Applications to Catalysis and Environmental Science

Chapter

Abstract

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.

6.1 Introduction

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 [1], and cromina-alumina catalysis by O’Reilly [2] 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 [23] 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 [5]. Modern ESR techniques such as pulsed ESR [25, 26, 27], ENDOR (Electron Nuclear Double Resonance) [26], and multi-frequency (X-, Q-, and W-band) ESR [28] 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 [29]. This subject is presented separately in Section 6.3.

6.2.1 NO-Na+ Complex Formed in Zeolites

Zeolites (Greek, ζέω (zeō), meaning “boil” and λίθος (lithos), meaning “stone”) are microporous aluminosilicate with the general formula {[Mn+]x/n.[H2O]m}{[AlO2]x[SiO2]y}x–, where Mn+ stands for cations, such as H+, Na+, K+, Ca2+, Mg2+, Cu2+, and other ions (Fig. 6.1). The Mn+ cations and H2O molecules bind inside the cavities, or pores, of the Al-O-Si framework. The Mn+ cations can be exchanged for other ions in a contact solution. Small molecules such as NOx(x = 1, 2), CO2, NH3, and hydrocarbons including aromatic and amine compounds can be adsorbed to the internal surfaces and this partially accounts for the utility of zeolites as catalysts, we refer to Section 6.4 for some studies of organic radicals in zeolites.
Fig. 6.1

A perspective view of part of Linde type Y (LTY) zeolite with the faujasite structure (Si/Al ratio of 2.73). Aluminum and silicium atoms lie on the corners, oxygen atoms near the mid-points of edges. Site I (S1) in hexagonal prisms and site II (S2) in supercage are indicated. The figure is adapted from [30] with permission from Elsevier. See Fig. 6.11 for other zeolites such as SAPO-37, SAPO-42 and Al-offretite

g-Tensor Anisotropy of Adsorbed NO

The NO molecule exhibits the degenerate molecular orbitals, 2pπ*(x,y), in the ground electronic state, see Fig. 6.2. The orbital degeneracy can be lifted by the electrostatic field due to a metal ion where NO is adsorbed on inorganic solid matrices. The unpaired electron occupies the 2pπ*(y) molecular orbital in the absence of any spin-orbit coupling, see Fig. 6.2. As a result of spin-orbit coupling, however, the perturbed wave function for the electron is a mixture of the 2pπ*(y) and the excited 2pπ*(x) orbital. The g-tensor for the NO adsorption complexes has been discussed by Lunsford [3], Kasai [4], and recently by Rudolf et al. [28] who proposed the following analytical expressions for the principal values gxx, gyy, and gzz of the g-tensor:
$$g_{xx} = g_e \frac{\varDelta }{{\sqrt {\lambda ^2 + \varDelta ^2 } }} - \frac{\lambda }{E}\left( {\frac{{\varDelta - \lambda }}{{\sqrt {\lambda ^2 + \varDelta ^2 } }} - 1} \right)$$
(6.1)
$$g_{yy} = g_e \frac{\varDelta }{{\sqrt {\lambda ^2 + \varDelta ^2 } }} - \frac{\lambda }{E}\left( {\frac{{\varDelta - \lambda }}{{\sqrt {\lambda ^2 + \varDelta ^2 } }} + 1} \right)$$
(6.2)
$$g_{zz} = g_e - \frac{{2\,l\lambda }}{{\sqrt {\lambda ^2 + \varDelta ^2 } }}$$
(6.3)
Fig. 6.2

(a)Energy levels of NO interacting with M+ located on a surface. (b) The x-y-z coordinate system used for NO adsorbed at metal ion (M+) in zeolite. The term Δ stands for the energy splitting between 2pπ*(x) and 2pπ*(y) orbitals, and the term E for that between 2pσ* and 2pπ*(y) orbitals. The unpaired electron resides in the 2pπ*(y) orbital in the absence of any spin-orbit coupling

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) [31]. 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 = gxxgyy, is of the order of 10–3 or even less, but can be resolved by multi-frequency ESR measurements [28]. 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 (gxxgyy) 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 X (9.3 GHz)-, Q (33.9 GHz)-, and W (93.9 GHz)-band ESR spectra of the NO/Na-LTA (Linde type A) zeolite system are shown in Fig. 6.3(a). The X-band spectrum shows a 14N (I = 1) hf coupling with three transitions (mI = –1, 0, 1) at the gxxgyy position. The gzz position lies in the high field region and does not exhibit any hf splittings. The gxx and gyy values are very close to each other and cannot be resolved both at X- and Q-band. In the W-band spectrum, however, the gxx spectral component is visibly separated from the 14N hf triplet at the gyy position. All three experimental spectra are satisfactorily simulated with an identical set of g and A principal values given in Table 6.1: see Fig. 6.3(a) vs (b). Thus, the frequency dependence of the powder ESR spectra with the overlapping gxx and gyy peaks at X- and Q-band and their successful separation at W-band were clearly demonstrated by T. Rudolf et al. [28]. The values of E, Δ and l for the Na-LTA/NO system, which were evaluated from the experimentally obtained g values, are given in Table 6.1 together with those for the Na-ZSM-5/NO zeolite system. The table also contains the distribution widths of δE, δΔ and δl, which are evaluated from the linewidth analysis at the gxx, gyy and gzz peaks. The readers can refer to ref. [28] for further details.
Fig. 6.3

(a) Experimental ESR spectra of NO adsorbed on Na-LTA zeolite at 10 K observed at X-, Q- and W-band microwave (MW) resonance frequencies. NO was adsorbed onto activated Na-LTA samples at room temperature with a gas pressure corresponding to approximately 10–1 molecules per unit cell. (b) ESR spectra simulated using an identical set of the A and g ESR parameters in Table 6.1. The figure is adapted from [28] with permission from the Royal Society of Chemistry

Table 6.1

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

NO-Na+ complex

(a)

Na-LTA

Multi-band ESR

(10 K)

(b)

Na-ZSM-5

Multi-band ESR

(10 K)

(c)

Na-LTA

Pulsed ENDOR

(d)

Na-LTA DFT

gxx

gyy

gzz

1.9993

1.9936

1.8842

1.9939

1.9914

1.8460

2.001

1.996

1.888

2.030

1.999

1.888

A(14N)/MHz

Axx

Ayy

Azz

16.2

91.6

0.0

32.5

102.0

0.0

X-band (5 K)

25.3 ± 0.2

91.0 ± 0.5

26.3 ± 0.2

16.9

82.0

16.9

A(23Na)/MHz

Axx

Ayy

Azz

  

W-band (4.3 K)

6.3 ± 0.2

6.3 ± 0.2

10.9 ± 0.2

5.16

5.16

11.56

Q(23Na)/MHz

Qxx

Qyy

Qzz

  

W-band (4.3 K)

–0.41

–0.23

0.64

–0.38

–0.19

0.57

E/eV

Δ/eV

l

5.67

0.272

1.051

13.45

0.165

0.848

  

δE/eV

δΔ/eV

δl

0.28

0.005

0.032

9.25

0.051

0.019

  

References

[28]

[28]

[26]

[32]

Notes: ESR g and A(14N) principal values in (a) and (b) were obtained from the simulation of CW ESR spectra observed at three different MW frequencies (multi-frequency ESR spectra). The E, Δ, l, δE, δΔ and δl values were evaluated from the g values and their distribution widths (δg). The computational DFT results correspond to the B3LYP/6-31+G(d) optimized geometry of the Na-NO complex in model 3A in [32].

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 3.4.2.3 (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. [26] 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% [34]. In addition, judging from the small anisotropic values of the A(23Na) tensor, Bzz(23Na) ≡ AzzAiso = 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.

From the experimental hf tensor of A(14N) the isotropic hf splitting of Aiso(14N) and the principal values of the dipolar coupling tensor, B(14N), are deduced: Aiso(14N) = 47.7 and (Bxx, Byy, Bzz) (14N) = (–22.4, 43.8, –21.4) MHz. The value of Aiso(14N) leads to a spin density of ρ2 s(N) = 0.031 in the nitrogen 2 s orbital [34]. From the average dipolar 14N hf coupling of Bxx(N) and Bzz(N) (i.e. –21.9 MHz) the spin density of ρ2p(N) = 0.458 is deduced for the nitrogen 2p orbital which forms, together with the oxygen 2p orbital, an anti-bonding 2pπ* SOMO (singly occupied molecular orbital) for NO. Assuming the relation of ρtotal = ρ2 s(N) + ρ2p(N) + ρ2p(O) + ρ3 s(Na) ≡1 the spin density in the oxygen 2p orbital can be evaluated to be ρ2p(O) = 0.502. The SOMO of the NO-Na+ complex is then given by a linear combination of the following atomic orbitals (χ):
$$\psi=(0.03)^{1/2}\chi_{2{\rm{s}}}({\rm{N}})+(0.46)^{1/2}\chi_{2{\rm{p}}}({\rm{N}})+(0.50)^{1/2}\chi_{2{\rm{p}}}({\rm{O}})+(0.01)^{1/2}\chi_{3{\rm{s}}}({\rm{Na}})$$
(6.4)
The geometrical structure of the NO-Na+ complex can be discussed based on the 23Na hf (axially symmetrical) dipolar principal values [(Bxx, Byy, Bzz) (23Na) = (–1.5, –1.5, 3.0) MHz] and their orientation with respect to the g and A(14N) principal coordinate systems. The 23Na dipolar hf couplings can be related to the electron spin densities of ρ2pπ(N) and ρ2pπ(O) and the Na+—N and Na+····O distances, r(Na+—N) and r(Na+····O) by an analysis based on the point dipole approximation proposed by Hutchison and McCay [26, 35]. The bond angle of Na+—N—O and the value of r(Na+—N) were evaluated to be 142° and 0.21 nm, respectively. The structure proposed for the NO-Na+ complex in Na-LTA zeolite is depicted in Fig. 6.4.
Fig. 6.4

Schematic drawing of the bent structure of the NO-Na+ adsorption complex in Na-LTA zeolite. The z principal axes of the g, A(23Na), and Q(23Na) tensors lie within the Na+-N-O complex plane and form an angle with the Na+-N(O) bond direction of 38, 3, and 8°, respectively. The unpaired electron is localized mainly in the anti-bonding 2pπ* molecular orbital of the NO molecule (refer to Fig. 6.2), which is also within the plane of the complex. The cation at site S2 is coordinated to the framework oxygens, Of, in the six-membered rings in a trigonal symmetry, refer to Fig. 6.1. Only the three oxygens in the first coordination sphere are shown. The figure is adapted from [26] with permission from the American Chemical Society

DFT Computations

Liu et al. [32] 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) [26].

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 [29]. ESR studies have accordingly continued to be of interest in recent decades.

X- and Q-Band ESR Spectra

The X-band spectrum of the NO/Na-LTA zeolite system is mainly due to the NO mono-radical when the pressure is low (PNO ≤ 0.1 kPa), while the (NO)2 bi-radical becomes dominant at higher NO pressure (PNO ≥ 10 kPa) [24, 36]. The ESR signals due to the NO mono-radical (NO-Na+ complex) and the (NO)2 bi-radical are superimposed at intermediate pressures. The Q-band ESR spectrum helped very much to resolve the individual spectrum and to evaluate the accurate ESR parameters of the (NO)2 bi-radical. As shown in Fig. 6.5, the Q-band spectral line-shape is well simulated using the following g tensor and the D and E parameters of the zero field splitting (ZFS) tensor for the (NO)2 bi-radical: (gxx, gyy, gzz) = (1.9120, 2.0042, 1.9770), |D| = 33.1 mT and |E| = 2.8 mT. The ESR spectra due to NO mono- and (NO)2 bi-radicals were also observed for NO adsorbed on other ion-exchanged zeolites such as partially lithium ion-exchanged Na-LTA zeolite [37] and on sulfated zirconia [38].
Fig. 6.5

(a)Q-band ESR spectra of NO (PNO = 13.2 kPa) introduced in Na-LTA zeolite at 5 K. (b) ESR spectrum simulated to (a). The theoretical spectra (c) for NO(I) and (d) for the (NO)2 bi-radical were calculated using Lorentzian lineshape with anisotropic line-width of 4.0, 7.5, and 6.0 mT for the x-, y-, and z-components, respectively. The simulation spectrum (b) is a superposition of (c) and (d) with the intensity ratio of 1:1. The figure is adapted from [36] with permission from the American Chemical Society

ESR simulations revealed that the spectral line-shape of (NO)2 bi-radical is very sensitive with respect to the relative orientations of the g and the D tensors and the principal axes of gZZ and DZZ cannot deviate by more than 30°(corresponding to angle “θ” in Scheme 6.1) from each other in this model. In the simulations the principal values of the g-tensors of each NO molecule were assumed to be the same as the experimental ones for the (NO)2 biradical with the principal axes parallel and perpendicular to the N—O bond and along the common x-axis (g = 2.0042) and the direction of DZZ was taken along the line connecting the midpoints of the two N—O bonds as shown below [13, 36].
Scheme 6.1

A geometrical structure proposed for the (NO)2 bi-radical in Na-LTA zeolite. The structure is adapted from [36] with permission from the American Chemical Society

The ZFS tensor of radical pairs generally originates from the dipolar-dipolar interaction between the two unpaired electrons of the radical units. As described in Chapter 4 the D-parameter can be related to the average distance between two radicals, R, by the following equation based on the point-dipole approximation,
$$D = \frac{{\mu _0 }}{{4\pi }}\frac{{3\,g\mu _B }}{{2R^3 }}$$
(6.5)
where g, μ0, and μB are the spectroscopic g-factor, the magnetic constant, and the Bohr magneton, respectively; for the numerical values of μ0, and μB, see Table G2 in General Appendix G, and refer to the H···CH3 radical pair in Section 5.4.2 where “d” is used in stead of “D”. The values of R, evaluated from the experimental D values, are summarized in Table 6.2.
Table 6.2

ESR g-tensor and zero-field splitting (ZFS) tensor for (NO)2 bi-radical formed in Na-LTA zeolite and other matrices

Zeolite

(a)

Na-LTA

CW Q-band ESR

(b)

Na-LTA

Pulsed ESR

(c)

Na-LTA

X-band ESR

(d)

Li-Na-LTAa

X-band ESR

(e)

Sulfated Zirconia

X-band ESR

g-tensors

gxx

gyy

gzz

2.0042

1.9770

1.9120

 

1.976

1.976

1.912

1.989

1.989

1.900

1.993

1.993

1.994

ZFS (mT)

|D|

|E|

33.1

2.8

 

28.8

41.0

19.5

A(23Na)/MHz

Axx

Ayy

Azz

 

4.6

4.6

8.2

   

Q(23Na)/MHzQxx

Qyy

Qzz

 

–0.3

–0.3

0.6

   

R(nm)b

0.45

0.45

0.45

0.42

0.52

References

[36]

[12]

[4c]

[37]

[38]

a The exchange level of lithium ion (Li+) is 66%.

b NO—NO distance (Scheme 6.1).

The NO molecule can be stabilized on a pair of adjacent cations (Na+ ions in the present case) as shown in Scheme 6.2(a). However, a structure with the two NO molecules bonded to the same Na+ ion and with nearly collinear N—O bonds (Scheme 6.2(b)) has also been proposed. The observation that the NO—NO distance became shorter from R = 0.45 nm in Na-LTA to 0.42 nm in Li-LTA (see Table 6.2) provided support for the latter model taking into account the Na+ ionic radius (r = 0.095 nm) in comparison with the Li+ ion (r = 0.060 nm). Thus, the structure shown in Scheme 6.2(b) can not be completely ruled out. Further studies are required to clarify the details of the geometrical structure of the (NO)2 bi-radical in the zeolite.
Scheme 6.2

Models for (NO)2 bi-radical. (a) Two NO molecules stabilized on a pair of adjacent Na+ ions. (b) Two NO molecules bounded to the same Na+ ion.

Pulsed 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 [12]. 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 [13].

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 [39]. 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 [40]; 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 [41]. 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 [44]. 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+ [45]. A detailed investigation of the NO2/Na-MOR system has suggested that there exists a distribution of exchange rates at each temperature [46].

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. [50] 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 [29].

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. [29] 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 [31]. 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 [53]: see Chapter 2 for pulsed ENDOR and HYSCORE.

6.3.1 Multifrequency ESR Spectra

Multifrequency ESR spectra of Cu(I)-NO complexes formed on Cu-ZSM-5 were extensively studied by Pöppl et al. [31]. Figure 6.6 shows the X-, Q-, and W-band ESR spectra of Cu-ZSM-5 with adsorbed NO. Although an auto-reduction of Cu(II) to Cu(I) takes place to a large extent during the activation process at an elevated temperature in vacuum, the ESR signal due to Cu(II) is still visible in the spectrum. In the X-band spectrum, the Cu(II) spectrum is superimposed by the intense spectra due to the Cu(I)-NO complexes, which makes it difficult to determine accurate values of the ESR parameters. In the Q-band spectrum, the Cu(II) spectrum is almost separated from that of the Cu(I)-NO complexes. Finally, in the W-band spectrum complete separation between the Cu(II) and Cu(I)-NO complex spectra is achieved. The Cu(I)-NO complexes formed in the Cu-ZSM-5 system show an anisotropic spectrum in which the 63Cu (I = 3/2) hf splitting with four lines is clearly visible in the gzz and gxx (= gyy) spectral regions. With increasing NO pressure electronic dipole-dipole interactions among the Cu(I)-NO complexes resulted in an ESR linewidth broadening so as to smear the 14N (I = 1) hf triplet. In fact the hf triplet is not resolved in the Q-band ESR spectra of Cu(I)-NO complexes at NO pressure of 5 Pa, but could be observed at lower NO pressures of ca. 0.5 Pa as a splitting of ca. 3 mT resolved in the gxx (= gyy) spectral regions.
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-, Q-bands) and 0.5 Pa (W-band) at 300 K. The stick diagrams indicate the 63Cu hf splitting of the Cu(I)-NO ESR spectrum in the gxx/yy and gzz spectral regions. (b’) Q-band spectra of the Cu(I)-NO complexes showing two different species A and B in the gzz spectral region. The hf splittings of both 63Cu (natural abundance 69.1 at.%) and 65Cu (30.9 at.%) isotopes with nuclea spin I = 3/2 have been included in the spectral simulation, but the splittings in the stick diagrams only refer to the 63Cu isotope. The ESR hf 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

Table 6.3

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

Cu(I)-NO

Cu-ZSM-5

Cu-MCM-22

Species

A

B

A

B

gxx

2.0050

2.0050

2.0050

2.0050

gyy

2.0050

2.0050

2.0050

2.0050

gzz

1.8900

1.8999

1.8900

1.8999

A(14N)/10–4 cm–1 b

    

Axx

Ayy

29

29

33

33

Azz

A(63Cu)/10–4 cm–1 b

    

Axx

158

158

157

157

Ayy

130

130

133

133

Azz

216

239

200

220

Aiso

168

175

163

170

Covalent parameter

    

lc

0.85

0.78

0.81

0.79

a The data taken from multi-band ESR results [31].

b 1 cm–1 = 4.6686 × 10–4g mT.

c Refer to Table 6.1 for parameter “l”.

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) 4 s atomic orbital to the SOMO of the Cu(I)-NO complex. That is, the observed relation of gzz < ge = 2.0023 < gxx= gyy suggests the presence of Cu 3d orbital contributions [56a] to the SOMO and the large isotropic Cu hf coupling shows that the structure of the complex is bent so as to allow an effective admixture of the 4 s 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. 6.7

Orientation of principal axes frames of the g, A(N), and A(Cu) tensors of the Cu(I)-NO moiety. The A(Cu) frame is rotated by the angle, β, about the common x principal axis, but the Azz(Cu) principal axis is not necessarily parallel to the Cu—N bond. The y-z plane of the g tensor is spanned by the N—O bond and the symmetry axis of the 2pπy* orbital of the NO molecule. See Table 6.3 for ESR parameters and structural data. The figure is adapted from [31] with permission from the American Chemical Society

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 [57]. 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 [58] 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. [53]. 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 [59]. 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 [63] and are used as electron donors [64]. 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 [65]. 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 [66], 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 [61]. 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

The X-band ESR spectrum of Et3N+ at 300 K consists of an intense and sharp hyperfine (hf) septet of 2.0 mT splitting with two weak additional lines at each wing which have almost the same splitting as the septet (Fig. 6.8(a)). The intense septet corresponds to the mI = 0 band of the central 14N(I = 1) atom and is attributed to the isotropic hf splittings due to six equivalent β-hydrogens of +N(α)—[C(β)H2—C(γ)H3]3. The weak wing lines have a spectral feature characteristic of hf anisotropy and are attributable to the parallel component, A||(14N), at the mI = ±1 lines of 14N(I = 1). The corresponding perpendicular (x and y) features are hidden beneath the mI(14N) = 0 lines because the value of A(14N) is less than the line width. The experimental spectrum is successfully simulated using the following ESR parameters: aiso(1H) = 2.0 mT for six equivalent β-hydrogens, A||(14N) = 4.4 mT, A(14N) = 0 ± 0.4 mT, giso= 2.0033 and Gaussian line-width of 0.18 mT. The lines corresponding to mI(14N) = ±1 are hidden beneath the intense septet of mI(14N) = 0, only weak shoulders of the parallel features are detected in the outer wings of the spectrum.
Fig. 6.8

X-band CW-ESR spectra of Et3N+ in AlPO4-5 observed at (a) 300 K and (b) 77 K together with the theoretical spectra (Cal.) calculated by assuming a “fast limit” case and a “rigid limit” case, respectively. The 1H and 14N hyperfine splittings used in the simulations are given in the text. The weak wing lines marked as a triangle (Δ) are attributable to A||(14N) at mI(14N) = ±1 transitions. (c)Two-site exchange model employed for ESR spectral line-shape simulations. Two methylene hydrogens at C(β) labeled as Ha and Hb are interchanged by the hindered rotation about the N(α)—C(β) bond. Only one methylene group is labeled. The figure is adapted from [61] with permission from the Royal Society of Chemistry

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

The 1H hf splitting due to the β-hydrogens of Et3N+ was reversibly changed from 3.6 mT (3H: three magnetically equivalent hydrogens) at 77 K to 2.0 mT (6H: six hydrogens) at 300 K. The anisotropic 14N hf splitting, however, remained constant and was not even partially averaged by the motion in the temperature range. This indicates that the temperature dependent spectral change is associated with a change in the number of magnetically equivalent β-hydrogens and their hf splitting. The temperature dependent ESR spectra were well simulated in terms of the two-site exchange model [67] in which the two inequivalent β-hydrogens of the Et or Pr groups interchange their positions with each other so as to become equivalent (Fig. 6.9). Two preferred conformations, (I) and (II) in Fig. 6.8(c), which are energetically equivalent with a mirror image structure, can undergo “exchange” with a temperature-dependent rate, k(s–1).
Fig. 6.9

(a) Temperature dependent ESR spectra observed for the Et3N+ radical generated in γ-ray irradiated AlPO4-5 containing Et3N. (b) Best-fit simulation spectra calculated using the two-site exchange model for the hindered rotation of two inequivalent β-hydrogens (see Fig. 6.8(c)). The exchange rates, k(s–1), are given in the figure. The rigid limit ESR parameters used in the calculations are given in the text. The spectra are adapted from [61] with permission from the Royal Society of Chemistry

A modified version of the Heinzer program [68] was employed for the line-shape simulations with the exchange rate constant, k(s–1), as a variable parameter. Based on the good correlation between the experimental and calculated spectra as shown in Fig. 6.9, the exchange rate of Et3N+ was evaluated to increase by more than two orders of magnitude from 1.8 × 107 to 6.6 × 109 s–1 when the temperature increased from 110 to 270 K. In a similar manner the temperature dependent experimental ESR spectra of Pr3N+ were well reproduced by the line-shape simulations. Furthermore, from a linear Arrhenius plot of the exchange rates, activation energies of 9.1 and 11.4 kJ mol–1 were evaluated for Et3N+ and Pr3N+, respectively, as shown in Fig. 6.10. DFT calculations for the isolated cations resulted in energy barriers of 8.7 and 7.7 kJ mol–1 for the exchange of Et3N+ and Pr3N+, respectively. The reason why the experimentally determined energy barrier for Pr3N+ in AlPO4-5 is higher than the calculated one is explained in terms of interaction with the zeolite wall. That is, in the calculations both Et3N+ and Pr3N+ are reasonably assumed to be located in the 12-ring channel (ca. 8 Å in diameter [69]) of the AlPO4-5 framework (Fig. 6.10(a) and (b)). The diameters of Et3N+ and Pr3N+ are ca. 6.5 and 8.3 Å, respectively, based on the optimized structures predicted by the DFT calculations. The diameter of Et3N+ is significantly smaller than the ring channel, while Pr3N+ has almost the same diameter as the channel. On the one hand a comparatively weak interaction with the zeolite channel wall is expected for the Et3N+ system because there is still a significantly large vacancy in the channel. On the other hand, for the Pr3N+ system there is essentially no vacancy in the channel so that Pr group can experience a stronger interaction with the wall. Thus, the reason why the experimentally determined energy barrier for Pr3N+ in AlPO4-5 is higher than the calculated one is explained in terms of interaction with the zeolite wall, refer to [61] for more details.
Fig. 6.10

AlPO4-5 framework viewed in the (001) plane. The Si or Al atoms occupy all corners and O atoms (not shown) are near the mid-points of edges. The incorporated (a) Et3N+ and (b) Pr3N+ are indicated by shaded and hatched circles. (c) The Arrhenius plots for the exchange rates, ln(k) vs. 1/T (K–1), for Et3N+ and Pr3N+ in AlPO4-5. The figure is adapted from [61] with permission from the Royal Society of Chemistry

6.4.2 Cage Effects on Stability and Molecular Dynamics of Amine Radicals in Zeolites

The ESR studies using amine radicals as a spin probe have been extended to investigate “cage effects” on stability and molecular dynamics of organic molecules in zeolites [62]. Three zeolites with different cages or channels were employed, i.e., SAPO-37 [70] with sodalite cages, Al-offretite with gmelinite cages and main channels and SAPO-42 [71] with α-cages, see Fig. 6.11. Two radical cations, [(CH3)3N]+ and [(CH3)3NCH2]+, were generated by ionizing radiation in cages of the zeolites containing [(CH3)4N]+ ions as an organic template. The [(CH3)3N]+ radical cation is stable at room temperature when it is generated in relatively small size cages such as sodalite cages of SAPO-37 and β-cages of SAPO-42. On the other hand, the [(CH3)3NCH2]+ radical cation is stable not only in the sodalite cages of SAPO-37, but also in relatively large size cages such as gmelinite cages or main channels of Al-offretite [72] and α-cages of SAPO-42.
Fig. 6.11

Framework structures of (a) SAPO-37, (b) SAPO-42 shown as perspective view, and (c) Al-offretite viewed parallel (I) and approximately perpendicular (II) to the c axis. Si, Al or P atoms occupy all corners and O atoms (not shown) are near the midpoints of the edges. Possible locations of Me4N+ [(CH3)4N+] and Pr4N+ [(CH3CH2CH2)4N+] are shown in the structures. The figure is adapted from [62] with permission from the Royal Society of Chemistry

Strongly temperature dependent ESR spectra were observed for the [(CH3)3NCH2]+ radical cation stabilized in Al-offretite and SAPO-42, see Fig. 6.12(a). The spectra are well simulated by assuming a three-site exchange model for the methyl protons (Fig. 6.12(b)). The three H-atoms labelled Ha, Hb and Hc of a specific methyl group in Fig. 6.12(d) are interchanged by the rotation cycle Ha → Hb→ Hc → Ha around the N—CH3 bond for sites 1, 2, and 3. Rotation of all three methyl groups is assumed. Only one methyl group is labelled, the other two are identical to the first and have the same 1H hf couplings. The best-fit mI and temperature dependent linewidths employed are: ΔBpp = 0.12 mT (mI = ±1) and 0.08 mT (mI = 0) for the 300 K spectrum; ΔBpp = 0.20 mT (mI = ±1) and 0.10 mT (mI = 0) for the spectra above 200 K; ΔBpp = 0.20 mT (mI = ±1) and 0.20 mT (mI = 0) below the 200 K spectrum. The other ESR parameters used are the same as those of [(CH3)3NCH2]+ in the SAPO-42 system. The evaluated exchange rates are in the order of SAPO-37 (with sodalite cages) < Al-offretite (with gmelinite cages or main channels) < SAPO-42 (with α-cages) in the temperature range 110–300 K.
Fig. 6.12

(a) Temperature dependent X-band ESR spectra of [(CH3)3NCH2]+ generated and stabilized in γ-irradiated Al-offretite. The lines marked as a closed circle (•) are attributed to [(CH3)3N]+. (b) ESR spectra simulated using the three-site exchange model for the hydrogens of the rotating methylgroup of [(CH3)3NCH2]+. (c)Arrhenius plots of the exchange rates, ln k(s–1) vs. T–1(K–1) for [(CH3)3NCH2]+ in SAPO-42 (▪) and Al-offretite (•). (d) Three-site exchange model for ESR lineshape simulation of [(CH3)3NCH2]+. The figure is adapted from [61] with permission from the Royal Society of Chemistry

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].

The advantage of TiO2 as a semiconductor photocatalyst comes from its ability of converting photon energy into chemical energy. The absorption of photons with band-gap energy excites an electron (e) from the semiconductor valence band (VB) to the conduction band (CB), leaving a positively charged hole (h+) in the valence band. Many of the charged pairs recombine with each other, accounting in part for low photo-efficiency and differences in the photo-activity of various catalysts [78]. A fraction of the electrons and holes moves to the surface and reacts with an adsorbed compound or migrates into trapping sites (et; ht) prior to surface reaction and/or recombination (Fig. 6.13).
Fig. 6.13

Conversion of photon energy into chemical energy by semiconductors. Absorbed photons (hv) lead to charged ion pairs (e; h+) that can be separated and transferred to electron acceptors (A) and electron donors (D) at the surface. The figure is adapted from [15] with permission from Elsevier

A number of TiO2 samples have been developed as photocatalysts and shown to exhibit various photoefficiencies and chemical selectivities [79]. 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 [103].

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) [16]. 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·)

The UV-visible diffuse reflectance spectrum of N-doped TiO2 (anatase), which was prepared by the sol-gel method, is compared with that of bare TiO2 (anatase) in Fig. 6.14. The two spectra essentially differ for the broad absorption in the visible region centered at about 450 nm (blue), which characterizes the doped material [96].
Fig. 6.14

UV-visible diffuse reflectance spectra of bare and N-doped TiO2 (anatase). The diagram is adapted from [96] with permission from the American Chemical Society

The ESR spectra of the N-doped TiO2 sample are shown in Fig. 6.15. The X-band spectra (ν : 9.5 GHz) were recorded for the sample containing either 14N(I = 1) or 15N(I = 1/2) isotope and the Q-band spectrum (ν : 35 GHz) for the 14N isotope species.
Fig. 6.15

Experimental (a–c) and simulated (a–c) ESR spectra of the N-containing paramagnetic species in N-TiO2. experimental and simulation. (a, a) X-band spectrum of the species containing 15N and 14N isotopes (70 and 30%, respectively). (b, b) X-band spectrum of the species containing 14N (99.63% in natural abundance). (c, c) Q-band spectrum of the species containing 14N. The spectra are adapted from [104] with permission from the American Chemical Society

The ESR spectrum is characterized by the rhombic g tensor (g1 = 2.0054, g2 = 2.0036, and g3 = 2.0030) and the hyperfine A tensor with a large splitting constant of A3(14N) = (±)32.3 G in the direction of the g3 component. The other two smaller hf splittings, A1(14N) and A2(14N), are given in Table 6.4. Both X- and Q-band spectra are well simulated by using the same set of g and A tensors as shown in Fig. 6.15.
Table 6.4

ESR parameters of various paramagnetic species identified in N-doped TiO2 systema

Radical

g-values

A-values/Gb

B-values/Gb

ρ2p

 

g1

g2

g3

A1

A2

A3

aiso

B1

B2

B3

 

Nb·

2.005

2.004

2.003

2.3

4.4

32.2

13.0

–10.7

–8.6

19.3

0.54

Nsub· (cal)

   

2.5

2.8

38.2

14.5

–12.0

–11.7

23.7

0.87

Nint· (cal)

   

0.2

1.8

33.4

11.8

–11.6

–10.0

21.6

0.67

O2· (1)c

2.023

2.009

2.004

        

O2· (2)c

2.025

2.009

2.003

        

NO·d

2.001

1.998

1.927

< 1

32.2

9.6

     

NO2·d

2.004

2.001

1.990

2.3

4.4

32.3

     

a Data taken from refs. [16, 104].

b 1 G (gauss) = 0.1 mT (T: tesla).

c The two signals are due to superoxide radicals stabilized on two distinct surface Ti4+ ions which slightly differ in terms of coordinative environment.

d By-products of the nitrogen incorporation in the solid and do not directly influence the electronic structure of the system.

Based on a close relationship with the theoretical ones by the DFT computations the observed hyperfine A tensor components are attributable to nitrogen centered radical (Nb·) (“b” stands for “bulk”, see below) located at either substitutional or interstitial site of anatase TiO2 lattice (Fig. 6.16) [16, 104]. Furthermore, the DFT computations suggest a localized electronic state of these species, whose energy levels are located somewhat above the top of the valence band for Ns and Ni, respectively [105]. The ESR spectra of Nb· in Fig. 6.15 show the presence of two slightly different species in the system, one of which is 3.5–4 times more intense than the other. The case of two or more species, which have the same chemical nature, but slightly different in the coordinative environment, is rather common in solid state chemistry.
Fig. 6.16

Partial geometry and schematic sketch (middle) and electron spin density (SD) maps (top) of the unpaired electron of the model for (a) substitutional and (b) interstitial N-doping in an anatase TiO2 crystal lattice. In the middle the N atom is represented by a black sphere, O atoms are represented by yellow spheres, and Ti atoms are represented by small grey spheres. The SD plot is in the plane perpendicular to that containing the three Ti atoms bound to the N species. The figure is adapted from [104] with permission from the American Institute of Physics

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 [34]. 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) [34]. 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) [106].

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 [16]. 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

To clarify the role of Nb· species in the photo-activity of N-doped TiO2, ESR spectra were measured under illumination in vacuum and in oxygen atmosphere, using visible light with two different wavelengths, one in the blue with λ = 437 nm (corresponding to the maximum absorption of the sample), and the other in the green with λ = 500 nm (corresponding to the tail of the absorption band), see Fig. 6.14 [96]. Changes in the ESR signal intensity of Nb· in various conditions are summarized together with those of the superoxide ion radical (O2· or O2) in Fig. 6.17.
Fig. 6.17

ESR signal intensity of Nb·and O2· in various conditions. I/I0 = 1 corresponds to the Nb· intensity in the non-illuminated sample. The diagram is adapted from [96] with permission from the American Institute of Physics

Illumination with green light causes only a small increase in the ESR signal of Nb·. With blue light illumination, however, the signal intensity grows by a factor of about 2. This higher intensity remains constant until the lamp is turned off, when the signal returns to the initial value. This behavior is reversible, and the signal grows again as the lamp is turned on. When the sample is illuminated with blue light in oxygen atmosphere, the increase in the Nb· signal is accompanied by the simultaneous appearance of new ESR signals due to O2· ion radical (see Section 6.6). The O2· ion radical was observed with the two different gzz values of 2.025 and 2.023 as shown in Fig. 6.18 (b and b). This suggests that the O2· radicals have been stabilized at two distinct surface Ti4+ ions, which slightly differ from each other in terms of coordinative environment [16, 104].
Fig. 6.18

Experimental (a–c) and simulated (a–c and c) ESR spectra of Nb· and surface adsorbed O2· radicals. (a, a) Nb· radical. (b, b) Nb· and O2· radicals. (c, c, c) Deconvolution of spectrum (b) (see ESR parameters of Nb·, O2·(1), and O2·(2) in Table 6.4). The figure is adapted from [96] with permission from the American Institute of Physics

A Mechanistic View

Based on the above experimental and theoretical results the following schematic picture can be drawn for the mechanism connected to the interaction of N-TiO2 with visible light (Fig. 6.19) [16, 96].
Fig. 6.19

Sketch of the proposed mechanism for the processes induced by visible light illumination of the N-doped TiO2 sample in O2 atmosphere. The diagram is adapted from [96] with permission from the American Institute of Physics

Upon illumination in vacuum or in O2 atmosphere the ESR signal intensity of Nb· increased. This can be explained in terms of electron transfer from diamagnetic Nb species, i.e., the illumination with visible light at 437 nm (2.84 eV) selectively promotes electrons from the Nb states to the conduction band according to the following process:
$$N^{-}_{{\rm{b}}}\rightarrow N^{\bullet}_{{\rm{b}}} +e^{-}$$
(6.6)
$$N^{\bullet}_{{\rm{b}}}\rightarrow N^{+}_{{\rm{b}}}+e^{-}$$
(6.7)
The visible light has not sufficient energy to excite electrons from the valence band, as the anatase TiO2 has a band gap of 3.2 eV. However, as noted above, the DFT calculations show that the N-induced defect states lie a few tenths of an eV above the valence band edge for both paramagnetic Nb· and diamagnetic charged Nb species. The Nb species are expected to be definitely more abundant because they are energetically favored and are thus preferentially excited (Eq. 6.11) so that under illumination the equilibrium attained involves an increase in the paramagnetic Nb· species (Fig. 6.17). The equilibrium conditions in the dark are recovered instantaneously when the light illumination is stopped.
The presence of oxygen molecules modifies the reactions as a fraction of photoexcited electrons is scavenged by O2, generating O2· radicals adsorbed on the surface:
$$N^{-}_{{\rm{b}}}+O_{2}{\rm{(gas)}}\rightarrow N^{\bullet}_{{\rm{b}}}+O^{\bullet {-}}_{2}{\rm{(surf)}}$$
(6.8)
$$N^{\bullet}_{{\rm{b}}}+O_{2}{\rm{(gas)}}\rightarrow N^{+}_{{\rm{b}}}+O^{\bullet {-}}_{2}{\rm{(surf)}}$$
(6.9)

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. [111] 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.

Figure 6.20(a) shows the 77 K CW-ESR spectrum of the rutile TiO2 sample, which has been thermally reduced under H2 atmosphere. The spectrum consists of a broad anisotropic singlet with a line-width of 3 mT at g ≈ 1.96, attributable to Ti3+ ion (with d1 electronic configuration) formed on the TiO2 surface [99, 108, 109, 112]. By exposing the sample to O2 at room temperature the spectral intensity of Ti3+ decreased to ca. 1/20 of the original, and signals of surface O2 radical with orthorhombic g-tensor (gx = 2.003, gy = 2.010, and gz = 2.020 and 2.023) appeared, as seen in Fig. 6.20(b). There are two gz peaks at 2.023 and 2.020 for the O2 ion, suggesting the presence of two kinds of O2 species, i.e. O2(A) and O2(B).
Fig. 6.20

ESR spectra observed at 77 K for rutile TiO2 nanoparticles treated in the following sequence: (a) preheated at 573 K in air followed by thermal treatment at 773 K under H2 atmosphere (27 kPa), (b) exposed to O2 (4.0 kPa) for 5 min at room temperature, and (c) illuminated with visible light for 10 min. The O2 line shapes around the gz component in spectra (b) and (c) are expanded for clarification. The peaks at gz(A) and gz(B) are designated by O2(A) and O2(B), respectively. Asterisks indicate signals due to a standard marker, Mn2+/MgO. (d) Relationship between the numbers of O2(B) radicals and of Ti3+ ions increased by visible-light illumination (λ > 500 nm) at 77 K. The figure is adapted from [111a] with permission from the American Chemical Society

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.

Two different Ti3+ sites were assumed to be formed in the thermally treated rutile TiO2 sample under H2 atmosphere; (a) five-coordinate Ti3+ site and (b) oxygen vacancy Ti3+ site consisting of two Ti3+ ions adjacent to each other, see Scheme 6.3. Then, the following reaction scheme was proposed for the O2 adsorption and photoinduced reactions by taking the experimental results into accounts.
Scheme 6.3

Schematic representation of (a) five-coordinate Ti3+ sites and (b) oxygen vacancy sites. O2(A) and O2(B) species are proposed to be formed by visible-light illumination to the O2 molecules adsorption on (a) and (b) sites, respectively. The diagram is adapted from [111a] with permission from the American Chemical Society

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 [113]. 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.

The ESR spectral response for the generation rate of O2(B) is shown in Fig. 6.21. The generation rate was found to increase with the photon energy and had a maximum at around 480 nm. On the other hand, the diffuse-reflectance vis-NIR(near infrared) spectra show a sharp absorption band around 460 nm for the TiO2 sample treated at 773 K under vacuum; the 460 nm band having been attributed to the oxygen vacancies (F-type color centers) in subsurface layers [114, 115, 116, 117, 118]. Based on the close correspondence between the 480 nm wave length giving the maximum ESR spectral response and the 460 nm absorption, it was concluded that F-type color centers generated in subsurface layers of TiO2 absorb the visible light to induce indirectly the electron transfer reaction from O22− to Ti4+ at the surface oxygen vacancy site.
Fig. 6.21

ESR intensity of O2(B) plotted against the wavelength of light in samples illuminated at 77 K. The wavelength was controlled by passing the source light through a series of interference filters combined with appropriate cutoff filters. The ESR intensities were measured after 50 s illumination at each wavelength. The diagram was obtained from Dr. K. Komaguchi [111b]

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. [121] 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 spectra shown in Fig. 6.22 were observed at 77 K for anatase, rutile, and P-25 TiO2 particles, which had been partially reduced in H2 atmosphere at 773 K. Both pure anatase and rutile TiO2 samples show a singlet ESR spectrum with a small g-anisotropy whose lineshapes are almost independent of the observation temperature in the range of 3.6–77 K. Another important spectral characteristic is that the linewidth is considerably broader for the anatase sample than for the rutile, which makes it possible to evaluate relative amount (concentration) of ESR signals from the anatase and rutile particles in the mixed phase TiO2 sample (for example, Degussa P-25) by the ESR spectral simulation method. The following ESR g-tensors and the peak-to-peak linewidth (ΔBpp) were evaluated by ESR spectral simulations: g = 1.953, g|| = 1.886, and ΔBpp = 8.0 mT for the anatase, and g = 1.963 and g|| = 1.931, and ΔBpp = 3.1 mT for the rutile. The observed g-values are attributable to the paramagnetic Ti3+ ions located on the surface of the anatase and rutile particles [119, 122]. The assignment to surface Ti3+ ions is supported by the experimental result showing that an electron is irreversibly moved from the Ti3+ to O2 to generate a superoxide (O2) anion radical when the O2 molecules are introduced to the sample.
Fig. 6.22

ESR spectra observed at 77 K for partially reduced TiO2 nanoparticles, which were prepared by heating the TiO2 sample at 773 K in vacuum and then in H2 gas atmosphere for 1 h. (a) Anatase, (b) Rutile, (c) Degussa P-25, (d) Degussa P-25 after exposure to air (i.e., a trace amount of air was admitted for several seconds and evacuated at room temperature prior to illumination). (e) Upon visible light (λ = 500–800 nm) illumination of (d). (f) In the dark after illumination of (e). The simulated ESR line shapes are shown by dotted lines. The ESR parameters used for the simulation are given in the text. The peaks due to the Mn2+/MgO standard marker are denoted by asterisks. The signals around the field denoted by an arrow are due to defects in the glass of the liquid N2 Dewar. The spectra are adapted from [121] with permission from Elsevier

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

The ESR spectral intensity and the line-shape of the Ti3+ ions in the partially reduced pure phase rutile and anatase samples changed upon visible light illumination in the ESR cavity at 77 K [121]. The signal intensity decreased and disappeared entirely after several minutes of illumination with white light. When the light was turned off, the signals started to reappear and increased in the dark at 77 K. Finally, the signals were restored completely to the initial intensity within 50 min. The overall photoresponse of the Ti3+ ESR signal intensity is shown in Fig. 6.23.
Fig. 6.23

Photoresponse of the ESR spectral intensity (I: the peak-to-peak height in the first derivative spectrum) of Ti3+ observed for partially reduced TiO2 nanoparticles at 77 K. (Δ) anatase (AMT-100), (Δ) rutile (STR-60C), (□) Degussa P-25 after air treatment. The samples were illuminated with white light from t = 0 min to t = 20 min. The diagram is adapted from [121] with permission from Elsevier

The Ti3+ signal restoration of the air contacted P-25 sample shows a striking contrast to those of the pure phases as shown in Fig. 6.23. The enhanced spectral intensity was attributed to the increase in the rutile Ti3+ ions with the narrow linewidth. In fact the relative concentration of rutile Ti3+ ions increased two times after the restoration, whereas the total amount of Ti3+ ions remained unchanged before and after the illumination. Taking the TiO2 band gap of 3.0–3.2 eV (λ: 380–410 nm) into account, the observed photo-response can be caused by the excitation of electrons from the surface Ti3+, but not from the valence band. This is in accord with the suggested energy level of the Ti3+ ions being 0.3–0.8 eV below the conduction band edge [123]. The electrons in the conduction band, which are generated from the Ti3+ ions by the photo excitation, are ESR silent until they relax back so as to be re-trapped by the Ti4+ ions as Ti3+ ions. Thus it was concluded that the electrons of Ti3+ are preferentially transferred from the anatase to the rutile by photoexcitation and retrapped at the rutile surface in dark as illustrated in Fig. 6.24 [121].
Fig. 6.24

Sketch of the proposed mechanism for the processes induced by visible light illumination of partially reduced a mixed phase TiO2 sample (P-25). The electrons are excited from the surface Ti3+ ion to the conduction band (CB) upon illumination of light energy lower than the band gap and are re-trapped as Ti3+ ions in the dark. For the partially reduced P-25 sample, a certain amount of the excited electrons in the anatase phase can transfer through the interfacial boundaries to be re-trapped by the surface Ti4+ ions in the rutile phase. The diagram is adapted from [121] with permission from Elsevier

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 [124] 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 [3], Kasai [4] and other scientists (for example, Shiotani [125]) 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

We start by comparing the electronic structure of the homonuclear O2 ion radical with that of the heteronuclear NO radical discussed in Section 6.2.1 because of their close similarity in electronic structure. The O2 radical is a 17 electron system having two additional electrons compared with the NO radical (15 electrons system). The degenerate anti-bonding 2pπg*(x,y) orbitals are occupied by three electrons in a “free” (not adsorbed) O2 radical, whereas the 2pπ*(x,y) orbitals contain one electron in the NO radical. Thus, for O2 a hole resides in a 2pπg*(x) orbital after the orbital degeneracy is lifted by the crystal field (Δ) due to the Ti4+ ion on which O2 is adsorbed (or bonded), see Fig. 6.25. For NO the unpaired electron resides in a 2pπ*(y) orbital after the orbital degeneracy being removed. This leads to a change in the sign of spin-orbit coupling (λ), i.e., minus sign for O2 instead of plus sign for NO. Thus the g-component along the inter-nuclear axis, gz, are shifted in opposite directions from the ge value, i.e., \(g_{zz} = g_e + \frac{{2\,l\left| \lambda \right|}}{{\sqrt {\lambda ^2 + \varDelta ^2 } }}\) for O2 and \(g_{zz} = g_e - \frac{{2\,l\left| \lambda \right|}}{{\sqrt {\lambda ^2 + \varDelta ^2 } }}\) for NO, refer to Eq. (6.3).
Fig. 6.25

The O2 ion radical adsorbed at Ti4+ ion on oxide supports. (a) Illustration of the unpaired electron 2pπg*(x) orbital in the ground electronic state in which the molecular coordinates and the principal axes of the g-tensor coincide. (b) Energy levels of O2 interacting with the Ti4+ ion. The term Δ stands for the energy splitting between 2pπg*(x) and 2pπg*(y) orbitals. The reader is referred to Fig. 6.2. for a comparison with the NO radical adsorbed at a metal ion

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.

The ESR spectrum of 17O enriched O2 adsorbed on Ti4+ ions on oxide supports is shown in Fig. 6.26(b). The ESR spectrum is well simulated by a superposition of three different spectra of (17O—18O), (17O—17O), and (18O—18O); the g-values and 17O hf splittings employed are given in Table 6.5. An expanded spectrum of the central part is shown together with the simulated ones of (17O—18O) and (17O—17O) species in Fig. 6.27.
Fig. 6.26

(a) X-band ESR spectrum of O2 adsorbed on Ti4+ ions on oxide supports recorded at 4.2 K. (a) The best fit simulated spectrum using one set of rhombic g-values listed in Table 6.5 and an orientation dependent Lorentzian linewidth 0.95, 0.18, and 1.35 G for the x, y, and z component, respectively. (a) The spectrum calculated using the same ESR parameters in (a), but an orientation independent linewidth of 0.2 G. (b) X-band ESR spectrum of 17O enriched O2; the experiments were carried out using 17O enriched O2 with 70% of 17O with I = 7/2 and 30% of 18O with I = 0). (b) Spectrum simulated by superposing three different spectra of (17O—18O), (17O—17O), and (18O—18O) with a relative intensity of 0.42: 0.49: 0.09. See Fig. 6.27 for the expanded spectra of the central part in (b). The spectra are adapted from [125] with permission from the American Institute of Physics

Fig. 6.27

Upper spectra: Expanded ESR spectra of the central part in Fig. 6.26(b). The peaks marked as (*), (**) and (•) correspond to resonances due to (17O—18O), (17O—17O) and (18O—18O) ion radicals, respectively. Lower spectra: The best fit simulation spectra calculated for the rigid limit spectra (including second-order corrections) with the two different sets of 17O hf splittings listed in Table 6.5. The spectra are adapted from [125] with permission from the American Institute of Physics

Table 6.5

ESR parameters of O2 adsorbed on Ti4+/oxide supportsa

 

gx, gy, gz

17O A1, A2, A3/Gb

Abundance/%

(1) (17O18O) –Ti3+

2.0027, 2.0092, 2.0268

74.9, 0, 0

50

(2) (18O—17O)–Ti3+

2.0027, 2.0092, 2.0268

80.3, 0, 0

50

a Data taken from ref. [125].

b 1 G (gauss) = 0.1 mT (T: tesla).

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°) [126]. (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 [125].

6.7 Summary

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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Copyright information

© Springer Science+Business Media B.V. 2011

Authors and Affiliations

  • Anders Lund
    • 1
  • Masaru Shiotani
    • 2
  • Shigetaka Shimada
    • 3
  1. 1.Department of Physics, Chemistry and Biology, IFMLinkoping UniversityLinkopingSweden
  2. 2.Graduate School of EngineeringHiroshima UniversityHigashi-HiroshimaJapan
  3. 3.Graduate School of EngineeringNagoya Institute of TechnologyOwari-AsahiJapan

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