Atomically Dispersed Pt–Sn Nanocluster Catalysts for Enhanced Toluene Hydrogenation in LOHC Systems

Abstract

Liquid organic hydrogen carriers (LOHCs) are promising materials for safe, reversible, and high-density hydrogen storage. Atomically dispersed bimetallic Pt–Sn nanocluster catalysts supported on TiO₂ (Pt–Sn/TiO₂) were developed to enhance the hydrogenation step in the toluene-methylcyclohexane cycle, a model LOHC system. Compared with monometallic Pt/TiO₂ and Sn/TiO₂, Pt–Sn/TiO₂ exhibited superior hydrogenation performance. Mechanistic studies, including X-ray photoelectron spectroscopy, kinetic analysis, and H₂-D₂ exchange experiments, revealed that Sn incorporation modulates the electronic structure of Pt, enhancing H₂ activation and spillover. These findings provide insights into the rational design of atomically dispersed bimetallic nanocluster catalysts for efficient and durable hydrogen storage in LOHC-based systems.

1. Introduction

Hydrogen energy has been recognized as one of the cleanest and most sustainable energy carriers due to its high gravimetric energy density, abundance, and environmentally benign combustion products [1,2,3]. The only by-product of hydrogen utilization is water, without the emission of greenhouse gases or pollutants, making hydrogen an ideal alternative to fossil fuels for achieving carbon neutrality [4,5]. In addition to its potential as a clean energy vector, hydrogen also serves as a critical feedstock in chemical, petrochemical, and metallurgical industries [6].

A complete hydrogen energy system generally consists of three essential components: hydrogen production, storage and transportation, and utilization [7]. Among these, hydrogen storage and transportation technologies remain key challenges for the large-scale deployment of hydrogen energy. Conventional storage methods, such as high-pressure gaseous storage and cryogenic liquid storage, are technologically mature but face issues including low volumetric density, high energy consumption, and potential safety risks such as leakage and material embrittlement [8,9]. These limitations highlight the urgent need for safer, more efficient, and more compact hydrogen storage technologies.

Liquid organic hydrogen carrier (LOHC) systems have emerged as a promising alternative, relying on the reversible hydrogenation and dehydrogenation of unsaturated organic molecules such as aromatics or olefins [10,11,12,13,14]. On 30 July 2021, Chiyoda Corporation and Mitsubishi Corporation jointly initiated a commercial-scale hydrogen import project based on LOHC technology, while companies in the Netherlands and Japan have also explored the use of LOHC systems for hydrogen transport. Compared with high-pressure or cryogenic storage, LOHC technology offers several advantages, including higher storage density, fully reversible hydrogenation/dehydrogenation cycles, simpler handling and transport, and improved safety [15,16,17]. Newson et al. evaluated various liquid organic hydrogen carriers and identified aromatic hydrocarbons as the most promising candidates, owing to their high storage capacity, favorable thermodynamics, and excellent reversibility during hydrogenation-dehydrogenation cycles [18]. Among these, the toluene-methylcyclohexane-hydrogen (MTH) cycle has been extensively investigated as a model LOHC system [19]. The hydrogenation of toluene to methylcyclohexane represents the hydrogen storage step in this reversible cycle and plays a crucial role in determining the overall system efficiency. Due to the inherent thermodynamic and kinetic stability of the aromatic ring, practical toluene hydrogenation often requires relatively high temperatures and elevated hydrogen pressures to reach desirable conversion levels [20]. Therefore, developing efficient and durable catalysts for toluene hydrogenation is essential for advancing LOHC-based hydrogen storage technologies.

Noble metal catalysts (e.g., Pt, Pd, Ru) have demonstrated excellent hydrogen activation capability and high catalytic activity in toluene hydrogenation [21,22]. Nevertheless, their high cost and limited natural abundance hinder large-scale application. To address these challenges, atomically dispersed metal catalysts have attracted growing attention due to their nearly 100% atomic utilization efficiency and tunable electronic properties [23,24,25]. Such catalysts—ranging from single-atom to dual-atom configurations—enable precise control of active sites and reaction pathways [26]. The introduction of a secondary metal can further modulate the electronic structure of the active center, enhance hydrogen adsorption and dissociation, and suppress undesired side reactions.

In this study, Pt–Sn/TiO₂ atomically dispersed nanocluster catalysts were synthesized via an impregnation method and comprehensively characterized to establish the correlation between structure and catalytic performance. The catalysts were evaluated in toluene hydrogenation, the key hydrogen storage step in the MTH cycle, to assess their potential for LOHC applications.

2. Materials and Methods

2.1. Chemicals

All reagents were used as received without further purification. Chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 99.9%), nano-TiO₂ (P25, 99%), tin(IV) chloride pentahydrate (SnCl₄·5H₂O, ≥99.9%), methanol (99.8%) and tridecane (>99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Toluene (99.8%) was purchased from Shanghai Anpu Experimental Technology Co., Ltd. (Shanghai, China). Methylcyclohexane (99%), anhydrous ethanol (≥99%), cyclohexane (chromatographic grade), and isopropanol (98%) were purchased from Shanghai Aladdin Biochemical Co., Ltd. (Shanghai, China). Hydrogen (99%) was purchased from Guangzhou Guangqi Gas Co., Ltd. (Shanghai, China).

2.2. Apparatus

Catalyst synthesis and characterization were conducted using standard laboratory equipment, including a muffle furnace (KSL-1750X, Hefei Kejing Material Technology Co., Ltd., Hefei, China), a magnetic stirrer bath (HWCL-1, Zhengzhou Great Wall Science & Trade Co., Ltd., Zhengzhou, China), a low-speed peristaltic pump (LHZW006, United Zhongwei Technology Co., Ltd., Zhengzhou, China), an analytical balance (ML204, Mettler-Toledo, Columbus, OH, USA), and a high-speed centrifuge (TG16-WT, Shenzhen Vector Scientific Instruments Co., Ltd., Shenzhen, China).

Structural and surface characterizations were performed using X-ray powder diffraction (XRD, D-MAX 2200 VPC, Rigaku Corporation, Tokyo, Japan), X-ray photoelectron spectroscopy (XPS, Escalab 250XPS, Thermo Fisher Scientific, Waltham, MA, USA), inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer Optima 8000, PerkinElmer Inc., Waltham, MA, USA), BET surface area analysis (ASAP2460, Micromeritics, Norcross, GA, USA), transmission electron microscopy (TEM, FEI Tecnai G2 F30, FEI, Hillsboro, OR, USA), and aberration-corrected high-angle annular dark-field scanning TEM (AC HAADF-STEM, JEM-ARM200P, JEOL Ltd., Tokyo, Japan).

Product analysis was performed with gas chromatography (GC2010 plus, Shimadzu, Kyoto, Japan) and GC-MS (GCMS-QP2010-Ultra, Shimadzu, Kyoto, Japan). The H₂-D₂ exchange reaction was conducted in a continuous-flow fixed-bed quartz reactor with an internal diameter of 7 mm under ambient temperature and atmospheric pressure. About 100 mg of the catalyst sample was loaded into the reactor and held in place with quartz cotton plugs. A mixture of 5% H₂ in N₂ was used as the carrier gas at a flow rate of 20 mL/min, while D₂ was introduced as a pulse gas flowing at 10 mL/min. The signal of HD (m/z = 3) was monitored in real time using a mass spectrometer (Hiden Analytical HPR-20 QIC benchtop gas analysis system, Warrington, UK).

2.3. Catalyst Preparation

2.3.1. Pt/TiO₂ Atomically Dispersed Nanocluster Catalyst

Chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 0.63 g) was accurately weighed and dissolved in 5 mL of deionized water to prepare a 0.1 g/mL H₂PtCl₆·solution. An aliquot of 210 μL of this solution was diluted with 100 mL of deionized water. Separately, nano-TiO₂ (P25, 1 g) was dispersed in 150 mL of deionized water and stirred at 800 rpm for 300 min. During stirring, the diluted H₂PtCl₆ solution was added dropwise to the TiO₂ suspension at a rate of 1.42 mL/min using a low-speed peristaltic pump. The resulting mixture was centrifuged, and the collected solid was dried overnight at 60 °C, followed by calcination in air at 400 °C for 2 h with a heating rate of 5 °C/min to obtain the Pt/TiO₂ catalyst.

2.3.2. Sn/TiO₂ Atomically Dispersed Nanocluster Catalyst

Tin(IV) chloride pentahydrate (SnCl₄·5H₂O, 29.5 mg) was dissolved in 100 mL of deionized water. Meanwhile, nano-TiO₂ (P25, 1 g) was dispersed in 150 mL deionized water and stirred at 800 rpm for 300 min. During stirring, the SnCl₄ solution was added dropwise to the TiO₂ suspension at a rate of 1.42 mL/min using a low-speed peristaltic pump. Subsequent centrifugation, drying, and calcination steps were identical to those described for Pt/TiO₂.

2.3.3. Pt–Sn/TiO₂ Atomically Dispersed Nanocluster Catalyst

Chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 0.63 g) was accurately weighed and dissolved in 5 mL of deionized water to prepare a 0.1 g/mL H₂PtCl₆ solution. An aliquot of 210 μL of this solution and tin(IV) chloride pentahydrate (SnCl₄·5H₂O, 3.7 mg) were added to 100 mL of deionized water. Nano-TiO₂ (P25, 1 g) was dispersed in 150 mL of deionized water and stirred at 800 rpm for 300 min. During stirring, the mixed metal precursor solution was added dropwise to the TiO₂ suspension at a rate of 1.42 mL/min using a low-speed peristaltic pump. The subsequent procedures were identical to those described above for the other catalysts.

2.4. Catalyst Performance Analysis Methods

2.4.1. Toluene Hydrogenation

The catalytic hydrogenation of toluene was conducted in a 10 mL stainless-steel autoclave. In a typical experiment, 20 mg of catalyst, 0.5 mmol of toluene, and 0.12 mmol of tridecane (internal standard) were dissolved in 2 mL of cyclohexane. The reactor was purged with H₂ five times before being pressurized to 1 MPa H₂. The reaction was carried out at 100 °C under magnetic stirring. After completion, the reaction mixture was filtered, and the liquid products were analyzed by GC and GC-MS.

2.4.2. Catalyst Stability

The used Pt–Sn/TiO₂ catalyst was recovered by filtration, washed with ethanol and deionized water, dried at 60 °C for 10 h, and re-calcined at 400 °C for 2 h. The regenerated catalyst was reused under identical reaction conditions for stability tests.

2.4.3. Data Analysis

Toluene conversion and product selectivity were calculated using the following equations:

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}(\mathrm{\%})={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_1}{{\mathrm{C}}_0}}$$

(1)

$$\mathrm{S}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}(\mathrm{\%})={\displaystyle \frac{{\mathrm{C}}_2}{{\mathrm{C}}_0-{\mathrm{C}}_1}}$$

(2)

where C₀ is the initial molar amount of toluene, C₁ is the molar amount of unreacted toluene, and C₂ is the molar amount of the product.

The activity of catalyst was calculated as:

$$\mathrm{A}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y} \left({\mathrm{h}}^{-1}\right)={\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{d}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e} \mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l} \left(\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}\right) \times \mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}}}$$

(3)

3. Results and Discussion

3.1. Catalyst Characterization

Efficient hydrogenation catalysts are critical not only for fundamental studies of reaction mechanisms, but also for applications in organic liquid hydrogen carriers (LOHCs) for reversible hydrogen storage. In this work, Pt–Sn/TiO₂ catalysts were synthesized by impregnating TiO₂ with chloroplatinic acid and tin(IV) chloride pentahydrate, followed by calcination at 400 °C for 2 h (Figure S1). Pt/TiO₂ and Sn/TiO₂ catalysts were prepared under identical conditions for comparison. The catalysts were characterized to evaluate their structural and morphological features. Metal loadings were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), as summarized in Table S1. Pt–Sn/TiO₂ contained 0.68 wt% Pt and 0.089 wt% Sn, whereas Pt/TiO₂ and Sn/TiO₂ contained 0.72 wt% Pt and 0.83 wt% Sn, respectively. The observed loadings were in good agreement with the theoretical values, indicating that the impregnation method is an effective approach for preparing supported catalysts.

The crystalline structures of the catalysts and the TiO₂ support were analyzed by X-ray diffraction (XRD), as shown in Figure 1a. All samples exhibited diffraction patterns consistent with the anatase phase of TiO₂, indicating that metal loading did not alter the crystal structure. No characteristic peaks of metallic Pt (PDF#04-0802) or Sn (PDF#18-1380) were detected in Pt/TiO₂, Sn/TiO₂, or Pt–Sn/TiO₂, implying that both Pt and Sn species are highly dispersed on the TiO₂ surface. The absence of metal peaks may also result from the low metal contents, which are below the detection limit of the instrument.

The textural properties were further investigated by N₂ physisorption (Figure 2b). All catalysts exhibited Type IV isotherms, characteristic of mesoporous materials. The BET surface areas of Pt/TiO₂, Pt–Sn/TiO₂, and Sn/TiO₂ were 38.5, 38.9, and 39.2 m²·g⁻¹, respectively (Table S2), exhibiting minor differences. The comparable pore volumes and average pore sizes across the series confirm that the mesoporous structure of the TiO₂ support was well-preserved after metal loading. The BET results, showing preserved mesoporosity, are consistent with the XRD analysis, indicating that metal loading did not significantly affect the TiO₂ framework.

To further investigate the spatial distribution of metal species, transmission electron microscopy (TEM) and aberration-corrected scanning transmission electron microscopy (AC-STEM) were performed. Representative TEM, STEM, AC-STEM, and energy-dispersive X-ray spectroscopy (EDS) mapping images of Pt–Sn/TiO₂, Pt/TiO₂, and Sn/TiO₂ are presented in Figure 2, Figures S2 and S3. The dark-field STEM images (Figure 2b, Figures S2b and S3b) display uniformly distributed bright spots at a 20 nm scale, indicating a homogeneous dispersion of metallic species. High-resolution AC-STEM images (Figure 2c, Figures S2c and S3c) further reveal that the metals are present as uniformly dispersed nanoclusters on the TiO₂ surface. The corresponding EDS mapping (Figure 2d, Figures S2d and S3d) further supports these observations, showing that Pt and Sn are distributed over the support. These results demonstrate the successful formation of Pt–Sn/TiO₂, Pt/TiO₂, and Sn/TiO₂ catalysts with atomically dispersed nanoclusters via the impregnation method.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the oxidation states of Pt and Sn in Pt/TiO₂ and Pt–Sn/TiO₂ catalysts. The Pt 4f spectra of Pt/TiO₂ and Pt–Sn/TiO₂ (Figure 1c) exhibits two peaks at 71.5 eV and 72.4 eV, assigned to Pt⁰ 4f_7/2 and Pt²⁺ 4f_7/2 in Pt/TiO₂, indicating the coexistence of metallic and oxidized Pt species. After introducing Sn, the Pt 4f peaks in Pt–Sn/TiO₂ shift slightly to lower binding energies (71.2 eV and 72.3 eV) without changing the valence states. This shift is attributed to electron transfer from Sn to the more electronegative Pt, which increases the electron density on Pt and generates an electron-rich state. The Sn 3d spectrum of Pt–Sn/TiO₂ (Figure 1d) exhibits four peaks: 483.6 eV and 489.1 eV for Sn⁰, and 486.3 eV and 494.7 eV for oxidized Sn species (Sn²⁺ and Sn⁴⁺) [27,28]. These findings indicate the coexistence of metallic and oxidized Pt and Sn, and demonstrate electronic interactions between the two metals in the bimetallic catalyst.

3.2. Catalytic Performance Evaluation

Toluene hydrogenation, representing the hydrogen storage step in the toluene-methylcyclohexane (MTH) cycle for LOHC systems, was employed to evaluate the catalytic performance of Pt–Sn/TiO₂ and Pt/TiO₂. The effect of solvent was tested at 100 °C and 1 MPa H₂ for 55 min in cyclohexane, ethanol, and isopropanol (IPA). Pt–Sn/TiO₂ showed the highest toluene conversion in cyclohexane (92.1%), significantly higher than in ethanol (42.1%) and IPA (55.2%) (Figure S4). Cyclohexane was thus selected as the reaction solvent for all subsequent experiments. Toluene conversion over Pt–Sn/TiO₂ and Pt/TiO₂ increased with reaction time, reaching >99.9% and 81.3% after 65 min, respectively, while Sn/TiO₂ was almost inactive (<1%) (Figure 3a). These results indicate that metallic Pt acts as the active center for toluene hydrogenation, and the incorporation of Sn promotes the reaction. At conversions below 30% and 100 °C, the activity of Pt–Sn/TiO₂ was 1992.1 h⁻¹, 1.4 times higher than Pt/TiO₂ (1462.7 h⁻¹) (Figure 3b). Temperature-dependent tests under 1 MPa H₂ from 80 to 110 °C showed that Pt–Sn/TiO₂ consistently outperformed Pt/TiO₂ (Figure 3c). Pt–Sn/TiO₂ also showed excellent recyclability, maintaining >99.9% toluene conversion and 97.4% methylcyclohexane selectivity over five cycles (Figure 3d). STEM images before and after reaction (Figure S5a,b) revealed that Pt remained as well-dispersed nanoclusters with only minor aggregation (average size increased from 1.15 nm to 1.5 ± 0.5 nm), indicating good structural stability under reaction conditions. Such durability is crucial for LOHC systems, which require catalysts capable of enduring repeated hydrogenation–dehydrogenation cycles. The high activity and stability of Pt–Sn/TiO₂ suggest its potential as an efficient catalyst for LOHC hydrogen storage, laying the groundwork for further investigation under solvent-free and extended cycling conditions.

The enhanced hydrogenation performance of Pt–Sn/TiO₂ highlights its potential for efficient hydrogen storage in LOHC systems, where rapid and reversible H₂ uptake is critical. To investigate the origin of the performance difference between Pt–Sn/TiO₂ and Pt/TiO₂ in toluene hydrogenation, the apparent activation energies were determined from Arrhenius plots (Figure 3e). Pt–Sn/TiO₂ exhibited a lower activation energy (28.6 kJ/mol) than Pt/TiO₂ (37.3 kJ/mol), indicating that Sn incorporation enhances the catalytic efficiency, consistent with the activity results shown in Figure 3b. The H₂ dissociation abilities of the two catalysts were further evaluated by H-D exchange experiments. The two peaks observed for each catalyst in Figure 3f correspond to sequential hydrogen activation events on different types of active sites, reflecting the catalyst’s ability to dissociate molecular hydrogen. The HD peak area of Pt–Sn/TiO₂ was 1.5 times larger than that of Pt/TiO₂, demonstrating that the bimetallic Pt–Sn catalyst has significantly improved H₂ activation. These results confirm that Sn doping enhances the hydrogenation performance of Pt for toluene. Overall, Pt–Sn/TiO₂ shows higher activity in toluene hydrogenation than most catalysts reported in the literature [21,22,29,30,31,32,33]. This superior activity can be attributed to the highly dispersed architecture of the catalyst and the consequent Pt–Sn synergy, which collectively enhance the hydrogen dissociation capability.

4. Conclusions

In summary, Pt–Sn/TiO₂ atomically dispersed nanocluster catalysts were successfully synthesized via an impregnation method and thoroughly characterized. Structural analyses (XRD, XPS, TEM, and AC-STEM) confirmed uniform dispersion of Pt and Sn nanoclusters with evident electronic interactions. Pt–Sn/TiO₂ exhibited superior activity in toluene hydrogenation (activity = 1992.1 h⁻¹) compared with Pt/TiO₂ (1462.7 h⁻¹), along with excellent recyclability. Kinetic analysis revealed a lower apparent activation energy, while H-D exchange experiments demonstrated enhanced H₂ dissociation, accounting for its improved catalytic performance. The high activity, stability, and electronic synergy of Pt–Sn/TiO₂ highlight its potential as an efficient hydrogenation catalyst for organic liquid hydrogen carriers (LOHCs). These findings provide valuable insights for designing robust, interface-engineered catalysts to enable efficient hydrogen storage and release in sustainable energy systems.