Applications to Catalysis and Environmental Science
Electronic and geometrical structures of NO-Na+ and Cu(I)-NO complexes formed in zeolites are discussed based on the g and the 14N and 23Na hf values evaluated by multi-frequency ESR, pulsed ENDOR and HYSCORE methods. The structure of (NO)2 bi-radical formed in zeolites is discussed based on X- and Q-band ESR spectra. Microenvironment effects on the molecular dynamics and the thermal stability of triethyl- and tripropyl-amine radical cations as spin probes are presented referring to the CW-X-band ESR results and theoretical DFT calculations. X- and Q-band ESR studies on nitrogen-doped TiO2 semiconductor reveal that the diamagnetic N– ion in the system absorbs visible light so as to excite an electron of N– to the conduction band. The photo-catalytic reactions of TiO2 are modified by introducing O2 molecules which scavenge a fraction of photoexcited electrons to generate O2–. ESR spectral characteristics of adsorbed O2–, g-tensor and hf structure of labeled 17O (I = 7/2), are presented.
To understand catalytic reactions it is indispensable to characterize catalytic materials and to clarify static and dynamic structures of reaction intermediates as well as active sites of reactions. Paramagnetic species are in general involved in many catalytic reactions as reaction intermediates and/or active sites, especially in heterogeneous catalytic reactions. Thus, the ESR method has played an important role to get valuable information on catalytic and/or surface reactions with high selectivity and high sensitivity, which has not been achieved by any other methods.
ESR applications to catalysis and solid surfaces have started in the beginning of the 1960s. The studies on Tigullar-Natta catalysis by Angelescu , and cromina-alumina catalysis by O’Reilly  are pioneer works in the field. Since then a large number of studies have been reported so far, including some important review papers and books cited as references [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. They cover a broad range of research subjects: (a) ESR characterization of oxide supported transition metal ions/complexes relevant to catalysis and/or environmental pollutant control, (b) ESR identification and quantitative measurements of reactive organic and inorganic radical species formed on catalytic surfaces, (c) catalytic and photo-catalytic reaction dynamics of radical species, (d) radicals on surfaces formed by ionizing radiation, (e) nature of surface centers and reactivity with adsorbed molecules, (f) chemical bonding or electronic structure of paramagnetic reaction intermediates like radical species, (g) diffusion and molecular dynamics of radicals on porous heterogeneous systems, etc.
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 (TiO2) semiconductor photo-catalysis, and (e) the superoxide (O2–) ion radical.
6.2 Surface Probing: Nitric Oxide Interactions with Metal Ions in Zeolites
Nitric oxide (NO) is an odd-electron molecule possessing one unpaired electron with electronic configuration, [(K2K2)–(2 sσ)2(2 sσ*)2(2pπ)4(2pσ)2(2pπ*)1]. The reactions of NO molecule with metal ions are one of the major topics in catalysis and environmental research as well as in biochemistry and coordination chemistry [13, 17]. In catalysis and environmental studies a large number of researchers have been interested in the decomposition of NO into N2 and O2 over transition metal ion exchanged zeolites [18, 19, 20, 21, 22]. The NO molecule has been used also as a paramagnetic (spin) probe to characterize the catalytic activity, particularly the structure, concentration and acid strength of Lewis acid surface sites and metal ions of nanoporous materials including zeolites [3, 4, 8, 23, 24, 25, 26, 27, 28]. Here ESR is the most appropriate spectroscopic method for the detection and identification of paramagnetic NO and can potentially provide valuable experimental information about the structure and dynamics of NO molecules interacting with metal ions and of the reactions involved.
Lunsford [3b] and Hoffman and Nelson  first reported the ESR spectra for adsorbed NO molecules. Then, Kasai [4b] revealed that ESR spectra of NO probe molecules are very sensitive to the interaction with metal ions and Lewis acid sites in zeolites. The earlier ESR studies of the NO/zeolite system have been summarized in several review papers [3a, 4a, 8]. A number of ESR studies have been also carried out for NO adsorbed on metal oxides such as MgO and ZnO as reviewed by Che and Giamello . Modern ESR techniques such as pulsed ESR [25, 26, 27], ENDOR (Electron Nuclear Double Resonance) , and multi-frequency (X-, Q-, and W-band) ESR  are especially useful for an unambiguous identification of the ESR magnetic parameters (g, hyperfine A, and quadrupole tensors, etc.) and, consequently, for a detailed characterization of structural changes and motional dynamics involved. Some recent advancements in ESR studies on NO adsorbed on zeolites are presented in this section.
The decomposition of nitric oxide, a process in which NO is converted to harmless nitrogen and oxygen, deserves considerable practical attention, as the oxides of nitrogen are regarded as the major cause of air pollution [18, 19, 20, 21, 22]. A number of transition metal ion exchanged zeolites have been reported to be active for NO decomposition. Among them, the copper exchanged high siliceous zeolites such as Cu-ZSM-5 have been observed to be highly active. The decomposition of NO has been reported to occur via the formation of a Cu(I)-(NO)2 dimer, whose precursor is a Cu(I)-NO monomer . This subject is presented separately in Section 6.3.
6.2.1 NO-Na+ Complex Formed in Zeolites
g-Tensor Anisotropy of Adsorbed NO
Here “ge” is the g-value for the “free” electron, “λ” the spin-orbit coupling constant of NO (123.16 cm–1), “E” the energy splitting between 2pσ* and 2pπ*(y) orbitals, and “Δ” that between 2pπ*(x) and 2pπ*(y) orbitals, see Fig. 6.2. The quantity “l” stands for a covalency factor, which equals one for free NO or for purely ionic bonding and offers the opportunity to correct the gzz principal value for possible spin delocalization (l < 1) . The energy splittings, “E” and “Δ”, are affected by the local electronic structure, leading to changes in the g tensors of NO adsorbed on different solids. The experimentally obtained principal values of the g tensor allow in principle to determine the three unknown parameters, “E”, “Δ”, and “l” using Eqs. (6.1), (6.2) and (6.3).
To evaluate accurate values of parameters E, Δ and l, three principal values of gxx, gyy and gzz have to be determined with a high degree of accuracy. The orthorhombic distortion, Δg = gxx–gyy, is of the order of 10–3 or even less, but can be resolved by multi-frequency ESR measurements . For the NO adsorption complexes at metal oxide surfaces, a distribution of the g tensor principal values and the corresponding splitting energies are expected because of the inhomogeneous properties of the surface and the varying orientations of the NO complexes. Thus, the linewidths of the ESR spectra can contain information about the g value distribution from which one can evaluate the distributions δE and δD of the splitting energies E and D.
Multi-Frequency ESR Spectra
An important advantage of using multi-frequency ESR spectroscopies is to separate the g and hyperfine (hf) A tensor components and to resolve the weak deviation (gxx–gyy) from the axially symmetry of the g tensor so as to evaluate accurate values of E, Δ and l according to Eqs. (6.1), (6.2) and (6.3).
Experimental and computed g, A(14N) and A(23Na), and Q(23Na) principal values of Na+-NO complexes formed in Na-LTA and Na-ZSM-5 zeolites together with the energy splitting E, the crystal field parameters Δ, l and their distribution widths δE, δΔ and δl
X-band (5 K)
25.3 ± 0.2
91.0 ± 0.5
26.3 ± 0.2
W-band (4.3 K)
6.3 ± 0.2
6.3 ± 0.2
10.9 ± 0.2
W-band (4.3 K)
The relative distribution of width, δΔrel (≡ δΔ/Δ) ≈ 0.2% for Na-LTA/NO is much narrower than that of δΔrel ≈ 31% for Na-ZSM-5/NO. This indicates that the LTA-type zeolite has a uniform structure, the local electric fields at the sodium ion adsorption sites do not significantly vary and the ion sites display uniform chemical properties in Na-LTA. In contrast, the relatively wide distribution of δΔrel for Na-ZSM-5/NO system suggests a variety of adsorption sites, which may originate from the more complicated structure for Na-ZSM-5 zeolites (possessing not only straight, but also zigzag channels) as well as the randomly distributed Al atoms in the Si-O-Al lattice. Thus, multifrequency ESR measurements with a NO probe molecule can be a very sensitive method to monitor the different site distributions in nanoporous materials.
14N and 23Na Hyperfine Couplings and Structure of NO-Na+ Complex
For the NO-Na+ complex in Na-LTA zeolite the ESR spectrum is characterised by a g-tensor with principal values of gxx = 1.999, gyy = 1.993, gzz = 1.884, and a 14N hyperfine coupling with Axx= Azz ≈ 0, Ayy = 91 MHz (Table 6.1); refer to Section 18.104.22.168 (Surface complex structures) in Chapter 3. The deviation of the g tensor from axial symmetry, although small, is resolved at W-band with the gxx spectral position distinguished from the gyy region as mentioned above. Pöppl et al.  have successfully employed pulsed ENDOR spectroscopy at X- and W-band frequencies to precisely evaluate the 14N(I = 1) and 23Na(I = 3/2) hf couplings and to characterize the geometrical and electronic structure of NO-Na+ complex in Na-LTA type zeolite at low temperature.
The principal values and even the orientation of the principal axes of the 23Na hyperfine coupling tensor with respect to axes of the g tensor could be determined from Mims’ and Davies’ pulsed ENDOR spectra, refer to Section 2.3.3 in Chapter 2. The values Axx(23Na) = Ayy(23Na) = 6.3 and Azz(23Na) = 10.9 MHz were obtained by simulation taking angular selection into account. The so-called hyperfine enhancement of ENDOR intensities due to the interaction between the radio frequency field and the electron spin could lead to pronounced differences in the ENDOR intensities between signals from different ms electron spin states in experiments at conventional MW frequencies such as in X-band, but also at the W-band. The 23Na (I = 3/2) nuclear quadrupole tensor is almost coaxial to the A tensor, Qzz = 0.48 MHz, Qyy = –0.07 MHz, and Qxx = –0.41 MHz. Simulation of orientation-selective ENDOR spectra as described in [26, 33] serves to refine the principal values of the hyperfine coupling tensors estimated from experiment. In addition the influence of the spectra on the orientation of the corresponding principal axes can be examined by simulation. The axes are specified in the principal axes system of the g-tensor.
In the X-band CW-ESR spectra of the NO-Na+ complex, the 14N (I = 1) hf splittings of Axx(14N) and Azz(14N) values were too small to be resolved and the third principal value of Ayy(14N) was only detected. Orientation selective ENDOR spectroscopy was therefore applied to determine the couplings along the x and z axes of the g tensor, yielding the principal values Axx(14N) = 25.3 and Azz(14N) = 26.3 MHz.
The 14N and the 23Na hyperfine interactions were finally employed to obtain the spin densities in the molecular orbitals of the NO-Na+ complex to give insight into the electronic structure of the adsorption complex. An isotropic hf coupling of Aiso(23Na) = 7.8 MHz was evaluated from the above principal values of the A(23Na) tensor. From the Aiso(23Na) value an unpaired electron spin density in the Na 3 s orbital is evaluated to be ρ3 s(Na) = 0.9% . In addition, judging from the small anisotropic values of the A(23Na) tensor, Bzz(23Na) ≡ Azz – Aiso = 3.1 ± 0.2 MHz, the spin density in Na 3p orbitals is negligible. Thus, the unpaired electron in the Na+-NO complex is concluded to be mainly localized at the NO molecule.
Liu et al.  have recently reported a computational study on the adsorption site and the ESR magnetic parameters of NO adsorbed in Na-LTA zeolite employing density functional theory (DFT); see Chapter 5 for “DFT”. A rather good correspondence was obtained between the experimental and computed electronic g and A(14N) tensors, and the A(23Na) and Q(23Na) tensors of the Na+-NO complex, as summarized in Table 6.1. For the computations the following model of zeolite network were employed: a six-membered ring terminated by hydrogen atoms with one Na+ ion above the ring, three additional Na+ ions located at the centers of three imagined four-membered rings adjacent to the six-membered ring, and three additional four-membered rings adjacent to the six-membered ring. The optimized geometry of the complex agrees nicely with that estimated experimentally, except for the Na-N distance, where the computations resulted in R(N-Na) = 0.266 nm which is longer by as much as 0.05 nm than that deduced from the previous ENDOR experiments (0.21 nm) .
6.2.2 Triplet State of (NO)2 Bi-Radical Formed in Zeolite
Kasai et al. [4c] have first reported that the CW X-band ESR spectrum of NO adsorbed on Na-LTA zeolite consists of two signals, one due to the NO-Na+ complex (NO mono-radical) as described in the above section and the other due to an unusual NO—NO dimer species with a triplet state (referred to as the (NO)2 bi-radical or radical pair in the following). The ESR spectrum of (NO)2 bi-radical shows the forbidden transition, ΔmS = 2, at ca. 170 mT (g ≈ 4), when the corresponding allowed transitions, ΔmS = 1, are observed for the same sample at ca. 340 mT (g ≈ 2). These verify the presence of the triplet electronic state. Thus the ESR study suggested that the zeolite can stabilize the (NO)2 dimer as the triplet rather than the usual singlet state, indicating a great affinity of the NO molecule for the zeolites. The (NO)2 bi-radical may play an important role as an intermediate species in the decomposition of NO . ESR studies have accordingly continued to be of interest in recent decades.
X- and Q-Band ESR Spectra
ESR g-tensor and zero-field splitting (ZFS) tensor for (NO)2 bi-radical formed in Na-LTA zeolite and other matrices
CW Q-band ESR
Pulsed ESR was employed to study the (NO)2 triplet-state bi-radical in Na-LTA type zeolite, with the purpose to resolve the interaction with surface groups, and to elucidate the role of the zeolite in stabilizing the triplet state rather than the usual singlet state . Measurements performed at 5 K gave rise to FT (Fourier Transformation, see Chapter 2) spectra that were assigned to the (NO)2 bi-radical interacting with one or two 23Na nuclei (with I = 3/2), with A(23Na) = (4.6, 4.6, 8.2) MHz and Q(23Na) = (–0.3, –0.3, 0.6) MHz for the hyperfine and nuclear quadrupole coupling tensors, respectively. The values are of similar magnitude as those determined for the NO-Na+ complex (see Table 6.1).
6.2.3 Other Nitrogen Oxides as Spin Probes
Other nitrogen oxides such as nitrogen dioxide (NO2) have been employed as a spin probe to characterize the zeolite structure, chemical properties of zeolites, and motional dynamics of small molecules in them by ESR .
NO2 is a stable paramagnetic gaseous molecule at normal temperatures. The ESR parameters of NO2 trapped in a solid matrix have been well established from single-crystal ESR measurements and have been related to the electronic structure by molecular orbital studies . Thus, the NO2 molecule has potential as a spin probe for the study of molecular dynamics at the gas-solid interface by ESR. More than two decade ago temperature-dependent ESR spectra of NO2 adsorbed on porous Vycor quartz glass were observed ; Vycor® is the registered trademark of Corning, Inc. and more information is available at their website. The ESR spectral line-shapes were simulated using the slow-motional ESR theory for various rotational diffusion models developed by Freed and his collaborators . The results show that the NO2 adsorbed on Vycor displays predominantly an axial symmetrical rotation about the axis parallel to the O—O inter-nuclear axis below 77 K, but above this temperature the motion becomes close to an isotropic rotation probably due to a translational diffusion mechanism.
In contrast to the NO2/Vycor glass system, translational diffusion (or Heisenberg type of spin exchange) of NO2 in Na-MOR, Na-MFI and K-LTL types of zeolites were suggested by analysis of the temperature dependence of the ESR spectra using the slow motional theory [13, 42, 43]. Broadening at increased temperature is assigned to the spin exchange between the NO2 molecules diffusing along zeolite channels. In Na-MFI the spin exchange rate increased rapidly with increasing Si/Al ratio of the zeolite, as expected if the hindrance against diffusion is caused by Na+, the amount of which increases with a decreased Si/Al ratio . The diffusion was also affected by the water content and by the channel structure, but not appreciably by replacing Na+ with Li+, Ca2+, Sr2+, K+ or Cs+ . A detailed investigation of the NO2/Na-MOR system has suggested that there exists a distribution of exchange rates at each temperature .
For the characterization of acid sites on solids such as silica-alumina, alumina, and zeolites, di-tert-butyl nitroxide (DTBN) , a stable organic nitroxide radical, was employed as a spin probe molecule by Hoffman et al. [47, 48] and others [49, 50]. Gutjahr et al.  studied the electron pair acceptor properties of the monovalent cations such as Li+, Na+, K+, and Cs+ in a faujacite (Y) type of zeolite by means of ESR using DTBN as a probe molecule. They reported a linear relationship between the nitrogen spin density estimated from the nitrogen hf constants of DTBN and the electro-negativity of the zeolite cations.
6.3 Cu(I)-NO Complexes Formed in Zeolites
Nitrogen oxides (NOx) are hazardous pollutants formed as byproducts during the combustion processes in industrial boilers and vehicle engines and are responsible for smog formation, acid rain, and global warming [18, 19, 20, 21, 22]. Many metal ion exchanged zeolites have been reported to be active for catalytic decomposition and reduction of nitrogen monoxide. Among them, copper ion exchanged zeolites have received much attention due to their high activity and selectivity toward the decomposition of NOx. High siliceous zeolites such as ZSM-5 and MCM-22 have been reported to be promising for the NO decomposition process [19, 29, 31, 51, 52, 53, 54, 55]. The Cu(I)-NO complexes have attracted special interest because they are important intermediates in the catalytic decomposition of nitric oxide over copper exchanged zeolites. The interaction of NO with Cu(I) is reported to be a complex redox (reduction-oxidation) process where the oxidation of the site is proposed to occur via the elimination of N2O from dinitrosyl through the formation of Cu(I)-(NO)2 from monomeric Cu(I)-NO complexes .
The Cu(I)-NO adsorption complexes formed on copper exchanged and auto-reduced Cu-ZSM-5 and Cu-MCM-22 and other zeolites were extensively studied by Giamello et al.  and by Pöppl et al. [31, 51, 52, 53, 54, 55] using multi frequency ESR and ENDOR spectroscopies. The increased spectral resolution and separation of the Cu(I)-NO signals from the Cu(II) signal at higher frequencies (Q- and W-bands) allowed an accurate determination of the 63,65Cu and 14N hf couplings and g-values from the powder ESR spectrum and successfully led to a detailed characterization of the Cu(I)-NO complexes . Furthermore, pulsed electron nuclear double resonance (ENDOR), and hyperfine sublevel correlation spectroscopy (HYSCORE) were employed to characterize the local structure of Cu(I)-NO adsorption complexes formed over Cu-L and Cu-ZSM-5 type of zeolites : see Chapter 2 for pulsed ENDOR and HYSCORE.
6.3.1 Multifrequency ESR Spectra
ESR parameters (g and Ahf tensors) and covalent parameter “l” of Cu(I)-NO complexes formed over Cu-ZSM-5 and Cu-MCM-22 zeolitesa
A(14N)/10–4 cm–1 b
A(63Cu)/10–4 cm–1 b
The ESR signal intensity at low temperature of the Cu(I)-NO species in ZSM-5 zeolite decreased as a function of increased NO loading suggesting the formation of diamagnetic Cu(I)-(NO)2 species at the expense of paramagnetic Cu(I)-NO. The Cu(I)-(NO)2 is ESR silent with a singlet (S = 0) ground state as predicted by quantum chemical calculations . This is in contrast to the triplet (S = 1) ground state that has been experimentally confirmed for Na+-(NO)2 complexes in Na-A zeolites as mentioned in Section 6.2. The ESR intensity due to the isolated Cu(II) cations was essentially independent of the NO loading at low temperatures, suggesting that the NO molecules do not form adsorption complexes with Cu(II) cations remaining after auto-reduction.
Two Cu(I)-NO complexes, A and B, were observed in Cu-ZSM-5 (and Cu-MCM-22) zeolites with similar ESR parameters, see Table 6.3. By comparing the experimental hf couplings of Aiso(63Cu; A) < Aiso(63Cu; B) with the results of theoretical computations  species A and B were tentatively assigned to Cu(I)-NO complexes with two and three oxygen neighbors, (RO)2Cu(I)-NO and (RO)3Cu(I)-NO, respectively. The assignment is qualitatively supported by the order of the covalent parameter of the two species, l(A) > l(B), derived from the corresponding gzz values, indicating a larger transfer of unpaired electron spin density from the 2pπy* orbital of the NO to the Cu(I) ion for species B than for A.
6.3.2 Pulsed ENDOR and HYSCORE Studies
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. . The 1H 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 1H 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 27Al ENDOR spectra were observed. The 27Al 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.
Less favorable experimental conditions were met for Cu(I)-NO complexes formed over Cu-ZSM-5 that prevented a determination of the 27Al hf coupling data because of short electron spin relaxation times and larger distributions of 27Al nuclear quadrupole couplings, probably due to an inhomogeneous distribution of Al framework atoms. Detailed local structures of the complexes in Cu-ZSM-5 zeolites, O2-Al-O2-Cu(I)-NO, were recently proposed on the basis of quantum chemical calculations . To experimentally verify the theoretically proposed structural properties of the Cu(I)-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.
6.4 Molecular Motion Probes: Radicals in Zeolites
In the foregoing sections the ESR studies of NO and other nitrogen oxides as a spin probe were shown to be useful for understanding the electronic and geometrical structure and the dynamical processes of molecules adsorbed on zeolites. The dynamics of NOx (x = 1, 2) radicals are strongly dependent on properties of zeolites such as channel structure (multiple-channel or single-channel) and channel size. These observations indicate that the microenvironment plays an important role for the molecular dynamics of molecules incorporated in it. In this section applications are presented of X-band ESR spectroscopy to investigate microenvironment effects on molecular dynamics and thermal stability of relatively large organic radicals such as triethylamine ((CH3CH2)3N+; Et3N+), and tripropylamine ((CH3CH2CH2)3N+; Pr3N+) cations used as spin probes [12, 60, 61, 62].
Amines have comparatively low ionization potentials (Ip1 = 7.82 and 7.50 eV for Me3N and Et3N, respectively  and are used as electron donors . Furthermore, amines are widely used as organic templates in synthesizing zeolites. Zeolites provide an appropriate microenvironment to retard back electron transfer and increase the lifetime of photoproduced radical ions, and long-living radicals could be observed even at room temperature in them . Thus, zeolites incorporated with amines have attracted interest to investigate the molecular dynamics of radical cations such as Et3N+ and Pr3N+ formed inside the void structures by ionizing radiation. By analyzing the temperature-dependent X-band ESR spectral line-shapes experimental information about molecular dynamics, especially rotational motion as well as electronic and geometrical structures, were obtained.
6.4.1 Structure and Dynamics of Et3N+ and Pr3N+ in AlPO4-5
AlPO4-5, which is typical of the AlPO4 family composed of AlO4 and PO4 tetrahedra , contains alternating Al and P atoms forming four- and six-membered rings. The Et3N and Pr3N encapsulated into AlPO4-5 take on a tripod shape in the cylindrical channel. The Et3N+ radicals were generated inside the void structure of the AlPO4-5 and gave well-resolved ESR spectra which allow to observe temperature-dependent ESR lineshapes over a wide temperature range, from 4 to 300 K . The temperature-dependent spectra were successfully explained assuming a two-site exchange model of inequivalent β-methylene hydrogens with respect to the central 14N 2pz orbital with the unpaired electron.
High Temperature ESR Spectra
Low Temperature ESR Spectra
The ESR spectrum of Et3N+ at 77 K corresponds to a rigid-limit (Fig. 6.8(b)). The spectrum consists of a broad hf sextet whose two outer-most lines have a 4.4 mT splitting, but the central quartet has a splitting of 3.6 mT. The quartet is attributed to three equivalent hydrogens, one from each β-methylene group, and the weak wing lines to the parallel component of mI(14N) = ±1 transitions. The experimental spectrum is well simulated using the following ESR parameters: aiso(1H) = 3.6 mT for three equivalent β-hydrogens (neglecting the contribution of other hydrogens), A||(14N) = 4.4 mT, A⊥(14N) = 0 ± 0.4 mT, giso = 2.003 and Gaussian linewidth of 1.2 mT.
Assuming the hyperconjugative mechanism [56b] for the β-hydrogen hf splittings the twist angles in Fig. 6.8(c) can be evaluated to θ(1) = 44° and θ(2) = 76°, which correspond to the 3.6 and 0.4 mT hf splittings, respectively, refer to Appendix A6.1.
Analysis of Temperature-Dependent ESR Line-Shapes
6.4.2 Cage Effects on Stability and Molecular Dynamics of Amine Radicals in Zeolites
6.5 Titanium Dioxide (TiO2)Semiconductor Photocatalysis
Semiconductor-based heterogeneous photocatalysts have been interested by a large number of scientists. Titanium dioxide (TiO2), which is inexpensive, nontoxic, resistant to photo-corrosion, and has high oxidative power, is the most widely used material for heterogeneous photocatalysis. The properties of TiO2 have made it a target for industrial uses including chemical synthesis, solar energy conversion and storage, environmental remediation, odor control, sensors, and protective coatings [15, 73, 74, 75, 76, 77].
A number of TiO2 samples have been developed as photocatalysts and shown to exhibit various photoefficiencies and chemical selectivities . The anatase phase of TiO2 has been in general considered to have the higher activity as an oxidative photocatalyst in comparison with the rutile phase [80, 81], see Appendix A6.2. Furthermore, in addition to the single phase TiO2, a mixed phase TiO2 is also commercially available. For example, Degussa P25 is produced by the addition of rutile to anatase with the ratio of ca. 1:4 and shows an unusually high activity [73, 74, 82]. Factors contributing to the increased activity include high surface areas, high adsorption affinity for organic compounds and lower recombination rates. Each of these surface and interface dependent factors increases the capability of chemical reaction by the photogenerated holes and electrons.
An important drawback of TiO2 for photocatalysis is that its band-gap is rather large, 3.0–3.2 eV (λ: 380–410 nm), and only a small fraction of the solar spectrum (λ < 380 nm, corresponding to the UV region) is absorbed. Sunlight can be more efficiently used in photocatalysis under visible light, rather than UV light. The potential technological impact of this system is huge and, to lower the threshold energy for photoexcitation, a large number of studies have focused on TiO2 doped with both transition metal and non-metal impurities [16, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96].
Due to its high sensitivity and its capability of an accurate characterization of paramagnetic species, ESR spectroscopy is particularly useful for the investigation of paramagnetic centers in the solid state. Thus, the ESR method has been extensively applied to the photo-catalytic TiO2 system and the identification and characterization of paramagnetic species such as electrons, holes and their reaction products generated by photocatalytic reactions [15, 16, 96, 97, 98, 99, 100, 101, 102]. We start with ESR applications to the nitrogen-doped TiO2 system.
6.5.1 Nitrogen-Doped TiO2
One of the most promising and widely investigated systems is nitrogen-doped titanium dioxide, N-TiO2, which shows a significant catalytic activity in various reactions performed under visible light irradiation [83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95]. A highly efficient dye-sensitized solar cell (DSC) has been recently fabricated using a nanocrystalline nitrogen-doped titania electrode .
Various preparation methods have been employed to dope nitrogen into TiO2 either based on chemical reactivity (sol-gel synthesis, chemical treatments of the bare oxide, oxidation of titanium nitride, etc.) or on physical methods (ion implantation, magnetron sputtering) . These different procedures lead, at least in some cases, to materials with somewhat different chemical and physical properties. In addition to the preparation method, many studies have addressed the electronic states associated to the N-impurities including the questions of localized or delocalized states.
Giamello et al. have extensively carried out ESR studies on paramagnetic species in N-doped anatase TiO2 powders obtained by sol-gel synthesis [16, 96, 102, 104]. Based on a combination of theoretical DFT calculations the paramagnetic N-impurity dopant has been successfully connected to the absorption of visible light and to the photoinduced electron transfer from the bulk to a surface-adsorbed electron scavenger such as molecular oxygen (O2).
ESR Spectra of Nitrogen Centered Radical (Nb·)
ESR parameters of various paramagnetic species identified in N-doped TiO2 systema
The electron spin density (SD) in the 2p orbitals, ρ2p, is evaluated to be ρ2p = 0.505 in one 2p nitrogen orbital and ρ2p′ = 0.035 in a second one (2p′) by comparison of the experimental anisotropic (or dipolar) hf values (B3) with the corresponding atomic value (Bo): ρ2p = B3/Bo, where B0 is = 39.62 G . Furthermore, a small spin density of ρ2 s = 0.02 in the 2s nitrogen orbital is evaluated from the isotropic hf splitting of a = 13 G with the corresponding atomic value (a0 = 646.2 G) . The total spin density on the nitrogen atom of the observed species amounts therefore to 0.56, with the larger contribution due to a single p orbital of the nitrogen atom in the center. The experimentally obtained hf value does not account for the whole unpaired electron spin density in the radical system. The unaccounted spin density is possibly delocalized on other atoms of the species having zero nuclear spin. It should be noted that the ESR parameters of the present Nb· radical are quite different from those of an isolated nitrogen atom with a quartet electronic ground state (with S = 3/2) .
No dipolar broadening was observed for the ESR spectra of the Nb· species when paramagnetic O2 molecules are adsorbed at low temperature on the surface . Furthermore, the Nb· radical is stable upon washing and calcination in air up to 773 K. These experimental results suggest a deep interaction of the Nb· species with the TiO2 matrix; for this reason the present nitrogen centered radical is written as “Nb·” with a subscript “b” referring to “bulk”. The intensity of the ESR signal from the Nb· species dramatically decreases when the N-doped TiO2 is outgassed at 773 K, the treatment being known to cause oxygen depletion and thus reduction of TiO2, and reversibly reappears after re-oxidation in O2 atmosphere at the same temperature. These results indicate that the energy levels of the N-species are part of the electronic structure of the solid and that their population is affected by the structural or electronic modifications induced by the reduction of TiO2.
ESR spectra of nitric oxide (NO) and nitrogen dioxide (NO2) radicals trapped in micro-voids of the solid were observed when the N-TiO2 samples were prepared and treated in different ways; the ESR parameters are listed in Table 6.4. The NO radical was found to be a product of the complex oxidation process of ammonium salts occurring upon calcinations of the solid. NO2 was formed only when nitrates or nitric acid were used as nitrogen source and could be thus considered to derive from their decomposition. Due to their nature of the trapped species, it is concluded that both NO and NO2 do not directly influence the electronic structure of the system.
Paramagnetic Nb· Species and Visible Light Illumination
A Mechanistic View
This shifts the equilibrium with formation of a larger amount of paramagnetic Nb· centers with respect to the illumination in vacuum. At this stage of the experiment the number of paramagnetic states observed in the system reaches its maximum value as the increase in Nb· concentration is accompanied by O2·− formation. In other words, a photo induced charge separation has occurred. Stopping illumination, the electrons scavenged by O2 remain in the adsorbed-layer so that the initial concentration of Nb· centers is not recovered.
The picture proposed here is based on the experimental results for the systems prepared by sol-gel reactions and, very likely, for other chemically prepared N-TiO2 systems. It cannot be excluded, however, that for systems prepared by radically different techniques other types of nitrogen centers are formed, and other mechanisms of photo-activation may apply.
6.5.2 Reversible Photoinduced Electron Transfer in TiO2 (Rutile)
It was shown in the foregoing section that the presence of oxygen molecules can modify the reactions as a fraction of photoexcited electrons is scavenged by O2 molecules to generate adsorbed superoxide (O2− or O2·−) ion radicals. The O2− ion radicals have been further suggested to play an important role in the photocatalytic oxidative and reductive degradations of organic pollutants under UV irradiation [74, 107, 108] and also in the cleavage of the conjugated structures of dyes on TiO2 illuminated with visible light [109, 110]. Komaguchi et al.  have recently studied photoinduced electron transfer reaction of O2 species formed on the H2-reduced surface of TiO2 (rutile) by monitoring the ESR spectra of the O2− species and Ti3+ ions. The ESR study demonstrates that the photoreaction occurs under sub-band gap illumination by the visible light, i.e. < 2.5 eV (> 500 nm), and the reverse process takes place after the illumination.
Upon visible-light illumination at 77 K, the ESR spectra of the TiO2 sample markedly changed (Fig. 6.20(c)): both O2−(B) and Ti3+ signals increased in intensity whereas the O2−(A) signal remained almost constant. Figure 6.20(d) depicts a correlation between the numbers of the O2−(B) and Ti3+ ions generated by illumination. A concomitant and equimolar formation of O2−(B) and Ti3+ ions on the TiO2 surface was suggested by a plot showing a straight line with a slope of unity. Once the illuminated sample was kept at room temperature for several minutes, the ESR spectra were perfectly restored to the original one, showing that this photoinduced (ESR) spectral change is reversible.
An O2 molecule attached to the oxygen vacancy site (b) forms a peroxide O22− ion by coordination with the two Ti3+ ions, in accord with theoretical studies . When one electron is transferred from O22− to one of the two oxidized Ti4+ ions by visible-light illumination, the corresponding O—Ti coordination is broken, and a pair of O2−(B) and Ti3+ species are concomitantly formed as observed. On the other hand, adsorption of O2 at the five-coordinate site (a) may generate O2−(A) by one-electron transfer from Ti3+ to the adsorbed O2 molecule. The paramagnetic O2−(A) species appears to be inactive under visible-light illumination.
6.5.3 Electron Transfer in Mixed Phase of Anatase and Rutile
Degussa P-25 consists of a mixed phase of anatase and rutile TiO2 nanoparticles in the ratio of ca. 4:1, and exhibits a higher photocatalytic activity than that of each of the pure phases [119, 120]. A number of studies have been carried out to elucidate the mechanism of the synergetic effect in the mixed TiO2 particles. Although it has been generally accepted that the enhanced activity is caused by an efficient charge separation, the detailed mechanism, however, has never been clarified. Recently Komaguchi et al.  have reported an “in situ” ESR study on the photo-effects of Ti3+ formed in partially reduced TiO2 nanoparticles. A preferential electron transfer from the Ti3+ (3d1) ion in anatase to the Ti4+ (3d0) ion in rutile phases in the TiO2 (P-25) particles took place upon light illumination with an energy lower than the band gap. An advantage of using partially reduced TiO2 is that the electron transfer is possible to observe by ESR without any serious interference due to the charge recombination with positive holes as the electron is released only from the paramagnetic Ti3+ ions by visible light illumination.
ESR Spectra of Ti3+ in Partially Reduced TiO2
The ESR spectrum of the P-25 sample was well simulated by the superposition of two distinct Ti3+ signals from the anatase and rutile phases with a 1:1 concentration ratio, see Fig. 6.22(c). The P-25 sample originally consists of a phase composition ratio of 4:1 for the anatase and rutile. The increased ratio of the rutile Ti3+ ions in comparison to the original phase composition indicates that in the mixed phase the Ti4+ ions in the rutile phase are more easily reduced to the Ti3+ ions than in the anatase phase. Note that the relative concentration of the Ti3+ ions was changed to a 3:1 ratio from the 1:1 ratio when the H2-reduced P-25 sample was contacted to a trace amount of air prior to illumination, see Fig. 6.22(d).
Synergetic Effects on Visible Light Illumination
6.6 Superoxide (O2−) Ion Radical
The superoxide (O2−) ion radical is one of the most important oxide radical intermediates in catalysis and has been extensively studied by means of ESR spectroscopy. Känzig and Cohen  have reported the first ESR spectrum of O2− trapped in a single crystal of alkali halides five decades ago. The reported g-value analysis has been well accepted and adapted by Lunsford , Kasai  and other scientists (for example, Shiotani ) in studies of the ion radicals adsorbed on various metal oxides and supported catalysts. Here we summarize some characteristics of the g-values and the hyperfine spectrum due to 17O (I = 7/2) labeling obtained from an ESR study on O2− adsorbed on Ti4+ ions on oxide supports [125, 126].
6.6.1 g-Values of O2−
The previously reported gz values of O2− range from 2.03 to 2.02. Consistent with this, for the present O2−/Ti4+/oxide support system, the following g-values were derived from the ESR spectrum recorded at 4.2 K: gx = 2.0027, gy = 2.0092 and gz = 2.0268. The small shift of gx and gy from ge and the order of gx < gy are also supported by the theory [3c, 4d].
6.6.2 17O Labeling Study
The 17O labeling is a powerful method for identifying oxygen species and their structure in the adsorbed phase on oxides and other catalysts by ESR. An observation of 17O (I = 7/2) hyperfine structure can give an unambiguous assignment of the ESR spectrum, e.g. if it is due to a superoxide ion O2−, and not other oxide radicals such as O− and O3−. Furthermore, it can provide important experimental evidence if the O2− consists of either equivalent or non-equivalent oxygen nuclei.
ESR parameters of O2− adsorbed on Ti4+/oxide supportsa
gx, gy, gz
17O A1, A2, A3/Gb
(1) (17O—18O)− –Ti3+
2.0027, 2.0092, 2.0268
74.9, 0, 0
2.0027, 2.0092, 2.0268
80.3, 0, 0
Comparing the experimental ESR spectra with the simulated ones the following conclusions were suggested. (1) The principal values of the 17O hf tensor are almost axially symmetric with the perpendicular component A┴(≈ A2 ≈ A3) being less than the linewidth of ca. 3 G. (2) Two different parallel components of 17O hf splittings were observed for both (17O—18O)− and (17O—17O)− radical ions with A1 (= A//) = 74.8 G and A1′ (= A//) = 80.3 G. (3) The observation of the nonequivalent 17O hf splittings suggests that the inter-nuclei axis (z-axis) of O2− is tilted slightly from the surface and/or one oxygen is closer to the Ti4+ ion. A molecular orbital study resulted in a small tilting of the O2 axis from the parallel conformation (i.e., less than 10°) . (4) The observed 17O hf splitting of A1 and A1′ correspond to the minimum g-tensor component, gx = 2.0027.
The ESR spectra of O2− adsorbed on supports sometimes show strong temperature dependency. Such ESR spectral changes are generally accompanied by shifting and/or broadening of certain features due to g-tensor anisotropy and give very rich information about the motional dynamics of the O2− on oxide surface .
Paramagnetic chemical species with unpaired electron(s) are involved as reaction intermediates and/or active sites in many catalytic reactions. Thus ESR spectroscopy has played an important role to obtain valuable experimental information on catalytic and/or surface reactions with high selectivity and high sensitivity, which has not been achieved by any other methods. With recent advancement in the measurement techniques and the data analysis methods the ESR method is an increasingly important tool in the studies on heterogeneous catalysis and solid surfaces. This chapter consisted of five topics relevant to ESR application to catalysis and environmental science.
The interaction of nitric oxide (NO) with metal ions in zeolites has been one of the major subjects in catalysis and environmental science and the first topic was concerned with NO adsorbed on zeolites. NO is an odd-electron molecule with one unpaired electron and can be used here as a paramagnetic probe to characterize the catalytic activity. In the first topic focus was on a mono NO-Na+ complex formed in a Na+-LTA type zeolite. The experimental ESR spectrum was characterized by a large g-tensor anisotropy. By means of multi-frequency ESR spectroscopies the g tensor components could be well resolved. The 14N and 23Na hyperfine tensor components were accurately evaluated by ENDOR spectroscopy. Based on these experimentally obtained ESR parameters the electronic and geometrical structures of the NO-Na+ complex were discussed. In addition to the mono NO-Na+ complex the triplet state (NO)2 bi-radical is formed in the zeolite and dominates the ESR spectrum at higher NO concentration. The structure of the bi-radical was discussed based on the ESR parameters derived from the X- and Q-band spectra. Furthermore the dynamical ESR studies on nitrogen dioxides (NO2) on various zeolites were briefly presented.
The second topic is an extension of the first one and was concerned with ESR studies of the Cu(I)-NO complexes. Copper ion exchanged high siliceous zeolites such as ZSM-5 and MCM-22 have been considered as a promising environmental catalyst for the NO decomposition. The Cu(I)-NO complex has attracted special interest because of its important intermediate in the catalytic NO decomposition. Pöppl and other scientists have extensively applied multi frequency ESR, pulsed ENDOR and HYSCORE methods to clarify the local structure of Cu(I)-NO adsorption complexes.
The third topic was concerned with ESR studies on the structure and the dynamics of organic radicals in zeolites. ESR methods were applied to investigate microenvironment effects on molecular dynamics and thermal stability of relatively large organic molecules such as triethylamine ((CH3CH2)3N+•) and tripropylamine ((CH3CH2CH2)3N+•) cation radicals used as spin probes. The cation radicals were generated by γ-ray irradiation of amines and related ammonium ions in various zeolites and subjected to X-band CW-ESR studies. The experimentally observed temperature dependent ESR spectral lineshapes were successfully analyzed by assuming a two-jump exchange process of the methylene hydrogens next to the nitrogen. The exchange rates and the barriers heights evaluated were discussed in terms of interaction with the surrounding zeolite wall by referring to theoretical DFT calculations. Furthermore the ESR studies using amine cation radicals as a spin probe were extended to investigate “cage effects” on stability and molecular dynamics of organic molecules in zeolites.
The fourth topic was concerned with titanium dioxide (TiO2) semiconductor photocatalysis. ESR spectroscopy has been extensively applied to the TiO2 systems and played an important role in the identification and characterization of paramagnetic species such as electrons, holes and their reaction products generated by photocatalytic reactions.The band-gap of pure TiO2 is 3.0–3.2 eV and only a small fraction of the solar spectrum (λ < 380 nm, corresponding to the UV region) is absorbed. This is an important drawback of TiO2 for photocatalysis using sunlight. A large number of modified TiO2 systems have been prepared to reduce the threshold energy for photoexcitation so as to use sunlight more efficiently in photocatalysis under visible light. One of the most promising and widely investigated systems is nitrogen-doped titanium dioxide (N-TiO2) . The N-TiO2 samples were illuminated by visible light and subjected to X- and Q-band ESR studies. Based on the ESR studies it was revealed that substitutionally or interstitually doped diamagnetic nitrogen ion (Nb−) absorbs visible light at 437 nm (2.84 eV) so as to promote an electron to the conduction band, generating N-centered neutral radical (Nb·). Furthermore, the ESR studies clarified that the presence of O2 molecules modifies the reactions as a fraction of photoexcited electrons is scavenged by O2, generating superoxide (O2−) ion radicals. In addition to the N-TiO2 system ESR spectroscopy was applied to the photoinduced electron transfer reaction of O2 species formed on partially reduced TiO2 (rutile). By monitoring ESR spectra of paramagnetic O2− species and Ti3+ ions it was revealed that the electron transfer could occur under sub-band gap illumination by visible light, and the reverse process takes place after the illumination. Furthermore, synergetic effects on the electron transfer upon illumination with visible light in a mixed phase of anatase and rutile were also presented by monitoring the ESR intensity of Ti3+ ions in partially reduced TiO2 nanoparticles.
The last topic was concerned with the ESR spectra of the superoxide ion radical (O2−) which is one of the most important oxide radical intermediates in catalysis and has been extensively studied by means of ESR spectroscopy. Some important characteristics of the g-values and the hyperfine structure due to 17O (I = 7/2) labeling were presented by exemplifying an ESR study on O2− adsorbed on Ti4+ ions on oxide supports.
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