Catalizadores de Pt-Sn para la reacción de oxidación de etanol
En los últimos años se ha incrementado el interés por el desarrollo de nuevos sistemas energéticos, alternativos al uso de combustibles fósiles, solidarios con el medio ambiente y sostenibles. Un ejemplo de este tipo de tecnologías son las pilas de combustible de etanol directo (DEFC), las cuales permiten la obtención de energía a partir de la oxidación electroquímica del etanol y cuya principal limitación radica en la búsqueda de catalizadores con una relación eficiencia catalítica/precio elevada. En este contexto, los catalizadores de Pt-Sn se han posicionado como firmes candidatos para la oxidación de etanol de forma eficiente. Sin embargo, para un desarrollo óptimo que los haga competentes en el mercado, es imprescindible una investigación y desarrollo más exhaustivo de los mismos. De esta forma, el objetivo principal de esta Tesis Doctoral se centra en el estudio fundamental de la influencia de la estructura superficial de los catalizadores de Pt-Sn en el mecanismo para la reacción de oxidación de etanol (ROE) y la oxidación de monóxido de carbono adsorbido (principal veneno catalítico durante la ROE), junto a la síntesis y caracterización de nanopartículas de Pt-Sn soportadas sobre carbón, altamente eficientes para la ROE.In the last few years, the interest in developing new and more environmentally friendly energy systems as an alternative to the use of fossil fuels has drastically increased. Direct ethanol fuel cells (DEFCs) represent a clear example, since these cells can provide energy from the electrochemical oxidation of an abundant material as ethanol. The main problem lies on finding a catalyst with a high catalytic efficiency/cost ratio. In this sense, Pt-Sn catalysts have been positioned as strong candidates for the efficient oxidation of ethanol. However, for an optimum development in order to make them commercially competitive, an exhaustive research about the behavior of these catalysts for the oxidation of ethanol is necessary. In this sense, the objective of this Doctoral Thesis is the fundamental study of the influence of Pt-Sn catalysts surface structure on the ethanol (EOR) and adsorbed carbon monoxide (main catalytic poison during the EOR) oxidation reactions, as well as the synthesis and characterization of highly efficient Pt-Sn nanoparticles supported on carbonaceous materials. Well-defined Pt single crystals (Pt (111), Pt (100) and Pt (110)) decorated with Sn adatoms were employed to investigate the influence of the surface catalyst structure on the reactions previously mentioned. The use of Sn modified Pt single crystals allowed to demonstrate, not only the high dependence of the EOR and the adsorbed carbon monoxide oxidation reaction mechanisms on the surface catalyst structure, but also the improvement in the electrocatalytic activity by adding Sn adatoms. The geometry of the adsorption of Sn adatoms on Pt depending on Pt surface structure, as well as the difference in the electroactivity of the catalyst with Sn coverage, were also studied by using these electrodes. The optimum Sn coverage (value that exhibits the highest activity for EOR) was established. Differences in the mechanism toward the EOR and the influence of surface adatoms on the mechanism itself were elucidated applying differential electrochemical mass spectrometry (DEMS). Although the role of Pt surface structure in the electrocatalytic activity of Pt-Sn catalysts toward EOR was investigated by the use of well-defined Pt single crystals, from a practical point of view, the use and commercialization of this type of electrodes is totally unrealistic, since catalysts with high catalytic efficiency but an appropriate cost are preferred. To fulfill this problem, the synthesis of nanocrystals with small sizes, to obtain large specific areas, has received substantial research interest over the past decades. In this sense, during this Doctoral Thesis, Pt-Sn nanoparticles supported on different carbon materials were synthesized by the formic acid reduction method (FAM). A series of techniques were employed for the physicochemical characterization of the synthesized catalysts in order to determine the crystallite size and the lattice parameter, to check the correct dispersion of the particles on the carbon support, as well as to elucidate the total and the surface catalyst composition. Different Pt-Sn atomic ratios and carbon supports were employed with the purpose of studying the influence of both, the nature of the support and the amount of Sn on the catalyst, for the adsorbed carbon monoxide oxidation reaction and the EOR. Conventional electrochemical techniques, such as cyclic voltammetry and chronoamperometry, were used to analyze the electrocatalytic activity of the synthesized materials. On the other hand, spectroclectrochemical in situ techniques were applied in order to establish the changes in the mechanism promoted by the employment of different carbon supports and Pt-Sn atomic ratios. Finally, the knowledge gained about the influence of the surface structure of the Pt-Sn electrodes on the activity toward the EOR and the Pt-Sn nanoparticles synthesis supported on carbon, was exploited to synthesize highly efficient shape-controlled Pt-Sn nanoparticles (cubic Pt-Sn nanoparticles).