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

Abstract

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 a^N_iso≈ 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 (τ_c) derived from the EPR lineshapes provides the kinetic parameters, and an Arrhenius representation gives access to the activation energy for the rotational motion.