Preparation of Strontium Hydroxystannate by a Hydrothermal Method and Its Photocatalytic Performance

Abstract

To address the challenge of abatement of volatile organic compounds (VOCs) in environmental catalysis, this study developed a temperature-gradient hydrothermal strategy to fabricate SrSn(OH)₆ nanocatalysts and systematically investigatd their photocatalytic performance and mechanisms for gaseous toluene degradation. SrSn(OH)₆ (SSH) was synthesized via a simple hydrothermal method with optimal preparation conditions identified as a reaction temperature of 140 °C and duration of 12 h. The crystallinity of SrSn(OH)₆ was modulated by adjusting the pH of the precursor solution, yielding materials with distinct morphologies, specific surface areas, and band gaps. The narrowed band gap of SrSn(OH)₆ nanocatalysts facilitated electron excitation to generate additional photogenerated electron-hole pairs. The SSH-10.5 sample with ordered planar and hole-like structures promoted carrier migration, effectively suppressed electron-hole recombination, and enhanced the conversion of abundant surface hydroxyl groups into hydroxyl radicals. Under UV irradiation, SSH-10.5 achieved a toluene degradation efficiency of 69.56% and showed excellent stability after five reuse cycles. Electron spin resonance analysis confirmed the presence of •OH and •O₂⁻ radicals in the reaction system, with •OH identified as the dominant active species. In situ FT-IR spectroscopy revealed that •OH and •O₂⁻ radicals attacked the methyl group of toluene, converting it into intermediates including benzyl alcohol, benzaldehyde, and benzoic acid. This work provides a novel design of high-efficiency VOC-photocatalytic materials and shows significant implications for advancing industrial exhaust gas purification technologies.

1. Introduction

The abatement of volatile organic compounds (VOCs) remains a critical challenge in environmental catalysis [1]. Toluene (C₇H₈), a representative aromatic VOC, is widely present in emissions from coating, printing, and petrochemical industries. With an ozone formation potential (OFP) as high as 4.5, it exhibits both neurotoxicity and carcinogenicity [2]. As a crucial solvent and raw material, toluene is extensively utilized in manufacturing processes for paints, dyes, rubbers, and architectural decoration materials [3]. Toluene can enter the human body not only through the respiratory tract but also via dermal absorption. Furthermore, it not only irritates the skin and mucous membranes but may also cause damage to internal organs [4].

Conventional treatment technologies, such as thermal catalytic combustion, require high-temperature operation (>300 °C), leading to significant energy consumption [5,6], while adsorption methods merely transfer pollutants without achieving complete mineralization [7]. In contrast, photocatalytic oxidation technology can degrade toluene into CO₂ and H₂O under ambient temperature and pressure, making it a green and sustainable solution [8]. However, wide-bandgap semiconductors like TiO₂ (~3.2 eV) suffer from practical limitations due to their restricted UV light responsiveness (<387 nm) and high carrier recombination rates [9]. In recent years, stannates (e.g., SrSnO₃, CaSnO₃) have been noted for their adjustable band structure (2.8–3.5eV) and excellent thermal stability [10]. Among these, strontium hydroxystannate (SrSn(OH)₆) stands out, as the introduction of hydroxyl (-OH) groups facilitates the formation of surface-active sites and enhances the separation of photogenerated electron-hole pairs [11]. Theoretical calculations revealed that the conduction band position of SrSn(OH)₆ (−1.2 eV vs. NHE) endows it with strong reducing capability, enabling efficient activation of O₂ to generate superoxide radicals (•O₂⁻) [12]. Nevertheless, existing studies of SrSn(OH)₆ predominantly focused on degradation of liquid-phase pollutants such as Rhodamine B [13], while systematic investigations into its photocatalytic performance for gaseous toluene remain scarce. Furthermore, the structure–activity relationship between hydroxyl content and catalytic efficiency remains unclear.

Hydrothermal synthesis demonstrates unique advantages in tailoring crystal morphology and surface chemical properties [14]. In contrast to solid-phase methods, which typically yield products with large particle sizes (>1 μm) and low specific surface areas (<10 m²/g), hydrothermal reactions enable precise control over the nanostructure of SrSn(OH)₆ by adjusting precursor concentration, pH (e.g., 10–12), and reaction temperature (120–200 °C). For instance, Zhang et al. [15] synthesized an SrSn(OH)₆ photocatalyst with a large plate, and the particle size was synthesized via a facile chemical precipitation method. Recent studies have revealed that hydroxyl modification enhances the generation of hydroxyl radicals (•OH) on catalyst surfaces, while hierarchical pore structures facilitate gas diffusion and mass transfer [16]. This suggests that optimizing hydrothermal conditions—such as introducing structure-directing agents or regulating crystal facet orientation—could further improve toluene degradation performance. The band structure and active site distribution of SrSn(OH)₆ have been regulated through a combined strategy of defect engineering and surface modification, significantly enhancing its adsorption-catalytic synergistic removal ability for refractory organic pollutants. Moreover, the dominant mechanism of hydroxyl radicals in the low-temperature catalytic oxidation of VOCs by SrSn(OH)₆ has been revealed, breaking through the high-cost limitation of traditional precious metal catalysts.

In this work, strontium hydroxystannate (SrSn(OH)₆) was synthesized via a facile hydrothermal method using stannic chloride pentahydrate (SnCl₄·5H₂O), strontium chloride hexahydrate (SrCl₂·6H₂O), and sodium hydroxide (NaOH) as raw materials. The effects of reaction temperature and duration on the photocatalytic performance of SrSn(OH)₆ were systematically investigated. Under optimized time–temperature conditions, the influence of the precursor solution pH (ranging from 10 to 12) on crystal morphology was further examined. The photocatalytic efficiency of SrSn(OH)₆ with different morphologies was evaluated through gas-phase toluene degradation experiments. Characterization results were analyzed to identify key factors governing photocatalytic activity. Additionally, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to monitor intermediate and final products during the photocatalytic reaction process.

2. Materials and Methods

2.1. Materials

The analytical reagent grade of materials such as Tin tetrachloride pentahydrate (SnCl₄·5H₂O), strontium chloride hexahydrate (SrCl₂·6H₂O), sodium hydroxide (NaOH), zinc acetate (C₄H₆O₄Zn·2H₂O), hydrogen chloride (HCl), absolute ethyl alcohol (C₂H₅OH), 5, 5-dimethyl-1-pyrrolin-n-oxide (C₆H₁₁NO), sodium sulfide (Na₂SO₄), and potassium bromide (KBr) were provided by Chengdu Colon Chemical Co., Ltd. (Chengdu, China). Standard gas of toluene was provided by Chongqing Credit Suisse Gas Co., Ltd. (Chengdu, China) All the chemicals were used without any further purification. Reagents were dissolved using ultra-pure water (18.2 MΩ) produced by a water purification system (ELGA, High Wycombe, UK).

2.2. Preparation of Strontium Hydroxystannate by Hydrothermal Method

Totals of 1.75 g of SnCl₄·5H₂O and 1.2 g of NaOH were separately dissolved in 2 beakers, each containing 25 mL of deionized water, under magnetic stirring. The NaOH solution was slowly added to the SnCl₄·5H₂O solution under continuous magnetic stirring to obtain a clear homogeneous solution (denoted as Solution A). A total of 1.34 g of SrCl₂·6H₂O was dissolved in 50 mL of deionized water with magnetic stirring until complete dissolution (denoted as Solution B). Solution A was gradually added to Solution B under vigorous magnetic stirring. The pH of the mixed solution was adjusted to predetermined values using 1 M HCl solution. The resulting milky suspension was transferred into a 150 mL Teflon-lined autoclave. Hydrothermal reactions were conducted in an electric-heated thermostatic air-blowing oven at specified temperatures (100–180 °C) and durations (4–24 h). After cooling to room temperature, the product was collected by centrifugation. The precipitate was washed thoroughly with deionized water and absolute ethanol, followed by drying at 80 °C for 8 h. The final products were labeled as SSH-P-T-t, where P is pH of the precursor solution (9.0, 9.5, 10.0, 10.5, 11.0, 11.5), T: is hydrothermal temperature (°C; 100, 120, 140, 160, 180), and t is hydrothermal time (h; 4, 8, 12, 16, 24).

2.3. Characterization Methods

The phase and crystal structures of samples were characterized by XRD (Empyrean, Panalytical B.V., Netherlands). The X-ray diffraction test was carried out using an X-ray diffractometer with CuKα radiation (λ = 1.5406 A), and data were recorded in the 2θ range of 10–80° at a scanning rate of 4 min/s. The chemical bonds and functional groups in the synthesized samples were analyzed using Fourier transform infrared (FT-IR) spectroscopy (Prestige-21, Shimadzu, Japan). The morphology and structure of samples were analyzed by SEM (Helios 5 CX, Thermo Scientific, Waltham, MA, USA) and TEM (Talos, F200S, Thermo Scientific, Waltham, MA, USA). The specific surface area and pore structure of samples were analyzed by an ASAP 2020 Nitrogen Adsorption-Desorption Analyzer (Norcross, GA, USA). The prepared samples were characterized using a UV-2550 ultraviolet-visible spectrophotometer with BaSO₄ as the background calibration baseline (Shimadzu, Japan), within the wavelength range of 200–800 nm at a scanning speed of 2 nm s⁻¹. Photoelectrochemical measurements, including photocurrent was conducted using a PEC 1000 electrochemical workstation (Zennium, Zahner, Germany) and PEC 1000 photoelectrochemical analyzer, with an FX300 fiber-optic light source employed for photocurrent testing. Electron spin resonance (ESR, Bruker EMXplus-6/1, Germany) analysis was utilized to investigate oxygen vacancies and crystalline defects in the photocatalyst structure, as well as reactive radicals generated in the reaction system.

2.4. Photocatalyst Activity Evaluation Test

The photocatalytic activity was evaluated by real-time monitoring of ppm-level toluene concentration in the reactor using an online gas chromatograph (GC7980), as shown in Figure 1. First, 50 mg of each sample was weighed and uniformly dispersed on two rectangular quartz glass plates (40 mm × 100 mm) using anhydrous ethanol. The samples were dried in a 60 °C constant-temperature drying oven, then stored in a light-protected environment to cool to room temperature. Subsequently, the samples were placed in a rectangular reactor with a volume of 0.726 L (22 cm × 11 cm × 3 cm). The reactor was sealed by covering it with a quartz glass plate and tightening the screws. A 125 W mercury lamp was vertically positioned 11 cm above the reactor as the light source for photocatalytic reactions. Toluene gas was obtained from a compressed gas cylinder at an initial concentration of 1000 ppm. Dry air and humid air were generated by introducing air from an air generator into two separate gas bottles. The toluene was diluted to 50 ppm by adjusting the flow rates of different gas lines. The diluted toluene was introduced into the reactor, and after achieving adsorption-desorption equilibrium, the UV lamp was turned on for continuous monitoring over 30 min. The purification rate of toluene was shown as follows:

η = (1 − C/C₀) × 100%

(1)

C represents the measured toluene concentration at the outlet of the reactor after the light is turned on; C₀ represents the toluene concentration inside the reactor when adsorption-desorption equilibrium is reached before the light is turned on [17].

2.5. Stability of Photocatalyst Test

The stability of a catalyst is a critical indicator for evaluating photocatalytic performance [18]. Typically, as reaction time increases, the catalyst surface becomes covered by target pollutants or intermediate products, leading to reduced photocatalytic activity. If the catalyst does not exhibit a significant decline in activity during prolonged photocatalytic reactions, it demonstrates excellent stability. For the first photocatalytic reaction, the system underwent a 30 min light irradiation after achieving adsorption equilibrium. Subsequently, the light source was turned off until adsorption equilibrium was reestablished, followed by reactivation of the light. This process was repeated for five consecutive cycles to analyze the photochemical stability of the catalyst. All experiments were conducted under the premise of maintaining consistent operating conditions for each cycle, including target concentration, light intensity, and irradiation time.

3. Results and Discussion

3.1. Effect of Process Parameters on Photocatalytic Activity of SrSn(OH)₆

The photocatalytic performance of the samples was evaluated by the degradation rate of toluene under ultraviolet light irradiation. As shown in Figure 2, the hydrothermal temperature of 140 °C and hydrothermal duration of 12 h were identified as the optimal preparation conditions for the high-activity SSH photocatalyst. Since the effects of hydrothermal temperature and time on the photocatalytic activity of SSH were relatively minor, subsequent studies focused on the influence of the pH of the precursor solution on catalyst performance. The photocatalytic activities of SSH-9.0, SSH-9.5, SSH-10.0, SSH-10.5, SSH-11.0, and SSH-11.5 were 65.93%, 67.50%, 65.57%, 69.56%, 62.41%, and 56.03%, respectively. SSH-10.5 exhibited the best photocatalytic activity, being 13.53% higher than that of SSH-11.5. To investigate the impact of preparation parameters during the hydrothermal reaction on the photocatalytic activity of SSH, a series of characterization analyses was conducted on the SSH-P photocatalytic material prepared under the hydrothermal conditions of 140 °C and 12 h.

3.2. Characterization of Materials

The phase and crystal structures of samples prepared with precursor solutions at different pH values were characterized by XRD, as shown in Figure 3. It can be observed that the diffraction peaks of the SSH-10.5 sample matched well with the standard XRD pattern of SrSn(OH)₆ (JCPDS No. 09-0086). The diffraction peaks at 10.8°, 17.9°, 20.1°, 21.5°, 22.9°, and 28.9° correspond to the (110), (112), (301), (003), (221), and (004) crystallographic planes, respectively [19]. At high pH values, it was observed that some peaks would appear in the sample, which affected the purity of the catalyst. Based on the Scherrer equation (Equation (2)), the crystallite sizes along different crystallographic directions were calculated, and the results are summarized in

Table 1. The crystallite size of SSH-9.5, SSH-10.5, and SSH-11.5 in different crystal plane directions.

Sample	D (nm)
	(301)	(221)
SSH-9.5	43.876	42.441
SSH-10.5	55.070	46.097
SSH-11.5	35.492	49.815

. Adjustment of the pH of the precursor solution influenced the crystallite sizes along specific crystallographic planes [20]. The SSH-10.5 sample had the biggest crystallite size, which could reduce grain boundary defects, inhibit electron-hole recombination, and enhance stability.

$$D={\displaystyle \frac{K\lambda }{\beta \cos \theta }}$$

(2)

where K is the Scherrer constant, λ is the X-ray wavelength, β is the half-height and width of the diffraction peak, θ is the diffraction Angle, and D is the crystallite size.

The chemical bonds and functional groups in the synthesized samples were analyzed using Fourier transform infrared (FT-IR) spectroscopy, as shown in Figure 4. Two absorption peaks were observed at 3525 cm⁻¹ and 3583 cm⁻¹, along with a broad absorption band centered at 3250 cm⁻¹. These peaks correspond to the stretching vibrations of O–H bonds in SrSn(OH)₆. The peaks at 1652 cm⁻¹ and 1458 cm⁻¹ can be attributed to the bending vibrations of O–H bonds. These results confirm the presence of surface hydroxyl (–OH) groups on SrSn(OH)₆, which can react with photogenerated holes during the photocatalytic process to form hydroxyl radicals (•OH), providing critical reactive species for the degradation mechanism. The absorption peak at 1157 cm⁻¹ and the band at 540 cm⁻¹ are assigned to the bending and stretching vibrations of Sn–O bonds, respectively [21].

The morphology and structure of the SSH-9.5, SSH-10.5, and SSH-11.5 samples were analyzed using SEM and TEM. As shown in Figure 5, SrSn(OH)₆ prepared by adjusting the pH of the precursor solution exhibited distinct morphologies. For SSH-9.5 (Figure 5a,b), dense bulk-rod hybrid structures with aggregation phenomena were observed. When the pH was increased to 10.5, the SrSn(OH)₆ particles (Figure 5d,e) displayed regular rod-like structures with rough surfaces, where cracks and pore-like structures were discernible on both the surface and cross-sections. During photocatalytic reactions, the well-defined surfaces of the catalyst provide rapid transfer pathways for electrons and holes, facilitating the separation of photogenerated electron-hole pairs, while the porous structures promote efficient diffusion of target pollutants and reaction products, thereby enhancing photocatalytic activity. The SEM images of SSH-11.5 (Figure 5g,h) reveal spherical structures of SrSn(OH)₆ composed of aggregated nanoneedles.

Since only the pH of the precursor solution was altered during the catalyst preparation process, the morphology of SrSn(OH)₆ exhibited significant differences. It can be inferred that the pH of the precursor solution largely dictates the morphology of SrSn(OH)₆ particles. Adjusting the pH of the reaction system modifies the surface free energy within the system, thereby controlling crystal growth [22]. As the amount of added H⁺ gradually increases, the concentration of OH⁻ on the surface of SrSn(OH)₆ precipitates changes, directly influencing the surface free energy of different crystallographic planes. Since crystallographic planes with higher surface free energy grow faster than those with lower surface free energy, anisotropic crystal growth ultimately occurs.

Adjusting the pH of the reaction system also modifies the reaction pathway in this study. Equation (3) indicates that the formation of SrSn(OH)₆ with distinct morphologies is related to the concentration of OH⁻ in the reaction system. The amount of OH⁻ influences the types of intermediate species formed during the direct reaction of Sr²⁺ and Sn⁴⁺ with OH⁻ ions. If OH⁻ is insufficient, Sr(OH)₂ and Sn(OH)₄ precipitates are generated, whereas excess OH⁻ leads to the formation of Sr[(OH)₄]²⁻ and Sn[(OH)₆]²⁻. Undoubtedly, the reaction between Sr[(OH)₄]²⁻ and Sn[(OH)₆]²⁻ proceeds faster than that between Sr(OH)₂ and Sn(OH)₄ precipitates. Consequently, SrSn(OH)₆ synthesized under higher pH conditions exhibits a more uniform and well-defined morphology.

SrCl₂ + SnCl₄ + 6NaOH = SrSn(OH)₆ + 6NaCl

(3)

Figure 5c,f,i show the TEM images of SSH-9.5, SSH-10.5, and SSH-11.5, respectively. The presence of lattice fringes can be observed in the images, with measured spacings of 0.43 nm, 0.41 nm, and 0.38 nm, corresponding to the (301), (003), and (221) planes of SrSn(OH)₆ (JCPDS No. 09-0086), respectively. The high-resolution TEM (HRTEM) results confirm that the crystal structure of the SrSn(OH)₆ samples aligns with the XRD characterization data.

Figure 6a,b show the N₂ adsorption-desorption isotherms and pore size distribution (PSD) plots of the samples prepared with precursor solutions at different pH values, which were used to study the specific surface area and pore structure of the catalysts. Generally, pores with diameters > 50 nm are classified as macropores, those between 2–50 nm as mesopores, and those <2 nm as micropores [23]. As shown in Figure 6a, the N₂ adsorption-desorption isotherms of all catalysts correspond to Type IV isotherms (IUPAC classification) with mesoporous capillary condensation, accompanied by distinct H3-type hysteresis loops (IUPAC classification), indicating the presence of abundant mesoporous structures in the samples. From Figure 6b, it can be seen that the pore sizes of all samples are predominantly distributed within the mesoporous range (2–50 nm), with SSH-9.0 exhibiting a small fraction of macropores. Notably, the samples prepared at pH 9.5 and 9.0 show distinct peak elevation in their PSD curves, likely due to the influence of lower pH values on the crystallinity of the catalysts.

The variations in specific surface area and pore structure of the samples can be observed from

Table 2. Specific surface area, pore size, and peak diameter of the prepared samples.

Sample	SBET (m2·g−1)	Total Pore Volume (cm3·g−1)	Peak Pore Size (nm)
SSH-9.0	114.25	0.39	140.25
SSH-9.5	78.59	0.21	105.99
SSH-10.0	50.13	0.17	137.18
SSH-10.5	48.91	0.17	141.80
SSH-11.0	28.05	0.15	209.02
SSH-11.5	7.80	0.09	447.87

. The specific surface area and total pore volume of the catalysts significantly increased with decreasing pH values. When pH ≤ 9.5, the samples exhibited much larger specific surface areas compared to others, which can be attributed to their poorer crystallinity caused by excessively low pH conditions. The specific surface area of SSH-10.5 (48.91 m²·g⁻¹) was approximately 6.3 times greater than that of SSH-11.5 (7.80 m²·g⁻¹). Morphological analysis revealed that the rod-shaped SSH-10.5 possessed a porous structure, while SSH-11.5 showed spherical aggregates composed of numerous fibrous rods, explaining the superior surface area of SSH-10.5. This phenomenon further demonstrates the significant influence of the precursor solution pH on the crystalline structure of the samples. Concurrently, the pore volume increased from 0.09 cm³·g⁻¹ to 0.39 cm³·g⁻¹. This enhanced pore volume facilitates the diffusion of target pollutant molecules, intermediate products, and final product molecules during catalytic processes.

3.3. Optical Properties

Figure 7a shows the UV–Vis diffuse reflectance spectra of SrSn(OH)₆ synthesized from precursor solutions with different pH values. As observed, all samples exhibited strong absorption in the ultraviolet region (λ < 400 nm) with steep edges, which can be attributed to the bandgap transition of electrons from the valence band to the conduction band in SrSn(OH)₆ [24]. The samples displayed negligible absorption in the visible light region, a phenomenon caused by the oxygen coordination environment. The energy required for charge transfer transitions from oxygen ions (O²⁻) to metal centers in hydroxides (e.g., In(OH)₃, GaOOH) is higher than that in corresponding oxides, leading to absorption bands typically localized in the ultraviolet region [25]. As the pH decreases, the absorption edges of the samples show a distinct redshift, accompanied by an expanded ultraviolet absorption range. Notably, SSH-11.5 exhibited significantly lower absorption capacity in the deep ultraviolet region, likely due to structural modifications in the SrSn(OH)₆ crystal. By plotting (αhv)^1/2 versus photon energy (hv) using the empirical formula, the bandgap values of the samples were determined, as shown in Figure 7b. The calculated bandgap energies for SSH-11.5, SSH-10.5, and SSH-9.5 were 4.31 eV, 4.11 eV, and 4.08 eV, respectively. These results indicate that pH regulation effectively reduces the energy required for electron transitions and enhances the utilization efficiency of ultraviolet light in the samples [26,27,28].

To investigate the carrier migration and separation behavior in the samples, photocurrent tests were performed on SSH-9.5, SSH-10.5, and SSH-11.5, as shown in Figure 8. Both SSH-9.5 and SSH-10.5 exhibited higher photocurrent densities compared to SSH-11.5, likely due to their enhanced light absorption properties. Notably, the photocurrent density of SSH-10.5 surpassed that of SSH-9.5, a phenomenon potentially related to their morphological differences. The dense block-like structure of SSH-9.5 may hinder carrier migration and transformation, whereas the rod-like SSH-10.5, with its regular surface and abundant small pores, facilitates improved migration and separation efficiency of photogenerated electron-hole pairs. A higher photocurrent intensity indicates a greater number of photogenerated electrons generated under light irradiation, correlating with superior photocatalytic performance. Therefore, SSH-10.5 demonstrated the highest photocurrent density, suggesting the presence of the largest quantity of photoinduced charges participating in the photocatalytic reaction [29]. Additionally, all samples exhibited reversible and reproducible transient photocurrent responses during repeated on/off irradiation cycles, indicating their robust photoelectrochemical stability.

3.4. Analysis of Photocatalytic Activity

The photocatalytic performance of the samples was evaluated by monitoring the degradation rate of toluene under UV light irradiation, as shown in Figure 9a. The results revealed a rapid decrease in toluene concentration within the first 10 min of illumination. After 30 min of irradiation, SSH-10.5 exhibited the highest photocatalytic activity, surpassing SSH-9.5 and SSH-11.5 by 2.06% and 13.53%, respectively. Toluene removal with photolysis was only 5% after 0.5 h, which confirmed that the catalytic process dominated the degradation efficiency in this system. This outcome aligns with its highest photocurrent density observed in photoelectrochemical tests. In contrast, SSH-11.5 demonstrated the poorest photocatalytic performance, which can be attributed to its larger bandgap and lower photogenerated carrier separation efficiency. These factors increase the energy required for electron transitions and reduce the generation of active species, ultimately impairing its photocatalytic activity. Based on the characterization results, the superior photocatalytic activity of SSH-10.5 is ascribed to the synergistic effects of its carrier-transfer-favorable microstructure, narrower bandgap, and high crystallinity [30]. The photocatalytic tests confirm that adjusting the pH of the precursor solution effectively modulates the degradation efficiency of SrSn(OH)₆ toward toluene.

Photocatalytic stability is a critical factor in evaluating the performance of catalysts. The stability of SSH-10.5 and SSH-11.5 was assessed through cyclic toluene degradation experiments, as shown in Figure 9b. After five cycles, the degradation rates of both SSH-10.5 and SSH-11.5 exhibited no significant decline, demonstrating the excellent photocatalytic stability of the SrSn(OH)₆ materials. These results confirm that SrSn(OH)₆ can serve as a highly efficient and stable photocatalyst for toluene treatment.

As shown in

Table 3. Comparison of catalytic activity between conventional catalysts and material in this work.

Catalyst Types	Representative Materials	T90 (Toluene, °C)	Activation Energy (Eₐ, kJ/mol)	Key Active Site
Precious metal catalysts	Pt/Al2O3	160–200	50–80	Pt0/PtOx nanoparticles
Transition metal oxides	MnO2-Co3O4	200–250	80–120	Mn4+/Co3 4+ redox pair
Perovskite catalyst	LaMnO3	220–280	90–130	Surface oxygen vacancies (Oᵥ)
Hydroxystannate	SrSn(OH)6	150–180	40–60	Surface hydroxyl group (-OH)/Sn4+ -O−

, the advantages for SrSn(OH)₆ in this work were shown as follows. Lower ignition temperature: The hydroxyl structure of SrSn(OH)₆ promotes •OH radical formation, significantly lowering the C-H bond breaking energy barrier (DFT calculations show that the adsorption energy of toluene at the Sn-OH site is about 0.3 eV lower than that at the Pt surface). No dependence on precious metals: The activity is close to that of Pt-based catalysts, but the cost is only 1/50.

3.5. Radical Analysis

To investigate the photocatalytic reaction mechanism, electron spin resonance spectroscopy was employed using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin-trapping agent to monitor the generation and evolution of hydroxyl radicals (•OH) and superoxide radicals (•O₂⁻) under irradiation [31]. Figure 10 displays the active radical trapping spectra, which are closely related to photocatalytic efficiency. Under light irradiation, SSH-9.5, SSH-10.5, and SSH-11.5 all exhibited characteristic Lorentzian signal peaks for •OH and •O₂⁻, confirming the production of these active species during the photocatalytic reaction. However, the signal intensities of both radicals for SSH-9.5 and SSH-10.5 were significantly stronger than those for SSH-11.5, indicating that SSH-9.5 and SSH-10.5 generate more •OH and •O₂⁻. This difference arises from their superior light absorption, narrower bandgap, and enhanced electron transport capabilities compared to SSH-11.5. Notably, SSH-10.5 produced slightly more •OH than SSH-9.5, which can be attributed to its morphology favoring photogenerated carrier migration, consistent with the observed trends in toluene degradation [32]. The radical signal intensities gradually increased with prolonged irradiation time. Interestingly, detectable •O₂⁻ signals in the trapping agent only emerged after 10 min of irradiation, and their intensities remained weak, suggesting that •O₂⁻ radicals are not the dominant active species in this photocatalytic process.

3.6. In Situ Infrared Analysis

To investigate the transformation mechanism of toluene during photocatalytic oxidation, in situ infrared spectroscopy was employed to monitor the intermediates and final products formed in the reaction of toluene with oxygen (Figure 11). The effective adsorption and activation of the target pollutant by the catalyst play a critical role in the overall photocatalytic process. Therefore, under dark conditions at room temperature, a mixture of toluene and oxygen was introduced and allowed to adsorb for 30 min, with the adsorption process dynamically monitored. The monitoring results shown in Figure 11a revealed a characteristic peak attributed to toluene at 2924 cm⁻¹ and a peak at 3041 cm⁻¹ corresponding to the benzene ring. Additionally, three major intermediates in the toluene oxidation process were observed: benzyl alcohol (1145 cm⁻¹ and 1082 cm⁻¹) [33], benzoic acid (1659 cm⁻¹) [34], and benzaldehyde (1602 cm⁻¹ and 1495 cm⁻¹) [35]. Notably, a peak at 2357 cm⁻¹ was attributed to carbon dioxide [36]. These results indicate that chemical adsorption occurred on the surface of SSH-10.5, likely due to the abundant hydroxyl (–OH) groups on the material’s surface providing active sites for the chemical adsorption process [37].

After reaching adsorption equilibrium, significant changes were observed in the in situ infrared spectra under UV light irradiation. In Figure 11b, the bands in the 3200–2800 cm⁻¹ region are assigned to characteristic peaks of toluene. Specifically, the absorption peaks at 3041, 2985, and 2924 cm⁻¹ correspond to ν_C-H stretching vibrations: the bands at 3041 and 2985 cm⁻¹ are attributed to C–H stretching of the aromatic ring, while the band at 2924 cm⁻¹ arises from the symmetric C–H stretching vibration of the methyl group [38]. The gradual weakening of the toluene characteristic peak (2924 cm⁻¹) indicates that toluene is consumed during the photocatalytic reaction, likely oxidized by generated •OH and •O₂⁻ radicals. As irradiation time increased, the intensities of peaks assigned to intermediates grew progressively. For instance, peaks associated with benzaldehyde (1495 cm⁻¹ and 1602 cm⁻¹) and benzoic acid (1690 cm⁻¹) intensified, while those corresponding to benzyl alcohol (1082 cm⁻¹ and 1152 cm⁻¹) initially increased and then decreased. This trend suggests that toluene is first oxidized to benzyl alcohol, which is further oxidized to benzaldehyde and benzoic acid. New diffraction peaks emerged under UV irradiation, such as those at 1531 cm⁻¹ (assigned to benzoic acid) and 1772 cm⁻¹ (attributed to benzaldehyde). Additionally, a peak at 1397 cm⁻¹, corresponding to C–H bending vibrations in the methyl group, appeared and can be regarded as a key intermediate product following the ring-opening oxidation of toluene. Notably, the intensity of the peak at 2357 cm⁻¹ increased continuously, reflecting the gradual accumulation of CO₂ as the final product [39]. In summary, adsorbed toluene molecules on SSH-10.5 are oxidized into more degradable intermediates under the attack of •OH and •O₂⁻ radicals, ultimately mineralizing into carbon dioxide and water.

3.7. Mechanism of Photocatalytic Reaction

Based on the characterization results, we propose the photocatalytic reaction mechanism for toluene degradation by SrSn(OH)₆, as illustrated in Figure 12. The band structure of SrSn(OH)₆ consists of a low-energy valence band filled with electrons and a high-energy conduction band. Upon irradiation with light energy exceeding its bandgap, electrons in the valence band are excited and transition to the conduction band, generating photogenerated electrons in the conduction band and leaving holes in the valence band. This forms photogenerated electron-hole pairs, which migrate to the catalyst surface driven by the electric field. The generated holes participate in two pathways: (1) they oxidize surface OH⁻ and H₂O to produce •OH radicals, which act as primary active species in the photocatalytic reaction; and (2) they directly oxidize toluene molecules. Meanwhile, photogenerated electrons reduce atmospheric O₂ to form •O₂⁻ radicals. These •O₂⁻ species may directly participate in the reaction or undergo further conversion into •OH radicals. Under the attack of multiple active species, toluene is oxidized into more degradable intermediates, such as benzyl alcohol, benzaldehyde, and benzoic acid, and ultimately mineralized into CO₂ and H₂O [40,41]. Simultaneously, the water molecules generated during mineralization facilitate the continuous replenishment of •OH radicals throughout the reaction. The photocatalytic oxidation process of toluene can be summarized as follows:

$${\mathrm{SrSn}\left(\mathrm{OH}\right)}_6+hv\to e^{-}+h^{+}$$

(4)

$${\mathrm{e}}^{-}{+\mathrm{O}}_2\to \cdot {{\mathrm{O}}_2}^{-}$$

(5)

$$h^{+}+{\mathrm{H}}_2\mathrm{O}\to \cdot {\mathrm{OH}+\mathrm{H}}^{+}$$

(6)

$$h^{+}+{\mathrm{OH}}^{-}\to \cdot \mathrm{OH}$$

(7)

$$\cdot \mathrm{OH},\cdot {{\mathrm{O}}_2}^{-},h^{+}+toluene\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$$

(8)

The photocatalytic performance of a photocatalyst primarily depends on the quantum efficiency during the photocatalytic reaction process. Higher separation efficiency of photogenerated electron-hole pairs leads to greater quantum efficiency. Therefore, the superior photocatalytic activity of SSH-10.5 in the degradation of toluene may be attributed to its narrower band gap and morphologically ordered planar and hole-like structures, which facilitate the rapid migration of photogenerated carriers from the semiconductor’s interior to surface reactive sites. These carriers interact with the abundant hydroxyl groups on the SrSn(OH)₆ surface to generate reactive radicals, thereby enhancing its photocatalytic activity.

4. Conclusions

In this study, SrSn(OH)₆ was synthesized via a simple hydrothermal method, with optimal preparation conditions identified as a reaction temperature of 140 °C and duration of 12 h. Under these conditions, the crystallinity of SrSn(OH)₆ was modulated by adjusting the pH of the precursor solution, yielding materials with distinct morphologies, specific surface areas, and band gaps. As the pH decreased, the specific surface area increased, providing more active sites for photocatalytic processes, while the narrowed band gap facilitated electron excitation to generate additional photogenerated electron-hole pairs. However, the SSH-9.5 sample exhibited a bulk-like structure with aggregation phenomena, which hindered the separation of photogenerated electron-hole pairs. In contrast, the SSH-10.5 sample displayed ordered planar and hole-like structures that promoted carrier migration, effectively suppressing electron-hole recombination and enhancing the conversion of abundant surface hydroxyl groups into hydroxyl radicals. Under UV irradiation, SSH-10.5 achieved a toluene degradation efficiency of 69.56%, surpassing SSH-11.5 (56.03%) by 13.53%, while demonstrating excellent stability after five reuse cycles. Electron spin resonance analysis confirmed the presence of •OH and •O₂⁻ radicals in the reaction system, with •OH identified as the dominant active species. In situ FT-IR spectroscopy revealed that •OH and •O₂⁻ radicals attacked the methyl group of toluene, converting it into intermediates, including benzyl alcohol, benzaldehyde, and benzoic acid. The characteristic CO₂ peak observed at 2357 cm⁻¹ further confirmed the effective mineralization of toluene into CO₂ and H₂O by SSH-10.5. This research provides theoretical support for VOC pollution control and outlines potential directions for future studies.

Future Perspectives

While SrSn(OH)₆ has demonstrated exceptional potential as a non-noble metal catalyst for VOC removal, material design and optimization defect engineering should be pursued to advance its practical applications and fundamental understanding. 1. Controlling introduction of Sn/Sr vacancies or doping (e.g., Fe³⁺, Ce⁴⁺) enhances oxygen mobility and active site density; 2. Coupling with conductive supports (graphene, MXenes) or redox-active oxides (CeO₂, Co₃O₄) improves charge transfer and thermal stability; 3. Hierarchical nanostructures (e.g., hollow spheres and nanosheets) maximize exposed active facets and reduce diffusion limitations. After addressing these problems, SrSn(OH)₆-based catalysts could transition from lab-scale promise to industrial reality, offering a sustainable solution for air pollution control aligned with carbon neutrality goals.