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7SOL-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: [email protected]; [email protected]
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 Ce0.45Zr0.45La0.10O2- /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 selectivity to carbon monoxide at 850900 °C. Both the CH4 conversion and CO selectivity approach the theoretical equilibrium values, calculated assuming negligible carbon formation, and
achieve 87 % and 84 %, respectively, at 900 °C and O2:CH4 ratio of 0.5. That improvement is, probably, due to reforming of residual methane on the catalyst surface with H2O and CO2 formed at the anode; as a result, the products mixture composition becomes closer to the thermodynami-
cally predicted. The catalytic performance of the reactor with Ce0.45Zr0.45La0.10O2- /Pt anode and Pt-
modified Ce0 45Zr0 45La0 10O2-S catalyst was found stable with no evidence of degradation with time
at 900 °C (Fig. 1,c).
As for Ce0.45Zr0.45La0.10O2- -containing electrodes, CH4 oxidation on Ce0.9Sm0.1O2- /Pt anode results in total combustion at 700-800 °C. Increasing temperature to 900 °C improves electrocatalytic performance; CH4 conversion and CO selectivity achieve 41 % and 55 %, respectively (Fig. 1,b). The conversion and selectivity to carbon monoxide are, however, lower compared to Ce0.45Zr0.45La0.10O2- /Pt
cermets and porous Pt anodes. The difference in the catalytic behavior of Ce0.45Zr0.45La0.10O2- and
Ce0 9Sm0 1O2-5 can be related to two major factors, namely lattice oxygen mobility and metastability of Ce0 45Zr0 45La0 10O2-S fluorite-type structure.
Typical CH4 TPR curves on the studied catalysts are presented in Fig. 2. In the reaction of TPR by methane for Ce0 45Zr0 45La0 10O2-S and
86
International Scientific Journal for Alternative Energy and Ecology № 8(52) 2007
© 2007 Scientific Technical Centre «TATA»
IotJ
E. V. Frolova, M. I. Ivanovskaya, A. A. Yaremchenko, V. V. Kharton, V. A. Sadykov
Sol-Gel derived nanocrystalline Ce0 45Z1Q 45La0 ^^-d/"^ and Ce0 9Sm0 1O2.d/Pt SOFC anodes: the peculiarities of the catalytic performance
Fig. 2. Typical CH4 TPR curves for Ce0A¡Zr0A¡La0MO2_s (a) and Ce0 9Sm01O2_s (b) samples
Ce0 9Sm0 1O2 5 samples the most reactive oxygen forms are mainly removed at low temperatures giving deep oxidation products (CO2), while syngas is mainly produced at higher temperatures. However, the observed curves of both CO and H2 formation have different shapes for the samples under investigation. One should note that the formation of CO starts at lower temperatures and with higher efficiency on the Ce0 45Zr0 45La0 10O2-5 catalysts (580 °C) possessing the higher concentration of the weakly bonded surface and near to the surface containing oxygen, rather than on a Ce0 9Sm0 1O2 5 sample (615 °C). In contrast, H2 lower temperature formation and higher performance is revealed for Ce0 9Sm0 1O2 5 and also almost the two times higher performance of CO formation at 880 °C. The latest can confirm the higher lattice oxygen mobility.
Mobility of oxygen ions in the ceria lattice may play an important role in the catalytic performance influencing oxygen supply from the bulk to active surface centers and the kinetics of redox process. For instance, inverse correlation between lattice oxygen mobility and selectivity towards POM was observed for Ce1-xGdxO2-5 fluorite-type solid solutions [A]. Higher oxygen mobility in Ce0 9Sm0 1O2 5 compared to that of Ce0 45Zr0 45La0 10O2-5 [B] promotes oxygen ions activation at the platinum/oxide interface and results in faster oxidation of methane and/or partial oxidation products. On the other hand, instability of Ce0 45Zr0 45La0 10O2-5 cubic lat-
tice, especially in the surface layers, is expected to increase the concentration of surface defects/active centers, thus improving the catalytic activity.
The crystallization of Ce0 45Zr0 45La0 10O2-5 and Ce0 9Sm0 1O2 5 samples resulting in cubic Ce1xMxO2_5 (M = Zr, Sm, La) fluorite-type (Fm3m) solid solution (SS). Also, the reflections are quite broad for both of the samples that indicate the fine particle nature of the product (~5-8 nm according to XRD and TEM data). According to XRD data analysis, the higher values of the unit cell parameter "a" comparing to the thermodynamically stable samples were observed for both samples. This phenomenon might be cause by "pushing apart" of oxygen vacancies as far as oxygen vacancy have some charge being in electro-neutral lattice. It results in "looseness" of the complex-oxides structures and this effect is maximal in the case of the isolated oxygen vacancies formation. But this state is unstable for the system and in case of individual oxides the structural relaxation by means of oxygen vacancies clusterisation occurs. It must be noticed that for complex oxides this structure metastable state is more stable and can be even detected if the oxygen vacancies being in the first shell of the different metals.
In addition, the changes in the particle size after thermal treatment at 600-1000 °C are minor. The variations in the unit cell parameter with high temperature treatment for Ce0 45Zr0 45La0 10O2-5 sample can be associated with partial separation of La(III) cations from the lattice and/or oxygen non-stoichiometry variations. On the other hand, in Ln-Zr-O system (where Ln = La, Ce) formation of the thermally stable up to high temperatures (1500 °C) LnxZr1-xO2-5 SS with fluorite-type is confirmed (100 % reflection 111 is marked at 35,52 2© that is close to the position of ZrxCe1-xO2 solid solution SS) [20]. It is possible to assume that type of structure for the fine particles. As the oxygen lattice of both Ce0 45Zr0 45La0 1O2-5 and LnxZr1-xO2-5 SS have a cubic symmetry the structural changes include moving or migration of the kations mostly. In the case of the fluorite-type SS formation in the system Ce-Zr-O there is an essential displacement of oxygen atoms out of their equilibrium positions, determining high oxygen capacity of this structure [21]. Therefore, it is possible to assume, that the structural relaxation of system occurs in the way of the morphotropic phase transformations with formation of very small LnxZr1-xO2-5 particles placed, probably, at/inter grain boundary [20]. Nevertheless, small amount of this "phase" and/or its high dispersiveness do not allow to fix it by XRD and furthermore to establish its structure. However, no evidence for extra reflections due to non-incorporated La2O3, Sm2O3, CeO2 or ZrO2 was observed in any XRD patterns of the samples even after calcination at high temperature (1200 °C).
The TEM patterns of
Ce0.45Zr0.45La0.1O2-8 sample
calcined in air at 600 and 1000 °C show the formation of spherical particles with average diameter of approximately 5 and 10 nm correspondingly. This is in agreement with the XRD data. On the TEM data (Fig. 3), after long heat treatment of
Ce0.45Zr0.45La0.1°2-8 sample at
1200 °C the particles of two types are formed: the aggregated particles (globules) with a size more than 20 nm and separate spherical particles of very small sizes less then 2 nm. The particles of small size are essentially less in number. The data of X-ray microanalysis (EDX) of the mentioned sample are resulted in the Fig. 3, where the ratio of basic elements (Ce, Zr, La), established is specified at scanning all sample and separately large and small particles. The ratio Ce:Zr:La on EDX data of the annealed at 1200 °C Ce0 45Zr0 45La0 1°2_5 sample, almost corresponds to the entered quantity in the case of large globules. In the particles of the small sizes contents of Zr is few times higher than for Ln (Ln = Ce, La). The idea of the formation of the separate zirconates groups in the complex-oxide sample, in other words LnxZr1-x°2-5 SS at/inter grain boundary was also confirmed by FTIR data [22]. Nevertheless, the XRD curves are not observed additional reflections or asymmetrical broadness of the present.
The explanation of unexpected increasing in unit cell parameter is in a good agreement with EPR
O
Fig. 3. TEM images of: a) Ce 4lZr 4lLa1O2_& sample calcined in air at 1200 °C. Insertion: Small particles. The chemical composition according to EDX results for the big and small particles shown by the arrow; b) Ce Sm1O2_s sample calcined in air at 1000 °C
data. The EPR measurements of Ce0 45Zr0 45.
Ce0.45Zr0.45
and not in volume (gl = 1,966, gjj = 1,947) like in untreated by Pt Ce0 45Zr0 45La0 10°2-s sample. In contrast, Pt deposition (with the same condition) on Ce0 9Sm0 1°2_5 sample does not bring any changes in [Ce3+-V ] defects concentration or location. However, the resolution of [Sm3+-Vo] defects sig-
iLa0.10°2-8
and Ce0 9Sm0 1°2-5 samples calcined at 600 °C in air, revealed basically two axial paramagnetic centers signals: first one (gl = 1,966 and gjj = 1,947) was assigned to the bulk Ce3+-centers (2F5/2) in pair with oxygen vacancy or ([Ce3+-Vo] defects); and the second one (g^ = 1,996, g3 = 1,998 and g\ = 1,973, gl = 1,974) was assigned to Sm3+-cent-ers (6H5/2) located at surface and bulk correspondingly. Those centers exist in Ce°2 structure also in pair with Vo ([Sm3+-Vo] defects). In other words EPR data confirms the occurrence of two type of Vo in Ce0 9Sm0 1°2-5 SS structure. It must be noticed that according to the EPR spectra analysis the paramagnetic centers concentration in general two times higher for the Ce0 9Sm0 1°2-5 sample.
After Pt deposition and thermal treatment in air at 700 °C the concentration of Ce3+ in La0 10°2-s sample became five times high-
f —^ / Sm*
|| B,mT
330
335
340
345
er as compared with untreated one. Moreover, as it can be conclude from g-factor value the Ce3+ centers are became surface-disposed (gl = 1,966, g2 = 1,940),
Fig. 4. Single scans (300 K) first-derivative EPR spectra
0f Ce0.46Zr0.46La0.10O2-'o (a) and Ce09Sm0JO2-'o (b) samPles
calcinated in air at 600 °C before (1) and after (2) Pt-treatment (deposition and thermal treatment in air at 700 °C)
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
nal became lower as well as its intensity. Also, it is possible that some part of [Sm3+-F ] defects became a surface located. Moreover, the difference
for Ce0.45Zr0.45La0.10°2-8/Pt and Ce0.9Sm0.1O2-S/Pt
samples in the signals satiety occurs at 77 K. For Ce0 45Zr0 45La0 10O2S/Pt sample it became and for Ce0 9Sm0 1O2 5 does not.
According to the EPR data it seems that Pt interacts with this complex-metal-oxides matrix very differently. In the case of Ce0 45Zr0 45La0 10O2-S sample Pt/support strong interactions occurs and in the case of Ce0 9Sm0 1O2 g — do not. It must be noted that strong chemical interaction between the components with charge transfer complexes formation ([Ptd+Zrd ] at Pt/Ln^Zr^O^ SS clusters grain boundary) for Ce0 45Zr0.45La010O2-S/Pt sample is confirmed by XPS analysis also [22].
4. Conclusion
The structural peculiarities investigation of the nano-crystalline Ce0 45Zr0 45La0 1O2 5/Pt and Ce0 9Sm0 1O2 5/Pt anodes, prepared via inorganic sol-gel method shows that colloidal transition of
Ce0.45Zr0.45La0.10°2-S and Ce0.9Sm0.1O2-S co-precipitates results in loose cubic Ce1xMxO2S (M = Zr, Sm, La) fluorite-type solid solution structure with higher concentration of structural defects [Ce3+-FJ [23]. Isolated oxygen vacancies arrangement caused the increasing in oxygen transport. Investigation of the nano-crystalline Ce0 45Zr0 45La0 1O2 S/Pt shows that he presence of La(III) enables to retain cubic fluorite structure of the nano-scale particles up to 1200 °C. The surface- and near-surface-located Ce3+ paramagnetic centers formation and also mor-photropic phase transformation with formation of LnxZr1-xO2S SS combined with strong metal-support interactions may also promote oxygen atoms activation by neighbouring Pt, incorporated into near-surface layers of the nano-structured catalyst. A mentioned structural peculiarity that caused by zirconium presence in the support structure seems responsible for higher electro-catalytic activity of Ce0 45Zr0 45La0 10O2S-based anodes in a solid oxide fuel cell (SOFC) type reactor towards partial oxidation of methane as compared to Ce0 9Sm0 1O2 S/Pt one.
Acknowledgements
Frolova E. V., Ivanovskaya M. I. and Sadykov V. A. grateful to the ISTC (Project No. 3234). Helpful discussions and experimental assistance of A. A. Valente is gratefully acknowledged.
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