Spent SCR catalysts have a high recovery value with components of over 80wt% consisting of V₂O₅, WO₃ and TiO₂ and should, therefore, be regenerated or recovered in order to recycle the resource [15,16,17]. Although spent SCR catalysts may contain oxides such as SiO₂, Al₂O₃ and CaO that constitute the ceramics, as well as compounds of elements including As and Pb deposited from the flue gas, the extraction of these materials is not reviewed in this article. Regeneration methods can extend the life of SCR catalysts by focusing on detoxification and reactivation. The major problems to be addressed during regeneration are the deactivation and loss of active sites as well as blocking of pores [4,18,19,20]. Specific regeneration methods for different types of deactivations can achieve promising results [21]. However, current regeneration processes are unable to achieve the expected results when the catalyst is severely deactivated or has undergone several regenerations. In this case, the SCR catalyst will reach the end of its life and be recycled by recovery [22]. Conventional recovery methods focused on the extraction and purification of metals (V, W and Ti) [23]. However, recent research has increasingly turned to the added value of the product with a view to increasing the economic benefits of recycling. For instance, after separation, NH₄VO₃, ammonium paratungstate and anatase TiO₂ are produced, respectively [24].

Under light conditions, photocatalysts produce photogenerated electrons and holes and, further, form active species such as hydroxyl (·OH) and oxygen (O₂⁻) radicals. Due to the strong redox properties of the active species, photocatalysts can address pollutions caused by heavy metals, organics and other substances [25]. Anatase TiO₂ is a classical photocatalyst with excellent photocatalytic activity. Studies have often used doping to build heterostructures to extend the wavelength range of light and to suppress the separation of photogenerated carriers [26,27]. On the other hand, to inhibit the agglomeration of nano TiO₂ and to assist recycling, the preference is to use carrier-loaded nano TiO₂ [28]. Spent SCR catalysts contain a large amount of anatase TiO₂ and are cost-effective; therefore, recovery as photocatalysts not only increases the recovery benefit, but also overcomes the high cost of conventional photocatalysts. In addition, V [29,30] and W [31,32] in spent SCR catalysts have the potential to build heterostructures with TiO₂.

Photocatalytic degradation is a promising technology to address environmental pollution with economic and environmental efficiency [64]. However, as most photocatalysts are nanoparticles, they are difficult to recycle and, therefore, present the potential risk of secondary pollution [65]. On the other hand, the tendency of nanoparticles to agglomerate can also limit their catalytic activity [66]. To solve these problems, photocatalysts can be loaded onto carriers, such as fly ash [67] and activated carbon [68]. The carriers not only provide dispersion and easier recycling, but specific carriers can also improve the catalytic activity of the catalyst by forming a specific heterostructure [69].

Spent SCR catalysts contain a variety of oxides with catalytic activity, which can be used as carriers to enhance catalytic activity. However, it is important to ensure that components such as V₂O₅ and As₂O₅ do not enter the environment during utilization and cause secondary contamination. Jin et al. [70] investigated spent SCR catalysts by grinding into powder and adding Al₂O₃, diatomite and agglomerant to calcine at 1000 °C to obtain ceramics. This was followed by impregnation and sintering in an 8wt% Ni(NO₃)₂ solution to obtain NiO loading. The prepared NiO-based catalysts were used for the reforming of formaldehyde and water vapour for hydrogen production with a selectivity of 100% for H₂, 31.9% for CO and 53.2% for CO₂ at 500 °C, and a conversion of formaldehyde above 93.0%. The analysis of the XPS data and the mechanism of the reforming reaction revealed that the presence of oxides on the surface had a positive effect on the performance of the catalysis. Based on the work of Jin et al., it can be demonstrated that spent SCR catalysts have potential as catalyst carriers. Particularly, TiO₂ shows typical photocatalytic activity and research on recycling spent SCR catalysts as photocatalyst carriers for this feature awaits further exploration.

V₂O₅, WO₃ and TiO₂ from spent SCR catalysts can be used for the preparation of photocatalysts. V₂O₅ and WO₃ can form crystals with cations such as Bi³⁺ in the form of VO₄³⁻ and WO₆⁶⁻, respectively [71], while TiO₂ exists as anatase TiO₂ mostly [72]. Zhang et al. [57] used a solution of H₂SO₄ and Na₂SO₃ to leach out V from spent SCR catalyst, followed by dissolving the Ti-containing residue in HF solution to obtain WO₃-TiO₂ photocatalyst after a hydrothermal reaction. In another case, V and W were leached out using a NaOH solution, followed by the addition of Bi(NO₃)₃ for a hydrothermal reaction to obtain BiVO₄/Bi₂WO₆ photocatalysts [71]. Wang et al. [73] also used NaOH to retain Ti in the leach residue for separation. Subsequently, Na₃PO₄ and Mg(NO₃)₂ were used to precipitate Ca²⁺, SiO₄⁴⁻, and PO₄³⁻, respectively. This was followed by the precipitation of Al³⁺ and Mg²⁺ using HNO₃ and NaOH to adjust the pH. The solution retains VO₃⁻ and WO₄²⁻, but additional NH₄VO₃ is required due to the low V. The addition of Zn(NO₃)₂ eventually leads to a visible light responsive Zn₃(VO₄)₂/ZnWO₄ photocatalyst by hydrothermal reaction.

Qian et al. [74] used NaOH to leach the V element from the catalyst, followed by a hydrothermal reaction using 96% H₂SO₄ to convert the TiO₂ to TiOSO₄ after roasting at 200 °C. V is removed after reaction with NaOH, while W remains in the residue. This method overcomes the difficulty of separating Ti and W and makes effective use of Ti. The prepared photocatalyst exhibits similar catalytic activity to P25 at a lower Ti content and demonstrates the feasibility of dissolving and converting Ti to nano TiO₂. Furthermore, loading TiO₂ onto the fly-ash surface also overcomes the shortcomings of nano TiO₂, which is difficult to recover and prone to agglomeration.

Current research has focused on the separation of TiO₂ from V₂O₅ and WO₃ with some progress being achieved. Figure 1 illustrates the transformation relationships of substances in these works. Since the goal is no longer to separate and purify V, W and Ti, the process can be simplified based on current separation processes. New extraction processes can also be developed, such as the dissolution of TiO₂ with sulphuric acid under hydrothermal conditions [75]. Mechanochemical methods also have potential applications in the preparation of TiO₂ [76]. Ball milling as a pre-treatment has proven to be effective in the recycling of spent SCR catalysts [59]. On the other hand, both BiVO₄ [77] and Bi₂WO₆ [78] can form corresponding heterostructures with TiO₂. Moreover, V and W can be extracted directly into solution as NH₄VO₃ and (NH₄)₂WO₄ [45], which would assist in the further synthesis of bismuth salt.