Theoretical modeling of heterogeneous catalysts based on platinum and cerium oxide

Abstract

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.

En aquesta tesi s’han utilitzat mètodes teòrics basats en la DFT per estudiar les propietats de sistemes formats per Pt i CeO2. Mitjançant un anàlisi de l’estructura electrònica i geomètrica dels models considerats, s’ha contribuït a la comprensió a nivell microscòpic de les interaccions entre Pt i CeO2.

Així, s’han descrit els processos de transferència de càrrega entre diferents espècies de Pt i substrats de CeO2, resultant en l’oxidació de les espècies metàl•liques i la reducció de cations Ce. A més, s’ha demostrat que la presència del platí facilita la reducció de l’òxid de ceri a través de processos de migració (spillover) d’àtoms d’oxigen i que aquest efecte és més pronunciat quan el CeO2 és nanoestructurat. Això ha permès explicar per primera vegada els mecanismes de formació de vacants en catalitzadors basats en Pt i CeO2, que són responsables de la seva capacitat d’emmagatzemar oxigen i per tant, de la seva activitat catalítica.

Amb els models de nanopartícules de CeO2, s’ha descrit l’existència de complexos d’adsorció on el Pt es troba dispersat atòmicament i en forma de catió a la superfície de les nanopartícules de cèria. A més, aquestes espècies es poden utilitzar com a electrocatalitzadors en els ànodes de cel•les de combustible, permetent el desenvolupament de dispositius que utilitzin platí de forma més eficient i, per tant, reduint-ne dràsticament els costos.

L’estudi de la reactivitat de diferents espècies de Pt envers la reacció WGS mitjançant la determinació del perfil energètic de la dissociació de l’H2O ha permès caracteritzar els efectes que tenen la mida de la partícula de Pt i les interaccions metall-suport amb l’òxid de ceri en l’activitat d’aquests catalitzadors. S’ha demostrat l’excepcional activitat catalítica d’aquests sistemes i s’han identificat les propietats que els fan més reactius, superant en rendiment als catalitzadors que s’utilitzen industrialment per a catalitzar la WGS.

Els estudis realitzats durant aquesta tesi han permès descriure detalladament propietats de sistemes formats per Pt i CeO2. Aquesta caracterització serveix per comprendre els sistemes catalítics basats en aquests materials i per guiar el disseny racional de nous materials amb les propietats catalítiques idònies per a cada aplicació.