**3. Catalyst preparation**

conduction band (CB) in the order of femtoseconds. Most of the photogenerated charge carriers are lost by the recombination within ~100 ns. The charge carriers surviving the recombination are trapped at the TiO2 surface to induce the redox reactions. In general, the CB electrons reduce oxygen (O2), whereas the VB holes oxidize organic pollutants. The VB holes have a highly positive potential (+2.67 V versus standard hydrogen electrode (SHE) at pH 7) to oxidize most organic compounds. Conversely, the driving force for one-electron O2 reduction (standard

is small. Consequently, the O2 reduction reaction (ORR) is much slower (~ms) than the oxidation process (~100 ns). TiO2 usually takes crystal forms of rutile and anatase. The flatband poten‐ tial of anatase is ~0.2 V, which is more negative than that of rutile, and anatase has a higher UVlight activity for the oxidation of organic compounds as compared with rutile [4]. This fact also points to the importance of the ORR in TiO2 photocatalytic reactions. Also, the coupling of anatase and rutile TiO2 can further increase the UV-light activity because of the enhancement

**Sheme 1.** The basic mechanism on the TiO2 photocatalytic reaction (the surface trapping processes for the CB electrons

Recently, the visible-light activation of TiO2 by its surface modification with metal oxide nanoparticles (NPs) or oxocomplexes has been developed [6,7]. This approach has a major advantage over the conventional doping [8–14], in that visible-light activation can be achieved by a simple procedure without the introduction of the impurity/vacancy levels into the bulk TiO2. To date, the impregnation method has been mainly used for the surface modification with metal oxide NPs, including chromium oxides [15], iron oxides [16–18], and copper oxides [19]. Unfortunately, the surface modification by the impregnation method is effective for rutile

This chapter deals with our recent studies on the surface modification of anatase TiO2 with the first (3d) transition metal oxocomplexes (MOCs) by the chemisorption–calcination cycle technique (MOCs/TiO2) [20] and the characterization and photocatalytic activities for the degradation of organic pollutants. We show that some MOCs/TiO2 fulfill the requirements for

of charge separation due to the interfacial electron transfer [5].

360 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

and VB holes are abbreviated) with the characteristic time for each step shown.

) = −0.284 V versus SHE) by the CB electrons (−0.53 V versus SHE at pH 7)

potential, *E*<sup>0</sup>

(O2/O2 −

but less effective for anatase.

the "solar environmental catalyst."

## **3.1. Chemisorption–calcination cycle technique**

The adsorption mechanism of metal acetylacetonates (acac) on TiO2 depends on the kind of complexes. As an example, Fe oxocomplex formation by the CCC technique is represented in Scheme 2. In the first step, Fe(acac)3 is chemisorbed on the TiO2 surface via the ligand ex‐ change between the acac ligand and the surface Ti-OH group from the nonaqueous solution (Eq. 1) [22]:

$$\text{Fe(accac)}\_{3}\text{ +l(Ti}\_{\text{s}}\text{-OH)}\rightarrow\text{Fe(accac)}\_{3\text{-l}}\text{(O-Ti}\_{\text{s}}\text{)}\_{\text{l}}\text{+lAcacH}\text{ (1}\approx\text{1)}\tag{1}$$

where the subscript s denotes the surface atom.

**Figure 1.** Solar spectrum (AM 1.5).

Conversely, [Sn(acac)2]Cl2 is adsorbed on TiO2 via the ion exchange between H+ and [Sn(acac)2] 2+ ion (Eq. 2) [23]. In each case, the adsorption apparently obeys the Langmuir model. The saturated adsorption amount and the adsorption constant for the adsorption of various metal acetylacetonates on TiO2 at 298 K are summarized in Table 1. The adsorption constants range from 102 to 104 , indicating that they are strongly adsorbed on the TiO2 surface by chemical bonds. Exceptionally, Cr(acac)3 is not adsorbed because of its large ligand-field stabilization energy (1.2*∆*0).

**Sheme 2.** Fe oxocomplex (Fe2O3) formation on the TiO2 surface by the CCC technique.


**Table 1.** Adsorption properties of 3d metal acetylacetonates on TiO2 at 298 K.

First-Transition Metal Oxocomplex–Surface-Modified Titanium(IV) Oxide for Solar Environmental Purification http://dx.doi.org/10.5772/62008 363

$$\left\lfloor \text{Sn} \text{(accac)}\_{2} \right\rfloor \text{Cl}\_{2} + 2 \text{(Ti}\_{s} \text{-OH)} \rightarrow \text{Sn} \text{(accac)}\_{2} \text{(O-Ti}\_{s} \text{)}\_{2} + 2 \text{HCl} \tag{2}$$

In the second step, the oxidation of the acac ligands by heating the samples in air at 773 K yields iron oxides on the TiO2 surface. Further, these procedures are repeated to control the Fe-loading amount. Chemical analysis confirmed that all the Fe was confirmed to be present only on the TiO2 surface. The Fe-loading amount is expressed by the number of Fe ions per unit TiO2 surface area (*Γ*/ions nm−2).

### **3.2. Control of loading amount**

Conversely, [Sn(acac)2]Cl2 is adsorbed on TiO2 via the ion exchange between H+

The saturated adsorption amount and the adsorption constant for the adsorption of various metal acetylacetonates on TiO2 at 298 K are summarized in Table 1. The adsorption constants

bonds. Exceptionally, Cr(acac)3 is not adsorbed because of its large ligand-field stabilization

**Sheme 2.** Fe oxocomplex (Fe2O3) formation on the TiO2 surface by the CCC technique.

**Table 1.** Adsorption properties of 3d metal acetylacetonates on TiO2 at 298 K.

2+ ion (Eq. 2) [23]. In each case, the adsorption apparently obeys the Langmuir model.

, indicating that they are strongly adsorbed on the TiO2 surface by chemical

[Sn(acac)2]

range from 102

energy (1.2*∆*0).

to 104

362 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

and

A feature of the CCC technique is precise control of the loading amount of metal oxides. As an example, the manner in which the Fe-loading amount is controlled in the iron oxide/TiO2 system is described. Figure 2A shows the relation between *Γ* and initial complex concentration: black, Fe(acac)3 and blue, Mn(acac)3. In each case, the *Γ* gradually increases with an increase in the initial concentration. Figure 2B shows plots of *Γ* versus CCC cycle number (*N*): black, Fe(acac)3]0 = 0.65 mmol dm−3 (black) and blue, Mn(acac)3]0 = 8 mmol dm−3. The *Γ* further increases in proportional to *N* in both the systems. The linear *Γ*–*N* relation is also observed in the other metal oxide/TiO2 systems. In this manner, the loading amount of metal oxides can be controlled in a wide range using the precursor complex concentration and the cycle number.

## **4. Structure of surface metal oxocomplexes**

Another feature of the CCC technique is the formation of molecular-scale metal oxide species on TiO2. Figure 3 shows transmission electron micrographs (TEMs) of iron oxide/TiO2 with varying *Γ*. No particulate deposits are observed on the TiO2 surface at *Γ* < 1 ions nm−2. This fact suggests that iron oxides exist as molecular-scale iron oxide species on the TiO2 surface.

**Figure 2.** (A) Plots of Fe-loading amount (*Γ*/ions nm−2) versus initial concentration of the complex ([M(acac)3]0): M = Fe (black) and M = Mn (blue). (B) Plots of *Γ* versus cycle number (*N*) at Fe(acac)3]0 = 0.65 mmol dm−3 (black) and Mn(acac)3]0 = 8 mmol dm−3 (blue).

To obtain the detailed structural information, Fe K-edge X-ray absorption fine-structure spectra were measured for the iron oxide/TiO2 samples with varying *Γ* [24]. Figure 4A shows X-ray absorption near-edge structure (XANES) spectra for the iron oxide/TiO2 samples, and authentic Fe, Fe3O4, and α-Fe2O3 for comparison. The absorption edge of the iron oxide/TiO2 sample is in agreement with that of α-Fe2O3, indicating the oxidation number of the iron to be +3 (Table 1). Figure 4B shows the Fourier transforms of the *k*<sup>3</sup> -weighted X-ray absorption fine structure (EXAFS) for the iron oxide/TiO2 samples. The peaks around 1.6 Ǻ and 2.8 Ǻ arise from the scattering from the nearest neighbor O and Fe, respectively. It is worth noting that the latter peak becomes very weak at *Γ* ≤ 0.5. Evidently, the iron oxides exist as a mononuclear Fe oxocomplex on TiO2 designated as Fe2O3/TiO2 below.

**Figure 3.** TEM images of Fe2O3/TiO2 with varying *Γ* : (A) *Γ* = 0 (P-25); (B) *Γ* = 0.23, (C) *Γ* = 0.38, (D) *Γ* = 0.54.

In this manner, MOCs are formed on the TiO2 surface in a highly dispersed state by the CCC technique, whereas the conventional impregnation method usually yields metal oxide NPs. As illustrated in Scheme 2, the molecular size of Fe(acac)3 (~0.5 nm2 complex−1) is much larger than the reciprocal number density of the Ti-OH groups on the TiO2 surface (~0.1 nm2 group −1) [25]. In the first step of the CCC process, Fe(acac)3 complexes are chemisorbed isolatedly each other owing to the bulky acac-ligands. Also, the strong bond between the complexes and the TiO2 surface suppresses the aggregation of the oxocomplexes during the second step. Consequently, mononuclear MOCs can be formed on the TiO2 surface.

**Figure 4.** XANES and EXAFS spectra. (A) XANES spectra for Fe, Fe3O4, α-Fe2O3, and Fe2O3/TiO2 with varying *Γ*. (B) Fourier transforms of the *k*<sup>3</sup> -weighted EXAFS spectra for Fe2O3/TiO2. The figures were taken from Ref. 24.
