Methane combustion over Pd/ZrO2/SiC, Pd/CeO2/SiC, and Pd/Zr0.5Ce0.5O2/SiC catalysts

doi:10.1016/j.catcom.2011.02.007

Catalysis Communications

Volume 12, Issue 10, 15 May 2011, Pages 870-874

https://doi.org/10.1016/j.catcom.2011.02.007 Get rights and content

Abstract

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.

Graphical Abstract

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. The catalytic activities of Pd/SiC, Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.

Research Highlights

► The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240oC and obtain a methane conversion of 100% at 340oC, and keep 100% methane conversion after 10 reaction cycles. ► The catalytic activities of Pd/SiC, Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. ► XRD and XPS results reveal that Zr⁴⁺ is incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. ►

Introduction

Supported palladium catalysts have been found to have excellent activity toward the catalytic combustion of methane, which has been found to be more environmentally friendly than traditional flame combustion due to lower emissions of NO_x, CO, and unburned hydrocarbons [1], [2]. Usually, the supports of Pd-based catalysts are thermal insulators, such as SiO₂ and Al₂O₃. The methane combustion is a strongly exothermic reaction, therefore the reaction heat accumulated on isolated metal nanoparticles makes them easily sintered [3], [4], [5]. It has been established that SiC can be a potentially excellent catalyst support for various reactions since SiC has admirable chemical stability and high thermal conductivity and stability [6], [7], [8]. Our previous study showed that Pd/SiC catalyst using etched SiC nanowires as the support can completely convert CH₄ at about 390 °C and run 10 reaction cycles without any decrease in the catalytic activity [9].

It is well known that CeO₂ and ZrO₂ are effective promoters for noble-based combustion catalysts because they can enhance the dispersion and stability of metallic active phases. However CeO₂ is easily sintered at high reaction temperatures, and ZrO₂ has the drawbacks of high cost and relatively low surface area [10], [11], [12], [13]. Therefore, researchers have suggested that formation of a Ce_0.5Zr_0.5O₂ solid solution by adding ZrO₂ to CeO₂ can improve oxygen storage capacity, redox properties, thermal resistance and catalytic activity of catalysts at lower temperatures [14], [15], [16], [17]. In this work, we investigate Pd/SiC catalysts modified by CeO₂, ZrO₂ and Zr_0.5Ce_0.5O₂ solid solution respectively, which show excellent activity and stability for methane combustion.

Section snippets

Catalyst preparation

The catalysts are 1 wt.% Pd supported on either SiC (S_BET = 50.8 m²/g)[18], [19] or SiC modified with 1% promoters (ZrO₂, CeO₂ and Zr_0.5Ce_0.5O₂) and were prepared by the impregnation method. First, 0.4 g SiC was added into 20 ml aqueous solution of Zr(NO₃)₄·5H₂O (0.05 wt.%), Ce(NO₃)₃·6H₂O (0.05 wt.%) separately and into the above two nitrate mixture (Zr: 0.025 wt.%; Ce: 0.025 wt.%) under stirring for 12 h. Afterwards all the mixtures were dried at 110 °C for 12 h and then calcined in air at 500 °C for 4 h.

XRD and XPS characterization of fresh catalysts

From the XRD patterns of different fresh catalysts shown in Fig. 1, the strong diffraction peak (2θ = 35.8°) can be indexed to β-SiC. All the catalysts present an obvious peak at 2θ = 33.8°, which attributed to the tetragonal PdO. Pd/ZrO₂/SiC presents two peaks at near 28.1° and 31.4° (2θ), assigned to monoclinic ZrO₂. The main peaks at 28.8° and 33.3° (2θ) in the XRD patterns of Pd/CeO₂/SiC correspond to the cubic, fluorite structures of CeO₂. The diffraction peaks ascribed to ZrO₂ were not

Conclusion

Pd-based catalysts using high surface area SiC (50.8m²/g) modified by CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution as the support could effectively increase the dispersion and activity of active metallic phases. XRD and XPS studies showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. T_10% and T_100% of Pd/Zr_0.5Ce_0.5O₂/SiC are 240 °C and 340 °C respectively and the reaction could be finished in a narrow temperature range (around 100 °C) certainly due to the

Acknowledgments

The work was financially supported by Shanxi Province (Ref: 2008011014-1), NSFC (20973190) and MOST (Ref: SKLCC-2008BWZ010 and 2011CB201405).

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Methane combustion over Pd/ZrO2/SiC, Pd/CeO2/SiC, and Pd/Zr0.5Ce0.5O2/SiC catalysts

Abstract

Graphical Abstract

Research Highlights

Introduction

Section snippets

Catalyst preparation

XRD and XPS characterization of fresh catalysts

Conclusion

Acknowledgments

Appl. Catal. B

Appl. Catal. B

J. Catal.

Appl. Catal. A

Catal. Commun.

Appl. Catal. A

J. Catal.

Catal. Commun.

Catal. Today

Catal. Commun.

J. Catal.

Appl. Catal. B

Catal. Commun.

Methane combustion over Pd/ZrO₂/SiC, Pd/CeO₂/SiC, and Pd/Zr_0.5Ce_0.5O₂/SiC catalysts