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The surface properties and the activities in catalytic wet air oxidation over CeO2–TiO2 catalysts
Applied Surface Science 252 (2006) 8499–8505 www.elsevier.com/locate/apsusc
The surface properties and the activities in catalytic wet air oxidation over CeO2–TiO2 catalysts Shaoxia Yang *, Wanpeng Zhu, Zhanpeng Jiang, Zhengxiong Chen, Jiangbing Wang Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 18 September 2005; received in revised form 11 November 2005; accepted 17 November 2005 Available online 27 December 2005
Abstract No-noble metal CeO2–TiO2 catalysts prepared by sol–gel method were developed and examined for catalytic wet air oxidation (CWAO) of acetic acid. The structure of the catalysts was measured by BET, SEM, XRD, XPS and DTA-TG. We investigated the effect of the interactions of Ce and Ti on the structure of CeO2–TiO2 catalysts. The mechanisms of the relationships between the different content of Ti and the activity of CeO2– TiO2 catalysts were discussed. The results showed that the average crystal size of CeO2 decreased and the surface areas increased; the low valence of Ce3+ increase, and the chemisorbed oxygen slightly decreased with the increase of Ti content on the surface of CeO2–TiO2 catalysts. The order of the activity in CWAO of acetic acid followed: Ce/Ti 1/1 > Ce/Ti 3/1 > Ce/Ti 1/3 > Ce/Ti 5/1 > CeO2 > TiO2 > no catalyst. In CWAO of acetic acid, the optimal atomic ratio of Ce and Ti was 1, and the highest COD removal was over 64% at 230 8C, 5 MPa and 180 min reaction time over Ce/ Ti 1/1 catalyst. The excellent activity and stability of CeO2–TiO2 catalysts was observed in our study. # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic wet air oxidation (CWAO); CeO2; TiO2; Acetic acid
1. Introduction The industrial wastewater originating from chemical, petrochemical, paper wills and textile plants contains toxic, hazardous and high concentration organic compounds. The wastewater can cause serious environmental problem, and is harmful to human health and aquatic life. Therefore, it is essential to treat the wastewater before it is discharged. The biological method is widely used to treat low chemical oxygen demand (COD) of the effluents (1 < COD < 20 g/L). If the effluents are toxic to bacteria or contain high concentration organic compounds, the method is not feasible [1]. Wet air oxidation (WAO) is an effective and potential treatment technology for toxic, hazardous and high concentration organic compounds (5 < COD < 100 g/L) [2]. In WAO process, organic compounds are oxidized into carbon dioxide, water and other innocuous end products, and there are not NOx, SO2, HCl, dioxins and fly ash. So WAO has received more attention
* Corresponding author. Tel.: +86 10 62784527/819; fax: +86 10 62785687. E-mail address: [email protected] (S. Yang). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.11.067
at last decades. But WAO process needs high temperature (125– 320 8C), high pressure (0.5–20 MPa) and high capital cost. This prevents the technology from being widely applied to treat the special industrial wastewater. The reaction under milder operation conditions run brings new strong challenge in WAO of organic compounds. Catalytic wet air oxidation (CWAO) seems to be better economically and technologically viable for treating the wastewater. Using catalysts, the reaction is operated at mild conditions, the reaction time is shortened, and much higher oxidation rate is achieved. For example, refractory organic compounds, such as carboxylic acid and ammonia, can be much easier oxidized than no-catalytic process. Homogenous catalysts, for example Cu, Fe and Mn salts, have good activity in WAO of organic compounds [3]. In Europe, homogenous catalysts in CWAO are already commercially exploited and some of them are in various stages of development. But additional steps to remove and recover metal ions are necessary. In the view of this, heterogeneous catalysts have more promising for treating the wastewater [4–9]. Now the development of active, stable and low-cost heterogeneous catalysts has been an important challenge to apply wildly the technology for treating industrial wastewater.
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Among organic compounds, lower carboxylic acid is refractory to be oxidized. Many studies have shown that acetic acid is often present at the end of reaction, and its oxidation is a rate-determining step in WAO of many organic compounds for complete mineralization into CO2 and H2O [10]. However, noble catalysts (Ru, Pt, etc.) need higher cost, they have been demonstrated that they are potentially more active and stable in WAO of high molecular weight polymers and refractory carboxylic acids, and they could effectively reduce the reaction operating conditions. For example, Ru/CeO2, Ru/MnO2/CeO2, Ru/C, Ru/graphite, Ru/TiO2 and Pt/C [11–15]. No-noble catalysts have good activity and potential application, especially CeO2-based catalysts have shown the effective activity in CWAO of acetic acid, for example Co–Bi, MnO2/CeO2, CeO2–ZrO2– CuO and CeO2–ZrO2–MnO2 catalysts [16–18]. But the leaching of metal ion had been observed in CWAO of acetic acid. TiO2 is a better stable metal oxide, but do not show the catalytic activity in CWAO of organic compounds. TiO2 is often used as a support of noble metals in CWAO of organic compounds. In the paper a novel catalyst of CeO2–TiO2 is prepared with sol–gel method. CWAO of acetic acid is investigated over CeO2–TiO2 catalyst. The effect of adding Ti content on the catalyst activity is discussed. Moreover, we further explore the effect of the surface structure on the activity of the different catalysts by BET, SEM, XRD, XPS and TG. In addition, the metal ions leaching concentration in the effluent has been measured with ICP-MS in CWAO of acetic acid over CeO2–TiO2 catalysts. 2. Experimental 2.1. Catalyst preparation TiO2 and CeO2–TiO2 catalysts were prepared by sol–gel method. The volumetric ratio of tetrabutylorthotitanate (Ti(OC4H9)4, A.R.), deionized water, anhydrous ethanol (CH3COOH, A.R.) and HNO3 (HNO3, A.R.) was 1:1:5:0.2. Firstly, Ti(OC4H9)4 and anhydrous ethanol were uniformly mixed with a volumetric ratio of 1:4. Anhydrous ethanol, deionized water and HNO3 were mixed, and then Ce(NO3)36H2O was uniformly dissolved into the ethanolic aqueous solution. Secondly, the Ti(OC4H9)4/ethanol solution was added drop wise to the ethanolic solution at room temperature under strongly stirring to carry out hydrolysis. After continuously stirring for 3 h, the yellowish transparent sol was obtained. Thirdly, the sol was dried at 80 8C for 24 h to give xerogel. The powder of the xerogel was calcinated under air at 500 8C for 3 h to obtain TiO2 and CeO2–TiO2 catalysts. The atom ratio of Ce and Ti is 5/1, 3/1, 1/1 and 1/3, respectively. CeO2 catalyst was prepared with coprecipitation by adding dropwise 0.2 M Ce(NO3)3 aqueous solution to excess ammonia solution. The precipitate was dried at 100 8C for 12 h and calcined under air flow at 500 8C for 3 h to get CeO2 catalyst. 2.2. Oxidation reaction The reaction of WAO was preformed in a 1 L autoclave equipped with a stirrer ensuring desirable mass transfer from
the gas to liquid phase and to the catalyst. A 5 g/L catalyst and 500 mL acetic acid aqueous solution (concentration: 0.083 M) were loaded into the reactor, then N2 was introduced into the reactor. When the reactor was heated to 230 8C, pure oxygen was added into it till a total pressure to 5 MPa. The point was taken as ‘‘zero time’’ under stirring, and the reaction started. The reaction time was 180 min. Samples from the reactor were taken at different reaction time, and COD concentration of the samples was measured. The experimental setup was shown in our study [19]. 2.3. Catalysts characterization 2.3.1. BET, XRD and SEM The surface area of the catalysts was estimated at 77 K by N2 adsorption using a Quantachrome Autosorb Automated Gas Sorption System. X-ray powder diffractometer (XRD) analysis was carried out in D/max-IIIA powder diffractometer using Cu ˚ ). The analyse was performed over an Ka radiation (1.5418 A scanning range of 2u = 10–908 at a speed of 68 min1. The surface elements of the used CeO2–TiO2 catalyst were analyzed with means of a scanning electron microscope (SEM) with JEOL JSM-6301F equipped with energy–dispersion microanalysis system. 2.3.2. XPS X-ray photoelectron spectroscope (XPS) was carried out to analyze the composition and the chemical state of the surface elements for the catalysts. PHI ESCA 5700 instrument, with a Al Ka X-ray source (1486.6 eV) and pass energy of 29.5 eV operating at a pressure of 7 1010 Torr, was used. The binding energies were calibrated with respect to the signal for contamination carbon (binding energy = 284.62 eV). An instrument error of 0.1 eV can be assumed for the measurement. Curve fitting was carried out using a Physical Electronics PC-ACCESS ESCA-V6.0E program with a Gaussian–Lorentzian sum function. The Gaussian–Lorentzian mixing ratio was kept in the range of 0.8–1.0. 2.3.3. The reaching of metal ions The metal leaching of the catalysts in the liquid phase was measured with the inductively coupled plasma (ICP). 3. Results and discussion 3.1. XRD of TiO2, CeO2 and CeO2–TiO2 catalysts The phase behavior of TiO2, CeO2 and CeO2–TiO2 catalysts was measured by X-ray diffractometer. The patterns of XRD for the catalysts are shown in Fig. 1. For TiO2 catalyst, the presence of peaks as an attributive indicator of anatase titania (2u = 25.28, 37.80, 48.05, 53.89) and of rutile titania (2u = 27.45, 41.23) is detected. It means that TiO2 catalyst calcined at 500 8C exists as the coexistence of anatase and rutile, and the anatase titania is a dominating structure. For CeO2 catalyst, only strong diffraction peaks of an attributive indicator as cubic CeO2 (2u = 28.57, 33.10, 47.53, 56.38) are
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[20]. On the other hand, the addition of Ti could promote the dispersion of CeO2 particles, and restrain the increase of CeO2 crystalline particles at high calcination temperature. Therefore, CeO2 peaks become much wider and weaker, and CeO2 particles become small. It is helpful to increase the surface area of CeO2–TiO2 catalysts. In Table 1, we see that the surface area of CeO2–TiO2 catalysts is much larger than that of CeO2 or TiO2 catalyst, and it increases with an increase of Ti content. This indicates that the interaction of Ce and Ti could make the crystal size lower and surface area increase for CeO2–TiO2 catalysts. It could lead to the effective active sites increase on the surface of CeO2–TiO2 catalysts. 3.2. XPS of TiO2, CeO2 and CeO2–TiO2 catalysts
Fig. 1. XRD patterns of different catalysts.
observed in the XRD pattern. It indicates that Ce exists as CeO2. In the CeO2–TiO2 catalysts, the patterns show the presence of CeO2 phase, while no relevant anatase or rutile phase of TiO2 is observed in the spectra of XRD. Moreover, the peaks of CeO2 become much wider and weaker with the increase of Ti content. When the ratio of Ce and Ti is 1:3, the faint scattering CeO2 peaks are observed. It indicates that CeO2 particles tend to not integrate crystalline phase, and CeO2 exists as amorphous and crystalline phase increasing Ti content. This means that the crystal size of CeO2 particles decreases with the increase of Ti content in the CeO2–TiO2 catalysts. The crystal size of the catalysts was evaluated using the Scherrer equation, and shown in Table 1. For CeO2 catalyst, the average crystal size of CeO2 particles is about 13 nm. With the increase of Ti content in CeO2–TiO2 catalysts, the average crystal size of CeO2 becomes smaller from 13 to 4 nm. This result is good consistent with that of XRD patterns. However, it is surprising that no relevant of Ti diffraction peaks are observed in the spectra of CeO2–TiO2 catalysts. The result is not same as that CeO2. In the mixture of Ti alkoxide and Ce nitrate, Ti and Ce ions reached the atomic distribution. During Ti4+ hydrolysis to form the gel, Ce ions dispersed into the gel web at the range of atomic scale. This might make the longrange order structure of TiO2 crystal particles destroyed in the drying and calcination process. So TiO2 exists as amorphous phase in the catalyst, and the diffraction peaks are not observed in the spectra of CeO2–TiO2 catalysts. The result is the same as
XPS analyse was preformed to get a better understanding the chemical state of all elements on the catalyst surface. Fig. 2 shows XPS survey spectra of the fresh CeO2–TiO2 catalyst. It can be seen clearly that there are Ce, Ti and O peaks except for contamination carbon on the catalyst surface. Fig. 3 presents Ti2p XPS spectra concerning double peaks (Ti2pi1/2 and Ti2p3/ 2). The binding energy of Ti2p3/2 at about 458.2–458.5 eV could be attributed to Ti(IV)O2 in the catalyst for TiO2 and CeO2–TiO2 catalysts [21,22]. The result indicates that Ti is present as amorphous phase TiO2 on the surface of CeO2–TiO2 catalysts. Ce3d XPS spectra of CeO2 and CeO2–TiO2 catalysts are shown in Fig. 4. Ce3d spectra are very complicated, and the series of core-level spectra exhibit three-lobed peaks. From these peaks, Ce4+ oxidation state is predominant, and Ce3+ oxidation state maybe is distinguishable through the Ce3d spectra. The series of v and u peaks are from the 3d5/2 and 3d3/2 states, respectively. The peak of v and v00 could be assigned to a mixing configuration of 3d94f2(O2p4) and 3d94f1(O2p5) Ce(IV) state, and v000 to 3d94f0(O2p6) Ce(IV) state. The peak of v0 is attributed to 3d94f1(O2p6) Ce(III) final state. The series of u
Table 1 The BET surface area and the crystal size of the different catalysts Samples
Crystallite size (nm)
The surface area (m2/g)
TiO2 CeO2 Ce/Ti Ce/Ti Ce/Ti Ce/Ti
17 13 10 8 5 4
21.3 66.5 81.0 90.5 94.1 145.7
5/1 3/1 1/1 1/3
Fig. 2. The XPS survey spectra of CeO2–TiO2 catalyst.
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S. Yang et al. / Applied Surface Science 252 (2006) 8499–8505 Table 2 XPS Ce3d data measured for the different catalysts Samples
CeO2 Ce/Ti Ce/Ti Ce/Ti Ce/Ti
Fig. 3. The XPS Ti2p spectra of the different catalysts.
structure can be explained in the same way [23]. For CeO2 catalyst, v0 and u0 feature peaks are seen in the CeO2 spectrum, but the intensity of two peaks is very weak. In the addition of Ti, the peaks of Ce3d for CeO2–TiO2 catalysts mildly change: (i) the decease in structure of v000 and u000 peaks, (ii) the decease of v and u peaks, (iii) the increase of v0 and u0 peaks in structure. This indicates that a fraction of the cerium exists as Ce3+ on the catalyst surface. For CeO2 and CeO2–TiO2 catalysts, the binding energy for u000 is about 916.5–917.0 eV, and for v peak it is around 882.5–882.9 eV [23]. According to Shyu, the ratio of intensity of u000 peak to the total intensity of Ce3d depends on the Ce4+/Ce3+ ratio [24]. With the rules of qualitative and quantitative analysis of XPS, Table 2 shows the result of the quantitative analysis with XPS analysis. As shown in Table 2, for CeO2 catalyst the ratio of the intensity of u000 peak to the total intensity of Ce3d is about
Fig. 4. The XPS Ce3d spectra of the different catalysts.
5/1 3/1 1/1 1/3
Ce3d (eV) v
u000
881.95 882.12 882.20 882.25 882.74
916.33 916.37 916.38 916.74 916.49
Atomic ratio Ce/(Ti + Ce)
u000 /Ce3d (%)
1 0.83 0.78 0.72 0.37
9.2 8.7 7.9 7.3 5.9
9.2%, and is larger than that of CeO2–TiO2 catalysts. With the increase of Ti content, the ratio gradually decreases from 9.2 to 5.9%. This means that the Ce3+ increases with increasing Ti content on the surface of CeO2–TiO2 catalysts. And the atomic ratio of Ce/(Ce + Ti) is also listed in Table 2. At the different Ti content, the surface concentration of Ce is larger than that of the as-prepared concentration of CeO2– TiO2 catalysts. This result is consistent with that of [25]. With increasing Ti content, Ce concentration slightly decreases, and does not obviously change except for Ce/Ti 1/3 catalyst. In Fig. 4, Ce3+ is present and its content increases with the increase of Ti content on the surface of CeO2–TiO2 catalysts. This could lead to O1s chemical state change on the surface catalysts. Therefore, O1s patterns of XPS also were measured. The spectra are shown in Fig. 5. It can be seen that O1s XPS spectra exhibit single-lobed peaks, and the peaks are asymmetric (the left sides are wider than the right), indicating that the different types of oxygen exist on the catalyst surface. The O1s peaks could be fitted into two peaks referred to as the lattice oxygen OI and the chemisorbed oxygen OII [9,25]. The fitted curves are shown in Fig. 5. The O1s peaks at about 529.0–530.0 eV could be attributed to the lattice oxygen (OI) for TiO2, CeO2 and CeO2–TiO2 catalysts [20,21,25]. Another component OII at about 531.0 eV belongs likely to the chemisorbed oxygen [9,26]. As shown in Fig. 5, the chemisorbed oxygen is present, but the lattice oxygen is much richer than the chemisorbed oxygen on the catalyst surface. As shown in Table 3, for TiO2 catalyst the percentage of the chemisorbed oxygen to the total oxygen (OT) is much smaller than CeO2 and CeO2–TiO2 catalysts. The content of chemisorbed oxygen does not obviously change, only slightly decreased for CeO2–TiO2 catalysts with the increase of Ti content. With the existence of Ce3+ on the surface of CeO2–TiO2 catalysts, it could create a charge imbalance, the vacancies and unsaturated chemical bonds on the catalyst surface. These make the chemisorbed oxygen exist on the surface. The increase of Ce3+ content is helpful for the chemisorbed oxygen on the catalyst surface to increase. However, it does not agreewith our result. The reason could be that: the proportion of Ce/(Ce + Ti) on the surface decreases with Ti content increasing. It could affect the chemisorbed oxygen content. Moreover, the ionic radius of Ce ˚ , while the ionic radius of Ti4+ is only 0.68 A ˚ [27]. ion is 0.93 A When Ti is added to CeO2 catalyst, a fraction of Ti4+ at the interface can enter the CeO2 lattice and substitute for Ce3+. This might make the charge imbalance weakened. It could lead to that the chemisorbed oxygen decreases on the catalyst surface. This
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Fig. 5. O1s curves fitted of the different catalysts.
result is agreement with [28]. The chemisorbed oxygen on the catalyst surface is the most active oxygen and plays an important role in oxidation reaction [9,29,30]. This means that CeO2–TiO2 catalysts might have good activity in WAO of organic compounds. And due to the different chemisorbed oxygen content of TiO2, CeO2 and CeO2–TiO2 catalysts, we predicate that the catalytic activity might be affected. Table 3 XPS O1s data measured for the different catalysts Sample
TiO2 CeO2 Ce/Ti Ce/Ti Ce/Ti Ce/Ti
5/1 3/1 1/1 1/3
O/OT (%)
E (eV) O1s
FW
OI
OII
OI/OT
OII/OT
529.75 528.87 529.12 529.16 529.25 529.40
1.60 1.74 1.80 1.84 2.01 2.04
529.87 528.84 529.11 529.16 529.28 529.35
531.47 531.28 531.43 531.26 531.20 531.11
88.86 78.19 78.58 78.96 79.32 83.52
11.14 21.81 21.42 21.04 20.68 16.48
3.3. The activity and stability in CWAO of acetic acid Fig. 6 shows the COD removal of different catalysts and the blank experiment in WAO of acetic acid. As shown in Fig. 6, it can be seen that only 4.5% COD is removed without catalyst, and 9% and 14% acetic acid are oxidized with TiO2 and CeO2 catalysts at 230 8C and 5 MPa after 180 min reaction time. It means that TiO2 and CeO2 catalysts do not have obvious activity in WAO of acetic acid. When CeO2– TiO2 catalysts are used, the COD removal is much higher than that of TiO2 and CeO2 catalysts. This indicates that CeO2–TiO2 catalysts have good activity in WAO of acetic acid. Moreover, the COD removal changes with the increase of Ti content. The order of the catalysis activity was obtained: Ce/Ti 1/1 > Ce/Ti 3/1 > Ce/Ti 1/3 > Ce/Ti 5/ 1 > CeO2 > TiO2 > no catalyst. Using the Ce/Ti 1/1 catalyst, which is the most active catalyst, 64% conversion of acetic acid is obtained at 230 8C and 5 MPa after 180 min reaction run.
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S. Yang et al. / Applied Surface Science 252 (2006) 8499–8505 Table 4 The metal ion concentration in the effluents after CWAO of acetic acid The leaching metal ions
Catalysts TiO2
CeO2
Ce/Ti 5/1
Ce/Ti 3/1
Ce/Ti 1/1
Ce/Ti 1/3
Ce (mg/L) Ti (mg/L)
– 0.08
0.67 –
0.71 0.05
0.65 0.04
0.61 0.05
0.61 0.05
WAO of organic compounds involves a free radical chain reaction, and hydroxyl radical (OH) and hydroperoxyl radical (HO2) are strong oxidative reactive species in aqueous solution [13,31]. With oxygen, HO2 could be produced by organic compounds (RH) and the catalyst according to the reactions followed: Fig. 6. The COD removal in WAO of acetic acid with the different catalysts at 230 8C and 5 MPa.
The metal leaching of catalysts is important, since continuous leaching of metal ions is one of the direct causes of catalyst deactivation. Therefore, leaching of metal ions for the different catalysts was measured with ICP. The concentration of the dissolved Ce and Ti ions in the solution after 180 min reaction time is shown in Table 4. In our study, the leaching concentration of metal ions is very low. This means that CeO2– TiO2 catalysts have good stability. The used Ce/Ti 1/1 catalyst was measured with SEM-EDX. It is seen that the surface of the used catalyst contains C element peak in Fig. 7. This indicates that the carbonaceous compounds deposit on the catalyst surface. By DTA-TG measure about l wt.% weight of the carbonaceous compounds deposits on the surface of the used Ce/Ti 1/1 catalyst. And with the washed and air-dried used Ce/ Ti 1/1 catalyst, about 58% COD removal was obtained in WAO of acetic acid at 230 8C and 5 MPa after 180 min reaction time. This means that the deposit does not obviously affect the activity of the catalyst. Therefore, CeO2–TiO2 catalysts have good activity and stability in WAO of acetic acid. Moreover, the carbonaceous compounds deposited on the surface were analyzed by XPS. We found that the carbonaceous polymeric compounds exit on the surface of the used Ce/Ti 1/1 catalyst.
RH þ Ce4þ ! RH Ce4þ
(1)
RH Ce4þ ! R þ Ce3þ þ Hþ
(2)
Ce3þ þ O2 ! Ce4þ þ O2
(3)
O2 þ Hþ ! HO2
(4)
The organic compound RH in the solution could firstly absorb on the catalyst surface (in Reaction (1)). Reaction (2) occurs on the catalyst surface and is a fast reaction. By the Reaction (3) the chemisorbed oxygen could trap the electron of Ce3+, and then produce superoxide radical (O2). Hydroperoxyl radical (HO2), the strong oxidative reactive specie, could be easily formed in the acidic solution from O2 through Reaction (4). So Reactions (1)–(4) indicate that the occurrence of O2 is the important step to produce HO2. For TiO2 catalyst, the content of the chemisorbed oxygen is very low on the surface catalyst. This leads to that the activity of TiO2 catalyst is very low, and almost same as no-catalyst. For the different Ti content of CeO2–TiO2 catalysts, there exist the higher content chemisorbed oxygen and the lower value of Ce3+
Fig. 7. The pattern of SEM-EDX of the used Ce/Ti 1/1 catalyst.
S. Yang et al. / Applied Surface Science 252 (2006) 8499–8505
on the catalyst surface. This could promote the process of Reaction (3), i.e. the chemisorbed oxygen could quickly accept the electron from Ce3+ and transform into O2. This enhances the occurrence of O2, i.e. the rate of the producing HO2 is quickened. Therefore, adding CeO2–TiO2 catalysts into the reaction, the activity obviously increases in WAO of acetic acid. For Ce/Ti 1/3 catalyst, the lower content of the chemisorbed oxygen is present on the surface. It inhibits the chemisorbed oxygen to get the electron, and then form O2. This leads to the lower activity for WAO of acetic acid with Ce/Ti 1/3 catalyst. On the other hand, when the addition of Ti into CeO2, the particle size decreases and the surface area increase for CeO2based catalysts. This makes the effective active sites increase on the surface of the catalyst. It is advantageous for the oxygen and organic compounds to be absorbed on the surface and to be oxidized. Therefore, Ce/Ti 1/1 catalyst has the highest activity in CWAO of acetic acid. The result is in good agreement with that of the activity of TiO2, CeO2 and CeO2–TiO2 catalysts in WAO of acetic acid. 4. Conclusions In our study CeO2–TiO2 catalysts prepared with sol–gel method have good activity and stability in WAO of acetic acid. The interaction of Ce and Ti affects the surface structural properties of CeO2–TiO2 catalysts. With the increase of Ti content, the average crystal size of CeO2 decreases and the surface area increases for CeO2–TiO2 catalysts. Adding Ti to CeO2-based catalyst, there exist higher content of the chemisorbed oxygen and the low valence of Ce3+ on the surface. In the addition of Ti, the activity of CeO2–TiO2 catalysts increases, and is much higher that of TiO2 and CeO2 catalysts in CWAO of acetic acid. Moreover, the optimal atom ratio of Ce and Ti is 1. With Ce/Ti 1/1 catalyst, over 64% COD removal is attained in WAO of acetic acid at temperature reaction of 230 8C and the total pressure of 5 MPa after 180 min reaction time. Acknowledgements This work was supported by the Natural High Tech. Research & Development Program (No. 2002AA601260) and the China Postdoctoral Science Foundation (2004-7).
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The surface properties and the activities in catalytic wet air oxidation over CeO2–TiO2 catalysts Shaoxia Yang *, Wanpeng Zhu, Zhanpeng Jiang, Zhengxiong Chen, Jiangbing Wang Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 18 September 2005; received in revised form 11 November 2005; accepted 17 November 2005 Available online 27 December 2005
Abstract No-noble metal CeO2–TiO2 catalysts prepared by sol–gel method were developed and examined for catalytic wet air oxidation (CWAO) of acetic acid. The structure of the catalysts was measured by BET, SEM, XRD, XPS and DTA-TG. We investigated the effect of the interactions of Ce and Ti on the structure of CeO2–TiO2 catalysts. The mechanisms of the relationships between the different content of Ti and the activity of CeO2– TiO2 catalysts were discussed. The results showed that the average crystal size of CeO2 decreased and the surface areas increased; the low valence of Ce3+ increase, and the chemisorbed oxygen slightly decreased with the increase of Ti content on the surface of CeO2–TiO2 catalysts. The order of the activity in CWAO of acetic acid followed: Ce/Ti 1/1 > Ce/Ti 3/1 > Ce/Ti 1/3 > Ce/Ti 5/1 > CeO2 > TiO2 > no catalyst. In CWAO of acetic acid, the optimal atomic ratio of Ce and Ti was 1, and the highest COD removal was over 64% at 230 8C, 5 MPa and 180 min reaction time over Ce/ Ti 1/1 catalyst. The excellent activity and stability of CeO2–TiO2 catalysts was observed in our study. # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic wet air oxidation (CWAO); CeO2; TiO2; Acetic acid
1. Introduction The industrial wastewater originating from chemical, petrochemical, paper wills and textile plants contains toxic, hazardous and high concentration organic compounds. The wastewater can cause serious environmental problem, and is harmful to human health and aquatic life. Therefore, it is essential to treat the wastewater before it is discharged. The biological method is widely used to treat low chemical oxygen demand (COD) of the effluents (1 < COD < 20 g/L). If the effluents are toxic to bacteria or contain high concentration organic compounds, the method is not feasible [1]. Wet air oxidation (WAO) is an effective and potential treatment technology for toxic, hazardous and high concentration organic compounds (5 < COD < 100 g/L) [2]. In WAO process, organic compounds are oxidized into carbon dioxide, water and other innocuous end products, and there are not NOx, SO2, HCl, dioxins and fly ash. So WAO has received more attention
* Corresponding author. Tel.: +86 10 62784527/819; fax: +86 10 62785687. E-mail address: [email protected] (S. Yang). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.11.067
at last decades. But WAO process needs high temperature (125– 320 8C), high pressure (0.5–20 MPa) and high capital cost. This prevents the technology from being widely applied to treat the special industrial wastewater. The reaction under milder operation conditions run brings new strong challenge in WAO of organic compounds. Catalytic wet air oxidation (CWAO) seems to be better economically and technologically viable for treating the wastewater. Using catalysts, the reaction is operated at mild conditions, the reaction time is shortened, and much higher oxidation rate is achieved. For example, refractory organic compounds, such as carboxylic acid and ammonia, can be much easier oxidized than no-catalytic process. Homogenous catalysts, for example Cu, Fe and Mn salts, have good activity in WAO of organic compounds [3]. In Europe, homogenous catalysts in CWAO are already commercially exploited and some of them are in various stages of development. But additional steps to remove and recover metal ions are necessary. In the view of this, heterogeneous catalysts have more promising for treating the wastewater [4–9]. Now the development of active, stable and low-cost heterogeneous catalysts has been an important challenge to apply wildly the technology for treating industrial wastewater.
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Among organic compounds, lower carboxylic acid is refractory to be oxidized. Many studies have shown that acetic acid is often present at the end of reaction, and its oxidation is a rate-determining step in WAO of many organic compounds for complete mineralization into CO2 and H2O [10]. However, noble catalysts (Ru, Pt, etc.) need higher cost, they have been demonstrated that they are potentially more active and stable in WAO of high molecular weight polymers and refractory carboxylic acids, and they could effectively reduce the reaction operating conditions. For example, Ru/CeO2, Ru/MnO2/CeO2, Ru/C, Ru/graphite, Ru/TiO2 and Pt/C [11–15]. No-noble catalysts have good activity and potential application, especially CeO2-based catalysts have shown the effective activity in CWAO of acetic acid, for example Co–Bi, MnO2/CeO2, CeO2–ZrO2– CuO and CeO2–ZrO2–MnO2 catalysts [16–18]. But the leaching of metal ion had been observed in CWAO of acetic acid. TiO2 is a better stable metal oxide, but do not show the catalytic activity in CWAO of organic compounds. TiO2 is often used as a support of noble metals in CWAO of organic compounds. In the paper a novel catalyst of CeO2–TiO2 is prepared with sol–gel method. CWAO of acetic acid is investigated over CeO2–TiO2 catalyst. The effect of adding Ti content on the catalyst activity is discussed. Moreover, we further explore the effect of the surface structure on the activity of the different catalysts by BET, SEM, XRD, XPS and TG. In addition, the metal ions leaching concentration in the effluent has been measured with ICP-MS in CWAO of acetic acid over CeO2–TiO2 catalysts. 2. Experimental 2.1. Catalyst preparation TiO2 and CeO2–TiO2 catalysts were prepared by sol–gel method. The volumetric ratio of tetrabutylorthotitanate (Ti(OC4H9)4, A.R.), deionized water, anhydrous ethanol (CH3COOH, A.R.) and HNO3 (HNO3, A.R.) was 1:1:5:0.2. Firstly, Ti(OC4H9)4 and anhydrous ethanol were uniformly mixed with a volumetric ratio of 1:4. Anhydrous ethanol, deionized water and HNO3 were mixed, and then Ce(NO3)36H2O was uniformly dissolved into the ethanolic aqueous solution. Secondly, the Ti(OC4H9)4/ethanol solution was added drop wise to the ethanolic solution at room temperature under strongly stirring to carry out hydrolysis. After continuously stirring for 3 h, the yellowish transparent sol was obtained. Thirdly, the sol was dried at 80 8C for 24 h to give xerogel. The powder of the xerogel was calcinated under air at 500 8C for 3 h to obtain TiO2 and CeO2–TiO2 catalysts. The atom ratio of Ce and Ti is 5/1, 3/1, 1/1 and 1/3, respectively. CeO2 catalyst was prepared with coprecipitation by adding dropwise 0.2 M Ce(NO3)3 aqueous solution to excess ammonia solution. The precipitate was dried at 100 8C for 12 h and calcined under air flow at 500 8C for 3 h to get CeO2 catalyst. 2.2. Oxidation reaction The reaction of WAO was preformed in a 1 L autoclave equipped with a stirrer ensuring desirable mass transfer from
the gas to liquid phase and to the catalyst. A 5 g/L catalyst and 500 mL acetic acid aqueous solution (concentration: 0.083 M) were loaded into the reactor, then N2 was introduced into the reactor. When the reactor was heated to 230 8C, pure oxygen was added into it till a total pressure to 5 MPa. The point was taken as ‘‘zero time’’ under stirring, and the reaction started. The reaction time was 180 min. Samples from the reactor were taken at different reaction time, and COD concentration of the samples was measured. The experimental setup was shown in our study [19]. 2.3. Catalysts characterization 2.3.1. BET, XRD and SEM The surface area of the catalysts was estimated at 77 K by N2 adsorption using a Quantachrome Autosorb Automated Gas Sorption System. X-ray powder diffractometer (XRD) analysis was carried out in D/max-IIIA powder diffractometer using Cu ˚ ). The analyse was performed over an Ka radiation (1.5418 A scanning range of 2u = 10–908 at a speed of 68 min1. The surface elements of the used CeO2–TiO2 catalyst were analyzed with means of a scanning electron microscope (SEM) with JEOL JSM-6301F equipped with energy–dispersion microanalysis system. 2.3.2. XPS X-ray photoelectron spectroscope (XPS) was carried out to analyze the composition and the chemical state of the surface elements for the catalysts. PHI ESCA 5700 instrument, with a Al Ka X-ray source (1486.6 eV) and pass energy of 29.5 eV operating at a pressure of 7 1010 Torr, was used. The binding energies were calibrated with respect to the signal for contamination carbon (binding energy = 284.62 eV). An instrument error of 0.1 eV can be assumed for the measurement. Curve fitting was carried out using a Physical Electronics PC-ACCESS ESCA-V6.0E program with a Gaussian–Lorentzian sum function. The Gaussian–Lorentzian mixing ratio was kept in the range of 0.8–1.0. 2.3.3. The reaching of metal ions The metal leaching of the catalysts in the liquid phase was measured with the inductively coupled plasma (ICP). 3. Results and discussion 3.1. XRD of TiO2, CeO2 and CeO2–TiO2 catalysts The phase behavior of TiO2, CeO2 and CeO2–TiO2 catalysts was measured by X-ray diffractometer. The patterns of XRD for the catalysts are shown in Fig. 1. For TiO2 catalyst, the presence of peaks as an attributive indicator of anatase titania (2u = 25.28, 37.80, 48.05, 53.89) and of rutile titania (2u = 27.45, 41.23) is detected. It means that TiO2 catalyst calcined at 500 8C exists as the coexistence of anatase and rutile, and the anatase titania is a dominating structure. For CeO2 catalyst, only strong diffraction peaks of an attributive indicator as cubic CeO2 (2u = 28.57, 33.10, 47.53, 56.38) are
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[20]. On the other hand, the addition of Ti could promote the dispersion of CeO2 particles, and restrain the increase of CeO2 crystalline particles at high calcination temperature. Therefore, CeO2 peaks become much wider and weaker, and CeO2 particles become small. It is helpful to increase the surface area of CeO2–TiO2 catalysts. In Table 1, we see that the surface area of CeO2–TiO2 catalysts is much larger than that of CeO2 or TiO2 catalyst, and it increases with an increase of Ti content. This indicates that the interaction of Ce and Ti could make the crystal size lower and surface area increase for CeO2–TiO2 catalysts. It could lead to the effective active sites increase on the surface of CeO2–TiO2 catalysts. 3.2. XPS of TiO2, CeO2 and CeO2–TiO2 catalysts
Fig. 1. XRD patterns of different catalysts.
observed in the XRD pattern. It indicates that Ce exists as CeO2. In the CeO2–TiO2 catalysts, the patterns show the presence of CeO2 phase, while no relevant anatase or rutile phase of TiO2 is observed in the spectra of XRD. Moreover, the peaks of CeO2 become much wider and weaker with the increase of Ti content. When the ratio of Ce and Ti is 1:3, the faint scattering CeO2 peaks are observed. It indicates that CeO2 particles tend to not integrate crystalline phase, and CeO2 exists as amorphous and crystalline phase increasing Ti content. This means that the crystal size of CeO2 particles decreases with the increase of Ti content in the CeO2–TiO2 catalysts. The crystal size of the catalysts was evaluated using the Scherrer equation, and shown in Table 1. For CeO2 catalyst, the average crystal size of CeO2 particles is about 13 nm. With the increase of Ti content in CeO2–TiO2 catalysts, the average crystal size of CeO2 becomes smaller from 13 to 4 nm. This result is good consistent with that of XRD patterns. However, it is surprising that no relevant of Ti diffraction peaks are observed in the spectra of CeO2–TiO2 catalysts. The result is not same as that CeO2. In the mixture of Ti alkoxide and Ce nitrate, Ti and Ce ions reached the atomic distribution. During Ti4+ hydrolysis to form the gel, Ce ions dispersed into the gel web at the range of atomic scale. This might make the longrange order structure of TiO2 crystal particles destroyed in the drying and calcination process. So TiO2 exists as amorphous phase in the catalyst, and the diffraction peaks are not observed in the spectra of CeO2–TiO2 catalysts. The result is the same as
XPS analyse was preformed to get a better understanding the chemical state of all elements on the catalyst surface. Fig. 2 shows XPS survey spectra of the fresh CeO2–TiO2 catalyst. It can be seen clearly that there are Ce, Ti and O peaks except for contamination carbon on the catalyst surface. Fig. 3 presents Ti2p XPS spectra concerning double peaks (Ti2pi1/2 and Ti2p3/ 2). The binding energy of Ti2p3/2 at about 458.2–458.5 eV could be attributed to Ti(IV)O2 in the catalyst for TiO2 and CeO2–TiO2 catalysts [21,22]. The result indicates that Ti is present as amorphous phase TiO2 on the surface of CeO2–TiO2 catalysts. Ce3d XPS spectra of CeO2 and CeO2–TiO2 catalysts are shown in Fig. 4. Ce3d spectra are very complicated, and the series of core-level spectra exhibit three-lobed peaks. From these peaks, Ce4+ oxidation state is predominant, and Ce3+ oxidation state maybe is distinguishable through the Ce3d spectra. The series of v and u peaks are from the 3d5/2 and 3d3/2 states, respectively. The peak of v and v00 could be assigned to a mixing configuration of 3d94f2(O2p4) and 3d94f1(O2p5) Ce(IV) state, and v000 to 3d94f0(O2p6) Ce(IV) state. The peak of v0 is attributed to 3d94f1(O2p6) Ce(III) final state. The series of u
Table 1 The BET surface area and the crystal size of the different catalysts Samples
Crystallite size (nm)
The surface area (m2/g)
TiO2 CeO2 Ce/Ti Ce/Ti Ce/Ti Ce/Ti
17 13 10 8 5 4
21.3 66.5 81.0 90.5 94.1 145.7
5/1 3/1 1/1 1/3
Fig. 2. The XPS survey spectra of CeO2–TiO2 catalyst.
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S. Yang et al. / Applied Surface Science 252 (2006) 8499–8505 Table 2 XPS Ce3d data measured for the different catalysts Samples
CeO2 Ce/Ti Ce/Ti Ce/Ti Ce/Ti
Fig. 3. The XPS Ti2p spectra of the different catalysts.
structure can be explained in the same way [23]. For CeO2 catalyst, v0 and u0 feature peaks are seen in the CeO2 spectrum, but the intensity of two peaks is very weak. In the addition of Ti, the peaks of Ce3d for CeO2–TiO2 catalysts mildly change: (i) the decease in structure of v000 and u000 peaks, (ii) the decease of v and u peaks, (iii) the increase of v0 and u0 peaks in structure. This indicates that a fraction of the cerium exists as Ce3+ on the catalyst surface. For CeO2 and CeO2–TiO2 catalysts, the binding energy for u000 is about 916.5–917.0 eV, and for v peak it is around 882.5–882.9 eV [23]. According to Shyu, the ratio of intensity of u000 peak to the total intensity of Ce3d depends on the Ce4+/Ce3+ ratio [24]. With the rules of qualitative and quantitative analysis of XPS, Table 2 shows the result of the quantitative analysis with XPS analysis. As shown in Table 2, for CeO2 catalyst the ratio of the intensity of u000 peak to the total intensity of Ce3d is about
Fig. 4. The XPS Ce3d spectra of the different catalysts.
5/1 3/1 1/1 1/3
Ce3d (eV) v
u000
881.95 882.12 882.20 882.25 882.74
916.33 916.37 916.38 916.74 916.49
Atomic ratio Ce/(Ti + Ce)
u000 /Ce3d (%)
1 0.83 0.78 0.72 0.37
9.2 8.7 7.9 7.3 5.9
9.2%, and is larger than that of CeO2–TiO2 catalysts. With the increase of Ti content, the ratio gradually decreases from 9.2 to 5.9%. This means that the Ce3+ increases with increasing Ti content on the surface of CeO2–TiO2 catalysts. And the atomic ratio of Ce/(Ce + Ti) is also listed in Table 2. At the different Ti content, the surface concentration of Ce is larger than that of the as-prepared concentration of CeO2– TiO2 catalysts. This result is consistent with that of [25]. With increasing Ti content, Ce concentration slightly decreases, and does not obviously change except for Ce/Ti 1/3 catalyst. In Fig. 4, Ce3+ is present and its content increases with the increase of Ti content on the surface of CeO2–TiO2 catalysts. This could lead to O1s chemical state change on the surface catalysts. Therefore, O1s patterns of XPS also were measured. The spectra are shown in Fig. 5. It can be seen that O1s XPS spectra exhibit single-lobed peaks, and the peaks are asymmetric (the left sides are wider than the right), indicating that the different types of oxygen exist on the catalyst surface. The O1s peaks could be fitted into two peaks referred to as the lattice oxygen OI and the chemisorbed oxygen OII [9,25]. The fitted curves are shown in Fig. 5. The O1s peaks at about 529.0–530.0 eV could be attributed to the lattice oxygen (OI) for TiO2, CeO2 and CeO2–TiO2 catalysts [20,21,25]. Another component OII at about 531.0 eV belongs likely to the chemisorbed oxygen [9,26]. As shown in Fig. 5, the chemisorbed oxygen is present, but the lattice oxygen is much richer than the chemisorbed oxygen on the catalyst surface. As shown in Table 3, for TiO2 catalyst the percentage of the chemisorbed oxygen to the total oxygen (OT) is much smaller than CeO2 and CeO2–TiO2 catalysts. The content of chemisorbed oxygen does not obviously change, only slightly decreased for CeO2–TiO2 catalysts with the increase of Ti content. With the existence of Ce3+ on the surface of CeO2–TiO2 catalysts, it could create a charge imbalance, the vacancies and unsaturated chemical bonds on the catalyst surface. These make the chemisorbed oxygen exist on the surface. The increase of Ce3+ content is helpful for the chemisorbed oxygen on the catalyst surface to increase. However, it does not agreewith our result. The reason could be that: the proportion of Ce/(Ce + Ti) on the surface decreases with Ti content increasing. It could affect the chemisorbed oxygen content. Moreover, the ionic radius of Ce ˚ , while the ionic radius of Ti4+ is only 0.68 A ˚ [27]. ion is 0.93 A When Ti is added to CeO2 catalyst, a fraction of Ti4+ at the interface can enter the CeO2 lattice and substitute for Ce3+. This might make the charge imbalance weakened. It could lead to that the chemisorbed oxygen decreases on the catalyst surface. This
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Fig. 5. O1s curves fitted of the different catalysts.
result is agreement with [28]. The chemisorbed oxygen on the catalyst surface is the most active oxygen and plays an important role in oxidation reaction [9,29,30]. This means that CeO2–TiO2 catalysts might have good activity in WAO of organic compounds. And due to the different chemisorbed oxygen content of TiO2, CeO2 and CeO2–TiO2 catalysts, we predicate that the catalytic activity might be affected. Table 3 XPS O1s data measured for the different catalysts Sample
TiO2 CeO2 Ce/Ti Ce/Ti Ce/Ti Ce/Ti
5/1 3/1 1/1 1/3
O/OT (%)
E (eV) O1s
FW
OI
OII
OI/OT
OII/OT
529.75 528.87 529.12 529.16 529.25 529.40
1.60 1.74 1.80 1.84 2.01 2.04
529.87 528.84 529.11 529.16 529.28 529.35
531.47 531.28 531.43 531.26 531.20 531.11
88.86 78.19 78.58 78.96 79.32 83.52
11.14 21.81 21.42 21.04 20.68 16.48
3.3. The activity and stability in CWAO of acetic acid Fig. 6 shows the COD removal of different catalysts and the blank experiment in WAO of acetic acid. As shown in Fig. 6, it can be seen that only 4.5% COD is removed without catalyst, and 9% and 14% acetic acid are oxidized with TiO2 and CeO2 catalysts at 230 8C and 5 MPa after 180 min reaction time. It means that TiO2 and CeO2 catalysts do not have obvious activity in WAO of acetic acid. When CeO2– TiO2 catalysts are used, the COD removal is much higher than that of TiO2 and CeO2 catalysts. This indicates that CeO2–TiO2 catalysts have good activity in WAO of acetic acid. Moreover, the COD removal changes with the increase of Ti content. The order of the catalysis activity was obtained: Ce/Ti 1/1 > Ce/Ti 3/1 > Ce/Ti 1/3 > Ce/Ti 5/ 1 > CeO2 > TiO2 > no catalyst. Using the Ce/Ti 1/1 catalyst, which is the most active catalyst, 64% conversion of acetic acid is obtained at 230 8C and 5 MPa after 180 min reaction run.
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S. Yang et al. / Applied Surface Science 252 (2006) 8499–8505 Table 4 The metal ion concentration in the effluents after CWAO of acetic acid The leaching metal ions
Catalysts TiO2
CeO2
Ce/Ti 5/1
Ce/Ti 3/1
Ce/Ti 1/1
Ce/Ti 1/3
Ce (mg/L) Ti (mg/L)
– 0.08
0.67 –
0.71 0.05
0.65 0.04
0.61 0.05
0.61 0.05
WAO of organic compounds involves a free radical chain reaction, and hydroxyl radical (OH) and hydroperoxyl radical (HO2) are strong oxidative reactive species in aqueous solution [13,31]. With oxygen, HO2 could be produced by organic compounds (RH) and the catalyst according to the reactions followed: Fig. 6. The COD removal in WAO of acetic acid with the different catalysts at 230 8C and 5 MPa.
The metal leaching of catalysts is important, since continuous leaching of metal ions is one of the direct causes of catalyst deactivation. Therefore, leaching of metal ions for the different catalysts was measured with ICP. The concentration of the dissolved Ce and Ti ions in the solution after 180 min reaction time is shown in Table 4. In our study, the leaching concentration of metal ions is very low. This means that CeO2– TiO2 catalysts have good stability. The used Ce/Ti 1/1 catalyst was measured with SEM-EDX. It is seen that the surface of the used catalyst contains C element peak in Fig. 7. This indicates that the carbonaceous compounds deposit on the catalyst surface. By DTA-TG measure about l wt.% weight of the carbonaceous compounds deposits on the surface of the used Ce/Ti 1/1 catalyst. And with the washed and air-dried used Ce/ Ti 1/1 catalyst, about 58% COD removal was obtained in WAO of acetic acid at 230 8C and 5 MPa after 180 min reaction time. This means that the deposit does not obviously affect the activity of the catalyst. Therefore, CeO2–TiO2 catalysts have good activity and stability in WAO of acetic acid. Moreover, the carbonaceous compounds deposited on the surface were analyzed by XPS. We found that the carbonaceous polymeric compounds exit on the surface of the used Ce/Ti 1/1 catalyst.
RH þ Ce4þ ! RH Ce4þ
(1)
RH Ce4þ ! R þ Ce3þ þ Hþ
(2)
Ce3þ þ O2 ! Ce4þ þ O2
(3)
O2 þ Hþ ! HO2
(4)
The organic compound RH in the solution could firstly absorb on the catalyst surface (in Reaction (1)). Reaction (2) occurs on the catalyst surface and is a fast reaction. By the Reaction (3) the chemisorbed oxygen could trap the electron of Ce3+, and then produce superoxide radical (O2). Hydroperoxyl radical (HO2), the strong oxidative reactive specie, could be easily formed in the acidic solution from O2 through Reaction (4). So Reactions (1)–(4) indicate that the occurrence of O2 is the important step to produce HO2. For TiO2 catalyst, the content of the chemisorbed oxygen is very low on the surface catalyst. This leads to that the activity of TiO2 catalyst is very low, and almost same as no-catalyst. For the different Ti content of CeO2–TiO2 catalysts, there exist the higher content chemisorbed oxygen and the lower value of Ce3+
Fig. 7. The pattern of SEM-EDX of the used Ce/Ti 1/1 catalyst.
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on the catalyst surface. This could promote the process of Reaction (3), i.e. the chemisorbed oxygen could quickly accept the electron from Ce3+ and transform into O2. This enhances the occurrence of O2, i.e. the rate of the producing HO2 is quickened. Therefore, adding CeO2–TiO2 catalysts into the reaction, the activity obviously increases in WAO of acetic acid. For Ce/Ti 1/3 catalyst, the lower content of the chemisorbed oxygen is present on the surface. It inhibits the chemisorbed oxygen to get the electron, and then form O2. This leads to the lower activity for WAO of acetic acid with Ce/Ti 1/3 catalyst. On the other hand, when the addition of Ti into CeO2, the particle size decreases and the surface area increase for CeO2based catalysts. This makes the effective active sites increase on the surface of the catalyst. It is advantageous for the oxygen and organic compounds to be absorbed on the surface and to be oxidized. Therefore, Ce/Ti 1/1 catalyst has the highest activity in CWAO of acetic acid. The result is in good agreement with that of the activity of TiO2, CeO2 and CeO2–TiO2 catalysts in WAO of acetic acid. 4. Conclusions In our study CeO2–TiO2 catalysts prepared with sol–gel method have good activity and stability in WAO of acetic acid. The interaction of Ce and Ti affects the surface structural properties of CeO2–TiO2 catalysts. With the increase of Ti content, the average crystal size of CeO2 decreases and the surface area increases for CeO2–TiO2 catalysts. Adding Ti to CeO2-based catalyst, there exist higher content of the chemisorbed oxygen and the low valence of Ce3+ on the surface. In the addition of Ti, the activity of CeO2–TiO2 catalysts increases, and is much higher that of TiO2 and CeO2 catalysts in CWAO of acetic acid. Moreover, the optimal atom ratio of Ce and Ti is 1. With Ce/Ti 1/1 catalyst, over 64% COD removal is attained in WAO of acetic acid at temperature reaction of 230 8C and the total pressure of 5 MPa after 180 min reaction time. Acknowledgements This work was supported by the Natural High Tech. Research & Development Program (No. 2002AA601260) and the China Postdoctoral Science Foundation (2004-7).
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