Síntesis de catalizadores basados en óxidos de metales de transición para la oxidación preferencial de monóxido de carbono en corrientes de hidrógeno

Resumen

[EN] The aim of this work is to study different active catalysts for the preferential oxidation (PROX) of CO in excess hydrogen. Nowadays this is the easiest and most efficient process to reduce residual concentrations of CO in the streams fed into polymeric fuel cells. It is well known that hydrogen streams need to be almost entirely CO-free in order to avoid platinum poisoning. As the reaction is heterogeneous, high surface area catalysts are preferred, since this involves higher surface/volume ratios for the gas-solid interaction and, consequently, higher catalytic activities can be achieved. Two types of high surface area nanocatalysts were prepared and analyzed in the PROX reaction: unsupported nanostructures and metal wire supported nanostructures. The silica confined co-precipitation (SACOP) technique, developed by the Functional Porous Materials Group at INCAR, was used as the reference method to obtain unsupported nanostructures. This is a modified silica-template route for obtaining high surface area nanostructures due to the collapse of the silica surrounding the metallic hydroxides during the precipitation stage. The SACOP procedure was optimized for the synthesis of cerium oxide, which contributed to our knowledge of the mechanisms involved. The procedure was then employed to prepare copper-based catalysts (CuOx/CeO2 and CuMn2O4), after a rigorous comparison of the activities of similar catalysts found in the literature tested for the PROX reaction. The catalysts used in this study exhibit a high activity for the PROX reaction but gradually undergo a slow deactivation during the reaction. The deactivation of CuOx/CeO2 nanocatalysts is caused mainly by carbonates formation, whereas for CuMn2O4 it seems to be produced by the progressive reduction of copper via the action of carbon monoxide. The SACOP method has produced the most active cobalt-based catalyst for the PROX reaction ever designed. The resulting nanocatalysts had a variable content in high surface area CoOOH, depending on the type of precipitation used. This species which was present in all the fresh samples and was the main contributor to catalyst performance, was gradually reduced to highly active Co3O4 under PROX conditions. Furthermore, supported Co3O4 mesoporous nanostructures were obtained by means of a mild template-free procedure known as the ammonia-evaporation-induced method. A stainless steel wire mesh was employed as the metallic support. Apart from the wellknown advantages of employing these monoliths over catalytic beds of particles (easy handling, low pressure drop and excellent heat transmission coefficients), these structured catalysts exhibited a good catalytic activity and a remarkable stability over the entire temperature range studied.