Ni-Pd-Incorporated Fe 3 O 4 Yolk-Shelled Nanospheres as Efﬁcient Magnetically Recyclable Catalysts for Reduction of N-Containing Unsaturated Compounds

: 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 Fe 3 O 4 (Ni-Pd/Fe 3 O 4 ) 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 Fe 3 O 4 support, as well as the uniformly dispersed active heterometals incorporated in the shell and yolk of spherical Fe 3 O 4 nanoparticles, the as-prepared Ni-Pd/Fe 3 O 4 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/Fe 3 O 4 and Pd/Fe 3 O 4 ). In the 4-nitrophenol reduction reaction, the catalytic activity of Ni-Pd/Fe 3 O 4 was superior to many Fe 3 O 4 -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.


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 (NaBH 4 ) 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., Fe 3 O 4 ) 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 Fe 3 O 4 -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 Fe 3 O 4 nanomaterials are ideal candidates as supports for catalysts [30].However, it is still challenging to stably anchor high-density active metal sites on Fe 3 O 4 materials due to the limited strength of the interaction between the iron oxide and heterometal species [31,32].Therefore, Fe 3 O 4 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 Fe 3 O 4 was functionalized with polydopamine [34].Our previous work demonstrated that the incorporation of active metal into Fe 3 O 4 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 Fe 3 O 4 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 Fe 3 O 4 (Ni-Pd/Fe 3 O 4 ) 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 Fe 3 O 4 , the Ni-Pd/Fe 3 O 4 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/Fe 3 O 4 and Pd/Fe 3 O 4 ).Furthermore, the catalytic activity of Ni-Pd/Fe 3 O 4 surpassed many Fe 3 O 4 -supported nanocatalysts reported within the last five years.

Preparation and Characterization of the Ni-Pd/Fe 3 O 4 Catalyst
Ni and Pd-incorporated Fe 3 O 4 (Ni-Pd/Fe 3 O 4 ) spherical yolk-shelled nanocatalyst was synthesized via a modified combination method [40].Fe(NO 3 ) 3 •9H 2 O and K 2 PdCl 4 or NiCl 2 •6H 2 O 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/Fe 3 O 4 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/Pdincorporated 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/Fe 3 O 4 yolk-shelled nanospherical catalyst.

Preparation and Characterization of the Ni-Pd/Fe3O4 Catalyst
Ni and Pd-incorporated Fe3O4 (Ni-Pd/Fe3O4) spherical yolk-shelled nanocatalyst w synthesized via a modified combination method [40].Fe(NO3)3•9H2O and K2PdCl4 NiCl2•6H2O were used as metal precursors and dissolved in a mixture of deionized wat isopropanol, and glycerol (Figure 1).On the basis of our experimental observations in t present study and previous works [40,41], the formation of the yolk-shelled structure Ni-Pd/Fe3O4 nanospheres could be explained by a self-templating mechanism.First, Ni and Pd ions coordinate with isopropanol to form Ni/Pd-incorporated Fe-isopropan solid nanospheres in the solvothermal process.The resulting Ni/Pd-incorporated Fe-i propanol solid nanospheres then gradually transform into a relatively thermodynam cally stable Ni/Pd incorporated Fe-glycerate composite.During the solvothermal transf mation process, Ni/Pd-incorporated Fe-glycerate grow on the surface of Ni/Pd incorp rated Fe-isopropyl alcohol nanospheres at the expense of the gradual consumption Ni/Pd-incorporated Fe-isopropanol.After the completion of this reaction, the Ni/Pdcorporated Fe-isopropanol solid nanospheres partially convert into yolk-shelled nan spheres consisting of a Ni/Pd-incorporated Fe-glycerate shell and a Fe-isopropyl alcoh solid core.Finally, the obtained Ni/Pd-incorporated Fe-glycerate composite was anneal and transformed into a Ni-Pd/Fe3O4 yolk-shelled nanospherical catalyst.The phase composition of the synthesized Ni-Pd/Fe3O4 catalyst was confirmed by ray diffraction (XRD).As illustrated in Figure 2a, the characteristic peaks at 18.3°, 30.35.5° 43.1°, 57.1°, and 62.6° match well with the (011), ( 112), ( 103), (004), (321), and (2 reflections of Fe3O4 (JCPDS No. 01-075-1609), respectively.We note the absence of meta Ni and Pd peaks in the XRD patterns of the Ni-Pd/Fe3O4 sample, which should be tributed to the small size and low loading of Ni and Pd incorporated in the Fe3O4 suppo In addition, the morphology of the synthesized Ni-Pd/Fe3O4 nanocatalysts was charact ized by scanning electron microscopy (SEM).As can be seen from the SEM images, N Pd/Fe3O4 presents a spherical nanostructure with a diameter of 500-800 nm (Figure 2b Transmission electron microscopy (TEM) and aberration-corrected high-angle annu dark-field scanning transmission electron microscopy (HAADF-STEM) images furth confirmed the spherical yolk-shelled structure.A yolk-like core was encapsulated in t thin shell for each Ni-Pd/Fe3O4 nanosphere (Figure 3a).The shell thickness of the N Pd/Fe3O4 particles is about 60 nm, consisting of stacked tiny nanoparticles (Figure 3b leading to the formation of a porous structure.In comparison, the pristine Fe3O4 nan spheres prepared without the Ni and Pd precursor show a hollow nanospherical structu (Figure S1).The STEM images and the energy dispersive X-ray (EDX) elemental mappi show that the Ni and Pd elements are uniformly distributed across the Fe3O4 nanosphe (Figure 3d-h).However, the crystallization degree of the Fe3O4 support is not high enou to discriminate Ni or Pd Fe species from the lattice of the Fe3O4 substrate (Figure S2).T SSA and porosity characteristics of the Ni-Pd/Fe3O4 hollow spherical nanocatalyst w analyzed by N2 adsorption-desorption measurements and determined via Brunauer-E mett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods.The H2-type N2 adso tion-desorption isotherm exhibited a distinct hysteresis loop in the desorption bran The SSA for the Ni-Pd/Fe3O4 was 140.4 m 2 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).The SSA for the Ni-Pd/Fe3O4 was 140.4 m 2 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).The SSA for the Ni-Pd/Fe3O4 was 140.4 m 2 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).

Catalytic Performance of the Ni-Pd/Fe3O4 Catalyst
The catalytic performance of Ni-Pd/Fe3O4 catalyst towards the reduction of N-con taining organic dyes (e.g., 4-NP, CR, and MO) was explored (Figure S3).Firstly, the cata lytic efficiency of 4-NP reduction with NaBH4 was investigated and quantitatively evalu ated by turnover frequency (TOF), which was defined as the amount of 4-NP (mmol) con verted into 4-AP per unit time catalyzed by per unit amount of active metal (mmol) [15] TOF4-NP = In order to probe the reaction kinetics, successive UV-vis detection of the reactio solution was conducted to monitor the reduction process.After adding the Ni-Pd/Fe3O catalyst into the mixture, the UV-vis absorbance peak of 4-NP-NaBH4 at ca. 400 nm de creased quickly and the absorption peak of 4-AP at ca. 300 nm increased with time (Figur 6a), indicating the successful conversion of 4-NP into 4-AP [45].Accordingly, a gradua decolorization of the bright yellow aqueous 4-NP/NaBH4 solution was observed durin the catalytic process (4 min) (inset of Figure 6a).When the reaction is completed, Ni

Catalytic Performance of the Ni-Pd/Fe 3 O 4 Catalyst
The catalytic performance of Ni-Pd/Fe 3 O 4 catalyst towards the reduction of Ncontaining organic dyes (e.g., 4-NP, CR, and MO) was explored (Figure S3).Firstly, the catalytic efficiency of 4-NP reduction with NaBH 4 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].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/Fe 3 O 4 catalyst into the mixture, the UV-vis absorbance peak of 4-NP-NaBH 4 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/NaBH 4 solution was observed during the catalytic process (4 min) (inset of Figure 6a).When the reaction is completed, Ni-Pd/Fe 3 O 4 catalyst can be quickly magnetically separated from the aqueous reaction medium, allowing facile recycling of the catalyst (Figure S4).The Ni-Pd/Fe 3 O 4 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/Fe 3 O 4 (TOF: 204 min −1 , Pd content: 0.87 wt.%) (Figures 7a and S5).We noted that Ni/Fe 3 O 4 (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 Fe 3 O 4 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/Fe 3 O 4 catalyst [46,47].It is worth noting that the catalytic activity of the Ni-Pd/Fe 3 O 4 catalyst surpassed many of the Fe 3 O 4 -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 the presence of a recycled Ni-Pd/Fe 3 O 4 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/Fe 3 O 4 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/Fe 3 O 4 catalyst had excellent reusability and stability in the 4-NP reduction process.The morphology and phase composition of the recycled Ni-Pd/Fe 3 O 4 were further characterized by SEM and XRD.We noted that a part of Ni-Pd/Fe 3 O 4 yolk-shelled nanospheres broke after recycling from the reaction mixture (Figure S7).It may be hydrogen gas that was produced from NaBH 4 hydrolysis ejected from the void of the Ni-Pd/Fe 3 O 4 hollow sphere and destroyed the yolk-shelled structure.XRD analysis of the recycled Pd-Fe 3 O 4 catalyst showed the same diffraction peaks as the freshly prepared one, indicating that no obvious redox reaction occurred between Ni-Pd/Fe 3 O 4 and NaBH 4 (Figure S8) and thus the magnetic property was maintained.
Ni-Pd/Fe 3 O 4 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/Fe 3 O 4 catalyst using NaBH 4 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/Fe 3 O 4 should arise from the uniformly incorporated dual heterometallic species on Fe 3 O 4 support, the high SSA of Fe 3 O 4 yolk-shelled nanosphere with a permeable porous shell, and its magnetically recyclable property.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 (λmax = 494 nm) and MO (λmax = 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 Kapp 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.

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).

Materials Preparation
Preparation of Ni-Pd/Fe 3 O 4 Yolk-Shelled Nanosphere Ni-Pd/Fe 3 O 4 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, K 2 PdCl 4 (1.7 mg) and NiCl 2 •6H 2 O (6.8 mg) were added to the mixture and stirred for 5 min to obtain a homogeneous mixture.Subsequently, Fe(NO 3 ) 3 •9H 2 O (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.

Recyclability or Durability Test
The durability of the Ni-Pd/Fe 3 O 4 catalyst was examined by measuring the conversion with a constant reaction time.In a typical reaction run, Ni-Pd/Fe 3 O 4 (5.0 mg) was added into the aqueous solution (5.0 mL) of 4-NP (20.0 mmol/L) and NaBH 4 (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/Fe 3 O 4 was washed with water and ethanol and then used for the next run.

Conclusions
In summary, we report a highly efficient magnetically recyclable catalyst with heterometals (Ni and Pd) uniformly incorporated in Fe 3 O 4 yolk-shelled nanospheres via solvothermal treatment and subsequent high-temperature annealing approaches.The high SSA, as well as the abundant mesopores on the spherical Fe 3 O 4 shell, facilitated the exposure and accessibility of active sites and promoted mass transportation of reactants, and thus boosted the catalytic activity.The Ni-Pd/Fe 3 O 4 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/Fe 3 O 4 and Pd/Fe 3 O 4 ).Furthermore, the kinetics of the catalytic reduction reaction were explored in detail.For the 4-NP reduction reaction, the catalytic efficiency of Ni-Pd/Fe 3 O 4 surpassed that of many Fe 3 O 4 -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.

Data Availability Statement:
The authors confirm that the data supporting the findings of this study are available within the article.Derived data supporting the findings of this study are available on request.

Conflicts of Interest:
The authors declare no conflict of interest.

Figure 1 .
Figure 1.Schematic illustration of the preparation process of Ni-Pd/Fe3O4 catalyst.

Figure 1 .
Figure 1.Schematic illustration of the preparation process of Ni-Pd/Fe 3 O 4 catalyst.The phase composition of the synthesized Ni-Pd/Fe 3 O 4 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 Fe 3 O 4 (JCPDS No. 01-075-1609), respectively.We note the absence of metallic Ni and Pd peaks in the XRD patterns of the Ni-Pd/Fe 3 O 4 sample, which should be attributed to the small size and low loading of Ni and Pd incorporated in the Fe 3 O 4 support.In addition, the morphology of the synthesized Ni-Pd/Fe 3 O 4 nanocatalysts was characterized by scanning electron microscopy (SEM).As can be seen from the SEM images, Ni-Pd/Fe 3 O 4 presents a spherical nanostructure with a diameter of 500-800 nm (Figure2b,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/Fe 3 O 4 nanosphere (Figure3a).The shell thickness of the Ni-Pd/Fe 3 O 4 particles is about 60 nm, consisting of stacked tiny nanoparticles (Figure3b,c), leading to the formation of a porous structure.In comparison, the pristine Fe 3 O 4 nanospheres prepared without the Ni and Pd precursor show a hollow nanospherical structure (FigureS1).The STEM images and the energy dispersive X-ray (EDX) elemental mapping show that the Ni and Pd elements are uniformly distributed across the Fe 3 O 4 nanospheres (Figure3d-h).However, the crystallization degree of the Fe 3 O 4 support is not high enough to discriminate Ni or Pd Fe species from the lattice of the Fe 3 O 4 substrate (FigureS2).The SSA and porosity characteristics of the Ni-Pd/Fe 3 O 4 hollow spherical nanocatalyst were analyzed by N 2 adsorption-desorption measurements and determined via Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods.The H2-type N 2 adsorption-desorption isotherm exhibited a distinct hysteresis loop in the desorption branch.The SSA for the Ni-Pd/Fe 3 O 4 was 140.4 m 2 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).

Figure 6 .
Figure 6.Time-dependent UV-vis spectra for (a) 4-NP, (c) CR, and (e) MO reduction by NaBH4 in the presence of Ni-Pd/Fe3O4 catalyst.Insets in Figure 4b,c are the photographs showing the color fading of the reaction mixture.Rate constant time for (b) 4-NP, (d) CR, and (f) MO reduction by NaBH4 in the presence of Ni-Pd/Fe3O4 catalyst.

Figure 6 .
Figure 6.Time-dependent UV-vis spectra for (a) 4-NP, (c) CR, and (e) MO reduction by NaBH 4 in the presence of Ni-Pd/Fe 3 O 4 catalyst.Insets in Figure 4b,c are the photographs showing the color fading of the reaction mixture.Rate constant versus time for (b) 4-NP, (d) CR, and (f) MO reduction by NaBH 4 in the presence of Ni-Pd/Fe 3 O 4 catalyst.

Figure 7 .
Figure 7. (a) The comparison of the catalytic activity (TOF) for the Ni-Pd/Fe3O4 catalyst and the Fe3O4 supported nanocatalysts reported within recent 5 years.Data adapted from Ref. [35,48-66] (b) Recycling of Ni-Pd/Fe3O4 catalyst for the 4-NP reduction by NaBH4 in each cycle.

Figure 7 .
Figure 7. (a) The comparison of the catalytic activity (TOF) for the Ni-Pd/Fe 3 O 4 catalyst and the Fe 3 O 4 supported nanocatalysts reported within recent 5 years.Data adapted from Refs.[35,48-66] (b) Recycling of Ni-Pd/Fe 3 O 4 catalyst for the 4-NP reduction by NaBH 4 in each cycle.
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 H 2 O 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 Fe 3 O 4 hollow nanospheres were synthesized similarly without adding any metal precursor.The mono metal samples (Ni/Fe 3 O 4 and Pd/Fe 3 O 4 ) were synthesized similarly by adding only one active metal precursor (e.g., NiCl 2 •6H 2 O or K 2 PdCl 4 ).

3. 2 .
Catalytic Measurements of Ni-Pd/Fe 3 O 4 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).NaBH 4 (378 mg) was then added into the aqueous 4-NP solution to obtain 4-NP-NaBH 4 mixture solution.After that, Ni-Pd/Fe 3 O 4 catalyst (1.0 mg) was added into the mixture under vigorous stirring at ambient conditions (ca. 25 • C).The bright yellow 4-NP-NaBH 4 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/Fe 3 O 4 or Pd/Fe 3 O 4 (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/Fe 3 O 4 catalyst (2.0 mg) and 0.2 mL ethanol were subsequently added into 3 mL of CR or MO (5.0 mmol/L) and NaBH 4 (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.
Figure S2: TEM image; Figure S3: reaction equations, Figure S4: photographs, Figure S5 and Figure S6: UV-vis spectra, Figure S7: SEM image; Figure S8: XRD pattern; Table S1: catalytic activity comparison of the prepared and previously reported catalysts.Author Contributions: J.X., P.L. (Pei Liu) and P.L. (Ping Li) conceived and supervised the research.D.W., Y.L., and L.W. carried out the materials synthesis and the chemical catalysis.D.W., P.L. (Pei Liu), T.W.H. and Y.L. performed the materials characterizations.J.X., P.L. (Pei Liu), P.L. (Ping Li), and T.W.H. write, review, and edit the manuscript.All authors discussed the results and contributed to the manuscript.All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by the Key Research and Development Program of Hubei Province (grant number 2022BAA026), the Major project of Hubei Provincial Department of Education (grant number D20211502), the Postgraduate Innovation Foundation from Wuhan Institute of Technology (grant number CX2022436), the Open/Innovation Project of Key Laboratory of Novel Biomass-Based Environmental and Energy Materials in Petroleum and Chemical Industry (grant number 2022BEEA06), and the Open Project of Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials and the Carlsberg Foundation (grant number CF20-0612).