The compound NiCo 2 O 4 , with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of z3.5 and z2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo 2 O 4 of {Ni 3z 0.1 Co 2z 0.9 } tet [Ni 3.5z 0.9 Co 2.5z 1.1 ] oct O 4 . This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co 3z and Ni 3z . The surface of NiCo 2 O 4 was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co 2z , Co 3z , Ni 2z , Ni 3z and, probably, Ni 4z .

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Two series of Fe−Ce catalysts were prepared following two different methods:  coprecipitation from Fe and Ce nitrate solutions and physical mixing of pure Fe and Ce precursors. Evidence of the presence of a chemical interaction between Fe and Ce was found in the calcined state of the coprecipitated catalysts. Such evidence was obtained with different techniques. The Fe−Ce interaction occurs through the formation of hematite-like and cubic ceria-like solid solutions. In the hematite-like solid solution, Ce cations are dissolved in the hematite structure, whereas in the cubic ceria-like solid solution Fe cations are dissolved in the ceria structure. Such interactions were absent in the samples prepared by the physical mixing. It is suggested that the Fe−Ce interaction present in the calcined state results in a strong Fe−Ce interaction in the final catalyst that defines their better catalytic properties. When tested in the Fischer−Tropsch synthesis of hydrocarbons from CO + H2 gas mixtures, the coprecipitation method series showed higher CO conversion rates, higher hydrocarbon formation rates, and a higher degree of olefinicity than the pure Fe catalyst sample and the Fe−Ce samples prepared by physical mixing.

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ABSTRACT.-Two different types of ancient bricks (XII-XIVth centuries) collected from historical buildings of Toledo (Spain) were characterized by Optical Microscopy, SEM/EDS, Electron microprobe, XRD, DTA and 57 Fe-Mössbauer spectroscopy. Physical properties such as water absorption and suction, porosity, density and compression strength were also determined.Several minerals found in the brick matrix, such as garnet, let us infer raw material sources; calcite, dolomite, illite and neo-formed gehlenite and diopside phases, on temperature reached in firing; secondary calcite, on first cooling scenarios; and manganese micro-nodules, on late pollution environments. XRD and DTA of original and re-fired samples supply information about firing temperatures. Additional data on firing conditions and type of the original clay are provided by the Mössbauer study. Physical properties of both types of bricks were compared and correlated with raw materials and fabric and firing technology employed. The physicochemical characterization of these bricks provides valuable data for restoration purposes to formulate new specific bricks using neighbouring raw materials.

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Two Fe-Ce catalysts were prepared by wet impregnation of Ce onto iron oxyhydroxide (FeOOH) and hematite iron oxide (a-Fe 2 O 3 ), respectively. Their performance in the Fischer-Tropsch (FT) synthesis was investigated and compared with that obtained with a Ce-free a-Fe 2 O 3 catalyst. It was observed that the behavior of the different catalysts changed along the course of the FT reaction. The catalysts were tested for different periods of time, carefully passivated, recovered from the reactor and characterized by different techniques. The FT activity of the Ce-loaded and Ce-free catalysts decreased initially, but at a certain point the catalytic activity started to increase. The time needed to reach this inflection point depended on the catalyst composition, being shorter for the Ce-promoted catalysts. The catalytic activity of the Ce-free catalyst increased when the Fe 3 C species were transformed into x-Fe 2.5 C, which are suggested to be the carbide phase present when polymerized carbon species (C b ) are formed. The addition of Ce to the iron oxyhydroxide developed solids with a higher BET surface area. Besides, these samples displayed a higher FT activity at long time-on-stream (TOS). Moreover, Ce addition also facilitated the formation of the C b species previous to the evolution of Fe 3 C into x-Fe 2.5 C, and therefore, promoted the FT synthesis reaction.

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