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.

Supramolecular Organization in Confined Nanospaces

“…However, the size of the confined clusters depends, in general, on the loading and preparation history of the sample. Operating at relatively mild conditions, cationic aggregates composed by 3, 4, 6 silver atoms – namely, Ag 3 n + , Ag 4 n + , Ag 6 + were obtained; such clusters were characterized by EPR spectroscopy, testing also their reactivity towards ethylene, dioxygen and nitrogen oxide …”

“…Operating at relatively mild conditions, cationic aggregates composed by 3, 4, 6 silver atoms -namely, Ag 3 n + , Ag 4 n + , Ag 6 + were obtained; such clusters were characterized by EPR spectroscopy, testing also their reactivity towards ethylene, dioxygen and nitrogen oxide. [467][468][469][470] Nowadays, silver-containing zeolites can be synthesized with tailored optical properties: [43] they can emit in the whole Figure 28. The void-space architecture of zeolite A features cages of different size, among which silver cluster of outstanding luminescence can be hosted.…”

Zeolite-driven Ag species during redox treatments and catalytic implications for SCO of NH3

“…However, the stabilization of metal atoms or clusters may be difficult because of the strong tendency to form large particles (d > 10 nm). 2 An approach to prevent the aggregation of sub-nanometer metal species is their encapsulation within the pores and cavities of microporous materials, as demonstrated for Pt, Pd, Au, Ag, and metal alloys. [3][4][5][6][7][8][9][10] The stabilization of metal clusters in zeolites hosts has been used to exploit their unusual structural, physical, electronic, magnetic, optical and biocidal properties, motivating several studies on their growth and stability under different conditions.…”

The Line Width of the EPR Signal of Gaseous Nitric Oxide as Determined by Pressure and Temperature-Dependent X-band Continuous Wave Measurements

“…This has been extensively done for NO adsorption on metal oxide surfaces and zeolites [1][2][3][4][5], where the characterization of inner surface sites with respect to applications such as adsorption and catalysis was of central interest. Recently, EPR has also been used to study the NO adsorption over metal organic framework (MOF) materials [6] which might be of interest in the development of NO-releasing drugs [7].…”