Gold Nanoparticles on Mesoporous SiO2-Coated Magnetic Fe3O4 Spheres: A Magnetically Separatable Catalyst with Good Thermal Stability

Fe3O4 spheres with an average size of 273 nm were prepared in the presence of CTAB by a solvothermal method. The spheres were modified by a thin layer of SiO2, and then coated by mesoporous SiO2 (m-SiO2) films, by using TEOS as a precursor and CTAB as a soft template. The resulting m-SiO2/Fe3O4 spheres, with an average particle size of 320 nm, a high surface area (656 m2/g), and ordered nanopores (average pore size 2.5 nm), were loaded with gold nanoparticles (average size 3.3 nm). The presence of m-SiO2 coating could stabilize gold nanoparticles against sintering at 500 °C. The material showed better performance than a conventional Au/SiO2 catalyst in catalytic reduction of p-nitrophenol with NaBH4. It can be separated from the reaction mixture by a magnet and be recycled without obvious loss of catalytic activity. Relevant characterization by XRD, TEM, N2 adsorption-desorption, and magnetic measurements were conducted.


Introduction
Gold was initially regarded as useless in catalysis, until Haruta and co-workers found that small gold nanoparticles supported on some reducible oxide supports can be highly active for CO oxidation [1][2][3].This finding triggered a great deal of interest in exploring the application of gold catalysts in other reactions [4][5][6], such as organic catalysis [7][8][9][10][11].Most of the heterogeneous gold catalysts reported so far involve oxide supports such as TiO 2 , ZrO 2 , Fe 2 O 3 , CeO 2 , Al 2 O 3 , and SiO 2 .These oxide supports are not magnetic, thus making the supported gold catalysts difficult to separate after conducting organic reactions.In addition, many gold catalysts tend to sinter under elevated temperatures due to the low melting points of gold nanoparticles.The sintering can occur even under mild reaction conditions in organic catalysis.Therefore, for the sake of practical applications, it is desirable to design magnetically separable gold catalysts with good thermal stability.
Fe 3 O 4 is a magnetic oxide.It can be used for designing magnetically separable catalysts and other functional materials [12][13][14][15][16][17][18][19][20].For instance, Yin and co-workers prepared SiO 2 /Au/Fe 3 O 4 catalysts by coating an Au/Fe 3 O 4 catalyst with a SiO 2 matrix, followed by controlled etching [21].That way, the gold nanoparticles were protected by the SiO 2 shell, and the SiO 2 shell was porous, allowing for the diffusion of reactants and products.Alternatively, Zhao and co-workers prepared SiO 2 /Au/Fe 3 O 4 catalysts by assembling a porous SiO 2 shell on top of an Au/Fe 3 O 4 catalyst, with the aid of a soft template [22].The resulting SiO 2 /Au/Fe 3 O 4 catalyst has enhanced stability against sintering.These SiO 2 /Au/Fe 3 O 4 catalysts are particularly useful in organic catalysis, because they can be separated from the liquid phase after reaction by simply using a magnet.
Here we prepare another catalyst, Au/m-SiO 2 /Fe 3 O 4 (Scheme 1).First, magnetic Fe 3 O 4 particles were prepared by a solvothermal method.The particles were treated by a small amount of TEOS in the absence of a soft template (CTAB), and subsequently coated by mesoporous SiO 2 (m-SiO 2 ) films with the aid of the soft template.Gold nanoparticles were then deposited onto the m-SiO 2 -coated Fe 3 O 4 support.The resulting catalyst is magnetically separable, thermally stable, and shows better catalytic activity than Au/SiO 2 in the catalytic reduction of p-nitrophenol with NaBH 4 .Scheme 1. Synthesis of Au/m-SiO 2 /Fe 3 O 4 that can be separated by a magnet.

Results and Discussion
Many references have reported the synthesis of Fe 3 O 4 spheres in the presence of a protecting agent [15,[23][24][25].The formation of Fe 3 O 4 spheres generally involves nanocrystal nucleation, crystal growth, and self-assembly [24].Our work used CTAB as a protecting agent.The synthesized Fe 3 O 4 particles are spherical, as seen from the TEM and SEM images in Figure 1.The particle size distribution obtained from TEM analysis of a number of particles is shown in Figure S1 in the Supplementary Materials.The average particle size is 273 nm.To synthesize m-SiO 2 /Fe 3 O 4 , the Fe 3 O 4 particles were first treated by a small amount of TEOS, resulting in the modification of Fe 3 O 4 particles with a thin layer of SiO 2 (see Figure S2 in the Supplementary Materials).Then the SiO 2 -modified Fe 3 O 4 particles were treated with more TEOS in the presence of a soft template (CTAB), followed by calcination to remove the soft template, resulting in the formation of m-SiO 2 /Fe 3 O 4 .The average size of m-SiO 2 /Fe 3 O 4 particles is about 320 nm, as seen from the TEM image in Figure 3. Figure 3 also shows that the thickness of the SiO 2 layer is about 27 nm, and the SiO 2 layer is porous.Figure S3 shows more TEM images of the sample.The material has a high surface area of 656 m 2 /g and an average pore size of 2.5 nm (Figure 4).For comparison, the surface area of Fe 3 O 4 particles is only 30 m 2 /g. ) Pore size (nm)

nm
The SiO 2 coating is amorphous, as shown by the XRD data (Figure 2b,c).Note that the material was calcined at 500 °C to remove the soft template and create mesopores.This calcination process increases the crystallinity of the Fe 3 O 4 particles, as seen from the sharper peaks in Figure 2c.
Figure 5 shows the magnetization curves of samples.The saturated susceptibility of Fe 3 O 4 spheres is 65.0 emu/g.The modification of Fe 3 O 4 spheres by a thin layer of SiO 2 leads to a negligible decrease in saturated susceptibility.The saturated susceptibility of m-SiO 2 /Fe 3 O 4 is 24.7 emu/g.The loading of gold nanoparticles onto m-SiO 2 /Fe 3 O 4 leads to a negligible decrease in saturated susceptibility.Thermal stability is an important factor for practical applications of gold catalysts.Figure 6 shows the TEM images of Au/m-SiO 2 /Fe 3 O 4 samples as prepared and calcined at 500 °C.For the as-prepared Au/m-SiO 2 /Fe 3 O 4 , the gold nanoparticles are highly dispersed, with an average particle size of 3.3 nm.The particle size distribution is shown in Figure S4.When the Au/m-SiO 2 /Fe 3 O 4 is calcined at 500 °C for 2 h, the average gold particle size increases only slightly to 3.8 nm.On the other hand, the average size of gold nanoparticles in Au/SiO 2 increases from 3.4 nm to 13.4 nm after calcination at 500 °C.Although the average size of gold nanoparticles (3.3 nm) is larger than the average pore size of the mesoporous SiO 2 coating, the presence of mesoporous SiO 2 still mitigates the sintering of gold nanoparticles.
Catalytic reduction of p-nitrophenol by NaBH 4 was chosen to compare the performance of different gold catalysts.The reaction was carried out in a cuvette (rather than a round-bottom flask reported previously [26]) to allow for in situ monitoring.The reaction progress was followed by UV-Vis as the peak around 400 nm corresponds to the absorption of p-nitrophenol. Figure 7 shows the decreases in p-nitrophenol concentrations as the reaction proceeds.A faster decrease indicates higher catalytic activity.The conversions of p-nitrophenol on Au/m-SiO 2 /Fe 3 O 4 and Au/SiO 2 after 90 s reaction are 72.5% and 28.2%, respectively.After calcination at 500 °C, these catalysts show conversions of 49.4% and 4.5%, respectively.The catalysis data (also seen in Figure S5 and Table S1) again show the advantage of using a mesoporous SiO 2 coating.As the catalytic activity drops greatly when the size of gold nanoparticles is increased [26], the low activity of the sintered Au/SiO 2 catalyst is justified.The catalyst recyclability was studied by testing the activity of Au/m-SiO 2 /Fe 3 O 4 after separation using a magnet (see the photos in Scheme 1).No additional catalyst was added into the liquid phase.As shown in Figure 8, Au/m-SiO 2 /Fe 3 O 4 shows stable activity in repeated runs.

Preparation of Fe 3 O 4 Spheres
Anhydrous FeCl 3 (0.54 g), CTAB (0.2 g) and sodium acetate (2.5 g) were dissolved in ethylene glycol (50 mL), transferred into an autoclave, and subjected to solvothermal treatment at 180 °C for 24 h.The obtained sample was washed by anhydrous ethanol and deionized water several times, and dried at room temperature.

Preparation of m-SiO 2 /Fe 3 O 4
Fe 3 O 4 spheres (0.1 g) were dispersed in deionized water (100 mL), then aqueous ammonia (3 mL) and TEOS (0.3 mL) were added, and the mixture was stirred mechanically for 1 h.The solid was collected by a magnet, washed with deionized water and ethanol, and dried at room temperature.
The solid product mentioned above (0.1 g) was mixed with deionized water (100 mL).CTAB (0.2 g) aqueous ammonia (3 mL) was added, and the mixture was stirred at room temperature for 30 min.TEOS (2 mL) was then added, and the mixture was continuously stirred for 8 h.The obtained product

Figure 7 .
Figure 7. Decrease in p-nitrophenol's relative concentration during the hydrogenation reaction using different catalysts.

Figure 8 .
Figure 8. Performance of Au/m-SiO 2 /Fe 3 O 4 in repeated runs, after recycling of the catalyst.