Ce 0.97 Cu 0.03 O 2 Nanocatalysts Synthesized via Microwave-assisted Hydrothermal Method: Characterization and CO-PROX C
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Ce0.97Cu0.03O2 Nanocatalysts Synthesized via Microwave-assisted Hydrothermal Method: Characterization and CO-PROX Catalytic Efficiency Vinícius D. Araújo1*, Waldir Avansi2, Artur J.S. Mascarenhas3, Heloysa M.C. Andrade3, Elson Longo2 and Maria I.B. Bernardi1 1. Instituto de Física de São Carlos, Universidade de São Paulo, Rio Claro, São Paulo, Brasil. 2. INCTMN, LIEC, IQ, Universidade Estadual Paulista, Araraquara, São Paulo, Brasil. 3. Instituto de Química, Universidade Federal da Bahia, Salvador, Bahia, Brasil. * correspondent author: [email protected] Phone: (55 16) 3373-9828 Fax: (55 16) 3373-9824 ABSTRACT In the work presented here, Ce0.97Cu0.03O2 nanoparticles were synthesized by a microwaveassisted hydrothermal method under different synthesis temperatures. The obtained nanoparticles were tested as catalysts in preferential oxidation of CO to obtain CO-free H2 (PROX reaction). The samples were characterized by X-ray diffraction, transmission electron microscopy (TEM), electron paramagnetic resonance spectroscopy (EPR) and temperature-programmed reduction (TPR). X-ray diffraction measurements detected the presence of pure cubic CeO2 for all synthesized samples. TEM images of the Ce0.97Cu0.03O2 nanoparticles revealed that samples synthesized at 80oC are composed mainly of nanospheres with an average size of 20 nm. The formation of some nanorods with an average diameter of 8 nm and 40 nm in length, and the size reduction of the nanoparticles from 20 to approximately 15 nm is observed with increasing synthesis temperature. EPR spectra indicated that copper is found well dispersed in sample synthesized at 160oC, located predominant in surface sites of ceria. For samples synthesized at 80 and 120oC, the species are less dispersed than in the other one, resulting in the formation of Cu2+−Cu2+ dimmers at the surface of ceria. TPR profiles presented two reduction peaks, one below 400oC attributed to the reduction of different copper species and a second peak around 800oC attributed to the reduction of Ce4+→ Ce3+ species located in the volume of the nanoparticles. The peak related to the reduction of copper species shifts to lower temperatures with increasing synthesis temperature, i.e., the sample synthesized at 160oC is more easily reduced than the ones synthesized at 120 and 80oC. The nanoparticles showed active as catalysts for the CO-PROX reaction. The microwave-assisted method revealed efficient for the synthesis of Ce0.97Cu0.03O2 nanoparticles with copper species selective for the CO-PROX reaction, which reaches CO conversions up to 92% for the sample synthesized at 160oC. INTRODUCTION The preferential oxidation (PROX) reaction is an attractive process for the removal of carbon monoxide, mainly of concern in fuel cell design, since the presence of CO impedes the H2/O2 reaction by the absorption of CO onto the anode [1]. Ceria (CeO2) has earned intensive interest in the past decade because it plays a vital role in emerging technologies for environmental and energy-related applications, such as fast-response
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