Structure and magnetic properties of six-atom silver clusters supported on LTA zeolite

Abstract

12\% (wt.) silver containing catalysts were prepared by aqueous ion-exchange of Ag$^+$ against Na$^+$ in NaA zeolite. Hydrogen reduction leads to the formation of silver clusters. The results of X-ray absorption spectroscopy experiments at the Ag $K$-edge using direct {\it in-situ} reduction yield a Ag-Ag coordination number of 4.00, which is compatible with a quite mono-disperse structure of ca. six-atomic clusters. $<$ 0.1\% of them is EPR active, revealing isotropic parameters with six equivalent silver nuclei. The magnetic susceptibility in a temperature range of 1.8 K to 300 K exhibits Curie-Weiss behavior, with a magnetization up to $\approx$ 0.81 $\mu_{\rm B}$ per Ag atom, dependent on reduction history. Thus, the overall magnetization reveals many more unpaired electrons than are observed by EPR, but is still less than a single unpaired electron per atom on average. This suggests that a mixture of diamagnetic, EPR active spin-1/2 and EPR silent high-spin species is present. Interestingly, neutral free Ag$_6$ clusters were predicted by theoretical calculations to be diamagnetic and of planar triangular geometry ({\it Phys. Rev. A.} {\bf 75} (2007) 063204).