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

The diffusion and bonding of NOx (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 Delta M-s = 1 and Delta M-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 H-1, H-2 and N-14 hyperfine interactions have been undertaken. The analysis indicates that the complex has a square planar structure with four ammonia ligands.