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New catalytic formulations for anodes of intermediate temperature direct fuel oxidation solid oxide fuel cells

Final Activity Report Summary - CATANITSOFC (New catalytic formulations for anodes of intermediate temperature direct fuel oxidation solid oxide fuel cells)

All these catalysts have been prepared by microemulsion method in order to achieve control of morphological/structural characteristics under conditions of optimum compositional homogeneity in comparison with other more contentional or simple preparation methods. For that purpose, n-Heptane, Triton-X-100, n-Hexanol and tetramethyl ammonium hydroxide (TMAH) were used as organic solvent, surfactant, co-surfactant and precipitating agent (base), respectively. Cu(NO3)2, Ni(NO3)2, Fe(NO3)2, Ce(NO3)2, Tb(NO3)2 and Gd(CH3COO)2 salts are used as metal precursors.All the catalysts were characterised by SBET, XRD, XPS, Raman, TPR-TPO, TEC. Calcined Cu-Ni/CeO2 catalyst produces total oxidation products whereas catalyst activated with H2 favor partial oxidation process. Moreover, activation through prereduction facilitates the oxidation of methane by reducing the starting temperature of the reaction to 623 K. In case of Cu-Fe catalyst where Fe is in metallic phase the same trend is observed although it exhibits higher onset temperature.

The addition of either Gd and Tb or Ni and Fe to the basic Cu-CeO2 formulation enhances the capability of the materials to oxidize methane as evidenced by the higher amounts of methane oxidation/decomposition products detection and the decrease of the reaction temperature onset. The initial calcined catalysts exhibit most thermodynamically stable oxidized form for all elements. Introduction of doping elements Gd or Tb into the fluorite structure of CeO2 is evidenced by analysis of the shift of corresponding XRD peaks towards positions which indicate formation of the corresponding mixed oxides as well as the absence of evidence of segregated phases or effects of tetragonal distortions.

There are series of diffraction peaks in case of Cu-Ni that could be attributable to the formation of alloys between Cu and Ni for systems tested after CH4-TPR runs up to 900 C. In contrast, the separation of metallic species is observed in case of bimetallic Cu-Fe systems. In turn, XRD analysis of such used catalysts after reaction with CH4 shows the presence of reduced ceria in undoped systems, whereas Ce3+ is not observed when Tb or Gd are incorporated into the support, indicating the stabilization of the fluorite structure by Tb and Gd. By TPO it could be seen that the introduction of both transition metals (Ni or Fe) enhances the carbon formation. However, these deposits have lower thermal resistance than those only residually formed for monometallic copper formulations.

As it is expected, Ni increases the amount of coke created. The same conclusions could be observed by Raman, in which the ID1/IG ratio between main peaks due to carbon-related species can give us a good approximation for the degree of graphitization of the carbon formed. This ratio shows us that the additives increase the limits of the range (considering analyses done at different micro-zones of the material), therefore different kinds of carbons are generated, and Ni favours graphitisation decreasing to lower values the inferior limit of the range. We could observe a similar behaviour also by XPS. Modifications in the support do not produce great effects in TEC value. In Tb doped CeO2 there is an increase of the slope which may be due to a phase shift experienced at that temperature or some other structural change involving a change of cell volume.

On the other hand, modifications in the metallic part state induce important variations in the TEC values. In any case, the values of TEC obtained for the different samples are close to those of 8YSZ and CGO, electrolytes commonly used in SOFC and IT-SOFC, so that the materials apparently present good characteristics in terms of thermal compatibility with those electrolytes.