Electron paramagnetic resonance studies of a platinum cluster in Linde L and faujasite zeolites

Abstract

Continuous wave and pulse EPR methods were used to investigate structure and chemical behaviour of paramagnetic platinum clusters in L and faujasite type zeolites. After reduction with hydrogen of a sample with a platinum content of 10% (wt.) a well resolved EPR spectrum reveals the presence of a unique, well defined cluster with 12 equivalent platinum nuclei and 12 hydrogen atoms chemisorbed to its surface. A plausible structure of this cluster is that of an icosahedron, Pt₁₃H₁₂ⁿ⁺, with n = 1 or 3, where the platinum atom in the center carries no significant spin density and is therefore EPR invisible. The hyperfine and g tensors are of axial symmetry, revealing a slight axial distortion of the icosahedron that is suggested to occur without noticeable loss of equivalence of the nuclei. The distortion reflects the influence of the cluster environment, conceivably a ring of 12 tetrahedral atoms (Si or Al), which is present in the large channels of Linde L as well as in the faujasite supercages. Chemisorbed D is readily exchanged for H upon exposure to H₂ at room temperature, but exposure to CO leads to the disintegration of the cluster. This is the first report of a Pt₁₃H₁₂ⁿ⁺ cluster, and the clear finding of one major well defined paramagnetic cluster species is in provocative disagreement with the widely accepted view that supported metal clusters are always of a non-uniform distribution of size and shape.