Density functional modelling of materials for fuel cell catalysts with reduced content of critical components
The thesis entiteled “Density Functional Modelling of Materials for Fuel Cell Catalysts with Reduced Content of Critical Components” deals with the following studies: • Adsorption sites at the {100} nanofacets of ceria nanoparticles can effectively anchor a wide range of transition metal atoms in th...
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| Formato: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2017 |
| País: | España |
| Recursos: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/462853 |
| Acesso em linha: | http://hdl.handle.net/10803/462853 |
| Access Level: | acceso abierto |
| Palavra-chave: | Catàlisi heterogènia Catálisis heterogénea Heterogeneus catalysis Teoria del funcional de densitat Funcionales de densidad Density functionals Metalls de transició Metales de transición Transition metals Ciències Experimentals i Matemàtiques 54 |
| Resumo: | The thesis entiteled “Density Functional Modelling of Materials for Fuel Cell Catalysts with Reduced Content of Critical Components” deals with the following studies: • Adsorption sites at the {100} nanofacets of ceria nanoparticles can effectively anchor a wide range of transition metal atoms in the form of Mn+ cations. Oxidation of the M centers takes place with the concomitant reduction of n Ce4+ cations to Ce3+. Higher oxidation states are favored by transition metals in later periods and in groups more to the left side of the periodic table. The preferred coordination mode of Mn+ cations at the {100} facets depends on the metal and on its oxidation state. Adsorption in each studied M1-ceria system is stronger than the binding of the corresponding M atom in a representative M79 particle. • Ceria nanoparticles show the ability to accommodate atomic Pt, Pd, Ni and Cu dopants more easily in surface positions than in bulk positions. Of the surface positions, under-coordinated corner ones are most prone to stabilizing the studied atomically dispersed transition metals. Upon partial reduction of the doped ceria nanoparticles, either via oxygen vacancy formation or homolytic dissociative adsorption of H2, surface Pt, Pd and Ni dopants feature +2 oxidation states, binding to four O atoms in square-planar fashion. Pt and Pd dopants inside ceria particles can be stabilized in the +4 state. In turn, Cu exhibits +2 or +3 state depending on its location in the ceria particle. • Pt dopants in bulk ceria feature +4 oxidation state and inherent octahedral coordination by six O atoms, leaving two more distant O atoms less-coordinated. Upon formation of a nearby oxygen vacancy bulk Pt4+ is reduced to Pt2+ and modifies its environment through a strong lattice distortion to become square- planar coordinated. Nanostructuring of Pt-doped ceria makes the +4 state of Pt energetically favorable up to two oxygen vacancies formed nearby. Formation of the third vacancy destroys the octahedral environment of Pt4+ and results in Pt2+ in the specific square-planar coordination. Nanostructuring of Pt-doped ceria also facilitates the formation of oxygen vacancies close to the dopant. Such improved oxygen storage capacity is related to the presence of surface Ce atoms accepting electrons of released O atoms more favorably than bulk Ce4+ species. • Atomic Pt2+ species adsorbed on {100} nanofacets of ceria are found to be resistant to reduction upon the formation of oxygen vacancies, increasing loading of the doping noble metal and deposition of Sn atoms. The onset of Pt2+ reduction to Pt0 is determined by the concentration of Ce3+ cations in the nanoparticle. To start the reduction, adsorption energy of the Pt2+ species needs to fall to around or below the cohesive energy of Pt metal. It is estimated to take place after formation of two oxygen vacancies per Pt adsorbate or adsorption in the vicinity of Pt2+ species of approximately three Sn atoms oxidized to Sn2+. • Our study addressing the usage of CO probe molecule for exploring Pt-CeO2 electrocatalysts for methanol oxidation revealed that the stretching frequency of on-top CO adsorbed on supported Pt particles correlates with the coordination number of the underlying Pt atom and reflects the particle size. Comparison with experimental results suggests that sub-nanometer particles of ca. 30 or fewer Pt atoms are formed at the applied electrochemical conditions. • Overall, the studies outlined in the thesis demonstrated new advantages of using dedicated nanoparticle models together with density-functional calculations to describe ceria-based nanomaterials for catalysis and related applications. This approach used in combination with experimental studies has been shown particularly successful for systems with properties strongly affected by their nanostructure and thus hardly accessible to conventional slab models. |
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