**3. The advanced structure and properties of blue TiO2**

Zhang et al. [9] discovered that the color and crystalline phase of white P25 (70% anatase and 30% rutile) changed into blue color by the treatment of lithium in an ethylenediamine (Li-EDA) solution, which is the first achievement in making blue TiO<sup>2</sup> under atmospheric


**Table 1.** Crystal structure data for TiO<sup>2</sup> Copyright (2014), Elsevier [15].

**Figure 1.** Bulk structures of anatase and rutile TiO<sup>2</sup> . Copyright (2003), Elsevier [18].

To date, TiO2

systems such as Pt-doped TiO2

tion methods. The blue TiO<sup>2</sup>

**Table 1.** Crystal structure data for TiO<sup>2</sup>

In this chapter, we focus on blue TiO<sup>2</sup>

228 Titanium Dioxide - Material for a Sustainable Environment

developed as visible light photocatalysts [3, 4].

ify TiO<sup>2</sup>

TiO2

TiO2

nanomaterials have attracted the interest of many scientists. The focus is to mod-

, or graphene/TiO2

nanomaterial contains Ti3+ with an abundant oxygen vacancy,

as a visible-light-driven photocatalyst and its prepara-

/carbon dot composites

[5]. In the future,

under atmospheric

under visible light.

structural properties or to combine supportive materials to demonstrate that TiO<sup>2</sup>

nanomaterials are excellent photocatalysts, which can be used as dopants in novel metal-TiO<sup>2</sup>

which can absorb visible and infrared light as well as UV light, producing more electrons and

we would like to further address the beneficial applications in clean energy storage media and protecting the environment, including the hydrogen evolution reaction, carbon dioxide

 belongs to the transition metal oxide family. There are four different polymorphs of TiO<sup>2</sup> found in nature such as anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and

 (B) (monoclinic) [6], the most important of which are anatase and rutile. With calcination at high temperatures exceeding ~600°C, the brookite and anatase polymorphs will transform

The tetragonal anatase bulk unit cell has dimensions of a = b = 0.3733 nm and c = 0.9370 nm, and the rutile bulk unit cell has dimensions of a = b = 0.4584 nm, and c = 0.2953 nm (**Table 1**). In both structures, the octahedral distortions create the basic building units [7, 8]. The lengths and angles of octahedral coordinated Ti atoms, therefore, dictate stacking in both structures, as shown in **Figure 1**.

Zhang et al. [9] discovered that the color and crystalline phase of white P25 (70% anatase and 30% rutile) changed into blue color by the treatment of lithium in an ethylenediamine

Copyright (2014), Elsevier [15].

[3], Au-doped TiO2

holes and also facilitating better electrical conductivity than pristine TiO<sup>2</sup>

reduction, and degradation of pollutants by using noble blue TiO<sup>2</sup>

**2. General structure and properties of TiO2**

into the thermodynamically stable rutile polymorph [5].

**3. The advanced structure and properties of blue TiO2**

(Li-EDA) solution, which is the first achievement in making blue TiO<sup>2</sup>

pressure at room temperature in solution and also the phase-selective reduction between anatase and rutile TiO2 phases. They showed that the white anatase TiO<sup>2</sup> phase was not changed, while the rutile TiO2 phase changed into black color. In the case of P25 TiO2 , the blue colored TiO2 appeared as a result of the combination of white and black colors (**Figure 2**) [9].

The unit cell parameters and nanocrystalline size profiles of white P25 and blue TiO2 are shown in **Table 2**. These results show that a slight change occurred along the a and b directions, but there was significant expansion in the c direction, and as a result, the unit cell volume expanded significantly as well [10].

**Figure 2.** Schematics of TiO<sup>2</sup> (white P25) (left) and blue TiO<sup>2</sup> crystals (right). The black color corresponds to the visual color of the reduced rutile TiO<sup>2</sup> . Copyright (2016), Royal Society of Chemistry [9].


the length and width of the peaks were calculated to confirm the percentages of the {101} and {001} facets (**Figure 3**). Their research well defined the optimum nanosize as well as the shape

to the excitation of conduction band electrons. Therefore, it should exhibit much better photocatalytic activity under visible light or the full spectrum of solar irradiation (**Figure 4**) [9, 14].

sis. As shown in **Figure 5**, the reduction potential of photogenerated electrons is defined by the energy level at the bottom of the conduction band (CB), while the oxidizing ability is the energy

Photocatalytic reactions occur as a material interacts with light, which provide higher energy than the bandgap of the semiconductor to create reactive oxidizing species, leading to the

(2) Separation and transport of electron-hole pairs with electrons excited from the valence

absorption of photons with sufficient energy and generation of electron-hole pairs.

and some photocatalysts with respect to the redox potential (vs. NHE) values of different

level at the top of the valence band (VB). Because the CB energy level of TiO<sup>2</sup>

reduction potential levels of NHE references, semiconductors as well as TiO<sup>2</sup>

The basics of the photocatalytic process can be summarized as follows:

(3) Chemical reaction on the surface-active sites with charge carriers.

chemical species measured at a pH of 7. Copyright (2014), Elsevier [11, 15, 16].

crystals, suggesting that the {101} facets are more photocatalytically active than the {001}

has excellent absorption over a much wider spectral range than white TiO<sup>2</sup>

 **in photocatalysis**

in particular, are widely used in the applications of photocataly-

conversion, or pollutant degradation [15].

(up to 2.1 mmol h−1 g−1) under simulated solar illumination, while

lattice [12].

Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis

http://dx.doi.org/10.5772/intechopen.73059

due

231

is higher than the

nanomaterials can

of TiO<sup>2</sup>

The blue TiO2

(1) TiO2

band (VB) to CB.

**Figure 5.** Bandgap of TiO<sup>2</sup>

facets for the evolution of H<sup>2</sup>

Semiconductor materials, TiO2

the blue coloration results from oxygen vacancies in the TiO<sup>2</sup>

**4. Electronic properties of blue TiO2**

be used as a catalyst for hydrogen evolution, CO<sup>2</sup>

photocatalytic transformation of a compound.

**Table 2.** Unit cell parameters of TiO<sup>2</sup> (white P25) and blue TiO2 Copyright (2014), American Chemical Society [10].

**Figure 3.** XRD patterns of different shapes. Experimental (thick black lines) and simulated (thin colored lines) plots for TiO2 nanocrystals. The insets showed accurate percentages of the {001} and {101} facets of atomistic models. Copyright (2012), American Chemical Society [12].

**Figure 4.** Color change of white P25 (left) to blue TiO<sup>2</sup> (right) and UV-vis absorption spectra of pristine TiO<sup>2</sup> (P-TiO2 ) and reduced anatase TiO2 (R-TiO<sup>2</sup> ). Copyright (2017), American Chemical Society [14].

Recent publications showed that the morphology of TiO<sup>2</sup> materials resulted in differences of the enhanced photocatalytic activity for the production of hydrogen between the {101} and {001} facets of anatase tetragonal bipyramidal nanocrystals [11–13]. Based on the XRD simulation, the length and width of the peaks were calculated to confirm the percentages of the {101} and {001} facets (**Figure 3**). Their research well defined the optimum nanosize as well as the shape of TiO<sup>2</sup> crystals, suggesting that the {101} facets are more photocatalytically active than the {001} facets for the evolution of H<sup>2</sup> (up to 2.1 mmol h−1 g−1) under simulated solar illumination, while the blue coloration results from oxygen vacancies in the TiO<sup>2</sup> lattice [12].

The blue TiO2 has excellent absorption over a much wider spectral range than white TiO<sup>2</sup> due to the excitation of conduction band electrons. Therefore, it should exhibit much better photocatalytic activity under visible light or the full spectrum of solar irradiation (**Figure 4**) [9, 14].

#### **4. Electronic properties of blue TiO2 in photocatalysis**

Semiconductor materials, TiO2 in particular, are widely used in the applications of photocatalysis. As shown in **Figure 5**, the reduction potential of photogenerated electrons is defined by the energy level at the bottom of the conduction band (CB), while the oxidizing ability is the energy level at the top of the valence band (VB). Because the CB energy level of TiO<sup>2</sup> is higher than the reduction potential levels of NHE references, semiconductors as well as TiO<sup>2</sup> nanomaterials can be used as a catalyst for hydrogen evolution, CO<sup>2</sup> conversion, or pollutant degradation [15].

Photocatalytic reactions occur as a material interacts with light, which provide higher energy than the bandgap of the semiconductor to create reactive oxidizing species, leading to the photocatalytic transformation of a compound.

The basics of the photocatalytic process can be summarized as follows:

**Figure 3.** XRD patterns of different shapes. Experimental (thick black lines) and simulated (thin colored lines) plots for

(white P25) and blue TiO2

). Copyright (2017), American Chemical Society [14].

enhanced photocatalytic activity for the production of hydrogen between the {101} and {001} facets of anatase tetragonal bipyramidal nanocrystals [11–13]. Based on the XRD simulation,

nanocrystals. The insets showed accurate percentages of the {001} and {101} facets of atomistic models. Copyright

(right) and UV-vis absorption spectra of pristine TiO<sup>2</sup>

Copyright (2014), American Chemical Society [10].

(P-TiO2

materials resulted in differences of the

) and

TiO2

(2012), American Chemical Society [12].

**Table 2.** Unit cell parameters of TiO<sup>2</sup>

230 Titanium Dioxide - Material for a Sustainable Environment

**Figure 4.** Color change of white P25 (left) to blue TiO<sup>2</sup>

Recent publications showed that the morphology of TiO<sup>2</sup>

(R-TiO<sup>2</sup>

reduced anatase TiO2


**Figure 5.** Bandgap of TiO<sup>2</sup> and some photocatalysts with respect to the redox potential (vs. NHE) values of different chemical species measured at a pH of 7. Copyright (2014), Elsevier [11, 15, 16].

**Figure 6.** Schematic diagram of Ti3+ self-doped TiO<sup>2</sup> mechanism for visible light photocatalysis.

Meanwhile, electron-hole recombination is also possible depending on the competition between these processes.

Blue TiO2 nanomaterials can overcome the limitations to enhance the photocatalytic performance due to the formation of oxygen vacancies (supports many free carriers charges). The oxygen vacancy is a positive charge. Then, Ti3+ from the center shifts away from the oxygen vacancy position, leading to an advanced sublevel electric state and excellently trapped holes, preventing the recombination of electrons and holes, even with the lower energy bandgap irradiation (~2.7 eV) compared to P25 (3.2 eV). Blue TiO<sup>2</sup> could generate electrons in the wide open region of irradiation such as solar light, which contains most visible and infrared wavelengths as well as UV light [17, 19] (**Figure 6**).

washing, TiH2

**Figure 7.** Photograph of H-aTiO<sup>2</sup>

TiO2

affecting the photocatalytic activity [21].

Copyright (2013) American Chemical Society [21].

reported the synthesis of novel blue colored TiO<sup>2</sup>

stable and is likely to be oxidized in air [28].

**5.2. Hydro(solvo)thermal method**

solvothermal method using TiCl3

was completely removed, and a well-crystallized bluish sample (TiO2−x: H)

with abundant defects through a one-step

gas flow at temperatures of 500–700°C. Gradual changes

Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis

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233

as precursors. The introduction of Ti4+ in the reac-

was obtained [4]. Qiu et al. found that the TiO2−x: H can efficiently enhance the visible- and infrared-light absorption and improve photocatalytic degradation of methyl orange (MO) and hydrogen production via water splitting by H doped into the well-crystallized lattice, which means that might be localized states in the bandgap was offered and has a relatively low recombination rate of electrons and holes. Moreover, we should note that the low concentration of hydrogen atoms in hydrogenated titania was found to be a unfavorable factor

in color from blue to gray to a different degree are observed, depending on annealing temperatures and annealing time.

Hydrothermal and solvothermal methods have received some attention due to their simple and low-cost production routes and are suitable for large-scale production [28, 29]. Zhu et al.

Ti3+ + oxygen species ⇄ Ti4+ (1)

This process is governed by the Le Chatelier's principle. The oxygen vacancy formation dominantly resulting from Ti3+ will not be completely oxidized during the solvothermal process. Moreover, leaving behind a high concentration of bulk Ti3+ defects is very favorable for visible light photocatalytic reactions [29]. In addition, Fang et al. synthesized a variety of reduced

 samples by using Zn powder as the reducing agent and HF as the solvent for the stabilization of the formed Ti3+ species and oxygen vacancies in a simple one-pot hydrothermal process. At the same time, it should be noted that the Ti3+ introduced by Zn reduction is not

and TiF<sup>4</sup>

samples prepared with a H2

tion system inhibits the oxidation of Ti3+ during the solvothermal treatment.

#### **5. Synthesis of blue TiO2 nanomaterials for photocatalysis**

#### **5.1. Hydrogenation synthesis**

H2 is the most common reagent used for the hydrogenation of TiO<sup>2</sup> , which can react with the lattice oxygen, leading to the formation of abundant oxygen vacancies and Ti3+ in TiO2 due to its facile activation by thermal or electromagnetic energy [4, 19]. The annealing time changes with the annealing temperature, where the blue color was maintained up to a longer time at 500°C. It readily changed to pale gray at 600°C due to the high concentration of Ti3+ in the bulk at the early stage of hydrogenation, which may absorb oxygen molecules and lead to O<sup>−</sup> as a major species on the surface after prolonged hydrogenation (**Figure 7**) [20, 21]. In addition, hydrogenation processes require harsh synthetic conditions and/or a dangerous production process [4, 10, 19, 21–25]. Therefore, H<sup>2</sup> is introduced using different reducing agents such as NaBH4 and TiH2 [4, 26, 27] instead of an external dose of hydrogen gas. TiH<sup>2</sup> as a solid solution of hydrogen in Ti and P25 was mixed and sealed in a quartz tube and calcined at 450°C for 10 h. After discarding most of the unreacted TiH<sup>2</sup> sediments, HCl and H2 O2 solutions were then introduced to completely remove the residual TiH<sup>2</sup> , during which the TiH2 dissolved and a yellow solution was formed. After centrifugation and thorough

**Figure 7.** Photograph of H-aTiO<sup>2</sup> samples prepared with a H2 gas flow at temperatures of 500–700°C. Gradual changes in color from blue to gray to a different degree are observed, depending on annealing temperatures and annealing time. Copyright (2013) American Chemical Society [21].

washing, TiH2 was completely removed, and a well-crystallized bluish sample (TiO2−x: H) was obtained [4]. Qiu et al. found that the TiO2−x: H can efficiently enhance the visible- and infrared-light absorption and improve photocatalytic degradation of methyl orange (MO) and hydrogen production via water splitting by H doped into the well-crystallized lattice, which means that might be localized states in the bandgap was offered and has a relatively low recombination rate of electrons and holes. Moreover, we should note that the low concentration of hydrogen atoms in hydrogenated titania was found to be a unfavorable factor affecting the photocatalytic activity [21].

#### **5.2. Hydro(solvo)thermal method**

Meanwhile, electron-hole recombination is also possible depending on the competition between

mance due to the formation of oxygen vacancies (supports many free carriers charges). The oxygen vacancy is a positive charge. Then, Ti3+ from the center shifts away from the oxygen vacancy position, leading to an advanced sublevel electric state and excellently trapped holes, preventing the recombination of electrons and holes, even with the lower energy bandgap

open region of irradiation such as solar light, which contains most visible and infrared wave-

the lattice oxygen, leading to the formation of abundant oxygen vacancies and Ti3+ in TiO2 due to its facile activation by thermal or electromagnetic energy [4, 19]. The annealing time changes with the annealing temperature, where the blue color was maintained up to a longer time at 500°C. It readily changed to pale gray at 600°C due to the high concentration of Ti3+ in the bulk at the early stage of hydrogenation, which may absorb oxygen molecules and lead

 as a major species on the surface after prolonged hydrogenation (**Figure 7**) [20, 21]. In addition, hydrogenation processes require harsh synthetic conditions and/or a dangerous

dissolved and a yellow solution was formed. After centrifugation and thorough

as a solid solution of hydrogen in Ti and P25 was mixed and sealed in a quartz tube and

irradiation (~2.7 eV) compared to P25 (3.2 eV). Blue TiO<sup>2</sup>

production process [4, 10, 19, 21–25]. Therefore, H<sup>2</sup>

and TiH2

is the most common reagent used for the hydrogenation of TiO<sup>2</sup>

calcined at 450°C for 10 h. After discarding most of the unreacted TiH<sup>2</sup>

solutions were then introduced to completely remove the residual TiH<sup>2</sup>

lengths as well as UV light [17, 19] (**Figure 6**).

**Figure 6.** Schematic diagram of Ti3+ self-doped TiO<sup>2</sup>

232 Titanium Dioxide - Material for a Sustainable Environment

**5. Synthesis of blue TiO2**

**5.1. Hydrogenation synthesis**

nanomaterials can overcome the limitations to enhance the photocatalytic perfor-

mechanism for visible light photocatalysis.

 **nanomaterials for photocatalysis**

could generate electrons in the wide

is introduced using different reducing

[4, 26, 27] instead of an external dose of hydrogen gas. TiH<sup>2</sup>

, which can react with

sediments, HCl and

, during which

these processes.

Blue TiO2

H2

to O<sup>−</sup>

H2 O2

the TiH2

agents such as NaBH4

Hydrothermal and solvothermal methods have received some attention due to their simple and low-cost production routes and are suitable for large-scale production [28, 29]. Zhu et al. reported the synthesis of novel blue colored TiO<sup>2</sup> with abundant defects through a one-step solvothermal method using TiCl3 and TiF<sup>4</sup> as precursors. The introduction of Ti4+ in the reaction system inhibits the oxidation of Ti3+ during the solvothermal treatment.

$$\text{Ti}^{\ast}\text{+}\text{oxygen species }\rightleftharpoons\text{Ti}^{\ast}\tag{1}$$

This process is governed by the Le Chatelier's principle. The oxygen vacancy formation dominantly resulting from Ti3+ will not be completely oxidized during the solvothermal process. Moreover, leaving behind a high concentration of bulk Ti3+ defects is very favorable for visible light photocatalytic reactions [29]. In addition, Fang et al. synthesized a variety of reduced TiO2 samples by using Zn powder as the reducing agent and HF as the solvent for the stabilization of the formed Ti3+ species and oxygen vacancies in a simple one-pot hydrothermal process. At the same time, it should be noted that the Ti3+ introduced by Zn reduction is not stable and is likely to be oxidized in air [28].

## **5.3. Electrochemical reduction synthesis**

Zhang et al. demonstrated that that electrochemical reduction method is a facile and effective strategy to induce *in situ* self-doping of Ti3+ into TiO2 and the self-doped TiO<sup>2</sup> photoelectrodes showed remarkably improved and very stable water splitting performance [30]. The hierarchical TiO2 NTs were fabricated by a two-step anodization process. In the first step of anodization, the as-prepared Ti sheet as an anode was anodized at 60 V for 30 min in electrolytes consisted of 0.5 wt% NH4 F in EG solution with 2 vol% water and a Pt mesh (Aldrich, 100 mesh) as a cathode, respectively. After the as-grown nanotube layer was ultrasonically removed in DI water, the second step of anodization was performed at 80 V for 5 min. Then, the prepared TiO2 NT samples were cleaned and annealed in air at 450 degree for 1 h with a heating rate of 5 degree min−1 [30]. In the electrochemical reduction processes, the TiO<sup>2</sup> NTs as the working electrode with an AgCl electrode and a Pt mesh formed a typical three-electrode system under a negative potential (0.4 V vs. the reversible hydrogen electrode (RHE)) in the supporting electrolyte of 1 M Na<sup>2</sup> SO<sup>4</sup> for 30 min [30]. The electronic transition from the valence band to the Ti3+ induced interbands and/or from the energy band levels to the conduction band was considered to contribute to enhance the absorption in the visible region in the self-doped TiO<sup>2</sup> , which helps explaining the observed color change from the prime white of the TiO2 NTs to the light blue of the ECR-TiO<sup>2</sup> NTs [30].

TiO2

modulatory TiO2

 was milled with Na/NaCl fine powders with different weight ratio at a series of milling rates such as 80, 120, 150, and 180 rpm at room temperature under argon atmosphere for 0.25–4 h. After the Na and NaCl was removed, the obtained TiO2−x products were dispersed in a small amount of deionized water and then vacuum-dried at room temperature to obtain TiO2−x powders [32]. Moreover, the obtained TiO2−x with a high surface area can be employed as an effective support for Ru particles and the Ru/TiO2−x catalyst exhibited superior activity

require high-temperature processing. Due to high-temperature processing, a phase-selective

temperature solution processing was demonstrated as a very effective method to prepare

reducing agent consists of lithium in ethylenediamine (Li-EDA), which can disorder only the

Li foil was dissolved in 20 ml ethanediamine to form a 1 mmol/ml solvated electron solution. Two hundred milligram of Degussa P25 (anatase, size: ~25 nm, rutile, size: ~140 nm, P25, size: 20–40 nm) was prepared after thorough drying and then added into the abovementioned solution and stirred for several days depending on the application. After sufficient reaction, the excess electrons and formed Li salts were quenched by slowly adding HCl into the mix-

absorption by induced abundant order/disorder junctions at the surface from selective disorder engineering, which means that it has well charge separation efficiency through type-II bandgap alignment and can effectively promote strong hydrogen evolution surface reaction

photocatalysts were used, they exhibited high stability and a high hydrogen evolution rate of 13.89 mmol h−1 g−1 using 0.5 wt% Pt (cocatalyst) and 3.46 mmol h−1 g−1 without using any

steps, in which the titanium dioxide powder was prepared via the sol-gelation approach

obtained by further annealing at 350°C in Ar [33]. Under visible light illumination, the degradation rate of RhB reached 97.01% by b-TR and the photocatalytic hydrogen evolution rate was as high as 149.2 μmol h−1 g−1 under AM 1.5 irradiation [33]. The mechanistic

have been reported, but all of them

nanoparticles using simple room

[9]. Firstly, 14 mg metallic

nanoparticles as

phases is almost impossible. For the first time,

Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis

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235

nanoparticles were obtained by using a strong

nanoparticles were thoroughly rinsed by deionized water

showed drastically enhanced visible and near-infrared light

(B) single-crystalline nanorods (b-TR) were synthesized *via* three

(B) single-crystalline nanorods were

in the catalytic hydrogenation of N-methylpyrrole [32].

Until now, numerous methods to prepare blue TiO<sup>2</sup>

phase-selective "disorder engineered" Degussa P25 TiO2

[9]. The blue-colored TiO<sup>2</sup>

white rutile phase of P25, while well maintaining white anatase TiO2

several times and dried at room temperature in a vacuum oven [9].

cocatalyst under simulated solar light (**Figure 9**) [9].

followed by hydrothermal treatment. Blue TiO<sup>2</sup>

[9]. Therefore, when the phase-selective disorder engineering of P25 TiO2

*5.6.1. Sol-gelation hydrothermal technique and subsequent reduction treatment method*

reduction between the anatase and rutile TiO2

ture. Finally, the blue-colored TiO<sup>2</sup>

In their study, the blue TiO<sup>2</sup>

**5.6. Other methods**

Ti3+ self-doped blue TiO<sup>2</sup>

**5.5. Phase-selective room-temperature solution processing**

#### **5.4. Metal reduction method**

Zheng et al. proposed an approach to synthesize blue TiO<sup>2</sup> nanoparticles with abundant oxygen deficiencies/Ti3+ species through Al reduction of TiO<sup>2</sup> nanosheets at 500°C [31]. Zhang et al. developed a reduction method to synthesize a series of TiO2−x samples with their color changing from white to dark blue, which possess a much higher surface area and visible light absorption compared to pristine TiO2 (**Figure 8**) [32]. In a typical reduction process, crystalline

**Figure 8.** The route for the preparation of Ru/TiO2−x; photographs of P25 nanocrystals and TiO2x. (a) P25 nanocrystals, (b) TiO-1-80-0.5, (c) TiO-1-80-1, (d) TiO-1-120-4, (e) TiO-1-150-4, (f) TiO-1-180-4, (g) TiO-2-180-4, (h) TiO-3-180-4, and (i) TiO-4-180-4. Reprinted with permission from [32]. Copyright (2017) The Royal Society of Chemistry.

TiO2 was milled with Na/NaCl fine powders with different weight ratio at a series of milling rates such as 80, 120, 150, and 180 rpm at room temperature under argon atmosphere for 0.25–4 h. After the Na and NaCl was removed, the obtained TiO2−x products were dispersed in a small amount of deionized water and then vacuum-dried at room temperature to obtain TiO2−x powders [32]. Moreover, the obtained TiO2−x with a high surface area can be employed as an effective support for Ru particles and the Ru/TiO2−x catalyst exhibited superior activity in the catalytic hydrogenation of N-methylpyrrole [32].
