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Article

Au-SnOx Hybrid Nanoparticles Encaged in Hollow Mesoporous Silica Nanoreactors for Catalytic Reduction of p-Nitrophenol

by
Qifan Zhao
1,
Kaijie Li
2,*,
Hongbo Yu
3 and
Hongfeng Yin
2,*
1
School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China
2
Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo 315201, China
3
School of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315211, China
*
Authors to whom correspondence should be addressed.
Catalysts 2026, 16(5), 480; https://doi.org/10.3390/catal16050480
Submission received: 28 April 2026 / Revised: 15 May 2026 / Accepted: 16 May 2026 / Published: 20 May 2026
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Abstract

p-nitrophenol (p-NP) is a pollutant with environmental persistence, bioaccumulation potential, and significant health risks, and is widely dispersed in wastewater, so efficient removal of p-NP is imperative. Among the various methods, the catalytic reduction of p-NP to p-aminophenol (p-AP) using sodium borohydride (NaBH4) is a particularly promising one and, herein, catalysts play a crucial role. Among the various metals, Au shows unique catalytic activity for p-NP reduction. However, nanosized Au often exhibit limited activity and stability due to their high surface free energy. To address this challenge, we designed and synthesized Au-SnOx hybrid nanoparticles confined within hollow mesoporous silica nanoreactors (Au-SnOx@hm-SiO2) via a soft-template-assisted co-adsorption strategy. The resulting bimetallic Au-SnOx@hm-SiO2 nanoreactor showed significantly enhanced catalytic activity toward the NaBH4-mediated reduction of p-nitrophenol (p-NP) compared with its monometallic Au@hm-SiO2 counterpart, owing to the synergistic effect between Au and SnOx. Among various Au/Sn ratios, the catalyst with an Au/Sn molar ratio of 1:0.1 demonstrated the highest activity, achieving complete conversion of p-NP within 5 min at a p-NP/Au molar ratio of 529:1—a tenfold improvement over Au@hm-SiO2. Moreover, the catalyst maintained high efficiency over six consecutive cycles, with only slight deactivation, benefiting from the protective silica shell.

Graphical Abstract

1. Introduction

With industrial and agricultural expansion, p-nitrophenol (p-NP)—a critical intermediate for pesticides, dyes, and rubber—inevitably contaminates wastewater [1,2,3]. Given its environmental persistence, bioaccumulation potential, and significant health risks, efficient removal of p-NP is imperative [4,5,6]. To this end, various strategies such as physical adsorption [7], photocatalytic degradation [8,9], catalytic oxidation [10,11], and catalytic hydrogenation [12,13,14] have been developed. Among the various methods, the catalytic reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) using sodium borohydride (NaBH4) is a particularly promising method [13,15,16]. The advantages of the NaBH4-based catalytic reduction method include simple operation, mild reaction conditions, and the complete conversion of p-NP to p-AP [17]. In this process, catalysts play a crucial role by significantly increasing the reaction rate and improving the utilization efficiency of NaBH4. Therefore, the development of an efficient catalytic system for this reduction reaction is of great importance.
To date, various metals, including Pd [18], Pt [19], Au [16,20], Ag [21], Cu [22], and Ni [23], have been explored as catalysts for reducing p-NP to p-AP. Among them, Au nanoparticles (NPs) exhibit unique catalytic activity. However, their high surface free energy also makes them prone to aggregation and loss during the reaction, compromising stability [24,25]. To enhance the catalytic stability of Au catalysts, various strategies have been employed, primarily focusing on enhancing the strong metal–support interaction [26,27], including confining Au NPs within the pores of mesoporous molecular sieves [28], or encapsulating them in a porous inorganic shell [29]. For instance, compared to conventional Au/SiO2 nanocatalysts, Au NPs encapsulated within hollow porous silica nanoreactors [20] have demonstrated significantly improved catalytic activity and stability for p-NP reduction.
On the other hand, to improve the catalytic efficiency of Au catalysts for p-NP reduction, various strategies have been extensively explored. For instance, Au nanoparticles stabilized by thiolated polyethylene glycol ligands [30] exhibit significantly better catalytic performance than their unmodified counterparts. Another effective approach involves alloying Au with a second metal to form bimetallic alloys—which often enhances catalytic activity. As an example, Bingwa and co-workers developed metal-oxide-supported PdAu alloy nanoparticles [31] for p-NP reduction, achieving markedly higher activity compared to monometallic Au catalysts. More recently, studies have shown that constructing metal-metal-oxide hybrid structures can also greatly boost catalytic performance in hydrogenation reactions. For instance, Zhou and co-workers reported that SiO2-supported Au-MOx (M = Fe, Co, Ni) hybrid nanostructures [32] deliver substantially enhanced activity in p-NP hydrogenation relative to conventional Au/SiO2 catalysts.
In this work, we designed a yolk-shell nanostructured composite composed of ~2.8 nm Au-SnOx hybrid NPs encapsulated in hollow mesoporous silica nanoreactors (denoted as Au-SnOx@hm-SiO2) for p-NP hydrogenation. The synthesis procedure is illustrated in Scheme 1. Micelles formed by the electrostatic interaction between poly(ethyleneimine) (PEI) and poly(acrylic acid) (PAA) were used as soft templates for silica deposition, yielding PEI-PAA@SiO2, which was then utilized to adsorb controlled amounts of Au3+ and Sn4+ ions. After washing and calcination, Au-SnOx@hm-SiO2 samples with various molar ratios of Au/SnOx were obtained. Compared with Au@hm-SiO2 and SnOx@hm-SiO2, the synthesized Au1-(SnOx)0.1@hm-SiO2 exhibited enhanced catalytic efficiency and stability in the reduction of p-NP to p-AP using NaBH4, which is attributed to the synergistic effect between Au and SnOx and the protection of Au-SnOx by SiO2 shell.

2. Results and Discussion

2.1. Synthesis and Characterization

X-ray diffraction (XRD) was employed to confirm the crystal structure of the as-synthesized nanomaterials, as shown in Figure S1. All materials only show the SiO2 diffraction at ~22°, and no Au or SnOx diffractions can be observed. However, small peaks of Au and Sn can be clearly observed from the energy-dispersive spectrometry (EDS) measurement of Au1-(SnOx)0.1@hm-SiO2 in Figure S2. The absence of distinct Au and Sn diffractions in XRD patterns may be related to the extremely low metal loading.
Figure 1 shows high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Au@hm-SiO2, SnOx@hm-SiO2, and Au-SnOx@hm-SiO2 with different Au/Sn molar ratios; corresponding high-resolution HAADF-STEM images are provided in the insets. As seen in Figure 1, all hm-SiO2 supports display a clear hollow structure with metal NPs located inside the cavities, although SnOx@hm-SiO2 (Figure 1f) shows no clearly visible SnOx NPs due to the low (below 0.5 wt‰) oxide loading. Based on these HAADF-STEM images, the sizes of 100 Au and Au-SnOx NPs with various Au/Sn ratios within the hm-SiO2 were measured. The size distributions of the Au and Au-SnOx hybrids are presented in Figure S3. The average diameter of the Au NPs was about 2.8 nm, while Au-SnOx hybrids with different Au/SnOx ratios ranged from 2.6 nm to 3.0 nm. Figure S4 displays HAADF-STEM images of Au1-(SnOx)0.1@hm-SiO2 prepared at different calcination temperatures. As shown in Figure S4c, calcination at 600 °C induced significant growth of the Au1-(SnOx)0.1 hybrid. The corresponding particle-size distribution after 600 °C calcination is given in Figure S5. When the calcination temperature was raised from 500 °C to 600 °C, the average diameter of the Au1-(SnOx)0.1 hybrid increased from 2.8 nm to 5.8 nm, as shown in Figures S3c and S5.
Due to the extreme low loading of Au (1.2 wt‰) and Sn (0.08 wt‰) in hm-SiO2, EDS line scans and phase-mapping for Au50-(SnOx)5@hm-SiO2 (with Au and Sn concentration a fiftieth of those of Au1-(SnOx)0.1@hm-SiO2) are shown in Figure 2 to characterize the dispersion of Au and Sn in hm-SiO2. As shown in Figure 2a,b, the highest concentration of Au and Sn occurred in the same area, indicating the close contact of the Au and Sn phase. Moreover, the corresponding phase-mapping of Au and Sn in Figure 2c,d further confirm the close interaction between Au and Sn phase.
The X-ray photoelectron spectroscopy (XPS) analysis of Au@hm-SiO2, SnOx@ hm-SiO2, and Au1-(SnOx)0.1@hm-SiO2 are shown in Figure S6, which were used to determine the valence states of Au and Sn in the Au-SnOx hybrid. As seen in Figure S6, no distinct peaks are observed for any of the materials, which can be attributed to the low metal loading (Au: 0.12 wt%, Sn: 0.08 wt%). To investigate the synergistic interaction between Au and SnOx, Figure S7 illustrates the XPS spectra of Au50-(SnOx)5@hm-SiO2 and Au50@hm-SiO2. For Au50@hm-SiO2 in Figure S7a), the binding energies at 83.2/86.9 eV belong to 4f7/2/4f5/2 of the Au0 species, and the binding energies at 84/87.6 eV are ascribed to 4f7/2/4f5/2 of the Au+ species. As for Au50-(SnOx)5@hm-SiO2, ~0.1 eV shift to high binding energies in Figure S7b) was observed for the Au0 species, which is similar to the reported Au metal oxides [33,34] indicating electron transfer from Au to SnOx due to the strong interaction of Au and SnOx. Due to 4 ‰ loading of SnOx in Au50-(SnOx)5@hm-SiO2, the XPS spectra of Sn 3d in Figure S7c) showed high noise/signal ratios, but the peaks of binding energies at 487.1/496.0 eV still can be founded, which are indexed to 3d5/2/3d3/2 of the Sn4+ species, indicating the majority of Sn element in Au50-(SnOx)5@hm-SiO2 is in the state of Sn4+.
Figure 3 shows N2 adsorption–desorption isotherms of Au1-(SnOx)0.1@hm-SiO2 at various calcination temperatures and their corresponding pore-size distributions by density functional theory method. As shown in Figure 3a, all synthesized Au1-(SnOx)0.1@hm-SiO2 presented type IV isotherms with hysteresis loops, confirming a mesoporous structure [35,36,37]. The sharp adsorption at P/P0 of ~0.90 is attributed to the aggregation cavities of hm-SiO2 [38]. In addition, 1–2 nm micropores and 2–5 nm mesopores can be clearly observed in Au1-(SnOx)0.1@hm-SiO2 calcined at 500 °C and 600 °C from Figure 3b. Moreover, Table 1 illustrates the textural properties of Au1-(SnOx)0.1@hm-SiO2 at various calcination temperatures. As the calcination temperature increased from 300 °C to 500 °C, the Brunauer–Emmett–Teller (BET) surface areas and pore volume of Au1-(SnOx)0.1@hm-SiO2 apparently increased from 188.3 m2/g to 871.7 m2/g and 0.55 cm3/g to 1.04 cm3/g, indicating the complete removal of organic compounds. When the calcination temperature further increased at 600 °C, the BET surface areas and pore volume had a slight increase compared to the one calcined at 500 °C, suggesting good thermal stability of hm-SiO2.
Figure 4a depicts the Fourier transform infrared (FT-IR) spectra of Au1-(SnOx)0.1@hm-SiO2 calcined at various temperatures. As shown in Figure 4a, the signal at 2854 and 2941 cm−1 are ascribed to the stretching vibration of C-H [39,40,41], while the peaks at 1559 and 1401 cm−1 can be assigned to the N-H bending vibration [39] and the COO stretching vibration [42], respectively, and the signal at 1456 cm−1 corresponded to the methylene bending vibration [39,42]. These signals were particularly invisible after Au1-(SnOx)0.1@hm-SiO2 was calcined at 400 °C, indicating the removal of most of organic components. When the calcination temperature further increased above 500 °C, those peaks completely disappeared, suggesting the complete removal of organic components at calcination temperatures above 500 °C. However, TG/DTG analysis of Au1-(SnOx)0.1@hm-SiO2 before calcination in Figure 4b shows the calcination temperature of complete removal of organic components at ~700 °C. A TG/DTG curve was obtained at a fast heating rate of 10 °C/min, while calcination temperature of materials at a slow heating rate of 2.6 °C/min and while maintaining a certain temperature at 3.0 h. Therefore, complete removal of the organic components at 500 °C is reasonable.
In this work, various hm-SiO2 were synthesized via a soft-template-assisted co-adsorption strategy. The TEM images of various hm-SiO2 with calcination at 500 °C show a distinct hollow structure and 2.6–3.0 nm Au-SnOx hybrids with different Au/SnOx ratios were highly dispersed in hollow mesoporous silica nanoreactors. Due to the extreme low loading of Au (1.2 wt‰) and Sn (0.08 wt‰) in hm-SiO2, XRD of various hm-SiO2 only show SiO2 diffraction, but the EDS measurement of Au1-(SnOx)0.1@hm-SiO2 confirms the existence of Au and Sn in hm-SiO2. EDS line scans and phase-mapping for Au50-(SnOx)5@hm-SiO2 indicate the close contact of the Au and Sn phase. And the XPS analysis of Au50-(SnOx)5@hm-SiO2 and Au50@hm-SiO2 further confirm the strong interaction between Au and Sn. The TG, FT-IR, and N2 adsorption–desorption isotherms of Au1-(SnOx)0.1@hm-SiO2 at various calcination temperatures and their corresponding pore size distribution confirm the complete removal of organic reagents in Au1-(SnOx)0.1@hm-SiO2 with calcination over 500 °C. And the synthesized Au1-(SnOx)0.1@hm-SiO2 mainly contains 1–2 nm micropores and 2–5 nm mesopores.

2.2. Catalytic Efficient of Various hm-SiO2 for p-NP Reduction

p-NP catalytic reduction with NaBH4 over various hm-SiO2 in water were studied at room temperature and atmospheric pressure. Table S1 lists the real Au and Sn loading of various hm-SiO2 by ICP-OES analysis. As shown in Table S1, the actual Au loading of various hm-SiO2 were ~0.12 wt% to be maintained the molar ratios of p-NP/Au of 529/1 in each catalytic experiment. In addition, p-NP demonstrates a distinct UV-vis adsorption peak at 400 nm, and the peak is reduced over time during catalytic reduction [43,44]. Therefore, the decrease in height at 400 nm can be used to calculate the conversion of p-NP [45,46].
Figure 5 presents the effect of various hm-SiO2 on the reduction of p-NP, while the UV-visible changes over the process of p-NP reduction using Au@hm-SiO2 and Au1-(SnOx)0.1@hm-SiO2 was shown in Figure S8. As shown in Figure 5a, the conversion of p-NP over Au@hm-SiO2 and SnOx@hm-SiO2 only achieved 83% and 3% within 10 min. Furthermore, the reduction of p-NP without a catalyst was also studied in Figure 5a. Compared with SnOx@hm-SiO2, p-NP reduction without a catalyst still can reach 5% conversion within 10 min, suggesting the slow interaction between p-NP and NaBH4. Therefore, the 5% p-NP conversion over SnOx@hm-SiO2 can be ascribed to the interaction between p-NP and NaBH4. In contrast, the conversion of p-NP over Au-SnOx@hm-SiO2 with various Au/Sn molar ratios all achieved above 95% within 10 min in Figure 5b, indicating the significant improvement of p-NP reduction efficiency. Moreover, when the molar ratios of SnOx/Au were increased from 0.05 to 0.1, the time of achieving 95% p-NP conversion decreased from 5 min to 4 min, indicating a synergistic effect between Au and SnOx. Further increasing SnOx/Au molar ratios to 0.2 and 0.3, the time of 95% p-NP conversion increased to 7 min and 9 min, respectively. This phenomenon is possibly ascribed to a less naked Au interface with excessive SnOx. Therefore, the best Sn/Au molar ratio to be further studied for the highest p-NP reduction performance was chosen as 0.1.
Figure S9 illustrates the effect of calcination temperature on the catalytic performance of Au1-(SnOx)0.1@hm-SiO2 in the reduction of p-NP with NaBH4. As the calcination temperature increased from 300 °C to 400 °C and then to 500 °C, the conversion of p-NP within 3 min rose from 33% to 37% and further to 90%, respectively, confirming the complete removal of organic species. However, when the temperature was further raised to 600 °C, the 3 min conversion dropped to 82%. This decrease can be attributed to the aggregation of the Au-SnOx hybrid at high temperatures, as observed in Figures S3c and S5. In summary, the best catalytic efficiency was achieved at a calcination temperature of 500 °C.
The effects of reaction temperature on the reduction of p-NP over Au@hm-SiO2 and Au1-(SnOx)0.1@hm-SiO2 are shown in Figure 6a,c. As shown in Figure 6a, the rate constants (k) values of Au@hm-SiO2 were 0.0013 s−1, 0.0035 s−1, and 0.010 s−1 at the reaction temperatures of 5 °C, 25 °C, and 45 °C, respectively. When the reaction temperatures were 5 °C, 25 °C, and 35 °C in Figure 6c, the k values of Au1-(SnOx)0.1@hm-SiO2 were 0.0073 s−1, 0.011 s−1, and 0.014 s−1, respectively. According to the literature, the active energy can be determined from the k and Arrhenius theory [47,48]. As shown in Figure 6b and Figure 6d, the activation energies for Au@hm-SiO2 and Au1-(SnOx)0.1@hm-SiO2 were 37.2 and 15.1 kJ/mol, respectively.
The effect of p-NP concentration on p-NP catalytic reduction at a fixed NaBH4/p-NP molar ratio is presented in Figure 7a. As shown in Figure 7a, a slower catalytic reduction rate was visible with the concentration of p-NP of 30 mM, which can be attributed to the low probability between p-NP collision and Au1-(SnOx)0.1@hm-SiO2. Further increasing the concentration of p-NP of 50 mM caused the reduction rate to increase rapidly, and the time of complete conversion p-NP to decrease to 5 min. As the concentration of p-NP continuously increased to 100 mM, the reduction rate slightly decreased, and complete conversion was still reached in 7 min, confirming the good catalytic efficiency of the mount of p-NP; when the concentration of NaBH4 increased from 100 mM to 200 mM and 300 mM, the catalytic reduction rate rapidly improved. Further increasing the concentration of NaBH4 to 400 mM, the catalytic reduction rate slightly increased, indicating the presence of sufficient reducing agent. In this work, the best catalytic reduction efficiency of 27,063 h−1 (defined as the moles of p-NP converted per total molar Au atoms per hour) was obtained. Compared with the existing reported Au-based catalytic system in Table S2, Au1-(SnOx)0.1@hm-SiO2 demonstrated excellent catalytic performance.
Due to the minimal catalyst usage, compared with a typical run of p-NP catalytic reduction, the amount of p-NP was enlarged five-fold with a fixed amount of catalyst and a constant molar ratio of p-NP/NaBH4 to evaluate the stability of catalyst. The catalytic efficient of Au1-(SnOx)0.1@hm-SiO2 on the catalytic reduction of p-NP at a molar ratio of NaBH4/p-NP of 30 is shown in Figure 8a. The complete conversion of p-NP achieved in 11 min. Figure 8b presents the catalytic performance of p-NP reduction over Au1-(SnOx)0.1@hm-SiO2 after five cycles of reactant treatment at normal conditions. Compared with the fresh Au1-(SnOx)0.1@hm-SiO2, the catalytic efficiency of the used Au1-(SnOx)0.1@hm-SiO2 showed a slight decrease, but the complete conversion of p-NP was still achieved in 5 min, suggesting the excellent reusability of Au1-(SnOx)0.1@hm-SiO2. In addition, the TEM image and size distribution of Au1-(SnOx)0.1@hm-SiO2 after six cycles are illustrated in Figure S10. As shown in Figure S10, the structure and size of the catalyst did not show significant changes, and Au and Sn ions were not detected in the solution of five cycles of reaction by ICP-OES, further confirming the excellent stability of Au1-(SnOx)0.1@hm-SiO2, which can be attributed to the protection of silica shell.
To further confirm the synergistic interaction between Au and SnOx, the hydrogen temperature-programmed desorption (H2-TPD) was performed on Au@hm-SiO2 and Au1-(SnOx)0.1@hm-SiO2. As shown in Figure 9, the TPD profile of Au@hm-SiO2 exhibits two H2-desorption peaks at around 459 °C and 657 °C, corresponding to two distinct types of hydrogen-desorption active sites. When SnOx is introduced into Au@hm-SiO2 to form Au1-(SnOx)0.1@hm-SiO2, the H2-desorption peak near 657 °C shifts to approximately 575 °C and 702 °C, indicating a strong interaction between Au and SnOx. Furthermore, all peaks located around 462 °C, 575 °C, and 657 °C show a pronounced increase in intensity compared with Au@hm-SiO2, confirming the higher H2-adsorption capacity of the Au1-(SnOx)0.1 hybrid nanoparticles.

3. Materials and Methods

3.1. Chemicals

Sodium borohydride (NaBH4, ≥96.0%), ethanol (AR, ≥99.7%), and ammonia (AR, 25.0–28.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). p-nitrophenol (≥98.0%), Polyethyleneimine (PEI, M.W~1800) and Chloroauric acid trihydrate (HAuCl4·3H2O, 99.99%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Polyacrylic acid (PAA, M.W~5000) and Tin(IV) chloride pentahydrate (SnCl4·5H2O, 99.0%) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). All reagents were used as received without purification.

3.2. Synthesis of PEI-PAA@SiO2

In a typical synthesis, 4.0 g of PEI, 1.4 g of PAA aqueous solution, 1.0 mL of deionized water and 15.0 mL of aqueous ammonia were mixed under vigorous stirring to get a transparent mixture. Subsequently, 2.0 mL of the above mixture was charged into 28.0 g of ethanol with magnetic stirring at 650 rpm. Following this, 170 μL of Tetramethoxysilane (TMOS) was slowly dropped into the above solution with magnetic stirring at 650 rpm. After further stirring for 15 min, the solids were collected by centrifugation and washed with deionized water three times to obtain PEI-PAA@SiO2.

3.3. Synthesis of Au-SnOx@hm-SiO2, Au@hm-SiO2, and SnOx@hm-SiO2

The above obtained PEI-PAA@SiO2 were re-dispersed in 15.0 mL of deionized water and then soaked in 15.0 mL of 0.1 mM HAuCl4 and 0.02 mM of SnCl4 in 0.01 M HCl solution. After stirring for another 4 h, the solids were collected by centrifugation and washed three times with deionized water and then dried in oven at 70 °C for 10 h. The obtained materials were calcined at 500 °C for 3 h to get Au1-(SnOx)0.1@hm-SiO2.
The production of Au1-(SnOx)0.05@hm-SiO2, Au1-(SnOx)0.2@hm-SiO2, and Au1-(SnOx)0.3@hm-SiO2 was the same as that of Au1-(SnOx)0.1@hm-SiO2 except for changing the concentration of SnCl4 aqueous solutions to 0.01, 0.04, and 0.06 mM, respectively.
The production of Au@hm-SiO2 and SnOx@hm-SiO2 was the same as that of Au1-(SnOx)0.1@hm-SiO2 except for adding 0.1 mM HAuCl4 and 0.02 mM SnCl4, respectively.

3.4. Catalytic Reduction of p-NP with NaBH4

In a typical run, 15.0 mg of Au-SnOx@hm-SiO2 (Au,0.12 wt%; Au/p-NP molar ratio, 1/529), 0.05 mmol of p-NP, and 1.50 mmol of NaBH4 were added in a 30.0 mL glass bottle containing 20.0 mL of deionized water with magnetic stirring at 700 rpm and room temperature.
A small aliquot of 250 μL was withdrawn from the reaction solution at regular time intervals. The collected sample was diluted 60-fold and then characterized by UV-visible spectroscopy to determine the conversion of p-nitrophenol.
For the experiment regarding the effect of p-NP concentration on catalytic reduction (Figure 7a), samples were diluted 36-fold and 120-fold at p-NP concentrations of 30 mM and 100 mM, respectively. For the five treatment times of p-NP in Figure 8a, the sample was diluted 300-fold prior to UV-visible measurement.

3.5. Catalytic Stability of p-NP Reduction

Amounts of 30.0 mg of catalyst, 15.0 mmol of NaBH4 and 0.5 mmol of p-NP and 40.0 mL deionized water were mixed in 65.0 mL glass bottle with magnetic stirring at 700 rpm and room temperature. The reaction was sampled for UV-vis spectroscopy measurement which was purchased from Shanghai Metash Instruments Co., Ltd. (Shanghai, China) to determine p-NP conversion. After 15 min of reaction, the catalysts were collected and washed with deionized water and ethanol several times, dried at 60 °C, and calcined at 500 °C for 3 h. The recovered catalyst underwent another cycle at the same reaction conditions as those in catalytic reduction of p-NP with NaBH4.

4. Conclusions

In summary, the polyelectrolyte complexed formed by electrostatic interaction of PEI and PAA were used as soft templates to synthesize PEI-PAA@SiO2, which was further employed to adsorb controlled amounts of Au3+ and Sn4+ ions. After thermal treatment, Au-SnOx@hm-SiO2 with different Au/SnOx molar ratios were obtained. Among the Au-based hollow mesoporous silica nanoreactors, the Au-SnOx@hm-SiO2 with a Au/SnOx molar ratio of 1:0.1 demonstrated the highest activity for catalytic reduction of p-NP due to the synergistic effect between Au and SnOx. Furthermore, owning to the protection of the silica shell, Au1-(SnOx)0.1@hm-SiO2 exhibited excellent catalytic stability in p-NP reduction. We believe that the developed synthetic method in this work can be extend to other bimetallic systems (such as Pt-SnOx, Pd-SnOx and Ag-SnOx), and such materials could find more applications in heterogeneous catalytic reactions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal16050480/s1, Figure S1. XRD patterns of materials, Figure S2. The energy-dispersive spectrometry (EDS) measurement of Au1-(SnOx)0.1@hm-SiO2, Figure S3. Size distributions of inner NPs, Figure S4. TEM images showing Au1-(SnOx)0.1@hm-SiO2 calcinated at different temperatures, Figure S5. Size distributions of inner NPs of Au1-(SnOx)0.1@hm-SiO2 with calcination at 600 °C, Figure S6. XPS spectra of Au@hm-SiO2 and Au1-(SnOx)0.1@hm-SiO2, Figure S7. XPS spectra of Au50@hm-SiO2 and Au50-(SnOx)5@hm-SiO2, Table S1. Au and Sn actual loading of various hm-SiO2, Figure S8. UV-visible spectra changes over the reduction process of p-NP, Figure S9. Catalytic reduction of p-NP with NaBH4 over Au1-(SnOx)0.1@hm-SiO2 at different calcination temperatures in 3 min, Table S2. Comparisons of catalytic performance between Au1-(SnOx)0.1@hm-SiO2 and various reported Au-based catalysts for p-NP reduction, Figure S10. TEM images of recycled Au1-(SnOx)0.1@hm-SiO2 and Size distribution of Au-SnOx NPs of recycled Au1-(SnOx)0.1@hm-SiO2. References [49,50,51,52,53,54,55,56] are cited in the Supplementary Materials.

Author Contributions

The project was conceived and designed by K.L., K.L. and Q.Z. carried out the materials synthesis/characterization as well as the catalytic experiments. K.L., H.Y. (Hongbo Yu) and H.Y. (Hongfeng Yin) supervised this project. All authors contributed to discussion on the project. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Ningbo Municipal Bureau of Science and Technology (2023Z152), National Key Research and Development Program (2023YFB4005902), and Ningbo Youth Science and Technology Leading Talents Project (2024QL017).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no competing interests.

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Catalysts 16 00480 sch001
Scheme 1. Synthetic procedures of Au-SnOx hybrid nanoparticles in hm-SiO2. Polyethylenimine (PEI) and polyacrylic acid (PAA) form clusters via electrostatic interaction. Tetramethoxysilane (TMOS) undergoes hydrolysis and polymerization to form the silica shell. Soaking facilitates the adsorption and complexation of Au3+ and Sn4+ ions with PEI and PAA. Subsequent centrifugation, drying and calcination remove the template agent and realize the reduction of metal particles.
Scheme 1. Synthetic procedures of Au-SnOx hybrid nanoparticles in hm-SiO2. Polyethylenimine (PEI) and polyacrylic acid (PAA) form clusters via electrostatic interaction. Tetramethoxysilane (TMOS) undergoes hydrolysis and polymerization to form the silica shell. Soaking facilitates the adsorption and complexation of Au3+ and Sn4+ ions with PEI and PAA. Subsequent centrifugation, drying and calcination remove the template agent and realize the reduction of metal particles.
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Catalysts 16 00480 g001
Figure 1. HAADF-STEM images showing: (a), Au@hm-SiO2; (b), Au1-(SnOx)0.05@hm-SiO2; (c), Au1-(SnOx)0.1@hm-SiO2; (d), Au1-(SnOx)0.2@hm-SiO2; (e), Au1-(SnOx)0.3@hm-SiO2; (f), SnOx@hm-SiO2. The insert images of (a–f) show the high-resolution HAADF-STEM of Au@hm-SiO2, Au-SnOx@hm-SiO2 with different Au/Sn molar ratios. Scales bars of (a–f) are 100 nm, and scale bars in the insert of (a–f) are 50 nm, respectively. All materials were calcined at 500 °C.
Figure 1. HAADF-STEM images showing: (a), Au@hm-SiO2; (b), Au1-(SnOx)0.05@hm-SiO2; (c), Au1-(SnOx)0.1@hm-SiO2; (d), Au1-(SnOx)0.2@hm-SiO2; (e), Au1-(SnOx)0.3@hm-SiO2; (f), SnOx@hm-SiO2. The insert images of (a–f) show the high-resolution HAADF-STEM of Au@hm-SiO2, Au-SnOx@hm-SiO2 with different Au/Sn molar ratios. Scales bars of (a–f) are 100 nm, and scale bars in the insert of (a–f) are 50 nm, respectively. All materials were calcined at 500 °C.
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Figure 2. (a) HAADF-STEM images of Au50-(SnOx)5@hm-SiO2, the yellow arrows suggesting the selected hm-SiO2 for EDS line scans; (b) the corresponding eds line scans of selected hm-SiO2; (c) the corresponding Au phase-mapping; (d) the corresponding Sn phase-mapping.
Figure 2. (a) HAADF-STEM images of Au50-(SnOx)5@hm-SiO2, the yellow arrows suggesting the selected hm-SiO2 for EDS line scans; (b) the corresponding eds line scans of selected hm-SiO2; (c) the corresponding Au phase-mapping; (d) the corresponding Sn phase-mapping.
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Catalysts 16 00480 g003
Figure 3. (a) N2 adsorption–desorption isotherms of Au1-(SnOx)0.1@hm-SiO2 calcined at various temperatures; (b) their corresponding pore size distributions by density functional theory method.
Figure 3. (a) N2 adsorption–desorption isotherms of Au1-(SnOx)0.1@hm-SiO2 calcined at various temperatures; (b) their corresponding pore size distributions by density functional theory method.
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Figure 4. (a), FT-IR spectra of Au1-(SnOx)0.1@hm-SiO2 before and after calcination at various temperatures; (b), TG/DTG analysis of Au1-(SnOx)0.1@hm-SiO2 before calcination.
Figure 4. (a), FT-IR spectra of Au1-(SnOx)0.1@hm-SiO2 before and after calcination at various temperatures; (b), TG/DTG analysis of Au1-(SnOx)0.1@hm-SiO2 before calcination.
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Catalysts 16 00480 g005
Figure 5. Catalytic reduction of p-NP with NaBH4 using (a) SnOx@hm-SiO2, Au@hm-SiO2 and without a catalyst, and (b) various Sn/Au molar ratios of hm-SiO2. Reaction conditions: 0.05 mmol of p-NP; p-NP/Au molar ratio 529/1 for Au-containing hm-SiO2; p-NP/Sn molar ratio 529/1 for SnOx@hm-SiO2; 1.50 mmol of NaBH4; 20.0 mL of H2O; room temperature; reaction time—10 min; stirring speed—700 rpm. All materials were calcined at 500 °C.
Figure 5. Catalytic reduction of p-NP with NaBH4 using (a) SnOx@hm-SiO2, Au@hm-SiO2 and without a catalyst, and (b) various Sn/Au molar ratios of hm-SiO2. Reaction conditions: 0.05 mmol of p-NP; p-NP/Au molar ratio 529/1 for Au-containing hm-SiO2; p-NP/Sn molar ratio 529/1 for SnOx@hm-SiO2; 1.50 mmol of NaBH4; 20.0 mL of H2O; room temperature; reaction time—10 min; stirring speed—700 rpm. All materials were calcined at 500 °C.
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Figure 6. (a), The effect of reaction temperature on rate constants (k) over Au@hm-SiO2 calcined at 500 °C; (b) the corresponding relationship between lnk and T−1; (c) the effect of reaction temperature on rate constants (k) over Au1-(SnOx)0.1@hm-SiO2 calcined at 500 °C; (d) the corresponding relationship between lnk and T−1. Reaction conditions: 0.05 mmol of p-NP; p-NP/Au molar ratio 529/1 for Au@hm-SiO2 or Au1-(SnOx)0.1@hm-SiO2; 1.5 mmol NaBH4; 20.0 mL H2O; stirring speed—700 rpm.
Figure 6. (a), The effect of reaction temperature on rate constants (k) over Au@hm-SiO2 calcined at 500 °C; (b) the corresponding relationship between lnk and T−1; (c) the effect of reaction temperature on rate constants (k) over Au1-(SnOx)0.1@hm-SiO2 calcined at 500 °C; (d) the corresponding relationship between lnk and T−1. Reaction conditions: 0.05 mmol of p-NP; p-NP/Au molar ratio 529/1 for Au@hm-SiO2 or Au1-(SnOx)0.1@hm-SiO2; 1.5 mmol NaBH4; 20.0 mL H2O; stirring speed—700 rpm.
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Catalysts 16 00480 g007
Figure 7. (a) Effect of p-NP concentration on catalytic reduction over Au1-(SnOx)0.1@hm-SiO2 calcined at 500 °C; (b) effect of NaBH4 concentration on catalytic reduction over Au1-(SnOx)0.1@hm-SiO2 calcined at 500 °C. Reaction conditions: 15 mg Au1-(SnOx)0.1@hm-SiO2; 20.0 mL of H2O; room temperature; stirring speed—700 rpm; NaBH4/p-NP molar ratio of 30/1 for (a), 0.05 mmol of p-NP for (b).
Figure 7. (a) Effect of p-NP concentration on catalytic reduction over Au1-(SnOx)0.1@hm-SiO2 calcined at 500 °C; (b) effect of NaBH4 concentration on catalytic reduction over Au1-(SnOx)0.1@hm-SiO2 calcined at 500 °C. Reaction conditions: 15 mg Au1-(SnOx)0.1@hm-SiO2; 20.0 mL of H2O; room temperature; stirring speed—700 rpm; NaBH4/p-NP molar ratio of 30/1 for (a), 0.05 mmol of p-NP for (b).
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Catalysts 16 00480 g008
Figure 8. (a) Influence of reaction time on p-NP conversion over Au1-(SnOx)0.1@hm-SiO2 at a p-NP/Au ratio of 2644 (five times); (b) effect of reaction time on for p-NP conversion over recycled Au1-(SnOx)0.1@hm-SiO2 after having treated five times p-NP. Reaction conditions: p-NP, 0.5 mmol for (a) and 0.05 mmol for (b); p-NP/Au ratio of 2644 for (a) and 529 for (b); H2O, 40 mL for (a) and 20 mL for (b); NaBH4/p-NP ratio of 30; room temperature; speed of agitation, 700 rpm.
Figure 8. (a) Influence of reaction time on p-NP conversion over Au1-(SnOx)0.1@hm-SiO2 at a p-NP/Au ratio of 2644 (five times); (b) effect of reaction time on for p-NP conversion over recycled Au1-(SnOx)0.1@hm-SiO2 after having treated five times p-NP. Reaction conditions: p-NP, 0.5 mmol for (a) and 0.05 mmol for (b); p-NP/Au ratio of 2644 for (a) and 529 for (b); H2O, 40 mL for (a) and 20 mL for (b); NaBH4/p-NP ratio of 30; room temperature; speed of agitation, 700 rpm.
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Catalysts 16 00480 g009
Figure 9. The hydrogen temperature-programmed desorption (H2-TPD) of Au1-(SnOx)0.1@hm-SiO2 and Au@hm-SiO2.
Figure 9. The hydrogen temperature-programmed desorption (H2-TPD) of Au1-(SnOx)0.1@hm-SiO2 and Au@hm-SiO2.
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Table 1. Textural properties of Au1-(SnOx)0.1@hm-SiO2 at various calcination temperatures.
Table 1. Textural properties of Au1-(SnOx)0.1@hm-SiO2 at various calcination temperatures.
Calcination Temperature (°C)SBET (m2/g) aPore Volume (cm3/g) b
3001880.6
4005830.8
5008721.0
6008781.1
a BET surface area; b The single point adsorption total pore volumes of pores.
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Zhao, Q.; Li, K.; Yu, H.; Yin, H. Au-SnOx Hybrid Nanoparticles Encaged in Hollow Mesoporous Silica Nanoreactors for Catalytic Reduction of p-Nitrophenol. Catalysts 2026, 16, 480. https://doi.org/10.3390/catal16050480

AMA Style

Zhao Q, Li K, Yu H, Yin H. Au-SnOx Hybrid Nanoparticles Encaged in Hollow Mesoporous Silica Nanoreactors for Catalytic Reduction of p-Nitrophenol. Catalysts. 2026; 16(5):480. https://doi.org/10.3390/catal16050480

Chicago/Turabian Style

Zhao, Qifan, Kaijie Li, Hongbo Yu, and Hongfeng Yin. 2026. "Au-SnOx Hybrid Nanoparticles Encaged in Hollow Mesoporous Silica Nanoreactors for Catalytic Reduction of p-Nitrophenol" Catalysts 16, no. 5: 480. https://doi.org/10.3390/catal16050480

APA Style

Zhao, Q., Li, K., Yu, H., & Yin, H. (2026). Au-SnOx Hybrid Nanoparticles Encaged in Hollow Mesoporous Silica Nanoreactors for Catalytic Reduction of p-Nitrophenol. Catalysts, 16(5), 480. https://doi.org/10.3390/catal16050480

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