J. Phys. IV France 7 (1997) C2-723-C2-725
QEXAFS Investigation of the Particle Growth of PtRh Clusters Supported on NaYF. Cimini and R. Prins
ETH-Zentrum, Universitätstr. 6, 8092 Zürich, Switzerland
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.
© EDP Sciences 1997