Catalytic combustion study of soot on Ce0.7Zr0.3O2 solid solution

JOURNAL OF RARE EARTHS, Vol. 26, No. 2, Apr. 2008, p. 250

Catalytic combustion study of soot on Ce0.7Zr0.3O2 solid solution FANG Ping (方 萍), LU Jiqing (鲁继青), XIAO Xiaoyan (肖小燕), LUO Mengfei (罗孟飞) (Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China) Received 24 September 2007; revised 3 January 2008

Abstract: The Ce0.7Zr0.3O2 solid solution and CeO2 were prepared using the sol-gel method. The phase structure, crystallite sizes and the reducibility of the catalysts were characterized by XRD and H2-TPR techniques. XRD results indicated that Zr4+ had replaced part of Ce4+ to form a fluorite-like solid solution, which was favorable to obtain ultrafine nanoparticles. The ratio of main H2 consumption for Ce0.7Zr0.3O2:CeO2 was 4.4:1.0, implying that the solid solution could improve the reducibility compared to the single CeO2. The Ce0.7Zr0.3O2 solid solution catalyst showed a sharp combustion peak at 397 ºC, which was 200 oC lower than that of the single soot. The good catalytic activity of the Ce0.7Zr0.3O2 was attributed to the formation of nano-CeO2-based solid solution, which enhanced the reducibility and then improved the combustion activity. As Ce0.7Zr0.3O2 could be easily reduced to Ce0.7Zr0.3O2-x, meanwhile, after oxygenation, the Ce0.7Zr0.3O2-x was recovered to Ce0.7Zr0.3O2 completely. A catalytic combustion reaction mechanism was proposed: the Ce0.7Zr0.3O2 was reduced to Ce0.7Zr0.3O2-x by the reaction with carbon and then it was recovered to Ce0.7Zr0.3O2-x by the interaction with O2. Keywords: Ce0.7Zr0.3O2; reoxidizing-reducing; soot combustion; catalytic activity; rare earths

Diesel engines emit larger quantities of particulate matter (PM), consisting of agglomerates of carbon nuclei and hydrocarbons, SO3 or sulfuric acid, and have been identified as hazardous to human health[1]. The normal way to reduce soot emissions is by the use of a diesel particulate filter placed through the exhaust stream, which can stagnate and burn the soot. For spontaneous regeneration under typical engine operating conditions, oxidation catalysts are required to enhance the soot combustion at diesel exhaust gas temperatures (250–400 oC). In recent times a number of catalysts have been developed for catalytic combustion of soot, such as, noble metal supported catalysts[2], and base metal oxides (V, Cu, Mn, Cr, Co, Fe, Mo, and their mixtures)[3]. As the cost is high, noble metal supported catalysts cannot be employed and extended in the present application. Nowadays, some results have reported that base metal oxide doped eutectic mixtures and low melting point catalysts, have superior performances for soot oxidation because of the extensive contact provided during the oxidation process[4]. However, these coatings are not suitable for application to real systems because of their poor stability[5]. Although CeO2 and CeO2-based catalysts have been extensively studied for the three-way catalyst (TWC) application[6], not many detailed studies have been carried out for soot oxidation on these materials. CeO2 has a potential to increase the oxidation rate of soot, because of

the creation of “oxygen vacancies”[7]. It may open a new route for the development of soot oxidation catalysts. In this study, Ce0.7Zr0.3O2 and CeO2 were prepared for diesel soot combustion. It is expected that the results can give some new insight on the development of catalytic removal of diesel soot.

1 Experimental 1.1 Catalyst preparation Ce0.7Zr0.3O2 solid solution and CeO2 were prepared by using a sol-gel method. Ce(NO3)3·6H2O (>99.5%), Zr(NO3)4·5H2O (>99.5%), and citric acid (>99.5%) were used as starting materials. 15 mmol Zr(NO3)4 and 35 mmol Ce(NO3)3 were dissolved in deionized water and mixed to form Ce-Zr nitrate solution. Citric acid with double the molar amount of the total metal cations was added, to obtain a mixed nitrate and citrate solution. After being stirred for a few minutes, the pellucid solution was heated at 90 ºC until a viscous gel was obtained. The gel was dried at 110 ºC, overnight, followed by calcination at 600 ºC for 4 h in air, to obtain the final sample, denoted as Ce0.7Zr0.3O2. The single CeO2 was prepared in the same manner. Al2O3 calcined at 600 oC was also prepared, to compare with the Ce0.7Zr0.3O2 for soot combustion activity.

Foundation item: Project supported by the Natural Science Foundation of Zhejiang Province (Z404383) Corresponding author: LUO Mengfei (E-mail: [email protected]; Tel.: +86-575-82283910)

FANG P et al., Catalytic combustion study of soot on Ce0.7Zr0.3O2 solid solution

The simulated soot (Printex-U) used in this study was purchased by Degussa AG. The reactive characters of the Printex-U have been determined to be similar to that of the actual diesel exhaust soot[4]. In this experiment, each catalyst and soot in a weight ratio of 9/1, was carefully mixed in an agate mortar to achieve a good contact before it was loaded in the reactor. 1.2 Catalyst characterization X-Ray Diffraction (XRD) patterns were collected on a Philips PW 3040/60 powder diffractometer using Cu Kα radiation. The working conditions were 40 kV and 40 mA. The intensity data were collected in a 2θ range from 20 º to 90º with a scan rate of 1.0 (º)/min at room temperature. The mean crystallite sizes of catalysts were calculated using the Scherrer equation, where the Scherrer constant (particle shape factor) was taken as 0.89. The lattice parameters were calculated by the least squares method according to the Cohen procedure. The reduction properties of the catalysts were measured by means of the H2-TPR technique. Fifty milligrams of sample was placed in a quartz reactor, and the reactor was heated from room temperature to 900 oC at a heating rate of 20 oC/min. Five percent H2 in N2 was used as a reducing agent with a flow rate of 25 ml/min. The amount of consumed H2 during the reduction was calculated based on the analysis with a Thermal Conductivity Detector (TCD). The purpose of the reoxidation treatment was to reveal the oxidation properties of the reduced sample. After reduction at 900oC and after the sample was cooled down to 50 oC, the oxidation took place under flowing air with a flow rate of 20 ml/min for 1.5 h. Subsequently, the reoxidated samples were cooled down to room temperature, followed by the TPR experiments under the above-mentioned conditions. 1.3 Catalytic testing

Fig.1 shows the XRD patterns of the catalysts. In the case of Ce0.7Zr0.3O2, the diffraction patterns are in good agreement with those of single ceria typical fluorite-like cubic structure (CeO2, JCPDS: 81-0792), and no diffraction peaks of zirconia are observed. It is found that the lattice parameter of the Ce0.7Zr0.3O2 (0.5320 nm) is smaller than that of CeO2 (0.5410 nm). As the radius of the Zr4+ ion (0.084 nm) is smaller than the Ce4+ ion (0.097 nm), the change in the lattice parameters of the Ce0.7Zr0.3O2 confirm that the Zr4+ ions have been incorporated into the ceria lattice to form the fluorite-like solid solution. For Al2O3 calcined at 600oC, the diffraction peaks of γ-Al2O3 (JCPDS: 48-0367) are observed. The mean crystallite sizes of all samples calculated by the Scherrer equation, based on the strongest peak FWHM, show that the crystallite size of Ce0.7Zr0.3O2 is 7.3 nm, which is much smaller than that of CeO2 (27.8 nm), suggesting that the formation of the solid solution is favorable to obtain ultrafine nanoparticles. 2.2 Reduction behavior of catalysts H2-TPR profiles of the catalysts are shown in Fig.2. For the single CeO2 there are two reduction peaks at about

Fig.1 XRD patterns of catalysts calcined at 600 oC

The catalytic combustion activity was carried out on a Temperature-Programed Desorption (TPD) apparatus, which was performed with a Balzers Omnistar 200 mass spectrometer by monitoring the m/e ratio 44 (CO2). A mixture of the catalyst and soot, of 20.0 mg, was located in a quartz microreactor and heated in air at 50 oC for 0.5 h with the air flow rate of 20 ml/min as pretreatment, and then cooled down to room temperature. Afterward, the reactor was heated to 800 oC at a heating rate of 10 oC/min with the same flow rate.

2 Results and discussion 2.1 Phase analysis of the catalysts

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Fig.2 H2-TPR profiles of catalysts calcined at 600 oC

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517 oC (peak α) and 875 oC (peak β), respectively. Peak α is attributed to the reduction at the surface region of CeO2, and peak β is attributed to the reduction of the bulk[8]. For Ce0.7Zr0.3O2, the peak α slightly shifts to a higher temperature, and the peak area is larger than a single ceria. The ratio of the main H2 consumption for peak α for Ce0.7Zr0.3O2: CeO2 is 4.4:1.0, implying that solid solution can improve the reduction ability compared to the single CeO2. The reduction peaks at higher temperature (ca. 800 oC) decline significantly, indicating that the reducible sites in the bulk are nearly as active as those at the surface region after forming solid solution. For the Al2O3, no obvious H2 consumption peak can be observed. 2.3 Catalytic testing In the TPD experiments, CO2 was the main gaseous product obtained by mass spectrometry analysis. The formation rate of CO2 (m/e: 44) was used as a measurement to evaluate the catalytic activity. To compare with the literature results, the performance of the catalysts was evaluated by taking the temperature corresponding to the maximum of the peak, called combustion temperature (Tmax), which represented the temperature of maximum soot combustion. The TPD profiles of the catalysts calcined at 600 oC mixed with soot are shown in Fig.3. It can be seen that the Tmax for soot combustion without catalyst is 594 oC, and after mixing with Ce0.7Zr0.3O2, the Tmax is about 200 oC lower, which shows very steep and sharp peaks at 397 ºC. The inlet of Fig.3 amplified from the shadowed part of the Ce0.7Zr0.3O2 sample shows a shoulder peak at 452 oC. The area of the shoulder peak is about 1/12 of the main peak. In recent studies[9], it has been demonstrated that the occurrence of two peaks is caused by a combination of heattransport and mass-transport limitation between the gaseous

phase and the soot sample surface. For the CeO2, the Tmax is at 485 oC, and the peak is broad, which may be the reason why the shoulder peak of the CeO2 cannot be observed as it is probably overlapped. For Al2O3, the Tmax is close to the single carbon combustion temperature. This indicates that the CeO2 and Ce0.7Zr0.3O2 have high catalytic activity, and the Ce0.7Zr0.3O2 shows the highest activity. H2-TPR results (Fig.2) clearly show that the order of the main H2 consumption peak areas of the solid solutions is Ce0.7Zr0.3O2>CeO2>Al2O3, which is consistent with the order of the catalytic combustion activity, suggesting that the activity is related to the reducibility of the sample well. Therefore, the relationship of the reducibility and activity indicates that the formation of Ce0.7Zr0.3O2 solid solution with a small crystallite size significantly improves the reducibility of the samples, and then the high reducibility can enhance the catalytic performance. As Al2O3 has nearly no reducibility, the catalytic combustion temperature is close to that of the single soot. Fig.4 shows the TPR profiles of CeO2 and Ce0.7Zr0.3O2 and

Fig.3 TPD profiles of catalysts calcined at 600 oC mixed with soot

Fig.4 H2-TPR profiles of CeO2 (a) and Ce0.7Zr0.3O2 (b) calcined at 600 oC and their reduced samples reoxidized at 50 oC

FANG P et al., Catalytic combustion study of soot on Ce0.7Zr0.3O2 solid solution

their reduced samples reoxidized at 50 oC. It can be seen that after CeO2 and Ce0.7Zr0.3O2 are reduced, as long as they are reoxidized at 50 oC for 1.5 h. In the second TPR experiments, the reoxidized samples show nearly the same reducibility as in the first test. Therefore, the Ce0.7Zr0.3O2 and CeO2 present a good reoxidizing-reducing ability. As Ce0.7Zr0.3O2 can be easily reduced to Ce0.7Zr0.3O2-x, after oxygenation, the Ce0.7Zr0.3O2-x will turn back to Ce0.7Zr0.3O2 completely. According to the soot combustion mechanism by KNO3[10], it is deduced that the mechanism by Ce0.7Zr0.3O2 shows that the Ce0.7Zr0.3O2 is reduced to Ce0.7Zr0.3O2-x by the reaction with carbon, and it is recovered to Ce0.7Zr0.3O2 by reaction with O2, namely, the lattice oxygen is the active oxygen. The reaction mechanism scheme is proposed as in Fig.5. Though the reoxidizing-reducing ability of CeO2 is nearly the same as Ce0.7Zr0.3O2, as the main H2 consumption peak areas of Ce0.7Zr0.3O2 are larger than CeO2, the combustion activity of Ce0.7Zr0.3O2 is better than CeO2. Moreover, the Ce0.7Zr0.3O2 is an extensively used TWC catalyst[6,11], which shows good stability. Therefore, the Ce0.7Zr0.3O2 catalysts could be potential candidates for catalytic soot combustion.

Fig.5 Scheme of reaction mechanism on Ce0.7Zr0.3O2 catalyst

3 Conclusion The Ce0.7Zr0.3O2 solid solution showed a good catalytic combustion activity, which could be attributed to the formation of the nanocrystalline CeO2-based solid solution. The

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solid solution could provide more active oxygen species. It could enhance the reoxidizing-reducibility, and further improve the performance for catalytic soot combustion.

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