3.1 Samples characterization
The prepared samples were qualitatively analyzed by XRD patterns, and the results are shown in Fig. 2. For the prepared Cu2O sample, the diffraction peaks at 29.5º, 36.4º, 42.3º, 61.3º, 73.5º and 77.3º assigned to (110), (111), (200), (220), (311) and (222) of Cu2O (PDF#99 − 0041), respectively, can be observed, indicating its successful preparation[39, 40]. It is interestingly found that the diffraction peaks at 27.5º, 54.4º, 56.7º, 69.1º and 69.9º corresponding to (110), (211), (220), (301) and (112) of rutile TiO2 (PDF#87–0710)[41] can be only formed for the prepared Cu2O@TiO2-y samples, suggesting the success of the surface coating. Compared to pure Cu2O sample, the Cu2O typical diffraction peaks intensities for the Cu2O@TiO2-5.5 sample would be promoted, suggesting that the surface coating by titanium species would be beneficial to the crystallinity. However, with the titanium species coating loading increasing, the relative crystallinity would be gradually dropped, which may be owing to the partial structural collapse by excessive titanium species coating[42].
SEM techniques were carried out to investigate the morphology for different samples, and the images are shown in Fig. 3. The microspheres for pure Cu2O sample with uniform size and slight smooth surface can be found from Fig. 3(a). The reason can be attributed to the Cu2O micronuclei gather together by reduction of glucose and adsorption by P123[43, 44]. After the titanium species surface coating, the rougher surface for the prepared Cu2O@TiO2-y samples can be observed, and the integrated microspheres with almost no impurities can be found for the Cu2O@TiO2-5.0 sample. However, with the titanium species increasing, the microspheres would be destroyed and large of impurities particles would be formed, suggesting the excessive titanium species coating would act adverse effect on the microspheres preservation, which is coincident with the XRD patterns[45, 46].
Table 1
Structural property for different samples
Samples
|
SBET
(m2·g− 1)
|
dp
(nm)
|
Vm
(cm3·g− 1)
|
Cu2O
|
3.4
|
3.28
|
0.004
|
Cu2O@TiO2-5.5
|
17.3
|
3.71
|
0.067
|
Cu2O@TiO2-5.0
|
19.0
|
3.71
|
0.082
|
Cu2O@TiO2-4.5
|
10.3
|
3.72
|
0.033
|
Cu2O@TiO2-4.0
|
4.5
|
3.29
|
0.003
|
Table 1 lists the structural property, such as specific surface area, pore size and pore volume for different samples. It can be seen from Table 1 that the surface area of 3.4 m2·g− 1 and pore volume of 0.004 cm3·g− 1 for pure Cu2O sample would be obtained, suggesting the stuffed microspheres formation resulting in the accumulation with the pore size of 3.28 nm[47]. Compared to the pure Cu2O sample, the above three values would be promoted owing to the successful surface coating by porous titanium species resulting in the formation of microspheres with rougher surface. The largest surface area of 19.0 m2·g− 1 and the maximum pore volume of 0.082 cm3·g− 1 for the Cu2O@TiO2-5.0 sample would be obtained, which would be beneficial to the catalytic performance probably[48]. With the further surface coating with more titanium species, they would be dropped because of the partial structural collapse based on the XRD patterns and SEM images.
To prove the core-shell structure, the prepared samples were characterized by TEM-EDX images. The darkness in the Cu2O microsphere with about 600 nm can be found from Fig. 4(a), further indicating that the stuffed microsphere was formed. The interplanar distance of 0.24 nm in the edge of Cu2O microsphere corresponding to its (111) plane can be measured from Fig. 4(b)[49]. Compared to the pure Cu2O sample, the Cu2O@TiO2-5.0 sample with slightly larger size of about 610 nm from Fig. 4(c) can be observed. And the interplanar distance of 0.35 nm assigned to the (101) plane of rutile TiO2 can be found in the edge of the Cu2O@TiO2-5.0 sample[50, 51], and the thickness for the TiO2 coating about 6.3 nm was also measured in the Fig. 4(d). The above analysis can be concluded that the core-shell structure with the TiO2 as shell and Cu2O as core for the Cu2O@TiO2-5.0 sample would be successfully prepared in this paper. At the same time, all the elements can be uniformly distributed in the Cu2O@TiO2-5.0 sample from Fig. 4(e).
The surface chemical state for titanium and copper species in the Cu2O@TiO2-5.0 sample was also investigated by XPS analysis. From Fig. 5(a), two main peaks with binding energy at 932.4 and 952.2 eV corresponding to Cu(I) 2p3/2 and Cu(I) 2p1/2, respectively, suggesting the monovalent copper coordination was present[52]. At the same time, the characteristic binding energies at 457.26 and 463.03 eV attributed to Ti4+ 2p3/2 and 2p1/2 can be also found in Fig. 5(b)[39]. The above results indicated that the Cu2O as core would not be oxidized by the protection of TiO2 coating as shell, which is coincident with the XRD patterns and TEM images resulting in the potential application in the naphthalene oxidation[53, 54].
3.2 Catalytic performance
Figure 6 was the catalytic performance of different catalysts in the liquid phase oxidation of naphthalene. It can be seen from Fig. 6 that naphthalene conversion would be promoted with the reaction time increasing. However, for the prepared pure Cu2O microspheres, naphthalene conversion was only 8.3% owing to the rapid deactivation. However, naphthalene conversion would be greatly improved by TiO2 coating to form the core-shell structure, which is may be owing to the catalytic activity protection of Cu2O probably[55]. The naphthalene conversion would be the maximum of 43.2% over the Cu2O@TiO2-5.0 catalyst under reaction at 8 h, which is about 5 times than that of pure Cu2O microspheres as catalysts. The above results can be attributed to the reason that the Cu2O@TiO2-5.0 catalyst has the largest surface area and the maximum pore volume resulting from relatively rougher surface with some pores based on the N2 adsorption-desorption isotherms, SEM and TEM characterizations probably.
Figure 7 showed the naphthalene conversion and phthalic anhydride and 1, 4-naphthoquinone selectivity over different catalysts reacted at 8 h, where the naphthalene conversion was the maximum, respectively. It is interestingly found that the 1, 4-naphthoquinone selectivity of 6.5% was much higher than phthalic anhydride selectivity of 4.3% over pure Cu2O microspheres. The above phenomenon indicated that it would rapidly deactivate resulting in the poor activity for the formation of phthalic anhydride by the continuous and deep reaction of 1, 4-naphthoquinone[56]. However, for the other four Cu2O@TiO2-y catalysts, the phthalic anhydride selectivity would much higher than that of 1, 4-naphthoquinone, suggesting that the deep oxidation would happen. The reason can be concluded that the core-shell structure by TiO2 coating on the Cu2O surface would be beneficial to the protection of the catalytic oxidation activity. At the same time, the Cu2O@TiO2-5.0 catalyst was the best candidate, where naphthalene conversion of 43.2%, 1, 4-naphthoquinone selectivity of 26.7% and phthalic anhydride selectivity of 53.4% can be obtained.
Figure 8 was the catalytic reusability for the Cu2O@TiO2-5.0 core-shell microsphere catalyst. It can be seen that the catalytic performance would slightly be a downward trend with the increase of reused times. The naphthalene conversion of 39.3%, 1, 4-naphthoquinone selectivity of 25.0% and phthalic anhydride selectivity of 48.3% can be also obtained even after repeated reaction for 5 times, which would be a potential application in the industrial process.