Mesoporous silica dispersed Co3O4-CuO nanocomposite and its catalytic reduction of 4-nitrophenol

Herein, a catalyst Co3O4-CuO nanocomposite uniformly dispersed on mesoporous silica nanospheres (MSN) has been successfully synthesized through hydrothermal method. The synthesis method is simple and convenient, the prepared Co3O4-CuO nanocomposites have high dispersibility, and the support SiO2 maintains the structure of mesoporous nanospheres. What’s more, the porous structure enables the obtained composite to have a high specific surface area (128.89 m2 g−1), which is easier to be contacted by catalytic substrates. The catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was investigated. The results show that the nanocomposite catalyst Co3O4-CuO@MSN present good catalytic performance, which can completely convert 4-NP to 4-AP in 200 s and the reaction rate constant k is up to 0.149 s−1. More important, the possible degradation mechanism was also proposed in the paper.


Introduction
Water pollution is the main environmental problem faced by today's society, in which 4-nitrophenol (4-NP) is one of the most toxic and refractory organic pollutants and difficult to be removed due to its stability against chemical and biological degradation [1][2][3][4][5]. Recently, different techniques have been developed for the removal of 4-NP pollutants, such as adsorption, membrane filtration, catalytic degradation and so on [6]. To date, the direct catalytic reduction of 4-NP to 4-aminophenol (4-AP) is becoming an important route, because 4-AP has a great importance in the pharmacy and dyeing industry as a potent intermediate [7,8]. Many reports are available on the reduction of 4-NP by noble metal nanoparticles as a catalyst [9][10][11][12]. However, noble metal nanoparticles are expensive and need organic polymers as a capping agent to prevent aggregation [13]. Therefore, to further optimize the composition and structure of catalyst is the focus of current research [14,15].
Transition metal oxides (such as Co 3 O 4 , CuO, Fe 3 O 4 , etc.) with empty electron orbitals can accept and transfer electrons to form relatively stable complexes for rapid reaction, so they are widely used in various catalytic fields. Because the structure of single component oxides is relatively simple, two-component and multicomponent composite oxides have been studied intensively due to their synergistic effects. For example, Chinnappan et al [16]. synthesized three-dimensional flower-like Co 3 O 4 /NiO microspheres for the reduction of 4-NP. Li et al [17]. prepared CuO-Co 3 O 4 @CeO 2 nanoparticles as heterogeneous catalysts by sol-gel method exhibited high catalytic performance. In addition, the existence of multivalent atoms and oxygen vacancies formed by the valence change can promote the hydrolysis of reducing agent NaBH 4 and the adsorption of −NO 2 , thus, it can be used as a catalytic active site for the reduction ofs 4-NP [18,19].
Mesoporous silica is widely used in various fields such as drug delivery [20][21][22], adsorption and separation [23,24], and heterogeneous catalysts [25][26][27][28][29], due to its stable physicochemical properties, non-toxicity, low cost, high specific surface area, high pore volume, easy surface modification, good adsorption capacity and continuously adjustable pore size, etc. So as a support for metal oxides, mesoporous silica is an excellent Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
candidate. Therefore, based on the advantages of transition metal oxides and mesoporous silica, nanocomposite metal oxides were designed to be uniformly dispersed by mesoporous silica with high specific surface area, which is expected to obtain catalysts with high catalytic activity and good stability for the reduction of 4-NP to 4-AP.
In this work, we successfully synthesized mesoporous silica nanospheres (MSN) dispersed nanocatalyst Co 3 O 4 -CuO by hydrothermal method, in which the Co 3 O 4 -CuO nanocomposite had high dispersibility and the support SiO 2 maintained the structure of mesoporous nanospheres. Furthermore, the obtained composite nanomaterial Co 3 O 4 -CuO@MSN was tested as a catalyst for the catalytic reduction of 4-NP, and its catalytic performance depends on its structure.

Preparation of Co 3 O 4 -CuO@MSN composite catalyst
As shown in figure 1, the composite catalyst was prepared by the precipitation method. 2.9860 g of CTAB and 150 ml of deionized water were mixed in the beaker and sonicated for 20 min. It was then magnetically stirred at room temperature and the pH was adjusted to 10 with NaOH solution (2 M). After 15 min of reaction, 2.6770 g of CoCl 2 ·6H 2 O and 0.5063 g of anhydrous CuCl 2 were added to the above solution, followed by continuing the reaction for 30 min. Then, 8.91 ml of TMB was dripped slowly into the beaker to promote the reaction for 15 min. After that, 11.2 ml of TEOS was trickled slowly into the beaker. After the reaction was continued for 24 h, all the suspension in the beaker was moved into a Teflon-lined stainless steel autoclave, which was placed in a blast drying oven at 120°C for 6 h. Finally, the product was centrifuged (8000 rpm, 5 min), washed with deionized water and absolute ethanol alternately. After the above operation was repeated 3 times, the centrifuged precipitate was vacuum-dried at 60°C. The dried solid was passed through a 100 μm sieve and then calcined at 550°C for 6 h to obtain the composite catalyst Co 3 O 4 -CuO@MSN.

Characterization
The morphologies, structures and compositions of all samples were characterized by Field-emission scanning electron microscopy (SEM, ZEISS, GeminiSEM 300) and fully automatic surface and porosity analyzer (BET, McMurray, Mc2460). Powder x-ray diffraction (XRD) patterns were carried out on a Bruker D8 Focus diffractometer (German) with Cu Kα radiation source (λ = 0.1541 Å) at 40 kV, 40 mA. Element type and composition were obtained by using x-ray Photoelectron Spectroscopy Analyzer (XPS, THERMO, NEXSA).    figure 4(B), the magnified Cu 2p peak shows two peaks at 935.19 and 955.13 eV, resulting from the spin-orbit splitting of 2p 3/2 and 2p 1/2 , respectively, which correspond to Cu 2+ . The two satellite peaks at 943.46 eV and 962.63 eV are the vibrational peaks of high binding energy Cu. It can be shown from the figure 4(C) that a strong peak was found at 105.88 eV, corresponding to the characteristic peak of the Si 2p orbital in the Co 3 O 4 -CuO@MSN structure. Furthermore, the strong peak at 533.8 eV corresponds to the lattice oxygen of the oxides in the composite as shown in figure 4(D), and the oxygen in each oxide has the same chemical state. As shown in figure 4(E), there are two salient peaks at the binding energies of 780.13 eV and 795.86 eV, corresponding to Co 2p 3/2 and Co 2p 1/2 , respectively. The two Co 2p 3/2 peaks at 779.52 eV and 781.38 eV are attributed to Co (III) and Co (II), and the two Co 2p 1/2 peaks at 797.87 eV and 796.86 eV are attributed to Co (III) and Co (II), respectively. Combined with the two satellite peaks at 785.92 and 801.75 eV, the successful loading of Co 3 O 4 was proved. Figure 4(F) shows the XPS spectra of C 1 s in Co 3 O 4 -CuO@MSN. The peaks of C 1 s are about 284.76 eV and 286.67 eV, which are related to the binding energies of C-C and C-OH, respectively [30,31]. The above results demonstrate that Co 3 O 4 -CuO@MSN has been successfully prepared, consistenting with the previous XRD test.  377.72 m 2 ·g −1 and 5.48 nm, 128.89 m 2 ·g −1 , respectively. The specific surface area, pore volume and pore size of Co 3 O 4 -CuO@MSN composites are lower than those of MSN due to Co 3 O 4 -CuO nanoparticles will block some of the pores. In general, Co 3 O 4 -CuO@MSN has a relatively high specific surface area, which is beneficial to the improvement of catalytic activity.

Catalytic performance of catalysts
When NaBH 4 was added to the reaction system, the 4-NP solution changed from light yellow to yellow, accompanied by the generation of a large number of air bubbles, as shown in figure 6(A), indicating the formation of sodium 4-NP. At the same time, the UV absorption characteristic peaks from 317 nm to 400 nm ( figure 6(B)). Therefore, the intensity of the peak at 400 nm can represent the concentration of the substrate 4-NP.
In order to test the catalytic activity of Co 3 O 4 -CuO@MSN composite, we explored its catalytic reduction performance of 4-NP at room temperature. To more intuitively see the advantages of Co 3 O 4 -CuO@MSN in catalytic reduction reaction, we also tested the catalytic reduction performance of the comparison material: Co 3 O 4 -CuO nanocomposites, and the obtained UV-vis absorption spectra are shown in figure 6. In the absence of catalyst, the absorption peak did not change. However, after adding the catalyst, the intensity of the  absorption peak at 400 nm was found to decrease, while a new absorption peak appeared at 300 nm, corresponding to 4-AP. As the catalytic reaction continued, it was found that the absorption peak at 400 nm decreased, while the absorption peak at 300 nm increased. When the reaction progressed to 200 s, the absorption peak of the solution with Co 3 O 4 -CuO@MSN at 400 nm disappeared (figure 6(C)), and the absorption peak at 300 nm became the strongest. So far, the catalytic reaction is over, that is, 4-NP has been completely reduced to 4-AP. In contrast, the reaction catalyzed by Co 3 O 4 -CuO cannot be completely degraded, indicating that the Co 3 O 4 -CuO@MSN composite has better catalytic performance. Figure 7(A) shows the catalytic activity of MSN, Co 3 O 4 -CuO and Co 3 O 4 -CuO@MSN on the reduction of 4-NP by NaBH 4 . It can be seen from Figure 7(A) that the conversion rate for the reduction of 4-NP by Co 3 O 4 -CuO@MSN composite has reached 99.3% at 150 s, while that of the Co 3 O 4 -CuO catalyst is only 58.2%. This is due to the large specific surface area of Co 3 O 4 -CuO@MSN, which facilitates the contact between reactants and the catalyst, and significantly improves the activity of the catalyst. As a comparison, the catalytic activity of MSN material was also tested. It can be seen that the 4-NP concentration is only slightly decreased, which may be due to the physisorption of the MSN mesoporous structure. The amount of degradation (D%) of 4-NP was measured using the standard formula, namely D% = (A 0 -A t )/A 0 ×100, where A 0 represented the initial absorbance and A t signified the absorbance at a time (t) [32].  Figure 7B shows the kinetic curves of the catalytic reduction of 4-NP by MSN, Co 3 O 4 -CuO and Co 3 O 4 -CuO@MSN. In the whole catalytic system, due to the large excess of NaBH 4 added, the concentration of BH 4can be considered to be constant during the reaction. Therefore, the catalytic reduction of 4-NP is equivalent to a pseudo-first-order reaction. As can be seen from figure 7, the fitted ln(c t /c 0 ) has a linear relationship with time t, indicating that the reduction of 4-NP by NaBH 4 is consistent with a first-order kinetic model: ln(c t /c 0 )=kt (where k is the rate constant). The reaction rate constants for the catalytic reduction  For comparison, the catalytic activities of various metal-based materials for 4-NP hydrogenation at room temperature are listed in table 1. By comparison, it can be found that Co 3 O 4 -CuO@MSN exhibits excellent catalytic activity. This may be because the composites use MSN as the carrier, which increases the contact area between the catalyst and the substrate. Meanwhile, in the catalytic reduction process, Co 3 O 4 -CuO and MSN exhibited a good synergistic effect.

Stability testing of catalysts
The stability and reusability of Co 3 O 4 -CuO@MSN are crucial for its practical application. Therefore, its recyclability was evaluated and the results are shown in figure 8(A). It can be seen that Co 3 O 4 -CuO@MSN maintains excellent catalytic performance for the degradation of 4-NP during 10 cycles. For the 10th use, approximately 84% of the 4-NP was still degraded within 200 s. Furthermore, compared to fresh catalysts, no obvious changes were observed in and SEM images ( figure 8(B)), XRD pattern (figure 8(C)) and XPS survey scans (figure 4) of the used Co 3 O 4 -CuO@MSN. All the results clearly demonstrate the excellent stability of Co 3 O 4 -CuO@MSN in the catalytic degradation of 4-NP.
Based on the above series of characterizations and anlysis of catalytic test results, the mechanism of catalytic reduction of 4-NP by Co 3 O 4 -CuO@MSN structure was studied (figure 9). For heterogeneous catalytic systems, the catalytic reaction usually occurs on the surface of the catalyst, so the catalytic activity of the catalyst is largely determined by its surface structure [45,46]. In the reaction system of catalytic reduction of 4-NP, after adding NaBH 4 , 4-NP was converted into sodium p-nitrophenolate ions adsorbed on the surface of

Conclusions
In this work, mesoporous silica was used as the support to disperse nanocomposite metal oxide catalysts, which have large specific surface area and abundant catalytic active sites, which effectively improves the catalytic reaction activity. The synthesized Co 3 O 4 -CuO @MSN composite can completely convert 4-NP to 4-AP in 200 s and the reaction rate constant k is up to 0.149 s −1 . This work provides meaningful guidance for the catalytic conversion application of 4-NP.