Tantos como para formar un tratado son los Causas Generativos de los Accidentes a que hemos aludido en este titulo, y por eso habremos de limitar su estudio a, las mas generalmente conocidas y sobre las cuales tiene efectividad practica el proceso etiologico, sin hacer caso omiso de los responsabilidades que surgen por el mal sostenimiento de los aerodromos y caminos de uso publico , asi como de los equipos de servicio disponibles para la finalidad que se propone llenar una empresa, sea cual fuere la entidad politico-administrativa que los expresados medios y vehiculos de transporte pertenecen. (…)
Heterogeneous catalysts are complex and dynamic multicomponent systems, usually consisting of several elements and phases. The catalyst active sites are very sensitive to the environmental conditions (gas atmosphere, temperature, and pressure) and may undergo major changes during the course of the chemical reaction. A good understanding of the active catalytic species and their role in the catalytic reaction can only be achieved by studying the catalyst under conditions comparable to those used in the industrial process.
The chemical state of the Cu promoter (1 wt %) in a Cu-promoted Cr-stabilized iron oxide based water−gas shift catalyst has been studied by in-situ X-ray absorption fine structure (XAFS) at 380 °C and elevated pressures, which is close to the conditions typically applied in industrial processes. The Cu promoter enhances the activity of the catalyst. In this study, the catalyst has been studied as fresh, after use for 2000 h, and after a further wash with an aqueous solution of ammonia. In all three cases, CuO and CuFe2O4-like phases are present; the used catalyst also contains some metallic Cu. X-ray absorption spectroscopy (XAS) shows that upon exposure to a typical feed gas for high-temperature shift containing CO, CO2, H2, and water, both the CuO and the CuFe2O4-phases are reduced to metallic Cu at 250 °C; Cr is present as Cr3+ and is slowly incorporated in the Fe3O4 matrix upon further heating to 380 °C and pressurizing to 12 or 24 bar. Washing a used catalyst with an aqueous solution of ammonia primarily removes the metallic Cu but also some CuO and CuFe2O4. Despite the fact that the Cu content in the catalyst after washing with ammonia varies, the activity is the same as that for the unwashed and fresh catalysts. This indicates that only a smaller part, estimated to <0.1 wt %, of the Cu is required to obtain the full promoter effect. These results indicate that the active sites for the water−gas shift reaction are provided by the magnetite surface and are promoted by metallic Cu.
Using the Fischer-Tropsch technology high molecular hydrocarbons are synthesized from CO/H2 mixtures through a surface-catalyzed polymerization reaction using Co-based catalysts. This process enables the transformation of natural gas into clean synthetic fuels. The work discribed in this thesis is the investigation of the pyhsicochemical and catalytic properties of TiO2-supported Co Fischer-Tropsch catalysts and the role played by MnO as a promoter. A detailed characterization of Co/Mn/TiO2 catalysts using a combination of spectroscopy and microscopy have provided new insights into the active site composition and its relationships with the catalytic performances.
