Ni-Pd-Incorporated Fe₃O₄ Yolk-Shelled Nanospheres as Efficient Magnetically Recyclable Catalysts for Reduction of N-Containing Unsaturated Compounds

Abstract

The use of metal-based heterogeneous catalysts for the degradation of N-containing organic dyes has attracted much attention due to their excellent treatment efficiency and capability. Here, we report the synthesis of heterometals (Ni and Pd)-incorporated Fe₃O₄ (Ni-Pd/Fe₃O₄) yolk-shelled nanospheres for the catalytic reduction of N-containing organic dyes using a facile combination of solvothermal treatment and high-temperature annealing steps. Benefiting from the magnetic properties and the yolk-shelled structure of the Fe₃O₄ support, as well as the uniformly dispersed active heterometals incorporated in the shell and yolk of spherical Fe₃O₄ nanoparticles, the as-prepared Ni-Pd/Fe₃O₄ composite shows excellent recyclability and enhanced catalytic activity for three N-containing organic dyes (e.g., 4-nitrophenol, Congo red, and methyl orange) compared with its mono metal counterparts (e.g., Ni/Fe₃O₄ and Pd/Fe₃O₄). In the 4-nitrophenol reduction reaction, the catalytic activity of Ni-Pd/Fe₃O₄ was superior to many Fe₃O₄-supported nanocatalysts reported within the last five years. This work provides an effective strategy to boost the activity of iron oxide-based catalytic materials via dual or even multiple heterometallic incorporation strategy and sheds new light on environmental catalysis.

1. Introduction

There are increasing concerns regarding the accumulation of organic pollutants such as N-containing dyes in the aquatic ecosystems [1]. These N-containing organic compounds are usually structurally stable and mostly refractory in the natural environment, thus threatening human health and the environment due to their intrinsic toxicity [2]. Therefore, effective techniques for dye-polluted wastewater treatment are highly desired but still need further investigation [3,4]. As commonly used dyes, N-containing organic compounds (e.g., nitroaromatics and azo compounds) usually have unsaturated chromophore groups, such as nitro- (O←N=O) and azo (-N=N-) moieties, along with aromatic rings in their structures. Considering that these unsaturated groups are reactive, it is possible to cleave the molecular structures of these toxic dyes by chemical hydrogenation or reduction reactions in the presence of an efficient catalyst [5]. In this way, N-containing organic dyes can be decolorized and converted into less harmful aminoaromatics [6], which are valuable intermediates for the industrial production of a variety of agrochemicals, pharmaceuticals, dyes, and pigments [7,8]. In recent years, the reduction of 4-nitrophenol, Congo red, and methyl orange by sodium borohydride (NaBH₄) in aqueous solution has become an ubiquitous model reaction used to test catalyst activity [2,9,10]. However, it is still a daunting task to achieve high activity, selectivity, as well as recyclability for the catalyst during harsh reaction processes.

In recent years, the use of metal-based heterogeneous catalysts for decolorizing and converting N-containing organic dyes to aminoaromatic substances has attracted much attention due to their excellent treatment efficiency and capability [11,12,13,14,15]. In order to gain high catalytic efficiency and favorable utilization of active metal, the metal species are usually loaded on support materials in the form of nanoparticles [16], clusters [17], or even single atoms [18,19]. However, their small sizes lead to obvious drawbacks, such as difficulty in separating and recycling the catalyst from the reaction systems efficiently by conventional methods (e.g., filtration and centrifugation) [20]. To address this issue, scientists are trying to combine active components with magnetic materials. They introduced active metal species (e.g., nanoparticles, clusters, and single-atoms) to magnetic support materials and synthesized magnetically recyclable catalysts [21,22,23]. Due to the magnetic property of the catalysts, quick separation, and recycling of the catalysts from the reaction system can easily be achieved by applying a permanent magnet externally [24,25]. It is believed that magnetically recyclable catalysts are cost-effective and potentially applicable for industrial applications.

As a representative magnetic material, iron oxide (e.g., Fe₃O₄) is considered one of the promising candidates as a catalyst-supporting material due to its abundance and excellent stability [26]. Since the high specific surface area (SSA) of the support material is crucial to the exposure of active metal sites and mass transportation, this significantly influences the catalytic performance [27,28]. Incorporating active metal into the Fe₃O₄-based material with specific micro-/nano architectures should be an option for fabricating efficient and magnetically recyclable catalysts [29]. As a result, hierarchically structured magnetic Fe₃O₄ nanomaterials are ideal candidates as supports for catalysts [30]. However, it is still challenging to stably anchor high-density active metal sites on Fe₃O₄ materials due to the limited strength of the interaction between the iron oxide and heterometal species [31,32]. Therefore, Fe₃O₄ materials are usually functionalized via surface modification, such as coating with polymer [33]. For instance, Duan et al. fabricated an effective recyclable nanocatalyst based on double-shelled hollow nanospheres-supported Pd nanoparticles, in which magnetic Fe₃O₄ was functionalized with polydopamine [34]. Our previous work demonstrated that the incorporation of active metal into Fe₃O₄ was an effective and facile strategy to synthesize magnetic catalysts [35]. Moreover, findings indicated that dual or multiple metal-based catalysts exhibited superior catalytic performance to their mono metal-based counterparts [13,15,36,37,38,39]. Based on the above-mentioned discussions and catalyst design rationales, the incorporation of heteroatom metals into hierarchical Fe₃O₄ should be an efficient magnetically recyclable catalyst for the catalytic decolorization of N-containing organic dyes.

In this work, we report the synthesis of Ni and Pd-incorporated Fe₃O₄ (Ni-Pd/Fe₃O₄) yolk-shelled nanospheres via a combination of solvothermal treatment and high-temperature annealing. Benefiting from magnetic properties, the yolk-shelled structure, and uniformly dispersed active heterometals incorporated in the shell and yolk of spherical Fe₃O₄, the Ni-Pd/Fe₃O₄ composite showed excellent recyclability and enhanced catalytic activity for three N-containing organic dyes [e.g., 4-nitrophenol (4-NP), Congo red (CR), and methyl orange (MO)] compared with its mono metal counterparts (e.g., Ni/Fe₃O₄ and Pd/Fe₃O₄). Furthermore, the catalytic activity of Ni-Pd/Fe₃O₄ surpassed many Fe₃O₄-supported nanocatalysts reported within the last five years.

2. Results and Discussions

2.1. Preparation and Characterization of the Ni-Pd/Fe₃O₄ Catalyst

Ni and Pd-incorporated Fe₃O₄ (Ni-Pd/Fe₃O₄) spherical yolk-shelled nanocatalyst was synthesized via a modified combination method [40]. Fe(NO₃)₃·9H₂O and K₂PdCl₄ or NiCl₂·6H₂O were used as metal precursors and dissolved in a mixture of deionized water, isopropanol, and glycerol (Figure 1). On the basis of our experimental observations in the present study and previous works [40,41], the formation of the yolk-shelled structure of Ni-Pd/Fe₃O₄ nanospheres could be explained by a self-templating mechanism. First, Fe, Ni and Pd ions coordinate with isopropanol to form Ni/Pd-incorporated Fe-isopropanol solid nanospheres in the solvothermal process. The resulting Ni/Pd-incorporated Fe-isopropanol solid nanospheres then gradually transform into a relatively thermodynamically stable Ni/Pd incorporated Fe-glycerate composite. During the solvothermal transformation process, Ni/Pd-incorporated Fe-glycerate grow on the surface of Ni/Pd incorporated Fe-isopropyl alcohol nanospheres at the expense of the gradual consumption of Ni/Pd-incorporated Fe-isopropanol. After the completion of this reaction, the Ni/Pd-incorporated Fe-isopropanol solid nanospheres partially convert into yolk-shelled nanospheres consisting of a Ni/Pd-incorporated Fe-glycerate shell and a Fe-isopropyl alcohol solid core. Finally, the obtained Ni/Pd-incorporated Fe-glycerate composite was annealed and transformed into a Ni-Pd/Fe₃O₄ yolk-shelled nanospherical catalyst.

The phase composition of the synthesized Ni-Pd/Fe₃O₄ catalyst was confirmed by X-ray diffraction (XRD). As illustrated in Figure 2a, the characteristic peaks at 18.3°, 30.1°, 35.5° 43.1°, 57.1°, and 62.6° match well with the (011), (112), (103), (004), (321), and (224) reflections of Fe₃O₄ (JCPDS No. 01-075-1609), respectively. We note the absence of metallic Ni and Pd peaks in the XRD patterns of the Ni-Pd/Fe₃O₄ sample, which should be attributed to the small size and low loading of Ni and Pd incorporated in the Fe₃O₄ support. In addition, the morphology of the synthesized Ni-Pd/Fe₃O₄ nanocatalysts was characterized by scanning electron microscopy (SEM). As can be seen from the SEM images, Ni-Pd/Fe₃O₄ presents a spherical nanostructure with a diameter of 500–800 nm (Figure 2b,c). Transmission electron microscopy (TEM) and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images further confirmed the spherical yolk-shelled structure. A yolk-like core was encapsulated in the thin shell for each Ni-Pd/Fe₃O₄ nanosphere (Figure 3a). The shell thickness of the Ni-Pd/Fe₃O₄ particles is about 60 nm, consisting of stacked tiny nanoparticles (Figure 3b,c), leading to the formation of a porous structure. In comparison, the pristine Fe₃O₄ nanospheres prepared without the Ni and Pd precursor show a hollow nanospherical structure (Figure S1). The STEM images and the energy dispersive X-ray (EDX) elemental mapping show that the Ni and Pd elements are uniformly distributed across the Fe₃O₄ nanospheres (Figure 3d–h). However, the crystallization degree of the Fe₃O₄ support is not high enough to discriminate Ni or Pd Fe species from the lattice of the Fe₃O₄ substrate (Figure S2). The SSA and porosity characteristics of the Ni-Pd/Fe₃O₄ hollow spherical nanocatalyst were analyzed by N₂ adsorption–desorption measurements and determined via Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. The H2-type N₂ adsorption–desorption isotherm exhibited a distinct hysteresis loop in the desorption branch. The SSA for the Ni-Pd/Fe₃O₄ was 140.4 m² g⁻¹, and the size of the pores (e.g., micropore and mesopore) mainly fall in the range from 1.5 to 10 nm (Figure 4a,b).

X-ray photoelectron spectroscopy (XPS) analysis was conducted to determine the elemental composition and chemical state of the Ni-Pd/Fe₃O₄ nanocatalyst. As illustrated in Figure 5a, the survey spectra showed Fe, O, C, Ni, and Pd in the sample. The high-resolution spectra of Fe 2p exhibited two prominent peaks, which are assigned to Fe 2p_3/2 (at around 712.5 eV and 710.6 eV) and Fe 2p_1/2 (at 727.8 eV and 724.3 eV) of Fe₃O₄ (Figure 5b) [35]. The fitted doublet peak of the Pd 3d spectra centered at 335.6 eV and 341.0 eV assigned to the Pd(I) oxidation state (Figure 5c) [35,42]. The fitted Ni 2p spectra are shown in Figure 5d. The peaks at 855.7 eV and 862.3 eV are assigned to Ni 2p_3/2 and Ni 2p_1/2, respectively [43,44]. The loading of Ni and Pd in Ni-Pd/Fe₃O₄ were 1.34 wt.% and 0.90 wt.% respectively, as determined by inductively coupled plasma mass spectrometry (ICP-MS) measurement.

2.2. Catalytic Performance of the Ni-Pd/Fe₃O₄ Catalyst

The catalytic performance of Ni-Pd/Fe₃O₄ catalyst towards the reduction of N-containing organic dyes (e.g., 4-NP, CR, and MO) was explored (Figure S3). Firstly, the catalytic efficiency of 4-NP reduction with NaBH₄ was investigated and quantitatively evaluated by turnover frequency (TOF), which was defined as the amount of 4-NP (mmol) converted into 4-AP per unit time catalyzed by per unit amount of active metal (mmol) [15].

$${\mathrm{TOF}}_{4-\mathrm{NP}}=\frac{4-\mathrm{NP} \mathrm{converted} \mathrm{into} 4-\mathrm{AP} \left(\mathrm{mmol}\right)}{ \mathrm{active} \mathrm{Pd} \mathrm{metal} \mathrm{in} \mathrm{the} \mathrm{catalyst} \left(\mathrm{mmol}\right) \times \mathrm{time} \left(\min \right)}$$

In order to probe the reaction kinetics, successive UV–vis detection of the reaction solution was conducted to monitor the reduction process. After adding the Ni-Pd/Fe₃O₄ catalyst into the mixture, the UV–vis absorbance peak of 4-NP-NaBH₄ at ca. 400 nm decreased quickly and the absorption peak of 4-AP at ca. 300 nm increased with time (Figure 6a), indicating the successful conversion of 4-NP into 4-AP [45]. Accordingly, a gradual decolorization of the bright yellow aqueous 4-NP/NaBH₄ solution was observed during the catalytic process (4 min) (inset of Figure 6a). When the reaction is completed, Ni-Pd/Fe₃O₄ catalyst can be quickly magnetically separated from the aqueous reaction medium, allowing facile recycling of the catalyst (Figure S4). The Ni-Pd/Fe₃O₄ exhibited remarkable activity towards 4-NP reduction with a TOF as high as 295 min⁻¹, which was superior to its mono metal counterpart, such as Pd/Fe₃O₄ (TOF: 204 min⁻¹, Pd content: 0.87 wt.%) (Figure 7a and Figure S5). We noted that Ni/Fe₃O₄ (Ni content: 1.16 wt.%) showed negligible catalytic activity for 4-NP reduction within 4 min (Figure S6), indicating a synergistic activity enhancement effect of the second heterometal (Ni). The O-containing Fe₃O₄ support could act as a ligand to stabilize the Ni and Pd species and facilitate their distribution. In addition, Ni could change the electronic structure of Pd and thus modulate the catalytic performance of the Ni-Pd/Fe₃O₄ catalyst [46,47]. It is worth noting that the catalytic activity of the Ni-Pd/Fe₃O₄ catalyst surpassed many of the Fe₃O₄-supported nanocatalysts reported within recent five years (Figure 7a and Table S1) [35,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66]. In addition, the magnetic recyclability and durability were studied by repeating the catalytic reduction of 4-NP in the presence of a recycled Ni-Pd/Fe₃O₄ catalyst. Figure 6b shows the relationship between ln(A) (A denotes the absorbance at ca. 400 nm) and reaction time, the linear correlation indicated that the reduction process followed pseudo-first-order reaction kinetics. The apparent rate constant (K_app) was determined to be 1.51 × 10⁻² s⁻¹ from the slope of the linear correlation [67]. We calculated the ratio of rate constant K over the total weight of the catalyst, k = K_app/m. The activity factor, k, of the Ni-Pd/Fe₃O₄ catalyst was 15.1 s⁻¹ g⁻¹. As shown in Figure 7b, the conversion was nearly 100% on the eighth run and was maintained to 84.3% on the eleventh run, which indicated that Ni-Pd/Fe₃O₄ catalyst had excellent reusability and stability in the 4-NP reduction process. The morphology and phase composition of the recycled Ni-Pd/Fe₃O₄ were further characterized by SEM and XRD. We noted that a part of Ni-Pd/Fe₃O₄ yolk-shelled nanospheres broke after recycling from the reaction mixture (Figure S7). It may be hydrogen gas that was produced from NaBH₄ hydrolysis ejected from the void of the Ni-Pd/Fe₃O₄ hollow sphere and destroyed the yolk-shelled structure. XRD analysis of the recycled Pd-Fe₃O₄ catalyst showed the same diffraction peaks as the freshly prepared one, indicating that no obvious redox reaction occurred between Ni-Pd/Fe₃O₄ and NaBH₄ (Figure S8) and thus the magnetic property was maintained.

Ni-Pd/Fe₃O₄ catalyst also exhibited catalytic activity for the reduction of CR and MO, which are anionic azo dyes containing N=N bonds in their molecular structures. As shown in Figure 6c,e, 5 mL of aqueous CR or MO solution (5.0 mM) could be reduced entirely and decolorized when catalyzed by Ni-Pd/Fe₃O₄ catalyst using NaBH₄ as a reducing agent (57.0 mg) within 5 min or 40 min, respectively. We observed a time-dependent decrease in the UV–vis absorbance peak of CR (λ_max = 494 nm) and MO (λ_max = 464 nm) within 5 min and 40 min, respectively [68]. The complete disappearance of the peak confirmed the cleavage of N=N bonds and the formation of aminoaromatics. The corresponding K_app values for CR and MO reduction reactions were determined as 7.85 s⁻¹ g⁻¹ and 0.76 s⁻¹ g⁻¹ (Figure 6d,f). The excellent catalytic performance (e.g., activity and recyclability) of Ni-Pd/Fe₃O₄ should arise from the uniformly incorporated dual heterometallic species on Fe₃O₄ support, the high SSA of Fe₃O₄ yolk-shelled nanosphere with a permeable porous shell, and its magnetically recyclable property.

3. Materials and Methods

3.1. Materials Preparation

Preparation of Ni-Pd/Fe₃O₄ Yolk-Shelled Nanosphere

Ni-Pd/Fe₃O₄ yolk-shelled nanospheres were synthesized according to the modified solvothermal-annealing method reported previously [35,40]. Firstly, isopropanol (52.5 mL) and glycerol (7.5 mL) were successively added to a Teflon container (80 mL) and stirred to obtain a mixed solvent. Secondly, K₂PdCl₄ (1.7 mg) and NiCl₂·6H₂O (6.8 mg) were added to the mixture and stirred for 5 min to obtain a homogeneous mixture. Subsequently, Fe(NO₃)₃·9H₂O (202.0 mg) was added and stirred for another 5 min until all metal precursors were completely dissolved. After that, deionized water (1.0 mL) was injected into the above solution followed by additional stirring for 10 min. Subsequently, the Teflon container was sealed and transferred to a Teflon-lined stainless steel autoclave. After heating in an oven at 190 °C for 13 h, the Teflon container was naturally cooled to room temperature. A yellowish precipitate was obtained by centrifugation separation and subsequently washed with DI H₂O and ethanol three times. Drying at 60 °C for 6 h resulted in the intermediate Pd/Fe-glycerate sample. Finally, the Pd/Fe-glycerate was annealed at 350 °C for 3 h in a nitrogen atmosphere at a heating rate of 5 °C/min. The Fe₃O₄ hollow nanospheres were synthesized similarly without adding any metal precursor. The mono metal samples (Ni/Fe₃O₄ and Pd/Fe₃O₄) were synthesized similarly by adding only one active metal precursor (e.g., NiCl₂·6H₂O or K₂PdCl₄).

3.2. Catalytic Measurements of Ni-Pd/Fe₃O₄ Catalyst

3.2.1. Reduction of 4-NP, CR, and MO

For the 4-NP reduction reaction, 4-NP was first dissolved in 5 mL of water to form an aqueous 4-NP solution (20 mmol/L). NaBH₄ (378 mg) was then added into the aqueous 4-NP solution to obtain 4-NP-NaBH₄ mixture solution. After that, Ni-Pd/Fe₃O₄ catalyst (1.0 mg) was added into the mixture under vigorous stirring at ambient conditions (ca. 25 °C). The bright yellow 4-NP-NaBH₄ mixture faded gradually and finally became colorless, indicating complete conversion of 4-NP into 4-AP. The reaction process and conversion of 4-NP were continuously monitored by UV-vis measurements of the reaction mixture (Note: the reaction mixture should be filtrated to remove the catalyst and diluted to a moderate concentration before analysis). For comparison, the catalytic performance of Ni/Fe₃O₄ or Pd/Fe₃O₄ (1.0 mg) for 4-NP (3 mL, 20.0 mmol/L) reduction were conducted under similar reaction conditions. For CR and MO reduction reactions, Ni-Pd/Fe₃O₄ catalyst (2.0 mg) and 0.2 mL ethanol were subsequently added into 3 mL of CR or MO (5.0 mmol/L) and NaBH₄ (57.0 mg) aqueous solution under vigorous stirring at ambient condition (ca. 25 °C). The conversion of CR and MO was continuously monitored by subjecting the filtrated reaction mixture to UV-vis measurements.

3.2.2. Recyclability or Durability Test

The durability of the Ni-Pd/Fe₃O₄ catalyst was examined by measuring the conversion with a constant reaction time. In a typical reaction run, Ni-Pd/Fe₃O₄ (5.0 mg) was added into the aqueous solution (5.0 mL) of 4-NP (20.0 mmol/L) and NaBH₄ (2.0 mol/L) with vigorous stirring under ambient conditions. The exact reaction time was considered as the constant reaction time for each catalytic run. The catalytic reduction process of the 4-NP was monitored by UV–vis spectroscopy analysis and color fading of the reaction mixture. The catalyst could be easily separated from the reaction mixture by a magnet. The recycled Ni-Pd/Fe₃O₄ was washed with water and ethanol and then used for the next run.

4. Conclusions

In summary, we report a highly efficient magnetically recyclable catalyst with heterometals (Ni and Pd) uniformly incorporated in Fe₃O₄ yolk-shelled nanospheres via solvothermal treatment and subsequent high-temperature annealing approaches. The high SSA, as well as the abundant mesopores on the spherical Fe₃O₄ shell, facilitated the exposure and accessibility of active sites and promoted mass transportation of reactants, and thus boosted the catalytic activity. The Ni-Pd/Fe₃O₄ catalyst showed excellent recyclability and high catalytic efficiency for the reduction of three N-containing organic dyes (e.g., 4-NP, CR, and MO) compared with its mono metal counterparts (e.g., Ni/Fe₃O₄ and Pd/Fe₃O₄). Furthermore, the kinetics of the catalytic reduction reaction were explored in detail. For the 4-NP reduction reaction, the catalytic efficiency of Ni-Pd/Fe₃O₄ surpassed that of many Fe₃O₄-supported nanocatalysts reported within the last five years. The present work provides a potential platform for designing and fabricating magnetically recyclable catalysts for various heterogeneous reactions.