Solar-to-Chemical Conversion

(2.3)

(2.4)

(2.5)

(2.6)

(2.7)

Figure 2.6A schematic illustration of the energy correlation between semiconductor catalysts and redox couples in water. CB and VB denote a conduction band and a valence band, respectively.

Source: Tu et al. [20].

To improve the selectivity of expected product, Anpo and coworkers reported a series of studies on photocatalytic reduction of CO 2with H 2O for producing methanol and methane with a high selectivity by designing various nanostructured photocatalysts, such as Cu/TiO 2[21], TiO 2/Y‐zeolites [22], Ti/FSM‐16 [23], and TiO 2/glass [24] in Figure 2.7. The mechanism investigation results inferred that the local structure of TiO 2tested by X‐ray absorption near‐edge structure (XANES) on the support has significant influences in product selectivity. In this section, different products and reaction mechanism are respectively discussed in detail.

Figure 2.7(a) Yields of the products produced during the photocatalytic reduction of CO 2with H 2O and photoluminescence of various Ti/FSM‐16 photocatalysts. (b) Product distribution of CO 2photoreduction over 1‐TiO 2, 2–10 wt% imp‐TiO 2/Y‐zeolite, 3–1.0 wt% imp‐TiO 2/Y‐zeolite, 4‐ex‐TiO 2/Y‐zeolite, and 5‐Pt‐loaded ex‐TiO 2/Y‐zeolite. (c) Schematic mechanisms of the photocatalytic CO 2reduction with H 2O on TiO 2.

Source: (a) Ikeuea et al. [23]; (b) Anpo et al. [22]; (c) From Anpo et al. [21]. © 1995 Elsevier.

As a high value‐added fuel, CH 4, a natural gas, is widely used in residential living and industrial activities. Reduction of CO 2to methane named by the methanation process is of substantial importance in industrial field as the following equation: CO 2+ 2H 2→ CH 4+ 2H 2O. In general, this reaction is proceeded at high temperatures and pressures in the presence of metal catalysts, for example, Ru, Mo, and Ni. However, inspired by natural photosynthesis, chemists expect to produce CH 4fuel under mild and environment‐friendly conditions. In 1986, Willner and coworker reported an exciting result, in which gaseous CH 4was successfully synthesized with a 0.0025% of quantum yield by using a Ru(bpz) 3 2+as sensitizer and Ru metal colloid as catalyst under visible‐light irradiation [25]. In Figure 2.8, the reaction mechanism displayed that Ru*(bpz) 3 2+produced from the light‐excited Ru(bpz) 3 2+can be reduced by triethanolamine (TEOA) as electron donor into Ru(bpz) 3 +that can pair with Ru(bpz) 3 2+with a redox potential of −0.86 V vs. Saturated Calomel Electrode (SCE). On the other hand, it is demonstrated that the redox potential of CH 4/CO 2is equal to −0.24 V vs. NHE, which is less negative than that of Ru(bpz) 3 +/Ru(bpz) 3 2+. As a result, CO 2molecular can be thermodynamically reduced into CH 4molecular by Ru(bpz) 3 +over Re metal catalyst in acidic media according to the Eq. (2.7).

Figure 2.8Schematic cycle for the photosensitized reduction of CO 2to CH 4.

Source: Maidan and Willner [25].

However, the chemical yield of CH 4from this expensive organic/inorganic photocatalytic system is low and unsustainable. In 1995, Kamber and coworkers took advantage of popular photocatalyst TiO 2to reduce gaseous CO 2into CH 4under ultraviolet (UV) illumination at moderate temperature while the chemical yield and selectivity is not satisfying due to the nonspecific reaction over semiconductor photocatalysts [26]. The chemical yield of CO and H 2is 27 and 14 times higher than that of CH 4, which indicated CH 4seemed the by‐products and the formation reactions of CO and H 2are the dominant reduction reaction.

Considering the commercial implication in future, the selectivity to CH 4is a key challenge in transforming CO 2into CH 4. Recently, Tasbihi et al. reported Pt/TiO 2/mesoporous SiO 2photocatalyst for significant enhancement of selectivity to CH 4[27]. The main motivation of constructing this system is to elucidate that the introduced platinum (Pt) not only increases the activity of TiO 2catalysts for CO 2reduction but also modifies the selectivity toward highly reduced products. It is reported that compared with other noble cocatalysts, such as palladium, gold, silver, and rhodium, platinum has shown exceptional selectivity to methane in CO 2photoreduction, and, therefore, the Pt nanoparticles are to be expected to improve the selectivity of catalytic processes [28]. To further reduce the cost of photocatalysts for CH 4production, transition metal elements are investigated to substitute noble metal as well as maintain high selectivity. For example, Zhao and coworkers prepared a surface La‐modified TiO 2photocatalyst through a facile sol–gel route [29]. It was found that the La species was deposited on the surface of TiO 2in the form of La 2O 3. More importantly, small part of La atoms replaced surface Ti atom forming a Ti—O—La bond, resulting in the generation of oxygen vacancies (OVs) and Ti 3+, which synergistically contributed to the enhanced CO 2adsorption, water vapor activation, and charge separation. Finally, the selectivity to CH 4in La‐modified TiO 2reached to ∼80% compared with ∼20% selectivity of TiO 2. Meanwhile, Ye and coworkers created a rich of oxygen vacancies on the (001) facet of Bi 2MoO 6(BMO), which significantly improved the special CO 2adsorption in a bidentate carbonate mode, thereby facilitating hydrogenation of intermediate *CO to form methane in a high selectivity of ∼88% [30], as present in Figure 2.9. Meanwhile, a broader density functional theory (DFT) study was performed to further explore the possible CO 2conversion pathways and intermediate species initiated by these two different CO 2adsorption configurations in Figure 2.9d,e. For the defect‐free BMO, only CO was produced in a rate of 0.12 μmol g −1h −1with total consumed electron number (TCEN) of 0.24 μmol g −1h −1in Figure 2.9g, while for BMO‐OVs, the introduced OVs served as the natural active sites, leading to remarkable production of CH 4with rate of 2.01 μmol g −1h −1as the main product, CO of 0.27 μmol g −1h −1, and TCEN of 16.62 μmol g −1h −1, respectively.

Figure 2.9(a) Calculated band structures and total density of states of BMO‐OVs. Absorptive formats of CO 2on BMO‐OVs (b) and BMO (c). CO 2reduction pathways on BMO‐OVs (d) and BMO (e). (f) The diagram of band positions. (g) CH 4production and the TCEN value for CO 2photoreduction over the BMO‐OVs and BMO during four hours visible‐light irradiation. (h) Typical time course of CO/CH 4generated over BMO‐OVs.