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Improving the surface properties of CeO2 by dissolution of Ce3+ to enhance the performance for catalytic wet air oxidation of phenol Changjian Ma, Jile Fu, Jiaxiang Chen, Yaoyao Wen, Paul Oluwaseyi Fasan, Hua Zhang, Nuowei Zhang, Jinbao Zheng, and Bing-Hui Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02121 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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Industrial & Engineering Chemistry Research

Improving the surface properties of CeO2 by dissolution of Ce3+ to enhance the performance for catalytic wet air oxidation of phenol Changjian Ma, Jile Fu, Jiaxiang Chen, Yaoyao Wen, Paul O Fasan, Hua Zhang, Nuowei Zhang,* Jinbao Zheng and Bing-Hui Chen*

Department of Chemical and Biochemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China

*Corresponding author: Tel: +86-592-2185253 Email addresses: [email protected] (Zhang), [email protected] (Chen)

Abstract: Surface properties of nano-ceria can strongly affect the catalytic performance in oxidation reactions. In this work, a simple but efficient post-treatment, where H2O2 is used as a complexing agent to dissolve Ce3+ from the surface of CeO2 nanorods with the help of ultrasonic treatment, is carried out to tune the surface properties and increase their catalytic performance for catalytic wet air oxidation (CWAO) of phenol. It is found that the dissolution of Ce3+ from the surface of CeO2 nanorods can create more surface defects and result in a much rougher surface. The H2O2-ultrasonic treatment can also increase Ce3+ concentration, create more surface oxygen vacancies, and narrow the band gap of CeO2 nanorods. These properties 1

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enable H2O2-ultrasonic treated CeO2 nanorods (CeO2-H2O2-sf) to possess strong oxidizing ability to effectively oxidize phenol. Additionally, for the sake of comparison, other post-treatments, including calcination, H2O2 and ultrasonic-H2O, are also imposed on CeO2 nanorods.

Keywords: CWAO; phenol oxidation; surface treatment; ceria; oxygen vacancy

1. Introduction

With the population and industrial growth, water, one of the most precious natural resources, have been heavily polluted by the wastewater containing organic and other kind of pollutants. Large amounts of wastewater containing phenol and its derivatives are produced especially in the chemical processes using coal as feedstock, where phenol and its derivatives should be removed before being discharged. The biological technologies, which are the most common methods to treat the wastewater, cannot effectively deal with phenolic wastewater due to its toxicity. Although other technologies, including incineration, adsorption and photo-catalysis, have also been used to treat wastewater,1-2 these methods usually are either high energy consumption or inefficient.

The heterogeneous catalytic wet air oxidation (CWAO) is considered as one of the most promising technologies for large-scale application to detoxify noxious wastewaters.3, 4 In the CWAO processes, phenol and its derivatives are oxidized and decomposed towards CO2 and H2O, in the presence of a catalyst and oxygen as the 2


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oxidant, at high temperature (130 to 250 oC ) and pressure (0.5 to 5 MPa). Catalysts play the key role for the complete oxidation of phenol to CO2 and H2O. Transition metal (Cu, Fe, Mn) have been demonstrated to be the excellent catalysts having satisfactory activity toward phenol oxidation.5-7 However, the problem of secondary pollution due to metal leaching has been reported for these types of catalysts.8-10 Supported noble metal catalysts, particularly supported Pt and Ru catalysts, are very active toward the oxidation of phenol.11-14 However, the limited reserves thus high cost hinders noble metal catalysts from the application for treating the phenol contaminated wastewater.

Oxygen solubility is very low in water, and the transfer of oxygen from gas phase to catalyst surface plays a decisive role in the whole CWAO process. The concentration of active oxygen species around the catalyst active sites is the key factor determining the CWAO reaction rate.15 Taking this into consideration, ceria-based materials find their applications as catalysts and/or supports for CWAO technology because of the special redox properties for the easiness of oxygen transfer.16-18 Lin et al reported that CeO2 was very effective for the CWAO of phenol and mentioned that the defects on CeO2 resulted in more reactive oxygen thus increased the catalytic activity of phenol oxidation.19, 20

CeO2 is a material that its catalytic performance strongly depends on the surface properties, especially the surface defects, which can promote the transition between Ce4+ ↔ Ce3+.21, 22 Extensive efforts have been made for tuning the surface defects of

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CeO2 in order to enhance its catalytic performance. Zhao et al successfully tuned the surface defects of CeO2 nanorods by calcination under different atmosphere.23 It was found that calcining at 800 oC under mixed Ar and H2 can effectively enhance the catalytic activity towards water oxidation under visible light, and the rich surface defects, including surface oxygen vacancies and Ce3+ ions, were the origin of the enhancement. Changing the synthetic pressure and/or oxygen partial pressure can also adjust the surface properties of CeO2 nanorods. The CeO2 synthesized at low oxygen partial pressure can lead to high concentration of oxygen vacancy, largest surface fraction of Ce3+, and thus exhibited the better catalytic performance for CO oxidation.24 Compared with small neutral Ce3+, larger size oxygen vacancy clusters was more favorable for CO oxidation.25 Although either chemical doping or high temperature annealing have been developed to adjust the surface properties of nano-ceria, controllable surface defect engineering of nano-ceria is still very challenging.

H2O2 has been employed to prepare nano-ceria in literature. Scholes et al studied the effect of H2O2 on preparing nano-crystalline CeO2 by introduction of different concentrations of H2O2 into the solutions of Ce(III) with NaOH.26 Their results indicated that increasing H2O2 concentration could decrease the CeO2 crystallite size. It was also proposed that O22- was coordinated with Ce(IV) and prevented the formation of a more extended crystalline CeO2 network. Gao et al used a redox chemical etching method to manipulate the surface properties of ceria nanorods, where L(+)-ascorbic acid and H2O2 were used as the reducing agent and oxidant, 4


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respectively.27 This method could produce more oxygen vacancies and surface Ce3+ fractions and thus improved the catalytic activity for CO oxidation. Zhou et al used H2O2 to dissolve Ce(OH)3 to prepare highly reducible CeO2 nanotubes.28

Herein, we developed a simple but efficient method to engineer the surface properties of CeO2 nanorods and enhance their catalytic performance for CWAO of phenol. H2O2 is used as a complexing agent to dissolve Ce3+ from the surface of CeO2 nanorods with the help of ultrasonic process.

2. Materials and methods

2.1 Reagents and materials

Cerium (III) nitrate hexahydrate (Ce(NO3)3•6H2O, purity > 99 wt %), acetonitrile (C3H3N, purity > 99.8 wt %), and phenol (C6H6O, purity > 99 wt %) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Sodium hydroxide (NaOH, purity > 96 wt %), hydrogen peroxide (H2O2, purity > 30 wt %), and acetic acid glacial (CH3COOH, purity > 99.5 wt %) were purchased from Guangdong Guanghua Sci-Tech Co. Ltd. (Shantou, China). Water used in all experiments was deionized water. All reagents were used as received.

2.2 Catalysts preparation

2.2.1 Synthesis of CeO2 nanorods

The CeO2 nanorods were synthesized using the hydrothermal method similar to

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the reported by Mai.29 Typically, 1.736 g of Ce(NO3)3•6H2O was dissolved in 10 mL of deionized water and stirred for 10 min. Then, 19.2 g of NaOH was dissolved in a separate container with 70 mL of deionized water and stirred for 10 min. The NaOH solution was added into the ceria precursor solution drop-wisely, under vigorous stirring. The above solution was stirred for 30 min, then transferred into a 100 mL Teflon-lined stainless autoclave and heated up to 100 oC for 24 h. The autoclave was allowed to cool naturally, and the precipitates were collected by centrifugation and washed by deionized water several times. The obtained clean precipitates were dried in the oven at 60 oC overnight. This light yellow colored sample was assigned as CeO2.

2.2.2 H2O2-ultrasonic treatment of CeO2 nanorods

The as-synthesized CeO2 nanorods (from 2.2.1) were treated using hydrogen peroxide under ultrasonic treatment. The typical steps are: 0.5 g of CeO2 as-synthesized sample was added into 40 mL of 15% H2O2, and sonicated for 30 min. After ultrasonic treatment, the mixture was centrifuged and the collected solid was washed using deionized water several times. The precipitates were further dried in oven at 60 oC for 12 h. The treated sample was assigned as CeO2-H2O2-sf.

2.2.3 H2O2 treatment of CeO2 nanorods

The procedure for H2O2 treatment of CeO2 nanorods was very similar with that of H2O2-ultrasonic treatment. The only difference was that the mixture was stirred rather than ultrasonic treatment for 30 min after 0.5 g of CeO2 as-synthesized sample was 6


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added into 40 mL of 15% H2O2. The obtained catalyst with these procedures was assigned as CeO2-H2O2.

2.2.4 H2O-ultrasonic treatment of CeO2 nanorods

The procedure for obtaining CeO2 nanorods with H2O- ultrasonic treatment was very similar with that of H2O2- ultrasonic treatment. The only difference was 0.5 g of CeO2-as synthesized sample was added into 40 mL of H2O, instead of H2O2 solution. The obtained catalyst was assigned as CeO2-H2O-sf.

2.2.5 Calcination of CeO2 nanorods

The as-synthesized CeO2 nanorods (from 2.2.1) were also treated at high temperature. A typical process includes: 0.8 g of CeO2-as synthesized sample was placed in the quartz tube. Then the quartz tube was put in a tubular furnace, and synthetic air was introduced into the tube at a flow speed of 40 mLmin-1. The temperature of the tubular oven was controlled by a temperature-programmed controller. The temperature was raised to 400 oC at a ramp of 5 oCmin-1. The treated sample was collected after it has been cooled to room temperature naturally. This whitish sample was assigned as CeO2-400.

2.3 Catalytic wet air oxidation of phenol

The catalytic activities were evaluated using a 100 mL temperature programmed autoclave reactor, and magnetic stirrer was used as mixer. Typically, 70 mL of phenol solution (1000 ppm) and 0.28 g of catalyst were added into the reactor. The reactor 7



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was purged with N2 at least 5 times in order to remove oxygen from the reactor. Then, the reactor was heated to the required temperature (140-180oC). After the desired temperature was reached, samples were taken to determine the amount of phenol absorbed on catalysts. Then 2 MPa of oxygen were charged into the reactor and time counting started. The stirring speed was set to 800 rotations per minute (rpm). Samples were periodically taken from the reactor by a sampling tube. The collected samples were centrifuged to separate reacted solution and the used catalyst. The used catalyst was washed using deionized water and dried in oven at 60 oC for 12 h. The reacted solution was further filtered using membrane to filter the fine particles, and allow all the reacted solution to pass through. The particle-free reacted solution was analyzed by the Agilent 1100 high performance liquid chromatography (HPLC) to monitor the phenol conversion. A UV detector at 254 nm was used to detect the phenol signal, and a Dionex C18 column was used to separate the organics within the samples. Acetonitrile (60%) and 0.2% acetic acid glacial solution (40%) were used as the mobile phase at a flow rate of 1 mLmin-1. Total organic carbon was measured by a Shimadzu TOC-L analyzer.

The conversion of phenol and total organic carbon were calculated using the following formulae:

X (%) = X (%) =

× 100%

! "#!$% %!"& ! "#!$% %!"& ! "#!$% %!"&

(1) × 100%

(2)

Where C0 is the initial concentration and the C is the current concentration. 8


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2.4 Characterization

X-ray diffraction (XRD) patterns were determined by a Rigaku X-ray diffractometer, where Cu Kα radiation (35 kV and 20 mA) was used as the X-ray source. The samples were scanned in the range of 10o to 80o at a scanning rate of 10 o

min-1. Transmission electron microscope (TEM) and high resolution transmission

electron microscope (HRTEM) were performed on a Tecnai F30 with an acceleration voltage of 300 kV. X-ray photoelectron spectra (XPS) was collected on a PHI Quantum 2000 Scanning ESCA Microprobe, and the spectra were calibrated with reference to C1s peak at 284.6 eV. Raman spectra were obtained by a Horiba Xplora under ambient condition. The power of laser line was 1.5 mW, and the excitation wavelength was set to 532 nm. The ultraviolet-visible (UV-vis) absorption spectra were obtained on a Cary 5000 UV-VIS-NIR spectrophotometer, and the absorption range was 350-800 nm. Hydrogen temperature programmed reduction (H2-TPR) was carried out under 5% H2/Ar flow with a rate of 40 mLmin-1, and the temperature was raised to 850 oC at a ramp of 10 oCmin-1. N2 adsorption-desorption isotherms were performed using a Micrometritics ASAP 2020 instrument. The specific surface areas were calculated by the Brunauer-Emmet-Teller (BET) method. The carbon content of the used catalysts was analyzed by elemental analysis, which is carried out on Vario EL III.

3. Results and discussion

3.1. Characterization of CeO2 and modified CeO2 catalysts 9



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The H2O2-ultrasonic treatment process was recorded and shown in Scheme 1. When the as-synthesized CeO2 nanorods were dispersed into deionized water (step 1), the mixture exhibited a light yellow color. After adding H2O2 (step 2), the color was changed from light yellow to orange immediately. This observation indicated that Ce(III) species were combined with hydrogen peroxide to form Ce(III) peroxo complexes, Ce(H2O2)3+, and then quickly deprotonated and oxidized into Ce(IV) peroxo complexes, Ce(O2)(OH)2 (orange color).26 The overall reaction can be summarized in Equation 3: Ce3+(aq) + (3/2)H2O2(aq) + 3OH-(aq) → Ce(O2)(OH)2(s) + 2H2O

(3)

The orange color retained after ultrasonic treatment for 30 min (step 3). All of the samples of each step were filtered and the concentration of Ce species in solution was analyzed by ICP. After step 1 and step 2, the Ce concentrations were 6.9 and 7.2 ppm, respectively. However, the Ce concentration was up to 269.4 ppm after ultrasonic process (Step 3). These results suggested that more surface Ce3+ has been dissolved by H2O2-ultrasonic treatment through Ce(IV) peroxo complexes.

The crystalline structures of CeO2 and CeO2-H2O2-sf were determined by XRD and shown in Figure 1. The two catalysts exhibited very similar XRD patterns, which implied that both catalysts were fluorite cubic structure (PDF # 34-0394). Noted that, compared with CeO2, the XRD peaks of CeO2-H2O2-sf was shifted to lower degrees. Lattice constants were calculated for the two catalysts based on the XRD results. After treatment, the lattice constant was increased from 5.4187 to 5.4343 Å. The ionic 10


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radius of Ce3+ (0.114 nm) is larger in size than that of Ce4+ (0.097 nm) and the presence of more Ce3+ can increase the bond-distances of CeO2.30 Therefore, the possible reason for the lattice constant increase is that H2O2-ultrasonic treatment has transformed some Ce4+ into Ce3+. This will be confirmed by XPS and UV-vis results.

The specific surface areas of CeO2 and CeO2-H2O2-sf were measured by physical adsorption of N2 (BET method). It was found that the H2O2-ultrasonic treatment can decrease the specific surface area to some extent. The specific surface areas of CeO2 and CeO2-H2O2-sf were 119 and 101 m2g-1, respectively.

To investigate the effect of H2O2-ultrasonic treatment on the surface properties, the Ce valence states of CeO2 and CeO2-H2O2-sf were analyzed using XPS. The Ce 3d spectra and corresponding peaks are given in Figure 2. Generally, two peaks at 916.6 and 898.2 eV are assigned to the 3d3/2, which are the Ce4+ states. Two peaks at 903.4 and 885.2 eV are attributed to the 3d5/2, which correspond to the Ce3+ states.31 Other peaks are due to the “shake down” processes.32 It can be seen that CeO2-H2O2-sf is more abundant in Ce3+. The relative portions of the surface Ce3+ of the CeO2 and CeO2-H2O2-sf are 44 and 57%, respectively. Clearly, H2O2-ultrasonic treatment can increase the surface Ce3+ fractions of CeO2. This result agreed with the XRD results, which suggested that the H2O2-ultrasonic treatment converted some Ce4+ into Ce3+ and increased the lattice constant of CeO2. It has also been reported that Ce3+ is directly related to the formation of oxygen vacancies, due to the following equation: 11



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2Ce4+ + O2- → 2Ce3+ + □ + 1/2O2

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(4)

Where the □ represents an oxygen vacant site generated by the removal of O2from the CeO2 lattice.25 Thus, it is believed that the presences of higher Ce3+ species would result greater amount of oxygen vacancies. This is confirmed by Raman spectra results, which is displayed in Figure 3(a). The peaks at 460 cm-1 are the Raman-active vibrational mode (F2g) of fluorite-type structure, and the peaks at 599 cm-1 are attributed to the oxygen vacancies due to Ce3+.33 The relative quantities of oxygen vacancies can be calculated by taking the peak area ratio of A599 and A460, which is shown in Figure 3(b). It was calculated that oxygen vacancy ratio for CeO2 and CeO2-H2O2-sf are 3.2% and 4.5%, respectively. CeO2-H2O2-sf contains 28% more oxygen vacancies than CeO2. This is consistent with XPS data, where CeO2-H2O2-sf contains 23% more Ce3+ species, compared to CeO2.

The UV-visible (UV-vis) diffuse reflectance spectroscopy was used to measure the UV-absorption of the two CeO2 samples, and their band gaps were calculated according to the method reported by Murphy.34 Figure 4 (a) shows the original UV-vis absorption spectra of CeO2 and CeO2-H2O2-sf. The CeO2-H2O2-sf showed a red shift of the UV absorption profile based on the untreated CeO2 catalyst. Their band gaps were further calculated and shown in the Figure 4 (b), where the band gap values of CeO2 and CeO2-H2O2-sf were 2.46 and 2.41eV, respectively. It has been reported that the increase of Ce3+ species and oxygen vacancies can lead to the red-shift of the UV absorption profile and narrow the band gap of CeO2.35, 36 Therefore, it is reasonable to

12


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conclude that the H2O2-ultrasonic treatment caused the increase of Ce3+ species and oxygen vacancies and thus led to the red-shift and narrowed band gap.

The TEM and HRTEM images of CeO2 and CeO2-H2O2-sf are shown in Figure 5. It can be seen that both catalysts showed a rod-like morphology with the similar diameter around 8.0 nm. Although the rod-like morphology was retained after treatment, more amorphous CeO2 was observed over CeO2-H2O2-sf catalyst. During the process of H2O2-ultrasonic treatment, Ce3+ was dissolved from the surface of CeO2 nanorods. The decomposition of Ce(IV) peroxo complex led to the formation of amorphous CeO2. As shown in Figure 5 (b) and (d), the dissolution of Ce3+ also enabled CeO2-H2O2-sf catalyst have a “rougher” surface comparing to the as-synthesized CeO2, where more black and white dots were observed. The results of XPS, Raman, and UV-vis spectroscopy indicated that H2O2-ultrasonic treatment can effectively enhance the formation of surface oxygen vacancies. Therefore, the presence of more surface oxygen vacancies might be another reason for the roughness of CeO2-H2O2-sf catalyst surface. These results proved the fact that H2O2-ultrasonic treatment can dissolve the Ce3+ from CeO2 surface, which can generate new surface defects, lead to a much rougher surface and increase Ce3+ concentration (surface oxygen vacancies), as shown previously in Scheme 1.

The redox properties of CeO2 and CeO2-H2O2-sf were studied by the H2-TPR experiments and the results are displayed in Figure 6, where two main reduction peaks were appeared for both catalysts. According to the literature, the low temperature

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reduction peak is the removal of surface oxygen, while the high temperature reduction peak is the removal of the bulk oxygen.37, 38 However, there are two shoulder peaks for low temperature reduction. This might due to the additional removal of the sub-surface capping oxygen that is caused by oxygen vacancies.39, 40 It can be seen that H2O2-ultrasonic treatment can improve the redox properties of CeO2 and the surface oxygen species over CeO2-H2O2-sf can be removed easier than that over CeO2. The reduction of surface Ce4+ of CeO2-H2O2-sf started at around 150 oC, while CeO2 started at around 210 oC. The enhanced reducibility of CeO2-H2O2-sf confirmed the fact that the higher Ce3+ fraction and surface oxygen vacancies concentration facilitated the oxygen diffusion,25 which is the rate-controlling step for the ceria reduction. Generally, the lower temperature at which the reduction peak appears, the higher oxidizing ability the catalyst possesses. Thus, the H2O2-ultrasonic treatment can effectively promote the oxidizing ability of CeO2. Furthermore, the lower reduction temperature of CeO2-H2O2-sf also suggests that the barrier between Ce4+ ↔ Ce3+ is lowered.

3.2. Catalytic wet air oxidation (CWAO) of phenol

Before each reaction, phenol absorption was evaluated (Table S1). It was found that CeO2 has the largest specific surface area, and it can absorb more phenol than CeO2-H2O-sf and CeO2-400. CeO2 treated with H2O2 show improved phenol absorption, where CeO2-H2O2-sf and CeO2-H2O2 can absorb 6% and 7% of phenol, respectively. This is important for CWAO processes, because catalyst’s surface

14


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constantly undergoes absorption, oxidation and desorption cycles. Higher absorption ability can promote this catalysis cycle, and give better catalytic activities, even though the fundamental effect of the absorption phenomenon here on catalytic activity is needed to investigate.

Figure 7 shows the catalytic performance of CeO2 and CeO2-H2O2-sf on CWAO of phenol, performed at 160 oC and 2 MPa of O2. It is clear that the H2O2-ultrasonic treatment can effectively enhance the catalytic performance of CeO2 for CWAO of phenol. The CeO2-H2O2-sf can convert 96.5% of phenol in 120 min, while only 70% of phenol was removed for CeO2. Even after 240 min, the conversion of phenol did not reach 90% and the total organic carbon conversion was only 72.2% for CeO2, much lower than that of the CeO2-H2O2-sf, where the total organic carbon conversion was 87.2%. It was found that total organic carbon conversion was always lower than phenol conversion, implying that some phenol was converted to small molecular intermediates rather than CO2 in CWAO. After 240 min of reaction, the gap between the phenol and total organic carbon conversions over CeO2 (16.3%) was larger than that of CeO2-H2O2-sf (12.4%), indicating that CeO2-H2O2-sf possessed stronger oxidizing ability than CeO2.

The promotional effect of H2O2-ultrasonic treatment on the catalytic performance of CeO2 for CWAO of phenol was further investigated at different temperatures. The O2 pressure was kept at 2 MPa and the reaction time was 120 min (Figure 8). At low reaction temperature (140 oC), the two catalysts showed similar activity. The

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conversions of phenol and total organic carbon were around 60%. Increasing temperature can improve the catalytic performances, however, a much more remarkable enhancement was observed for the CeO2 treated by H2O2-ultrasonic process. For CeO2-H2O2-sf, when the temperature was increased from 140 to 160 oC, the conversion of phenol and total organic carbon were sharply increased from 63.6 and 61.4% to 96.5 and 84.2%, respectively. With further increasing temperature, the catalytic performance remained high and almost unchanged. For CeO2, the increase of temperature only result a slightly increase in the phenol conversion and total organic carbon removal. Even if the temperature was up to 180 oC, the phenol conversion and total organic carbon removal were only 84.8 and 70.0%. As discussed above, CeO2-H2O2-sf starts to reduce at 150 oC. At this temperature, the transition between Ce4+ and Ce3+ process was promoted, and the oxidation ability of catalyst surface was also increased. This might be the cause for the sudden increase of catalytic activity when temperature was raised from 140 to 160 oC. These observations clearly show that the H2O2-ultrasonic treatment enabled CeO2 a higher catalytic performance for CWAO of phenol at a much lower reaction temperature, as compared to the CeO2 without treatment. The Ce3+ species and surface oxygen vacancies play a very important role in the catalytic performance, especially for oxidation reaction, because they can improve the oxygen diffusion of CeO2.41 For CWAO system, the low solubility of O2 in water lifts up the importance of the effect of oxygen transfer on the catalytic performance. The characterization results of XRD, XPS, Raman, UV-Vis, HRTEM and H2-TPR 16


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indicated that the dissolution of Ce3+ from the surface of CeO2 nanorods promoted the formation of Ce3+ and surface oxygen vacancies, and enabled CeO2-H2O2-sf catalyst to possess higher oxidizing ability and thus better catalytic performance.

The effect of other treatments, including calcination (CeO2-400), H2O2 (CeO2H2O2) and ultrasonic-H2O (CeO2-H2O-sf), on the catalytic performance for CWAO of phenol were further investigated and the results are shown in Figure 9. For the sake of comparison, the catalytic performance of CeO2 and CeO2-H2O2-sf were also shown. The post-treatments strongly affected the catalytic performance of CeO2. It was found that the catalytic activity of CeO2 was the second highest in the beginning. This might due to the large BET surface area of CeO2, which enables CeO2 to have more active centers for catalytic oxidation reaction. In the beginning of the reaction, more active center is expected to faster phenol conversion. However, the oxidizing ability of the catalyst plays a more important role in later stage. Since CeO2-H2O2 and CeO2-H2O-sf have smaller specific surface area, it is reasonable that they have slower initial phenol conversion.

Overall,

the

calcination

lowered

the

reactivity,

while

H2O2,

ultrasonic-H2O and ultrasonic-H2O2 enhanced the catalytic performance. However, the increase of activity for H2O2 and ultrasonic-H2O was less remarkable than that of ultrasonic-H2O2. CeO2-400 presented the lowest catalytic activity, which can only convert 51.7 % of phenol after 120 min of reaction. These results indicated that both H2O2 and ultrasonic was required for the best catalytic performance. Both the fraction of Ce3+ species and the concentration of surface oxygen

17



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vacancies are essential for the catalytic behavior for CWAO of phenol, because oxygen vacancies and Ce3+ can enhance the reversible Ce4+/Ce3+ cycles and oxygen diffusion.41 The surface properties of the five catalysts were studied by XPS to explore the reasons for the different catalytic performance. The results are shown in Figure 10. It can be seen that the relative portion of Ce3+ depended on the post-treatment methods to a great extent. Calcination can oxidize some Ce3+ into Ce4+ species, while H2O2, ultrasonic-H2O and ultrasonic-H2O2 favored the transformation from Ce4+ to Ce3+ species. After calcination at 400 oC, Ce3+ fraction was decreased from 44 to 38%. In contrast with calcination, ultrasonic-H2O and H2O2 treatments can increase Ce3+ fraction, from 44 to 46 and 47%, respectively. Among the studied treatments, the ultrasonic-H2O2 treatment was confirmed to be the most effective way to increase Ce3+ concentration. The effects of different treatments on phenol conversion and Ce3+ fraction are shown in Figure 11 to reveal the relationship between the catalytic performance and surface properties. Apparently, the higher the value of Ce3+ fraction was, the more phenol was converted. For CeO2-400, the Ce3+ fraction was 38% and the phenol conversion was 48.3%, much lower than the other four catalysts. With the increase of Ce3+ fraction, the phenol conversion was enhanced. Among the studied treatments, ultrasonic-H2O2 created the highest Ce3+ fraction (57%), thus CeO2-H2O2-sf possessed the best phenol conversion (87.0%).

CeO2-H2O2-sf catalyst was also recycled and tested for stability (Figure S1). It

18


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was found that CeO2-H2O2-sf was deactivated slowly after the first use. Phenol conversions for the second and third cycles are 81.7% and 62.6%, respectively. Meanwhile, the amount of carbon content on CeO2-H2O2-sf was increasing after each cycle. The amount of carbon deposition for first, second and third uses are 6.2%, 10.4% and 11.9%, respectively. It has been reported that the formation of carbonaceous deposit can limit the catalysts surface accessibility, and inhibit catalytic activities.42 The clear relationship between phenol conversion and carbon deposition also suggests that CeO2-H2O2-sf was deactivated due to the formation of carbonaceous deposit on its surface.

4. Conclusion

A simple, but efficient H2O2-ultrasonic treatment has been reported to modify the surface properties of CeO2 nanorods and enhance its catalytic performance for CWAO of phenol to CO2. It was found that the H2O2-ultrasonic treatment dissolved Ce3+ from CeO2 nanorods surface, which can roughen the surface of CeO2, increase Ce3+ fraction and create more oxygen vacancies. Higher Ce3+ fraction and surface oxygen vacancies concentration promoted the oxygen diffusion and lowered the barrier for Ce4+/Ce3+ cycles. These properties enable the CeO2 catalyst treated by H2O2-ultrasonic to possess more oxidizing surface and highest catalytic performance for CWAO of phenol.

Acknowledgement

The authors would like to thank the financial supports from the National Key 19



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Technology Support Program of China (2014BAC10B01) and Natural Science Foundation of China (21336009 and 21673187).

Supporting information

The supplementary material includes the summary of phenol absorption and carbon deposition data.

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Table of Contents graphic

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Figure captions: Scheme 1. The dissolution of Ce3+ species by H2O2-ultrasonic treatment. Figure 1. XRD patterns of CeO2 and CeO2-H2O2-sf. Figure 2. XPS spectra of CeO2 and CeO2-H2O2-sf. Figure 3. Raman spectra of CeO2 catalysts (a) and the area ratio of Raman peaks at 599 and 460 cm-1 (b). Figure 4. UV-vis absorption spectra of CeO2 and CeO2-H2O2-sf. (a) The original spectrum, (b) The converted spectrum. Figure 5. TEM and HRTEM images of CeO2 (a, b) and CeO2-H2O2-sf (c, d). Figure 6. H2-TPR patterns of CeO2 and CeO2-H2O2-sf. Figure 7. CWAO of phenol on CeO2 and CeO2-H2O2-sf. (a) phenol conversion, (b) total organic carbon conversion. Reaction condition: temperature = 160 oC, oxygen pressure = 2 MPa. Figure 8. The effect of reaction temperature for CWAO of phenol on CeO2 and CeO2-H2O2-sf. (a) phenol conversion, (b) total organic carbon conversion. Reaction condition: oxygen pressure = 2 MPa, reaction time = 120 min. Figure 9. CWAO of phenol on different CeO2 catalysts. (a) phenol conversion, (b) total organic carbon conversion. Reaction condition: temperature = 160 oC, oxygen pressure = 2 MPa. Figure 10. XPS spectra (a) and Ce3+ fraction (b) of different CeO2 catalysts. a: CeO2-400, b: CeO2, c: CeO2-H2O-sf, d: CeO2-H2O2 and e: CeO2-H2O2-sf. Figure 11. The relationship between phenol conversion and Ce3+ fraction. a: CeO2-400, b: CeO2, c: CeO2-H2O-sf, d: CeO2-H2O2 and e: CeO2-H2O2-sf. Reaction condition: temperature = 160 oC, oxygen pressure = 2 MPa, reaction time = 90 min.

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Figures

Scheme 1.

Figure 1.

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Figure 2.

Figure 3.

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Figure 4.

Figure 5.

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Figure 6.

Figure 7.

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Figure 8.

Figure 9.

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Figure 10.

Figure 11.

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Improving the Surface Properties of CeO2 by ... - ACS Publications

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