**1. Introduction**

Environmental pollution is beyond limits, and the development of environmental catalysts is a critical subject for scientists and engineers all over the world. Here, the catalysts for purify‐ ing polluted water and air by utilizing the solar energy are termed as "solar environmental catalyst." The TiO2 photocatalyst possesses great potential as the "solar environmental catalyst" owing to its strong oxidation ability, high physicochemical stability, abundance in nature, and nontoxicity [1,2]. Scheme 1 shows the fundamental reaction mechanism on the TiO2-photocata‐ lyzed decomposition of organic pollutants with the characteristic time for each process [3]. UVlight absorption by TiO2 causes the excitation of electrons in the valence band (VB) to the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 potential, *E*<sup>0</sup> (O2/O2 − ) = −0.284 V versus SHE) by the CB electrons (−0.53 V versus SHE at pH 7) 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 of charge separation due to the interfacial electron transfer [5].

**Sheme 1.** The basic mechanism on the TiO2 photocatalytic reaction (the surface trapping processes for the CB electrons and VB holes are abbreviated) with the characteristic time for each step shown.

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 but less effective for anatase.

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 the "solar environmental catalyst."
