Theoretical modeling of heterogeneous catalysts based on platinum and cerium oxide

• Autores: Albert Bruix Fusté
• Directores de la Tesis: Frances Illas Riera (dir. tes.), Ramón Sayós Ortega (dir. tes.), Konstantin Neyman (dir. tes.)
• Lectura: En la Universitat de Barcelona ( España ) en 2014
• Idioma: inglés
• Tribunal Calificador de la Tesis: Jordi Llorca Piqué (presid.), Carmen Sousa Romero (secret.), Alexis Markovits (voc.)
• Materias:
  • Física
    • Física molecular
    • Química física
    • Física del estado sólido
• Enlaces
  • Tesis en acceso abierto en:  TDX  Dipòsit Digital de la UB 
• Resumen

  • This thesis focuses on the computational study of models for platinum catalysts supported on cerium oxide (CeO2) which are of technological relevance. In these catalysts, ceria is often found acting as a non-inert support, leading to complex metal-support interactions (MSI) that modify the properties of both the oxide and the supported metal. First principles computational methods based on the Density functional Theory (DFT) have been used to study the nature of these interactions and their effect on the atomic and electronic structure and on the chemical activity of these catalytic systems. In particular, charge transfer phenomena have been studied and how the oxygen storage capacity of CeO2 is affected by the presence of deposited Pt particles. The effect of the MSI in the activity towards the WGS has also been addressed, as well as the During the fulfillment of these studies, close collaboration with world leading experimental groups from different countries has been crucial to broaden the understanding of these systems. The interaction of single Pt atoms with the CeO2(111) surface was studied first. By using the DFT+U approach in combination with hybrid functionals the validity of the DFT+U methodology for studying Pt/CeO2 systems was assessed and a suitable value for the U parameter was established for further studies dealing with similar systems. It was found that in its most stable adsorption site, Pt atoms can be found in a neutral form or oxidized to +1 formal oxidation state, with the concomitant reduction of one Ce4+ cation to Ce3+. The formation and stability of Pt dimers on CeO2(111) was studied next. It was shown that Pt atoms can easily diffuse until forming the more stable Pt2 moieties. In turn, the dimers will also diffuse before dissociating, thus describing the initial phase of Pt particle nucleation in Pt/CeO2 catalysts. The effect of the MSI in the reducibility of CeO2 was also addressed. The Oxigen Storage Capacity (OSC) of CeO2 is a consequence of its inherent reducibility, and it is believed that tuning the OSC leads to improving the catalytic performance of ceria-based catalysts. In collaboration with the experimental partners, it was found that the presence of deposited Pt particles facilitates the release of oxygen atoms from ceria by enabling the migration of an oxygen atom from the ceria substrate to the deposited metal. This phenomenon is known as reverse oxygen spill-over mechanism and is only thermodynamically favorable when the ceria substrate is nanostructured. The unprecedented experimental observation of this process and its rationalization through theoretical calculations merited the publication of these results in the prestigious Nature Materials journal. It is also found that very stable cationic Pt2+ species are formed upon the adsorption of Pt atoms on ceria nanoparticles. These adsorption complexes are so stable that they can resist harsh conditions leading to bulk-diffusion and sintering to form larger Pt species. The effect of strong electronic MSI on the activity of Pt/CeO2 catalysts toward the Water-Gas Shift Reaction was also investigated. It was found that the intimate contact between the small metal particles and the oxide substrate triggers electronic perturbations that dramatically enhance the ability of Pt to dissociate water, leading to increased WGS activity. This investigation was performed in collaboration with Jose Rodriguez’s experimental group and was initiated by the research visit that AB carried out to Brookhaven National Laboratory. These landmark results were published in the Journal of the Americal Chemical Society. In addition, the effect of the size and shape of Pt nanoparticles towards their capacity to dissociate water was investigated for lone-standing nanoparticles of different sizes. It was shown that, for Pt particles still under the scalable-to-bulk regime, the size of the particle as well as the type of sites exposed by these play an important role on their reactivity. Smaller particles with uncoordinated corner Pt atoms were found to be most active.