EPR study of NO adsorption-desorption behaviour on Lewis acid sites in NaA zeolites

Nitrogen monoxide (NO) introduced into partially and fully lithium ion (Li + )-exchanged A-type zeolite was studied by X-band EPR measurements. Two types of NO species, NO monoradical and NO-NO biradical, were detected for the partially ion-exchanged zeolite, while the latter was less detectable for the fully ion-exchanged one. The EPR parameters of the NO monoradical provided evidence that the electrostatic field associated with Li + ions in A-type zeolites is weaker than that with Na + ions. It was found that the molecular distance of NO biradical formed in partially Li + -exchanged zeolite was shorter than that in sodium ion-exchanged A-type zeolite.

Section: Introductionsupporting

confidence: 62%

Section: Introductionmentioning

confidence: 99%

EPR study on NO introduced into lithium ion-exchanged LTA zeolites

Yahiro

Kurohagi

Okada

et al. 2002

“…Adsorption behavior of NO in various zeolites was extensively studied by means of continuous wave and pulse EPR spectroscopy. [13][14][15][16][17][18] The orbital degeneracy and the orientation of the unpaired electron with respect to the orbital angular momentum in the external magnetic field leads to molecular ground states 2 P 1/2 and 2 P 3/2 which are only slightly split by spin-orbit interaction in the free NO molecule. In the 2 P 1/2 state, the spin and orbital angular momenta are antiparallel, thus the spin magnetism is compensated.…”

Section: Introductionmentioning

confidence: 99%

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

Baldansuren

Eichel

Roduner

2009

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.

“…[122][123][124][125] The NO molecule has also attracted considerable interest since it can be used as a paramagnetic probe molecule to characterize the structure, concentration, and acid strength of Lewis acid sites on the basis of its unique chemical and spectroscopic properties. [126][127][128][129][130][131][132] Such Lewis acid sites in zeolites can be cations or aluminum defect centers (Al x O y ) n1 , both of which form adsorption sites for NO molecules. The particular adsorption sites and coordination geometry of the NO adsorption complexes can be obtained from the hyperfine interaction between the unpaired electron spin of the NO molecules and the nuclear spin of the metal ions which form the Lewis acid sites.…”

Section: No In Zeolitesmentioning

confidence: 99%

“…Unfortunately, metal ion hyperfine couplings have only been resolved in the CW EPR spectra of a few NO adsorption complexes, including (Al x O y ) n1 -NO [126][127][128][129][130][131] and Cu 1 -NO. [133][134][135] In the case of Na 1 -NO adsorption complexes, the CW EPR spectra do not resolve the sodium hyperfine coupling, [127][128][129]132,136 and ENDOR had to be applied. 137 A series of orientation-selective W-band Davies ENDOR spectra of the Na 1 -NO adsorption complex in zeolite NaA is shown in Fig.…”

Section: No In Zeolitesmentioning

confidence: 99%

High field ENDOR as a characterization tool for functional sites in microporous materials

Goldfarb

2006

The determination of the details of the spatial and electronic structure of functional sites (centers) in any system, be it in materials chemistry or in biology, is the first step towards understanding their function. When such sites happen to be paramagnetic in any point of their activity cycle, the tool box offered by a variety of high resolution electron paramagnetic resonance (EPR) spectroscopic techniques becomes very attractive for their characterization. This tool box has been considerably expanded by the developments in high field (HF) EPR in general, and HF electron nuclear double resonance (ENDOR), in particular. These have led to numerous new applications in the fields of biology, physics, chemistry and materials sciences. This overview focuses specifically on recent applications of pulsed HF ENDOR spectroscopy to microporous materials, such as zeotype materials, presenting the new opportunities it offers. First, a brief description of the theoretical basis required for the analysis of the HF ENDOR spectrum is given, followed by a description of the pulsed techniques used to record spectra and assign the signals, along with a brief presentation of the required instrumentation. Next, specific applications are given, including transition metal ions and complexes exchanged into zeolite cages, transition metal substitution into frameworks of zeolites, aluminophosphate molecular sieves, and silicious mesoporous materials, the interaction of NO with Lewis sites in zeolite cages and trapped S. We end with a discussion of the advantages and the shortcomings of the method and conclude with a future outlook.