Cu I -NO Complexes Formed in Zeolites

With recent advancement in the measurement techniques and the data analysis methods ESR spectroscopy is an increasingly important tool in the studies on catalysis and solid surfaces. This chapter focuses on the following five specific subjects relevant to the ESR applications in catalysis and environmental science (a) nitric oxide (NO) adsorbed on zeolites, (b) Cu(I)-NO complexes formed in zeolites, (c) structure and dynamics of organic radicals in zeolites, (d) titanium dioxide (Ti02) semiconductor photo-catalysis, and (e) the superoxide (O2 ) ion radical. 

The ESR parameters of the Cu(I)-NO species are indicative of a N-centered complex with a bent geometry and a significant contribution of the Cu(I) 4s atomic orbital to the SOMO of the Cu(I)-NO complex. That is, the observed relation of fe < Se = 2.0023 orbital contributions [56a] to the SOMO and the large isotropic Cu /i/coupling shows that the structure of the complex is bent so as to allow an effective admixture of the 4s orbital of Cu(I) to the SOMO. Based on the experimental ESR parameters the geometrical structure shown in Fig. 6.7 was proposed for the Cu(I)-NO complex formed in the ZSM-5 zeolite. 

Fig. 6.6 Experimental (solid lines) and simulated (dotted lines) (a) X-, (b) Q-, and (c) W-band ESR spectra of Cu(I)-NO species formed in Cu-ZSM-5 zeolite after activation at 673°C in vacuum and subsequent NO adsorption at pressures of 5 Pa (X-, (2-bands) and 0.5 Pa (IF-band) at 3(X) K. The stick diagrams indicate the Cu h/splitting of the Cu(I)-NO ESR spectrum in the gxxlyy and 22 spectral regions, (b ) Q-band spectra of the Cu(I)-NO complexes showing two different species A and B in the 2z spectral region. The h/splittings of both Cu (natural abundance 69.1 at.%) and Cu (30.9 at.%) isotopes with nuclea spin I = ill have been included in the spectral simulation, but the splittings in the stick diagrams only refer to the Cu isotope. The ESR if/parameters and g-values used for the simulations are listed in Table 6.3. The spectra are adapted from [31] with permission from the American Chemical Society...

Less favorable experimental conditions were met for Cu(I)-NO complexes formed over Cu-ZSM-5 that prevented a determination of the Al hf coupling data because of short electron spin relaxation times and larger distributions of Al nuclear quadrupole couplings, probably due to an inhomogeneous distribution of Al framework atoms. Detailed local structures of the complexes in Cu-ZSM-5 zeolites, 02-Al-02-Cu(l)-N0, were recently proposed on the basis of quantum chemical calculations [59]. To experimentally verify the theoretically proposed structural properties of the Cu(l)-NO species formed in ZSM-5, it is highly desirable to develop improved synthesis strategies for high siliceous zeolites that lead to a better statistical Al distribution in the crystallites. 

The local structure of Cu(I)-NO adsorption complexes formed over Cu-L and Cu-ZSM-5 zeolites were studied by pulsed ENDOR and HYSCORE methods by Umamaheswari et al. [53]. The H ENDOR signals from residual distant protons were not detected in completely copper ion exchanged Cu-ZSM-5 zeolites. Such signals were, however, observed for the Cu-L zeolite, where the H form of the zeolite was 30% ion exchanged with Cu(II) ions and subsequently dehydrated to (auto)reduce Cu(II) to Cu(I). For both systems, very broad A1 ENDOR spectra were observed. The Al hf couplings were estimated using the point dipole approximation for the Cu(I)-NO center in Cu-L. The result shows that an aluminum framework atom is located in the third coordination sphere with respect to the NO molecule adsorbed on a Cu(I) cation site. 

In order to know whether the Pd ions or complexes are anchored to the zeolite framework or not, the IR framework vibrations of Pd-H-ZSM-5(0.49) were investigated (Figure 5). After activation under O2, a weak band at 930 cm" forms. Upon NO adsorption, the 930 cm band disappear while a new band appears at 980 cm". These bands are attributed to asymmetric internal stretching vibrations of T-O-T bonds (T = Si or Al) perturbed by Pd ions. The higher the perturbation, the lower the frequency. Therefore, the 930 cm band could be related to anchored Pd(II) ions or complexes formed upon decomposition of exchanged complexes, and the 980 cm band could be due to Pd(I) nitrosyl entities formed upon NO contact. Similar observations were found on Cu-ZSM-5 catalysts (34). 

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