Cu2O@TiO2 core-shell microspheres for naphthalene oxidation

New Cu2O@TiO2 core-shell microspheres were successfully prepared for the first time in this paper. The XRD, N2 adsorption-desorption, SEM, TEM, EDX and XPS characterizations were utilized to investigate the physical and chemical properties. The liquid phase oxidation of naphthalene was also carried out to test their catalytic performance. The characterization results indicating that the Cu2O microspheres were firstly formed by hydrothermal treatment and the rutile TiO2 coating on the surface would be formed by the hydrolysis of tetrabutyl titanate. The Cu2O@TiO2-5.0 catalyst with the molar ratio of copper to titanium species as high as 5.0 has the largest surface area and maximum pore volume resulting from the integrated microspheres with rougher surface thickness of about 6.3 nm, and it showed higher catalytic performance in the liquid phase oxidation of naphthalene. Naphthalene conversion of 43.2%, 1, 4-naphthoquinone selectivity of 26.7% and phthalic anhydride selectivity of 53.4% can be obtained, and it only slightly decreased even after repeated use for 5 times. The method would provide a valuable theoretic reference for the hindrance of Cu2O rapid deactivation and the industrial application of the naphthalene oxidation to produce high valuable chemicals.

such as 1, 4-naphthoquinone and phthalic anhydride. Many strategies, including adsorption, biodegradation, oxidation and photocatalysis, etc. have been made, where the oxidation process would be the best candidate owing to its simple device, facile operation and high economic benefits [11][12][13][14]. Up to now, a large series of catalysts with high effective oxidation performance have been explored. Among of them, although silicon-based materials such as modified SBA-15, MCM-41, ZSM-22 and SiO 2 have a relatively good oxidation activity, the complicated catalyst preparation procedure and the harsh reaction conditions limit its application [15][16][17][18]. Instead, non-siliceous materials, especially metal oxides have been widely reported as the effective catalysts in many oxidation processes [19][20][21][22][23].
In our previous reports, Ti-based mesoporous metal oxides or little titanium species introduction would be beneficial to the high catalytic performance in the liquid phase oxidation of naphthalene [24][25][26]. At the same time, the Cu 2 O species with unsaturated monovalence would be the appropriate catalyst in the oxidation process [19,27,28]. It can be found in our reported literature that little copper species incorporated into the Sn-Ti mesoporous oxides would be beneficial to the catalytic activity in the B-V oxidation of cyclohexanone [29]. Therefore, based on the above

Introduction
In recent years, with the increasingly strict environmental protection standards in the chemical industry, the discharged polycyclic aromatic hydrocarbons (PAHs) from petroleum resources consumption have attracted widespread concerns owing to the toxicity, teratogenicity, carcinogenicity and mutagenicity to human body [1][2][3]. In addition, the PAHs would be long-term preserved because it is highly structural stable resulting in its difficulty to degradation [4,5]. Therefore, as a typical substance, naphthalene has been listed as one of the sixteen of PAHs by the European Union (EU) and US Environmental Protection Agency (EPA) [6][7][8][9][10], which would cause an urgent need to develop an effective technology to convert it into other chemicals with high value, analysis, the oxidation performance would show good over the synergistic effect by combination with copper and titanium species.
Up to now, some Cu 2 O/TiO 2 catalysts with heterojunction structure, nanorods or nanoparticles etc., have been explored and they show good catalytic performance simultaneously [30][31][32]. However, the catalytic stability would be severely affected by the Cu 2 O deactivation, which can be attributed that it was directly exposed in the reaction system. Therefore, Cu 2 O microspheres with unique and regular spherical shape resulting in the closed packing nature and lowest surface energy [33] would be the ideal candidate to form core-shell structure by its surface coating [34,35]. The prepared core-shell structural catalyst with the Cu 2 O as the core can effectively protect it to be oxidized resulting in the potential catalytic application in the oxidation process. Therefore, based on the above analysis, the Cu 2 O microspheres as core and titanium species as shell coating on its surface would be expected to achieve the desired results in the oxidation process by their synergistic oxidation effect. However, no relative literature on the work has been reported.
In this paper, the pure Cu 2 O microspheres with copper chloride dehydrate as precursor in the presence of anhydrous glucose as reducer and P123 as template would be firstly prepared by hydrothermal method. After that, the surface coating would be formed by the hydrolysis of tetrabutyl titanate, and the core-shell Cu 2 O@TiO 2 microspheres with different molar ratio of copper to titanium species by annealing treatment under nitrogen atmosphere would be obtained. The detailed schematic diagram for the preparation process was shown in Fig. 1. The prepared Cu 2 O@TiO 2 microspheres were also characterized and it was applied in the liquid phase oxidation of naphthalene to investigate the catalytic activity.

Cu 2 O microspheres
In a typical process, a transparent solution was obtained after 0.268 g of CuCl 2 ·2H 2 O was added into 20 mL of ethylene glycol by continuously stirring for 30 min. A solution containing 0.2 g of P123 dissolved in a 3 mL of ethanol was dropwise introduced by stirring for further 5 min [36]. Subsequently, another solution containing 1.6 g of NaOH dissolved in 3 mL of deionized water was also dropwise added, and it was kept to stand for another 10 min. At last, 0.36 g of anhydrous glucose was added and stirred for 5 min. After that, the mixed solution was transferred into a 50 mL of crystallization kettle with polytetrafluoroethylene lining and treated at 60 ºC for 1 h. The samples was collected by filtration after the kettle was cooled down to room temperature and washed with ethanol and deionized water for three times, respectively. The Cu 2 O microspheres would be obtained when the precipitate was dried under vacuum at 60 ºC for 4 h [37].

Cu 2 O@TiO 2 core-shell microspheres
Firstly, the obtained Cu 2 O microspheres were dissolved in a 40 mL of ethanol followed by ultrasonic treatment for 30 min. Then, 10 mL ethanol solution containing a certain amount of tetrabutyl titanate was added into the above solution and stirred for 2 h. Subsequently, another 10 mL of ethanol solution (containing 5 mL of ethanol and 5 mL of water) was further introduced and stirred for another 2 h [38,39]. And then, the samples were collected by centrifugation, washed with deionized water for three times and dried under vacuum at 60 ºC for 4 h. Finally, the core-shell microspheres denoted as Cu 2 O@TiO 2 -y can be obtained when they were annealed at 1000 ºC for 2 h under nitrogen atmosphere in a tubular furnace, where y represents the molar ratio of copper to titanium species. The detailed preparation conditions for different samples are listed in Table 1.

Characterization of catalysts
The XRD patterns were obtained on a Rigaku Smartlab instrument with Ni-filtered Cu Kα radiation (λ = 0.154 nm) and operated at 40 kV and 40 mA. The scanning range was from 10 º to 80 º. The samples morphologies were recorded using field emission scanning electron microscopy (FE-SEM) on a Hitachi S-4800 instrument. N 2 adsorptiondesorption isotherms were measured at -196 ºC using a BELSORP-MINI volumetric adsorption analyzer. The samples were degassed at 200 ºC for 2 h under vacuum before measurement. The surface area was calculated according to BET method, and the pore size distribution was determined based on the BJH adsorption model. TEM image was recorded on the JEOL JEM-2010 EX transmission electron microscope under 200 kV after the sample was dispersed in ethanol and assisted by an ultrasonic technique. The dispersion of the semi-quantitative elemental composition (Ti, Cu, O) was verified by EDX spectrometer analysis in the Oxford INCA EDAX Detecting Unit. The surface chemical composition was analyzed by XPS on the Thermo Scientific Escalab 250 Xi spectrometer with Al Kα X-ray source (1486 eV).

Catalytic test
The catalytic performance was investigated in the naphthalene oxidation with hydrogen peroxide, which was conducted in a three neck flat bottom flask equipped with a reflux condenser. In a typical procedure, 0.05 g of catalyst was added into the solution containing 1.0 g of naphthalene and 10 mL acetonitrile as solvent. The solution was raised to 60 ºC in a water bath by magnetic stirring, and 2.2 g of hydrogen peroxide was pumped within 30 min. The products would be collected every hour, and it was analyzed on a SP-6890 gas chromatograph equipped with OV-1701 column and a flame ionization detector. The catalytic stability was also investigated when the catalyst was reused in the next test by filtration without any treatment.

Samples characterization
The prepared samples were qualitatively analyzed by XRD patterns, and the results are shown in Fig. 2 attributed to the Cu 2 O micronuclei gather together by reduction of glucose and adsorption by P123 [44,45]. With the titanium species surface coating, the rougher surface for the Cu 2 O@TiO 2 -y samples can be observed, and the integrated microspheres with almost no impurities can be found for the Cu 2 O@TiO 2 -5.0 sample. The calculated average particle size based on the SEM images listed in Table 2 would be promoted from 600.4 to 609.6 nm, which is the further evidence for the success of the surface coating. However, with the titanium species increasing, the microspheres would be partially destroyed and large of irregular particles would be formed resulting in the serious drop of average particle size, suggesting the excessive titanium species coating would act adverse effect on the microspheres preservation, which is coincident with the XRD patterns [46,47]. Table 2 lists the structural properties, such as specific surface area, pore size and pore volume for different samples. It can be seen from Table 2 that the surface area of 3.4 m 2 ·g − 1 and pore volume of 0.004 cm 3 ·g − 1 for pure Cu 2 O sample would be obtained, suggesting the stuffed microspheres formation resulting in the accumulation with the pore size of 3.28 nm [48]. Compared to the pure Cu 2 O 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 m 2 ·g − 1 and the maximum pore volume of 0.082 cm 3 ·g − 1 for the Cu 2 O@TiO 2 -5.0 sample would be obtained, which would be beneficial to 73.5º and 77.3º assigned to (110), (111), (200), (220), (311) and (222) of Cu 2 O (PDF#99 − 0041), respectively, can be observed [40,41]. 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 TiO 2 (PDF#87-0710) [42] can be only observed for the Cu 2 O@TiO 2 -y samples, suggesting the success of the surface coating. Compared to pure Cu 2 O sample, the Cu 2 O typical diffraction peaks intensities for the Cu 2 O@TiO 2 -5.5 sample would be promoted, suggesting that the surface coating would be beneficial to the crystallinity. However, with the titanium species coating loading increasing, the relative crystallinity would be gradually dropped owing to the partial structural collapse probably [43].
The SEM images are shown in Fig. 3. The microspheres for pure Cu 2 O sample with uniform size and slight smooth surface can be found from Fig. 3(a). The reason can be  XPS analysis. From Fig. 5(a), two main peaks with binding energy at 932.4 and 952.2 eV corresponding to Cu(I) 2p 3/2 and Cu(I) 2p 1/2 , respectively, suggesting the monovalent copper coordination [53]. At the same time, the characteristic binding energies at 457.26 and 463.03 eV attributed to Ti 4+ 2p 3/2 and 2p 1/2 can be also found in Fig. 5(b) [40]. The above results indicated that the Cu 2 O as core would not be oxidized by the protection of TiO 2 coating as shell, which is coincident with the XRD patterns and TEM images resulting in the potential application in the naphthalene oxidation [54,55]. Figure 6 was the effect of reaction time on the naphthalene conversion for 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 pure Cu 2 O microspheres, naphthalene conversion was only 8.3% owing to the rapid deactivation. However, naphthalene conversion would be greatly improved by TiO 2 coating to form the core-shell structure, which is may be owing to the catalytic activity protection of Cu 2 O probably [56]. The naphthalene conversion would the catalytic performance probably [49]. 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.

Catalytic performance
To prove the core-shell structure, the prepared samples were characterized by TEM-EDX images. The darkness in the Cu 2 O 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 Cu 2 O microsphere corresponding to its (111) plane can be measured from Fig. 4(b) [50]. Compared to the pure Cu 2 O sample, the Cu 2 O@TiO 2 -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 TiO 2 can be found in the edge [51,52] and its thickness about 6.3 nm was also measured in the Fig. 4(d). The above analysis can be concluded that the core-shell structure with rutile TiO 2 as shell and Cu 2 O as core for the Cu 2 O@TiO 2 -5.0 sample would be successfully prepared in this paper. At the same time, all the elements can be uniformly distributed in the Cu 2 O@TiO 2 -5.0 sample from Fig. 4(e).
The surface chemical state for titanium and copper species in the Cu 2 O@TiO 2 -5.0 sample was also investigated by  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 Cu 2 O 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 [57]. However, for the other four Cu 2 O@TiO 2 -y catalysts, the phthalic anhydride selectivity would much higher than that of 1, 4-naphthoquinone, suggesting that the deep oxidation would happen. The Cu 2 O@TiO 2 -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.
Above the obtained results and the proposed reaction mechanism in our previous literatures [24][25][26], the excellent catalytic performance for the Cu 2 O@TiO 2 -y catalyst in the liquid phase oxidation of naphthalene can be attributed to the synergistic effect of the titanium and copper species with the core-shell structure. The pumped hydrogen peroxide can be easily adsorbed on the shell of rutile TiO 2 and hydroxyl radical would be subsequently generated. At the same time, naphthalene would be attacked to form 1, 4-naphthoquinone. After that, 1, 4-naphthoquinone would be diffused into the core of Cu 2 O and further oxidized to phthalic anhydride.
In order to investigate the novelty in this work, Table 3 listed the comparison of catalytic performance between Cu 2 O@TiO 2 -5.0 and other previous reported catalysts. It can be seen that although the naphthalene conversion for Cu 2 O@TiO 2 -5.0 catalyst was slightly lower than that of Fe/ Beta(100)-hy catalyst, the selectivity and yield of 1, 4-naphthoquinone and phthalic anhydride was much higher than   that of other catalyst, suggesting the remarkable catalytic performance and potential application. Figure 8 was the catalytic reusability for the Cu 2 O@TiO 2 -5.0 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.

Conclusion
In this paper, a series of Cu 2 O@TiO 2 core-shell microspheres with different molar ratio of copper to titanium species were successfully prepared by rutile TiO 2 coating on the pre-synthetic Cu 2 O microspheres. The rough surface with the thickness of about 6.3 nm of rutile TiO 2 on the regular Cu 2 O microspheres for the Cu 2 O@TiO 2 -5.0 sample would be formed. The Cu 2 O@TiO 2 -5.0 sample showed highest catalytic oxidation activity in the liquid phase oxidation of