Nanoflake-assembled Al2O3-supported CeO2-ZrO2 as an efficient catalyst for oxidative dehydrogenation of ethylbenzene with CO2

Accepted Manuscript Title: Nanoflake-assembled Al2 O3 -supported CeO2 -ZrO2 as an efficient catalyst for oxidative dehydrogenation of ethylbenzene with CO2 Author: Tehua Wang Xiaolin Guan Huiyi Lu Zhongwen Liu Min Ji PII: DOI: Reference:

S0169-4332(16)32623-X http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.180 APSUSC 34485

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APSUSC

Received date: Revised date: Accepted date:

11-10-2016 21-11-2016 23-11-2016

Please cite this article as: Tehua Wang, Xiaolin Guan, Huiyi Lu, Zhongwen Liu, Min Ji, Nanoflake-assembled Al2O3-supported CeO2-ZrO2 as an efficient catalyst for oxidative dehydrogenation of ethylbenzene with CO2, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.180 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nanoflake-assembled Al2O3-supported CeO2-ZrO2 as an efficient catalyst for oxidative dehydrogenation of ethylbenzene with CO2

Tehua Wang a, Xiaolin Guan a, Huiyi Lu b,*, Zhongwen Liu c, Min Ji a,*

a

College of Chemistry, Faculty of Chemical, Environmental and Biological Science and

Technology, Dalian University of Technology, Dalian 116023, China b

Department of Pharmacy, The Second Affiliation Hospital of Dalian Medical University, Dalian

116027, China c

Key Laboratory of Applied Surface and Colloid Chemistry MOE, School of Chemistry &

Materials Science, Shaanxi Normal University, Xi’an 710062, China.

Highlights • The slit-shape pores of nanoflake-assembled Al2O3 favored the dispersion of CexZr1-x(OH)4. • The dispersion and Ce/Zr surface ratio of CeO2-ZrO2 species were improved. • The catalytic efficiency of CeO2-ZrO2 species was significantly enhanced.

Abstract An Al2O3 material assembled by nanoflakes was used to prepare supported CeO2-ZrO2 catalyst via a deposition-precipitation method for oxidative dehydrogenation of ethylbenzene with CO2. Both unsupported and commercial Al2O3-supported CeO2-ZrO2 were prepared for comparison. It was

found that the CeO2-ZrO2/nanoflake-assembled Al2O3 catalyst exhibited the best catalytic activity. The characterization results revealed that the slit-shape pores existing in nanoflake-assembled Al2O3 were responsible for the small particle size and high Ce/Zr surface ratio of supported CeO2-ZrO2 species. The dispersion of Ce1-xZrx(OH)4 precursors onto Al2O3 support surface during the deposition-precipitation process was proposed. The high dispersion and large numbers of surface oxygen vacancies of the CeO2-ZrO2 species on nanoflake-assembled Al2O3 contributed to the excellent catalytic performance in oxidative dehydrogenation of ethylbenzene with CO2. This kind of special Al2O3 is expected to be a promising support for preparing highly dispersed metal/metal oxide catalysts. Key words: nanoflake-assembled alumina; ceria-zirconia; ethylbenzene; dehydrogenation; carbon dioxide. 1 Introduction The utilization of CO2 as a soft oxidant in the oxidative dehydrogenation of light alkanes and ethylbenzene (EB) has received significant attentions in recent years [1-4]. Styrene (ST), an important monomer for synthetic polymers, is commercially produced by EB dehydrogenation in the presence of a large quality of superheated steam, which serves as a diluent, a heat carrier, and a decoking reactant [5-7]. However, the unavoidable loss of heat in the steam condensation leads this process to be highly energy-consuming. Oxidative dehydrogenation of ethylbenzene with CO2 (CO2-ODEB) is a promising route to replace the above commercial process, since it has been estimated that the amount of required energy for yielding per ton ST can be decreased from 1.5×106 kcal via the commercial process to 1.9×105 kcal via the CO2-ODEB route [6]. Furthermore, several advantages such as accelerating the reaction rate, alleviating the chemical

equilibrium and enhancing the ST yield are also proposed [7]. Therefore, the development of efficient catalysts for this energy-saving process is highly desirable. Ce1-xZrxO2 solid solutions are important materials as catalysts or catalyst supports for many redox reactions due to the excellent redox properties [8-11]. The presence of cerium can provide a better reduction capacity for catalyst at all the temperatures studied, in contrast with the other non-cerium catalysts. This is consistent with the well-known oxygen storage/release capacity of ceria owing to oxygen vacancies in the catalyst [12]. In the field of CO2-ODEB, Ce1-xZrxO2-based materials have been extensively investigated and show satisfactory performances [13-17]. The catalytic performance of Ce1-xZrxO2 highly depends on its physicochemical properties, including the crystalline phase, particle size, special surface area, surface composition and density of oxygen vacancy. Unfortunately, unsupported Ce1-xZrxO2 catalysts are susceptible to sintering at high temperatures [18]. An effective method to overcome this drawback is to stabilize Ce1-xZrxO2 on thermally stable supports such as SiO2, Al2O3, TiO2 and MFI [19-22]. Al2O3 materials have been widely used as catalyst supports owing to the large surface area and both chemical and thermal stability. Their surface properties not only influence the dispersion of active species but also tailor the chemical environment of active sites. In past several years, a special Al2O3 assembled by nanosheets, nanoflakes or nanoplates, which usually presents hierarchically flower-like microstructure, has been developed and used as support for metal catalysts [23-25]. It exhibits some unexpected advantages especially in the improvement of metal dispersion. In a previous work [23], a flower-like Al2O3@Co-Cu-Al catalyst was designed via a layered double hydroxide precursor route for the oxidation of EB. This heterogeneous Co-based catalyst exhibited a high dispersion of cobalt species due to the interactions between the Co-Cu

species in the well-developed three-dimensional flower-like platelets. Zhang et al. [24] synthesized flower-like Ni/γ-Al2O3 composites by a facile hydrothermal method for methane dry reforming. The hierarchical spinel intermediates embedded Ni nanoparticles in nanoflakes of the hollow Al2O3 microspheres, and the strong metal-support interactions resulted in a high dispersion of Ni. More recently, Zhao et al. [25] prepared hierarchical core-shell Al2O3@Pd-CoAlO microspheres with a flower-like shell assembled by nanosheets. The excellent catalytic efficiency of this novel catalyst for toluene combustion was attributed to the homogeneous distribution of catalytically active Pd-CoAlO nanosheets on the Al2O3 supports. The above reports indicate that this kind of hierarchical Al2O3 support is beneficial to the formation of highly dispersed metal species on catalyst surface. Herein, a nanoflake-assembled Al2O3 synthesized by a hydrothermal method was used to prepare supported CeO2-ZrO2 catalyst via a deposition-precipitation method. It was found that the special pore structure of nanoflake-assembled Al2O3 can effectively improve the dispersion and Ce/Zr surface ratio of CeO2-ZrO2 species, which further determined its excellent catalytic performance in CO2-ODEB reaction. 2 Experiments 2.1 Catalyst preparations Nanoflake-assembled Al2O3, labeled as NFA, was prepared via a hydrothermal method according to a previous literature [26]. A commercial Al2O3 support, labeled as CA, was purchased from Sinopharm Chemical. Supported CeO2-ZrO2 catalysts were prepared by a previous deposition-precipitation method with some modifications [20]. Typically, 1 g of NFA or CA powders was dispersed into 400 ml of

deionized water under vigorous stirring for 2 h. 40 ml of aqueous solution containing 1.45 mmol of (NH4)2Ce(NO3)6 and 1.45 mmol of Zr(NO3)4·5H2O was dropped into the suspension. Then, 360 ml of additional deionized water was added and the resultant suspension was stirred for another 1 h. Thereafter, the pH of the solution was adjusted to ~8 using diluted NH3·H2O (2.5 wt%) under vigorous stirring. The light yellow solids were immediately filtered and washed with deionized water for several times, followed by drying at 100 oC overnight. Finally, the supported CeO2-ZrO2 catalysts, labeled as CZ/NFA or CZ/CA, were obtained by calcination of above solids at 600 oC for 4 h in static air. The percentage of CeO2-ZrO2 in the catalysts was 30 wt% and the nominal Ce/Zr ratio was 1. For comparison, unsupported CeO2-ZrO2 catalyst, labeled as CZ, was prepared following the above procedure except that no support was introduced. 2.2 Characterization techniques Field emission scanning electron microscopy (FESEM) images were obtained on a Zeiss Supra 55 microscope. High resolution transmission electron microscopy (HRTEM) images were recorded on a FEI Tecnai G2 20 S-TWIN. X-ray diffraction (XRD) patterns were acquired on a Panalytical X’Pert Pro MPD diffractometer with Cu Kα radiation (0.15406 nm) at 40 kV and 40 mA. N2 physisorption experiments were performed on an automatic analyzer (3H-2000PM1, Beishide, China) at -196 oC. All samples were outgassed under vacuum at 200 oC for 2 h prior to the measurement. The special surface area was calculated by the BET method. The pore volume was taken at the P/P0=0.98 single point. The pore size distribution was obtained by applying the BJH formalism to both the adsorption and desorption branch of the isotherm. H2 temperature programmed reduction (H2-TPR) experiments were performed on a chemical adsorption instrument (PCA-1200, Builder, China). 50 mg of sample was pretreated at 300 oC for

30 min in N2 (40 ml/min). After cooling to room temperature, TPR experiment was carried out in H2/N2 (5%/95% in volume, 30 ml/min) in the range of 100 to 900 oC with a heating rate of 10 oC/min.

UV-Vis diffuse reflectance (UV-Vis DR) spectra were recorded on a Jasco V-550 spectrophotometer in the range of 200 to 800 nm referenced to BaSO4. UV Raman spectra were collected in the anti-Stokes range of 100-2000 cm-1 on a DL-2 Raman spectrometer using a He-Gd laser (excitation wavelength of 325 nm). X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo VG ESCALAB 250 Microprobe instrument using Al Kα radiation as the X-ray source. The binding energy of the element was calibrated using a C 1s photoelectron peak at 284.6 eV. 2.3 Catalytic tests CO2-ODEB reaction was carried out in a fixed-bed quartz tubular reactor with an inner diameter of 4 mm at 600 oC under an atmospheric pressure. Before the reaction, 0.3 g of catalyst was placed at the center of the reactor with quartz wool plugs and then purged with N2 at 600 oC for 1 h. The reaction was started by changing the atmosphere from N2 to a reaction mixture gas of CO2 and EB vapours with a molar ratio of 20, which was produced by bubbling a CO2 flow (20 ml/min) through a liquid EB held at 50 oC in a thermostat. The cooling products were analyzed by gas chromatography on Agilent-6890 equipped with a FID detector and capillary column of HP-5. 3 Results and discussions 3.1 Physicochemical properties of the catalyst materials The morphologies of CA and NFA supports were determined using FESEM. Fig. 1a shows the FESEM image of CA support, which indicates that the sample is composed by a large number of

irregular particles. By contrast, NFA support consists of numerous nanoflake-assembled architectures, as shown in Fig. 1b. A closer inspection in Fig. 1c reveals that the thickness of these nanoflakes is about 40 nm.

Fig. 2 shows the HRTEM images of CZ, CZ/CA and CZ/NFA catalysts. The lattice fringes with the interplanar spacing of 0.31 nm associated with the cubic (111) or tetragonal (111) facet of Ce1-xZrxO2 solid solutions suggests that the CeO2-ZrO2 species of these catalysts were well crystallized after calcination [27, 28]. The average particle sizes are 7.0, 4.5 and 4.0 nm for CZ, CZ/CA and CZ/NFA, respectively. The significant decrease in the particle size of Ce1-xZrxO2 nanoparticles in the supported catalysts, especially in CZ/NFA, implies the improved dispersion of CeO2-ZrO2 species.

Fig. 3 shows the N2 adsorption-desorption isotherms of both the supports and the CeO2-ZrO2 catalysts. All samples display non-reversible adsorption-desorption isotherms with hysteresis loops characteristic of capillary condensation in mesopores. The hysteresis loops of CZ, CA support, and CZ/CA are classified as type H2 and suggest the presence of intraparticle mesopore [29]. By comparison, NFA support and CZ/NFA exhibit the hysteresis loops of type H3, reflecting the slit-shape pores associated with the space between the neighboring nanoflakes of the nanostructures [29]. The loading of CeO2-ZrO2 results in different changes in the isotherms of the two supports. There is a remarkable decrease in the adsorption volume at P/P0>0.9 in the isotherm

of CZ/CA, compared to CA support. Whereas, CZ/NFA shows the isotherm similar to NFA support.

Fig. 4 depicts the pore size distributions (PSD) curves of the samples in the 1.7-100 nm range. In the aspect of desorption PSD curves (Fig. 4a-c), CZ catalyst displays a unimodal PSD with a pore diameter at 3.5 nm, resulted from the aggregation of nanoparticles. For the two supports, CA presents a pore diameter of around 8.0 nm. By contrast, the PSD of NFA exhibits a hierarchical porosity consisting of a narrow pore centered at 3.4 nm, derived from the inter space among primary Al2O3 particles, and a broad interpenetrating meso-macropore, indicative of the slits between the adjacent nanoflakes. Generally, the particle size of nuclei formed in solution during precipitation process ranges from 4 to 20 nm [30]. Thus it is difficult to deposit precipitates into the pore of a support, of which the diameter is smaller than the particle size of precipitate nuclei. Comparing the PSD curves of CA and NFA, it can be inferred that the slit-shape pores of NFA is more accessible to Ce1-xZrx(OH)4 precipitates than the intraparticle pores of CA during the deposition-precipitation process. After loading CeO2-ZrO2 species, the PSD of CZ/CA shows the shape similar to CA and an additional narrow peak around 3.5 nm, meaning that the loaded CeO2-ZrO2 species hardly locate into the intraparticle pores of CA support but instead aggregate on the surface. In the case of CZ/NFA, a newly broad peak at 6.4 nm appears in its PSD curves, which may be resulted from the incorporation of CeO2-ZrO2 species into the slit-shape pores of NFA. Considering that the peak around 3.5 nm in Fig. 4a-c might be caused from tensile strength effect of the adsorbed phase [31], the adsorption PSD curves for these materials are also provided and displayed in Fig. 4d-f. As observed in Fig. 4d, unsupported CZ represents a single peak

around 3.0 nm. Comparing the curves of CA and CZ/CA in Fig. 4e, it is found that the loading of CeO2-ZrO2 leads to an obvious decrease in the intensity of the curve for CA, except for the pore around 3.0 nm due to the aggregated CeO2-ZrO2 nanoparticles on the surface. By contrast, the incorporation of CeO2-ZrO2 into NFA results in a newly shown shoulder peak at around 8.0 nm (Fig. 4f). These results are consistent with the observations in the desorption PSD curves. Thus, the presence of false pore in these materials can be ruled out.

The textural data of the samples are gathered in Table 1. Compared to the supported CeO2-ZrO2 catalysts, CZ has the lowest special surface area (SBET) and pore volume (Vp) due to the aggregation characteristic. As expected, the loading of CeO2-ZrO2 species leads to decreases in SBET and Vp for CA support because of partial pore blockage as well as a dilution effect by CeO2-ZrO2 species [29]. However, CZ/NFA shows textural data similar to NFA support. This suggests that the hierarchical porosity of NFA is hardly closed off by the metal oxide particles compared to the exclusively intraparticle porosity of CA.

The surface compositions for the catalysts were analyzed using XPS technique. As seen in Table 1, all CeO2-ZrO2 catalysts show Ce/Zr surface ratios below 1, a nominal ratio for them. This result reflects the Zr-rich surface and the Ce-rich core of their CeO2-ZrO2 species [32]. Compared to CZ, CZ/CA and CZ/NFA possess the relatively higher ratios of 0.56 and 0.60, respectively, indicating that the core-shell structure of their CeO2-ZrO2 species is partially disrupted. The higher value for CZ/NFA than CZ/CA suggests that NFA support shows an advantage over CA support in

disturbing the core-shell structure. The (Ce+Zr)/Al surface ratios of CZ/CA and CZ/NFA are also displayed in Table 1. It can be seen that the value of CZ/NFA is lower than that of CZ/CA. Combining this result with the higher CeO2-ZrO2 dispersion for CZ/NFA observed by HRTEM, it can be deduced that many CeO2-ZrO2 species of CZ/NFA are inside the slit-shape pores rather than preferentially on the outer surface of NFA support [33], in accord with the observation of its PSD in Fig. 4c and 4f. The XRD patterns of the CeO2-ZrO2 catalysts are shown in Fig. 5. In addition to the two diffraction peaks of γ-Al2O3 (JCPDS 10-0425) at 245.8o and 66.9o for the two supported catalysts, all samples present four main characteristic diffraction peaks at 229.4o, 33.8o,48.8o, and 58.0o and no separated single oxides of CeO2 and ZrO2 are detected. These results demonstrate the formation of Ce1-xZrxO2 solid solutions [34], in agreement with the observation of HRTEM. For CZ, a Ce0.4Zr0.6O2 phase can be distinguished [34]. It is worth noting that the peak at 2=29.4o is slightly asymmetric. Generally, the asymmetric shape of diffraction peaks for Ce1-xZrxO2 means the segregation of the Ce-rich and Zr-rich phases due to the heterogeneous distribution of Ce4+ and Zr4+ cations in the oxides [35, 36]. Herein, the slight asymmetric peak of CZ reflects that the metal cations are inhomogeneously distributed, in accord with the fact that the main phase of CZ differs from an ideal Ce0.5Zr0.5O2 phase with the Ce/Zr ratio of 1. In the case of the supported CeO2-ZrO2 catalysts, the asymmetry of this diffraction peak becomes more evident, to the extent that it splits into two peaks at 2=29.0o and 29.6o for CZ/NFA, corresponding to Ce0.5Zr0.5O2 and Ce0.3Zr0.7O2 phases, respectively [34]. This result indicates that the NFA support has a more significant effect on the Ce4+/Zr4+ distribution than CA support, in agreement with the XPS results.

Based on the above results, it is noted that the Al2O3 supports significantly influence both the dispersion and the surface compositions of CeO2-ZrO2 species. Compared to CA, NFA support is a superior support that not only enhances the dispersion of CeO2-ZrO2 species, but also increases the Ce/Zr surface ratio. The differences in the dispersion and the atomic distribution of CeO2-ZrO2 species in the catalysts can be explained by the formation mechanism of Ce and Zr precursors during the precipitation processes. Fig. 6 shows the schematic of synthesis processes for the three catalysts. It is necessary to take into account that the solubility constant (Ksp) of Ce(OH)4 is several orders of magnitude lower than that of Zr(OH)4, with Ksp of 410-51 and 210-48, respectively [32]. Thus, the precipitation rate of Ce(OH)4 is faster than that of Zr(OH)4 during the coprecipitation process of Ce4+ and Zr4+ in alkali media. In the coprecipitation process for preparing CZ, Ce-rich hydroxide precipitates are initially formed and act as nucleating agents for Zr-rich precipitates at the final stage. This fact results in the core/shell structure of CZ with a low Ce/Zr surface ratio, as confirmed by XPS. The large particles and intraparticle pore of CZ, as observed in the HRTEM image and the PSD curve, respectively, are inevitably formed due to the high-temperature sintering during the transformation of these Ce1-xZrx(OH)4 precursors into oxides. For

the

Al2O3-supported

CeO2-ZrO2

catalysts,

a

different

situation

in

the

deposition-precipitation process can be inferred. In general, a supersaturated solution is necessary for the formation of precipitate nuclei in the solution in order to compensate for the considerable surface energy of nuclei particles [37]. When supports are introduced into the solution, although precipitates can form in the solution and on the support surface simultaneously, the nucleation rate

of precipitates on support surface is much higher because the relatively lower nucleation barrier [37, 38]. In the present work, it is rational that the initial Ce-rich precipitates are deposited preferentially onto the Al2O3 support surface, accompanied by the formation of a few Ce-rich nuclei in the solution. Those Ce-rich nuclei formed in the solution are severely defected [39], and tend to adhere to the Ce-rich precipitates deposited on Al2O3 or to uncovered Al2O3 surface under vigorous stirring [37, 40, 41]. With the concentration of Ce4+ in the solution decreasing, Zr-rich hydroxides are formed and deposited on the Ce-rich precipitate surface to form a core/shell structure or directly on the uncovered Al2O3 surface. This model of dispersing Ce1-xZrx(OH)4 precursors onto the surface of Al2O3 supports can account for the higher Ce/Zr surface ratios and higher dispersions of CeO2-ZrO2 species in the supported catalysts than in unsupported CZ, as substantiated by XPS and HRTEM. In this respect, the accessible surface of Al2O3 support is essential to the dispersion of Ce1-xZrx(OH)4 precursors. Since the slit-shape pores of NFA are much more exposed to the hydroxide precursors than intraparticle pores of CA, as suggested by their PSD curves, CZ/NFA exhibits both higher Ce/Zr surface ratio and higher dispersion of CeO2-ZrO2 species than CZ/CA.

Fig. 7 shows the H2-TPR profiles of the three CeO2-ZrO2 catalysts. Generally, the TPR profile of Ce1-xZrxO2 samples shows two main reduction peaks around 550 and 800 oC, attributed to the reduction of surface and bulk Ce4+ cations, respectively [8, 42]. The surface Ce4+ ions are easily reduced because the unsaturated surface oxygen anions can be removed by hydrogen at low temperature, while the bulk Ce4+ ions are reduced only when the bulk oxygen species have been transported to the surface region at high temperature [8]. In this work, only one broad asymmetric

reduction peak is displayed in the range of 300 to 700 oC for these CeO2-ZrO2 catalysts. It could be attributed to the overlap of reduction peaks of both surface and bulk Ce4+ cations due to the small nanoparticles of their CeO2-ZrO2 species, which are beneficial to the diffusion of bulk oxygen species. Compared with CZ, CZ/CA and CZ/NFA exhibit moderate decreases in reduction temperature to 565 and 551 oC, respectively, reflecting the smaller particle sizes of CeO2-ZrO2 species on the supported catalysts. The CeO2-ZrO2 species dispersed on NFA surface may have the smallest particle size.

The UV-Vis DR spectra of the catalysts were shown in Fig. 8. All catalysts show broad bands in the range of 200 to 600 nm. It is known that pure CeO2 presents three absorption maxima centered at ~255, 285, and 340 nm in its UV-Vis DR spectrum [43]. The former one is attributed to the Ce3+O2- charge transfer transitions, while the latter two are ascribed to Ce4+O2- charge transfer and inter-band transitions, respectively. On the other hand, the spectrum of pure ZrO2 exhibits a main absorption at 211 nm with a shoulder in the region of 290 to 400 nm [44]. Thus, the broad bands of the CeO2-ZrO2 catalysts are the result of the superposition of the above absorption bands. Remarkably, the absorption edges of CZ/CA and CZ/NFA are significantly blue-shifted with respect to CZ, pointing out the decrease in particle size of the CeO2-ZrO2 species [45]. CZ/NFA shows a more obvious blue-shift of absorption edge than CZ/CA, indicating that the CeO2-ZrO2 species in the former case possess a smaller particle size. The results of H2-TPR and UV-Vis DR further substantiate the observation of HRTEM that the CeO2-ZrO2 species supported on NFA are better dispersed than on CA.

Fig. 9 displays the UV Raman spectra of the CeO2-ZrO2 catalysts. UV Raman spectroscopy using excitation wavelength of 325 nm mainly reflects the surface information on the catalyst [46]. In Fig. 9, both CZ and CZ/NFA show a main band around 630 cm-1 and a weak band around 460 cm-1, corresponding to the oxygen vacancies and the symmetric stretching F2g mode of Ce-O, respectively [34, 43]. The defect-related band around 630 cm-1 of CZ/NFA is more intensive than that of CZ, suggesting that there are more oxygen vacancies on CZ/NFA surface. The spectrum of CZ/CA (not given) is difficult to be analyzed due to strong fluorescence interference.

The Ce 3d XPS spectra of the CeO2-ZrO2 catalysts are plotted in Fig. 10. These Ce 3d spectra consist of eight peaks related to four different spin-orbit doublets. Among them, the peaks labeled as u, u´´ and u´´´ are attributed to Ce4+ 3d3/2 and those remarked as v, v´´ and v´´´ arise from Ce4+ 3d5/2, while the doublet u´ and v´ are characterized of Ce3+ 3d3/2 and Ce3+ 3d5/2, respectively [47]. The proportion of Ce3+ in the total Ce species based on the area ratios of v´/(v+v´+v´´+v´´´) are shown in Table 1. It can be seen that the values of Ce3+/(Ce3++Ce4+) of CZ/CA and CZ/NFA are 21.9% and 23.8%, respectively, which are higher than that of CZ. Since the surface oxygen vacancy of Ce1-xZrxO2 materials locates between two neighbouring Ce3+ ions in the surface region [48]. The higher relative concentrations of Ce3+ for the supported CeO2-ZrO2 catalysts signify the more surface oxygen vacancies, in agreement with the UV Raman result. Although Zr-doping into CeO2 facilitates the generation of oxygen vacancies, the excess amount of Zr will decrease the amount of neighbouring Ce ions and hinder the formation of oxygen vacancies [49]. Therefore, the increased Ce/Zr surface ratios of CZ/CA and CZ/NFA, as listed in Table 1, may contribute to their higher Ce3+/(Ce3++Ce4+) ratios and thereby the more surface oxygen vacancies. Among the two

supported catalysts of CZ/CA and CZ/NFA, the higher proportion of Ce3+ for CZ/NFA indicates that the CeO2-ZrO2 species on NFA surface possess the larger amount of oxygen vacancies.

The above-mentioned characterizations of FESEM, HRTEM, N2 physisorption, XPS, XRD, H2-TPR, UV-Vis DR and UV Raman indicate that the pore structure of Al2O3 support has great influences on the precipitation process of metal cations and subsequently on the dispersion and surface properties of CeO2-ZrO2 species in the catalysts. During the preparation process for CZ in which Al2O3 support is absent, the Ce1-xZrx(OH)4 precursors aggregate into Ce-rich core/Zr-rich shell structure, resulting in the low dispersion and Ce/Zr surface ratio of its CeO2-ZrO2 species. Moreover, the Zr-rich surface hinders the formation of surface oxygen vacancies. For the preparation of the supported CeO2-ZrO2 catalysts, the Al2O3 supports disperse the Ce1-xZrx(OH)4 precursors and disturb the formation of core/shell structure, leading to significant improvements in the dispersion and Ce/Zr surface ratio of the CeO2-ZrO2 species, as well as the generation of surface oxygen vacancies. Among the two supported catalysts of CZ/CA and CZ/NFA, the nanoflake-assembled support of CZ/NFA possesses slit-shape pores, which expose more accessible surface than the intraparticle pores of CA support for the dispersion of Ce1-xZrx(OH)4 precursors during the deposition-precipitation process. Therefore, CZ/NFA has both the higher CeO2-ZrO2 dispersion and the larger number of surface oxygen vacancies than CZ/CA. 3.2 Catalytic performance in CO2-ODEB The catalytic performances of these CeO2-ZrO2 catalysts in CO2-ODEB are shown in Fig. 11. In accordance with the characteristics of CO2-ODEB, all catalysts show high steady-state selectivity

of >96% towards ST. However, the EB conversions over the supported catalysts of CZ/CA and CZ/NFA are much higher than unsupported CZ. The enhanced activities can be related to the improvements in the dispersion and the formation of surface oxygen vacancies of the supported CeO2-ZrO2 species, since high CeO2-ZrO2 dispersion results in a high density of active species and surface oxygen vacancies are important sites involving with the redox process for EB dehydrogenation over Ce-based catalysts [14, 50, 51]. Most importantly, CZ/NFA catalyst exhibits a superior activity than CZ/CA, indicating the CeO2-ZrO2 species supported on NFA are more efficient than on CA. As aforementioned, the slit-shape pores of NFA are more accessible to the Ce1-xZrx(OH)4 precursors than the intraparticle pores of CA. This leads to the higher CeO2-ZrO2 dispersion and more surface oxygen vacancies present on CZ/NFA, thereby contributing to the superior activity.

4 Conclusions The pore structure of Al2O3 support was a key factor influencing the physicochemical properties and catalytic activity of supported CeO2-ZrO2 species. It was found that numerous slit-shape pores existed in NFA support, while exclusive intraparticle pores were present in CA support. Since the slit-shape

pores

were

more

accessible

to

Ce1-xZrx(OH)4

precursors

during

the

deposition-precipitation process, NFA was a superior support that not only dispersed the hydroxides precursors but also disturbed the Ce-rich core/Zr-rich shell structure. The resultant CZ CZ/NFA catalyst exhibited both the highest dispersion and the highest Ce/Zr surface ratio among the catalysts investigated. Furthermore, the increased Ce/Zr surface ratio of CZ/NFA endowed this

catalyst with the largest number of surface oxygen vacancies. Therefore, the CeO2-ZrO2 species of CZ/NFA were catalytically efficient in CO2-ODEB. This kind of nanoflake-assembled Al2O3 may be an excellent support to prepare highly dispersed metal/metal oxide catalysts. Acknowledgement This work was supported by the Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University (Grant 2016015) and the Key Laboratory of Petrochemical Technology and Equipment, Department of Education of Liaoning Province, China (Grant LZ2015017). References [1] W. Yan, Q. Y. Kouk, S. X. Tan, J. Luo, Y. Liu, Effects of Pt0-PtOx particle size on 1-butene oxidative dehydrogenation to 1,3-butadiene using CO2 as soft oxidant, J. CO2 Util. 15 (2016) 154-159. [2] F. Rahmani, M. Haghighi, M. Amini, The beneficial utilization of natural zeolite in preparation of Cr/clinoptilolite nanocatalyst used in CO2-oxidative dehydrogenation of ethane to ethylene, J. Ind. Eng. Chem. 31 (2015) 142-155. [3] R. Yuan, Y. Li, H. Yan, H. Wang, J. Song, Z. Zhang, W. Fan, J. Chen, Z. Liu, Z. Liu, Insights into the vanadia catalyzed oxidative dehydrogenation of isobutane with CO 2, Chinese J. Catal. 35 (2014) 1329–1336. [4] Q. Ling, M. Yang, R. Rao, H. Yang, Q. Zhang, H. Liu, A. Zhang, Simple synthesis of layered CeO2-graphene hybrid and their superior catalytic performance in dehydrogenation of ethylbenzene, Appl. Surf. Sci., 274 (2013) 131-137. [5] M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga, T. Suzuki, Oxidative dehydrogenation of ethylbenzene with carbon dioxide, Appl. Catal. A: Gen. 121 (1995) 125-137.

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Fig. 1

Fig. 2

FESEM images of (a) CA, (b and c) NFA.

HRTEM images of (a) CZ, (b) CZ/CA and (c) CZ/NFA.

Fig. 3 N2 adsorption-desorption isotherms of (a) CZ, (b) CA and CZ/CA, (c) NFA and CZ/NFA.

Fig. 4 Pore size distribution curves of (a, d) CZ, (b, e) CA and CZ/CA, (c, f) NFA and CZ/NFA calculated from (a-c) desorption and (d-f) adsorption branch of the isotherms.

Fig. 5

Fig. 6

XRD patterns of CZ, CZ/CA and CZ/NFA.

Schematic of synthesis processes of the catalysts.

Fig. 7

H2-TPR profiles of CZ, CZ/CA and CZ/NFA.

Fig. 8

UV-Vis DR spectra of CZ, CZ/CA and CZ/NFA.

Fig. 9

UV Raman spectra of CZ and CZ/NFA.

Fig. 10

Ce 3d XPS spectra of (a) CZ, (b) CZ/CA and (c) CZ/NFA.

Fig. 11

EB conversion and ST selectivity over the CeO2-ZrO2 catalysts.

Table 1 Textural data and surface compositions of the catalyst materials. Sample

a

Ce/Zra

(Ce+Zr)/Ala

Ce3+/(Ce3++Ce4+)a

SBET

Vp

(m2/g)

(cm3/g)

CZ

80

0.07

0.40

-

18.8

CA

302

0.75

-

-

-

CZ/CA

252

0.51

0.54

0.057

21.9

NFA

149

0.53

-

-

-

CZ/NFA

142

0.56

0.60

0.036

23.8

Determined by XPS analysis.

(%)