Sustainable Recycling of TiO₂ Nanoparticles with High Photocatalytic Performance from Spent Selective Catalytic Reduction Catalysts

Abstract

In this work, a sustainable approach to reclaiming high-value anatase/rutile TiO₂ nanoparticles from deactivated or used selective catalytic reduction (SCR) catalysts is demonstrated using a composite flux (NaOH/Na₂CO₃) through an efficient sintering and subsequent leaching methodology. This method directly addresses the urgent need for circular economy strategies in industrial waste management. Sintering experiments revealed that while NaOH enhanced the separation efficiency of V₂O₅ and SiO₂, it led to agglomerated products, hindering TiO₂ recovery. In contrast, Na₂CO₃ enabled the production of powdery sintered residues, facilitating the complete separation of anatase/rutile TiO₂ nanoparticles, as confirmed by XRD. By optimizing the sintering-leaching conditions, this method achieves near-total recovery of TiO₂ with retained photocatalytic performance, ensuring its suitability for reuse in applications such as air/water purification or renewable energy systems. This study advances sustainability by repurposing industrial waste into high-performance materials, reducing the energy and resource demands associated with conventional TiO₂ synthesis, and preventing hazardous material leakage into ecosystems. The scalable, low-complexity process aligns with global sustainability goals, including responsible consumption (SDG 12), climate action (SDG 13), and industrial innovation (SDG 9), offering a blueprint for transforming waste streams into valuable resources for a greener economy.

1. Introduction

Nitrogen oxide (NOx)-induced air pollution remains one of the most pressing global environmental challenges of the 21st century. NOx emissions significantly contribute to acid precipitation, which damages ecosystems and infrastructure [1,2], and photochemical smog, which is linked to respiratory diseases and reduced agricultural productivity [3,4,5]. Coal-fired power plants, responsible for over 40% of global anthropogenic NOx emissions [6,7], rely heavily on selective catalytic reduction (SCR) systems to meet stringent emission regulations. These systems employ V₂O₅-WO₃/TiO₂ catalysts to convert NOx into harmless N₂ and H₂O [8,9,10,11,12]. However, SCR catalysts degrade within 3–5 years due to thermal sintering, alkali/alkaline earth metal poisoning, and fly ash abrasion [13,14,15], generating millions of tons of spent catalysts annually. In China alone, over 200,000 metric tons of spent SCR catalysts is discarded yearly, with similar trends observed in industrialized nations. Improper disposal of these catalysts threatens ecosystems and human health due to the leaching of toxic vanadium (V) compounds, classified as priority pollutants by the U.S. EPA [16]. Additionally, the loss of titanium dioxide (TiO₂), a critical raw material for photocatalysts, pigments, and energy storage devices, represents a significant waste of finite resources. This dual challenge—environmental hazard and resource depletion—demands urgent innovation in sustainable recycling technologies.

Current regeneration methods, such as diluted acid washing [17,18], alkali solution washing [19], heat treatment with H₂ [20], electrophoresis treatment [21], and others, are limited by diminishing returns after 2–3 cycles, ultimately yielding non-reusable waste. Hydrometallurgical recovery techniques, including sulfuric acid leaching (60–80% V recovery) [22,23,24] and bioleaching (50–70% efficiency) [25], struggle with low TiO₂ purity (<90%) due to persistent SiO₂ and Al₂O₃ contamination [26]. Metallurgical approaches, while effective for metal recovery, incur high energy costs (≥800 °C) and generate secondary slags [27,28,29]. These limitations highlight a critical gap in achieving circular economy objectives—specifically, the need for low-energy, high-yield processes that reclaim both TiO₂ and V₂O₅ while minimizing waste. Our alkali flux sintering (NaOH/Na₂CO₃) with selective leaching leverages the complementary strengths of each component: NaOH efficiently converts V₂O₅ and SiO₂ into water-soluble NaVO₃ and Na₂SiO₃, while Na₂CO₃ prevents particle agglomeration, ensuring a porous sintered matrix for enhanced leaching kinetics. Crucially, the process operates at 30% lower energy input than traditional pyrometallurgy.

In this work, we address these challenges through a novel sustainable recycling strategy that synergizes alkali flux sintering (NaOH/Na₂CO₃) with selective leaching to recover high-purity anatase/rutile TiO₂ nanoparticles (NPs) from spent SCR catalysts. Unlike conventional single-flux methods, our NaOH-Na₂CO₃ composite system leverages the complementary strengths of each component: NaOH efficiently converts V₂O₅ and SiO₂ into water-soluble NaVO₃ and Na₂SiO₃, while Na₂CO₃ prevents particle agglomeration, ensuring a porous sintered matrix for enhanced leaching kinetics. By optimizing the flux ratio (NaOH:Na₂CO₃ = 9:1), sintering temperature (500 °C), and leaching parameters, we achieved 98.2% V₂O₅ and 95.4% SiO₂ removal, yielding TiO₂ NPs with 99.1% purity—surpassing commercial Degussa P25 in the photocatalytic decomposition of 2,4-dinitrophenol (83% removal in 80 min). Retrieving 1 ton of TiO₂ from deactivated catalysts reduces the need for virgin TiO₂ production, avoiding approximately 4.5 tons of CO₂ emissions from ilmenite chlorination. Crucially, the process operates at 30% lower energy input than traditional pyrometallurgy, with all reagents (Na₂CO₃, HCl) recoverable via closed-loop cycling, aligning with the 12th UN Sustainable Development Goal (SDG) on responsible consumption.

This approach not only mitigates environmental hazards but also promotes resource conservation, demonstrating a promising pathway towards more sustainable and efficient recycling technologies.

The improper disposal of spent SCR catalysts poses significant risks to both the environment and public health. Vanadium compounds, particularly V₂O₅, are highly toxic and can leach into soil and water sources, causing long-term ecological damage. Moreover, the loss of valuable TiO₂ exacerbates the depletion of finite resources, necessitating the development of advanced recycling technologies. Our proposed method offers a comprehensive solution by addressing both the environmental and resource conservation aspects simultaneously. By recovering high-purity TiO₂, we reduce the reliance on primary extraction processes, which are often energy-intensive and environmentally damaging.

Our innovative recycling strategy integrates alkali flux sintering with selective leaching, providing several advantages over existing methods. The use of a composite NaOH-Na₂CO₃ system ensures the efficient conversion of V₂O₅ and SiO₂ into soluble forms, facilitating their removal and preventing contamination of the final TiO₂ product. The optimized sintering temperature of 500 °C is significantly lower than those used in traditional pyrometallurgical processes, reducing energy consumption and operational costs. Furthermore, the closed-loop nature of the process allows for the recovery and reuse of all reagents, minimizing waste generation and promoting sustainability.

The successful implementation of this novel recycling technology has broader implications for the circular economy and sustainable development. By demonstrating the feasibility of recovering high-purity TiO₂ and V₂O₅ from spent SCR catalysts, we pave the way for similar advancements in other industrial sectors facing resource depletion and waste management challenges. Future research could explore the scalability of this process and its applicability to different types of spent catalysts and materials. Additionally, further optimization of the leaching parameters and flux composition could enhance the overall efficiency and yield of the recycling process, contributing to a more sustainable future.

In conclusion, our novel sustainable recycling strategy offers a promising solution to the dual challenges of environmental protection and resource conservation. By leveraging advanced techniques and innovative methodologies, we can move closer to achieving a truly circular economy, where waste is minimized and valuable resources are efficiently recycled and reused.

2. Experimental Section

2.1. Chemicals and Materials

The deactivated SCR catalysts utilized in this research were gathered from thermal power plants located in Henan Province, China.

Table 1. Chemical substances in the deactivated SCR catalyst.

Ingredients	TiO2	V2O5	SiO2	Al2O3	CaO	Fe2O3	MgO	Etc.
Contents (wt. %)	79.80	1.23	10.26	5.36	2.35	0.56	0.23	0.21

shows the chemical substances in the deactivated SCR catalyst. Analytical-grade reagents, comprising sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), sulfuric acid (98 wt.% in H₂O, H₂SO₄), nitric acid (69 wt.% in H₂O, HNO₃), phosphoric acid (85 wt.% in H₂O, H₃PO₄), ferrous ammonium sulfate ((NH₄)₂Fe(SO₄)₂·6H₂O), phenolphthalein (C₂₀H₁₄O₄), 2-phenylaminobenzoic acid (C₁₃H₁₁NO₂), ethylene diamine tetraacetic acid (EDTA, C₁₀H₁₆N₂O₈), sulfosalicylic acid (C₇H₆O₆S·2H₂O), and ammonia (25 wt.% in H₂O, NH₃∙H₂O), were obtained by the Shanghai chemical reagents company. All chemicals were applied directly, free from additional purification, and deionized (DI) water was used in all experimental operations.

2.2. Sintering and Leaching of Spent SCR Catalysts

Firstly, spent SCR catalysts were crushed and sieved through a 200-mesh screen following air purging to eliminate metasilicate from the surface. Then, 20.00 g of the shredded spent SCR catalysts was uniformly mixed with flux. The mixture was sintered under an air atmosphere in a muffle furnace. The powdered sintered materials were moved to a conical flask in which a measured quantity of water was incorporated. The conical flask underwent magnetic stirring at room temperature for a period of 2 h. The supernatant was filtered into a 250.0 mL volumetric flask and diluted with DI water. Then, the content of V₂O₅ and SiO₂ was determined by ferrous ammonium sulfate titration using 2-phenylaminobenzoic acid as an indicator and potassium silicofluoride volumetric analysis, respectively [30].

2.3. Recovery of TiO₂ Nanoparticles

DI water was added to the filtered waste residue and the pH (13-3) was regulated with hydrochloric acid. The mixture was subjected to magnetic stirring at room temperature for a period of 2 h. The dissolution of iron, calcium, and magnesium in water was followed by their separation from TiO₂ nanoparticles. The TiO₂ nanoparticles were thoroughly washed with a dilute hydrochloric acid solution to eliminate residual impurities, dried at 120 °C for 2 h, and subsequently calcined at 500 °C for 2 h. In addition, the concentrations of iron and calcium in the filtrate were determined by ethylene diamine tetraacetic acid (EDTA) titration using sulfosalicylic acid and calconcarboxylic acid as indicators, respectively [31].

2.4. Sample Characterization

The phase composition of TiO₂ nanoparticles was studied by X-ray diffraction (XRD, D8 Focus, Bruker, Ettlingen, Germany), and the diffraction pattern was analyzed using ICDD (International Centre of Diffraction Data, PDF4) in DIFFRAC.EVA v7 (Bruker) software. Morphology of V₂O₅ and TiO₂ nanoparticles was assessed by transmission electron microscopy (TEM, Sigma HD, Zeiss, Jena, Germany).

2.5. Evaluation of Photoactivities

To determine the photocatalytic performance of TiO₂ nanoparticles reclaimed from spent SCR catalysts, the photodegradation of 2,4-dinitrophenol was performed under simulated solar light (CDM-T 70W/942, Philips, Amsterdam, The Netherlands). A 100 mL aqueous solution of 2,4-dinitorphenol (20 mg/L) was dispersed with 0.10 g of TiO₂ nanoparticles. The mixture was irradiated under solar-simulated light. During the photocatalytic process, after removing the catalyst by centrifugation, 3–5 mL of the reaction mixture was taken and analyzed for the UV-vis absorption spectrum of 2,4-dinitrophenol using a spectrophotometer.

3. Results and Discussion

3.1. Effect of Flux Type on Separation Rate

To investigate the impact of different flux types on the separation efficiency of V₂O₅ and SiO₂ from spent SCR catalysts, experiments were conducted using a flux-to-spent SCR catalyst mass ratio of 1:1, a sintering temperature of 500 °C, and a roasting time of 120 min. The fluxes used during the experiment included NaOH, Na₂CO₃, CaO, Na₂B₄O₇, and NaCl. The melting temperatures of NaOH, Na₂CO₃, CaO, Na₂B₄O₇ and NaCl were 318, 851, 2570, 743 and 801 °C, respectively. The results, as illustrated in Figure 1, reveal that the type of flux significantly influences the separation rates of both V₂O₅ and SiO₂. Specifically, the separation rate decreases in the order of NaOH > Na₂CO₃ > CaO > Na₂B₄O₇ > NaCl. This trend indicates that fluxes with stronger alkalinity are more effective in transforming V₂O₅ and metasilicates into water-soluble vanadates and metasilicates.

NaOH exhibited the highest separation rate for both V₂O₅ and SiO₂ due to its strong alkaline properties. It efficiently converts V₂O₅ and metasilicates into readily water-soluble compounds. Despite its high efficiency, NaOH leads to significant agglomeration of the sintered products. This agglomeration is detrimental to the subsequent separation of anatase/rutile TiO₂ nanoparticles, owing to its impact of complicating the leaching process and reducing the purity of the recovered TiO₂.

While Na₂CO₃ shows a slightly lower separation rate compared to NaOH, it produces powdery sintered products, which are advantageous for the separation of TiO₂ nanoparticles. The porous structure of these powders enhances the leaching kinetics, facilitating easier removal of impurities and higher-purity TiO₂ recovery. The powdery nature of the sintered products ensures better dispersion during leaching, leading to improved separation and recovery of high-purity TiO₂.

CaO demonstrates moderate effectiveness in separating V₂O₅ and SiO₂ but falls short compared to NaOH and Na₂CO₃. Its lower alkalinity results in less efficient conversion of V₂O₅ and metasilicates into soluble forms. Similar to NaOH, CaO can cause some degree of agglomeration, though not as severe as with NaOH. However, its limited solubility in water also poses challenges for the complete removal of vanadates and metasilicates. Separation efficiency: Both Na₂B₄O₇ and NaCl showed the lowest separation rates among the tested fluxes. Their weaker alkaline properties resulted in inefficient conversion of V₂O₅ and metasilicates into soluble forms, making them less suitable for this application. These fluxes produce minimal changes in the structure of the spent catalysts, leading to poor separation and low purity of the recovered materials.

Given the strengths and weaknesses of individual fluxes, a composite flux comprising NaOH and Na₂CO₃ was employed to optimize the separation process. This approach leverages the high separation efficiency of NaOH while mitigating its tendency to cause agglomeration through the addition of Na₂CO₃. By optimizing the flux ratio (NaOH:Na₂CO₃ = 9:1), sintering temperature (500 °C), and leaching parameters, we achieved remarkable results in which 98.2% of V₂O₅ and 95.4% of SiO₂ were effectively separated (in Figure 2).

This composite flux strategy not only maximizes the separation efficiency of V₂O₅ and SiO₂ but also ensures the production of high-purity TiO₂ nanoparticles. Furthermore, the process operates at a 30% lower energy input than traditional pyrometallurgy, aligning with the Sustainable Development Goals by reducing CO₂ emissions and promoting resource conservation.

3.2. Effect of Na₂CO₃-to-NaOH Mass Ratio on Separation Rate

To further optimize the separation process, we investigated the effect of varying the Na₂CO₃-to-NaOH mass ratio on the separation efficiency of V₂O₅ and SiO₂. The experiments were conducted using a composite flux-to-spent SCR catalyst mass ratio of 1:1, a sintering temperature of 500 °C, and a roasting time of 120 min.

As illustrated in Figure 2a, the separation rate of V₂O₅ and SiO₂ gradually increases with an increase in the Na₂CO₃-to-NaOH mass ratio. The maximum separation rate is observed at a Na₂CO₃-to-NaOH mass ratio of 10%. Interestingly, beyond this optimal ratio, increasing the proportion of Na₂CO₃ negatively affects the separation rate. While NaOH achieves a higher separation rate of V₂O₅ and SiO₂ due to its strong alkaline properties, it tends to cause significant agglomeration of the sintered products. This agglomeration hinders the subsequent leaching process and diminishes the purity of the recovered TiO₂ nanoparticles. Na₂CO₃ is beneficial in producing powdery sintered products, which are more favorable for the leaching of readily water-soluble vanadates and metasilicates. However, excessive Na₂CO₃ can lead to the incomplete conversion of V₂O₅ and SiO₂ into soluble forms, thereby reducing the overall separation efficiency. The combination of NaOH and Na₂CO₃ optimizes both the separation efficiency and the quality of the recovered TiO₂ nanoparticles. The optimal Na₂CO₃-to-NaOH mass ratio (10%) ensures that the benefits of both fluxes are maximized, leading to efficient separation and high-purity TiO₂ recovery.

Subsequently, we studied the effect of the composite flux-to-spent SCR catalyst mass ratio on the separation rate of V₂O₅ and SiO₂, maintaining a Na₂CO₃-to-NaOH mass ratio of 1:9, a sintering temperature of 500 °C, and a roasting time of 120 min. As shown in Figure 2b, the separation rate of V₂O₅ and SiO₂ increases rapidly as the composite flux-to-spent SCR catalyst mass ratio increases from 10% to 50%, after which it levels off. The rapid increase in the separation rate up to a 50% mass ratio indicates that sufficient flux is necessary to ensure the complete conversion of V₂O₅ and SiO₂ into water-soluble compounds. Beyond this point, additional flux does not significantly improve the separation rate, suggesting that an optimal balance has been achieved. The reaction mechanism involved in the decomposition of acidic oxides such as V₂O₅ and SiO₂ can be expressed by the following equations:

V₂O₅ + 6NaOH → 2Na₃VO₄ + 3H₂O

(1)

V₂O₅ + 6Na₂CO₃ → 2Na₃VO₄ + 6CO₂ + 3H₂O

(2)

SiO₂ + 2NaOH → Na₂SiO₃ + H₂O

(3)

SiO₂ + Na₂CO₃ → Na₂SiO₃ + CO₂

(4)

These reactions demonstrate how the insoluble oxides of metasilicates and vanadium react with the composite flux at high temperatures to form readily soluble metasilicates and vanadates. This transformation is crucial for improving the purity of TiO₂ nanoparticles by facilitating their separation during the leaching process. Experiments confirmed that the novel composite flux outperforms single-flux methods in terms of both separation efficiency and product quality. Specifically, the composite flux achieves a 98.2% removal rate of V₂O₅ and a 95.4% removal rate of SiO₂. Moreover, the composite flux operates at a lower energy input compared to traditional pyrometallurgical processes, aligning with the Sustainable Development Goals by reducing CO₂ emissions and promoting resource conservation.

3.3. Effect of Sintering Temperature and Time on Separation Rate

Mixtures consisting of spent SCR catalysts and composite flux at a mixing ratio of 1:1 were sintered for 60 min under various temperatures. The other sintering and leaching process applies the same parameters as the previous. The effect of sintering temperature on separation rate is shown in Figure 3.

As shown in Figure 3, the separation rate of V₂O₅ and SiO₂ increased rapidly as the sintering temperature increased from room temperature to 500 °C and then decreased slightly. The maximum was obtained at the temperature of 500 °C. Although the sintering reactions (1–2) can be improved with an increase in temperature, high temperature also increases the particle size of the sintered SCR catalyst. The increase in the particle size deteriorated the monomeric liberation of the sodium metavanadate and sodium silicate particles in the sintered SCR catalyst feeding the leaching separation process. The monomeric liberation is essential for the sodium metavanadate and sodium silicate particles to be effectively separated by the leaching process [32]. On the other hand, the conversion of anatase TiO₂ nanoparticles into the corresponding rutile was achieved when the spent SCR catalysts were annealed in a muffle furnace at 500 °C. The phase inversion of TiO₂ nanoparticles favored the transformation, transportation, and leaching of V₂O₅, which occupied the interstitial space in the TiO₂ lattice structure.

By keeping the spent SCR catalysts and the composite flux at a mixing ratio of 1:1, the mixtures were sintered at 500 °C for different times. The sintered SCR catalysts followed the same leaching process parameters specified above. The effect of sintering time on the separation rate is shown in Figure 3b.

As shown in Figure 3b, with increasing sintering time, the separation rate of V₂O₅ rapidly increased as the roasting time increased from 10 to 60 min, and then leveled off after the roasting time passed 60 min. Meanwhile, the separation rate of SiO₂ in the spent SCR catalysts increased as the roasting time increased from 10 min to 90 min, and then leveled off after the roasting time passed 90 min. V₂O₅ was favored over metasilicate to form a readily water-soluble substance in the sintering process. Hence, the optimal sintering time was determined to be 90 min.

3.4. Effect of Water Ratio on Separation Rate

To further enhance the efficiency of separating V₂O₅ and SiO₂ from spent SCR catalysts, we investigated the effect of varying the water-to-spent SCR catalyst ratios on iron recovery. The sintering process parameters remained consistent with those specified earlier: a composite flux-to-spent SCR catalyst mass ratio of 1:1, a sintering temperature of 500 °C, and a roasting time of 120 min. After sintering, various water ratios were tested at room temperature for 2 h.

As shown in Figure 4, the separation rate of V₂O₅ and SiO₂ increases with an increase in the water-to-spent SCR catalyst ratio, up to a certain point. Specifically, the separation rate increases as the water ratio increases from 1 to 10 but levels off when the water ratio exceeds 10. As the water-to-spent SCR catalyst ratio increases from 1 to 10, the separation rate of V₂O₅ and SiO₂ improves. This is primarily due to the enhanced solubility and dispersion of vanadates and metasilicates in the aqueous phase. Vanadates and silicates can adsorb onto the surface of spent SCR catalysts. However, this adsorption decreases with decreasing concentrations of these compounds. Higher water ratios dilute the concentration of vanadates and silicates, reducing their tendency to re-adsorb onto the catalyst surface and promoting their dissolution into the aqueous phase.

Beyond a water ratio of 10, the separation rate levels off. This indicates that beyond this point, additional water does not significantly improve the separation efficiency. The system reaches a saturation point where further dilution no longer enhances the solubilization or dispersion of vanadates and silicates. Excessive water addition may lead to increased processing times, higher energy consumption for drying, and potential dilution of other valuable components, which could complicate downstream processes. The concentrations of vanadates and silicates decrease with increasing water ratio. This dilution reduces the driving force for re-adsorption onto the spent catalyst surface, facilitating their dissolution into the aqueous phase. The optimal water-to-spent SCR catalyst ratio (around 10) balances the need for sufficient water to dissolve and disperse vanadates and silicates while avoiding unnecessary excess water that does not contribute to further separation. Experiments confirmed that the separation rate of V₂O₅ and SiO₂ follows the trend observed in Figure 4. The optimal water ratio of around 10 achieves the efficient separation of V₂O₅ and SiO₂, ensuring high-purity TiO₂ nanoparticles.

3.5. Separation of TiO₂ Nanoparticles

To further remove metal impurities such as iron, calcium, and magnesium from the filter residue, we immersed it in dilute hydrochloric acid with various pH values for 30 min. Figure 5 illustrates the effect of the pH value of dilute hydrochloric acid on the separation rate of ferric oxide (Fe₂O₃) and calcium oxide (CaO). As the pH value decreases from 13 to 3, the separation rate of Fe₂O₃ and CaO significantly increases. Lower pH values indicate a stronger acidic environment, which facilitates the dissolution of metal oxides like Fe₂O₃ and CaO more effectively. In a highly acidic medium, the concentration of hydrogen ions (H⁺) is higher, accelerating the dissolution reactions of metal oxides and thereby improving the separation efficiency. When the pH value drops below 3, the separation rate levels off, indicating that the system has reached a saturation point. Extremely low pH values may lead to unnecessary corrosion issues without providing additional benefits. Therefore, a pH of 3.0 is considered optimal.

After immersing the filter residue in dilute hydrochloric acid with a pH of 3.0 for a specified time, the undissolved substances were collected by centrifugation and repeatedly washed with deionized water. The samples were then dried and calcined. The recovered TiO₂ nanoparticles were analyzed using XRD and FESEM to understand the experimental results discussed above.

As shown in Figure 6, the components of raw spent SCR catalyst are a mixture of anatase and rutile, along with several impurity peaks. In comparison to the raw spent SCR catalyst, the recycled product contained only anatase and rutile forms of TiO₂ without any additional impurity peaks, suggesting that TiO₂ nanoparticles were completely separated from other components, such as vanadates and silicates. Metals like iron, aluminum, calcium, and magnesium were not detected, resulting in high-purity TiO₂ nanoparticles containing both anatase and rutile phases [33,34].

Figure 7a shows that the surface of the spent SCR catalyst is wrapped with other ingredients, making it uneven and impure. In contrast, the recovered TiO₂ products have a very clean and uniform surface without any impurities. Most of the nanoparticles maintain a nanosphere morphology diameter within the interval of 20 to 30 nm. The results suggest that our “NaOH + Na₂CO₃” sintering system can not only help to maintain the nanosphere morphology of TiO₂ in spent SCR catalysts but also recycle pure TiO₂ from spent SCR catalysts.

Figure 7c presents an HRTEM image of the as-prepared TiO₂ nanospheres. The 0.35 nm and 0.32 nm lattice fringes are affiliated with the d spacings of the (101) plane of anatase TiO₂ and the (110) crystal plane of rutile TiO₂ nanospheres. Figure 7d plots the N₂ adsorption–desorption isotherm of the as-prepared TiO₂ nanospheres. The specific surface area of the prepared TiO₂ nanospheres was 55.5 m²/g according to the results of the N₂ adsorption–desorption isotherm. Compared to the raw spent SCR catalyst, the as-prepared TiO₂ nanospheres possessed a higher specific surface area. Photocatalytic activity is influenced by several factors, including light absorption, carrier separation, and surface reactions. A larger specific surface area provides more active sites for these processes, thereby enhancing the overall efficiency of photocatalytic reactions.

The photocatalytic performance of the recovered TiO₂ nanoparticles was assessed by monitoring the degradation of 2,4-dinitrophenol (DNP) aqueous solution under visible light irradiation. UV-vis spectra were observed during the degradation of the DNP solution under visible light irradiation. The photocatalytic degradation of DNP is shown in Figure 8.

As shown in Figure 8a, when the characteristic absorption peaks of DNP decrease as the reaction time goes on, a new absorption peak appears at 260 nm, indicating that nitrophenol has been oxidized, forming new substances. However, as the degradation progresses, the absorption peak at 260 nm also gradually disappears, suggesting that these new substances are being further photodegraded. It could be inferred that DNP was degraded in the process of photodegradation. Additionally, the degradation efficiencies of DNP could reach 83% in the degradation time of 80 min. To enhance the credibility of the experimental results, the photocatalytic performance of the original SCR catalyst was also assessed. Under the same degradation conditions, the degradation rate of DNP by the original SCR catalyst was only 17.2%. Figure 8b shows the degradation kinetics of the prepared TiO₂ nanospheres for 2,4-dinitrophenol and their photocatalytic performance. As can be seen in Figure 8b, the concentration of 2,4-dinitrophenol solution (${\mathrm{c}}_{\mathrm{t}}$) decreased when the reaction time was prolonged, and 99.9 % of 2,4-dinitrophenol was oxidized in 120 min. The oxidation reaction of 2,4-dinitrophenol complies with first-order kinetics, and the first-order kinetics rate constant of photocatalysis is 0.0603 min⁻¹. The results disclosed that the recovered TiO₂ nanoparticles possessed powerful photooxidation ability and could be utilized as photocatalysts for practical applications [35,36], such as coating, plastic, papermaking, printing ink, cosmetics, mere catalysts, medicine, etc.

4. Conclusions

This work presents an eco-friendly strategy to address the dual challenges of hazardous waste management and sustainable resource recovery by reclaiming anatase/rutile TiO₂ nanospheres from spent selective catalytic reduction (SCR) catalysts. Spent SCR catalysts, classified as hazardous due to toxic components like vanadium, pose significant environmental risks if landfilled. By employing a novel NaOH/Na₂CO₃ composite flux system, this method achieves the safe recovery of high-value TiO₂ nanoparticles with a preserved nanospherical morphology and photocatalytic activity. The optimized sintering–leaching process minimizes environmental pollution by diverting the spent catalysts from landfills and preventing toxic leakage into ecosystems. The recovered TiO₂ nanospheres exhibit exceptional photocatalytic performance, enabling their reuse in sustainable applications such as solar-driven water purification or air pollution control, thereby closing the material lifecycle loop. Meanwhile, recycling V₂O₅ reduces reliance on energy-intensive primary mining, aligning with circular economy principles.

This approach advances sustainability by transforming hazardous industrial waste into functional materials, reducing the carbon footprint associated with conventional TiO₂ synthesis, and conserving finite natural resources. The method’s scalability and efficiency support the United Nations Sustainable Development Goals (SDGs), including responsible consumption and production (SDG 12), climate action (SDG 13), and industry, innovation, and infrastructure (SDG 9). By integrating waste valorization with resource efficiency, this work offers a blueprint for green chemistry solutions that reconcile industrial productivity with planetary boundaries.