Chemical reactivity of oxide surfaces
(Matteo Farnesi, Min Huang, Lucie Szabova, and Stefano Fabris)

Highly-active catalysts based on precious-metal nano-particles (Au, Pt, Rh, Pd) supported by reducible metal oxides are key-components for abating the emissions from hydrocarbons combustion and are of fundamental importance in the production and purification of hydrogen. The mechanisms responsible for the superior catalytic activity of these devices rely on subtle electronic effects following the creation of oxygen vacancies and on the interaction between the metal nano-particles, the defective surfaces, and the molecular species entering the main chemical reactions to be catalyzed (water-gas shift, steam reforming, CO oxidation). This project aims at providing a microscopic understanding of these complex highly-active catalysts by DFT calculations.

Electron localization determines defect formation on ceria surfaces

The high performance of ceria (CeO2) as an oxygen buffer and active support for noble metals in catalysis relies on an efficient supply of lattice oxygen at surface reaction sites, governed by the formation of oxygen vacancies and by the high mobility of oxygen adspecies. The origin of these unique properties are studied by comparing reducible and non-reducible oxide substrates, and by combining STM experiments and DFT calculations suitable for modeling reduction processes. The major defects present on the most stable surface of ceria upon oxygen release are single vacancies on the 1st and 2nd surface layers, and linear vacancy clusters. Electron localization controls defect cluster formation since, as a general rule, defects tend to maximize the number of exposed reduced Ce3+ ions.

Mechanisms of oxygen buffering at low-temperature

Besides vacancies, superoxides appear as key factors in controlling the O buffering capacity of ceria, being weakly bound, highly mobile and reactive. The local mechanisms for their activation are shown to involve both electronic and structural effects, such as electron localization, electron transfer, vacancy sealing, and surface relaxation. A rationalization on the basis of the calculated energies and electronic structures is proposed in the context of recent spectroscopic measurements.

CO adsorption and oxidation to ceria surfaces

In CO-rich atmosphere and above 550-600 K, the prevalent mechanism for CO oxidation is of the Mars-van Krevelen type that requires the adsorption of CO as an intermediate reaction state. The actual CO oxidation step involves the participation of lattice oxygen leading to CO₂ desorption and surface oxygen vacancy formation. The vacancy is then sealed by molecular oxygen that reoxidizes the surface. This project focuses on the CO adsorption and oxidation steps of this redox reaction mechanism, by studying with density functional theory (DFT) calculations the interaction between CO molecules and ceria surfaces. The limiting steps of CO oxidation catalyzed by ceria via the Mars-van Krevelen reaction mechanism are identified and investigated. We address the adsorption of CO on the (111) and (110) surfaces, and its oxidation via participation of lattice oxygen leading to vacancy formation and CO₂ desorption. CO adsorption shows a strong surface selectivity: CO physisorbs on the (111) ceria surface, while it chemisorbs on the more open (110) surface yielding carbonate formation and surface reduction.

Experimental partners:

• INFM-CNR TASC National Laboratory (F. Esch, C. Africh, and G. Comelli)
• ELETTRA - Materials Science Beamline (V. Matolin)
• Università di Trieste (G. Balducci, P. Fornasiero, and Graziani)

M. Huang and S. Fabris
CO adsorption and oxidation on ceria surfaces from DFT+U calculations
to appear in J. Phys. Chem. C

M. Huang and S. Fabris
Role of surface peroxo and superoxo species in the low-temperature oxygen buffering of ceria: Density functional theory calculations
Phys. Rev. B 75, 081404, (2007)

G. Vicario, G. Balducci, S. Fabris, S. de Gironcoli, and S. Baroni
Interaction of Hydrogen with Cerium Oxide Surfaces: a Quantum Mechanical Computational Study
J. Phys. Chem. B 110, 19380 (2006)

F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, G. Comelli, and R. Rosei
Electron localization determines defect formation on ceria substrates
Science 309, 752 (2005)

S. Fabris, S. de Gironcoli, S. Baroni, G. Vicario, and G. Balducci
Taming multiple valency with density functionals: a case study of defective ceria''
Phys. Rev. B 71, 041102 (2005)

S. Fabris, G. Vicario, G. Balducci, S. de Gironcoli, and S. Baroni
Electronic and atomistic structures of clean and reduced ceria surfaces
J. Phys. Chem B 109, 22860 (2005)