Characterization and catalysis of a SiO2-supported [Au6Pt] cluster [(AuPPh3)(6)Pt(PPh3)](2+)/SiO2

Abstract

Structures and catalytic properties of a SiO2-supported [Au6Pt] cluster, [(AuPPh3)(6)Pt(PPh3)](NO3)(2) (1), were characterized by FT-IR, in-situ EXAFS, and TPR, and also by the reactions such as H-2-D-2 equilibration, ethene hydrogenation, and CO oxidation to get insight into the key issues of metal catalysis. The cluster 1 was supported on SiO2 without fragmentation of the cluster framework at room temperature under Ar atmosphere. The cluster framework of 1/SiO2 was stable up to 400 K under vacuum. EXAFS analysis revealed that after heat-treatment of 1/SiO2 at 473 K under vacuum the coordination numbers of Pt-Au and Au-Au(Pt) decreased compared to those for the original clusters due to cluster deformation and after treatment of 1/SiO2 at 773 K the Au particles and Pt-P-# species (P-#: phosphine species) were produced. The combination of TPR and EXAFS showed that the reduction at 603 K caused complete cleavage of Pt-Au and Au-P-# bonds accompanied with the formation of Au particles, while Pt-P-# bonds remained at 773 K. The incipient 1/SiO2 showed catalytic activity in H-2-D-2 and ethene hydrogenation at 303 K without change of the cluster framework. The catalysis of 1/SiO2 was suggested to be referred to the platinum atom which was embedded in the six gold cluster by the results of CO-adsorption, EXAFS, and pulse reaction analysis. This is contrasted to the fact that both Pt(PPh3)(4)/SiO2 and [Au-9(PPh3)(8)](NO3)(3)/SiO2 showed no catalytic activity for these reactions.