The Rh-0 signal slowly decreased between 100 and 2W0C, simultaneous with the appearance and slow growth of a small Rh-Pt signal. The Pt edge EXAFS changed less abruptly than the Rh edge EXAFS. Above 80°C, the Pt-0 signal started to decrease, and at lOO0C the Pt-Pt and Pt-Rh signals appeared simultaneously, pointing to the formation of small bimetallic particles. These signals grew slowly until 300°C. even after the Pt-0 signal had already disappeared. This indicates that small bimetallic particles move through the channels of the zeolite and merge to larger particles. This merging should also involve the big monometallic Rh clusters formed at 40°C, probably situated in the supercages of the zeolite. At the end of the reduction, two kinds of clusters are present: one having a big Rh core (maybe also completely monometallic), the other consisting of Pt-rich bimetallic PtRh clusters. It is concluded that the different reduction behaviour of Rh and Pt drastically influences the final structure of the metal particles

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The Quick EXAFS technique was used to study the formation and growth of bimetallic PtRh particles in zeolite NaY during reduction. Before reduction, only metal-oxygen signals were detectable. While increasing the temperature at a rate of 5°C/min and flowing H2 through the in-situ cell, 1 minute EXAFS scans were recorded at the Rh and Pt edges. At 40°C, an intense Rh-Rh signal appeared within few degrees, indicating the formation of monometallic Rh clusters (20-25 atoms). The Rh-O signal slowly decreased between 100 and 200°C, simultaneous with the appearance and slow growth of a small Rh-Pt signal. The Pt edge EXAFS changed less abruptly than the Rh edge EXAFS. Above 80°C, the Pt-O signal started to decrease, and at 100°C the Pt-Pt and Pt-Rh signals appeared simultaneously, pointing to the formation of small bimetallic particles. These signals grew slowly unti1 300°C, even after the Pt-O signal had already disappeared. This indicates that small bimetallic particles move through the channels of the zeolite and merge to larger particles. This merging should also involve the big monometallic Rh clusters formed at 40°C, probably situated in the supercages of the zeolite. At the end of the reduction, two kinds of clusters are present : one having a big Rh core (maybe also completely monometallic), the other consisting of Pt-rich bimetallic PtRh clusters. It is concluded that the different reduction behaviour of Rh and Pt drastically influences the final structure of the metal particles

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