*3.3.4 CO2 adsorption*

One of the potential options for lowering CO2 emissions and utilizing CO2 as a building block to produce valuable goods is the photochemical conversion of CO2 into solar fuels by photocatalysts like TiO2. The first stage in CO2's photo-reduction is its adsorption on TiO2 [39, 111]. According to theoretical research, the physisorption and the most stable chemisorption of CO2 on the neutral charge of perfect anatase TiO2 (001) have adsorption energies of 9.03 and 24.66 kcal mol1, respectively, on the spinunpolarized TiO2 with V0. This suggests that CO2 is tightly bound by V0 on a TiO2 surface that is deficient. Additionally, it is shown that the CO2 activation barrier on TiO2 with V0 is lower than it is on TiO2 with flawless anatase (001) [112]. Furthermore, it is discovered that the energetically favored conversion of CO2 to CO occurs on the surface of flawed TiO2, with V0 acting as the photocatalyst. Surprisingly, Li et al. [83] have shown that CO2 spontaneously dissociates into CO on a Cu(I)/TiO2x surface created by thermal annealing under an inert atmosphere even when it is dark. According to **Figure 7**, the surface V0 that provides the electrical charge as well as the locations for the adsorption of oxygen atoms from CO2 is mostly responsible for the spontaneous dissociation of CO2 in the dark. In addition, compared to those in the dark, CO2 activation and dissociation may be markedly enhanced by photoirradiation.

In conclusion, the reactant molecule's dissociative adsorption would be facilitated by V0 on the TiO2 surface. In the photocatalytic processes, it seems that the dissociative adsorption of the reactant molecule on the TiO2 surface lowers its activation energy and influences the reaction mechanism at the molecular level. Additionally, compared to physisorbed species, chemically separated compounds have higher photocatalytic reactivity. It should be emphasized, nevertheless, that the surface of TiO2 goes through re-oxidation often in conjunction with the dissociated adsorption of the adsorbates. Therefore, before investigating its dissociation adsorption capabilities for photocatalytic application, we first need to stabilize the V0 in reduced TiO2. This may be accomplished by doping the Fe into the TiO2 nanoparticles, a procedure we covered in Section 2.5.

### **3.4 Reductive properties**

In addition to changing the properties of adsorbates, V0 on catalyst surfaces also plays a role in the reduction of a number of these adsorbates. As demonstrated by Lu et al., one can observe the reactivity of thermally created V0 sites for the reduction of

### **Figure 7.**

*Proposed mechanism for CO2 dissociation on the surface of Cu/TiO2 under ambient temperature in dark. Copied with permission from Ref. [83] Copyright American Chemical Society.*

NO, CH2O, or D2O by adsorbing a test molecule on the defective surface as well as a fully stoichiometric surface and comparing the results of temperature programming desorption (TPD) [113]. By measuring the TPD, the reductive products (N2O, C2H4, and D2) are identified after adsorbing these adsorbates on the flawed surface. The oxidation of surface defect sites occurs concurrently with the deoxygenation of adsorbates. Therefore, the coverage of surface V0 is inversely correlated with the yield of reduction products. On the surface with no defects, there are no deoxygenation processes seen. It has also been shown that surface V0 sites are active in the reduction of metal ions. Our most recent study has shown that V0 plays a crucial role in the charge transfer from the damaged surface to gold ions. As a result, a very quick, direct development of metallic gold nanoparticles was accomplished on the surface of the semiconductor TiO2 containing V0. Ye and colleagues also saw the metal ions spontaneously reducing on damaged surfaces. They have shown that on the flawed surface of WO2.72, the ions of noble metals are engaged in redox processes where the metal ions partly oxidize the reduced V0 states [114]. As a result, the metal ions are quickly reduced and nucleated on their surface, where they develop into clusters and then nanoparticles. The controlled synthesis of metal/semiconductor hybrid nanomaterials may be accomplished using this approach in a single step without the need for external reducing agents, stabilizing molecules, or pretreatment of the precursors. It is interesting to note that Li et al. [115] found that the sub-stoichiometric WO3-x, which is produced by utilizing hydrogen treatment to create V0 in WO3, is stable thermodynamically at room temperature and has a strong resistance to re-oxidation. So, in the absence of catalysts for oxygen evolution reaction, hydrogen-treated WO3 is stabilized to be used for water oxidation in a neutral media. These ground-breaking findings imply that by carefully selecting the preparation techniques, the property of V0 states may be precisely regulated.
