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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In this paper, we have studied, using X-ray photoelectron spectroscopy (XPS), the chemical stability of TiN, TiAlN and AlN layers produced by magnetron sputtering on Si wafer substrates, against humid, SO 2 -polluted atmospheres. The results have indicated that the TiN layers suffer almost no degradation after seven days of exposure to the aggressive environment. The degradation of the TiAlN layers is small, and this is evidenced by the appearance of signals corresponding to N-O bonds in the N 1s spectra, after seven days of exposure to the corrosive atmosphere and the increase in the Ti 2p spectra of the contribution corresponding to TiO 2 . In contrast, the degradation experienced by the AlN layers is quite large. The spectra reveal dramatic changes after just one day of exposure to the aggressive conditions. This is related to the preferential growth of the AlN layers, which gives rise to an open, columnar structure.

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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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A series of Fe-Ce mixed oxides (95 atom % Fe-5 atom % Ce) has been prepared by different methods: coprecipitation, impregnation, and physical mixture of Ce and Fe oxides. These solids have been tested in the Fischer-Tropsch synthesis. The characterization of the catalytic precursors was carried out by X-ray diffraction (XRD), Raman, Mössbauer, and X-ray photoelectron (XPS) spectroscopic techniques. When the preparation method ensures a microscopic contact between Fe and Ce cations in the solid, several types of Fe-Ce interactions are present in the calcined solids. The interactions take the shape of Fe-O-Ce bridges that can exist either in the hematite-like solid solution or in the interphase between the Fe oxide covered by microcrystals of Ce oxide. In the case of the hematite-like solid solution, Ce(IV) cations are dissolved in the alpha-Fe2O3 network. The promotion by Ce of the catalytic properties observed in the final catalysts can be directly related with the detection of these Fe-O-Ce bridges in the calcined solids. The Ce promotion results in a larger yield to hydrocarbons, a higher production of olefins, and a higher selectivity to medium and large chain hydrocarbons (larger than six carbon atoms). It is proposed that the Ce promotion is due to the presence of Fe0-Ce(III) ensembles in the final catalysts arising from the initial Fe-O-Ce bridges developed in the parent calcined samples.

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