The preparation of a highly dispersed nanosphere with the desired properties has been intensively pursued not only for the fundamental scientific interest of the nanomaterials, but also for their wide technological applications. Up to the present, different methods, such as the Stöber method, a layer-by-layer deposition, a sol-gel process, or a hydrothermal method, etc., have been developed to prepare a highly dispersed nanosphere 12345. Various monocomponent nanospheres including SiO₂, Fe₂O₃, CuO, ZnS, or metal materials Au and Pt could be successfully obtained 45678. These materials showed good properties during utilization in gas sensors, biomedicine, electrochemistry, catalysis, etc. Furthermore, for the demand of the application, much effort has been devoted to prepare a bi- or multicomponent nanocomposite 91011121314. Among these materials, silica was often utilized as a carrier to disperse the active phase on its surface or in its matrix because silica can not only be easily obtained from several precursors, but also remains stable in most chemical and biological environments. What's more is that the rapid development of the modern nanotechnolgy has supplied flexible methods to modulate the morphology and structure of silica, which could be adopted for the preparation of the SiO₂-based nanocomposite 1516.

Cobalt oxide system or cobalt-silicon mixed oxide is a widely studied system in material domain, which could be used as catalyst for many reactions involving hydrogen transfer, such as methane reforming, oxidation of hydrocarbon, Fischer-Tropsch synthesis, and hydrogenation of aromatics 171819202122. For the bi-component cobalt-silicon mixed oxide, it was acknowledged in the recent studies that the preparation method could show an obvious effect on the type and dispersion of cobalt oxide species, and thus on the catalytic performance of the derived catalysts 232425. For the traditional two-step method, silica was firstly prepared as a support, and then, cobalt species were introduced through ion-exchange, impregnation, or grafting techniques. Compared with this method, one-step condensation method owns it's predominance in that it allows a better control of the textural properties of the silica matrix and a more effective dispersion of cobalt oxide in the matrix on a nanometric scale.

From a particle-preparation point of view, microemulsion method is such a good method to prepare a uniform-sized nanosphere 26272829. The water nanodroplets present in the bulk oil phase serve as nanoreactors to control the size and the distribution of the nanoparticles. While for cobalt-silicon mixed oxide, it seems that the uniform particle size distribution remains a delicate task with the normal sol-gel method or microemulsion methods 3031323334. In our previous work, a modified reverse-phase microemulsion method was successfully adopted to prepare a highly dispersed SiO₂-based nanocomposite 3536. Herein, a similar method was used to prepare cobalt-silicon mixed oxide materials, and the obtained material presents as a kind of highly dispersed, uniform-sized nanosphere. In the catalytic application, this novel nanosphere showed a good activity for the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone.

TEM and SEM were utilized to study the morphology of the material Co-SiO₂. It can be observed in Figure 1a and 1b that the obtained material Co-SiO₂ presented as a highly dispersed, uniform-sized nanosphere, which was further proved by the characterization of SEM (Figure 1c). The distribution of the particle size was centered at about 110 nm (Figure 1d). By comparison, in our previous work, the highly dispersed nanosphere could not be obtained with the normal operation of blending two microemulsions before adding a silicon source 38. A similar situation also happened during the preparation of silica-supported cobalt materials 3031. As pointed out by Boutonnet et al., there are two main ways of preparing nanoparticles from the microemulsion method: (1) by mixing two microemulsions, one containing the precursor and the other, the precipitating agent; and (2) by adding the precipitating agent directly to the microemulsion containing the metal precursor 26. Different with the above two methods, in the present work, the metal precursor was firstly prepared as an aqueous solution of a cobalt ammonia complex, which acted as the water phase in the microemulsion and could also supply a base environment for the hydrolysis of TEOS. No more bases are necessary to be added during the preparation process. This method can also avoid the blending of two microemulsions that might affect the properties of the water droplet in the microemulsion and then affect the morphology of the prepared materials. With the same method, highly dispersed Cu-SiO₂, Ni-SiO₂, and Zn-SiO₂ nanospheres could also be successfully prepared.

The composition of the material Co-SiO₂ was primarily recognized through the XRD pattern measurement, which was shown in Figure 2. As a comparison, the pattern of the C-Co₃O₄ was also supplied in which eight peaks corresponding with the cubic structure of Co₃O₄ with the Fd3m space group can be clearly observed 21. These peaks do not emerge in the pattern of Co-SiO₂, and it shows only a broad peak at 2θ = ca. 22°, which is assigned to the amorphous silica. These results indicate that Co species in Co-SiO₂ are amorphous and/or the particle size is too small 33.

The FTIR spectrum of the material Co-SiO₂ is illustrated in Figure 3. Strong absorption bands at 1,090, 800, and 473 cm^-1 agree well with the SiO₂ bond structure. The band at 1,090 cm^-1 was assigned to the asymmetric stretching vibration of the bond Si-O-Si in the SiO₄ tetrahedron. The band at 800 cm^-1 was assigned to the vibration of the Si-O-Si symmetric stretching vibration. The band at 473 cm^-1 is related to the bending modes of the Si-O-Si bonds 3739. Besides these three bands, one weak shoulder band emerged at 960 cm^-1 that was usually attributed to the Si-OH stretching vibration. The absorption bands at 3,440 and 1,635 cm^-1 were caused by the absorbed water molecules 40. For the as-prepared sample without solvent extraction, intense characteristic absorption bands associated with C-H bond (about 1,500 and 3,000 cm^-1) are evident for the presence of the organic surfactant, which almost disappeared for the spectrum of Co-SiO₂. This indicates that the surfactant could be totally removed with the solvent extraction method.

UV-Vis DRS is a powerful characterization method to study the coordination geometry of cobalt incorporated in the materials, and the spectrum of Co-SiO₂ was shown in Figure 4. Between 450 and 750 nm, this spectrum displays three absorption peaks (525, 584, and 650 nm), which can be unambiguously assigned to the ⁴A₂(F) → ⁴T₁(P) transition of Co(II) ions in tetrahedral environments 4142. Moreover, a broad band in the UV region centered at 224 nm is also observed. This has been assigned to a low-energy charge transfer between the oxygen ligands and central Co(II) ion in tetrahedral symmetry 43. Besides the above absorption, another broad absorption was centered at 356 nm, which was assigned to Co(III) species 44. It could be found in the literature that Co(III) was usually obtained through a heating treatment such as calcination 213233. In the present work, however, Co(II) salt precursor was firstly converted to cobalt(II) ammonia complex during the preparation process. The formation of a Co(II) ammonia complex would decrease the standard potential of Co(III)/Co(II) from 1.84 to 0.1 v, and then Co(III) ions were formed via the automatic oxidation of the Co(II) ammonia complex by dissolved dioxygen. As identified in a previous study 42, the emergence of this absorption was taken as a strong evidence for the presence of a distinct Co₃O₄ phase. So, it can be deduced from the above results that a Co₃O₄ phase exists in the material Co-SiO₂.

In addition, from the characterization result of X-ray absorption spectroscopy near-edge structure [XANES] measurement (Figure 5), the information about the valence state of cobalt ions could be further acknowledged. It was believed that the main-edge should be shifted to a higher energy with the mixing of Co(III) with Co(II), and the distance between the pre-edge peak and the main edge can be used to measure the oxidation state of cobalt ions. Compared with the reference data, Co-SiO₂ has an edge position that is consistent with cobalt ions aligning with Co₃O₄ that contains both oxidation states, not with CoO or CoAl₂O₄ 45. The main-edge emerged at a higher energy (7,726.9 ev) for Co-SiO₂, and the distance between the pre-edge peak and the main edge (E_main-edge - E_pre-edge) reached 17.2 ev. These situations are quite similar with those of Co₃O₄, manifesting that cobalt ions in Co-SiO₂ own a close coordination environment with the cobalt ions in Co₃O₄ 45. This is consistent with the result of UV-Vis DRS.

Selective oxidation of cyclohexane to cyclohexanone and cyclohexanol (the so-called K-A oil) is the centerpiece of the commercial production of Nylon. Although many attempts have been made to develop various catalytic systems for this reaction, it continues to be a challenge 464748. The present industrial process for cyclohexane oxidation is usually carried out above 423 K and 1 to approximately 2 MPa pressure without catalyst or with metal cobalt salt as homogeneous catalyst. For obtaining higher selectivity of K-A oil (about 80%), the conversion of cyclohexane is always controlled by about 4% 48. It is one of the lowest efficient technologies that have been put into application among the present petrochemical domain. The main reason for the low yield of K-A oil is that it is easily overoxidized to the acids and further transformed to other by-products.

In the present work (Table 1), when Co-SiO₂ was used as catalyst for the selective oxidation of cyclohexane, encouraging results were obtained. Under more mild conditions (388 K, which is 35 K lower than that of the industrial process), the conversion reached 6.0%, while the selectivity of K-A oil reached as high as 85.7% at the same time. As a comparison, the commercial C-Co₃O₄ could give a moderate activity with a conversion of 3.8% and a K-A oil selectivity of 78.4%. In addition, compared with the reported data, the predominance of the present Co-SiO₂ is evident. Under the same conditions, when cobalt acetate was used, which was a homogeneous catalyst being widely used in the industrial process, the conversion was only 3.3% and the selectivity of K-A oil was also below 80% 19. Moreover, the activity of Co-SiO₂ is higher than that of the cobalt-containing mesoporous silica [Co-HMS] system (Table 1). Through N₂ physical adsorption-desorption measurement, it could be acknowledged that the BET surface area of Co-SiO₂ is 60 m²/g and average pore size is about 17 nm, respectively, which manifest that most of the pores come from the aggregation of the nanospheres. So, the accessible catalytic active sites of Co-SiO₂ should exist all on the outerface of the nanospheres, which is contrary with the situation for the porous materials such as mesoporous silica or molecular sieves. For those porous materials, most of the catalytic active sites exist on the interface of the pore. Though the surface area of Co-SiO₂ is much lower than that of Co-HMS (682 m²/g) 37, the absence of a long channel of inner pore may facilitate the fast diffusion of the substrate and the oxygenated products. Thus, the primary oxygenated products such as cyclohexanone and cyclohexanol are easily desorbed from the surface of the catalyst, which would decrease the chance for them to be overoxided. This might be the main reason for the evident enhancement of the selectivity for K-A oil. The deeper study of the relationship between the structure of the material and the activity is underway.

With a modified reverse-phase microemulsion method, highly dispersed cobalt-silicon mixed oxide nanosphere was successfully prepared for the first time. The utilization of cobalt ammonia complex as metal source is favorable not only for controlling of the morphology, but also for obtaining a high valence state cobalt without calcination. These two factors are fascinating for the catalytic application, and Co-SiO₂ was found to act as an efficient catalyst for the selective oxidation of cyclohexane. Considering that many kinds of metal ions can be converted to metal ammonia complex, we can extend this method to prepare such highly dispersed SiO₂-based nanocomposite, which might show good application properties for its specific morphology and structure.

The authors declare that they have no competing interests.

JX and CX designed the experiment. QZ and CC carried out the experiment and drafted the manuscript. MW and JC participated in some of the characterizations and performed the data analysis. All authors read and approved the final manuscript.