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 Ag(6)(+) clusters are isolated and all silver atoms are close to equivalent. Upon addition of NO gas at room temperature, the spectrum of the Ag(6)(+) cluster disappears immediately. A new spectrum has the character of adsorbed NO. Its line is split by hyperfine interaction between the unpaired electron with (14)N of the adsorbed NO, and by the nuclear spin of one Ag nucleus to which NO is bound. The experimental isotropic hyperfine coupling a approximately 11.0 G is too large for being observed in the HYSCORE spectrum, but a small (14)N-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 NO(2), but no further activity was observed at room temperature. The NO(2) 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 NO(2). Above 50 K, it becomes rapidly nearly isotropic. Analysis of the correlation time (tau(c)) derived from the EPR lineshapes provides the kinetic parameters, and an Arrhenius representation gives access to the activation energy for the rotational motion.