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

Abstract

The diffusion and bonding of NO_x (x=1, 2) and Cu²⁺ species in zeolites are reviewed, based mainly on our own research. The molecular motion of adsorbed NO₂ has been examined with EPR in several zeolite samples 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 diffusion model in agreement with earlier results in Vycor glass and Cu-metal. For the diffusion of NO₂ in Na-mordenite and Na-ZSM-5 the 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 NO₂ molecules diffusing along the zeolite channels. In Na-ZSM-5 the spin exchange rate increased rapidly with an increasing Si/Al ratio of the zeolite. The effect was attributed to the hindrance against diffusion by Na⁺, the amount of which increases with a decreased Si/Al ratio. The rates increased with increasing number of H₂O molecules adsorbed on the zeolite because of weaker interaction between NO₂ and the surface. A more detailed slow-motional analysis indicated that at each temperature a distribution of diffusion 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 ΔM_s=1 and ΔM_s=2 features became dominant at higher NO pressure (>100 mbar). The triplet was absent in calcium ion-exchanged A-type zeolite indicating that the NO–NO 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 Cu²⁺ complexes with ammonia and deuterated ammonia have been investigated with electron nuclear double resonance (ENDOR). Simulations of the ENDOR spectra with ¹H, ²H and ¹⁴N hyperfine interactions have been undertaken. The analysis indicates that the complex has a square planar structure with four ammonia ligands.

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