Interaction and Reaction of Ethylene and Oxygen on Six-Atom Silver Clusters Supported on LTA Zeolite

Abstract

Mixed silver−sodium containing zeolite LTA was prepared by aqueous ion exchange. Reduction of oxidized Ag/NaA (12 wt %) using hydrogen leads to the formation of a paramagnetic cluster possessing a well-defined structure of six equivalent Ag nuclei. The hyperfine structure of the silver clusters appeared only in Ag+-exchanged zeolite. Continuous wave and pulse EPR techniques revealed that the unpaired spin density distribution was strongly affected by adsorption of C2H4 at room temperature, and the initially symmetric structure of the reduced Ag6+ cluster turned less symmetric or less compact, lifting the equivalence of the silver atoms. The extremely weak isotropic hyperfine coupling of the protons was resolved by Mims 1H-ENDOR and HYSCORE. The temperature dependence of the electron spin−lattice relaxation was studied by pulse EPR applying a two-pulse spin−echo method. The inversion recovery of the C2H4/Ag6+ system was on the order of milliseconds, exhibiting a biexponential character. There was no effect on the hyperfine coupling of the Ag6+ cluster when it was exposed to 16O2 at room temperature. The interaction of 17O2 with the surface of the Ag6+ cluster was monitored using CW EPR. This revealed some anisotropy of the hyperfine structure of the cluster at 3.0 K. The sudden disappearance of the anisotropy in the range 7−10.5 K is ascribed to motional effects by 17O2. The hyperfine structure of the Ag6+ cluster disappeared after coadsorbing C2H4 and 16O2, and a new spectrum appeared, suggesting that an intermediate or product of a catalytic reaction is observed. In-situ EXAFS measurements were performed at the Ag K-edge of the hydrogen reduced silver clusters and after adsorbing C2H4 and coadsorbing C2H4 and O2 on the reduced clusters at room temperature. The local structural parameters were derived from a three-shell fitting model including a split Ag−Ag shell and a single Ag−O shell. Formation of a quite monodisperse structure was favored by in-situ reduction, and the structure of the reduced cluster was compatible with six silver atoms, based on the Ag−Ag coordination of four. The interatomic distance was contracted by <10% as compared to the bulk distance. The multiple-shell fitting proved the increase of the carbon plus oxygen coordination number after coadsorbing C2H4 and O2 which accounts for about one added oxygen molecule per cluster. EPR methods prove the electronic structure alterations of the clusters, seen as shifts of the confined unpaired electron spin density.