SOL-GEL DERIVED NANOCRYSTALLINE Ce045Zr045La010O2 S/Pt AND Ce09Sm01O2 S/Pt SOFC ANODES: THE PECULIARITIES
' of'the catalytic performance
E. V. Frolova1, M. I. Ivanovskaya1, A. A. Yaremchenko2, V. V. Kharton12, V. A. Sadykov3
1 Research Institute for Physical Chemical Problems, BSU Leningradskaya str., 14, Minsk, 220030, Belarus Phone: +375-29-5071365; fax: +375-17-2264696 E-mail: frolova@bsu.by; evfrolova@yahoo.com
2 Department of Ceramics and Glass Engineering, CICECO, University of Aveiro
3810-193 Aveiro, Portugal
3 Boreskov Institute of Catalysis of SO RAN Novosibirsk, Russia
Фролова Елена Владимировна
Сведения об авторе: канд. хим. наук, научный сотрудник НИИ физико-химических проблем Белорусского государственного университета (БГУ).
Область научных интересов: нанотехнология сложно-оксидных систем, химия твердого тела, природа структурных дефектов.
Публикации: более 80 работ.
Ивановская Мария Ивановна
Сведения об авторе: канд. хим. наук, ведущий научный сотрудник НИИ физико-химических проблем БГУ.
Область научных интересов: химия твердого тела, роль структурных факторов в определении электрофизических и адсорбционных свойств метал-локсидных материалов.
Публикации: более 300 работ.
Сведения об авторе: канд. хим. наук, ведущий научный сотрудник Университета Авей-ро (Португалия), тематический редактор «Журнала электрохимии твердого тела» (Германия), член редакционных коллегий 4 международных журналов.
Область научных интересов: высотемпературная электрохимия и физическая химия твердого тела. Публикации: автор более 280 статей в международных и национальных изданиях, включая 7 обзоров, 2 книги и 2 патента.
Хартон Владислав Вадимович
Яремченко Алексей Александрович
Сведения об авторе: канд. хим. наук, сотрудник Университета Авейро (Португалия).
Область научных интересов: высотемпературная электрохимия ионных и смешанных проводников, топливные элементы, электрокаталитические реакторы.
Публикации: автор более 100 статей в международных и национальных изданиях, включая 3 обзорных статьи.
Садыков Владислав Александрович
Сведения об авторе: доктор хим. наук, профессор, зав. лабораторией Института катализа им. Борескова СО РАН.
Область научных интересов: химия твердого тела, гетерогенный катализ, кинетика и механизм каталитических реакций окислительно-восстановительного типа. Публикации: более 150 работ.
Статья поступила в редакцию 03.09.2007 г. на конкурс «Катализ в альтернативной энергетике» Ред. per. № 123. The article has entered in publishing office for contest "Catalysis in renewable energy" 03.09.2007. Ed. reg. No. 123.
International Scientific Journal for Alternative Energy and Ecology № 8(52) 2007
© 2007 Scientific Technical Centre «TATA»
E. V. Frolova, M. I. Ivanovskaya, A. A. Yaremchenko, V. V. Kharton, V. A. Sadykov
Sol-Gel derived nanocrystalline Ce0 45Zr0 45La0 10O2-d/Pt and Ce0 9Sm0 ^2^/Pt SOFC anodes: the peculiarities of the catalytic performance
The present work focused on the evaluation of electro-catalytic activity in a solid oxide fuel cell (SOFC) type reactor of nano-crystalline Ce0 45Zr0 45La0 jO2-5 and Ce0 9Sm0 jO2-5 samples, prepared via inorganic sol-gel method, and their characterization. Structural peculiarities of samples were investigated by means of TEM, XRD, EDXA and EPR methods. It seems that the colloidal transition results in the loose cubic Ce1xMxO2-fl (M = Zr, Sm, La) fluorite-type solid solution structure with higher concentration of structural defects [Ce3+-Fo]. A isolated oxygen vacancies arrangement caused the increasing in oxygen transport. The presence of La(III) enables to retain the cubic fluorite structure of the nano-scale particles up to 1200 °C. The morphotropic phase transformation combined with strong metal-support interactions seems to be responsible for the attractive catalytic activity of Ce0.45Zr0.45La0.10O2-s-based anodes towards partial oxidation of CH4.
1. Introduction
Ceria-based oxides are widely used as catalysts for carbon monoxide oxidation, water-gas shift and hydrogenation reactions, and as a key component of three-way catalysts for automotive exhaust gas treatment [1-3]. Recently, a significant attention was focused on the application of ceria and its derivatives for the conversion of methane to synthesis gas, a mixture of CO and H2 used as a feedstock for the commercial methanol and Fischer-Tropsch synthesis [4-9]. Particular interest relates to use of the ceria-based materials as components of solid oxide fuel cells (SOFCs) anodes [10-16]. SOFCs are an attractive electrical energy generation technology which has many advantages over existing technologies including high fuel conversion efficiency, low pollutant emissions, low noise, and fuel flexibility with prospects for operation directly on natural gas. Direct oxidation of methane in SOFCs provides a higher open circuit voltage, elimination of the reformer, and the possibility to co-generate electrical power and useful products such as synthesis gas [11, 14-17].
Since the defective structure and high temperature stability and dispersity of the samples are the most essential peculiarities of materials under consideration it is very important to develop the methods allowing to control these structural features. Processing of oxide catalysts by the sol-gel and related soft chemistry methods, using solutions at the initial step of synthesis, leads to numerous advantages including great surface area and highly defective structure of the product [18].
The present work is focused on the evaluation of electro-catalytic activity in a solid oxide fuel cell (SOFC) type reactor of nano-crystalline Ce045Zr045La01O2-5 and Ce0,9Sm01O2-s samples, prepared via inorganic sol-gel method, and their characterization.
2. Experimental
The powders of Ce0 45Zr0 45La0 10O2-S and Ce0 9Sm0 jO2-5 were synthesized by inorganic modification of the sol-gel method. The samples were prepared by co-precipitation of the hydroxides from aqueous solutions of Zr(IV), Ce(III), La(III) and
Sm(III) nitrates with proper molar ratio. A co-precipitation of complex hydroxides was preceded by a drop-wise addition of aqueous ammonia under continuous stirring. After precipitation, the samples were thoroughly washed by distilled water. After washing the precipitates were transformed into colloidal solution under short (less that 1 minute) ultrasonic treatment and then air-dried at 50 °C and calcined at elevated temperatures in air. The fabrication of anodes and the studies of
Ce0.45Zr0.45La0.1O2-8/Pt and Ce0.9Sm0.1°2-8/Pt composites anodes electrocatalytic activity for direct oxidation of dry CH4 were described in [16, 17]. The structural peculiarities of Ce0 45Zr0 45La0 1O2-5 and Ce0 9Sm0 1O2-5 samples were characterized by means of XRD, TEM, BET, EPR and XPS spec-troscopy. X-ray diffraction (XRD) patterns were collected using a HZG 4A instrument (CoKa radiation). The particle size was estimated from the XRD data using the Scherer equation, and from the TEM images (LEO 906E). EPR spectra were recorded at 300 K in the X-band range on a VARIAN E112 spectrometer. The g values were determined using DPPH and Mn2+/MgO as standards. FTIR-spectra in the range 400-4000 cm-1 were recorded using Avatar 330 XPS studies were performed with ES-2401 spectrometer (MgKa = = 1253,6 eV). Core level energies were calibrated by the C 1s line with BE = 284.6 eV.
3. Results and discussion
Methane oxidation over nanocrystalline
Ce0.45Zr0.45La0.1O2-8/Pt and Ce0.9Sm0.1O2-8/Pt samples at 900oC in a SOFC-type reactor shows well-
developed but weary different electro-catalytic performance as anodes towards partial oxidation of methane (POM). Such a behavior is rather unusual for ceria-based catalysts active to total CH4 combustion [19]. Therefore, the main task of present work is to elucidate the origin of the difference in the catalytic performance of Ce0 45Zr0 45La0 1O2-5 and Ce0 9Sm0 1O2-5 samples.
The conversion of dry methane over Ce0.45Zr0 45La010O2-5/Pt anodes at 700-800 °C results in dominant CO2 formation even at low O2:CH4 ratio (Fig. 1). Increasing temperature to 900 °C
Fig. 1. Catalityc performance of Ce1SmO2_i /Pt and (CelZr )1LaiO2_h/Pt in the SOFC-type reactor with YSZ-membrane: a) methane conversion and CO selectivity on Ce045Zr045La010Os/Pt (900 °C) versus inlet O2:CH4 ratio in comparison with c Pt- u Ce0 sGd0 2O2 /Pt-anodes: 1 — theoretical yield; 2 — system "anode + catalyst" formatted of Ce0 45Zr0 45La010O2/Pt; 3 — Ce0 45Zr0 45La0 l0O2_JPt-anode; 4 — Ce0 sGd0 2O2_JPt-anode; 5 — Pt-anode. b) Temperature dependence of CH4 conversion and selectivity to CO formation on Ce09Sm01O2 /Pt-anode at O2:CH4 = 0,5 ratio on inlet in comparison with data for Ce0SGd02O2 S/Pt-anode (OO). c) 'Time dependent CH4 conversion and CO selectivity in reactor with Ce045Zrg45La01gO2_JPt anode and Pt-modified Ce045Zrg45La01gO2& catalyst.
leads to a sharp increase of the catalytic activity towards partial oxidation reaction. At O2:CH4 ratio of 0.5, stoichiometric for the POM, CH4 conversion and selectivity to carbon monoxide achieves 54 and 73%, respectively. Under similar conditions methane oxidation over porous Pt anode results in lower conversion but higher selectivity to CO, which may indicate a contribution of the reverse water-gas shift reaction. The practical use of platinum anodes is, however, impossible due to extensive carbon deposition; the coking limited the operation of these anodes to 35-40 h even at the O2:CH4 ratio close to 0.5. On the contrary, no carbon imbalance between the influent and effluent gas mixtures was observed for Ce0 45Zr0 45La0 10O25/Pt composite anodes within the limits of experimental uncertainty (Fig. 1,c).
Further improvement in the catalytic performance was achieved by packing the reactor with Pt-modified Ce0 45Zr0.45La010O2-5 catalyst (Fig. 1,a). Incorporation of additional catalyst has rather small effect at 650-800 °C, but increases conversion efficiency approximately twice and also improves selec
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