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Surfactant-Assisted Synthesis, Characterizations, and Catalytic Oxidation Mechanisms of the Mesoporous MnOx-CeO2 and Pd/MnOx-CeO2 Catalysts Used for CO and C3H8 Oxidation Zhi-Qiang Zou, Ming Meng,* and Yu-Qing Zha Tianjin Key Laboratory of Applied Catalysis Science & Engineering, Department of Catalysis Science & Technology, School of Chemical Engineering & Technology, Tianjin UniVersity, Tianjin 300072, P. R. China ReceiVed: September 09, 2009; ReVised Manuscript ReceiVed: October 26, 2009
A series of mesoporous MnOx-CeO2 binary oxide catalysts with high specific surface areas were prepared by surfactant-assisted precipitation. The CO and C3H8 oxidation reactions were used as model reactions to evaluate their catalytic performance. The techniques of N2 adsorption/desorption, XRD, XPS, TPR, TPO, TPD, and in situ DRIFTS were employed for catalyst characterization. It is found that the activity for CO and C3H8 oxidation of the catalysts exhibits a volcano-type behavior with the increase of Mn content. The catalyst with a Mn/Ce ratio of 4/6, possessing a high specific surface area of 215 m2/g, exhibits the best catalytic activity, which is related not only to its highest reducibility and oxygen-activation ability, as revealed by TPR and TPO, but also to the formation of more active oxygen species on the MnOx-CeO2 interface as identified by TPD. After the addition of a small amount of Pd to the MnOx-CeO2 catalyst, its activity for CO oxidation is greatly enhanced, due to the acceleration of gas-phase oxygen activation and transferring via spillover. However, the activity for C3H8 oxidation is hardly promoted due to the different reaction pathways for CO and C3H8 oxidation. For CO oxidation, the gas-phase oxygen activated by Pd can directly react with the adsorbed CO to form CO2, while, for C3H8 oxidation, which takes place at a much higher temperature than CO oxidation, the C-H bond activation and cleavage may be mainly driven by the active oxygen species on the interface between MnOx and CeO2. The addition of Pd shows little effect on the active interface oxygen species, so no promotion upon C3H8 oxidation is observed. 1. Introduction Up to now, the three-way catalysts (TWCs) have been successfully developed to remove the CO, NOx, and hydrocarbons in the gasoline engine exhaust during the normal running period.1 However, three-way catalysts often show very low activity for the oxidation of CO and hydrocarbons during the cold-start period due to the low temperatures of the catalystbed and exhaust itself ( Mn3Ce7 = Mn5Ce5 > Mn7Ce3 > Mn2Ce8 > Mn1Ce9. The
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Figure 12. Proposed CO reaction pathways over the catalysts Mn4Ce6 (a) and Pd/Mn4Ce6 (b).
higher activity of the sample Mn4Ce6 should result from the following facts. First, compared with the sample with the lowest Mn/Ce ratio of 1/9 and those with a ratio above 5/5, the sample Mn4Ce6 possesses a larger specific surface area (215 m2/g), which can provide more active sites during oxidation reaction. Second, compared with the sample with a lower Mn/Ce ratio of 2/8 and that with a higher Mn/Ce ratio of 7/3, the sample Mn4Ce6 possesses a moderate surface Mn/Ce atomic ratio, as indicated in the XPS section, which favors the contact and interaction between Mn and Ce oxides. Third, the redox property is very crucial for the oxidation catalyst. The H2-TPR and TPO results indicate that the sample Mn4Ce6 with a Mn/Ce ratio of 4/6 possesses not only the highest reducibility but also the better capability for oxygen activation. Furthermore, O2-TPD results show that the samples with Mn/Ce ratios from 3/7 to 5/5 possess more interface oxygen vacancies compared with the samples with lower or higher Mn/Ce ratios. The interface oxygen vacancy can effectively capture gas-phase oxygen to form active interface oxygen species. The interface oxygen between MnOx and CeO2 could be extracted by the adsorbed CO, forming CO2 and recovering oxygen vacancy, as shown in Figure 12a. The activation of C3H8 over metal oxides is thought to occur by abstraction of a H atom from the weakest C-H bond, with a simultaneous reduction of the surface sites and successive formation of the surface hydroxide ions.36 Therefore, for C3H8 oxidation, it is inferred that the active interface oxygen can also preferentially extract the H atom from the weakest C-H bond in the C3H8 molecule to form surface hydroxide ions, which is shown in Figure 14. More active interface oxygen species should favor the activation of C3H8. Thus, the activated oxygen species
Mesoporous MnOx-CeO2 and Pd/MnOx-CeO2 Catalysts
Figure 13. Catalytic activities of the MnOx-CeO2 catalysts with different Mn/Ce atomic ratios for CO oxidation (a) and C3H8 oxidation (b).
Figure 14. Proposed C3H8 reaction pathway over the catalysts Mn4Ce6 and Pd/Mn4Ce6.
forming on the interface are also considered as the main active oxygen species for C3H8 oxidation. Figure 15 shows the light-off curves for CO and C3H8 oxidation of the MnOx-CeO2 catalysts calcined at different temperatures and that promoted by Pd. It is apparent that Pd doping has greatly enhanced CO oxidation. The temperature for CO full conversion is decreased by 35 °C. Such promotion may result from the oxygen spillover effect, as confirmed by TPO, and the spilled over oxygen species can directly react with CO, as discussed in section 3.4. However, for C3H8 oxidation, the situation is far different. It can be seen from Figure 15 that Pd doping does not improve the activity of the catalyst at all; on the contrary, it causes a little decrease. It is mentioned above that the function of Pd is to activate gas-phase oxygen. For C3H8 oxidation, it occurs at much higher temperatures than CO oxidation, so the oxygen activation does not seem difficult. It is reported that the light-off temperatures of the saturated
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Figure 15. Catalytic activities of the MnOx-CeO2 catalysts calcined at different temperatures and the Pd-promoted catalyst for CO oxidation (a) and C3H8 oxidation (b).
hydrocarbon oxidation are closely related to the strength of the C-H bond.37 Therefore, the C-H bond activation in the C3H8 molecule is more crucial for C3H8 oxidation. As analyzed above, the active interface oxygen species between MnOx and CeO2 are the active sites to C-H bond activation, as shown in Figure 14; however, in the TPD test, neither the desorption temperature nor the amount of the active oxygen species is changed by the addition of Pd. Different from CO oxidation, the spilled over oxygen species from Pd may not be effective on C-H bond activation. The active oxygen species on the interface between MnOx and CeO2 may have played a main role in C-H bond activation and cleavage. Since such oxygen species are not influenced by Pd doping, it is understandable that the C3H8 oxidation is hardly promoted. As for the little decrease in activity, the reduction in the specific surface area of the sample caused by Pd doping may account for this logically. The elevation of calcination temperature of the catalyst MnOx-CeO2 always decreases its oxidation activity, irrespective of CO oxidation or C3H8 oxidation, which is especially evident for the sample calcined at a high temperature of 800 °C. The unavoided sintering of active phase results in a dramatic decrease in the specific surface area of the sample, leading to a decrease of active sites, so the activity decrease is natural. In future work, we will aim to improve the thermal stability of MnOx-CeO2 catalyst by adding the dopants La and Zr to the catalytic system, which have shown excellent ability to inhibit the sintering of Co-based mixed oxide catalysts.38
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TABLE 4: Catalytic Performance of Various Catalysts for CO and C3H8 Oxidation catalysts Co/γ-Al2O3 Pd-Co/γ-Al2O3 Pd/TGFd Pt/TGF Mn4Ce6 Pd/Mn4Ce6
T100a
T90b
experimental conditions
125
400 420 261
90
283
0.5% CO/5.0% O2/N2, SV ) 4500 h 0.5% CO/5.0% O2/N2, SV ) 4500 h-1 0.5% C3H8/10% O2/Ar, SV ) 242 cm3 g-1 min-1 0.5% C3H8/10% O2/Ar, SV ) 242 cm3 g-1 min-1 2% CO/10% O2/N2, SV ) 30000 h-1 0.3% C3H8/5% O2/N2, SV ) 30000 h-1 2% CO/10% O2/N2, SV ) 30000 h-1 0.3% C3H8/5% O2/N2, SV ) 30000 h-1 c
200 145
a The temperature for the full conversion of CO. glass fiber.
b
reference -1
The temperature for the 90% conversion of C3H8. c Space velocity.
In order to compare the MnOx-CeO2 catalysts with other typical combustion catalysts, Table 4 gives the performance data for CO and C3H8 oxidation of the reported Co and noble metal based combustion catalysts. It can be seen that the temperatures for the full conversion of CO and 90% conversion of C3H8 over the MnOx-CeO2 catalyst or the Pd-promoted one are much lower than those on Co or noble metal based catalysts. This result indicates that the MnOx-CeO2 mixed oxides prepared by surfactant-assisted precipitation do possess excellent oxidation performance. In order to investigate the stability of the proposed catalysts, the sample Mn4Ce6 and the Pd-promoted one are selected to perform a stability test. It is found that the activities for CO and C3H8 oxidation of the catalysts are well maintained after 10 h of reaction (see the Supporting Information), indicating the high stability of the MnOx-CeO2 and Pd/ MnOx-CeO2 catalysts.
32 32 39 39 this work this work d
Titania modified
Acknowledgment. This work is financially supported by the “863 Program” of the Ministry of Science & Technology of China (No. 2006AA06Z348) and the National Natural Science Foundation of China (Nos. 20676097, 20876110). The authors are also grateful for the support of the Program for New Century Excellent Talents in University of China (NCET-07-0599), the Cheung Kong Scholar Program for Innovative Teams of the Ministry of Education (No. IRT0641), and the Program for Introducing Talents of Discipline to Universities of China (No. B06006). Supporting Information Available: Figure showing a stability test of the catalysts for CO and C3H8 oxidation. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
4. Conclusions (1) The MnOx-CeO2 mixed oxide catalysts with mesoporous pore structure were successfully prepared by surfactant-assisted precipitation, whose specific surface areas follow a volcanotype behavior with the increase of Mn content. The largest specific surface area of 237 m2/g is achieved on the sample with a Mn/Ce atomic ratio of 3/7. Higher Mn content favors the formation of Mn species with higher oxidation state. The catalyst with a moderate Mn/Ce ratio of 4/6 possesses a moderate binding energy of Mn 2p, a moderate surface Mn/Ce atomic ratio, and the best redox property, as a result showing the highest catalytic activity for both CO and C3H8 oxidation. A kind of active oxygen species, namely, the active interface oxygen species formed on the interface oxygen vacancies between MnOx and CeO2, is identified in the mixed oxide catalysts. The oxidation performance of the catalysts seems more dependent on such oxygen species. (2) CO oxidation is via bidentate carbonate routes over MnOx-CeO2 catalysts. The gas-phase oxygen can be captured by the oxygen vacancy on the interface between MnOx and CeO2, forming a kind of active interface oxygen species which readily reacts with adsorbed CO to form CO2. For Pd/ MnOx-CeO2, the gaseous oxygen adsorption and activation mainly take place on Pd sites. The activated oxygen species are spilled over to the vicinal adsorbed CO and directly react with it to form CO2. As a result, the CO oxidation activity of MnOx-CeO2 is greatly enhanced by the addition of Pd. However, for C3H8 oxidation, the active oxygen species on the interface between MnOx and CeO2 play a more important role in C-H bond activation and cleavage. The addition of Pd can hardly promote the C3H8 oxidation activity of the catalyst, since the active interface oxygen species are not affected at all by Pd doping.
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