EPR and ENDOR studies of NOx and Cu2+ in zeolites: bonding and diffusion

Abstract

The di †usion and bonding of (x \ 1, 2) and Cu2`species in zeolites are reviewed, based mainly on our NOx own research. The molecular motion of adsorbed has been examined with EPR in several zeolite samples NO 2 and analyzed using the slow-motional EPR theory. In X-and Y-type zeolites the broadening of the spectra with temperature could be analyzed by simulations using a rotational di †usion model in agreement with earlier results in Vycor glass and Cu-metal. For the di †usion of in Na-mordenite and Na-ZSM-5 the NO 2 broadening of the spectra with increased temperature could be better simulated with the Heisenberg type of spin exchange model. The exchange was attributed to the interaction between molecules di †using along NO 2 the zeolite channels. In Na-ZSM-5 the spin exchange rate increased rapidly with an increasing Si/Al ratio of the zeolite. The e †ect was attributed to the hindrance against di †usion by Na`, the amount of which increases with a decreased Si/Al ratio. The rates increased with increasing number of molecules adsorbed on the H 2 O zeolite because of weaker interaction between and the surface. A more detailed slow-motional analysis NO 2 indicated that at each temperature a distribution of di †usion rates occurs. NO adsorbed on Na-A zeolite was found to be present as monomer at low pressure (\1 mbar) while a dimer characterised by a triplet state ESR spectrum involving and features became dominant at higher NO pressure ([100 mbar). *M s \ 1 *M s \ 2 The triplet was absent in calcium ion-exchanged A-type zeolite indicating that the NOÈNO species depends on Na`for its stability. The detailed structure of a Cu2`complex with water molecules in Cu-ZSM-5 zeolite was characterised with ENDOR at 4 K. The complex has an axial distorted octahedral structure with two water molecules at a longer distance in axial position and four molecules in the equatorial positions. The Cu2c omplexes with ammonia and deuterated ammonia have been investigated with electron nuclear double resonance (ENDOR). Simulations of the ENDOR spectra with 1H, 2H and 14N hyperÐne interactions have been undertaken. The analysis indicates that the complex has a square planar structure with four ammonia ligands.

Visible light induced paramagnetic sites in nitrogen-doped TiO2prepared by a mechanochemical method

“…The observed broadening of the g zz component in figure 2(b) indicates the distribution of the trapped sites of NO. The EPR spectra of NO adsorbed on alkali metal-zeolites show that the g zz values are smaller than 1.90 [25]. Relatively small Ág zz (¼g e À g zz ) in the present system indicates the large splitting of the p à y and p Ã…”

“…We tentatively assigned this species to the NO radical site, because one principal g-value is nearly identical to that of free spin and others are smaller than g e [21,25]. The observed spectral change corresponds to the following reaction:…”

Interaction of CH2OH with silver cation in Ag–A/CH3OH zeolite: A DFT study

“…They can form complexes with many organic molecules -including radicals or ions. Such processes are based on specific interactions between the adsorbates and exchangeable cations [8][9][10][11][12][13][14][15].…”

Adsorption of NO in Li-exchanged zeolite A. A density functional theory study

“…The EPR parameters in those studies differ from each other. Biglino et al have summarized systematically these EPR parameters indicating that, among the cation-exchanged zeolites, Na-exchanged zeolites yield the weakest electrostatic field [8].…”