Preparation of Superhighly Dispersed Co3O4@SBA-15 with Different

Jun 5, 2014 - Preparation of Superhighly Dispersed Co3O4@SBA-15 with Different Morphologies in Supercritical CO2 with the Assistance of Dilute Acids...
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Preparation of Superhighly Dispersed Co3O4@SBA-15 with Different Morphologies in Supercritical CO2 with the Assistance of Dilute Acids Qin-Qin Xu,† Gang Xu,† Jian-Zhong Yin,*,† Ai-Qin Wang,‡ Yu-Ling Ma,† and Jin-Ji Gao† †

State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

‡

ABSTRACT: In previous reports, organic solvents have usually been used as cosolvents in supercritical fluid applications. In this study, dilute acids were introduced into supercritical CO2 as cosolvents for the first time to improve the adsorption and dispersion of metal precursors on the substrate. Superhighly dispersed Co3O4 nanoparticles were confined in the nanochannels of SBA-15 after deposition and calcination when a small amount of nitric acid was added as the cosolvent. As the concentration of nitric acid increased, more individual small nanoparticles aggregated in adjacent nanochannels through the micropores in the walls of the substrates, which favored the formation of large nanoparticles. Furthermore, only Co3O4 nanoparticles were obtained when nitric acid was added to supercritical CO2−ethanol solution, whereas nanowires were found instead when hydrochloric acid was used. This indicates that H+ favors the adsorption and dispersion of precursors on the substrates and Cl− plays a key role in dictating the nanowire morphology.

1. INTRODUCTION Cobalt oxide nanostructures exhibit promising catalytic behaviors in many reactions such as CO oxidation,1,2 methane combustion,3 propane combustion,4 cyclohexane oxidation,5 and ethyl acetate oxidation.2 In addition, cobalt metallic nanoparticles, obtained by reduction of Co3O4, are involved in hydrogenation reactions, such as the Fischer−Tropsch reaction.6,7 As is well-known, the catalytic performance of a nanocatalyst is dependent not only on its intrinsic electronic properties but also on the size and morphology of the nanostructure. Therefore, the size- and shape-controllable synthesis of cobalt oxide nanostructures is very important. Mesoporous materials with well-defined pore geometries and large surface areas have often been used as hosts to confine nanoparticles or nanowires.8,9 However, it is still challenging to make the size and morphology of nanostructures controllable, and it is even harder to obtain homogeneous metal dispersion and distribution because of low support−precursor interactions in the most commonly used impregnation method.10,11 To achieve high dispersion of the active phase on the support, a change of precursors, for example, using acetates12 or chelated metal complexes13−15 instead of nitrates, and the innovation of post-treatment processes16 were taken into consideration by some researchers. Moreover, some new impregnation methods have been put forward such as the double-solvent method17 and supercritical fluid deposition (SCFD) method.18,19 SCFD is a new, effective, and environmentally benign technique that has attracted growing attention in recent years. Superior to traditional impregnation methods, SCFD can usually avoid the formation of large aggregates of nanoparticles outside the nanochannels as a result of the near-zero surface tension of supercritical fluids.20−24 Supercritical carbon dioxide (scCO2) is the most commonly used solvent in a typical SCFD process, and the dissolution of the precursor in scCO2 is an essential prerequisite for this process. Generally speaking, expensive organometallic compounds have typically been used © 2014 American Chemical Society

as precursors in this method because they have relatively high solubilities in scCO2.19,23,25 Recently, several researchers have used inexpensive inorganic salts instead of expensive organometallic compounds.26−30 Inorganic salts are not soluble in scCO2, so cosolvents such as methanol, ethanol, and acetone are usually applied to modify the polarity of the solvent and to enhance the dissolution of the precursor. In this study, a small amount of dilute nitric acid (HNO3) or hydrochloric acid (HCl) was introduced into scCO2 as the cosolvent for the first time to synthesize Co3O4@SBA-15, using Co(NO3)2·6H 2O as the precursor. The aim of adding dilute acid is not limited to the enhancement of the dissolution of the precursor but also focuses on the change in the interaction between the precursor and the substrates, which can consequently influence the dispersion and morphology of nanostructures. To confirm whether the pathway is much more versatile for other nitrate precursors, we describe Co3O4 on SBA-15 as an example, but we also prepared Ag@SBA-15 using AgNO3 as the precursor with a mixture of ethanol and a small amount of dilute acid as the cosolvent.

2. EXPERIMENTAL SECTION 2.1. Materials. Co(NO3)2·6H2O and CoCl2·6H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. Carbon dioxide (>99%) was obtained from Dalian GuangMing Special Gas Products Co., Ltd. Ethanol, ethylene glycol, hydrochloric acid, and nitric acid were all of analytical grade and were supplied by Tianjin Fuyu Fine Chemical Co., Ltd. Mesoporous SBA-15 with a hexagonally ordered pore structure was prepared using the method reported by Zhao et al.,31 and it had a Brunauer−Emmett−Teller (BET) surface area of 751 m2/g and Received: Revised: Accepted: Published: 10366

March 25, 2014 May 29, 2014 June 5, 2014 June 5, 2014 dx.doi.org/10.1021/ie501241f | Ind. Eng. Chem. Res. 2014, 53, 10366−10371

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Figure 1. Co3O4@SBA-15 prepared using 2 mL of ethanol and 0.5 mL of 1 mol/L HNO3 as the cosolvent: (a,b) TEM images, (c) XRD pattern, (d) N2 adsorption−desorption isotherm.

metal loading was measured on an Optima 2000 DV inductively coupled plasma-atomic emission spectrometer. The surface areas, pore volumes, and pore sizes of the substrate and nanocomposite were determined from N2 adsorption− desorption isotherms obtained at 77 K on a Micromeritics ASAP2010 analyzer.

an average pore diameter of 4.8 nm according to the N2 adsorption−desorption isotherm. 2.2. Deposition Process. Co(NO3)2·6H2O (100 mg) and cosolvent were loaded in the bottom of the reactor (effective volume of 84 mL). A stainless steel basket containing 200 mg of SBA-15 was fixed in the upper part of the reactor to avoid the direct contact of the precursor and substrate. The reactor was subsequently sealed and connected to the lines, preheated at 50 °C for 1 h, and then charged with CO2 using a piston pump until the pressure of the system got up to 18 MPa. The pressure and temperature were held constant for 1 h for the deposition process, and then the system was slowly depressurized. Finally, the samples in the basket were calcined in air for 4 h at 350 °C to obtain Co3O4@SBA-15 composites. Every experiment was repeated at least three times, and the results including the metal loading and TEM images were investigated to evaluate the reproducibility of the experiments. 2.3. Characterization. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and inductively coupled plasma-atomic emission spectroscopy (ICP). XRD was used to obtain the phase composition of the composites in continuous mode from 10° to 90° (2θ) at a scanning speed of 5°/min on a PW3040/60 X’Pert PRO (PANalytical) diffractometer using a Cu Kα radiation source (λ = 0.15432 nm), operated at 40 kV and 30 mA. The morphologies of the Co3O4 nanostructures on the substrates were examined by TEM on a JEOL 2000EX electron microscope operating at an accelerating voltage of 120 kV. The

3. RESULTS AND DISCUSSION 3.1. Preparation of Co3O4@SBA-15 Using Ethanol and Nitric Acid as the Cosolvent. Ethanol is the most commonly used cosolvent in supercritical fluid processes; however, no Co3O4 nanoparticles or nanowires were found in TEM images when only ethanol was used as the cosolvent. Similar results were obtained using the dual cosolvent of ethanol and ethylene glycol, which was reported to be a very effective cosolvent for the synthesis of Ag@SBA-15 using the SCFD method.26,27 Based on the traditional impregnation method, the addition of a small amount of dilute acid to the precursor-containing solution was expected as the competing adsorption agent that could change the adsorption of the precursor onto the substrate. To the best of our knowledge, this is the first time dilute acid has been introduced in the SCFD method. For the sample prepared using 2 mL of ethanol and 0.5 mL of 1 mol/L HNO3 as the cosolvent, small individual particles were well-distributed on SBA-15, as can been seen in Figure 1a,b. Some small particles in adjacent pores were connected with each other, but no particles were found outside the nanochannels. A representative XRD pattern of Co3O4/SBA-15 10367

dx.doi.org/10.1021/ie501241f | Ind. Eng. Chem. Res. 2014, 53, 10366−10371

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Figure 2. TEM images of Co3O4@SBA-15 using 2 mL of ethanol and 0.5 mL of dilute HNO3 at different concentrations: (a,b) 0.5, (c,d) 1.5, and (e,f) 2 mol/L.

is presented in Figure1c. It clearly shows characteristic diffraction peaks at 31.3°, 36.8°, 44.8°, 59.4°, and 65.2°, corresponding to the (220), (311), (400), (511), and (400) reflections, respectively, of crystalline Co3O4 (PDF card 731701). The N2 adsorption−desorption isotherms of SBA-15 and Co3O4@SBA-15 (Figure 1d) show a typical step in the relative pressure (p/p0) range of 0.5−0.8 that is characteristic of a highly ordered mesoporous porous lattice. Compared with the parent SBA-15, the BET surface area of Co3O4@SBA-15 decreased from 751 to 498 m2/g, and the total pore volume of

the sample directly derived from the adsorption isotherm (p/p0 = 0.995) decreased from 0.793 to 0.546 cm3/g. The inset in Figure1d indicates the size distribution of the parent SBA-15 and Co3O4@SBA-15 derived from the Barrett−Joyner− Halenda (BJH) adsorption branch. It can be seen that the average pore size of the substrate decreased from 4.8 to 4.5 nm. The decrease of these parameters demonstrates that Co3O4 was uniformly dispersed inside the pores of SBA-15. The metal loading was 5.78 wt % according to the ICP analysis. 10368

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Figure 3. (a) TEM image and (b) N2 adsorption−desorption isotherms of Co3O4@SBA-15 prepared using ethanol and 0.5 mL of 1 mol/L HCl as the cosolvent. (c) TEM image and (d) XRD pattern of Co2(OH)3Cl@SBA-15 using 1.5 mL of ethanol and 1.5 mL of ethylene glycol as the cosolvent.

interactions between the cations and the supports.12 Because of the low isoelectric point of the silica support,32 higher pH was beneficial to the dispersion of the active phase on the substrates.12 When the pH value was below the isoelectric point of the substrate, the surface of the substrate was positively charged and adsorbed anions first.33 Then, the anions absorbed metal cations, leading to a weak interaction between the substrates and the precursor and, consequently, to the aggregation of more nanoparticles. In this study, no aggregates were found outside the channels mainly because of the nearzero surface tension of scCO2. 3.3. Preparation of Co3O4@SBA-15 Using Ethanol and Hydrochloric Acid as the Cosolvent. In comparison to HNO3, HCl was chosen as an alternative to introduce H+ into the scCO2−ethanol solution. In this case, 2 mL of ethanol was used with 0.5 mL of 1 mol/L HCl as the cosolvent, while the other parameters were kept constant to prepare Co3O4@SBA15 using the SCFD method. It was interesting that discontinuous nanowires instead of nanoparticles were formed, as shown in the TEM image in Figure 3a. The N2 adsorption− desorption isotherm of this sample is shown in Figure 3b. Compared with the parent SBA-15, the BET surface area decreased from 751 to 601m2/g, and the total pore volume decreased from 0.793 to 0.723 cm3/g. It can be clearly seen from the inset in Figure 3b that the average pore size of the substrate decreased from 4.8 to 4.6 nm. The metal loading was 5.63 wt % according to the ICP analysis. Although the metal loading was similar to that of the sample prepared using 2 mL

3.2. Influence of H+ Concentration on the Size of Co3O4 Nanoparticles. To further investigate the influence of H+ on the deposition of Co(NO3)2·6H2O into the channels of SBA-15, a series of experiments were conducted using 2 mL of ethanol plus 0.5 mL of dilute HNO3 at different concentrations of 0.5, 1, 1.5, and 2 mol/L. Co3O4 nanoparticles were highly dispersed on the substrates of the four samples seen in the TEM images in Figures 1a and 2a,c,e. Note that the uniformity of these samples extended beyond the TEM images presented in this article. When the TEM analysis was performed, all of the images for each position of each sample observed by camera were the same as those shown here. In addition, as the concentration of nitric acid increased, the amount of small individual nanoparticles in single pores decreased, and increasing numbers of small nanoparticles in adjacent pores connected with each other. There were almost no small nanoparticles in a single nanochannel at a nitric acid concentration of 2 mol/L. It can be seen from the inset in the upper right of Figure 2f that the size of the nanoparticles was much larger than the diameter of the parent SBA-15. However, ordered channels could be distinguished on the large nanoparticles, which indicated that the large nanoparticles were inside the nanochannels and were composed of the small nanoparticles in adjacent channels due to the micropores in the walls of SBA-15. It seems that the increase of H+ concentration led to the aggregation of precursors. In the traditional impregnation method, the pH of the impregnation solution is often taken into account because it influences the electrostatic 10369

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Figure 4. (a) TEM image and (b) XRD pattern of Ag@SBA-15 using 200 mg of AgNO3 as the precursor, 200 mg of SBA-15 as the substrate, 5 mL of ethanol and 0.5 mL of 1 mol/L HNO3 as the cosolvent.

15 was highly reproducible, so this pathway has great potential in the synthesis of nanocomposites.

of ethanol and 0.5 mL of 1 mol/L HNO3, the decrements of the BET surface area and the pore volume were slightly smaller than those of the other sample. The main reason can be attributed to several large nanoparticles outside the channels of SBA-15 when ethanol was used with HCl as the cosolvent, which can be seen in the TEM images in Figure 3a. It seems that Cl− had a great influence on the morphology of the nanostructure. We tried to apply CoCl2·6H2O instead of Co(NO3)2·6H2O as the precursor and 1.5 mL of ethanol plus 1.5 mL of ethylene glycol as the cosolvent to verify the function of Cl−. The deposition process was conducted at 50 °C and 18 MPa for 1 h, and the obtained sample was calcined at 350 °C. It was found that the morphology of the nanostructure was discontinuous nanowires, as shown in Figure 3c, but the XRD (Figure 3d) analysis indicated that the supported nanowires were Co2(OH)3Cl instead of Co3O4, mainly because of the relatively low calcination temperature. This is auxiliary evidence that Cl− dictates the nanowire morphology. 3.4. Preparation of Ag@SBA-15 Using Ethanol and Nitric Acid as the Cosolvent. Nitrates are the salts most commonly used for the preparation of supported metal oxide catalysts because of their low cost, high solubilities in water, and decomposition at moderate temperatures. However, supported metal oxide catalysts prepared from nitrates generally display heterogeneous dispersions and distributions of the active phase owing to the low support−precursor interactions. In this study, the obtained highly dispersed Co3O4 nanostructures inspired our interest in the universality of this dilute acid-assisted method, although the role of H+ is not very clear at present. An SCFD experiment was conducted using 200 mg of AgNO3 as the precursor, 200 mg of SBA-15 as the substrate, 5 mL of ethanol, and 0.5 mL of 1 mol/L HNO3 as the cosolvent [more ethanol was added than for the Co(NO3)2·6H2O system because AgNO3 has a lower solubility in ethanol]. The TEM image in Figure 4a shows that a large amount of discontinuous silver nanowires were well-defined in the nanchannels of SBA15. The XRD pattern of this sample is shown in Figure 4b and presents four peaks at 2θ values of 38.2°, 44.2°, 64.4°, and 77.1°, demonstrating the formation of metallic Ag. The metal loading was 10.6 wt % according to the ICP analysis. This dilute-acid-assisted SCFD method should therefore be a promising technique for the preparation of a wide range of metal (oxide) supported nanocomposites using different nitrates as precursors and silica as the substrate. Furthermore, in our work, the preparation of Co3O4@SBA-15 and Ag@SBA-

4. CONCLUSIONS In summary, we have developed a new and versatile method that allows the preparation of supported nanocomposites with superhighly dispersed metal or metal oxide nanoparticles using metal nitrates as precursors. The success of this method can be attributed to the near-zero surface tension of scCO2, followed by the addition of a small amount of dilute acid as the cosolvent. As for the preparation of Co 3 O4 @SBA-15, nanostructures with different morphologies were obtained when different kinds of acid were used, although the metal loadings were similar. It seems that H+ favors the adsorption of Co2+ on the nanoscale channels of the mesoporous silica and Cl− plays a key role in dictating the nanowire morphology. More work is underway to determine the underlying reason for the influence of dilute acid on the morphology of the nanostructure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. and Fax: +86-411-84986274. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (20976026, 20976028, 21376045) and the Doctoral Fund of the Ministry of Education of China (20120041110022).

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