Nitrogen oxide reaction with six-atom silver clusters supported on LTA zeolite

Silver containing catalysts were prepared by aqueous ion exchange of Ag+ against Na+ in an LTA zeolite. A well-defined paramagnetic cluster consisting of six equivalent silver nuclei was obtained after oxidation and hydrogen reduction. Continuous wave EPR demonstrates that the reduced Ag6+ clusters are isolated and all silver atoms are close to equivalent. Upon addition of NO gas at room temperature, the spectrum of the Ag6+ cluster disappears immediately. A new spectrum has the character of adsorbed NO. Its line is split by hyperfine interaction between the unpaired electron with 14N of the adsorbed NO, and by the nuclear spin of one Ag nucleus to which NO is bound. The experimental isotropic hyperfine coupling aNiso≈ 11.0 G is too large for being observed in the HYSCORE spectrum, but a small 14N-hyperfine interaction is observed and assigned to a second nitrogen spin in the vicinity of the silver cluster. In the presence of trace oxygen NO transforms slowly into NO2, but no further activity was observed at room temperature. The NO2 molecule exhibits dynamics in the temperature range of 30 to 125 K. The rotational diffusion is axially symmetric about the molecular y axis of the adsorbed NO2. Above 50 K, it becomes rapidly nearly isotropic. Analysis of the correlation time (τc) derived from the EPR lineshapes provides the kinetic parameters, and an Arrhenius representation gives access to the activation energy for the rotational motion.